Patent Publication Number: US-10317785-B2

Title: Optical film

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of and claims the priority benefit of U.S. application Ser. No. 15/490,766, filed on Apr. 18, 2017, now pending. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an optical film using a composite material. 
     2. Description of Related Art 
     Projector is a display device used for producing large size images. A projector is to convert an illumination beam produced by a light source module into an image beam by using a light valve, and project the image beam onto a screen or a wall through a projection lens to form an image. 
     In order to produce illumination beams of three primary colors (red, blue, green), some projectors are configured with color wheels. The projector color wheel has a plurality of light converting regions (for example, filter regions or phosphor regions). The light converting regions are able to convert, for example, an incident beam produced by the light source into the required color beam. 
     When in operation, the projector color wheel is exposed to high temperature (up to 180° C.) and intensive light within the projector. Similarly, the surface temperature of a light emitting diode (LED) chip can be up to 120° C., with intensive illumination of light emitted from the LED chip. Currently, phosphors are used in combination with a blue light source to produce a white light. In order to improve the color gamut, it is desirable to use quantum dots (QDs) instead of the conventional phosphors. However, the high temperature and the intensive light will damage the QDs very quickly. In the situation, the luminance of the QDs will be lost permanently. 
     SUMMARY OF THE INVENTION 
     The invention provides an optical film using a composite material. The composite material includes a plurality of quantum dots and a silica material encapsulating surfaces of the quantum dots. Therefore, the thermal stability is enhanced by using silica (SiO 2 ) material to encapsulate the quantum dots. 
     In an embodiment of the invention, an optical film includes a first component, a second component, and a third component. The first component includes a plurality of quantum dots and a silica material encapsulating surfaces of the quantum dots. The second component includes a UV curable composite material. The first component is dispersed in the second component. The third component includes two PET (polyethylene terephthalate) substrate sheets. The first component and the second component are disposed between the two PET substrate sheets. 
     In an embodiment of the invention, the silica material includes an inorganic polymer made from tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). 
     In an embodiment of the invention, the UV curable composite material includes 2-phenylethyl methacrylate, triaryl isocyanurate (TAIC), diallyl phthalate, and a photoinitiator. 
     In an embodiment of the invention, the optical film is used in a light emitting apparatus, such as a backlight unit in a flat-panel display apparatus, a liquid crystal display monitor or other similar visual display apparatus. 
     Based on the above, the invention provides an optical film using a composite material. The optical film includes a first component, a second component and a third component. The first component includes the composite material having a plurality of quantum dots encapsulated by a silica material. The first component is dispersed in the second component including a UV curable composite material. The first component and the second component are disposed between the third component including two PET substrate sheets. The thermal stability of the composite material is enhanced by using the silica material to encapsulate the quantum dots. Moreover, the quantum dots are uniformly dispersed in the silica material (e.g., encapsulating material). As a result, the luminescence efficacy of the composite material is improved which is suitable for a backlight unit, projector color wheels and surface packaging of photoelectric devices, such as LED devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a front view illustrating a projector color wheel according to a first embodiment of the invention. 
         FIG. 2A  is a schematic drawing illustrating a composite material according to a second embodiment of the invention. 
         FIG. 2B  is a schematic drawing illustrating a composite material according to a third embodiment of the invention. 
         FIG. 3  is a flow-chart drawing illustrating a manufacturing flow of a composite material according to a fourth embodiment of the invention. 
         FIG. 4A  and  FIG. 4B  are schematic drawings illustrating the manufacturing flow of the composite material according to  FIG. 3 . 
         FIG. 5  is a transmission electron microscopy (TEM) image of a composite material of Experimental example 1. 
         FIG. 6A  is a green test screen from DLP projector of Experimental example 2. 
         FIG. 6B  is a red test screen from DLP projector of Experimental example 3. 
         FIG. 7  is a LED output spectrum of Experimental example 4. 
         FIG. 8  is a LED output spectrum of Experimental example 8. 
         FIG. 9  is a luminescence intensity of Experimental example 9. 
         FIG. 10  is a plot of LED efficacy of Experimental example 5 and Comparative example 1. 
