Low-Temperature, Plasma-based Synthesis of MoS2
The majority of synthesis methods explored for transition metal dichalcogenides (TMDs), including chemical vapor deposition and thin film alloying processes, require high temperatures which are not compatible with the thermal budget of a conventional CMOS process. This work explores the use of plasma-based processing to lower the synthesis temperature of MoS2 using two approaches: (1) sulfurization of an e-beam evaporated film of MoOx/Mo, and (2) vapor phase growth using phase MoO3 and an H2S plasma. Current results demonstrate that MoS2 growth is possible at temperatures as low as 400 °C, and the resulting material is highly uniform across large areas. This suggests that plasma processing is a promising method for lowering the required synthesis temperature for TMDs without sacrificing the quality of the material.
Resistive Random Access Memories (RRAMs)
Memristors can be used for various applications, including RRAMs and electrical synapse. RRAMs are very promising not only because they can replace the flash memory but also meet the requirements of smaller size and better performance, while the flash memory device has already reached its limits. A memristor consists of a Metal-Insulator-Metal nanostructure: Two metallic electrodes and one dielectric (also called an “active layer”) as shown in the figures. In most cases, the switching from high resistance to low resistance state is caused by the creation of a localized conductive path (also known as conducting filament or CF). The origin of the resistive switching (or insulator to metal transition) is well accepted as due to the formation of a filament in the active layer when a voltage is applied between the two electrodes. However, the mechanism is still not fully explained in the literature. As such, we are interested to study the formation of CFs in the state-of-the-art materials (for e.g. HfO2 and HfTiOx) by examining the effects of 1) applied voltage, 2) oxygen vacancies/ ions (in the active layers), 3) local heating, and 4) metal-electrode interfaces. In this work, a new strategy is used to design, prepare, and characterize the memristor, and to compare its performance for different applications. Our aim is to understand the atomic scale behavior of a memristor, including the ionic conduction and transport when forming the CF across the metal-dielectric, and to develop an efficient memristor with high endurance and stability.
Radiation Effects in 2D Heterostructures
Irradiation effects such as defects generation have been studied for single layers of 2D materials, but the effects of exposing a stack of different 2D materials to ionizing radiation has yet to be investigated. By exposing 2D vertical heterostructure devices to ionizing radiation, we can explore the impact radiation has on materials quality, electrical behavior, and device performance of various 2D stacks. The effect of electron, ion, and X-ray radiation will be studies on different 2D stacks such as graphene/hBN/graphene, graphene/TMD/graphene, and TMD/hBN/TMD. Graphene/high-k/graphene will also be studied to look at the effect on devices with paired 2D/3D interfaces.
2D Materials as Corrosion Barriers
2D materials like graphene and hBN can be used as extremely thin corrosion barriers because they are extremely chemically inert and impermeable. Graphene and hBN are grown onto the metal by CVD before corrosion begins. Depending on the environment different properties like grain size, crystal structure, adhesion to metal, and conductivity can play different roles in the overall mechanism for corrosion. By changing these properties in a controlled fashion, a quantitative analysis can be achieved to study the corrosion mechanism and be able to make recommendations for different applications.
The ability of graphene to store an electric charge coupled with a large surface area to mass ratio make graphene an ideal candidate for energy storage applications. The use of graphene for energy storage applications is currently being limited by the quality of synthesized graphene. Defects in the graphene result in high irreversible capacity due to trapping of ions. A systematic study of how the graphene structure can be designed to further improve the Coulombic efficiency of graphene electrodes for battery applications is being conducted to take advantage of graphene’s electrochemical properties, large surface area, high electronic conductivity, and excellent mechanical properties for energy storage applications.
ArgiSense: Multichannel Disposable Sensors for Animal Health Disease Diagnostics
Bovine respiratory disease is responsible for more than half billion dollars of economy loss in the U.S. cattle industry annually. Development of rapid on-site point-of-care diagnostic tools is desired. However, conventional animal diseases screening and diagnostic tools require very long turnover time in a centralized lab which delays the disease control, prevention and treatment of among the herd. To solve this problem, label-free potentiometric biochemical sensors provide a rapid diagnosis by detecting the charged biomolecules that attached to the sensing surface and converting it to electrical signal instantaneously. Monitoring specific antigen-antibody interactions, target antigens, for example, viral proteins, in the animal blood can be therefore detected. Both ion-sensitive field effect transistors (ISFET) and extended-gate FETs (EGFET) are promising candidates for potentiometric sensing. In AgriSense project, we explore the properties of sensing materials, the design and construction of sensors, and the practical applications. Furthermore, next generation flexible biosensors using two-dimensional (2D) materials on non-conventional substrates, for example, paper, are being investigated.
Biosensors for Infectious Disease Detection
With funding from AVX, we are developing reliable biosensors for clinical applications. These sensors utilize antibody-antigen interactions to detect infectious disease. Thiol-based self-assembled monolayers are used to covalently attach a receptor protein to a gold surface. The gold surface is then used as an extended gate for a field-effect transistor, which translates the antibody-antigen binding into a current. The current readout is sensitive to the surface charge of the gold, and is therefore proportional to how much antibody has bound. Currently, our focus is on engineering these sensors such that they are both as sensitive as possible and stable over the measurement timescale.
Mattress-based Sweat Monitoring using Flexible Graphene Sensors
From electrolytes to metabolites, sweat contains a vast amount of real-time medical information. Given the amount of time an individual spends sleeping, sweat monitoring in mattresses presents a novel opportunity for monitoring health. In this work, field-effect transistors (FETs) are used as chemical and biological sensors, offering rapid and label-free potentiometric detection of sweat-based biomarkers. We employ graphene, a 2D carbon sheet, as the FET’s active channel. Its reliable performance in response to bending enables sensor fabrication on flexible substrates, such as plastic or paper. Sensor arrays, composed of specifically functionalized surfaces, will be integrated into mattresses for sweat diagnostics.