Tunable Modal Birefringence in a Low‐Loss Van Der Waals Waveguide

van der Waals (vdW) crystals are promising candidates for integrated phase retardation applications due to their large optical birefringence. Among the two major types of vdW materials, the hyperbolic vdW crystals are inherently inadequate for optical retardation applications since the supported polaritonic modes are exclusively transverse‐magnetic (TM) polarized and relatively lossy. Elliptic vdW crystals, on the other hand, represent a superior choice. For example, molybdenum disulfide (MoS₂) is a natural uniaxial vdW crystal with extreme elliptic anisotropy in the frequency range of optical communication. Both transverse‐electric (TE) polarized ordinary and TM polarized extraordinary waveguide modes can be supported in MoS₂ microcrystals with suitable thicknesses. In this work, low‐loss transmission of these guided modes is demonstrated with nano‐optical imaging at the near‐infrared (NIR) wavelength (1530 nm). More importantly, by combining theoretical calculations and NIR nanoimaging, the modal birefringence between the orthogonally polarized TE and TM modes is shown to be tunable in both sign and magnitude via varying the thickness of the MoS₂ microcrystal. This tunability represents a unique new opportunity to control the polarization behavior of photons with vdW materials.     

Gate-Coupling-Enabled Robust Hysteresis for Nonvolatile Memory and Programmable Rectifier in Van der Waals Ferroelectric Heterojunctions

Ferroelectric field-effect transistors (FeFETs) are one of the most interesting ferroelectric devices; however, they, usually suffer from low interface quality. The recently discovered 2D layered ferroelectric materials, combining with the advantages of van der Waals heterostructures (vdWHs), may be promising to fabricate high-quality FeFETs with atomically thin thickness. Here, dual-gated 2D ferroelectric vdWHs are constructed using MoS₂, hexagonal boron nitride (h-BN), and CuInP₂S₆ (CIPS), which act as a high-performance nonvolatile memory and programmable rectifier. It is first noted that the insertion of h-BN and dual-gated coupling device configuration can significantly stabilize and effectively polarize ferroelectric CIPS. Through this design, the device shows a record-high performance with a large memory window, large on/off ratio (10⁷), ultralow programming state current (10⁻¹³ A), and long-time endurance (10⁴ s) as nonvolatile memory. As for programmable rectifier, a wide range of gate-tunable rectification behavior is observed. Moreover, the device exhibits a large rectification ratio (3 × 10⁵) with stable retention under the programming state. This demonstrates the promising potential of ferroelectric vdWHs for new multifunctional ferroelectric devices.     

Highly Confined and Tunable Hyperbolic Phonon Polaritons in Van Der Waals Semiconducting Transition Metal Oxides

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid‐infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α‐MoO₃. The α‐MoO₃ supports strong hyperbolic PhPs in the mid‐infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ₀/120, which can be tailored by altering the thicknesses of the α‐MoO₃ 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal‐ion‐intercalation strategy. The results demonstrate α‐MoO₃ as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid‐infrared nanophotonic devices.     

Thickness‐Dependent In‐Plane Polarization and Structural Phase Transition in van der Waals Ferroelectric CuInP2S6

van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intralayer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high‐density storage devices. With the thickness becoming thinner, the competition between the surface energy, depolarization field, and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, the existence of the intrinsic in‐plane polarization in vdW ferroelectrics CuInP₂S₆ single crystals is reported, whereas below a critical thickness between 90 and 100 nm, the in‐plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, these behaviors are ascribed to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. These findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.     

Piezoelectricity Across a Strain‐Induced Isosymmetric Ferri‐to‐Ferroelectric Transition

We identify a first‐order, isosymmetric transition between a ferrielectric (FiE) and ferroelectric (FE) state in A‐site ordered LaScO₃/BiScO₃ and LaInO₃/BiInO₃ superlattices. Such a previously unreported ferroic transition is driven by the easy switching of cation displacements without changing the overall polarization direction or crystallographic symmetry. Epitaxial strains less than 2% are predicted to be sufficient to traverse the phase boundary, across which we capture a ≈5× increase in electric polarization. Unlike conventional Pb‐based perovskite ceramics with a morphotropic phase boundary (MPB) that show polarization rotation, we predict an electromechanical response up to 102 pC/N in the vicinity of the FiE‐FE phase boundary due to polarization switching without any change in symmetry. We propose this transition as an alternative ferroic transition to obtain a piezoelectric response, with the additional advantage of occurring in benign chemistries without chemical disorder.     

