Co-reporter:Yun Chen, Cheng Zhang, Liyi Li, Chia-Chi Tuan, Fan Wu, Xin Chen, Jian Gao, Yong Ding, and Ching-Ping Wong
Nano Letters July 12, 2017 Volume 17(Issue 7) pp:4304-4304
Publication Date(Web):June 14, 2017
DOI:10.1021/acs.nanolett.7b01320
Silicon (Si) zigzag nanowires (NWs) have a great potential in many applications because of its high surface/volume ratio. However, fabricating Si zigzag NWs has been challenging. In this work, a diffusion-controlled metal-assisted chemical etching method is developed to fabricate Si zigzag NWs. By tailoring the composition of etchant to change its diffusivity, etching direction, and etching time, various zigzag NWs can be easily fabricated. In addition, it is also found that a critical length of NW (>1 μm) is needed to form zigzag nanowires. Also, the amplitude of zigzag increases as the location approaches the center of the substrate and the length of zigzag nanowire increases. It is also demonstrated that such zigzag NWs can help the silicon substrate for self-cleaning and antireflection. This method may provide a feasible and economical way to fabricate zigzag NWs and novel structures for broad applications.Keywords: diffusion-controlled method; metal-assisted chemical etching; Silicon zigzag nanowires;
Co-reporter:Yun Chen, Liyi Li, Cheng Zhang, Chia-Chi Tuan, Xin Chen, Jian Gao, and Ching-Ping Wong
Nano Letters February 8, 2017 Volume 17(Issue 2) pp:
Publication Date(Web):January 19, 2017
DOI:10.1021/acs.nanolett.6b04410
Kinked silicon (Si) nanowires (NWs) have many special properties that make them attractive for a number of applications, such as microfluidics devices, microelectronic devices, and biosensors. However, fabricating NWs with controlled three-dimensional (3D) geometry has been challenging. In this work, a novel method called alternating metal-assisted chemical etching is reported for the fabrication of kinked Si NWs with controlled 3D geometry. By the use of multiple etchants with carefully selected composition, one can control the number of kinks, their locations, and their angles by controlling the number of etchant alternations and the time in each etchant. The resulting number of kinks equals the number times the etchant is alternated, the length of each segment separated by kinks has a linear relationship with the etching time, and the kinking angle is related to the surface tension and viscosity of the etchants. This facile method may provide a feasible and economical way to fabricate novel silicon nanowires, nanostructures, and devices for broad applications.Keywords: alternating MACE; controlling kink geometry; Kinked nanowires;
Co-reporter:Bo Song, Chia-Chi Tuan, Xiaogu Huang, Liyi Li, Kyoung-Sik Moon, Ching-Ping Wong
Materials Letters 2016 Volume 166() pp:12-15
Publication Date(Web):1 March 2016
DOI:10.1016/j.matlet.2015.11.108
•Formation of sulfonated polyaniline intercalated graphene nanostructure•Highly conductive and water-soluble rGO/SPANI composite•Electrode material with high charge storage and excellent cycling stabilityA sulfonated polyaniline (SPANI)/graphene nanocomposite has been fabricated as the electrode material for supercapacitors. The SPANI nanostructure that was uniformly embedded within the graphene network can effectively facilitate the charge transfer between graphene layers and improve the overall electrochemical performance. The as-synthesized nanocomposite has delivered high specific capacitance up to 262 F/g, excellent rate performance, and cycling stability with over 87% of capacitance retained after 10000 charge/discharge cycles.
Co-reporter:Binghe Xie, Yang Wang, Wenhui Lai, Wei Lin, Ziyin Lin, Zhexu Zhang, Peichao Zou, Yang Xu, Shuang Zhou, Cheng Yang, Feiyu Kang, Ching-Ping Wong
Nano Energy 2016 Volume 26() pp:276-285
Publication Date(Web):August 2016
DOI:10.1016/j.nanoen.2016.04.045
•Just one single process can both reduce and ablate GO electrode arrays with great convenience.•The first benchmark study of LPG-MPS with commercial-available counterparts.•Well-packaged LPG-MPS devices can be tailored into alien shapes to cater to various applications and integrated with printed circuits.With the development of wearable/flexible electronics, a formidable challenge is to integrate electronic components which were large in their original size into a flexible, thin, and arbitrary layout. As an indispensible component in electronics, commercial micro-supercapacitors are disadvantageous in their clumsy cuboid geometry and limited capacity, and are not promising for future applications. In comparison, film-like micro-supercapacitors are superior in miniaturized system integration since they can be folded to fit in restricted spaces while maintaining a high level of volumetric energy density. Here, we carried out a benchmark study of a state-of-the-art well-packaged thin film micro-supercapacitor toward commercial micro-supercapacitor and aluminum electrolyte capacitor. The micro-planar supercapacitor not only exhibits 3.75 times of a commercial micro-supercapacitor and 8785 times of an aluminum electrolytic capacitor in volumetric energy density under 1000 mV s−1 scan rate, but can also be tailored into diversified shapes, rolled up, and plugged into tiny interstitial spaces inside a device. Such ultrathin (18 µm) micro-supercapacitor component with high volumetric energy density (0.98 mWh cm−3 in LiCl-PVA gel, 5.7 mWh cm−3 in ionic liquid), can be integrated into an electronic device system and shows a series of superior performance characteristics over current commercial benchmarks, which may find vast applications.The laser processed graphene based micro-planar supercapacitor (LPG-MPS) component showed 3.75 and 8785 times in volumetric energy density to the commercial surface mountable supercapacitor (SMS) and aluminum electrolyte capacitor (AEC) under 1000 mV s−1. It can be fabricated in a large scale and is readily applicable for wearable and flexible electronics energy management.
Co-reporter:Liyi Li, Bo Song, Luke Maurer, Ziyin Lin, Gang Lian, Chia-Chi Tuan, Kyoung-Sik Moon, Ching-Ping Wong
Nano Energy 2016 Volume 21() pp:276-294
Publication Date(Web):March 2016
DOI:10.1016/j.nanoen.2016.01.028
•Graphene-amine compounds were synthesized through one-pot hydrothermal reactions.•The compounds showed high specific capacitance up to 612 F/g.•The amine molecules were covalently bonded to graphene sheets as spacers.In this work, chemical compounds of graphene oxide (GO) and amine molecules as spacers were synthesized by one-step hydrothermal reactions. The products of the reactions, referred to as rGO-Spacers samples, were fabricated into electrodes for supercapacitors. By engineering the molecular structures of the spacers, the specific capacitance (Cs) of the rGO-Spacers can reach up to 612 F/g at 2 mV/s in cyclic voltammetry (CV) measurement, 597 F/g at 0.5 A/g and 512 F/g at 10 A/g in galvanostatic charge–discharge measurement using two-electrode setup with 1 M sulfuric acid aqueous solution at the electrolyte. 97% of the high Cs was retained after 10 000 charge–discharge cycles. In comparison, the control sample without spacers presents a Cs of 194 F/g measured in CV measurement under the same conditions. The increased interlayer distance up to 1.9 nm between graphene sheets caused by addition of the amine spacers was confirmed by X-ray diffraction. The morphology of rGO-Spacers samples was characterized by scanning electron microscope, transmission electron microscope and atomic force microscope, where crumpled thin sheets were observed. The spacer molecules were found to disperse uniformly on the sheets by energy dispersive spectroscope mapping. Chemical reactions and covalent bonding between GO and the spacers are evidenced by thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), UV–visible spectroscopy, Fourier-transform infrared spectroscope and Raman spectroscopy. Based on the characterization results, the amide formation reactions and epoxide ring opening reactions between amine spacers and GO are proposed. The weight measurement and the atomic ratio information from XPS further reveal different reactivity of the spacers to GO, which could account for the different Cs of the samples with spacers of different molecular structures. From the results of these comparative studies, the engineering of the molecular structures was found to play a pivotal role in determining the value of Cs. The findings also provide guidance to achieve even higher Cs by optimization of the molecular structures of the spacers, as well as other experimental parameters which can facilitate the reaction between the spacers and GO.
Co-reporter:Zhuo Li;Taoran Le;Zhenkun Wu;Yagang Yao;Liyi Li;Manos Tentzeris;Kyoung-Sik Moon;C. P. Wong
Advanced Functional Materials 2015 Volume 25( Issue 3) pp:464-470
Publication Date(Web):
DOI:10.1002/adfm.201403275
Stretchable radio-frequency electronics are gaining popularity as a result of the increased functionality they gain through their flexible nature, impossible within the confines of rigid and planar substrates. One approach to fabricating stretchable antennas is to embed stretchable or flowable conductive materials, such as conductive polymers, conductive polymer composites, and liquid metal alloys as stretchable conduction lines. However, these conductive materials face many challenges, such as low electrical conductivity under mechanical deformation and delamination from substrates. In the present study, a silicone-based electrically conductive adhesive (silo-ECA) is developed that have a conductivity of 1.51 × 104 S cm−1 and can maintain conductivity above 1.11 × 103 S cm−1, even at a large stain of 240%. By using the stretchable silo-ECAs as a conductor pattern and pure silicone elastomers as a base substrate, stretchable antennas can be fabricated by stencil printing or soft-lithography. The resulting antenna's resonant frequency is tunable over a wide range by mechanical modulation. This fabrication method is low-cost, can support large-scale production, has high reliability over a wide temperature range, and eliminates the concerns of leaking or delamination between conductor and substrate experienced in previously reported micro-fluidic antennas.
Co-reporter:Zhen-Kun Wu, Ziyin Lin, Liyi Li, Bo Song, Chia-Chi Tuan, Zhuo Li, Kyoung-sik Moon, Shu-Lin Bai and Ching-Ping Wong
RSC Advances 2015 vol. 5(Issue 102) pp:84113-84118
Publication Date(Web):23 Sep 2015
DOI:10.1039/C5RA18108A
Various aromatic molecules have been reported to improve the performance of reduced graphene oxide (rGO)-based supercapacitors. However, the mechanism for this improvement remains unclear. Here we design a facile approach that clearly identifies the main reason for the enhancement. Benzene derivatives, namely p-phenylenediamine (PPD), m-phenylenediamine (MPD), o-phenylenediamine (OPD), hydroquinone (HQ), phenol, aniline and p-aminophenol (PAP) are incorporated into graphene oxide (GO) layers during their reduction and assembly at room temperature. We find that the capacitance increase mainly arises from the pseudocapacitance of specific benzene derivative molecules rather than their spacing effect. Moreover, the para and ortho substituted benzene derivatives contribute much more than the meta substituted ones do. With a small amount of PPD (∼11 wt%), the specific capacitance reaches 273 F g−1, much higher than that of pure rGO electrodes (113 F g−1). The hybrid electrode also shows great stability with a capacitance retention of up to 86% after 10000 charge/discharge cycles.
Co-reporter:Xiangning Lu, Liyi Li, Bo Song, Kyoung-sik Moon, Ningning Hu, Guanglan Liao, Tielin Shi, Chingping Wong
Nano Energy 2015 Volume 17() pp:160-170
Publication Date(Web):October 2015
DOI:10.1016/j.nanoen.2015.08.011
•p-Phenylenediamine modified GO/rGO were synthesized by a facile solution processing.•Propose routes of ‘Reaction first then Reduction’ and ‘Reduction first then Reaction’.•PPD molecules are covalently bonded onto the surface of GO sheets acting as spacers.•GPPDH exhibits excellent performance for supercapacitor application.•Provide fundamental insights of grafting organic molecules on graphene sheets.The p-phenylenediamine (PPD) modified graphene oxide (GO)/reduced graphene oxide (rGO) composite materials were synthesized by a facile solution processing with two different routes: ‘Reaction first then Reduction’ and ‘Reduction first then Reaction’, which are referred to as GPPDH and GHPPD, respectively. The two composites were employed as active supercapacitor electrode materials, their electrochemical properties were studied, and the mechanism how PPD improves the electrochemical performance is investigated systematically. GPPDH with the PPD molecules incorporated between the graphene sheets demonstrated a loose and crumpled microstructure. However, for GHPPD, the graphene sheets were stacked and cross-linked through PPD molecules. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) also provided quantitative information about the newly formed chemical constituents in the synthesized materials. The specific capacitance values of GPPDH and GHPPD are 316.54 and 249.24 F g−1 at 10 mV s−1, respectively. The synthesized materials also exhibited an excellent cycling stability as 93.66% (GPPDH) and 87.14% (GHPPD) retentions after 4000 cycles at current density of 2 A g−1.The results revealed the role of amine-based organic additives in graphene supercapacitor materials and validated the GPPDH as the electrodes of supercapacitor with an excellent electrochemical performance.The p-phenylenediamine (PPD) modified GO/rGO composite materials were synthesized by a facile solution processing with two different routes: ‘Reaction first then Reduction’ and ‘Reduction first then Reaction’, which are referred to as GPPDH and GHPPD, respectively. The mechanism is investigated by a systematic study. GPPDH with the PPD molecules inserted into the space of graphene sheets demonstrated a loose and crumpled microstructure. For GHPPD, the graphene sheets were restacked and cross-linked through PPD molecules. The synthesized material of GPPDH with high specific capacitance and good electrochemical cycling stability was considered to be a promising electrode material for supercapacitors.
Co-reporter:Bo Song, Liyi Li, Ziyin Lin, Zhen-Kun Wu, Kyoung-sik Moon, Ching-Ping Wong
Nano Energy 2015 Volume 16() pp:470-478
Publication Date(Web):September 2015
DOI:10.1016/j.nanoen.2015.06.020
•A solution-processible method to fabricate MSC electrodes with direct thin film deposition.•The rG/SP electrode material is highly conductive and aqueous dispersible.•Formation of interdigital patterns via shadow masking and plasma etching.•The device shows superior energy density, cycling and electromechanical stability.Lightweight and mechanically compliant energy storage devices have attracted tremendous interest due to the increasing demand for flexible and miniaturized electronics. Recently, micro-supercapacitors (MSCs) have advanced rapidly as potential micro-sized power sources, but it is still challenging to build the micro-device using cost-effective technologies for large-scale manufacture. Here, we developed a simple solution-processible method to fabricate microelectrode patterns using a water-dispersible graphene/sulfonated polyaniline (rG/SP) as the active MSC material. The highly stabilized rG/SP dispersion in aqueous solution enables the direct thin-film deposition on flexible substrates and the formation of interdigital patterns by plasma etching. The as-fabricated solid-state MSC delivers an ultrahigh areal capacitance of 3.31 mF/cm2 and volumetric stack capacitance of 16.55 F/cm3 with excellent rate and cycling performance. Furthermore, the rG/SP based MSC demonstrates a superior energy density of 1.51 mW h/cm3 while maintaining high power density. The micro-device also shows superior mechanical stability with 96.5% of capacitance retained under different bending and twisting conditions, which makes it suitable specifically for portable and wearable electronic applications.
