Portland, Ore. - All the fuss over nanoparticles might lead one to assume they are routinely being used to prepare thin films. But insolubility and the clumping it causes thwarted early attempts to turn nanoparticle laboratory results into usable devices. Now, researchers at Sandia National Laboratories and the University of New Mexico say they've perfected a commercially feasible way for orderly arrays of nanoparticles to self-assemble, each insulated from the others by silicon dioxide.
The technique will not only enable new devices, the researchers said, but could also solve one of the longest-standing problems with nanoparticles: forming orderly connections between micro- and nanoscale.
"We are showing engineers how to make use of the nanoparticles that physicists have only been able to measure in the lab," said Jeff Brinker, who is both a Sandia National Laboratories fellow and an engineering professor at the University of New Mexico. "With our self-assembly technique you can stop nanoparticles from clumping, plus they are insulated from each other with silicon dioxide."
By spin-coating precisely controlled thicknesses of silicon dioxide with embedded nanoparticles, the researchers hope to reduce to nanoscale the applications that until now have resisted downsizing. For instance, besides using the nanoparticles for biological labeling and as large-surface-area catalysts, they could be formed into thin films for nanoscale lasers whose frequency would depend on the nanoparticles' size, or they could be used as ultradense arrays of nanoscale flash memories.
Moreover, the orderliness of the arrays should ease the transition from the micron-sized connections on today's commercial chips to the orders-of-magnitude denser nanoscale structures.
"One problem that is at the heart of nanotechnology," said Brinker, is how to establish connections between circuits on different scales, which is known as the fan-in problem. "We think our orderly arrays will help chip makers design a solution," said Brinker.
By using self-assembly techniques compatible with standard microelectronics processing, Brinker hopes to bridge huge gaps in scale by integrating nanocrystal arrays into standard silicon chips. If that can solve the fan-in problem, the nanoparticles embedded in silicon dioxide could become a massive number of stored charge cells.
In the test material, the researchers demonstrated a kind of choreographed transmission among nanoparticles that is called a coulomb blockade. At low voltages, no current passes, because each nanoparticle is separated from adjacent ones by a layer of silicon dioxide several nanometers thick. But at high voltages, current jumps by the cube of the voltage.
In addition, because nanoparticles typically range from 1 to 10 nm in diameter, their electrical properties are dominated by quantum confinement effects. Coulomb interactions in nanoparticles form excitons-electron-hole pairs-when they are pumped with optical energy from a laser. The distance between the electron and hole is called the Bohr radius of the exciton, and the resulting energized nanoparticle is called a quantum dot.
"We have found that we can get different frequencies of emission, when [laser pumping is used], by merely changing the size of a quantum dot," said Sandia researcher Hongyou Fan. "That we can get them to emit light could make them useful adjuncts to molecules we create to bind to cancer cells." Sandia Labs, an arm of the National Nuclear Security Administration, has applied for a patent on identifying cancer cells early, before their numbers increase, with such fluorescent markers.
The patented process created at Sandia uses an organic surfactant layer that ordinarily makes it difficult to process nanoparticles. Acting as a kind of grease, the patented approach scrubs the surfactants off the nanoparticles with an ozone compound and instead embeds them in oxide.
In the two-step process, first a detergent solution is mixed with the nanoparticles, scrubbing off the grease and thereby making them water-soluble. In the second step, silica is introduced into the solution, causing the nanoparticles to embed themselves into a silicon dioxide lattice when the compound solidifies.
"You end up with an artificial solid, one that doesn't exist in nature," said Brinker. "Many researchers have speculated about making designer materials where the properties can be tailored to an application by controlling the composition, size and ligands of embedded nanoparticles. With our process, they can start trying those ideas."
The-3-D films and solids created with the process are stable indefinitely, the researchers said, and can have application-specific ligands attached for biomedical, optical or even magnetic storage devices. Even nanoparticles of different types could be combined to create specialized nano-molecules. "Table salt has the useful properties that it does because it is made from two very different-sized atoms," said Brinker. "We think that using very different-sized nanoparticles could result in novel properties in an artificial solid."
For electronics, such artificial solids can now be consistently, repeatedly fabricated so that their voltage-to-current characteristics can be characterized and thus designed into production-line devices. In addition, engineers should have an easier time fitting theory to practice than when using natural materials whose properties must be discovered rather than designed.
"Our artificial solid will allow materials engineers to test out their theories with real materials," said Brinker.