Laser scribing on HOPG for graphene stamp printing on silicon wafer

Highly oriented pyrolytic graphite (HOPG) was scribed by pulsed laser beam to produce square patterns. Patterning of HOPG surface facilitates the detachment of graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted stamp printing of a few-layers graphene was compared. Printed graphene sheets were visualized by optical and scanning electron microscopy. The number of graphene layers was measured by atomic force microscopy. Glue-assisted stamp printing allows printing relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method is easier to implement and is more flexible than the majority of existing ways of placing graphene sheets onto a substrate.     

Scanning probe microscopy study of exfoliated oxidized graphene sheets

Exfoliated oxidized graphene (OG) sheets, suspended in an aqueous solution, were deposited on freshly cleaved HOPG and studied by ambient AFM and UHV STM. The AFM images revealed oxidized graphene sheets with a lateral dimension of 5–10 μm. The oxidized graphene sheets exhibited different thicknesses and were found to conformally coat the HOPG substrate. Wrinkles and folds induced by the deposition process were clearly observed. Phase imaging and lateral force microscopy showed distinct contrast between the oxidized graphene and the underlying HOPG substrate. The UHV STM studies of oxidized graphene revealed atomic scale periodicity showing a (0.273 ± 0.008) nm × (0.406 ± 0.013) nm unit cell over distances spanning few nanometers. This periodicity is identified with oxygen atoms bound to the oxidized graphene sheet. I(V) data were taken from oxidized graphene sheets and compared to similar data obtained from bulk HOPG. The dI/dV data from oxidized graphene reveals a reduction in the local density of states for bias voltages in the range of ±0.1 V.     

Few-layer graphene film fabricated by femtosecond pulse laser deposition without catalytic layers

The films of few-layer graphene are formed through laser exfoliation of a highly ordered pyrolytic graphite(HOPG), without a catalytic layer for the growth process. The femtosecond(fs) laser exfoliation process is investigated at different laser fluences and substrate temperature. For fs laser exfoliation of HOPG, the few-layer graphene is obtained at 473 K under an optimal laser fluence. The formation of few-layer graphene is explained by removal of intact graphite sheets occurred by an optimal laser fluence ablation. The new insights may facilitate the controllable synthesis of large area few-layer graphene.     

Folding and cracking of graphene oxide sheets upon deposition

Graphene Oxide (GO) sheets, suspended in an aqueous solution, were deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG) and studied using Raman spectroscopy, atomic force microscopy (AFM) and scanning tunneling microscopy (STM). AFM phase imaging shows a distinct contrast between GO and the underlying HOPG substrate. Raman spectroscopy clearly showed the presence of GO sheets on the top of HOPG substrate. The AFM and STM images also reveal wrinkling, folding, and tearing of individual GO sheets after depositing onto an HOPG substrate. We have also observed a distinct cracking of a GO sheet after folding. We attribute this new cracking phenomenon to a weakening of C–C bonds during the oxidation of a graphene sheet.     

Diamond-to-graphite conversion in nanodiamond and the electronic properties of nanodiamond-derived carbon system

Heat-treatment-induced conversion of nanodiamond to nanographite is investigated. Graphitization starts at the surface region around a heat-treatment temperature of 900°C, then it proceeds inward in the particle, and finally it is completed around 1600°C, where nanographite particles form a polyhedron with a hollow inside. The change in the electronic feature is subjected to the structural change induced by the heat treatment. In the intermediate stage of graphitization, where graphene sheets are small and defective, charge transfer takes place from graphitic π-band to nonbonding edge states. Electrophoretic deposition of nanodiamond particles provides a technique for fabricating isolated single nanodiamond particles on a substrate. Successive heat treatment at 1600°C converts a nanodiamond particle to a single nanographene sheet laying flat on a highly oriented pyrolytic graphite substrate. Weak interaction between the nanographene sheet and the substrate is expected to give a model system of nanographene, for which theory predicts the presence of nonbonding \gv-electron states of the edge origin and its related unconventional nanomagnetism.     

