Effect of Gd3+ doping on structural and optical properties of ZnO nanocrystals

In this work, the study of Gd doped ZnO nanocrystals synthesized through sol-gel method is reported. Gd: ZnO system has been investigated here to understand effect of Gd doping on the structure and photoluminescence characteristics of ZnO nanocrystals. Main highlights of our studies can be summarized as below.
- Crystallinity of Gd doped ZnO nanocrystals decrease due to the microstress developed by Gd presence on surface of nanocrystals.
- A1 symmetry phonon modes of wurtzite crystal structure are found to be more Raman active than the usual E1 and E2 modes in doped system.
- Dominant contribution to visible emission arises mainly due to the surface states.
- Surface states have more pronounced effect on long lifetimes of visible emission than the interstitial defects upon Gd doping.
- Collective excitation of doped system results in enhanced visible emission due to energy transfer from Gd centers to defect states.

To investigate the effect of rare earth ions on the excitation energy redistribution between the UV and visible photoluminescence, the Gd3+ doped ZnO nanocrystals prepared through Sol-Gel method were studied. Different structural characterizations and spectroscopic studies were carried out to establish a correlation between the structural effect and the photoluminescence. Our study indicates that the incorporation of Gd3+ in ZnO nanocrystals prepared through this synthesis takes place predominantly on the surface up to 0.18 mole fraction of Gd3+. This is manifested by the observed lattice contraction due to the presence of Gd3+ on the surface. This structural defect introduces various defects that modifies the photoluminescence properties. For these modifications in crystal system, the A1 symmetry phonon modes of wurtzite structure are found to be gradually more Raman active than the other modes as the Gd3+ concentration is increased while the E modes are suppressed. These doping induced modification of phonon spectrum strongly influences the exciton phonon coupling and thereby the photophysical properties. Resulting crystal imperfections strongly enhance the defect band luminescence. It is shown that energy transfer from selectively excited Gd3+ centers to surface states can be effectively exploited for the sensitization of defect band luminescence in various fields such as bio imaging, light emitting devices etc.

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