Metal Nanocluster Self-assembly Adjustment And Application In Light-emitting Diodes

The unique size and structure of metal nanocluster enable them to have discrete energy level,show molecular energy gaps,and possess special physical and chemical properties,such as fluorescence,chirality,catalysis,magnetism and electrochemistry.Of all the properties,fluorescence is almost the most interesting,fascinating and promising one.Fluorescent metal nanoclusters show good stability,biocompatibility and low toxicity,so they are expected to be used in lighting and display,chemical sensing,cell labeling,biological imaging,light therapy,drug delivery and so on.However,compared with organic molecules,semiconductor quantum dots,perovskite quantum dots,carbon dots and rare earth materials,their biggest shortcoming is that the photoluminescence quantum yield is relatively low at the present stage.In recent years,researchers have developed some synthesis and fluorescence enhancement methods to improve their fluorescence performance,aggregation induced enhancement and alloying are two typical ones.Self-assembly is controllable aggregation,which can influence the fluorescence performance of metal nanoclusters through the adjustment of self-assembly.In this paper,we studied the adjustment methods of Cu and Au nanocluster self-assembly,then engineered the self-assembly induced emission of Cu nanoclusters by Au?I?doping,and finally applied these fluorescent metal nanoclusters in light-emitting diodes.In chapter 2,we demonstrate the analogous chloride-directed Cu nanocluster self-assembly and subsequent growth of Cu_2-xS nanocrystal.As to self-assembly,the adsorbed Cl^-lowers the dipolar attraction between the neighboring nanoclusters and leads to the redistribution of the DTs on nanoclusters.As a result,the morphology of Cu nanocluster assemblies changes from 1D nanowires to 2D nanoribbons and sheets with the increase of chloride.The following growth of Cu_2-xS nanocrystals is attributed to the fusion of Cu nanoclusters and the reaction with the S from DT decomposition.The Cl^-selectively adsorbs on the?002?facets of Cu_2-xS nanocrystals and suppresses the growth along the[002]direction.Consequently,the morphology of Cu_2-xS changes from 1D nanorods to 2D nanodisks and nanosheets with the increase of chloride.The Cu nanocluster self-assembly architecture and Cu_2-xS nanocrystal shape change from one-dimension to two-dimension with the increasing of chlorine content.The analogous Cu nanocluster self-assembly and the growth of Cu_2-xS nanocrystals in the presence of chloride give the hope to find new methods for directing nanocluster self-assembly by virtue of the existing approaches to control the morphologies of colloidal nanocrystals.Then we study the effect of ligand length as a single variable on the self-assembly structure and morphology of Au nanoclusters.By extending the ligand length,the self-assembly architecture of Au nanoclusters changes from ultra-long ribbons and hexagonal sheets to twisted ribbons and broken twisted ribbons.The length of the ligand will bring flexibility to self-assembly structure.Through the use of long flexible ligand,the linking between Au nanoclusters by ligand can be bent,then the self-assembly structure will also be flexible,from rigid ribbons into three dimensional twisted ribbons.In chapter 3,on the basis of self-assembly of Cu nanoclusters,we demonstrate the preparation of Au?I?-doped Cu nanocluster self-assembled snanoheets?NSASs?by virtue of the emission dynamics of self-assembly induced emission?SAIE?,which significantly improved the PLQY and emission stability.Au is likely doped across the whole Cu NSASs rather than in individual Cu NCs in the form of Au?I?ions.Therefore,0.3%Au is enough to generate a 4-fold PL enhancement and 100nm red-shift of the emission spectrum.The doping of Au induces an additional Au?I?-Cu?I?metallophilic interaction,which leads to a Cu to Au charge transfer,facilitating the relaxation of excited electrons via a radiative pathway.Because Au doping lowers the energy of the original Cu-centered triplet state by introducing a Au?I?-centered state,a red-shift of emission spectra is achieved.In comparison with the previously reported Cu?I?-centered defect state,the Au?I?-centered state is more stable.Thus,Au?I?-doped Cu NSASs can be separated from solution and applied as phosphors for fabricating LEDs.The current approach of doping a metal impurity into the whole NC self-assembled architecture rather than in individual NCs provides a new pathway for tuning the emission properties of metal nanoclusters within the field of SAIE.In chapter 4,we demonstrate the electrophoretic deposition of fluorescent Cu and Au sheets into film materials,which are employed to fabricate WLEDs as a color conversion layer.By studying the deposition dynamics,the factors that affect the deposition efficiency and thickness of the EPD films are well revealed.The key for governing the deposition efficiency is the lateral size of the sheets.The sheets with a large lateral size possess high electrophoretic mobility and strong face-to-face vdW interactions,thus leading to a high deposition efficiency.In our experiments,the Cu sheets have a larger lateral size than the Au sheets.Thus their the deposition is easier than for the Au sheets.To increase the deposition efficiency of the Au sheets,a secondary solvent with a high dielectric constant was added,which greatly enhanced the electrophoretic mobility and therefore the deposition efficiency.Such an understanding permits the adjustment of the deposition ratio of the Cu and Au sheets for tuning the emission colors of LEDs.A WLED prototype exhibits the color coordinates?x,y?=?0.31,0.36?,color temperature of 6577 K,and CRI of 88.The current work gives hope for fabricating complicated films from functional 2D nanomaterials via electrophoretic deposition.Subsequently,we demonstrate the application of aqueous Au nanoclusters in light-emitting diodes as emitting layer.By modifying the ZnO interface with hydrophilic polymer to improve aqueous wettability and engineering the Au nanocluster solution with surfactant to enhance film-forming ability,we successfully apply the aqueous Au nanocluster into light-emitting diodes.After optimizing the device structure,its performance exceeds that reported in literature.