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1. Van der Waals interaction affects wrinkle formation in two-dimensional materials

   https://doi.org/10.1073/pnas.2025870118  

   Ares, Pablo ; Wang, Yi Bo ; Woods, Colin R. ; Dougherty, James ; Fumagalli, Laura ; Guinea, Francisco ; Davidovitch, Benny ; Novoselov, Kostya S. ( March 2021 , Proceedings of the National Academy of Sciences)

   null (Ed.)

   Nonlinear mechanics of solids is an exciting field that encompasses both beautiful mathematics, such as the emergence of instabilities and the formation of complex patterns, as well as multiple applications. Two-dimensional crystals and van der Waals (vdW) heterostructures allow revisiting this field on the atomic level, allowing much finer control over the parameters and offering atomistic interpretation of experimental observations. In this work, we consider the formation of instabilities consisting of radially oriented wrinkles around mono- and few-layer “bubbles” in two-dimensional vdW heterostructures. Interestingly, the shape and wavelength of the wrinkles depend not only on the thickness of the two-dimensional crystal forming the bubble, but also on the atomistic structure of the interface between the bubble and the substrate, which can be controlled by their relative orientation. We argue that the periodic nature of these patterns emanates from an energetic balance between the resistance of the top membrane to bending, which favors large wavelength of wrinkles, and the membrane-substrate vdW attraction, which favors small wrinkle amplitude. Employing the classical “Winkler foundation” model of elasticity theory, we show that the number of radial wrinkles conveys a valuable relationship between the bending rigidity of the top membrane and the strength of the vdW interaction. Armed with this relationship, we use our data to demonstrate a nontrivial dependence of the bending rigidity on the number of layers in the top membrane, which shows two different regimes driven by slippage between the layers, and a high sensitivity of the vdW force to the alignment between the substrate and the membrane. 

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2. Magnetic properties of intercalated quasi-2D Fe3-xGeTe2 van der Waals magnet

   https://doi.org/10.1038/s41699-023-00417-w  

   Iturriaga, Hector ; Martinez, Luis M ; Mai, Thuc T ; Biacchi, Adam J ; Augustin, Mathias ; Hight_Walker, Angela R ; Sanad, Mohamed Fathi ; Sreenivasan, Sreeprasad T ; Liu, Yu ; Santos, Elton_J G ; et al ( December 2023 , npj 2D Materials and Applications)

   Among several well-known transition metal-based compounds, cleavable van der Waals (vdW) Fe_3-xGeTe₂(FGT) magnet is a strong candidate for use in two-dimensional (2D) magnetic devices due to its strong perpendicular magnetic anisotropy, sizeable Curie temperature (T_C~154 K), and versatile magnetic character that is retained in the low-dimensional limit. While the T_Cremains far too low for practical applications, there has been a successful push toward improving it via external driving forces such as pressure, irradiation, and doping. Here we present experimental evidence of a room temperature (RT) ferromagnetic phase induced by the electrochemical intercalation of common tetrabutylammonium cations (TBA+) into quasi-2D FGT. We obtained Curie temperatures as high as 350 K with chemical and physical stability of the intercalated compound. The temperature-dependent Raman measurements, in combination with vdW-corrected ab initio calculations, suggest that charge transfer (electron doping) upon intercalation could lead to the observation of RT ferromagnetism. This work demonstrates that molecular intercalation is a viable route in realizing high-temperature vdW magnets in an inexpensive and reliable manner, and has the potential to be extended to bilayer and few-layer vdW magnets.

