二鍗化鈦與二硒化鈦於雙層石墨稀上的橫向應變異質接面

TiTe2-TiSe2 Laterally Strained Heterojunctions

陳筠婷(Yun-Ting Chen)

指導教授 ： 林登松

國立清華大學/理學院/物理學系/碩士(2018年)

摘要

本研究主要是探討過渡金屬二硫族化物中具有電荷密度波特性的單層二鍗化鈦與單層二硒化鈦的混和與置換。實驗方法為使用掃描穿隧式顯微鏡(STM)，在低溫(LT)及室溫(RT)的STM下觀察兩種樣品：其一為透過分子磊晶之方式蒸鍍硒元素，將二鍗化鈦置換為各占一半比例之二鍗化鈦與二硒化鈦樣品，我們將透過LT-STM於77 K下觀察其表面形貌；其二為單層二鍗化鈦/雙層石墨烯樣品，以分子磊晶之方式多次蒸鍍硒元素，將二鍗化鈦置換為二硒化鈦，並以RT-STM觀察其表面變化。

在LT-STM結果如下：(1)於77 K掃描溫度，二硒化鈦的CDW特性在佔據態影像中顯得較為強烈，而二鍗化鈦的相變溫度為92 ± 3 K，其電荷密度波現象較難觀測；(2)單層二鍗化鈦與二硒化鈦的橫向應變異質接面存在因晶格失配而產生的錯位核心，其深度約300 pm，考慮二鍗化鈦與二硒化鈦具有6.5％的晶格失配，我們在此核心處發現錯排，錯位核心的發現不同於以往橫向應變異質接面情況；(3)二硒化鈦幾乎分布在薄膜邊緣處，而二鍗化鈦邊緣像是手指形狀般地被二硒化鈦所置換；(4)STM具有局域性特性，由此發現二鍗化鈦或二硒化鈦的單位晶格向量沒有一定與石墨稀的單位晶格向量一致，此在ARPES或是RHEED上並沒有觀察到。

在RT-STM實驗結果如下：透過蒸鍍硒元素，表面出現隨時間變動的不穩定二維島嶼，經過統計，這些二維島嶼的總面積與蒸鍍硒量成正比，我們認為其為已置換的二硒化鈦。總而言之，單層中獨特的電荷密度波現象與橫向應變異質接面引發了有趣的現象，這些成果將提供給未來研究過渡金屬二硫化物的學者們有用的幫助。

關鍵字

過渡金屬二硫族化合物 ； 二維材料 ； 異質接面 ； 二鍗化鈦 ； 二硒化鈦 ； 掃描穿隧顯微術 ； 分子束磊晶 ； 橫向應變 ； 晶格失配 ； 錯位核心

並列摘要

Here we report a study of the single-layer TiTe2 and TiSe2 mixed film. Both TiTe2 and TisSe2 are transition metal dichalcogenide (TMD) materials and characterized by charge density wave (CDW) in TMD materials. Using scanning tunneling microscopy (STM) at both 77 and 300 K, we observed the atomic details how TiTe2 domains on a bi-layer graphene are replaced by TiSe2, under the Se deposition.

The results in LT-STM experiment are as follows: (1) At 77 K scanning temperature, the appearance of the CDW on TiSe2 (TCDW = 232 K) is more obvious in occupied-states images. In contrast, the lower phase transition temperature of TiTe2 (TCDW = 92 K) makes the charge density wave phenomenon more difficult to observe. (2) There exists a lattice-misfit core between at the heterojunction of TiTe2 and TiSe2 domains due to a lattice mismatch of 6.5%. The core depth measures about 300 pm. These cores are to release the lattice misfit and have not been found at the interfaces of TMD films before. (3) Most of the TiSe2 areas are at the edge of the TiTe2 domains. (4) STM images also show that the unit cell vectors of TiTe2 and TiSe2 are not all be in the same direction as the unit cell vector of graphene.

On the other hand, the results in RT-STM experiment are as follows: Upon the evaporation of selenium, unstable islands appear on the surface and they change with time. The surface areas of these unstable islands increases monotonically with the amount of Se deposition; we considered they are TiSe2. The unique charge density wave phenomenon and the lattice-misfit lateral strained heterojunctions in the mixed TMD single-layer cause interesting phenomenon. The results of this study will shine light to the further understanding of TMD two-dimensional materials.

並列關鍵字

Transition Metal Dichalcogenide ； heterojunction ； TiTe2 ； TiSe2 ； STM ； Molecular Beam Epitaxy ； MBE ； Single-layer ； Dislocation ； Lattice misfit ； Laterally Strained Heterojunction

參考文獻

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