Soluble, Chloride-Terminated CdSe Nanocrystals: Ligand Exchange

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Soluble, Chloride-Terminated Cadmium Selenide Nanocrystals: Ligand Exchange Monitored by 1H and 31P NMR spectroscopy Jonathan S. Owen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm303219a • Publication Date (Web): 07 Dec 2012 Downloaded from http://pubs.acs.org on December 11, 2012

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Chemistry of Materials

Soluble, Chloride-Terminated CdSe Nanocrystals: Ligand Exchange Monitored by 1H and 31P NMR Spectroscopy Nicholas C. Anderson and Jonathan S. Owen* Department of Chemistry, Columbia University, 3000 Broadway, MC 3121, New York, NY 10027. [email protected]

KEYWORDS: CdSe quantum dots, nanocrystals, ligand exchange, 31P NMR, stoichiometry ABSTRACT: Chloride-terminated, tri-n-butylphosphine (Bu3P) bound CdSe nanocrystals were prepared by cleaving carboxylate ligands from CdSe nanocrystals (2.5 carboxylate/nm2) with chlorotrimethylsilane in Bu3P solution. 1H and

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P{1H} nuclear magnetic resonance (NMR)

spectra of the isolated nanocrystals allowed assignment of distinct signals from several free and bound species, including surface-bound Bu3P (δ = -13 ppm, fwhm = 908 Hz) and [Bu3P–H]+[Cl]ligands as well as a Bu3P complex of cadmium chloride. NMR spectroscopy supports complete cleavage (> 99%) of the X-type carboxylate ligands. Primary n-alkylamines rapidly displace the bound Bu3P on mixing, leading to amine-bound nanocrystals with higher dative ligand coverages (1.8 RNH2/nm2 vs. 0.5 Bu3P/nm2) and greatly increased photoluminescence quantum yields (33±3% vs. 0.1 M CdSe and Bu3P) remain transparent indefinitely (> 24 months) under nitrogen. However, slow precipitation over 24 hours is observed if the concentration of Bu3P is low ( 99%) is confirmed by the lack of aliphatic signals from tetradecyl and oleyl chains in isolated samples of CdSe-CdCl2/Bu3P (Figure 3).67 Comparing the integral of the methyl group versus a ferrocene standard reveals 19– 38 Bu3P ligands bind each chloride-terminated nanocrystal for an average surface coverage of 0.5 phosphines/nm2 in the range of sizes (d = 3.3-3.7 nm) used in this study (Table 1), which is significantly fewer organic ligands compared to the carboxylate coverage in CdSe-Cd(O2CR)2 (2.6 carboxylates/nm2). Transmission electron microscope (TEM) images of CdSe-CdCl2/Bu3P are consistent with this analysis where a decrease in the shortest distances between nanocrystals is visible; from 2-3 nm for CdSe-Cd(O2CR)2 to ~1 nm or less (Figure 4). The small separation between nanocrystals can lead to increased conductivity within nanocrystal solids, the details of which will be reported in a forthcoming manuscript. Typical 1H and

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P NMR spectra of purified CdSe-CdCl2/Bu3P display broad, but

distinct signals characteristic of several Bu3P fragments (Figure 3). In addition to free Bu3P (δ = -31 ppm), three major phosphorus-containing species are visible in the 31P NMR spectrum of CdSe-CdCl2/Bu3P. One of these resonances appears in the range of a [Bu3P-H]+[BF4]- standard (δ = 10ppm, fwhm = 21 Hz, d6-benzene, Figure S2) though with a significantly broadened linewidth, which results from it being associated with the nanocrystal. Another resonance compares well with an independently prepared mixture of Bu3P and cadmium chloride (1:3) in

