Method for straightforward identification of graphene domains on topologically challenging substrates

Besides the production of large-areas of graphene, forthcoming industrial needs require a large-scale contactless method for testing its electrical properties. Currently, only time-consuming, complicated and expensive methods of measurements of the electrical properties of graphene are available, including patterning of devices and subsequent transport measurements.

The obtained information is generalized over the whole device and not correlated with the exact morphology of graphene and presence of structural defects or local adsorbates. Electrical modes of scanning probe microscopy, e.g. Electrostatic Force Microscopy (EFM) and Scanning Kelvin Force Microscopy (SKPM), provide a contactless (and, therefore, simple, widely accessible and cheap) electrical mapping of epitaxial and exfoliated graphene and extracting crucial information about graphene thickness, distribution of the electrical potential and charge, work function, etc. at the microscale.

We perform local electrical characterization of epitaxial graphene grown on 4H-SiC(0001) using EFM and SKPM techniques in ambient conditions and at elevated temperatures ^[1]. These methods provide a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by complex morphology and adsorbates (Fig. 1).

Fig. 1: Epitaxial graphene on 4H-SiC(0001). 1LG layer is decorated with 0D adsorbates. a) Topography, b) topography deflection error signal highlighting the adsorbates found on the 1LG, c) SKPM image corresponding to a). The frame indicates the area where the SKPM histogram (d) was taken. The solid line in d) is a double-peak fit using Lorentzian distribution. Measurements performed at room temperature. The scale bar in a-c) panels is 1 μm.

We show that the quality of imaging significantly improves at elevated temperatures, following partial evaporation of the surface water layer and change of the screening effects. We study the variation of the local graphene contrast in dependence on the the tip biases, attributed to the change in the capacitive coupling between the tip and graphene domain (Fig. 2). We have also developed a novel EFM spectroscopy measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.

Fig. 2: a-d) EFM phase images of epitaxial graphene taken in ambient conditions with the different tip biases. IFL,1LG and 3LG domains are labelled in (a). The phase range is 2° for images a) and d) and 1° for images b) and c). The frame highlights the location of magnified areas. e) Comparison of the EFM phase line profiles taken in the centre of each EFM image as indicated by a straight line.

We believe that these results can serve as a quick guide particularly valuable for identification of epitaxial graphene for industrial applications.