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Preface to selected papers from EMRS 2011 Symposium Q : Engineering of wide bandgap semiconductor materials for energy saving

In this paper, we demonstrate the fabrication of gel beads composed of ionic liquid (IL) and polymer. The IL droplets are kept spherical during the deposition process by placement onto chromatography paper coated with fluoropolymer. The deposition process then occurs in two steps. In the first step, the monomer is absorbed into the IL droplet. In the second step, the initiator radicals are introduced. This sequential deposition process allows polymerization to primarily occur within the liquid droplet and therefore the beads are not attached to the underlying substrate and can be easily removed.

Graphene is a 2D material with extraordinary electronic and mechanical properties and has a wide range of applications. Among existing synthesis methods, chemical vapor deposition (CVD) onto Cu foil is the most prominent because it allows large area high quality graphene to be prepared at relatively low costs and then subsequently wet transferred to arbitrary substrates. Typically, PMMA and FeCl 3 are used as a polymeric support layer and a metal etchant respectively during wet transfer. Unfortunately, this method leaves graphene susceptible to contamination, degrading transferred graphene's extraordinary properties. In this paper, contamination in the form of black particulates is identified to be Fe wrapped in graphene and a model of how it originates was developed. Then, a procedure to reduce this contamination during wet transfer was developed and its effectiveness was systematically studied. Transfers were characterized using Raman spectroscopy and optical microscopy with image segmentation. Experimental data obtained indicates that this simple and economic method is very effective in reducing Fe contamination on transferred CVD graphene, offering a way to improve the wet transfer process.

In this paper, we report graphene on Ge(111) by using an electron beam evaporation under appropriate conditions in ultra high vacuum. Surface morphology, structural and electronic properties of graphene on Ge(111) substrate are investigated in detail by low energy electron diffraction, Auger electron spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, optical microscopy, scanning electron microscopy, atomic force microscopy and scanning tunneling microscopy. We found that the crystalline quality of graphene reduces with increasing the amount of deposited carbon atoms during graphene formation at near melting temperature of the Ge substrate.

Graphene has attracted a great deal of interest due to its fascinating properties and a wide variety of potential applications. Several methods have been used to achieve high-quality graphene films on different substrates. However, there have been only a few studies on graphene growth on iron (Fe) and the growth mechanism remains unclear. This paper systematically investigates temperature-dependent growth of graphene on Fe substrate by gas-source molecular beam epitaxy. Two-dimensional (2D), large-area graphene samples were grown on Fe thin films, and characterized by Raman, X-ray photoelectron spectroscopy, X-ray diffraction, optical microscopy, transmission electron microscopy and atomic force microscopy. It is found that graphene flakes can be grown on Fe at a growth temperature as low as 400 °C and the optimized large-area graphene growth temperature is relatively low between 500 °C and 550 °C. The graphene growth on Fe that undergoes the formation and decomposition of iron carbide is discussed.

A facile fabrication strategy of multi-functional hybrid thin films of reduced graphene oxide (RGO) and TiO 2 was developed by using layer-by-layer (LBL) assembly of titanium(IV) bis(ammonium lactato)dihydroxide (TALH) and polyallylamine grafted reduced graphene oxide (PAA-RGO) as negatively and positively charged components. The fabrication process of hybrid films was thoroughly characterized by using UV–vis spectroscopic, ellipsometric and atomic force microscopic analysis. This fabrication strategy provided precisely-controlled thickness, surface roughness through 1–10 LBL assembly cycles. By thermal treatment, the PAA-RGO and TALH in the hybrid films were respectively converted into RGO and TiO 2 with the restoration of their electrical conductivity, UV-resistance and photo-catalytic self-cleaning properties.

Highly oriented CuInS2 films were successfully prepared on graphene electrodes for solar-cell applications. CuInS2 films were deposited on high-quality graphene via reactive sputtering using a system with facing targets. This high-quality graphene was grown via chemical vapour deposition. The obtained CuInS2 films on graphene were highly (112)-oriented, and their crystallite size was enhanced compared with that of films grown on conventional Mo electrodes. The graphene/CuInS2 interfaces clearly exhibited ohmic characteristics comparable to those of Mo/CuInS2. Our experimental results demonstrate the high potential of graphene electrodes for use in CuInS2 thin-film and multi-junction solar cells.

