Rozprawy doktorskie na temat „Dimensional Nanostructure”

Ce manuscrit de thèse présente les recherches et les applications potentielles en tant que plate-forme (bio) capteur des couches minces conformes de ZnO et / ou Al2O3 / ZnO nanolaminates, déposées par dépôt de couche atomique (ALD) sur les différents substrats. Tout d'abord, une étude des propriétés optiques des films minces ZnO (20 et 50 nm) déposés par la technique ALD sur les grandes zones de nanofils de silicium ordonné (SiNW), réalisée en combinant la lithographie à la nanosphère et la gravure chimique à base de métal, a été réalisée. Ces méthodes ont permis la morphologie et le contrôle organisationnel des SiNW sur une grande surface. L'étude détaillée des propriétés structurales et optiques de l'hétérostructure SiNWs / ZnO à noyau-coquille a été réalisée en utilisant respectivement la spectroscopie XRD, SEM, de réflectance et de photoluminescence. L'intégration des tableaux SiNWs en tant que noyau et ZnO comme coque peut avoir un impact important sur le développement d'éléments de détection avec des propriétés améliorées. Dans les recherches ultérieures, des films ZnO formés par ALD en tant que plate-forme de biocapteur optique pour la détection des protéines de type A du virus Grapevine (antigènes GVA) ont été représentés. La détection de l'antigène GVA a été effectuée en utilisant les changements dans le comportement de la bande PL liée à la GVA. La sélectivité du biocapteur a été prouvée. La possibilité de détecter les antigènes GVA sans étiquettes supplémentaires a été démontrée. Ainsi, on a développé un biosensor à base de photoluminescence à base de photoluminescence libre pour les antigènes GVA. Une autre partie de notre étude est un contrôle spécifique de l'ancrage des protéines par le développement d'une surface multifonctionnelle avec une grande gamme de sphères de polystyrène (PSS), produite par la lithographie de nanosphère et bloquant davantage l'adsorption non spécifique des protéines à la surface du PSS par SAM de PEG. La microscopie d'épifluorescence a été utilisée pour confirmer qu'après l'immersion de l'échantillon sur la protéine cible (avidine et anti-avidine), ces dernières sont spécifiquement situées sur une sphère de polystyrène. Ces résultats sont significatifs pour l'exploration de dispositifs basés sur un nanoarray à grande échelle de sphères de PS et peuvent être utilisés pour la détection de protéines cibles ou simplement pour structurer une surface avec des protéines spécifiques. Notre recherche comprend également l'ajustement des propriétés structurelles et l'amélioration des propriétés électroniques et optiques des nanolaminés 1D PAN ZnO / Al2O3 conçus par dépôt de couche atomique (ALD) et électrospinning. Les propriétés structurelles et optiques de Al2O3 / ZnO déterminées à partir des analyses XPS, TEM, FTIR, XRD et PL. L'amélioration des propriétés électroniques et optiques permettrait l'application dans différents domaines de tels capteurs et biosensors
This thesis manuscript presents the investigations and potential applications as a (bio)sensor platform of the conform thin layers of ZnO and/or Al2O3/ZnO nanolaminates, deposited by atomic layer deposition (ALD) on the various substrates. First, a study of the optical properties of ZnO thin films (20 and 50 nm) deposited by ALD technique on the large areas of ordered silicon nanowires (SiNWs), produced by combining nanosphere lithography and metal-assisted chemical etching, was performed. These methods allowed the morphology and the organization control of SiNWs on a large area. The detailed study of structural and optical properties of core-shell SiNWs/ZnO heterostructure was done by utilizing XRD, SEM, reflectance and photoluminescence spectroscopy, respectively. Integration of SiNWs arrays as core and ZnO as shell can have a strong impact on the development of sensing elements with improved properties. In the further investigations, ZnO films formed by ALD as an optical biosensor platform for the detection of Grapevine virus A-type proteins (GVA-antigens) were represented. The GVA-antigen detection was performed using the changes in the GVA related PL band behavior. The biosensor selectivity has been proved. The possibility to detect GVA-antigens without additional labels has been demonstrated. Thus, label free and sensitive photoluminescence based biosensor for GVA-antigens has been developed. Another part of our study is a specific control of protein anchoring by the development of multifunctional surface with large-scale array of polystyrene spheres (PSS), which produced by nanosphere lithography and further blocking the unspecific adsorption of protein on the surface of the PSS by PEG SAMs. The epifluorescence microscopy was used to confirm that after immersion of sample on target protein (avidin and anti-avidin) solution, the latter are specifically located on polystyrene sphere. These results are meaningful for exploration of devices based on large-scale nanoarray of PS spheres and can be used for detection of target proteins or simply to pattern a surface with specific proteins. Our research also includes the tuning of structural properties and the enhancement of electronic and optical properties of 1D PAN ZnO/Al2O3 nanolaminates designed by atomic layer deposition (ALD) and electrospinning. The structural and optical properties of Al2O3/ ZnO determined from the XPS, TEM, FTIR, XRD and PL analysis. The enhancement of electronic and optical properties would allow application in different fields such sensors and biosensors

Thesis (Ph.D)--Chemical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Deng,Yulin; Committee Member: Hsieh, Jeffery S.; Committee Member: Nair, Sankar; Committee Member: Singh, Preet; Committee Member: Yao, Donggang. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The interest for plasmonic nanolasers has been growing in the last ten years, since they are one of the most promising ways to reach the miniaturization of lasers. In fact, these devices could break the limit of physical confinement of light thanks to the virtual cavity given by plasmonic nanostructures which substitutes the current macroscopic optical cavities. These plasmonic devices can also support high speed operation mode, low lasing threshold and a narrow directional emission. For this reason, during this project, we focused on the design, the synthesis and the characterization of plasmonic nanolasers based on Au nanodome arrays and Ag nanodisk arrays. In order to synthesize highly ordered nanostructure arrays, we used Nanosphere Lithography (NSL), which is a cost effective and high throughput technique based on the self-assembling of polystyrene nanospheres. Thanks to the versatility of NSL, we have developed different nanofabrication protocols, combining NSL with Reactive Ion Etching (RIE) and Physical Vapor Deposition (PVD). Therefore, we investigated the optical properties of our synthesized arrays, recreating the optical band structure along the high symmetry directions of the reciprocal space. Suitable dye emitters (Pyridine 2 and Styryl 9M) were selected in order to couple their emission with the optical modes of the nanoarrays, on the basis of optical band structure information. In addition, in order to optimize the plasmonic properties and the local field enhancement of the metallic nanostructures, numerical simulations by COMSOL Multiphysics were performed. The interaction between dye and plasmonic structure generated an amplified emission. In particular, for Au nanodome arrays coupled with Pyridine 2 dissolved in ethanol, an amplification on the emission arises at 720 nm with a threshold behavior at 0.9 mJ/cm^2 and the FWHM of 14 nm. Furthermore, a highly directional emission was obtained at 17° with an angular divergence of 3° which takes place along the Rayleigh anomaly mode. By comparing the results of Au nanodome arrays and silica nanodome arrays, we concluded that lattice modes give a contribution to the emission directionality, while plasmonic modes provide a reduced lasing threshold overcoming the energy loss. Ag hexagonal nanodisk array showed a similar behavior to the Au nanodome arrays: we found a lasing threshold at 1.6 mJ/cm^2 , with also a similar FWHM. In this case, the emission is directed at 65° and presents an angular divergence of about 14° . Moreover, we investigated a nanolaser with a solid-state gain medium for the interest in applications and for the device integration on a chip. The Styryl 9M laser dye is embedded in a PMMA film and coupled with an Au nanodome array. This solid-state system presents an amplified emission at 795 nm with a threshold of 1.2 mJ/cm^2 and a FWHM of about 26 nm. The sample shows also a directional emission at 24° and with an angular divergence of 6° . Further investigations have shown the possibility to eliminate the substrate, creating a self-standing device, which exhibits an amplified emission with similar properties of that with the substrate. Finally, in order to discern the spontaneous or stimulated nature of the emission, we performed coherence measurements of the emitted beam. By a modified Michelson interferometer, a coherence length of about 29 um was determined for Au nanodome arrays above threshold. This result demonstrated that a coherent, low-threshold and highly directional emission can be obtained by coupling a suitable fluorescent dye to a properly designed virtual cavity realized by an ordered array of plasmonic nanostructures.
Nell'ultima decina di anni, l'interesse per i nanolaser plasmonici è cresciuto siccome sono uno tra i modi più promettenti per la miniaturizzazione dei laser. Infatti, questi dispositivi possono superare il limite di confinamento fisico della luce, grazie alla cavità virtuale data dalle nanostrutture plasmoniche che sostituiscono la convenzionale cavità ottica macroscopica. Inoltre, questi dispositivi plasmonici possono supportare modalità di funzionamento ad alta velocità, bassa soglia di emissione laser e una direzionalità ben definita. Per questa ragione, durante questo progetto, ci siamo concentrati sulla progettazione, la sintesi e la caratterizzazione di nanolasers plasmonici basati su array di nanocupole di oro e array di nanodischi di argento. Al fine di sintetizzare reticoli di nanoparticelle con un ordine elevato, abbiamo utilizzato la Nanosphere Lithography (NSL), una tecnica economica e ad alta produttività basata sull'autoassemblaggio di nanosfere di polistirene. Grazie alla versatilità della NSL, abbiamo sviluppato diversi protocolli di nanofabbricazione, combinando la NSL con i processi di Reactive Ion Etching (RIE) e deposizione fisica da vapore (PVD). Successivamente, abbiamo studiato le proprietà ottiche dei campioni sintetizzati, ricostruendo la struttura a bande ottica lungo le direzioni di alta simmetria dello spazio reciproco. Abbiamo selezionato due adeguati emettitori coloranti, la Pyridine 2 e lo Styryl 9M, al fine di accoppiare la loro emissione con le modalità ottiche dei reticoli nanostrutturati, sulla base delle informazioni della struttura a bande ottica. Inoltre, per ottimizzare le proprietà plasmoniche e l'amplificazione del campo locale delle nanostrutture metalliche, delle simulazioni numeriche sono state effettuate tramite il software COMSOL Multiphysics. L'interazione tra il colorante e la struttura plasmonica ha generato un'emissione amplificata. In particolare, nel reticolo di nanocupole di oro accoppiato alla piridina 2 disciolta in etanolo, un'amplificazione dell'emissione si presenta a720nm con un comportamento a soglia a 0.9 mJ/cm^2 . Inoltre, è stata ottenuta un'emissione direzionale a 17° con una divergenza angolare di 3° che avviene lungo l'anomalia di Rayleigh. Confrontando i risultati dei reticoli di nanocupole di oro con quelli dei reticoli di nanocupole di silice, abbiamo concluso che i modi di reticolo danno un contributo alla direzionalità dell'emissione, mentre i modi plasmonici forniscono una riduzione della soglia laser superando così la perdita di energia. Il reticolo esagonale di nanodischi di argento mostra un comportamento simile a quello con le nanocupole di oro: abbiamo trovato una soglia laser a 1.6 mJ/cm^2 , con anche una simile FWHM. In questo caso, questo fascio è diretto a 65° e presenta una divergenza angolare di circa 14° . Inoltre, abbiamo studiato anche un nanolaser con un mezzo di guadagno a stato solido per l'interesse nelle applicazioni e nell'integrazione di dispositivi su chip. Il colorante laser Styryl 9M è incorporato in un film di PMMA e accoppiato con un reticolo di nanocupole di oro. Questo sistema a stato solido presenta un'emissione amplificata a 795 nm con una soglia di 1.2 mJ/cm^2 e una FWHM di circa 26 nm. Questo campione manifesta anche un'emissione direzionale a 24° con una divergenza angolare di 6° . Ulteriori ricerche hanno dimostrato la possibilità di eliminare il substrato, creando un dispositivo autoportante, che presenta un'emissione amplificata con proprietà simili a quella con il substrato. Infine, per discernere la natura spontanea o stimolata dell'emissione, abbiamo misurato la coerenza del raggio emesso. Tramite un interferometro di Michelson dedicato, la lunghezza di coerenza è stimata a circa 29 um per i reticoli di nanocupole d'oro sopra la soglia. Questo risultato ha dimostrato che è possibile ottenere un'emissione coerente, a bassa soglia e altamente direzionale, accoppiando un colorante fluorescente adeguato con una cavità virtuale opportunamente progettata e realizzata da una reticolo ordinato di nanostrutture plasmoniche.

