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Abstract The mechanical exfoliation of naturally occurring layered materials has emerged as an easy and effective method for achieving ultrathin van der Waals (vdW) heterostructures with well-defined lattice orientations of the constituent two-dimensional (2D) material layers. Cylindrite is one such naturally occurring vdW heterostructure, where the superlattice is composed of alternating stacks of SnS 2 -like and PbS-like layers. Although the constituent 2D lattices are isotropic, inhomogeneous strain occurring from local atomic alignment for forcing the commensuration makes the cylindrite superlattice structurally anisotropic. Here, we demonstrate the highly anisotropic optical responses of cylindrite thin flakes induced by the anisotropic crystal structure, including angle-resolved polarized Raman scattering, linear dichroism, and polarization-dependent anisotropic third-harmonic generation. Our results provide a promising approach for identifying various natural vdW heterostructure-based 2D materials with tailored optical properties and can be harnessed for realizing anisotropic optical devices for on-chip photonic circuits and optical information processing.
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Exfoliation of boron carbide into ultrathin nanosheets
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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- Award ID(s):
- 1906030
- PAR ID:
- 10346667
- Date Published:
- Journal Name:
- Nanoscale
- Volume:
- 13
- Issue:
- 3
- ISSN:
- 2040-3364
- Page Range / eLocation ID:
- 1652 to 1662
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/D0NR07971E
