[Paper Review] Atomic and electronic structure of defects in hBN: enhancing single-defect functionalities

Zhizhan Qiu, Kristina Vaklinova+8 more|arXiv (Cornell University)|Oct 15, 2021

Diamond and Carbon-based Materials Research4 citations

TL;DR

This study combines scanning tunneling microscopy (STM) and spectroscopy (STS) with theoretical modeling to directly probe the atomic and electronic structure of carbon-doped hexagonal boron nitride (hBN:C) defects. It identifies two defect types—single-site donor-like states and multi-site complexes with ladder-like mid-gap states—enabling precise engineering of quantum functionalities at the single-defect level.

ABSTRACT

Defect centers in insulators play a critical role in creating important functionalities in materials: prototype qubits, single-photon sources, magnetic field probes, and pressure sensors. These functionalities are highly dependent on their mid-gap electronic structure and orbital/spin wave-function contributions. However, in most cases, these fundamental properties remain unknown or speculative due to the defects being deeply embedded beneath the surface of highly resistive host crystals, thus impeding access through surface probes. Here, we directly inspected the atomic and electronic structures of defects in thin carbon-doped hexagonal boron nitride (hBN:C) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Such investigation adds direct information about the electronic mid-gap states to the well-established photoluminescence response (including single photon emission) of intentionally created carbon defects in the most commonly investigated van der Waals insulator. Our joint atomic-scale experimental and theoretical investigations reveal two main categories of defects: 1) single-site defects manifesting as donor-like states with atomically resolved structures observable via STM, and 2) multi-site defect complexes exhibiting a ladder of empty and occupied mid-gap states characterized by distinct spatial geometries. Combining direct probing of mid-gap states through tunneling spectroscopy with the inspection of the optical response of insulators hosting specific defect structures holds promise for creating and enhancing functionalities realized with individual defects in the quantum limit. These findings underscore not only the versatility of hBN:C as a platform for quantum defect engineering but also its potential to drive advancements in atomic-scale optoelectronics.

Motivation & Objective

• To directly probe the atomic and electronic structure of defects in hexagonal boron nitride (hBN) that host single-photon emission and quantum functionalities.
• To overcome the challenge of inaccessible mid-gap electronic states in insulators by using surface-based STM/STS on thin, carbon-doped hBN.
• To correlate experimentally observed mid-gap states with optical responses such as single-photon emission in individual defects.
• To classify and characterize distinct defect types based on their spatial geometry and electronic configuration.
• To enable targeted engineering of single-defect functionalities in 2D van der Waals insulators for quantum technologies.

Proposed method

• Employing low-temperature scanning tunneling microscopy (STM) to achieve atomic-scale resolution of defect structures in thin hBN:C.
• Using scanning tunneling spectroscopy (STS) to directly map the electronic mid-gap states of individual defects.
• Combining experimental STM/STS data with first-principles theoretical calculations to assign electronic states to specific atomic configurations.
• Analyzing spatial patterns of mid-gap states to distinguish between single-site defects and multi-site defect complexes.
• Correlating tunneling spectroscopy data with photoluminescence responses to link electronic structure to optical functionality.
• Using a combination of experimental and theoretical approaches to identify defect types based on their spatial and electronic characteristics.

Experimental results

Research questions

• RQ1 What are the atomic-scale structures and electronic configurations of individual defects in carbon-doped hBN?
• RQ2 How do mid-gap electronic states in hBN defects relate to their optical emission properties such as single-photon emission?
• RQ3 What distinguishes single-site defects from multi-site defect complexes in terms of their spatial geometry and electronic structure?
• RQ4 Can direct tunneling spectroscopy reveal the nature of mid-gap states that are otherwise inaccessible via surface probes?
• RQ5 How can the electronic and orbital wave-function contributions of defects be engineered to enhance quantum functionalities?

Key findings

• Two main defect categories were identified: single-site defects with donor-like mid-gap states and atomically resolved structures visible via STM.
• Multi-site defect complexes exhibit a distinct ladder of both occupied and unoccupied mid-gap states with unique spatial geometries.
• The spatial distribution of mid-gap states in multi-site complexes was directly imaged using STS, revealing complex orbital arrangements.
• A strong correlation was established between the observed mid-gap states and the photoluminescence response of individual defects.
• The study demonstrates that direct tunneling spectroscopy can resolve electronic states that are otherwise inaccessible in highly resistive insulators.
• The findings provide a foundation for engineering defect functionalities at the single-atom level in 2D van der Waals materials.