Alkali chalcogenides-assisted vapor-liquid-solid growth of WX2 (X = S, Se, Te)

Abstract

Promoter-assisted chemical vapor deposition (CVD) has emerged as a robust strategy for the low-temperature synthesis of diverse transition metal dichalcogenides (TMDs). In these processes, promoter-induced intermediates facilitate specific reaction pathways, enabling controlled growth via vapor-solid-solid (VSS) or vapor-liquid-solid (VLS) modes. While previous studies have primarily focused on transition metal precursors, growth pathways involving engineered chalcogen-based intermediates remain underexplored due to their volatility and low melting points. Here, we demonstrate a stabilized chalcogen strategy that enables the scalable growth of highly crystalline tungsten-based (W-) TMDs through the formation of alkali-chalcogen mixtures within the VLS regime. Atomically resolved scanning tunneling microscopy (STM) of transferred WTe₂ confirms ultraclean surfaces, attributed to the salt-like alkali-chalcogen interfacial layer that enables support-free film delamination. This work demonstrates a versatile route toward the scalable synthesis and clean manipulation of high-quality TMD.

Keywords: Alkali chalcogenides; Promoter-assisted CVD; STM; Transition metal dichalcogenides; Ultraclean manipulation; WTe2.

Fig. 1

Alkali-Chalcogen-assisted growth. a Schematic illustration of the two representative growth regimes in CVD of the W-based TMD (WX₂). b Raman spectra of the synthetic WX₂ (X = S, Se, and Te)

Fig. 2

Verification of the interfacial alkali chalcogens. a Optical microscopy images of the two-step growth process, showing the formation of a salt-like K₂Seₐ intermediate layer during Step 1, prior to the introduction of tungsten precursors. In Step 2, the WSe₂ emerges upon the supply of W-based reactants. b SEM image of the WSe₂ film after PDMS-assisted delamination, revealing a well-defined boundary between the remaining WSe₂ and the exposed SiO₂ substrate, serving as a contrast to highlight the interfacial region. c AES spectra acquired from the red (substrate) and blue (WSe₂) regions marked in (b), showing clear compositional differences between the two areas

Fig. 3

Water-assisted transfer of the as-grown films. Schematic illustration, AFM images and height of (a) the as-grown and b the transferred TMDs. c XPS spectra of Na 1s and Cl 2p core levels for NaCl-assisted WX₂ films before and after the transfer. d XPS spectra of K 2p and Cl 2p core levels for KCl-assisted WX₂ films before and after the transfer

Fig. 4

Atomically resolved STM imaging of the synthetic WTe₂. a Constant-current scanning tunneling microscopy (STM) image of a transferred WTe₂ film on highly ordered pyrolytic graphite (HOPG), showing a well-ordered atomic lattice. b Corresponding fast Fourier transform (FFT) image, with purple circles highlighting the characteristic Bragg diffraction vectors