Co-reporter:Zhuo Li, Bo Song, Zhenkun Wu, Ziyin Lin, Yagang Yao, Kyoung-Sik Moon, C.P. Wong
Nano Energy 2015 Volume 11() pp:711-718
Publication Date(Web):January 2015
DOI:10.1016/j.nanoen.2014.11.018
•Capacitive deionization (CDI) is an emerging technology to supply deionized water as liquid coolant for the thermal management in microelectronics.•KOH-activated graphene has an ultrahigh specific surface area of 3513 m2/g and electrical conductivity of 104 S/m.•KOH-activated graphene-based CDI can achieve an electrosorption capacity of 11.86 mg/g, which is the highest value among graphene-based electrode reported for CDI applications at the same conditions.Capacitive deionization (CDI) is an emerging technology to supply deionized water as liquid coolant for the thermal management in microelectronics. Graphene has been demonstrated as a promising candidate for CDI electrode. However, the performance of current graphene-based CDI is far below expectation due to the limited specific surface area (SSA) and electrical conductivity of the chemically reduced graphene. Here we presented a KOH-activated graphene that has ultrahigh SSA of 3513 m2/g and electrical conductivity of 104 S/m. With improved materials properties, an ultrahigh electrosorption capacity of 11.86 mg/g and a significant adsorption rate of 20 min are achieved.
Co-reporter:Chia-Yun Chen and Ching-Ping Wong
Nanoscale 2014 vol. 6(Issue 2) pp:811-816
Publication Date(Web):18 Oct 2013
DOI:10.1039/C3NR04956F
Highly-sensitive, reliable and reproducible Raman-active substrates via a facile and organic-free method are reported. These intriguing hierarchical structures are formed through the uniform incorporation of silver (Ag) nanoflowers with aluminium (Al) micro-supporters. The underlying mechanism is systematically investigated, visualizing that the solvents used in galvanic displacement have a major effect on diversifying the reaction kinetics of Ag deposition. Moreover, the exploration of AgNO3 concentrations reveals a drastic transition of Ag morphologies, driven by the elimination of high-energy surfaces of Ag. In addition, the surface-modified Al@Ag structures with octadecyltrichlorosilane demonstrate both the non-wetting (contact angle = 157.2°), as well as easy droplet roll-off (contact angle hysteresis = 5.4°) characteristics, which further enables the tested targets to avoid being pinned at a static position upon detection. Finally, we find that the Ag nanoflower surfaces are corrugated with numerous nanogaps at interparticle sites, in such a way that allows the abundant active sites (referred to as “hot spots”) to amplify the Raman signal, and simultaneously maintain the sound reliability and reproducibility of Raman detection. These designs along with the fabrication strategy are anticipated to benefit versatile optical, optoelectronic and energy devices.
Co-reporter:Liyi Li, Yan Liu, Xueying Zhao, Ziyin Lin, and Ching-Ping Wong
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 1) pp:575
Publication Date(Web):November 21, 2013
DOI:10.1021/am4046519
Recently, metal-assisted chemical etching (MaCE) has been proposed as a promising wet-etching method for the fabrication of micro- and nanostructures on silicon with low cost. However, uniform vertical trench etching with high aspect ratio is still of great challenge for traditional MaCE. Here we report an innovated MaCE method, which combined the use of a nanoporous gold thin film as the catalyst and a hydrofluoric acid (HF)–hydrogen peroxide (H2O2) mixture solution with a low HF-to-H2O2 concentration ratio (ρ) as the etchant. The reported method successfully fabricated vertical trenches on silicon with a width down to 2 μm and an aspect ratio of 16. The geometry of the trenches was highly uniform throughout the 3D space. The vertical etching direction was favored on both (100)- and (111)-oriented silicon substrates. The reported method was also capable of producing multiple trenches on the same substrate with individually-tunable lateral geometry. An etching mechanism including a through-catalyst mass-transport process and an electropolishing-favored charge-transport process was identified by a comparative study. The novel method fundamentally solves the problems of distortion and random movement of isolated catalysts in MaCE. The results mark a breakthrough in high-quality silicon trench-etching technology with a cost of more than 2 orders of magnitude lower than that of the currently available methods.Keywords: deep trenches; electropolishing; high aspect ratio; metal-assisted chemical etching; nanoporous catalyst; uniformity;
Co-reporter:Zhuo Li;Kyoung-sik Moon;Ziyin Lin;Yagang Yao;Stewart Wilkins;C. P. Wong
Journal of Applied Polymer Science 2014 Volume 131( Issue 12) pp:
Publication Date(Web):
DOI:10.1002/app.40355
ABSTRACT
Carbon nanotubes (CNTs) are found to inhibit the free-radical cross-linking of vinyl-terminated polydimethylsiloxane (PDMS) by trapping the free radicals in a CNT/PDMS composite. The cross-linking density values measured by swelling test, equilibrium stress-strain test, glass-transition temperature and Raman spectroscopy all decrease with the addition of CNTs. The inhibition effect is shown to be more significant with increased functionality and curvature in the CNTs used. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40355.
Co-reporter:Yongcun Zhou, Yining Chen, Hong Wang, C.P. Wong
Materials Letters 2014 Volume 119() pp:64-67
Publication Date(Web):15 March 2014
DOI:10.1016/j.matlet.2014.01.009
Co-reporter:Yan Liu, Arnab Das, Ziyin Lin, Ian B. Cooper, Ajeet Rohatgi, C.P. Wong
Nano Energy 2014 Volume 3() pp:127-133
Publication Date(Web):January 2014
DOI:10.1016/j.nanoen.2013.11.002
•Black Si with a front average weighted reflectance of 1.4%.•Cost effective screen-printed solar cell with a conversion efficiency of 16.5%.•The hierarchical structure features mechanical robustness and self-cleaning properties.•Nano-scale texture plays an important role in low light reflection and superhydrophobicity.Hierarchically textured Si is explored for low reflection, and self-cleaning solar cells, which was fabricated from micropyramids with nanostructures that were added by Au-assisted electroless chemical etching. Such a structure creates a graded effective refractive index from air to Si and leads to a black appearance due to broadband reflection suppression in the 300–1200 nm range. A front average weighted reflectance of 1.4% was achieved, which is one of the lowest values reported for black Si solar cells. Variation in the size of the nano-scale texture was found to have greater impact on the reflectance of the hierarchal structure than the size of the micro-scale pyramid texture. The hierarchial texturing was applied to a cost effective screen-printed, aluminum back-surface field solar cell resulting in a conversion efficiency of 16.5%. In addition, the hierarchical structure features mechanical robustness and self-cleaning properties.
Co-reporter:Tianchi Wang, Lijing Chang, Benjamin Hatton, Jian Kong, Guang Chen, Yang Jia, Dangsheng Xiong, Chingping Wong
Materials Science and Engineering: C 2014 Volume 43() pp:310-316
Publication Date(Web):1 October 2014
DOI:10.1016/j.msec.2014.07.022
•Plant leaves as templates to make a superhydrophobic ZnO/carbon surface.•The ZnO/carbon surface copied the macro and micro morphology of the leaf surface.•The ZnO/carbon surface had an good superhydrophobicity similar to the templates.A lotus leaf was used as a template to fabricate superhydrophobic biomorphic ZnO on a carbon substrate (ZnO/C). First, a carbon substrate with the microstructure of a leaf surface was obtained by sintering a lotus leaf in argon. A biomorphic ZnO/C material was then obtained by immersing this carbon substrate into a Zn(NO3)2 solution and sintering. Finally, the hydrophobicity of the surfaces of the products thus obtained was investigated. This ZnO/C material exhibited excellent superhydrophobicity and low adhesion after it was modified with fluorine silane. The water contact angle of the resulting product was 162°, which exceeds that of the lotus leaf (157°) and is much higher than that of smooth carbon covered with the same fluorine silane (114°). Moreover, this surface displayed a stable superhydrophobic performance even after exposure to ambient air for several months.
Co-reporter:Zhuo Li;Rongwei Zhang;Kyoung-Sik Moon;Yan Liu;Kristen Hansen;Taoran Le;C. P. Wong
Advanced Functional Materials 2013 Volume 23( Issue 11) pp:1459-1465
Publication Date(Web):
DOI:10.1002/adfm.201202249
Abstract
Flexible interconnects are one of the key elements in realizing next-generation flexible electronics. While wire bonding interconnection materials are being deployed and discussed widely, adhesives to support flip-chip and surface-mount interconnections are less commonly used and reported. A polyurethane (PU)-based electrically conductive adhesive (ECA) is developed to meet all the requirements of flexible interconnects, including an ultralow bulk resistivity of ≈1.0 × 10−5 Ω cm that is maintained during bending, rolling, and compressing, good adhesion to various flexible substrates, and facile processing. The PU-ECA enables various interconnection techniques in flexible and printed electronics: it can serve as a die-attach material for flip-chip, as vertical interconnect access (VIA)-filling and polymer bump materials for 3D integration, and as a conductive paste for wearable radio-frequency devices.
Co-reporter:Yagang Yao;Lorenzo Tolentino;Zhongzheng Yang;Xiaojuan Song;Wen Zhang;Yongsheng Chen
Advanced Functional Materials 2013 Volume 23( Issue 28) pp:3577-3583
Publication Date(Web):
DOI:10.1002/adfm.201201843
Abstract
Molybdenum disulfide (MoS2) nanosheets have been attracting increasing research interests due to their unique material properties. However, the lack of a reliable large-scale production method impedes their practical applications. Here a facile, efficient, and scalable method for the fabrication of high-concentration aqueous dispersion of MoS2 nanosheets using combined grinding and sonication is reported. The 26.7 ± 0.7 mg/mL concentration achieved is the highest concentration in an aqueous solution reported up to now. Grinding generates pure shear forces to detach the MoS2 layers from the bulk materials. Subsequent sonication further breaks larger crystallites into smaller crystallites, which promotes the dispersion of MoS2 nanosheets in ethanol/water solutions. The exfoliation process establishes a new paradigm in the top-down fabrication of 2D nanosheets in aqueous solution. In the meantime, MoS2-based sensing film produced using this approach has successfully demonstrated the feasibility of a low-cost and efficient NH3 gas sensor using inkjet printing as a viable method.
Co-reporter:Owen J. Hildreth, Konrad Rykaczewski, Andrei G. Fedorov and Ching P. Wong
Nanoscale 2013 vol. 5(Issue 3) pp:961-970
Publication Date(Web):04 Dec 2012
DOI:10.1039/C2NR32293E
Metal-assisted Chemical Etching of silicon has recently emerged as a powerful technique to fabricate 1D, 2D, and 3D nanostructures in silicon with high feature fidelity. This work demonstrates that out-of-plane rotational catalysts utilizing polymer pinning structures can be designed with excellent control over rotation angle. A plastic deformation model was developed establishing that the catalyst is driven into the silicon substrate with a minimum pressure differential across the catalyst thickness of 0.4–0.6 MPa. Force–displacement curves were gathered between an Au tip and Si or SiO2 substrates under acidic conditions to show that Derjaguin and Landau, Verwey and Overbeek (DLVO) based forces are capable of providing restorative forces on the order of 0.2–0.3 nN with a calculated 11–18 MPa pressure differential across the catalyst. This work illustrates that out-of-plane rotational structures can be designed with controllable rotation and also suggests a new model for the driving force for catalyst motion based on DLVO theory. This process enables the facile fabrication of vertically aligned thin-film metallic structures and scalloped nanostructures in silicon for applications in 3D micro/nano-electromechanical systems, photonic devices, nanofluidics, etc.
Co-reporter:Zhuo Li, Kristen Hansen, Yagang Yao, Yanqing Ma, Kyoung-sik Moon and C. P. Wong
Journal of Materials Chemistry A 2013 vol. 1(Issue 28) pp:4368-4374
Publication Date(Web):13 May 2013
DOI:10.1039/C3TC30612G
The conduction development mechanism of silicone-based electrically conductive adhesives (Silo-ECAs) is studied. The reduction of surfactants on silver flakes by hydride in the silicone backbone and the subsequent sintering of the generated silver nanoparticles between micron-sized silver flakes are found to be the major contributor to the conductivity development in Silo-ECAs; this is in contrast with the mechanism observed in most polymer–metal ECAs, where the curing shrinkage of the polymer matrix is the major cause of conductivity development. The conductivity development in Silo-ECAs ceases when the polymer curing is completed. Hence, in order to enhance the resulting electrical conductivity, the curing process is prolonged by using a long-chain prepolymer, lowering the platinum catalyst concentration, or adding curing inhibitors. A bulk resistivity of 8.82 × 10−5 Ω cm is achieved, which is 55% lower than the best values reported previously for Silo-ECAs.
Co-reporter:Ziyin Lin, Yan Liu, Sathyanarayanan Raghavan, Kyoung-sik Moon, Suresh K. Sitaraman, and Ching-ping Wong
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 15) pp:7633
Publication Date(Web):July 1, 2013
DOI:10.1021/am401939z
We report magnetic alignment of hexagonal boron nitride (hBN) platelets and the outstanding material properties of its polymer composite. The magnetically responsive hBN is produced by surface modification of iron oxide, and their orientations can be controlled by applying an external magnetic field during polymer curing. Owing to the anisotropic properties of hBN, the epoxy composite with aligned hBN platelets shows interesting properties along the alignment direction, including significantly reduced coefficient of thermal expansion, reaching ∼28.7 ppm/°C, and enhanced thermal conductivity, 104% higher than that of unaligned counterpart, both of which are observed at a low filler loading of 20 wt %. Our modeling suggests the filler alignment is the major reason for these intriguing material properties. Finite element analysis reveals promising applications for the magnetically aligned hBN-based composites in modern microelectronic packaging.Keywords: coefficient of thermal expansion; hexagonal boron nitride; magnetic alignment; polymer composite; thermal conductivity;
Co-reporter:Chia-Yun Chen and Ching-Ping Wong
Chemical Communications 2013 vol. 49(Issue 66) pp:7295-7297
Publication Date(Web):12 Jul 2013
DOI:10.1039/C3CC43466D
Multiple etching pathways for the formation of porous Si nanostructures are visualized with the addition of ethylene glycol in metal-assisted chemical etching. The monotonic transition from solid to porous morphologies of Si nanostructures along with remarkable photoluminescence (PL) emission efficiency and distinct wetting phenomena can be observed.
Co-reporter:Ziyin Lin, Zhuo Li, Kyoung-sik Moon, Yunnan Fang, Yagang Yao, Liyi Li, Ching-ping Wong
Carbon 2013 Volume 63() pp:547-553
Publication Date(Web):November 2013
DOI:10.1016/j.carbon.2013.07.033
We developed a facile method to transfer vertically aligned carbon nanotubes (VACNT) onto a carbon fiber paper (CFP) substrate using silicone as an adhesion layer. The resulting VACNT–CFP hybrid structure displays a very low interface resistance of 0.006 Ω as a result of the Van der Waals interaction at the VACNT–CFP interface. We demonstrated in this work that the VACNT–CFP can be used as a supercapacitor electrode with excellent rate performance and cycling stability, which could be attributed to the vertically alignment and chemical stability of CNTs in the hybrid structure. Further, we found that VACNT–CFP showed good resistance to the impact of flowing water when tested as a capacitive deionizer electrode, owing to superior mechanical robustness of the hybrid structure.
Co-reporter:Ziyin Lin, Gordon H. Waller, Yan Liu, Meilin Liu, Ching-ping Wong
Carbon 2013 Volume 53() pp:130-136
Publication Date(Web):March 2013
DOI:10.1016/j.carbon.2012.10.039
We prepared nitrogen-doped graphene (NG) by simple pyrolysis of graphene oxide and polyaniline, which was selected as the N source. The resulting NG contains 2.4 at.% N, of which as high as 1.2 at.% is quaternary N. Electrochemical characterizations reveal that the NG has excellent catalytic activity toward oxygen reduction reaction (ORR) in an alkaline electrolyte, including a desirable four-electron pathway for the formation of water, large kinetic-limiting current density, long-term stability and good tolerance to methanol crossover. In addition, we demonstrate that the NG also has high catalytic activity toward oxygen evolution reaction (OER), rendering its potential application as a bi-functional catalyst for both ORR and OER.
