Patent Publication Number: US-2012044564-A1

Title: Switchable imaging device using mesoporous particles

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/375,194 filed on Aug. 19, 2010, entitled “MESOPOROUS PARTICLES, CHARGE CONTROLLING AGENTS AND SWITCHABLE IMAGING DEVICE USING MESOPOROS PARTICLES AND CHARGE CONTROLLING AGENTS,” which application is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switchable display device. More particularly, the present invention relates to a switchable display device using mesoporous particles. 
     2. Description of the Related Art 
     As an information display device substitutable for liquid crystal displays (LCDs), information display devices applying technology such as an electrophoretic, electro-chromic, a thermal, dichroic-particles-rotary, electrodeposition, or cholesteric liquid crystals have been proposed to replace LCDs. 
     For information display devices, it is highly desirable to have an inexpensive visual display device having a wide viewing angle which is close to normal printed matter that is readable under various lighting conditions including sunlight. Also, when compared to LCDs, it is advantageous to have smaller power consumption and higher image bistability that maintains a readable image, even when power is turned off, and operation costs as low as that of traditional paper. Electronic paper (E-paper) is a display technology designed to mimic the appearance of ordinary ink on paper. Compared to a conventional flat panel display which uses a backlight to illuminate its pixels, electronic paper reflects light like ordinary paper and is capable of holding text and images without requiring electricity, while allowing the image to be changed later. 
     Particle-based displays, such as an electrophoretic display device and a dry powder type display device, are widely used in E-papers. Particle-based displays comprise a plurality of independently addressable display cells arranged in an array, where each display cell comprises a plurality of pigment particles that are held between a pair of opposing, spaced-apart electrodes. 
     The electrophoretic display device influences the movement of charged pigment particles suspended in a colored dielectric solution based on electrophoresis phenomon. However, the main problem occurred in the electrophoretic display device is a low response rate because high viscosity resistance would be arisen from the charged pigment particles. Furthermore, pigment particles with high specific gravity such as titanium oxide are typically used as the white pigment particles and dispersed in the colored dielectric solution of low specific gravity. Thus, the large difference in specific gravity between the white pigment particles and the colored dielectric solution tends to result in undesirable sedimentation and aggregation or flocculation upon aging, which makes it difficult for the dispersion state and the display characteristics to be stably maintained. While using microencapsulating, a cell size is diminished to a microcapsule level in the range of about 50 to 100 μm in order to reduce the probability of excessive sedimentation or flocculation, but the underlying problem is not overcome at all. 
     The dry powder type display device is a particle-based display device without using a liquid solution. The typical dry powder type display device comprises two kinds of dry pigment particles with contrast colors and charges disposed between a pair of electrodes having different potentials. An electrostatic field produced by the two electrodes is applied the pigment particles to make them move for imaging. In addition, the attractive force (electrical and non-electrical) between the electrodes and the dry pigment particles enable us to store the image with “no electric power”, thereby leading to ultra-low power consumption of such dry powder type E-paper. 
     There are, however, some problems associated with the dry powder type display device. First of all, charge density of the pigment particles is the most important parameter in controlling the force generated by the electric field and an adhesive force between the pigment particles and the electrodes. However, due to the low charge density of pigment particles, the dry powder type display device needs a higher voltage than the electrophoretic display device to work. For example, the voltage of controlling the particle movement of the dry powder type display device is usually at around several tens of volts, and the driving voltage is near hundreds volts. Although the charge density of dry powder may be increased or stabilized by triboelectric interactions among the pigment particles or by using suitable charge controlling agents, the driving voltage and the time needed to reach a given contrast ratio are still hard to be reduced. As predicted by the DLVO (Derjaguin, Landau, Verwey and Overbeek) theory, low charge density particles also tend to aggregate or flocculate through a secondary potential minimum because the van der Waals force may become the prevailing particle-particle interaction, as compared to Columbic repulsion. Both the reduction of charge density and the particle aggregation or flocculation result in an increase of the driving voltage or time needed to reach a given contrast ratio. Furthermore, they also result in changes in the threshold voltage and operation temperature latitude and consequently cause difficulties in image modulation, and image stickiness or ghost images. 
     In addition, it is difficult to achieve precision control of charge amounts or to significantly increase charge density of the pigment particles. 
