Highly Luminescent Lead Sulfide Nanocrystals in Organic Solvents

Langmuir 2008, 24, 8215-8219

8215

Highly Luminescent Lead Sulfide Nanocrystals in Organic Solvents and Water through Ligand Exchange with Poly(acrylic acid) Wanjuan Lin,† Karolina Fritz,† Gerald Guerin,† Ghasem R. Bardajee,†,§ Sean Hinds,‡ Vlad Sukhovatkin,‡ Edward H. Sargent,‡ Gregory D. Scholes,*,† and Mitchell A. Winnik*,† Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Canada M5S 3H6, and Department of Electrical and Computer Engineering, UniVersity of Toronto, 10 King’s College Circle, Toronto Canada, M5S 3G4 ReceiVed February 21, 2008. ReVised Manuscript ReceiVed April 22, 2008 Hydrophobic lead sulfide quantum dots (PbS/OA) synthesized in the presence of oleic acid were transferred from nonpolar organic solvents to polar solvents such as alcohols and water by a simple ligand exchange with poly(acrylic acid) (PAA). Ligand exchange took place rapidly at room temperature When a colloidal solution of PbS/OA in tetrahydrofuran (THF) was treated with excess PAA, the PbS/PAA nanocrystals that formed were insoluble in hexane and toluene but could be dissolved in methanol or water, where they formed colloidal solutions that were stable for months. Ligand exchange was accompanied by a small blue shift in the band-edge absorption, consistent with a small reduction in particle size. While there was a decrease in quantum yield associated with ligand exchange and transfer to polar solvents, as is commonly found for colloidal quantum dots, the quantum yields determined were impressively high: PbS/OA in toluene (82%) and in THF (58%); PbS/PAA in THF (42%) and in water (24%). The quantum yields for the PbS/PAA solutions decreased over time as the solutions were allowed to age in the presence of air.

Introduction Semiconductor nanocrystals, or quantum dots (QDs), are attracting much attention, owing to their size-tunable properties and their wide range of potential applications, such as light emitting diodes,1 biological labeling2 and sensors,3 and solar cells.4 PbS QDs absorb and emit light in the near-infrared (NIR) region, making them potentially useful for telecommunication5 and biotechnology applications.6 For example, the use of QDs for deep-tissue imaging in vivo requires QDs with emission in the NIR.7 The tumor imaging sensitivity is optimized if the QD excitation and emission can occur at wavelengths where the major absorption peaks of blood and water are absent. PbS QDs with the highest photoluminescence (PL) emission quantum yield (QY) are normally synthesized by a high temperature organometallic route in the presence of surface * To whom correspondence should be addressed. E-mail: mwinnik@ chem.utoronto.ca (M.A.W.); [email protected] (G.D.S.). † Department of Chemistry. ‡ Department of Electrical and Computer Engineering. § Current address: Department of Chemistry, Payame Noor University, Qazvin Branch, Qazvin, Iran. (1) (a) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (b) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (c) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. AdV. Mater. 2000, 12, 1102. (d) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (e) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S.; Banin, U. Science 2002, 295, 1506. (f) Rogach, A. L.; Eychmu¨ller, A.; Hickey, S. G.; Kershaw, S. V. Small 2007, 3, 536. (2) (a) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (b) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (c) 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. (d) Tran, P. T.; Goldman, E. R.; Anderson, G. P.; Mauro, J. M.; Mattoussi, H. Phys. Status Solidi B 2002, 229, 427. (3) Zhang, C.-Y.; Yeh, H.-C.; Kuroki, M. T.; Wang, T.-H. Nat. Mater. 2005, 4, 826. (4) (a) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phy. ReV. B 1996, 54, 17628. (b) Huynh, W. U.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (c) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (5) (a) Bekueva, L.; Musikhin, S.; Hines, M. A.; Chang, T.-W. F.; Tzolov, M.; Scholes, G. D.; Sargent, E. H. Appl. Phys. Lett. 2003, 82, 2895. (b) Bakueva, L.; Konstantatos, G.; Levina, L.; Musikhin, S.; Sargent, E. H. Appl. Phys. Lett. 2004, 84, 3459. (6) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nat. Mater. 2005, 4, 435.

