Suchergebnisse

7 pages, 4 figures.-- PACS nrs.: 73.20.-r. ; Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of ~1.8–2.0 eV with respect to the vacuum for the unoccupied resonance of the ns valence electron of alkali atoms (Li-Cs) chemisorbed at low coverage (less than 0.1 monolayer) on noble metal [Cu(111) and Ag(111)] surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the ns electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces. ; This work was supported by the U.S. Department of Energy Grant No. DE-FG02-03ER15434. Some calculations were performed in the Environmental Molecular Sciences Laboratory, a user facility sponsored by the DOE Office of Biological and Environmental Research. N.P. thanks the Alexander von Humboldt Foundation for support. H.P. thanks Ikerbasque for the support of his stay at DIPC. The San Sebastián group acknowledges support by the Spanish MEC grant, the European FP6-NoE grant "Nanoquanta", the UPV/EHU and the projects "NANOMATERIALES" and "NANOTRON" of the Basque Government and the Diputación de Guipúzcoa. ; Peer reviewed

We investigated the adsorption of pentacene on the (111) surface of platinum, which is an archetypal system for a junction with a low charge-injection barrier. We probed the structural and electronic configurations of pentacene by scanning tunnelling microscopy (STM), X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. We simulated the interface by means of ab initio methods based on the density functional theory (DFT) framework, while including the dispersion forces. We found that the molecules adsorb at the bridge site of the close-compact atom rows with the long axis parallel to the substrate's directions, in a slightly distorted geometry, driven by the good match between the position of the carbon atoms of the molecule and the underlying lattice of the surface. Most importantly, a chemical bond is formed at the interface, which we attribute to the high chemical reactivity of the Pt substrate. ; This work was performed in the PCAM network of European doctorate. S.S.H. acknowledges support from the European Union Seventh Framework Programme under grant agreement no. 607232 (THINFACE) and the computing infrastructure at the Vienna Scientific Cluster. L.F. acknowledges financial support from CNR-INFN national project (PREMIALE 2012) EOS 'Organic Electronics for Innovative research instrumentation'. ESISNA group acknowledge funding from MAT2014-54231-C4-1-P. ; Peer reviewed

Aluminum nitride (AlN), gallium nitride (GaN),and indium nitride (InN) form a family of technologicallyimportant semiconductors of high importance to light-emittingdiodes and high-frequency electronics. Although thinfilms of thesematerials are routinely manufactured by chemical vapor deposition(CVD) and atomic layer deposition (ALD), these methods are farfrom optimal and knowledge of the underlying chemical processesis lacking. In this work, we performed ab initio investigations of thesurface coverage of these materials under an ammonia-richatmosphere. Periodic density functional theory calculations wereused to test the probable surface structures, and their electronicand thermal energies were used to calculate their contribution tothe surface composition under the temperature and pressureconditions relevant for CVD and ALD processes of these materials. The results show similarities between the group of materials witha similar NHxsurface structure present for all three. Comparison of the coverage showed that at low growth temperatures, thesurface is expected to be covered by NH2, while at high temperatures, most surface sites would be vacant. The surface structureswere all found to be the most stable on AlN and least stable on InN. These results are important for further investigations of thematerial growth mechanisms. ; Funding Agencies|Swedish foundation for Strategic Research through the project "Time-resolved low temperature CVD for III-nitrides" [SSF-RMA 15-0018]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at the Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; Swedish Research Council (VR)Swedish Research Council

