Dissertations / Theses on the topic 'Dioxide de vanadium'
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1
Thery, Virginie. "Etude de la microstructure et des transitions de phases électroniques et cristallines de couches épitaxiales de VO₂ déposées sur différents substrats." Thesis, Limoges, 2017. http://www.theses.fr/2017LIMO0059/document.
Abstract:
Les travaux de recherche présentés dans ce manuscrit concernent l'étude du rôle des déformations (épitaxiale et d'origine thermique) sur les transitions structurales et électriques du dioxyde de vanadium. A cet effet, nous avons synthétisé des films minces de VO₂ par évaporation à faisceau d'électrons et par ablation laser. La géométrie des déformations est contrôlée en modifiant, d'une part, la nature des substrats et, d'autre part, l'épaisseur des dépôts. Dans le cas de la croissance sur des substrats de saphir (Al₂ O₃ ) orientés (001), le fort désaccord de réseau entraîne une croissance par coïncidence de domaine, de sorte que les déformations résiduelles résultent exclusivement du désaccord de coefficient de dilatation entre la couche et le substrat. L'étude de la transition structurale par diffraction des rayons X et l'étude de la résistivité électrique via un dispositif 4 pointes ont montré que la déformation en tension selon l'axe cᵣ conduit à une augmentation de la température de transition (> 68◦ C). L'apparition d'une phase intermédiaire a été mise en évidence au cours de l'étude structurale en température. La croissance sur des substrats de TiO₂ orientés (001) et (111) est caractérisée par un désaccord de réseau de plus faible (∼ 1%) avec une épaisseur critique de 4 nm, à partir de laquelle des dislocations sont créées en vue de relaxer l'énergie élastique. L'étude des transitions électriques et structurales a mis en évidence que l'évolution des transitions résulte d'une compétition entre les déformations épitaxiales, les déformations d'origine thermique et la présence de lacunes d'oxygène à l'interfaceThe research presented in this manuscript deals the study of the effect of strain (epitaxial or thermal) on the structural and the electrical transitions of vanadium dioxide. VO₂ thin films have been synthesized by e-beam deposition and Pulsed Laser Deposition methods. The strain geometry is controlled by modifying, on the one hand, the nature of the substrates and, on the other hand, the thickness of thin films. In the case of (001) sapphire substrates (Al₂ O₃ ), the important lattice mismatch leads to a domain matching epitaxial growth mechanism, so that the residual strain solely result from the film/substrate thermal expansion mismatch. The study of the structural phase transition, using X-ray diffraction, and the study of the metal-insulator transition, using a 4-probes device, showed that the tensile strain along the cᵣ axis leads to an increase of the transition temperature (> 68◦ C). The appearance of an intermediate phase was demonstrated during the study of the structural phase transition. Growth on (001)- and (111)-TiO₂ substrates is characterized by a weaker lattice mismatch (∼ 1%), with a critical thickness of 4 nm, from which dislocations are created to relax the elastic energy. The study of electrical and structural transitions has shown that the evolution of transitions results from a competition between epitaxial distorsion, thermal distorsions and the presence of oxygen vacancies at the interface
2
Pan, Kuan-Chang. "Vanadium Dioxide Based Radio Frequency Tunable Devices." University of Dayton / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=dayton154341840843132.
3
Safi, Taqiyyah(Taqiyyah Sariyah). "Tunable spin-charge conversion in vanadium dioxide." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/122767.
Abstract:
Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 52-58).
Spin-based electronic devices rely on the interplay of spin and charge degree of freedom of electrons and are emerging as a promising beyond CMOS technology. Fast, scalable, low energy consumption magnetic memories and nonvolatile spin logic devices have been demonstrated utilizing spin-orbit-torque based magnetization switching. A large, pure spin current is crucial for these applications and significant effort is geared towards finding materials with large charge-to-spin conversion efficiency to exploit the full potential of spintronics. The charge-to-spin conversion efficiency is an inherent property of the spintronics materials and cannot be easily modified without changing the chemical or structural properties of the material. To date most of the explored materials, have significant electrical conductivity and are in their pure, stable, intrinsic structural form. Most importantly, they exhibit negligible variation in the electrical and structural properties.
In this thesis we investigate spin-charge conversion efficiency in the transition metal oxide, vanadium dioxide (VO₂), which exhibits structural phase transition subject to external stimuli. We demonstrate tunable charge-to-spin conversions in this material across the phase transition. Vanadium dioxide is a prototypical metal-insulator transition material and has the unique property of a dramatic and abrupt structural phase transition under external stimuli such as heat, strain, and electric field etc. Due to its unique properties, it has gained much interest from both fundamental research and applications perspective but its spin related properties remain largely unexplored. In this thesis, we demonstrate the successful tuning of charge-spin conversion efficiency via the metal-insulator transition in this quintessential strongly correlated electron compound.
We inject a pure spin current through ferromagnetic resonance driven spin pumping and measure the temperature dependent inverse spin Hall effect voltage across VO₂ We found a swift, dramatic change in the spin pumping signal (decrease by >80%) and charge-spin conversion efficiency (increase by five times) upon transition. The swift, dramatic change in the structural and electrical properties of this material therefore provides additional knobs to modulate the conversion efficiency. This work leads to extra flexibilities in spintronic device design and opens up new avenues for variable spintronics.
by Taqiyyah Safi.
S.M.
S.M. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science
4
Meling, Artur. "Scattering of vibrationally excited NO from vanadium dioxide." Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2020. http://hdl.handle.net/21.11130/00-1735-0000-0005-12F9-E.
5
Gaudin, Michael. "Ablation laser impulsionnelle : source de nanoparticules en vol et de films minces : Développement de matériaux nanostructurés à base d'argent, de vanadium et de dioxyde de vanadium." Thesis, Limoges, 2017. http://www.theses.fr/2017LIMO0025/document.
Abstract:
Ces travaux de thèse portent sur le développement d’un dispositif de synthèse de nanoparticules (NPs) par une voie physique basée sur la pulvérisation laser d’une cible suivie d’une trempe du panache plasma ainsi formé. L’association de cette source à une enceinte d’ablation laser conventionnelle a permis de synthétiser des NPs d’argent et de vanadium empilées sur des substrats ou noyées dans des matrices synthétisées par ablation laser. Des analyses par microscopie électronique en transmission (MET) et microscope à force atomique (AFM) ont révélé des NPs cristallisées en vol, de forme sphérique et de tailles relativement monodisperses (~ 3 nm de diamètre) fonction de leur temps de séjour dans la cavité de nucléation. La réalisation de nanocomposites Al2O3 amorphe dopée par des NPs d’argent métallique de différentes tailles a montré l’influence de la taille de ces entités nanométriques sur la position et la largeur de la résonance plasmon de surface (RPS) du matériau nanostructuré. Les NPs gardent leur forme originelle après impact sur le substrat ce qui conduit à des empilements de nanoparticules fortement poreux (de l’ordre de 50%). Des NPs de dioxyde de vanadium ont pu être synthétisées par recuit d’empilements de NPs de vanadium. Du fait de leur individualité, les NPs de VO2 présentent une température de transition plus faible (~50°C) et une largeur d’hystérésis plus importante (~10-30°C) qu’un film mince (température de transition d’environ 68°C et largeur d’hystérésis d’environ 3°C). En associant un film mince synthétisé par PLD à un empilement de NPs il est alors possible de combiner leurs propriétés et d’obtenir un matériau nanocomposite présentant une transition par palierThe work presented in this thesis is focused on the development of an experimental setup for the synthesis of nanoparticles (NPs) by a physical route, based on the laser vaporization of a target and followed by the rapid quenching of the plasma plume. Combining such a NP source with conventional laser ablation makes possible to synthesize silver and vanadium NPs in stacks on substrates or embedded in different matrices synthesized by laser ablation. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) analysis revealed crystallized spherical NPs relatively monodisperse in size (~ 3 nm in diameter) depending on the residence time in the nucleation cavity. The synthesis of amorphous Al2O3 nanocomposites doped with metallic silver NPs of different sizes showed the influence of the size on the position and the width of the surface plasmon resonance (SPR) of the nanostructured material. The NPs keep their original shape during impact on the substrate, leading to highly porous NPs stacks (approximately 50%). Vanadium dioxide nanoparticles (VO2 NPs) have been synthesized by annealing vanadium NPs stacks. Due to their individual behaviour, VO2NPs exhibit lower transition temperature (~ 50°C) and larger hysteresis width (~ 10-30°C) than thin films (transition temperature around 68°C and hysteresis width around 3°C). By coupling a PLD thin film and a NPs stack, it is possible to combine their properties and obtain a nanostructured material having a step transition
6
Huffman, Tyler J. "Shining Light on The Phase Transitions of Vanadium Dioxide." W&M ScholarWorks, 2017. https://scholarworks.wm.edu/etd/1499450049.
Abstract:
The salient feature of the familiar structural transition accompanying the thermally-driven metal-insulator transition in bulk vanadium dioxide (VO2) is a pairing of all the vanadium ions in the monoclinic M¬1 insulating phase. Whether this pairing (unit cell doubling) alone is sufficient to open the energy gap has been the central question of a classic debate which has continued for almost sixty years. Interestingly, there are two less familiar insulating states, monoclinic M2 and triclinic, which are accessible via strain or chemical doping. These phases are noteworthy in that they exhibit distinctly different V-V pairing. With infrared and optical photon spectroscopy, we investigate how the changes in crystal structure affect the electronic structure. We find that the energy gap and optical inter-band transitions are insensitive to changes in the vanadium-vanadium pairing. This result is confirmed by DFT+U and HSE calculations. Hence, our work conclusively establishes that intra-atomic Coulomb repulsion between electrons provides the dominant contribution to the energy gap in all insulating phases of VO2. VO2 is a candidate material for novel technologies, including ultrafast data storage, memristors, photonic switches, smart windows, and transistors which move beyond the limitations of silicon. The attractiveness of correlated materials for technological application is due to their novel properties that can be tuned by external factors such as strain, chemical doping, and applied fields. For advances in fundamental physics and applications, it is imperative that these properties be measured over a wide range of regimes. Towards this end, we study a single domain VO2 crystal with polarized light to characterize the anisotropy of the optical properties. In addition, we study the effects of compressive strain in a VO2 thin film in which we observe remarkable changes in electronic structure and transition temperature. Furthermore, we find evidence that electronic correlations are active in the metallic rutile phase as well. VO2 films exhibit phase coexistence in the vicinity of the metal-insulator transition. Using scanning near-field infrared microscopy, we have studied the patterns of phase coexistence in the same area on repeated heating and cooling cycles. We find that the pattern formation is reproducible each time. This is an unexpected result from the viewpoint of classical nucleation theory that anticipates some degree of randomness. The completely deterministic nature of nucleation and growth of domains in a VO2 film with imperfections is a fundamental finding. This result also holds promise for producing reliable nanoscale VO2 devices.
7
Madaras, Scott. "Insulator To Metal Transition Dynamics Of Vanadium Dioxide Thin Films." W&M ScholarWorks, 2020. https://scholarworks.wm.edu/etd/1616444322.
Abstract:
Vanadium Dioxide (VO2) is a strongly correlated material which has been studied for many decades. VO2 has been proposed for uses in technologies such as optical modulators, IR modulators, optical switches and Mott memory devices. These technologies are taking advantage of VO2’s insulator to metal transition (IMT) and the corresponding changes to the optical and material properties. The insulator to metal transition in VO2 can be accessed by thermal heating, applied electric field, or ultra-fast photo induced processes. Recently, thin films of VO2 grown on Titanium Dioxide doped with Niobium (TiO2:Nb), have shown promise as a possible UV photo detector with high quantum efficiency which utilizes a heterostructure between these two materials. In this work, the dynamics of the IMT on thin films of VO2 is explored. We show that surface plasmons generated in an Au thin film can induce the insulator to metal transition in a thin film of VO2 due to the enhanced electric field as well as help detect the IMT via changes in its resonance condition. Time resolved pump probe studies were also done on thin films of VO2 grown on TiO2 and TiO2:Nb, using UV photon energy of 3.1 eV (400nm wavelength). The fluence threshold of the IMT at 3.1 eV was significantly lower than published values for the 1.55 eV pump fluence. The time response of the IMT shows uncommon reflectivity dynamics in these samples. The response was partially attributed to internal interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2, and can be elucidated by a diffusion model with respect to its optical properties. Finally, the photocurrent generation time constants for the sample with highest quantum efficiency are given and compared to its ultrafast photo induced IMT time constants.
8
Rivera, Felipe. "Electron Microscopy Characterization of Vanadium Dioxide Thin Films and Nanoparticles." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/2975.