Fabrication and characterization of nanometre scale devices consisting of a suspended nanotube/nanowire and self-aligned electrodes is reported. An electromechnical switch and a field effect transistor have been realised using the nano device technology developed. The electromechanical switch has a triode structure and is designed so that a suspended carbon nanotube is mechanically switched to one of two self-aligned electrodes by repulsive electrostatic forces between the nanotube and the self-aligned electrode. The electrical measurements show well defined On and Off states with change of gate voltage. The measured threshold voltage for electromechanical switching is 3.6 V. A field effect transistor (FET) using a zinc oxide nanowire with significantly enhanced performance is demonstrated. The fabricated FET exhibits superior electrical performance with a transconductance of 3.06 μS, a mobility of 928 cm²/Vs and an On/Off ratio 10⁶. The electrical characteristics are the best obtained to date for a ZnO nanowire transistor. The results are close to those reported previously for p-type Carbon Nanotube FETs. This raises the possibility of using ZnO as the n-type FET with a CNT as the p-type FET in nanometre scale complimentary logic circuits.

At the beginning of this thesis, basic and advanced device fabrication process which I haveexperienced during study such as top-down and bottom-up approach for the nanoscale devicefabrication technique have been described. Especially, lithography technology has beenfocused because it is base of the modern device fabrication. For the advanced device structure,etching technique has been investigated in detail.The characterization of FET has been introduced. For the practical consideration in theadvanced FET, several parameter extraction techniques have been introduced such as Yfunction,split C-V etc.FinFET is one of promising alternatives against conventional planar devices. Problem ofFinFET is surface roughness. During the fabrication, the etching process induces surfaceroughness on the sidewall surfaces. Surface roughness of channel decreases the effectivemobility by surface roughness scattering. With the low temperature measurement andmobility analysis, drain current through sidewall and top surface was separated. From theseparated currents, effective mobilities were extracted in each temperature conditions. Astemperature lowering, mobility behaviors from the transport on each surface have differenttemperature dependence. Especially, in n-type FinFET, the sidewall mobility has strongerdegradation in high gate electric field compare to top surface. Quantification of surfaceroughness was also compared between sidewall and top surface. Low temperaturemeasurement is nondestructive characterization method. Therefore this study can be a propersurface roughness measurement technique for the performance optimization of FinFET.As another quasi-1 D nanowire structure device, 3D stacked SiGe nanowire has beenintroduced. Important of strain engineering has been known for the effective mobility booster.The limitation of dopant diffusion by strain has been shown. Without strain, SiGe nanowireFET showed huge short channel effect. Subthreshold current was bigger than strained SiGechannel. Temperature dependent mobility behavior in short channel unstrained device wascompletely different from the other cases. Impurity scattering was dominant in short channelunstrained SiGe nanowire FET. Thus, it could be concluded that the strain engineering is notnecessary only for the mobility booster but also short channel effect immunity.Junctionless FET is very recently developed device compare to the others. Like as JFET,junctionless FET has volume conduction. Thus, it is less affected by interface states.Junctionless FET also has good short channel effect immunity because off-state ofjunctionless FET is dominated pinch-off of channel depletion. For this, junctionless FETshould have thin body thickness. Therefore, multi gate nanowire structure is proper to makejunctionless FET.Because of the surface area to volume ratio, quasi-1D nanowire structure is good for thesensor application. Nanowire structure has been investigated as a sensor. Using numericalsimulation, generation-recombination noise property was considered in nanowire sensor.Even though the surface area to volume ration is enhanced in the nanowire channel, devicehas sensing limitation by noise. The generation-recombination noise depended on the channelgeometry. As a design tool of nanowire sensor, noise simulation should be carried out toescape from the noise limitation in advance.The basic principles of device simulation have been discussed. Finite difference method andMonte Carlo simulation technique have been introduced for the comprehension of devicesimulation. Practical device simulation data have been shown for examples such as FinFET,strongly disordered 1D channel, OLED and E-paper
At the beginning of this thesis, basic and advanced device fabrication process which I haveexperienced during study such as top-down and bottom-up approach for the nanoscale devicefabrication technique have been described. Especially, lithography technology has beenfocused because it is base of the modern device fabrication. For the advanced device structure,etching technique has been investigated in detail.The characterization of FET has been introduced. For the practical consideration in theadvanced FET, several parameter extraction techniques have been introduced such as Yfunction,split C-V etc.FinFET is one of promising alternatives against conventional planar devices. Problem ofFinFET is surface roughness. During the fabrication, the etching process induces surfaceroughness on the sidewall surfaces. Surface roughness of channel decreases the effectivemobility by surface roughness scattering. With the low temperature measurement andmobility analysis, drain current through sidewall and top surface was separated. From theseparated currents, effective mobilities were extracted in each temperature conditions. Astemperature lowering, mobility behaviors from the transport on each surface have differenttemperature dependence. Especially, in n-type FinFET, the sidewall mobility has strongerdegradation in high gate electric field compare to top surface. Quantification of surfaceroughness was also compared between sidewall and top surface. Low temperaturemeasurement is nondestructive characterization method. Therefore this study can be a propersurface roughness measurement technique for the performance optimization of FinFET.As another quasi-1 D nanowire structure device, 3D stacked SiGe nanowire has beenintroduced. Important of strain engineering has been known for the effective mobility booster.The limitation of dopant diffusion by strain has been shown. Without strain, SiGe nanowireFET showed huge short channel effect. Subthreshold current was bigger than strained SiGechannel. Temperature dependent mobility behavior in short channel unstrained device wascompletely different from the other cases. Impurity scattering was dominant in short channelunstrained SiGe nanowire FET. Thus, it could be concluded that the strain engineering is notnecessary only for the mobility booster but also short channel effect immunity.Junctionless FET is very recently developed device compare to the others. Like as JFET,junctionless FET has volume conduction. Thus, it is less affected by interface states.Junctionless FET also has good short channel effect immunity because off-state ofjunctionless FET is dominated pinch-off of channel depletion. For this, junctionless FETshould have thin body thickness. Therefore, multi gate nanowire structure is proper to makejunctionless FET.Because of the surface area to volume ratio, quasi-1D nanowire structure is good for thesensor application. Nanowire structure has been investigated as a sensor. Using numericalsimulation, generation-recombination noise property was considered in nanowire sensor.Even though the surface area to volume ration is enhanced in the nanowire channel, devicehas sensing limitation by noise. The generation-recombination noise depended on the channelgeometry. As a design tool of nanowire sensor, noise simulation should be carried out toescape from the noise limitation in advance.The basic principles of device simulation have been discussed. Finite difference method andMonte Carlo simulation technique have been introduced for the comprehension of devicesimulation. Practical device simulation data have been shown for examples such as FinFET,strongly disordered 1D channel, OLED and E-paper

The key challenge for a nanomaterial based sensor is how to synthesize in bulk quantity and fabricate an actual device with insightful understanding of operational mechanisms during performance. I report here effective, controllable methods that exploit the concepts of the "green approach" to synthesize two different one-dimensional nanostructures, including tin oxide nanowires and carbon nanotubes. The syntheses are followed by product characterization and sensing device fabrications as well as sensor performance understanding at the molecular level. Sensor-analyte response and recovery kinetics are also presented. The first part of the thesis describes bulk-scale synthesis and characterization of tin oxide nanowires by the molten salt synthetic method and the nanowire doping with antimony (n-types) and lithium. The work builds on the success of using n-doped SnO2 nanoparticles to selectively detect chlorine gas at room temperature. Replacing n-doped nanoparticles with n-doped nanowires reduces the number of inter-particle electron hops between sensing electrodes. The nanowire based sensors show unprecedented 5 ppb detectability of corrosive Cl2 gas concentration in air. At the higher range, 10 ppm of Cl2 gas leads to a 250 fold increase in the device resistance. During sensor recovery, FT-IR studies show that dichlorine monoxide (Cl2O) and chlorine dioxide (ClO2) are the desorbing species. Long term stability of devices is affected by lattice oxygen vacancies replaced by chlorine atoms. Bulk-scale synthesis of multiwall carbon nanotube (MWCNTs) was achieved by a novel inexpensive synthetic method. The green chemistry method uses the non-toxic and easy to handle solid carbon source naphthalene. The synthesis is carried out by simply heating naphthalene and organometallic precursors as catalysts in a sealed glass tube. Synthesis at 610º C leads to MWCNTs of 50 nm diameter and lengths exceeding well over microns. MWCNT doping is attempted with nitrogen (n-type) and boron (p-type) precursors. Palladium nanoparticles decorated on as-synthesized MWCNTs are employed for specific detection of explosive hydrogen gas with concentrations far below the explosive concentration limits. During performance, the sensor exhibits abnormal response behaviors at hydrogen gas concentrations higher than 1%. A model of charge carrier inversion, brought about by reduction of MWCNT by hydrogen molecules dissociated by Pd nanoparticles is proposed.