ligands such as oleic acid (OA).8 These surface ligands passivate the QD surface states and provide colloidal stability (solubility) in nonpolar organic solvents. These materials, however, are insoluble in protic solvents such as methanol and water. Water solubility is important for many applications, especially in biological systems. Thus, postsynthesis surface modification of the as-prepared PbS QDs is required. Ligand exchange is a versatile way to modify the surfaces of QDs for end applications. Over the past decade, surface modification for the group II-VI nanocrystals such as CdSe, CdTe, CdS, ZnS, and ZnSe have been studied extensively. Group II-VI QDs synthesized by the traditional organometallic method result in QDs covered with organic ligands containing bulky alkyl groups. For example, QDs synthesized in mixtures containing trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) contain a mixture of TOP/TOPO ligands bound to the surface.9 Surface modification strategies to obtain water-soluble colloids involve the substitution of the native TOP/TOPO molecules with bifunctional ligands, with each presenting a surface-anchoring moiety to bind to the inorganic QD surface (for example, thiol) and a hydrophilic end group (for example, hydroxyl, carboxyl) to achieve water compatibility. Nie and Chan2b reported the use of mercaptoacetic acid for transferring TOPO-capped CdSe/ZnS QDs into water. Peng and co-workers10 demonstrated modification of the surface of CdSe or CdSe/ZnS QDs with organic dendron ligands, which not only provide compatibility with water but also can sterically hinder the diffusion of small molecules or ions from the bulk solution to the interface between the QDs and its ligands. Bawendi and co-workers11 examined a poly(ethylene glycol) (PEG)-phosphine oxide copolymer for obtaining water soluble CdSe/ZnS nanoparticles. Polymers (7) (a) Cheong, W.-F.; Prahl, S. A.; Welch, A. J. IEEE J. Quantum Electron. 1990, 26, 2166. (b) Ntziachristos, V.; Bremer, C.; Weissleder, R. Eur. Radiol. 2003, 13, 195. (8) Hines, M. A.; Scholes, G. D. AdV. Mater. 2003, 15, 1844. (9) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (10) Wang, Y. A.; Li, J. J.; Chen, H.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2293. (11) Kim, S.-W.; Kim, S.; Tracy, J. B.; Jasanoff, A.; Bawendi, M. G. J. Am. Chem. Soc. 2005, 127, 4556.

10.1021/la800568k CCC: $40.75  2008 American Chemical Society Published on Web 07/03/2008

8216 Langmuir, Vol. 24, No. 15, 2008 Scheme 1. The Ligand Exchange Process

Lin et al.

and after ligand exchange. Small changes in size detected in this way were confirmed by shifts in the absorption and photoluminescence (PL) spectra. We showed that PAA-capped PbS QDs maintain stable photoluminescence properties in methanol and in aqueous solution and that the photoluminescence quantum yield in water approaches that of the thiol-MTEG PbS QDs reported in ref 17.

