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Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display

Nanoparticles reduce switching time of encapsulated electrophoretic display

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Introduction
Encapsulated electrophoretic displays exhibit a high degree of contrast and stability against coagulation of their electrophoretic pigment particles. Under the influence of an electric field applied across the addressing electrodes, the pigment particles move to the upper electrode and provide for the coloring of a display pixel microcapsule into one of the contrasting colors. Another color may be provided (for example) by a colored dielectric liquid. As a rule, the particle size ranges from 0.25 to 2 microns. The particles are substantially large and thus inherit the bulk optical properties of the material from which they are formed. The optical properties of the particles in the display remain constant. With the constant properties of the electrophoretic particles, the appearance of the display is changed only by moving the electrophoretic particles toward the upper or lower electrode within the suspending fluid under the influence of the applied electric field. However, the number of colors produced by conventional electrophoretic displays is limited. In addition, the time necessary to switch the colors is longer than that in other displays and amounts to hundreds of milliseconds. It is necessary to reduce the switching time of an encapsulated electrophoretic display, while simultaneously increasing the number of produced colors.
 
Description
To reduce the switching time of an encapsulated electrophoretic display, using colored nanoparticles instead of common microparticles is proposed. The nanoparticle diameters range from 1 to 100 nanometers. Due to their nanosize, the nanoparticles do not practically scatter incident light (or rather, the light-scattering power is proportional to the sixth power of the particle radius) until they are concentrated into cluster particles the sizes of which are comparable to the light wavelength. A nanoparticle-based display changes its state (for example) from transparent to opaque depending upon whether the nanoparticles are dispersed or aggregated. The thickness of the layer may be no more than 1 micron, so the time necessary for a nanoparticle to travel the distance between the electrodes is much less than that in an encapsulated display based upon electrophoretic particles of micron size. Therefore, nanoparticles reduce the switching time.
 
Additional information
The number of produced colors may be increased by adding nanoparticles of another color with a different electrophoretic mobility. Due to the mobility difference, the lower mobility nanoparticles reach one of the electrodes more slowly than more mobile nanoparticles. Thus, when the particles are aggregated at the upper, transparent electrode, coloring is provided by the more mobile particles, as they reach the electrode earlier than the less mobile ones. When the nanoparticles are aggregated at the bottom electrode, the less mobile particles reach the electrode later than the more mobile particles. They coat the more mobile particles and make them obscure. The coloring is thus provided by the less mobile particles. When in the third, dispersed state, the particles expose the lower electrode for viewing. If the electrode has a third color, this third color becomes the color of the pixel. The preferred nanoparticle size is below 50 nm. If the nanoparticles consist of a semiconductor as opposed to dielectric nanoparticles, the color of the clusters of the semiconductor nanoparticles also depends upon the distance between the nanoparticles within a cluster. For example, for gold, the color of a cluster may change from blue to black depending upon the interparticle distance in the cluster. In this case, dispersions of gold nanoparticles are typically ruby red.
 
Reference
US Patent 6323989; Link >>
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