When materials approach the size of few nanometers, they show properties which are significantly different from their bulk counterpart. Such unique/improved properties make them potential candidate for several emerging applications. At the reduced dimension, controlling the shape of nanocrystals provides an effective way to tune several material properties. In this regard, wet chemical synthesis has been established as the ultimate route to synthesize nanocrystals at ultra-small dimensions with excellent control over the morphology. However, the use of surfactant poses a barrier into efficient realization of its application as it requires a clean interface for better performance. Exercise of available cleaning protocols to clean the surface often leads to coarsening of the nanoparticles due to their inherent high surface curvature. For anisotropic nanomaterials, rounding of the shape is an additional problem. Anchoring nanomaterials onto substrates provides an easy way to impart stability. In this thesis, ultrathin Au nanowires, that are inherently unstable, have been shown to grow over a wide variety of substrates by in-situ functionalization. Use of nanomaterials as device component holds promise into miniaturization of electronics. But device fabrication in such cases require manipulation of nanomaterials with enhanced control. Dielectrophoresis offers an easy way to assemble nanomaterials in between contact pads and hence evolved as a promising tool to fabricate device with a good level of precision. Herein, directed assembly of ultrathin Au nanowires by dielectrophoresis, has been shown as an efficient strategy to fabricate devices based on the wires. Combining more than one nanocrystal, to form a heterostructure, often has the advantage of synergism and/or multifunctionality. Therefore, synthesis of heterostructure is highly useful in enhancing and/or adding functionalities to nanomaterials. There are several routes available in literature for synthesis of heterostructures. Newer strategies are being evolved to further improve performance in an application specific way. In that regard, a good understanding of mechanism of formation is crucial to form the desired product with the required functionality. For example, Au due to high electron affinity has been known to undergo reduction rather than cation exchange with chalcogenides. In this thesis, it has been shown that the final product depends on the delicate balance of reaction conditions and the system under study using CdS-Au as the model system. In yet another case, PdO nanotubes have been shown to form, on reaction of PdCl2 with ZnO at higher starting ratio of the precursors. In-situ generation of HCl provides an effective handle for tuning of the product from the commonly expected hybrid to hollow. Graphene has evolved as a wonder material due to its wide range of practical applications. Its superior conductivity with high flexibility has made it an important material in the field of nanoelectronics. In this thesis, an interesting case of packed crumpled graphene has been shown to sense a wide variety of strain/pressure which has applications in day to day life. The study reported in the thesis is organized as follows: Chapter 1 presents a general introduction to nanomaterials followed by the review of the available strategies to synthesize various 1D nanomaterials. Subsequently, a section on the classification of hybrid followed by the different synthetic protocols adopted in literature to synthesize them, have been provided. A review on the available methodologies for directed assembly of nanomaterials has been presented. Chapter 2 provides a summary of the materials synthesized and the techniques used for characterization of the materials. A brief description of all the synthetic strategy adopted has been provided. The basic principle of all the characterization techniques used, has been explained. A section explaining the principle of dielectrophoresis has also been presented. Chapter 3 presents a general method to grow ultrathin Au nanowires over a variety of substrates with different nature, topography and rigidity/flexibility. Ultrathin nanowires of Au (~2 nm in diameter) are potentially useful for various catalytic, plasmonic and device applications. Extreme fragility on polar solvent cleaning was a limitation in realizing the applications. Direct growth onto substrate was an alternative but poor interfacial energy of Au with most commercial substrates lead to poor coverage. In this chapter, in-situ functionalization of the substrates have been shown to improve Au nucleation dramatically which lead to growth of dense, networked nanowires over large area. Catalysis and lithography-free device fabrication has been