Investigation of van der Waals ferroelectric CuInP2S6 by scanning probe microscopy

Advanced characterization methods can promote the development of materials science, including the structural characterization of materials and the functional characterization of the response to external fields. Scanning probe microscopy (SPM) can measure and collect rich multi-dimensional information and has become a powerful tool for the characterization of micro-nanoscale materials. Ferroelectric materials play an important role in non-volatile memory devices. With the increasing demand for device miniaturization, van der Waals (vdW) ferroelectrics that feature atomically thin ferroelectricity and complementary metal-oxide-semiconductor (CMOS)-compatible defect-free interfaces are highly promising for the construction of post-Moore’s law electronics. Among the ferroelectric two-dimensional materials reported so far, CuInP2S6 (CIPS) exhibits the highest out-of-plane polarization value (4.93 μC/cm^2), making it an excellent “active” dielectric in two-dimensional electronic devices. Here, we investigate inhomogeneous friction behaviour existing in copper-deficient CIPS (Cu0.2In1.26P2S6), which exhibits a nanoscale phase separation. The paraelectric In4/3P2S6 (IPS) phase exhibits higher friction than the ferroelectric CIPS phase, while phase boundaries display the lowest friction. We also observe a thickness-dependent friction behaviour in Cu0.2In1.26P2S6. The friction in this vdW material gradually decreases with increasing thickness within the range of 20 nm to 230 nm. We also investigate the softer domain walls of stochiometric CIPS and found that flexoelectricity increases the stiffness of downward domains, while increasing electrostatic force leads to an apparent reduction of mechanical response. In copper-deficient CIPS, the CIPS phase is stiffer than the IPS phase, while phase boundaries display the lowest elastic modulus. We then investigate the anomalous time-dependent and downward-to-upward domains switching in CIPS, and discuss the competition between flexoelectric and piezoelectric fields, together with the unique quadruple-well state in CIPS. In summary, these studies explore the friction behaviour, mechanical properties and flexoelectric engineering of CIPS. This knowledge is highly interesting for an improved understanding of ferroelectric two-dimensional materials and may pave the way for the design of electromechanical devices, a new concept that could be explored for other vdW material systems.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2025

Author dc:creator

• Wang, Lei ; https://orcid.org/0000-0002-2113-4193

• Ferroelectricity
• Scanning probe microscopy
• CuInP2S6
• anzsrc-for: 401605 Functional materials
• anzsrc-for: 5104 Condensed matter physics

Statement dc:rights

• open access
• CC BY 4.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

en

Identifier

https://doi.org/10.26190/unsworks/30828

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/104001