Electronic and structural properties of graphene-based transparent and conductive thin film electrodes

We demonstrate that graphene-based transparent and conductive thin films (GTCFs), fabricated by thermal reduction of graphite oxide, have very similar electronic and structural properties as highly oriented pyrolytic graphite (HOPG). Electron spectroscopy results suggest that the GTCFs are also semi-metallic and that the individual graphene sheets of the film are predominantly oriented parallel to the substrate plane. These films may therefore be considered as a technologically relevant analogue to HOPG electrodes, which cannot be easily processed into thin films.     

Giant Faraday and Kerr rotation with strained graphene

Polarized electromagnetic waves passing through (reflected from) a dielectric medium parallel to a magnetic field undergo Faraday (Kerr) rotation of their polarization. Recently, Faraday rotation angles as much as 0.1?rad were observed for terahertz waves propagating through graphene over a SiC substrate. We show that the same effect is observable with the magnetic field replaced by an in-plane strain field which induces a pseudomagnetic field in graphene. With two such sheets a rotation of π/4 can be achieved, which is the required rotation for an optical diode. Similarly a Kerr rotation of 1/4 rad is predicted from a single reflection from a strained graphene sheet.     

扫描隧道显微镜研究石墨表面的莫尔图

利用扫描隧道显微镜研究石墨表面的大尺度周期性图样．研究结果表明,莫尔图起源于石墨深层的缺陷,实验结果与理论完全吻合,并且第一次在实验上证明了纳米波可以穿透多层石墨而没有明显衰减．     

Neutron polarization induced by interfering pseudospins between two graphene sheets inserted into a cavity

We investigate the neutron polarization induced by interfering pseudospins between two graphene sheets inserted into a cavity with a circularly polarized photon field. We find that only when two graphene sheets are located at the coordinate positions of some antinodes of standing photon field, the neutron polarization is most remarkable. This is caused by an interference effect of pseudospins between two graphene sheets induced by photons and neutron. This neutron polarization mechanism has potential application on neutron polarization device.     

Sonochemical synthesis of TiO(2 nanoparticles on graphene for use as photocatalyst

Using ultrasonication we succeed in a controlled incorporation of TiO(2) nanoparticles on the graphene layers homogeneously in a few hours. The average size of the TiO(2) nanoparticles was controlled at around 4-5 nm on the sheets without using any surfactant, which is attributed to the pyrolysis and condensation of the dissolved TiCl(4) into TiO(2) by ultrasonic waves. The photocatalytic activity of the resultant graphene-TiO(2) composites containing 25 wt.% TiO(2) is better than that of commercial pure TiO(2). This is partly due to the extremely small size of the TiO(2) nanoparticles and partly due to the graphene-TiO(2) composite structure consisting of homogeneous dispersion of crystalline TiO(2) nanoparticles on the graphene sheets. As the graphene in the composites has a very good contact with the TiO(2) nanoparticles it enhances the photo-electron conversion of TiO(2) by reducing the recombination of photo-generated electron-hole pairs.     

Magnetism of nanographene network influenced by the interaction with acid species

The magnetism of nanographite (stacked nanographene sheets)-based nanoporous carbon is investigated in relation to the interaction with acid guest species. The concentration of the localized spins of non-bonding π-electron state (edge state) localized in the nanographene edges decreases upon the sulfonation of nanographene edges through charge-transfer interaction with sulfonic groups. The sulfonation of nanographene edges enhances the hydrophilic nature of the edges, resulting in the easiness in the water adsorption into the nanopores. This enhances the mechanical compression effect of water molecules condensed in the nanopores on the nanographite domains, resulting in the decrease in the spin concentration. The change in the magnetism upon water uptake reveals ferrimagnetic nature of individual nanographene sheets. The adsorption of HCl having no oxidation ability shows a mechanical effect on the edge-state spins similar to water adsorption. The spin concentration is reduced in two-step manner by the charge-transfer interaction with guest concentrated HNO₃ that is strong oxidant. In the presence of H₂O molecules in diluted HNO₃ the cooperation of mechanical and charge-transfer interactions creates also a two-step change in the magnetism.     