Abstract:
Vanadium dioxide (VO_2) is a material of particular interest due to its exhibited metal to insulator phase transition at 68°C that is accompanied by an abrupt and significant change in its electronic and optical properties. Since this material can exhibit a reversible drop in resistivity of up to five orders of magnitude and a reversible drop in infrared optical transmission of up to 80%, this material holds promise in several technological applications. Solid phase crystallization of VO_2 thin films was obtained by a post-deposition annealing process of a VO_{x,x approx 2} amorphous film sputtered on an amorphous silicon dioxide (SiO_2) layer. Scanning electron microscopy (SEM) and electron-backscattered diffraction (EBSD) were utilized to study the morphology of the solid phase crystallization that resulted from this post-deposition annealing process. The annealing parameters ranged in temperature from 300°C up to 1000°C and in time from 5 minutes up to 12 hours. Depending on the annealing parameters, EBSD showed that this process yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. In addition to these films on SiO_2, other VO_2 thin films were deposited onto a-, c-, and r-cuts of sapphire and on TiO_2(001) heated single-crystal substrates by pulsed-laser deposition (PLD). The temperature of the substrates was kept at ~500°C during deposition. EBSD maps and orientation imaging microscopy were used to study the epitaxy and orientation of the VO_2 grains deposited on the single crystal substrates, as well as on the amorphous SiO_2 layer. The EBSD/OIM results showed that: 1) For all the sapphire substrates analyzed, there is a predominant family of crystallographic relationships wherein the rutile VO_2{001} planes tend to lie parallel to the sapphire's {10-10} and the rutile VO_2{100} planes lie parallel to the sapphire's {1-210} and {0001}. Furthermore, while this family of relationships accounts for the majority of the VO_2 grains observed, due to the sapphire substrate's geometry there were variations within these rules that changed the orientation of VO_2 grains with respect to the substrate's normal direction. 2) For the TiO_2, a substrate with a lower lattice mismatch, we observe the expected relationship where the rutile VO_2 [100], [110], and [001] crystal directions lie parallel to the TiO_2 substrate's [100], [110], and [001] crystal directions respectively. 3) For the amorphous SiO_2 layer, all VO_2 crystals that were measurable (those that grew to the thickness of the deposited film) had a preferred orientation with the the rutile VO_2[001] crystal direction tending to lie parallel to the plane of the specimen. The use of transmission electron microscopy (TEM) is presented as a tool for further characterization studies of this material and its applications. In this work TEM diffraction patterns taken from cross-sections of particles of the a- and r-cut sapphire substrates not only solidified the predominant family mentioned, but also helped lift the ambiguity present in the rutile VO_2{100} axes. Finally, a focused-ion beam technique for preparation of cross-sectional TEM samples of metallic thin films deposited on polymer substrates is demonstrated.
9
Kumar, Sachin. "Gas Phase Oxidation of Dimethyl Sulfide by Titanium Dioxide Based Catalysts." Miami University / OhioLINK, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=miami1081780904.
10
Vernardou, Dimitra. "The growth of thermochromic vanadium dioxide films by chemical vapour deposition." Thesis, University of Salford, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.419082.
11
Creeden, Jason Andrew. "Growth Engineering And Characterization Of Vanadium Dioxide Films For Ultraviolet Detection." W&M ScholarWorks, 2020. https://scholarworks.wm.edu/etd/1593091709.
Abstract:
There is a need for efficient ultraviolet (UV) detectors in many fields, such as aerospace, automotive manufacturing, biology, environmental science, and defense, due to photomultiplier tubes (the currently available technology) often not meeting application constraints in weight, robustness, and power consumption. In my thesis, I demonstrate that high quality vanadium dioxide (VO2) thin films, epitaxially grown on niobium doped titanium dioxide substrates (TiO2:Nb), display a strong photoconductive response in the UV spectral range, making them promising candidates for photomultiplier-free UV photodetection. By adjusting the characteristics of the substrate and VO2 film, the samples achieve external quantum efficiency exceeding 100% (reaching beyond 1,000% for optimized samples) superior to that of current wide band gap UV detectors at room temperature. The mechanism for photocurrent production in VO2/TiO2:Nb heterostructure is a space-charge region, engineered in the heterojunction, yielding favorable conditions for hole tunneling from TiO2:Nb into VO2. Improving upon the heterostructure, I demonstrate up to an order of magnitude improvement in parameters such as responsivity, external quantum efficiency, detectivity, and dark current density by applying Au films to the VO2/TiO2:Nb heterostructure. Ultimately, my work proved that the VO2/TiO2:Nb heterostructure is a promising alternative technology for UV detection in high demand fields, with great potential for scalable device production.
12
Xiao, Wanyao. "Asymmetric Capacitor Based on Vanadium Dioxide/Graphene/Nickle and Carbon Nanotube Electrode." University of Akron / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1396870651.
13
Mandiwana, KL, and N. Panichev. "The leaching of vanadium(V) in soil due to the presence of atmospheric carbon dioxide and ammonia." Elsevier, 2009. http://encore.tut.ac.za/iii/cpro/DigitalItemViewPage.external?sp=1001493.
Abstract:
The natural leaching of vanadium(V) with CO2 from soil-water in the presence of ammonia, a known
precursor to atmospheric aerosols, has been tested by bubbling carbon dioxide through soil suspension
with varying amount of ammonia. It was found that the leaching of V(V) is enhanced in the presence
of ammonia. From the results of the investigation, it could be concluded that atmospheric CO2 in the
presence of ammonia (the only atmospheric gas that increases the pH of soil-water) could naturally leach
V(V) from soil. Furthermore, it was also shown that the presence of (NH4)2CO3 in soil could enhance the
leaching of toxic V(V) species thereby making it bioavailable for both plants and animals.
14
Rivera, Felipe. "Solid Phase Crystallization of Vanadium Dioxide Thin Films and Characterization Through Scanning Electron Microscopy." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd2233.pdf.
15
García, Wong Alexis. "Oxidation of vanadium nitrides thin films : a novel method to form efficient thermochromic VO₂." Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0098.
Abstract:
Dans cette thèse, nous étudions la formation de dioxyde de vanadium (VO₂) à partir d'un procédé en deux étapes. Nous démontrons pour la première fois que l'oxydation contrôlée de films polycristallins VN et V₂N déposés par pulvérisation cathodique conduit à la formation de VO₂ thermochromique de haute qualité. L'optimisation de la durée du recuit s'avère fondamentale pour former la phase désirée. VO₂ et V₂O₅ sont, en général, les seules phases formées lors du processus d'oxydation des films polycristallins de VN et V₂N. Le premier oxyde obtenu est VO₂(M), qui coexiste avec le précurseur azoté pendant une période prolongée à des températures comprises entre 450 et 550°C. Les performances thermochromiques des films oxydés sont affectées lorsque l'oxydation n'est pas complète ou lorsqu’elle est totale en raison de la formation de V₂O₅ pour des temps de recuit dépassant la valeur optimale. L'analyse MET indique que les deux précurseurs suivent des cinétiques d'oxydation différentes. Concernant l'oxydation de VN, il existe une interface abrupte entre les films polycristallins de VN et le VO₂ formé. D'autre part, les résultats EELS pour l'oxydation de V₂N mettent en évidence la formation d'une couche de VN à l'interface entre la couche de V₂N restante et la couche de VO₂ en croissance. De plus, l'influence de l'épaisseur de V₂N sur le signe de la variation d’émissivité de VO₂(M) formé a été étudiée. L'oxydation de films de V₂N de 100 nm d’épaisseur déposés sur un substrat en Si donne un changement d'émissivité négatif (Δε). Au contraire, le recuit d'échantillons de V₂N de 445 nm produit une valeur positive pour Δε. Ces résultats indiquent pour la première fois qu’il est possible de contrôler le signe de la variation d’émissivité de VO₂ formé à partir de l'épaisseur initiale du précurseur V₂N. Par ailleurs, nous avons également étudié l'oxydation des couches minces épitaxiées de VN, qui suivent un chemin différent des films polycristallins de VN, causé par l'absence de joints de grains dans les couches épitaxiées. Ce processus conduit à la croissance épitaxiée d'une couche de VO₂(B) sur le VN restant, comme le révèlent les résultats TEM. Enfin, nous avons appliqué un critère simple pour transposer les conditions de dépôt entre les deux machines de pulvérisation utilisées durant cette thèse à partir des performances thermochromiques des filmsIn this thesis, we study the formation of vanadium dioxide (VO₂) from a two-step procedure. We demonstrate for the first time that high-quality thermochromic VO₂ can be obtained from the air oxidation of sputter-deposited VN and V₂N polycrystalline films. Depending on the annealing conditions (duration and temperature) VO₂ and V₂O₅ are the two principal phases formed during the oxidation process of the nitride films. The first oxide obtained is VO₂(M), which coexists with the precursor for a prolonged period at 450°C or suddenly disappeared at 525 and 550°C. For optimized durations at each temperature VO₂(M) is the predominant phase. For higher durations, the VO₂ phase is oxidized into V₂O₅. The thermochromic performance of the vanadium oxide films is strongly influenced by the oxidation duration. TEM analysis indicates that both VN and V₂N precursors follow different oxidation kinetics. Concerning the VN oxidation, there is an abrupt interface between the polycrystalline VN films and the formed VO₂. On the other hand, EELS results for the V₂N oxidation evidence a formation of a VN layer at the interface between the remaining V₂N layer and the growing VO₂ one. Besides, the influence of the V₂N thickness on the optical modulation properties of the formed VO₂(M) is studied. The oxidation of 100 nm-thick V₂N films on Si substrate yields a negative emissivity switch (Δε). On the contrary, the annealing of 445 nm V₂N samples produces a positive value for Δε. These findings indicate the possibility to control the sign of the optical modulation of the formed VO₂ depending on the initial thickness of the V₂N precursor. Furthermore, we investigate the oxidation of epitaxial VN thin films, which follow a different path than polycrystalline VN films, caused by the absence of grain boundaries in the epitaxial layers. This process depicts the epitaxial growth of a VO₂(B) layer on the remaining VN, as shown by TEM results. Finally, we apply a simple criterion to transpose the deposition conditions between the two sputtering machines used in the thesis based on the thermochromic performance
16
Sechogela, Thulaganyo P. "Vanadium dioxide nanocomposite thin film embedded in zinc oxide matrix as tunable transparent conductive oxide." Thesis, University of the Western Cape, 2013. http://hdl.handle.net/11394/4529.
Abstract:
Philosophiae Doctor - PhDThis project is aimed at fabricating a smart material. Zinc oxide and vanadium dioxide have received a great deal of attention in recent years because they are used in various applications. ZnO semiconductor in particular has a potential application in optoelectronic devices such as light emitting diodes (LED), sensors and in photovoltaic cell industry as a transparent electrode. VO2 also has found application in smart windows, solar technology and infrared smart devices. Hence the need to synthesis or fabricate a new smart material using VO2 and an active ZnO based nano-composites family in which ZnO matrix will be hosting thermally active VO2 nano-crystals is the basis of this study. Since VO2 behave as an MIT Mott’s type oxides and exhibits a thermally driven semiconductor-metal phase transition at about 68 oC and as a direct result ZnO:VO2 nano-composites would exhibit a reversible and modulated optical transmission in the infra-red (IR) while maintaining a constant optical transmission in the UV-Vis range. The synthesis is possible by pulsed laser deposition and ion implantation. Synthesis by pulsed laser deposition will involve thin films multilayer fabrication. ZnO buffer layer thin film will be deposited on the glass and ZnO single crystals and subsequent layer of VO2 and ZnO will be deposited on the substrate. X-ray diffraction (XRD) reveals that the series of ZnO thin films deposited by Pulsed Laser Deposition (PLD) on glass substrates has the hexagonal wurtzite structure with a c-axis preferential orientation. In addition the XRD results registered for VO2 samples indicate that all thin films exhibits a monoclinic VO2 (M) phase. UV-Vis NIR measurements of multilayered structures showed the optical tunability at the near-IR region and an enhanced transparency (>30 %) at the visible range.
17
Simo, Aline. "Physical properties of vanadium dioxide nanoparticles: application as 1-d nanobelts room temperature for hydrogen gas sensing." Thesis, University of the Western Cape, 2013. http://hdl.handle.net/11394/4581.
Abstract:
Philosophiae Doctor - PhDTransition metal oxides magneli phases present crystallographic shear structure which is of great interest in multiple applications because of their wide range of valence, which is exhibited by the transition metals. The latter affect chemical and physical properties of the oxides. Amongst them we have nanostructures VO2 system of V and O components which are studied including chemical and physical reactions based on non-equilibrium thermodynamics. Due to their structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure, these oxides are generally used as catalytic materials which are prepared by common methods under mild conditions presenting distortion or defects in the case of VO2. Existence of an intermediate phase is proved using an x-ray thermodiffraction experiment providing structural information as the nanoparticles are heated. Potential application as gas sensing device has been the first time obtained due to the high surface to volume ratio, and good crystallinity, purity of the material and presence of suitable nucleating defects sites due to its n-type semiconductor behavior. In addition, annealing effect on nanostructures VO2 nanobelts shows a preferential gas reductant of Ar comparing to the N2 gas. Also, the hysteresis loop shows that there is strong size dependence to annealing treatment on our samples. This is of great interest in the need of obtaining high stable and durable material for Mott insulator transistor and Gas sensor device at room temperature.
18
Madiba, Itani Given. "Thermochromic properties of VO2 nano-coatings by inverted cylindrical magnetron sputtering." Thesis, University of the Western Cape, 2012. http://hdl.handle.net/11394/4381.
Abstract:
>Magister Scientiae - MScVanadium dioxide (VO2) films have been known as the most feasible thermochromic nano-coatings for smart windows which self control the solar radiation and heat transfer for energy saving and comfort in houses and automotives. Such an attractive technological application is due to the fact that VO2 crystals exhibit a fast semiconductor-to-metal phase transition at a transition temperature TM of about 68°C, together with sharp optical changes from high transmitive to high reflective coatings in the IR spectral region. The phase transition has been associated to the nature of the microstructure, stoichiometry and some other surrounding parameters of the oxide. This study reports on the effect of the crystallographic quality controlled by the substrate temperature on the thermochromic properties of VO2 thin films synthesized by inverted cylindrical magnetron sputtering. Vanadium dioxide thin films were deposited on glass substrate, at various temperatures between 350 to 600 0C, deposition time kept constant at 1 hour. Prior the experiment, deposition conditions such as base pressure, oxygen pressure, rf power and target-substrate distance were carefully optimized for the quality of VO2 thin films. The reports results are based on AFM, XRD, RBS, ERDA and UV-VIS. The atomic force microscopy (AFM) was used to study the surface roughness of the thin films. Microstructures and orientation of grain size within the VO2 thin films were investigated by the use of X-ray diffraction technique. The stoichiometry and depth profiles of the films were all confirmed by RBS and ERDA respectively. The optical properties of VO2 were observed using the UV-Vis spectrophotometer.
19
Johnson-Eusebio, Alejandro. "60 GHz 4-Bit Phase Shifter Design with VO2 Switches." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1524207662285961.
