Optical phonons in colloidal CdSe nanorods

摘要

In this work the vibrational properties of colloidal CdSe nanorods (NRs) and CdSe-ZnS core–shell NRs are investigated via Raman spectroscopy. The longitudinal optical (LO) phonons in NRs are confined to the NR volume. The confinement of the phonon wave function leads to a relaxation of the q?0 rule and the phonon frequency is found to depend on the NR diameter. The coupling strength between LO phonons and excitons also depends on the NR diameter. The total coupling strength is much lower than in bulk material due to a decrease of the influence of the Coulomb interaction in nanoparticles. However, the coupling strength is found to rise for decreasing diameters. This is due to the increasing contributions of higher frequency phonons for smaller nanoparticle sizes. A radial breathing mode with a diameter dependent frequency is deduced from ab initio calculations and its existence in NRs is verified experimentally. The diameter-dependence of the modes’ frequency can be used to estimate theNRdiameter from a Raman measurement. In core–shell structures, the lattice mismatch between core and shell leads to a compressive strain of the core lattice. The exciton wave function changes with the modified boundary. This is reflected in an altered exciton– phonon coupling strength.