Flexoelectric Engineering of Bulk Photovoltaic Photodetector.

The bulk photovoltaic effect (BPVE) offers an interesting approach to generate a steady photocurrent in a single-phase material under homogeneous illumination, and it has been extensively investigated in ferroelectrics exhibiting spontaneous polarization that breaks inversion symmetry. Flexoelectricity breaks inversion symmetry via a strain gradient in the otherwise nonpolar materials, enabling manipulation of ferroelectric order without an electric field. Combining these two effects, we demonstrate active mechanical control of BPVE in suspended 2-dimensional CuInP2S6 (CIPS) that is ferroelectric yet sensitive to electric field, which enables practical photodetection with an order of magnitude enhancement in performance. The suspended CIPS exhibits a 20-fold increase in photocurrent, which can be continuously modulated by either mechanical force or light polarization. The flexoelectrically engineered photodetection device, activated by air pressure and without any optimization, possesses a responsivity of 2.45 × 10-2 A/W and a detectivity of 1.73 × 1011 jones, which are superior to those of ferroelectric-based photodetection and comparable to those of the commercial Si photodiode.

Multistate Ferroelectric Diodes with High Electroresistance Based on van der Waals Heterostructures.

Some van der Waals (vdW) materials exhibit ferroelectricity, making them promising for novel nonvolatile memories (NVMs) such as ferroelectric diodes (FeDs). CuInP2S6 (CIPS) is a well-known vdW ferroelectric that has been integrated with graphene for memory devices. Here we demonstrate FeDs with self-rectifying, hysteretic current-voltage characteristics based on vertical heterostructures of 10 nm thick CIPS and graphene. By using vdW indium-cobalt top electrodes and graphene bottom electrodes, we achieve a high electroresistance (on- and off-state resistance ratios) of ∼106, an on-state rectification ratio of 2500 for read/write voltages of 2 V/0.5 V, and a maximum output current density of 100 A/cm2. These metrics compare favorably with state-of-the-art FeDs. Piezoresponse force microscopy measurements show that stabilization of intermediate net polarization states in CIPS leads to stable multibit data retention at room temperature. The combination of two-terminal design, multibit memory, and low-power operation in CIPS-based FeDs is potentially interesting for compute-in-memory and neuromorphic computing applications.

Pd@CuInP2S6 Core-Shell Nanospheres with Exceptional Hydrogen Evolution Capability and Stability in Both Alkaline and Acidic Media under Large Current Density Exceeding 1000 mA cm-2.

By combining Pd with 2D layered crystal CuInP2S6 (CIPS) via laser irradiation in liquids, low-loading Pd@CIPS core-shell nanospheres are fabricated as an efficient and robust electrocatalysts for HER in both alkaline and acidic media under large current density (⩾1000 mA cm-2). Pd@CIPS core-shell nanosphere has two structural features, i) the out-shell is the nanocomposite of PdHx and PdInHx, and ii) there is a kind of dendritic structure on the surface of nanospheres, while the dendritic structure porvides good gas desorption pathway and cause the Pd@CIPS system to maintain higher HER activity and stability than that of commercial Pt/C under large current densities. Pd@CIPS exhibits very low overpotentials of -218 and -313 mV for the large current density of 1000 mA cm-2, and has a small Tafel slope of 29 and 63 mV dec-1 in 0.5 m H2SO4 and 1 m KOH condition, respectively. Meanwhile, Pd@CIPS has an excellent stability under -10 and -500 mA cm-2 current densities and 50 000 cycles cyclic voltammetry tests in 0.5 m H2SO4 and 1 m KOH, respectively, which being much superior to that of commercial Pt/C. Density functional theory (DFT) reveals that engineering electronic structure of PdHx and PdInHx nanostructure can strongly weaken the Pd─H bonding.

Effect of Intrinsic Ferroelectric Phase Transition on Hydrogen Evolution Electrocatalysis.

Heterogeneous electrocatalysis closely relies on the electronic structure of the catalytic materials. The ferroelectric-to-paraelectric phase transition of the materials also involves a change in the state of electrons that could impact the electrocatalytic activity, but such correlation remains unexplored. Here, we demonstrate experimentally and theoretically that the intrinsic electrocatalytic activity could be regulated as exampled by hydrogen evolution reaction catalysis over two-dimensional ferroelectric CuInP2S6. The obvious discontinuity in the overpotential and apparent activation energy values for CuInP2S6 electrode are illustrated during the ferroelectric-to-paraelectric phase transition caused by copper displacement around Tc point (318 K), revealing the ferroelectro-catalytic effect on thermodynamics and kinetics of electrocatalysis. When loading Pt single atom on the CuInP2S6, the paraelectric phase one showed an improved hydrogen evolution activity with smaller apparent activation energy over the ferroelectric phase counterpart. This is attributed to the copper hopping between two sulfur planes, which alternate between strong and weak H adsorption at the Pt sites to simultaneously promote H+ reactant adsorption and H2 product desorption.

