Electrically Driven Reversible Phase Changes in Layered In2 Se3 Crystalline Film

Soret-Effect Induced Phase-Change in a Chromium Nitride Semiconductor Film

Phase-change materials such as Ge-Sb-Te (GST) exhibiting amorphous and crystalline phases can be used for phase-change random-access memory (PCRAM). GST-based PCRAM has been applied as a storage-class memory; however, its relatively low ON/OFF ratio and the large Joule heating energy required for the RESET process (amorphization) significantly limit the storage density. This study proposes a phase-change nitride, CrN, with a much wider programming window (ON/OFF ratio more than 105) and lower RESET energy (one order of magnitude reduction from GST). High-resolution transmission electron microscopy revealed a phase-change from the low-resistance cubic CrN phase into the highly resistive hexagonal CrN2 phase induced by the Soret-effect. The proposed phase-change nitride could greatly expand the scope of conventional phase-change chalcogenides and offer a strategy for the next-generation of PCRAM, enabling a large ON/OFF ratio (∼105), low switching energy (∼100 pJ), and fast operation (∼30 ns).

Characteristic Plasmon Energies for 2D In2Se3 Phase Identification at Nanoscale

Two-dimensional (2D) materials with competing polymorphs offer remarkable potential to switch the associated 2D functionalities for novel device applications. Probing their phase transition and competition mechanisms requires nanoscale characterization techniques that can sensitively detect the nucleation of secondary phases down to single-layer thickness. Here we demonstrate nanoscale phase identification on 2D In2Se3 polymorphs, utilizing their distinct plasmon energies that can be distinguished by electron energy-loss spectroscopy (EELS). The characteristic plasmon energies of In2Se3 polymorphs have been validated by first-principles calculations, and also been successfully applied to reveal phase transitions using in situ EELS. Correlating with in situ X-ray diffraction, we further derive a subtle difference in the valence electron density of In2Se3 polymorphs, consistent with their disparate electronic properties. The nanometer resolution and independence of orientation make plasmon-energy mapping a versatile technique for nanoscale phase identification on 2D materials.

Nanoscale fabrication of heterostructures in thermoelectric SnTe

Enhancing the performance of thermoelectric materials is demanded to develop strategies for introducing multidimensional microstructures into materials to induce full-scale phonon scattering while ensuring electrical transport performance. Herein, a previously unreported rhombohedral h-SnTe (R3̄m) has been achieved in the nanoscale dimension by the electron beam irradiation of β-SnTe (Fm3̄m) materials. The h-SnTe structure contains interlayer van der Waals gaps and exhibits metallic behavior evaluated by density-functional theory calculations, which coherently appears in the narrow-band semiconductor β-SnTe matrix. Our results provide a strategy for modifying the properties of SnTe-based thermoelectric materials and designing nanostructured chalcogenide heterostructures via electron beam irradiation.

Two-Dimensional Ferroelectric C2N/In2Se3 Heterobilayer with Tunable Electronic Property and Photovoltaic Effect

Two-dimensional ferroelectric monolayer materials with reversible spontaneous polarization provide more regulatory dimensions for their relevant van der Waals heterostructures. Using first-principles calculations, we construct the C2N/In2Se3 bilayer heterostructure and study its physical properties as well as the effects of E-field and strain. The results indicate that the intrinsic polarization of the component In2Se3 monoalyer can significantly adjust the electronic properties of the C2N/In2Se3 heterobilayer. When the polarization of the In2Se3 monolayer points to the interface (up-In2Se3), the C2N/In2Se3 bilayer behaves as the type-I indirect band gap heterostructure, while it transforms to the type-II direct band gap heterostructure after reversing the polarization of the In2Se3 monolayer (dp-In2Se3). Furthermore, the two C2N/In2Se3 heterostructures both have enhanced optical absorption in the visible region than the isolated In2Se3 and C2N monolayers. More importantly, the external electric field and strain can easily regulate the electronic properties of the C2N/In2Se3 heterostructures. The power conversion efficiency (PCE) of the type-II C2N/dp-In2Se3 heterostructure is 8.16%, and the electric field of 0.1 V/Å and the strain of -2% can transform the C2N/up-In2Se3 heterostructure into type-II one, conducive to the high PCE up to 24.03 and 24%, respectively. Our proposed C2N/In2Se3 heterostructure is promising in future luminescent and photovoltaic fields, and our findings also provide a strategy for functionalizing 2D monolayer materials by the intrinsic polarization property of ferroelectric materials.

