  The ability to form silk films on semiconductors, metals, and oxides or as
free-standing membranes has motivated research into silk-based electronic,
optical, and biomedical devices. However, the inherent disorder of native silk
limits device performance. Here we report the creation of highly ordered
two-dimensional (2D) silk fibroin (SF) layers on van der Waals solids. Using in
situ atomic force microscopy, synchrotron-based infrared spectroscopy, and
molecular dynamics simulations, we develop a mechanistic understanding of the
assembly process. We show that the films consist of lamellae having an
epitaxial relationship with the underlying lattice and that the SF molecules
exhibit the same Beta-sheet secondary structure seen in the crystallites of the
native form. By increasing the SF concentration, multilayer films form via
layer-by-layer growth, either along a classical pathway in which SF molecules
assemble directly into the lamellae or, at sufficiently high concentrations,
along a two-step pathway beginning with formation of a disordered monolayer
that subsequently converts into the crystalline phase. Kelvin probe
measurements show that these 2D SF layers substantially alter the surface
potential. Moreover, the ability to assemble 2D silk on both graphite and MoS2
suggests that it may provide a general platform for silk-based electronics on
vdW solids.