     In general, the pigment particles used in the dry powder type display device are by either pulverization or chemical polymerization. Pulverization involves a milling process in which polymer resins, pigments, and charge controlling agents (hereinafter referred as to the “CCAs”) are fused and kneaded, and then crushed and classified. There are, however, problems associated with the pulverized particles manufactured by pulverization. A desired charge density of the pulverized particles may not be easily obtained since it is difficult to control the amount of CCAs attached on the surface of the particle, and which also results in the low charge density. Another problem associated with the pulverization is that the size of pulverized particles is usually big (e.g. &gt;8 μm) and the size distribution is relatively wide. 
     Although spherical particles having a narrow particle size distribution may be manufactured by a polymerization method such as suspension polymerization, emulsion polymerization or dispersion polymerization, the CCAs would hinder polymerization during particle preparation because the ionic characteristic of CCAs acts as extra surfactants. 
     Secondly, when dense inorganic pigment particles such as TiO 2  (specific gravity ˜4) is employed as a white pigment, it is very difficult for gravity densities to be reduced. This problem may be eliminated or alleviated by mixing or coating the dense inorganic pigment particles with a suitable polymer to reduce the specific gravity to that of the air. However, dielectric medium in dry powder type image display device, e.g. air, has a relatively low refractive index compared to most polymers. As a result, specific gravity reduced pigment microcapsules having a thick polymeric shell or matrix typically show a low hiding power or low light scattering efficiency, as compared to non-capsulated pigment particles having high specific gravity. 
     Thirdly, the typical dry powder type display device shows unsatisfactory reflectance or whiteness. In practice, the white pigment particles are manufactured through pulverization or chemical polymerization by filling white pigment such as titanium oxide (TiO 2 ), zinc oxide or zirconium oxide into a base polymer resin. Although a larger amount of the pigments such as titanium oxide can be added for achieving excellent whiteness of pigment particles, scattering becomes insufficient resulting in a decreased white refraction index to, whereby a high gravity density issue will also arise which may deteriorate bistability of the device. For the poor reflectance issue of current dry powder systems, the hiding power of the white particles is largely determined by the packing density and the colloidal stability of the particles electrically attracted to the electrode plate. For narrow particle size distribution particles, the maximum packing densities for cubical and tetrahedral packing structures are about 52% and about 74% by volume, respectively. The particle packing density of a current dry powder device is much lower than the maximum because the particle size is large and size distribution for the particles is wide, which results in a significant deterioration of minima in reflectance (Dmin). 
     Therefore, there exists a need for pigment particles with optimal characteristics for application in all-types of particle-based switchable imaging displays. Desirable particle characteristics include high charge density, low gravity density, stability against agglomeration, good hiding power, high contrast ratio, and other particle characteristics which provide for a wider latitude in the control of switching rate. 
     BRIEF SUMMARY OF THE INVENTION 
     One of the broader forms of an embodiment of the present invention involves a switchable imaging device. The switchable imaging device includes a plurality of particles suspended in a dielectric medium, at least part of the particles being charged, at least part of the particles being mesoporous particles. 
     Another one of the broader forms of an embodiment of the present invention involves a full color switchable imaging device. The full color switchable imaging device the switchable imaging device described above and a color filter disposed adjacent to the switchable imaging device. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be further understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a schematic diagram showing a cross-section view of a switchable imaging device according to an embodiment of the present invention; and 
         FIG. 2  shows a schematic diagram showing a top view of a switchable imaging device according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over, above, below, or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. 
     A switchable imaging device using mesoporous particles is provided according to embodiments of the present invention. Mesoporous materials may provide several than normal materials due to their large surface area, open porosity, small pore sizes, and the ability to coat the surface of the mesoporous structure with one or more compounds. That is, mesoporous particles are able to contain more charges than that of typical nonporous particles by adsorption of charging species or a charge controlling agent onto the mesoporous particles, resulting in substantially improved charge density. Furthermore, the bulk density would be greatly reduced due to their open porosity, and significantly enhanced light scattering because they have the highest difference of refractive index between the pigment and the dispersing medium, e.g., air. Thus, the switchable device using the mesoporous particles according to embodiments of the present invention would have high charge density, low gravity density, stability against agglomeration, good hiding power and high contrast ratio. The problems occurred in the conventional switchable imaging device would be overcome. The switchable device may be an electronic display, signage, a bulletin board, a price tag, a digital barcode, a digital coupon, an e-paper device, or an e-reader device. 
