Direct observation of motion of interstitial atoms at van der Waals bonded interfaces

Diffusion processes in two-dimensional materials govern phenomena such as phase transformations, growth by chemical vapor depositions and doping by substitutional and interstitial atoms. Furthermore, intercalation of foreign atomic species into van der Waals (vdW) bonded materials has been studied for tuning of physical properties (electronic, optoelectronic, magnetic), and for uses in energy and battery applications. Here, we directly image the diffusion of W atoms using high-angle annular scanning transmission electron microscopy inside hexagonal boron nitride (BN)/WSe₂/BN vdW heterostructures and BN/WSe₂ heterostructures. Using thin (~2 nm thick) BN crystals consisting of light B and N atoms allows us to image the heavier W atoms in these heterostructures and allows us to study the electron beam-induced motion of W atoms at three interfaces: BN/vacuum, BN/BN, and WSe₂/BN. In combination with density functional theory calculations, we find that the motion is governed by interfacial defect formation. This leads to similar diffusion coefficients at the BN/vacuum and BN/BN interface. Furthermore, we find that the diffusion coefficient is twice as large at the WSe₂/BN interface compared to the BN/BN and BN/vacuum interfaces, and we attribute this to differences in defect formation rates or different interlayer spacings. This work highlights the importance of interfacial defects for diffusion properties of atoms at van der Waals interfaces.