Chemical Vapor Deposition Growth of Monolayer WSe<sub>2</sub> with Tunable Device Characteristics and Growth Mechanism Study

Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe<sub>2</sub>) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe<sub>2</sub> and their growth mechanism studies are important steps toward applications of WSe<sub>2</sub>. Here, we report systematical studies on ambient pressure chemical vapor deposition (CVD) growth of monolayer and few layer WSe<sub>2</sub> flakes directly on silica substrates. The WSe<sub>2</sub> flakes were characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, and photoluminescence spectroscopy. We investigated how growth parameters, with emphases on growth temperatures and durations, affect the sizes, layer numbers, and shapes of as-grown WSe<sub>2</sub> flakes. We also demonstrated that transport properties of CVD-grown monolayer WSe<sub>2</sub>, similar to mechanically exfoliated samples, can be tuned into either <i>p</i>-type or ambipolar electrical behavior, depending on the types of metal contacts. These results deepen our understandings on the vapor phase growth mechanism of WSe<sub>2</sub>, and may benefit the uses of these CVD-grown monolayer materials in electronic and optoelectronics.