Investigating the effect of gas absorption on the electromechanical and electrochemical behavior of graphene/ZnO structure,suitable for highly selective and sensitive gas sensors

Graphene/ZnO hybrid was used, for the first time, to fabricate a highly selective and sensitive graphene based gas sensor by a combination of electromechanical and electrochemical characteristics of the graphene. ZnO nanowires in our fabricated sensor have two important roles: as the reductant of graphene oxide to obtain graphene and as an efficient electromechanical actuator due to their piezoelectric properties. To investigate the operation of the fabricated sensor as a gas sensor, a selected set of chemical vapors were introduced to the structure. It was found that chemical vapors change the resonance frequency of the graphene/ZnO structure, as well as the electrical resistivity of the sensor. The observed variation of the mechanical and electrical characteristics of the graphene/ZnO in response to gas exposure entitles the graphene/ZnO based sensor as a highly selective/sensitive device for gas sensing applications with distinctive signatures for different gas species.     

硅基底石墨烯器件的现状及发展趋势

石墨烯是一种由单层碳原子紧密排列而形成的具有蜂窝状结构的二维晶体材料,特殊的结构赋予了其优异的性能,如高载流子迁移率、电导率、热导率、力学强度以及量子反常霍尔效应.由于石墨烯优异的特性,迅速激起了人们对石墨烯研究以及应用的热情.石墨烯沉积或转移到硅片后,其器件构建与集成和传统硅基半导体工艺兼容.基于石墨烯的硅基器件与硅基器件的有机结合,可以大幅度提高半导体器件的综合性能.随着石墨烯制备工艺和转移技术的优化,硅基底石墨烯器件将呈现出潜在的、巨大的实际应用价值.随着器件尺寸的纳米化,器件的发热、能耗等问题成为硅基器件与集成发展面临的瓶颈问题,石墨烯的出现为解决这些问题提供了一种可能的解决方案.本文综述了石墨烯作为场效应晶体管研究的进展,为解决石墨烯带隙为零、影响器件开关比的问题,采用了量子限域法、化学掺杂法、外加电场调节法和引入应力法.在光电器件研究方面,石墨烯可以均匀吸收所有频率的光,其光电性能也受到了广泛的关注,如光电探测器、光电调制器、太阳能电池等.同时,石墨烯作为典型的二维材料,其优越的电学性能以及超高的比表面积,使其作为高灵敏度传感器的研究成为纳米科学研究的前沿和热点领域.     

Growth of ZnO Nanostructures on Porous Silicon and Oxidized Porous Silicon Substrates

We have investigated an oxidation of substrate effect on structural morphology of zinc oxide (ZnO) rods. ZnO rods are grown on porous silicon (PS) and on thermally oxidized porous silicon substrates by carbothermal reduction of ZnO powder through chemical vapour transport and condensation. Porous silicon is fabricated by electrochemical etching of silicon in hydrofluoric acid solution. The effects of substrates on morphology and structure of ZnO nanostructures have been studied. The morphology of substrates is studied by atomic force microscopy in contact mode. The texture coefficient of each sample is calculated from X-ray diffraction data that demonstrate random orientation of ZnO rods on oxidized porous silicon substrate. The morphology of structures is investigated by scanning electron microscopy that confirms the surface roughness tends to increase the growth rate of ZnO rods on oxidized PS compared with porous silicon substrate. A green emission has been observed in ZnO structures grown on oxidized PS substrates by photoluminescence measurements.     

MOCVD法生长SAWF用ZnO/Diamond/Si多层结构

使用等离子体辅助MOCVD系统在金刚石,硅衬底上成功地制备了氧化锌多层薄膜材料,通过两步生长法对薄膜质量进行了优化。XRD测试显示优化后的样品具有c轴的择优取向生长,PL谱测试表明样品经优化后不仅深能级发射峰消失,同时紫外发射峰增强。对优化后的样品的表面测试显示出较低的表面粗糙度。比较氧化锌多层薄膜结构的声表面波频散曲线,ZnO薄膜声表面滤波器受膜厚和衬底材料的影响较大。当ZnO薄膜较薄时,在它上面的传播速度将与衬底上的传播速度接近,与其他衬底上生长的薄膜相比,以金刚石这种快声速材料为衬底的ZnO多层薄膜结构,声表面波滤波器的中心频率将提高1倍左右。     

HfO_2-based ferroelectric modulator of terahertz waves with graphene metamaterial

Tunable modulations of terahertz waves in a graphene/ferroelectric-layer/silicon hybrid structure are demonstrated at low bias voltages. The modulation is due to the creation/elimination of an extra barrier in Si layer in response to the polarization in the ferroelectric Si:HfO_2 layer. Considering the good compatibility of HfO_2 with the Si-based semiconductor process, the highly tunable characteristics of the graphene metamaterial device under ferroelectric effect open up new avenues for graphene-based high performance integrated active photonic devices compatible with the silicon technology.     

