Water-assisted controllable growth of atomically thin WTe₂ nanoflakes by chemical vapor deposition based on precursor design and substrate engineering strategies

Abstract

WTe2 nanostructures have intrigued much attention due to their unique properties, such as large non-saturating magnetoresistance, quantum spin Hall effect and topological surface state. However, the controllable growth of large-area atomically thin WTe2 nanostructures remains a significant challenge. In the present work, we demonstrate the controllable synthesis of 1T’ atomically thin WTe2 nanoflakes (NFs) by water-assisted ambient pressure chemical vapor deposition method based on precursor design and substrate engineering strategies. The introduction of water during the growth process can generate a new synthesized route by reacting with WO3 to form intermediate volatile metal oxyhydroxide. Using WO3 foil as the growth precursor can drastically enhance the uniformity of as-prepared large-area 1T’ WTe2 NFs compared to WO3 powders. Moreover, highly oriented WTe2 NFs with distinct orientations can be obtained by using a-plane and c-plane sapphire substrates, respectively. Corresponding precursor design and substrate engineering strategies are expected to be applicable to other low dimensional transition metal dichalcogenides, which are crucial for the design of novel electronic and optoelectronic devices.

Substrate Effects on Growth Dynamics of WTe2 Thin films

“…It also shows that while direct synthesis is a viable method for the layer‐by‐layer growth of TMDCs, more work needs to be done regarding the reactivity of W with Te. Previous attempts have included increasing the Te supply by converting WO x films on Ni y Te x substrates, [ 31 ] using H 2 O to convert WO 3 to WO 2 (OH), [ 35 ] making the precursor film more reactive to form WTe 2 , and utilizing H 2 Te. [ 30 ] Nevertheless, these methods have proven ineffective for synthesizing large‐area uniform WTe 2 at the thin film limit.…”

“…[27,28] There are several synthesis studies on WTe 2 which have investigated the reactivity of W with Te through direct deposition of W metal or amorphous WO x . [30][31][32][33][34][35] In this work, we investigated the role of substrate on the growth of large area, thickness-controlled WTe 2 films, which was not investigated in these prior studies. We synthesize WTe 2 by tellurizing ALD-grown WO x precursor thin films of varying thicknesses using sapphire and SiO 2 substrates and focus on the growth of WTe 2 with thickness <5 nm to study the effects of the growth substrate on the resulting morphology of WTe 2 .…”

Large-area single-crystal TMD growth modulated by sapphire substrates

“…Due to the twofold symmetry of the a-plane sapphire, the diffusion barriers along the step and across the step directions are distinct, which results in the growth of materials that exhibit anisotropy. [78][79][80]94 In addition, the a-plane sapphire is a nonpolar surface and the growth on the non-polar surface can make the luminescence efficiency increase, which provides ideas for device luminescence efficiency improvement.…”

“…(b) OM images of WTe 2 NFs on the a-plane sapphire substrate (top) and the a-plane sapphire substrate (bottom). 80 Copyright 2022, IOP Publishing Ltd. (c) OM images of triangle MoS 2 grains grown on the c-plane sapphire substrate (top) and OM images of rectangle MoS 2 grains grown on the a-plane sapphire substrate (bottom). 78 Copyright 2020, Wiley-VCH.…”

“…In addition, the appropriate amount of water can increase the nucleation density, and regulate the nucleation and growth process. 80 Modified reverse flow CVD was employed to ensure high control over the nucleation and growth of TMDs at high temperatures. Reverse flow is used during the temperature ramping stage to prevent undesired nucleation and growth before the optimal growth temperature was reached.…”