# Atomic Imaging of Phase Transformation and Self-Intercalation of Two-Dimensional CrS2 by In Situ TEM

## TL;DR

Researchers used atomic imaging to observe how 2D CrS2 transforms into a 3D structure when heated, revealing a new way to control magnetic and electronic properties.

## Contribution

The study provides the first atomic-scale visualization of thermal-driven self-intercalation and phase transformation in 2D CrS2.

## Key findings

- Heating 1T-CrS2 nanosheets to 500°C triggers irreversible transformation into Cr2S3 with a change in magnetic order.
- Thermal depletion of sulfur atoms initiates self-intercalation, forming covalent bonds in van der Waals gaps.
- The transformation represents a dimensional crossover from 2D to 3D bonding at the nanoscale.

## Abstract

Controlling phase transitions in two-dimensional (2D)
materials
offers a powerful route for engineering novel electronic and magnetic
functionalities. However, atomically resolved visualization of these
dynamic processes remains a significant challenge. Herein, we report
the synthesis of ultrathin (1.6 nm), single-crystal 1T-CrS2 nanosheets via atmospheric-pressure chemical vapor deposition (APCVD)
and uncover their thermal transformation pathway using in
situ heating transmission electron microscopy (TEM). Real-time
atomic-scale imaging reveals that upon heating to 500 °C, the
material undergoes an irreversible structural and magnetic transformation
from a layered, antiferromagnetic 1T-CrS2 structure into
a nonlayered, ferrimagnetic Cr2S3 structure.
The driving mechanism is identified as a unique self-intercalation
process initiated by the thermal depletion of S atoms, which promotes
the migration of lattice Cr atoms into van der Waals (vdW) gaps to
form new interlayer covalent bonds. This transformation represents
a fundamental dimensional crossover from a 2D vdW crystal to a three-dimensionally
bonded material at the nanoscale. Our findings elucidate a critical
thermal transformation pathway in Cr-based 2D magnets and demonstrate
a mechanism for irreversibly switching both the crystal structure
and the magnetic order, thereby providing crucial insights for the
design of thermally stable phase-change memory devices. a reduction
in the slope of the curve was observed compared with the lower-voltage
regime