Article pubs.acs.org/cm
Synthesis of Highly Luminescent and Photo-Stable, Graded Shell CdSe/CdxZn1−xS Nanoparticles by In Situ Alloying Klaus Boldt,† Nicholas Kirkwood,† Gary A. Beane, and Paul Mulvaney*
School of Chemistry and Bio21 Institute, University of Melbourne, Parkville, Victoria 3010, Australia S Supporting Information *
ABSTRACT: We report a facile and robust synthesis of CdxZn1−xS graded shells on CdSe nanoparticles that are prepared by interface alloying between CdS and ZnS shells at elevated temperatures. Alloying provides systematic control over the electronic structure and enables switching between Type-I and quasi-Type-II configurations. Good control of particle shape, shell thickness, and composition is achieved by slowly adding zinc oleate and octane thiol via syringe pump to readily prepared CdSe/CdS particles. The resultant quantum dots exhibit PL quantum yields of up to 97% and superior robustness toward environmental influences and quenching agents. Alloying promotes a blue-shift of both the absorption and PL spectra compared to pure CdSe/CdS particles and an increased Stokes shift, opening a new synthetic pathway to stable, green-emitting core/shell/shell quantum dots. High PL quantum yields are correlated to a narrow distribution of single-particle lifetimes and suppressed fluorescence intermittency. We introduce a new method to characterize the PL intermittency of single quantum dots based on the autocorrelation function of their PL time trajectories. KEYWORDS: quantum dots, alloy, synthesis, core−shell, CdSe he wet chemical synthesis of highly fluorescent semiconductor nanocrystals or quantum dots (QDs) has been a topic of intense research since the development of the hot injection synthesis 20 years ago made available a facile route to cadmium chalcogenide particles of high crystallinity and low size dispersity, stabilized by organic surfactants.1 The quality of the synthetic routes is constantly being improved. The main focus of these efforts has been on the use of more environmentally friendly and less toxic precursors,2,3 as well as the improvement of the photoluminescence quantum yield (PL QY) and fluorescence stability.4 The predominant strategy for achieving higher PL is to epitaxially grow a shell of a larger band gap material around the CdSe cores, creating core−shell particles with an electronic structure commonly referred to as a Type-I configuration.5,6 The potential step at the core−shell interface separates both electron and hole wave functions from the dangling bonds or excess charges at the particle surface, which are regarded as the predominant cause for nonradiative pathways of exciton recombination.7−10 In this work we provide a new method for creating photostable, luminescent QDs with close to unity PL quantum yield. The method builds on previous work in this field but exploits the in situ formation of alloys at high temperatures as a means to provide minimal lattice strain and a smooth potential well. The materials that are commonly employed for shells are ZnS4 and CdS.11−13 While the former provides a deeper potential well for both charge carriers, the large lattice mismatch between core and shell material of ∼12% introduces additional crystal faults
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© 2013 American Chemical Society
that may limit the ultimate PL.2 CdS introduces less lattice strain, but the excited electron is delocalized over the whole structure due to the small difference in the conduction band energies of the core and shell material.7,14 This configuration in which only one charge carrier wave function is confined, while the other extends over the whole particle, is referred to as a quasi-Type-II or TypeI1/2 configuration.5 Improved approaches to shell growth include the synthesis of giant shells of 10−20 monolayers (ML) of CdS deposited on the CdSe cores15−18 and a stepwise increase of the band gap with more than one material in a CdSe/CdS/ZnS or CdSe/ZnSe/ZnS heterostructure.19 The growth of giant CdS shells has been found to yield virtually nonblinking particles with suppressed Auger recombination but comes at the cost of large red-shifts of the PL emission and the loss of strong charge carrier confinement. A further improvement has been the introduction of a graded shell with a gradual instead of stepwise increase of the confinement potential.20 By mixing the precursors for the CdS and ZnS shells, alloys with a tunable band gap can be formed, while the gradient is controlled by different precursor reactivities.21−24 Apart from forming a smooth band gap gradient, alloying also relaxes the lattice strain of unmatched crystal structures.25−27 Furthermore, it has been shown that alloying suppresses Auger recombination in CdSe/CdSexS1−x/ CdS quantum dots and thereby increases PL QY.28 Received: August 5, 2013 Revised: October 10, 2013 Published: November 5, 2013 4731
