Scientists from Columbia's Materials Research Science and Engineering Center (MRSEC) and IBM have created a new, three-dimensional designer material assembled from two different types of particles only billionths of a meter across. This experiment demonstrates the ability to bring more materials together than previously realized.
In the June 26 issue of Nature, the research team described the precision chemistry methods developed to tune the particles' sizes in increments of less than one nanometer and to tailor the experimental conditions so the particles would assemble themselves into repeating 3-D patterns. Designing new materials with otherwise unattainable properties, sometimes referred to as "metamaterials," is one of the promises of nanotechnology.
Columbia's Stephen O'Brien, assistant professor of applied physics and applied mathematics, and Franz Redl, an adjunct associate research scientist affiliated with Columbia and IBM, worked with Christopher Murray, manager of nanoscale materials and devices at IBM Research, and Kyung Sang Cho, a researcher affiliated with IBM and supported by the Advanced Materials Research Institute of the University of New Orleans.
"What excites us the most is that this is a modular assembly method that will let us bring almost any materials together," said Murray. "We've demonstrated the ability to bring together complementary materials with an eye to creating materials with interesting custom properties."
The research team believes that these multi-component nanoparticle assemblies bring together novel tunable magnetic properties and tunable optical properties wavelengths in the infra-red important for fiber optic communications. For example, there may be magneto-optic phenomena in these materials that could be the basis of new optical modulators and switches that in turn may find use in future telecommunication technologies. These assemblies also have a unique combination of magnetic and semiconductor properties that may find applications in the emerging area of magneto-electronics.
The work was conducted at the IBM Watson Research Center in Yorktown Heights, New York. It was supported in part by the National Science Foundation (NSF) through MRSEC, and by the Defense Advanced Research Projects Agency (DARPA). The Columbia University MRSEC is an interdisciplinary team of university, industrial, and national laboratory scientists and engineers working together to develop and examine new types of nanocrystals and ways of assembling them into thin films.
The scientists chose the materials for the experiments specifically because of their dissimilar, yet complementary properties. Lead selenide is a semiconductor that has applications in infrared detectors and thermal imaging and can be tuned to be more sensitive to specific infrared wavelengths. The other material, magnetic iron oxide, is best known for its use in the coatings for certain magnetic recording media.
"This was a demonstration of the ability to create such materials," O'Brien said. "Given the unique combination of these nanoscale materials, we're in uncharted territory with respect to the properties, which we will be working on in the future. The greatest challenge was in the precise control of the size and structure of the constituent particles. Only the most uniform particles will permit this multi-component assembly. Dr. Franz Redl, the bridging post-doc between the Columbia group and the IBM group, did a superb job of bringing it all to together, by carefully tuning the conditions in order to induce the formation of the superlattice."
The first step was to create the nanoparticles. The particle sizes were calculated from the mathematical ideal of the structures they wanted to create. In addition to fine-tuning the sizes, the particles had to be very uniform, all within 5 percent of the target size. They settled on iron oxide particles 11 nanometers in diameter, which were created by Redl, and lead selenide particles 6 nanometers in diameter, created by Cho. There are approximately 60,000 atoms in one of the iron oxide nanoparticles and approximately 3,000 atoms in the lead selenide particles.
Next, Redl assembled the nanoparticles -- or more to the point, had the particles assemble themselves -- into three different repeating 3-D patterns by tailoring the experimental conditions. Forming these so-called "crystal structures," as opposed to random mixtures of nanoparticles, is essential for the composite material to exhibit consistent, predictable behaviors. Various other materials are known to assemble spontaneously into these structures of close-packed particles, but none has been made of two components in three dimensions and at the length scales reported in the Nature paper.
According to Irving P. Herman, director of the MRSEC, this project exemplifies the kind of collaborative interdisciplinary research that is one of the Center's key goals.
"From the beginning of the Columbia MRSEC in 1998, we have been bridging postdoctoral research scientists with IBM, enabling them to do research at both places. Faculty from six different academic departments at Columbia are also involved in the MRSEC. Collaboration between industry and academia is key to the Center's work, and we are pleased to partner with IBM, which is one of the pioneers in nanotechnology."
"When the NSF renewed the MRSEC in 2002, the focus of our research turned to complex metal oxide nanocrystals and how to make films of them -- such as these new three-dimensional materials. There is a lot of excitement about the results of this experiment."