发信人: buckyx (buckyx), 信区: NanoST 标 题: 石墨烯真的与众不同吗？ 发信站: 水木社区 (Wed Dec 1 13:59:16 2010), 转信 终于有人出来泼冷水了。 ACS Nano上曾经有一篇石墨烯/二氧化钛复合材料的论文，认为复合之后的光催化活性强于二氧化钛和碳纳米管的复合材料。 刚刚ACS Nano又出来一篇同样内容的论文，题目很直接：Is TiO2-Graphene Truly Different from Other TiO2-Carbon Composite Materials? 这篇论文的表征更详细，认为在提高催化活性方面，石墨烯跟碳纳米管没什么大的差别。论文里说It is hoped that our current work could avert the misleading message to the readership ，文献37就是前面那篇论文。这两个工作都是国内的组做的。 客观的说，石墨烯和碳纳米管在提高性能方面到底有没有差别，也要取决于具体的材料，石墨烯和碳纳米管有很多种类，不同种类的性能肯定有差别。所以不能说上面两篇论文一个是对的另一个是错的。不过，像后面这一篇这样的论文还是挺给力的，尤其是在大家众口一辞说石墨烯怎么怎么好的时候。 发信人: Synthon (合成子信件请发给smartpolymer), 信区: NanoST 标 题: Re: 石墨烯真的与众不同吗？ 发信站: 水木社区 (Wed Dec 1 14:34:04 2010), 转信 第一篇文章，graphene是氧化过的，CNT是没氧化的，这样solvothermal合成出来的复合物，跟TiO2的结合差别很大的。那个含氧基团留下的空位可以增进复合。我觉得没准换个氧化的CNT就跟graphene的性能差不多了。 第二篇文章，没有提供CNT样品制备的细节。。。
Monolayer Graphene as Saturable Absorber in Mode-locked Laser Authors: Qiaoliang Bao , Han Zhang , Zhenhua Ni , Yu Wang , Lakshminarayana Polavarapu , Kian Ping Loh , Zexiang Shen , Qing-Hua Xu , Ding Yuan Tang (Submitted on 14 Jul 2010) Abstract: We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkable large modulation depth of 95.3%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 ps) can be generated using monolayer graphene as saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability and output energy. http://arxiv.org/abs/1007.2243
Vector dissipative solitons in graphene mode locked fiber lasers Han Zhang a , Dingyuan Tang a , , , Luming Zhao a , Qiaoliang Bao b and Kian Ping Loh b a School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore b Department of Chemistry, National University of Singapore, Singapore 117543, Singapore Received 2 March 2010; revised 20 April 2010; accepted 21 April 2010. Available online 7 May 2010. Abstract Vector soliton operation of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated. Either the polarization rotation or polarization locked vector dissipative solitons were experimentally obtained in a dispersion-managed cavity fiber laser with large net cavity dispersion, while in the anomalous dispersion cavity fiber laser, the phase locked nonlinear Schrdinger equation (NLSE) solitons and induced NLSE soliton were experimentally observed. The vector soliton operation of the fiber lasers unambiguously confirms the polarization insensitive saturable absorption of the atomic layer graphene when the light is incident perpendicular to its 2-dimentional (2D) atomic layer.
