石墨烯在GaN表面的直接制备及其在GaN-LED中的应用研究

石墨烯具有优良的光电特性,它能够替代传统的ITO材料用作GaN-LED的透明导电层。为了使上述应用实现工业化生产,利用热壁CVD研究了石墨烯在GaN表面直接生长的最佳条件,解释了直接生长的机理,直接生长的最佳条件为生长温度800℃,生长时间60 min,CH_4和H_2的分压比分别为1.59%和3.17%,该条件下得到具有明显2D峰的多层石墨烯。利用冷壁CVD研究了石墨烯在GaN表面的低温生长并制造了相应的GaN-LED,测试了其性能,结果表明生长温度高于700℃时器件的性能明显降低。该研究对实现石墨烯在LED中的工业化应用具有积极意义。     

化学气相沉积法制备大面积石墨烯及其光谱学表征

化学气相沉积（CVD）法是近年来发展起来的制备石墨烯的新方法。该方法产物具有生长面积大、质量高等优点,逐渐成为制备石墨烯的主要方法。用CVD法在常压下通过全面优化实验参量,以镍箔为基底制备了大面积少数层和单层石墨烯,用拉曼光谱,场发射扫描电子显微镜（SEM）和原子力显微镜（AFM）手段表征,通过分析常压下不同温度、不同载气成分比等实验参数,最终获得制备高质量、大面积、少数层石墨烯的最佳参量,用双共振理论解释少数层和单层石墨烯的拉曼光谱中2D峰强度随石墨烯层数变化而变化的原理。CVD法制备的石墨烯具有面积大、低成本、可测量性强、可用于大批量生产的优点,为工业用途石墨烯的制备提供了有效途径。     

三嗪对CVD石墨烯n型掺杂的研究

以化学气相沉积(CVD)制备的单层石墨烯为原料, 小分子三嗪为掺杂剂, 采用吸附掺杂的方式, 在低温下对石墨烯实现n型掺杂。利用拉曼光谱(Raman)、X射线光电子能谱分析(XPS)、原子力显微镜(AFM)、紫外分光光度计(UV)和霍尔效应测试仪(Hall)对样品的形貌、结构及电学性能进行表征。结果表明: 该方法简单安全, 能够对石墨烯实现均匀的n型掺杂, 掺杂石墨烯的透光率达到95%。掺杂后石墨烯的特征峰G峰和2D峰向高波数移动。掺杂180 min后, 载流子浓度达到4×10¹²/cm², 接近掺杂前的载流子浓度, 掺杂后的石墨烯在450℃的退火温度下具有可逆能力, 其表面电阻在300℃以下具有较好的稳定性。     

基于激光选区熔化成形Ni-Cu合金模板的Ni-Cu-石墨烯复合材料的制备

采用优化的SLM成形参数,用激光选区熔化(SLM)增材制造技术制备了三维Ni-Cu合金。使用三维Ni-Cu合金基底材料用化学气相沉积法(CVD)制备Ni-Cu合金/石墨烯复合材料,研究了CVD法生长反应温度对石墨烯结构的影响并分析其原因。结果表明,石墨烯层的厚度随着反应温度的提高而减小。与未生长石墨烯的样品相比,在100℃石墨烯复合使复合材料的热扩散系数提高了12.5%。用SLM增材制造技术和金属模型结构设计成形三维Ni-Cu合金,实现了对石墨烯片层取向的控制,结合CVD法优化在Ni-Cu合金表面生长石墨烯工艺可调控石墨烯的结构。     

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.     

MPCVD工艺参数对石墨烯性能影响的研究

实验采用MPCVD装置，以氢气和甲烷为主要气源，氮气和氩气为辅助气源在镍片上生长石墨烯薄膜，并对不同条件下制备样品进行拉曼光谱仪表征，通过拉曼光谱图中D峰和D′峰峰强来分析石墨烯缺陷含量；2D峰峰强和半高宽来分析薄膜层数。结果显示氮气等离子体离解率低，会增加成膜缺陷不利于成膜；氩气离解率较高，适量的氩气会减少缺陷含量提高膜层质量；较低功率会加速石墨的沉积，较高功率会增加sp3杂化的碳碳键的形成。     

球状纳米二氧化钛/石墨烯复合材料的合成及导电性能

采用改进的水热法制备二氧化钛/石墨烯(TiO2/G)复合导电材料,并研究水热温度以及石墨烯用量对TiO2/G复合材料导电性的影响。利用傅里叶变换红外(FTIR)光谱、X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)和电化学阻抗谱等测试手段对复合材料的结构,微观形貌以及导电性能进行表征,并确定最佳的水热温度以及石墨烯的最佳添加量。结果表明:石墨烯添加量为5%(质量分数),水热温度为160℃,TiO2/G复合材料的导电性最佳,其电阻率为13.46Ω·cm。复合材料中TiO2纳米颗粒为球状的锐钛矿相,直径为100~200nm左右,且均匀生长在石墨烯片层表面。其中,TiO2纳米颗粒生长于石墨烯片层上,有效地阻止石墨烯片层的聚集,有利于石墨烯片层间形成导电网络,提高电子迁移效率,赋予二氧化钛复合材料优异的导电性能。     

