Carbon nanotubes (CNTs) offer enormous potential in a wide-ranging variety of applications by virtue of their high current-carrying capability, high thermal conductivity and high mechanical strength. They consists of sheets of graphite which have been "rolled up" with a diameter as small as a few nanometres. Since electron transport in CNTs is ballistic over macroscopic lengthscales and yet their cross-sections are on the nanometre scale they are ideal candidates for interconnects in integrated circuits. In order to incorporate CNTs into any integrated device technology it will be necessary to control the CNT growth. At present neither the CNT chirality and diameter (which determine its electronic properties), nor its orientation and location on the substrate can be reliably controlled. Currently the leading method for controlling the CNT location is to manipulate it into position using a scanning probe tip – clearly a painstaking and non-scaleable technique. In this project the student will experimentally investigate novel ion-beam techniques for directing the growth of CNTs. Our long-term goal is the ability to grow CNTs in precisely defined locations on a substrate, with the ability to attach metallic contacts and/or additional CNTs, with each CNT having the required (i.e. metallic or semiconducting) electronic properties.

The Josephson junction is the basic building block of all superconducting circuits. Its operation is based on the interference between the two electronic wavefunctions in two superconductors on either side of a thin insulating barrier. If one makes a stack of many (~10³) such Josephson junctions, the interference between many such electronic wavefunctions leads to a variety of new phenomena which are ripe for exploitation in a range of quantum electronic applications. We are particularly interested in energy-level quantisation and quantum tunnelling - phenomena which are well understood for single junctions, but which should be observable at much higher temperatures in stacked junctions. This may lead to important developments in the burgeoning field of quantum computation.

The student working on this project will develop our existing highly successful technology for fabricating nanoscale self-assembled Josephson junction stacks. Fabrication and characterisation of the stacks will be done at UCL in our existing Clean Room Suite and the Quantum Nanoelectronics Lab. It is expected that the student working on this project will further develop our existing collaborations with the Universities of Oxford (thin film growth), Erlangen - Nuremberg (measurements in high magnetic fields).

When an electron is transferred to a conductor, the energy of the conductor is raised by e²/(2C), where e is the electronic charge and C is the capacitance of the conductor. For most conductors this effect is negligible. If the capacitance is made very small, however, the increase in energy when a single electron is transferred is sufficient to create an energy barrier to prevent the transfer of any further electrons. This "Coulomb blockade" effect allows us to manipulate single electrons through carefully designed circuits with very low capacitance. One application of this is the single-electron memory, in which the presence or absence of a single electron on a metallic island with very low capacitance is used to code digital ones and zeros.

The question is: how to fabricate such low capacitance islands? One increasingly important technique used widely in nanotechnology is self-assembly. Here atoms or molecules assemble themselves into the required device structure as it is grown. At UCL we have developed a novel self-assembly technology for fabricating islands which are only 0.3 nm thick! At present, however, the cross-sectional dimensions are too large to observe single-electron effects. We therefore are seeking a student to work on nanoscale self-assembled islands and to investigate single-electron effects in these devices.

The proliferation of multi-media mobile telecommunications devices has led to tremendous pressure on the limited bandwidth allocated to mobile service providers. This has driven major advances in digital modulation and compression techniques to optimise usage of the channel capacity. Similar advances are also required in the physical layer if full development of mobile applications is not to be hindered by the limited capacity. One way to achieve this is by using adaptive antennas which are spatially agile, coupled with tuneable filters which provide spectral agility.

We have been experimentally investigating three distinct techniques for obtaining the tuneable phase-shift which is required for both adaptive antenna feeds and tuneable filters. Two of these are based on current-biased modulation of the line inductance and one on voltage-biased modulation of the line capacitance. The key technological goal is to obtain a phase shift without adversely affecting the magnitude of the device S-parameters and losses. We have most spectacularly demonstrated this using microstrip tuneable resonators on ferrite substrates – at 2.4 GHz the resonators have 10% tuneability and a quality factor of Q = 450.

Future work in this area will include development of microstrip filters by coupling many such resonators together, and a demonstration of beam-steering using patch antennas integrated with our inductively-tuneable phase shifters.

For further information on any of these projects, please contact Paul Warburton, whose contact details may be found on his webpage.

For more general information on applying to study for a PhD in Electrical Engineering at UCL, please follow this link.