Controlled physical vapor growth of WSe$_{2}$ and other MX$_{2}$ monolayers

Although exfoliated monolayers of two-dimensional semiconductors such as WSe$_{2}$ show extraordinary and potentially useful optical properties, the ability to grow them in a controlled way will be critical for tuning their properties, incorporating dopants, and making devices on larger scales and with high yield. We are investigating their growth by physical vapor deposition on insulators such as silicon dioxide without catalyst, systematically varying the growth parameters (gas flow and type, sources, temperature, and substrate), with a focus on WSe$_{2}$ which has the smallest gap and strongest spin-orbit coupling of the MX$_{2}$s. While MoS$_{2}$ monolayers of high optical quality can easily be grown as triangular single crystals tens of microns in size using a simple MoS$_{2}$ source, WSe$_{2}$ proves to be much more sensitive to the growth parameters, as well as to air leaks and contamination of the furnace tube. Nevertheless we have reproducibly grown monolayer WSe$_{2}$ crystals up to 15 microns in size showing excellent optical properties using a WSe$_{2}$ source and pure hydrogen carrier gas.