Reversible Amorphous-Crystalline Phase Transformation in an Ultrathin van der Waals FeTe System

Searching for new phase-change materials for memory and neuromorphic device applications and further understanding the phase transformation mechanism are attracting wide attention. Phase transformation from the amorphous phase to the crystal phase has been unraveled in iron telluride (FeTe) bulk film deposited by pulsed laser deposition, recently. However, the van der Waals-layered feature of FeTe in the crystal form was not noted, which will benefit the scaling of the memory devices and shine light on phase-change heterostructures or interfacial phase-change materials. Moreover, the demonstration of advanced memory or neuromorphic device applications is lacking. Here, we investigate the phase transformation of FeTe starting from mechanically exfoliated van der Waals layers from a bulk single crystal. Surficial amorphization is revealed at the surface layers of FeTe flakes after exfoliation under ambient conditions, which could be transformed back to the crystalline phase with laser irradiation or heating. The conductance drop of the flake devices near 400 K verifies the phase transformation electrically. Memristor behavior of the amorphous surface in FeTe has been further demonstrated, proving the reversibility of the phase transformation and shining light on the possible applications of neuromorphic devices.

Reversible Thermally Driven Phase Change of Layered In2Se3 for Integrated Photonics

Two-dimensional In₂Se₃, an unconventional phase-change material, has drawn considerable attention for polymorphic phase transitions and electronic device applications. However, its reversible thermally driven phase transitions and potential use in photonic devices have yet to be explored. In this study, we observe the thermally driven reversible phase transitions between α and β' phases with the assistance of local strain from surface wrinkles and ripples, as well as reversible phase changes within the β phase family. These transitions lead to changes in the refractive index and other optoelectronic properties with minimal optical loss at telecommunication bands, which are crucial in integrated photonic applications such as postfabrication phase trimming. Additionally, multilayer β'-In₂Se₃ working as a transparent microheater proves to be a viable option for efficient thermo-optic modulation. This prototype design for layered In₂Se₃ offers immense potential for integrated photonics and paves the way for multilevel, nonvolatile optical memory applications.

Discovery of a metastable van der Waals semiconductor via polymorphic crystallization of an amorphous film

Here we report on the growth of thin crystalline films of the metastable phase GeTe₂. Direct observation by transmission electron microscopy revealed a Te-Ge-Te stacking with van der Waals gaps. Moreover, electrical and optical measurements revealed the films exhibted semiconducting properties commensurate with electronics applications. Feasibility studies in which device structures were fabricated demonstrated the potential application of GeTe₂ as an electronic material.

Neuromorphic Photonics Based on Phase Change Materials

Neuromorphic photonics devices based on phase change materials (PCMs) and silicon photonics technology have emerged as promising solutions for addressing the limitations of traditional spiking neural networks in terms of scalability, response delay, and energy consumption. In this review, we provide a comprehensive analysis of various PCMs used in neuromorphic devices, comparing their optical properties and discussing their applications. We explore materials such as GST (Ge₂Sb₂Te₅), GeTe-Sb₂Te₃, GSST (Ge₂Sb₂Se₄Te₁), Sb₂S₃/Sb₂Se₃, Sc_0.2Sb₂Te₃ (SST), and In₂Se₃, highlighting their advantages and challenges in terms of erasure power consumption, response rate, material lifetime, and on-chip insertion loss. By investigating the integration of different PCMs with silicon-based optoelectronics, this review aims to identify potential breakthroughs in computational performance and scalability of photonic spiking neural networks. Further research and development are essential to optimize these materials and overcome their limitations, paving the way for more efficient and high-performance photonic neuromorphic devices in artificial intelligence and high-performance computing applications.

