Interstitial defects in the van der Waals gap of Bi2Se3. Issue 4 (9th August 2019)

• Record Type: Journal Article
• Authors: Callaert, Carolien; Bercx, Marnik; Lamoen, Dirk; Hadermann, Joke
• Is Part Of: Acta crystallographica. Volume 75:Issue 4(2019:Aug.)
• Page Start: 717
• Page End: 732
• Publication Date: 2019-08-09
• Subjects: Bi2Se3 -- defect -- transmission electron microscopy (TEM) -- density‐functional theory (DFT) -- electron diffraction tomography (EDT)
• DOI: 10.1107/S2052520619008357
• Languages: English
• ISSNs: 2052-5206

Abstract

Abstract : The use of scanning transmission electron microscopy (STEM) and energy‐dispersive X‐ray spectroscopy uncovered interstitial defects with mainly Bi character in Bi2 Se3 . This finding is supported by precession electron diffraction tomography and density functional theory calculations. The electron beam used during the STEM experiment induced atomic displacements across and along the van der Waals gap, though climbing‐image nudged elastic band calculations show that some paths could also occur spontaneously at room temperature. Abstract : Bi2 Se3 is a thermoelectric material and a topological insulator. It is slightly conducting in its bulk due to the presence of defects and by controlling the defects different physical properties can be fine tuned. However, studies of the defects in this material are often contradicting or inconclusive. Here, the defect structure of Bi2 Se3 is studied with a combination of techniques: high‐resolution scanning transmission electron microscopy (HR‐STEM), high‐resolution energy‐dispersive X‐ray (HR‐EDX) spectroscopy, precession electron diffraction tomography (PEDT), X‐ray diffraction (XRD) and first‐principles calculations using density functional theory (DFT). Based on these results, not only the observed defects are discussed, but also the discrepancies in results or possibilities across the techniques. STEM and EDX revealed interstitial defects with mainly Bi character in an octahedral coordination in the van der Waals gap, independent of the applied sample preparation method (focused ion beam milling or cryo‐crushing). The inherent character of these defects is supported by their observation in the structure refinement of the EDT data. Moreover, the occupancy probability of the defects determined by EDT is inversely proportional to their corresponding DFT calculated formation energies. STEM also showed the migration of some atoms across and along the van der Waals gap. The kinetic barriers calculated using DFT suggest that some paths are possible at room temperature, while others are most probably beam induced.