Shell Quantum Dots - The

Article pubs.acs.org/JPCC

Lanthanide Modification of CdSe/ZnS Core/Shell Quantum Dots Johannes R. Dethlefsen,†,§ Alexander A. Mikhailovsky,‡ Peter T. Burks,‡ Anders Døssing,*,† and Peter C. Ford*,‡ †

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States

‡

S Supporting Information *

ABSTRACT: Lanthanide-modified CdSe quantum dots (CdSe(Ln) QDs) have been prepared by heating a solution of Cd(oleate)2, SeO2, and Ln(bipy)(S2CNEt2)3 (bipy = 2,2′-bipyridine) to 180−190 °C for 10−15 min. The elemental compositions of the resulting CdSe(Ln) cores and CdSe(Ln)/ZnS core/shell QDs show this route to be highly reproducible. The optical absorption spectra of these composite materials are similar to those of the unmodified nanocrystals, but the QD-centered band edge photoluminescence (PL) is partially quenched. The time-gated emission and excitation spectra of the CdSe(Ln) cores display sensitized lanthanide-centered PL upon higher energy excitation of the nanocrystal host but not upon excitation at the lowest energy QD absorption band. Growth of the ZnS shell led to the depletion of about 60% of the lanthanide ions present together with depletion of nearly all of the lanthanide-centered PL. On these bases, we conclude that the lanthanide-centered PL from the CdSe(Ln) cores originates with Ln3+-related trap states associated with the QD surface.

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INTRODUCTION The special position held by CdSe quantum dots (QDs) in the field of colloidal semiconductor nanocrystals is due to the ease of preparation of high-quality samples,1 the tunability of their luminescence over most of the visible spectrum, the high photoluminescence quantum yields that approach unity for crystals with well-passivated surfaces,2 and the well-understood electronic structure.3 The applications of CdSe QDs are limited by the inaccessibility of dots with near-infrared emission and the difficulty with isolating high-quality dots with blue emission. If they could be modified with lanthanide ions, the QDs could potentially inherit the attractive properties of the lanthanides, that is, sharp emission bands at characteristic wavelengths ranging from the ultraviolet to the near-infrared and long luminescence lifetimes (milliseconds for the visible-lightemitting lanthanides).4,5 Major obstacles to achieving efficient lanthanide emission are the low molar absorption coefficients of the Laporte-forbidden f−f transitions (ε < 13 M−1 cm−1)6 and the need to protect the Ln3+ ions from the high-energy oscillators O−H, N−H, and C−H that quench the luminescence through nonradiative energy transfer to the vibrational overtones.7 Both could potentially be overcome by incorporating lanthanide ions in CdSe nanocrystals. Because of the high absorption coefficients of these nanocrystals (ε > 105 M−1 cm−1),8 the QD could sensitize lanthanide emission through the antenna effect. In addition, lanthanide ions in a nanocrystal lattice would be wellprotected from high-energy oscillators. The incorporation of lanthanides in nanomaterials was reviewed in 2007.9 The most elaborate work has been done on doping of insulating nanocrystals used for upconversion of © 2012 American Chemical Society

near-infrared light, while modification of group II−VI semiconductor nanocrystals,10−13 and of CdSe in particular,14−16 is more scarce. Two early results are found in the 2002 paper by Strouse et al.,14 who showed that CdSe(Eu) nanocrystals can be prepared with a Cd:Eu ratio of at least 1:0.60 and that the europium ions have the +3 oxidation state. Thus, Eu3+ was apparently incorporated into the CdSe forming a solid solution, presumably because the ionic radii for Cd2+ and Eu3+ are the same (0.109 nm for coordination number six).17 Although the additional charge on Eu3+ might have been expected to be a problem, it is notable that unmodified CdSe nanocrystals rarely have a Cd:Se stoichiometry of 1:1. Instead, there is generally surplus Cd2+, the typical Cd:Se ratio being between 1:1 and 2:1,1 although a higher ratio was reported by Weiss et al.18 The additional positive charge would be balanced by negatively charged surface ligands like phosphonates and carboxylates.19 Described here are the synthesis and characterization of the photophysical properties of CdSe(Ln) core and CdSe(Ln)/ ZnS core/shell nanocrystals that were prepared with the goal of achieving efficient lanthanide-centered emission from Lnmodified QDs. We discuss the location of the lanthanide ions in these CdSe(Ln) cores, the effect of introducing a ZnS shell, and energy transfer from the QD host to the lanthanide ion.

