Direct Growth of Nanographene on Silicon with Thin Oxide Layer for High‐Performance Nanographene‐Oxide‐Silicon Diodes

Graphene‐silicon based configurations are attracting great attention for their potential application as electronics and optoelectronics. For their practical use, it is still limited by the configuration fabrication process. In this paper, a catalyst‐free method is reported to directly grow nanographene on silicon covered with a thin oxide layer to form nanographene‐oxide‐silicon configurations. Compared with previously reported nanographene‐silicon Schottky junctions, the nanographene‐oxide‐silicon structures exhibit a high performance on electronic and photovoltaic properties. The reverse leakage current of the nanographene‐oxide‐silicon is suppressed from over 10^?5 A down to 10^?8 A and the rectifier ratio is greatly enhanced from less than 5 up to 10³. The photovoltage is enhanced over 50 times. The nanographene‐oxide‐silicon structures exhibit especially ultrasensitive to weak light at a photovoltage working mode, which exceeds up to 10⁶ V/W at the light power of 0.025 μW. Due to the source material for nanographene is photoresist and the fabrication process is mainly based on the current‐used photolithography and silicon technique, the developed nanographene‐oxide‐silicon structures are very easy for device fabrication, integration, and miniaturization, and could be a promising way to produce metal‐free graphene‐silicon based electronics and optoelectronics for commercial use.     

Novel Cu Nanowires/Graphene as the Back Contact for CdTe Solar Cells

Cu‐nanowire‐doped graphene (Cu NWs/graphene) is successfully incorporated as the back contact in thin‐film CdTe solar cells. 1D, single‐crystal Cu nanowires (NWs) are prepared by a hydrothermal method at 160 °C and 3D, highly crystalline graphene is obtained by ambient‐pressure CVD at 1000 °C. The Cu NWs/graphene back contact is obtained from fully mixing the Cu nanowires and graphene with poly(vinylidene fluoride) (PVDF) and N‐methyl pyrrolidinone (NMP), and then annealing at 185 °C for solidification. The back contact possesses a high electrical conductivity of 16.7 S cm^?1 and a carrier mobility of 16.2 cm² V^?1 s^?1. The efficiency of solar cells with Cu NWs/graphene achieved is up to 12.1%, higher than that of cells with traditional back contacts using Cu‐particle‐doped graphite (10.5%) or Cu thin films (9.1%). This indicates that the Cu NWs/graphene back contact improves the hole collection ability of CdTe cells due to the percolating network, with the super‐high aspect ratio of the Cu nanowires offering enormous electrical transport routes to connect the individual graphene sheets. The cells with Cu NWs/graphene also exhibit an excellent thermal stability, because they can supply an active Cu diffusion source to form an stable intermediate layer of CuTe between the CdTe layer and the back contact.     

Metal‐Free Growth of Nanographene on Silicon Oxides for Transparent Conducting Applications

Conventional methods to prepare large‐area graphene for transparent conducting electrodes involve the wet etching of the metal catalyst and the transfer of the graphene film, which can degrade the film through the creation of wrinkles, cracks, or tears. The resulting films may also be obscured by residual metal impurities and polymer contaminants. Here, it is shown that direct growth of large‐area flat nanographene films on silica can be achieved at low temperature (400 °C) by chemical vapor deposition without the use of metal catalysts. Raman spectroscopy and TEM confirm the formation of a hexagonal atomic network of sp²‐bonded carbon with a domain size of about 3–5 nm. Further spectroscopic analysis reveals the formation of SiC between the nanographene and SiO₂, indicating that SiC acts as a catalyst. The optical transmittance of the graphene films is comparable with transferred CVD graphene grown on Cu foils. Despite the fact that the electrical conductivity is an order of magnitude lower than CVD graphene grown on metals, the sheet resistance remains 1–2 orders of magnitude better than well‐reduced graphene oxides.     

