A Selective Cation Exchange Strategy for the Synthesis of Colloidal

Jul 27, 2017 - Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f–4...
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A Selective Cation Exchange Strategy for the Synthesis of Colloidal Yb3+-Doped Chalcogenide Nanocrystals with Strong Broadband Visible Absorption and Long-Lived Near-Infrared Emission Sidney E. Creutz, Rachel Fainblat, Younghwan Kim, Michael C. De Siena, and Daniel R. Gamelin* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States S Supporting Information *

ABSTRACT: Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f−4f emission with the strong broadband absorption and low-phonon-energy crystalline environment of semiconductors to make new solutionprocessable spectral-conversion nanophosphors, but synthesis of this class of materials has proven extraordinarily challenging because of fundamental chemical incompatibilities between lanthanides and most intermediate-gap semiconductors. Here, we present a new strategy for accessing lanthanide-doped visible-light-absorbing semiconductor nanocrystals by demonstrating selective cation exchange to convert precursor Yb3+doped NaInS2 nanocrystals into Yb3+-doped PbIn2S4 nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb3+ is both incorporated within the PbIn2S4 nanocrystals and sensitized by visible-light photoexcitation of these nanocrystals. This combination of strong broadband visible absorption, sharp near-infrared emission, and long (>400 μs) emission lifetimes in a colloidal nanocrystal system opens promising new opportunities for both fundamental-science and next-generation spectral-conversion applications such as luminescent solar concentrators.

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INTRODUCTION The characteristic optical properties of the trivalent lanthanide ionsespecially their efficient and sharp “atomic-like” emission from internal f-f transitionshave made them the longstanding targets of extensive research and development for a multitude of photonics applications ranging from phosphors, lasers, and fiber amplifiers to upconverters and bioimaging probes.1−9 Ytterbium(III), in particular, is attractive for its potential as a luminophore in luminescent solar concentrators (LSCs), because the ∼1000 nm emission from its 2F5/2 → 2F7/2 transition is well-matched to the band gap of silicon.10−14 The narrow absorption features and very low extinction coefficients of these formally parity-forbidden f-f transitions present a challenge for this and other new phosphor applications, however. Whereas in traditional applications such as color-conversion phosphors for fluorescent lighting, high-energy (e.g., UV) photons can generate f-f emission by exciting strongly allowed lanthanide f-d or charge-transfer (CT) transitions, new applications such as LSCs will require strong visible-light absorption to capture sufficient solar radiation. In principle, this limitation can be overcome by using a ligand “antenna” to sensitize the lanthanide emission. Various strongly absorbing aromatic ligand chromophores have been successfully demonstrated in this capacity.15,16 Molecular Yb3+ complexes frequently display short emission lifetimes and poor emission quantum yields due to nonradiative quenching of the emissive © 2017 American Chemical Society

F5/2 excited state, however, resulting primarily from coupling of this state to high-energy C−H, N−H, and O−H vibrations of the nearby ligands and solvent molecules.17,18 This quenching can in part be mitigated through the use of bulky, encapsulating, perdeuterated or perfluorinated ligands and solvents, allowing for the demonstration of lifetimes up to 712 μs and a quantum yield of 63% in one Yb3+ complex in deuterated dichloromethane,19 the current record among molecular Yb3+ species in solution at room temperature. This proof-of-concept result demonstrates a high ceiling for this class of luminophores and motivates further research and development in this direction. In contrast with molecules, Yb3+ and other trivalent lanthanide (Ln3+) ions incorporated as dopants into nanocrystalline materials can easily exhibit luminescence lifetimes near their radiative limit (e.g., ∼1−2 ms for Yb3+),20,21 and correspondingly high luminescence quantum yields, if the host crystal lattice is effective at protecting the Ln3+ dopant from high-energy vibrations in solution and at the nanocrystal surfaces.19 Moreover, at sufficiently small dimensions, such nanocrystals retain the important advantages of solution processability and negligible light scattering that make Received: May 12, 2017 Published: July 27, 2017 11814

DOI: 10.1021/jacs.7b04938 J. Am. Chem. Soc. 2017, 139, 11814−11824

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Limited literature precedent exists for the synthesis of welldefined colloidal NaInS2 nanocrystals. Relatively large (100− 600 nm diameter) hexagonal nanoplatelets have recently been synthesized through an ultrasonic spray pyrolysis method and via a solvothermal reaction;46,47 other examples of nanocrystalline NaInS2 have largely been limited to poorly defined nanostructures or large (∼1 μm) platelets.45,48,49 Here we demonstrate the successful synthesis of small colloidal Yb3+:NaInS2 nanocrystals. We then exploit selective cation exchange within these doped nanocrystals to form Yb3+:PbIn2S4 nanocrystals that strongly absorb visible light while retaining their Yb3+ dopants. These nanocrystals show the target property of nanocrystal-sensitized near-IR luminescence with long decay times at room temperature. The new strategy demonstrated here of using selective cation exchange to tune visible light absorption in pre-formed, pre-doped nanocrystals without loss of dopants introduces new opportunities for the synthesis and spectroscopic investigation of a portfolio of lanthanide-doped colloidal nanomaterials that simultaneously display broad and intense visible light absorption as well as narrow sensitized near-IR luminescence, with promising ramifications for the development of this class of materials as solution-processable spectral-conversion nanophosphors.

