4,155 research outputs found
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
Highly Sensitive and Selective Gas Sensors Based on Vertically Aligned Metal Oxide Nanowire Arrays
Mimicking the biological olfactory systems that consist of olfactory receptor arrays with large surface area and massively-diversified chemical reactivity, three dimensional (3D) metal oxide nanowire arrays were used as the active materials for gas detection. Metal oxide nanowire arrays share similar 3D structures as the array of mammal\u27s olfactory receptors and the chemical reactivity of nanowire array can be modified by surface coatings. In this dissertation, two standalone gas sensors based on metal oxide nanowire arrays prepared by microfabrication and in-situ micromanipulation, respectively, have been demonstrated. The sensors based on WO3 nanowire arrays can detect 50 ppb NO2 with a fast response; well-aligned CuO nanowire array present a new detection mechanism, which can identify H2S at a concentration of 500 ppb. To expand the material library of 3D metal oxide nanowire arrays for gas sensing, a general route to polycrystalline metal oxide nanowire array has been introduced by using ZnO nanowire arrays as structural templates. The effectiveness of this method for high performance gas sensing was first investigated by single-nanowire devices. The polycrystalline metal oxide coatings showed high performance for gas detection and their sensitivity can be further enhanced by catalytic noble metal decorations. To form electronic nose systems, different metal oxide coatings and catalytic decorations were employed to diversify the chemical reactivity of the sensors. The systems can detect low concentrated H2S and NO2 at room temperature down to part-per-billion level. The system with different catalytic metal coatings is also capable of discriminiating five different gases (H2S, NO2, NH3, H2 and CO)
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Nanowire Photoelectrochemistry.
Recent applications of photoelectrochemistry at the semiconductor/liquid interface provide a renewable route of mimicking natural photosynthesis and yielding chemicals from sunlight, water, and air. Nanowires, defined as one-dimensional nanostructures, exhibit multiple unique features for photoelectrochemical applications and promise better performance as compared to their bulk counterparts. This article reviews the use of semiconductor nanowires in photoelectrochemistry. After introducing fundamental concepts essential to understanding nanowires and photoelectrochemistry, the review considers answers to the following questions: (1) How can we interface semiconductor nanowires with other building blocks for enhanced photoelectrochemical responses? (2) How are nanowires utilized for photoelectrochemical half reactions? (3) What are the techniques that allow us to obtain fundamental insights of photoelectrochemistry at single-nanowire level? (4) What are the design strategies for an integrated nanosystem that mimics a closed cycle in artificial photosynthesis? This framework should help readers evaluate the salient features of nanowires for photoelectrochemical applications, promoting the sustainable development of solar-powered chemical plants that will benefit our society in the long run
Nanofabrication
We face many challenges in the 21st century, such as sustainably meeting the world's growing demand for energy and consumer goods. I believe that new developments in science and technology will help solve many of these problems. Nanofabrication is one of the keys to the development of novel materials, devices and systems. Precise control of nanomaterials, nanostructures, nanodevices and their performances is essential for future innovations in technology. The book "Nanofabrication" provides the latest research developments in nanofabrication of organic and inorganic materials, biomaterials and hybrid materials. I hope that "Nanofabrication" will contribute to creating a brighter future for the next generation
Characterization and Synthesis of Nanoscale Materials
This dissertation focuses on the systematic study of techniques for characterization and synthesis of nanoscale materials. We have achieved several goals. Firstly, high number density uniform zinc oxide nanostructure growth has been achieved using thermal evaporation, through control of experimental parameters that include source material temperature, substrate temperature, substrate material, gas flow rate, and choice of catalyst. Aligned zinc oxide nanowires, randomly oriented zinc oxide nanowires, zinc oxide container-shaped structures, and zinc oxide nanobelts have been synthesized with high yield. Secondly, using a one parameter family of lattice fringe geometry curves, we show how to examine the epitaxial relationship between catalyst particles and a cylindrical support. Using digital darkfield techniques, this investigation can be automated. Thirdly, the structure relationship between catalyst particles and zinc oxide nanowires has been investigated using scanning and high resolution scanning transmission electron microscopes. A vapor-solid-solid growth model involving a hexagonal array of aligned growth regions is proposed in zinc oxide nanowire formation. Evidence indicates in particular that gold catalyst particles remain solid during ZnO nanowire growth. Finally, the effect of tin catalyst thickness on nanostructure formation has been investigated. The catalyst abundance on the substrate has a direct impact on its ability to absorb ZnO. The thicker coated substrates can absorb more source vapor, and form larger structures, than can thinner coated substrates
Characterization and synthesis of nanoscale materials
This dissertation focuses on the systematic study of techniques for characterization and synthesis of nanoscale materials. We have achieved several goals. Firstly, high number density uniform zinc oxide nanostructure growth has been achieved using thermal evaporation, through control of experimental parameters that include source material temperature, substrate temperature, substrate material, gas flow rate, and choice of catalyst. Aligned zinc oxide nanowires, randomly oriented zinc oxide nanowires, zinc oxide container-shaped structures, and zinc oxide nanobelts have been synthesized with high yield. Secondly, using a one parameter family of lattice fringe geometry curves, we show how to examine the epitaxial relationship between catalyst particles and a cylindrical support. Using digital darkfield techniques, this investigation can be automated. Thirdly, the structure relationship between catalyst particles and zinc oxide nanowires has been investigated using scanning and high resolution scanning transmission electron microscopes. A vapor-solid-solid growth model involving a hexagonal array of aligned growth regions is proposed in zinc oxide nanowire formation. Evidence indicates in particular that gold catalyst particles remain solid during ZnO nanowire growth. Finally, the effect of tin catalyst thickness on nanostructure formation has been investigated. The catalyst abundance on the substrate has a direct impact on its ability to absorb ZnO. The thicker coated substrates can absorb more source vapor, and form larger structures, than can thinner coated substrates --Abstract, page iv
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