  Understanding and controlling the electronic structure of thin layers of
quantum materials is a crucial first step towards designing heterostructures
where new phases and phenomena, including the metal-insulator transition (MIT),
emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth
and local site environment through selection of the number of atomic layers
deposited. We take CaVO3, a correlated metal in its bulk form that has only a
single electron in its V4+ 3d manifold, as a representative example. We find
that thick films and ultrathin films (6 unit cells, uc, and below) are metallic
and insulating, respectively, while a 10 uc CaVO3 film exhibits a clear thermal
MIT. Our combined X-ray absorption spectroscopy and resonant inelastic x-ray
scattering (RIXS) study reveals that the thickness-induced MIT is triggered by
electronic bandwidth reduction and local moment formation from V3+ ions, that
are both a consequence of the thickness confinement. The thermal MIT in our 10
uc CaVO3 film exhibits similar changes in the RIXS response to that of the
thickness-induced MIT in terms of reduction of bandwidth and V 3d - O 2p
hybridization.
