Direct Evidence of Significant Cation Intermixing in Upconverting Core

Article Cite This: Chem. Mater. 2017, 29, 9238-9246

pubs.acs.org/cm

Direct Evidence of Significant Cation Intermixing in Upconverting Core@Shell Nanocrystals: Toward a New Crystallochemical Model Damien Hudry,*,† Dmitry Busko,† Radian Popescu,‡ Dagmar Gerthsen,‡ A. M. Milinda Abeykoon,§ Christian Kübel,∥ Thomas Bergfeldt,⊥ and Bryce Sydney Richards*,†,# †

Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡ Laboratory of Electron Microscopy, Karlsruhe Institute of Technology, Engesserstrasse 7, D-76131 Karlsruhe, Germany § Photon Science Division, National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ⊥ Institute of Applied Materials, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany # Light Technology Institute, Karlsruhe Institute of Technology, Engesserstrasse 13, D-76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: Core@shell design represents an important class of architectures because of its capability not only to dramatically increase the absolute upconversion quantum yield (UCQY) of upconverting nanocrystals (UCNCs) but also to tune energy migration pathways. A relatively new trend toward the use of very thick optically inert shells affording significantly higher absolute UCQYs raises the question of the crystallographic and chemical characteristics of such nanocrystals (NCs). In this article, local chemical analyses performed by scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDXS) and X-ray total scattering experiments together with pair distribution function (PDF) analyses were used to probe the local chemical and structural characteristics of hexagonal β-NaGd0.78Yb0.2Er0.02F4@NaYF4 core@shell UCNCs. The investigations lead to a new crystallochemical model to describe core@shell UCNCs that considerably digresses from the commonly accepted epitaxial growth concept with sharp interfaces. The results obtained on ultrasmall (4.8 ± 0.5 nm) optically active cores (β-NaGd0.78Yb0.2Er0.02F4) surrounded by an optically inert shell (NaYF4) of tunable thickness (roughly 0, 1, 2, and 3.5 nm) clearly indicate the massive dissolution of the starting seeds and the interdiffusion of the shell element (such as Y) into the Gd/Yb/Er-containing core giving rise to the formation of a nonhomogeneous solid solution characterized by concentration gradients and the lack of sharp interfaces. Independently of the inert shell thickness, core/interface/shell architectures were observed for all synthesized UCNCs. The presented results constitute a significant step toward the comprehensive understanding of the “structure−property” relationship of upconverting core@shell architectures, which is of prime interest not only in the development of more efficient structures but also to provide new physical insights at the nanoscale to better explain upconversion (UC) properties alterations.

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guaranteed, and a comprehensive understanding of their upconversion (UC) properties is needed. RE-based UCNCs such as hexagonal ternary alkali metal fluorides (β-NaREF4) codoped with optically active ion pairs based on sensitizers (such as Yb3+, Nd3+) and activators (for example, Er3+, Ho3+, Tm3+) represent, to date, the most widely studied family of UCNCs due to the relatively high absolute

or more than a decade, rare-earth-based (RE-based) upconverting nanocrystals (UCNCs) attracted considerable interest due to their capability to convert multiple (two or more) low-energy photons into a higher-energy photon. Such a nonlinear optical phenomenon, predicted by Bloembergen in 19591 and first observed by Auzel in 1966,2 is of major interest for a wide range of potential applications impacting technological fields as various as energy harvesting, medical imaging, anticounterfeiting, solid-state lighting, and solid-state lasers.3,4 Nevertheless, the widespread penetration of UCNCs within devices of commercial interest is still far from being © 2017 American Chemical Society

Received: July 24, 2017 Revised: October 17, 2017 Published: November 2, 2017 9238

DOI: 10.1021/acs.chemmater.7b03118 Chem. Mater. 2017, 29, 9238−9246

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Chemistry of Materials

Figure 1. (a) Design of core@shell upconverting nanocrystals using a single unique synthesis of optically active β-NaGd0.78Yb0.2Er0.02F4 (βNaGdF4:Yb:Er) core protected by an optically inert β-NaYF4 shell with tunable thickness (upper left corner). The characteristic crystal structure of the β-phase (space group P6̅, 174) is shown along the c-axis (upper right corner). (b) Upconversion emission spectra (λex = 980 nm), (c) powerdependent absolute upconversion quantum yield (λex = 980 nm), and (d−f) time-dependent upconversion photoluminescence measurements of the starting β-NaGdF4:Yb:Er core (4.8 ± 0.5 nm) nanocrystals (orange) and β-NaGdF4:Yb:Er@NaYF4 core@shell nanocrystals characterized by an overall diameter [shell thickness] of 7.2 nm [1.2 nm] (core@shell-1, green), 8.9 nm [2.0 nm] (core@shell-2, red), and 11.7 nm [3.5 nm] (core@ shell-3, blue).

