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Surfaces, Volume 1, Issue 1 (December 2018)

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Open AccessArticle The Work Function of TiO2
Surfaces 2018, 1(1), 73-89; https://doi.org/10.3390/surfaces1010007
Received: 13 July 2018 / Revised: 31 August 2018 / Accepted: 1 September 2018 / Published: 7 September 2018
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Abstract
Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO2 single crystals and (001)- and (110)-oriented rutile TiO2 single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation
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Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO 2 single crystals and (001)- and (110)-oriented rutile TiO 2 single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of 7.96 ± 0.15 eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies. Full article
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Open AccessArticle Material Structure and Mechanical Properties of Silicon Nitride and Silicon Oxynitride Thin Films Deposited by Plasma Enhanced Chemical Vapor Deposition
Surfaces 2018, 1(1), 59-72; https://doi.org/10.3390/surfaces1010006
Received: 9 May 2018 / Revised: 22 August 2018 / Accepted: 27 August 2018 / Published: 30 August 2018
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Abstract
Silicon nitride and silicon oxynitride thin films are widely used in microelectronic fabrication and microelectromechanical systems (MEMS). Their mechanical properties are important for MEMS structures; however, these properties are rarely reported, particularly the fracture toughness of these films. In this study, silicon nitride
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Silicon nitride and silicon oxynitride thin films are widely used in microelectronic fabrication and microelectromechanical systems (MEMS). Their mechanical properties are important for MEMS structures; however, these properties are rarely reported, particularly the fracture toughness of these films. In this study, silicon nitride and silicon oxynitride thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) under different silane flow rates. The silicon nitride films consisted of mixed amorphous and crystalline Si3N4 phases under the range of silane flow rates investigated in the current study, while the crystallinity increased with silane flow rate in the silicon oxynitride films. The Young’s modulus and hardness of silicon nitride films decreased with increasing silane flow rate. However, for silicon oxynitride films, Young’s modulus decreased slightly with increasing silane flow rate, and the hardness increased considerably due to the formation of a crystalline silicon nitride phase at the high flow rate. Overall, the hardness, Young modulus, and fracture toughness of the silicon nitride films were greater than the ones of silicon oxynitride films, and the main reason lies with the phase composition: the SiNx films were composed of a crystalline Si3N4 phase, while the SiOxNy films were dominated by amorphous Si–O phases. Based on the overall mechanical properties, PECVD silicon nitride films are preferred for structural applications in MEMS devices. Full article
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Open AccessArticle Diazonium Salts: Versatile Molecular Glues for Sticking Conductive Polymers to Flexible Electrodes
Surfaces 2018, 1(1), 43-58; https://doi.org/10.3390/surfaces1010005
Received: 20 June 2018 / Revised: 20 July 2018 / Accepted: 22 July 2018 / Published: 8 August 2018
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Abstract
Adhesion of polymers to surfaces is of the upmost importance in timely applications such as protective coatings, biomaterials, sensors, new power sources and soft electronics. In this context, this work examines the role of molecular interactions in the adhesion of polypyrrole thin films
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Adhesion of polymers to surfaces is of the upmost importance in timely applications such as protective coatings, biomaterials, sensors, new power sources and soft electronics. In this context, this work examines the role of molecular interactions in the adhesion of polypyrrole thin films to flexible Indium Tin Oxide (ITO) electrodes grafted with aryl layers from various diazonium salts, namely 4-carboxybenzenediazonium (ITO-CO2H), 4-sulfonicbenzenediazonium (ITO-SO3H), 4-N,N-dimethylbenzenediazonium (ITO-N(CH3)2), 4-aminobenzenediazonium (ITO-NH2), 4-cyanobenzenediazonium (ITO-CN) and 4-N-phenylbenzenediazonium (ITO-NHPh). It was demonstrated that PPy thin layers were adherent to all aryl-modified surfaces, whereas adhesive failure was noted for bare ITO following simple solvent washing or sonication. Adhesion of polypyrrole was investigated in terms of hydrophilic/hydrophobic character of the underlying aryl layer as probed by contact angle measurements. It was found that sulfonic acid-doped polypyrrole (PPy-BSA) thin films were preferably deposited on the most hydrophobic surfaces. More importantly, the redox properties and electrochemical impedance of PPy were closely related to the hydrophobic character of the aryl layers. This work demonstrates that diazonium compounds are unique molecular glues for conductive polymers and permit to tune their interfacial properties. With robust, diazonium-based architectured interfaces, one can design high performance materials for e.g., sensors, printed soft electronics and flexible thermoelectrics. Full article
(This article belongs to the Special Issue Electrochemical Surface Science: basics and applications)
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Open AccessArticle On the Contrasting Effect Exerted by a Thin Layer of CdS against the Passivation of Silver Electrodes Coated with Thiols
Surfaces 2018, 1(1), 29-42; https://doi.org/10.3390/surfaces1010004
Received: 14 June 2018 / Revised: 27 July 2018 / Accepted: 27 July 2018 / Published: 31 July 2018
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Abstract
