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SG -South African Group

BRICS Project meeting, 27/12/2017

    Initially B-NCD samples will be synthesized in Belgium in the lab of our Flemish partner and we shallmake the quantum devices and also measure the electronic and spintronic properties. Carbon samples and devices are produced in our laboratory and fully characterized at low temperatures, highmagnetic fields, and at high frequency regimes.

    (I) Synthesis: B-NCD films can be synthesized using a new microwave Chemical Vapor Deposition (MWCVD) chamber obtained from NRF/NNEP 2015. Device grade samples will be produced with high power and dense plasma to achieve a high level of doping of diamond films. Also in our laboratory we operate routinely a HFCVD system to grow nitrogen-doped nanodiamond films and have produced a number of important publications.
    Boron doping can be achieved by adding boron to the plasma using dense plasma which will help us to incorporate a large amount of boron in nanodiamond systems. Factors which can be controlled experimentally in CVD synthesis include the deposition rate (controls grain size) and ratio of injected gasses in plasma (dopant concentration). In such a process, various gasses are injected into an evacuated chamber (~35 mbar) and heated by the filament to form plasma. We use CH4 (carbon source), H2 and Ar. The amounts of the gasses are controlled using mass flow controllers. Ions in the plasma are then deposited onto the substrate. The optimal parameters for diamond growth in our system have already been established and so the growth of NCD films is anticipated to be straightforward.
    Carbon nanotubes as well as semiconducting nanowires such as Silicon and Germanium will be synthesized using a pulsed laser assisted chemical vapor technique (PLCVD). This involves ablating a target which is prepared by mixing the high purity carbon/Si powders with calculated trace amounts of metal catalyst and pressed. This target, which is situated inside a quartz tube furnace and heated to temperatures in the range of 900 C – 1000 C, is ablated using a Nd-YAG pulsed laser whilst carrier gas mixture comprising of Ar and H2 is passed through the furnace. The nanowires or nanotubes are carried downstream and deposit on the inner surface of the quartz tube in the cooler region. The structures are collected from the inside of the tube when the system has cooled down.
    After synthesis purified carbon nanotubes will be chemically functionalized by using a wet chemistry method. This is achieved by functionalizing the cnts through a nitric acid reflux process in order to add carboxylic acid functional groups to the outer walls. These functional groups are highly reactive and can be used to form covalent bonds between magnetic molecules, such as gadolinium-DTPA complexes, to the cnts thus adding magnetic domains to the cnts.
We have shown that it is possible to mechanically exfoliate thin mesoscopic flakes of multilayered graphene using nano-manipulating probes. This method is achieved by using the nano-manipulators to peel off the graphene samples and maneuver them onto predefined electrodes. The manipulators also serve as a means of inducing structural changes in the Graphene layers, this includes folding or straightening flakes. Advantage of such a method lies in the fact that no chemicals are involved such as in the conventional electron beam lithography technique.
    AFM and STM measurements can be performed and Raman analysis and TEM facilities are available at the University of the Witwatersrand. Quantum Devices will be made in our laboratory with the electron beam lithography (NNEP 2012).
(II) Developing nano-scale diamond devices working at GHz frequency: Microwave absorption of superconducting diamond nanowires: In order to determine the (BCS) gap and related superconducting state microwave absorption of carbon superconducting devices will be studied. We have worked extensively the high frequency transport in the ballistic regime for nanostructured carbon.
    Some of our results show the excitation of so-called Luttinger liquid which was predicted theoretically. in 1D materials however all predicted features have remained unobserved clearly in nanotubes.
Luttinger liquid is a collective excitation of electrons repelling each other whereas an attractive force between electrons should be present to explain superconductivity. A crossover from Fermi liquid to non-fermi liquid will be studied in the carbon superconducting materials.
    Once the nanostructes are incorporated into devices with the correct structure, that is co-planar waveguide geometry, high frequency response can be measured. This is carried out using a custom built unique analysis setup that includes a low temperature cryostat coupled to a high frequency signal generator and analyser. Samples are placed on measurement chuck and cooled down to 4 K in high vacuum environment. High frequency probes are then carefully landed on the device allowing for the ac signal to be passed through the channel and the response to be measured. This system also has
the capabilities of applying a magnetic field of up to 3 T perpendicular to the sample.

Experimental investigation of we will be connected with:
Quantum transport in two dimensional nano-carbon, nanotubes at low temperature in to 300 mK high
magnetic fields at a magnetic field up to 12 T.
Magneto-resistance high magnetic fields at various angles and Hall measurements.
Fabrication of diamond devices using electron beam lithography
High frequency transport of superconducting diamond devices in the frequency range up to 67 GHz at
various temperatures down to 4 K and also in the presence of 3 T magnetic fields.