First-Principles Investigations on the Semiconductor-to-Metal Phase Transition of 2D Si2Te3 for Reversible Resistive Switching

Si₂Te₃ is attracting attention due to its compatibility with Si technology while still showing advantages as a two-dimensional layered material. Although recent experimental studies have observed the resistive switching process in Si₂Te₃-based memristors, the mechanism has not been clearly identified. In this study, first-principles density functional theory calculations are employed to understand the relationship between the phase transition of Si₂Te₃ and the reversible resistive switching of the Si₂Te₃-based memristor. Our calculation results show that although semiconducting Si₂Te₃ is energetically more stable than two metallic Si₂Te₃ phases (α and β), two metallic Si₂Te₃ can be energetically stabilized by excess holes. The enhanced energetic preference of two metallic Si₂Te₃ by excess holes is explained by the reduced occupation of antibonding states between Si and Te. Our study finds that the energy barrier for the phase transition between semiconducting Si₂Te₃ and α-metallic Si₂Te₃ varies significantly by excess charge carriers so the phase transition can be directly connected to the reversible resistive switching of the Si₂Te₃-based memristor under external bias. Our finding will serve as a cornerstone for optimizing the resistive switching process of the Si₂Te₃-based memristor.

Molecular precursor mediated selective synthesis of phase pure cubic InSe and hexagonal In2Se3 nanostructures: new anode materials for Li-ion batteries

Indium selenides (InSe and In₂Se₃) have earned a special place among the 2D layered metal chalcogenides owing to their nontoxic nature and favourable carrier mobility. Additionally, they are also being projected as next generation battery anodes with high theoretical lithium-ion storage capacities. While the development of indium selenide-based batteries is still in its embryonic stage, a simple and easily scalable synthetic pathway to access these materials is highly desirable for energy storage applications. This study reports a controlled synthetic route to nanometric cubic InSe and hexagonal In₂Se₃ materials through proper choice of coordinating solvents from a structurally characterized air and moisture stable single source molecular precursor: tris(4,6-dimethyl-2-pyrimidylselenolato)indium(III). The crystal structure, phase purity, composition, morphology and band gap of the nanomaterials were thoroughly evaluated by pXRD, energy dispersive X-ray spectroscopy (EDS), electron microscopy (SEM and TEM), and diffuse reflectance spectroscopy (DRS), respectively. The pristine InSe and In₂Se₃ nanostructures have been employed as anode materials in lithium-ion batteries (LIBs). Both the cells deliver reasonably high initial discharge capacities with a cyclability of 200 and 620 cycles for cubic InSe and hexagonal In₂Se₃ respectively with ∼100% coulombic efficiency.

Selective Enhancement of Photoresponse with Ferroelectric-Controlled BP/In2 Se3 vdW Heterojunction

Owing to the large built-in field for efficient charge separation, heterostructures facilitate the simultaneous realization of a low dark current and high photocurrent. The lack of an efficient approach to engineer the depletion region formed across the interfaces of heterojunctions owing to doping differences hinders the realization of high-performance van der Waals (vdW) photodetectors. This study proposes a ferroelectric-controlling van der Waals photodetector with vertically stacked two-dimensional (2D) black phosphorus (BP)/indium selenide (In₂ Se₃ ) to realize high-sensitivity photodetection. The depletion region can be reconstructed by tuning the polarization states generated from the ferroelectric In₂ Se₃ layers. Further, the energy bands at the heterojunction interfaces can be aligned and flexibly engineered using ferroelectric field control. Fast response, self-driven photodetection, and three-orders-of-magnitude detection improvements are achieved in the switchable visible or near-infrared operation bands. The results of the study are expected to aid in improving the photodetection performance of vdW optoelectronic devices.

Giant piezoresistivity in a van der Waals material induced by intralayer atomic motions

The presence of the van der Waals gap in layered materials creates a wealth of intriguing phenomena different to their counterparts in conventional materials. For example, pressurization can generate a large anisotropic lattice shrinkage along the stacking orientation and/or a significant interlayer sliding, and many of the exotic pressure-dependent properties derive from these mechanisms. Here we report a giant piezoresistivity in pressurized β'-In₂Se₃. Upon compression, a six-orders-of-magnitude drop of electrical resistivity is obtained below 1.2 GPa in β'-In₂Se₃ flakes, yielding a giant piezoresistive gauge π_p of -5.33 GPa^-1. Simultaneously, the sample undergoes a semiconductor-to-semimetal transition without a structural phase transition. Surprisingly, linear dichroism study and theoretical first principles modelling show that these phenomena arise not due to shrinkage or sliding at the van der Waals gap, but rather are dominated by the layer-dependent atomic motions inside the quintuple layer, mainly from the shifting of middle Se atoms to their high-symmetric location. The atomic motions link to both the band structure modulation and the in-plane ferroelectric dipoles. Our work not only provides a prominent piezoresistive material but also points out the importance of intralayer atomic motions beyond van der Waals gap.

