Christopher Coleman

Controlling the activation energy of graphene-like thin films through disorder induced localization

The influence of disorder on the activation energy in few layer graphitic films is demonstrated through combined Raman and temperature dependent transport studies. A laser ablation technique is employed which allows the level of disorder in the sp2-C phase to be controlled and conditions for minimizing the level of disorder are determined. As conditions vary from optimal, Raman measurements show increasing D and G bandwidths while the activation energy, negligible for optimal growth conditions, can be correlated with the level of disorder. This laser ablation technique allows the specific effects of structural disorder in the sp2 phase to be probed while defects act as effective barriers resulting in localization of charge carriers. Electron transmission spectra, calculated with a tight-binding model, account for the change of localization length as a result of disorder in the sp2 hybridized phase. This tandem experimental and theoretical approach shows that the localization length of the thin graphitic films can be tuned with the level of disorder which is controlled through synthesis parameters. This study, which addresses the role of disorder in graphene-like materials, is a prerequisite for device applications.

Nanoscale deformations in graphene by laser annealing

We investigate a method of inducing nano to micron scale strained regions in graphene using a laser treatment monitored by Raman spectroscopy. The Raman G-peak of the strained region shows a splitting and redshift for graphene exposed to a laser power density above a certain threshold limit (20 mW). We also note blue-shifting of the positions of both Raman D and 2D-peaks and the decrease of both their intensities relative to the G-peak with increasing laser power. These features correspond to p-type doping effects that are believed to be caused by gas adsorbates released from the substrate during the laser treatment. The induced strain is verified by AFM analysis, which shows the blister-like deformations of the treated area and the corresponding strength of the induced gauge fields in the deformed region. We find that, depending on the exact size and geometry of the blisters, the gauge fields can range between 0.4 mT and 300 T. This laser treatment procedure establishes an effective method for the local ...

Nanomanipulation device fabrication: multilayerd graphene and OFET devices

We utilize nano-manipulting probes for the fabrication of a range of devices including multilayered graphene coplanar waveguides, suspended multilayered graphene hall bars and air-gap single crystal organic field effect transistors. We find that devices fabricated using this technique are of high quality and can be used to probe not only application based phenomena (such as transistor behaviour), but also fundamental quantum transport properties. Magnetoresistance measurements show that the multilayered graphene devices exhibit either quantum linear magnetoresistance (QLMR) or Shubnikov de-Haas oscillations depending on the topology (i.e. either wrinkled or smooth) of the graphene sheet. From this data we calculate the carrier density (ns) in the wrinkled graphene to be in the range 5.2 × 109 cm-2 (at B~0) to 3.5 × 1013 cm-2 (at B=12 T) with effective masses of 0.001me and 0.121me, respectively. The smooth multilayer graphene devices have a carrier density 1.39 - 2.85 × 1012 cm-2 and effective mass (0.022me ≤ m*≤ 0.032me) as calculated from the analysis of the Shubnokov de-Haas oscillations. The high frequecny coplanar waveguide devices fabricated using this technique demonstrated high transmission up to 50 GHz, highlighting the potential for HF application. Organic field effect transistors were also fabricated using the manipulation technique, the transfere characteristics were measured, it was found that the devcies with channel length of 1 μm have non-linear transfere characterisitcs and pass a maximum current of between 0.1 and 10 nA. These OFET devices showed pronounced switching behaviour with mobilities of up to 3 cm2V-1s-1 in the best devices.

Low temperature magneto transport features of rare earth element functionalized carbon nanotube network devices for spintronic applications

In this paper we report on the fabrication of a spintronic device based on multiwalled carbon nanotubes functionalized/coupled with a rare earth element complex. The spin valve behavior is verified by magneto-resistance fluctuations at low temperatures. We report the magneto-transport features of spin valve devices based on multiwalled carbon nanotube covalently functionalized with a gadolinium complex. The switching is encoded in the composite from which devices are fabricated by di-electrophoresis. The magnetic field dependent electronic transport characteristics are investigated through the Cryogenic high field measurement system at 300 mK. Structural characterization of the material through transmission and scanning electron microscopy and Raman spectroscopy of the pristine and composite is done. The electronic transport shows a magnetic field dependence which is characteristic of spin valve at 300 mK and furthermore this spin valve feature is temperature dependent.

Possible observation of the Berezinskii-Kosterlitz-Thouless transition in boron-doped diamond films

The occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated in heavily boron-doped nanocrystalline diamond films through a combination of current-voltage and resistance measurements. We observe transport features suggesting a robust BKT transition along with transport features related to vortex pinning in nanocrystalline diamond films with smaller grain size. The vortex core energy determined through analysis of the resistance temperature curves was found to be anti-correlated to the BKT transition temperatures. It is also observed that the higher BKT temperature is related to an increased vortex-antivortex binding energy derived from the activated transport regions. Further, the magnetic field induced superconductor insulator transition shows the possibility of the charge glass state. The consequences of granularity such as localization and vortex pinning can lead to tuneable BKT temperatures and strongly affects the field induced insulating state.