Strain-Induced Modification of Optical Selection Rules in Lanthanide

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Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles Michael David Wisser, Maverick Chea, Yu Lin, Di Meng Wu, Wendy L Mao, Alberto Salleo, and Jennifer A. Dionne Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl504738k • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 7, 2015

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Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles Michael D. Wisser,∗,† Maverick Chea,‡ Yu Lin,¶ Di M. Wu,§ Wendy L. Mao,¶,k,⊥ Alberto Salleo,∗,# and Jennifer A. Dionne∗,# Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA, Department of Chemistry, Stanford University, Stanford, CA 94305, USA, Stanford Institute for Materials and Energy Sciences, Stanford, CA 94305, USA, Photon Science, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA, and Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA E-mail: [email protected]; [email protected]; [email protected]

∗

To whom correspondence should be addressed Dept. of Materials Science and Engineering, Stanford University ‡ Dept. of Chemical Engineering, Stanford University ¶ Dept. of Geological and Environmental Sciences, Stanford University § Dept. of Chemistry, Stanford University k Stanford Institute for Materials and Energy Sciences ⊥ Photon Science, SLAC National Accelerator Laboratory # Dept. of Materials Science and Engineering, Stanford University †

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KEYWORDS: upconversion, lanthanides, optical selection rules, excited-state lifetimes, diamond anvil cell, crystal field theory Abstract NaYF4 :Yb3+ ,Er3+ nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. While hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly twofold increase in efficiency. In-situ x-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.

Advances in the field of upconversion 1–4 as well as increased demand for innovation in the realms of bioimaging 2,3,5–10 and solar energy generation 4,11–18 have led to significant interest in and enthusiasm for the development of high-efficiency upconverting materials. Upconversion (UC) is the process of converting low-energy photons into higher-energy photons. 19 To date, most work has focused on bimolecular systems 20 - in which UC is accomplished by pairs of organic compounds which undergo triplet-triplet annihilation - and lanthanide-doped materials, 1,2 wherein trivalent lanthanide ions are multiply excited via sequential absorption and energy transfer events before decaying radiatively. While bimolecular upconverters primarily absorb and emit visible-frequency photons, 20 lanthanide-doped systems are capable of absorbing near-infrared (NIR) light and emitting visible photons. This NIR-to-visible UC capability makes lanthanide-based systems not only well-matched to current solar cell technologies 11,12 but also particularly well-suited to biological applications 3 due to the ability of NIR light to penetrate tissue more deeply than that of other wavelength regimes. 2 ACS Paragon Plus Environment

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Despite their promise as NIR upconverters, lanthanide-based systems have historically been plagued by low quantum efficiencies. For example, hexagonal-phase NaYF4 doped with Yb3+ and Er3+ exhibits one of the best efficiencies to date for nanostructured systems at 0.3%. 1,21,22 While several factors contribute to this low efficiency, two of the most detrimental are the Laporte-forbidden nature of the constituent transitions as well as the high probability of nonradiative quenching via coupling to phonon modes of the host lattice. 1,2,23,24 The forbidden nature of the UC transitions can be overcome by carefully engineering the crystal field environments of the lanthanide ions. 25 Briefly, the Laporte (or parity) selection rule states that electric dipole transitions which maintain parity cannot occur. 26 However, this rule is only strictly applicable to states of definite parity (i.e. those belonging to a centrosymmetric point group), meaning that it can be relaxed by altering the symmetry of the electronic states such that they do not have definite parity. Because the states involved in lanthanide UC are molecular orbitals formed by the superposition of the atomic orbitals of the lanthanide ions and those of the host matrix ions, this symmetry modulation can be accomplished by manipulating the geometry of the host lattice. Relaxing this selection rule could lead to more probable transitions, increased radiative rate constants, and, with careful control and optimization, improved UC quantum efficiency. 27,28 In parallel, quenching by lattice phonons can be mitigated by increasing the rate of energy transfer between Yb3+ and Er3+ ions. Because phonon-assisted nonradiative relaxation directly competes with the desired Yb-Er energy transfer steps, decreasing the ion separation can increase the rate of energy transfer. 19,23,29 In other words, tuning the separation between nearby Yb3+ and Er3+ ions can reduce the probability of phonon quenching and, like modifying the parity of the electronic states, represents a promising avenue toward increasing the efficiency of lanthanide-based upconverters. Here, we report relaxation of the Laporte selection rule and simultaneous modification of 3 ACS Paragon Plus Environment

