4D electron diffraction reveals correlated unidirectional Behavior in zinc oxide nanowires

The confined electronic structure of nanoscale materials has increasingly been shown to induce behavior quite distinct from that of bulk analogs. Direct atomic- scale visualization of nanowires of zinc oxide was achieved through their unique pancake- type diffraction by using four- dimensional (4D) ultrafast electron crystallography. After electronic excitation of this wide- gap photonic material, the wires were found to exhibit colossal expansions, two orders of magnitude higher than that expected at thermal equilibrium; the expansion is highly anisotropic, a quasi- one- dimensional behavior, and is facilitated by the induced antibonding character. By reducing the density of nanowires, the expansions reach even larger values and occur at shorter times, suggesting a decrease of the structural constraint in transient atomic motions. This unanticipated ultrafast carrier- driven expansion highlights the optoelectronic consequences of nanoscale morphologies

Transport Properties and Nanosensors of Oxide Nanowires and Nanobelts

ZnO is one of the most important materials for electronics, optoelectronics, piezoelectricity and optics. With a wide band gap of 3.37eV and an exiton binding energy of 60meV, ZnO 1D nanostructures exhibit promising properties in a lot of optical device applications. It is also an important piezoelectric material and has applications in a new category of nanodevices, nano-piezotronics. Demonstrated prototype of devices includes nanogenerators, piezoelectric-FET, and a series of evolutive devices based on the concept of nanogenerator. This is based on working principle of a semiconductor and piezoelectric coupled property. This thesis is about the growth, characterization and device fabrication of ZnO nanowires and nanobelts for sensors and UV detectors. First, the fundamental synthesis of ZnO nanostructurs is investigated, particularly polar surface dominated nanostructues, to illustrate the unique growth configurations of ZnO. Detail study in this part includes nanobelts, nanorings, nanocombs, nanonetworks, and nanodiskettes synthesis. Important factors in driving the nanostructure synthesis mechanism are analyzed, such as the chemical activities of different surface of ZnO and the polar surface dominated effects. Then, the devices fabricated methods using individual nanowires/nanobelts and their electrical transport properties were carefully characterized. In this part, dominant factors which are critical for nanobelt device performance are investigated, such as the contact properties, interface effects, and durability testing. Also, a metal doping method is studied to explore the controlling and modification of nanowire electric and optical properties. Further more, I will present the surface functionalization of nanobelt for largely improving its electrical, optoelectronic and chemical performance. Surface functionalization of nanobelts is proven to be an effective method in enhancing the semiconductor and metal contact. Piezoelectric field-effect transistors will be demonstrated as a powerful approach as chemical sensors. Finally, a technique is illustrated for functionalizing the surfaces of ZnO nanobelts for enhancing its UV sensitivity by over five orders of magnitude. This demonstrates an effective approach for fabricatiing ultrasensitive UV detectors. The research results presented in this thesis have made great contribution to the growth, device fabrication and novel applications of ZnO nanostructures for photonics, optoelectronics and sensors.Ph.D.Committee Chair: Wang, Zhong L. (ZL); Committee Member: Deng, Yulin; Committee Member: Hesketh, Peter; Committee Member: Snyder, Robert; Committee Member: Wong, C.P

Density-controlled growth of aligned ZnO nanowire arrays by seedless chemical approach on smooth surfaces

©2008 Materials Research Society. The original publication is available at: http://www.mrs.org/A novel ZnO seedless chemical approach for density-controlled growth of ZnO nanowire (NW) arrays has been developed. The density of ZnO NWs is controlled by changing the precursor concentration. Effects of both growth temperature and growth time are also investigated. By this novel synthesis technique, ZnO NW arrays can grow on any substrate (polymer, glass, semiconductor, metal, and more) as long as the surface is smooth. This technique represents a new, low-cost, time-efficient, and scalable method for fabricating ZnO NW arrays for applications in field emission, vertical field effect transistor arrays, nanogenerators, and nanopiezotronics

