Three-Dimensional Imaging of Buried Interfaces in Twisted Hexagonal Boron Nitride
Year: 2024
Type: article
Abstract: The development of twisted van der Waals (vdW) heterostructures, where individual layers of 2D materials are successively stacked at relative rotation angles, has led to exciting applications in optoelectronics, sensing, energy storage, quantum computing and more [1]. Visualizing individual defect sites using atomic-resolution scanning transmission electron microscopy (STEM) could provide great insight into the structure-property relationships of quantum materials, particularly at vdW heterointerfaces. However, while STEM enables sub-Ångstrom resolution imaging in two dimensions, it remains a major challenge to determine the three-dimensional locations of point defects at the buried interfaces of twisted 2D heterostructures. Firstly, the light-element composition of many vdW heterostructures is limited by poor imaging sensitivity of existing techniques such as annular dark-field (ADF-) STEM. Furthermore, probing these vertical interfaces requires high resolution parallel to the electron beam, which is currently limited by poor axial depth resolution limits (5-10 nm) and strict alignment requirements of electron tomography. Here we demonstrate the application of multislice ptychography (MSP) [2], a highly sensitive coherent diffractive imaging technique [3], to a twisted hexagonal boron nitride (hBN) heterointerface from a single-view data set [4]. As illustrated in Fig. 1, an array of 2D diffraction patterns is acquired across a 2D raster scan, resulting in a four-dimensional data set. By employing MSP, the propagation from the top flake, through the interface, to the bottom flake is visualized from separate slices of the reconstruction, without performing any tilting during data acquisition. Using knife-edge calculations on the phase of the atomic columns, the depth resolution of the reconstruction is determined to be 2.74 nm, as shown in Figure 2 [5], which is a significant improvement over the aperture-limited depth resolution of 6.73 nm. This is attributed to the diffraction signal extending beyond the aperture edge with the depth resolution set by the curvature of the Ewald sphere [6]. Future advances to this approach could improve the depth resolution to the near-atomic level and enable the identification of individual dopants, defects and color centers in twisted heterointerfaces and other materials. Left: Experimental setup for electron ptychography. Right: Phase reconstructions of the top, interfacial and bottom 4-nm slices of the vdW heterointerface. Scale bar: 0.5 nm. Knife-edge depth resolution calculation for the MSP reconstruction. Right: subsections of 1-nm slice phase reconstructions corresponding to specific sections of the knife-edge curve.
Source: Microscopy and Microanalysis
Language: English
Subfield: Structural Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W4401005564