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Several methods for shell growth have been successfully implemented, most notably the continuous addition of one or both precursor solutions by syringe pump,18,19 thermal cycling,29,30 and the successive ion layer adsorption and reaction (SILAR) method, which gives accurate control over the number of monolayers.12,13,31 It has been demonstrated by Raman26,32 and fluorescence line narrowing studies33 that the formation of a graded shell between CdSe and CdS occurs naturally during shell growth. To avoid separate nucleation of the shell materials, shell deposition reactions are usually performed at lower temperatures than those employed in the core synthesis (250 °C and below), so that interdiffusion between the core and the shell materials is mitigated. Most recently Chen et al. have published a method to synthesize CdSe/CdS particles at elevated temperatures (310 °C) with octane thiol as the sulfur source.34 This method combines a robust and facile synthesis with a high reproducibility, yields extremely narrow size distributions after the growth of several monolayers of CdS, and PL QY above 80%. However, due to the lack of confinement of the electron wave function, particles with up to 4 ML of CdS are easily quenched by oxygen, moisture, or electron and hole scavengers. It is highly desirable to further improve Chen’s protocol with regard to environmental stability without losing the high PL QY. In this work we present the one-pot synthesis of graded shell CdSe/CdxZn1−xS QDs starting from CdSe particles by in situ alloying of a freshly prepared CdS shell with zinc and sulfur precursors. The particles retain the excellent PL properties and exhibit superior photostability. In addition alloying of the outer CdS layers leads to stronger confinement of the electron, converting the quasi-Type-II structure into a Type-I configuration. For extended alloying times this conversion is reversed, allowing for precise engineering of the electronic structure. We employ single-particle PL data to demonstrate the high quality and homogeneity of these particles. There have been many comprehensive studies on the phenomenon of fluorescence intermittency or “blinking” in quantum dots; these have identified the likely cause of blinking to be related to the ionization of the nanocrystal and subsequent nonradiative recombination via Auger recombination.35−37 Despite our increasing knowledge of the likely origins of blinking, there is still room for further qualifying this understanding and devising a method to quantify the amount of blinking in individual samples. The two most prevalent analytical methods that have been used to quantify blinking in quantum dots are “on/off” statistics38 and power spectral density.39 While the data manipulation in each case is quite different, fitting of the data to power law models provides a way of comparing the amount of blinking between samples, though the fit parameters will depend on the bin time of the time traces. The Mandel Q parameter has been proposed as an alternative method to quantify blinking, which does not rely on fitting of the data but is instead tied to the number of PL counts in a certain bin time.36 While this method provides additional information about the nature of the noise (Poissonian or super-Poissonian) and complements existing techniques, there is still scope to further quantify blinking beyond this. To characterize the blinking behavior in this study, we present an alternative approach employing the dimensionless parameter κ for the amount of blinking. It is related to the fractional on-time, calculated using the autocorrelation time of single-particle time trajectories, and is therefore intrinsic to the sample in much the same way as the radiative lifetime. Importantly, it does not depend on the bin time of the measurement.
