Temperature-dependent photoionization thresholds of alkali-metal nanoparticles reveal thermal expansion and the melting transition

TL;DR

This study uses high-resolution photoionization measurements to track how the work function of sodium and potassium nanoparticles varies with temperature, revealing thermal expansion effects and detecting melting transitions at the nanoscale.

Contribution

It demonstrates that photoemission thresholds can serve as sensitive probes for structural phase transitions in metal nanoparticles, linking electronic properties to melting behavior.

Findings

Work function decreases gradually with temperature due to thermal expansion.

A sharp drop in work function indicates the onset of nanoparticle melting.

Melting temperature of 7-9 nm nanoparticles is nearly 100 K lower than bulk values.

Abstract

A precision measurement of the photoionization of pure sodium and potassium nanoparticles isolated in a beam enabled an accurate determination of their work functions as a function of temperature. In addition to resolving and quantifying the initial gradual decrease of the work function with temperature, which is associated with thermal expansion, the experiment revealed that the work function then undergoes a distinct drop both in magnitude and in slope that signifies the onset of nanoparticle melting. This establishes that a structural phase transition can be detected via a high-resolution measurement of the photoemission threshold. The melting temperature of nanoparticles with diameters of 7-9 nm is reduced by nearly 100 K relative to the bulk value. This suppression aligns with predictions from the Gibbs-Thomson equation which describes finite-size phase transitions.