interest for their further successful development and deployment. In particular, the chemical and structural characteristics of core@shell UCNCs are necessary to correctly explain their UC properties and ultimately pave the way toward the rational design of more efficient upconverting architectures. The motivation for the present work was based on recently published investigations dealing with core@shell UCNCs. First, it was observed that the notion of epitaxial growth (derived from thin-film technology) is oversimplified and very often in disagreement with the corresponding published structural data. Second, recently published investigations clearly showing the correlation between the thickness of inert shell and absolute UCQY of core@shell UCNCs strongly suggest that the existence of sharp chemical and structural interfaces in core@shell UCNCs is very unlikely.14,15 Hence, we decided to further push the local chemical and structural characterization of core@shell UCNCs using single outer shell architecture with tunable thickness as a model system. The investigations are based on local chemical analyses by scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy (STEM-EDXS). These results are complemented with selected area electron diffraction (SAED) and X-ray total scattering experiments together with pair distribution function (PDF) analyses to develop a new crystallochemical model that is in good agreement with the alterations of UC properties of core@shell UCNCs reported in the literature. Results presented in this article rely on the use of an optically active core (β-NaGd0.78Yb0.2Er0.02F4 or β-NaGdF4:Yb:Er) onto which an optically inert shell (β-NaYF4) was grown (Figure 1a). The thickness of the inert shell was tuned between 0 (pure core) and 3.5 nm with steps of about 1 nm. The corresponding samples will be referred to as core (pure optically active core

upconversion quantum yield (UCQY) of their bulk microcrystalline counterpart. Compared to the latter, UCNCs exhibit a much lower UCQY (up to several orders of magnitude depending on the size of the optically active region) because of their extremely high surface-to-volume (SV) ratio. The direct consequence is that excitation energy can easily and rapidly migrate toward the surface of UCNCs where it is quenched because of surface defects and surface oscillators (stabilizing ligands, solvent molecules),5 thus resulting in nonradiative relaxation of excited-state electronic energy levels. Similar to semiconductor NCs, the synthesis of core@shell upconverting architectures was rapidly investigated and massively developed since 2007.6 Although various strategies were pursued within the last couple of years to further enhance the absolute UCQY of UCNCs, including plasmon-enhanced UC,7 crystal field engineering,8−11 and organic dye sensitized UC,12 the use of core@shell architectures remains vital. Indeed, the use of an optically inert shell wrapped around an optically active core acts as an efficient energy migration barrier leading to energy confinement and tremendous effects in terms of absolute UCQY. The simple core@shell (single shell) concept is so powerful that it was further extended in 2011 to the design and synthesis of multishell upconverting architectures leading to the fine control of energy migration (both for the excitation and relaxation) pathways within a single particle of just a few tens of nanometers.13 To date, the confinement of various sensitizer/activator pairs into different shells of a single UCNC constitutes the main pillar onto which scientists all around the world develop innovative upconverting architectures. Due to the importance of core@shell (single-, and multishell) upconverting architectures, a comprehensive understanding of their “structure−property” relationship is of critical 9239


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Figure 2. (a1−d1) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) micrographs, (a2−d2) high-resolution transmission electron microscopy (HRTEM) micrographs, and (a3−d3) the two-dimensional Fourier transform patterns of the single particle located in the center of each HRTEM images of the starting β-NaGdF4:Yb:Er core (column a1−3) nanocrystals and β-NaGdF4:Yb:Er@NaYF4 core@shell 1 (column b1−3), 2 (column c1−3), and 3 (column d1−3) nanocrystals. For each sample, size distribution histograms (calculated from at least 500 particles) are shown as overlay images of the corresponding HAADF-STEM micrographs. All histograms have been plotted with the same scale for the x-axis (in nm) so that direct visual comparisons are possible.

without inert shell), core@shell 1 (CS1), 2 (CS2), and 3 (CS3). To be rigorous and to avoid any fluctuation of the optical characteristics between the different investigated samples, the exact same active core NCs obtained from a single unique synthesis were used to grow the different core@ shell (CS1, CS2, and CS3) systems. The optical properties of the synthesized core and core@ shell samples are shown in Figure 1. For all samples the UC emission spectra (Figure 1b), power-dependent absolute UCQY evolution (Figure 1c), and time-dependent upconversion (Figure 1d,e) or downshifting (Figure 1f) photoluminescence (PL) spectra were measured. As expected, the UC efficiency increases as a function of the thickness of the inert shell. This is confirmed by both the power-dependent absolute UCQY evolution and the increased lifetime of various emitting levels. Note that such observations are in perfect agreement with the ones recently reported by Fisher et al.15 Note also that, in the present work, the best sample (CS3) exhibits a maximum absolute UCQY of 1.8% with irradiance around 700 W.cm−2. Such a UCQY value is mainly due to the size of the starting core UCNCs, which are about five times smaller compared to the one reported by Fisher et al. Interestingly, because of the relatively small size of the core@shell particles (11.7 nm), the synthesized core@shell

