Heavy strain conditions in colloidal core-shell quantum dots and their consequences on the vibrational properties from \emph{Ab initio} calculations

TL;DR

This study uses large-scale ab initio calculations to analyze how heavy strain in colloidal core-shell quantum dots affects their vibrational properties, revealing core-induced strain and its impact on vibrational frequency shifts.

Contribution

It provides new insights into strain distribution and vibrational behavior in core-shell quantum dots, highlighting the counteracting effects of compression and surface undercoordination.

Findings

Core shells remain unstrained while cores adapt to lattice mismatch.

Both cores and shells can be under compressive strain up to 20 GPa.

Strain influences vibrational frequency shifts observed in Raman spectra.

Abstract

We preform large-scale \emph{ab initio} density functional theory calculations to study the lattice strain and the vibrational properties of colloidal semiconductor core-shell nanoclusters with up to one thousand atoms (radii up to 15.6~\AA). For all the group IV, III-V and II-VI semiconductors studied, we find that the atom positions of the shell atoms, seem unaffected by the core material. In particular, for group IV core-shell clusters the shell material remains unstrained, while the core adapts to the large lattice mismatch (compressive or tensile strain). For InAs-InP and CdSe-CdS, both the cores and the shells are compressively strained corresponding to pressures up to 20 GPa. We show that this compression, which contributes a large blue-shift of the vibrational frequencies, is counterbalanced, to some degree, by the undercoordination effect of the near-surface shell, which contributes a red-shift to the vibrational modes. These findings lead to a different interpretation of the frequency shifts of recent Raman experiments, while they confirm the speculated interface nature of the low-frequency shoulder of the high frequency Raman peak.