Heavy strain conditions in colloidal core-shell quantum dots and their consequences on the vibrational properties fromab initiocalculations

Abstract

We preform large-scale 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 15.6Å). 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.

Measuring and Predicting the Internal Structure of Semiconductor Nanocrystals through Raman Spectroscopy

“…With this bonding symmetry, 2 distinguishable Raman peaks are observed as the longitudinal optical (LO) and transverse optical (TO) modes, shown for CdS and CdSe core QDs in Figure 2a with their first harmonic peaks (2 × LO). 29,39,40 LO peaks dominate the spectra and the TO peaks on the low frequency sides are almost undetectable, as they are buried in surface-related modes (see below). 27,41 The significant frequency (or wavenumber) difference between LO CdS and LO CdSe derives from differences in bond vibrational frequencies in each nanocrystal, as Cd–S bonds are shorter and stronger (i.e., larger spring constant) than Cd–Se bonds.…”

Band gap renormalization of diamondoids: vibrational coupling and excitonic effects

“…consider the fraction of bulk-type carbon atoms (the ones connected to four neighboring carbon atoms, labeled as C-C) does not exceed 23%. Hence, a bulk-type vibrational DOS with acoustic and optical branches can hardly be recognized in these structures, in contrast to larger quantum dots [33,37]. The ZPR (DE g ) shows contributions mainly from the frequency range between 800 and 1400 cm −1 , which correspond to bending C-H modes and rotation/shear H-C-H modes.…”

Comparison of three empirical force fields for phonon calculations in CdSe quantum dots

“…51 While these are still smaller than the structures on which most experiments are performed, they are large enough to be relevant to experimental systems. These calculations properly reproduce several features of the experimental resonance Raman spectra 32 including the shift to higher frequency of the CdSe core optical modes due to compression by the shell and the distribution of the CdS shell optical modes over a wide range of frequencies.…”