demonstrated. Using the same concept of functionalization, SiO2 coating of the nanowires have been shown. A comparative study of thermal stability of these ultrafine Au nanowires in the uncoated and coated form, has been presented. Chapter 4 demonstrates an ultrafast device fabrication strategy with Au nanowires using dielectrophoresis. While dense growth of Au nanowires is beneficial for some applications, it is not so for some others. For example, miniaturization of electronics require large number of devices in a small area. Therefore, there is a need for methods to manipulate nanowires so as to place them in the desired location for successful fabrication of device with them. In this chapter, dielectrophoresis has been used for assembling nanowires in between and at the sides of the contact pads. Alignment under different conditions lead to an understanding of the forces. Fabrication of a large number of devices in a single experiment has been demonstrated. Chapter 5 presents a simple route to synthesize CdS-Au2Sx hybrid as a result of cation-exchange predominantly. Au due to high electron affinity has been shown in literature to undergo reduction rather than cation exchange with CdS. In this chapter, it has been shown that cation exchange may be a dominant product. The competition between cation exchange and reduction in the case of CdS-Au system has been studied using EDS, XRD, XPS and TEM. Thermodynamic calculation along with kinetic analysis show that the process may depend on a delicate balance of reaction conditions and the system under study. The methodology adopted, is general and may be applied to other systems. Chapter 6 presents an one pot, ultrafast microwave route to synthesize PdO hollow/hybrid nanomaterials. The common strategy to synthesize hollow nanomaterials had been by nucleation of the shell material on the core and subsequent dissolution of the core. In this chapter, a one step method to synthesize hollow PdO nanotubes, using ZnO nanorods as sacrificial template, has been shown. By tuning the ratio of the PdCl2 (PdO precursor) to ZnO, ZnO-PdO hybrid could be obtained using the same method. The PdO nanotubes synthesized could be converted to Pd nanotubes by NaBH4 treatment. Study of thermal stability of the PdO nanotubes has been carried out. Chapter 7 demonstrates a simple strategy to sense a variety of strain/pressure with taped crumpled graphene. Detection of ultralow strain (10-3) with high gauge factor is challenging and poorly addressed in literature. Taped crumpled graphene has been shown to detect such low strain with high gauge factor (> 4000). An ultra-fast switching time of 20.4 ms has been documented in detection of dynamic strain of frequency 49 Hz. An excellent cyclic stability for >7000 cycles has been demonstrated. The same device could be used to detect gentle pressure pulses with consistency. Slight modification of the device configuration enabled detection of high pressure. Simplicity of the device fabrication allowed fabrication of the device onto stick labels which could be pasted on any surface, for instance, floor. Hard pressing, stamping with feet and hammering shocks do not alter the base resistance of the device, indicating that it is extremely robust. Sealed arrangement of the graphene allowed operation of the device under water in detection of water pressure. Presence of trapped air underneath the tape enabled detection of air pressure both below and above atmospheric pressure.

Submitted by Thais Silva do Amaral null (thais_rpss@yahoo.com) on 2016-06-03T19:14:32Z No. of bitstreams: 1 Thais Silva do Amaral-Dissertação.pdf: 3804617 bytes, checksum: 8083e47078da73695bac6f8aa62e3778 (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-06-08T13:24:26Z (GMT) No. of bitstreams: 1 amaral_ts_me_araiq_par.pdf: 1221750 bytes, checksum: e72ea0e2cee4ebb7654b4a0d18d9289f (MD5) / Made available in DSpace on 2016-06-08T13:24:26Z (GMT). No. of bitstreams: 1 amaral_ts_me_araiq_par.pdf: 1221750 bytes, checksum: e72ea0e2cee4ebb7654b4a0d18d9289f (MD5) Previous issue date: 2015-05-12 / Polímeros derivados do petróleo como polietileno tereftalato (PET) e polietileno naftalato (PEN), são utilizados em larga escala como substratos em diversos dispositivos eletrônicos. A crescente preocupação com o meio ambiente nos leva a buscar alternativas sustentáveis na utilização de materiais para fabricação de novas tecnologias. Neste trabalho, com o intuito de avaliar a viabilidade da substituição destes substratos por polímeros naturais, foi explorada uma biocelulose, a celulose bacteriana (CB), secretada por bactérias Acetobacter xylinum, que é um polímero de obtenção ―verde‖, não gerando resíduos ou altos impactos ambientais para ser produzida, além de possuir características