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The Effects of Chemisorption on the Luminescence of CdSe Quantum Dots C. Bullen† and P. Mulvaney*,†,‡ Chemistry School, UniVersity of Melbourne, ParkVille, Victoria 3010, Australia, and Stiftung Caesar, Ludwig-Erhard Allee 2, Bonn 53175, Germany ReceiVed July 13, 2005. In Final Form: December 13, 2005 We report on the effects of Lewis bases and other ligands on radiative recombination in CdSe quantum dots (QDs) in several solvents. Long-chain primary amines are found to be the most efficacious capping agents for CdSe QDs in nonpolar solvents. Primary alkylamines are superior to secondary and tertiary alkylamines. The kinetics of chemisorption and desorption in less polar solvents, such as hexane or chloroform, are temperature controlled and obey a Langmuir isotherm. Mercaptan adsorption also obeys a Langmuir isotherm, and alkylmercaptans rapidly displace amines, leading to luminescence quenching. In more polar solvents, such as toluene, ligands desorb, leading to luminescence quenching. It is proposed that surface Cd vacancies function as nonradiative recombination centers. The adsorption of a Lewis base to the QD raises the surface vacancy energy close to, or above, the conduction band edge and eliminates electron capture by the surface vacancies. Solvent polarity has a strong effect on luminescence since the solvent determines the extent of ligand adsorption to the QD surface.
Introduction The processes determining the luminescence quantum yields in semiconductor quantum dots (QDs) have been scrutinized for some 20 years. A range of nanocrystals emitting luminescence from the near-UV to the near-infrared (NIR) have been developed. However, the creation of photostable fluorophores is possible only if all the relaxation processes in these materials can be elucidated. The surface chemistry plays a central role in the manipulation of these materials because it not only determines the dispersion interactions of the particles in the embedding medium, but it is increasingly apparent that high quantum yields and long term photostability can be achieved only when surface recombination processes are better understood. Consequently, the ligands and adsorbates present during and after nanocrystal synthesis are integral to the overall electronic function of the particles. They play at least four distinct roles. First, they are present during the nucleation process and determine the reactivity and availability of the crystal precursors and monomers. Second, they control the rate of growth and final particle size distribution. Third, they provide colloid stability, preventing aggregation and growth. Finally, they interact electronically with surface sites and may passivate surface defects and intrinsic mid-band-gap energy states. The fundamental importance of surface states on bulk semiconductors has long been recognized.1 Surface passivation was demonstrated for CdS nanoparticles by Spanhel et al.2 These authors demonstrated that polyphosphate ions at high pH caused a drastic enhancement of the band-edge luminescence. Excess Cd ions were necessary for this process to be efficacious. McLendon et al. reported strong enhancement of the trap emission from CdS colloids upon addition of amines,3 while several authors showed that mercaptans quench the luminescence. Following * Author for correspondence. E-mail:
[email protected]. † University of Melbourne. ‡ Stiftung Caesar. (1) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (2) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. Photochemistry of Colloidal Semiconductors 20. Surface Modification and Stability of Strongly Luminescing CdS Particles. J. Am. Chem. Soc. 1987, 109, 5649. (3) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. L. Photophysics of Quantized Colloidal Semiconductors: Dramatic Luminescence Enhancement by Binding of Simple Amines. J. Phys. Chem. 1986, 90, 6074.