Original Paper Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion to all normal dispersion H. Zhang 1 , D.Y. Tang * , L.M. Zhao 1 , Q.L. Bao 2 , K.P. Loh 2 , B. Lin 1 , S.C. Tjin 1 1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore 2 Department of Chemistry, National University of Singapore, Singapore 117543, Singapore email: D.Y. Tang ( edytang@ntu.edu.sg ) * Correspondence to D.Y. Tang, 1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore setDOI("ADOI=10.1002/lapl.201010025") Funded by: National Research Foundation of Singapore; Grant Number: NRF-G-CRP 2007-01 NRF-CRP Graphene Related Materials and Devices; Grant Number: R-143-000-360-281 Keywords fiber laser graphene mode-locked soliton nanophotonics Abstract Soliton operation and soliton wavelength tuning of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated under various cavity dispersion conditions. It was shown that not only wide range soliton wavelength tuning but also soliton pulse width variation could be obtained in the fiber lasers. Our results show that the graphene mode locked erbium-doped fiber lasers provide a compact, user friendly and low cost wavelength tunable ultrashort pulse source. ( 2010 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH Co. KGaA) Compact graphene mode-locked wavelength-tunable er
国兴做出这个真不容易，很赞！ http://www.rsc.org/Publishing/Journals/CC/article.asp?doi=b922733d Chem. Commun. , 2010, 46 , 3256 - 3258, DOI: 10.1039/b922733d Architecture of graphdiyne nanoscale films Guoxing Li, Yuliang Li, Huibiao Liu, Yanbing Guo, Yongjun Li and Daoben Zhu We have demonstrated a methodology to generate large area graphdiyne films with 3.61 cm 2 on the surface of copper via a cross-coupling reaction using hexaethynylbenzene. The device based on graphdiyne films for measurement of electrical property is fabricated and shows conductivity of 2.516 10 -4 S m -1 indicating a semiconductor property. PDF下载： Architecture of graphdiyne nanoscale films http://news.sciencenet.cn//htmlnews/2010/5/231845.shtm 作者：张巧玲 来源： 科学时报 发布时间：2010-5-7 9:15:52 选择字号： 小 中 大 我国科学家成功合成新的碳同素异形体 饱受重视的碳材料家族又诞生了一个新成员 最近，中科院化学所有机固体院重点实验室科研人员在国家自然科学基金委、科技部和中国科学院的资助下，在石墨炔研究方面取得了重要突破。研究人员利用六炔基苯在铜片的催化作用下发生偶联反应，成功地在铜片表面上通过化学方法合成了大面积碳的新的同素异形体石墨炔（graphdiyne）薄膜，研究结果发表在2010年的《化学通讯》（ Chem. Commun ）上。 据悉，近20年来，科学家们一直致力于发展新的方法合成新的碳同素异形体，探索其新的性能，先后发现了富勒烯、碳纳米管和石墨烯等新的碳同素异形体，并成为国际学术研究的前沿和热点，形成了交叉科学的独立研究领域。碳具有sp 3 、sp 2 和sp三种杂化态，通过不同杂化态可以形成多种碳的同素异形体，如通过sp 3 杂化可以形成金刚石，通过sp 3 与sp 2 杂化则可以形成碳纳米管、富勒烯和石墨烯等。由于sp杂化态形成的碳碳三键具有线性结构、无顺反异构体和高共轭等优点，人们一直渴望能获得有sp杂化态的新的碳同素异形体，并认为该类碳材料具备优异的电学、光学和光电性能而成为下一代新的电子和光电器件的关键材料。石墨炔是第一个以sp、sp 2 和sp 3 三种杂化态形成的新的碳同素异形体，最有可能被人工合成的非天然的碳同素异形体。 化学所有机固体院重点实验室科研人员长期致力于碳材料的合成、聚集态结构和性能的研究。他们成功研究出石墨炔薄膜后， Chem. Commun 的审稿人在评价这一研究成果时表示：这是碳化学的一个令人瞩目的进展，大面积的石墨炔薄膜的制备是一个真正的重大发现，研究结果非常让人振奋，并将为大面积石墨炔薄膜在纳米电子的应用开辟一条道路。 研究结果表明，石墨炔是由1，3-二炔键将苯环共轭连接形成二维平面网络结构的全碳分子，具有丰富的碳化学键，大的共轭体系、宽面间距（4.1913）、优良的化学稳定性和半导体性能。所获得的石墨炔薄膜面积可达3.61cm 2 ，是高晶化的单晶薄膜，拉曼光谱显示了其特征峰在1382、1569、1926和2189cm -1 ，并证实该薄膜具有较高的有序度和较低的缺陷，薄膜电导率为：10 -3 -10 -4 S m -1 。 这种新的碳同素异形体的发现，使得受国际科学界高度重视的碳材料家族又诞生了一个新的成员。石墨炔特殊的电子结构将在超导、电子、能源以及光电等领域具有潜在、重要的应用前景。 《科学时报》 (2010-5-7 A1 要闻) 更多阅读 《化学通讯》发表论文摘要（英文）
graphene_nature_photonics GRAPHENE Under high energy Appl. Phys. Lett. 96, 051122 (2010) Despite intense research into the preparation and characterization of graphene, there have been few studies into its suitability for applications in photonics. Graphene may have significant advantages over carbon nanotubes for ultrafast photonic applications, including a much lower level of saturable absorption, ultrafast recovery times (~200 fs) and a wide operating spectral range that covers the whole telecommunications band. Now, Yong-Won Song and colleagues from the Korea Institute of Science and Technology have used graphene as an intracavity passive power-modulating element, demonstrating efficient laser pulsation in the high-pulse-energy regime. The researchers have overcome the problem of optical power-induced thermal damage by ensuring evanescent field interaction between the propagating light and the graphene layer. The resulting passively mode-locked fibre laser has a central wavelength of 1,561.6 nm, a spectral width of 1.96 nm, a repetition rate of 6.99 MHz and an estimated pulse duration of 1.3 ps. These results suggest that graphene could be useful in ultrafast photonics.
