Synthesis And Optical Properties Of ZnO Micro-/Nanoheterostructures

ZnO/CdS heterostructures porous microspheres, CdS/ZnO core-shell quantum dots, ZnO/ZnMgO heterostructures porous prisms, have been synthesized via simple and facile methods. The structures and optical properties of as-prepared products were thoroughly characterized using X-ray diffraction (XRD), thermogravimetry differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), UV-Vis spectra, electron paramagnetic resonance (EPR), and photoluminescence (PL), respectively. The results are summarized as follows.ZnO/CdS heterostructures porous microspheres were obtained by directly annealing of commercial ZnS and CdCl2 powders at 700°C in air. The average diameter of as-prepared microspheres is about 3μm, with many nanoparticles (100-300 nm) around their surface. A self-assembled heterostructure has been formed with ZnO and CdS atomic layers randomly alternately growing along the c axis. The as-obtained ZnO structures show good UV absorption properties in the range of 250 to 540 nm, exhibiting strong green emission centered at 530 nm under the excitation wavelength of 400 nm.CdS/ZnO core-shell structures with average diameter of ZnO shell about 10 nm were fabricated by annealing of the precursor contained CdS/ZnO core-shell quantum dots prepared via sol-gel route in air. There is no obvious crystal boundary between core and shell, only connected by a transition layer. A clear shrinkage in the optical band gap is observed, which depends on the properties of the incorporated transition layer. At the same time, the PLE and PL peaks shift to the long wavelength, and then the PL spectrum exhibiting strong red emission centered at 620 nm under the excitation wavelength of 460 nm.ZnO/ZnMgO heterostructures porous prisms were prepared by annealing of the mixtures of ZnS and MgCl2 at 1000℃in air. A new UV absorption peak at～400 nm and red shift of 30 nm are found due to the incorporation of ZnMgO heterostructures into ZnO lattice. A clear expansion in the optical band gap is also observed, which depends on the properties of the incorporated ZnMgO heterostructures and the annealing temperature. Furthermore, the excitation band at 390 nm is highly enhanced as the ZnMgO heterostructures were induced into ZnO. A new energy level is proposed, and the possible mechanisms are also suggested, which is used to explain the enhancement of green emission. The correlation of the annealing temperature the green PL emission of the heterostructures products is also discussed.