     Referring to  FIG. 1 , illustrated is a schematic diagram of a cross-section view of a switchable imaging device  100  according to an embodiment the present invention. In a preferred embodiment, the switchable device  100  may be an e-paper device. In particular, the switchable imaging device  100  may comprise a particle-based e-paper, such as a dry powder type e-paper device or an electrophoresis e-paper device. As shown in  FIG. 1 , the switchable imaging device  100  may comprise a top substrate  11  and a bottom substrate  13  opposite to each other with a predetermined distance therebetween. A plurality of particles  21  and  23  are suspended or dispersed in a medium which is disposed within the space defined by the top substrate  11  and the bottom substrate  13 . At least part of the particles  21  and  23  may be mesoporous particles, and at least part of the particles  21  and  23  may be charged. In this embodiment, only one of the particles  21  and particles  23  may be particles, and the other one may be other type pigment powders, such as carbon black. Alternatively, both of particles  21  and  23  may be mesoporous particles. Preferably, at least part of the mesoporous particles may be charged and at least part of charged mesoporous particles surrounded are charged or statically charged. In an embodiment, as the switchable imaging device being the dry powder type e-paper device, the medium may be air. In another embodiment, as the switchable imaging device being the electrophoresis e-paper device, the medium may be a dielectric solution. The top substrate  11  and the bottom substrate  13  may comprise electrodes having different potentials formed thereon. 
     In the present embodiment, the particles  21  and  23  may be mesoporous particles which may include but are not limited to porous metal oxides, inorganic dielectrics, and inorganic semiconductors having pores of substantially uniform diameter, shape of cross-section, and/or orientation. The particles  21  and  23  may be used as pigment particles for imaging colors in the switchable imaging device. 
     In an embodiment, the mesoporous particles may be expressed by the formula: M m Y y , where M may be an inorganic element selected from Ti, Mn, Mg, Co, Ni, Al, Cr, Si, Cu, Ag, Zn, Ba, Ca, Fe, Zr, Sn, Pb, Ta, Cd, V, Nb, W, Hf, Ge, Sb, or Mo, m is the number of moles or mole fraction of M, Y may be nitrogen, oxygen, sulphur, or hydroxyl, and y is the number of moles or mole fraction of Y. Furthermore, in order to enhance the light scattering efficiency or hiding power of the mesoporous particles  21  and  23  in various of the switchable imaging devices, the mesoporous particles are preferably formed from a material of high refractive index which is preferably greater than about 2, or more preferably greater than about 2.5. Suitable high refractive index materials for the mesoporous particles may include, but are not limited to, metal oxides such as oxides of Ti, Zn, Zr, Ba, Ca, Mg, Fe, Al, or the like. For example, the mesoporous particles M m Y y  having high refractive index may be TiO 2 , NiO, MgO, Cr 2 O 3 , or Fe 2 O 3 . In particular, rutile TiO 2  mesoporous particles are preferred because of their superior whiteness and light fastness. 
     In another embodiment, the mesoporous particles may be expressed by the formula: M m N n Y y , where M and N are independently inorganic elements which may be independently selected from the group consisting of Ti, Mn, Mg, Co, Ni, Al, Cr, Si, Cu, Ag, Zn, Ba, Ca, Fe, Zr, Sn, Pb, Ta, Cd, V, Nb, W, Hf, Ge, Sb, Mo, In, C, N, S, and F, m is the number of moles or mole fraction of M, n is the number of moles or mole fraction of N, Y is nitrogen, oxygen, sulphur or hydroxyl, or a combination thereof, and y is the number of moles or mole fraction of Y. 
