  Charge density wave (CDW) is a collective quantum phenomenon in metals and
features a wave-like modulation of the conduction electron density. A
microscopic understanding and experimental control of this many-body electronic
state in atomically thin materials remain hot topics in materials physics. By
means of material engineering, we realized a dimensionality and Zr
intercalation induced semiconductor-metal phase transition in 1T-ZrX$_2$ (X =
Se, Te) ultra-thin films, accompanied by a commensurate 2 $\times$ 2 CDW order.
Furthermore, we observed a CDW energy gap up to 22 meV around the Fermi level.
Fourier-transformed scanning tunneling microscopy and angle-resolved
photoemission spectroscopy reveal that 1T-ZrX$_2$ films exhibit the simplest
Fermi surface among the known CDW materials in TMDCs, consisting only of Zr
4d-derived elliptical electron conduction band at the corners of the Brillouin
zone.
