Multicolor tunable upconversion luminescence from sensitized seed

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Multicolor tunable upconversion luminescence from sensitized seed-mediated grown LiGdF4:Yb,Tm-based core/triple-shell nanophosphors for transparent displays Jeehae Shin, Ji-Hoon Kyhm, A-Ra Hong, Jin Dong Song, Kwangyeol Lee, Hyungduk Ko, and Ho Seong Jang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02497 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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Chemistry of Materials

Jeehae Shin,1,† Ji-Hoon Kyhm,2,† A-Ra Hong,1,3 Jin Dong Song,2 Kwangyeol Lee,3 Hyungduk Ko,4 and Ho Seong Jang*,1,5 1

Materials Architecturing Research Center, 2Center for Optoelectronic Materials, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea 3

Department of Chemistry, Korea University, 145, Anam-ro, Seoul 02841, Republic of Korea

4

Nanophotonics Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea 5

Korea University of Science and Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

ABSTRACT: Red, green, blue, and natural white upconversion (UC) luminescence colors are realized from the tetragonalstructured LiGdF4-based core/triple-shell (C/T-S) upconversion nanophosphors (UCNPs) and the C/T-S UCNPincorporated polymer composites. The LiYF4:Yb cores are used as sensitized seeds for the formation of LiGdF4:Yb,Tm UC shell followed by the growth of LiGdF4:Tb,Eu color tuning shell. Finally, LiYF4 inert shell is grown on the core/shell/shell UCNPs and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs exhibit enhanced UC luminescence. The single tetragonal-phased C/T-S UCNPs exhibit blue, green, and red UC luminescence which is attributed to the electronic transitions in Tm3+ via energy transfer UC process and Tb3+ and Eu3+ via energy migration UC process, respectively. The multicolor UC emissions including natural white, medium aquamarine, purple, and thistle color are created by fine tuning of the ratio of Tb3+ and Eu3+ in the color tuning shell. The transparent polymer composites are prepared by incorporating the C/T-S UCNPs into polydimethylsiloxane and the polymer composites also exhibit red, green, blue, and natural white light UC emissions, indicating that these multicolor tunable LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs have potential to be applied to transparent volumetric displays.

Over the past decade, there has been a great development in researches on synthesis and applications of upconversion nanophosphors (UCNPs).1-14 Like other luminescent materials such as conventional phosphors, quantum dots, and organic dyes, the UCNPs can be applied to optoelectronic and display devices, security field, sensors, and bio-imaging, etc.15-26 In particular, because UCNPs are excited by invisible near infrared (NIR) light and emit visible light, they are advantageous for realization of transparent displays.17 Park et al. reported transparent flexible displays using the UCNPs and Liu’s group realized 3D volumetric displays which solve ghost voxel problem using the UCNP-polymer composites.17-19 While various lanthanide ions such as Ce3+, Eu3+, Eu2+, Pr3+, Dy3+, Tb3+, Tm3+, and Sm3+ are doped as activator ions into host crystals in downshifting phosphors, Yb3+ and Er3+/Ho3+/Tm3+ ions are mainly used as sensitizer and activator ions in most upconversion phosphors.27 Because Er3+, Ho3+, and Tm3+ ions have long-lived intermediate energy levels whose positions are similar to the 2F5/2 state of Yb3+, resulting in efficient energy transfer from Yb3+ to

Er3+/Ho3+/Tm3+, these Er3+, Ho3+, and Tm3+ exhibit bright visible light under NIR light excitation via successive energy transfer from Yb3+ to those ions (sometimes by the help of phonon assistance).28 On the other hand, Eu3+ and Tb3+ ions, representative activator ions in the conventional downshifting phosphors, do not have intermediate energy states well-matched with 2F5/2 state of the Yb3+ ion.27 Thus, efficient upconversion luminescence (UCL) is generally observed from aforementioned Er3+, Ho3+, or Tm3+ ion-doped UCNPs and it has been challenging that efficient UCL is realized from other lanthanide ions such as Eu3+ and Tb3+ ions. Recently, Chen’s group reported that Eu3+ ions emit upconversion red light in core/shell (C/S) structured UCNPs.29 Moreover, Liu’s group reported upconversion color tuning via energy migration upconversion (EMU) process which enables Tb3+, Eu3+, Sm3+, and Dy3+ ions to exhibit UCL.27 These results opened up new horizons for tailoring UCL color without restriction to use the specific lanthanide ions, e.g., Er3+, Ho3+, and Tm3+ ions. After Liu’s group reported the UCL color tuning via EMU process, there were several reports on the UCL emitted from Eu3+ or Tb3+ ion-doped UCNPs.30-35 However, effi-

