In the recent issue of Nature, scientists from Empa and the Max
Planck Institute for Polymer Research report how they have managed
for the first time to grow graphene ribbons that are just a few
nanometres wide using a simple surface-based chemical method.
Graphene ribbons are considered to be «hot
candidates» for future electronics applications as their
properties can be adjusted through width and edge shape.
Transistors on the basis of graphene are considered to be
potential successors for the silicon components currently in use.
Graphene consists of two-dimensional carbon layers and possesses a
number of outstanding properties: it is not only harder than
diamond, extremely tear-resistant and impermeable to gases, but it
is also an excellent electrical and thermal conductor. However, as
graphene is a semi-metal it lacks, in contrast to silicon, an
electronic band gap and therefore has no switching capability which
is essential for electronics applications. Scientists from Empa,
the Max Planck Institute for Polymer Research in Mainz (Germany),
ETH Zürich and the Universities of Zürich und Bern have
now developed a new method for creating graphene ribbons with band
gaps.
Extremely narrow graphene ribbons
To date, graphene ribbons have been «cut» from
larger graphene sheets, akin to tagliatelle being cut from pasta
dough. Or carbon nanotubes were slit open lengthwise and unfurled.
This gives rise to a band gap via a quantum mechanical effect - the
gap being an energy range that cannot be occupied by electrons and
which determines the physical properties, such as the switching
capability. The width (and edge shape) of the graphene ribbon
determines the size of the band gap and thereby influences the
properties of components constructed from the ribbon.
If extremely narrow graphene ribbons (well under 10 nanometres
wide) that also have well-defined edges could be manufactured, so
the reasoning, then they might allow for components exhibiting
specific optical and electronic properties: depending on
requirements, adjustment of the band gap could be used to fine-tune
the switching characteristics of a transistor. This is no mean
feat, as the lithographic methods that have been used until now,
for example for cutting graphene layers, come up against
fundamental barriers; they yield ribbons that are too wide and have
diffuse edges.
Growing graphene ribbons
In the issue of Nature published on 22 July 2010, scientists led
by Roman Fasel, Senior Scientist at Empa and Professor for
Chemistry and Biochemistry at the University of Bern, and Klaus
Müllen, Director at the Max Planck Institute for Polymer
Research, describe a simple surface-based chemical method for
creating such narrow ribbons without the need for cutting, in a
bottom-up approach, i.e. from the basic building blocks. To achieve
this, they spread specifically designed halogen-substituted
monomers on gold and silver surfaces under ultrahigh vacuum
conditions. These are linked to form polyphenylene chains in a
first reaction step.
In a second reaction step, initiated by slightly higher heating,
hydrogen atoms are removed and the chains interconnected to form a
planar, aromatic graphene system. This results in graphene ribbons
of the thickness of a single atom that are one nanometre wide and
up to 50 nm in length. The graphene ribbons are thus so narrow that
they exhibit an electronic band gap and therefore, as is the case
with silicon, possess switching properties - a first and important
step for the shift from silicon microelectronics to graphene
nanoelectronics. And if this wasn't enough, graphene ribbons with
different spatial structures (either straight lines or with zig-zag
shapes) are created, depending on which molecular monomers the
scientists used.
Further studies will help identify properties
As the scientists can now (almost) produce graphene ribbons at
will, they want to start investigating their properties, for
instance how the magnetic properties of the graphene ribbons can be
influenced by different edge structures. The surface-based chemical
method also opens up interesting possibilities with regard to the
targeted doping of graphene ribbons: the use of monomer components
with nitrogen or boron atoms in well-defined positions or the use
of monomers with additional functional groups should enable the
creation of positively and negatively doped graphene ribbons.
A combination of different monomers is also possible and may
permit, for example, the creation of so called heterojunctions -
interfaces between different types of graphene ribbons, such as
ribbons with small and large band gaps - which could be used in
solar cells or high frequency components. The scientists have
already demonstrated that the underlying principle for this works:
they have connected three graphene ribbons to each other at a nodal
point by means of two suitable monomers.
To date, the scientists have focused on graphene ribbons on
metal surfaces. However, to be usable in electronics the graphene
ribbons need to be created on semi-conductor surfaces or methods
must be developed to transfer the ribbons from metal to
semi-conductor surfaces. And first results in this direction also
give the scientists good reasons to be optimistic.
Source: http://www.empa.ch/
Posted July 22nd, 2010
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