Flexible, Photopatterned, Colloidal Cdse Semiconductor Nanocrystal Integrated Circuits

As semiconductor manufacturing pushes towards smaller and faster transistors, a parallel goal exists to create transistors which are not nearly as small. These transistors are not intended to match the performance of traditional crystalline semiconductors; they are designed to be significantly lower in cost and manufactured using methods that can make them physically flexible for applications where form is more important than speed. One of the developing technologies for this application is semiconductor nanocrystals. We first explore methods to develop CdSe nanocrystal semiconducting “inks” into large-scale, high-speed integrated circuits. We demonstrate photopatterned transistors with mobilities of 10 cm2/Vs on Kapton substrates. We develop new methods for vertical interconnect access holes to demonstrate multi-device integrated circuits including inverting amplifiers with ~7 kHz bandwidths, ring oscillators with \u3c10 µs stage delays, and NAND and NOR logic gates. In order to produce higher performance and more consistent transistors, we develop a new hybrid procedure for processing the CdSe nanocrystals. This procedure produces transistors with repeatable performance exceeding 40 cm2/Vs when fabricated on silicon wafers and 16 cm2/vs when fabricated as part of photopatterned integrated circuits on Kapton substrates. In order to demonstrate the full potential of these transistors, methods to create high-frequency oscillators were developed. These methods allow for transistors to operate at higher voltages as well as provide a means for wirebonding to the Kapton substrate, both of which are required for operating and probing high-frequency oscillators. Simulations of this system show the potential for operation at MHz frequencies. Demonstration of these transistors in this frequency range would open the door for development of CdSe integrated circuits for high-performance sensor, display, and audio applications. To develop further applications of electronics on flexible substrates, procedures are developed for the integration of polychromatic displays on polyethylene terephthalate (PET) substrates and a commercial near field communication (NFC) link. The device draws its power from the NFC transmitter common on smartphones and eliminates the need for a fixed battery. This allows for the mass deployment of flexible, interactive displays on product packaging

Interface Engineering to Control Charge Transport in Colloidal Semiconductor Nanowires and Nanocrystals

Colloidal semiconductor nanocrystals (NCs) are a class of materials that has rapidly gained prominence and has shown the potential for large area electronics. These materials can be synthesized cheaply and easily made in high quality, with tunable electronic properties. However, evaluating if colloidal nanostructures can be used as a viable semiconducting material for large area electronics and more complex integrated circuits has been a long standing question in the field. When these materials are integrated into solid-state electronics, multiple interfaces need to be carefully considered to control charge transport, these interfaces are the: metal contact/semiconductor, dielectric/semiconductor and the nanocrystal surface. Here, we use colloidal nanowire (NW) field-effect transistors (FETs) as a model system to understand doping and hysteresis. Through controllable doping, we fabricated PbSe NW inverters that exhibit amplification and demonstrate that these nanostructured materials could be used in more complex integrated circuits. By manipulating the dielectric interface, we are able to reduce the hysteresis and make low-voltage, low-hysteresis PbSe NW FETs on flexible plastic, showing the promise of colloidal nanostructures in large area flexible electronics. In collaboration, we are able to fabricate high-performance CdSe NC FETs through the use of a novel ligand, ammonium thiocyanate to enhance electronic coupling, and extrinsic atom in indium to dope and passivate surface traps, to yield mobilities exceeding 15 cm2V-1s-1. Combining high-mobility CdSe NC FETs with our low-voltage plastic platform, we were able to translate the exceptional devices performances on flexible substrates. This enables us to construct, for the first time, nanocrystal integrated circuits (NCICs) constructed from multiple well-behaved, high-performance NC-FETs. These transistors operate with small variations in device parameters over large area in concert, enabling us to fabricate NCIC inverters, amplifiers and ring oscillators. Device performance is comparable to other emerging solution-processable materials, demonstrating that this class of colloidal NCs as a viable semiconducting material for large area electronic applications

