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Two-dimensional (2D) heterostructured or alloyed monolayers composed of transition metal dichalcogenides (TMDCs) have recently emerged as promising materials with great potential for atomically thin electronic applications. However, fabrication of such artificial TMDC heterostructures with a sharp interface and a large crystal size still remains a challenge because of the difficulty in controlling various growth parameters simultaneously during the growth process. Here, a facile synthetic protocol designed for the production of the lateral TMDC heterostructured and alloyed monolayers is presented. A chemical vapor deposition approach combined with solution-processed precursor deposition makes it possible to accurately control the sequential introduction time and the supersaturation levels of the vaporized precursors and thus reliably and exclusively produces selective and heterogeneous epitaxial growth of TMDC monolayer crystals. In addition, TMDC core/shell heterostructured (MoS2/alloy, alloy/WS2) or alloyed (Mo1-xWxS2) monolayers are also easily obtained with precisely controlled growth parameters, such as sulfur introduction timing and growth temperature. These results represent a significant step toward the development of various 2D materials with interesting properties.

Zinc (Zn) ion based capacitors (ZICs) have been considered to be a new emerging energy storage device due to their high energy density and long cycle life. However, it is still challenging to overcome the limitation of ZICs with the low power densities and poor rate performance because of their low ion diffusion ability at high current densities during the cycling. Hence, we propose a novel surface modification strategy for the development of the rough surface and high surface area of Zn electrodes as the anode material so as to improve the ion diffusion ability at the high current densities. The energy storage performance of the surface-tailored Zn electrodes is significantly improved with the high specific capacitance (353 F g−1 at the current density of 0.5 A g−1), excellent high-rate performance (62 F g−1 at the current density of 10.0 A g−1), and remarkable cycling stability of 99% after 2000 cycles. This improved electrochemical performance is discussed and explained in terms of surface roughness and surface area. Our fining suggests that the surface engineering is a powerful approach for the potential applications of ZICs.

Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal–semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.

Solution-processed quantum dot (QD) white light-emitting diodes (WLEDs) have received much attention as a viable light source in the next-generation large-area ambient lighting, flexible photonics and full-colour display backlighting technologies. Attributable to their solution processibility, tunable colour temperature, high quantum efficiency and high photostability, considerable research efforts have been spent on accumulating new insights into the materials and device architecture design for high-performance QD WLEDs. At present, prevalent research on WLEDs focuses on using QDs as a photoluminescence colour converter or using purely narrow linewidth QD LEDs to complement white colour spectra, which is not energy efficient as well as challenging to achieve Eye Comfort ambient colour coding. Herein, a quantum composite made from hybridising perovskite (CsPbBr3 and CsPb(Br1-xClx)3) and CuInS2 (CIS) QDs is proposed as a colour-encoded electroluminescence layer, which underpins good white colour temperature and bias stability for ambient lighting. Instead of using solely sharp emission QDs to match the white light spectra, broad light emission spectrum CIS QDs are used for colour temperature toning. In addition, the mixed-halide perovskite CsPb(Br1-xClx)3 QDs are successfully synthesised through a noninvasive halide ion exchange method associated with trimethylsilyl chloride additives, which provide a good colour purity in blue and green electroluminescence region. This rational-designed QD composites enable the as-prepared WLED electroluminescence spectra to match the ideal ambient light Commission International de l’Éclairage 1931 (CIE) colour coordinates with a mean value of 0.33, 0.34. Moreover, as-prepared WLEDs show a turn-on voltage of 4 V with negligibly small leakage current, good colour stability and electrical bias tolerance even under a broad range of driving voltages. Our results herald the advent of molecular level hybridisation of different quantum materials for high-performance electroluminescence white colour toning.

The development of highly conductive electrodes with robust mechanical durability and clear transmittance in the visible to IR spectral range is of great importance for future wearable/flexible electronic applications. In particular, low resistivity, robust flexibility, and wide spectral transparency have a significant impact on optoelectronic performance. Herein, we introduce a new class of covellite copper monosulfide (CuS) nanosheet films as a promising candidate for soft transparent conductive electrodes (TCEs). An atmospheric sulfur adsorption-corrosion phenomenon represents a key approach in our work for the achievement of wafer-scale CuS nanosheet films through systematic control of the neat Cu layer thickness ranging from 2 to 10 nm multilayers at room temperature. These nanosheet films provide outstanding conductivity (∼25 Ω sq-1) and high transparency (> 80%) in the visible to infrared region as well as distinct flexibility and long stability under air exposure, yielding a high figure-of-merit (∼60) that is comparable to that of conventional rigid metal oxide material-based TCEs. Our unique room temperature synthesis process delivers high quality CuS nanosheets on any arbitrary substrates in a short time (< 1 min) scale, thus guaranteeing the widespread use of highly producible and scalable device fabrication.