Mode locking of Ceramic Nd: YAG with graphene as a saturable absorber The mode-locking of a ceramic Nd: YAG solid-state laser (SSL) with solution processed graphene as the saturable absorber (SA) was demonstrated. Transform-limited pulses of 4 ps with an average power of 100 mW centered at 1064 nm were generated, for a non-dispersion compensated Nd: YAG SSL. Z-scan studies revealed that the graphene SA had a saturation intensity of 0.87 MW cm -2 and a normalized modulation depth of 17 %. The results illustrate the potential of using graphene as a mode locker for SSLs. Appl. Phys. Lett. 96 , 031106(2010); doi:10.1063/1.3292018 Mode locking of Ceramic Nd: YAG with graphene as a
我们跟鲍师兄今年下半年才报道的石墨烯锁模的工作，11月就得到了曼彻斯特大学 物理学与天文学院的科斯特雅诺沃塞洛夫博士的关注 （见转载）。能引起石墨烯研究中心的科学家们的注意应该是对我们工作的一种肯定，所以倍感荣幸和挑战呀！不得不感叹石墨烯研究发展之迅速，世界资讯之发达。未来研究石墨烯在锁模激光器的应用也会越来越广，竞争自然会变得越来越激烈；不比一年前，只有 我们跟鲍师兄 暗地里做 石墨烯锁模 ，现在国际上相关的课题组都正在或者即将做石墨烯在超快激光中的应用；有几个课题组甚至还有文章即将发表。看来我们如果想保持领先地位，还必须继续创新，仍需继续努力呀！加油吧！ 转载自： http://www.azonano.com/details.asp?ArticleId=2456 Graphene: From Physics to Applications Dr. Kostya S. Novoselov , School of Physics Astronomy , University of Manchester Corresponding author: kostya@manchester.ac.uk Graphene - one layer of carbon atoms arranged in a hexagonal lattice - is the newest member in the family of carbon allotropes. Although isolated graphene was reported for the first time only in 2004 1 , the progress it made over these years is enormous, and it rightly has been dubbed the wonder material. There are three major areas of excitement about graphene. Firstly, it is the first example of two-dimensional atomic crystal, which very existence improves our understanding about thermodynamic stability of low-dimensional systems. Secondly, the electronic properties of graphene are very peculiar: electrons in graphene obey linear dispersion relation (just like photons), thus mimicking massless relativistic particles 2 . And the last but not least, many properties of graphene are superior to those in all other materials, so it is very tempting to use it in a variety of applications, ranging from electronics to composite materials. Historically, it is the electronic properties which attracted most of attention. Electrons in graphene behave like massless relativistic particles, which governs most of its electronic properties. Probably one of the most spectacular consequences of such unusual dispersion relation is the observation of half-integer Quantum Hall Effect and the absence of localisation 2 . The later might be very important for graphene-based field effect transistors. Generally crystals of graphene could be prepared with very few defects (consequence of ultra strong carbon-carbon bonds), which, in conjunction with the absence of localisation and high Fermi velocity ensures very high mobility of the charge carriers and short time of flight in ballistic regime. First prototypes of high-frequency transistors have been recently developed and demonstrated very encouraging characteristics 3 . Also peculiar are graphene's optical properties. It has been measured that graphene absorbs 2.3% of light 4 - quite a sizable fraction for an ultimately thin material. What is even more exciting is the fact that this number is given solely by the combination of fundamental constants 4 : (=e 2 /hc1/137 is the fine structure constant). Do it at home, multiply 3.14 by 1/137 and you will get something close to 0.023. Such combination of high conductivity (sheet resistance of doped graphene can be as low as 10 Ohm) and low light adsorption makes this material an ideal candidate for transparent conductive coating. Graphene utilisation for this type of applications has been recently demonstrated