  Through ${in~situ}$ photoemission spectroscopy, we investigated the change in
the electronic and crystal structures of dimensionality-controlled VO$_2$ films
coherently grown on TiO$_2$(001) substrates. In the nanostructured films, the
balance between the instabilities of a bandlike Peierls transition and a Mott
transition is controlled as a function of thickness. The characteristic
spectral change associated with temperature-driven metal-insulator transition
in VO$_2$ thick films holds down to 1.5 nm (roughly corresponding to five V
atoms along the [001] direction), whereas VO$_2$ films of less than 1.0 nm
exhibit insulating nature without V-V dimerization. These results suggest that
the delicate balance between a Mott instability and a bandlike Peierls
instability is modulated at a scale of a few nanometers by the dimensional
crossover effects and confinement effects, which consequently induce the
complicated electronic phase diagram of ultrathin VO$_2$ films.