Theoretical understanding of adlayer structure,thermal stability and electronic property of graphene molecules

The geometry of hexafluorotribenzo[a,g,m]coronene with n-carbon alkyl chains [FTBC-Cn (n = 4, 6, 8, 12)] and their supramolecule self-assembly on a highly oriented pyrolytic graphite (HOPG) surface has been optimized by molecular dynamics simulations using COMPASS force field at 0 K, 298 K, 333 K and 353 K. Electronic properties and intermolecular interactions in graphene supramolecule assembly have been studied by the first principle methods based on the density functional theory (DFT). It is indicated that the thermal stability and electronic properties of graphene molecules can be tunable by attaching alkyl chains to a triangular graphene sheet, and changing the length of the alkyl chain, and self-assembling on a certain substrate. The main results are as follows. The geometry and energy gap of the FTBC-Cn single molecule and their supramolecule self-assembly on HOPG are both stable with the changes of the temperature from 0 K to 353 K and the number of carbon atoms on the alkyl chain. The simulation results of geometry, energy gap as well as STM images of graphene supramolecule assembly are in good agreement with the corresponding experimental results in room temperature. Furthermore, the electronic properties of graphene supramolecule assembly at the temperatures of 0 K, 333 K and 353 K are also predicted. When a triangular graphene molecule attached with six alkyl chains, the energy gaps are increased and stabilized at the temperature from 0 K to 353 K. After FTBC-Cn molecule self-assembly on a HOPG substrate, the energy gap is reduced but still stable.     

基于石墨烯的太赫兹波散射可调谐超表面

设计了一个可调谐的太赫兹超表面,由在随机反射超表面基底中嵌入可偏置的双层石墨烯构成,可以实现对太赫兹波散射特性的动态调控.全波仿真试验结果证实了所预期的超表面散射可调性能.通过增大偏置电压提升石墨烯的费米能级,使得该超表面的太赫兹波散射样式从漫反射逐渐向镜面反射过渡,从而实现散射特性的连续调控,且该超表面具有对电磁波极化角度不敏感的特点.这些特性使得该超表面能很好地融合到变化的环境中,在太赫兹隐身方面具有潜在的应用价值.     

Current transport across the pentacene/CVD-grown graphene interface for diode applications

We investigate the electronic transport properties across the pentacene/graphene interface. Current transport across the pentacene/graphene interface is found to be strikingly different from transport across pentacene/HOPG and pentacene/Cu interfaces. At low voltages, diodes using graphene as a bottom electrode display Poole–Frenkel emission, while diodes with HOPG and Cu electrodes are dominated by thermionic emission. At high voltages conduction is dominated by Poole–Frenkel emission for all three junctions. We propose that current across these interfaces can be accurately modeled by a combination of thermionic and Poole–Frenkel emission. Results presented not only suggest that graphene provides low resistive contacts to pentacene where a flat-laying orientation of pentacene and transparent metal electrodes are desired but also provides further understanding of the physics at the organic semiconductor/graphene interface.     

Quantum friction controlled by plasmons between graphene sheets

We theoretically study quantum friction between two infinite graphene sheets, which is controlled by plasmons excited at the interfaces of graphenes and dielectrics. In near-field regime, quantum friction can be enhanced due to the coupling of plasmons between two graphene sheets. Dependences of friction coefficient on distance, chemical potential of graphene, temperature of environment, and dielectric constant of substrate have been investigated in detail. Friction coefficient can be increased by increasing temperature or dielectric constants of substrates, and can be reduced by increasing distance or chemical potential.     

Quantum friction

We investigate the van der Waals friction between graphene and an amorphous SiO(2) substrate. We find that due to this friction the electric current is saturated at a high electric field, in agreement with experiment. The saturation current depends weakly on the temperature, which we attribute to the quantum friction between the graphene carriers and the substrate optical phonons. We calculate also the frictional drag between two graphene sheets caused by van der Waals friction, and find that this drag can induce a voltage high enough to be easily measured experimentally.     