by constructing graphene-based liquid crystal 5 and solar cells 6 . Furthermore, the general issue of graphene mass-production (until recently only research-size graphene samples have been available) has been resolved for these sort of applications with the introduction of a novel technique: large area thin films of micrometer-size graphene flakes can be produced by chemical exfoliation of graphite 5 . It is very tempting to use the unique properties of graphene for applications. The already mentioned examples do not even nearly exhaust the list of technologies which would benefit from using graphene. Composite materials and photodetectors, support for bio-objects in TEM and mode-lockers for ultrafast lasers - all those and many more areas would gain strongly from using graphene. The issue, however, was always the mass-production of this material. Since the very first experiments 1 , the technique of choice for graphene production for many researchers was the very nave Scotch-tape method 1 , 2 - simple peeling of graphene monolayers from bulk graphite with an adhesive tape. However, recent months seen a dramatic progress in development of truly mass-production techniques for graphene synthesis. Ranging from aforementioned chemical exfoliation to epitaxial growth (for a review see 7 ), these techniques give us a realistic hope that soon we will see products based on this exciting two-dimensional material. Reference 1. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V. Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films Science 306, 666-669 (2004). 2. Geim, A. K. Novoselov, K. S. The Rise of Graphene Nature Mater. 6, 183-191 (2007). 3. Yu-Ming Lin, Keith A. Jenkins, Alberto Valdes-Garcia, Joshua P. Small, Damon B. Farmer Phaedon Avouris, Operation of Graphene Transistors at Gigahertz Frequencies Nano Lett., 9 (1), 422-426 (2009). 4. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim Fine Structure Constant Defines Visual Transparency of Graphene Science 320, 1308 (2008). 5. Peter Blake, Paul D. Brimicombe, Rahul R. Nair, Tim J. Booth, Da Jiang, Fred Schedin, Leonid A. Ponomarenko, Sergey V. Morozov, Helen F. Gleeson, Ernie W. Hill, Andre K. Geim, Kostya S. Novoselov Graphene-Based Liquid Crystal Device Nano Letters 8(6) 1704 - 1708 (2008). 6. X. Wang, L. Zhi, K. Mullen, Transparent, conductive graphene electrodes for dye-sensitized solar cells Nano Letters 8(1), 323-327 (2008) 7. A. K. Geim Graphene: Status and Prospects Science 324, 1530-1534 (2009). Copyright AZoNano.com, Dr. Kostya Novoselov (University of Manchester) Date Added: Nov 29, 2009
Applied Physics Letters , Vol. 95, pp. 141103. 原文下载 http://www3.ntu.edu.sg/home2006/ZHAN0174/apl.pdf Due to its unique electronic property and the Pauli blocking principle, atomic layer graphene possesses wavelength-independent ultrafast saturable absorption, which can be exploited for the ultrafast photonics application. Through chemical functionalization, a graphene-polymer nanocomposite membrane was fabricated and first used to mode lock a fiber laser. Stable mode locked solitons with 3 nJ pulse energy, 700 fs pulse width at the 1590 nm wavelength have been directly generated from the laser. We show that graphene-polymer nanocomposites could be an attractive saturable absorber for high power fiber laser mode locking.
石墨烯可以产生高能量超短脉冲 OPTICS EXPRESS,Vol. 17, P17630. http://www3.ntu.edu.sg/home2006/zhan0174/OE_graphene.pdf Abstract: We report on large energy pulse generation in an erbium-doped fiber laser passively mode-locked with atomic layer graphene. Stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser. Our results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking.
