Abstract:
A particle display device having a monolayer of a bistable polarizable molecule on either side of a gap filled with a liquid containing suspended particles. The monolayers and the liquid filled gap are disposed between patterned electrodes that are used to apply an electric field across the layers of the gap, thus influencing the orientation or the distribution of the suspended particles. The optical transmission or reflectance of the gap is dependent upon the applied electric field. The polarizable monolayers respond to the applied electric field by assuming one of two stable polarized states. The field provided by the dipoles of the molecules in the monolayer remains after the external field is removed, thereby sustaining the electric field influence on the suspended particles of the display device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to display devices. In particular, the invention relates to a structure for maintaining the state of a display device using an induced electric field.  
           [0003]    2. Related Art  
           [0004]    Flat-Panel Displays (FPDs), which are commonly used in laptop computers and other portable applications require low power consumption and compact size, in addition to good visual display characteristics. One class of technology used for FPDs involves the manipulation of particles that are suspended in a liquid medium to alter the reflective and transmissive properties of the medium.  
           [0005]    There are two general types of particle displays, the first is often referred to as a suspended particle display (SPD). In SPDs the orientation of the particles is selectively controlled to produce the optical contrast required for a display. In an electrophoretic image display (EPID) the distribution of a particle population is selectively controlled in order to produce the optical contrast required for a display. In both cases an electric field is used to control the particles. It should be noted that particles in both display types are suspended in a liquid medium, and in one case the response to the electric field is with respect to orientation, and in the other with respect to distribution.  
           [0006]    SPDs are attractive due to their wide viewing angle, high optical transmission and ease of fabrication. With SPDs, light valve action is obtained when sub-micron sized particles with an asymmetric, plate-like shape align with an externally-applied electric field, and thus permit light to pass through (the “light” state). This alignment occurs because the external field induces a dipole moment in the molecules of the particles. In the absence of the external field, the particles orient randomly due to brownian motion, and consequently block light (the “dark” state).  
           [0007]    For example, crystals of iodoquinine sulfate or related compounds may be dispersed in an organic liquid, and since the crystals are dichroic, there is a large difference between the absorption in the unaligned state in comparison to the aligned state. In the absence of an applied field, the random orientation produces a bluish-black appearance, and in the aligned state there is very little absorption and good contrast can be obtained with a white background. A significant disadvantage of SPDs is that the light areas of the display must be continuously energized with the external electric field to maintain the display, thus consuming energy even when the image on the display is static. SPDs also typically lack a clear voltage threshold, and require active-matrix addressing for high resolution.  
           [0008]    In EPIDs, the particles used in the display are electrically charged and may either have a color that contrasts with the liquid used to suspend them, for example white particles in a dark blue dye, or may be divided into particles of two contrasting colors with opposite charge. The particles migrate under the influence of an applied electric field to the front or back of the display, producing a light or dark region when viewed from the front. The EPID operates by reflection and absorption as opposed to transmission. Although EPIDs have inherent memory, there is no voltage threshold, making multiplexed displays difficult.  
           [0009]    Thus, the need exists for a particle display that is able to retain a displayed image without an applied external field. There is also a need for a particle display that has a voltage threshold that enables multiplexing.  
         SUMMARY OF THE INVENTION  
         [0010]    A particle display device having a monolayer of a bistable polarizable molecule on either or both sides of a gap filled with a liquid containing suspended particles is disclosed. The monolayers and the liquid filled gap are disposed between patterned electrodes that are used to apply an electric field across the layers of the gap, thus influencing the orientation or the distribution of the suspended particles. The optical transmission or reflectance of the gap is dependent upon the applied electric field. The polarizable monolayers respond to the applied electric field by assuming one of two stable polarized states. The field provided by the dipoles of the molecules in the monolayer remains after the external field is removed, thereby sustaining the electric field influence on the suspended particles of the display device.  
           [0011]    In one embodiment of the present invention, an SPD having plate-like or tabular particles suspended in a liquid filled gap between two transparent substrates has a monolayer of a bistable polarizable molecule applied to the surface of each of the substrates. The bistable polarizable molecule has two stable states separated by an electric field threshold, with one state having a higher electric dipole moment than the other, or a reversed dipole. When an external field of sufficient strength is applied across the substrates, the dipoles on opposite sides of the gap are in opposite complementary states and provide an internal field for the device that remains after the external field is removed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:  
         [0013]    [0013]FIG. 1 illustrates a representative molecule having a bistable dipole in accordance with an embodiment of the present claimed invention.  
         [0014]    [0014]FIG. 2 shows a bistable molecular monolayer on a conductive film in accordance with an embodiment of the present claimed invention.  
         [0015]    [0015]FIG. 3 shows a bistable molecular monolayer on a dielectric film in accordance with an embodiment of the present claimed invention.  
