A simple method for large scale synthesis of highly monodisperse gold nanoparticles at room temperature and their electron relaxation properties.

Here we demonstrate a simple method for large scale preparation of monodisperse gold nanoparticles by simple mixing of chloroauricacid (HAuCl(4)) with oleylamine (OA) at room temperature. The as-prepared gold nanoparticles have high monodispersity with an average diameter of 13 nm and can self-organize into two-dimensional (2D) hexagonal close-packed arrays. The size of the gold nanoparticles can be experimentally controlled. The capping agent, oleylamine, can be easily replaced with other capping agents such as thiol groups for further functionalization. The electron relaxation dynamics of these gold nanoparticles in toluene was studied by femtosecond pump-probe measurements, in comparison with the citrate-stabilized gold nanoparticles in water. The phonon-phonon relaxation time of gold nanoparticles in toluene is slower than that of citrate-capped gold nanoparticles in water, due to the lower thermal conductivity of toluene than water. The electron-phonon relaxation of the gold nanoparticles in toluene was found to display weaker pump energy dependence, compared to that of citrate-capped gold nanoparticles in water. The different electron-phonon relaxation dynamics is ascribed to the extra vibrational states provided by gold- NH(2), which serves as an extra nonradiative relaxation pathway for the e-ph relaxation in oleylamine-capped gold nanoparticles in toluene.