Single photon emitters in hBN: Limitations of atomic resolution imaging and potential sources of error

TL;DR

This paper investigates the limitations of atomic resolution imaging in identifying single-photon emitters in hBN, highlighting how sample thickness and imaging aberrations can lead to misinterpretation of atomic defects.

Contribution

The study demonstrates that at typical thicknesses used in photonic studies, atomic contrast differences become indistinguishable, and aberrations can cause misleading defect identification in STEM images.

Findings

Contrast differences vanish beyond 17 atomic layers in hBN.

Residual aberrations can artificially alter STEM contrast.

Detection of vacancy-type defects becomes unreliable at typical sample thicknesses.

Abstract

There is a growing interest in identifying the origin of single-photon emission in hexagonal boron nitride (hBN), with proposed candidates including boron and nitrogen vacancies as well as carbon substitutional dopants. Because photon emission intensity often increases with sample thickness, hBN flakes used in these studies commonly exceed 30 atomic layers. To identify potential emitters at the atomic scale, annular dark-field scanning transmission electron microscopy (ADF-STEM) is frequently employed. However, due to the intrinsic AA' stacking of hBN with vertically alternating boron and nitrogen atoms, this approach is complicated even in few-layer systems. Here, we demonstrate using STEM image simulations and experiments that, even under idealized conditions, the intensity differences between boron- and nitrogen-dominated columns and carbon substitutions become indistinguishable at thicknesses beyond 17 atomic layers (ca. 6 nm). While vacancy-type defects can remain detectable at somewhat larger thicknesses, also their detection becomes unreliable at thicknesses typically used in photonic studies. We further show that common residual aberrations, particularly threefold astigmatism, can lead to artificial contrast differences between columns, which may result in misidentification of atomic defects. We systematically study the effects of non-radially symmetric aberrations on multilayer hBN and demonstrate that even small residual threefold astigmatism can significantly distort the STEM contrast, leading to misleading interpretations.