Progress in Nanoporous Templates: Beyond Anodic Aluminum Oxide and Towards Functional Complex Materials

Successful synthesis of ordered porous, multi-component complex materials requires a series of coordinated processes, typically including fabrication of a master template, deposition of materials within the pores to form a negative structure, and a third deposition or etching process to create the final, functional template. Translating the utility and the simplicity of the ordered nanoporous geometry of binary oxide templates to those comprising complex functional oxides used in energy, electronic, and biology applications has been met with numerous critical challenges. This review surveys the current state of commonly used complex material nanoporous template synthesis techniques derived from the base anodic aluminum oxide (AAO) geometry

Boosting solar energy harvesting with ordered nanostructures fabricated by anodic aluminum oxide templates

To date, technical development has boosted the efficiencies of solar energy conversion devices with conventional planar architectures to be close to the respective theoretical values, which are hard to be further improved without reforming the device structures. Alternatively, ordered nanostructure arrays have recently emerged as efficacious scaffolds to construct devices for converting energy more efficiently due to their advantageous optical effects. To meet the global energy requirements for producing renewable energy efficiently, a general approach is needed to fabricate diverse ordered nanostructure arrays. In the meantime, the approach should allow for fine tuning in every set of nanounits towards obtaining desired properties. Herein, we utilized anodic aluminum oxide (AAO) templates to provide a versatile method for constructing ordered nanostructure arrays from one to two dimensions. Firstly, arrays of one-dimensional Au nanowires comprising two components of pillar and truncated pyramid were fabricated. Then, periodic one-dimensional Janus hetero-nanostructures with programmable morphologies, compositions, dimensions, and interfacial junctions were realized. Finally, two-dimensional superlattice photonic crystals with two sets of nanopores were constructed via a combination of the AAO template and the structural replication technique. Subsequently, these as-obtained nanostructures were integrated into photoelectrochemical water-splitting cells and solar-to-thermal conversion systems, which significantly boosted solar energy harvesting performance. In conjunction with theoretical simulations, we further elucidated that the enhanced light harvesting ability can be ascribed to twofold facts: photonic effects and surface plasmon resonance which thus provide a route to manipulate light at the nanoscale.In dieser Dissertation habe ich drei Arten von hochgeordneten Nanostrukturen realisiert, einschließlich 1D-PTP-Au-Core / CdS-Shell-Array, Au-NW / TiO2-NT-Janus-Hetero-Nanostruktur-Array und 2D-Metall-SPhCs. Diese fortschrittlichen Architekturen könnten als vielseitige Gerüste zum Aufbau energiebezogener Geräte eingesetzt werden und haben ein großes Potenzial, die Gesamtleistung drastisch zu verbessern und die durch die planare Konfiguration auferlegten Grenzen zu durchbrechen. Insbesondere die geordneten Nanostruktur-Arrays mit mehreren Komponenten sind von großer Bedeutung, und die entsprechenden Geräte können die Vorteile dieser nanostrukturierten Komponenten kombinieren, wodurch die relevante Leistung systematisch verbessert wird. Darüber hinaus ermöglichen die hohe Regelmäßigkeit der Nanostrukturverteilung, die Gleichmäßigkeit der Nanounits sowie die steuerbaren Größen und Profile der Nanostruktur die resultierenden Architekturen als leistungsfähige Plattform, um die spezifischen Energieumwandlungsreaktionen mikroskopisch zu untersuchen. Diese Ergebnisse könnten wiederum die weitere Entwicklung der relevanten Geräte leiten

An Integrated Gas Sensing System Based on Surface-Functionalized Gallium Nitride Nanowires with Embedded Micro-Heaters