硅晶体表面石墨烯褶皱形貌的分子动力学模拟研究

本文利用分子动力学的方法和模拟退火技术从原子尺度分析研究了Si (100), Si (111)和Si (211)表面单原子层石墨烯的褶皱形貌及其演化特点. 研究表明, 分别置于Si晶体的三种不同原子表面的石墨烯都展现出原子尺度的褶皱形貌. 石墨烯与Si晶体表面原子的晶格失配是引起石墨烯褶皱的主要原因. 研究发现, Si晶体表面石墨烯的褶皱形貌强烈的依赖于退火温度. 石墨烯的褶皱形貌还将直接影响其在Si晶体表面的吸附稳定性. 这些研究结果有助于人们认识基于Si晶体衬底的石墨烯的结构形貌及其稳定性, 为石墨烯的进一步应用提供理论参考.     

Comparative study on graphene growth mechanism using Ni films,Ni/Mo sheets,and Pt substrates

We demonstrate a comparative study on graphene growth mechanism using various catalytic metal substrates such as Ni thin films, Ni-deposited Mo (Ni/Mo) sheets, and Pt sheets during chemical vapor deposition (CVD). Depending on the substrates, two kinds of graphene growth mechanisms that involve either precipitation or surface adsorption of carbon have been reported. We synthesized graphene, focusing especially on the initial growth stage during CVD, by varying synthesis parameters such as synthesis time, amount of feedstock, and cooling rate after synthesis. We concluded that precipitation-driven synthesis is dominant in the case of Ni substrates whereas adsorption-driven growth is dominant in the Ni/Mo system. In the case of the Pt substrate, which is generally believed to grow by carbon precipitation, graphene growth by adsorption was found to be dominant. We believe that our results will contribute to a clearer understanding of the graphene synthesis mechanism, and development of manufacturing routes for controllable synthesis of high-quality graphenes.     

Comparison of Goos–Hänchen shifts of the reflected beam from graphene on dielectrics and metals

The Goos–Hänchen shifts of the reflected beam from graphene-on-dielectric (or metal) in the optical wavelength are investigated by using the stationary-phase method. For the graphene-on-dielectric substrates, it is found that the pseudo-Brewster angle and Goos–Hänchen shift are influenced greatly by the introduced graphene sheets for TM polarization. By changing number of graphene sheets, the lateral shifts can be large positive or negative near the pseudo-Brewster angle. For TE polarization, the lateral shift is still small; however it can also be positive or negative by changing the number of graphene sheets. For the graphene-on-metal substrates, graphene sheets exert a great impact on the reflectance while has little effect on the lateral shifts of both polarizations. Finally, the role of the graphene sheets on the lateral shifts for the different visible wavelengths is discussed.     

纳米微结构表面与石墨烯薄膜的界面黏附特性研究

石墨烯性能的发挥受石墨烯表面形貌的影响,而石墨烯表面形貌则与基底密切相关.石墨烯在纳米微结构表面的吸附与剥离可以为石墨烯的功能化制备和转移提供理论基础.分子动力学模拟能提供石墨烯在纳米微结构表面的吸附构型和剥离特性等详细信息,可以弥补实验的不足.本文利用LAMMPS分子动力学模拟软件,从吸附能角度研究了石墨烯在矩形微结构表面的黏附特性,并进一步探讨了石墨烯从矩形微结构表面剥离的行为.研究表明:石墨烯的吸附构型在矩形微结构表面的转变是连续的,但由部分贴合状态向完全贴合状态的转变是一个反复的过程,当石墨烯完全贴合微结构表面时吸附能最大;从微结构表面剥离石墨烯时,剥离力会出现周期性的波动.剥离过程表现为两种形式:完全贴合时,石墨烯是直接滑过槽底;而当悬浮构型或部分贴合构型时,石墨烯是直接从微结构表面分离.本文给出了平均剥离力随微结构尺寸参数变化的理论公式,该公式与模拟结果拟合较好.此外,随着剥离角度的变大,平均剥离力先变大后变小,从平整基底表面剥离具有Stone-Wales缺陷结构的石墨烯会使剥离力变大.研究结果可为探究石墨烯在纳米微结构表面的剥离行为、揭示其黏附机理提供理论参考.