MICROSCOPY & IMAGING
INVESTIGATING GRAPHENE
Ilka M. Hermes & Simonas Krotkus on correlating graphene’s functional properties via AFM
to the step edges, visible in the enlarged image detail in Fig. 1. Te surface potential displayed a clear contrast between two discrete states: a low potential state at the elevations close to the sapphire steps as well as on the graphene wrinkles, and a high potential state on the bulk film on underlying sapphire terraces with a potential difference of up to 0.7 V.
FIG. 1. Topography and surface potential captured via sideband KPFM on graphene on a sapphire substrate. Line profiles of topography and surface potential visualise the correlation of the two signals
T
he implementation of graphene in high-performance devices for electronic or energy applications requires a homogeneous electronic
property distribution on the wafer-scale. Te influence of morphological effects in graphene on the local electronic properties can be investigated by atomic force microscopy (AFM), which unites real-space topography imaging with the detection of functional surface properties. Here, we demonstrate the capabilities of
Park Systems’ AFM by corelating surface potential imaging via sideband Kelvin probe force microscopy (KPFM) with PinPoint nanomechanical measurements for the characterisation of a wafer-scale CVD-grown graphene on sapphire produced in an Aixtron CCS R&D reactor.
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SURFACE POTENTIAL IMAGING VIA SIDEBAND KPFM KPFM captures the surface potential as a conductive cantilever scans a conductive or semiconductive sample. To improve the resolution and accuracy of the KPFM signal, Park Systems has recently implemented an easy-to-use sideband KPFM mode in its NX research AFMs. Sideband KPFM improves spatial resolution and local potential sensitivity significantly. On graphene, sideband KPFM
resolved a distinct potential contrast that directly correlates with the topography (Fig. 1). Te topography showed underlying sapphire terraces and steps as well as graphene wrinkles. Interestingly, each sapphire terrace featured finer substructures with elevations in proximity
CORRELATION OF SURFACE POTENTIAL AND NANOMECHANICS To further investigate the substructures on the terraces, the scientists imaged adhesion force and modulus at the same scan area using Park System’s PinPoint mode. PinPoint provides quantitative nanomechanical images via fast force spectroscopy mapping (Fig. 2): the cantilever approaches and retracts at each pixel to acquire topography and nanomechanics. Te automated analysis of each force curve in Park’s SmartScan
FIG.2. Schematic diagram of Park Systems’ PinPoint nanomechanical mode based on fast force spectroscopy
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