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Supplementary Information

Tuning Upconversion through Energy Migration in

Core-Shell Nanoparticles

Feng Wang1, Renren Deng1, Juan Wang1, Qingxiao Wang3, Yu Han3, Haomiao Zhu4, Xueyuan Chen4 & Xiaogang Liu1,2,5*

1Department of Chemistry, Faculty of Science, National University of Singapore, Singapore 117543 2Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 3Advanced Membrane and Porous Materials Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia 4Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. 5Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576. *To whom correspondence should be addressed, e-mail: [email protected]

Contents

I. Materials and Methods S2 II. Core-Shell Structures for EMU S4 III. Dopant Concentration Optimization S8 IV. Multicolor Upconversion Imaging S11 V. Heterogeneous vs Homogeneous Doping in Core-Shell Nanoparticles S13 VI. Mechanistic Investigation S16 VII. Inter-Particle Energy Migration (I-PEM) S24 VIII. Prospects of the EMU process S30 IX. Appendix: Notes on the Energy Level Diagram of Lanthanide Ions S33 X. References S35

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT3149

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I. Materials and Methods Reagents: Gd(CH3CO2)3•xH2O, (99.9%), Y(CH3CO2)3•xH2O (99.9%), Yb(CH3CO2)3•4H2O (99.9%), Tm(CH3CO2)3•xH2O (99.9%), Tb(CH3CO2)3•xH2O (99.9%), Eu(CH3CO2)3•xH2O (99.9%), Dy(CH3CO2)3•xH2O (99.9%), Sm(CH3CO2)3•xH2O (99.9%), NaOH (98+%), NH4F (98+%), 1-octadecene (90%), oleic acid (90%), fluorescein 5(6)-isothiocyanate (FITC), tetramethylrhodamine-5-isothiocyanate (TRITC), polyoxyethylene (5) nonylphenylether (CO-520), were purchased from Sigma-Aldrich. Polystyrene beads (3.55 μm) in 10% w/v aqueous dispersion were purchased from Microparticles GmbH. Unless otherwise noted, all chemicals were used as received without further purification. General procedure for the synthesis of core nanoparticles: In a typical procedure to the synthesis of NaGdF4:Yb/Tm nanoparticles, 2 mL water solution of Ln(CH3CO2)3 (0.2 M, Ln = Gd, Yb, and Tm) was added to a 50-mL flask containing 4 mL of oleic acid. The mixture was heated at 150 oC for 30 min to remove the water content from the solution. A solution of 1-octadecene (6 mL) was then quickly added to the flask and the resulting mixture was heated at 150 oC for another 30 min before cooling down to 50 oC. Shortly thereafter, 5 mL of methanol solution containing NH4F (1.36 mmol) and NaOH (1 mmol) was added and the resultant solution was stirred for 30 min. After the methanol was evaporated, the solution was heated to 290 oC under argon for 1.5 h and then cooled down to room temperature. The resulting nanoparticles with a yield of 80 mg were precipitated by addition of ethanol, collected by centrifugation at 6000 rpm for 5 min, washed with ethanol several times, and re-dispersed in 4 mL of cyclohexane. General procedure for the synthesis of core-shell nanoparticles: The NaGdF4:Ln shell precursor was first prepared by mixing 2-mL water solution of Ln(CH3CO2)3 (0.2 M, Ln = Gd, Tb, Eu, Dy, and Sm) and 4 mL of oleic acid in a 50-mL flask followed by heating at 150 oC for 30 min. Then 1-octadecene (6 mL) was added and the mixed solution was heated at 150 oC for another 30 min before cooling down to 50 oC. Subsequently, NaGdF4:Yb/Tm core nanoparticles (40 mg) dispersed in 2 mL of cyclohexane were added along with a 5-mL methanol solution of NH4F (1.36 mmol) and NaOH (1 mmol). The resulting mixture was stirred at 50 oC for 30 min, at which time the solution was heated to 290 oC under argon for 1.5 h and then cooled down to room temperature. The resulting nanoparticles were precipitated by addition of ethanol, collected by centrifugation at 6000 rpm for 5 min, washed with ethanol several times, and re-dispersed in 4 mL of cyclohexane. General procedure for the synthesis of core-shell-shell nanoparticles: The procedure is identical to the synthesis of core-shell nanoparticles. The as-synthesized core-shell nanoparticles were isolated and used as seeds to induce a subsequent shell coating. General procedure for the preparation of ligand-free nanoparticles: The as-prepared oleic acid-capped nanoparticles were dispersed in a 2-mL HCl solution (0.1 M) and ultrasonicated for 15 min to remove the surface ligands. After the reaction, the nanoparticles were collected via centrifugation at 16500 rpm for 20 min, and further purified by adding an acidic ethanol solution (pH 4; prepared by adding 0.1 M HCl aqueous solution to absolute ethanol). The resulting products were washed with ethanol and deionized water several times, and re-dispersed in deionized water. 2 NATURE MATERIALS | www.nature.com/naturematerials

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General procedure for the preparation of nanoparticle-tagged polystyrene beads: In a typical experiment, a solution of ligand-free nanoparticles (50-100 μL, 2 wt%) was added into a mixture of polystyrene bead dispersion (5 μL) and butanol (200 μL). The mixture was kept at room temperature for 30 min, at which time nanoparticle-tagged beads were collected by centrifugation at 6000 rpm for 5 min, washed with deionized water, and re-dispersed in deionized water. General materials characterization: Low-resolution transmission electron microscopy (TEM) measurements were carried out on a JEL-1400 transmission electron microscope (JEOL) operating at an acceleration voltage of 120 kV. Scanning electron microscopy (SEM) was performed on a FEI NOVA NanoSEM 230 scanning electron microscope operated at 5 kV. Powder X-ray diffraction (XRD) data were recorded on a Bruker D8 Advance diffractometer with a graphite-monochro-matized CuKα radiation (1.5406 Å). UV-vis transmission spectrum was recorded on a SHIMADZU UV-2450 spectrophotometer. Fourier transform infrared (FTIR) spectroscopy spectra were obtained on a Varian 3100 FT-IR spectrometer. Photoluminescence spectra were recorded at room temperature with a DM150i monochromator equipped with a R928 photon counting photomultiplier tube (PMT), in conjunction with a 980-nm diode laser. Unless otherwise specified, the emission spectra were normalized to maximum Tm3+ emission either at 450 nm or 475 nm, whichever is strong. The decay curves were measured with a customized UV to mid-infrared phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band OPO laser as an excitation source (410-2400 nm, Vibrant 355II, OPOTEK). The effective lifetimes were determined by

