Cation-eutectic transition via sublattice melting in CuInP2S6/In4/3P2S6 van der Waals layered crystals

Single crystals of the van der Waals layered ferrielectric material CuInP2S6 spontaneously phase separate when synthesized with Cu deficiency. Here we identify a route to form and tune intralayer heterostructures between the corresponding ferrielectric (CuInP2S6) and paraelectric (In4/3P2S6) phases through control of chemical phase separation. We conclusively demonstrate that Cu-deficient Cu1–xIn1+x/3P2S6 forms a single phase at high temperature. We also identify the mechanism by which the phase separation proceeds upon cooling. Above 500 K both Cu+ and In3+ become mobile, while P2S64– anions maintain their structure. We therefore propose that this transition can be understood as eutectic melting on the cation sublattice. Such a model suggests that the transition temperature for the melting process is relatively low because it requires only a partial reorganization of the crystal lattice. As a result, varying the cooling rate through the phase transition controls the lateral extent of chemical domains over several decades in size. At the fastest cooling rate, the dimensional confinement of the ferrielectric CuInP2S6 phase to nanoscale dimensions suppresses ferrielectric ordering due to the intrinsic ferroelectric size effect. Intralayer heterostructures can be formed, destroyed, and re-formed by thermal cycling, thus enabling the possibility of finely tuned ferroic structures that can potentially be optimized for specific device architectures

The strain-induced transitions of the piezoelectric, pyroelectric and electrocaloric properties of the CuInP2_2S6_6 films

The low-dimensional ferroelectrics, ferrielectrics and antiferroelectrics are of urgent scientific interest due to their unusual polar, piezoelectric, electrocaloric and pyroelectric properties. The strain engineering and strain control of the ferroelectric properties of layered 2D Van der Waals materials, such as CuInP$_2$(S,Se)$_6$ monolayers, thin films and nanoflakes, are of fundamental interest and especially promising for their advanced applications in nanoscale nonvolatile memories, energy conversion and storage, nano-coolers and sensors. Here, we study the polar, piezoelectric, electrocaloric and pyroelectric properties of thin strained films of a ferrielectric CuInP$_2$S$_6$ covered by semiconducting electrodes and reveal an unusually strong effect of a mismatch strain on these properties. In particular, the sign of the mismatch strain and its magnitude determine the complicated behavior of piezoelectric, electrocaloric and pyroelectric responses. The strain effect on these properties is opposite, i.e., "anomalous", in comparison with many other ferroelectric films, for which the out-of-plane remanent polarization, piezoelectric, electrocaloric and pyroelectric responses increase strongly for tensile strains and decrease or vanish for compressive strains.Comment: 16 pages, 5 figures, to be presented at the VI Lithuanian-Polish Meeting on Physics of Ferroelectric

Screening-Induced Phase Transitions in Core-Shell Ferroic Nanoparticles

Using the Landau-Ginzburg-Devonshire approach, we study screening-induced phase transitions in core-shell ferroic nanoparticles for three different shapes: an oblate disk, a sphere, and a prolate needle. The nanoparticle is made of a ferroic CuInP2S6 core and covered by a "tunable" screening shell made of a phase-change material with a conductivity that varies as the material changes between semiconductor and metallic phases. We reveal a critical influence of the shell screening length on the phase transitions and spontaneous polarization of the nanoparticle core. Since the tunable screening shell allows the control of the polar state and phase diagrams of core-shell ferroic nanoparticles, the obtained results can be of particular interest for applications in nonvolatile memory cells.Comment: 22 pages, 6 figures, 1 Appendi

Anomalous Polarization Reversal in Strained Thin Films of CuInP2_2S6_6

Strain-induced transitions of polarization reversal in thin films of a ferrielectric CuInP$_2$S$_6$ (CIPS) with ideally-conductive electrodes is explored using the Landau-Ginzburg-Devonshire (LGD) approach with an eighth-order free energy expansion in polarization powers. Due to multiple potential wells, the height and position of which are temperature- and strain-dependent, the energy profiles of CIPS can flatten in the vicinity of the non-zero polarization states. This behavior differentiates these materials from classical ferroelectrics with the first or second order ferroelectric-paraelectric phase transition, for which potential energy profiles can be shallow or flat near the transition point only, corresponding to zero spontaneous polarization. Thereby we reveal an unusually strong effect of the mismatch strain on the out-of-plane polarization reversal, hysteresis loops shape, dielectric susceptibility, and piezoelectric response of CIPS films. In particular, by varying the sign of the mismatch strain and its magnitude in a narrow range, quasi-static hysteresis-less paraelectric curves can transform into double, triple, and other types of pinched and single hysteresis loops. The strain effect on the polarization reversal is opposite, i.e., "anomalous", in comparison with many other ferroelectric films in that the out-of-plane remanent polarization and coercive field increases strongly for tensile strains, meanwhile the polarization decreases or vanish for compressive strains. We explain the effect by "inverted" signs of linear and nonlinear electrostriction coupling coefficients of CIPS and their strong temperature dependence. For definite values of temperature and mismatch strain, the low-frequency hysteresis loops of polarization may exhibit negative slope in the relatively narrow range of external field amplitude and frequency.Comment: 26 pages, including 8 figures and 1 Appendi

Bending-induced isostructural transitions in ultrathin layers of van der Waals ferrielectrics

