Nanocluster ionization energies and work function of aluminum, and their temperature dependence

TL;DR

This study measures ionization energies of aluminum nanoclusters across temperatures, revealing small shifts due to thermal expansion and providing insights into surface and electronic properties of nanomaterials.

Contribution

It provides precise measurements of temperature-dependent ionization energies and work function shifts in aluminum nanoclusters, linking experimental data with theoretical predictions.

Findings

Thermal shifts in work function agree with theory

Small clusters show larger thermal expansion effects

Photoionization follows Fowler law above threshold

Abstract

Ionization threshold energies of Al$_n$ (n=32-95) nanoclusters are determined by laser ionization of free neutral metal clusters thermalized to several temperatures in the range from 65 K to 230 K. The photoion yield curves of cold clusters follow a quadratic energy dependence above threshold, in agreement with the Fowler law of surface photoemission. Accurate data collection and analysis procedures make it possible to resolve very small (few parts in a thousand) temperature-induced shifts in the ionization energies. Extrapolation of the data to the bulk limit enables a determination of the thermal shift of the polycrystalline metal work function, found to be in excellent agreement with theoretical prediction based on the influence of thermal expansion. Small clusters display somewhat larger thermal shifts, reflecting their greater susceptibility to thermal expansion. Ionization studies of free size-resolved nanoclusters facilitate understanding of the interplay of surface, electronic, and lattice properties under contamination-free conditions.