My work presents a novel approach to fabricate binder free three-dimensional carbon nanotubes/sulfur (3DCNTs/S) hybrid composite by a facile and scalable method increasing the loading amount from 1.86 to 8.33 mg/cm2 highest reported to date with excellent electrochemical performance exhibiting maximum specific energy of ~1233Wh/kg and specific power of ~476W/kg, with respect to the mass of the cathode. Such an excellent performance is attributed to the fact that 3DCNTs offers higher loading amount of sulfur, and confine polysulfide within the structure. In second part of the thesis, molybdenum disulfide (MoS2) is typically studied for three electrochemical energy storage devices including supercapacitors, Li-ion batteries, and hybrid Li-ion capacitors. The intrinsic sheet like morphology of MoS2 provides high surface area for double layer charge storage and a layered structure for efficient intercalation of H+/ Li+ ions. My work demonstrates the electrochemical analysis of MoS2 grown on different substrates including copper (conducting), and carbon nanotubes. MoS2 film on copper was investigated as a supercapacitor electrode in three electrode system exhibiting excellent volumetric capacitance of ~330F/cm3 along with high volumetric power and energy density in the range of 40-80 W/cm3 and 1.6-2.4 mWh/cm3, respectively. Furthermore, we have developed novel binder-free 3DCNTs/ MoS2 as an anode materials in half cell Li-ion batteries. The vertically oriented morphology of MoS2 offers high surface area and active electrochemical sites for efficient intercalation of Li+ ions and demonstrating excellent electrochemical performance with high specific capacity and cycling stability. This 3DCNTs/ MoS2 anode was coupled with high surface area southern yellow pine derived activated carbon (SYAC) cathode to obtain hybrid 3DCNTs/ MoS2 || SYAC Li-ion capacitor (LIC), which delivers large operating voltage window of 1-4.0V with excellent cycling stability exhibiting capacitance retention of ~80% after 5000 cycles.

Low dimensional semiconductors are promising materials with diverse range of applications in a variety of fields. Specifically, in recent times low dimensional oxide and sulfide based semiconductors are regarded as materials that can have potential applications in chemical gas sensor, optoelectronic devices and memristor. How ever, in some cases it is envisioned that appropriate doping as well as phase stabilization is important in enhancing their material properties. This work presents the synthesis, characterization and application of various (pristine and doped) quasi-one dimensional metal oxides (TiO2, VO2) and two-dimensional materials (CuO thin film, MoS2). Some practical protocols for stabilization of specific phases at ambient conditions via a new method of doping in VO2 nanostructures with aluminum, is demonstrated. Similarly, a temperature-doping level phase diagram for the free-standing nanostructures in the temperature range close to the ambient conditions was presented. TiO2 nanowire was doped during growth and electrical measurements on individual TiO2 single crystal nanowires indicate that light in visible range can induce electron-hole pair formation. Furthermore, gas sensing (CO, H2) measurements taken under visible light irradiation imply that photo-activated chemical oxidization on the surface of TiO2 nanowires occurs, which is responsible for the observed measurements. Further, the effect of self heating in some nanostructures was also explored. Since self-heating is a prospective power-efficient energy delivery channel to the conductometric chemical sensors that require elevated temperatures for their operation, the unprecedentedly low power consumption can be achieved via minimizing the heat dissipation in the optimized device architecture. By investigating the heat dissipation in these devices we show that the thermal, electrical and chemical properties of the self-heated semiconducting nanowires appear to be strongly coupled with each other at nanoscale. This opens up unique opportunity to fabricate low power nanoscopic sensing leading to an ultra-small and power efficient single nanostructure gas recognition system. The CuO film based lateral devices were fabricated and studied for its resistive switching behavior. A good, stable and reproducible threshold RS performance of CuO film was obtained by electrical measurement. Finally, the micro-flake MoS2 based FET photoelectronic device was fabricated (using mechanically exfoliated MoS2) and its electronic and photoelectronic properties were investigated. We show that though the FET mobility values of MoS2 microflake is in the average range, but the photo-responsivity is much higher compared to most of others similar sulfide based 2D layered materials.

Because of its excellent and unique physical properties, ZnO nanowires have been widely used in numerous scientific fields such as sensors, solar cells, nanogenerators, etc. Although it is believed that single crystal ZnO has a much higher electron transfer rate than TiO₂, it was found that ZnO nanowire-based dye-sensitized solar cells (DSSCs) have lower efficiencies than TiO₂ nanoparticle-based DSSCs because the density and surface area of ZnO nanowires are usually lower than that of TiO₂ nanoparticles, limiting the cell's light absorption, and because the open-root structure of ZnO nanowires results in electron back transfer that causes charge shortage of the cell. Here, experimental studies were performed that utilize strategic manipulations of the design of the ZnO nanowire based DSSCs in efforts to address and solve its key challenges. It was shown that by incorporating various blocking layers into the design of the cell, the performance of the DSSC can be improved. Specifically, by placing a hybrid blocking layer of TiO₂-P4VP polymer between the substrate and the ZnO nanowires, the conversion efficiency of the cell was 43 times higher than that of a cell without this blocking layer due to the reduction of electron back transfer. Furthermore, in efforts to improve the surface area of the ZnO nanowire array, unique three dimensional structures of ZnO nanowires were fabricated. It was found that by significantly improving the overall density and surface area of the ZnO nanowire array through distinctive hierarchal nanowire structures, the light harvesting efficiency and electron transport were enhanced allowing the DSSC to reach 5.20%, the highest reported value for 3D ZnO NW based DSSCs. Additionally, the development of a theoretical model was explored in efforts to investigate how the geometry of ZnO nanowires affects the incident photon-to-current conversion efficiency of 1D ZnO nanowire-based N719-sensitized solar cells at the maximum absorption wavelength of 543 nm.

Materials with one-dimensional (1D) nanostructure are important for catalysis. They are the preferred building blocks for catalytic nanoarchitecture, and can be used to fabricate designer catalysts. In this thesis, one such material, alumina nanofibre, was used as a precursor to prepare a range of nanocomposite catalysts. Utilising the specific properties of alumina nanofibres, a novel approach was developed to prepare macro-mesoporous nanocomposites, which consist of a stacked, fibrous nanocomposite with a core-shell structure. Two kinds of fibrous ZrO2/Al2O3 and TiO2/Al2O3 nanocomposites were successfully synthesised using boehmite nanofibers as a hard temperate and followed by a simple calcination. The alumina nanofibres provide the resultant nanocomposites with good thermal stability and mechanical stability. A series of one-dimensional (1D) zirconia/alumina nanocomposites were prepared by the deposition of zirconium species onto the 3D framework of boehmite nanofibres formed by dispersing boehmite nanofibres into a butanol solution, followed by calcination at 773 K. The materials were characterised by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscope (TEM), N2 adsorption/desorption, Infrared Emission Spectroscopy (IES), and Fourier Transform Infrared spectroscopy (FT-IR). The results demonstrated that when the molar percentage, X, X=100*Zr/(Al+Zr), was > 30%, extremely long ZrO2/Al2O3 composite nanorods with evenly distributed ZrO2 nanocrystals formed on their surface. The stacking of such nanorods gave rise to a new kind of macroporous material without the use of any organic space filler\template or other specific drying techniques. The mechanism for the formation of these long ZrO2/Al2O3 composite nanorods is proposed in this work. A series of solid-superacid catalysts were synthesised from fibrous ZrO2/Al2O3 core and shell nanocomposites. In this series, the zirconium molar percentage was varied from 2 % to 50 %. The ZrO2/Al2O3 nanocomposites and their solid superacid counterparts were characterised by a variety of techniques including 27Al MAS-NMR, SEM, TEM, XPS, Nitrogen adsorption and Infrared Emission Spectroscopy. NMR results show that the interaction between zirconia species and alumina strongly correlates with pentacoordinated aluminium sites. This can also be detected by the change in binding energy of the 3d electrons of the zirconium. The acidity of the obtained superacids was tested by using them as catalysts for the benzolyation of toluene. It was found that a sample with a 50 % zirconium molar percentage possessed the highest surface acidity equalling that of pristine sulfated zirconia despite the reduced mass of zirconia. Preparation of hierarchically macro-mesoporous catalyst by loading nanocrystallites on the framework of alumina bundles can provide an alternative system to design advanced nanocomposite catalyst with enhanced performance. A series of macro-mesoporous TiO2/Al2O3 nanocomposites with different morphologies were synthesised. The materials were calcined at 723 K and were characterised by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscope (TEM), N2 adsorption/desorption, Infrared Emission Spectroscopy (IES), and UV-visible spectroscopy (UV-visible). A modified approach was proposed for the synthesis of 1D (fibrous) nanocomposite with higher Ti/Al molar ratio (2:1) at lower temperature (<100oC), which makes it possible to synthesize such materials on industrial scale. The performances of a series of resultant TiO2/Al2O3 nanocomposites with different morphologies were evaluated as a photocatalyst for the phenol degradation under UV irradiation. The photocatalyst (Ti/Al =2) with fibrous morphology exhibits higher activity than that of the photocatalyst with microspherical morphology which indeed has the highest Ti to Al molar ratio (Ti/Al =3) in the series of as-synthesised hierarchical TiO2/Al2O3 nanocomposites. Furthermore, the photocatalytic performances, for the fibrous nanocomposites with Ti/Al=2, were optimized by calcination at elevated temperatures. The nanocomposite prepared by calcination at 750oC exhibits the highest catalytic activity, and its performance per TiO2 unit is very close to that of the gold standard, Degussa P 25. This work also emphasizes two advantages of the nanocomposites with fibrous morphology: (1) the resistance to sintering, and (2) good catalyst recovery.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008.
Committee Chair: El-Sayed, Mostafa A.; Committee Member: Perry, Joseph W.; Committee Member: Srinivasarao, Mohan; Committee Member: Whetten, Robert L.; Committee Member: Zhang, Z. John.