Article
EXPERIMENTAL METHODS
Chemicals. Cadmium oxide, zinc acetate, n-octadecene, oleic acid, tri-n-octylphosphine, tri-n-octylphosphine oxide, octane thiol, tri-nbutylphosphite, and selenium were purchased from Sigma-Aldrich. Oleylamine was purchased from Acros and Sigma-Aldrich, tri-nbutylphosphine from Capotchem, and n-octadecylphosphonic acid from PCI Synthesis. All chemicals and solvents were used as received without further purification. Preparation of Stock Solutions. Cadmium oleate (85.77 mM) was prepared by mixing CdO (0.128 g), oleic acid (0.847 g, 3 equiv), and ODE (10.1 mL) in a three-neck flask. The mixture was degassed under vacuum at 50 °C for one hour and heated to 350 °C under nitrogen. When the CdO had dissolved to form a clear solution the mixture was cooled to 100 °C, degassed again under vacuum, and transferred into a nitrogen glovebox for storage. Oleylamine (0.535 g, 2 equiv) can be added to prevent gelation during cooling. Comparable results were obtained by reacting cadmium acetate with 2 equiv of oleic acid in ODE and stabilizing with 2 equiv oleylamine. Zinc oleate (208.8 mM) was prepared by mixing zinc acetate (367 mg), oleic acid (1.13 g, 2 equiv), and ODE (7.0 mL) in a glass bottle inside a nitrogen glovebox. The mixture was heated to 200 °C until it became clear and then cooled to room temperature. At ∼ 150 °C oleylamine (1.07 g, 2 equiv) was added and the solution was stored until further use. Particle Synthesis. CdSe particles of varying sizes with a wurtzite crystal structure were prepared in a TOPO/ODPA mixture following the protocol of Carbone et al.,40 washed three times with methanol, and stored in hexane. The epitaxial growth of CdS was performed according to the method published by Chen et al.34 Briefly, ODE (3 mL), oleylamine (3 mL), and 100 nmol of CdSe particles in hexane were loaded into a three-necked flask and degassed at 50 °C for 45 min followed by 15 min at 120 °C under vacuum. The temperature was raised to 310 °C under a nitrogen atmosphere at a heating rate of 12 °C/ min. Starting at 230 °C, solutions of cadmium oleate and octane thiol, each diluted with ODE to give a final volume of 3 mL, were injected from separate syringes with a syringe pump. The precursor amounts were calculated from the core particle sizes and desired shell thickness. A 1.2fold excess of thiol was used, and the injection rate was adjusted to add the equivalent of 2 ML of CdS per hour. After the addition of precursors was completed, the temperature was lowered to 200 °C and 1 mL of oleic acid was added dropwise. The reaction mixture was annealed for 1 h at this temperature. Before the growth of the ZnS shell the temperature was lowered to 120 °C and the reaction mixture was degassed for 30 min under vacuum to remove volatiles added or created during the reaction. The solution was again heated to 310 °C. Starting when the temperature reached 230 °C, zinc oleate and octane thiol in ODE were added by syringe pump. For reactions with faster addition rates the starting temperature was set higher so that the reaction temperature was reached after the same delay. A twofold excess of thiol to Zn acetate was used. After the precursors were added the reaction mixture was allowed to cool to room temperature. The crude solution was washed by adding acetone until the solution became turbid, usually by doubling the volume. The addition of small amounts of ethanol or freezing the centrifuge vial in liquid nitrogen for 10 s helped to quantitatively flocculate the particles. The precipitate was collected after centrifugation at 3.3 × g and redispersed in a minimal amount of chloroform. After repeating this procedure three times the final product was dried to a powder under a nitrogen stream, redispersed in hexane, passed through a syringe filter, and stored in the dark. Single Particle Measurements. The samples for single particle detection were prepared by spin coating small amounts of dilute nanocrystal dispersions in hexane on top of clean glass cover slides. The excitation of the single particles and detection of PL was achieved using a confocal microscope in epi-illumination configuration, with an oilimmersion objective (Olympus, PlanApo NA 1.4) and a 400 nm, 10 MHz repetition rate laser diode (PicoQuant, LDH-P-C-405). A low pump intensity level was employed with an excitation power of 70 W/ cm2, measured after the objective, to ensure single exciton excitation. 4732
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Figure 1. (A) Absorption and PL spectra taken during the synthesis of CdSe/CdS/ZnS particles with 4 CdS and 0−4 ZnS ML, starting from 3.7 nm cores. The dotted line is a guide to the eye. (B) TEM micrograph of the sample after 60 min of ZnS shell growth. (C) Particle size distribution of CdSe cores, CdSe/CdS particles, and CdSe/CdS/ZnS after 60 and 120 min of shell growth. (D) PL QY of washed samples before (■) and after (•) quenching of the fluorescence with 50 μL of octane thiol. The sample at 0 min reaction time consists of CdSe particles with 4 ML CdS.