20
Mabakachaba, Boitumelo Mafalo. "Carbon, magnesium implantation and proton irradiation on pulsed laser deposited thermochromic thin film of VO2." University of Western Cape, 2020. http://hdl.handle.net/11394/7723.
Abstract:
>Magister Scientiae - MScWhen the spacecrafts orbit in space, it is subjected to significant thermal cycling variation. Thermal regulation of the spacecraft temperature is required to ensure a good operation of the small crafts such as CubeSats and the on-board equipment while minimizing the weight. Three methods employed for the Smart Radiator Devices (SRD) are (i) mechanical louvers, (ii) electrochromic coatings and (iii) thermochromic coatings (which is of interest in this study). Based on the characteristics of the thermochromic coatings, the passive smart radiator device is by far the most efficient option since there are no mechanical moving components and also no electric energy needed for the craft to operate.
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Sirjita, Eduard-Nicolae. "Elaboration and correlation of structure/optical and electrical properties of phase transition oxide materials (VO2, W-doped VO2 and NbO2)." Electronic Thesis or Diss., Limoges, 2024. https://aurore.unilim.fr/theses/nxfile/default/cfd27744-856a-4645-a183-cbf2ff31ef34/blobholder:0/2024LIMO0005.pdf.
Abstract:
Cette thèse vise la réalisation et la caractérisation structurale, optique et électrique de couches minces de matériaux oxydes ayant des propriétés de transition isolant-métal (MIT), plus particulièrement des oxydes de vanadium (VO2 et VO2 dopé avec W) et du dioxyde de niobium (NbO2). Le dépôt de ces couches minces a été effectué par pulvérisation magnétron réactive dans une atmosphère Ar/ O2. Le processus d'optimisation des films minces a permis d'obtenir des informations précieuses sur les effets du processus de fabrication sur les caractéristiques structurelles, optiques et électriques. En outre, les films ont été intégrés dans divers dispositifs tels que des modulateurs THz activés thermiquement/électriquement, des métacanvases reconfigurables optiquement et des dispositifs oscillants. Les informations fournies dans ce manuscrit sont d'un grand intérêt pour l'intégration de films de VO2 et de VO2 dopé avec W dans des dispositifs pouvant fonctionner dans différents domaines. Ce travail permet également d'approfondir notre compréhension des films de NbO2, un domaine relativement moins étudié mais particulièrement intéressant pour les applications nécessitant une plus grande stabilité à la températureThis thesis aims at the realization and structural, optical, and electrical characterization of thin films of oxide materials with insulator-to-metal transition (MIT) properties, more specifically vanadium oxides (VO2 and W-doped VO2) and niobium dioxide (NbO2). The deposition of these thin films was performed using reactive magnetron sputtering in an Ar/ O2 atmosphere. During the optimization process of the thin films, valuable insights were gained into the effects the fabrication process has over the structural, optical and electrical characteristics. Furthermore, the films were integrated into various devices such as thermally/ electrically activated THz modulators, optically reconfigurable metacanvases and oscillating devices. The information provided in this manuscript is of high interest for the integration of VO2 and W-doped VO2 films into devices that can operate in various fields. This work also deepens our understanding of NbO2 films, an area with comparatively less research but of particular interest for applications requiring higher temperature stability
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Didelot, Aurélien. "Films d’oxydes de vanadium thermochromes dopés aluminium obtenus après un recuit d’oxydation-cristallisation pour applications dans le solaire thermique." Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0307/document.
Abstract:
Ces travaux sont issus d’une thèse CIFRE et de la collaboration entre la société Viessmann Faulquemont et le laboratoire de recherche l’Institut Jean Lamour. Ayant pour objectif de fortement réduire les problèmes liés aux hautes températures de stagnation dans les panneaux solaires thermiques, nous présentons une nouvelle génération d’absorbeur solaire intelligent à base de dioxyde de vanadium. Le dioxyde de vanadium, noté VO2, est un matériau présentant une transition métal-isolant (MIT) à une température critique (Tc) de 68°C. Cette transition s’accompagne d’une modification de la structure cristallographique. Le VO2 se trouve sous une forme monoclinique VO2(M) à basse température, et sous une forme rutile VO2(R) à haute température. Ce changement de structure s’accompagne d’une forte modification des propriétés optiques. La synthèse de ces films est réalisée à partir d’une couche de vanadium métallique déposée par pulvérisation. Un recuit d’oxydation-cristallisation est ensuite effectué pour obtenir une couche d’environ 400 nm de dioxyde de vanadium. Afin d’optimiser et d’augmenter la variation d’émissivité (Δε), la température et la durée du recuit sont étudiées. Dans un second temps, un dopage aluminium est réalisé afin d’augmenter l’effet de la transition thermochrome. Après optimisation, le passage au niveau industriel est un succès et des prototypes de taille 1 sont réalisés à partir de la couche thermochrome et de la couche standard afin d’être comparés dans des conditions normales d’utilisationThis work is a CIFRE thesis between VIESSMANN Faulquemont SAS society and the laboratory Institut Jean Lamour. In order to strongly reduce the problems associated with high stagnation temperature, we present a new generation of solar absorbent layers based on a smart thermochromic vanadium dioxide thin film. Vanadium dioxide (VO2) is a material which exhibit a metal insulator transition (MIT) at a critical temperature of 68°C (Tc). The transition is accompanied by a change in crystallographic structure VO2(M), while a rutile-like structure VO2(R) is obtained at high temperature. This structural change induces a drastic modification of the optical properties. The synthesis of vanadium-based films is performed using magnetron sputtering. We proceed to a subsequent annealing in air to form crystalline films of about 400 nm thickness. In order to increase the thermochromic effect of our thin film (Δε) we study the temperature and duration of the annealing. In a second time we try to increase the emissivity switch between the low and high temperature phase by adding an aluminum doping. After optimization, scale up have been successfully done and the optimized parameters have been used to build a prototype of thermochromic selective layer that has been compared to the standard industrial solar absorber
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Sadiq, Mohammad Nikhian. "Conception et développement de dispositifs hyperfréquences à reconfiguration rapide à partir de matériaux à transition isolant-métal (MIT) : application au dioxyde de vanadium (VO2)." Thesis, Brest, 2019. http://www.theses.fr/2019BRES0109.
Abstract:
Ce travail de thèse, réalisé au Lab−STICC, dans le cadre du projet ANR MUFRED, porte sur l’étude, la conception et le développement de dispositifs hyperfréquences reconfigurables à base de dioxyde de vanadium (matériau à transition isolant-métal). Ce projet, multidisciplinaire – allant du dépôt et l’étude du matériau à la conception et la caractérisation de dispositifs RF en passant par une commande optique – vise à démontrer les performances du VO2 en tant qu’élément d’accord pour une commutation rapide (une dizaine de nanosecondes) à ultra-rapide (une centaine de picosecondes, selon la littérature). Pour ce faire, ce travail débute par une caractérisation du dioxyde de vanadium en tant qu’élément de commutation avant de l’intégrer dans des dispositifs reconfigurables.Ainsi, les premiers commutateurs SPST, SP2T et SP4T à base de VO2 sont conçus pour un contrôle de la transition isolant-métal par commande électrique et optique. Ces commutateurs sont par la suite utilisés dans la conception de déphaseurs reconfigurables 1-bit (déphasage relatif de 0° et − 45°) et 2-bits (déphasage relatif de 0°, − 90°, − 180°, − 270°) de type « True Time Delay » à lignes commutées.La suite du travail porte sur le démonstrateur ciblé par le projet MUFRED, i.e. un réseau d’antennes phasé reconfigurable à base de commutateurs de VO2. Les performances de chacun des blocs RF intervenant dans sa conception sont décrites, présentées et analysées.Les premiers démonstrateurs réalisés ont permis d’envisager des perspectives d’amélioration à court et long termeThis thesis, conducted at Lab−STICC as part of the ANR MUFRED project, focuses on the study, the design and the development of reconfigurable microwave devices based on vanadium dioxide (a metal-insulator transition material). This multidisciplinary project – from material deposition and study to the design and characterization of RF devices by way of optical control – aims to demonstrate the VO2 performances as a tuning element for fast (about ten nanoseconds) to ultra-fast (about hundred picoseconds) switching.With this aim in mind, this work begins with a characterization of vanadium dioxide as a tuning element before integrating it into reconfigurable RF devices.Thus, the first VO2 based switches, SPST, SP2T and SP4T are designed for control of the metal-insulator transition with an electrical or optical command. These switches are subsequently used in the design of reconfigurable 1-bit (relative phase shift of 0° and − 45°) and 2-bits (relative phase shift of 0°, − 90°, − 180° and − 270°) switched lines True Time Delay phase shifters.Then this study focuses on the proof-of-concept targeted by the MUFRED project, i.e. a reconfigurable phased array antennas based on VO2 switches. The performances of each RF blocks involved in its design are described, presented and analyzed.The first demonstrators carried out make it possible to foresee prospects for improvement in the short and long term
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Koussi, Erieta-Katerina. "Micro patterning of complex Waveguide Resonant Gratings (WRG)." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSES027.
Abstract:
Cette thèse de doctorat intitulée “Micro structuration de Réseaux Résonants complexes” étudie les dispositifs de détection optique, qui impliquent diverses techniques de photolithographie et outils de micro/nanotechnologie pour leur fabrication. Ces dispositifs, dans leur forme classique, consistent en un réseau de diffraction formé de lignes microscopiques gravées périodiquement sur une surface photosensible, qui est déposée sur une couche diélectrique, le guide d'onde. Les deux couches sont supportées sur un substrat. Pour activer les fonctions de détection, le réseau de diffraction doit être extrêmement sélectif, c'est-à-dire qu'il doit avoir la capacité de rejeter toutes les composantes spectrales reçues, en sélectionnant qu’une seule longueur d'onde pour la coupler au guide d'onde. Après le découplage, une réflexion dite résonante liée à sa très grande amplitude et sélectivité spectrale et angulaire se produit. Différents types de composants en fonction de l'application envisagée peuvent être réalisés sur différents types de substrats, matériaux ou géométries (plan, cylindrique). L'un des projets de cette thèse consiste à concevoir des WRG (Waveguide Resonant Gratings) sur les parois intérieures d'un tube pour coupler les modes TE et TM à l’intérieur du guide d’onde. La fabrication est réalisée par un masque de phase radial spécialement conçu, tandis que la fonction optique est mise en évidence en utilisant un miroir conique, capable de réfléchir la lumière de manière isotrope pour l'excitation des modes. De plus, des matériaux innovants peuvent être utilisés pour leur intégration dans des structures résonantes planaires. L'un des matériaux à l'étude est le dioxyde de vanadium (VO2), qui subit respectivement des transitions de phase de premier ordre (isolant vers métal) à basses et hautes températures. La fabrication d'un composé aussi délicat est complétée par deux méthodes de synthèse différentes, le dépôt par laser pulsé et la pulvérisation cathodique magnétron. La capacité d'induire une résonance en déclenchant thermiquement le dispositif est destinée aux applications de sécurité laser afin d'éviter l’endommagement lors d'une surchauffeThis PhD thesis entitled “Micro patterning of complex Waveguide Resonant Gratings (WRG)” studies the optical sensing devices, which involve various photolithography techniques and nanotechnology tools with clean room processes for their fabrication. These devices, in their classic form, consist of a diffraction grating formed by microscopic lines engraved periodically on a photosensitive surface, which is deposited on a dielectric layer, the waveguide. Both layers are supported on a substrate. To enable sensing functions, the diffraction grating must be extremely selective, i.e. it must have the ability to reject all the received spectral components, while selecting only one wavelength to couple it into the waveguide. After the out coupling, a reflection with a very large amplitude and great finesse occurs. Different types of components depending on the intended application can be produced on different types of substrates, materials or geometries (plane, cylindrical).One of the projects of this thesis engineers WRG on the interior walls of a tube to couple TE and TM modes into the waveguide. The fabrication is achieved by a specially designed radial phase mask, whereas the optical function is highlighted by the use of a conical mirror, able to reflect light isotropically for mode excitation. In addition, innovative materials can be used for their integration into flat WRG. One of the materials under study is the Vanadium Dioxide (VO2), which undergoes first-order phase transitions (Insulator to Metal) at low and high temperatures respectively. The fabrication of such a delicate compound is completed by two different synthesis methods, the Pulsed Laser Deposition and Magnetron Sputtering. The ability to induce resonance by thermally triggering the device is intended for laser safety applications to avoid damage during overheating
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Duncan, Morris. "Surface and sensor studies of doped titanium dioxide." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.365772.
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Zaabi, Rafika. "Films minces intelligents à propriétés commandables pour des applications électriques et optiques avancées : dopage du dioxyde de vanadium." Thesis, Limoges, 2015. http://www.theses.fr/2015LIMO0119/document.
