First-principles Study Of The Electronic,magnetic,optical And Photocatalytic Properties Of Sn-based Monolayer Dichalcogenides And Van Der Waals Heterostructures

The successful exfoliation of single sheet of graphene by Andre Geim and Konstantin Novoselov in 2004 created an explosion of single layered materials research in the fields of chemistry,materials science,condensed matter physics,electrical engineering,etc.Then,the two-dimensional(2D)graphene has attracted significant attention due to its interesting physical and chemical properties.However,the intrinsic zero band gap of graphene limits its application in logic electronic devices,led researchers to look elsewhere in the periodic Table for other 2D materials with rich physics and an even broader application space.Hence,new 2D materials with sizable band gaps such as transition metals dichalcogenides(MoS2,WS2,etc),post-transition metals dichalcogenides(SnS2,SnSe2,etc),group-? monochalcogenide(GaS,InS,etc),group?(SiC,GeC,etc),and so on,have attracted extensive research interest with efforts to overcome the limitations of graphene in electronic device.Owing to the quantum confinement effect,the 2D materials show novel physical properties and have real technological applications in future electronics,high-efficiency solar cell,transistors,and energy harvesting.In addition to the use of individual 2D materials,the accelerated field of 2D van der Waals(vdW)heterostructures enables even greater functionality.The different electronic band alignments of the vdW heterostructures have different applications.The type-?(staggered gap)heterostructure can effectively separate the photo-induced electrons and holes pairs and extend their life time,which is highly desirable for photovoltaic and photocatalytic devices.The type-?(straddling gap)and type-?(broken gap)heterostructures can be used for light-emitting diodes and tunnel field-effect transistors,respectively.In this thesis,by using first-principles density functional calculations,we modulate the structural,electronic,magnetic and optical properties of SnX2(X=S,Se)monolayers by defects.We also investigate the electronic,optical and photocatalytic properties of SiC/CrS2,MoSSe/BSe and CdO/As vdW heterostructures.The main contents and results are included as following:(1)We explore the structural,electronic and magnetic properties of the SnS2 monolayer doped with 5d transition metals(TM)atoms X(X=W,Re,Os,Ir,Pt,Au or Hg).The formation energies show that these materials can be easily fabricated under the S-rich experimental condition.The spin-polarized phases are more favorable than non spin-polarized phases.The pristine SnS2 monolayer is a nonmagnetic semiconductor with an indirect band gap of 1.579 eV,the 5d TM atoms X(X=W,Re,Os,Ir,Pt,Au or Hg)doped SnS2 monolayer have the total magnetic moments of 2,3,2,1,0,1,2 ?B,respectively,because the octahedral coordinated six S ligands bring in a larger crystal field splitting than exchange splitting onto the 5d TM atoms X.Furthermore,the spinpolarized band structures show that the W,Re or Hg-doped SnS2 monolayer are magnetic semiconductors,but the Os,Ir or Au-doped SnS2 monolayer are magnetic half-metallic with the wider band gaps of 1.509,1.632 or 1.383 eV respectively in one of the two spin channels,and thus are useable in spintronic fields.(2)We systematically studied the effects of strain on the electronic structure,magnetic states and optical characteristics of Mn-doped SnS2 monolayer.Ab-initio molecular dynamics simulations and formation energy reveal that the Mn-doped SnS2 monolayer is stable at 500 k and can be fabricated under the S-rich environment.The substitution of a Mn for a Sn atom induces 3 ?B magnetic moment in nonmagnetic SnS2 monolayer,which is consistent with the Hund's rule and Aufbau principle.Based on classical Heisenberg model and mean-field approximation the Curie temperature is calculated to be 476.13 K.The net magnetic moment of Mn-doped SnS2 monolayer is maintained even if considerable strains(-10 to 10%)are applied.The blue-shift and red-shift of the absorption peaks are observed in the optical spectrum by biaxial compressive and tensile strains,respectively.The biaxial strain effectively improves the optical characteristics of Mn-doped SnS2 monolayer,particularly in the visible light region.This study provide useful guidance for practical application in magnetic and optoelectronic devices.