Experimental Section bearing pendant groups that can act as multidentate ligands are normally much more effective than small molecule ligands at conferring robust colloidal stability and are often compatible with a variety of different solvents.12 In this regard, we have examined poly(dimethylaminoethyl methacrylate) (PDMA) as well as a PEGPDMA block copolymer for rendering CdSe and CdSe/ZnS QDs colloidally soluble in polar solvents including water.13 In contrast, fewer successful surface modification strategies have been reported for IV-VI QDs, such as PbS and PbSe QDs.14 To date, these strategies have relied on thiol-based ligands.15,16 A particularly effective strategy reported by Sargent and co-workers involves treating the PbS/OA QDs directly in water with (1mercaptoundec-11-yl)tetraethylene glycol (MTEG).17 The thiol groups bind to the surface of the nanoparticles, and the TEG units confer solubility in water, even in the presence of (20 mM HEPES) buffer. An added feature of this system is that the 11carbon spacer provides a hydrocarbon encapsulation around the nanoparticle, which may confer protection against ions or oxidants in the solution. We are particularly interested in polymers as multidentate ligands for inorganic nanoparticles. We seek systems where the polymer and the nanoparticles can undergo well-defined ligand exchange in a common solvent. These circumstances can open the door to direct study of the ligand exchange process, either by light scattering or by pulsed gradient NMR.18 In this paper, we report that poly(acrylic acid) (PAA) in tetrahydrofuran solution rapidly replaces the surface ligands of PbS/ OA to yield surface passivated QDs soluble in a variety of polar and semipolar organic solvents as well as water. Scheme 1 illustrates the general principle of this ligand exchange process. PAA, as a polycarboxylate, was chosen on the basis of its anticipated strong coordination ability to the PbS QD surfaces as a multidentate ligand. Tetrahydrofuran (THF) was selected as the reaction solvent because it provides solubility at room temperature for both PAA and the OA-capped PbS QDs. The PAA sample was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. High-quality PbS colloidal quantum dots were prepared as reported by Hines and Scholes.8 We used a combination of transmission electron microscopy (TEM) and dynamic light scattering (DLS) to examine the size of QDs before (12) Wang, X.-S.; Dykstra, T. E.; Salvador, M. R.; Manners, I.; Scholes, G. D.; Winnik, M. A. J. Am. Chem. Soc. 2004, 126, 7784. (13) (a) Wang, M.; Oh, J. K.; Dykstra, T. E.; Lou, X.; Scholes, G. D.; Winnik, M. A. Macromolecules 2006, 39, 3664. (b) Wang, M.; Felorzabihi, N.; Guerin, G.; Haley, J. C.; Scholes, G. D.; Winnik, M. A. Macromolecules 2007, 40, 6377. (14) For a detailed discussion of the spectroscopy of PbS and PbSe quantum dots, see (a) Ferne´e, M. J.; Thomsen, E.; Jensen, P.; Rubinsztein-Dunlop, H. Nanotechnology 2006, 17, 956. (b) Ferne´e, M. J.; Jensen, P.; Rubinsztein-Dunlop, H. Appl. Phys. Lett. 2007, 91, 043112. (c) Lifshitz, E.; Brumer, M.; Kigel, A.; Sashchiuk, A.; Bashouti, M.; Sirota, M.; Galun, E.; Burshtein, Z.; Le Quang, A. Q.; Ledoux-Rak, I.; Zyss, J. J. Phys. Chem. B 2006, 110, 25356. (15) Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L. Chem. Mater. 2004, 16, 3318. (16) Zhao, X.; Gorelikov, I.; Musikhin, S.; Cauchi, S.; Sukhovatkin, V.; Sargent, E. H.; Kumacheva, E. Langmuir 2005, 21, 1086. (17) Hinds, S.; Myrskog, S.; Levina, L.; Koleilat, G.; Yang, J.; Kelley, S. O.; Sargent, E. H. J. Am. Chem. Soc. 2007, 129, 7218. (18) Shen, L.; Soong, R.; Wang, M.; Lee, A.; Wu, C.; Scholes, G. D.; Macdonald, P. M.; Winnik, M. A. J. Phys. Chem. B. 2008, 112, 1626.

