First Principles Studies On Electronic And Transport Properties Of Two-dimensional Materials

The discovery of graphene has given birth to the field of two-dimensional(2D)materials.After that,more and more 2D materials have been explored successfully and attracted great attention due to their unique properties and potential applications in future devices.However,the practical applications of 2D materials still limited by defect,doping,substrate,molecule adsorption as well as contact,which arise from the process of materials preparation and device fabrication.In this paper,first principles calculations are carried out to investigate the above effect on the intrinsic electronic and transport properties of 2D materials.We hope our studies contribute to designing more efficient electronic devices in experiment.The main contents and results are as follows:(1)Electronic and transport properties of graphene with grain boundariesGrain boundaries(GBs)can hardly avoid in the preparation of graphene.To understand the effect of GBs on the electronic transport properties of graphene,we have performed first principles studies on the electronic structure and transport properties of graphene with four single GBs and two crossed GBs.Calculations indicate that the zero band gap nature of graphene is not destroyed by introducing GBs.The localized states introduced by GBs have positive contribution to the transport ability in the vicinity of the Fermi level.The transport properties across and alongthe GBs show an obvious discrepancy.Compared with the pristine graphene,an at least 50%current suppression is found in the transport across the GBs,while less current degradation presents along the GBs.The transports along other directions show that the transmission ability can be efficiently enhanced by avoiding the transport direction across the GBs.Moreover,the transport behavior of the crossed GB is akin to that of the transport across a single GB.(2)Gas adsorption on MoS2/WS2 in-plane heterojunction and the I-V responseNew artificial in-plane heterojunctions based on two-dimensional transition metal dichalcogenides fabricated in recent reports are considered to be able to offer great scope for applications.However,their responds to ambient conditions are still unknown.Here,we study by first princples calculations the adsorption of CO,H2O,NH3,NO,and NO2 gas molecules on the MoS2/WS2 heterojunction.We have determined the optimal adsorption positions and the adsorption strength,which is driven by charge transfer between the molecules and the heterojunction.Except NH3,which performs as the charge donor,all the other studied molecules act as charge accepters to the heteroj unction.The charge transfer mechanism has been discussed by analyzing the electronic structure of the molecules and the heterojunction.Further calculations show that the molecule adsorption significantly affects the electronic transport properties of the heterojunction.Both the rectification behavior and the value of the passing current can be altered by adsorption,and such sensitivity to adsorption makes the heterojunction a superior gas sensor that promises wide-ranging applications.(3)The electronic structure,mechanical flexibility and carrier mobility of black arsenic-phosphorusThe new artificial layered semiconductors-black arsenic-phosphorus(b-AsP)-which have tunable band gaps owing to good tunability of the chemical compositions,have been synthesized in a recent experiment.However,the mechanical and electrical properties of its two dimensional form are still little involved.In present work,first principles calculations are performed to systematically study the structure,mechanical,electrical,and transport properties of b-AsP monolayers.The mechanical analysis demonstrates that the exfoliation of single-layer b-AsP systems from bulk form is more difficult compared with that of pure black phosphorus(BP).In addition,the breaking strain of the b-AsP monolayer is comparable with other widely studied two dimensional materials,indicating their excellent mechanical flexibility and good potentials for flexible device applications.Besides,the electronic structures of b-AsP system monolayers are not sensitive to their specific compositions,which however,can be flexibly modulated by strain effect.The predicted carrier mobilities of b-AsP systems are directional anisotropic,which are similar with pure BP.However,the degradation of their carrier mobilities may become a practical limitation in real electronic device applications.