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EXPERIMENTAL SECTION Caution. The cadmium and selenium compounds as well as carbon disulfide are toxic. Skin contact should be avoided, and Received: July 12, 2012 Revised: October 12, 2012 Published: October 29, 2012 23713

dx.doi.org/10.1021/jp306915p | J. Phys. Chem. C 2012, 116, 23713−23720

The Journal of Physical Chemistry C

Article

precipitated upon addition of 1 mL of 1-butanol, 2 mL of acetone, and 5 mL of methanol, isolated by centrifugation, washed with 5 mL of acetone, and redissolved in 4 mL of hexane for storage. ICP-AES Experiments. Elemental analysis experiments were carried out using a Thermo iCAP 6300 inductively coupled plasma atomic emission spectrophotometer (ICPAES). The QDs were digested by adding 0.5 mL of the nanocrystal stock solution to a mixture of 4 mL of 10% H2O2 and 1 mL of 1 M NaOH in a 20 mL open vial and stirring the solution at 80−85 °C overnight in a small water bath (i.e., a beaker containing 4−5 cm of water). The next day, all water had evaporated from the vial, and the residue was dissolved in 20 mL of 1 M HNO3 upon heating. Any undissolved organic residues were removed by centrifugation or filtration. The final solutions had Cd and Se concentrations of ∼0.6 mM and a Ln concentration of ∼0.06 mM. Optical Absorption and Steady-State Luminescence Spectroscopy. The optical absorption and steady-state emission and excitation spectra were recorded on standard research quality instruments. The energy of the first absorption band (Eg) and its half-width at half-maximum (ΔEhwhm) were determined by Gaussian fits to the low-energy side of the optical absorption spectra. The energy of the emission band (Elum) and its full width at half-maximum (ΔEfwhm) were determined by Gaussian fits to the steady-state emission spectra. The photoluminescence quantum yields were determined against Rhodamine 6G (95% in ethanol).25 Time-Gated Luminescence Spectroscopy. The timegated emission spectra were recorded on a Cary Eclipse fluorescence spectrophotometer (CEFS) with tdelay = 0.10 ms (the instrument’s lower time limit), tgate = 1.0 ms for the Eu samples, and tgate = 5.0 ms for the Tb samples. The total decay was 20 ms, the excitation and emission slit widths were set to 10 nm resolution, and the excitation wavelength was 300 nm. The excitation spectra were recorded with the same parameters; the emission wavelength was 617 nm for the Eu samples and 545 nm for the Tb samples. Time-Resolved Luminescence Spectroscopy. The lifetimes of the Ln-centered luminescence were measured on the CEFS with tdelay = 100 μs. For each decay, 1000 data points were recorded, and the time between two adjacent data points, tgate, was 4.0 μs for the Eu samples and 10.0 μs for the Tb samples. The excitation and emission slit widths were set to 20 nm resolution, and the excitation wavelength was 300 nm. To check the quality of the CEFS, the luminescence lifetimes of the Eu3+ and Tb3+ aqua ions as triflate salts in dilute, aqueous triflic acid were determined to be 114 and 434 μs, respectively, which is in accord with the literature values.6 The lifetimes of the QD-centered luminescence were measured at the UCSB Optical Characterization Facility using time-correlated single-photon counting (TCSPC) techniques. The repetition rate of the laser was 1 MHz. The excitation wavelength was 400 nm, and the instrument response function was ∼70 ps.26 For each decay, about 900 data points were recorded, the time between two adjacent data points, tgate, was 50 ps, and the total decay was 50 ns. The lifetimes were determined by fitting a sum of two (for the Ln-centered emission) or three (for the QD-centered emission) exponentials to the decay curve.