UCNCs (CS3) constitute a very good optical reference for the biological field. The measured optical properties of the asprepared core and core@shell UCNCs and their coherent evolution as a function of the inert shell thickness were used to confirm the quality of the samples to justify investigations of their local chemical and structural characteristics. Overview high-angle annular dark-field (HAADF)-STEM (Figure 2a1−d1) and transmission electron microscopy (TEM; Figure S8) micrographs reveal that all synthesized core and core@shell UCNCs are highly monodisperse in terms of both size and shape distributions. In a first approximation, all UCNCs can be considered as being spherical in shape. The starting core UCNCs are ultrasmall with an average diameter of 4.8 ± 0.5 nm (Figure 2a1). When the inert shell is grown on the optically active core, the overall size increases up to 7.2 ± 0.6, 8.9 ± 0.3, and finally 11.7 ± 0.4 nm leading to inert shell thicknesses of 1.2, 2.1, and 3.5 nm for CS1 (Figure 2b1), CS2 (Figure 2c1), and CS3 (Figure 2d1), respectively. Note that contrast of HAADF-STEM micrographs, which is related to the atomic number (Z) of chemical elements in the material as well as to the local thickness of the particles, makes them well-suited to distinguish the core and shell regions of core@shell structures containing atoms with different Z. Accordingly, regions consisting of heavier elements (Gd, Yb, Er) appear 9240


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Figure 3. (a1−d1) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) micrographs together with (a2−d2) the corresponding elemental composition line-profiles from energy dispersive X-ray spectroscopy (EDXS) of (a1, a2) single β-NaGdF4:Yb:Er core and (b1, b2) β-NaGdF4:Yb:Er@NaYF4 core@shell 1, (c1, c2) 2, and (d1, d2) 3 nanocrystals. Red arrows on HAADF-STEM micrographs indicate EDXS scan directions. (a3) Extracted gadolinium (□), ytterbium (▽), and erbium (◊) concentration profiles of single β-NaGdF4:Yb:Er core nanocrystals. (b3−d3) Extracted Y (blue) and lanthanides (∑31Gd, Yb, Er; orange) concentration profiles of single β-NaGdF4:Yb:Er@NaYF4 core@ shell (b3) 1, (c3) 2, and (d3) 3 nanocrystals. (a4−d4) Schematic representation showing the experimentally determined chemical compositions of various regions of (a4) single β-NaGdF4:Yb:Er core and β-NaGdF4:Yb:Er@NaYF4 core@shell (b4) 1, (c4) 2, and (d4) 3 nanocrystals.

structure as the core as demonstrated by the agreement of their FT patterns with the calculated diffraction patterns in the [001]- (Figure 2b3), [001]- (Figure 2c3), and [211]- (Figure 2d3) zone axis, respectively. Average lattice parameters are determined to be a = 6.0 Å and c = 3.5 Å (CS1), a = 6.0 Å and c = 3.5 Å (CS2), and a = 6.0 Å and c = 3.5 Å (CS3) with an accuracy of 0.1 Å, which is consistent with the lattice parameters of bulk hexagonal NaGdF4:Yb:Er (a0 = 6.02 Å and c0 = 3.60 Å)16 and bulk hexagonal NaYF4 (a1 = 5.96 Å and c1 = 3.53 Å).16 The structural analysis based on the FT of single core@shell UCNCs reveals the well-defined orientation relationship between the crystal structures of the core and shell materials. It is obvious that the shell pseudomorphically grows over the core. Because of the oriented growth and the limited resolution regarding lattice parameter determination (0.1 Å) from the experimental FTs, in depth structural characterization of core@shell UCNCs is not possible and will be subsequently pushed by using X-ray total scattering experiments (see the last part of the article). STEM-EDXS was used to investigate the average chemical composition of core and core@shell UCNCs as well as the