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cient UCL via EMU process was reported from only βNaGdF4-based host lattice, i.e., NaGdF4 with hexagonal structure, and UCNPs with other compositions except βNaGdF4, which exhibit efficient UCL color tuning via EMU process, is still lacking. In this study, we report multicolor tunable LiGdF4-based core/triple-shell (C/T-S) UCNPs via energy migration and energy transfer process. The LiREF4 (RE: rare earth) with tetragonal structure can serve as a host material for efficient UCNPs.36-38 Previously, Capobianco’s group reported intense ultraviolet (UV) to NIR-emitting LiYF4:Yb,Tm UCNPs.38 However, the LiYF4 is not suitable for EMU process because the Y3+ ion is optically inert.27 On the other hand, when Y is substituted with Gd, LiGdF4 can be a good host material for EMU process because LiGdF4 has the same crystal structure as the LiYF4 and Gd3+ ions in the LiGdF4 act as mediators for energy migration.39 In a point of view of chemical composition, β-NaGdF4 and LiGdF4 are similar to each other and it seems that LiGdF4 can be obtained by just substituting Na with Li. However, the crystal structure of β-NaGdF4 is different from that of LiGdF4 (The former has hexagonal structure, while the latter has tetragonal structure.40,41), resulting in development of different morphologies during the growth (The former shows hexagonal shape and the latter exhibits tetragonal bipyramidal shape.27,36). Thus, they are completely different from the viewpoint of crystal structure and morphology despite their similar chemical composition. Moreover, it was reported that the synthesis of single tetragonal phase LiGdF4 nanocrystals (NCs) is difficult via conventional solution chemical route such as co-precipitation and co-thermolysis through which single phase β-NaGdF4 NCs are synthesized.36,42 Although single tetragonal phase LiGdF4-based UCNPs can be synthesized via Y3+-doping,36 EMU process may not be efficient due to lower concentration of Gd3+ mediators in the host NC as a result of Y3+ doping.27 To synthesize single tetragonal LiGdF4-based UCNPs, we used seedmediated growth method. Yb3+ was doped into LiYF4 to achieve bright UCL and the Yb3+-doped LiYF4 NCs were used as sensitized seeds for the growth of LiGdF4:Yb,Tm shell followed by the growth of LiGdF4:A (A = Tb and Eu) luminescent shell and the outermost LiYF4 inert shell. In addition, multicolor tunable UCL was realized from our designed LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs via EMU through the Gd3+ mediator and energy transfer from Tb3+ to Eu3+. Moreover, red-, green-, blue-, and natural white-emitting transparent UCNPpolymer composites were prepared to investigate the feasibility of application to transparent volumetric displays and various colored UCL images were realized in the transparent UCNP-polymer composites.

Figure 1a shows schematic diagram showing our strategy to synthesize LiGdF4:Yb,Tm UCNPs with tetragonal structure. As mentioned above, when rare earth (RE) element is Gd in LiREF4, GdF3 with orthorhombic structure is apt to be formed.36 On the other hand, LiYF4 tetragonal

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Figure 1. (a) Schematic illustration showing the formation of LiREF4 NCs (RE = Gd and Y) and LiYF4/LiGdF4 C/S NCs. TEM images of (b) GdF3:Yb,Tm, (c) LiYF4, and (d) LiYF4/LiGdF4:Yb,Tm. (e) XRD patterns of GdF3:Yb,Tm UCNPs, LiYF4 core NCs, and LiYF4/LiGdF4:Yb,Tm C/S UCNPs. (f) EDS spectra of LiYF4 core NCs and LiYF4/LiGdF4:Yb,Tm C/S UCNPs. phase is formed when RE element is Y in LiREF4.36 Thus, we grew LiGdF4:Yb,Tm as a shell on the LiYF4 NCs. When we used 74% Gd(oleate)3, 25% Yb(oleate)3, and 1% Tm(oleate)3 as RE precursors for the synthesis of the UCNPs, rhombic plate-shaped NCs were synthesized as shown in the transmission electron microscopy (TEM) image of Figure 1b. On the other hand, when 100% Y(oleate)3 was used as the RE precursor, small polyhedral NCs were synthesized. The rhombic and polyhedral NCs were revealed to have orthorhombic and tetragonal structures, respectively (see below). Thus, LiYF4 small polyhedra were used as seeds for growth of LiGdF4:Yb,Tm with tetragonal structure. Due to the growth of LiGdF4:Yb,Tm shell, larger polyhedral NCs with C/S structure were synthesized compared with LiYF4 cores (Figure 1c and 1d). The discrepancy in morphologies of rhombic NCs and polyhedral NCs is attributed to the different crystal structures. X-ray diffraction (XRD) pattern of rhombic NCs synthesized with Gd, Yb, and Tm precursors is matched with that of orthorhombic GdF3 (JCPDS No. 12-0788) (Figure 1e). The XRD pattern of small polyhedral NCs synthesized with the Y precursor is well matched with that of tetragonal LiYF4 (JCPDS No. 81-2254). The XRD peaks of C/S polyhedral NCs were slightly shifted to smaller angle and they are in agreement with those of tetragonal LiGdF 4 (JCPDS No. 27-1236). Since the relative XRD peak intensities of the NCs are affected by the orientation of the NCs on a substrate,43 the strong XRD peak of the C/S NCs at about 50° in Figure 1e can be attributed to the orientation of the NCs on the substrate where the (204) and (220) planes of some NCs were aligned parallel to the substrate.


