Electron spin-lattice relaxation of silver nanoparticles embedded in SiO2 and TiO2 matrices

Metallic silver nanoparticles were prepared by the sol-gel method in amorph ous SiO2 and crystalline TiO2 matrices. The metal particles were monodisper sed in size and their mean diameters varied between 1 and 10 nm, depending upon the processing conditions. The spin-lattice relaxation (SLR) times wer e investigated by pulsed electron paramagnetic resonance (EPR) spectroscopy in a temperature range between 4 K and 300 K. The spin echo recoveries wer e slow enough (of the order of some ms), and demonstrated a biexponential c haracter. This study showed that for silver nanoparticles in the SiO2 matri x, the temperature dependence of T-1 can be described by the relation (1/T- 1)proportional to T-n, where 0.4 < n < 1. Contrary to this behavior, the te mperature dependence of T-1 has a Raman-type character (1/T(1)proportional to T-2) for silver nanoparticles in the TiO2 matrix. The unusual behavior f or the SiO2 samples is attributed to the amorphous phase of the SiO2 matrix . The results are discussed in terms of possible relaxation mechanisms sugg ested by Khaliullin and Khusainov taking into account the energy levels cor relation functions. This study showed that the spin relaxation due to stati c defects is in agreement with our results. In addition, a satisfactory agr eement between theory and experiment can only be obtained in the case of re pulsion between the energy levels indicative for the quantum size effect. ( C) 1999 American Institute of Physics. [S0021-9606(99)70741-5].