CuInP2S6 (CIPS) is an emerging 2D ferroelectric material known for disrupting spatial inversion symmetry due to Cu(I) position switching. Its ferroelectricity strongly relies on the Cu(I) atom/ion occupation ordering and dynamics. Nevertheless, the accurate Cu(I) occupations and correlated migration dynamics under the electric field, which are key to unlocking ferroelectric properties, remain controversial and unresolved. Herein, an atomic-level direct imaging through aberration-corrected scanning transmission electron microscopy is performed to precisely trace the Cu(I) dynamic behaviours under electron-beam irradiation along (100)-CIPS. It clearly demonstrates that Cu(I) possesses multiple occupations, and Cu(I) could migrate to the lattice, vacancy and interstitial sites between the InS6 octahedral skeletons of CIPS to form local CuxInP2S6 (x = 2–3) structure. Cu(I) multi-occupations induced lattice stress results in a layer sliding along the b-axis direction with generating a sliding size of 1/6 b axis. The CuxInP2S6 (x = 2–3) exists in a type of dynamic structure, only metastable with electron dose over 50 e− Å−2, thus generating a dynamic process of CuxInP2S6 (x=2-3) ⇌ CuInP2S6, a completely new phenomenon. These findings shed light on the novel mechanism underlying the Cu(I) migration in CIPS, providing crucial insights into the fundamental processes governing its ferroelectric properties.
Article
Atomic-level direct imaging for Cu(Ⅰ) multiple occupations and migration in 2D ferroelectric CuInP2S6
https://doi.org/10.21203/rs.3.rs-4487714/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted
You are reading this latest preprint version
ion migration
CuInP2S6
ferroelectric materials
crystal structure
2D Van der Waals ferroelectric materials are emerging with intriguing physical and quantum phenomena for a broad span of applications in e.g. field-effect transistors (FETs), non-volatile memory, ferroelectric tunnel junctions (FTJs), and beyond. A prominent representative one is 2D monoclinic CuInP2S6 (CIPS). It manifests room-temperature and tunable quadruple-well ferroelectricity,1–4 and therefore, many electronic devices based on CIPS have been rapidly developed.5–9 With regard to the ferroelectric polarizations, great efforts have been devoted, with having been uncovering the dominated role from the lattice Cu(I) site occupations. To clearly illustrate it, Fig. 1 demonstrates the Cu(Ⅰ) occupation and polarization characteristics. Cu(Ⅰ) ions are usually located between two InS6 octahedral skeletons (positions I or III in Fig. 1a). The InS6 skeletons with Cu(Ⅰ) locked inside combine with P-P ion pairs to form a continuous layer. The layers are connected by Van der Waals forces stacking in the ABAB stack, allowing it to reach the low dimensional limit by separating layers from each other experimentally.
The primitive ferroelectricity of 2D CIPS originates from the spatial inversion symmetry breaking with the Cu(Ⅰ) ions occupying either I or III position (Fig. 1a) where the feasible mobility of Cu(Ⅰ) ions within the in-plane (IP) and out-of-plane (OOP) orientations in CIPS crystal structure also happens. But more crucially, when an external field (e.g. electric field, stress, thermal, etc.) is applied, the Cu(I) occupations may switch to align the polarization direction with the electric field direction (e.g. all the Cu(I) ions switch to position III to make the polarization is up when an upward electric field applied in Fig. 1a). This is related to the Cu(I) migration and similar phenomena have been well explored in CIPS. However, several fundamental questions raise, e.g. how many possible positions that could accommodate Cu(I) switching or additional migration? Are there any other Cu(I) occupations except the I and III positions (Fig. 1a)? Whether Cu(I) possibly migrates to occupy the interstitial site (position II in Fig. 1a) or interlayer site (position IV in Fig. 1a) exists or not?10,11 Definitely, these basically structural and physical pictures are fundamentally important to understanding the ferroelectric nature and polarization switching of CIPS.
