超高真空构筑新型二维材料及其异质结构

由于量子受限效应,二维材料表现出很多三维材料所不具备的优异电学、光学、热学以及力学性能,为研究人员所关注.材料的优异物性离不开高质量材料的制备,超高真空环境可以减少杂质分子的污染与影响,提高二维材料的质量与性能.本文介绍基于超高真空环境的新型二维原子晶体材料的原位制备方法,包括利用分子束外延构筑新型二维材料、利用石墨烯插层构筑新型二维原子晶体材料异质结构以及利用扫描探针原位操纵构筑二维材料异质结构三大类.文章回顾利用这三类方法构筑的二维材料及其物理化学性质,比较三种方法各自的优势与局限性,对未来二维材料制备提供一定的指引.     

原子层厚度超表面光场调控原理及应用

超表面由亚波长尺度二维人工微结构构成,可以实现对光场振幅、相位、偏振等多参量进行调控,为光场调控提供了优良平台。二维材料作为一种新型层状结构材料,相对于三维体材料有着十分独特的光学和电学特性,其与超表面结合为纳米尺度平面光学器件的发展提供了新的可能。本文综述了基于原子层厚度的二维材料超表面发展,介绍了多种二维材料超表面光场调控机制、制备以及应用,最后对原子层厚度超表面发展面临的挑战和潜在应用进行展望。     

大面积二维磁性材料的制备及居里温度调控

当前,尽管集成电路制造工艺水平不断提高,但受到量子效应的限制,器件尺寸的缩小使业界遇到了可靠性低、功耗大等瓶颈,微电子行业延续了近50年的"摩尔定律"将难以持续. 2004年二维材料—石墨烯的问世,为突破集成电路的功耗瓶颈带来了新的机遇.由于低维特性,二维材料在一层或者几层原子厚度中表现出丰富多样的电学、磁学、力学和光学等物理特性.其中,铁磁性在信息处理、存储等技术上有着广泛的应用价值.然而,目前在实验上合成的具有铁磁性的二维材料屈指可数.同时,在二维系统中长程有序磁态会因为热涨落的因素在有限温度内受到强烈的抑制,无法在室温下保持铁磁性,这为后续工作带来了不可忽视的限制与挑战.因此实现二维磁性材料室温下的铁磁有序及其调控是现阶段需要解决的重大问题.本综述详细地介绍了二维磁性材料的发展过程、制备方法及其优越性能,并着重阐述了调控二维磁性材料居里温度的方法.最后,扼要地分析并展望了二维磁性材料在未来的应用前景.     

二维半导体微纳光腔中光与物质的耦合

二维半导体具有独特的二维材料属性、新奇的谷电子能带结构和丰富的调控自由度,为凝聚态物理、光学等领域的研究带来了机遇。然而,这些研究依然存在许多根本问题,例如光的利用效率低、量子特性易受环境扰动等。将二维半导体和精密微纳光腔进行耦合不仅为这些问题的解决提供了合适的方案,还展现了前所未有的新颖光学效应,从而为二维半导体的基础物理研究和光电应用开拓了新的研究方向。对近10年来微纳光腔中二维半导体中的光与物质耦合的研究进展进行梳理,重点讨论了二维半导体的光学特性,以及二维半导体与微纳光腔的不同耦合区域的研究进展、调控机制及其在纳米激光光源、谷电子学、量子光学等方面的潜在应用,并对未来的发展方向和机遇进行展望。     

二维过渡金属硫族化合物中的缺陷和相关载流子动力学的研究进展

原子级厚度的单层或者少层二维过渡金属硫族化合物因其独特的物理特性而被寄希望成为下一代光电子器件的重要组成部分。然而,二维材料的缺陷在很大程度上影响着材料的性质。一方面,缺陷的存在降低了材料的荧光量子效率、载流子迁移率等重要参数,影响了器件的性能。另一方面,合理地调控和利用缺陷催生了单光子源等新的应用,因此,表征、理解、处理和调控二维材料中的缺陷至关重要。本文综述了二维过渡金属硫族化合物中的缺陷以及缺陷相关的载流子动力学研究进展,旨在梳理二维材料中的缺陷及其超快动力学与材料性能之间的关系,为二维过渡金属硫族化合物材料特性和高性能光电子器件的相关研究提供支持。     

