With the current development of modern electronics toward miniaturization, high-degree integration and multifunctionalization, considerable heat is accumulated, which results in the thermal failure or even explosion of modern electronics. The thermal conductivity of materials has thus attracted much attention in modern electronics. Although polymer composites with enhanced thermal conductivity are expected to address this issue, achieving higher thermal conductivity (above 10 W m–1 K–1) at filler loadings below 50.0 wt % remains challenging. Here, we report a nanocomposite consisting of boron nitride nanotubes and cellulose nanofibers that exhibits high thermal conductivity (21.39 W m–1 K–1) at 25.0 wt % boron nitride nanotubes. Such high thermal conductivity is attributed to the high intrinsic thermal conductivity of boron nitride nanotubes and cellulose nanofibers, the one-dimensional structure of boron nitride nanotubes, and the reduced interfacial thermal resistance due to the strong interaction between the boron nitride nanotubes and cellulose nanofibers. Using the as-prepared nanocomposite as a flexible printed circuit board, we demonstrate its potential usefulness in electronic device-cooling applications. This thermally conductive nanocomposite has promising applications in thermal interface materials, printed circuit boards or organic substrates in electronics and could supplement conventional polymer-based materials.Keywords: boron nitride nanotubes; cellulose nanofibers; interfacial thermal resistance; nanocomposites; thermal conductivity;

The piezoelectric effect is widely applied in pressure sensors for the detection of dynamic signals. However, these piezoelectric-induced pressure sensors have challenges in measuring static signals that are based on the transient flow of electrons in an external load as driven by the piezopotential arisen from dynamic stress. Here, we present a pressure sensor with nanowires/graphene heterostructures for static measurements based on the synergistic mechanisms between strain-induced polarization charges in piezoelectric nanowires and the caused change of carrier scattering in graphene. Compared to the conventional piezoelectric nanowire or graphene pressure sensors, this sensor is capable of measuring static pressures with a sensitivity of up to 9.4 × 10–3 kPa–1 and a fast response time down to 5–7 ms. This demonstration of pressure sensors shows great potential in the applications of electronic skin and wearable devices.Keywords: flexible device; graphene; piezoelectric nanowire; pressure sensor; static measurements;

Recently, there are increasing interests in two-dimensional materials, as a result of their outstanding electrical and optical properties and numerous potential applications in optoelectronic devices. Here, we first report on a bipolar phototransistor based on WSe2-BP-MoS2 van der Waals heterostructure, showing its broadband photoresponse from visible to the infrared spectral regions. Broadband photoresponsivities for visible (532 nm) and the infrared (1550 nm) light waves reach up to 6.32 and 1.12 A W1–, respectively, which are both improved by tens of times in comparison with similar photodiode devices composed of WSe2-BP. The phototransistor also exhibits ultrasensitive shot noise limit specific detectivities which are 1.25 × 1011 Jones for visible light at wavelength λ = 532 nm and 2.21 × 1010 Jones for the near-infrared light at wavelength λ = 1550 nm at room temperature. It is a promising candidate for progressive development of photodetector, with implementation of smaller sensor elements, large sensing area, super-high integration, and broadband photoresponse.Keywords: amplification capacity; bipolar phototransistor; infrared; optoelectronic; two-dimensional material;

With the rapid development of modern electronics toward miniaturization, high-degree integration, and multifunctionalization, increased heat is generated during the operation of devices, which seriously limits the performance, lifetime, and reliability of electronic devices. Polymer-based composites with high thermal conductivity have attracted much attention in solving the heat dissipation issue. However, conventional polymer-based composites can hardly achieve a thermal conductivity of over 10 W m−1 K−1, due to high interfacial thermal resistance. Herein, engineering interfacial thermal resistance in boron nitride nanosheet/nanofibrillated cellulose nanocomposites by constructing nanoscale silver “bridges” between fillers is reported, aiming at achieving a high thermal conductivity. The highest in-plane thermal conductivity is up to 65.7 ± 3.0 W m−1 K−1, which is one order magnitude higher than those of conventional polymer-based composites. By fitting the experimental data with theoretical models, it is quantitatively demonstrated that silver nanoparticles can help to sharply decrease the interfacial thermal resistance between adjacent boron nitride nanosheets. In addition, the small amount of silver hardly affects the electrical insulation of boron nitride nanosheet/nanofibrillated cellulose nanocomposites. This strategy can potentially pave the way for the design and preparation of highly thermally conductive materials in the future.

The mixed perovskite (FAPbI3)1−x(MAPbBr3)x, prepared by directly mixing different perovskite components, suffers from phase competition and a low-crystallinity character, resulting in instability, despite the high efficiency. In this study, a dual ion exchange (DIE) method is developed by treating as-prepared FAPbI3 with methylammonium brodide (MABr)/tert-butanol solution. The converted perovskite thin film shows an optimized absorption edge at 800 nm after reaction time control, and the high crystallinity can be preserved after MABr incorporation. More importantly, it is found that the threshold electrical field to initiate ion migration is greatly increased in DIE perovskite thin film because excess MABr on the surface can effectively heal structural defects located on grain boundaries during the ion exchange process. It contributes to the over-one-month moisture stability under ≈65% room humidity (RH) and greatly enhanced light stability for the bare perovskite film. As a result of preserved high crystallinity and simultaneous grain boundary passivation, the perovskite solar cells fabricated by the DIE method demonstrate reliable reproducibility with an average power conversion efficiency (PCE) of 17% and a maximum PCE of 18.1%, with negligible hysteresis.

Deliberate halide exchange between unstable intermediate HPbI2Br and nonstoichiometric FAI is employed to produce high-quality FAPbI3–xBrx (x ≈ 0.44), which eliminates the use of antisolvent dripping and other post-treatment techniques. The obtained perovskite thin film demonstrates high crystallinity and a large and compact crystal domain up to 2–3 µm. The corresponding device shows power conversion efficiency of up to around 19.0%, with reliable stability and reproducibility.

•The newly developed three-dimensional (3D) nanobowl arrays act as light-trapping substrates for constructing 3D hematite-based photoanodes for photoelectrochemical (PEC) water splitting.•A self-driven tandem cell is assembled with the 3D NTO nanobowl array photoanode and a bandgap-tunable perovskite solar cell.•The tandem cell could deliver up to 3.25% solar-to-hydrogen conversion efficiency, higher than those of state-of-the-art hematite-based PEC cells.Solar-driven photoelectrochemical (PEC) water splitting is a clean and powerful approach for renewable hydrogen production. The PEC performance of hematite (α-Fe2O3) is largely limited by its short hole diffusion length, which imposes restrictions on increasing the thickness for enough light absorption. In this work, we report a well-engineered three-dimensional (3D) Fe2O3/NTO (Nb-doped SnO2) nanobowl heterojunction for PEC electrode, driven by a high-photovoltage perovskite solar cell (PSC), has boosted the efficiency greatly. The 3D heterojunction is made of an ultrathin Ti-doped hematite layer deposited on a periodic NTO nanobowl array. This PEC electrode, not only significantly improves the light absorption of the ultrathin hematite, but also enhances the interface contact. As a result, it exhibits ∼3.16 mA cm−2 photocurrent at 1.23 V vs the reversible hydrogen electrode with the Co–Pi cocatalyst. The tandem cell self-biased with a high-photovoltage PSC delivers up to 3.25% solar-to-hydrogen conversion efficiency, higher than those of state-of-the-art hematite-based PEC cells.Download high-res image (141KB)Download full-size image

We report a facile and low-temperature aqueous route for the fabrication of various oxide thin films (Al2O3, In2O3 and InZnO). A detail study is carried out to reveal the formation and properties of these sol-gel-derived thin films. The results show that the water-based oxide thin films undergo the decomposition of nitrate group as well as conversion of metal hydroxides to form metal oxide framework. High quality oxide thin film could be achieved at low temperature by this aqueous route. Furthermore, these oxide thin films are integrated to form thin-film transistors (TFTs) and the electrical performance is systematically studied. In particular, we successfully demonstrate In2O3/Al2O3 TFTs with high mobility of 30.88 cm2 V−1 s−1 and low operation voltage of 4 V at a maximum processing temperature of 250 °C.

Correction for ‘The physics and chemistry of graphene-on-surfaces’ by Guoke Zhao, Xinming Li, Meirong Huang et al., Chem. Soc. Rev., 2017, 46, 4417–4449.

