Dramatic Enhancement of Quantum Cutting in Lanthanide-Doped

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Dramatic Enhancement of Quantum Cutting in Lanthanide-Doped Nanocrystals Photosensitized with an Aggregation Induced Enhanced Emission Dye Wei Shao, Chang-Keun Lim, Qi Li, Mark T. Swihart, and Paras N. Prasad Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01724 • Publication Date (Web): 24 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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Dramatic Enhancement of Quantum Cutting in Lanthanide-Doped Nanocrystals Photosensitized with an Aggregation Induced Enhanced Emission Dye †

∥

†∥

‡

‡

Wei Shao, ,#, Chang-Keun Lim, , Qi Li, Mark T. Swihart, Paras N. Prasad*, †

†

Department of Chemistry and Institute for Lasers, Photonics, and Biophotonics, University at

Buffalo, The State University of New York, Buffalo, New York 14260, United States ‡

Department of Chemical and Biological Engineering, University at Buffalo, The State

University of New York, Buffalo, New York 14260, United States #

Department of Chemical Engineering, Zhejiang University of Technology, Hangzhou 3100314,

P. R. China KEYWORD. Aggregation induced enhanced emission (AIEE), Förster (or fluorescence) resonance energy transfer (FRET), Photosensitization, Photovoltaics, Quantum cutting.

ACS Paragon Plus Environment

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ABSTRACT.

Applications of multiphoton processes in lanthanide-doped nanophosphors (NPs) are often limited by relatively weak and narrow absorbance. Here, the concept of an ultimate photosensitization by aggregation induced enhanced emission (AIEE) dyes to dispel this limitation is introduced. Because AIEE dyes do not suffer from concentration quenching, they can fully cover the NP surface with large number density to maximize absorbance, while passivating the surface. This concept is applied to multiphoton downconversion by quantum cutting. Specifically, coating Yb3+/Tb3+-doped NPs with an AIEE dye designed for efficient energy transfer and attachment to the NPs, produces a 2260-fold enhancement of multiphoton downconversion by quantum cutting with remarkable photostability. In a prototypical application, quantum cutting of UV photons to near-IR photons that are matched to the band gap of a silicon solar cell produces an average 4% increase in efficiency under concentrated solar illumination. This provides a general strategy for NP photosensitization that can be applied to both multiphoton up- and down-conversion.


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Photosensitization, the process of triggering or amplifying a physicochemical process using a substance that absorbs light and transfers the energy to desired acceptors, has been a versatile tool

photoreaction,1

in

photodynamic

therapy,2,

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and

bio-/nano-photonics.4,

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A

photosensitization strategy is particularly valuable for exploiting the unique emission properties of lanthanide-doped nanophosphors (NPs) including cascade up-/down-conversion and quantum cutting.6,

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The relatively weak (molar extinction coefficient: ~170 M-1cm-1) and narrow

(FWHM: 5~10 nm) absorbance of NPs limits their practical applications in biotechnology and optoelectronics. Several recent studies have reported improvement of luminescence intensity by coupling to localized surface plasmon resonance,8, 9 minimizing the surface quenching,10-13 and sensitizing with organic dyes6,

14-16

. Use of organic dyes, which have the highest extinction

coefficient (2.5×104 ~ 2.5×105 M-1cm-1) per mass,17 as an antenna to sensitize the NPs can efficiently enhance the photoluminescence (PL) intensity. Moreover, they provide a broader absorption band than the NPs, which is essential in applications involving broad-spectrum light sources, such as photovoltaics.18 Förster (or fluorescence) resonance energy transfer (FRET)-based dye-sensitization depends upon the number lanthanide ions (acceptors) within the FRET radius of each dye molecule (donor) and the number of dye molecules within the FRET radius of each lanthanide ion. The FRET radius is sufficiently small that only dye molecules bound to a particular NP can transfer energy to lanthanide ions within it. However, for conventional luminophores, the number of the dye molecules per particle is limited by the onset of self-quenching by energy transfer or reabsorption between the dye molecules themselves.13,

19

Because the energy transfer and

emission quantum yield must be lower than 100%, this energy circulation between dye molecules in close proximity leads to self-quenching of luminescence.