molecular luminophores attractive. Most commonly, lanthanides are incorporated into wide-band-gap fluoride, oxide, or oxyanion insulator lattices (e.g., NaYF4)5,22−29 that do not sensitize the lanthanide to visible excitation.5,22−31 An ideal combination of these two approachesencapsulation in a nanocrystalline host and sensitization by a strongly absorbing speciescould be achieved if it were possible to incorporate lanthanides into semiconductor nanocrystals with band gaps in the visible or near-IR region. The most prominent candidate hosts are III−V materials such as InP32 and II−VI or IV−VI chalcogenide materials such as CdS, CdSe, ZnSe, and PbS. Efforts to incorporate Ln3+ ions, especially the later lanthanides such as Yb3+, into such nanocrystals have largely served to highlight the synthetic hurdles involved, however, and have met with limited success.33−40 In most cases, Ln3+ ions have been segregated at the nanocrystal surfaces, and/or sensitized lanthanide luminescence was not clearly demonstrated. In a notable advance, Yb3+ was recently incorporated into the surface layers of CdSe nanocrystals, and the CdSe excitonic features were demonstrated in the Yb3+ photoluminescence excitation spectrum, providing clear evidence for the first time of Yb3+ dopant sensitization by a visible-bandgap semiconductor nanocrystal.41 Even in this case, however, Yb3+ was still only incorporated within a surface selenide layer, as reflected by the retention of primarily CdSe excitonic luminescence and by the relatively short Yb3+ luminescence decay time (∼160 μs). Photoluminescence quantum yields were not reported. There are several fundamental reasons behind the general difficulty of incorporating Ln3+ ions in intermediate-gap chalcogenide and pnictide semiconductor lattices: first, the Ln3+ ions are hard, oxophilic Lewis acids with little affinity for sulfides, selenides, phosphides, or other soft Lewis bases; second, Ln3+ ions exhibit a strong preference for coordination numbers of six or greater, making incorporation at tetrahedral lattice sites such as those occupied by Cd2+, Zn2+, or In3+ in their common binary chalcogenide/pnictide semiconductors thermodynamically unfavorable; third, substitution of Ln3+ for Cd2+, Zn2+, or Pb2+ necessitates some form of charge compensation (although this is likely the least challenging of these issues to overcome42). The generally poor affinity of Ln3+ ions for chalcogenide lattices is also reflected in the dearth of reported binary or ternary lanthanide chalcogenide nanocrystals, especially compared to the considerable body of work on rare-earth oxides and fluorides. One notable exception is the recently reported synthesis of a series of Na(RE)S2 (RE = Y3+, La3+ to Nd3+, Sm3+ to Lu3+) nanocrystals with edge-lengths of ∼12−200 nm, where the addition of sodium is proposed to facilitate the incorporation of sulfide rather than oxide during nanocrystal growth.43,44 Although most of these materials are wide-band-gap insulators with no visible absorbance, and although mixed compositions and their luminescent properties have not yet been examined, we were inspired by these results to explore the synthesis and Yb 3+ doping of NaInS 2 nanocrystals by a similar route. NaInS2 is reported to be a semiconductor, albeit with a relatively large band gap and limited absorbance at wavelengths longer than 400 nm.45 Importantly, the In3+ ions in this lattice occupy octahedral sites, which should be far more favorable for Yb3+ doping than the tetrahedral cation sites of II−VI and III−V nanocrystals. Indeed, NaInS2 is isostructural to NaYbS2, which bodes well for formation of alloyed compositions.

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METHODS

General. Pb(oleate)2 was prepared as previously described and stored in a desiccator over anhydrous calcium sulfate.50 In(acac)3 and Na(oleate) were purchased from TCI, and octadecene (90%), hexadecylamine (90%), oleic acid (90%), and oleylamine (70%) were purchased from Sigma Aldrich and used without further purification. Unless stated otherwise below, all other reagents and solvents were purchased from commercial sources and used without further purification. Yb(acac)3·xH2O. The synthesis of ytterbium(III) 2,4-pentanedionate hydrate (Yb(acac)3·xH2O) was adapted from a previously reported synthesis of rare earth acetylacetonates.51 YbCl3·6H2O (4.13 g) was dissolved in water (100 mL) and vigorously stirred while a solution of acetylacetone (4.8 g) and ammonium hydroxide (3.3 mL) in 50 mL of water was added slowly. A copious white microcrystalline product formed, and the reaction flask was sealed and stirred for 1 h. The white precipitate was then isolated atop a sintered glass frit, washed with water, and then dried in air. The isolated material was further dried in a desiccator over anhydrous calcium sulfate in the presence of an open vial of acetylacetone.52 Synthesis of Yb3+-Doped NaInS2 Nanocrystals. The synthesis of Yb3+:NaInS2 nanocrystals was performed using reaction conditions adapted from the syntheses of Na(RE)S2 nanocrystals,43,44 with modifications to reflect the different reactivity of In3+ than of Ln3+ ions, to reduce the nanocrystal dimensions, and to narrow the nanocrystal size distributions. Sodium oleate (600 mg), In(acac)3 (100 mg), Yb(acac)3·xH2O (30 mg), hexadecylamine (4.8 g), and octadecene (10 g) were combined under N2 in a dried 100 mL three-neck flask equipped with a reflux condenser and temperature probe. These reagents were dried at 120 °C under vacuum for at least 90 min until the solution became clear and highly viscous. The flask was then backfilled with N2 and allowed to cool to 80 °C before sulfur powder (160 mg) was added as a solid. The flask was then heated quickly (20 °C/min) to 315 °C. Once the reaction mixture reached 315 °C, heating was maintained for 2 min, and then the heating mantle was removed and the flask was allowed to cool toward room temperature. The reaction mixture was orange and transparent or slightly cloudy. After the temperature fell below 100 °C, ethanol (50 mL) was added, and the mixture was gently refluxed until residual sodium oleate was fully dissolved, giving a yellow-orange solution with a suspended pale yellow precipitate. The crude product solution was centrifuged (10 min, 6500 rpm) and the supernatant discarded. The precipitate was resuspended in 10 mL of hexane, and then ethanol (40 11815

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refinement against the model using Jade 9 (see SI for further information). Quantum-Yield Determination. Quantum yield measurements were performed on sample j by comparison to a sample with a known quantum yield. A sample of Ag+-doped CdSe nanocrystals53 was used as a standard for the quantum yield measurements due to its absorbance in a similar region to j and red-to-near-IR emission, solubility in similar solvents (toluene), and anticipated similar quantum yield. Dilute samples of j and Ag+:CdSe nanocrystals were prepared in toluene (O.D. < 0.15 at 405 nm). The absolute quantum yield of the Ag+:CdSe nanocrystal standard was measured using an integrating sphere (Hamamatsu C9920-12) to be 16% (405 nm excitation). The total emission intensity values of the sample and standard when excited at 405 nm (CW, 20 μW total power) were then measured on a liquid-nitrogen-cooled silicon CCD. The spectra were corrected for the instrument setup and detector response using an OceanOptics calibrated light source, and the quantum yield (Φ) of j was calculated from the relative integrated emission intensities (I) and absorbance (A) at 405 nm: Φj = (Aref/Aj) × (Ij/Iref) × Φref.