the syntheses should be carried out in a well-ventilated fume hood. Chemicals. Tri-n-octylphosphine (TOP, technical grade, 90%), oleylamine (OLA, technical grade, 70%), and 1octadecene (ODE, technical grade, 90%) were purchased from Sigma-Aldrich. The solvents were used as received, but TOP was stored under argon. Cadmium oleate, Cd(C17H33COO)2,1 and 1,2-octadecanediol20 were prepared by literature procedures. Selenium dioxide (99.8%) was purchased from ChemPur. Zinc diethyldithiocarbamate, Zn(S2CNEt2)2, was prepared by methanolic metathesis of zinc acetate and (NH2Et2)(S2CNEt2) and recrystallized from CH2Cl2. The lanthanide triflates, Ln(CF3SO3)3·9H2O, were prepared from lanthanide oxides purchased from Stanford Materials Corp. (≥99.9%).21 Synthesis of (NH2Et2)(S2CNEt2). The dithiocarbamate salt was prepared by a slightly modified literature procedure.22 A mixture of 25 mL (18 g, 0.24 mol) of diethylamine and 75 mL of acetone was cooled to 5 °C, and 8.0 mL (10 g, 0.13 mol) of carbon disulfide was slowly added. The temperature of the reaction mixture was kept below 10 °C. During the addition of CS2, the product slowly precipitated as a white, crystalline solid, which was subsequently isolated, recrystallized from hot acetone, and washed twice with diethyl ether. Synthesis of Ln(bipy)(S2CNEt2)3. The lanthanide precursor Ln(bipy)(S2CNEt2)3 (bipy = 2,2′-bipyridine) was prepared by a modified literature procedure.23 A solution of 0.94 g (6.0 mmol) of bipy and 4.00 g (18.0 mmol) of (NH2Et2)(S2CNEt2) in 100 mL of acetonitrile was mixed with a solution of 6.0 mmol of Ln(CF3SO3)3·9H2O in 75 mL of acetonitrile. Shortly after the mixing, the product precipitated and was isolated by filtration and washed twice with diethyl ether. Synthesis of CdSe(Ln). The 50 mL reaction flask was loaded with 168.8 mg (0.250 mmol) of Cd(C17H33COO)2, 27.7 mg (0.250 mmol) of SeO2, 71.6 mg (0.25 mmol) of 1,2octadecanediol, 0.01−0.08 mmol of crystalline Ln(bipy)(S2CNEt2)3, 0.1 mL (0.3 mmol) of OLA, and 15 mL of ODE. Within 15 min, the reaction mixture was heated to 180 °C, at which temperature the nucleation began, and kept at 180−190 °C for 10−15 min. The reaction was quenched by adding 25 mL of 1-butanol, and the QDs were precipitated by adding 25 mL of acetone and 25 mL of methanol. The QDs were isolated by centrifugation, washed with 15 mL of 1butanol, and redissolved in 4 mL of hexane. After removal of insoluble impurities by centrifugation, the nanocrystals were precipitated upon addition of 1 mL of 1-butanol, 2 mL of acetone, and 5 mL of methanol, isolated by centrifugation, washed with 5 mL of acetone, and redissolved in 4 mL of hexane for storage. Synthesis of CdSe(Ln)/ZnS. A 50 mL three-necked roundbottomed flask was loaded with a solution of the purified CdSe(Ln) cores in 10 mL of ODE, 3 mL of OLA, 3 mL of TOP, and the amount of crystalline Zn(S2CNEt2)2 needed24 for the growth of two monolayers of ZnS. Under nitrogen, the reaction mixture was slowly (within ∼75 min) heated to and maintained at 110−120 °C. After at least 2 h, the mixture was allowed to cool to room temperature, and the QDs were precipitated upon addition of 25 mL of 1-butanol, 20 mL of acetone, and 20 mL of methanol. The QDs were isolated by centrifugation, washed with 20 mL of 1-butanol and 20 mL of methanol, and redissolved in 4 mL of hexane. After removal of insoluble impurities by centrifugation, the nanocrystals were 23714



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RESULTS Synthetic Procedure. The synthesis of the CdSe(Ln) nanocrystals is based on the route to CdSe QDs published by Cao et al. in 2008,1 where equimolar amounts of cadmium oleate, selenium dioxide, and 1,2-hexadecanediol are mixed in the solvent 1-octadecene and heated to 220 °C. The modifications consist of the addition of 0.05−0.2 equiv of the crystalline, air-stable lanthanide precursor Ln(bipy)(S2CNEt2)3 and 1.2 equiv of oleylamine to the reaction mixture and the resultant lowering of the nucleation temperature to 180 °C. This new synthetic procedure is highly reproducible, but control of temperature (T) is crucial. If T is lower than 180 °C, nucleation is very slow and incomplete; if T is higher than 190 °C, the nanocrystals become larger but have relatively lower lanthanide contents. The reason for this counterintuitive observation (cf. Table S3 in Supporting Information) probably is that lanthanide modification is thermodynamically unfavorable, for which reason the higher temperature favors the kinetic equilibrium, where relatively fewer Ln3+ ions are incorporated in the lattice. The temperature range of 180−190 °C was therefore chosen as a compromise between the smallest possible QDs (with the highest possible band gap) and the quality of the isolated nanocrystals. The dithiocarbamate lanthanide precursors were chosen because they are air-stable, crystalline compounds that decompose upon heating. The complexes Ln(bipy)(S2CNEt2)3 and Ln(phen)(S2CNEt2)3 (phen = 1,10-phenanthroline) have been used to prepare lanthanide sulfides in general27 and EuS nanocrystals in particular.28 The crystalline compounds decompose in a single step at temperatures between 250 and 400 °C depending on the nature of the lanthanide ion.29 The Ln precursors do complicate the QD syntheses, since the composition of the Ln-modified nanocrystals is probably CdSeS(Ln) instead of CdSe(Ln). However, alloyed CdSeS QDs behave like CdSe QDs apart from having a higher band gap,30 a property that would be advantageous for the energy transfer to the lanthanide ions. The addition of the amine is a crucial modification of the procedure that gives much more reproducible lanthanide content and nucleation temperature. The role of the amine is probably threefold: First, it helps to dissolve the lanthanide complex, and having all of the Ln precursor in solution ensures a uniform decomposition process. Second, Lee et al.31 recently demonstrated that primary amines lower the decomposition temperature of another dithiocarbamate complex, Zn(S2CNEt2)2, and we suggest that a similar mechanism accounts for the beneficial role of the oleylamine. Third, in a parallel project we observed a reaction between oleylamine and SeO2 in the absence of any metal ions, and it seems likely that oleylamine facilitates the SeO2 reduction at a temperature 40 deg lower than that (220 °C) typically used. This view is supported by the observation that adding more amine can result in nucleation at 150 °C. Characterization. One challenge is reproducibility; thus, we decided to characterize these new Ln-modified nanocrystals by preparing four independent samples each of the CdSe(Eu) and CdSe(Tb) QDs by heating reaction mixtures with almost identical Cd:Ln ratios (∼1:0.11) to the same temperature for the same time interval (15 min). Samples of the eight CdSe(Ln) preparations were then coated with ZnS shells by heating with the amount of Zn(S2CNEt2)2 corresponding to 2 monolayers of ZnS to 110−120 °C for the same length of time

