In Situ Atomistic Insight into Magnetic Metal Diffusion across Bi_0.5Sb_1.5Te₃ Quintuple Layers

Abstract

Diffusion and occupancy of magnetic atoms in van der Waals (VDW) layered materials have significant impact on applications such as energy storage, thermoelectrics, catalysis, and topological phenomena. However, due to the weak VDW bonding, most research focus on in‐plane diffusion within the VDW gap, while out‐of‐plane diffusion has rarely been reported. Here, to investigate out‐of‐plane diffusion in VDW‐layered Bi2Te3‐based alloys, a Ni/Bi0.5Sb1.5Te3 heterointerface is synthesized by depositing magnetic Ni metal on a mechanically exfoliated Bi0.5Sb1.5Te3 (0001) substrate. Diffusion of Ni atoms across the Bi0.5Sb1.5Te3 quintuple layers is directly observed at elevated temperatures using spherical‐aberration‐corrected scanning transmission electron microscopy (STEM). Density functional theory calculations demonstrate that the diffusion energy barrier of Ni atoms is only 0.31–0.45 eV when they diffuse through Te3(Bi, Sb)3 octahedron chains. Atomic‐resolution in situ STEM reveals that the distortion of the Te3(Bi, Sb)3 octahedron, induced by the Ni occupancy, drives the formation of coherent NiM (M = Bi, Sb, Te) at the heterointerfaces. This work can lead to new strategies to design novel thermoelectric and topological materials by introducing magnetic dopants to VDW‐layered materials.

Theoretical and Experimental Investigation on 3d Transition Metal Anisotropic Diffusion in van Der Waals Layered Sb2Te3

“…For each peak, the TM atom location and surrounding lattice are illustrated. The highest energy barrier during transquituple-layer diffusion occurs when the TM atom squeezes across the Sb 4 parallelogram, which agrees with previous work on Ni transquituple layer diffusion . All 3d TM elements exhibit a lower diffusion barrier along the vdW gap than along the quintuple layer (Figure e), which agrees with our impression that the vdW gap has more space for atoms to diffuse.…”

“…The highest energy barrier during transquituple-layer diffusion occurs when the TM atom squeezes across the Sb 4 parallelogram, which agrees with previous work on Ni transquituple layer diffusion. 44 All 3d TM elements exhibit a lower diffusion barrier along the vdW gap than along the quintuple layer (Figure 4e), which agrees with our impression that the vdW gap has more space for atoms to diffuse. However, for Fe, Co, and Ni, the diffusion barrier crossing the quintuple layers is less than 0.8 eV, and the energy difference between two diffraction modes is less than 0.3 eV.…”

“…Te 6 octahedrons at the vdW gap provide the largest interstitial space in the lattice (Figure a). Previous work suggests that Mn and K atoms might prefer to sit at Te 6 octahedrons. , The slightly smaller distorted Te 3 -Sb 3 octahedrons are located between the Sb sublayer and Te sublayer (Figure a) and have been identified to contribute to the transquintuple layer diffusion of Ni atoms . The Sb 3 -Te­(II) 3 octahedron is slightly larger than the Sb 3 -Te­(I) 3 octahedron.…”

“…20,43 The slightly smaller distorted Te 3 -Sb 3 octahedrons are located between the Sb sublayer and Te sublayer (Figure 1a) and have been identified to contribute to the transquintuple layer diffusion of Ni atoms. 44 The Sb 3 -Te(II) 3 octahedron is slightly larger than the Sb 3 -Te(I) 3 octahedron. The smallest are Te 4 tetrahedrons that fill the empty space between Te 6 octahedrons in the vdW gap (Figure 1a).…”

“…Here, the BST (0001) surface was obtained by exfoliating bulk BST, and the metal thin film was deposited using evaporation. More details can be found in ref 44. The thin films of evaporated 3d TM atoms Fe, Co, and Ni were annealed in a tube furnace for 2 h at 250 °C.…”