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the interionic distances in the NaYF4 :Yb,Er UC system. Such control is accomplished by using mechanical compression to tune the geometry of the host NaYF4 matrix in a systematic and continuous fashion. To help separate the effects of the two factors, we study both the cubic (α) and hexagonal (β) phases of NaYF4 . We choose NaYF4 not only for its historical use as a host material for lanthanide-based UC, 1,30 but also because one of its two phases (the α phase) coordinates dopant lanthanide ions in a centrosymmetric geometry. Therefore, compressing the α-NaYF4 lattice will affect both the parity of the orbitals as well as the separations between Yb3+ and Er3+ ions, while compressing the β lattice will primarily influence the latter. By comparing the induced changes in UC emission and excited-state lifetimes as the two materials are compressed, we can qualitatively deduce the relative impacts of parity distortion and interionic distance reduction. Correspondingly, in situ pressure-dependent x-ray diffraction (XRD) measurements enable quantitative understanding of the manner in which both host lattice geometries distort under compression. Together, these experiments provide insight into the complex relationship between UC characteristics and host matrix geometry in the NaYF4 :Yb,Er system. Assemblies of α-NaYF4 nanoparticles doped with 20 at. % Yb3+ and 2 at. % Er3+ were synthesized using the procedure outlined by Li et al. 31 β-phase NaYF4 nanoparticles doped with 18 at. % Yb3+ and 2 at. % Er3+ were colloidally synthesized following a separate procedure described by Li and Zhang. 32 We choose to study nanoparticles for their technological importance; the chosen doping ratios have been shown to yield the most efficient UC in nanoparticles of these materials systems. 1 The synthesized α-phase and β-phase nanoparticles are approximately 200 nm and 90 nm in diameter, respectively, as demonstrated by the electron micrographs in Figure 1. While absolute UC efficiency is known to vary to some degree with particle size, 21,33,34 our studies suggest that the results described herein are size-independent at least to scales as small as 25 nm (see supplemental information). XRD patterns for both 4 ACS Paragon Plus Environment

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Figure 1: NaYF4 structure and characterization a) Illustration of the experimental approach used in this work. Upconverting nanoparticles (UCNPs) are inserted into the sample volume (in addition to a ruby used for pressure calibration) of the diamond anvil cell and compressed. The particles are then illuminated with 980-nm NIR light or synchrotron x-rays and UC emission spectra and excited-state lifetimes or XRD patterns, respectively, are collected as a function of pressure. b) Transmission electron microscope (TEM) image displaying representative assemblies of α-NaYF4 :Yb,Er nanoparticles and (inset) an increased-magnification micrograph showing a typical assembly in greater detail. c) Schematic depiction of the α-NaYF4 unit cell; all edges are of length a. Faces and vertices are occupied by Na+ and Y3+ in a 1:1 ratio while the F− ions fill the tetrahedral interstitial sites. d) TEM micrograph showing typical β-phase NaYF4 :Yb,Er particles as well as (inset) a zoomed-in view of a single nanoparticle. e) Unit cell of β-NaYF4 ; basal edges are of length a and the height is defined as c. One internal site is occupied by Na+ and Y3+ (1:1) while the second contains a Na+ ion. Y3+ and F− ions are located at the cell vertices and interstices, respectively.


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materials were obtained to confirm sample phase purity by comparison to International Centre for Diffraction Data (ICDD) standards; these patterns alongside the corresponding standards are included in the supplemental information (SI). A diamond anvil cell (DAC) was used to access pressure regimes as high as 25 GPa. All measurements were performed using a silicone oil pressure medium and diamond culets 500 µm in diameter. A Ti:sapphire laser operating in continuous wave mode at 980 nm was used to excite the particles within the DAC as the pressure was incrementally increased to 25 GPa before being subsequently decreased. At each pressure, UC emission from the particles was collected and analyzed with a spectrometer while an avalanche photodiode coupled to a multichannel scaler was used to determine lifetimes. Synchrotron radiation was used to perform in situ XRD over the same range of pressures. Further details regarding each experimental procedure can be found in the SI. The structures of α-NaYF4 and β-NaYF4 are shown in Figures 1c and 1e, respectively. Due to the similarity of their ionic radii and equivalence of their valence states, it is generally assumed that the dopant lanthanide ions occupy the Y3+ sites in both structures. 22,30,35 As such, the dopant ions are coordinated by eight F− ions in a simple cubic geometry in the α phase and by nine F− ions in a trigonal tricapped prismatic geometry in the β phase of NaYF4 . Nominally, the former arrangement is centrosymmetric while the latter is not, contributing to the observation that the β-phase lattice yields 10x more efficient Yb-Er UC than does the α material. 1,35 However, in both host lattices the complexes are in actuality slightly distorted from these perfect geometries due to the random occupation of cation sites by Na+ and Y3+ . 22,35 Notably, Y-F bonds are slightly shorter and Na-F bonds are slightly longer than the average cation-F bond length of the lattice, yielding a local geometric distortion and thus a symmetry reduction. We hypothesized that this symmetry reduction could be enhanced by hydrostatically compressing the matrix to reduce the distance between lanthanide and host ions. For the α-phase particles, increasing the strength of the crystal field would continuously distort the orbitals to pronounced 6 ACS Paragon Plus Environment

