LETTER pubs.acs.org/NanoLett
A High Quantum Efficiency Preserving Approach to Ligand Exchange on Lead Sulfide Quantum Dots and Interdot Resonant Energy Transfer Zachary Lingley,† Siyuan Lu,‡ and Anupam Madhukar*,†,‡ †
Mork Family Department of Chemical Engineering and Materials Science and ‡Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, United States
bS Supporting Information ABSTRACT: We present a new approach to ligand exchange on lead sulfide (PbS) quantum dots (QDs) in which the QDs are reacted with preformed Pb cationligand exchange units designed to promote reactions that replace surface Pb and oleate groups on the as-grown QDs. This process introduces negligible surface defects as the high quantum efficiency (∼55%) of the as-grown QDs is maintained. Infrared spectroscopy and electron microscopy are used to confirm the replacement of ligands and time-resolved photoluminescence to demonstrate the expected inverse sixth power dependence of the nonradiative resonant energy transfer rate on inter-QD spacing. KEYWORDS: Quantum dot, ligand exchange, energy transfer, lead sulfide, quantum efficiency
C
olloidal semiconductor nanocrystal quantum dots (QDs) have been a subject of interest for many years due to their tunable optical properties arising from size-dependent quantum confinement and the flexibility of synthesis in solution phase growth.1,2 An essential component of all single phase, colloidal QDs is the ligands that prevent aggregation and largely determine how the QDs interact with their environment.3 Such interactions are controlled by tailoring the chemical functionalization through ligand exchange for specific bonding, solubility in a variety of solvents,2 or their contribution to the confinement energy barriers, thus influencing electronic coupling between neighboring QDs in electronic, optoelectronic, and photonic devices.4 Moreover, the ligands influence the density of surface defects by nullifying the adverse effect of the dangling bonds on the nanocrystal surface.5 Efforts to manipulate ligands on as-grown colloidal QDs have been an integral part of the development of nanocrystal QDs. It was first reported that trioctylphosphine oxide (TOPO) ligands on CdSe QDs could be replaced by pyridine2,6 and a variety of other ligands4,7 by extended exposure to solutions containing excess ligand at room temperature and has since become the standard approach to ligand exchange (Figure 1a). This approach became particularly popular following the demonstration that the as-grown QDs could, through such surface modification, be functionalized with chemical moieties that impart water solubility and permit binding to specific biological receptors.810 However, realizing high quantum efficiency (QE) as-grown QDs and maintaining or improving QE after the desired ligand exchange are of central importance to their practical use as fluorescent markers and in devices. The above-noted standard ligand exchange approach does not necessarily maintain QE, and thus there have been other approaches,11 including demonstration r 2011 American Chemical Society
that the QE of CdSe QDs can be preserved during a one-step simultaneous ZnS shell growthligand exchange process by exposing the as-grown CdSeligand cores simultaneously to Zn precursor, S precursor, and the new ligand molecules,12 and ligand exchange on CdSe/ZnS core/shell QDs with Zn-ligand complexes.13 For a wide variety of electronic, optoelectronic, and photonic devices, dense-packed films of QDs and the transfer of charge and/or energy between them are at the core of basic physics underlying device performance. Thus the ability to tailor the 2-D or 3-D spacing by exchanging the ligands used during growth with more desirable ones from the viewpoint of the desired thin film characteristics but without degrading the high QE is of considerable importance. Here we introduce such an approach utilizing as a vehicle the as-grown PbS QDs with oleate ligands. Lead chalcogenide QDs are of particular interest for use in QD-based devices such as photodetectors,14 Schottky-type solar cells,15 and hybrid QDhigh mobility channel solar cells.16,17 The standard approach to solution phase ligand exchange has been applied to PbS QDs to replace oleate ligands with octadecylamine, dodecylamine, and octylamine in solution, but the influence of ligand exchange on the QE or emission spectrum has not been reported.18 A variation of the standard approach to ligand exchange is to expose films of QDs on a substrate to a solution containing the new ligands but one which will not dissolve the deposited QDs.19 This approach, originally developed for IIVI QDs,20 has been used successfully for a variety of new ligands on lead chalcogenide QDs.2124 Although this solid Received: April 22, 2011 Revised: May 25, 2011 Published: June 27, 2011 2887
dx.doi.org/10.1021/nl201351f | Nano Lett. 2011, 11, 2887–2891
Nano Letters
Figure 1. (a) Conventional ligand exchange method in which the asgrown QDs are treated in a solution of excess new ligands (red, with functional group symbolized by the cross). (b) Our approach wherein the as-grown QDs are treated with excess of preconjugated cationligand unit for exchange.
