Time-Resolved Laser-Induced Incandescence for Sizing Metal Nanoparticles

Metal nanoparticles have generated considerable interest in multiple disciplines due to their unique chemical, mechanical, and electromagnetic properties. Since these properties are strong functions of nanoparticle size, and gas phase synthesis is often the most efficient route for industrial-scale production, there is a pressing need for a diagnostic which can accurately measure the size of aerosolized nanoparticles within gas phase reactors.

Time-resolved laser induced incandescence (TiRe-LII), a combustion diagnostic used to characterize soot primary particles, appears to be a promising candidate to fulfill this need. In this technique, a laser pulse rapidly heats the nanoparticles in a sample volume of aerosol, and their combined spectral incandescence is recorded as they return to the gas temperature. Most often, an effective temperature decay is derived from spectral incandescence measurements at multiple wavelengths. Since larger nanoparticles cool more slowly than smaller ones, nanoparticle sizes can be inferred by regressing simulated effective temperature decays, generated with a heat transfer model, to experimental data.

Unfortunately, there remains considerable uncertainty in this model, especially concerning the thermal accommodation coefficient, which defines the average energy transferred when a gas molecule scatters from the laser-energized nanoparticle. This uncertainty propagates into inferred nanoparticle sizes. Furthermore, most aerosols are polydisperse, in which case recovering the nanoparticle size distribution is mathematically ill-posed since many candidate distributions exist that could explain the observed TiRe-LII data within measurement resolution.

 

This talk reports recent theoretical and experimental research aimed at extending TiRe-LII into a tool for sizing aerosolized silicon and metal nanoparticles in a variety of gases. Optical properties, needed to interpret the spectral incandescence measurements, are calculated using Drude dispersion theory. Heat transfer models for these aerosols include TACs calculated using molecular dynamics, with interatomic potentials found through ab initio techniques. Bayesian analysis is used to derive posterior densities of the nanoparticle size distribution parameters, and corresponding credibility intervals. The talk concludes with a discussion of planned research aimed at quantifying and reducing the uncertainty associated with this diagnostic using Bayesian analysis.