Liquid crystalline properties of symmetric and asymmetric end-grafted cellulose nanocrystals

The hydrophilic polymer poly[2-(2-(2-methoxy ethoxy)ethoxy)ethylacrylate] (POEG3A) was grafted onto the reducing end-groups (REGs) of cellulose nanocrystal (CNC) allomorphs, and their liquid crystalline properties were investigated. The REGs on CNCs extracted from cellulose I (CNC-I) are exclusively located at one end of the crystallite, whereas CNCs extracted from cellulose II (CNC-II) feature REGs at both ends of the crystallite, so that grafting from the REGs affords asymmetrically and symmetrically decorated CNCs, respectively. To confirm the REG modification, several complementary analytical techniques were applied. The grafting of POEG3A onto the CNC REGs was evidenced by Fourier transform infrared spectroscopy, atomic force microscopy, and the coil–globule conformational transition of this polymer above 60 °C, i.e., its lower critical solution temperature. Furthermore, we investigated the self-assembly of end-tethered CNC-hybrids into chiral nematic liquid crystalline phases. Above a critical concentration, both end-grafted CNC allomorphs form chiral nematic tactoids. The introduction of POEG3A to CNC-I does not disturb the surface of the CNCs along the rods, allowing the modified CNCs to approach each other and form helicoidal textures. End-grafted CNC-II formed chiral nematic tactoids with a pitch observable by polarized optical microscopy. This is likely due to their increase in hydrodynamic radius or the introduced steric stabilization of the end-grafted polymerPeer ReviewedPostprint (author's final draft

Index to NASA Tech Briefs, 1972

Abstracts of 1972 NASA Tech Briefs are presented. Four indexes are included: subject, personal author, originating center, and Tech Brief number

Cumulative index to NASA Tech Briefs, 1986-1990, volumes 10-14

Tech Briefs are short announcements of new technology derived from the R&D activities of the National Aeronautics and Space Administration. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This cumulative index of Tech Briefs contains abstracts and four indexes (subject, personal author, originating center, and Tech Brief number) and covers the period 1986 to 1990. The abstract section is organized by the following subject categories: electronic components and circuits, electronic systems, physical sciences, materials, computer programs, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Flexible Electronics Based on Solution Processable Organic Semiconductors and Colloidal Semiconductor Nanocrystals

Solution-processable semiconductors hold great potential for the large-area, low-cost fabrication of flexible electronics. Recent advances in flexible electronics have introduced new functional devices such as light-weight displays and conformal sensors. However, key challenges remain to develop flexible devices from emerging materials that use simple fabrication processes and have high-performance. In this thesis, we first use a solution-processable organic semiconductor to build field-effect transistors on large-area plastic with mobility of 0.1 cm^2/Vs. Combined with passive components, we are able to build voltage amplifiers to capture few mV amplitude bio-signals. This work provides a proof of concept on applying solution processable materials in flexible circuits. In the second part of the thesis, we introduce colloidal CdSe nanocrystals (NCs) as solution-processable inks of semiconductor thin film devices. By strongly coupling and doping the CdSe NC thin films, we demonstrate high-performance, flexible nanocrystal field-effect transistors (NC-FETs) with mobility greater than 20 cm^2/Vs under 2V supply. Using these NC-FETs as building blocks, we demonstrate the first flexible nanocrystal integrated circuits (NCICs) with switching speed of 600 µsec. To design reliable integrated circuits with low-noise, we characterize the flicker noise amplitude and origin. We find the figure of merit for noise, the Hooge parameter, to be 3 x 10^-2 for CdSe NC-FETs, comparable to other emerging solution processable organic semiconductors and promising for low-noise circuit applications.As most of NCs are reactive and their devices tend to degrade in air, we develop processes that allow manipulation of the NCs in ambient atmosphere without compromising device performance. These processes open up opportunities for NC-based devices to be fabricated over large area using photolithography. By scaling the devices and reducing device parasitics, we are able to fabricate hundreds of NC-FETs on wafer-scale substrates and integrate them as circuits. We demonstrate voltage amplifiers with bandwidths of a few kHz and ring-oscillators with a stage delay of 3 µsec. We also show functional NCICs NOR and NAND logic. This thesis demonstrates the use of colloidal NCs to realize flexible, large-area circuits and the feasibility of more advanced analog and digital NCICs built on flexible substrates for various applications

