ZnO Nanostructures Synthesized by Vapor Transport and Liquid Phase Synthesis Techniques: Growth and Properties

In this review, we briefly describe work devoted in recent years towards the effective control of morphology, structure and optical properties of ZnO nanostructures, with particular focus on cost effective and simple methods for ZnO nanowires (NWs) fabrication. For the vapor transport technique, we describe in detail mechanisms for growth precursors generation, their transport in inert and forming gas, as well as their reactions on different pretreated substrates and corresponding growth mechanisms. As for low temperature synthesis methods, three techniques are outlined: sol-gel, solvothermal and electrophoretic deposition, with emphasis on effective morphology, structure and optical properties control. In this context, we discuss recent attempts to understand the role of solvent and alkaline agents used during solvothermal synthesis of ZnO nanostructures on their morphology and photoluminescence properties. Recent success of electrophoretic deposition of ZnO nanoparticles on pre-patterned silicon substrates in the form of NWs and NW bunches is highlighted over many previous attempts to fabricate ZnO NWs with inconvenient sacrificial templates. Finally, we present a critical discussion on the current understanding of passivation mechanisms of ZnO NW surfaces by MgO shells.Fil: Marín Ramírez, Oscar Alonso. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; ArgentinaFil: Real, Silvina Claudia. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; ArgentinaFil: Vega, Nadia Celeste. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; ArgentinaFil: Tirado, Monica Cecilia. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; ArgentinaFil: Comedi, David Mario. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; Argentin

Reaction pathways and catalyst phase transformations during synthesis of nanotubes and nanowires by the vapor-liquid-solid mechanism.

The discovery of carbon nanotubes and semiconductor nanowires has initiated an exploding research field in which enormous efforts have been invested due to their fundamental significance to the study of size- and dimensionality-dependent chemical and physical properties. Extensive research work is now focused on the study of the processes that occur during synthesis of these materials since they are key essentials needed to establish a method that provides large quantities of nanotubes and nanowires with well-determined and reproducible characteristics. A clear understanding of their formation mechanism is therefore a key issue for the development of further advances in this topic.This thesis is focused on the study of the processes that take place on the catalytic particle during the growth of SWNT and SiNW using VSL processes over a cobalt molybdate and gold supported catalyst respectively. A series of analytical techniques have been employed to characterize the structure of the materials obtained and to appraise the physicochemical state of the catalytic particle. The state of Co and Mo has been investigated using laser Raman spectroscopy, X-ray photoelectron spectroscopy and Transmission electron microscopy. The chemical state of gold was studied using extended X-ray absorption fine structure spectroscopy, X-ray absorption near-edge spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. In a similar way, the SWNT and SiNW materials have been characterized using laser Raman spectroscopy, transmission electron microscopy, temperature programmed oxidation, scanning electron microscopy and optical absorption spectroscopy. The information obtained revealed that in for both SiNW and SWNT different synthesis conditions result in regimes in which different controlling rate-limiting steps determine the quantity and quality of the products obtained.To get a deeper insight onto the kinetics of SWNT growth a detailed electron microscopy study was performed. By using a model system based on silica coated TEM grids as substrate, SWNT were grown under similar reaction condition as in the commercial CoMoCatRTM process. The observations from the model system showed that CO and CH4 have different reactivity towards the cobalt molybdate catalytic particles species. Phase separation process of the pre-reduced cobalt molybdate species was observed when CO was used as feed. However, in the case of CH4 a complex alloy seems to be formed on the pre-reduced cobalt molybdate before the growth of SWNTs starts, in this case a phase separation process was not observed. These phenomena were used to explain the differences in behavior of real powder catalysts under different reaction conditions reported on previous literature. Moreover it is demonstrated how this processes affect nanotube quality, carbon yield and the chiral distribution of nanotubes structures. Indeed the (n, m) population distribution of single-walled carbon nanotubes can be controlled by varying the gaseous feed composition and the reaction temperature. The clearly different distributions obtained when varying reaction conditions demonstrate that the (n, m) distribution is a result of differences in the growth kinetics, which in turn depends on the nanotube cap-metal cluster interaction

