Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching

Abstract

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are, however, compromised by nanometer-scale edge roughness and resolution variability issues, which especially affect the performance of 2D materials. Here, we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF6) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS2), tungsten diselenide (WSe2), molybdenum disulfide (MoS2), and molybdenum ditelluride (MoTe2). Scanning electron microscopy and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS2 > WSe2 > MoTe2 ∼ MoS2. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA′ stacking order into account. Patterns in WS2 are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy but shows consistent vertical etch angles, independent of crystal orientation. Using an hBN crystal as the base, ultrasharp corners can be created in lithographic patterns, which are then transferred to a graphite crystal underneath. We find that the anisotropic SF6 reactive ion etching process makes it possible to downsize nanostructures and obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch masks to pattern other nanomaterials.

Optical Constants of Several Multilayer Transition Metal Dichalcogenides Measured by Spectroscopic Ellipsometry in the 300–1700 nm Range: High Index, Anisotropy, and Hyperbolicity

“…These observations have led to a recently renewed interest in TMD optics and nanophotonics. This interest has grown even more after publication of several nanopatterning methods of TMDs and studies of optical phenomena in resulting TMD nanostructures. , Recent observations include high-index Mie resonances and anapole states in WS 2 nanodisks, optical anisotropy in TMD slabs , and nanocones, self-hybridization in TMD slabs , and nanotubes, optical modes in lattices of TMD nanostructures, improved second-harmonic generation in WS 2 and MoS 2 nanodisks, , high-index metamaterials, nanoholes down to ∼20 nm, dimer nanoantennas, and TMD metamaterials with atomically sharp edges …”

Giant Magnetoresistance in a Chemical Vapor Deposition Graphene Constriction

“…The magnitude of the change in resistance for the insulating behavior is reduced compared to the unintentional constriction, which can be attributed to a combination of the higher base temperature (1.5 K vs 300 mK) and increased disorder and doping in the etched edges. 27 , 28 It is expected that devices with higher disorder will show a later onset of the insulating state such that R is lower at the same B and T . 1 , 11 , 53 For comparison, Figure 2 b shows R = ∼860 kΩ at B = 10 T and T = 2 K. We speculate that less disorder is present in the unintentional constriction since it forms as a result of tearing, 25 , 26 such that the edges are never exposed to plasma etching.…”

“…We speculate that less disorder is present in the unintentional constriction due to its formation as a result of tearing, where edges are never exposed to plasma etching, which can cause greater edge disorder and contamination compared to cleaner alternatives 25 , 26 thus motivating further studies toward developing clean processes for constriction edges such as h -BN etch masks. 27 , 28 The observation of the insulating phase at charge neutrality in wet transferred CVD graphene on amorphous oxide suggests the intriguing potential of developing systems to investigate exotic quantum phenomena using conventional and scalable fabrication techniques. A primary application of multiplexing is the statistical analysis of large data sets and identifying trends in data ensembles.…”

Fermi Level Depinning in Two-Dimensional Materials Using a Fluorinated Bilayer Graphene Barrier

“…Metal contacts of Cr/Au (20/80 nm) are made on pristine and fluorinated graphene using standard electron-beam lithography and e-beam evaporation 21 . The current-voltage characteristics of the BLG device shows a linear behavior with high conductivity, while it is nonlinear with much lower current in the FBLG device, as seen in Figure 1c.…”