Optoelectronic Properties Of Graphyne Based Van Der Waals Heterostructures

With the development of the science and technology,electronic devices need higher performance,smaller size as well as lower energy consumption.However,as the feature size decreases,the traditional transistor would encounter numerous problems,such as short channel effect,quantum tunneling effect,more complex processes,et al.These problems can not only severely deteriorate the device performance but also sharply increase the fabrication cost.Two-dimensional(2D)materials have got great attention in microelectronic industry due to its no dangling bond and rich physico-chemical properties.Especially,2D van der Waals heterostructures(2D vd WHs)are combined two different 2D materials.They can not only take fully the advantages of individual 2D materials but also emerge abnormal phenomena by interfacial coupling.Researches on vd WHs provide a new opportunity for functionalized materials and devices.Graphyne(Gyne),a novel 2D carbon material with sp²-and sp-hybridized carbon atom,have a broad application prospect in photoelectric device because of the excellent properties,including unique semiconductor character,high carrier mobility and wide band optical absorption.Nevertheless,the research relevant to photodetector is still rare because the fabrication of the Gyne is still in primary stage.Thus,studying and exploring the performances of the Gyne-based photodetector by the first-principles calculation can promote both academic research and experimental progress.Furthermore,the intrinsic mechanism of experimental phenomena that the introduction of Gyne can significantly enhance the photocatalytic performance of the g-C₃N₄ is considerable ambiguity:whether forming 2D/2D heterostructure with built-in electric field or yielding new C-N bond for transport carrier?In this dissertation,the Gyne-related issues are systematically studied by the first-principles calculation.The specific research contents and main results are as follows:?.Firstly,the electronic and optical properties of the Gyne monolayer and Gyne bilayers are investigated.The Gyne monolayer and Gyne bilayers show narrow direct band gap semiconductor,both of the conduction band minimum(CBM)and valence band maximum(VBM)are located in M point.While the band gaps of Gyne bilayer(0.094e V?0.418 eV)are smaller than that of Gyne monolayer(0.450 eV)due to the lattice symmetry breaking by different stacking sequences.Moreover,both of Gyne monolayer and Gyne bilayers show wide band optical absorption from infrared(IR)to ultraviolet(UV),although the absorption intensity of Gyne bilayer is stronger than that of Gyne monolayer.Further research found that the band gap of Gyne bilayer increases under biaxial tensile strain,and decreases under biaxial compressive strain and uniaxial strain.This band gap modulation character is similar with Gyne monolayer.?.The electronic properties of Gyne/BNyne vd WHs with different stacking sequences,external strain and electric field are investigated by the first-principles simulations.It is found that Gyne/BNyne vd WHs possesses type I band alignment regardless of the stacking sequence and external strain.Only under strong positive electric field,the transition of type I/type II band alignment takes place.This Gyne/BNyne vd WHs with type II band alignment shows both direct band gap and significant optical absorption of wide band range,which is significant for wide band photodetector with high quantum efficiency.?.Hydrogenation(H-)and halogenation(M-)are proposed to modify the type of the band alignment of Gyne/BNyne.It is found that partial H-Gyne,F-Gyne and(C-chain1+ring1)Cl-Gyne can change the type of the band alignment of Gyne/BNyne to type II,which can realize spacial separation of the electrons and holes.Furthermore,all the H-or M-Gyne/BNyne vd WHs with type II band alignment show strong light absorption in UV range,besides,(C-chain1+ring1)Cl-Gyne/BNyne also shows optical absorption in visible region.Our research can not only guide the fabrication of the UV photodetector,but also develop a novel method to artificially manipulate the electronic properties of 2D vd WHs.?.A novel Gyne/BP vd WHs is designed for near-infrared(NIR)self-powered photodetector with high quantum efficiency and high robustness.The self-powered photoresponsivity of Gyne/BP vd WHs can reach up to 120 m A/W.This superior photoresponsivity can be contributed by the interfacial properties,including type II band alignment,high carrier mobility,big conduction band offset(CBO)and valence band offset(VBO),as well as strong absorption in NIR.Surprisingly,the high self-powered photoresponsivity is robust against external elastic strain and the thickness of Gyne layer due to its robust type-II band alignment,large band offset and intrinsic build-in electric field.Compared with previous reported 2D vd WHs NIR self-powered photodetector,Gyne/BP vd WHs not only possess robust type II band alignment to ensure effective carrier separation but also have high carrier mobility to yield outstanding current transport,paving the road towards NIR photodetector with high performance.?.The inner mechanism of the enhanced photocatalytic performance by introducing Gyne to g-C₃N₄ was studied by the first-principles calculation,and a new method via BN-pair doping for further enhancing the photocatalytic performance of the Gyne/g-C₃N₄vd WHs is proposed.Our calculations indicate that a 2D/2D hetero-interface is formed between Gyne and g-C₃N₄,and Gyne/g-C₃N₄ vd WHs naturally show type I band alignment,thus the high carrier mobility and strong optical absorption in visible light of the Gyne can be effectively utilized.All those explain the mechanisms of the enhanced photocatalytic performance.Particularly,the type I band alignment can be changed to type II band alignment by doping 2BN-pair into Gyne layer.In this case,the photogenerated-electrons and photogenerated-holes can sweep to different layers,and may further enhance the photocatalysis performance.As expected,the photocurrent of the 2BN-Gyne/g-C₃N₄ vd WHs increases to 7 times than that of pure g-C₃N₄ and 1.8 times than that of Gyne/g-C₃N₄ vd WHs.Compared with the ordinary method to modulate the band alignment,like strain or electric filed,BN-pair doping can realized in the process of material preparation,avoiding additional energy consumption in use.Our research supplies a new pathway for enhancing the photocatalytic performance.