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Figure 2. TEM images of (a) LiYF4:Yb (sensitized core NCs), (b) LiYF4:Yb/LiGdF4:Yb,Tm (C/S UCNPs), (c) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb (C/D-S UCNPs), (d) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu (C/D-S UCNPs), (e) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb/LiYF4 (C/T-S UCNPs), and (f) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu/LiYF4 UCNPs (C/T-S UCNPs). (g) PL spectra of LiYF4:Yb/LiGdF4:Yb,Tm, LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb C/D-S UCNPs, and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb/LiYF4 C/T-S UCNPs, and (h) PL spectra of LiYF4:Yb/LiGdF4:Yb,Tm, LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu C/D-S UCNPs, and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu/LiYF4 C/T-S UCNPs under excitation with a 980 nm NIR light-emitting CW diode laser. (The PL spectra were normalized to maximum Tm3+ emission.) As shown in Figure S1, high resolution TEM (HR-TEM) images of the polyhedral C/S NCs exhibit highly clear lattice fringes in individual NCs, showing that the C/S NCs have high crystallinity. Lattice spacing of rhombic NCs was measured to be 0.36 nm which is well matched with d-spacing of (101) planes of orthorhombic GdF3 crystals (d101 = 0.365 nm), while lattice spacings of polyhedral core and C/S NCs were measured to be 0.47 nm corresponding to d-spacing of (101) planes of tetragonal LiYF4 (d101 = 0.465 nm) and LiGdF4 (d101 = 0.472 nm), respectively. Energy dispersive X-ray spectroscopy (EDS) spectra of core and C/S NCs prove that Gd3+ and Yb3+ are present after coating of LiGdF4:Yb,Tm on LiYF4 core (Figure 1f). In addition, EDS maps and selected area electron diffraction (SAED) pattern shown in Figure S2 confirm the formation of LiYF4/LiGdF4:Yb,Tm C/S UCNPs. These results indicate that GdF3:Yb(25%),Tm(1%) (GdF3:Yb,Tm) was indeed synthesized when Li, F, Gd, Yb, and Tm precursors were used without tetragonal-structured seeds, while LiGdF4:Yb(25%),Tm(1%) (LiGdF4:Yb,Tm) was successfully synthesized by seed-mediated growth method. When photoluminescence (PL) spectrum of GdF3:Yb,Tm was compared with that of LiYF4/LiGdF4:Yb,Tm, the Li-

YF4/LiGdF4:Yb,Tm showed much stronger UCL intensity than the GdF3:Yb,Tm (Figure S3). In addition, luminescent properties of LiGdF4:Yb,Tm UCNPs and well-known NaGdF4:Yb,Tm UCNPs were compared. To precisely compare their UCL properties, we synthesized LiYF4/LiGdF4:Yb(25%),Tm(1%) and NaGdF4/NaGdF4:Yb(25%),Tm(1%) C/S UCNPs because LiGdF4:Yb,Tm UCNPs with single tetragonal phase was not synthesized via conventional solution chemical synthesis method, as shown in Figure 1. As exhibited in Figure S4, PL intensity of LiYF4/LiGdF4:Yb,Tm UCNPs was nearly the same as that of NaGdF4/NaGdF4:Yb,Tm UCNPs in UV spectral region and was much stronger than that of NaGdF4/NaGdF4:Yb,Tm UCNPs in blue spectral region, indicating that LiGdF4 is a superior host lattice for efficient UCL. Although the LiYF4/LiGdF4:Yb,Tm UCNPs showed higher PL intensity than GdF3:Yb,Tm UCNPs, Yb3+ was doped into LiYF4 core to further enhance Tm3+ luminescence. As shown in Figure S5, when 80% Yb3+ was sensitized into LiYF4 core, the LiYF4:Yb/LiGdF4:Yb,Tm showed the strongest UCL intensity under 980 nm illumination. In addition, the thickness of LiGdF4:Yb,Tm shell was ad-



justed and UCL intensity of C/S UCNPs was increased as the thickness of LiGdF4:Yb,Tm shell increased (Figure S6). However, too much amount of shell precursors induced the phase separation besides increase of the shell thickness. As shown in Figure 2a-2f, LiGdF4:Tb and LiGdF4:Eu shells were coated on the LiYF4:Yb/LiGdF4:Yb,Tm C/S UCNPs, respectively and then LiYF4 shells were coated on the LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu core/double-shell (C/DS) UCNPs, respectively. Due to the growth of LiGdF4:Tb and LiGdF4:Eu shells, particle sizes were increased (Figure 2a-2d). The synthesis of the uniform polyhedral NCs was also verified by the high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images as shown in Figure S7. XRD patterns combined with HR-TEM images for core, C/S, C/D-S, and C/T-S UCNPs confirm that all synthesized NCs have tetragonal structure with high crystallinity (Figure S8 and S9). In addition, the HR-TEM images of C/S, C/D-S, and C/T-S UCNPs and high resolution STEM (HR-STEM) images of the C/T-S UCNPs demonstrate that each layer was epitaxially grown on the respective inner layer (Figure S9 and S10). Although the contrast in the HAADF STEM images of the C/T-S UCNPs was not clearly observed (Figure S7), the EDS spectra of LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(Eu)/LiYF4 NCs shown in Figure S11 support the successful formation of C/T-S structure. The representative EDS maps of the UCNP samples also support the formation of the C/T-S structure (Figure S12). For Yb3+ in the LiGdF4:Yb,Tm layer, optimized Yb3+ concentration was 25 mol% (Figure S13). As mentioned above, high Yb3+ concentration in the LiYF4 core favored strong Tm3+ luminescence from the LiYF4:Yb/LiGdF4:Yb,Tm C/S UCNPs and this result is consistent with previous results reported by Chen et al. and Qiu et al.44-46 However, Tb3+ intensity decreased with increasing Yb3+ concentration in the LiGdF4:Yb,Tm layer, as shown in Figure S13. The weaker Tb3+ emission at higher Yb3+ concentration in the LiGdF4:Yb,Tm shell can be explained as follows. When Yb3+ concentration increased in the LiGdF4:Yb,Tm layer, Gd3+ concentration decreased. Since excitation energy is transferred from Tm3+ to Tb3+ through Gd3+-mediated energy migration, decreased concentration of Gd3+ may lead to inefficient energy migration, resulting in weak Tb3+ emission. According to Liu’s group, low Gd3+ concentration in the NaGdF4:Yb,Tm core increases interionic distance between Gd3+ ions and it suppresses energy migration process, resulting in weakened emission from the activators doped into the NaGdF4:A (A = Tb or Mn) shell on the NaGdF4:Yb,Tm core.27,47 The weak Tb3+ emission at high Yb3+ concentration is consistent with the previous results.27,47 Thus, there is an optimal Yb3+ concentration and in our experimental condition, optimized Yb3+ concentration in the LiGdF4:Yb,Tm layer was 25%. In this study, since high concentration of Yb3+ (80%) was already sensitized to the LiYF4 core and the Yb3+ ions in the core can efficiently absorb NIR light and transfer the energy to the LiGdF4:Yb,Tm shell, the Yb3+ concentration in the LiGdF4:Yb,Tm shell could be optimized at 25%. To opti-

