Flexible transparent displays based on core/shell upconversion nanophosphor-incorporated polymer waveguides

Core/shell (C/S)-structured upconversion nanophosphor (UCNP)-incorporated polymer waveguide-based flexible transparent displays are demonstrated. Bright green- and blue-emitting Li(Gd,Y)F4:Yb,Er and Li(Gd,Y)F4:Yb,Tm UCNPs are synthesized via solution chemical route. Their upconversion luminescence (UCL) intensities are enhanced by the formation of C/S structure with LiYF4 shell. The Li(Gd,Y)F4:Yb,Er/LiYF4 and Li(Gd,Y)F4:Yb,Tm/LiYF4 C/S UCNPs exhibit 3.3 and 2.0 times higher UCL intensities than core counterparts, respectively. In addition, NaGdF4:Yb,Tm/NaGdF4:Eu C/S UCNPs are synthesized and they show red emission via energy transfer and migration of Yb3+ → Tm3+ → Gd3+ → Eu3+. The C/S UCNPs are incorporated into bisphenol A ethoxylate diacrylate which is used as a core material of polymer waveguides. The fabricated stripe-type polymer waveguides are highly flexible and transparent (transmittance > 90% in spectral range of 443–900 nm). The polymer waveguides exhibit bright blue, green, and red luminescence, depending on the incorporated UCNPs into the polymer core, under coupling with a near infrared (NIR) laser. Moreover, patterned polymer waveguide-based display devices are fabricated by reactive ion etching process and they realize bright blue-, green-, and red-colored characters under coupling with an NIR laser.

images in real 3D spatial form 18 , the realization of 3D volumetric displays indispensably requires a thicker nature of the display devices. On the other hand, when the thickness of the transparent display devices is thin enough to be bendable, transparent and flexible display devices using the UCNPs can be actualized. Polymer waveguides have a capability of application to flexible displays and Park et al. recently reported polymer waveguide-based thin film displays 22,23 . These results encourage us to develop transparent and flexible display devices by combining the UCNPs with polymer waveguide. We previously reported bright green-emitting Li(Gd,Y)F 4 :Yb,Er UCNPs which have high color purity of 98.9% 24 . Small-sized Li(Gd,Y)F 4 :Yb,Er UCNPs can be readily synthesized by adjusting the ratio of Gd to Y, and it can reduce light scattering for the waveguide containing the UCNPs because light scattering coefficient increases with particle size 25 . Whereas smaller size of the UCNPs decreases light scattering, leading to higher transparency of the devices, it causes PL intensity of the UCNPs to decrease 26,27 . The issue of weaker PL intensity of smaller UCNPs is mainly attributed to large surface defect density and it can be overcome by suppressing the surface quenching effect via formation of shell on the UCNP core 3,[26][27][28][29] . In this study, Li(Gd,Y)F 4 :Yb,Er/LiYF 4 core/shell (C/S) UCNPs were synthesized to enhance green UC luminescence (UCL) and applied to the polymer waveguide-based flexible transparent displays. Recently, arrayed waveguide gratings on green-emitting UCNP layer by micromolding in capillaries were reported 30 . In the transparent film, patterned discrete UCNP agglomerates were placed between arrayed waveguide gratings surrounded by polydimethyl siloxane polymer with millimeter scale thickness and it shows that UCNPs can be applied to waveguides 30 . In this study, C/S-structured UCNPs were incorporated into a polymer, and highly thin and bright flexible transparent displays were fabricated using UCNP-incorporated polymer waveguides. To fabricate blue-emitting flexible transparent displays, Li(Gd,Y)F 4 :Yb,Tm/LiYF 4 C/S UCNPs were synthesized because Tm 3+ is known as one of the efficient blue-emitting activators 31,32 . Additionally, for red-emitting UCNPs, we synthesized NaGdF 4 :Yb,Tm/NaGdF 4 :Eu C/S UCNPs by adopting the method previously reported by Wang et al. 33 because Eu 3+ emits strong red emission peak via energy migration process under excitation with NIR light 33 . These green-, blue-, and red-emitting C/S UCNPs were incorporated into the polymer waveguides and the UCNP-incorporated polymer waveguide-based bright flexible transparent displays with highly thin thickness are demonstrated here for the first time.