Graphene has demonstrated great potential in next-generation electronics due to its unique two-dimensional structure and properties including a zero-gap band structure, high electron mobility, and high electrical and thermal conductivity. The integration of atom-thick graphene into a device always involves its interaction with a supporting substrate by van der Waals forces and other intermolecular forces or even covalent bonding, and this is critical to its real applications. Graphene films on different surfaces are expected to exhibit significant differences in their properties, which lead to changes in their morphology, electronic structure, surface chemistry/physics, and surface/interface states. Therefore, a thorough understanding of the surface/interface properties is of great importance. In this review, we describe the major “graphene-on-surface” structures and examine the roles of their properties and related phenomena in governing the overall performance for specific applications including optoelectronics, surface catalysis, anti-friction and superlubricity, and coatings and composites. Finally, perspectives on the opportunities and challenges of graphene-on-surface systems are discussed.

•A new never-before achieved photogate heterojunction based on BP-on-WSe2 used as photodetector exhibits a broadband photoresponsivity in a spectral range of 400–1550 nm; and ultrasensitive visible and infrared specific detectivities are obtained up to ~1014 and ~1010 Jones, respectively, with excellent external quantum efficiency as high as ~106 and ~102 at a low bias of 0.5 V, at room temperature. Such photodetector based on BP-on-WSe2 structure affords new opportunities especially for infrared detection or imaging at room temperature by utilizing two-dimensional materials.•The high infrared polarization-dependent photoresponse of the photodetector is successfully demonstrated by spatially scanning photocurrent mapping, based on the BP photogate which both can sufficiently collect the photoinduced carriers isotropically with photoresponsivity enhancement and eliminate the confused effect of polarization photoresponse resulted from BP/metal junction. Inspired by the photogate mechanism that is a superior method in terms of photoresponsivity enhancement and ease of fabrication, the BP-on-WSe2 photodetectors heralds a promising beginning for practical use in high-resolution infrared imaging applications under complex environment.•We further investigate the photoresponse dynamics of the BP-on-WSe2 photodetectors. In comparison with the response speed of other photodetectors based on 2D materials with the same photoconductance mechanism, its relatively fast response speed is attributed to the BP-on-WSe2 junction which accelerates the separation of photoinduced electron-hole pairs. The reversible and stable photoresponse behavior of the detector indicates its outstanding reversibility and stability performance.The ability to use infrared imaging systems with multicolor capabilities, high photoresponsivity and polarization sensitivity, is central to practical photodetectors and has been demonstrated with conventional devices based on Ⅲ-Ⅴ or Ⅱ-Ⅵ semiconductors. However, the photodetectors working at room temperature with high responsivity for polarized infrared light detection remains elusive. Here, we first demonstrate a broadband photodetector using a vertical photogate heterostructure of BP-on-WSe2 (black phosphorus-on-tungsten diselenide) in which BP serves as the photogate and WSe2 as the conductive channel. Ultrahigh visible and infrared photoresponsivity at room temperature can reach up to ~103 A/W and ~5×10−1 A/W, respectively, and ultrasensitive visible and infrared specific detectivity is obtained up to ~1014 and ~1010 Jones respectively at room temperature. Moreover, the high sensitivity to infrared polarization is about 40 mA/W with incident light polarized along the horizontal axis (defined as 0° polarization). This performance is due to the strong intrinsic linear dichroism of BP and the device design which can sufficiently collect the photoinduced carriers isotropically, as well as the influence from the orientation of the edge of the BP-on-WSe2 overlapped area which is the same for all polarizations. The high responsivity, good sensitive detectivity and highly polarization-sensitive infrared photoresponse suggest that the photodetectors based on photogate structure afford new opportunities for infrared detecting or imaging at room temperature by using two-dimensional materials.The highly polarization-sensitive infrared photodetector based on BP-on-WSe2 photogating vertical heterostructure is demonstrated for high infrared identification with a broadband visible and infrared photoresponsivities R of ~103 A/W and ~5×10−1 A/W, respectively and highly polarization infrared photoresponsivity R of ~40 mA/W.Download high-res image (348KB)Download full-size image

In recent years, ultrathin two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as MX2 (M = Mo, W; X = S, Se, etc.) have become the flagship materials after graphene. 2D-MX2 have attracted significant attention due to their novel properties arising from their strict dimensional confinement as well as strong spin–orbit coupling effects, which provides an ideal platform for exploring new fundamental research and realizing technological innovation. The 2D nature and the small lattice mismatch between MX2 make them ideal templates for construction of vertical and lateral heterojunctions at atomic scale by means of CVD epitaxial growth. This feature article aims to introduce current advances in the preparation of vertical or lateral epitaxial heterostructures based on 2D MX2 nanosheets as well as their potential applications in electronics, and optoelectronics. Firstly, various epitaxial CVD strategies for synthesis of vertical or lateral 2D MX2 heterostructures are comprehensively reviewed. Meanwhile, the advantages of these epitaxial methods as well as several applications of 2D MX2 heterostructures, such as photodiodes and photovoltaic devices are highlighted. Then the remaining challenges facing the controllable syntheses and the future perspectives of this promising area are discussed.

Controllable creating of wafer-scale homogeneous vertical or parallel 2D heterostructures with low cost by the van der Waals stacking or covalently bonded stitching of 2D layered materials, such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides, is of great challenge. In this paper, a new green growth strategy for the fabrication of high-quality large-area and low-cost vertical MoS2/graphene heterostructures has been successfully demonstrated via electrochemical deposition in water solution, followed by an annealing process in chemical vapour deposition system for the first time. The vertical MoS2/graphene heterostructures have been systematically investigated by the combined use of Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. This simple, reliable, and environmentally friendly growth strategy on conducting monolayer graphene in a controlled manner opens up a new way for producing low-cost, large-area, and high-quality vertical MoS2/graphene heterostructures, which have promising applications not only in electronics and optoelectronics but also in the fields of catalysis and renewable energy.

Graphene’s unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 μm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications.Keywords: fast photoresponse; graphene; plasmonics; short-wave infrared photodetector; ultrahigh responsivity;

Layered α-MoO3 is a multifunctional material that has significant application in optoelectronic devices. In this study, we show the growth of large-scale, large-size, few-layered (FL) α-MoO3 nanosheet directly on technical substrates (SiO2 and Si) by physical vapor deposition. We suggest that the growth is self-limiting in the [010] direction because of the re-evaporation and high diffusion capacity of MoOx species at high temperature. As-prepared FL α-MoO3 is nonconductive and shows poor response to photoillumination with wavelength of 405 and 630 nm. Its work function is strongly altered by the substrate. Improvement of conductivity and photoresponse is observed after the FL device is annealed in vacuum. Line defects along the [001], [100], and [101] directions belonging to the generation of Os and Oa vacancy states appear, and the interfacial effect is suppressed. Scanning near-field optical microscope shows that the defects are absorption sites. Kelvin probe force microscope reveals decrease of apparent work function under illumination, which confirms that electrons are excited from defects states. Our findings show that intense studies on defect engineering are required to push forward the application of two-dimensional metal oxides.Keywords: MoO3; photoresponse; physical vapor deposition; surface defects; two-dimensional materials;

Owing to the miniaturization of power electronics and the development of portable and flexible devices, demands for highly thermally conductive, mechanically flexible, and electrically insulating composites have substantially increased. However, the conventional method to improve thermal conductivity usually yields both an undesired value (usually below 10 W m–1 K–1) and poor flexibility. Thus, a combination of all the desired properties together remains a technical challenge. Bioinspired engineering offers great promise in the synthesis and fabrication of thermal materials that are different from engineering through conventional approaches. Inspired by the interface and orientation of natural nacre, we report on thermally conductive and mechanically flexible papers based on boron nitride nanosheets (BNNSs) and graphene oxide (GO) via a simple vacuum-assisted filtration process. We experimentally show that the papers possess high thermal conductivity of 29.8 W m–1 K–1, excellent mechanical flexibility, and satisfactory electrical insulation. We attribute the high thermal conductivity to the well-designed BNNS–GO interface as well as the advantageous orientation in layered structure. This approach to constructing thermally conductive composites provides a creative opportunity for design and fabrication of high-performance materials in the near future, and this kind of paper has great potential application in next-generation commercial portable electronics.