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non-centrosymmetry. The β-phase particles, which already lack definite parity, effectively serve as a control experiment in this regard. Figure 2 illustrates the influence of lattice compression on the UC spectra and lifetimes of the α-NaYF4 :Yb,Er particles. The nanoparticles were compressed to a maximum of 24.3 GPa before the pressure was decreased in finite steps back to 0.2 GPa. Figure 2a depicts how the probability of each emissive transition is affected throughout the pressure cycle (see SI for an illustration of the involved transitions), while Figure 2b shows the relationship between integrated emission intensity and applied pressure. An increase in UC emission intensity is observed at low pressures, reaching an experimental maximum at a pressure of 2.1 GPa before additional pressure engenders a sharp emission decrease. The maximum emission corresponds to an enhancement of 1.7x compared to the initial data point collected. It is worth noting, however, that due to practical considerations associated with loading the sample into the DAC, the initial pressure studied is 0.4 GPa rather than ambient pressure (approximately 10−4 GPa). As such, it is likely that 1.7x is an underestimate of the actual enhancement obtained with respect to α-NaYF4 :Yb,Er at ambient conditions. Also of interest are the precise peak positions within the UC emission spectrum. Figure 2c presents this information for the 4 F9/2 →4 I15/2 transition, where the intensity of each spectrum (denoted by its color) is expressed as a function of pressure and wavelength. Each spectrum is normalized so as to make the peak energy shifts more apparent. The positions of the individual peaks within the 4 F9/2 manifold (the two strongest initially occurring at 652 and 670 nm) redshift (to 652.4 and 671.2 nm, respectively) as pressure is increased from 0.5 to 7.0 GPa. As the peak energies result directly from the crystal field environment of the emitting ion, 22,28 these shifts demonstrate the utility of using mechanical compression for crystal field engineering. Note that data above 7.0 GPa are omitted as a pressure-induced phase transition is observed (and will be discussed shortly); data for the 4 S3/2 emission manifold are included in the SI. 7 ACS Paragon Plus Environment

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Figure 2: α-NaYF4 :Yb,Er UC emission and lifetimes a) UC emission upon excitation with 980-nm light as pressure is increased. Spectra obtained at points during the subsequent pressure relaxation are omitted for clarity (see SI). The emitting states which correspond to the three radiative relaxation pathways are labeled; for each, the final state is 4 I15/2 . b) Integrated emission intensity summed over all three transitions as the pressure inside the DAC is first increased (solid circles) then released (open circles); guides to the eye (solid line, dashed line) are included to emphasize the trends. c) Color map depicting the spectral dependence of emission from the 4 F9/2 manifold on pressure. Each spectrum is normalized to highlight shifts in peak position. Intensity values are given base 10. d) Lifetime values with increasing (solid circles) and decreasing (open circles) pressure for the 2 H11/2 , 4 S3/2 , and 4 F9/2 emitting states. As before, guides (solid and dashed lines) highlight the pressure-induced behavior. Intensity-time traces for each of the transitions can be found in the SI. 8 ACS Paragon Plus Environment

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In addition to UC emission spectra, excited-state lifetimes were also collected throughout a range of applied pressures. These lifetimes encompass both radiative and nonradiative relaxation pathways 36 and cannot be quantitatively decomposed into individual contributions without knowledge of the quantum yield of the material at each pressure. However, simultaneously studying how both the UC emission intensity and lifetimes are affected by pressure enables qualitative elucidation of which rate component is responsible for the observed trends. For example, a lifetime reduction accompanied by enhanced UC emission signifies an increased radiative rate. Conversely, a lifetime reduction accompanied by decreased UC emission implies an increased nonradiative rate. Figure 2d depicts the excited-state lifetime values (herein defined as the time at which the UC luminescence intensity falls to half of its initial value) from the three emitting manifolds as the lattice is compressed and decompressed. Increasing pressure is seen to yield a steady and substantial decrease in the lifetime of each of the three emitting states. Because the vast majority of emitters in α-NaYF4 :Yb,Er relax nonradiatively (as evidenced by the very low UC efficiency of the material), the lifetimes measured are likely dominated by the nonradiative component. That said, the concurrent decrease in excited-state lifetimes and increase in UC emission intensity demonstrate that the radiative lifetime must be reduced. 36 This result is, to our knowledge, the first successful dynamic modification of the Laporte selection rule in any UC system. To determine the lattice distortion necessary to achieve the observed selection rule modification, we collected in situ pressure-dependent XRD patterns from the assemblies; these data are shown in Figure 3. As pressure is increased, the peaks in the diffraction patterns (Figure 3a) shift to larger values of 2θ, indicating a reduction in the interplanar spacings of the crystals. Additionally, as shown in Figure 3b, each of the diffraction peaks broadens substantially with pressure application. Because peak width is related to the periodicity of interplanar separations 9 ACS Paragon Plus Environment

