Laser ablation of a bulk ZnTe target in liquids for nanoparticle synthesis

Abstract

Laser ablation of a bulk ZnTe target in de-ionized water or ethanol for nanoparticle synthesis was performed using a femtosecond laser beam (190 fs) with UV wavelength (343 nm). ZnTe nanoparticles with the cubic crystal structure are obtained. The largest percentage of the nanoparticles has sizes from ∼4 to ∼30 nm. Their size distribution histogram follows a lognormal function with a median diameter of ∼10 nm. The nanoparticles have a bandgap energy of ∼2.5 eV and larger up to ∼3 eV for a decrease of the laser ablating pulse energy from 50 to 10 μJ. Resonant Raman spectra of the nanoparticles are characterized by the fundamental longitudinal-optical and multiphonon peaks of ZnTe that exhibit a broadening and shift to lower frequencies compared to the bulk material attributed to phonon confinement and surface optical phonons. The ratio of the second to the fundamental longitudinal-optical phonon mode integrated intensity peaks exhibits a decrease in the nanoparticles compared to the bulk material, which indicates a weaker electron–phonon coupling. Time-resolved optical emission spectra of the laser-induced plasma plume are characterized by neutral Zn and Te lines as well as singly ionized Te lines observed at the early times of plume expansion. ZnI emission in de-ionized water decays faster than that in the air due to plume compression by the liquid and its effective cooling. At longer time delays, molecular emission from ZnO in the liquids is observed.