         FIG. 11  is a schematic drawing illustrating an optical film according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, the invention is illustrated more comprehensively referring to the drawings. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Thicknesses of layers and regions in the drawings may be enlarged for clarity. The same or similar reference numbers represent the same or similar components, and are not repeated again in the following paragraphs. 
     In the present specification, ranges represented by “a numerical value to another numerical value” are schematic representations to avoid listing all of the numerical values in the range in the specification. Therefore, the recitation of a specific numerical range discloses any numerical value in the numerical range and a smaller numerical range defined by any numerical value in the numerical range, as is the case with any numerical value and a smaller numerical range stated expressly in the specification. For instance, the range of “a size of 100 nm to 500 nm” discloses the range of “a size of 200 nm to 350 nm”, regardless of whether other numerical values are listed in the specification. 
       FIG. 1  is a front view illustrating a projector color wheel according to a first embodiment of the invention. 
     Referring to  FIG. 1 , the projector color wheel  10  of the first embodiment includes a plurality of different color converter regions  12 ,  14  and  16  and a rotating portion  18 . The color converters  12 ,  14  and  16  are arranged to form a disk configuration. The rotating portion  18  is disposed at a center of the disk configuration. That is, the color converters  12 ,  14  and  16  are arranged as an annular shape which surrounds a circumference of the rotating portion  18 . Rotation of the rotating portion  18  may drive the projector color wheel  10  to rotate. Specifically, by controlling rotation of the projector color wheel  10  according to the time sequence, an illumination beam from a light source device (not shown) is converted or filtered into various light beams of different colors according to the time sequence when passing through the different color converter regions of the projector color wheel  10 . 
     In some embodiments, the illumination beam from a light source device may be blue illumination beam. The converter regions would separately convert blue illumination beam to green and red beams. In alternative embodiments, illumination beam from a light source device may be short visible (e.g., a wavelength thereof is about 405 nm) or ultraviolet (UV) illumination beam. The converter regions would separately convert Vis or UV illumination beam to blue, green, and red beams. 
     In some embodiments, the color converter  12  may be an opening, a transparent region without color for letting the illumination beam (for example, a blue illumination beam) to pass through directly without altering color. In alternative embodiments, the color converter  12  may be a reflective surface for letting the illumination beam (for example, a blue illumination beam) to reflect without altering color. The color converters  14  and  16  may include different composite materials, which are called as a first color converter  14  and a second color converter  16  hereinafter. The first color converter  14  may include a first composite material for converting the illumination beam into a first color illumination beam (i.e., to change a wavelength of the illumination beam). On the other hand, the second color converter  16  may include a second composite material for converting the illumination beam into a second color illumination beam. In one embodiment, the first color illumination beam differs from the second color illumination beam. Namely, a wavelength of the first color illumination beam differs from a wavelength of the second color illumination beam. For example, the first color converter  14  may convert the illumination beam into red beam, while the second color converter  16  may convert the illumination beam into green beam. However, the invention is not limited thereto. In another embodiment, the composite materials may be chosen according to the needs of color. In alternative embodiment, a particle size and composition of a quantum dot of the first composite material differs from a particle size and composition of a quantum dot of the second composite material. Although only three color converters are illustrated in  FIG. 1 , but the invention is not limited thereto. In other embodiments, a number of the color converters and the color of the converters may be adjusted according to the design. 
       FIG. 2A  is a schematic drawing illustrating a composite material according to a second embodiment of the invention.  FIG. 2B  is a schematic drawing illustrating a composite material according to a third embodiment of the invention. 
     Referring to  FIG. 2A  and  FIG. 2B , each of the foregoing composite materials includes at least one quantum dot and a silica material. In detail, as shown in  FIG. 2A , one quantum dot  102   a  is located in a silica material  104   a  to form a composite material  100   a . That is, a surface of the one quantum dot  102   a  is encapsulated by the silica material  104   a . Similarly, as shown in  FIG. 2B , a plurality of quantum dots  102   b  are dispersed uniformly in a silica material  104   b  to form a composite material  100   b . Surfaces of the quantum dots  102   b  are encapsulated by the silica material  104   b . Although only four quantum dots  102   b  are illustrated in  FIG. 2B , but the invention is not limited thereto. In other embodiments, a number of the quantum dots may be adjusted according to the design. 
