Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions

TL;DR

This study demonstrates that fractal TiO2 nanostructures formed by femtosecond laser ablation at ambient conditions are created through a diffusive aggregation process, supported by experiments and Monte Carlo simulations, advancing control over nanostructure production.

Contribution

The paper introduces a minimal diffusion-based model that accurately reproduces experimental fractal nanostructures of TiO2 formed by femtosecond laser ablation, elucidating the formation mechanism.

Findings

Fractal dimensions match between model and experiments.

Diffusive aggregation explains nanostructure formation.

Substrate thermal conductivity influences nanostructure properties.

Abstract

The specific mechanisms which leads to the formation of fractal nanostructures by pulsed laser deposition remain elusive despite intense research efforts, motivated mainly by the technological interest in obtaining tailored nanostructures with simple and scalable production methods. Here we focus on fractal nanostructures of titanium dioxide, $TiO_2$, a strategic material for many applications, obtained by femtosecond laser ablation at ambient conditions. We model the fractal formation through extensive Monte Carlo simulations based on a set of minimal assumptions: irreversible sticking and size independent diffusion. Our model is able to reproduce the fractal dimensions and the area distributions of the nanostructures obtained in the experiments for different densities of the ablated material. The comparison of theory and experiment show that such fractal aggregates are formed after landing of the ablated material on the substrate surface by a diffusive mechanism. Finally we discuss the role of the thermal conductivity of the substrate and the laser fluence on the properties of the fractal nanostructures. Our results represent an advancement towards controlling the production of fractal nanostructures by pulsed laser deposition.