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for the corresponding set of planes, this broadening suggests the introduction of non-uniform strain, or, equivalently, lattice defects. 37–39 The increase in width occurs rapidly (compressing to 5 GPa yields at least a twofold width increase in all peaks), and much of the induced width remains upon complete pressure relaxation. This increase in peak width reveals the onset of deformation even at low pressures, and its irreversibility confirms this deformation to be plastic. Also noteworthy is the appearance of additional reflections beginning at 13 GPa and becoming more pronounced with further compression, suggesting a pressure-induced phase transition. This same transition was observed in a bulk powder of undoped α-NaYF4 by Grzechnik et al., who concluded that α-NaYF4 transforms at high pressures into a polymorph structurally related to but distinct from unperturbed β-NaYF4 . 40 Figure 3c depicts the manner in which the α-NaYF4 lattice constant evolves throughout the cycle. As shown, the lattice parameter is reduced by approximately 6% at the maximum pressure (23.8 GPa), and at 2.1 GPa (the value corresponding to the maximum integrated UC emission achieved experimentally), it is only 1% less than at ambient conditions. This magnitude of lattice modulation is certainly attainable through doping with a lanthanide of ionic radius slightly smaller than that of Y3+ . 41 Combining the observations of the pressure-dependent XRD with the UC emission intensity and lifetime trends, we can deduce a comprehensive account of the effects of hydrostatic compression. At low pressures (i.e. < 5 GPa), we observe an emission enhancement, lifetime reduction, and irreversible XRD peak broadening. The peak broadening indicates the occurrence of plastic deformation, while the concurrent emission increase and lifetime decrease reveal a selection rule relaxation. At intermediate pressures (i.e. 5-12 GPa), further XRD peak broadening and lifetime reduction occur, but the UC emission falls dramatically. These UC observations reveal that the dominant effect in this pressure regime is a shortening of the nonradiative component of the excited-state lifetime, leading to increased phonon decay and reduced emission. At pressures above 12 GPa, the α-phase particles begin to transform into a high-pressure poly10 ACS Paragon Plus Environment

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Figure 3: α-NaYF4 :Yb,Er XRD data a) XRD spectra (λ = 0.539 Å) revealing how the structure and phase of the α-phase sample is affected by pressure application. The patterns are arranged in order (from bottom to top) of increasing to the maximum pressure studied (23.8 GPa) before subsequently relaxing back to ambient conditions. Reflections from α-NaYF4 are indexed using the standard ICDD 013-7404, and peaks associated with the high-pressure polymorph are denoted with solid triangles. b) Full widths at half the maximum intensity (FWHMs) of XRD peaks; data beyond 11.7 GPa and all data for the (222) reflection are not included due to the phase transition and low intensity, respectively, preventing accurate width determination. Peaks are fit to Gaussian functions, and guides to the eye (solid lines) serve to emphasize the overall evolution of peak width. c) Percent strain in the α-NaYF4 lattice (defined relative to the lattice parameter at 0.22 GPa) as pressure is increased (solid circles) and decreased (open circles). A guide to the eye (solid line) is included for the compression data. The decompression data also fall along this curve, highlighting the elasticity of the sample fraction which does not undergo a phase transition. The lattice constant was calculated using the (111) reflection of the α phase, the only one distinguishable at all pressures.


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morph. Upon pressure release, elastic relaxation occurs; the observation that UC emission increases as the nanoparticles are decompressed indicates the deleterious effect of elastic deformation. Ultimately, we conclude that some plastic deformation (i.e. that engendered by 2 GPa of hydrostatic pressure) yields a beneficial selection rule relaxation which outweighs the effects of crystal damage in terms of UC emission, but that the deformation induced by further compression produces a sharp reduction in UC efficiency, likely resulting from defect formation and increased phonon mode energies (see SI for a discussion of the pressure dependence of phonon modes). The UC intensity and lifetimes measured in β-NaYF4 :Yb,Er particles are shown in Figure 4. In contrast to the α-phase particles, these nanoparticles exhibit UC intensities nearly 10x stronger, while the lifetimes are approximately 10x longer. This increased lifetime highlights the reduced contribution of fast nonradiative decay channels (i.e. phonon modes) to the overall lifetime. Although we do not anticipate any substantial efficiency improvement by affecting the orbital symmetry, hydrostatically compressing the β-phase particles could increase the strength of the interactions between lanthanide and host ions; it will also certainly affect the lanthanide interionic separations. Modifying either of these parameters will likely yield changes in UC intensity and lifetimes. Indeed, with increasing pressure, the β-phase particles exhibit a monotonic decrease in UC emission, shown in Figure 4a. The excited-state lifetimes of the UC transitions in β-NaYF4 :Yb,Er show a similar trend. Figure 4c shows the evolution of the UC emission energies with pressure for the 4 S3/2 →4 I15/2 transition (see SI for data corresponding to the 4 F9/2 →4 I15/2 transition). The energies again show significant pressure dependence which, due to the absence of a phase transition in the β-phase particles, can be monitored throughout the full range of pressures studied. The highest energy peak in the cluster, initially at 539 nm, can be seen to blueshift by 1 nm as the applied pressure reaches 27 GPa. The next most-energetic peak blueshifts by 0.4 nm, while the next 12 ACS Paragon Plus Environment