(2.5 h). The 16 samples of CdSe(Ln) core and CdSe(Ln)/ZnS core/shell QDs were then characterized by ICP-AES, optical absorption spectroscopy, and steady-state, time-gated, and time-resolved luminescence spectroscopy. In addition to these 16 samples, other CdSe(Ln) nanocrystals with both higher and lower lanthanide contents and other lanthanides were prepared and characterized by analogous techniques. Characterization with ICP-AES. The ICP results, normalized with respect to cadmium content, are compiled in Table 1. For the CdSe(Ln) cores, it is evident that there is a Table 1. Compilation of the Elemental Composition of the Eight CdSe(Ln) and Eight CdSe(Ln)/ZnS Nanocrystals Determined with ICP-AESa CdSe(Ln) Cd Zn Se Sb Ln (added) Ln (found) Nmlc

1.00 0.82 ± 0.02 0.15 ± 0.02 0.110 ± 0.004 0.100 ± 0.010

CdSe(Ln)/ZnS 1.00 2.13 ± 0.09 0.55 ± 0.03 2.76 ± 0.13 0.042 ± 0.008 1.69 ± 0.03

a

Each value is an average of measurements on up to eight independent samples. A comprehensive list of these data for the 16 individual samples is presented in the Supporting Information Table S1. All concentrations have been normalized with respect to the concentration of Cd2+ (printed in bold). bThe concentrations of sulfur (printed in italic) reflect the sulfur-containing ligands. cNml denotes the calculated number of ZnS monolayers with an estimated thickness of 0.31 nm.

one-to-one correspondence between the Cd:Ln ratio in the synthesis and that found, although this may be coincidental. The reproducibility of the synthesis is evidenced by the small standard deviations of all concentrations. The observed Cd:Se ratio of 1:0.82 was expected, since this is generally found. The sulfur concentrations were less reproducible but do not necessarily indicate the presence of sulfur in the nanocrystal, since decomposition of the lanthanide precursor might lead to the formation of thioureas or thiols, whose coordination to the QD surface would give artificially high sulfur concentrations. For the CdSe(Ln)/ZnS core/shell quantum dots, the growth of the ZnS shell was conducted as described previously.24 The amount of ZnS deposited can be described in terms of the estimated number of monolayers, Nml, with a thickness of 0.31 nm.32 The amount of ZnS deposited was highly reproducible as was the selenium depletion relative to cadmium. This confirms the reproducibility of the ZnS shell preparation as well as the suggestion that TOP removes selenium ions at the CdSe surface upon growth of the shell.24 That said, the most interesting observation is that roughly 60% of the lanthanide ions were lost during the shell growth. Before shell growth, it is possible for the lanthanide ions in the CdSe(Ln) cores to be located in two fundamentally different environments: incorporated inside the nanocrystal lattice or bound to surface sites. Exclusion of Ln dopants inside the nanocrystal, a process that Peng et al.33 named “lattice ejection”, seems unlikely due to the low T of the shell growth (110−120 °C) compared to that of core preparation (180−190 °C). It seems much more likely that depletion occurs during the shell growth. This is analogous to the process discussed by 23715



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Erwin et al.34 where Mn2+ ions are removed from nanocrystal surfaces by surfactants. Since the hard Ln3+ ions are in a soft coordination environment at the CdSe surface, these may be labilized by coordination to the excess oleylamine with its relatively hard nitrogen donor. Cation exchange in lanthanide fluoride nanoparticles has been observed to take place.35 However, in the present study significant cation exchange of core-localized Ln3+ by Zn2+ is unlikely since the formation of alloyed CdZnSe QDs would lead to a larger band gap and a blue shift of the absorption and emission band. The opposite was observed upon shell growth. Furthermore, as discussed below, elemental analysis demonstrated that a substantial fraction of the Ln3+ ions initially present in the cores remain in the core/shell QDs. The observation that ∼60% of Ln3+ ions present are lost upon preparing the ZnS shell suggests that these are located at the surface of the CdSe core while the rest of the lanthanide ions are incorporated in the CdSe lattice. One might expect that the large Ln3+ ions with an odd charge would localize on the QD surfaces: However, Strouse et al.14 prepared CdSe(Eu) QDs with a Cd:Eu ratio as high as 1:0.60 and concluded that the Eu3+ ions were incorporated into the CdSe nanocrystal with random displacement of Cd2+ ions. In contrast, Bol et al. in 200236 prepared lanthanide-doped ZnS and CdS QDs by precipitation and microemulsion techniques and concluded from the spectroscopic properties that the Ln3+ ions were entirely at the surface for nanomaterials prepared in this manner. Thus, it appears that the location and distribution of lanthanide ions in such semiconductor QDs is very much a function of the synthetic methods used. Characterization with Optical Absorption and SteadyState Emission Spectroscopy. The absorption and steadystate emission spectra were recorded for each of the 16 QD samples dissolved in hexane. The spectra for a CdSe(Tb) sample and the corresponding CdSe(Tb)/ZnS sample are shown in Figure 1, and the data obtained from Gaussian fits to the spectra are compiled in Table 2. The very small standard deviations are further evidence of the reproducibility of the syntheses.