MAX 相陶瓷的结构、制备及物理性能研究

MAX 相陶瓷因具有独特的 MX 片层与A 片层交替堆叠的晶体结构,使其兼具金属和陶 瓷的优良特性,如良好的导热导电性、可加工性,同时具有良好的抗氧化性、耐腐蚀性以及耐 摩擦磨损等性能,具有非常广泛的应用前景。本文首先介绍了 MAX 相陶瓷材料的种类与晶体 结构,并简述了近几年新发现的 MAX 相陶瓷材料以及制备手段的发展动态。之后从 MAX 相 物理性能的角度出发,重点综述了几种典型 MAX 相陶瓷材料的弹性性能、电学性能、热学性 能、磁性能以及抗辐照性能的研究进展。此外,进一步介绍了MAX 相的二维衍生物 MXene 的 衍生过程、超导性以及其在电化学储能、催化领域的研究进展。最后,本文从探索 MAX 相材 料新结构的多样性、MAX 相物理性能及相关理论计算、MXene 二维材料以及相应的制备、表 征和应用等方面,展望了 MAX 相陶瓷材料的潜在研究方向及应用前景,为 MAX 相和 MXene 材料的深入研究提供了新的思路。     

二维过渡金属硫化物二次谐波:材料表征、信号调控及增强

二维过渡金属硫化物(transition metal dichalcogenides, TMDCs)由于可实现从间接带隙到直接带隙半导体的转变,能带宽度涵盖可见光到红外波段,及二维限域所带来的优异光电特性,在集成光子以及光电器件领域受到了广泛的关注.最近随着二维材料基础非线性光学研究的深入,二维TMDCs也展现出了在非线性光学器件应用上的巨大潜能.本综述聚焦于二维层状TMDCs中关于二次谐波的研究工作.首先简述一些基本的非线性光学定则,然后讨论二维TMDCs中原子层数、偏振、激子共振、能谷等相关的二次谐波特性.之后将回顾这些材料二次谐波信号的调制及增强工作,讨论外加电场、应变、表面等离激元结构、纳米微腔等方法和手段的影响机理.最后进行总结和对未来本领域工作的展望.理解二维TMDCs二次谐波的产生机制及材料自身结构与外场调控机理,将对未来超薄的二维非线性光学器件的发展产生深远的意义.     

单光子源及在量子信息领域的应用

光量子比特是量子计算和量子通信的理想候选体系之一。高效率、高品质、确定性的单光子源是实现光学量子计算和绝对安全量子通信的重要前提条件。自组装半导体量子点,又称“人造原子”,具有优良的单光子性和光子全同性,是理想的单光子源。此外,量子点可以通过外加电场,囚禁单个原子或空穴,作为光子-自旋比特的界面,构建可扩展光量子网络。微柱腔耦合的量子点,拥有很强的Purcell效应,在保持单光子性和光子全同性的同时,大大地提高了提取效率,且具有很好的相干性,可用于大规模量子计算。近年来,人们在二维单原子层材料中发现了非经典的单光子发射,使二维材料和量子光学领域得到了结合,开辟了新的研究路线：探索单原子层材料在量子技术的潜在应用。和传统固态单光子源系统相比,二维材料更易于与其他光电平台结合,可人为控制缺陷位置,有利于推动高品质、低成本单光子源的发展,得到了科学家的广泛关注。本报告首先从量子计算和量子通信两方面提出发展单光子源的意义,接着介绍单光子源的性质和产生原理,然后介绍单光子源在自组装半导体量子点和二维单原子层材料中的实现和发展,最后从光子-自旋量子隐形传态和玻色采样实验中讨论单光子源在量子计算和量子网络方面的应用前景。     