0

0

)(1 dttIIeff

where I0 and I(t) represents the maximum luminescence intensity and luminescence intensity at time t after cutoff of the excitation light, respectively. Upconversion luminescence microscopy imaging was performed on an Olympus BX51 microscope with the xenon lamp adapted to a diode laser. Luminescence micrographs were recorded with a Nikon DS-Ri1 imaging system. Digital photographs were taken with a Nikon D700 camera. Experimental conditions for scanning transmission electron microscopy: High-resolution scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) were performed on an FEI aberration-corrected Titan Cubed S-Twin transmission electron microscope operated at 200 kV. Probe Cs corrector was applied to get better spatial resolution. In a typical experiment, high-resolution STEM imaging was conducted at a 2-s/pixel scanning rate with 70 m C2 aperture, spot size 9, a high-angle annular dark-filed (HAADF) detector, and 146 mm camera length. Under such conditions a spatial resolution of ~1.0 Å was obtained. EDX spectrum was collected with 150 m C2 aperture, spot size 6, and 240 s collection time. EELS point analysis was performed with 150 m C2 aperture, spot size 6, 29.6 mm camera length, 5 mm entrance aperture (collection angle = 54 mrad) and 1s collection time, while EELS line scan was conducted using 70 m C2 aperture, spot size 9, 29.6 mm camera length, 5 mm entrance aperture (collection angle = 54 mrad) and 0.1 s/pixel collection time. NATURE MATERIALS | www.nature.com/naturematerials 3



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II. Core-Shell Structures for EMU

In the realization of efficient EMU processes, the crystal structure and optical property of host materials play important roles and thus require careful consideration. To date, NaYF4 in its hexagonal phase is known as the most efficient upconversion host. In this work, hexagonal phase NaGdF4, which is isostructural to NaYF4, was chosen as the host material to render EMU. The NaGdF4 nanoparticles were synthesized by a wet chemical route adapted from our previous report (1). The as-prepared nanoparticles were characterized by XRD analysis to be hexagonal phase (Figure S1).

Figure S1. (A) Schematic presentation and crystallographic data of hexagonal-phase NaREF4 structure with a space group of P63/m, Z=1.5. Note that Na occupies every other (2b) sites along the c-axis. (B) X-ray powder diffraction patterns of the as-prepared NaGdF4:Yb/Tm (49/1 mol%) nanoparticles showing that all peaks can be well indexed in accordance with hexagonal-phase structure (JCPDS file number 27-0699).

The core-shell nanoparticles were synthesized by an epitaxial growth process, which involves the use of pre-synthesized core nanoparticles as seeds to mediate the growth of the shell layer. By adjusting the sample volume of shell precursors used in the synthesis, we can control the shell thickness of the nanoparticles (Figure S2). Notably, an increase in shell thickness is likely to increase the amount of doped activators, which results in enhanced emission intensities from the activators (Figure S2, top and middle panels). However, at a high volume (20 mL) of shell precursors, we obtained a binary sized mixture of nanoparticles as a result of phase separation (Figure S2, bottom panel). The smaller particles were composed of NaGdF4:Tb. We ascribed the phase separation during the shell growth process to the low concentration of core particles in which two different growth pathways dominate the reaction. The phase-separation effect was also observed for control experiments with low concentration of the core particles featuring large particle sizes and dimensions (Figure S3).

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Figure S2. Control experiments investigating core-shell growth as a function of shell precursor volume. As an epitaxial growth process, the core particles (~15 nm) act as substrates to seed the growth of shell layers. Our results showed that by controlling the volume of shell precursors with respect to the number of core particles, the shell thickness can be precisely controlled (top and middle panels). However, phase separation occurs at a considerably low concentration of core particles (bottom panel). Note that emission intensities from Tb3+ activator increased with increasing shell thickness.

To investigate the core-size effect on EMU, we further carried out a series of control experiments through use of different sized core particles. Despite having similar shell thickness, large-sized core particles exhibited a slight decrease in activator emission relative to small core particles (Figure S3, top and middle panels). This phenomenon can be attributed to depletion of the excitation energy over long-distance migration in large-sized particles. In case of using nanorods as the core particles, phase separation typically occurred under our standard experimental conditions. As examined by electron microscopy, a large amount of NaGdF4:Tb nanoparticles formed in addition to the formation of core-shell nanorods (Figure S3, bottom panel). The phase separation is also responsible for suppressed Tb3+ emission from the resulting sample mixture as physically separated core-shell nanorods and NaGdF4:Tb nanoparticles can hardly exchange energy due to dissipation of excitation energy at the particle interface (see Figure S21 for more discussion). Building on the results from extensive control experiments, the optimum reaction conditions for the growth of uniform core-shell particles that feature maximum Tb3+ emission were determined to be 10 mL of shell precursors and 40 mg of 15-nm core particles as seeds.





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Figure S3. Control experiments investigating core-shell growth as a function of core particle size. The large spherical core particles (~20 nm) were synthesized by adjusting the molar ratio of NH4F to metal ions (1). The large-sized nanorods (~50x120 nm) were synthesized by a different protocol using water/ethanol as solvent (2). Our results showed that under the standard conditions it is difficult to grow uniform core-shell nanoparticles by using large-sized particles or nanorods as the core. This effect can be partly attributed to the phase separation that dominates the reaction at low concentrations of core particles.

The lanthanide-doped nanoparticles with different compositions were further investigated by electron microscopy combined with EELS analysis to verify the designed core-shell structure. Figure S4 shows the imaging results of nanoparticles with a NaGdF4:Yb/Tm (49/1 mol%) core and a NaGdF4:Tb (15 mol%) shell. A representative high-resolution STEM image clearly shows the highly crystalline structure of hexagonal phase NaGdF4 (Figure S4, A). The EDS spectrum, collected over a large illumination area covering many nanoparticles, confirms the presence of all expected elements in the nanoparticles (Figure S4, B). An EELS line scan across a single nanoparticle indicates that the particle edge contains a higher Gd content than the center region (Figure S4, C and D), which is consistent with the designed element distribution for the core-shell structure.





