Probing the Crystal Structure and Formation Mechanism of Lanthanide

Nov 23, 2016 - Lanthanide (Ln)-doped upconverting nanocrystals (UCNCs), such as NaLnF4 (with Ln = lanthanide), constitute an important class of nanosc...
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Probing the Crystal Structure and Formation Mechanism of Lanthanide-Doped Upconverting Nanocrystals D. Hudry,*,†,∥ A. M. M. Abeykoon,‡ E. Dooryhee,‡ D. Nykypanchuk,§ and J. H. Dickerson*,†,§ †

Department of Physics, Brown University, Providence, Rhode Island 02912, United States Photon Science Division, National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, United States § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ‡

S Supporting Information *

ABSTRACT: Lanthanide (Ln)-doped upconverting nanocrystals (UCNCs), such as NaLnF4 (with Ln = lanthanide), constitute an important class of nanoscale materials due to their capacity to convert near-infrared photons into nearultraviolet or visible light. Although under intense investigation for more than a decade, UCNCs have been relatively underexplored especially regarding their crystal structure and mechanisms of formation in organic media. The former is needed to explain the relationship between atomic scale structure and upconversion (UC) properties of UCNCs (i.e., local symmetry for 4f−4f transition probability, Ln3+ distances for energy migration), while the latter is essential to finely tune the size, morphology, chemical composition, and architecture of well-defined upconverting nanostructures, which constitute the experimental levers to modify the optical properties. In this contribution, we use synchrotron-based diffraction experiments coupled to Rietveld and pair distribution function (PDF) analyses to understand the formation of NaGdF4:Yb:Er UCNCs in organic media and to investigate their crystal structure. Our results reveal a complex mechanism of the formation of NaGdF4:Yb:Er UCNCs based on chemical reactions involving molecular clusters and in situ-generated, crystalline sodium fluoride at high temperature. Additionally, a detailed crystallographic investigation of NaGdF4:Yb:Er UCNCs is presented. Our Rietveld and PDF analyses show that the space group P6̅ is the one that best describes the crystal structure of NaGdF4:Yb:Er UCNCs contrary to what has been recently proposed. Further, our Rietveld and PDF data reveal the formation of bulk-like crystal structure down to 10 nm with limited distortions. The results presented in this paper constitute an important step toward the comprehensive understanding of the underlying picture that governs UC properties of lanthanide-doped nanostructures.

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The majority of the UCNCs developed to date are based on a sensitizer-activator pair where the UC process relies on several aspects, including the following: (i) the absorption of the excitation energy by the sensitizer; (ii) the energy transfer from the sensitizer to the activator; (iii) the radiative transitions of the excited activator; and (iv) luminescence quenching. Hence, over the past decade, various strategies, which can be gathered into three main approaches, were developed to enhance photon UC in UCNCs.7 Initially, the core−shell approach (inspired from quantum dots) was developed to passivate UCNCs’ surface and, hence, prevent luminescence quenching due to surface structural defects or surface oscillators (e.g., stabilizing ligands or solvent molecules).8 Subsequently, the efficacy of absorption by the sensitizer was addressed by harnessing the effect of surface plasmon resonance in noble metal nanoparticles or by using organic dyes.9,10 More recently,

hoton upconversion (UC) is a nonlinear optical phenomenon that converts multiple (two or more) lowenergy photons into one photon of higher energy.1 This phenomenon is characterized by the absorption of near-infrared (NIR) radiation followed by radiative emission within the nearultraviolet to visible/NIR range. In 2006, Yan’s group published the first synthetic approach to prepare high-quality lanthanidedoped upconverting nanocrystals (Ln-doped UCNCs) within the family of ternary alkali metal fluorides (NaLnF4 with Ln = Y, La−Lu).2 Less than a decade later, Ln-doped UCNCs have risen to be one of the most important classes of nanoscale materials due to their potential applications in technological fields as diverse as solid-state lasers, optical data storage, biological imaging and therapy, or solar energy conversion.3,4 However, UCNCs suffer from low absolute quantum yield,5,6 which is currently inhibiting their widespread use in commercial devices. As a consequence, a better understanding of all features that influence the UC process in UCNCs is of prime importance to boost their efficiency. © XXXX American Chemical Society

Received: September 28, 2016 Revised: November 11, 2016

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DOI: 10.1021/acs.chemmater.6b04140 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Scheme 1. Crystal Structures of β-NaLnF4, Viewed along the c-Axis, Described by Three Different Space Groupsa

