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前沿|六方氮化硼:量子发射器的辐射特性

前沿|六方氮化硼:量子发射器的辐射特性 两江科技评论
2021-07-26
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导读：二维材料六方氮化硼中基于点缺陷的单光子发射体的发现，激发了人们对相关材料中光学活性缺陷的研究。但如何确定其原子结构和性质尚有很多挑战。美国科学家采用密度泛函理论等计算方法，研究了六方氮化硼中原子缺陷的

来源：npj计算材料学

Fig. 1 DFT transition energy of candidate defect structures.

二维材料作为一个可以应用于光量子信息技术的载体，近年来得到快速发展。其中，六方氮化硼中基于点缺陷的单光子发射体的发现，激发了人们对相关材料中光学活性缺陷的大量研究。六方氮化硼的缺陷发射器目前面临的挑战是如何确定它们的原子结构。基于密度泛函理论的第一性原理计算能够提出各种可能的缺陷结构，且能与实验对比，但不能处理单光子发射器的一些关键特性，如激发态和辐射过程。基于量子化学和密度矩阵重整化组等方法也已被用于研究特定的缺陷结构，但迄今很少有报道将不同结构进行对比。在本工作中，来自美国加州理工学院应用物理与材料科学系的Marco Bernardi教授等人采用密度泛函理论和GW+Bethe-Salpeter方程的计算方法，研究了六方氮化硼中原子缺陷的光学性质、跃迁偶极子、激子和辐射寿命等性质。研究发现，不同的量子发射器的辐射寿命横跨6个数量级，发射能量涉及红外到紫外光区。通过贝叶斯统计分析，他们确认了各种高度可能的电荷中性缺陷发射体，其中本征缺陷VNNB的发射能量和辐射寿命与实验数据一致。通过对VNNB缺陷的进一步计算，作者发现其辐射特性对其原子结构的小扰动有很强的依赖性。该工作推进了对基于六方氮化硼单光子发射体的微观理解，并引入了一个计算框架来表征和识别二维材料中的量子发射体。该文近期发表于npj Computational Materials7: 85 (2021)。

Fig. 2 Radiative lifetime and energy of the lowest bright exciton of candidate defect SPEs in hBN from GW-BSE calculations.  

Editorial Summary

Quantum emitters in hBN: Radiative properties 
As a platform for photon based quantum information technologies, two-dimensional (2D) materials have been growing rapidly in recent years. The discovery of single-photon emitters (SPE) at point defect in hBN has spurred an intense search for optically active defects in 2D materials. Currently, a pressing challenge for defect emitters in hBN is identifying their atomic structure. Density functional theory (DFT) based first-principles calculations are able to identify various possible structures that can be compared to experiments, but DFT cannot address key aspects of point-defect SPEs such as their excited states and radiative processes. Quantum chemistry approaches and density matrix renormalization group have also been used to investigate specific defect structures, but wide comparisons among different structures are still missing. In this work, Prof. Marco Bernardi from the Department of Applied Physics and Material Science, California Institute of Technology, used ab initio density functional theory and GW plus Bethe-Salpeter equation calculations to investigate the optical properties, transition dipoles, excitons and radiative lifetimes of atomic defects in hBN. They found that different quantum emitters exhibit radiative lifetimes spanning six orders of magnitude and emission energies from infrared to ultraviolet. Through the Bayesian statistical analysis, they identified various high-likelihood charge-neutral defect emitters, among which the native VNNB defect possesses emission energy and radiative lifetime in agreement with experiments. In-depth calculations on the VNNB defect highlight the strong dependence of radiative properties on small perturbations to the atomic structure. This work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials. This article was recently published in npj Computational Materials 7: 85 (2021).

Fig. 3 Bayesian inference workflflow.

原文Abstract及其翻译

Radiative properties of quantum emitters in boron nitride from excited state calculations and Bayesian analysis (氮化硼中量子发射体辐射特性的激发态计算和贝叶斯分析)

Shiyuan Gao, Hsiao-Yi Chen and Marco Bernardi

Abstract Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structure and radiative properties remains incomplete. Here we study the excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN using ab initio density functional theory and GW plus Bethe-Salpeter equation calculations, generating a large data set of their emission energy, polarization and lifetime. We find a wide variability across quantum emitters, with exciton energies ranging from 0.3 to 4 eV and radiative lifetimes from ns to ms for different defect structures. Through a Bayesian statistical analysis, we identify various high-likelihood charge-neutral defect emitters, among which the native VNNB defect is predicted to possess emission energy and radiative lifetime in agreement with experiments. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials.

Fig. 4 Properties of the VNNB defect.

摘要六方氮化硼(hBN)中的点缺陷作为单光子发射体越来越受到人们的关注。然而，目前对它们的原子结构和辐射特性的理解仍然不完整。在这里，我们使用从头算密度泛函理论和GW+Bethe-Salpeter方程的计算方法，研究了hBN中20多种本征缺陷和碳或氧杂质缺陷的激发态和辐射寿命，给出了它们的一个包含发射能量、极化和寿命的大数据集。我们发现，量子发射器的可变性很大，对于不同的缺陷结构，激子能量在0.3至4 eV范围内，辐射寿命在ns 至ms范围内。通过贝叶斯统计分析，我们确定了各种高度可能的电荷中性缺陷发射体，其中对本征VNNB缺陷预测的发射能量和辐射寿命与实验数据一致。我们的工作推进了对hBN单光子发射器的微观理解，并引入了一个计算框架来表征和识别二维材料中的量子发射体。

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