Boron Dangling Bonds as Single Photon Emitters in Hexagonal Boron Nitride*

First-principles calculations have proven invaluable in the identification of novel quantum defects in a variety of semiconductor hosts. Hexagonal boron nitride (h-BN) is one such host that exhibits promising properties: h-BN has an ultra-wide band gap, two-dimensional crystal structure, and excellent stability. Single-photon emission has been observed in h-BN, and the emitters around 2 eV possess a wealth of interesting features that are promising for quantum applications. However, a conclusive identification of the microscopic origin has proven elusive. In this work, we employ hybrid density functional theory to demonstrate that the properties of boron dangling bonds are consistent with the experimental reports. Specifically, doubly occupied boron dangling bonds give rise to optical emission at 2.06 eV with a Huang-Rhys factor of 2.3. The emission is linearly polarized, with indirect excitation into the conduction band explaining the lack of dipole alignment seen in experiment. The boron dangling bond possesses a metastable triplet state, which can explain the observed spin dependence. We will discuss recent developments to the model, including the behavior of the boron dangling bond in a monolayer of h-BN. Our work demonstrates that dangling bonds may also be efficient quantum defects in other materials.