Investigating Graphene’s Functional Properties

Ilka M. Hermes & Simonas Krotkus on correlating graphene’s functional properties via AFM

The implementation of graphene in high-performance devices for electronic or energy applications requires a homogeneous electronic property distribution on the wafer-scale. The 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.

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). The topography showed underlying sapphire terraces and steps as well as graphene wrinkles. Interestingly, each sapphire terrace featured finer substructures with elevations in proximity to the step edges, visible in the enlarged image detail in Fig. 1. The 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.

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. The automated analysis of each force curve in Park’s SmartScan software allows for real-time visualisation of modulus, adhesion force, etc.

On graphene, adhesion force and modulus showed a contrast between the underlying sapphire terraces, the steps and graphene wrinkles (Fig. 3). Compared to the terraces, the elevations around the steps exhibit a lower adhesion force and a higher modulus suggesting that the film becomes harder at these positions. The correlation of the nanomechanical properties with the surface potential and the sample topography indicates a possible connection of the material’s mechanical and electronic properties.

The combination of different AFM techniques including sideband KPFM and PinPoint mode available on Park Systems’ research AFMs allows a holistic and in-depth characterisation of 2D materials as demonstrated on a wafer-scale CVD-grown graphene on sapphire produced in an Aixtron CCS R&D reactor. The measurements showed a distinct correlation of the sample’s surface potential with the adhesion force and modulus indicating a connection of graphene’s electronic and mechanical properties. Characterising the highly localised mechanical and electronic properties of 2D materials enable a tailored development for future nanoelectronics applications.

Ilka M. Hermes is with Park Systems & Simonas Krotkus is with Aixtron

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