Results and Discussion
Morphologies of UCNPs. Because the shell formation on the core UCNPs significantly enhances UCL as mentioned above 34,35 , C/S structure is believed to be crucial for UCNPs' practical applications. Thus, there have been many studies on the C/S-structured UCNPs and, in particular, C/S UCNPs based on NaREF 4 :Yb,Er(Tm) where RE represents rare earth elements such as Y, Gd, and Lu etc. were mainly reported 3 . It was also reported that LiREF 4 -based C/S UCNPs exhibit bright luminescence 31,32,36 . In the case of LiGdF 4 , there are some reports on the difficulty of the synthesis of LiGdF 4 nanocrystals with tetragonal structure via solution chemical route 24,37 . Thus, we modified material composition by substituting Gd with Y for the formation of single tetragonal-phased nanocrystals 24 . In this study, we report bright green-and blue-emitting LiYF 4 -coated Li(Gd,Y)F 4 :Yb,Er(Tm) C/S UCNPs with tetragonal structure, inducing tetragonal bipyramidal shape 38 . The transmission electron microscopy (TEM) images of the core and C/S UCNPs are shown in Fig. 1. Truncated parallelogram or hexagonal shapes are observed in the TEM images of Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm core UCNPs. In the TEM images of Fig. 1a-d, truncated parallelogram shape is mostly observed and the shape is attributed to 2D projection of the tetragonal bipyramidal morphology of the synthesized Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm UCNPs with blunt tips 38 . The synthesized Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm cores, and Li(Gd,Y)F 4 :Yb,Er/LiYF 4 and Li(Gd,Y)F 4 :Yb,Tm/LiYF 4 C/S UCNPs have tetragonal structure judging from the measured spacing of 0.47 nm between two adjacent lattice fringes, which are in good agreement with d-spacing between {101} planes (d 101 = 0.465 nm) of LiYF 4 with tetragonal structure (Figures S1-S4). The high-resolution TEM (HR-TEM) and high angle annular dark field (HAADF) high-resolution scanning transmission electron microscopy (HR-STEM) images indicate that the synthesized Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm cores and Li(Gd,Y)F 4 :Yb,Er/LiYF 4 and Li(Gd,Y)F 4 :Yb,Tm/LiYF 4 C/S UCNPs are single crystalline, as shown in Figures S1-S4. In the HR-TEM and HR-STEM images of C/S UCNPs, lattice mismatch between core and shell was not observed, indicating that the LiYF 4 shells were epitaxially grown on the Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm cores. The epitaxial growth of LiYF 4 shell on the core is due to the same crystal structure of core composition and shell composition with similar lattice parameter 39 . On the other hand, highly ordered hexagonal-shaped particles are viewed in the TEM image of the NaGdF 4 :Yb,Tm core UCNPs and the observed shape is attributed to the alignment of NaGdF 4 :Yb,Tm UCNPs with hexagonal prism morphology along < 0001 > direction (Fig. 1e). The rectangular shape observed in TEM image of Fig. 1f is ascribed to the alignment of NaGdF 4 :Yb,Tm/NaGdF 4 :Eu C/S UCNPs along < 101 0 > direction. As shown in HR-TEM and HR-STEM images of Figures S5 and S6, NaGdF 4 -based core and C/S UCNPs have single crystalline nature with high crystallinity based on highly clear lattice fringes. The measured lattice spacings between two lattice fringes of NaGdF 4 :Yb,Tm-based core and C/S UCNPs were in agreement with d-spacing between (1010) planes (d 1010 = 0.521 nm) of NaGdF 4 with hexagonal structure. In addition, crystal structures of the core and C/S UCNPs were characterized by using X-ray diffraction (XRD) patterns. The XRD patterns shown in Figure S7 support that Li(Gd,Y)F 4 :Yb,Er-and Li(Gd,Y)F 4 :Yb,Tm-based core and C/S UCNPs have tetragonal structure and NaGdF 4 :Yb,Tm-based core and C/S UCNPs have hexagonal structure.