phase ligand exchange approach has provided a way to control QDQD spacing and thus manipulate the electrical characteristics of the QD film (such as carrier mobility25,26), the fact that the post-ligand-exchange QDs are necessarily deposited on a substrate makes it infeasible to evaluate the effects of ligand exchange on QE which is of critical importance for light energy harvesting applications.16 The lack of a solution based, high QE preserving ligand exchange method for lead chalcogenides QDs in the literature thus motivated us to develop one. The nature of the surface chemistry of oleate ligand capped PbS QDs suggests that the standard approach to ligand exchange will have a detrimental effect on the QE of these QDs as described below. High QE PbS QDs are expected to be terminated by Pb atoms bonded by ligands.27 The binding energy between a Pb atom and a typical carboxylate ligand, estimated to be ∼1.7 eV (∼168 kJ/mol),28 is on the same order of magnitude as the binding energy between a Pb atom and nearby S atoms on a highly curved PbS QD surface. We estimate that the binding energy associated with a Pb atom with one of its S neighbors in a PbS crystal is ∼1 eV using the known PbS enthalpy of formation of 98.12 kJ/mol (1.02 eV per PbS pair),29 the cohesive energy of Pb (2.03 eV/atom) and S (2.85 eV/atom),30 and the coordination number (6). Because the surface Pb atoms are bound with comparable strength to the oleate ligands as to the neighboring S atoms, when the as-grown oleate capped PbS QDs are heated (either dispersed in solution or in the form of aggregated solid on a substrate) in excessive new ligands, we anticipate that the dissociation of the ligand from the surface Pb atoms will occur in parallel with the dissociation of Pbligand units from the QD which leaves behind an S terminated surface that leads to deep level surface traps and degradation of the QE.27 Indeed the above anticipation is supported by the observations that (1) the oxidation of PbS QD is accompanied by the loss of the oleate ligands and their bonded surface Pb atoms together into the solution31,32 and (2) the presence of oleic acid in PbS QD
LETTER
solution significantly accelerates the removal of Pb atoms from the QDs surface.32 To combat problems related to unwanted removal of Pb atoms with their as-grown ligand, we introduce an alternative approach to ligand exchange, as illustrated in Figure 1b. In this approach, the desired new ligands are first conjugated to Pb cations to form cationligand units for exchange to which the asgrown QDs are then exposed. As exchange conjugates, we use Pb cations bound to carboxylate anion ligands of varying lengths synthesized by reacting lead oxide (PbO) and carboxylic acid at ratios that are kept just below the stoichiometric ratio of 1:2 (typically ∼1:1.9) so that the presence of free carboxylic acid ligands is minimized in the reacting solution. Thus the reaction is likely dominated by the Pb cationnew carboxylate ligand conjugate exchanging with the Pb cationoleate conjugate on the QD surface, a process in which at least one important guiding rule for carrying out ligand exchange—the conservation of QD surface charge33—is well maintained. The detailed mechanism of such an exchange involving cationligand conjugate is unknown, though to some extent, one may extrapolate from the general knowledge of the mechanism of cation exchange reaction in nanocrystals34,35 and, most relevantly, the study of reaction between oleate capped PbX (X = S, Se, or Te) QDs and Cdoleate to create oleate capped PbX/CdX coreshell QDs via CdPb cation exchange.36,37 We hope that the current work establishing the usefulness of conjugated cationligand units in ligand exchange reaction to core-only nanocrystal QDs while preserving the initial high QE in the technologically important lead chalcogenide QDs will motivate studies that uncover the underlying reaction mechanism. PbS Nanocrystal Synthesis. Lead sulfide nanocrystal QDs were synthesized by following a modified version of the method of Hines and Scholes.38 Two sizes of PbS QDs were created for use in these conjugated cationligand exchange experiments. Small (∼2.6 nm diameter) PbS QDs were created as follows. Ninety milligrams of PbO and 220 mg of oleic acid (99%) were dissolved in octadecene (90%) at 90 °C in an argon environment to form lead oleate, and then the solution was degassed for 10 min at 50 mTorr. After degassing, the lead oleateoctadecene solution was heated in argon to 120 °C and 35.5 mg of hexamethyldisilythiane in 2 mL of trioctylphoshine at 4 °C was rapidly injected. At these growth conditions, the final PbS QD size is reached within about 1 min of injection. Large (∼6.2 nm diameter) PbS QDs were synthesized by dissolving 180 mg of PbO in 1.4 g of oleic acid and 2.6 g of octadecene. The solution is degassed at 100 °C for 10 min. The solution is heated to 150 °C, and 35.5 mg of hexamethyldisilythiane in 2 mL of trioctylphoshine at room temperature is rapidly injected. After 10 min, another 35.5 mg of hexamethyldisilythiane in 2 mL of trioctylphoshine is slowly added dropwise over about 10 min and the QDs are allowed to grow for an additional 10 min. Both the small and large PbS QDs are separated from their respective growth solutions by precipitation with acetone and finally dissolved in toluene. The as-grown PbS QDs are hereafter referred to as C18-capped. Ligand Exchange and Quantum Efficiency. The procedure for conjugated cation-ligand exchange consists of injecting asgrown PbS QDs into a separately prepared solution of the conjugated Pb-ligand units for exchange in octadecene. For examination of the exchange process on small PbS QDs, the Pbligand conjugates were prepared as follows: 140 mg of PbO and the new ligand, either 180 mg of octanoic acid or 250 mg of dodecanoic acid, were dissolved in octadecene in an argon environment at a molar ratio of slightly less than the 2888
dx.doi.org/10.1021/nl201351f |Nano Lett. 2011, 11, 2887–2891
Nano Letters
Figure 2. PL of C8-, C12-, and C18-capped average diameter 2.6 nm PbS QDs in solution illustrating the small change in QE and emission wavelength and spectrum shape. PbS QDs used in PL measurements are not exposed to air.
stoichiometric ratio of 1:2 (typically ∼1:1.9) at 100 °C to form lead octanoate or lead dodecanoate respectively and then degassed. Ten milligrams of PbS QDs in 0.5 mL of octadecene were then injected without exposure to air, and the Pbligand conjugate exchange reaction was allowed to proceed for 10 min at 100 °C. PbS QDs were removed and immediately centrifuged to produce a light brown precipitate that contains mostly excess cationligand complexes and a clear, dark brown supernatant that contains PbS QDs with new ligands. The supernatant was decanted, and PbS QDs with new ligands were separated from the remaining solvent by precipitation with acetone and dissolved in toluene. Cationligand exchange on the large 6.2 nm QDs follows the same procedure but with a reduced amount of the new ligand to adjust for a lower ratio of the surface area to volume as compared with the 2.6 nm PbS QDs. In a typical synthesis, 10 mg of PbO and 13 mg of octanoic acid or 19 mg of dodecanoic acid were dissolved in octadecene under argon at 100 °C to form lead octanoate or lead dodecanote, respectively. The solution was degassed for 10 min, then 5 mg of 6.2 nm PbS QDs in 0.5 mL of octadence was injected at constant temperature and allowed to react for 10 min. PbS QDs with new ligands were separated from the remaining growth solution by precipitation with acetone and finally dissolved in toluene. The PbS QDs after exposure to lead octanoate or lead dodecanoate are hereafter referred to as C8capped and C12-capped, respectively. Comparisons of photoluminescence (PL) in solution from the as-grown PbS QDs and PbS QDs after the cationligand unit exchange demonstrate the success of our approach in preserving both size distribution and the high QE. Figure 2 shows the typical PL behavior of the 2.6 nm diameter as-grown PbS QDs (i.e., capped with C18) and the C8- and C12-capped PbS QDs. The PL curves are normalized by the excitation power at 700 nm. The QE of the as-grown PbS QDs is 59 ( 4% and the QE of the C8- and C12-capped QDs after cationligand exchange is 55 ( 4%. The benefits of using cationligand complexes can be seen by comparison to the QDs that are exposed to octanoic acid without Pb cation in which case the QE drops to 33% (see Figure S1, Supporting Information). The QE values were obtained by comparison to emission from the dye IR 125 dissolved in dimethyl sulfoxide with excitation at 700 nm.
