Study On The Synthesis Of Large-sized Blue-emitting ZnSe-Based Nanocrystals For Novel Display Applications

Zinc selenide(ZnSe)is a typical II-VI semiconductor material with a bulk bandgap of 2.7 eV,which is considered to be promising blue emitters with eco-friendly and low-cost features.Hence,ZnSe nanocrystals(NCs)are an alternative to toxic cadmium-based blue NCs materials(quantum dots).In recent years,ZnSe NCs have attracted extensive attention from academia and industry community due to their blue-emitting spectra,narrow spectral width,and high fluorescence quantum efficiency.According to the size-dependent quantum confinement effects,large-sized ZnSe NCs with weak confinement have the potential to realize pure-blue(455-475 nm)emission.Based on chemical-solution synthesis methods,it is difficult to synthesize large-sized Se-based semiconductor NCs with narrow size distribution,and it is more challenging for synthesizing large ZnSe NC due to its higher formation energy.As a result,the PL wavelength of ZnSe NCs remains limited to blue-violet region in previous literatures.The emission wavelength of electroluminescent devices based on ZnSe/ZnS NCs limited to 445 nm.To overcome the size limitation of existing synthesis methods,further synthesize large-sized ZnSe NCs to realize pure-blue emission.In this work,we investigated the nucleation and growth mechanism of ZnSe NCs to understand the limiting factors of fabricating large ZnSe NCs.In addition,based on the theoretical guidance,continuous epitaxial growth of ZnSe crystal seeds was achieved via controlling the precursor reactivity,and their emission wavelengths can be tuned to the pure-blue region.In addition,to improve the fluorescence efficiency of large-sized ZnSe NCs,we epitaxially grown heterogeneous ZnS on their surface as passivation shell,and investigated the growth mechanism of ZnS shells on large-sized ZnSe NCs.The main research and innovative achievements of the thesis include the following aspects.(1)First,the reactivity of Zn and Se precursors is regulated by ligand engineering.The reactivity of precursors can be determined by nuclear magnetic resonance(NMR)spectra,nucleation temperature,reaction kinetics and growth kinetics.The results show that the reactivity of OA-[Zn(OAc)2]-OLA complex decreases with the increase of OA/OLA ratio,and the reactivity order of Se precursors is:Se-DPP>>Se-TOP>Se-ODE.Based on the above precursor reactivity regulation strategy,we further investigated the influence of reactivity on the nucleation process of ZnSe NCs via one-step hot-injection synthesis.As a result,a nucleation number model based on LaMer theory was proposed to establish the relationship between precursor reactivity,the number of nuclei and the final size of ZnSe NCs.The higher reactivity precursors make the system have a larger critical nucleation radius,resulting in a smaller number of nuclei,thus producing larger ZnSe NCs.However,the maximum average size of ZnSe NCs limits to 5 nm.(2)By building a diffusion growth model of ZnSe NCs,we demonstrated that the growth process of ZnSe NCs is limited by the critical diffusion radius during the synthesis of ZnSe NCs via one-step hot-injection method.Due to the large number of nuclei generated in the burst nucleation stage,the growth diffusion spheres of NCs gradually become larger and eventually overlap each other with the decrease of monomer concentration in the solution.The further growth of NCs is limited and eventually approaches to a critical value.Nevertheless,under the condition of continuous injection of precursors,the growth of ZnSe NCs would not be limited by the diffusion radius(3)Based on the theoretical study of the nucleation and growth of ZnSe NCs,epitaxial growth was used to prepare larger sized ZnSe NCs.In the epitaxial growth process,the effects of surface reactivity of crystal seeds and the reactivity of the injected precursors on the growth and spectral properties of ZnSe NCs were investigated.According to the above results,we propose a reactivity-controlled epitaxial growth(RCEG)strategy to achieve continuous epitaxial growth of ZnSe NCs with precisely tunable size in the range of 3-35 nm.The PL wavelength of ZnSe NCs can be tuned to 470 nm,which is the first report.In addition,the RCEG strategy is versatile to synthesize large-sized CdSe and PbSe semiconductor NCs with an average size of over 75 nm,which provides new insights on the synthesis of large-sized NCs and the study of optoelectronic properties.(4)In order to improve the fluorescence efficiency and chemical stability of ZnSe NCs,heterologous ZnS shells were grown as passivated shells on their surface.By changing the reactivity of the shell precursors,it is found that the ZnS shell has different effects on the optical properties and morphologies of ZnSe NCs under different reactivity conditions.When low reactivity shell precursors are used,it leads to blue-shifts of the spectrum of ZnSe nanocrystal,while the high reactivity precursors result in red-shifts in their spectrum.The results show that the reactivity of Zn and S precursors is significantly different,resulting in corresponding vacancy defects,thereby releasing the strain of ZnS shell on ZnSe core.We further reveal the influence of strain effect on the spectral characteristics and shell growth mode of ZnSe/ZnS core/shell NCs.The strain between ZnSe core and ZnS shell can convert the band alignment of ZnSe/ZnS NCs from type-I into type-II core/shell structure,resulting in red-shifts of the PL emission,which is more obvious in smaller ZnSe NCs.By optimizing the use of highly reactive zinc oleamine(ZnOLA)and octanethiol(OT)as shell precursors,the fluorescence quantum efficiency of ZnSe NCs with PL peak at 455 nm increased from 20%to 60%after coating ZnS shell,which is the highest record of ZnSe NCs in pure-blue region.