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Figure 4: β-NaYF4 :Yb,Er UC emission and lifetimes a) UC emission from β-phase particles showing only spectra obtained with increasing pressure for clarity. The final state for each of the three transitions is 4 I15/2 . b) Integrated emission including pressure-increasing (solid circles) and pressure-decreasing data (open circles); guides to the eye (solid line and dashed line, respectively) are included. c) Color map depicting the manner in which peaks within the 4 S3/2 manifold change in energy with pressure application. Spectra are normalized to highlight these energy shifts, all of which are entirely reversible. d) Excited-state lifetimes measured in the β-phase particles for each of the three emitting states. Experimental data (solid and open circles) with corresponding guides to the eye (solid and dashed lines, respectively) are shown. Intensity-time traces at each pressure studied are included in the SI. 13 ACS Paragon Plus Environment

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remains constant. For lower energies, each successive peak is observed to redshift by an increasing amount, with the lowest-energy peak in the cluster shifting by as much as 4 nm. In short, the peak cluster broadens in energy, as does the 4 F9/2 group of peaks (see SI), revealing the inhomogeneity of the manner in which specific microstate energies are influenced. This inhomogeneity is desirable in the context of modifying the strength of the crystal field and therefore the radiative rates of the system. 2,27 And yet, only an emission decrease is observed. As confirmed via XRD, the beneficial changes to the radiative rate are outweighed by an increase in nonradiative relaxation engendered by defect formation. Accordingly, synthetic routes toward lattice compression may prove more beneficial toward enhanced UC emission.

Figure 5: XRD characterization of β-NaYF4 :Yb,Er nanoparticles a) XRD patterns obtained at several different pressures using x-rays of wavelength λ = 0.539 Å. The spectra are arranged (from bottom to top) in order of increasing pressure until the maximum (23.9 GPa) is reached, above which the pattern corresponding to the pressure-released sample is shown. Indices of higher-order peaks are omitted for clarity. b) FWHMs for each of the first seven lowest-order reflections as pressure is increased (solid circles) then released (open circles). The XRD peaks are approximated as Gaussian functions, and guides to the eye (solid lines) are again included. c) Percent strain using the two β-phase lattice parameters (defined relative to the values measured at 0.27 GPa) with increasing (solid circles) and decreasing (open circles) pressure; the evolution of the lattice parameters are highlighted with guides to the eye (solid lines). The lattice constants were determined by averaging the values given by the seven lowest-order reflections, which remain distinct at all pressures studied. 14 ACS Paragon Plus Environment

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Figure 5 shows pressure-dependent XRD patterns for these β-phase nanoparticles. Similar to those of the α-phase sample, peaks in the diffraction spectra (Figure 5a) shift to higher values of 2θ and, as shown in Figure 5b, broaden with increasing pressure. Unlike the αphase sample, the β-NaYF4 nanoparticles do not undergo a phase transition and the reflections revert to their initial values upon pressure relaxation; the absence of a phase transformation is consistent with the findings of Grzechnik et al. for undoped β-NaYF4 . 42 Additionally, much of the pressure-induced peak width increase in the β-NaYF4 :Yb,Er XRD spectra is lost upon pressure relaxation, suggesting little irreversible large-scale deformation and indicating that the broadening was likely due to elastic, non-uniform strain. That said, the large hystereses in UC emission and lifetimes demonstrate that some lasting damage is done to the material throughout the cycle. We hypothesize that this irreversibility results from localized point defects precipitated by pressure application. Point defects yield only short-range deviations from the perfect crystal lattice which typically do not cause significant peak broadening in XRD spectra, thus explaining the reversibility in peak breadth. Ions in the vicinity of these pressure-induced defects are likely quenched nonradiatively, yielding the net hysteresis in UC emission intensity and lifetime. Conversely, Er3+ ions sufficiently isolated from any pressure-induced defects (or, equivalently, those which experience a local reversible lattice compression) can still emit radiatively. The reversibility of the UC emission peak shifts (see Figure 4c) therefore arises from the fact that only those ions which experience reversible changes to the surrounding crystal field environments emit radiatively upon pressure relaxation. It is also worth noting that these emission peak energies are extremely sensitive to the local environments of the emitting ions; therefore, the local lattice geometry, like the overall structure, varies purely elastically. The two hexagonal lattice parameters can be extracted from the XRD spectra; these are shown in Figure 5c. Similar to that of the α phase, the lattice constants in the β-phase matrix change by roughly 5% at the maximum pressure, though 15 ACS Paragon Plus Environment