Electrochemically active redox mediators have been widely investigated in energy conversion/storage system to improve overall catalytic activities and energy storing ability by inducing favorable surface redox reactions. However, the enhancement of electrochemical activity from the utilization of redox mediators (RMs) is only confirmed through theoretical computation and laboratory-scale experiment. The use of RMs for practical, wearable, and flexible applications has been scarcely researched. Herein, for the first time, a wearable fiber-based flexible energy storage system (f-FESS) with hydroquinone (HQ) composites as a catalytically active RM is introduced to demonstrate its energy-storing roles. The as-prepared f-FESS-HQ shows the superior electrochemical performance, such as the improved energy storage ability (211.16 F L−1 and 29.3 mWh L−1) and long-term cyclability with a capacitance retention of 95.1% over 5000 cycles. Furthermore, the f-FESS-HQ can well maintain its original electrochemical properties under harsh mechanical stress (bending, knotting, and weaving conditions) as well as humid conditions in water and detergent solutions. Thus, the strategical use of electrochemically active RMs can provide the advanced solution for future wearable energy storage system.

To improve quantum dot solar cell performance, it is crucial to make efficient use of the available incident sunlight to ensure that the absorption is maximized. The ability of metal nanoparticles to concentrate incident sunlight via plasmon resonance can enhance the overall absorption of photovoltaic cells due to the strong confinement that results from near-field coupling or far-field scattering plasmonic effects. Therefore, to simultaneously and synergistically utilize both plasmonic effects, the placement of different plasmonic nanostructures at the appropriate locations in the device structure is also critical. Here, we introduce two different plasmonic nanoparticles, Au and Ag, to a colloidal PbS quantum dot heterojunction at the top and bottom interface of the electrodes for further improvement of the absorption in the visible and near-infrared spectral regions. The Ag nanoparticles exhibit strong scattering whereas the Au nanoparticles exhibit an intense optical effect in the wavelength region where the absorption of light of the PbS quantum dot is strongest. It is found that these dual-plasmon layers provide significantly improved short-circuit current and power conversion efficiency without any form of trade-off in terms of the fill factor and open-circuit voltage, which may result from the indirect contact between the plasmonic nanoparticles and colloidal quantum dot films.

We investigate the effect of applied gate and drain voltages on the charge transport properties in a zinc oxide (ZnO) nanowire field effect transistor (FET) through temperature- and voltage-dependent measurements. Since the FET based on nanowires is one of the fundamental building blocks in potential nanoelectronic applications, it is important to understand the transport properties relevant to the variation in electrically applied parameters for devices based on nanowires with a large surface-to-volume ratio. In this work, the threshold voltage shift due to a drain-induced barrier-lowering (DIBL) effect was observed using a Y-function method. From temperature-dependent current-voltage (I-V) analyses of the fabricated ZnO nanowire FET, it is found that space charge-limited conduction (SCLC) mechanism is dominant at low temperatures and low voltages, in particular, variable-range hopping dominates the conduction in the temperature regime from 4 to 100 K, whereas in the high-temperature regime (150&ndash, 300 K), the thermal activation transport is dominant, diminishing the SCLC effect. These results are discussed and explained in terms of the exponential distribution and applied voltage-induced variation in the charge trap states at the band edge.

Tactile sensors mimicking the human sense of touch have been studied and various technologies for the sensing of external stimuli have been suggested as well. Humans detect certain external stimuli and become aware of related sensations, such as roughness or smoothness. Among the various physical parameters, surface information is the most informative type of perception to impart these sensations onto an electromechanical system, such as an android robot or a smart phone. Here, an array sensor, which uses a sliding method for the precise perception of surface information, such as shapes and structures, is demonstrated. The suggested array sensor design with the excellent dynamic response of a piezoelectric material results in enhanced spatial resolutions with sliding motions and detects variable sliding speeds. A soft material was employed to the sensor to enhance the capability of shape distinction. Color mapping was applied to translate surface patterns into visual images. The reconfigured surface information had high accuracy compared to actual information. The demonstrated sliding speed, pattern detection, and shape detection capabilities as well as the higher spatial resolutions allow the sensor to be utilized as an artificial tactile sensor.

In this work, we develop a Ag@Al2O3@Ag plasmonic core–shell–satellite (PCSS) to achieve highly sensitive and reproducible surface-enhanced Raman spectroscopy (SERS) detection of probe molecules. To fabricate PCSS nanostructures, we employ a simple hierarchical dewetting process of Ag films coupled with an atomic layer deposition (ALD) method for the Al2O3 shell. Compared to bare Ag nanoparticles, several advantages of fabricating PCSS nanostructures are discovered, including high surface roughness, high density of nanogaps between Ag core and Ag satellites, and nanogaps between adjacent Ag satellites. Finite-difference time-domain (FDTD) simulations of the PCSS nanostructure confirm an enhancement in the electromagnetic field intensity (hotspots) in the nanogap between the Ag core and the satellite generated by the Al2O3 shell, due to the strong core–satellite plasmonic coupling. The as-prepared PCSS-based SERS substrate demonstrates an enhancement factor (EF) of 1.7 × 107 and relative standard deviation (RSD) of ~7%, endowing our SERS platform with highly sensitive and reproducible detection of R6G molecules. We think that this method provides a simple approach for the fabrication of PCSS by a solid-state technique and a basis for developing a highly SERS-active substrate for practical applications.

