铜基板上CVD法生长单晶石墨烯及研究现状

石墨烯是一种以SP2键结合的二维碳的同素异形体,其独一无二的优异性能,使得其在过去几十年里受到了石墨烯研究工作者的极大兴趣。但石墨烯不同于自然界的石墨,并且受限于小尺寸和低产率。化学气相沉积法(CVD)的出现解决了这些问题,并逐渐发展为一种规模生产大面积、大尺寸、多应用石墨烯的重要方法。但化学气相沉积法生长石墨烯是多晶石墨烯并且由于晶界会产生降解性能。因此,石墨烯生长研究的下一个关键问题是如何让大晶粒单晶石墨烯生长。本文主要叙述了4种代表性预处理铜基板来生长毫米级单层石墨烯的方法:电化学抛光后高温退火、盒状铜箔基板、融化再结晶成新的铜基板、让铜基板富氧。以及现在发展的石墨烯晶粒的特殊空间结构,这些特殊晶粒包括雪花、六瓣鲜花、金字塔和六角形的石墨烯洋葱圈形状。综述了利用不同预处理铜基板的工艺得到毫米级单晶石墨烯的方法。尽管CVD生长单晶石墨烯已经有了空前的进步,但仍然有潜在的挑战,例如,晶元尺寸单晶石墨烯的生长和器件的制作,以及对石墨烯生长机制和生长动力学的进一步了解。     

Crystallographic etching of few-layer graphene

We demonstrate a method by which few-layer graphene samples can be etched along crystallographic axes by thermally activated metallic nanoparticles. The technique results in long (>1 microm) crystallographic edges etched through to the insulating substrate, making the process potentially useful for atomically precise graphene device fabrication. This advance could enable atomically precise construction of integrated circuits from single graphene sheets with a wide range of technological applications.     

石墨烯片的制备与表征

通过微机械剥离高定向热解石墨(HOPG)法和化学气相沉积法(CVD)分别制备了不同层数的石墨烯片,并将其转移到硅片上.利用石墨烯片在不同厚度SiO2硅片上光学显微图像颜色及对比度存在的差异,对其层数进行了识别与区分.采用原子力显微镜(AFM)和拉曼(Raman)光谱判定了所制石墨烯片的层数.结果表明:所制石墨烯片有单层、少数层和多层.与双层石墨烯片的Raman谱图比较,多层石墨烯片的2D模线宽变宽,G模强度增大.此外,CVD法可生长出大面积(～cm2)的石墨烯片.     

Hydrothermally grown ZnO nanostructures on few-layer graphene sheets

This study describes the hydrothermal growth of ZnO nanostructures on few-layer graphene sheets and their optical and structural properties. The ZnO nanostructures were grown on graphene sheets of a few layers thick (few-layer graphene) without a seed layer. By changing the hydrothermal growth parameters, including temperature, reagent concentration and pH value of the solution, we readily controlled the dimensions, density and morphology of the ZnO nanostructures. More importantly, single-crystalline ZnO nanostructures grew directly on graphene, as determined by transmission electron microscopy. In addition, from the photoluminescence and cathodoluminescence spectra, strong near-band-edge emission was observed without any deep-level emission, indicating that the ZnO nanostructures grown on few-layer graphene were of high optical quality.     

Synthesis of graphene together with undesired CuxO nanodots on copper foils by low-pressure chemical vapor deposition

The synthesis of graphene on Cu foils has been carried out using a low-pressure chemical vapor deposition (LPCVD) process. Under certain growth conditions apart from the graphene flakes, undesired Cu_xO nanodots appear. The samples were characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray photoemission spectroscopy. On the basis of the results, we investigated the effect of growth parameters such as pressure, methane-to-hydrogen ratio and cooling atmosphere on the growth rate, the composition, especially the cleanliness of graphene by scanning electron microscopy in detail. It is shown that the obtained film is quite sensitive on the preparation conditions and the appearance of Cu_xO nanodots is preventable.     

Direct imaging of atomic-scale ripples in few-layer graphene

Graphene has been touted as the prototypical two-dimensional solid of extraordinary stability and strength. However, its very existence relies on out-of-plane ripples as predicted by theory and confirmed by experiments. Evidence of the intrinsic ripples has been reported in the form of broadened diffraction spots in reciprocal space, in which all spatial information is lost. Here we show direct real-space images of the ripples in a few-layer graphene (FLG) membrane resolved at the atomic scale using monochromated aberration-corrected transmission electron microscopy (TEM). The thickness of FLG amplifies the weak local effects of the ripples, resulting in spatially varying TEM contrast that is unique up to inversion symmetry. We compare the characteristic TEM contrast with simulated images based on accurate first-principles calculations of the scattering potential. Our results characterize the ripples in real space and suggest that such features are likely common in ultrathin materials, even in the nanometer-thickness range.     