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the absence of nanocrystals (δ = -9ppm, Figure S3). We assign the remaining signal (δ = -13 ppm, fwhm = 908 Hz, d6-benzene) to the neutral Bu3P ligand bound directly to the nanocrystal surface. The presence of [Bu3P–H]+ in CdSe-CdCl2/Bu3P arises from hydrogen chloride produced from Me3Si–Cl and a protic species, like oleic acid. This hydrogen chloride protonates the phosphine forming [Bu3P–H]+[Cl]- which adsorbs to the nanocrystal surface via the chloride anion. Thus CdSe-CdCl2/Bu3P has a net negative charge that is balanced by the assoiciated phosphonium counter ion. Larger quantities of [Bu3P–H]+[Cl]- form if carboxylic acids, primary amines, and alcohols are present during the cleavage reaction. Attempts to minimize the production [Bu3P–H]+ by extensive purification of CdSe-Cd(O2CR)2 were not effective, and CdSe-CdCl2/Bu3P was consistently isolated with ~2-3 [Bu3P–H]+[Cl]- per nanocrystal. The >150-fold excess of Bu3P present during ligand exchange suggests that [Bu3P–H]+[Cl]- binds the nanocrystal with much greater affinity than Bu3P. Extensive drying of the solvents used during ligand exchange or during nanocrystal purification did not influence our results, and bistrimethylsilyl ether was not detected in any of our in situ reactions, arguing against reaction with water as a source of [Bu3P–H]+. We tentatively propose that hydrogen chloride is produced by a small percentage of oleic acid ligands present in CdSe-Cd(O2CR)2 (1-2%) that are not removed during purification. These results strongly indicate that oleic acid is not present in the starting material (CdSe-Cd(O2CR)2) in significant amounts and supports a structure where carboxylate ligands balance the charges of excess cadmium ions (Scheme 1). The NMR line-widths and chemical shifts observed in this study are affected by rapid and reversible chemical exchange of free Bu3P with other Bu3P fragments. The dynamic nature of L-


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type ligands bound to nanocrystals has been discussed previously, both in experimental studies that use NMR spectroscopy to monitor pyridine and amine exchange68 as well as theoretical studies that calculate binding energies of dative ligands.69 Similarly, exchange of Bu3P bound to CdCl2 is known to be rapid at room temperature producing a single 31P NMR signal from a mixture of free Bu3P, (Bu3P)3CdCl2, (Bu3P)2CdCl2 and [(Bu3P)CdCl2]2 (Figure S3).70,71 The observed chemical shift in this case is an average of each component weighted by its respective mole fraction. However, when the chloride exchange is monitored in situ and in the presence of a large excess of Bu3P the 31P NMR resonance of free Bu3P is observed in addition to and separate from signals of Bu3P bound to the nanocrystal (Figure S4). This observation requires the exchange between free and bound Bu3P is slow on the NMR timescale. These 31P NMR signals appear between -30 and +10 ppm, which in a 11.7 Tesla magnetic field (31P frequency = 212 MHz) places an upper limit on the exchange rate (< ~103 sec-1). The distinct signals from bound and free phosphines makes it convenient to monitor exchange of Bu3P with

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P NMR spectroscopy. For example, we can directly monitor the

displacement Bu3P from CdSe-CdCl2/Bu3P by adding n-alkylamines. At five equivalents of nbutylamine per phosphine the broad NMR signatures of bound Bu3P and [Bu3P–H]+[Cl]- are replaced by the sharp signal of free Bu3P upon mixing (Scheme 3 and Figure 5, top). At one equivalent partial displacement is observed and rapid exchange between free and bound Bu3P is evident from its broadened and shifted signal (Figure 5, middle). In support of this proposed dynamic exchange process, the free phosphine chemical shift and line-width depends on the total concentration and the chain length of the amine ligand (Figure S5). Similar results were obtained with n-octyl and n-dodecyl amine providing access to amine-bound, chloride-terminated CdSe nanocrystals (CdSe-CdCl2/RNH2, R= C4H9, C8H17, or C12H25). 1H NMR and UV-visible


