Theoretical Research On The Electronic Properties Of The Layered MX₂?M=Mo,W;X=S,Te? With Point Defect

Transition metal dichalcogenides(TMDs)as a kind of novel family of layered materials,which was reported to have many unique and fascinating optoelectronic properties that present a broad application prosperity in photoelectric devices,spin-related,catalysis and new energy sources.Resembled to traditional semiconductors,whether or not the TMDs can be applied to design high-performance electric and optoelectronic devices depends critically on their dupability.Therefore,a full inquiry and a profound understanding of TMDs' doping properties are necessary and significant.In this paper,using the first-principles density-functional theory calculations,we investigate systematically the doping properties of layered MoS2,WS2,MoTe2 and WTe2 by replacing M or X with substitutional impurities.The main conclusions are summarized as follow:In this study,we replace anion sites or cation sites of the host with the groups ?,? and ? elements to simulate n-type/p-type doping in MX?(M=Mo,W;X=S,Te).There are 16 substitutional doping cases in each materials.It is found that substituting by B on M site BM is hard to form in MX2.This is mainly because B-doped MX2 materials produce large crystal distortion and then lead to large defect formation energies.While the Al atom which belongs to the same group with B atom dopes on M site AlM is easy to realize compared to the former.In MoS2,WS2 and MoTe2,Al is the most desirable p-type dopant under anion-rich conditions among the group ? components,since Alm has relatively low transition and formation energies.Group V impurities can dope into the host materials through two different ways.According to the computations,it is found that the substitutions on the cation sites have deeper defect levels than those on the anion sites due to the strong electronegativity of the impuries.As and Sb as acceptor impurities are easy to dope into MoTe2 and WTe2 under cation-rich conditions,and AsTe and SbTe introduce shallow acceptor levels in MoTe2 and WTe2,respectively.This indicates high hole-concentrations for p-type materials.Sbs in MoS2 and PTe in WTe2 both have low defect formation energies under cation-rich conditions,which turns out Sb and P are suitable p-type candidates.In despite of that the substitutions of group VII on X site have low formation energies,the transition energy levels are too deep to achieve well-performance n-type MoS2 and WS2.Nevertheless,for MoTe2,the substitutions with the group VII elements on the anion sites are suitable for n-type doping on account of the shallow donor levels and low formation energies under Mo-rich condition.As to WTe2,F is the only potential donor due to the shallow transition energy of FTe.Our findings of filtering out unfavorable and identifying favorable dopants in MX2 are very valuable for experimental implementations.Furthermore,we make a comparison of the new defect calculation method with the traditional method in chapter five.The traditional method is only suitable to study the doping properties in bulk materials,however,the new method can be used for calculating the defects in monolayer MX2(M=Mo,W;X=S,Te).In this chapter,we take the S vacancy as an example to study the intrinsic defect in MoS2.It turns out that one can obtain converged defect transition energy by performing the new method.