二维锡烯拓扑材料研究取得进展
《Nature》子刊重磅：中美合作制备出石墨烯兄弟——单层锡烯！
2018-11-09 17:48
近日，中国科学技术大学合肥微尺度物质科学国家研究中心教授王兵和副教授赵爱迪研究团队与清华大学助理教授徐勇、教授段文晖以及美国斯坦福大学教授张首晟合作，成功制备出具有纯平蜂窝结构的单层锡烯，并结合第一性原理计算证实了其存在拓扑能带反转及拓扑边界态。相关研究成果11月5日在线发表在《自然-材料》（Nature Materials）杂志上。
类石墨烯结构的IV族元素二维晶体材料及其物性研究，是当前凝聚态物理学和材料科学领域的重要焦点。其中，基于元素锡(Sn)的二维类石墨烯晶体锡烯(Stanene)因其具有很强的电子自旋-轨道耦合，被认为是继石墨烯后又一种具有优越物理性质的新型量子材料。2013年前后理论物理学家们预言，锡烯中由于pxy轨道具有远强于pz轨道的自旋轨道耦合效应，因此s-p轨道的能带反转可以在布里渊区中心打开数百毫电子伏的巨大能隙；更巧妙的是，由于pxy轨道是平面内的，所以其拓扑性更为鲁棒，不易受到衬底和吸附物的影响和破坏。因此，锡烯是一种理想的大能隙二维拓扑绝缘体，有望实现室温量子自旋霍尔效应，在拓扑电子学器件应用方面具有重要意义。理论同时还预言了锡烯有可能被调控实现拓扑超导态、优越的热电效应、近室温的量子反常霍尔效应等新奇特性。过去几年中，国内外多个研究组在不同的衬底表面制备了单层锡烯，但由于受衬底影响，这些已制备出的锡烯都具有非平面的翘曲结构且均未表现出拓扑物性。如何制备出具有拓扑特性的锡烯，成为二维类石墨烯材料物性研究亟待突破的重要难题。
经过近三年反复摸索，研究团队利用低温分子束外延技术成功制备出了具有拉伸晶格结构的单层锡烯。该研究工作首次发现单层锡烯可以表现出与石墨烯完全一致的平面蜂窝状结构，其单胞中AB位原子无高度差，形成理想的纯平六角蜂窝晶格，为碳基石墨烯家族添加了锡基成员。实验中观测到纯平锡烯的化学惰性以及缺陷结构，也证实了其与碳基石墨烯具有诸多相似性，有望为平面蜂窝结构的材料提供新的研究平台。更为重要的是，由于衬底的外延作用，这一纯平锡烯的晶格常数高达0.51纳米，故存在因晶格拉伸导致的s-p轨道拓扑能带反转，即具有拓扑特性。超高真空扫描隧道显微学以及角分辨光电子能谱学结果与第一性原理计算的能态结构一致，充分证实了其由于自旋-轨道耦合和拓扑能带反转所导致的拓扑能隙以及拓扑边界电子态。其中，角分辨光电子能谱结果表明，锡烯由于自旋轨道耦合打开的拓扑能隙约0.3电子伏特，远超室温热涨落能量，使其具备应用于近室温的拓扑量子器件的潜质。进一步的理论计算还预言了在纯平蜂窝结构的锗烯和铅烯中也存在类似的拓扑特性，从而构成了一类新型的二维拓扑量子材料家族。
具有拓扑能带反转和大拓扑能隙的纯平锡烯的实验实现，为类石墨烯的拓扑物性研究开辟了一条新的研究路线，将对二维量子材料的研究和应用开发起到重要推动作用。后续拟开展的研究工作将通过优化衬底和增加栅极以隔绝衬底电子相互作用并实现拓扑能隙的调控，为最终制备可实用的室温拓扑器件提供研究基础。
中国科大博士生邓家良、清华大学博士生夏炳煜以及中国科大博士生马晓川为论文的共同第一作者。此项研究得到科技部、教育部、中组部、国家自然科学基金委、中国科大、清华大学等机构的大力支持。
论文链接：https://www.nature.com/articles/s41563-018-0203-5
纯平蜂窝结构锡烯的制备和原子尺度形貌图（1-3）、结构模型（4-5）、理论计算（6）和实验观测到的电子能带结构（7-8）。
Epitaxial growth of ultraflat stanene with topological band inversion
Article | Published: 05 November 2018
Epitaxial growth of ultraflat stanene with topological band inversion
- Jialiang Deng, Bingyu Xia, Xiaochuan Ma, Haoqi Chen, Huan Shan, Xiaofang Zhai,
- Bin Li, Aidi Zhao, Yong Xu, Wenhui Duan,
- Shou-Cheng Zhang, Bing Wang & J. G. Hou
Nature Materials volume 17, pages1081–1086 (2018) | Download Citation
Abstract
Two-dimensional (2D) topological materials, including quantum spin/anomalous Hall insulators, have attracted intense research efforts owing to their promise for applications ranging from low-power electronics and high-performance thermoelectrics to fault-tolerant quantum computation. One key challenge is to fabricate topological materials with a large energy gap for room-temperature use. Stanene—the tin counterpart of graphene—is a promising material candidate distinguished by its tunable topological states and sizeable bandgap. Recent experiments have successfully fabricated stanene, but none of them have yet observed topological states. Here we demonstrate the growth of high-quality stanene on Cu(111) by low-temperature molecular beam epitaxy. Importantly, we discovered an unusually ultraflat stanene showing an in-plane s–p band inversion together with a spin–orbit-coupling-induced topological gap (~0.3 eV) at the Γ point, which represents a foremost group-IV ultraflat graphene-like material displaying topological features in experiment. The finding of ultraflat stanene opens opportunities for exploring two-dimensional topological physics and device applications.