LETTER
Figure 3. FTIR of large (average diameter 6.2 nm) PbS QDs with C8, C12, and C18 illustrating the absence of CdC—H stretch absorption in C8 and C12 and presence of CdC—H stretch absorption in as-grown C18 (indicated by dashed box). Other absorption peaks due to C—H stretch at methyl groups or C—C single bonds are, as expected, present in all curves. Curves are offset vertically for clarity. Note that the FTIR spectra for small PbS QDs with C8, C12, and C18 ligands are nominally the same as those shown here for their large counterparts.
PL spectra of the C8-, C12-, and C18-capped PbS QDs in solution illustrate that the size distribution has not changed as a result of the cationligand exchange process. In this range of PbS QD diameters, the removal of a single monolayer of PbS would result in a blue shift of about 50 nm. The strong sensitivity of the PL emission wavelength to the PbS diameter contrasted against the lack of variation of PL curves before and after ligand exchange suggests that there is negligible gain or loss of Pb cations. Having established that our exchange procedure using Pbconjugated ligands results in minimal changes in the QE and the emission spectrum, we checked that oleate ligands have indeed been replaced from the as-grown samples. To this end we used Fourier transform infrared (FTIR) spectroscopy to investigate the changes in the chemical bonds present in the samples (Figure 3). Specifically, we used the presence and absence of the absorption peak due to C—H stretch at a carbon in a CdC double bond at 3005 cm1 that is present in oleic acid39 but not in octanoic or dodecanoic acids. In Figure 3 the absence of a peak at 3005 cm1 for samples after exposure to lead octanoate or lead dodecanoate (C8- and C12-capped) indicate that the oleate ligands are removed as a result of our conjugated cationligand exchange procedure. QD Thin Films and Interdot Nonradiative Resonant Energy Transfer. As noted, the primary motivation for the ligand replacement is to create solid-state QD assemblies of varying and known inter-QD separations to tailor the electronic response of the assembly. As a first step toward this aim, we created QD assemblies of the C8-, C12-, and C18-capped 2.6 nm PbS QDs by drop casting from toluene solutions onto glass substrates. Drop casting was performed in a glovebox in an argon environment. The toluene was allowed to dry for 30 min, and then the substrates were loaded into a cryostat in the glovebox and the cryostat was sealed without exposure to air. The cryostat was removed from the glovebox and pumped to ∼1 106 Torr within 30 min of substrate mounting. The change in ligand length should result in a change in the inter-QD spacing in aggregated QD assemblies that is observable in transmission electron microscopy (TEM) of PbS QDs that 2889
dx.doi.org/10.1021/nl201351f |Nano Lett. 2011, 11, 2887–2891
Nano Letters
LETTER
Figure 5. Estimate of NRET rates of the 880 nm emission QDs in the QD solids made of the as-synthesized oleate capped QDs (nc = 18) and in QDs after ligand exchange with dodecanoate and octanoate ligands (nc = 12, 8) as a function of r 6 (r: QD diameter plus QDQD surfaceto-surface spacing as determined by TEM images). The line is a linear fit passing the origin.
