Role of Strain on Ferroelectricity in Ultrathin CuInP2S6

CuInP2S6 (CIPS) is a layered room-temperature ferroelectric material prime for next-generation electronics applications, and it is important to understand its thickness-dependent ferroelectric properties and surface chemistry. Some reports have described a critical thickness below which ferroelectric or phase behavior changes; however, none agree regarding the value of the critical thickness or present an explanation of its origin. Here, we explore the thickness-dependent properties of mechanically exfoliated CIPS using Raman spectroscopy and piezoresponse force microscopy (PFM). Raman spectra from the as-exfoliated flakes revealed blue-shifted peak frequencies below a certain thickness that we attribute to substrate-induced compressive strain. These effects were also observed in thickness-dependent PFM phase morphologies. Upon thermal annealing, the compressive strains were reversed, and we observed concomitant and significant variations in both domain morphologies and coercive voltages. Our work reveals the significant role of strain in tuning the ferroelectric behavior of ultrathin CIPS and highlights opportunities afforded by strain engineering to manipulate its ferroelectric properties through the flexoelectric effect.