There are many studies on Cu(I) migration and associated mechanisms, mainly including two categories: (1) Cu(I) ions migration and ferroelectric polarization switching under the external electric field. As shown in Fig. 1b, both OOP and IP modes (i.e. applying external electric field styles) have been performed. Under OOP migration path (Fig. 1b-i), some Cu(I) ions could be occasionally trapped in the vdW gap and even extracted out of the lattice because of the coexistence of polarization switching and ionic conduction.12 Under IP migration path (Fig. 1b-ii), Cu(I) ions are proven to move along the electric field by in-situ scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements.13 The Cu(I) ions migration toward the negative electrode leads to a typical diode-like resistive switching behavior in the homojunction device.14 Further, the robust threshold-switching behavior assisted by Cu(I) migration is also found in CIPS heterostructure even if using asymmetrical electrodes (e.g. graphene, Au vs. Cu).15 To conclude, the Cu(I) ions migration under the external electric field is a universal behavior unaffected by the electrodes. (2) Ferroelectric domain distributions and Cu(I) ions location under the flexoelectric effect. Polarization switching based on the flexoelectric effect can effectively avoid triggering longer-range ionic redistribution (Cu(I) ions just move their positions confined between InS6 octahedral skeletons). As depicted in Fig. 1c, when stress is applied on OOP mode, the free energies for occupying position up and position down are not equal, and Cu(I) ions tend to move and occupy positions in the direction gradient, thus exhibiting polarization (Pup and Pdown in Fig. 1c). The synergistic effect of Cu ion migration and interfacial Schottky barrier controls the current and device performance.16–21
These above studies combine with the existing cognition inferred from multiple characterizations have played a great role in promoting the understanding of the Cu(I) migration ferroelectric polarization. However, the basically physical picture with showing accurate Cu(I) lattice-site occupations and Cu(I) configurations to the surrounding arrangement in CIPS is far beyond established. In particular, in addition to the primitive Cu(I) lattice positions I and III (Fig. 1a), whether the positions II and IV (Fig. 1a) that are only predicted theoretically really occur and how they contribute to the observed overall ferroelectricity are also the fundamental questions to be resolved.
Herein, an atomic-level direct imaging to precisely trace the Cu(I) migration and its local dynamics in CIPS is performed through aberration-corrected scanning transmission electron microscopy (AC-STEM). The cleavage plane (100) based direction (i.e. a-axis direction) is chosen for the imaging view because the out-of-(100) plane best reflects the atom occupying situation as well as the clear Cu(I) migration along c direction (Fig. 1d). Accordingly, the configuration and moving dynamics (speed, energy, and structural stability) of Cu(I) ions in the CIPS under an electric field (i.e. electron beam irradiation) are well monitored, with finding the presence of Cu(I) interstitial occupations i.e. positions II presented in Fig. 1a. The Cu(I) outside the field of view converges towards the irradiation location and crowds in the possible location of Cu(I), eventually forming an unstable CuxInP2S6 (x = 2–3) local structure.
Atomic-level imaging observation along (100) plane
The pivotal criterion to get accurate and sufficient atomic-level structural information on Cu(I) occupations is the exposed plane aligning flawlessly with a-axis, i.e. (100) cleavage plane, as proposed in Fig. 1d. For this, the as-grown CIPS single crystal is precisely cut into nanosheet along a-axis by focused-ion-beam (FIB) technology (Figure S1). The low-dose integrated differential phase-contrast coupled scanning transmission electron microscopy (iDPC-STEM) is performed to achieve atomic-level imaging. Figure 2a shows a dark field (DF) mode image where all the Cu(I)/In/P/S atoms and their positions are clearly seen. Strikingly, the atoms between the InS6 octahedral skeletons are not solitary, but instead there occur obvious multiple lattice sites for Cu(I) occupations, e.g. Cu(I) ions occupy both position I and III (Fig. 2b-i); Cu(I) ions occupy simultaneously position I, II and III (Fig. 2b-ii). For convenience, we here refer to the local crystal configuration situation of Region I and Region II by Cu2InP2S6 and Cu3InP2S6, respectively. It is worth noting that the local structure CuxInP2S6 (x = 2, 3) only occurs in a tiny region but does not change the overall chemical formula of the CuInP2S6 single-crystal. The structural configurations with forming local CuxInP2S6 (x = 2, 3) can be universally observed from different areas or regions in CIPS (Figure S2), which is definitely attributed to the Cu(I) migration and multiple occupations under the electric field.