Using Landau-Ginzburg-Devonshire (LGD) phenomenological approach we analyze the bending-induced re-distribution of electric polarization and field, elastic stresses and strains inside ultrathin layers of van der Waals ferrielectrics. We consider a CuInP2S6 (CIPS) thin layer with fixed edges and suspended central part, the bending of which is induced by external forces. The unique aspect of CIPS is the existence of two ferrielectric states, FI1 and FI2, corresponding to big and small polarization values, which arise due to the specific four-well potential of the eighth-order LGD functional. When the CIPS layer is flat, the single-domain FI1 state is stable in the central part of the layer, and the FI2 states are stable near the fixed edges. With an increase of the layer bending below the critical value, the sizes of the FI2 states near the fixed edges decreases, and the size of the FI1 region increases. When the bending exceeds the critical value, the edge FI2 states disappear being substituted by the FI1 state, but they appear abruptly near the inflection regions and expand as the bending increases. The bending-induced isostructural FI1-FI2 transition is specific for the bended van der Waals ferrielectrics described by the eighth (or higher) order LGD functional with consideration of linear and nonlinear electrostriction couplings. The isostructural transition, which is revealed in the vicinity of room temperature, can significantly reduce the coercive voltage of ferroelectric polarization reversal in CIPS nanoflakes, allowing for the curvature-engineering control of various flexible nanodevices.Comment: 26 pages, 7 figures and Appendices A-

Stress-Induced Transformations of Polarization Switching in CuInP2_2S6_6 Nanoparticles

Using the Landau-Ginzburg-Devonshire approach, we study stress-induced transformations of polarization switching in ferrielectric CuInP2S6 nanoparticles for three different shapes: a disk, a sphere, and a needle. Semiconducting properties of a nanoparticle are modeled by a surface charge layer, whose effective screening length can be rather small due to the field-effect. We reveal a very strong and unusual influence of hydrostatic pressure on the appearance of polarization switching in CuInP2S6 nanoparticles, hysteresis loops shape, magnitude of the remanent polarization, and coercive fields, and explain the effects by the anomalous temperature dependence and "inverted" signs of CuInP2S6 linear and nonlinear electrostriction coupling coefficients. In particular, by varying the sign of the applied pressure (from tension to compression) and its magnitude (from zero to several hundreds of MPa), quasi-static hysteresis-less paraelectric curves can transform into double, triple, pinched, or single hysteresis loops. Due to the sufficiently wide temperature and pressure ranges of double, triple, and pinched hysteresis loop stability (at least in comparison with many other ferroelectrics), CuInP2S6 nanodisks can be of particular interest for applications in energy storage (in the region of double loops), CuInP2S6 nanospheres maybe suitable for dynamic random access multibit memory, and CuInP2S6 nanoneedles are promising for non-volatile multibit memory cells (in the regions of triple and pinched loops). The stress control of the polarization switching scenario allows the creation of advanced piezo-sensors based on CuInP2S6 nanocomposites.Comment: 43 pages, 8 figures, including Supplementary Material with 12 figure

Ferroelectric Materials for Synaptic Transistors and Their Neuromorphic Applications

After more than a hundred years of development, ferroelectric materials have demonstrated their strong potential to people, and more and more ferroelectric materials are being used in the research of ferroelectric transistors (FeFETs). As a new generation of neuromorphic devices, ferroelectric materials have attracted people's attention due to their powerful functions and many characteristics. This article summarizes the development of ferroelectric material systems in recent years and discusses the simulation of artificial synapses. The mainstream ferroelectric materials are divided into traditional perovskite structure, fluorite structure, organic polymer, and new 2D van der Waals ferroelectricity. The principles, research progress, and optimization for brain like computers of each material system are introduced, and the latest application progress is summarized. Finally, the scope of application of different material systems is discussed, with the aim of helping people screen out different material systems based on different needs.Comment: 44 pages, 7 figures

Influence of van der Waals interfaces on electrical and optical properties of two-dimensional materials

The interactions between two-dimensional (2D) materials and surrounding surfaces are critical for electronic devices. Interfacial effects become increasingly important as electronic devices continue to shrink in size. This thesis investigates the influence of van der Waals (vdW) interfaces on electrical and optical properties of 2D materials. I began by studying the nucleation and growth of indium and gold metal films on 2D materials such as molybdenum disulfide (MoS2) and graphene. Indium and gold metal films were chosen because previous research in the group has demonstrated that indium forms vdW interfaces with 2D materials while gold forms defective non-vdW interfaces. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that indium metal deposition on MoS2 and graphene follows a 2D growth mechanism to form large grains. The layer-dependent study of MoS2 and graphene shows that increasing the number of layers reduces the influence of SiO2 substrate roughness so that the diffusivity and diffusion length of indium atoms increase and the activation energy for indium atom diffusion decreases - as reflected in the fivefold decrease in nucleation density (~12 μm-2 ). The influence of vdW interface between graphene and ferroelectric CuInP2S6 (CIPS) was also explored. Graphene field effect transistors (FETs) with CIPS as the top gate were studied. Polarisation-induced hysteresis in CIPS-gated graphene FETs was observed – indicating the realisation of ferroelectric FETs (FeFETs). We show that interfacial remote doping influences the macroscopic polarisation and performance of CIPS-based graphene FeFETs. Finally, Raman and photoluminescence (PL) measurements on MoS2 with indium metal film of thicknesses ranging from 2 to 25 nm were performed. The results suggest that the vdW gap between indium and MoS2 leads to a strong enhancement of Raman and PL signals. Strong Raman enhancement was also observed in graphene. In the absence of vdW gap with gold film, the plasmon-mediated enhancement of Raman and PL signals was not observed