Optical phonons in colloidal CdSe nanorods

Abstract

Colloidal CdSe nanorods are nanoparticles synthesized in solution. The synthesis allows the growth of CdSe nanorods with well defined diameters and aspect ratios. They have many potential applications in the field of optoelectronics and biotechnology. The nanorods can be epitaxially covered with a graded CdS/ZnS shell of a few monolayers in thickness. The shell leads to an increased quantum efficiency and improved photostability of the nanorods. However, the lattice mismatch between the nanorod core material and the shell material introduces strain into the core lattice. In this work Raman spectroscopy, accompanied by ab-initio calculations, is used to determine the amount of this strain, the exciton-phonon coupling strength in the nanorods and to investigate confinement effects. The longitudinal optical phonons in a nanorod are confined to the nanorod volume. The confinement of a phonon wavefunction leads to a neutralization of the q = 0 rule and the phonon frequency is found to depend on the nanorod diameter. The coupling strength between longitudinal optical phonons and excitons is investigated. It also depends on the nanorod 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 nanorods is verified experimentally. The diameter-dependence of the modes' frequency can be used to estimate the nanorod diameter from a Raman measurement. In core-shell structures, the coverage of a CdSe nanorod with a ZnS shell leads to a compressive strain of the CdSe core due to the smaller lattice parameter of ZnS compared to CdSe. Ab-initio calculations show that all bonds of the CdSe core are shortened. The bonds in the lateral direction are much more strongly compressed than the bonds parallel to the c-axis. The amount of strain is estimated from the Raman spectra. The compressive nature of the shell decreases for thicker nanorods. The exciton wave function changes with the modified boundary from air to ZnS. This is reflected in a altered exciton-phonon coupling strength and can be monitored in the Raman spectra. ZusammenfassungCdSe Nanorods sind Nanopartikel, die in einer kolloidalen Lösung synthetisiert werden. Für diese existieren viele mögliche Anwendungen im Bereich der Optoelektronik und der Biotechnologie. Der Durchmesser und die Länge der Nanorods lässt sich durch die Syntheseparameter festlegen. Die Nanorods können in eine epitaktische Hülle aus einem Halbleitermaterial mit einer größeren Bandlücke, ZnS, eingebettet werden. Diese Hülle verbessert die optischen Eigenschaften und ermöglicht eine weitere Funktionalisierung der Oberfläche. Der Unterschied der Gitterparameter zwischen ZnS und CdSe führt jedoch zu Verspannung...

Morphology-induced phonon spectra of CdSe/CdS nanoplatelets: core/shell vs. core–crown

“…The latter mode is expected in the frequency range between bulk CdS TO (240 cm −1 ) and LO (305 cm −1 ) 55 and was repeatedly observed for CdS nanostructures. 15,21,53,57 This feature is also observed for CdSe/CdS core/shell NPLs discussed later in this work. Increasing the nominal volume of CdS crown causes only proportional increase of the CdS phonon peak intensity, which corroborated by the stronger CdS absorption peak at 2.85 eV in UV-vis spectra ( Fig.…”

“…17,26,53,54 At T = 35 K ν CdSe TO = 170 cm −1 and ν CdSe LO = 213 cm −1 and about 3 cm −1 lower frequencies are at room temperature. 55 Even though only one SO component was usually reported in the previous studies of spherical NCs and nanorods, 53 a good fit of the NPL spectra requires at least two components in the relevant spectral region. The combination of quasi-zero size distribution of the NPL samples and the low temperature of Raman experiments could be the reason which allowed us to resolve the fine structure of the SO phonon spectra in this work, which was not possible in studies of other NC morphologies which all possess a certain size distribution.…”

“…17,18,53 The SO modes are known to occur between the frequencies of TO (transverse optical) and LO phonons of the corresponding bulk material. 17,26,53,54 At T = 35 K ν CdSe TO = 170 cm −1 and ν CdSe LO = 213 cm −1 and about 3 cm −1 lower frequencies are at room temperature. 55 Even though only one SO component was usually reported in the previous studies of spherical NCs and nanorods, 53 a good fit of the NPL spectra requires at least two components in the relevant spectral region.…”

Light Induced Nanowire Assembly: The Electrostatic Alignment of Semiconductor Nanowires into Functional Macroscopic Yarns

“…Figure a shows Raman spectra from one such thread taken at various positions along its length [denoted by the numbered positions in Figure 3a (right)]. The spectra show both CdSe and CdTe modes, namely CdSe's (CdTe's) fundamental longitudinal optical (LO) phonon at 203 cm −1 (163 cm −1 ) and its first overtone at 410 cm −1 (330 cm −1 ) 21, 22. This demonstrates the homogeneity of the resulting composite.…”

One‐Dimensional Semiconductor Nanowires: Synthesis and Raman Scattering

“…Recently, this mode was revisited again in Si nanowires by the ab initio method. [132] Experimentally, the RBM was also reported in CdSe nanorods [131,134,135] but the RBM modes in SiNWs have not yet been observed by Raman scattering.…”