To gain the insights on the microscopical results, standard atomic structure and lattice occupations are compared. The theoretical angular/distance relationship between adjacent In ions, as the (001) plane demonstrated in Fig. 2c, is ∠ABC = 59.969 º (~ 60 º) and BC = 0.6099 nm, respectively. Experimentally, the projected distance between the two nearest In ions in a layer from the (100) plane is 0.5500 nm (highlighted by red line in Fig. 2a), which yields an actual distance of 0.55/cos(59.969/2) = 0.6350 nm according to the geometric relationship. This value is 4.12% larger than the theoretical In-In distance (i.e. BC = 0.6099 nm in Fig. 2c), which suggests the existence of the multiple Cu(I) occupations between the InS6 octahedral skeletons that thus leads to lattice expansion in the < 010 > direction. The occurrence of the atomic multi-occupation and lattice constant increase is to release the local stress and maintain the structural stability when the electron-beam irradiation continues.
Atomic distance relationships for P-P in one layer and In-In between adjacent two layers are further analyzed. As shown in Fig. 2d, the theoretical P-P distance is 0.2218 nm, almost analogous to the experimental value of 0.2256 nm (highlighted by blue line in Fig. 2a). The theoretical In-In distance between the nearest layers in Fig. 2e is 0.6812 nm, while the experimentally measured distance is 0.6812 nm (highlighted by yellow line in Fig. 2a). It is worth noting that the lattice constant c (= 1.3524 nm) experimentally determined also matches well the theoretical one (1.3623 nm). These percentage differences, i.e. 1.7% in P-P, 0.0058% in In-In in layers, and − 0.73% in lattice constant c, respectively, are small enough to be considered no variations in the c-axis direction. To conclude, the Cu(I) atomic multi-occupations do not cause the out-of-ab-plane structural fluctuations but apparent in-ab-plane structural expansion. Then, what is the structural reason?
This can be attributed to the sliding mis-arrangements for the CIPS layers. As indicated in Fig. 2f, the theoretical In ions between the CIPS layers should be arranged in a straight line from the perspective of the (100) plane, but the microscopical observation clearly exhibits that the In ions in CIPS layers are not situated in a straight line along the c-axis direction. The array paths of In ions are shown in Fig. 2b-iii dotted orange lines, which is a broken line, containing lines segment parallel to the c-axis. Accordingly, the sliding boundary position can be deduced, which is rightly the interface of the two adjacent CIPS layers, as indicated by the green line in Fig. 2a. This suggests that the smallest unit in which the glide occurs is a single cell, without compromising its double-layer structure. The sliding distance between the layers is carefully confirmed, and its magnitude is 1/6 lattice constant b. This further confirms that in order to keep the stability of the crystal structure induced by the Cu(I) multiple occupations, the interlayer sliding releases the lattice stress but does not cause changes in lattice constant c.
In addition to the atom positions or occupations, more atomic-level information e.g. atomic distribution/intensity/shift etc. are further extracted. To this, precise analysis and transformation are performed on the iDPC-STEM images (Figs. 3a, S3) by optimized Multiple-Ellipse Fitting method with the detailed calculations are seen in Supporting information section (Figures S3-7).22 The noise reduction image in Figure S3a is obtained by BM3D method and the position of the atoms are determined by threshold method.23 Fig. 3a and Fig. 3b show respectively the pseudo-color display raw image and fitting image that exhibit quite high matchiness, thus giving a very small difference in intensity, morphology and strength errors (Figs. 3c, S3) with all the fitting parameters can be obtained in Table S1). Accordingly, the 2D intensity distributions of Cu, In, P, and S based on the elemental mapping and statistical analysis (corresponding to the highlighted area in Fig. 3d, Figure S8) are deduced, as shown in Fig. 3e, and meanwhile, the distribution of the atom shift is also obtained (Fig. 3f).