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absorbance spectroscopy reveals a higher surface coverage for CdSe-CdCl2/RNH2 (~1.8 nalkylamine/nm2) (Table 1, Figure S6). In addition, at 1 equivalent n-butylamine, [Bu3P–H]+ undergoes complete deprotonation leading to free Bu3P in dynamic exchange with bound Bu3P (Figure 5). Similarly, addition of excess N,N-diisopropylethylamine causes deprotonation of [Bu3P–H]+ due to the higher pKa of N,N-diisopropylethylammonium (pKa = 11.4 in dimethylsulfoxide) versus [Bu3P–H]+ (pKa = 8.43 in dimethylsulfoxide). However, in this case the amine is too sterically hindered to displace bound Bu3P from the nanocrystal (Figure S7). Precipitation and isolation of the product three times produces a sample with associated N,N-diisiopropylammonium and Bu3P (Figure S8). In only a few cases has 31P NMR spectroscopy been used to analyze the ligand shell of nanocrystals, despite the large number of studies which report nanocrystals bound by phosphines and phosphine oxides.38,52 More recent literature supports a different assignment, namely that phosphonate and phosphinate ligands are responsible for the

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P NMR signals in these

samples.17,20,22,53,54 In the present study displacement of Bu3P by primary amines produces a single 31

P NMR signal for free Bu3P, confirming our assignment. In addition, while the exchange of

phosphine for amine ligands has been claimed previously,34,38,40,51,72 more recent literature assigns the phosphorus-containing ligands as anionic, X-type phosphonate.17,20,22,53,54 Prior studies using CdSe nanocrystals synthesized in TOPO and TOP clearly demonstrate displacement of the ligand shell upon addition of amines, but the mechanism for this exchange is uncertain, presuming that the ligand shell is composed of X-type phosphonate and phosphinate ligands. This discrepancy may also explain the difficulty achieving complete ligand displacement using amine ligands.35,42-


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In the present study, we were able to achieve complete exchange of the starting carboxylate

ligands using Me3Si–Cl cleavage and manipulating the L-type ligands separately. Converting the L-type ligands from phosphines to amines (CdSe-CdCl2/Bu3P to CdSeCdCl2/RNH2) causes a dramatic increase in the photoluminescence provided that the nanocrystals are dissolved in concentrated (0.1 M) amine solution (~1000-fold, PLQY = 33±3%).53 At the same time, the optical absorption blue-shifts slightly (3 nm) and the apparent intensity of the 1Se-2S3/2h transition is recovered (Figure 6). The 1H NMR spectrum of a sample isolated from concentrated n-dodecylamine solution (1 M), shows broad features characteristic of bound n-dodecylamine, which is present in ~3x greater quantity than the starting Bu3P ligands (Figure S5, Table 1). These observations suggest that the greater surface coverage may be related to the dramatic changes in the optical properties upon ligand exchange. In particular, the weak photoluminescence and somewhat unusual absorption spectrum of CdSe-CdCl2/Bu3P may derive from coordinative unsaturation. However, the ~1000-fold increase in quantum yield upon amine exchange cannot be simply related to surface coverage, which only changes 3-fold on converting CdSe-CdCl2/Bu3P to CdSe-CdCl2/RNH2. Ligand exchange reactions are known to influence the energy of the first excitonic absorption feature which is often attributed to changes in carrier confinement.34,38,40 In particular, several studies have reported 3-5 nm blue-shifts on addition of amines and pyridine.40,43,68 Fewer studies, however, document changes to higher energy absorption features like those visible in Figures 1 and 6. Recently Cao and coworkers concluded that displacement of carboxylate ligands by amines can influence the difference between the 1S3/2h and 2S3/2h hole states leading to a change in splitting between the first and second absorption features.66 Although similar effects


