Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study

TL;DR

This study uses advanced quantum Monte Carlo methods to analyze the optical properties of diamondoids, revealing size-dependent gaps, exciton binding energies, and negative electron affinity, with implications for electron emission applications.

Contribution

It provides the first QMC calculations of diamondoids' optical gaps and electron affinity, clarifying size effects and quantum confinement phenomena.

Findings

Quantum confinement effects vanish in diamondoids larger than 1 nm.

Diamondoids exhibit negative electron affinity up to 1 nm.

QMC gaps are 1-2 eV higher than DFT predictions.

Abstract

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.