Optoelectronic Property Of The Group-VA Halides Single Layers And Their VdW Heterostructures From First-principles

Bulk silicon is not suitable for the channel material in the future transistor as the size of electronic device is gradually reduced.On the other hand,the power conversion efficiency of the silicon-based solar cell is approaching its theoretical limit.Thus,looking for the alterative semiconductor materials has become the first task in the field of condensed matter physics and the semiconductor industry.Because of their novel physical properties,the two-dimensional(2D)single layers and van der Waals(vdW)heterostructures are considered as the ideal candidates for the next generation electronic,optoelectronic and energy conversion applications.The Group-VA tri-iodides AI3(A=As,Sb and Bi)we mainly study here are a kind of layered intrinsic semiconductors with tunable electronic and optical properties.They have been applied in y ray detectors,solid state battery and photonic devices.In the present paper,we have systematically studied structure,stability,electronic and optical properties of AI3(A=As,Sb)single layers,SbI3BiI3 and ASI3/SbI3 vdW heterostructures on the basis of first-principle study.Moreover,we also predicted the power conversion efficiency(PCE)of excitonic solar cell(XSC)based on heterostructures and tuned the PCE by changing the interlayer distance and the external electric field.In addition,we study the bending rigidity of transition metal dichalcogenide(TMDC)monolayers.Specifically,this paper includes:(1)We have investigated systematically the structure,stability,and electronic and optical properties of the group-VA trihalides AI3(A = As,Sb)single layers.Our results suggest that the AI3(A = As,Sb)single layers can be exfoliated from their bulk crystal easily and are dynamically stable.Standard PBE predicts that the band gap of AI3 increases with element number of A,which is in conflict with the experimental results of the bulk.This unreasonable trend can be corrected when the spin-orbit coupling(SOC)effect is considered.The inconsistence between PBE and PBE+SOC calculations can be understood by the competition of two contrary effects for gap variation induced by lattice expansion and relativistic effect.Our PBE+SOC calculations indicate the ASI3 and SbI3 monolayers are potential photocatalysts for water splitting with indirect band gaps of 2.00 and 1.89 eV?(2)Two-dimensional vdW heterostructures are considered as a kind of competitive materials in future optoelectronic and energy conversion applications.We have investigated the structure,stability,electronic structure and band edge of SbI3BiI3 and AsI3/SbI3 vdW heterostructure and calculated the power conversion efficiency of the excitonic solar cell based on the heterostructures.We find SbI3 and BiI3 single layers can be stacked to a lattice-matched vdW heterostructure with type-II band alignment and strong interlayer coupling.It possesses an indirect band gap with 1.34 eV and an indirect-quasidirect gap transition can be found by tuning its interlayer distance.AsI3/SbI3 is also a type II heterostructure with an indirect band gap of 1.63 eV.Its band edges are sufficient for hydrogen production from photocatalytic water splitting.The PCE of XSC based on SbI3/BiI3 and ASI3/SbI3 heterostructure are 14.42%and 18.44%,respectively.The PCE of SbI3/BiI3 can be enhanced by changing the interlayer distance.Furthermore,we tune the SbI3/BiI3 heterostructure from normal to inverted type-II band alignment via external electric field and the PCE of XSC based on inverted type-II SbI3/BiI3 is up to 21.63%.(3)Due to the presence of a sizeable direct band gap,three-atom-thick transition metal dichalcogenide(TMDC)monolayers have been suggested as important candidates for flexible electronic and optoelectronic devices recently.The in-plane elasticity of TMDC monolayers has been investigated extensively,however,little is known about their bending rigidity.Here,we have determined bending rigidities of single-layer MX2(M = Mo,W;X = S,Se)by fitting the energetics of single wall nanotubes from first-principles to the Helfrich Hamiltonian for the configurational energy of membranes.This parameter-free approach can avoid the controversy induced by ambiguous definition of the thickness of monolayers,which are required in the empirical determination of bending rigidity using classical shell theory.The obtained direction-dependent bending rigidities of single-layer MoS2 are 9.10 and 9.61 eV along the armchair and zigzag directions,which are larger than that estimated using shell theory but similar to the previous analytic formula based on an empirical potential.Moreover,the relative magnitude of bending rigidities for different TMDCs are found to be MoS2<MoSe2<WS2<WSe2,which is in conflict with shell theory.Further analysis indicates that this inconsistence can be understood by a competition mechanism between two-dimensional elastic modulus of monolayers and the structural relaxation of nanotubes.