Reducing Competition by Coordinating Solvent Promotes

Article pubs.acs.org/cm

Reducing Competition by Coordinating Solvent Promotes Morphological Control in Alternating Layer Growth of CdSe/CdS Core/Shell Quantum Dots Rui Tan,† Yi Shen,† Stephen K. Roberts,† Megan Y. Gee,† Douglas A. Blom,‡ and Andrew B. Greytak*,†,‡ †

Department of Chemistry and Biochemistry, and ‡USC Nanocenter, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: The formation of core/shell structures in colloidal semiconductor nanocrystals is important in maintaining the spectroscopic properties of colloidal quantum dots (QDs) and defining new functions. When using selective ionic layer adhesion and reaction (SILAR)-based techniques, conversion of shell precursors to surface-adsorbed equivalents should be maximized for effective control of shell growth. In this work, we monitored precursor conversion and shell growth on CdSe QDs in the presence of three different amine solvents in an effort to increase the synthetic yield of shell growth. UV−vis and photoluminescence spectroscopy are applied to monitor shell growth. Photoluminescence excitation spectroscopy was applied to confirm the presence/absence of precursor nucleation. Additionally, during shell growth, the free precursor concentration was measured by inductively coupled plasma mass spectrometry (ICP-MS), and fit with a Langmuir isotherm-based model, which reveals the influence of the different solvents on the fractional occupation of shell precursor equivalents on the QD surface. The binding affinities of the solvent molecules to the QD surface are also studied to understand the influence of such interactions on shell growth. We find that a tertiary amine solvent is effective in increasing precursor conversion and suppressing nucleation of side products when compared to primary and secondary amines at similar solvent mole fraction. The difference appears to be associated with competition for surface sites between the metal carboxylate precursor and the primary amine. This study is important for understanding the mechanism of growing the core/shell nanoparticles via SILAR technique and further could be applied to synthesize isotropic/ anisotropic core/shell nanoparticles in an advanced and controllable manner.

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core/shell QDs4,10,12,30,32−34 in an effort to (1) saturate available surface binding sites in each half-cycle to enforce conformal growth; and (2) avoid the simultaneous presence of both precursors in the solution to prevent uncontrolled surface growth or homogeneous nucleation of the shell material. A comprehensive understanding of the mechanism of shell growth under SILAR conditions will benefit a wide range of materials systems and heterostructure geometries for which SILAR growth could be applicable. Prospective systems for SILAR-mediated growth include, but are not limited to, QDsensitized nanorod arrays,35−40 synthesis of ZnO/CdS,41,42 TiO2/In2S3,43 nanorod/shell heterostructures, and core/shell nanowires.44−46 SILAR techniques are highly dependent on self-limiting reactions between shell/film precursors and the substrate (QD core) surface. High temperatures8,30,32 and/or shell precursors with high reactivity3,47 have been employed to drive these reactions to completion within reasonable cycle times, and for QDs, precise amounts of shell precursors are

INTRODUCTION Colloidal semiconductor nanoparticles are of interest because of their high photoluminescence quantum yields (QY),1−4 sizetunable absorption and emission spectra,5 and high photostability.6,7 Forming an isotropic shell is desirable for many fluorescence applications because it isolates the core from surface-associated trap states and leads to enhanced QY.4,8,9 Increasing the thickness or varying the composition of isotropic shells can further result in suppressed blinking/photobleaching10−14 and decreased Auger recombination rates enabling enhanced biexciton QY,15−17 which are advantageous qualities for bioimaging,6,7,18−20 lighting, and display applications.21−27 Lowering the density of defects at the core−shell interface(s) using materials with low lattice mismatch28,29 or forming a gradient shell30,31 with a transitional layer have been demonstrated as effective strategies to decrease the nonradiative decay rate and maintain high QY. Growing high-quality shells with controlled thickness requires shell precursor(s) that are soluble under reaction conditions and can be converted to epitaxial growth of the shell material on the existing cores in high synthetic yield. The selective ionic layer adhesion and reaction (SILAR) technique has been applied to growth of © 2015 American Chemical Society

