Beijing Univ Aeronaut & Astronaut, Sch Mat Sci & Engn, Beijing 100083, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
; Chinese Acad Sci, Inst Chem, Beijing 100080, Peoples R China
; Peking Univ, Sch Phys, Electron Microscopy Lab, Beijing 100871, Peoples R China
; Peking Univ, Sch Phys, State Key Lab Mesoscop Phys, Beijing 100871, Peoples R China
Optical properties of star-shaped ZnO nanostructures were studied. The temperature-dependent photoluminescence (PL) was examined up to fourth-order longitudinal optical (LO) phonon assisted emissions of free excitons and confirmed that the nature of the room temperature PL in ZnO is 1-LO phonon assisted emission of free excitons. Low threshold ultraviolet stimulated emissions (SE) were obtained for our powder samples at room temperature. Picosecond time-resolved PL measurements detected a bi-exponential decay behavior which is strongly dependent on the excitation intensity: the slow decay term decreased faster than the fast decay term as the excitation intensity increased and the emission decays were dominated by the fast one. We also found that the emission decays decreased super-linearly before the appearance of the SE. This behavior may be used to deduce the threshold of SE or lasing. (c) 2006 Elsevier Ltd. All rights reserved.