Professor Joshua Agar
Drexel University
Publications
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Kinetically controlled assembly of terpheny-4, 4”-dithiol self-assembled monolayers (SAMs) for highly conductive anisotropically conductive adhesives (ACA)
Anisotropically conductive adhesives (ACA) are a promising alternative to solder interconnects for high performance electronic devices due to their increased I/O capabilities and reduced form factor. Previous studies have shown that modification of Au coated Ni/Cu bumps with conjugated self-assembly monolayers (SAMs) increases conductivity, current carrying capacity and reliability of ACA interconnects[1-3]. In this study, we kinetically control the assembly of p-Terphenyl-4,4”-dithiol (TPD) monolayers on Au bumps. Using a custom designed test vehicle we show how TPD SAMs can either increase or decrease the single bump resistance depending on the kinetics of the monolayer formation and its resulting structure. Future studies focusing on controlling monolayer assembly will determine the efficacy of conjugated SAMs at enhancing the conductivity and current carrying capacity of ACA interconnects.
Highly conductive stretchable electrically conductive composites for electronic and radio frequency devices
The electronics industry is shifting its emphasis from reducing transistor size and operational frequency to increasing device integration, reducing form factor and increasing the interface of electronics with their surroundings. This new emphasis has created increased demands on the electronic package. To accomplish the goals to increase device integration and interfaces will undoubtedly require new materials with increased functionality both electrically and mechanically. This thesis focuses on developing new interconnect and printable conductive materials capable of providing power, ground and signal transmission with enhanced electrical performance and mechanical flexibility and robustness. More specifically, we develop: 1.) A new understanding of the conduction mechanism in electrically conductive composites (ECC). 2.) Develop highly conductive stretchable silicone ECC (S-ECC) via in-situ nanoparticle formation and sintering. 3.) Fabricate and test stretchable radio frequency devices based on S-ECC. 4.) Develop techniques and processes necessary to fabricate a stretchable package for stretchable electronic and radio frequency devices. In this thesis we provide convincing evidence that conduction in ECC occurs predominantly through secondary charge transport mechanism (tunneling, hopping). Furthermore, we develop a stretchable silicone-based ECC which, through the incorporation of a special additive, can form and sinter nanoparticles on the surface of the metallic conductive fillers. This sintering process decreases the contact resistance and enhances conductivity of the composite. The conductive composite developed has the best reported conductivity, stretchability and reliability. Using this S-ECC we fabricate a stretchable microstrip line with good performance up to 6 GHz and a stretchable antenna with good return loss and bandwidth. The work presented provides a foundation to create high performance stretchable electronic packages and radio frequency devices for curvilinear spaces. Future development of these technologies will enable the fabrication of ultra-low stress large area interconnects, reconfigurable antennas and other electronic and RF devices where the ability to flex and stretch provides additional functionality impossible using conventional rigid electronics.
Electrically conductive silicone nano-composites for stretchable RF devices
The objective of this paper is to show how stretchable conductive composites can be utilized for the fabrication of ultra-low cost stretchable RF devices. We show a method to produce biocompatible highly conductive stretchable silicone composites via an in-situ nanoparticle formation and sintering process. Furthermore, we develop a simple, low cost, processing technique to fabricate stretchable RF transmission lines. These RF transmission lines are highly flexible, stretchable and robust. The S-parameter measurements show stable performance during mechanical deformation up to 6 GHz. Future development of this technology will enable ultra low cost consumer RF devices serving as a platform for future stretchable electronic devices.
Controlled growth of multilayer, few-layer, and single-layer graphene on metal substrates
The effects of graphene growth parameters on the number of its layers were systematically studied and a new growth mechanism on Cu substrate was thus proposed. Through the investigation of the graphene growth parameters, including growth substrate types, carrier gases, types of carbon sources, growth temperature, growth time, and cooling rates, we found that graphene grows on Cu substrates via a surface-catalyzed process, followed by a templated growth. We can obtain either single layer graphene (SLG) or few-layer graphene (FLG) by suppressing the subsequent templated growth with a low concentration of carbon source gases and a high concentration of H2. Our findings provide important guidance toward the synthesis of large-scale and high-quality FLGs and SLGs. This is expected to widen both the research and applications of graphene.
A simple, low-cost approach to prepare flexible highly conductive polymer composites by in situ reduction of silver carboxylate for flexible electronic applications
In recent years, efforts to prepare flexible highly conductive polymer composites at low temperatures for flexible electronic applications have increased significantly. Here, we describe a novel approach for the preparation of flexible highly conductive polymer composites (resistivity: 2.5 × 10−5 Ω cm) at a low temperature (150 °C), enabling the wide use of low cost, flexible substrates such as paper and polyethylene terephthalate (PET). The approach involves (i) in situreduction of silver carboxylate on the surface of silver flakes by a flexible epoxy (diglycidyl ether of polypropylene glycol) to form highly surface reactive nano/submicron-sized particles; (ii) the in situ formed nano/submicron-sized particles facilitate the sintering between silver flakes during curing. Morphology and Raman studies indicated that the improved electrical conductivity was the result of sintering and direct metal–metal contacts between silver flakes. This approach developed for the preparation of flexible highly conductive polymer composites offers significant advantages, including simple low temperature processing, low cost, low viscosity, suitability for low-cost jet dispensing technologies, flexibility while maintaining high conductivity, and tunable mechanical properties. The developed flexible highly conductive materials with these advantages are attractive for current and emerging flexible electronic applications.