Optical Properties of Zincblende Cadmium Selenide Quantum Dots

Mar 10, 2010 - Cadmium carboxylates (cadmium to carboxylic acid ratio 1:3) were ... After the reactions were performed, typically about 10 mL of tolue...
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J. Phys. Chem. C 2010, 114, 6371–6376

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Optical Properties of Zincblende Cadmium Selenide Quantum Dots ˇ apek,† Iwan Moreels,† Karel Lambert,† David De Muynck,‡ Qiang Zhao,§ Richard Karel C Andre´ Van Tomme,§ Frank Vanhaecke,‡ and Zeger Hens*,† Physics and Chemistry of Nanostructures, Ghent UniVersity, Krijgslaan 281-S3, B-9000 Gent, Belgium, and Department of Analytical Chemistry, Ghent UniVersity, Krijgslaan 281-S12, B-9000 Gent, Belgium, and Instituut Voor Kern-en Stralingsfysica and INPAC, K.U.LeuVen, Celestijnenlaan 200D, B-3001 LeuVen, Belgium ReceiVed: January 8, 2010; ReVised Manuscript ReceiVed: March 2, 2010

Although wurtzite cadmium selenide quantum dots (wz-CdSe QDs) are one of the best explored colloidal nanomaterials, no detailed investigation of the optical properties of zincblende cadmium selenide quantum dots (zb-CdSe QDs) has been performed until now. Typically, it is assumed that this material shows the same behavior as the wurtzite modification. To investigate this, we present a study on the optical properties of zb-CdSe QDs, yielding the electronic band gap to size relation (sizing curve), the extinction coefficient at short wavelengths, and the oscillator strength of the band gap transition. Comparing these results with literature data on wz-CdSe QDs we observe, despite a deviation of the sizing curve for diameters above 4 nm, a similar extinction coefficient at short wavelengths and a similar oscillator strength. Introduction Semiconductor nanoparticles are a relatively young class of fluorescent dyes with a tunable band gap due to the size quantization effect.1 Among the different materials synthesized, especially wurtzite cadmium selenide quantum dots (wz-CdSe QDs) came into the focus of recent research because of their good accessibility by colloidal synthesis and their outstanding properties as light emitters and harvesters. Therefore, they have been used as bare or as coated QDs in various applications like, e.g., biolabeling, LEDs, and solar cells.2-5 More recently, syntheses of CdSe QDs with a zincblende (zb) structure were reported as well.6-8 In the particular size range regarded in literature, these QDs show a similar band gap to size relation (sizing curve) as wz-CdSe QDs.6,7 Consequently, the sizing curve and the extinction coefficients valid for wz-CdSe QDs have also been used for the characterization of zb-CdSe QDs by absorption spectroscopy.9 Although an argument for this procedure could be found in the similarity of the bulk bandgap, the bulk effective masses, and the bulk optical constants,10,11 this approach is still questionable due to the differences in energy band structure of both modifications close to the Γ-point. More in particular, while zb-CdSe has a 4-fold degenerate valence band level in the Γ point, the valence band is split into two 2-fold degenerate levels by the crystal field in wz-CdSe.12 Upon reduction of the crystal size into the strong confinement regime, this implies that hole states in zb-CdSe and wz-CdSe are described by a different Hamiltonian.13 Early studies led to the conclusion that this only leads to minor differences in the sizing curve,13,14 yet more recent theoretical work on III-V nitrides indicates that wurtzite and zincblende polymorphs can show a different sizing curve, with a more gentle reduction of the band gap with increasing size in the case of wurtzite.15 * To whom correspondence should be addressed. Phone: 0032-92644863. Fax: 0032-9-2644983. E-mail: [email protected]. † Physics and Chemistry of Nanostructures, Ghent University. ‡ Department of Analytical Chemistry, Ghent University. § Instituut voor Kern-en Stralingsfysica and INPAC.

