Work Function Trends and New Low Work Function Boride and Nitride Materials for Electron Emission Applications

TL;DR

This study uses DFT calculations to analyze work function trends in hexaborides, tetraborides, and nitrides, predicting new low work function materials like alloyed LaB6 and HfN for electron emission.

Contribution

It provides a comprehensive theoretical analysis of work function trends across multiple material families and predicts new low work function materials for electron emission applications.

Findings

Alloying LaB6 with Ba lowers work function by ~0.2 eV.

Alloyed (La, Ba)B6 can have lower work functions than pure LaB6 or BaB6.

HfN has a low work function of about 2.2 eV.

Abstract

LaB6 has been used as a commercial electron emitter for decades. Despite the large number of studies on the work function of LaB6, there is no comprehensive understanding of work function trends in the hexaboride materials family. In this study, we use Density Functional Theory (DFT) calculations to calculated trends of rare earth hexaboride work function and rationalize these trends based on the electronegativity of the metal element. We predict that alloying LaB6 with Ba can further lower the work function by ~0.2 eV. Interestingly, we find that alloyed (La, Ba)B6 can have lower work functions than either LaB6 or BaB6, benefitting from an enhanced surface dipole due to metal element size mismatch. In addition to hexaborides we also investigate work function trends of similar materials families, namely tetraborides and transition metal nitrides, which, like hexaborides, are electrically conductive and refractory and thus may also be promising materials for electron emission applications. We find that tetraborides consistently have higher work functions than their hexaboride analogues as the tetraborides having less ionic bonding and smaller positive surface dipoles. Finally, we find that HfN has a low work function of about 2.2 eV, making HfN a potentially promising new electron emitter material.