Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-03-21
2004-08-17
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S021000, C257S098000, C438S069000
Reexamination Certificate
active
06777706
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical devices, especially devices comprising particles.
BACKGROUND OF THE INVENTION
Nanoparticles are particles of very small size, typically less than 100 nm across. The preparation of well-defined nanoparticles via colloid chemistry was demonstrated at least as early as the 1980s. A review of the current technology in this field is given in M P Pileni, Langmuir, 13, 1997, 3266-3279. There are three principal established routes for the formation of nanoparticles: a microemulsion route, a sol-gel route and a high temperature process used principally for semiconducting nanoparticles such as CdSe.
The synthetic principles of microemulsions have been widely described in the literature. For recent work see for example: F. J. Arriagada and K. Osseo-Asare, “Synthesis of Nanosized Silica in Aerosol CT Reverse Microemulsions,” Journal of Colloid and Interface Science 170 (1995) pp. p17; V. Chhabra, V. Pillai, B. K. Mishra, A. Morrone and D. O. Shah, “Synthesis, Characterization, and Properties of Microemulsion-Mediated Nanophase TiO
2
Particles,” Langmuir 11 (1995) pp. 3307-3311; H. Sakai, H. Kawahara, M. Shimazaki and M. Abe, “Preparation of Ultrafine Titanium Dioxide Particles Using Hydrolysis and Condensation Reactions in the Inner Aqueous Phase of Reversed Micelles: Effect of Alcohol Addition,” Langmuir 14 (1998) pp. 2208-2212; J. Tanori and M. P. Pileni, “Control of the Shape of Copper Metallic Particles by Using a Colloidal System as Template,” Langmuir 13 (1997) pp. 639-646. A microemulsion is a sufficiently thermodynamically stable solution of two normally immiscible liquids (for example, oil and water) consisting of nanosized droplets (or cores) of one phase in another “continuous” phase, stabilised by an interfacial film of a surfactant with or without a co-surfactant. Examples of surfactants include ionic ones such as Aerosol OT and cetyldimethylethylammonium bromide, and non-ionic ones such as the polyoxyethylene ether and ester surfactants. Examples of co-surfactants include medium to long alkyl-chain alcohols such as 1-hexanol. Examples of oils include hydrocarbons such as cyclohexane and isooctane. The surfactant and co-surfactant molecules reduce the interfacial tension so that stable dispersions can be formed.
Forming nanoparticles by the microemulsion route typically involves preparing a reaction mixture as a water-in-oil reverse micellar system using a ternary phase mixture containing high oil and surfactant contents, but low water content. This allows discrete but thermodynamically-stable nanometer-sized “water pools” or “water cores” to develop in the reaction mixture. In a typical water-in-oil microemulsion, the water cores are around 1 to 10 nm in diameter. One reactant for the nanoparticle formation can be initially housed in these water cores. The second reactant can subsequently diffuse into and react inside these “nano-reactors” in the normal course of microemulsion dynamics. In this way, microemulsions provide a versatile route to the controlled synthesis of a wide array of oxide and non-oxide types of nanoparticle. In the water pools a metal salt can be reduced to the free metal, or metathesis reactions can be included, to obtain a controlled nucleation and growth of the desired nanoparticle material. The surfactant also acts as a coating to prevent unwanted flocculation (agglomeration) of the growing particles. Many of the fundamental principles governing such micellar chemistry, such as reaction rate and final growth size, are still largely unknown. Most experiments are done by trial-and-error and the data interpreted empirically.
Much of the work on nanoparticles has concentrated either: (i) on demonstrating that nanometer-sized particles have indeed been created (for instance by using transmission electron microscopy (TEM) or ultraviolet-visible (UV-Vis) spectroscopy); or (ii) on subsequently sintering the nanoparticles to prepare a sintered body. This work has involved relatively crude techniques for handling the nano-sized material. For aspect (i) there has generally been no need to isolate or further manipulate the nano-size material. For aspect (ii) the formed material has typically been recovered from the emulsion by bulk precipitation upon addition of a destabilising solvent, or by vacuum removal of the reaction solvent. The material is then sintered at high temperatures to obtain the desired nano-grained article after “burning off” of the surfactant coating. Since the particles are to be sintered into a solid mass there is no need to counteract their tendency to aggregate.
Some work has been done on other uses for nanoparticles. S Carter, J C Scott and P J Brock, Appl. Phys. Lett, 71, 1997, 1145-1147 describe the use of polydispersed TiO
2
, SiO
2
and Al
2
O
3
nanoparticles in the form of a blend in polymer LED devices with the aim of enhancing the forward emission of light generated in the LED and/or improving carrier injection and recombination. The route by which the nanoparticles are obtained is not described, but the particles are described as having relatively large sizes: 30 to 80 nm, especially in comparison to the device thickness of 110 nm. It appears from the presence of light scattering that the nanoparticle material suffers from agglomeration. Thus the nanoparticular nature of the material cannot be fully exploited.
In some other works, for instance V. L. Colvin, M. C. Schlamp & A. P. Alivisatos Nature 370, 6488 (1994) “Light-emittng-diodes made from cadmium diselenide nanocrystals and a semiconducting polymer”, the use of CdSe nanocrystals as a form of transport layer (deposited neat either by spin-coating or electrostatic self-assembly) in a multilayered device with organic light-emitting polymers has also been described. Another reference relating to nanoparticle polymer composites is J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow, R. E. Geer, R. Shashidhar, and J. M. Calvert, “Metal nanoparticle/polymer superlattice films: fabrication and control of layer structure,”
Adv. Mater
., vol. 9, pp. 61, 1997.
Organic materials are used for a wide range of applications, including the formation of light emissive devices (see PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference). There is often a need to tune the properties of such an organic material. For example, in the manufacture of optoelectronic devices there is a need for control over various properties of the materials to be used, including conductivity (and/or mobility), refractive index, bandgap and morphology. Some examples of known techniques for tuning various properties are as follows:
1. Conductivity. This has been tuned by adding a chemical compound that acts as a donor or acceptor (namely an electronic dopant), see C. K. Chiang, C. R. Fincher, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, “Electrical Conductivity in Doped Polyacetylene,”
Phys. Rev. Lett
., vol. 39, pp. 1098-1101, 1977.
2. Charge generation and photo-voltaic response. This has been tuned by blending two materials with appropriate electronic levels so that electrons prefer to reside on one and holes on the other. The blends have been either of two organic materials such as polymers (J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, and A. B. Holmes, Nature 376, 498 (1995), “Efficient photodiodes from interpenetrating polymer networks”) or of organic material with a nano-particle (N. C. Greenham, X. G. Peng, and A. P. Alivisatos, “Charge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity ,”
Phys. Rev. B—Cond. Matt
., vol. 54, pp. 17628-17637, 1996) to achieve exciton dissociation at the interface.
3. Band-gap and emission colour. This has been tuned by mixing organic compounds in the form of blends or co-polymers (see co-pending UK patent application number 9805476.0).
4. Scattering. Highly aggregated or very large s
Friend Richard Henry
Ho Peter
Tessler Nir
Cambridge Display Technologies
Jackson Jerome
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