Abstract:
Cette thèse concerne l’étude de l’effet du dopage au chrome sur les propriétés structurales, électriques et optiques des films de dioxyde de vanadium. Ces films V(1-x)CrxO2 (x allant de 0 à 25%) de 110 nm d’épaisseur ont été déposés par dépôt par ablation laser (PLD) multicibles sur substrat saphir c. Ils ont été caractérisés grâce à des techniques d’analyse morphologique, structurale, électrique et optique. Les différentes phases présentes dans les films V(1-x)CrxO2 ont été identifiées par DRX, spectroscopie Raman et comparées au diagramme de phase du matériau massif. Les phases M1, M2 et M3, un mélange M2 + M3 et la phase R ont été identifiées. En revanche la phase M4 n’a pas été détectée pour des dopages supérieurs à 8%, montrant une réelle différence entre diagrammes de phase du matériau massif et des films. Le dopage au chrome a permis d’augmenter la température de transition isolant-métal de 68 à 102°C. En revanche, la dynamique de cette transition, déterminée par mesure de transmission optique ou par mesure de résistivité électrique, est souvent diminuée. Enfin, des dispositifs à deux terminaux à base de films V(1-x)CrxO2 ont été réalisés. Leurs caractérisations courant-tension montrent que le dopage au chrome influence fortement le seuil d’activation de la transition entre les états isolant et métalliqueThis thesis presents a study of the effect of chromium doping on structural, electrical and optical properties of thin films of vanadium dioxide. These V(1-x)CrxO2 thin films (x from 0 to 25%) of 110 nm thick have been deposited on c sapphire substrate by multi target Pulsed Laser Deposition method. Their morphological, structural, electrical and optical properties have been studied. Different phases for V(1-x)CrxO2 have been identified by XRD and Raman analysis and compared to those of bulk material. M1, M2, M3, a mixture M2 + M3 and R phases are present. The M4 phase has not been detected for doping above 8%, showing a real difference between phase diagram of bulk and thin films. Chromium doping also increases the metal-insulator transition temperature from 68°C to 102°C. Moreover, the transition dynamics, determined using optical transmission and electrical resistivity measurements, decreases. Finally, two terminal switches based on V(1-x)CrxO2 thin films have been fabricated. Their current-voltage characterization showed that chromium doping affects the activation threshold voltage of the metal to insulator transition
27
Victor, Jean-Louis. "Étude de revêtements thermochromes à base de VO2 élaborés par voie PVD/HiPIMS." Thesis, Bordeaux, 2019. http://www.theses.fr/2019BORD0275.
Abstract:
Le dioxyde de vanadium (VO2) est un matériau thermochrome capable de changer de propriétés optiques avec la température suite à un changement de phase cristallographique associé à une transition de Mott. Cette capacité lui permet de moduler les flux thermiques en tant que revêtement de parois ou de vitrages. Il peut ainsi servir à réguler la température d’un habitacle ou encore être utilisé dans le domaine de la discrétion infrarouge. Le dioxyde de vanadium passe d’un état semi-conducteur vers un état métallique aux alentours de 68 °C. Cette température de transition, trop élevée pour l'obtention d'un confort thermique, peut être abaissée par dopage de type n. D’autre part, une température de recuit généralement supérieure à 450 °C est nécessaire à la stabilisation de la phase VO2 thermochrome. Son élaboration est donc difficilement envisageable sur des substrats thermiquement sensibles tels que les polymères.Les travaux présentés dans ce manuscrit concernent l’étude de ce matériau élaboré en couches minces par pulvérisation cathodique magnétron réactive. Les propriétés thermochromes résultantes ont été mises en évidence par plusieurs techniques d’analyses telles la diffraction des rayons X (DRX), les spectroscopies de rétrodiffusion de Rutherford (RBS) et photo-électronique d’émission X (XPS), l’ellipsométrie, ainsi que par des mesures en température de résistivité corrélées à des mesures spectrophotométriques dans la gamme 2,5 μm à 25 μm du rayonnement infrarouge. En raison de son efficacité démontrée dans la littérature, le tungstène a été choisi comme dopant entre 0 et 4% at. Un taux de 3,6 % a alors permis d’abaisser la température de commutation du matériau jusqu’à 9 °C après recuit à 500 °C. Pour finir, une couche de VO2 performante autour de 59 °C a été obtenue à seulement 300 °C via l’emploi d’une alimentation à haute puissance pulsée (HiPIMS). L’ensemble de ces résultats ouvre des perspectives quant à une application industrielle dans le domaine de l’efficacité énergétiqueVanadium dioxide (VO2) is a thermochromic material exhibiting tunable optical properties, due to its structural change associated with a Mott transition. Thus, this material is capable to modulate heat flows when coated on walls orwindows. Consequently, it can be used for a wide range of applications, like cabin’s temperature control and adaptive thermal camouflage. Around 68 °C, VO2 shifts from a semiconductor to a metallic state. However, this temperature is too high to bring a thermal comfort, which can be adjusted by n doping. Moreover, in order to stabilize the thermochromic VO2 phase, an annealing step over 450 °C is required, preventing its deposition on thermally sensible substrates such as polymers.This work focuses on the study of this material which was synthesized by reactive magnetron sputtering. Resulting thermochromic properties were evidenced by X-Rays Diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS) and X-Rays Photoelectron Spectroscopy (XPS), ellipsometry and by resistivity and spectrophotometric measurements with temperature in the infrared domain ranging from 2.5 μm to 25 μm. The doping was made with tungsten (W) due to its reported efficiency. VO2 thin films were doped with various W amounts (0 to 4% at.). Especially, the 3.6% W doping rate yielded a lowering of the commutation temperature up to 9 °C after annealing treatment at 500 °C. Endly, a performing VO2 thin film switching around 59 °C was obtained at only 300 °C by using HiPIMS mode synthesis, paving the way to industrial applications for energetic efficiency
28
Johansson, Daniel. "VO2 films as active infrared shutters." Thesis, Linköping University, The Department of Physics, Chemistry and Biology, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-6458.
Abstract:
An active optical shutter for infrared light (3-5 μm) has been designed, exploiting the phase transition in thermochromic vanadium dioxide (VO2). A spin coating processing route for VO2 films has been adapted to manufacture reproducible depositions onto sapphire (Al2O3) substrates. The VO2 films have been characterized by X-ray powder diffraction (XRPD) and infrared spectroscopy (FTIR), showing 55 % transmittance in the open mode and 0.1 % in the closed mode.
The VO2 film temperature determines the operating mode of the shutter, and a resistive circuit of gold was deposited on top of the film for heating purposes. Switching times from the open to the closed mode down to 15 ms have been measured.
This work is a part of a comprehensive project at the Swedish Defence Research Agency (FOI), aiming to improve active components for protection against lasers. The shutter within this work is at this stage an early prototype, and needs further development and complementary systems such as a control unit to be implemented in an optical system.
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Johansson, Daniel. "VO2 films as active infrared shutters." Thesis, Linköpings universitet, Institutionen för fysik, kemi och biologi, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-6458.
Abstract:
An active optical shutter for infrared light (3-5 μm) has been designed, exploiting the phase transition in thermochromic vanadium dioxide (VO2). A spin coating processing route for VO2 films has been adapted to manufacture reproducible depositions onto sapphire (Al2O3) substrates. The VO2 films have been characterized by X-ray powder diffraction (XRPD) and infrared spectroscopy (FTIR), showing 55 % transmittance in the open mode and 0.1 % in the closed mode. The VO2 film temperature determines the operating mode of the shutter, and a resistive circuit of gold was deposited on top of the film for heating purposes. Switching times from the open to the closed mode down to 15 ms have been measured. This work is a part of a comprehensive project at the Swedish Defence Research Agency (FOI), aiming to improve active components for protection against lasers. The shutter within this work is at this stage an early prototype, and needs further development and complementary systems such as a control unit to be implemented in an optical system.
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Viasus, Pérez Camilo Javier. "Modeling Study on Reverse Combustion Promoted by m-BiVO4." Thesis, Université d'Ottawa / University of Ottawa, 2019. http://hdl.handle.net/10393/38892.
Abstract:
Reverse combustion is a process converting CO2 into its different reduced/hydrogenated forms while, ideally, oxygen is being released. Understanding how CO2 is interacting/reacting with vanadium (main component in the CB of m-BiVO4) in different oxidation states was our main goal. In this thesis we have attempted to contribute to the ongoing efforts for overcoming the formidable challenges posed by H2 production and CO2 activation.
In the process to prove the role of each metal during a reverse combustion process mediated by m-BiVO4, several strategies were followed to prepare pure monoclinic BiVO4 using different starting materials (Chapter 2). Hydrothermal processes in an autoclave were determined as the most efficient way to obtain m-BiVO4. Photoirradiation experiments were performed in-situ and analyzed by EPR, demonstrating that a photoexcited species was generated. EPR spectra were compared with VO2, which suggested that one electron is being transferred from the VB to the CB in the photoexcitation process, in this case forming a vanadium(IV). This experiment suggested that the reduction process of CO2 is possibly occurring through a one-electron transfer process. Several attempts were made unsuccessfully to prepare a bismuth vanadate-like compound containing only vanadium(IV) in its structure. Bi4V2O10 was obtained where the vanadium atom was present in a lower oxidation state but with different Bi/V ratio than in BiVO4. This species does not present any photo-catalytic activity. Instead, it presented mild reactivity in hydrogen formation from formaldehyde in basic media. Photocatalytic experiments on pure m-BiVO4 in the presence of water and CO2 were performed and methanol was obtained as a product. In this process, vanadium leached out from the structure affording a mixture of V(IV) and V(V). On the surface of the remaining m-BiVO4, Bi2O4−x was deposited as a result of the loss of vanadium.
The initial idea behind the preparation of a compound different to BiVO4 was to produce a new photocatalyst that preserves the electronic characteristics of vanadium(V) as well as being a semiconductor (Chapter 3). In addition, a higher oxidation state than the vanadium +5 could provide longer electron-hole recombination times and increase lifetime of the photogenerated electrons. By having a +6-oxidation state, such as provided by a Cr atom, it could give a better chance to improve the reduction of CO2 by facilitating oxygen release. Unfortunately, photochemical activity was not observed under any conditions. On the other hand, both monoclinic and orthorhombic BiOHCrO4 were tested for formic acid thermal decomposition. These two unique crystal structures were analyzed by single crystal XRD. The monoclinic isomer displays a much higher thermal resilience and was chosen for the degradation of formic acid studies. During the process, an active species of BiCrO4 was formed and identified.
When using vanadium aryloxide compounds in an oxidation state lower than +5 as possible reagents to reduce CO2, interesting results were obtained (Chapter 4). These compounds were prepared aiming at mimicking the reduction of CO2 as performed by hypothetically formed lower valent vanadium.
As presented in chapter 2, during the photoirradiation of BiVO4 a new vanadium species is formed. EPR experiments indicated that it could be V(IV). As a result, while vanadium(IV) showed negligible reduction/interaction with CO2, vanadium(III) aryloxide was a powerful reductant. Experiments attempting to control the electron transfer to CO2 resulted in two different outcomes. Firstly, a two-electron transfer from the metal center to CO2 was obtained affording CO and vanadyl(V) tris-aryloxide. Secondly the introduction of a halogen in the metal coordination sphere of a vanadium(III) compound triggered a radical behavior.
The use of a compound of vanadium(II) with polydentate oxygen-donor based ligand still yielded CO. However, an intermediate V-O-V moiety was formed in turn performing radical H atom extraction from the solvent through an unprecedented pattern of reactivity. DFT calculations confirmed the nature of the electronic transfer and the formation of V-O-V that acted as an intermediate for the second CO2 interaction.
We successfully arrested the reaction to isolate an intermediate and an unprecedented (ONNO)V(OH)-OCO compound was isolated and fully characterized. This CO2 complex provide the second example of a linearly end on bonded CO2 and the first case of such a bonding mode to a transition element.
A further study of the reactivity of the vanadium trivalent state was carried out by modifying the ligand to H2ONOO and secondly, by introducing a Cl atom as in LV-Cl (L = ONNO or ONOO) to enable the formation of derivative such as p-methoxy-phenoxide and methoxide ligands via simple ligand substitution.
Unfortunately, the (ONOO)2- ligand quenched the reducing power such that no reaction was observed with CO2. Halogen replacement afforded (ONNO)V(p-methoxy-phenoxide)(THF) which displayed no reactivity with CO2, but once the p-methoxy-phenoxide ligand was replaced by a methoxide group, formaldehyde and formate were formed. The DFT proposed mechanism presented an interesting interaction wherein the cis- position in [V(ONNO)]+ is responsible for the H transfer to occur
Finally, we have prepared a heterobimetallic system containing Bi-V atoms (Chapter 6). The oxidation states of Bi and V were +3 and +5 respectively. One pot reaction was the most adequate procedure to obtain the heterobimetallic structure. Trasmetallation on Bi compounds by V atoms was observed when attempting to build the heterobimetallic structure using more rational reaction pathways. Attempts to obtain a heterobimetallic structure in oxidation states different than that presented in m-BiVO4 were unsuccessful. When oxidation states lower than +5 for vanadium (vanadium(III-II)) and +3 in bismuth were used, metallic bismuth and untreatable materials with a mixed-valence vanadium were formed.
31
Wang, Shu. "Experimental Investigation of New Inductor Topologies." University of Dayton / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1460733373.
32
Radue, Elizabeth Lee. "Study of Variations of the Dynamics of the Metal-Insulator Transition of Thin Films of Vanadium Dioxide with An Ultra-Fast Laser." W&M ScholarWorks, 2016. https://scholarworks.wm.edu/etd/1477068473.
Abstract:
Vanadium dioxide is an intensely studied material, since it goes through an insulator-metal transition at a critical temperature just above room temperature at 340~K. The dramatic change in conductivity and the easily accessible transition temperature makes it an attractive material for novel technologies. Thin films of VO2 have a reversible transition without any significant degradation in contrast, and depending on the microstructure of the films, the properties of the transition are tunable. In this work, I study the dynamics of the insulator-transition in thin films grown on different substrates using a pump-probe configuration. The energy needed to trigger the transition, as well as the time constants of the change in reflectivity are affected by the strain in the VO2 films. I also characterized the samples using Raman spectroscopy and XRD measurements in order to identify what underlies the differences in behavior. Finally, in collaboration with Dr. Yamaguchi's group at RPI, I show that it is possible to trigger the transition using a THz pulse that directly pumps energy into the lattice, and at lower energies than needed to pump films by photoinducing the electrons across the band gap.
33
Guan, Shian. "Étude de la synthèse de dioxyde de vanadium : propriétés physico-chimiques, dopages élémentaire et applications sous forme de films." Thesis, Bordeaux, 2019. http://www.theses.fr/2019BORD0443.
