posted on 2022-12-27, 13:35authored byZhehao Zhu, Joon-Seok Kim, Michael J. Moody, Lincoln J. Lauhon
Inks based on two-dimensional (2D) materials could be
used to tune
the properties of printed electronics while maintaining compatibility
with scalable manufacturing processes. However, a very wide range
of performances have been reported in printed thin-film transistors
in which the 2D channel material exhibits considerable variation in
microstructure. The lack of quantitative physics-based relationships
between film microstructure and transistor performance limits the
codesign of exfoliation, sorting, and printing processes to inefficient
empirical approaches. To rationally guide the development of 2D inks
and related processing, we report a gate-dependent resistor network
model that establishes distinct microstructure-performance relationships
created by near-edge and intersheet resistances in printed van der
Waals thin-film transistors. The model is calibrated by analyzing
electrical output characteristics of model transistors consisting
of overlapping 2D nanosheets with varied thicknesses that are mechanically
exfoliated and transferred. Kelvin probe force microscopy analysis
on the model transistors leads to the discovery that the nanosheet
edges, not the intersheet resistance, limit transport due to their
impact on charge carrier depletion and scattering. Our model suggests
that when transport in a 2D material network is limited by the near-edge
resistance, the optimum nanosheet thickness is dictated by a trade-off
between charged impurity screening and gate screening, and the film
mobilities are more sensitive to variations in printed nanosheet density.
Removal of edge states can enable the realization of higher mobilities
with thinner nanosheets due to reduced junction resistances and reduced
gate screening. Our analysis of the influence of nanosheet edges on
the effective film mobility not only examines the prospects of extant
exfoliation methods to achieve the optimum microstructure but also
provides important perspectives on processes that are essential to
maximizing printed film performance.