vanadium selenide is a member of transition metal dichalcogenides (TMDs) with interesting properties such as magnetic properties, electronic band structure and ultrafast electron transport. It has a sandwiched layer structure that is isolated by weak van de Waals forces and is typically prepared as monolayers or via mechanical or chemical exfoliation.

In this article, we investigate the structural transformation of V selenide on Au(111) during MBE growth. Initially, a honeycomb structure is observed with the moire periodicity as depicted in Figure 1b. Subsequently, defective architectures with streaked patterns appear due to depletion of Se. However, pristine hexagonal VSe2 can be restored by redepositing sufficient Se.

Several different V selenide complexes are obtained by varying the deposition parameters and post annealing treatments. Among them, single-layer (SL) VSe2 is only possible in the condition of excessive Se relative to V. In this way, a hexagon-defect phase is formed on Au(111) which has no bulk analogue.

Interestingly, a long-range ordered hexagonal pattern is found with regularly alternating bright and dim dots on the substrate. Furthermore, a rectangular unit cell is recognized with lattice parameters of 1.36 nm and 0.31 nm as depicted by STM images in Figure 2b. Moreover, the distance between streaks is preserved, despite depletion of Se after intense annealing, and the moire periodicity along streaks is retained as well. This observation highlights the structural stability of SL VSe2 at 300 degC, and may suggest that the hexagon-defect phase is metastable from the thermodynamic point of view.