Keynote Talk  

Among all state-of-the-art characterisation techniques, atomic force microscopy (AFM) is particularly powerful tool in the study of layered materials since it can be used to measure both atomic registry of such layered materials heterostructures and their associated functional properties at sub-nanometre length scales. In this talk, we provide a brief overview of the state-of-the-art in AFM-based characterisation of layered materials before proceeding to discuss two examples of interest both for fundamental studies and applications.

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In the case of our first example, controlling the twist angle between flakes of layered materials leads to interfacial ferroelectricity in the case of parallel stacked hexagonal boron nitride (hBN) [1-3]. We provide an account of the fabrication of such parallel stacked hBN samples using a home-built transfer setup [4] and the mapping of ferroelectric domains in these samples using both electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). We discuss different ways in which such domains may be mapped and manipulated using different AFM modalities. In particular, we address the measurement of these domains with different derivatives of KPFM (both amplitude modulated and sideband KPFM inside and outside of vacuum) with higher contrast extracted for sideband KPFM. We conclude by studying the manipulation of domains both electrically by applying bias via the tip and mechanically using contact mode AFM [5].

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Fig. 1. Parallel stacked hBN may be formed by breaking and restacking exfoliated layers mechanically (a). Once formed, these structures may be characterised using AFM (b) to map the morphology of ferroelectric domains (c).

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In our second example, we use high voltage conductive AFM to study the dielectric breakdown of hBN and mica in order to extract the dielectric strength of these materials and study the morphology and current voltage characteristics of layers before and after dielectric breakdown. We go on to apply this technique to large area thin films and demonstrate the ability of this technique to measure both the ‘intrinsic’ dielectric properties of thin films and at defects addressed individually via the probe. In summary, we exemplify the exceptional utility of AFM to provide insightful information for layered systems of both fundamental and applied interest.

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[1] C. R. Woods et al. Nat Commun. 12, 347 (2021).

[2] K. Yasuda et al. Science 372, 6549 1458 (2021)

[3] M. Vizner Stern et al. Science 372, 6549, 1462, (2021).

[4] Q. Zhao et al. J. Phys. Mater. 3 016001 (2020)

[5] R. Ribeiro-Palau et al. Science 361, 6403, 690 (2018)