Charging Induced Emission of Neutral Atoms from NaCl Nanocube Corners

TL;DR

This study uses first principles calculations to show that neutral atom emission from NaCl nanocube corners can be energetically feasible through electron donation or removal, with potential exothermic processes.

Contribution

It provides the first detailed energetic analysis of neutral atom emission from NaCl nanocube corners upon electron donation or removal.

Findings

Neutral Cl corner atoms can be detached with about 0.8 eV energy.

Na atoms can be extracted at about 0.6 eV energy.

Electron donation to the nanocube is exothermic due to the surface state energy level.

Abstract

Detachment of neutral cations/anions from solid alkali halides can in principle be provoked by donating/subtracting electrons to the surface of alkali halide crystals, but generally constitutes a very endothermic process. However, the amount of energy required for emission is smaller for atoms located in less favorable positions, such as surface steps and kinks. For a corner ion in an alkali halide cube the binding is the weakest, so it should be easier to remove that atom, once it is neutralized. We carried out first principles density functional calculations and simulations of neutral and charged NaCl nanocubes, to establish the energetics of extraction of neutralized corner ions. Following hole donation (electron removal) we find that detachment of neutral Cl corner atoms will require a limited energy of about 0.8 eV. Conversely, following the donation of an excess electron to the cube, a neutral Na atom is extractable from the corner at the lower cost of about 0.6 eV. Since the cube electron affinity level (close to that a NaCl(100) surface state, which we also determine) is estimated to lie about 1.8 eV below vacuum, the overall energy balance upon donation to the nanocube of a zero energy electron from vacuum will be exothermic. The atomic and electronic structure of the NaCl(100) surface, and of the nanocube Na and Cl corner vacancies are obtained and analyzed as a byproduct.