Mechanically exfoliated graphene layers deposited on SiO2 substrate were irradiated with Ar+ ions in order to experimentally study the effect of atomic scale defects and disorder on the low-energy electronic structure of graphene. The irradiated samples were investigated by scanning tunneling microscopy and spectroscopy measurements, which reveal that defect sites, besides acting as scattering centers for electrons through local modification of the on-site potential, also induce disorder in the hopping amplitudes. The most important consequence of the induced disorder is the substantial reduction in the Fermi velocity, revealed by bias-dependent imaging of electron-density oscillations observed near defect sites

We have measured how irradiation by Ar + and N + ions modifies electronic conduction in single-wall carbon nanotube (SWNT) networks, finding dramatically different effects for different thicknesses. For very thin transparent networks, ion irradiation increases localization of charge carriers and reduces the variable-range hopping conductivity, especially at low temperatures. However, for thick networks (SWNT paper) showing metallic conductivity, we find a relatively sharp peak in conductivity as a function of irradiation dose. Our investigation of this peak reveals the important role of thermal annealing extending beyond the range of the irradiating ions, and shows the dependence on the morphology of the samples. We propose a simple model that accounts for the temperature-dependent conductivity. Recently, the effect of defects on the conducting properties of individual metallic single-wall carbon nanotubes (SWNTs) was shown [1,2] by the controlled reduction of the electronic localization length by irradiation with Ar + ions. For these SWNTs in the strong Anderson localization regime, the exponential increase of resistance with length of SWNTs between electrodes was greatly enhanced by di-vacancies created by the Ar + ions. In this paper, we report the first measurements (to our knowledge) of the effect of irradiation by ions on conduction in thin transparent SWNT networks, finding that resistance always increases on ion irradiation. In contrast to these results, however, we show that ion irradiation initially decreases the resistance of our thick SWNT networks.Many applications proposed for carbon nanotubes [3][4][5][6] are likely to be sensitive to irradiation effects, which u Fax: +49-711-689-1010, E-mail: v.skakalova@fkf.mpg.de would be particularly significant, for example, for devices used in space or other high-radiation environments. Controlled irradiation also has many applications related to carbon nanotubes. It has been demonstrated by experimental and theoretical studies that irradiation can be used for nano-engineering of carbon nanotubes [7,8]. For example, focused electron beams were shown to result in welding of crossed single-wall carbon nanotubes to form molecular junctions [9], and molecular dynamics simulations showed how such junctions might also be produced by irradiation by Ar + ions [10]. Irradiation by 1.25-MeV electrons at high temperature (800 • C) was able to coalesce neighbouring Ndoped SWNTs as a result of defect formation followed by thermal annealing [11]. Using irradiation by Ar + ions, Stahl et al. [12] introduced defects in the SWNTs in the upper part of a bundle of loosely coupled SWNTs before depositing gold contacts on top of the bundle. They found that to avoid the defects, the current switched to undamaged nanotubes in the lower part of the bundle, tunnelling being the transfer mechanism between nanotubes.In previous work [13], we observed local modifications of the electronic structure in atomically resolved scanning tunnelling microscope (STM) images of multi-wal...

Multiwall carbon nanotubes ͑MWCNTs͒ dispersed on graphite on highly oriented pyrolytic graphite ͑HOPG͒ substrate were irradiated with Ar + ions of 30 keV. The irradiated samples were investigated by scanning tunneling microscopy ͑STM͒ and spectroscopy ͑STS͒ in air. The irradiation-induced defects appear as hillocklike protrusions on the nanotube walls, similar to the hillocks observed earlier on ion irradiated HOPG. The results are in agreement with recent predictions, which attribute the STM features produced by ion irradiation to local modifications of the electronic structure. " ͱ 3 ϫ ͱ 3R" type superstructures are also observed near some of the defects. After annealing at 450°C in nitrogen atmosphere, the irradiated MWCNTs were investigated again by STM. The effect of heat treatment on the irradiation-induced nanotube defects is also discussed.

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