# Aggregation of BiTe Monolayer on Bi$_2$Te$_3$(111) Induced by Diffusion of Intercalated Atoms in van der Waals Gap

## TL;DR

This study reveals how intercalated atoms in Bi$_2$Te$_3$ layers diffuse and aggregate over time, forming a BiTe monolayer, with implications for understanding vdW layered material behaviors.

## Contribution

It demonstrates a diffusion-induced aggregation mechanism of intercalated atoms in Bi$_2$Te$_3$ using combined microscopy and theoretical calculations.

## Key findings

- Intercalated atoms diffuse through vdW gaps.
- Aggregated monolayer forms with squared lattice symmetry.
- Diffusion and aggregation may be common in vdW layered materials.

## Abstract

We report a post-growth aging mechanism of Bi$_2$Te$_3$(111) films with scanning tunneling microscopy in combination with density functional theory calculation. It is found that a monolayered structure with a squared lattice symmetry gradually aggregates from surface steps. Theoretical calculations indicate that the van der Waals (vdW) gap not only acts as a natural reservoir for self-intercalated Bi and Te atoms, but also provides them easy diffusion pathways. Once hopping out of the gap, these defective atoms prefer to develop into a two dimensional BiTe superstructure on the Bi$_2$Te$_3$(111) surface driven by positive energy gain. Considering the common nature of weakly bonding between vdW layers, we expect such unusual diffusion and aggregation of the intercalated atoms may be of general importance for most kinds of vdW layered materials.