Raman and Infrared Phonon Spectra of Ultrasmall Colloidal CdS

Article pubs.acs.org/JPCC

Raman and Infrared Phonon Spectra of Ultrasmall Colloidal CdS Nanoparticles Volodymyr M. Dzhagan,*,†,‡ Mykhailo Ya. Valakh,‡ Cameliu Himcinschi,§ Alexander G. Milekhin,∥,⊥ Dmytro Solonenko,† Nikolay A. Yeryukov,∥ Oleksandra E. Raevskaya,# Oleksandr L. Stroyuk,# and Dietrich R. T. Zahn† †

Semiconductor Physics, Technische Universität Chemnitz, D-09107 Chemnitz, Germany V. E. Lashkaryov Institute of Semiconductor Physics, Nat. Acad. Sci. of Ukraine, Nauky Av. 45, 03028 Kyiv, Ukraine § Institute of Theoretical Physics, TU Bergakademie Freiberg, Leipziger Str. 23, D-09596 Freiberg, Germany ∥ A. V. Rzhanov Institute of Semiconductor Physics, Pr. Lavrentieva, 13, Novosibirsk 630090, Russia ⊥ Novosibirsk State University, Pirogov Str. 2, 630090, Novosibirsk, Russia # L. V. Pysarzhevsky Institute of Physical Chemistry, Nat. Acad. Sci. of Ukraine, Nauky Av. 31, 03028 Kyiv, Ukraine ‡

ABSTRACT: Raman and infrared phonon spectra of ultrasmall (1.8 nm) colloidal CdS nanoparticles (usNPs) are presented. Multiphonon scattering by optical phonons up to the third order is observed in the Raman spectra at low temperature and resonant (325 nm) excitation. The first-order optical phonon peak is a superposition of several components, two of which can be assigned to surface optical (SO) and longitudinal optical (LO) modes, respectively. The LO mode, being markedly broadened compared to that of spectra of regular (>2 nm) NPs, is related to phonon confinement and bond distortion induced by a significant structural relaxation in usNPs. A shoulder observed above the LO frequency is either due to the density of phonon states induced by distorted surface bonds or due to higher-order scattering processes involving optical and acoustic phonons. The Raman peaks of usNPs do not reveal the upward shift and narrowing upon decreasing temperature from 300 K down to 85 K typical for crystalline semiconductors, even though their intensity increases as expected. The abnormal thermal behavior of phonon peaks is likely related to the significant structural reorganization of the usNP lattice. A broad feature in the range of 200−300 cm−1 observed in the infrared phonon spectrum of usNPs correlates with the Raman data and is distinct from the SO mode previously reported for NPs of larger size.

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INTRODUCTION Ultrasmall semiconductor nanoparticles (usNPs), or “magicsize” clusters, have recently attracted much attention due to their unique physical properties.1−6 The growth rate of colloidal NPs deviates from the prediction of the classic thermodynamic growth theory as the NPs attain the magic closed-shell structure resulting from the existence of a chemical potential well.6 For CdSe, CdS, and several other II−VI compounds, the most frequently observed magic structure comprises 32−34 molecular units corresponding to a diameter of around 1.7−1.9 nm.1,6,7 The usNPs reveal either narrow blue8 or broad-band quasi-white photoluminescence (PL), making this kind of NP promising for light-emission applications.2−4,9,10 Studying NPs of such a small size is also of fundamental importance, as it allows the existing models to be verified in the regime of strong electron and phonon confinement.11−15 Understanding and controlling the electronic and optical properties of usNPs is, however, complicated due to a considerable contribution from surface atoms, the number of which is comparable to the number of fully coordinated inner atoms, as well as due to the coupling between the electronic states in the NP and ligands.16 The lattice vibration (phonon) spectra can provide valuable © 2014 American Chemical Society

information on the structure and physical properties of semiconductor NPs and their interaction with the environment.15−22 Here we report on resonant Raman scattering and infrared (IR) studies of phonons in CdS usNPs prepared by “wet” colloidal chemistry under mild conditions. We observe for the first time multiple Raman scattering by optical phonons up to the third order and resolve the structure of the first feature in the range of 250−300 cm−1. The Raman spectra at various temperatures and room-temperature IR absorption spectra are also reported for the first time for ultrasmall NPs.