应力调控单层MoSe2的光致发光光谱特性

为了探究二维材料在片上可调有源光学器件领域的应用潜力,通过干法转印将由机械剥离法得到的高品质单层二硒化钼转移到正面涂有150nm厚聚甲基丙烯酸甲酯的双轴压电陶瓷上,制作出可应力调控发光性质的单层二硒化钼光源.对双轴压电陶瓷施加驱动电压,使电信号转化为应力信号,观察低温下(～5K)二硒化钼光致发光光谱中本征激子态、带电激子态信号峰随应力变化的规律.结果表明:在应力由拉伸应力转变为压应力并逐渐增大的过程中,本征激子态、带电激子态信号峰分别出现了～3.8meV、～3.7meV的波长蓝移.增大压应力、拉伸应力都会导致本征激子态、带电激子态信号峰光强线性降低.同时,与泵浦光圆极化相关的圆偏振度也随应力变化表现出规律性改变.此项研究证明了应力调控与单层二硒化钼光学性质之间的紧密关系,为开发各类基于二维材料的片上可调有源光学器件提供支持.     

深度神经网络和高光谱显微图像的二维材料纳米片识别

近年来,二维材料由于其独特的性质而受到了广泛关注。在制备二维层状晶体的各种方法中,机械剥离法获得的薄层二维材料晶体质量高,适用于基础研究及性能演示。然而用机械剥离法从衬底上获得的材料具有一定的随机性,可能包含了少许相对较厚的部分。实现对这些二维薄层材料有效、快速且智能化的表征有利于促进二维材料性能的进一步研究。提出了一种基于深度学习的表征方法,通过搭建的编解码结构的卷积神经网络语义分割算法,可以根据光学显微镜图像进行分割和快速识别二维材料纳米片。卷积神经网络作为深度学习在图像处理领域中的典型算法,能够对光学显微镜图像中的复杂信息进行特征提取。首先采用机械剥离制备MoS₂纳米片样本,通过光学显微镜采集高光谱图像并对样本进行标记,根据样本的厚度范围标记出不同的区域,对标记后的图像进一步处理,包括图像的颜色校准和剪切操作,得到用于网络训练和测试的数据集。针对光学图像中二维纳米薄片存在的低对比度、碎裂等特点,编码时加入残差结构和金字塔池化模型,有助于特征信息的提取;解码时融合编码路径中提取的浅层特征信息,以提高网络分割精度。实验中采用带权重的交叉熵损失函数解决类别数量不平衡问题和采用数据增强扩大数据集。对训练后的网络测试结果表明,模型像素精度为97.38%,平均像素精度为90.38%,均交并比为75.86%。之后通过迁移学习成功地对剥离的单层和双层石墨烯纳米片样本进行了识别,均交并比达到了81.63%,表明该方法具有普适性。通过MoS₂和石墨烯纳米片的识别演示,实现了深度学习在二维材料的光学显微镜图像中的成功应用。该方法有望在更多的二维材料上得到扩展并突破自动动态处理光学显微镜图像的问题,同时为其他纳米材料的高光谱图像处理提供参考。     

Two‐Dimensional Au Nanocrystals: Shape/Size Controlling Synthesis,Morphologies, and Applications

The fascinating roles of two‐dimensional (2D) nanomaterials in natural super‐strong bio‐composites (i.e. aragonite platelets in nacre and apatite platelets in bone) have aroused great interest in scientific communities to synthesize their artificial counterparts with controllable geometric and physical properties. The quantum 2D confinement of electrons recently revealed in graphene nanosheets further inspires to explore 2D metal nanocrystals with intrinsic anisotropic properties and quantum size effect. Among them, 2D Au nanocrystals stand out not only due to their unique structure‐ and environmental‐dependent properties, but also due to their rapid development in synthesis approaches and emerging applications in electronics, optics, sensing and biomedicines. In this Review the aim is to rationalize numerous newly published studies on 2D Au nanocrystals and to gain insight into their shape/size controlling synthesis, diverse morphologies, properties, and applications, potentially serving a handy guide to achieve on‐demand 2D Au nanocrystals.     

