The effect of the target structure and composition on the ejection and transport of polymer molecules and carbon nanotubes in matrix-assisted pulsed laser evaporation

Abstract

Matrix-assisted pulsed laser evaporation (MAPLE) is a prominent member of a broad and expanding class of laser-driven deposition techniques where a matrix of volatile molecules absorbs laser irradiation and provides the driving force for the ejection and transport of the material to be deposited. The mechanisms of MAPLE are investigated in coarse-grained molecular dynamic simulations focused on establishing the physical regimes and limits of the molecular transfer from targets with different structures and compositions. The systems considered in the simulations include dilute solutions of polymer molecules and individual carbon nanotubes (CNTs), as well as continuous networks of carbon nanotubes impregnated with solvent. The polymer molecules and nanotubes are found to be ejected only in the ablation regime and are incorporated into matrix-polymer droplets generated in the process of the explosive disintegration of the overheated matrix. The ejection and deposition of droplets explain the experimental observations of complex surface morphologies in films deposited by MAPLE. In simulations performed for MAPLE targets loaded with CNTs, the ejection of individual nanotubes, CNT bundles, and tangles with sizes comparable or even exceeding the laser penetration depth is observed. The ejected CNTs align along the flow direction in the matrix plume and tend to agglomerate into bundles at the initial stage of the ablation plume expansion. In a large-scale simulation performed for a target containing a network of interconnected CNT bundles, a large tangle of CNT bundles with the total mass of 50 MDa is separated from the continuous network and entrained with the matrix plume. No significant splitting and thinning of CNT bundles in the ejection process is observed in the simulations, suggesting that fragile structural elements or molecular agglomerates with complex secondary structures may be transferred and deposited to the substrate with the MAPLE technique.