Fabrication and Characterization of 3D-Printed Highly-Porous 3D LiFePO4 Electrodes by Low Temperature Direct Writing Process

LiFePO4 (LFP) is a promising cathode material for lithium-ion batteries. In this study, low temperature direct writing (LTDW)-based 3D printing was used to fabricate three-dimensional (3D) LFP electrodes for the first time. LFP inks were deposited into a low temperature chamber and solidified to maintain the shape and mechanical integrity of the printed features. The printed LFP electrodes were then freeze-dried to remove the solvents so that highly-porous architectures in the electrodes were obtained. LFP inks capable of freezing at low temperature was developed by adding 1,4 dioxane as a freezing agent. The rheological behavior of the prepared LFP inks was measured and appropriate compositions and ratios were selected. A LTDW machine was developed to print the electrodes. The printing parameters were optimized and the printing accuracy was characterized. Results showed that LTDW can effectively maintain the shape and mechanical integrity during the printing process. The microstructure, pore size and distribution of the printed LFP electrodes was characterized. In comparison with conventional room temperature direct ink writing process, improved pore volume and porosity can be obtained using the LTDW process. The electrochemical performance of LTDW-fabricated LFP electrodes and conventional roller-coated electrodes were conducted and compared. Results showed that the porous structure that existed in the printed electrodes can greatly improve the rate performance of LFP electrodes

Effect of Hot Isostatic Pressing on Microstructures and Mechanical Properties of Ti6Al4V Fabricated by Electron Beam Melting

This study investigated the effects of hot isostatic pressing (HIP) on the microstructures and mechanical properties of Ti6Al4V fabricated by electron beam melting (EBM). The differences of surface morphologies, internal defects, relative density, microstructures, textures, mechanical properties and tensile fracture between the as-built and HIPed samples were observed using various characterization methods including optical metallography microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) and tensile tests. It was found that the main effects of HIP on microstructures include—the increase of average grain size from 7.96 ± 1.21 μm to 11.34 ± 1.89 μm, the increase of α lamellar thickness from 0.71 ± 0.15 μm to 2.49 ± 1.29 μm and the increase of β phase ratio from 4.7% to 10.5% in terms of area fraction on the transversal section. The combinatorial effects including densification, increase of grain size, α lamellar thickness, β phase ratio, reduction of dislocation density and transformation of dislocation patterns contributed to the improvement of elongation and ductility of EBM-fabricated Ti6Al4V. Meanwhile, these effects also resulted in a slight reduction of the yield strength and UTS mainly due to the coarsening effect of HIP

3D Printing Technologies for Flexible Tactile Sensors toward Wearable Electronics and Electronic Skin

3D printing has attracted a lot of attention in recent years. Over the past three decades, various 3D printing technologies have been developed including photopolymerization-based, materials extrusion-based, sheet lamination-based, binder jetting-based, power bed fusion-based and direct energy deposition-based processes. 3D printing offers unparalleled flexibility and simplicity in the fabrication of highly complex 3D objects. Tactile sensors that emulate human tactile perceptions are used to translate mechanical signals such as force, pressure, strain, shear, torsion, bend, vibration, etc. into electrical signals and play a crucial role toward the realization of wearable electronics and electronic skin. To date, many types of 3D printing technologies have been applied in the manufacturing of various types of tactile sensors including piezoresistive, capacitive and piezoelectric sensors. This review attempts to summarize the current state-of-the-art 3D printing technologies and their applications in tactile sensors for wearable electronics and electronic skin. The applications are categorized into five aspects: 3D-printed molds for microstructuring substrate, electrodes and sensing element; 3D-printed flexible sensor substrate and sensor body for tactile sensors; 3D-printed sensing element; 3D-printed flexible and stretchable electrodes for tactile sensors; and fully 3D-printed tactile sensors. Latest advances in the fabrication of tactile sensors by 3D printing are reviewed and the advantages and limitations of various 3D printing technologies and printable materials are discussed. Finally, future development of 3D-printed tactile sensors is discussed