Large-Scale Domain Engineering in Two-Dimensional Ferroelectric CuInP2S6 via Giant Flexoelectric Effect.

Room-temperature ferroelectricity in two-dimensional (2D) materials is a potential for developing atomic-scale functional devices. However, as a key step for the technology implementations of 2D ferroelectrics in electronics, the controllable generation of uniform domains remains challenging at the current stage because domain engineering through an external electric field at the 2D limit inevitably leads to large leakage currents and material breakdown. Here, we demonstrate a voltage-free method, the flexoelectric effect, to artificially generate large-scale stripe domains in 2D ferroelectric CuInP2S6 with single domain lateral size at the scale of several hundred microns. With giant strain gradients (∼106 m-1), we mechanically switch the out-of-plane polarization in ultrathin CuInP2S6. The flexoelectric control of polarization is understood with a distorted Landau-Ginzburg-Devonshire double well model. Through substrate strain engineering, the stripe domain density is controllable. Our results highlight the potential of developing van der Waals ferroelectrics-based flexible electronics.

Nonvolatile Reconfigurable 2D Schottky Barrier Transistors.

Nonvolatile reconfigurable transistors can be used to implement highly flexible and compact logic circuits with low power consumption in maintaining the configuration. In this paper, we build nonvolatile reconfigurable transistors based on 2D CuInP2S6/MoTe2 heterostructures. The ferroelectric polarization-induced electron and hole doping in the heterostructure are investigated. By introducing the ferroelectric doping into the source/drain contacts, we demonstrate reconfigurable Schottky barrier transistors, whose polarity (n-type or p-type) can be dynamically programmed, where the configuration is nonvolatile in nature. These transistors exhibit a tunable photoresponse, where the n-n doping state leads to negative photocurrent, whereas the p-p doping state gives rise to a positive photocurrent. The transistor with asymmetric (n-p or p-n) contacts exhibits a strong photovoltaic effect. These reconfigurable logic and optoelectronic transistors will enable a new type of device fabric for future computing systems and sensing networks.

Linear Electro-Optic Effect in 2D Ferroelectric for Electrically Tunable Metalens.

The advent of 2D ferroelectrics, characterized by their spontaneous polarization states in layer-by-layer domains without the limitation of a finite size effect, brings enormous promise for applications in integrated optoelectronic devices. Comparing with semiconductor/insulator devices, ferroelectric devices show natural advantages such as non-volatility, low energy consumption and high response speed. Several 2D ferroelectric materials have been reported, however, the device implementation particularly for optoelectronic application remains largely hypothetical. Here, the linear electro-optic effect in 2D ferroelectrics is discovered and electrically tunable 2D ferroelectric metalens is demonstrated. The linear electric-field modulation of light is verified in 2D ferroelectric CuInP2S6. The in-plane phase retardation can be continuously tuned by a transverse DC electric field, yielding an effective electro-optic coefficient rc of 20.28 pm V-1. The CuInP2S6 crystal exhibits birefringence with the fast axis oriented along its (010) plane. The 2D ferroelectric Fresnel metalens shows efficacious focusing ability with an electrical modulation efficiency of the focusing exceeding 34%. The theoretical analysis uncovers the origin of the birefringence and unveil its ultralow light absorption across a wide wavelength range in this non-excitonic system. The van der Waals ferroelectrics enable room-temperature electrical modulation of light and offer the freedom of heterogeneous integration with silicon and another material system for highly compact and tunable photonics and metaoptics.

Revealing Strong Flexoelectricity and Optoelectronic Coupling in 2D Ferroelectric CuInP2S6 Via Large Strain Gradient.

The interplay between flexoelectric and optoelectronic characteristics provides a paradigm for studying emerging phenomena in various 2D materials. However, an effective way to induce a large and tunable strain gradient in 2D devices remains to be exploited. Herein, we propose a strategy to induce large flexoelectric effect in 2D ferroelectric CuInP2S6 by constructing a 1D-2D mixed-dimensional heterostructure. The strong flexoelectric effect is induced by enormous strain gradient up to 4.2 × 106 m-1 resulting from the underlying ZnO nanowires, which is further confirmed by the asymmetric coercive field and the red-shift in the absorption edge. The induced flexoelectric polarization efficiently boosts the self-powered photodetection performance. In addition, the improved photoresponse has a good correlation with the induced strain gradient, showing a consistent size-dependent flexoelectric effect. The mechanism of flexoelectric and optoelectronic coupling is proposed based on the Landau-Ginzburg-Devonshire double-well model, supported by density functional theory (DFT) calculations. This work provides a brand-new method to induce a strong flexoelectric effect in 2D materials, which is not restricted to crystal symmetry and thus offers unprecedented opportunities for state-of-the-art 2D devices.