the development of an organometallic route for CdSe synthesis by Murray and colleagues in 1993, attention has focused on CdSe.4 There have been a number of influential papers on the effects of the adsorption of ligands on the luminescence from CdSe single crystals, particularly the extensive work by the Ellis group.5-9 A key issue in such studies is the state of the native crystal surface. For adsorption studies on bulk CdSe single crystals, the reference point may be a freshly cleaved surface or a crystal face etched using a bromine/methanol solution.10 However colloid particles must be stabilized against particle aggregation before, during, and after adsorption by a ligand. Hence, it is not possible to prepare “pure” CdSe nanocrystal surfaces with no adsorbates as a reference for studying ligand effects in solution. Nearly “naked” nanocrystals can be prepared by the surface exchange of trioctylphosphine oxide (TOPO) for the weakly coordinating solvent pyridine,11 but this system is not ideal for adsorption studies. In particular, Mattousi et al. reported small-scale aggregation of nanocrystals capped with small molecules such as pyridine.12 This aggregation severely com(4) Murray, C. B.; Nirmal, M.; Norris, D. J.; Bawendi, M. G. Synthesis and Structural Characterization of II-VI Semiconductor Nanocrystallites (Quantum Dots). Z. Phys. D: At., Mol. Clusters 1993, 26, S231. (5) Fujii, T.; Tanaka, N.; Tai, H.; Obara, S.; Ellis, A. B. Influence of a Naphthalene Derivative on the Luminescence Properties of CdS Particles Prepared by the Sol-Gel Method. Bull. Chem. Soc. Jpn. 2000, 73, 809. (6) Murphy, C. J.; Lisensky, G. C.; Leung, L. K.; Kowach, G. R.; Ellis, A. B. Photoluminescence-Based Correlation of Semiconductor Electric Field Thickness with Adsorbate Hammett Substituent Constants. Adsorption of Aniline Derivatives onto Cadmium Selenide. J. Am. Chem. Soc. 1990, 112, 8344. (7) Murphy, C. J.; Ellis, A. B. Evidence for Adduct Formation at the Semiconductor-Solution Interface: Photoluminescent Properties of Cadmium Selenide in the Presence of Lanthanide β-Diketonate Complexes. J. Phys. Chem. 1990, 94, 3082. (8) Neu, D. R.; Olson, J. A.; Ellis, A. B. Photoluminescence as a Probe of the Adsorption of Gaseous Boranes onto the Surface of Cadmium Selenide Crystals. J. Phys. Chem. 1993, 97, 5713. (9) Cohen, R.; Kronik, L.; Shanzer, A.; Cahen, D.; Liu, A.; Rosenwaks, Y.; Lorenz, J. K.; Ellis, A. B. Molecular Control over Semiconductor Surface Electronic Properties: Dicarboxylic Acids on CdTe, CdSe, GaAs, and InP. J. Am. Chem. Soc. 1999, 121, 10545. (10) Zhang, J. Z.; Geselbracht, M. J.; Ellis, A. B. Binding of Fullerenes to Cadmium Sulfide and Cadmium Selenide Surfaces, Photoluminescence as a Probe of Strong, Lewis Acidity-Driven, Surface Adduct Formation. J. Am. Chem. Soc. 1993, 115, 7789. (11) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019.
10.1021/la051898e CCC: $33.50 © 2006 American Chemical Society Published on Web 02/23/2006
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plicates the interpretation of fluorescence-based adsorption studies, since aggregation affects luminescence.13 Moreover, pyridine-capped nanocrystals do not form stable dispersions in solvents such as chloroform and hexane, and this limits the range of adsorbates that can be investigated. Recently, Kalyuzhny et. al. carefully investigated the effect of various additives on the luminescence of very small CdSe nanocrystals prepared in TOPO from cadmium salt precursors.14 They have proposed that trioctylphosphine (TOP) adsorption leads to surface-state emission from CdSe nanocrystals. In this study, we have elected to use CdSe nanocrystals stabilized by TOPO. While not ideal, such CdSe nanocrystals prepared by the organometallic method of Murray et al.15 are colloidally stable in a range of nonpolar organic solvents.12 The sterically bulky and weakly bound TOPO molecules are readily substituted by a variety of other ligand molecules. In the first part of the study, we focus on the effects of aging on the luminescence of CdSe QDs. This provides an essential reference point for understanding the perturbations caused by ligands. We then attempt to model the adsorption process more closely once we are confident that equilibrium can be established. Experimental Procedures Chemicals. Unless otherwise indicated, chemicals were sourced from Aldrich, stored in ambient laboratory conditions and used without further purification. Aliphatic primary amines (CnH2n+1NH2; n ) 2-18) were all above 95% purity. Technical-grade (90%) TOPO was obtained from both Cytec and Aldrich, while TOP (90%) was procured from Aldrich and was stored in a sealed bottle under nitrogen. Oleic acid (tech.) was sourced