Magnetic properties of dense graphitic filaments formed via thermal decomposition of mesitylene in an applied electric field

摘要

All known carbon allotropes are diamagnetic: diamond, graphite (with a very high diamagnetic anisotropy, due to its high crystalline anisotropy), fullerenes and carbon nano-tubes, either single wall (one rolled-up sp~2-bonded graphene sheet) or multiwail exhibit significant diamagnetism [1]. Disordered non-crystalline forms of solid carbon, such as diamond-like carbon or tetrahedral amorphous carbon (ta-C) and amorphous carbon (a-C) generally exhibit also diamagnetic behavior. To our knowledge, paramagnetic behavior has not been observed before in dense graphitic carbon. Increasing the disorder brings about the reduction of diamagnetism [2], Paramagnetic behavior of carbon has been reported previously in disordered carbon materials with surface areas similar to the surface area of a single graphene sheet: thin films of non-graphitic disordered carbon [3], nanofoams of extremely low density [4], pitch-based activated carbon filaments with specific surfaces of 1500-3000 m~2/g [5] or in heat treated nanopowders produced from explosion-induced techniques and disordered at the atomic level with mixed sp~2:sp~3 bonding [6], The complete disordering of the crystalline structure can result in the observation of paramagnetic behavior in non-clustered carbon materials disordered at the atomic-scale obtained by pulsed laser deposition or high-repetition-rate pulsed laser ablation [3,4], The presence of non-bonding pi-electrons at the edges of nanosized graphene layers have been proposed to explain this paramagnetic behavior [6], but the presence of localized edge states of graphene layers in the absence of graphitic clusters is still an open question in non-graphitic disordered carbon. In microporous activated carbon fibers the presence of non-carbon elements must be carefully considered: Enoki has shown that the presence of guest elements (i.e., oxygen molecules) in the pores results in significant low-temperature paramagnetic susceptibility, due to the magnetic contribution of the condensed oxygen in the micropores [7]. A weak paramagnetic contribution from edge states localized at zig-zag edge-carbon atoms has been predicted by a tight-binding analytical model [8] in nanosized graphene ribbons, but the low-temperature paramag-netism predicted effect is about one order of magnitude lower than the values observed in our study. A low-temperature upturn (decrease in the absolute value) in the susceptibility has been observed for diamagnetic multiwall carbon nanotubes [9], giving further evidence of the existence of paramagnetic contributions associated with certain sp~2-bonded carbon structures.