Rugged Nano Films to Enable Applications
Stronger nanoparticle films that are easier to handle have been developed by Vanderbilt University researchers. "Our films are so resilient that we can pick them up with a pair of tweezers and move them around on a surface without tearing," said James Dickerson, assistant professor of physics at Vanderbilt.
Alexander E. Braun, Senior Editor -- Semiconductor International, 11/16/2009
Nanoparticle films hold great potential, but are difficult to handle — the slightest touch can damage or destroy them. Vanderbilt University (Nashville, Tenn.) researchers said they have discovered a way to make them strong enough so they will not disintegrate when handled.
Over the past quarter century, scientists have been developing nanoparticles for use in semiconductors, flexible displays and medical applications. However, molding them into thin films has been challenging because of a lack of cohesion. Nanoparticles typically consist of an inorganic core coated with a thin layer of organic molecules. The particles are not very sticky and do not easily form into coherent thin films. To stick nanoparticles together, they must be encapsulated in a polymer coating, or mixed with chemical cross-linker molecules that act like glue. Adding this extra material not only complicates nanoparticle film fabrication and makes it more expensive, but the added material itself — usually a polymer — can modify the very physical properties that make these films attractive.
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AFM image of the surface of an iron oxide nanoparticle film, showing individual nanoparticles. (Source: Vanderbilt University) |
"Our films are so resilient that we can pick them up with a pair of tweezers and move them around on a surface without tearing," said James Dickerson, assistant professor of physics at Vanderbilt, who heads the research group. "This makes it particularly easy to put them into microelectronic devices, such as computer chips." Dickerson expects that the most immediate and straightforward applications for the films will be in semiconductor manufacturing to continue scaling, as well as in the production of flexible television and computer screens.
A key component in transistor fabrication is the insulating layer that separates the gate from the channel. Traditionally, SiO2 has been used for this purpose. As scaling has continued, however, this insulating layer has had to be made increasingly thinner, and the stage was reached at which electrons leak through and sap power. This led to a retooling of the fabrication process for the use of high-k dielectric materials, such as hafnium oxide. "We have made high-k nanoparticle films that could be cheaper and more effective than the high-k materials that manufacturers are currently using," Dickerson said. The physicist added that the films developed by his research team display properties that make them ideal for flexible television and computer screens. "They're very flexible and don't show any signs of cracking when they are flexed repeatedly," he said.
The Vanderbilt films are made using an electrophoretic deposition (EPD) technique that is well-suited for creating patterned material and is compatible with fluorescent materials that can form the red, green and blue pixels used in flat panel TV screens and computer displays. EDP is a wet method, mixing particles 10-1000× larger than nanoparticles, which is widely used to apply coatings to complex metal parts such as automobile bodies, prosthetic devices, appliances and beverage containers. The nanoparticles are placed in a solution along with a pair of electrodes. When a current is applied, it creates an electrical field in the liquid that attracts the nanoparticles, which coat the electrodes. It is only recently that researchers like Dickerson have begun applying the technique to nanoparticles.
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The sacrificial layer electrophoretic deposition method used to organize nanoparticles to produce more rugged films. (Source: Vanderbilt University) |
"Colloidal EDP is well known, but the particles are substantially larger than the solvent molecules," Dickerson explained. "Many nanoparticles, however, are about the same size as the solvent molecules, which makes the process considerably more complicated and difficult to control." To get the method to work, the Vanderbilt researchers essentially had to invent a new form of EDP, which they call "sacrificial layer electrophoretic deposition." They added a spun-cast layer of polymer to the electrodes that serves as a pattern that organizes the nanoparticles as they are deposited. Once the deposition process is completed, they dissolve (sacrifice) the polymer layer to free the nanoparticle film.
According to the researchers, films made in this way stick together because the electrical field slams the nanoparticles into the film with sufficient force to pack the particles together tightly enough to allow naturally attractive inter-particle forces to bind them. So far, Dickerson's group has used the technique to make films out of two different types of nanoparticles — iron oxide and cadmium selenide — and they are convinced that the technique can be used with a wide variety of other nanoparticles.
"The technique is liberating because you can make these films from the materials you want and use them where you want," Dickerson said.
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