Department of Chemistry , National University of Singapore , 3 Science Drive 3 117543 , Singapore.
Centre for Advanced 2D Materials and Graphene Research Centre , National University of Singapore 117546 , Singapore.
Nano Lett. 2018 Aug 8;18(8):5085-5090. doi: 10.1021/acs.nanolett.8b01953. Epub 2018 Jul 18.
Two-dimensional (2D) transition metal dichalcogenides (TMDs) manifest in various polymorphs, which deliver different electronic properties; the most prominent among them include the semiconducting 2H phase and metallic 1T (or distorted 1T' phase) phase. Alkali metal intercalation or interface strain has been used to induce semiconductor-to-metal transition in a monolayer MoS sheet, leading to exotic quantum states or improved performance in catalysis. However, the direct growth of 1T or 1T' phase MoS is challenging due to its metastability. Here, we report MBE growth of isolated 1T' and 2H MoS nanocrystals on a Au substrate; these nanocrystals can be differentiated unambiguously by their electronic states using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). By studying the initial stages of nucleation during molecular beam epitaxy (MBE) of MoS, we could identify atomic clusters (30-50 atoms) with intralayer stacking corresponding to 1T' and 2H separately, which suggests a deterministic growth mechanism from initial nuclei. Furthermore, a topological insulator type behavior was observed for the 1T' MoS crystals, where an energy gap opening of 80 meV was measured by STS in the basal plane at 5 K, with the edge of the nanocrystals remaining metallic.
二维（2D）过渡金属二卤化物（TMDs）表现出多种多晶型，从而提供不同的电子特性；其中最突出的包括半导体 2H 相和金属 1T（或扭曲的 1T'相）相。碱金属插层或界面应变已被用于诱导单层 MoS 片中的半导体-金属转变，从而导致奇异的量子态或在催化中改善性能。然而，由于其亚稳性，直接生长 1T 或 1T'相 MoS 具有挑战性。在这里，我们报告了在 Au 衬底上通过分子束外延（MBE）生长孤立的 1T'和 2H MoS 纳米晶体；这些纳米晶体可以通过扫描隧道显微镜（STM）和扫描隧道光谱学（STS）使用其电子状态明确地区分。通过研究 MoS 的分子束外延（MBE）过程中核的初始阶段，我们可以分别识别出具有层内堆叠对应于 1T'和 2H 的原子团簇（30-50 个原子），这表明存在从初始核的确定性生长机制。此外，观察到 1T' MoS 晶体的拓扑绝缘体型行为，其中在 5 K 时通过 STS 在基面中测量到 80 meV 的能隙打开，纳米晶体的边缘保持金属性。
