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Inkjet Printed Thin films and Graphene Nanohybrid Photodetectors and Sensors
Cook, Brent
Cook, Brent
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Abstract
The discovery of two-dimensional (2D) atomic materials, starting with graphene in 2004, and now including MoS2, WS2, and hBN, has led to great interest in their potential use in microelectronic devices. These atomic dimension materials have unique properties that differ considerably from the three-dimensional (3D) materials that currently dominate in the construction of microelectronics and are based primarily on complementary metal-oxide semiconductors (CMOS). The 2D atomic materials are typically under a nanometer in thickness, for example graphene is ~0.3 nm in thickness, while they can have lateral dimension on the order of centimeters. The atomic thickness of the 2D atomic materials results in a strong quantum confinement on the charge carriers, leading to novel electronic structures that differ fundamentally from their 3D counterparts. In graphene the carbon atoms form a periodic honeycomb lattice, which exhibits a linear energy dispersion of density of states described by a modified Dirac equation. This results in the energy band structure called a "Dirac-cone." In this configuration the conduction and valence bands meet at a "Dirac point," without having a gap between the two bands. This so-called "zero bandgap" configuration leads to a Fermi energy of graphene, that is, the highest occupied energy level of the charges, that can be continuously tuned under an electric potential from dominate positive charge carriers (holes) to negative charge carriers (electrons), enabling a bipolar conductivity. The charges in graphene are massless fermions near the Dirac point with a fermi velocity of ~106 m/s and, hence, graphene has an extremely high charge carrier mobility of 15000 cm2 V-1 s-1 at room temperature. In addition, graphene also exhibits a minimal quantum conductance and a half integer quantum hall conductance. These unique properties of graphene make it ideal to combine with nanostructures (nanostructure/graphene) to form nanohybrids for the exploration of new electronics beyond CMOS through a combination of the iv quantum confinement effects in both graphene and the nanostructures. Examples of these new nanohybrids include quantum dots (QDs), nanowires (NWs), nanoparticles (NPs) or their mesoporous thin films combined with graphene. These nanostructures act as a sensitizer for graphene such that they lend their ability to generate excitons (electron-hole pairs) through external excitation such as light. They could also generate an electric field through mechanical deformation or molecule attachment in the nanostructure. The excitons then can equivalently produce an electric gating effect on graphene. The electric gating is driven by the interface built-in electric field determined by the electronic band edge alignment of the nanostructure with graphene. This causes charge transfer of either a hole or electron across the nanostructure/graphene interface. This will leave one charge trapped in the sensitizer for an exciton lifetime of a millisecond or more, which is orders of magnitude longer than typical for larger 3D devices, which do not benefit from the charge quantum confinement. The transferred charge may make multiple trips between the source and drain electrodes in the graphene device as a result of a short transit time (ttransit) that is inversely related to the carrier mobility in graphene. This enables an external quantum efficiency (EQE) or “gain” up to 1010 in nanostructure/graphene nanohybrid devices, in contrast to the capped EQE1 of conventional semiconductor electronics. This thesis is motivated by the extraordinary properties of these atomic and nanoscale materials. The goal is to explore nanohybrids and their nanocomposites using QDs (ZnO QDs, PbS QDs, FeS2 QDs), ZnO NWs and ZnO NP-network films as sensitizers for graphene. The objectives of this thesis include: (1) to achieve a control of the sensitizer morphology and nanostructure/graphene interfaces for high device performance using inkjet printing; and (2) to further develop inkjet printing for on-chip generation of the nanostructure/graphene nanohybrids devices and circuits that are multi-functional, low-cost and scalable. The ultimate goal is to achieve nanohybrid optoelectronic devices and sensors that are high performance, flexible, wearable and self-powered.
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Date
2020-08-31
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University of Kansas
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Keywords
Condensed matter physics, graphene, inkjet printing, nanomaterials, nanosensor, nanowires, photodetector