mL) was added to precipitate the nanocrystals, the mixture centrifuged (10 min, 6500 rpm), and the supernatant again discarded. Next, 100 μL each of oleic acid and oleylamine were added to the solution, and the nanocrystals were similarly precipitated and washed with hexane/ ethanol twice more. Finally, the nanocrystals were suspended in 10 mL of hexane, and the suspension was centrifuged to remove insoluble material (20 min, 6500 rpm). The resulting clear yellow nanocrystal solution can be stored under ambient conditions at room temperature. Further discussion of the synthetic parameters and the effects of time, temperature, and concentration is provided in the SI. Size-Selective Precipitation. Ethanol was added to the nanocrystals in 10 mL of hexane until the solution became slightly cloudy (∼3 mL). The suspension was centrifuged at 6500 rpm for 2 h, the precipitate was discarded, and ethanol was added to the supernatant to precipitate the purified nanocrystals. The sample was centrifuged again (10 min, 6500 rpm), and the supernatant was discarded. The precipitated nanocrystals were redissolved in hexane (5 mL) for storage. See the Supporting Information (SI) for further discussion. Cation Exchange with Pb2+. A 0.05 M Pb(oleate)2 stock solution was prepared by combining 192 mg of Pb(oleate)2 with 82 μL (2 equiv) of octylamine in 5 mL of octadecene and stirring under vacuum at room temperature for 1 h, and then backfilling with nitrogen. Separately, in a 50 mL three-neck flask, 7 mL of octadecene and 3 mL of oleylamine were combined and degassed by heating to 110 °C under vacuum for 1 h. After cooling to room temperature and backfilling with N2, an amount of 2% Yb3+:NaInS2 nanocrystals corresponding to 0.09 mmol of In3+, or approximately 0.0125 μmol of nanocrystals with an average diameter of 11 nm, was added as a hexane solution, and the volatile solvent was removed under vacuum. The reaction solution was further dried under vacuum at 110 °C for an additional 20 min and then backfilled with N2. A 100 μL aliquot (aliquot a) was withdrawn and diluted with 2 mL of hexane for spectroscopic analysis. Next, Pb(oleate)2 was added as 50 μL aliquots of the stock solution. After each addition, the reaction was allowed to proceed for 15 min before a 100 μL aliquot (labeled b−s) was withdrawn and diluted with 2 mL of hexane for spectroscopic analysis. At aliquots d, g, i, k, m, o, q, and s, a 1 mL sample was also withdrawn, and the nanocrystals from this larger sample were purified by precipitating with ethanol, centrifuging, and then washing twice more with hexanes/ethanol; this purified sample was used for XRD, TEM, and ICP-AES analysis. During the course of the reaction, the reaction mixture remained transparent and darkened in color from pale yellow to orange-red and eventually to brown. General Characterization. UV−vis−NIR absorbance measurements were performed on a Varian Cary 500 spectrometer on hexane solutions unless otherwise stated. Emission spectra were acquired using 405 nm laser excitation and a liquid-nitrogen-cooled CCD. Unless otherwise stated, all luminescence spectra were corrected for the instrument response using an OceanOptics LS-1-CAL calibrated light source for reference. TEM samples were prepared by dropcasting suspensions of nanocrystals onto 400 mesh carbon-coated copper grids from TED Pella, Inc. and dried in air; TEM images were obtained on an FEI TECNAI F20 microscope operated at 200 kV at the UW Molecular Analysis Facility. Size distributions were determined by analysis of >200 individual nanocrystals. Elemental composition was determined on nitric-acid-digested samples using inductively coupled plasma−atomic emission spectroscopy (ICP-AES) with a PerkinElmer 8300 spectrometer. Time-resolved emission traces were collected using a silicon photodiode with built-in amplifier and a Tektronix digital oscilloscope and excited with a 405 nm laser with a 100 Hz square wave pulse. Excitation spectra were collected using a halogen lamp with wavelength selection through a monochromator with a bandwidth of 5 nm and corrected for the wavelength dependence of the excitation intensity. Excitation intensities were measured separately using a silicon photodiode. Crystallographic Analysis. Samples were prepared for powder Xray diffraction (XRD) by depositing from solution on a silicon substrate and analyzed on a Bruker D8 Discover diffractometer at the UW Molecular Analysis Facility. For analysis of the crystal structure of PbIn2S4, the diffraction data of sample o were subjected to Rietveld

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RESULTS AND ANALYSIS Synthesis of Yb3+-Doped NaInS2 Nanocrystals. Given the chemical and structural similarity of NaInS2 to the recently reported series of Na(RE)S2 colloidal nanocrystals,43,44 we pursued the synthesis of Yb3+:NaInS2 nanocrystals using reaction conditions adapted from the Na(RE)S2 syntheses. We were able to determine optimized reaction conditions that reliably provide colloidal Yb3+:NaInS2 nanocrystals with diameters 400 nm, and the absorbance sharply increases approaching the UV. Literature values for the band gap of NaInS2 vary widely (2.3−3.1 eV) and its value seems to depend on sample treatment, possibly due to alloying with H+ or the presence of other impurities.47 The band gaps of our samples appear to be closer to the high end of this range. Despite this large band gap, visible-light excitation (405 nm) gives rise to near-IR emission from the 2F5/2→2F7/2 transition of the Yb3+ dopants (Figure 1c). The Yb3+ emission maximum is located at 993 nm, significantly red-shifted from the emission frequently observed for Yb3+ in oxide and fluoride nanocrystals (∼970−980 nm),54−57 and more consistent with that observed for Yb3+ implanted in bulk CdS (990 nm).58 Although the energies of the f-f transitions in lanthanides are largely independent of the chemical environment, emission redshifts occur in lattices with increasingly polarizable anions because of reduced spin−orbit coupling and electron−electron repulsion (nephelauxetic effect).59,60 The luminescence spectrum observed here thus provides preliminary evidence that the 11816

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host (Scheme 1b) is much more unusual, likely because complete cation exchange in this case causes loss of the dopant cations, and even partial cation exchange will generally preferentially displace dopant cations relative to host cations.69 Cation exchange in ternary doped nanocrystals such as Yb3+:NaInS2 could conceivably circumvent this problem, because Yb3+ ions are likely substitutionally doped only at In3+ sites. If selective exchange of only the Na+ cation sublattice could be achieved, then the optical properties of the host nanocrystals could be modified without loss of Yb3+, creating a new composition of doped nanocrystals. Figure 1d shows the optical properties of the material that results from treating Yb3+:NaInS2 nanocrystals with a Pb2+ source at 110 °C in octadecene/oleylamine (Scheme 2). Scheme 2. Synthesis of Yb3+-Doped NaInS2 Nanocrystals and Na+-to-Pb2+ Cation Exchange

