Role of Strain on Ferroelectricity in Ultrathin CuInP<sub>2</sub>S<sub>6</sub>

CuInP<sub>2</sub>S<sub>6</sub> (CIPS) is a layered room-temperature\nferroelectric material prime for next-generation electronics applications,\nand it is important to understand its thickness-dependent ferroelectric\nproperties and surface chemistry. Some reports have described a critical\nthickness below which ferroelectric or phase behavior changes; however,\nnone agree regarding the value of the critical thickness or present\nan explanation of its origin. Here, we explore the thickness-dependent\nproperties of mechanically exfoliated CIPS using Raman spectroscopy\nand piezoresponse force microscopy (PFM). Raman spectra from the as-exfoliated\nflakes revealed blue-shifted peak frequencies below a certain thickness\nthat we attribute to substrate-induced compressive strain. These effects\nwere also observed in thickness-dependent PFM phase morphologies.\nUpon thermal annealing, the compressive strains were reversed, and\nwe observed concomitant and significant variations in both domain\nmorphologies and coercive voltages. Our work reveals the significant\nrole of strain in tuning the ferroelectric behavior of ultrathin CIPS\nand highlights opportunities afforded by strain engineering to manipulate\nits ferroelectric properties through the flexoelectric effect.