posted on 2021-08-25, 14:47authored byDorte
R. Danielsen, Anton Lyksborg-Andersen, Kirstine E. S. Nielsen, Bjarke S. Jessen, Timothy J. Booth, Manh-Ha Doan, Yingqiu Zhou, Peter Bøggild, Lene Gammelgaard
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.