spectra did not display bands characteristic of Eu3+ and Tb3+, but this was expected given the forbidden nature of the f−f transitions. Neither did the steady-state emission spectra display Ln-centered emission; this is evidently much weaker than the QD-centered emission and must be detected with time-gated spectroscopy. The approximate QD radii, molar absorption coefficients, and concentrations were calculated from the absorption spectra, according to Mulvaney et al.8 In this manner, the core radii were estimated to be 1.4−1.5 nm. Such QDs would contain approximately 250 cadmium and 25 lanthanide atoms. The calculated core radii were used for calculating the amount of Zn(S2CNEt2)2 needed for the growth of a ZnS shell with a thickness of 2 monolayers. Since the actual thickness of the shells was calculated to be only 1.7 monolayers (based on the ICP results), the resulting core/shell quantum dots were estimated to have radii of 1.9−2.0 nm. The steady-state luminescence spectra of the CdSe(Ln) QDs appear to be nearly the same as those of comparable undoped QDs: A single Gaussian-shaped emission band is observed with a λmax of 563 nm and a ΔEfwhm value of 0.167 eV (Figure 1), which are the expected values. However, the overall photoluminescence quantum yields (PL QYs) of the modified QDs (0.64% for CdSe(Eu) and 1.5% for CdSe(Tb)) are much lower than those of undoped dots (typically 10−40%).1,37 Furthermore, the degree of emission quenching is qualitatively judged to increase with higher lanthanide levels (Table S2). More surprisingly, the quenching appears to be independent of the nature of the lanthanides: Similar levels of Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Tm3+ resulted in similar low luminescence intensities (Table S3). Thus, the quenching must be attributed to a mechanism that is independent of the 4f energy levels of the lanthanide ions. The growth of the ZnS shell results in a red shift of the absorption and luminescence bands accompanied by marked decreases in PL QYs, for example, from 0.64 to 0.04% for CdSe(Eu)/ZnS and from 1.5 to 0.11% for CdSe(Tb)/ZnS. This observation will be discussed in connection with the lifetimes of the QD-centered emission. The fact that the modified QDs apparently do not display obvious steady-state lanthanide-centered emission is noteworthy. Petoud et al.13,15 observed steady-state emission from terbium in ZnS(Tb) and, to a much smaller extent, in CdS(Tb) but reported no steady-state lanthanide emission from ZnSe(Tb), CdSe(Tb), or ZnS(Eu). On the other hand, Planelles-Aragó et al.11,12 observed steady-state Eu emission from both ZnS(Eu) and CdS(Eu) together with a large, broad band assigned to emission from trap states. In addition, Bol et al.36 observed steady-state lanthanide-centered emission from ZnS(Eu), ZnS(Tb), and CdS(Eu) but concluded that the lanthanide-centered emissions were most efficient when the lanthanide ion was excited directly. The difference is puzzling but can probably be attributed to differences in the preparation methods, dispersion media, and composition. That said, neither Petoud et al. nor Bol et al. reported any lanthanide levels, while Planelles-Aragó et al. determined the Eu content with less certain techniques; thus, it is difficult to compare the various systems. Characterization with Time-Gated Luminescence Spectroscopy. Time-gated emission and excitation spectra were recorded for the 16 QD samples dissolved in hexane. The time-gated emission spectra of CdSe(Eu) and CdSe(Tb) recorded for the excitation wavelength 300 nm and a 0.1 ms

Figure 1. Optical absorption and normalized steady-state emission spectra of CdSe(Tb) and CdSe(Tb)/ZnS QDs in hexane recorded with an excitation wavelength of 400 nm.

From Figure 1, it is evident that the optical absorption spectra of the CdSe(Tb) QDs do not display the well-resolved bands seen in the spectra of analogous undoped QDs (Figure S1 in Supporting Information). The same is true for the CdSe(Eu) QDs. This can be attributed to the lower quality of the modified dots (evidenced by the relatively large ΔEhwhm values that indicate a larger size distribution) and/or alteration of the electronic structure by the dopants. The absorption 23716



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Table 2. Compilation of the Results Obtained from the Gaussian Fits to the Optical Absorption and Steady-State Luminescence Spectra of the Eight CdSe(Ln) and Eight CdSe(Ln)/ZnS Nanocrystals Dissolved in Hexane (Ln = Eu, Tb)a CdSe(Eu) Eg/eVb (λmax/nm) ΔEhwhm/eVb Elum/eVc (λmax/nm) ΔEfwhm/eVc

