**3. Results and Discussion**

### *3.1. Structural Characterization*

The room-temperature crystal structure of CIPS was first discovered in 1995 using single-crystal X-ray diffraction [24]. Bulk CIPS is composed of vertically stacked, weakly interacting layers bound together by vdW interactions. The metal cations and P–P pairs fill the octahedral voids in the sulfur framework in a CIPS crystal (Figure 1a). A complete unit cell is reported to consist of two adjacent monolayers due to the site exchange between Cu and the P–P pairs from one layer to another [25]. Figure 1b shows two typical Raman spectra measured in different regions of a CIPS flake on a Pt/Si substrate. In general, we observed five Raman active modes, including δ(S–P–P) modes at 162.9 cm−1, δ(S–P–S) modes around 226.1 cm−1, and 263.3 cm<sup>−</sup>1, an active mode from the cations at 303.2 cm−1, andυ(P–P) and υ(P–S) modes at 374.3 cm−<sup>1</sup> and 436.4 cm<sup>−</sup>1, respectively. The tested Raman results are consistent with the former reports on CIPS crystals [26,27]. P–S bond stretching vibrations (υ) can be resolved to A1g + A2u + Eu + Eg. The S–P–S modes change the S–P–S angles, resulting in another set of A1g + A2u + Eu + Eg. The P–P bond is associated with bending the PS3 groups, which accounts for Eu + Eg. Twisting (P2S6) <sup>4</sup><sup>−</sup> oscillation is related to the A1u mode [27]. The Raman spectra in orange showed a strong peak at ~303 cm−1, which corresponds to the cations [26]. The blue curve showed a weak and broad peak at ~303 cm<sup>−</sup>1, which is mainly related to the reduction of Cu content. It is believed to originate from the paraelectric phase of CIPS–IPS, where IPS stands for In4/3P2S6, appearing in the flake due to Cu+ deficiency [26,27]. The transmission electron microscope (TEM)

images show two structures on the same CIPS flakes (Figure 1c), which correspond to the monoclinic and triclinic phases, consistent with the Raman spectrum.

**Figure 1.** Material characterization of CIPS flake. (**a**) Top and side views of the CIPS crystal structure. In the atomic model, the yellow networks are S triangular networks, and the green, purple, and blue balls are P, In, and Cu atoms, respectively. (**b**) Raman spectra of CIPS flakes, including ferroelectric and paraelectric phases on the Pt substrate with 532 nm laser excitation. (**c**) The TEM characterizations of CIPS crystal include fast Fourier transform (FFT, **right**) and filtered inverse FFT (**middle**) patterns of the selected areas, respectively.
