1,721,318 research outputs found
Semiconducting nanocrystals: synthesis, colloidal stability, and charge carrier dynamics
Full text linkColloidal nanocrystals (NCs) with controllable morphologies and sizes are at the core of nanomaterials research. Among different NCs (zero-dimensional quantum dots, one-dimensional nanorods, etc.), two-dimensional cadmium selenide (CdSe) nanoplatelets (NPLs) with lateral extension of over 100 nm2 and atomically precise thickness are one of the most promising semiconductors for optoelectronic applications. However, several issues remain for implementing CdSe NPLs for applications, including: (1) containing toxic and heavy elements, and (2) lacking a fundamental understanding of the colloidal stability and the process of charge carrier generation in solids. This thesis aims to synthesize environmentally friendly alternatives, such as indium phosphide (InP) NCs with varied shapes, and shed light on colloidal stability and charge carrier generation and dynamics using conventional CdSe NPLs as the model system.
The introductory Chapter 1 briefly presents an overview of nanomaterials and nanotechnology, including quantum confinement in NCs, quantum dots, InP NCs, the synthesis status of CdSe NPLs, colloidal NCs stabilization in dispersion, photophysics of CdSe NPLs, ligand exchange method, and the thermal annealing method. In Chapter 2, we introduce terahertz (THz) spectroscopy basics, including the generation and detection mechanism, experimental setups, and the conductivity models for data analysis to infer charge transport properties in materials of interest. Chapters 3-5 present the key results of this thesis. In Chapter 3, we synthesize InP NCs with different shapes and sizes by utilizing triphenyl phosphite as the phosphorus source. We show that InP NCs can be synthesized and formed from In2O3 NCs. Our findings open new synthetic possibilities by utilizing a cost-effective, non-pyrophoric, and non-toxic phosphorus precursor. In Chapter 4, we combine THz photoconductivity measurements with simulations to study the colloidal stability and aggregation process in CdSe NPL dispersion. We demonstrate that increasing the facet area of NPLs and the solvent length can reduce the critical concentration at which NPLs start aggregating. Our simulation attributes the effect to the solvation force-dominated colloidal stability for the NPL system; the solvation force is enhanced by increasing the NPL lateral area and the length of solvent molecules. In Chapter 5, we report that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. We observe a direct correlation between the temperature-dependent ligand decomposition and the NPL-NPL distance shortening (by TEM). This leads to a strong red-shift in the absorption band edge (by UV-vis studies) and enhanced electrical transport
Abstract
in NPL films. Our results illustrate a straightforward manner to control the interfacial electronic coupling strength for developing functional optoelectronics through thermal treatments.III, 97 Seiten ; Illustrationen, Diagramm
Single-Atom Catalysts through Pressure-Controlled Metal Diffusion
Full text linkSingle-atom catalysts (SACs) open up new possibilities for advanced technologies. However, a major complication in preparing high-density single-atom sites is the aggregation of single atoms into clusters. This complication stems from the delicate balance between the diffusion and stabilization of metal atoms during pyrolysis. Here, we present pressure-controlled metal diffusion as a new concept for fabricating ultra-high-density SACs. Reducing the pressure inhibits aggregation substantially, resulting in almost three times higher single-atom loadings than those obtained at ambient pressure. Molecular dynamics and computational fluid dynamics simulations reveal the role of a metal hopping mechanism, maximizing the metal atom distribution through an increased probability of metal-ligand binding. The investigation of the active site density by electrocatalytic oxygen reduction validates the robustness of our approach. The first realization of Ullmann-type carbon-oxygen couplings catalyzed on single Cu sites demonstrates further options for efficient heterogeneous catalysis
Investigating the water-titanium dioxide interface using sum frequency generation spectroscopy
Full text linkIn order to fight global warming, clean and cheap alternatives to fossil fuels are needed. A promising candidate is hydrogen, which can be obtained by photocatalytic splitting of water utilizing sunlight and titanium dioxide as photocatalyst. To make this process profitable, its fundamentals need to be understood. The research presented in this work aims at a better understanding of the structure of the water-TiO2-interface, using the surface sensitive Sum-Frequency-Generation (SFG) spectroscopy technique.