石墨烯 (graphene)作为近年来的研究热点，有很多优异的物理性质。利用紧束缚方法(Tight-binding approach)计算出的能量色散关系(energy dispersion relation) 可以发现很多有趣的东西，同时对碳纳米管的研究也有一定的指导意义。 The energy dispersion relation formula. Honeycomb lattice and its Brillouin zone.( 以上公式和图见 Rev. Mod. Phys., Vol. 81,No. 1,109-162,2009 .) The values of Brillouin zone A number of interesting and peculiar features emerge from this figure. First it is clear that the valence and the conduction bands touch each other at a number of finite momentum values. The momentum values at which the two bands touch are termed Dirac points (there are two in the Brillouin zone) and are represented by the momentum vectors K and K’. As a consequence, graphene’s spectrum does not have an energy gap. On the other hand, since the bands only touch at two momentum points the density of states is zero at the corresponding energy. Therefore, grapheme is sometimes termed a zero-gap semiconductor with vanishing density of states at the Fermi energy.( J. Phys.: Condens. Matter 21 (2009) 323201 ) Dirac points附近的数值。 附Matlab程序 %单层石墨（graphene）的色散关系 %Copyright 2009 Zheng-Wei Zuo clc;clear all;close all %清除之前的内存空间变量、图形等 syms x y %d定义自变量，x代表Kx，以此类推; t=2.7;%the nearest-neighbor hopping energy (hopping between different sublattices). m=-0.2*t;%the next nearest-neighbor hopping energy (hopping in the same sublattices). a=1.42;%the carbon-carbon distance. E1=t*sqrt(3+2*cos(sqrt(3)*y*a)+4*cos(sqrt(3)/2*y*a)*cos(3/2*x*a))-m*(2*cos(sqrt(3)*y*a)+4*cos(sqrt(3)/2*y*a)*cos(3/2*x*a));%the upper band E2=-t*sqrt(3+2*cos(sqrt(3)*y*a)+4*cos(sqrt(3)/2*y*a)*cos(3/2*x*a))-m*(2*cos(sqrt(3)*y*a)+4*cos(sqrt(3)/2*y*a)*cos(3/2*x*a));%the lower band ezsurf(E1, )%plot the upper hold on %继续在当前图形上画图 ezsurf(E2, )%plot the lower shading interp; % 每个等高面用不同的颜色进行区分 box on%加外框 axis tight figure; %另外画一图形 ezsurf(E2, )%plot the upper hold on %继续在当前图形上画图 ezsurf(E1, )%plot the lower shading interp; % 每个等高面用不同的颜色进行区分 box on%加外框 axis tight
The first artificial graphene has been created at the NEST laboratory of the Italian Institute for the Physics of Matter (INFM-CNR) in Pisa. It is sculpted on the surface of a gallium-arsenide semiconductor, to which it grants the extraordinary properties of the original graphene. Published as a Rapid Communication on Phys.Rev.B, the research has been highlighted by the American Physical Society ( Engineering artificial graphene in a two-dimensional electron gas). They envisioned it at the NEST laboratory in Pisa (a joint INFM-CNR and Scuola Normale Superiore di Pisa lab), and then they sculpted it like a work of art on the surface of a gallium-arsenide semiconductor. They named it artificial graphene, the very first ever created, ready to raise the interests of both industry and research. This amazing copy promises to render available the incredible electronic qualities of graphene, and thus, it offers a way to overcome the closing physical limits that plague silicon. An exceptional result of Marco Gibertini, Achintya Singha, Marco Polini and Vittorio Pellegrini of INFM-CNR and Scuola Normale Superiore di Pisa, with the cooperation of Giovanni Vignale (University of Missouri), Aron Pinczuk (Columbia University) Loren Pfeiffer and Ken West (Alcatel-Lucents Bell Labs). 6 {2 O5 H8 e$ J: j, R2 w {; A0 W. @. @' M( m Natural graphene is an interesting but elusive material, observed for the first time in 2004. It has a very peculiar structure, being composed of a single layer of carbon atoms (only one atom thick) arranged in a grid which resambles common chicken wire. This structure grants graphene its exciting electronic properties: over this two-dimensional carbon nano world, electrons move almost freely at very high speeds, acting like massless p articles . For the electronic industry, this means more efficient devices that will be able to be built a lot smaller than what silicon allows. Such an innovation, however, is yet far away to come, because production of graphene with sizes and reproducibility needed by the semiconductor industry is not possible yet. 9 / J* c7 M - S* l( d V5 X V 9 @: K5 v# `jZ- ]1 ?. H And the idea proved to be a complete success: modified in this way, the nanosculptured semiconductor exhibits the properties of the famous material it imitates, thus becoming the very first artificial graphene. With an added advantage: the overall procedure does not rely on exotic equipments, but on tools and instruments that the nanofabrication industry already possesses and masters, meaning that the artificial graphene can already enable the development of high-mobility transistors and lasers. # I8 K/ |! a+ E; ?; ^! v' d1 | . F) M% [1 q6 L We are extremely happy commented Vittorio Pellegrini and Marco Polini to have been first in creating artificial graphene. This line of research has a great strategic importance, and for this reason very intense competition had sparkled between many research groups around the world. Being the first to create this material means in fact gaining a significant advantage in exploiting its extraordinary characteristics. And we believe that this artificial copy, already part of a semiconductor, may finally make graphenes properties available to be implemented in industrial projects and products. - w- B# J* [7 bv4 F) M8 H ( zg+ x# F( h' `! ~+ @ Source: National Institute for the Physics of the Matter
