Photoelectric Properties And Regulation Of Several Low Dimensional Semiconductor Materials Using The Density Functional Theory

Photon-to-electron conversion（photoelectric conversion）is a process in which semiconductor materials absorb clean solar energy and produce a series of effects,including photovoltaic andphotoconductive effect.Photoelectric conversion applications mainly include solar cells,photoluminescence,photodetectors and photocatalytic applications.As a carrier,low dimensional semiconductor materials are favored by researchers owing to unique photoelectric properties.Common low dimensional semiconductor materials include two-dimensional（2D）layered materials,metal halogen perovskite materials and transition metal oxide materials.Therefore,how to improve the performance of photoelectric conversion applications is the main problem.In this study,the electronic and optical properties including band structures,carrier mobility,optical absorption spectra and other photoelectrical properties for 2D Ruddlesden-Popper（RP）phase perovskite,2D polymeric carbon nitride（PCN）,zero dimensional（0D）Ti O₂clusters are investigated systemically with density functional theory（DFT）for recognizing the applicability of low-dimensional semiconductor materials in photoelectric conversion applications.In order to regulate the band structure and enhance the light absorption coefficient,improve the carrier separation and change the photocatalytic reaction thermal kinetics,these methods including changing of number of layers（dimension）,interfacial engineering,solvent effect and doping engineering are used to improve the performance of low dimensional semiconductor photoelectric conversion applications.The main original results obtained are as follows.1.The electronic and optical properties including band structures,carrier mobility,optical absorption spectra and exciton-binding energies for the all-inorganic perovskite Cs₂Pb I₄with a RP structure are investigated systemically with DFT calculations.The calculated results demonstrate the thickness-dependence of electronic properties in the all-inorganic 2D RP perovskite Cs₂Pb I₄and its carrier mobility which is comparable to that of Cs Pb I₃thin films.The exciton-binding energies of perovskite Cs₂Pb I₄with a RP structure increase with the decrease of the number of layers.Besides,the value of exciton-binding energy for monolayers（181.70 me V）is more than 3 times larger than that of Cs Pb I₃（59.12 me V）.Moreover,the calculated results show that 2D layered RP perovskite Cs₂Pb I₄may not be a good material for photovoltaic applications due to its low carrier mobility and poor visible light absorption,but may be a good material for light-emission applications due to its larger thickness-dependent exciton binding energy.This work would provide a theoretical basis for other ultrathin 2D perovskite materials with potential application for photoluminescent devices or solar absorbers.2.The interfacial engineering method is used to improve the photoelectric performance of the low-dimensional metal halogen perovskite-2D RP phase Cs₂XI₂Cl₂（X=Pb,Sn,and Ge）system,and the limitations and shortcomings of its application in photodetectors are solved.DFT calculations of the electronic and optical properties are performed for In Se/Cs₂XI₂Cl₂（X=Pb,Sn,and Ge）heterostructures.The results demonstrate that the photodetection response spectrum of the heterostructures is significantly broadened as the bandgap decreases from 2.17 to 0.40 e V for the In Se/Cs₂Sn I₂Cl₂heterostructure.Moreover,the electron effective mass,m_e^*,is reduced from 1.13 m₀to 0.41 m₀for the In Se/Cs₂Ge I₂Cl₂heterostructure.The significant reductions in both the band gap and effective mass are determined to be related to the type II band alignment,wherein the In Se monolayer contributes to the conduction band minimum（CBM）while the valence band maximum（VBM）results from the all-inorganic 2D RP perovskites,which favors the carrier separation at the interface.Band unfolding further supports such a band alignment for this type of heterostructure.The proposed III-VI semiconductor In Se and all-inorganic 2D RP perovskite Cs₂XI₂Cl₂（X=Pb,Sn,and Ge）heterostructures are promising candidates for use in high-performance photodetectors.This study provides a theoretical basis for the design of high-performance III-VI In Se/Cs₂XI₂Cl₂（X=Pb,Sn and Ge）heterojunction photodetectors.3.This study investigates the photocatalytic selective oxidation of toluene adsorbed on Ti₃O₉H₆cluster in two different solvents（water and toluene）with DFT calculations.Besides,the influences of the OH groups on catalysts（Ti-OH bonds）are also considered.The results clearly demonstrate the water-induced double-edged sword effects in the photocatalytic selective oxidation of toluene.On the one hand,not only do the photocatalytic water splitting reactions and the OH groups（Ti-OH bonds）induced by photocatalytic water splitting reactions substantially restrain the holes involved activation processes,but the existence of·OH radicals mainly from water leads to the formation of byproducts（C₆H₅COOH）,which is detrimental to the high selectivity.On the other hand,the promotion effects of water are also presented owing to the solvent effect and Grotthuss mechanism in photocatalytic toluene selective oxidation reactions.Compared with toluene solvent,the hole activation process of toluene in water solvent will release more energy,and the barrieres of dehydration reactions are significantly reduced,which could promote the generation of C₆H₅CHO.This work can not only shed light on the mechanisms of photocatalytic selective oxidation of toluene into benzaldehyde and other activation reactions of sp³C-H bonds,but also design and modulate highly efficient photocatalysts.4.The effects of two stable and cooperative F-doping types(one is the formation of sp³C-F bond(F_cornertype)and another is F atom replacing-NH₂(F_N3type)were explored,and their respective roles were analyzed.The main contents are as follows:through DFT calculation method,the co-doped PCN system（F-codoped）can be found more stable,and the formation energy increase from-0.499 e V(F_corner)and-0.330 e V(F_N3)to-0.924 e V（F-codoped）.In F-codoped PCN system,F_cornerdoping reduces the valence band level,improves toluene photocatalytic oxidation capacity,polarized excited electron-hole distribution,increases electron-hole separation capacity,increases the hole distribution of F atoms,further strengthens the photocatalytic oxidation capacity,while F_N3doping reduces the band gap and improves light absorption.In addition,under the action of two synergistic F doping,the adsorption capacity of F-codoped PCN is greatly enhanced,from 0.225 e V（original PCN）to-0.445 e V,thus improving the ability of photocatalytic activated toluene.This work develops a new understanding of F doping and reveals the role of different types of F doping,providing theoretical evidence for designing and regulating more efficient low-dimensional photocatalysts and improving photocatalytic properties.