The Electronic Properties Of Transition-metal Dichalcogenides: 1T-TaS₂

The demand for new materials is increasing with the development.Over the past decade,graphene is attractive owing to its outstanding optical,electrical,thermal and mechanical properties and has become a star material in preparation of semiconductor devices.However,the band gap of graphene is zero,and field effect transistors with an ideal high switching ratio require a considerable band gap at room temperature.Therefore the switching of field effect transistors made of graphene cannot be controlled by the gate,which greatly limits the application of graphene in the electronics industry.This issue not only promotes researchers to explore various approaches to open the gap of graphene,but also stimulates general interest in two-dimensional semiconductors,such as dichalcogenides.Many transition metal chalcogenides?TMCs?present nonlayered structures such as zinc blende and wurtzite.The layered TMCs are commonly restricted to metals in groups IV–VI.Typical two-dimensional TMCs are transition metal dichalcogenides?TMDs?with a common formula of MX₂.TMCs present abundant electronic properties including metals,semimetals,insulators and semiconductors with direct and indirect band gaps and tunable electron and hole mobilities depending on the composition.Benefiting the unique physical and chemical properties,TMDs are promising and attractive in high-performance nanomaterial.In many transition-metal oxides and dichalcogenides,the electronic and lattice degrees of freedom are strongly coupled,giving rise to remarkable phenomena such as the metal-insulator transition?MIT?and charge-density wave?CDW?order.We study this interplay by tracing the instant electronic structure under ab initio molecular dynamics.Applying this method to a 1T-TaS₂ layer,we show that the CDW-triggered Mott gap undergoes a continuous reduction as the lattice temperature rises,despite a nearly constant CDW amplitude.Before the CDW order undergoes a sharp first-order transition around the room temperature,the dynamical CDW fluctuation shrinks the Mott gap size by half.The gap size reduction is one order of magnitude larger than the lattice temperature variation.Our calculation not only provides an important clue to understanding the thermodynamic behavior in 1T-TaS₂,but also demonstrates a general approach to quantify the lattice entropy effect in the MIT.