mize Tb and Eu concentrations for intense green and red luminescence, various quantities of Tb and Eu were doped into LiGdF4 shells, respectively and the optimized Tb and Eu concentrations were 15 mol%, respectively (Figure S14). As depicted in Figure 2g, Tb3+ characteristic peaks due to 5D4 → 7FJ (J = 0, 1, 2, 3, 4, 5, and 6) transitions are evidently shown as a result of EMU process under 980 nm NIR irradiation. The formation of LiYF4 outermost shell on the LiGdF4:Tb layer efficiently prevents surface quenching effect32 and thus, emission intensity from Tb3+ was significantly increased, resulting in green UCL from the C/T-S UCNPs. In addition, Eu3+ characteristic peaks were observed in PL spectra of LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu C/D-S and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu/LiYF4 C/T-S UCNPs under 980 nm laser irradiation (Figure 2h). Sharp PL peaks in red spectral range are ascribed to 5D0 → 7FJ (J = 1-4) transitions in Eu3+ ions. Similarly to the increase of Tb3+ luminescence in the C/T-S UCNPs, the formation of the outermost LiYF4 shell led to the significant enhancement of Eu3+ emission by suppression of the surface quenching.32 When Eu3+ is co-doped with Tb3+ into the LiGdF4 shell, strong red emission can be created from low concentration of Eu3+ ions via Tb3+ → Eu3+ energy transfer.48 The excited energy can be transferred from Tb3+ to Eu3+ via interfacial energy transfer in C/S UCNPs in which Tb3+ and Eu3+ are separately doped into core and shell, respectively.49 However, in this case, high Eu3+ concentration is necessary to achieve strong red emission intensity.49 Recently, we reported that energy transfer efficiency for Tb3+ → Eu3+ is high when Tb3+ and Eu3+ were doped into the same region and small concentration of Eu3+ exhibited strong red emission intensity.50 In addition to intense red emission from small concentration of Eu3+ ions, UCL color can be easily tuned by slightly adjusting the Eu3+ amount. Thus, in this study, LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu UCNPs were synthesized instead of LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb/LiGdF4:Eu UCNPs. Codoping of the Tb and Eu into the second shell did not affect the formation of C/D-S and C/T-S structures, and the Tb and Eu co-doped UCNPs with C/D-S and C/T-S structure exhibited uniform size and shape as shown in Figure S15. As depicted in Figure 3a, excited energy can be migrated from the LiGdF4:Yb,Tm shell to the outer LiGdF4:Tb,Eu shell. The Yb3+ ions in the sensitized seed core and the first shell absorb 980 nm NIR light followed by energy transfer to Tm3+. The upconverted energy is then transferred to Tb3+ ions via energy migration through Gd3+ sublattice and finally transferred to Eu3+ (5D1 state) ions from the Tb3+ (5D4 state) ions.49,50 Some of the excited energy can be also transferred from Tm3+ to Eu3+ via Gd3+mediated energy migration. The emission is enhanced by suppressing the surface quenching effect due to the presence of the outermost LiYF4 shell (Figure S16). The Tb3+ → Eu3+ energy transfer was confirmed by shortened decay of Tb3+ luminescence as shown in Figure S17. Because the decay profiles doesn’t exhibit remarkable change and have the multiexponential decay feature, we evaluated the


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Figure 3. (a) Schematic energy level diagram showing luminescence from LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu-based C/D-S and C/T-S UCNPs (b) PL spectra of LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 UCNPs under NIR light irradiation. Activator emissions are highlighted with blue, green, and red colors, respectively (Tm: blue, Tb: green, and Eu: red). (c) CIE color coordinates of various C/D-S and C/T-S UCNPs. Inset shows photographs showing the UCL from the LiGdF4:Yb,Tm-based C/D-S and C/T-S UCNPs under irradiation with 980 nm NIR light [(i) LiYF4:Yb/LiGdF4:Yb,Tm/LiYF4, (ii)LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%), (iii) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%)/LiYF4, (iv) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%),Eu(1%)/LiYF4, (v) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%),Eu(2%)/LiYF4, (vi) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%),Eu(3%)/LiYF4, (vii) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu(15%), (viii) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu(15%)/LiYF4]. (The PL spectra were normalized to maximum Tm3+ emission.) effective experimental lifetime (τeff) which can be expressed by the following equation:51