(4)The electronic properties of black phosphorus tuned by Al2O3 dielectricChemically degradation of exfoliated black phosphorus(BP)exposure to ambient conditions can be effectively suppressed by deposition of Al2O3 dielectric on BP surface.A good understanding on the interaction between BP layer and Al2O3 dielectric is important to practical deceive application.In present paper,first principles calculations have been performed to the study the structural,energetic,and electronic properties of BP on Al-terminated and hydroxylated Al2O3(0001)surface.Our calculations indicate the band gap of monolayer BP has been enlarged about 160 meV and 92 meV after deposited respectively on Al-terminated and hydroxylated Al2O3 surface,which is mainly due to the interlayer charge transfer between BP and Al2O3 surface.However,this increasing trend of the band gap is inversed with increasing the number of BP layers.Besides,the valence-band offsets of few-layer(2-4 layer)BP/Al2O3 system are about 0.5-0.9 eV larger than that of monolayer BP/Al2O3 system,which are more suitable for creating an injection barrier.Moreover,the band gaps of BP/Al2O3 systems could be tuned by an external electric field for practical applications.(5)Electronic and transport properties of edge contact graphene-MoS2 heterostructureNanodevice based on MoS2 channel lateral connecting with graphene electrode was fabricated in recent experiment.In present paper,first principles calculations are carried out to reveal the relationship between contact geometries and electrical properties of graphene-MoS2 heterostructure.Four different contact edges between graphene and MoS2,namely,Armchair-Armchair,Zigzag-Armchair,Armchair-Zigzag,Zigzag-Zigzag,are investigated.Calculations indicate that MoS2 will be metalized as a consequence of junction formation with graphene.The metallic states located at Fermi level are mainly laid at the contact interface and dominated by 4d states of Mo atom as well as 2p states of both S and C atoms.Different contact geometries of graphene-MoS2 result in different charge transfer values in contact interfaces.Investigation on band alignments reveals that n-type Schottky contacts are formed in four graphene-MoS2 lateral heterostructures with barrier heights of 0.45?0.75 eV,which are larger than those of edge contact with Sc and Ti metals.The transmission gap of each configuration obtained using a two-probe system is unexpectedly larger than the intrinsic band gap of MoS2.The discrepancies of current-voltage behavior in two represented configurations demonstrate that contact geometries play an important role in electronic transport properties of graphene-MoS2 junctions.(6)Electronic and transport properties of 2H1-x1TxMoS2 hybrid structureThe impure 1T phase(mixed with 2H phase)MoS2 electrodes adopted in 2H phase MoS2-based Field effect transistors(FETs)has been demonstrated efficiently decreasing the contact resistance in recent experiment.Here,first principles calculations are carried out to reveal the electronic and transport properties of the hybrid electrode 2H1-x1Tx MoS2 with different 1T compositions.Our calculated results show that the stability of hybrid 2H1-x1Tx MoS2 system is decreased as increasing the concentration of 1T phase.The band gaps are largely reduced as introducing 1T phase to 2H phase MoS2.The occurrence of flat band around the Fermi level of the hybrid 2H1-x1Tx MoS2 system is not beneficial for the electronic transport.Different transmission performances of each configuration indicate that transmission probabilities are not only subjected to the concentration of 1T phase,but also depending on the specific arrangement of the two phase structure.It is found that binding energies of O2 molecule adsorbed on the phase boundaries of the hybrid 2H1-x1Tx MoS2 is enlarged compared with that on pure 2H and 1T MoS2,accompanying with charge transfer from the substrate to adsorbate.To protect the hybrid 2H1-x1Tx MoS2 state from molecule adsorption,we predict that BN sheet can be used as an effective encapsulating material.This thesis consists of nine chapters as follows.Chapter 1 gives a brief research progress of 2D materials.Chapter 2 shows a description of calculation methods based on the density functional theory and the nonequilibrium Green's function.From chapter 3 to chapter 8,the main works are present by using above methods.In chapter 3,the grain boundaries effect on the electronic and transport properties of graphene are investigated.In chapter 4,the transport prosperities of MoS2/WS2 in-plane heterojunction under ambient conditions are studied.In chapter 5,we predict the electronic structure,mechanical flexibility and carrier mobility of black arsenic-phosphorus.In chapter 6,the substrate effects on the electronic structure of black phosphorus are revealed.In chapter 7,the electronic and transport properties of graphene-MoS2 heterostructure with different contact configurations are investigated.In chapter 8,we show the electronic and transport properties of2H1-x1Tx MoS2 hybrid structure.The last chapter draws conclusions for the whole work and gives a prospect on the future work.