Feasible Structure Manipulation of Vanadium Selenide into VSe2 on Au(111)

Abstract

Vanadium diselenide (VSe₂), a member of the transition metal dichalcogenides (TMDs), is proposed with intriguing properties. However, a comprehensive investigation of VSe₂ (especially regarding on the growth mechanism) is still lacking. Herein, with the molecular beam epitaxy (MBE) measures frequently utilized in surface science, we have successfully synthesized the single-layer VSe₂ on Au(111) and revealed its structural transformation using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT). Initially, formation of the honeycomb structure is observed with the moiré periodicity, which is assigned to VSe₂. Followed by stepwise annealing, defective structures with streaked patterns start to emerge due to the depletion of Se, which can be reversed to the pristine VSe₂ by resupplying Se. With more V than Se deposited, a new compound that has no bulk analogue is discovered on Au(111), which could be transformed back to VSe₂ after providing excessive Se. As the realization of manipulating V selenide phases is subtly determined by the relative ratio of V to Se and post-annealing treatments, this report provides useful insights toward fundamental understanding of the growth mechanism of TMDs and might promote the wide application of VSe₂ in related fields such as catalysis and nanoelectronics.

Keywords: controlled structural manipulation; density functional theory calculations; phase transition; scanning tunneling microscopy; transition metal dichalcogenides.

Figure 1

(a) The overview STM of the V selenide grown on Au(111). Black arrows: high-symmetry directions of the Au(111) substrate. (b) High-resolution STM with the moiré pattern resolved. Unit cells of the V selenide as well as the moiré pattern are indicated by colored rectangles, while FFT is shown inset indicating the periodicity of moiré structure. (c) DFT predicated configuration of VSe₂ on Au(111) with top and side views. Au: yellow; Se: green; V: red. Scanning parameters: (a) U_bias = −1.0 V, I_{tunneling (t)} = 0.6 nA; (b) U_bias = −1.2 V, I_t = 1.2 nA.

Figure 2

Formation of the streaked structure after annealing the pristine VSe₂ at 450 °C. (a) STM image of linear chains with three orientations corresponding to the three-fold symmetry of Au(111). (b) Atomically resolved STM of the striped pattern with two different periodicities. (c) Configuration for the typical streaked structure from DFT calculations with top and side views. The unit cell is highlighted by the black parallelogram. Scanning parameters: (a) U_bias = 1.2 V, I_t = 5.6 nA; (b) U_bias = −1.2 V, I_t = 3.0 nA.

Figure 3

The observation of clusters formation on Au(111) after annealing at 480 °C. (a) The overview STM. (b) Zoom-in view showing the identical adsorption configuration. (c) Top and side views of the optimized model of the V cluster on Au(111). The cluster is marked with the white triangle. Scanning parameters: (a) U_bias = −0.9 V, I_t = 0.6 nA; (b) U_bias = −0.6 V, I_t = 4.2 nA.

Figure 4

The hexagon-like pattern formed on Au(111) with a varied ratio of V to Se. (a) The overview and (b) zoom-in view of the hexagonal defect structures with the white rhombus highlighting the unit cell. (c) Top and side views of the relaxed model of the hexagonal structures. Scanning parameters: (a) U_bias = −1.5 V, I_t = 0.92 nA; (b) U_bias = −1.5 V, I_t = 1.05 nA.