拓樸絕緣體硒化鉍聲子能量在缺陷上之變化

Abstract

近年來，拓撲絕緣體（Topological insulators, TIs）因其獨特的拓撲性質而成為熱門研究課題。由於具有較大的能帶間隙和高穩定性，硒化鉍被視為應用潛力極大的拓樸材料之一。材料中的熱學及電學性質與聲子相關，材料中聲子的性質對材料的應用有很大的影響。儘管理論計算和實驗測量已經對理想結構中的振動模式和聲子進行了深入研究，但對於缺陷引起的性質的影響仍然缺乏研究。尤其在拓樸絕緣體中，因為 TIs 的能帶間隙較窄，缺陷可能會對材料性質產生顯著影響。拉曼光譜是一種可以檢測材料的振動模式和聲子相互作用的工具。然而，拉曼光譜只能在較大的區域內進行測量，無法在奈米尺度的視角內測量，包括缺陷附近的變化。我們使用掃描穿隧顯微鏡（scanning tunneling microscopy, STM）並應用非彈性電子隧道光譜學（inelastic electron tunneling spectroscopy, IETS）在硒化鉍上進行電學性質的檢測。藉由 STM 我們成功地識別了Bi2Se3 表面上不同類型的缺陷。在掃描穿隧光譜（scanning tunneling spectroscopy, STS）的 dI/dV 曲線中觀察到靠近費米能級的能隙，我們認為這是由於聲子振動能量的結果。我們應用了 IETS 來觀察 BiSe 缺陷區域中聲子能量的變化，並觀察到缺陷處的振動能量增加。這個變化可能來自於缺陷所造成的晶體結構變化以及粒子之間的交互作用。這一結果為拓樸絕緣體在費米能級附近的電學性質提供了新的變因，並為研究拓樸絕緣體缺陷上的奈米尺度聲子相互作用開啟了新的研究方向。

Topological insulators (TIs) have been a popular study topic in recent years for their unique nontrivial topology properties. Bismuth selenide is one of the most potential TIs in application due to its large band gap and high stability. Phonons can influence the material's heat and electronic transport properties. Although the vibration modes and the induced phonons are well studied in ideal TI structure through theoretical calculations and experimental measurements, the influence from defects is still lacking. Defects may significantly influence the properties of a material, especially for TIs, since TIs have narrow band gaps and can be affected more by defects. Raman spectroscopy is a powerful tool to detect the material's vibration modes and phonon interactions. However, Raman spectroscopy can only measure the vibrations in a large area but cannot measure in a nano-scale view, including the changes near a defect. Here, we use scanning tunneling microscopy (STM) and apply inelastic electron tunneling spectroscopy (IETS) on bismuth selenide to detect electrical changes at defect sites. We successfully identified different types of defects on the Bi2Se3 surface by STM/S. A gap near the Fermi level is observed in the scanning tunneling spectroscopy (STS) dI/dV curve, which we considered as a result of phonon vibration energy. We applied IETS to see the variation of the phonon energy through a BiSe defect and observed that the vibration energy increases on the defect. The IETS change may result from the atomic structural change of crystal and particle interactions near the defect. This result suggests a new variation to electronic properties near the Fermi level on TIs and also opens a new study to nano-scale phonon interactions at defects on TIs.