Synthesis of GaN nanowires and nano-pyramids in a two-hot-boat chemical vapor deposition system via an in-doping technique

A two-hot-boat chemical vapor deposition system was modified from a thermal evaporation equipment. This system has the advantage of high vacuum, rapid heating rate and temperature separately controlled boats for the source and samples. These are in favor of synthesizing compound semiconducting nano-materials. By the system, we have synthesized high-quality wurtzite single crystal GaN nanowires and nanotip triangle pyramids via an in-situ doping indium surfactant technique on Si and 3C-SiC epilayer/Si substrates. The products were analyzed by x-ray diffraction, field emission scanning electron microscopy, highresolution transmission electron microscopy, energy- dispersive x-ray spectroscopy, and photoluminescence measurements. The GaN nanotip triangle pyramids, synthesized with this novel method, have potential application in electronic/ photonic devices for field-emission and laser.

High and electric field tunable Curie temperature in diluted magnetic semiconductor nanowires and nanoslabs

The Curie temperature of diluted magnetic semiconductor (DMS) nanowires and nanoslabs is investigated using the mean-field model. The Curie temperature in DMS nanowires can be much larger than that in corresponding bulk material due to the density of states of one-dimensional quantum wires, and when only one conduction subband is filled, the Curie temperature is inversely proportional to the carrier density. The T-C in DMS nanoslabs is dependent on the carrier density through the number of the occupied subbands. A transverse electric field can change the DMS nanowires from the paramagnet to ferromagnet, or vice versae. (c) 2007 American Institute of Physics.

Semiconductor-metal transition in InSb nanowires and nanofilms under external electric field

The electronic structures, Rashba spin-orbit couplings, and transport properties of InSb nanowires and nanofilms are investigated theoretically. When both the radius of the wire (or the thickness of the film) and the electric field are large, the electron bands and hole bands overlap, and the Fermi level crosses with some bands, which means that the semiconductors transit into metals. Meanwhile, the Rashba coefficients behave in an abnormal way. The conductivities increase dramatically when the electric field is larger than a critical value. This semiconductor-metal transition is observable at the room temperature. (c) 2006 American Institute of Physics.

Electronic structure and g factors of narrow-gap zinc-blende nanowires and nanorods

The Hamiltonian in the framework of eight-band effective-mass approximation of the zinc-blende nanowires and nanorods in the presence of external homogeneous magnetic field is given in the cylindrical coordinate. The electronic structure, optical properties, magnetic energy levels, and g factors of the nanowires and nanorods are calculated. It is found that the electron states consist of many hole-state components, due to the coupling of the conduction band and valence band. For the normal bands which are monotone functions of |k(z)|, long nanorods can be modeled by the nanowires, the energy levels of the nanorods approximately equal the values of the energy band E(k(z)) of the nanowires with the same radius at a special k(z), where k(z) is the wave vector in the wire direction. Due to the coupling of the states, some of the hole energy bands of the nanowires have their highest points at k(z)=0. Especially, the highest hole state of the InSb nanowires is not at the k(z)=0 point. It is an indirect band gap. For these abnormal bands, nanorods can not be modeled by the nanowires. The energy levels of the nanorods show an interesting plait-like pattern. The linear polarization factor is zero, when the aspect ratio L/2R is smaller than 1, and increases as the length increases. The g(z) and g(x) factors as functions of the k(z), radius R and length L are calculated for the wires and rods, respectively. For the wires, the g(z) of the electron ground state increases, and the g(z) of the hole ground state decreases first, then increases with the k(z) increasing. For the rods, the g(z) and g(x) of the electron ground state decrease as the R or the L increases. The g(x) of the hole ground state decreases, the g(z) of the hole ground state increases with the L increasing. The variation of the g(z) of the wires with the k(z) is in agreement with the variation of the g(z) of the rods with the L.

Electrochemical polymerization and characterization of polypyrrole nanowires and nanotubules

Polypyrrole nanostructure arrays, including simultaneously large quantities of nanowires and small quantities of partially filled nanotubules have been electrochemically synthesized in home-made etched ion-track polycarbonate (PC) templates. Diameter of the prepared nanostructures varies from 45 to 320 nm with their lengths up to 30 microns. Morphological studies of these nanostructures were performed by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. While optical absorption properties were studied by ultraviolet-visible-near infrared spectrophotometry (UV-vis-NIR). It has been observed that the absorption maximum of polypyrrole shifts to the longer wavelength side as the diameter of these nanostructures (nanowires and nanotubules) increases. (C) 2010 Elsevier B.V. All rights reserved.

