pubs.acs.org/Langmuir © 2010 American Chemical Society
Tuning the Emission of CdSe Quantum Dots by Controlled Trap Enhancement David R. Baker and Prashant V. Kamat* Radation Laboratory, Departments of Chemical and Biomolecular Engineering, and Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received February 8, 2010. Revised Manuscript Received March 25, 2010 Ligand exchange with 3-mercaptopropionic acid (MPA) has been successfully used to tune the emission intensity of trioctylphosphineoxide/dodecylamine-capped CdSe quantum dots. Addition of 3-mercaptopropionic acid (MPA) to CdSe quantum dot suspension enhances the deep trap emission with concurrent quenching of the band edge emission. The smaller sized quantum dots, because of larger surface/volume ratio, create a brighter trap emission and are more easily tuned. An important observation is that the deep trap emission which is minimal after synthesis is brightened to have a quantum yield of 1-5% and can be tuned based on the concentration of MPA in solution with the quantum dots. Photoluminescence decay and transient absorption measurements reveal the role of surface bound MPA in altering the photophysical properties of CdSe quantum dots.
Introduction Semiconductor nanocrystal-based solar cells are gaining interest because of the ability to tune their photoresponse by size and surface manipulation.1-3 Of particular interest are quantum dot sensitized solar cells which typically employ a metal chalcogenide nanocrystal (e.g., CdSe) anchored onto an oxide semiconductor (e.g., TiO2).4-9 A bifunctional linker molecule, such as 3-mercaptopropionic acid (MPA), assists in joining the two semiconductors close together with a uniform coverage.10,11 By varying the alkyl chain length of the linker molecule, it was shown that electron injection yields increase with decreasing interparticle separation.12 However, thiols, such as MPA, can alter the *To whom correspondence should be addressed. (1) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. (2) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M. K.; Kamat, P. V. Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture. J. Am. Chem. Soc. 2008, 130, 4007-4015. (3) Weiss, E. A.; Chiechi, R. C.; Geyer, S. M.; Porter, V. J.; Bell, D. C.; Bawendi, M. G.; Whitesides, G. M. Size-dependent charge collection in junctions containing single-size and multi-size arrays of colloidal CdSe quantum dots. J. Am. Chem. Soc. 2008, 130, 74-82. (4) Liu, L.; Hensel, J.; Fitzmorris, R. C.; Li, Y.; Zhang, J. Z. Preparation and Photoelectrochemical Properties of CdSe/TiO2 Hybrid Mesoporous Structures. J. Phys. Chem. Lett. 2009, 155-160. (5) Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778-17787. (6) Bang, J. H.; Kamat, P. V. Quantum Dot Sensitized Solar Cells. CdTe versus CdSe Nanocrystals. ACS Nano 2009, 3, 1467-1476. (7) Kim, J. Y.; Choi, S. B.; Noh, J. H.; HunYoon, S.; Lee, S.; Noh, T. H.; Frank, A. J.; Hong, K. S. Synthesis of CdSe-TiO2 Nanocomposites and Their Applications to TiO2 Sensitized Solar Cells. Langmuir 2009, 25, 5348-5351. (8) Baker, D. R.; Kamat, P. V. Disassembly, Reassembly and Photoelectrochemistry of Etched TiO2 Nanotubes. J. Phys. Chem. C 2009, 113, 17967-17972. (9) Liu, L.; Hensel, J.; Fitzmorris, R. C.; Li, Y.; Zhang, J. Z. Preparation and Photoelectrochemical Properties of CdSe/TiO2 Hybrid Mesoporous Structures. J. Phys. Chem. Lett. 2009, 1, 155-160. (10) Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848-1857. (11) Guijarro, N.; Lana-Villarreal, T.; Mora-Sero, I.; Bisquert, J.; Gomez, R. CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment. J. Phys. Chem. C 2009, 113, 4208-4214. (12) Dibbell, R. S.; Watson, D. F. Distance-Dependent Electron Transfer in Tethered Assemblies of CdS Quantum Dots and TiO2 Nanoparticles. J. Phys. Chem. C 2009, 113, 3139-3149.