Materials. Acrylic acid (99%), 4,4′-azobis(4-cyanopentanoic acid) (98%), 1,1′-thiocarbonyldiimidazole (95%), benzyl mercaptan (99%), triethylamine (99%), dichloromethane (ACS reagent grade), tetrahydrofuran (ACS reagent grade), and methanol (ACS reagent grade) were purchased from Sigma-Aldrich Co. and used without further purification. All aqueous solutions were prepared with water from a MilliQ water purification system. Synthesis of PbS Nanocrystals. PbS QDs were synthesized as previously reported by Hines and Scholes.8 A typical reaction is as follows: the lead precursor lead oxide (PbO) was dissolved in oleic acid (OA) at 150 °C under an argon atmosphere. During this step, lead oleate was formed; bis(trimethylsilyl)sulfide (as the sulfur precursor) in octadecene (ODE) was then injected, while the temperature was reduced to a value between 50 and 150 °C, depending on the desired growth rate and final particle size. After the reaction mixture was cooled to room temperature, the PbS QDs were isolated by adding methanol to precipitate them. The PbS QDs were then redispersed in toluene. The experiments reported here were carried out with a colloidal suspension of OA-capped PbS QDs with a mean size of approximately 2 nm (as determined by TEM) and a band-edge absorption at around 1200 nm. Synthesis of Poly(acrylic acid). Poly(acrylic acid) (PAA) was synthesized through reversible addition fragmentation transfer (RAFT) polymerization using trithiocarbonic acid dibenzyl ester (DBT) as the RAFT agent. DBT was synthesized following the procedure reported by Claverie et al.19 A detailed description of the synthesis of DBT and PAA is provided in the Supporting Information (S1). Ligand Exchange. The PAA-capped PbS QDs (PbS/PAA) were prepared by mixing a solution of an excess amount of PAA with a solution of PbS/OA in tetrahydrofuran (THF) at room temperature and stirring overnight. The product was isolated and purified from the free OA ligands by precipitation into hexane followed by redispersion in THF. As a typical example, PAA (158.4 mg) was mixed with PbS/OA (7.77 mg) in THF (6.0 mL) in a 20 mL scintillation vial and this mixture was stirred at room temperature overnight. The resulting solution was homogeneous and black in color. After evaporation of the THF by a gentle flow of nitrogen gas, a black solid (167 mg) was obtained. It was identified (see below) as PAA-capped PbS nanocrystals (PbS/PAA). This black solid was then washed by stirring for 5 min in hexane (6 mL). The black solid was not soluble in hexane; most of the solid stayed at the bottom of the vial, and the rest settled quickly after the stirring was ended. The purified PbS/PAA could readily be transferred to methanol or water to form robust colloidal solutions that remained stable for weeks. The pH of aqueous solutions of PbS/PAA was adjusted to neutral with triethylamine. Characterization of the QDs. UV-vis-NIR absorption spectra were recorded at room temperature on a Cary 5000 UV-vis-NIR spectrophotometer using 1.00 cm quartz cuvettes. The spectra were baseline corrected. The PbS/OA samples in toluene were diluted with additional toluene. PbS/PAA QDs in THF were characterized after purification by repeated precipitation with hexane. All PbS/PAA solutions were concentrated to 4 mL by a gentle nitrogen flow prior to the measurements. Photoluminescence spectra were recorded with an Ocean Optics NIR-512 spectrometer via a 100 µm core VIS-NIR fiber collimator assembly and excited by a λex ) 831 nm laser diode. Samples were diluted such that their optical densities were less than 0.05 at the excitation wavelength to minimize reabsorption and measured using 1.00 mm quartz cuvettes. (19) Loiseau, J.; Doe¨rr, N.; Suau, J. M.; Egraz, J. B.; Llauro, M. F.; Ladavie`re, C.; Claverie, J. Macromolecules 2003, 36, 3066. In the Supporting Information, the synthesis of DBT was accidentally omitted. Professor Claverie sent us his synthesis as a personal communication.

Ligand Exchange of PbS/OA with PAA

Langmuir, Vol. 24, No. 15, 2008 8217

Table 1. Characteristic Radii of PbS Quantum Dots As a Function of Their Surface Ligand and Solvent sample name

radius, nm, TEM

radius, nm,b DLS (polydispersity)c

PbS/OA in/from toluene PbS/PAA1 in/from THF PbS/PAA1 in/from methanol PbS/PAA1 in/from H2O

2.9 ( 0.5a 2.2 ( 0.6 2.5 ( 0.6 2.3 ( 0.5

4.7 (1.7) 7.0 (1.4) 6.0 (0.7) 4.5d (N/A)

a Calculated as one standard deviation from TEM image analysis. b Calculated from the first cumulant Γ of the autocorrelation decay. c Calculated from (µ2/ Γ), where µ is the second cumulant. d Estimated from the maximum of the peak corresponding to the fast component in the CONTIN plot (see Figure S2 in the Supporting Information). There is a large uncertainty in this value.