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at a 1% reduction the integrated UC intensity has already fallen to half of its initial value. In the β-NaYF4 :Yb,Er nanoparticles especially, this result highlights just how sensitive Yb,Er-based UC systems can be to the structure of the host material. Finally, in the context of the insight provided by the XRD and UC measurements conducted for both materials, we can comment on the respective influences of orbital parity modification and interionic distance reduction. Recall that the crystal field environments experienced by lanthanide ions in unperturbed α-NaYF4 are centrosymmetric (or very nearly so given the cation-F− bond length variation), so both an interionic distance decrease and a relaxation of the Laporte selection rule are likely products of lattice compression. In contrast, the β-NaYF4 lattice accommodates the dopant Yb3+ and Er3+ ions in non-centrosymmetric environments even at ambient conditions; therefore, the main parameter modified with hydrostatic pressure is the separation between ions. Due to the cation disorder depicted in Figures 1c and 1e, we cannot say with certainty that the average lanthanide interionic distances afforded by the two matrices evolve identically. However, the minimum possible separation in each can be extracted from the corresponding lattice parameter(s) and subsequently plotted (see Figure SI9). The plot shows a surprising consistency between the two NaYF4 phases, suggesting that the average lanthanidelanthanide distances in α- and β-NaYF4 are quite similar. We therefore conclude that reducing the interionic distances of the lanthanides in both lattices has relatively little effect while engineering the crystal fields can significantly alter UC emission. Similar crystal field engineering without introduction of defects (i.e. through colloidal synthetic techniques) could yield even greater improvements in both phases. In summary, we have successfully achieved an approximately twofold enhancement in UC emission from α-NaYF4 :Yb,Er nanoparticles by manipulating the host lattice structure to relax the Laporte selection rule and increase the radiative rates of the involved electronic transitions. In situ pressure-dependent XRD revealed that only a 1% reduction in lattice constant 16 ACS Paragon Plus Environment

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is necessary for such enhancement, suggesting the feasibility of reproduction through a synthetic approach. Performing the same experiments with β-NaYF4 upconverting nanoparticles yielded only an efficiency decrease, though the emission peak energies exhibited pressuredependent shifts indicative of crystal field modification. Such decreased UC efficiency in β-NaYF4 nanoparticles likely arises from an increase in nonradiative relaxation caused by pressure-induced defects and/or higher-energy lattice phonon modes. From the markedly different trends in UC emission intensity exhibited by the α- and β-phase particles, we conclude that engineering the crystal fields to relax the Laporte selection rule is a much more impactful and promising approach to improving the NaYF4 :Yb,Er system than is reducing the lanthanide interionic distances. Work is underway to gain a greater understanding of how the β-NaYF4 :Yb,Er system is influenced by lattice modulation toward ultimately achieving an enhancement similar to or greater than that demonstrated in the α-phase. Our approach represents a novel method of exploring the nuanced structure-property relations in lanthanide-based upconverters, and further research may lead to record upconversion efficiencies through the elucidation of the ideal host lattice for Yb,Er-based systems.

Acknowledgement The authors gratefully acknowledge the assistance of and discussions with Tarun Narayan, Ashwin Atre, and Greyson Christoforo. XRD measurements were performed at The Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Funding for this research was provided by the Global Climate and Energy Project (GCEP) at Stanford University, by the Department of Energy under Contracts No. DE-EE0005331 and No. DE-AC02-76SF00515, and by Stanford’s TomKat Center for Sustainable Energy.


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Supporting Information Available Experimental procedures and equipment used, ancillary data, and further discussion of observations and hypotheses. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Wang, F.; Liu, X. Chem. Soc. Rev. 2009, 38. (2) Chen, G.; Qiu, H.; Prasad, P. N.; Chen, X. Chem. Rev. 2014, 114. (3) Zhou, J.; Liu, Z.; Li, F. Chem. Soc. Rev. 2011, 41. (4) Strümpel, C.; McCann, M.; Beaucarne, G.; Arkhipov, V.; Slaoui, A.; Švrˇcek, V.; del Cañizo, C.; Tobias, I. Sol. Energ. Mat. Sol. C. 2007, 91. (5) Carpenter, C. M.; Sun, C.; Pratx, G.; Liu, H.; Cheng, Z.; Xing, L. Opt. Express 2012, 20. (6) Xiong, L.; Chen, Z.; Tian, Q.; Cao, T.; Xu, C.; Li, F. Anal. Chem. 2009, 81. (7) Naczynski, D. J.; Tan, M. C.; Zevon, M.; Wall, B.; Kohl, J.; Kulesa, A.; Chen, S.; Roth, C. M.; Riman, R. E.; Moghe, P. V. Nat. Commun. 2013, 4. (8) Liu, C.; Gao, Z.; Zeng, J.; Hou, Y.; Fang, F.; Li, Y.; Qiao, R.; Shen, L.; Lei, H.; Yang, W.; Gao, M. ACS Nano 2013, 7. (9) Liu, Q.; Yang, T.; Feng, W.; Li, F. J. Am. Chem. Soc. 2012, 134. (10) Idris, N. M.; Gnanasammandhan, M. K.; Zhang, J.; Ho, P. C.; Mahendran, R.; Zhang, Y. Nat. Med. 2012, 18.


