Theoretical Study On The Effect Of Functionalization Of Two-Dimensional Black Phosphorus On Photoelectric Properties

As a new type of two-dimensional?2D?material,monolayer black phosphorus?MLBP?has broad application in the field of optoelectronics due to its unique topological structure,and has become a hot topic at home and abroad.However,MLBP is unstable in nature,so the research on its modification is favored.The common methods of modification mainly include covalent grafting and van der Waals heterojunction.At present,covalent grafting is mainly concentrated on inorganic non-metal molecules and organic molecules,and the grafting of transition metal is rarely reported.Moreover,the adsorption energy of metal on MLBP surface is larger than that of graphene or carbon nanotube,indicating that the combination of metal and MLBP is easier to realize.In addition,Van der Waals heterojunction that combined MLBP with graphene,BN and MoS₂ have been reported,but the study of grafting transition metal to form intercalation compounds has not been reported.The grafting of transition metals may induce new optical properties.In this dissertation,density functional theory?DFT?and non-equilibrium Green's function?NEGF?methods were used to study the effects of the covalent grafting of MLBP with PdCl₂ and PtCl₂ and forming three different kinds of MLBP heterojunctions?including Pt and PtCl₂ intercalation composites?on the electronical and optical properties of MLBP.The calculations show that MLBP could form strong bond with the metal atom while maintaining its own structural integrity,which combines the advantages of graphene and silene.MLBP has an intrinsic band gap,and the surface modification can introduce impurity bands in the valence band or conduction band,which can significantly affects the photoelectric properties.Therefore,this dissertation focuses on the impact of the above two functional modifications on the photoelectric properties of MLBP.The main research contents are as follows:First,the electronic structures and photoelectric properties of the complexes formed by grafting different density PdCl₂,?n PdCl₂/BP?n=1,2 and 4??,were investigated.The results show that n PdCl₂ groups tend to be located at the furrow site of MLBP and form stable planar quadridentate structure[PdCl₂P₂].The n PdCl₂ introduce localized energy levels between MLBP valence band and Fermi level?E_f?,which make the conductivity reduce.The conductivity decreases with the grafting density,and a threshold voltage of 0.6 V appears in 4PdCl₂/BP system with the density of 12.5%,which is a valuable clue for exploring current switches.The conductivity of the systems after grafted n PdCl₂ still keep anisotropic,and electrons permeate easier through the armchair direction compared with the zigzag direction.Under linear illumination,the photocurrent of n PdCl₂/BP is about 20 times higher than that of MLBP,and it also exhibits anisotropic characteristic.Unlike conductivity,the zigzag direction shows much stronger photoresponse than the armchair direction.All above results indicate that n PdCl₂/BP may have potential applications in optoelectronic devices.Then,the electronic structures and optoelectronic properties of the complexes formed by grafting different density of PtCl₂,?n PtCl₂/BP?n=1,2 and4??,were studied.The results show that the n PtCl₂ groups also tend to be located at the furrow site of MLBP and form stable planar quadridentate structure[PtCl₂P₂].The systems exhibit opposite results in terms of electron transport and linear photogalvanic effect:the conductivity of the composite systems along the armchair direction and zigzag direction is reduce,but the photoresponse is increase.The higher the density,the lower the conductivity,but the larger the photoresponse;the conductivity in the armchair direction is larger than that in the zigzag direction,while the photoresponse in the zigzag direction is larger than that in the armchair direction.In functionalized systems,due to the valence band is derived from n PtCl₂ and the conduction band is derived from MLBP,type-II band is formed,which indicated that nPtCl₂/BP is expected to be a candidate for solar cells.Finally,the electronic structures and transport properties of the heterojunctions?X/BP?formed by MLBP with graphene?zero band gap?,single layer BN?wide bandgap?,single layer MoS₂?semiconductor?and their intercalation compounds X/Pt/BP and X/PtCl₂/BP?X=graphene?G?,BN,and MoS₂?were studied,and the linear photogalvanic effect of Pt and PtCl₂ grafting systems were further investigated.The X and MLBP were stacked by the way of zigzag-zigzag?armchair-armchair?.The results show that these systems are thermodynamic stability.The band of BN/BP is mainly controlled by MLBP,which is type I band.The valence band of MoS₂/BP is contributed by MLBP,and the conduction band is dominated by MoS₂ to form type II band.In G/BP,the graphene and MLBP control the band structure of the zigzag direction and the armchair direction,respectively.After introducing Pt between layers,there is not obvious influence on the band structure and conductivity for BN/Pt/BP and MoS₂/Pt/BP.While PtCl₂ is introduced,the conductivity is enhanced and the composites have a strong photoresponse than that grafting Pt.The maximum photocurrents generated by BN/PtCl₂/BP and MoS₂/PtCl₂/BP are approximately10² and 10³ times greater than that grafting Pt.For G/Pt/BP and G/PtCl₂/BP,there are different effects on conductivity in the zigzag direction and the armchair direction:the order along the armchair direction is G/Pt/BP>G/PtCl₂/BP and the sequence along the zigzag direction is G/PtCl₂/BP>G/Pt/BP.Under linear illumination,the maximum photocurrent produced by PtCl₂ is about 10⁴ times larger than that of Pt.