Electrical characteristics of TiB2 for ULSI applications

The work function of TiB₂ was measured using Fowler-Nordheim tunneling in MOS capacitors, Schottky diode current measurements, capacitance-voltage techniques, and contact resistance. The resulting data place the Fermi level of TiB₂ about 0.9 eV below the silicon conduction band. Given this barrier height, Schottky diodes of TiB₂/p-Si exhibit ohmic characteristics, but the contact resistance of TiB₂ to n⁺ junctions is an order of magnitude higher than the generally desired value. Boron outdiffusion from TiB₂ into underlying silicon was observed at temperatures of 1000°C and greater. Boron diffusion from TiB₂ into silicon above 1000°C is enhanced compared to the conventionally accepted value of the boron diffusivity     

Current transport studies of amorphous n/p junctions and its application in a‐Si:H/HIT‐type tandem cells

This paper presents an understanding of the fundamental carrier transport mechanism in hydrogenated amorphous silicon (a‐Si:H)‐based n/p junctions. These n/p junctions are, then, used as tunneling and recombination junctions (TRJ) in tandem solar cells, which were constructed by stacking the a‐Si:H‐based solar cell on the heterojunction with intrinsic thin layer (HIT) cell. First, the effect of activation energy (E_a) and Urbach parameter (E_u) of n‐type hydrogenated amorphous silicon (a‐Si:H(n)) on current transport in an a‐Si:H‐based n/p TRJ has been investigated. The photoluminescence spectra and temperature‐dependent current–voltage characteristics in dark condition indicates that the tunneling is the dominant carrier transport mechanism in our a‐Si:H‐based n/p‐type TRJ. The fabrication of a tandem cell structure consists of an a‐Si:H‐based top cell and an HIT‐type bottom cell with the a‐Si:H‐based n/p junction developed as a TRJ in between. The development of a‐Si:H‐based n/p junction as a TRJ leads to an improved a‐Si:H/HIT‐type tandem cell with a better open circuit voltage (V_oc), fill factor (FF), and efficiency. The improvements in the cell performance was attributed to the wider band‐tail states in the a‐Si:H(n) layer that helps to an enhanced tunneling and recombination process in the TRJ. The best photovoltage parameters of the tandem cell were found to be V_oc = 1430 mV, short circuit current density = 10.51 mA/cm², FF = 0.65, and efficiency = 9.75%. Copyright © 2015 John Wiley & Sons, Ltd.     

Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density

Hierarchical flowerlike nickel hydroxide decorated on graphene sheets has been prepared by a facile and cost‐effective microwave‐assisted method. In order to achieve high energy and power densities, a high‐voltage asymmetric supercapacitor is successfully fabricated using Ni(OH)₂/graphene and porous graphene as the positive and negative electrodes, respectively. Because of their unique structure, both of these materials exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high‐voltage region of 0–1.6 V and displays intriguing performances with a maximum specific capacitance of 218.4 F g^?1 and high energy density of 77.8 Wh kg^?1. Furthermore, the Ni(OH)₂/graphene//porous graphene supercapacitor device exhibits an excellent long cycle life along with 94.3% specific capacitance retained after 3000 cycles. These fascinating performances can be attributed to the high capacitance and the positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.     

Janus‐Structured Co‐Ti3C2 MXene Quantum Dots as a Schottky Catalyst for High‐Performance Photoelectrochemical Water Oxidation

MXene materials have attracted increasing attention in electrochemical energy‐storage applications while MXene also becomes photo‐active at the quantum dot scale, making it an alternative for solar‐energy‐conversion devices. A Janus‐structured cobalt‐nanoparticle‐coupled Ti₃C₂ MXene quantum dot (Co‐MQD) Schottky catalyst with tunable cobalt‐loading content serving as a photoelectrochemical water oxidation photoanode is demonstrated. The introduction of cobalt triggers concomitant surface‐plasmon effects and acts as a water oxidation center, enabling visible‐light harvesting capability and improving surface reaction kinetics. Most importantly, due to the rectifying effects of Co‐MQD Schottky junctions, photogenerated carrier separation/injection efficiency can be fundamentally facilitated. Specifically, Co‐MQD‐48 exhibits both superior photoelectrocatalysis (2.99 mA cm^?2 at 1.23 V vs RHE) and charge migration performance (87.56%), which corresponds to 194% and 236% improvement compared with MQD. Furthermore, excellent photostability can be achieved with less than 6.6% loss for 10 h cycling reaction. This fills in gaps in MXene material research in photoelectrocatalysis and allows for the extension of MXene into optical‐related fields.     

