Study Of Plasmon-enhanced Zinc Oxide Hybrid Structrue Optical Properties

As a kind of wide direct bandgap（3.37 eV）semiconductor materials,ZnO has been considered as a promising candidate for short-wavelength optoelectronic devices.Moreover,due to its high exciton binding energy（60 meV）,ZnO presents a very broad application prospects in the field of ultraviolet optoelectronic devices,especially UV lasers.Recently,researches on the ultraviolet（UV）performances of ZnO especially the whispering gallery mode（WGM）lasings of ZnO have attracted intense interest among scientists from many fields.Though ZnO microrod always has hexagonal cross-section,which facilitate the occurrence of the WGM lasing,the big optical loss is inevitable with decreasing of the diameter of the microrod.So it is significant to reduce the optical loss,as well as improve the lasing performance of an optical cavity.With the development of plasmonics,the surface plasmon（SP）ehnaced the performance of semiconductor material has attracted much attention due to its characteristics of altitudinal spatial localization and intense near-field enhancement.Graphene has been considered as a plasmonic material alternative to noble metals based on its unique conductivity and optical properties.More importantly,plasmon in graphene can be tuned by chemical doping and gating potentials,thus has more wide potential applications than that of the metal SP.Utilizing of this unique physical effect of graphene and metal has been proposed as a promising method to improve the intrinsic luminous efficiency of ZnO and develop SPs-coupled novel optoelectronics devices.In this thesis,SP coupled ZnO WGM microcavities will be designed and fabricated.In this case,both WGM optic field and SPs of the metal and graphene are confined near the cavity surface and provide a favorable physical space for their coupling,while ZnO will compensate efficiently the SP response in short wavelength based on its superior gain in UV region.The plasmon-enhanced spontaneous and stimulated emission from ZnO decorated with metal nanoparticles and monolayer graphene（thehybridstructureofZnO/metal,ZnO/metal/Graphene,ZnO/Graphene/metal）have been investigated by a chemical vapor deposition（CVD）method and a radio frequency magnetic sputtering system,respectively.The main contents are listed as following:1.Using vapor transport method and ion sputtering method,the shape-controlled ZnO microstructures and graphene/metal nanoparticles（NPs）were fabricated by controlling the ratio of raw materials,air condition,sputtering temperature,time,and electric current etc.The growth mechanisms and crystal structures were investigated by SEM,EDS,XRD,TEM and so on.Using theμ-PL and Raman testing,the optical characteristics were systematically studied in the ZnO microstructures and graphene and metal NPs.2.Further using a variety of spectroscopy technology,combined with the time-resolved PL and temperature-dependent PL,the enhanced excitonic emission intensity were observed in the PL spectra of the ZnO/Au-NPs hybrid structures at room temperature,and the plasmon-assisted electron transfer mechanism was also proposed.The Au-sputtered ZnO presents the strongest NBE with more than 20 times enhancement compared with the as-grown sample,while the defect-related emission is completely suppressed.Also,the interaction of excitons,electrons and photons in this hybrid ZnO/metal microcavity was analyzed.The investigation on the absorption spectra and temperature-dependent PL spectra demonstrate the Burstein-Moss effect behind the optical phenomena.3.A hybrid WGM microcavity of Graphene/Al-NPs/ZnO（GAZ）microrod was constructed by sputtering Al-NPs with magnetron sputtering method and transferring a piece of monolayer graphene on the same Al/ZnO microrod to investigate the spontaneous and stimulated emission properties systemically.More than 50-fold lasing intensity enhancement was observed in the GAZ hybrid WGM cavity due to the resonant energy coupling synergistically between the Graphene/Al SPs and the excitons of ZnO.The Al NPs in the hybrid structures not only corrugated ZnO microrod surface,but also contributed to LSPs itself through resonant coupling with ZnO UV emission.In this GAZ microrod hybrid WGM cavity,the improved lasing performance was obtained due to the synergistic energy coupling between the graphene/Al SPs and the excitons of ZnO.4.Combined with ZnO optical cavity and the graphene and silver nanoparticles（Ag NPs）,a novel biosensor for Raman signal detection based on ZnO/Graphene/Ag-NPs hybrid microcavity was designed and fabricated.The hybrid SERS substrate is displayed an ultrahigh SERS sensitivity with the enhancement factor of 0.95×10¹² and an ultralow detection limit down to 10^-15 M for the probe molecule detection.It can be attributed to not only the WGM-enhanced light-matter interaction of ZnO geometric cavity structure but also the graphene-assisted charge transfer and Ag SPs induced local field enhancement,thus giving rise to an ultrahigh enhancement of Raman signal.