Laser cleaning of exfoliated graphene
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Laser cleaning of exfoliated graphene Oliver Ochedowski1, Benedict Kleine Bußmann1 and Marika Schleberger1 Fakultät für Physik and CeNIDE, Universität Duisburg-Essen, D-47048 Duisburg, Germany
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ABSTRACT We have employed atomic force and Kelvin-Probe force microscopy to study graphene sheets exfoliated on TiO2 under the influence of local heating achieved by laser irradiation. Exfoliation and irradiation took place under ambient conditions, the measurements were performed in ultra high vacuum. We show that after irradiation times of 6 min, an increase of the surface potential is observed which indicates a decrease of p-type carrier concentration. We attribute this effect to the removal of adsorbates like water and oxygen. After irradiation times of 12 min our topography images reveal severe structural modifications of graphene. These resemble the nanocrystallite network which form on graphene/SiO2 but after much longer irradiation times. From our results we propose that short laser heating at moderate powers might offer a way to clean graphene without inducing unwanted structural modifications. INTRODUCTION Since its isolation 2004 by Geim and Novoselov graphene, a single layer of carbon atoms arranged in a honeycomb lattice, has caught much attention in the scientific community [1]. Because of its unusual physical properties like extremely high charge carrier mobility, graphene is expected to play a major role as a part of future semiconductor devices [2, 3]. It has already been shown that photoresist residues or adsorbates like water introduced from ambient conditions can strongly affect the electronic properties of graphene [4, 5]. Controlling the presence or being able to locally accumulate adsorbates might offer a way to manipulate future graphene devices. In this paper we investigate in which way local laser irradiation of graphene can be used to achieve this. Krauss et al. have shown that laser irradiation causes both, the removal of dopants as well as the disassembly of the single-crystal graphene on SiO2 [6]. We chose TiO2 as a substrate because here adsorbate effects are even more pronounced than on SiO2 (see below). To simultaneously measure the effects of laser irradiation on the topography and the surface potential of graphene we employ atomic force microscopy (AFM) in combination with Kelvin-Probe force microscopy (KPFM) [7]. KPFM is a widely used tool to measure the local contact potential difference (LCPD) between the AFM tip and the sample surface. For this an ac voltage with 1-2 kHz and about 1 V is applied to the oscillating tip during the AFM measurement. A lock-in amplifier is used to compensate the resulting electrostatic force with a dc voltage which corresponds to the LCPD or difference in work function between two materials. This method has been employed to investigate charge carrier characteristics of graphene on various substrates and the influence of water and adsorbates on its electronic properties [8, 9, 10, 11]. RESULTS AND DISCUSSION Samples were prepared by means of mechanical exfo