Improved Electrical Contact Properties of MoS2‐Graphene Lateral Heterostructure

2D materials have been extensively investigated in view of their excellent electrical/optical properties, with particular attention directed at the fabrication of vertical or lateral heterostructures. Although such heterostructures exhibit unexpected or enhanced properties compared to those of singly used 2D materials, their fabrication is challenged by the difficulty of realizing spatial control and large area integration. Herein, MoS₂ is grown on patterned graphene at variable temperatures, combining the concept of lateral heterostructure with chemical vapor deposition to realize large area growth with precise spatial control, and probe the spatial distribution of graphene and MoS₂ by a number of instrumental techniques. The prepared MoS₂‐graphene lateral heterostructure is employed to construct field effect transistors with graphene as the source/drain and MoS₂ as the channel, and the performance of these transistors (on/off ratio ≈10⁹, maximum field effect mobility = 8.5 cm² V^?1 s^?1) is shown to exceed that of their MoS₂‐only counterparts.     

Flexible Cellulose Paper‐based Asymmetrical Thin Film Supercapacitors with High‐Performance for Electrochemical Energy Storage

Cellulose paper (CP)‐based asymmetrical thin film supercapacitors (ATFSCs) have been considered to be a novel platform for inexpensive and portable devices as the CP is low‐cost, lightweight, and can be rolled or folded into 3D configurations. However, the low energy density and poor cycle stability are serious bottlenecks for the development of CP‐based ATFSCs. Here, sandwich‐structured graphite/Ni/Co₂NiO₄‐CP is developed as positive electrode and the graphite/Ni/AC‐CP as negative electrode for flexible and high‐performance ATFSCs. The fabricated graphite/Ni/Co₂NiO₄‐CP positive electrode shows a superior areal capacitance (734 mF/cm² at 5 mV/s) and excellent cycling performance with ≈97.6% C_sp retention after 15 000 cycles. The fabricated graphite/Ni/AC‐CP negative electrode also exhibits large areal capacitance (180 mF/cm² at 5 mV/s) and excellent cycling performance with ≈98% C_sp retention after 15 000 cycles. The assembled ATFSCs based on the sandwich‐structured graphite/Ni/Co₂NiO₄‐CP as positive electrode and graphite/Ni/AC‐CP as negative electrode exhibit large volumetric C_sp (7.6 F/cm³ at 5 mV/s), high volumetric energy density (2.48 mWh/cm³, 80 Wh/kg), high volumetric power density (0.79 W/cm³, 25.6 kW/kg) and excellent cycle stability (less 4% C_sp loss after 20 000 cycles). This study shows an important breakthrough in the design and fabrication of high‐performance and flexible CP‐based electrodes and ATFSCs.     

Simultaneous Electrochemical Dual‐Electrode Exfoliation of Graphite toward Scalable Production of High‐Quality Graphene