Physicists announce graphene’s latest cousin: stanene
First observation of 2D tin can't confirm whether material can conduct electricity without heat loss.
Nature News524 , 18–1803 August 2015 [backcolor=url(&]Clarified:
- 07 August 2015
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石墨烯又出来一个“兄弟”：二维锡烯出炉
上海交通大学与斯坦福大学的合作研究组首次制备出锡原子构成的二维材料——锡烯，但尚未证实它是否有理论所预测的超高导电效率
在物理学家预言锡原子能形成一种单原子厚度的二维网格的两年后，研究人员称他们已经在实验中制备出了这种结构。文章发表在8月3日的《自然•材料》（Nature Materials）上，但研究组还没能确认锡烯是否具有理论所语言的奇异电子性质，比如在导电过程中完全不产生耗散热量等等。
锡烯的英文名为stanene，来自于锡的拉丁文名stannum（锡的原子符号Sn也来源于此），它是石墨烯最新诞生的“小弟弟”。石墨烯是由碳原子构成的二维蜂窝状晶格材料，对与之相关的其他二维材料的研究也已经有很多了：其中包括由硅原子组成的硅烯、由磷原子组成的磷烯，以及由锗原子组成的锗烯，甚至还有由不同的单层原子材料堆叠成的功能材料（见《环球科学》2015年8月号《超越石墨烯——从二维到三维》）。
许多这类单层材料都有着超高的导电性，而锡烯（在理论上）或许还更为特别：在室温下，锡烯中的电子可以沿着网格的边缘快速运动，而不会与其他电子进行碰撞，这意味着锡烯在导电的过程中不会以热量的形式耗散能量。这是斯坦福大学物理学家张首晟2013年所做的理论工作，他也是这项最新研究的共同作者之一。
有预测表明锡烯是一种拓扑绝缘体（topological insulator），其载流子（如电子）不能穿过该材料的中间部分，但可以沿边缘自由运动，运动方向与它们的自旋（一种量子性质，自旋“向上”或者“向下”）相关。电流之所以不会耗散，是因为大多数晶体缺陷都不会影响电子的自旋，因此也不会阻挡电子的运动，这意味着锡烯薄膜或许能成为运载电流的完美高速通道——来自普渡大学的物理学家、电子工程师叶培德说。“我一直在寻找着不仅有科研价值，还有应用潜力的材料，”他说，“这是非常有意思的工作。”
衬底的干扰
虽然张首晟和他在中国的合作者已经成功制备出了锡烯，但他们还未能确定它的拓扑绝缘体性质。他们在真空中将锡蒸发变成气体，并让锡原子一个一个地沉积在碲化铋衬底的表面上。尽管这个衬底促进了二维锡烯晶体的形成，但它也会与锡烯产生相互作用，这样的条件可不利于拓扑绝缘体的形成，张首晟说。他同合作者已经写出了另一篇论文，描述了表面条件更好的样品情况，发布在论文预印本网站arXiv上。
德国维尔茨堡大学的物理学家Ralph Claessen认为，我们还不能清楚地认为发表在《自然•材料》上的论文的作者是否真正地制成了锡烯。理论预测这种二维材料应当形成一种扭曲的蜂窝状结构，原子有上有下，形成波纹脊状结构，但张首晟和他的研究组只能通过扫描隧道显微镜（STM）看到上层原子。然而，研究组相信他们的确得到了波纹结构，或许是因为“脊”与“脊”之间的距离与理论预测相符合。
Claessen认为研究组需要通过X射线衍射等实验来直接测量材料的晶格结构，这样才能确认他们的确制成了锡烯。要做这类实验，张首晟和他的合作者还需要制备出更多、尺寸更大的材料。
来自复旦大学的张远波对于该研究则抱有更多的信心。“我认为这项工作是一个重要突破，它再次扩展了二维材料的世界，”他说，“让我们激动地期待这种材料能否满足人们的预期。”张远波因去年从黑磷中剥离出二维单层原子材料“磷烯”（phosphorene）而闻名于世。
法国艾克斯-马赛大学的物理学家居伊•勒莱是首次制备出硅烯和锗烯的研究者之一，他也对确定锡烯的电子性质充满乐观：“正如登月一样，最重要的是迈出第一步。”（撰文：Chris Cesare 翻译：丁家琦）
Ref. 1
Physicists say they have produced stanene - a 2D layer of tin (Sn) atoms. It forms a honeycomb structure 'buckled' on top of a bismuth telluride support (centre: top view; right: side view). Microscope images pick out only the upper ridges of the sheet (left).Two years after physicists predicted that tin should be able to form a mesh just one atom thick, researchers say that they have made it. The thin film, called stanene, is reported on 3 August in
Nature Materials1. But researchers have not been able to confirm whether the material has the predicted exotic electronic properties that have excited theorists, such as being able to conduct electricity without generating any waste heat.Stanene (from the Latin
stannum meaning tin, which also gives the element its chemical symbol, Sn), is the latest cousin of graphene, the honeycomb lattice of carbon atoms that has spurred thousands of studies into related 2D materials. Those include sheets of
silicene, made from silicon atoms;
phosphorene, made from phosphorus; germanene, from germanium; and thin stacks of sheets that combine different kinds of chemical elements (see
