AFM for the Manipulation and Metrology of Layered Materials and their Heterostructures

Park Systems UK Ltd, Medicity Nottingham, D6 Building Thane Road, Nottingham, NG90 6BH

James Kerfoot

Atomic force microscopy (AFM) is a go-to technique for measurements of the nanoscale surface morphology and functional properties of layered materials (LMs) and their heterostructures (LMHs) due to its excellent spatial resolution and surface specificity. In this talk we will introduce the key concepts in the operation of AFM and look at how it enables advanced nanoscale characterisation (both topographic and electrical) and improvement of LMH based devices. We will start by reviewing the different modes that can be used to routinely measure moiré patterns in different layered materials heterostructures, including topography, lateral force microscopy (LFM), torsional force microscopy (TFM) and electrical modes including conductive AFM (C-AFM) and Kelvin probe force microscopy (KPFM). In addition to providing an overview and comparison of these techniques on different layered samples, with a particular focus on how they can be automated at multiple specific locations, we will also present a method to electrically contact isolated heterostructures on insulating stamps to enable electrical AFM characterisation of partially assembled heterostructures and interfaces that go on to be buried upon further fabrication steps.  

In the next part of the talk, we will then look at the characterisation of interfacial ferroelectricity both by kelvin probe force microscopy and piezoresponse force microscopy. In particular, we will show how bias applied via the tip enables deterministic switching by layer sliding [1]. In the final part of the talk, we will demonstrate how the use of in-situ heating during contact mode AFM cleaning [2,3] leads to clean interfaces within layered materials heterostructures. By forming marginally twisted homostructures of monolayer molybdenum disulphide (1L-MoS2) on graphite using PDMS stamps, we show a dramatic increase in the efficacy of the removal of trapped interfacial contamination when we do contact mode in-situ cleaning versus low temperature contact mode cleaning and annealing. The effectiveness of cleaning is determined by both the absence of trapped interfacial contamination in topographic channels and the observation of moiré patterns between layers after cleaning.  

By combining versatile methods to mechanically manipulate and clean layered materials and their heterostructures and probe features such as moiré patterns with high resolution across a range of imaging modalities, all with an automated platform, we showcase the unique opportunities AFM offers to discover and refine new materials platforms.  

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

[2] M. Rosenberger et al. ACS Appl. Mater. Interfaces 10, 12, 10379–10387 (2018)

[3] Y. Kim et al. ACS Nano 13, 12, 14182–14190 (2019)

Presenter Biography

James received his PhD in Physics from the University of Nottingham in 2018, studying the morphology and optical properties of monolayers of self-assembled molecules and their heterostructures. He then went on to work as a postdoctoral researcher, also at the University of Nottingham, working on the formation of hybrid heterostructures of molecular assemblies and layered materials demonstrating both electroluminescence and selective triplet excitation. In 2020, James took up a position as a postdoctoral researcher at the Cambridge Graphene Centre, using scanning probe microscopy and optical spectroscopy to study electrostatics and optical properties of layered materials heterostructures with controlled twist angle and their scalable incorporation into integrated photonic circuits. Since January 2022, James has been a member of the Park Systems team as an applications scientist, supporting customers with interest ranging from fundamental physics to industrial scale production in the application of a diverse range of scanning probe microscopy techniques to gain insightful results.