After the formation of LiYF 4 shells on the Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm core UCNPs, the sizes of the UCNPs increased from 32.6 nm × 36.5 nm (short edge × long edge) to 36.8 nm × 41.5 nm and from 19.6 nm × 22.8 nm to 24.7 nm × 28.4 nm, respectively. Similarly, NaGdF 4 :Yb,Tm/NaGdF 4 :Eu C/S UCNPs showed larger particle size (39.5 nm) than the NaGdF 4 :Yb,Tm core UCNPs (34.6 nm) due to the growth of the NaGdF 4 :Eu layer on the NaGdF 4 :Yb,Tm core. The increase of particle size means that the shell was successfully formed on the core UCNP. Furthermore, the formation of the shell on the Li(Gd,Y)F 4 :Yb,Er, Li(Gd,Y)F 4 :Yb,Tm, and NaGdF 4 :Yb,Tm UCNP cores was directly confirmed by energy dispersive X-ray spectroscopy (EDS) analysis. The EDS maps shown in Fig. 2 apparently indicate that the C/S-structured UCNPs were successfully synthesized. It is Scientific RepoRts | 7:45659 | DOI: 10.1038/srep45659 noteworthy that the core element, Yb is located in the core region and shell elements (Y for the LiYF 4 shell and Eu for the NaGdF 4 :Eu shell) spread out into wider region than core region, indicating successful growth of the shell on the core UCNP ( Figures S8-S10).
Luminescence properties of UCNPs. Figure 3 shows UC photoluminescence (PL) spectra of green-, blue-, and red-emitting core and C/S UCNPs under excitation with 980 nm NIR light. In all cases, C/S UCNPs exhibited much stronger UC PL intensity than core UCNPs. In Fig. 3a, characteristic emission peaks of Er 3+ ions are observed due to 2 H 11/2 and 4 S 3/2 → 4 I 15/2 transitions in green spectral region and 4 F 9/2 → 4 I 15/2 transition in red spectral region 24 . By the formation of LiYF 4 shell, the Li(Gd,Y)F 4 :Yb,Er/LiYF 4 C/S UCNPs exhibited 3.3 times higher PL intensity than Li(Gd,Y)F 4 :Yb,Er core UCNPs. Although emission peaks are shown in both green and red spectral region, the Li(Gd,Y)F 4 :Yb,Er/LiYF 4 UCNPs exhibit green light due to intense emission peak attributed to 4 S 3/2 → 4 I 15/2 transition, as shown in Fig. 3b. The Li(Gd,Y)F 4 :Yb,Tm/LiYF 4 C/S UCNPs exhibited 2.0 times higher PL intensity than Li(Gd,Y)F 4 :Yb,Tm core UCNPs and showed bright blue light via electronic transitions from 1 D 2 to 3 F 4 and from 1 G 4 to 3 H 6 in Tm 3+ ions, as shown in Fig. 3c and d 31,40 . When optically inert LiYF 4 shell was grown on the green-and blue-emitting core UCNPs, PL intensities of green and blue emissions were significantly enhanced without change of the spectral shape by suppressing surface quenching due to the decrease of surface defects such as dangling bonds 26 . However, when NaGdF 4 :Eu was coated on the NaGdF 4 :Yb,Tm core, red emission peaks were created from the Eu 3+ ions via energy migration process through a network of the Gd sublattice in addition to PL enhancement of Tm 3+33, 41 . For UC red emission, excited energy is transferred from 1 I 6 level of Tm 3+ to 6 P 7/2 level of Gd 3+ through five-step UC process via efficient energy transfer of Yb 3+ → Tm 3+ followed by energy transfer from Gd 3+ to Eu 3+ 33 . Finally, sharp emission peaks at red spectral region are generated via strong electronic transitions of 5 D 0 → 7 F J (J = 1, 2, and 4) in Eu 3+ ions 42,43 . As a result, the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu C/S UCNPs emit reddish purple light because blue emission peaks due to 1 D 2 → 3 F 4 and 1 G 4 → 3 H 6 transitions of Tm 3+ exist together with Eu 3+ emission peaks, as shown in Fig. 3e and f. Although the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs do not emit pure red light, they were adopted as a red-emitting UC material due to its strong reddish UC emission under excitation with 980 nm NIR light. In our experimental condition, NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs showed stronger red light than other red-emitting UCNPs such as NaGdF 4 :Yb,Ho,Ce/NaYF 4 ( Figure S11). In addition, when we compared luminescence intensity of Li(Gd,Y) F 4 :Yb,Er/LiYF 4 with that of NaGdF 4 :Yb,Tm/NaGdF 4 :Eu, the former exhibited slightly higher PL intensity than the latter ( Figure S12). When we consider UC luminescence mechanism of both UCNPs, the Li(Gd,Y)F 4 :Yb,Er/LiYF 4 UCNPs emit green light via two-step UC process, whereas the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs exhibit blue emission from Tm 3+ via three-step UC process and red emission from Eu 3+ via five-step UC process, as shown in Figure S13. Therefore, it is expected that Li(Gd,Y)F 4 :Yb,Er/LiYF 4 UCNPs show much higher PL intensity than the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs under the same excitation condition. However, the difference of PL intensity between the Li(Gd,Y)F 4 :Yb,Er/LiYF 4 and the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs was not large as shown in Figure S12. This result is attributed to the fact that the size of NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs is larger than that of the Li(Gd,Y)F 4 :Yb,Er/LiYF 4 UCNP, because larger UCNPs exhibit stronger luminescence intensity than smaller UCNPs 26 . The related green-, blue-, and red-emitting UCL mechanisms are illustrated using energy level diagram as shown in Figure S13.