Flexible polymer-based dielectric materials that are used to store dielectric energy have widely been used in modern electronics and electric power systems, due to their relatively high energy density, light weight, low cost, etc. However, owing to the growing global environmental issues and a rapid consumption of nonrenewable polymer resources, there exists a strong desire to fabricate flexible dielectric materials using biodegradable materials. Here, we report on flexible dielectric papers based on biodegradable cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) for dielectric energy storage. Highly ordered, homogeneous CNF/CNT papers have been fabricated using a facile vacuum-assisted self-assembly technique. The obtained paper possesses a high dielectric constant of 3198 at 1.0 kHz, thus leading to enhanced dielectric energy storage capability (0.81 ± 0.1 J cm−3), which is attributed to the presence of a low loading of CNTs (4.5 wt%). Moreover, the CNF/CNT papers are mechanically flexible and show improved mechanical strength. These findings enable feasible fabrication of high-performance flexible dielectric materials using ecofriendly materials.

We developed a molecule/polymer composite hole transporting material (HTM) with a periodic microstructure for morphology replication of a corrugated Au electrode, which in combination plays a dual role in the optical and electronic enhancement of high performance perovskite solar cells (PSCs). The electro-optics revealed that perovskite couldn't readily extinct the red light even though the thickness increased to 370 nm, but we found that the quasi periodic microstructure composite (PMC) HTM in combination with the conformal Au electrode could promote the absorption through the enhanced cavity effects, leading to comparable absorption even using much thinner perovskite (240 nm). We identified that the cavity was the combination of Fabry–Pérot interferometer and surface plasmonic resonance, with light harvesting enhancement through surface plasmon polariton or waveguide modes that propagate in the plane of the perovskite layer. On the other hand, the PMC HTM increased hole conductivity by one order of magnitude with respect to standard spiro-OMeTAD HTM due to molecular packing and self-assembly, embodying traceable hole mobility and density elevation up to 3 times, and thus the hysteresis was greatly avoided. Owing to dual optical and electronic enhancement, the PMC PSC afforded high efficiency PSC using as thin as 240 nm perovskite layer, delivering a Voc of 1.05 V, Jsc of 22.9 mA cm−2, FF of 0.736, and efficiency amounting to 17.7% PCE, the highest efficiency with ultrathin perovskite layer.

•Efficient PTB7:PCDTBT:PC71BM ternary solar cells was fabricated.•The PCDTBT cascaded energy alignment lead to enhanced charge separation.•Voc is limited by the work function pinning of donor blends to PTB7.Ternary bulk hetero-junction (BHJ) architecture for organic solar cells has been developed to enhance the power conversion efficiency (PCE) by expanding the light absorption range and smoothing the energy level at the BHJ interface. In this work, we report on a ternary polymer blend solar cell with two donor materials, polythieno[3,4-b]-thiophene/benzodithiophene (PTB7), poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and using C71-butyric acid methyl ester (PC71BM) as acceptor material. The resultant device shows an optimized PCE of 7.81% (control sample, 7.06%), with an open circuit voltage (Voc) of 0.76 V, a short circuit current (Jsc) of 15.4 mA/cm2 and a fill factor (FF) of 66.7%. The improved device performance is mainly attributed to the enhanced charge separation, improved hole mobility, and better film morphology. And we find that the third component PCDTBT can reduce charge recombination in the ternary blend system as well. The results bring new insight into the future development of high efficiency ternary organic solar cells.

Quantification of intergrain length scale properties of CH3NH3PbI3 (MAPbI3) can provide further understanding of material physics, leading to improved device performance. In this work, we noticed that two typical types of facets appear in sequential deposited perovskite (SDP) films: smooth and steplike morphologies. By mapping the surface potential as well as the photoluminescence (PL) peak position, we revealed the heterogeneity of SDP thin films that smooth facets are almost intrinsic with a PL peak at 775 nm, while the steplike facets are p-type-doped with 5-nm blue-shifted PL peak. Considering the reaction process, we propose that the smooth facets have well-defined crystal lattices that resulted from the interfacial reaction between MAI and PbI2 domains containing low trap states density. The steplike facets are MAI-rich originated from the grain boundaries of PbI2 film and own more trap states. Conversion of steplike facets to smooth facets can be controlled by increasing the reaction time through Ostwald ripening. The improved stability, photoresponsivity up to 0.3 A/W, on/off ratio up to 3900, and decreased photo response time to ∼160 μs show that the trap states can be annihilated effectively to improve the photoelectrical conversion with prolonged reaction time and elimination of steplike facets. Our findings demonstrate the relationship between the facet heterogeneity of SDP films and crystal growth process for the first time, and imply that the systematic control of crystal grain modification will enable amelioration of crystallinity for more-efficient perovskite photoelectrical applications.Keywords: facet heterogeneity; MAI rich; Ostwald ripening; sequential deposited perovskite; trap states annihilation;

Two-dimensional (2D) materials present their excellent properties in electronic and optoelectronic applications, including in ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation, high mobility, transparency, flexibility, and the ability to confine electromagnetic energy to extremely small volumes. Herein, we demonstrate a photodetector with visible to near-infrared detection range, based on the heterojunction fabricated by van der Waals assembly between few-layer black phosphorus (BP) and few-layer molybdenum disulfide (MoS2). The heterojunction with electrical characteristics which can be electrically tuned by a gate voltage achieves a wide range of current-rectifying behavior with a forward-to-reverse bias current ratio exceeding 103. The photoresponsivity (R) of the photodetector is about 22.3 A W–1 measured at λ = 532 nm and 153.4 mA W–1 at λ = 1.55 μm with a microsecond response speed (15 μs). In addition, its specific detectivity D* is calculated to have the maximum values of 3.1 × 1011 Jones at λ = 532 nm, while 2.13 × 109 Jones at λ = 1550 nm at room temperature.

The precursor of solution-processed perovskite thin films is one of the most central components for high-efficiency perovskite solar cells. We first present the crucial colloidal chemistry visualization of the perovskite precursor solution based on analytical spectra and reveal that perovskite precursor solutions for solar cells are generally colloidal dispersions in a mother solution, with a colloidal size up to the mesoscale, rather than real solutions. The colloid is made of a soft coordination complex in the form of a lead polyhalide framework between organic and inorganic components and can be structurally tuned by the coordination degree, thereby primarily determining the basic film coverage and morphology of deposited thin films. By utilizing coordination engineering, particularly through employing additional methylammonium halide over the stoichiometric ratio for tuning the coordination degree and mode in the initial colloidal solution, along with a thermal leaching for the selective release of excess methylammonium halides, we achieved full and even coverage, the preferential orientation, and high purity of planar perovskite thin films. We have also identified that excess organic component can reduce the colloidal size of and tune the morphology of the coordination framework in relation to final perovskite grains and partial chlorine substitution can accelerate the crystalline nucleation process of perovskite. This work demonstrates the important fundamental chemistry of perovskite precursors and provides genuine guidelines for accurately controlling the high quality of hybrid perovskite thin films without any impurity, thereby delivering efficient planar perovskite solar cells with a power conversion efficiency as high as 17% without distinct hysteresis owing to the high quality of perovskite thin films.

Piezoelectric materials used in the development of nanoscale mechanical sensors, actuators and energy harvesters have received much attention. More recently, devices made of graphene are of particular interest because of graphene’s intriguing electronic and mechanical properties. Intrinsic graphene has long been considered devoid of the piezoelectric effect, although flexoelectricity has been exploited to demonstrate piezoelectricity in functionalized graphene and graphene nanoribbons. The perceived lack of this property has restricted graphene’s use in nanoelectromechanical systems (NEMS) for electromechanical coupling purposes. Here an unprecedented two-dimensional (2D) piezoelectric effect on a strained/unstrained graphene junction is reported. In stark contrast to the bulk piezoelectric effect that results from the occurrence of electric dipole moments in solids, the 2D piezoelectric effect arises from the charge transfer along a work function gradient introduced by the biaxial-strain-engineered band structure. The observed effect, termed the band-piezoelectric effect, exhibits an enormous magnitude due to the ultrathin structure of graphene. On the basis of the band-piezoelectric effect, a graphene nanogenerator and a pressure gauge were fabricated. The results not only provide a versatile NEMS platform for sensing, actuating and energy harvesting, but also pave the way for efficiently modulating graphene via strain engineering.

As a structural analogue of graphene, boron nitride nanosheets (BNNSs) have attracted ever-growing research interest in the past few years, due to their remarkably mechanical, electrical, and thermal properties. The preparation of BNNS aerogels is considered to be one of the most effective approaches for their practical applications. However, it has remained a great challenge to fabricate BNNS aerogels with superelasticity by a facile method. Here, we report the preparation of BNNS aerogels via a facile method involving polymer-assisted cross-linking and freeze-casting strategies. The resulting aerogels exhibit a well-ordered and anisotropic microstructure, leading to anisotropic superelasticity, high compressive strength, and excellent energy absorption ability. The unique microstructure also endows the aerogels with ultralow dielectric constant (1.24) and loss (∼0.003). The successful fabrication of such fascinating materials paves the way for application of BNNSs in energy-absorbing services, catalyst carrier, and environmental remediation, etc.

