Size-Dependent Absolute Quantum Yields for Size-Separated

Letter pubs.acs.org/NanoLett

Size-Dependent Absolute Quantum Yields for Size-Separated Colloidally-Stable Silicon Nanocrystals Melanie L. Mastronardi,† Florian Maier-Flaig,‡ Daniel Faulkner,†,§ Eric J. Henderson,† Christian Kübel,∥,⊥ Uli Lemmer,‡ and Geoffrey A. Ozin*,† †

Materials Chemistry and Nanochemistry Research Group, Center for Inorganic and Polymeric Nanomaterials, Chemistry Department, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S3H6 ‡ Light Technology Institute, Karlsruhe Institute of Technology, 76133 Karlsruhe, Germany § Department of Materials Science and Engineering, Faculty of Applied Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada, M5S3E4 ∥ Institute of Nanotechnology and ⊥Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1−5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of “green” silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts. KEYWORDS: Silicon nanocrystals, size-selective precipitation, size-dependent absolute quantum yield, photoluminescence, lifetime

T

attention toward developing alternative materials such as silicon nanocrystals (ncSi), which were found to exhibit intense room temperature photoluminescence in 1990.12 Many methods were subsequently developed for the synthesis of ncSi including the annealing of SiOx powders followed by etching with HF,13 plasma synthesis,14 solution reduction of SiCl4,15 and electrochemical dispersion of bulk silicon.16 However, most ncSi synthetic methods produce samples with considerable size polydispersity. The measurement of size-dependent properties, such as AQY, requires monodispersions rather than polydispersions of colloidally stable nanocrystals, since nonnegligible nanocrystal size distributions lead to inhomogeneous broadening of spectral features and average effects in measured properties. The previously observed photoluminescence AQYs of ncSi apply to polydispersions and therefore refer to an ensemble average for the nanocrystal size distribution.9 In order to reliably establish the existence and understand the nature of quantum size effects on the photoluminescence efficiency, monodisperse ncSi fractions must be studied over a broad range and below the exciton size.

he size-dependent optical and electronic properties associated with semiconductor nanocrystals or quantum dots make them useful materials for tailoring optoelectronic and biomedical devices with specific optical, electrical, and biological requirements. Particularly, the quantum and spatial confinement effects that result in a widening of the electronic bandgap and a blue shift in photoluminescence as the particle size decreases has allowed for the archetypical (II−VI) and (IV−VI) nanocrystals, such as CdSe and PbS, to show great promise in various optoelectronic and biomedical devices.1−4 Well-documented scaling laws have been reported for the properties of semiconductor nanocrystals (e.g., electronic bandgaps, extinction coefficients, melting points) whose physical dimensions are small compared to their exciton size.5−7 Size-dependent trends have also been observed for the photoluminescence absolute quantum yields (AQY) of semiconductor nanocrystals.8,9 The AQY for photoluminescence of semiconductor nanocrystals, defined as the ratio of the number of emitted photons to the number of absorbed photons, determines the performance of nanocrystals in light-emitting diodes (LEDs) and lasers and their usefulness in biomedical diagnostics, therapeutics and imaging. A major concern with the archetypal semiconductor materials, however, is their cytotoxicity and effect on the environment.10,11 Therefore, research has begun to turn its © 2011 American Chemical Society

Received: October 13, 2011 Revised: December 11, 2011 Published: December 23, 2011 337