Fabrication of flexible transparent displays based on UCNP-incorporated polymer waveguide. These blue-, green-, and red-emitting C/S UCNPs were incorporated into bisphenol A ethoxylate diacrylate to fabricate polymer waveguide-based flexible transparent display devices. It was reported that bisphenol A ethoxylate diacrylate can be applied to the polymer waveguide core material for transparent thin film displays 23 , and we used bisphenol A ethoxylate diacrylate as a core material constituting polymer waveguides for application to the flexible transparent displays. The C/S UCNP-polymer mixtures also exhibited bright blue, green, and red UCL like the C/S UCNP solutions under the illumination with 980 nm NIR light, as shown in Figure S14. The UC blue-, green-, and red-emitting polymer waveguides were fabricated by spin-coating the blue-, green-, and red-emitting C/S UCNP-incorporated bisphenol A ethoxylate diacrylate core materials on the lower clad material (tetra(ethylene glycol) diacrylate), respectively, followed by formation of tetra(ethylene glycol) diacrylate upper clad. As shown in Fig. 4a, the fabricated C/S UCNP-incorporated polymer waveguides were highly transparent. All the fabricated waveguides (thickness ~38 μ m) containing blue-, green-, and red-emitting UCNPs exhibited high transparency with transmittance values of 86.7-93% in visible spectral region (400-800 nm) (Fig. 4a). In particular, the fabricated polymer waveguides exhibited high transmittance over 90% in the spectral range from 443 to 900 nm. Considering the transmittance value (89-93% in visible spectral range) of the bare polymer waveguide without the C/S UCNPs, it should be noted that there was little influence of the incorporation of C/S UCNPs into the polymer waveguides on the reduction of the transparency of the waveguides in visible spectral region. (Also, mechanical property of the polymer substrate was investigated and it was shown in Figure S15.) Due to this high transparency of the UCNP-incorporated polymer waveguides, background letters under the polymer waveguide are clearly seen (Fig. 4a-i). When the fabricated stripe-type polymer waveguides were coupled with a fiber laser emitting 980 nm NIR light, the waveguides exhibited uniform blue, green, and red light, respectively, indicating uniform dispersion of the C/S UCNPs in the bisphenol A ethoxylate diacrylate (Fig. 4a-ii). As shown in Fig. 4b, the blue-, green-, and red-emitting C/S UCNPs exhibited the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.1170, 0.1251), (0.3012, 0.6835), and (0.4290, 0.2329), respectively. Thus, any luminescence color from the waveguide can be generated in the triangle which is formed by connecting the CIE color coordinates of blue-, green-, and red-emitting C/S UCNPs by simply blending three C/S UCNP components. The color space generated by these blue-, green-, and red-emitting C/S UCNPs exhibited relatively narrow color gamut range compared with National Television System Committee (NTSC) color space (~49% NTSC). The narrow color gamut is attributed to the low color purity of the red light from the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu UCNPs and the color gamut can be widened if another efficient red-emitting UCNPs with higher color purity are used. As shown in the scanning electron microscopy (SEM) image of cross-section of the fabricated stripe-type waveguide, the thickness of the fabricated waveguide was observed to be ~38 μ m ( Figure S16). Because the polymer waveguides are very thin and have flexible nature of the polymer, they are easily bendable and the bended waveguides also exhibited bright and uniform UC blue, green, and red lights like flat waveguides without bending shown in Fig. 4a-ii (see Figure S17). Furthermore, UC blue, green, and red lights are well observed along the severely bended waveguides and there was no discrepancy of UCL between before and after bending (Fig. 4c-e). It is noted that the brightness of the UCL emitted from the fabricated waveguides can be easily enhanced by increasing input laser power ( Figure S18).
