| Layered transition metal dichalcogenides(TMDs)have been recognized as novel two-dimensional semiconducting materials.As the thickness of TMDs reduces to sub-nanometer scale,TMD monolayers possess appealing electronic and optical properties due to coulomb screening and quantum confinement effects,such as indirect-direct band gap transition,efficient luminescence of 2D excitons,ultrafast charge transfer,coupled spin-valley physics and so on.In addition,although TMD monolayers are thin,they have strong interaction with light in very wide spectrum.Because of these extraordinary physical properties and strong light-matter interaction,TMDs have not only served as an ideal platform for 2D physics researches,but also become promising candidates for novel logical electronic devices,photonic detectors,spin-valley devices and so on.In recent years,tremendous efforts have been made to research on properties of TMDs.However,dynamical mechanisms involving TMDs interacting with light,such as behaviours of 2D excitons,interlayer coupling and dynamics of carriers across the heterostructure interface,are still unclear.Direct and further study on these dynamical mechanisms is crucial not only for manipulating optical properties of TMDs but for understanding as well as evaluating the performances of devices constructed by TMDs.Hence,in this thesis,we comprehensively study 2D excitonic photoluminescence of TMD monolayers,interactions between TMDs and dielectric environments,interlayer interactions and ultrafast dynamics of charge transfer in van der Waals(vdWs)heterostructures,employing extensive spectroscopy ranging from static to ultrafast.The contents of this thesis are organized as followings:1.The effect of substrates on MoS2 Photoluminescence.(1)Monolayer and few-layer MoS2 were prepared by mechanical exfoliation and then transferred on SiO2/Si,PDMS,LaAlO3 and SrTiO3 substrates.The effect of different kinds of dielectric substrates on MoS2 was studied by Raman and PL spectroscopy systematically.The Raman results showed that substrates had small effect on Raman-active phonon modes,indicating that substrates had small strain effect on MoS2.However,PL spectra showed strong substrate-dependent properties.Different substrates changed the ratio of excitons and trions largely and resulted in optical properties of MoS2 changing which is attributed to different density of electrons doping from different substrates.According electron density estimated,SrTiO3 substrate transferred electrons to MoS2 the least while SiO2/Si the most.Meanwhile,we proposed the band structure at the interface between MoS2 and substrate to explain this electron doping effect.All the results indicated that the charge transfer between MoS2 and substrate affect the optical properties of MoS2 which can be used as a way to change optical properties of TMDs just by substrate engineering.(2)Utilizing the Insulator to Metal Transition(IMT)of VO2 drived by temperature,MoS2/VO2 structure was designed and prepared to research on the effect of dielectric environment on PL properties of MoS2.Temperature-varying Raman spectra indicated that with the temperatue increasing,VO2 changed from insulator to metal while the phonon modes of MoS2 did not change showing that phase transition of VO2 had small strain effect on MoS2.However,the PL spectra showed that the IMT of VO2 enhanced the PL intensity of MoS2 a lot.The IMT of VO2 changed the surrounding dielectric environment of MoS2 and thus the interference between substrate and MoS2 mainly enhanced the PL intensity.These results provided a method to manipulate PL of MoS2 by changing substrate in si-tu.2.Modulations of PL and carrier dynamics in MoS2 by Re-doping.Using absorption spectra,PL,Raman and ultrafast laser Pump-Probe techniques,the effect of Re doping on MoS2 were studied systematically.Results showed that Re-doping could make E2g1 phonon mode redshift and A1g mode broaden,the PL quench a lot,and the exciton peak in absorption spectra redshift and broaden.All these results indicated that Re-doping did not change the crystal structure of MoS2 but actually introduced perturbation.Re-doping could introduce defect energy level and make electrons and holes back to ground state without radiation.Lifetime of carriers in Re-doping MoS2 was about 1 ps while that was about 20 ps in pure MoS2 indicating the Re-doping suppressed the formation of excitons and scattered the carriers without radiation.3.Interlayer interactions in vdWs heterostructures studied by optical spectra.By Raman and PL spectroscopy,we studied the interaction between layers in MoS2/graphene,WSe2/MoSe2,MoSe2/MoS2 and WSe2/MoS2 heterostructures.The interlayer phonon modes were observed by Raman spectra in all these heterostructures indicating strong interlayer coupling in all these heterostructures.The interface band alignment in WSe2/MoSe2,MoSe2/MoS2 and WSe2/MoS2 heterostructures was type-II alignment and interlayer exciton were observed by PL spectra.In MoS2/Graphene heterostructure,after excited by laser,the electrons-holes were detached and electrons were transferred to graphene to make the system back to ground state without radiation,thus PL quenching.4.Interlayer carrier dynamics in homo/hetero-structure studied by ultrafast optical spectroscopy.Using graphene as fast recombination channel,TMD/TMD/graphene trilayer structure was designed and fabricated to research on charge transfer dynamics between homo-bilayers systematically by ultrafast laser pump-probe techniques.All the results showed that the interlayer electron transfer in MoSe2/MoSe2,WS2/WS2 happened on picosecond timescale while in WSe2/WSe2 that was faster and on sub-picosecond timescale.The efficient interlayer electron transfer indicated that such homo-bilayers can be used as building blocks for vdWs multilayers to achieve desired optical thicknesses without scarifying interlayer transport performance.These results also provide information for understanding charge transfer across different types of vdWs interfaces. |
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