Monolayer graphene/hexagonal boron nitride heterostructure

A buried metal-gate field-effect transistor (FET) using a stacked hexagonal boron nitride (h-BN) and chemically vapor deposited (CVD) graphene heterostructure is demonstrated. A thin h-BN multilayer serves as both gate dielectric and supporting layer for the monolayer graphene channel. It is observed that electrical stressing could significantly improve graphene conduction, similar to the effect reported in the graphene/SiO₂ system. In the graphene/h-BN/TiN FET structure, p-type doping behavior in graphene is observed, possibly attributed to spontaneous doping due to the work function difference between the graphene channel and the metal gate electrode. At a high-level of stress, graphene exhibits n-type doping behavior due to charge transfer across the thin h-BN multilayer. The dielectric strength and tunneling behavior of h-BN are investigated, showing the robust nature of the layer-structured insulator.     

Electrothermal Actuator on Graphene Bilayer Film

Electrodriven bilayer actuator is designed and fabricated by spin‐coating a reduced graphene oxide solution onto a polymer substrate. The bonded interface properties, electrical and thermal conductivities are characterized through scanning electron microscopy and X‐ray diffraction. The bilayer actuator exhibits fast and large bending response when a direct current voltage is applied to the graphene layer. Whereas it exhibits oscillation when an alternating current voltage is applied to the bilayer actuator. The effects of the layer structure and the electro‐operation methods on the bending motions are studied. Two new actuation modes are explored to broaden the applications of electrodriven bilayer actuator, which are achieved by the actuator structure design and the driven current control. Electrically responsive graphene bilayer soft actuator provides the potential applications in the fields of thin‐film electronics, sensors, and energy conversion.     

Interface traps and quantum size effects on the retention time in nanoscale memory devices

Based on the analysis of Poisson equation, an analytical surface potential model including interface charge density for nanocrystalline (NC) germanium (Ge) memory devices with p-type silicon substrate has been proposed. Thus, the effects of P_b defects at Si(110)/SiO₂, Si(111)/SiO₂, and Si(100)/SiO₂ interfaces on the retention time have been calculated after quantum size effects have been considered. The results show that the interface trap density has a large effect on the electric field across the tunneling oxide layer and leakage current. This letter demonstrates that the retention time firstly increases with the decrease in diameter of NC Ge and then rapidly decreases with the diameter when it is a few nanometers. This implies that the interface defects, its energy distribution, and the NC size should be seriously considered in the aim to improve the retention time from different technological processes. The experimental data reported in the literature support the theoretical expectation.     

Fabrication of diamond in-plane-gated field effect transistors using oxygen plasma etching

Diamond dual in-plane-gated field effect transistors with very low gate leakage current have been fabricated on an undoped hydrogen-terminated diamond p-type surface using oxygen plasma etching. Adjusting the threshold voltage optimally by one side gate, lateral electric field from the other side gate modulates the channel conductance. The oxygen plasma etching of 60 nm in depth fully isolated the channel of the hydrogen-terminated diamond surface conductive layer from the side gates resulting very low gate leakage current (<1 pA at −60 V) at room temperature. This feature provides a necessary condition for the fabrication of diamond single-hole transistors operated at room temperature.     

Field-induced recovery of massless Dirac fermions in epitaxial graphene on SiC

We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-terminated 4H-SiC(0 0 0 1) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model consisting of the single-layer and the buffer-layer graphene only. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior, despite the band-gap closure under electric field, makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications.     

Preparation and electrochemical characterization of MnOOH nanowire–graphene oxide

MnOOH nanowire–graphene oxide composites are prepared by hydrothermal reaction in distilled water or 5% ammonia aqueous solution at 130 °C with MnO₂–graphene oxide composites which are synthesized by a redox reaction between KMnO₄ and graphene oxide. Powder X-ray diffraction (XRD) analyses and energy dispersive X-ray analyses (EDAX) show MnO₂ is deoxidized to MnOOH on graphene oxide through hydrothermal reaction without any extra reductants. The electrochemical capacitance of MnOOH nanowire–graphene oxide composites prepared in 5% ammonia aqueous solution is 76 F g⁻¹ at current density of 0.1 A g⁻¹. Moreover, electrochemical impedance spectroscopy (EIS) suggests the electrochemical resistance of MnOOH nanowire–graphene oxide composites is reduced when hydrothermal reaction is conducted in ammonia aqueous solution. The relationship between the electrochemical capacitance and the structure of MnOOH nanowire–graphene oxide composites is characterized by cyclic voltammetry (CV) and field emission scanning electron microscopy (FESEM). The results indicate the electrochemical performance of MnOOH nanowire–graphene oxide composites strongly depends on their morphology.     