purification. When the temperature reached 230 °C solutions of zinc oleate and octane thiol in ODE were introduced from separate syringes by syringe pump. Samples were taken at various times during shell growth and subjected to the same cleaning steps as the final product. The 1S transition energy was determined from the absorption spectra of each sample by fitting the first two visible transitions with two Gaussians. Growth of a CdS shell caused the 1S transition to red-shift 40 nm after the deposition of 4 ML of CdS, in agreement with earlier work from our group.13 The higher transitions in the absorption spectrum remained clearly defined, and an increased absorption of the 2S transition compared to the 1S peak could be observed (Figure 1A). The 2S/1S transition intensity ratio continued to increase with the growth of 4 ML ZnS, while the spectrum blue-shifted 25% for the first 2.4 ML relative to the red-shift caused by the CdS layer. Further addition of ZnS caused a broadening and again a red-shift of the signal. The PL spectra generally exhibited smaller blue-shifts. The magnitudes of the PL blue-shifts correlate with the particle size and number of CdS shells. They can be comparable to the shifts of the absorption peak (25%, 17 nm) when starting with 2 nm CdSe cores and 2 ML of CdS (Figure 2A), but are lower for larger particles and thicker CdS layers, maximizing at 13% (4 nm) for the sample shown in Figure 1. This corrensponds to a 57% increase of the Stokes shift from 8 nm for CdSe/CdS particles to 14 nm after growth of 2.4 ML of ZnS.
After passing through the confocal optics, the signal was sent to a Hanbury-Brown-Twiss interferometer consisting of a nonpolarizing 50/ 50 beam splitter and two avalanche photodiodes (Perkin-Elmer, SPCMAQR-15). For pulsed antibunching measurements, the outputs of these detectors were connected to the start and stop channels of a photoncounting card (PicoQuant, TimeHarp 200), with one of the detectors having an electronic delay (about 80 ns) in the output. This allowed the measurement of interarrival times. For the measurement of the PL decay times of the individual particles, the photon-counting card was set to time-tagged time-resolved mode (TTTR). The total photon arrival times of the TTTR data were binned with a bin width of 40 ms to obtain time trajectories. PL decays were generated by time-correlated single photon counting (TCSPC).
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RESULTS AND DISCUSSION An important challenge for the preparation of core/shell nanoparticles has been to determine to what extent alloying occurs between the core and shell materials. Effort has been put into either completely avoiding any phase mixing5,29 or intentionally producing alloyed shells to yield a defined composition and improved optical properties. In this study we prepared CdSe/CdS/ZnS particles at elevated temperatures, which would favor the interdiffusion of shell materials. Starting from CdSe/CdS particles synthesized by the protocol of Chen et al.,34 we prepared CdSe/CdS/ZnS particles at elevated temperatures in a 1:1 mixture of ODE and oleylamine. After removing volatiles that were added or produced during the shell growth, the reaction mixture was heated to 310 °C without 4733
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the blue shift of both absorption and PL as well as an increased Stokes shift, which is the expected behavior for decreasing exciton size.42,43 The subsequent red-shift cannot be caused by assuming an increasing ZnS shell thickness, which would not influence the charge carrier confinement inside the core. The data instead indicates strong ion diffusion at high temperatures. It is known that cations in II−VI semiconductors diffuse rapidly, enabling complete cation exchange even at room temperature,44,45 and that mixing of CdSe and CdS occurs during CdS shell growth at temperatures below those employed here.32 After prolonged heating, mixing of the CdSe/CdS and CdS/ZnS diffusion zones takes place, causing the formation of a graded alloy involving all four ions, with a larger band gap than pure CdSe (Figure 3). By converting the electronic structure to quasi-Type-II with a confined hole and a delocalized electron, the PL emission is redshifted again.46 This is further supported by an increase in fluorescence lifetime that is observed for particles after extended shell growth at 310 °C. By fitting a stretched exponential function to PL decays, lifetimes of ⟨τ⟩ = 24.0 ns (τ = 23.6 ns, β = 0.96) after 60 min and ⟨τ⟩ = 33.5 ns (τ = 32.6 ns, β = 0.94) after 120 min are obtained. This indicates a smaller overlap integral of the electron and hole wave functions, which is characteristic of a quasi-Type-II system (see Supporting Information, Figure S2). Photoluminescence