**1. Introduction**

Two-dimensional (2D) ferroelectric materials are gaining extensive attention. It can effectively improve device performance when applied to devices such as memory, capacitors, actuators, and sensors [1–4]. Nowadays, people regard such kinds of 2D materials as var der Waals (vdW) layered ferroelectric materials, which benefit from covalently bonded polar or non-polar monolayers by Van der Waals forces and exhibit ferroelectric properties. However, reports of ferroelectric 2D materials at room temperature are rare. Because of the depolarization field with decreasing thickness, there is an enormous challenge in maintaining ferroelectricity in ultrathin ferroelectric films. Ferroelectricity is remained elusive in the 2D material library [5,6] Van der Waals (vdW) layered ferroelectric materials has become a promising research branch in condensed matter physics [7,8], among which copper indium thiophosphate, CuInP2S6 (CIPS), is one of the most representative materials because of its room temperature ferroelectricity [9]. CIPS is promised to play an important role in nonvolatile memory. A recent experiment reported that the vdW ferroelectric tunnel junction

**Citation:** Jia, T.; Chen, Y.; Cai, Y.; Dai, W.; Zhang, C.; Yu, L.; Yue, W.; Kimura, H.; Yao, Y.; Yu, S.; et al. Ferroelectricity and Piezoelectricity in 2D Van der Waals CuInP2S6 Ferroelectric Tunnel Junctions. *Nanomaterials* **2022**, *12*, 2516. https://doi.org/10.3390/ nano12152516

Academic Editor: Goran Drazic

Received: 4 July 2022 Accepted: 15 July 2022 Published: 22 July 2022

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(FTJ) device based on CIPS achieved a high tunneling electroresistance (TER) ratio [10,11]. Quantum transport device simulations of an FTJ based on CIPS and graphene demonstrate that scaling of the ferroelectric layer thickness exponentially not only significantly builds up the ferroelectric tunneling ON current but also reduces the read latency, in addition to enabling the FTJs with CIPS bilayers or trilayers to read speed in nanoseconds [12]. There are also outstanding endurance and retention characteristics for FTJ devices [3].

It was reported that giant intrinsic negative longitudinal piezoelectricity was observed in 2D layered CIPS. Lu You et al. [12] tested the converse piezoelectric effects of poly(vinylidene difluoride (PVDF), CIPS, and lead zirconate titanate (PZT) using a piezoelectric microscope and concluded that the electromechanical properties of CIPS are the same as those of PVDF with a negative longitudinal piezoelectric effect but opposite to those of PZT. This abnormal electromechanical phenomenon is caused by the significant deformation sensitivity of the weak interlayer interaction and is mediated by the high displacive instability of Cu ions. Several groups have discussed the origins of negative piezoelectricity. Yubo Qi and Andrew M. Pappe [13] attributed the negative piezoelectricity to the "lag of Wannier center" effect by proposing a negative clamped-ion term in the low-dimensional layered materials. John A. Brehm et al. [14] combined first-principles calculations with local electromechanical material characterization. They predicted and verified the existence of a uniaxial quadruple potential well for Cu displacements achieved by the van der Waals gap in CIPS. This led to the explanation that the negative longitudinal piezoelectric coefficient stems from the low polarization and very high sensitivity to a strain of the Cu atoms within the layer. Due to the negative piezoelectricity, CIPS has more complex piezoelectric properties, which gives it potential application prospects for 2D vdW materials with the same complex piezoelectric behavior in calculation and energy conversion. Therefore, it is crucial to study the piezoelectric behavior of CIPS.

Recently, to achieve resistance changes by ferroelectric switching, a ferroelectric fieldeffect transistor (FeFET) has been proposed, which uses ferroelectric materials instead of the oxide layer in a FET [15–17]. The causesFeFET exhibits two resistive states due to the hysteresis of ferroelectric switching. Furthermore, ferroelectric tunneling junctions and ferroelectric diodes were also investigated. It is reported that the polarization-modulation of Schottky-like barriers realizes the resistive change [18,19]. Yet, the vast majority of these devices use conventional ferroelectric materials such as PbTiO3 and BaTiO3 [20]. Affected by the three-dimensional nature of the ferroelectric oxide lattices, it is necessary for epitaxially grown high-quality films to select the substrates with a small lattice mismatch [21]. This seriously limits the possible application of materials in ferroelectric heterostructure devices. Therefore, it may be fundamentally and practically beneficial to study weakly bonded non-oxide ferroelectric compounds. Beyond that, the pioneering work on graphene has attracted an intense search for other 2D materials [22,23].

In this work, the crystal structure and ferroelectricity in crystalline CIPS nanoflakes are investigated at room temperature. Thin layer crystals in their pristine state show ferroelectric domains that are visualized directly. Piezoresponse force microscopy (PFM) measurements show that the polarization is stable and switchable in different layers of CIPS, and a negative piezoelectric effect is observed. Furthermore, we observed the ion migration phenomenon in CIPS via electrochemical strain microscopy (ESM). Finally, to obtain insight into the nature of the TER phenomena in the CIPS—based FTJs, we investigated their current—voltage (*I*–*V*) characteristics with a conductive probe microscope.