In this work, we present the sizing curve, the molar extinction coefficient at short wavelengths, and the oscillator strength at the bandgap transition for zb-CdSe QDs as determined by combining transmission electron microscopy, elemental analysis, and UV-vis spectroscopy. The resulting data are used for a comparison with literature data of wz-CdSe QDs,16-18 leading to the conclusion that the differences between both polymorphs are small indeed, except for a steeper drop in the band gap energy with increasing size for zb-CdSe QDs larger than 4 nm. Experimental Section Chemicals. Methanol and 2-propanol were purchased from VWR BDH Prolabo and were of Rectapur grade. Toluene and chloroforme were also purchased from VWR BDH Prolabo and were of technical and Normapur grade, respectively. Oleic acid (90%) and cadmium oxide (CdO; 99.99+%) were purchased from Aldrich. Selenium (99.999%) and 1-octadecene (ODE; technical) were purchased from Alfa Aesar. Behenic acid (98%) was purchased from Fluka. Hexadecylamine (HDA, 90%) was purchased from Merck. Precursor Preparation. Cadmium carboxylates (cadmium to carboxylic acid ratio 1:3) were prepared by mixing CdO and the particular carboxylic acid in a 1:3 molar ratio, degassing for 1 h at 100 °C under a nitrogen flow, followed by dissolving the cadmium oxide under a nitrogen atmosphere between 250 and 300 °C until the mixture became clear. ODE-Se was typically prepared by dissolving 2 mmol of selenium in 20 mL of ODE at temperatures of about 195 °C and reaction times between 2 and 4 h. The reaction time was varied in order to tune the ODE-Se reactivity. Synthesis of zb-CdSe QDs. Three different procedures were used to prepare zb-CdSe QDs. For particles with a diameter of 2.2 nm and above, recipes similar to that of Jasieniak et al.8 have been used (procedures A and B). The synthesis of smaller zb-CdSe is based on the prefocused approach that we published earlier (procedure C).19 Procedure A: in a 25 mL four-necked flask, 0.3 mmol of the cadmium behenate precursor (quantity calculated on cadmium), 0.9 mmol of behenic acid, and 10 mL of ODE were degassed

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under a nitrogen flow for 1 h at room temperature and 1 h at 100 °C. The nitrogen flow was stopped, the temperature was raised to 260 °C keeping the mixture under nitrogen atmosphere, and 3 mL of an 0.1 M ODE-Se solution was injected. Then the temperature was reduced to 235 °C. For short reaction times, the reaction was quenched by the injection of 10 mL of ODE. Otherwise, the reaction flask was cooled in a water bath. The product was dissolved in a mixture of toluene and oleic acid (5:1) and precipitated with methanol. This procedure was repeated, until the product became soluble in pure toluene. Afterward, the QDs in toluene were precipitated with methanol and redispersed in toluene at least two more times. The QD size was controlled by the reaction time and by use of differently reactive ODESe precursors. Procedure B: A sample of 0.36 mmol of CdO, 3.6 mmol of oleic acid, and 12 mL of ODE were mixed and degassed for 1 h at 100 °C under a nitrogen flow, followed by heating to 250 °C until all CdO dissolved under nitrogen. Then the solution was heated to 265 °C and 3.6 mL of an 0.1 M ODE-Se solution was injected. The temperature of the mixture dropped after injection and the reaction continued at 235 °C. The product was dissolved in toluene and precipitated with a 1:1 mixture of 2-propanol and methanol. The precipitate was dissolved in toluene, precipitated, and redissoved in toluene. The QD size was controlled by the reaction time. Procedure C: In a 25 mL 4-necked flask, 0.2 mmol of the cadmium oleate precursor (calculated on cadmium), 1.2 mmol of HDA, and 10 mL of ODE were degassed for 1 h at room temperature and 1 h at 100 °C under a nitrogen flow. Still under a nitrogen flow, the temperature was raised to the injection temperature and 2 mL of a solution of 1 M TOPSe in TOP (2 mmol selenium dissolved in 2 mL of TOP) was injected. Depending on the injection temperature, the reaction was stopped after 5 s by injection of 10 mL of a certain solvent (230 °C/ODE; 230 °C/toluene; 210 °C/heptane; 190 °C/hexane; 170 °C/hexane). After the reactions were performed, typically about 10 mL of toluene was added to the raw material and the nanoparticles were precipitated by adding a 1:1 to 1:2 mixture of 2-propanol and methanol. The precipitate was separated by centrifugation and redissolved in hexane.19 Materials Characterization. UV-vis absorption spectra were recorded with a Perkin-Elmer λ-950 spectrometer. X-ray powder diffraction (XRD) patterns were measured with a Bruker D8 Discover. Transmission electron microscopy (TEM) images were recorded with a Cs-corrected JEOL 2200 FS electron microscope. Inductively coupled plasma-mass spectrometry (ICP-MS) analysis was performed with a PerkinElmer SCIEX Elan 5000 inductively coupled plasma mass spectrometer. Rutherford backscattering spectrometry (RBS) was performed by measuring backscattered He+ ions accelerated to an energy of 1.57 MeV with an NEC 5SDH-2 Pelletron tandem accelerator with a semiconductor detector at a backscattering angle of 168°. Results and Discussion Structural Properties. For this investigation, nanoparticles with a close to isotropic shape, a narrow size dispersion, and a clear zb modification are required. In Figure 1A-C, TEM images of representative CdSe QDs, which were made according to procedure A, are shown. Only for the larger nanoparticles is a slight shape anisotropy observed. The XRD patterns (Figure 1D) demonstrate that all QDs possess the zincblende modification. As shown in the literature, the QDs obtained by procedures B and C meet the same standards.8,19