Co-reporter:Zhuo Li, Kyoung-Sik Moon, Yagang Yao, Kristen Hansen, Kenneth Watkins, Liliane Morato, C.P. Wong
Carbon 2013 Volume 65() pp:71-79
Publication Date(Web):December 2013
DOI:10.1016/j.carbon.2013.07.105
Carbon nanotube (CNT)/polymer composites with filler loading close to percolation threshold are demonstrated as sensor materials to monitor the thermal aging of insulation components. In order to represent the real aging conditions of the monitored component, the sensor is made of the same polymer material as the one used in the monitored component and placed in close proximity to the component. During thermal aging, the polymer matrix shrinks, leading to a slight increase of volume percentage of CNTs and a substantial decrease in resistivity of the sensors. The sensors are aged at different temperatures and the resistivity change at each temperature is fitted into the modified Arrhenius equation to yield the activation energy of the aging reactions. The activation energy obtained from the sensor signals is similar to those obtained by conventional elongation measurements and the thermal evaluation of the insulation component according to IEEE standard 275. Therefore, the resistivity value of the sensor can be used to accurately predict the remaining life of the monitored parts. These sensors provide many advantages over conventional conditioning monitoring methods, including no damage to the insulation system, real-time monitoring, facile operation, low cost, and much higher sensitivity.
Co-reporter:Ziyin Lin, Gordon H. Waller, Yan Liu, Meilin Liu, Ching-ping Wong
Nano Energy 2013 Volume 2(Issue 2) pp:241-248
Publication Date(Web):March 2013
DOI:10.1016/j.nanoen.2012.09.002
Nitrogen-doped graphene (NG) is a promising metal-free catalyst for oxygen reduction reaction (ORR) in fuel cells and metal–air batteries. However, its practical application hinges on significant cost reduction by using novel synthetic methods and further improvement of the catalytic activity by increasing the density of catalytically active site. Here we report a low-cost, scalable, synthetic method for preparation of NG via pyrolysis of graphene oxide with a rationally selected N source polypyrrole. Because of the large number of N atoms in pyrrole ring, polypyrrole can facilitate the formation of graphitic N, which is considered vital for high catalytic activity. The resulting 3D porous structure of NG has an N doping level of 2–3 at%, of which as high as 44% are graphitic N. Electrochemical characterizations show that NG has high catalytic activity toward ORR in an alkaline electrolyte via a favorable four-electron pathway for the formation of water, leading to high performance and low polarization loss. The NG also displays excellent long-term stability and resistance to methanol crossover, offering performance characteristics superior to those of a commercial Pt/C catalyst. The effect of pyrolysis temperature on the structure and property of NG are revealed using X-ray photoelectron spectroscopy and electrochemical measurements, providing important insights into the rational optimization of electrocatalytic activity for ORR. In addition, the NG also shows high catalytic activity toward oxygen evolution reaction (OER), rendering its potential application as a bifunctional electrocatalyst for both ORR and OER.Graphical abstractHighlights► Low-cost and scalable preparation of nitrogen-doped graphene. ► Enriched graphitic nitrogen functionality that is vital for high catalytic activity. ► Excellent catalytic performance toward oxygen reduction and oxygen evolution reactions.
Co-reporter:Yan Liu;Arnab Das;Sheng Xu;Ziyin Lin;Chen Xu;Zhong Lin Wang;Ajeet Rohatgi;Ching Ping Wong
Advanced Energy Materials 2012 Volume 2( Issue 1) pp:47-51
Publication Date(Web):
DOI:10.1002/aenm.201100287
Co-reporter:Yagang Yao, Ziyin Lin, Zhuo Li, Xiaojuan Song, Kyoung-Sik Moon and Ching-ping Wong
Journal of Materials Chemistry A 2012 vol. 22(Issue 27) pp:13494-13499
Publication Date(Web):04 Apr 2012
DOI:10.1039/C2JM30587A
Two-dimensional (2D) nanomaterials such as graphene, boron nitride (BN), and molybdenum disulfide (MoS2) have been attracting increasing research interest in the past few years due to their unique material properties. However, the lack of a reliable large-scale production method is an inhibiting issue for their practical applications. Here we report a facile, efficient, and scalable method for the fabrication of monolayer and few-layer BN, MoS2, and graphene using combined low-energy ball milling and sonication. Ball milling generates two forces on layered materials, shear force and compression force, which can cleave layered materials into 2D nanosheets from the top/bottom surfaces, and the edge of layered materials. Subsequent sonication would further break larger crystallites into smaller crystallites. These fabricated 2D nanosheets can be well dispersed in aqueous solutions at high concentrations, 1.2 mg mL−1 for BN, 0.8 mg mL−1 for MoS2, and 0.9 mg mL−1 for graphene, which are highly advantageous over other methods. These advantages render great potential in the construction of high-performance 2D material-based devices at low cost. For example, a prototype gas sensor is demonstrated in our study using graphene and MoS2, respectively, which can detect several ppm of ammonia gas.
Co-reporter:Yan Liu, Ziyin Lin, Wei Lin, Kyoung Sik Moon, and C. P. Wong
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 8) pp:3959
Publication Date(Web):July 5, 2012
DOI:10.1021/am300778d
Tuning the surface wettability is of great interest for both scientific research and practical applications. We demonstrated reversible transition between superhydrophobicity and superhydrophilicity on a ZnO nanorod/epoxy composite film. The epoxy resin serves as an adhesion and stress relief layer. The ZnO nanorods were exposed after oxygen reactive ion etching of the epoxy matrix. A subsequent chemcial treatment with fluoroalkyl and alkyl silanes resulted in a superhydrophobic surface with a water contact angle up to 158.4° and a hysteresis as low as 1.3°. Under UV irradiation, the water contact angle decreased gradually, and the surface eventually became superhydrophilic because of UV induced decomposition of alkyl silanes and hydroxyl absorption on ZnO surfaces. A reversible transition of surface wettability was realized by alternation of UV illumination and surface treatment. Such ZnO nanocomposite surface also showed improved mechanical robustness.Keywords: reversible wettability; UV irradiation; ZnO nanocomposite films;
Co-reporter:Yagang Yao, Ching-ping Wong
Carbon 2012 Volume 50(Issue 14) pp:5203-5209
Publication Date(Web):November 2012
DOI:10.1016/j.carbon.2012.07.003
The synthesis of monolayer graphene is the key to graphene’s practical applications. Herein we report a facile and scalable technique to grow monolayer graphene on Cu, Ni, Co, and Fe surfaces using an etching-aided chemical vapor deposition (CVD) process. The growth was performed using an additional step of hydrogen etching in atmospheric pressure CVD after stopping the carbon supply. The etching of formed multi-layer graphene for Cu substrates assists the formation of monolayer graphene. The etching of excessive dissolved carbon for Ni, Co, and Fe substrates really helps to suppress the troublesome carbon precipitation which is believed to cause the non-uniform thickness of the produced graphene. We believe this technique is not only limited to Cu, Ni, Co, and Fe surfaces but also can be extend to other metal substrates such as Pt, Au, Pd, and Ru if choosing appropriate carbon precursors. We also found out that by varying the time of hydrogen exposure both monolayer and bilayer graphene were successfully synthesized on Ni surfaces. Metal substrates with high carbon solubility in them seem to hold great advantages in the layer-controlled synthesis of graphene. Our findings open a new pathway for an efficient growth of monolayer graphene and will facilitate graphene research.
Co-reporter:Ziyin Lin, Min-kyu Song, Yong Ding, Yan Liu, Meilin Liu and Ching-ping Wong
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 10) pp:3381-3387
Publication Date(Web):17 Jan 2012
DOI:10.1039/C2CP00032F
Nitrogen-doped graphene (nG) is a promising metal-free catalyst for oxygen reduction reaction (ORR) on the cathode of fuel cells. Here we report a facile preparation of nGviapyrolysis of graphene oxide with melamine. The morphology of the nG is revealed using scanning electron microscopy and transmission electron microscopy while the successful N doping is confirmed by electron energy loss spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The resulting nG shows high electrocatalytic activity toward ORR in an alkaline solution with an onset potential of −0.10 V vs.Ag/AgCl reference electrode. The nG catalyzed oxygen reduction exhibits a favorable formation of watervia a four-electron pathway. Good stability and anti-crossover property are also observed, which are advantageous over the Pt/C catalyst. Furthermore, the effect of pyrolysis temperature on the structure and activity of nG is systematically studied to gain some insights into the chemical reactions during pyrolysis.
Co-reporter:Tianchi Wang, Song Kong, Lijing Chang, Chingping Wong
Ceramics International 2012 Volume 38(Issue 8) pp:6783-6788
Publication Date(Web):December 2012
DOI:10.1016/j.ceramint.2012.05.074
Abstract
Natural silk fibers were used as the template to prepare biomorphic ZrO2 fibers. Silk fibers were first immersed into a Zr(NO3)4 solution and then sintered in air at high temperatures to produce the final ZrO2 fibers. Their microstructures, phases, synthesis process, infrared absorption spectra and thermal conductivity were analyzed. The results show that these synthesized fibers retained the morphologies of silk faithfully. These ZrO2 fibers also obtained the ability of absorbing infrared from the silk, so that they possessed better heat-insulating property than the traditional ZrO2 fibers.
Co-reporter:Zhuo Li, Yi Gao, Kyoung-Sik Moon, Yagang Yao, Allen Tannenbaum, C.P. Wong
Polymer 2012 Volume 53(Issue 7) pp:1571-1580
Publication Date(Web):22 March 2012
DOI:10.1016/j.polymer.2012.01.048
A facile and objective method is introduced to automatically quantify the filler dispersion in polymer composites through image analysis. This method consists of automatic identification of the fillers in the image and a rigorous measurement of the filler dispersion within the space of functions. Compared with previous methods, this method has the advantages of 1) automatically recognizing the fillers, 2) minimizing the subjectivity induced by the inhomogeneity and noise of the background in the images, 3) a mathematically more rigorous definition of the deviation of the filler dispersion from uniformity, 4) a single performance metric reflecting both the distribution and the size of fillers. Both synthetic and real images from model compounds are used to demonstrate the sensitivity of the proposed method to the dispersion and aggregation of fillers. The computed dispersion index shows good agreement with visual observation of synthetic images and mechanical properties of the model compounds.
Co-reporter:Dr. Qizhen Liang;Sinsar Alec Hsie; Ching Ping Wong
ChemPhysChem 2012 Volume 13( Issue 16) pp:3700-3706
Publication Date(Web):
DOI:10.1002/cphc.201200450
Abstract
Microwaves (MWs) are applied to initialize deoxygenation of graphene oxide (GO) in the solid state and at low temperatures (∼165 °C). The Fourier-transform infrared (FTIR) spectra of MW-reduced graphene oxide (rGO) show a significantly reduced concentration of oxygen-containing functional groups, such as carboxyl, hydroxyl and carbonyl. X-ray photoelectron spectra confirm that microwaves can promote deoxygenation of GO at relatively low temperatures. Raman spectra and TGA measurements indicate that the defect level of GO significantly decreases during the isothermal solid-state MW-reduction process at low temperatures, corresponding to an efficient recovery of the fine graphene lattice structure. Based on both deoxygenation and defect-level reduction, the resurgence of interconnected graphene-like domains contributes to a low sheet resistance (∼7.9×104 Ω per square) of the MW-reduced GO on SiO2-coated Si substrates with an optical transparency of 92.7 % at ∼547 nm after MW reduction, indicating the ultrahigh efficiency of MW in GO reduction. Moreover, the low-temperature solid-state MW reduction is also applied in preparing flexible transparent conductive coatings on polydimethylsiloxane (PDMS) substrates. UV/Vis measurements indicate that the transparency of the thus-prepared MW-reduced GO coatings on PDMS substrates ranges from 34 to 96 %. Correspondingly, the sheet resistance of the coating ranges from 105 to 109 Ω per square, indicating that MW reduction of GO is promising for the convenient low-temperature preparation of transparent conductors on flexible polymeric substrates.
Co-reporter:Owen J. Hildreth, Andrei G. Fedorov, and Ching Ping Wong
ACS Nano 2012 Volume 6(Issue 11) pp:10004
Publication Date(Web):October 4, 2012
DOI:10.1021/nn303680k
The ability to fabricate 3D spiraling structures using metal-assisted chemical etching (MaCE) is one of the unique advantages of MaCE over traditional etching methods. However, control over the chirality of the spiraling structures has not been established. In this work, a systematic parametric study was undertaken for MaCE of star-shaped catalysts, examining the influence of arm shape, arm length, number of arms, center core diameter, and catalyst thickness on the rotation direction. This data was used to identify a set of geometric parameters that reliably induce rotation in a predefined direction such that large arrays of 3D spiraling structures can be fabricated with the same chirality. Electroless deposition into the MaCE template was used to examine the full etch path of the catalyst and an experimental fit was established to control rotation angle by adjusting the catalyst’s center core diameter. The ability to fabricate large arrays of 3D spiraling structures with predefined chirality could have important applications in photonics and optoelectronics.Keywords: 3D; chiral; etching; metal-assisted chemical etching; nanotechnology; silicon
Co-reporter:Owen J. Hildreth;Devin Brown;Ching P. Wong
Advanced Functional Materials 2011 Volume 21( Issue 16) pp:3119-3128
Publication Date(Web):
DOI:10.1002/adfm.201100279
Abstract
Pinned structures in conjunction with shaped catalysts are used in metal-assisted chemical etching (MACE) of silicon to induce out-of-plane rotational etching. Sub-micro- and nanostructures are fabricated in silicon, which include scooped-out channels and curved subsurface horns, along with vertically oriented thin metal structures. Five different etching modes induced by catalyst and pinning geometry are identified: 1) fully pinned–no etching, 2) rotation via twist, 3) rotation via delamination, 4) in-plane bending, and 5) swinging. The rotation angle is roughly controlled through catalyst geometry. The force and pressure experienced by the catalyst are calculated from the deformation of the catalyst and range between 0.5–3.5 μN and 0.5–3.9 MPa, respectively. This is a new, simple method to fabricate 3D, heterogeneous sub-micro- and nanostructures in silicon with high feature fidelity on the order of tens of nanometers while providing a method to measure the forces responsible for catalyst motion during MACE.
Co-reporter:Cheng Yang;Wei Lin;Zhongyu Li;Rongwei Zhang;Haoran Wen;Bo Gao;Guohua Chen;Ping Gao;Matthew M. F. Yuen;Ching Ping Wong
Advanced Functional Materials 2011 Volume 21( Issue 23) pp:4582-4588
Publication Date(Web):
DOI:10.1002/adfm.201101433
Abstract
This paper reports the first high-performance water-based isotropically conductive adhesives (WBICAs) – a promising material for both electrical interconnects and printed circuits for ultralow-cost flexible/foldable printed electronics. Through combining surface iodination and in situ reduction treatment, the electrically conductivity of the WBICAs are dramatically improved (8 × 10-5 Ω cm with 80 wt% of silver); moreover, their reliability (stable for at least 1440 h during 85 °C/85% RH aging) meets the essential requirements for microelectronic applications. Prototyped applications in carrying light emitting diode (LED) arrays and radio frequency identification (RFID) antennas on flexible substrates were demonstrated, which showed satisfactory performances. Moreover, their water-based character may render them more environmentally benign (no volatile organic chemicals involved in the printing and machine maintenance processes), more convenient in processing (reducing the processing steps), and energy economic (thermally sintering the silver fillers and curing the resin is not necessary unlike conventional ICAs). Therefore, they are especially advantageous for mass-fabricating flexible electronic devices when coupled with paper and other low-cost substrate materials such as PET, PI, wood, rubber, and textiles.