dx.doi.org/10.1021/nl2036194 | Nano Lett. 2012, 12, 337−342

Nano Letters

Letter

Various methods exist for the postsynthetic size-separation of different nanomaterials,17−21 and recently we reported the preparation of monodisperse ncSi fractions using density gradient ultracentrifugation.22 In this work, however, the simple and effective method of size-selective precipitation,13,23 proven to purify and narrow the size distribution of passivated ncSi, was used to prepare monodisperse, colloidally stable ncSi with dimensions that smoothly traverse the quantum size effect range. In this paper, for the first time we report the sizedependent photoluminescence AQY and radiative lifetimes for various monodisperse fractions of allylbenzene-capped ncSi. Allylbenzene-passivated silicon nanocrystals (AB-ncSi) were synthesized using a method previously developed by Henderson et al. with slight modifications (see the Supporting Information for detailed experimental methods).24 The hightemperature thermal processing of sol−gel hydridosilicate (HSiO1.5) glasses derived from trichlorosilane (HSiCl3), was used to initiate a reductive disproportionation nucleation and growth reaction, forming ncSi with a distribution of sizes within a SiO2 matrix. Etching with HF was used to liberate the ncSi from the encapsulating SiO2 matrix and form hydrideterminated ncSi. The hydrophobic hydride-terminated ncSi were extracted into mesitylene, a nonpolar high boiling organic solvent, followed by a thermally initiated hydrosilylation reaction to passivate the nanocrystals with allylbenzene. The ncSi that were too large to be stabilized with allylbenzene caps were removed from the reaction mixture by centrifugation and the colloidally stable AB-ncSi were isolated by vacuum distillation and redispersed in toluene. Extensive characterization indicates that an ensemble of spherical, colloidally stable, allylbenzene-capped ncSi with limited surface oxidation was synthesized with a broad distribution of sizes, similar to our previously reported work (see the Supporting Information for complete characterization).22 The ensemble’s photoluminescence (PL) spans the yellow to near-infrared region (Figure 1A) with two broad emission bands centered at ca. 670 and 985 nm, corresponding to the smaller visible-emitting and larger NIR-emitting ncSi, respectively. The excitation and UV−vis absorption spectrum both show a rising edge into the UV, with the absorption onset at ca. 700 nm (Figure 1A). Z-contrast high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of the AB-ncSi ensemble sample (Figure 1B) shows spherical particles with considerable polydispersity and was used to determine the ensemble’s measured mean diameter of 2.05 nm with a standard deviation of 0.77 nm (Supporting Information Figure S3). Highresolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) experiments (Figure 1B, insets) provide evidence that the sample contains some large crystalline particles but indicate that most of the smaller particles (1−3 nm) contained in the ensemble may be amorphous or nanocluster Si. This result is consistent with our previous report examining size-separated ncSi,22 and will be discussed in greater detail in the later sections of this work. To examine the size-dependence of the PL AQY, sizeselective precipitation was used to fractionate the as-prepared polydisperse AB-ncSi ensemble. Methanol was added to the clear ncSi dispersion as the antisolvent, causing the largest nanocrystals to aggregate and turning the solution cloudy. The aggregated large particles were then precipitated with centrifugation, while the medium and smaller sized ncSi remained in the supernatant. The collected precipitate was redissolved in toluene and the size-selective precipitation

Figure 1. Characterization of polydisperse allylbenzene-capped silicon nanocrystal (AB-ncSi) ensemble. (A) Normalized photoluminescence, excitation, and UV−vis absorption spectrum of AB-ncSi ensemble in toluene. (B) Z-contrast HAADF-STEM image of the ncSi ensemble shows particles varying in size from approximately 1−5 nm, verifying the polydispersity of the sample. Inset, selected-area electron diffraction (top) and high-resolution transmission electron microscopy (bottom) indicate that the ensemble AB-ncSi sample contains some larger crystalline particles.

process was repeated on the supernatant several times to isolate fractions of the medium and small ncSi. In this manner, the clear orange as-prepared AB-ncSi polydispersion, which showed orange-red PL under photoexcitation at 365 nm (Figure 2A), was separated into 18 fractions that spanned the NIR to yellow range. For the study of the size-dependent AQY and lifetimes, we examine only the 14 visible emitting fractions, labeled P1 through P14, that span the red to yellow region (Figure 2B). The PL of each fraction was measured at an excitation wavelength of 365 nm, and notably the PL maximum or center of all fractions shifted monotonically from a wavelength around 1000 to 600 nm (Figure 3). The PL blue shift observed with increasing fractionation steps is consistent with quantum size effects expected for ncSi that decrease in size. The shift in PL center is summarized for the visible emitting fractions in the inset of Figure 3. To confirm the success of the size-selective precipitation signaled by PL spectroscopy, HAADF-STEM, powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS) measurements were obtained for selected visible emitting fractions of AB-ncSi. Z-contrast HAADF-STEM images of fractions P2, P6, P9, and P12 (Figure 4) show a dramatic decrease in particle size with increasing fraction number, consistent with previous reports on size-selective precipitation separations of organically 338


