Falling into Place: Atom mist yields nanobricks and mortar

Nanotechnologists envision using tiny structures to create ultrastrong materials and to build memory chips that store entire libraries. But these visions require making matter behave in exceptionally orderly ways.

Now, materials scientists Jagdish Narayan and Ashutosh Tiwari of North Carolina State University (NCSU) in Raleigh have induced tiny particles, or nanodots, of nickel to spontaneously assemble into exceptionally uniform, three-dimensional arrays of macroscopic size.

With this method, they’ve also created blends of copper nanodots and tin that they say are harder than steel. The company Kopin in Taunton, Mass., is already applying the technique to semiconductors that they use to manufacture unusually efficient light-emitting diodes.

Narayan and Tiwari describe their work in the September Journal of Nanoscience and Nanotechnology.

Many other scientists “will now investigate this approach” to make such orderly 3-D arrays of nanoparticles, comments William H. Butler, director of the Center for Materials for Information Technology at the University of Alabama in Tuscaloosa.

Nanodots, which are particles made of only hundreds to thousands of atoms, can exhibit extraordinary properties compared with those of bulk materials. For instance, some nanodots of semiconductor materials, also called quantum dots, emit light of a color determined by the size of the clump (SN: 8/7/04, p. 94: Available to subscribers at Quantum dots light up cancer cells in mice).

Many researchers have devised methods for making two-dimensional arrays of nanodots. Achieving precise, three-dimensional arrangements has proved more elusive.

Narayan and Tiwari use a laser to vaporize two targets in a vacuum chamber. The atoms of one target are designated for nanodots, and atoms of the other will end up in matrix material surrounding the nanodots.

As the vaporized atoms land on a surface in the chamber, they typically migrate toward other atoms of the same type. Unchecked, such migrations produce massive, irregularly spaced islands of each material. However, by fine-tuning the temperature in the chamber and the rates at which the target materials vaporize, the North Carolina team can control the atomic migration, leading to uniform nanodot arrays surrounded by matrix material.

To extend the nanodot order to the third dimension, the team exploits the pattern of stresses in the nanodot-matrix layer. When the scientists provide successive rounds of the two types of vapor, the previous layer’s deformation guides positioning of the new nanodots in direct register with the ones beneath and makes the new matrix molecules line up with the old.

The NCSU researchers used sturdy compounds such as aluminum oxide and titanium nitride for the matrix material.

The new arrays may be useful in developing close-packed magnetic nanodots that could serve as extraordinarily dense data-storage devices for computers, Butler says.