         [0016]    [0016]FIG. 4 shows a suspended particle display structure for use with the bistable molecular monolayer in accordance with an embodiment of the present claimed invention.  
         [0017]    [0017]FIG. 5 shows a schematic of a display using aligned tabular particles in accordance with an embodiment of the present claimed invention.  
         [0018]    [0018]FIG. 6 shows a schematic of a display using a homogeneous population of aligned spherical particles in accordance with an embodiment of the present claimed invention.  
         [0019]    [0019]FIG. 7 shows a schematic of a display using a heterogeneous population of aligned spherical particles in accordance with an embodiment of the present claimed invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    In the following detailed description of the present invention, a particle display device using bistable molecular monolayers, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances well known processes, procedures, components, and structures have not been described in detail so as not to unnecessarily obscure aspects of the present invention.  
         [0021]    [0021]FIG. 1 shows a bistable polarizable molecular species for producing the bistable molecular monolayer of the present invention. Con 1  and Con 2  are connecting units that provide a site or sites for attaching the molecule to a substrate (e.g, metal electrode, dielectric substrate, etc.). For Con 1  and Con 2  the substrate bond may be made between a carbon atom in the ring by dehydrogenation, or by subsitution of a ring carbon with with an atom such as nitrogen, oxygen, sulfur, silicon, etc. The molecule shown in FIG. 1 meets the following requirements:  
         [0022]    a) The molecule must have at least two segments.  
         [0023]    b) All segments must have π- and/or p-electrons.  
         [0024]    c) The molecule can be either symmetrical or asymmetrical with a donor group on one side and an acceptor group on another side.  
         [0025]    d) At least two segments of the molecule have some functional groups that will help to stabilize both states of folding and stretching through intra- or inter-molecular forces such as hydrogen bonding, Van der Waals forces, coulomb attraction or metal complex.  
         [0026]    e) The folding or stretching states of the molecule must be e-field addressable.  
         [0027]    f) At least in one state (presumably on a fully stretched-out state), the π- and/or p-electrons of the molecule will be well delocalized, and the π- and/or p-electrons of the molecule will be localized or only partially delocalized in other state(s).  
         [0028]    In FIG. 1, the stretched-out configuration  10  of the molecule arises in response to an external field of sufficient strength. In this configuration, the whole molecule is in a perfect planar conformation and the π- and-or p-electrons delocalized through the entire molecule. Because of the extended conjugation (π- and/or p-electrons delocalization) of the molecule makes participation of the donor (R 1 R 2 N—) and the acceptor (Nitro group) to the polarization of the molecule possible, the molecular dipole is greatly increased.  
         [0029]    When in the folded configuration  11  of FIG. 1, the molecule is nonplanar, and the extended conjugation (π- and/or p-electrons delocalization) is cut off. In this state, there is no participation from the donor (R 1 R 2 N—) and the acceptor (Nitro group) in polarization, that is, the route between donor (R 1 R 2 N—) and acceptor (Nitro group) is cut off, and the dipole of the molecule is greatly reduced.  
         [0030]    [0030]FIG. 2 shows a bistable molecular monolayer  20  on a conductive film  21  in accordance with an embodiment of the present claimed invention. In this example, a sulfur atom  22  is shown as a bridge between a terminal carbon atom  23  and the surface of the conductive film  21 . The conductive film  21  is supported by a substrate  24 .  
         [0031]    For particle display devices, the front substrate is transparent for both transmissive and reflective devices, and the conductive film may be either a transparent conductive oxide (e.g. doped indium or tin oxide or ruthenium oxide), or a thin metal film. For thin metal films, a stable metal such as gold or a platinum group metal is preferred. Since the metal film must be very thin to provide transparency, chemical stability is desirable.  
         [0032]    The fabrication of a molecular monolayer on a gold film can be accomplished by using a molecule having thiol functional group (S—H) on one end. The monolayer can be assembled from solution or by vapor deposition. The process may be performed in a two step sequence in which the species are first physisorbed on the substrate and then mildly heated to produce the sulfur-gold bond, with the chemisorbed species being more tightly bound than the physisorbed species. Alkanethiols, having a linear carbon chain terminated with a thiol group may serve as the binding end of a bistable molecule, with functional groups being added to binding group to provide the required electronic properties for the molecule as a whole.  
         [0033]    For a reflective particle display such as an EPID, transparency is not required for the back substrate, and a thicker metal film may be used. The ability to use a thicker film provides more flexibility in the selection of the metal used and the species used to bond to the metal film.  