     In one embodiment, a particle size W 1  of the composite material  100   a  is less than a particle size W 2  of the composite material  100   b . The particle size W 1  of the composite material  100   a  and the particle size W 2  of the composite material  100   b  may respectively range from 10 nm to 500 μm. In another embodiment, the particle size W 1  of the composite material  100   a  and the particle size W 2  of the composite material  100   b  may respectively range from 10 nm to 1 μm. 
     In some embodiments, the quantum dots  102   a  and  102   b  refer to nanostructures that are substantially monocrystalline. However, the invention is not limited thereto. In other embodiment, the quantum dots  102   a  and  102   b  refer to nanostructures that are polycrystalline or amorphous. 
     In some embodiments, the quantum dots  102   a  and  102   b  may be core structure, core-shell structure, core-multishell structure, alloy structure, core-alloy layer-shell structure, core-alloy layer-multishell structure, core-gradient alloy-shell structure, or a combination thereof. When the quantum dots  102   a  and  102   b  are core-shell structure, each of the quantum dots  102   a  and  102   b  includes a core structure and a shell structure covering the core structure. The core structure may be selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, GaN, GaP, GaAs, InP, InAs, InSb, AlN, AlP, AlAs, AlSb, SiC, Fe, Pt, Ni, Co, Al, Ag, Au, Cu, FePt, Si, Ge, PbS, PbSe, PbTe, and alloys thereof. The shell structure may be selected from the group consisting of ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe and PbTe, and alloys thereof. However, the invention is not limited thereto. In other embodiment, the quantum dots  102   a  and  102   b  may be perovskite. 
     In some embodiments, the silica materials  104   a  and  104   b  may be a network of alternating crosslinked silicon (Si) and oxygen (O) extending in all possible directions, which may be respectively represented by formula I: 
     
       
         
         
             
             
         
       
     
     wherein the dotted lines represent continual crosslinked networks of —Si—O—. 
     In some embodiments, each of the silica materials  104   a  and  104   b  may have a particle size of 10 nm to 500 μm. However, the invention is not limited thereto. In other embodiment, the size of the silica materials  104   a  and  104   b  may be adjusted according to the needs. 
     It should be noticed that the quantum dots  102   a  and  102   b  are respectively covered by the silica materials  104   a  and  104   b , thus the thermal stability of the composite materials  100   a  and  100   b  are enhanced with respect to the conventional quantum dots without silica material covering. In addition, since the projector color wheel of the present invention includes the composite material having quantum dots, the color gamut of the light produced by the projector color wheel of the present invention is enhanced compared with the color gamut using conventional phosphors. Moreover, as shown in  FIG. 2B , since the quantum dots  102   b  are uniformly dispersed in the silica material  104   b  without aggregation, the luminescence efficacy of the composite material  100   b  is improved, which is suitable for surface package of photoelectric devices, such as LED devices, and digital light processing (DLP) devices, such as DLP projector color wheels. 
     In some embodiments, each of the composite materials  100   a  and  100   b  further include a silicon resin (not shown) covering a surface of each of the silica materials  104   a  and  104   b . Hence, the thermal stability the composite materials  100   a  and  100   b  are further enhanced and enable the composite materials  100   a  and  100   b  to be easily moulded into various shapes and sizes depending on the application. 
       FIG. 3  is a flow-chat drawing illustrating a manufacturing flow of a composite material according to a fourth embodiment of the invention.  FIG. 4A  and  FIG. 4B  are schematic drawings illustrating the manufacturing flow of the composite material according to  FIG. 3 . 
     Referring to  FIG. 3 , a method  300  for manufacturing the composite material of the fourth embodiment is as follows. First, a step  302  is performed. A mixture is prepared. The mixture includes a plurality of quantum dots and a non-polar solvent, wherein the quantum dots are dissolved in the non-polar solvent. In some embodiments, the mixture is stirred or sonicated at room temperature (e.g., 25° C.) to fully dissolve the quantum dots. The non-polar solvent may be but not limited to cyclohexane, hexane, toluene, or a combination thereof. In some embodiments, a content of the quantum dots is 0.8-20 wt % and a content of the non-polar solvent is 80-99.2 wt % based on the weight of the mixture (i.e., a sum of the quantum dots and the non-polar solvent). 