In the last few decades, significant improvements have been made in gas sensor technologies. Metal-oxide sensors have been used for low-cost detection of combustible and toxic gases. However, hurdles relating to sensitivity, stability and selectivity still remain. Recently, nanotechnology has helped tremendously through the introduction of nano-engineered materials like nanowires and nanoclusters. Nanowire sensors have much better sensitivity as compared with thin-film devices due to the larger detecting surface-to-volume ratio. But clearly, improvements are still needed. For real-world applications, selectivity between different classes of compounds, such as combustible and toxic gases, is highly desirable. An ideal chemical sensor should distinguish between the individual analytes from a single class of compounds. For example, in detection of benzene or toluene, a good sensor will not be disturbed by other aromatic compounds present in the environment. This is a huge challenge for semiconductor based metal-oxide sensors, such as TiO2, SnO2, Fe2O3 and ZnO, which have inherent non-selective surface adsorption sites. Recently, a new class of nanowire-nanocluster (NWNC) based gas sensors has gained interest. This type of sensor represents a new method of functionalizing the surface for selective adsorption and detection. The adjustable sensitivity can be achieved by tuning the density, size or composition of the nanoparticles that decorate the nanowires. These advantages make the NWNC sensors a good alternative to conventional thin-film sensors. So far, research into NWNC sensors has demonstrated the potential in sensing many important classes of compounds. However, most of these NWNC devices require elevated working temperatures. They also have long response/recovery times and must function in an inert atmosphere. All these limitation will be the obstacles in real-world usage for domestic, environmental or industrial applications. And finally, the sensors thus developed must be manufacturable. That is, they must be batch fabricated with high yield. To remedy these problems, my thesis was divided into the following tasks, 1. Develop dry etching techniques to fabricate horizontally aligned GaN nanowires (NW), combining these techniques with wet etching treatment for surface damages removal. I call this a “top-down approach” using a subtractive process that fabricates NWs from thin-films and adding sensitive nanocrystals after the initial NW definition. This is to be compared to the additive “bottom-up” nanowire growth by MBE/HVPE/Sol-gel, in which NWs are grown, harvested from the growth surface and subsequently re-attached to a new surface. The top-down approach enhances the yield and homogeneity of the NW and it is mass-production oriented. 2. Study the metal-oxide nanoclusters (NCs) deposition method by physical vapor deposition (PVD) and rapid thermal annealing (RTA) for TiO2, SnO2, WO3, Fe2O3, etc. Develop the metal nanoparticle deposition method by PVD for Au, Ag, Pt, Pd, etc. 3. Study the crystalline phases and gas adsorption sites formed by the method and establish a database connecting metal-oxide bonding sites with different target chemicals. 4. Utilize Si doped n-type and unintentionally doped GaN nanowires functionalized with different metal-oxide and metal-oxide/metal composite nanoclusters to create a series of highly selective and sensitive gas sensing nanostructure devices. 5. Develop a low-cost micro-heater (MH) for local high temperature generation with low power consumption. This allows the rapid chemical desorption cycles as we anticipate frequently re-use or reset of the sensor. It also enables the use of these NWs in high temperature sensor applications. 6. Integrate the NW, NCs and MH into one working sensor, and integrate multiple types of gas sensors on a single chip. The chip can simultaneously sense many types of gases without interference. In this study, the potential of multicomponent NWNC based sensors for developing the next-generation of ultra-sensitive and highly selective chemical sensors was explored. We have achieved uA and nA levels of baseline detector current and we have shown that low UV illumination enhances sensitivity for some cases. These sensors have low power consumption making them suitable for portable devices

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

Facile integration of ordered nanowires in functional devices

The integration of one-dimensional (1D) nanostructures of non-industry-standard semiconductors infunctional devices following bottom-up approaches is still an open challenge that hampers the exploita-tion of all their potential. Here, we present a simple approach to integrate metal oxide nanowires inelectronic devices based on controlled dielectrophoretic positioning together with proof of conceptdevices that corroborate their functionality. The method is flexible enough to manipulate nanowiresof different sizes and compositions exclusively using macroscopic solution-based techniques in conven-tional electrode designs. Our results show that fully functional devices, which display all the advantagesof single-nanowire gas sensors, photodetectors, and even field-effect transistors, are thus obtained rightafter a direct assembly step without subsequent metallization processing. This paves the way to lowcost, high throughput manufacturing of general-purpose electronic devices based on non-conventionaland high quality 1D nanostructures driving up many options for high performance and new low energyconsumption devices

Suspended 1D metal oxide nanostructure-based gas sensor

Department of Materials Science and EngineeringWe developed a novel batch fabrication technology for the ultralow-power-consumption metal oxide gas sensing platform consisting of a suspended glassy carbon heating nanostructure and hierarchical metal oxide nanostructures forests fabricated by the carbon-micro electromechanical systems (carbon-MEMS) and selective nanowire growth process. We have developed a new manufacturing process for suspended glass carbon nanostructures such as single nanowire, nano-mesh and nano-membranes fabricated using carbon-MEMS consisting of the UV-lithography and the polymer pyrolysis processes. We designed a gas sensing platform consisting of suspended glassy carbon heating nanostructures and suspended hierarchical metal oxide nanostructure forests for the sensing part. Glassy carbon structure produced by the carbon-MEMS has many advantages such as high thermal & chemical stabilities, good hardness, and good thermal & electrical characteristics. The electrical conductivity of glassy carbon nanostructures has been increased more than three times by using rapid thermal annealing (RTA) process owing to the inferior heating property of glassy carbon nano-heater in the electrical conductivity. In order to divide the suspended glassy carbon nano-heater and the suspended hierarchical metal oxide nanostructures forests, the insulating layer of HfO2 materials is a high dielectric constant and is deposited uniformly using a atomic layer deposition (ALD) process on a suspended glassy carbon nano-heater. Suspended hierarchical metal oxide nanostructures forests were grown circumferentially on the suspended HfO2/glassy carbon nano-heater using a hydrothermal method consisting of the seed deposition and the growth processes. For selective metal oxide seed layer deposition process, a short-time exposed polymer patterning process was performed using the positive photoresist. After the polymer patterning process, a metal oxide seed layer is deposited using the rf-sputtering system, followed by a metal oxide nanostructure growth process. The distinguishing architecture of a suspended hierarchical metal oxide nanostructures forests/HfO2/glassy carbon nanostructure ensures efficient mass transport to the metal oxide nanostructure detection point of the gas analyte, resulting in highly sensitive gas detection. In the absence of an external heating system, the ultralow-power-consumption gas sensing platform of a suspended hierarchical metal oxide nanostructures forests/HfO2/glassy carbon nanostructure has excellent the gas sensing characteristics.ope

