Bandgap Modulation And Optical Property Research Of 2D-Layered Materials

In recent years,two-dimensional atomic crystal materials,especially graphene,have attracted widespread concern and attention,due to the advantages of application in flexible integration devices.Graphene has very excellent electrical,mechanical and optical properties,but graphene's zero bandgap is a congenital deficiency in optoelectronic devices.Although scientists have made great efforts to make a synthetic bandgap for graphene,the bandgap value is always very less,which is far away from the requirement.The rapid development of grapheme and the method progress of the preparation of thin layer accelerated the research of other two-dimensional(2D)materials.In particular,the single layer structure of transition-metal dichalcogenides(TMDs)is similar to that of graphite,which has received significant attention,because some of them are semiconductors with quite large bandgaps and are abundant in the natural world.Transition-metal dichalcogenides,such as MoS2,MoSe2,WS2,and WSe2,have recently attracted considerable interest as a new class of atomically thin 2D layered materials,due to their atomically thin geometry,unique electronic and optical properties and potential applications integrated nano systems.In the past two years,2D atomic crystal semiconductors,particularly in MoS2,have been found successively.Because monolayer MoS2 has a direct band gap,and excellent photonic and optoelectronic properties,it has raised hopes for the 2D application in functional nanoelectronic and optoelectronic devices.Since the application of semiconductor materials is closely related to their bandgaps,a vital task in TMDs research is to realize the growth of new 2D materials with tunable bandgaps for potential applications in functional electronic/optoelectronic devices.With these ideas in mind,in this dissertation,we well designed and improved the traditional chemical vapor deposition growth method of 2D nanostructures and accordingly various special semiconductor nanostructures were fabricated.Ultimately,the simultaneous growth of alloy nanosheets with complete composition tunability was realized,and bandgap modulation was also successfully achieved within single-nanostructures via variation of alloy composition.More importantly,through an effective control of the atomic substitution process,we realized the controllable chemical composition modulation of 2D semiconductors,and proposed a self-catalyzed strain-driven atomic substitution mechanism in 2D atomic crystals.The main achievements are summarized as follows.(1)Atomically thin uniform 2D MoS2xSe2(i-x)nanosheets have been simultaneously synthesized with complete composition(0<x<1)tunability using a very simple one-step temperature gradient assisted CVD route.The achieved samples exhibit triangular shape with edge length up to 80 microns.Under laser excitation,the nanosheets collected from difference growth temperature(different positions along the length of the tube)show a consistent composition related Raman shift and PL emission.The nanosheets with variable compositions all show single bandedge emission band,with the spectral peak continuously tunable from-668 nm(for pure MoS2)to-795 nm(for pure MoSe2).(2)Lateral composition graded atomic layered 2D MoS2(1-x)Se2x nanosheets have been successfully synthesized using a simple moving-source thermal evaporation method by an improved CVD route.Both microstructure and spectral characterizations demonstrate that the achieved nanosheets are highly crystallized,with the composition being continuously tuned from the pure MoS2 at the center(x=0)to highly Se doped ternary alloy at the edge(x=0.68).These alloy nanosheets can give position related PL emission,with the peak position broadly tunable from 680 nm at the center to 755 nm at the edge.(3)Lateral composition tuned atomic layered heterostructures have been successfully prepared through an effective control of the layer dependent atomic substitution process.Both microstructure and spectral characterizations demonstrate that the achieved nanosheets after substitution are lateral heterostructures,with the composition at peripheral monolayer region being continuously tuned to ternary alloy while the composition at central bilayer region keeping originally as before.The lateral heterostructures with tunable compositions can give composition-related optical modulations,with the PL peak positions broadly tunable at the periphery while fixed at the center.(4)We successfully realized the precise control on the gradient substitution of monolayer MoS2 and MoSe2.By comparing with our experimental data with DFT and KMC results,we proposed the mechanism for such substitution process.It was found that a Se atom is prone to adsorb on the top site of S atom of MoS2.After that,it sinks into the hollow site of three S atoms,pushes an adjacent S atom up and takes the original site of the said S atom,while the S atom being pulled out from the surface adsorbs on an in-surface S atom next to the Se atom.We name this process as Se-S exchange transfer process.As a result of this exchange transfer process,an S atom of MoS2 is replaced by a Se atom.It turns out the exchange transfer barrier,thus the rate,is highly correlated with the local strain applied around the S atom being substituted,which provides a key parameter allowing us to control the substitution process.The inverse gradient of S-substituted monolayer MoSe2 further confirms our suggestion.As for WS2 and WSe2,because of their similar structure and the same relationship between barrier and lattice constant,we can predict that they will show the same results in substitution according to our theory.Namely,the Se-substitution of WS2 will show large gradient where high occupation at edge and low occupation in center.We believe that the same process not only happen in TMDs but all 2D heterostructure since it is not relevant to specific electronic structure.