Organic small molecules have played a significant role in the field of organic light-emitting diodes (OLEDs) technology. Improving the stability of OLEDs continues to be a major task in organic electronics. To overcome this issue, it is essential to enhance the carrier injection into the emissive layer for better charge balance. Though OLEDs are highly desirable for the solution process, enabling simple and cost-effectively manufacturing to fabricate large-area devices, the development of hole-transporting materials (HTMs) still a considerable challenge. Herein the IMzN (1-(4-methoxyphenyl)-2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole) and IMzB (4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)benzoic acid) with excellent solution processability and stability suitable for efficient OLED devices using a simple and efficient microwave-assisted synthetic procedure. Thermogravimetric analysis of IMzN and IMzB suggested that the material be thermally stable up to 320–345 °C. No glass transition has occurred as confirmed by DSC indicating the amorphous nature of the materials, which is well supported by XRD analysis. Materials acquire the appropriate HOMO levels for favorable hole injection as well as superior hole mobility. As a result, both HTMs show better device performance than the commercially used TAPC. The IMzB as HTL shows remarkable current efficiency of 91.13 cd A-1 and EQE of 21.11%. This is the highest current efficiency reported using solution-processed green phosphorescent OLEDs. Devices with IMzB as HTL also exhibit relatively better stability under continuous stress.

Phototransistors that are based on a hybrid vertical heterojunction structure of two-dimensional (2D)/quantum dots (QDs) have recently attracted attention as a promising device architecture for enhancing the quantum efficiency of photodetectors. However, to optimize the device structure to allow for more efficient charge separation and transfer to the electrodes, a better understanding of the photophysical mechanisms that take place in these architectures is required. Here, we employ a novel concept involving the modulation of the built-in potential within the QD layers for creating a new hybrid MoS2/PbS QDs phototransistor with consecutive type II junctions. The effects of the built-in potential across the depletion region near the type II junction interface in the QD layers are found to improve the photoresponse as well as decrease the response times to 950 μs, which is the faster response time (by orders of magnitude) than that recorded for previously reported 2D/QD phototransistors. Also, by implementing an electric-field modulation of the MoS2 channel, our experimental results reveal that the detectivity can be as large as 1 × 1011 jones. This work demonstrates an important pathway toward designing hybrid phototransistors and mixed-dimensional van der Waals heterostructures.

Anisotropic nanowires are promising candidates for electronic thermal management due to their unique electrical and thermal properties. However, eco-friendly solution-processed nanomaterials with an elaborate morphology and microstructure for modulating thermal and charge transfer are still a considerable challenge. Herein, we present a simple but effective approach for synthesizing pseudo core-shell nanowires through quantum dot (QD)-like nanostructure coating (p-NW@QD) to generate exceptional electron-phonon transport properties. With the assistance of diphenyl ether as a coordination solvent, high crystallinity lead sulfide NWs can be fabricated with a large aspect ratio together with uniform QD coating. This p-NW@QD exhibits high electronic mobility (30.65 cm2/Vs) as well as a diameter independent low thermal conductivity (1.53 ± 1 W/m K). Direct charge/heat carrier flow measurements and computational simulations demonstrate that the unusual electrical and thermal transport phenomenon is strongly dependent on the fast charge transport through the QD shell, and a slow phonon migration across the Umklapp process dominated NW cores. These findings indicate a significant step toward colloidal synthesis nanostructures for future high-performance nanoelectronics and thermal energy devices.

We demonstrate silver nanowire -network-film (AgNWNF)-coated soft polydimethyl siloxane (PDMS) substrates with wrinkled surfaces for use as stretchable SERS sensors. The AgNWNF-coated stretchable substrates showed strong SERS effects compared to those of Ag film-deposited substrates, which were likely attributed to the electromagnetic field enhancement induced by the relatively high density of hot spots in the AgNWNF. To investigate the stability of the stretchable SERS sensors, we examined the adhesion strength of AgNWNFs on the ultraviolet-ozone (UVO)-treated PDMS substrates, which was much stronger than the adhesion strength of AgNWNFs on the PDMS without prior UVO treatment. In addition, the strain-dependent SERS activity of the AgNWNF-coated stretchable sensor showed the detection of 10−7 M Rhodamine 6G (R6G) molecules despite the decreased Raman intensity under strain increased to 100%, resulting from the surface plasmon resonance tuning by the modulation of the active gap distance between neighboring AgNWs through the application of strain. This was further supported by finite-difference time domain (FDTD) numerical evaluation.

High-performance cascaded-junction quantum dot solar cells (CJQDSCs) are fabricated from as-prepared highly monodispersed lead sulfide QDs. The cells have a high power conversion of 9.05% and a short-circuit current density of 32.51 mA cm-2. A reliable and effective stratagem for fabricating high-quality lead sulfide quantum dots (QD) is explored through a "monomer" concentration-controlled experiment. Robust QDSC performances with different band gaps are demonstrated from the as-proposed synthesis and processing stratagems. Various potential CJQDSCs can be envisioned from the band edge evolution of the QDs as a function of size and ligands reported here.

