No abstract

Metal thiophosphates are attracting growing attention in the context of quasi-two-dimensional van der Waals functional materials. Alkali thiophosphates are investigated as ion conductors for solid electrolytes, and transition-metal thiophosphates are explored as a new class of ferroelectric materials. For the latter, a representative copper indium thiophosphate is ferrielectric at room temperature and, despite low polarization, exhibits giant negative electrostrictive coefficients. Here, we reveal that ionic conductivity in this material enables localized extraction of Cu ions from the lattice with a biased scanning probe microscopy tip, which is surprisingly reversible. The ionic conduction is tracked through local volume changes with a scanning probe microscopy tip providing a current-free probing technique, which can be explored for other thiophosphates of interest. Nearly 90 nm-tall crystallites can be formed and erased reversibly on the surface of this material as a result of ionic motion, the size of which can be sensitively controlled by both magnitude and frequency of the electric field, as well as the ambient temperature. These experimental results and density functional theory calculations point to a remarkable resilience of CuInP 2 S 6 to large-scale ionic displacement and Cu vacancies, in part enabled by the metastability of Cu-deficient phases. Furthermore, we have found that the piezoelectric response of CuInP 2 S 6 is enhanced by about 45% when a slight ionic modification is carried out with applied field. This new mode of modifying the lattice of CuInP 2 S 6 , and more generally ionically conducting thiophosphates, posits new prospects for their applications in van der Waals heterostructures, possibly in the context of catalytic or electronic functionalities.

No abstract

A material with reversible temperature change capability under an external electric field, known as the electrocaloric effect (ECE), has long been considered as a promising solid-state cooling solution. However, electrocaloric (EC) performance of EC materials generally is not sufficiently high for real cooling applications. As a result, exploring EC materials with high performance is of great interest and importance. Here, we report on the ECE of ferroelectric materials with van der Waals layered structure (CuInP2S6 or CIPS in this work in particular).Over 60% polarization charge change is observed within a temperature change of only 10 K at Curie temperature. Large adiabatic temperature change (|ΔT|) of 3.3 K, isothermal entropy change (|ΔS|) of 5.8 J kg -1 K -1 at |ΔE|=142.0 kV cm -1 at 315 K (above and near room temperature) are achieved, with a large EC strength (|ΔT|/|ΔE|) of 29.5 mK cm kV -1 . The ECE of CIPS is also investigated theoretically by numerical simulation and a further EC performance projection is provided.Electrocaloric refrigerators using electrocaloric materials are low noise, environmentfriendly and can be scaled down to small dimensions, compared to the common vaporcompression refrigerators. 1-13 Electrocaloric cooling is also much easier and lower cost to realize compared to other field induced cooling techniques such as magnetocaloric and mechanocaloric cooling, because the electric field is easily to be realized and accessible. Thus, electrocaloric effect is promising for future cooling applications, especially in micro-or nano-scale such as onchip cooling. Electrocaloric effect in ferroelectric materials is of special interest because of the large polarization change near the ferroelectric-paraelectric (FE-PE) phase transition temperature 21 TOC.

No abstract

The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we quantitatively unravel giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), which exhibits an electrostrictive coefficient Q33 as high as -3.2 m 4 /C 2 and a resulting bulk piezoelectric coefficient d33 up to -85 pm/V. As a result, the electromechanical response of CIPS is comparable in magnitude to established perovskite ferroelectrics despite possessing a much smaller spontaneous polarization of only a few µC/cm 2 . In the paraelectric state, readily accessible owing to low transition temperatures, CIPS exhibits large dielectric tunability, similar to widely-used barium strontium titanate, and as a result both giant and continuously tunable electromechanical 3 response. The persistence of electrostrictive and tunable responses in the paraelectric state indicates that even few layer films or nanoparticles will sustain significant electromechanical functionality, offsetting the inevitable suppression of ferroelectric properties in the nanoscale limit. These findings can likely be extended to other ferroelectric transition metal thiophosphates and (quasi-) two-dimensional materials and might facilitate the quest towards novel ultrathin functional devices incorporating electromechanical response.

Polar van der Waals chalcogenophosphates exhibit unique properties, such as negative electrostriction and multi-well ferrielectricity, and enable combining dielectric and 2D electronic materials. Using low temperature piezoresponse force microscopy, we revealed coexistence of piezoelectric and non-piezoelectric phases in CuInP 2 Se 6 , forming unusual domain walls with enhanced piezoelectric response. From systematic imaging experiments we have inferred the formation of a partially polarized antiferroelectric state, with inclusions of structurally distinct ferrielectric domains enclosed by the corresponding phase boundaries. The assignment is strongly supported by optical spectroscopies and density-functionaltheory calculations. Enhanced piezoresponse at the ferrielectric/antiferroelectric phase boundary and the ability to manipulate this entity with electric field on the nanoscale expand the existing phenomenology of functional domain walls. At the same time, phase-coexistence in chalcogenophosphates may lead to rational strategies for incorporation of ferroic functionality into van der Waals heterostructures, with stronger resilience toward detrimental size-effects.

The rapid development of spectral-imaging methods in scanning probe, electron, and optical microscopy in the last decade have given rise for large multidimensional datasets. In many cases, the reduction of hyperspectral data to the lower-dimension materialsspecific parameters is based on functional fitting, where an approximate form of the fitting function is known, but the parameters of the function need to be determined. However, functional fits of noisy data realized via iterative methods, such as least-square gradient descent, often yield spurious results and are very sensitive to initial guesses. Here, we demonstrate an approach for the reduction of the hyperspectral data using a deep neural network approach. A combined deep neural network/least-square approach is shown to improve the effective signal-to-noise ratio of band-excitation piezoresponse force microscopy by more than an order of magnitude, allowing characterization when very small driving signals are used or when a material's response is weak.