Robin Nicholas

Abstract:

We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.

Abstract:

We report the phase space defined by the quantum Hall effect breakdown in polymer gated epitaxial graphene on SiC (SiC/G) as a function of temperature, current, carrier density, and magnetic fields up to 30 T. At 2 K, breakdown currents (I(c)) almost 2 orders of magnitude greater than in GaAs devices are observed. The phase boundary of the dissipationless state (ρ(xx)=0) shows a [1-(T/T(c))2] dependence and persists up to T(c)>45  K at 29 T. With magnetic field I(c) was found to increase ∝B(3/2) and T(c)∝B2. As the Fermi energy pproaches the Dirac point, the ν=2 quantized Hall plateau appears continuously from fields as low as 1 T up to at least 19 T due to a strong magnetic field dependence of the carrier density.

Abstract:

We report studies of the magnetospectroscopy of graphite into a new regime of high energies and ultrahigh magnetic fields which allows us to perform the first spectroscopic studies of the interlayer split-off bands, E1 and E2. These bands can be well described by an asymmetric bilayer model and have only a small interlayer band gap asymmetry. We show that all of the properties of the electrons and holes can be described by a simple relativistic behavior determined by γ0 and γ1.

Abstract:

AbstractWe describe two methods in which we manipulate the binding of multiple conjugated polymers to single‐walled carbon nanotubes (SWNTs) to produce new and novel nanostructures. One method first utilizes the selective binding of poly(9,9‐dioctylfluorenyl‐2,7‐diyl) (PFO) to a narrow distribution of semiconducting SWNTs and then uses a polymer exchange to transfer this purity to other nanotube‐polymer combinations, using technologically useful polymers such as poly(3‐hexylthiophene) (P3HT) and poly(9,9′‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) as first examples. The other method involves controlling the competitive binding of P3HT and F8BT to SWNTs to produce coaxial nanostructures consisting of both polymers simultaneously bound in ordered layers. We show that these two simple solution‐processing techniques can be carried out sequentially to afford new dual‐polymer nanostructures comprised of a semiconducting SWNT of a single chirality. This allows the favorable properties of both polymers and purified semiconducting SWNTs to be implemented into potentially highly efficient organic photovoltaic devices.

Abstract:

A scalable method to coat monochiral (7,5) semiconducting single-walled carbon nanotubes with a monolayer coating of a range of technologically useful polymers such as poly(3-hexylthiophene) (P3HT) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) is presented. Optical spectroscopy and atomic force microscopy measurements show that the semiconducting tube purity (>99%) obtained from the selective wrapping of nanotubes by polymers such as poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) can be transferred to these other nanotube-polymer combinations by polymer exchange.