Study On High Efficient Photoluminescence Of MoS₂

Two-dimensional Transition metal dichalcogenines?TMDCs?,especially MoS₂ are currently attracting increasing interest among scientists,due to their outstanding electrcal and optoelectrcal properties and their promising applications in various fields.In contrast to traditional semiconductors,2-dimensional semiconductors show a lot of benefits of ultrathin thickness,high surface-to-volume ratio,and high compatibility with flexbale devices.2-dimensional semiconductors is a promising post-silicon material.MoS₂ is composed of strong covalently bond S-Mo-S sheets which are bound by relatively weak van der Waals interactions.Micromechanical cleavage technique is a common method used to separate monolayer crystals from bulk MoS₂.The bulk MoS₂ crystal is an indirect-gap seminconductor with a band gap of 1.2eV.With decreasing thickness,the indirect band gap of MoS₂ increases gradually,and becomes most high in the monolayer that the material changes into 2-dimensional direct band gap semiconductor with a band gap of 1.9eV.Monolayer MoS₂ crystal is a direct-gap semiconductor,which make it promising candidate in the field of optoelectronic,such as high performance photo detecter,photoswitch,light emitter and so on.However,the as-exfoliated monolayer MoS₂ crystal is usually n-type.Charged excitons?i.e.trions?are extremely easy to form,which result in low light-emitting efficiency.In this case,it is a very meaningful research subject to improve the photoluminescence efficiency of monolayer MoS₂.Based on the above disscussion,we have carried a series of studies in improving photoluminescence efficiency.Main content as follows:?1?We found a strong photoluminescence enhancement of monolayer MoS₂ by simple ambient annealing treatment at 300?.The photoluminescence intensity of monolayer MoS₂ could be greatly improved by a factor up to two orders of magnititude.Combined with micro-Raman,TEM,XPS,time-resolved photoluminescence and other characterization,the conclusion is that the formation of Mo-O bonds during ambient annealing treatment introduces an effective p-doping effect in the monolayer MoS₂ and localized electrons formed around Mo-O bonds related defective sites avoids non-radiation recombination.In addition,we repaired the sulfur vacancies that is the main type instrinsic defects in monolayer MoS₂ by chemistry effectively.The difference between the photoluminescence of the MoS₂ sample that before and after repairment further validates our claims.?2?We studied the photoluminescence properties and behavior of the monolayer MoS₂ by dipping into the H2O2 aqueous solution at different time.We found that the photolunescence of the monolayer MoS₂ can be effectively enhanced by this simple approach and at the same time,we analysed influence of exciton and trion on photoluminescence.Combined with micro-Raman,AFM,XPS and other characterization,we found the cause of this phenomenon.The one reason is that H2O2 extract electrons from mono lyer MoS₂ effectively,resulting in the conversion from trion to exciton;another is that formation of Mo-o bonds during oxidizing reaction between H2O2 and monolayer MoS₂ introduces heavy p-doping;at the same time,localized electrons formed at defect sites on monolayer MoS₂ is also an important factor.?3?We synthesized the gold nanoparticles which have a localized surface plasmon resonance absorption peak around 514nm.Than we dispersed the gold nanoparticles on the surface of monolayer,bi-layer and multilayer MoS₂ by the method of spin-coating and measured the photoluminescence and Raman of these samples before and after spin-coating.A great enhancement in photoluminescence intensity of monolyer MoS₂ could be observed.At the same time,photoluminescence intensity of bi-layer and multilayer were slightly enhanced.The conclusion is that gold nanoparticles can cause an effective p-doping to the samples to enhance the photoluminescence intensity,and a localized surface plasmon effect of gold nanoparticles is also another positive factor for the enhancement.