Laser direct written silicon nanowires for electronic and sensing applications

Silicon nanowires are promising building blocks for high-performance electronics and chemical/biological sensing devices due to their ultra-small body and high surface-to-volume ratios. However, the lack of the ability to assemble and position nanowires in a highly controlled manner still remains an obstacle to fully exploiting the substantial potential of nanowires. Here we demonstrate a one-step method to synthesize intrinsic and doped silicon nanowires for device applications. Sub-diffraction limited nanowires as thin as 60 nm are synthesized using laser direct writing in combination with chemical vapor deposition, which has the advantages of in-situ doping, catalyst-free growth, and precise control of position, orientation, and length. The synthesized nanowires have been fabricated into field effect transistors (FETs) and FET sensors. The FET sensors are employed to detect the proton concentration (pH) of an aqueous solution and highly sensitive pH sensing is demonstrated. Both top- and back-gated silicon nanowire FETs are demonstrated and electrically characterized. In addition, modulation-doped nanowires are synthesized by changing dopant gases during the nanowire growth. The axial p-n junction nanowires are electrically characterized to demonstrate the diode behavior and the transition between dopant levels are measured using Kelvin probe force microscopy

Large-Scale Patterned Oxide Nanostructures: Fabrication, Characterization and Applications

Nanotechnology is experiencing a flourishing development in a variety of fields covering all of the areas from science to engineering and to biology. As an active field in nanotechnology, the work presented in this dissertation is mostly focused on the fundamental study about the fabrication and assembly of functional oxide nanostructures. In particular, Zinc Oxide, one of the most important functional semiconducting materials, is the core objective of this research, from the controlled growth of nanoscale building blocks to understanding their properties and to how to organize these building blocks. Thermal evaporation process based on a single-zone tube furnace has been employed for synthesizing a range of 1D nanostructures. By controlling the experimental conditions, different morphologies, such as ultra-small ZnO nanobelts, mesoporous ZnO nanowires and core-shell nanowire were achieved. In order to pattern the nanostructures, a large-scale highly-ordered nanobowl structure based on the self-assembly of submicron spheres was created and utilized as patterning template. The growth and patterning techniques were thereafter integrated for aligning and patterning of ZnO nanowires. The aligning mechanisms and growth conditions were thoroughly studied so as to achieve a systematic control over the morphology, distribution and density. The related electronic and electromechanical properties of the aligned ZnO nanowires were investigated. The feasibility of some potential applications, such as photonic crystals, solar cells and sensor arrays, has also been studied. This research may set a foundation for many industrial applications from controlled synthesis to nanomanufacturing.Ph.D.Committee Chair: Wang, Zhong Lin; Committee Co-Chair: Summers, Christopher J.; Committee Member: Dupuis, Russell D.; Committee Member: Wagner, Brent; Committee Member: Wong, C. P

Engineering thin films of magnetic alloys and semiconductor oxides at the nanoscale