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Figure 1. Schemes of AIEE LP sensitized quantum cutting. a) Schematic illustration of the difference between conventional and AIEE LPs in sensitization of quantum cutting. b) Energy transfer pathway from the AIEE LPs to NaYF4:Yb3+,Tb3+ nanocrystals. c) Absorption (green) and emission (red) spectra of the AIEE LP, absorption spectrum of Tb3+ (black), and emission spectrum of Yb3+ (pink). The dashed blue area highlights the overlap between the AIEE LP emission and absorption of Tb3+. Recently, a novel class of luminophores showing aggregation induced enhanced emission (AIEE) has been developed to overcome the concentration quenching problem.20 The working principles of AIEE luminophores (AIEE LPs) mainly include (i) restriction of energy dissipating rotational relaxation (e.g. in tetraphenylethene),20 (ii) inducing planarization through twisted conformations (e.g. in bis-(styryl) anthracene derivatives),21 (iii) head-to-tail molecular arrangement (e.g. in cyanostilbene derivatives),22 and (iv) protecting intramolecular hydrogen bonds from interaction with solvent molecules (e.g. in hydroxyphenylbenzoxazole)23 in the aggregated state. For a light-emitting device, the emission brightness of a material is proportional to the product of absorbance and quantum yield, and the absorbance is proportional to the


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number of absorbing units. In AIEE LPs, which are immune to concentration quenching, enhancement of absorbance and brightness can be increased by increasing LP concentration until pure LP is obtained. Therefore, bulk AIEE LPs and nanoparticles of pure AIEE LPs have been explored for applications in bioimaging and light-emitting diodes, where high brightness and high concentration of optically active material is needed.20 Here, we propose and demonstrate the feasibility of dye-sensitization with AIEE LPs to drive efficient quantum cutting (QC) emission from NPs co-doped with Tb3+ and Yb3+ ions. The AIEE LPs can fully cover the surface of NPs, while conventional LPs must be separated by at least their own FRET radius to limit concentration quenching. Thus, all acceptor ions within one FRET radius of surface of the NPs can receive energy from dye molecules, but conventional LPs could sensitize only a fraction of the acceptor ions (Figure 1a). We exploited this superior sensitization capability to design and implement a cascade energy transfer quanum cutting process consisting of four steps: (i) UV absorption by AIEE LPs, (ii) FRET between excited AIEE LPs and Tb3+ ions, (iii) quantum cutting by energy transfer from one excited Tb3+ ion to two excited Yb3+ ions, (iv) emission from excited Yb3+ ions (Figure 1b and 1c). As shown in Figure 1c, the well matched spectral overlap between emission of our AIEE LPs and absorption of Tb3+ ions (5D4) can facilitate the FRET process, while Tb3+ and Yb3+ ions are a well-known pair for quantum cutting because of their perfect energy matching (5D4 state of Tb3+: 2.53 eV and 2

F7/2 state of Yb3+: 1.265 eV).24-26 The QC emission of Yb3+ at 1.265 eV is perfectly matched

with the most efficient wavelength of photocurrent generation by a silicon solar cell for the further application below. This QC produces two lower-energy photons from one higher-energy photon, with a theoretical quantum efficiency exceeding 100%.27-29 As an example of the practical utility of AIEE sensitized QC, we demonstrated improved photovoltaic efficiency of a


