Fabrication of SiO2 by anisotropic etching of silicon

Bakalářská práce se zabývá přípravou membrán z oxidu křemičitého (SiO2) na křemíkovém (Si) substrátu pomocí anizotropního leptání křemíku. Masky pro anizotropní leptání křemíku byly vytvořeny pomocí elektronové litografie a chemického leptání SiO2. V práci jsou popsány jednotlivé kroky postupu přípravy membrán včetně použitých experimentálních podmínek. Za účelem optimalizace pracovního postupu bylo provedeno několik měření rychlosti leptání Si/SiO2 v různých roztocích. Výsledky těchto měření jsou zde rovněž uvedeny. Připravené membrány byly charakterizovány pomocí optické mikroskopie, rastrovací elektronové mikroskopie a spektroskopické reflektometrie. Součástí práce je i popis metod použitých pro přípravu a analýzu definovaných struktur připravených pomocí anizotropního leptání křemíku.The aim of the bachelor's thesis is the fabrication of silicon dioxide (SiO2) membranes on silicon (Si) substrate by anisotropic etching of silicon. Masks for anisotropic silicon etching were prepared by electron beam litography and SiO2 wet etching. Individual steps of membrane fabrication are described, including used experimental conditions. In order to optimize the fabrication process, etch rates of Si/SiO2 in several solutions were measured. Results of the measurements are included in the thesis. Fabricated membranes were characterised by optical microscopy, scanning electron microscopy and spectroscopic reflectometry. Methods used for fabrication and analysis of defined structures created by anisotropic silicon etching are briefly summarized.

Switching vortex chirality in magnetostatically coupled permalloy nanodisks

Diplomová práce se zabývá přepínáním cirkulace vortexů v magnetických nanodiscích. V práci jsou uvedeny výsledky mikromagnetických simulací hysterezních smyček disků s různými stupni asymetrie. Je zde diskutován vliv geometrické asymetrie disku na tvar hysterezní smyčky. Je zde také popsáno přepínání cirkulace vortexů v asymetrických discích pomocí vnějšího magnetického pole v rovině disku. Dále byly provedeny simulace hysterezních smyček dvojic magnetostaticky svázaných nanodisků v různých vzdálenostech a pro různé stupně asymetrie. Analýzou výsledků simulací byl porovnán vliv magnetostatické interakce a tvarové asymetrie na výslednou cirkulaci jednotlivých vortexů a odhadnut dosah magnetostatické interakce pro disky daných rozměrů a dané asymetrie. Součástí práce je stručný popis experimentálních technik použitých pro přípravu a měření připravených vzorků.The diploma thesis is concerned with switching of vortex circulation in magnetic nanodisks. The results of micromagnetic simulations of hysteresis loops of individual disks with different degrees of asymmetry are presented. The influence of geometric asymmetry of the disk on the shape of the hysteresis loop is discussed as well as switching of vortex circulation in asymmetric nanodisks by external in-plane magnetic field. Simulations of pairs of magnetostatically coupled nanodisks were carried out for different interdisk distances and degrees of asymmetry. By analysing the results of the simulations, the effects of magnetostatic coupling and the asymmetry on resultant circulation of individual vortices were compared and the range of magnetostatic interaction between nanodisks of given dimensions and asymmetry was estimated. Experimental techniques used for fabrication and measurement of the samples are briefly summarized.

Apparatus for dosing liquid water in ultrahigh vacuum

The structure of the solid-liquid interface often defines the function and performance of materials in applications. To study this interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows bringing a surface in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice. The ice is at cryogenic temperature, which reduces its vapor pressure to the UHV range. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. In test experiments, a rutile TiO2(110) single crystal exposed to liquid water showed unprecedented surface purity, as established by X-ray photoelectron spectroscopy and scanning tunneling microscopy. These results enabled us to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed. (C) 2018 Author(s)