The passivation of metal electrodes covered by self-assembled monolayers of long-chain thiols is well known. The disappearance of the voltammetric peak of redox species in solution is a classical test for the formation of full layers of thiols. Similar studies on semiconductors are
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The passivation of metal electrodes covered by self-assembled monolayers of long-chain thiols is well known. The disappearance of the voltammetric peak of redox species in solution is a classical test for the formation of full layers of thiols. Similar studies on semiconductors are still very limited. We used silver surfaces covered by an ultrathin layer of CdS as substrate for self-assembling of n-hexadecanethiol (C16SH), and we compared the experimental results with those obtained by using the bare silver surface as substrate. The strong insulating effect of C16SH deposited on Ag(III) is shown by the inhibition of the voltammetric peak of Ru(NH3)63+/2+. On the contrary, the voltammogram obtained on CdS-covered Ag(III) is very similar to that obtained on the bare Ag(III) electrode, thus suggesting that the presence of CdS exerts a contrasting effect on the passivation of the silver electrode. A crucial point of our work is to demonstrate the effective formation of C16SH monolayers on Ag(III) covered by CdS. The formation of full layers of C16SH was strongly suggested by the inhibition of the stripping peak of Cd from the CdS deposit covered by C16SH. The presence of C16SH was confirmed by electrochemical quartz crystal microbalance (EQCM) measurements as well as by Auger electron spectroscopy (AES) analysis. Full article
(This article belongs to the Special Issue Electrochemical Surface Science: basics and applications)
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Open AccessArticle Potential Driven Non-Reactive Phase Transitions of Ordered Porphyrin Molecules on Iodine-Modified Au(100): An Electrochemical Scanning Tunneling Microscopy (EC-STM) Study
Surfaces 2018, 1(1), 12-28; https://doi.org/10.3390/surfaces1010003
Received: 3 July 2018 / Revised: 22 July 2018 / Accepted: 24 July 2018 / Published: 25 July 2018
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Abstract
The modelling of long-range ordered nanostructures is still a major issue for the scientific community. In this work, the self-assembly of redox-active tetra(N-methyl-4-pyridyl)-porphyrin cations (H2TMPyP) on an iodine-modified Au(100) electrode surface has been studied by means of Cyclic Voltammetry
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The modelling of long-range ordered nanostructures is still a major issue for the scientific community. In this work, the self-assembly of redox-active tetra(N-methyl-4-pyridyl)-porphyrin cations (H2TMPyP) on an iodine-modified Au(100) electrode surface has been studied by means of Cyclic Voltammetry (CV) and in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) with submolecular resolution. While the CV measurements enable conclusions about the charge state of the organic species, in particular, the potentio-dynamic in situ STM results provide new insights into the self-assembly phenomena at the solid-liquid interface. In this work, we concentrate on the regime of positive electrode potentials in which the adsorbed molecules are not reduced yet. In this potential regime, the spontaneous adsorption of the H2TMPyP molecules on the anion precovered surface yields the formation of up to five different potential-dependent long-range ordered porphyrin phases. Potentio-dynamic STM measurements, as a function of the applied electrode potential, show that the existing ordered phases are the result of a combination of van der Waals and electrostatic interactions. Full article
(This article belongs to the Special Issue Electrochemical Surface Science: basics and applications)
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Open AccessArticle Switchable Interfaces: Redox Monolayers on Si(100) by Electrochemical Trapping of Alcohol Nucleophiles
Surfaces 2018, 1(1), 3-11; https://doi.org/10.3390/surfaces1010002
Received: 29 June 2018 / Accepted: 17 July 2018 / Published: 20 July 2018
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Abstract
Organic electrosynthesis is going through its renaissance but its scope in surface science as a tool to introduce specific molecular signatures at an electrode/electrolyte interface is under explored. Here, we have investigated an electrochemical approach to generate in situ surface-tethered and highly-reactive carbocations.
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Organic electrosynthesis is going through its renaissance but its scope in surface science as a tool to introduce specific molecular signatures at an electrode/electrolyte interface is under explored. Here, we have investigated an electrochemical approach to generate in situ surface-tethered and highly-reactive carbocations. We have covalently attached an alkoxyamine derivative on an Si(100) electrode and used an anodic bias stimulus to trigger its fragmentation into a diffusive nitroxide (TEMPO) and a surface-confined carbocation. As a proof-of-principle we have used this reactive intermediate to trap a nucleophile dissolved in the electrolyte. The nucleophile was ferrocenemethanol and its presence and surface concentration after its reaction with the carbocation were assessed by cyclic voltammetry. The work expands the repertoire of available electrosynthetic methods and could in principle lay the foundation for a new form of electrochemical lithography. Full article
(This article belongs to the Special Issue Electrochemical Surface Science: basics and applications)
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Open AccessEditorial Welcome to Surfaces—A New Open Access Journal for an Interdisciplinary Scientific Community
Surfaces 2018, 1(1), 1-2; https://doi.org/10.3390/surfaces1010001
Received: 27 March 2018 / Accepted: 27 March 2018 / Published: 30 March 2018
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Abstract
Surfaces are ubiquitous. Everyday life gives plenty of examples where surfaces and interfaces play a leading role[...] Full article
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