拓扑绝缘体二维纳米结构与器件

拓扑绝缘体是一种全新的量子功能材料, 具有绝缘性体能带结构和受时间反演对称性保护的自旋分辨的金属表面态, 属于Dirac 粒子系统, 将在新原理纳电子器件、自旋器件、量子计算、表面催化和清洁能源等方面有广泛的应用前景. 理论和实验相继证实Sb₂Te₃, Bi₂Se₃和Bi₂Te₃单晶具有较大的体能隙和单一Dirac 锥表面态, 已经迅速成为了拓扑绝缘体研究中的热点材料. 然而, 利用传统的高温烧结法所制成的拓扑绝缘体单晶块体样品常存在大量本征缺陷并被严重掺杂, 拓扑表面态的新奇性质很容易被体载流子掩盖. 拓扑绝缘体二维纳米结构具有超高比表面积和能带结构的可调控性, 能显著降低体态载流子的比例和凸显拓扑表面态, 并易于制备高结晶质量的单晶样品, 各种低维异质结构以及平面器件. 近年来, 我们一直致力于发展拓扑绝缘体二维纳米结构的控制生长方法和物性研究. 我们发展了拓扑绝缘体二维纳米结构的范德华外延方法, 实现了高质量大比表面积的拓扑绝缘体二维纳米结构的可控制备, 并实现了定点与定向的表面生长. 开展拓扑绝缘体二维纳米结构的谱学研究, 利用角分辨光电子能谱直接观察到拓扑绝缘体狄拉克锥形的表面电子能带结构, 发现了拉曼强度与位移随层数的依赖关系. 设计并构建拓扑绝缘体纳米结构器件, 系统研究其新奇物性, 观测到拓扑绝缘体Bi₂Se₃表面态的Aharonov-Bohm (AB)量子干涉效应等新奇量子现象, 通过栅电压实现了拓扑绝缘体纳米薄片化学势的调控, 并将拓扑绝缘体纳米结构应用于柔性透明导电薄膜. 本文首先简单介绍拓扑绝缘体的发展现状, 然后系统介绍我们开展的拓扑绝缘体二维纳米结构的范德华外延生长、谱学、电学输运特性以及透明柔性导电薄膜应用的研究, 最后对该领域所面临的机遇和挑战进行简要的展望.

Two-Dimensional Nanostructures of Topological Insulators and Their Devices

Three-dimensional (3D) topological insulators are a new state of quantum matter that are insulating in the bulk but have current-carrying massless Dirac surface states. Nanostructured topological insulators, such as quasi-two-dimensional (2D) nanoribbons, nanoplates, and ultrathin films with extremely large surface-to-volume ratios, distinct edge/surface effects, and unique physicochemical properties, can have a large impact on fundamental research as well as in applications such as electronics, spintronics, photonics, and the energy sciences. Few-layer topological insulator nanostructures have very large surface-to-volume ratios that can significantly enhance the contribution of exotic surface states, and their unique quasi-2D geometry also facilitates their integration into functional devices for manipulation and manufacturing. Here, we present our recent results on the controlled growth of quasi-2D nanostructures of topological insulators, as well as their novel functional devices. High quality quasi-2D nanostructures of Bi₂Se₃ and Bi₂Te₃ topological insulators have been synthesized by vapor-phase growth. Ultra-thin nanoplates of the topological insulators with uniform thickness down to a single layer have been grown on various substrates, including conductive graphene. A facile, high-yield method has been developed for growing single-crystal nanoplate arrays of Bi₂Se₃ and Bi₂Te₃ with well-aligned orientations, controlled thickness, and specific placement on mica substrates by van der Waals epitaxy. A systematic spectroscopic study, including angle-resolved photoemission spectroscopy (ARPES), micro-Raman spectroscopy, and micro-infrared spectroscopy, was carried out to investigate the quasi-2D nanostructures of topological insulators. Pronounced Aharonov-Bohm (AB) interference effects were observed in the topological insulator nanoribbons, providing direct transport evidence of the robust, conducting surface states. Transport measurements of a single nanoplate device, with a high-k dielectric top gate, showed a significant decrease in the carrier concentration and a large tuning of the chemical potential with electrical gating. We also present the first experimental demonstration of near-infrared transparent flexible electrodes based on few-layer topological insulator Bi₂Se₃ nanostructures that was epitaxially grown on a mica substrate by van der Waals epitaxy. Topological insulator nanostructures show promise as transparent flexible electrodes because of their good near-infrared transparency and excellent conductivity, which is robust against surface contamination and bending. Our studies suggest that quasi-2D nanostructures of topological insulators show promise for future electronic and optoelectronic applications.