  Imposing additional confinement in two-dimensional (2D) materials can yield
further control over the associated electronic, optical, and topological
properties. However, synthesis of ultra-narrow nanoribbons (NRs) remains a
challenge, particularly for the transition metal dichalcogenides (TMDs), and
synthesizing TMD NRs narrower than 50 nm has remained elusive. Here, we report
the vapor-phase synthesis of ultra-narrow TaS2 NRs. The NRs are grown within
the hollow cavity of carbon nanotubes, thereby limiting their lateral
dimensions and layer number, while simultaneously stabilizing them against the
environment. The NRs reach the monolayer (ML) limit and exhibit widths as low
as 2.5 nm. Atomic-resolution scanning transmission electron microscopy (STEM)
reveals the detailed atomic structure of the ultra-narrow NRs and we observe a
hitherto unseen atomic structure supermodulation phenomenon of ordered defect
arrays within the NRs. First-principles calculations based on density
functional theory (DFT) show the presence of flat bands, as well as edge- and
boundary-localized states, and help identify the atomic configuration of the
supermodulation. Nanotube-templated synthesis represents a unique,
transferable, and broadly deployable route toward ultra-narrow TMD NR growth.
