Excitation wavelength dependent fluorescence behaviour of nano colloids of ZnO

Abstract

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