Received: September 14, 2015 Revised: October 16, 2015 Published: October 28, 2015 7468

DOI: 10.1021/acs.chemmater.5b03588 Chem. Mater. 2015, 27, 7468−7480

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Chemistry of Materials Table 1. Matched Core/Shell Growth Results with Different Amine Solvents entry

core radiusa (nm)

shell (ML equiv)

amine solventb

abs peak (nm)

abs widthc (meV)

PL peak (nm)

PL widthd (meV)

CdSe/CdS_OAM CdSe/CdS_DOM CdSe/CdS_THM

1.69 1.70 1.69

6 6 6

oleylamine dioctylamine trihexylamine

586 586 583

106 86 82

600 597 593

93 84 79

From absorption based on calibration curve. b1:2 v/v with ODE. cTwice hwhm from Gaussian fit to first exciton absorption peak. dfwhm from Gaussian fit to PL peak.

a

course of the growth was monitored by UV−visible absorption and photoluminescence (PL) emission spectroscopy. Emission peaks at wavelengths shorter than the effective band gap (“blue peaks”) appeared in the PL spectrum when QDs were grown in primary amine, suggesting nucleation of small CdS particles as a result of cross-reaction of the shell precursors as seen previously, and such nucleation was suppressed and no CdS particles were present in the QDs grown in tertiary amine. Further evidence of such small particles was obtained from photoluminescence excitation (PLE) measurements. Timecorrelated single photon counting (TCSPC) measurements indicated shorter average lifetimes and increased rate dispersion in samples prepared with secondary and tertiary amines, when compared to oleylamine. This finding indicates a difference in interactions/passivation between the different amines and the QD surface. Proton NMR was also applied to understand the interaction of different amines with the CdSe surface. Scanning transmission electron microscopy (STEM) proved the yield of the shell was highest when using the tertiary amine (trihexylamine) as the growth solvent. Titration experiments,50 in which the metal precursor was introduced incrementally in small doses and unreacted species were monitored by inductively coupled plasma mass spectrometry (ICP-MS), were designed to study the conversion efficiency and completion of shell formation during the growth under different amines. We demonstrated that the interaction between the solvent molecules and the nanoparticle surface is an issue influencing shell growth by SILAR, because the shell precursor must compete with such interactions to saturate the surface prior to introduction of the complementary precursor for growth of the shell compound.

introduced for each half-reaction to avoid the presence of excess precursor at the beginning of the next cycle.4,32 Other attempts to enforce self-limiting surface reactions have included colloidal atomic layer deposition (SILAR coupled with phase transfer between immiscible solvents)48 and monitoring of the saturation end point for each addition cycle by in situ potentiometry;49 however, the QYs of the final core/shell particles are limited (∼40%).48 We have previously demonstrated that the commonly used cadmium precursor Cd(oleate)2 has low conversion yield when added in monolayer-equivalent quantities during the growth of CdSe/CdS core/shell QDs via the SILAR technique.50 The growth solvent could potentially play an important role in governing precursor conversion. Primary amines such as oleylamine,4,10,34 octadecylamine,30,32,33 and hexadecylamine51 have long been used as coordinating solvents for nanocrystal growth, with oleylamine being a common choice for shell growth on CdSe QDs by SILAR.4,10,50 The role of the primary amine in the nanocrystal growth has been studied extensively, and there has been contradiction between conclusions, which have been well summarized recently by Garcia-Rodriguez et al.52 Additionally, Hollingsworth’s and Vela’s groups have reported that switching to a secondary amine (dioctylamine) improved the synthetic yield when growing CdS shells on CdSe QDs, especially for larger shell thicknesses.12,53 Foos et al. have shown that the use of secondary and tertiary amines can result in an improved size distribution during growth of CdSe nanocrystals.54 One possible mechanism is that the reactivity of the Cd precursor was reduced due to the strong coordination of the primary amine. Liu’s and Vela’s groups have suggested53,55 that primary amines such as oleylamine may stabilize Cd(oleate)2 in solution through the formation of six-coordinate complexes. Solution-phase complexes could be sterically restricted in the case of secondary or tertiary amines.53 However, it is also known that amines can improve the fluorescence quantum yield by coordination to the nanocrystal surface, and it is possible that such surface coordination is competing with precursor conversion.56−59 It is known that Cd carboxylates such as Cd(oleate)2 can associate to the surface of CdSe nanocrystals.59 The equilibrium constant for conversion of such compounds to surface-bound equivalents must be high under reaction conditions if these compounds are to be used as metal precursors for growth under SILAR control. Amines can bind to and electronically passivate the nanoparticle surface by acting as two-electron donating (L-type) ligands.59−61 Occlusion of neighboring Cd(oleate)2 precursor binding sites by amines could decrease, to varying extents, the effective equilibrium constant for binding of Cd(oleate)2 (negative cooperativity). This mechanism has not been explored directly, making the role of amine solvents in influencing SILAR shell growth unclear. In this work, we grew CdSe/CdS core/shell quantum dots in solvent mixtures with three different representative amines, primary, secondary, and tertiary, via a SILAR technique. The