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Above: (a) P6̅ (#174-Structure I); (b) P63/m (#176-Structure II); and (c) P6̅2m (#189-Structure III). Lanthanide (Ln3+) and sodium (Na+) cations are represented in red and blue, respectively, while fluoride (F−) anions are in light gray. The Wyckoff positions, related to the different cationic sites, are identified for each space group. All structures are represented with isotropic atomic displacement parameters.

an idea based on crystal field (CF) engineering was proposed to enhance photon UC by tuning local environments and local symmetry of the optically active centers.11−16 Not only does the CF influence intra-4f transition probability, but the CF also has an impact on energy transfer processes, which play a crucial role in photon UC.17 Note that such an approach is of high interest because it can change the fundamental behavior of 4f electrons (energy transfer efficiency, intra-4f transitions probability, electron−phonon coupling). Of course, one can consider coupling the CF engineering approach with surface passivation and enhanced absorption for better results. Although the importance of the crystal structure on photon UC has been recently underlined in several reviews,8,18−20 structural data regarding hexagonal NaLnF4 UCNCs are very sparse. Once focused on the CF engineering approach to modify fundamental physics of both 4f electrons and the UC process, detailed structural investigations remain of major interest to elucidate the relationship between the crystal structure in which optically active Ln3+ ions are embedded and their resulting UC properties. The determination of the crystal structure of the bulkcounterpart of UCNCs has been a matter of debate for more than half a century. In the literature, the hexagonal phase (aka, the β phase) is usually described by three different space groups, namely, P6̅,21 P63/m,22 and P6̅2m,23 whose corresponding crystal structures mainly differ by the number, occupation, and symmetry of cationic sites. Surprisingly, little interest has been given to an in-depth structural characterization of βNaLnF4 UCNCs. To the best of our knowledge, the problem associated with the space group identification observed for bulk materials has only been discussed in the literature very recently for UCNCs.24 Indeed, the recent study by Schurko and coworkers focusing on core−shell NaYF4-NaLuF4 NCs (27 and 37 nm) was the first report on the structural problem related to β-NaLnF4 UCNCs.24 Schurko and co-workers concluded that P63/m was the only space group compatible with their NMR data (23Na, 19F, 89Y for the inorganic part). Note that the authors only considered P63/m and P6̅2m as possible space groups; yet, historically, the confusion was between P6̅ and P63/m. The authors considered that the structural models of the β phase derived from space groups P6̅ and P6̅2m were the same, led by a recent computational study.25 In the latter, the authors concluded that the space group P6̅2m was a better candidate to represent the hexagonal structure of NaYF4. To add to the confusion, experimental data based on diffuse X-ray scattering and polarized absorption (both performed on single crystals) are in favor of the existence of P6̅ with various Ln3+

ions.21,26 The existence of the different reports attributing βNaLnF4 to each of the three space groups (P6̅, P63/m, and P6̅2m) necessitates that the structural data be interpreted without any prior assumptions by considering all space groups independent and equally probable. Because of the importance of the relationship between crystal structure and UC properties, and due to the recent strategy of enhanced photon UC efficiency based on the CF engineering, the effort initiated by Schurko and co-workers should be pursued much further. Rietveld analysis is the standard powder diffraction data analysis technique. Although widely used to characterize bulklike crystalline materials, Rietveld analysis appears to be underappreciated when dealing with nanocrystals (NCs). Although Bragg peak broadening, due to the finite size of the coherent domains (i.e., crystallite size), can be a problem, Rietveld analysis has been shown to have been successfully used to determine the average structure of well-ordered ultrasmall nanocrystals down to 4 nm.27,28 One major limitation of Rietveld analysis is that deviations from the average structure (if any) cannot be directly captured because such information is buried, between Bragg peaks, in the diffuse scattering. On the other hand, pair distribution function (PDF) analysis has emerged within the past decade as a powerful tool to tackle the nanostructure problem.29 Compared to classical XPD, PDF analysis uses both Bragg and diffuse scattering30−32 to extract structural information on the short- and long-range order of the atomic structure. For that reason, PDF analysis is of great interest to reveal deviations (if any) from the average structure. Additionally, PDF analysis is well-adapted to investigate the mechanisms governing the nucleation and growth of UCNCs. Such information is particularly relevant to understanding the formation of hexagonal UCNCs because the crystallization path ultimately controls elements of symmetry within UCNCs. In this contribution, we report on the use of synchrotronbased diffraction experiments coupled to Rietveld and PDF analyses to probe (i) the crystal structure and (ii) the formation of β-NaGdF4 doped with Yb and Er acting as the sensitizer and the activator, respectively (β-NaGdF4:Yb:Er). Our experimental data show that the crystal structure of spherical βNaGdF4:Yb:Er UCNCs, as small as 10 nm (mean diameter), is well-described by the bulk crystalline structure with limited distortions. Additionally, our Rietveld and PDF models indicate that the space group P6̅ best describes the structure of 10 nm βNaGdF4:Yb:Er UCNCs. Finally, PDF analyses provided insight into the nucleation and growth of β-NaGdF4:Yb:Er UCNCs in organic media (oleate route), which are governed by chemical reactions between molecular clusters and in situ-generated, B

