'Nanotools' - Self-assembling durable nanocrystal arrays

A wish list for nanotechnologists would likely include a simple, inexpensive means of self-assembling nanocrystals into robust, orderly arrangements, like soup cans on a shelf or bricks in a wall, each separated from the next by an insulating layer of silicon dioxide.

The silica matrix could be linked to compatible semiconductor devices. The trapped nanocrystals might function as a lasing medium, their frequency dependent on their size, or as a very fine catalyst with unusually large surface area, or perhaps a memory device tunable by particle size and composition.

Or perhaps the technologist might want to stop nanocrystals from clumping. Agglomeration prevents them from being used as lightemitting tagging mechanisms to locate cancer cells in the body and from being used in light-emitting devices needed for solid-state lighting.

A simple, commercially feasible method for doing both these things was described in the April 23, 2004, issue of Science magazine, in an article titled "Self-Assembly of Ordered, Robust, Three-Dimensional Gold Nanocrystal/ Silica Arrays."

"The question in nanotechnology isn't 'where's the beef," says Jeff Brinker, Sandia Fellow and University of New Mexico (UNM) chemical engineering professor. Brinker, with Sandia's Hongyou Fan, led the self-assembly effort. "It's 'where are the connectors?' How does one make connections from the macroscale to the nanoscale? This question lies at the heart of nanotechnology."

Bridging huge gaps

The Sandia/UNM self-assembly approach allows nanocrystal arrays to be integrated into devices using standard microelectronic processing techniques, bridging huge gaps in scale.

"One thing that's nice is that these are hard materials," says IBM staff researcher Chuck Black of T. J. Watson Research Center in Yorktown Heights, NY. "Often they come with an organic surfactant layer that makes it difficult to process materials, like a kind of grease." The Sandia/UNM approach scrubs the surfactants with an ozone compound. "This material is embedded in oxide. It sounds like a neat thing and a new approach."

"Quantum dots (another term for nanocrystals) can also be important for bio-labeling and bio-sensing," says Fan, who initiated the effort to use the nanocrystals for those purposes. "Our approach makes quantum dots both watersoluble and bio-compatible, which are two essential qualities for in-vivo imaging.

The functional organic groups on the quantum dots can link with a variety of peptides, proteins, DNA, antibodies, etc., so that the dots can bind to and help locate targets like cancer cells." Sandia has applied for a patent on this approach, which should aid attempts to identify individual cancer cells before they increase in number. Researchers have found that, at the nanoscopic realm, changing the size of a material changes the frequency it emits when "pumped" by outside energy. Thus, quantum dots of particular sizes and material will emit at predictable frequencies, which makes them useful adjuncts when bound to particular cancer molecules.

The process uses a simple surfactant similar to dishwashing soap to surround the nanocrystals -- in this case, made of gold -- to make them water soluble.

Further processing involving silica causes the gold nanocrystals to arrange themselves within a silica matrix in a lattice with adjustable properties.

Physicists' dream A further use allows physicists to go beyond computer modeling. "Previously," says Brinker, "there was no way to make precisely ordered 3-D nanocrystalline solids, integrate them in devices, and characterize their behavior. There was no theoretical model."

The new material can be used as an artificial solid to test out theories. "It should be a dream for physicists," says Brinker.

A kind of choreographed transmission possibility exists with the so-called "Coulomb blockade," Brinker says. No current is passed at low voltages because each crystal is separated by a thin (several nanometer-thick) layer of silica dioxide. This creates an insulator between the stored charges and each nanocrystal charges separately. "This could be configured into a flash memory with a huge number of charges stored in an array of nodes," says Brinker.

Researchers at UNM's Center for High Technology Materials performed experiments to establish the current/ voltage-scaling characteristics of the gold/silica arrays as a function of temperature. Sandia researcher Tim Boyle made and provided nanocrystal semiconductor (cadmium selenide) quantum dots.

The full-text Science article is posted at http://www.sciencemag.org/cgi/content/ full/304/5670/567

Media Contact: Neal Singer nsinger@sandia.gov, 505-845-7078