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EXPERIMENTAL SECTION Materials. CdCl2, Na2S·9H2O, linear polyethylenimine (PEI) with a molecular mass of 50 000 g/mol, mercaptoacetic acid, hydrazine hydrate N2H4·H2O, and NaOH were supplied by Sigma-Aldrich. Received: June 25, 2014 Revised: July 12, 2014 Published: July 23, 2014 19492

dx.doi.org/10.1021/jp506307q | J. Phys. Chem. C 2014, 118, 19492−19497

The Journal of Physical Chemistry C

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Figure 1. Optical absorption (a, curves 1 and 2) and PL spectra (a, curves 1′ and 2′) and XRD patterns (b) of CdS usNPs with d = 1.8 nm (curves 1) and 3.5 nm (curve 2). The spectral position of the Raman excitation wavelength is indicated in (a). Also shown in (b) are the XRD patterns of bulk wurtzite (JCPDS no. 75-0581) and zinc blende (JCPDS no. 80-0019) CdS.

Synthesis of the NPs. The ultrasmall (1.8 nm) CdS NPs stabilized with polyethylenimine (PEI) were synthesized similarly to the procedure used in ref 2. In a typical procedure, to 9.5 mL of water were added 0.1 mL of a 1.0 M CdCl2 aqueous solution, 0.25 mL of a 10 wt % aqueous PEI solution (PEI molecular weight 50 000 g/mole), and then 0.1 mL of a 1.0 M Na2S aqueous solution with vigorous stirring at room temperature. The NP size dispersion does not exceed 10%.2 CdS NPs of larger size, 4 nm, used in this work for comparison with the usNPs were formed in a slightly modified way in order to promote the growth of larger NCs. To 9.0 mL of water were added 0.1 mL of a 1.0 M CdCl2 aqueous solution, 0.25 mL of a 10 wt % PEI aqueous solution, 0.1 mL of a 1.0 M aqueous solution of mercaptoacetic acid, and 0.5 mL of hydrazine hydrate N2H4·H2O with vigorous stirring. To this mixture, 0.1 mL of 1.0 M Na2S was added with vigorous stirring. Solutions of both NP samples were aged in the dark for 2 days. Optical Measurements. Absorbance spectra were recorded with a Specord 220 spectrophotometer. The photoluminescence (PL) spectra were taken using a PerkinElmer LS55 spectrometer. The spectral resolution was less than 1 nm in both cases. Raman spectra were excited with the 325 nm line of a He−Cd laser and recorded with a LabRam microRaman system (15× objective). The spectral resolution was 3 cm−1. The temperature-dependent Raman measurements were made using a Linkam THMS-600 cooling−heating stage. For Raman scattering measurements the NPs were deposited by dropcasting onto a Si substrate. Room-temperature nonpolarized FTIR reflection spectra were recorded at an angle of light incidence of ⊖ = 75° in the spectral range of 90−600 cm−1 using a Vertex 80v FTIR spectrometer with a resolution of 2 cm−1 over the whole spectral range, with data collected over 300 scans. For FTIR measurements the NPs were deposited by drop-casting onto an Au-capped (∼100 nm) Si substrate. XRD spectra were registered using a Bruker D8 Advance diffractometer with a Cu Kα irradiation source in Bragg− Brentano geometry at a scanning rate of 1 grade/min.

and allows the mean NP diameter, d, to be estimated. In the previous detailed study of synthesis and absorption spectra of these PEI-stabilized CdS usNPs,2 we derived d = 1.8 nm and a size dispersion of ∼10%. The observed E1 peak energy, size homogeneity, and stability of this particular NP size allowed these usNPs to be assigned to magic-sized clusters.1,6−8 The second absorption peak can be well identified at about 325 nm and assigned to the (1Se−2Sh) electronic transition.20 The aging of CdS usNPs in the presence of mercaptoacetic acid and hydrazine hydrate results in a decrease in main absorption peak E1 and the formation of an additional band with the longerwave edge at around 430 nm indicating that the partial transformation of usNPs into larger “regular” NPs takes place under such conditions. The size of aged CdS NPs estimated from the absorption threshold was found to be 3 nm. It was found that the presence of mercaptoacetic acid and N2H4 is essential for growth to stop at this stage and not proceed to the formation of larger NPs. Such behavior and growth mechanism are currently under study and will be reported elsewhere. The relatively intense (quantum yield 5−10% in PEI solutions and 15−20% in PEI films at room temperature) broadband PL strongly red-shifted from the first absorption maximum is obviously related to deeply trapped charge carriers.9,10,23,24 The deep-trap emission often occurs in the metal chalcogenide NPs produced by low-temperature aqueous-phase syntheses.2,9 Nevertheless, in the present case of the ultrasmall NPs, in which almost half of the atoms can be regarded as surface atoms, the key factor determining the PL emission can be the surface states rather than internal defects. However, the nature of this kind of broadband and strongly red-shifted PL emission in metal chalcogenide NPs is still a matter of discussion.2,3,5 The transformation of 1.8 nm usNPs into larger 3 nm NPs is accompanied by a red shift of the PL band (compare curves 1 and 2, Figure 1b) concomitant with a narrowing of the band gap from 3.49 eV for 1.8 nm NPs to 2.75 eV for 3.5 nm CdS NPs and a red shift of the absorption threshold. Because of the large broadening of the XRD reflections, it is not possible to identify the crystalline structure of the PEIstabilized CdS usNPs as wurtzite or zinc blende (Figure 1b). Nevertheless, on the basis of the X-ray pattern one can conclude that the usNPs preserve the partially crystalline structure of bulk CdS. For the amorphous phase, a single similarly broad peak is expected around the (111) peak position