𝜏𝑒𝑓𝑓 =

∫ 𝐼(𝑡)𝑡𝑑𝑡 ∫ 𝐼(𝑡)𝑑𝑡

(1)

where I(t) is the PL intensity at the time t after initial decay process. The normalized decay profile of Tb3+:5D4 → 7F5 without Eu3+ co-doping showed a near single exponential feature whereas the Tb3+ decay curves with varying Eu3+ concentration exhibited multiexponential decay behavior and the τeff decreased gradually. This lifetime alteration can be explained by the nonradiative energy transfer from Tb3+ to Eu3+. Due to this energy transfer of Tb3+ → Eu3+, strong red emission peaks are generated from the LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs in which Eu3+ concentration varied from 1 to 3%. The UC emission peaks from the C/T-S UCNPs in blue, green, and red spectral region could be

dramatically tuned by slight variation of Eu3+ concentration in the second shell which is noted as color tuning shell (see Figure 3b). The ratio of red to green peak intensities increased from 0.30 to 1.21 as Eu3+ concentration increased from 1 to 3% in the C/T-S UCNPs. The UC emission color were easily tuned by adjusting dopants and their concentration in the color tuning shell as shown in the Commission Internationale de l’Éclairage (CIE) color coordinates of each UCNP (Figure 3c). The emission color varied from medium aquamarine for 1% of Eu3+ co-doping to thistle color for 3% of Eu3+ co-doping as the ratio of red to green peak intensity increased. In particular, when 2% of Eu3+ was co-doped with 15% of Tb3+, daylight-like white (or natural white) light was realized from the C/T-S UCNPs by balanced combination of blue (from Tm3+), green (from Tb3+), and red (from Eu3+) peaks. The white light exhibits CIE color coordinates of



(0.3107, 0.3272) and correlated color temperature (CCT) of 6631 K, which are close to those of standard illuminant C [(CIE x, CIE y) = (0.3101, 0.3162) and CCT = 6770 K].52 Consequently, we successfully realized a wide range of emission color from the core/multishell-structured UCNPs by controlling the type and concentration of activators, and structure of the LiGdF4-based core/multi-shell UCNPs, as shown in Figure 3c inset and S18.

Figure 4. (a) Photographs of UCNP-PDMS composites under indoor room light, (b) schematic of experimental set-up for transparent display, (c) 980 nm NIR light-driven luminescence images of the UCNP-PDMS composites under (c) indoor room light and (d) dark condition [(i) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Eu(15%)/LiYF4, (ii) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%)/LiYF4, (iii) LiYF4:Yb/LiGdF4:Yb,Tm/LiYF4, and (iv) LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb(15%),Eu(2%)/LiYF4]. To investigate feasibility of the color-tunable C/T-S UCNPs for transparent display applications, the C/T-S UCNP-polydimethyl siloxane (PDMS) composites were fabricated. The prepared UCNP-PDMS composites are highly transparent as shown in Figure 4a. The transmittance spectra of the UCNP-PDMS composites were shown in Figure S19 and they support high transparency of the UCNP-PDMS composites. It is noted that mixing and thermal curing process for preparation of C/T-S UCNP-PDMS composites has not significantly decreased PL properties of the C/T-S UCNP (Figure S20). Figure 4b exhibits schematic of the experimental set-up for the transparent display. The UCNP-PDMS composites displayed R/G/B/Natural white-emitting luminescence under 980 nm irradiation. When we investigated UC quantum yields (QYs) of our R/G/B/Natural white-emitting UCNPs, we used an integrating sphere (Experimental detail can be found in supporting information and our previously published work.11). The UC QY values were calculated by following equation:53 QY =

𝑡ℎ𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑒𝑚𝑖𝑡𝑡𝑒𝑑 𝑡ℎ𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑

=

𝐿𝑠𝑎𝑚𝑝𝑙𝑒 𝐴𝑟𝑒𝑓 −𝐴𝑠𝑎𝑚𝑝𝑙𝑒

,

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where Lsample is the integrated UC PL intensity of the sample, Aref and Asample are the integrated intensities of the excitation NIR light which is not absorbed by the reference and sample, respectively. The obtained UC QYs of the R/G/B/Natureal white-emitting UCNPs ranged from 1.14 to 2.51% under 106 Wcm-2 condition (Table S1). Due to the high brightness of red, green, blue, and white UCL, the UCL characters were apparently visible under the condition of indoor room light (Figure 4c). When the luminescent images were taken under dark condition, the red, green, blue, and white UCL characters were more conspicuous as shown in Figure 4d.

In summary, LiGdF4:Yb,Tm-based multicoloremitting core/multi-shell UCNPs were successfully realized by sensitized seed-mediated growth, and energy migration and energy transfer mechanism. LiYF4:Yb acted as a seed for the LiGdF4:Yb,Tm growth and increased NIR light absorption via Yb sensitization. By adjusting activators and their concentrations in LiGdF4:Tb/Eu color tuning shell, we could tailor the color of the UCL from the C/D-S and C/T-S-structured UCNPs. When we controlled small quantity of the Eu3+ in the LiGdF4:Tb,Eu layer, UCL color dramatically varied from aquamarine to thistle. Transparent UCNPPDMS polymer composites were fabricated and they also displayed various emission colors under excitation with 980 nm NIR light. These results indicate that the multicolor-emitting LiGdF4-based C/D-S or C/T-S UCNPs have potential for transparent volumetric display applications.

Detail experimental methods can be found in the supporting information.

Supporting Information Experimental methods, HR-TEM, STEM, and HR-STEM images, additional PL spectra and TEM images, XRD patterns, EDS maps, decay profiles, photographs showing multicolor emission from the UCNP solutions, transmittance spectra of UCNP-PDMS composites. This material is available free of charge via the Internet at http://pubs.acs.org.

*E-mail: [email protected].

