Sandia LabNews

The gold standard: Self-assembly process forms durable nanocrystal arrays, independent nanocrystals

The gold standard: Self-assembly process forms durable nanocrystal arrays, independent nanocrystals

A wish list for nanotechnologists might consist of a simple, inexpensive means — actually, any means at all — 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 casing 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.

Tracking hostile cells

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

In the April 23 Science, Sandia and University of New Mexico researchers report a simple, commercially feasible method for doing both these things.

"The paper overcomes barriers to using nanocrystals routinely," says Jeff Brinker, Sandia Fellow and UNM chemical engineering professor, who with Hongyou Fan (1846) led the self-assembling effort. "The question in nano- technology isn’t ‘where’s the beef,’ it’s ‘where’re the connectors’? How does one make connections from the macroscale to the nanoscale? This question lies at the heart of nanotechnology."

It is, Brinker says, "the raison d’etre for CINT." (CINT is the Sandia/

Los Alamos joint Center for Integrated Nanotechologies, now under construction.)

Bridging huge gaps in scale

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

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

Biolabeling, biosensing

"Also, quantum dots [another term for nanocrystals] can be important for biolabeling and biosensing," says Hongyou, who is the paper’s lead author and who initiated the effort to use the nanocrystals for those purposes. "The beauty of our approach is that it makes these quantum dots both water-soluble and biocompatible, two essential qualities if we want to use them 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, a critical issue in biomedicine."

Sandia has applied for a patent on this approach, which should aid attempts at several major universities to identify individual cancer cells before they increase in number.

Researchers have found that in the nanoscopic realm, changing merely the size of a material changes the frequency it emits when "pumped" by outside energy; so, quantum dots of particular sizes and material will emit at predictable frequencies, which makes them useful adjuncts when bound to molecules created to bind to particular cancer molecules.

How it works

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 — a kind of artificial solid with properties that can be adjusted through control of nanocrystal composition, diameter, properties of the surfactant, or stabilizing ligands used in formation of the water soluble nanocrystals.

The robust 3-D solids, which are stable indefinitely, demonstrate the incorporation of nanocrystalline arrays into device architectures.

Relief for physicists

A further use allows physicists to go beyond modeling to determine how current scales with voltage in nanodevices. "Before," says Jeff, "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. How does the current decide which way to hop between crystals?"

The new material can be used as an artificial solid to test theories. "It should be a dream for physicists; they don’t just have to model anymore," says Jeff.

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

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 (1846) made and provided nanocrystal semiconductor (cadmium selinide) quantum dots.