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crystalline silicon solar cell by conversion of one UV photon, at an energy far above the bandgap of silicon, into two NIR photons that match the silicon bandgap and are thus efficiently utilized. We designed and synthesized a new AIEE active dicyanostilbene derivative, 4,4’-((1Z,1’Z)1,4-phenylbis(2-cyanoethene-2,1-diyl)dibenzoic acid, DCDCS, that has two anchoring carboxylic acid groups for attachment to the surface of NPs (Figure 2a and Scheme S1, Supporting Information). This type of dicyanostilbene (DCS) derivative has previously been of interest for its piezochromic AIEE emission property.30, 31 Based on those reports, we examined the AIEE activity of DCDCS by inducing aggregation in a mixture of dimethylformamide (DMF, good solvent, polarity index: 6.4) and trichloroethylene (TCE, poor solvent, polarity index: 1.0). Because DCDCS exhibits some solubility in water (a typical poor solvent for AIEE LPs), we screened various less polar solvents and chose TCE to induce aggregation. DCDCS in 90% TCE showed clear Mie scattering under illumination by a 630 nm laser, while the solution in DMF showed no scattering (Figure 2b and 2c). Accordingly, we measured photoluminescence spectra of DCDCS in TCE/DMF mixed solvents with varying TCE fraction. DCDCS aggregated with increasing TCE content, producing a bathochromic shift from 450 nm to 540 nm (Figure 2d), as is typical of a cyanostilbene-based AIEE active luminophore. The dramatic growth of PL intensity with increasing poor solvent fraction is typical AIEE behavior (Figure 2e). The fluorescence quantum yield of aggregated (in 90% TCE) DCDCS was enhanced to 47% from 13% for the solution in DMF (Figure S1).


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Figure 2. AIEE behavior of DCDCS. a) Schematic illustration of AIEE of DCDCS. Photographs under b) room light and, c) 630 nm laser illumination of DCDCS in DMF and 90% TCE. The dotted white arrow in c shows the laser propagation direction. d) Photoluminescence spectra of DCDCS in mixed solvents of varying TCE to DMF ratio. The inset is a photograph of DCDCS in DMF and in 90% TCE under 365 nm light illumination. e) PL intensity of DCDCS as a function of TCE fraction in the solvent. Having established the AIEE behavior of DCDCS, we proceeded to test its ability to photosensitize quantum cutting in Yb3+/Tb3+ co-doped NPs (QCNPs). We synthesized NaYF4:50%Yb3+,x%Tb3+ (x = 2, 5, 8, 12, 15) NPs by published methods.32 We varied the contents of Tb3+ ion, since it is the direct acceptor of energy transfer from the AIEE dye. Transmission electron microscopy (TEM) images (Figure 3a and Figure S2) showed average nanoparticle sizes of 34, 33, 32, 30, and 29 nm for Tb3+ doping levels of 2%, 5%, 8%, 12%, and 15%, respectively. A slight variation in the nanoparticle size is due to the ionic size of Tb3+


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(0.923 Å) being larger than Y3+ (0.9 Å)33. X-ray diffraction (Figure S3) showed that all NPs were in the expected hexagonal phase. Under 405 nm laser excitation the emission intensities of 4 mg of each nanoparticle sample were almost negligible (Tb3+, 5D3). Among them, 8% Tb3+ doped NPs showed stronger emission than the others (Figure S6a). We then added 100 µg/mL of AIEE LP (DCDCS) into each sample and measured the PL spectra in visible region (for DCDCS emission) and near 1000 nm (for QC emission). The emission spectrum of DCDCS was clearly red-shifted and enhanced by about a factor of 5 upon mixing with the Tb3+ doped NPs (Figure S4), which indicates formation of DCDCSs aggregates on the surface of the NPs. With the sensitization, we observed remarkably enhanced absorption in the ultraviolet region and PL at 980 nm for all the Tb3+-doped NPs (Figure 3b and Figure S5, S6). Plotting the integrated near IR (NIR) emission peak area of each sample (with and without DCDCS), demonstrated the extraordinary enhancement achieved with AIEE sensitization. For the 12% Tb3+-doped NPs we observed 2260-fold enhancement of emission intensity. For possible further optimization, we coated NaYF4 shells of different thicknesses onto NaYF4:50%Yb3+,12%Tb3+ cores, and measured their emission with and without DCDCS. The PL intensity of pristine nanoparticles increased with increasing shell thickness (Figure S7b). However, this trend was reversed upon adding DCDCS (Figure S7c), illustrating the strong (r6) distance dependence of FRET. From the relationship between the shell thickness and the FRET-induced QC emission intensity, we estimated the FRET radius between the DCDCS aggregate and NaYF4:50%Yb3+,12%Tb3+ NPs as 2.56 nm (Figure S8). As the QC emission intensity depends on the doping concentration of Tb3+ (Figure 3c) and the inert shell thickness (Figure S7), we can rule out another possible energy transferring pathway in this system such as direct Dexter transfer from DCDCS to Yb3+. The result of stronger emission without shell suggests that the AIEE sensitizers improve the