The first experimental part of this work demonstrates the potency of SFG. It is used as a complementary technique to learn more about the behaviour of the surfactant CTAB on the water-air-interface in the presence of high amounts of dissolved NaCl.
One part of this work was the deposition of TiO2-thin films on CaF2-windows using the magnetron-RF-sputter technique. The target was to produce samples for measurements at the TiO2-D2O-interface. The main problems of this task are presented and it is shown why no reproducible results could be obtained.
The first main part of this thesis describes SFG experiments on the TiO2-D2O-interface using solutions of pD 3, 5, 7, 9 and 11. The SFG-signal shows three peaks for all pD values. The high-frequency peak originates from weakly hydrogen-bonded interfacial Ti-OD groups that are pointing away from the TiO2. The low-frequency peak is caused by D2O molecules pointing towards the semiconductor, forming strong donating hydrogen-bonds with the aforementioned Ti-OD groups. The central-frequency peak originates from D2O molecules in the second water layer. When the solution pD is below 5 (point of zero charge (pzc) of TiO2), the water molecules point away from the TiO2 due to a positive charge caused by protonation of the surface. At pD 5, the over all intensity of the SFG-spectra decreases, indicating that the orientation of the water molecules is random. With rising pD value, the intensity increases, caused by deprotonation of the surface, which results in a negative charge. At higher pD, the water molecules in the second layer are oriented with their deuterium atoms pointing towards the semiconductors surface. Additional measurements at pD 2 showed a coupling e˙ect between the low and the central frequency peaks. This is not the case for pD 11.
In the second main part of this work, femtosecond time-resolved SFG-experiments are presented, conducted on the TiO2-D2O-interface with solutions of pD 3, 7 and 11. Using pump frequencies of 2550 and 2400 cm−1, it was found that with increasing pD-value the lifetime of the excited state decreases, probably caused by the stronger hydrogen-bonding network for high pD-values. It is also shown that for the 2550 cm−1 pump the coupling from the direct to the cross peak is dominated by a down-hill energy transfer, whereas the coupling for the 2400 cm−1 pump includes an instantaneous frequency shift of the cross peak. Additionally, experiments using isotopically diluted solutions of pD 3, 7 and 11 showed, that for all three pD-values the relaxation time strongly increases, due to the suppression of coupling e˙ects in the hydrogen-bonding network. For pD 11 this finding stands in contrast to results of the static SFG-experiments conducted before. This hints to an unknown coupling mechanism between the water molecules and the semiconductor.IV, 119 Seiten, Illustrationen, Diagramm
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
On the vibrational and photodesorption dynamics of carbon monoxide on the silicon(100) surface
No full textDie Rolle schwingungsangeregter Adsorbatmoleküle bei Oberflächenreaktionen ist bis dato noch wenig geklärt. Im Unterschied zum Gasphasenmolekül ist hier vor allem der Energieaustausch mit dem Festkörper bestimmend für Anregungsdauer und -art des Moleküls und damit auch für dessen Reaktivität. Diese Arbeit widmet sich der Untersuchung des Energieflusses am Beispiel des Systems „Kohlenstoffmonoxid auf Silizium(100)“. Mithilfe der Reflektions-Absorptions-Infrarotspektroskopie (RAIRS) und der Summenfrequenzerzeugungsspektroskopie wurden die spektroskopischen Eigenschaften und die Lebensdauer der internen Streckschwingung des CO-Moleküls auf dieser Oberfläche untersucht. Die spektroskopischen Eigenschaften des CO-Moleküls auf der Si(100)-Oberfläche ändern sich nur wenig in Abhängigkeit von Oberflächenbedeckungsgrad und Probentemperatur. Die experimentell beobachtete Adsorptionskinetik wird qualitativ erklärt mithilfe einer kinetischen Monte-Carlo-Simulation. Bei den Experimenten zur Schwingungslebensdauermessung wurde der Einfluss diverser Faktoren wie Ladungsträgerdichte im Substrat, von Oberflächendefekten und Inhomogenitäten der Adsorbatschicht (Isotopomerengemische) auf die Schwingungslebensdauer beleuchtet. Die Schwingungslebensdauer bewegt sich mit rund 2,5 ns in einem für Adsorbate auf Halbleitern typischen Bereich und nimmt als Folge einer Störung der Adsorbatschicht ab. Aus den Ergebnissen wird die Schlussfolgerung gezogen, die