Nanoscale Near-Field Tomography of Surface States on (Bi0.5Sb0.5)2Te3.
Nanoscale Near-Field Tomography of Surface States on (Bi0.5Sb0.5)2Te3.
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DOI：
10.1021/acs.nanolett.8b03008
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发表时间：
2018-11
期刊：
影响因子：
10.8
通讯作者：
F. Mooshammer;F. Sandner;M. Huber;M. Zizlsperger;H. Weigand;M. Plankl;Christian Weyrich;M. Lanius;J. Kampmeier;G. Mussler;D. Grützmacher;J. Boland;T. Cocker;R. Huber
中科院分区：
文献类型：
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作者：
F. Mooshammer;F. Sandner;M. Huber;M. Zizlsperger;H. Weigand;M. Plankl;Christian Weyrich;M. Lanius;J. Kampmeier;G. Mussler;D. Grützmacher;J. Boland;T. Cocker;R. Huber
三维拓扑绝缘体（TI）由于其在拓扑保护表面态（TSS）中容纳无质量狄拉克费米子的可能性而引起了极大的兴趣，这可能使新型高速电子学成为可能。然而，最近的报道已经概述了这些材料中的能带弯曲效应的重要性，这导致在表面具有有限质量的额外二维电子气（2DEG）。TI表面在纳米尺度上也是高度不均匀的，这在传统的远场研究中被掩盖了。在这里，我们使用近场显微镜在中红外光谱范围内探测定制的（Bi0.5Sb0.5）2 Te 3结构的局部表面特性，在所有三个空间维度上具有纳米精度。应用nanotomography和nanospectroscopy，我们揭示了几个纳米厚的层的高表面电导率和检索其本地的介电函数，而不假设任何模型的光谱响应。这使我们能够直接区分不同类型的表面状态。弯曲导带中量子限制所形成的大量2DEG内的子带间跃迁表现为尖锐的、表面束缚的洛伦兹形共振。介电函数虚部的额外宽带背景可能由TSS引起。跟踪子带间共振与纳米空间精度，我们观察到其频率的变化，可能源于掺杂或/和Bi和Sb之间的混合比的局部变化。我们的研究结果突出了研究这些新型材料的表面在纳米尺度上的重要性，直接访问本地的光学和电子性能通过介电函数。
Three-dimensional topological insulators (TIs) have attracted tremendous interest for their possibility to host massless Dirac Fermions in topologically protected surface states (TSSs), which may enable new kinds of high-speed electronics. However, recent reports have outlined the importance of band bending effects within these materials, which results in an additional two-dimensional electron gas (2DEG) with finite mass at the surface. TI surfaces are also known to be highly inhomogeneous on the nanoscale, which is masked in conventional far-field studies. Here, we use near-field microscopy in the mid-infrared spectral range to probe the local surface properties of custom-tailored (Bi0.5Sb0.5)2Te3 structures with nanometer precision in all three spatial dimensions. Applying nanotomography and nanospectroscopy, we reveal a few-nanometer-thick layer of high surface conductivity and retrieve its local dielectric function without assuming any model for the spectral response. This allows us to directly distinguish between different types of surface states. An intersubband transition within the massive 2DEG formed by quantum confinement in the bent conduction band manifests itself as a sharp, surface-bound, Lorentzian-shaped resonance. An additional broadband background in the imaginary part of the dielectric function may be caused by the TSS. Tracing the intersubband resonance with nanometer spatial precision, we observe changes of its frequency, likely originating from local variations of doping or/and the mixing ratio between Bi and Sb. Our results highlight the importance of studying the surfaces of these novel materials on the nanoscale to directly access the local optical and electronic properties via the dielectric function.