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Chemistry of Materials J.S.: Department of Chemical & Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291, Daehak-ro, Yuseong-gu, Daejeon 305-338, Republic of Korea

J.H.K.: Quantum functional semiconductor research center, Dongguk University, Joong-gu, Seoul 04620, Republic of Korea

The manuscript was written through contributions of all authors. H.S.J. designed the concept and the experimental method of the research. J.S and A.R.H carried out synthesis of materials, optical and structural characterization of the synthesized samples. J.H.K. conducted experiments for transparent displays and decay time measurement. J.D.S and H.K supported decay time measurement and analyzed decay profiles. K.L analyzed TEM data. All authors have given approval to the final version of the manuscript. †These authors contributed equally. The authors declare no competing financial interest.

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A2B5A03023239), Future Key Technology Program (project No. 2E28020) by Korea Institute of Science and Technology, and a grant from the Bio & Medical Technology Development Programs (NRF2016M3A9B6902060) through the Ministry of Science, ICT & Future Planning.

(1) Gai, S.; Li, C.; Yang, P.; Lin, J. Recent Progress in Rare Earth Micro/Nanocrystals: Soft Chemical Synthesis, Luminescent Properties, and Biomedical Applications. Chem. Rev. 2014, 114, 2343-2389. (2) Chen, X.; Peng, D.; Ju, Q.; Wang, F. Photon upconversion in core-shell nanoparticles. Chem. Soc. Rev. 2015, 44, 1318-1330. (3) Haase, M.; Heer, S.; Kömpe, K.; Güdel, H. U. Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-doped NaYF4 Nanocrystals. Adv. Mater. 2004, 16, 2102-2105. (4) Wang, F.; Liu, X. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev. 2009, 38, 976-989. (5) Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling upconversion nanocrystals for emerging applications. Nat. Nanotechnol. 2015, 10, 924-936. (6) Mai, H.-X.; Zhang, Y.-W.; Si, R.; Yan, Z.-G.; Sun, L.-D.; You, L.-P.; Yan, C.-H. High-Quality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties. J. Am. Chem. Soc. 2006, 128, 6426-6436. (7) Chan, E. M.; Levy, E. S.; Cohen, B. E. Rationally Designed Energy Transfer in Upconverting Nanoparticles. Adv. Mater. 2015, 27, 5753-5761. (8) Yang, D.; Ma, P.; Hou, Z.; Cheng, Z.; Li, C.; Lin, J. Current advances in lanthanide ion (Ln3+)-based upconversion nanomaterials for drug delivery. Chem. Soc. Rev. 2015, 44, 14161448. (9) Na, H.; Woo, K.; Lim, K.; Jang, H. S. Rational morphology control of β-NaYF4:Yb,Er/Tm upconversion

nanophosphors using a ligand, an additive, and lanthanide doping. Nanoscale 2013, 5, 4242-4251. (10) Ye, X.; Collins, J. E.; Kang, Y.; Chen, J.; Chen, D. T. N.; Yodh, A. G.; Murray, C. B. Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shapedirected self-assembly. Prod. Natl. Acad. Sci. U.S.A. 2010, 107, 22430-22435. (11) Kim, S. Y.; Jeong, J. S.; Mkhoyan, K. A.; Jang, H. S. Direct observation of the core/double-shell architecture of intense dual-mode luminescent tetragonal bipyramidal nanophosphors. Nanoscale 2016, 8, 10049-10058. (12) Liu, B.; Li, C.; Yang, P.; Hou, Z.; Lin, J. 808-nm-LightExcited Lanthanide-Doped Nanoparticles: Rational Design, Luminescence Control and Theranostic Applications. Adv. Mater. 2017, 29, 1605434. (13) Hou, Z.; Zhang, Y.; Deng, K.; Chen, Y.; Li, X.; Deng, X.; Cheng, Z.; Lian, H.; Li, C.; Lin, J. UV-Emitting UpconversionBased TiO2 Photosensitizing Nanoplatform: Near-Infrared Light Mediated in Vivo Photodynamic Therapy via MitochondriaInvolved Apoptosis Pathway. ACS Nano 2015, 9, 2584-2599. (14) Yang, G.; Yang, D.; Yang, P.; Lv, R.; Li, C.; Zhong, C.; He, F.; Gai, S.; Lin, J. A Single 808 nm Near-Infrared LightMediated Multiple Imaging and Photodynamic Therapy Based on Titania Coupled Upconversion Nanoparticles. Chem. Mater. 2015, 27, 7957-7968. (15) Yan, C.; Dadvand, A.; Rosei, F.; Perepichka, D. F. NearIR photoresponse in new up-converting CdSe/NaYF4:Yb,Er nanoheterostructures. J. Am. Chem. Soc. 2010, 132, 8868-8869. (16) Wang, H.-Q.; Batentschuk, M.; Osvet, A.; Pinna, L.; Brabec, C. J. Rare-Earth Ion Doped Up-Conversion Materials for Photovoltaic Applications. Adv. Mater. 2011, 23, 2675-2680. (17) Park, B. J.; Hong, A.-R.; Park, S.-T.; Kyoung, K.-U.; Lee, K.; Jang, H. S. Flexible transparent displays based on core/shell upconversion nanophosphor-incorporated polymer waveguides Sci. Rep. 2017, 7, 45659. (18) Deng, R.; Qin, F.; Chen, R.; Huang, W.; Hong, M.; Liu, X. G. Temporal full-colour tuning through non-steady-state upconversion. Nat. Nanotechnol. 2015, 10, 237-242. (19) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping. Nature 2010, 463, 1061-1065. (20) You, M.; Zhong, J.; Hong, Y.; Duan, Z.; Lin, M.; Xu, F. Inkjet printing of upconversion nanoparticles for anticounterfeit applications. Nanoscale 2015, 7, 4423-4431. (21) Wang, L.; Yan, R.; Huo, Z.; Wang, L.; Zeng, J.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. Fluorescence Resonant Energy Transfer Biosensor Based on Upconversion-Luminescent Nanoparticles. Angew. Chem. Int. Ed. 2005, 44, 6054-6057. (22) Vetrone, F.; Naccache, R.; Zamarrón, A.; Juarranz de la Fuente, A.; Sanz-Rodríguez, F.; Martinez Maestro, L.; Martín Rodriguez, E.; Jaque, D.; García Solé, J.; Capobianco, J. A. Temperature Sensing Using Fluorescent Nanothermometers. ACS Nano 2010, 4, 3254-3258. (23) Dong, B.; Cao, B.; He, Y.; Liu, Z.; Li, Z.; Feng, Z. Temperature Sensing and In Vivo Imaging by Molybdenum Sensitized Visible Upconversion Luminescence of Rare-Earth Oxides. Adv. Mater. 2012, 24, 1987-1993. (24) Tian, G.; Gu, Z.; Zhou, L.; Yin, W.; Liu, X.; Yan, L.; Jin, S.; Ren, W.; Xing, G.; Li, S.; Zhao, Y. Mn2+ Dopant-Controlled Synthesis of NaYF4:Yb/Er Upconversion Nanoparticles for in vivo Imaging and Drug Delivery. Adv. Mater. 2012, 24, 1226-1231. (25) Dai, Y.; Ma, P.; Cheng, Z.; Kang, X.; Zhang, X.; Hou, Z.; Li, C.; Yang, D.; Zhai, X.; Lin, J. Up-Conversion Cell Imaging and pH-Induced Thermally Controlled Drug Release from NaYF4:Yb3+/Er3+@Hydrogel Core–Shell Hybrid Microspheres. ACS Nano 2012, 6, 3327-3338.



