Raman analysis of CdSe/CdS core–shell quantum dots with different CdS shell thickness

Liu, Lu; Xu, Xiu‐Hua; Liang, Wentao; Lu, Haifei

doi:10.1088/0953-8984/19/40/406221

Cited by 61 publications

(89 citation statements)

References 37 publications

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“…It is related either with phonons at k / = 0 [33], surface optical (SO) [23], zone-edge (ZE) [22] phonons, or quantized optical phonons (vibrons) with a quantum number n > 1 [17,34,35]. Fitting the Raman spectrum in the range of LO mode with two Lorentzians, corresponding to SO (or ZE) and LO phonon or to vibrons with different quantum numbers, is widely applicable for II-VI NCs [15,18,23,36]. However, the similar shoulder has been observed in bulk alloys [37].…”

supporting

confidence: 67%

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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supporting

confidence: 67%

Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

Dzhagan

Valakh

Mel’nik

et al. 2012

International Journal of Spectroscopy

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“…[18][19][20][21][22][23] Closer examination of Raman spectra obtained with a high signal-to-noise ratio reveals an additional, low-frequency shoulder on the LO phonon. This shoulder is seen quite consistently in nanocrystals with different sizes and shapes and with different surface chemistries, 6,[24][25][26][27][28][29][30][31][32][33][34][35][36] and also in many semiconductor materials other than CdSe. It was assigned as a "surface optical" (SO) phonon by comparison with predictions based on dielectric continuum theory for the optical phonon modes of ionic crystals.…”

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confidence: 76%

“…With a few exceptions, 33 the frequency of the "SO" mode is found to shift by far less than predicted upon replacing the organic ligands with a semiconductor shell, 6,28,29 changing the medium refractive index from about 2 to about 9. This indicates that the purely ionic, dielectric continuum model is not a good approximation for CdSe NCs.…”

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confidence: 82%

“…37,38 Although alternative explanations involving only the dispersion of the LO phonons have been presented, 39 the SO assignment seems to have been widely accepted and there have been a number of efforts to correlate the frequency and/or intensity of this mode with surface properties of core-shell nanocrystals such as shell thickness and degree of alloying at the surface. 6,26,27,33,40 However, the phonon mode assignments seem tenuous given that surface modification often affects the LO phonon about as much as the "SO". 6,[26][27][28]35 The frequencies of the surface modes of a small spherical nanocrystal are given by dielectric continuum theory as 37 …”

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confidence: 99%

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The “Surface Optical” Phonon in CdSe Nanocrystals

et al. 2014

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The origin of the ubiquitous low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature is usually assigned as a "surface optical" (SO) phonon, but it is only slightly affected by modifying the surface through exchanging ligands or adding a semiconductor shell. Here we present excitation profile data showing that the lowfrequency shoulder loses intensity as the excitation is tuned to longer wavelengths, closer to resonance with the lowest-energy 1S e À1S 3/2 excitonic transition. Calculations of the resonance Raman spectra are carried out using a fully atomistic model with an empirical force field to calculate the phonon modes and the standard effective mass approximation envelope function model to calculate the electron and hole wave functions.When a force field of the Tersoff type is used, the calculated spectra closely resemble the experimental ones in showing mainly the higher-frequency LO phonon with 1S e À1S 3/2 resonance but showing intensity in lower-frequency features with 1P e À1P 3/2 resonance. These calculations indicate that the main LO phonon peak involves largely motion of the interior atoms, while the low-frequency shoulder is more equally distributed throughout the crystal but not surface-localized. Interestingly, very different results are obtained with the widely used Coulomb plus Lennard-Jones force field developed by Rabani, which predicts far more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.

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“…The frequency of the CdS LO phonon has been reported to increase by anywhere from 6 to 24 cm -1 upon increasing the shell thickness in CdSe/CdS core/shell structures. 1,10,15,22 Most studies on CdSe/CdS core/shells also report that the CdSe LO fundamental shifts to higher frequencies by 3-7 cm -1 between bare CdSe cores and thick CdS shells. Most of the previously reported Raman spectra for CdSe/CdS core/shell structures show significant intensity in the CdS overtone, but in all cases these spectra were excited >3500 cm -1 above the lowest exciton.…”

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confidence: 99%

Electron–Phonon Coupling in CdSe/CdS Core/Shell Quantum Dots

et al. 2015

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Resonance Raman spectra and excitation profiles have been measured and semiquantitatively modeled for core/shell quantum dots consisting of 2.7 nm diameter zincblende CdSe cores and thin (0.5 nm) or thick (1.6 nm) CdS shells. The Raman spectra show previously reported trends of increased peak frequency for both the CdSe and the CdS longitudinal optical (LO) phonons with increasing shell thickness. We also find a strong dependence of the peak CdS frequency on excitation energy and a large discrepancy between the experimental frequency of the CdSe+CdS combination band and the sum of the corresponding fundamental frequencies. This suggests that the dominant transitions at high excitation energies are localized on either the CdSe core or the CdS shell and thereby cannot enhance combination band transitions between core and shell. The CdS to CdSe Raman intensity ratios at high excitation energies further support this picture. The electron-phonon coupling for the CdSe LO phonon in the lowest excitonic transition is slightly weaker in the core/shell structures than in pure CdSe quantum dots, contrary to expectations for the Fröhlich coupling mechanism. Possible explanations for this discrepancy are discussed.

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Raman analysis of CdSe/CdS core–shell quantum dots with different CdS shell thickness

Cited by 61 publications

References 37 publications

Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

The “Surface Optical” Phonon in CdSe Nanocrystals

Electron–Phonon Coupling in CdSe/CdS Core/Shell Quantum Dots

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