Developing scalable methods to produce large quantities of high‐quality and solution‐processable graphene is essential to bridge the gap between laboratory study and commercial applications. Here an efficient electrochemical dual‐electrode exfoliation approach is developed, which combines simultaneous anodic and cathodic exfoliation of graphite. Newly designed sandwich‐structured graphite electrodes which are wrapped in a confined space with porous metal mesh serve as both electrodes, enabling a sufficient ionic intercalation. Mechanism studies reveal that the combination of electrochemical intercalation with subsequent thermal decomposition results in drastic expansion of graphite toward high‐efficiency production of graphene with high quality. By precisely controlling the intercalation chemistry, the two‐step approach leads to graphene with outstanding yields (85% and 48% for cathode and anode, respectively) comprising few‐layer graphene (1–3 layers, >70%), ultralow defects (I_D/I_G < 0.08), and high production rate (exceeding 25 g h^?1). Moreover, its excellent electrical conductivity (>3 × 10⁴ S m^?1) and great solution dispersibility in N‐methyl pyrrolidone (10 mg mL^?1) enable the fabrication of highly conductive (11 Ω sq^?1) and flexible graphene films by inkjet printing. This simple and efficient exfoliation approach will facilitate the development of large‐scale production of high‐quality graphene and holds great promise for its wide application.     

Face‐to‐Face Contact and Open‐Void Coinvolved Si/C Nanohybrids Lithium‐Ion Battery Anodes with Extremely Long Cycle Life

To develop high‐performance anode materials of lithium‐ion batteries (LIBs) instead of commercial graphite for practical applications, herein, a layer of silicon has been well‐anchored onto a 3D graphene/carbon nanotube (CNT) aerogels (CAs) framework with face‐to‐face contact and balanced open void by a simple chemical vapor deposition strategy. The engineered contact interface between CAs and Si creates high‐efficiency channels for the rapid electrons and lithium ions transport, and meanwhile, the balanced open‐void allows the free expansion of Si during cycling while maintaining high structural integrity due to the robust mechanical strength of 3D CAs framework. As a consequence, the as‐synthesized Si/CAs nanohybrids are highly stable anode materials for LIBs with a high reversible discharge capacity (1498 mAh g^?1 at 200 mA g^?1) and excellent rate capability (462 mAh g^?1 at 10 000 mA g^?1), which is much better than Si/graphene‐CNTs‐mixture (51 mAh g^?1 at 10 000 mA g^?1). More significantly, it is found that the Si/CAs nanohybrids display no obvious capacity decline even after 2000 cycles at a high current density of 10 000 mA g^?1. The present Si/CAs nanohybrids are one of the most stable Si‐based anode materials ever reported for LIBs to date.     

Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors

The high‐bias electrical characteristics of back‐gated field‐effect transistors with chemical vapor deposition synthesized bilayer MoS₂ channel and Ti Schottky contacts are discussed. It is found that oxidized Ti contacts on MoS₂ form rectifying junctions with ≈0.3 to 0.5 eV Schottky barrier height. To explain the rectifying output characteristics of the transistors, a model is proposed based on two slightly asymmetric back‐to‐back Schottky barriers, where the highest current arises from image force barrier lowering at the electrically forced junction, while the reverse current is due to Schottky‐barrier‐limited injection at the grounded junction. The device achieves a photoresponsivity greater than 2.5 A W^?1 under 5 mW cm^?2 white‐LED light. By comparing two‐ and four‐probe measurements, it is demonstrated that the hysteresis and persistent photoconductivity exhibited by the transistor are peculiarities of the MoS₂ channel rather than effects of the Ti/MoS₂ interface.     

Scalable Production of Graphene Inks via Wet‐Jet Milling Exfoliation for Screen‐Printed Micro‐Supercapacitors

The miniaturization of energy storage units is pivotal for the development of next‐generation portable electronic devices. Micro‐supercapacitors (MSCs) hold great potential to work as on‐chip micro‐power sources and energy storage units complementing batteries and energy harvester systems. Scalable production of supercapacitor materials with cost‐effective and high‐throughput processing methods is crucial for the widespread application of MSCs. Here, wet‐jet milling exfoliation of graphite is reported to scale up the production of graphene as a supercapacitor material. The formulation of aqueous/alcohol‐based graphene inks allows metal‐free, flexible MSCs to be screen‐printed. These MSCs exhibit areal capacitance (C_areal) values up to 1.324 mF cm^?2 (5.296 mF cm^?2 for a single electrode), corresponding to an outstanding volumetric capacitance (C_vol) of 0.490 F cm^?3 (1.961 F cm^?3 for a single electrode). The screen‐printed MSCs can operate up to a power density above 20 mW cm^?2 at an energy density of 0.064 µWh cm^?2. The devices exhibit excellent cycling stability over charge–discharge cycling (10 000 cycles), bending cycling (100 cycles at a bending radius of 1 cm) and folding (up to angles of 180°). Moreover, ethylene vinyl acetate‐encapsulated MSCs retain their electrochemical properties after a home‐laundry cycle, providing waterproof and washable properties for prospective application in wearable electronics.     

