The influence of shell parameters on phonons in core–shell nanoparticles: a resonant Raman study

Dzhagan, Volodymyr; Valakh, M. Ya.; Raevska, O E; Stroyuk, A. L.; Kuchmiy, S Ya; Zahn, Dietrich R. T.

doi:10.1088/0957-4484/20/36/365704

Cited by 54 publications

(86 citation statements)

References 25 publications

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“…This should also be valid for nanorods. The shift of the LO frequency due to a decreased phonon confinement because of its (partial) propagation into the shell is not likely because the LO phonon dispersion curves of bulk CdSe and ZnS are not overlapping, and the ZnS shell is thus not expected to support the CdSe-like LO vibrations [96]. The shift of the LO frequency can explained by compressive strain.…”

Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

“…It could be attributed to the ZnS transverse optical phonon [28]. Otherwise, the ZnS shell deposition could be accompanied by core-shell material interdiffusion and the formation of an intermixed interface layer, manifesting itself in the Raman spectra as a peak at 280 − 290 cm −1 [96]. Dzhagan et al investigated the influence of different shell parameters on phonons in spherical CdSe core-shell nanoparticles.…”

Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

“…There, the peak at 280 cm −1 was observed for both CdS and ZnS shells. The authors of reference [96] attribute it thus to the Cd-S vibration in an interface layer.…”

Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

“…An estimation of the relative contributions of the two effects to the reduced coupling is not possible on the available experimental data. Dzhagan et al, however, included a discussion of the exciton-LO-phonon coupling in their investigation of the influence of different shell parameters on the phonons in spherical CdSe core-shell particles [96]. By comparing CdS and ZnS capped particles.…”

Section: Shell-influence On the Exciton-phonon Couplingmentioning

confidence: 99%

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Optical phonons in colloidal CdSe nanorods

Lange

Mohr

Artemyev

et al. 2010

Physica Status Solidi (b)

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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...

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Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

“…There, the peak at 280 cm −1 was observed for both CdS and ZnS shells. The authors of reference [96] attribute it thus to the Cd-S vibration in an interface layer.…”

Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

Section: Shell-influence On the Exciton-phonon Couplingmentioning

confidence: 99%

See 2 more Smart Citations

Optical phonons in colloidal CdSe nanorods

Lange

Mohr

Artemyev

et al. 2010

Physica Status Solidi (b)

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“…Furthermore, the analysis of the literature shows that the relative intensity of the SO mode can be very close for different NC morphologies and dimensions (dots, rods, tetrapods, and wires) with various S/V . The nonsystematic sensitivity of the SO peak parameters to capping of the CdSe core with a ZnS (or CdS) shell [40] introduces further ambiguity in understanding of the nature of this mode. Based on similarity of the SO mode parameters for different NC morphologies, as well as existence of similar low-frequency asymmetry for bulk alloys [37]; one may assume it to be induced by the presence of atoms with coordination differing from the rest of (bulk) atoms.…”

mentioning

confidence: 99%

Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

Dzhagan

Valakh

Mel’nik

et al. 2012

International Journal of Spectroscopy

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Resonant Raman spectroscopy has been employed to explore the first-and higher-order phonon spectra of several kinds of II-VI nanocrystals (NCs), with the aim of better understanding of the nature of phonon modes and forming a unified view onto the vibrational spectrum of semiconductor NCs. Particularly, besides the previously discussed TO, SO, LO, and 2LO modes, the combinational modes of TO+LO and SO+LO can be assumed to account for the lineshape of the spectrum below 2LO band. No trace of 2TO or 2SO band was detected, what can be the result of the dominance of Fröhlich mechanism in electron-phonon coupling in II-VI compounds. The resonant phonon Raman spectrum of NCs smaller than 2 nm is shown to be dominated by a broad feature similar to the SO mode of larger NCs or phonon density of states of a bulk crystal.The understanding of the phonon spectrum of semiconductor nanocrystals (NCs), electron-phonon coupling (EPC), dependence of the phonon spectra on the NC size, surfacebound moieties, and extended environment is of a large fundamental and application significance [1][2][3][4][5][6][7][8][9][10][11] due to the determinant role phonons play in the optical and electrical properties of semiconductor nanostructures [12,13]. Though a number of in-depth studies have addressed the phonon spectra of colloidal [14][15][16][17][18][19][20], glass-embedded [21][22][23] NCs, and epitaxial nanostructures [24][25][26], a significant divergence of the results exists concerning both the nature of the modes and their response to such factors as size, surface conditions, properties of the hosting medium and so forth. In particular, the apparently similar Raman spectra of a wide range of compounds (e.g., II-VI, III-V, and IV) and NC morphologies (dots, rods, tetrapods, wires, and discs) have been assigned differently.Recently, attempts were made to build a general picture of the phonon spectrum of various semiconductor NCs, based on analysis of the experimental Raman data and appropriate models [14,27,28]. This paper further explores the first-and higher-order phonon spectra of several kinds of II-VI NCs, as well as their similarity to other compounds, with the aim of better understanding of the vibrational spectrum of small NCs. High-quality NC samples were selected for this study in order to resolve Raman features, which are usually smeared by size dispersion, structural disorder, or low signal-to-noise ratio.The NC samples studied in this work have been prepared by means of colloidal chemistry according to protocols reported elsewhere [29][30][31][32], and CdSe, CdS, and CdTe NC samples of the same mean diameter (∼4 nm) were selected. Another sort of NCs studied were ultrasmall NCs of about 1.8 nm mean diameter, which revealed distinct Raman spectra. The attempts to measure the size and shape of these NCs directly by electron microscopy were unsuccessful [19], obviously due to ultrasmall NC size and charging of the stabilizing polymer. The broad X-ray features of these ultrasmall NCs [3] indicated their small size or/and n...

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