Important issues in optoelectronic devices based on exfoliated graphene are the electronic noise due to the interaction between graphene flakes and the substrate, and the accumulation of electrical charge at the flake edges. These effects, in addition to the distribution of flakes in the composites, being random and locally showing multiple overlaps, cause significant Inhomogeneity in the measured work function of the graphene-based materials. To shed light on the above effects, we investigated both graphene flakes and single-layer graphene by means of scanning Kelvin probe microscopy in ambient conditions. The graphene materials were deposited on rough substrates of silver with a top layer of titanium dioxide. The roughness caused the random disorder of interest for characterization, while the thin dielectric layer of titanium dioxide helped in keeping the electric field localized and provided better contrast in the surface potential images. It is demonstrated that proper engineering of the substrate can minimize the work function inhomogeneity. Additionally, SKPM allows for local mapping of graphene flakes overlaps and thus effective thickness, for possible quality control in produced devices.

In this paper the studies on structural, mechanical and corrosion properties of the hybrid graphene multilayer system (graphene/SiN/graphene) coupled with titanium alloy (Ti6Al4V) have been investigated. We have shown that the graphene/SiN/graphene multilayer hybrid system deposited on titanium alloy surfaces, improves surface hardness (23 GPa) and the corrosion properties of the Ti6Al4V alloy in very aggressive environments. Furthermore, this multilayer hybrid system has a very stable course of electrochemical potential in comparison with pure silicon nitride thin film with the same thickness. Unfortunately, this system does not maintain structural and mechanical stability during the corrosion process. The structure of the coating system becomes porous, which is characterised by low hardness (0.62 GPa) and low Young's modulus — 69 GPa.

This paper demonstrates some aspects on the synthesis and characterisation of nanoparticles of metallic alloys using polyvinyl alcohol as a stabiliser, which combines high surface area and superior hybrid properties. The present experimental design was to synthesise a nanocomposite of reduced graphene oxide and antimony nanoparticles to be used as thin films for macro- and micro-carbon electrodes for enhancing sensing of different toxic metal pollutants in the environment. The synthetic process of reduced graphene oxide was done using the modified Hummers method while antimony pentachloride was reduced with sodium borohydride into nanoparticles of antimony using polyvinyl-alcohol as a stabiliser. The systematic investigation of morphology was done by scanning electron microscopy and high resolution-transmission electron microscope, which revealed the synthesis of a product, consists of reduced graphene oxide antimony nanoparticles. The electrochemical behaviour of the reduced graphene oxide antimony nanoparticles coated on a glassy carbon electrode was performed using voltammetric and impedance techniques. Electrochemical impedance measurements showed that the overall resistance, including the charge–transfer resistance, was smaller with reduced graphene oxide antimony nanoparticles than reduced graphene oxide and antimony nanoparticles, on their own. Evaluation of the reduced graphene oxide antimony nanoparticle sensor in the stripping voltammetry has shown a linear working range for concentration of platinum (II) between 6.0 × 10− 6–5.4 × 10− 5 μg L− 1 with limit of detection of 6 × 10− 6 μg L− 1 (signal-to-noise ratio = 3), which is below the World Health Organisation guidelines for freshwater.

This work demonstrates the formation of few-layer graphene flake structures directly from graphite particles. A nano-particle deposition system, which is a dry spray deposition method generally used for deposition of metals and ceramics, has been introduced to deposit graphite particles at room temperature. In this study, graphite powder was deposited on a Cu substrate without using binders and the deposited thin film was characterized. The deposited thin film contained few-layer graphene flake structures, as observed using field effect scanning electron microscopy and confirmed using X-ray diffraction, Raman spectroscopy and field emission transmission electron microscopy. We also suggested the critical impact velocity for deposition of few-layer graphene flake structured thin film and a mechanism for forming few-layer graphene flake structures during thin film preparation. The suggested mechanism was interlayer separation of micron-sized graphite particles and fragmentation of particles into small pieces during deposition due to the impact of graphite particles on the substrate.