Absorbance in the visible region is greatly increased, resulting in a color change of the sample from light yellow to orange-red. Strong Yb3+ emission at 993 nm is still observed upon 405 nm excitation, demonstrating that the Yb3+ dopants are still incorporated in the nanocrystals. Figure 2 compares TEM data for a representative sample of Yb3+:NaInS2 starting material and the same material after Pb2+ cation exchange (to Pb/S = 0.14). The nanocrystals retain their overall shapes and sizes, and a small increase in the average diameter is observed after cation exchange. Further study of this reaction (vide inf ra) demonstrates that under these reaction conditions, Pb2+ is initially exchanging only with Na+ and not with In3+, thus allowing Yb3+ doping to be maintained even as the host material changes (Scheme 1), as hypothesized above. This selective cation exchange therefore provides a novel strategy for the preparation of Yb3+-doped chalcogenide nanocrystals that display strong visible absorption. The successful demonstration of this general synthesis strategy is one of the central new findings of this study. To better understand the cation exchange process and the optical properties of the resulting materials, we have characterized Yb3+:PbxNa2(1‑x)In2S4 nanocrystals at different stages of Pb2+ incorporation (0 ≤ x ≤ 1). A Pb2+ cationexchange reaction (Scheme 2) was carried out on a sample of 0.012 μmol of nanocrystals with an average diameter of 11 nm, containing a total of 0.068 mmol of Na+ ions. The nanocrystals were suspended in a degassed 7:3 (v/v) mixture of octadecene and oleylamine and heated to 110 °C under an N2 atmosphere. Next, 50 μL portions of a 0.05 M solution of Pb(oleate)2 in octadecene were added, and before each addition an aliquot of the reaction solution was withdrawn for analysis. Nineteen such aliquots, herein referred to sequentially as a−s, were studied. Aliquot a represents the starting material (Yb3+:NaInS2 nanocrystals) before addition of any Pb2+. Fifteen minutes elapsed between each addition; the reaction mixture appears to stop changing within ca. 5 min after addition of a portion of Pb2+. The UV−visible absorption spectra of the nanocrystals collected during the cation exchange reaction from NaInS2 to

Figure 1. Characterization of colloidal Yb3+-doped nanocrystals. (a) TEM image of Yb3+:NaInS2 nanocrystals, with scale bar representing 100 nm (inset: close-up image with scale bar representing 20 nm), and (b) corresponding size distribution and Gaussian fit, yielding a mean diameter of 8.8 ± 1.1 nm. (c,d) Room-temperature electronic absorption (blue) and photoluminescence spectra (red, excited at 405 nm) of colloidal (c) Yb 3+ :NaInS 2 and (d) Yb 3+ :PbIn2 S 4 nanocrystals. Absorption spectra represent samples with the same nanocrystal concentration.

Yb3+ dopants in this material are indeed incorporated within the sulfide lattice rather than just adsorbed to the nanocrystal surfaces. In addition to the Yb3+ emission, a broad and weak higher-energy emission feature is observed centered at around 550 nm, which is also present in undoped NaInS2 nanocrystals; we tentatively attribute this emission to surface trap states. Cation Exchange in Yb 3+ :NaInS 2 Nanocrystals. Although these initial results with Yb3+:NaInS2 nanocrystals are quite promising, we ultimately seek materials with greater visible absorbance. We hypothesized that cation exchange61−64 might prove useful in this case as a synthetic technique to modify the Yb3+ host lattice and improve the optical properties of the Yb3+-doped nanocrystals. Cation exchange has emerged as an important tool for the synthesis of doped semiconductor nanocrystals, but typically it is used to introduce the dopants themselves (Scheme 1a).65−68 The use of post-doping cation exchange to modify the properties of the doped nanocrystal Scheme 1. Two Orthogonal Cation-Exchange Approaches to the Synthesis of Novel Doped Nanocrystals

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Figure 3. (a) Electronic absorption and (b) Yb3+ luminescence spectra collected at various stages of cation exchange (reaction progresses from blue to red; first curve is prior to the addition of Pb2+). The bold curve in (a) corresponds to sample j. All data were collected at room temperature. Luminescence spectra were collected with 405 nm excitation, and were not corrected for instrument response. All spectra were measured on samples with the same nanocrystal concentration. Figure 2. TEM data comparing representative nanocrystals before and after a Pb2+ cation exchange reaction. Wide-field and magnified views of Yb3+:NaInS2 nanocrystals (top) and the partial-cation-exchange product (bottom, Pb/S = 0.14, measured by ICP-AES). These nanocrystals are not spherical, but instead appear as anisotropic platelets; in both samples, the thinner dimension of a few nanocrystals can be observed. Size distributions (histograms shown on right) of the nanocrystal diameters before and after Pb2+ cation exchange are very similar. Scale bars represent 100 nm in the wide-field images and 10 nm in the higher-resolution insets. Additional data for this sample are given in the SI.

are evident: from sample a to sample m, the characteristic diffraction peaks of rhombohedral NaInS2 (structure shown in Figure 4b) gradually shift in position and intensity; in samples m and o, the characteristic NaInS2 diffraction peaks are no longer discernible and conversion to a new nanocrystalline species appears to be essentially complete. We assign this new species as PbIn2S4. After sample o, the formation of cubic PbS becomes apparent. Similarly, elemental analysis of purified aliquots (Figure 4c) shows that early in the reaction (up to m), the Na+ content decreases and is replaced by Pb2+ (at a 2:1 ratio) while the In3+ content remains nearly constant. After m, the In3+ content decreases along with a further increase in Pb2+, consistent with cation exchange to form some PbS. We note that this cation-exchange reaction requires an excess of Pb2+ to reach completion under the conditions employed. Notably, although the Yb3+/In3+ ratio decreases by about half with the first addition(s) of Pb2+ (Figure 4d), perhaps due to loss of some surface Yb3+ ions, it then remains almost constant throughout the rest of the cation-exchange reaction, averaging 1.0 ± 0.2 mol% with a slight downward trend. The Yb3+:In3+ ratio changes little even during PbS formation, suggesting that this stage of the exchange forms PbS and not Yb3+-doped PbS as the final product. TEM data (see SI) illustrate that the nanocrystals remain largely intact at least through sample m (Pb2+/S2− = 0.21), but by sample q (Pb2+/S2− = 0.37), where PbS formation becomes evident, they appear to be degrading into smaller particles. The diffraction pattern observed for m (Pb2+/S2− = 0.21) and o (Pb2+/S2− = 0.24) does not match the known orthorhombic structure of PbIn2S4 (ICSD 640215, see SI). Instead, we were able to model the diffraction data using a rhombohedral structure where the Na+ cations of the NaInS2