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RESULTS AND DISCUSSION To investigate the impact of the amines on the growth of CdSe/CdS core/shell quantum dots, three parallel experiments were performed under identical conditions but with three different alkyl amines composing a portion of the shell growth solvent. We selected oleylamine (OAM), dioctylamine (DOM), and trihexylamine (THM) for our studies in this Article. The three amines were chosen to (1) represent primary/secondary/ tertiary amines, and (2) have similar molecular weight and molar volume, so that similar amine:QD ratios (∼50 000:1) could be achieved at similar QD concentrations. A common batch of CdSe cores was used to minimize differences. Detailed procedures are described in the Experimental Section. Shell growth solvents were prepared with the amine and 1octadecene (ODE) in a 2:1 volume ratio. Purified CdSe cores were introduced as a solution in hexane; hexane was subsequently removed under vacuum, and the system was brought to the reaction temperature under nitrogen. Shell precursor solutions (containing Cd(oleate)2 and TMS2S) were freshly prepared for each synthesis and were introduced into the reaction flask in an alternating fashion according to the SILAR technique, one monolayer equivalent (1 ML equiv) 7469


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Chemistry of Materials

Figure 1. Scaled absorption and emission spectra over the course of CdSe/CdS core/shell QDs growth in three amines. (A,D) CdSe/CdS_OAM grown in oleylamine; (B,E) CdSe/CdS_DOM grown in dioctylamine; (C,F) CdSe/CdS_THM grown in trihexylamine. Absorptions and emissions are normalized to the concentration of QDs in each aliquot, so that all of the absorption and emission represent the absorption and intensity of the same amount of QDs; the dashed lines represent the upper and lower bands of 25% error for the QD concentration in each aliquot. The insets zoomed in the region of emission where “blue peaks” appeared for CdSe/CdS_OAM and CdSe/CdS_DOM, and no “blue peaks” in CdSe/ CdS_THM.

applied to the evolution of the 1S absorbance in the CdSe/CdS core/shell particles is indicated by the black curves in Figure 1A−C, with 25% error indicated by dashed lines. Figure 1D−F shows that in all three growths, the PL emission intensity of CdSe/CdS core/shell particles continuously increased with increasing CdS shell thickness; this is a result of an increasing quantum yield as well as an increasing excitation rate at the same concentration due to enhanced absorption at short wavelengths because of the CdS shell. Despite superficially similar absorption spectra and bandedge PL spectra among the three samples, a close examination of the emission spectra reveals a PL peak appearing between 400 and 500 nm (“blue peak”) that is present in the oleylamine case (Figure 1D), greatly diminished (∼50× less intense) with dioctylamine, and nearly absent with trihexylamine. The blue peaks are absent prior to introduction of shell precursors, are centered at wavelengths shorter than the emission of the CdSe cores used, and shift to longer wavelengths as additional shell precursors are introduced. These characteristics are all consistent with the appearance of a CdS nanoparticle side product. We have previously shown that nucleation of CdS nanoparticles can occur when growing CdSe/CdS core/shell QDs via the SILAR technique under 1 ML equiv dose per cycle.50 The wavelengths of the blue peaks fall within the range of emissions for CdS nanoparticles with diameters of 3.5−4.5 nm.63,64 We offer additional evidence below, in the form of photoluminescence excitation (PLE) and STEM imaging, to support the conclusion that the blue peaks are PL from a CdS nanoparticle side product that is abundant in the case of oleylamine, but much reduced in the presence of the secondary and tertiary amines. A close examination of the 1S absorption and PL peaks of the CdSe cores before and after being brought to the reaction temperature in the shell growth solvent (Figure S2) reveals a small blue shift of ∼3 nm (∼15 meV) in the case of the oleylamine solvent that is absent in the dioctylamine and trihexylamine cases. Blue shifts attributed to nanocrystal etching in primary amine solvents have been reported previously.53,65