Synthesis and Testing of Tin-oxide Nanowires for Flux Pinning in Superconductors

In this work, we explore the application of pre-fabricated nanowires in order to pin magnetic flux lines in superconducting films. We have synthesized SnO2 nanowires in a chemical vapor deposition (CVD) process using Au colloids and sputter coating on flexible CeO2 substrates. Several growth methods are explored in order to attain full control over nanowire arrays and reduce surface defects. For microstructure modification, we employed ion bombardment which can straighten nanowires as well as remove surface defects. With the optimized growth processes, the results show reductions in surface defects and increases in uniformity and repeatability. Superconducting films were grown on the nanowire-embedded substrates using trifluoroacetate-based metal-organic deposition (TFA-MOD) and metal-organic chemical vapor deposition (MOCVD) systems. Several optimizations were performed on the TFA-MOD growth process. After deposition of the films, decent critical current (Ic) values were attained from samples with pre-fabricated nanowires in the B || c field direction, strongly indicating flux pinning.Mechanical Engineering, Department o

Controlled Synthesis and Characterization of Metal Oxide Nanowires by Chemical Vapor Deposition on Silicon and Carbon Substrates

Nanotechnology and nanomaterials have attracted considerable interest and are predicted to revolutionize many materials and technologies that we use in everyday life. In the past few years, significant research has focused on one dimensional metal oxide nanostructures due to their unique properties and potential applications in various fields from nanoelectronics to energy. However, controlled synthesis of these nanostructures is still a challenge. The objective of this thesis is to synthesize metal oxide nanowires by chemical vapour deposition directly on various substrates. The nanostructures include (i) silicon oxide nanostructures on silicon substrate, (ii) manganese oxide nanostructures on silicon substrate, and (iii) manganese oxide nanostructures on carbon paper substrate. Firstly, silicon oxide nanowires were synthesized on silicon substrate by a VO2 assisted chemical vapor deposition. Networked features of silicon oxide nanowires were found. Systematic study on the nanowire growth has indicated that morphology and composition of the final products are considerably sensitive to the catalyst components, reaction atmosphere and temperature. These results will help in better understanding the growth process of silicon oxide nanowires. Secondly, manganese oxide nanostructures were synthesized on silicon substrate by chemical vapor deposition method. It was found that MnO nanowires are high density and single crystalline with average diameter of 150 nm. These nanowires were characterized using FESEM, EDX, TEM and XRD. The synthesis process and effects of growth parameters such as temperature, heating rate and source/substrate distance on the morphology, composition and structure of the products were systematically studied. Finally, manganese oxide nanostructures were synthesized on carbon paper substrate by chemical vapor deposition method. It was revealed that manganese oxide nanowires and nanobelts can be selectively grown on carbon paper substrate by using a catalyst (gold) assisted or catalyst free thermal evaporation of manganese powder under an argon gas atmosphere. Various effects of growth parameters such as temperature, catalyst and buffered substrate on the growth product were also systematically investigated by using SEM, TEM and XPS

Controlled Synthesis and Characterization of Metal Oxide Nanowires by Chemical Vapor Deposition on Silicon and Carbon Substrates

Nanotechnology and nanomaterials have attracted considerable interest and are predicted to revolutionize many materials and technologies that we use in everyday life. In the past few years, significant research has focused on one dimensional metal oxide nanostructures due to their unique properties and potential applications in various fields from nanoelectronics to energy. However, controlled synthesis of these nanostructures is still a challenge. The objective of this thesis is to synthesize metal oxide nanowires by chemical vapour deposition directly on various substrates. The nanostructures include (i) silicon oxide nanostructures on silicon substrate, (ii) manganese oxide nanostructures on silicon substrate, and (iii) manganese oxide nanostructures on carbon paper substrate. Firstly, silicon oxide nanowires were synthesized on silicon substrate by a VO2 assisted chemical vapor deposition. Networked features of silicon oxide nanowires were found. Systematic study on the nanowire growth has indicated that morphology and composition of the final products are considerably sensitive to the catalyst components, reaction atmosphere and temperature. These results will help in better understanding the growth process of silicon oxide nanowires. Secondly, manganese oxide nanostructures were synthesized on silicon substrate by chemical vapor deposition method. It was found that MnO nanowires are high density and single crystalline with average diameter of 150 nm. These nanowires were characterized using FESEM, EDX, TEM and XRD. The synthesis process and effects of growth parameters such as temperature, heating rate and source/substrate distance on the morphology, composition and structure of the products were systematically studied. Finally, manganese oxide nanostructures were synthesized on carbon paper substrate by chemical vapor deposition method. It was revealed that manganese oxide nanowires and nanobelts can be selectively grown on carbon paper substrate by using a catalyst (gold) assisted or catalyst free thermal evaporation of manganese powder under an argon gas atmosphere. Various effects of growth parameters such as temperature, catalyst and buffered substrate on the growth product were also systematically investigated by using SEM, TEM and XPS