     In still another embodiment, the mesoporous particles may be expressed by the formula: M m Y y Z z , where M is an inorganic element selected from the group consisting of Ti, Mn, Mg, Co, Ni, Al, Cr, Si, Cu, Ag, Zn, Ba, Ca, Fe, Zr, Sn, Pb, Ta, Cd, V, Nb, W, Hf, Ge, Sb, Mo, In, C, N, S, and F, m is the number of moles or mole fraction of M, Y is nitrogen, oxygen, sulphur or hydroxide, or a combination thereof, y is the number of moles or mole fraction of Y, Z is nitrogen, oxygen, sulphur or hydroxide, or a combination thereof, and z is the number of moles or mole fraction of Z. Preferably, the mesoporous particles may be TiO a (OH) b , NiO a (OH) b , or MgO a (OH) b , where a and b are integers and a sum of a and b is 2 or 3. 
     For example, in the present embodiment, in addition to the superior characteristics of TiO 2  mesoporous particles, addition of transition metal ions, such as V, Cd, Al, Zr, Fe, Ag, Co or Cu selected from the periodic table, or organic atoms, such as F, Si, N, S, C or the like, as dopants may be used to alter the band gap and color appearance of the TiO 2  mesoporous particles. In a preferred embodiment, these doped mesoporous TiO 2  particles may directly serve as colorful pigments to assembly a color switchable imaging and may show different charge density through heteroatom doping. 
     The mesoporous particles may be free-flowing dry particles, and may have an average particle size of from, for example, about 0.05 μm to 20 μm, or about 0.1 μm to 5 μm, or other suitable ranges, depending on requirement. Alternatively, the mesoporous particles are aggregates of primary particles having an average diameter of, for example, from 0.001 μm to 0.5 μm, or from 0.01 μm to 0.3 μm, or from 0.1 μm to 0.3 μm, or other suitable ranges, depending on requirement. The mesoporous particles may have an average pore size of, for example, from 1 nm to 100 nm, or from 3 nm to 50 nm, or other suitable ranges, depending on requirement. BET (Brunauer-Emmett-Teller) surface area may be ranged from about 1 to 500 m 2 /g. The mesoporous particles may comprise pores in a form of column, disc, sheet, or aggregates thereof. 
     In an embodiment, the mesoporous particles may be formed by using typical synthetic method by means of a structure-directing or liquid crystal templating technique. In the typical synthetic method, the mesoporous particles are prepared with the aid of an ionic or non-ionic (polymeric or small molecular) structure-directing agent, such as cetyltrimethylammonium bromide (CTAB) or poly(ethylene oxide)-poly(propylene oxide) triblock copolymer (P123). In another embodiment, the mesoporous particles may be synthesized by using inorganic precursors and structure-directing agents different from the precursors and structure-directing used in the typical method. For example, the structure-directing agent may be organic acids, urea, or long chain amine such as hexadecylamine, or a combination thereof. The inorganic precursor may be titanium tetra-isopropoxide, TiCl 4  and TiOSO 4 , or a combination thereof. The mesoporous particles may be synthesized through a combination of sol-gel and hydrothermal process. For example, in an embodiment of synthesis of TiO 2  mesoporous particles, the TiO 2  mesoporous particles may be synthesized through a sol-gel process in combination with a hydrothermal reaction from carboxylic acids (e.g., butyric or valeric acid) as templates and titanium tetra-isopropoxide in presence of water. A solution comprising 0.1 to 100 equivalents, or preferably 1 to 30 equivalents, of carboxylic acid and 1 equivalent titanium tetra-isopropoxide and a solvent of ethanol is prepared. The solution then may be heated at 30 to 150° C., preferably at 50 to 125° C., for several hours, for example, 0.1 to 24 hours. Subsequently, a solution of deionized water and ethanol with a ratio of 0.01 to 100 may be added to the heated solution for precipitating TiO 2  particles. The precipitate is collected by centrifuging to yield a sphere precursor. Then, the sphere precursor is hydrothermaled at an elevated temperature, preferably between 50 and 250° C. and calcined at high temperature 200 to 1200° C. The resulted TiO 2  mesoporous particles may have a spherical morphology with an average particle size ranging from several tens nm to micron meters and a BET area ranging from 1 to 500 m 2 /g. In this embodiment, the mesoporous particles, more particularly TiO 2  mesoporous particles, may be spheres and can provide a higher surface area and a larger pore size than the mesoporous particles formed using typical synthetic method. 
