Design Of Several Two-dimensional Energy Conversion Semiconductor Materials Based On First-principle Calculation

Two-dimensional(2D)materials span the entire range of electronic structures from insulator to metal and show a vast array of distinct physical properties such as topological insulator behavior,spontaneous magnetization and anisotropic transport properties.In particular,some inherent properties of 2D materials provide great opportunities for their application in energy conversion.First of all,the electronic band structure changes as the dimension of 2D materials reduces,which results in a tunable band structure.Secondly,a 2D material is entirely made up of its surface,and its tremendous surface area is of benefit to energy conversion.Additionally,the material's inherent properties can dramatically alter due to the interface between the surface and the substrate as well as the presence of adatoms and defects,which provides more methods for the modification of band structure.In brief,2D materials exhibit potentially useful properties including high electron mobilities,tremendous surface area and tunable band structures,which prove them promising candidates for energy conversion.To this end,this thesis presents a few 2D systems for energy conversion as follows:(1)we systematically studied the electronic structures and potential photocatalytic properties of single-layer Group-IVB nitride halides(MNX,M = Ti,Zr,Hf;X = Cl,Br,I)in different forms by first-principles calculations.The results show that the single-layer MNX have very low formation energies,which indicates that isolation of the single layer MNX should be not difficult.The calculated band structures reveal that all single-layer MNX are semiconductors,while each of them shows a distinct type of electronic properties.Among these semiconducting nitride halides,ten members of single-layer MNX are feasible photocatalysts for splitting water.Interestingly,single-layer ?-ZrNX(X = Cl,Br,I)and ?-HfNI are direct band gap semiconductors with desirable band gaps(2.23-2.83eV),and the calculated optical absorption further convinces the excellent light absorption in visible light zone.Finally,the electronic properties and optical absorption in visible light region of single-layer MNX can be easily tuned through hybrid or doping between them because the similarity of the MNX.The high stability,versatile electronic properties,and high optical absorption make the single layer Group-IVB nitride halides promising candidates for applications in photocatalytic water-splitting.(2)A systematical study of single-layer metal phosphorus trichalcogenides(APX3,A =M?,M0.5?M0.5?;X = S,Se;M?,M? and M? represent Group-?,Group-? and Group-? metals,respectively)has been performed by density functional theory calculations.The results show that the single layer metal phosphorus trichalcogenides have very low formation energies,which indicates that the exfoliation of single layer APX3 should be not difficult.The family of single layer metal phosphorus trichalcogenides exhibits a large range of bandgaps from 1.77 to 3.94 eV,and the electronic structures are greatly affected by the metal or the chalcogenide atoms.The calculated band edges of metal phosphorus trichalcogenides further reveal that single-layer ZnPSe3,CdPSe3,Ag0.5Sc0.5PSe3 and Ago.5Ino.5PX3(X=S and Se)have both suitable band gaps for visible-light driving and sufficient over-potentials for water splitting.More fascinatingly,single-layer Ag0.5Sc0.5PSe3 is a direct band gap semiconductor,and the calculated optical absorption further convinces that such materials own outstanding properties for light absorption.Such results demonstrate that the single layer metal phosphorus trichalcogenides own high stability,versatile electronic properties,and high optical absorption,thus such materials have great chances to be high efficient photocatalysts for water-splitting.(3)We present a systematical study of new type 2D tetragonal materials of ScN and YN by density functional theory calculations.Several thermodynamically stable 2D tetragonal structures were firstly determined,and such novel tetragonal structures exhibit extraordinary electronic and optical properties.Bulk ScN and YN are indirect band gap semiconductors,while the electronic structures of 2D ScN and YN change from indirect gap to direct gap,and the band gaps are 0.62?2.21 eV for the different structures.The calculated optical spectrum suggests that 2D tetragonal ScN and YN nanosheets have high visible light absorption efficiency.These remarkable electronicproperties provide 2D ScN and YN great potential for the applications in photovoltaics and photocatalysis.(4)A systemical study of the thermoelectric properties of both bulk BiCuSeO and 2D BiCuSeO nanosheets has been performed by density functional theory calculations.Due to its nature structure of superlattice,bulk BiCuSeO possesses low thermal conductivity as well as low electric conductivity.The calculated band structures indicate that alternately stacking of Bi2O2 hole-conductive layers and Cu2Se2 electron-conductive layers appears as excited electrons translate from Cu2Se2 layer to Bi2O2 layer.The calculated thermal properties show that the electric conductivity and figure of merit(ZT)of bulk BiCuSeO can be improved by doping.2D BiCuSeO nanosheets can be prepared by mechanical exfoliation.The calculated electronic structures display that 2L-BCSO and 4L-BCSO are metallic while 3L-BCB and 3L-CBC are n type and p type semiconductor,respectively.Obviously,the 2D structure itself introduces carriers to BiCuSeO without doping.The calculated densities of states(DOS)of multilayer BCSO indicate that the conduction type of multilayer BCSO can be easily switched between P-type and N-type by controlling the number of layers and the surface layer.