Figure 4. Transmission electron microscope images of arrays of PbS QDs with (a) C18 ligands with average 2.1 ( 0.5 nm spacing and (b) C12 ligands with average 1.4 ( 0.4 nm spacing. (c) C8 ligands with average spacing 1.0 ( 0.2 nm. Scale bars are 10 nm on all three images.
have self-assembled into close packed two-dimensional arrays and thin films. Characteristic TEM images for illustrative selfassembled C18-, C12-, and C8-capped PbS QDs are shown in Figure 4. Images were obtained in bright field at 200 kV with no objective aperture in place. The QDs are supported on a ∼3 nm thick carbon film. Such images reveal that the QDQD separation scales with the number of carbons in the ligands. The surface-to-surface spacing averaged over about 80 pairs of the asgrown C18 capped PbS QD, C12 capped PbS QDs, and C8 capped PbS QDs is 2.1 ( 0.5, 1.4 ( 0.4, and 1.0 ( 0.2 nm, respectively. As the ligands determine the QDQD spacing when in arrays,4 such measurements suggest that the effective ligand length is approximately 0.06 nm times the number of carbon atoms in the ligand, nc. The reduction in QDQD spacing due to ligand exchange observed in the TEM images is independently supported by the measured time-resolved photoluminescence (TRPL) behavior of the drop cast solid assemblies on glass substrates and the estimates of inter-QD nonradiative energy transfer (NRET) rate they provide. The pairwise NRET rate (kQDQD NRET ) from a smaller donor QD (shorter emission wavelength) to a neighboring larger acceptor QD (longer emission wavelength) is strongly dependent on the QDQD center to center separation, r, and expected to follow the F€orster expression ¼ krad ðR0 =rÞ6 kQDQD NRET
ð1Þ
where krad is the donor QD radiative decay rate and R0 is the F€orster radius defined as the distance at which the pairwise NRET rate is equal to the donor QD radiative decay rate.40 The occurrence of dipoledipole interaction mediated inter-QD NRET is well established in densely packed QD solids,4143 but the influence of QDQD spacing on inter-QD NRET rate in solid state is largely uninvestigated save a few examples. In solution, the reduction of QDQD spacing by increasing the concentration of QDs has been shown to enhance inter-QD NRET efficiency.44 In the solid phase, energy transfer between monolayers of QDs separated by polymer spacer layers has been studied as a function of spacer layer thickenss.45 However, the use of ligand exchange to control inter-QD spacing in QD solids and thereby control interQD NRET rates is first demonstrated as described below. The TRPL decay rate of the subset of QDs emitting at a fixed wavelength in a QD solid is the sum of their radiative decay rate, nonradiative decay rate, and the NRET rates to all larger neighboring QDs. The NRET contribution to the PL decay is prominent for the small QDs and is negligible for the largest QDs because the probability of finding even larger neighboring QDs is low. Thus, as argued in ref 16, assuming the radiative and nonradiative decay rates are independent of QD size, the difference between the TRPL decay rate of the small QDs and that of the largest QDs can be used as a measure of the NRET rate (kNRET) for the small QDs to their neighbors. As an illustrative example, estimates of NRET rates for the 880 nm emitting QDs, kNRET (880 nm), for the three different ligand lengths C8, C12, and C18 are shown in Figure 5. Note that the NRET rate is the PL decay rate of smallest (880 nm emitting) QDs minus the decay rate of the largest (1080 nm emission) QDs. The PL decay rates are extracted from the measured TRPL curves as discussed in ref 16 and are provided in the Supporting Information. We observe that the NRET rates increase with decreasing number of carbons, nc (Figure 5) confirming the occurrence of ligand exchange and the corresponding reduction of the QDQD separation. In fact, as plotted in Figure 5, the estimated NRET rates are indeed linearly proportional to r 6 for r measured via TEM, as expected from eq 1. In conclusion, we have presented a new approach for ligand exchange on nanocrystals based on exchanging conjugated cationligand exchange units rather than the common approach 2890
dx.doi.org/10.1021/nl201351f |Nano Lett. 2011, 11, 2887–2891
Nano Letters of exchanging only the organic ligands. The approach is motivated by considerations of the characteristics of lead chalcogenide QDs, specifically the necessity of Pb termination and of suppressing the undesirable removal of Pb atoms on the QD surface with oleate ligands on as-grown samples. We successfully employed cationligand unit exchange to replace oleate ligands on the surface of the as-grown PbS QDs with octanoate and dodecanoate ligands with a very small reduction in QE (from 59 ( 4% to 55 ( 4%) and negligible change in emission spectra. We demonstrated that inter-QD NRET rates in QD solids can be enhanced by controllably reducing the ligand length and found, as expected, that the measured NRET rate as a function of QDQD separation determined independently via TEM follows the F€orster expression. We think that this approach of exploiting desired cationdesired ligand as a unit exchange should be suitable for ligand exchange on a variety of species of QDs with a variety of new ligands.
