Artificial intelligence guided search for van der Waals materials with high optical anisotropy.

Origins of heat transport anisotropy in MoTe2 and other bulk van der Waals materials

Germanene, a 2D honeycomb germanium crystal, is grown at graphene/Ag(111) and hexagonal boron nitride (h‐BN)/Ag(111) interfaces by segregating germanium atoms. A simple annealing process in N2 or H2/Ar at ambient pressure leads to the formation of germanene, indicating that an ultrahigh‐vacuum condition is not necessary. The grown germanene is stable in air and uniform over the entire area covered with a van der Waals (vdW) material. As an important finding, it is necessary to use a vdW material as a cap layer for the present germanene growth method since the use of an Al2O3 cap layer results in no germanene formation. The present study also proves that Raman spectroscopy in air is a powerful tool for characterizing germanene at the interfaces, which is concluded by multiple analyses including first‐principles density functional theory calculations. The direct growth of h‐BN‐capped germanene on Ag(111), which is demonstrated in the present study, is considered to be a promising technique for the fabrication of future germanene‐based electronic devices.

Van der Waals (vdW) materials consisting of two-dimensional (2D) building blocks have strong in-plane covalent bonding and weak interlayer interactions. While monolayer 2D materials exhibit impressive fracture resistance, as demonstrated in hexagonal boron nitride (h-BN), preserving these remarkable properties in vdW materials remains a challenge. Here we reveal an anomalous mechanical interlayer coupling that involves interlayer-friction toughening and edge-reconstruction embrittlement during the fracture of multilayer h-BN. Both asynchronous and synchronous fracture modes and their flaw-size dependence are identified. Edge reconstruction in the synchronous fracture mode can eliminate a toughening mechanism induced by lattice asymmetry in monolayer h-BN, leading to embrittlement of the multilayer h-BN, while the asynchronous fracture mode results in greater fracture resistance. Such findings will provide fundamental guidelines for engineering interlayer interactions in vdW materials including heterostructures and layered architectures for better mechanical and functional performances.

Many fascinating properties and applications of van der Waals (vdW) materials arise from how their layers slide against each other. Quantifying friction at the nanoscale is essential for further advancing the assembling of sophisticated heterostructures and their applications. A unified model and experimental validation for such friction forces remain elusive. A simple linear relationship between friction force and the energy barrier encountered by an atomic force microscope (AFM) tip interacting with crystal atoms has been proposed but not experimentally validated. Here, we present a unified understanding of nanoscale friction in vdW materials through lateral force microscopy measurements using a single AFM tip on four commonly studied vdW materials: MoS2, MoSe2, WS2, and graphene. Remarkably, the measured friction not only follows a proportional relationship with respect to the energy barriers calculated using the Lennard-Jones potential but agrees with predicted values within 5% across all examined materials. This agreement is achieved by accounting for variations in the number of contact-edge tip atoms that likely settle simultaneously into hollow sites in each material. Our findings provide critical insights to support a wide range of research efforts that rely on an advanced understanding of nanotribology in vdW materials.

Recent progress in strong spin-orbit coupling van der Waals materials and their heterostructures for spintronic applications

This paper investigates the importance of explainability versus accuracy of machine learning (ML) models. We propose that greater task uncertainty makes people want to interact more with the ML model, which increases the importance of explainability relative to accuracy. We focus on the use of ML models for product cost estimation during new product development. The paper provides mixed-methods evidence on the trade-off between explainability and accuracy of ML models. Specifically, we find support for an inverse relationship between explainability and accuracy from the perspective of cost experts. We also find that the accurate but complex and less explainable ML model of gradient boosted regression (GBR) was preferred in only a few situations; mostly, the more basic, better explainable models of multiple linear regression (MLR) and case-based reasoning (CBR) were preferred, although these were less accurate. This suggests that lack of explainability can indeed be a major limitation for the application of ML models. Furthermore, we investigate specific characteristics that could increase task uncertainty and the importance of explainability in our context: project unpredictability, product cost granularity, predecessor product availability, target cost gap, and product development phase.

Magnetic van der Waals (vdW) materials have emerged as promising candidates for spintronics applications, especially after the recent discovery of intrinsic ferromagnetism in monolayer vdW materials. There has been a critical need for tunable ferromagnetic vdW materials beyond room temperature. Here, we report a real-space imaging study of itinerant ferromagnet Fe3GeTe2 and the enhancement of its Curie temperature well above ambient temperature. We find that the magnetic long-range order in Fe3GeTe2 is characterized by an unconventional out-of-plane stripe-domain phase. In Fe3GeTe2 microstructures patterned by a focused ion beam, the out-of-plane stripe domain phase undergoes a surprising transition at 230 K to an in-plane vortex phase that persists beyond room temperature. The discovery of tunable ferromagnetism in Fe3GeTe2 materials opens up vast opportunities for utilizing vdW magnets in room-temperature spintronics devices.

The advancement of integrated nanophotonics requires the development and characterization of materials with high refractive indices and low optical losses. Van der Waals (vdW) materials, particularly transition metal dichalcogenides (TMDs) such as WSe2 and MoSe2, exhibit giant optical anisotropy and high refractive indices (n > 4) in the near-infrared range, making them promising candidates for next-generation photonic circuits. However, accurately determining their anisotropic optical constants, especially the out-of-plane component, is challenging due to the limited size of exfoliated flakes and the limitations of far-field techniques. This study details the application of scattering-type scanning near-field optical microscopy (s‑SNOM) for the nanoscale characterization of these materials. The principles of s-SNOM, including the excitation and detection of waveguide modes in planar vdW structures, are discussed. By analyzing the interference fringes generated by propagating modes, the effective mode index is determined. We demonstrate this methodology using WSe2 and MoSe2 flakes, where the experimental effective indices are compared with theoretical models. This comparison confirms the necessity of accounting for giant optical anisotropy and validates the out-of-plane dielectric constants, illustrating the efficacy of s-SNOM for the comprehensive optical characterization of vdW materials.