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emission of the NPs through not only enhanced absorbance but also through passivation of trap sites on the NP surface, a function typically performed by an undoped shell.7 The contribution of passivation to enhanced emission was studied by exciting NaYF4:50%Yb3+,12%Tb3+ NPs, with and without AIEE LP, at 920 nm, a wavelength that directly excites Yb3+ ions. These measurements showed ~ 1.6-fold enhancement after coating the NPs with AIEE LP (Figure S9). Because the AIEE LP does not absorb at 920 nm (Figure1c), we attribute the 1.6-fold enhancement to passivation of the NP surface. Therefore, of the total 2260-fold enhancement, around 1.6-fold originates from surface passivation, and the remaining 1410-fold enhancement is attributed to AIEE LP sensitization. To further elucidate the mechanism of AIEE dye sensitized quantum cutting, we measured the emission intensity at 980 nm of DCDCS-Tb3+/Yb3+ NPs under 405 nm laser illumination as a function of the excitation power. When plotted on a log-log scale the power dependence was fit well by a line with slope 0.69 (Figure 3d), consistent with a QC process, for which typical values range from 0.5 to 1.34 Subsequently, we studied the QC emission intensity per nanoparticle as a function of the number of DCDCS molecules per NP. The PL intensity per NP was calculated by simply dividing the overall PL intensity by the number of nanoparticles. As shown in Figure 3e, single nanoparticle QC emission intensity increases exponentially with increasing dye content, from 0 to around 8000 molecules per NP, then begins to saturate. This clearly differentiates AIEE sensitization from sensitization with conventional LPs, for which PL intensity per NP declines beyond some critical number of sensitizers per NP. For AIEE dye to NP ratios from 0 to around 8000, the surface of the nanocrystals may not be fully covered by AIEE dye. Thus the exponential growth can be attributed to aggregation-induced PL enhancement as the dye coverage on the QCNPs increases. This nonlinear PL enhancement is consistent with the typical


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AIEE enhancement upon aggregation in a mixed solvent system. Somewhere near an AIEE dye to NP ratio of 8000, the surface of QCNPs is fully covered by DCDCS, with each dye molecule occupying ~0.4 nm2 on the NP surface (Figure S10). A rough calculation (semi-empirical (PM3)) for aggregated adsorbed DCDCS yields footprints of 0.65, 0.16, and 1.05 nm2 for lateral, vertical, and parallel adsorption, respectively (Figure S11). Thus, combined experimental and theoretical considerations suggest that DCDCS are mainly adsorbed on NPs laterally, anchored by the two carboxylic acid groups. The slightly smaller average area observed experimentally could be attributed to the dynamic interaction between the dye and NP. The increase in FRET between AIEE dye and Tb3+ increases more slowly beyond the point where full coverage of laterally-adsorbed dyes is reached.

