Edge and Interface Resistances Create Distinct Trade-Offs When Optimizing the Microstructure of Printed van der Waals Thin-Film Transistors

Abstract

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.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks

“…Importantly, the printed high-density transistor arrays (∼47 000 transistors·cm –2 ) deliver an average mobility of ∼4.2 cm 2 ·V –1 ·s –1 and an on/off ratio of ∼10 6 , representing the highest device density to date in the printed TMD materials (Figure c). Zhu et al found that the presence of fixed negative charges at nanosheet edges could induce a large local increase in resistance that limits effective mobility . Hence, passivating the edges of nanosheets may be an effective method to further improve the mobility of the printed vdW films.…”

“…Zhu et al found that the presence of fixed negative charges at nanosheet edges could induce a large local increase in resistance that limits effective mobility. 319 Hence, passivating the edges of nanosheets may be an effective method to further improve the mobility of the printed vdW films.…”

Inkjet-printed thin-film transistors using surfactant-based transition-metal dichalcogenide nanocomposites suspended in polymeric semiconductors

“…Previous investigations have shown that deposited MoS 2 layers (from bulk flakes) formed from a drop-cast suspension showed very limited electrical conductivity between the interflakes of the MoS 2 [16]. This degraded charge transport may be due to carrier depletion and scattering at the edge of these overlapping MoS 2 flakes [17]. Spin-coated hybrid films from MoS 2 suspension in a polymeric semiconductor have been reported where the suspension was found to improve the transport properties of the TFT devices [11][12][13]18], but it is still unclear whether the suspension only affects the electrical characteristics through changes to the electronic structure.…”

All-Solution-Processed High-Performance MoS2 Thin-Film Transistors with a Quasi-2D Perovskite Oxide Dielectric

“…In recent years, two-dimensional layered materials (2DLMs) have gained significant attention as alternative candidates for preparing colloidal inks, offering several potentials in the fabrication of solution-processed TFTs. − These atomically thin materials, consisting of van der Waals ( vdW ) bonded atomic layers, exhibit a diverse range of electronic properties, including semimetallic graphene, semiconducting transition metal dichalcogenides (TMDCs), and insulating hexagonal boron nitride ( h -BN). , The 2D layered structure allows for precise control over their thickness and uniformity in film formation, enabling the fabrication of ultrathin devices with distinct properties and functionalities. − Additionally, 2DLMs exfoliated (electro)­chemically in solution exhibit stability and processability. They can be effectively dispersed in solvents to form inks or suspensions, allowing for solution-based deposition techniques such as spin coating, inkjet printing, and spray coating .…”