Schwingungsenergieübertragung verlaufe über die simultane Anregung von niederfrequenten Schwingungsmoden und Phononen. Der Einfluss von Störungen der Adsorbatmonolage auf die Schwingungslebensdauer wird qualitativ erklärt durch die Ausbildung eines Phononenbandes mit Delokalisierung der Schwingungsenergie. Eine Wechselwirkung mit dem elektronischen System des Substrats wird nicht festgestellt. Umgekehrt werden Indizien dafür präsentiert, daß die Anregung höherer Schwingungsniveaus von Adsorbaten durch mittels UV-Bestrahlung erzeugte „heiße“ Elektronen im Substrat erfolgen kann.The role of vibrationally excited adsorbate molecules in surface reactions has not been investigated in depth yet. In contrast to gas phase molecules the energy exchange with the substrate determines duration and type of excitation of the molecule and consequently its reactivity. This work is dedicated towards the examination of the energy flow in the model system “carbon monoxide on silicon(100)”. The spectroscopic properties und the vibrational lifetime of the CO molecule on this surface have been examined by mains of reflection-absorption infrared spectroscopy and sum frequency generation spectroscopy. The spectroscopic properties of CO on a Si(100) surface vary only little depending on surface coverage and sample temperature. The experimentally observed adsorption kinetics is explained qualitatively by means of kinetic Monte-Carlo simulations. In experiments concerning the vibrational lifetime the influence of factors like substrate carrier density, surface defects and inhomogenities in the adsorbate layer (isotopomeric mixtures) has been examined. The lifetime exhibits a value of 2.5 ns and thus is within the typical range of vibrational lifetimes on semiconductors. In addition it decreases if the adsorbate layer is disturbed. From these results the conclusion is drawn that the dissipation of energy takes place via the simultaneous excitation of low-frequency vibrational modes and phonons. The influence of disturbances of the adsorbate monolayer on the vibrational lifetime is explained qualitatively by the formation of a phonon band, leading to delocalisation of the vibrational energy. An interaction with the electronic system of the substrate was not found. In contrast evidence is presented that the excitation of higher vibrational levels of adsorbates can take place via interaction with UV-generated hot electrons from the substrate.El papel de los adsorbidos excitados en vibración en reacciones sobre superficies está hasta ahora poco clarificado. A distinción de la molécula gas, el intercambio de energía con el cuerpo solido decide sobre la duración y el modo de la excitación y con ello de su reactividad. Este trabajo se ocupa de la investigación del trasporte de la energía en el sistema ejemplar “monóxido de carbono sobre silicio (100)”. Las cualidades espectroscópicas y el tiempo de duración de la vibración fueron investigados con la espectroscopia de reflexión y absorción infrarrojo y la espectroscopia de generación de frecuencia suma. Las cualidades espectroscópicas cambian poco dependiente del grado de cubierta de la superficie y la temperatura de la muestra. La cinética de la adsorción está explicada cualitativamente con una simulación cinética del tipo Monte-Carlo. En los experimentos de la determinación del tiempo de duración de la vibración la influencia de factores diversos de influencia, como la densidad de porteadores de carga en el sustrato, la densidad de defectos de la superficie e inhomogenidades de la capa del adsorbato, fue alumbrada. El tiempo de duración con un valor de 2.5 ns representa un valor típico para adsorbatos sobre semiconductores y decrece como consecuencia de un desarreglo del capa del adsorbato. De estos resultados se concluye que el traspaso de energía de vibración tiene lugar para la excitación simultánea de modas de vibración de frecuencia baja y fonones. La influencia de desarreglos del mono capa del adsorbato para el tiempo de duración está explicada con la formación de una banda de fonones con deslocalización de la energía de la vibración. No se encuentra una interacción con el sistema electrónico del sustrato. En contrario, se presentan indicios que la excitación de niveles de vibración más altos de adsorbatos puede tener lugar para electrones “calientes” generados en el sustrato con la luz ultravioleta
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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