Folding Graphene with Swift Heavy Ions
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Folding Graphene with Swift Heavy Ions Sevilay Akcöltekin1 , Hanna Bukowska1 , Ender Akcöltekin1 , Henning Lebius2 and Marika Schleberger1 1 2
University of Duisburg-Essen, 47048 Duisburg, Germany. CIMAP (CEA-CNRS-ENSICAEN-UCBN), 14070 Caen Cedex 5, France.
ABSTRACT Swift heavy ion induced modifications on graphene were investigated by means of atomic force microscopy and Raman spectroscopy. For the experiment graphene was exfoliated onto different substrates (SrTiO3 (100), TiO2(100), Al2O3(1102) and 90 nm SiO2/Si) by the standard technique. After irradiation with heavy ions of 93 MeV kinetic energy and under glancing angles of incidence, characteristic folding structures are observed. The folding patterns on crystalline substrates are generally larger and are created with a higher efficiency than on the amorphous SiO2. This difference is attributed to the relatively large distance between graphene and SiO2 of d § 1 nm. INTRODUCTION Folding of graphene has attracted enormous interest in recent times. Several groups have studied self-folding or stimulated folding effects of graphene [1-9], either on suspended sheets or on exfoliated flakes. Several possible driving forces for those folding effects have been identified, e.g. ultrasonic excitation [3], chemical interactions [4] or side effects from the preparation. Detailed investigations have shown that the folded parts as well as the edges are mainly oriented along either zig-zag or armchair directions following the structure of the single layer of carbon atoms arranged in a two-dimensional sp²-bonded honeycomb network [3,10]. Another method to induce folding on single- (SLG) or bilayer graphene (BLG) is the irradiation of graphene by swift heavy ions (SHI) in the kinetic energy range of some tens of MeV [8]. In contrast to the above-mentioned folding techniques SHI irradiation of graphene provides the opportunity to control the folding shape and its dimension and also the number of folded parts. The present paper will demonstrate how to fold graphene by SHI irradiation and discuss on which type of substrate it can be applied. Generally, if a projectile ion hits a surface one can distinguish roughly between two ionmatter interaction mechanisms: (1) electronic stopping Se which is the dominating mechanism in the higher energy range (> 1 MeV) and described by the initial excitation of the electronic system of the target, (2) nuclear stopping Sn denoting the elastic collision of the projectile with the target atoms. Figure 1 shows the stopping power of a typical projectile ion (Xenon) impinging on a SrTiO3 substrate without graphene. The dotted line represents the total stopping power which consists of Se (solid line) and Sn (dashed line). It can be seen that the second mechanism plays a rather minor role in the SHI energy regime and can therefore be neglected in the following discussion. The typical energy for the projectiles in our experiment was 93 MeV, yielding an electronic stopping power of SSrTiO3 § 21 keV/nm in the crystal close to the surface.
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