Tunneling Spectroscopy for the Study of Adsorption and Reaction on Model Catalysts

Abstract

Inelastic electron tunneling spectroscopy measures the vibrational modes of a small quantity of molecules that are included in or near the insulating layer of a metal-insulator-metal tunnel junction [l, 21. Figure 1 stows two metal electrodes separted by a thin insulating layer. If this layer is thin enough (of order 20 A) and if a voltage V is applied between the two electrodes, an electron current will flow from one electrode to the other. The maximum energy of the tunneling electrons will be of order eV. If there is a molecule in or near the insulating layer with a characteristic vibrational mode energy hv, this vibrational mode may or may not be excited by the tunneling electron. The most important factor turns out to be simply, does the tunneling electron have enough energy? In particular, is eV  hv?     

Tunneling Spectroscopy for the Study of Adsorption and Reaction on Model Catalysts

Inelastic electron tunneling spectroscopy measures the vibrational modes of a small quantity of molecules that are included in or near the insulating layer of a metal-insulator-metal tunnel junction [l, 21. Figure 1 stows two metal electrodes separted by a thin insulating layer. If this layer is thin enough (of order 20 A) and if a voltage V is applied between the two electrodes, an electron current will flow from one electrode to the other. The maximum energy of the tunneling electrons will be of order eV. If there is a molecule in or near the insulating layer with a characteristic vibrational mode energy hv, this vibrational mode may or may not be excited by the tunneling electron. The most important factor turns out to be simply, does the tunneling electron have enough energy? In particular, is eV ≥ hv?     

Structural and electrical properties of H-terminated diamond field-effect transistor

A dielectric barrier separating hydrogen induced p-type channel and Al gate metal contact of diamond FET has been investigated. The separation barrier is necessary to prevent tunneling current between the H-induced channel and the gate contact. In this investigation, CV measurements, fitting of forward IV characteristics, TEM and SIMS profiles have been used to obtain a more detailed picture of this barrier layer. While the composition of this layer is not clear, TEM and SIMS measurements indicate that this layer may be connected to a diamond phase or aluminium oxide. Using material properties of these materials, thickness of the separation layer extracted from the CV measurements was between 5–10 nm and the channel sheet change density was above 1 × 10¹³ cm^? 2. This thickness is in good agreement with the TEM observations. Frequency dependent CV measurements showed almost no frequency dependence, and no UV light dependence has been observed. Temperature dependent CV measurements showed a decrease of the dielectric constant at 100 °C. Fitting of the forward tunnelling current indicated a thickness of the barrier layer of about 5 nm with a barrier height of 2.4 eV.     

Graphene p–n–p junctions controlled by local gates made of naturally oxidized thin aluminium films

Graphene structures with both top- and bottom-electrostatic gates are studied. The top gate is made of thin aluminium (Al) film deposited directly onto graphene, with no prior dielectric layer in between. Natural oxidation of Al at the interface with graphene results in an insulating barrier proving useful in making top gates to graphene. For electrically disconnected top gate, graphene resistance as a function of the slowly-varying back-gate voltage shows hysteresis which reveals dielectric properties of the barrier. The estimated barrier thickness is only 2 nm allowing for very sharp profiles of the electric field in graphene devices. By applying voltages to both back- and top gates, effective p–n–p junctions with sharp interfaces can be created.     