     Next, steps  304  and  306  are performed. A polar solvent and a surfactant are prepared. In some embodiments, the polar solvent may be but not limited to water. The surfactant may include organic compounds that are amphiphilic. That is, the organic compounds contain both hydrophobic groups (e.g., tails) and hydrophilic groups (e.g., heads). Therefore, the surfactant contains both a polar-insoluble (e.g., water-insoluble) or non-polar-soluble (e.g., oil-soluble) component and a polar-soluble (e.g., water-soluble) component. For example, the surfactant will diffuse in water and adsorb at the interface between oil and water, in the case where water is mixed with oil. The water-insoluble hydrophobic group may extend out of the bulk water phase or into the oil phase, while the water-soluble head group remains in the water phase. 
     Incidentally, the order of the step  302 ,  304  and  306  can be adjusted according to the needs. That is, after preparing the mixture (i.e., the step  302 ), the polar solvent and the surfactant are prepared (i.e., the steps  304  and  306 ); however, the invention is not limited thereto. In one embodiment, the steps  304  and  306  may be performed first, and then the step  302  is performed. In alternative embodiment, the steps  302 ,  304  and  306  may be performed at the same time. 
     Step  308  is then performed. The mixture, the polar solvent and the surfactant are mixed to form an emulsion. Referring to  FIG. 4A , after mixing the mixture, the polar solvent and the surfactant in a container  203 , an emulsion  201  is formed in the container  203 . The emulsion  201  may include a plurality of dispersed phases  206  and a continuous phase  204 . The dispersed phases  206  are dispersed in the continuous phase  204 . As shown in the enlarged view of  FIG. 4A , one of the dispersed phases  206  and part of the continuous phase  204  constitute single micelle  205 . A plurality of micelles  205  are dispersed uniformly in the emulsion  201 . In addition, the micelle  205  also includes a plurality of surfactant compounds  208 . Each of the surfactant compounds  208  includes a hydrophilic group  208   a  and a hydrophobic group  208   b . Since the dispersed phases  206  include the foregoing polar solvent, the hydrophilic groups  208   a  face toward interfaces  210  between the dispersed phases  206  and the continuous phase  204 . On the other hand, since the continuous phase  204  includes the foregoing non-polar solvent, the hydrophobic groups  208   b  are away from the interfaces  210 . As a result, the surfactant compounds  208  are arranged as a spherical shape. Moreover, a plurality of quantum dots  202  are also included in the emulsion  201 . The quantum dot surface changes from non-polar to polar when surrounded by surfactant molecules. During mixing the mixture, the quantum dots with polar surfaces tend to move from the less polar continuous phase  204  to the more polar dispersed phase  206  that is surrounded by the hydrophilic groups of the surfactants. 
     Referring back to  FIG. 3  and  FIG. 4B , a step  310  is then performed. A catalyst and a silica precursor are added into the emulsion  201  to form a plurality of composite materials  200  in the dispersed phases  206  respectively. Each of the composite materials  200  includes quantum dots  202  and a silica material  212  (i.e., the silica precursor after performing the step  310 ). The quantum dots  202  are dispersed uniformly in the silica material  212 . In particular, steps of adding the catalyst and the silica precursor may include a hydrolysis, a condensation and a polymerization. During the hydrolysis, the silica precursor is fully hydrolyzed or partially hydrolyzed by H 2 O from the foregoing polar solvent, so as to form siloxane compounds with hydroxyl groups (OH groups) attached on Si of the siloxane compounds. The hydrolysis scheme (1) is represented as below. 
                         
wherein each R is independently selected from the group consisting of C 1-8  alkyl, cycloalkyl and aryl.
 
     Then, during the condensation, two or more of the fully hydrolyzed or partially hydrolyzed siloxane compounds are bonded together. The condensation scheme (2) or (3) is represented as below. 
                         
wherein each R is independently selected from the group consisting of C1-8 alkyl, cycloalkyl and aryl.
 
     During the polymerization, one hydrolysis and one condensation are repeated one or more times to form the foregoing silica materials. 