PbIn2S4 (Figure 3a) show that as the color changes from pale yellow to orange-red, there is a significant increase in absorption throughout the visible region up to ∼650 nm, and a distinct absorption peak centered at 462 nm becomes apparent even very early in the process. This peak grows in intensity and broadens with increasing Pb2+ content, but it does not shift significantly in position. After sample j, it begins to broaden substantially with added Pb2+. Similar reactions carried out on other nanocrystal samples show that this peak’s energy does not depend strongly on nanocrystal size, and that the same absorption features are present in Yb3+-free samples (see SI). This band shows a per-Pb2+ molar extinction coefficient of εmax ≈ 8.1 × 103 M−1 cm−1 at its maximum (462 nm), corresponding to a per-nanocrystal extinction coefficient of εmax(NC) ≈ 1.4 × 107 M−1 cm−1 for sample j, comparable to the first-excitonic extinction coefficients of other well-known colloidal semiconductor nanocrystals.70,71 Structural Changes with Cation Exchange. Figure 4a shows the evolution of the powder X-ray diffraction pattern measured for the Yb3+:NaInS2 nanocrystals during the course of their cation exchange with Pb(oleate)2. Two distinct processes 11818

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Figure 4. Analysis of structural and compositional changes of Yb3+:NaInS2 nanocrystals during cation exchange with Pb2+. (a) Powder XRD data, illustrating the evolution of the crystal structure from NaInS2 to PbIn2S4, and subsequently toward PbS. The reference pattern shown for PbIn2S4 is calculated from a modified NaInS2 structure (see discussion in text). (b) Depiction of the rhombohedral NaInS2 structure. (c) Analytical stoichiometries of In3+ (brown squares), Na+ (purple diamonds), and Pb2+ (red crosses) plotted relative to S2− (yellow circles, fixed at 4 to reflect the stoichiometry PbxNa2(1‑x)In2S4), demonstrating that Pb2+ replaces Na+ throughout the reaction, whereas In3+ is not displaced until after Na+ has been exchanged. (d) Analytical ratio of Yb3+/In3+ concentrations in the nanocrystals. For reference, the dashed gray line shows the average ratio during cation exchange, neglecting the first aliquot. Standard deviations for the ICP-AES measurements (c,d) are smaller than the symbol sizes, but error from potential incomplete removal of byproducts (e.g., Na(oleate)) from the nanocrystal aliquots may still exist.

structure (Figure 4b) were replaced with Pb2+ cations at half occupancy. Refinement of this model against the data produced small changes in the sulfur-atom coordinate and lattice parameters. Most notably, the c axis lengthens from 19.89 Å in NaInS2 to 20.53 Å in the PbIn2S4 model. The result of these changes is an increase from a Na−S bond length of 2.88 Å to a Pb−S bond length of 2.94 Å, while the In−S bond lengths remain unchanged (2.64 Å). The diffraction pattern calculated for this model is shown in Figure 4a, overlaid against sample m (Pb2+/S2− = 0.21), which it matches well. This structure effectively models the Pb2+ cations as randomly occupying half of the former Na+ sites within each layer. Although there may be some Pb2+ ordering, such ordering cannot be determined from these data. This structural model is consistent with a picture in which the anion sub-lattice of NaInS2 as well as the S−In−S layers themselves remain intact during cation exchange, facilitating selective exchange of just the inter-layer Na+ cations. A consequence of this selective cation exchange is that Yb3+ dopants that reside within the S−In−S layers are unaffected. It is a hallmark of nanocrystal cation-exchange reactions that the anion sub-lattice preserves its structure,61−64 often resulting in retention of the overall crystal symmetry and hence sometimes yielding unusual or completely new product phases, as observed here. Selective exchange of just one cation population in a ternary lattice is rather uncommon and has limited precedent in the colloidal nanocrystal literature.72−74 In the present case, selective cation exchange is likely facilitated by several features of the NaInS2 nanocrystal structure and composition; in particular, the layered structure of NaInS2 (Figure 4b) allows facile diffusion and exchange of Na+ ions without disruption of the In3+ cations. The high concentration of Na+ vacancies suggested by elemental analysis should further promote rapid ion diffusion within the Na+ layers. Evolution of Photoluminescence during Cation Exchange. Concomitant with the rise in visible absorption, the Yb3+ luminescence intensity observed upon excitation at 405 nm increases dramatically during the first phase of the cation exchange (Figure 3b), increasing 45-fold in sample j

relative to the starting Yb3+:NaInS2 nanocrystals (sample a). This increase is only partially explained by the increased absorbance at 405 nm, which is only 6 times greater in sample j than in sample a (Figure 5a). The remaining change must be due to an increase in the quantum yield for Yb3+ emission with 405 nm photoexcitation. As shown in Figure 5a (green diamonds), the relative Yb3+ emission quantum yield, parametrized by the ratio of the Yb3+ emission to the absorbance at the excitation wavelength (405 nm), increases about 7-fold from a (Yb3+:NaInS2) to a rough plateau for samples f to j (Pb2+/S2− ≈ 0.07−0.15) before dropping sharply with further cation exchange. This dramatic improvement could be due to more efficient excitation energy transfer from the host lattice to the Yb3+ dopants, to a higher internal quantum efficiency for the Yb3+ emission, or to a combination of these factors. As described below, the Yb3+ luminescence lifetime changes relatively little from a to j, suggesting that this increase in nanocrystal quantum yield cannot be explained primarily by a change in the internal quantum efficiency of emission from the Yb3+ 2F5/2 excited state, and is instead likely primarily due to an increase in the quantum efficiency of Yb3+ sensitization by the host lattice as Pb2+ is incorporated. Photoluminescence excitation spectra collected on three representative aliquots (e, j, m) provide strong evidence that the Yb3+ emission is indeed sensitized by energy transfer from the PbxNa2(1‑x)In2S4 nanocrystal host (Figure 5b). These excitation spectra show the same distinct feature at 462 nm as seen in the absorption spectra. Furthermore, the broadening of this absorption feature with increasing Pb2+ content from e to m is mirrored in the excitation spectra. Importantly, the excitation spectra track the nanocrystal absorption well not just at this peak but throughout the entire absorption spectrum. Luminescence lifetime measurements provide additional evidence for successful incorporation of Yb3+ dopants inside the internal volumes of the host nanocrystals. Because the nearIR luminescence of Yb3+ is readily quenched by high-energy vibrations (e.g., C−H, N−H, or O−H bonds), close proximity to such bonds typically causes shortened excited-state lifetimes (e.g., ∼10 μs for typical Yb3+ complexes with organic ligands)75 11819

