Stable organic light emitting coating

Abstract

The invention concerns a stable white light emitting diode (WLED) coating, composed of biological and organic materials and free of rare earth elements.

Claims

1. An optically excitable film comprising an optically excitable composition being a composite comprising crystalline nano-cellulose (CNC), at least one mucin and at least one organic dye selected from R-dyes, G-dyes and B-dyes, wherein the film is formed on a substrate material, the substrate being of a material selected from carbonaceous materials, metallic materials, oxides, glass, silicon-based materials, ceramic materials, polymeric materials, hybrid materials, biomimetic material, biomaterials, dielectric crystalline or amorphous materials, fibers, paper and any combination thereof.

2. The film according to claim 1, wherein the at least one mucin is PGM and the composite is in a form selected from CNC-PGM-R, CNC-PGM-G, CNC-PGM-B, CNC-PGM-RG, CNC-PGM-RB, CNC-PGM-GB and CNC-PGM-RGB.

3. A device comprising an optically excitable film comprising an optically excitable composite comprising crystalline nano-cellulose (CNC), at least one mucin and at least one organic dye selected from R-dyes, G-dyes and B-dyes, wherein the device is a multicolored organic light emitting diode (OLED).

4. The device according to claim 3, wherein the at least one mucin is PGM and the composite is in a form selected from CNC-PGM-R, CNC-PGM-G, CNC-PGM-B, CNC-PGM-RG, CNC-PGM-RB, CNC-PGM-GB and CNC-PGM-RGB.

5. A white-emitting LED, the LED comprising a UV-LED having on at least a region of its surface a film of an optically excitable film comprising an optically excitable composite comprising crystalline nano-cellulose (CNC), at least one mucin and at least one organic dye selected from R-dyes, G-dyes and B-dyes.

6. The LED according to claim 5, wherein the film comprises the G-dye 8-hydroxypyrene-1,3,6- trisulfonic acid trisodium salt and the R-dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran.

7. The LED according to claim 5, wherein the at least one mucin is PGM and the composite is in a form selected from CNC-PGM-R, CNC-PGM-G, CNC-PGM-B, CNC-PGM-RG, CNC-PGM-RB, CNC-PGM-GB and CNC-PGM-RGB.

8. A process for the preparation of a film according to claim 1, the process comprising incorporating a product of association of mucin and the at least one organic dye within a matrix composed of CNC to form a composite; and forming a film of the composite on a surface.

9. The process according to claim 8, wherein the product of association of mucin and the at least one organic dye is obtained separately and in advance to the combination step with CNC.

10. The process according to claim 8, wherein the product of association is a mixture of at least two products of association, wherein the first product is PGM-R and the second is PGM-G; or wherein the product of association to be combined with CNC is a combination of PGM-R and PGM-G.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 depicts an exemplary fabrication process of highly-stable REE-free WLED coating according to the invention. In the fabrication process (a) a hydrophobic red dye e.g., 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran and a hydrophilic green dye e.g., 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt are separately encapsulated within the porcine gastric mucin (PGM) structure, (b) the red dye is absorbed into the hydrophobic pocket of PGM, whereas the green dye is incorporated within the hydrophilic oligosaccharides. Thereafter, (c) the two complexes (PGM-R and PGM-G) are blended together to form a mixture (PGM-RG) that is then (d) incorporated within a matrix made of cellulose nanocrystals (CNC) forming a layered structure CNC-PGM-RG. (e) When the composite (CNC-PGM-RG) is coated onto a blue LED, white light is generated.

(3) FIGS. 2A-F present SEM and TEM micrographs of the CNC-PGM-RG coating. SEM micrographs in FIG. 2A, FIG. 2B show top view and FIGS. 2C and 2D show tilted view—featuring the conformal coverage of the protein complex of CNC. FIGS. 2E-F are TEM images of the CNC-PGM-RG composites.

(4) FIGS. 3A-D present fluorescence spectra of the (FIG. 3A) CNC-PGM-G-, (FIG. 3B) CNC-PGM-R- and (FIG. 3C) CNC-PGM-RG-films. FIG. 3D provides an optical image of the UV-illuminated CNC-PGM-RG coating.

