Design and development of the growth-process for the production of wafer-scale spatially homogeneous thickness controlled atomically thin transition metal dichalcogenides (TMDs) is one of the key challenges to realize modern electronic devices. Here, we demonstrate rapid and scalable synthesis of MoS2 films with precise thickness control via gas-phase chemical vapor deposition approach. We show that a monolayer MoS2 can be synthesized over a 2-in. sapphire wafer in a growth time as low as 4 min. With a linear growth rate of 1-layer per 4 min, MoS2 films with thicknesses varying from 1- to 5-layers with monolayer precision are produced. We propose that, in addition to Raman spectroscopy, the energy splitting of exciton bands in optical-absorbance spectra may be another choice for layer thickness identification. With suitable precursor selection, our approach can facilitate the rapid synthesis of spatially homogeneous atomically thin TMDs on a large scale.

Atomically thin MoS 2 hosts rich and distinct vibrational spectral features, which are prominent under selective excitation energies near the excitonic transitions. In this work, we have investigated the resonant Raman scattering of the MoS 2 layers of different thicknesses, from monolayer to five-layer samples, measured near resonance with the A excitonic transition. We show that the near-resonance excitation (1.96 eV) resulted in a Davydov splitting of the out-of-plane A-like phonon mode (A 1g ) around 406 cm −1 caused by the weak interlayer interaction. The number of Davydov splitting components (N) equals the number of layers (NL) of the MoS 2 , suggesting that it can be used as a thickness indicator. The origin of various Davydov components is understood based on a simple nearest-interlayer interaction. We extend our investigation to identify some acoustic phonon modes associated with characteristic second-order double-resonance Raman and disorderinduced bands.

Band gap engineering via 2D alloying is a vital strategy for three-atom-thick transition metal dichalcogenides based optoelectronics, valleytronics and nanophotonics. Here we demonstrate the growth of Mo1−xWxS2 ternary alloy monolayers and precise compositional tuning for the entire range of x from 0 to 1 using the gas-phase precursor approach. By means of Raman spectroscopy we show that W alloying in MoS2 lattice can lead to a tensile strain of ∼0.8%. The alloying-induced tensile strain plays a key role in observing redshift in optical absorption and photoluminescence (PL) bands and resulted an unusual bandgap bowing. The coupling of tensile strain and alloying effect allowed us to tune the overall PL emission energy to as large as 185 meV. Our optical spectroscopy results indicate three different phase-regions for the Mo1−xWxS2 alloy system. For x < 0.37, the alloys exhibit MoS2-like nature, whereas, WS2-like behavior is observed for x > 0.64, and a mixed behavior for 0.37 ≤ x ≤ 0.64.

A viable solution for the large-scale production of MoS2 thin films directly on SiO2/Si with relatively larger growth rates is demonstrated via a gas-phase precursor-assisted chemical vapor deposition approach. Comprehensive Raman and photoluminescence measurements reveal the excellent spatial homogeneity and high optical quality of the MoS2 thin films. The electrical properties of the MoS2 layers were tested by fabricating arrays of back-gated monolayer MoS2 field-effect transistors. Our findings suggest that the electrical properties are influenced by the grain size of the MoS2 monolayers.

Due to the lowest formation energies, sulfur vacancies are inevitable in the vapor-phase chemical vapor deposition (CVD) of MoS2, which act as deep donors and induce midgap defect states, making the material intrinsically n-type. The postgrowth oxygen passivation of such defects has been the subject of a large number of recent studies because passivation of defects augments the photoluminescence quantum yield by several orders. In this study, by introducing an SiO2/Si wafer in close proximity to the growth substrate, we were able to supply trace oxygen in situ during the growth while simultaneously enabling chemisorption of oxygen at defect sites on the basal plane of large-area MoS2 monolayers. Low-temperature photoluminescence spectroscopy allowed us to distinguish clearly the nature of oxygen bonding in defective MoS2 grown with and without the trace oxygen. Chemisorption of oxygen enabled elimination of defect-related bound exciton emission at the near band edge transition of MoS2, leading to about 300% enhancement in the photoluminescence. We observed unusual splitting of the first-order A1g Raman mode in monolayer MoS2 films when the sulfur vacancies are not compensated by oxygen. The present study provides new experimental evidence to better distinguish between chemisorption and physisorption of oxygen and may serve as an effective way to tune the optical properties of van der Waals crystals during the large-area CVD process.