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RESULTS AND DISCUSSION The optical absorption and PL spectra of the CdS usNPs studied are shown in Figure 1a. The lowest-energy absorption peak (E1) at 356 nm, related to the (1Se−1Sh) electronic transition,20 is strongly blue-shifted from the bulk CdS band gap of 520 nm. This indicates a strong electronic confinement 19493



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Figure 2. (a) Resonant Raman spectra of CdS usNPs at various temperatures (PL background subtracted). The right inset shows normalized Raman spectra of usNPs in comparison to larger (3.5 nm) NPs at 85 and 250 K. The first-order phonon region of these spectra is shown in detail in the left inset. (b) Normalized PL spectra of CdS usNPs at various temperatures (spectra shifted vertically for better comparison).

near 30°,25 while for usNPs a weaker additional feature is observed at 40−50° (Figure 1b).15 The width of the main experimental XRD peak being about 10° (Figure 1b) is almost 2 times larger than predicted by calculations for 1.8 nm NPs26 indicates that additional factors of broadening besides the small size of NPs exist. Most probably the additional broadening originates from the distortion of the usNP lattice due to its energy minimization.14,16,27,28 Due to the large surface-tovolume ratio in usNPs, this surface-driven optimization of the lattice structure may spread over the whole volume of the NP. Attempts to characterize usNPs directly by electron microscopy were precluded by the strong interaction of the stabilizing polymer with the electron beam resulting in the rupture of the sample.15 Figure 1b (curve 2) shows the XRD pattern of 3.5 nm CdS NPs precipitated from colloidal solution exhibiting broadened peaks at 2⊖ = 27, 44, and 52°, characteristic of cubic cadmium sulfide. The average size of the coherent X-rays scattering domain was estimated using the Scherer formula as ∼3.0 nm, which corresponds well to the estimations made from the absorption spectrum of the original colloidal solution. The XRD-derived NP size value is somewhat smaller than that obtained from absorption spectra, probably as a result of the partial conversion of 1.8 nm usNPs into 3.5 nm NPs and the contribution of usNPs to peak broadening. Raman spectra of the usNPs (Figure 2a) reveal a dominant band in the range of optical vibrations of CdS, near 300 cm−1, and weaker features around 600 and 900 cm−1, which can be assigned to the overtones of the main phonon peak.23,24,29,30 The temperature-dependent measurement, from 250 down to 85 K (Figure 2, inset), reveals no noticeable spectral change, except for the expected increase in the peak intensities with decreasing temperature.31,32 The temperature dependence of the optical phonon frequency is known to be determined by two main factors: the change in the lattice constant (bond length) and phonon−phonon coupling.31,32 The temperatureinduced change in the lattice constant in usNPs takes place, as can be concluded, from the blue shift and narrowing of the PL band (Figure 2b). Note, however, that in this respect the phonon spectrum is a more reliable probe of the lattice than PL because the PL band position can be affected by a temperaturedependent energy distribution of the charge carriers. With respect to the effect of anharmonicity, it was shown previously that the contributions from two-phonon processes decrease with decreasing NP diameter while the contributions from higher-order processes increase.17 However, in the studies reported up to now no trend in decreasing NP size was tracked, which could indicate that no or a very weak temperature