Transmission electron microscopy images were obtained using a TEI Technai 20 instrument equipped with a Gatan camera. The TEM images were analyzed for particle size distribution using Gatan Digital Micrograph 3.6.4 software. To prepare each specimen, a small drop of a solution of the PbS QDs in water or in an organic solvent (ca. 1 wt %) was placed onto a carbon-coated copper TEM grid (200 mesh, purchased from Electron Microscopy Science). PbS/PAA samples from THF, methanol, or water solution were used in these measurements. Each grid was dried in air. Dynamic light scattering measurements were carried out at 90° using a (multiangle) light scattering apparatus from ALV that has previously been described.12 Samples were examined at a single concentration, 0.5 mg/mL. Photoluminescence quantum yields (QY) were measured using the two-port integrating sphere approach as described by Chang et al.20 Two sample positions, corresponding to direct and indirect excitation regimes, were used, with collected power spectra acquired using a calibrated monochromator, lock-in amplifier, and liquid nitrogen-cooled infrared-sensitive germanium detector. Samples were excited at 831 nm, which is slightly blue shifted relative to the first excitonic peak of the nanocrystals studied.

Results and Discussion Size Analysis of the QDs by DLS and TEM. The colloidal QD solutions were characterized by dynamic light scattering, which provided the apparent hydrodynamic radii, Rhapp, of the particles. CONTIN plots21 (see Figure S2 in the Supporting Information) show a monomodal distribution for PbS/OA QDs in toluene centered at 4.7 nm, whereas the polymer-capped particles are shifted to larger values: Rhapp ) 7.0 nm for the THF solution, 6.0 nm for the solution in methanol, but only 4.5 nm in water (see Table 1). Analysis of the DLS data for the sample in water is complicated by the formation of a small amount of large aggregates, detected by a second peak in the CONTIN plot. No evidence for these aggregates can be found in TEM images such as that shown in Figure 1D. Additional images are provided in the Supporting Information. The hydrodynamic radius is largely determined by the size of the organic layer on the surfaces of the nanocrystals. Ligand exchange leads to replacement of the thin OA shell in PbS/OA with the solvent-swollen corona of PAA. Compared to OA, PAA acts as a multidentate ligand. Multidentate ligands provide enhanced coordination interactions due to the cooperative, amplifying effect of multiple binding sites. This type of ligand exchange, in which a multidentate ligand such as PAA replaces a larger number of monodentate ligands such as OA, is normally driven by the entropy increase in the system. The core size of the PbS QDs in the solid state was monitored by TEM. Representative examples of the images obtained are shown in Figure 1. We observed that ligand exchange led to a small (20) Chang, T.-W. F.; Maria, A.; Cyr, P. W.; Sukhovatkin, V.; Levina, L.; Sargent, E. H. Synth. Met. 2005, 148, 257. (21) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213.

