New research paves way for the nanoscale self-assembly of
organic building blocks, a promising new route towards the next generation of
ultra-small electronic devices.
Ring-like molecules with unusual five-fold symmetry bind
strongly to a copper surface, due to a substantial transfer of charge, but
experience remarkably little difficulty in sideways diffusion, and exhibit
surprisingly little interaction between neighboring molecules. This
unprecedented combination of features is ideal for the spontaneous creation of
high-density stable thin films, comprising a pavement of these organic
pentagonal tiles, with potential applications in computing, solar power, and
novel display technologies.
Currently, commercial electronics use a top-down approach, with
the milling or etching away of inorganic material, such as silicon, to make a
device smaller. For many years the computing power of a given size of computer
chip has been doubling every eighteen months (a phenomenon known as Moore’s law) but a limit
in this growth is soon expected. At the same time, the efficiency of coupling
electronic components to incoming or outgoing light (either in the generation
of electricity from sunlight, or in the generation of light from electricity in
flat-screen displays and lighting) is also fundamentally limited by the
development of fabrication techniques at the nanometer scale.
Researchers are therefore looking for ingenious solutions in the
creation of ever smaller electronics. The field of nanotechnology is taking a
bottom-up approach of creating electronics using naturally self-assembling
organic components, such as polymers, which will be capable of spontaneously
forming devices with the desired electronic or optical characteristics.
The latest findings are from scientists at the Univ. of Cambridge
and Rutgers Univ. who are working on the development
of new classes of organic thin films on surfaces. By studying the fundamental
forces at play in self-assembling thin films, they are developing the knowledge
that will allow them to tailor these films into molecular-scale
organic-electronic devices, creating smaller components than would ever be
possible with conventional fabrication techniques.
Dr Holly Hedgeland, of the Department of Physics at the Univ. of Cambridge, one of the co-authors of the
paper, said: “With the semiconductor industry currently worth an estimated $249
billion per year there is a clear motivation towards a molecular scale
understanding of innovative technologies that could come to replace those we
use today.”
It is not simply the electronic properties of a molecule on a
surface that will control its potential to form part of a device, but also
whether it will move by itself into the required structural configuration and
remain stable in that position even if the device becomes heated in use.
Molecules that are strongly bound to the substrate with a high
degree of transfer of charge offer a range of new possibilities, though little
is currently known of their behavior. A number of organic molecules, usually
featuring carbon rings across which electronic charge can conduct, potentially
demonstrate the right electronic properties, but the long-range forces which
will govern their self-assembly during the first phases of growth often remain
a mystery.
Now the interdisciplinary team based in the Departments of
Physics and Chemistry at the Univ. of Cambridge, and the Department of
Chemistry and Chemical Biology at Rutgers Univ., have reported the first
dynamical measurements for a new class of organic thin film where
cyclopentadienyl molecules (C5H5) receive significant
electronic charge from the surface, yet diffuse easily across the surface and
show interactions with each other that are much weaker than would typically be
expected for the amount of charge transferred.
Hedgeland explained: “By coupling the experimental helium spin
echo technique with advanced first-principles calculations, we were able to
study the dynamic behavior of a cyclopentendienyl layer on a copper surface,
and to deduce that the charge transfer between the metal and the organic
molecule was occurring in a counter-intuitive sense.”
Dr Marco Sacchi, of the Department of Chemistry at the Univ. of
Cambridge, who carried out the calculations that helped explain the startling
new experimental results, said that “the key to the unique behavior of
cyclopentadienyl lies in its pentagonal (five-fold) symmetry, which prevents it
latching onto any one site within the triangular (three-fold) symmetry of the
copper surface through directional covalent bonds, leaving it free to move
easily from site to site; at the same time, its internal electronic structure
is just one electron short of an extremely stable ‘aromatic’ configuration,
encouraging a high degree of charge transfer from the surface and creating a
strong non-directional ionic bond.”
The researchers’ findings, reported in Physical Review Letters, highlight the potential of a new category
of molecular adsorbate, which could fulfill all the criteria required for
useful application.
Hedgeland concluded: “The unusual character of
the charge transfer in this case prevents the large repulsive interactions
between adjacent molecules that would otherwise have been expected, and hence
should enable the formation of unusually high-density films. At the same time,
the molecules remain highly mobile and yet strongly bound to the surface, with
a large degree of thermal stability. In all, this is a combination of physical
properties that offers huge potential benefit to the development of new classes
of self-assembled organic films relevant for technological applications.”