Abstract:
L’oxyde VO2 est un matériau prometteur pour son utilisation en tant que film pour fenêtres intelligentes en raison de sa transition réversible métal isolant à 68°C. Cette transition est accompagnée par d’importantes modifications des propriétés optiques avec passage d'un état transparent à basse température à un état plus opaque du proche infrarouge. D’un point de vue cristallographique, la transition implique une transformation de deux phases cristallines: la phase monoclinique (M) et la phase rutile (R).Les verrous technologiques limitant l'utilisation commerciale de cet oxyde sont sa température de transition élevée et sa faible stabilité thermique et chimique. Plus important encore, il n’existe toujours pas de méthode reproductible et peu coûteuse pour la synthèse des particules de VO2.Deux méthodes de synthèse de poudre de VO2 sont proposées. Dans la première méthode, des poudres de VO2 (M) à cristallinité ajustée ont été préparées par un traitement thermique en deux étapes: une décomposition rapide du VEG dans l'air conduisant à la formation de cristaux de VO2 de qualité cristalline médiocre, suivie d'un traitement de post-recuit à des températures plus élevées sous vide dynamique afin que la cristallinité augmente. La deuxième méthode, appelée méthode de carbo-réduction, repose sur le rôle réducteur du carbone. Ici, la suie est choisie pour effectuer une réduction directe d’un oxyde précurseur V2O5 élaboré préalablement et de taille de cristallite nanométrique. La comparaison de scellé et dynamique permet une meilleure compréhension du mécanisme de réduction. La taille moyenne des particules de VO2 obtenues vari de 5,3 um à 415 nm selon la température et la durée du recuit opérées. De plus, les ions métalliques Al3+, Ti4+ et Nb5+ ont été avec succès au sein des oxyde VO2. Les ions Nb5+ présentent l'effet de diminution de la température de transition le plus efficace (Tc= 25°C).Les effets du dopage en Nb sur les propriétés des oxydes VO2, sous tous leurs aspects : morphologie, distorsion des cristaux, stabilité thermique, capacité calorifique, résistivité électrique et susceptibilité magnétique, ont été pleinement étudiés. Enfin, une étude approfondie des propriétés magnétiques est aussi présentée, notamment en se concentrant sur les' impacts de la concentration en dopant et de la taille des particules.La mise en forme en film mince à base de VO2 a été entreprise avec la volonté, de conserver un bon compromis entre la transmittance lumineuse (Tlum) et la capacité de modulation de l’énergie solaire (ΔTsol) de ces films. Par dip-coating, des films VO2 à « structure en îles » ont été fabriqués en suivant notre stratégie de carbo-réduction. Un surfactant : PVP, est utilisé à la fois comme stabilisant de surface et agent réducteur. Pour moduler le taux de couverture par les îles fonctionnels (VO2) de la surface des substrats transparents, trois paramètres d’élaboration ont été modifiés : (i) la concentration de la suspension ; (ii) l’épaisseur de couches de film de V2O5@PVP; (iii) le nombre de couches de VO2 lors de la répétition de la boucle de revêtement par centrifugation / recuit.Enfin, la force motrice de la transition de phase des oxydes de type VO2 n’est toujours pas claire : certains auteurs proposent à cette transition une origine structurale (modèle de Peierls), d’autres énoncent que la transition procède d’une corrélation complexe électron-électron (modèle de Mott). Nous avons également exposé notre point de vue sur cette transition métal-isolant VO2. Dans la limite d'une approche géométrique simple, mais avec une approche originale, les principales forces motrices impliquées dans la transition MIT de VO2 ont été évaluées. Notre approche basée sur le modèle de valence de liaison, nous a permis de montrer que la forme allotropique la plus stable (monoclinique ou rutile) à une température donnée, est celle présentant pour une température donnée le volume de maille unitaire le plus importantVO2 oxide is a promising material for energy-saving smart windows due to its reversible metal-to-insulator transition at 68℃, accompanied by large optical changes from a low temperature transparent state to a more blocking state at high temperature in the near-infrared region. From a crystallographic point of view, the VO2 transition occurs between two crystal phases: monoclinic (M) and rutile (R) phases.The limiting factors for a commercial use are the high transition temperature, unpopular yellow color (film) and poor thermal and chemical stability. Most importantly, there is still lack of a reproducible and low-cost method for VO2 particles synthesis.Two synthesis methods for VO2 powder are proposed. Firstly, VO2 (M) powders with tuned crystallinity were prepared through a two-step thermal treatment: a fast VEG decomposition in air leads to the formation of poor crystal VO2, followed by a post-annealing treatment at higher temperatures in vacuum for a full-crystallization. The second method, named as carbo-thermal reduction method, is based on the reducing role of carbon. Herein soot is chosen for a direct reduction of homemade V2O5 nano-powder. The comparison of sealed or dynamic vacuum systems allows further understanding of the reductive mechanism. The obtained VO2 particles size varied from 5.3 um to 415 nm by adjusting the annealing temperature and time. In addition, Al3+, Ti4+ and Nb5+ metal ions can be doped into VO2, successfully. Nb5+ shows the most efficient effect on decreasing of the transition temperature down to Tc= 25°C.The doping effects of Nb on VO2 are fully investigated on several aspects, including the morphology, crystal distortion, thermal stability, heat capacity and resistivity. Finally, a deep investigation of the magnetic properties with respect to the impact of the doping concentration and the particle size is proposed.The shaping step into thin films, is performance using a dip coating process from VO2 suspensions. This step is investigated to tackle down the trade-off relationship between luminous transmittance (Tlum) and solar-energy modulation ability (ΔTsol) in VO2-based thin films. Zero dimensional island structured VO2 films are fabricated by taking advantage of the reductive strategy using carbon as developed in previous part. PVP is used as a surface stabilizer and reducing agent. To tune the functional materials VO2’s surface coverage on substrate, three elaboration parameters were changed: (i) the suspension concentration; (ii) the V2O5@PVP film layer thickness; (iii) the number of VO2 layers through repeating the spin-coating/annealing loop.Finally, the driving force of the phase transition of VO2 type oxides is still unclear: as far as distortion is concerned, some authors propose to explain this transition a distortional induced structural origin (Peierls model), other state that the transition proceeds by a complex electron-electron correlation (Mott model). We discuss our point of view on this metal-insulator transition of VO2 oxides. In the limits of a simple geometrical approach, we describe here with a new approach, the main driving forces involved in the MIT transition of VO2 oxides. This approach, based on the binding valence model, a well-established model, has allowed us to show that the phase transition is certainly consistent with a maximization of the lattice energy due to a maximization of the unit-cell volume of the VO2 oxides, in other words, the most stable allotropic form is that presenting for a given temperature the largest unit-cell volume
34
Fan, Xiaofei. "Contrôle ultrarapide de l'aimantation dans des hétérostructures à base de VO₂." Electronic Thesis or Diss., Université de Lorraine, 2022. http://www.theses.fr/2022LORR0271.
Abstract:
(1) Nous aVO₂vons étudié la transition de phase dans le VO₂ amorphe ultrafin et son mécanisme physique : Nous avons préparé avec succès des films amorphes ultraminces (à l'échelle nanométrique) de VO₂ avec une transition de phase significative par pulvérisation magnétron et démontré la transition de phase du VO₂ amorphe - EGT. En outre, nous avons modélisé quantitativement la transition de phase du VO₂ amorphe et classé différentes épaisseurs de VO₂ en "système fort" (>5 nm) et "système fragile" (0-2 nm). Pour le système fort, les propriétés du matériau sont moins affectées par la température, et le modèle d'Arrhenius est utilisé pour décrire le transport d'électrons de la transition de phase du VO₂. Alors que pour le système fragile, les propriétés du matériau sont plus affectées par les fluctuations de température, et le modèle de Vogel-Tammann-Fulcher peut être utilisé pour l'analyse. Les résultats démontrent le mécanisme de transition de phase des matériaux amorphes et fournissent une nouvelle idée pour comprendre la transition de phase. En outre, cette méthode directe de croissance de VO₂ ultra-mince par pulvérisation magnétron est pratique et rapide, et il peut être cultivé dans le même lot avec d'autres matériaux dans l'hétérostructure, ce qui devrait promouvoir l'application de matériaux à transition de phase dans des dispositifs pratiques. (2) Nous avons exploré une méthode permettant de réguler dynamiquement le couplage d'échange entre couches par transition de phase : nous avons introduit le VO₂ dans la couche ferromagnétique/non magnétique et nous avons réussi à réaliser la transformation réversible du couplage antiferromagnétique et du couplage ferromagnétique en régulant les électrons de conduction par le MIT de VO₂. En même temps, à partir de l'analyse du changement des propriétés magnétiques, nous clarifions que le CEI induit par le VO₂ dans différents états électroniques est dominé par le RKKY et l'effet tunnel dépendant du spin. En outre, nous étudions en détail la racine physique derrière la régulation de l'IEC par le VO₂, et nous révélons le mécanisme de régulation de l'effet de spin de l'interface par la régulation des états électroniques de l'espaceur non magnétique. Cette partie du travail propose une nouvelle approche de la régulation dynamique de l'IEC, qui fournit de nouvelles idées pour l'application de l'IEC dans les dispositifs spintroniques. (3) Nous étudions la régulation dynamique du transport d'électrons chauds polarisés en spin par transition de phase : Dans une hétérostructure ferrimagnétique/non magnétique à canal de diffusion/ferromagnétique, nous introduisons du VO₂ dans le canal de diffusion pour contrôler les propriétés électriques du canal par MIT, puis nous régulons dynamiquement le transport des électrons chauds polarisés en spin générés par la désaimantation ultrarapide de GdCo. En régulant l'activation et la désactivation des électrons chauds dans le canal, nous obtenons une régulation dynamique de l'aimantation des couches ferromagnétiques adjacentes. Parallèlement, grâce aux changements de propriétés optiques introduits par le VO₂, nous avons réussi à commuter l'aimantation de matériaux ferromagnétiques sans AOS en ferrimagnétisme excité par un laser femtoseconde à impulsion unique. De plus, nous avons vérifié et analysé le mécanisme de cette modulation ultrarapide. Dans ce travail, nous utilisons le matériau de transition de phase VO₂ comme canal de diffusion avec des propriétés électriques contrôlables pour contrôler le transport des électrons chauds à travers le MIT. Les résultats montrent que les matériaux non magnétiques jouent un rôle important dans différents types d'hétérostructures(1) We have investigated the phase transition in ultrathin amorphous VO₂ and its physical mechanism: We have successfully prepared ultrathin (nano-scale) amorphous VO₂ films with significant phase transition by magnetron sputtering and demonstrated the phase transition of amorphous VO₂ - EGT. In addition, we quantitatively modeled the phase transition of amorphous VO₂ and classified different thicknesses of VO₂ into "strong system" (>5 nm) and "fragile system" (0-2 nm). For the strong system, the material properties are less affected by temperature, and the Arrhenius model is used to describe the electron transport of VO₂ phase transition. While for the fragile system, the material properties are more affected by temperature fluctuations, and the Vogel-Tammann-Fulcher model can be used for analysis. The results demonstrate the phase transition mechanism of amorphous materials and provide a new idea for understanding phase transition. In addition, this direct method of growing ultrathin VO₂ using magnetron sputtering is convenient and fast, and it can be grown in the same batch with other materials within the heterostructure, which is expected to promote the application of phase transition materials in practical devices.(2) We explored a method to dynamically regulate the interlayer exchange coupling by phase transition: we introduced the VO₂ into the ferromagnetic/nonmagneticspacer/ferromagnetic heterostructure, and successfully realized the reversible transformation of the antiferromagnetic coupling and ferromagnetic coupling through regulating conduction electrons by MIT of VO₂. At the same time, from the analysis of the change of magnetic properties, we clarify that the IEC induced by VO₂ in different electronic states is dominated by the RKKY and spin dependent tunneling. Furthermore, we fully investigate the physical root behind the regulation of IEC by the VO₂, and reveal the regulation mechanism of the interface spin effect by the regulation of electronic states of non-magnetic spacer. This part of the work proposes a novel approach to the dynamic regulation of IEC, which provides new ideas for the application of IEC in spintronic devices.(3) We study the dynamic regulation of spin-polarized hot electron transport by phase transition: In a ferrimagnetic/nonmagnetic diffusion channel/ferromagnetic heterostructure, we introduce VO₂ into the diffusion channel to control the electrical properties of the channel by MIT, and then dynamically regulate the transport of spin-polarized hot electrons generated by the ultrafast demagnetization of GdCo. By regulating the on/off of hot electrons in the channel, we achieve dynamic regulation of the magnetization of adjacent ferromagnetic layers. Meanwhile, with the optical property changes introduced by VO₂, we have successfully achieved the switching of the magnetization of ferromagnetic materials without AOS in ferrimagnetism excited by a single-pulse femtosecond laser. Furthermore, we have verified and analyzed the mechanism of this ultrafast modulation. In this work, we use the phase transition material VO₂ as a diffusion channel with controllable electrical properties to control the hot electron transport through MIT. The results show that the non-magnetic materials play an important role in various types of heterostructures
35
Kaya, Sarp. "Structural and catalytic investigations on vanadium oxide nanoparticles supported on silica films grown an a Mo(112) substrate." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2007. http://dx.doi.org/10.18452/15639.
