Research On The Surface-enhanced Fluorescence Of Quantum Dot-sensitized Nano-alumina

Controlling the fluorescence of the nano-radiator and its surface/interface can effectively increase the electron transfer rate on the interface,reduce the non-radiative reconstruction of electron-hole pairs,improve the utilization of light energy,have important applications in photovoltaic,photodetection,nano-sensing,molecular imaging and many other fields,and it is one of the hot spots in the current scientific research.Compared with the aluminum oxide of wide band gap bulk material,nano-alumina（such as oxygen vacancies,aluminum fill,etc.）with many surface defects can be used in semiconductor material,and the fluorescence in the visible light band is radiated in the case of photoelectric excitation,which provides new ideas for energy band.Semiconductor quantum dots（QDs）produce a series of unique physicochemical properties（such as high fluorescence quantum yield,high activity surface,size effect,etc.）due to the size reduction to nanometers,provide new raw materials for the sensitization of heterojunction devices,biomarkers,bioimaging and other fields,especially quantum dot solar cells are called the new hope of the third generation solar industry.Combining the adjustable properties of nano-alumina band and the unique photoelectric properties of quantum dots,and their applications in surface enhanced fluorescence（SEF）,This paper proposes the use of quantum dot-sensitized nano-alumina and regulates the surface-enhanced fluorescence properties of their interfaces.It provides a new research method for increasing light absorption and improving the utilization of photo-generated carriers in quantum dots,and It also lays a foundation for new quantum dot heterojunction photovoltaic devices,photodetector devices,and optical communicators.In this paper,a new sensitizer system was constructed using colloidal CdSe and ZnSe as sensitizers.We studied the surface-enhanced fluorescence properties of nano-Al₂O₃ thin films,and we explored Several methods for regulating the surface-enhanced fluorescence of nano-Al₂O₃,and established a photo-carrier transport model for heterojunction composite systems.The main work and results of this paper are as follows:1.The CdSe/Al₂O₃ heterojunction was prepared by Microfluids Electrochemical Deposition Methods（M-ECD）.The near-field optical microscopy imaging method was proposed to study the three CdSe quantum dot sensitized nano-Al₂O₃ films.Surface enhanced fluorescence properties.The results show that（1）For the unipolar distribution,the main peak is located at 411nm linear Al₂O₃film,using CdSe quantum dots as sensitizer to realize the directional transport of photogenerated carriers to Al₂O₃ of CdSe quantum dots,so that Al₂O₃ The single-peak enhancement of the defect fluorescence is more than 50 times.（2）For the linear-like Al₂O₃ film with multimodal distribution,the broad-spectrum enhanced surface-enhanced fluorescence effect of Al₂O₃ is obtained.（3）The photo-generated carriers by controlling the CdSe quantum dots are obtained.TheconcentrationreachestheregulationofCdSe/Al₂O₃nano-heterojunction photoluminescence enhancement.On the heterojunction of different CdSe quantum dot deposition time（5 minutes,10 minutes,15 minutes）,the spectral enhancement and peak shift of the nano-Al₂O₃ film are different.When the deposition time is short,the spectral intensity of the heterojunction increases and the spectral shape does not change.As the deposition time increases,the defect fluorescence enhancement of nano-Al₂O₃ gradually changes from multi-peak enhancement to single-peak enhancement.2.Gold nanoparticles are used as the intermediate carrier of CdSe QDs/porous Al₂O₃ electron transfer.On the one hand,it can stabilize the defect level position of the defect Al₂O₃,non-radiative reconstruction of the defect center,and on the other hand,the electrons on the CdSe QDs conduction band can transfer to the gold nanoparticles surface to achieve fluorescence enhancement and regulation of porous Al₂O₃ surface.The results show that CdSe QDs with narrow band gap low photon energy can transfer to defective porous Al₂O₃ surface with wide band gap high photon energy,occur composite radiation at their interfaces.The number of interfaces increases after introduction the gold nanoparticles with high conductance as the intermediate layer,and the interface radiation also changes greatly.The interface radiation of the two types of heterojunctions plays a major role,one is the interface radiation of CdSe/Au connected with Al₂O₃/Au is enhanced,and the enhancement factor is 4.4 times.Gold nanoparticles act as electron guides in this process.The interface of another type of