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Figure S4. Characterization of the core-shell nanoparticles comprising a NaGdF4:Yb/Tm (49/1 mol%) core and a NaGdF4:Tb (15 mol%) shell. (A) High-resolution STEM image showing single crystalline nature of the nanoparticle with high structural uniformity. (B) EDS spectrum confirming the presence of all expected elements in the NaGdF4 core-shell nanoparticles. Note that the copper peak is due to the supporting TEM grid. (C) EELS line scan showing the signal intensity variation of Gd across a randomly selected nanoparticle. The Gd content at particle edge is clearly higher than that in the interior, thus confirming the core-shell structure of the nanoparticle. (Inset) STEM image of the nanoparticle under investigation. The area of line scan is highlighted. Note that the core-shell structure cannot be determined directly from the STEM image by virtue of closely matched lattice constant and composition of the core and shell layers.





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III. Dopant Concentration Optimization

The dopant concentration dictates the average distance between neighboring dopant ions, which in turn determines dopant–dopant interaction and thus imposes a strong impact on optical properties of the nanoparticles (3). For example, we previously demonstrated that introduction of an elevated amount of Yb3+ dopants in the NaYF4:Yb/Er host lattice can lead to emission color tuning from yellow to red (4). This effect can be attributed to facilitation of back-energy-transfer from Er3+ to Yb3+, caused by decreased Yb•••Er interionic distance. As a result, the population in excited states of 2H9/2, 2H11/2 and 4S3/2 was suppressed by the energy transfer, resulting in the decrease of blue (2H9/2→4I15/2) and green (2H11/2, 4S3/2→4I15/2) light emissions with respect to the red (4F9/2→4I15/2).

Figure S5. EMU emission spectra for a series of NaGdF4@NaGdF4 core-shell nanoparticles as a function of different Yb/Tm concentration ratios (Sensitized emissions of Tb3+ are highlighted with color). The emission spectra were obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. The results indicate that the optimum Yb3+ and Tm3+ dopant concentrations for maximum Tb3+ emission in reference to visible upconversion emission of Tm3+ are 49 and 1 mol%, respectively.

To optimize dopant concentrations of the sensitizer/accumulator pair for efficient EMU, we synthesized a series of NaGdF4@NaGdF4 core-shell nanoparticles. The particle core was doped with varying quantities of Yb/Tm (39-59/0.5-1.5 mol%), while the shell had a fixed Tb3+ dopant concentration of 15 mol%. By assessing the emission intensity of Tb3+ and using visible emission of Tm3+ as a reference, the dopant concentrations of Yb3+ and Tm3+ for maximum Tb3+ emission were estimated to be 49 and 1 mol%, respectively (Figure S5). Similarly, we also prepared another series of NaGdF4@NaGdF4 core-shell nanoparticles with different amounts (2.5-20 mol%)





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of Tb3+ activators. From the photoluminescence data obtained from these samples, the optimum Tb3+ concentration was estimated to be 15 mol% (Figure S6, A). In addition, we also obtained optimum activator concentrations of 15, 5, and 5 mol% for Eu3+, Dy3+, and Sm3+, respectively (Figure S6, B-D).

Figure S6. The effect of activator concentration on optical properties of the NaGdF4@NaGdF4 core-shell nanoparticles. (A-D) EMU emission spectra obtained in cyclohexane solutions comprising 1 wt% of the core-shell particles with different activators under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. (E) Compiled luminescence photograph of colloidal solutions of representative samples showing the upconversion multicolor tuning by varying activator composition and concentration.

Prior to the addition of activators in the shell layer, the excitation energy of Gd3+ can readily migrate to the sites of crystal defects and surface contaminations where it is quenched nonradiatively. Introducing activators in the shell layer would convert the excitation energy of Gd3+ into visible emission before it can be quenched. The optimization of dopant concentrations enables efficient trapping of migrating energy while avoiding luminescence quenching of the activators through deleterious cross-relaxation. As the activators are spatially isolated from





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Yb/Tm by the core-shell structure, the change in shell composition has marginal influence on the emission process of Tm3+ in the core layer. Therefore, the total emission intensities could also be maximized with optimized activator concentrations. As a separate note, these results also suggest a general EMU approach to fine-tuning the emission colors by selection of different dopant combinations at precisely controlled dopant concentrations (Figure S6, E). As an added benefit, the new emitters (Tb3+ and Eu3+) offer much longer luminescence lifetime (Figure S7) than the accumulator (Tm3+), partly owing to the existence of large energy gaps in the energy level diagram (See Appendix for more discussion). This feature could provide new opportunities for time-resolved optical studies.

Figure S7. Upconversion luminescence decay curves of Tm3+, Tb3+, and Eu3+ (centered at 452 nm, 544 nm, and 615 nm, respectively) in NaGdF4:Yb/Tm (49/1 mol %)@NaGdF4:Tb(or Eu) (15 mol %) core-shell nanoparticles.





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IV. Multicolor Upconversion Imaging

Importantly, the as-synthesized core-shell nanoparticles with different types of activators can be simultaneously excited by a home-made luminescence microscope equipped with a 980-nm diode laser (Figure S8). This capability provides a convenient platform for multicolor luminescence imaging. As a demonstration of this concept, we utilized upconversion nanoparticle-tagged polystyrene beads (~3.55 μm). The assembly of nanoparticles on the beads enables amplification of upconverted signals. In contrast to non-modified polystyrene beads, nanoparticle-tagged beads showed a uniform coating of the nanoparticles under SEM (Figure S9, A-C). Remarkably, luminescence photographs at different magnifications showed that different activator emissions can be clearly distinguished from each other in a single set of experimental conditions (Figure S9, D and E). No background autofluorescence was noticeable, which is a unique advantage of using lanthanide-doped upconversion nanoparticles. These characteristics, in parallel with the benefits of large penetration depth and low photo-damage of near-infrared irradiation in biological samples (5, 6), suggest the potential use of the nanoparticles as luminescent probes for multiplexed biological labeling and imaging studies. We note that the strong emission from the peripheral region of the polystyrene beads indicates a high surface coverage of upconversion nanoparticles on the bead, which is consistent with the observation by SEM.

Figure S8. Schematic drawing of the experimental setup for EMU luminescence imaging of lanthanide-doped core-shell nanoparticles. A 980-nm diode laser was used to excite the samples. The laser was directed into an Olympus BX51 microscope through a beam expansion lens to offer uniform illustration of the samples. The upconverted luminescence image was recorded by a Nikon DS-Ri1 imaging system equipped with a CCD camera.