brighter compared to the ones made of lighter elements (Y). On HAADF-STEM micrographs, the core@shell structure is visible for all samples except for the pure core (Figure 2a1−d1). Regions of bright contrast in the HAADF-STEM image of the pure core NCs in Figure 2a1 result from particles stacked on top of each other. Another important point is that various geometrical core localizations relative to inert shell can be observed. In other words, the core is not always perfectly centered relative to its surrounding shell. High-resolution transmission electron microscopy (HRTEM) micrographs of the synthesized core and core@ shell UCNCs (Figure 2a2−d2) together with their corresponding two-dimensional Fourier transform (FT) patterns (Figure 2a3−d3) can be used to extract structural information. The core NC (Figure 2a2) is single crystalline. Its crystal structure corresponds to its bulk counterpart as revealed by the agreement between its FT pattern and the calculated diffraction pattern of the bulk hexagonal NaGdF4:Yb:Er (space group P6̅, a0 = 6.02 Å and c0 = 3.60 Å)16 structure in the [001]-zone axis (Figure 2a3). Similarly, all single core@shell NCs (Figure 2b2− d2) are single crystalline and consist entirely of the same 9241


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Figure 4. (a, b) Experimental pair distribution functions (PDFs) of β-NaGdF4:Yb:Er core (orange) nanocrystals and β-NaGdF4:Yb:Er@NaYF4 core@shell 1 (green), 2 (red), and 3 (blue) nanocrystals. (b) Focus on the short (top panel) and long (bottom panel) interatomic distance regions to highlight the regular shift of the PDF toward shorter interatomic distances as a function of the shell thickness. Dashed lines are given as guide for the eyes. PDF refinements (space group P6̅, isotropic displacement parameters) and their corresponding Rw values (bottom right corners) of (c) the starting β-NaGdF4:Yb:Er core nanocrystals, and (d−f) β-NaGdF4:Yb:Er@NaYF4 core@shell 1, 2, and 3 nanocrystals. The color circles and black solid lines represent the experimental and modeled PDFs, respectively. The difference between experimental and modeled PDFs (gray solid lines) is offset for clarity. Note that all PDF refinements are based on a single-phase model (see explanation in the text).

Yb, Er) of different regions of CS1 (Figure 3b3), CS2 (Figure 3c3), and CS3 (Figure 3d3) NCs was determined according to this method. To better visualize the results, a schematic crosssection model of each core@shell UCNC is given together with average compositions of their different regions (Figure 3b4−d4) where the Y concentration is indicated by blue lines and the sum of the Gd, Yb, and Er concentrations by orange lines. The first observation is that all synthesized core@shell NCs are characterized by the existence of an outer shell of pure NaYF4 with thicknesses of about 1, 2, and 3 nm for CS1, CS2, and CS3 UCNCs, respectively. Second, the evaluated concentration profiles clearly reveal the presence of extended interface regions between shell and core for all core@shell NCs, which are characterized by the formation of nonhomogeneous solid solutions. The latter are characterized by the generic chemical composition NaLnxY1−xF4, where Ln (Gd, Yb, and Er) atoms are statistically replaced by Y atoms. Interestingly, while the thickness of the interface regions seems to be independent of the outer shell thickness, their chemical composition is not. Indeed, when the outer shell is thinner, the Y content in the interface is lower. For instance, from data presented in Figure 3, the Y content in the interface region increases from 28 to 45, and 76 atom % (relative to the total amount of RE) for CS1, CS2, and CS3 UCNCs, respectively. Additionally, for all core@ shell UCNCs, the Y content in the corresponding interface is not constant from the outer region of the interface to its inner region clearly revealing the existence of concentration gradients within single core@shell UCNCs. The third observation indicates that the initial geometrical characteristics of the starting core UCNCs (acting as seeds for the growth of the shell) have been strongly altered for all core@shell UCNCs. Indeed, the characteristic 4.8 ± 0.5 nm initial core consisting of pure NaGdF4:Yb:Er is not visible anymore. At best, a core

elemental distributions within single particles. The quantification of EDX spectra either recorded on an ensemble of particles (average composition) or on single particles (elemental distributions) was obtained after substrate correction (details given in the Supporting Information). Within the error bars, the determined average chemical compositions are in good agreement with the formation of NaGdF4:Yb:Er core and NaGdF4:Yb:Er@ NaYF4 core@shell UCNCs (Figures S2−S5 and Table S2) and match well with compositions obtained from elemental chemical analyses (Table S3). A HAADFSTEM micrograph of a single-core NC (Figure 3a1) together with its corresponding chemical composition (Figure 3a2,3) extracted from an EDXS line-profile (red arrow in Figure 3a1) clearly reveal the formation of an homogeneous solid solution. HAADF-STEM micrographs of CS1 (Figure 3b1), CS2 (Figure 3c1), and CS3 (Figure 3d1) UCNCs clearly show the formation of a core@shell architecture with a contrast difference between the Gd/Yb/Er-containing core (brighter regions on HAADFSTEM micrographs) on one hand, and the Y-containing shell (darker regions) on the other hand. Quantitative data on the chemical composition of the shell and core regions of core@ shell UCNCs were extracted from EDXS line-profiles across single NCs (details given in the Supporting Information). It is worth mentioning that the evaluated Na, F, Y, Gd, Yb, and Er concentration profiles (Figure 3b2−d2) along a line through the center of single core@shell NCs (red arrows in Figure 3b1−d1) are averaged values along the electron-beam direction. This means that the whole volume along the electron trajectory contributes to the detected X-ray signal. However, following the procedure described in detail by Kind et al.,17 it is possible to determine the elemental composition of the core and shell regions of a single core@shell NC. The normalized composition (relative to the total content of RE, ∑41Y, Gd, 9242