Figure 1. (A-C) TEM images of zb-CdSe QDs with a mean diameter of 4.9 (a), 3.5 (b), and 2.2 nm (c). (D) XRD patterns of zb-CdSe QDs (bars indicate bulk data).

Sizing Curve. Figure 2 shows representative spectra of zbCdSe QDs, prepared according to procedures A (Figure 2A) and B and C (Figure 2B).8,19 Syntheses A and B give access to a similar range of peak wavelengths of the first transition (λ1s-1s), from about 480 nm to above 630 nm. However, for QD solutions made according to procedure A, the first transition peak typically has a half-width at half-maximum (hwhm) of 11-14 nm (size dispersion 4.5-6%) while hwhms between 15 and 18 nm are obtained by procedure B (size dispersion 6.5-8%). Therefore, we mainly used procedure A in this study, while zb-CdSe QDs made according to procedure B were added to show the consistency of the data. Finally, procedure C was used to shift λ1s-1s further down to 420 nm (size dispersion 7-10%). The band gap energy as a function of the QD diameter is displayed in Figure 2C. Clearly, the three different procedures yield zb-CdSe QDs that fall on a single sizing curve. The black line represents a best fit to the experimental data. With the diameter d (in nanometers), this yields the bandgap energy Eg (in eV) according to

Eg ) 1.74 +

1 0.89 - 0.36d + 0.22d2

(1)

The gray lines in Figure 2C correspond to the sizing curves presented by Yu et al.17 and Donega et al.18 for wz-CdSe QDs. The sizing curve for zb-CdSe is in good agreement with these wz-CdSe curves for QD diameters up to 3.8 nm, yet significant deviations are observed between both modifications for diameters above 4 nm. For example, for 5 nm zb-CdSe QDs, a diameter of 6.4 nm would be expected according to the wzCdSe sizing curves. This corresponds to about two monolayers of cadmium selenide (lattice spacing in the 111-direction in zbCdSe 0.35 nm, respectively, and in the 100-direction in wzCdSe 0.37 nm). Interestingly, this sharper drop of the zincblende sizing curve is in line with calculations on the band gap of zincblende and wurtzite III-V QDs.15 The Intrinsic Absorption Coefficient. Besides the mean size, also the concentration of semiconductor material or nanoparticles in solution can be determined from the absorption spectra by using the intrinsic absorption coefficient (µi) or the molar extinction coefficient (ε) at short wavelengths, respectively. For

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Figure 3. Intrinsic absorption coefficient spectra of differently sized zb-CdSe QDs in chloroform (colored lines): black line, bulk intrinsic absorption coefficient spectrum of zb-CdSe in chloroform. Inset: Zoom of part A.

with MCdSe and MCd the molar mass of cadmium selenide and cadmium, respectively, and FCdSe the density of zb-CdSe. The experimental µi can be compared to a theoretical value µi,th calculated by using the bulk optical constants of the semiconductor:23-25

µi,th )

4πnk|fLF | 2 nsλ

(4)

Here, n and k stand for the real and imaginary part of the refractive index of bulk zb-CdSe,10 ns is the refractive index of the surrounding medium,26,27 λ is the observation wavelength, and fLF denotes the local field factor. The latter represents the ratio between the light intensity inside the QDs and in the medium around the QDs and is given by:25 Figure 2. (A) Typical absorption spectra of zb-CdSe QDs made according to procedure A. (B) Typical absorption spectra of zb-CdSe QDs made according to procedures B (upper three spectra) and C (lower three spectra). (C) Sizing curve of zb-CdSe QDs: open squares, CdSe samples made according to procedure C; filled circles, CdSe-QDs made according to procedure A; and open circles, CdSe-QDs made according to procedure B. A best fit and wz-CdSe sizing curves according to Yu17 and Donega´18 have been added.