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Figure S9. (A) SEM image of the polystyrene beads used as the substrates for nanoparticle modification. (B) Corresponding SEM image of the as-synthesized nanoparticle-tagged beads. (C) An enlarged SEM view of a single particle marked by a yellow box in B, clearly showing a uniform coating of nanoparticles on the bead. (D, E) Luminescence photographs of the nanoparticle-tagged polystyrene beads at low and high magnifications showing that multicolor emissions can be simultaneously imaged under single-wavelength excitation at 980 nm. The blue, green, red and yellow colors were generated from NaGdF4:Yb/Tm@NaGdF4, NaGdF4:Yb/Tm@NaGdF4:Tb, NaGdF4:Yb/Tm@NaGdF4:Eu, and a binary mixture of NaGdF4:Yb/Tm@NaGdF4:Tb and NaGdF4:Yb/Tm@NaGdF4:Eu nanoparticles, respectively.





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V. Heterogeneous vs Homogeneous Doping in Core-Shell Nanoparticles

Although energy migration is not unique to core-shell nanoparticles, the spatial confinement of dissimilar dopant ions within specific regions of the core-shell nanoparticles is critical for generating efficient EMU. The core-shell structure was designed to eliminate cross-relaxation-induced energy loss due to direct interaction between the Yb/Tm pair and the activator (2).

To verify the need of a core-shell structure separating the Yb/Tm pair and the activator, we prepared a series of NaGdF4@NaGdF4 core-shell nanoparticles with the core homogeneously doped with Yb3+, Tm3+, and Tb3+ at different doping levels. The emission spectra of these nanoparticles were compared to that of the core-shell nanoparticles with the Yb/Tm pair and Tb3+ activators at separated layers. As shown in Figure S10, the homogenously doped nanoparticles with different Tb3+ dopant concentrations (2.5-15 mol%) all exhibited weak emission intensities for both Tm3+ and Tb3+, in stark contrast to strong emissions from the heterogeneously doped counterpart. The decreased emission intensity can be attributed to cross-relaxation between Tm3+ and Tb3+. The direct interaction between Tm3+ and Tb3+ is also supported by the suppressed emission peaks of Tm3+ centered at 360 nm and 450 nm, corresponding to 1D2→3H6 and 1D2→3F4 transitions. Note that the cross- relaxation between Tm3+ and Tb3+ can be slightly eliminated at a low Tb3+ dopant content (Figure S10). However, the low concentration of Tb3+ is also unfavorable for trapping the excitation energy of Gd3+

. As a result, a suppressed EMU process occurs with significant luminescence quenching. Similar quenching effects have also been observed for nanoparticles with Eu3+, Dy3+, and Sm3+ activators doped homogeneously with the Yb/Tm pair (Figure S11). The nanoparticles doped with Dy3+ and Sm3+ showed much more quenched emissions than the particles doped with Eu3+ and Tb3+. These results can be ascribed to the depletion of excitation energy of Tm3+ associated with the existence of larger numbers of low energy electronic states in Sm3+ and Dy3+ (See Appendix for more discussion).





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Figure S10. Non-normalized upconversion emission spectra of a series of NaGdF4@NaGdF4 core-shell nanoparticles with Yb/Tm and Tb3+ doped at separated layers or doped homogeneously in the core level. Emission spectra were obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. The results revealed a strong quenching in upconversion emission when Yb/Tm and the activators were homogenously doped in the host lattice. Note that without the NaGdF4 shell around the homogeneously doped nanoparticles almost no emission was observed for these nanoparticles, largely due to surface quenching by solvent molecules and ligands (1).





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Figure S11. Control experiments showing quenching of upconversion emission for core-shell nanoparticles with Yb/Tm doped homogeneously with Eu3+, Dy3+, and Sm3+, respectively. Emission spectra without normalization were obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2.





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VI. Mechanistic Investigation

As discussed in the manuscript, we define EMU as an optical process that involves energy exchange interactions between four types of lanthanide ions, namely sensitizers (type I), accumulators (type II), migrators (type III), and activators (type IV). A notable feature of the EMU process is that excitation energy is not directly transferred from accumulator Tm3+ to activators. Instead, the energy transfer first occurs from Tm3+ to a migrator ion (e.g., Gd3+), followed by energy migration through the Gd sublattice and subsequent trapping by the activators. The energy exchange interaction from Tm3+ to Tb3+ via Gd3+ in the core-shell nanoparticles was supported by the observations of the reduced decay time in Gd3+ emission (but not Tm3+) and of similar pump-power intensity dependence between Tb3+ and Tm3+ at ~290 nm (Figure S12). To verify the energy transfer from Tm3+ to Gd3+, we prepared NaGdF4:Yb/Tm nanoparticles coated with an optically inert layer of NaYF4. The coating of NaYF4 prevents the excitation energy of Gd3+ from trapping by surface quenching sites. As expected, we observed radiative Gd3+ emission at ~310 nm, which corresponds to 6P7/2 → 8S7/2 transition of Gd3+ (Figure S13, top panel). In contrast, when all Gd3+ ions were replaced by Y3+ in the host lattice, we observed recovered Tm3+ emission at 290 nm, corresponding to 1I6 → 3H6 transition of Tm3+ (Figure S13, bottom panel). These data provide direct evidence for the energy transfer from Tm3+ to Gd3+.

Figure S12. (A) A comparison of upconversion luminescence lifetimes of Gd3+ and Tm3+ in NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4 and NaGdF4:Yb/Tm (49/1 mol%)@NaGdF4:Tb (15 mol%) nanoparticles. (B) Log-log plots of the upconversion emission intensity versus NIR excitation power for the 1I6→3H6 transition of Tm3+ at 290 nm and the 5D4 →7F5 transitions of Tb3+ at 544 nm. Note that the nanoparticles used in this study were pre-treated with concentrated HCl to remove the surface oleate ligands that typically quench the upconversion emission around 300 nm (see Figure S21 for more discussion).





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Figure S13. Luminescence investigation confirming the energy transfer from Tm3+ to Gd3+ by evaluating NaGdF4:Yb/Tm@NaYF4 and NaYF4:Yb/Tm@NaYF4 core-shell nanoparticles. (Left column) TEM images of the NaGdF4 and NaYF4 core particles. (Middle column) Corresponding TEM images of the core-shell particles after epitaxial growth of a NaYF4 shell layer. (Right column) Upconversion emission spectra of the core-shell nanoparticles obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. The emission spectra provide clear evidence that the energy transfer occurs from Tm3+ to Gd3+. Note that under our standard conditions it is difficult to grow a uniform NaYF4 shell around large-sized (~ 40 nm) NaYF4:Yb/Tm core particles. To achieve the shell coating, a reduced concentration of shell precursors (half of the standard concentration) was used to avoid the phase separation of the particles.