With regard to the Cu(I) occupation, Cu(I) ions are present between the InS6 octahedral skeletons in the form of CuxInP2S6 (x = 2–3) local structure. As a matter of fact, the STEM image contrast predominantly depends on the atomic number (Z) and the atom count.24,25 The intensities of Cu(I) are primarily distributed in 5–7×106 and 9–16×106 for positions II and I & III, respectively in the intensity distribution histogram of Fig. 3e. These results show that Cu(I) in CIPS have their own filling rules: Under electron irradiation, Cu(I) will preferentially occupy positions I and III to form Cu2InP2S6 local structure. When positions 1 and 3 are filled, Cu(I) then will occupy position II to further form Cu3InP2S6 local structure. These results are very significant for the understanding of the kinetic process of copper ion occupation in CIPS.
The Cu(I) preferential occupation might induce the atom/ion shift/displacement in CIPS crystal structure, shown in Fig. 3f, which favors the understanding of lattice distortion and polarization. There occur two types of the atom shift (the shift vectors are highlighted by yellow arrows). One shifts along the < 010 > directions ([010] or [0–10]) distinguished from the middle position, which contributes to the lattice distortion by atomic multiple occupancies between the InS6 octahedral skeletons. The other shifts along < 001 > direction, which is due to the intrinsic out-of-plane (OOP) polarization in the lattice. Because the atomic shift vector in < 001 > direction is overall upward, it does not affect the lattice constant of the c-axis. The fitting result corresponding to the iDPC-STEM image indicates the dependability of the structural evolution process of Cu(I) multiple occupancies between the InS6 octahedral skeletons of CIPS under the electron-beam irradiation along < 100 > direction.
Dynamic and stability of Cu x InP 2 S 6 (x = 2–3) local structure
The above precise iDPC-STEM imaging and analysis prove the Cu(I) multiple occupations and the structural changes of CIPS under electron irradiation, but the dynamic process and stability for the local structural configurations on CuxInP2S6 (x = 2–3) remain to be investigated clearly. Herein, a facile strategy is devised to observe the CuxInP2S6 (x = 2–3) configurations. Specifically, a large current (50 pA) is used to irradiate partial region (red box in Fig. 4a) for 30 seconds, and EDS mapping is then carried out over a larger area with a small current (33 pA). The CuxInP2S6 (x = 2–3) configurations can be judged by the fluctuations in elemental contents in the red box. The resulting EDS mapping (Fig. 4b) shows that there is no clear distinction of element contrast. Another similar experiment by line scanning (Fig. 4c, d) for element mapping also gives the same conclusion, i.e. the electron-irradiated area exhibits the uniform elemental distribution. As a matter of fact, the Cu(I) contents (26.01% for Fig. 4a and 29.52% for Fig. 4c) are much higher than the theoretical value from the standard structure (Table S2 and Table S3), which means the occurrence of the Cu(I) multi-occupancy with forming CuxInP2S6 (x = 2–3). However, the CuxInP2S6 (x = 2–3) is quite unstable because of undergoing the extremely severe lattice distortion, unless continuous injection energy (electron-beam irradiation) maintains CuxInP2S6 (x = 2–3) configurations. Otherwise, when the energy (electron-beam irradiation) is removed, the Cu(1) would move back to its original position to retain the standard CuInP2S6 structure (Figure S9). This dynamic process that clearly uncovers locally structural configuration fluctuations i.e.
is obviously a result of the Cu(I) migration under the electric field behavior.
The Cu(I) migration dynamics is further investigated by monitoring the elemental fluctuations during the continuous electron-beam irradiations. To figure out the energy dependence or threshold for the Cu(I) migration, the varied current dose rates are performed. Here Fig. 4e-h only show the cases of two typical current rates, i.e. 30 pA and 72 pA. Note that too small dose (e.g. 2 pA) can not drive the Cu(I) migration (Figure S9) while too large dose makes Cu(I) migration too quick for inaccurate detection. It can be seen that the detected elemental amounts exhibit different changes with increasing the irradiation time. Clearly from both the EDS imaging (Fig. 4e) and quantitive analysis (Fig. 4g), the Cu(I) amount gradually increases with the irradiation time at a low current (30 pA), from initial 25.9% to final 27.09% (irradiation for 30 mins) and keeping unchanged upon longer irradiation in atomic proportion. While, high current irradiation e.g. 72 pA (Figs. 4f and 4h), might be due to the quite fast Cu(I) migration, does not bring about any apparent fluctuations in elemental distributions that nearly undergo unchanged at about 28%.