from BDH, and stearic acid (tech.) was obtained from Aldrich. Chloroform, toluene, hexane, methanol, and ethanol solvents were analytical grade and all sourced from BDH. Dimethylcadmium was prepared from methylmagnesium bromide and cadmium bromide in diethyl ether using standard inert atmosphere techniques. Nanocrystal Samples. The nanocrystal samples are labeled herein according to the position of the first excited state in the absorption spectrum and with a letter to denote the surface ligand on the surface. Since all the as-prepared samples are synthesized in TOP/TOPO, we denote such samples with a T. For consistency of presentation, most of the data presented here are from two samples, T543 and T570, with average particle sizes of 2.9 and 3.5 nm, respectively, based on the sizing relationship of Yu et al.16 Sample T570 was prepared by injecting a 5 mL TOP solution containing 0.1 mL dimethyl cadmium (1.4 mmol) and selenium (1 mmol) into 5 g of TOPO at 350 °C. This was followed 20 min later by a further identical injection of precursors at 250 °C. Sample T543 was similarly prepared, with the second injection made 25 min after nucleation using 0.7 mmol dimethyl cadmium and 0.5 mmol trioctylphosphine selenide in 1 mL TOP. The as-prepared nanocrystal samples were thrice washed in methanol and redispersed in a small amount of chloroform. These stock solutions were stored in a freezer at -20 °C in the dark when not in use. Ligand Comparison and Aging Experiments. An aliquot of washed nanocrystals was diluted with solvent (usually chloroform) to yield a diluted crystallite concentration of approximately 0.5-1 (12) Mattoussi, H.; Cumming, A. W.; Murray, C. B.; Bawendi, M. G.; Ober, R. Characterization of CdSe Nanocrystallite Dispersions by Small-Angle X-ray Scattering. J. Chem. Phys. 1996, 105, 9890. (13) Komoto, A.; Maenosono, S.; Yamaguchi, Y. Oscillating Fluorescence in an Unstable Colloidal Dispersion of CdSe/ZnS Core/Shell Quantum Dots. Langmuir 2004, 20, 8916. (14) Kalyuzhny, G.; Murray, R. W. Ligand Effects on Optical Properties of CdSe Nanocrystals. J. Phys. Chem. B 2005, 109, 7012. (15) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse Cde (e ) S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706. (16) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854.
Figure 1. Band-edge photoluminescence intensity as a function of time after the addition of 5 mM of the respective ligand to (a) 2 µM solutions of T543 CdSe nanocrystals in chloroform, and (b) 1.2 µM solutions of T570 CdSe nanocrystals in chloroform. µM. A 3 mL portion of the diluted nanocrystals was distributed into each of a number of fluorescence cuvettes. The ligand concentration was fixed at 5 mM for these experiments. This represents at least a 10-fold excess over the concentration of nanocrystal surface atoms, which was always less than 0.5 mM. Absorbance and fluorescence spectra of each sample were collected before accurately measured quantities of a range of different ligands were added to the cuvettes, which were subsequently sealed with Teflon stoppers. Fluorescence-Based Ligand Adsorption Isotherms. In a typical experiment, 2-5 µL of concentrated nanocrystal solution was injected into the chosen solvent, and the system was stirred to mix. The final nanocrystal concentration was about 1 µM. The solution temperature was controlled to (0.1 °C using a water-jacketed quartz cuvette. Ligand stock solutions were injected into the nanocrystal dilution in 2 µL aliquots, with thorough mixing. The total volume change in a given titration experiment was always less than 3%, and titrations were usually completed within 40 min of dilution. Final ligand concentrations of approximately 2000 µM were used for each isotherm experiment, which corresponds to approximately 2000 adsorbate molecules per QD.
Results I. Effects of Different Adsorbates. A screening of aliphatic organic surfactants was first made. In Figure 1, the luminescence intensity from the T543 and T570 samples is shown as a function of time after adding 5 mM of the respective ligands. The T543 sample in chloroform had an initial quantum yield of approximately 2%, and this value was increased to 15-20% after the introduction of primary amine molecules in dilute chloroform solutions. The older, and slightly larger T570 nanocrystals
Effects of Chemisorption on CdSe QDs
Figure 2. (a) Relative photoluminescence intensity of 2 µM solutions of T543 CdSe nanocrystals in chloroform in the presence of added 1, 2, and 3° aliphatic amine molecules after equilibration for 22 h. (b) Fluorescence intensities of 3.1 nm CdSe nanocrystals in chloroform 22 h after the addition of 5 mM of various ligands. [CdSe] ) 1 µM, excitation wavelength was 450 nm.
