Vladimir Samuilov

Conducting MWNT/poly(vinyl acetate) composite nanofibres by electrospinning

Electrospinning is a relatively simple and versatile method to produce polymer nanofibres and their composites. In this work, functionalized multiwalled carbon nanotubes (f-MWNTs) were used for the fabrication of conducting nanocomposite fibres, in comparison with the composite nanofibres made of unfunctionalized MWNTs (u-MWNTs). Our results showed that the addition of f-MWNTs could improve the dispersion of carbon nanotubes in the polymer solution and therefore result in composite nanofibres with uniform diameters by electrospinning. Alignment of the composite nanofibres was achieved by using a rotating drum as the collector. F-MWNTs were found to align parallel to the axis direction of the nanofibres. DC electrical properties of a single composite fibre were investigated at room temperature as well as cryogenic states (100-300 K). An electrical percolation phenomenon was observed for nanofibres with different mass fractions of MWNTs. It was shown that the conductivity of the material could be significantly improved above the percolation threshold. The conductivity could be of several orders of magnitude higher than the pure PVAc.

DNA separation at a liquid-solid interface.

We demonstrate that it is possible to separate a broad band of DNA on a solid substrate without topological obstacles. The mobility was found to scale with molecular size (N) as N–0.25, while the resolution scaled as N0.75 indicating that diffusivity on this substrate was minimal. By varying the buffer concentration we were able to show that the mobility for a given chain length scaled with the persistent length (p) as p1/2. This could be shown to be related to the Gaussian conformation of the chains adsorbed on the surface. A two‐dimensional corrugated surface of nonporous silica beads was produced using a self‐assembling process at the air/water interface. Even though the surface corrugations were comparable to persistence length we show that they do not affect the mobility, indicating that surface friction rather than topological constraints are the predominant mechanism of separation on a surface.

Preparation and Electrical Properties of the MWNT/Polymer Nanocomposite Fibers

An oxidation method has been applied to functionalize multiwalled carbon nanotubes with carboxylic acid (-COOH) group. Functionalized carbon nanotubes (f-MWNT) were used for the fabrication of conducting nanocomposite fibers by electrospinning, in comparison with the composite nanofibers made of un-functionalized carbon nanotubes (u-MWNT). Our results showed that the addition of f-MWNTs into polymer solution could increase the compatibility of MWNTs with the polymer matrix, and thus result in composite nanofibers with uniform diameters. Alignment of the composite nanofibers was achieved by using a rotating drum as the collector. F-MWNTs were found to align parallel to the axis direction of the nanofibers. Temperature-dependent DC electrical properties of a single composite fiber were investigated by a two-probe method. It was shown that the conductivity of the material could be significantly improved above a percolation threshold. The conductivity could be of ten orders of magnitude higher than the pure PVAc.

Charge Transport in Carbon Nanotube Films and Fibers

Electrical properties and magnetotransport in carbon nanotubes (CNTs) have attracted much attention due to their importance in verification of existing theories of modern condensed matter physics and a number of possible applications (Robertson, 2007; Dai, 2002). Single-wall carbon nanotube (SWCNT) is a graphene sheet rolled up into a hollow cylinder and show metallic or semiconducting properties dependently upon their diameter and chirality. Due to their unique structure SWCNTs allows to study a large variety of different quantum phenomena like single-electron tunneling (Bockrath et al., 1997), Luttinger liquid behaviour (Bockrath et al., 1999), ballistic transport (Krstic et al., 2000), etc. Multi-wall carbon nanotubes (MWCNTs) are more complicated systems. They consist of a several shells of different diameter and chirality. Due to weak coupling between the shells the conductivity in bulk-contacted MWCNTs is defined mostly by the outermost shells. Diffusive transport in majority of experiments for individual MWCNTs was observed. Therefore quantum interference effects inherent for mesoscopic systems (weak localization and universal conductance fluctuation) were reported (Schonenberger et al., 1999). Besides that, MWCNTs with large diameter allows to observe Aahronov-Bohm effect at experimentally available values of magnetic fields (Bachtold et al., 1999; Lassagne et al., 2007). Ballistic transport even at room temperatures was observed by some authors (Frank et al., 1998; Urbina et al., 2003). Possibility to switch between ballistic and diffusive transport regime in the same MWCNTs sample using gate voltage by virtue of an electrostatic change of electron density was reported as well (Strunk et al., 2006; Nanot et al., 2009). Processing of nanotubes on macroscopic scale and investigation of their synergetic properties is a most important task for realistic application of these materials, especially for fabrication of carbon nanotubes-based gas-, bioand chemical sensors where signal from the sensor output depends on the conductivity of device (Stetter & Maclay, 2004). Different examples of morphology of the samples of arrays of nanotubes involve definitions of bundles (ropes) (Fischer et al., 1997; Krstic et al., 2000), mats (Fischer et al., 1997; Kaiser et al., 7

Magnetotransport properties of carbon nanotube fibers

Temperature dependencies of the resistance R(T) and magnetoresistance (MR) of singlewall carbon nanotube (SWCNT) fibers have been investigated. R(T) shows negative dR/dT at 4.2-300 K and can be well fitted by Mott’s law for 3-D variable range hopping (VRH) up to ~80 K. The negative MR at low fields with a positive upturn observed in the system can be explained by Shklovskii-Efros and Sivan models of MR for hopping conductivity.

Charge Transport in Mesoscopic Carbon Network Structures

The charge transport and quantum interference effects in low-dimensional mesoscopic carbon networks prepared using self-assembling were investigated. The mechanism of conduction in low-dimensional carbon networks was found to depend on the annealing temperature of the nitrocellulose precursor. The charge transport mechanism for carbon networks obtained at T ann =750 0 C was found to be the hopping conductivity in the entire investigated temperature range. The Coulomb gap near the Fermi level in the density of states was observed in the investigated carbon networks. The width of the Coulomb gap was found to be decreased with the annealing temperature of the carbon structures. The crossover from the strong localization to the weak localization regime of the charge transport in the carbon structures, obtained at T ann =950 0 C and T ann =1150 0 C, was observed in the temperature range T>100 K and T>20 K, respectively.