Big things come in small packages

Nanotechnology is receiving increasing attention and research dollars as its potential for revolutionizing science and technology becomes more apparent. As selected aspects of nanotechnology deliver new and unexpected results, new applications emerge, while current applications are transformed and improved. For example, one can imagine nanorobotic devices delivering many forms of healing to explicit sites in a living body without affecting adjacent healthy cells in the process. Computers will benefit from nanotechnology, not only in size but also in memory, speed, functionality, reliability, and cost. Although these applications might be in the distant future, many other applications could benefit immediately as researchers work to develop the tools to build the next set of tools and so on. One area of development involves nano building blocks such as nanopowders.

As the name implies, nanopowders are particles with sizes ranging from a few to several tens of nanometers. They can be made from fully organic materials, inorganic materials, or organic-inorganic hybrids. Nanopowders can either directly, or with further functionalization, offer novel electronic transport, light emission, light absorption, sensing, or magnetic properties, among others. Fabrication of nanopowders is a nontrivial task. Once made, processing the powder to obtain desired properties can be an even more daunting challenge. For example, sorting particles by size and into addressable arrays represents formidable difficulties for today's materials scientists. Thereafter, creating robust, useful devices using low-cost processing and assembly techniques represents another set of tangible challenges. In this article, we discuss the synthesis and some applications of mixed-metal-oxide nanopowders, based on work performed by the collaboration of groups at the University of Arizona (Tucson, AZ) and the University of Michigan (Ann Arbor, MI). Although many applications will benefit from the synthesis of high-quality nanopowders, we limit ourselves to a few examples.

We produce the nanopowders using liquid-feed flame spray pyrolysis (LF-FSP).¹ In this process, we dissolve single- and mixed-metal metallo-organic species in alcohol in the exact composition we want in the final oxide nanopowders. We then aerosolize this solution with oxygen and ignite the aerosol mist. The resulting flame reaches temperatures of approximately 2000°C depending on the precursor, flow rates, and oxygen content. The flame is quickly quenched to 400°C about 1.5 m below the combustion chamber, and the powders are collected either electrostatically or in a bag filter.

The resulting nanopowders typically mirror the composition of the solution to the level of parts per million. The rapid quenching allows us to obtain single-crystal particles of materials such as cerium-doped sapphire (Ce³⁺:δ-Al₂O₃), often with average particle sizes of 15 nm to 100 nm, depending on the combustion conditions.² The smaller the average particle sizes, the smaller the size distribution. We call the fabrication process "shooting." Currently, we can shoot powders with average particle sizes of 15 nm at rates of 50 to 100 g/h, and particle sizes of just under 30 nm at rates as fast as 4 kg/h (Tal Materials Inc.; Ann Arbor, MI).

Using the techniques outlined above, it is possible to make large quantities of mixed-metal oxide nanopowders of diverse chemical composition for various applications. For example, our group has fabricated nanopowders with up to five metals in a single phase. We find that under certain conditions, we can obtain phase segregation that allows us to make novel catalyst materials, including photocatalysts that offer potential for photo-oxidation of organics and bacteria using visible (400-nm) light.

Figure 1. Plots of current dependence of ultraviolet and visible emission originating from the ²F_5/2 or ²F_7/2 states of Nd³⁺ show that all curves but one increase monotonically at low currents and quench rapidly above a current of approximately 3 µA, where emission intensity on the ²F_5/2 to ⁴F_9/2 transition undergoes an abrupt change in slope.

Figure 2. Cathodoluminescence spectra of 3000-ppm Ce³⁺:δ-Al₂O₃ shows emission intensity versus current at 5 keV.

In another example, we demonstrated continuous laser action near 405 nm δ–sapphire (Al₂O₃) nanopowders doped with neodymium (Nd³⁺) at 3000 ppm (see figure 1). The nanopowders were packed (at ~30 p.s.i.) into recesses in an oxygen-free copper platen and placed in an ultrahigh vacuum chamber at a pressure of <10^-9 Torr. Likewise, by focusing a steerable beam of 2 to 10 keV electrons to a spot diameter of 1 to 2 mm on Ce³⁺ nanopowder samples, we have generated UV and visible laser light at room temperature for what we believe is the first time (see figure 2)³. Highly scattering powders are normally difficult to pump and study optically because incident light does not penetrate the medium well; therefore, we used the electron beam. The mild pumping conditions necessary to achieve this result lend support to the idea that the threshold for laser action is lowered by the onset of strong scattering in the nanopowder media.

By suspending nanopowders in solution, it is possible to print structures using an ink-jet printer.⁴ Numerous applications with low fabrication cost can be envisioned using the nanopowders in conjunction with printing techniques. Consider, for example, energy upconverting nanopowders that transform 980-nm illumination to visible light. Such powders can be used for security and anticounterfeiting applications.

Figure 3. Patterns of upconverting nanopowders formed using an ink-jet printer emit red light when illuminated with 980-nm light. (Photo credit: University of Michigan)

To demonstrate, we used a solution to suspend the nanopowder and load it into the ink-jet cartridge. We designed the device on the computer, then sent it to the printer, where the device fabrication was carried out. The ink-jet printing technique allows us to deposit the nanoparticles in controlled amounts and fashion anywhere on a given substrate. We printed some logos on flexible plastic substrate. Upon illumination with 980 nm, the nanoparticles emit red light (see figure 3).

We envision that many applications can benefit from the nanopowders and especially from using them in conjunction with printing techniques such as ink-jet printing. However, with all the progress we have made so far, there is still a need to optimize the printing process and to find new nanopowders. In this regard, the team is attempting to develop Nd³⁺-doped yttrium aluminum garnet nanopowders for processing polycrystalline transparent laser materials. oe

Acknowledgements

We would like to thank our colleagues and students, G. Williams, B. Bayram, and S.C. Rand, T. Hinklin, B. Li, Y. Yoshioka, P. Calvert, and N. Peyghambarian, for their help. We also gratefully acknowledge the financial support of NSF, ARO, DOD, and DARPA.

References

1. R. Baranwal, M. P. Villar, et al., J. Am. Ceram. Soc. 84, 951-61 (2001).

2. B. Li, G. Williams, et al., Optics Lett. 27, 394-6 (2002).

3. G. Williams, S.C. Rand, et al., Phys. Rev. A. 65, 013807 (2002).

4. J. Marchal, T. Hinklin, et al., "Yttrium aluminum garnet nanopowders by LF-FSP flame spray pyrolysis," to be submitted.