‘The super materials that could trump graphene’).Many of these sheets are excellent conductors of electricity, but stanene is — in theory — extra-special. At room temperature, electrons should be able to travel along the edges of the mesh without colliding with other electrons and atoms as they do in most materials. This should allow the film to conduct electricity without losing energy as waste heat, according to predictions
2 made in 2013 by Shou-Cheng Zhang, a physicist at Stanford University in California, who is a co-author of the latest study.
Related stories
That means that a thin film of stanene might be the perfect highway along which to ferry current in electric circuits, says Peide Ye, a physicist and electrical engineer at Purdue University in West Lafayette, Indiana. “I'm always looking for something not only scientifically interesting but that has potential for applications in a device,” he says. “It’s very interesting work.”Stanene is predicted to be an example of a
topological insulator, in which charge carriers (such as electrons) cannot travel through a material’s centre but can move freely along its edge, with their direction of travel dependent on whether their spin — a quantum property — points ‘up’ or ‘down’. Electric current is not dissipated because most impurities do not affect the spin and cannot slow the electrons, says Zhang.
Substrate interference
But even after making stanene, Zhang and his colleagues at four universities in China have not been able to confirm that it is a topological insulator. Experimentalists at Shanghai Jiao Tong University created the mesh by vaporizing tin in a vacuum and allowing the atoms to waft onto a supporting surface made of bismuth telluride. Although this surface allows 2D stanene crystals to form, it also interacts with them, creating the wrong conditions for a topological insulator, says Zhang. He has already co-authored another paper3 examining which surfaces would work better.Ralph Claessen, a physicist at the University of Würzburg in Germany, says that it is not completely clear that the researchers have made stanene. Theory predicts that the 2D tin lattice should form a buckled honeycomb structure, with alternate atoms folding upwards to form corrugated ridges; Zhang and his team mostly saw only the upper ridge of atoms with their scanning tunnelling microscope, except in a small spot where that ridge disappeared and a lower layer of tin atoms was exposed. However, they are confident that they have created a buckled honeycomb, partly because the distance between upper and lower layers matches predictions.Claessen says that he would need to see direct measurements of the lattice’s structure — from X-ray diffraction — to be confident that the team has made stanene, and not some other arrangement of tin. This would require larger amounts of the material than Zhang and his co-authors have grown.Yuanbo Zhang, a physicist at Fudan University in Shanghai, China, who was not involved in the study, is more convinced. “I think the work is a significant breakthrough that once again expands the 2D-material universe,” he says. “It’ll be exciting to see how the material lives up to its expectations.”And Guy Le Lay, a physicist at Aix-Marseille University in France who was among the first to produce both silicene4 and germanene5, preaches optimism in the attempt to verify stanene’s electronic properties. “It’s like going to the Moon,” he says. “The first step is the crucial step.”Nature 524, 18 (06 August 2015) [p]doi[/p]:10.1038/nature.2015.18113