desejáveis para ser utilizado como substrato em novos materiais, como resistência mecânica com módulo de Young de 134 GPa, tamanho nanométrico das fibras e transparência. Membranas funcionais foram obtidas pelo crescimento de nanofios de óxido de zinco na sua superfície. Os nanofios de ZnO foram obtidos com comprimento médio de 1,69 ± 0,08 μm e diâmetro de 37,2 ± 4,2 nm. Os materiais foram avaliados estruturalmente pela Difratometria de Raios-x (DRX) e Microscopia Eletrônica de Transmissão de Alta Resolução (HRTEM), e quimicamente utilizando Espectroscopia de Espalhamento Raman e Espectroscopia Vibracional na Região do Infravermelho (FT-IR). Também foram realizadas de medidas de Impedância Elétrica e Análise termogravimétrica (TG/DTG). Por fim os materiais foram testados em três diferentes aplicações: como membrana para fotodegradação de corantes, sensor piezoelétrico e substrato removível para obtenção de fios de ZnO não suportados que se mostraram aplicações viáveis para o material. / Petroleum-derived polymers such as Polyethylene Terephthalate (PET) and Polyethylene Naphthalate (PEN), are largely used as substrates in various electronic devices. The growing concern with the environment leads us to seek sustainable alternatives in the use of materials for the manufacture of new technologies. In this work, in order to assess the feasibility of replacing these substrates by natural polymers, bacterial cellulose (BC) was explored, secreted by bacteria Acetobacter xylinum is a ―green‖ polymer that don’t generate waste or high environmental impacts to be produced, and has desirable characteristics for use as new substrate materials, such as mechanical strength with Young's modulus of 134 Gpa, nano-sized fibers and transparency. Functional membranes were prepared by growing ZnO nanowires on the BC dried membranes surface. The obtained ZnO nanowires presented an average length of 1.69 ± 0.08 m and diameter of 37.2 ± 4.2 nm. Materials were evaluated structurally by X-ray Diffraction (XRD) and High-resolution Transmission Electron Microscopy (HRTEM), chemically using Raman Scattering spectroscopy and Vibrational Spectroscopy in the Infrared Region (FT-IR). Electrical Impedance measurements and thermal gravimetric analysis (TG / DTG) were performed as well. Finally the materials were tested in three different applications: as a membrane for dyes photodegradation, piezoelectric sensor and removable substrate for obtaining unsupported ZnO nanowires that are viable applications for the material.

This thesis reports transport in ultrathin single crystalline nanowires of gold (∼ 2nm). These nanowires were fabricated using an oriented attachment process whereby nanoparticles of appropriate dimensions join in a linear fashion to form long and stable wires. The main motivation was to study the role of electron-electron interactions on the transport mechanism in case of a metallic system, as one approaches dimensions closer to the Fermi wavelength. The study forms the first of its kind in a simple metallic system of this dimension. Indeed, several new features have been obtained in this regard: Chapter 4 reports a breakdown of Fermi liquid state in such a system opening up possibilities of exotic states constituted by a strongly correlated Tomonaga-Luttinger liquid. We report consistent scaling of current-voltage curves, characteristic of such a phase and even resonant tunneling in such structures. The study reports the first observation of a correlated electron liquid in a metal, which has been observed only in semiconductors and polymer wires till date. Chapter 5 discusses the possibility of tuning the transport mechanism in these wires via a controlled change in the growth process. We show that using appropriate growth mechanisms, we can have a localized ground state as well, where variable range hopping is the dominant transport mechanism. Possibility of structural transitions in ultrathin wires is a field that has garnered considerable interest due to simulations. We present a highly sensitive tool in the form of electrical noise and its higher order statistics that can act as a detector of structural transitions. This has been thoroughly studied in case of conventional shape memory systems in Chapter 6. Preliminary noise studies on the nanowires have been reported in Chapter 7.

Cílem diplomové práce je popsat oxidaci nanotrubic sulfidu wolframičitého za zvýšených teplot v přítomnosti vodní páry. Na jejich povrchu se nejprve vytvoří nanočástice oxidu wolframu, ze kterých potom vyrůstají nanodráty. Na základě in-situ experimentů v rastrovacím elektronovém mikroskopu je navržen mechanismus reakce a ten je zjednodušeně popsán analyticky. Ukazuje se, že elektronový svazek má zásadní vliv na reakci.