We report a novel MoS2-based fluorescent biosensor for DNA detection via hybridization chain reactions (HCRs). As an emerging nanomaterial, MoS2 has excellent fluorescence quenching ability and distinct adsorption properties toward single- and double-stranded DNA. In the sensing method, MoS2 nanosheets are used to suppress the background signal and control the “on” and “off” of fluorescence emission of the detection system with and without the presence of the target DNA. In addition, the signal generation is amplified through the target-triggered HCRs between two hairpin probes. The employment of MoS2 and HCRs guarantees the high sensitivity of the detection strategy with a detection limit of 15 pM. The biosensor also exhibits very good selectivity over mismatched DNA sequences. The detection takes place in solutions and requires only one “mix-and-detect” step. The high sensitivity, selectivity, and operational simplicity demonstrate that MoS2 can be a promising nanomaterial for versatile biosensing.

We developed a facile and environmentally friendly solution-processed method for aluminum oxide (AlOx) dielectrics. The formation and properties of AlOx thin films under various annealing temperatures were intensively investigated by thermogravimetric analysis–differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD), spectroscopic ellipsometry, atomic force microscopy (AFM), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, and leakage current measurements. The sol–gel-derived AlOx thin film undergoes the decomposition of organic residuals and nitrate groups, as well as conversion of aluminum hydroxides to form aluminum oxide, as the annealing temperature increases. Finally, the AlOx film is used as gate dielectric for a variety of low-temperature solution-processed oxide TFTs. Above all, the In2O3 and InZnO TFTs exhibited high average mobilities of 57.2 cm2 V–1 s–1 and 10.1 cm2 V–1 s–1, as well as an on/off current ratio of ∼105 and low operating voltages of 4 V at a maximum processing temperature of 300 °C. Therefore, the solution-processable AlOx could be a promising candidate dielectric for low-cost, low-temperature, and high-performance oxide electronics.Keywords: aluminum oxide; environmentally friendly; high-performance; low-temperature; oxide thin-film transistors; solution process

We reported a novel aqueous route to fabricate Ga2O3 dielectric at low temperature. The formation and properties of Ga2O3 were investigated by a wide range of characterization techniques, revealing that Ga2O3 films could effectively block leakage current even after annealing in air at 200 °C. Furthermore, all aqueous solution-processed In2O3/Ga2O3 TFTs fabricated at 200 and 250 °C showed mobilities of 1.0 and 4.1 cm2 V–1 s–1, on/off current ratio of ∼105, low operating voltages of 4 V, and negligible hysteresis. Our study represents a significant step toward the development of low-cost, low-temperature, and large-area green oxide electronics.Keywords: aqueous route; gallium oxide dielectric; green oxide electronics; indium oxide; low-temperature; oxide thin-film transistors;

Organic solar cells based on bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]2NH) bulk doped poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT):C71-butyric acid methyl ester (PC71BM) were fabricated to study the effect of molecular doping. By adding TFSA (0.2–0.8 wt %, TFSA to PCDTBT) in the conventional PCDTBT:PC71BM blends, we found that the hole mobility was increased with the reduced series resistance in photovoltaic devices. The p-doping effect of TFSA was confirmed by photoemission spectroscopy that the Fermi level of doped PCDTBT shifts downward to the HOMO level and it results in a larger internal electrical field at the donor/acceptor interface for more efficient charge transfer. Moreover, the doping effect was also confirmed by charge modulated electroabsorption spectroscopy (CMEAS), showing that there are additional polaron signals in the sub-bandgap region in the doped thin films. With decreased series resistance, the open-circuit voltage (Voc) was increased from 0.85 to 0.91 V and the fill factor (FF) was improved from 60.7% to 67.3%, resulting in a largely enhanced power conversion efficiency (PCE) from 5.39% to 6.46%. Our finding suggests the molecular doping by TFSA can be a facile approach to improve the electrical properties of organic materials for future development of organic photovoltaic devices (OPVs).Keywords: carrier mobility; charge transfer; Fermi level; polymer solar cells; TFSA doping;

The viscoelasticity of boron nitride nanosheet (BNNS) aerogel has been observed and investigated. It is found that the BNNS aerogel has a high damping ratio (0.2), while it exhibits lightweight and negligible temperature dependence below 180 °C. The creep behavior of the BNNS aerogel markedly demonstrates its strain dependence on stress magnitude and temperature, and can be well simulated by the classical models.

Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS2–WS2 heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX2 heterostructure with monolayer MoS2 core wrapped by multilayer WS2 is obtained by the technique, which is entirely different from MX2 heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS2 (monolayer)/WS2 (multilayer) heterojunctions serve as intrinsic lateral p–n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor.Keywords: built-in electric field; built-in potential; depletion-layer width; KPFM; MoS2−WS2 heterostructure; two-step epitaxial CVD method;

We developed a simple and environmentally friendly spin-coating method for high-κ dielectrics (AlOx, ZrOx, YOx and TiOx). These materials were used as gate dielectrics for solution-processed nanocrystalline In2O3 or amorphous InZnO TFTs with a maximum processing temperature of 300 °C. The role of high-κ dielectrics in device performance was systematically studied. Among the high-κ dielectrics, the AlOx-based devices showed the best performance with mobilities of 21.7 cm2 V−1 s−1 in an In2O3 TFT and 11.6 cm2 V−1 s−1 in an InZnO TFT with the on/off current ratio exceeding 106. Furthermore, the devices exhibited ultra-low operating voltages (<3 V) and negligible hysteresis. A comprehensive study suggests that the high performance of the AlOx-based devices could be attributed to the smooth dielectric/semiconductor interface and the low interface trap density besides its good insulating properties. Therefore, the solution-processed AlOx can be used as a promising high-κ dielectric for low cost, low voltage, high-performance oxide electronic devices.

p–n junction is a fundamental building block in modern electronic circuits. We report graphene p–n junctions formed by a one-step thickness-dependent surface treatment of mono-/bilayer graphene steps. The junction electronic properties are systemically studied by means of Kelvin probe force microscopy (KPFM) and transport measurements. Because of the dissimilar modifications to graphene electronic properties, the junctions behave distinctly, i.e., two-component resistance-like for organic charge transfer doping and Shottky-junction-like for covalent doping. By exploring the spatially potential distribution, we clarify the potential profiles as well as the transport attributes across the graphene p–n junction interface under lateral bias and electrical gating. Our results not only unveil the detailed properties of graphene p–n junction interface, but also gain an insight into its practical applications in nanoelectronics.Keywords: electronic properties; graphene; KPFM; p−n junction;

•Solar cell performance enhanced by adding dichloromethane to 1,2 o-dichlorobenzene.•The crystallization of P3HT increased in competing solvent evaporation process.•The film surface composition was tuned from P3HT-rich to more favored PCBM-rich.•The charge transfer at the active layer-cathode interface was optimized.In this work, the effects of mixed solvents on donor–acceptor vertical phase separation and light absorption was investigated. By using mixed orthogonal solutions of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM), a PCBM([6,6]-phenyl-C61-butyric acid methyl ester)-rich top layer was induced in typical poly(3-hexylthiophene-2,5-diyl)(P3HT):PCBM bulk heterojunction structure. By carefully adjusting the o-DCB:DCM volume ratio, the contact between active layer and the Al cathode was significantly improved due to the precipitation of PCBM on the top surface, which resulting in an electron transport preferable interface between the active layer and cathode. Meanwhile, light absorption was also effectively improved due to the increased crystallinity of polymers under mixed solvents. Overall, the short circuit current was greatly increased, and the efficiency was improved from 3.07% in the control sample to 3.97% by adding 30% DCM. The detailed mechanism of the formation of PCBM-rich layer and enhanced light absorption with o-DCB:DCM solution was expatiated. Our findings suggest a facile spin coating method to fabricate efficient BHJ solar cells, which can pave the way for the large scale application of organic photovoltaic devices (OPVs) in the future.Graphical abstract