Thesis (Ph.D)--Materials Science and Engineering, Georgia Institute of Technology, 2010.
Committee Co-Chair: Snyder, Robert L.; Committee Co-Chair: Wang, Zhong Lin; Committee Member: Garmestani, Hamid; Committee Member: Summers, Christopher; Committee Member: Wilkinson, Angus. Part of the SMARTech Electronic Thesis and Dissertation Collection.

As the limitations of current technology and the possibility of scaling down of technology becomes ever more apparent the drive for smaller, faster, cheaper and more sensitive devices gains momentum. Recent literature reports new and exciting possibilities with zinc oxide based one-dimensional nanomaterials rather than the popular carbon nanotubes. The attractiveness of these one-dimensional nanomaterials is the increased surface to volume ratios and the ability of this increased surface area to exhibit sensitivity to a range of gases by altering the conductivity upon absorption of molecules on the surface. The work in this thesis demonstrated the effectiveness of the electric force microscopy technique in imaging conducting and semi-conducting samples. The technique is extremely useful in charging nanomaterials and imaging the sample discharging. This technique allows for the imaging of nanomaterials with varying applied tip bias and the results allowed the determination of a method to calculate the dielectric constant of one dimensional nanomaterials by examining the phase data. The second part of this thesis illustrates the intriguing nature of zinc oxide one dimensional nanomaterials by exploring the gas sensing capabilities of single nanowire devices. The sensitivity observed is mostly likely due to the absorption of electron donating molecules to the surface of the nanowire and hence donating charge carriers into the bulk increasing the conduction. This sensitivity can also be due to electron withdrawing molecules being absorbed onto the surface of the nanowire which reduces the conduction.

We have performed a first principles density functional theory (DFT) calculations to study the thermal conductivity in ZnO nanotubes, ZnO nanowires, and Si/Ge shell-core nanowires. We found the equilibrium configuration and the electric band structure of each nanostructure using DFT, the interatomic force constants and the phonon dispersion relations were calculated using DFPT as implemented in Quantum Espresso. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach using a Green's function method. All ZnO nanostructures studied exhibit semiconducting behavior, with direct bandgap at the Gamma point. The calculated values for the bandgaps were larger than the value of the bandgap of the bulk ZnO. We were able to identify phonon modes in which the motion of Zn atoms is significant when it is compared with the motion of oxygen atoms. The thermal conductivity depends on the diameter of the nanowires and nanotubes and it is dramatically affected when the nanowire or nanotube is doped with Ga. For Si/Ge nanowires, the slope and the curvature of acoustic modes in the phonon dispersion relation increases when the diameter increases. For nanowires with the same number of atoms, the slope and curvature of acoustic modes depends on the concentration of Si atoms. We were able to identify phonon modes in which the motion of core atoms is significant when it is compared with motion of atoms on the nanowire's shell. The thermal conductivity in these nanostructures depends on the nanowire's diameter and on the Si atoms concentration.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2005.
Vita.
Includes bibliographical references
The general concept of a replicating monolayer system is introduced as a new method of nanostructure synthesis. One possible implementation of a 2-D replicating system is pursued which uses a diacetylene moiety for cross-linking and amide hydrogen bonding for molecular recognition between replicates and templates. The synthesis of several monomers for amide hydrogen-bonded adlayer formation is described. The assembly and crosslinking of diacetylene monomers on an underlying amide-capped self- assembled monolayer (SAM) was studied on unpatterned thermally evaporated gold films. Raman and Fourier-transform infrared spectroscopies, as well as ellipsometry and contact angle data, indicate that amide hydrogen bonding interactions are sufficient to organize an adlayer of diacetylene-containing molecules on the underlying SAM which can be polymerized with ultraviolet light. In order to obtain gold substrates suitable for cross-linking of bis(diacetylene) monomers, new methods of producing ultraflat gold surfaces were developed.
(cont.) A solid- state bonding technique using only gold was developed, yielding ultraflat gold surfaces, with root-mean-square roughnesses of -0.5 nm, on glass slides which are free of impurities from epoxies or other bonding agents. The patterning and cross-linking of poly(diacetylene) adlayers on ultraflat gold surfaces was investigated by atomic force microscopy (AFM). Soft lithography was suitable for adlayer structures down to about 500 nm. Electron beam lithography for patterning of polymerizable adlayers was demonstrated for the first time. The polymerized adlayer patterns were significantly more difficult to remove from the gold surface than unpolymerized adlayer patterns, indicating cross-linking. Studies to remove adlayer patterns as intact 2-D polymers failed, due either to poor cross-linking or robustness of the resulting 2-D polymers. In another approach to nanostructure synthesis, the synthesis of monofunctionalized gold nanoparticles by a solid phase synthetic route was described. This represents a versatile method of producing monofunctionalized nanoparticles, which can be used to produce and study more elaborate nanoparticle structures.
by David W. Mosley.
Ph.D.

Nanotechnology is one of the most innovative and multidisciplinary fields in modern research. Techniques to manipulate and control matter at the nanometric scale, giving the possibility to change the morphology and the physical and chemical properties are growing in number. At the nano scale, changing the morphology implies changing also the properties of matter: many properties are no longer intrinsic but they depend on the size, shape, and even on the environment. One of the most striking example is given by gold and silver colour when they are in the nanoparticle form. Gold, for example ould be vine-red or green, or bluish or black simply by changing the morphology of the particles. Appropriate manipulation of the matter could also give birth at new proprieties such as the transmission of light through holes much smaller that the light wavelength, allowing the possibility to control at a very intimate scale the propagation of light. It is clear that to exploit the great potential of nanotechnology it is important to have nanofabrication techniques that have a very precise control on the production of nanometric materials or materials with nanometric structures. There are many technologies that can produce structures with an outstanding resolution of just a few nanometer (Electron Beam Lithography, Focused Ion Beam). These technologies are ”serial” fabrication tool, they can produce one object at time and this means high costs and low throughput. On the other hand parallel technologies derive from the semiconductor industry and are mostly optical lithographic methods that are limited by the diffraction limit of light (200nm). In this thesis work the need for a nanofabrication tool that can allow the production of smaller nanostructures will be addressed by using a nanofabrication tool that meets the following criteria: • parallel processing • low cost • large area processing (cm2) • scalability • reproducibility • easy implementation We choose to exploit the ability of self aggregation of matter in ordered structures. In particular we exploited the tendency of spherical monodisperse nanoparticles to assemble in ordered, close packed structures known as colloidal crystals. One monolayer of such colloidal crystal is a very interesting structure because it has a well ordered array of pores among the particles that have a well defined size and shape, that could be tuned by simply changing the size of the self assembled colloidal particles. A simple and easy method to create and deposit on different substrate these self assembled monolayer of polystyrene nanoparticles will be presented. Monolayers will be used to synthesize arrays of monodisperse plasmonic nanoparticles with a very good control on their size and shape, allowing the tuning of the plasmonic proprieties on the desired application. We will use the array of plasmonic nanoparticles to realize molecular sensors and to amplify the Raman signal by the Surface Enhanced Raman Scattering effect. We will study the rise in temperature induced by illumination with a laser light resonant with the nanoparticle’s plasmonic transition. This information could be very interesting for the biological application of the nanoparticles arrays since temperature variation in such a very complex environment could have relevant effect. Moreover we will use these 2D colloidal crystal to synthesize different kinds of nanostructures like an array of holes in a metal film. This nanostructure is very interesting since the discovery of its ability to transmit light even if the hole size is much smaller than the light’s wavelength. A synthesis method based on self assembled nanospheres could be useful for the fabrication of such nanostructures because of its high flexibility in changing the nanoparticles size and so the array geometric parameters like hole size and the lattice period. Self assembled monolayers will be used as a template for the synthesis of nanostructured thin films of TiO2. Titania is a semiconductor of great technological interest in many different fields: catalysis, energy conversion, gas sensing. We will fabricate using the same technology two different nanostructured thin film: a macroporous thin films and a surface patterned with a nanobowl pattern. Finally we will demonstrate the use of self assembled monolayers coupled with a standard technology used in the semiconductor industry such the ion implantation. Nanometric patterns will be produced on Si wafers using the ordered monolayer as a mask for the ion beam.
Il campo delle nanotecnologie è uno dei più innovativi e multidisciplinari della ricerca moderna. Sempre pi`u numerose diventano le tecniche per manipolare la materia su scala nanometrica, modificando così le proprietà fisico, chimiche e morfologiche a livelli mai raggiunti prima. Alla nano scala la manipolazione morfologica è accompagnata dal cambiamento delle proprietà che smettono di essere intrinseche della materia ma diventano dipendenti da altri fattori come la forma, la dimensione e l’ambiente in cui le nanostrutture sono immerse. Uno dei casi più eclatanti è il colore dell’oro e dell’argento quando sono sottoforma di particelle nanometriche. L’oro, ad esempio, può essere di colore rosso-vino, verde, blu e nero, semplicemente cambiando la forma o l’ambiente attorno ad esso. Manipolando la materia opportunamente possono comparire nuove proprietà come la trasmissione della luce attraverso aperture che sono molto più piccole della lunghezza d’onda della luce, dando la possibilità di ottenere il controllo della propagazione della luce ad un livello molto intimo. Si può capire quindi come per poter sfruttare le enormi potenzialità offerte dalle nanotecnologie sia importante avere tecnologie di fabbricazione che permettano un preciso controllo nella produzione di oggetti nanometrici o con strutture nanometriche. Le tecnologie al momento disponibili che permettono di creare strutture con precisione molto elevata (pochi nanometri) sono tecnologie ”seriali” come l’Electron Beam Lithography o il Focused Ion Beam. Queste tecniche sono limitate alla produzione di un oggetto alla volta e quindi comportano costi elevati e lunghi tempi. Le tecnologie ”parallele” derivano dall’industria dei semiconduttori e sono tecniche litografiche che hanno come limite la risoluzione della luce utilizzata ( 200nm). In questo lavoro di tesi si cercherà di dare risposta alla domanda di tecniche di fabbricazione di strutture nanometriche utilizzando una tecnica che abbia le seguenti caratteristiche: • quickness • low cost • ability to synthesize very small nanostructures • reproducibility • easy implementation Si è scelto di utilizzare la capacità della materia di organizzarsi spontaneamente in strutture ordinate. In particolare si è sfruttata la tendenza di nanoparticelle sferiche di polistirene ad impaccarsi in strutture compatte ed ordinate costituendo dei ”cristalli colloidali”. Un singolo strato di nanosfere autoassemblate è una struttura interessante perchè presenta dei pori tra le particelle di forma e dimensioni ben definite, che possono essere modificate cambiando le dimensioni delle sfere che costituiscono il cristallo bidimensionale. Verrà illustrato un metodo semplice e rapido per ottenere questi monostrati di particelle ordinate e per poterli depositare su vari substrati. Questi cristalli bidimensionali verranno utilizzati per depositare una matrice ordinata di nanoparticelle plasmoniche, con un ottimo controllo sulla loro forma e dimensioni, consentendo di realizzare particelle con proprietà su misura per l’applicazione desiderata. Verranno anche studiate applicazioni di queste nanoparticelle come sensori di molecole e per amplificare il segnale Raman grazie all’effetto SERS. Verrà inoltre studiato l’aumento di temperatura di queste nanoparticelle quando vengono illuminate da un laser risonante con la loro risonanza di plasma di superficie. Per applicazioni spettroscopiche applicate a sistemi biologici il cambiamento di temperatura può avere effetti rilevanti in un ambiente complesso come quello biologico. In seguito verrà dimostrato come questi cristalli colloidali bidimensionali possono essere utilizzati per creare altre classi di nanostrutture, come ad esempio una matrice di buchi nanometrici in un film metallico. Queste strutture sono studiate da quando è stato scoperta la loro capacità di far trasmettere attraverso strutture che sono molto minori del limite di diffrazione per le lunghezze d’onda trasmesse. Una sintesi che si basa sulle nanosfere autoassemblate può essere interessante per queste strutture grazie alla sua intrinseca flessibilità. Si possono infatti cambiare in modo molto semplice i parametri geometrici che caratterizzano la matrice di buchi quali le dimensioni dei buchi e il periodo degli stessi. Un’altra tipologia di nanostrutture che verrà realizzata sono film sottili nanostrutturati di TiO2. La titania è un semiconduttore di grande interesse tecnologico in molti campi diversi: dalla catalisi, alla conversione di energia ai sensori di gas. Verranno fabbricati, con la stessa tecnologia, dei film con una porosità ordinata e delle superfici nanostrutturate con un motivo a incavi. Infine verrà dimostrata la possibilità di utilizzare i cristalli colloidali 2D accoppiati con una tecnologia molto utilizzata dall’industria dei semiconduttori quale l’impiantatore ionico. Pattern nanometrici verranno realizzati su silicio utilizzando le nanoparticelle autoassemblate come maschera per il fascio ionico.

