28 research outputs found

    Thermolubricity of gas monolayers on graphene

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    Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. Graphene was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates

    Dual photo- and redox- active molecular switches for smart surfaces

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    Moleculaire schakelaars zijn organische moleculen waarvan de structuur of conformatie verandert onder interactie met licht of elektrische potentiaal. In de inleiding van dit proefschrift wordt een route naar de ontwikkeling van slimmen oppervlakten geïllustreerd op basis van een aantal belangrijke voorbeelden van zulke moleculaire schakelaars die zowel op licht als op potentiaal reageren. Van bis-thiaxanthylidene overcrowded alkene schakelen door licht en een redox-potentiaal in oplossing de lichtgevende eigenschappen. Opvallend genoeg, vertoont het een hogere stabiliteit van de instabiele conformationele toestanden wanneer het geïmmobiliseerd als een monolaag op goud of indiumtinoxide, hetgeen wordt verklaard door de intermoleculaire reacties in zo`n monolaag. Verder maakt oxidatie naar een di-kationische toestand (2+) de reversibele verandering van de bevochtigingseigenschappen van het oppervlak (een verandering van de contacthoek van water > 30°) mogelijk, hetgeen gebruikt kan worden als een vluchtig moleculair geheugen. Spiropyrans zijn fotochemisch en elektrochemisch schakelende moleculen die al meer dan 60 jaar bekend zijn. Fotochemisch schakelen door UV- of zichtbaar licht wordt typisch gebruikt om dit molecuul in oplossing heen en weer te laten gaan tussen de vorm met de open, respectievelijk de gesloten ring. In dit onderzoek is het ook gelukt om dit reversibel schakelen mogelijk te maken in zelf-geassembleerde monolagen op goud; ook een bundel van nabij IR-licht is succesvol toegepast om ring-opening te veroorzaken. Volgens de literatuur is oxidatie van spiropyrans irreversibel doordat de ring opent en er vervolgens dimerisatie optreedt. Op basis van verschillende spectroscopische technieken laten we in dit proefschrift zien dat dimerisatie van geöxideerd spiropyran in oplossing inderdaad wel gebeurt; echter niet in de vorm met de open ring, maar juist in die met de gesloten ring. Opvallend genoeg zijn ook in monolagen op goud intermoleculaire interacties gunstig voor zulke oxidatieve dimerisatie. Daarom kunnen monolagen van spiropyran die zowel reageren op licht, als op redox-potentiaal, gebruikt worden om een moleculaire schrijf-lees-wis geheugenstoepassing te ontwikkelen

    Near-Surface Alloys of PtRh on Rh(111) and Pt(111) Characterized by STM

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    The use of Pt–Rh catalysts is central in a number of industrial processes, and to explain their performance it is essential to have a solid understanding of their nanoscopic surface structure. Here we use scanning tunneling microscopy to investigate the growth, elemental nanostructuring, and reconstruction behavior of various Pt–Rh near-surface alloys (NSA) on fcc Rh(111) and Pt(111). We document the formation of a novel lamellar island reconstruction atop Pt/Rh(111) islands as well as network reconstructions on Rh/Pt(111), similar to those previously observed for Pt/Pt(111). The extended roadmap for preparation of PtRh NSAs allows comparison with other homo/heteroepitaxial metal/fcc surfaces and provides a facile guideline for producing tailor-made model catalytic surfaces

    Preparation and Post-Functionalization of Hyperbranched Polyurea Coatings

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    Postfunctionalizable hyperbranched polyurea coatings were prepared by the bulk polycondensation of AB(2) monomers on preactivated silicon substrates. As previously shown, AB(2) monomers were prepared, comprising a secondary amino group (A) and two blocked isocyanates (B) connected by hexyl spacers, in a single step and in quantitative yields. Covalent anchoring of the coatings on substrates was accomplished by reacting the secondary amino group in the focal point of the polymers with the blocked isocyanates (BIs) of the covalently attached coupling agent. The BIs in the top layer of the coatings were storage-stable under ambient conditions but well-modifiable with amino- or hydroxyl-functional compounds on heating. Attachment of polyethylene glycol or perfluoro-1-decanol afforded hydrophilic or hydrophobic surfaces. Immobilization and quaternization of polyethylenimines yielded highly charged surfaces. The coatings were extensively characterized by a number of techniques, such as Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, ellipsometry, and contact -angle measurements

    A Remarkable Multitasking Double Spiropyran; Bidirectional Visible Light Switching of Polymer Coated Surfaces with dual Redox and Proton Gating

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    Smart or functional surfaces that exhibit complex multi-modal responsivity, e.g., to light, heat, pH etc., although highly desirable, require a combination of distinct functional units to achieve each type of response, and present a challenge in achieving combinations that can avoid cross-talk between the units, such as excited state quenching. Compounds that exhibit multiple switching modalities help overcome this challenge and drastically reduce the synthetic cost and complexity. Here, we show that a bis-spiropyran photochrome, which is formed through coupling at the indoline 5-position using redox chemistry, exhibits pH gated photochromism, with opening of the spiro moiety by irradiation with UV light and the expected reversion both by heating and irradiation with visible light gated by protonation/deprotonation. Remarkably, when oxidized to its dicationic form, bis-spiropyran2+, visible light can be used instead of UV light to switch between the spiro and merocyanine forms, with locking and unlocking of each state achieved by protonation/deprotonation. The formation of the bis-spiropyran unit by electrochemical coupling is exploited to generate 'smart surfaces', i.e. polymer modified electrodes, avoiding the need to introduce an ancillary functional group for polymerization and the concomitant potential for cross-talk. The approach taken means that the multi-repsonsive properties of the bis-spiropyran are not only retained upon immobilization, but in fact that the effective switching rate can be enhanced dramatically

