A basic premise that informs the diverse work on nanostructure synthesis, and a premise of this WTEC study in particular, is that the length scales defining structure and organization determine the fundamental characteristics of the material. Hence, if we can control structures at the level of those length scales, we can control the material. Aksay posits nested levels of structural hierarchy that together yield improved properties in the total material (Aksay et al. 1992): this is a useful framework to employ in a comparative description of the various techniques of synthesis and assembly presented in this study.
Hierarchical organization begins at the nanometer length scale and can comprise the unit cell of a crystalline structure. Succeeding levels of the hierarchy might include the scale of the grain size within a polycrystalline or composite material. We may choose to synthesize the nanoscaled building blocks of our materials through the aerosol techniques described by Friedlander (Wu et al. 1993); the controlled reaction of organo-metallic precursors to form semiconductor "nanocrystals" described by Brus et al. (1995); or the formation of powder particles by gas condensation of metal vapors in a low pressure inert atmosphere described by Schwarz (Wasz et al. 1995). Those building blocks can subsequently be further organized through incorporation into a matrix material (e.g., a polymer) or through compaction of component heterogeneous nanoparticles into a composite whole.
Another approach is to create uniform macroscopic layers of the desired material and, separating materials synthesis from subsequent processing, pattern and "carve out" the desired nanostructures. This is the approach that characterizes the formation of "functional" electronic and optical nanostructures fashioned largely from single crystal semiconductor materials; it requires a battery of pattern formation (such as electron beam lithography) and pattern transfer processes (such as reactive ion etching) that have the requisite spatial resolution to achieve creation of structures at the nanoscale. This particular area of nanostructure formation has tremendous scope, warranting its own separate study. It is a driving issue for the electronics industry and is not a principal theme of this WTEC study. We simply mention here that there are tremendous problems associated with the viability of present techniques for nanometer scale fabrication: some of these issues are addressed through Whitesides' "soft lithographic" techniques (Wang et al. 1997). Based on the study of self-assembling surfaces (e.g., alkylthiols and other biomembrane mimics on gold), soft lithography incorporates an elastomeric stamp that enables multiple-use, rapid pattern transfer, extendible to non-flat surfaces. Soft lithography has been shown to be viable at dimensions < 25 nm (Chou, Krauss, and Renstrom 1996).
Yet another approach, the most potentially powerful one, is the one Aksay describes as pertaining to biogenic materials (Sarikaya and Aksay 1994), in which nanostructural design is achieved through the self-assembly of organics (1-100 nm), which forms the scaffolding for the deposition of inorganic materials. The organic structures form a natural, ordered structure that can in turn catalytically or epitaxially induce growth of specifically oriented inorganic thin films. Ying's work follows this theme, with the use of surfactant-metal interactions to form self-assembled structures. These two themes, self-assembly and the formation of natural templates at the nanoscale for the further nucleation of material, may be key concepts in the controlled engineering of materials from the nanostructure level. For example, zeolite cages or carbon nanotubes could serve as the templates for the nucleation of nanostructured synthesis of metal or semiconductor materials.
"Self-assembly" is a term that arose from and best pertains to biological synthesis of molecules that undergo exquisitely selective binding and obey highly specific rules for self-assembly. Because of these properties, research is now being focused on the specific utilization of such highly ordered, complex biological materials, such as DNA, as the building blocks of more complex, three-dimensional nanostructures (Du, Stollar, and Seeman 1992). Research frontiers that exploit the capacity of biomolecules and cellular systems to undergo self-assembly have been identified in two recent National Research Council (NRC) reports (1994, 1996). The general importance and desirability of this degree of control of composition, size, and structure is so critical to the area of nanostructure synthesis that self-assembly has been an increasing hallmark of the field, whether applied to the synthesis of semiconductor structures or polymers employed as templates for lithography. The term has therefore taken on a spectrum of meanings: in many cases chemical specificity leads to the self-assembled formation of zeolite, mesoporous structures, or carbon nanotubules. The pseudomorphic growth of lattice-mismatched semiconductors and the drive of the system to minimize its free energy leads to the islanded growth of self-assembled quantum dots, as detailed by Petroff (Leonard et al. 1993).
