Spontaneous strain in quasi-two-dimensional Janus CdSe nanoplatelets and its microscopic mechanisms

TL;DR

This study uses first-principles calculations to uncover microscopic mechanisms behind spontaneous strain and folding in Janus CdSe nanoplatelets, highlighting the dominant role of surface strain in their structural transformations.

Contribution

It identifies three microscopic mechanisms of spontaneous strain in Janus CdSe nanoplatelets, emphasizing the surface strain mechanism as the primary cause of folding and complex structures.

Findings

Surface strain by bridging bonds causes spontaneous folding.

Bulk inverse piezoelectric effects contribute partially to strain.

Surface stresses influence the formation of scrolls, spirals, and twisted ribbons.

Abstract

Spontaneous strain and spontaneous folding of thin nanoplatelets are known phenomena whose microscopic mechanisms are still debating. In this work, first-principles calculations are used to study the mechanical stresses that arise in Janus CdSe nanoplatelets and result in their spontaneous strain. Calculations reveal the existence of three microscopic mechanisms of this phenomenon. Two bulk mechanisms are associated with the inverse piezoelectric effect in an electric field created by the difference in electronegativities of ligands and by the depolarizing field resulting from the difference in the potential jumps in electrical double layers on the surfaces of nanoplatelets. These mechanisms account for 5-25% of the observed effect. The third mechanism is associated with the surface strain of nanoplatelets by bridging bonds, and its influence is predominant. It is shown that the latter mechanism cause spontaneous folding of thin CdSe nanoplatelets and, depending on the values of surface stresses and lateral orientation of nanoplatelets, can result in formation of their experimentally observed structures such as scrolls, spirals, and twisted ribbons.