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may influence the spectra in Figures 1 and 6, significant broadening of the 1Se-2S3/2h absorption and/or a change in its extinction are also evident and not readily explained by confinement effects alone. To explain these results, we hypothesize that coordinative unsaturation results in a surface reconstruction and/or new electronic states that become resonant with the 2S3/2h level. These effects could cause a decrease in the extinction or lead to lifetime broadening of the 1Se2S3/2h transition. Density functional theory calculations have shown that binding of ligands to nanocrystal surfaces prevents their reconstruction, and those without ligands are significantly distorted from the usual tetrahedral configuration.73 Tretiak and coworkers studied the binding of amines and phosphines to CdSe clusters and found that phosphine-bound clusters have localized surface states near the band edge.69 Calculated absorbance spectra also show differences in the higher electronic transitions accompanied by only minor shifts in the HOMO-LUMO transition. Surface reconstruction under low ligand coverage has been suggested as a possible mechanism for the observed changes to higher energy electronic transitions.74 This proposal has also been suggested by Dzhagan et al. who report a correlation between the 1Se-1S3/2h absorption and changes to the nanocrystals Raman spectrum that they attribute to a surface reconstruction.43 Although the link between changes in the optical properties in the nanocrystals described in the present study is in need of additional study, the low surface coverage measured for CdSe-CdCl2/Bu3P is likely to cause surface reconstruction and low PLQY. The bright photoluminescence and narrow spectral features of CdSe-CdCl2/RNH2 indicate that surface bound chloride ligands can support a nonstoichiometric nanocrystal without introducing surface trap states as long as the coverage of L-type ligands is high. This makes


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CdSe-CdCl2/RNH2 unlike metal-chalcogenide-, thiocyanate-, or sulfide-bound nanocrystals used to make thin-film devices. Recent reports have also demonstrated that cadmium chloride and cadmium carboxylate are effective passivating layers for lead sulfide nanocrystals, leading in one case to high photovoltaic efficiencies.75,76 Furthermore, cadmium chloride is well known to cause grain growth and passivate trap states in cadmium telluride photovoltaic cells.77 Both of these features suggest that cadmium chloride passivated nanocrystals are promising materials for optoelectronic devices. By separately manipulating the L-type and X-type ligands, we can prepare gram quantities of chloride terminated nanocrsyatls with volatile organic ligands that may be removed under mild conditions. Conclusion: Using a combination of 1H and

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P NMR and UV-Visible absorption

spectroscopy we have demonstrated both X-type and L-type ligand exchange. Our approach makes it possible to cleave >99% of the anionic carboxylate ligands and replace them with a mixture of chloride ligands, which balance the charge of the nonstoichiometric nanocrystals, and dative Bu3P ligands, which maintain solubility.

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P NMR spectroscopy of CdSe-CdCl2/Bu3P

revealed a complex reaction mixture where a small concentration of [Bu3P-H]+[Cl]- is bound to the nanocrystal. Hydrogen chloride produced from reaction of Me3Si–Cl with protic species like oleic acid leads to this minor impurity (2-3/nanocrystal). Our assignment verifies the low concentration of protic species in our reaction mixtures. It is also consistent with a nanocrystal where carboxylate ligands balance the charges of the nonstoichiometric crystal (X-type), rather than adsorption of carboxylic acid ligands. The resultant phosphine ligands are labile and readily replaced by primary amines, although the 31P NMR spectrum allows us to distinguish signals of bound and free ligands and


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directly monitor the displacement reaction. By measuring the ligand surface coverage for both the primary amine (1.8/nm2) and Bu3P (0.5/nm2) bound nanocrystals, we demonstrate a significant difference in the binding of these two molecules. The greater surface coverage of CdSe-CdCl2/RNH2 leads to a much higher PLQY (33±3%) and an increase in the apparent intensity of the 1Se-2S3/2h transition as compared to CdSe-CdCl2/Bu3P. We suggest these changes may derive from a surface reconstruction, which is pronounced for nanocrystals that are coordinatively unsaturated. Studies to verify this hypothesis are underway in our lab. Control over nanocrystal surface chemistry in this study was possible by addressing ligands that balance charge with excess Cd2+ (X-type ligands). By considering the importance of a nanocrystals stoichiometry, new strategies will be developed that improve the implementation of colloidal semiconductor nanocrystals in a wide range of applications. In particular, the nanocrystals developed in the current study utilize reversibly bound and volatile organic molecules, like n-butylamine, to maintain solubility and their cadmium-enriched surface structure. These features are valuable for the processing of thin films where the organic ligands can be desorbed at low temperature under vacuum. Studies of electrical transport in thin films composed of these nanocrystals as well as the details of L-type ligand exchange monitored by 31P NMR spectroscopy are forthcoming.


