Large-scale growth and characterizations of 2D ultrathin Niobium disulfides and oxides

There have been recent attempts to produce transition metal dichalcogenide (TMDC) layers via chemical vapor methods or mechanical exfoliation from bulk materials and their properties have been studied extensively in contrast to the less reported NbS2. The challenge in the synthesis of NbS2 is the delicate balance of the structure and stoichiometry of the films. We demonstrate a controlled scalable fabrication of 2D NbS2 films by synthesizing sputter-deposited ultrathin Nb films on SiO2/Si substrates under high sulfur vapor pressure. This growth method is advantageous in that the NbS2 thickness can be controlled by the starting Nb film thickness. Analyses based on the Grazing incidence X-ray diffraction, Raman scattering and UV-Vis spectroscopy indicate that the films have highly crystalline rhombohedral 3R-NbS2 structures with an optical bandgap of 0.90 ± 0.02 eV. The ultrathin NbS2 film is semiconducting and differs from the metallic nature of bulk NbS2. The morphological and compositional properties were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). This growth method can be applied to other TMDCs in which the metal has a high oxide heat of formation. We also fabricated the wide bandgap (~ 3.65 eV) orthorhombic Niobium pentoxide (Nb2O5) by systematic annealing of the sputter deposited Nb films under controlled Argon-oxygen environment. The transmittance spectrum of the Nb2O5 film shows over 90 % transmittance below the band gap energy in the visible wavelength range and decreases to less than 20 % in the ultraviolet regime. The optical properties of the films in the UV-Vis range show potential applications as UV detectors.