dependence of phonon frequency and bandwidth should be expected below 2 nm. Therefore, the absence of marked spectral changes in the Raman spectra of usNPs with temperature as well as the large width of the phonon peaks can be related to the significant structural reorganization in usNPs, as compared to larger NPs and bulk crystals. In particular, if the optical phonon bandwidth is dominated by a temperature-independent inhomogeneous broadening, then the relatively small spectral changes induced by temperature variation may not be resolved. Despite the significant structural reorganization, the usNPs preserve the parental crystalline structure of CdS in general, as follows from the XRD data discussed above and the good matching of the E1 value of the usNPs with the E1(d) dependence derived for a large d range.2 The results of Raman spectroscopy corroborate this assumption. Indeed, in comparing the spectrum of the 1.8 nm usNPs with the spectrum of 3.5 nm NPs measured under the same experimental conditions (Figure 2, inset), we see the correspondence of the bands for both types of NPs; in particular, the first-order Raman band (Figure 3) can be satisfactorily fitted with two Lorentzian

Figure 3. Fitting of the main Raman band for 1.8 and 3.5 nm CdS NPs. The dotted vertical lines indicate the position of the zone-center TO and LO bulk phonons and the SO mode frequency calculated for spherical NPs.30

components. The frequencies (full widths at half maximum (fwhm)) of the components amount to 280(36) and 306(33) cm−1 for 1.8 nm NPs and 285(45) and 305(16) cm−1 for 3.5 nm NPs, correspondingly. These two components can be assigned to surface optical (SO) and longitudinal optical (LO) 19494



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phonons in agreement with previous observations for NPs with diameters larger than 2 nm.23,24,29,30 The LO phonon frequency in usNPs being almost as high as for bulk CdS can be related to compressive strain in the usNPs, with the corresponding upward shift compensating for the downward shift due to phonon confinement. The net compressive strain is a result of the structural reconstruction in usNPs, accompanied by changes in the bond lengths and elastic properties, as predicted theoretically33,34 and verified experimentally.28,34,35 Another distinct feature of the Raman spectra of usNPs is the high-frequency shoulder (HFS) of the main peak (Figure 3), which is more pronounced than for previously studied 2 to 3 nm CdSe NPs.36 In the initial study of the HFS,36 two origins of the HFS appearance were suggested as most probable: the phonon density of states (PDOS) and combinations of optical phonons with acoustical modes activated by structural disorder.36 The latter hypothesis has recently been further developed in the work on ZnTe nanorods37 and CdSe/Co NPs, with the most favorable mechanism assumed to be exciton scattering by acoustic phonons, followed by LO phonon emission (in the general case nLO, with n = 0, 1, 2,..., because a similar HFS is observed for LO overtones as well; see Figure 2b and refs 36−39). The second of two suggested origins of the HFSPDOS is based on the ab initio calculations of phonons in CdS and other semiconductor NPs,10,14 which revealed a PDOS intensity above the bulk LO phonon frequency exactly in the region where the HFS is observed. The PDOS obtained in ref 10 was assigned to vibrations of (near) surface bonds distorted by the surface reconstruction. These distorted bonds can cause a noticeable contribution to the Raman spectrum for 1.8 nm usNPs with their large portion of (near) surface bonds. The weaker HFS intensity for 2 to 3 nm CdSe NPs36 and its complete absence for 3.5 nm NPs (Figure 2b and ref 34) is in agreement with this assumption. The PDOS version of the HFS origin is also corroborated by HFS suppression upon passivation of NPs by a shell of another semiconductor.15,36 Indeed, coherently grown inorganic shells can preclude uncoordinated atoms at the (core) NP surface, and thus the corresponding contribution to the Raman spectrum would be eliminated. Besides the PDOS intensity in the optical phonon range discussed above, the calculated PDOS13,14,27 contains features near 50 cm−1 and 120−150 cm−1 which were also observed experimentally in the Raman spectra of 2−3.5 nm CdSe NPs36 and can be seen in the IR spectrum of 1.8 nm NPs as discussed below (Figure 4). It should be noted that the notation “surface optical phonon” is used in our work based on its traditional implementation to the low-frequency shoulder of the LO mode, observed for nanostructures,13,15,29 while the applicability of the underlying dielectric continuum approximation to NPs as small 1.8 nm can be questionable. Nevertheless, recently reported calculations of the Raman spectra using the fully atomic model reproduce the same low-frequency shoulder of the main optical phonon peak.37 Because the “SO”-mode vibration, considered theoretically in ref 37, involves all of the atoms of the NP including the surface ones, it is reasonable that the ratio of this mode to LO is very high in the case of our ultrasmall NPs with a large surfaceto-volume ratio. The observation of phonon overtones up to third order (Figure 2a) indicates that the CdS usNPs preserve the crystalline structure of the bulk crystal, though strongly