We demonstrate the charge transport characteristics of MoS2-based vertical heterojunction devices through the formation of interfacial strain. Atomically thin MoS2 bilayers were directly synthesized on a p-type Si substrate by using chemical vapor deposition to introduce an interfacial tensile strain in the vertical heterojunction diode structure, which was confirmed by Raman, X-ray and ultraviolet photoelectron spectroscopy techniques. The electrical and optoelectronic properties of the heterojunction devices with the as-grown MoS2 (A-MoS2) on p-Si were compared with those of transferred MoS2 (T-MoS2)/p-Si devices. To clearly understand the charge transport characteristics induced by the interfacial tensile strain, the Fowler–Nordheim (FN) analysis of the electrical properties of the diode devices was conducted with the corresponding energy band diagrams. All of the fabricated MoS2-based vertical diodes exhibited clearly rectifying behaviors, but the photoresponse properties of the A-MoS2-based and T-MoS2-based heterojunctions exhibited distinct differences. Interestingly, we found that the tunneling barrier heights of the A-MoS2-based heterojunction devices were relatively higher than those of the T-MoS2-based devices and were almost the same before and after illumination due to the interfacial tensile strain, whereas those of the T-MoS2-based devices were lowered after illumination. Our study will help further understand the charge transport properties of 2D material-based heterojunction devices in the presence of interfacial strain, ultimately enabling the design of electronic and optoelectronic devices with novel functionalities.

To achieve the further development of supercapacitors (SCs), which have intensively received attention as a next-generation energy storage system, the rational design of active electrode materials with electrochemically more favorable structure is one of the most important factors to improve the SC performance with high specific energy and power density. We propose and successfully grow copper sulfide (CuS) nanowires (NWs) as a chalcogenide-based electrode material directly on a Cu mesh current collector using the combination of a facile liquid-solid chemical oxidation process and an anion exchange reaction. We found that the as-prepared CuS NWs have well-arrayed structures with nanosized crystal grains, a high aspect ratio and density, as well as a good mechanical and electrical contact to the Cu mesh. The obtained CuS NW based electrodes, with additional binder- and conductive material-free, exhibit a much higher areal capacitance of 378.0 mF/cm2 and excellent cyclability of an approximately 90.2 percentage retention during 2000 charge/discharge cycles due to their unique structural, electrical, and electrochemical properties. Furthermore, for practical SC applications, an asymmetric supercapacitor is fabricated using active carbon as an anode and CuS NWs as a cathode, and exhibits the good capacitance retention of 91% during 2000 charge/discharge processes and the excellent volumetric energy density of 1.11 mW h/cm3 compared to other reported pseudo-capacitive SCs., 18 pages, 4 figures

We investigate the current-dependent negative differential resistance (NDR) behavior in a single-crystalline vanadium dioxide (VO2) nanobeam without epitaxial interfaces. VO2 nanobeams are grown out of the basal r plane of sapphire and then mechanically transferred onto an oxidized silicon substrate using a stamping transfer technique. Interestingly, compared to a previous report on an epitaxially grown VO2 nanobeam with a strong adhesion strain at interfaces, the transferred VO2 nanobeam without epitaxial interfaces exhibits distinctly different NDR phenomena. We observe three different NDR behaviors along with the appearance of an intermediate M2 phase in the current–voltage (I–V) characteristics of the transferred VO2 nanobeam device obtained using the current-sweeping mode. This result is well supported by temperature-dependent Raman spectra and corresponding optical microscope images showing the appearance of an intermediate M2 phase and the spatial distribution of insulating and metallic domains along the length of the transferred nanobeam without epitaxial interfaces, which is likely attributed to different surface stresses, including interfacial stress.

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport.

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride-trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect.

Polyvinylidene fluoride (PVDF) has great potential for its use as an energy harvesting material as it exhibits not only outstanding piezoelectric and electrostatic characteristics resulting from ferroelectric effects, but also remarkably robust stability against repeated mechanical stress compared to inorganic materials. We report enhanced performances of poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) based energy generators with wider range of selections of flexible substrates through a surface morphology engineering using solvent annealing method as the key technology for simple and cost-effective fabrication at room temperature. It is clearly revealed that a solvent annealed P(VDF-TrFE) film is crystallised at room temperature and that the surface morphology is changed from a rough surface into a smooth and flat surface with increasing annealing time. This surface morphology engineering results in 8 times enhanced output voltage and current of the energy generators because of well-aligned electrical dipoles. We also demonstrate a highly transparent and flexible energy generator by employing graphene electrodes with the solvent annealed P(VDF-TrFE) film, which can be effectively harvesting various mechanical energy sources.

Direct growth of graphene on silicon (Si) through chemical vapor deposition has predominantly focused on surface-mediated processes due to the low carbon (C) solubility in Si. However, a considerable quantity of C atoms was incorporated in Si and formed Si1−xCx alloy with a reduced lattice dimension even in the initial stage of direct graphene growth. Subsequent high temperature annealing promoted active C out-diffusion, resulting in the formation of a graphitic layer on the Si surface. Furthermore, the significantly low thermal conductivity of the Si1−xCx alloy shows that the incorporated C atoms affect the properties of a semiconductor adjacent to the graphene. These findings provide a key guideline for controlling desirable properties of graphene and designing hybrid semiconductor/graphene architectures for various applications.

