Deconstructing the Nanoscale Electromechanical Response of Mixed Conductors

Abstract

Piezoresponse force microscopy (PFM) techniques, which probe nanoscale electromechanical coupling, are often difficult to interpret. For ferroelectric mixed conductors useful for computation elements, this becomes more difficult due to non-piezoelectric electromechanical effects. The measured PFM signal can have tip-local and nonlocal contributions associated with multiple coupling mechanisms, and it is often unclear what material property is truly being probed. Here, using antimony sulfide (Sb2S3) as a model system, an emerging phase change material for optical and electrical memory and computation elements, we show that the PFM signal can be separated into a fast, capacitive contribution, and a slow, ionic contribution whose time-domain characteristics match well with diffusion-controlled kinetics. This ionic contribution, rather than ferroelectric switching, leads to electromechanical hysteresis in Sb2S3, with no clearly measurable ferroelectric switching effects. Uniquely, electromechanical behavior has a very clear spatial contrast that is decoupled from surface topography, showing that the signal arises from surface charging that reflects bulk-like properties and allowing a self-consistent interpretation in terms of dielectric behavior. This protocol establishes a clear interpretation of a non-piezoelectric signal, often mischaracterized as an indication of the ferroelectric state. Additionally, because all electromechanical experiments show a strong, low-frequency electric relaxation in crystalline Sb2S3 while the response of glassy Sb2S3 is static, we conclude that the crystalline phase of Sb2S3 is a mixed conductor, while the amorphous phase remains strictly insulating.