Preparation And Measurement Of Single Colloidal Quantum Dot Sample For Near-field Optical Spectroscopy

As a new kind of optoelectronic nanomaterial,colloidal quantum dots(QDs)have potential applications in many different fields due to their favorable optical properties such as broad absorption bandwidth,narrow and tunable emission peaks,high photo-luminescence(PL)quantum efficiency,etc..Traditionally,properties of single QDs are studied through far-field spectroscopy,where the excitation or emission spot size is diffraction limited.Such spectroscopic technique neither offer higher spatial resolutions and nor be used to study near-field optical excitation and emission control of the QDs.With the development of nano-optics in the past two decades,near-field spectroscopic method have became applicable to study optical properties and light-matter interaction with single QDs at nanometer length scale.However,sample problems typically occur for near-field spectroscopy,since the QDs must be very close(a few nm)to near-field probe and the photostability of QDs usually becomes a problem.In this thesis,we carried out experimental studies on the preparation and measurement of single colloidal quantum dot sample for near-field optical spectroscopy.Firstly,we studied the preparation of CdSe/CdS QD samples to achieve stable luminescence at ambient conditions by exposing the QDs in air and also embedding the QDs into organic thin matrices.Secondly,we tunned the resonance of the gold nanoparticles(GNPs)to match the emission properties of QDs by shaping the GNPs with the laser.Finally,we demonstrated high-brightness non-classical light sources based on single QDs.The research content of this thesis covers the following:1.Making stable QD samples exposed in air.Colloidal QDs are highly sensitive to water in the surrounding,and thus usually shielded by polymer films.While in situations such as single QD nano-manipulation,bare QD samples exposed on coverslip are needed.To this end,we prepared the desired luminescence-stable QD sample by properly processing the substrate and strictly controlling the water content of the QD solution.We tested and characterized the sample’s optical properties with an inverted fluorescence microscopy imaging system.2.Preparation of single QD samples in ultrathin films.Generally,the polymer films used to protect QDs are thick.This not only reduces the collection efficiency of single QD fluorescence,but also hinders the near-field regulation of single QDs by GNPs.In this part we focus on the preparation of the ultrathin polymer films,i.e.PMMA and Zeonex,in which QDs are embedded.We controlled the film thickness by adjusting the concentration of polymer solution and the spinning speed of spin coater.The film quality and stability are tested by atomic force microscopy(AFM).Then,we compared the luminescence stability of single QDs that are embedded in films and that exposed slides,highlighting the importance of the protection of ultrathin films.3.Near-field spectroscopy measurement on single QDs.Using a QD sample in which QDs are embedded in an approximately 10nm-thick polymer film,we conducted near-field control and near-field microscopy on single-QD luminescence with the help of GNPs.Here the peaks of GNPs’ scattering spectra can be adjusted by heat-melting shaping.Once the spectra match those of single-QD luminescence,single-QD biexciton quantum efficiency will be boosted greatly.In this way high brightness non-classical nano light sources can be obtained.Based on the experimental apparatus which combines inverted fluorescence microscopy and the near-field scanning probe technology,we successfully prepared ultrathin samples with excellent optical stability for near-field microscopy.We observed that under the influence of single GNP the ratio between biexciton quantum efficiency and single-exciton in single QDs can exceed 1.