Figure 4. Experimental IR reflection/absorption spectrum of ultrasmall CdS NPs and its multipeak fitting.

distorted by energy-minimization-driven reconstructions reflected in the discussed broadening and upward shift of the LO mode. The intensity ratio of the overtones to the fundamental one is commonly considered to be a measure of the electron− phonon coupling strength in polar semiconductors.12 The 2LO/LO ratio in our study is about 0.4 for both 1.8 and 3.5 nm NPs (Figure 2). The independence of the 2LO/LO ratio on the NP size is in agreement with previous works.36,40 In other works, however, both increases41 and decreases42 in the 2LO/ LO ratio with NP size were reported. Obviously the electron− phonon coupling is not solely determined by the geometrical size of the NPs but also by its internal structure and the electronic coupling between the electronic states of the NPs and ligands to be taken into account. Unlike the Raman spectra reported in numerous works for larger sizes17,18,26−30,32,36 and in a few works for ultrasmall NPs,1,15,22 there are only a few reports on IR spectra of larger NPs30,43−45 and no reports on usNPs. Raman and IR spectroscopic techniques provide complementary information because the selection rules allow different phonons to be investigated by Raman and IR spectroscopy. In particular, while the resonant Raman spectrum of II−VI NPs is dominated by longitudinal optical modes,17,18,26−30,32,36 features due to TO and SO modes are expected to appear in the IR spectra.30,43−45 Indeed, the IR reflection/absorption spectrum of usNPs (Figure 4) reveals a broad feature between 200 and 300 cm−1, where the TO and SO modes occur. The absorption feature of the usNPs is distinctly broader than the SO peak at 270−280 cm−1 observed previously for larger (3−6 nm) CdS NPs.30 Unlike the first-order Raman feature, which we identify as an overlap of SO and LO modes, the broad IR feature is most probably a superposition of TO and SO modes, represented in the fit in Figure 4 by dotted blue and solid blue curves, respectively. Both broadening of the SO mode and activation of the TO mode, as compared to those of larger-size CdS NPs,30 can be related to lattice deformations in the usNPs due to their energy minimization. On the basis of the calculated PDOS, a feature at about 150 cm−1 can be assigned to disorder-activated LA phonons.30 These modes are typically IR-inactive in larger NPs but can be activated in ultrasmall NPs with noticeable bond distortion due to the reduction in crystal symmetry and consequent lifting of IR selection rules. Note that both disorder-activated TA and LA features were observed in the 19495



