Department of Materials Science and Engineering, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Advanced Institute for Materials Research, Tohoku University , Sendai 980-8577, Japan.
ACS Nano. 2018 Feb 27;12(2):1571-1579. doi: 10.1021/acsnano.7b08149. Epub 2018 Jan 30.
1T-1H metal-semiconductor interfaces in two-dimensional (2D) transition-metal dichalcogenides (TMDs) play a crucial role in utilizing the band gaps of TMDs for applications in electronic devices. Although the 1T-1H two-phase structure has been observed in exfoliated 2D nanosheets and chemically or physically treated TMDs, it cannot in principle be achieved in large-scale TMD monolayers grown by chemical vapor deposition (CVD), which is a fabrication method for electronic device applications, because of the metastable nature of the 1T phase. In this study we report CVD growth of 1T-1H two phase TMD monolayers by controlling thermal strains and alloy compositions. It was found that in-plane thermal strains arising from the difference in thermal expansion coefficients between TMD monolayers and substrates can drive the 1H to 1T transition during cooling after CVD growth. Moreover, grain boundaries in the 2D crystals act as the nucleation sites of the 1T phase and the lattice strain perturbations from alloying noticeably promote the formation of the metastable 1T phase. This work has an important implication in tailoring structure and properties of CVD grown 2D TMDs by phase engineering.
二维（2D）过渡金属二卤化物（TMD）中的 1T-1H 金属-半导体界面在利用 TMD 的带隙应用于电子器件方面起着至关重要的作用。尽管在剥离的 2D 纳米片中以及经过化学或物理处理的 TMD 中已经观察到 1T-1H 两相结构，但由于 1T 相的亚稳性，原则上不能在通过化学气相沉积（CVD）生长的大规模 TMD 单层中实现，因为 CVD 是电子器件应用的制造方法。在这项研究中，我们通过控制热应变和合金组成报告了 CVD 生长的 1T-1H 两相 TMD 单层。研究发现，CVD 生长后冷却过程中，TMD 单层和衬底之间的热膨胀系数差异引起的面内热应变可以驱动 1H 向 1T 转变。此外，二维晶体中的晶界作为 1T 相的成核位点，合金引起的晶格应变扰动显著促进了亚稳 1T 相的形成。这项工作对于通过相工程来调整 CVD 生长的 2D TMD 的结构和性能具有重要意义。