dose for each addition, with growth conducted up to 6 monolayer (ML) equivalent thickness of CdS shell in total. Each addition was slowly applied over 3 min, with 12 min waiting time before the next addition. Aliquots for monitoring of cores were drawn just prior to shell growth; aliquots for core/shell particles at series of shell thickness in terms of ML were drawn at the end of each complete ML addition for both Cd and S precursors. The experimental results are summarized in Table 1. Shell Growth As Monitored by Absorption and Photoluminescence Spectroscopy. During the course of the growth, aliquots with a consistent volume of 50 ± 5 μL were drawn and diluted in 2.0 ± 0.2 mL of hexane for monitoring by absorption and PL spectroscopy. This method resulted in diluted samples with KDOM > KOAM, indicating that the different amines as solvents strongly influence the equilibrium. Indeed, KTHM is more than 5 times larger than KOAM. Although some re-equilibration of bound and dissolved Cd species may occur in the aliquot dilution step, the trend in relative effective K is consistent with the growth results. The lower association constant in oleylamine could be brought about by stabilization of Cd(oleate)2 in solution and/ or by stronger interactions of oleylamine with the CdSe surface. Although the nucleophilic primary amine and electrophilic Cd presumably bind to different sites on the NC surface, unfavorable steric and/or electronic effects could result in effective inhibition of Cd binding in the presence of the amine. The PL lifetime results support a stronger interaction of oleylamine with the CdSe/CdS surface as compared to the more highly substituted amines as a contributing factor to the difference in K. The results from our titration experiments indicate that growing core/shell particles in solvent mixture of oleylamine will lead to a large amount of free Cd(oleate)2 at monolayer equivalency, which could promote cross reaction of shell precursors and CdS nucleation at the expense of surface reactions when TMS2S is introduced. STEM Images of the Core/Shell QDs Grown in Three Amines. Figure 7 shows the STEM images and radius distribution histograms for CdSe cores as well as the three core/shell products described in Table 1. The radius histograms are determined by analysis of STEM images of the same magnification at 6−7 randomly selected regions; N is the number of particles analyzed. In comparing STEM images of Figure 7A−D and the radius histograms of Figure 7E−H, the differences in particle sizes and distributions are clearly displayed. We characterize the average radius and peak radius for particles; the average radius is obtained directly from the distribution (including small particles), while the peak radius is the center of a Gaussian fit (red curve, Figure 7E−H) to the distribution and represents a characteristic radius for core/shell particles in the sample. Table 2 summarizes the STEM imaging results. The average radius and the peak radius of the CdSe cores are ravg = 1.70 ± 0.16 nm and rpeak = 1.70 ± 0.00(1) nm based on the analysis of N = 1997 different particles, in agreement with the core radius that was determined by the CdSe size calibration curve,4,50,73 7475


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toluene without QDs were recorded for comparison. The QDs were purified by gel permeation chromatography74 (GPC, toluene/polystyrene) to minimize the impact of any impurities and isolate interactions between the amines and the CdSe QD surface. In the presence of the QDs, the α-proton and olefin peaks of oleylamine are significantly broadened, which is evidence of a strong interaction between oleylamine and the nanoparticle surface.75 In contrast, a mixture of dioctylamine with the CdSe cores shows only a small degree of broadening and a small downfield shift, and a mixture of trihexylamine with the CdSe cores shows almost no change versus the free molecule. These results indicate that the interaction affinities of amines to the nanoparticle surface are in the order of oleylamine > dioctylamine > trihexylamine from strong to weak, in agreement with the PL lifetime results. This implies that replacing oleylamine with dioctylamine or trihexylamine in SILAR shell growth effectively destabilizes the neutral nanocrystal surface, which facilitates binding of the Cd(oleate)2 precursor independently of differences in solution-phase interactions that may also be present.

