Electronic and Electrochemical Devices
Much of our research focuses on the use of semiconductor thin films in electronic devices, particularly thin film transistors (TFTs) and electrolyte-gated transistors (EGTs). Transistors are not only important switching and amplifying elements for electronics, but they are also outstanding platforms for probing fundament conduction properties of materials as a function of continuously tunable charge density. Electrolyte-gated transistors are a class of TFTs that employ gel electrolytes as high capacitance gate insulators; the high capacitance allows both low voltage operation and enormous charge accumulations in the source-drain channels simultaneously. We are developing device architectures and materials for EGTs that improve their performance and increase the chances for practical application. One set of applications lies in chemical and biomolecular sensing, where the low voltage sensitivity and amplification characteristics of EGTs are a major advantage. Another set of applications is in printed electronics, where the device architecture of EGTs facilitates rapid printing of circuits on plastic using roll-to-roll manufacturing methods. Our fundamental work with EGTs includes the first Hall effect mobility measurement in a polymer semiconductor transistor as a function of charge density.
A particularly intriguing and unusual direction is the application of transistor concepts to control or enhance electocatalytic reactions on the surfaces of 2D semiconductor materials (e.g., MoS2, graphene, ZnO, etc). Transistors operate by controlling carrier concentrations in a thin semiconductor channel adjacent to the gate dielectric. If the semiconductor is thin enough, the charge induced by the underlying gate electrode is exposed on the top surface of the semiconductor. We exploit this by coupling the transistor to an electrochemical cell and then employing the exposed ultrathin semiconductor channel as a gate-tunable working electrode. That is, we adjust the Fermi level position in the 2D semiconductor with the back-gate and examine the impact on electrochemical performance. We hope that this electrochemical platform will allow new insight into the role of electron occupation in the semiconductor density of states on electrocatalysis, e.g., the reduction of oxygen to water at thin metal oxide electrodes, or the reduction of H+ to H2 gas on MoS2.
Crystals and films of pi-conjugated molecules transport charge and can be used as functional semiconductors in thin film transistors, photovoltaic cells and light-emitting diodes. This research focuses on understanding connections between structure and electrical transport behavior in organic semiconductors. We are particularly interested in the dependence of electron and hole mobility (the velocity per unit electric field) on molecular structure, crystal packing, intermolecular bonding, and defects in organic crystals and films. A theme of our experimental investigations is the development of methods for measuring transport behavior on length scales spanning nanometers to microns, so that we can accurately characterize the effects of specific structural features on transport. For example, we have used high resolution scanning probe microscopy techniques to measure electrical resistances and potential variations associated with individual grain boundaries in organic semiconductor films. Students are also actively involved in the fabrication and electrical characterization of organic transistors using electron beam lithography and other semiconductor processing equipment in the Minnesota Nano Center. In collaboration with students and faculty in the chemistry and physics departments, we are actively exploring the synthesis and characterization of novel organic semiconductor materials with enhanced transport properties.
Currently there is great interest in developing manufacturing methods for integrating electronic circuitry into flexible and stretchable substrates for a spectrum of applications including roll-up displays, wearable biosensors, smart labels, and electronic skins (‘e-skins’) for robotics, for example. One fabrication strategy that has captured imaginations involves the use of digital or analog printing techniques to pattern electronic inks onto paper, plastic, rubber, or metal foils. However, printed electronics has a number of significant challenges, including spatial resolution, pattern registration, and printed circuit performance. In this project, we are pursuing a multi-pronged approach to address these challenges in order to bring roll-to-roll (R2R) printed electronics closer to reality. First, we focus on innovations in materials that allow printing of low voltage thin film transistors (TFTs), the key building blocks of flexible circuits. Specifically, we are experts in a particular kind of TFT, known as the electrolyte gated transistors (EGT), which can be easily printed and operates at sub-2V supply voltages. Here the key enabling material is a gel electrolyte that provides ultrahigh capacitance necessary for low voltage transistor operation. Second, we are developing a University of Minnesota patented printing strategy that we term SCALE, or Self-Aligned Capillarity-Assisted Lithography for Electronics. SCALE offers both high spatial resolution and precision registration of multiple layers of electronic ink. Specifically, the SCALE process employs a combination of digital printing and in-substrate capillary flow to produce self-aligned devices with feature sizes that are currently as small as 1 µm. We have pilot R2R lines for implementing imprint lithography and ink jet printing in order to develop and improve the SCALE process. We are currently perfecting our approach to making all circuit building blocks including transistors, diodes, resistors, capacitors, and interconnects with the ultimate aim of printing full electronic systems on paper and plastic.
We are interested in electrical transport through individual molecules. A key issue in molecular electronics is how one "wires up" single molecules or groups of molecules. We use conducting probe atomic force microscopy to contact small numbers of molecules and to test their electrical properties. Questions of interest include how the current-voltage characteristics of these molecular junctions depend on molecular size, bonding and functional group architecture, and the nature of the metal-molecule contacts.