O. Osmani

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.

Laser damage in silicon: Energy absorption, relaxation, and transport

Silicon irradiated with an ultrashort 800 nm-laser pulse is studied theoretically using a two temperature description that considers the transient free carrier density during and after irradiation. A Drude model is implemented to account for the highly transient optical parameters. We analyze the importance of considering these density-dependent parameters as well as the choice of the Drude collision frequency. In addition, degeneracy and transport effects are investigated. The importance of each of these processes for resulting calculated damage thresholds is studied. We report damage thresholds calculations that are in very good agreement with experimental results over a wide range of pulse durations.

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.

Graphitic nanostripes in silicon carbide surfaces created by swift heavy ion irradiation

The controlled creation of defects in silicon carbide represents a major challenge. A well-known and efficient tool for defect creation in dielectric materials is the irradiation with swift (E(kin) ≥ 500 keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.

Calculation of electronic stopping power along glancing swift heavy ion tracks in perovskites using ab initio electron density data

In recent experiments the irradiation of insulators of perovskite type with swift (E~100 MeV) heavy ions under glancing incidence has been shown to provide a unique means to generate periodically arranged nanodots at the surface. The physical origin of these patterns has been suggested as stemming from a highly anisotropic electron density distribution within the bulk. In order to show the relevance of the electron density distribution of the target we present a model calculation for the system that is known to produce the aforementioned surface modifications. On the basis of the Lindhard model of electronic stopping, we employ highly-resolved ab initio electron density data to describe the conversion of kinetic energy into excitation energy along the ion track. The primary particle dynamics are obtained via integration of the Newtonian equations of motion that are governed by a space- and time-dependent frictional force originating from Lindhard stopping. The analysis of the local electronic stopping power along the ion track reveals a pronounced periodic structure. The periodicity length varies strongly with the particular choice of the polar angle of incidence and is directly correlated to the experimentally observed formation of periodic nanodots at insulator surfaces.

Energy dissipation in insulators induced by swift heavy ions: A parameter study

The irradiation of solids with high energy laser or particle beams has led to a deeper understanding of the relaxation processes inside the target material. However, a lot of open questions remain. In the present paper, we will examine the irradiation of the model system Xe23+ @ 93 MeV → SrTiO3 within the framework of the two-temperature-model and study the electron-phonon-coupling g and the electron diffusivity De as well as the lattice diffusivity Dp. These are crucial parameters for which no experimental data is available. Experimentally, g is very difficult to measure and therefore theoretical predictions are of great importance. With the approach presented here it is possible to determine the coupling-constant by one order of magnitude.