Investigating Strain between Phase-Segregated Domains in Cu-Deficient CuInP₂S₆

Abstract

CuInP 2 S 6 (CIPS) is an emerging layered ferroelectric material with a Curie temperature above room temperature. When synthesized with Cu deficiencies (i.e., Cu 1−x In 1+x/3 P 2 S 6 ), the material segregates into CIPS and In 4/3 P 2 S 6 (IPS) self-assembled heterostructures within the same single crystal. This segregation results in significant in-plane and out-of-plane strains between the CIPS and IPS phases as the volume fraction of the CIPS (IPS) domains shrinks (grows) with a decreasing Cu fraction. Here, we synthesized CIPS with varying amounts of Cu (x = 0, 0.2, 0.3, 0.4, 0.5, 0.7, 0.8, and 1) and measured the strains between the CIPS and IPS phases through the evolution of the respective Raman, infrared, and optical reflectance spectra. Density functional theory calculations revealed vibrational modes that are unique to the CIPS and IPS phases, which can be used to distinguish between the two phases through two-dimensional Raman mapping. A comparison of the composition-dependent frequencies and intensities of the CIPS and IPS Raman peaks showed interesting trends with a decreasing CIPS phase fraction (i.e., Cu/In ratio). Our data reveal red-and blueshifted Raman and infrared peak frequencies that we correlate to lattice strains arising from the segregation of the material into CIPS and IPS chemical domains. The strain is highest for a Cu/In ratio of 0.33 (Cu 0.4 In 1.2 P 2 S 6 ), which we attribute to the equal and opposite strains that the CIPS and IPS phases exert on each other. In addition, the bandgaps we extracted from the optical reflectance spectra revealed a decrease in values, with Cu 0.4 In 1.2 P 2 S 6 having the lowest value of ∼2.3 eV.

CuInP2S6‐Based Electronic/Optoelectronic Synapse for Artificial Visual System Application

“…The Raman spectroscopy of CIPS in Figure 1c presents the characteristic peaks of CIPS, that is, peaks located at 101, 161, 263, 318, and 376 cm −1 , corresponding to P 2 S 6 4− liberations, δ (S–P–P) mode, δ (S–P–S) mode, cation oscillation, and ν (P–P) mode, respectively. [ 35 ] Figure 1d shows the Raman spectroscopy of graphene. The intensity of G peak (1582 cm −1 ) is significantly higher than that of 2D peak (2685 cm −1 ), which confirms the nature of ML graphene, [ 36 ] and the negligible D peak (≈1350 cm −1 ) suggests that the transferred ML graphene is defect‐free.…”