Lattice dynamics of quasi-two-dimensional CdSe nanoplatelets and their Raman and infrared spectra

Abstract

Phonon spectra of CdSe nanoplatelets (2-6 ML) with the zinc-blende structure were calculated from first principles within the density-functional theory. It turned out that the Lamb modes in nanoplatelets are in fact optical rather than acoustic vibrations. Phonon spectra of the nanoplatelets show the appearance of a large number of low-frequency modes inherited from TA phonons in bulk CdSe. Calculations of the Raman spectra indicate a need to revise the interpretation of available experimental data. The largest contribution to the Raman spectra is provided by the quasi-Lamb modes with the A1 symmetry. The B2 modes whose frequencies depend on the environment of nanoplatelets and whose properties are closest to the properties of LO phonons explain the results obtained in the "nanoparticle-on-mirror" geometry. The features in Raman spectra previously attributed to surface optical (SO) modes should be interpreted as a manifestation of lower-order quasi-Lamb A1 modes. Calculations of the infrared spectra find, in addition to the TO phonon line, the appearance of intense lines from surface modes originating from terminating F(Cl) atoms on the surface of nanoplatelets and true SO-modes.

Ligand-dependent nano-mechanical properties of CdSe nanoplatelets: calibrating nanobalances for ligand affinity monitoring

“…When considering this additional monolayer, a good agreement between the approach in ref. 34 and the Lamb model is observed. These results thus confirm that the sulfur atom constituting the thiol group is involved in a molecular bond with the cadmium atoms present at the surface of the NPLs.…”

“…Indeed, the origin of this vibration mode at low frequency has also been discussed by A. I. Lebedev in ref. 34. In this publication, the author uses first-principles calculations and modeling of the Raman spectra to determine the origin of this vibrational peak.…”

Negative thermal expansion in CdSe quasi-two-dimensional nanoplatelets

“…As was shown in Ref. 37, the low-frequency optical E modes arise from the folding of TA phonon mode which is responsible for a negative thermal expansion in bulk CdSe. From Fig.…”

“…3. 37 The twodimensional character of the phonon spectrum is confirmed by the absence of a dispersion of the phonon modes in the q z direction normal to the plane of the nanoplatelet.…”

“…A detailed analysis of phonon spectra of CdSe nanoplatelets was carried out in Ref. 37. It was shown that the phonon spectrum of a CdSe nanoplatelet with a thickness of n ML consists of three types of optical modes: n symmetric quasi-Lamb A 1 modes, n antisymmetric quasi-Lamb B 2 modes (in both modes the atomic displacements are out-of-plane), and 2n modes of symmetry E with the in-plane atomic displacements.…”