Nanoparticles typically exhibit low cohesion, making their use in thin films problematic. Scientists at Vanderbilt University have found a way of fabricating all-nanoparticle thin films that do not easily disintegrate, making them suitable for a wider range of applications than was previously the case [Hasan, et al., Chemical Communications 2009 25 3723].
Much interest has been shown in colloidal nanoparticles because of size-dependent characteristics that, potentially, make them suitable for use in electronic, optical and magnetic devices. “One of the central goals for the successful implementation of these nanomaterials in various applications is establishing facile mechanisms to fabricate nanoparticulate materials made in size scalable, reproducible, and transportable configurations,” comments James Dickerson, assistant professor of physics at Vanderbilt, who headed the research group. Until now, limitations including lack of structural integrity and lack of reversible adhesion have prevented the commercial use of such materials.
EDP, or electrophoretic deposition, was used to create the nanoparticle films but a new method had to be used. In colloidal EDP, the useful particles are much larger than those of the solvent but nanoparticles are the same size, so the process is difficult to control and considerably more complicated. A process they call ‘sacrificial layer electrophoretic deposition’ was invented by the team, adding a spun-cast layer of polymer that coats the electrodes. This acts as a pattern to organise the deposition of nanoparticles and the polymer layer is dissolved on completion.
“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,” says Dickerson. “This makes it particularly easy to put them into microelectronic devices, such as computer chips.” The films are becoming more relevant as semiconductor devices shrink, thinner layers of silicon dioxide leaking electrons. High-k dielectric materials have higher electrical resistance and nanoparticle films from such materials as hafnium oxide could contribute to further chip miniaturization.
James Dickerson continues, “Our research efforts proffer a versatile approach to the fabrication of a new class of colloidal solids: freestanding, binder-free all-nanoparticle thin films.” This technology can be used to construct wholly novel materials for device applications, such as flexible, ‘roll-up’ video displays, transferable anti-corrosion and anti-fogging coatings, anti-reflection films for optical components, and freestanding carbon nanotube-based electrodes. “Future trajectories in this area include explorations of the mechanical characteristics of the films and the fabrication of macroscopic ordered arrays of nanoparticles via EPD, so called ‘bulk crystals of nanocrystals’,” he concludes.