Electrical ac and dc behavior of epoxy nanocomposites containing graphene oxide

This article deals with the investigation of electrical properties of epoxy‐based nanocomposites containing graphene oxide nanofillers dispersed in the polymer matrix through two‐phase extraction. Broadband dielectric spectroscopy and dc electrical conductivity as a function of electric field have been evaluated in specimens containing up to 0.5 wt % of nanofiller. Nanocomposites containing pristine graphene oxide do not show significant changes of electrical properties. On the contrary, the same materials after a proper thermal treatment at 135°C, able to provoke the in situ reduction of graphene oxide, exhibit higher permittivity and electrical conductivity, without showing large decrease of breakdown voltage. Moreover, a nonlinear behavior of the electrical conductivity is observed in the range of electric fields investigated, i.e. 2–30 kV mm^?1. A new relaxation phenomenon with a very low temperature dependence is also evidenced at high frequency in reduced graphene oxide composites, likely associated to induced polarization of electrically conductive nanoparticles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41923.     

Fabrication and properties of under-gated triode with CNT emitter for flat lamp

We investigated under-gate type carbon nanotube field emitter arrays (FEAs) for back light unit (BLU) in liquid crystal display (LCD). Gate oxide was formed by wet etching of ITO coated glass substrate instead of depositing SiO₂ on the glass substrate. Wet etching is easier and simpler than depositing and etching thick gate oxide to isolate the gate metal from cathode electrode in triode. To optimize the triode, we simulated the electric field distribution and electron trajectory in triode structures by the SIMION simulator. CNT emitters were formed using screen printing of photosensitive CNT paste. Field-emission characteristics of triode structure were measured. The maximum current density of 92.5 μA/cm² was when the gate and anode voltage was 95 and 2500 V, respectively, at the anode–cathode spacing of 1500 μm.     

Ultra-low-damage radical treatment for the highly controllable oxidation of large-scale graphene sheets

We have used low-damage oxygen radical treatment (O-LDRT)—with chemically reactive radicals generated in a plasma system equipped with a complementary filter—to prepare large-scale graphene oxide sheets with highly controllable levels of oxidation. Raman spectroscopy and X-ray photoelectron spectroscopy revealed that oxidative functionalities were formed on the graphene surfaces in a highly controllable manner and with a wide process window. Contact angle measurements revealed the high hydrophilicity of the large-scale graphene after O-LDRT. We measured the current–voltage characteristics of oxidized double-layer graphene (DLG) sheets to confirm the low degree of damage after using O-LDRT for the oxidation process. For treatment times of up to 5 min, the current passing through the DLG sheets remained almost identical to that passing through the corresponding sample that had not been subjected to O-LDRT, indicating that the bottom layer of the graphene sheet remained almost unmodified, whereas a massive number of oxidative functionalities had been formed on the top layer of the graphene sheet; in addition, the adhesion energy increased to approximately twice that of the untreated graphene sheet.     

氧化石墨烯膜间距对电渗析空气除湿特性影响的分子动力学研究

电渗析空气除湿技术利用高压电场为气体分子荷电,通过多孔氧化石墨烯膜实现水分子和其他带电分子,如氧气分子的分离。双层膜间距将影响到气体分子的传输特性和热力学性质,本文采用分子动力学研究了带电后的水分子和氧气分子在不同膜间距下通过膜的扩散及吸附现象。结果表明：膜层间距主要影响层间氢键形成方式,继而影响多孔氧化石墨烯膜与气体分子间吸附能,存在最佳层间距离1.25 nm,水分子渗透效果最佳,气体选择性达到最高,为1.14,较无电场时扩大了3倍。     

Influence of graphene oxide on metal-insulator-semiconductor tunneling diodes

ABSTRACT: In recent years, graphene studies have increased rapidly. Graphene oxide, which is an intermediate product to form graphene, is insulating, and it should be thermally reduced to be electrically conductive. We herein describe an attempt to make use of the insulating properties of graphene oxide. The graphene oxide layers are deposited onto Si substrates, and a metal-insulator-semiconductor tunneling structure is formed and its optoelectronic properties are studied. The accumulation dark current and inversion photocurrent of the graphene oxide device are superior to the control device. The introduction of graphene oxide improves the rectifying characteristic of the diode and enhances its responsivity as a photodetector. At 2 V, the photo-to-dark current ratio of the graphene oxide device is 24, larger than the value of 15 measured in the control device.     