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(11) Atre, A. C.; Dionne, J. A. J. Appl. Phys. 2011, 110. (12) Briggs, J. A.; Atre, A. C.; Dionne, J. A. J. Appl. Phys. 2013, 113. (13) Wang, H.-Q.; Stubhan, T.; Osvet, A.; Litzov, I.; Brabec, C. J. Sol. Energ. Mat. Sol. C. 2012, 105. (14) Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Nat. Photonics 2012, 6. (15) Nattestad, A.; Cheng, Y. Y.; MacQueen, R. W.; Schulze, T. F.; Thompson, F. W.; Mozer, A. J.; Fückel, B.; Khoury, T.; Crossley, M. J.; Lips, K.; Wallace, G. G.; Schmidt, T. W. J. Phys. Chem. Lett. 2013, 4. (16) Baluschev, S.; Miteva, T.; Yakutkin, V.; Nelles, G.; Yasuda, A.; Wegner, G. Phys. Rev. Lett. 2006, 97. (17) Shalav, A.; Richards, B. S.; Green, M. A. Sol. Energ. Mat. Sol. C. 2007, 91. (18) Wang, H.-Q.; Batentschuk, M.; Osvet, A.; Pinna, L.; Brabec, C. J. Adv. Mater. 2011, 23. (19) Auzel, F. Chem. Rev. 2004, 104. (20) Singh-Rachford, T. N.; Castellano, F. N. Coord. Chem. Rev. 2010, 254. (21) Boyer, J.-C.; van Veggel, F. C. J. M. Nanoscale 2010, 2. (22) Renero-Lecuna, C.; Martín-Rodríguez, R.; Valiente, R.; González, J.; Rodríguez, F.; Krämer, K. W.; Güdel, H. U. Chem. Mater. 2011, 23. (23) Chan, E. M.; Gargas, D. J.; Schuck, P. J.; Milliron, D. J. J. Phys. Chem. B 2012, 116.


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(24) Saboktakin, M.; Ye, X.; Chettiar, U. K.; Engheta, N.; Murray, C. B.; Kagan, C. R. ACS Nano 2013, 7. (25) Brunold, T. C.; Güdel, H. U. In Inorganic Structure and Spectroscopy; Solomon, E. I., Lever, A. B. P., Eds.; Wiley-Interscience: New York, USA, 1999; pp 259 – 306. (26) Laporte, O.; Meggers, W. F. J. Opt. Soc. Am. 1925, 11. (27) Yu, H.; Cao, W.; Huang, Q.; Ma, E.; Zhang, X.; Yu, J. J. Solid State Chem. 2013, 207. (28) Werts, M. H. V. Sci. Prog. 2005, 88. (29) Wang, J.; Deng, R.; MacDonald, M. A.; Chen, B.; Yuan, J.; Wang, F.; Chi, D.; Hor, T. S. A.; Zhang, P.; Liu, G.; Han, Y.; Liu, X. Nat. Mater. 2014, 13. (30) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Nature 2010, 463. (31) Li, Z.; Wang, Z.; Wang, L.; Qian, H. Cryst. Eng. Comm. 2011, 13. (32) Li, Z.; Zhang, Y. Nanotechnology 2008, 19. (33) Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Altoe, M. V. P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D. J.; Cohen, B. E.; Schuck, P. J. Nat. Nanotechnol. 2014, 9. (34) Yuan, D.; Tan, M. C.; Riman, R. E.; Chow, G. M. J. Phys. Chem. C 2013, 117. (35) Aebischer, A.; Hostettler, M.; Hauser, J.; Krämer, K.; Weber, T.; Güdel, H. U.; Bürgi, H.B. Angew. Chem. Int. Ed. 2006, 45. (36) Berezin, M. Y.; Achilefu, S. Chem. Rev. 2010, 110. 20 ACS Paragon Plus Environment

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(37) Chen, B.; Lutker, K.; Raju, S. V.; J. Yan, W. K.; Lei, J.; Yang, S.; Wenk, H.-R.; Mao, H.; Williams, Q. Science 2012, 338. (38) Zak, A. K.; Majid, W. H. A.; Abrishami, M. E.; Yousefi, R. Solid State Sci. 2011, 13. (39) Mote, V. D.; Purushotham, Y.; Dole, B. N. J. Theor. Appl. Phys. 2012, 6. (40) Grzechnik, A.; Bouvier, P.; Crichton, W. A.; Farina, L.; Köhler, J. Solid State Sci. 2002, 4. (41) Xiang, G.; Zhang, J.; Hao, Z.; Zhang, X.; Luo, Y.; Lü, S.; Zhao, H. CrystEngComm 2014, 16. (42) Grzechnik, A.; Bouvier, P.; Mezouar, M.; Mathews, M. D.; Tyagi, A. K.; Köhler, J. J. Solid State Chem. 2002, 165.