One-dimensional CaWO4 and CaWO4:Tb3+ nanowires and nanotubes: electrospinning preparation and luminescent properties

One-dimensional CaWO4 and CaWO4:Tb3+ nanowires and nanotubes have been prepared by a combination method of sol-gel process and electrospinning. X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), low voltage cathodoluminescence (CL) and time-resolved emission spectra, as well as kinetic decays were used to characterize the resulting samples. The results of XRD, FT-IR, TG-DTA indicate that the CaWO4 and CaWO4: Tb3+ samples begin to crystallize at 500 degrees C with the scheelite structure. Under ultraviolet excitation and low-voltage electron beams excitation, the CaWO4 samples exhibit a blue emission band with a maximum at 416 nm originating from the WO42- groups, while the CaWO4:Tb3+ samples show the characteristic emission of Tb3+ corresponding to (D4-F6,5,4,3)-D-5-F-7 transitions due to an efficient energy transfer from WO42- to Tb3+.

Facile chemical conversion synthesis and luminescence properties of uniform Ln3+ (Ln = Eu, Tb)-doped NaLuF4 nanowires and LuBO3 microdisks

Uniform NaLuF(4) nanowires and LuBO(3) microdisks have been successfully prepared by a designed chemical conversion method. The lutetium precursor nanowires were first prepared through a simple hydrothermal process. Subsequently, uniform NaLuF(4) nanowires and LuBO(3) microdisks were synthesized at the expense of the precursor by a hydrothermal conversion process. The whole process was carried out in aqueous condition without any organic solvents, surfactant, or catalyst. The conversion processes from precursor to the final products have been investigated in detail. The as-obtained Eu(3+) and Tb(3+)-doped LuBO(3) microdisks and NaLuF(4) nanowires show strong characteristic red and green emissions under ultraviolet excitation or low-voltage electron beam excitation. Moreover, the luminescence colors of the Eu(3+) and Tb(3+) codoped LuBO(3) samples can be tuned from red, orange, yellow, and green-yellow to green by simply adjusting the relative doping concentrations of the activator ions under a single wavelength excitation, which might find potential applications in the fields such as light display systems and optoelectronic devices.

Fabrication and characterisation of semiconductor nanowires and metal interconnects

One-dimensional semiconductor nanowires are considered to be promising materials for future nanoelectronic applications. However, before these nanowires can be integrated into such applications, a thorough understanding of their growth behaviour is necessary. In particular, methods that allow the control over nanowire growth are deemed especially important as it is these methods that will enable the control of nanowire dimensions such as length and diameter (high aspect ratios). The production of nanowires with high-aspect ratios is vital in order to take advantage of the unique properties experienced at the nanoscale, thus allowing us to maximise their use in devices. Additionally, the development of low-resistivity interconnects is desirable in order to connect such nanowires in multi-nanowire components. Consequently, this thesis aims to discuss the synthesis and characterisation of germanium (Ge) nanowires and platinum (Pt) interconnects. Particular emphasis is placed on manipulating the nanowire growth kinetics to produce high aspect ratio structures. The discussion of Pt interconnects focuses on the development of low-resistivity devices and the electrical and structural analysis of those devices. Chapter 1 reviews the most critical aspects of Ge nanowire growth which must be understood before they can be integrated into future nanodevices. These features include the synthetic methods employed to grow Ge nanowires, the kinetic and thermodynamic aspects of their growth and nanowire morphology control. Chapter 2 outlines the experimental methods used to synthesise and characterise Ge nanowires as well as the methods used to fabricate and analyse Pt interconnects. Chapter 3 discusses the control of Ge nanowire growth kinetics via the manipulation of the supersaturation of Ge in the Au/Ge binary alloy system. This is accomplished through the use of bi-layer films, which pre-form Au/Ge alloy catalysts before the introduction of the Ge precursor. The growth from these catalysts is then compared with Ge nanowire growth from standard elemental Au seeds. Nanowires grown from pre-formed Au/Ge alloy seeds demonstrate longer lengths and higher growth rates than those grown from standard Au seeds. In-situ TEM heating on the Au/Ge bi-layer films is used to support the growth characteristics observed. Chapter 4 extends the work of chapter 3 by utilising Au/Ag/Ge tri-layer films to enhance the growth rates and lengths of Ge nanowires. These nanowires are grown from Au/Ag/Ge ternary alloy catalysts. Once again, the supersaturation is influenced, only this time it is through the simultaneous manipulation of both the solute concentration and equilibrium concentration of Ge in the Au/Ag/Ge ternary alloy system. The introduction of Ag to the Au/Ge binary alloy lowers the equilibrium concentration, thus increasing the nanowire growth rate and length. Nanowires with uniform diameters were obtained via synthesis from AuxAg1-x alloy nanoparticles. Manifestation of the Gibbs-Thomson effect, resulting from the dependence of the mean nanowire length as a function of diameter, was observed for all of the nanowires grown from the AuxAg1-x nanoparticles. Finally, in-situ TEM heating was used to support the nanowire growth characteristics. Chapter 5 details the fabrication and characterisation of Pt interconnects deposited by electron beam induced deposition of two different precursors. The fabrication is conducted inside a dual beam FIB. The electrical and structural characteristics of interconnects deposited from a standard organometallic precursor and a novel carbon-free precursor are compared. The electrical performance of the carbon-free interconnects is shown to be superior to that of the organometallic devices and this is correlated to the structural composition of both interconnects via in-situ TEM heating and HAADF-STEM analysis. Annealing of the interconnects is carried out under two different atmospheres in order to reduce the electrical resistivity even further. Finally, chapter 6 presents some important conclusions and summarises each of the previous chapters.

