退火时间对6H-SiC(0001)表面外延石墨烯形貌和结构的影响

在分子束外延(MBE)设备中,采用高温退火的方法在6H-SiC(0001)表面外延石墨烯,并研究了退火时间对外延石墨烯形貌和结构的影响.利用反射式高能电子衍射(RHEED)、原子力显微镜(AFM)、激光拉曼光谱(Raman)和近边X射线吸收精细结构(NEXAFS)等实验技术对制备的样品进行了表征.RHEED结果发现,不同退火时间的样品在SiC衍射条纹的外侧都出现了石墨烯的衍射条纹.AFM测试表明,外延石墨烯的厚度随退火时间增加而增大,且样品孔洞减少、表面更加平整.Raman测试表明,外延石墨烯拉曼谱中2D峰和G峰的位置相对高定向热解石墨(HOPG)中2D峰和G峰的位置蓝移,且退火时间增加,峰的蓝移量减小.对样品中碳原子K边NEXAFS谱测量显示,样品中存在sp2杂化的碳原子,退火时间增加使碳原子的1s→π以及1s→σ吸收的强度增大,且1s电子到π态的吸收峰相对HOPG的向高能偏移.     

基于石墨烯的样品制备技术研究亮点

自2004年被首次制备出来,石墨烯就受到了全世界科学家的普遍关注。石墨烯是由单层碳原子组成的六方蜂巢状结构的二维物质,如果以每平方纳米石墨烯片层结构上容纳的碳原子个数为38来计算,石墨烯具有很大的比表面积（2630 m2/g）,再加上其生产成本低,使其成为样品预处理中理想的富集材料之一。     

具有SiC缓冲层的Si衬底上直接沉积碳原子生长石墨烯

石墨烯是近年发现的一种新型多功能材料.在合适的衬底上制备石墨烯成为目前材料制备的一大挑战.本文利用分子束外延(MBE)设备,在Si 衬底上生长高质量的SiC 缓冲层,然后利用直接沉积C原子的方法生长石墨烯,并通过反射式高能电子衍射(RHEED)、拉曼(Raman)光谱和近边X 射线吸收精细结构谱(NEXAFS)等实验技术对不同衬底温度(800、900、1000、1100 °C)生长的薄膜进行结构表征.实验结果表明,在以上衬底温度下都能生长出具有乱层堆垛结构的石墨烯薄膜.当衬底温度升高时,碳原子的活性增强,其成键的能力也增大,从而使形成的石墨烯结晶质量提高.衬底温度为1000 °C时结晶质量最好.其原因可能是当衬底温度较低时,碳原子活性太低不足以形成有序的六方C-sp2环.但过高的衬底温度会使SiC 缓冲层的孔洞缺陷增加,衬底的Si 原子有可能获得足够的能量穿过SiC薄膜的孔洞扩散到衬底表面,与沉积的碳原子反应生成无序的SiC,这一方面会减弱石墨烯的生长,另一方面也会使石墨烯的结晶质量变差.     

拉曼光谱在石墨烯结构表征中的应用

石墨烯是sp2碳原子紧密堆积形成的二维原子晶体结构,因其独特的结构与性质引起了科学家们的广泛关注.拉曼光谱是一种快速而又简洁的表征物质结构的方法.主要综述了拉曼光谱技术在石墨烯结构表征中应用的一些最新进展.首先,在系统分析石墨烯声子色散曲线的基础上介绍了石墨烯的典型拉曼特征(G'峰、G峰和D峰),讨论了G'峰、G峰和D峰在石墨烯层数的指认和石墨烯边缘与缺陷态分析中的应用;然后,通过对石墨烯拉曼G峰和G'峰的峰位、峰型以及强度的分析,讨论了石墨烯的层间堆垛方式、掺杂、基底、温度和应力等对石墨烯的电子能带结构的影响;最后,介绍了石墨烯中的二阶和频与倍频拉曼特征以及石墨烯的低频拉曼特征(剪切和层间呼吸振动模),并讨论了其对石墨烯结构的依赖性.     