To further probe the role of Gd sublattice in mediating energy transfer to activators, we prepared a series of NaGdF4:Yb/Tm/Y (49/1/10–30 mol%) nanoparticles with varied Gd3+ contents (50–20 mol%) in the core level. Note that the Gd3+ core content was exchanged by optically inactive Y3+ ions. XRD analysis reveals that all nanoparticles crystallize in pure hexagonal phase (Figure S14). The peak shifts towards higher diffraction angles in the XRD patterns as a function of Y3+ ions arises as a result of shrink in unit-cell volume owing to the substitution of Gd3+ by smaller Y3+ in the host lattice.





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Figure S14. X-ray powder diffraction patterns of the NaGdF4:Yb/Tm/Y nanoparticles in the presence of 10, 20, and 30 mol% Y3+ dopant ions. Diffraction peaks of the samples are consistent with those of pure hexagonal phase NaGdF4 materials ( JCPDS file number 27-0699).

To initiate the energy migration process, each Gd3+ needs roughly two Gd3+ neighbors within a

reasonable distance (7). In hexagonal phase NaGdF4 (space group P63/m), Gd3+ ions only occupy the (2c) sites with occupancy of 75 %. The arrangement of the (2c) sites, depicted in Figure S15, shows each site surrounded by 14 neighboring sites that can be classified into three families based on their distances from the center site. The critical distance (dc) between Gd3+ ions for energy migration to occur can thus be estimated from the critical Gd3+ concentration (30 mol%; see Figure S17). To provide two Gd3+ neighbors for each Gd3+ ion at such concentration, a minimum number of neighboring (2c) sites N = 2/(0.3×0.75) ≈ 9 is required. Considering that 8 out of the 14 neighboring sites have a distance of not greater than 3.9 Å while the other 6 sites are 6.0 Å away from the center site, we derived a critical distance of ~6.0 Å between Gd3+ ions.

Figure S15. Schematic presentation showing the arrangement of the (2c) cation sites in hexagonal phase NaGdF4. Note that 3/4 of the sites are occupied by Gd3+ and the others are occupied by Na+. All sites are identical and the colors are used to identify different packing layers.





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Figure S16 shows the high-resolution STEM analysis of the core-shell nanoparticles comprising a NaGdF4:Yb/Tm/Y (49/1/20 mol%) core and a NaGdF4:Tb (15 mol%) shell. EELS point analysis reveals that the center (marked at spot 1) of the nanoparticle contains Gd and Yb while the edge (marked at spot 2) contains Gd and Tb (Figure S16, D and E). Moreover, the particle edge contains a higher Gd content than the center region. Taken together, these data clearly support the core-shell structure of the NaGdF4 nanoparticles.

Figure S16. Characterization of the core-shell nanoparticles comprising a NaGdF4:Yb/Tm/Y (49/1/20 mol%) core and a NaGdF4:Tb (15 mol%) shell. (A) High-resolution STEM image showing single crystalline nature of the nanoparticle with high structural uniformity. (B) Selected area electron diffraction (SAED) pattern confirming hexagonal phase of the nanoparticles. (C) High-resolution STEM image of a single nanoparticle. The electron beam damage to the sample (marked at spot 1 and 2) is evident due to prolonged exposure time for acquiring EELS signals. (D, E) EELS spectra collected respectively from spot 1 and 2 in C, showing that (i) Yb is only present at particle center; (ii) Tb is only present at particle edge, and (iii) more Gd content exists at particle edge as compared to particle center, and thus confirming the core-shell structure of the nanoparticle. Note that Y was not detected because of overlap of its major peak with other elements.





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In a further set of experiments, we investigated photoluminescence properties of the as-prepared NaGdF4:Yb/Tm/Y@NaGdF4:Tb core-shell nanoparticles. As shown in Figure S17, we observed a dampening effect on emission intensity of Tb3+ activators as the Gd3+ core content is partially exchanged by optically inactive Y3+ ions. Therefore, the decreased emission intensity must be due to the suppression of energy migration process because of increased Gd–Gd interionic distance. Based on previous analysis, we derived a critical energy transfer distance (dc) of ~6 Å between Gd3+ ions needed for an efficient EMU process. The results also imply that direct energy transfer from Tm3+ to Tb3+ is unlikely to occur in the core-shell nanoparticles. Importantly, our control experiments showed that Eu3+, Dy3+, and Sm3+ activators all exhibited suppressed EMU emissions when the Gd3+ core content was partially replaced by Y3+ (20 mol%) (Figure S18). Taken together, the data clearly indicate a general Gd sublattice-mediated EMU mechanism for all the activators studied here.

Figure S17. Photoluminescence investigation of core-shell nanoparticles doped with different Gd core content (40, 30, 20 mol%), confirming Gd sublattice-mediated energy transfer to the activator. The NaGdF4@NaGdF4 core-shell nanoparticles comprise a NaGdF4:Yb/Tm/Y (49/1/10–30 mol%) core and a NaGdF4:Tb (15 mol%) shell. (Left column) TEM images of the NaGdF4:Yb/Tm/Y (49/1/10–30 mol%) core nanoparticles. (Middle column) Corresponding TEM images of the core-shell particles after epitaxial growth of NaGdF4:Tb (15 mol%) layers. (Right column) Upconversion emission spectra of the core-shell nanoparticles obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. Upconversion emission spectra revealed suppressed Tb3+ emission intensities (highlighted with color) with reduced Gd content in the core level. Note that variation of Gd content in the core particle can induce significant change in particle size. To ensure the formation of similar sized core particles, the ratio of F- to RE precursors was accordingly increased (3.6, 3.8, and 4) for preparing particles with deceasing Gd content (40, 30, 20 mol%).





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Figure S18. Comparative emission spectrum analysis confirming Gd sublattice-mediated energy transfer to Eu3+, Dy3+, and Sm3+ activators doped in core-shell nanoparticles (Sensitized emissions of activators are highlighted with color). The emission spectra were obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. Similar to Tb3+ activated samples, suppressed EMU emissions were observed for Eu3+, Dy3+, and Sm3+ with decreased Gd content in the core level. The results suggest that a general upconversion mechanism is shared by all the activators under investigation.





