Frequency Response of Graphene Electrolyte-Gated Field-Effect Transistors

This work develops the first frequency-dependent small-signal model for graphene electrolyte-gated field-effect transistors (EGFETs). Graphene EGFETs are microfabricated to measure intrinsic voltage gain, frequency response, and to develop a frequency-dependent small-signal model. The transfer function of the graphene EGFET small-signal model is found to contain a unique pole due to a resistive element, which stems from electrolyte gating. Intrinsic voltage gain, cutoff frequency, and transition frequency for the microfabricated graphene EGFETs are approximately 3.1 V/V, 1.9 kHz, and 6.9 kHz, respectively. This work marks a critical step in the development of high-speed chemical and biological sensors using graphene EGFETs.United States. Office of Naval Research (Grant N00014-12-1-0959)United States. Office of Naval Research (Grant N0014-16-1-2230)United States. National Aeronautics and Space Administration (Award NNX14AH11A)United States. Army Research Office (Contract W911NF-13-D-0001

Large scale applications of 2D materials for sensing and energy harvesting

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.Cataloged from PDF version of thesis.Includes bibliographical references.In this project we demonstrate the fabrication and characterization of printed reduced graphene oxide strain sensors, Chemical Vapor Deposition (CVD) 2D material transistors, and tungsten diselenide (WSe₂) photovoltaic devices that were produced through a combination of printing and conventional microfabrication processes. Each of these components were designed with the purpose of fitting into a "smart skin" system that could be discretely integrated into and sense its environment. This thesis document will describe the modification-of a 3D printer to give it inkjet capabilities that allow for the direct deposition of graphene oxide flakes onto a 3D printed surface. These graphene oxide flake traces were then reduced, making them more conductive and able to function as strain sensors. Next, this thesis will discuss the development of CVD molybdenum disulfide (MoS₂) and CVD graphene transistors and how they can be modified to function as chemical sensors. Finally, this work will detail steps taken to design, fabricate, and test a WSe₂ photovoltaic device which is composed of a printed active layer. In summary, these devices can fit into the sensing, communication, and energy harvesting blocks required in realizing a ubiquitous sensing system.by Elaine D. McVay.S.M

Impact of Al2O3 Passivation on the Photovoltaic Performance of Vertical WSe2 Schottky Junction Solar Cells

Transition metal dichalcogenide (TMD) materials have emerged as promising candidates for thin-film solar cells due to their wide bandgap range across the visible wavelengths, high absorption coefficient, and ease of integration with both arbitrary substrates and conventional semiconductor technologies. However, reported TMD-based solar cells suffer from relatively low external quantum efficiencies (EQE) and low open circuit voltage due to unoptimized design and device fabrication. This paper studies Pt/WSe₂ vertical Schottky junction solar cells with various WSe₂ thicknesses in order to find the optimum absorber thickness. Also, we show that the devices' photovoltaic performance can be improved via Al₂O₃ passivation, which increases the EQE up to 29.5% at 410 nm wavelength incident light. The overall resulting short circuit current improves through antireflection coating, surface doping, and surface trap passivation effects. Thanks to the Al₂O₃ coating, this work demonstrates a device with an open circuit voltage (VOC) of 380 mV and a short circuit current density (JSC) of 10.7 mA/cm². Finally, the impact of Schottky barrier height inhomogeneity at the Pt/WSe2 contact is investigated as a source of open circuit voltage lowering in these devices.NSF (Grant DMR-1231319)AFOSR (Grant FA9550-15-1-0514

Additive manufacturing of patterned 2D semiconductor through recyclable masked growth

The 2D van der Waals crystals have shown great promise as potential future electronic materials due to their atomically thin and smooth nature, highly tailorable electronic structure, and mass production compatibility through chemical synthesis. Electronic devices, such as field effect transistors (FETs), from these materials require patterning and fabrication into desired structures. Specifically, the scale up and future development of “2D”-based electronics will inevitably require large numbers of fabrication steps in the patterning of 2D semiconductors, such as transition metal dichalcogenides (TMDs). This is currently carried out via multiple steps of lithography, etching, and transfer. As 2D devices become more complex (e.g., numerous 2D materials, more layers, specific shapes, etc.), the patterning steps can become economically costly and time consuming. Here, we developed a method to directly synthesize a 2D semiconductor, monolayer molybdenum disulfide (MoS₂), in arbitrary patterns on insulating SiO₂/Si via seed-promoted chemical vapor deposition (CVD) and substrate engineering. This method shows the potential of using the prepatterned substrates as a master template for the repeated growth of monolayer MoS₂ patterns. Our technique currently produces arbitrary monolayer MoS₂ patterns at a spatial resolution of 2 μm with excellent homogeneity and transistor performance (room temperature electron mobility of 30 cm²V⁻¹s⁻¹ and on–off current ratio of 10⁷. Extending this patterning method to other 2D materials can provide a facile method for the repeatable direct synthesis of 2D materials for future electronics and optoelectronics. Keywords: 2D semiconductor; monolayer MoS₂; patterned growth; growth mechanism; recyclable; masked growthAir Force Office of Scientific Research (Grant FA9550-15-1-0514)National Science Foundation (U.S.) (Grant 0939514)National Science Foundation (U.S.) (Grant DMR-1231319