2.317 0.089 2.197 0.158

± ± ± ±

0.004 (535) 0.002 0.005 (564) 0.002

CdSe(Tb) 2.312 0.087 2.207 0.176

± ± ± ±

0.013 (536) 0.002 0.008 (562) 0.007

CdSe(Eu)/ZnS 2.228 0.083 2.153 0.174

± ± ± ±

0.010 (556) 0.002 0.015 (576) 0.004

CdSe(Tb)/ZnS 2.236 0.085 2.152 0.200

± ± ± ±

0.013 (555) 0.003 0.009 (576) 0.007

a

Every reported value is the average of measurements on eight independent samples together with the standard deviation. bEg is the energy of the maximum, and ΔEhwhm is the half-width at half-maximum of the first absorption band. cElum is the energy of the maximum, and ΔEfwhm is the full width at half-maximum of the emission band.

to energies that are sufficiently high to excite the Ln cations into the lowest luminescent levels ∼2.54 eV for Tb3+ and ∼2.14 eV for Eu3+.38,39 For CdSe(Ln) QDs with Ln = Sm, Gd, Dy, and Tm (added Cd:Ln ratio of 1:0.10), only CdSe(Dy) displayed very weak, Ln-centered time-gated emission when excited at 300 nm. The absence of the emission for these examples may be attributed to inefficient energy transfer to the very high energy levels of Gd3+ and the faster decay of emission from Sm3+, Dy3+, and Tm3+.6 The time-gated emission spectra of the modified core/shell QDs CdSe(Eu)/ZnS and CdSe(Tb)/ZnS in Figure 4 were

delay (Figure 2) display sharp Ln-centered emission bands at the expected wavelengths and with the expected relative

Figure 2. Time-gated emission spectra of CdSe(Eu) and CdSe(Tb) recorded with an excitation wavelength of 300 nm and a delay of 0.1 ms.

intensities. A similar observation was made by Petoud and coworkers15 with regard to Tb3+-modified CdSe QDs. The excitation spectra (Figure 3) for these time-gated emissions from CdSe(Eu) and CdSe(Tb)recorded for Figure 4. Time-gated emission spectra of CdSe(Eu)/ZnS and CdSe(Tb)/ZnS recorded with an excitation wavelength of 300 nm and a delay of 0.1 ms. The QD-centered emission is not real but the result of detector ringing.

featureless. In order to record spectra with a decent signal-tonoise ratio, the slits were wide open (20 nm resolution), the PMT voltage was set to the maximum value (1000 V), and 100 scans were averaged; hence, the quantum yields for the Lncentered emission in the CdSe(Ln)/ZnS QDs must be significantly lower than in the CdSe(Ln) cores. Although both samples seemed to display a long-lived QD-centered emission band, this may be an artifact resulting from detector ringing,40 that is, saturation of the detector by the prompt QD PL. This ringing can create dead times of more than 100 μs, where the instrument displays QD PL, even though it has already decayed. This is in particular a problem, when weak long-lived lanthanide emission needs to be discriminated against strong background luminescence.40 The only real bands observable are the weak Tb-centered emission from CdSe(Tb)/ZnS (weak europium bands would be obscured by the ringing artifact). Thus, while more efficient Ln emission was anticipated from passivating the surface of the CdSe(Ln) cores with the ZnS shell, the opposite was observed. Although 60% of the lanthanide ions were lost during the shell growth, the lanthanide content still remained relatively high: Since we have observed Eu-centered luminescence from CdSe(Eu) core QDs with a Cd:Eu ratio as low as 1:0.02 (Figure S2), the failure

Figure 3. Time-gated and steady-state excitation spectra of CdSe(Eu) and CdSe(Tb). The time-gated spectra were recorded with an emission wavelength of 617 nm for Eu and 545 nm for Tb and a delay of 0.1 ms. The steady-state spectra were recorded with an emission wavelength of 565 nm.

emission at 617 nm for the former and at 545 nm for the latterdisplay broad, featureless bands in the UV that are very different from the absorption spectra (Figure 1) and from the excitation spectra for the QD band gap emission (Figure 3). The latter roughly parallel the absorption spectra. For CdSe(Tb), the excitation band has practically no fine structure and reaches a maximum intensity close to 300 nm, while almost no terbium emission is observed upon excitation at 400 nm (3.10 eV). For CdSe(Eu), the band is located at a slightly lower energy, but almost no europium emission is observed upon excitation at 450 nm (2.76 eV). Notably, the QDs have high absorption coefficients at these wavelengths, which correspond 23717