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The role of energy migration in this novel upconversion process was further demonstrated in core-shell-shell nanoparticles comprising a NaGdF4:Yb/Tm core, an undoped NaGdF4 spacing layer, and a NaGdF4:Tb outer shell. The particle morphologies were examined by TEM and shown in Figure S19. Interestingly, despite having an increased Tm–Tb interionic distance caused by the NaGdF4 spacing layer (5 nm), photoluminescence studies showed only slight decrease in emission intensity of Tb3+ in reference to that of Tm3+. Consistent with the result for Tb3+, a slight dampening effect on emission intensity was observed for Eu3+, Dy3+, and Sm3+ activators doped in the core-shell-shell nanoparticles containing a 3-nm NaGdF4 spacing layer (Figure S20). The important point is that energy migration through Gd sublattice can travel a substantial length without losing most of the excitation energy.

Figure S19. Distance-dependent energy migration studies of core-shell-shell nanoparticles comprising a NaGdF4:Yb/Tm (49/1 mol%) core, a NaGdF4 spacing layer, and a NaGdF4:Tb (15 mol%) shell. To prepare the core-shell-shell structure, shell precursors of different compositions were sequentially grown on the core particles (top panel). To make a 5-nm thick spacing layer of NaGdF4, we repeated the second growth step rather than increasing the volume of the shell precursor to avoid phase separation, as evidenced by TEM characterizations (bottom panel). Further spectral characterization indicates that the EMU process can be generated in the core-shell-shell nanoparticles even with a relatively large Tm–Tb distance (Sensitized emissions of Tb3+ are highlighted with color). Emission spectra were obtained in cyclohexane solutions comprising 1 wt% particle under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2.





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Figure S20. Distance-dependent energy migration studies of core-shell-shell nanoparticles containing Eu3+, Dy3+, and Sm3+ activators. Sensitized emissions of activators are highlighted with color. Similar to Tb3+ activated samples, the presence of a NaGdF4 spacing layer (~3 nm) of NaGdF4 showed a negligible effect on the EMU for Eu3+, Dy3+, and Sm3+ activated samples. Emission spectra were obtained in cyclohexane solutions comprising 1 wt% particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. NATURE MATERIALS | www.nature.com/naturematerials 23




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VII. Inter-Particle Energy Migration (I-PEM)

During the course of our mechanistic investigations, we have also discovered that the excitation energy can migrate across nanoparticles, leading to observation of inter-particle energy migration (I-PEM) and sensitized lanthanide emission from energy acceptor particles. Two sets of lanthanide-doped NaGdF4 nanoparticles were used for this study. One set of particles were doped with the Yb/Tm pair as energy donors while the other set of particles were doped with different activators. Note that the I-PEM process is sensitive to surrounding environment of the nanoparticles. For example, sensitized emission of acceptor particles was not observed both in solution and in the solid state for oleate ligand-stabilized nanoparticles (Figure S21). This result is probably due to quenching of the excitation energy of Tm3+ and Gd3+ by oleate ligands.

Figure S21. I-PEM studies of oleate-capped NaGdF4 nanoparticles. (A) Schematic presentation showing that excitation energy fails to migrate across the oleate-capped NaGdF4 nanoparticles because of energy quenching by oleate ligand. (B) Upconversion emission spectra of solution and solid sample mixtures obtained under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. No acceptor emissions were observed in both cases. (C) Absorption spectra of oleic acid and lanthanide (Gd/Yb/Tm 50/49/1 mol%)-oleate complex obtained in 100 μM of cyclohexane solutions. The strong absorption band of the oleate overlaps with the emission of Tm3+ and Gd3+ in the ultraviolet spectral region, contributing to unsuccessful demonstration of I-PEM process.





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To avoid the quenching of excitation energy by oleate ligand molecules, energy donor and acceptor particles were thoroughly washed with concentrated HCl solutions to yield ligand-free particles (Figure S22, A) (8). The removal of the surface ligands was confirmed by FTIR analysis (Figure S22, B). The ligand-free donor and acceptor particles were then mixed in ethanol/water (4:1; v/v) solutions at different molar ratios. Following near-infrared excitation we observed clear energy migration from the donor to acceptor particles, as evidenced by the appearance of sensitized emissions for all the activators studied (Tb3+, Eu3+, Dy3+, and Sm3+) both in solution and in the solid state (Figure S23). Notably, small nanoparticles (~15 nm) resulted in more efficient I-PEM than large-sized nanorods, which is likely caused by dissipation of excitation energy of Gd3+ over long-distance migration in large particles. The energy transfer efficiency judged by the emission intensity of activators was found to be dependent on the distance between the donor and acceptor particles. For example, NaGdF4:Yb/Tm and NaGdF4:Eu nanoparticles mixed in solid form exhibited a more than five-fold enhancement in acceptor (Eu3+) emission in comparison with that in colloidal dispersion (Figure S23, D).

Figure S22. Synthesis and characterization of oleate-free NaGdF4 nanoparticles. (A) Schematic presentation showing the removal of oleate ligands from the particle surface in acid environment. (B) FTIR spectra reveal that the oleate-related absorption bands disappeared after washing the nanoparticles with HCl solution, confirming the successful removal of oleate ligands from the particle surface. Note that strong O-H absorption bands from the ligand-free nanoparticles are due to water molecules physically absorbed on the particle surface.





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We would like to emphasize that this is, to the best of our knowledge, the first successful demonstration of inter-particle energy transfer involving lanthanide-doped crystalline nanoparticles as energy acceptors (emitters). In conventional Förster resonance energy transfer (FRET) process (9), energy acceptors developed to date are generally limited to quantum dots and organic dyes because of their broad absorption bands and high extinction coefficients (typically in the range of 104-105 M-1 cm-1) (10). In comparison, lanthanide ions typically feature narrower absorption bands and substantially lower molar extinction coefficients (~ 1 M-1 cm-1) (11). Addition of chelating agents can result in an increase in the lanthanide emission by several orders of magnitude (“antenna effect”). However, there is significant nonradiative deactivation in lanthanide chelate complexes due to high-energy vibrations of organic ligands and solvents (12).