Wearable energy harvesting devices are of increasing importance for the realization of flexible smart electronics, as the basic building blocks of power sources are able to convert the mechanical energy induced from body movement to electricity. Here, we report the electrical responses of a textile substrate-based triboelectric nanogenerator (T-TENG), including nanostructured surface configurations provided by Al nanoparticles and PDMS, in which no intricate fabrication process was performed or required. Along with analysis of the working principle and finite element simulation, the textile-based output power density of 33.6 mW/cm2 was obtained from the periodic mechanical stress of an adjustable bending machine, and this was used to activate commercially available light emitting diode (LED) bulbs. Additionally, we demonstrated the generation of sufficient energy from clothes attached to a commercial arm sleeve to power the LED devices from the activity of a human arm. Our practical cloth approach may contribute to a general framework for developing functional and self-powered wearable electronic devices.

We report a facile method for preparing hybrid nanostructured films composed of poly(3-hexlythiophene) (P3HT) and [6,6]-phenylC61-butyric acid methyl ester (PCBM) with silver (Ag) nanorods (AgNRs) array. The AgNRs were synthesized by an electrochemical deposition method using an anodic aluminum oxide template with 50-nm-pore-diameter and 10-μm-thickness. The nanostructured P3HT/PCBM film was formed by the intercalation of AgNRs into the P3HT/PCBM film. The nanostructured P3HT/PCBM film with AgNRs showed enhanced optical absorption in the spectral range of 300–650 nm due to localized surface plasmon resonance and scattering effects around the AgNRs compared with spin-coated and nanopatterned P3HT/PCBM films without AgNRs. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0021504ssl] All rights reserved.

We demonstrate the interplay between surface recombination dynamics and environmental temperature in UV photoresponse of ZnO nanowire field effect transistors (FETs) with and without a passivation layer. We find that optoelectronic performance and photoresponse mechanism can be significantly altered by temperature, influencing surface chemical interaction and reaction associated with charge trapping. Particularly, regardless of surface passivation on nanowires, photocurrent gains and decay rate are drastically enhanced with increasing temperature. Furthermore, the temperature dependence of photoresponse behavior in nanowire FETs is discussed in terms of surface recombination rates, carrier concentration, surface potential barrier height, and charge trapping rates on the surface.

Vertically stacked atomic layers from different layered crystals can be held together by van der Waals forces, which can be used for building novel heterostructures, offering a platform for developing a new generation of atomically thin, transparent and flexible devices. The performance of these devices is critically dependent on the layer thickness and the interlayer electronic coupling, influencing the hybridisation of the electronic states as well as charge and energy transfer between the layers. The electronic coupling is affected by the relative orientation of the layers as well as by the cleanliness of their interfaces. Here, we demonstrate an efficient method for monitoring interlayer coupling in heterostructures made from transition metal dichalcogenides using photoluminescence imaging in a bright-field optical microscope. The colour and brightness in such images are used here to identify mono- and few-layer crystals, and to track changes in the interlayer coupling and the emergence of interlayer excitons after thermal annealing in mechanically exfoliated flakes as well as a function of the twist angle in atomic layers grown by chemical vapour deposition. Material and crystal thickness sensitivity of the presented imaging technique makes it a powerful tool for characterisation of van der Waals heterostructures assembled by a wide variety of methods, using combinations of materials obtained through mechanical or chemical exfoliation and crystal growth., 24 pages, 5 figures

Recently, artificial tactile sensors have been studied in attempts to mimic the human sense of touch for various applications, from the simple input function of mobile devices to the complicated finger systems of android robots. To produce artificial psychological feeling, especially ‘pain’, electrical mimic structure of human skin has been studied. Array type tactile sensor can verify the input object which may be sharp, blunt, and hot or cool by simple signal processing. This capability means that tactile sensor can convey psychological feeling from physical parameters. Imitating this phenomenon, the skin mimic device employing the piezoelectric nanowire sensor arrays can generate the electrical ‘pain’ signal with signal processing when a sharp or a hot object touch to the device. The electrical ‘pain’ signal is expected to enhance the protection mechanism of android robot or mobile phone from harsh environment.

Visible emission colloidal quantum dots (QDs) have shown promise in optical and optoelectronic applications. These QDs are typically composed of relatively expensive elements in the form of indium, cadmium, and gallium since alternative candidate materials exhibiting similar properties are yet to be realized. Herein, for the first time, we report red green blue (RGB) photoluminescences with quantum yields of 18% from earth-abundant lead sulfide (PbS) QDs. The visible emissive property is mainly attributed to a high degree of crystallinity even for the extremely small QD sizes (1–3 nm), which is realized by employing a heterogeneous reaction methodology at high growth temperatures (>170 °C). We demonstrate that the proposed heterogeneous synthetic method can be extended to the synthesis of other metal chalcogenide QDs, such as zinc sulfide and zinc selenide, which are promising for future industrial applications. More importantly, benefiting from the enlarged band gaps, the as-prepared PbS solar cells show an impressive open circuit voltage (∼0.8 V) beyond that reported to date.

