  Layered quasi-two-dimensional transition metal dichalcogenides (TMDCs), which
can be readily made in ultrathin films, offer excellent opportunities for
studying how dimensionality affects electronic structure and physical
properties. Among all TMDCs, NbSe2 is of special interest; bulk NbSe2 hosts a
charge-density-wave phase at low temperatures and has the highest known
superconducting transition temperature, and these properties can be
substantially modified in the ultrathin film limit. Motivated by these effects,
we report herein a study of few-layer NbSe2 films, with a well-defined
single-domain orientation, epitaxially grown on Gallium Arsenide (GaAs).
Angle-resolved photoemission spectroscopy (ARPES) was used to determine the
electronic band structure and the Fermi surface as a function of layer
thickness; first-principles band structure calculations were performed for
comparison. The results show interesting changes as the film thickness
increases from a monolayer (ML) to several layers. The most notable changes
occur between a ML and a bilayer, where the inversion symmetry in bulk NbSe2 is
preserved in the bilayer but not in the ML. The results illustrate some basic
dimensional effects and provide a basis for further exploring and understanding
the properties of NbSe2.