Pursuing the development and implementation of novel synthesis techniques to produce nanostructures with an interesting set of properties is a goal that advances the frontiers of nanotechnology. Also of fundamental importance is to revisit well-established synthesis techniques employing a new set of materials as precursors, substrates and catalysts. Fundamental breakthroughs in the field of nanotechnology can be achieved by developing new synthesis procedures as well as by adapting known procedures to new materials. This thesis focuses on both kinds of experiments. A variant of chemical vapor deposition (CVD) has been used to produce Al5BO9 nanowires out of sapphire wafers without the need of a catalyst material. The novelty of the work relies on the formation mechanism of the Al5BO9 nanowires. Essentially, the process can be described as a large-scale topological transformation taking place on the substrate’s surface as its chemical composition changes due to the arrival of precursor molecules. Dense mats of Al5BO9 nanowires cover large areas of the substrate that were previously relatively flat. The process is enhanced by a high temperature and the presence of pre-existing superficial defects (cracks, terraces, etc.) on the substrates. Al5BO9 nanowires as well as B/BOX nanowires and BOX nanotubes were also produced via a novel in-situ electron beam-induced synthesis technique. The process was carried out at room temperature and inside a transmission electron microscope. Au nanoparticles were used as catalyst for the case of B/BOX nanowires and BOX nanotubes, while the Al5BO9 nanowires were synthesized without the need of a catalyst material. The formation and growth of the nanostructures is solely driven by the electron beam. The growth mechanism of the B/BOX nanowires and BOX nanotubes relies on interplay between electrostatic charging of the precursor material (to produce and transport feedstock material) and electron stimulated desorption of oxygen which is able to activate the catalytic properties of the Au nanoparticles. For the case Al5BO9 nanowires a nucleation process based on massive atomic rearrangement in the precursor is instigated by the e-beam, afterwards, the length of some of the nanowires can be extended by a mechanism analogous to that of the growth of the B/BOX nanowires.

No description available.

ZnO nanowires are a promising material with great semiconductor properties. ZnO nanowires were prepared by carbothermal reduction and vapor-liquid-solid growth mechanism. Altering a variety of parameters ranging from mole to mole ratio of ZnO to C all the way to gas flow rate was examined. The nanowires were then characterized and their morphology examined under a SEM to observe what effect the parameter had on the morphology of the nanowires. From the experiments and the parameters tested it was observed that in order to produce the highest quality straight nanowires one should use a mole to mole ratio of ZnO to C graphite of 1 to 3. With a dwell temperature and time of 900 °C for 3 hours. A gold seed catalyst of 4nm and a gas flow rate of 50 to 100sccm of Ar provides the straightest nanowires. Understanding the effect of each parameter on the morphology of ZnO nanowires is vital for the current research. This will only lead to further the research and provide a better understanding of the growth mechanism of these wires and how the production of specific wires with certain morphologic features and characteristics can be achieved.

