Confinement‐, surface‐ and disorder‐related effects in the resonant Raman spectra of nanometric CdS1−xSex crystals

Gomonnai, A. V.; Azhniuk, Yu. M.; Yukhymchuk, V.О.; Kranjčec, M.; Lopushansky, V. V.

doi:10.1002/pssb.200301838

Cited by 55 publications

(67 citation statements)

References 41 publications

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“…1b). The difference of the LO phonon frequencies is known to be a good measure of the nanocrystal chemical composition [6,7,[10][11][12]. The observed slight increase of selenium content in the nanocrystals with heat treatment temperature is consistent with the earlier reported effect of segregation of minority chalcogen atoms in glass-embedded CdSe 1Àx S x nanocrystals with increasing heat treatment temperature and/or duration [18].…”

Section: Methodssupporting

confidence: 89%

Precipitates of selenium and tellurium in II–VI nanocrystal‐doped glass probed by Raman scattering

Azhniuk

Lopushansky

Hutych

et al. 2011

Physica Status Solidi (b)

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, Phone: þ380 312 643822, Fax: þ380 312 643650As follows from Raman studies, the formation of cadmium chalcogenide nanocrystals in borosilicate glass under thermal treatment at 625-700 8C can be accompanied by precipitation of elemental Se or Te in the form of molecular Se 2 or Te 2 clusters or larger aggregations. The clusters are in most cases formed when the thermal treatment temperature and/or duration values are beyond the range most suitable for the formation of the II-VI nanocrystals. Larger aggregations are formed at intense thermal treatment, precipitation of tellurium being achieved much easier than that of selenium.1 Introduction II-VI (mostly CdSe 1Àx S x ) semiconductor nanocrystal-doped glasses were extensively studied by Raman spectroscopy and the excellent applicability of this technique to determine the nanocrystal parameters was demonstrated. Low-wavenumber Raman scattering by acoustic phonons due to the confinement-related selection rules relaxation in CdSe 1Àx S x nanocrystals was used to estimate the average nanocrystal size [1][2][3][4][5]. Raman spectroscopy was also proven to be a very efficient tool to examine the chemical composition of ternary II-VI nanocrystals diluted in the glass matrix, based on the LO phonon frequencies taking into account the specific effects related to glass matrix pressure, phonon confinement, surface phonon scattering and compositional and size dispersion of nanocrystals within the ensemble [6][7][8][9][10][11][12][13][14][15][16][17][18].Fabrication of II-VI semiconductor nanocrystals in glass matrices is performed by a well-elaborated diffusion-limited growth technique [19][20][21]. Recently Raman scattering studies revealed that not only the nanocrystal size, but also their composition is affected by the growth conditions (thermal treatment temperature and duration) [16,18]. Moreover, it was reported that at certain thermal treatment conditions molecular clusters of selenium can be formed in CdSe and CdSe 1Àx S x nanocrystal-doped glasses [22].

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Section: Methodssupporting

confidence: 89%

Precipitates of selenium and tellurium in II–VI nanocrystal‐doped glass probed by Raman scattering

Azhniuk

Lopushansky

Hutych

et al. 2011

Physica Status Solidi (b)

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“…On the contrary, the strain arising in the glass-embedded nanocrystals due to the glass matrix pressure, results in an upward shift of the phonon frequencies [15]. All these factors should be taken into account for a correct determination of the average nanocrystal composition for a ternary compound [8]. However, since all these factors are expected to act in the same way for both CdSe-like and CdTe-like phonons, it seems reasonable, similarly to CdS 1-x Se x [1], to use the difference of the CdSe-like and CdTe-like LO phonon frequencies ν 2 -ν 1 as a measure of the nanocrystal composition.…”

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confidence: 96%

“…It should be noted that phonon confinement in nanocrystals also results in a relaxation of the selection rules, leading to scattering by nonzero-wavevector phonons revealed in the Raman spectra which, due to the phonon dispersion, is observed as an asymmetric broadening and a slight downward shift of the corresponding LO phonon band with the size decrease [3,4,8]. Besides, the Raman band lineshape and position in nanocrystals can be also affected by marked surface phonon contribution due to a high surface-to-volume ratio in nanometric systems [2][3][4]8]. Since the frequency of a surface phonon is between the corresponding TO and LO phonon frequencies, one can also expect the surface phonon scattering to lead to a downward shift of the observed Raman peak and its asymmetric broadening at the low-frequency side.…”

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confidence: 96%

Phonon spectroscopy of CdSe_1–xTe_x nanocrystals grown in a borosilicate glass

Azhniuk

Hutych

Lopushansky

et al. 2009

Phys. Status Solidi (c)

