Accessing the fractal dimension of free clusters in supersonic beams

Here a method for the quantitative determination of a morphology descriptor of free cluster with complex nanostructure is presented and applied to transition metal nanoparticles produced by a pulsed vaporization source. T. Mazza et al New J. Phys. 13 (2011) 023009

In this paper a method for the quantitative determination of a morphology descriptor of free clusters with complex nanostructure is presented and applied to transition metal nanoparticles produced by a pulsed vaporization source. The method, which is based on the low-pressure aerodynamic mobility of neutral particles, can be applied as a characterization tool to a broad class of gas-phase nanoparticle sources for on-line investigation of particle growth and for quantifying coalescence versus agglomerate aggregation. We report on the application of this method for the characterization of free titanium clusters produced by a pulsed microplasma cluster source in the size range of approximately 300–6000 atoms. The clusters have an open fractal-like structure, with the fractal dimension depending on their thermal history during growth and evolving towards softer aggregates for longer residence times where lower-temperature conditions characterize the growth environment.

GENERAL SCIENTIFIC SUMMARY
Introduction and background. Clusters, i.e. aggregates of atoms or molecules whose size can range from a few units up to several thousands of constituents, are nano-scale objects with peculiar properties arising generally from their quantum-mechanical character. Despite the fact that nano-particles at larger sizes are typically observed as fractal-like random aggregates, clusters are most often modelled as compact objects whose properties are described by referring simply to the number of constituents. Yet this assumption can be a crude oversimplification, depending on material and growth conditions, clusters can show a ramified or fractal-like structure with peculiar properties and functionality, which can strongly depend on the morphology. The neglecting of this complexity can be ascribed to the difficulty of morphological characterization at a sub-nanometric scale.

Main results. In this paper we present an experimental approach for a quantitative morphological characterization of free, isolated clusters accelerated in a supersonic beam. The method is based on the same concepts of mobility measurements, with the advantage of working with neutral particles: the fractal dimension of the particles described as random fractal-like objects is inferred from the relationship between particle mass and its velocity. The results demonstrate that, under suitable growth conditions, metal clusters in the size-range of a few hundreds of atoms can exhibit a complex fractal-like morphology.

Wider implications. We believe that this technique represents a powerful tool to inspect the morphology of free ultra-fine nanoparticles, thus providing access to real-time feedback for the improvement of gas-phase particle formation and manipulation processes, as well as for the development of nanoparticle growth and transport models.