Doping, Self-assembly And Tunable Optical Properties Of ZnO Nanocrystals

Colloidal Zinc oxide(Zn O) nanocrystals(NCs) have been considered as important semiconductor materials due to some advantages over conventional thin film materials, such as remarkable optoelectronic properties and convenience of fabrication and processing. The properties of the Zn O depend closely on the doping, microstructures of the materials, including crystal size, orientation, morphology, dimensionality and crystallinity. Consequently, various synthetic approaches have been explored to tune the size, morphology of Zn O nanocrystals. Among them, thermolysis of metal salts in non-aqueous medium has been proven to be an attractive and versatile method for the synthesis of colloidal Zn O nanocrystals with good crystallinity. Techniques, including use of appropriate capping agents, seeded growth, and oriented attachment of nanoparticles have been developed to produce nanocrystals with controllable sizes and anisotropic shapes for specific applications.Doping or alloying is very important for controlling the electronic and optical properties of semiconductor nanocrystals. For traditional doping, trace amount of impurities or defects are introduced and many efforts have been paid. However, knowledge on the formation of alloyed nanocrystals is still limited, especially for oxide nanocrystals. In addition, if two basic oxides possess different crystal structures, alloying will be more difficult due to low solid solubility and possible phase separation. Thus, deep understanding of doping mechanism and growth behavior is very important for increasing the solid solubility and inhibiting the phase separation.In this paper, we choose Mg Zn O and Be Mg Zn O alloy nanocrystals as an example and try to draw a deep understanding of their growth behavior. Interest in Mg Zn O and Be Mg Zn O alloy nanocrystals frequently stems from its potential applications in deep ultraviolet photodetectors, barrier layers and Zn O-based laser devices. The work included: 1. Mg Zn O alloy nanocrystals(NCs) were prepared using Zn(St)2, Zn(acac)2, Mg(St)2 and Mg(acac)2 by a one-pot synthesis method. Structural and optical properties of Mg Zn O NCs were investigated by X-ray diffraction(XRD) and optical transmittance spectra. Regardless of precursors, the products undergo a phase transition from hexagonal to cubic structure when the nominal concentration of Mg increases to a certain value. Different precursors have different decomposition rates, which would influence the crystallinity and phase separation of Mg Zn O alloy. Mg Zn O alloy nanocrystals obtained by using Zn(acac)2 and Mg(acac)2 had the highest crystallinity and the widest optical band gap of 3.9 e V. 2. Zero dimensional(0D) spherical, 2D flower-like and 3D bouquet-like Mg Zn O nanostructures were synthesized in solution with an increase in the Mg dopant. The results of high resolution transmission electron microscopy(HRTEM) indicated that primary Mg Zn O nanoparticles directionally attached on specific crystal face to produce 2D and 3D nanostructures. A face-selective attachment mechanism based on the relationship between dopant, ligand, and growth was proposed and verified by a series of fourier transform infrared spectrum(FTIR) and thermal gravity analysis(TGA). 3. We introduced small quantities of Be into Mg Zn O alloy and obtained single-phase wurtzite Be Mg Zn O alloy NCs. Based on XRD and ICP-AES analysis, Be doping was found able to improve solubility of Mg and suppress the phase segregation in wurtzite Mg Zn O alloy. The band gap of as-prepared quaternary alloys can be raised to the solar-blind range(4.35 e V).