     In some embodiment, the catalyst may be a base. The base may be but not limited to ammonia, ammonium fluoride, sodium hydroxide or a combination thereof. The silica precursor may include tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), or a combination thereof. The catalyst are polar molecules situated in the polar dispersed phase  206 , which is generally separated from the non-polar silica precursor in the non-polar continuous phase  204 . During mixing, silica precursor and catalyst will come into contact at the polar and non-polar interface due to the random movements of the molecules and initiate the polymerization reaction. Thus the silica materials  212  tend to surround the quantum dots  202  in the polar dispersed phase  206 , forming the foregoing composite materials  200 . It should be noted that a number of the quantum dots  202  in each of the silica materials can be controlled by the weight ratio of the quantum dots  202  and the silica precursor. In addition, the particle size of each of the composite materials  200  can also be controlled by the size of each of the micelles  205 . That is, the composite materials  200  having similar or the same particle size can be obtained according to the foregoing method of the present invention. Each of the composite materials  200  may have similar or the same number of the quantum dots  202  therein. In some embodiments, the weight ratio of the quantum dots  202  to the silica precursor ranges from 1:1 to 1:200. 
     After performing the step  310 , a step  312  is performed. A centrifugation process and at least one cleaning process are performed to collect the precipitate of the composite materials  200 . In some embodiments, the cleaning process may include using ethanol to wash resulting products after performing the centrifugation process. 
     In one embodiment, the composite materials  200  may be mixed with silicone and disposed onto a LED chip. The silicone may be a commercially available silicone (e.g., Dow Corning® OE-6370). More specifically, the LED chip is one component of a LED device. The LED device may include a substrate with a cavity and the LED chip disposed in the cavity of the substrate. The composite material formed from foregoing method  300  is mixed with silicone and filled in the cavity of the substrate and covers a surface of the LED chip. The composite material includes a plurality of quantum dots. The quantum dots may emit red, green, blue or any other colored lights desired. The quantum dots are able to absorb light energy emitted from the LED chip and re-emit the absorbed energy as light of a different wavelength. The quantum dots may have variously-regulated light emitting wavelengths. For example, one white LED device may be fabricated by combining red and green quantum dots with a blue LED chip. Alternatively, another white LED device may be fabricated by combining red, green, and blue quantum dots with a short visible (Vis) or an ultraviolet (UV) LED chip. 
     It should be noted that the silica material used in the LED device may be a modification-free silica material. Specifically, the modification-free silica material is able to react with the silicone without any additional functional groups, such as methacrylate, vinyl, vinyl acetate, alkene, thiol, or a combination thereof. If the silica material is modified with an additional functional group, propylmethacrylate (as in TMOPMA) for example, the thermal stability of the composite material formed is decrease (see  FIG. 10 ). That is, the composite material having the modified silica material is not suitable for surface package of the light emitting apparatus, such as the LED device. On the other hand, the composite material of the embodiment having the modification-free silica material, thus the thermal stability of the composite material formed is enhanced (see  FIG. 10 ). 
     In alternative embodiment, the composite materials  200  may be used as color converters of a projector color wheel. Since the quantum dots  202  are encapsulated by the silica material  212 , the thermal endurance of the composite material  200  are enhanced. Therefore, the color gamut of a projector is not only improved, the luminance property of the quantum dots  202  disposed in the projector color wheel of the projector is also maintained. 
     In addition, the composite materials  200  may be mixed with luminescent phosphor material, so as to supplement the deficiency of the luminescent phosphor material and thus increase the color gamut of the light emitting apparatus, such as the projector color wheel or the LED device. In detail, a mixture includes the luminescent phosphor material and the composite material. The luminescent phosphor material is capable of emitting light with a first color. The composite material includes a plurality of quantum dots capable of emitting light with a second color and a silica material encapsulating surfaces of the quantum dots. The first color and the second color are complementary to each other. 
