Misfit-generated structural and optical anisotropies of the natural ${\mathrm{MoS}}_2\text{−}\mathrm{PbS}$ van der Waals heterostructure merelaniite

Significant efforts developing ${\mathrm{MoS}}_2\text{−}\mathrm{PbS}$-based heterostructure devices with several different architectures show promise for photosensor, solar cell, and chemical sensor applications. Merelaniite $\left({\mathrm{Mo}}_4{\mathrm{Pb}}_4{\mathrm{VSbS}}_{15}\right)$ is a newly discovered natural van der Waals heterostructure of the cylindrite type, composed predominantly of heavily modulated pseudotetragonal PbS layers and pseudohexagonal ${\mathrm{MoS}}_2$ layers with large misfit-induced anisotropy. For an incommensurate modulated structure, the refined structural model from single-crystal x-ray diffraction analysis is in reasonable agreement with the results obtained by high-resolution scanning transmission electron microscopy, especially in light of the fact that that the two isolated single-crystal domains used for the x-ray and electron diffraction experiments were extracted from two different whiskers and subjected to different sample preparation methods. The effects of the misfit-induced structural anisotropy are studied using angle-resolved polarized Raman spectroscopy. The intensities of 12 Raman modes are studied as a function of incident polarization angle relative to merelaniite's whisker axis, and show maximal intensity with the polarization direction perpendicular to the whisker axis. Polarization-dependent anisotropic third-harmonic generation from ultrathin mechanically exfoliated flakes reveals the anisotropy of the third-order nonlinear susceptibility tensor. Merelaniite demonstrates an expanded structure-chemistry space for engineering stable layered materials for potential device applications.

Figure 1

Morphology of merelaniite. (a) SEM image of merelaniite whiskers showing the circumferentially wound layers. (b) Cylindrical whiskers and comblike thin layers of merelaniite enclosed in calcite (8.75-mm-wide field of view). (c) SEM image of flat merelaniite layers on the (001) surface of a graphite crystal. (d) A 1.4-mm-long section of cylindrical merelaniite in calcite with attached partially nonwound thin merelaniite layers. The sample shown in (d) is in the Natural History Museum (London) collection, no. BM2017,111).

Figure 2

Crystal structure of merelaniite. (a) View as seen down [100] with ${\mathbf{a}}_{13}^{*}$ vertical. (b) The $H$ layer as seen down [001]. (c) The $Q$ layer as seen down [001]. Green, gray, and yellow circles refer to Pb, Mo, and S atoms, respectively.

Figure 3

Structure modulations. (a) Variation of the bond-valence sums (BVS) of Mo as a function of the fourth coordinate ($t$) in the superspace in the $H$ layer of the merelaniite structure. (b) Variation of the Pb–S bond distances as a function of the fourth coordinate ($t$) in the superspace in the $Q$ layer of the merelaniite structure.

Figure 4

Atomic resolution imaging of merelaniite. (a) HAADF-STEM image of a merelaniite crystal projected along [100] zone axis. In this mode, brighter contrast comes from elements with higher atomic number. Here, the brightest columns consist of Pb atoms, and the less-bright columns consist of Mo atoms. A bend in the sample is visible at the top right. The section outlined in red is presented at higher magnification in (b) with the refined structure model projected on it. Pb, Mo, and S atoms are shown in green, gray, and yellow spheres, respectively.

Figure 5

Atomic resolution chemical mapping of merelaniite. (a) Atomic-resolution EDS x-ray maps of Pb, Mo, S, and all three superimposed on HAADF-STEM image of a merelaniite crystal projected along [100] zone axis. Scale is 5 nm. (b) Line profiles of the atoms from the boxed area shown in (a).

Figure 6

High-resolution XPS spectra around the respective binding energy regions of (a) Mo $3d$, S $2s$, (b) Pb $4f$, and (c) S $2p$.

Figure 7

Anisotropic Raman scattering from merelaniite crystal. (a) Recorded Raman spectrum from the top surface area of merelaniite cylinder (spot 1) in parallel polarization configuration, where the observed Raman peaks are labeled. (b) Contour map of the angle-resolved polarized Raman spectra obtained in parallel polarization configuration. (c)–(n) Polar plots for different Raman modes at 142, 163, 189, 215, 230, 286, 326, 354, 383, 392, 404, and $459\mathrm{c}{\mathrm{m}}^{-1}$. The measured values are indicated with black points, while the theoretical fits are shown with red solid curves.

Figure 8

Mechanical exfoliation of merelaniite thin flakes. (a), (c), (e), (g), (i) Optical reflection microscope images of mechanically exfoliated merelaniite crystals with thicknesses of 8, 17, 49, 61, and 121 nm, respectively. The yellow dashed boxes indicate the scanned regions for AFM images. (b), (d), (f), (h), (j) Corresponding AFM images in the scanned regions. AFM line profiles confirm the thicknesses of the probed crystals. Scale bars are 2 μm.

Figure 9

Anisotropic THG emission from merelaniite thin flakes. (a) Recorded THG spectrum from the 8-nm-thick merelaniite flake, showing a peak emission at 520 nm. Inset shows the transmission optical image of the THG emission from the flake. (b) Log-scale plot of the measured average THG power as a function of the incident laser power. (c) Polar plot of the Raman $A_g$ mode at $190\mathrm{c}{\mathrm{m}}^{-1}$ for the 8-nm-thick merelaniite flake. (d)–(h) Angular evolution of the average THG emission power as a function of the incident pump-polarization angle for merelaniite flakes with thicknesses of 8, 17, 49, 61, and 121 nm. Black square $\left(I_x^{\left(3\omega \right)}\right)$, blue circle $\left(I_y^{\left(3\omega \right)}\right),$ and red triangle $\left(I_{\mathrm{tot}}^{\left(3\omega \right)}\right)$ data points are the measured data, while the solid curves are the corresponding theoretical fits. (i) Evolution of the average THG emission power vs the flake thickness.