报告题目（Title）：Single photon emission from defects in 2D hexagonal Boron Nitride: evalution of computational approaches
报 告 人（Speaker）：Michael John Ford
报告时间（Time）：2018年10月9日（周二）9:00
报告地点（Place）：校本部E106
邀请人（inviter）：Jeffrey Reimers 教授
报告摘要（Abstract）：
Defect states in semiconductors have a range of possible uses arising from their light emission properties. For example, the well-known NV-defect in bulk and nano-diamond provides a source of quantum emission that is becoming a key component in many quantum technologies. Understanding the atomic origin of this emission has required advances in both the experimental and theoretical domains. The recent discovery of single-photon emission from hBN has generated considerable interest and efforts are underway by a number of groups to characterise the spectroscopy of this emission.The most widely used approach to this problem computationally is to use Density Functional Theory based methods offering a good balance between computational expediency and reliability. The HSE06 functional reproduces bandgaps in semiconductors well and, combined with methods to constrain orbital occupation, is perhaps the most favoured approach for calculating defect excited states. In this paper we evaluate the performance of DFT methods by benchmarking against high level coupled-cluster and multi-reference methods. Many of the electronic states for these defects are inherently multi-reference open-shell and closed-shell states and involve broken chemical bonds and charge-transfer states. We find that some DFT methods can perform very poorly, for example PBE. HSE06 works well for excitations among the triplet states but underestimates triplets relative to closed-shell singlets, and fails with regard to open-shell singlet states. Long-range corrected functionals such as CAM-B3LYP perform much better, but their implementations so far have been restricted to non-periodic codes. We also show that there can be significant differences between cluster and periodic-slab models of these defects implying that implementation of such functionals in periodic codes is needed.