     In an embodiment, according to the use of the switchable imaging device, the particles  21  and  23  may be optionally coated with a colorant. The colorant may be a dye, pigment or a precursor of a dye or pigment. In particular, the colorant may have pair of contrast colors selected from black and white, blue and white, red and white, or green and white, or the pair of contrast colors is selected from black and white, black and cyan, black and magenta, or black and yellow, respectively. Thus, particles  21  and  23  may have contrast colors to each other. For example, in the present embodiment, the particles  21  may be coated with white colorant and the particles  23  may be coated with black colorant. In another embodiment, the particles  21  and  23  may be doped mesoporous particles which may directly serve as colorful pigments without coating with colorants as described above. 
     Furthermore, the surfaces of the particles  21  and  23  may be distributed with a plurality of positive and negative charges, respectively. For example, the particles  21  may be distributed with a plurality of positive charges, and the particles  23  may be distributed with a plurality of negative charges, or vice versa. When an electric field formed between the top substrate  11  and the bottom substrate  13 , the particles  21  and  23  may migrate toward the bottom substrate  13  and the top substrate  11 , respectively. As a result, a designed frame can be shown due to proper control of the potential of each pair of electrodes on relative locations on the top substrate  11  and the bottom substrate  13 . 
     In an embodiment, the particles may be mesoporous particles  21  and  23  which are charged with charging species or a charge controlling agent (CCA) other than doping with hetero-atoms. Compared to conventional polymeric colloid particles, the mesoporous particles loaded with the charging species or the CCA may have lighter weights and higher charge densities. Thus, the switchable imaging device using mesoporous particles with the charging species or the CCA according to embodiments of the present invention would have a reduced operation voltage and an enhanced performance as well as a reduced manufacturing cost. In an embodiment, the mesoporous particles may be charged by tribo-electric interaction, electron transfer, proton transfer, or acid-base reaction on the plurality of mesoporous particles. For example, the mesoporous particles may be charged by physical adsorption or chemisorption of the charging species or the CCA onto the plurality of mesoporous particles for performing electron transfer, proton transfer or directly carrying the charges. The charging species or the CCA may be a donor of electron or proton. Alternatively, the charging species or the CCA may be an acceptor of electron or proton. The mesoporous particles with the charging species or the CCA may have lighter weights and higher charge densities than conventional polymeric colloid particles, thereby the operation voltage switchable imaging device according to embodiments of the present would be significantly reduced. Also, in addition to carry the charging species or the CCA, the mesoporous particles may be coated with a polymer layer for performing tribo-electric interaction therebetween. Thus, the charge density of such mesoporous particles can be further adjusted, such as high charge density to give faster switching performance. 
     According to the present invention, the electron accepting or proton donating compounds of the charging species may include, but not limited to, alkyl, aryl, alkylaryl or arylalkyl carboxylic acids and their salts, alkyl, aryl, alkylaryl or arylalkyl sulfonic acids and their salts, tetra-alkylammonium and other alkylaryl ammonium salts, pyridinium salts and their alkyl, aryl, alkylaryl or arylalkyl derivatives, sulfonamides, perfluoroamides, alcohols, phenols, salicylic acids and their salts, acrylic acid, sulfoethyl methacrylate, styrene sulfonic acid, itaconic acid, maleic acid, hydrogen hexafluorophosphate, hydrogen hexafluoroantimonate, hydrogen tetrafluoroborate, hydrogen hexafluoroarsenate (V), or the like. Alternatively, the electron accepting or proton donating compounds of the charging species may include organometallic compounds or complexes containing an electron deficient metal group such as tin, zinc, magnesium, copper, aluminum, cobalt, chromium, titanium, zirconium or derivatives or polymers thereof. 
     According to the present invention, the electron donating or proton accepting compounds of the charging species may include, but are not limited to, amines, particularly tert-amines or tert-anilines, pyridines, guanidines, ureas, thioureas, imidazoles, tetraarylborates, or the alkyl, aryl, alkylaryl or arylalkyl derivatives thereof. Alternatively, the electron donating or proton accepting compounds of the charging species may include a copolymer reacted from at least two monomers of 2-vinyl pyridine, 4-vinyl pyridine, 2-N,N-dimethylaminoethyl acrylate, styrene, alkyl acrylates, alkyl methacrylates, or aryl acrylate. For example, the charging species may be poly(4-vinylpyridine-co-styrene), poly(4-methacrylate), poly(4-vinylpyridine-co-butyl methacrylate), or the like. 
