  Two dimensional layered materials exhibit versatile electronic properties in
their different phases. The intrinsic electronic properties of these materials
can be modulated through doping or intercalation. In this study, we
investigated the electronic properties of Hf doped ZrSe2 single crystals using
angle-resolved photoemission spectroscopy (ARPES) combined with first
principles density functional theory (DFT) calculations. It is observed that
the valence band maxima of ZrSe2, located below the Fermi level, undergo a
significant change with the introduction of Hf substitution. Hf can introduce
extra charges into the conduction band, rather than making a mixed structure of
HfSe2 and ZrSe2 band structure, which can cross the Fermi level. Compared to
the semiconducting band structure of ZrSe2, we observed that the conduction
band crosses the Fermi level at the high symmetry M point in Hf-doped ZrSe2.
This suggests an increase of electron type carriers around the Fermi level,
resulting in an extrinsic charge carrier density in the conduction band, which
can form a metallic behaviour. It can be noticed that the Hf cations can create
disorder in the form of excess atoms of Zr, which yields more carriers in the
conduction band in the shape of smeared bands. The tails of the smeared band
occupied the d-orbitals extended into the Fermi level and left the d band
below. Similarly, the electrical resistance measurements further confirm the
metallic-like character of Hf doped ZrSe2 compared to the semiconductor ZrSe2,
indicating increased carriers. This metallic like behavior is suggested to be
predisposed by the extrinsic electrons induced by the substitutional disorder.
This study further demonstrates the possibility of band gap engineering through
heavy metal doping in 2D materials.
