Nanocrystalline materials for photovoltaic and nanomedicine applications

The aim of this thesis is to investigate the property of some typical photovoltaic and luminescent materials, and the possible application of these materials in the fields of solar energy, light emitting diode and biomedical imaging. Earth abundant and nontoxic material quaternary semiconductor copper zinc tin sulfide (CZTS) provides a photovoltaic capability with favorable optical and electronic properties. Crystalized into either kesterite or wurtzite phases in aqueous or organic solution, CZTS represents potential application in the field of solar cell. Low cost and easy producing material methylammonium lead halide perovskite CH3NH3PbX3 (X=I, Br and Cl) possess superior optoelectronic properties, it represents a great potential for a variety of applications, such as high-efficiency photovoltaic cells and light-emitting diodes, but the involvement of toxic element and the instability is a concerning issue. Some efforts are in progress in the recent years to replace it with nontoxic and stable varieties such as cesium tin halide (CsSnX3, X=Cl, Br, I) and the derivative Cs2SnX6 (X=Cl, Br, I). Ternary I–III–VI semiconductor CuInS2 possesses unique properties such as large Stokes shift and high absorption coefficient. By doping with lanthanide ion Gd3+, the dual-modal MRI contrast agent with good relaxivity and fluorescent probe-CuInS2@ZnS:Gd3+ quantum dots have been investigated. Synthesized in a very simple way, the copper thiolate compounds exhibit bright emission in the optical region of green, yellow to white. The life time of copper thiolate is in the range of micrometers. These compounds possess interesting mechanochromic and thermochromism luminescent properties. The α-NaYbF4, β-NaGdF4, β-NaGdF4:(Yb,Tm), orthorhombic KYb2F7, α-KGdF4, α-KGdF4:(Yb,Tm), MGdF4:Ce@MGdF4:Eu/Tb, MGdF4:(Yb,Tm)@MGdF4:Eu/Tb, and MGdF4:Yb,Tm@MGdF4:Eu/Tb@MGdF4:Ce (M: Na or K) NPs have been synthesized hydrothermally. Under 365-400 nm UV excitation, the Eu3+/Tb3+ doped NaYbF4, NaGdF4 and KGdF4 samples emit red or green light. Not only by longer UV excitation, MGdF4:Ce@MGdF4:Eu3+/Tb3+ NPs and MGdF4:Yb,Tm@MGdF4:Eu3+/Tb3+@MGdF4:Ce NPs can also emit red or green light under short UV excitation 254 nm by a Ce3+-> Gd3+ -> Eu3+/Tb3+ energy transfer process. Under 980 nm NIR laser excitation, these potassium and sodium based core shell and core-shell-shell NPs emit red or green light by an energy transfer process of Yb3+ -> Tm3+-> Gd3+ -> Eu3+/Tb3+. The potassium based NPs exhibit brighter UCPL than sodium based NPs.