    

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3. Nanophotonic biosensors harnessing van der Waals materials

   https://doi.org/10.1038/s41467-021-23564-4  

   Oh, Sang-Hyun ; Altug, Hatice ; Jin, Xiaojia ; Low, Tony ; Koester, Steven J. ; Ivanov, Aleksandar P. ; Edel, Joshua B. ; Avouris, Phaedon ; Strano, Michael S. ( June 2021 , Nature Communications)

   Low-dimensional van der Waals (vdW) materials can harness tightly confined polaritonic waves to deliver unique advantages for nanophotonic biosensing. The reduced dimensionality of vdW materials, as in the case of two-dimensional graphene, can greatly enhance plasmonic field confinement, boosting sensitivity and efficiency compared to conventional nanophotonic devices that rely on surface plasmon resonance in metallic films. Furthermore, the reduction of dielectric screening in vdW materials enables electrostatic tunability of different polariton modes, including plasmons, excitons, and phonons. One-dimensional vdW materials, particularly single-walled carbon nanotubes, possess unique form factors with confined excitons to enable single-molecule detection as well as in vivo biosensing. We discuss basic sensing principles based on vdW materials, followed by technological challenges such as surface chemistry, integration, and toxicity. Finally, we highlight progress in harnessing vdW materials to demonstrate new sensing functionalities that are difficult to perform with conventional metal/dielectric sensors.

    

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4. Naturally occurring van der Waals heterostructure lengenbachite with strong in-plane structural and optical anisotropy

   https://doi.org/10.1038/s41699-021-00271-8  

   Dasgupta, Arindam ; Yang, Xiaodong ; Gao, Jie ( November 2021 , npj 2D Materials and Applications)

   Lengenbachite is a naturally occurring layered mineral formed with alternating stacks of two constituent PbS-like and M₂S₃-like two-dimensional (2D) material layers due to the phase segregation process during the formation. Here, we demonstrate to achieve van der Waals (vdW) heterostructures of lengenbachite down to a few layer-pair thickness by mechanical exfoliation of bulk lengenbachite mineral. The incommensurability between the constituent isotropic 2D material layers makes the formed vdW heterostructure exhibit strong in-plane structural anisotropy, which leads to highly anisotropic optical responses in lengenbachite thin flakes, including anisotropic Raman scattering, linear dichroism, and anisotropic third-harmonic generation. Moreover, we exploit the nonlinear optical anisotropy for polarization-dependent intensity modulation of the converted third-harmonic optical vortices. Our study establishes lengenbachite as a new natural vdW heterostructure-based 2D material with unique optical properties for realizing anisotropic optical devices for photonic integrated circuits and optical information processing.

    

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5. Large in-plane vibrational and optical anisotropy in natural 2D heterostructure abramovite

   https://doi.org/10.1038/s41598-022-21042-5  

   Dasgupta, Arindam ; Belakovskiy, Dmitriy I. ; Chaplygin, Ilya V. ; Gao, Jie ; Yang, Xiaodong ( December 2022 , Scientific Reports)

   Abstract The design and formation of van der Waals (vdW) heterostructures with different two-dimensional (2D) materials provide an opportunity to create materials with extraordinary physical properties tailored toward specific applications. Mechanical exfoliation of natural vdW materials has been recognized as an effective way for producing high-quality ultrathin vdW heterostructures. Abramovite is one of such naturally occurring vdW materials, where the superlattice is composed of alternating Pb 2 BiS 3 and SnInS 4 2D material lattices. The forced commensuration between the two incommensurate constituent 2D material lattices induces in-plane structural anisotropy in the formed vdW heterostructure of abramovite, even though the individual 2D material lattices are isotropic in nature. Here, we show that ultrathin layers of vdW heterostructures of abramovite can be achieved by mechanical exfoliation of the natural mineral. Furthermore, the structural anisotropy induced highly anisotropic vibrational and optical responses of abramovite thin flakes are demonstrated by angle-resolved polarized Raman scattering, linear dichroism, and polarization-dependent third-harmonic generation. Our results not only establish abramovite as a promising natural vdW material with tailored linear and nonlinear optical properties for building future anisotropic integrated photonic devices, but also provide a deeper understanding of the origin of structural, vibrational and optical anisotropy in vdW heterostructures. 

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