Co-reporter:Zhuo Li, Wei Lin, Kyoung-Sik Moon, Stewart J. Wilkins, Yagang Yao, Ken Watkins, Liliane Morato, Chingping Wong
Carbon 2011 Volume 49(Issue 13) pp:4138-4148
Publication Date(Web):November 2011
DOI:10.1016/j.carbon.2011.05.042
An anomalous decrease in the thermal stability of silicone was observed when carbon nanotubes (CNTs) were added as fillers. The decreased thermal stability is found to result from the residues of cobalt nanoparticles in CNTs, whereas CNTs synthesized with other metal catalysts do not show such a phenomenon. The analysis of thermal degradation products indicates that CNT fillers do not change the mechanism of the thermal degradation of silicone but cobalt nanoparticles within CNTs may accelerate the degradation through free radical generation. Radical scavengers such as hindered amines and impurity-free CNTs, or removal of cobalt nanoparticles by acid treatment, can mitigate the accelerated thermal degradation.
Co-reporter:Rongwei Zhang, Kyoung-sik Moon, Wei Lin, Josh C. Agar, Ching-Ping Wong
Composites Science and Technology 2011 Volume 71(Issue 4) pp:528-534
Publication Date(Web):28 February 2011
DOI:10.1016/j.compscitech.2011.01.001
In recent years, efforts to prepare flexible highly conductive polymer composites at low temperatures for flexible electronic applications have increased significantly. Here, we describe a novel approach for the preparation of flexible highly conductive polymer composites (resistivity: 2.5 × 10−5 Ω cm) at a low temperature (150 °C), enabling the wide use of low cost, flexible substrates such as paper and polyethylene terephthalate (PET). The approach involves (i) in situ reduction of silver carboxylate on the surface of silver flakes by a flexible epoxy (diglycidyl ether of polypropylene glycol) to form highly surface reactive nano/submicron-sized particles; (ii) the in situ formed nano/submicron-sized particles facilitate the sintering between silver flakes during curing. Morphology and Raman studies indicated that the improved electrical conductivity was the result of sintering and direct metal–metal contacts between silver flakes. This approach developed for the preparation of flexible highly conductive polymer composites offers significant advantages, including simple low temperature processing, low cost, low viscosity, suitability for low-cost jet dispensing technologies, flexibility while maintaining high conductivity, and tunable mechanical properties. The developed flexible highly conductive materials with these advantages are attractive for current and emerging flexible electronic applications.
Co-reporter:Qizhen Liang, Xuxia Yao, Wei Wang, Yan Liu, and Ching Ping Wong
ACS Nano 2011 Volume 5(Issue 3) pp:2392
Publication Date(Web):March 8, 2011
DOI:10.1021/nn200181e
Thermally conductive functionalized multilayer graphene sheets (fMGs) are efficiently aligned in large-scale by a vacuum filtration method at room temperature, as evidenced by SEM images and polarized Raman spectroscopy. A remarkably strong anisotropy in properties of aligned fMGs is observed. High electrical (∼386 S cm−1) and thermal conductivity (∼112 W m−1 K−1 at 25 °C) and ultralow coefficient of thermal expansion (∼−0.71 ppm K−1) in the in-plane direction of A-fMGs are obtained without any reduction process. Aligned fMGs are vertically assembled between contacted silicon/silicon surfaces with pure indium as a metallic medium. Thus-constructed three-dimensional vertically aligned fMG thermal interfacial material (VA-fMG TIM) architecture has significantly higher equivalent thermal conductivity (75.5 W m−1 K−1) and lower contact thermal resistance (5.1 mm2 K W−1), compared with their counterpart from A-fMGs that are recumbent between silicon surfaces. This finding provides a throughout approach for a graphene-based TIM assembly as well as knowledge of vertically aligned graphene architectures, which may not only facilitate graphene’s application in current demanding thermal management but also promote its widespread applications in electrodes of energy storage devices, conductive polymeric composites, etc.Keywords: alignment; anisotropy; assembly; multilayer graphene; thermal interfacial materials
Co-reporter:Yagang Yao ; Zhuo Li ; Ziyin Lin ; Kyoung-Sik Moon ; Josh Agar ;Chingping Wong
The Journal of Physical Chemistry C 2011 Volume 115(Issue 13) pp:5232-5238
Publication Date(Web):March 11, 2011
DOI:10.1021/jp109002p
The effects of graphene growth parameters on the number of its layers were systematically studied and a new growth mechanism on Cu substrate was thus proposed. Through the investigation of the graphene growth parameters, including growth substrate types, carrier gases, types of carbon sources, growth temperature, growth time, and cooling rates, we found that graphene grows on Cu substrates via a surface-catalyzed process, followed by a templated growth. We can obtain either single layer graphene (SLG) or few-layer graphene (FLG) by suppressing the subsequent templated growth with a low concentration of carbon source gases and a high concentration of H2. Our findings provide important guidance toward the synthesis of large-scale and high-quality FLGs and SLGs. This is expected to widen both the research and applications of graphene.
Co-reporter:Ziyin Lin ; Yan Liu ; Yagang Yao ; Owen J. Hildreth ; Zhuo Li ; Kyoungsik Moon
The Journal of Physical Chemistry C 2011 Volume 115(Issue 14) pp:7120-7125
Publication Date(Web):March 14, 2011
DOI:10.1021/jp2007073
We report the superior capacitance of functionalized graphene prepared by controlled reduction of graphene oxide (GO). In a solvothermal method, GO dispersed in dimethylformamide (DMF) was thermally treated at a moderate temperature (150 °C), which allows a fine control of the density of functionalities. Surface functionalities on graphene would enable a high pseudocapacitance, good wetting property, and acceptable electric conductivity. A specific capacitance up to 276 F/g was achieved based on functionalized graphene at a discharge current of 0.1 A/g in a 1 M H2SO4 electrolyte, which is much higher than the benchmark material. The excellent performance of the functionalized graphene signifies the importance of controlling the surface chemistry of graphene-based materials.
Co-reporter:Wei Lin ;C. P. Wong
Advanced Materials 2010 Volume 22( Issue 11) pp:1177-1179
Publication Date(Web):
DOI:10.1002/adma.200902189
No abstract is available for this article.
Co-reporter:Cheng Yang;Yu-Tao Xie;Matthew Ming-Fai Yuen;Bing Xu;Bo Gao;Xiaomin Xiong;C. P. Wong
Advanced Functional Materials 2010 Volume 20( Issue 16) pp:2580-2587
Publication Date(Web):
DOI:10.1002/adfm.201000673
Abstract
The electrical conductivity of a silver microflake-filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF-SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF-SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four-point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the −40 ∼ 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component.
Co-reporter:Cheng Yang;Yu-Tao Xie;Matthew Ming-Fai Yuen;Bing Xu;Bo Gao;Xiaomin Xiong;C. P. Wong
Advanced Functional Materials 2010 Volume 20( Issue 16) pp:
Publication Date(Web):
DOI:10.1002/adfm.201090071
Abstract
The electrical conductivity of a silver microflake-filled conductive composites is dramatically improved after a filler surface treatment. By a simple iodine solution treatment, nonstoichiometric silver/silver iodide nanoislands form on the silver filler surface. Evidence of the decrease of surface silver oxide species is provided by TOF-SIMS and the redox property of the nanoclusters is studied using cyclic voltammetry and TOF-SIMS depth profile analyses. The redox property of the nanoclusters on silver flakes helps enhance the electrical conductivity of the conductive composites. The electrical resistivity of the improved conductive composites is measured by four-point probe method; the reliability of the printed thin film resistors is evaluated by both the 85 °C/85% relative humidity moisture exposure and the −40 ∼ 125 °C thermal cycling exposure. The conductive composite printed radio frequency identification (RFID) antennas with 27.5 wt% of the modified silver flake content show comparable performance in the RFID tag read range versus copper foil antennas, and better than those commercial conductive adhesives that require much higher silver content (i.e., 80 wt%). This work suggests that a surface chemistry method can significantly reduce the percolation threshold of the loading level of the silver flakes and improve the electrical conductivity of an important printed electronic passive component.
Co-reporter:Zhuo Li, Yagang Yao, Ziyin Lin, Kyoung-Sik Moon, Wei Lin and Chingping Wong
Journal of Materials Chemistry A 2010 vol. 20(Issue 23) pp:4781-4783
Publication Date(Web):04 May 2010
DOI:10.1039/C0JM00168F
Through direct absorption of microwave irradiation by GO film, we developed a rapid, dry approach to synthesize reduced graphene.
Co-reporter:Rongwei Zhang, Kyoung-sik Moon, Wei Lin and C. P. Wong
Journal of Materials Chemistry A 2010 vol. 20(Issue 10) pp:2018-2023
Publication Date(Web):20 Jan 2010
DOI:10.1039/B921072E
Highly conductive polymer nanocomposites with very low resistivity (4.8 × 10−5 Ω cm) were prepared by thermal sintering of silver nanoparticles with silver flakes dispersed in a polymer matrix at 180 °C. By comparative studies of thermal behavior of Ag nanoparticles, the critical processing temperature required to obtain very low resistivity of polymer nanocomposites has been identified for Ag nanoparticles with different surface properties. The results indicate that the decomposition temperature of surface residues on Ag nanoparticles plays a key role in the sintering of Ag nanoparticles and thus the electrical resistivity of the polymer nanocomposites. Electrical measurements of the polymer nanocomposites showed that morphological changes induced by sintering of Ag nanoparticle with Ag flakes considerably contribute to the reduction of the contact resistance between conductive fillers, increasing the nanocomposite conductivity.
Co-reporter:Rongwei Zhang, Wei Lin, Kyoung-sik Moon, and C. P. Wong
ACS Applied Materials & Interfaces 2010 Volume 2(Issue 9) pp:2637
Publication Date(Web):August 24, 2010
DOI:10.1021/am100456m
We show the fast preparation of printable highly conductive polymer nanocomposites for future low-cost electronics. Highly conductive polymer nanocomposites, consisting of an epoxy resin, silver flakes, and incorporated silver nanoparticles, have been prepared by fast sintering between silver flakes and the incorporated silver nanoparticles. The fast sintering is attributed to: 1) the thermal decomposition of silver carboxylate—which is present on the surface of the incorporated silver flakes—to form in situ highly reactive silver nanoparticles; 2) the surface activation of the incorporated silver nanoparticles by the removal of surface residues. As a result, polymer nanocomposites prepared at 230 °C for 5 min, at 260 °C for 10 min, and using a typical lead-free solder reflow process show electrical resistivities of 8.1 × 10−5, 6.0 × 10−6, and 6.3 × 10−5 Ω cm, respectively. The correlation between the rheological properties of the adhesive paste and the noncontact printing process has been discussed. With the optimal rheological properties, the formulated highly viscous pastes (221 mPa s at 2500 s−1) can be non-contact-printed into dot arrays with a radius of 130 μm. The noncontact printable polymer nanocomposites with superior electrical conductivity and fast processing are promising for the future of printed electronics.Keywords: conductive adhesive; interconnect; nanocomposite; printable; rheology; sintering
Co-reporter:Wei Lin, Rongwei Zhang, Kyoung-Sik Moon, C.P. Wong
Carbon 2010 Volume 48(Issue 1) pp:107-113
Publication Date(Web):January 2010
DOI:10.1016/j.carbon.2009.08.033
A novel assembling process of incorporating carbon nanotubes as thermal interface materials for heat dissipation has been developed by synthesizing vertically aligned carbon nanotubes on a copper substrate and chemically bonding the carbon nanotubes to a silicon surface. The assembling process and the copper/carbon nanotubes/silicon structure are compatible with current flip-chip technique. The carbon nanotubes are covalently bonded to the silicon surface via a thin but effective bridging layer as a “molecular phonon coupler” at the CNT–silicon interface to mitigate phonon scattering. Experimental results indicate that such an interface modification improves the effective thermal diffusivity of the carbon nanotube-mediated thermal interface by an order of magnitude and conductivity by almost two orders of magnitude. The interfacial adhesion is dramatically enhanced as well, which is significant for reliability improvement of the thermal interface materials.
Co-reporter:Cheng Yang, Yu-Tao Xie, Matthew M. F. Yuen, Xiaomin Xiong and C. P. Wong
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 43) pp:14459-14461
Publication Date(Web):30 Sep 2010
DOI:10.1039/C0CP00414F
This communication describes a new surface-enhanced Raman scattering (SERS) active silver substrate prepared by iodination of the evaporated silver foil. After iodination, the morphology of the silver substrate undergoes a self-evolution process in which it displays accordingly the UV-vis absorption shift as well as the AFM topological test. Rhodamine 6G (R6G) is used as the probe molecule to evaluate the enhancement efficiency of the silver substrate at different self-evolution time intervals. The SERS intensity of R6G increases up to ∼29-fold and reaches a maximum after the substrate evolved for 24 h. This method is feasible for the production of an efficient SERS silver substrate.
Co-reporter:Rongwei Zhang, Wei Lin, Kevin Lawrence, C.P. Wong
International Journal of Adhesion and Adhesives 2010 Volume 30(Issue 6) pp:403-407
Publication Date(Web):September 2010
DOI:10.1016/j.ijadhadh.2010.01.004
One of the main hurdles for the wide use of current Ag-filled, isotropically conductive adhesives (ICAs) is the high cost of Ag fillers, while the challenge for low cost copper-filled ICAs is their poor reliability. In this paper, highly reliable, low cost ICAs in which the copper flakes used as filler are coated with silver (Ag-coated Cu flakes) have been developed. With Ag-coated Cu flakes modified by an amine-based silane coupling agent (SCA), the ICAs with the resistivity (2.4×10−4 Ω cm) comparable to that of commercially available Ag-filled ICAs have been achieved. Moreover, the contact resistance of the ICAs filled with the modified Ag-coated Cu flakes on a Ni/Au surface can be stabilized (less than 10% increase) for more than 1000 h of aging at 85 °C and 85% RH and after three reflows with a peak reflow temperature of 255 °C. The causes leading to the higher conductivity and the better reliability of the ICAs filled with Ag-coated Cu flakes modified by amine-based silane coupling agent are discussed.
Co-reporter:Ziyin Lin, Yagang Yao, Zhuo Li, Yan Liu, Zhou Li and Ching-Ping Wong
The Journal of Physical Chemistry C 2010 Volume 114(Issue 35) pp:14819-14825
Publication Date(Web):August 19, 2010
DOI:10.1021/jp1049843
The thermal behavior of graphite oxide (GO) is essential to study and design GO reduction and functionalizion reaction. We provide a detailed description of the thermal reduction of GO dispersed in solvent (H2O and dimethylformamide (DMF)) at temperatures of 100 and 150 °C, respectively. The thermal stability and structure change of GO during the thermal treatment were characterized using UV−vis spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). It was found that at temperatures of 100 and 150 °C the carboxylic and carbonyl groups decomposed. GO dispersed in H2O and DMF at temperatures of 100 and 150 °C exhibited increased rates of reduction than in dry condition. Moreover, the reduction rate was found to be highly dependent on the solvent used. At 150 °C, DMF accelerates the GO reduction rate significantly, while dimethyl sulfoxide (DMSO) has less acceleration effect. However, ethylene glycol (EG) reduces the reduction rate compared to dry conditions. The solvent−GO interaction, including polar−polar interaction and hydrogen bonding, was discussed as one possible reason for the solvent-dependent reduction.