Persistent luminescence of SrAl2O4:Ce(III), Dy, Eu nanotubes, nanowires and core-shells for people with disabilities: nano-emergency

Este estudo transversal está focado na propriedade de luminescência persistente do aluminato de estrôncio co-dopado com cério (III), disprósio (III) e európio (II), SrAl2O4:Ce3+, Dy3+, Eu2+, em sistemas de sinalização de áreas de risco e emergências para pessoas com deficiências. Na área da ciência e engenharia dos materiais, foram desenvolvidos novos materiais com características nanométricas, nanotubos, nanoarames e nanobastões luminescentes de SrAl2O4:Ce3+, Dy3+, Eu2+ para aplicações na área da reabilitação e acessibilidade de pessoas com deficiências. Os nanotubos foram obtidos a partir de micro- e nano-partículas precursoras sintetizadas por reacção do estado-sólido e tratamento térmico de recozedura (1273-1473 K). Os nanoarames e nanobastões foram preparados por moagem, sonificação e recozedura (373 K). Novas nanocápsulas de aluminatos luminescentes dopados com cério (III) e encapsulados com TiO2 foram criadas de modo a obter-se materiais multifuncionais, designadamente com acção fotocatalítica antimicrobiana, antibacteriana e resistência à água. Tais aluminatos podem ser amplamente aplicados como superfícies higiénicas, auto-limpantes, em biomateriais, no domínio de medicamentos antibióticos, na formulação de vacinas, e com ênfase à aplicação em cerâmicas fotoluminescentes. As metodologias de obtenção de tais nanoestruturas de aluminato de estrôncio dopado com cério (III) e do seu encapsulamento, desenvolvidas no âmbito desta tese, são aplicáveis a diversos aluminatos dopados com outros iões lantanídeos (Ln consiste em La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Tm ou Lu) com a fórmula M(1-x-y)N2O4:Cex, Lny, onde M é Be, Mg, Ca, Sr ou Ba. Na área da oftalmologia, foi desenvolvido um equipamento médico para o diagnóstico de biofuncionalidade das células retinais fotoreceptoras, e como suporte à telemedicina oftalmológica. Este equipamento foi utilizado para realizar testes de visão cromática FM100HUE em fundo branco/preto para a personalização de materiais luminescentes. Os resultados demonstraram uma biofuncionalidade celular à visibilidade fotópica das cores em fundo preto superior no grupo de tratamento, composto por pessoas com retinopatia diabética (n=38), em comparação ao grupo de referência (n=38). Estes resultados sugerem a recomendação de materiais com fotoluminescência persistente (λem=485-555 nm), incluindo SrAl2O4:Ce3+, Dy3+, Eu2+, para o referido grupo de tratamento, em sinalização de emergência e em ambientes de baixa iluminação. Na área da arquitectura, foi proposta uma nova aplicação dos referidos nanomateriais luminescentes à base de SrAl2O4:Ce3+, Dy3+, Eu2+ em cerâmica de revestimento, tendo em vista a sua boa visibilidade e uso por pessoas com deficiências. Novos pavimentos, cerâmicos, fotoluminescentes, foram desenhados com propriedades multisensoriais (contraste táctil, sonoro e visual) e antimicrobianas, para pessoas portadoras de deficiências utilizarem, no escuro, com a prioridade de salvar vidas em emergências. Tais pisos, com relevos, podem ser combinados de modo a compor um sistema exclusivo de sinalização fotoluminescente multisensorial que possibilita a rápida evacuação mediante o uso de auxílios de mobilidade (e.g. bengala, cadeira de rodas, andadores, muletas). A solução integrada de tais inovações que potencializa a propriedade de luminescência persistente de SrAl2O4:Ce3+, Dy3+, Eu2+ de modo acessível para as pessoas com deficiências, pode contribuir para salvar vidas, no escuro, em emergências.