’ ASSOCIATED CONTENT
bS
Supporting Information. Comparison of PL spectra for as-grown oleate capped PbS QDs and as-grown PbS QDs after treatment by the standard approach to ligand exchange with plain octanoic acid and TRPL spectra of aggregated QD solids made of QDs with C8, C12, and C18 ligands and the extraction of decay rates. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by AFOSR Grant No. FA9550-08-1-0146. The TEM and PL studies were carried out at the USC supported Center for Electron Microscopy and Microanalysis (CEMMA) and the NanoBiophyics Core Facility, respectively. ’ REFERENCES (1) Brus, L. J. Phys. Chem. 1986, 90, 2555–2560. (2) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (3) Alivisatos, A. P. Science 1996, 271 (5251), 933–937. (4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270 (5240), 1335–1338. (5) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66 (11), 1316–1318. (6) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98 (15), 4109–4117. (7) Lee, J.-K.; Kuno, M.; Bawendi, M. G. Mater. Res. Soc. Symp. Proc. 1997, 452, 323–328. (8) Chan, W. C. W.; Nie, S. Science 1998, 281 (5385), 2016–2018. (9) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281 (5385), 2013–2016. (10) 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 (5709), 538–544. (11) Dubois, F.; Mahler, B. T.; Dubertret, B. T.; Doris, E.; Mioskowski, C. J. Am. Chem. Soc. 2006, 129 (3), 482–483. (12) Wang, Q.; Xu, Y.; Zhao, X.; Chang, Y.; Liu, Y.; Jiang, L.; Sharma, J.; Seo, D.-K.; Yan, H. J. Am. Chem. Soc. 2007, 129 (20), 6380–6381. (13) Liu, D.; Snee, P. T. ACS Nano 2011, 5 (1), 546–550.