exhibited an initial quantum yield of 3.5%, which was enhanced with primary amine treatment to 6%. Two clear trends are evident. First, there is no clear effect of the alkylamine ligand chain length on the luminescence intensity for alkyl chains ranging from C2 to C18. More significantly, the luminescence clearly follows this trend: primary . secondary > tertiary amines. This is highlighted by the data presented in the bar graph of Figure 2a. The photoluminescence of the reference samples, to which only chloroform was added, decreased gradually during the course of the experiments. This gradual drop is probably due to slow aggregation, ligand desorption, or slow surface oxidation. Quenching was observed in all samples to which thiols were added. This is consistent with previous studies indicating the hole-trapping capacity of mercaptan groups on CdSe nanocrystals.17,18 In Figure 2b, the effects of various other ligands are shown. No other functional group investigated had significant effects on the nanocrystal luminescence. Amide and nitrile moieties had no significant effect on the luminescence of these samples, possibly because the nitrogen atoms of amides and nitriles are poor electron donors compared to primary amine nitrogen atoms. The addition (17) Dollefeld, H.; Hoppe, K.; Kolny, J.; Schilling, K.; Weller, H.; Eychmuller, A. Investigations on the Stability of Thiol Stabilized Semiconductor Nanoparticles. Phys. Chem. Chem. Phys. 2002, 4, 4747. (18) Berrettini, M.; Braun, G.; Hu, J. G.; Strouse, G. F. NMR Analysis of Surfaces and Interfaces in 2 nm CdSe Nanocrystals. J. Am. Chem. Soc. 2004, 126, 7063.
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Figure 3. (a) Position of the luminescence emission peaks for 1.2 µM T570 nanocrystals dispersed in chloroform as a function of time after the addition of 5 mM hexanethiol, 5 mM hexylamine, and chloroform (control). (b) Emission intensity and peak position of a 0.7 µM T570 nanocrystal solution in chloroform to which 5 mM DA was added, then, 15 min later, 5 mM octanethiol was added. The excitation wavelength was 450 nm. Squares: emission intensity; circles: peak position.
of the Y(phenyl)3 (Y ) N, P, As, Sb) series of ligands did not affect luminescence appreciably. Triphenylamine modestly enhanced the luminescence relative to the reference TOPO/ TOP crystallites, while the other ligands produced little or no change. II. Spectral Shifts due to Ligand Adsorption. In Figure 3a, the position of the luminescence emission peak is plotted as a function of time after ligand addition. A 4 nm red-shift in the fluorescence peak position of the control sample was observed after 150 h for the T543 sample, but the T570 sample showed no discernible shift in position. Samples treated with primary amines had stable peak positions at slightly higher energy than the control samples. A similar blue-shift in emission following amine addition has been observed in some other studies.19,20 Thiol-treated samples also had stable absorbance spectra; however, the fluorescence peak position was red-shifted. Figure 3a shows that the peak position is stable for days after ligand addition. In Figure 3b, the spectral shifts are shown on shorter time scales. The amines cause a blue-shift, while mercaptans cause a red-shift as well as partial quenching. Figure 3b also shows that the addition of mercaptan to an amine-capped CdSe surface causes an immediate red-shift and quenching of the luminescence. These spectral shifts are not due to coalescence or particle growth, but to the electronic effects of the ligand. (19) Balogh, L.; Zhang, C.; O’Brien, S.; Turro, N. J.; Brus, L. Surface Modification of CdSe Nanocrystals with Organic Ligands. Chim. Oggi 2002, 20, 45. (20) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture. Nano Lett. 2001, 1, 207.
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Figure 4. (a) Photoluminescence of 2 µM chloroform solutions of 2.9 nm CdSe nanocrystals as a function of time after the addition of 5mM DA. The different curves indicate the age of the stock solution of 1 mM CdSe nanocrystals in chloroform. (b) Emission intensity as a function of the age of the stock solution with and without added 5 mM decylamine. The decylamine has a decreased effect as the stock solution ages. The luminescence of the stock solution itself decreases with long-term storage.
III. Stock Solution Aging in Ambient Conditions. A fundamental difficulty in these studies is that batches of CdSe nanocrystals undergo slow aging, even in the absence of added ligands. In Figure 4, the effects of stock solution age on the luminescence properties of freshly diluted stock solutions, with and without added primary amines, are demonstrated. As is evident from Figure 4b, the photoluminescence of the native TOPO/TOP crystallites (diluted to 2 µM in chloroform) decreased by 70% over 120 days, while the effectiveness of the amine capping on the luminescence of these nanocrystals decreased even more drastically. The luminescence quantum yield for aminetreated aliquots dropped from 20 to 100 monolayers of amine are present in solution at the plateau. This clearly indicates that most (26) Qu, L.; Peng, X. Control of Photoluminescence Properties of CdSe Nanocrystals in Growth. J. Am. Chem. Soc. 2002, 124, 2049.