storage after synthesis, while particles in trihexylamine remained well dissolved and showed no precipitation. Proton NMR Characterization of Amine Interactions with QD Surface. The titration experiments confirm that the Cd(oleate)2 conversion efficiency improves when the more highly substituted amines are used as the solvent, and the PL lifetime results provided a strong indication that the amines bind differently to the nanocrystal surface. To probe this ligand/nanocrystal interaction directly under mild and controlled conditions, we recorded 1H NMR spectra of mixtures of CdSe cores with each of the amine solvents diluted in d8-toluene (Figure 8). Reference spectra of the amines in d8-

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CONCLUSIONS We have confirmed that replacing oleylamine with a secondary amine, dioctylamine, suppresses nucleation and improves core/ shell growth, and we have shown that moving to a tertiary amine, trihexylamine, is even more effective. We have also shown through measurement of Cd fractionation in collected aliquots, time-resolved PL, and NMR spectroscopy that the more highly substituted amines bind less strongly to the CdSe QD surface and permit greater precursor conversion under experimental conditions. On the basis of these results, we can conclude that oleylamine effectively competes with the precursor Cd(oleate)2 for occupation of nanocrystal surface sites, leading to a significant amount of cross-reaction and nucleation of CdS particles during CdS shell growth by SILAR (Scheme 1). Studies that separately quantify QD−amine56,76,77 and Cd precursor−amine interactions could be used to differentiate their relative contributions to control of precursor conversion and growth. Measurements of ligand binding equilibria through techniques including resonant energy transfer,78 NMR,75 and calorimetry56,79,80 are important to the development of sequential chemistries for nanocrystal surface reactions,

Figure 8. 1H NMR for α-proton for the three amines studied in the presence and absence of CdSe cores. Solvent is d8-toluene. Full spectra are provided in Figure S6.