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(4) Schreuder, M. A.; Xiao, K.; Ivanov, I. N.; Weiss, S. M.; Rosenthal, S. J. White Light-Emitting Diodes Based on Ultrasmall CdSe Nanocrystal Electroluminescence. Nano Lett. 2010, 10, 573−576. (5) Pennycook, T. J.; Mcbride, J. R.; Rosenthal, S. J.; Pennycook, S. J.; Pantelides, S. T. Dynamic Fluctuations in Ultrasmall Nanocrystals Induce White Light Emission. Nano Lett. 2012, 12, 3038−3042. (6) Kilina, S. V.; Kilin, D. S.; Prezhdo, O. V. Breaking the Phonon Bottleneck in PbSe and CdSe Quantum Dots: Time- Charge Carrier Relaxation. ACS Nano 2009, 3, 93−99. (7) Chen, H. S.; Kumar, R. V. Discontinuous Growth of Colloidal CdSe Nanocrystals in the Magic Structure. J. Phys. Chem. C 2009, 113, 31−36. (8) Li, M.; Ouyang, J.; Ratcliffe, C. I.; Pietri, L.; Wu, X.; Leek, D. M.; Moudrakovski, I.; Lin, Q.; Yang, B.; Yu, K. CdS Magic-Sized Nanocrystals Exhibiting Bright Band Gap Photoemission via Thermodynamically Driven Formation. ACS Nano 2009, 3, 3832− 3838. (9) Kosmella, S.; Venus, J.; Hahn, J.; Prietzel, C.; Koetz, J. LowTemperature Synthesis of Polyethyleneimine-Entrapped CdS Quantum Dots. Chem. Phys. Lett. 2014, 592, 114−119. (10) Ozel, T.; Soganci, I. M.; Nizamoglu, S.; Huyal, I. O.; Mutlugun, E.; Sapra, S.; Gaponik, N.; Eychmüller, A.; Demir, H. V. Selective Enhancement of Surface-State Emission and Simultaneous Quenching of Interband Transition in White-Luminophor CdS Nanocrystals Using Localized Plasmon Coupling. New J. Phys. 2008, 10, 083035− 1−8. (11) Richter, H.; Wang, Z.; Ley, L. The One Phonon Raman Spectrum in Microcrystalline Silicon. Solid State Commun. 1981, 21, 625−629. (12) Kelley, A. M. Electron-Phonon Coupling in CdSe Nanocrystals from an Atomistic Phonon Model. ACS Nano 2011, 5, 5254−62. (13) Han, P.; Bester, G. Confinement Effects on the Vibrational Properties of III-V and II-VI Nanoclusters. Phys. Rev. B 2012, 85, 041306(R). (14) Mohr, M.; Thomsen, C. Phonons in Bulk CdSe and CdSe Nanowires. Nanotechnology 2009, 20, 115707−1−6. (15) Silva, A. C. A.; Neto, E. S. F.; da Silva, S. W.; Morais, P. C.; Dantas, N. O. Modified Phonon Confinement Model and Its Application to CdSe/CdS Core-Shell Magic-Sized Quantum Dots Synthesized in Aqueous Solution by a New Route. J. Phys. Chem. C 2013, 117, 1904−1914. (16) Fischer, S. A.; Crotty, A. M.; Kilina, S. V.; Ivanov, S. A.; Tretiak, S. Passivating Ligand and Solvent Contributions to the Electronic Properties of Semiconductor Nanocrystals. Nanoscale 2012, 4, 904− 14. (17) Kusch, P.; Lange, H.; Artemyev, M.; Thomsen, C. SizeDependence of the Anharmonicities in the Vibrational Potential of Colloidal CdSe Nanocrystals. Solid State Commun. 2011, 151, 67−70. (18) Todescato, F.; Minotto, A.; Signorini, R.; Jasieniak, J. J.; Bozio, R. Investigation into the Heterostructure Interface of CdSe-Based Core−Shell Quantum Dots Using Surface-Enhanced Raman Spectroscopy. ACS Nano 2013, 7, 6649−6657. (19) Sun, Z.; Zhao, B.; Lombardi, J. R. ZnO Nanoparticle SizeDependent Excitation of Surface Raman Signal from Adsorbed Molecules: Observation of a Charge-Transfer Resonance. Appl. Phys. Lett. 2007, 91, 221106. (20) Ekimov, A. I.; Hache, F.; Schanne-Klein, M. C.; Ricard, D.; Flytzanis, C.; Kudryavtsev, I. A.; Yazeva, T. V.; Rodina, A. V.; Efros, Al. L. Absorption and Intensity-Dependent Photoluminescence Measurements on CdSe Quantum Dots: Assignment of the First Electronic Transitions. J. Opt. Soc. Am. B 1993, 10, 100−107. (21) Dworak, L.; Matylitsky, V.; Braun, M.; Wachtveitl, J. Coherent Longitudinal-Optical Ground-State Phonon in CdSe Quantum Dots Triggered by Ultrafast Charge Migration. Phys. Rev. Lett. 2011, 107, 247401−1−5. (22) Dzhagan, V.; Mel’nik, N.; Rayevska, O.; Grozdyuk, G.; Strelchuk, V.; Plyashechnik, O.; Kuchmii, S.; Valakh, M. Vibrational Raman Spectra of CdSxSe1‑x Magic-Size Nanocrystals. Phys. Status Solidi RRL 2011, 5, 250−252.

Raman spectra of NPs with a slightly larger size (around 2 nm).36

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CONCLUSIONS The phonon spectra of ultrasmall colloidal CdS NPs are reported. We observe for the first time multiple Raman scattering (325 nm excitation) by optical phonons up to the third order and resolve the structure of the first feature in the range of 250−300 cm−1. The Raman spectra at various temperatures and room-temperature IR absorption spectra are also obtained for the first time for ultrasmall NPs. The firstorder optical phonon peak that was allowed to be well deconvoluted into two components assigned to SO and LO modes significantly broadened due to phonon confinement and structural deformation of the usNP lattice. This observation is in agreement with a broad feature in the range of 200−300 cm−1 in the IR absorption spectrum. The NP size dispersion is supposed to be of minor importance to the broadening of the phonon modes, as it does not exceed 10% for the usNPs studied. The high-frequency shoulder above the LO frequency in the Raman spectra is related directly either to the distorted surface bonds or to the surface-activated acoustic phonons. The Raman peaks of usNPs do not reveal an upward shift and narrowing for decreasing temperature from 300 K to 85 K typical for crystalline semiconductors, even though their intensity increases as expected. The abnormal thermal behavior of phonon peaks can be due to the domination of the inhomogeneous broadening in the phonon bandwidth, related to the significant structural reorganization of the usNPs lattice.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49 (0)371 531-35790. Fax: +49 (0)371 531-835790. Email: [email protected]. Author Contributions

This manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was partially supported by the Alexander von Humboldt Foundation, Cluster of Excellence “MERGE” (EXC 1075), the Fund for Basic Research of Ukraine (F40.2/068, F40.3/040, and F54.1/013), the Russian Foundation for Basic Research (14-02-904410-Ukr_a), and the Ministry of Education and Science of the Russian Federation. C.H. thanks the German Research Foundation for financial support (HI 1534/ 1-2).

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REFERENCES

(1) Park, Y.-S.; Dmytruk, A.; Dmitruk, I.; Kasuya, A.; Okamoto, Y.; Kaji, N.; Tokeshi, M.; Baba, Y. Aqueous Phase Synthesized CdSe Nanoparticles with Well-Defined Numbers of Constituent Atoms. J. Phys. Chem. C 2010, 114, 18834−18840. (2) Rayevska, O. E.; Grodzyuk, G. Y.; Dzhagan, V. M.; Stroyuk, O. L.; Kuchmiy, S. Y.; Plyusnin, V. F.; Grivin, V. P.; Valakh, M. Y. Synthesis and Characterization of White-Emitting CdS Quantum Dots Stabilized with Polyethylenimine. J. Phys. Chem. C 2010, 114, 22478−22486. (3) Gallagher, S. A.; Moloney, M. P.; Wojdyla, M.; Quinn, S. J.; Kelly, J. M.; Gun’ko, Y. K. Synthesis and Spectroscopic Studies of Chiral CdSe Quantum Dots. J. Mater. Chem. 2010, 20, 8350−8355. 19496



