SELF-ASSEMBLY IN SPACE

Geoffrey A. Ozin
Materials Chemistry Research Group

An exciting new direction in the Ozin materials chemistry research group concerns their groundbreaking work on the synthesis, characterization and properties of a novel class of materials that are referred to as nanoporous electronic materials. These materials have open-framework crystal lattices that can be structurally viewed as semiconductors filled with regular arrays of molecular dimension pores (3-20 angstroms size range). They can also be regarded as the geometrical complement of semiconductor clusters arranged in periodic lattices, the latter being known in physics as quantum dot lattices and the former as quantum antidot lattices. The spatial confinement of charge-carriers in these nanostructures, to length scales that are small compared to the wavelength of an electron or hole, confers unique "size and composition-tunable" electronic and optical properties on the materials that are not found in their bulk semiconductor analogs.

Specifically, the Ozin group has learned how to synthesize metal sulfide and metal selenide open-framework materials, through the use of a synergistic integration of organic and inorganic chemistry. In essence, this involves the spatially controlled co-assembly of inorganic building-blocks with organic templating molecules to create a "molecularly imprinted inorganic replica" material. In these solids the patterning of the pore structure is facilitated by the organic additive whose templating function is to fill space and balance charge. The properties of such materials are most interesting. They have, for instance, stimulated the group to investigate the idea of nanoporous electronic and optical devices, such as, a synthetic photonic-gap material, which has recently been described in the semiconductor physics literature to have the intriguing property of being able to trap light of the right wavelength.

Intensive research on these nanoporous electronic materials in the Ozin laboratory over the last few years has provided much information on their combined synthesis-structure-property-function relationships (i.e., the trade of the solid state chemist).^1-10 The group has learned a lot about their materials, including knowledge of their single crystal and Rietveld powder X-ray diffraction structures, mode of formation, atomic force microscopy-based images of surface topographies (with molecular resolution), thermochemical properties, adsorbate-induced framework flexibility, compositional tuning of their electronic band properties, electrical transport (ac/dc conductivity) behaviour, Fourier Transform-IR/Raman vibrational spectroscopy, growth as lattice matched and oriented thin films, synthesis of epitaxial junctions and studies of their potential for chemical sensing. Most of this research in the group has been published in materials chemistry journals and some was recently selected by the German Chemical Society as one of the materials research highlights of 1995.

The Ozin group are optimistic that this new class of metal chalcogenide semiconductors, with regular arrays of size-tunable nanometer dimension holes, will be able to function as discerning hosts and discriminate between different guest molecules based on their size and shape-selective adsorption behaviour. The Ozin group is currently exploring the potential of their new materials for chemoselective sensing applications.

An extension of this research has led the Ozin group to earth-based 1G and space-based µG investigations of the synthesis and crystal growth of their self-assembling nanoporous electronic materials. This work has been funded by the Canadian Space Agency over the period 1993-1996. The essence of this project, is to take advantage of the fact that deleterious sedimentation and convection effects that abound in their earth-based synthesis-crystal growth experiments, do not exist under the quiescent state of a microgravity-based analogous experiment. In this way the group hopes to be able to "harvest in space", larger and higher quality crystals of their nanoporous semiconductors, that are as free as possible of the unwanted crystal defects that generally form in their earth-grown crystal samples. Achievement of this goal is expected to enhance the electrical conductivity and optical properties of their crystals which should improve their efficacy for future semiconductor device applications envisaged for these materials.

This materials chemistry microgravity experiment will be one of the first of its kind to perform a simultaneous synthesis and crystal growth procedure in space. The experiment will be fully automated. The specially designed reactor will perform 38 synthesis-crystal growth experiments with comprehensive on-board computer recording of the progress of the entire system. Prototype experiments have been optimized in the Ozin group laboratory. Once the space-grown samples are recovered, the reaction profiles of the experiments will be evaluated and the syntheses will be immediately duplicated under earth-based conditions. Analysis of a wide range of the physicochemical properties of the space and earth-generated nanoporous semiconductor crystals will allow the group to evaluate the effects of microgravity on the synthesis-crystal growth process.

This microgravity experiment has the official name NANOGAS (in the jargon of the Canadian Space Agency, the experiment is referred to as a "Get-Away-Special"). It is a joint venture between the Canadian Space Agency (funding, technical support), Communication Development in New Brunswick (flight hardware), the U. of T. Chemistry Department (facilities, services) and the Ozin group. The experiment was carried on a NASA Space Shuttle flight in May 1996, STS-77. The samples were recovered later and the ground and space-based samples will be analyzed over a period of about six months. The results of this research will be announced towards the end of 1996.

The team of researchers in the Ozin group that have made invaluable contributions to this exciting self-assembly experiment in space, is headed by Dr. David Young with the expert assistance of Dr. Atul Verma, Dr. Srebri Petrov, Dr. Neil Coombs, Dr. Alan Lough, Mr. Holmes Ahari, Ms. Carol Bowes, Ms. Wendy Huynh, Ms. Tong Jiang and Mr. Scott Kirkby, and the outstanding technical assistance of Mark Cornish in the departmental machine shop. Bruce Morton at Communication Development has also made invaluable contributions to the project. The technical and financial support of the scientific and management team at the Canadian Space Agency, in particular Ed Sloot, Phil Gregory and Jack Fisher, is also appreciated.

¹Ozin, G. A., Bowes, C. L., Adv. Mater. (in press)

²Ozin, G. A., Adv. Mater., 4, 612-649 (1992); Ozin, G. A., in "Materials Chemistry: An Emerging Subdiscipline," A.C.S. Symposium, Washington, March 1992, A.C.S. Symp. Ser., Ed. Interrante, L., Washington, 1994; Ozin, G. A., Bowes, C. L., Mater. Res. Soc. Symp. Proc., 286, 93 (1992)

³Ozin, G. A., Supramolecular Chemistry, 6, 125-134 (1995)

⁴Jiang, T., Lough, A. J., Ozin, G. A., Young, D., Chem. Mater., 7, 245-248 (1995)

⁵Jiang, T., Ozin, G. A., Bedard, R. L., Adv. Mater. 6, 860-865 (1994)

⁶Bedard, R. L., Enzel, P., Henderson, G. S., Ozin, G. A., Adv. Mater., 7, 64-68 (1995)

⁷Jiang, T., Ozin, G. A., Bedard, R. L., Adv. Mater., 7, 166-170 (1995)

⁸Ahari, H., Bedard, R. L., Bowes, C. L., Jiang, T., Lough, A., Ozin, G. A., Petrov, S., Young, D., Adv. Mater., 7, 375-378 (1995)

⁹Ahari, H., Bedard, R. L., Lough, A., Petrov, S., Ozin, G. A., Young, D., Adv. Mater., 7, 370-374 (1995)

¹⁰Ahari, H., Bedard, R. L., Bowes, C. L., Jiang, T., Kirkby, S. J., Kuperman, A., Lough, A., Ozin, G. A., Petrov, S., Verma, A., Young, D., Adv. Mater. (in press)