vanadium selenide is a promising member of transition metal dichalcogenides (TMDs) with intriguing properties that can be exploited in various applications, such as catalysis, nanoelectronics and energy storage. However, a fundamental understanding of the growth mechanism of TMDs and their defect structures remains to be established.
In this report, we have successfully fabricated mono-layer VSe2 on Au(111) via molecular beam epitaxy (MBE) and investigated the structural transformation of VSe2 by utilizing scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Intriguingly, a reversible structural transition from the homogeneous SL VSe2 to defective striped patterns with alternating bright protrusions is achieved with controllable structural manipulation using subtle regulation of deposition ratios and thermal treatment temperatures.
Detailed STM recordings of the as-synthesized VSe2 on Au(111) indicate that V atoms are first close packed in array (triangular shape) on Au(111) as dim dots while Se atoms are selectively adsorbed afterwards on top of V arrays as bright protrusions. As a result, the nearest Se-Se distance of 0.55 nm is observed in the lattice unit cell, and the lattice constant is measured to be 0.91 nm.
Intriguingly, annealing a close packed VSe2 to 450 degC induces the conversion of the pristine hexagonal phase to a defective structure with streaked appearances as a result of regular loss of Se in columns from the superficial layer. Moreover, it is observed that the unit cell vector of 1.39 nm is shrunk from its original value after depletion of Se by shrinking. Additionally, the moire periodicity along streaks is preserved, as shown in Figure 2b. Finally, it is demonstrated that the defective phase can be transformed back to the pristine VSe2 by resupplying Se. This shows the structural reversibility between V selenide compounds and may facilitate the widespread use of VSe2 in TMDs.