1. Field of the Invention
This invention, generally relates to plasmonic displays and, more particularly, to a low-cost method for fabricating metallic nanostructures used in plasmonic displays.
2. Description of the Related Art
Reflective display or color-tunable device technology is attractive primarily because it consumes substantially less power than liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. A typical LCD used in a laptop or cellular phone requires internal (backlight) illumination to render a color image. In most operating conditions the internal illumination that is required by these displays is in constant competition with the ambient light of the surrounding environment (e.g., sunlight or indoor overhead lighting). Thus, the available light energy provided by these surroundings is wasted, and in fact, the operation of these displays requires additional power to overcome this ambient light. In contrast, reflective display technology makes good use of the ambient light and consumes substantially less power.
A number of different reflective display technologies have been developed, such as electrophoretic, electrowetting, electrochromic displays, and interference-based MEMS display. These display technologies all have disadvantages or challenges that must be overcome to obtain greater commercial success. Many existing technologies rely upon phenomena that are intrinsically slow. For example, electrophoretic or electrochemical techniques typically require particles to drift or diffuse through liquids over distances that create a slow response. Some other technologies require high power to operate at video rates. For example, many reflective displays must switch a large volume of material or chromophores from one state to another to produce an adequate change in the optical properties of a pixel. At video switching rates, currents on the order of hundreds of mA/cm2 are necessary if a unit charge must be delivered to each dye molecule to affect the change. Therefore, display techniques that rely on reactions to switch dye molecules demand unacceptably high currents for displaying video. The same holds true for electrochromic displays.
A second challenge for reflective displays is the achievement of high quality color. In particular, most reflective display technologies can only produce binary color (color/black) from one material set. Because of this, at least three sub-pixels using different material sets must be used when employing a side-by-side sub-pixel architecture with fixed colors. This limits the maximum reflected light for some colors to about ⅓, so that the pixels of this type cannot produce saturated colors with a good contrast.
Finally, some reflective displays face reliability problem over a long lifetime. In particular, to sustain video rate operation for a few years requires at least billions of reversible changes in optical properties. Achieving the desired number of cycles is particularly difficult in reflective displays using techniques based on chemical reactions, techniques that involve mixing and separation of particles, or MEMS technology that involves repeated mechanic wear or electric stress.
In polymer-networked liquid crystal (PNLC) or polymer dispersed liquid crystal (PDLC) devices, liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. The curing conditions affect the size of the droplets that in turn affect the final operating properties of the “smart window”. Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that includes a thin layer of a transparent, conductive material followed by curing of the polymer, thereby forming the basic sandwich structure of the smart window. This structure is in effect a capacitor.
Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the smart window assembly. This scattering results in a translucent “milky white” appearance. When a voltage is applied to the electrodes, the electric field formed between the two transparent electrodes on the glass causes the liquid crystals to align, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the applied voltage. This is possible because at lower voltages, only a few of the liquid crystals align completely in the electric field, so only a small portion of the light passes through while most of the light is scattered. As the voltage is increased, fewer liquid crystals remain out of alignment, resulting in less light being scattered. It is also possible to control the amount of light and heat passing through, when tints and special inner layers are used. It is also possible to create fire-rated and anti X-Ray versions for use in special applications. Most of the devices offered today operate in on or off states only, even though the technology to provide for variable levels of transparency is available. This technology has been used in interior and exterior settings for privacy control (for example conference rooms, intensive-care areas, bathroom/shower doors) and as a temporary projection screen.
The full range of colors produced by plasmon resonances resulting from metal nanostructures has been known since ancient times as a means of producing stained colored glass. For instance, the addition of gold nanoparticles to otherwise transparent glass produces a deep red color. The creation of a particular color is possible because the plasmon resonant frequency is generally dependent upon the size, shape, material composition of the metal nanostructure, as well as the dielectric properties of the surroundings environment. Thus, the optical absorption and scattering spectra (and therefore the color) of a metal nanostructure can be varied by altering any one or more of these characteristics. The parent applications listed above describe means of electronically controlling these color-producing characteristics.
The properties of metallic nanoparticles have drawn significant attention due to their application in photonics and electro-optics, as well as their potential application in biological/chemical sensors and renewable energy. Moreover, the fabrication of periodic metallic nanoparticle arrays for applications in photonics utilizing their localized surface plasmon resonance (LSPR) properties has been extensively studied in recent years. Among various processing techniques, depositing a film of metal on a nano-size patterned mask and using a lift-off process to remove the sacrificial layer is becoming a widely used technique, because it allows for fabricating nanoparticles with precisely controlled shape, size, and particle spacing. Moreover, advanced research has revealed that ordered array nanostructures have improved quantum characteristics utilized in LSPR properties, or photoluminescence and electroluminescence properties of semiconductor nanophosphors. Therefore, a method to achieve ordered nanoparticles and nanostructures is of significant importance.
One method of forming nanoparticles in an ordered array which has been proposed is a nano-imprinting process using a mold to generate ordered dot patterns on poly methyl methacrylate (PMMA) mask layers. After forming an array of nanostructures to a predefined depth, the residual PMMA layers at the bottom of the pattern are removed to reveal the surface underneath the mask. The metallic nanoparticles are formed by depositing the desired metals and removing the PMMA mask layers in a solution to lift off the residual metals deposited on the mask. According to this technique, the size and spacing of the nanostructures are limited by the photolithography patterning capability of the mold. Also, the manufacturing cost is high due to expensive capital costs associated with the nanoimprint lithography process. Alternatively, nano-scale structures can be patterned using e-beam lithography. However, this process is extremely slow and costly.
Among other processing techniques, forming an array of ordered nanostructures using self-assembly solution process is desired due to low capital cost without sacrificing manufacturing scalability. It is possible to anodize an aluminum layer in an electrolyte solution to form aluminum oxide pores with diameters of several nanometers to several hundred of nanometers arranged in hexagonal order. This anodized aluminum oxide (AAO) template can then be used as a mask to generate an array of nanoparticles on a substrate by depositing desired materials and lifting off the mask to form the nanostructures. However, in order to produce a pore that extends all the way through the AAO template, an electrically conductive layer must lie beneath the AAO. Otherwise, the pore formation process will stop with a thick and variable insulating barrier layer beneath each pore, which is difficult to subsequently remove in a uniform fashion. Such a structure would be difficult to use as a mask for reliably patterning nano-scale structures. Thus, it is currently not possible to use an AAO template fabricated directly on an insulating substrate as a mask for producing a high quality array of nanostructures, such as metal nano-dots. It is, however, possible to form the AAO template using a separate Al foil and transfer the template to any desired substrate. However, this process is difficult to scale to large size, due to the delicate nature of the template and its transfer process.
Plasmonic effects are larger if the metal particles are electrically isolated from each other, i.e. they are on an insulating substrate. Also, due to the nature of the liquid crystal used in these displays, it is best to allow a switching electric field parallel to the surface containing the metallic nanoparticle array. Consequently, it is best to have electrodes embedded in this surface, but electrically insulated from the metallic nanoparticles.
It would be advantageous if a nanoparticle array could be fabricated using a low-cost AAO template process which can be scaled to large sizes, and in which the AAO is formed directly on the surface rather than in a separate Al foil. It would also be advantageous if the AAO template fabrication process could be used to control the shape the nanoparticles formed.