Analysis of luminescence spectra of substrate-free icosahedral and crystalline clusters of argon

TL;DR

This study introduces a new spectral analysis method for substrate-free argon nanoclusters, revealing how luminescence varies with cluster size and structure, and identifying the transition point from icosahedral to fcc crystalline forms.

Contribution

A novel approach to analyze cathodoluminescence spectra of argon nanoclusters, linking spectral features to cluster size, structure, and emission regions, and identifying the size at which structural transition occurs.

Findings

Luminescence intensity proportional to logarithm of cluster size.

Neutral excimer emission from entire cluster volume.

Charged complex emission from near-surface layers.

Abstract

We propose a new approach to analysis of cathodoluminescence spectra of substrate-free nanoclusters of argon produced in a supersonic jet expanding into a vacuum. It is employed to analyze intensities of the luminescence bands of neutral and charged excimer complexes (Ar2)* and (Ar4+)* measured for clusters with an average size of 500 to 8900 atoms per cluster and diameters ranging from 32 to 87 {\AA}. Concentration of the jet substance condensed into clusters, which determines the absolute values of the integrated band intensities, is shown to be proportional to the logarithm of the average cluster size. Analysis of reduced intensities of the (Ar2)* and (Ar4+)* bands in the spectra of crystalline clusters with an fcc structure allows us to conclude that emission of the neutral molecules (Ar2)* comes from within the whole volume of the cluster, while the charged complexes (Ar4+)* radiate from its near-surface layers. We find the cluster size range in which the jet is dominated by quasicrystalline clusters with an icosahedral structure and demonstrate that the transition from icosahedral to fcc clusters occurs when the average cluster size is N = 1400+-400 atoms per cluster.