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Experimental General Considerations: Cadmium nitrate tetra-aquo (99%), sodium hydroxide, myristic acid (99%), selenium dioxide (99.8%), anhydrous oleic acid

(99%), n-dodecylamine (99%) and 1-

octadecene (90%) were purchased from Sigma Aldrich and used as received. d6-Benzene (99.6%) and anhydrous methyl acetate (99.5%) were purchased from Sigma Aldrich, shaken with activated alumina, filtered, and stored over 4 Å molecular sieves at least 24 hours prior to use. Pentane and toluene were dried over alumina columns, shaken with activated alumina, filtered and stored over 4 Å molecular sieves at least 24 hours prior to use. Tri-n-butylphosphine (99%) was purchased from Strem and used without further purification in a nitrogen glove box. Chlorotrimethylsilane (99%), nbutylamine (99%), and n-octylamine were purchased from Sigma Aldrich and dried over CaH2, distilled, and stored in a nitrogen glove box. All manipulations were performed under air-free conditions unless otherwise indicated using standard Schlenk techniques or within a glove box. Cadmium tetradecanoate was synthesized from cadmium nitrate and tetradecanoic acid on 25 mmol scale following the procedure reported by Chen et al.65 NMR spectra were recorded on Bruker Avance III 400 MHz and 500 MHz instruments. 1H NMR spectra were acquired with sufficient relaxation delay to allow complete relaxation between pulses (30 seconds).

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P NMR spectra were recorded with 2 second delays between pulses. UV-

Visible data was obtained using a Perkin Elmer Lambda 650 spectrophotometer equipped with deuterium and tungsten halogen lamps. Photoluminescence spectra were recorded using a FluoroMax-4 equipped with an Integrating Sphere from Horiba Scientific. RBS measurements were performed in the Shepherd Lab at the University of Minnesota. Transmission electron microscopy was performed at Brookhaven National Laboratory on a 200keV JEOL 2100F.

Synthesis and isolation of CdSe-Cd(O2CR)2. Nanocrystals were synthesized under an argon atmosphere using a modification of the procedure reported by Chen et al. typically beginning with 10 mmol cadmium tetradecanoate under an argon atmosphere.65 After cooling the reaction from the growth temperature, volatile organics were removed by vacuum distillation (130-135 °C, 20-50 mbar). Once the still pot was near dryness, the dark residue was cooled to room temperature and 30 ml of pentane was added via cannula. The dark slurry was transferred via cannula to a Teflon sealable Schlenk tube and taken into a nitrogen glove box. The slurry was divided into equal parts in two 45 ml centrifuge tubes and centrifuged at 7000 rpm for 10 minutes. The clear solution of nanocrystals was decanted while the remaining grey precipitate was discarded. Methyl acetate was


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added to the solution until the nanocrystals precipitated and could be isolated by centrifugation (5 min at 7000 RPM). The dark residue was dissolved in a minimal amount of pentane, and methyl acetate was added until the nanocrystals precipitated. This process was repeated until 5-10 ml of pentane was sufficient to dissolve the nanocrystals. The cycle of precipitation from pentane with methyl acetate was repeated 3-5 additional times and the nanocrystals stored as a solution in 5 ml of pentane. If these solutions produce a white precipitate over 24 hours the suspension was centrifuged and the solids discarded and the nanocrystals isolated by precipitation with methyl acetate and centrifugation and re-dissolved in 5 ml of pentane. If no precipitate formed after 24 hours in the pentane solution, the solvent was distilled and the residue dried under vacuum for 24 hours. The residue was dissolved in d6-benzene and the vinyl region of the 1H NMR used to analyze the sample purity. Samples were considered to be impure when features indicative of free oleyl chains were identified by a sharp feature in the vinyl region (δ = 5.5 ppm). Solutions with sharp signals from free oleyl chains were purified by an additional methyl acetate wash prior to re-dissolving in d6-benzene for purity analysis with 1H NMR. Solutions without signals from free carboxyl chains were used as stock for ligand exchange studies.