•Charge barrier at anode interface was reduced by TFSA doping.•More efficient charge injection at anode/active layer interface lead to enhanced PCE.•The work function of PCDTBT moved downward to its HOMO results in higher Voc.•TFSA could be a new anode buffer layer to replace PEDOT:PSS.Organic solar cells based on bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]2NH) doped poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) were fabricated to investigate the effect of molecular doping. By replacing poly(3,4-ethylenedioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS) with a thin TFSA layer, we have found more efficient charge injection at anode/active interface enhanced photovoltaic performance. The doping effect is confirmed by photoemission spectroscopy that the Fermi level of doped PCDTBT shifts downward to its HOMO level and results in higher carrier concentration. The reduced injection barrier also evidented by impedance spectroscopy that the real impedance of the TFSA doped PCDTBT solar cell decreases more than 50%. Using the molecular doping approach, the overall power conversion efficiency (PCE) was largely increased from 4.70% to 5.98%. Our results suggest that TFSA functions not only as a surface doping molecule, but also an anode interfacial layer to replace the conventional PEDOT:PSS.Graphical abstract

In this work, acetylacetone assisted solution-processed In–Ga–Zn–O (IGZO) thin film transistors (TFTs) using Al2O3 as gate dielectrics were investigated. Normally, fully covered Al2O3 thin films are difficult to achieve by spin coating with conventional solvent, such as 2-methoxyethanol, due to the poor wettability of highly doped silicon. Here, a conventional aluminum nitrate solution with an additive was designed to spin coat robust continuous Al2O3 thin films, resulting from improved solution hydrophilic with a contact angle of 17°. For active layer fabrication, we utilized the previous reported combustion process to lower treatment temperature, which could be confined in the range from 220 °C to 300 °C, without losing the device performance. Results show that all the devices performed well. Especially, after 240 °C annealing of both Al2O3 (in thickness of around 45 nm) and IGZO thin films (in thickness of around 30 nm), we have obtained the following device parameters, namely a Al2O3 dielectric breakdown electric field at 7.8 MV cm−1, a current density of around 1 × 10−6 A cm−2 in the voltage range of −3 V to 3 V, a areal capacitance of 291 nF cm−2 at 100 Hz, a carrier mobility of 0.74 cm2 V−1 s−1, a threshold voltage of −0.4 V, a current on–off ratio of 6 × 103, a subthreshold swing of 375 mV per decade. Fabrication of combustion-processed active layers and our facile solution processed high-k dielectrics provides a feasible approach for low cost oxide flexible TFTs applications.

•High-performance C60 based OTFTs with low operation voltage.•Low-cost Cu modified by PEI as S/D electrodes.•Electron mobility up to 3.2 cm2/V s is obtained on modified device.•Improved interface property between Cu and C60 upon interface modification.Exploring suitable electrode materials with sufficiently low work function, ambient stability and low-cost is of great technological importance to the development of n-channel OTFTs. Here, we show that the work function of Cu can be effectively reduced from 4.65 eV to 4.28 eV through surface modification via simply spin-coating a thin layer of branched polyethylenimine (PEI). By exploiting a high-capacitance density gate dielectric (200 nF/cm2), low-voltage (3 V) C60 TFTs with electron mobility (μe) of 3.2 cm2/V s are demonstrated with PEI modified Cu as source–drain (S/D) electrodes. In contrast, the device with Cu S/D electrodes possesses μe of only 1.0 cm2/V s. The improvement in electrical performance of the PEI modified device is attributed to the efficient electron injection at the Cu/C60 interface which resulted from the reduction in work function of Cu. Moreover, upon PEI modification, the bias stability of the device can be obviously enhanced as compared to the unmodified one, and the resultant device exhibits an excellent thermal stability up to 200 °C without appreciable degradation in mobility. The facile modification of low-cost Cu as S/D electrodes for high-performance n-channel OTFTs as well as the low-voltage operation will pave the way for large scale manufacturing of organic electronics.

To explore the potential of ternary blend bulk heterojunction (BHJ) solar cells as a general platform for improving the performance of organic photovoltaics, we studied a ternary BHJ system based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), and DTDCTB. The optimized ternary structure containing a weight ratio of 20% DTDCTB as the cascade material demonstrates a ∼25% improvement of the power conversion efficiency (PCE) as compared to the binary P3HT/PC61BM solar cells. A systematic spectroscopic study is carried out to elucidate the underlying mechanism of charge transfer in the ternary system. Wavelength-dependent external quantum efficiency measurement confirms the contribution of DTDCTB to the enhanced photocurrent. Photoinduced absorption spectroscopy and transient photovoltage measurement reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and subsequently transported in P3HT and PC61BM. The results also suggest that despite the realization of cascade charge transfer, the bimolecular charge recombination process in the ternary system is still dominated by the P3HT/PC61BM interface.

Low-voltage, flexibility and low-cost are essential prerequisites for large scale application of organic thin film transistors (OTFTs) in future low-end electronics. Here, we demonstrate a low-voltage flexible OTFT by using a low-temperature, solution-processed gate dielectric. Such a dielectric can be well integrated with an Au coated polyimide film, and exhibits a low leakage current density of less than 10−6 A cm−2 and a high capacitance density of 180 nF cm−2. Pentacene films deposited onto the solution-processed dielectric show a highly ordered “thin film phase”. The source–drain (S/D) electrodes are made of in situ modified Cu encapsulated by Au (Au/M-Cu). The obtained flexible OTFT exhibits outstanding electrical characteristics under a gate voltage of only −2 V, which include an on/off ratio of 2 × 104, a mobility (μ) of 1.5 cm2 V−1 s−1, a threshold voltage (VT) of −0.3 V and a subthreshold slope (SS) of 161 mV dec−1. The obtained mobility value is among the highest achieved in flexible pentacene OTFTs. The mechanical flexibility and reliability of the OTFTs are also studied and discussed in detail, and the observed degradation of the device performance under strains is attributed to the damage induced in the electrodes giving rise to increased contact resistance and the phase transition from the thin film phase to bulk phase of the pentacene films.

In this letter, the different scattering mechanisms of triphenylene-derived graphene on conventional SiO2/Si substrates and octadecyltrimethoxysilane (OTMS) self-assembled monolayer (SAM) functionalized SiO2/Si substrates were systematically studied at room temperature. In comparison with the devices on conventional SiO2/Si substrates, triphenylene-derived GFETs with OTMS–SAM modified SiO2/Si substrate exhibit the marked carrier-density-dependent field-effect mobility. Quantitative analyses reveal that at ambient temperature, the predominant scattering sources affect the carrier mean free path for graphene devices on bare SiO2 substrates and for those on OTMS passivated SiO2 substrates are charged impurity induced long-range scattering (∼5.34 × 1011 cm−2 in carrier density) and resonant scattering (short-range scattering ∼9.77 × 1010 cm−2 carrier in density), respectively. Our findings elucidate the underlying dominant factors for achieving significantly improved device performance of GFETs at room temperature.

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of D-X/G/H-D, Si-X/S/H-Si and D-X/S/H-D can be modulated effectively by tuning the pressure (interlayer spacing) and stacking arrangement. The electron effective mass (EEM) is proportional to the band gap. The band gap of confined silicene is more sensitive to pressure than that of confined graphene. Moreover, a heterogeneous interface structure would be beneficial for effectively regulating the band gap and carrier effective masses of confined graphene and silicene. Using a confinement technique and pressure, the integrity of the honeycomb structure of graphene and silicene will be preserved so that the small effective masses and high mobility of graphene and silicene will be retained during compression. The tunable band gap and high carrier mobility of the sandwich structures are promising for building high-performance nanodevices.

There has recently been a great deal of interest and excitement in applying graphene field effect transistors (GFETs) in digital and radio frequency (RF) circuits and systems. Peculiar output characteristics such as kinks and negative differential resistance (NDR) in a strong field are the unique transport properties of GFETs. Here we demonstrate that these unusual features are attributed to a carrier sheet density constrained transport framework. Simulation results based on a simple analytic model which includes the linear DOS structure are in very good agreement with experimental data. The kernel mechanism of NDR is ascribed to the fact that the total current increase of a channel with a high average carrier density is constrained by its minimum sheet density. Utilizing in situ Kelvin probe force microscopy (KPFM), the principle which naturally distinguishes NDR from kinks is further verified by studying the spatially resolved surface potential distribution along the channel. The influence and potential application of GFETs' unique output characteristics in the digital and RF fields are also proposed.