On the Growth Mechanism of Nickel and Cobalt Nanowires and Comparison of Their Magnetic Properties

Magnetic nanowires (NWs) are ideal materials for the fabrication of various multifunctional nanostructures which can be manipulated by an external magnetic fi eld. Highly crystalline and textured nanowires of nickel (Ni NWs) and cobalt (Co NWs) with high aspect ratio (~330) and high coercivity have been synthesized by electrodeposition using nickel sulphate hexahydrate (NiSO4·6H2O) and cobalt sulphate heptahydrate (CoSO4·7H2O) respectively on nanoporous alumina membranes. They exhibit a preferential growth along〈110〉. A general mobility assisted growth mechanism for the formation of Ni and Co NWs is proposed. The role of the hydration layer on the resulting one-dimensional geometry in the case of potentiostatic electrodeposition is verified. A very high interwire interaction resulting from magnetostatic dipolar interactions between the nanowires is observed. An unusual low-temperature magnetisation switching for fi eld parallel to the wire axis is evident from the peculiar high fi eld M(T) curve

Computer simulations of copper and gold nanowires and single-wall nanowires

Copper and gold nanowires under tension evolve to form linear atomic chains (LACs), and the study and understanding of this evolution is an important subject for the development of nanocontacts. Here we study the differences and similarities between copper and gold nanowires (NWs) under stress along the [111] crystallographic direction until their rupture using tight-binding molecular dynamics. In both metals, the first significant rearrangement occurs due to one inside atom that goes to the NW` surface. In an attempt to better understand this effect, for both metals we also consider hollow NW`s where the inside atoms were excluded after the initial relaxation to create single-wall NW`s (SWNWs). The dynamical evolution of these SWNWs provides insight on the formation of the constriction that evolves to form LACs. Studying the calculated forces supported by the NW`s we show that SWNWs can sustain larger forces before the first major rearrangement in the copper and gold when compared to the original NW`s.

Structural Effects of Nanotubes, Nanowires, and Nanoporous Ti/TiO2 Electrodes on Photoelectrocatalytic Oxidation of 4,4-Oxydianiline

The present work illustrates the effect of electrolyte composition on the self-organized TiO2 nanotube arrays electrode preparation. The influence of structural and surface morphology of the TiO2 nanotube-like anode on their photoactivity and photoelectrocatalytic performance was also investigated. TiO2 nanotubular array electrodes are grown by anodization of Ti foil in 0.25wt % NH4F/glycerol/water, but nanowires can be obtained in 4% HF-DMSO as supporting electrolyte, even when both are subjected to electrochemical anodization at 30V during 50 h. The morphological characteristics are analyzed by X-ray diffraction (XRD) and field emission scanning electron microscope (FEG-SEM). The electrodes were successfully applied in photoelectrocatalytic oxidation of 4,4'-oxydianiline (ODAN) in aqueous solution, as a model of a harmful pollutant. Complete removal of the aromatic amine was obtained after 3 hours of photoelectrocatalytic treatment on nanotubular arrays electrodes.

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

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