decrease in the size of the inorganic core, from a mean radius of 2.9 ( 0.5 nm for PbS/OA from toluene solution to values for PbS/PAA of 2.2 ( 0.6 nm from THF, 2.5 ( 0.6 nm from methanol, and 2.3 ( 0.5 nm from water solution. While these differences are small and within experimental error, they are also reflected in the shift in the band-edge absorption maxima for the sample in THF before (λabs ) 1186 nm) and after (λabs ) 1159 nm) ligand exchange with PAA. It is well-known that one can relate the size of a QD to its band-edge absorbance wavelength. This small decrease in size is consistent with etching of QDs accompanying ligand exchange. More pronounced etching in the presence of excess ligand has been reported in ref 8. Ostwald ripening observed for PbS/OA QDs16 also likely involves ligandassisted solubilization of inorganic material. In our system, prior to exchange, the surface layer of PbS nanocrystals contained oleate groups coordinated to Pb atoms. We take the blue shift of the band-edge absorption maxima as evidence for the decrease in core size and infer that, during the ligand exchange, the oleic groups that were lost from the particles carried with them some of the Pb atoms, and possibly some of the S atoms as well, from the core surface. The characteristic radii determined by TEM and DLS for these QD samples, before and after ligand exchange, are summarized in Table 1. Optical Properties of the PbS QDs. We begin by comparing the band-edge absorption and photoluminescence spectra of PbS/ OA QDs in various solvents. These spectra are presented in Figure 2. In the figure, one can see that the band-edge absorption of PbS/OA QDs in hexane, toluene, THF, and chloroform solutions are very similar, with the absorption peak positions, λabs, at 1185 ( 3 nm. Photoluminescence spectra were taken for an excitation wavelength of 831 nm, which is blue shifted relative to the first excitonic peak of the PbS QDs. The PL spectra of all PbS/OA solutions exhibit sharp, symmetrical emission peaks. The emission spectra show a slight red shift in the THF solution, with a peak λem at 1299 nm, and in the chloroform solution, with λem at 1289 nm, compared to the toluene and hexane solutions, both with λem at 1284 nm. The red shifts in the emission spectra may reflect the higher solvent polarity of THF and the higher polarizability of chloroform, compared to toluene and hexane. The absorption and photoluminescence spectra of PbS QDs after ligand exchange are presented in Figure 3. The corresponding spectra for PbS/OA in toluene are presented for comparison. One can note that, after ligand exchange with PAA, the bandedge absorption spectra of PbS QDs show a blue shift in the THF solution, with the λabs value of 1159 nm, relative to the value of 1185 nm in THF before ligand exchange. As mentioned above, the shift of the band-edge to higher energies indicates that the size of the QDs decreased.22 In methanol, PbS/PAA shows λabs at 1162 nm, which is similar to the value in THF, while in water solution the absorption band-edge blue shift is more pronounced, with λabs ) 1138 nm. The blue shifts in the emission spectra are more significant than those in the absorption spectra. Prior to ligand exchange, the emission of PbS/OA in the THF solution was centered at 1299 nm, while following ligand exchange the PbS/PAA QDs in the THF solution exhibited a λem value of 1247 nm, a 52 nm shift to the blue. PbS/ PAA in methanol shows a blue shift in its PL spectrum similar to that in THF, with λem ) 1250 nm. The most significant blue shift in the emission spectra appears in the aqueous solution, where λem (22) Wise, F. W. Acc. Chem. Res. 2000, 33, 773.

8218 Langmuir, Vol. 24, No. 15, 2008

Lin et al.

Figure 1. TEM images of PbS/OA QDs from toluene solution (A) and of PbS/PAA QDs from THF (B), methanol (C), and water (D).

Figure 2. Normalized absorption and emission spectra of PbS/OA in various solvents. The curves have been shifted vertically for clarity, and the line at 1200 nm is a guide for the eye to shifts in λmax. The arrow at 831 nm indicates the excitation wavelength for the PL measurements.

) 1209 nm. The blue shifts in the emission peak are consistent with the smaller particle core size, and this effect is consistent with results reported in the literature.23 Note that in Figures 2 and 3 we have plotted the spectra on a wavelength scale, which tends to distort the line shapes. When plotted on an energy scale, the main absorption and emission bands are narrow bands with Gaussian profiles indicative of inhomogeneous line broadening.24 Surface trap states are present, but they are dark. While they do not contribute to the photoluminescence spectra, trap states do play a significant role in lowering the photoluminescence quantum yield. (23) Gallardo, S.; Gutierrez, M.; Henglein, A.; Janata, E. Ber. Bunnsen-Ges. 1989, 93, 1080. (24) (a) Salvador, M. R.; Graham, M. W.; Scholes, G. D. J. Chem. Phys. 2006, 125, 184709. (b) Fernee, M. J.; Jensen, P.; Rubinsztein-Dunlop, H. J. Phys. Chem. C 2007, 111, 4984–4989.