Nano Letters

Letter

Figure 2. Allylbenzene-capped silicon nanocrystals (AB-ncSi). (A) Ensemble solution of polydisperse AB-ncSi under ambient light (left) and under photoexcitation at 365 nm (right). (B) Visible emitting AB-ncSi fractions obtained using size-selective precipitation under ambient light (top) and under photoexcitation at 365 nm (bottom).

diameters were 2.01, 1.61, 1.31, and 0.99 nm with standard deviations of 0.41, 0.26, 0.24, and 0.21 nm, respectively. To verify the crystallinity of the fractions, HRTEM and SAED experiments were performed. Only a small amount of crystalline particles were observed in HRTEM images of P2, the fraction containing the largest particles (Supporting Information Figure S4A), and no crystalline particles could be easily imaged in the subsequent fractions analyzed. Similarly, the SAED of P2 showed a weak crystalline silicon ring pattern, confirming that the sample contains some crystalline silicon (Supporting Information Figure S4B). However, in PXRD experiments performed to further confirm the size-separation, the signal from crystalline silicon was observed for fractions P1, P4, and P7, indicating that there are some crystalline particles contained in the fractions up to at least P7, even if there are not enough to easily locate and image using HRTEM. No usable PXRD signal could be detected for the fractions measured after P7, which along with the HRTEM and SAED results suggests that as the Si particle size decreases, so does the crystallinity. This may be inherent to the synthetic method used, as a crystalline lattice structure has been observed for ncSi particles as small as 1.1 nm,25 and the lack of crystallinity we observe for small Si particles is the topic of an ongoing investigation. PXRD spectra obtained for fractions P1, P4, and P7 show an increase in peak broadening with increasing fraction number, which confirms the decrease in size of the crystalline ncSi present in the fractions (Supporting Information Figure S5). Calculated Rietveld size approximations also fit relatively well with the mean sizes determined from HAADF-STEM images; the approximate sizes determined from PXRD for fractions P1, P4, and P7 are 1.9−2.5, 1.3−1.8, and 1.2−1.8 nm, respectively. The outcome of the fractionation of AB-ncSi was also confirmed by XPS. The Si 2p peak associated with the surface Si species dominates the spectra, while the core Si peak decreases in intensity as the fraction number increases (Supporting Information Figure S6), which is consistent with the increase in surface to volume ratio expected with decreasing particle size. With size-separation established and the mean particle sizes of select visible emitting AB-ncSi fractions determined from HAADF-STEM, the AQY was measured for each fraction. The integration sphere method, described by de Mello and coworkers,26 which quantitatively accounts for absorption and emission, reflection and scattering effects, was employed in this work (see Supporting Information section for detailed experimental methods). AQY measurements were performed on toluene solutions of AB-ncSi with a standard deviation of about 0.02%. The system was calibrated using a standard solution of Rhodamine 6G, the results of which showed good

Figure 3. AB-ncSi photoluminescence (PL) blue shift with decreasing size for the visible emitting fractions (solid lines) and the NIR emitting fractions (dashed lines). Inset shows summary of PL center versus fraction number for the visible emitting fractions P1−P14.

Figure 4. HAADF-STEM images of AB-ncSi fractions P2, P6, P9, and P12.

capped ncSi,13,23 indicating that the AB-ncSi ensemble was successfully separated into fractions of decreasing size. Particle size statistics were obtained for fractions P2, P6, P9, and P12 (Supporting Information Figure S3); the measured mean 339


Nano Letters

Letter

agreement with reported literature values, thereby confirming the accuracy of the AQY measurements recorded on size fractionated AB-ncSi solutions. The results show that the AQY of AB-ncSi smoothly decreases from a maximum of about 43% for fraction P1 containing the largest visible emitting fraction to a minimum value of around 5% for fraction P14 containing the smallest particles (Figure 5A). The AQY observed for the as-

Figure 6. Size-dependent lifetime measurements of AB-ncSi. (A) Integrated lifetime plotted against wavelength and energy. (B) Quantum yield plotted against wavelength and energy, as measured (black squares) and calculated using lifetime values (red circles).