     In one embodiment, the luminescent phosphor material may be a yellow-emitting or green-emitting phosphor material with dominant emission wavelength of 500 nm to 600 nm, such as yttrium aluminium garnet (YAG; Y 3 Al 5 O 12 :Ce 3+ ), silicate ((Ba, Sr) 2 SiO 4 :Eu 2+ ), β-SiAlON (Si 6-z Al z O z N 8-z :Eu 2+ ), or γ-AlON (Al 1.7 O 2.1 N 0.3 :Mn 2+ , Eu 2+ ). Due to its deficiency in emitting red color, a red-emitting quantum dot encapsulated in silica material (dominant emission wavelength of 600 nm to 650 nm) may be used to supplement this deficiency and thus increase the color gamut of the final luminescent composite material. For example, when the luminescent phosphor material is a yellow-emitting phosphor, and the quantum dots may be red quantum dots, green quantum dots or a combination thereof. In some embodiments, when the luminescent phosphor material is a green-emitting phosphor, and the quantum dots may be red quantum dots. In alternative embodiments, the luminescent phosphor material may be a red-emitting phosphor material with dominant emission wavelength of 590 nm to 680 nm, such as CaAlSiN 3 :Eu 2+  or KSF (K 2 SiF 6 :Mn 4+ ). Due to its deficiency in emitting green color, a green-emitting quantum dot encapsulated in silica material (dominant emission wavelength of 500 nm to 550 nm) may be used to supplement this deficiency and thus increase the color gamut of the final luminescent composite material. 
       FIG. 11  is a schematic drawing illustrating an optical film according to an embodiment of the invention. 
     Furthermore, the composite materials  200  may exist in the form of a film. Specifically, as shown in  FIG. 11 , an optical film  400  includes a first component  402 , a second component  408  and a third component  410 . The first component  402  includes one or more quantum dots  404  and a silica material  406  encapsulating surfaces of the one or more quantum dots  404 . The second component  408  includes a UV curable composite material. The third component  410  includes two PET (polyethylene terephthalate) substrate sheets  410   a  and  410   b  that sandwich the first component  402  and the second component  408  in between. The first component  402  is dispersed in the second component  408 . In some embodiments, the UV curable composite material may include 2-phenylethyl methacrylate, triaryl isocyanurate (TAIC), diallyl phthalate, and a photoinitiator. In addition, the one or more quantum dots  404  are encapsulated by the silica material  406 , thus, the silica material  406  is able to prevent oxygen and water vapor from contacting the dispersed quantum dots  404 . Compared with organic material, such as polymer or resin, the silica material  406  has a better effect of preventing oxygen and water vapor. Therefore, the optical film  400  is able to be directly used in a light emitting apparatus without using expensive gas barrier layers on the third component (PET sheets)  410  to protect the optical film  400 . 
     In order to improve reliability of the invention, the following lists several examples and several comparative examples to illustrate the composite material of the invention further. Although the following experiments are described, the material used and the amount and ratio of each thereof, as well as handling details and handling procedures, etc., can be suitably modified without exceeding the scope of the invention. Accordingly, restrictive interpretation should not be made to the invention based on the embodiments described below. 
     Experimental Example 1 
     CdSe/ZnS quantum dots (200 mg; prepared according to literature procedures) are dissolved in 50 ml of cyclohexane to form a mixture. Igepal CO-520 (purchased from Sigma-Aldrich) is added in the mixture, wherein a weight ratio of quantum dot to Igepal CO-520 is 1:5. Water (50 ml) is added and stirred at room temperature (about 25° C.) until half of the liquid evaporated. Ammonia (384 μl) and ethyl acetate (2 ml) is then added and mixed homogenously before 320 μl of tetraethyl orthosilicate (TEOS) is added and reacted for 8 hours. After the reaction, the precipitate is collected by centrifugation and then washed with ethanol three to five times. The precipitate is dried in the oven at 70° C. for one hour to form a composite material of Experimental example 1. The composite material of Experimental example 1 is characterized by transmission electron microscopy (TEM). As shown in  FIG. 5 , the CdSe/ZnS quantum dots are encapsulated by the silica. 
     Experimental Example 2 
     In the following, the manufacturing method of composite materials of Experimental example 2 to Experimental example 4 are conducted in similar methods described as Experimental example 1. 
