  We conducted a tip-enhanced Raman scattering spectroscopy (TERS) and
photoluminescence (PL) study of quasi-1D TaSe3 nanoribbons exfoliated onto gold
substrates. At a selenium deficiency of ~0.25 (Se/Ta=2.75,), the nanoribbons
exhibit a strong, broad PL peak centered around ~920 nm (1.35 eV), suggesting
their semiconducting behavior. Such nanoribbons revealed a strong TERS response
under 785-nm laser excitation, allowing for their nanoscale spectroscopic
imaging. Nanoribbons with a smaller selenium deficiency of ~0.15 (Se/Ta=2.85)
did not show any PL or TERS response. The confocal Raman spectra of these
samples agree with the previously-reported spectra of metallic TaSe3. The
differences in the optical response of the nanoribbons examined in this study
suggest that even small variations in Se content can induce changes in
electronic structure, causing samples to exhibit either metallic or
semiconducting character. The temperature-dependent electrical measurements of
devices fabricated with both types of materials corroborate these observations.
The density-functional-theory calculations revealed that incorporation of an
oxygen atom in a Se vacancy can result in band gap opening and thus enable the
transition from a metal to a semiconductor. However, the predicted bandgap is
substantially smaller than that derived from PL data. These results indicate
that the properties of van der Waals materials can vary significantly depending
on stoichiometry, defect types and concentration, and possibly environmental
and substrate effects. In view of this finding, local probing of nanoribbon
properties with TERS becomes essential to understanding such low-dimensional
systems.
