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Hagerty, PhillipPhysical Vapor Deposition of Materials for Flexible Two Dimensional Electronic Devices
Master of Science (M.S.), University of Dayton, 2016, Chemical Engineering
Molybdenum Disulfide (MoS2) and Tungsten Disulfide (WS2) are two materials in a larger class of materials known as Transition Metal Dichalcogenides (TMDs) that have begun emerge as semiconducting materials. When their horizontal length scale is reduced from bulk to monolayer they demonstrate surprising combinations of properties including a direct electronic band gap and mechanical flexibility. Two dimensional (2D) materials have the potential to revolutionize performance and tailorability of electro-optical devices fabricated entirely from molecularly thin materials. In a departure from traditional exfoliation or high temperature chemical vapor deposition approaches for 2D materials synthesis, novel plasma-based physical vapor (PVD) techniques were used to fabricate uniform films over large areas. This experimental approach allowed unique studies. For example, vapor phase growth allowed systematically variation of the sulfur vacancy concentration in MoS2 and WS2 and subsequent correlation to electronic properties. This effort leads to controlled bottom-up assembly of 2D devices on flexible and standard substrates to experimentally couple the remarkable intrinsic mechanical and electronic properties of ultrathin materials, which are particularly appealing for molecular sensing. The pursuit of an all physical vapor deposited field effect transistor (FET) is the main priority for the 2D materials community as definitive demonstration of the feasibility of physical vapor deposition as a scalable technique for consumer electronics. PVD sputtered Titanium Nitride (TiN) and Tungsten (W) were experimentally characterized as potential back gated materials, Plasma Vapor Deposited (PLD) a-BN was electrically characterized as a uniform ultra-thin low temperature dielectric, and sputtered MoS2 and WS2 were electrically characterized as a semiconductor material. Tungsten deposition methods were previously researched and mimicked for smooth and conductive back gate material depositions. TiN was parameterized and the best room temperature deposition conditions were 70V applied to the sputtering gun with 25 sccm gas flow of 90% N2 and 10% Ar for 60 minutes. The best high temperature depositions were done at 500oC, 70V applied to the sputtering gun with 25 sccm gas flow of 90% N2 and 10% Ar for 30 minutes. Dielectric a-BN electrical characterization began to occur after 6nm which equated to 100 pulses, while 200 pulses equated to 16.5nm thickness. A dielectric constant of 5.90 ± .65 is reported for a-BN for under 20nm thickness. Soft probing techniques by conductively pasted gold wires on the probe tips were required to obtain true electrical measurements of 2D materials in a stacked structure, otherwise scratching would occur and uniformity would cease to exist in the film. Chemical Vapor Deposition (CVD) and mechanical exfoliation have provided the only working TMD semiconductor 2D materials in MOSFET structure to date with lithographic electrical connections. PVD sputtering as a new synthesis method for crystalline TMD with a stoichiometric ratio is achievable over large areas. Though, reduced area depositions are required for doped Silicon and Silicon Oxide (SiO2) based FET structures to limit the chance of encountering a pinhole. With reduced area and stoichiometric enhancement control, sputtered TMD films exhibit high sensitivity to oxygen and are electrically conductive even when exposed to a field effect. Increasing the grain size of the sputtered materials is the next driving force towards a fully recognizable TMD thin film transistor.

Committee:

Christopher Muratore, PhD (Committee Chair); Terrence Murray, PhD (Committee Member); Kevin Myers, DSc (Committee Member)

Subjects:

Aerospace Engineering; Aerospace Materials; Electrical Engineering; Engineering; Materials Science

Keywords:

PVD; materials; 2D materials; Nanoelectronics; TMDs; 2D Transistors; Molybdenum Disulfide; MoS2; WS2; Tungsten Disulfide

Ponsot, Amanda EileenSYNTHESIS OF TETRABENZO[18]CYCLYNE CROSS-CONJUGATED MACROCYCLES WITH FOCUS ON THE DONOR-ACCEPTOR INDUCED FUNCTIONALITY
Master of Science, Miami University, 2010, Chemistry
The exploitation of organic compounds as key materials in the nanotechnology field is a growing science that is always demanding creation of new materials with specific functions in mind. A synthetic route that allows for “tuning” of the functional groups of a product is an important part in designing a material for a specific purpose. Using the well-known Sonogashira cross-coupling reaction, three new tetrabenzo[18]cyclyne macrocycles and six new trimer precursors were successfully synthesized with a focus on creating a donor-bridge-acceptor material for study. Characterization of these materials were done using 1HNMR, 13CNMR, mass spectrometry, UV/VIS spectrophotometry, and/or fluorescence. This fundamental study is a start in exploring tunable materials for potential use in the nanotechnology field (i.e. electronic devices, light-harvesting systems, etc.).