Facile preparation of nitrogen-doped few-layer graphene via supercritical reaction

To achieve the applications of graphene, the modulation of its electrical properties is of great significance. The element doping might give a promising approach to produce fascinating properties of graphene. Herein we report a facile chemical doping method to obtain nitrogen-doped (N-doped) few-layer graphene sheets through supercritical (SC) reaction in acetonitrile at temperature as low as 310 °C, using expanded graphite as starting material. X-ray photoelectron spectroscopy analysis revealed that the level of nitrogen-doping (N-doping) increased from 1.57 to 4.56 at % when the reaction time was tuned from 2 to 24 h. Raman spectrum confirmed that the resulting N-doped few-layer graphene by SC reaction maintain high quality without any significant structural defects. Electrical measurements indicated that N-doped few-layer graphene sheets exhibit a typical n-type field-dependent behavior, suggesting the N-doping into the lattice of graphene. This work provides a convenient chemical route to the scalable production of N-doped graphene for potential applications in nanoelectronic devices.     

Spatially resolved Raman spectroscopy of single- and few-layer graphene

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry.     

Direct laminating of silver foils on copper substrates

A novel laminating process of Ag foils on Cu substrates is successfully developed. The Ag foil is 280 μm thick. After slight polishing to obtain shining surface, the Ag foil is laminated to Cu substrate with a static pressure of 260 psi at 400 °C in 50 millitorr vacuum to suppress metal oxidation. No bonding medium is used. The Ag foil is directly bonded to the Cu substrate. Laminated samples are cut into halves for cross-section examination using scanning electron microscopy (SEM). Nearly void-free bonding is achieved. The Ag–Cu bonding interface is very sharp. This metal-to-metal lamination technique can be applied to various electronic packages. Ag is much ductile than Cu. The yield strength of Ag is only one-tenth of Cu. Ag layer on Cu substrate functions as a buffer to absorb thermal expansion mismatch between semiconductor chips and Cu substrates through plastic strain. Progress is underway to refine the laminating process.     

基于乙烯的化学气相沉积法制备少层石墨烯

采用乙烯作为碳源,利用化学气相沉积法(CVD法),在1 000℃条件下在铜箔上制备了少层石墨烯;采用不引入PMMA和PDMS等杂质的直接转移方法将石墨烯薄膜转移到Si/SiO2基底上,对石墨烯薄膜进行了表征。实验研究表明,减小乙烯的进气量可以提高石墨烯的质量;减少反应时间可以降低无定形碳的含量;增加退火时间可以提高Cu表面结晶质量,更加有利于石墨烯的生长。通过优化各项参数,使用乙烯已经可以制备I2D/IG=0.88的少层石墨烯。     

Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes

We report on a method to fabricate and measure gateable molecular junctions that are stable at room temperature. The devices are made by depositing molecules inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. The molecular junctions display gateable I-V-characteristics at room temperature.     

The preparation and properties of single crystal copper phosphide

Journal of Materials Science -     

Plastic deformation of copper thin foils

According to one suggested model, bending of a single crystal introduces edge dislocations of the same sign. In the present study, this model is examined by computer simulation using molecular dynamics. When a notch is present on the tension surface, Heidenreich-Shockley partial dislocations are created near the tip of the notch. In the compression surface, partial dislocations are created due to wrinkling of the crystal plane. The results of simulation shows that dislocations are more easily created in a compressive bending region than in a tension bending region or simple tension region. For shear deformation, partial dislocations are created on the highest resolved shear stress slip plane {1 1 1} and slip in the direction of highest resolved shear stress.     

Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour deposition

If graphene is ever going to live up to the promises of future nanoelectronic devices, an easy and cheap route for mass production is an essential requirement. A way to extend the capabilities of plasma-enhanced chemical vapour deposition to the synthesis of freestanding few-layer graphene is presented. Micrometre-wide flakes consisting of four to six atomic layers of stacked graphene sheets have been synthesized by controlled recombination of carbon radicals in a microwave plasma. A simple and highly reproducible technique is essential, since the resulting flakes can be synthesized without the need for a catalyst on the surface of any substrate that withstands elevated temperatures up to 700?°C. A thorough structural analysis of the flakes is performed with electron microscopy, x-ray diffraction, Raman spectroscopy and scanning tunnelling microscopy. The resulting graphene flakes are aligned vertically to the substrate surface and grow according to a three-step process, as revealed by the combined analysis of electron microscopy and x-ray photoelectron spectroscopy.     

Spinodal decomposition of mono- to few-layer graphene on Ni substrates at low temperature

Mono to few-layer graphene were prepared on pre-annealed polycrystalline nickel substrates by chemical vapor deposition at a relatively low temperature of 800 degrees C using fast cooling rate. It was observed that the reduced solubility of Carbon in Ni at low temperature and an optimum gas mixing ratio (CH4:H2 = 60/80 (sccm)) can be used to synthesize mano-layer graphene that covers about 100 microm2 area. The number of graphene layers strongly depends upon the hydrogen and methane flow rates. An increase in the methane flow is found to increase the growth density of the single-layer graphene. The number of graphene layers was identified from micro-Raman spectra. The thinnest areas containing mono-layer graphene formed at small Ni grains surrounded by large Ni Grains can be explained in terms of Spinodal decomposition. Scanning tunneling microscopy observations of the graphene samples indicate that the graphene structure exhibits no defects, and extremely symmetry hexagon carbon at flat graphene surface is observed.