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as a starting point, but the site occupancy of all crystallographic sites occupied by Gd3+, Yb3+, and Er3+ was modified by considering the formation of a solid solution Na(Gd/Yb/ Er)1−xYxF4 with a statistical distribution fixed according to the results derived from elemental analyses. Results of the corresponding structural refinements are presented in Figure 4d−f, for CS1, CS2, and CS3, respectively. Note that the exact same strategy was adopted for all structural refinements presented in Figure 4d−f, and the following parameters were refined: (1) scale factor, (2) correlated motion peak sharpening factor, (3) Q-dependent peak broadening factor, (4) lattice parameters (a and c), (5) a single isotropic atomic displacement parameter (ADP) for each atom in the structure, (6) fractional atomic coordinates when allowed by symmetry, and (7) particle size. Site occupations were not refined but fixed to average values extracted from elemental analyses. A key initial observation is that difference curves (Figure 4d−f, gray solid lines) and values of the residual function (Rw), which are used to quantify the agreement between experimental and calculated PDFs, show a reasonable agreement between calculated (Figure 4d−f, black solid lines) and experimental (Figure 4d−f, color circles) PDFs. Refined particle sizes of 7.3 (CS1), 8.7 (CS2), and 10.6 (CS3) nm are in perfect agreement with size distributions obtained from STEM (Figure 2). Refined isotropic ADPs are physically reasonable and of the same order of magnitude as the ones determined for core NCs (Table S5). The evolution of cell parameters decreasing from CS1 (a = b = 5.9967 Å, c = 3.5454 Å) to CS2 (a = b = 5.9907 Å, c = 3.5372 Å) and CS3 (a = b = 5.9825 Å, c = 3.5272 Å) compared to the starting core NCs (a = b = 6.0209 Å, c = 3.5847 Å) indicates that the whole core@shell structure is influenced by the growth of the shell. These lattice parameters agree well with the lattice parameters obtained from SAED pattern evaluation (see the Supporting Information, Table S4). The fact that cell parameters tend to be closer to those of pure NaYF4 (a = b = 5.96 Å, c = 3.53 Å) as a function of the shell thickness is a solid proof in favor of the formation of solid solutions. Nevertheless, it is important to keep in mind that the lattice parameters extracted from PDF refinements must be seen as averaged values for each core@shell structure. The synthesized core@shell structures are a very good example to the concept of modulated multishell structure introduced by Palosz and co-workers for core NCs.21 The consequence is that, to better describe core@shell UCNCs, a continuous modification of cell parameters, and hence of interatomic distances, should be considered all the way from the center of the particles toward their outer surface region. Results extracted from local chemical analyses and structural investigations can be used to propose a new crystallochemical model (Figure 5) of core@shell UCNCs synthesized by the controlled hot injection method. As a reminder, core@shell UCNCs were synthesized by first heating up core NCs at 300 °C, and then by slowly injecting the shell precursor solution containing all necessary elements (Na, F, and Y). The total number of seeds and shell precursor solution concentration are adjusted to control the final shell thickness. For instance, thick (thin) shells are obtained by decreasing (increasing) the number of seeds and increasing (decreasing) the shell precursor solution concentration. Local chemical analyses performed on single core@shell UCNCs revealed that, independently of the inert shell thickness, the overall size of the core and interface regions not only appears to be constant but also very well matches the average diameter of the seeds (core