二维原子晶体的低电压扫描透射电子显微学研究

二维原子晶体材料,如石墨烯和过渡金属硫族化合物等,具有不同于其块体的独特性能,有望在二维半导体器件中得到广泛应用.晶体中的结构缺陷对材料的物理化学性能有直接的影响,因此研究结构缺陷和局域物性之间的关联是当前二维原子晶体研究中的重要内容,需要高空间分辨率的结构研究手段.由于绝大部分二维原子晶体在高能量高剂量的电子束辐照下容易发生结构损伤,利用电子显微方法对二维原子晶体缺陷的研究面临诸多挑战.低电压球差校正扫描透射电子显微(STEM)技术的发展,一个主要目标就是希望在不损伤结构的前提下对二维原子晶体的本征结构缺陷进行研究.在STEM下,多种不同的信号能够被同步采集,包括原子序数衬度高分辨像和电子能量损失谱等,是表征二维原子晶体缺陷的有力工具,不但能对材料的本征结构进行单原子尺度的成像和能谱分析,还能记录材料结构的动态变化.通过调节电子束加速电压和电子辐照剂量,扫描透射电子显微镜也可以作为电子刻蚀二维原子晶体材料的平台,用于加工新型纳米结构以及探索新型二维原子晶体的原位制备.本综述主要以本课题组在石墨烯和二维过渡金属硫族化合物体系的研究为例,介绍低电压扫描透射电子显微学在二维原子晶体材料研究中的实际应用.     

二维有机拓扑绝缘体的研究进展

新材料的发现促进了科学与技术的进步.拓扑绝缘体是近期材料领域新的研究热点,相关研究的进一步深入,不仅加深了人们对材料物理性质的理解,也为其在自旋电子学和量子计算机等领域的潜在应用提供了有价值的参考.近年来,理论工作预测了一系列由金属和有机物构筑的二维有机拓扑绝缘体,本文主要介绍六角对称的金属有机晶格与Kagome金属有机晶格两类典型的二维有机拓扑绝缘体的研究进展,其中重点介绍了理论预测的氰基配位二维本征有机拓扑绝缘体.除了理论计算方面的工作,还简要介绍了关于二维有机拓扑绝缘体材料合成方面的实验工作.二维有机拓扑绝缘体的理论与实验研究不仅拓展了拓扑绝缘体的研究体系,还为寻找新的拓扑绝缘体材料提供了思路.     

Epitaxial fabrication of AgTe monolayer on Ag(111) and the sequential growth of Te film

Transition-metal chalcogenides (TMCs) materials have attracted increasing interest both for fundamental research and industrial applications. Among all these materials, two-dimensional (2D) compounds with honeycomb-like structure possess exotic electronic structures. Here, we report a systematic study of TMC monolayer AgTe fabricated by direct depositing Te on the surface of Ag(111) and annealing. Few intrinsic defects are observed and studied by scanning tunneling microscopy, indicating that there are two kinds of AgTe domains and they can form gliding twin-boundary. Then, the monolayer AgTe can serve as the template for the following growth of Te film. Meanwhile, some Te atoms are observed in the form of chains on the top of the bottom Te film. Our findings in this work might provide insightful guide for the epitaxial growth of 2D materials for study of novel physical properties and for future quantum devices.     

Photodetectors based on junctions of two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides(TMDCs) have gained considerable attention because of their novel properties and great potential applications. The flakes of TMDCs not only have great light absorption from visible to near infrared, but also can be stacked together regardless of lattice mismatch like other two-dimensional(2D) materials. Along with the studies on intrinsic properties of TMDCs, the junctions based on TMDCs become more and more important in applications of photodetection. The junctions have shown many exciting possibilities to fully combine the advantages of TMDCs, other2 D materials, conventional and organic semiconductors together. Early studies have greatly enriched the application of TMDCs in photodetection. In this review, we investigate the efforts in photodetectors based on the junctions of TMDCs and analyze the properties of those photodetectors. Homojunctions based on TMDCs can be made by surface chemical doping,elemental doping and electrostatic gating. Heterojunction formed between TMDCs/2D materials, TMDCs/conventional semiconductors and TMDCs/organic semiconductor also deserve more attentions. We also compare the advantages and disadvantages of different junctions, and then give the prospects for the development of junctions based on TMDCs.     

