Radhakrishnan, P; Nampoori, V P N; Litty, Irimpan; Bindu, Krishnan; Deepthy, A(Journal of Physics D : Applied Physics, August 30, 2007)
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Abstract:
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.