region made of pure NaGdF4:Yb:Er is observed (CS1 and CS2) but with dramatically reduced dimensions of about 1 nm in diameter. In the case of CS3, the initial NaLnF4 core is entirely replaced by NaLnxY1−xF4 solid solution. However, the Y content in the small core region remains lower than that in the interface region. Finally, the fourth observation is related to the overall size of the core and interface regions (without the outer shell) that seems to be constant (5.0 ± 0.5 nm) for all core@ shell UCNCs. It is extremely well-correlated to the mean diameter (4.8 ± 0.5 nm) measured for the starting core UCNCs. The lack of sharp chemical interfaces in core@shell UCNCs raises an issue regarding their atomic-scale structure. Hence, the crystal structure of core and core@shell UCNCs was further investigated by X-ray total scattering experiments combined with PDF analysis.18,19 The experimental PDFs are given in Figure 4a,b (overview; and highlight of short and long interatomic distance regions; respectively). Note that the main advantage of X-ray total scattering experiments combined with PDF analysis compared to classical Rietveld analysis is that (i) size effects do not broaden PDF peaks, and (ii) deviations from the average structure can be observed. The PDF of core NCs can be used as a reference starting point regarding the structural characterization of core@shell NCs. The experimental PDF of core NCs was modeled by using the corresponding bulk hexagonal structure (space group P6̅) of NaGdF4 as starting point. As can be seen in Figure 4c, the crystal structure of the ultrasmall core NCs is very welldescribed by the bulk structure with a goodness-of-fit value (Rw) of 11.4%. This is in perfect agreement with HRTEM results previously described as well as with the recently published data on the crystal structure of 10 nm UCNCs,20 confirming that UCNCs down to about 5 nm are bulklike materials from a structural point of view. The experimental PDFs of core@shell NCs (CS1, CS2, and CS3) are all isostructural to the starting core UCNCs in agreement with results from HRTEM and SAED (see the Supporting Information). However, two noticeable differences compared to the PDF of the starting core NCs should be mentioned here. First, core@shell PDFs damping (amplitude of features at high interatomic distances), which is directly correlated to the size of the coherent domains, constantly decreases as a function of the shell thickness (Figure 4a). Such a behavior is expected because the size of the NCs increases from 4.8 (pure core) to 7.2 (CS1), 8.9 (CS2), and 11.7 (CS3) nm (see Figure 3a1−d1). As a consequence, interatomic distance correlations increase. Second, all PDF peaks are shifted toward shorter interatomic distances when the thickness of the inert shell increases (Figure 4b). Such a behavior is characteristic of the alteration of cell parameters and should not be observed in the case of pure epitaxial growth because (at least) the in-plane cell parameters of the epitaxial layer should be constrained by the substrate (core UCNCs) until the critical thickness is reached. At this point, it is worth noting that PDF (and even classical X-ray diffraction) modeling of core@shell NCs is extremely challenging because chemical gradients and interphase correlations (core−shell arrangement and associated structural constraints) are a complicated problem. PDF modeling of CS1, CS2, and CS3 NCs was performed by using a single-phase model based on results obtained from SAED patterns (see the Supporting Information, Figure S6 and Table S4). The structural model obtained for core NCs (Figure 4c) was used 9243


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used an active core surrounded by a very thick inert shell (10 nm), Chen et al. used a heavily “doped” active shell (surrounding an inert core) protected by a thin (2 nm) inert shell. A strong intermixing/interdiffusion phenomenon together with the random geometrical localization of active cores relative to their inert shells contribute to the formation of preferential paths for energy migration toward the surface of particles in the case reported by Chen et al. On the contrary, in the case reported by Johnson et al., the very thick protective shell (potentially combined to a dilution effect that still needs to be quantified) passivates all or most of energy migration pathways promoting radiative deactivation and hence contributing to a higher UC efficiency. This clearly explains the need of thick protective shells but also questions the whole core@shell decoupling theory that has been proposed in particular for multishell upconverting architectures28−30 where energy migration pathways and optical interactions might be more complicated than initially thought. In the absence of cation intermixing and solid solution formation, sharp interfaces (active versus inert regions) would be observed, and efficient confinement of optically active elements should be possible with a relatively thin inert shell of just a few nanometers (1−2). It is still premature to consider the new crystallochemical model derived from our experimental results as being universal. Nevertheless, early works not only by Alivisatos et al. on quantum dots31 (core only) but also by van Veggel et al. on lanthanide trifluorides32 (core only) demonstrated that inorganic NCs are chemically more dynamic systems compared to their bulklike counterparts. Indeed, their experimental data clearly prove that extremely fast (just a few minutes), complete, and reversible cation exchange reactions do exist at room temperature. Back in 2009, van Veggel even questioned the real nature of core@shell architectures explaining that “the chemical composition of the core nanoparticles could thus have been changed due to the cation exchange before the formation of the core-shell architecture”. The magnitude of the effects reported in this article is most likely multi-parameter-dependent. For instance, the size of the starting core NCs can change the diffusion length into which cation intermixing is significant. Indirect proofs were given by Chen et al.33 as well as by Dühnen et al.34 for 40 (NaYF4:Ce@NaYF4:Tb) and 8 (NaEuF4@NaGdF4) nm starting core NCs, respectively. While significant cation intermixing was reported for the latter case, it was considered as being nonexistent in the former case. Note that in the case of 40 nm starting core NCs, local chemical analyses were not reported, and cation intermixing cannot be completely ruled out for the first (few nanometers) outermost layers. Regarding NCs of intermediate size (≈10−25 nm), numerous examples reported by several authors show obvious signs of significant cation intermixing. This is true in the recently published contribution by Fischer et al.15 based on NaYF4:Yb:Er@NaLuF4 (starting core NCs with a mean diameter of 24 nm). Indeed, the corresponding structural data (Figure S10 in Fischer’s Supporting Information) clearly show a continuous shift of Bragg peaks toward higher 2θ angles (expected because Lu3+ is smaller than Y3+) with no sign of Bragg peaks broadening. This effect is noticeable for very thin shells down to 1.7 nm, and is then continuous for shell thickness up to 13 nm. Such a behavior is not in agreement with the epitaxial growth model for thin shells and the subsequent relaxation of constraints for thicker shells leading to the formation of a 2-phase core@shell system. Those observations combined to the fact that the absolute UCQY of

