Nanocomposites and nanomaterials

Transport properties of carbon nanotubes with different degrees of structural perfection
Shpylka D.O.1, Ovsiienko I.V.1, Len T.A.1, Matzui L.Yu.1, Semen’ko M.P.1, Naumova D.D. 2
Taras Shevchenko National University of Kyiv, 01601, Kyiv, Volodymyrska st. 64/13, Department of Physics1 and Chemistry2
E-mail: denys8600@ukr.net

The paper presents the results of investigations of transport properties of multiwall carbon nanotubes (CNTs) with different degrees of structural perfection. For investigations two specimens of CNTs have been chosen. These are technical CNTs (specimen #1, crystallite size L ~ 20 nm, distance between neighboring layers d002 = 0.340 nm) and purified CNTs (specimen #2, L ~ 65 nm, d002 = 0.335 nm). The structure of CNTs has been investigated by X-R diffraction method. The defect of the graphite layer also has been evaluated by the degree of CNTs surface functionalization under the ultraviolet radiation. The resistivity of bulk specimens obtained from CNTs powder has been measured in the temperature interval from 4.2 K to 293 K and in magnetic field up to 2 T. 
The Fig. 1 presents the field dependences of magnetoresistance (Dr/r(B) for bulk specimens of technical CNTs (Fig. 1a) and purified CNTs (Fig. 1b).
a)	b)	As can be seen from Figure for specimen #1 magnetoresistance at room temperature is positive and quadratic for the magnetic field. At T = 77 K the magnetoresistance is a quadratic for the magnetic field also, but it becomes negative. 	
Fig. 1. Dr/r(B) for CNTs specimens #1 (a) and #2 (b). 1 – T = 293 K, 2 – T = 77 K.		
For specimen #2 the magnetoresistance is positive at all temperatures, but the logarithmic dependence of the magnetoresistance on the magnetic field is observed at T = 77 K. 
In the work it is shown that for CNTs of imperfect structure the main mechanism of the magnetoresistance is the weak localization of charge carriers, while for the CNTs with perfect structure the main contribution to the magneto-resistance is the interaction of charge carriers.