Phase Instability in van der Waals In2 Se3 Determined by Surface Coordination

van der Waals In₂ Se₃ has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In₂ Se₃ have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and β'-In₂ Se₃ originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In₂ Se₃ in air to form amorphous In₂ Se_3-3x O_3x layers and Se hemisphere particles. Both O₂ and H₂ O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In₂ Se_3-3x O_3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In₂ Se₃ performance for device applications.

Oscillatory Order-Disorder Transition during Layer-by-Layer Growth of Indium Selenide

It is important to understand the polymorph transition and crystal-amorphous phase transition in In₂Se₃ to tap the potential of this material for resistive memory storage. By monitoring layer-by-layer growth of β-In₂Se₃ during molecular beam epitaxy (MBE), we are able to identify a cyclical order-disorder transition characterized by a periodic alternation between a glassy-like metastable subunit cell film consisting of n < 5 sublayers (nth layers = the number of subunit cell layers), and a highly crystalline β-In₂Se₃ at n = 5 layers. The glassy phase shows an odd-even alternation between the indium-cluster layer (n = 1, 3) and an In-Se solid solution (n = 2, 4), which suggests the ability of In and Se atoms to diffuse, aggregate, and intermix. These dynamic natures of In and Se atoms contribute to a defect-driven memory resistive behavior in current-voltage sweeps that is different from the ferroelectric switching of α-In₂Se₃.

Strong Anisotropic Two-Dimensional In2Se3 for Light Intensity and Polarization Dual-Mode High-Performance Detection

Detecting the light from different freedom is of great significance to gain more information. Two-dimensional (2D) materials with low intrinsic carrier concentration and highly tunable electronic structure have been considered as the promising candidate for future room-temperature multi-functional photodetectors. However, current investigations mainly focus on intensity-sensitive detection; the multi-dimensional photodetection such as polarization-sensitive photodetection is still in its early stage. Herein, the intensity- and polarization-sensitive photodetection based on α-In₂Se₃ is studied. By using angle-resolved polarized Raman spectroscopy, it is demonstrated that α-In₂Se₃ shows an anisotropic phonon vibration property indicating its asymmetric structure. The α-In₂Se₃-based photodetector has a photoelectric performance with a responsivity of 1936 A/W and a specific detectivity of 2.1 × 10¹³ Jones under 0.2 mW/cm² power density at 400 nm. Moreover, by studying the polarized angle-resolved photoelectrical effect, it is found that the ratio of maximum and minimum photocurrent (dichroic ratio) reaches 1.47 at 650 nm suggesting good polarization-sensitive detection. After post-annealing, α-In₂Se₃ in situ converts to β-In₂Se₃ which has similar in-plane anisotropic crystallinity and exhibits a dichroic ratio of 1.41. It is found that the responsivity of β-In₂Se₃ is 6 A/W, much lower than that of α-In₂Se₃. The high-performance light intensity- and polarization-detection of α-In₂Se₃ enlarges the 2D anisotropic materials family and provides new opportunities for future dual-mode photodetection.

Phase and polarization modulation in two-dimensional In2Se3 via in situ transmission electron microscopy

Phase transitions in two-dimensional (2D) materials promise reversible modulation of material physical and chemical properties in a wide range of applications. 2D van der Waals layered In₂Se₃ with bistable out-of-plane ferroelectric (FE) α phase and antiferroelectric (AFE) β' phase is particularly attractive for its electronic applications. However, reversible phase transition in 2D In₂Se₃ remains challenging. Here, we introduce two factors, dimension (thickness) and strain, which can effectively modulate the phases of 2D In₂Se₃. We achieve reversible AFE and out-of-plane FE phase transition in 2D In₂Se₃ by delicate strain control inside a transmission electron microscope. In addition, the polarizations in 2D FE In₂Se₃ can also be manipulated in situ at the nanometer-sized contacts, rendering remarkable memristive behavior. Our in situ transmission electron microscopy (TEM) work paves a previously unidentified way for manipulating the correlated FE phases and highlights the great potentials of 2D ferroelectrics for nanoelectromechanical and memory device applications.

