Stress state of silver nanoparticles embedded in a silicate glass matrix investigated by HREM and EXAFS spectroscopy

Ag particles of 3.9 and 5.1 mn mean size in silicate glasses were produced by ion exchange and subsequent annealing at 480 and 600 degreesC. These the rmal treatments may induce stresses in matrix and particles in addition to the well known effect of surface atoms because of the thermal expansion mis match of both materials. Structural characterisation of the particles by hi gh-resolution electron microscopy revealed a size-dependent lattice dilatat ion quite opposite to the so far observed lattice contraction of similar me tal/glass composites. This result, confirmed by X-ray absorption spectrosco py at the Ag K-edge, is discussed in terms of an Ag-Ag bond length increase near the particle surface. The temperature-dependent EXAFS spectra (10-300 K) indicate an increased thermal expansion coefficient of the particles wi th an increased mean particle size calculated on the basis of an anharmonic Einstein model. With that the bond length increase can be explained. The r esults can be interpreted by a combination of both the particle size effect s and the influence of the surrounding matrix.