Interstitial defects in the van der Waals gap of Bi₂Se₃

Abstract

Bi2Se3 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 Bi2Se3 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.

Identifying Native Point Defects in the Topological Insulator Bi2Te3

“…Bi interstitial defects in the van der Waals gaps of Bi 2 Se 3 crystals have also been identified by HR-STEM measurements. 28 A reasonable assumption is to assign the adatoms as Bi adatoms that are formed according to the first mechanism mentioned above.…”

“…Bi interstitial defects in the van der Waals gaps of Bi 2 Se 3 crystals have also been identified by HR-STEM measurements. 28 A reasonable assumption is to assign the adatoms as Bi adatoms that are formed according to the first mechanism mentioned above. However, experimental and simulated STM images of the adatoms cannot unequivocally distinguish between Bi and Te adatoms, implying the possibility that the adatoms are Te adatoms cannot be excluded.…”

“…Up to date this information remains restricted to defects located below the top surface QLs. 27,28 On the other hand, scanning tunneling microscopy (STM) and spectroscopy (STS) allow direct visualization of the atomicscale surface topography and investigation of the two-dimensional (2D) TSSs that appear within the top surface QLs. 23,29,30 For conclusively identifying the origin of the native defects in Bi 2 Te 3 , a direct comparison between high-resolution STM images and simulated STM images is required.…”

“…Advances in high-resolution scanning transmission electron microscopy (HR-STEM) have made it possible for this technique to provide structural and chemical information down to the atomic scale about defects in tetradymite crystals and thin films. To date, this information remains restricted to defects located below the top surface QLs. , On the other hand, scanning tunneling microscopy (STM) and spectroscopy (STS) allow direct visualization of the atomic-scale surface topography and investigation of the two-dimensional (2D) TSSs that appear within the top surface QLs. ,, For conclusively identifying the origin of the native defects in Bi 2 Te 3 , a direct comparison between high-resolution STM images and simulated STM images is required.…”

The Vertical Position of Sr Dopants in the Sr$_x$Bi$_2$Se$_3$ Superconductor

“…In the case of TEM, it was suggested [9] that lateral movement of the dopants (due to the low diffusion barrier [8]) inside the vdW gap at room temperature precludes an observation. However, a recent TEM study of native defects in Bi 2 Se 3 demonstrated that Bi atoms inside the vdW gap can be observed [15]. STM data of Cu-doped Bi 2 Se 3 thin-films grown by molecular beam epitaxy showed Cu atoms inside the quintuple layer, but no direct evidence of Cu inside the vdW gap [8].…”