Optical trapping as a morphologically selective tool for in-situ LIBS elemental characterization of single nanoparticles generated by laser ablation of bulk targets in air

Abstract

In the present work, authors introduce a shape-specific methodology for evaluating the full elemental composition of single micro and nanoparticles fabricated by laser ablation of bulk targets. For this purpose, bronze samples were directly ablated within an ablation cell, originating dry aerosols consisting of multielemental particles. The in-situ generated particles were first optically trapped using air at atmospheric pressure as medium, and then probed by LIBS. A key aspect of this technology is the circumvention of possible material losses owed to transference into the inspection instrument while providing the high absolute sensitivity of single-particle LIBS analysis. From results, we deepen the knowledge in laser-particle interaction, emphasizing fundamental aspects such as matrix effects and polydispersity during laser ablation. The dual role of air as the atomization and excitation source during the laser-particle interaction is discussed on the basis of spectral evidences. Fractionation was one of the main hindrances as it led to particle compositions differing from that of the bulk material. To address possible preferential ablation of some species in the laser-induced plasma two fluence regimes were used for particle production, 23 and 110 J/cm2. LIBS analysis revealed a relation between chemical composition of the individual particles and their sizes. At 110 J/cm2, 65% of the dislodged particles were distributed in the range 100-500 nm, leading to higher variability of LIBS spectra among the inspected nanoparticles. In contrast, at 23 J/cm2, around 30 % of the aerosolized particles were larger than 1 m. At this regime, the composition resembled better to the bulk material. Therefore, we present a pathway to evaluate how adequate the fabrication parameters are towards yielding particles of a specific morphology while preserving compositional resemblance to the parent bulk sample.