The interfacial transport properties and density of states (DOS) of CuPc near the dielectric surface in an operating organic field-effect transistor (OFET) are investigated using Kelvin probe force microscopy. We find that the carrier mobility of CuPc on high-k Al2Oy/TiOx (ATO) dielectrics under a channel electrical field of 4.3 × 102 V/cm reaches 20 times as large as that of CuPc on SiO2. The DOS of the highest occupied molecular orbital (HOMO) of CuPc on the ATO substrate has a Gaussian width of 0.33 ± 0.02 eV, and the traps DOS in the gap of CuPc on the ATO substrate is as small as 7 × 1017 cm–3. A gap state near the HOMO edge is observed and assigned to the doping level of oxygen. The measured HOMO DOS of CuPc on SiO2 decreases abruptly near EVGS = VT, and the pinning of DOS is observed, suggesting a higher trap DOS of 1019–1020 cm–3 at the interface. The relationships between DOS and the structural, chemical, as well as electrical properties at the interface are discussed. The superior performance of CuPc/ATO OFET is attributed to the low trap DOS and doping effect.Keywords: density of state; high-k; interface; Kelvin probe force microscopy; organic field-effect transistor;

We demonstrate low-voltage pentacene thin film transistors (TFTs) using in situ modified low-cost Cu (M-Cu) as source–drain (S/D) electrodes and solution-processed high capacitance (200 nF/cm2) gate dielectrics. Under a gate voltage of −3 V, the device with M-Cu electrodes shows a much higher apparent mobility (1.0 cm2/V s), a positively shifted threshold voltage (−0.62 V), a lower contact resistance (0.11 MΩ) and a larger transconductance (12 μS) as compared to the device with conventional Au electrodes (corresponding parameters are 0.71 cm2/V s, −1.44 V, 0.41 MΩ, and 5.7 μS, respectively). The enhancement in the device performance is attributed to the optimized interface properties between S/D electrodes and pentacene. Moreover, after encapsulation the M-Cu electrodes with a thin layer of Au in the aim of suppressing unfavorable surface oxidation, the electronic characteristics of the device are further improved, and highly enhanced apparent mobility (2.3 cm2/V s) and transconductance (19 μS) can be achieved arising from the increased conductivity of the electrode itself. Our study provides a simple and feasible approach to achieve high performance low-voltage OTFTs with low-cost S/D electrodes, which is desirable for large area applications.Graphical abstractHighlights► Low-voltage pentacene OTFTs with solution-processed dielectric. ► In situ modification of low-cost Cu (M-Cu) as S/D electrodes. ► M-Cu based device show higher performance than Au and Cu. ► Mobility reaches 2.0 cm2/V s with Au encapsulated M-Cu (Au/M-Cu) electrodes. ► The mechanism of this phenomenon is studied in detail.

The performance of graphene field effect transistors (GFETs) strongly depends on the interface between graphene sheets and the underlying substrates. In this work, we report that an octadecyltrimethoxysilane (OTMS) SAM modified conventional SiO2/Si substrate can consistently enhance the performance of coronene-derived large-area graphene FETs. The improved transport properties in terms of boosted carrier mobility (up to 10 700 ± 300 cm2 V–1 s–1), long mean free path, nearly vanished hysteretic behavior, and remarkably low intrinsic doping level are mainly attributed to the strong suppression of interfacial charge impurity scattering and remote interfacial phonon (RIP) scattering, less adsorption of dipolar adsorbates, and the attenuated charger transfer at the interface of graphene and dielectric. The intrinsic doping levels (the Fermi energy) of graphene on OTMS-modified and bare SiO2 have been quantitatively estimated and confirmed by the Dirac points of GFETs, the Raman mapping of G-peak positions, and the surface potential maps by KPFM. The facile fabrication of a graphene device over a large area provides an unprecedented combination of high performance and low cost for the future application of all carbon-based nanoelectronics.

Recent studies show that, at the initial stage of chemical vapor deposition (CVD) of graphene, the isolated carbon monomers will form defective carbon clusters with pentagons that degrade the quality of synthesized graphene. To circumvent this problem, we demonstrate that high-quality centimeter-sized graphene sheets can be synthesized on Cu foils by a self-assembled approach from defect-free polycyclic aromatic hydrocarbons (PAHs) in a high vacuum (HV) chamber without hydrogen. Different molecular motifs, namely coronene, pentacene, and rubrene, can lead to significant difference in the quality of resulting graphene. For coronene, monolayer graphene flakes with an adequate quality can be achieved at a growth temperature as low as 550 °C. For the graphene obtained at 1000 °C, transport measurements performed on back-gated field-effect transistors (FETs) with large channel lengths (∼30 μm) exhibit a carrier mobility up to ∼5300 cm2 V–1 s–1at room temperature. The underlying growth mechanism, which mainly involves surface-mediated nucleation process of dehydrogenated PAHs rather than segregation or precipitation process of small carbon species decomposed from the precursors, has been systematically investigated through the first-principles calculations. Our findings pave the way for optimizing selection of solid carbon precursors and open up a new route for graphene synthesis.Keywords: coronene; first-principles; graphene; growth; PAHs; pentacene; rubrene;

In this communication, we report a simple method for a covalent band-forming reaction to change the hybridization of carbon atoms from sp2 to sp3, to introduce a band gap in pristine graphene. Graphene with well-defined functionality was synthesized, and the resulting material was soluble in organic solvent. The LUMO of functionalized graphene was below the LUMO of poly(3-hexylthiophene) (P3HT) and close to that of [6,6]-phenyl C61-butyric acid methyl ester (PCBM), indicating its suitability as an electron acceptor for organic photovoltaic (OPV) applications. We further show the P3HT/functionalized graphene composite as the active layer in a solar cell, with device efficiency of 1.1%.

The electronic properties of graphene supported by the (0001) SiO2 surface are theoretically studied using density functional theory. It is found that the electronic attributes of graphene on (0001) SiO2 strongly depend on the underlying SiO2 surface properties and the percentage of hydrogen passivation. By applying methyl to passivate the oxygen-terminated (0001) SiO2 surface, one can further reduce the interaction between the graphene sheet and oxygen-terminated surface. This can improve the charge carrier mobility of graphene supported by SiO2 substrate and reduce the influence of residues of interfacial molecules. In addition, the external electric field modulates the charge transfer between graphene and the SiO2 surface when graphene layers are physisorbed on the oxide surface. This phenomenon will enhance the built-in electric field of bilayer graphene so as to effectively modify its band structure. Our results shed light on a better atomistic understanding of the recent experiments on graphene supported by SiO2.

We demonstrate the correlation between intermolecular coupling and electrical transport in VOPc molecular bilayers. The intermolecular electrical interaction is estimated by scanning tunneling spectroscopy. We find that the stronger coupling of LUMO–LUMO between VOPc molecular bilayers leads to thickness-independent LUMO positions and contact-dominating electron transport. On the other hand, intermolecular coupling of HOMO–HOMO is less; therefore, the measured HOMO positions are thickness-dependent, and transition from thermionic emission to Frenkel-Poole transport and to field emission can be observed with increasing thickness and applied bias. The findings show that the intermolecular coupling and electrical transport behavior can be revealed in situ at the same time, which is helpful in understanding the fundamental transport mechanism in organic semiconductors.

Single crystals of TIPS-TAP were grown from solution using poor solvents. With gluing silver-films as source and drain electrodes, the crystals exhibited field-effect mobility up to 1.77 cm2 V−1s−1, which is one of the highest values reported for solution-processed n-channel single crystal OFETs.

In this study, low-voltage copper phthalocyanine (CuPc)-based organic field-effect transistors (OFETs) are demonstrated utilizing solution-processed bilayer high-k metal-oxide (Al2Oy/TiOx) as gate dielectric. The high-k metal-oxide bilayer is fabricated at low temperatures (< 200 °C) by a simple spin-coating technology and can be controlled as thin as 45 nm. The bilayer system exhibits a low leakage current density of less than 10-5 A/cm2 under bias voltage of 2 V, a very smooth surface with RMS of about 0.22 nm and an equivalent k value of 13.3. The obtained low-voltage CuPc based OFETs show high electric performance with high hole mobility of 0.06 cm2/(V s), threshold voltage of −0.5 V, on/off ration of 2 × 103 and a very small subthreshold slope of 160 mV/dec when operated at −1.5 V. Our study demonstrates a simple and robust approach that could be used to achieve low-voltage operation with solution-processed technique.Keywords: Al2Oy/TiOx; CuPc; high-k; low-voltage; OFETs; solution-processed;

We have observed a rectifying behavior in a lateral symmetrical device structure based on rubrene single crystals. Our analysis shows that the rectifying characteristics are not due to formation of Schottky junction between the electrode and the organic semiconductor, but should be attributed to a mechanism that is similar to the super-linear operation regime in organic field-effect transistors. Furthermore, we have demonstrated that this rectifying behavior can be turned on and off via modulation of the density of space charge in the organic semiconductor/substrate interface, which essentially affects the threshold voltage (VT). Our results demonstrate a simple approach that can potentially be used to fabricate organic rectifiers without application of multi-layer device structure such as MOSFET diode.Graphical abstractHighlights► Rectifying behavior is observed in a lateral symmetrical device. ► The rectifying property is not due to formation of Schottky contact. ► The rectifying property should be ascribed a mechanism similar to super-linear operation regime in OFETs. ► The rectifying behavior can be turned on and off. ► This demonstrates a simple approach that can potentially be used in organic rectifiers.