Applying innovative measurement techniques, research ers from the Georgia Institute of Technology and the National Institute of Standards and Technology have directly measured the unusual energy spectrum of graphene, a technologically promising, two-dimensional form of carbon that has tantalized and puzzled scientists since its discovery in 2004. Drawing represents a probe scanning and mapping the atomic contours of graphene, a single layer of carbon atoms arranged in a honeycomb-like array. Simultaneously applying a magnetic field causes electrons (ball) to organize in circular orbits, like a dog chasing its tail. Orbits hold clues to the materials exotic properties . Credit: Kubista, Georgia Institute of Technology. Published in this week's issue of Science , their work adds new detail to help explain the unusual physical phenomena and properties associated with graphene, a single layer of carbon atoms arrayed in a repeating, honeycomb-like arrangement. Graphene 's exotic behaviors present intriguing prospects for future technologies, including high-speed, graphene-based electronics that might replace today's silicon-based integrated circuits and other devices . Even at room temperature, electrons in graphene are more than 100 times more mobile than in silicon. Graphene apparently owes this enhanced mobility to the curious fact that its electrons and other carriers of electric charges behave as though they do not have mass. In conventional materials , the speed of electrons is related to their energy, but not in graphene. Although they do not approach the speed of light, the unbound electrons in graphene behave much like photons, massless p articles of light that also move at a speed independent of their energy. NIST-built STM shuttle module contains the atomic-scale position-and-scan system. Graphene sample and probe tip are in the center opening. Shuttle moves between a room-temperature vacuum environment for loading to an ultracold environment for measuring. Model in background shows graphene s honeycomb structure. Credit: Holmes, NIST This weird massless behavior is associated with other strangeness. When ordinary conductors are put in a strong magnetic field, charge carriers such as electrons begin moving in circular orbits that are constrained to discrete, equally spaced energy levels. In graphene these levels are known to be unevenly spaced because of the massless electrons. The Georgia Tech/NIST team tracked these massless electrons in action, using a specialized NIST instrument to zoom in on the graphene layer at a billion times magnification, tracking the electronic states while at the same time applying high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum scanning tunneling microscope allowed them to sweep an adjustable magnetic field across graphene samples prepared at Georgia Tech, observing and mapping the peculiar non-uniform spacing among discrete energy levels that form when the material is exposed to magnetic fields. The team developed a high-resolution map of the distribution of energy levels in graphene. In contrast to metals and other conducting materials, where the distance from one energy peak to the next is uniformly equal, this spacing is uneven in graphene.The researchers also probed and spatially mapped graphene's hallmark zero energy state, a curious phenomenon where the material has no electrical carriers until a magnetic field is applied. The measurements also indicated that layers of graphene grown and then heated on a substrate of silicon-carbide behave as individual, isolated, two-dimensional sheets. On the basis of the results, the researchers suggest that graphene layers are uncoupled from adjacent layers because they stack in different rotational orientations. This finding may point the way to manufacturing methods for making large, uniform batches of graphene for a new carbon-based electronics. More information: D.L. Miller, K.D. Kubista, G.M. Rutter, M. Ruan, W.A. de Heer, P.N. First and J.A. Stroscio. Observing the quantization of zero mass carriers in graphene. Science. May 15, 2009. Source: National Institute of Standards and Technology.
标签: graphene