3D Bicontinuous Nanoporous Reduced Graphene Oxide for Highly Sensitive Photodetectors

Reduced graphene oxide (RGO) is an important graphene derivative for applications in photonics and optoelectronics because of the band gap created by chemical oxidation. However, most RGO materials made by chemically exfoliated graphite oxide are 2D flakes. Their optoelectronic performance deteriorates significantly as a result of weak light‐matter interaction and poor electrical contact between stacking flakes. Here we report a bicontinuous 3D nanoporous RGO (3D np‐RGO) with high optoelectronic performance for highly sensitive photodetectors. 3D np‐RGO demonstrates a over 40 times higher light absorption than monolayer graphene materials and at least two orders of magnitude higher electron mobility than conventional RGO from discrete RGO flakes. The np‐RGO with an optimal reduction state shows ultrahigh photoresponse of 3.10 3 10⁴ A W^?1 at room temperature, approximately four orders of magnitude higher than graphene and other graphene derivatives at similar levels of light intensity radiations, and the excellent external quantum efficiency of 1.04 3 10⁷% better than commercial silicon photodetector. The ultrahigh capability of conversing photons to photocurrent originates from strongly enhanced light absorption, facilitated photocarrier transport, and tunable oxygenous defects and reduction states in the 3D interconnected bicontinuous RGO network.     

Nitrogen‐Doped Nanoporous Carbon/Graphene Nano‐Sandwiches: Synthesis and Application for Efficient Oxygen Reduction

A zeolitic‐imidazolate‐framework (ZIF) nanocrystal layer‐protected carbonization route is developed to prepare N‐doped nanoporous carbon/graphene nano‐sandwiches. The ZIF/graphene oxide/ZIF sandwich‐like structure with ultrasmall ZIF nanocrystals (i.e., ≈20 nm) fully covering the graphene oxide (GO) is prepared via a homogenous nucleation followed by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high‐temperature treatment (800 °C). After carbonization and acid etching, N‐doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m² g^?1). These N‐doped nanoporous carbon/graphene nanosheets are used as the nonprecious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V vs reversible hydrogen electrode; RHE) and a large limiting current density (5.2 mA cm^?2 at 0.60 V). To further increase the oxygen reduction performance, nanoporous Co‐N_x/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as cometal sources, which reveal higher onset potential (0.96 V) than both commercial Pt/C (0.94 V) and N‐doped nanoporous carbon/graphene nanosheets. Such nanoporous Co‐N_x/carbon nanosheets also exhibit good performance such as high activity, stability, and methanol tolerance in acidic media.     

基于石墨烯的肖特基结辐射伏特同位素电池

从传统肖特基结辐射伏特同位素电池金属电极存在的对放射源衰变粒子的阻挡及导电性不理想的问题出发,借鉴石墨烯肖特基结太阳电池结构,将石墨烯/硅肖特基结引入辐射伏特同位素电池中,在63Ni放射源的照射下验证石墨烯/硅肖特基结换能单元在辐射伏特同位素电池中应用的可行性.研究结果发现,基于硝酸掺杂,当少层(3?5层)石墨烯经过4...     

