Janelle Leger

Precise Color Tuning via Hybrid Light-Emitting Electrochemical Cells

We report color-tunable light-emitting devices employing CdSe/ZnS quantum dots (QDs) blended into a polymer light-emitting electrochemical cell (LEC) architecture. This novel structure circumvents the charge-tunneling barrier of QDs to achieve bright, uniform, and highly voltage-independent electroluminescence, with nearly all emission generated by the QDs. By blending varying ratios of two QD materials that emit at different wavelengths, we demonstrate precise color control in a single layer device structure.

Characterizing ion profiles in dynamic junction light-emitting electrochemical cells.

Organic semiconductors have the unique ability to conduct both ionic and electronic charge carriers in thin films, an emerging advantage in applications such as light-emitting devices, transistors, and electrochromic devices, among others. Evidence suggests that the profiles of ions and electrochemical doping in the polymer film during operation significantly impact the performance and stability of the device. However, few studies have directly characterized ion profiles within LECs. Here, we present an in-depth study of the profiles of ion distributions in LECs following application of voltage, via time-of-flight secondary ion mass spectrometry. Ion distributions were characterized with regard to film thickness, salt concentration, applied voltage, and relaxation over time. Results provide insight into the correlation between ion profiles and device performance, as well as potential approaches to tuning the electrochemical doping processes in LECs.

Characterization of Ion Profiles in Light-Emitting Electrochemical Cells by Secondary Ion Mass Spectrometry

Ion profiles in polymer light-emitting electrochemical cells are known to significantly affect performance and stability, but are not easily measured. Here, secondary ion mass spectrometry is used to investigate ion profiles in both dynamic and chemically fixed junction devices. Results indicate lower reversibility of dynamic junctions and a more significant time delay for ion redistribution than previously expected, but confirm the complete immobilization of ions in chemically fixed junction devices. When compared with prior studies analyzing the electric field profiles in similar devices, these results help to elucidate the roles of ion distribution and electrochemical doping in LECs.

Versatile Method for Producing 2D and 3D Conductive Biomaterial Composites Using Sequential Chemical and Electrochemical Polymerization.

Flexible and conductive biocompatible materials are attractive candidates for a wide range of biomedical applications including implantable electrodes, tissue engineering, and controlled drug delivery. Here, we demonstrate that chemical and electrochemical polymerization techniques can be combined to create highly versatile silk-conducting polymer (silk-CP) composites with enhanced conductivity and electrochemical stability. Interpenetrating silk-CP composites were first generated via in situ deposition of polypyrrole during chemical polymerization of pyrrole. These composites were sufficiently conductive to serve as working electrodes for electropolymerization, which allowed an additional layer of CP to be deposited on the surface. This sequential method was applied to both 2D films and 3D sponge-like silk scaffolds, producing conductive materials with biomimetic architectures. Overall, this two-step technique expanded the range of available polymers and dopants suitable for the synthesis of mechanically robust, biocompatible, and highly conductive silk-based materials.

Mimicking muscle fiber structure and function through electromechanical actuation of electrospun silk fiber bundles

Here we detail the fabrication and testing of artificial muscles fabricated from composites of the natural biopolymer silk fibroin and conducting polymers. Aligned nanofiber bundles of silk that mimic the structure of skeletal muscles were produced via electrospinning, and the fibers were infused with conducting polymers using chemical and electrochemical in situ polymerization methods. The resulting bundles of individual, electroactive fibers underwent electromechanical actuation in biologically-relevant electrolyte solutions when low potentials were applied, thus mimicking the contractile function of native muscles. The fabrication methods, bulk mechanical properties, stress and strain generation, and stability under repeated actuation for fiber bundles coated with different conducting polymer formulations are presented.

Detection of guided-wave plasmon polariton modes in a high-index dielectric MIM structure

Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These modes are supported by a simple physical structure that results from a minor symmetry modification of the traditional MIM structure, specifically the use of a central insulator layer with a higher refractive index than the supporting substrate. However, they display fundamental properties that are distinctly different from those of standard SPPs and from recently reported hybrid plasmonic modes. While GW-PPMs have been explored theoretically, they have not yet been realized experimentally. In this article, we present the first experimental demonstration of GW-PPMs. Specifically, we excite and detect GW-PPMs at visible frequencies and match model predictions to experimental results with remarkable accuracy using minimal parameter fitting. In addition to the experimental detection, we calculate and report on other interesting and relevant features of the detected modes, including the associated electric field profiles, confinement values, and propagation lengths, and discuss in terms of the applications-relevance of GW-PPMs.Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These mode...