Figure 3. Normalized absorption and emission spectra of PbS/PAA in water, methanol, and THF compared to the spectra of PbS/OA in toluene. The curves have been shifted vertically for clarity, and the line at 1200 nm is a guide for the eye for shifts in λmax. The arrow at 831 nm indicates the excitation wavelength for the PL measurements.

The spectral shifts observed here are different from those reported by Hinds et al.17 for PbS/MTEG QDs, whose absorption and emission spectra were red shifted relative to that of PbS/OA in toluene. In their sample, the tetraethylene glycol groups that formed the corona were separated from the thiol binding group by 11 CH2 groups. This hydrocarbon layer in water should collapse around the PbS surface, placing it in a nonpolar environment. In the case of PbS/PAA, we expect an open, solvent-swollen corona, exposing the PbS surface to a polar environment. Band Broadening and Stokes Shifts. Another effect of modifying the PbS surface with PAA is a slight broadening of the absorption and emission spectra. The size distribution of QDs in solution can be estimated by comparing the peak width of the absorption or photoluminescence spectra of given samples.25 In Table 2, we report HWHM (half-width at the half-maximum) values on the long (25) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854.

Ligand Exchange of PbS/OA with PAA

Langmuir, Vol. 24, No. 15, 2008 8219

Table 2. Comparison of the Absorption and Emission Maxima (in nm and in eV), the Stokes Shifts in meV, and the PL Quantum Yields (QY) for PbS/OA in Various Solvents and for PbS/PAA in THF, Methanol, and Water solutions

λabs, nm

HWHM, meV

λabs, eV

λem, nm

FWHM, meV

λem, eV

Stokes shift, meV

QY, %

PbS/OA/hexane PbS/OA/toluene PbS/OA/THF PbS/OA/chloroform PbS/PAA/THF PbS/PAA/methanol PbS/PAA/water