Pseudo-capacitive transition metal chalcogenides have recently received considerable attention as a promising class of materials for high performance supercapacitors (SCs) due to their superior intrinsic conductivity to circumvent the limitations of corresponding transition metal oxides with relatively poor conductivity. However, the important challenge associated with the utilization of such high-capacitive electrode materials is the development of desirably structured electrode materials, enabling efficient and rapid Faradaic redox reactions and ultra long-term cycling. Here, we propose a hierarchically integrated hybrid transition metal (Cu-Ni) chalcogenide shell-core-shell (HTMC-SCS) tubular heterostructure using a facile bottom-up synthetic approach. The resultant HTMC-SCS electrode exhibits a high volumetric capacitance of 25.9 F cm−3 at a current density of 2 mA cm−2. Furthermore, asymmetric SCs based on an HTMC-SCS heterostructured electrode demonstrate a high power density (770 mW cm−3) and an energy density (2.63 mW h cm−3) as well as an ultrahigh reversible capacity with a capacitance retention of 84% and a long-term cycling stability of over 10,000 cycles. Based on experimental results and density functional theory calculations, these remarkably improved electrochemical features are discussed and explained in terms of the unique combination of the conductive CuS core and active NiS shell materials, hierarchical tubular open geometry with nanoscale inner/outer shell structure, and mechanical stress-mitigating interlayer on shell-core-shell interface, allowing highly reversible and efficient electrochemical redox processes coupled with fast charge transfer kinetics and an electrochemically stable structure.

Diverse functionalities of liquid crystals (LCs) offer enormous opportunities for their potential use in advanced mobile and smart displays, as well as novel non-display applications. Here, we present snapshots of the research carried out on emerging applications of LCs ranging from electronics to holography and self-powered systems. In addition, we will show our recent results focused on the development of new LC applications, such as programmable transistors, a transparent and active-type two-dimensional optical array and self-powered display systems based on LCs, and will briefly discuss their novel concepts and basic operating principles. Our research will give insights not only into comprehensively understanding technical and scientific applications of LCs, but also developing new discoveries of other LC-based devices. © 2014 by the authors.

Two-dimensional (2D) semiconductors offer great potential in nanoelectronics due to their unique electrical and optical properties mainly attributed to their atomically thin nature. To make use of 2D semiconductors for practical applications, it is highly desired to develop scalable methods for integration of complementary circuits rather than a discrete device. We report a simple, scalable method for fabricating complementary inverters based on n-type monolayer MoS2, grown by a chemical vapor deposition (CVD), and inkjet-printed p-type SWCNTs. The CVD and inkjet printing methods are effectively combined to show the feasibility in terms of the integration of low-dimensional materials for complementary circuits. In the complementary circuits, cost-effectively printed Ag is utilized as the source and drain electrodes for both MoS2 and SWCNT transistors. Both the n- A nd p-type transistors show decent device characteristics at low operating voltages under ambient conditions. As a result, the complementary inverter composed of MoS2 and SWCNT transistors exhibits excellent performance with a gain over 10, low power dissipation, high noise margins, and switching threshold of 1/2 V DD at a low operating voltage. This successful demonstration of the complementary inverter, the most basic building block in digital circuits, will lead promising 2D semiconductors to practical applications.

Colloidal quantum dots (CQDs) are extremely promising as photovoltaic materials. In particular, the tunability of their electronic band gap and cost effective synthetic procedures allow for the versatile fabrication of solar energy harvesting cells, resulting in optimal device performance. However, one of the main challenges in developing high performance quantum dot solar cells (QDSCs) is the improvement of the photo-generated charge transport and collection, which is mainly hindered by imperfect surface functionalization, such as the presence of surface electronic trap sites and the initial bulky surface ligands. Therefore, for these reasons, finding effective methods to efficiently decorate the surface of the as-prepared CQDs with new short molecular length chemical structures so as to enhance the performance of QDSCs is highly desirable. Here, we suggest employing hybrid halide ions along with the shortest heterocyclic molecule as a robust passivation structure to eliminate surface trap sites while decreasing the charge trapping dynamics and increasing the charge extraction efficiency in CQD active layers. This hybrid ligand treatment shows a better coordination with Pb atoms within the crystal, resulting in low trap sites and a near perfect removal of the pristine initial bulky ligands, thereby achieving better conductivity and film structure. Compared to halide ion-only treated cells, solar cells fabricated through this hybrid passivation method show an increase in the power conversion efficiency from 5.3% for the halide ion-treated cells to 6.8% for the hybrid-treated solar cells.

Structural design factors of sensor units have been studied in order to enhance the sensitivity of pressure sensors based on utilizing a piezoelectric material for an artificial tactile sensor. In this study, we have primarily demonstrated the effect of a square pattern array design in a pressure sensor using ZnO nanowires. Nanowires grown on the edge of cells can be bent easily because of growth direction, density control, and buckling effect. Since smaller square pattern arrays induce a higher circumference to cell area ratio, if one sensor unit consists of many micro-level square pattern arrays, the design enhances the piezoelectric efficiency and the sensitivity. As a result, 20 $\mu {\mathrm { m}}\times 20\,\,\mu {\mathrm { m}}$ cell arrays showed three times higher pressure sensitivity than $250~\mu {\mathrm { m}}\times 250\,\,\mu {\mathrm { m}}$ cell array structures at a pressure range from 4 kPa to 14 kPa. The induced piezoelectric voltage with the same pressure level also increased drastically. Therefore, the square pattern array design is more appropriate for a high-sensitive pressure sensor than a simple one-body cell design for tactile systems, and it has the advantage of better power efficiency, which is also important for artificial tactile systems. This suggested cell array design can be applied to various systems using piezoelectric nanowires.