Figure 3. AIEE sensitized QC. a) TEM and histogram of size distribution for NaYF4:50%Yb3+,12%Tb3+ nanoparticles. b) QC emission spectra of NaYF4:50%Yb3+,12%Tb3+ nanoparticles with or without 100 µg/mL of DCDCS in CHCl3/DMF (1/1) solution. c) Integrated QC emission of NaYF4:50%Yb3+,x%Tb3+ (x = 2, 5, 8, 12, 15) nanoparticles with or without 100


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µg/mL of DCDCS in CHCl3/DMF (1/1) solution. d) Excitation (405 nm) power dependence of QC emission at 980 nm for NaYF4:50%Yb3+,12%Tb3+ nanoparticles with 100 µg/mL of DCDCS. e) QC emission intensity as a function of the number of DCDCS molecules per NaYF4:50%Yb3+,12%Tb3+ nanoparticle. For all the QC emission measurements, a 405 nm laser was used as the excitation source. f) Photostability of AIEE sensitized QC. The graph represents time dependent QC emission intensity upon continuous illumination with a 405 nm laser (190 W/cm2) in CHCl3/DMF (1/1) solution. g) Relative PCE of bare and PDMS (with or without DCDCS-NPs) covered Si SC under focused (780 mW/cm2) or unfocused (100 mW/cm2) AM1.5G illumination. The FRET process was characterized with DCDCS and the NaYF4 NPs with or without doping of 50% Yb3+/12% Tb3+. The fluorescence of DCDCS on Yb3+/Tb3+ doped NPs was weaker than that on undoped (blank) NP, demonstrating the energy transfer from DCDCS to NaYF4:50%Yb3+,12%Tb3+ QCNPs (Figure S12a). To estimate the energy transfer efficiency, the lifetime variation was fitted, showing a reduction from 3.7 ns to 1.8 ns (Figure S12b). We can estimate the energy transfer efficiency from ET = 1 − τDA/τD (τDA and τD are the effective lifetimes of DCDCS in the absence and the presence of Tb3+/Yb3+ doping),13 to determine an energy transfer efficiency of 51 %. Based on the PLQY of DCDCS (47%), the FRET efficiency (51%) and theoretical efficiency of QC between Tb and Yb (188%), we could estimate the whole energy conversion efficiency to be up to 45%. Notably, the AIEE sensitized QC system showed good stability; the QC emission intensity decreased by only 14% upon continuous illumination with a 405 nm laser (power density of 190 W/cm2) for 5.5 hours (Figure 3f). Furthermore, the emission intensity of DCDCS upon continuous UV illumination (365 nm, 0.3 mW/cm2) and heating at 100 °C for more than 20 hours decreased less than 20% as well (Figure S13 and S14).


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This stability is far superior to the well-known indocyanine green sensitization system,15 and, hence, increases the potential for practical applications. We next demonstrated that the high UV to 980 nm photon conversion efficiency by AIEE sensitized QC is sufficient to improve photovoltaic performance by converting under-utilized UV/blue photons to near-IR photons that are more efficiently used by a silicon solar cell (Si SC). As shown in Figure S15a, a crystalline silicon solar cell (Si SC) can convert 800~1000 nm light to electric current more efficiently than visible light, however, much of energy in the solar spectrum is at shorter wavelengths that are less efficiently utilized. This mismatch is one key factor limiting the conversion efficiency of photovoltaic cells. For instance, the maximum possible conversion efficiency (Shockley-Queisser limit) of Si SC with a bandgap of 1.1 eV under AM1.5 illumination is approximately 31%.35 With AIEE-sensitized QC, one UV high energy photon can be converted into two NIR photons where the Si SC utilizes more effectively, in principle allowing efficiencies that exceed the Shockley-Queisser limit.35 As a first test of this application, we prepared thick films (2 mm) of DCDCS-NP doped polydimethylsiloxane (PDMS).36 The transmittance spectrum of PDMS film with DCSCS-NPs showed 2% lower extinction of incident light than that of the pristine PDMS in the efficient photocurrent generating wavelength range (400~1100 nm) of the Si SC (Figure S16), which is attributable to the light scattering by the NPs. It clearly showed absorption band below 400 nm by DCDCS. We put the film on a commercial Si SC, and measured photovoltaic performance. Under unfocused (1-sun) AM1.5G simulated solar illumination, we did not observe any improvement in efficiency. In this case, nondirectional scattering by the nanoparticles caused light-loss (Figure S17). We overcame this problem by focusing light upon the center of the device (Figure S15b and Figure S17). Notably, with the focused light (AM1.5G) illumination, the device with a