The energy injected for Cu(I) migration induced crystal structure distortion is further quantified, which is referred to the current dose (D) that can be obtained by Eq. (1) is 50 e− Å−2 and 120 e− Å−2 for 30 pA (Fig. 4e) and 72 pA (Fig. 4f), respectively. Although the energy dose increases doubly, the Cu(I) content is not increased all the way, but finally fixed at about 28%. This is, again, indicative of the Cu(I) migration but the available positions in the lattice sites (i.e. positions I, II, III, IV illustrated in Fig. 1a) for Cu(I) occupation are in a certain level. Even if forming the Cu3InP2S6 structural configuration, Cu(I) has an atomic percentage content of 25%, still less than the value obtained from experimental determinations. One possibility is Cu(I) may occupy the interlayer position IV (Fig. 1a) that has been predicted by the theoretical calculation, but here from the atomic-level microscopy experiment, no direct evidence that Cu(I) could occupy the interlayer position.
The ferroelectricity of CuInP2S6 (CIPS) strongly relies on the Cu(I) atom/ion occupation ordering and dynamics. When Cu(I) only occupies either the position I or position III (Fig. 1a) orderly, the spatial inversion symmetry is broken, thus exhibiting apparent ferroelectricity. Such a state is structurally and thermally stable at room temperature that has been proven by various experimental and theoretical studies. Up to date, there is no doubt on the Cu(I) migration, but the direct evidences especially the atomic-level imaging that demonstrate the specific occupation sites as well as the locally structural configurations under the electric field are not explored previously. Therese questions are fundamental for understanding ferroelectric mechanism and exploiting ferroelectric-based applications.
This room-temperature ferroelectricity must allow the occurrence of the intrinsic and ordered Cu(I) vacancy (conversely, position III/I vs. occupied I/III), and meanwhile, endow the adjacent Cu(I) a chance for migrating to occupy the Cu(I) vacancy, which is structurally responsible of the Cu(I) migration. When the Cu(I) has migrated to occupy the vacancy, new locally structural configurations would form, with the theoretically possible configurations could be CuxInP2S6 (x = 2–4). Here, we noted that no any traces from our results indicate the Cu(I) could migrate to the interlayer site (Position IV in Fig. 1a), despite of this possibility has been predicted by John A. Brehm et al. that Cu(I) will migrates to the interlayer site (position IV in Fig. 1a) and bond with the S atoms in the next layers under the external field to reveal tunable quadruple-well ferroelectric by DFT calculation,4 so it only leaves the possibility of the CuxInP2S6 (x = 2–3) that have been observed in our experiment. When the three positions I/II/III (Fig. 1a) are simultaneously occupied (Fig. 2b-ii) with forming local Cu3InP2S6, or both positions I and III (Fig. 1a) are occupied (Fig. 2b-i, Cu2InP2S6), these formed locally structural configurations should not exhibit the ferroelectricity because the occupied Cu(I) causes the structural symmetry. The case of the Cu(I) occupies the interstitial site (position II) and one of lattice sites (positions I/III) but not simultaneously occupies vacancy site should also generate/maintain the ferroelectricity because the spatial inversion symmetry is still broken, but unfortunately, it is not observed by current atomic imaging that may be due to the Cu(I) fast migration effect. Anyway, these locally structural configurations exist only at a very tiny scale instead of generating long-range migration, as indicated from the relatively small change in Cu(I) atom proportion (Fig. 4), which should not influence the ferroelectricity to a large extent.
In summary, through iDPC-STEM direct imaging on (100)-CIPS, we obtained the atomic-level structure clearly. We confirm that Cu(I) possesses multiple occupations (lattice, vacancy and interstitial sites) and Cu(I) will migrate to positions I, III, and II between the InS6 octahedral skeletons of CIPS to form local CuxInP2S6 (x = 2–3) structure when electron-beam irradiation along < 100 > direction. As a result of the Cu(I) multiple-occupation, lattice distortion occurs for stress release to induce layer sliding along the b-axis direction with generating a sliding size of 1/6 b axis. The dynamic process of the locally formed CuxInP2S6 (x = 2–3) configurations monitored as a function of the irradiation times and currents indicates that CuxInP2S6 (x = 2–3) is an unstable structure, which just exists by external energy injection (i.e. electron-beam irradiation), but it will return back to standard CuInP2S6 immediately when the external energy vanishes. With new findings of Cu(I) multiple occupations and migration dynamics, this work provides the deep insights on the Cu(1) migration origin and ferroelectric mechanism in CIPS, which are significant for guiding device design and performance improvement based on CIPS.
