Intra-excitonic relaxation dynamics in ZnO Appl. Phys. Lett. 99, 231910 (2011) Thermal diffusion of nitrogen into ZnO film deposited on InN/sapphire substrate by metal organic chemical vapor deposition J. Appl. Phys. 110, 113509 (2011) Manifestation of spin-spin interaction between oxygen vacancy and magnesium in ZnMgO nanorods by electron paramagnetic resonance studies Appl. Phys. Lett. 99, 194101 (2011) Selective pair luminescence in the 1.4-eV band of CdTe:In J. Appl. Phys. 110, 093103 (2011) Evidence of cation vacancy induced room temperature ferromagnetism in Li-N codoped ZnO thin films Appl. Phys. Lett. 99, 182503 (2011) Additional information on J. Appl. Phys. In this article we present size dependent spectroscopic observations of nanocolloids of ZnO. ZnO is reported to show two emission bands, an ultraviolet ͑UV͒ emission band and another in the green region. Apart from the known band gap 380 nm and impurity 530 nm emissions, we have found some peculiar features in the fluorescence spectra that are consistent with the nanoparticle size distribution. Results show that additional emissions at 420 and 490 nm are developed with particle size. The origin of the visible band emission is discussed. The mechanism of the luminescence suggests that UV luminescence of ZnO colloid is related to the transition from conduction band edge to valence band, and visible luminescence is caused by the transition from deep donor level to valence band due to oxygen vacancies and by the transition from conduction band to deep acceptor level due to impurities and defect states. A correlation analysis between the particle size and spectroscopic observations is also discussed.

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Solid-immersion-lens-enhanced nonlinear frequency-variation mapping of a silicon integrated-circuit Appl. Phys. Lett. 99, 193103 (2011) A microfiber cavity with minimal-volume confinement Appl. Phys. Lett. 99, 051105 (2011) Lead silicate glass microsphere resonators with absorption-limited Q Appl. Phys. Lett. 98, 181105 (2011) Full band structure calculation of two-photon indirect absorption in bulk silicon Appl. Phys. Lett. 98, 131101 (2011) Geometry and quadratic nonlinearity of charge transfer complexes in solution: A theoretical study J. Chem. Phys. 134, 044534 (2011) Additional information on J. Appl. Phys. We have investigated the third-order nonlinearity in ZnO nanocolloids with particle sizes in the range 6-18 nm by the z-scan technique. The third-order optical susceptibility ͑3͒ increases with increasing particle size ͑R͒ within the range of our investigations. In the weak confinement regime, an R 2 dependence of ͑3͒ is obtained for ZnO nanocolloids. The optical limiting response is also studied against particle size.

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In this paper, the fluorescence behaviour of nano colloids of ZnO has been studied as a function of the excitation wavelength. We have found that excitation at the tail of the absorption band gives rise to an emission that shifts with the change of the excitation wavelength. The excitation wavelength dependent shift of the fluorescence maximum is measured to be between 60 and 100 nm. This kind of excitation wavelength dependent fluorescence behaviour, which may appear to be in violation of Kasha's rule of excitation wavelength independence of the emission spectrum, has been observed for nano ZnO colloids prepared by two different chemical routes and different capping agents. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited state relaxation processes, namely, solvation and energy transfer, are responsible for the excitation wavelength dependent fluorescence behaviour of the systems.