The role of SiC as a diffusion barrier in the formation of graphene on Si(111)

In this paper, we investigate the role of SiC as a diffusion barrier for Si in the formation of graphene on Si(111) via direct deposition of solid-state carbon atoms in ultra-high vacuum. Therefore, various thicknesses of the SiC layer preformed on the Si substrates were produced in order to evaluate its influence on the quality of graphene formation at different substrate temperatures from 900 °C to 1100 °C. At a given temperature of 1100 °C, we found that a thicker SiC layer can suppress silicon-out diffusion from the substrate and improve the structural quality of the graphene layer. The samples were analyzed by low energy electron diffraction, Auger electron spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and scanning tunneling microscopy.     

Graphene oxide as dyestuffs for the creation of electrically conductive fabrics

Graphene oxide (GO) was immobilized on the surfaces of acrylic yarns through a conventional dyeing approach. The GO dyed yarns and/or the fabric were immersed in an aqueous sodium hydrosulfite solution at around 363 K for 30 min, which converted the GO into graphene. The graphene created a graphitic-coloured and electrically conductive thin layer over each yarn in the fabric. Data on the electrical conductance of the yarns versus temperature (30-300 K) fit well with the so-called fluctuation-induced tunneling model, which suggests that the graphene layer belongs to a continuously interconnected network. Values of the electrical resistivity ranged from 10² to 10¹⁰ Ohm/cm, as verified by the content of graphene in the conductive layer.     

One-pot synthesis of CdS sensitized TiO2 decorated reduced graphene oxide nanosheets for the hydrolysis of ammonia-borane and the effective removal of organic pollutant from water

A hybrid material of reduced graphene oxide (RGO) sheets decorated with CdS-TiO₂ NPs was prepared through a facile one-pot hydrothermal method. The assembly of CdS-TiO₂ nanoparticles (NPs) on RGO sheets was in-situ produced. As-synthesized nanocomposites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy disperse X-ray spectrum (EDS), fourier transform infrared spectroscopy (FTIR), and photoluminescence spectroscopy (PL). The obtained nanocomposites exhibited a good photocatalytic activity for the visible-light-induced decomposition of methylene blue (MB) dye and hydrolysis of ammonia borane. The results showed that by incorporation of CdS and TiO₂ NPs on graphene oxide sheets the photocatalytic efficiency was enhanced. The significant enhancement in the photocatalytic activity of CdS-TiO₂/RGO nanocomposites under visible light irradiation can be ascribed to the effect of CdS by acting as electron traps in TiO₂ band gap. Reduced graphene oxide worked as the adsorbent, electron acceptor and a photo-sensitizer to efficiently enhance the dye photo decomposition. Such nanocomposite photocatalyst might find potential application in a wide range of fields, including hydrogen energy generation, air purification, and wastewater treatment.     

Synthesis of enhanced hydrophilic and hydrophobic graphene oxide nanosheets by a solvothermal method

We report that the hydrophilic affinity of graphene oxide nanosheets can be significantly increased by reacting with allylamine. High resolution transmission electron microscopy and electron diffraction analysis confirmed that the graphene oxide nanosheets were amorphous in structure. Hydrophobic graphene oxide nanosheets were also prepared via functionalising with phenylisocynate (C₆H₅NCO) through a solvothermal synthesis process. Hydrophobic graphene oxide nanosheets can be used as additives in polymer-based composites and other functional applications.     

Ge/Si异质键合界面纳米级氧化层对Ge/Si异质结光电特性调控机制

由于Ge/Si存在4.2%晶格失配,Si基外延Ge中的穿透位错密度(TDD)极高,导致器件暗电流偏大。低温Ge/Si异质键合可以通过抑制失配位错传播降低Ge薄膜中的TDD,在高质量大失配薄膜制备方面展现出巨大的潜力。然而,Ge/Si键合界面由于亲水反应形成的纳米级氧化层会对异质结性能产生影响。基于载流子基本方程,在键合界面构建载流子非局域量子隧穿模型,通过半经典近似解法研究Ge/Si异质键合界面氧化层厚度(dO)对异质结暗电流、总电流、光谱响应、带宽等性能的影响,并揭示异质结性能影响机制。研究表明:随着dO的增加,氧化层对载流子的阻挡效应增强,导致器件暗电流、总电流和光谱响应下降。由于氧化层的分压效应,dO的增加导致Ge层中电场下降,载流子速率降低,进而导致异质结高频特性变差。为确保键合Ge/Si异质结优异的光电特性,dO必须控制在0.50 nm以内。