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Figure 6. Room-temperature luminescence decay data measured for colloidal Yb3+-doped PbxNa2(1‑x)In2S4 nanocrystals at various stages of Na+-to-Pb2+ cation exchange. (a) Normalized photoluminescence decay curves for select samples, and fits to a double-exponential function; the fit function is also shown (I is the emission intensity, τ1 and τ2 are the long and short lifetimes, respectively, and A1 is the amplitude of the τ1 component). (b) Plot of the two-component lifetimes from samples a−p, and the fraction of the amplitude corresponding to the τ1 component (blue circles, τ1; purple circles, τ2; and green diamonds, A1). Error bars represent 95% confidence bounds for the fit parameters; where not visible, confidence bounds are smaller than the size of the plotted symbols. The solid symbols indicate samples for which the molar Pb2+/S2− ratio was determined analytically (Figure 4c), and the open symbols indicate samples for which it was interpolated from the analytical data.

Figure 5. (a) Plot of the change in absorbance at 405 nm (blue circles) and the ratio of near-IR emission to 405 nm absorbance (green diamonds) at various stages of cation exchange from Yb3+:NaInS2 nanocrystals with Pb2+. The solid symbols indicate samples for which the molar Pb2+/S2− ratio was determined analytically (Figure 4c), and the open symbols indicate samples for which it was interpolated from the analytical data. (b) Room-temperature photoluminescence excitation spectra (solid lines) collected on samples e, j, and m, overlaid with corresponding absorption spectra (dashed lines).

and low luminescence quantum yields, making lifetime measurements a useful probe of Yb3+ proximity to the crystallite surfaces. Photoluminescence decay curves were collected for samples a−o. Representative normalized decay curves (for a, j, and n) are shown in Figure 6a. These data were modeled as double exponential decays, and the resulting fit parameters (long and short lifetimes (τ1 and τ2), and the fractional amplitude (A1) of the long lifetime component) are plotted vs aliquot number in Figure 6b. For most of the series (c−k, Pb2+/ S2− ≈ 0.03−0.17), each component accounts for about half of the total amplitude. Notably, the longer luminescence lifetime (τ1) exceeds 500 μs for most of the aliquots, reaching a maximum of 815 μs in sample c, indicative of Yb3+ in a crystalline lattice well isolated from the ligand and solvent environment, as well as from other defects that may introduce nonradiative decay pathways. The faster decay component has a lifetime of τ2 ≈ 100−160 μs and may arise from Yb3+ ions positioned closer to the nanocrystal surfaces, where nonradiative relaxation by coupling to solvent or ligand vibrations becomes more probable; the increased contribution from τ2 at the end of the series (Pb2+/S2− > 0.19) may reflect some degradation of the nanocrystals, as observed by TEM (see SI). Indeed, this shorter luminescence lifetime (τ2) is similar to the one reported for Yb3+ located in the outer layers of CdSe quantum dots.41 Overall, the long lifetimes observed here reflect the presence of Yb3+ ions that are well isolated from

solvent and ligand vibrationsi.e., embedded within the internal volumes of the nanocrystals. The photoluminescence quantum yield of sample j was measured to be ∼5% with 405 nm excitation. While promising, improvement is certainly needed before this material could be viable for practical applications. Nevertheless, given its large extinction coefficient (ε405 nm = 8.5 × 103 M−1 cm−1 per Pb2+ cation, or 1.5 × 107 M−1 cm−1 per nanocrystal for sample j) throughout much of the visible spectrum, this material can still be considered a fairly bright solution-phase Yb3+ emitter under visible excitation even with a ∼5% quantum yield. 19 Interestingly, preliminary results show a notable improvement of the quantum yield upon shelling with ZnS (see SI). The quantum yield alternates with alternating cation and anion surface termination, further suggesting that this improvement involves passivation of electronically active traps, likely at the nanocrystal surfaces. These results indicate that at least some nonradiative losses can be eliminated in such nanocrystals with further synthetic optimization, and they suggest promising routes for future materials development. Cation Exchange Using Cd2+, Ag+, or Cu+. We have also found that this selective cation exchange within NaInS2 nanocrystals can be performed with other cations besides Pb2+. Addition of Cd2+ to Yb3+-doped NaInS2 nanocrystals 11820

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Journal of the American Chemical Society generates Yb3+-doped CdIn2S4 nanocrystals (see SI). This material still shows strong sensitized luminescence from the Yb3+ dopants. Photoluminescence lifetime data and excitation spectra are provided in the SI. Likewise, cation exchange using Ag+ generates an unusual rhombohedral delafossite form of AgInS2 (see SI) that had previously only been generated by high-pressure treatment of bulk orthorhombic or tetragonal AgInS2.76 With Cu+, cation exchange led to some increase in absorption but also to Yb3+ PL quenching, and product conversion was not clean (see SI); no well-defined product could be identified in this case.