Co-reporter:Ziyin Lin, Yan Liu, and Ching-ping Wong
Langmuir 2010 Volume 26(Issue 20) pp:16110-16114
Publication Date(Web):September 21, 2010
DOI:10.1021/la102619n
We demonstrated a facile strategy of producing superhydrophobic octadecylamine (ODA)-functionalized graphite oxide (GO) films. ODA was chemically grafted on GO sheets by the nucleophilic substitution reaction of amine groups with epoxy groups. The long hydrocarbon chain in ODA reduces the surface energy of the GO sheet. The fabricated ODA-functionalized GO film exhibited a high contact angle (163.2°) and low hysteresis (3.1°). This method is promising in terms of low-cost and large-scale superhydrophobic coatings and has potential applications for surface modification of GO paper or other GO-based composite materials.
Co-reporter:Wei Lin;Rong-Wei Zhang;Seung-Soon Jang; Ching-Ping Wong ;Jung-Il Hong
Angewandte Chemie International Edition 2010 Volume 49( Issue 43) pp:7929-7932
Publication Date(Web):
DOI:10.1002/anie.201001244
Co-reporter:Yan Liu, Yonghao Xiu, Dennis W. Hess and C. P. Wong
Langmuir 2010 Volume 26(Issue 11) pp:8908-8913
Publication Date(Web):March 5, 2010
DOI:10.1021/la904686c
Superoleophobic surfaces display contact angles >150° with liquids that have lower surface energies than does water. The design of superoleophobic surfaces requires an understanding of the effect of the geometrical shape of etched silicon surfaces on the contact angle and hysteresis observed when different liquids are brought into contact with these surfaces. This study used liquid-based metal-assisted etching and various silane treatments to create superoleophobic surfaces on a Si(111) surface. Etch conditions such as the etch time and etch solution concentration played critical roles in establishing the oleophobicity of Si(111). When compared to Young’s contact angle, the apparent contact angle showed a transition from a Cassie to a Wenzel state for low-surface-energy liquids as different silane treatments were applied to the silicon surface. These results demonstrated the relationship between the re-entrant angle of etched surface structures and the contact angle transition between Cassie and Wenzel behavior on etched Si(111) surfaces.
Co-reporter:Ching-Ping Wong;Wei Lin;Ling-Bo Zhu
Frontiers of Optoelectronics 2010 Volume 3( Issue 2) pp:139-142
Publication Date(Web):2010 June
DOI:10.1007/s12200-010-0009-9
This paper addresses the state-of-the-art nanoscience and technology regarding next generation high density microelectronics and photonics packaging applications, including carbon nanotubes (CNTs) for electrical/thermal devices, and molecular wires for electrical interconnects, etc.
Co-reporter:Wei Lin, Kyoung-Sik Moon, Shanju Zhang, Yong Ding, Jintang Shang, Mingxiang Chen and Ching-ping Wong
ACS Nano 2010 Volume 4(Issue 3) pp:1716
Publication Date(Web):February 17, 2010
DOI:10.1021/nn901621c
An ultrafast microwave annealing process has been developed to reduce the defect density in vertically aligned carbon nanotubes (CNTs). Raman and thermogravimetric analyses have shown a distinct defect reduction in the CNTs annealed in microwave for 3 min. Fibers spun from the as-annealed CNTs, in comparison with those from the pristine CNTs, show increases of ∼35% and ∼65%, respectively, in tensile strength (∼0.8 GPa) and modulus (∼90 GPa) during tensile testing; an ∼20% improvement in electrical conductivity (∼80000 S m−1) was also reported. The mechanism of the microwave response of CNTs was discussed.Keywords: carbon nanotube; defect; electrical conductivity; mechanical properties; microwave
Co-reporter:Wei Lin;Kyoung-Sik Moon ;C. P. Wong
Advanced Materials 2009 Volume 21( Issue 23) pp:2421-2424
Publication Date(Web):
DOI:10.1002/adma.200803548
Co-reporter:Owen James Hildreth, Wei Lin and Ching Ping Wong
ACS Nano 2009 Volume 3(Issue 12) pp:4033
Publication Date(Web):December 2, 2009
DOI:10.1021/nn901174e
Metal-assisted chemical etching (MaCE) of silicon in conjunction with shaped catalysts was used to fabricate 3D nanostructures such as sloping channels, cycloids, and spirals along with traditional vertical channels. The investigation used silver nanorods, nanodonuts along with electron beam lithography (EBL)-patterned gold nanodiscs, nanolines, squares, grids, and star-shaped catalysts to show how catalyst shape and line width directly influence etching direction. Feature sizes ranging from micrometers down to 25 nm were achieved with aspect ratios of at least 10:1 and wall roughness of 10 nm or less. This research demonstrates the potential of MaCE as a new, maskless nanofabrication technology.Keywords: 3D nanostructures; etching; metal-assisted chemical etching; nanofabrication; silicon
Co-reporter:Yonghao Xiu, Fei Xiao, Dennis W. Hess, C.P. Wong
Thin Solid Films 2009 Volume 517(Issue 5) pp:1610-1615
Publication Date(Web):1 January 2009
DOI:10.1016/j.tsf.2008.09.081
A eutectic liquid (choline chloride and urea) that served as a templating agent in sol–gel processing was used to prepare thin silica films on glass microscope slides. Subsequent extraction of the eutectic liquid yielded a film with a rough surface. After treating the film surface with a fluoroalkyl silane, the surface became superhydrophobic with a contact angle ∼ 170° and a contact angle hysteresis < 10°. The optical transmittance of the film coated on the glass slide was comparable to that of the microscope glass slide. Atomic Force Microscopy (AFM) was used to characterize the surface structures; a tipless probe allowed measurement of the force of interaction with superhydrophobic surfaces. The interaction force between the AFM probe and the superhydrophobic surface was reduced greatly compared to that between the probe and the flat surface treated with fluoroalkyl silane.
Co-reporter:Jiongxin Lu, Kyoung-Sik Moon and C. P. Wong
Journal of Materials Chemistry A 2008 vol. 18(Issue 40) pp:4821-4826
Publication Date(Web):28 Aug 2008
DOI:10.1039/B807566B
A silver (Ag)-polymer nanocomposite has been developed by in-situ formation of metal nanoparticles within the polymer matrix and utilized as a high-dielectric constant (k) polymer matrix to enhance the dielectric properties of high-k composite materials. By using an in-situ photochemical reduction method, uniformly dispersed Ag nanoparticles of size of around 10 nm were generated in polymer matrices. Self-passivated aluminium (Al) particles were incorporated into this Ag-epoxy matrix and the dielectric properties of the as-prepared composite materials were investigated. The composites showed more than 50% increase in k values as compared with an Al/neat epoxy composite with the same filler loading of Al. The dielectric loss tangent of the Al/Ag-epoxy composites was below 0.1, which meets the requirement for embedded decoupling capacitors. These results suggest that the Ag-epoxy high-kpolymer matrix effectively enhances the dielectric constant while maintaining the low dielectric loss of the high-k composites. In addition, detailed dielectric property measurements revealed that the dielectric properties and their frequency dispersion as well as the breakdown behaviors of the Al/Ag-epoxy composites were related to the incorporation and concentration of Ag nanoparticles in the high-kpolymer matrix.
Co-reporter:Yonghao Xiu, Dennis W. Hess, C.P. Wong
Journal of Colloid and Interface Science 2008 Volume 326(Issue 2) pp:465-470
Publication Date(Web):15 October 2008
DOI:10.1016/j.jcis.2008.06.042
A method for the preparation of inorganic superhydrophobic silica coatings using sol–gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol–gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 °C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 °C and UV testing times of 5500 h. Contact angles remained >160° with contact angle hysteresis ∼2°.The UV stability was significantly improved after the hydrophobic surface groups (isobutyl) were replaced by fluoroalkyl groups.
Co-reporter:Yangyang Sun;Lingbo Zhu;Hongjin Jiang;Jiongxin Lu
Journal of Electronic Materials 2008 Volume 37( Issue 11) pp:1691-1697
Publication Date(Web):2008 November
DOI:10.1007/s11664-008-0533-1
Carbon nanotube (CNT) arrays/films were transferred onto copper substrates via eutectic tin/lead (SnPb) solder pastes. The morphologies, thermal stabilities, adhesion to substrates, and electrical properties of the as-transferred CNT arrays were studied. The CNT arrays generated negligible expansion or contraction below 250°C. The adhesion of CNT arrays to the substrate was significantly improved by the transfer process. An ohmic contact was formed between the transferred CNT arrays and the Sn-Pb solder. Four-probe electrical measurements yielded the resistance of the as-transferred CNT films under the electrode to be around 0.0056 Ω, from which the resistivity of each individual CNT tube was calculated to be 2.44 × 10−4 Ω cm.
Co-reporter:Hongjin Jiang;Kyoung-sik Moon;Yangyang Sun
Journal of Nanoparticle Research 2008 Volume 10( Issue 1) pp:41-46
Publication Date(Web):2008 January
DOI:10.1007/s11051-007-9234-6
Shape and size controlled gram level synthesis of tin/indium (SnIn) alloy nanoparticles and nanobundles is reported. Poly(N-vinylpyrrolidone) (PVP) was employed as a capping agent, which could control the growth and structure of the alloy particles under varying conditions. Transmission electron microscopy showed that unique SnIn alloy nanobundles could be synthesized from the bulk materials above a certain concentration of PVP and below this concentration, discrete spherical nanoparticles of variable size were evolved. The morphology and the composition of the as-synthesized SnIn alloy nanobundles were investigated by high-resolution transmission electron microscopy (TEM). The possible mechanisms on the formation of these structures were discussed.
Co-reporter:Yonghao Xiu ; Lingbo Zhu ; Dennis W. Hess ;C. P. Wong
The Journal of Physical Chemistry C 2008 Volume 112(Issue 30) pp:11403-11407
Publication Date(Web):July 2, 2008
DOI:10.1021/jp711571k
Low contact angle hysteresis is an important characteristic of superhydrophobic surfaces for nonstick applications involving the exposure of these surfaces to water or dust particles. In this article, a relationship is derived between the surface work of adhesion and the dynamic contact angle hysteresis, and the resulting predictions are compared with experimental data. Superhydrophobic surfaces with different contact angles and contact angle hysteresis were prepared by generating silicon pillars with varying pillar size and pitch. Surfaces were subsequently treated with fluoroalkyl silanes to modify further the hydrophobicity. The three-phase contact line established for such systems was related to the Laplace pressure needed to maintain a stable superhydrophobic state.
Co-reporter:Yonghao Xiu, Shu Zhang, Vijay Yelundur, Ajeet Rohatgi, Dennis W. Hess and C. P. Wong
Langmuir 2008 Volume 24(Issue 18) pp:10421-10426
Publication Date(Web):August 19, 2008
DOI:10.1021/la801206m
Silicon is employed in a variety of electronic and optical devices such as integrated circuits, photovoltaics, sensors, and detectors. In this paper, Au-assisted etching of silicon has been used to prepare superhydrophobic surfaces that may add unique properties to such devices. Surfaces were characterized by contact angle and contact angle hysteresis. Superhydrophobic surfaces with reduced hysteresis were prepared by Au-assisted etching of pyramid-structured silicon surfaces to generate hierarchical surfaces. Consideration of the Laplace pressure on hydrophobized hierarchical surfaces gives insight into the manner by which contact is established at the liquid/composite surface interface. Light reflectivity from the etched surfaces was also investigated to assess application of these structures to photovoltaic devices.
Co-reporter:Lingbo Zhu, Jianwen Xu, Fei Xiao, Hongjin Jiang, Dennis W. Hess, C.P. Wong
Carbon 2007 Volume 45(Issue 2) pp:344-348
Publication Date(Web):February 2007
DOI:10.1016/j.carbon.2006.09.014
A CNT stack formation technique is used for the pseudo in-situ monitoring of CNT growth in the kinetics-controlled regime. CNT stacks are fabricated by water-assisted selective etching and the cyclic introduction of ethylene into the chemical vapor deposition (CVD) reactor. By varying the growth temperature and the ethylene flow rate within growth cycles, the reaction was shown to be first order with an activation energy of 201.2 kJ/mol. This technique can be implemented to monitor the initial stage of CNT growth by deceasing the ethylene flow time in growth cycles. Within one minute, the CNT growth reaches steady state. The formation of CNT stacks could be tailored to investigate the effects of growth parameters, such as pressure, temperature, and carbon source flow rate, on the CNT growth.
Co-reporter:Hongjin Jiang, Lingbo Zhu, Kyoung-sik Moon, C.P. Wong
Carbon 2007 Volume 45(Issue 3) pp:655-661
Publication Date(Web):March 2007
DOI:10.1016/j.carbon.2006.10.006
Metal nanoparticles were synthesized by using surface-modified carbon nanotubes (m-CNTs) and pristine CNTs (p-CNTs) as stabilizing substrates. The surfaces of the m-CNTs were modified by the addition of hydrogen peroxide during the chemical vapor deposition synthesis. X-ray photoelectron spectroscopy showed oxygen peaks, which came from C–OH and C–O–C functional groups on the m-CNT walls. Unlike the p-CNTs, the m-CNTs could be used as excellent stabilizing substrates for metal nanoparticles which were produced by using resorcinol as a reducing agent via a sonochemical method. They could stabilize the nanoparticles and prevent them from agglomeration.