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Table 4. Lifetimes (τ3, τ4, and τ5) and Normalized Amplitudes (A3, A4, and A5) Derived by Analysis of the Time-Dependent Decay of the QD-Centered Luminescence from CdSe(Ln) and CdSe(Ln)/ZnS QDs (Ln = Eu, Tb) by Fitting to a Sum of Three Exponentials (Excitation Wavelength = 400 nm)a

to observe Eu-centered emission in the CdSe(Eu)/ZnS QDs with a Cd:Eu ratio of 1:0.04 cannot be attributed simply to a depleted number of lanthanide chromophores. One explanation would be that sensitization of Ln emission by the QD host is limited to Ln3+ ions at surface sites and that this does not occur for dopants within the nanocrystal host. Thus, if formation of the ZnS shell leads to stripping surfacebound lanthanide ions, this pathway for energy transfer to the Ln centers is no longer available. Characterization with Time-Resolved Emission Spectroscopy. The time-gated transient decays of the Eu- and Tbcentered luminescence from the 8 CdSe(Ln) QD core samples were analyzed by fitting a sum of two exponentials. The lifetime (τ1 and τ2) and amplitude (A1 and A2) values are compiled in Table 3 (Table S4 for all results). The observed double-

CdSe(Eu)b τ3/ ns τ4/ ns τ5/ ns A3 A4 A5

Table 3. Lifetimes (τ1 and τ2) and Normalized Amplitudes (A1 and A2) Derived by Analysis of the Time-Dependent Decay of the Ln-Centered Luminescence from CdSe(Eu) and CdSe(Tb) by Fitting to a Sum of Two Exponentials (Excitation Wavelength = 300 nm)a CdSe(Eu)b τ1/ms τ2/ms A1 A2

0.75 0.18 0.59 0.41

± ± ± ±

0.06 0.05 0.08 0.08

± ± ± ±

CdSe(Eu)/ ZnSc

CdSe(Tb)/ ZnSc

0.19 ± 0.01

0.21 ± 0.02

0.11 ± 0.02

0.14 ± 0.02

1.55 ± 0.08

1.74 ± 0.07

0.68 ± 0.07

0.83 ± 0.13

11.3 ± 0.5

13.1 ± 0.2

5.3 ± 0.3

6.0 ± 0.7

0.63 ± 0.01 0.28 ± 0.01 0.092 ± 0.003

0.59 ± 0.01 0.30 ± 0.01 0.11 ± 0.01

0.73 ± 0.02 0.22 ± 0.02 0.045 ± 0.004

0.74 ± 0.03 0.22 ± 0.03 0.044 ± 0.006

a

Each value is an average of measurements at one wavelength for four samples. bEmission wavelength 565 nm. cEmission wavelength 580 nm.

do not change much because core dimension and composition do not change.43 That said, a shortening of the lifetimes upon shell growth is precedented and has been reported in cases where the charge recombination process is complicated by the presence of trap states37 that in our case would be the lanthanide ions. Petoud et al.15 found that the three lifetimes of the QDcentered emission in ZnS(Eu) and ZnS(Tb) were independent of the lanthanide ion, whereas the three normalized amplitudes were different. This observation is in agreement with their conclusion that the sensitization of Eu3+ and Tb3+ emission follows different mechanisms. We also find that the lifetimes of the QD-centered emission in CdSe(Eu) and CdSe(Tb) are independent of the lanthanide ion, but given the small differences in the normalized amplitudes, we have no reason to interpret this observation in terms of different types of energy transfer mechanisms. Finally, it should be noted that the time-gated emission spectra and the lifetimes of the lanthanide-centered emission were recorded using 300 nm excitation, while the lifetimes of the QD-centered emission were recorded using 400 nm excitation. The lower excitation energy was insufficient to simultaneously sensitize lanthanide luminescence, but 300 nm was not accessible on the TCSPC instrumentation.

CdSe(Tb)c 1.33 0.24 0.43 0.57

CdSe(Tb)b

0.09 0.03 0.04 0.04

a

Each value is an average of measurements at two wavelengths for four samples. bEmission wavelengths 617 and 699 nm. cEmission wavelengths 490 and 545 nm.

exponential decay would be consistent with the Ln cations being present in (at least) two distinct sites in the modified nanocrystals. Double-exponential decay has been seen for other Ln-modified semiconductors: Petoud et al.13,15 found τ1 and τ2 of 2.02 and 4.7 ms for CdSe(Tb), 0.92 and 2.50 ms for ZnS(Tb), and 2.0 and 3.6 ms for ZnS(Eu); Planelles-Aragó et al.12 found 0.33 and 0.63 ms for CdS(Eu), and Chowdhury and Patra10 found 0.042 and 0.28 ms or 0.057 and 0.38 ms (depending on the presumed lanthanide content) for an analogous system. However, it is difficult to compare these systems since the amounts and locations of the lanthanide ions are unknown. In the present case, the changes in the photophysical behavior upon growth of the ZnS shell leads us to conclude that Ln-centered emission occurs from Ln3+ ions at the surface of the CdSe core. Since these ions are more sensitive to the nature of the surfactants and the solvent than Ln3+ ions within the nanocrystal lattice, one might expect different lifetimes. The time-resolved behavior of the QD band edge PL was probed by TCSPC for all 16 CdSe(Ln) and CdSe(Ln)/ZnS samples. These data were analyzed by fitting a sum of three exponentials to the decay curves (three lifetimes on the 70 ps to 50 ns time scale is a reasonable number of variables for QDcentered emission). The values are compiled in Table 4 (Table S5 for individual samples), and it is seen that the lifetimes for the QD-centered emission from CdSe(Ln) are significantly shorter than for undoped systems.41,42 Upon growth of the ZnS shell, the surface passivation and the removal of at least one-half of the lanthanide ions might be expected to result in higher PL QYs and longer emission lifetimes, but the opposite was observed. Usually, one observes a change of amplitudes of PL decay components upon shell growth, but the time constants

