  Two-dimensional materials constitute a promising platform for developing
nanoscale devices and systems. Their physical properties can be very different
from those of the corresponding three-dimensional materials because of extreme
quantum confinement and dimensional reduction. Here we report a study of
TiTe$_2$ from the single-layer to the bulk limit. Using angle-resolved
photoemission spectroscopy and scanning tunneling microscopy and spectroscopy,
we observed the emergence of a (2 x 2) charge density wave order in
single-layer TiTe$_2$ with a transition temperature of 92 $\pm$ 3 K. Also
observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K.
Surprisingly, no charge density wave transitions were observed in 2- and
multi-layer TiTe$_2$, despite the quasi-two-dimensional nature of the material
in the bulk. The unique charge density wave phenomenon in the single layer
raises intriguing questions that challenge the prevailing thinking about the
mechanisms of charge density wave formation.