Vacancy filling effect of graphene on photoluminescence behavior of ZnO/graphene nanocomposite

We report on the aerosol synthesis and optical characterization of ZnO/unoxidized graphene (UG) platelets nanocomposite films with high optical transparency (>85% at visible wavelengths). The ZnO/UG composite films, in which UG nanoplatelets are embedded in nano‐grained ZnO, were fabricated from colloidal suspensions of UG platelets with an aqueous zinc precursor. From photoluminescence (PL) spectra of the UG composite films, it was found that PL intensity decreases with the addition of UG platelets. The features of PL intensity in the UG composites are in contrast to that of ZnO/graphene oxide (G‐O) platelets composites, and can be explained by the absence of an oxygen vacancy filling effect, due to the unoxidized nature of UG and an increase in defect sites in its composites. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Copper and Graphene activated ZnO nanopowders for enhanced photocatalytic and antibacterial activities

ZnO, ZnO:Cu and ZnO:Cu:Graphene nanopowders were synthesized via a facile wet chemical method. The XRD studies show that the synthesized samples have hexagonal wurtzite structure. It is found that graphene addition induces a decrease in crystallite size. UV–vis absorption spectra of the samples show sharp absorption edges around 380 nm. Photoluminescence studies reveal that the incorporation of copper and graphene in ZnO facilitates the efficient photo generated electron–hole pair separation. It is found that the ZnO:Cu and ZnO:Cu:Graphene nanopowder exhibit improved photocatalytic efficiency for the photodegradation of Methylene Blue (MB) under visible light irradiation. Moreover, improved antibacterial activity of ZnO:Cu:Graphene nanopowder against Escherichia coli and Staphylococcus aureus bacteria is observed.     

Dependence of the solar cell performance on nanocarbon/Si heterojunctions

Solar cells that combine single-crystalline silicon(Si) with graphene(G) have been widely researched in order to develop next-generation photovoltaic devices. However, the power conversion efficiency(PCE) of G/Si solar cell without chemical doping is commonly low due to the relatively high resistance of graphene. In this work, through combining graphene with carbon nanotube(CNT) networks, we fabricated three kinds of hybrid nanocarbon film/Si heterojunction solar cells in order to increase the PCE of the graphene based Si solar cell. We investigated the characteristics of different nanocarbon film/Si solar cells and found that their performance depends on the heterojunctions. Specifically, a doping-free G-CNT/Si solar cell demonstrated a high PCE of 7.9%, which is nearly equal to the combined value of two individuals(G/Si and CNT/Si). This high efficiency is attributed to the synergistic effect of graphene and CNTs, and can be further increased to 9.1% after applying a PMMA antireflection coating. This study provides a potential way to further improve the Si based heterojunction solar cells.     

SiO2/Si衬底上石墨烯的制备与结构表征

在分子束外延(MBE)设备中,利用直接沉积C原子的方法在覆盖有SiO₂的Si衬底(SiO₂/Si)上生长石墨烯,并通过Raman光谱和近边X射线吸收精细结构谱等实验技术对不同衬底温度(500℃,600℃,700℃,900℃,1100℃,1200℃)生长的薄膜进行结构表征.实验结果表明,在衬底温度较低时生长的薄膜是无定形碳,在衬底温度高于700℃时薄膜具有石墨烯的特征,而且石墨烯的结晶质量随着衬底温度的升高而改善,但过高的衬底温度会使石墨烯质量降低.衬底温度为1100℃时结晶质量最好.衬底温度较低时C原子活性较低,难以形成有序的C-sp²六方环.而衬底温度过高时(1200℃),衬底表面部分SiO₂分解,C原子与表面的Si原子或者O原子结合而阻止石墨烯的形成,并产生表面缺陷导致石墨烯结晶变差.     

悬浮石墨烯摩擦特性

石墨烯作为固体润滑剂在微/纳米机电系统中具有巨大的应用潜力.本文在SiO_2/Si基底上制备了微孔阵列,将石墨烯剥离在微孔上,形成悬浮结构.使用原子力显微镜研究悬浮石墨烯和支撑石墨烯的摩擦特性,结果表明:悬浮石墨烯表面摩擦力比基底支撑石墨烯明显减小,同时在支撑石墨烯上出现的摩擦增强效应也消失.随着石墨烯厚度的增大,面外刚度逐渐增大,悬浮石墨烯与支撑石墨烯的摩擦力差异逐渐减小.此外,使用预磨损探针时,悬浮石墨烯和支撑石墨烯的摩擦力都显著增大,且悬浮石墨烯的摩擦力依然比支撑石墨烯小.通过对比不同厚度石墨烯,不同针尖半径时悬浮石墨烯与支撑石墨烯表面摩擦力的变化,揭示了面外变形对石墨烯摩擦力的影响,为有效提高石墨烯固体润滑剂的摩擦性能提供了理论指导.     

