  By combining first principles theoretical calculations and experimental
optical and structural characterization such as spectroscopic ellipsometry,
X-ray spectroscopy, and electron microscopy, we study the dielectric
permittivity and plasmonic properties of ultrathin TiN films at an atomistic
level. Our results indicate a remarkably persistent metallic character of
ultrathin TiN films and a progressive red shift of the plasmon energy as the
thickness of the film is reduced. The microscopic origin of this trend is
interpreted in terms of the characteristic two-band electronic structure of the
system. Surface oxidation and substrate strain are also investigated to explain
the deviation of the optical properties from the ideal case. This paves the way
to the realization of ultrathin TiN films with tailorable and tunable plasmonic
properties in the visible range for applications in ultrathin metasurfaces and
flexible optoelectronic devices.