Electronic Transport and Thermopower in 2D and 3D Heterostructures—A Theory Perspective

The impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials is discussed herein. Recent progress in understanding electronic transport in heterostructures of 2D materials ranging from graphene to transition metal dichalcogenides, their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS₂ lateral interfaces), and vertical van der Waals heterostructures is reviewed. Work on thermopower in 2D heterojunctions, as well as their applications in creating devices such as resonant tunneling diodes (RTDs), is also discussed. Last, the focus turns to work in 3D heterostructures. While transport in 3D heterostructures has been researched for several decades, here recent progress in theory and simulation of quantum effects on transport via the Wigner and non‐equilibrium Green's functions approaches is reviewed. These simulation techniques have been successfully applied toward understanding the impact of heterojunctions on transport properties and thermopower, which finds applications in energy harvesting, and electron resonant tunneling, with applications in RTDs. In conclusion, tremendous progress has been made in both simulation and experiments toward the goal of understanding transport in heterostructures and this progress will soon be parlayed into improved energy converters and quantum nanoelectronic devices.     

Epitaxy of III-nitrides on two-dimensional materials and its applications

III-nitride semiconductor materials have excellent optoelectronic properties, mechanical properties, and chemical stability, which have important applications in the field of optoelectronics and microelectronics. Two-dimensional (2D) materials have been widely focused in recent years due to their peculiar properties. With the property of weak bonding between layers of 2D materials, the growth of III-nitrides on 2D materials has been proposed to solve the mismatch problem caused by heterogeneous epitaxy and to develop substrate stripping techniques to obtain high-quality, low-cost nitride materials for high-quality nitride devices and their extension in the field of flexible devices. In this progress report, the main methods for the preparation of 2D materials, and the recent progress and applications of different techniques for the growth of III-nitrides based on 2D materials are reviewed.     

Stanene: A Promising Material for New Electronic and Spintronic Applications

The attractive mechanical and electronic properties of freestanding graphene has led to the exploration of two‐dimensional (2D) materials which can be integrated with contemporary electronics. As a 2D analog of graphene, stanene has become a hopeful candidate for 2D films due to its excellent quantum effects, superconductivity, and thermoelectric properties. Focusing on the promising 2D elemental material stanene, the fundamental electronic properties and experimental preparation of this material are reviewed. The prospects of utilizing the ability to manipulate the electronic properties of stanene for nanoelectronic and optoelectronic applications are determined.     

High-throughput identification of one-dimensional atomic wires and first principles calculations of their electronic states

Low dimensional materials are suitable candidates applying in next-generation high-performance electronic, optoelectronic, and energy storage devices because of their uniquely physical and chemical properties. In particular, onedimensional(1 D) atomic wires(AWs) exfoliating from 1 D van der Waals(vd W) bulks are more promising in next generation nanometer(nm) even sub-nm device applications owing to their width of few-atoms scale and free dandling bonds states. Although several 1 D AWs have been experimentally prepared, few 1 D AW candidates could be practically applied in devices owing to lack of enough suitable 1 D AWs. Herein, 367 kinds of 1 D AWs have been screened and the corresponding computational database including structures, electronic structures, magnetic states, and stabilities of these 1 D AWs has been organized and established. Among these systems, unary and binary 1 D AWs with relatively small exfoliation energy are thermodynamically stable and theoretically feasible to be exfoliated. More significantly, rich quantum states emerge,such as 1 D semiconductors, 1 D metals, 1 D semimetals, and 1 D magnetism. This database will offer an ideal platform to further explore exotic quantum states and exploit practical device applications using 1 D materials. The database are openly available at http://www.dx.doi.org/10.11922/sciencedb.j00113.00004.     

Fabrication of three‐dimensional photonic crystals in quantum‐dot‐based materials

Controlling spontaneous emission (SE) is of fundamental importance to a diverse range of photonic applications including but not limited to quantum optics, low power displays, solar energy harvesting and optical communications. Characterized by photonic bandgap (PBG) property, three‐dimensional (3D) photonic crystals (PCs) have emerged as a promising synthetic material, which can manipulate photons in much the same way as a semiconductor does to electrons. Emission tunable nanocrystal quantum dots (QDs) are ideal point sources to be embedded into 3D PCs towards active devices. The challenge however lies in the combination of QDs with 3D PCs without degradation of their emission properties. Polymer materials stand out for this purpose due to their flexibility of incorporating active materials. Combining the versatile multi‐photon 3D micro‐fabrication techniques, active 3D PCs have been fabricated in polymer‐QD composites with demonstrated control of SE from QDs. With this milestone novel miniaturized photonic devices can thus be envisaged.