Figure S23. Energy transfer from NaGdF4:Yb/Tm nanoparticles to NaGdF4 nanoparticles doped with varying activators. (A) Schematic presentation showing I-PEM between nanoparticles. (B) Emission spectra of ethanol/water (4:1; v/v) solutions containing NaGdF4:Yb/Tm nanoparticles (2 wt%) and NaGdF4:Tb nanoparticles of varying particle concentrations. (C) Emission spectra of ethanol/water (4:1; v/v) solutions containing NaGdF4:Tb (4 wt%) and NaGdF4:Yb/Tm nanoparticles (2 wt%) of varying particle size. (Inset) TEM images of the NaGdF4:Yb/Tm particles. Scale bars are 100 nm. (D-F) Emission spectra of different donor-acceptor particle systems obtained in ethanol/water (4:1; v/v) solutions and in the corresponding solid state upon drying. The emission spectra were obtained at room temperature under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. 26 NATURE MATERIALS | www.nature.com/naturematerials




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Our successful demonstration of inter-particle energy transfer via lanthanide-doped particle acceptors is enabled by the Gd sublattice as indicated by time decay studies (Figure S24). In particular, the unique ability of the Gd sublattice provides resonant energy transfer at a shortened interaction distance (Figure S25, A and B). When Gd3+ ions were removed by optically inactive Y3+ in the host lattice, we did not observe the energy transfer from Tm3+ to Tb3+, potentially due to large separation of the ion pairs and mismatched resonant frequencies (Figure S25, C and D). The role of energy migration through Gd sublattice in mediating the unprecedented I-PEM process was further confirmed by the preserved acceptor emission when NaGdF4:Yb/Tm@NaGdF4 core-shell nanoparticles were used as the energy donor (Figure S25, E). To verify that the interaction between the nanoparticles is governed by non-radiative energy transfer other than radiative reabsorption, we increased the interaction distance by using NaGdF4:Yb/Tm@NaYF4 core-shell nanoparticles as the energy donor (Figure S25, F). The substantially suppressed acceptor emission clearly revealed negligible contribution of the reabsorption that can efficiently operate at a considerably long distance range (in the order of the wavelength of the photons involved) (13). The insignificance of reabsorption effect in our particle system is primarily owing to low extinction coefficients of the lanthanide ions.

Figure S24. A comparison of upconversion luminescence lifetimes of Gd3+ and Tm3+ in NaGdF4:Yb/Tm (49/1 mol%) nanoparticles and in a binary mixture (1:1; wt/wt) of NaGdF4:Yb/Tm (49/1 mol%) and NaGdF4:Tb (15 mol%) nanoparticles. The essentially unaltered lifetimes of Tm3+ indicate that the energy transfer is mediated by Gd3+.





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Figure S25. Comparative analysis of energy transfer processes through an I-PEM design or a conventional design without involving Gd3+ energy migration. (A) Schematic design of the Gd sublattice-mediated I-PEM, showing the ability of the Gd sublattice to provide the resonance energy transfer between the donor and acceptor particles at a shortened interaction distance. (B) Representative emission spectrum showing the realization of efficient I-PEM as a result of shortened interaction distance and perfect resonance between the donor and acceptor particles. (C) Schematic presentation showing unsuccessful energy transfer without use of Gd sublattice. The ineffective energy transfer is attributed to the mismatched resonance energy between Tm3+ and Tb3+. (D) Representative emission spectrum obtained from a mixture of NaYF4:Yb/Tm and NaGdF4:Tb nanoparticles. (E) Representative emission spectrum obtained from a mixture of NaGdF4:Yb/Tm@NaGdF4 and NaGdF4:Tb nanoparticles. (F) Representative emission spectrum obtained from a mixture of NaGdF4:Yb/Tm@NaYF4 and NaGdF4:Tb nanoparticles. The emission spectra were obtained under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. Colloidal particles (2 wt% donor and 4 wt% acceptor nanoparticles) were dispersed in ethanol/water (4:1; v/v) solutions.

We also demonstrated the EMU across two sets of NaGdF4 nanoparticles in the solid state, as directly monitored by luminescence microscopy. To image the I-PEM process, we first tagged polystyrene beads with energy donor particles (NaGdF4:Yb/Tm) and immobilized them on a microscope glass slide. Energy acceptor particles (NaGdF4:Tb) were added from one side of the glass slide and allowed to diffuse toward the NaGdF4:Yb/Tm-tagged beads (Figure S26, A). By correlating the emission colors of the beads that are in contact or without contact with the acceptor particles under the microscope, we clearly identified the occurrence of an I-PEM process. As a result of I-PEM, sensitized emission of Tb3+ from the acceptor particles showed the green luminescence, which was further confirmed from the emission spectrum (Figure S26, B and C).





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Figure S26. Characterization of I-PEM process in the solid state. NaGdF4:Yb/Tm energy donor nanoparticles were first attached to polystyrene beads (3.55 m) for signal amplification. NaGdF4:Tb energy acceptor nanoparticles were brought in contact with donor particle-tagged beads by slow nanoparticle diffusion to the beads pre-immobilized on a glass substrate. (A) SEM image of the NaGdF4:Yb/Tm particle-tagged beads in contact with the NaGdF4:Tb nanoparticles. (Inset) Schematic design of the energy transfer experiment. (B) Photoluminescence micrograph showing the I-PEM-induced Tb3+ emission at the donor-acceptor particle interface. (C) Emission spectrum taken from the sample shown in B, confirming the presence of Tb3+ emission via I-PEM. Emission spectrum was obtained under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2.





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VIII. Prospects of the EMU process

In addition to being useful for realizing I-PEM for lanthanide-doped particle acceptors, we noted that energy migration is useful for achieving dual-activator emissions in nanoparticles. For instance, we showed that by sequentially doping Eu3+, Tm3+ and Tb3+ ions into different shell layers of a core-shell-shell structure, sensitized emissions from both Tb3+ and Eu3+ can be realized (Figure S27). Importantly, the emission of dually activated nanoparticles covers almost the whole visible spectral range, making them particularly attractive for use as universal energy donors in conventional FRET studies involving organic dyes as energy acceptors. As a demonstration of the concept, we showed simultaneous excitation of FITC and TRITC dyes by using a single set of NaGdF4@NaGdF4@NaGdF4 core-shell-shell nanoparticles doped with dual Tb3+ and Eu3+ activators (Figure S28). Of related interest, EMU in different Gd-based host materials was observed. By utilizing the core-shell structure we were able to obtain sensitized emission of Tb3+ or Eu3+ in a tetragonal phase LiGdF4 host lattice (Figure S29). Given the large range of options available in the choice of hosts and dopant combinations, our findings described here may stimulate new concepts for the development of nanomaterials displaying exciting optical properties.