1183 1185 1186 1184 1159 1162 1138

47 46 46 47 58 57 57

1.048 1.046 1.046 1.047 1.070 1.067 1.090

1284 1284 1299 1289 1247 1250 1209

106 109 107 116 122 132 150

0.966 0.966 0.955 0.962 0.994 0.992 1.026

82 80 91 85 76 75 64

62 82 58 81 42 9 24

wavelength side of the first absorption peak and FWHM (the full width at the half-maximum) values of the PL spectrum for all PbS QD samples. We observe that, prior to ligand exchange, the HWHM values for PbS/OA are 46-47 meV, while after ligand exchange the HWHM values for PbS/PAA increase to 57-58 meV. PbS QD samples with the same surface ligand possess similar HWHM values in different solvents. Compared to changes in the HWHM for the absorption spectra, the changes in the FWHM in the PL spectra are more sensitive to the solvents employed. For PbS/OA, the emission spectra show a FWHM value of 106 meV in hexane, 109 meV in toluene, 107 meV in THF solution, and 116 meV in chloroform. For PbS/PAA, the emission spectra show a FWHM value of 122 meV in THF, 132 meV in methanol, and 150 meV in water. All these results indicate a small increase in the width of the size distribution of the PbS QDs following ligand exchange. Another interesting feature of the optical spectra of the PbS QDs following ligand exchange is a decrease in the Stokes shift. The origin and nature of the Stokes shift in PbS QDs have been examined in detail by Ferne´e and co-workers.14a We evaluate the Stokes shifts as the energy difference (in meV) between λabs and λem maxima for each solution. For PbS/OA, the magnitude of the Stokes shift is 91 meV in THF, 85 meV in chloroform, 82 meV in hexane, and 80 meV in toluene. For PbS/PAA, the magnitude of the Stokes shift value drops to 76 meV in THF, 75 meV in methanol, and 64 meV in water. One explanation for the decrease in the Stokes shift is related to possible surface alterations leading to defects that create deep red radiative and nonradiative pathways for recombination of the electron and hole upon photoexcitation of the nanocrystals.26 Quantum Yields. To examine the influence of ligand exchange and chemical environment on the photoluminescence efficiency of the QDs, we measured the PL quantum yields (QY) in the infrared using the two-port integrating sphere approach proposed by de Mello et al.27,19 Two sample positions were used, corresponding to direct and indirect excitation regimes, with collected power spectra acquired using a calibrated monochromator, lock-in amplifier, and liquid nitrogen-cooled infrared-sensitive germanium detector. Sample excitation was at 831 nm. Values of the quantum yields obtained are listed in the right-hand column of Table 2. The quantum yields for original OA-capped PbS QDs are high and attest to the quality of the quantum dots synthesized. Small variations are noted between toluene as the solvent, compared to hexane or THF. Ligand replacement in THF led to a small decrease in the quantum yield. The measured quantum yield in methanol was low, on the order of 9%. In contrast, the quantum yield in water is significant (24%) and comparable to that reported for PbS/MTEG QDs by Hinds et al.17 We also note that the smaller Stokes shift of the PbS/PAA (26) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. J. Chem. Phys. 1997, 106, 9869. (27) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9, 230.

QDs in water leads to some reabsorption of emission in the samples used for the PLQY measurements. Thus, the value in water reported in Table 2 may be somewhat smaller than the actual quantum yield. Following ligand exchange and phase transfer, all of the PbS/ PAA samples gradually experienced a decrease in QY, accompanied by blue shifts of both the absorption and emission spectra. These comments pertain to samples exposed to air, and no attempts were made to handle samples in an inert atmosphere. These results suggest oxidative etching of the QD surface. Quantum dot quantum yields are highly sensitive to the surface characteristics of the nanocrystals, especially with respect to ligand chemistry. In the PAA-coated PbS QDs, we expect an open, solvent-swollen corona, exposing the PbS surface to a polar environment. Thus, oxygen in solution should have access to the QD surface. In contrast, the PbS/MTEG QDs reported in ref 17 are protected in water by the hydrophobic shell associated with 11 CH2 groups of the undecyl spacer separating the thiol from the tetraethylene glycol spacer.

Summary In this study, we describe a ligand exchange reaction of PbS/OA with poly(acrylic acid) (PAA) for the preparation of stable colloidal solutions of PbS quantum dots in polar solvents such as tetrahydrofuran, alcohols, and water. PAA serves as a multidentate ligand to displace oleic acid from the surface of PbS/OA QDs. Ligand exchange takes place in THF at room temperature and is accompanied by a small blue shift in the absorption spectrum (from λabs ) 1185 to 1159 nm for the band-edge maximum) and a small broadening of the band-edge absorption peak. These results (along with TEM measurements) indicate a small decrease in the size of the PbS nanoparticles and a slight broadening in the size distribution. Photoluminescence spectra of the PbS/PAA QDs show a pronounced blue shift in water compared to THF, indicating that the solventswollen PAA corona chains leave the surface of the PbS nanoparticles exposed to solvent. The as-prepared PbS/OA QDs have a quantum yield of 82% in toluene. After transfer to water, the quantum yield dropped to 24%, which is still substantial for these QDs in water. Solutions of PbS/PAA experienced slow degradation of the quantum yields and small blue shifts in the absorption and emission spectra upon aging in the presence of air. Acknowledgment. The authors thank the NSERC Canada for financial support and Dr. Neil Coombs for assistance with the TEM measurements. Supporting Information Available: Experimental details on the synthesis of DBT and poly(acrylic acid), CONTIN plots from dynamic light scattering measurements on the PbS/OA and PbS/PAA quantum dots, plus additional TEM images of PbS/PAA from aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA800568K