wz-CdSe,16,17 InAs,20 PbSe,21 and PbS QDs,22 both quantities have already been related to bulk values. Experimentally, µi can be derived from the absorbance (A) of the sample and the volume fraction of semiconductor material in the solution (f), taking the path length of the light through the sample (L) into account:

µi )

A ln(10) fL

(2)

For this study, the volume fraction was determined from the cadmium weight concentration (cCd), determined by ICP-MS, and the cadmium-to-selenium ratio (RCd/Se, see the Supporting Information 1), determined by RBS:

f ) cCd

(

MCdSe 1 1 1+ MCd 2FCdSe RCd/Se

)

(3)

|fLF | ) 2

9ns4 (n2 - k2 + 2ns2)2 + 4(nk)2

(5)

Equation 4 does not take the refractive index of the ligand shell into account. Therefore it applies in principle to a situation where the refractive index of the ligand shell and the solvent match. Taking the preparation of the QDs used here into account, we can safely assume that they have oleic acid ligands. As the refractive index of oleic acid almost matches that of chloroform (nd20chloroforme) 1.445; nd20oleic acid) 1.459), the latter was used as a solvent for the quantitative analysis and the comparison with theoretical values. Figure 3 shows the spectrum of the intrinsic absorption coefficient determined for six differently sized zb-CdSe QD samples in chloroform, together with the theoretical value calculated following eqs 4 and 5. For the largest QDs (λ1s-1s ≈ 600 nm) the spectra almost coincide with the theoretical bulk spectrum at wavelengths shorter than 400 nm. On the other hand, the µi spectrum of the smallest QDs (λ1s-1s ≈ 470 nm) shows features down to 300 nm. Since these are not present in the bulk spectrum, this indicates that confinement effects are still present in this case. For all samples, an increasing deviation from the bulk behavior is observed at wavelengths shorter than 300 nm. In Figure 4A the measured absorption coefficients at wavelengths of 300, 320, and 340 nm are shown and compared to the µi,th values and the respective mean values µi,mean. These

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Figure 4. (A) Intrinsic absorption coefficient µi at 300, 320, and 340 nm of differently sized nanoparticles in chloroform (dots), together with the theoretical value µth (full lines) and the mean values µi,mean at the respective wavelengths (dashed lines). The typical measurement error on µi amounts to 5% to 10%. (B) Comparison of µi values of zb-CdSe and wz-CdSe QDs. Experimental points are indicated with dots, µi,th values with full lines, and µi,mean values with dashed lines. Experimental wurtzite values are taken from Jasieniak et al.,28 µi,th is calculated according to the n and k values given by Adachi10 using chloroform as the solvent. All data at 350 nm.

TABLE 1: µi,th and µi,mean Value for zb-CdSe QDs in Chloroform λ (nm)

µi,th (cm-1)

µi,mean (cm-1)

340 320 300

150 100 188 400 257 900

141 100 173 600 231 500

latter have been determined from the µi values of particles with a diameter above 2.8 nm, since for smaller QDs still additional features are present in the absorption spectra in this wavelength range. Depending on the particular wavelength, µi,mean differs from µi,th by 5% to 10%. Taking into account that the bulk absorption data from literature may also contain experimental errors, we recommend the use of the µi,mean, given in Figure 4 and summarized in Table 1, for the determination of the concentration of zb-CdSe QDs in chloroform. Entering the refractive index of the solvent as a value for ns in eq 5, one finds that a reduction of µi by about 10% is predicted upon substituting hexane for chloroform as a solvent. However, similar to the investigation of Leatherdale et al.16 and Jasieniak et al.,28 we observe that the intrinsic absorption coefficients in hexane almost match those in chloroform. Although the hexane values are smaller by 2% to 3%, the deviations fall within the precision of the measurements (see the Supporting Information). This shows that eq 4 should not be applied directly if the refractive indices of the solvent and the ligand shell are considerably different. Following Leatherdale et al.,16 this insensitivity of µi to the solvent is probably due to the ligand shell that surrounds the CdSe quantum dot. As no difference is observed between data collected in chloroform and in hexane, the values given in Table 1 can be used for both solvents. However, as the solvent dependence of µi remains poorly understood for larger refractive index solvents, we insist that these data should not be extended to other solvents without proper calibration. Comparison of the Intrinsic Absorption Coefficient of wzand zb-CdSe QDs. In Figure 4B, µi values for CdSe-QDs at λ1s-1s ) 350 nm are shown. To avoid deviations due to confinement effects at this particular wavelength, only QDs with a diameter larger than 2.8 nm are considered. For wz-CdSe QDs, the µi values are in almost perfect agreement with the bulk values according to Adachi,10 while a deviation of about 7% is found for the zb-CdSe QDs. The difference between the experimental data for wz- and zb-QDs is only in the range of