Figure 5. Crystallochemical model of a single core@shell upconverting structure derived from chemical and structural analyses. Contrary to the classical representation of core@shell upconverting nanocrystals, the model highlights the lack of sharp chemical interface (chemical gradients) as well as the existence of a continuous modification of cell parameters (apparent lattice parameters) all the way from the center of the particle toward its surface.

NCs). Additionally, the formation of nonhomogeneous solid solutions (confirmed by both EDXS and X-ray total scattering experiments) with concentration gradients was clearly identified. When the outer shell is thicker, the Y contents are higher in both the core and interface regions. All elements clearly indicate that the formation mechanism of core@shell UCNCs is absolutely not compatible with a simple epitaxial growth process of shell precursors onto the seeds. Instead, a partial seed dissolution process followed by recrystallization together with a strong intermixing/interdiffusion mechanism can better explain the observed chemical and structural characteristics of the synthesized core@shell UCNCs. Note that such an interpretation is in good agreement with recently published data regarding interdiffusion phenomena in UCNCs at both low and high temperatures.22,23 Once the reservoir of chemical elements from the initial seeds is exhausted, a “pure” inert shell starts growing (constrained by the core) until the critical thickness is reached. Note that the notion of a chemically “pure” inert shell is relative because trace levels of optically active elements into the inert shell cannot be totally excluded. Only the growth of a relatively thick inert shell can guarantee the absence or a low concentration of optically active ions near to the surface. Recently published data suggest that such a shell thickness is in the range 5−10 nm for 20 nm optically active cores. The new crystallochemical model proposed for core@ shell UCNCs (Figure 5) can explain the evolution of their UC properties as well as several behaviors recently reported in the literature. First, the loss of optically active elements due to the partial dissolution of the active core induces deviations from the optimum doping concentration leading to a lower UC efficiency. Several authors reported on the use of a much higher concentration of the sensitizer−activator pairs in active core UCNCs protected by an inert shell without concentration quenching effects.24−26 Johnson et al. even reported a maximum UC efficiency of 5.2 ± 0.3% (at 10 W cm−2) by using a 100% optically active matrix combined with an extremely thick (10 nm) inert shell (NaErF4@NaLuF4).26 Unfortunately, no local chemical and structural characterizations have been reported for such heavily “doped” core@ shell systems but will be of significant interest in the near future to better understand the concentration quenching effect in NCs compared to their bulk counterpart. It is worth pointing out that discrepancies exist between the results reported by Johnson et al.26 on one hand and Chen et al.27 on the other hand. The former observed no concentration quenching while the latter observed a quenching effect. While Johnson et al. 9244


