Quantum size effects in Mg epilayers on Si(111): Enhancement of photoelectron yield and oxidation rate

The subject of this thesis are non-adiabatic e ects during the oxidation of di erent magnesium and silicon surfaces. In the chemicurrent setup different Schottky diodes served as high-pass filters to detect highly energetic electron-hole pair excitations as a current through the device. This detection of the current allowed monitoring of the reaction kinetics. Crystalline Mg films, with a thickness of only a few monolayers (ML), were grown on the p-doped Si(111) 7x7 reconstruction, forming thin film Schottky diodes. Quantum size effects as a result of the confinement of the electron system in the z-direction lead to a thickness dependent increase in the oxidation rate of the Mg surface. Valence band photoemission spectroscopy (UPS) measurements indicate that the electronic density of states at the Fermi edge, which oscillates due to the existence of Quantum well states, is the driving force behind this change in reactivity. X-ray photoemission spectroscopy (XPS) reveals that for these highly reactive films, not only the first but also the second atomic layer reacts to MgO. The low sample temperature of 110 K hinders bulk oxidation for samples that do not show quantum size effects. Thin Mg films that were deposited on Ag/p-Si(111) diodes showed current intensities, upon exposure to molecular oxygen, that depend on the Ag film thickness. A maximum in the detected current is found for an Ag layer with a thickness of 45 nm. It was possible to attribute this phenomenon to the effect of surface plasmon coupled chemiluminescence (SPCC). MgO formation emits chemiluminescence photons, which excite surface plasmons in the silver film. These decay radiatively at the Ag/Si interface and are detected as a current. Simulations of this interface in the attenuated total reflection (ATR) geometry reveal a maximum of absorption of the silver film at a thickness of 46 nm. Oxidation of the hydrogen passivated p-Si(111) surface and the depassivated p-Si(111) surface do not show a detectable chemicurrent. XPS measurements reveal that, while the former does not react with oxygen, the latter does show oxygen uptake. However, as the surface has virtually no electronic surface conductivity, no current can be measured. The p-Si(111) 7x7 reconstruction, which does react with oxygen, has a large enough surface conductivity to allow detection of a chemicurrent. The chemicurrent trace can be interpreted to be a displacement current. Strong Lyman α emission from the UV-source is found when it is operated with Ar, Ne or He. Combined with the regular noble gas photon lines, this allows the measurement of the total photoyield for thin, crystalline Mg films and investigation of the thickness dependence of the photoemission cross-section. The Lyman α emission can be explained by a resonant energy transfer process between noble gas excimers and hydrogen molecules.Quantengrößeneffekt bei kristallinen Mg Schichten auf Si(111): Erhöhung der Photoelektronenausbaute und der Oxidationsrate In dieser Arbeit wurden nicht-adiabatische Effekte bei der Oxidation verschiedener Magnesium- und Siliziumoberflächen untersucht. Dünnschicht-Schottkydioden ermöglichen die Detektion von den, von der Reaktion chemisch angeregten, Elektron-Loch Paaren als Chemoströme und erlauben so die Aufzeichnung der Reaktionskinetik. Kristalline Mg-Filme von nur wenigen Monolagen (ML) Dicke wurden auf die p-dotierte Si(111) 7x7 Rekonstruktion aufgedampft. Die Beschränkung des Elektronensystems auf wenige ML führt zu einer Oszillation der elektronischen Zustandsdichte an der Fermienergie (DOS (EF)). Diese wurde mit Valenzbandspektroskopie (UPS) gemessen. Hohe Zustandsdichten führen zu einem Anstieg der Reaktionsrate und zu einer Oxidation der zweiten atomaren Lage des Mg, wie mittels Röntgenphotoemissionsspektroskopie (XPS) gezeigt werden konnte. Die niedrige Probentemperatur von 110 K verhindert, bei Proben ohne erhöhte DOS (EF), eine Tiefenoxidation über die erste Lage hinaus. Mg-Schichten, die auf Ag/p-Si(111) Schottkydioden aufgedampft wurden, zeigen eine starke Abhängigkeit des detektierten Stroms von der Dicke des Silberfilms. Für eine Schichtdicke von 45 nm wird ein Maximum des Stroms detektiert. Dies kann mit dem Effekt der Oberflächenplasmonen gekoppelten Chemoluminiszenz (SPCC) erklärt werden. Die Reaktion von Mg mit O 2 emittiert Photonen, die Oberflächenplasmonen im Silberfilm anregen. Diese zerfallen radiativ an der Ag/Si Grenzfläche und werden im Silizium als Strom detektiert. Eine Simulation der Ag/Si Grenzfläche in der abgeschwächten Totalreflexion Geometrie (ATR) ergibt ein Absorptionsmaximum für die emittierte Strahlung für eine Silberschichtdicke von 46 nm. Drei verschiedene Siliziumoberflächen wurden elektrisch kontaktiert und dann O2 ausgesetzt. Die Wasserstoff-passivierte p-Si(111) Oberfläche zeigte keine Sauerstoffaufnahme und daher auch keinen Chemostrom durch die Diode. Die reine p-Si(111) Oberfläche zeigt hingegen eine Sauerstoffaufnahme. Da jedoch an dieser Oberfläche keine elektrische Leitfähigkeit existiert, kann kein Strom detektiert werden. Da hingegen die Oberfläche der p-Si(111) 7x7 Rekonstruktion sowohl elektrisch leitfähig als auch chemisch sehr reaktiv ist, ist für diese Oberfläche ein Strom und Sauerstoffaufnahme nachweisbar. Es kann gezeigt werden, dass dieser Strom ein Verschiebungsstrom ist. Während der UPS Messungen an kristallinen Mg-Schichten wurde eine starke Lyman α Emissionaus der UV-Quelle gefunden, obwohl reines Neon-, Argon- oder Heliumgas verwendet wurden. Diese Lyman α Linie wurden verwendet, um die anregungsenergieabhängigkeit der Photoelektronenemission des Magnesiumfilms zu untersuchen. Die Erzeugung der Wasserstoffinien kann durch einen quasi-resonanten Energietransfer zwischen den Wasserstoffmolekülen und Edelgasexzimeren erklärt werden

