Structure and exfoliation mechanism of two-dimensional boron nanosheets

Abstract

Exfoliation of two-dimensional (2D) nanosheets from three-dimensional (3D) non-layered, non-van der Waals crystals represents an emerging strategy for materials engineering that could significantly increase the library of 2D materials. Yet, the exfoliation mechanism in which nanosheets are derived from crystals that are not intrinsically layered remains unclear. Here, we show that planar defects in the starting 3D boron material promote the exfoliation of 2D boron sheets-by combining liquid-phase exfoliation, aberration-corrected scanning transmission electron microscopy, Raman spectroscopy, and density functional theory calculations. We demonstrate that 2D boron nanosheets consist of a planar arrangement of icosahedral sub-units cleaved along the {001} planes of β-rhombohedral boron. Correspondingly, intrinsic stacking faults in 3D boron form parallel layers of faulted planes in the same orientation as the exfoliated nanosheets, reducing the {001} cleavage energy. Planar defects represent a potential engineerable pathway for exfoliating 2D sheets from 3D boron and, more broadly, the other covalently bonded materials.

Fig. 1. Liquid phase exfoliation of boron.

a Schematic depiction of products through liquid-phase exfoliation from van der Waals (vdW) crystals, isotropic polycrystalline materials, and boron. b Scanning electron microscopy (SEM) images of the starting boron material: Sample A (Sigma-Aldrich), Sample B (Alfa-Aesar), and Sample C (Yamanaka Advanced Materials). c Converged beam transmission electron microscopy (TEM) image, and corresponding high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of 2D boron sheets suspended on a holey carbon TEM grid.

Fig. 2. Phase and elemental characterization of 3D boron and exfoliated 2D boron.

a Atomic-resolved center bright-field (C-BF) STEM image of 3D boron revealing the β-rhombohedral phase viewed along the [100] zone-axis. b Electron energy loss spectroscopy (EELS) spectra of 3D and exfoliated 2D boron, with the B K edge marked by the dashed line, and the B-O near-edge structure marked by the arrow. c Schematic unit cell of β-rhombohedral boron (projected in the [11$\overline{2}$] orientation), which can be represented separately in terms of (d) B₁₂ icosahedra located at the vertices (light green) and middle of each edge (dark green) of the rhombohedron, and (e) two B₂₈ triple-fused icosahedra (brown) + one boron atom (red) centrally located in the middle of the rhombohedron. f X-ray diffraction (XRD) patterns of 3D Samples A, B and C and exfoliated Sample C. The simulated pattern (in black) is shown indicating Miller indices of rhombohedral boron. The curves have been vertically shifted for clarity. g Raman spectra of 3D boron and exfoliated Sample C. Regions corresponding to the vibrations of the individual unit cell subunits are segmented by the vertical dashed lines and displayed at the bottom. The shaded regions denote the A_1g modes. The curves have been vertically shifted for clarity.

Fig. 3. Atomic structure of liquid-phase exfoliation (LPE) derived boron.

Schematic illustrations showing the differences in the STEM projection for boron nanosheets derived from {001} icosahedral planes with (a) 1 layer, and (b) 4 layers. c Atomic-resolved C-BF STEM of an exfoliated nanosheet. d–f STEM simulations with probe direction perpendicular to the {001} plane of β-rhombohedral boron with 2, 4, and 10 layers of the {001} plane, respectively. The same atomic patterns in the experimental image in c and the simulated image in e are outlined to guide the eye.

Fig. 4. Atomic-structure of 3D β-rhombohedral boron from various zone-axes.

Simulated STEM images and atomic models are shown on the right of the experimental C-BF STEM images (β = 0−15 mrad) images. The B₁₂ icosahedrons are depicted in green, B₂₈ triple-fused icosahedrons in brown, and the center boron atom in red. The rhombohedrons on the left of the images demonstrate the relation of the electron beam direction (arrow) with the rhombohedral unit cell. (a–c) show the three projections with the largest spacing between atomic columns where the B₁₂ icosahedra columns are aligned parallel to the beam direction. (d–j) show viewing orientations of lower crystal symmetry. For clarity, bonds are omitted in the models for the lower symmetry projections.

Fig. 5. Role of planar defects in the exfoliation mechanism of boron.

a TEM BF image of 3D boron (Sample C) with a high density of planar defects in the bottom right crystal grain 2. b C-BF STEM image of β-rhombohedral boron projected along the [001] zone-axis showing a sequence of parallel planar defects, including twins and stacking faults (dashed lines indicate the defect plane). The icosahedral columns are represented as red-ringed circles for easier visualization of the symmetry breaking due to the defects. The magnified images of the twin boundary and stacking fault (SF), boxed in light blue and red, respectively, are shown on the right with the faulted plane marked by the black arrows. c Model of the unit cell projected in the [001] depicting the atoms (red circles) forming the defect planes. d Density functional theory (DFT) calculated energies for the formation of the {001}-I and {001}-II surfaces from the pristine crystal and from the planar defects. e Schematic of the defect-mediated exfoliation process (viewed from the [1$\overline{1}$0] orientation). The icosahedral columns are represented as red-ringed circles, stacking faults as dashed lines, and symmetry breaking of the crystal as solid lines. The black arrows represent the cleaving of the crystal from the stacking faults.