Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots

Баранов, А. В.; Rakovich, Yu. P.; Donegan, John F.; Perova, Tatiana S.; Moore, Robert A.; Talapin, Dmitri V.; Rogach, Andrey L.; Masumoto, Yasuaki; Nabiev, Igor

doi:10.1103/physrevb.68.165306

Cited by 249 publications

(263 citation statements)

References 21 publications

(53 reference statements)

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“…The Huang-Rhys factors varied from 0.02 to 0.72, roughly the range that has been reported for the LO phonon of CdSe nanocrystals through different experimental techniques. 5,17,20,[41][42][43][44][45][46] The Brownian oscillator parameter κ, which has the effect of varying the homogeneous lineshape from Lorentzian to Gaussian (exponential to…”

Section: Resultsmentioning

confidence: 99%

“…The matrix element for a 0 → phonon transition in a resonance Raman spectrum depends on the EPC strength for each phonon. It is often assumed that the ratio of the integrated area of the first overtone transition to that of the fundamental, 0→2 0→1 ⁄ , is equal to or directly proportional to S, [20][21][22][23][24][25][26] as it is in one-photon absorption or emission as described above. However, this is not generally the case for resonance Raman scattering, a coherent twophoton process in which paths through different intermediate states interfere.…”

Section: Introductionmentioning

confidence: 99%

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Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

Kelley

2013

J. Phys. Chem. A

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For linear electron-phonon coupling, the Huang-Rhys factor, S, gives the intensity ratio of the one-quantum vibronic transition (0→1) to the purely electronic origin transition (0→0) in a vibrationally resolved, zero-temperature absorption or emission spectrum. It is often assumed that the overtone to fundamental integrated intensity ratio in resonance Raman scattering of semiconductor nanocrystals is equal to or proportional to S, or that S may be determined from the overtone intensity in some other straightforward manner. In fact, this is not generally possible because of different excitation profiles for overtones and fundamentals, differential sensitivity of overtones and fundamentals to electronic dephasing, and interference effects from partially overlapping electronic transitions. Here we examine the relationship between the Huang-Rhys factor and the overtone to fundamental intensity ratio through spectroscopic simulations using parameters appropriate to II-VI semiconductor nanocrystals such as CdSe. A simple equation relating the overtone to fundamental Raman intensity ratio to the Huang-Rhys factor is obtained only in the case of a single resonant electronic state, excitation at the maximum of the inhomogeneously broadened absorption band, and a homogeneous linewidth small compared with the phonon frequency.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

Kelley

2013

J. Phys. Chem. A

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“…The ZnS LO is not observed in the Raman spectra, propably due to the small shell thickness of only a few monolayers. In investigations of spherical CdSe nanocrystals with ZnS shells, the ZnS LO can only be well separated from the CdSe related features for shell thicknesses above 3 monolayers [68]. A further increase in shell thickness then leads to an increase of the ZnS LO bands integral intensity, which is roughly proportional to the ZnS volume.…”

Section: Influence Of the Shell On The Raman Spectramentioning

confidence: 99%

“…The fundamental, first-order, Raman band is located around 200 cm −1 . The band has an asymmetric shape, which is common for low-dimensional CdSe structures [68,69] and similar materials [70]. The fundamental Raman band is assembled from two bands, each with different origin.…”

Section: Raman Spectroscopy Of Cdse Nanorodsmentioning

confidence: 99%

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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“…12,13 It is established that capping a core material with a shell reduces nonradiative recombination, when the shell inorganic material is a semiconductor with wider band gap than the core semiconductor. 14,15 Fluorescence quantum efficiency and related aging and photodegradation effects are therefore of utmost importance in the development and applications of the QDs materials. 16,17 QDs were also used to probe local-field effects on the spontaneous emission rate.…”

mentioning

confidence: 99%

Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry

Cruz

Pilla

Catunda

2010

Journal of Applied Physics

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A recently developed thermal lens spectrometry configuration has been used to study CdSe/ZnS core-shell quantum dots ͑QDs͒ suspended in toluene and tetrahydrofuran ͑THF͒ solvents. The special features of this configuration make it very attractive to measure fluorescence quantum yield ͑͒ excitation spectrum since it simplifies the measurement procedure and consequently improve the accuracy. Furthermore, the precision reached is much higher than in conventional photoluminescence ͑PL͒ technique. Two methods, called reference sample and multiwavelength have been applied to determine , varying excitation wavelength in the UV-visible region ͑between 335-543 nm͒. The and PL spectra are practically independent of the excitation wavelength. For CdSe/ZnS QDs suspended in toluene we have obtained =76Ϯ 2%. In addition, the aging effect on and PL has been studied over a 200 h period for QDs suspended in THF.

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Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots

Cited by 249 publications

References 21 publications

Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

Optical phonons in colloidal CdSe nanorods

Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry

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