Precise one-dimensional nanochannels in transition metal dichalcogenides as building blocks for advanced nanophotonics

Technical Summary: Precise one-dimensional nanochannels in transition metal dichalcogenides as building blocks for advanced nanophotonics

Problem Statement
The fabrication of high-index nanophotonic structures requires atomically sharp edges to ensure optical confinement and reproducibility. Conventional top-down methods, specifically electron-beam lithography (EBL) followed by dry etching, inevitably introduce edge roughness, sidewall damage, and structural imperfections. These defects cause increased scattering, reduced diffraction efficiency, and lower quality factors (Q-factors), particularly in high-index materials. While anisotropic wet etching of van der Waals crystals like transition metal dichalcogenides (TMDs) can produce crystallographically defined, atomically sharp edges, this process is inherently limited to isolated, confined geometries (e.g., triangles in 3R-phase or hexagons in 2H-phase) dictated by crystal stacking symmetry. This limitation prevents the formation of extended, 180°-aligned features necessary for continuous nanophotonic elements such as waveguides, gratings, and photonic cavities.

Methodology
The authors propose a lithography-guided anisotropic etching framework to overcome the geometric constraints of isolated etching. The process involves three main steps:

1. Lithography: Nanohole arrays are defined in an e-beam resist covering exfoliated multilayer 2H-WS₂ flakes. The authors systematically optimize the nanohole radius (selected at 20 nm for reproducibility), center-to-center spacing (optimized at 40 nm to minimize proximity effects), and array pitch.
2. Dry Etching: The lithographic pattern is transferred into the TMD layers using reactive ion etching (RIE).
3. Anisotropic Wet Etching: The patterned nanoholes undergo anisotropic wet etching. By lithographically enforcing the spatial arrangement of nanoholes along crystallographic axes, the authors drive the etching process into a regime where neighboring etch fronts interact and merge before reaching individual equilibrium shapes. This controlled interaction suppresses vertex formation and enables the sustained, long-range propagation of zigzag facets, creating continuous 180°-aligned edges.

Key Contributions

• Unlocking 180° Edge Alignment: The study demonstrates that merging anisotropic etch fronts in multilayer 2H-WS₂ enables the formation of extended, atomically sharp edges with 180° alignment, a geometry previously inaccessible in isolated wet-etching processes.
• Crystallographically Defined Design Space: The approach establishes a design space combining intrinsic 120° junctions (from hexagonal equilibrium) and lithographically enabled 180° junctions, allowing for the deterministic construction of complex 2D photonic structures.
• Fabrication of Extended Nanophotonic Structures: The method is used to fabricate sub-100-nm-gap 1D gratings, waveguides, defect-engineered photonic cavities, angle-programmed photonic lattices, and radially patterned diffractive zone plates with ultrasharp sidewalls.
• Ultralong Nanoribbons: The technique produces ultrathin, ultranarrow, and ultralong WS₂ nanoribbons with record-high aspect ratios, limited primarily by the lateral size of the exfoliated flakes rather than the fabrication process.

Results

• Structural Characterization: Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm the fabrication of atomically sharp, straight, and uniform edges. Gratings exhibit uniform widths (107 nm) and gaps (103 nm) over micrometer-length scales. The method is robust across a range of initial nanohole radii (from 25 nm to 500 nm), with the final geometry governed by anisotropic etch-front propagation rather than initial feature size.
• Optical Performance: Back-focal-plane reflection spectroscopy of 1D periodic 2H-WS₂ gratings reveals symmetry-protected bound states in the continuum (SP-BICs) and strong exciton–photon coupling. The measured dispersion spectra show excellent agreement with Rigorous Coupled Wave Analysis (RCWA) simulations, validating the structural precision. Comparison with dry-etched benchmarks shows that wet etching removes excess material and roughness, improving geometric uniformity.
• Complex Geometries: The authors successfully fabricated 2D photonic lattices and aperiodic diffractive zone plates composed exclusively of crystallographically allowed 120° and 180° edge connections, maintaining edge quality without accumulation of line-edge roughness.
• Nanoribbons: WS₂ nanoribbons with lengths up to ~50 µm (and up to 125 µm in supplementary data), widths of 40–70 nm, and thicknesses of 12–22 nm were fabricated, achieving aspect ratios significantly higher than those reported for other synthesis methods.

Significance
The paper claims to demonstrate, for the first time, edge merging as a generic route to fabricate edge-defined, atomically sharp nanophotonic and nanoelectronic architectures in layered van der Waals platforms. By overcoming the limitation of isolated equilibrium shapes, this lithography-guided anisotropic etching approach provides crystallographic precision and scalable pattern transfer. It establishes a robust structural foundation for low-scattering nanophotonic elements, enabling the realization of advanced devices such as gratings, waveguides, and photonic cavities with minimal scattering losses. The work highlights that edge continuity and connectivity, rather than lithographic resolution alone, govern the achievable geometry once equilibrium termination is overcome.