To confirm the accuracy of the measured lifetime (τmeas) and quantum yield (QY) values, the following relationships were examined

Figure 5. Size-dependent photoluminescence AQY of AB-ncSi. (A) AQY plotted against fraction number. (B) AQY plotted against mean particle diameter, as determined from HAADF-STEM for fractions P2, P6, P9, and P12.

QY =

prepared ensemble of approximately 24% falls in the middle of this range, representing an average AQY value for the various sized particles contained in the polydisperse sample. The same monotonic decrease in AQY with decreasing particle size is observed when the AQY is plotted against mean particle size (Figure 5B) for P2, P6, P9, and P12. What is the origin of the monotonic decrease for the visible PL AQY as the size of the AB-ncSi decreases from around 2 to 1 nm? One might have expected the opposite trend where the probability of radiative relaxation in the nanometer space of the ligand-capped ncSi core should increase as size diminishes. To probe this trend, PL lifetimes were measured at the PL emission center for fractions P2, P6, P9, and P12 (see Supporting Information for further details). Since each fraction still contains a small size distribution, the lifetimes were also determined at additional wavelengths (all within ±10 nm of the fraction’s PL center) to complete the data set. The AB-ncSi exhibit measured lifetimes in the range of 20−60 μs, which is consistent with previously reported ncSi lifetimes.27,28 A decreasing monotonic trend with decreasing particle size was observed, as expected for an increased probability of recombination in Si cores with decreasing size (Figure 6A).27

with

τ kr = τmeask r = meas τr k r + k nr

(1)

1 k r + k nr

(2)

τmeas = τr + nr =

A constant value for the radiative lifetime (τr) of 160 μs was used to calculate QY values, which is comparable to radiative lifetimes measured for decyl-capped ncSi, similar to the AB-ncSi discussed here, at low temperature when the surface defect states are “frozen out” (Supporting Information Figure S7).29 These values fit well with the measured AQY results (Figure 6B). Equations 1 and 2 were also used to estimate the variation in radiative recombination rate (kr), nonradiative recombination rate (knr), radiative lifetime (τr), and nonradiative lifetime (τnr) with decreasing particle size. From eq 2, the measured lifetime (τmeas) is the inverse sum of kr and knr, and examining the contribution of each parameter as a function of size should identify the role these processes play in the decreasing AQY trend observed. Using the measured AQY and lifetime values, kr and knr, as well as τr and τnr, were determined at various wavelengths (Figure 7), showing significant separation between radiative and nonradiative components with decreasing size. The nonradiative decay constant is more than 1 order of 340


Nano Letters

Letter

efficiencies.8,31 Since with decreasing AB-ncSi size the surface to volume ratio increases, the contribution of higher frequency Si−C surface and C−C capping organic vibrational modes, relative to lower frequency nanocrystal core Si−Si vibrational modes, grow in importance and may eventually dominate for the smallest AB-ncSi. In nonradiative relaxation, energy transfer to surface ligands is expected to become more efficient with enhanced coupling between the frequency of vibrational modes and the electronically excited photogenerated state. Hence, this effect can progressively contribute to the decreasing AQY with diminishing AB-ncSi size. The competing nonradiative effect of a transition from the nanocrystalline Si to the nanoamorphous Si form with decreasing size (iii) would also be expected to contribute to the decrease observed in the AQY.32 HRTEM, SAED, and XRD measurements of the medium and small size fractions suggest that the gradual transition to a nanoamorphous phase could be contributing to the decreasing AQY. Although in this very small size regime a transformation from quantum size effect nanocrystal to molecular nanocluster33 cannot be discounted with the available data and will require ongoing investigations to clarify this interesting question. However, since there is no visible variation in the AQY and lifetime trends where the crystalline to amorphous transition seems to occur, it is unlikely that this effect contributes significantly to the decrease observed. While we are unable to definitively disentangle the contributions of opposing quantum and spatial confinement radiative relaxation effects relative to nonradiative pathways on the PL AQY and lifetimes of AB-ncSi without further investigation, the monotonically decreasing behavior with decreasing size can be satisfactorily reconciled based upon the above proposed considerations. It is extraordinary that the efficiency of visible PL for an indirect bandgap semiconductor can be as high as 43% for the larger size AB-ncSi fractions, an observation that speaks positively for the displacement of toxic heavy metal chalcogenide nanocrystals in optoelectronic devices by “green” silicon nanocrystals.