(5) FIGS. 4A-C present a WLED device. FIG. 4A provides a photograph of the original UV-purple LED (left) and after deposition of the CNC-PGM-RG coating (right). In FIG. 4B shown is the original purple light emitted from an operating UV-purple LED in comparison to FIG. 4C where the emission of white light is shown after being coated with the CNC-PGM-RG composite.

(6) FIGS. 5A-B present fluorescence spectra under (FIG. 5A) humidity (90% RH), and heat conditions (85° C.) over time, and under (FIG. 5B) constant UV illumination.

(7) FIGS. 6A-C show the effect of an external stimuli on (FIG. 6A) the correlated color temperature (CCT), (FIG. 6B) the color rendering index (CRI) and (FIG. 6C) the CIE values.

DETAILED DESCRIPTION OF EMBODIMENTS

(8) Composite Characterization

(9) A complete WLED device is also provided that is composed of a blue/UV LED chip and a REE-free coating. The concept of a film fabrication and operable as a coating for WLED is shown in FIG. 1, described hereinabove.

(10) Optical and confocal images of a coating and of the free-standing film were obtained. It was evident that the films lose some of their transparency upon introduction of the PGM-bound dyes. While the addition of the hydrophobic R-dye substantially decreased transmittance, the hydrophilic G-dye only affected it slightly. The RG-film (white emission) exhibited average transmittance properties of these two (˜40% transparency).

(11) To evaluate the homogeneity of the composites, three-dimensional (3D) confocal microscopy images of the white film were taken. Each of the 3D images was composed of multiple two-dimensional layers taken at different depths, thus the overall picture was correlated to 3D distribution of the dyes. Images of the white film (λ.sub.excitation=405 nm) and of the 3D spatial distribution of each of its RGB bands provided evidence that the guest material (PGM-dyes) was homogeneously distributed within the CNC host matrix. Morphological characterizations of the films were performed using scanning electron and transmission electron microscopy (SEM, TEM).

(12) FIGS. 2A-D show top and tilted SEM micrographs of the films. The SEM results indicate that the PGM complex uniformly covers the fibrous network of the CNC host, further supporting the confocal microscopy observations. The TEM analysis also supports previous studies pointing towards the formation of a layered structure of the composites.

(13) Additionally, results indicate that multiple layers with ellipsoid components of an average size of approximately 250 nm are formed in the CNC and the composite materials.

(14) Next, the mechanical properties of the stand-alone films were evaluated. Qualitative bending test taken for the CNC-PGM-RG film, exhibits pronounced elastic properties. An optical investigation revealed that the bending action did not cause any deformation or cracks.

(15) In most cases, the two-dye composite showed average characteristics of the G-based complex (highest tensile stress/strain and toughness) and the R-complexes (the lowest values).

(16) Optical Properties

(17) Fluorescence spectra (λ.sub.excitation−420 nm) and a corresponding image of a white film are shown in FIG. 3. The fluorescence data (FIG. 3A-C) shows a relatively broad spectrum for the CNCPGM-R (λ.sub.max=605 nm) and the CNC-PGM-G (λ.sub.max=548 nm), indicating that each dye maintains its optical properties when incorporated in the host matrix. The emission spectra of the CNC-PGM-RG film (λ.sub.max=562 nm and 625 nm) is shown in FIG. 3C. As expected, a continuous broadband spectrum was obtained, indicating the existence of a white-emitting coating. When the film was illuminated (FIG. 3D, λ.sub.excitation=365 nm), uniform and bright white light was obtained from the film.

(18) Further optical characterization of the light emitting coating revealed that the material exhibits a quantum yield of 38.1%, a color rendering index (CRI) of 84.4, and a correlated color temperature (CCT) ranging between 3543 K and 4150 K, which indicates the formation of a warm-white light LED (CCT<4500 K). To form a complete white-emitting solid LED, the composite was deposited on the surface of a commercial UV-LED (λ=380 nm), which was used as an excitation light source. FIG. 4 shows that the fabricated LED exhibits conformal and bright white light emission.

(19) Stability Tests

(20) The effect of humidity, temperature, and UV on the luminescent properties of the CNC-PGMRG coating was investigated (FIGS. 5 and 6). All films were optically examined after the stability tests. No cracks or other defects were found and the coatings kept their transparency and exhibited white light emission under UV illumination. After a 24-hour exposure to relative humidity (RH) conditions of 90%, no significant change in the spectra was observed (FIG. 5A). However, after additional 10 d under the same conditions, some photobleaching was detected. This is manifested by a narrow- and a blue shift of the emission spectrum.