Distribution, ecology, and population structure of Senecio lopezii (Asteraceae) in the Serra de Monchique (SW Portugal)

The Serra de Monchique, a mountain chain in the hinterland of the Algarve Province, is characterized by a remarkable degree of floristic richness and endemism. Senecio lopezii, a tall-growing hemicryptophyte, is one of these endemic plants. We studied the local distribution, the ecological requirements, and the size and vitality of the populations in order to evaluate the vulnerability of this species with regard to disturbance by man and landuse changes. Between April and June 2001, populations observed in an area of 84 km² (= the central part of the Serra) were analysed in detail by non-destructive methods. A synopsis of the phytosociology and ecology of the species, based on literature including the Spanish populations in the Campo de Gibraltar (SW Andalusia), is presented. Senecio lopezii was discovered in the Serra de Monchique at only 5 localities, all in mid altitude (500 to 600 m a. s. l.) in the mesomediterranean bioclimatic belt on the northwestern, windward side of the Serra. The species is associated with the Sanguisorbo-Quercetum suberis quercetosum canariensis and its shaded fringes. It persists in Castanea sativa coppices, cultivated on sites with semi-deciduous oak forests as climax, when the topsoil remains intact. Population size ranges from 30 to 350 individuals, the total number is 920 specimens. Plant size and phenological stage were recorded for 258 individuals. Most of them (89 %) are sterile adults, which persist in dark conditions under a closed tree canopy. Flowering starts at a specific plant size and increases with growing rosette diameter, respectively, increasing number of leaves. To get information about the age structure, the life cycle, the role of the seed bank, and the effect of clearings of the shrub layer, further investigations are necessary. Senecio lopezii is a rare species as regards distribution, habitat specification and population size. It must be considered as a potentially endangered species, because it will not tolerate the transformation of oak forests into Eucalyptus plantations

Improving the zT value of thermoelectrics by nanostructuring: Tuning the nanoparticle morphology of Sb2Te3 by using ionic liquids

A systematic study on the microwave-assisted thermolysis of the single source precursor (Et2Sb)2Te (1) in different asymmetric 1-alkyl-3-methylimidazolium- and symmetric 1,3-dialkylimidazolium-based ionic liquids (ILs) reveals the distinctive role of both the anion and the cation in tuning the morphology and microstructure of the resulting Sb2Te3 nanoparticles as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). A comparison of the electrical and thermal conductivities as well as the Seebeck coefficient of the Sb2Te3 nanoparticles obtained from different ILs reveals the strong influence of the specific IL, from which C4mimI was identified as the best solvent, on the thermoelectric properties of as-prepared nanosized Sb2Te3. This work provides design guidelines for ILs, which allow the synthesis of nanostructured thermoelectrics with improved performances

Arctic Seafloor Integrity Cruise No. MSM95 – (GPF 19-2_05)