There has been a shift in focus towards systems that integrate “nanomagnetic” components with existing device technologies, particularly for the further miniaturisation of magnetic storage devices. This thesis presents results, analysis and conclusions of studies on novel hybrid magnetic/metallic (Au/Co/Au) compositions in conjunction with low dimensional structures AlGaAs/GaAs quantum point contacts (QPC) and quantum dots (QD). The Au/Co/Au gates are used as surface confinement gates and also to apply a local inhomogeneous magnetic strayfield. Magneto-transport measurements were performed in low temperature cryostats with a minimum temperature of 350mK. It was concluded that the gating properties of these magnetic/metallic structures was significantly poor to allow for accurate characterisation of the devices. In addition, the QPC and QD themselves showed inhomogeneous effects that were dependent on the fabrication processes and material impurities. Novel broadband microwave spectroscopy measurements were used to study the transport mechanisms of a QPC. A continuous frequency range of 1-20GHz was studied where there have been no previous investigations on such devices. The experimental setup utilizes a non-invasive microwave source via a coaxial transmission line, based on the Corbino approach. The response of the device was seen to be very sensitive to these microwaves and showed ‘aperiodic’ fluctuations in the source-drain current as a function of the frequency and the onset of negative current features. These fluctuations are determined to be a result of semi-classical chaotic trapped electrons states that can be modified by the electric field of the microwave at these frequencies.

Thesis: Ph. D. in Medical Engineering and Medical Physics, Harvard-MIT Program in Health Sciences and Technology, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 137-148).
Patterning complex 3D features at the nanoscale offers potential applications for a wide range of fields from materials to medicine. While numerous methods have been developed to manipulate nanoscale materials, these methods are typically limited by their difficulty in creating arbitrary 3D patterns. Self-assembly of nucleic acids has emerged as a promising method for addressing this challenge due to the predictability and programmability of the material and its structure. While a diversity of DNA nanostructures have been designed by specifying complementarity rules between strands, creation of 3D nanostructures requires careful design of strand architecture, and patterns are often limited to a volume of 25 x 25 x 25 nm³ Here, we address the challenges in structural DNA nanotechnology by developing a modular DNA "brick" approach. These bricks are short, single-stranded oliogomers that can self-assemble in a single-pot reaction to a prescribed 3D shape. Using this modular approach, we demonstrate high efficiency in 3D design by generating 100 distinct, discrete 3D structures from a library of strands. We also created long-range ordering of channels, tunnels, and pores by growing micron-sized 3D periodic crystals made from DNA bricks. Finally, we applied this approach to control over 30,000 unique component strands to selfassemble into cuboids measuring over 100 nm in each dimension. These structures were further used to pattern highly complex cavities. Together, this work represents a simple, modular, and versatile method for 3D nanofabrication. This unique patterning capability of DNA bricks may enable development of new applications by providing a foundation for intricate and complex control of an unprecedented number of independent components.
by Luvena Le-Yun Ong.
Ph. D. in Medical Engineering and Medical Physics

In the last 20 years, computer simulations, based on the micromagnetic model, have become an important tool for the characterisation of ferromagnetic structures. This work mainly uses the finite-element (FE) based micromagnetic solver Nmag to analyse the magnetic properties of ferromagnetic shell structures of different shapes and with dimensions below one micrometre. As the magnetic properties of structures in this size regime depend crucially on their shape, they have a potential towards engineering by shape manipulation. The finite-element method (FEM) discretises the micromagnetic equations on an unstructured mesh and, thus, is suited to model structures of arbitrary shape. The standard way to compute the magnetostatic potential within FE based micromagnetics is to use the hybrid finite element method / boundary element method (FEM/BEM), which, however, becomes computationally expensive for structures with a large surface. This work increases the efficiency of the hybrid FEM/BEM by using a data-sparse matrix type (hierarchical matrices) in order to extend the range of structures accessible by micromagnetic simulations. It is shown that this approximation leads only to negligible errors. The performed micromagnetic simulations include the finding of (meta-)stable micromagnetic states and the analysis of the magnetic reversal behaviour along certain spatial directions at different structure sizes and shell thicknesses. In the case of pyramidal shell structures a phase diagram is delineated which specifies the micromagnetic ground state as a function of structure size and shell thickness. An additional study demonstrates that a simple micromagnetic model can be used to qualitatively understand the magnetic reversal of a triangular platelet-shaped core-shell structure, which exhibits specific magnetic properties, as its core material becomes superconducting below a certain critical field Hcrit.

My work has been focussed on the synthesis of titanium oxide (Ti02) nanostructures. Commercially available Ti02 is widely used in applications such as self-cleaning surfaces, water purification devices and solar cells. However, a new generation of Ti02particles offers high crystallinity, monodispersity, and well defined . geometry. High aspect ratio particles are likely to offer advantages in terms of transport properties and accessibility to other phases. I have investigated three distinct strategies for the preparation of Ti02 nanorods. The first involves a hydrolysis of titanium tetraisopropoxide (TrIP), with oleic acid as a surfactant, which prodUces anatase Ti02nanorods of -3 nm diameter and -30 nm in length. This reaction was performed both on the bulk seale, with a standard flask and manifold set-up, and also in the highly controlled environment of microfluidic chips. The use of continuous-flow, microfluidic devices confers several advantages over conventional macroscale techniques, including high surface area-ta-volume ratios and reduced diffusional dimensions. Our studies show a roughly ten-fold increase in reaction rate when the hydrolysis is performed onchip, as opposed to in a flask. As an alternative, a non-hydrolytic synthesis of Ti02nanorods was performed, via the reaction of TrIP and TiCI 4 in the presence of oleic acid. The non-hydrolytic reaction provides scope for higher reaction temperatures, different surface functionalities, and different reaction rates. The anatase nanorods obtained were -S nm in diameter and -SO nm in length. The reaction conditions (time of reaction, temperature, mode of addition of TiCI4) were optimised for purity, crystallinity and monodispersity. The third synthetic technique relied on the conversion of aligned multi-walled carbon nanotubes (MWCNTs) into rutile Ti02nanorods. The MWCNTs were grown via a standard CVD process in which a -3% ferrocenein xylene solution is gradually pumped into a quartz tube at -760��?���°C. The MWCNTs were then reacted with a volatile titanium iodide, generated in situ, at high temperature (800 -1200 ��?���°C) and reduced . pressure, to produce titanium carbide (TiC) nanorods with similar dimensions. Oxidation of the TiC at 800��?���°C, converts it to rutile Ti02nanorods. The resulting nanostructures are -SO nm in diameter and up to -SOO /-1m in lellgth, and composed of pure, polycrystaJline rutile .The process preserves the unidirectional alignment of the original nanotubes, which may be useful for applications.

