*2.1. Sample Preparation and Structural Characterization*

All CIPS crystals were purchased from 6Carbon Technology (Shenzhen, China). The purchased synthetic bulk crystals were mechanically exfoliated onto platinized silicon substrates to obtain the flakes. The flake thickness was measured with a three-dimensional (3D) laser confocal microscope (VK-X1100, KEYENCE CORPORATION., Itasca, IL, USA) and an atomic force microscope (AFM, Cypher S, OXFORD INSTRUMENTS, Abingdon, Oxfordshire, UK). The microscope objective lenses used in our experiments are ×5, ×10, ×20, ×50, ×100, and the microscopic ocular is ×5. The numerical aperture and working distance of the microscope objective lens with ×100 magnification are 0.3 mm and 4.7 mm, respectively. The confocal Raman system (LabRAM HR, HORIBA Instuments(SHANGHAI) co. LTD, Shanghai, China) with 532 nm laser excitation was used to collect Raman spectra. The morphologies and structures of the as—prepared samples were collected by transmission electron microscopy (TEM, JEM-3200FS, JEOL (BEIJING) Co., Ltd. GUANGZHOU BRANCH, Beijing, China).

### *2.2. Piezoelectric Force Microscopy (PFM) Measurements*

The PFM measurements were performed on μm—sized CIPS flakes on the crystal surface. The silver paste was used to attach the sample to the sample stage, which served as an electric back contact. PFM image scanning measurements were performed using a piezoelectric force microscope (PFM) with Au- and diamond-coated Si cantilever tips (FM-LC, 100 kHz and 8 N/m), respectively. The cantilever tip acted as a local variable electrode. The electric voltage was applied to the sample surface via the conductive PFM tip. The sample was cleaved using Scotch tape directly before the measurements. The topography of CIPS flakes was probed in AC mode, while the piezoelectric and ferroelectric responses were measured using dual AC resonance tracking PFM (DART-PFM) mode. The local piezoresponse hysteresis loops were measured 10 times at each position at multiple different arbitrary points.
