Nanoscience: carbon nanotubes, fullerenes and metal surfaces
Nanoelectronics: Semiconductor Nanowire Simulation for Technology Design
The recent fabrication of semiconductor wires with just a few nanometers
in cross-section has upheld their strong position for applications
in nanoelectronics and nanophotonics. As an example, atomic-level
control of the electronic properties of a single-dopant has been achieved
in a top-down fabricated multigate field effect transistor (FET).
Also, semiconductor nanowires synthesised via a bottom-up route
offer an alternative for the fabrication of "end-of-the-roadmap"
transistor technologies. The realisation of FETs, p-i-n photodiodes
and biosensors is just a small demonstration of their potential.
Nevertheless, independent of the fabrication route electrons are
naturally confined in such geometries and quantum effects are expected
due to both small sizes and long coherence times.
In the Electronics Theory Group at Tyndall we use simulations
to design nanoelectronic components by addressing the following
key issues:
confinement effects in the electronic structure
of semiconductor nanowires;
band engineering via chemical modification;
quantum-effect length scales for electron device operation.
Metal-phthalocyanines (MPc) are stable molecules which exhibit a
wide range of optoelectronic, magnetic, and mechanical properties.
Deposited and/or self-assembled on metal electrodes,
they are attractive candidates for novel molecular sensors, memory,
and light-harvesting components.
Understanding the interactions between metal-phthalocyanines
and surfaces is a critical element
that is required for optimizing their use in many applications.
Of particular interest is the charge transfer characteristic
and geometry configuration of the MPc-surface system.
We address these issues via electronic structure theory.
Schematic illustration of MPc molecules encapsulated in a carbon nanotube.
Single walled carbon nanotube chirality (picture courtesy of Intel).
Carbon nanotubes (CNTs) are being considered as interconnects,
transistor channels, and thermal interface material (TIM)
by the semiconductor industry.
Most promising are the single-walled carbon nanotubes (SWNTs).
The broad range of uses for SWNTs is connected to the different nanotube
properties according to how the hexagonal lattice is oriented
(see figure on left), which together with the nanotube diameter decides
the chirality.
The problem is that all production techniques give a mixture of nanotubes
with different chiralities.
One of the major obstacles to the use of CNTs in technology is
that chirality controlled growth can not be achieved at present,
i.e. controlled production of only metallic SWNTs, or
only semiconducting SWNTs is the end goal.
We have studied the catalytic growth of CNTs by looking at the
bonding between the growing nanotube and the metal nanoparticle
that catalyzes the growth (see figure on right)
using first-principles calculations.
We have found that the SWNT-metal bonds need to be strong enough to
stabilize the open end of the nanotube.
This criterion is fulfilled by Fe, Co, and Ni, but not by Cu, Pd, and Au
On the other hand, too strong carbon-metal bonds leads to metal
carbides (e.g. Mo, W) rather than CNT growth.
Bonding between the growing nanotube and the metal catalyst.
Prediction of charge contraction of the endohedral dopants N and P
in C60 and the preservation of atomic spin states.
The calculated anti-bonding (repulsive) interaction between a phosphorus
atom trapped inside a C60 fullerene when the phosphorus
atom is forced off-centre.
To relate the electronic structure of molecules adsorbed on to surfaces to
experimental STM images, first principle calculations are performed.
Shown is a single buckminsterfullerene electronic state
projected onto a constant current STM surface.