In this paper, both n-type and p-type doped exfoliated graphene sheets are presented by virtue of adsorbing organic molecules. Flat organic layers are uniformly grown on graphene sheets by the technique. Meanwhile, the high-mobility attribute of graphene is largely preserved, as adsorption on the graphene surface does not significantly modify the π-bonding networks of graphene. By employing Kelvin probe force microscopy, we show that the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules obtain electrons from graphene whereas vanadyl−phthalocyanine (VOPc) molecules donate electrons to it. The amount of charge transfer by F4-TCNQ and VOPc to bilayer graphene on silicon dioxide substrate is estimated to be approximately 0.4 and 0.1 electron/molecule by a tight-binding self-consistent model, respectively. The consistent theoretical and experimental results demonstrate that doping of graphene by organic molecular charge transfer has great implications for future large-scale applications of graphene-based nanoelectronics.

We report on thermally evaporated chromium oxide (CrOx) as cathode interfacial layer to improve the efficiency and stability in air for the bulk heterojunction solar cells of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Devices with CrOx interfacial layers show higher power conversion efficiency (PCE) and stability than those without interfacial layer. Devices with CrOx show improved stability more than 100 times that of devices without interfacial layer or with LiF interfacial layer. We tentatively attributed the CrOx interfacial layer as an electronic tunneling layer for electron collection and a protective layer of Al assumably by minimizing the organic−Al interfacial areas caused by the evaporation of Al and blocking diffusion of oxygen and water.Keywords: bulk heterojunction; cathode interfacial layer; polythiophene; solar cells

The band gap opening of bilayer graphene with one side surface adsorption of F4-TCNQ is reported. F4-TCNQ doped bilayer graphene shows p-type semiconductor characteristics. With a F4-TCNQ concentration of 1.3 × 10−10 mol/cm2, the charge transfer between each F4-TCNQ molecule and graphene is 0.45e, and the built-in electric field, Ebi, between the graphene layers could reach 0.070 V/Å. The charge transfer and band gap opening of the F4-TCNQ-doped graphene can be further modulated by an externally applied electric field (Eext). At 0.077 V/Å, the gap opening at the Dirac point (K), ΔEK = 306 meV, and the band gap, Eg = 253 meV, are around 71% and 49% larger than those of the pristine bilayer under the same Eext.

A systematic investigation of the correlation between bonding geometries and electronic structures of mercapto-acetic acid molecule on the ZnO(101̅0) nonpolar surface is reported. The geometric structure calculation results are consistent with the recent Fourier transform infrared attenuated total reflectance (FT-IR-ATR) findings. The mercapto-acetic acid molecule can contribute an abundance of band gap states to ZnO. Monolayer functionalized ZnO(101̅0) is on the verge of a metal to insulator transition, which is consistent with the experimental findings of the conductivity increase by 6 orders of magnitude. The electrostatic net charge transfer from the molecule to ZnO is around 0.3 electrons for all configurations, but the electronic structure and adsorption energy of carboxylic molecules on ZnO(101̅0) show strong configuration dependence. This is also the magic of the organic molecule-oxide interface. The mercapto-acetic acid molecule functionalized ZnO also shows facet-dependent characteristics while the monolayer functionalized ZnO(21̅1̅0) does not show metal to insulator transition. Acetic acid does not contribute to the band gap states of ZnO(101̅0), whereas benzoic acid and 9-anthracenecarboxylic acid do contribute an abundance of band gap states to ZnO(101̅0). 9-Anthracenecarboxylic acid functionalized ZnO(101̅0) shows a smaller energy difference between the conduction band minimum (CBM) and highest occupied molecular orbital (HOMO), compared to mercapto-acetic acid under the same situation. Our findings are useful to understand the effect of surface functionaliztion on ZnO-based solar cells, biosensor applications, oxide surface nanofabrications, and molecular electronics.

The self-assembly of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on graphene with coverage in the range of 0.3−3 monolayers (MLs) is characterized by a DFT-based ab initio calculation method. For α modification mode, with a critical thickness of 1 ML, the growth of PTCDA on graphene follows the Stranski−Krastanov (SK) mode. For β modification mode, the PTCDA can form two complete MLs on a graphene substrate. From the thermodynamic viewpoint, α modification mode is more stable than β modification mode. At 1 ML, the PTCDA follows a planar configuration on graphene, which is also almost unperturbed by typical defects in the graphene sheet. The PTCDA adlayer remains its planar and continuous herringbone structure on graphene with three typical defects. For α modification mode with 2 or 3 ML coverage, the molecular planes incline to the substrate plane with angles around 9° and 13°, respectively, which indicates that a bulk-like phase appears. This also enhances the lateral intrinsic charge transport characteristics. For α modification mode, the total amount of charge transfer between PTCDA and graphene per 5√3 × 5 super cell at 2 MLs saturates with 0.42e, which is 0.1 and 0.06e larger than those of 1 and 3 MLs, respectively.

We report on the thickness-dependent morphology and surface potential of vanadyl phthalocyanine (VOPc) ultrathin films due to the variations between lying-down and tilted molecular packing configurations at the interface of VOPc film/highly ordered pyrolytic graphite (HOPG). Transition from a bilayer-thickness-dependent distribution to an orientation-dependent inhomogeneity of surface potential is observed in the same film. The surface potential change in the initial lying-down bilayers can be described by the abrupt junction model. In areas thicker than ca. 6 nm, a potential energy difference of about 100 meV between differently oriented VOPc molecular layers is found, indicating that the boundaries act as significant barriers for hole transport.

Transition-metal compound TiC60 thin films were grown by co-deposition from two separated sources of fullerene C60 powder and titanium. Study of structural properties of the films, by Raman spectroscopy, atomic force microscopy, and scanning tunneling spectroscopy reveals that the films have a deformed C60 structure with certain amount of sp3 bonds and a rough surface with a large number of nanoclusters. z–V tunnelling spectroscopic measurements suggest that several charge transport mechanisms are involved in as the tip penetrates into the thin film. Conventional field electron emission (FEE) measurements show a high emission current density of 10 mA/cm2 and a low turn-on field less than 8 V/μm, with the field enhancement factors being 659 and 1947 for low-field region and high-field region, respectively. By exploiting STM tunneling spectroscopy, local FEE on nanometer scale has also been characterized in comparison with the conventional FEE. The respective field enhancement factors are estimated to be 99–355 for a gap varying from 36 to 6 nm. The enhanced FEE of TiC60 thin films can be ascribed to structural variation of C60 in the films and the electrical conducting paths formed by titanium nanocrystallites embedded in C60 matrix.

A single CdS nanoribbon-based photoelectric detector was fabricated by the shadow mask technique and conventional lithography. Atomic force microscopy (AFM) and micro-Raman techniques were applied to acquire the morphology and structure of a single CdS nanoribbon. Transmission electron microscopy reveals a single crystalline interior with a few local defects. From the current–voltage (I–V) measurements, it is found that the maximum current reached 15 μA, and the photoconductivity variation could be as high as 25,000, as well as the corresponding current density is estimated to be about 7.0×105 A/cm2. Besides the ohmic characteristic of the I–V curve by photoelectric effect, the nonlinear I–V curve owing to the Schottky contact is also found. The transient photocurrent response indicates the slow process by carrier trapping.

By exploiting variable-temperature tapping mode atomic force microscopy (VT-AFM), along with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy, we have systematically investigated the nature of the bright clusters, which can be classified as protrusion and grown grain, in/on the moisture-exposed tris(8-hydroxyquinoline) aluminum (Alq3) thin film. The experimental results suggest that the protrusion is most likely to be a hydrated Alq3 species, whereas the grown grain is a crystalline Alq3 structure. Moreover, the evolution of a hydrated Alq3 species into a dark hole and/or a crystalline Alq3 structure was in-situ visualized.