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photophysical properties of semiconductor quantum dots by interacting with surface atoms.13 Thiols, as well as amines, are known to act as hole scavengers which quench photoluminescence of CdSe nanocrystals.14,15 However, when they attach onto quantum dots, the ligands change the surface properties including emission decay rates and surface passivation of dangling bonds.16,17 Amines, for example, will increase the quantum yield if bound to the surface despite their hole scavenging properties in solution, a primary reason for their use in synthesis.18 If not properly passivated, selenium can easily oxidize and be removed from the surface of the nanocrystal, creating a selenium vacancy.19 Chalcogenide vacancies in quantum dots have been shown to be the origin of deep trap emission,20-22 and if ligand exchange can cause selenium vacancy, there should be an associated rise in deep trap emission. Size control of quantum dots in the synthesis procedure allows for different peak emission wavelengths. Tuning the emission (13) Kalyuzhny, G.; Murray, R. W. Ligand effects on optical properties of CdSe nanocrystals. J. Phys. Chem. B 2005, 109, 7012-7021. (14) Klimov, V. I.; Mikhailovsky, A. A.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Mechanisms for intraband energy relaxation in semiconductor quantum dots: The role of electron-hole interactions. Phys. Rev. B 2000, 61, R13349-R13352. (15) Sharma, S.; Pillai, Z. S.; Kamat, P. V. Photoinduced charge transfer between CdSe nanocrystals and p-phenelenediamine. J. Phys. Chem. B 2003, 107, 10088-10093. (16) Hill, N. A.; Whaley, K. B. A Theoretical-Study of the Influence of the Surface on the Electronic-Structure of Cdse Nanoclusters. J. Chem. Phys. 1994, 100, 2831-2837. (17) Haesselbarth, A.; Eychmueller, A.; Weller, H. Detection of shallow electron traps in quantum sized CdS by fluorescence quenching experiments. Chem. Phys. Lett. 1993, 203, 271-6. (18) Xie, R. G.; Kolb, U.; Li, J. X.; Basche, T.; Mews, A. Synthesis and characterization of highly luminescent CdSe-Core CdS/Zn0.5Cd0.5S/ZnS multishell nanocrystals. J. Am. Chem. Soc. 2005, 127, 7480-7488. (19) Henglein, A. Physicochemical properties of small metal particles in solution: “Microelectrode” reactions, chemisorption, composite metal particles, and the atom-to-metal transition. J. Phys. Chem. 1993, 97, 5457-71. (20) Landes, C. F.; Braun, M.; El-Sayed, M. A. On the nanoparticle to molecular size transition: Fluorescence quenching studies. J. Phys. Chem. B 2001, 105, 10554-10558. (21) Ramsden, J. J.; Graetzel, M. Photoluminescence of small cadmium sulphide particles. J. Chem. Soc., Faraday Trans. 1 1984, 80, 919-33. (22) Lambe, J. J.; Klick, C. C.; Dexter, D. L. Nature of Edge Emission in Cadmium Sulfide. Phys. Rev. 1956, 103, 1715-1720.
