  Two-dimensional (2D) materials, such as graphene, are seen as potential
candidates for fabricating electronic devices and circuits on flexible
substrates. Inks or dispersions of 2D materials can be deposited on flexible
substrates by large-scale coating techniques, such as inkjet printing and spray
coating. One of the main issues in coating processes is nonuniform deposition
of inks, which may lead to large variations of properties across the
substrates. Here, we investigate the role of surface morphology on the
performance of graphene ink deposited on different paper substrates with
specific top coatings. Substrates with good wetting properties result in
reproducible thin films and electrical properties with low sheet resistance.
The correct choice of surface morphology enables high-performance films without
postdeposition annealing or treatment. Scanning terahertz time-domain
spectroscopy (THz-TDS) is introduced to evaluate both the uniformity and the
local conductivity of graphene inks on paper. A paper-based strain gauge is
demonstrated and a variable resistor acts as an on-off switch for operating an
LED. Customized surfaces can thus help in unleashing the full potential of
ink-based 2D materials.