Structural Phase Transitions between Layered Indium Selenide for Integrated Photonic Memory

The primary mechanism of optical memoristive devices relies on phase transitions between amorphous and crystalline states. The slow or energy-hungry amorphous-crystalline transitions in optical phase-change materials are detrimental to the scalability and performance of devices. Leveraging an integrated photonic platform, nonvolatile and reversible switching between two layered structures of indium selenide (In₂ Se₃ ) triggered by a single nanosecond pulse is demonstrated. The high-resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With interlayer "shear glide" and isosymmetric phase transition, switching between the α- and β-structural states contains low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline-crystalline phase transition are experimentally characterized in molecular-beam-epitaxy-grown thin films and compared to ab initio calculations. The nonlinear resonator transmission spectra measure of incremental linear loss rate of 3.3 GHz, introduced by a 1.5 µm-long In₂ Se₃ -covered layer, resulted from the combinations of material absorption and scattering.

Nonvolatile Ferroelectric Memory with Lateral β/α/β In2Se3 Heterojunctions

The electric dipole locking effect observed in van der Waals (vdW) ferroelectric α-In₂Se₃ has resulted in a surge of applied research in electronics with nonvolatile functionality. However, ferroelectric tunnel junctions with advantages of lower power consumption and faster writing/reading operations have not been realized in α-In₂Se₃. Here, we demonstrate the tunneling electroresistance effect in a lateral β/α/β In₂Se₃ heterojunction built by local laser irradiation. Switchable in-plane polarizations of the vdW ferroelectric control the tunneling conductance of the heterojunction device by 4000% of magnitude. The electronic logic bit can be represented and stored with different orientations of electric dipoles. This prototype enables a new approach to rewritable nonvolatile memory with in-plane ferroelectricity in vdW 2D materials.

Temperature-Driven α-β Phase Transformation and Enhanced Electronic Property of 2H α-In2Se3

In recent years, thin layered indium selenide (In2Se3) has attracted rapidly increasing attention due to its fascinating properties and promising applications. Here, we report the temperature-driven α-β phase transformation and the enhanced electronic property of 2H α-In2Se3. We find that 2H α-In2Se3 transforms to β-In2Se3 when it is heated to a high temperature, and the transformation temperature increases from 550 to 650 K with the thickness decreasing from 67 to 17 nm. Additionally, annealing the sample below the phase transformation temperature can effectively improve the electronic property of a 2H α-In2Se3 field-effect transistor, including increasing the on-state current, decreasing the off-state current, and improving the subthreshold swing. After annealing, not only the contact resistance decreases significantly but also the mobility at 300 K increases more than 2 times to 45.83 cm2 V-1 s-1, which is the highest among the reported values. Our results provide an effective method to improve the electrical property and the stability of the In2Se3 nanodevices.

Synthesis of photo-responsive indium selenides (InSe and In2Se3) from tris(4,6-dimethyl-2-pyrimidylselenolato)indium(iii) as a molecular precursor

Facile and selective synthesis of phase pure photo-responsive InSe and In2Se3 nanostructures employing air-stable In[Sepym(Me-4,6)2]3 as a novel molecular precursor.