CdSe/Al₂O₃ also achieved fluorescence enhancement,and the enhancement factor was also 4.4 times.Which effect plays a leading role depends on the synergistic effect of electron transfer between them.3.The ZnSe/Au energy transfer system was constructed by using wider band gap ZnSe quantum dots.Then,the Langmuir-Blodgett method was used to self-assemble the ZnSe/Au composite film on the porous Al₂O₃ surface to control the fluorescence enhancement of the system.The fluorescence enhancement effect of sensitized porous alumina by Au/ZnSe energy transfer system was Studied.The results showed:（1）In the ZnSe/Au energy transfer system,a 1:1 mixture solution of ZnSe quantum dots and gold nanoparticles was spin-coated on TiO₂,and three samples of 1,2,and 3 layers of thin films were prepared.Owing to the fractional transfer of photo-generated electrons from the conduction band of ZnSe QDs to the s-p band of gold NPs,the excitonic and enhanced fluorescence on FETS were simultaneously and in succession emerged on its optical spectrum to broaden and enhance fluorescence.In addition,when the gold NPs absorbed the excitonic fluorescence of ZnSe QDs,the energy level of s-p band raised toward CB direction of ZnSe QDs not only to result in a blue-shift of enhanced fluorescence but also increase the quantity of s-p band electron to induce the enhanced fluorescence surpass the excitonic fluorescence.With the increase of interaction of ZnSe QDs with the gold NPs on the vertical direction via coating the multi-layers film of FETS,the enhanced fluorescence was observed instead of the excitonic fluorescence.（2）Five groups of gold particles and quantum dots with different concentration ratios were prepared by Langmuir-Blodgett method,and the fluorescence of composite structure was enhanced and broadened to the visible range of 400-800 nm.Emission of ZnSe QDs could be enhanced or quenched with varying the amount of gold NPs,resulting in the distance between ZnSe QDs and gold NPs.When gold NPs were at a long distance from the ZnSe QDs,the PL intensity were expected to increase as the concentration of added gold NPs increases due to the increased local field by gold surface plasmons.While as the concentration of gold NPs reached a certain threshold,ZnSe QDs and gold NPs would get closer to one another that electrons could now easily transfer from QDs to Au particles,which resulting in a quenching of the emission.With the gold concentration increased sequentially,the distance between ZnSe QDs and gold NPs became the closest.An enhanced and broadened defect fluorescence spectrum effect was observed owing to the increase of the built-in electric field to hinder the electron transfer to gold NPs and is trapped by the defect states of ZnSe QDs.（3）When Au/ZnSe energy transfer system is used to sensitize porous alumina,the energy of ZnSe quantum dots is further enhanced,and the spectrum shape is basically unchanged,and the enhancement factor is 2.8 times.The results of this study can further study the energy transfer of two fluorescent peaks in the Au/ZnSe energy transfer system,and which peak energy can be further adjusted by the preparation process,and can be applied to photovoltaic devices,LED illumination,light detection and the like.The innovation of this thesis lies in:1.The method for enhancing the surface-enhanced fluorescence of a nano-alumina substrate with weak fluorescence radiation by a quantum dot sensitization method is proposed for the first time,and in particular,conversion enhancement of low-energy photons to high-energy photons is realized;2.The preparation of QDs/Al₂O₃ heterojunction by M-ECD is proposed,which is characterized by the positive charge of the surface hydration layer strongly adsorbing the carboxyl quantum dots suspended in the microfluid.And the variable-frequency alternating load is used to transform the positive and negative charge in the hydration layer,and the small molecules of the non-quantum dots are removed,so that the quantum dots are uniformly deposited on the surface of the substrate,and the gaps can be controlled.3.After introducing gold nanoparticles with high conductance as the intermediate layer of CdSe/Al₂O₃,the surface enhanced fluorescence of CdSe/Al₂O₃heterojunction controlled by gold nanoparticles was realized,and the physical mechanism of fluorescence enhancement was proposed.4.The Langmuir-Blodgett method is used to self-assemble the ZnSe/Au composite film on the porous Al₂O₃ surface to enhance the spectral structure of the composite structure and broaden it to the visible region.At the same time,the Au/ZnSe energy transfer system is used to sensitize the defect aluminum oxide to further enhance the energy of the composite system.