We investigate time-dependent inorganic ligand exchanging effect and photovoltaic performance of lead sulfide (PbS) nanocrystal films. With optimal processing time, volume shrinkage induced by residual oleic acid of the PbS colloidal quantum dot (CQD) was minimized and a crack-free film was obtained with improved flatness. Furthermore, sufficient surface passivation significantly increased the packing density by replacing from long oleic acid to a short iodide molecule. It thus facilities exciton dissociation via enhanced charge carrier transport in PbS CQD films, resulting in the improved power conversion efficiency from 3.39% to 6.62%. We also found that excess iodine ions on the PbS surface rather hinder high photovoltaic performance of the CQD solar cell.

We have shown the enhanced piezoelectricity by electrostatic effects on a textile based platform. The electrostatic and piezoelectric effects were hybridized by integrating piezoelectric ZnO nanowires and a charged dielectric film on a wearable textile substrate. The hybrid textile nanogenerator produced an output voltage of 8 V and an output current of 2.5 μA. Using a simple AC-DC converter circuit, we operated the green organic light-emitting diode and a liquid crystal display panel using a 100 dB sonic wave. © 2012 The Royal Society of Chemistry.

We investigate strain-induced optical modulation in a ZnO microwire with wavy geometries induced by mechanical strains. Curved sections of the wavy ZnO microwire show red-/blue-shifts of near-band-edge emission and broadening of full width at half maximum in cathodoluminescence spectra along the length of the wavy ZnO microwire, compared with straight sections. The observed variations indicate that local strains in the wavy ZnO microwire lead to strain-dependent local changes of its energy band structure. The local bending curvature calculations using a geometric model also provide correlation between the shift of the near-band-edge emission peaks and the bending strain.

We carried out temperature-dependent photoluminescence measurements to investigate the influence of surface roughness on the phonon-assisted emission of the surface-tailored ZnO nanowires (NWs). For the rough ZnO NWs, the observation of strong defect emission with vibration peaks by the exciton-phonon coupling reflects the presence of a high density of surface defects, resulting in a rapid shift to lower energy region of free exciton emission and a strong contribution of the first order phonon-assisted free exciton in ultraviolet emission. This investigation indicates that the surface defects associated with roughness have a significant influence on the phonon-assisted exciton emission. © 2009 American Institute of Physics.

We have investigated the Ag (1 nm)indium tin oxide (ITO) (200 nm) contacts by scanning transmission electron microscopy (STEM), Auger electron spectroscopy (AES), and X-ray photoemission spectroscopy (XPS) to understand its ohmic mechanism. The Ag/ITO contacts exhibit ohmic behaviors, when annealed at 400-600°C. The effective Schottky barrier heights depend on the annealing temperatures. STEM and AES results reveal the formation of Ag nanodots (5-35 nm across) and Ga-Ag solid solution. Based on the STEM, AES, and XPS results, the ohmic contact formation is described in terms of the formation of the Ga-Ag solid solution and the inhomogeneous interfaces with nanodots. © 2007 The Electrochemical Society.

We demonstrate modulations of electrical conductance and hysteresis behavior in ZnO nanowire transistors via electrically polarized switching of ferroelectric liquid crystal (FLC). After coating a nanowire channel in the transistors with FLCs, we observed large increases in channel conductance and hysteresis width, and a strong dependence of hysteresis loops on the polarization states associated with the orientation of electric dipole moments along the direction of the gate electric field. Furthermore, the reversible switching and retention characteristics provide the feasibility of creating a hybrid system with switch and memory functions. © 2013 American Institute of Physics.

For biomedical implanted devices, a wireless power or a signal transmission is essential to protect an infection and to enhance durability. In this study, we present a magnetic induction technique for a power transmission without any wire connection between transmitter (Tx) and receiver (Rx) in a micro scale. Due to a micro size effect of a flat spiral coil, a magnetic inductance is not high. To enhance the magnetic inductance, a three dimensional magnetic core is added to an antenna structure, which is consisted of ZnO nano wires coated by a nickel (Ni) layer. ZnO nano wires easily supply a large effective surface area with a vertical structural effect to the magnetic core structure, which induces a higher magnetic inductance with a ferro-magnetic material Ni. The magnetic induction antenna with the magnetic core shows a high inductance value, a low reflection power and a strong power transmission. The power transmission efficiencies are tested under the air and the water medium are almost the same values, so that the magnetic induction technique is quite proper to body implanted systems.

