Unveiling the photo-physical properties of NaCl promoted large area WS2 monolayers

Abstract

Atomically thin layers of tungsten disulfide (WS2) have attracted much attention due to their unique optical and electronic properties. The controllable, scalable, and cost-effective development of defect-free monolayers of WS2 for the fabrication of electronic devices remains a challenge. A few experiments have indicated the efficacy of halide salts as growth promoters for the low temperature growth of WS2 monolayers. However, the optical and electronic behavior of materials grown with halide assistance remains to be addressed fully. Herein, a large-area synthesis of WS2 on Si/SiO2 substrates with sodium chloride (NaCl) as a growth promoter in a chemical vapor deposition process is reported. Temperature-dependent Raman and photoluminescence (PL) spectroscopy of monolayer WS2 carried out from 77 to 433 K reveals phonon softening and a redshift in the energy bandgap at higher temperatures. The effect of the laser power on the Raman and PL spectra has also been investigated, demonstrating spectral changes and peak shifts due to laser-induced heating and enhanced electronic excitations across the gap. A comparative study of the temperature and power dependent spectra has yielded a threshold laser power beyond which sample damage may occur due to excessive heating. Electrostatic gating of the sample during PL measurements reveals distinct effects of electronic doping on the spectral content of the trionic emission. Furthermore, the photodetection capability of monolayer WS2 is demonstrated under 532 nm laser illumination, exhibiting good responsivity (∼400 A/W) and detectivity (4.89 × 109 Jones). This comprehensive analysis of monolayer WS2 provides valuable insights into its potential applications in optoelectronic devices.