Abstract:
Die breite Anwendung von Modellsystemen, um heterogene katalytische Prozesse zu verstehen, basiert darauf, die Lücke der strukturellen Komplexität zu überbrücken zwischen heutigen technischen Katalysatoren, bestehend aus einem Metalloxid sowie einem darauf geträgerten Metall, sowie kristallinen Metallen und planaren Metall/Oxid-Systemen, welche dazu benutzt werden, Struktur-Reaktivitäts-Beziehungen mittels einer Fülle von Surface Science-Methoden zu untersuchen. In der vorliegenden Arbeit liegt das Hauptaugenmerk auf so genannten Vanadiumoxid-‚Monolagen’-Katalysatoren, die insbesondere für Oxidationsreaktionen von Methanol eingeführt wurden. Mittels eines ‚bottom-up’-Ansatzes wurden Silica-geträgerte Vanadiumoxid-Modellkatalysatoren untersucht. Durch Kombination einer Reihe experimenteller Techniken wurde die Oberfläche von Mo(112), die als Substrat für den Silica-Film diente, im Detail untersucht und die atomare Struktur des Silica-Films wurde ermittelt. Adsorption von Wasser und das Wachstum von Vanadiumoxid-Nanopartikeln auf dem Silica-Film und schließlich die Reaktivität von Vanadiumoxid/Silica-Systemen gegenüber Methanol wurden untersucht. Im Gegensatz zu früher vorgeschlagenen Modellen sollte eine Sauerstoff-induzierte p(2×3)-Überstruktur, die sich auf einer Mo(112)-Oberfläche ausbilded, angenommen werden als ein eindimensionales Oberflächenoxid, bei dem sich Mo=O-Gruppen bevorzugt entlang der [-1-11]-Richtung der Mo(112)-Oberfläche ausbilden. Monolagen-Silica-Filme, die auf Mo(112) gewachsen wurden, bestehen aus einem zweidimensionalen Netz von SiO4-Tetraedern. In Abhängigkeit der Bedingungen, unter denen der Film präpariert wurde, kann die Struktur durch zusätzlich auf dem Mo-Substrat adsorbierte Sauerstoff-Atome verändert werden. Die Defekt-Struktur schließt Antiphasen-Domänengrenzen ein, die durch eine Verschiebung um die halbe Gitterkonstante entlang der [-110]-Richtung gebildet werden, und eine geringe Dichte von Punkt-Defekten, die höchstwahrscheinlich Silizium-Fehlstellen darstellen. Wasser dissoziiert nicht auf dem Monolagen-Silica-Film. Eine Wasser-Struktur, die geordnet bezüglich des Silica-Films ist, wurde bei 140 K beobachtet, was der guten Übereinstimmung der Gitterkonstanten von Silica-Film und hexagonalem Eis geschuldet ist. Amorphe Lagen festen Wassers, die die Oberfläche bei 100 K homogen bedecken, wurden als reaktive Lagen für Vanadiumoxid-Partikel benutzt, um die ‚Nasschemie’ nachzubilden, wie sie in der Präparation technischer Katalysatoren zum Einsatz kommt. Die Ergebnisse verdeutlichen, dass die Eis-Lagen die Bildung von hydratisierten Vanadiumoxid-Nanopartikeln, welche teilweise von V=O und V-OH-Gruppen terminiert werden, begünstigen. Die Dehydratisierung geschieht oberhalb 500 K, wobei eine V-terminierte Oberfläche entsteht. Methanol dissoziiert auf dehydratisierten Vanadiumoxid-Partikeln, und Methoxy-Spezies sind auf der Oberfläche stabil bis 500 K, allerdings nur in der Gegenwart von V-Plätzen. Die Produktion von Formaldehyd, die bei etwa 550 K stattfindet, ist stark abhängig von der Struktur der Oberfläche der Vanadiumoxid-Partikel und weist ein Maximum bei einem spezifischen Verhältnis zwischen V- und V=O-Oberflächenplätzen auf. Die hier vorgestellten Ergebnisse könnten unser Verständnis von katalytischen Reaktionen auf molekularer Ebene bedeutend vorantreiben.The widespread use of model systems for understanding the heterogeneous catalytic processes is based on bridging the structural complexity gap between present generation of supported metal and metal oxide technical catalysts and crystalline metal and planar metal/oxide systems, which are utilized to investigate structure-reactivity relationships by a large variety of surface science techniques. In this thesis, we focused on a concept of so-called ''monolayer'' vanadium oxide catalysts, which have been introduced particularly for methanol oxidation reactions. Following a bottom-up approach, silica supported vanadium oxide model catalysts were investigated. Combining a number of experimental techniques, the surface of Mo(112) used as a substrate for the silica films was characterized in detail and the atomic structure of the silica film was determined. Adsorption of water and growth of vanadium oxide nanoparticles on the silica films, and finally the reactivity of vanadium oxide/silica systems towards methanol were studied. In contrast to the previously suggested models, an oxygen induced p(2×3) superstructure formed on a Mo(112) surface should be considered as one dimensional surface oxide where Mo=O groups are formed preferentially along the [-1-11] direction of the Mo(112) surface. Monolayer silica films grown on Mo(112) surfaces are composed of two-dimensional network of SiO4 tetrahedra. Depending on the film preparation conditions, the structure can be altered by additional oxygen atoms adsorbed on the Mo substrate. The defect structure includes antiphase domain boundaries which form by a half-lattice shift along the [-110] direction and a low density of point defects, most probably silicon vacancies. Water does not dissociate on the monolayer silica film. An ordered structure of water with respect to silica film was observed at 140 K owing to good lattice matching between the silica film and hexagonal ice. Amorphous solid water layers homogenously covering the surface at 100 K were used as reactive layers for vanadium oxide particles in order to mimic ''wet chemistry'' used in preparation of technical catalysts. The results revealed that ice layer assisted the formation of hydrated vanadium oxide nanoparticles partially terminated by V=O and V-OH groups. The dehydration takes place above 500 K, thus exposing V-terminated surface. Methanol dissociates on dehydrated vanadium oxide particles and methoxy species are stable on the surface up to 500 K only in the presence of vanadium terminated surface sites. Formaldehyde production which takes place at ~550 K is strongly affected by the surface structure of the vanadium oxide particles and exhibits a maximum at specific ratio between V- and V=O sites on the surface. The results presented may have a strong impact on our understanding of the catalytic reactions at the molecular level.
36
Boulley, Laurent. "Développement de dispositifs à base de dioxyde de vanadium VO ₂ et de méta-surfaces dans le moyen infrarouge : applications passives et intégration sur des lasers à cascade quantique." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS150.
Abstract:
Le travail de thèse présenté dans ce manuscrit traite du développement de dispositifs à base de dioxyde de vanadium VO₂ et de méta-surfaces dans le moyen infrarouge pour des applications passives et une intégration sur des lasers à cascade quantique (QCL). Ce travail a permis l'élaboration de nouvelles conditions de dépôt du matériau à changement de phase VO₂ par ablation laser pulsé à des températures compatibles avec les hétérostructures de III-V utilisées en optoélectronique. Les caractérisations des couches minces déposées montrent un changement de la réflectivité et de la conductivité électrique entre l'état isolant à basse température et l'état métallique à haute température autour de 68°C (341 K). Des développements ont ensuite été menés sur l'emploi d'un réseau de méta-surfaces permettant d'obtenir une couche homogène d'indice de réfraction effectif résonnant. Ces méta-surfaces sont constituées de résonateurs à anneau fendu dont la fréquence de résonance peut être ajustée par le choix de leurs paramètres géométriques et des matériaux les constituant. Une modulation optique de plus de 100cm-1 du pic de la résonance a été obtenue lors de la transition de phase avec des méta-surfaces déposées sur un film de VO₂. Ce résultat est très prometteur pour la conception de dispositifs monolithiques, robustes, compacts, accordables en fréquence et dont les propriétés optiques ne dépendent que de la température de la couche de VO₂.Enfin, ce travail étudie la fonctionnalisation des QCL dans le moyen infrarouge (7-8 µm) par des couches de VO₂ et de méta-surfaces. Il vise à comprendre l’influence des couches intégrées sur les propriétés d’émission. Afin de garantir une bonne interaction entre ces couches et le mode guidé du laser tout en ayant des pertes optiques faibles, des nouveaux guides d'onde ont été modélisés, puis les premiers QCL à base de VO₂ ont été démontrés et une température maximale de fonctionnement de 334 K a été mesuréeThe thesis work presented in this manuscript deals with the development of vanadium dioxide VO₂ and meta-surfaces based devices in the mid-infrared for passive applications and an integration on quantum cascade lasers (QCL).This work enabled the elaboration of new deposition conditions for the VO₂ phase change material by pulsed laser ablation at temperatures compatible with III-V heterostructures used in optoelectronics. The characterizations of the VO₂ coated thin films show a change in reflectivity and in electric conductivity between the insulating state at low temperature and the metallic state at high temperature around 68°C (341 K).Developments were then carried out on the use of a meta-surfaces array in order to obtain an homogeneous layer of resonant effective refractive index. These meta-surfaces are constituted by split-ring resonators, whose resonance frequency can be adjusted by choosing their geometric parameters and the materials they are made of. An optical modulation of more than 100cm-1 of the resonance peak has been obtained during the phase transition with meta-surfaces deposited on a VO₂ layer. This result is very promising for the conception of monolithic, robust, compact, frequency tunable devices and whose optical properties only depend on the VO₂ layer temperature.Finally, this work studies the functionalization of mid-infrared QCL (7-8 µm) with VO₂ and meta-surfaces layers. It aims at understanding the influence of the integrated layers on the emission properties. In order to ensure a good interaction between these layers and the laser guided mode while having low optical losses, new waveguides were modeled, then the first VO₂ based QCL have been demonstrated and a maximum operating temperature of 334 K has been measured
37
Kepič, Peter. "Návrh a výroba laditelných dielektrických metapovrchů pro viditelné a infračervené vlnové délky." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2021. http://www.nusl.cz/ntk/nusl-443746.
Abstract:
Metapovrchy sú nanoštruktúrované povrchy vytvorené za účelom špecifického ovládania propagácie svetla. Predstavujú revolúciu v oblastiach ultratenkých optických prvkov a nanofotonických obvodov. Zakomponovaním laditeľných dielektrických materiálov do metapovrchov sa otvára možnosť aktívne ovládať ich optické vlastnosti aj po tom, čo boli vyrobené. Oxid vanadičitý (VO2) takéto ladenie umožňuje vďaka svojej fázovej premene už pri teplote okolo 67°C a preto sa radí k najsľubnejším z laditeľných dielektrických materiálov. Nakoľko je možné postupnú fázovú premenu vo VO2 vybudiť opticky a lúč svetla je možné fokusovať do stopy s veľkosťou pár stoviek nanometrov, laditeľné metapovrchy obsahujúce VO2 by mohli byť ladené postupne a dokonca s nanometrovým rozlíšením. V tejto práci skúmame fázu a amplitúdu svetla po prechode VO2 nanoštruktúrami usporiadanými do metapovrchu navrhnutého pre viditeľnú zložku elektromagnetického žiarenia. Výskum fáze a amplitúdy je založený na numerických simuláciách VO2 nanoštruktúr (stavebných kameňov metapovrchov), ktoré sú následne overené experimentálnymi výsledkami. VO2 nanoštruktúry vykazujú taktiež Mieho dielektrické rezonancie, ktoré sú v závere tejto práce využité v postupne laditeľnom metapovrchu fungujúcom vo viditeľnej oblasti. Okrem termálneho ladenia je možné vyrobený metapovrch ovládať taktiež opticky, čo dokazuje možnosť postupného ladenia na nanometrových rozmeroch.
38
Meling, Artur [Verfasser], Alec M. [Akademischer Betreuer] Wodtke, Alec M. [Gutachter] Wodtke, Jürgen [Gutachter] Troe, Sebastian [Gutachter] Kruss, Dirk [Gutachter] Schwarzer, Martin [Gutachter] Suhm, and Igor [Gutachter] Rahinov. "Scattering of vibrationally excited NO from vanadium dioxide / Artur Meling ; Gutachter: Alec M. Wodtke, Jürgen Troe, Sebastian Kruss, Dirk Schwarzer, Martin Suhm, Igor Rahinov ; Betreuer: Alec M. Wodtke." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2020. http://d-nb.info/1203875517/34.
39
Mennai, Amine. "Conception et réalisation de commutateurs RF à base de matériaux à transition de phase (PTM) et à changement de phase (PCM)." Thesis, Limoges, 2016. http://www.theses.fr/2016LIMO0035/document.
Abstract:
Ces travaux de recherche portent sur la conception et la réalisation de commutateurs RF basées sur l’intégration de matériaux innovants fonctionnels tels que le dioxyde de vanadium (VO2) et les alliages de chalcogénures de types Ge2Sb2Te5 (GST) et GeTe. Le principe de fonctionnement de ces composants repose sur le changement de résistivité que présentent ces matériaux. Le VO2 possède une transition Isolant-Métal (MIT) autour de 68°C à travers laquelle le matériau passe d’un état isolant (forte résistivité) à un état métallique (faible résistivité). La transition MIT présente l’intérêt de pouvoir être initiée sous l’effet de plusieurs types de stimuli externes (thermique, électrique et optique) avec de faibles temps de commutation. Les alliages de types GST et GeTe ont la particularité de commuter réversiblement entre un état amorphe à forte résistivité à un état cristallin à faible résistivité suite à un traitement thermique spécifique. Les commutateurs à base de GST ou de GeTe présentent l'avantage de pouvoir opérer en mode bistable car le changement de résistivité présenté par ces matériaux est de type non volatile. Les composants réalisés ont de bonnes performances électriques (isolation et pertes d’insertion) sur une large bande. Nos travaux de recherche visent à proposer une solution alternative aux solutions classiques (semi-conducteurs et MEMS-RF) pour réaliser des commutateurs RF qui peuvent être par la suite utilisés dans la conception des dispositifs reconfigurables (filtres, Antennes)This research work focuses on the design and realization of RF switches based on the integration of new materials such as vanadium dioxide (VO2), Ge2Sb2Te5 (GST) and GeTe chalcogenides alloys. The operating principle of these devices is based on the resistivity change presented by these materials. VO2 exhibits a Metal-Insulator transition (MIT) around 68°C for which the material changes from an insulating state (high resistivity) to a metallic one (low resistivity). The MIT transition can be triggered in different ways (thermally, electrically and optically) with low switching time. GST and GeTe alloys have the particularity to be reversibly switched between a high resistive-amorphous state to low resistive-crystalline state, under a specific heat treatment. Thanks to the non-volatile resistivity change presented by these materials, GST/GeTe-based switches are able to operate in bistable mode. The fabricated devices exhibit good electrical performances (insertion loss and isolation) over a broadband. The aim of our work is to propose an alternative solution to conventional technologies (semiconductors and RF-MEMS), to design RF switches that can be used afterward in the design of reconfigurable devices (filters, antennas)
40
Kana, Jean Bosco Kana. "Towards stimuli-responsive functional nanocomposites : smart tunable plasmonic nanostructures Au-VO2." Thesis, University of the Western Cape, 2010. http://etd.uwc.ac.za/index.php?module=etd&action=viewtitle&id=gen8Srv25Nme4_8032_1299494958.
