Blue Quantum Emitter in Hexagonal Boron Nitride and a Carbon Chain Tetramer: a First-Principles Study

Single-photon emitters in hexagonal boron nitride (h-BN) exhibit compelling optical properties, such as high brightness and photostability, making them promising candidates for applications in emerging nanophotonic and quantum technologies. However, their further progress would greatly benefit from the identification of their chemical nature, which in most cases is still under debate. Among the various emitters observed in h-BN, blue quantum emitters (435 nm), also known as B-centers, are unique not only due to their promising optical properties but also because they can be fabricated at specific locations via electron-beam irradiation. Herein, ab initio calculations are used to demonstrate that the properties of a carbon chain tetramer are in excellent agreement with the characteristics of blue quantum emitters. The calculated zero-phonon line (ZPL) energy (3.13 eV) and radiative lifetime (1.62 ns) of the carbon chain tetramer align well with experimental observations. The relatively weak electron-phonon coupling indicates intense emission at the ZPL, and the Debye-Waller factor of 0.25 is consistent with experimental data. We demonstrate that, despite the absence of an inversion center in the carbon chain tetramer, it exhibits a negligible linear Stark effect, consistent with experimental findings. Additionally, our hypothesis explains the experimental observation that the formation of blue emitters is only possible in samples containing numerous ultraviolet emitters, which are probably identical to carbon dimers.