Published on Web 04/07/2010
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Baker and Kamat Scheme 1. Exchange of Dodecylamine with 3-Mercaptopropionic Acid and Its Effect on Charge Recombination Dynamics of TOPOCapped CdSe Quantum Dots
band thus requires preparation of different batches of quantum dots. Variations in quantum yield can be accomplished by changing the suspension solvent after formation,23-25 but emission wavelengths are confined nearly to the original spectrum. Several groups have investigated the idea of white emitting CdSe quantum dots and have succeeded by synthesizing “magic sized” dots which have a high deep trap contribution due to their relatively high surface-to-volume ratios.26-30 Recent success in doping of ZnSe with Mn2þ has also provided new ways to achieve bright emission in the visible region.31,32 The amount of deep traps on the surface is populous enough to be in similar brightness to the band edge emission. The deep trap emission peaks are broad due to the deep trap energy level being affected by many factors, most significantly variations in the surface environment.33,34 If the deep trap peak is sufficiently broad, and the band edge emission is not too bright, the quantum dot can emit what appears to the eye as “white light”. By controlling the deep trap population after formation, one would be able to change the relative intensity of the emission peaks without synthesizing a new batch. We have now investigated the surface interaction of MPA with different size quantum dots and the effect of ligand exchange on the photophysical properties. The emission and transient absorption measurements that elucidate the role of deep traps in modulating the emission properties of CdSe quantum dots are described. (23) Aryal, B. P.; Benson, D. E. Electron donor solvent effects provide biosensing with quantum dots. J. Am. Chem. Soc. 2006, 128, 15986-15987. (24) Liang, J. G.; Zhang, S. S.; Ai, X. P.; Ji, X. H.; He, Z. K. The interaction between some diamines and CdSe quantum dots. Spectrochim. Acta, Part A 2005, 61, 2974-2978. (25) Selmarten, D.; Jones, M.; Rumbles, G.; Yu, P. R.; Nedeljkovic, J.; Shaheen, S. Quenching of semiconductor quantum dot photoluminescence by a pi-conjugated polymer. J. Phys. Chem. B 2005, 109, 15927-15932. (26) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y. Q.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. Sequential growth of magic-size CdSe nanocrystals. Adv. Mater. 2007, 19, 548. (27) Bowers, M. J.; McBride, J. R.; Rosenthal, S. J. White-light emission from magic-sized cadmium selenide nanocrystals. J. Am. Chem. Soc. 2005, 127, 15378-15379. (28) Sapra, S.; Mayilo, S.; Klar, T. A.; Rogach, A. L.; Feldmann, J. Bright white-light emission from semiconductor nanocrystals: by chance and by design. Adv. Mater. 2007, 19, 569. (29) Qian, L.; Bera, D.; Holloway, P. H. Temporal evolution of white light emission from CdSe quantum dots. Nanotechnology 2008, 19. (30) Ouyang, J.; Zaman, M. B.; Yan, F. J.; Johnston, D.; Li, G.; Wu, X.; Leek, D.; Ratcliffe, C. I.; Ripmeester, J. A.; Yu, K. Multiple families of magic-sized CdSe nanocrystals with strong bandgap photoluminescence via noninjection one-pot syntheses. J. Phys. Chem. C 2008, 112, 1380513811. (31) Acharya, S.; Sarma, D. D.; Jana, N. R.; Pradhan, N. An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals. J. Phys. Chem. Lett. 2009, 1, 485-488. (32) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. High-quality manganese-doped ZnSe nanocrystals. Nano Lett. 2001, 1, 3-7. (33) Eychmueller, A.; Haesselbarth, A.; Katsikas, L.; Weller, H. Photochemistry of semiconductor colloids. 36. Fluorescence investigations on the nature of electron and hole traps in Q- sized colloidal CdS particles. Ber. Bunsenges. Phys. Chem. 1991, 95, 79-84. (34) Leung, K.; Whaley, K. B. Surface relaxation in CdSe nanocrystals. J. Chem. Phys. 1999, 110, 11012-11022.