Figure S27. Demonstration of dual-activator emissions using NaGdF4@NaGdF4@NaGdF4 core-shell-shell nanoparticles doped with Yb/Tm (49/1 mol%), Tb3+ (5 mol%), and Eu3+ (10 mol%) at different shell layers. Note that no detectable Tb3+ emission can be observed when Tb3+ and Eu3+ are doped in the same shell layer (Top panel). This indicates strong quenching of Tb3+ emission by Eu3+. However, by doping Tb3+ and Eu3+ in two separate shell layers, we obtained both Tb3+ and Eu3+ emission peaks (Middle panel). Note that the weak Eu3+ emission is a result of a large separation distance between Tm3+ and Eu3+. To enhance the activator emission, we modified the layout of the core-shell-shell structure without changing the overall composition of the particles. Eu3+ and Tb3+ were separately doped in neighboring layers of Tm3+. Remarkably, we observed strong emissions from both activators (Bottom panel). Emission spectra were obtained under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2.





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Figure S28. Demonstration of simultaneous excitation of two different dyes using a single set of core-shell-shell nanoparticles doped with dual lanthanide activators (Tb3+ and Eu3+). The core-shell-shell particles, composed of a inner NaGdF4:Eu (10 mol%) core, a NaGdF4:Yb/Tm (49/1 mol%) spacing layer, and a NaGdF4:Tb (5 mol%) outmost shell layer, serve as model energy donors. Common dye molecules of FITC and TRITC were used as model energy acceptors. Emission spectra were obtained in CO-520/ethanol (1:1; v/v) solutions comprising 0.5 wt% upconversion nanoparticles and 0.1 mM dye molecules under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2. Dual emissions from FITC and TRITC are evident following the near-infrared excitation.





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Figure S29. Preliminary investigation of EMU in LiGdF4-based host lattices. (A) Schematic presentation showing the core-shell design for EMU in a lanthanide-doped LiGdF4 nanoparticle. (B) Schematic presentation of tetragonal-phase LiGdF4 (I41/a, Z=4) structure. Note that fluorine ions are not shown for clarity. (C) Room temperature upconversion emission spectra of the as-synthesized LiGdF4:Yb/Tm (49/1 mol%)@LiGdF4:Tb (15 mol%) and LiGdF4:Yb/Tm (49/1 mol%)@LiGdF4:Eu (15 mol%) nanoparticles. The emission spectra were obtained in cyclohexane solutions comprising 1 wt% respective core-shell particles under excitation of a 980-nm CW diode laser at a power density of 15 W cm-2, showing sensitized emissions of Tb3+ and Eu3+ activators.





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IX. Appendix: Notes on the Energy Level Diagram of Lanthanide Ions

Lanthanides (Ln) are a family of 15 chemically similar elements from lanthanum (La) to lutetium (Lu) characterized by the filling of the 4f-shell. The lanthanides, essentially existing in their most stable oxidation state as trivalent ions (Ln3+), are extensively investigated for their optical properties. The lanthanide ions feature an electron configuration of 4fn (0< n <14) and the arrangements of electrons within this configuration are substantially diverse. This results in a fairly large number of energy levels. Most of the fascinating optical properties, such as photon upconversion emission, of lanthanide ions can be ascribed to the electron transitions within the 4fn configuration. Most lanthanide ions generally show sharp line spectra, much narrower and more distinct than those for transition metal ions. The spectra are associated with weak f–f electronic transitions. The narrow spectral bands indicate that the f-orbitals have a smaller radial extension than the outer 5s- and 5p-orbitals, thus leading to smaller electron-phonon coupling strengths and a lower susceptibility to crystal-field and exchange perturbations.

The energy levels of free lanthanide ions in 4f-orbitals are determined by, in order of importance, the Coulombic interaction and the spin–orbit coupling between f-electrons. The Coulombic interaction, which represents the mutual repulsion of the electrons, generates the total orbital angular momentum (L) and total spin angular momentum (S). Furthermore, the spin–orbit coupling makes the total angular momentum (J) of the f-electrons. Each set of L, S, and J corresponds to a specific distribution of electrons within the 4f-shell and defines a particular energy level. We can now derive the free ion levels using the term symbols of 2S+1LJ according to the Russell–Saunders notation, where 2S+1 represents the total spin multiplicity (14). With the Russell–Saunders notation, the letters S, P, D, F, G, H, I, . . . are used to denote total orbital angular momentum 0, 1, 2, 3, 4, 5, 6, . . ., respectively.

When we dope a lanthanide ion into a crystal lattice, the electric field of the surroundings produces a crystal field which splits the multiplets into crystal-field levels (“Stark levels”). The pattern of splitting is determined by crystal-field strength and the site symmetry of the lanthanide ion. Because of the shielding by the 5s- and 5p-electrons, the crystal-field splitting of an f-electron term is rarely more than a few hundred cm-1. As a result, their luminescence, in particular their emission spectra, closely resemble to those of free ions and lanthanide chelate complexes. However, significant non-radiative deactivation occurs in lanthanide complexes due to molecular high-energy vibrations in organic ligands and solvents. Indeed, efficient upconversion and quantum cutting have not been observed in these lanthanide complexes. Figure A1 displays the important energy levels for the trivalent lanthanide ions investigated in this work. Note that lanthanide ions typically feature low molar extinction coefficients and long luminescence lifetimes, largely attributing to the Laporte forbidden nature of the f–f electronic transitions.

The suitability of lanthanide ions as activators relies on their unique energy level structures. For instance, the existence of a large energy gap in the energy level diagram promotes the preservation of excitation energy against non-radiative deactivation by high energy phonons in the host lattice. To this regard, it is clear that Gd3+, Tb3+, and Eu3+ are good candidates for use as activators, as judged by their large energy gaps with ΔE = 32 200 (6P7/2 → 8S7/2), 14 800 (5D4 → 7F0), and 12 300 (5D0 → 7F6) cm-1, respectively. Despite enormous research efforts, however, these ions have not been effective as common activators for upconversion. This is largely due to a lack of long-lived intermediary energy states of ladder-like arrangement in these ions to complete the excited state absorption and energy transfer upconversion processes. Although several research groups have demonstrated that upconversion emissions of Gd3+ and Eu3+ can be obtained





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through direct energy transfer from conventional upconverting ions (Er3+, Tm3+, and Ho3+), these methods generally require extremely stringent control over the host composition and often suffer from low emission intensities (15-18). The task of achieving efficient upconversion emissions, via a general approach, for lanthanide ions without long-lived intermediary energy states remains a daunting challenge.

Figure A1. Partial energy level diagram in the range from 0 to 37 500 cm-1 for the trivalent lanthanide ions investigated in our study. The large energy gaps for Gd3+, Tb3+, and Eu3+ are highlighted in the diagram. 34 NATURE MATERIALS | www.nature.com/naturematerials




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