Different strategies for the synthesis of graphene/ZnO composite and its photocatalytic properties

Owing to its unique physical and chemical properties, graphene has attracted tremendous attention in the preparation of graphene-based composites for various applications. In this study, two different strategies have been developed to load zinc oxide (ZnO) nanorods onto reduced graphene oxide (RGO) sheets, i.e., in situ growth and a self-assembly approach. The microstructure and morphology of the synthesized RGO/ZnO nanocomposites was investigated by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM) and Brunauer–Emmett–Teller (BET) measurements. Fluorescence emission spectra (PL) of RGO/ZnO composites were performed to attribute quality of combination between RGO and ZnO. Significantly enhanced photocatalytic activity of RGO/ZnO nanocomposites in comparison to bare ZnO nanoparticles was revealed by the degradation of methylene blue under irradiation, which can be attributed to the inhibition of electron–hole pair recombination and enhanced adsorption due to the presence of RGO sheets.     

Photovoltaic performance of Gallium-doped ZnO thin film/Si nanowires heterojunction diodes

In this work, photovoltaic performance of Ga-doped ZnO thin film/Si NWs heterojunction diodes was investigated. Highly dense and vertically well-aligned Si NW arrays were successfully synthesised on a p-type (1?0?0)-oriented Si wafer through cost-effective metal-assisted chemical etching technique. Ga-doped ZnO thin films were deposited onto Si NWs via radio frequency magnetron sputtering to construct three-dimensional heterostructures. Photovoltaic characteristics of the fabricated diodes were determined with current density (J)–voltage (V) measurements under simulated solar irradiation of AM 1.5 G. The optimal open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were found to be 0.37 V, 3.30 mA cm^?2, 39.00 and 0.62%, respectively. Moreover, photovoltaic diodes exhibited relatively high external quantum efficiency over the broadband wavelengths between 350 and 1100 nm interval of the spectrum. The observed photovoltaic performance in this study clearly indicates that the investigated device structure composed of Ga-doped ZnO thin film/Si NWs heterojunctions could facilitate an alternative pathway for optoelectronic applications in future, and be a promising alternative candidate for high-performance low-cost new-generation photovoltaic diodes.     

Temperature dependent electron transport in graphene

We have investigated the electron transport in graphene at different carrier densities. Single layer graphene was fabricated into Hall bar shaped devices by mechanical extraction onto a silicon oxide/silicon substrate followed by standard microfabrication techniques. From magnetoresistance and Hall measurements, we measure the carrier density and mobility at different gate voltages. Different temperature dependent resistivity behaviors are found in samples with high and low mobilities.     

ZnO薄膜/金刚石在不同激励条件下声表面波特性的计算与分析

本文首先以刚度矩阵法为基础, 给出了ZnO薄膜/金刚石在四种不同激励条件下的有效介电常数计算公式. 然后以此为工具, 分别计算了多晶ZnO(002) 薄膜/多晶金刚石和单晶ZnO(002) 薄膜/多晶金刚石的声表面波特性, 并根据计算结果及设计制作声表面波器件的要求, 对ZnO膜厚的选择进行了详细地分析. 最后讨论了ZnO/金刚石/Si复合晶片可以忽略Si衬底对声表面特性影响时对金刚石膜厚的要求. 关键词： 声表面波 压电多层结构 有效介电常数 刚度矩阵法     

Direct formation of graphene layers on top of SiC during the carburization of Si substrate

We grow graphene film on silicon substrates having various orientations by simple heating in the presence of carbon source gas. We observed that a 3C-SiC (111) film would form upon carburizing silicon with carbon deposited from a carbon source because it is well lattice-matched with Si (110) (less than 2%). Graphene grew on the buffer layer of 3C-SiC (111). The surface consists of hexagonal arrays that can act as a template for graphene growth. This simple and inexpensive method of forming graphene on silicon wafer in situ is compatible with silicon technology.     