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Chemistry of Materials core@shell NCs stabilizes for a shell thickness of ≈4 nm are in perfect agreement with our newly proposed crystallochemical model. The model is also supported by Chen et al.35 who recently reported on NaGdF4:Yb:Er@NaYF4:Yb:Nd@NaGdF4 dual-shell UCNCs (starting core NCs with an average diameter of ≈16 nm). Once again, the corresponding structural data (Figure S1 in Chen’s Supporting Information) show clear evidence of Bragg peaks shifting. Interestingly, Bragg peaks are shifted toward higher 2θ angles after the deposition of the first shell (NaYF4:Yb:Nd) while the same Bragg peaks are shifted back (intermediate positions between those of core and core@ shell) after the deposition of the second shell (NaGdF4). Note that, after each shell deposition, Bragg peaks are getting sharper indicating that (i) coherent domains are getting bigger and/or (ii) microstrains within the newly formed NCs are less important compared to their corresponding seeds). This is the expected behavior in the case of the formation of a solid solution. More importantly, STEM-HAADF micrographs (Figure 2e,f in Chen’s article) show clear evidence of dissolution. Indeed, UCNCs are smaller after the deposition of the second shell (NaGdF4) with a mean diameter of ≈22 nm (Figure 2e in Chen’s article) compared to the size of the NCs after the deposition of the first shell (NaYF4:Yb:Nd) with a mean diameter of ≈30 nm (Figure 2f in Chen’s article). Similar signs of strong cation intermixing and dissolution are also obvious for NaGdF4:Yb:Er@NaYF4 UCNCs (starting core NCs with a mean diameter of 17 nm) reported by Chen et al.36 The size of the starting core NCs decreases by a factor of 2 as made evident by the STEM-HAADF micrographs given by the authors (Figure 1a−d in Chen’s article). Additionally, elemental mapping data reported by Chen et al. 37 for NaGdF4:Yb:Ho:Ce@NaYF4:Yb:Nd (starting core NCs with a mean diameter of 18 nm) show also clear evidence of strong cation intermixing as indicated by the Ce3+ and Gd3+ distributions (Figure 1e−j in Chen’s article) although the latter were not quantified. The same behavior is strongly suspected in NaGdF4:Yb:Er@NaYF4 UCNCs (starting core NCs with mean diameters of 10, 17, 20, and 32 nm) reported by Chen and Huang.38 Unfortunately, structural and chemical data provided by the authors are not sufficient to perform a quantitative comparison based on the size of the starting cores. Finally, note that electron energy loss spectroscopy and X-ray photoelectron spectroscopy were used by Van Veggel and coworkers to characterize core@shell UCNCs.39−41 The lack of quantitative data given by the authors with nanometer-scale resolution prevents drawing any conclusion for the reported NCs. Additional to size effects, it is also legitimate to question the influence of the shell growth method on cations intermixing. Indeed, although core@shell UCNCs have been widely synthesized by the heating up or controlled hot injection methods based on the use of two different precursors (NaOH and NH4F) as sodium and fluorine sources,42 the controlled hot injection of a single-source precursor (NaOOCCF3),43 or the use of sacrificial seeds44 are now common synthetic methods to grow dual- and multishell upconverting architectures. Because formation mechanisms are dramatically different between all of those synthetic methods, we still have to figure out whether intermixing (i) is an intrinsic property of nanocrystals whose magnitude is governed by the size of the starting seeds, (ii) simply depends on the shell growth method, or (iii) is just a combination of the first two points. As a consequence, there is still a massive amount of work to be done

not only to optimize the design of core@shell upconverting architectures as a function of the targeted size regime, chemical composition, or synthetic method but also to quantify cation intermixing effect in single- and multishell architectures with different size and doping concentration. To conclude, the UC community is facing a major challenge regarding the “structure−property” relationship of complex upconverting nanoarchitectures. Results presented in this article pave the way in such a direction. Fundamental breakthroughs are needed first to better understand energy migration pathways and second to substantially improve UC efficiency. Such a basic understanding will ultimately enable assessment of the real potential of UCNCs to compete with reference materials such as quantum dots in newly emerging economical markets. There is no doubt that basic fundamental knowledge will help to rationally determine whether the hexagonal family of ternary fluoride UCNCs can massively and durably impact the industrial and commercial landscapes related to the use of luminescent nanomaterials.

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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/acs.chemmater.7b03118. Synthesis of core and core@shell nanocrystals, TEM data acquisition, synchrotron data acquisition, data modeling, and chemical analyses (PDF)

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AUTHOR INFORMATION

ORCID

Damien Hudry: 0000-0002-5167-7471 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.H. would like to thank Dr. Andrey Turshatov and Dr. Ian Howard for discussions. D.H. would like to thank Marion Lenzner for TG analyses. The authors would also like to acknowledge the financial support provided by Helmholtz Recruitment Initiative Fellowship (B.S.R.) and the Helmholtz Association’s research program Science and Technology of Nanosystems (STN). The authors would like to thank the Karlsruhe Nano Micro Facility (KNMF) for TEM access. This research used resources of the National Synchrotron Light Source II (beamline 28-ID), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract DE-SC0012704.

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REFERENCES

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Direct Evidence of Significant Cation Intermixing in Upconverting Core

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