物理与天文系在二维晶体新材料---锡烯研究方面取得重大突破
随着石墨烯研究的巨大成功，类石墨烯结构的IV族元素二维晶体材料成为凝聚态物理和材料科学领域关注的焦点。其中，基于锡(Sn)的二维类石墨烯晶体锡烯(Stanene)被认为具有极其优越的物理特性。2012到2013年间，理论学家们预言，锡烯是一种可以在室温下工作的大能隙二维拓扑绝缘体，可实现室温下无损耗的电子输运，在未来更高集成度的电子学器件应用方面具有重要的意义。同时，通过对锡烯的调控，还能够实现拓扑超导态、优越的热电效应、室温下的反常量子霍尔效应等新奇特性。
锡烯在理论上是一种非常理想的新型量子材料，如何在实验上实现锡烯材料成为凝聚态物理和材料物理学家的重要目标。锡烯的晶体结构是基于金刚石结构的α-锡。和石墨不同，α-锡不是层状结构，无法用机械剥离的方法获得单层的锡烯。室温下，体材料的α-锡不能稳定存在。稳定的α-锡厚薄膜能够在晶格失配度非常小的半导体InSb基底上高质量的生长。但是，在InSb基底上生长单层锡烯却失败了。过去几年中，单层锡烯成为二维晶体材料和拓扑态材料领域的重大挑战之一，进展非常缓慢。2015年，上海交通大学物理与天文系凝聚态物理研究所低维物理和界面工程实验室研究团队与美国斯坦福大学张首晟团队合作首次实现了锡烯实验研究的重大突破。该实验室博士生朱锋锋在钱冬、贾金锋两位教授指导下，经过近两年的反复实验，终于找到了合适的基底材料和生长条件，利用分子束外延生长技术第一次实现了锡烯二维晶体薄膜。在整个研究过程中，研究团队面临的另外一个关键问题是如何确定外延的薄膜是锡烯薄膜，而不是其他晶体结构的锡薄膜。为了回答这个问题，研究团队克服了两大难题。第一个难题是如何确定单个锡烯薄膜中双原子层的相对高度(buckling)。通常情况下，扫描隧道显微镜只能看到最表面的一层原子，无法看到下面的第二层原子。通过大量的实验，博士生陈维炯同学终于成功观察到双原子层内部结构，精确测定了双原子层的相对高度。第二个难题是如何确定外延薄膜的电子能带结构。由于薄膜厚度不到0.2纳米，用来确定电子能带结构的角分辨光电子能谱信号中包含了众多的基底信号，造成了极大的混淆。为了解决这个问题，研究团队将锡烯的生长设备搬到同步辐射光源，利用同步辐射光源光子能量和光子偏置可变的特性，实现了锡烯的电子能带结构和基底信号的完全分离，还进一步利用原位表面电子掺杂的方法，确定了空态的部分能带结构。研究团队发现，实验精确确定的原子结构及电子能带结构和第一性原理计算的结果具有非常好的一致性，从而真正地证实外延生长的确是二维锡烯薄膜。美国斯坦福大学物理系张首晟教授和清华大学徐勇教授对锡烯的研究提供了第一性原理计算支持。该项研究成果在Nature Materials在线发表【03 Aug. 2015 http://dx.doi.org/10.1038/nmat4384 】。
锡烯薄膜的实验实现，为开展其物性研究打来了大门，将对二维拓扑电子学材料的发展起到重要的推动作用。低维物理和界面工程实验室的研究团队将进一步深入开展锡烯薄膜晶体结构和电子结构的调控、量子输运特性测量等一系列后续研究。凝聚态所低维物理和界面工程实验室隶属于教育部人工结构和量子调控重点实验室，是国家先进微结构创新中心的成员。此项研究得到了科技部、中组部、国家自然科学基金委、上海市科委、上海市教委等机构的大力支持。
Epitaxial growth of two-dimensional stanene
Nature Materials volume14, pages1020–1025 (2015) | Download Citation
Abstract
Following the first experimental realization of graphene, other ultrathin materials with unprecedented electronic properties have been explored, with particular attention given to the heavy group-IV elements Si, Ge and Sn. Two-dimensional buckled Si-based silicene has been recently realized by molecular beam epitaxy growth, whereas Ge-based germanene was obtained by molecular beam epitaxy and mechanical exfoliation. However, the synthesis of Sn-based stanene has proved challenging so far. Here, we report the successful fabrication of 2D stanene by molecular beam epitaxy, confirmed by atomic and electronic characterization using scanning tunnelling microscopy and angle-resolved photoemission spectroscopy, in combination with first-principles calculations. The synthesis of stanene and its derivatives will stimulate further experimental investigation of their theoretically predicted properties, such as a 2D topological insulating behaviour with a very large bandgap, and the capability to support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall effect.[/p]
来源：石墨烯资讯 《Nature》子刊重磅：中美合作制备出石墨烯兄弟——单层锡烯！
http://www.sohu.com/a/274338930_100014686
文章来源：中国科学技术大学 发布时间：2018-11-07 二维锡烯拓扑材料研究取得进展
http://www.cas.cn/syky/201811/t20181107_4669428.shtml
物理与天文系在二维晶体新材料---锡烯研究方面取得重大突破
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