化学气相沉积制备大面积高质量石墨烯的研究进展

石墨烯是由单层碳原子紧密堆积形成的一种碳质新材料,具有优良的电学、光学、热学及力学等性质。在众多的石墨烯制备方法中,化学气相沉积(Chemical vapor deposition,CVD)最有可能实现大面积、高质量石墨烯的可控制备。综述了CVD方法制备大面积、高质量石墨烯的影响因素,包括衬底、碳源及生长条件(气体流量、生长温度、等离子体功率、生长压强、沉积时间、冷却速率等)。最后展望了CVD方法制备石墨烯的发展方向。     

低温一步制备氧化石墨烯及微波还原研究

以天然鳞片石墨为原料,通过低温一步氧化制备氧化石墨烯,经微波热还原得到低缺陷的还原氧化石墨烯。讨论了低温氧化过程中氧化剂用量、氧化时间对氧化石墨烯层间距、氧化程度的影响。结果表明:在高锰酸钾与天然鳞片石墨的质量比为1∶3,氧化温度为0℃,氧化时间为48h的条件下,制备出碳氧原子比为1.98、高C—O结构、低缺陷结构( I D∶ I G=0.63)的氧化石墨烯,避免了Hummers制备过程中由于CO 2的形成导致六元环断裂以及碳原子的缺失而使得氧化石墨烯的缺陷增加;经微波热还原后,得到的还原氧化石墨烯的两点平均缺陷距离 L D=12nm,缺陷密度 n D=2.21×10 11 cm -2 , I D∶ I G仅为0.85(Γ G=32.1cm -1 ),制备出低缺陷的还原氧化石墨烯。     

拉曼光谱表征石墨烯结构的研究进展

石墨烯是一种只有一个原子层的二维原子晶体,它是构成零维富勒烯、一维碳纳米管和三维石墨等其他碳同素异形体的基本结构单元,具有很多独特的电子及力学性能,因而吸引了化学、材料及其他领域众多科学家的高度关注。拉曼光谱作为一种灵敏便捷的表征方法,在石墨烯的研究中起到重要的作用。该综述总结了近年来拉曼光谱在石墨烯表征中的应用,在对单层石墨烯的典型特征峰作详细介绍的基础上,通过对拉曼谱图中D峰、G峰和2D峰的强度、位置和半峰宽变化情况的分析,可以快速而准确地表征出石墨烯的层数,并可以对石墨烯的堆垛方式、边缘手性和掺杂程度进行判定。同时,也系统地分析了在石墨烯制备与测试过程中基底、掺杂、温度和激光功率等因素对拉曼谱图中D峰、G峰和2D峰的强度、位置和半峰宽的影响。     

Understanding Interlayer Contact Conductance in Twisted Bilayer Graphene

Bilayer or few‐layer 2D materials showing novel electrical properties in electronic device applications have aroused increasing interest in recent years. Obtaining a comprehensive understanding of interlayer contact conductance still remains a challenge, but is significant for improving the performance of bilayer or few‐layer 2D electronic devices. Here, conductive atomic force microscope (C‐AFM) experiments are reported to explore the interlayer contact conductance between bilayer graphene (BLG) with various twisted stacking structures fabricated by the chemical vapor deposition (CVD) method. The current maps show that the interlayer contact conductance between BLG strongly depends on the twist angle. The interlayer contact conductance of 0° AB‐stacking bilayer graphene (AB‐BLG) is ≈4 times as large as that of 30° twisted bilayer graphene (t‐BLG), which indicates that the twist angle–dependent interlayer contact conductance originates from the coupling–decoupling transitions. Moreover, the moiré superlattice‐level current images of t‐BLG show modulations of local interlayer contact conductance. Density functional theory calculations together with a theoretical model reproduce the C‐AFM current map and show that the modulation is mainly attributed to the overall contribution of local interfacial carrier density and tunneling barrier.     

Critical Annealing Temperature for Stacking Orientation of Bilayer Graphene

It is rarely reported that stacking orientations of bilayer graphene (BLG) can be manipulated by the annealing process. Most investigators have painstakingly fabricated this BLG by chemical vapor deposition growth or mechanical means. Here, it is discovered that, at ≈600 °C, called the critical annealing temperature (CAT), most stacking orientations collapse into strongly coupled or AB‐stacked states. This phenomenon is governed (i) macroscopically by the stress generation and release in top graphene domains, evolving from mild ripples to sharp billows in certain local areas, and (ii) microscopically by the principle of minimal potential obeyed by carbon atoms that have acquired sufficient thermal energy at CAT. Conspicuously, evolutions of stacking orientations in Raman mappings under various annealing temperatures are observed. Furthermore, MoS₂ synthesized on BLG is used to directly observe crystal orientations of top and bottom graphene layers. The finding of CAT provides a guide for the fabrication of strongly coupled or AB‐stacked BLG, and can be applied to aligning other 2D heterostructures.     