     In accordance with one embodiment of the present invention, the charge controlling agent is a positive charge controlling agent selected from the group consisting of quaternary ammonium salts, pyridinium salts, onium salts, squarium salts, metal salts, nigrosine dye, polyamine resin, triphenylmethane compound, imidazole derivatives, amine derivatives, and phosphonium salt. In accordance with another embodiment of the present invention, the charge controlling agent is a negative charge control agent selected from the group consisting of metal complexes of salicylic acid, alkyl-salicylic acid, azo dye, calixarene compound, benzyl acid boron complex, sulfonate salt, and fluorocarbon derivatives. Preferably, the metal complexes of salicylic acid may comprise a metal selected from the group consisting of Cr, Zn, Mg, Co, Al, B, Ni, Fe, and Cu. 
     Furthermore, the mesoporous particles may be further overcoated with a polymer layer to improve tribo-electric interaction and/or to prevent charge leakage from highly charged mesoporous particles to electrode when contacting with electrode during switching. This charge leakage could reduce charge density of charged mesoporous particles resulting in slower switching speed and performance deterioration. In an embodiment, the polymers to enhance tribo-electric interaction between polymer-coated mesoporous particles may include, but are not limited to, polytetrafluoroethylene, poly(vinyl chloride), polypropylene, polyethylene, polystyrene, poly(vinylidene chloride), poly(bisphenol A carbonate), polyacrylonitrile, epoxy resin, poly(ethylene terephthalate), poly(methyl methacrylate), poly(vinyl acetate), poly(vinyl alcohol), polyamide, or the like. 
     In summary, the embodiment according to the present invention provides a switchable device comprising a plurality of particles. At least part of the particles  21  and  23  are mesoporous particles, and at least part of the particles  21  and  23  are charged. The particles is prepared by reducing a mixture comprising: (1) a solvent or continuous phase, (2) a source of metal dissolved in the solvent or continuous phase, and (3) a structure-directing agent present in an amount sufficient to form a liquid crystalline phase in the mixture, to form a composite of metal-based material and organic matter, or by reducing a mixture comprising: (1) a solvent or continuous phase; (2) a source of metal dispersed in the solvent or continuous phase; and (3) a structure-directing agent present in an amount sufficient to form a liquid crystalline phase in the mixture, to form a composite of metal-based material and organic matter. Optionally, the organic matter may be removed from the composites. Then, the formed mesoporous particles may be treated or doped with additives, a colorant, charging species, a charge controlling agent, or overcoating with dielectric materials. The charging species or the charge controlling agent may be a donor of electron or proton, an acceptor of electron or proton, metallic, or non-metallic. 
     According to another embodiment of the present invention, a full color switchable imaging device is also provided. In this embodiment, the switchable imaging device comprises a plurality of microcups comprising charged particles confined therein, wherein each of the microcups is separately filled with a pair of particles having contrast colors and carrying opposite charges, and only one pair of the contrast colors is associated with one of the microcups. That is, each of the microcups of the switchable imaging device may comprise particles of a pair of contrast colors having opposite charges, wherein at least one of the particles is mesoporous. Preferably, the pair of contrast colors is selected from black and white, blue and white, red and white, or green and white, or the pair of contrast colors is selected from black and white, black and cyan, black and magenta, or black and yellow. The charged particles may be mesoporous particles similar or the same with the mesoporous particles described in the above embodiment. For example, the mesoporous may be treated or doped with an additive, a colorant, charging species or a charge controlling agent as mentioned. 
       FIG. 2  shows a schematic diagram of a top view of the full color switchable imaging device. In this embodiment, the switchable imaging device  200  is similar with the switchable imaging device  100  shown in  FIG. 1  except that an array of microcups  30   a ,  30   b ,  30   c  comprising pigment particles confined therein are used in the switchable imaging device  200 . In an embodiment, each of the microcups  30   a ,  30   b ,  30   c  may comprise a top substrate and bottom substrate with electrodes formed thereon and two kinds pigment particles which comprise contrast colors and charges disposed therebetween. 