Low-Dimensional In2Se3 Compounds: From Material Preparations to Device Applications

Nanostructured In₂Se₃ compounds have been widely used in electronics, optoelectronics, and thermoelectrics. Recently, the revelation of ferroelectricity in low-dimensional (low-D) In₂Se₃ has caused a new upsurge of scientific interest in nanostructured In₂Se₃ and advanced functional devices. The ferroelectric, thermoelectric, and optoelectronic properties of In₂Se₃ are highly correlated with the crystal structure. In this review, we summarize the crystal structures and electronic band structures of the widely interested members of the In₂Se₃ compound family. Recent achievements in the preparation of low-D In₂Se₃ with controlled phases are discussed in detail. General principles for obtaining pure-phased In₂Se₃ nanostructures are described. The excellent ferroelectric, optoelectronic, and thermoelectric properties having been demonstrated using nanostructured and heterostructured In₂Se₃ with different phases are also summarized. Progress and challenges on the applications of In₂Se₃ nanostructures in nonvolatile memories, photodetectors, gas sensors, strain sensors, and photovoltaics are discussed in detail. In the last part of this review, perspectives on the challenges and opportunities in the preparation and applications of In₂Se₃ materials are presented.

Multi-level phase-change memory with ultralow power consumption and resistance drift

By controlling the amorphous-to-crystalline relative volume, chalcogenide phase-change memory materials can provide multi-level data storage (MLS), which offers great potential for high-density storage-class memory and neuro-inspired computing. However, this type of MLS system suffers from high power consumption and a severe time-dependent resistance increase ("drift") in the amorphous phase, which limits the number of attainable storage levels. Here, we report a new type of MLS system in yttrium-doped antimony telluride, utilizing reversible multi-level phase transitions between three states, i.e., amorphous, metastable cubic and stable hexagonal crystalline phases, with ultralow power consumption (0.6-4.3 pJ) and ultralow resistance drift for the lower two states (power-law exponent < 0.007). The metastable cubic phase is stabilized by yttrium, while the evident reversible cubic-to-hexagonal transition is attributed to the sequential and directional migration of Sb atoms. Finally, the decreased heat dissipation of the material and the increase in crystallinity contribute to the overall high performance. This study opens a new way to achieve advanced multi-level phase-change memory without the need for complicated manufacturing procedures or iterative programming operations.

Focusing and defocusing switching of an indium selenide-silicon photonic metalens

With a fixed geometric design, homogeneous change of Indium Selenide (In₂Se₃) switches the focusing length of a silicon photonic metalens between positive and negative values. This unique functionality of the hybrid metasurface is attributed to the fact that the silicon's refractive index is in the middle of the two convertible states in the optical phase change material. The infrared transparency of In₂Se₃ in both states enables near phase-only metasurface structures. The design is foundry compatible and feasible for implementing nonvolatile adaptive transformation optic systems on-chip.

Emerging 2D Memory Devices for In-Memory Computing

It is predicted that the conventional von Neumann computing architecture cannot meet the demands of future data-intensive computing applications due to the bottleneck between the processing and memory units. To try to solve this problem, in-memory computing technology, where calculations are carried out in situ within each nonvolatile memory unit, has been intensively studied. Among various candidate materials, 2D layered materials have recently demonstrated many new features that have been uniquely exploited to build next-generation electronics. Here, the recent progress of 2D memory devices is reviewed for in-memory computing. For each memory configuration, their operation mechanisms and memory characteristics are described, and their pros and cons are weighed. Subsequently, their versatile applications for in-memory computing technology, including logic operations, electronic synapses, and random number generation are presented. Finally, the current challenges and potential strategies for future 2D in-memory computing systems are also discussed at the material, device, circuit, and architecture levels. It is hoped that this manuscript could give a comprehensive review of 2D memory devices and their applications in in-memory computing, and be helpful for this exciting research area.

Two-dimensional ferroelasticity in van der Waals β'-In2Se3

Two-dimensional (2D) materials exhibit remarkable mechanical properties, enabling their applications as flexible and stretchable ultrathin devices. As the origin of several extraordinary mechanical behaviors, ferroelasticity has also been predicted theoretically in 2D materials, but so far lacks experimental validation and investigation. Here, we present the experimental demonstration of 2D ferroelasticity in both exfoliated and chemical-vapor-deposited β'-In₂Se₃ down to few-layer thickness. We identify quantitatively 2D spontaneous strain originating from in-plane antiferroelectric distortion, using both atomic-resolution electron microscopy and in situ X-ray diffraction. The symmetry-equivalent strain orientations give rise to three domain variants separated by 60° and 120° domain walls (DWs). Mechanical switching between these ferroelastic domains is achieved under ≤0.5% external strain, demonstrating the feasibility to tailor the antiferroelectric polar structure as well as DW patterns through mechanical stimuli. The detailed domain switching mechanism through both DW propagation and domain nucleation is unraveled, and the effects of 3D stacking on such 2D ferroelasticity are also discussed. The observed 2D ferroelasticity here should be widely available in 2D materials with anisotropic lattice distortion, including the 1T' transition metal dichalcogenides with Peierls distortion and 2D ferroelectrics such as the SnTe family, rendering tantalizing potential to tune 2D functionalities through strain or DW engineering.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Shape-control growth of 2D-In2Se3 with out-of-plane ferroelectricity by chemical vapor deposition