DOI: 10.1021/acs.chemmater.6b04140 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. (a) Transmission electron microscopy micrograph and its size distribution histogram; (b) the raw total scattering data and its corresponding background contribution (quartz capillary with a mixture of oleic acid and octadecene); and (c) the reduced structure function F(Q) = Q[S(Q) − 1] obtained from the background subtracted diffraction pattern, where S(Q) represents the total scattering structure function, of the asprepared β-NaGdF4:Yb:Er UCNCs, synthesized by the oleate route. a.u. stands for arbitrary units.

Figure 2. Structure refinement results of the as-prepared β-NaGdF4:Yb:Er UCNCs based on bulk crystal structures related to space groups: P6̅ (a,b), P63/m (c,d), and P6̅2m (e,f). The left and right columns show the results obtained by the Rietveld and PDF analyses, respectively. For Rietveld analyses (i.e., left column), the black circles represent the experimental XPD (X-ray powder diffraction) pattern, the red lines the modeled XPD patterns, and the blue lines (shifted for clarity) the difference between experimental and modeled XPD patterns. Bragg peak positions are indicated by gray tick marks. For PDF analyses (i.e., right column), the blue circles represent the experimental PDF, the red lines the modeled PDFs, and the green lines (shifted for clarity) the difference between experimental and modeled PDFs. For all Rietveld and PDF analyses, the goodness of fit is indicated (bottom-right). Note that within a column, all graphics have been plotted with the same scales for the x and y axes so that direct visual juxtapositions (especially regarding the difference curves) are possible.

crystalline NaF at high temperature, in contrast with the low temperature mechanism usually promoted in the literature and based on the precipitation of amorphous NaLnF4. Synchrotronbased X-ray diffraction experiments coupled to Rietveld and PDF analyses, which are ubiquitous in structural characterization of nanocrystals but have never been applied to UCNCs, represent an interesting and complementary toolbox: (i) to understand the relationship between the crystal structure of UCNCs and their corresponding UC properties; and (ii) to

shed light on the formation of UCNCs in organic media. Indepth structural characterizations based on diffraction experiments pave the way toward a comprehensive understanding of size-dependent UC properties and will be of major interest when dealing with multishell architectures.

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CRYSTAL STRUCTURE OF NAGDF4:YB:ER UCNCS The crystal structures that result from space groups P6̅, P63/m, and P6̅2m are depicted in Scheme 1. Structure I (Scheme 1a) is C