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Phonon spectra of CdSe1–x Tex nanocrystals grown in a borosilicate glass matrix, are studied by Raman spectroscopy. The experimental data are compared to those obtained by other authors for bulk and nanoscale CdSe1–x Tex. With the account of the factors specific for glass‐embedded nanocrystals, Raman spectroscopy is shown to be a reliable and non‐destructive method to evaluate the ternary nanocrystal composition. Raman measurements performed at different excitation wavelengths enabled the compositional dispersion of the nanocrystals to be estimated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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“…It is noted that these modes show an negligible dependence with the dot-size. The small frequency variation on ∆ω corresponding to ±2 cm −1 can be attributed to strain or small spatial confinement effects on the phonon modes [4]. The positions of the peaks corresponding to the SO-modes, as a function of the heating times, are presented in Fig.…”

mentioning

confidence: 97%

“…To estimate the LO and SO frequencies of the ternary [5]. The dielectric constants are ε ∞,1 = 1.61, ε ∞,2 = 5.32 while for the host material, we are taking ε D = 2.25 [4]. Figure 2 shows the calculated frequencies for CdSe-like and CdSlike TO modes as a functions of the concentration x.…”

mentioning

confidence: 99%

Surface phonons modes: a tool to determine the quantum dot morphology

Alcalde¹,

Ribeiro²,

Dantas³

et al. 2006

Braz. J. Phys.

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We report here theoretical and experimental studies on the spatial confinement of phonons in ternary CdS x Se 1−x nanocrystals embedded in a glass matrix formed by the composites SiO 2 -Na 2 CO 3 -B 2 O 3 -Al 2 O 3 doped with CdO, S and Se. We determined the morphologic characteristics of the nanocrystals by analyzing the dependence of surface phonon modes on the geometrical parameters. The calculated frequencies are compared with values from Raman spectra of CdS x Se 1−x nanocrystals grown under different thermal treatments. A good correlation between experimental and calculated CdS-like and CdSe-like surface optical modes is observed. Raman selection rules and their connection with the nature of the surface optical phonons is discussed.Keywords: Quantum dots; Raman scatteringThe quantum confinement effects on the optical properties of nanocrystallites or quantum dots (QD's) grown over semiconductor-doped glasses are subject of current researches. The interest in these materials arises from their great potential for use in device applications due to their nonlinear optical properties. Experimental and theoretical studies showing the confinement effects on the QD electronic states and their energy levels are now well established. Nonetheless, the size effects on their phonon modes and on the electron-phonon interaction are still somewhat controversial. In this theoretical scenario, one aspect has not yet been sufficiently addressed: how the geometrical shape effects can modify the electronic structure and other physical properties in small QD's. In II-VI QD's, the morphological effects can modify the energies, symmetry properties of the wave functions and, thus, their optical transitions.In this work, we explore theoretically and experimentally the connection between the surface optical (SO) phonon modes and the geometrical shape of a semiconductor QD. By analyzing the behavior of SO modes in ternary alloy based QD's, we may determine the alloy composition fluctuation and the geometrical parameter r, that measures the degree of deviation from sphericity. To do this, we have extended the Comas et al. [1] model for binary spheroidal dots to study ternary alloys of A 1 1−x A 2 x B QD type. Thus, we calculated the dependence of the A 1 B-like and A 2 B-like SO-modes as a function of x and r. Our extended theory recovers the binary compound limits x → 0 and x → 1 of Ref.[1] and verifies the conditions ω TO < ω SO < ω LO .Samples were prepared over the glass matrix SiO 2 -Na 2 CO 3 -B 2 O 3 -Al 2 O 3 doped with CdO, S and Se. The mixture was melted in an aluminum crucible at 1200 • C, for 2h. Then, it was quickly cooled down to room temperature. In order to enhance the diffusion of the Cd 2+ , S 2− , and Se 2− ions, we have performed 600 • C thermal treatments on each samples using different annealing times of t a =3, 5, 6, 10, 12, 14, 16, 18, 20, and 22 h, respectively. As a result of these thermal treatments, the QD's of CdS x Se 1−x are formed over the glass matrix. The QD Raman spectra were recorded at room tempe...

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Confinement‐, surface‐ and disorder‐related effects in the resonant Raman spectra of nanometric CdS_1−xSe_x crystals

Cited by 55 publications

References 41 publications

Precipitates of selenium and tellurium in II–VI nanocrystal‐doped glass probed by Raman scattering

Precipitates of selenium and tellurium in II–VI nanocrystal‐doped glass probed by Raman scattering

Phonon spectroscopy of CdSe_1–xTe_x nanocrystals grown in a borosilicate glass

Surface phonons modes: a tool to determine the quantum dot morphology

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Confinement‐, surface‐ and disorder‐related effects in the resonant Raman spectra of nanometric CdS1−xSex crystals

Cited by 55 publications

References 41 publications

Precipitates of selenium and tellurium in II–VI nanocrystal‐doped glass probed by Raman scattering

Precipitates of selenium and tellurium in II–VI nanocrystal‐doped glass probed by Raman scattering

Phonon spectroscopy of CdSe1–xTex nanocrystals grown in a borosilicate glass

Surface phonons modes: a tool to determine the quantum dot morphology

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Confinement‐, surface‐ and disorder‐related effects in the resonant Raman spectra of nanometric CdS_1−xSe_x crystals

Phonon spectroscopy of CdSe_1–xTe_x nanocrystals grown in a borosilicate glass