Silicon Neural Probes for Stimulation of Neurons and the Excitation and Detection of Proteins in the Brain

This thesis describes the development of a number of novel microfabricated neural probes for a variety of specific neuroscience applications. These devices rely on single mode waveguides and grating couplers constructed from silicon nitride thin films, which allows the use of planar lightwave circuits to create advanced device geometries and functions. These probes utilize array waveguide gratings to select an individual emitter from a large array of emitters using the wavelength of incoming light, allowing for spatial multiplexing of optical stimulation. These devices were tested in the laboratory and in living tissue to verify their efficacy. This technology was then modified to create steerable beam forming for stimulation of neurons using optical phase arrays. This technology was also tested for use in fluoresence lifetime imaging microscopy and the first application of pulsed light through the photonic circuits. Finally, this technology was again modified to create laminar illumination patterns for light sheet fluorescence microscopy applications. These devices were further improved by adding embedded microfluidics to the probes. The process of creating embedded microfluidic channels by the dig and seal method is described in detail, including modifications to the procedure that were added to address potential pitfalls in the fabrication process. Next, two projects which combine microfluidics with the optical devices described in the previous chapter are detailed. One project involves combining the use of optical emitters with microfluidic injections containing caged neurotransmitters to stimulate neurons is described. The other project involves microfluidic sampling of the extracellular space for neuropeptides which are detected using ring resonator biosensors. The sensitivity of these biosensors was analyzed in detail, determining both the physical limit of detection and the effect of biological noise due to non-specific binding on the sensors

Ultra-violet lithography of thick photoresist for the applications in BioMEMS and micro optics

UV lithography of thick photoresist is widely used in microelectromechanical systems (MEMS) and micro-optoelectromechanical systems (MOEMS). SU-8 is a typical negative tone thick photoresist for micro systems, and can be used for both structural material and pattern transfer. This dissertation presents an effort to comprehensively study these important subjects. The first part, and the most fundamental part of this dissertation concentrated on the numerical analysis and experimental study of the wavelength dependent absorbance of SU-8 and the diffraction effects on the sidewall profiles of the microstructures made using UV lithography of SU-8. This study has laid the foundation for all the designs and analysis for the BioMEMS and Micro-optic components and systems using UV lithography of SU-8 in the following chapters of the dissertation. After a full discussion of UV lithography of SU-8, the applications of SU-8 in BioMEMS and micro optics were presented in the following areas: 1) design, analysis, and molding fabrication of biodegradable PLGA microstructures for implanted drug delivery application; 2) design, fabrication, and test of a novel three-dimensional micro mixer/reactor based on arrays of spatially impinging micro-jets; 3) design, analysis, fabrication, and test of a novel new type of truly three-dimensional hydro-focusing unit for flow cytometry applications based on SU-8; 4) Study on a new technology to fabricate out-of-plane pre-aligned microlens and microlens array, and the application of the microlens in a fiber bundle coupler. Finally, a new negative tone thick photoresist based on the composition of EPON resins 165 and 154 were introduced. The synthesis, physical properties, and UV-lithography properties of this new photoresist have been completed. The experimental results have proved it can be a better alternative to SU-8 and can be used in various MEMS and MOEMS applications. Most of the contents have been published or accepted for publications in technical journals or international conferences. Two US patent applications are pending and two more disclosures have been filed for the new technologies presented in this dissertation. There are obviously more work to be done in this promising area and these are presented in the section for future work