Selective cation exchange of Na+ for Pb2+ changes the optical properties of NaInS2 nanocrystals by introducing new mid-gap electronic transitions and an intense absorption maximum at ∼462 nm (εmax = 8.1 × 103 M−1 cm−1 per Pb2+ cation, εmax(NC) ≈ 1.4 × 107 M−1 cm−1 for sample j) that is evident even at low levels of Pb2+. Pb2+ is itself a well-known luminescence activator in wide-gap crystal lattices (e.g., Pb2+:BaSi2O5). In oxides, its absorption typically occurs at ∼250 nm and gives rise to broad, Stokes-shifted luminescence maximizing at ∼350−400 nm.2 The luminescent excited state arises from an intra-ionic Pb2+ 1S0→3P1 (“A-band”) transition involving a 6s2→6s16p1 electronic promotion. Although formally spin-forbidden, this transition carries substantial oscillator strength because of large spin−orbit coupling in Pb2+ (ζ ≈ 0.95 eV).2,78 Absorption features (∼470−480 nm) very similar to the one we observe here have been reported for isolated Pb2+ dopants in bulk ZnS crystals, as well as in matrixisolated “monomolecular” PbS diatomics, and have been assigned in this way.79−82 Alternatively, the large extinction coefficient of this absorption band in the PbxNa2(1‑x)In2S4 nanocrystals would also be consistent with its assignment as an electric-dipole allowed S2−→Pb2+ charge-transfer transition, analogous to the so-called “D-band” of Pb2+-doped oxides and halides. Such a charge-transfer transition has been proposed to occur at close to this energy in other sulfide lattices.82 This assignment would appear to be more consistent with sulfur Kβ emission spectra of bulk PbS crystals, which show predominant S(3p) character at the top of the valence band.83 The Pb(6s) orbitals appear only much deeper below the band edge,83 suggesting that S2−→Pb2+ charge-transfer excited states should be lower in energy than intra-Pb2+ 6s2→6s16p1 excitations. In either case, the broadening of this absorption feature at elevated Pb2+ concentrations suggests increasing delocalization of this excited state as Pb2+−Pb2+ coupling increases. The sensitized Yb3+ photoluminescence quantum yield of the nanocrystals increases ∼7-fold with Pb2+ cation exchange to form Yb3+:PbxNa2(1‑x)In2S4 (x ≈ 0.8) nanocrystals, attributed to an increase in the quantum efficiency of Yb3+ sensitization as Pb2+ is incorporated. A curious aspect of the new Yb3+:PbIn2S4 composition, however, is the absence of any clear mechanism for energy transfer from the photoexcited nanocrystals to the Yb3+ dopants. In other analogous luminescent doped nanocrystals, such as Mn2+-doped II−VI or III−V nanocrystals,84 Dexter-type energy transfer is extremely efficient because of reasonably strong dopant-semiconductor sp-d electronic coupling. Additionally, although Mn2+ d-d emission in many lattices occurs deep within the optical gap, Mn2+ possesses an almost continuous series of higher-energy d-d excited states such that resonant energy-transfer conditions can be met in essentially every instance. For Yb3+ dopants, neither of these factors is present in any obvious way. The valence f orbitals of lanthanides are highly shielded and hence support only minimal direct electronic coupling with the surrounding matrix. Likewise, Yb3+ ions have no higher-energy f-f excited states to provide spectral overlap with the nanocrystal donor states for resonant or near-resonant energy transfer. The nanocrystalYb3+ energy gaps described here are sufficiently large that simple phonon-mediated energy transfer processes appear improbable. Energy transfer in these nanocrystals could conceivably involve participation of Yb3+ charge-transfer excited states, in which a photogenerated charge carrier within the nanocrystal is formally captured by Yb3+, followed by internal conversion to

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DISCUSSION The crystallographic and analytical data collected during the addition of Pb2+ to colloidal Yb3+:NaInS2 nanocrystals present a coherent picture of a selective cation-exchange reaction in which each incoming Pb2+ ion displaces two Na+ ions to form the new lattice composition, Yb3+:PbIn2S4. This process is summarized in Scheme 3. At the earliest stage of the reaction, Scheme 3. Selective Exchange of Pb2+ for 2Na+ Cations in Yb3+:NaInS2 Nanocrystals

the Yb3+ content decreases by about half. Given that the reaction conditions involve heating in an octadecene/oleylamine mixture, this decrease could be due in part to stripping of some Yb3+ ions from the nanocrystal surfaces by the amine ligands. After sample d, there is relatively little further change in the Yb3+/In3+ ratio, confirming the selective displacement of just Na+ by Pb2+. This high selectivity derives in part from the large difference between Na+−S2− and In3+(or Yb3+)−S2− bond formation enthalpies, the former being an entirely ionic interaction with a monovalent cation, and the latter involving substantial covalency and a higher cation charge. Further Pb2+ addition eventually leads to loss of both In3+ and Yb3+ as PbS is formed. Selective cation exchange in ternary or multinary materials is unusual and deserves further consideration as a synthetic strategy for accessing new colloidal nanocrystal materials. For example, the layered form of PbIn2S4 generated here has not been previously described; similarly, cation exchange using Cd2+ and Ag+ generates unusual nanocrystalline CdIn2S4 and AgInS2 phases.77 It is highly likely that similar selective ionexchange processes with other cations or starting materials will provide access to a wealth of new nanomaterials not previously explored. Moreover, as demonstrated here, selective cation exchange within ternary chalcogenide nanocrystals offers a powerful approach for generating these new materials while retaining the doping of the parent nanocrystals, thereby opening doors to accessing particularly challenging doped-nanocrystal compositions and their unique physical properties, such as the Yb3+-doped PbIn2S4 and CdIn2S4 nanocrystals described here. 11821

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Journal of the American Chemical Society populate the emissive f-f excited state. Such a process has been proposed for Yb3+ sensitization in bulk InP crystals, for example.85,86 In this case, InP excitation is proposed to be followed by electron localization to form a state that can be described (formally) as Yb2+ with a delocalized but Coulombically bound hole, i.e., a VB→Yb3+ charge-transfer (LVBMCT) excited state. From here, the system then relaxes to the Yb3+ 2 F5/2 excited state via nonradiative internal conversion, followed by radiative decay back to the ground state. The broad LVBMCT band profile, which reflects the breadth of the VB itself, allows energy matching with the InP excitonic states for rapid electron capture. The energies of the LVBMCT excited states for Yb3+ in the NaInS2 and PbIn2S4 lattices are not known, but in the similar bulk lattice, NaYS2, the analogous transition has been reported to occur at ∼2.85 eV (∼435 nm).87 Assuming a similar LVBMCT energy in NaInS2, this state is then anticipated to fall within the host’s energy gap (∼3.0 eV) and provide a viable pathway for Yb3+ luminescence sensitization. Figure 7a illustrates this potential sensitization mechanism for Yb3+ in NaInS2 nanocrystals.

help to guide future optimization of this class of visible-lightabsorbing Yb3+-doped nanophosphors. Finally, we note the exciting challenges and opportunities associated with the aim of developing such Yb3+-doped chalcogenide nanocrystals for use as spectral-conversion nanophosphors for luminescent solar concentrators (LSCs) or related technologies.89 From the spectroscopy side, the narrow near-IR emission of Yb3+ is extremely attractive for coupling with Si photovoltaics. Figure 8 illustrates the