Measurement of the ligand and CdSe concentration. The concentrations of nanocrystals and carboxylate ligands in d6-benzene stock solutions of purified nanocrystals were determined using a combination of NMR spectroscopy and UV-Visible absorption spectroscopy. Ferrocene dissolved in d6-benzene (10 μl, 0.05 M) was added to a known volume of the nanocrystal stock solution and used as an internal standard. CdSe-Cd(O2CR)2 and CdSe-CdCl2/RNH2: The integral of the methyl peak was determined and compared with the integral of the ferrocene standard to determine the concentration of carboxyl ligands in the stock solution. CdSe-CdCl2/Bu3P: The entire spectrum from δ = 3.99 - 0.3 ppm was integrated and compared with the ferrocene standard to determine the concentration of Bu3P or RNH2 fragments. The molar concentration of CdSe in these stock solutions was determined by diluting 1-3 μl to a known volume with toluene and measuring the absorbance at λ = 350 nm where the extinction coefficient is independent of size as determined by Bawendi78 and verified by Mulvaney.79 Precision in the sampling volumes were evaluated by measuring the mass of the sample prior to dilution and used, along with uncertainty in the integration of NMR spectra to calculate the error shown in Table 1.


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Calculation of ligand per particle and CdSe units per ligand ratios. The wavelength of the 1Se2Sh1/2 transition maximum was used to determine the average nanocrystal diameter according to Yu et al.,80 which is similar to values determined by Mulvaney in this size range.79 From this diameter the number of CdSe units per crystal were calculated, assuming a spherical shape and the molar volume of bulk cadmium selenide. The concentration of nanocrystals, the ratio of ligands per nanocrystal and the ligand surface density assuming a spherical shape were calculated from the number of CdSe units per nanocrystal, the molar concentration of CdSe, and ligands in the stock solution.

Photoluminescence quantum yield. PLQY were measured using a Fluoromax-4 Fluorometer equipped with an integrating sphere. Samples were diluted to concentrations below 0.1 absorbance units at the 1Se-2Sh1/2 to minimize reabsorption. A blank sample of toluene was used to adjust the excitation and emission slits to avoid saturating the detector, and the photoluminescence spectra that include the excitation light were recorded for both the toluene blank and the nanocrystal solution. The number of photons absorbed by the sample was determined by measuring the difference between the blank and the sample at the excitation wavelength, and the photoluminescence spectrum of the nanocrystal solution was integrated from 500-650 nm to determine the photons emitted. PLQY was calculated from the ratio of photons emitted and photons absorbed. The validity of this approach was evaluated using freshly prepared solutions of Coumarin-153 in ethanol (PLQY = 53%).

Synthesis of CdSe-CdCl2/Bu3P. All manipulations were conducted in the glove box at room temperature. In a typical synthesis, a d6-benzene stock solution of CdSe-Cd(O2CR)2 (1.0 ml, 0.5-2.0 mmol ligand) with a known carboxylate concentration was transferred to a 20 ml vial with a magnetic stir bar. The solution was diluted to a total volume of 5 ml with toluene to which Bu3P (0.506 g, 0.624 ml, 2.5 mmol) was added. Me3Si–Cl (6.0-24 mmol, 12 equiv.) was added and the solution stirred for 3 hours. After this time, the volatiles were distilled off under vacuum and the red solid dissolved in toluene (5 ml) and pentane was added to precipitate the nanocrystals, which were separated by centrifugation (7000 RPM for 5 minutes). This process was repeated twice more, after which the red powder was dried overnight under vacuum. The nanocrystals were dissolved in d6benzene to a concentration of 0.5-1.0 M cadmium selenide and analyzed with UV-Visible and NMR spectroscopy.