Abstract:
The fascinating optical properties of metallic nanostructures, dominated by collective oscillations of free electrons known as plasmons, open new opportunities for the development of devices fabrication based on noble metal nanoparticle composite materials. This thesis demonstrates a low-cost and versatile technique to produce stimuli-responsive ultrafast plasmonic nanostructures with reversible tunable optical properties. Albeit challenging, further control using thermal external stimuli to tune the local environment of gold nanoparticles embedded in VO2 host matrix would be ideal for the design of responsive functional nanocomposites. We prepared Au-VO2 nanocomposite thin films by the inverted cylindrical reactive magnetron sputtering (ICMS) known as hollow cathode magnetron sputtering for the first time and report the reversible tuning of surface plasmon resonance of Au nanoparticles by only adjusting the external temperature stimuli. The structural, morphological, interfacial analysis and optical properties of the optimized nanostructures have been studied. ICMS has been attracting much attention for its enclosed geometry and its ability to deposit on large area, uniform coating of smart nanocomposites at high deposition rate. Before achieving the aforementioned goals, a systematic study and optimization process of VO2 host matrix has been done by studying the influence of deposition parameters on the structural, morphological and optical switching properties of VO2 thin films. A reversible thermal tunability of the optical/dielectric constants of VO2 thin films by spectroscopic ellipsometry has been intensively also studied in order to bring more insights about the shift of the plasmon of gold nanoparticles imbedded in VO2 host matrix.
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Hariri, Ahmad. "Etude de commutateurs hyperfréquences bistables à base des matériaux à changement de phase (PCM)." Thesis, Limoges, 2019. http://www.theses.fr/2019LIMO0013/document.
Abstract:
Les travaux présentés dans ce manuscrit portent sur la conception, simulation et réalisation des nouvelles structures des commutateurs hyperfréquences basées sur l’intégration des couches minces des matériaux innovants fonctionnels tels que les matériaux à changement de phase (PCM) et les matériaux à transition de phase (PTM). Le principe de fonctionnement de ces composants repose sur le changement de résistivité présenter par ces matériaux. Nous avons exploité le changement de résistivité réversible du GeTe de la famille des matériaux à changement de phase (PCM) entre les deux états : amorphe à forte résistivité et cristallin à faible résistivité, pour réaliser une nouvelle structure d’un simple commutateur SPST. Ensuite, nous avons intégré ce commutateur dans une nouvelle structure de la matrice de commutation DPDT (Double Port Double Throw) à base de PCM pour l’application dans la charge utile du satellite. Nous avons utilisé la transition isolant-métal présenté par le dioxyde de vanadium (VO2) de la famille des matériaux à transition de phase, pour réaliser une nouvelle structure de commutateur simple à deux terminaux sur une très large bande de fréquence (100 MHz–220 GHz)The work presented in this manuscript focuses on the design, simulation and realization of new microwave switches structures based on the integration of thin layers of innovative functional materials such as phase change materials (PCM) and phase transition materials. (PTM). The operating principle of these components is based on the change of resistivity present by these materials. We exploited the reversible resistivity change of GeTe of phase change materials family between the two states: amorphous with high resistivity and crystalline with low resistivity to realize a new structure of SPST switch. Then, we have integrated this switch structure on a new structure of DPDT (Double Port Double Throw) switch matrix based on phase change materials for application in satellite payload. We have used the insulatingmetal transition presented by the vanadium dioxide (VO2) of phase transition materials family to realize a new two terminals simple switch structure on a very wide frequency band (100 MHz–220 GHz)
42
Rosa, Escutia Álvaro. "High-efficient electrodes for novel optoelectronic devices in silicon photonics." Doctoral thesis, Universitat Politècnica de València, 2018. http://hdl.handle.net/10251/110364.
Abstract:
La fotónica de silicio es actualmente la tecnología mejor posicionada para reemplazar las conexiones electrónicas tanto dentro de los mismos chips, como entre ellos mismos, con el fin de mejorar su rendimiento. Las principales ventajas de la tecnología fotónica de silicio residen en su bajo coste y en su compatibilidad con las actuales técnicas de fabricación desarrolladas por la industria microelectrónica. Dicha compatibilidad permitiría fabricar tanto chips ópticos como chips híbridos que incluyan componentes ópticos y electrónicos. Los moduladores y los conmutadores optoelectrónicos resultan dispositivos fundamentales en aplicaciones de telecomunicaciones. Las principales funciones de los conmutadores y moduladores optoelectrónicos son el enrutamiento y la transmisión de datos de alta velocidad.
Esta tesis aborda el diseño y la optimización de la parte eléctrica y óptica (en menor medida) con el fin de optimizar el rendimiento de tales dispositivos desde el punto de vista optoelectrónico. Además, también se tratará la introducción de nuevos materiales compatibles con el silicio y sus procesos de fabricación, como el dióxido de vanadio o el titanato de bario con el fin de demostrar sus propiedades y aplicarlas a los dispositivos optoelectrónicos con el fin de mejorar su rendimiento.Silicon photonics is nowadays the most promising technology to replace electrical inter- and intra-connections of the chips, increasing the performance in this way. The main advantages of silicon photonics technology lie on its low cost and its compatibility with the fabrications processes of microelectronics industry developed during years which allows the mass production of silicon photonics chips as well hybrid electronic and photonic devices in the same chip. Optoelectronics switches and modulators are key building blocks in photonic devices for tele/datacom applications. Such switches and modulators are devices which provides routing functionalities and the transmission of high speed data respectively. The work of this thesis delves with the design and optimization of silicon based switches and modulators spotlighting the electrical elements. Additionally, the work of this theses deals with the introduction of new silicon-compatible materials as vanadium dioxide and barium titanate, with the aim of demonstrating its functionalities and develop high-performance optoelectronic devices.
La fotònica de silici és actualment la tecnologia millor posicionada per a reemplaçar les connexions electròniques tant dins del propis xips, com entre ells mateixos, amb la finalitat de millorar el seu rendiment. Els principals avantatges de la tecnologia fotònica de silici resideixen en el seu baix cost i en la seua compatibilitat amb les actuals tècniques de fabricació desenvolupades per la indústria microelectrònica. Aquesta compatibilitat permetria fabricar tant xips òptics com a xips híbrids que incloguen components òptics i electrònics. Els moduladors i els commutadors optoelectrònics resulten dispositius fonamentals en aplicacions de telecomunicacions. Les principals funcions dels commutadors i moduladors optoelectrònics són l'encaminament i la transmissió de dades d'alta velocitat. Aquesta tesi aborda el disseny i l'optimització de la part elèctrica i òptica (en menor mesura) amb la finalitat d'optimitzar el rendiment de tals dispositius des del punt de vista optoelectrònic. A més, també es tractarà la introducció de nous materials compatibles amb el silici i els seus processos de fabricació, com el diòxid de vanadi o el titanato de bari amb la finalitat de demostrar les seues propietats i aplicar-les als dispositius optoelectrònics amb la finalitat de millorar el seu rendiment.
Rosa Escutia, Á. (2018). High-efficient electrodes for novel optoelectronic devices in silicon photonics [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/110364
TESIS
43
Callegari, Gustavo Luiz. "Propriedades extrínsecas em filmes finos de VO2." Universidade Federal de Santa Maria, 2010. http://repositorio.ufsm.br/handle/1/3917.
Abstract:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorIn earlier work done at the Laboratory of Magnetism and Magnetic Materials in Santa Maria, spectroscopy measurements of electrical impedance between 100 kHz and 1 GHz were a function of temperature in VO2 thin films along the metal insulator transition undergone by this material. These results suggested [127] that the Impedance Spectroscopy can be a useful tool to separate the contributions from those intrinsic material generated by morphological characteristics. For that such possibility could be actually realized, technological improvements were introduced in the system deposited by Magnetron Sputtering at Laboratory of Magnetism and Magnetic Materials as the addition of a Residual Gas Analyzer and improvements in the heating substrate holder. Spectroscopy measurements were performed in a wider range of frequencies and relaxation times extracted from them were compared with the structural characteristics of the samples obtained by X-ray diffraction and Atomic Force Spectroscopy.
Em trabalhos anteriores realizados no Laboratório de Magnetismo e Materiais Magnéticos em Santa Maria, medidas de espectroscopia de impedância elétrica entre 100 kHz e 1 GHz foram realizadas em função da temperatura em filmes finos de VO2, ao longo da transição metal isolante sofrida por este material. Estes resultados sugeriram [127] que a Espectroscopia de Impedância pode ser uma ferramenta útil para separar as contribuições intrínsecas do material daquelas geradas por características morfológicas. Para que tal possibilidade pudesse ser realmente efetivada, foram introduzidas melhorias técnicas no sistema de deposição por Magnetron Sputtering do Laboratório de Magnetismo e Materiais Magnéticos como a adição de um Analisador de Gás Residual e aperfeiçoamentos no sistema de aquecimento do porta substrato. Foram realizadas medidas de espectroscopia numa faixa mais ampla de freqüências e os tempos de relaxação delas extraídos. Essas medidas foram comparadas com as características estruturais das amostras obtidos por difração de Raios X e Espectroscopia de Força Atômica.
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Oliveira, João Tiburcio Dias de. "Espectroscopia de Impedância Elétrica do VO2." Universidade Federal de Santa Maria, 2010. http://repositorio.ufsm.br/handle/1/3894.
Abstract:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorImpedance Spectroscopy has been performed at the metal-semiconductor transition of vanadium dioxide thin films. The samples have been deposited by reactive magnetron sputtering onto heated glass substrates. The crystallographic properties and morphology of the samples have been established by X-ray diffraction. By the spectra measured between 30 and 100 celsius degrees, the volume fractions of the monoclinic and tetragonal phases were quantified. The real and imaginary parts of the electrical impedance have been measured in VO2 thin films as function of the frequency (100 kHz to 1GHz) and temperature (30ºC to 90ºC). For fixed frequencies larger than 100 MHz, the hysteresis presented by the real part of the impedance when the critical temperature is surpassed became inverted. Below the semiconductor/metal transition, the Argand plots can be well reproduced assuming a Debye-like system with one relaxation time. After the beginning of the transition this approach fails, and at least two relaxation times are needed. We attribute one of them to the intrinsic processes taken place in the material, and the other to extrinsic properties like grain boundaries.
Foi realizada uma análise, por espectroscopia de impedância, das propriedades de filmes finos de dióxido de vanádio. As amostras foram depositadas por magnetron sputtering sobre substratos de vidro aquecidos em atmosfera reativa de argônio e oxigênio. Características cristalográficas e morfológicas dos filmes foram estabelecidas por difração de RX. Através de medidas de espectros de difração em temperaturas compreendidas entre 30 e 100 graus Celsius, foram quantificadas as frações volumétricas das fases monoclínica e tetragonal. As partes real e imaginária da impedância elétrica foram medidas em filmes policristalinos de VO2 tanto em função da frequência (100 KHz a 1 GHz) quanto da temperatura (30 ºC a 90ºC). Para frequências fixas maiores que 100 MHz, o ciclo de histerese apresentado pela parte real da impedância é suprimido e invertido quando a temperatura da amostra ultrapassa a temperatura crítica. Abaixo da transição semicondutor/metal, os diagramas de Argand podem ser bem reproduzidos assumindo-se um sistema, tipo Debye, com um único tempo de relaxação. Após o início da transição, este aproximação falha e são necessários pelo menos 2 tempos de relaxação. Um destes tempos de relaxação foi atribuído a processos intrínsecos que ocorrem no material e o outro a propriedades extrínsecas, como fronteiras de grãos.
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Ligmajer, Filip. "Pokročilé plazmonické materiály pro metapovrchy a fotochemii." Doctoral thesis, Vysoké učení technické v Brně. CEITEC VUT, 2018. http://www.nusl.cz/ntk/nusl-387738.
Abstract:
Plazmonika, tedy vědní obor zabývající se interakcí světla s kovovými materiály, nabízí ve spojení s nanotechnologiemi nezvyklé možnosti, jak světlo ovládat a využívat. Výsledkem tohoto spojení může být například zaostřování světla pod difrakční limit, zesilování emise nebo absorbce kvantových zářičů, či extrémně citlivá detekce molekul. Tato práce se zabývá zejména možnostmi využití plazmoniky pro vývoj plošných optických prvků, tzv. metapovrchů, a pro fotokatalytické aplikace založené na plazmonicky generovaných elektronech s vysokou energií, tzv. horkých elektronech. Nejprve jsou vysvětleny teoretické základy plazmoniky a je poskytnut přehled jejích nejvýznamnějších aplikací. Poté jsou představeny tři studie zabývající se využitím plazmonických nanostruktur pro ovládání fáze a polarizace světla, pro vytváření dynamicky laditelných metapovrchů, a pro foto-elektrochemii s horkými elektrony. Společným prvkem těchto studií je pak používání pokročilých, resp. v rámci těchto oblastí netradičních, materiálů, jako např. oxidu vanadičitého nebo dichalkogenidů přechodných kovů.
46
Valmalette, Jean-Christophe. "Composites thermochromes a base de dioxyde de vanadium." Toulon, 1994. http://www.theses.fr/1994TOUL0014.