准二维金属硫属化合物类石墨烯结构的化学合成与组装

无机类石墨烯属于准二维纳米结构体系,因其具有比纯碳石墨烯本身更多的调控参数,比如带隙类型及其带隙宽度可调节等,在电子器件构筑和能量转化存储等领域有着重要的科学意义和广阔的应用前景．近年来,具有准二维特征的金属硫属化合物（metalchalcogenides,MCs）作为类石墨烯结构的重要材料体系受到了广泛关注．本文概述了近年来金属硫属化合物类石墨烯结构的化学制备方法及其可能的组装应用,提出了通过系列方法影响层间作用力及利用晶体各向异性等材料设计与合成策略来实现类石墨烯的化学合成,并展望了类石墨烯的组装结构在能量存储与智能传感领域的应用前景．     

新型碳材料——石墨烯的制备及其在电化学中的应用

作为单原子厚度的二维碳原子材料,石墨烯由于其特殊的结构和物化性能而成为目前碳基材料中的一个研究热点.本文主要介绍了石墨烯及其复合纳米材料制备的一些新方法.结合石墨烯优良的导电性和生物相容性,论述了石墨烯在电化学分析领域,以及电催化、超级电容器和电化学电池等方面的应用.     

氧化石墨与石墨烯的制备及其在样品前处理中的应用

石墨烯是继碳纳米管之后出现的新一代碳同素异形体,是碳原子以SP2杂化轨道组成的具有六角形蜂巢晶格结构的二维纳米材料,具有良好的化学稳定性、热稳定性和高比表面积.氧化石墨是用强氧化剂氧化鳞片石墨而获得的石墨衍生物,同样是一层状共价化合物,其表面有丰富的含氧宫能团.近年来,氧化石墨和石墨烯在有机小分子、金属离子和生物大分子分离富集方面的应用已有文献报道.该文简要介绍了氧化石墨以及石墨烯的制备方法,并详细概述了其在固相萃取、固相微萃取、磁性固相萃取、搅拌棒吸附萃取等样品前处理技术中的应用.     

类石墨烯化学中的挑战与机遇

类石墨烯结构超薄的厚度、较大的比表面积和优良的柔韧性为满足人们对便携式透明纳米器件的需求带来了希望.将类石墨烯结构的制备从典型层状材料扩展到层间具有较强作用力的准层状材料以及非层状材料,不但能够丰富类石墨烯材料的种类,而且还可能带来一系列革新的性能和广泛的应用.但是,由于类石墨烯材料在第三维度上缺乏长程有序,使其低维结构的表征和清晰构效关系的建立较为困难.本文系统总结了典型层状结构、准层状结构以及非层状结构类石墨烯材料的制备方法,调研了最近对类石墨烯材料精细结构和缺陷结构的研究进展以及对类石墨烯材料电子结构的调控方法,强调了它们结构-性质之间的关系.另外,描述了基于类石墨烯结构的透明、柔性器件在光电化学催化、光探测器、光电转换、超级电容器等领域中表现出的优异性能和广阔的应用前景.期望本文能够加深人们对这一领域的理解,为今后高效能源器件的设计提供指导.     

石墨烯的功能化及其在储能材料领域中的应用

石墨烯是由sp~2杂化的碳原子紧密堆积成的单原子层二维碳材料,由于其优异的物理和化学性质被视为最有前景的新型材料之一。但由于石墨烯片层之间在范德华力的作用下易发生不可逆团聚,丧失其单层二维纳米片的结构特性,以及石墨烯表面呈现惰性状态,致使其与其他介质的相互作用较弱,难以均匀分散在极性或非极性的溶剂中,因而石墨烯的应用受到限制。对石墨烯进行功能化可以调控其分子结构、电子能级和化学性质,不仅可以有效抑制石墨烯的团聚而且能够改善其在溶剂中的分散性和稳定性,从而实现石墨烯基材料的多元化应用。本文综述了近年来共价键和非共价键功能化石墨烯以及其复合材料在储能领域的研究进展,并对功能化石墨烯的发展前景进行了展望。     

新型碳材料——石墨烯的制备及其在电化学中的应用

作为单原子厚度的二维碳原子材料,石墨烯由于其特殊的结构和物化性能而成为目前碳基材料中的一个研究热点。本文主要介绍了石墨烯及其复合纳米材料制备的一些新方法。结合石墨烯优良的导电性和生物相容性,论述了石墨烯在电化学分析领域,以及电催化、超级电容器和电化学电池等方面的应用。     

Effect of Irradiation with a Continuous Beam of Accelerated Electrons on the Texture and Nanostructure of Carbon Black: a Study by Adsorption and High-Resolution Transmission Electron Microscopy

Textural, structural, and morphological changes occurring in globular carbon black of various particle size fractions under continuous irradiation with a continuous beam of 2.5-MeV accelerated electrons were studied by the methods of adsorption and high-resolution transmission electron microscopy. The electron irradiation leads to pronounced transformation of the solid globular framework of carbon black mainly into graphite-like nanocapsules of 10–50 nm size with the spacing between the graphene layers of the order of 0.355 nm. The observed effect leads to a decrease in the porosity (by a factor of approximately 1.65) and in the specific surface area (by a factor of 3–4) of the irradiated samples. This may be due to the transformation of the turbostratic (practically amorphous) form of carbon black into the nanostructured state with denser packing of carbon particles. The physicochemical properties of such carbon should be primarily determined by the structure of the solid framework.     