Figure 5. Molar extinction coefficient spectra of differently sized zbCdSe QDs. Inset: Development of the molar extinction coefficient at 340 nm of differently sized zb-CdSe-QDs. The trend line shows the best fit of the data to a d3 power law. Error bars of (10% have been added.

4%. This means that within experimental error, no distinction can be made between both modifications. The Molar Extinction Coefficient at Short Wavelengths. For QDs with a given diameter d, the molar extinction coefficient ε can be directly calculated from µi (NA: Avogadro’s number):

ε)

πd3NA µ 6 ln(10) i

(6)

Figure 5A shows the resulting molar extinction coefficient spectra of different colloidal QD solutions. At 340 nm, ε increases with almost the third power of the diameter of the QDs (inset of Figure 5), obviously confirming the fact that an almost size independent intrinsic absorption coefficient was found at this wavelength. Therefore a determination of the QD concentration is possible in the wavelength range 300-340 nm, using molar extinction coefficients derived from eq 6 and Table 1. The molar extinction coefficient (in cm-1 mol-1 L) can be determined by use of the expression

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TABLE 2: Values for Determination of the Molar Extinction Coefficients Based on the µi,th and Mean µi Value for zb-CdSe QDs in Chloroforma λ (nm)

ath

amean

340 320 300

20 550 25 710 35 040

19 300 23 800 31 700

a With the diameter in nm, these values yield the molar extinction coefficient in cm-1 mol-1 L.

below, with a the constant given in Table 2 and d the QD diameter (in nm):

ε ) ad3

(7)

Comparison of the Oscillator Strength of zb- and wz-CdSe QDs. At the first exciton transition, the extinction coefficient of a QD ensemble is strongly affected by the size dispersion. Therefore, many authors use an integrated or a normalized molar extinction coefficient εgap to determine concentrations from the absorbance at the band gap transition. Here, we define εgap as the integral on an energy scale of the first electronic transition in the molar extinction coefficient spectrum. Often, it is calculated by doubling the integral of the low energy half of the absorbance peak.21,22,29

εgap ) 2

∫0E

1s-1s

ε(E) dE

(8)

Like µi, εgap is solvent dependent. Therefore, the oscillator strength fif is a more suitable quantity for characterizing the first exciton transition and for comparing zb- and wz-CdSe QDs as it is an intrinsic QD property. It can be determined from the integrated extinction coefficient εgap, estimating fLF by taking the refractive index n of CdSe at the bulk band gap and assuming k , n:21,22

fif )

2 ln(10) meε0nsc ε π epN |f | 2 gap

(9)

A LF

Here, ε0 is the permittivity of the vacuum, c the velocity of light, and p the Planck constant. Figure 6 shows the resulting values for the oscillator strength of zb-CdSe QDs as a function of size. We see that fif slightly decreases with decreasing size, from a value of 6 for 4 nm QDs to a value of 4 for 2 nm QDs. Comparing these results with the fif of wz-CdSe-QDs calculated from the literature data of Jasieniak et al.,28 we see, as for the short wavelength extinction coefficient, no significant differences. Conclusions We present an experimental analysis of the optical properties of zb-CdSe QDs, providing the sizing curve, the intrinsic absorption coefficient at short wavelengths, and the oscillator strength of the first exciton transition. The sizing curve of zbCdSe coincides with the published curves for wz-CdSe for particle diameters below ∼4 nm. For larger particles a significant deviation exists, namely a weaker confinement for zb-CdSe QDs in comparison to wz-CdSe QDs. The intrinsic absorption coefficients at short wavelengths (300-340 nm) of zb-CdSe QDs are almost size independent, although for diameters smaller

Figure 6. Oscillator strength of the band gap transition for zb-CdSe quantum dots (black dots) and wz-CdSe QDs (gray dots), calculated according the data of Jasieniak et al.28 The error bars indicate an estimated error of (10%.

