Cu-base Ternary And Zn-base Binary Green Semiconductor Nanocrystals: Synthesis And Properties

In the past two decades, semiconductor nanocrystals have been rapidly developedand attracted more attention. When the particle size is reduced to the nano-scale range,the nanoparticles often exhibit very interesting properties: quantum confinement effect,surface effect, quantum tunneling effect and so on. These properties cannot beachieved by their bulk counterparts. Therefore, the semiconductor nanomaterials showunique size- and shape- dependent physical and chemical properties. They are em-ployed for various applications including biomedical tags, diagnostics, light emittingdiodes, lasers and solar cell. Accordingly, synthesis of various nanocrystals with con-trollable structures, sizes and shapes is the basis of these potential applications. To date,the majority of semiconductor nanocrystals research have been focused on Cd- orPb-base nanocrystals. However, many benefits of the applications are linked to adoubtful future, due to concerns about the toxicity of the raw materials. More and moreresearchers are currently integrating green chemistry principles into their synthetic ap-proaches. Utilization of nontoxic or low toxicity starting materials and environmentallybenign solvents to synthesize green semiconductor nanocrystals. In our work, we pre-sent a green approach toward the synthesis of Cu-base ternary and Zn-base binarygreen semiconductor nanocrystals. Moreover, we study on their optical properties, highpressure structural transition and so on.CuGaS2and CuInS2are important ternary I-III-VI2semiconductor compounds. Theenergetically most stable phase of them is the chalcopyrite structure at ambient condi-tions. The advanced synthetic techniques, the predominant surface energy, unsaturatedbonds on surfaces of nanocrystals, together with the aid of solvent and/or surfactantsallow the synthesis of nanoscale metastable materials at ambient conditions, though the istence in bulk forms.We have synthesized the metastable wurtzite nanocrystals of CuGaS2through a fac-ile and effective one-pot solvothermal approach. The size and shape of the CuGaS2nanocrystals could be elaborately controlled by varying the additional ligands andother experimental parameters.Through a Rietveld refinement on experimental X-raydiffraction patterns, we have unambiguously determined the structural parameters andthe disordered nature of this wurtzite phase. The metastablility of wurtzite structurewith respect to the stable chalcopyrite structure was testified by a precise theoreticaltotal energy calculation. Subsequent high-pressure experiments at room temperaturewere performed to establish the isothermal phase stability of this wurtzite phase below15.9 GPa, above which another disordered rock salt phase crystallized and remainedstable up to 30.3 GPa, the highest pressure studied. Upon release of pressure, the sam-ple was irreversible and intriguingly converted into the energetically more favorableand ordered chalcopyrite structure as revealed by the synchrotron X-ray diffraction andthe high-resolution transmission electron microscopic measurements. The observedphase transitions were rationalized by first-principles calculations. The current researchsurely establishes a novel phase transition sequence of disorder→disorder→order,where pressure has played a significant role in effectively tuning stabilities of thesedifferent phases.The metastable wurtzite nanocrystals of CuInS2have also been synthesized and con-sidered as a disordered superstructure of chalcopyrite structure, where Cu and In atomsrandomly occupy the cation sublattice positions with equal occupancy (0.5 for Cu and0.5 for Ga). Further experiments indicate that sulfur sources also strongly influnencethe structure of wurtzite CuInS2. When CH3CSNH2and S were used as copper source,mixed phase (chalcopyrite phase and wurtzite phase) CuInS2nanocrystals were ob-tained. Using CH4N2S as copper source, we obtained wurtzite phase CuInS2nanocrys-tals.In addition, we have successfully synthesized ZnS and ZnSe nanocrystals using green chemistry principle, in which olive oil was used as solvent and ligand. By con-trolling the experimental conditions, we were able to tune the band edge photolumi-nescences and trap state photoluminescences of ZnS nanocrystals and obtained pureexcitonic photoluminescence that was rarely observed in literature. The trap stateemission was derived from sulfur vacancies and would be eliminated when an excessof sulfur was used during the synthesis. Additionally, the morphology of ZnSnanocrystals could be tuned to appear like flowers, where the limited ligand protection(LLP) was employed to explain the formation mechanism. The ZnSe nanocrystals andnanoflowers could be controlled by varying the Zn/Se ratio and reaction time.