(21) Heat stability tests clearly indicate that the CNC matrix efficiently protects the PGM-bound dyes. Interestingly, an unexpected increase of the fluorescence intensity was measured. This finding suggests the presence of heat-induced conformational changes of the PGMs' ternary structure, which in turn, result in a decrease in the efficiency of the unwanted UV absorption by the PGM. This leads to a subsequent increase in the dye emission efficiency.

(22) Exposure of the complex to UV radiation (λ=365 nm) induces some decrease in the emission of the CNC-PGM-RG coating. It can be seen that with an increase in exposure time, the contribution of R to the spectra dye decreases.

(23) Next, the effect of the external stimuli on the optical properties was studied. FIG. 6 shows the results of these test as calculated and measured by means of CRI, CCT, and CIE diagram (1931) chart. Although some decrease in the optical quality was observed (especially in the case of humidity), the light coordinates did not change significantly (yellow region), giving a good indication on the stability of the coating. Further investigation of the optical characteristics revealed that the quantum yield dropped only by 0.7% after heat treatment for 11 days, but a substantial drop of 13.7% was observed with respect to humidity.

(24) Discussion

(25) The invention disclosed herein provides (bio)composite for white light-emitting applications. It is evident that although the film is made of biological and organic materials it exhibits excellent optical and mechanical properties and found to be resilient to external stimuli. Few insights on the composite of the invention may be pointed out:

(26) (i) The mechanical properties of the films, as well as its transparency, are affected by the chemical nature of the dyes and of the incorporation of the PGM. In general, except for some decrease in Young's modulus of the composite films compared with pristine CNC, ultimate strain ultimate stress and toughness were improved in the case of addition of the PGM-dyes. This can be explained by the plasticizing effect of the PGM-dyes. Similar effects have been shown in the case of CNC-Resilin composites. Notably, the incorporation of the PGM-G (hydrophilic) with the hydrophilic CNC allows the formation of strong interactions within the composite, which increase its mechanical properties. The film exhibits significant improvement in the average moduli and tensile stress and strain values at break, resulting in tougher films compared to CNC-PGM-R film. In the case of the CNC-PGM-R, hydrophilic-hydrophobic interactions take place which reduces the complex's mechanical behavior. In all cases, the addition of the PGM-R/G improved the mechanical properties of the films indicating strong interactions between the CNC and the PGM.

(27) (ii) The composite was found to be relatively resilient to heat and UV, but less to humidity stimuli. Being a hydrophilic polymer, CNC can absorb water molecules, which might affect the hydrogen bonds composing the CNC network, leading to oxygen and water permeability. Water permeation through the composite can facilitate oxidation and photobleaching of the fluorescent organic dyes causing a decrease in their emission intensity and thus, decrease in internal quantum efficiency. Chemical modifications of the CNC preventing further oxygen penetration, as well replacement of the R dye by better anti-oxidant one might resolve this problem.

(28) (iii) The CNC-PGM-RG coating showed high chromaticity stability. After exposure to UV, humidity and heat stimuli, CIE coordinates remained very close to initial state value.

(29) The device of the invention is a REE-free bio-organic phosphor applicable for the use in WLED applications made out of CNC-PGM-RG biocomposite material. White color emission was observed when the complex was excited by UV/blue LED, and when implanted in a UV working LED. The coating was prepared by incorporation of PGM with embedded R- and G dyes into CNC. The coating revealed high thermal and UV stability, which play a crucial role in LED performance Nevertheless, the coating exhibits a reduced stability under high humidity conditions which requires further research into solving this issue.

Experimental Section

(30) Composite preparation: G, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (Sigma-Aldrich), and R, (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran), (Sigma-Aldrich) were introduced to a 20 mg/ml of PGM(Sigma-Aldrich) solution to prepare the following complexes:

(31) Single color complexes: (1) G dye—4 mg, (2) R-3 mg, were added to 1 ml of PGM solution and stirred at 600 rpm for 72 hours. RG complexes: two single-color complex solutions (PGM-R and PGM-G) were mixed with ratio 1:1. The solution was then stirred at 600 rpm for 24 hours.

