# Negative thermal expansion in CdSe quasi-two-dimensional nanoplatelets

## TL;DR

This study calculates the negative in-plane thermal expansion coefficient of CdSe nanoplatelets, revealing enhanced effects compared to bulk material, mainly due to specific phonon modes, using first-principles quasiharmonic approximation.

## Contribution

It provides the first-principles calculation of negative thermal expansion in CdSe nanoplatelets and compares it with bulk structures, highlighting the phonon modes responsible.

## Key findings

- Negative CTE is significantly larger in nanoplatelets.
- Negative thermal expansion occurs over a broader temperature range.
- Out-of-plane flexural and in-plane optical phonon modes dominate the effect.

## Abstract

The in-plane coefficient of thermal expansion (CTE) for CdSe nanoplatelets with the zinc-blende structure containing from two to five monolayers is calculated from first principles within the quasiharmonic approximation. A comparison of the obtained results with those for bulk CdSe with both the zinc-blende and wurtzite structures finds a significant increase in the magnitude of negative CTE and the temperature range of its observation in nanoplatelets. The main contribution to the negative thermal expansion in CdSe nanoplatelets is given by the out-of-plane flexural ZA mode and in-plane optical $E$ modes that arise from the folding of TA phonon of bulk CdSe.