     The composite material of Experimental example 2 includes green CdSe/ZnS quantum dots and silica (SiO 2 ), wherein the green CdSe/ZnS quantum dots are encapsulated by silica. Then, 5 wt % of composite material of Experimental example 2 and 95 wt % of two-part silicone (Dow Corning® OE-6370) are mixed, then applied onto a section of a digital light processing (DLP) projector color wheel and cured at 100° C. for 3 hours. The projector color wheel was then installed into a DLP projector to produce green test screen. As shown in  FIG. 6A , a green test screen was produced successfully using silica encapsulated green quantum dots, showing that the silica encapsulation allowed the green quantum dots to survive the extreme environment (e.g., high temperature up to 180° C.) inside the projector. 
     Experimental Example 3 
     The composite material of Experimental example 3 includes red CdSe/ZnS quantum dots and silica (SiO 2 ), wherein the red CdSe/ZnS quantum dots are encapsulated by silica. Then, 5 wt % of composite material of Experimental example 3 and 95 wt % of two-part silicone (Dow Corning® OE-6370) are mixed, then applied onto a section of a digital light processing (DLP) projector color wheel and cured at 100° C. for 3 hours. The projector color wheel was then installed into a DLP projector to produce red test screen. As shown in  FIG. 6B , a red test screen was produced successfully using silica encapsulated red quantum dots, showing that the silica encapsulation also allowed the red quantum dots to survive the extreme environment (e.g., high temperature up to 180° C.) inside the projector. 
     Experimental Example 4 
     4 wt % of green CdSe/ZnS quantum dots encapsulated by silica, 1 wt % of red CdSe/ZnS quantum dots encapsulated by silica and 95 wt % of two-part silicone (Dow Corning® OE-6370) are mixed to form a mixture. The mixture is dispensed onto a blue LED chip (i.e., an emission wavelength thereof is 440˜460 nm) and cured at 100° C. for 3 hours. The LED output spectrum is shown in  FIG. 7 . As shown in  FIG. 7 , the silica encapsulated quantum dots are able to convert the source blue light from the LED chip to green and red lights. 
     Comparative Example 1 
     Reverse Emulsion Synthesis of Silica Beads Embedded with Quantum Dots 
     A solution of CdSe/ZnS core/shell quantum dots (containing 70 mg of inorganic material) was subjected to evaporation to remove most of the solvent, which in this case was toluene, and then mixed with silane monomers (e.g., 0.1 mL of 3-(trimethoxysilyl)propylmethacrylate (TMOPMA) and 0.5 mL of tetramethoxy silane (TEOS)) until a clear solution was obtained. 
     10 mL of degassed cyclohexane/Igepal™ CO-520 (CO-520 is C 9 H 19 -Ph-(OCH 2 CH 2 ) n —OH where n=5) (18 mL/1.35 g) was prepared in a 50 mL flask and 0.1 mL of 4% NH 4 OH injected to form a stable reverse emulsion. The Cyclohexane/CO-520/NH 4 OH mixture was then mixed with the quantum dots/silane monomers mixture and stirred at 500 rpm under N 2  overnight. Modified silica beads containing quantum dots were recovered by centrifugation and washed with cyclohexane twice. The resulting sediment was then dried under vacuum. 
     Experimental Example 5 
     CdSe/ZnS quantum dots (200 mg; prepared according to literature procedures) are dissolved in 50 ml of cyclohexane to form a mixture. Igepal CO-520 (purchased from Sigma-Aldrich) is added in the mixture, wherein a weight ratio of quantum dot to Igepal CO-520 is 1:5. Water (50 ml) is added and stirred at room temperature (about 25° C.) until half of the liquid evaporated. Ammonia (384 μl) and ethyl acetate (2 ml) is then added and mixed homogenously before 210 μl of tetramethyl orthosilicate (TMOS) is added and reacted for 8 hours. After the reaction, the precipitate is collected by centrifugation and then washed with ethanol three to five times. The precipitate is dried in the oven at 70° C. for one hour to form a composite material of Experimental example 5. 
     LED Lifetime Test 
     Quantum dots encapsulated by modification-free silica were synthesized according to Experimental example 5 and were mixed with a two-part silicone (Dow Corning® OE-6370). The mixture was dispensed onto a blue LED chip (i.e., an emission wavelength thereof is 440˜460 nm) and cured at 100° C. for 3 hours. The fabricated LEDs were tested and continuously powered with a forward current of 20 mA. The LED efficacy (e.g., blue light from the LED plus green and red lights emitted by quantum dots) in lumen and quantum dots photoluminescence (PL) intensity (e.g., green and red lights emitted by quantum dots only) was measured periodically using a photometric integrating sphere (diameter=10 cm) with a spectral light meter (OPTIMUM SRI-2000). 
