ScienceWeek

MATERIALS SCIENCE: ON THE GROWTH OF NANOFIBERS

The following points are made by Pulickel M. Ajayan (Nature 2004 427:402):

1) In his historic monograph On Growth and Form(1), D'Arcy Thompson (1860-1948) wrote that the form of an object is a diagram of its growth forces. Often, however, this relationship between growth and form is too complex to determine: the growth of filamentous carbon in the vapour phase is a case in point. Despite a large body of literature on the subject, replete with fascinating and varied examples, no definitive model for the growth of carbon nanofibers has evolved, owing to a lack of consistent experimental data(2,3). Considering the substantial impact that these materials are likely to have on technology(4), it seems imperative that their growth mechanisms be understood, so that nanofibers can be manufactured with well-defined characteristics. A long-awaited solution to the mystery of nanofiber growth was recently presented by Helveg et al(5).

2) Nanoscale carbon fibers are grown through the interaction of metal-catalyst nanoparticles with hydrocarbon vapour at high temperature. The hydrocarbon molecules dissociate at the interface between catalyst and vapour, and carbon atoms precipitate into a graphite trail in the shape of a cylindrical, multi-walled nanofiber. Exactly how the nanofiber forms is unknown. The catalyst particle might stay at the growing end of the nanofibers (called "tip growth"), or it might sit at the starting end ("base growth"). The state of the particle itself (such as its structure or shape) during the growth process is also unknown, but the particle size and fibre diameter are similar.

3) Experimentally, it has proved difficult to track the dynamics of this high-temperature catalytic reaction with spatial and temporal resolution sufficient to observe the growth of the nanofibers directly at the atomic scale. The images presented by Helveg et al(5), capturing the early stages of nanofibers growth, are clearly the result of some impressive developments in high-resolution transmission electron microscopy in situ. Helveg et al(5) have managed to record in real time the catalytic reaction at about 500 C between methane and supported nickel nanoparticles (5-20nm in diameter) inside a high-resolution transmission electron microscope. Analyzed frame by frame, the images reveal the events that lead to nanofibers growth.

4) It seems that the catalyst particle moves with the growing nanofibers, following the tip-growth prescription. What is surprising is that the nanofibers growth is promoted by abrupt shape changes in the catalyst particle itself, which are in turn driven by the reaction between the catalyst and the vapour. These shape changes, between spherical and elongated, are sudden and repeated. In its elongated form the particle serves as a template, facilitating the formation of aligned graphite layers as carbon atoms diffuse across its surface. A closer look at the images reveals that the nucleation and growth of these graphite layers occur at "bumps" on the nickel nanoparticle -- single-atom step-edges that develop and then disappear continuously. This step-edge mechanism is also backed up by theoretical calculations.

References (abridged):

1. Thompson, D'A. On Growth and Form (Cambridge Univ. Press, 1917)

2. Baker, R. T. K. & Harris, P. S. Chem. Phys. Carbon 14, 83-165 (1978)

3. Tibbets, G. G. in Carbon Filaments and Nanotubes: Common Origins, Differing Applications (eds Biro, L. P. et al.) 63-73 (Kluwer, Dordrecht, 2001)

4. Terrones, M. Annu. Rev. Mater. Res. 33, 419-501 (2003)

5. Helveg, S. et al. Nature 427, 426-429 (2004)

Nature http://www.nature.com/nature

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SELF-ASSEMBLY AND MINERALIZATION OF NANOFIBERS

The following points are made by J.D. Hartgerink et al (Science 2001 294:1684):

1) Self-assembly and biomineralization are used in biological systems for natural fabrication of many composite materials. Bone tissue is a particularly complex example of such a composite because it contains multiple levels of hierarchical organization. At the lowest level of his hierarchy is the organization of collagen fibrils with respect to hydroxyapatite crystals. The collagen fibrils are formed by self-assembly of collagen triple helices and hydroxyapatite crystals grow within these fibrils in such a way that their (c) axes are oriented along the long axes of the fibers.

2) The preparation of any material with structure on the nanoscale is a challenging problem. Fabrication of materials that resemble bone, even at the lowest level of hierarchical organization, is even more difficult because it involves two dissimilar organic and inorganic nanophases that have a specific spatial relation with respect to one another. One way to accomplish this in an artificial system is to prepare an organic nanophase designed to exert control over crystal nucleation and growth of the inorganic component.

3) The authors report they have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of the extracellular matrix of living tissue. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease mineralization of hydroxyapatite to form a composite material in which the crystallographic (c) axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

Science http://www.sciencemag.org

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ON THE NEW SCIENCE OF NANOTECHNOLOGY

The following points are made by J-M. Lehn and P. Ball (citation below):

1) Technology is getting smaller. The average size of microelectronic components has been decreasing steadily for the past few decades, as has the bit size of memory devices. Between 1985 and 1995, for example, the storage density of commercial disc drives increased by almost two orders of magnitude. The rule of thumb coined by Gordon Moore, cofounder of Intel, is that the power of commercial computers doubles every year and a half. Since the earliest days of computing in the 1950s, this power has increased by a factor of around ten billion. The driving force behind these advances has been miniaturization. Today you can hold in your hand a 20-cm silicon wafer containing more electronic components than there are people in the world. So far, the reduction in scale has been made possible by photolithographic procedures that use photochemical patterning followed by selective etching to carve microscopic structures into semiconductor wafers.

2) However, this approach has fundamental limits of resolution, set by the wave optics of the patterning beams: at present, a resolution of 0.1 microns for commercial photolithography remains the Holy Grail. Electron-beam, hard ultra-violet and X-ray lithographies offer still finer resolution, but currently at greater cost; these are not yet routine industrial techniques. Yet, if current trends prevail, the scale of miniaturization will approach the scale of large molecules -- a nanometer or so -- in just a few decades. Once this happens, completely new technologies will be needed.

3) As an alternative to the "top-down" techniques of conventional semiconductor engineering, it now seems expedient to take seriously the option of "bottom-up" approaches that build nanostructures from molecular components. That is, rather than building devices and patterning materials by "reduction" -- by carving them from larger, monolithic blocks -- we can think of achieving this goal by synthesis, putting the structures together molecule by molecule, atom by atom. The methods of standard chemical synthesis provide one option; but by exploiting self-assembly and self-organization, we might attain the same ends in a spontaneous, pre-programmed and less labor-intensive manner. The creation of organized molecular structures and devices is the objective of the nascent field of nanotechnology, whose adherents propose to conduct engineering on the scale of nanometers.

Adapted from: J-M. Lehn and P. Ball: in The New Chemistry, Nina Hall (ed.). Cambridge University Press 2000, p.347. More information at: http://www.amazon.com/exec/obidos/ASIN/0521452244/scienceweek

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