聚丙烯腈/石墨烯纳米复合物的制备、表征及其热性能

采用以水和N,N-二甲基甲酰胺的混合溶剂作反应介质的沉淀聚合法制备了聚丙烯腈/石墨烯纳米复合物。利用傅里叶红外光谱(FT-IR)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和广角X射线衍射(XRD)研究了聚丙烯腈/石墨烯复合物的组成、结构、形貌及两组份的相互作用。利用差式扫描量热分析(DSC)研究了聚丙烯腈及纳米复合物的热性能。结果表明,强极性的聚丙烯腈与石墨烯之间存在较强的非共价相互作用;由于石墨烯的加入,聚丙烯腈的玻璃化转变温度提高了30℃;石墨烯添加量为3%(质量分数)时,聚丙烯腈在氮气和空气中的环化反应放热峰值分别提高了3和11℃;石墨烯使聚丙烯腈在热稳定化过程中的环化反应和氧化反应放热峰宽化、缓和。     

Twinning and twisting of tri- and bilayer graphene

The electronic, optical, and mechanical properties of bilayer and trilayer graphene vary with their structure, including the stacking order and relative twist, providing novel ways to realize useful characteristics not available to single layer graphene. However, developing controlled growth of bilayer and trilayer graphene requires efficient large-scale characterization of multilayer graphene structures. Here, we use dark-field transmission electron microscopy for rapid and accurate determination of key structural parameters (twist angle, stacking order, and interlayer spacing) of few-layer CVD graphene. We image the long-range atomic registry for oriented bilayer and trilayer graphene, find that it conforms exclusively to either Bernal or rhombohedral stacking, and determine their relative abundances. In contrast, our data on twisted multilayers suggest the absence of such long-range atomic registry. The atomic registry and its absence are consistent with the two different strain-induced deformations we observe; by tilting the samples to break mirror symmetry, we find a high density of twinned domains in oriented multilayer graphene, where multiple domains of two different stacking configurations coexist, connected by discrete twin boundaries. In contrast, individual layers in twisted regions continuously stretch and shear independently, forming elaborate Moiré patterns. These results, and the twist angle distribution in our CVD graphene, can be understood in terms of an angle-dependent interlayer potential model.     

The shear mode of multilayer graphene

The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. Few-layer graphene (FLG) flakes with less than ten layers each show a distinctive band structure. Thus, there is an increasing interest in the physics and applications of FLGs. Raman spectroscopy is one of the most useful and versatile tools to probe graphene samples. Here, we uncover the interlayer shear mode of FLGs, ranging from bilayer graphene (BLG) to bulk graphite, and suggest that the corresponding Raman peak measures the interlayer coupling. This peak scales from ~43 cm(-1) in bulk graphite to ~31 cm(-1) in BLG. Its low energy makes it sensitive to near-Dirac point quasiparticles. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions.     

Recent Advances in 3D Graphene Architectures and Their Composites for Energy Storage Applications

Graphene is widely applied as an electrode material in energy storage fields. However, the strong π–π interaction between graphene layers and the stacking issues lead to a great loss of electrochemically active surface area, damaging the performance of graphene electrodes. Developing 3D graphene architectures that are constructed of graphene sheet subunits is an effective strategy to solve this problem. The graphene architectures can be directly utilized as binder‐free electrodes for energy storage devices. Furthermore, they can be used as a matrix to support active materials and further improve their electrochemical performance. Here, recent advances in synthesizing 3D graphene architectures and their composites as well as their application in different energy storage devices, including various battery systems and supercapacitors are reviewed. In addition, their challenges for application at the current stage are discussed and future development prospects are indicated.     

Percolation effect and thermoplasticity of conducting [poly(acrylic acid)/C16TAB-modified graphene oxide]n multilayer films

The feasibility and effectiveness of the electrostatic self-assembly technique are demonstrated for the fabrication of thermoplastically conducting multilayer films. The layer-by-layer self-assembly process is based on the alternating adsorption of low molecular weight (M _n) poly(acrylic acid) (PAA) and cetyltrimethyl-ammonium bromide-modified graphene oxide (GO) with three carbon layers. A unique conductivity percolation effect is observed at a percolation threshold (percolation bilayer number) because the carbon–carbon interlayer can be expanded by the diffusion of PAA molecular chains. The resultant multilayer films show typical positive/negative temperature coefficient effects because of the thermoplasticity of the PAA with low M _n. After being reduced from GO to graphene (G), the electrical conductivity of the resulting (PAA/G)_n multilayer film is dramatically enhanced, and the percolation threshold occurs at a high bilayer number. The reasonable conductivities and the percolation effect make these films inherently interesting and potentially useful as components of advanced electronic devices.     

Temperature dependence of the Raman spectra of graphene and graphene multilayers

We investigated the temperature dependence of the frequency of G peak in the Raman spectra of graphene on Si/SiO2 substrates. The micro-Raman spectroscopy was carried out under the 488 nm laser excitation over the temperature range from -190 to +100 degrees C. The extracted value of the temperature coefficient of G mode of graphene is chi = -0.016 cm-1/ degrees C for the single layer and chi = -0.015 cm-1/ degrees C for the bilayer. The obtained results shed light on the anharmonic properties of graphene.