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1. Top-Down Processing Towards Ångström-Thin Two-Dimensional (2D) Elemental Metals

   https://doi.org/10.1115/MSEC2020-8495  

   Rubayat-E Tanjil, Md; Agbakansi, Stanley; Suero, Keegan Phayden; Douglas, Ossie; Jeong, Yunjo; Yin, Zhewen; Panaccione, Wyatt; Wang, Michael Cai (January 2021, Top-Down Processing Towards Ångström-Thin Two-Dimensional (2D) Elemental Metals)

   Abstract Two-dimensional (2D) materials have recently garnered significant interest due to their novel and emergent properties. A plethora of 2D materials have been discovered and intensively studied, such as graphene, hexagonal boron nitride, transitionmetal dichalcogenides (TMDCs), and other metallic compound MXenes (nitrides, phosphides, and hydroxides), as well as elemental 2D materials (borophene, germanene, phosphorene, silicene, etc.). Considering the widespread interest in conventional van der Waals 2D materials, two-dimensional metallic nanosheets (2DMNS), a recent addition to the 2D materials family, have exhibited diverse potential spanning optics, electronics, magnetics, catalysis, etc. However, the close-packed, non-layered structure and non-directional, isotropic bonding of metallic materials make it difficult to access metals in their 2D forms, unlike 2D van der Waals materials, which have intrinsically layered structure (strong in-plane bonding in addition to the weak interlayer interaction). Until now, conventional top-down and bottom-up synthesis schemes of these 2DMNS have encountered various limitations such as precursor availability, substrate incompatibility, difficulty of control over thickness and stoichiometry, limited thermal budget, etc. To overcome these manufacturing limitations of 2DMNS, here we report a facile, rapid, large-scale, and cost-effective fabrication technique of nanometer-scale copper (Cu) 2DMNS via iterative rolling, folding, and calendering (RFC) that is readily generalizable to other conventional elemental metallic materials. Overall, we successfully show a scalable fabrication technique of 2DMNS. 

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2. Polariton Photonics Using Structured Metals and 2D Materials

   https://doi.org/10.1002/adom.201901090  

   Cai, Ziqiang; Xu, Yihao; Wang, Chuangtang; Liu, Yongmin (October 2019, Advanced Optical Materials)

   Abstract Polaritons are quasiparticles originating from strong interactions between photons and elementary excitations that could enable high tunability, tight electromagnetic field confinement, and large density of photonic states, making it possible to achieve novel and otherwise inaccessible functionalities. For these reasons, polaritons spawn great interest in the fields of physics, materials science, and optics for both fundamental studies as well as potential applications (e.g., modulators, photodetectors, photoluminescence, etc.). In recent years, the explosive growth of research in graphene and other 2D van der Waals materials is witnessed because they provide a new platform that substantially complements conventional metals, dielectrics, and semiconductors to investigate different polariton modes. This review highlights the works published in recent years on the topic of polariton photonics based on structured metals, graphene, and transition‐metal dichalcogenides (TMDs). The exotic optical properties of the polaritons in metallic structures and 2D van der Waals materials offer bright prospects for the development of high‐performance photonic and optoelectronic devices. 

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3. Modeling and simulations for 2D materials: a ReaxFF perspective

   https://doi.org/10.1088/2053-1583/acd7fd  

   Nayir, Nadire; Mao, Qian; Wang, Tao; Kowalik, Malgorzata; Zhang, Yuwei; Wang, Mengyi; Dwivedi, Swarit; Jeong, Ga-Un; Shin, Yun Kyung; van Duin, Adri (June 2023, 2D Materials)

   Abstract Recent advancements in the field of two-dimensional (2D) materials have led to the discovery of a wide range of 2D materials with intriguing properties. Atomistic-scale simulation methods have played a key role in these discoveries. In this review, we provide an overview of the recent progress in ReaxFF force field developments and applications in modeling the following layered and nonlayered 2D materials: graphene, transition metal dichalcogenides, MXenes, hexagonal boron nitrides, groups III-, IV- and V-elemental materials, as well as the mixed dimensional van der Waals heterostructures. We further discuss knowledge gaps and challenges associated with synthesis and characterization of 2D materials. We close this review with an outlook addressing the challenges as well as plans regarding ReaxFF development and possible large-scale simulations, which should be helpful to guide experimental studies in a discovery of new materials and devices. 

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4. Direct measurements of interfacial adhesion in 2D materials and van der Waals heterostructures in ambient air

   https://doi.org/10.1038/s41467-020-19411-7  

   Rokni, Hossein; Lu, Wei (November 2020, Nature Communications)

   Abstract Interfacial adhesion energy is a fundamental property of two-dimensional (2D) layered materials and van der Waals heterostructures due to their intrinsic ultrahigh surface to volume ratio, making adhesion forces very strong in many processes related to fabrication, integration and performance of devices incorporating 2D crystals. However, direct quantitative characterization of adhesion behavior of fresh and aged homo/heterointerfaces at nanoscale has remained elusive. Here, we use an atomic force microscopy technique to report precise adhesion measurements in ambient air through well-defined interactions of tip-attached 2D crystal nanomesas with 2D crystal and SiO_xsubstrates. We quantify how different levels of short-range dispersive and long-range electrostatic interactions respond to airborne contaminants and humidity upon thermal annealing. We show that a simple but very effective precooling treatment can protect 2D crystal substrates against the airborne contaminants and thus boost the adhesion level at the interface of similar and dissimilar van der Waals heterostructures. Our combined experimental and computational analysis also reveals a distinctive interfacial behavior in transition metal dichalcogenides and graphite/SiO_xheterostructures beyond the widely accepted van der Waals interaction. 

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5. Two-dimensional material templates for van der Waals epitaxy, remote epitaxy, and intercalation growth

   https://doi.org/10.1063/5.0090373  

   Ryu, Huije; Park, Hyunik; Kim, Joung-Hun; Ren, Fan; Kim, Jihyun; Lee, Gwan-Hyoung; Pearton, Stephen J. (September 2022, Applied Physics Reviews)

   Epitaxial growth, a crystallographically oriented growth induced by the chemical bonding between crystalline substrate and atomic building blocks, has been a key technique in the thin-film and heterostructure applications of semiconductors. However, the epitaxial growth technique is limited by different lattice mismatch and thermal expansion coefficients of dissimilar crystals. Two-dimensional (2D) materials with dangling bond-free van der Waals surfaces have been used as growth templates for the hetero-integration of highly mismatched materials. Moreover, the ultrathin nature of 2D materials also allows for remote epitaxial growth and confinement growth of quasi-2D materials via intercalation. Here, we review the hetero-dimensional growth on 2D substrates: van der Waals epitaxy (vdWE), quasi vdWE, and intercalation growth. We discuss the growth mechanism and fundamental challenges for vdWE on 2D substrates. We also examine emerging vdWE techniques that use epitaxial liftoff and confinement epitaxial growth in detail. Finally, we give a brief review of radiation effects in 2D materials and contrast the damage induced with their 3D counterparts. 

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