     Similarly, quantum dot-containing silica microbeads prepared as Comparative example 1 can be mixed with an LED encapsulant (e.g., Shin Etsu SCR1011 or Shin Etsu SCR1016) using sufficient stirring to ensure good dispersion within the encapsulating polymer. The encapsulant mixture was dispensed then onto a blue LED chip (i.e., an emission wavelength thereof is 440˜460 nm) and cured under an inert atmosphere using standard conditions for the LED encapsulant used. The fabricated LEDs were tested and continuously powered with a forward current of 20 mA. The LED efficacy (e.g., blue light from the LED plus green and red lights emitted by quantum dots) in lumen and quantum dots photoluminescence (PL) intensity (e.g., green and red lights emitted by quantum dots only) was measured periodically using a photometric integrating sphere (diameter=10 cm) with a spectral light meter (OPTIMUM SRI-2000). 
       FIG. 10  is a plot of LED efficacy and quantum dot photoluminescence (PL) intensity of LED with modification-free silica encapsulated quantum dots (Experimental examples 5) or LED with modified silica encapsulated quantum dots (Comparative example 1). As can be seen in  FIG. 10 , the efficacy of LED with modification-free silica encapsulated quantum dots does not exhibit a sharp initial decrease as shown by LED with modified silica encapsulated quantum dots. Furthermore, LED with modification-free silica encapsulated quantum dots exhibited less decrease in efficacy and photoluminescence (PL) throughout the whole test period. This clearly illustrated that quantum dots encapsulated by modification-free silica material (Experimental examples 5) are more stable than quantum dots encapsulated by modified silica material (Comparative example 1). 
     Experimental Example 7 
     Color Wheel Application 
     Red CdSe/ZnS QDs encased in silica (10 wt %) and yellow yttrium aluminium garnet (YAG) phosphor (40 wt %) mixed with two-part silicone (Dow Corning® OE-6370; 50 wt %) was applied on a section of the color wheel and cured at 100° C. for 3 hours. The color wheel was then installed into a DLP projector. The silica encapsulated red quantum dots are able to supplement the deficiency of red color from YAG phosphor, and improve the color gamut of the projector. 
     Experimental Example 8 
     LED Application 
     Silica encapsulated red quantum dots (3 wt %), yellow yttrium aluminium garnet (YAG) phosphor (5 wt %), and silicone (Dow Corning® OE-6370; 92 wt %) are mixed to form a mixture. The mixture is dispensed onto a blue LED chip (i.e., an emission wavelength thereof is 440˜460 nm) and cured at 100° C. for 3 hours. The LED output spectrum is shown in  FIG. 8 . As shown in  FIG. 8 , the silica encapsulated red quantum dots are able to supplement the deficiency of red color from YAG phosphor (see the dotted line in  FIG. 8 ), leading to an improved color gamut. 
     Experimental Example 9 
     Silica encapsulated red quantum dots (3 wt %) and green quantum dots (10 wt %) are mixed with a UV curable composite material comprising AG-F-26729 (48 wt %; Angene), TAIC™ (24 wt %; Nippon Kasei Chemical), DAISO DAP™ (14 wt %; Osaka Soda) and Darocur® TPO (1 wt %; Ciba Specialty Chemicals Inc.). The mixture (100 μm thick) is sandwiched between two polyethylene terephthalate (PET) sheets (100 μm thick each) and exposed to UV light (370 nm, 400 W, 2 minutes) to form an optical thin film. The optical thin film was installed into a 7-inch blue (445 nm) backlight unit and stored at 85° C. with the light power constantly switched on (2.4 mW/cm 2 ) for 1000 hours. The luminescence intensity test of the backlight unit is measured with a Konica Minolta CS-100 meter at different time interval. 
     As shown in  FIG. 9 , the luminescence intensity of the backlight unit remained constant throughout the test. This clearly illustrated that there was no decay in the photoluminescence of the optical film inside the backlight unit. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.