Nanotubes (inner diameter of 8nm and outer diameter of 10nm with a length of up to several hundred nm) and nanowires (diameter 20 – 50nm and up to several μm in length) of TiO₂-B have been synthesised and characterised for the first time. These exhibit excellent properties as a host for lithium intercalation and are able to accommodate lithium up to a composition of Li₀.₉₈TiO₂-B for the nanotubes and Li₀.₈₉TiO₂-B for the nanowires. Following some irreversible capacity on the first cycle, which could be reduced to 4% for the nanowires, capacity retention for the nanowires is 99.9% and for the nanotubes is 99.5% per cycle. In both cases, the cycling occurs at ~1.6V versus lithium. The cycling performance was compared with other forms of bulk and nano-TiO₂, all of which were able to intercalate less lithium. Nanowires of VO₂-B (50 – 100nm in diameter and up to several μm in length) were synthesised by a hydrothermal reaction and characterised. By reducing the pressure inside the hydrothermal bomb, narrower VO₂-B nanowires with a diameter of 2 – 5nm and length of up to several hundred nm were created - some of the narrowest nanowires ever made by a hydrothermal reaction. These materials are isostructural with TiO₂-B and were also found to perform well in rechargeable lithium ion batteries, being able to intercalate 0.84Li for the ultra-thin nanowires and 0.57Li for the standard nanowires. The standard VO₂-B nanowires have a capacity retention of 99.8% and the ultra-thin nanowires have 98.4% per cycle after some irreversible capacity on the first cycle. This was found to improve markedly when different electrolytes were used. Macroporous Co₃O₄ (pore size 400nm with a surface area of 208m²/g) was prepared and cycled in rechargeable lithium cells with capacities of 1500mAh/g being achieved. The structure was found to break down on the first cycle and after this the material behaved in the manner of Co₃O₄ nanoparticles. Finally a new candidate for next generation rechargeable lithium batteries was examined; Li/O₂ cells. The cathode is composed of porous carbon in which Li⁺, e⁻ and O₂ meet to form Li₂O₂ on discharge. The reaction is reversible on charge. Capacities of 2800mAh/g can be achieved when 5%mole of αMnO₂ nanowires catalyst is used. Fade is high at 3.4% per cycle meaning that there is much work to do to develop these into a commercial prospect.

Οι μονοδιάστατες μεταλλικές νανοδομές, όπως νανοσύρματα και νανοσωλήνες, έχουν ελκύσει το ενδιαφέρον της επιστημονικής κοινότητας τα τελευταία χρόνια λόγο του σημαντικού ρόλου που παίζουν στην κατασκευή νανοσυσκευών, όπως ανιχνευτές, ηλεκτρονικά και οπτικά συστήματα και συστήματα αποθήκευσης πληροφορίας. Μια απλή και αποτελεσματική τεχνική για την κατασκευή μεγάλου αριθμού νανοσυρμάτων και νανοσωλήνων με μεγάλη αναλογία μήκους προς διάμετρο είναι η ονομαζόμενη «σύνθεση μήτρας», η οποία βασίζεται στην ηλεκτροχημική εναπόθεση μετάλλου μέσα στους πόρους κατάλληλου υλικού το οποίο λειτουργεί σαν μήτρα. Η πορώδης αλουμίνα είναι ένα υλικό που χρησιμοποιείται ευρέως για τον σκοπό αυτό λόγο της χημικής και μηχανικής της σταθερότητας και της αντοχής της σε υψηλές θερμοκρασίες. Επίσης, τα γεωμετρικά της χαρακτηριστικά μπορούν να ελέγχουν εύκολα κατά την διάρκεια της διαδικασίας παρασκευής της. Στην εργασία περιγράφεται η διαδικασία κατασκευής μεμβρανών πορώδους αλουμίνας με εξαγωνική κατανομή πόρων, ανοιχτών και στις δύο επιφάνειες και με συγκεκριμένα γεωμετρικά χαρακτηριστικά. Για να κατασκευασθούν οι μήτρες οι οποίες θα χρησιμοποιηθούν στην ηλεκτροεναπόθεση και για άλλες εφαρμογές εξήχθη η σχέση μεταξύ του ρυθμού ανάπτυξης της μεμβράνης πορώδους αλουμίνας και των παραμέτρων της ανοδίωσης (πυκνότητα ρεύματος, θερμοκρασία) για ανοδίωση σε υδατικό διάλυμα με περιεκτικότητα 0.3Μ οξαλικό οξύ. Επιπλέον, μελετήθηκε ο απαραίτητος χρόνος για την διάλυση του συμπαγούς διαχωριστικού στρώματος (barrier layer) της πορώδους αλουμίνας με την χρήση υδατικού διαλύματος 5%wt. φωσφορικού οξέος. Τέλος, στην εργασία αυτή μελετήθηκε η ανάπτυξη νανοσυρμάτων νικελίου σε πορώδης αλουμίνα με μέση διάμετρο πόρων 240nm. Για την καλύτερη κατανόηση των μηχανισμών ανάπτυξης των νανοσυρμάτων μελετήθηκε η σχέση της κρυσταλλογραφικής δομής των νανοσυρμάτων με την εφαρμοζόμενη τάση κατά την ανάπτυξη τους με την χρήση της τεχνικής dc ηλεκτροεναπόθεσης. Κατά την μελέτη αυτή αναπτύχθηκαν μονοκρυσταλλικά νανοσύρματα νικελίου προσανατολισμένα κατά την διεύθυνση [110] και