Fabrication and synthesis of nanostructured materials are essential aspects of nanoscience and nanotechnology. Although researchers are now able to create and tailor different nanostructured materials, the ability to precisely control the materials' sizes, shapes, and properties at the nanoscale level remains challenging. The aim of this dissertation was to develop new methods to aid researchers in overcoming these challenges. The study investigated two different methods used to create one-dimensional carbon nanostructures, i.e. carbon nanotubes and carbon nanopillars.In the first section, chemical vapor deposition method was used to grow carbon nanotubes (CNTs). Studies examining the effects of methane and hydrogen flow rates on the growth of CNTs were conducted. Results indicated that multi-walled CNTs with metallic properties could be obtained at a methane flow rates as low as 300 cc/min. At higher methane flow rates, i.e. 600-700 cc/min, semiconducting single-walled CNTs and double-walled CNTs were produced. Another phase of this section developed a new and simple CNT growth method using a solid carbon source and indicated polyacrylonitrile and nanosized SiO₂ were effective in producing MWCNTs. In the second part, a new nanoimprint technique was developed to enable printing of nanostructures at sub-100nm level using various polymers. This technique inherited its high-resolution feature from traditional nanoimprint lithography, but without the use of pressure. To demonstrate, PAN nanopillar structures were printed and converted to carbon. In another phase of the part, the use of our imprint technique resulted in the creation and conversion of polysilazane nanostructures to ceramic for the first time.The final section of this dissertation is devoted to study the impact of porosity in gas diffusion layers (GDLs) on the performance of fuel cells. In one study, a new technique using SEM images to determine GDL porosity was developed. The difference between SEM calculated porosities and mercury intrusion porosimetry measurements were less than 2%. The second study characterized fuel cell performances using GDLs constructed with additional micro porous layers (MPLs) and treated with different wet proofing treatments (WPT). Results showed that when MPL is added, cell performance decreases. However, the increase in WPT in the MPL improved cell performance.

This research aims to explore the optimization strategies of two-dimensional (2D) nanostructures for high-performance rechargeable batteries. Three effective strategies, including 2D-based phase engineering, component engineering and van der Waals (vdW) heterostructures, were proposed for improving electrochemical properties of 2D nanomaterials. These effective strategies will offer good references for researchers to develop practical next-generation rechargeable batteries using the emerging 2D nanomaterials.

This work entails the preparation of various polyanilines with different morphologies and their application in photovoltaic solar cells. Zinc oxide (ZnO) with one-dimensional and flower-like morphology was also prepared by microwave irradiation and used as electron acceptors in photovoltaics devices. The morphological, structural, spectroscopic and electrochemical characteristics of these materials were determined by scanning electron microscopy (SEM), X-Ray diffraction (XRD), Raman, Fourier-transformed infrared spectroscopy (FTIR), ultraviolet and visible spectroscopy (UV-Vis), photoluminescence(PL), thermal gravimetric analysis (TGA) and cyclic voltammetry (CV) experiments. Devices fabricated from these materials were characterized under simulated AM 1.5 at 800 mW.

Miniaturization has become a driving force in different areas of technology including microelectronics, sensoric- and bio-technologies and in fundamental science. Because of the well-known limitations of conventional lithographic methods, newly emerging bottom-up approach, utilizing self-assembly of various nanoobjects including single polymer molecules and carbon nanotubes constitutes a very promising alternative for fabrication of ultimately small devices. Carbon nanotubes are attractive materials for nanotechnology and hold much promise to revolutionize fundamental science in a investigation of phenomena, associated with the nanometer–sized objects.It was found in this work that grafted chains of poly(2-vinylpyridine) form a shell covering the carbon nanotubes that makes them dispersible in organic solvents and in acidic water (CNTs-g-P2VP).The positively charged poly(2-vinylpyridine) shell is responsible for the selective deposition of carbon nanotubes onto oppositely charged surfaces. It was established that the deposition CNTs-g-P2VP from aqueous dispersions at low pH is an effective method to prepare ultra-thin films with a tunable density of carbon nanotubes.It was shown that poly(2-vinylpyridine) grafted to carbon nanotubes is a universal support for the immobilization of various nanoclusters at the carbon nanotube's surface. Prussian Blue nanoparticles were selectively attached to the surface of CNTs-g-P2VP.Conducting polymer nanowires are another very promising kind of nanomaterials that could be also suitable for applications in nanodevices and nanosensors. In this work was developed a simple method to control the conformation and orientation of single adsorbed polyelectrolyte molecules by co-deposition with octylamine. A simple chemical route to conductive polypyrrole nanowires by the grafting of polypyrrole from molecules of polystyrensulfonic acid was developed. The dc conductivity of individual polypyrrole nanowires approaches the conductivity of polypyrole in bulk.The conductivity can be described using variable-range hopping model.

A challenge exists for understanding the origin of color for structurally colored, 3-dimensional bioorganic nanostructures, such as the scales of butterflies, beetles, and moths. Complex, hierarchical structures found within such scales create the overall scale appearance. The controlled alteration of color through material deposition and the addition of new optical functionalities to such structures are other areas of scientific interest. This dissertation addresses these challenges with a first-of-its-kind, systematic isolation (deconstruction) of scale component nanostructures, followed by evaluation of optical property/structure correlations. The additive deposition (constructive alteration) of emissive materials to structurally-colored templates complements this deconstructive approach towards understanding the origin of color in butterfly scales. Discoveries made through this work may help advance the bioinspired design of synthetic optical structures and subsequent color control through the addition of multilayered, emissive optical components.

碩士
義守大學
電子工程學系碩士班
94
Schrodinger equation is the quantum mechanics foundation，in which involves a specific potential energy function,But actual problem potential energy function is suitable complex,therefore we needed to develop the numerical method solve schrodinger equation.Transfer Matrix Method can solve any potential energy function in schrodinger equation to be able to energy level and wave function.Again analyzes potential energy of the physics significance,then can analyzes schrodinger equation of physical property.

碩士
中原大學
應用物理研究所
94
In this thesis, we introduce the principle and application of the Langmuir-Blodgett technique. By this method, we fabricate the ordered Polystyrene 2D structure, and discuss the relation between the surface pressure and the area of the LB-film in the process of forming the 2D structure. The surface pressure increases as compressing the bipolar molecules on the air/water interface continuously. As the separation distance between molecules decreasing, the surface pressure goes through two phase transitions (2D-gas to 2D-liquid and 2D-liquid to 2D-solid). If PS micro-balls have the similar behavior on the air/water interface, such two phase transitions should be observed. In the case of 1�慆 in diameter, we observe two phase transitions successfully. In the 150nm and 250nm cases, we only discover one phase transition (2D-gas to2D-liquid). As the phase transition occurs, the ratio of separation distance to micro-ball diameter (D/R value) is less than unity. This phenomenon is possibly due to the non-uniformity caused by the thermal fluctuation. More over, we try to disperse silver nano-powder into HPLC grade chloroform to get the suspension with ultrasonic cleaner. However, the concentration of the suspension is far thinner than 1mg/ml, so it is difficult to apply in this LB technique.

碩士
國立臺灣科技大學
電子工程系
100
There are many technologies to fabricate one-dimensional silicon nanostructure. The silicon nanostructures were used for many applications. For example, the silicon nanostructures have high reflection property in solar cell. In the electron field emission, the silicon nanostructures possess the higher aspect ratio and numerous emission sites. For sensor researching, it has higher surface area to volume ratio that can improve the sensitivity. We fabricated the one dimension silicon nanostructure for hydrogen sensor. In this research, we synthesized different morphology of silicon nanostructure using wet electroless etching technique. The field emission scanning electron microscopy (FE-SEM) was used to observe the surface morphology of silicon nanostructure. The micro-Raman and Fourier transform infrared (FT-IR) was used to investigate the bonding of silicon nanostructure. Finally, we carried out the electrical analyze of gas sensor system with hydrogen sensing. In this study, we use the two-step wet electroless etching technique to form the straw-like silicon nanowire. It can be observed that some different layers in SEM photographs. The top layer was straw-like silicon nanowire, and the middle layer was straight aligned silicon nanowire, and the bottom layer was silicon base which is the bulk silicon. The straw-like silicon nanowire was found the the bonding of Si-O-Si about 1173cm-1 in FT-IR spectrum. And it also observed some red shift in Raman spectrum. Because of the higher surface area to volume ratio and Si-O-Si bonding, the performance of this straw-like structure hydrogen sensor was improved. Moreover, this hydrogen gas sensor was also modified with Pt nanoparticles, which can enhance the ratio of hydrogen gas dissolve into the metal-semiconductor interface. Finally we tried to make the porous structure with high density and high aspect ratio also by wet electroless etching technique. This porous possessed the high surface area which had the superior sensitivity.

碩士
國立東華大學
物理學系
100
Potassium titanate (M2TinO2n+1，M=K，n=6) has been taken as an exemplar system to disclose conditions for directly forming one-dimensional (1D) oxide nanostructures on titanium substrates with modifiable optical properties. Morphology and spatial distribution of the surface-layer oxides were found to depend on the temperature and duration of anneals, whereas the potassium-containing oxides were confined in a thin surface layer. Results of XRD and Raman scattering showed the phase formation scenarios of anatase, rutile, and potassium hexatitanate. Transmission electron microscopy was performed to reveal the 1D nanostructures as monoclinic K2Ti6O13 single crystals extending along <010>. Ring-like patterns observed in selected-area electron diffraction reflected the fact thatthe 1D nanocrystals derived from a multi-crystalline precursor exhibiting hexatitanate-similarlattice d-spacings. A growth model was proposed for the K2Ti6O13 crystals, signifying the role of the early-stage crystallizationwhich dominates the following 1D growth. Photoluminescence spectra showed a broad peak within the range of 2-3 eV, wherein the intensity maximum systematically shifted due to competitive contributions from the phases of anatase, rutile and potassium hexatitanate.