Applying the Wake-Up-like Effect to Enhance the Capabilities of Two-Dimensional Ferroelectric Field-Effect Transistors.

For traditional ferroelectric field-effect transistors (FeFETs), enhancing the polarization domain of bulk ferroelectric materials is essential to improve device performance. However, there has been limited investigation into the enhancement of polarization field in two-dimensional (2D) ferroelectric material such as CuInP2S6 (CIPS). In this study, similar to bulk ferroelectric materials, CIPS exhibited enhanced polarization field upon application of external cyclic voltage. Moreover, unlike traditional ferroelectric materials, the polarization enhancement of CIPS is not due to redistribution of the defect but rather originates from a mechanism: the long-distance migration of Cu ions. We termed this mechanism the "wake-up-like effect". After incorporating the wake-up-like effect into the graphene/CIPS/WSe2 FeFET device, we successfully increased the hysteresis window and enhanced the current on/off ratio by 4 orders of magnitude. Moreover, the FeFET yielded remarkable achievements, such as multilevel nonvolatile memory with 21 distinct conductance levels, a high on/off ratio exceeding 106, a long retention time exceeding 103 s, and neuromorphic computing with 93% accuracy at recognizing handwritten digits. Introducing the wake-up-like effect to 2D CIPS may pave the way for innovative approaches to achieve advanced multilevel nonvolatile memory and neuromorphic computing capabilities for next-generation micro-nanoelectronic devices.

Ferroelectric Polarization Enhanced Optoelectronic Synaptic Response of a CuInP2S6 Transistor Structure.

Neuromorphic computing can simulate brain function and is a pivotal element in next-generation computing, providing a potential solution to the limitations brought by the von Neumann bottleneck. Optoelectronic synaptic devices are highly promising tools for simulating biomimetic nervous systems. In this study, we developed an optoelectronic neuromorphic device with a transistor structure constructed using ferroelectric CuInP2S6. Essential synaptic behaviors in this device are observed in response to light and electrical stimuli. The optoferroelectric coupling is revealed, and the highly tunable gate modulation of the charge carrier is realized in a single device. On this basis, the light adaptation of the biological eyes and smarter Pavlovian dogs was implemented successfully and enhanced by ferroelectric polarization. The gate voltage application promotes the migration of additional Cu+ ions in the in-plane direction, thus enhancing the synaptic performance of electrical stimulation. Meanwhile, the processing ability of convolutional kernel noise images in ferroelectric devices has been achieved. Our results offer the important observation and application of ferroelectric polarization-enhanced synaptic properties of a transistor structure and have great potential in promoting the development of two-dimensional van der Waals materials and devices.

Interface-Tuning of Ferroelectricity and Quadruple-Well State in CuInP2S6 via Ferroelectric Oxide.

Ferroelectric van der Waals CuInP2S6 possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk TC ∼ 42 °C) and the lack of precise domain control. Here we show that CuInP2S6 can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when it is interfaced with ferroelectric oxide PbZr0.2Ti0.8O3 films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP2S6 fully conform to those of the underlying PbZr0.2Ti0.8O3, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature in situ domain imaging points to a significantly enhanced TC of >200 °C for 13 nm CuInP2S6 on PbZr0.2Ti0.8O3. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and TC can be attributed to interface-mediated structure distortion in CuInP2S6. Our study provides an effective material strategy to engineer the polar properties of CuInP2S6 for flexible nanoelectronic, optoelectronic, and mechanical applications.

Eliminating Ferroelectric Hysteresis in All-Two-Dimensional Gate-Stack Negative-Capacitance Transistors.

