Optical Properties and Spin States of Inter-layer Carbon Defect Pairs in Hexagonal Boron Nitride: A First-Principles Study

TL;DR

This study uses density functional theory to explore inter-layer carbon defect pairs in hexagonal boron nitride, revealing their potential for stable spin states and optical emissions relevant for quantum technologies.

Contribution

It provides the first detailed theoretical analysis of inter-layer C$_X$C$_X$ defect pairs, highlighting their spin states, optical properties, and vibrational characteristics in hBN.

Findings

Inter-layer C$_X$C$_X$ pairs can have stable triplet states at room temperature.

The zero-phonon line of these defects is within the visible spectrum.

Certain C$_B$C$_B$ configurations show low-energy phonon replicas, explaining vibronic features.

Abstract

Substitutional carbon defects in hexagonal boron nitride (hBN) are prominent single photon emitters (SPEs), and their potential for spin activity ($S\geq1$) is particularly intriguing. While studies have largely focused on intra-layer defects, we employ density functional theory (DFT) to investigate inter-layer dimers of identical carbon species (C$_X$C$_X$). We demonstrate that these C$_X$C$_X$ pairs can exhibit a stable triplet spin state at room temperature when closely spaced (e.g., within 3.5-7.1 {\AA}) across hBN layers. As their separation increases beyond this range (e.g., $>7$ {\AA}), they transition into weakly interacting $S=1/2$ pairs, characterized by singlet-triplet degeneracy. This regime is predicted to result in a very small zero-field splitting for the triplet manifold, offering a potential explanation for certain optically detected magnetic resonance (ODMR) signals. The zero-phonon line (ZPL) energy of these inter-layer C$_X$C$_X$ pairs is found to be practically monochromatic and within the visible range. Furthermore, we identify specific C$_B$C$_B$ inter-layer configurations exhibiting atypical low-energy phonon replicas due to out-of-plane vibrational coupling, a finding that may clarify the vibronic structure of other hBN emitters, such as the 'yellow emitters'.