  We have performed angle-resolved photoemission spectroscopy on
transition-metal dichalcogenide 1$T$-HfTe$_2$ to elucidate the evolution of
electronic states upon potassium (K) deposition. In pristine HfTe$_2$, an
in-plane hole pocket and electron pockets are observed at the Brillouin-zone
center and corner, respectively, indicating the semimetallic nature of bulk
HfTe$_2$, with dispersion perpendicular to the plane. In contrast, the band
structure of heavily K-dosed HfTe$_2$ is obviously different from that of bulk,
and resembles the band structure calculated for monolayer HfTe$_2$. It was also
observed that lightly K-dosed HfTe$_2$ is characterized by quantized bands
originating from bilayer and trilayer HfTe$_2$, indicative of staging. The
results suggest that the dimensionality-crossover from 3D (dimensional) to 2D
electronic states due to systematic K intercalation takes place via staging in
a single sample. The study provides a new strategy for controlling the
dimensionality and functionality of novel quantum materials.