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DCDCS-NP doped PDMS film exhibited enhanced power conversion efficiency (PCE), maximum: 8% relative increase and average: 4% relative increase (Figure 3g, Figure S15c and S18) of the original PCE, while pristine PDMS reduced the performance of the Si SC. We further measured the photostability of DCDCS-NPs again under continuous illumination with focused AM1.5G for 6 hours, and observed less than 10% attenuation of QC emission over this time (Figure S19). This demonstrates the potential to achieve PCE improvement through spectral matching by AIEE-sensitized QC. In summary, we have described a new strategy for sensitization of QC emission of nanophosphors by anchoring of AIEE LPs on the nanocrystal surface. We developed a new dicyanostilbene derivative (DCDCS) with two anchoring group (carboxylic acid), that is highly emissive in an aggregated state. The AIEE behavior of the dye allows us to fully cover the Yb3+/Tb3+ NP surface with dye, without any concentration quenching effect. Doing so produced an extraordinary 2260-fold enhancement of QC emission intensity relative to the dye-free NPs. The passivation of surface traps on NPs by AIEE dyes contributed a 1.6-fold emission enhancement. Thus, AIEE sensitization produced a 1410-fold enhancement. The organicinorganic hybrid showed extremely high stability compared to previously reported NIR dyes for sensitizing upconversion in lanthanide-doped NPs. With the DCDCS-Yb3+/Tb3+ NPs converting one photon below 400 nm (3 eV) to two photons of 1000 nm (1.2 eV), we demonstrated improvement of photovoltaic performance by spectral matching and photon multiplication. This sensitization strategy opens the door to use of lanthanide doped nanophosphors in many applications where their brightness was previously insufficient.

ASSOCIATED CONTENT


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Chemicals and instrumentation, synthetic procedures,sample preparations, estimations of molecularweights, number of dyes per core/shell nanoparticle, supporting figures for the quantum yield for DCDCS in DMF (isolated dyes) and DCDCS in 90% TCE (aggregated dyes), supporting figures for TEM images of NaYF4:50%Yb3+,x%Tb3+ (x = 2, 5, 8, 12, 15), supporting figure for XRD patterns of NaYF4:50%Yb3+,x%Tb3+ (x = 2, 5, 8, 12, 15), supporting figure for the emission spectra of DCDCS (0.1 mg) with and without 12%Tb3+/Yb3+ NPs in 2 mL of CHCl3/DMF (1/1), supporting figure for absorbance spectra of NaYF4:50%Yb3+,12%Tb3+ NP suspension with (red) or without (black) DCDCS, supporting figure for the QC emission spectra of NaYF4:50%Yb3+,x%Tb3+ (x = 2, 5, 8, 12, 15) without and with 100 µg/mL of DCDCS, supporting figure for the TEM images and QC emission spectra of ore-shell nanoparticles with varied shell thickness, supporting figure for the calculation of FRET radius, supporting figure for the emission spectra of NaYF4:50%Yb3+,12%Tb3+ nanoparticles with and without DCDCS, supporting figure for the simulation of area covered by DCDCS on a NP surface, supporting figure for the FRET induced fluorescence alteration of DCDCS, supporting figure for time dependent spectra of DCDCS-NP suspension, supporting figure for time dependent spectra of DCDCS powder, supporting figure for the improvement of photovoltaic performance through quantum cutting, supporting figure of light scattering of NP-doped PDMS film upon illumination of focused and defocused laser (405 nm), supporting figure for time dependent QC emission spectra (excited by 405 nm laser). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions


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∥W. Shao and C.-K. Lim contributed equally to this work

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by a grant from the U.S. Air Force Office of Scientific Research (No. FA9550-15-1-0358).

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Dramatic Enhancement of Quantum Cutting in Lanthanide-Doped

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