Co-reporter:Nathan Cernetic;Tobias Weidner;Joe E. Baio;Hao Lu;Alex K.-Y. Jen
Advanced Functional Materials 2015 Volume 25( Issue 33) pp:5376-5383
Publication Date(Web):
DOI:10.1002/adfm.201501263
Low-voltage self-assembled monolayer field-effect transistors (SAMFETs) that operate under an applied bias of less than −3 V and a high hole mobility of 10−2 cm2 V−1 s−1 are reported. A self-assembled monolayer (SAM) with a quaterthiophene semiconducting core and a phosphonic acid binding group is used to fabricate SAMFETs on both high-voltage (AlOx/300 nm SiO2) and low-voltage (HfO2) dielectric platforms. High performance is achieved through enhanced SAM packing density via a heated assembly process and through improved electrical contact between SAM semiconductor and metal electrodes. Enhanced electrical contact is obtained by utilizing a functional methylthio head group combined with thermal annealing post gold source/drain electrode deposition to facilitate the interaction between SAM and electrode.
Co-reporter:Nathan Cernetic;Sanfeng Wu;Joshua A. Davies;Benjamin W. Krueger;Daniel O. Hutchins;Xiaodong Xu;Alex K.-Y. Jen
Advanced Functional Materials 2014 Volume 24( Issue 22) pp:3464-3470
Publication Date(Web):
DOI:10.1002/adfm.201303952
Recent reports have shown that self-assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next-generation graphene-based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z-component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio-terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.
Co-reporter:Daniel Orrin Hutchins, Tobias Weidner, Joe Baio, Brent Polishak, Orb Acton, Nathan Cernetic, Hong Ma and Alex K.-Y. Jen
Journal of Materials Chemistry A 2013 vol. 1(Issue 1) pp:101-113
Publication Date(Web):06 Nov 2012
DOI:10.1039/C2TC00378C
A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ∼(CH2)12 PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm2 V−1 s−1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm2 V−1 s−1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.
Co-reporter:Hong Ma, Orb Acton, Daniel O. Hutchins, Nathan Cernetic and Alex K.-Y. Jen
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 41) pp:14110-14126
Publication Date(Web):14 Jun 2012
DOI:10.1039/C2CP41557G
Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source–drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec−1, high Ion/Ioff ratios of 105–107, contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1–4.6 cm2 V−1 s−1, and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.
Co-reporter:Nathan Cernetic, Orb Acton, Tobias Weidner, Daniel O. Hutchins, Joe E. Baio, Hong Ma, Alex K.-Y. Jen
Organic Electronics 2012 Volume 13(Issue 12) pp:3226-3233
Publication Date(Web):December 2012
DOI:10.1016/j.orgel.2012.09.018
Low-voltage, n-type organic field effect transistors (OFETs) with simultaneously modified bottom-contact (BC) electrodes and dielectric were compared to their top-contact (TC) counterparts. The devices modified with 6-phenoxyhexylphosphonic acid (Ph6PA) self-assembled monolayer (SAM) showed similar performance, morphology, and contact resistance. Electron mobility of C60 devices were 0.212 and 0.320 cm2 V−1 s−1 and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) devices were 0.04 and 0.06 cm2 V−1 s−1 for TC and BC devices, respectively. Low contact resistance between 11 and 45 kΩ cm was found regardless of device architecture or n-type semiconductor used. This work shows it is possible to fabricate solution processable low-voltage bottom-contact devices with performance that is similar or better than their top-contact counterparts without the addition of complex and time-consuming processing steps.Graphical abstractHighlights► Simultaneous surface energy control of electrode and dielectric layers. ► Compatible with thermally evaporated and solution processed organic semiconductors. ► Comparable n-type top- and bottom-contact device performance. ► Low-contact resistance for all tested device geometries between 11 and 45 kΩ cm.
Co-reporter:By Orb Acton, Guy G. Ting, Patrick J. Shamberger, Fumio S. Ohuchi, Hong Ma and Alex K.-Y. Jen
ACS Applied Materials & Interfaces 2010 Volume 2(Issue 2) pp:511
Publication Date(Web):January 15, 2010
DOI:10.1021/am9007648
In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO2) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO2 with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO2 hybrid dielectrics, pentacene-based OTFTs operate under −2.0 V with low hysteresis, on-off current ratios above 1 × 106, threshold voltages below −0.6 V, subthreshold slopes as low as 100 mV dec−1, and field-effect mobilities as high as 1.8 cm2 V−1 s−1.Keywords: hybrid materials; organic field-effect transistor; organic semiconductor; self-assembled monolayer
Co-reporter:Hong Ma, Orb Acton, Daniel O. Hutchins, Nathan Cernetic and Alex K.-Y. Jen
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 41) pp:NaN14126-14126
Publication Date(Web):2012/06/14
DOI:10.1039/C2CP41557G
Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source–drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec−1, high Ion/Ioff ratios of 105–107, contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1–4.6 cm2 V−1 s−1, and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.
Co-reporter:Daniel Orrin Hutchins, Tobias Weidner, Joe Baio, Brent Polishak, Orb Acton, Nathan Cernetic, Hong Ma and Alex K.-Y. Jen
Journal of Materials Chemistry A 2013 - vol. 1(Issue 1) pp:NaN113-113
Publication Date(Web):2012/11/06
DOI:10.1039/C2TC00378C
A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ∼(CH2)12 PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm2 V−1 s−1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm2 V−1 s−1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.
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