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Nanotechnological Self-Assembly
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TAU Research Briefs
Nanotechnological Self-Assembly
Nanostructures, with dimensions on the order of a billionth (10-9) of a meter, will be the building blocks of many exciting future electronic, mechanical and optical devices. Such structures will be so small that quantum effects can no longer be ignored. For example, such structures can constrain their electrons to behave in different ways than they would in larger structures ("quantum confinement") and both their electrical and thermal conductance will be quantized (change in discrete steps). One can even imagine futuristic memory elements that could store information with the input of a single electron. Nanotechnology could allow packing incredibly more circuit elements onto a computer chip and permit the development of ultra-small lasers. However, the high technical barriers to realizing such visions, in a way consistent with commercially viable mass production, are commensurate with their high potential payoff.
There are several ways of creating nanostructures. Etching or growing them using lithographic methods is a familiar extension of techniques already used in microelectronics. Resolution, however, is limited by the wavelength of light used; and a switch to short wavelength X-rays and electron-rays presents other formidable challenges. Molecular manipulation by the ultra-fine tip of a scanning probe (STM, AFM) microscope can produce sufficiently small structures; but such methods are currently far too slow and cumbersome to be industrially viable. Inducing atoms and molecules to self-assemble and self-organize by utilizing natural materials' tendencies for clustering themselves into useful nanostructures appears to be a far more elegant approach, which often involves familiar "wet" chemical techniques. While promising, such bottom-up techniques are still in their infancy.
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Scanning tunneling microscope (STM) image of a complex heterogeneous Cobalt-Germanium (Co-Ge) surface consisting of two different nanocrystal types: pyramidal ones and smaller interspersed equi-axial ones. The insert shows a reflection high-energy electron diffraction image from this surface. |
Dr. Ilan Goldfarb of TAU's Faculty of Engineering has been pursuing a different approach, growing self-assembled nanostructures using physical epitaxy in ultra-high vacuum (UHV). In epitaxy, a layer of atoms of one substance is added to a single crystal of another in such a way that the new layer conforms to the structure of the substrate. This approach provides a far cleaner environment and better control over the process than current "wet" chemical methods (atoms could even be put on the substrate surface one by one). In Goldfarb's approach, nanostructure nucleation and growth can be carefully monitored in real-time at every stage of evolution. This is imperative since self-assembling systems may also develop tendencies contradictory to the desired results, and to adequately control the outcome requires better understanding the physical factors involved. The TAU group achieves this degree of insight by combining scanning probe microscopy and spectroscopy with electron spectroscopy and surface electron diffraction methods during deposition (see figures).
 | A bare titanium oxide surface after 0 sec (first row), 20 sec (second row), 50 sec (third row) and 110 sec (fourth row) of nickel deposition. |
The group's research concentrates on six principal topics. First, the crystalline and electronic structure and topography of surfaces and the search for new self-assembling nanocrystalline materials (for example, by exploiting deliberate mismatches between the epitaxially deposited layer and its substrate). Second, investigations of self-assembled nanostructure size, shape and uniformity, their statistical distributions, and how to control them. Third, the development of high spatial resolution methods capable of probing ultra-small surface regions for crystallographic, electronic, chemical, mechanical and other information. And finally, nanostructure modeling, the nanomechanics of surface-tip interactions, and the development of real devices.