Silicon-basedI nanostructures: Growth and Characterizations of Si2Te3 nanowires and nanoplates

Silicon-basedI nanostructures: Growth and Characterizations of Si2Te3 nanowires and nanoplate

Inorganic and Organic Photovoltaic Materials for Powering Electrochromic Systems

abstract: ABSTRACT Autonomous smart windows may be integrated with a stack of active components, such as electrochromic devices, to modulate the opacity/transparency by an applied voltage. Here, we describe the processing and performance of two classes of visibly-transparent photovoltaic materials, namely inorganic (ZnO thin film) and fully organic (PCDTBT:PC70BM), for integration with electrochromic stacks. Sputtered ZnO (2% Mn) films on ITO, with transparency in the visible range, were used to fabricate metal-semiconductor (MS), metal-insulator-semiconductor (MIS), and p-i-n heterojunction devices, and their photovoltaic conversion under ultraviolet (UV) illumination was evaluated with and without oxygen plasma-treated surface electrodes (Au, Ag, Al, and Ti/Ag). The MS Schottky parameters were fitted against the generalized Bardeen model to obtain the density of interface states (Dit ≈ 8.0×1011 eV−1cm−2) and neutral level (Eo ≈ -5.2 eV). These devices exhibited photoconductive behavior at λ = 365 nm, and low-noise Ag-ZnO detectors exhibited responsivity (R) and photoconductive gain (G) of 1.93×10−4 A/W and 6.57×10−4, respectively. Confirmed via matched-pair analysis, post-metallization, oxygen plasma treatment of Ag and Ti/Ag electrodes resulted in increased Schottky barrier heights, which maximized with a 2 nm SiO2 electron blocking layer (EBL), coupled with the suppression of recombination at the metal/semiconductor interface and blocking of majority carriers. For interdigitated devices under monochromatic UV-C illumination, the open-circuit voltage (Voc) was 1.2 V and short circuit current density (Jsc), due to minority carrier tunneling, was 0.68 mA/cm2. A fully organic bulk heterojunction photovoltaic device, composed of poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyli2’,1’,3’-benzothiadiazole)]:phenyl-C71-butyric-acidmethyl (PCDTBT:PC70BM), with corresponding electron and hole transport layers, i.e., LiF with Al contact and conducting/non-conducting (nc) PEDOT:PSS (with ITO/PET or Ag nanowire/PDMS contacts; the illuminating side), respectively, was developed. The PCDTBT/PC70BM/PEDOT:PSS(nc)/ITO/PET stack exhibited the highest performance: power conversion efficiency (PCE) ≈ 3%, Voc = 0.9V, and Jsc ≈ 10-15 mA/cm2. These stacks exhibited high visible range transparency, and provided the requisite power for a switchable electrochromic stack having an inkjet-printed, optically-active layer of tungsten trioxide (WO3), peroxo-tungstic acid dihydrate, and titania (TiO2) nano-particle-based blend. The electrochromic stacks (i.e., PET/ITO/LiClO4/WO3 on ITO/PET and Ag nanowire/PDMS substrates) exhibited optical switching under external bias from the PV stack (or an electrical outlet), with 7 s coloration time, 8 s bleaching time, and 0.36-0.75 optical modulation at λ = 525 nm. The devices were paired using an Internet of Things controller that enabled wireless switching.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201