Suppression of Mirror Domains in the Epitaxial Growth of Single Layer WS$_2$

The need for a versatile, scalable protocol to grow single layer transition-metal dichalcogenides (TMDCs) beyond exfoliation is an essential requirement for expanding the knowledge about the features of these materials and thus implementing devices based on them. However, TMDCs grown by bottom-up methods usually exhibit two mirror orientations, a condition that invalidates the possibility of applications in devices using the valley degree of freedom. We present herein a bottom-up approach to grow epitaxial single layer tungsten disulfide (WS$_2$) suppressing the growth of a detrimental mirror orientation. By means of fast X-ray photoelectron spectroscopy we show that a low W evaporation rate and high substrate temperature lead to crystalline islands with a single orientation, as demonstrated by photoelectron diffraction experiments. The morphology and size of the WS$_2$ domains were probed by employing low-energy electron microscopy. Our results can pave the way to the realization of valleytronic and optoelectronic applications based on single layer tungsten disulfide.