For potential applications in ferroelectric switching and piezoelectric nano-generator devices, the promising ferroelectric properties of two dimensional (2D) layered In2Se3 attracted much attention. In the present study, 2D In2Se3 flakes down to monolayers are grown by the chemical vapor deposition (CVD) technique on a mica substrate with their structural, optical and ferroelectric properties being studied. The effect of growth parameters (time of growth and Ar flow rate) on the shape and size of the deposited flakes was studied. The optical microscopy study revealed that the flake changed from a circular shape to a sharp face triangle as the Ar flow rate and growth time increased. Raman spectroscopy and high-resolution scanning transmission electron microscopy (HR-STEM) studies revealed that the flakes were of α and β phases, each of which has a hexagonal crystal structure. Strong second harmonic generation (SHG) was observed from α-In2Se3, demonstrating its non-centrosymmetric structure. The piezo-force microscopic (PFM) study showed the presence of out of plane (OOP) ferroelectricity with no in plane (IP) ferroelectricity in CVD grown α-In2Se3 indicating its vertically confined piezoresponse, which was tuned by the applied electric bias and the flake thickness. The present result of shape-controlled growth of In2Se3 with OOP ferroelectricity would open new pathways in the field of 2D ferroelectric switching devices.

Two-Dimensional Antiferroelectricity in Nanostripe-Ordered In_{2}Se_{3}

Two-dimensional (2D) layered materials have been an exciting frontier for exploring emerging physics at reduced dimensionality, with a variety of exotic properties demonstrated at 2D limit. Here, we report the first experimental discovery of in-plane antiferroelectricity in a 2D material β^{'}-In_{2}Se_{3}, using optical and electron microscopy consolidated by first-principles calculations. Different from conventional 3D antiferroelectricity, antiferroelectricity in β^{'}-In_{2}Se_{3} is confined within the 2D layer and generates the unusual nanostripe ordering: the individual nanostripes exhibit local ferroelectric polarization, whereas the neighboring nanostripes are antipolar with zero net polarization. Such a unique superstructure is underpinned by the intriguing competition between 2D ferroelectric and antiferroelectric ordering in β^{'}-In_{2}Se_{3}, which can be preserved down to single-layer thickness as predicted by calculation. Besides demonstrating 2D antiferroelectricity, our finding further resolves the true nature of the β^{'}-In_{2}Se_{3} superstructure that has been under debate for over four decades.

Novel Type of Synaptic Transistors Based on a Ferroelectric Semiconductor Channel

Three-terminal synaptic transistors are basic units of neuromorphic computing chips, which may overcome the bottleneck of conventional von Neumann computing. So far, most of the three-terminal synaptic transistors use the dielectric layer to change the state of the channel and mimic the synaptic behavior. For this purpose, special dielectric layers are needed, such as ionic liquids, solid electrolytes, or ferroelectric insulators, which are difficult for miniaturization and integration. Here, we report a novel type of synaptic transistors using a two-dimensional ferroelectric semiconductor, i.e., α-In₂Se₃, as the channel material to mimic the synaptic behavior for the first time. The essential synaptic behaviors, such as single-spike response, paired-spike response, and multispike response have been experimentally demonstrated. Most importantly, the conventional gate dielectric material of our transistors may facilitate the miniaturization and batch manufacture of synaptic transistors. The results indicate that the three-terminal synaptic transistors based on two-dimensional ferroelectric semiconductors are very promising for neuromorphic systems.