We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (N-ad) on VN(001) favor epitaxial atop-V positions and diffuse among them by transiting through 4-fold hollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V -amp;gt; FFH -amp;gt; atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (N-surf) atom. Frequent N-ad/N-surf pair exchange reactions lead to N-2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane (110) jumps and act as efficient catalysts for the dissociative chemisorption of incident N-2 molecules. During exposure of VN(001) to incident atomic N gas atoms, N-ad/N-ad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N-2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with N-ad or N-surf atoms. Similarities and differences between: N/VN(001) vs. previous N/TiN(001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning. ; Funding Agencies|Knut and Alice Wallenberg Foundation (Isotope Project) [2011.0094]; Swedish Research Council (VR) Linkoping Linnaeus Initiative LiLi-NFM [2008-6572, 2014-5790, 2013-4018]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials (through Swedens innovation agency VINNOVA) [MatLiU 2009-00971]

Электронная структура приповерхностных областей наночастиц оксидного материала особого типа и возможность на этой основе получения специфических выпрямляющих свойств контактаизучаются теоретически, а затем проверяются экспериментально. Построены модели поверхностных состояний типа Тамма с учетом кулоновского дальнодействия. Обнаруженная дисперсия и еезависимость от кривизны поверхности наночастиц позволяют модельно изучить условия формирования контактной разности потенциалов для случаев наночастиц одинакового радиуса (синергетический эффект), разных радиусов (легированный и нелегированный варианты), а также обнаружить возможность описания группы частиц порошка из данного материала в рамках модели Андерсона. Некоторые следствия предложенной концепции были исследованы в рамках эксперимента по характеру вольтамперных характеристик пары контактирующих частиц порошка с различными комбинациями радиусов, и они показывают наличие свойств, характерных для выпрямляющего контакта. Выдвигаются перспективы для дальнейших экспериментальных испытаний, вытекающих из предложенной теории.

Since the European Union has tightened its environmental legislation requirements to the maximum allowable industrial emissions, there is a necessity to devise efficient treatment technologies for exhaust SO2-containing gases. The study presents a critical analysis of industrial methods of disposal of sulfur (IV) oxide from exhaust gases. It is proved that the liquid-oxidative method using NaOH as an intermediate absorbent and air oxygen as an oxidant ensures optimal utilization of gases with low concentration of SO2. The analysis of the related references and the performed calculations show that chemical adsorption of SO2 requires high pH and low temperature, whereas oxidation of the chemisorbed SO2 – low pH and high temperature.The study justifies economic efficiency of combining the operations of dust collection, chemisorption and oxidation of the chemisorbed SO2 in a single unit. A thorough analysis of the existing mass-exchange equipment and the made calculations allow implementing the process. We have suggested new technical solutions and an efficient main unit of a horizontal apparatus with scoop-shaped dispersants that fully meets the physical and chemical characteristics of the above processes. The proposed device facilitates an upstream phase movement, thereby providing the required modes for dust collection, chemisorptions, and SO2 oxidation. The unit has low resistance to mass transfer from the gas and the fluid films. Due to the direct supply of energy to the fluid and the non-stationary mode of dispersion, the apparatus has little hydraulic resistance and low energy costs. The theoretical studies minimize the number of experimental research and scientifically justify the choice of a technologically appropriate mode of purifying exhaust gases from SO2. ; Показано, что низкококонцентрированные по SO2 газы целесообразно утилизировать жидкостно–окислительним способом с использованием NaOH в качестве промежуточного абсорбента и кислорода воздуха, как окислителя. Установлены технологически целесообразные режимы проведения хемосорбции и окисления SO2. На основе критического анализа современной массообменной аппаратуры обоснована эффективность горизонтального аппарата с ковшеобразными диспергаторами для совместного проведения процессов пылеулавливания, хемосорбции и окисления SO2. ; Показано, що низькоконцентровані за SO2 гази доцільно утилізувати рідинно-окисним способом з використанням NaOH у якості проміжного абсорбента та кисню повітря, як окисника. Встановлено технологічно доцільні режими проведення хемосорбції та окиснення SO2. На основі критичного аналізу сучасної масообмінної апаратури обґрунтовано ефективність горизонтального апарата з ковшоподібними диспергаторами для сумісного проведення процесів пиловловлення, хемосорбції та окиснення SO2.