Atomically thin two-dimensional nanomaterials such as graphene and transition metal dichalcogenides (TMDCs) have seen a rapid growth of exploration since the isolation of monolayer graphene. These materials provide a rich field of study for physics and optoelectronics applications. Many applications seek to combine a two dimensional (2D) material with another nanomaterial, either another two dimensional material or a zero (0D) or one dimensional (1D) material. The work in this thesis explores the consequences of these interactions from 0D to 2D. We begin in Chapter 2 with a study of energy transfer at 0D-2D interfaces with quantum dots and graphene. In our work we seek to maximize the rate of energy transfer by reducing the distance between the materials. We observe an interplay with the distance-dependence and surface effects from our halogen terminated quantum dots that affect our observed energy transfer. In Chapter 3 we study supercapacitance in composite graphene oxide- carbon nanotube electrodes. At this 2D-1D interface we observe a compounding effect between graphene oxide and carbon nanotubes. Carbon nanotubes increase the accessible surface area of the supercapacitors and improve conductivity by forming a conductive pathway through electrodes. In Chapter 4 we investigate effective means of improving sample quality in TMDCs and discover the importance of the monolayer interface. We observe a drastic improvement in photoluminescence when encapsulating our TMDCs with Boron Nitride. We measure spectral linewidths approaching the intrinsic limit due to this 2D-2D interface. We also effectively reduce excess charge and thus the trion-exciton ratio in our samples through substrate surface passivation. In Chapter 5 we briefly discuss our investigations on chemical doping, heterostructures and interlayer decoupling in ReS₂. We observe an increase in intensity for p-doped MoS₂ samples. We investigated the charge transfer exciton previously identified in heterostructures. Spectral observation of this interlayer exciton remained elusive in our work but provided the motivation for our work in Chapter 4. We also discuss our preliminary results on interlayer decoupling in ReS₂.

碩士
國立成功大學
口腔醫學研究所
103
The rapid development of nanomedicine based on theranostic has gradually evolved into practical clinical applications. Recently, researchers have reported that certain inorganic nanomaterial may induce cytotoxicity and biological dysfunction. DNA is an intrinsic biological macromolecule thus their nanostructures were regarded as excellent carriers with low toxicity and high biocompatibility. In my study, I would like to develop an advanced theranostic platform based on a self-assembled DNA nanostructure as scaffold with targeting aptamer and carrying antibiotics. Antimicrobial molecules (Actinomycin D, AMD) will be attached to minor groove of DNA double helix through hydrogen bond. The DNA nanostructure system serves as carriers for targeting delivery of AMD to attack bacteria. Currently, the DNA nanostructure system has been assembled and was confirmed by DNA agarose gel electrophoresis. Cryo-electron microscopy (cryo-EM) and three-dimensional (3D) reconstruction technique showed the DNA nanostructure system was a hollow spherical structure. In addition, we found a single DNA nanostructure could carry 2409.47 AMD molecules when saturated. We further found that the anti-bacteria ability of our system had a higher efficacy than free AMD. In the aptamer binding assays, we demonstrated the aptamer target specifically to Streptococcus mutans.

博士
國立中正大學
化學工程所
96
In this thesis, we prepared high purity and uniform one-dimensional zinc oxide nanostructures by thermal evaporation and thermal decomposition methods. The research includes the studies of the structure of these materials and explores their applications. Fundamentals of one-dimensional zinc oxide nanomaterials were reviewed in chapter 1. The structure of these one-dimensional zinc oxide nanomaterials and their field emission, photocatalytic activity and UV photodetector applications were systemaically studied and reported in chapter 2-4. In chapter 2, one-dimensional ZnO nanostructures were fabricated on a tin-doped indium oxide (ITO) glass substrate by thermal evaporation of Zn powder at 400 oC. ZnO nanocones, ZnO nanorods, and ZnO nanowires can be synthesized with a catalyst-free ITO glass, a Pt-coated ITO glass, and an Au-coated ITO glass, respectively. The XRD and TEM investigations show that ZnO nanostructures possess good crystallinity with growth direction along the c-axis of the crystal plane. Field emission studies reveal ZnO nanorod arrays with a turn-on voltage of 3.4 V/μm at a current density of 10 μA/cm2. This study finds that morphology and growth direction of the ZnO nanostructures affect the field emission property. In chapter 3, high-purity single crystal ZnO nanowires were synthesized by the thermal decomposition of zinc acetate dihydrate at 300 oC in air without the presence of catalyst. The zinc acetate dihydrate was characterized by thermogravimetry-differential scanning calorimetry and mass spectrometry (TG-DSC-MS) to determine its thermal decomposition and crystallization temperature. The ZnO nanowires were generated by a dehydration, vaporization/decomposition, and deposition/formation process. In addition, the ZnO nanowires showed a relatively good photocatalytic activity in the degradation of NOx. The NO concentration decreased from 1 ppm to 0.1 ppm. In chapter 4, high purity, vertically aligned ZnO nanowires were synthesized uniformly on a tin-doped indium oxide (ITO) glass substrate. The ZnO nanowire arrays with a uniform diameter distribution of 30~50 nm and a length of about 5 μm were formed by the thermal decomposition of zinc acetate at 300 oC in air. The study of growth mechanism found that it is a vapor-solid (VS) growth mechanism with a sequence of the processes including dehydration, vaporization, decomposition and oxidation of the zinc acetate, deposition of ZnO clusters for the formation of the seeds and finally selectively epitaxial growth of the ZnO nanowires. The photocurrent characteristics and UV photoresponse of the ZnO nanowire photodetectors were investigated. Under illumination using the UV light with a wavelength of 365 nm, photo-generated current was measured at 15 μA at a bias of 2 V. The fabrication method is a simple, catalyst-free and cost-effective (low temperature, one chemical agent used only) for the growth of high quality ZnO nanowire arrays. The study on the optical properties and the UV photodetector device found that the as-produced ZnO nanowires are of great potential on the applications of photodetector/sensor in the UV region. In addition, the turn-on field for the ZnO nanowires growth at 300 oC was found about 3.5 V/μm at current density of 10 μA/cm2. The emission current density from the ZnO nanowires growth at 300 oC reached 1 mA/cm2 at an applied field of 7.0 V/μm.

博士
國立交通大學
材料科學與工程學系
100
One-dimensional nanostructers are a new class of advanced materials that have been receiving a lot of research interest in the last decade due to their superior physical and chemical properties. Among one-dimensional materials, zinc oxide (ZnO) is one of the most important materials and has attracted much interest in recent years, due to its unique optical, electrical, and piezoelectric properties and versatile applications. In this dissertation, we propose new methods or technologies to improve the applications of 1D ZnO nanostructure. Macrostructure of the samples was characterized by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The optical and electrical properties were investigated by photoluminescence (PL) and current–voltage (I–V) characterization. The main focus of this dissertation can be divided into four parts. In first part, we demonstrate a simple method to fabricate ZnO nanotip array, which exhibit low turn-on field, high field enhancement factor and stable field emission properties at 25-100 ℃. The good field emission properties are attributed to reduced oxygen vacancy concentration and small tip angle of ZnO emitters, which shows good potential for developing field emission and light emiting devices. In the second part, we provide another interesting route of fabricating ZnO-SnO2 core-shell nanowires for gas sensor applications. The ZnO-SnO2 core-shell nanowires exhibited good hydrogen sensor performance, such as the sensitivity is up to 89% against 200 ppm hydrogen at 250℃. Such high sensitivity was believed to be controlled by the nanoscale SnO2 layer, which was determined from pinch-off and fully conductive state. The ZnO-SnO2 core-shell nanostructures made by two-step chemical growth have high potential for gas sensor application. In the third part, vertical well-aligned and uniform ZnO nanorods were successfully prepared on low cost and flexible PET polymer substrate by aqueous solution method under various growth conditions. The photocurrents can be repeatly and reproducibly switched by modulating UV exposure with power densities of 25-70 μW/cm2. The fast response time (100 sec) and rapid recovery time (120 sec) are achieved in UV turn-on/off switching measurements. Owing to the mechanical flexibility, nondestructive properties, high reliability and multilevel photoresponse, the well-aligned ZnO nanorods grown on transparent and flexible PET polymer substrates have high potential for UV photodetector applications. In the fourth part, vertically well-aligned and uniform Ga¬-doped ZnO (GZO) nanorod thin films were successfully grown on Au/Ti/SiO2/p-Si substrates, which used to make resistive switching memory devices. Such memory devices can be reversibly switched between ON and OFF states, with a stable resistance ratio of 10 times, narrow dispersion of ON and OFF voltages, and good endurance performance of over 100 cycles. The resistive switching mechanism in this design is related to the formation and rupture of conducting filaments consisting of oxygen vacancies, occurred at interfaces between GZO nanorods (grain boundaries). Results show that the resulting compact GZO nanorod thin films have a high potential for resistive memory applications.