Ultra-fast diode structures based on non-semiconductive materials employing tunneling mechanism have been investigated. Applying the structurally asymmetric effect of multi-wall carbon nanotube (MWCNT) to a vertical metal-insulator-MWCNT (MIC) tunneling diode structure, the ‘on-off’ ratio (∼10^4) and the current density (38.2 MA/cm^2) are drastically enhanced compared to those of conventional metal-insulator-metal (MIM) tunneling diodes. The electrical characteristics are stable up to 423 K. Experimentally, rectifying performance of the MIC diode is good up to 10 MHz and the cut-off frequency of the MIC diode is estimated to be 6.47 THz. The growth process of MWCNT is more controllable for the number and the position than that of SWCNT. Therefore, it has a high probability of realization. The vertically aligned single MWCNT design can guarantee an ultra-high integration density, as well. Therefore, the MIC diode can be applied to various high frequency applications, such as communication devices, high speed electrical switches, and high performance control process units (CPUs), or other new concept devices.

Enhancing the output power of a nanogenerator is essential in applications as a sustainable power source for wireless sensors and microelectronics. We report here a novel approach that greatly enhances piezoelectric power generation by introducing a p-type polymer layer on a piezoelectric semiconducting thin film. Holes at the film surface greatly reduce the piezoelectric potential screening effect caused by free electrons in a piezoelectric semiconducting material. Furthermore, additional carriers from a conducting polymer and a shift in the Fermi level help in increasing the power output. Poly(3-hexylthiophene) (P3HT) was used as a p-type polymer on piezoelectric semiconducting zinc oxide (ZnO) thin film, and phenyl-C(61)-butyric acid methyl ester (PCBM) was added to P3HT to improve carrier transport. The ZnO/P3HT:PCBM-assembled piezoelectric power generator demonstrated 18-fold enhancement in the output voltage and tripled the current, relative to a power generator with ZnO only at a strain of 0.068%. The overall output power density exceeded 0.88 W/cm(3), and the average power conversion efficiency was up to 18%. This high power generation enabled red, green, and blue light-emitting diodes to turn on after only tens of times bending the generator. This approach offers a breakthrough in realizing a high-performance flexible piezoelectric energy harvester for self-powered electronics.

We demonstrate the fabrication of solution processed highly crystalline p-type PbS nanowires via the oriented attachment of nanoparticles. The analysis of single nanowire field effect transistor (FET) devices revealed a hole conduction behaviour with average mobilities greater than 30 cm2 V−1 s−1, which is an order of magnitude higher than that reported to date for p-type PbS colloidal nanowires. We have investigated the response of the FETs to near-infrared light excitation and show herein that the nanowires exhibited gate-dependent photo-conductivities, enabling us to tune the device performances. The responsivity was found to be greater than 104 A W−1 together with a detectivity of 1013 Jones, which benefits from a photogating effect occurring at negative gate voltages. These encouraging detection parameters are accompanied by relatively short switching times of 15 ms at positive gate voltages, resulting from a combination of the standard photoconduction and the high crystallinity of the nanowires. Collectively, these results indicate that solution-processed PbS nanowires are promising nanomaterials for infrared photodetectors as well as p-type nanowire FETs.

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior. © 2008 American Chemical Society.

A wireless thin film transistor (TFT) structure in which a source/drain or a gate is connected directly to a micro antenna to receive or transmit signals or power can be an important building block, acting as an electrical switch, a rectifier or an amplifier, for various electronics as well as microelectronics, since it allows simple connection with other devices, unlike conventional wire connections. An amorphous indium gallium zinc oxide (α-IGZO) TFT with magnetic antenna structure was fabricated and studied for this purpose. To enhance the induction coupling efficiency while maintaining the same small antenna size, a magnetic core structure consisting of Ni and nanowires was formed under the antenna. With the micro-antenna connected to a source/drain or a gate of the TFT, working electrical signals were well controlled. The results demonstrated the device as an alternative solution to existing wire connections which cause a number of problems in various fields such as flexible/wearable devices, body implanted devices, micro/nano robots and sensors for the ‘internet of things’ (IoT).

Tactile sensors have been received with tremendous attention and developed rapidly for the last few decades. There have been various technologies and transduction methods to innovate the tactile sensor; however, the ability of tactile sensors has left much to be desired. For example, a resistive and a capacitive type sensor, most widely used techniques for touch screen technology and tactile sensor have a limit of detecting temperature. In this study, Zinc Oxide (ZnO) nanowire-based tactile sensors which can detect both the pressure and temperature is demonstrated when the sensor is under pressure or heat. Pressure or temperature sensitivity from dynamic tactile sensing is also discussed from the results.

We report on the formation of reflective and low resistance Ag-based contacts to p-GaN using Ir interlayers for GaN-based near UV flip-chip light emitting diodes (FCLEDs). The as-deposited Ir/Ag contacts give non-linear current–voltage ( I – V ) behaviors. However, the contacts become ohmic with specific contact resistance of ∼10 −4 Ω cm 2 when annealed at temperatures 330–530 °C for 1 min in air. The 530 °C-annealed Ir/Ag contacts give a reflectance of ∼75% at a wavelength of 405 nm, which is somewhat higher than that (∼71%) of annealed single Ag contacts. It is also shown that the output power of the LEDs made with the annealed Ir/Ag contact is higher than that with the annealed single Ag contact.