Synthesis of Single Crystal CuInP2S6
The raw materials and chemicals used in this study were sourced from reputable suppliers. Copper powder (99.9%), Indium powder (99.99%), and Sulfur powder (99.999%) were purchased from Zhongnuo New Material Co., LTD. Phosphorus particle (99.999%) was obtained from Sichuan Khaqikun Technology Co., LTD. I (99.8%) was procured from Macklin Co., LTD.
The CuInP2S6 crystals were synthesized by a chemical vapor transport method. The raw materials Cu, In, P, and S were ground into powder with a mortar for homogenization purposes according to the stoichiometric ratio and iodine was used as the transport agent (50 mg/25 mL). Then the mixture into the bottom of a quartz test tube (inner diameter, 16 mm; length, 30 cm) subjected to evacuation via a mechanical pump and sealed via a Partulab MRVS-1002 system. The temperature for the evaporation and crystallization zones was initially set at 650 ℃ and 750 ℃, respectively, and the chemical reaction was allowed to proceed for 2 days. Following this, the temperature in the evaporation and crystallization zones was adjusted to 700°C and 665°C, respectively. Then the process was continued for 5 days. Finally, cool down to room temperature at a cooling rate of 10 °/h.
Crystal structure characterizations
The CIPS crystal was cut along the (100) plane by Focus Ion Beam (FIB) double-beam electron microscopy (Thermofisher Helios 5 CX) for electron microscopy measurement. A thickness of 5 µm platinum is deposited on the surface of CIPS as a protective layer, leaving 1 µm remaining after cutting. iDPC-STEM imaging were performed on Thermofisher Spectra 300, equipped with Gatan 1069 camera for collecting EDS mapping data.
Plan-view STEM Imaging
The integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) imaging was conducted using Cs-corrected (S) TEM (Thermofisher Spectra 300), operated at 300 kV. The collection semi-angles for the high-angle annular dark field (HAADF) STEM images were 51–200 mrad and the accumulated electron dose D in iDPC-STEM imaging was calculated by
$$\text{D}=\frac{I\times t}{A}$$
1
Where I was the electron-beam current, t was the irradiation time, and A was the area of irradiation. The electron-beam current was 30 pA and 72 pA for Fig. 4e and Fig. 4f, respectively. The dwell time is 20 µs. The pixel size of Fig. 4e, f was 8.67 Å, and the electron dose D is about 50 e− Å−2 and 120 e− Å−2, respectively.
Analysis of atom shift in CIPS
iDPC-STEM images of CIPS were analyzed by CalAtom Software to extract the atomic position by multiple-ellipse fitting. Atomic position and species are determined by fitting the intensity of the iDPC image. The atomic shift is determined by comparing the fitting results with the standard CIPS structure.