(26) Dai, Y.; Xiao, H.; Liu, J.; Yuan, Q.; Ma, P.; Yang, D.; Li, C.; Cheng, Z.; Hou, Z.; Yang, P.; Lin, J. In Vivo Multimodality Imaging and Cancer Therapy by Near-Infrared Light-Triggered trans-Platinum Pro-Drug-Conjugated Upconverison Nanoparticles. J. Am. Chem. Soc. 2013, 135, 18920-18929. (27) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning upconversion through energy migration in core-shell nanoparticles. Nat. Mater. 2011, 10, 968973. (28) Chen, G.; Å gren, H.; Ohulchanskyy, T. Y.; Prasad, P. N. Light upconverting core-shell nanostructures: nanophotonic control for emerging applications. Chem. Soc. Rev. 2015, 44, 1680-1713. (29) Liu, Y.; Tu, D.; Zhu, H.; Li, R.; Luo, W.; Chen, X. A Strategy to Achieve Efficient Dual-Mode Luminescence of Eu3+ in Lanthanides Doped Multifunctional NaGdF4 Nanocrystals. Adv. Mater. 2010, 22, 3266-3271. (30) Wen, H.; Zhu, H.; Chen, X.; Hung, T. F.; Wang, B.; Zhu, G.; Yu, S. F.; Wang, F. Upconverting Near-Infrared Light through Energy Management in Core–Shell–Shell Nanoparticles. Angew. Chem. Int. Ed. 2013, 52, 13419-13423. (31) Huang, X. Realizing efficient upconversion and downshifting dual-mode luminescence in lanthanide-doped NaGdF4 core–shell–shell nanoparticles through gadolinium sublatticemediated energy migration. Dyes Pigment. 2016, 130, 99-105. (32) Su, Q.; Han, S.; Xie, X.; Zhu, H.; Chen, H.; Chen, C.-K.; Liu, R.-S.; Chen, X.; Wang, F.; Liu, X. The Effect of Surface Coating on Energy Migration-Mediated Upconversion. J. Am. Chem. Soc. 2012, 134, 20849-20857. (33) Ding, M.; Chen, D.; Wan, Z.; Zhou, Y.; Zhong, J.; Xi, J.; Ji, Z. Achieving efficient Tb3+ dual-mode luminescence via Gdsublattice-mediated energy migration in a NaGdF4 core-shell nanoarchitecture. J. Mater. Chem. C 2015, 3, 5372-5376. (34) Zhang, C.; Yang, L.; Zhao, J.; Liu, B.; Han, M.-Y.; Zhang, Z. White-Light Emission from an Integrated Upconversion Nanostructure: Toward Multicolor Displays Modulated by Laser Power. Angew. Chem. Int. Ed. 2015, 54, 11531-11535. (35) Zhong, Y.; Tian, G.; Gu, Z.; Yang, Y.; Gu, L.; Zhao, Y.; Ma, Y.; Yao, J. Elimination of Photon Quenching by a Transition Layer to Fabricate a Quenching-Shield Sandwich Structure for 800 nm Excited Upconversion Luminescence of Nd3+-Sensitized Nanoparticles. Adv. Mater. 2014, 26, 2831-2837. (36) Na, H.; Jeong, J. S.; Chang, H. J.; Kim, H. Y.; Woo, K.; Lim, K.; Mkhoyan, K. A.; Jang, H. S. Facile synthesis of intense green light emitting LiGdF4:Yb,Er-based upconversion bipyramidal nanocrystals and their polymer composites. Nanoscale 2014, 6, 7461-7468. (37) Huang, P.; Zheng, W.; Zhou, S.; Tu, D.; Chen, Z.; Zhu, H.; Li, R.; Ma, E.; Huang, M.; Chen, X. Lanthanide-Doped LiLuF4 Upconversion Nanoprobes for the Detection of Disease Biomarkers. Angew. Chem. Int. Ed. 2014, 126, 1252-1257. (38) Mahalingam, V.; Vetrone, F.; Naccache, R.; Speghini, A.; Capobianco, J. A. Colloidal Tm3+/Yb3+-Doped LiYF4 Nanocrystals: Multiple Luminescence Spanning the UV to NIR Regions via Low-Energy Excitation. Adv. Mater. 2009, 21, 4025-4028. (39) Tu, L.; Liu, X.; Wu, F.; Zhang, H. Excitation energy migration dynamics in upconversion nanomaterials. Chem. Soc. Rev. 2015, 44, 1331-1345. (40) Aebischer, A.; Hostettler, M.; Hauser, J.; Krämer, K.; Weber, T.; Güdel, H. U.; Bürgi, H.-B. Structural and Spectroscopic Characterization of Active Sites in a Family of Light-Emitting Sodium Lanthanide Tetrafluorides. Angew. Chem. Int. Ed. 2006, 45, 2802-2806. (41) Kim, H. J.; Song, J. S.; Kim, S. S. Efficiency Enhancement of Solar Cell by Down-Conversion Effect of Eu3+ Doped LiGdF4. J. Korean Phys. Soc. 2004, 45, 609-613.