quantum yield (PL QY) was measured relative to one of three organic dyes, Rhodamine 101, Rhodamine 6G, or Coumarin 153, in air-saturated, spectroscopic grade ethanol, depending on the PL of the respective particles.8 The PL QY remained above 90% for washed samples with 4 ML of CdS and up to 4 ML of added ZnS, going from 98% for pure CdSe/CdS to 91% at the end of the reaction (Figure 1D). To test the charge carrier isolation due to the added ZnS shell a fluorescence quencher was added in excess. A total of 50 μL (∼250 equiv) of octane thiol were added to 3 mL of a particle solution at an optical density of 0.03 to act as a hole scavenger. PL QY was compared before and after adding the quencher. The emission QY dropped from 98% to 41% for pure CdSe/CdS particles. Adding the equivalent of 1 ML of ZnS per particle did not affect the PL QY, while the stability against the quenching agent increased significantly with a drop from 95% to 84%. For 2.4 ML the PL loss was even smaller (94% to 90%). Further shell growth did not significantly improve the particles’ resistance to quenching agents. p-Phenyl benzoquinone (∼35 equiv) was employed as an electron scavenger that is reduced to the hydroquinone upon electron transfer from the nanocrystals. For particles with only CdS shells the quenching efficiency was comparable to that of the thiol (37% retained PL intensity). The addition of a 1.3 ML ZnS shell decreased the quenching efficiency twofold (75% retained PL) with no significant change for thicker shells (see Supporting Information, Figure S3). The larger quenching efficiency can be expected from the less confined electron in the particle. The particle morphology changed with increasing ZnS precursor addition rates. Adding sufficient precursors to form 4 ML ZnS over 120 min resulted in particles with a spherical or oval shape (Figure 4C). Increasing addition rates appeared to favor growth along the [002] axis, resulting in increasingly irregular, tadpole-shaped particles (Figure 4A,B). The fastest addition rate (15 min) also resulted in a 25% PL QY reduction compared to shell growth over 30 and 120 min. Effects of Reaction Temperature. To examine the alloying process in more detail 4 ML of ZnS was grown onto the same batch of CdSe/CdS particles at three different temperatures: 260
Figure 2. (A) Absorption and PL spectra of 2 nm sized CdSe cores (blue) and the same particles after adding 2 ML CdS (orange) and 2 ML ZnS (green) to yield green emitting core/shell/shell quantum dots. (B) Photo of a selection of samples of CdSe/CdS/ZnS particles with alloyed shells under ambient light (top) and UV irradiation (bottom). The samples emit between 530 and 628 nm (right). The spectra above correspond to the second from left sample.
The particles appear spherical and single crystalline in the TEM micrographs and are slightly elongated along the [002] axis of the wurtzite CdSe cores (Figure 1B−C). Analysis of the TEM data reveals a continuous growth of the particles during precursor addition, ruling out surface etching to explain the observed blueshift. Correlation of the measured particle sizes from TEM data with the expected shell thickness calculated from bulk crystal parameters confirms a complete turnover of added precursors (see Supporting Information, Figure S1). Consequently we explain the observations through interfacial alloying between the CdS shell and newly deposited ZnS at high temperatures.41 The formation of a graded CdxZn1−xS layer with a band gap between that of CdS and ZnS causes the electron to become more strongly confined than in the case of a pure CdS shell, leading to 4734
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Figure 3. Schematic of the electronic structure of core/shell/shell particles during ZnS shell growth and alloying. The ZnS shell initially confines both electron and hole to the core (center), until extended alloying smoothes out the potential well and the electron wave function spreads out over the whole structure in a quasi-Type-II configuration (right).
Figure 4. TEM micrographs of CdSe/CdS/ZnS particles with 2 ML CdS and 4 ML ZnS grown at 310 °C. The injection rates for the ZnS shell were (a) 15 min, (b) 60 min, and (c) 120 min. Scale bars are 10 nm.
Figure 5. ZnS shell growth at different reaction temperatures: 260 °C (■), 280 °C (▲), and 310 °C (★). Peak positions of the first absorption maxima and PL emission lines for particles with (A) 2 and (B) 4 ML of CdS, starting from 2.9 and 4.2 nm CdSe cores, respectively. (C) Relative PL stability of the particles with (left) 2 and (right) 4 ML of CdS after addition of octane thiol in excess and (D) PL peak width for the particles with 4 ML of CdS. Zero minutes growth time denotes CdSe/CdS QDs immediately before precursor addition.
