Ultrafast transient absorption spectroscopic studies on the Impact of growth time on size, stability, and optical characteristics of colloidal gold nanoparticles

Abstract

Metallic gold (Au) nanoparticles (NPs) with an average size of less than 20 nm exhibit surface plasmon resonance (SPR), which makes them intriguing candidates for optical and optoelectronic applications. In this work, Turkevich approach is used to synthesize different samples of Au NPs by tuning the growth times during synthesis. Transmission electron micrographs depict that the Au NPs are highly crystalline, monodisperse, and spherical, with an average size of ~13–30 nm. Moreover, a shift in the ultraviolet-visible (UV–visible) absorption spectrum is observed from 525 nm to 542 nm. The change in the SPR band corroborates with the growth time-dependent size enhancement. Ultrafast transient absorption spectroscopy (UTAS) has been employed to characterize the optical properties as well as the stability of NPs, and it is worth noticing that as the growth time increases, the intensity of the ground-state bleach and the intraband transitions reduces and increases at 400 fs and 5 ps, respectively, demonstrating the size dependence of the hot-electron distribution (e--e- scattering). These heated electrons subsequently relax into the lattice through two different forms of scattering, namely electron-phonon (e--ph) and phonon-phonon (ph-ph) scatterings. The stability of the Au NPs has been evaluated by kinetic parameters, with a longer average decay time (τavg) showing enhanced stability of NPs with longer growth durations. Transient absorption spectroscopy reveals the size-dependent dynamics of interfacial charge/energy transfer between the Au NPs and the lattice.