Study On Optoelectronic And Spintronic Properties Of Transition Metal Dichalcogenides

After the discovery of the world’s thinnest two-dimensional(2D)material,graphene,other graphene-like 2D materials have gained huge attention in the scientific community due to their fascinating physical and chemical properties.Among all the 2D materials,the single atomic thick transition metal dichalcogenides(TMDs),for example the MoS2 and WS2 monolayers,are a central part of research because of their outstanding structural,optoelectronic,chemical,and spintronics properties in different applications.In addition,the exploration of ferromagnetic TMDs,for example chromium(CrX2)and vanadium(VX2)based dichalcogenides,is also evolving very fast after the first predication of 2D magnetism recently.Although TMDs have remarkable optical and electronic properties,the modulation in carrier concentrations of TMDs is still the thrust of the scientific community to further explore new and functional applications by making hybrid structure of TMDs with other materials such as organic semiconductors.Inspired by these perspectives,we extend our scientific curiosity in exploring the study of optoelectronic and spintronic properties of 2D TMDs and their heterostructure with organic semiconductors.The summary of the research work can be written one by one as follows:The 2D TMDs can be obtained by a variety of methods,but chemical vapor deposition(CVD)is one of the best synthesis methods to get monolayer TMDs.These materials can be generally synthesized from transition metal oxides(TMOs)and chalcogens(X)as precursors in the CVD technique.However,unfortunately,the high and low melting points of TMOs and chalcogens respectively make it difficult to get highly scalable size TMDs easily,and need extra consumption of energy and time.Thus,one of the key challenges for the scalable growth of TMD monolayers is how to achieve uniform nucleation and growth of the layers in a short time at lower temperature.The use of growth promoters(organic small molecules and molten salt)is employed in the TMDs(MoS2 and WS2)growth and it is observed that using organic small molecules,for example,DIP and PTCDI-Ph and inorganic salt(NaCl)have a very huge impact on the synthesis of TMDs,for example,their scale,size,quality,uniformity,and yield at lower temperature and growth time as compare to without using promoters.We observe that both organic small molecules and salt promoted the growth of scalable size thin films of ML-MoS2 and WS2.The family of 2D TMDs has been growing at a rapid rate for the past few years.The ferromagnetism in TMDs is being explored after the first report of ferromagnetism in 2D Cr2Ge2Te6(CGT)and chromium trioxide(CrI3)in 2017.Based on the idea of ferromagnetism in TMDs,we focus our attention on a new 2D compound of MnS2 first time experimentally which is also one of the members of TMDs family.The theoretical calculations showed that 2D MnS2 is an indirect bandgap semiconductor having both spins up and spin down band values in their monolayer limit.Furthermore,layer-dependent studies showed that the material changes its semiconducting nature and turns into a metallic phase at bi-layer and tri-layer thickness.The structure exhibits ferromagnetic behavior at its equilibrium state from monolayer to tri-layer thickness.The calculated magnetic moment of the 2D ML-MnS2 is≥ 3 μB,which is the highest value of the reported magnetic TMD materials yet.Based on the calculations,this material would be one of the potential candidates for future spin filters and other spin-based device applications.There has been much interest in introducing organic molecules directly onto the surfaces of TMDs to build heterostructures in order to manipulate the performance through tailoring interfacial properties.We investigated the photoluminescence(PL)properties of type-II vdW heterostructure made of ML-MoS2 and DIP organic semiconductor molecules experimentally and theoretically in chapter 5.The density functional theory(DFT)calculations predicted the hybridization between the DIP molecules and MoS2 and form heterostructure via vdW interfacial interaction.A nearly 300%drastic quenching of PL was observed with redshift in DIP/MoS2 as compared to the bare MoS2.The PL quenching and redshift can be explain by the appeared trap-like electronic states in the density of states(DOS)and suppression of exciton to trion ratio due to the electron transfer from the lowest unoccupied molecular orbital(LUMO)of DIP to the conduction band(CB)of MoS2.Furthermore,we also discussed the role of weak coupling strength and dielectric screening effect induced by the organic layer in the shifting of the PL of the heterostructure as compared to MoS2.