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Experimental Section CdSe quantum dots were made using a procedure described in previous papers.2,35 Trioctylphosphine oxide (4 g), tetradecylphosphonic acid (0.6 g), dodecylamine (various amounts), and cadmium oxide (0.1 g) were degassed under vacuum while stirring (1 h) followed by introduction of a nitrogen atmosphere. The mixture was then heated to 300 °C; 1 M trioctylphosphine selenide (0.5 mL) and trioctylphosphine (8 mL) were then injected into the cadmium precursor solution. When the desired color was reached, the reaction vessel was cooled to stop crystal growth. Quantum dots were “washed” by adding methanol to precipitate them from solution. After centrifugation the quantum dots were resuspended in toluene, and the washing procedure was conducted a total of three times and then stored for at least 24 h before use. Particle sizes were determined from the position of the first excitonic absorbance peak.36 A 1 M MPA stock solution was made in acetonitrile, and from this stock solution dilutions of 0.1 and 0.01 M were made also using acetonitrile as the solvent. Quantum dot/MPA mixtures were made by creating a master CdSe solution of a predetermined concentration and then split into the various samples to minimize sample-to-sample variation. 10 or 20 μL of the MPA solutions was added to the quantum dot solutions to create the desired MPA concentration. The samples were left to equilibrate in the dark for a minimum of 2 h before any tests were performed. Photoluminescence steady state measurements were made with a Jobin Yvon Fluorlog-3. Samples were excited at a wavelength of 410 nm. Photoluminescence decay measurements were made with a Jobin Yvon single photon counting system with a 373 nm LED excitation source. Both steady state and decay measurements were conducted in a 1 cm quartz cell. Ultrafast transient absorption spectra were recorded using a Clark MXR-2010 775 nm fundamental laser system as described in previous studies.35,37 The samples were excited with 150 ps pulses of the second harmonic (387 nm) at 1 kHz in a 2 mm quartz cell. The incident power was adjusted to keep carrier densities at 0.1 excitons/quantum dot/s, minimizing the probability of Auger recombination. Changes in absorbance were measured using an Ocean-Optics S2000 UV-vis CCD spectrograph; the data were compiled and analyzed using Ultrafast System’s Helios software.
Results and Discussion Addition of MPA to a toluene solution of CdSe quantum dots causes significant changes to their emission properties. The emission spectra recorded in Figure 1A exhibit two distinct features arising from the mixture of MPA and CdSe (2.6 nm diameter): (i) quenching of the band edge emission and (ii) appearance of deep trap emission in the 600-750 nm region. The deep trap emission, arising from Se vacancies, increases with increasing MPA concentration and is concurrent with a decrease in the band edge emission. At higher concentrations of MPA (>1 mM) one observes the quenching of both band edge and deep trap emission bands, though the deep trap peak intensity continues to grow with respect to the band edge emission. The deep trap emission arising from the MPA interaction is sufficiently bright with a quantum yield in the range of 1-5%. The quantum yield of the entire sample spectrum shown in Figure 1A was initially 1.3%, which increased at first due to the deep trap enhancement until a maximum of 2.4% at 100 μM MPA but then decreased to 1.1% at (35) Robel, I.; Kuno, M.; Kamat, P. V. Size-Dependent electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 4136-4137. (36) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals. Chem. Mater. 2003, 15, 2854-2860. (37) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. J. Am. Chem. Soc. 2006, 128, 2385-2393.
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Figure 1. (A) Photoluminescence spectra and (B) absorbance spectra (normalized to the excitonic peak) of CdSe quantum dots (2.6 nm diameter) with different concentrations of MPA in solution: (a) 0 M, (b) 50 μM, (c) 100 μM, (d) 500 μM, (e) 1 mM, (f) 5 mM, and (g) 10 mM. The inset of (A) is a double reciprocal plot representing the quenching of the band edge emission at 497 nm.