Reversible displacive transformation in MnTe polymorphic semiconductor

Displacive transformation is a diffusionless transition through shearing and shuffling of atoms. Diffusionless displacive transition with modifications in physical properties can help manufacture fast semiconducting devices for applications such as data storage and switching. MnTe is known as a polymorphic compound. Here we show that a MnTe semiconductor film exhibits a reversible displacive transformation based on an atomic-plane shuffling mechanism, which results in large electrical and optical contrasts. We found that MnTe polycrystalline films show reversible resistive switching via fast Joule heating and enable nonvolatile memory with lower energy and faster operation compared with conventional phase-change materials showing diffusional amorphous-to-crystalline transition. We also found that the optical reflectance of MnTe films can be reversibly changed by laser heating. The present findings offer new insights into developing low power consumption and fast-operation electronic and photonic phase-change devices.

Metallic contact induced van der Waals gap in a MoS2 FET

Electrical metal contacts formed with 2D materials strongly affect device performance. Here, we used scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) to characterize the interfacial structure formed and physical damage induced between MoS2 and the most commonly used metals, Ti, Cr, Au, and Pd. We further correlated the electrical performance with physical defects observed at the 2D interfacial structure. The contact resistances were higher in the order of Ti, Au, Pd, and Cr contacts, but all 4-point probe mobilities measured with metals in contact with identical quadrilayer MoS2 were ∼65 cm2 V-1 s-1, confirming the reliability of the devices. According to the STEM and EDS analyses, the Ti contact gave rise to a van der Waals gap between the clean quadrilayer MoS2 and the Ti contact. By contrast, Cr migrated into MoS2 while Mo and S counter-migrated into the SiO2 substrate. Au and Pd formed glassy layers that resulted in the migration of Mo and S into the Au and Pd electrodes. These interfacial structures between MoS2 and contact metals strongly correlated with the electrical performance of 2D MoS2 FETs, providing practical guidelines to form van der Waals contacts.

Atomic-Scale Observation of Reversible Thermally Driven Phase Transformation in 2D In2Se3

Phase transformation in emerging two-dimensional (2D) materials is crucial for understanding and controlling the interplay between structure and electronic properties. In this work, we investigate 2D In₂Se₃ synthesized via chemical vapor deposition, a recently discovered 2D ferroelectric material. We observed that In₂Se₃ layers with thickness ranging from a single layer to ∼20 layers stabilized at the β phase with a superstructure at room temperature. At around 180 K, the β phase converted to a more stable β' phase that was distinct from previously reported phases in 2D In₂Se₃. The kinetics of the reversible thermally driven β-to-β' phase transformation was investigated by temperature-dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, density functional theory calculations reveal in-plane ferroelectricity in the β' phase. Scanning tunneling spectroscopy measurements show that the indirect bandgap of monolayer β' In₂Se₃ is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In₂Se₃ with thickness down to the monolayer limit and the associated electronic properties will provide insights to tune the functionalities of 2D In₂Se₃ and other emerging 2D ferroelectric materials and shed light on their numerous potential applications (e.g., nonvolatile memory devices).

Phase-Engineering-Driven Enhanced Electronic and Optoelectronic Performance of Multilayer In2Se3 Nanosheets

Here, we report electronic and optoelectronic performance of multilayer In₂Se₃ are effectively regulated by phase engineering. The electron mobility is increased to 22.8 cm² V^-1 s^-1 for β-In₂Se₃ FETs, which is 18 times higher than 1.26 cm² V^-1 s^-1 of α-In₂Se₃ FETs. The enhanced electronic performance is attributed to larger carrier sheet density and lower contact resistance. Multilayer β-In₂Se₃ photodetector exhibits an ultrahigh responsivity of 8.8 × 10⁴ A/W under 800 nm illumination, which is 574 times larger than 154.4 A/W of α-In₂Se₃ photodetector. Our results demonstrate phase-engineering is a valid way to tune and further optimize electronic and optoelectronic performance of multilayer In₂Se₃ nanodevices.

New monolayer ternary In-containing sesquichalcogenides BiInSe3, SbInSe3, BiInTe3, and SbInTe3 with high stability and extraordinary piezoelectric properties

We predicted several novel monolayers consisting of ternary sesquichalcogenides with extraordinary piezoelectric properties, which may be alternatives to the conventional piezoelectric materials such as PZT.