πολυκρυσταλλικά νανοσύρματα με ισχυρό προσανατολισμό κατά την διεύθυνση [111]. / One-dimensional metallic nanostructured materials, like nanowires and nanotubes, have attracted extensive attention in recent years because of their importance in the fabrication of nanometer-scale devices such as sensors, electronics, and optics and information storage systems. A simple and effective technique to fabricate large number of metallic nanowires and nanotubes with high aspect ratio is the so called “template synthesis”, which involves electrochemically depositing metal into nanopores of a suitable material used as a template. Porous alumina membrane is a commonly used material for this purpose because of its chemical and mechanical stability and durability at high temperatures. Moreover, its geometrical characteristics are easily controlled during the fabrication process. In this work the fabrication process of free- standing porous alumina membranes with highly organized hexagonal structure and with specific geometrical characteristics is described. In order to fabricate templates for use in electrodeposition and other applications the dependence relation between the thickness growth rate and the anodization parameters (current density and temperature) for anodization in aqueous solution of 0.3M oxalic acid was derived. Furthermore, the time needed for barrier layer dissolution of porous alumina membrane with the use of hydrate solution of 5%wt. phosphoric acid. Finally, nickel nanowires were fabricated inside porous alumina membranes with mean pore diameter of 240nm. For better understanding of the nanowires growth mechanism the dependence of the crystal structure of nickel nanowire fabricated with dc electrodeposition from the applied voltage was studied. As a result, single crystal nickel nanowires oriented along [110] and polycrystalline nickel nanowires with a strong orientation along [111] have been fabricated.

In order to completely assess the potential of semiconductor nanowires for multifunctional applications such as flexible electronics, nanoelectromechanical systems (NEMS), and composites, a full characterization of their properties must be obtained. While many of their physical properties have been well studied, explorations of mechanical, electromechanical, and optical properties of semiconductor nanowires remain relatively sparse in the literature. Two major hurdles to the elucidation of these properties are: (1) the development of experimental techniques which are capable of mechanical and electromechanical measurements coupled with detailed structural analysis, and (2) the synthesis of high quality nanowires with the high yields necessary to produce the quantities needed for composite fabrication. These issues are addressed in this dissertation by utilizing the supercritical fluid-liquid-solid (SFLS) synthesis method to produce germanium (Ge) nanowire specimens for mechanical and electromechanical measurements coupled with high-resolution transmission electron microscopy (HRTEM). In addition, excellent dispersibility and large quantities allow for optical measurements of dispersions and composites. Ge cantilever nanoelectromechanical resonators were fabricated and induced into resonance. From the frequency response, the Young's modulus of the nanowires was determined to be insensitive to diameter and on par with the literature values for bulk Ge. The mechanical quality factors of the resonators were found to decrease with decreasing diameter. The data indicate that energy dissipation from the oscillating cantilevers occurs predominantly via surface losses. The mechanical strengths of individual Ge nanowires were measured by in situ nanomanipulation in a scanning electron microscope (SEM). The nanowires were found to tolerate diameter-dependent flexural strains more than two orders of magnitude higher than bulk Ge. Corresponding bending strengths were in agreement with the ideal strength of a perfect Ge crystal, indicative of a reduced presence of extended defects. The nanowires also exhibited plastic deformation at room temperature, becoming amorphous at the point of maximum strain. The optical absorbance spectra of Ge nanowires were measured and found to exhibit spectra markedly different from bulk Ge. Simulations using a discrete dipole approximation (DDA) model suggest that the difference in light absorption results from light trapping within the nanowires. / text