(32) CNC suspension: A clear and transparent CNC suspension manufactured and kindly supplied by Melodea Ltd (% S=0.15, Z-Potential=−29±2), dialyzed for 5 days against distilled water. The CNC suspension was then sonicated (80% amplitude, 100% pulse time) for 5 minutes (500 V UIP1000hd industrial sonicator, Hielscher, Teltow, Germany).

(33) CNC coating: PGM-RG complex solution were dispersed in CNC (2 wt. %) suspensions at ratio 1:12. The mixtures were vigorously stirred. 5 mL of CNC-PGM-RG suspensions were cast onto a glass substrate (7 cm×2.5 cm) in order to form 50 μm±10 μm thick layer.

(34) Stand-alone film preparation: Stand-alone films. PGM-RG complex solution were dispersed in 13 ml CNC (2 wt. %) suspensions at ratio 1:12. The mixtures were vigorously stirred and cast onto a Sigmacote® treated glass substrate (7 cm×5 cm). The CNC/PGM-RG suspensions were dried for 48 h under ambient conditions. Finally, dried films were detached and cut with sharp scissors to the required form.

(35) Confocal microscopy: Nikon A1.sub.+confocal setup was used to scan the stand-alone film using 405 nm laser. For evaluation of the RGB bands contributions to the spectrum, Red (570-620 nm and 663-738 nm) Green (500-550 nm) and Blue (425-475 nm) filters were used.

(36) SEM: Measurements were carried Out in Dual FIB-SEM (model FEI Helios Nanolab 600 Ion milling). The images were taken at top- and tilted. (52°)—views with an electron beam acceleration voltage of 1-2 kV and a current of 86 pA, while using a secondary electron detector. Prior to imaging, 14 nm of gold was sputtered on the samples to improve their conductivity.

(37) TEM: Imaging was carried out using a computer-controlled TEM (JEM-2100F Jeol Pty Ltd.) fitted with a field emission gun. Samples were diluted in distilled water (×5) and deposited on a TEM grid (Gilder TEM Grids 600 mesh, Ted-Pella Inc.). Experiments were operated at 200 kV accelerating voltage and images were acquired with a Gatan UltraScan 1000 (2 k×2 k) CCD camera.

(38) Mechanical tests: CNC-PGM-RG films were cut into rectangle 5 mm×20 mm stripes. The films' average thickness was obtained by measuring 4 random positions along the film using a Mitutoyo Digimatic Indicator (Type ID-S112 MB, Mitutoyo Manufacturing Co. Ltd).

(39) The Caliber: Tensile properties of the films were determined by using an Instron 3345 universal testing machine equipped with a 100 N load cell, utilizing a cross-head speed of 2 mm/min, at 25° C. Five measurements were performed for each sample. The stress-strain curves were plotted and related parameters were calculated from the obtained results.

(40) Optical properties: Fluorescence spectra were measured using a Fuorolog-322 spectrometer (Jobin Yvon). The measurements were done by excitation at 420 nm and emission was recorded over a spectral range of 435-700 nm (1 nm steps) with an integration time of 0.3 sec. Absolute PL quantum yield measurements were performed using an absolute quantum yield characterization system (Hamamatsu Quantaurus QY).

(41) LED Fabrication: In order to obtain white LED, first, an epoxy layer of commercial UV LED was removed. Then the diode was coated with CNC-PGM-RG suspension by drop casting method. The coated device was operated by 3V battery.

(42) CRI and CCT: CRI was calculated from the fluorescence spectra CCT of the phosphor was calculated by using the McCamy's approximation algorithm.

(43) Stability Tests

(44) Humidity: Samples were placed in a home-built humidity cell at 90% humidity for 24 hours. Then the samples were removed from the cell and examined. Afterwards, the samples were placed back for additional 10 days and then re-examined.

(45) Heat: Samples were placed into the oven at 85° C. for 24 hours. Then the samples were removed from the oven and examined Afterwards, the samples were placed back for additional 10 days and then re-examined.

(46) UV: UV lamp (Spectroline, ENF-260C/FE) was used. The samples were exposed to UV during 11 days (λ=365 nm) and were investigated every 24 hours. The emission spectra were obtained after every 24 hours exposure during first three days, then after 5,7,9 and 11 days.