Rose M. Mutiso

Integrating Simulations and Experiments To Predict Sheet Resistance and Optical Transmittance in Nanowire Films for Transparent Conductors

Metal nanowire films are among the most promising alternatives for next-generation flexible, solution-processed transparent conductors. Breakthroughs in nanowire synthesis and processing have reported low sheet resistance (Rs ≤ 100 Ω/sq) and high optical transparency (%T > 90%). Comparing the merits of the various nanowires and fabrication methods is inexact, because Rs and %T depend on a variety of independent parameters including nanowire length, nanowire diameter, areal density of the nanowires and contact resistance between nanowires. In an effort to account for these fundamental parameters of nanowire thin films, this paper integrates simulations and experimental results to build a quantitatively predictive model. First, by fitting the results from simulations of quasi-2D rod networks to experimental data from well-defined nanowire films, we obtain an effective average contact resistance, which is indicative of the nanowire chemistry and processing methods. Second, this effective contact resistance is used to simulate how the sheet resistance depends on the aspect ratio (L/D) and areal density of monodisperse rods, as well as the effect of mixtures of short and long nanowires on the sheet resistance. Third, by combining our simulations of sheet resistance and an empirical diameter-dependent expression for the optical transmittance, we produced a fully calculated plot of optical transmittance versus sheet resistance. Our predictions for silver nanowires are validated by experimental results for silver nanowire films, where nanowires of L/D > 400 are required for high performance transparent conductors. In contrast to a widely used approach that employs a single percolative figure of merit, our method integrates simulation and experimental results to enable researchers to independently explore the importance of contact resistance between nanowires, as well as nanowire area fraction and arbitrary distributions in nanowire sizes. To become competitive, metal nanowire systems require a predictive tool to accelerate their design and adoption for specific applications.

Resistive switching in silver/polystyrene/silver nano-gap devices

In this paper, we demonstrate reversible resistive switching in silver/polystyrene/silver nano-gap devices comprising Ag nano-strips separated by a nanoscale gap and encapsulated in polystyrene (PS). These devices show highly reversible switching behavior with high on-off ratios (>103) during cyclic switching tests over many cycles. We also observe evolution of the gap after extensive testing, which is consistent with metal filament formation as the switching mechanism in these Ag/PS/Ag nano-gap devices. The reversible electrical bistability demonstrated here was accomplished with an electrically inactive polymer, thereby extending the range of polymers suitable for organic digital memory applications.

Electrical Conductivity of Polymer Nanocomposites

Electrically conductive polymer nanocomposites with rodlike fillers are a versatile class of materials that are being explored for a wide range of applications. Realizing the full commercial potential of these novel materials hinges upon our ability to produce composites with well-defined and controllable properties. This, in turn, requires an in-depth understanding of the structure-property relations for the electrical properties of polymer nanocomposites. In this chapter, we review major theoretical and fundamental experimental studies of polymer nanocomposites, with particular emphasis on the main factors that determine their electrical properties. We will also discuss the major gaps in our current understanding of these materials and the greatest opportunities in the field.