First-principles Investigation Of Electron-phonon Couplings And Charge Carrier Transport In Two-dimensional Nanomaterials

Charge carrier mobility is the central issue for field-effect transistors. In thisdissertation, we studied the electronic structure, electron-phonon couplings, and chargecarrier transport in the two-dimensional nanomaterials from first-principles.The key factor for intrinsic mobility is electron-phonon coupling. In the first part ofthis dissertation, charge carrier mobilities for the single-layer and bilayer graphenes arecalculated by using Boltzmann transport equation and acoustic phonon deformationpotential theory. It is found that charge mobility of single-layer graphene is about3×105cm2V-1s-1at room temperature. This result is good agreement with that of experimentsand the deformation potential theory is reasonable in this case. Due to the inter-layerinteractions, charge mobility of bilayer graphene is smaller than that of single-layergraphene. Further, the electron-phonon couplings and charge transport properties of α-and γ-graphynes are investigated from first-principles by using the density-functionalperturbation theory and Boltzmann transport equation. Wannier function-basedinterpolation techniques are applied to obtain the ultra-dense electron-phonon couplingmatrix elements throughout the Brillouin zone. The main scattering mechanisms byphonon modes with dispersion are examined beyond the deformation potential theory.Meanwhile, we benchmark the computational approach of Wannier interpolationmethod taking graphene as an example. We demonstrate that the intrinsicelectron-phonon scatterings in these two-dimensional carbon materials are dominatedby low-energy longitudinal-acoustic phonon scatterings over a wide range oftemperatures. In contrast, the high-frequency optical phonons play appreciable roles athigh temperatures, due to the significant coupling strength and increasing excitations ofoptical phonons. The electron mobilities of α-and γ-graphynes are predicted to be~104cm2V-1s-1at room temperature. The lower mobilities of α-and γ-graphynes compared tographene are due to the stronger longitudinal acoustic phonon scatterings of graphynes.Due to the fine electronic structure, the MoS2monolayer exhibits excellentelectronic and optelectronic properties. In the second part of the dissertation, weinvestigate the composition-dependent electronic properties of two-dimensionaltransition-metal dichalcogenide alloys (WxMo1-xS2) based on first-principles calculations by applying the supercell method and effective band structureapproximation. It is found that (i) the bandgaps of WxMo1-xS2monolayers first decreaseslowly as W composition increases, then a turning point shows up, after that, the bandgap rises quickly until that of WS2is reached;(ii) the hole effective mass decreaseslinearly with increasing W composition, and electron effective mass of alloys is alwayslarger than that of their binary constituents. These findings in alloys are attributed tothat (i) metal d-orbitals have different contributions to conduction bands of MoS2andWS2but almost identical contributions to valence bands;(ii) The frontierorbitals’energy levels of WS2are higer than those of MoS2. Finally, we present a briefdiscussion of the stability of WxMo1-xS2alloys with different W compositions. The freeenergies of mixing are negative for these monolayer alloys, indicating that they arethermodynamically stable at room temperature. The predicted tunable electronicstructure of monolayer WxMo1-xS2alloys shows the potential for improving theperformance of electronic and optelectronic devices based on them.