Figure 7. Calculated size-dependent lifetime and recombination rates of AB-ncSi. Radiative and nonradiative recombination rates plotted against wavelength and energy (red), and radiative and nonradiative lifetimes plotted against wavelength and energy (black).

magnitude larger than its radiative counterpart at the shortest wavelength of 600 nm, which corresponds to the smallest ncSi and a reduced AQY of about 5%. On the other side of the spectrum, at 700 nm nonradiative and radiative recombination rates are much closer in value, which corresponds to larger ncSi and an AQY value of 37%. The above-mentioned is also valid for the calculated lifetimes, at 600 nm the radiative lifetime is over 1 order of magnitude larger than its nonradiative counterpart, whereas at 700 nm both values are closer in range. In the latter case, radiative and nonradiative decay channels are both comparable leading to a high value for the AQY (if they were the same, the AQY would be 50%). The trends for both lifetime and recombination rate suggest that nonradiative processes dominate with decreasing size. In order to completely understand the decreasing trend of AQY that likely arises from the domination of nonradiative processes as the AB-ncSi size diminishes, one must examine the competing nonradiative relaxation pathways that suppress PL. An extensive study of the nonradiative pathways that might affect the PL of ncSi is far beyond the scope of this report; however, we suggest the following proposals for the effects most likely to dominate, (i) surface defects and traps, (ii) vibrational relaxation, and (iii) the existence of a transition from nanocrystalline to nanoamorphous forms. On the basis of the results of low-temperature studies of decyl-capped ncSi (see Supporting Information Figure S7) showing that the radiative lifetime increases dramatically at low temperature,29 it is likely that the effects of (i) and (ii), which would both be minimized at low temperature, are the most probable nonradiative pathways. The competing nonradiative relaxation surface trap and defect pathways (i) become more important with decreasing ncSi size as the effect of imperfections at the surface become dominant with increasing proportions of nanocrystal surface sites compared to the core. Since the surface curvature of ncSi increases with decreasing particle size, resulting in lower surface group density, it is likely that oxidative species can more easily reach the surface of small nanoparticles.30 As such, an increase in oxidation-related defects with decreasing size likely plays an important role in the observed AQY trend. For organically capped nanocrystals, nonradiative relaxation pathways (ii) that involve energy transfer to the vibrational modes of the surface-capping ligands have been suggested as an explanation for low PL

■

ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and characterization information. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

■

ACKNOWLEDGMENTS G.A.O. is a Government of Canada Research Chair in Materials Chemistry and Nanochemistry. He is deeply indebted to the Natural Sciences and Engineering Research Council of Canada (NSERC) for strong and sustained support of his research. This work was partially carried out with the support of the Karlsruhe Nano Micro Facility, a Helmholtz Research Infrastructure at the Karlsruhe Institute of Technology. M.L.M. and E.J.H. are grateful to NSERC for graduate and postgraduate scholarships. F.M.-F. acknowledges generous support by the Karlsruhe School of Optics and Photonics (KSOP). 341