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Figure 3. Normalized photoluminescence spectra of (A) CdSe (a, c) with and (b, d) without DDA used in synthesis (a, b) before and (c, d) after addition of 1 mM MPA. (B) Photoluminescence spectra of different sized CdSe quantum dots with 1 mM MPA in solution. Samples were excited at 410 nm.
quantum dot. The method described in this paper provides a route to tune the emission intensity and spectrum solely by manipulating the concentration of a chemical species to prepared samples, a flexibility previously unseen. Since the decrease in the band edge emission reflects surface interaction with MPA, we can estimate the association constant (Kapp) of the surface complexation phenomenon (equilibrium 1) by monitoring the emission intensity (I) at different MPA concentrations. Kapp
CdSe þ MPA S ðCdSe 3 3 3 MPAÞ Figure 2. CdSe quantum dots (2.6 nm diameter) fluorescing under 366 nm UV irradiation with different concentrations of MPA in solution (a) 0 M, (b) 50 μM, (c) 100 μM, (d) 500 μM, (e) 1 mM, (f) 5 mM, and (g) 10 mM.
10 mM MPA almost entirely consisting of deep trap emission. Brighter dots, with initial quantum yields of 5-10%, showed similar trends as those in Figure 1. Figure 2 shows how the balance between band edge and deep trap intensities changes the apparent emission to the eye; at 100 μM MPA the emission is close to white. In addition to intensity changes in the emission spectra, a small red shift was observed in the absorption maximum (first excitonic peak), indicating the influence of surface interactions between MPA and CdSe quantum dots (Figure 1B). The absorbance changes associated with surface complexation have been noted in earlier studies13,38 where the authors describe how Cd-thiol bonds affect the electronic structure of the CdSe (38) Koole, R.; Luigjes, B.; Tachiya, M.; Pool, R.; Vlugt, T. J. H.; Donega, C. D. M.; Meijerink, A.; Vanmaekelbergh, D. Differences in cross-link chemistry between rigid and flexible dithiol molecules revealed by optical studies of CdTe quantum dots. J. Phys. Chem. C 2007, 111, 11208-11215.
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ð1Þ
Kapp can be determined from the slope and intercept of the double reciprocal plot using eq 2.39 I° I° I° ¼ þ I° - I I° - I 0 Kapp ðI° - I 0 Þ½MPA
ð2Þ
The inset of Figure 1A further supports the existence of association equilibrium between CdSe and MPA. From the slope and intercept of the double reciprocal plot we obtain Kapp as 3730 M-1. This value is of a similar order of magnitude as previously determined for other complexing species with CdS nanoparticles.39-41 (39) Kamat, P. V.; Dimitrijevic, N. M.; Fessenden, R. W. Photoelectrochemistry in particulate systems. 6. Electron-transfer reactions of small CdS colloids in acetonitrile. J. Phys. Chem. 1987, 91, 396-401. (40) Kamat, P. V.; Dimitrijevic, N. M. Photoelectrochemistry in semiconductor particulate systems. 13. Surface modification of CdS semiconductor colloids with diethyldithiocarbamate. J. Phys. Chem. 1989, 93, 4259-63. (41) Mann, J. R.; Watson, D. F. Adsorption of CdSe Nanoparticles to Thiolated TiO2 Surfaces: Influence of Intralayer Disulfide Formation on CdSe Surface Coverage. Langmuir 2007, 23, 10924-10928.
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Figure 4. (A) Photoluminescence decay of CdSe (3.1 nm) with 1 mM MPA in solution at various wavelengths (500-700 nm). (B) Fitted lifetimes for the traces from (A) versus wavelength both with and without MPA.