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DISCUSSION In CdSe(Eu) and CdSe(Tb), excitation of the CdSe QD host to its first excited state 1S3/21Se results in band edge emission that is substantially quenched by the presence of the lanthanide ions. However, there appears to be little sensitization of PL from the Ln centers unless the excitation energy is considerably higher, that is, above at least 3 eV. A possible explanation for this observation could be that energy transfer involves 4f−5d excitations. Dorenbos et al.44 have, however, shown that the energies of the first 4f−5d transitions of terbium and europium dopants in LiYP4O12 are 4.86 and 8.50 eV, respectively. Although the 5d orbitals are more sensitive to the chemical environment than are the 4f orbitals, it is unlikely that the 5d orbitals are involved in the energy transfer mechanism in a CdSe host. Alternatively, the Ln3+ excitation might involve ligand-to-metal charge transfer (LMCT) states. An argument against this, however, is that the difference in excitation 23718



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energies for the sensitized europium and terbium luminescence is much smaller than the difference of >3 eV expected for the LMCT states given that Eu3+ is the much more readily reduced.44 A combination of 4f−5d transitions and charge transfer is unlikely since the amplitudes of the lifetimes of the QD-centered emission in CdSe(Eu) and CdSe(Tb) are almost identicaldifferent mechanisms should result in different amplitudes. Instead, it is plausible that the europium and terbium emissions are sensitized by a mechanism similar to that proposed by Klik et al.45 This involves localization of the photogenerated electron−hole pair at an Ln3+-related trap followed by nonradiative recombination. If the energy is high enough, the result might lead to excitation of the lanthanide ion. We suggest that these Ln3+-related traps are independent of the nature of the lanthanide ion. This would explain the similar lifetimes and amplitudes of the QD-centered emission in CdSe(Eu) and CdSe(Tb) as well as the observed quenching of the QD-centered emission in CdSe(Gd), despite the inaccessibility of 4f energy levels in Gd3+. We also propose that the Ln3+-related traps are associated with the surface of the CdSe(Ln) core, which explains the strongly diminished Ln3+centered emission in the core/shell analogues. While typical surface trap states are at energies lower than the QD band edge, this cannot be the case for trap states leading to Ln3+ PL, since this required much shorter excitation wavelengths. For energy transfer from the QD host to the surface Ln3+ to occur from higher energy excitation requires the sensitization to be fast enough to compete with the relaxation of the exciton to the band edge excited state. In a recent review, Kambhampati46 discussed the relaxation dynamics of hot excitons and concluded that hot trapping, that is, direct transfer of a hot exciton to a surface trap, can be competitive with relaxation to the first excited state if the surface is poorly passivated. One result of such hot-carrier dynamics is that quantum yields of QD PL often decrease at shorter wavelength excitation. The relative efficiency of exciton relaxation to the QD band edge state vs trapping at a surface state is dependent on the number and nature of such trap states, whose exact nature and energy levels are typically unknown. It is our hypothesis that the presence of numerous Ln3+ ions at the surface will quench the QD PL (hence the much lower emission quantum yields from the modified nanoparticles) but that arrival of a hot exciton at such a site may lead to sensitization of and resulting emission from a Ln3+ ion. Such an explanation can account for the sometimes very different observations in other papers on Lnmodified QDs and the decreased lanthanide-centered emission upon shell growth.

Article

ASSOCIATED CONTENT

S Supporting Information *

Additional data on elemental composition, luminescence intensities, and lifetimes for the investigated quantum dots. 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] (A.D.), [email protected] (P.C.F.); Ph +45 353 201 14; Fax +45 353 202 12 (A.D.). Present Address §

Department of Chemistry, Technical University of Denmark, Building 201, DK-2800 Kgs. Lyngby, Denmark. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Danish Natural Science Research Council grant to A.D. (272-08-0491) and by the US National Science Foundation (CHE-0749524).

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REFERENCES

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CONCLUSIONS Lanthanide-modified CdSe and CdSe/ZnS quantum dots, CdSe(Ln) and CdSe(Ln)/ZnS QDs, have been prepared by a new, reproducible route and characterized with ICP-AES, optical absorption spectroscopy, and steady-state, time-gated, and time-resolved luminescence spectroscopy. The results show that sensitization of Ln3+ PL in CdSe(Eu) and CdSe(Tb) can be achieved through high-energy excitation of the CdSe host. The sensitization is concluded to occur through hot exciton excitation of Ln3+-associated trap states at the surface of the CdSe core. The conclusion is supported by the observation that growth of a ZnS shell on the Ln-modified CdSe cores leads to fractional depletion of the Ln3+ content but to nearly complete loss of sensitized emission from the Ln sites. 23719



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