The main aim of the MSM95 research expedition was to investigate and map physical impacts on the arctic seafloor in two distinct and contrasting Arctic areas (The Svalbard shelf edge and the HAUSGARTEN time series stations in the FRAM strait) with a range of research equipment. A ‘nested’ data approach was conducted in each research area, with broad seafloor mapping conducted initially with the R/V MARIA S. MERIAN onboard acoustic systems (The EM122 and EM712 bathymetric systems), followed by focused subsequent mapping conducted by PAUL 3000 automated underwater vehicle (AUV) sidescan and camera deployments, Ocean Floor Observation and Bathymetry System (OFOBS) towed sidescan and camera trawls and finally with very high resolution investigations conducted with a new mini-ROV launched directly from the OFOBS for close seafloor visual analysis. These data will be used to produce spatial distribution maps of iceberg and fishery impacts on the seafloor at three locations to the north, south and west of the Svalbard Archipelago, as well as maps of drop stone and topography variations across several of the HAUSGARTEN stations

Mono‐, Di‐, and Tri‐Valent Cation Doped BiFe0.95Mn0.05O3 Nanoparticles: Ferroelectric Photocatalysts

The ferroelectricity of multivalent codoped Bismuth ferrite (BiFeO3; BFO) nanoparticles (NPs) is revealed and utilized for photocatalysis, exploiting their narrow electronic bandgap. The photocatalytic activity of ferroelectric photocatalysts BiFe0.95Mn0.05O3 (BFM) NPs and mono‐, di‐, or tri‐valent cations (Ag+, Ca2+, Dy3+; MDT) coincorporated BFM NPs are studied under ultrasonication and in acidic conditions. It is found that such doping enhances the photocatalytic activity of the ferroelectric NPs approximately three times. The correlation of the photocatalytic activity with structural, optical, and electrical properties of the doped NPs is established. The increase of spontaneous polarization by the mono‐ and tri‐valent doping is one of the major factors in enhancing the photocatalytic performance along with other factors such as stronger light absorption in the visible range, low recombination rate of charge carriers, and larger surface area of NPs. A‐site doping of BFO NPs by divalent elements suppresses the polarization, whereas trivalent (Dy3+) and monovalent (Ag+) cations provide an increase of polarization. The depolarization field in these single domain NPs acts as a driving force to mitigate recombination of the photoinduced charge carriers.The ferroelectricity of Ag/Ca/Dy‐doped BiFe0.95Mn0.05O3 nanoparticles are utilized for photocatalysis under ultrasonic conditions. The mitigated recombination of photoinduced charge‐carriers in the nanoparticles due to the depolarization field, is one of the important factors for the photocatalytic rate. The piezoresponse becomes a crucial parameter under ultrasonic conditions for ferroelectric photocatalysts. The pink dye (rhodamine B) is photodegraded using MDT doped nanoparticles. The ease of photoinduced charge carrier separation in single domain nanoparticles using the depolarization field as a driving force is shown. imageDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Peer Reviewe

Scientific and human errors in a snow model intercomparison

International audienceTwenty-seven models participated in the Earth System Model - Snow Model Intercomparison Project (ESM-SnowMIP), the most data-rich MIP dedicated to snow modelling. Our findings do not support the hypothesis advanced by previous snow MIPs: evaluating models against more variables, and providing evaluation datasets extended temporally and spatially does not facilitate identification of key new processes requiring improvement to model snow mass and energy budgets, even at point scales. In fact, the same modelling issues identified by previous snow MIPs arose: albedo is a major source of uncertainty, surface exchange parametrizations are problematic and individual model performance is inconsistent. This lack of progress is attributed partly to the large number of human errors that led to anomalous model behaviour and to numerous resubmissions. It is unclear how widespread such errors are in our field and others; dedicated time and resources will be needed to tackle this issue to prevent highly sophisticated models and their research outputs from being vulnerable because of avoidable human mistakes. The design of and the data available to successive snow MIPs were also questioned. Evaluation of models against bulk snow properties was found to be sufficient for15 some but inappropriate for more complex snow models whose skills at simulating internal snow properties remained untested. Discussions between the authors of this paper on the purpose of MIPs revealed varied, and sometimes contradictory, motivations behind their participation. These findings started a collaborative effort to adapt future snow MIPs to respond to the diverse needs of the communit

Proposta de Roteiro Estratégico para Implantação de Teletrabalho em Organizações Públicas: Um Estudo de Caso no Ministério Público do Trabalho.

Teletrabalho é uma forma de atividade laboral desenvolvida em local diverso daquele em que se situa estabelecimento empregador ou tomador dos serviços e para cuja execução há utilização de ferramentas tecnológicas de informação e comunicação para produção, armazenamento e intercâmbio de resultados. A compreensão ampla deste fenômeno, notadamente quanto aos fatores tecnológicos, processos organizacionais e relações socioprofissionais que o viabilizam, acompanhada de análise do modo como vem sendo incorporado às atividades públicas, é a base desta dissertação, que pretende oferecer uma proposta de roadmapping para implantação de teletrabalho em organizações públicas, em integração com o planejamento e a gestão estratégica. Palavras-chave: teletrabalho; roadmapping; organizações públicas