The morphological evolution and nature of the degradation of the moisture-exposed tris(8-hydroxyquinoline) aluminum (Alq3) thin films have been microscopically investigated by variable-temperature tapping mode atomic force microscopy and complementarily by thickness-shear mode acoustic wave sensor. It is plausibly ascertained that the dark hole in the Alq3 films results from the release of the volatile 8-hydroxyquinoline, a product of the reaction of Alq3 with moisture.

MAPbI3 perovskite is an important component for high-performance perovskite solar cell (PSC) but its own thin film stability is challenging in PSC community. Herein, we report a high crystallinity perovskite MAPbI3 with texture structure prepared from HPbI3 reacted with low partial pressure (LPP) MA gas, that has substantially higher both thermal and moisture stability than polycrystalline perovskite (PP) prepared from MAI+PbI2. A prototype reactor is developed to perform coordination engineering between MA vapor and HPbI3 solid and facilitate the large-scale fabrication. The large Pb-N binding energy (~80.04 kJ mol−1) results in the liquefied state after MA adhesion. Finally, a high texture perovskite (TP) is formed after excess MA expeditious releasing. The MA-rich passivation through Pb-N bonding at interface and boundary contributes to the substantial improved stability. Besides, MA-rich species trigger an anti-degradation reaction in presence of moisture and thus endow stability above two months under ~65% humidity. The textured PSCs (TPSCs) deliver power conversion efficiency (PCE) between 15.5% and champion 18.9% in the batch deposition. Therefore, the coordination engineering improves the efficiency, stability, scalability and ease of fabrication together.A texture perovskite (TP) MAPbI3 thin film was developed through the reaction between diluted MA vapor and HPbI3 solid. The XRD intensity of the prepared perovskite without annealing generally can be as high as 106 to 107 which is rarely reported. The high crystallinity of TP is attributed to the excess MA strongly coordinating to Pb (II) and crosslinking adjacent crystals. This excess MA can passivate perovskite surface and grain boundary, leading to 2 months stability under 65% humidity and also substantially improved thermal stability. And the texture perovskite delivered champion PCE 18.9% in the batch production. Therefore, the excess MA coordination in the TP is beneficial for the crystallinity, PCE, stability and scalability together.

Graphene has demonstrated great potential in next-generation electronics due to its unique two-dimensional structure and properties including a zero-gap band structure, high electron mobility, and high electrical and thermal conductivity. The integration of atom-thick graphene into a device always involves its interaction with a supporting substrate by van der Waals forces and other intermolecular forces or even covalent bonding, and this is critical to its real applications. Graphene films on different surfaces are expected to exhibit significant differences in their properties, which lead to changes in their morphology, electronic structure, surface chemistry/physics, and surface/interface states. Therefore, a thorough understanding of the surface/interface properties is of great importance. In this review, we describe the major “graphene-on-surface” structures and examine the roles of their properties and related phenomena in governing the overall performance for specific applications including optoelectronics, surface catalysis, anti-friction and superlubricity, and coatings and composites. Finally, perspectives on the opportunities and challenges of graphene-on-surface systems are discussed.

Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

The viscoelasticity of boron nitride nanosheet (BNNS) aerogel has been observed and investigated. It is found that the BNNS aerogel has a high damping ratio (0.2), while it exhibits lightweight and negligible temperature dependence below 180 °C. The creep behavior of the BNNS aerogel markedly demonstrates its strain dependence on stress magnitude and temperature, and can be well simulated by the classical models.

Single crystals of TIPS-TAP were grown from solution using poor solvents. With gluing silver-films as source and drain electrodes, the crystals exhibited field-effect mobility up to 1.77 cm2 V−1s−1, which is one of the highest values reported for solution-processed n-channel single crystal OFETs.

Low-voltage, flexibility and low-cost are essential prerequisites for large scale application of organic thin film transistors (OTFTs) in future low-end electronics. Here, we demonstrate a low-voltage flexible OTFT by using a low-temperature, solution-processed gate dielectric. Such a dielectric can be well integrated with an Au coated polyimide film, and exhibits a low leakage current density of less than 10−6 A cm−2 and a high capacitance density of 180 nF cm−2. Pentacene films deposited onto the solution-processed dielectric show a highly ordered “thin film phase”. The source–drain (S/D) electrodes are made of in situ modified Cu encapsulated by Au (Au/M-Cu). The obtained flexible OTFT exhibits outstanding electrical characteristics under a gate voltage of only −2 V, which include an on/off ratio of 2 × 104, a mobility (μ) of 1.5 cm2 V−1 s−1, a threshold voltage (VT) of −0.3 V and a subthreshold slope (SS) of 161 mV dec−1. The obtained mobility value is among the highest achieved in flexible pentacene OTFTs. The mechanical flexibility and reliability of the OTFTs are also studied and discussed in detail, and the observed degradation of the device performance under strains is attributed to the damage induced in the electrodes giving rise to increased contact resistance and the phase transition from the thin film phase to bulk phase of the pentacene films.

We developed a simple and environmentally friendly spin-coating method for high-κ dielectrics (AlOx, ZrOx, YOx and TiOx). These materials were used as gate dielectrics for solution-processed nanocrystalline In2O3 or amorphous InZnO TFTs with a maximum processing temperature of 300 °C. The role of high-κ dielectrics in device performance was systematically studied. Among the high-κ dielectrics, the AlOx-based devices showed the best performance with mobilities of 21.7 cm2 V−1 s−1 in an In2O3 TFT and 11.6 cm2 V−1 s−1 in an InZnO TFT with the on/off current ratio exceeding 106. Furthermore, the devices exhibited ultra-low operating voltages (<3 V) and negligible hysteresis. A comprehensive study suggests that the high performance of the AlOx-based devices could be attributed to the smooth dielectric/semiconductor interface and the low interface trap density besides its good insulating properties. Therefore, the solution-processed AlOx can be used as a promising high-κ dielectric for low cost, low voltage, high-performance oxide electronic devices.

Flexible polymer-based dielectric materials that are used to store dielectric energy have widely been used in modern electronics and electric power systems, due to their relatively high energy density, light weight, low cost, etc. However, owing to the growing global environmental issues and a rapid consumption of nonrenewable polymer resources, there exists a strong desire to fabricate flexible dielectric materials using biodegradable materials. Here, we report on flexible dielectric papers based on biodegradable cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) for dielectric energy storage. Highly ordered, homogeneous CNF/CNT papers have been fabricated using a facile vacuum-assisted self-assembly technique. The obtained paper possesses a high dielectric constant of 3198 at 1.0 kHz, thus leading to enhanced dielectric energy storage capability (0.81 ± 0.1 J cm−3), which is attributed to the presence of a low loading of CNTs (4.5 wt%). Moreover, the CNF/CNT papers are mechanically flexible and show improved mechanical strength. These findings enable feasible fabrication of high-performance flexible dielectric materials using ecofriendly materials.

We report a novel MoS2-based fluorescent biosensor for DNA detection via hybridization chain reactions (HCRs). As an emerging nanomaterial, MoS2 has excellent fluorescence quenching ability and distinct adsorption properties toward single- and double-stranded DNA. In the sensing method, MoS2 nanosheets are used to suppress the background signal and control the “on” and “off” of fluorescence emission of the detection system with and without the presence of the target DNA. In addition, the signal generation is amplified through the target-triggered HCRs between two hairpin probes. The employment of MoS2 and HCRs guarantees the high sensitivity of the detection strategy with a detection limit of 15 pM. The biosensor also exhibits very good selectivity over mismatched DNA sequences. The detection takes place in solutions and requires only one “mix-and-detect” step. The high sensitivity, selectivity, and operational simplicity demonstrate that MoS2 can be a promising nanomaterial for versatile biosensing.

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of D-X/G/H-D, Si-X/S/H-Si and D-X/S/H-D can be modulated effectively by tuning the pressure (interlayer spacing) and stacking arrangement. The electron effective mass (EEM) is proportional to the band gap. The band gap of confined silicene is more sensitive to pressure than that of confined graphene. Moreover, a heterogeneous interface structure would be beneficial for effectively regulating the band gap and carrier effective masses of confined graphene and silicene. Using a confinement technique and pressure, the integrity of the honeycomb structure of graphene and silicene will be preserved so that the small effective masses and high mobility of graphene and silicene will be retained during compression. The tunable band gap and high carrier mobility of the sandwich structures are promising for building high-performance nanodevices.