Synthesis of metallic mixed 3R and 2H Nb1+xS2 nanoflakes by chemical vapor deposition

Abstract

In this work, we report the synthesis and characterization of mixed phase Nb_1+xS₂ nanoflakes prepared by chemical vapor deposition. The as-grown samples show a high density of flakes (thickness ∼50 nm) that form a continuous film. Raman and X-ray diffraction data show that the samples consist of both 2H and 3R phases, with the 2H phase containing a high concentration of Nb interstitials. These Nb interstitials sit in between the NbS₂ layers to form Nb_1+xS₂. Cross-sectional Energy Dispersive Spectroscopy analysis with transmission electron microscopy suggests that the 2H Nb_1+xS₂ region is found in thinner flakes, while 3R NbS₂ is observed in thicker regions of the films. The evolution of the phase from 2H Nb_1+xS₂ to 3R NbS₂ may be attributed to the change of the growth environment from Nb-rich at the start of the growth to sulfur-rich at the latter stage. It was also found that the incorporation of Nb interstitials is highly dependent on the temperature of the NbCl₅ precursor and the position of the substrate in the furnace. Samples grown at high NbCl₅ temperature and with substrate located closer to the NbCl₅ source show higher incorporation of Nb interstitials. Electrical measurements show linear I–V characteristics, indicating the metallic nature of the Nb_1+xS₂ film with relatively low resistivity of 4.1 × 10⁻³ Ω cm.