A complex interplay of different energy scales involving Coulomb repulsion, spin-orbit coupling, and Hund's coupling energy in 2D van der Waals (vdW) material produces a novel emerging physical state. For instance, ferromagnetism in vdW charge transfer insulator CrGeTe_{3} provides a promising platform to simultaneously manipulate the magnetic and electrical properties for potential device implementation using few nanometers thick materials. Here, we show a continuous tuning of magnetic and electrical properties of a CrGeTe_{3} single crystal using pressure. With application of pressure, CrGeTe_{3} transforms from a ferromagnetic insulator with Curie temperature T_{C}∼66 K at ambient condition to a correlated 2D Fermi metal with T_{C} exceeding ∼250 K. Notably, absence of an accompanying structural distortion across the insulator-metal transition (IMT) suggests that the pressure induced modification of electronic ground states is driven by electronic correlation furnishing a rare example of bandwidth-controlled IMT in a vdW material.

Nanophotonics relies on precise nanoscale structuring, yet conventional fabrication techniques remain complex and costly. Layered van der Waals (vdW) materials, with their intrinsic anisotropy and high refractive indices, offer a promising route toward simplified nanostructuring and tunable optical functionality. However, no vdW material has previously been shown to exhibit a strong photorefractive effect-a key requirement for light-based modulation. Here, we report a giant photorefractive response (Δn up to 0.3) in crystalline arsenic trisulfide (As2S3), observed at low optical intensities. In addition to refractive-index modulation, light exposure enables controlled thickness tuning of As2S3. The material exhibits a giant photoexpansion of up to 7%, depending on the illumination intensity, which may originate from light-induced generation of point defects, consistent with molecular-dynamics modeling. Building on this photoexpansion effect, we introduce a maskless nanopatterning technique based on continuous-wave laser writing, achieving ~500 nm pitch (~50,000 dpi) without the need for ultrafast lasers. The combination of high photosensitivity, anisotropy, ease of exfoliation and transfer, and optical transparency positions vdW As2S3 as a practical platform for integrated photonics, adaptive optics, reconfigurable photonic elements, and dense optical encoding.

Recent progress in Van der Waals (vdW) heterojunction-based electronic and optoelectronic devices

The objective of this document is to identify the most accurate machine learning (ML) models for predicting the value of a real estate property, based on a systematic literature review (SLR). It was conducted on research published between 2022 and 2023 that analyzed the accuracy of ML models in real estate valuation. Information was extracted regarding the ML models used, the databases employed, and the models highlighted for their accuracy. A variety of ML models used in real estate valuation were identified, including Random Forest (RF), XGBoost, Gradient Boosting Machine (GBM), Linear Regression (LR), and Lasso Regression. RF and LR models stood out as the most accurate in the analyzed research, showing that the accuracy of ML models varies depending on the database, property characteristics, and valuation context. It was concluded that ML models like RF and LR are promising tools for improving the accuracy of real estate valuation. Additionally, the choice of the best ML model depends on factors such as the database, property characteristics, and specific valuation objectives. Further research is needed on the application of ML in real estate valuation, considering aspects such as model interpretability and the real estate market context.

A confined electronic system can host a wide variety of fascinating electronic, magnetic, valleytronic and photonic phenomena due to its reduced symmetry and quantum confinement effect. For the recently emerging one-dimensional van der Waals (1D vdW) materials with electrons confined in 1D sub-units, an enormous variety of intriguing physical properties and functionalities can be expected. Here, we demonstrate the coexistence of giant linear/nonlinear optical anisotropy and high emission yield in fibrous red phosphorus (FRP), an exotic 1D vdW semiconductor with quasi-flat bands and a sizeable bandgap in the visible spectral range. The degree of photoluminescence (third-order nonlinear) anisotropy can reach 90% (86%), comparable to the best performance achieved so far. Meanwhile, the photoluminescence (third-harmonic generation) intensity in 1D vdW FRP is strong, with quantum efficiency (third-order susceptibility) four (three) times larger than that in the most well-known 2D vdW materials (e.g., MoS2). The concurrent realization of large linear/nonlinear optical anisotropy and emission intensity in 1D vdW FRP paves the way towards transforming the landscape of technological innovations in photonics and optoelectronics.

The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials.

The area of two-dimensional (2D) materials research would benefit greatly from the development of synthetically tunable van der Waals (vdW) materials. While the bottom-up synthesis of 2D frameworks from nanoscale building blocks holds great promise in this quest, there are many remaining hurdles, including the design of building blocks that reliably produce 2D lattices and the growth of macroscopic crystals that can be exfoliated to produce 2D materials. Here we report the regioselective synthesis of the cluster [trans-Co6Se8(CN)4(CO)2]3-/4-, a "superatomic" building block designed to polymerize and assemble into a 2D cyanometalate lattice whose surfaces are chemically addressable. The resulting vdW material, [Co(py)4]2[trans-Co6Se8(CN)4(CO)2], grows as bulk single crystals that can be mechanically exfoliated to produce flakes as thin as bilayers, with photolabile CO ligands on the exfoliated surface. As a proof of concept, we show that these surface CO ligands can be replaced by 4-isocyanoazobenzene under blue light irradiation. This work demonstrates that the bottom-up assembly of layered vdW materials from superatoms is a promising and versatile approach to create 2D materials with tunable physical and chemical properties.