Titanium nitride as a typical transition metal nitride
(TMN) has
attracted increasing interest for its fascinating characteristics
and widespread applications. However, the synthesis of two-dimensional
(2D) atomically thin titanium nitride is still challenging which hinders
its further research in electronic and optoelectronic fields. Here,
2D titanium nitride with a large area was prepared via in
situ topochemical conversion of the titanate monolayer. The
titanium nitride reveals a thickness-dependent metallic-to-semiconducting
transition, where the atomically thin titanium nitride with a thickness
of ∼1 nm exhibits an n-type semiconducting behavior and a highly
sensitive photoresponse and displays photoswitchable resistance by
repeated light irradiation. First-principles calculations confirm
that the chemisorbed oxygen on the surface of the titanium nitride
nanosheet depletes its electrons, while the light irradiation induced
desorption of oxygen leads to increased electron doping and hence
the conductance of titanium nitride. These results may allow the scalable
synthesis of ultrathin TMNs and facilitate their fundamental physics
research and next-generation optoelectronic applications.