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1. TaCo 2 Te 2 : An Air‐Stable, High Mobility Van der Waals Material with Probable Magnetic Order

   https://doi.org/10.1002/adfm.202108920  

   Singha, Ratnadwip ; Yuan, Fang ; Cheng, Guangming ; Salters, Tyger H. ; Oey, Yuzki M. ; Villalpando, Graciela V. ; Jovanovic, Milena ; Yao, Nan ; Schoop, Leslie M. ( November 2021 , Advanced Functional Materials)

   Van der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low‐dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high‐mobility charge carriers, which is an essential requirement for high‐speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, TaCo₂Te₂is introduced as an air‐stable, high‐mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. It is found that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after 4 months of monitoring using an optical microscope. Signatures of canted antiferromagntic behavior are also observed at low‐temperature. TaCo₂Te₂shows a metallic character and a large, nonsaturating, anisotropic magnetoresistance. Furthermore, the Hall data and quantum oscillation measurements reveal the presence of both electron‐ and hole‐type carriers and their high mobility.

    

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2. Controlling Magnetic and Optical Properties of the van der Waals Crystal CrCl 3− x Br x via Mixed Halide Chemistry

   https://doi.org/10.1002/adma.201801325  

   Abramchuk, Mykola ; Jaszewski, Samantha ; Metz, Kenneth R. ; Osterhoudt, Gavin B. ; Wang, Yiping ; Burch, Kenneth S. ; Tafti, Fazel ( May 2018 , Advanced Materials)

   Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy—a tendency of spins to align in a certain crystallographic direction known as easy‐axis. In layered materials, two limits of easy‐axis are in‐plane (XY) and out‐of‐plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin‐orbitronic functions. Here, a unique experiment is designed to control the easy‐axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl_3−xBr_xcrystals between CrCl₃withXYand CrBr₃with Ising anisotropy. The easy‐axis is controlled between the two limits by varying spin–orbit coupling with the Br content in CrCl_3−xBr_x. The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly withx. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.

    

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3. Data-driven studies of magnetic two-dimensional materials

   https://doi.org/10.1038/s41598-020-72811-z  

   Rhone, Trevor David ; Chen, Wei ; Desai, Shaan ; Torrisi, Steven B. ; Larson, Daniel T. ; Yacoby, Amir ; Kaxiras, Efthimios ( September 2020 , Scientific Reports)

   We use a data-driven approach to study the magnetic and thermodynamic properties of van der Waals (vdW) layered materials. We investigate monolayers of the form$$\hbox {A}_2\hbox {B}_2\hbox {X}_6$$${\text{A}}_2{\text{B}}_2{\text{X}}_6$, based on the known material$$\hbox {Cr}_2\hbox {Ge}_2\hbox {Te}_6$$${\text{Cr}}_2{\text{Ge}}_2{\text{Te}}_6$, using density functional theory (DFT) calculations and machine learning methods to determine their magnetic properties, such as magnetic order and magnetic moment. We also examine formation energies and use them as a proxy for chemical stability. We show that machine learning tools, combined with DFT calculations, can provide a computationally efficient means to predict properties of such two-dimensional (2D) magnetic materials. Our data analytics approach provides insights into the microscopic origins of magnetic ordering in these systems. For instance, we find that the X site strongly affects the magnetic coupling between neighboring A sites, which drives the magnetic ordering. Our approach opens new ways for rapid discovery of chemically stable vdW materials that exhibit magnetic behavior.

    

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4. Large piezoelectric response of van der Waals layered solids

   https://doi.org/10.1039/C8TC02560F  

   Manna, Sukriti ; Gorai, Prashun ; Brennecka, Geoff L. ; Ciobanu, Cristian V. ; Stevanović, Vladan ( October 2018 , Journal of Materials Chemistry C)

   The bulk piezoelectric response, as measured by the piezoelectric modulus tensor ( d ), is determined by a combination of charge redistribution due to strain and the amount of strain produced by the application of stress (stiffness). Motivated by the notion that less stiff materials could exhibit large piezoelectric responses, herein, we investigate the piezoelectric modulus of van der Waals (vdW) layered materials using first-principles calculations. From a pool of 869 known binary and ternary quasi-2D layered materials, we have identified 135 non-centrosymmetric crystals of which 51 are found to have piezoelectric modulus tensor ( d ) components larger than the longitudinal piezoelectric modulus of AlN, a commonly used piezoelectric material for resonators. We have also identified three materials with d components larger than that of PbTiO 3 , which is among the materials with the largest known piezoelectric modulus. None of the identified materials have previously been considered for piezoelectric applications. Furthermore, we find that large d components are always coupled to the shear or axial deformations of the vdW gap between the layers and are indeed enabled by the weak inter-layer interactions. 

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5. Exfoliation of boron carbide into ultrathin nanosheets

   https://doi.org/10.1039/D0NR07971E  

   Guo, Yuqi ; Gupta, Adway ; Gilliam, Matthew S. ; Debnath, Abhishek ; Yousaf, Ahmed ; Saha, Sanchari ; Levin, Mark D. ; Green, Alexander A. ; Singh, Arunima K. ; Wang, Qing Hua ( January 2021 , Nanoscale)

   Liquid phase exfoliation (LPE) is a method that can be used to produce bulk quantities of two-dimensional (2D) nanosheets from layered van der Waals (vdW) materials. In recent years, LPE has been applied to several non-vdW materials with anisotropic bonding to produce nanosheets and platelets, but it has not been demonstrated for materials with strong isotropic bonding. In this paper, we demonstrate the exfoliation of boron carbide (B 4 C), the third hardest known material, into ultrathin nanosheets. B 4 C has a structure consisting of strongly bonded boron icosahedra and carbon chains, but does not have anisotropic cleavage energies to suggest that it can be readily cleaved into nanosheets. B 4 C has been widely studied for its very high melting point, high mechanical strength, and chemical stability, as well as its zero- and one-dimensional nanostructured forms. Herein, ultrathin nanosheets are successfully prepared by sonication of B 4 C powder in organic solvents and are characterized by microscopy and spectroscopy. Density functional theory (DFT) simulations reveal that B 4 C can be cleaved along several different crystallographic planes with similar energetic favourability, facilititated by an unexpected mechanism of breaking boron icosahedra and forming new boron-rich cage structures at the surface. Atomic force microscopy (AFM) shows that the nanosheets produced by LPE are as thin as 5 nm, with an average thickness of 31.4 nm and average area of 16 000 nm 2 . Raman spectroscopy shows that many of the nanosheets exhibit additional carbon-rich peaks that change with laser irradiation, which are attributed to atomic rearrangements and amorphization at the nanosheet surfaces, consistent with the diverse cleavage planes. High-resolution transmission electron microscopy (HRTEM) demonstrates that many different cleavage planes exist among the exfoliated nanosheets, in agreement with DFT simulations. This work elucidates the exfoliation mechanism of 2D B 4 C and suggests that LPE can be applied to generate nanosheets from a variety of non-layered and non-vdW materials. 

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