Nano Letters

■

Letter

REFERENCES

(1) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110, 389. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (3) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (4) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538. (5) Li, M.; Li, J. C. Mater. Lett. 2006, 60, 2526. (6) Cademartiri, L.; Montanari, E.; Calestani, G.; Migliori, A.; Guagliardi, A.; Ozin, G. A. J. Am. Chem. Soc. 2006, 128, 10337. (7) Goldstein, A. N. Appl. Phys. A 1996, 62, 33. (8) Semonin, O. E.; Johnson, J. C.; Luther, J. M.; Midgett, A. G.; Nozik, A. J.; Beard, M. C. J. Phys. Chem. Lett. 2010, 1, 2445. (9) Mangolini, L.; Jurbergs, D.; Rogojina, E.; Kortshagen, U. J. Lumin. 2006, 121, 327. (10) Kirchner, C.; Liedl, T.; Kudera, S.; Pellegrino, T.; Javier, A. M.; Gaub, H. E.; Stölzle, S.; Fertig, N.; Parak, W. J. Nano Lett. 2005, 5, 331. (11) Derfus, A. M.; Chan, W. C. W.; Bhatia, S. N. Nano Lett. 2004, 4, 11. (12) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (13) Liu, S. M.; Yang, Y.; Sato, S.; Kimura, K. Chem. Mater. 2006, 18, 637. (14) Pi, X. D.; Liptak, R. W.; Deneen Nowak, J.; Wells, N. P.; Carter, C. B.; Campbell, S. A.; Kortshagen, U. Nanotechnology 2008, 19. (15) Zou, J.; Sanelle, P.; Pettigrew, K. A.; Kauzlarich, S. M. J. Cluster Sci. 2006, 17, 565. (16) Belomoin, G.; Therrien, J.; Nayfeh, M. Appl. Phys. Lett. 2000, 77, 779. (17) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (18) Anand, M.; Odom, L. A.; Roberts, C. B. Langmuir 2007, 23, 7338. (19) Krueger, K. M.; Al-Somali, A. M.; Falkner, J. C.; Colvin, V. L. Anal. Chem. 2005, 77, 3511. (20) Bai, L.; Ma, X.; Liu, J.; Sun, X.; Zhao, D.; Evans, D. G. J. Am. Chem. Soc. 2010, 132, 2333. (21) Shirahata, N.; Hirakawa, D.; Sakka, Y. Green Chem. 2010, 12, 2139. (22) Mastronardi, M. L.; Hennrich, F.; Henderson, E. J.; Maier-Flaig, F.; Blum, C.; Reichenbach, J.; Lemmer, U.; Kübel, C.; Wang, D.; Kappes, M. M.; Ozin, G. A. J. Am. Chem. Soc. 2011, 133, 11928. (23) Li, X.; He, Y.; Swihart, M. T. Langmuir 2004, 20, 4720. (24) Henderson, E. J.; Kelly, J. A.; Veinot, J. G. C. Chem. Mater. 2009, 21, 5426. (25) Shirahata, N. Phys. Chem. Chem. Phys. 2011, 7284. (26) De Mello, J. C.; Wittmann, H. F.; Friend, R. H. Adv. Mater. 1997, 9, 230. (27) Garcia, C.; Garrido, B.; Pellegrino, P.; Ferre, R.; Moreno, J. A.; Morante, J. R.; Pavesi, L.; Cazzanelli, M. Appl. Phys. Lett. 2003, 82, 1595. (28) Wilson, W. L.; Szajowski, P. F.; Brus, L. E. Science 1993, 262, 1242. (29) Maier-Flaig, F., Henderson, E. J., Valouch, S., Klinkhammer, S., Kübel, C., Ozin, G. A., Lemmer, U. 2011, manuscript submitted for publication. (30) Mei, B. C.; Oh, E.; Susumu, K.; Farrell, D.; Mountziaris, T. J.; Mattoussi, H. Langmuir 2009, 25, 10604. (31) Guyot-Sionnest, P.; Wehrenberg, B.; Yu, D. J. Chem. Phys. 2005, 123, 074709. (32) Anthony, R.; Kortshagen, U. Phys. Rev. B 2009, 80, 115407. (33) Rao, S.; Sutin, J.; Clegg, R.; Gratton, E.; Nayfeh, M. H.; Habbal, S.; Tsolakidis, A.; Martin, R. M. Phys. Rev. B: Condens. Matter 2004, 69, 205319.

342


Size-Dependent Absolute Quantum Yields for Size-Separated

Recommend Documents