°C, 280 °C, and 310 °C. The experiment was performed on particles coated with 2 and 4 ML of CdS, starting with absorption maxima at 528 and 615 nm, respectively (Figure 5A,B). The shift toward higher energy initially occurs after the growth of approximately one full monolayer and remains small for
reactions at 260 °C (4 nm) and 280 °C (6 nm). At 310 °C the blue-shift increases strongly up to 15 nm, pointing toward a temperature dependent alloying process (Figure 5A,B). Zhong et al. have demonstrated an onset of alloying for CdSe core particles with a ZnSe shell above 270 °C, which is in close agreement with 4735
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Figure 6. Representative time traces of a single particle from the sample shown in Figure 5A with a 2 ML CdS shell only (A) and after 45 min (B) and 90 min (C) of ZnS shell growth at 310 °C. (D) Correlation plot of the total fractional correlation time κ and the fluorescence lifetime of 15 individual CdSe/ CdS/ZnS particles initially (A) (green squares), after 45 min (red triangles) and 90 min (blue circles) of ZnS shell growth.
We introduce the metric κ to quantify fluorescence intermittency or “blinking” of the particles, which is the fraction of the measurement window over which the fluorescence is correlated. This can be represented mathematically as
the parameters observed here.47 The blue-shift of the PL is less pronounced than that of the absorption peak and reverses after the deposition of more than 3 ML ZnS. The highest PL QY was achieved at 280 °C. At the highest temperature the spectra again showed signs of deterioration due to Ostwald ripening, including a 10 nm broadening of the PL peak from 22 to 32 nm as well as reduced fluorescence (Figure 5D). However, even considerably broadened particles spontaneously formed superlattices when dried on a TEM grid, demonstrating that the uniformity of the particle size and shape is maintained (see Supporting Information, Figure S4). The percentage of PL QY retained after the addition of octane thiol is plotted in Figure 5C. The resistance to the quenching agent increases strongly during the reaction with 60% fluorescence intensity being retained for CdSe/CdS particles and the core/shell/shell particles being virtually unaffected after the growth of two complete shells. Initially after starting the reaction the PL of the larger particles is more strongly reduced by the quencher than the purely CdS-capped samples (Figure 5C). This can be attributed to the presence of small ZnS islands on the surface that act as recombination sites, as well as unreacted octane thiol that binds to the particle surface. This increase in quenching efficiency is only resolved in the data for the reaction at 260 °C. For epitaxial growth the PL stability can be employed as a measure of the shell thickness needed for complete isolation of the charge carriers to be achieved. This limit is reached for the reaction at 310 °C after 30 min, whereas the same PL stability is reached after 60 min when adding the precursors at 280 °C. The differences between 280 and 310 °C vanish for the smaller particles, which have higher surface energies and form a complete shell more quickly. At lower reaction temperatures the same quenching efficiency is not reached, pointing to incomplete deposition of ZnS during the reaction time. Single Particle Spectroscopy. The conclusions from the ensemble data are supported by PL data collected from single nanocrystals. Time trajectories of the PL of 15 single particles on a glass cover slide were taken for samples of CdSe/CdS/ZnS with 2 ML CdS and 0, 2.2, and 4 ML of ZnS grown at 310 °C (0, 45, and 90 min reaction time), using a confocal microscope setup (Figure 6A−C).
R (τ ) =
⟨I(t ) − ⟨I ⟩⟩⟨I(t + τ ) − ⟨I ⟩⟩ σ2
(1)
where R(τ) is the autocorrelation function, I(t) is the time trajectory of a single nanocrystal, and σ is the standard deviation. The fluorescence intermittency is then defined as t
κ=
∫0 w R(τ ) dτ twR(0)
(2)
where tw is the time window over which the autocorrelation is measured and R(0) the autocorrelation when τ = 0. κ will be unity for particles that are nonblinking, i.e., always remain in their on-state, whereas for nonluminescent particles κ falls to zero. For QD time trajectories, where there is an inherent “signal” among noise, a method of removing both shot noise and 1/f noise is ideal. While binning of the time trajectory removes some of the shot noise, to remove the 1/f noise what is needed is a multiple time averaging (MTA) approach, where the properties of segments of the signal are calculated and then averaged over the total number of segments. This is indeed how the power spectral density (PSD) of the signal is calculated. The problem with the PSD is that it will always depend on the bin size of the time trajectory, so any fits to this data will not be solely intrinsic to the signal but will depend on the bin size used. While comparison of different particles and different samples provides a way of comparing relative amounts of blinking in different systems, it does not provide an absolute measure of the amount of blinking. In comparison, as κ is a unitless parameter that is normalized to both R(0) and the bin size of the measurement, it is a truly intrinsic measurement independent of bin size. CdSe/CdS particles without any added ZnS were subject to fast photobleaching and exhibited a high degree of fluorescence intermittency. Representative time traces show that the fluorescence intermittency is initially improved upon addition of 2.3 ML ZnS (45 min). These particles exhibit binary blinking 4736
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Figure 7. Representative time traces of a single particle from the sample shown in Figure 1 after 2.4 (A) and 4 ML (B) of ZnS shell growth. (C) Correlation plot of the total fractional correlation time κ and the fluorescence lifetime of 15 individual CdSe/CdS/ZnS particles with 2.4 (red squares) and 4 ML (blue circles) of ZnS.
