nº25/Septiembre 2012 Abstract We probe the local inhomogeneities of the electronic properties of graphene at the nanoscale using scanning probe microscopy techniques. First, we focus on the study of the electronic inhomogeneities caused by the graphene-substrate interaction in graphene samples exfoliated on silicon oxide. We find that charged impurities, present in the graphene- substrate interface, perturb the carrier density significantly and alter the electronic properties of graphene. This finding helps to understand the observed device-to-device variation typically observed in graphene-based electronic devices. Second, we probe the effect of chemical modification in the electronic properties of graphene, grown by chemical vapour deposition on nickel. We find that both the chemisorption of hydrogen and the physisorption of porphyrin molecules strongly depress the conductance at low bias indicating the opening of a bandgap in graphene, paving the way towards the chemical engineering of the electronic properties of graphene. Resumen Hemos estudiado las inhomogeneidades locales de las propiedades electrónicas del grafeno a escala nanométrica utilizando técnicas de microscopía de sonda próxima. En primer lugar, nos centramos en el estudio de las inhomogeneidades electrónicas causadas por la interacción del grafeno con el sustrato en muestras de grafeno exfoliado sobre óxido de silicio. Encontramos que las impurezas cargadas, presentes en la interfaz entre el grafeno y el sustrato, perturban considerablemente la densidad de portadores y alteran las propiedades electrónicas del grafeno. Este hallazgo ayuda a comprender la gran variabilidad entre distintos dispositivos que se observa típicamente en dispositivos electrónicos basados en grafeno. En segundo lugar, investigamos el efecto de la modificación química de las propiedades electrónicas de grafeno, crecido sobre níquel por depósito por vapor químico. Encontramos que tanto la quimisorción de hidrógeno como la fisisorción de moléculas de porfirina logran reducir fuertemente la conductancia a bajo voltaje, lo que indica la apertura de un gap en el grafeno, allanando el camino hacia el diseño químico de las propiedades electrónicas de grafeno. 1. Introduction Since the first experimental realization of graphene [1], its unique properties [2-4] have boosted the research in this novel material. Due to the high sensitivity of graphene to external electric fields, it can be used for sensing applications or as the channel in field effect transistors. This high sensitivity, however, makes graphene very vulnerable to charged impurities in the surroundings and to chemical doping [5, 6], leading to a large device-to-device variation in the electronic performance [6]. This effect is especially critical because graphene is all surface and thus when it lays on a surface or it is covered by a layer of adsorbates almost every single carbon atom of the graphene layer can be altered by the graphene-substrate interaction or the chemical doping. In this work we review experimental results on the local electronic properties of graphene, focused first on the study of the electronic inhomogeneities caused by the graphene-substrate interaction (based on Ref. [7]) and second on the variation of the local electronic properties of graphene by chemical modification of the surface (based on Ref. [8]). 2. Graphene-substrate interaction: exfoliated graphene on SiO 2 When graphene is exfoliated on insulating substrates, the presence of charged impurities typically generates electric fields strong enough to change the doping level of the graphene layer at the nanoscale [9, 10]. As a consequence, graphene-based electronic devices show a large device-to-device variation in electrical performance and reproducibility [5, 6]. We have developed a combined scanning tunnelling and atomic force microscope (STM/AFM) to characterize the electronic properties of graphene layers even when they are deposited on top of insulating substrates. This scanning probe microscopy (SPM) tool can operate as an AFM, without the need of a conductive substrate, to locate the graphene flake. The microscope relies on an STM which has been supplemented with a piezoelectric quartz tuning fork force sensor [11, 12] in the so-called qPlus configuration [13] (spring constant k ~ 12500 N/m, resonance frequency f0 ~ 32.1 kHz and quality factor Q ~ 4200). The use of a carbon fiber tip has found to optimize the performance of this type of combined STM/AFM microscopes [14-16]. The measurement starts by positioning the carbon fiber tip on top of a graphene flake (prepared by cleavage of highly oriented pyrolytic graphite on a SiO 2 /Si substrate using silicone stamps [17]) with the help of a long working distance optical microscope (see Figure 1a). Then the AFM capability of our combined STM/AFM is used to scan the region under study (see Figure 1b) and thus to determine thickness of the different areas of the flake. Before starting the STM measurements the tip is positioned onto the flake, which is electrically contacted by a gold electrode (deposited by shadow mask evaporation), and the scan range is reduced (dashed square in Figure 1b) in order to avoid the tip reaching the insulator substrate which would result in a tip crash A. Castellanos-Gomez 1,2,+,* , Arramel 2 , M. Wojtaszek 2 , R.H.M. Smit 1,3 , N. Tombros 2 , N. Agraït 1 , B.J. van Wees 2 , G Rubio-Bollinger 1 1 Física de la Materia Condensada. Universidad Autónoma de Madrid, Campus de Cantoblanco, E-28049 Madrid, Spain. 2 Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, The Netherlands. 3 Kamerlingh Onnes Laboratorium, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands. + Present address: Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands. * Corresponding author: a.castellanosgomez@tudelft.nl 18