Furthermore, patterned waveguides were constructed through reactive ion etching (RIE) process as depicted in Fig. 5a (also see Figure S19). First, the lower clad layer (tetra(ethylene glycol) diacrylate) was spin-coated on a Si wafer and UV-cured (2 kW, 5 min), and the core layer (C/S UCNP-incorporated bisphenol A ethoxylate diacrylate) was spin-coated on the lower clad layer and UV-cured (2 kW, 5 min) (Fig. 5a-i). Second, optical waveguide was patterned by photolithography and RIE process (Fig. 5a-ii-5a-vi). Third, the upper clad layer was spin-coated on the patterned optical waveguide and UV-cured at 2 kW for 5 min (Fig. 5a-vii). Finally, the patterned optical waveguide was detached from the Si wafer (Fig. 5a-viii and 5a-ix). The patterned waveguide-based display devices were also highly transparent as shown in Fig. 5b and c. Patterned letter "J" of the free-standing waveguide is not distinguishable to the naked eyes although the pattern is evidently seen in the optical microscope image shown in Fig. 5d. However, blue, green, and red capital "J"s were distinctly observed owing to UCL along the patterned optical waveguides when the waveguides were coupled with 980 nm NIR light (Fig. 5e). The color of the patterned letter was easily tailored by adjusting the kinds of the incorporated UCNPs into the core Scientific RepoRts | 7:45659 | DOI: 10.1038/srep45659 polymer constituting the waveguides. Since these patterned waveguide-based display devices were also flexible, UCL images can be displayed from the largely curved waveguides. While the waveguides were severely bended, blue, green, and red UCL images (i.e., blue, green, and red capital "J"s) were clearly seen without any difference between UCL uniformities before and after bending (Fig. 5f). It is worthy to note that all these UCL images from the patterned waveguides are clearly seen under ambient indoor light condition.

Conclusion
In summary, highly transparent and flexible display devices based on C/S UCNP-incorporated polymer waveguide have been demonstrated. The C/S-structured Li(Gd,Y)F 4 :Yb,Er/LiYF 4 and Li(Gd,Y)F 4 :Yb,Tm/LiYF 4 UCNPs were successfully synthesized and they showed strong UCL intensities enhanced by the factors of 3.3 and 2.0 compared with Li(Gd,Y)F 4 :Yb,Er, and Li(Gd,Y)F 4 :Yb,Tm cores, respectively. As a result, they exhibited bright green and blue emission under illumination with 980 nm NIR light, respectively. The synthesized NaGdF 4 :Yb,Tm/ NaGdF 4 :Eu C/S UCNPs showed red emission via 5 D 0 → 7 F 2 transition in Eu 3+ in the shell through energy migration UC process under excitation with 980 nm NIR light. These blue-, green-, and red-emitting C/S UCNPs were incorporated into the bisphenol A ethoxylate diacrylate to fabricate flexible transparent waveguides. The polymer waveguide-based devices were highly transparent (transmittance > 90% in the spectral range of 443-900 nm). The flexible transparent monochromatic display devices were fabricated by patterning polymer waveguide through RIE process. The patterned polymer waveguide-based devices exhibited bright blue-, green-, and red-colored letters under coupling with a 980 nm NIR laser, irrespective of severe bending. These results can be a cornerstone to realize multicolor-emitting flexible transparent display devices utilizing NIR light.  A ethoxylate diacrylate (average molecular weight ~468) and tetra(ethylene glycol) diacrylate (molecular weight 302.32) were obtained from Aldrich.

Syntheses of the Li(Gd,Y)F 4 :Yb,Er and Li(Gd,Y)F 4 :Yb,Tm UCNPs.
