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Keeney et al. Microstructures 2023;3:2023041 Microstructures
DOI: 10.20517/microstructures.2023.41

Research Article Open Access

What lies beneath? Investigations of atomic force

microscopy-based nano-machining to reveal sub-
surface ferroelectric domain configurations in

ultrathin films

Lynette Keeney , Louise Colfer, Debismita Dutta, Michael Schmidt, Guannan Wei
Tyndall National Institute, University College Cork, Cork T12 R5CP, Ireland.
Correspondence to: Dr. Lynette Keeney, Tyndall National Institute, University College Cork, Lee Maltings Complex, Dyke Parade,
Cork T12 R5CP, Ireland. E-mail: lynette.keeney@tyndall.ie

How to cite this article: Keeney L, Colfer L, Dutta D, Schmidt M, Wei G. What lies beneath? Investigations of atomic force
microscopy-based nano-machining to reveal sub-surface ferroelectric domain configurations in ultrathin films. Microstructures
2023;3:2023041. https://dx.doi.org/10.20517/microstructures.2023.41
Received: 13 Aug 2023 First Decision: 24 Aug 2023 Revised: 31 Aug 2023 Accepted: 5 Sep 2023 Published: 20 Oct 2023

Academic Editor: Shujun Zhang Copy Editor: Fangyuan Liu Production Editor: Fangyuan Liu

Abstract
Multiferroic materials, encompassing simultaneous ferroelectric and ferromagnetic polarization states, are enticing
multi-state materials for memory scaling beyond existing technologies. Aurivillius phase B6TFMO
(Bi Ti Fe Mn O ) is a unique room temperature multiferroic material that could ideally be suited to future
y
x
6
18
z
production of revolutionary memory devices. As miniaturization of electronic devices continues, it is crucial to
characterize ferroelectric domain configurations at very small (sub-10 nm) thickness. Direct liquid injection
chemical vapor deposition allows for frontier development of ultrathin films at fundamental (close to unit cell)
dimensions. However, layer-by-layer growth of ultrathin complex oxides is subject to the formation of surface
contaminants and 2D islands and pits, which can obscure visualization of domain patterns using piezoresponse
force microscopy (PFM). Herein, we apply force from a sufficiently stiff diamond cantilever while scanning over
ultrathin films to perform atomic force microscopy (AFM)-based nano-machining of the surface layers.
Subsequent lateral PFM imaging of sub-surface layers uncovers 45° orientated striped twin domains, entirely
distinct from the randomly configured piezoresponse observed for the pristine film surface. Furthermore, our
investigations indicate that these sub-surface domain structures persist along the in-plane directions throughout
the film depth down to thicknesses of less than half of an Aurivillius phase unit cell (< 2.5 nm). Thus, AFM-based
nano-machining in conjunction with PFM allows demonstration of stable in-plane ferroelectric domains at
thicknesses lower than previously determined for multiferroic B6TFMO. These findings demonstrate the

© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0
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