Electron beam coater for reduction of charging in ice-embedded biological specimens using Ti(88)Si(12) alloy.

Biological macromolecules embedded in vitreous ice are known to suffer from charging while being imaged in an electron transmission cryomicroscope. We developed an electron beam coater that deposits conductive films onto the surface of frozen-hydrated specimens. The conductive films help to dissipate charge during electron irradiation of poorly conductive ice-embedded biological samples. We observed significant reduction in charging of ice-embedded catalase crystals suspended over holes in a holey carbon film after coating them with a 30-A-thick layer of an amorphous alloy, Ti(88)Si(12). Images of the crystals after coating showed diffraction spots of up to 3 A resolution.     

Highly Intact and Pure Oxo‐Functionalized Graphene: Synthesis and Electron‐Beam‐Induced Reduction

Controlling the chemistry of graphene is necessary to enable applications in materials and life sciences. Research beyond graphene oxide is targeted to avoid the highly defective character of the carbon framework. Herein, we show how to optimize the synthesis of oxo‐functionalized graphene (oxo‐G) to prepare high‐quality monolayer flakes that even allow for direct transmission electron microscopy investigation at atomic resolution (HRTEM). The role of undesired residuals is addressed and sources are eliminated. HRTEM provides clear evidence for the exceptional integrity of the carbon framework of such oxo‐G sheets. The patchy distribution of oxo‐functionality on the nm‐scale, observed on our highly clean oxo‐G sheets, corroborates theoretical predictions. Moreover, defined electron‐beam irradiation facilitates gentle de‐functionalization of oxo‐G sheets, a new route towards clean graphene, which is a breakthrough for localized graphene chemistry.     

A viable way to tailor carbon nanomaterials by irradiation-induced transformations

Since the discovery of carbon nanotubes (CNT), transmission electron microscopy (TEM) has been the most important tool in their investigation. It is possible to use electron irradiation in a TEM to construct a single-walled carbon nanotube (SWCNT) from an amorphous carbon film. Here we show that such a synthesis method creates a large number of carbon ad-atoms, which after some critical amount of radiation act to restore the system by reconstructing the carbon film. The behavior of the ad-atoms can be controlled by adjusting the current density in the microscope, suggesting that carbon nanomaterials can be tailored by electron irradiation.     

Ion irradiation induced structural and electrical transition in graphene

The relationship between the electrical properties and structure evolution of single layer graphene was studied by gradually introducing the gallium ion irradiation. Raman spectrums show a structural transition from nano-crystalline graphene to amorphous carbon as escalating the degree of disorder of the graphene sample, which is in correspondence with the electrical transition from a Boltzmann diffusion transport to a carrier hopping transport. The results show a controllable method to tune the properties of graphene.     

基于透射电子显微镜的沸石分子筛结构研究进展

透射电子显微镜是解析沸石分子筛新结构、 分析结构缺陷和研究活性位点等的有力工具. 应用于分子筛研究的透射电子显微术总体上可以分为图像法和衍射法, 包括透射电子显微镜和扫描透射电子显微图像、 选区电子衍射和三维电子衍射, 通常结合其中的几种方法进行分析. 近年来, 随着电子显微镜硬件性能的不断提升, 特别是球差矫正器的广泛应用及各种适用于分子筛等电子束敏感材料的探测器和图像处理技术的不断革新, 在原子尺度观察分子筛的结构已成为可能. 此外, 利用原位电子显微镜技术研究分子筛的生长和催化反应机理也在逐步展开. 本文按电子显微镜方法分类, 综述了近些年基于电子显微镜的分子筛研究, 包括新结构解析、 手性确认和金属负载等的最新进展.     