A High‐Performance Nitro‐Explosives Schottky Sensor Boosted by Interface Modulation

A high‐performance Schottky sensor boosted by interface modulation is fabricated for the detection of trace nitro‐explosives vapors. The interface modulation strategy results in a silicon nanowires (SiNWs) array/TiO₂/reduced graphene oxide (rGO) sensor with sensitive and selective response toward nitro‐explosives vapors. The response of the SiNWs array/TiO₂/rGO sensor toward nitro‐explosives vapors, such as 9 ppb 2,4,6‐trinitrotoluene, 4.9 ppt hexogen, and 0.25 ppq octagon, is boosted by 2.4, 7.5, and 5 times with the insertion of TiO₂. Superior selectivity is shown even compared with interfering gases of 10 ppm. Such good sensing performance can be attributed to the good sensing performance of the Schottky heterojunction‐based sensor, the Schottky barrier height modulation with the insertion of TiO₂, SiNWs array structure enhanced diffusion, and TiO₂ nanoparticles enhanced adsorption. This is believed to be the first Schottky heterojunction‐based sensor for nitro‐explosives vapors detection. This work would open a new way to develop highly sensitive and selective sensors.     

A Controllable Self‐Assembly Method for Large‐Scale Synthesis of Graphene Sponges and Free‐Standing Graphene Films

A simple method to prepare large‐scale graphene sponges and free‐standing graphene films using a speed vacuum concentrator is presented. During the centrifugal evaporation process, the graphene oxide (GO) sheets in the aqueous suspension are assembled to generate network‐linked GO sponges or a series of multilayer GO films, depending on the temperature of a centrifugal vacuum chamber. While sponge‐like bulk GO materials (GO sponges) are produced at 40 °C, uniform free‐standing GO films of size up to 9 cm² are generated at 80 °C. The thickness of GO films can be controlled from 200 nm to 1 µm based on the concentration of the GO colloidal suspension and evaporation temperature. The synthesized GO films exhibit excellent transparency, typical fluorescent emission signal, and high flexibility with a smooth surface and condensed density. Reduced GO sponges and films with less than 5 wt% oxygen are produced through a thermal annealing process at 800 °C with H₂/Ar flow. The structural flexibility of the reduced GO sponges, which have a highly porous, interconnected, 3D network, as well as excellent electrochemical properties of the reduced GO film with respect to electrode kinetics for the [Fe(CN)₆]^3?/4? redox system, are demonstrated.     

Hafnium-ntype silicon Schottky barriers

This paper describes the electrical properties of hafnium-/n-type/silicon contacts. These contacts were found to be Schottky barriers with a low barrier height. Polished and chemically cleaned 〈111〉 silicon wafers with a donor concentration N_d = 7 × 10²² m^?3 were used to fabricate experimental Schottky barrier structures. For the Schottky barrier height φ_bn and the ideality factor n values were found of 0.47 V and 1.07–1.11, respectively. It is concluded that due to their low forward voltage drop and good rectifying properties, Hf-nSi contacts can be applied in microwave Schottky barrier diodes.     

Graphite/<Emphasis Type="Italic">p</Emphasis>-SiC Schottky Diodes Prepared by Transferring Drawn Graphite Films onto SiC

Graphite/p-SiC Schottky diodes are fabricated using the recently suggested technique of transferring drawn graphite films onto p-SiC single-crystal substrates. The current–voltage and capacitance–voltage characteristics are measured at different temperatures and at different frequencies of a small-signal AC signal, respectively. The temperature dependences of the potential-barrier height and of the series resistance of the graphite/p-SiC junctions are measured and analyzed. The dominant mechanisms of the charge–carrier transport through the diodes are determined. It is shown that the dominant mechanisms of the transport of charge carriers through the graphite/p-Si Schottky diodes at a forward bias are multi-step tunneling recombination and tunneling described by the Newman formula (at high bias voltages). At reverse biases, the dominant mechanisms of charge transport are the Frenkel–Poole emission and tunneling. It is shown that the graphite/p-SiC Schottky diodes can be used as detectors of ultraviolet radiation since they have the open-circuit voltage V_oc = 1.84 V and the short-circuit current density I_sc = 2.9 mA/cm² under illumination from a DRL 250-3 mercury–quartz lamp located 3 cm from the sample.