(4)
At low amine coverage, desorption can be neglected. In that case, the half-life for the increase in luminescence is approximately
τ1/2 ads ≈
1 kads[RNH2]
(5)
Inserting experimental values of τ1/2 ≈ 60 s and [RNH2] ) 5 mol m-3 gives a value for kads of 3.3 × 10-3 m3 mol-1 s-1. Moreover, since Keq ) (kads/kdes) ) 22 m3/mol for DA at 20 °C, kdes ≈ 1.5 × 10-4 s-1. This yields a half-life for ligand desorption of around 110 min. While these are only estimates from a simple model, they clearly demonstrate that amine adsorption is a facile process. Factors that affect the ligand adsorption behavior will directly impact the nanocrystal luminescence. The effects of solvent exchange follow directly from the primary assumption that the luminescence is proportional to the coverage. At equilibrium, the chemical potential of the ligand is the same on the surface and in solution, that is, 2 2 j RNH µ j RNH solv ) µ ads (θ)
µosolv + RT ln[RNH2]solv ) µoads + RT ln
(6) θ 1-θ
(7)
When the QDs are phase-transferred, the chemical potential of the ligand in the solvent changes. Within the Langmuir model, the standard chemical potential of the adsorbed ligand, µoads is constant; consequently, the surface coverage, θ, is controlled by the standard chemical
Effects of Chemisorption on CdSe QDs
potential of the ligand in the solvent bath, µosolv. Hence, as intuitively expected, when the solvent in which the QD and ligand is altered, the adsorption isotherm adjusts. If the ligand is more soluble in the new medium, desorption occurs. The response time is minutes to hours for primary alkylamines. If the ligand is less soluble, it will tend to adsorb from solution and may enhance the luminescence even further. A final important point concerns the spectral shifts plotted in Figure 3. A consistent blue-shift is observed when alkylamines are added to the nanocrystals. This has been interpreted as a dissolution of the crystals. Yet the absorption spectra of the particles are inconsistent with this interpretation. There is no evidence for a reduction in particle concentration. Instead, it may be attributed to the blocking of surface states and an effective reduction in the electron wave function in the nanocrystal. Likewise, the red-shift observed when thiols adsorb may be due to an increase in the delocalization of the hole wave function due to the availability of new accessible energy states on the adsorbed chalcogenide atoms.
Conclusions The focus in this paper has been on the effects of Lewis bases and other ligands on radiative recombination in CdSe nanocrystals in organic media. Aliphatic primary amines are the most suitable capping agents for CdSe nanocrystals in nonpolar solvents. Primary amines are superior to bulkier secondary or tertiary amines. The kinetics of chemisorption and desorption can be modeled using a Langmuir isotherm. Mercaptans rapidly displace amines leading to quenching. It has been known for nearly two decades that amines can enhance the luminescence of semiconductor particles;3 we have
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demonstrated that amines in particular seem to be the optimal ligands for CdSe nanocrystals. Many of the effects of aging, solvent, and temperature can be related to changes in the adsorption isotherm for the ligand. Optimal activation occurs for ligands that are basically insoluble in the solvent; smaller ligands have higher adsorption densities and give stronger enhancements. The particular efficacy of amines in comparison to numerous other ligands is due to the high oxidation potential of alkylamines (cf. thiols, phosphines, etc.). They are not photooxidized by valence band holes. We have not discussed the effects of oxygen or photolysis on the luminescence quantum yield, nor the role of lower temperatures on the luminescence lifetime. Indeed, an important aspect of this work has been separating slow redox reactions involving oxygen from ligand effects, which also show slow time-dependent influences. The role of redox reactions will be presented in a future manuscript. Acknowledgment. C.B. acknowledges the support of an Australian Postgraduate Award. This work was supported by Quantum Dot Corporation and the Australian Research Council through Grant C0002034. P.M. thanks the Alexander von Humboldt Foundation and Stiftung Caesar for support during the writing of the paper. Supporting Information Available: Schematic view of ligand coverage on a spherical nanocrystal, graph showing calculated ligand coverage on CdSe nanocrystals, and a table listing the physical and optical data for selected solvents. This material is available free of charge via the Internet at http://pubs.acs.org. LA051898E