Wechselwirkung von Titanoxidoberflächen mit flüssigem Wasser

Arbeit an der Bibliothek noch nicht eingelangt - Daten nicht geprueftAbweichender Titel nach Übersetzung der Verfasserin/des VerfassersMetalloxide sind eine wichtige Materialklasse, da sich auf den meisten Metalle unter Umgebungsbedingungen spontan eine Oxidschicht ausbildet. Diese Materialien sind nicht nur reichlich vorhanden, sondern bieten auch eine breite Palette interessanter Eigenschaften. Ein prototypisches Beispiel ist Titandioxid (TiO2), das in vielen industriellen Anwendungen wie der Fotokatalyse, in Farbstoffsolarzellen und der heterogenen Katalyse eingesetzt wird. Zur Zeit sind die Oberflächen von TiO2 unter hoch idealisierten Ultrahochvakuumbedingungen (UHV) gut verstanden. In Realbedingungen sind die Oberflächen jedoch von Gasen umgeben und mit Flüssigkeiten bedeckt. Viele praktische Verfahren verwenden die Oberfläche eines Festkrpers, der in eine Wasserlösung eingetaucht ist. In der Luft sind Oberflächen automatisch von einem dnnen Kondenswasserfilm bedeckt. Es ist oft diese Fest-Flüssig-Grenzfläche, die das Verhalten eines Materials in der Anwendung definiert. Das Verständnis von Prozessen auf atomaren Maßstab ist wesentlich für eine rationale Gestaltung von Materialien, um die Effizienz in Anwendungen weiter zu steigern und zu verbessern. Die Untersuchung von Oberflächen unter Umgebungsbedingungen bleibt aufgrund der begrenzten Anzahl verfgbarer experimenteller Methoden und einer hohen Wahrscheinlichkeit einer Kontamination eine Herausforderung. In dieser Dissertation wurde eine Versuchsapparatur entwickelt, die es ermöglicht, hochreine Flüssigkeiten ohne Lufteinwirkung auf die Oberfläche einer Probe (typischerweise eines Einkristalls) zu dosieren. Diese Apparatur ist mit einer vorhandenen Ultrahochvakuum (UHV)- Kammer gekoppelt, die eine reproduzierbare Probenpräparation und Probencharakterisierung auf UHV basierende Analysemethoden ermöglicht. In dieser Arbeit wurde die Wechselwirkung von flüssigem Wasser mit den beiden niederenergetischen Orientierungen von Rutil TiO2 untersucht. Es stellte sich heraus, dass die (110)-Oberfläche ihre bulk-terminierte (11) Struktur beim Eintauchen in reines flüssiges Wasser beibehält. Darüber hinaus wurde den Ursprung einer molekular geordneten Überstruktur geklärt, über die bereits von mehreren Forschungsgruppen berichtet wurde. Diese Überstruktur besteht aus einer Mischung aus Carboxylatgruppen und bildet sich bei Kontakt mit Luft spontan auf der Obe fläche. Trotz ihrer relativ niedrigen Konzentration in der Atmosphäre adsorbieren die Carbonsäuren an TiO2 (110) mit einer hohen Affinität und blockieren die unterkoordinierten Oberflächenkationen. Im Gegensatz dazu wurde auf der (011)-Orientierung von Rutil TiO2 eine Veränderung der Oberflächenstruktur bei Kontakt mit flüssigem Wasser gefunden. Wie durch auf Dichtefunktionaltheorie basierende Berechnungen vorhergesagt,wird die ursprüngliche (21)-Rekonstruktion aufgehoben und dissoziiertes Wasser adsorbiert auf der unrekonstruierten (11)-Oberfläche in einer geordnete (21) Überstruktur. Neben den detaillierten Versuchsergebnissen umfasst die Arbeit auch das Design der UHV-kompatiblen Apparatur zur Dosierung von reinem flüssigem Wasser, eine kurze Beschreibung der verwendeten Methoden und eine kurze Einführung in die untersuchten Oberflächen.Metal oxides are an important class of materials as most metals spontaneously develop an oxide layer in ambient conditions. These materials are not only abundant, but also offer a wide range of interesting properties. A prototypical example is titanium dioxide (TiO2), which is utilized in many industrial applications such as in photocatalysis, dye-sensitized solar cells, and heterogeneous catalysis. At present, the surfaces of TiO2 are well understood under highly idealized ultrahigh vacuum (UHV) conditions. In reality, however, surfaces are surrounded by gases and covered with liquids. Many practical processes involve a solid surface immersed in an aqueous solution. In air, surfaces are automatically covered by a thin film of condensed water. It is often the solid-liquid interface that defines the performance of a material in application. Understanding atomic scale processes closer to real conditions is essential for a rational design of materials, and further improvement and increased efficiency in applications. The investigation of surfaces under ambient conditions remains a challenge due to the restricted number of available experimental techniques and a high chance of contamination. We have designed an experimental apparatus that allows dosing ultrapure liquid water on the surface of a sample (typically a single crystal) without exposure to air. The apparatus is coupled to an existing surface-science chamber, which enables reproducible sample preparation and sample characterization by UHV-based analytical techniques. Within the thesis, the interaction of liquid water with the two lowest-energy terminations of TiO2 rutile was studied. The (110) surface was found to retain its bulkterminated (11) structure upon immersion in pure liquid water. In addition, we clarified the origin of a molecularly ordered overlayer previously reported by several research groups. This overlayer consists of a mixture of carboxylates, and, upon exposure to air, spontaneously forms on the surface. Despite their relatively low atmospheric concentration, carboxylic acids adsorb on TiO2(110) with a high affinity and block the undercoordinated surface cation sites. In contrast, the (011) termination of TiO2 rutile was identified to change its surface structure upon contact with liquid water. As predicted by DFT calculations, the original (2 1) reconstruction was lifted, and dissociated water remained on the unreconstructed (11) surface in the form of an ordered (21) overlayer. Apart from detailed experimental results, the thesis also covers the design of the UHV-compatible apparatus for dosing pure liquid water, a brief description of the used techniques and a short introduction into the investigated surfaces.12