Co-reporter:Jianwen Xu;C.P. Wong
Journal of Applied Polymer Science 2007 Volume 103(Issue 3) pp:1523-1528
Publication Date(Web):8 NOV 2006
DOI:10.1002/app.24957
A novel, photodefinable, high dielectric constant (high-k) nanocomposite material was developed for embedded capacitor applications. It consists of SU8 as the polymer matrix and barium titanate (BT) nanoparticles as the filler. The UV absorption characteristics of BT nanoparticles were studied with a UV-Vis spectrophotometer. The effects of BT nanoparticle size, filler loading, and UV irradiation dose on SU8 photopolymerization were systematically investigated. The dielectric properties of the photodefined SU8 nanocomposites were characterized. Embedded capacitors using the novel high dielectric constant SU8 composite photoresist were demonstrated on a flexible polyimide substrate by the UV lithography method. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1523–1528, 2007
Co-reporter:Yonghao Xiu;Yangyang Sun;C. P. Wong;Fei Xiao;Yangyang Sun;C. P. Wong;Yonghao Xiu;Fei Xiao
Journal of Applied Polymer Science 2007 Volume 104(Issue 4) pp:2113-2121
Publication Date(Web):7 FEB 2007
DOI:10.1002/app.25746
Polyhedral oligomeric silsesquioxanes (POSS) epoxy hybrid composites have attracted much research interest because of their unique structure, versatile synthetic approaches, and changeable properties through molecular tailoring. Octakis(dimethylsiloxypropylglycidyl ether)silsesquioxane and octakis(dimethylsiloxyethylcyclohexenyl epoxide)silsesquioxane were synthesized and cured with 4,4′-methylenebis(cyclohexylamine) and 4-methylhexahydrophthalic anhydride to prepare the highly crosslinked hybrid materials. The thermochemical data from DSC analysis and FTIR show that the curing reactions of POSS epoxy are more difficult than diglycidyl ether of bisphenol A (DGEBA) resin because of steric hindrance. The multifunctional structure of POSS can form highly crosslinked network throughout the composites, therefore the polymer main framework is frozen and cannot move freely. Some POSS composites do not show glass transitions and the observed relaxation for other POSS composites is likely due to the motion of tethers between POSS cores. The moduli of POSS composites, which decrease slowly with temperature increasing, are much higher than that of DGEBA resins at high temperatures. Although the coefficients of thermal expansion of POSS composites are larger than that of DGEBA resins at low temperatures, they are less dependent on temperature and relatively low at high temperatures. The unique thermal and mechanical properties of POSS composites make them potential candidates for applications in high temperature and temperature variable environments. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007
Co-reporter:Myung Jin Yim;Yi Li;Kyoung Sik Moon;C.P. Wong
Journal of Electronic Materials 2007 Volume 36( Issue 10) pp:1341-1347
Publication Date(Web):2007 October
DOI:10.1007/s11664-007-0204-7
This paper describes the development and characterization of isotropically conductive adhesives (ICAs) incorporating copper (Cu) powders as electrically conductive fillers, along with a silane coupling agent for oxidation protection of copper powders, for environmentally friendly, low cost and high thermal reliability applications in microelectronics packaging. The effect of silane coupling agent materials and concentration on the electrical conductivity, thermal stability and reliability of Cu-filled ICAs was investigated for potential alternatives of conventional silver-filled ICAs. The surface characteristics of silane thin films on copper surfaces, such as their hydrophobicity and thermal stability, were also evaluated to compare the performance of antioxidant behaviors of different silane coupling agents for Cu-filled ICAs. The low contact resistance and high thermal stability of the contact resistance of Cu-filled ICAs were achieved by addition of an optimized silane coupling agent. Greater thermal stability and improved reliability of Cu-filled ICAs under high temperature and humidity conditions were achieved with a silane coupling agent of high molecular weight and hydrophobicity. The bulk resistivity of ∼10−4 Ωcm of Cu-filled ICAs was achieved with bimodal filler loading.
Co-reporter:Yi Li;Myung Jin Yim;C.P. Wong
Journal of Electronic Materials 2007 Volume 36( Issue 5) pp:549-554
Publication Date(Web):2007 May
DOI:10.1007/s11664-007-0101-0
In this paper, we introduce a novel approach of using π-conjugated molecular wires to improve the electrical properties of nonconductive films (NCFs) for the electronic interconnects. Two thiol (-SH) terminated conjugated molecular wires, 1,4-benezenedithiol (BDT) and biphenyldithiol (BPD), are incorporated into an NCF formulation and investigated. The molecular wires can be well assembled on the gold (Au) electrodes and improve the interfacial properties of the NCF joints. The BPD exhibits higher thermal stability after curing at 150°C than BDT due to the higher aromaticity (more aromatic rings) and thus more rigid structure. Formation of a self-assembled monolayer on Au electrodes can tune the effective work function (Φ) of Au electrodes and reduce the tunnel resistivity. Therefore, the joint resistance of NCF joints, which is the sum of tunnel resistance and constriction resistance, can be significantly reduced from 0.15 × 10−3 Ω to 0.1 × 10−3 Ω and 0.05 × 10−3 Ω with BDT and BPD, respectively. The improved electrical conduction and current carrying capability enables the application of the NCF in fine pitch and high performance electronic interconnects in microelectronics.
Co-reporter:Hai Dong;Adam Meininger;Hongjin Jiang;Kyoung-Sik Moon
Journal of Electronic Materials 2007 Volume 36( Issue 5) pp:593-597
Publication Date(Web):2007 May
DOI:10.1007/s11664-007-0112-x
Cobalt ferrite nanocomposites were investigated as example materials for microelectronic applications in the ultrahigh frequency range. Both static magnetic properties (magnetization versus applied field curve) and dynamic properties (complex permeability and permittivity at frequencies up to 1 GHz) were studied. When the ferritic density reached a certain level, the coercivity of the composite material decreased and the permeability increased noticeably. This phenomenon indicated the establishment of magnetic coupling between particles, which is necessary for magnetic nanocomposites to be used in electronic applications. Compared to magnetic composites made of micron-sized fillers, the magnetic loss of the nanocomposites was lower. However, the permeability of the nanocomposite was also lower. Cobalt zinc ferrite nanocomposites were prepared to explore the effect of composition, which revealed that the permeability of the magnetic nanocomposite could be improved by modifying the composition.
Co-reporter:Jiongxin Lu, Kyoung-Sik Moon, Jianwen Xu and C. P. Wong
Journal of Materials Chemistry A 2006 vol. 16(Issue 16) pp:1543-1548
Publication Date(Web):08 Feb 2006
DOI:10.1039/B514182F
Dielectric properties of in-situ formed silver (Ag) incorporated carbon black (CB)/polymer composites were studied. In-situ formed Ag nanoparticles in the Ag/CB/epoxy composites increased the dielectric constant (K) value and decreased the dissipation factor (Df). The remarkably increased dielectric constant of the nanocomposite is due to the piling of charges at the extended interface of the interfacial polarization-based composites. The reduced dielectric loss might be due to the Coulomb blockade effect of the contained Ag nanoparticles, the well-known quantum effect of metal nanoparticles. The size, size distribution and loading level of metal nanoparticles in the nanocomposite were found to have significant influences on the dielectric properties of the nanocomposite system.
Co-reporter:Lingbo Zhu, Jianwen Xu, Yonghao Xiu, Yangyang Sun, Dennis W. Hess, C.P. Wong
Carbon 2006 Volume 44(Issue 2) pp:253-258
Publication Date(Web):February 2006
DOI:10.1016/j.carbon.2005.07.037
The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we describe a microelectronics compatible process for growing high-aspect-ratio CNT arrays with application to vertical electrical interconnects. A lift-off process was used to pattern catalyst (Al2O3/Fe) islands to diameters of 13 or 20 μm. After patterning, chemical vapor deposition (CVD) was involved to deposit highly aligned CNT arrays using ethylene as the carbon source, and argon and hydrogen as carrier gases. The as-grow CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs have high purity, and form densely-aligned arrays with controllable array size and height. Two-probe electrical measurements of the CNT arrays indicate a resistivity of ∼0.01 Ω cm, suggesting possible use of these CNTs as interconnect materials.
Co-reporter:Hongjin Jiang, Kyoung-sik Moon, Hai Dong, Fay Hua, C.P. Wong
Chemical Physics Letters 2006 Volume 429(4–6) pp:492-496
Publication Date(Web):5 October 2006
DOI:10.1016/j.cplett.2006.08.027
Tin nanoparticles with various sizes were synthesized by the chemical reduction method and their thermal properties were first studied by differential scanning calorimetry. Both particle size dependent melting temperature and latent heat of fusion have been observed. The experimental results in this study showed reasonable agreement with the thermodynamics model.Different sized tin nanoparticles were synthesized by the chemical reduction method. Differential scanning calorimetry results showed that the melting point of the synthesized Sn nanoparticles depended nonlinearly on the particle radius, which were in reasonable agreement with Lai et al. model.
Co-reporter:Yi Li;C. P. Wong;Kyoung-Sik Moon
Journal of Applied Polymer Science 2006 Volume 99(Issue 4) pp:1665-1673
Publication Date(Web):6 DEC 2005
DOI:10.1002/app.22509
The electrical properties of anisotropically conductive adhesives (ACAs) joints through low temperature sintering of nano silver (Ag) particles were investigated and compared with that of the submicron-sized Ag-filled ACA and lead-free solder joints. The nano Ag particles used exhibited sintering behavior at significantly lower temperatures (<200°C) than at the bulk Ag melting temperature (960°C). The sintered nano Ag particles significantly reduced the joint resistance and enhanced the current carrying capability of ACA joints. The improved electrical performance of ACA was attributed to the reduced interfaces between the Ag particles and the increased interfacial contact area between nano Ag particles and bond pads by the particle sintering. The reduced joint resistance was comparable to that of the lead-free (tin/3.5 silver/0.5 copper) metal solder joints. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1665–1673, 2006
Co-reporter:Yi Li, C.P. Wong
Materials Science and Engineering: R: Reports 2006 Volume 51(1–3) pp:1-35
Publication Date(Web):30 January 2006
DOI:10.1016/j.mser.2006.01.001
Tin–lead solder alloys are widely used in the electronic industry. They serve as interconnects that provide the conductive path required to achieve connection from one circuit element to another. There are increasing concerns with the use of tin–lead alloy solders in recognition of hazards of using lead. Lead-free solders and electrically conductive adhesives (ECAs) have been considered as the most promising alternatives of tin-lead solder. ECAs consist of a polymeric resin (such as, an epoxy, a silicone, or a polyimide) that provides physical and mechanical properties such as adhesion, mechanical strength, impact strength, and a metal filler (such as, silver, gold, nickel or copper) that conducts electricity. ECAs offer numerous advantages over conventional solder technology, such as environmental friendliness, mild processing conditions (enabling the use of heat-sensitive and low-cost components and substrates), fewer processing steps (reducing processing cost), low stress on the substrates, and fine pitch interconnect capability (enabling the miniaturization of electronic devices). Therefore, conductive adhesives have been used in liquid crystal display (LCD) and smart card applications as an interconnect material and in flip–chip assembly, chip scale package (CSP) and ball grid array (BGA) applications in replacement of solder. However, no currently commercialized ECAs can replace tin–lead metal solders in all applications due to some challenging issues such as lower electrical conductivity, conductivity fatigue (decreased conductivity at elevated temperature and humidity aging or normal use condition) in reliability testing, limited current-carrying capability, and poor impact strength. Considerable research has been conducted recently to study and optimize the performance of ECAs, such as electrical, mechanical and thermal behaviors improvement as well as reliability enhancement under various conditions. This review article will discuss the materials, applications and recent advances of electrically conductive adhesives as an environmental friendly solder replacement in the electronic packaging industry.
Co-reporter:Yi Li;Fei Xiao;Kyoung-Sik Moon;C. P. Wong
Journal of Polymer Science Part A: Polymer Chemistry 2006 Volume 44(Issue 2) pp:1020-1027
Publication Date(Web):7 DEC 2005
DOI:10.1002/pola.21239
A basic type of amino acid, lysine, was used as a novel ecofriendly curing agent of epoxy in electronic materials. Differential scanning calorimetry and in situ Fourier transform infrared spectroscopy characterizations suggested a different reactivity and reaction mechanism for amino acid cured epoxy than for a typical amine or carboxylic acid cured epoxy. The primary amine in the lysine participated in the curing reaction first and was followed by the protonation of the secondary amine NH on another epoxy group and the esterification of the carboxylic group in lysine with epoxide. The crosslinked epoxy with amino acid was thermally degradable and could be used as a reworkable resin as well.
Co-reporter:Yangyang Sun;Zhuqing Zhang;Ching Ping Wong
Macromolecular Materials and Engineering 2005 Volume 290(Issue 12) pp:1204-1212
Publication Date(Web):21 NOV 2005
DOI:10.1002/mame.200500149
Summary: As a special epoxy resin material used in the electronics packaging, wafer level underfill (WLU) is studied using a chemorheology approach to provide a fundamental understanding of its reaction dependent rheology behaviors. In this work, the relationship between the molecular weight () and the viscosity of the epoxy resins at fixed temperatures has been established. Subsequently an Arrhenius-Erying equation was used to fit the relationship between viscosity and temperature for given molecular weight epoxies. By combining the two relationships, the viscosity could be modeled as a function of temperature and molecular weight. To obtain the viscosity change of the WLU during the reflow process, the molecular weight change of the underfill was calculated from the degree of curing through the kinetics modeling. A semi-empirical model was developed to predict the viscosity of the underfill as a function of time and temperature during the curing process. Modeled predictions were compared with experimental data under isothermal and ramping temperatures during curing experiments. The model showed good agreement with the experiments in the early stage of curing reaction. The critical viscosity of the underfill for solder wetting was obtained by wetting experiments, which can be used as the criteria to determine the flowability of the wafer level underfill.
Co-reporter:Zhuqing Zhang;Tsuyoshi Yamashita;Ching P. Wong
Macromolecular Chemistry and Physics 2005 Volume 206(Issue 8) pp:
Publication Date(Web):19 APR 2005
DOI:10.1002/macp.200400466
Summary: The gelation behavior of an epoxy/anhydride system was studied using the differential scanning calorimeter (DSC) and a stress rheometer. It was found that the gelation was dependent on the curing temperature. At a higher curing temperature, the resin tended to gel at lower degree of cure (DOC). Monte Carlo (MC) simulation was performed, to investigate the effect of curing kinetics on the network formation and the gelation. Both the effects of the propagation rate and the catalyst concentration was studied. The temperature dependent gelation was related to the change of propagation/initiation ratio with temperature according to the simulation results.
Co-reporter:Yangyang Sun, Shijian Luo, Ken Watkins, C.P. Wong
Polymer Degradation and Stability 2004 Volume 86(Issue 2) pp:209-215
Publication Date(Web):November 2004
DOI:10.1016/j.polymdegradstab.2004.04.013
The main objectives of this work are (1) to establish the correlation between changes in the electrical conductivity (resistivity) and the mechanical properties (ultimate tensile elongation) of a carbon black filled ethylene propylene rubber (EPR) during thermal oxidative oven aging at 125 °C and (2) to show the feasibility of using the resistivity change as a non-destructive monitor method of the aging process. Elongation, weight, density and resistivity were followed versus aging time for EPR samples loaded with 21.6 and 26.2 wt% carbon black. Assuming a typically chosen mechanical failure criterion (elongation reaches 50% absolute), the drop in resistivity at “failure” is found to be on the order of a factor of 10 for both materials. Analyses of weight and density changes indicated that the substantial resistivity decreases were due to increases in volume fraction of the conductive carbon black caused mainly by the mass loss of polymer matrix and sample shrinkage during the thermal oxidation aging process. The results offer strong evidence that resistivity measurements could represent a very useful and non-destructive cable monitor approach.
Co-reporter:Yang Rao, Daoqiang Lu, C.P. Wong
International Journal of Adhesion and Adhesives 2004 Volume 24(Issue 5) pp:449-453
Publication Date(Web):October 2004
DOI:10.1016/j.ijadhadh.2003.12.003
Electrically conductive adhesive is an environment- and user-friendly alternative to traditional lead-bearing solders in electronics packaging. However, current commercial conductive adhesives are not suitable in electronics packages, such as surface mount applications, due to some critical reliability issues. One of the critical issues is that current conductive adhesives do not have desirable impact strength. In the study described here, finite element analysis (FEA) was used to conduct the modal analysis of the packages and to estimate their natural vibration frequencies. Then, a series of conductive adhesives were formulated, and the dynamic properties and impact performance of these materials were studied. The experimental results were correlated to the results of the modal analysis. Constrained-layer damping theory was used to explain the results of the drop tests. It was found that (1) finite element modal analysis accurately predicts the vibration frequencies experienced by the components and (2) conductive adhesive materials with high damping properties in the vibration frequency range estimated by FEA show high impact performance.