The thesis aims to exploit properties of thin films for applications such as spintronics, UV detection and gas sensing. Nanoscale thin films devices have myriad advantages and compatibility with Si-based integrated circuits processes. Two distinct classes of material systems are investigated, namely ferromagnetic thin films and semiconductor oxides. To aid the designing of devices, the surface properties of the thin films were investigated by using electron and photon characterization techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and energy-dispersive X-ray spectroscopy (EDS). These are complemented by nanometer resolved local proximal probes such as atomic force microscopy (AFM), magnetic force microscopy (MFM), electric force microscopy (EFM), and scanning tunneling microscopy to elucidate the interplay between stoichiometry, morphology, chemical states, crystallization, magnetism, optical transparency, and electronic properties. Specifically, I studied the effect of annealing on the surface stoichiometry of the CoFeB/Cu system by in-situ AES and discovered that magnetic nanoparticles with controllable areal density can be produced. This is a good alternative for producing nanoparticles using a maskless process. Additionally, I studied the behavior of magnetic domain walls of the low coercivity alloy CoFeB patterned nanowires. MFM measurement with the in-plane magnetic field showed that, compared to their permalloy counterparts, CoFeB nanowires require a much smaller magnetization switching field , making them promising for low-power-consumption domain wall motion based devices. With oxides, I studied CuO nanoparticles on SnO2 based UV photodetectors (PDs), and discovered that they promote the responsivity by facilitating charge transfer with the formed nanoheterojunctions. I also demonstrated UV PDs with spectrally tunable photoresponse with the bandgap engineered ZnMgO. The bandgap of the alloyed ZnMgO thin films was tailored by varying the Mg contents and AES was demonstrated as a surface scientific approach to assess the alloying of ZnMgO. With gas sensors, I discovered the rf-sputtered anatase-TiO2 thin films for a selective and sensitive NO2 detection at room temperature, under UV illumination. The implementation of UV enhances the responsivity, response and recovery rate of the TiO2 sensor towards NO2 significantly. Evident from the high resolution XPS and AFM studies, the surface contamination and morphology of the thin films degrade the gas sensing response. I also demonstrated that surface additive metal nanoparticles on thin films can improve the response and the selectivity of oxide based sensors. I employed nanometer-scale scanning probe microscopy to study a novel gas senor scheme consisting of gallium nitride (GaN) nanowires with functionalizing oxides layer. The results suggested that AFM together with EFM is capable of discriminating low-conductive materials at the nanoscale, providing a nondestructive method to quantitatively relate sensing response to the surface morphology

Chemical Vapor Deposition of One Dimensional Tin Oxide Nanostructures: Structural Studies, Surface Modifications and Device Applications

One-dimensional (1D) metal oxide nanostructures such as wires, rods, belts and tubes have become the focus of intensive research for investigating structure-property relationship under diminishing dimensions and probing their possible scientific and technological applications. Chemical vapor deposition (CVD), based on catalyzed vapor-liquid-solid (VLS) growth mechanism, is an efficient way to synthesize 1D metal oxide nanostructures, which can be implored by combining molecular precursors with CVD-VLS growth. This thesis contains results obtained on a molecule-based CVD approach to grow metal oxide nanowires, elaboration of experimental parameters enabling control over random and orientated growth. (1) Controlled synthesis, growth mechanism and plasma-treatment of SnO2 nanowires. Uniform and high-density single crystalline SnO2 NWs were fabricated by optimization of deposition temperature, precursor temperature, size of catalyst and angle of graphite holder, and the electrical, photoluminescence, gas sensing and field emission properties were also systematically investigated, it enabled us to have a better understanding of SnO2 nanowires. The technical highlights of this work include the successful demonstration of oriented growth of SnO2 nanowires arrays on TiO2(001) substrates by MB-CVD method for the first time. A growth model for the nanowire morphology based upon crystallographic relation between the substrate and NW material is proposed. Electrical and gas sensing properties of SnO2 [101] single nanowire showed that oriented nanowire arrays can be potentially used towards diameter- and orientation-dependent sensing unit for detection of gas molecules. Surface modification of SnO2 nanowires in an argon-oxygen (Ar/O2) plasma treatment caused preferential etching of the oxygen atoms from surface and the inner volume (lattice) producing a non-stoichiometric overlayer, resulting in the higher sensitivity for ethanol gas at lower operating temperature and exhibited improved transducing response towards changing gas atmospheres. (2) New architectures of SnO2 nanowire based 1D heterostructure: Synthesis and properties. New morphological SnO2 nanowire based heterostructures (such as SnO2@TiO2, SnO2@SnO2, SnO2@VOx and SnO2@CdS) were fabricated by chemical surface modification via a two-step process. Structural characterization of SnO2/TiO2 core-shell structures revealed the formation of mixed-cation phases of composition SnxTi1-xO2 (x = 0.857 ~ 1.0) depended on the annealing temperatures, the excellent electrical property and gas sensing performance of SnO2/TiO2 core-shell structures are attributed to nanowire based sensor applications. The SnO2@SnO2 heterostrucutres with contact angle (CA) of 133° exhibited a superhydrophobic property in comparison with the superhydrophilic SnO2 nanowires (CA = 3°). Switchable surface wettability of SiOx coated SnO2@SnO2 heterostructure (CA = 155.8°) was observed by alternation of UV irradiation, dark storage and O2 annealing. Geometric microstructure was the major determinant in the switchable wettability from superhydrophilic to superhydrophobic. The SnO2@CdS QDs heterostructures were fabricated by a chemical bath deposition (CBD) method via hydroxide cluster growth mechanism, and had a remarkably enhancement in photoconductivity than non-coated SnO2 nanowires when the wavelength was below 450 nm. The work carried out in this thesis is supported by Federal Ministry of Education and Research (BMBF) in the frame of the priority program "BMBF-NanoFutur" (FKZ 03X5512)