DOI: 10.1021/acs.chemmater.6b04140 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials described by space group P6̅ (#174) and is characterized by the existence of three cationic sites. Two of them (Wyckoff positions 1a and 1f) are 9-fold coordinated by F− anions and form tricapped trigonal prisms. Both sites are characterized by the same symmetry (C3h). While site 1a is fully occupied by Ln3+ cations, site 1f shows occupational disorder involving Na+ and Ln3+ cations with a 1:1 ratio. The third cationic site (Wyckoff position 2h), only half occupied by Na+ cations (the other half being vacant), is 6-fold coordinated by F− anions forming an irregular octahedron with C3 symmetry. In Structure I, F− anions occupy two nonequivalent sites (Wyckoff positions 3j and 3k) both with Cs symmetry. Structure II (Scheme 1b) is described by the space group P63/m (#176) and characterized by only two cationic sites. The first one (Wyckoff position 2b), 6-fold coordinated by F− anions, is solely occupied by Na+ cations with C3i symmetry, and the corresponding site occupancy is always 0.5 or less due to steric constraints. As for Structure I, the second site (Wyckoff position 2c), 9-fold coordinated by F− anions, forms tricapped trigonal prisms with C3h symmetry where Na+ and Ln3+ cations are randomly distributed. Contrary to Structure I, F− anions occupy a single unique site (Wyckoff position 6h) with Cs symmetry. Finally, Structure III (Scheme 1c) is described by the space group P6̅2m (#189). As for Structure II, only two cationic sites exist (Wyckoff positions 1a and 2d); however, contrary to Structure II, both sites are 9-fold coordinated by F− anions (tricapped trigonal prisms). Site 1a with D3h symmetry is fully occupied by Ln3+ cations, while site 2d with C3h symmetry is occupied by Ln3+ and Na+ cations with a 3/4:1/4 ratio. F− anions occupy two nonequivalent sites (Wyckoff positions 3f and 3g) with C2v symmetry. Spherical NaGdF4:Yb:Er UCNCs were synthesized according to a previously reported method, which was slightly modified (see Supporting Information for details), 33 leading to monodisperse NCs with a mean diameter of 10.3 ± 1.2 nm (Figure 1a). Diffraction experiments were performed on the asprepared UCNCs, and the experimental data were analyzed both by the Rietveld and PDF methods. The integrated data (Figure 1b) were corrected and normalized to obtain the total scattering structure function S(Q) and the reduced structure function F(Q) = Q[S(Q) − 1]34 (Figure 1c) from which the experimental PDF (i.e., the function G(r)) was extracted (PDF analysis). All necessary information regarding data acquisition, PDF extraction, and modeling (i.e., structural refinements) are provided in the Supporting Information. The three possible space groups (Scheme 1) reported in the literature, namely, P6̅, P63/m, and P6̅2m, were successively used as the starting models to perform structural refinements. First, results obtained in the case of Rietveld refinements are presented in Figure 2 (left column). The experimental XPD pattern (black circles, left column) of the as-prepared βNaGdF4:Yb:Er UCNCs has been successively refined (red solid lines) by using the bulk-structural models derived from space groups P6̅ (Figure 2a), P63/m (Figure 2c), and P6̅2m (Figure 2e). A rapid inspection of the difference curves (blue solid lines, left column) and values of goodness of fit (RF2) clearly show differences between the three structural models. Indeed, both models that are related to space groups P6̅ and P63/m more favorably describe the crystal structure of β-NaGdF4:Yb:Er UCNCs compared to P6̅2m, which can be eliminated from consideration. Note that the latter (P6̅2m) has been proposed for the bulk counterpart under high pressure only.23 The slightly better fit with P6̅ compared to P63/m, combined with

the analysis of the refined structural parameters such as the isotropic atomic displacement parameters (Table S1, Supporting Information) show that the structural model derived from the space group P6̅ correctly describes the crystal structure of βNaGdF4:Yb:Er UCNCs in contrast with recently published results.24 Our space group identification for β-UCNCs also is well-supported by the fact that investigations related to the bulk β-phase, and performed with single crystals,21,26 identified P6̅ as being the correct space group instead of P63/m. Finally, it is important to note that the bulk crystal structure describes extremely well the crystal structure of small NCs down to 10 nm. The high quality of the Rietveld refinement, presented in Figure 2a, clearly indicates that UCNCs down to 10 nm can be considered, on average, as being bulk-like materials from a crystallographic point view. Rietveld analyses were complemented by PDF analyses first (i) to verify whether the space group identification can be confirmed and second (ii) to detect potential deviations from the average structure. The same strategy as for Rietveld analyses was adopted for PDF analyses. P6̅, P63/m, and P6̅2m space groups were successively used as the starting models to perform structural refinements (Figure 2, right column, red lines) of the experimental PDF (Figure 2, right column, blue circles). A key initial observation is that difference curves (Figure 2, right column, green lines) and values of the residual function (Rw), which are used to quantify the agreement between experimental and calculated PDFs, show differences for the three considered space groups. Once again, the structural model related to space group P6̅2m (Rw = 16%) can definitely be ruled out to describe the crystal structure of UCNCs, in perfect agreement with the Rietveld analysis. The second observation is that, similarly to Rietveld analyses, a non-negligible difference is observed between P6̅ and P63/m space groups with Rw values of 11% and 15%, respectively. To the best of our knowledge, the final Rw value of 11% is one of the best ever reported for NCs of this size.35−42 Note also that the comparison of the refined isotropic atomic displacement parameters (Table S2, Supporting Information) indicates that the structural model derived from P6̅ is more realistic compared to P63/m. As a consequence, based on both Rietveld and PDF results, it is reasonable to consider P6̅ as the most appropriate space group to describe the crystal structure of βNaGdF4:Yb:Er UCNCs. One of the interests of the PDF analysis relative to the Rietveld analysis is that the former can be performed on various length scales. To get additional information about the crystal structure of the as-prepared β-NaGdF4:Yb:Er UCNCs, PDF refinements (space group P6̅) were performed by considering the longrange order from 10 to 30 Å and the short-range order from 1.8 to 10 Å. The structural model extracted from the Rietveld refinement (Figure 2a) was used as the starting point. Results obtained from the PDF long-range order (Figure 3a) are in perfect agreement with the structural model obtained from the Rietveld refinement with an Rw value of 9.2%. Additionally, bond distances and bond angles obtained from the Rietveld and PDF long-range order refinements match very well (Figure 4a1−c1 and 4a2−c2, respectively). On the other hand, when the exact same structural model extracted from the PDF long-range order is applied to the short-range order, the lack of agreement is clearly revealed by a dramatic increase of the Rw value from 9.2% (Figure 3a) up to 19.5% (Figure 3b). Once the shortrange order is refined, the Rw value decreases to 11.2% (Figure 3c). It is important to note that the structural model obtained D