     In this embodiment, the pigment particles may be same or similar with the pigment particles  21  and  23  shown in  FIG. 1 . The array of microcups  30   a ,  30   b ,  30   c  may provide a full color by at least three different colors. For example, the microcup  30   a  may comprise pigment particles having contrast colors of red and white and opposite charges, the microcup  30   b  may comprise pigment particles having contrast colors of green and white and opposite charges, and the microcup  30   c  may comprise pigment particles having contrast colors of blue and white and opposite charges. Note that, in addition to the three different pair of contrast colors, a microcup having contrast colors of black and white (not shown) also can be further added to the array of the microcups. Alternatively, the microcups  30   a ,  30   b  and  30   c  may have contrast colors selected from cyan and black, magenta and black, and yellow and black, respectively. Note that, in addition to the three different contrast colors, a microcup having contrast colors selected from black and white (not shown) also can be further added to the array of the microcups. 
     In another embodiment, color filters (not shown) may be disposed on the top substrate or the bottom substrate of each of the microcups for providing the desired colors. The color filters may comprise at least three colors such as red, green and blue. As such, if the microcups in the switchable imaging device can only image one pair of contrast colors such as black and white, the switchable imaging device can still image full color depending on the use of color filters. 
     In summary, embodiments of the present invention provide a switchable imaging device using mesoporous particles is provided. The mesoporous particles according to embodiments of the present invention would have high charge density, low gravity density, stability against agglomeration, good hiding power and high contrast ratio, and therefore the problems occurs in the conventional switchable imaging device would be overcome. 
     The following are examples of the present invention which are directed to the preparation of various kinds of mesoporous particles which may be treated or doped with charging species or the charge controlling agent or overacted with the polymer. 
     Example 1 
     9.9 ml valeric acid (Aldrich) was injected into the 750 mL ethanol, and then 15 mL titanium isopropoxide (Aldrich) was added. The mixture was then heated to above 85° C. for 5 hours. Then, a solution of deionized water and ethanol with ratio of 1 was added to the heated mixture and precipitate of particles was formed. The precipitate was then collected and washed with ethanol to yield TiO 2  particles. Following by hydrothermal process with 0.2 M NH 4 OH solution at 160° C. and calcined at high temperature of 500° C. to give desired TiO 2  mesoporous particles having an average size of 450 nm, a BET surface area of 68 m 2 /g, and an average pore size of 14 nm (characterized by ASAP2020 from Micromeritics). 
     Example 2 
     1 g of the TiO 2  mesoporous particles obtained from Example 1 are reacted with 0.042 g of 3-(trihydroxysilyl)-1-propanesulfonic acid in a 80% methanol/water solution at 90° C. for 3 hours. After completion of the reaction, the modified mesoporous TiO 2  particles were washed thoroughly with ethanol and dried with a stream of N 2 . 
     Example 3 
     A dispersion formed of 1 g of the TiO 2  mesoporous particles obtained from Example 1 and 3 ml of THF/Ethanol solvent was prepared. Then, 0.15 g of Bontron E-84 (Orient Chemical) was added to the dispersion and mixed under sonication for half hour. Then, powder of the charged TiO 2  mesoporous particles was collected by vaporization of solvent, and dried with a stream of N 2 . 
     Example 4 
     1 g of the TiO 2  mesoporous particles obtained from Example 1 were reacted with 0.08 g of trichloro(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl) silane (Alfa-Aesar) in a 80% methanol/water solution at 90° C. for 3 hours. After completion of the reaction, the modified TiO 2  mesoporous particles were washed thoroughly with ethanol and dried with a stream of N 2 . 
     Example 5 
     1 g of the TiO 2  mesoporous particles obtained from Example 1 was reacted with 0.043 g of 3-(trichlorosilyl)propyl methacrylate (Aldrich) in a 95% ethanol at room temperature for 2 hours. After completion of the reaction, the modified TiO 2  mesoporous particles were washed thoroughly with ethanol and dried with a stream of N 2 . Then, the acrylate-functionalized TiO 2  mesoporous particles were transferred to another flask containing 30 mL of water, 0.1 g of potassium persulfate (Acros), and 1.5 g of methyl methacrylate (Acros). A graft polymerization was carried out at 80° C. for 24 h with vigorous stirring under N 2 . Finally, the resulting was filtered and washed with methanol, and then dried in air. After that, the poly(methyl methacrylate)-coated TiO 2  mesoporous were obtained. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.