Scheme 1

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Synthesis of Core/Shell Nanoparticles in Different Amines. The method for CdSe/CdS core/shell particle growth was modified from our previous work.4,50 The difference was switching different types of amines (oleylamine, dioctylamine, trihexylamine) in the solvent mixture. The Cd precursor was prepared by diluting Cd(oleate)2 stock solution in a solvent of 50:50 ODE and TOP with 2 equiv of the same amine in the solvent mixture (vs Cd) added to yield a Cd concentration of 0.1 M. The sulfur precursor was 0.1 M solution of (TMS)2S dissolved in TOP. The CdS shell was grown by alternatively introducing Cd and sulfur precursors into the reaction flask, 1 ML equiv of precursors added per cycle, and forming 6 ML of CdS shell in total after six cycles. Reaction progress was monitored by periodically withdrawing a small aliquot of a measured volume (typically 50 μL) from the reaction flask and diluting it in hexanes at room temperature; these aliquots were analyzed for UV−vis absorption and fluorescence emission in hexanes solution. The absorption spectra were recorded by a Thermo Scientific Evolution Array UV−visible spectrophotometer with hexane as the solvent as well as the blank in a 1 cm path quartz cuvette. The emission spectra were recorded by an Ocean Optics USB 4000 spectrometer under 365 nm LED excitation. Ensemble QY for as-synthesized core/shell samples with 6 ML equiv CdS was measured with an integrating sphere and found to be 60−70% in each case. Photoluminescence Excitation Spectroscopy. PLE scans were recorded via a FS5 fluorescence spectrofluorometer from Edinburgh Instruments, with a scanning wavelength from 300 to 640 nm with a step of 0.5 nm. Time-Resolved Photoluminescence Measurement. The PL decays of QDs in hexane were collected in front-face mode with 1 cm quartz cuvette in a lifetime spectrometer (Edinburgh Mini-τ) equipped with a 368 nm picosecond-pulsed-light-emitting diode. A stirring stage was set under the Mini-τ, and a mini stirring bar was placed in the cuvette to stir the QD solution to avoid accumulation of photoproducts during the measurement. The instrument response function (IRF) is recorded using Rayleigh scattering of pure water. Titration of Cd(oleate)2 to CdSe Cores in Different Amines. CdSe cores were washed via two cycles of precipitation/redissolution as described previously, and then brought into a known volume of hexane for absorption measurements to determine the size and quantity. They were then injected into the overcoating solvent and degassed at 80 °C for 2 h to remove hexane. The solvent was a mixture of amines and ODE with a volume ratio of 1:2, 9 mL in total. Different amines (oleylamine/dioctylamine/trihexylmaine) were used for the solvent mixture. After degassing under vacuum, the system was placed under nitrogen and brought to growth temperature (180 °C) before titrating with Cd precursor. The Cd precursor was prepared by introducing Cd(oleate)2 stock solution with 2 equiv (vs Cd) of the same amine as in the solvent mixture, balanced by TOP to yield a Cd concentration of 0.1 M. A computer-controlled syringe pump (J-KEM Scientific Dual Syringe Pump, model 2250) was used to introduce reagents according to the dose and timing. Briefly, a series of additions of 0.2 ML equiv dose each, up to 1 ML equiv, followed by 5 additions of 0.48 ML equiv of Cd precursor (0.1 M) was added periodically at a constant rate over a 3 min injection time, and a total of 15 min was allowed to elapse for each addition. A small aliquot of 25.0 μL (Valiquot) was withdrawn by microsyringe from the reaction flask and injected into a small amount of hexane (2 mL, dispensed by J-KEM syringe pump to minimize variability) at room temperature. The aliquots then were treated by adding 1 mL of acetone and 3 mL of methanol to precipitate QDs, followed by centrifuging at 5000 rpm (∼3000g) for 5 min. The supernatant was transferred into 20 mL sealed vials, and checked with UV light to make sure that it showed no absorption or fluorescence indicative of QDs left in solution. The samples were dried by removing the solvent by vacuum pump. A 1 mL portion of aqua regia (3:1 hydrochloric acid/nitric acid; Caution! Highly corrosive; oxidizer) was introduced and was allowed to digest the sample for 2 h. Each of the samples was then quantitatively transferred into a volumetric flask and brought to 50.0 mL with 2% HNO3, and the concentration of Cd2+ was measured by a Themo-Finnigan Element XR ICP-MS. A control experiment using Cd(oleate)2 in ODE was

including shell growth. The SILAR shell growth mechanism effectively requires a ligand exchange associated with each halfcycle. The identity, binding constants, and interactions of the competing ligands at the nanocrystal surface are important in determining the extent to which this ligand exchange can be achieved. While highlighting the limitations of oleylamine as a solvent for SILAR shell growth on CdSe, our result suggests that solvents that promote greater precursor conversion can indeed steer the course of the reaction toward layer by layer growth without solution-phase cross reactions. Among the coordinating amine solvents studied here for CdSe/CdS core/ shell growth, trihexylamine permitted the greatest precursor conversion at near-monolayer equivalencies. This greater conversion was associated with improved morphological control. As an added benefit, trihexylamine is less expensive than the widely used oleylamine at comparable purity levels. The high extent of precursor conversion per half-cycle is likely to be a benefit in forming thick shells requiring multiple cycles, in which accumulation of byproducts over multiple cycles is undesirable,53 and in extending SILAR techniques to higherdimensional substrates in which a high dose per cycle is favored to drive saturation of all available surface growth sites.

