Iodization of rf sputter induced disordered Ag thin films reveals volume plasmon-exciton “transition”

Abstract

Quasiamorphous Ag films of thicknesses ranging from 5to30nm were prepared using rf magnetron sputtering technique and their controlled iodization was carried out for selected durations in the range of 15min–60h at room temperature. As deposited Ag and iodized films were characterized using x-ray diffraction (XRD), atomic force microscope (AFM), and optical absorption techniques. From XRD, γ and β+γ (mixed) phases of AgI nanoparticles have been observed for 5–10 and 20–30nm thick films, respectively. Lattice parameters (a and c) and average strain (ε) were calculated versus iodization time for γ and β-AgI nanoparticles. Uniform and nonuniform spherically shaped AgI nanoparticles (∼20–130nm) are realized through AFM for 5–10 and 20–30nm thick films. Optical absorption shows volume plasmons (classified as PR1) for short duration iodization, which “decay” upon further iodization to convert to Z1,2 and Z3 excitons at 420 and 330nm, respectively, in the manner of a metal-semiconductor/dielectric phase transition. Ag “colloidal” particles (classified as PR2) are formed for 5–10nm thick films and thereby control the γ phase—a significant and applicable effect attributed to critical film thickness. With increasing thickness, a surface strain field lifting the degeneracy of the valence band results in Z1,2 and Z3 exciton formation at room temperature. Blueshift in the exciton absorption with decreasing film thickness implies the progressive quantum confinement due to decrease in the particle size. A thickness induced phase transition from γ-AgI to β-AgI is discussed by means of x-ray diffraction and optical absorption studies.