碩士
國立成功大學
化學工程學系碩博士班
94
Abstract Three main subjects on the growth of Zinc oxide with one-dime nsional nanostructure have be studied. The first part is the effect of vapor species in controlling ZnO nanoarchitectures. The second one is the influence of substrates upon the vertical aligned ZnO nanowires. The last section is the effect of electrical field on the direction growth of ZnO NW. At first, the architectures of ZnO nanomaterials could be controlled by varying the composition of O2 gas and substrate temperature during growths. At higher O2 concentration, ZnO nanowires growing along [0001] were synthesized. At lower O2 concentration, ZnO nanobelts growing along [0001] or were synthesized. The width of ZnO nanobelts increased with the reduction of O2 concentration. Photo- lumen escence(PL) spectra measurement indicated that ZnO nanobelts contained a lower concentration of structure defects than ZnO nanowires. During the substrate temperature decreased to 550℃, the morphology of ZnO became thin-film structure. However, the substrate temperature increased to 900℃, the densities of ZnO nanowires decreased. Regardless of temperature increased or decreased, ZnO nanowires grow along [0001]. PL spectra analysis indicated that ZnO nanowire, fabricated at high temperature, contained a lower concentration of structure defects Secondly, the vertical aligned ZnO nanowires could be controlled by varying the conditions and substrates during growth. At different conditions, the vertical aligned ZnO nanowire grown on the surface of Si substrates containing Au catalyst. The major factor is the ZnO buffer layer formed on Si substrate. Then, growing the vertical aligned ZnO nanowires on the different substrates, including Si substrates containing Au catalyst, c-plane sapphire containing Au catalyst, and Al-doped ZnO thin film glass substrate. Beside, nanowire grew in specific direction on the c-plane sapphire containing Au catalyst. The vertical aligned ZnO nanowires was also grew on the other substrates. The variation of morphology was caused by lattice mismatch. Furthermore, we also studied the influence of different seed layer on the nanostructure of ZnO of ZnO nanowire. Obviously, the nanowire tip was sharp when the ZnO nanoparticle as the seed layer ; when the Au nanoparticle as seed layer, the nanowire tip was formed hexagonal. We deduced that the growth rate of different crystal surfaces were main parameter on the formation of ZnO nanostructure. The last part is to study the effect of electrical field on the growth direction of ZnO nanowires. In this study, dielectrophoresis force was used align nanowires, fabricated by furnace, between inter-digital electr- odes. When E-field increased to 8.8 Vp-p and the frequency increased to 1MHz, the alignment of NWs become obviously. But, when the aligned ZnO nanowires were applied on the manufacture of transistor, the electrical behavior ZnO nanowires and electrode was schottky contact , and the transistor characteristics could not be observed. In order to directional growth ZnO nanowires, an electrical field was applied on the inter-digital electrodes during ZnO nanowires growth. When the electrical field was applied between the inter-digital electrodes, the growth of ZnO nanowire focused on the edge of electrodes. Although the densities of ZnO nanowires on the edge of electrodes were decreased, when the magnitude of electrical field were decreased, many nucleation sites were existed on the edge of electrodes. After growth , we used the aligned ZnO nanowires to fabricate transistor. The results of transistor characteristics shown that the electrodes and nanowires was ohimc contact. However, the gate voltage still could not be controlled.

碩士
國立臺灣科技大學
化學工程系
95
Gallium nitride(GaN) nanostructure were grown by metalorganic chemical vapor deposition using trimethylgallium(TMGa) as source material for Ga, and Ge nanowire、ZnO nanorod、Si(111) with 1 nm of Au as substrates. Si(111) coated with 1 nm Au was acted as a substrate. Heating the substrate until its temperature was 800℃ would form a sheet-like nanostructures. In the other hand, heating the substrates until its temperature was 550℃ would form a rod-like nanostructures. Well-aligned GaN nanorod structure were formed by increasing the feed ratio of Ⅴ/Ⅲ to 15000. GaN nanodots were formed on Ge nanowire substrate. Core-shell structure of GaN/ZnO nanorod、GaN/Ge nanowire were formed by atomic layer epitaxy(ALE) technique. The selective growth process of GaN nanorod on ZnO nanorod, which was coated with SiO2 and acted as substrate, was failed due to high temperature(850℃) and high NH3 concentration.

在本論文中，我們利用簡單的濕化學氧化鋅(ZnO)納米線陣列模板法成功地製備了一系列具有不同化學成份、晶體結構和形貌的準一維鋅-鐵-氧納米結構陣列。
垂直排列的ZnO納米線陣列首先生長在不同的襯底上，然后进一步被用作其他納米結構陣列的生長模板。ZnO納米線不僅僅起到骨架定型的作用，最終還可以为后續納米結構提供原料组分。通過控制ZnO和氯化鐵溶液的反應時間，在煅燒后，我們可以製備ZnO/鐵酸鋅(ZnFe₂O₄)納米線纜陣列，以及化學/非化學計量的ZnFe₂O₄、ZnFe₂O₄/α-三氧化二鐵(α-Fe₂O₃)和α-Fe₂O₃納米管陣列。ZnFe₂O₄和α-Fe₂O₃納米管陣列都表現出了對可見光的吸收，它們的帶隙經估算分別是2.3 eV和1.7 eV。
通過電子能量損失譜(EELS)，可以得到ZnFe₂O₄納米管陣列的一些細節的結構信息。我們分別研究了兩個不同系列(溫度和化學計量)的ZnFe₂O₄納米管。研究發現，樣品的磁性和它們的晶體結構有著非常緊密的關係。首先，對於溫度系列的樣品，當樣品的燒結溫度從600 °C降到400 °C時，更多的三價鐵離子(Fe³⁺)佔據了尖晶石結構中的A位置(四面體位置)而並非它們本應佔據的平衡B位置(八面體位置)。這種偏離了正常尖晶石結構的情況使得A和B位置上的Fe³⁺的超交換作用增加，進而增加了樣品的阻隔溫度(TB)，磁各向異性常數(K)，3K和300 K下的飽和磁化強度(MS)和3K下的矯頑力(HC)。同時使3K和300K下的MS的比值變小。其次，對於化學計量系列的樣品，通過比較在同一燒結溫度下製備的化學計量和非化學計量的ZnFe₂O₄納米管，我們發現在鐵鋅比大於2的納米管中，Fe³⁺佔據A和B位置的比例和化學計量的樣品是类似的。這些多出的Fe³⁺也會增加超交換作用，從而導致較大的TB, K, MS(3K和300 K)，HC(3K)和較小的MS(3 K)/MS(300 K)比值。最後，作為非化學計量的極端情況，α-Fe₂O₃納米管在小的外加磁場下表現出了典型的Morin相變，在大的外加磁場下出現了場致spin-flop轉變。
另一方面，我們發現，當使用羅丹明B(RhB)作為指示劑時，ZnO/ZnFe₂O₄納米線纜陣列表現出了優於纯ZnO和纯ZnFe₂O₄納米管陣列的可見光降解活性，但是它們的降解路徑各不相同。ZnO由於染料敏化機制而具有可見光降解能力，但是其降解活性最差。ZnO/ZnFe₂O₄納米線纜陣列和ZnFe₂O₄納米管陣列的基本降解原理是相同的，那就是，利用有可見光活性的ZnFe₂O₄中的光生電子和空穴所生成的活性自由基降解RhB。但是，ZnO/ZnFe₂O₄納米線纜陣列的降解能力明顯優於ZnFe₂O₄納米管陣列，這是由於ZnO與ZnFe₂O₄之間的II型能帶匹配顯著地促進了光生電子和空穴的分離。
In the present thesis, several kinds of pseudo-one-dimensional Zn-Fe-O nanostructure arrays with tunable chemical compositions, crystal structures and morphologies are successfully synthesized via a simple wet-chemical ZnO-nanowire-array templating method.
Vertically-aligned ZnO nanowire arrays are firstly fabricated on several different substrates and then serve as templates for other nanostructured arrays growth. The ZnO nanowires not only act as morphology-defining skeleton but also contribute chemically to the final composition of the nanostructures. By controlling the reaction time between ZnO and FeCl₃ solution, ZnO/ZnFe₂O₄ nanocable arrays, stoichiometric ZnFe₂O₄ nanotube arrays, nonstoichiometric ZnFe₂O₄ nanotube arrays, ZnFe₂O₄/α-Fe₂O₃ nanotube arrays and α-Fe₂O₃ nanotube arrays can be synthesized in a controlled manner after calcination. Both ZnFe₂O₄ and α-Fe₂O₃ nanotube arrays exhibit visible light absorption and their bandgap are estimated to be ~2.3 eV and ~1.7 eV, respectively.
The detailed structural information of the ZnFe₂O₄ nanotube arrays are obtained by electron energy loss spectroscopy (EELS). In particular, EELS are carried out for two different series (i.e., temperature and stoichiometric series). The magnetic properties of these samples are found to closely correlate to their structural characteristics. Firstly, with the decrease of the calcination temperature from 600 °C to 400 °C, more Fe³⁺ions occupy A sites (tetrahedral sites in spinel structure) rather than their equilibrium B sites (octahedral sites in spinel structure). The deviation from the normal spinel structure leads to the enhancement of superexchange interactions between Fe³⁺ions in A and B sites, and thus results in an increase in blocking temperature (TB), magnetic anisotropic constant (K), saturation magnetization (MS, at 3 K and 300 K), coercivity (HC, at 3 K) and a decrease in MS(3 K)/MS(300 K) ratios. Secondly, by comparing stoichiometric and nonstoichiometric ZnFe₂O₄ nanotubes calcinated at the same temperature, we found that the nonstoichiometric nanotubes (Fe:Zn > 2) shows similar ratios of Fe³⁺in A and B sites to that of the stoichiometric one. The extra Fe³⁺in the crystal also enhances the superexchange interactions of Fe³⁺, which results in larger TB, K, MS(at 3 K and 300 K) and HC(at 3 K), and smaller MS(3 K)/MS(300 K) ratio. Lastly, α-Fe₂O₃ nanotubes, as an extreme case of the nonstoichiometric sample, show typical Morin-transition characterization under small external field, and field-induced spin-flop transition at large external field.
On the other hand, we found that the visible-light-driven photodegradation activities of ZnO/ZnFe₂O₄ nanocable arrays are superior to those of the ZnO nanowire arrays and ZnFe₂O₄ nanotube arrays using RhB as the probe molecules. All the three nanostructures show degradation of RhB molecules under visible light irradiation, but they take different degradation pathways. The degradation of RhB in the presence of ZnO nanowire arrays is attributed to the dye-sensitized mechanism, and the photodegradation activity is the worst. ZnO/ZnFe₂O₄ nanocable arrays and ZnFe₂O₄ nanotube arrays have the same degradation mechanism, that is, reactive radicals produced by photogenerated electron-hole pairs in the visible-light-active ZnFe₂O₄ are responsible for the photodegradation of RhB. However, the nanocable arrays show much higher degradation capability. This is owing to the type II band alignment between ZnO and ZnFe₂O₄, which greatly promotes the separation of photogenerated electronsand holes in ZnFe₂O₄.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Guo, Xuan = 準一維鋅-鐵-氧納米結構陣列 : 控制製備, 磁學性質以及光催化方面的應用 / 郭璇.
Thesis (Ph.D.) Chinese University of Hong Kong, 2013.
Includes bibliographical references (leaves 107-117).
Abstracts also in Chinese.
Guo, Xuan = Zhun yi wei xin-tie-yang na mi jie gou zhen lie : kong zhi zhi bei, ci xue xing zhi yi ji guang cui hua fang mian de ying yong / Guo Xuan.