Cross‐Plane Carrier Transport in Van der Waals Layered Materials

The mechanisms of carrier transport in the cross‐plane crystal orientation of transition metal dichalcogenides are examined. The study of in‐plane electronic properties of these van der Waals compounds has been the main research focus in recent years. However, the distinctive physical anisotropies, short‐channel physics, and tunability of cross layer interactions can make the study of their electronic properties along the out‐of‐plane crystal orientation valuable. Here, the out‐of‐plane carrier transport mechanisms in niobium diselenide and hafnium disulfide are explored as two broadly different representative materials. Temperature‐dependent current–voltage measurements are preformed to examine the mechanisms involved. First principles simulations and a tunneling model are used to understand these results and quantify the barrier height and hopping distance properties. Using Raman spectroscopy, the thermal response of the chemical bonds is directly explored and the insight into the van der Waals gap properties is acquired. These results indicate that the distinct cross‐plane carrier transport characteristics of the two materials are a result of material thermal properties and thermally mediated transport of carriers through the van der Waals gaps. Exploring the cross‐plane electron transport, the exciting physics involved is unraveled and potential new avenues for the electronic applications of van der Waals layers are inspired.     

Metal Thio‐ and Selenophosphates as Multifunctional van der Waals Layered Materials

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow‐on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase‐transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate‐ to wide‐bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought‐after properties of complex oxides with electronic functions of 2D and quasi‐2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.     

High‐Performance Photoinduced Memory with Ultrafast Charge Transfer Based on MoS2/SWCNTs Network Van Der Waals Heterostructure

Photoinduced memory devices with fast program/erase operations are crucial for modern communication technology, especially for high‐throughput data storage and transfer. Although some photoinduced memories based on 2D materials have already demonstrated desirable performance, the program/erase speed is still limited to hundreds of micro‐seconds. A high‐speed photoinduced memory based on MoS₂/single‐walled carbon nanotubes (SWCNTs) network mixed‐dimensional van der Waals heterostructure is demonstrated here. An intrinsic ultrafast charge transfer occurs at the heterostructure interface between MoS₂ and SWCNTs (below 50 fs), therefore enabling a record program/erase speed of ≈32/0.4 ms, which is faster than that of the previous reports. Furthermore, benefiting from the unique device structure and material properties, while achieving high‐speed program/erase operation, the device can simultaneously obtain high program/erase ratio (≈10⁶), appropriate storage time (≈10³ s), record‐breaking detectivity (≈10¹⁶ Jones) and multibit storage capacity with a simple program/erase operation. It even has a potential application as a flexible optoelectronic device. Therefore, the designed concept here opens an avenue for high‐throughput fast data communications.     

Tunable and Ultraefficient Microwave Absorption Properties of Trace N‐Doped Two‐Dimensional Carbon‐Based Nanocomposites Loaded with Multi‐Rare Earth Oxides

A high efficiency and great tunability of bandwidth and absorption‐range electromagnetic wave absorber is proposed without precedent. A series of 2D carbon‐based nanocomposites with the loading of cerium oxide (CN‐Ce) and other types of rare earth oxides (CN‐REOs) can be successfully synthesized by a simple solvothermal‐sintering method. As‐synthesized 2D nanocomposites with local graphite‐like C₃N₄ structure and trace N‐doped are identified by transmission electron microscopy, X‐ray photoelectron spectroscopy, X‐ray powder diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. The CN‐REOs and polyvinylidene fluoride composite absorbers with reflection loss values above ?40 dB are obtained in C‐band, X‐band, and Ku‐band, respectively. The empirical rules on effective bandwidth and frequency range are discovered and summarized, which can be successfully realized by simply tuning the doping amount or type of REO. The mechanism is explained by enhanced attenuation and tunable impedance matching. In addition co‐filled samples by two types of CN‐REOs nanocomposites are prepared to support these findings and inspire the preparation of absorber with desirable frequency band in the range of 2–18 GHz.     

Emergent Low‐Symmetry Phases and Large Property Enhancements in Ferroelectric KNbO3 Bulk Crystals

The design of new or enhanced functionality in materials is traditionally viewed as requiring the discovery of new chemical compositions through synthesis. Large property enhancements may however also be hidden within already well‐known materials, when their structural symmetry is deviated from equilibrium through a small local strain or field. Here, the discovery of enhanced material properties associated with a new metastable phase of monoclinic symmetry within bulk KNbO₃ is reported. This phase is found to coexist with the nominal orthorhombic phase at room temperature, and is both induced by and stabilized with local strains generated by a network of ferroelectric domain walls. While the local microstructural shear strain involved is only ≈0.017%, the concurrent symmetry reduction results in an optical second harmonic generation response that is over 550% higher at room temperature. Moreover, the meandering walls of the low‐symmetry domains also exhibit enhanced electrical conductivity on the order of 1 S m^?1. This discovery reveals a potential new route to local engineering of significant property enhancements and conductivity through symmetry lowering in ferroelectric crystals.