DOI: 10.1021/acs.chemmater.6b04140 Chem. Mater. XXXX, XXX, XXX−XXX

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was investigated by performing ex situ X-ray total scattering experiments. The oleate synthesis route, which has been one of the most successful techniques to prepare high quality UCNCs, was employed. This technique, first proposed by Li and Zhang,33 is based on the use of a stoichiometric amount (relative to lanthanides) of fluoride reagents that are supposed to react entirely at low temperature (50 °C) to form small amorphous NaLnF4 nuclei. Then an annealing treatment at high temperature (up to 280 °C-300 °C) is used to improve the crystallinity and uniformity of the NCs. According to the authors, no more fluoride reagents exist during the annealing treatment. Although no experimental evidence was given, the mechanism proposed by Li and Zhang has been massively promoted in the literature to the extend that it is now taken for granted. To obtain a deeper insight into the nucleation and growth of NaGdF4:Yb:Er UCNCs, a series of solutions was prepared (Scheme 2) to further perform ex situ synchrotron-based total scattering measurements to extract structural information. First, Solution A was prepared by dissolving lanthanide acetates (Ln(OAc)3.xH2O with Ln = Gd, Yb, Er) in a mixture of oleic acid (OA) and octadecene (ODE), while Solution B consisted of freshly mixed methanol solutions containing sodium hydroxide (NaOH) and ammonium fluoride (NH4F). Once Solution B was obtained, it was immediately injected into Solution A at room temperature, giving rise to Solution C. The latter was heated up to 50 °C for 30 min (Solution D), at which point methanol was eliminated under reduced pressure giving rise to Solution E. The latter was heated up to various target temperatures (Solutions E1 to E4). Note that to prepare Solutions E1 to E4, the entire process was independently repeated for each solution. Once the target temperature was reached (i.e., 175 °C, 215 °C, 250 °C, and 285 °C), the solution was immediately cooled to room temperature within a minute. All solutions identified on Scheme 2a, with the exception of solution B, were submitted to the extraction and purification process depicted on Scheme 3. For all reactive solutions (i.e., solutions C to E4), two different compounds were extracted and will be referred to as powdery precipitate and waxy compounds, respectively (left and right sides in Scheme 3). A white powdery precipitate (insoluble in nonpolar solvents) has been extracted for all reactive solutions (i.e., solutions C to E4). The biggest observed difference between the reactive solutions corresponds to the final quantity of extracted powdery precipitate. The higher the temperature is, the smaller the quantity of powdery precipitate is extracted. This difference is qualitatively highlighted with the room temperature (i.e., after cooling) photographs of solutions E1 (175 °C: 0 min) and E4 (285 °C: 0 min), as seen in Figure 5a. While Solution E1 is highly turbid, Solution E4 is optically clear. Only a very small quantity of powdery precipitate (