First principles study of Ag absorption mechanism in amorphous large silica clusters

Ag nanoparticles cause interesting modifications to the electronic and optical properties of amorphous silica scaffold that is used in optical fibers. The present study is first of its kind to implement first principles based calculation of large amorphous dielectric host with metal inclusions in it. Ground state structural parameters and equilibrium geometries of amorphous (SiO2)n clusters (n = 8,16,24,32,40) with Ag atoms doped inside the interstitial space of the scaffold have been calculated using DMol3 DFT program. We have shown that the configuration of small Ag clusters depend strongly on the structural characteristics of the scaffold. This creates a unique opportunity to control the microscopic properties of small metal clusters via Ag impregnation. Our study reveals that small sized silica clusters have lower level of absorption due to self-purification effect and Ag absorption efficiency depends on the nature of amorphicity of clusters having different geometries but same number of atoms. The nature of amorphicity of two different geometries of equal sized silica clusters has been verified via two independent approaches viz. (a) pair correlation and bond length/angle distributions plot of Si atoms and (b) the quasi-entropy calculated using USPEX code. More amorphous network of host leads to incorporation of more number of Ag atoms. The effect of host silica matrix on the structures of Ag nanoclusters are evaluated and the results show that bond distances in Ag clusters increase when encapsulated in silica matrix. The effect of doped Ag atoms on the structural characteristics of silica clusters are also evaluated and correlated with the shift in frequencies and change in Raman spectral intensities.