"While one would like to control materials entirely by self-assembly, sometimes this may not be possible due to a variety of factors such as solubility of components," Todd Emrick and Tom Russell told nanotechweb.org. "Thus, it is highly desirable to develop routes by which sequential deposition processes can be used, for example using templates whereby one can spatially select the deposition of nanoparticles on a surface."
The electrophoretic deposition process uses an electric field to attract charged particles in solution onto an electrode. In this case, the researchers attached templates containing nanopores or nanotrenches to the anode - the nanoparticles deposited inside the nanopores or nanotrenches and not on the surface of the template.
To make the templates, Emrick, Russell and colleagues prepared diblock copolymers from polystyrene and poly(methyl methacrylate). In one case, they used a PMMA volume fraction of 0.25; the polymer self-assembled into a structure containing 15 nm-diameter cylinders of PMMA. The other copolymer had a PMMA volume fraction of 0.5 and contained alternating lamellae of PMMA and polystyrene.
The team removed the PMMA phase of the templates by exposing them to ultraviolet radiation. This created a structure containing nanopores for the polymer with cylinders of PMMA, or nanotrenches for the polymer with a lamellar structure. The scientists attached the templates to a copper grid and linked the grid to a silicon wafer coated with gold to form an anode. The cathode was a bare gold-coated silicon wafer.
To provide the CdSe nanoparticles with a charge and make them soluble, the researchers functionalized them with carboxylate molecules. Next, they made a solution of the nanoparticles in methanol and carried out the electrophoretic deposition process.
"By placing the template on a conducting surface, it was obvious that charge could be used to drive the nanoparticles into the hollow cylinders of diblock copolymer templates," said Emrick and Russell. "However, the deposition would only occur in areas where the electrode was exposed to the nanoparticle solution. Hence, a selective deposition is achieved and, with these polymer templates, one can place the nanoparticles virtually anywhere on the surface over macroscopic distances."
The researchers believe that the next step is using the technology for device fabrication. "Attracting the interest of scientists in this area is clearly a step we must take," they said. "Of course, the exact nature of the nanoparticles, ligands attached to the nanoparticles, nanoparticle density on the surface, nanoparticle size and surface patterning will need to be tailored for the device."
The team is also "combining nanoparticles with block copolymers to make materials that will self-direct and self-assemble".
The researchers reported their work in Nano Letters.