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Synthesis of CdSe-CdCl2/RNH2 In a nitrogen filled glovebox, n-alkylamines (R= n-butyl, n-octyl, or n-dodecyl, 5 mmol) were added to a d6-benzene stock solution of CdSe-CdCl2/Bu3P (5 ml, 0.51.0 M in CdSe, 37.5-75.0 mM in Bu3P) and the solution was stirred for 1 hour. An increase in the fluorescence is immediately apparent on mixing. Small amounts of insoluble material formed during upon stirring that were removed by centrifugation. The nanocrystals are precipitated from the solution by addition of methylacetate (R= n-octyl and n-dodecyl) or pentane (R= n-butyl), and the solids isolated by centrifugation. This process is repeated a second and third time, after which the red powder was dried under vacuum. Samples prepared with n-butylamine, can result in insoluble materials if they are left under vacuum for prolonged periods. Samples prepared with n-octylamine and n-dodecylamine may be dried under vacuum for several hours without compromising their solubility. The nanocrystals were dissolved in d6-benzene to a concentration of 0.5-1.0 M cadmium selenide and analyzed with UV-Visible and NMR spectroscopy.

Supporting Information. RBS data and NMR control experiments are available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This research was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE07-07425 for NCA and the Department of Energy under Grant No. DESC0006410. The authors also thank Columbia University and the Department of Chemistry for support. We thank Dr. Abraham Wolcott for TEM images taken with support from the Center for Functional Nanomaterials at Brookhaven National Laboratory. REFERENCES (1) Kamat, P. V. Society 2008, 18737-18753. (2) Kramer, I. J.; Sargent, E. H. ACS nano 2011. (3) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chemical reviews 2010, 110, 389-458. (4) Drndić, M.; Jarosz, M. V.; Morgan, N. Y.; Kastner, M. a.; Bawendi, M. G. Journal of Applied Physics 2002, 92, 7498.


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Scheme 1. Stoichiometry Dependent Surface Coordination Chemistry Nonstoichiometric (Metal Rich)

Stoichiometric

-anionic ligands terminate cation rich surface

-dative binding to Lewis acidic surface

S R,

XX XX X X X X XX X X

Cl

O P R, O OH

O R

O

L R NH2 , R SH O O L P R, R P R HO L OH R

(CdSe)m(CdX2)n

L

L L

(CdSe)mLp

Scheme 2: Synthesis of CdSe-CdCl2/Bu3P O R

O OO

R

R O

Me3Si–Cl, Bu3P

R O O

O O

Bu3P

toluene, 298K, 3hr.

O

Cl Cl

O

R R O (CdSe-Cd(O2CR)2)

Cl Cl

[HPBu3]+ Cl

–

Cl Cl PBu3

+ +

RCO2SiMe3 (Bu3P)m(CdCl2)n (m = 1,2,...) (n = 1,2,...)

(CdSe-CdCl2/Bu3P)

R = C17H33, C13H27

Scheme 3: Synthesis of CdSe-CdCl2/RNH2

Bu3P Cl Cl

Cl Cl

[HPBu3]+ Cl

–

Cl Cl PBu3

(CdSe-CdCl2/Bu3P)

H2NR toluene, 298K, 1 hr.

RNH2

Cl Cl

[H3NR]+ Cl

–

Cl Cl

Cl Cl

+

H2NR (CdSe-CdCl2/H2NR)

R = C4H9, C8H17, C12H25


Bu3P


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Figure 1: Absorbance (left) and photoluminescence (right) spectra of CdSe-Cd(O2CR)2 (red) and CdSe-CdCl2/Bu3P (blue) after isolation. For clarity, the photoluminescence spectrum CdSeCdCl2/Bu3P was magnified ten-fold.

Figure 2: 1H NMR spectra of CdSe-Cd(O2CR)2 (top) and CdSe-Cd(O2CR)2 after adding Bu3P and Me3Si–Cl (bottom).


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Figure 3: 1H (top) and 31P (bottom) NMR spectra of isolated and purified CdSe-CdCl2/Bu3P in d6-benzene (0.56 M CdSe, 1.6 mM nanocrystals, and 34 mM Bu3P, corresponding to 22 ± 5 Bu3P per nanocrystal; entry 6, Table 1). The ferrocene standard (0.2 mM, δ = 4 ppm), toluene (δ = 2.1 ppm), silicon grease (δ = 0.21 ppm), as well as an unidentified terminal olefinic impurity at

δ = 5.6 ppm (< 0.5/nanocrystal) that may be a byproduct of the reaction between SeO2 and 1octadecene are visible in the 1H NMR spectrum.