Figure 8. AM1.5 solar irradiance distribution throughout the visible and near-IR (W m−2 eV−1, dashed blue) and EQE of a c-Si solar cell (dashed red),93 compared to the absorption (solid blue) and photoluminescence (solid red) spectra of Yb3+:PbIn2S4 nanocrystals. Each curve is normalized.

relationship of the Yb3+:PbIn2S4 nanocrystal spectral properties to the external quantum efficiency (EQE) of crystalline Si photovoltaics, as well as to the AM1.5 solar spectrum. These Yb3+:PbIn2S4 nanocrystals absorb well in the high-energy region of the solar spectrum, and convert this energy to near-IR photons emitted near the band edge of crystalline Si. The narrow band shape of the f-f luminescence avoids losses incurred with broad-band near-IR emitters such as CuInS2 or related materials,90−92 and should allow the nanocrystal absorption threshold to be moved deeper into the near-IR with appropriate modifications of the overall composition. In principle, Yb3+-based nanophosphors can have very high luminescence quantum yields, meaning their potential in such applications is high, but much more work on shelling, defect control, and composition tuning is required to achieve these quantum yields in the colloidal Yb3+:PbIn2S4 and related nanocrystals described here. Experiments in these directions are presently underway in our laboratories. Overall, the results presented here offer a promising glimpse into potential future uses of intermediate-gap semiconductor nanocrystals for broadband visible sensitization of Yb3+ luminescence in LSCs and related spectral-conversion applications.

Figure 7. Potential mechanisms for energy transfer from photoexcited chalcogenide nanocrystals to Yb3+ dopants, proceeding via (a) an LVBMCT excited state of Yb3+ or (b) nanocrystal midgap trap states, both leading to sensitized near-IR Yb3+ f-f luminescence.

In contrast, Pb2+ incorporation introduces excited states at 2.68 eV (462 nm) that would appear to reside below the LVBMCT absorption of Yb3+, and it is therefore unclear precisely how the Yb3+ 2F5/2 excited state is populated. Perhaps the lowest-energy Yb3+ LVBMCT state in Yb3+:PbIn2S4 occurs below that of the reference Yb3+:NaYS2 lattice. Alternatively, it is conceivable (albeit improbable) that Yb3+ sensitization could proceed by simultaneous Yb3+−Yb3+ pair excitation (∼2.6 eV, ∼475 nm) in a so-called “quantum cutting” process.88 A possibly more likely scenario could involve participation of lowoscillator-strength trap states, such as those responsible for the broad mid-gap luminescence observed in Figure 1c (∼2.25 eV, ∼550 nm; see also SI). One such process, involving direct relaxation from trap states to Yb3+, is illustrated schematically in Figure 7b, but Auger-type coupling of Yb3+ excitation to carrier trapping could also be envisioned. The observation that surface shell growth increases rather than decreases sensitization of Yb3+ luminescence suggests that surface traps may not be likely candidates for this mechanism, but other defect-related traps could be involved. At this time, the most effective mechanism(s) remains unclear. Understanding the mechanism of energy relaxation from the nanocrystals to the Yb3+ dopants is clearly an important fundamental-science objective for ongoing investigations of these new materials, and should

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CONCLUSION We have developed a procedure for synthesizing monodisperse, colloidal ∼9 nm diameter Yb3+-doped NaInS2 nanocrystals that exhibit sensitized Yb3+ emission around 1000 nm. Cation exchange using Pb2+ results in the selective replacement of Na+ within these nanocrystals, to form a new set of Yb3+:PbxNa2(1‑x)In2S nanocrystals (0 ≤ x ≤ 1). Significant Yb3+ doping at In3+ sites is maintained during this cation exchange. The effectiveness of this selective cation exchange is attributed to the high Na+ mobility within the layered NaInS2 lattice and the thermodynamic driving force of Pb2+−S2− bond formation. The resulting Yb3+:PbxNa2(1‑x)In2S4 nanocrystals exhibit broadband visible absorption with a large extinction 11822

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Journal of the American Chemical Society coefficient, as well as an ∼7 times improved quantum yield for Yb3+ emission relative to the parent Yb3+:NaInS2 nanocrystals. Long Yb3+ emission lifetimes are observed that indicate the presence of Yb3+ deep within the internal volumes of the nanocrystals, rather than just at their surfaces, and photoluminescence excitation spectra confirm that Yb3+ emission is sensitized through photoexcitation of the PbxNa2(1‑x)In2S4 host nanocrystals. Preliminary results suggest that the Yb3+:PbIn2S4 nanocrystal quantum yields can be improved by careful shell growth, offering promising directions for future work. Finally, the selective cation-exchange approach used to synthesize these Yb3+:PbIn2S4 nanocrystals has also been demonstrated using other cations (Cd2+, Ag+), providing proof-of-concept support for the hypothesis that this general strategy can have broad applicability in the synthesis of novel luminescent doped nanomaterials useful in emerging spectral-conversion applications such as luminescent solar concentrators.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04938. Further information about the synthesis of Yb3+:NaInS2 nanocrystals; characterization of undoped NaInS2 nanocrystals; procedures and data for cation exchange with Cd2+, Ag+, and Cu+; additional TEM and spectral characterization data on Pb2+ exchange and ZnS shelling; details on the crystallographic modeling of PbIn2S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Daniel R. Gamelin: 0000-0003-2888-9916 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the U.S. National Science Foundation (DMR-1505901 to D.R.G.) is gratefully acknowledged. R.F. was supported by the German Academic Exchange Service (DAAD) with funds from the German Federal Ministry of Education and Research (BMBF) and the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA Grant Agreement No. 605728 (P.R.I.M.E. − Postdoctoral Researchers International Mobility Experience). Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington that is supported in part by the National Science Foundation (grant ECC-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, and the National Institutes of Health. Dr. Emily Tsui and Dr. Carl Brozek are acknowledged for assistance with quantum yield measurements, and Tyler Milstein is acknowledged for photography.

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REFERENCES

(1) Blasse, G.; Grabmaier, B. C. Luminescent Materials; SpringerVerlag: Berlin, 1994. 11823

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Journal of the American Chemical Society

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DOI: 10.1021/jacs.7b04938 J. Am. Chem. Soc. 2017, 139, 11814−11824