Committee:

Scott Hartley, PhD (Advisor); Michael Novak, PhD (Committee Chair); Hong Wang, PhD (Committee Member); Michael Crowder, PhD (Committee Member)

Subjects:

Organic Chemistry

Keywords:

tetrabenzocyclyne; nanoelectronics; light-harvesting; sonogashira coupling; materials; donor-bridge-acceptor; electronic devices; shape-persistent; macrocycle; organic; synthesis; functionality; cross-conjugation; molecular wire; molecular design;

Bonifas, Andrew PaulSpectroscopy, Fabrication, and Electronic Characterization of Molecular Electronic Devices
Doctor of Philosophy, The Ohio State University, 2011, Materials Science and Engineering
Molecular electronics is the study of charge transport through single molecules or molecular ensembles. Molecular electronic junctions consist of single molecules or an ensemble of molecules positioned between two conducing contacts. To fabricate and measure the electronic properties of molecular junctions, several techniques have been employed such as scanning tunneling microscopy, conducting probe atomic force microscopy, and vapor deposition of top contacts. Charge transport observed through molecular junctions has been shown to exhibit technologically important phenomena such as rectification, conductance switching, and orbital gating. The primary focus of the field of molecular electronics is to understand the effect of molecular properties, such as structure and molecular orbitals, on charge transport mechanisms through molecular junctions. In this dissertation, the various techniques to fabricate and characterize molecular junctions are discussed, along with an introduction to charge transport mechanisms expected to control transport through molecular junctions. More specifically, this dissertation is primary focused on the fabrication and characterization of molecular junctions fabricated through the formation of an electronic contact on a molecular layer through physical vapor deposition. A common problem with this technique is structural damage to the molecular layer or metal penetration through the molecular layer during the contact formation. To overcome these limitations, a novel fabrication technique was developed and employed to fabricate reproducible molecular junctions through a physical vapor deposition technique without molecular damage or metal penetration. Termed surface diffusion mediated deposition (SDMD), the technique remotely deposits a metallic contact adjacent to and about 10 – 100 nm away from the molecular layer. Surface diffusion causes the metallic contact to migrate towards and onto the molecular layer to form an electronic contact. With SDMD, single molecule and many-molecule junctions are fabricated and electronically characterized. To probe electronic states and molecular structure in molecule/oxide junctions, an in-situ optical absorbance spectroscopy technique was developed and employed to monitor bias induced molecular redox events in solid-state molecular junctions. Correlation of the observed spectral changes with molecular redox events allows characterization of the electronic properties of molecules which are critical in understanding charge transport through molecules. In a related application, the developed in-situ optical absorbance spectroscopy technique was used to probe doping events in polypyrrole/oxide junctions. Doping reactions in polypyrrole are shown to strongly depend on the surrounding environment. For application to both molecular and conjugated polymer junctions, in-situ absorbance spectroscopy is shown to be a useful analytical tool to determine charge transport mechanisms. Finally, a thermal oxidation technique is introduced to increase the resolution of nanoimprint lithography to fabricate nanogap electrodes for molecular junctions. The advantage of this technique is the ability to use a simple, fast, and reliable oxidation process to increase the resolution of standard nanofabrication techniques.

Committee:

Gerald S. Frankel, ScD (Advisor); Richard L. McCreery, PhD (Advisor); Roberto Myers, PhD (Committee Member); Nitin P. Padture, PhD (Committee Member)

Subjects:

Materials Science

Keywords:

molecular electronics; electronics; nanotechnology; nanoelectronics; spectroscopy; fabrication; surface diffusion mediated deposition; nanoimprint lithography