ScienceWeek

MATERIALS SCIENCE: NANORINGS AND NANOBELTS

The following points are made by X.Y. Kong et al (Science 2004 303:1348):

1) Self-assembly of nanocrystals can be driven by van der Waals forces and hydrogen bonding among the passivating organic molecules on particle surfaces (1-3). For inorganic nanostructures that expose charge-polarized surfaces, such as nanobelts of oxides like ZnO (4), electrostatic forces can drive self-assembly, especially in gas-phase environments where these forces are unscreened by solvents. For crystalline nanomaterials grown in a solid-vapor environment, one type of polar charge-induced helical and spiral ZnO structure was previously reported (5).

2) The authors report a distinct nanoring structure that is formed by spontaneous self-coiling of a polar nanobelt during growth. Nanoring growth appears to be initiated by circular folding of a nanobelt driven by long-range electrostatic interactions. Short-range chemical bonding among the loops leads to the final single-crystalline structure. The self-coiling is driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation.

3) Single-crystal nanorings of ZnO were grown by a solid-vapor process. The raw material was a mixture of ZnO (melting point 1975 deg C), indium oxide, and lithium carbonate powders at a weight ratio of 20:1:1, and it was placed at the highest temperature zone of a horizontal tube furnace. Before heating to a desired temperature of 1400 deg C, the tube furnace was evacuated to ~10^(-3) torr to remove the residual oxygen. The source materials were then heated to 1400 deg C at a heating rate of 20 deg C/min.

4) In summary: Freestanding single-crystal complete nanorings of zinc oxide were formed via a spontaneous self-coiling process during the growth of polar nanobelts. The nanoring appeared to be initiated by circular folding of a nanobelt, caused by long-range electrostatic interaction. Coaxial and uniradial loop-by-loop winding of the nanobelt formed a complete ring. Short-range chemical bonding among the loops resulted in a single-crystal structure. The self-coiling is likely to be driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. The authors suggest that zinc oxide nanorings formed by self-coiling of nanobelts may be useful for investigating polar surface-induced growth processes, fundamental physics phenomena, and nanoscale devices.

References (abridged):

1. C. B. Murray, C. R. Kagan, M. G. Bawendi, Science 270, 1335 (1995)

2. R. L. Whetten et al., Adv. Mater. 8, 428 (1996)

3. J. F. Banfield, S. A. Welch, H. Z. Zhang, T. T. Ebert, R. L. Penn, Science 289, 751 (2000)

4. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947 (2001)

5. X. Y. Kong, Z. L. Wang, Nano Lett. 3, 1625 (2003)

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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ON NANOFABRICATION AND NANOTECHNOLOGY

The following points are made by G.M. Whitesides and J.C. Love (Scientific American 2001 September):

1) "Make it small!" is a technological edict that has changed the world. The development of microelectronics -- first the transistor and then the aggregation of transistors into microprocessors, memory chips and controllers -- has brought forth a cornucopia of machines that manipulate information by streaming electrons through silicon. Microelectronics rests on techniques that routinely fabricate structures almost as small as 100 nanometers across... This size is tiny by the standards of everyday experience -- about one thousandth the width of a human hair -- but it is large on the scale of atoms and molecules. The diameter of a 100-nanometer-wide wire would span about 500 atoms of silicon.

2) The idea of making "nanostructures" that comprise just one or a few atoms has great appeal, both as a scientific challenge and for practical reasons. A structure the size of an atom represents a fundamental limit: to make anything smaller would require manipulating atomic nuclei -- essentially, transmuting one chemical element into another. In recent years, scientists have learned various techniques for building nanostructures, but they have only just begun to investigate their properties and potential applications. The age of nanofabrication is here, and the age of nanoscience has dawned, but the age of nanotechnology -- finding practical uses for nano-structures -- has not really started yet.

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