Modulating The Photoluminescence Of 2D Transition Metal Dichalcogenide Monolayers Via Light/Electric Field

Two dimensional(2D)layered semiconductor materials have attracted intense research interest from both industry and academia owing to their exceptional electrical and optical properties.Especially,2D transition metal dichalcogenide(TMD)monolayers,with the single molecular layer thickness,show great potential in light emitting devices and optoelectronic communication systems due to their stable and effective excitonic emission at visible frequencies.However,for TMD based optoelectronic devices,it is still difficult to control the excitonic emission characteristics and to control the excitonic transport.For example,as a fundamental characteristic of light,polarization largely influences the data ratio of the optical communication system.In fact,the excitonic emission of TMDs does not have obvious polarization dependency,which limits their applications in polarizationsensitive optical systems.Up to date,the polarized excitonic emission of TMDs has been achieved by coupling anisotropic plasmonic structures with the monolayer TMDs,but with an insufficient polarization anisotropy,which hinders the development of TMD-based polarized light source.On the other hand,the large excitonic binding energy renders monolayer TMDs to be the excellent candidate of the excitonic devices.Unfortunately,the excitons in monolayer TMDs have very short transport distance and are difficult to be modulated through extra electrical field,which restricts their potential as information carrier.In this thesis,we have shown our trials for the above two problems.By embedding the monolayer WSe2 a plasmonic nanocavity,we have achieved highly polarized excitonic emission.Moreover,with a lateral electrical field,we have realized the high-speed modulation of in-plane excitonic distribution in monolayer TMDs.The detailed results are as follows:(1)To achieve highly polarized excitonic emission of the monolayer TMDs,we have embedded the monolayer tungsten selenide(WSe2)in a plasmonic nanocavity,consisted of a top Ag nanowire(NW)and a bottom Au film.With the highly polarization-sensitive gapplasmon mode of the plasmonic nanocavity,the polarization anisotropic ratio of the monolayer WSe2 excitonic emission reaches 200:1.Correspondingly,the degree of polarization(DoP)is more than 99%.Furthermore,we have controlled the gap-plasmon resonance peak position by changing the width of the Ag NW,and have finely tuned the polarization anisotropic ratio of excitonic emission.Our findings pave the way for highly linear polarized emission devices of monolayer semiconductors and will boost the development of functional polarization-sensitive 2D optoelectronic devices.(2)To promote the development of exciton-based optoelectronic devices,we have designed and verified a strategy for high-speed modulating the in-plane exciton distribution of monolayer TMDs at room temperature by the lateral alternating electrical field.The alternating charge trapping/detrapping at the two monolayer/electrode interfaces has induced a non-uniform carrier distribution,leading to controlled spatial variations of excitonic populations,and mimicking a bias-driven excitonic flow.This modulation increases with the AC bias and eventually saturates,relating to the energetic distribution of trap density of states.The switching time of the modulation is down to 60 ns,enabling high-speed excitonic devices.Our findings reveal the trap-assisted exciton engineering in monolayer semiconductors and offer great potential for the development of twodimensional excitonic devices and circuits.