Enhanced Second-Order Nonlinearities at Strained Ultrasharp Zigzag Edges in Multilayer MoS₂

Abstract

Transition metal dichalcogenide (TMD) materials attract significant research attention thanks to their exceptional excitonic and optical properties. In this work, we analyze the formation of strained ultrasharp zigzag edges in MoS2 multilayers produced by anisotropic wet etching. The topography of the edges is determined by the relative stability of the different crystallographic directions of the multilayer as well as the interlayer interactions. Furthermore, we study the linear (Raman) and nonlinear (second-harmonic generation) spectroscopic characteristics of such edges and observe enhanced second-order nonlinearity originating from the strained zigzag edges. We also confirm that ultrasharp hexagonal nanoholes in MoS2 grow along the most stable crystallographic directions despite potential stacking faults or instabilities in the crystal quality. Our results open the way to exploit a broad range of phenomena occurring at the edges of MoS2 material, including the unique determination of crystal orientation for moiré engineering and strongly correlated phenomena in 2D material-based systems, as well as potential applications in TMD-based electrocatalysis and gas sensing.

Anisotropic Wet Etching of WSe2 and MoS2 for Twist-Angle Extraction of Heterobilayers

“…The hexagonal hole size obtained from a WSe 2 multilayer is much larger than that of a MoS 2 multilayer. This is attributed to the faster wet-etching rate of WSe 2 by the H 2 O 2 -based solution, which is consistent with previous studies. , …”

“…This is attributed to the faster wet-etching rate of WSe 2 by the H 2 O 2 -based solution, which is consistent with previous studies. 24,26 Figure 2a depicts atomic force microscopy (AFM) images of the WSe 2 and MoS 2 hexagonal hole structures fabricated by anisotropic wet etching for 6 and 180 min, respectively. The lengths from the center to the sides and vertices of a hexagonal hole are R ZZ and R AC , respectively.…”

“…Many methods have been used to identify the crystal orientation in TMDs, including transmission electron microscopy, , nonlinear optics, − ,, and edge engineering. − However, these techniques have limitations such as destructive observation, a lack of precision, crystal damage, and some complexities under experimental conditions. In contrast, simple techniques based on a combination of standard top-down lithography and a subsequent anisotropic etching process have recently been used to control the edge state in TMD crystals. − Among a variety of anisotropic etching techniques, a wet etching process using only abundant chemicals allows for the fabrication of nanostructures of various sizes and shapes over large areas. However, there are few studies on the elucidation of anisotropic wet etching mechanisms in TMD crystals.…”

Top-Down Nanostructured Multilayer Mos2 With Atomically Sharp Edges for Electrochemical Hydrogen Evolution Reaction

“…The etching pit shapes are the same within a single flake but show a remarkable flake-to-flake variation. We provide an extended description of the wet etching mechanism in the Supporting Information, Chapter S2 [51,53,[60][61][62][71][72][73][74][75][76][77][78]; it is also addressed in our recent publication on second-order nonlinearities at strained MoS 2 zz edges [79]. In brief, the employed edge-enrichment technique generates precisely periodic arrays of the etching pits bordered by numerous zz edges.…”