Figure 4: Transmission electron micrographs of CdSe-Cd(O2CR)2 (A) and CdSeCdCl2/Bu3P (B). The inset in Figure 2B is shown with a 5 nm scale bar.



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Figure 5:

31

P NMR spectra of CdSe-CdCl2/Bu3P (bottom, black) upon addition of 1

equivalent (blue, middle) and 5 equivalents of n-butylamine (orange, top). 31P NMR signals for [Bu3P-H]+ and Bu3P shift upfield toward the chemical shift of free Bu3P (δ = -31 ppm) as nbutylamine deprotonates [Bu3P-H]+ and displaces Bu3P from the nanocrystals.

Figure 6: Absorbance (left) and photoluminescence (right) spectra of CdSe-CdCl2/Bu3P (blue) and CdSe-CdCl2/BuNH2 (green) after isolation. For clarity, the fluorescence spectrum CdSe-CdCl2/Bu3P was magnified ten-fold.


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Table 1: Composition of isolated CdSe nanocrystals Nanocrystals

Diametera

CdSe:NCb

Cd:Sec

Ligand:NCd

Ligand Densitye

100(15) 70(11) 80(12) 170(25) 190(25) 22(3) 17(3) 27(4) 35(5) 19(3) 38(5) 64(10) 59(9)

(nm-2) 2.9(5) 2.0(4) 2.1(4) 3.7(6) 2.6(4) 0.5(1) 0.3(1) 0.5(1) 0.6(1) 0.3(1) 0.7(1) 1.9(3) 1.6(3)

(nm) CdSe-Cd(O2CR)2 CdSe-Cd(O2CR)2 CdSe-Cd(O2CR)2 CdSe-Cd(O2CR)2 CdSe-Cd(O2CR)2 CdSe-CdCl2/Bu3P CdSe-CdCl2/Bu3P CdSe-CdCl2/Bu3P CdSe-CdCl2/Bu3P CdSe-CdCl2/Bu3P CdSe-CdCl2/Bu3P CdSe-CdCl2/RNH2f CdSe-CdCl2/RNH2g

3.3 3.3 3.5 3.8 4.8 3.3 3.5 3.6 3.7 3.7 3.8 3.3 3.4

335(35) 335(35) 400(40) 511(47) 1030(74) 335(35) 400(40) 435(42) 472(44) 472(44) 511(47) 335(35) 366(37)

1.09(23) 1.28(27) 1.11(23) -

a) Diameters are calculated from the energy of the lowest energy absorption and estimated to have an error of approximately 1 Cd-Se bond distance (~0.2 nm) b) CdSe units per nanocrystal (NC) are calculated assuming a spherical shape and the molar volume of zinc-blende CdSe (17.80 nm-3). Errors shown are propagated from the uncertainty in the diameter (see Supporting Information). c) Cd to Se ratios are determined by integrating Rutherford backscattering spectra. Errors shown are an estimate of the uncertainty in the integration of each peak (10%) and the precision of RBS (5%) (see Figure S1 and the error analysis in the Supporting Information) d) The number of organic ligands per nanocrystal (NC) is measured by comparing the concentration of ligands determined with 1H NMR spectroscopy and the concentration of CdSe determined using absorption spectroscopy; see experimental section for details. The Bu3P:NC ratio includes signal from all Bu3P-containing compounds in the sample, e.g. [Bu3P–H]+[Cl]–, free Bu3P, and Bu3P complexes of CdCl2 that amount to 25% of the total signal intensity in the 31P NMR. e) Ligand densities were calculated by dividing the number of ligands per nanocrystal by its surface area assuming a spherical shape. In the case of Bu3P the ligand density was corrected for the 25% of Bu3P in other forms not bound to the nanocrystal surface. f) R = n-dodecyl. g) R = n-octyl



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TOC Graphic


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Soluble, Chloride-Terminated CdSe Nanocrystals: Ligand Exchange

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