(3)We investigate the effect of vacancies and doping on the electronic,magnetic and optical properties of SnSe2 monolayer.The semiconducting SnSe2 monolayer becomes a half-metal on creating one tin vacancy(V1sn),while it is still a semiconducting on creating one selenium vacancy(V1Se),two selenium vacancy(V2se),as well as one tin and one selenium vacancy(V1Sn+1Se).The V1Sn and V1Sn+1Se vacancies induce magnetic ground states with net magnetic moments of 4 and 2 ?B respectively,while the Vise and V2Se vacancies induce nonmagnetic ground states.In Mo-doped SnSe2 monolayer,crystal-field splitting of Mo-4d orbital in the octahedral environment induces net magnetic moment of 2 ?B.The band structure and density of states also show that the Mo-doped SnSe2 monolayer is a magnetic semiconductor.In addition,the defective SnSe2 monolayer can also enhanced the absorption efficiency of the solar energy,particularly in the infrared region.An interesting red-shift phenomenon has been observed in the curves of imaginary part of the dielectric function and absorption spectra for the defect systems,suggesting their quite promising applications in optoelectronic devices.(4)The structural,electronic,and optical properties of SiC/CrS2 vdW heterostructure have been investigated.The SiC/CrS2 vdW heterostructure is energetically and thermodynamically stable indicating its great promise for experimental realization.The SiC/CrS2 vdW heterostructure has a direct-band gap and type-II(staggered)band alignment,which can effectively separate the photo-induced electrons and holes pairs and extend their life time.The carrier mobilities of electrons and holes along the armchair and zigzag directions are as high as 6.621×103 and 6.182×104 cm2 V-1 s-1,respectively.Besides,the charge difference and potential drop across the interface can induce a large built-in electric field across the heteroj unction,which will further hinder the electron and hole recombination.The SiC/CrS2 vdW heterostructure has enhanced optical absorption capability compared to SiC or CrS2 monolayers.This study demonstrates that the SiC/CrS2 vdW heterostructure is a good candidate for application in the nanoelectronic and optoelectronic devices.(5)We study the electronic,optical and photocatalytic properties of MoSSe/BSe(Model-1 and Model-2)vdW heterostructures.The stabilities of these hetero structures are confirmed through phonon spectra and ab initio molecular dynamic simulations.The Model-1 and Model-2 heterostructures have indirect band gaps of 1.95 and 1.54 eV respectively by HSE06 hybrid functional.Interestingly,the transition from indirect to direct band gap occurs in Model-1 heterostructure after including spin-orbit coupling effect.Remarkably,the high carrier mobilities are quantitatively explored by means of deformation potential theory.Furthermore,the transition from type-? to type-? band alignment happens at compressive strain in both Model-1 and Model-2,which effectively slows down the recombination of electron-hole pairs.Compared to isolated MoSSe or BSe monolayers,the MoSSe/BSe(Model-1 and Model-2)heterostructures harvest maximum portion of visible spectrum,revealing the outstanding paybacks of high efficiency utilization of solar spectrum.Most intriguingly,the band edges of MoSSe/BSe vdW heterostructures meet the redox potential requirements for water splitting.These results will be valuable for easing the investigation and applications of MoSSe/BSe heterostructures for optoelectronics and photocatalytic water splitting.(6)We investigate the optical and photocatalytic properties of the As/CdO vdW heterostructure.The stability of the heterostructure is verified by calculating the binding energy and ab-initio molecular dynamics simulations.We find that the As/CdO vdW heterostructure is a semiconductor with a direct band gap of 1.96 eV and type-?(staggered)band alignment,which can effectively reduce the recombination of photogenerated electron-hole pairs.Compared with two isolated As or CdO monolayers,As/CdO vdW heterostructure shows suitable band edge positions and high absorption efficiency in the visible and ultraviolet light regions,making it a good candidate for optical devices and photocatalytic water splitting.The calculated higher carrier mobility of holes and electrons along the armchair direction are 932.510 cm2 V-1 s-1 and 90.808 cm2 V-1 s-1,respectively.Biaxial strain can modulate the electronic structure and optical characteristics of As/CdO vdW heterostructure.In particular,compressive strain maintains type-? band alignment and improve the optical performance of As/CdO vdW heterostructure in the visible light region.Our results are beneficial for designing optoelectronic and photocatalytic devices.