Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting
great attention because of their interesting low-temperature properties such as
superconductivity, magnetism, and charge density waves (CDW). However, further studies
and practical applications are being slowed down by difficulties in synthesizing
high-quality materials with a large grain size and well-determined thickness. In this
work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D
NbS
2
crystals on a sapphire substrate, with a thickness-dependent
structural phase transition. NbS
2
crystals are epitaxially aligned by the
underlying c-plane sapphire resulting in high-quality growth. The thickness of
NbS
2
is well controlled by growth parameters to be between 1.5 and 10 nm
with a large grain size of up to 500 μm. As the thickness increases, we observe in
our NbS
2
a transition from a metallic 3R-polytype to a superconducting
2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission
electron microscopy (STEM) and electrical transport measurements. A
Berezinskii–Kosterlitz–Thouless (BKT) superconducting transition occurs in
the CVD-grown 2H-phase NbS
2
below the transition temperature
(
T
c
) of 3 K. Our work demonstrates thickness and
phase-controllable synthesis of high-quality superconducting 2D NbS
2
, which
is imperative for its practical applications in next-generation TMDC-based electrical
devices.