Previous work by Koole et al. has shown that amines are readily displaced by thiols on the surface of CdSe while TOPO and TOP are not as easily removed.42 The effect amines play on the surface ligand exchange was further assessed by preparing CdSe quantum dots without DDA as one of the capping constituents. Addition of MPA to the suspension of CdSe capped with only TOPO failed to show emission in the red region arising from Se vacancies (Figure 3A). On the other hand, a similarly prepared CdSe QD suspension capped with DDA and TOPO shows an emission growth in the red region, thus demonstrating the importance of the ligand exchange of thiol with amine in the enhancement of deep trap emission. When CdSe quantum dots are made with the procedure used herein the Se surface atoms are passivated with TOP. Though MPA will not exchange with TOP42 the surface environment will still change due to existence of a new species. In this case the thiol attached to surface cadmium is expected to form an R-S-Cd bond stronger than the original amine R-NH-Cd bond.42 The surface Cd-Se bonds are thus thought to be weakened or removed, resulting in exposed Se which can be oxidized and removed from the crystal. If indeed the surface interactions play a major role in inducing deep trap emission, one would expect to see particle size dependence since surface to bulk atom ratio decreases with increasing particle size. MPA was therefore added to three different suspensions of CdSe QD of varying particle size. CdSe quantum dots of particle diameter 2.6, 2.8, and 3.5 nm were prepared using the same proportions of TOPO, TOP, and DDA for surface capping agents. Figure 3B shows emission spectra recorded following addition of MPA (1 mM) to each of these suspensions. The intensity ratio of the band edge:trap emission strongly depends on the size of the CdSe quantum dots. As the particle size decreases, the relative contribution of deep trap emission increases, indicating a varying degree of MPA influence on the excited state dynamics of CdSe based on the surface-to-volume ratio. We further probed the influence of MPA interactions using photoluminescence decay (Figure 4). Emission decays monitored at different wavelengths were analyzed using stretched exponential decay kinetics (see Supporting Information for detailed analysis). The fitted lifetimes at various wavelengths are summarized in Figure 4B. The band edge exhibits a lifetime noticeably shorter (42) Koole, R.; Schapotschnikow, P.; Donega, C. D.; Vlugt, T. J. H.; Meijerink, A. Time-dependent photoluminescence spectroscopy as a tool to measure the ligand exchange kinetics on a quantum dot surface. ACS Nano 2008, 2, 1703-1714.
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than the deep trap wavelengths; refer to the Supporting Information for analyzed values. At the band edge peak wavelength (530 nm) the presence of 1 mM MPA caused the fitted lifetime to decrease by a factor of ∼3 from 8.0 to 2.6 ns. The lifetimes in the deep trap (625-700 nm) are almost equal, implying that MPA induces electron scavenging from the band edge states, but the deep trap states remain relatively unaffected. These results suggest that the nature of trap emission remains the same with or without MPA, but the population of these vacancies increases in the presence of MPA as evidenced by the increase in deep trap emission intensity. Earlier studies have employed changes in the band edge emission to probe the charge transfer process in CdS and CdSe systems.2,43,44 If the decrease in band edge emission lifetime is attributed to electron transfer from band edge states to trap states, we can employ eq 3 to determine the rate constant of interstate electron transfer (ket). ket ¼ 1=τCdSe þ
MPA - 1=τCdSe
ð3Þ
The τCdSe and τCdSeþMPA are the lifetimes of CdSe in the absence and presence of MPA. By substituting the lifetime values obtained from the band edge emission (530 nm), we obtain a ket of 2.6 108 s-1. When a double-exponential decay is applied to the photoluminescence decay, the contribution of the fast component increases with increasing concentration of MPA (see Figure S3) while the slow component similarly decreases, implying that there are two types of emission which change population based on concentration of MPA, as would be suggested by expression 1. The change in photoluminescence decay has serious implications for electron transfer studies which rely on photoluminescence decay to determine electron transfer rates. Since most decay measurements are taken of the band edge peak, the contribution of MPA to the decrease in lifetime must be accounted for to obtain an accurate value of electron transfer to other species such as TiO2. Deep trap enhancement by MPA addition also manifests itself in ultrafast transient absorption spectroscopy. The transient absorption spectra recorded following excitation of 2.4 nm CdSe by 387 nm laser pulses, in the absence and presence of MPA, are (43) Sant, P. A.; Kamat, P. V. Inter-Particle Electron Transfer between SizeQuantized CdS and TiO2 Semiconductor Nanoclusters. Phys. Chem. Chem. Phys. 2002, 4, 198-203. (44) Farrow, B.; Kamat, P. V. CdSe Quantum Dot Sensitized Solar Cells. Shuttling Electrons Through Stacked Carbon Nanocups. J. Am. Chem. Soc. 2009, 131, 11124-11131.