than 2.8 nm confinement effects persist down to 300 nm. Overall, the mean µi value is about 7% smaller than that expected for bulk zb-CdSe and coincides within experimental errors with experimental literature values and calculated bulk values of µi for wz-CdSe. On the basis of the size-independent average absorption coefficients, we provide experimental extinction coefficients. Finally, the oscillator strength of the band gap transition was determined by using the integrated extinction coefficient. Again, a close correspondence between the values for zb-CdSe QDs determined here with literature data on wzCdSe is found. ˇ . and Z.H. acknowledge BelSPO (IAP Acknowledgment. R.C VI.10, photonics@be) and the FWO-Vlaanderen (G.0.144.08). Z.H and AVT acknowledge the IWT (SBO-Metacel). I.M. is a postdoctoral fellow with the FWO-Vlaanderen. Supporting Information Available: Additional data on the stoichiometry of zb-CdSe quantum dots and a comparison of absorption coefficients in chloroform and hexane. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (2) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 12142. (3) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (4) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (5) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (6) Mohamed, M. B.; Tonti, D.; Al-Salman, A.; Chemseddine, A.; Chergui, M. J. Phys. Chem. B 2005, 109, 10533. (7) Yang, Y. A.; Wu, H. M.; Williams, K. R.; Cao, Y. C. Angew. Chem., Int. Ed. 2005, 44, 6712. (8) Jasieniak, J.; Bullen, C.; van Embden, J.; Mulvaney, P. J. Phys. Chem. B 2005, 109, 20665. (9) Jasieniak, J.; Mulvaney, P. J. Am. Chem. Soc. 2007, 129, 2841. (10) Adachi, S. Optical Constants of Crystalline and Amorphous Semiconductors; Kluwer: Norwell, MA, 1999. (11) Ro¨ssler, U., Ed. Semiconductors: II-VI and I-VII Compounds; Semimagnetic Compounds; Landolt-Bo¨rnstein, New Series Vol. III/41B; Springer: Berlin, Germany, 1999. (12) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Phys. ReV. B 1996, 54, 4843. (13) Zorman, B.; Ramakrishna, M. V.; Friesner, R. A. J. Phys. Chem. 1995, 99, 7649. (14) vonGrunberg, H. H. Phys. ReV. B 1997, 55, 2293. (15) Bagga, A.; Chattopadhyay, P. K.; Ghosh, S. Phys. ReV. B 2003, 68, 155331.

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(16) Leatherdale, C. A.; Woo, W. K.; Mikulec, F. V.; Bawendi, M. G. J. Phys. Chem. B 2002, 106, 7619. (17) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Chem. Mater. 2003, 15, 2854. (18) Donega, C. D.; Koole, R. J. Phys. Chem. C 2009, 113, 6511. (19) Capek, R. K.; Lambert, K.; Dorfs, D.; Smet, P. F.; Poelman, D.; Eychmuller, A.; Hens, Z. Chem. Mater. 2009, 21, 7. (20) Yu, P. R.; Beard, M. C.; Ellingson, R. J.; Ferrere, S.; Curtis, C.; Drexler, J.; Luiszer, F.; Nozik, A. J. J. Phys. Chem. B 2005, 109, 7084. (21) Moreels, I.; Lambert, K.; De Muynck, D.; Vanhaecke, F.; Poelman, D.; Martins, J. C.; Allan, G.; Hens, Z. Chem. Mater. 2007, 19, 6101. (22) Moreels, I.; Lambert, K.; Smeets, D.; De Muynk, D.; Mollet, T.; Martins, J. C.; Vanhacke, F.; Vantomme, A.; Delerue, C.; Allan, G.; Hens, Z. ACS Nano 2009, 3, 3023.

(23) Maxwell Garnett, J. C. Philos. Trans. R. Soc. London., Ser. A 1904, 203, 385. (24) Sihvola, A. Journal of Electromagnetic WaVes and Applications 2001, 15, 715. (25) Ricard, D.; Ghanassi, M.; Schanneklein, M. C. Opt. Commun. 1994, 108, 311. (26) Samoc, A. J. Appl. Phys. 2003, 94, 6167. (27) Kerl, K.; Varchmin, H. J. Mol. Struct. 1995, 349, 257. (28) Jasieniak, J.; Smith, L.; van Embden, J.; Mulvaney, P. J. Phys. Chem. C 2009, 113, 19468. (29) Cademartiri, L.; Montanari, E.; Calestani, G.; Migliori, A.; Guagliardi, A.; Ozin, G. A. J. Am. Chem. Soc. 2006, 128, 10337.

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