Controllable n-type doping in WSe2 monolayer via construction of anion vacancies

The successful applications of two-dimensional (2D) transition metal dichalcogenides highly rely on rational regulation of their electronic properties. The nondestructive and controllable doping strategy is of great importance to implement 2D materials in electronic devices. Herein, we propose a straightforward and effective method to realize controllable n-type doping in WSe₂ monolayer by electron beam irradiation. Electrical measurements and photoluminescence (PL) spectra verify the strong n-doping in electron beam-treated WSe₂ monolayers. The n-type doping arises from the generation of Se vacancies and the doping degree is precisely controlled by irradiation fluences. Due to the n-doping-induced narrowing of the Schottky barrier, the current of back-gated monolayer WSe₂ is enhanced by an order of magnitude and a ∼8× increase in the electron filed-effect mobility is observed. Remarkably, it is a moderate method without significant reduction in electrical performance and severe damage to lattice structures even under ultra-high doses of irradiation.     

Electron-beam-induced topographical, chemical, and structural patterning of amorphous titanium oxide films

Electrolytically deposited amorphous TiO2 films on steel are remarkably sensitive to electron beam (e-beam) irradiation at moderate energies at 20 keV, resulting in controlled local oxide reduction and crystallization, opening the possibility for local topographical, chemical, and structural modifications within a biocompatible, amorphous, and semiconducting matrix. The sensitivity is shown to vary significantly with the annealing temperature of as-deposited films. Well-defined irradiation conditions in terms of probe current IP (5 microA) and beam size were achieved with an electron probe microanalyzer. As shown by atomic force and optical microscopy, micro-Raman spectroscopy, wavelength-dispersive X-ray (WDX), and Auger analyses, e-beam exposure below 1 Acm-2 immediately leads to electron-stimulated oxygen desorption, resulting in a well-defined volume loss primarily limited to the irradiated zone under the electron probe and in a blue color shift in this zone because of the presence of Ti2O3. Irradiation at 5 Acm(-2) (IP = 5 microA) results in local crystallization into anatase phase within 1 s of exposure and in reduction to TiO after an extended exposure of 60 s. Further reduction to the metallic state could be observed after 60 s of exposure at approximately 160 Acm(-2). The local reduction could be qualitatively sensed with WDX analysis and Auger line scans. An estimation of the film temperature in the beam center indicates that crystallization occurs at less than 150 degrees C, well below the atmospheric crystallization temperature of the present films. The high e-beam sensitivity in combination with the well-defined volume loss from oxygen desorption allows for precise electron lithographic topographical patterning of the present oxides. Irradiation effects leading to the observed reduction and crystallization phenomena under moderate electron energies are discussed.     

Compact ILU-type electron accelerators as a base for industrial 4-sided irradiation systems for cable and tubes

The ILU-type industrial electron accelerators are developed in BINP sins 1967. Their energy range is 0.7–4.0 MeV at beam power of 20–50 kW. The comparison of the irradiation results after bilateral and four-sided irradiation of cables and tubes is given. It is shown that the required electron energy and beam power in the case of four-sided irradiation are sufficiently lower than in the case of bilateral irradiation, resulting in an increase of productive rate of the process and improvement of treatment quality. The installations for four-sided irradiation of cables and tubes are based on the industrial electron accelerators type ILU.     

Enhanced performance as a lithium-ion battery cathode of electrodeposited V2O5 thin films by e-beam irradiation

The influence of electron beam irradiation on the electrochemical properties of electrodeposited V₂O₅ thin films was investigated. V₂O₅ thin films were deposited electrochemically onto indium tin oxide-coated glass from an aqueous vanadyl sulfate hydrate (VOSO₄⋅nH₂O) solution using Pt and Ag/AgCl as the counter electrode and reference electrode, respectively. Electrodeposition was performed potentiostatically at 1.7 V vs. Ag/AgCl. Electrodeposited samples were then subjected to a 1-MeV electron beam using an electron beam accelerator at the Korea Atomic Energy Research Institute. For comparison, a control sample was not irradiated with the electron beam. Crystallinity change before and after electron beam irradiation was investigated by X-ray diffraction and the oxidation state of vanadium determined by X-ray photoelectron spectroscopy. Scanning electron microscopy was utilized to examine surface morphology. It was observed that electron beam irradiation altered the oxidation state of vanadium and increased crystallinity. Significant morphological changes of V₂O₅ thin films were also observed with electron beam irradiation. Cyclic voltammetry was employed to evaluate the electrochemical properties of the synthesized V₂O₅ films in terms of their application as electrodes of lithium-ion battery. Compared with the control sample, which was not irradiated with an electron beam, the electron beam-irradiated V₂O₅ specimens showed much higher capacitance.