excitons are strongly confined with negligible wave function leakage after growth of 2−3 ML ZnS. The alloying between CdS and ZnS reduces the size of the electron wave function by narrowing the potential well of the intermediate CdSe/CdS particles, thereby inducing a 25% blueshift of the PL relative to the red-shift from growing the CdS shell. This opens an easy synthetic route to highly stable, greenemitting particles with a fully graded shell, which has been difficult to achieve with previous methods. The plot of mean lifetime versus κ provides insight into the photophysical quality of the particles. It can be seen that during the alloying process particles with a superior quality can be synthesized by engineering the electronic structure of the graded shell. Further heating and alloying leads to a slow degradation of the optical properties.
behavior and remain in the on-state 80% of the time, characterized by a large κ value (>0.8). The κ values are narrowly distributed around the mean, pointing to a very homogeneous sample and suggesting that the single particle data are indeed representative of the QD ensemble (Figure 6D). After further addition of ZnS (4 ML, 90 min) the particles blinked more and the average quality factor of the sample decreased again, with a broader distribution similar to CdSe/CdS particles. When the CdS shell thickness was increased from 2 to 4 ML the optical properties of the single particles improved, as can be expected. Adding 2.4 ML of ZnS greatly enhanced the photostability and produced particles that could be continuously irradiated for ∼30 min without a change in the blinking behavior. A subset of this 30 min time trajectory is shown in Figure 7A. The distribution of κ values was extremely narrow with an average above 0.9, signifying particles that virtually did not blink. Adding further monolayers of ZnS again led to a significant broadening of κ, mirroring the broadening observed in the ensemble spectra. The time trace for larger particles was more complex, showing a distribution of gray states with 60% or less of the maximum emission intensity (Figure 7B). This corroborates the interpretation that the exciton becomes more strongly confined after alloying the particle surface with ZnS in a change from quasiType-II to Type-I structure. After further alloying, the potential well becomes wider again and the electron therefore less confined. The fluorescence for these particles exhibited monoexponential decays with a mean lifetime of ∼34.8 ns (see Supporting Information, Figure S5). For thin ZnS layers the PL lifetime and κ were strongly correlated. Upon further growth the mean lifetime increased to ∼37.1 ns and became much more broadly distributed (Figure 7C and Supporting Information, Figure S6). A similar trend was observed for thinner CdS layers (Supporting Information, Figure S7), pointing toward the loss of insulation between the exciton and the particle surface.
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ASSOCIATED CONTENT
* Supporting Information S
Excel spreadsheet with detailed information about the synthesis, correlation of measured and calculated particle sizes, fluorescence decay curves of core/shell particles after different reaction times, TEM images showing particle superlattices, and additional data on single particle measurements. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
K.B. and N.K. contributed equally to this work. K.B. and N.K. developed and performed ZnS shell synthesis. G.B. performed and analyzed single particle spectroscopy. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Notes
CONCLUSIONS We have demonstrated a one-pot procedure for the epitaxial growth of a graded CdS/ZnS shell onto CdSe quantum dots. These particles exhibit superior PL QY above 90% due to a smooth confinement potential caused by strong interface alloying between both shells at 310 °C. The method avoids a lengthy procedure of multiple injections and is therefore highly reproducible. PL quenching with octane thiol has shown that
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
K.B. acknowledges the support of the Alexander von Humboldt Foundation through a Feodor Lynen research fellowship. N.K. would like to thank the Melbourne Materials Institute for support through an MMI/CSIRO scholarship. 4737
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dx.doi.org/10.1021/cm402645r | Chem. Mater. 2013, 25, 4731−4738