(42) Du, Y.-P.; Zhang, Y.-W.; Sun, L.-D.; Yan, C.-H. Optically active uniform potassium and lithium rare earth fluoride nanocrystals derived from metal trifluroacetate precursors. Dalton Trans. 2009, 8574-8581. (43) Ho, J.-Y.; Huang, M. H. Synthesis of SubmicrometerSized Cu2O Crystals with Morphological Evolution from Cubic to Hexapod Structures and Their Comparative Photocatalytic Activity. J. Phys. Chem. C 2009, 113, 14159-14164. (44) Chen, G.; Ohulchanskyy, T. Y.; Kumar, R.; Å gren, H.; Prasad, P. N. Ultrasmall Monodisperse NaYF4:Yb3+/Tm3+ Nanocrystals with Enhanced Near-Infrared to Near-Infrared Upconversion Photoluminescence. ACS Nano 2010, 4, 3163-3168. (45) Qiu, H.; Yang, C.; Shao, W.; Damasco, J.; Wang, X.; Å gren, H.; Prasad, P. N.; Chen, G. Enhanced Upconversion Luminescence in Yb3+/Tm3+-Codoped Fluoride Active Core/Active Shell/Inert Shell Nanoparticles through Directed Energy Migration. Nanomaterials 2014, 4, 55. (46) Qiu, H.; Chen, G.; Fan, R.; Yang, L.; Liu, C.; Hao, S.; Sailor, M. J.; Å gren, H.; Yang, C.; Prasad, P. N. Intense ultraviolet upconversion emission from water-dispersed colloidal YF3:Yb3+/Tm3+ rhombic nanodisks. Nanoscale 2014, 6, 753-757. (47) Li, X.; Liu, X.; Chevrier, D. M.; Qin, X.; Xie, X.; Song, S.; Zhang, H.; Zhang, P.; Liu, X. Energy Migration Upconversion in Manganese(II)-Doped Nanoparticles. Angew. Chem. Int. Ed. 2015, 54, 13312-13317. (48) Kim, S. Y.; Woo, K.; Lim, K.; Lee, K.; Jang, H. S. Highly bright multicolor tunable ultrasmall β-Na(Y,Gd)F4:Ce,Tb,Eu/βNaYF4 core/shell nanocrystals. Nanoscale 2013, 5, 9255-9263. (49) Zhou, B.; Yang, W.; Han, S.; Sun, Q.; Liu, X. Photon Upconversion Through Tb3+-Mediated Interfacial Energy Transfer. Adv. Mater. 2015, 27, 6208-6212. (50) Kim, S. Y.; Won, Y.-H.; Jang, H. S. A Stratege to enhance Eu3+ emission from LiYF4:Eu nanophosphors and greento-orange multocolor tuanble, transparent nanophosphorpolymer composites. Sci. Rep. 2015, 5, 7866. (51) Chen, D.; Yu, Y.; Wang, Y.; Huang, P.; Weng, F. Cooperative Energy Transfer Up-Conversion and Quantum Cutting Down-Conversion in Yb3+:TbF3 Nanocrystals Embedded Glass Ceramics. J. Phys. Chem. C 2009, 113, 6406-6410. (52) Wyszecki, G.; Stiles, W. S. Color Science: Concepts and Methods, Quantitative Data and For-mulae; John Wiley & Sons, Inc.: New York, 1967, 269-275. (53) Wang, J.; Deng, R.; MacDonald, M. A.; Chen, B.; Yuan, J.; Wang, F.; Chi, D.; Andy Hor, T. S.; Zhang, P.; Liu, G.; Han, Y.; Liu, X. Enhancing multiphoton upconversion through energy clustering at sublattice level. Nat. Mater. 2013, 13, 157-162.


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Multicolor tunable upconversion luminescence from sensitized seed

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