S. Akcöltekin

Unzipping and folding of graphene by swift heavy ions

We show that graphene on a dielectric substrate sustains major modifications if irradiated with swift heavy ions under oblique angles. Due to a combination of defect creation in the graphene layer and hillock creation in the substrate, graphene is split and folded along the ion track yielding double layer nanoribbons. The folded parts are up to several 100 nm in length. Our results indicate that the radiation hardness of graphene devices is questionable but also open up a new way of introducing extended low-dimensional defects in a controlled way.

Swift heavy ion irradiation of SrTiO3 under grazing incidence

The irradiation of SrTiO3 single crystals with swift heavy ions leads to modifications of the surface. The details of the morphology of these modifications depend strongly on the angle of incidence and can be characterized by atomic force microscopy. At glancing angles, discontinuous chains of nanosized hillocks appear on the surface. From the variation of the length of the chains with the angle of incidence the latent track radius can be determined. This radius is material specific and allows the calculation of the electron–phonon coupling constant for SrTiO3. We show that a theoretical description of the nanodot creation is possible within a two-temperature model if the spatial electron density is taken into account. The appearance of discontinuous features can be explained easily within this model, but it turns out that the electronic excitation dissipates on a femtosecond timescale, too rapidly to feed sufficient energy into the phonon system in order to induce a thermal melting process. We demonstrate that this can be solved if a temperature-dependent diffusion coefficient is introduced into the model.

Graphene on insulating crystalline substrates

We show that it is possible to prepare and identify ultra-thin sheets of graphene on crystalline substrates such as SrTiO(3), TiO(2), Al(2)O(3) and CaF(2) by standard techniques (mechanical exfoliation, optical and atomic force microscopy). On the substrates under consideration we find a similar distribution of single layer, bilayer and few-layer graphene and graphite flakes as with conventional SiO(2) substrates. The optical contrast C of a single graphene layer on any of those substrates is determined by calculating the optical properties of a two-dimensional metallic sheet on the surface of a dielectric, which yields values between C = -1.5% (G/TiO(2)) and C = -8.8% (G/CaF(2)). This contrast is in reasonable agreement with experimental data and is sufficient to make identification by an optical microscope possible. The graphene layers cover the crystalline substrate in a carpet-like mode and the height of single layer graphene on any of the crystalline substrates as determined by atomic force microscopy is d(SLG) = 0.34 nm and thus much smaller than on SiO(2).

Swift heavy ion irradiation of SrTiO 3 under grazing incidence

The irradiation of SrTiO3 single crystals with swift heavy ions leads to modifications of the surface. The details of the morphology of these modifications depend strongly on the angle of incidence and can be characterized by atomic force microscopy. At glancing angles, discontinuous chains of nanosized hillocks appear on the surface. From the variation of the length of the chains with the angle of incidence the latent track radius can be determined. This radius is material specific and allows the calculation of the electron–phonon coupling constant for SrTiO3. We show that a theoretical description of the nanodot creation is possible within a two-temperature model if the spatial electron density is taken into account. The appearance of discontinuous features can be explained easily within this model, but it turns out that the electronic excitation dissipates on a femtosecond timescale, too rapidly to feed sufficient energy into the phonon system in order to induce a thermal melting process. We demonstrate that this can be solved if a temperature-dependent diffusion coefficient is introduced into the model.

Energy threshold for the creation of nanodots on SrTiO3 by swift heavy ions

We present experimental and theoretical data on the threshold behaviour of nanodot creation with swift heavy ions. A model calculation based on a two-temperature model that takes into account the spatially resolved electron density gives a threshold of 12 keV nm−1 below which the energy density at the end of the track is no longer high enough to melt the material. In the corresponding experiments, we irradiated SrTiO3 surfaces under grazing incidence with swift heavy ions. The resulting chains of nanodots were analysed by atomic force microscopy (AFM). In addition, some samples of SrTiO3 irradiated under normal incidence were analysed by transmission electron microscopy (TEM). Both experiments showed two thresholds, which were connected with the appearance of tracks and the creation of fully developed tracks. The threshold values were similar for surface and bulk tracks, suggesting that the same processes occur at both glancing and normal incidence. 5 Author to whom any correspondence should be addressed.

Scanning probe microscopy investigation of nanostructured surfaces induced by swift heavy ions

Swift heavy ions can be used to modify material surfaces on the nanometer scale. In particular, the irradiation of a target surface under grazing angle of incidence offers new possibilities to create chains of individual nanodots with different lengths. The length of these chains can be controlled by the angle of incidence. So far, this method could be successfully applied for insulating materials. The present work dealt with nanosized tracks on the well-known highly oriented pyrolytic graphite surface. By using atomic force microscopy and scanning tunneling microscopy, comparative studies of two different ion beam energies and ion types have been performed. From the analysis of the scanning probe microscopy results, the same track length–angle relation was found, similar to earlier studies on other materials such as SrTiO3.

Conductive nanodots on the surface of irradiated CaF2

CaF2(111) single crystal surfaces have been irradiated with fast heavy ions under oblique angles resulting in chains of nanosized hillocks. In order to characterize these nanodots with respect to their conductivity we have applied non-contact atomic force microscopy using a magnetic tip. Measurements in ultra high vacuum as well as under ambient conditions reveal a clearly enhanced electromagnetic interaction between the magnetic tip and the nanodots. The dissipated energy per cycle is comparable to the value found for metals, indicating that the interaction of the ion with the target material leads to the creation of metallic Ca nanodots on the surface. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)