Fluoride-free wet-chemical preparation of oxide single crystal surfaces:66th Annual Meeting of the Austrian Physical Society

The ultimate goal to perform surface science studies under technologically relevant conditions includes wet-chemical methods to prepare well-defined oxide surfaces [1]. The most widely practised approach is hydrofluoric acid etching, even though this chemical poses serious health risks and may inadvertently dope the surface with fluorine, an efficient electron donor [2]. Here, we present a rational yet versatile wet-chemical alternative to lengthy sputtering–annealing cycles in ultrahigh vacuum for preparing single crystal oxide samples for surface science investigations. The method does not require hydrofluoric acid, is environmentally benign and is demonstrated on rutile TiO2 (110), rutile TiO2 (011) and SrTiO3 (100), but may have much wider application potential, also for surfaces that are quickly destroyed by acids. The procedure consists of (i) ultrasonication in the presence of a dispersing agent to remove polishing debris; (ii) thermal annealing to produce equilibrium-shaped steps and terraces determined by the crystal miscut; and (iii) oxidative cleaning in an alkaline mixture to remove adsorbed organic contaminants from the surface. Each of the steps is optimised based on AFM and characterisation in ultrahigh vacuum, including by LEED and XPS. Following this wet-chemical preparation, we demonstrate atomically resolved electrochemical scanning tunnelling microscopy on TiO2 (110), on a sample that was never treated by sputtering–annealing

Monodisperse tungsten oxide cluster deposition from solution:66th Annual Meeting of the Austrian Physical Society

Perfectly monodisperse clusters of oxides are critically important model systems for catalysis studies because they allow the rigorous analysis of reaction mechanisms, and variations at the single-atom level can already be reflected in their reactivity. The generation and intact immobilisation on a suitable substrate of such clusters is quite challenging, and usually requires mass spectrometric size selection and sophisticated soft landing protocols to make such studies successful. Tungsten (VI) oxide in particular holds promise as a visible-light photocatalyst, but is quite reactive and can be challenging to immobilise in a well-defined manner in vacuum [1]. Here, we present a solution-based protocol for the preparation of monodisperse cyclic tris (tungsten (VI) trioxide) clusters, (WO3) 3. The clusters can be harvested efficiently on the boron nitride nanomesh [2], an atomically thin layer of hexagonal boron nitride on Rh (111) with strong corrugation, and a promising platform for self-assembly [3] and electrochemical functionality [4]. The triangular (WO3) 3 clusters adsorb in the ‚pores‘ of the nanomesh, where they were imaged with submolecular resolution using electrochemical scanning tunnelling microscopy. The decorated surface was transferred to vacuum where the chemical identity of the clusters was confirmed with XPS. To our knowledge, this is the first successful example of self-assembly on the nanomesh from solution. We expect that proper control over deposition conditions will allow tuning of the number of clusters per pore, making this a promising model system for on-surface catalysis studies. We contrast this finding with deposition of the same source material on rutile TiO2 (110) in liquid, on which the clusters appear to react and form chains, akin to some observations of sublimated WO3 in vacuum [5]. Even though the clusters are likely hydroxylated in aqueous solution, this behaviour indicates surprising parallels with UHV and suggests that, in many cases, solution-based procedures complement vacuum methods