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EXPERIMENTAL SECTION

Materials. The following chemicals were used as received. Cadmium oxide (CdO; 99.999%), trioctylphosphine (TOP; 97%), and trioctylphosphine oxide (TOPO; 99%) were purchased from Strem Chemicals. Oleic acid (OA; 99%), 1-octadecene (ODE; 90% technical grade), 1-tetradecylphosphonic acid (TDPA; 98%), and Se (99.999%) were purchased from Alfa Aesar. Di-n-octylamine (98%) and tri-n-hexylamine (97%) were purchased from Alfa Aesar. Decylamine (95%) was purchased from Sigma-Aldrich. Oleylamine (80−90%) and bis(trimethylsilyl) sulfide ((TMS)2S; 95%) were purchased from Acros Organics. Toluene-d8 (D, 99.5%) was purchased from Cambridge Isotope Laboratories, Inc. 200 proof ethyl alcohol (ethanol) was obtained from Decon Laboratories, Inc. Methanol (99.8%) was purchased from BDH. Acetone (99.9%) was purchased from VWR. Ethanol (99.9%) was purchased from Fisher Scientific. TOPSe (2.2 M) was prepared by dissolving Se in TOP. A stock solution of Cd(oleate)2 (0.2 M) in ODE was prepared by heating CdO in ODE with 2.2 equiv of oleic acid at 260 °C under nitrogen, followed by degassing under vacuum at 100 °C for 20 min. The sulfur precursor was 0.1 M solution of (TMS)2S dissolved in TOP. Nanocrystal core and shell growth was carried out under nitrogen (N2) using Schlenk line techniques; air-sensitive reagents were prepared in a nitrogen filled glovebox. Optical Spectroscopy. The optical absorption spectrum was recorded using a Thermo Scientific Evolution Array UV−visible spectrophotometer with hexane as the solvent as well as the blank in a 1 cm path quartz cuvette. Routine emission spectra were recorded by an Ocean Optics USB 4000 spectrometer under ∼365 nm excitation. Synthesis of CdSe Cores. A hot-injection technique was applied for synthesis of CdSe nanocrystals (NCs) cores.4 For a representative synthetic route, CdO (0.12 g) was heated with TDPA (0.5500 g) at 330 °C in a solvent of TOP (6 mL) and TOPO (6 g) under nitrogen flow until the solution became colorless. Following removal of evolved H2O under vacuum at 130 °C, the solution was heated again to 360 °C under nitrogen. As-prepared TOPSe (1.3 mL) was injected rapidly into the reaction pot, which was immediately allowed to cool to room temperature and stored as a yellow waxy solid. The Cd:TDPA:Se molar ratio is 1:2:3. The core radius was estimated by a calibration curve4,73 describing the radius as a function of the position of the lowest-energy absorption peak. One batch of cores provided sufficient material for several core/shell growth experiments; all core/shell particles were made on the basis of the CdSe QD cores taken from the same batch. 7477


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Chemistry of Materials designed to investigate the accuracy of this method in quantifying the amount of Cd2+. The error was determined to be less than 6%. Scanning Transmission Electron Microscopy Imaging. After purification, the CdSe or CdSe/CdS core/shell QDs were brought into hexane to form a dilute solution (1.1 μM), and one drop of the solution was drop-casted on a clean TEM grid (400 mesh Cu grid with ultrathin carbon support film, Type-A, Ted Pella, Inc.) and pumped dry under vacuum for 2 h. The STEM samples were imaged by JEOL 2100F 200 kV FEG-STEM/TEM equipped with a CEOS CS corrector on the illumination system. Prior to high magnification observation, a large specimen area was preirradiated with electrons for 10 min to polymerize surface hydrocarbons and therefore prevent their diffusion to the focused probe. The geometrical aberrations were measured and controlled to provide less than a π/4 phase shift of the incoming electron wave over the probe-defining aperture of 17.5 mrad. High angle annular dark-field (HAADF) STEM images were acquired on a Fischione model 3000 HAADF detector with a camera length such that the inner cutoff angle of the detector was 75 mrad. A pixel dwell time of 16 μs was chosen.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03588. Supplementary figures and tables, including PL decay analysis and method for calculating QD surface coverage (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the University of South Carolina. We give thanks to L. He for ICP-MS analysis, and to P. J. Pellechia for assistance with NMR spectroscopy. Timeresolved PL was enabled by a College of Arts and Sciences Small Research Instrumentation Grant. Photoluminescence excitation spectroscopy was enabled by an Aspire III grant from the USC Office of Research. We thank N. Shustova and D. Williams for assistance with PLE.

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REFERENCES

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