A Facile Strategy for the Growth of High-Quality Tungsten Disulfide Crystals Mediated by OxygenDeficient Oxide Precursors

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Abstract

Chemical vapor deposition (CVD) has been established as a versatile route for the large-scale synthesis of transition metal dichalcogenides, such as tungsten disulfide (WS2). Yet, the role of the precursor composition on the efficiency of the CVD process remains largely unknown and remains to be explored. Here, we employ Pulsed Laser Deposition (PLD) in a two-stage approach to tune the oxygen content in the tungsten oxide (WO3-x) precursors and demonstrate that the presence of oxygen vacancies in the oxide films leads to a more facile conversion from WO3-x to WS2. Using a joint study based on ab initio density functional theory (DFT) calculations and experimental observations, we unravel that the oxygen vacancies in WO3-x can serve as niches through which sulfur atoms enter the lattice and facilitate an efficient conversion into WS2 crystals. By solely modulating the precursor stoichiometry, the photoluminescence emission of WS2 crystals can be significantly enhanced. Atomic resolution scanning transmission electron microscopy imaging (STEM) reveals that tungsten vacancies are the dominant intrinsic defects in mono- and bilayers WS2. Moreover, bi- and multilayer WS2 crystals derived from oxides with a high V0 content exhibit dominant AA' /AB or AA(A..) stacking orientations. The atomic resolution images reveal local strain buildup in bilayer WS2 due to competing effects of complex grain boundaries. Our study provides means to tune the precursor composition to control the lateral growth of TMDs while revealing insights into the different pathways for the formation of grain boundaries in bilayer WS2. Tungsten disulfide (WS2) is an intriguing material belonging to the layered transition metal dichalcogenide (TMD) family. WS2 can exist in two phases with distinct electronic properties, i.e., the 2H semiconducting with a trigonal prismatic geometry and the 1T metallic phase with tetragonal symmetry.1 In particular, the 2H-WS2 phase in its monolayer form is a direct bandgap semiconductor with an energy gap of ~2 eV, as demonstrated theoretically and experimentally.2,3 WS2 monolayers exhibit high photoluminescence (PL) quantum yield (> 6.3%), which can be further enhanced via doping,4,5,6 excellent thermal stability, mechanical flexibility7 and access to valley degree of freedom.8 Several approaches have been exploited to obtain the wafer-scale synthesis of TMD monolayers, including chemical vapor deposition (CVD),9,10 metal-organic chemical vapor deposition (MOCVD), 11,12 molecular beam epitaxy (MBE),13 atomic layer deposition (ALD),14 pulsed laser deposition (PLD).15,16,17,18,19 For instance, the conventional CVD synthesis of 2D-TMDs involves evaporating stoichiometric tungsten trioxide (WO3) 4,9,20 or molybdenum trioxide (MoO3) powders. 21 The narrow optimum window requires precise control of the precursor composition, chalcogen flow rate, etc. It has been shown that the conversion of MoO3 to MoS2 evolves via an intermediate step, during which MoO3 is first partially reduced to a sulfur-oxide phase and then sulfurized into MoS2. 22 Moreover, based on previous findings, variation in the oxide precursor composition can significantly impact the growth of WS2. 23,24. Recently, it was demonstrated that the direct conversion of epitaxial MoO2 films grown by PLD into MoS2 could be exploited to produce high-quality TMDs with good device performance.19 However, despite a vast number of studies on CVD synthesis, the role of the oxygen vacancies of the precursors on the growth of TMD domains remains largely unexplored. In this work, we unravel how the presence of intrinsic oxygen vacancies in the non-stoichiometric tungsten oxides (WO3-x, 0< x<1) grown by PLD leads to a more facile conversion from WO3-x to 4 WS2 films. A two-stage growth process was developed, employing tunability of the oxygen vacancies in uniform WO3-x precursors to independently control the nucleation, lateral growth, and ultimately, the WS2 domain size. Our study suggests that native oxygen vacancies in the oxide films can serve as active sites through which sulfur atoms enter the lattice and facilitate the growth of WS2 crystals with high PL emissions and large domain sizes. Atomic resolution imaging reveals how the single and bilayer WS2 crystals develop for low-temperature and high-temperature grown precursors. By solely modulating the precursor composition, the effect of precursor composition on the lateral growth can be discerned, providing insights into the parameters that control WS2 domain growth.
Original languageEnglish
JournalNanoscale
Number of pages53
ISSN2040-3364
Publication statusAccepted/In press - 2022

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