| Laser Ablation of Microparticles (LAM) is a process of nanoparticle formation in which microparticles in a flowing aerosol are continuously ablated by high power laser pulses. For the first time, we have produced CdSe/ZnS core/shell nanoparticles using a double ablation apparatus, designed to undergo a two step LAM process. This process can be inverted to produce ZnS/CdSe core/shell nanoparticles. The present work focuses on the range around ~15 nm radius heterostructures and uses high resolution transmission electron microscopy (HRTEM) to image core and shells. For smaller particles core shell structures have been detected with a energy dispersive spectroscopy (EDS) 5 nm spot size beam, and fast Fourier transform (FFT) spectra. Differences in the ablation behavior were measured between the two IIB-VIA type semiconductors.;We investigate the temperature and size distribution of Ag, Ge, CdSe and ZnS nanoparticles undergoing UV excimer laser pulses. A two laser pulse experiment is designed to monitor nanoparticle size before and after laser interaction. We study HRTEM images and measure the ablation and fluorescence spectra of particles before and after evaporation. Results show that the nanoparticle mean radius decreases from 3.4 +/- 0.2 nm to 2.6 +/- 0.2 nm, from 4.3 +/- 0.1 nm to 3.5 +/- 0.1 nm, and from 3.1 +/- 0.2 nm to 2.6 +/- 0.2 nm for Ag, Ge and CdSe, respectively. No ZnS nanoparticle size reduction was observed. Theoretical models for nanoparticles undergoing laser heating show that temperatures above the bulk and nanoparticle material melting point reduce the nanoparticles size by a factor of 0.3 and suggest recondensation before collection. For CdSe nanoparticles collected on dry substrates and solvents, blue shifted fluorescence (PL) peaks support the size reduction. |
