First Principles Studies Of Heterostructures For Application In The Optoelectronic And Energy Storage Devices

Low dimensional 2D semiconductor heterostructures and CSNWs have attracted great research and industrial interest due to their interesting physical properties and potential applications in photo detector,solar cell,light emitting diode,sensor,battery,photoelectron-chemical cell,field effect transistor(FET),electrochromic device,laser,multistate memory and so on.In this dissertation we used a well-known quantum mechanical approach,i.e.density functional theory(DFT),to predict such new hybrid materials for applications in optoelectronics(transistor,p-n junction diode,LED,FET)and energy storage devices(Li ion battery).The first Chapter is about the background,application and importance of NWs,CSNWs and 2D semiconductor heterostructures,the second Chapter introduces the employ methodology,i.e.DFT.In the 3rd Chapter we have studied the electronic structure and band gap nature of hydrogen passivated wurtzite ZnS/Si and Si/ZnS CSNWs in the[0001]direction by using DFT calculations.We have studied the effects of size and core-to-shell chemical composition ratio on the lattice parameter and band gap.Almost all of these CSNWs have the indirect band gap nature for different sizes and different chemical composition ratios;however,in the case of ZnS/Si CSNWs for x=0.4 with size of 2.7 nm and 3 nm,they show a direct band behavior.This indirect to direct band gap transition is attributed to the direct band gap nature of the core region in their bulk counterpart and strain effect on the core/shell interface.The band gap also demonstrates an increase when the size of both CSNWs is reduced from 3 nm to 2.5 nm,as a consequence of quantum confinement.The band gap modulation is in perfect agreement with the experimental results.The cohesive energy indicates that CSNWs with larger diameters are energetically more stable,and Si/ZnS CSNWs are more stable than ZnS/Si CSNWs.The 4th Chapter is about the structural stability and electronic structures of hydrogen passivated wurtzite CdSe/ZnS and ZnS/CdSe CSNWs in the[0001]direction.The calculated cohesive energy shows that ZnS/CdSe CSNWs are more stable than CdSe/ZnS CSNWs and the stability of ZnS/CdSe CSNWs increases with increasing the thickness of ZnS shell.The modulated electronic band gap demonstrates an increase when the size of both CSNWs is reduced,as a result of the quantum confinement effect.The core-to-shell chemical composition of atoms shows that a strong composition effect also exists in these CSNWs,which in turn affects their electronic properties.Our simulated results show that the photoemission spectra of the CSNWs can be significantly improved by tuning the energy gap of CSNWs.In the 5th Chapter we have investigated the structural,electronic and optical properties of three transition metal dichalcogenide(TMD)heterobilayers,ZrS2/HfS2,ZrSe2/HfSe2 and SnS2/SnSe2 using density functional theory.These heterobilayers are predicted energetically and dynamically stable structure.The band structure calculation result shows that ZrS2/HfS2,ZrSe2/HfSe2 and SnS2/SnSe2 heterobilayers are semiconductors with indirect band gaps.The efficient charge carrier separation in ZrS2/HfS2 and ZrSe2/HfSe2 heterobilayers indicates that they can be employed in energy harvesting devices.Contrary to the previous report on ZrS2/HfS2 heterobilayer,we found it an intrinsic type-II band alignment which is required in p-n junction diode and tunnel field effect transistor,and the same behavior in ZrSe2/HfSe2 and SnS2/SnSe2.The ZrS2/HfS2 and ZrSe2/HfSe2 heterobilayers reveal enhanced optical absorption both in the ultraviolet and visible regions as compared to their respective monolayers;whereas the parallel and perpendicular part of the optical absorption of SnS2/SnSe2 heterobilayer revealed an anisotropic behavior;the perpendicular part is largely improved in the higher energy region,and the parallel part of the optical absorption are improved in the ultraviolet region.In the 6th Chapter a DFT based study is carried out to examine the electrochemical performance of ZrS2 and graphene monolayers and ZrS2/graphene heterobilayer for anodic applications in rechargeable Li ion batteries(LIBs).The energetic and dynamic stabilities of the heterobilayer have been confirmed first.The low Li diffusion barrier(0.24 eV),low half-cell voltage(0.7 V)and high storage capacity(350 mAh/g)of ZrS2 monolayer makes it a prospective anode material for the LIBs.The large energy gap of ZrS2 monolayer may reduce the reversibility and this issue can be overcome by making a contact with graphene.A significant improvement in the Li binding energy for the ZrS2/graphene heterobilayer as compared with that graphene and monolayer ZrS2 has been found.