Electrical transport crossover and large magnetoresistance in selenium deficient van der Waals HfSe2−x (0≤x≤0.2)

Transition-metal dichalcogenides (TMDs) have received much attention in the past decade not only due to the new fundamental physics, but also due to the emergent applications in these materials. Currently chalcogenide deficiencies in TMDs are commonly believed either during the high-temperature growth procedure or in the nanofabrication process resulting in significant changes of their reported physical properties in the literature. Here, we perform a systematic study involving pristine stochiometric ${\mathrm{HfSe}}_{2}$, Se-deficient ${\mathrm{HfSe}}_{1.9}$, and ${\mathrm{HfSe}}_{1.8}$. Stochiometric ${\mathrm{HfSe}}_{2}$ transport results show semiconducting behavior with a gap of 1.1eV. Annealing ${\mathrm{HfSe}}_{2}$ under high vacuum at room temperature causes the Se loss resulting in ${\mathrm{HfSe}}_{1.9}$, which shows unconventionally large magnetoresistivity following the extended Kohler rule at low temperatures below 50 K. Moreover, a clear electrical resistivity crossover, mimicking the metal-insulator transition, is observed in the ${\mathrm{HfSe}}_{1.9}$ single crystal. Further increasing the degree of deficiency in ${\mathrm{HfSe}}_{1.8}$ results in complete metallic electrical transport at all temperatures down to 2 K. Such a drastic difference in the transport behaviors of stoichiometric and Se-deficient ${\mathrm{HfSe}}_{2}$ further emphasizes the defect control and engineering could be an effective method that could be used to tailor the electronic structure of 2D materials, potentially unlock new states of matter, or even discover new materials.