Co-reporter:Yang Rao;C. P. Wong
Journal of Applied Polymer Science 2004 Volume 92(Issue 4) pp:2228-2231
Publication Date(Web):9 MAR 2004
DOI:10.1002/app.13690
Embedded capacitor technology can improve electrical performance and reduce assembly cost compared with traditional discrete capacitor technology. Polymer–ceramic composites have been of great interest as embedded capacitor materials because they combine the processability of polymers with the desired electrical properties of ceramics. We have developed a novel nanostructure polymer–ceramic composite with a very high dielectric constant (εr ≈ 150, a new record for the highest reported εr value of a nanocomposite) in a previous work. RF applications of embedded capacitors require that the insulating material have a high εr at a high frequency (in the gigahertz range), low leakage current, high breakdown voltage, and high reliability. A set of electrical tests were conducted in this study to characterize the electrical properties of the novel high-εr polymer–ceramic nanocomposite developed in- house. The results show that this material had a fairly high εr in the RF range, low electrical leakage, and high breakdown voltage. An 85°C/85% thermal humidity aging test was been performed, and it showed that this novel high-K material had good reliability. An embedded capacitor prototype with a capacitance density of 35 nF/cm2 was manufactured with this nanocomposite with spin-coating technology. This novel nanocomposite can be used for the integral capacitors for RF applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2228–2231, 2004
Co-reporter:Yangyang Sun;Lianhua Fan;Ken Watkins;Jonathan Peak;C. P. Wong
Journal of Applied Polymer Science 2004 Volume 93(Issue 2) pp:513-520
Publication Date(Web):20 APR 2004
DOI:10.1002/app.20464
Polymeric materials are widely used as insulation and jacketing materials in wire and cable. When such materials are used for long-term applications, they undergo thermal oxidation aging in the environment. It is necessary to develop an in situ and nondestructive condition monitoring (CM) method to follow the aging of cable materials. The main objective of this work was to investigate low-density polyethylene/carbon black (LDPE/CB) conductive polymer composites as potential sensor materials for this purpose. LDPE/CB composites with a carbon black loading below the percolation threshold underwent accelerated thermal oxidation aging experiments. The results indicated that the substantial resistivity decreases of the LDPE/CB composites could be directly related to the increases in volume fraction of the conductive carbon black, which was mainly caused by the mass loss of polymer matrix and sample shrinkage during the thermal oxidation aging process. Compared to existing CM method based on density change, the electrical resistivity is more explicit regarding its absolute changes throughout the thermal oxidation aging. The change in resistivity spanned over four orders of magnitude, whereas the composite density only increased 10%. The results offer strong evidence that resistivity measurements, which reflect property changes under thermal aging conditions, could represent a very useful and nondestructive CM approach as well as a more sensitive method than density CM approach. Crystallinity changes in materials investigated by modulated DSC and TGA measurements indicated deterioration of crystalline regions in polymer during the thermal oxidation aging. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 513–520, 2004
Co-reporter:Suresh Pothukuchi;Yi Li;C. P. Wong;C. P. Wong;Suresh Pothukuchi;Yi Li
Journal of Applied Polymer Science 2004 Volume 93(Issue 4) pp:1531-1538
Publication Date(Web):18 MAY 2004
DOI:10.1002/app.20626
Metal nanoparticles exhibit a number of interesting characteristics, including unique physical, chemical, optical, magnetic, and electric properties. Numerous investigations have exploited their properties in a readily usable form by incorporating them into polymers. The current focus of interest is the behavior of such polymer nanocomposites near the percolation loading levels of the metal nanoparticles. This material is particularly suitable for the new integral passive technology. Discrete capacitors are used in many applications, such as noise suppression, filtering, tuning, decoupling, bypassing, termination, and frequency determination, and they occupy a substantial amount of surface area on a substrate. Thus there are limitations in the number of capacitors that can be placed around the chip. Integral passive components are gradually replacing discrete components because of the inherent advantages of improved electrical performance, increased real estate on the printed wiring board, miniaturization of interconnect distance, reduced processing costs, and efficient electronics packaging. For integral capacitors, polymer composite material has emerged as a potential candidate because it meets the requirements of low processing temperature and reasonably high dielectric constant. Yang and Wong, whose patent was filed in 2001, demonstrated novel integral passive component materials with extraordinarily high dielectric constants (K > 1000) and high reliability performance. These materials are characterized by high dielectric constant based on the mechanism of interfacial polarization, although they need precision filler concentration control. The current study overcomes this drawback and produces the composite through an in situ reduction in an epoxy matrix. Material characterization was done through TEM, SEM, X-ray analysis, and energy-dispersive analysis for X rays. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1531–1538, 2004
Co-reporter:Zhuqing Zhang;Erin Beatty;C. P. Wong
Macromolecular Materials and Engineering 2003 Volume 288(Issue 4) pp:365-371
Publication Date(Web):8 APR 2003
DOI:10.1002/mame.200390029
No-flow underfill is used in the assembly of microelectronics to increase the productivity and to decrease the cost of the flip-chip manufacturing. The curing process, especially the gelation of the no-flow underfill, is essential for the yield and reliability of the flip-chip assembly. A differential scanning calorimeter (DSC) and a stress rheometer are used to study the curing process of epoxy/anhydride system at different curing rates and different isothermal temperatures. The gel point is found when the storage modulus and the loss modulus of the resin measured by the rheometer equals to each other. The degree of cure (DOC) at gelation is calculated according to the results from DSC. The results indicate a strong dependence of the DOC at gelation on the heating rates and the curing temperatures. In order to further investigate the difference in the curing process at various heating rates, FTIR spectra of the resin are taken during curing. The change of different functional groups is recorded and compared. The results do not show a significant difference in the chemical structure at different heating rates. The early gelation at high heating rate/ high temperature can be caused by the structure difference in the epoxy network at the early stage of curing due to the chain initiation and propagation of the molecules in the curing process.
Co-reporter:C. P. Wong;Kyoung-Sik Moon;Lianhua Fan;Kyoung-Sik Moon;Lianhua Fan;C. P. Wong
Journal of Applied Polymer Science 2003 Volume 88(Issue 10) pp:2439-2449
Publication Date(Web):17 MAR 2003
DOI:10.1002/app.11988
Fillet cracking of no-flow underfill in a flip-chip device during a reliability test such as thermal shock or thermal cycling has been a serious reliability problem. The effect of toughening agents and modification of epoxy on fillet cracking of no-flow underfill was investigated. The best epoxy formulation and the appropriate loading level of toughening agent regarding the antifillet cracking performance were found. In the case where the epoxy was modified with polysiloxanes, the second-phase particle with a submicron particle size was formed and the size of the particle depended on the kind of toughening agent. The morphology was observed by a scanning electron microscopy and confirmed by a dynamic mechanical analyzer measurement. The physical properties such as the fracture toughness, flexual modulus, coefficient of thermal expansion, and adhesion were measured, and the liquid–liquid thermal shock (LLTS) test under −55 to 125°C was performed with different formulations. One of the formulations toughened by amine/epoxy-terminated polysiloxane, which has higher die shear strength, lower modulus, and higher toughness, passed 1000 cycles of the LLTS test. In order to obtain a high reliable no-flow underfill, the physical properties of the no-flow underfill should be well controlled and balanced. Finally, a correlation between physical properties of the no-flow underfill and the fillet cracking capability for those approaches was discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2439–2449, 2003
Co-reporter:Yang Rao, Atsushi Takahashi, C.P Wong
Composites Part A: Applied Science and Manufacturing 2003 Volume 34(Issue 11) pp:1113-1116
Publication Date(Web):November 2003
DOI:10.1016/S1359-835X(03)00202-1
Embedded capacitor technology can improve electrical performance and reduce assembly cost compared with traditional discrete capacitor technology. Polymer-ceramic composites have been of great interest as embedded capacitor material because they combine the processability of polymers with the desired electrical properties of ceramics. Dispersion of ceramic particles is a critical factor to affect the effective dielectric constant of polymer-ceramic composite. Di-block copolymer surfactants have been used to prevent agglomeration of the ceramic particles. It was found that di-block copolymer surfactant could improve the ceramic dispersion better than monomer surfactant. Using di-block copolymer surfactant, higher dielectric constant was achieved at lower ceramic loading level. This high dielectric constant polymer-ceramic composite material has much better mechanical properties and can be used for the integral capacitors in the SOP (system on a package) substrate.
Co-reporter:Johannes Leisen;Shijian Luo;C. P. Wong
Journal of Applied Polymer Science 2002 Volume 85(Issue 1) pp:1-8
Publication Date(Web):23 APR 2002
DOI:10.1002/app.10473
This paper presents a study on the mobility of water and polymer chains in epoxy materials and its influence on the rate of adhesion degradation in a humid environment. Solid state nuclear magnetic resonance (NMR) techniques (both 1H NMR and 2H NMR) were used to study the binding states of water within two epoxy formulations along with the possible plasticizing effects of moisture affecting the mobility of polymer chains. Absorbed water reduces the glass transition temperature of polymeric materials. However, the presence of moisture has no significant effect on the polymer chain mobility at temperatures below the reduced glass transition temperature. Water in an epoxy in its rubbery state above the glass transition has a much higher mobility than in a polymer in its glassy state. The mobility of water absorbed by a polymer in its rubbery state is similar to that of pure water. The translational mobility of water within epoxies was studied by measuring the diffusion coefficient of water in epoxies through the water uptake. Higher rotational mobilities of water and polymer chains in the rubbery state lead to a significant increase of the water diffusion coefficient in the rubbery state polymer matrix as compared to a polymer in its glassy state. The higher mobility of absorbed water and the higher mobility of polymer chains in epoxy lead to faster adhesion degradation during aging in a humid environment. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1–8, 2002
Co-reporter:Zhuqing Zhang;C. P. Wong
Journal of Applied Polymer Science 2002 Volume 86(Issue 7) pp:1572-1579
Publication Date(Web):4 SEP 2002
DOI:10.1002/app.11025
Metal acetylacetonates are effective latent catalysts for epoxy/anhydride systems. A screen test on the catalytic behavior of metal acetylacetonate shows that this system offers a wide range of curing latency and material properties. It can be inferred that the curing behavior is closely related to the bonding strength of the metal ion to the ligand. Isothermal kinetic study on the catalytic behavior of metal chelates with first-row transition metal ions is conducted and analyzed by using an autocatalytic model. It is found that the activation energy of the system containing the divalent metal chelates follows the Irving and Williams rule. The activation energies of the reaction obtained in the kinetic study are compared with the dissociation energies of the metal/ligand bond and the results are discussed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1572–1579, 2002
Co-reporter:Yang Rao;S. Ogitani;Paul Kohl;C. P. Wong
Journal of Applied Polymer Science 2002 Volume 83(Issue 5) pp:1084-1090
Publication Date(Web):28 NOV 2001
DOI:10.1002/app.10082
Embedded capacitor technology can increase silicon packing efficiency, improve electrical performance, and reduce assembly cost compared with traditional discrete capacitor technology. Developing a suitable material that satisfies electrical, reliability, and processing requirements is one of the major challenges of incorporating capacitors into a printed wiring board (PWB). Polymer–ceramic composites have been of great interest as embedded capacitor material because they combine the processability of polymers with the high dielectric constant of ceramics. A novel nanostructure polymer–ceramic composite with a very high dielectric constant (εr ∼110, a new record for the highest reported εr value of a nanocomposite) was developed in this work. A high dielectric constant is obtained by increasing the dielectric constant of the epoxy matrix (εr >6) and using the combination of lead magnesium niobate–lead titanate (PMN–PT)/BaTiO3 as the ceramic filler. This nanocomposite has a low curing temperature (<200°C); thus, it is multichip-module laminate (MCM-L) process-compatible. An embedded capacitor prototype with a capacitance density of 50 nF/cm2 was manufactured using this nanocomposite and spin-coating technology. The effect of the composite microstructure on the effective dielectric constant was studied. This novel nanocomposite can be used for integral capacitors in PWBs. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1084–1090, 2002
Co-reporter:Haiying Li;Lejun Wang;Karl Jacob;C. P. Wong
Journal of Polymer Science Part A: Polymer Chemistry 2002 Volume 40(Issue 11) pp:1796-1807
Publication Date(Web):15 APR 2002
DOI:10.1002/pola.10258
In flip-chip technology, the development of reworkable underfill materials has been one of the keys to the recovery of highly integrated and expensive board assembly designs through the replacement of defective chips. This article reports the syntheses, formulations, and characterizations of two new diepoxides, one containing secondary ester linkages and the other containing tertiary ester linkages, that are thermally degradable below 300 °C. The secondary and tertiary ester diepoxides were synthesized in three and two steps, respectively. Both compounds were characterized with NMR and Fourier transform infrared spectroscopy and formulated into underfill materials with an anhydride as the hardener and an imidazole as the catalyst. A dual-epoxy system was also formulated containing the tertiary ester diepoxide and a conventional aliphatic diepoxide, 3,4-epoxy cyclohexyl methyl-3,4-epoxycyclohexyl carboxylate (ERL-4221E), with the same hardener and catalyst. The curing kinetics of the formulas were studied with differential scanning calorimetry (DSC). Thermal properties of cured samples were characterized with DSC, thermogravimetric analysis, and thermomechanical analysis. The dual-epoxy system showed a viscosity of 18.7 and 0.87 P at 25 and 100 °C, respectively. The cured secondary, tertiary, and dual-epoxy formulas showed decomposition temperatures around 265, 190, and 220 °C, glass-transition temperatures around 120–140, 110–157, and 140–157 °C, and coefficients of thermal expansion of 70, 72, and 64 ppm/°C below their glass-transition temperatures, respectively. The shear strength of the cured dual-epoxy system decreased quickly with aging at 230 °C. The reworkability test showed that the removal of a chip underfilled with this material from the board was quite easy, and the residue on the board could be thoroughly removed with a mechanical brush without obvious damage to the solder mask. In summary, the synthesized tertiary epoxide can be used as a reworkable underfill for flip-chip applications. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1796–1807, 2002
Co-reporter:Lejun Wang;C.P. Wong
Journal of Applied Polymer Science 2001 Volume 81(Issue 8) pp:1868-1880
Publication Date(Web):6 JUN 2001
DOI:10.1002/app.1620
Underfill is the material used in a flip-chip device to dramatically enhance its reliability as compared to a nonunderfilled device. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which is an obstacle in flip-chip technology developments, where unknown bad die is a concern. Reworkable underfill is the key to address the nonreworkability of the flip-chip devices. The ultimate goal of this study is to develop epoxy-based thermally reworkable underfills. Our previous work showed that when incorporated into epoxy matrix, special additives could provide the epoxy formulation with die-removal capability around solder reflow temperature. The additive-epoxy interactions were studied and the results show that the additives do not adversely affect the epoxy properties. Moreover, when the additive decomposition temperature is reached, the decomposition of the additive causes a microexplosion within the epoxy matrix. Subsequently, the adhesion of the epoxy matrix is greatly reduced. Among the four additives studied, Additive1 and Additive2 may be used in reworkable underfills that can be reworked around solder reflow temperature, Additive3 cannot be used in underfill because it greatly reduces the shelf life of the underfill, and Additive4 may be used to develop reworkable underfill that withstands multiple reflows. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1868–1880, 2001
Co-reporter:Daoqiang Lu, C.P Wong
International Journal of Adhesion and Adhesives 2000 Volume 20(Issue 3) pp:189-193
Publication Date(Web):2000
DOI:10.1016/S0143-7496(99)00039-1
Our previous study proved that intimate contact between conductive fillers caused by resin cure shrinkage, rather than lubricant removal, was the main mechanism for establishment of conductivity of isotropic conductive adhesives (ICAs). The purpose of the present study was to investigate the changes in properties, especially in dimension (cure shrinkage), of an ICA during cure and to correlate them with establishment of conductivity. An ICA was cured nonisothermally by a temperature increase from 30 to 250°C; and its heat flow, storage modulus, dimension change, and electrical conductivity were studied with a differential scanning calorimeter (DSC), rheometer, thermomechanical analyzer (TMA), and electrical multimeter, respectively. It was found that all of these properties changed dramatically over the same small range of temperature. Changes in these four properties of this ICA with time in the course of an isothermal cure were also investigated. It was found that all of the properties showed significant changes within the same period of time. In addition, the conductivities of three different ICA formulations which were filled with a blank Ag powder and whose resins had different cure shrinkages were measured and compared. From this study, it was concluded that (a) conductive adhesives achieved high conductivity only when enough cure shrinkage was achieved, and (b) ICAs with higher cure shrinkage showed higher conductivity.
