Size distribution and characterization of tungsten nanoparticles generated by laser-assisted chemical vapor deposition and pulsed laser ablation

Abstract

Two different laser techniques for producing tungsten-based nanoparticles have been compared in this investigation. The particles were produced by Laser assisted Chemical Vapor Deposition (LCVD) from WF₆/H₂/Ar gas mixtures, and by Pulsed Laser Ablation (PLA) of a solid W target in N₂ atmosphere, respectively. An ArF excimer laser (λ = 193 nm) was used as the light source for both methods. The ablation was performed at atmospheric pressure which allowed for direct size-distribution determination by a Differential Mobility Analyzer (DMA), and a subsequent deposition of size-selected monodisperse particles. The deposits were characterized by using TEM, SEM, XRD, and XPS techniques. Optical emission spectroscopy was capable of monitoring the temperature of the particles by measuring the emitted thermal radiation from the laser-excited nanoparticles during LCVD. Strong evaporation, due to high temperature of the particles, affects the size-distribution as the laser fluence is increased. Coagulation widens and alters the size-distribution as the partial pressure of WF₆ is increased; according to TEM analysis of the deposits. Lognormal distribution was found at low WF₆ partial pressures. However, laser ablation at fluences below the ablation threshold yielded a non-lognormal size-distribution with a continuous decreasing occurrence as the particle diameter increased in the observed size-window (7-133 nm in diameter). Crystalline (β-phase) particles could be formed by LCVD, but only amorphous WN_x (x~0.3) particles were obtained by PLA in a N₂ ambient. The differences in size-distributions and crystallinity for LCVD- and PLA-produced particles are discussed on a basis that diverse mechanisms lead to particle formation.