Boltzmann distribution thermal tails of carriers restrain the subthreshold swing (SS) of field-effect transistors (FETs) to be lower than 60 mV/decade at room temperature, which restrains the reduction of operate-voltage and power consumption of transistors. The negative-capacitance FET (NC FET) is expected to break through this physical limit and obtain a steep SS by amplifying the gate voltage through the negative capacitance effect of the ferroelectric thin film, providing a new way to further reduce the power consumption of the transistor at the end of Moore's law. Here, we show a MoS2 NC FET with a CuInP2S6 ferroelectric, exhibiting a large on/off ratio of 108, a steep SS as low as 6 mV/decade, and a wide sub-60 mV/decade drain current range of more than 4 orders of magnitude while sacrificially inducing a huge hysteresis larger than 500 mV. Furthermore, we found that by inserting the h-phase boron nitride (h-BN) layer with suitable thickness, the dielectric capacitance matches the ferroelectric negative capacitance better, and thus the hysteresis on the transfer curve is reduced, and the ideal switching-behavior transistors with SS as low as 62 mV/decade and only 5 mV negligible hysteresis were obtained. Our work demonstrates that under the capacitance-matching condition, the hysteresis-free negative-capacitance transistors do not act as the predicted steep-slope transistors, but their voltage-saving still occurs instead as a type of effective transconductance booster with more than 20 times transconductance amplification.

Stress and Curvature Effects in Layered 2D Ferroelectric CuInP2S6.

Nanoscale ferroelectric 2D materials offer the opportunity to investigate curvature and strain effects on materials functionalities. Among these, CuInP2S6 (CIPS) has attracted tremendous research interest in recent years due to combination of room temperature ferroelectricity, scalability to a few layers thickness, and ferrielectric properties due to coexistence of 2 polar sublattices. Here, we explore the local curvature and strain effect on polarization in CIPS via piezoresponse force microscopy and spectroscopy. To explain the observed behaviors and decouple the curvature and strain effects in 2D CIPS, we introduce the finite element Landau-Ginzburg-Devonshire model, revealing strong changes in hysteresis characteristics in regions subjected to tensile and compressive strain. The piezoresponse force microscopy (PFM) results show that bending induces ferrielectric domains in CIPS, and the polarization-voltage hysteresis loops differ in bending and nonbending regions. These studies offer insights into the fabrication of curvature-engineered nanoelectronic devices.

Low-Thermal-Budget Ferroelectric Field-Effect Transistors Based on CuInP2S6 and InZnO.

In this paper, we demonstrate low-thermal-budget ferroelectric field-effect transistors (FeFETs) based on the two-dimensional ferroelectric CuInP2S6 (CIPS) and oxide semiconductor InZnO (IZO). The CIPS/IZO FeFETs exhibit nonvolatile memory windows of ∼1 V, low off-state drain currents, and high carrier mobilities. The ferroelectric CIPS layer serves a dual purpose by providing electrostatic doping in IZO and acting as a passivation layer for the IZO channel. We also investigate the CIPS/IZO FeFETs as artificial synaptic devices for neural networks. The CIPS/IZO synapse demonstrates a sizable dynamic ratio (125) and maintains stable multilevel states. Neural networks based on CIPS/IZO FeFETs achieve an accuracy rate of over 80% in recognizing MNIST handwritten digits. These ferroelectric transistors can be vertically stacked on silicon complementary metal-oxide semiconductor (CMOS) with a low thermal budget, offering broad applications in CMOS+X technologies and energy-efficient 3D neural networks.

Ferroelectric control of pseudospin texture in CuInP2S6monolayer.

Spin-orbit coupling (SOC) plays an important role in condensed matter physics and has potential applications in spintronics devices. In this paper, we study the electronic properties of ferroelectric CuInP2S6(CIPS) monolayer through first-principles calculations. The result shows that CIPS monolayer is a potential for valleytronics material and we find that the in-plane helical and nonhelical pseudospin texture are induced by the Rashba and Dresselhaus effect, respectively. The chirality of helical pseudospin texture is coupled to the out-of-plane ferroelectric polarization. Furthermore, a large spin splitting due to the SOC effect can be found atKvalley, which can be regarded as the Zeeman effect under a valley-dependent pseudomagnetic field. The CIPS monolayer with Rashbaet aleffects provides a good platform for electrically controlled spin polarization physics.

Ferroelectric Tuning of ZnO Ultraviolet Photodetectors.

The ferroelectric field effect transistor (Fe-FET) is considered to be one of the most important low-power and high-performance devices. It is promising to combine a ferroelectric field effect with a photodetector to improve the photodetection performance. This study proposes a strategy for ZnO ultraviolet (UV) photodetectors regulated by a ferroelectric gate. The ZnO nanowire (NW) UV photodetector was tuned by a 2D CuInP2S6 (CIPS) ferroelectric gate, which decreased the dark current and enhanced the responsivity and detectivity to 2.40 × 104 A/W and 7.17 × 1011 Jones, respectively. This strategy was also applied to a ZnO film UV photodetector that was tuned by a P(VDF-TrFE) ferroelectric gate. Lower power consumption and higher performance can be enabled by ferroelectric tuning of ZnO ultraviolet photodetectors, providing new inspiration for the fabrication of high-performance photodetectors.

