Materials Simulation And Rational Design Of Two-dimensional Electronic And Optoelectronic Semiconductors

In 2004,graphene was successfully exfoliated in experiment,which has triggered the great attention due to its unique structural characteristics and excellent physical properties.In addition to graphene,people also studied other types of two-dimensional（2D）materials.Such as,highly heat-resistant hexagonal boron nitride,high mobility and anisotropic black phosphorus（BP）,and various transition material chalcogenides,high mobility and stableⅢ-ⅥA semiconductors（InSe-type）,topological bismuth telluride（Bi₂Se₃）,and photo catalytic materials like bismuth oxychloride（BiOCl）.The thin thickness in 2D semiconductor materials confines the carrier in a narrow channel,which can better control the motion behavior of the carrier and enhance the application in the field of electronic devices.In addition,2D materials have a large specific surface area and are uniquely advantageous for applications in catalysis.Therefore,the systematic study of structures and physical properties will help to deepen the physical cognition of the two-dimensional materials.For the optimization of photoelectric properties of materials,materials design and the discovery of new structures will be of great significance to the development of 2D semiconductor materials.By using ab-initio density functional theory calculation,the following works are carried out for several typical 2D materials:1.Revealing the importance of interlayer coupling effect in tuning the electronic properties of indium selenide（InSe）.InSe exhibits an extremely wide band-gap variation with varying thickness,generally believed from quantum confinement effect.However,by using the one-dimensional finite potential well model（based on quantum confinement effect）,we found that it can’t describe the band-gap variations well,especially for the few-layer region.In addition,quantum confinement effect can’t explain the indirect-to-direct band-gap transition with increasing thickness.As the direct evidence for interlayer coupling,the frequencies variation for interlayer shear and breathing modes show great splitting with increasing thickness.The electron mobility also has a strong layer-dependent behavior.The above findings confirm the interlayer coupling effect should play an important role in tuning the electronic properties of InSe.2.Combiningtheswarm-intelligencestructuralmethodand first-principles calculation,a 2D centrosymmetric monolayer InSe and three new bulk structures.Under the same elemental composition,we can have a variety of monolayer structures,such as transition metal dichalcogenides（TMDCs,2H phase,1T phase,1T’phase）and phosphorene（black phosphorus,blue phosphorus）.InSe has three known structural phases,but the only building unit is a mirror-symmetric(D_3h)monolayer structure.Combining the swarm-intelligence structural method and first-principles calculation,we successfully identified a centrosymmetric(D_3d)monolayer structure,which can construct three new bulk InSe phases under thermodynamic-stable and kinetic-stable structures.Among those new phases,there is a structure shows a wider band-gap variation and higher electron mobility than the known phases.The distinction between new structures and the known structures can be achieved by using X-ray diffraction,Raman spectroscopy,and second harmonic generation signals.3.Design of a binary structure search using phosphorus and carbon as components,which successfully identified a 2D layered structure with direct band-gap transition and high mobility.Graphene and black phosphorus have high mobility and good On/off ratio,but the zero band-gap behavior of graphene and the easy decomposition of black phosphorus restricts the applications in the field of electronic devices.To this end,we designed a 2D layered structure search using phosphorus and carbon as components,and successfully identified a phosphorus-carbon（PC₆）binary layered structure with direct band-gap（0.84 eV）transition.The electron and hole mobility can reach¹0⁵ cm²V^-1s^-1 and high absorption efficiency(10⁵ cm^-1)in an extremely wide wavelength range（300²000nm）.Based on theoretical calculation,PC₆ structure shows great resistance to water and oxygen.In summary,the newly identified phosphorus-carbon binary layered structure possesses the physical properties not inferior to graphene and black phosphorus materials,and successfully overcomes their shortcomings,providing a new chance for the application of 2D semiconductor materials in the field of electronics.4.Investigation and adjusting the band-edge effective mass in 2D transition metal dechalcogenides and their superlattice structure.Band-edge effective mass is an important physical quantity for measuring the electron transport capability of semiconductor materials.Combining the band-edge effective mass of 2D transition metal dechalcogenides and one-dimensional finite potential well model,we demonstrated that the band-gap variations are controlled by both quantum confinement effect and interlayer coupling effect with varying thickness.In addition,the values show different trends with increasing thickness at different band-edge states,but mostly under 1.0 m₀.For the superlattice structure,the values can be tuned by changing the composition ratio.The regulation of effective mass provides the opportunity for the applications in electronics and optoelectronic devices.5.Demonstrating on the enhancement of hydrogen evolution reaction（HER）efficiency under alkaline condition by doping of transition metal and oxygen into 1T molybdenum disulfide（MoS₂）.Experimenter successfully synthesized a small molecule precursor structure centered with transition metal element（iron,cobalt,nickel）,the transition metal element was uniformly incorporated into the 1T-MoS₂.After testing,we found that the catalytic performance is improved,and the catalytic performance can be further elevated after oxygen doping.Theoretical calculations show that the main reason for the poor hydrogen evolution performance in 1T-MoS₂ is the high reaction barrier,and the barrier can be effectively reduced when different transition metal elements are incorporated.After analysis,we found that the oxygen doping at different positions in the structure reduce the reaction barrier in the hydrogen generation process.The synergy between oxygen and transition metal elements not only improve the sluggish alkaline HER kinetics,but also provides abundant HER active sites.6.Demonstrating that the moirésuperlattice in bismuth oxychloride（BiOCl）spiral nanosheets increases the interlayer coupling at local region and causes different distribution of electrons and holes in real space,which brings the optoresponse and photocatalytic activity under visible light.Layered BiOCl material has been widely used in the field of ultraviolet photocatalysis,the experimenters successfully synthesis a new spiral BiOCl material for the first time.The angle between adjacent layers（the angle range is 1.6°³.0°）causes the appearance of moirésuperlattice.Comparing to the non-spiral BiOCl materials,its band-gap shows⁰.6 eV decrease,which brings optoresponse and photocatalytic activity under visible light.Theoretical calculations show that the small twist angles broken the original structure symmetry,and the local area forms a pile-up structure with enhanced interlayer coupling,which causes the enhanced interaction of the Cl-2p_z orbital in the interlayer region.The formed anti-bonding energy level rises to a new valence band maximum,which causes the band gap decrease to the visible region.In addition,the moirésuperlattice changes the distribution of electrons and holes,then increases the carrier lifetimes,which in turn increases the photocatalytic properties of BiOCl materials.