Abstract
Unlike MoS2 ultra-thin films, where solution-based single source precursor synthesis for electronic applications has been widely studied, growing uniform and large area few-layer WS2 films using this approach has been more challenging. Here, we report a method for growth of few-layer WS2 that results in continuous and uniform films over centimetre scale. The method is based on the thermolysis of spin coated ammonium tetrathiotungstate ((NH4)2WS4) films by two-step high temperature annealing without additional sulphurization. This facile and scalable growth method solves previously encountered film uniformity issues. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to confirm the few-layer nature of WS2 films. Raman and X-Ray photoelectron spectroscopy (XPS) revealed that the synthesized few-layer WS2 films are highly crystalline and stoichiometric. Finally, WS2 films as-deposited on SiO2/Si substrates were used to fabricate a backgated Field Effect Transistor (FET) device for the first time using this precursor to demonstrate the electronic functionality of the material and further validate the method.
Highlights
IntroductionEnvelopeSiO2/Si substrate Al2O3 substrateExperimental data S 2p3/2 S 2p1/2. EnvelopeBinding energy (eV)Interestingly, after the second annealing step all the peaks from E12g (Γ) to 2LA (M)-2E22g (M) are shifted to lower wavenumbers compared to their peak positions after the first annealing step at 500 °C
EnvelopeSiO2/Si substrate Al2O3 substrateExperimental data S 2p3/2 S 2p1/2
In solution-based synthesis of semiconducting WS2 films, the main defects are pinholes and de-wetted areas over the substrate that occur in the precursor deposition step
Summary
EnvelopeSiO2/Si substrate Al2O3 substrateExperimental data S 2p3/2 S 2p1/2. EnvelopeBinding energy (eV)Interestingly, after the second annealing step all the peaks from E12g (Γ) to 2LA (M)-2E22g (M) are shifted to lower wavenumbers compared to their peak positions after the first annealing step at 500 °C. The separation of the in-plane E12g (Γ) and the out-of-plane peak A1g (Γ) Raman peaks after the first annealing step is smaller compared to after the high temperature annealing step (60.8 cm−1 for SiO2/Si and 59.9 cm−1 for sapphire). This is due to blue shifts that E12g (Γ) peaks experience in poor crystalline films as stated previously. The layer number estimation of poor crystalline WS2 films (500 °C) using Raman spectra with 532 nm excitation wavelength might be not accurate. The reason behind this is the correlation between Raman peaks difference and the intensity ratio is not valid for poor crystalline WS2 films (500 °C) as opposed to the higher crystalline WS2 films (1000 °C) which show a clear correlation between Raman peak difference and intensity ratio when resonant excitation wavelength is used for Raman spectroscopy[33]
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