LETTER
(14) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4 (2), 138–142. (15) Johnston, K. W.; Pattantyus-Abraham, A. G.; Clifford, J. P.; Myrskog, S. H.; MacNeil, D. D.; Levina, L.; Sargent, E. H. Appl. Phys. Lett. 2008, 92 (15), 151115. (16) Lu, S.; Madhukar, A. Nano Lett. 2007, 7 (11), 3443–3451. (17) Lu, S.; Lingley, Z.; Asano, T.; Harris, D.; Barwicz, T.; Guha, S.; Madhukar, A. Nano Lett. 2009, 9 (12), 4548–4552. (18) Chang, T.-W. F.; Musikhin, S.; Bakueva, L.; Levina, L.; Hines, M. A.; Cyr, P. W.; Sargent, E. H. Appl. Phys. Lett. 2004, 84 (21), 4295–4297. (19) Yu, D.; Wang, C.; Guyot-Sionnest, P. Science 2003, 300 (5623), 1277–1280. (20) Jarosz, M. V.; Porter, V. J.; Fisher, B. R.; Kastner, M. A.; Bawendi, M. G. Phys. Rev. B 2004, 70 (19), 195327. (21) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86–89. (22) Murphy, J. E.; Beard, M. C.; Nozik, A. J. J. Phys. Chem. B 2006, 110 (50), 25455–25461. (23) Klem, E. J. D.; MacNeil, D. D.; Cyr, P. W.; Levina, L.; Sargent, E. H. Appl. Phys. Lett. 2007, 90 (18), 183113. (24) Zarghami, M. H.; Liu, Y.; Gibbs, M.; Gebremichael, E.; Webster, C.; Law, M. ACS Nano 2010, 4 (4), 2475–2485. (25) Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Nano Lett. 2010, 10 (5), 1960–1969. (26) Law, M.; Luther, J. M.; Song, Q.; Hughes, B. K.; Perkins, C. L.; Nozik, A. J. J. Am. Chem. Soc. 2008, 130 (18), 5974–5985. (27) Tang, J.; Brzozowski, L.; Barkhouse, D. A. R.; Wang, X.; Debnath, R.; Wolowiec, R.; Palmiano, E.; Levina, L.; PattantyusAbraham, A. G.; Jamakosmanovic, D.; Sargent, E. H. ACS Nano 2010, 4 (2), 869–878. (28) Bala, T.; Prasad, B. L. V.; Sastry, M.; Kahaly, M. U.; Waghmare, U. V. J. Phys. Chem. A 2007, 111, 6183–6190. (29) Deore, S.; Navrotsky, A. Am. Mineral. 2006, 91, 400–403. (30) Kittel, C. Introduction to Solid State Physics, 7th ed.; John Wiley & Sons, Inc.: New York, 1996. (31) Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. J. Am. Chem. Soc. 2008, 130 (45), 15081–15086. (32) Dai, Q.; Zhang, Y.; Wang, Y.; Wang, Y.; Zou, B.; Yu, W. W.; Hu, M. Z. J. Phys. Chem. C 2010, 114 (39), 16160–16167. (33) Owen, J. S.; Park, J.; Trudeau, P.-E.; Alivisatos, A. P. J. Am. Chem. Soc. 2008, 130 (37), 12279–12281. (34) Mews, A.; Eychmueller, A.; Giersig, M.; Schooss, D.; Weller, H. J. Phys. Chem. 1994, 98 (3), 934–941. (35) Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Science 2004, 306 (5698), 009–1012. (36) Pietryga, J. M.; Werder, D. J.; Williams, D. J.; Casson, J. L.; Schaller, R. D.; Klimov, V.; Hollingsworth, J. A. J. Am. Chem. Soc. 2008, 130 (14), 4879–4885. (37) Lambert, K.; Geyter, B. D.; Moreels, I.; Hens, Z. Chem. Mater. 2009, 21 (5), 778–780. (38) Hines, M. A.; Scholes, G. D. Adv. Mater. 2003, 15 (21), 1844–1849. (39) Mielczarski, J. A.; Cases, J. M.; Bouquet, E.; Barres, O.; Delon, J. F. Langmuir 1993, 9 (9), 2370–2382. (40) F€orster, T. Discuss. Faraday Soc. 1959, 27, 7–17. (41) Kagan, C. R.; Murray, C. B.; Bawendi, M. G. Phys. Rev. B 1996, 54 (12), 8633. (42) Crooker, S. A.; Hollingsworth, J. A.; Tretiak, S.; Klimov, V. I. Phys. Rev. Lett. 2002, 89 (18), 186802. (43) Clark, S. W.; Harbold, J. M.; Wise, F. W. J. Phys. Chem. C 2007, 111 (20), 7302–7305. (44) Xu, L.; Xu, J.; Ma, Z.; Li, W.; Huang, X.; Chen, K. Appl. Phys. Lett. 2006, 89 (3), 033121. (45) Kim, D.; Okahara, S.; Nakayama, M.; Shim, Y. Phys. Rev. B 2008, 78 (15), 153301.
2891
dx.doi.org/10.1021/nl201351f |Nano Lett. 2011, 11, 2887–2891