    Light Emission as a Probe of Energy Losses in Molecular Junctions

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    Visible light emission was observed for molecular junctions containing 5–19 nm thick layers of aromatic molecules between carbon contacts and correlated with their current–voltage behaviors. Their emission was compared to that from Al/AlOx/Au tunnel junctions, which has been previously attributed to transport of carriers across the AlOx layer to yield “hot carriers” which emit light as they relax within the Au contact. The maximum emitted photon energy is equal to the applied bias for the case of coherent tunneling, and such behavior was observed for light emission from AlOx and thin (<5 nm) molecular junctions. For thicker films, the highest energy observed for emitted photons is less than <i>eV</i><sub>app</sub> and exhibits an energy loss that is strongly dependent on molecular layer structure and thickness. For the case of nitroazobenzene junctions, the energy loss is linear with the molecular layer thickness, with a slope of 0.31 eV/nm. Energy loss rules out coherent tunneling as a transport mechanism in the thicker films and provides a direct measure of the electron energy after it traverses the molecular layer. The transition from elastic transport in thin films to “lossy” transport in thick films confirms that electron hopping is involved in transport and may provide a means to distinguish between various hopping mechanisms, such as activated electron transport, variable range hopping, and Poole Frankel transport

    Oxidative electrochemical aryl C–C coupling of spiropyrans

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    The isolation and definitive assignment of the species formed upon electrochemical oxidation of nitro-spiropyran (SP) is reported. The oxidative aryl C–C coupling at the indoline moiety of the SP radical cation to form covalent dimers of the ring-closed SP form is demonstrated. The coupling is blocked with a methyl substituent para to the indoline nitrogen.

    UV/Vis and NIR Light-Responsive Spiropyran Self-Assembled Monolayers

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    <p>Self-assembled monolayers of a 6-nitro BIPS spiropyran (SP) modified with a disulfide-terminated aliphatic chain were prepared on polycrystalline gold surfaces and characterized by UV/vis absorption, surface-enhanced Raman scattering (SEAS), and X-ray photoelectron spectroscopies (XPS). The SAMs obtained are composed of the ring-closed form (i.e., spiropyran) only. Irradiation with UV light results in conversion of the monolayer to the merocyanine form (MC), manifested in the appearance of an N+ contribution in the N Is region of the XPS spectrum of the SAMs, the characteristic absorption band of the MC form in the visible region at 555 nm, and the C-O stretching band in the SERS spectrum. Recovery of the initial state of the monolayer was observed both thermally and after irradiation with visible light. Several switching cycles were performed and monitored by SERS spectroscopy, demonstrating the stability of the SAMs during repeated switching between SP and MC states. A key finding in the present study is that ring-opening of the surface-immobilized spiropyrans can be induced by irradiation with continuous wave NIR (785 nm) light as well as by irradiation with UV light. We demonstrate that ring-opening by irradiation at 785 nm proceeds by a two-photon absorption pathway both in the SAMs and in the solid state. Hence, spiropyran SAMs on gold can undergo reversible photochemical switching from the SP to the MC form with both UV and NIR and the reverse reaction induced by irradiation with visible light or heating. Furthermore, the observation of NIR-induced switching with a continuous wave source holds important consequences in the study of photochromic switches on surfaces using SERS and emphasizes the importance of the use of multiple complementary techniques in characterizing photoresponsive SAMs.</p>

    How Surface Species Drive Product Distribution during Ammonia Oxidation: An STM and Operando APXPS Study

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    The oxidation of ammonia is a key reaction for the production of artificial fertilizers and for environmental protection. Depending on the area of application, the catalytic reaction needs to be tuned toward the production of either N2 or NO, and this selectivity is controlled by temperature, pressure, reactant ratio, and the type of catalyst. PtRh alloys are highly useful catalytic materials for the oxidation of ammonia, and they can be employed at different reaction conditions. In contrast to pure Pt and Rh catalysts, for which a large number of studies of ammonia oxidation reaction mechanism are available, for PtRh alloys, direct spectroscopic evidence for structure–performance relationship is still lacking. To understand the behavior of PtRh alloys, namely, what is their active phase under reaction conditions and how the alloy composition leads to a particular product distribution, we study the oxidation of ammonia over PtRh/Pt(111) surfaces by simultaneous operando ambient pressure X-ray photoelectron spectroscopy and mass spectrometry at 1 mbar total reaction pressure. These data are complemented by a catalyst surface characterization by scanning tunneling microscopy in ultrahigh vacuum. We establish that the predominant surface structure during NH3 oxidation strongly depends on the degree of Pt enrichment and the O2/NH3 mixing ratio. At the nanoscale, the selectivity toward N2 or NO production is driven by the surface populations of N and O species. These, in turn, are controlled by the nature of the alloying of Pt with Rh
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