The synthesis techniques discussed in this report span a wide number of applications and different levels of maturity and utilization in manufacturing. The applications in turn dictate different levels of tolerance in the size distribution and control over the component nanostructure, although process and structure control remain important issues even for the more mature technologies, such as those used in powder production (see, for example, Pratsinis and Kodas 1993; also, Table 1 in WTEC 1997, 93). For example, nanostructured thermal barrier coatings offer the advantage of lower thermal conductivity, but the change in thermal conductivity may vary only slightly for an order of magnitude change in the nanostructure dimension (Gell 1997; also p. 126 in Chapter 6 of this volume). However, certain electronic device applications may require a uniformity in the nanostructure size of less than ± 1 nm in order to realize the expected device benefits.
Concerning the diversity of approaches to nanostructure synthesis that are treated in this study, the broad questions are as follows:
- What are the issues of control of the nanostructure formation (level 1 in the hierarchy)? Brus has mentioned the importance of the separation of nucleation from growth process; is this an important strategy to adopt?
- What are the issues of control of compaction/consolidation process (level 2 in hierarchy)?
- Will present nanostructure synthesis techniques be able to evolve to the greater hierarchical (more sophisticated) structures characteristic of biogenic systems? Will we be able to devise economical means of integrating synthesis and consolidation as in biogenic systems -- that is, use self-assembled structures to form templates for further growth, etc.?
Aksay, I.A., et al., eds. 1992. Hierarchically structured materials. In MRS Proceeding 255. Pittsburgh, PA: Materials Research Society.
Brus, L.E., P.F. Szajowski, W.L. Wilson, T.D. Harris, S. Schuppler, and P.H. Citrin. 1995. Electronic spectroscopy and photophysics of Si nanocrystals: Relationship to bulk c-Si and porous Si. J. Amer. Chem. Soc. 117: 2915.
Chou, S.Y., P.R. Krauss, P.J. Renstrom. 1996. Imprint lithography with 25-nanometer resolution. Science 272: 85.
Du, S.M., B.D. Stollar, N.C. Seeman. 1992. A synthetic DNA molecule in three knotted topologies. J. Am. Chem. Soc. 117: 9652.
Gell, M. 1997. Oral Presentation at the WTEC Workshop on R&D Status and Trends in Nanoparticles, Nanostructured Materials, and Nanodevices in the United States, May 8-9, Rosslyn, VA.
Leonard, D., M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, and P.M. Petroff. Direct formation of quantum-sized dots from uniform coherent islands of InGaAs on GaAs surfaces. Applied Physics Letters 63(23): 3203-5.
NRC (National Research Council). 1994. Hierarchical structures in biology as a guide for new materials technology. NMAB-464. Washington D.C.: National Academy Press.
_____. 1996. Biomolecular self-assembling materials: Scientific and technological frontiers. Washington D.C.: National Academy Press.
Pratsinis, S.E., and T.T. Kodas. 1993. Manufacturing of materials by aerosol processes. In Aerosol measurement, ed. K. Willeke and P.A. Baron. New York: Van Nostrand Reinhold.
Sarikaya, M., and I.A. Aksay, eds. 1994. Design and processing of materials by biomimicking. New York: Am. Inst. Physics.
Wang, D., S.G. Thomas, K.L. Wang, Y. Xia, and G.M. Whitesides. 1997. Nanometer scale patterning and pattern transfer on amorphous Si, crystalline Si, and SiO2 surfaces using self-assembled monolayers. Appl. Phys. Lett. 70: 1593.
Wasz, M.L., R.B. Schwarz, S. Srinivasan, and M.P.S. Kumar. 1995. Sn-substituted LaNi2 alloys for metal hydride electrodes. Mater. Res. Soc. Symp. Proc. 393: 237.
Wu, M.K., R.S. Windeler, C.K. Steiner, T. Bors, and S.K. Friedlander. 1993. Controlled synthesis of nanosized particles by aerosol processes. Aerosol Sci. Technol. 19: 527.
WTEC. 1997. Viewgraphs of the WTEC workshop on R&D status and trends in nanoparticles, nanostructured materials, and nanodevices in the United States, May 8-9, 1997, Rosslyn, VA. ITRI, Loyola College in Maryland (Baltimore). Photocopy.