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(23) Xu, W.; Liao, Y.; Akins, D. L. Formation of CdS Nanoparticles within Modified MCM-41 and SBA-15. J. Phys. Chem. B 2002, 106, 11127−11131. (24) Zeiri, L.; Patla, I.; Acharya, S.; Golan, Y.; Efrima, S. J. Raman Spectroscopy of Ulranarrow CdS Nanostructures. J. Phys. Chem. C 2007, 111, 11843−11848. (25) Levichev, S.; Rolo, A. G.; Chahboun, A.; Conde, O.; Kovalev, A. I.; Wainstein, D. L.; Gomes, M. J. M. Confinement Effect in CdTe Nanocrystals Embedded in Silica Thin Films. Phys. Status Solidi A 2008, 205, 1500−1504. (26) Murray, C. B.; Noms, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706− 8715. (27) Vasilevskiy, M. I.; Rolo, A. G.; Gomes, M. J. M.; Vikhrova, O. V.; Ricolleau, C. Impact of Disorder on Optical Phonons Confined in CdS Nano-crystallites Embedded in a SiO2 Matrix. J. Phys.: Condens. Matter 2001, 13, 3491−3509. (28) Kumpf, C.; Neder, R. B.; Niederdraenk, F.; Luczak, P.; Stahl, A.; Scheuermann, M.; Joshi, S.; Kulkarni, S. K.; Barglik-Chory, C.; Heske, C.; Umbach, E. Structure Determination of CdS and ZnS Nanoparticles: Direct Modeling of Synchrotron-Radiation Diffraction Data. J. Chem. Phys. 2005, 123, 224707. (29) Mlayah, A.; Brugman, A. M.; Carles, R.; Renucci, J. B.; Valakh, M. Ya.; Pogorelov, A. V. Surface phonons and alloying effects in (CdS)x(CdSe)1‑x nanospheres. Solid State Commun. 1994, 90, 567− 570. (30) Milekhin, A.; Friedrich, M.; Zahn, D. R. T.; Sveshnikova, L.; Repinsky, S. Optical Investigation of CdS Quantum Dots in LangmuirBlodgett Films. Appl. Phys. A: Mater. Sci. Process. 1999, 69, 97−100. (31) Hart, T. R.; Aggarwal, R. L.; Lax, B. Temperature Dependence of Raman Scattering in Silicon. Phys. Rev. B 1970, 1, 638−641. (32) Freitas Neto, E. S.; Dantas, N. O.; da Silva, S. W.; Morais, P. C.; Pereira-da-Silva, M. A.; Moreno, A. J. D.; López-Richard, V.; Marques, G. E.; Trallero-Giner, C. Temperature-Dependent Raman Study of Thermal Parameters in CdS Quantum Dots. Nanotechnology 2012, 23, 125701−1−8. (33) Kilina, S.; Ivanov, S.; Tretiak, S. Effect of Surface Ligands on Optical and Electronic Spectra of Semiconductor Nanoclusters. J. Am. Chem. Soc. 2009, 131, 7717−26. (34) Crowe, I. F.; Halsall, M. P.; Hulko, O.; Knights, A. P.; Gwilliam, R. M.; Wojdak, M.; Kenyon, A. J. Probing the Phonon Confinement in Ultrasmall Silicon Nanocrystals Reveals a Size-Dependent Surface Energy. J. Appl. Phys. 2011, 109, 083534−1−8. (35) Wu, P.-J.; Stetsko, Y. P.; Tsuei, K.-D.; Dronyak, R.; Liang, K. S. Size Dependence of Tetrahedral Bond Lengths in CdSe Nanocrystals. Appl. Phys. Lett. 2007, 90, 161911−1−3. (36) Dzhagan, V. M.; Lokteva, I.; Valakh, M. Y.; Raevska, O. E.; Kolny-Olesiak, J.; Zahn, D. R. T. Spectral Features above LO Phonon Frequency in Resonant Raman Scattering Spectra of Small CdSe Nanoparticles. J. Appl. Phys. 2009, 106, 084318−1−6. (37) Lin, C.; Kelley, D. F.; Rico, M.; Kelley, A. M. The “Surface Optical” Phonon in CdSe Nanocrystals. ACS Nano 2014, 8, 3928− 3938. (38) Zhang, Q.; Zhang, J.; Utama, M.; Peng, B.; de la Mata, M.; Arbiol, J.; Xiong, Q. Exciton-Phonon Coupling in Individual ZnTe Nanorods Studied by Resonant Raman Spectroscopy. Phys. Rev. B 2012, 85, 085418. (39) Freitas Neto, E. S.; Silva, A. C. A.; da Silva, S. W.; Morais, P. C.; Gómez, J. A.; Baffa, O.; Dantas, N. O. Raman Spectroscopy of Very Small Cd1‑xCoxS Quantum Dots Grown by a Novel Protocol: Direct Observation of Acoustic-Optical Phonon Coupling. J. Raman Spectrosc. 2013, 44, 1022−1032. (40) Klein, M. C.; Hache, F.; Ricard, D.; Flytzanis, C. Size Dependence of Electron-Phonon Coupling in Semiconductor Nanospheres: The Case of CdSe. Phys. Rev. B 1990, 42, 11123. (41) Shiang, J. J.; Risbud, S. H.; Alivisatos, A. P. Resonance Raman Studies of the Ground and Lowest Electronic Excited State in CdS Nanocrystals. J. Chem. Phys. 1993, 98, 8432−8442.

(42) Scamarcio, G.; Spagnolo, V.; Ventruti, G.; Lugará, M.; Righini, G. C. Size Dependence of Electron-LO-Phonon Coupling in Semiconductor Nanocrystals. Phys. Rev. B 1996, 53, R10489(R). (43) Vasiliev, R. B.; Vinogradov, V. S.; Dorofeev, S. G.; Kozyrev, S. P.; Kucherenko, I. V.; Novikova, N. N. IR-Active Vibrational Modes of CdTe and CdSe Colloidal Quantum Dots and CdTe/CdSe Core/ Shell Nanoparticles. Phys. Solid State 2007, 49, 547−551. (44) Manciu, F. S.; Sahoo, Y.; Macrae, D. J.; Furis, M.; Mccombe, B. D. Optical Phonon Spectra of GaP Nanoparticles Prepared by Nanochemistry. Appl. Phys. Lett. 2012, 82, 4059−4061. (45) Vasilevskiy, M. I.; Rolo, A. G.; Artemyev, M. V.; Filonovich, S. A.; Gomes, M. J. M.; Rakovich, Y. P. FIR Absorption in CdSe Quantum Dot Ensembles. Phys. Status Solidi B 2001, 224, 599−604.

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Raman and Infrared Phonon Spectra of Ultrasmall Colloidal CdS

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