This article focuses on the influence of surface topography (roughness), surface chemistry (carbon and oxygen functional groups) and physical (film thickness) parameters on the electrical conductivity of graphene oxide (GO) and reduced graphene oxide (rGO) films. This study was carried out to understand how changes in chemistry, roughness and film thickness, arising from the reduction process, alter the electrical properties of the films. Improved understanding is needed to control and optimise these parameters in graphene/rGO films for future applications where targeted property performance is needed. Films with smooth surfaces, measured using confocal imaging, and lower thicknesses have been shown to possess higher electrical conductivity. X-ray diffraction patterns shows minor changes in d-spacing, though improvements in crystal perfection, orientation and crystallinity could be concluded. X-ray photoelectron spectroscopy shows a significant decrease in the oxygen present at the surface as the films are exfoliated to reduce their thickness. Conductivity improves as the materials become increasingly defect-free, achieved by careful control of reduction and post-processing techniques. Ideal practical conductivity is achieved for films of 4 μm thickness: beyond this point, no practical gains are made (about 25 exfoliation trials from the bulk cast film).

The demand to fabricate graphene film in large wafer-scale with minimum cost is crucial for device applications, and one of the promising techniques is the reduction of graphene oxide (GO) films. We have investigated morphological changes of large-area, monolayer GO films during methane (CH 4 )-assisted thermal annealing process and the role of CH 4 in the restoration of their electrical conductivity. We have discovered that long reduction process in high CH 4 flow rates can contribute to the partial reparation of lattice defects and formation of new multilayer graphene that are stacked at the boundary areas between the reduced GO (rGO) flakes. These multilayer graphene layers with graphitic domains operate as the electrical connection between the separated rGO flakes and create new conduction pathways across the entire films. This phenomenon is more important than the sole reduction process (elimination of oxygen functional groups) in the restoration of electrical conductivity of the continuous rGO films. We have achieved in the fabrication of highly conductive rGO films with a minimum sheet resistance value of 1.01 kΩ/sq. This study exhibits a promising reduction method for the mass production of large-area, continuous rGO films for thin film device applications.

We report transfer-free graphene growth on insulated substrates and the fabrication of a sensor array from the synthesized graphene. The insulated substrate coated with amorphous carbon and catalyst metals was annealed under an Ar atmosphere. After annealing, graphene was synthesized between the metal layer and the insulated substrate. We fabricated a sensing array based on the graphene growth method and used it for pH measurements. Our sensor had as a high resolution of pH as that of graphene synthesized by chemical vapor deposition. This technique allows graphene arrays to be synthesized simply and is suitable for industrial sensing applications.

Surface chemical modification and functionalization are common strategies used to provide new properties or functionalities to a material or to enhance existing ones. In this work, graphene oxide prepared using Hummers' method has been chemically modified with calcium ions by immersion in a calcium carbonate solution. Transmission electron microscopy analyses showed that graphene oxide (GO) and calcium incorporated graphene oxide have a morphology similar to an ultra-thin membrane composed of overlapping sheets. X-ray diffraction and Fourier-infrared spectroscopy show that calcium carbonate residue was completely removed by hydrochloric acid washes. Energy dispersive X-ray spectroscopy mapping showed spatially homogeneous calcium in Ca-incorporated graphene oxide sample after HCl washing. This Ca is mainly ionic according to X-ray photoelectron spectroscopy, and its incorporation promoted a small reduction in the graphene oxide structure, corroborated also by four-point probe measurements. A thermal study shows a remarkable increase in the GO stability with the presence of Ca 2 + ions.