For the synthesis of Li(Gd,Y) F 4 :Yb,Er UCNPs, rare earth oleate (RE(oleate) 3 , RE = Gd, Y, Yb, and Er) complexes were firstly prepared by adopting previously reported method by Hyeon's group 44 . Then 1 mmol of RE(oleate) 3 complexes (RE = Gd (35%), Y (45%), Yb (18%), and Er (2%)) were loaded into three-neck flask containing mixed solvents of OA (10.5 mL) and ODE (10.5 mL). The mixture was heated to 150 °C and maintained for 40 min to yield a transparent solution. After the reaction solution was cooled to 50 °C, a methanol (MeOH) solution (10 mL) containing LiOH·H 2 O (2.5 mmol) and NH 4 F (4 mmol) was injected into the reaction flask and then the reaction mixture was stirred for 40 min. After the MeOH was removed, the solution was heated to 320 °C and maintained for 90 min under Ar atmosphere. The as-synthesized Li(Gd,Y)F 4 :Yb,Er UCNPs were washed several times with ethanol (EtOH) and dispersed into chloroform. In the case of the synthesis of Li(Gd,Y)F 4 :Yb,Tm UCNPs, 1 mmol of RE(oleate) 3 complexes (RE = Gd (34.5%), Y (40%), Yb (25%), and Tm (0.5%)) were used as precursors and other synthetic procedures were the same as those for Li(Gd,Y)F 4 :Yb,Er UCNPs. The mixed solution was heated to 150 °C and maintained for 40 min. After the reaction mixture was cooled to 50 °C, 10 mL of MeOH solution containing NaOH (2.5 mmol) and NH 4 F (4 mmol) was added to the reaction solution. After stirring at 50 °C for 40 min, MeOH was removed and the reaction solution was heated to 310 °C and maintained for 90 min under Ar atmosphere. The as-synthesized UCNPs were washed with EtOH several times and dispersed in 10 mL of chloroform.
Synthesis of the NaGdF 4 :Yb,Tm/NaGdF 4 :Eu C/S UCNPs. To synthesize red-emitting C/S UCNPs, GdCl 3 ·6H 2 O (0.85 mmol) and EuCl 3 ·6H 2 O (0.15 mmol) were dissolved in mixed solvents of OA (6 mL) and ODE (15 mL) by heat-treatment at 150 °C for 40 min. After the reaction mixture was cooled to 80 °C, 10 mL of NaGdF 4 :Yb,Tm UC core chloroform solution was injected into the reaction flask, and then 10 mL of MeOH solution containing NaOH (2.5 mmol) and NH 4 F (4 mmol) was added to the reaction solution. After stirring at 50 °C for 40 min, MeOH was removed and the reaction solution was heated to 310 °C and maintained for 110 min under Ar atmosphere. The as-synthesized C/S UCNPs were washed with EtOH several times and dispersed in 10 mL of chloroform.

Fabrication of the flexible transparent display devices based on C/S UCNP-incorporated polymer waveguides.
To fabricate polymer waveguide-based flexible transparent display devices, the synthesized C/S UCNPs were first incorporated into bisphenol A ethoxylate diacrylate (refractive index, n a = 1.5647 at 632 nm) polymer core (0.12 wt%). The C/S UCNP-incorporated polymer was spin-coated on the lower clad (tetra(ethylene glycol) diacrylate, n a = 1.5031 at 632 nm), which was spin-coated on a Si wafer at 500 rpm for 20 s and UV-cured at 2 kW for 5 min, at 3700 rpm for 30 s and UV-cured at 2 kW for 5 min. Then, patterned optical waveguide was formed by photolithography and RIE process. Next, the upper clad was formed by spin-coating tetra(ethylene glycol) diacrylate on the patterned optical waveguide and UV-cured under the same condition as that for the formation of the lower clad layer. Finally, the blue-, green-, and red-emitting patterned waveguides were detached from the Si wafers.
Characterization. All PL spectra of the blue-, green-, and red-emitting UCNPs were collected with a Hitachi F-7000 spectrophotometer. The TEM images of the UCNPs were obtained by using an FEI Tecnai F20 G 2 transmission electron microscope operating at 200 kV and EDS maps on the C/S UCNPs were obtained on an FEI Talos F200X scanning transmission electron microscope (S/TEM) operating at 200 kV. The crystal structures of the synthesized UCNPs were investigated by XRD using a Bruker D8 ADVANCE diffractometer with Cu K α radiation under the condition of 40 kV and 40 mA. The HR-STEM images were obtained with an aberration-corrected FEI Titan 80-300 S/TEM operating at 300 kV. Transmittance of the fabricated devices were measured by using a Shimadzu UV-2600 spectrophotometer.