Two-Dimensional Tungsten Disulfide Monolayers Synthesized from Solid Oxide Precursor Grown by Pulsed Laser Deposition

Description

Two-dimensional (2D) WS2 is among the most intriguing representatives of the semiconducting transition metal dichalcogenide (TMD) family with a direct bandgap ~2 eV. WS2 material shows extraordinary properties at the monolayer limit, such as strong light absorption, one of the lowest electron effective masses, high (>6.4%) photoluminescence yield, which can be further enhanced via doping, excellent thermal stability, mechanical flexibility and access to valley degree of freedom. Strong light absorption and emission at visible wavelengths opens great opportunities for its integration in ultrathin optoelectronic devices, such as light-emitting diodes, photodetectors, photovoltaic cells, and microcavity lasers.
However, for any practical application of WS2, it is required to find a method to grow uniform and highly oriented WS2 monolayers with a precise control over the layers number on a large scale. Tremendous efforts were devoted to study 2D TMD growth and examine their electronic and structural properties. Recently, several approaches were suggested to obtain a wafer-scale production of TMD monolayers. These methods include chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD)1, magnetron sputtering molecular beam epitaxy (MBE), etc. Pulsed laser deposition (PLD) is one of the prominent ways to synthesize TMD monolayers, which has demonstrated its potential for 2D materials synthesis both via direct growth2 and two-step process3.
In this paper, we report on two-step synthesis of WS2 mono- and multilayers by high-temperature sulfurization of oxygen-deficient tungsten oxide films obtained by PLD on sapphire. PLD is an established method to grow high-quality oxides with sharp interfaces, and it offers great opportunities to control crystalline quality of the material and its composition. This is achieved by deposition of precursors at different substrate temperatures and background gases (oxygen and argon). The X-ray photoelectron spectroscopy (XPS) results reveal that by changing the growth conditions in the PLD process, the properties of WOx precursors can be significantly varied.
Next, we unravel how the presence of intrinsic oxygen vacancies in the tungsten suboxide (WOx) precursor leads to a more facile conversion from WOx to WS2 films in a CVD process. Our study suggests that native oxygen vacancies in the PLD-grown precursors can serve as niches through which sulfur atoms enters the lattice and facilitates the growth of WS2 crystals with high photoluminescence (PL) emission and large domain size. Indeed, the overall PL emission increases ~40 fold while the full width of half maximum (FWHM) of the PL peak decreases significantly for WS2 crystals synthesized from precursors with a high content of oxygen vacancies as compared to that obtained from nearly stoichiometric counterpart. Based on atomic resolution images we will discuss intrinsic, grain boundaries, bilayer WS2 crystal orientation evolve from nearly stoichiometric oxides to highly reduced oxide precursors.
Our studies reveals how the epitaxial WOx precursors with tunable properties grown by PLD can be used to independently control the nucleation, lateral growth and ultimately the WS2 domain size.

1. Cun, H. et al. Wafer-scale MOCVD growth of monolayer MoS2 on sapphire and SiO2. Nano Res 12, 2646–2652 (2019).
2. Bertoldo, F. et al. Intrinsic Defects in MoS2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers. ACS Nano 15, 2858–2868 (2021).
3. Xu, X., Wang, Z., Lopatin, S., Quevedo-Lopez, M. A. & Alshareef, H. N. Wafer scale quasi single crystalline MoS2 realized by epitaxial phase conversion. 2D Mater 6, (2019).

Period	6 Dec 2021 → 8 Dec 2021
Event title	2021 Materials Research Society Fall Meeting and Exhibit
Event type	Conference
Location	Boston, United States, MassachusettsShow on map
Degree of Recognition	International