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Figure 5. Transient absorption spectra for CdSe (2.4 nm) quantum dots (A) without and (B) with 1 mM MPA at various times after the pump pulse. (C) Decay traces measured at 464 nm of (A) and (B).
shown in Figure 5A,B. The bleaching recoveries, which correspond to the depopulation of excited electrons via recombination or interfacial charge transfer, are shown in Figure 5C. It is interesting to note that the binding of MPA to CdSe quantum dots slows the bleaching recovery. The slower recovery stems from newly formed Se vacancies trapping electrons that otherwise would recombine quickly to the valence band. Since the recombination mediated by the trap sites is slower than the direct recombination of charge carriers in the valence and conduction bands, we observe a slower bleaching recovery in the monitoring time scale of 0.5 ns. These results further confirm that the primary effect of MPA interaction with CdSe quantum dots is to introduce deep traps (Se vacancies). The generation of many traps on the surface of CdSe quantum dots has serious implications for semiconductor sensitized solar cells. Limiting carrier losses from interfacial transport has been the subject of numerous studies.5,10,35 Maximizing the electron transfer from the quantum dot to the metal oxide support is one of the common ways to improve a cell’s efficiency. Though MPA causes better surface coverage and closer linking of CdSe on TiO2, the introduction of traps on the CdSe surface may hinder the maximum electron transfer rate to the wide bandgap semiconductor. A new linker molecule may need to be designed for attachment and distribution of quantum dots onto TiO2 or other oxide supports because the losses at the traps are substantial enough to encompass the entire emission spectrum with a substantial quantum yield (Figure 1A). The binding of MPA to CdSe was found to be quite strong as the bound MPA molecules are retained even after precipitation and resuspension. The quantum dots, following MPA treatment, were precipitated from the toluene suspension by the addition of methanol, similar to the washing steps in the synthesis procedure. The supernatant was removed, and the dots were resuspended in toluene. Since unbound MPA remaining in solution is removed with the supernatant during the washing process, the emission spectrum reflects the effects arising from surface-bound MPA. The normalized emission spectra recorded before and after the
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“washing” procedure show extremely good overlap (see Figure S1 in the Supporting Information). These results further ascertain that the deep trap emission enhancement arises solely from the binding of MPA to the surface of the CdSe quantum dots and not from the solvent effects. Peng and co-workers have shown that MPA bound to the surface of CdSe is susceptible to degradation from excited CdSe.45 Though ligand exchange may not appear to be reversible, the removal of MPA from the CdSe surface may still be achieved. This raises an interesting question of whether the surface bound MPA can survive during the operation of a quantum dot solar cell. Efforts are currently underway to address this question and examine the role of rigid versus flexible linker molecules.
Conclusions The results presented here show that MPA interaction with CdSe creates emissive deep trap sites (Se vacancies) by exchanging with surface bound amines (DDA). The intensity of the enhanced deep trap and quenched band edge emission are readily tuned by controlling the concentration of MPA. Smaller sized quantum dots exhibit a higher level of deep trap recombination due to a larger surface to volume ratio. The introduction of MPA also resulted in changes to transient absorption and photoluminescence decay, tests typically used to measure electron transfer rates to TiO2. These effects must be accounted for when characterizing systems with electron acceptors. Acknowledgment. The research described herein was supported by the Department of Energy, Office of Basic Energy Sciences. This is Contribution NDRL 484X from the Notre Dame Radiation Laboratory. Supporting Information Available: Emission spectra of unwashed and washed samples and the analysis of emission (45) Aldana, J.; Wang, Y. A.; Peng, X. G. Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols. J. Am. Chem. Soc. 2001, 123, 8844-8850.
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