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Figure 1: NaYF4 structure and characterization a) Illustration of the experimental approach used in this work. Upconverting nanoparticles (UCNPs) are inserted into the sample volume (in addition to a ruby used for pressure calibration) of the diamond anvil cell and compressed. The particles are then illuminated with 980-nm NIR light or synchrotron x-rays and UC emission spectra and excited-state lifetimes or XRD patterns, respectively, are collected as a function of pressure. b) Transmission electron microscope (TEM) image displaying representative assemblies of αNaYF4:Yb,Er nanoparticles and (inset) an increased-magnification micrograph showing a typical assembly in greater detail. c) Schematic depiction of the α-NaYF4 unit cell; all edges are of length a. Faces and vertices are occupied by Na+ and Y3+ in a 1:1 ratio while the F- ions fill the tetrahedral interstitial sites. d) TEM micrograph showing typical β-phase NaYF4:Yb,Er particles as well as (inset) a zoomed-in view of a single nanoparticle. e) Unit cell of β-NaYF4; basal edges are of length a and the height is defined as c. One internal site is occupied by Na+ and Y3+ (1:1) while the second contains a Na+ ion. Y3+ and F- ions are located at the cell vertices and interstices, respectively. 85x99mm (300 x 300 DPI)


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Figure 2: α-NaYF4:Yb,Er UC emission and lifetimes a) UC emission upon excitation with 980-nm light as pressure is increased. Spectra obtained at points during the subsequent pressure relaxation are omitted for clarity (see SI). The emitting states which correspond to the three radiative relaxation pathways are labeled; for each, the final state is 4I15/2. b) Integrated emission intensity summed over all three transitions as the pressure inside the DAC is first increased (solid circles) then released (open circles); guides to the eye (solid line, dashed line) are included to emphasize the trends. c) Color map depicting the spectral dependence of emission from the 4F9/2 manifold on pressure. Each spectrum is normalized to highlight shifts in peak position. Intensity values are given base 10. d) Lifetime values with increasing (solid circles) and decreasing (open circles) pressure for the 2H11/2, 4S3/2, and 4 F9/2 emitting states. As before, guides (solid and dashed lines) highlight the pressure induced behavior. Intensity-time traces for each of the transitions can be found in the SI.


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Figure 3: α -NaYF4:Yb,Er XRD data a) XRD spectra ( λ = 0.539 Å) revealing how the structure and phase of the α -phase sample is affected by pressure application. The patterns are arranged in order (from bottom to top) of increasing to the maximum pressure studied (23.8 GPa) before subsequently relaxing back to ambient conditions. Reflections from α -NaYF4 are indexed using the standard ICDD 013-7404, and peaks associated with the high-pressure polymorph are denoted with solid triangles. b) Full widths at half the maximum intensity (FWHMs) of XRD peaks; data beyond 11.7 GPa and all data for the (222) reflection are not included due to the phase transition and low intensity, respectively, preventing accurate width determination. Peaks are fit to Gaussian functions, and guides to the eye (solid lines) serve to emphasize the overall evolution of peak width. c) Percent strain in the α-NaYF4 lattice (defined relative to the lattice parameter at 0.22 GPa) as pressure is increased (solid circles) and decreased (open circles). A guide to the eye (solid line) is included for the compression data. The decompression data also fall along this curve, highlighting the elasticity of the sample fraction which does not undergo a phase transition. The lattice constant was calculated using the (111) reflection of the phase, the only one distinguishable at all pressures. 177x71mm (300 x 300 DPI)


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Figure 4: β-NaYF4:Yb,Er UC emission and lifetimes a) UC emission from β-phase particles showing only spectra obtained with increasing pressure for clarity. The final state for each of the three transitions is 4I15/2. b) Integrated emission including pressure-increasing (solid circles) and pressure-decreasing data (open circles); guides to the eye (solid line and dashed line, respectively) are included. c) Color map depicting the manner in which peaks within the 4S3/2 manifold change in energy with pressure application. Spectra are normalized to highlight these energy shifts, all of which are entirely reversible. d) Excited-state lifetimes measured in the β-phase particles for each of the three emitting states. Experimental data (solid and open circles) with corresponding guides to the eye (solid and dashed lines, respectively) are shown. Intensity-time traces at each pressure studied are included in the SI. 86x210mm (300 x 300 DPI)


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Figure 5: XRD characterization of β-NaYF4:Yb,Er nanoparticles a) XRD patterns obtained at several different pressures using x-rays of wavelength λ = 0.539 Å. The spectra are arranged (from bottom to top) in order of increasing pressure until the maximum (23.9 GPa) is reached, above which the pattern corresponding to the pressure-released sample is shown. Indices of higher-order peaks are omitted for clarity. b) FWHMs for each of the first seven lowest-order reflections as pressure is increased (solid circles) then released (open circles). The XRD peaks are approximated as Gaussian functions, and guides to the eye (solid lines) are again included. c) Percent strain using the two βphase lattice parameters (defined relative to the values measured at 0.27 GPa) with increasing (solid circles) and decreasing (open circles) pressure; the evolution of the lattice parameters are highlighted with guides to the eye (solid lines). The lattice constants were determined by averaging the values given by the seven lowest-order reflections, which remain distinct at all pressures studied. 177x73mm (300 x 300 DPI)


Strain-Induced Modification of Optical Selection Rules in Lanthanide

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