         [0034]    [0034]FIG. 3 shows an alternative structure for supporting the bistable molecular monolayer of the present invention. In this example, the monolayer  30  is deposited on a dielectric film  31  that is used as an overcoating for a conductive film  32 . the dielectric film  31  and the conductive film  32  are supported by a substrate  33 . The dielectric film  31  may be used to prevent undesirable charge transfer leading to electrochemical reactions. A preferred material for the dielectric film  31  is silicon dioxide. Since the monolayer of FIG. 3 is bonded to an oxide, a silicon atom  34  replaces the sulfur atom of FIG. 2 as the bridge between the terminal carbon and the surface of the dielectric film  31 . The silicon atom is typically provided on the molecular species as a silane (Si—H) or silanol (Si—O—H) group. The silicon functional group may also be used to bond to the surface of the previously mentioned transparent conductive oxide film that may be used in place of a conductive metal film.  
         [0035]    [0035]FIG. 4 shows a typical suspended particle display structure  40  that can be used with the present invention. A top substrate  41  and a bottom substrate  42  provide support for a top conductive coating  43  and a bottom conductive coating  44 , respectively. The top substrate  41  and the top conductive coating  43  are transparent. The bottom substrate  42  and the bottom conductive coating  44  may be either transparent as in the case of a SPD, or non-transparent as in the case of an EPID. The transparent materials suitable for use as the top substrate  41  are organic polymers, glass, and crystalline materials such as sapphire and quartz. The suitable materials for the bottom substrate include those for use in the top substrate  41  as well as other opaque dielectric materials.  
         [0036]    As shown in FIG. 4, the substrates are separated by a gap  46  and sealed along the perimeter by a seal  45 . The gap  46  is filled with a liquid containing suspended particles. The properties of the liquid and particles are dependent upon the operational characteristics desired in the display.  
         [0037]    [0037]FIG. 5 shows a schematic closeup of the display of FIG. 4 with aligned tabular particles  50 , such as those used in a SPD, suspended in the gap between the substrates  51  and  52  in a transparent liquid medium  53 . The tabular particles are flat with a shape that can be approximated by a regular polygon. The shape of the particles allows for the maximum transmission of light through the gap when the particles are aligned with the electric field. In the absence of an electric field, the particles are randomly oriented and the transmission is reduced. The contrast between the aligned and random orientations is a function of the properties of the materials in the optical path, and the number and distribution of the particles. Ideally, the index of refraction of the substrate and coatings is well matched to the suspending liquid, and poorly matched to the particles.  
         [0038]    [0038]FIG. 6 shows a schematic closeup of the display of FIG. 4 with spherical particles  60 , such as those used in an EPID, suspended in the gap between the substrates  61  and  62  in an opaque liquid medium  63 . The particles  60  have the same charge and will migrate to either the top substrate  61  or the bottom substrate  62  in response to an applied field, depending on the polarity of the charge and the direction of the applied field. The charge on the particles may be produced an interaction with the liquid medium  63  or by an electret process, or dielectric absorption.  
         [0039]    The suspended particles  60  and the liquid medium  63  have contrasting colors, for example, the particles  60  may be white and the liquid medium  63  a dark blue. In FIG. 6, the particles  60  are shown positioned on the top substrate  61 . In regions having this distribution, the display will appear light due to reflection from the particles  60 . A reversal in polarity causes the particles  60  to migrate to the bottom surface  62 , making the display appear dark due to absorption by the liquid medium  63 .  
         [0040]    [0040]FIG. 7 shows a schematic closeup of a display similar to that of FIG. 6, having a heterogeneous population of spherical particles, suspended in the gap between the substrates  71  and  72  in a liquid medium  73 . The population of particles is composed of two groups of particles  70 A and  70 B having contrasting colors and opposite charge.  
         [0041]    Since the two groups of particles  70 A and  70 B have opposite charge, they will migrate to opposite substrates in the presence of an applied electric field. Depending upon the field orientation in a given region of the display, one group or the other will be resident on the surface of the substrate being viewed and the other will be obscured. The liquid medium  73  may be either transparent or opaque.  
         [0042]    Since the field produced by the bistable molecular monolayers is a DC field, it is important that the particles be prevented from agglomerating or “plating out” in response to the field. In addition to the monolayers that are tailored for the substrate surfaces, monolayers or partial layers of molecules designed to provide steric hindrance may be used on the particle surfaces to prevent irreversible binding of the particles to the bistable molecular monolayers, or to each other. The charge associated with the particles used in an EPID will help prevent agglomeration in an EPID; however, the particles in a SPD are not typically charged, and may require the assistance of steric hindrance to prevent Van der Waals forces from becoming strong enough to resist the electrostatic forces that enable operation of the display device.  
         [0043]    In order to serve as a standoff, the molecule providing steric hindrance must be able to inhibit close approach between particles and the bistable molecular monolayer so that the radius of approach does not become small enough to allow Van der Waals forces to dominate.  
         [0044]    The preferred embodiment of the present invention, a particle display device using bistable molecular monolayers, is thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.