Highly oriented crystalline hybrid thin films primarily consisting of Magneli-phase VO2 and conductive graphene nanoplatelets are fabricated by a sol–gel process via dipping pyrolysis. A combination of chemical, microstructural, and electrical analyses reveals that graphene oxide (GO)-templated vanadium oxide (VOx) nanocomposite films exhibit a vertically stacked multi-lamellar nanostructure consisting of horizontally aligned vanadium oxide nanowire (VNW) arrays along the (hk0) set of planes on a GO template, with an average crystallite size of 41.4 A and a crystallographic tensile strain of 0.83%. In addition, GO-derived VOx composite semiconducting films, which have an sp3/sp2 bonding ratio of 0.862, display thermally induced electrical switching properties in the temperature range of − 20 °C to 140 °C, with a transition temperature of approximately 65 °C. We ascribe these results to the use of GO sheets, which serve as a morphological growth template as well as an electrochemically tunable platform for enhancing the charge-carrier mobility. Moreover, the experimental studies demonstrate that graphene-based Magneli-phase VOx composite semiconducting films can be used in advanced thermo-sensitive smart sensing/switching applications because of their outstanding thermo-electrodynamic properties and high surface charge density induced by the planar-type VNWs.

Due to its atomic thickness (thinness), the wear of graphene in nanoscale devices or as a protective coating is a serious concern. It is highly desirable to develop effective methods to reduce the wear of graphene. In the current paper, the effect of a nano-lubricant, perfluoropolyether, on the wear of graphene on different substrates is investigated. Graphene was synthesized by chemical vapor deposition (CVD) and characterized by Raman spectroscopy. The nano-lubricant is applied on the graphene by dip-coating. The friction and wear of graphene samples are characterized by nanotribometer, AFM, optical microscopy and Raman spectroscopy. The results showed that lubricating silicon/graphene with nano-lubricant reduces the friction but increases the wear. However, lubricating nickel/graphene with nano-lubricant has little effect on the friction but reduce the wear significantly. The underlying mechanism has been discussed on the basis of the graphene–substrate adhesion and the roughness. The current study provides guidance to the future design of graphene-containing devices.

FeCo-reduced graphene oxide (FeCo-RGO) composite film was fabricated on indium tin oxide substrate using one-step electrodeposition method. Raman spectroscopy and field emission scanning electron microscope results show that the reduced graphene oxide is coprecipitated with the FeCo film. The energy-dispersive spectrometer results demonstrate that the atomic ratio of Fe/Co in FeCo-RGO composite film is larger than that of the FeCo film under the same fabrication condition. As a result, the FeCo-RGO composite film exhibits good soft magnetic properties and high-frequency properties as well as the FeCo film. The magnetic anisotropy field and saturation magnetization of FeCo-RGO composite film are increased when compared with FeCo film. Furthermore, the ferromagnetic resonance frequency is improved from 2.15 GHz for the FeCo film to 3.9 GHz for the FeCo-RGO composite film.

Graphene/silicon heterostructures present a Schottky characteristic and have potential applications for solar cells and photodetectors. Here, we fabricated graphene/silicon heterostructures by using chemical vapor deposition derived graphene and n-type silicon, and studied the electronic and optoelectronic properties through varying their interface and silicon resistivity. The results exhibit that the properties of the fabricated configurations can be effectively modulated. The graphene/silicon heterostructures with a Si (111) interface and high resistivity show a better photovoltaic behavior and should be applied for high-performance photodetectors. With the combined atomic force microscopy and theoretical analysis, the possible origination is discussed. The work here should be helpful on exploring high-performance graphene/silicon photoelectronics.

In this work, we have synthesized Graphene Oxide (GO) using modified Hummers method and investigated its electrical properties using parallel plate impedance spectroscopic technique. Graphene Oxide films were prepared using drop casting method on Indium Tin Oxide (ITO) coated glass substrate. Atomic force microscopy was used to characterize the films' microstructure and surface topography. Electrical characterization was carried out using LCR meter in frequency regime (100 Hz to 10 MHz) at different temperatures. AC conductivity σac of the films was observed to be varied with angular frequency, ω as ωS, with S