Co-reporter:Lejun Wang;Haiying Li;C. P. Wong
Journal of Polymer Science Part A: Polymer Chemistry 2000 Volume 38(Issue 20) pp:3771-3782
Publication Date(Web):31 AUG 2000
DOI:10.1002/1099-0518(20001015)38:20<3771::AID-POLA80>3.0.CO;2-4
Flip-chip technology is a face-down attachment of the active side of the silicon device onto the substrate. It is the ultimate packaging solution to integrated circuit devices used in 21st century electronic systems to meet the requirements of small size, high performance, and low cost. Underfill technology enhances the flip chip on board cycle fatigue life and thus dramatically extends the application of flip-chip technology in electronics from high-end to cost-sensitive commodity products. Reworkable underfill is the key to addressing the nonreworkability of the underfill, so it is very important to electronic packaging. To meet the need for reworkable epoxy resins, four cycloaliphatic epoxides containing thermally cleavable carbonate linkages have been synthesized and characterized. These materials are shown to undergo curing reactions with cyclic anhydride similarly to a commercial cycloaliphatic diepoxide. Furthermore, these cured epoxides start to decompose at temperatures lower than 350 °C, the decomposition temperature for the cured sample of the commercial cycloaliphatic diepoxide. Two formulations based on two carbonate-containing diepoxides start network breakdown around 220 °C, which is the targeted rework temperature. Moreover, these two formulations have similar properties, including the glass-transition temperature, coefficient of thermal expansion, storage modulus, viscosity, and adhesion, compared to the standard commercial diepoxide formulation. As such, these two formulations are potential candidates for a successful reworkable underfill. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3771–3782, 2000
Co-reporter:Daoqiang Lu;C. P. Wong
Journal of Applied Polymer Science 1999 Volume 74(Issue 2) pp:399-406
Publication Date(Web):28 JUL 1999
DOI:10.1002/(SICI)1097-4628(19991010)74:2<399::AID-APP22>3.0.CO;2-F
Electrically conductive adhesives (ECAs) have been explored as a tin/lead (Sn/Pb) solder alternative for attaching encapsulated surface mount components on rigid and flexible printed circuits. However, limited practical use of conductive adhesives in surface mount applications is found because of the limitations and concerns of current commercial ECAs. One critical limitation is the significant increase of joint resistance with Sn/Pb finished components under 85°C/85% relative humidity (RH) aging. Conductive adhesives with stable joint resistance are especially desirable. In this study, a novel conductive adhesive system that is based on epoxy resins has been developed. Conductive adhesives from this system show very stable joint resistance with Sn/Pb-finished components during 85°C/85% RH aging. One ECA selected from this system has been tested here and compared with two popular commercial surface mount conductive adhesives. ECA properties studied included cure profile, glass transition temperature (Tg), bulk resistivity, moisture absorption, die shear adhesion strength, and shift of joint resistance with Sn/Pb metallization under 85°C/85% RH aging. It was found that, compared to the commercial conductive adhesives, our in-house conductive adhesive had higher Tg, comparable bulk resistivity, lower moisture absorption, comparable adhesion strength, and most importantly, much more stable joint resistance. Therefore, this conductive adhesive system should have better performance for surface mount applications than current commercial surface mount conductive adhesives. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 399–406, 1999
Co-reporter:C. P. Wong;Jiali Wu;Ry T. Pike
Journal of Applied Polymer Science 1999 Volume 73(Issue 6) pp:997-1005
Publication Date(Web):25 MAY 1999
DOI:10.1002/(SICI)1097-4628(19990808)73:6<997::AID-APP18>3.0.CO;2-J
The need to have a high-temperature adhesive that can withstand temperatures in excess of 350°C for MCM-D silicon substrate process application, yet which can be reworkable at slightly high temperature ∼ 400°C for the removal from the glass pallet, is important. A novel, reworkable, high-temperature adhesive based on polyimide–amide–epoxy (PIAE) copolymer was developed and investigated using modulated differential scanning calorimetry (MDSC), thermal gravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR) and solid-probe pyrolysis mass spectroscopy (MS). Compared with commercial polyimide–amide (PIA) adhesives, FTIR spectra reveal that the thermally degradative ester groups contribute to the reworkability of the PIAE adhesive at a specific temperature (400°C), yet they remain thermally stable at a lower working temperature (350°C). FTIR spectrum comparison of the residuals of PIAE and PIA are similar after exposure to 400°C. MS spectra of outgassed products identify that the components of radical fragmentation from PIAE are due to polymeric chain degradation at 400°C, while only volatile trace water and N-methyl pyrolidone (NMP) are evolved from the commercial PIA adhesive. TGA results suggest a complementary explanation for the variation of total ion current (TIC) curves on these two adhesives. MDSC curves further verify that the reworkable PIAE adhesive is a copolymer. Furthermore, a reasonable thermal degradation mechanism is presented on the adhesive reworkability. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 997–1005, 1999
Co-reporter:Cheng Yang, Yu-Tao Xie, Matthew M. F. Yuen, Xiaomin Xiong and C. P. Wong
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 43) pp:NaN14461-14461
Publication Date(Web):2010/09/30
DOI:10.1039/C0CP00414F
This communication describes a new surface-enhanced Raman scattering (SERS) active silver substrate prepared by iodination of the evaporated silver foil. After iodination, the morphology of the silver substrate undergoes a self-evolution process in which it displays accordingly the UV-vis absorption shift as well as the AFM topological test. Rhodamine 6G (R6G) is used as the probe molecule to evaluate the enhancement efficiency of the silver substrate at different self-evolution time intervals. The SERS intensity of R6G increases up to ∼29-fold and reaches a maximum after the substrate evolved for 24 h. This method is feasible for the production of an efficient SERS silver substrate.
Co-reporter:Ziyin Lin, Min-kyu Song, Yong Ding, Yan Liu, Meilin Liu and Ching-ping Wong
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 10) pp:NaN3387-3387
Publication Date(Web):2012/01/17
DOI:10.1039/C2CP00032F
Nitrogen-doped graphene (nG) is a promising metal-free catalyst for oxygen reduction reaction (ORR) on the cathode of fuel cells. Here we report a facile preparation of nGviapyrolysis of graphene oxide with melamine. The morphology of the nG is revealed using scanning electron microscopy and transmission electron microscopy while the successful N doping is confirmed by electron energy loss spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The resulting nG shows high electrocatalytic activity toward ORR in an alkaline solution with an onset potential of −0.10 V vs.Ag/AgCl reference electrode. The nG catalyzed oxygen reduction exhibits a favorable formation of watervia a four-electron pathway. Good stability and anti-crossover property are also observed, which are advantageous over the Pt/C catalyst. Furthermore, the effect of pyrolysis temperature on the structure and activity of nG is systematically studied to gain some insights into the chemical reactions during pyrolysis.
Co-reporter:Chia-Yun Chen and Ching-Ping Wong
Chemical Communications 2013 - vol. 49(Issue 66) pp:NaN7297-7297
Publication Date(Web):2013/07/12
DOI:10.1039/C3CC43466D
Multiple etching pathways for the formation of porous Si nanostructures are visualized with the addition of ethylene glycol in metal-assisted chemical etching. The monotonic transition from solid to porous morphologies of Si nanostructures along with remarkable photoluminescence (PL) emission efficiency and distinct wetting phenomena can be observed.
Co-reporter:Qing Guo Chi, Jiu Feng Dong, Chang Hai Zhang, Ching Ping Wong, Xuan Wang and Qing Quan Lei
Journal of Materials Chemistry A 2016 - vol. 4(Issue 35) pp:NaN8188-8188
Publication Date(Web):2016/07/12
DOI:10.1039/C6TC01655C
In this paper, we report promising dielectric characteristics of iron oxide-deposited nanosized calcium copper titanate (nano CCTO–Fe3O4)/polyimide (PI) hybrid films induced by an external magnetic field. Nano CCTO particles with a pure perovskite cubic structure and without impurity phases were prepared, and then Fe3O4 nanoparticles were deposited as discrete units on the surface of CCTO to fabricate nano CCTO–Fe3O4 hybrid particles. The effects of magnetic field treatments on the dielectric properties of the composites were investigated in detail. The nano CCTO–Fe3O4/PI hybrid films annealed under an applied magnetic field exhibited a substantial increase in dielectric permittivity, a slight increase in dielectric loss, and a decrease in the percolation threshold. For the hybrid composites subjected to a magnetic field treatment for 30 min at 90 °C, a high dielectric permittivity of 308 and relatively low dielectric loss of 0.60 at 100 Hz were simultaneously achieved when the content of nano CCTO–Fe3O4 was only 12 vol%. Such composites are promising for applications in electronic devices. The experimental results are well explained by percolation theory, which indicated that the enhanced dielectric properties of the hybrid films mainly originated from the large interfacial area and interfacial polarization induced by the external magnetic field.
Co-reporter:Zhuo Li, Kristen Hansen, Yagang Yao, Yanqing Ma, Kyoung-sik Moon and C. P. Wong
Journal of Materials Chemistry A 2013 - vol. 1(Issue 28) pp:NaN4374-4374
Publication Date(Web):2013/05/13
DOI:10.1039/C3TC30612G
The conduction development mechanism of silicone-based electrically conductive adhesives (Silo-ECAs) is studied. The reduction of surfactants on silver flakes by hydride in the silicone backbone and the subsequent sintering of the generated silver nanoparticles between micron-sized silver flakes are found to be the major contributor to the conductivity development in Silo-ECAs; this is in contrast with the mechanism observed in most polymer–metal ECAs, where the curing shrinkage of the polymer matrix is the major cause of conductivity development. The conductivity development in Silo-ECAs ceases when the polymer curing is completed. Hence, in order to enhance the resulting electrical conductivity, the curing process is prolonged by using a long-chain prepolymer, lowering the platinum catalyst concentration, or adding curing inhibitors. A bulk resistivity of 8.82 × 10−5 Ω cm is achieved, which is 55% lower than the best values reported previously for Silo-ECAs.
Co-reporter:Yagang Yao, Ziyin Lin, Zhuo Li, Xiaojuan Song, Kyoung-Sik Moon and Ching-ping Wong
Journal of Materials Chemistry A 2012 - vol. 22(Issue 27) pp:NaN13499-13499
Publication Date(Web):2012/04/04
DOI:10.1039/C2JM30587A
Two-dimensional (2D) nanomaterials such as graphene, boron nitride (BN), and molybdenum disulfide (MoS2) have been attracting increasing research interest in the past few years due to their unique material properties. However, the lack of a reliable large-scale production method is an inhibiting issue for their practical applications. Here we report a facile, efficient, and scalable method for the fabrication of monolayer and few-layer BN, MoS2, and graphene using combined low-energy ball milling and sonication. Ball milling generates two forces on layered materials, shear force and compression force, which can cleave layered materials into 2D nanosheets from the top/bottom surfaces, and the edge of layered materials. Subsequent sonication would further break larger crystallites into smaller crystallites. These fabricated 2D nanosheets can be well dispersed in aqueous solutions at high concentrations, 1.2 mg mL−1 for BN, 0.8 mg mL−1 for MoS2, and 0.9 mg mL−1 for graphene, which are highly advantageous over other methods. These advantages render great potential in the construction of high-performance 2D material-based devices at low cost. For example, a prototype gas sensor is demonstrated in our study using graphene and MoS2, respectively, which can detect several ppm of ammonia gas.
Co-reporter:Zhuo Li, Yagang Yao, Ziyin Lin, Kyoung-Sik Moon, Wei Lin and Chingping Wong
Journal of Materials Chemistry A 2010 - vol. 20(Issue 23) pp:NaN4783-4783
Publication Date(Web):2010/05/04
DOI:10.1039/C0JM00168F
Through direct absorption of microwave irradiation by GO film, we developed a rapid, dry approach to synthesize reduced graphene.
Co-reporter:Rongwei Zhang, Kyoung-sik Moon, Wei Lin and C. P. Wong
Journal of Materials Chemistry A 2010 - vol. 20(Issue 10) pp:NaN2023-2023
Publication Date(Web):2010/01/20
DOI:10.1039/B921072E
Highly conductive polymer nanocomposites with very low resistivity (4.8 × 10−5 Ω cm) were prepared by thermal sintering of silver nanoparticles with silver flakes dispersed in a polymer matrix at 180 °C. By comparative studies of thermal behavior of Ag nanoparticles, the critical processing temperature required to obtain very low resistivity of polymer nanocomposites has been identified for Ag nanoparticles with different surface properties. The results indicate that the decomposition temperature of surface residues on Ag nanoparticles plays a key role in the sintering of Ag nanoparticles and thus the electrical resistivity of the polymer nanocomposites. Electrical measurements of the polymer nanocomposites showed that morphological changes induced by sintering of Ag nanoparticle with Ag flakes considerably contribute to the reduction of the contact resistance between conductive fillers, increasing the nanocomposite conductivity.
Co-reporter:Jiongxin Lu, Kyoung-Sik Moon and C. P. Wong
Journal of Materials Chemistry A 2008 - vol. 18(Issue 40) pp:NaN4826-4826
Publication Date(Web):2008/08/28
DOI:10.1039/B807566B
A silver (Ag)-polymer nanocomposite has been developed by in-situ formation of metal nanoparticles within the polymer matrix and utilized as a high-dielectric constant (k) polymer matrix to enhance the dielectric properties of high-k composite materials. By using an in-situ photochemical reduction method, uniformly dispersed Ag nanoparticles of size of around 10 nm were generated in polymer matrices. Self-passivated aluminium (Al) particles were incorporated into this Ag-epoxy matrix and the dielectric properties of the as-prepared composite materials were investigated. The composites showed more than 50% increase in k values as compared with an Al/neat epoxy composite with the same filler loading of Al. The dielectric loss tangent of the Al/Ag-epoxy composites was below 0.1, which meets the requirement for embedded decoupling capacitors. These results suggest that the Ag-epoxy high-kpolymer matrix effectively enhances the dielectric constant while maintaining the low dielectric loss of the high-k composites. In addition, detailed dielectric property measurements revealed that the dielectric properties and their frequency dispersion as well as the breakdown behaviors of the Al/Ag-epoxy composites were related to the incorporation and concentration of Ag nanoparticles in the high-kpolymer matrix.
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