Abstract:
Vo#2 presente vers tc = 68c une transition structurale semi-conducteur-metal reversible qui s'accompagne d'une modification importante des proprietes electriques, magnetiques et optiques. En outre, par dopage, il est possible d'abaisser la temperature de transition. L'utilisation d'un tel materiau comme filtre adaptatif du rayonnement infrarouge est envisagee depuis de nombreuses annees. Nos travaux ont montre qu'il est possible d'utiliser les proprietes du dioxyde de vanadium en l'inserant dans une matrice polymere et en respectant certaines conditions: distribution granulometrique du pigment et repartition au sein du polymere. Dans ce but, nous avons developpe une nouvelle synthese du dioxyde de vanadium par decomposition thermique reactive de l'hexavanadate d'ammonium. Cette transformation comporte de nombreuses etapes ou interviennent des reductions successives du v#5#+ en v#4#+. Nous mettons en evidence la formation de n#2o (selon les conditions cinetiques) durant la pyrolyse. La transformation de la phase metastable vo#2(b) en vo#2 thermochrome est etudiee. Le dioxyde de vanadium thermochrome obtenu presente une distribution granulometrique centree sur 1 micron. Le dopage de vo#2 a ete realise a partir de melanges d'hexavanadate d'ammonium et de tungstate ou de molybdate d'ammonium. Les films polymeres thermochromes obtenus par insertion de vo#2 ont ete caracterises par meb et spectroscopie ir-tf. La mise au point d'un dispositif de mesure pyranometrique a permis une mesure globale de l'efficacite de la transition dans la gamme du rayonnement solaire. Enfin, une modelisation de la transmittance infrarouge du film thermochrome a permis de degager les conditions optimales de l'efficacite de la transition pour un composite polymere/pigment thermochrome. Ces resultats permettent d'envisager l'utilisation future de tels revetements pour la regulation thermique (habitat, automobile, serres agricoles etc. ) ou pour tout autre application interessant des changements brutaux de proprietes en fonction de la temperature
47
Silva, Luciane Janice Venturini da. "Transição semicondutor-metal em nanocristais de VO2 termoeletricamente ativada." Universidade Federal de Santa Maria, 2015. http://repositorio.ufsm.br/handle/1/3933.
Abstract:
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorIn this thesis, structural and electrical characteristics are investigated around the thermally triggered semiconductor to metal transition in VO2 thin films. The films, the metallics electrodes, as well as SiO2 buffer layers have been deposited by reactive magnetron sputtering onto Si substrates. The crystallographic and morphological characteristics have been observed through measurements of X-Ray diffraction as a function of the temperature, and atomic force microscopy (AFM). The nanoscale electrical characterization have been performed using a measurement system via nano-tips. The results of X-ray diffraction at room temperature revealed that the samples are polycrystalline and are strongly textured in the < 011 > direction, which is almost perpendicular to the substrate plane. The X-Ray diffraction spectra have been extracted at different temperatures to follow the crystallographic transition experienced by VO2 near the transition temperature. For films deposited on SiO2 (without electrodes) and the Ta electrode at temperatures below the critical temperature for the transition, the material presented in the monoclinic phase M1. Within the range of temperatures that comprises the transition occurs progressive appearance of the peak corresponding to the (110) plane of R rutile phase. Within a range at relatively higher temperatures, there is a coexistence of phases R and M1 and M2 may be the M2 monoclinic. As would be expected, the peak of rutile structure grows to the point of being virtually the only present when the temperature reaches about 80°C. The transition from one crystallographic film VO2 with Pd electrode was accompanied by diffraction measured at room temperature. The peak (011) of phase M1 is much smaller compared to the samples deposited on Ta electrode. However, contrary to the Ta electrode film which is likely to have grown in the shape of very small nano-grain or even amorphous form, the Pd electrode film is polycrystalline and highly textured. The transport properties during the electrical phase transition were investigated using injection of electrical current perpendicular to the sample plane. Films grown on Ta electrodes showed abrupt semiconductor-metal phase transitions in different nano-crystallites VO2. The IV characteristics of the film on the Pd electrode had an S-NDR region, specifically attributed to the formation of a filamentary current flow between the Pd probe and the electrode. The details of this phenomenon could not be established definitively, but if in fact the electrical transition is present in nano-crystallites measured, it was suggested that the origin of this conducting channel may be related to reminiscent earlier phase transitions.
Nesta tese, realizou-se uma investigação estrutural e elétrica em torno da transição semicondutor-metal desencadeada termicamente em filmes finos de VO2. Os filmes foram depositados por magnetron sputtering reativo, os eletrodos metálicos, bem como camadas buffers de SiO2 sobre os substratos de Si foram depositados por magnetron sputtering. As características cristalográficas e morfológicas foram evidenciadas através de medidas de difração de raios-X em função da temperatura e microscopia de força atômica (AFM), respectivamente. A caracterização elétrica, em nanoescala foi realizada utilizando-se um sistema de medidas via nano-ponteiras. Os resultados de difração de raios-X à temperatura ambiente revelaram que as amostras são policristalinas e estão fortemente texturizados com a direção < 011 > praticamente perpendicular ao plano do substrato. Os difratogramas em função da temperatura foram realizados para acompanhar a transição cristalográfica que o VO2 apresenta próximo a temperatura de 68°C. Para os filmes depositados sobre SiO2 (sem eletrodo) e sobre o eletrodo de Ta, em temperaturas abaixo da temperatura crítica para a transição, o material apresentou-se na fase monoclínica M1. Na faixa de temperaturas que compreende a transição, ocorre o surgimento progressivo do pico correspondente ao plano (110) da fase rutila R. Para uma faixa relativamente grande de temperaturas, há uma coexistência das fases M1 e R e, eventualmente da monoclínica M2. Como seria de se esperar, o pico da estrutura rutila cresce até o ponto de ser praticamente o único presente, quando a temperatura atingiu cerca de 80°C. A transição cristalográfica de um filme de VO2 com eletrodo de Pd foi acompanhada por medidas de difração à temperatura ambiente. O pico (011) da fase M1 é muito menor comparado ao das amostras depositadas sobre eletrodo de Ta. Porém, contrariamente ao eletrodo de Ta, que provavelmente tenha crescido na forma de nano-grãos muito pequenos ou mesmo na forma amorfa, o filme de Pd depositado é policristalino e bastante texturizado. As propriedades de transporte durante a transição de fase elétrica forma investigadas utilizando-se injeção de corrente elétrica perpendicular ao plano da amostra. Esta investigação, para os filmes crescidos sobre eletrodo de Ta, mostraram abruptas transições de fase semicondutor-metal em diferentes nano-cristalitos de VO2. As características I-V do filme com eletrodo de Pd apresentaram uma região com S-NDR, especificamente atribuída à formação de um regime filamentar de corrente entre a ponteira e o eletrodo de Pd. Os detalhes deste fenômeno não puderam ser estabelecidos de forma definitiva, mas se de fato a transição elétrica está presente nos nano-cristalitos medidos, sugeriu-se que a origem deste canal condutor pode estar relacionada com transições de fase anteriores e remanescentes.
48
Cymes, Brittany Allison. "Catalytic Properties of Novel Microporous Minerals." Miami University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=miami1587730697926361.
49
Sánchez, Diana Luis David. "High performance photonic devices for switching applications in silicon photonics." Doctoral thesis, Universitat Politècnica de València, 2017. http://hdl.handle.net/10251/77150.
Abstract:
El silicio es la plataforma más prometedora para la integración fotónica, asegurando la compatibilidad con los procesos de fabricación CMOS y la producción en masa de dispositivos a bajo coste. Durante las últimas décadas, la tecnología fotónica basada en la plataforma de silicio ha mostrado un gran crecimiento, desarrollando diferentes tipos de dispositivos ópticos de alto rendimiento.
Una de las posibilidades para continuar mejorando las prestaciones de los dispositivos fotónicos es mediante la combinación con otras tecnologías como la plasmónica o con nuevos materiales con propiedades excepcionales y compatibilidad CMOS. Las tecnologías híbridas pueden superar las limitaciones de la tecnología de silicio, dando lugar a nuevos dispositivos capaces de superar las prestaciones de sus homólogos electrónicos. La tecnología híbrida dióxido de vanadio/ silicio permite el desarrollo de dispositivos de altas prestaciones, con gran ancho de banda, mayor velocidad de operación y mayor eficiencia energética con dimensiones de la escala de la longitud de onda.
El objetivo principal de esta tesis ha sido la propuesta y desarrollo de dispositivos fotónicos de altas prestaciones para aplicaciones de conmutación. En este contexto, diferentes estructuras basadas en silicio, tecnología plasmónica y las propiedades sintonizables del dióxido de vanadio han sido investigadas para controlar la polarización de la luz y para desarrollar otras funcionalidades electro-ópticas como la modulación.Silicon is the most promising platform for photonic integration, ensuring CMOS fabrication compatibility and mass production of cost-effective devices. During the last decades, photonic technology based on the Silicon on Insulator (SOI) platform has shown a great evolution, developing different sorts of high performance optical devices. One way to continue improving the performance of photonic optical devices is the combination of the silicon platform with another technologies like plasmonics or CMOS compatible materials with unique properties. Hybrid technologies can overcome the current limits of the silicon technology and develop new devices exceeding the performance metrics of its counterparts electronic devices. The vanadium dioxide/silicon hybrid technology allows the development of new high-performance devices with broadband performance, faster operating speed and energy efficient optical response with wavelength-scale device dimensions. The main goal of this thesis has been the proposal and development of high performance photonic devices for switching applications. In this context, different structures, based on silicon, plasmonics and the tunable properties of vanadium dioxide, have been investigated to control the polarization of light and for enabling other electro-optical functionalities, like optical modulation.
El silici és la plataforma més prometedora per a la integració fotònica, assegurant la compatibilitat amb els processos de fabricació CMOS i la producció en massa de dispositius a baix cost. Durant les últimes dècades, la tecnologia fotònica basada en la plataforma de silici ha mostrat un gran creixement, desenvolupant diferents tipus de dispositius òptics d'alt rendiment. Una de les possibilitats per a continuar millorant el rendiment dels dispositius fotònics és per mitjà de la combinació amb altres tecnologies com la plasmònica o amb nous materials amb propietats excepcionals i compatibilitat CMOS. Les tecnologies híbrides poden superar les limitacions de la tecnologia de silici, donant lloc a nous dispositius capaços de superar el rendiment dels seus homòlegs electrònics. La tecnologia híbrida diòxid de vanadi/silici permet el desenvolupament de dispositius d'alt rendiment, amb gran ample de banda, major velocitat d'operació i major eficiència energètica en l'escala de la longitud d'ona. L'objectiu principal d'esta tesi ha sigut la proposta i desenvolupament de dispositius fotònics d'alt rendiment per a aplicacions de commutació. En este context, diferents estructures basades en silici, tecnologia plasmònica i les propietats sintonitzables del diòxid de vanadi han sigut investigades per a controlar la polarització de la llum i per a desenvolupar altres funcionalitats electró-òptiques com la modulació.
Sánchez Diana, LD. (2016). High performance photonic devices for switching applications in silicon photonics [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/77150
TESIS
50
John, Jimmy. "VO2 nanostructures for dynamically tunable nanophotonic devices." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEI044.
Abstract:
L'information est devenue le bien le plus précieux au monde. Ce mouvement vers la nouvelle ère de l'information a été propulsé par la capacité à transmettre l'information plus rapidement, à la vitesse de la lumière. Il est donc apparu nécessaire de mener des recherches plus poussées pour contrôler plus efficacement les supports d'information. Avec les progrès réalisés dans ce secteur, la plupart des technologies actuelles de contrôle de la lumière se heurtent à certains obstacles tels que la taille et la consommation d'énergie et sont conçues pour être passives ou sont limitées technologiquement pour être moins actives (technologie Si-back). Même si rien ne voyage plus vite que la lumière, la vitesse réelle à laquelle les informations peuvent être transportées par la lumière est la vitesse à laquelle nous pouvons la moduler ou la contrôler. Ma tâche dans cette thèse visait à étudier le potentiel du VO2, un matériau à changement de phase, pour la nano-photonique, avec un accent particulier sur la façon de contourner les inconvénients du matériau et de concevoir et démontrer des dispositifs intégrés efficaces pour une manipulation efficace de la lumière à la fois dans les télécommunications et le spectre visible. En outre, nous démontrons expérimentalement que les résonances multipolaires supportées par les nanocristaux de VO2 (NC) peuvent être réglées et commutées dynamiquement en exploitant la propriété de changement de phase du VO2. Et ainsi atteindre l'objectif d'adaptation de la propriété intrinsèque basée sur le formalisme de Mie en réduisant les dimensions des structures de VO2 comparables à la longueur d'onde de fonctionnement, créant un champ d'application pour un métamatériau accordable défini par l'utilisateurInformation has become the most valuable commodity in the world. This drive to the new information age has been propelled by the ability to transmit information faster, at the speed of light. This erupted the need for finer researches on controlling the information carriers more efficiently. With the advancement in this sector, majority of the current technology for controlling the light, face certain roadblocks like size, power consumption and are built to be passive or are restrained technologically to be less active (Si- backed technology). Even though nothing travels faster than light, the real speed at which information can be carried by light is the speed at which we can modulate or control it. My task in this thesis aimed at investigating the potential of VO2, a phase change material, for nano-photonics, with a specific emphasis on how to circumvent the drawbacks of the material and to design and demonstrate efficient integrated devices for efficient manipulation of light both in telecommunication and visible spectrum. In addition to that we experimentally demonstrate the multipolar resonances supported by VO2 nanocrystals (NCs) can be dynamically tuned and switched leveraging phase change property of VO2. And thus achieving the target tailoring of intrinsic property based on Mie formalism by reducing the dimensions of VO2 structures comparable to the wavelength of operation, creating a scope for user defined tunable metamaterial