- Maisonneuve V, Evain M, Payen C, Cajipe VB, MoliniC P (1995) Room-temperature crystal structure of the layered phase CuIInIIIP2S6. J Alloys Compd 218:157–164
- Belianinov A et al (2015) CuInP2S6 Room Temperature Layered Ferroelectric. Nano Lett 15:3808–3814. 10.1021/acs.nanolett.5b00491
- Liu F et al (2016) Room-temperature ferroelectricity in CuInP2S6 ultrathin flakes. Nat Commun 7. 10.1038/ncomms12357
- Brehm JA et al (2019) Tunable quadruple-well ferroelectric van der Waals crystals. Nat Mater 19:43–48. 10.1038/s41563-019-0532-z
- Neumayer SM et al (2020) Alignment of Polarization against an Electric Field in van der Waals Ferroelectrics. Phys Rev Appl 13. 10.1103/PhysRevApplied.13.064063
- Li P et al (2022) Electrostatic Coupling in MoS2/CuInP2S6 Ferroelectric vdW Heterostructures. Adv Funct Mater 32. 10.1002/adfm.202201359
- Taylor PD, Tawfik SA, Spencer MJS (2022) Ferroelectric van der Waals heterostructures of CuInP2S6 for non-volatile memory device applications. Nanotechnology 34. 10.1088/1361-6528/aca0a5
- Li W et al (2022) A Gate Programmable van der Waals Metal-Ferroelectric‐Semiconductor Vertical Heterojunction Memory. Adv Mater 35. 10.1002/adma.202208266
- Wu J et al (2020) High tunnelling electroresistance in a ferroelectric van der Waals heterojunction via giant barrier height modulation. Nat Electron 3:466–472. 10.1038/s41928-020-0441-9
- Maisonneuve V, Cajipe VB (1997) Ferrielectric ordering in lamellar CuInP2S6. Phys Rev B 56:10860–10868
- Zhang D et al (2021) Anisotropic Ion Migration and Electronic Conduction in van der Waals Ferroelectric CuInP2S6. Nano Lett 21:995–1002. 10.1021/acs.nanolett.0c04023
- Zhou Z et al (2023) Unconventional polarization fatigue in van der Waals layered ferroelectric ionic conductor CuInP2S6. Nat Commun 14:8254. 10.1038/s41467-023-44132-y
- Sun Y et al (2023) Internal ion transport in ionic 2D CuInP2S6 enabling multi-state neuromorphic computing with low operation current. Mater Today 66:9–16. 10.1016/j.mattod.2023.04.013
- Zhu H et al (2023) Highly Tunable Lateral Homojunction Formed in Two-Dimensional Layered CuInP2S6 via In-Plane Ionic Migration. ACS Nano 17:1239–1246. 10.1021/acsnano.2c09280
- Zhong Z et al (2023) Robust Threshold-Switching Behavior Assisted by Cu Migration in a Ferroionic CuInP2S6 Heterostructure. ACS Nano 17:12563–12572. 10.1021/acsnano.3c02406
- Chen C et al (2022) Large-Scale Domain Engineering in Two-Dimensional Ferroelectric CuInP2S6 via Giant Flexoelectric Effect. Nano Lett 22:3275–3282. 10.1021/acs.nanolett.2c00130
- Huang Y et al (2023) Cu+ Migration and Resultant Tunable Rectification in CuInP2S6. ACS Appl Electron Mater 5:5625–5632. 10.1021/acsaelm.3c00973
- Jiang X et al (2023) Strong Piezoelectricity and Improved Rectifier Properties in Mono- and Multilayered CuInP2S6. Adv Funct Mater 33. 10.1002/adfm.202213561
- Jiang X et al (2022) Manipulation of current rectification in van der Waals ferroionic CuInP2S6. Nat Commun 13. 10.1038/s41467-022-28235-6
- Liu Y et al (2023) Stress and Curvature Effects in Layered 2D Ferroelectric CuInP2S6. ACS Nano 17:22004–22014. 10.1021/acsnano.3c08603
- Ming W et al (2022) Flexoelectric engineering of van der Waals ferroelectric CuInP2S6. Sci Adv 8. 10.1126/sciadv.abq1232
- Zhang Q, Zhang LY, Jin CH, Wang YM, Lin F, CalAtom (2019) A software for quantitatively analysing atomic columns in a transmission electron microscope image. Ultramicroscopy 202:114–120. 10.1016/j.ultramic.2019.04.007
- Dabov K, Foi A, Katkovnik V, Egiazarian K (2007) Image denoising by sparse 3-D transform-domain collaborative filtering. IEEE Trans Image Process 16:2080–2095. 10.1109/tip.2007.901238
- Shibata N et al (2017) Electric field imaging of single atoms. Nat Commun 8. 10.1038/ncomms15631
- Seki T, Ikuhara Y, Shibata N (2021) Toward quantitative electromagnetic field imaging by differential-phase-contrast scanning transmission electron microscopy. Microscopy (Oxf) 70:148–160. 10.1093/jmicro/dfaa065
There is NO Competing Interest.