Microstructure and optical properties of ZnO/porous silicon nanocomposite films

In this study, porous silicon (PS) templates were formed by electrochemical anodization on p-type (100) silicon wafer and ZnO films were deposited on PS substrates using radio frequency (RF) reactive magnetron sputtering technique. The effects of oxygen partial pressures of growth ZnO films and annealing ambience on the microstructure and photoluminescence (PL) of the ZnO/PS nanocomposite films were systematically investigated by X-ray diffraction and fluorescence spectrophotometry. The results indicated that all ZnO/PS nanocomposite films were polycrystalline in nature with a hexagonal wurtzite structure and the (002) oriented ZnO films had the best crystal quality under O₂:Ar ratio of 10:10 sccm and annealing in vacuum. PL measurements at room temperature revealed that ZnO/PS nanocomposite systems formed a broad PL band including the blue and green emissions from ZnO and red-orange emission from the PS. The mechanism and interpretation of broadband PL of the nanocomposites were discussed in detail using an oxygen-bonding model in PS and a native defects model in ZnO.     

Direct fabrication of graphene/zinc oxide composite film and its characterizations

Graphene-based composites represent a new class of materials with potential for many applications. Graphene can be attached to a metal, a semiconductor, or any polymer for enhancing properties. In this work, a new mixed dispersion approach for graphene-based composite has taken on. Graphene flakes (<4 layers) and a well-known semiconductor zinc oxide (ZnO) (<50 nm particle size) have dispersed in N-methyl-pyrrolidone. We deposited graphene/ZnO composite thin film by a simple, low-cost, environmentally friendly and non-vacuum electrohydrodynamic atomization process on silicone substrate. Experiments have been carried out by changing flow rate and applied potential while keeping stand-off distance and substrate velocity constant, to discover the optimum conditions for obtaining a high-quality thin film. It has been explored that high-quality thin composite film is obtained at optimum flow rate of 300 μl/h at 6.3 kV applied potential after curing for 2 h at 300 °C. Graphene/ZnO thin composite film has been characterized using Field emission scanning electron microscopy, Ultra-violet Visible near Infra Red spectroscopy, X-ray diffraction, Raman Spectroscopy and 3D-Nanomap. For electrical behavior analysis, a simple diode Indium tin oxide/(poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/polydioctylfluorene-benzothiadiazole(F8BT)/(Graphene/ZnO) has fabricated. It is observed that at voltage of 0.3 V, the current in organic structure is at low value of 1.20 × 10^?3 Amp/cm² and after that as further voltage was applied, the device current increased by the order of 110 and reaches up to 1.32 × 10^?1 Amp/cm² at voltage 2 V.     

ZnO/Ag/ZnO multilayer transparent electrode for highly-efficient ITO-Free polymer solar cells

Ultrathin metal film (UTMF) with a ZnO/Ag/ZnO hybrid structure was used as transparent electrode in a high-efficiency bulk heterojunction system for the fabrication of ITO-free polymer solar cells. The performance of the devices was carefully tuned through optical simulation using transfer matrix method by varying the thickness of ZnO seed layer and thin absorber layer. By employing appropriate device architecture, polymer solar cells fabricated using this UTMF-based electrode show efficiency as high as 9.49%, which is slightly higher compared to that of ITO-based device. From good agreement between the external quantum efficiency and optical modeling, it was found that the optimized microcavity configuration formed in UTMF-based device can greatly enhance the absorbance of the BHJ layer at longer wavelength as well as the favored exciton distribution for better charge transport and collection.     

Effect of graphene grain boundaries on MoS_2/graphene heterostructures

The grain boundaries of graphene are disordered topological defects, which would strongly affect the physical and chemical properties of graphene. In this paper, the spectral characteristics and photoresponse of MoS_2/graphene heterostructures are studied. It is found that the blueshift of the G and 2 D peaks of graphene in Raman spectrum is due to doping. The lattice mismatch at the graphene boundaries results in a blueshift of MoS_2 features in the photoluminescence spectra, comparing to the MoS_2 grown on SiO_2. In addition, the photocurrent signal in MoS_2/hexagonal single-crystal graphene heterostructures is successfully captured without bias, but not in MoS_2/polycrystalline graphene heterostructures.The electron scattering at graphene grain boundaries affects the optical response of MoS_2/graphene heterostructures. The photoresponse of the device is attributed to the optical absorption and response of MoS_2 and the high carrier mobility of graphene. These findings offer a new approach to develop optoelectronic devices based on two-dimensional material heterostructures.     

Morphology and properties of ZnO films obtained by repeated spin coating on porous silicon substrates

Layers of porous silicon (PS), multilayered ZnO films, and heterostructures based on them are obtained. The surface morphology, chemical and phase composition of the PS layers and ZnO films, and the transverse cleavage of ZnO–PS nanocomposite, are investigated via energy-dispersive X-ray spectral analysis (EDX), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The current–voltage characteristics of Al/Ag/p-Si(100)/PS/ZnO/Ag/Al and Al/Ag/p-Si(100)/PS/ZnO/SiC/Ag/Al heterostructures are studied.