First-Principles Calculations of Nanoplatelet Heterostructures: Surface Ligands and Strain Fields

Nanoplatelets (NPLs) – highly anisotropic, quasi-two-dimensional semiconductor nanostructures – exhibit optoelectronic properties that are governed by their precisely tunable thickness of a few atomic layers. However, the atomistic structure of NPLs is not completely known and is expected to depend on the passivating ligands. Here we determine the structure of both CdSe-core and CdSe/CdS-core/shell NPLs, using Density Functional Theory (DFT) and the Qbox code[1]. For both core and core/shell NPLs with different thicknesses and ligands, we determined the equilibrium positions and the in-plane strain. We found that NPL thickness, different ligands and the presence of a shell are all factors affecting the axial atomistic structure and they are responsible for inducing an in-plane strain field, which in many studies has been neglected. We show that, compared to CdSe NPL of the same thickness, the lattice constants of the core-shell NPLs expand in the axial direction, in agreement with recent experiments[2]. We rationalize our first principles findings by a continuum elastic model including surface-stress terms that account for the surface passivation of NPLs.

[1] Gygi F, IBM J. Res. Dev. 52.1.2 (2008): 137-144.
[2] Hazarika et al. Submitted.