Self-Enhancing Photoelectrochemical Properties in van der Waals Ferroelectric CuInP2S6 by Photoassisted Acid Hydrolysis.

Transition metal thiophosphate, CuInP2S6 (CIPS), has recently emerged as a potentially promising material for photoelectrochemical (PEC) water splitting due to its intrinsic ferroelectric polarization for spontaneous photocarrier separation. However, the poor kinetics of the hydrogen evolution reaction (HER) greatly limits its practical applications. Herein, we report self-enhancing photocatalytic behavior of a CIPS photocathode due to chemically driven oxygen incorporation by photoassisted acid oxidation. The optimal oxygen-doped CIPS demonstrates a >1 order of magnitude enhancement in the photocurrent density compared to that of pristine CIPS. Through comprehensive spectroscopic and microscopic investigations combined with theoretical calculations, we disclose that oxygen doping will lower the Fermi level position and decrease the HER barrier, which further accelerates charge separation and improves the HER activity. This work may deliver a universal and facile strategy for improving the PEC performance of other van der Waals metal thiophosphates.

Nonvolatile Electric Control of Exciton Complexes in Monolayer MoSe2 with Two-Dimensional Ferroelectric CuInP2S6.

Monolayer transition-metal dichalcogenides (TMDs) have provided a platform to investigate the excitonic states at the two-dimensional (2D) limit. The inherent properties of excitons in TMDs, such as the photoluminescence quantum yield, the charge states, and even the binding energy, can be effectively controlled via electrostatic gating, selective carrier doping, or substrate dielectric engineering. Here, aiming for the nonvolatile electrical tunability of excitonic states and thereby the optical property of TMDs, we demonstrate a 2D ferroelectric heterostructure with monolayer MoSe2 and ultrathin CuInP2S6 (CIPS). In the heterostructure, the electric polarization of CIPS results in continuous, global, and large electronic modulation in monolayer MoSe2. With the saturated ferroelectric polarization of CIPS, electron-doped or hole-doped MoSe2 is realized in a single device. The carrier density tunability in the heterostructure is as high as 5 × 1012 cm-2. The nonvolatile behavior of these devices up to 3 months is also characterized. Our results provide a new and practical strategy for low-power consumption and agelong tunable optoelectronic devices.

Strong Temperature Effect on the Ferroelectric Properties of CuInP2S6 and Its Heterostructures.

Van der Waals (vdW) ferroelectric insulator CuInP2S6 (CIPS) has attracted intense research interest due to its unique ferroelectric and piezoelectric properties. In this paper, we systematically investigate the temperature and frequency dependence of the ferroelectric properties of CIPS. We find that there is a large imprint in the CIPS capacitor, which can be attributed to the fixed dipoles induced by defects. At high temperatures and low frequencies, the amplitude and direction of the imprint become tunable by the preset pulse, as the copper ions are more mobile and these dipoles become switchable. When the polarization in CIPS changes direction, the graphene/CIPS/graphene ferroelectric diode exhibits switchable resistance since the Fermi level in graphene is modulated by the polarization in CIPS. For CIPS/MoTe2 dual-gate transistor, a temperature-dependent nonvolatile memory window is observed, which can be attributed to the interplay between ferroelectric polarization and interface traps. This research provides experimental groundwork for vdW ferroelectric materials, expands the understanding of ferroelectricity in CIPS, and opens up exciting opportunities for novel electronic devices based on vdW ferroelectric materials.

Ferroelectric Field-Effect Transistors Based on MoS2 and CuInP2S6 Two-Dimensional van der Waals Heterostructure.

We demonstrate room-temperature ferroelectric field-effect transistors (Fe-FETs) with MoS2 and CuInP2S6 two-dimensional (2D) van der Waals heterostructure. The ferroelectric CuInP2S6 is a 2D ferroelectric insulator, integrated on top of MoS2 channel providing a 2D/2D semiconductor/insulator interface without dangling bonds. The MoS2- and CuInP2S6-based 2D van der Waals heterostructure Fe-FETs exhibit a clear counterclockwise hysteresis loop in transfer characteristics, demonstrating their ferroelectric properties. This stable nonvolatile memory property can also be modulated by the back-gate bias of the MoS2 transistors because of the tuning of capacitance matching between the MoS2 channel and the ferroelectric CuInP2S6, leading to the enhancement of the on/off current ratio. Meanwhile, the CuInP2S6 thin film also shows resistive switching characteristics with more than four orders of on/off ratio between low- and high-resistance states, which is also promising for resistive random-access memory applications.