Effect of resonant tunneling on exciton dynamics in coupled dot-well nanostructures

Excitonic dynamics in a hybrid dot-well system composed of InAs quantum dots (QDs) and an InGaAs quantum well (QW) is studied by means of femtosecond pump-probe reflection and continuous wave (cw) photoluminescence (PL) spectroscopy. The system is engineered to bring the QW ground exciton state into resonance with the third QD excited state. The resonant tunneling rate is varied by changing the effective barrier thickness between the QD and QW layers. This strongly affects the exciton dynamics in these hybrid structures as compared to isolated QW or QD systems. Optically measured decay times of the coupled system demonstrate dramatically different response to temperature change depending on the strength of the resonant tunneling or coupling strength. This reflects a competition between purely quantum mechanical and thermodynamical processes.

Multiwavelenght study of cluster mergers and consequences for the radio emission properties of galaxy clusters

In the present thesis a thourough multiwavelength analysis of a number of galaxy clusters known to be experiencing a merger event is presented. The bulk of the thesis consists in the analysis of deep radio observations of six merging clusters, which host extended radio emission on the cluster scale. A composite optical and X–ray analysis is performed in order to obtain a detailed and comprehensive picture of the cluster dynamics and possibly derive hints about the properties of the ongoing merger, such as the involved mass ratio, geometry and time scale. The combination of the high quality radio, optical and X–ray data allows us to investigate the implications of the ongoing merger for the cluster radio properties, focusing on the phenomenon of cluster scale diffuse radio sources, known as radio halos and relics. A total number of six merging clusters was selected for the present study: A3562, A697, A209, A521, RXCJ 1314.4–2515 and RXCJ 2003.5–2323. All of them were known, or suspected, to possess extended radio emission on the cluster scale, in the form of a radio halo and/or a relic. High sensitivity radio observations were carried out for all clusters using the Giant Metrewave Radio Telescope (GMRT) at low frequency (i.e. ≤ 610 MHz), in order to test the presence of a diffuse radio source and/or analyse in detail the properties of the hosted extended radio emission. For three clusters, the GMRT information was combined with higher frequency data from Very Large Array (VLA) observations. A re–analysis of the optical and X–ray data available in the public archives was carried out for all sources. Propriety deep XMM–Newton and Chandra observations were used to investigate the merger dynamics in A3562. Thanks to our multiwavelength analysis, we were able to confirm the existence of a radio halo and/or a relic in all clusters, and to connect their properties and origin to the reconstructed merging scenario for most of the investigated cases. • The existence of a small size and low power radio halo in A3562 was successfully explained in the theoretical framework of the particle re–acceleration model for the origin of radio halos, which invokes the re–acceleration of pre–existing relativistic electrons in the intracluster medium by merger–driven turbulence. • A giant radio halo was found in the massive galaxy cluster A209, which has likely undergone a past major merger and is currently experiencing a new merging process in a direction roughly orthogonal to the old merger axis. A giant radio halo was also detected in A697, whose optical and X–ray properties may be suggestive of a strong merger event along the line of sight. Given the cluster mass and the kind of merger, the existence of a giant radio halo in both clusters is expected in the framework of the re–acceleration scenario. • A radio relic was detected at the outskirts of A521, a highly dynamically disturbed cluster which is accreting a number of small mass concentrations. A possible explanation for its origin requires the presence of a merger–driven shock front at the location of the source. The spectral properties of the relic may support such interpretation and require a Mach number M < ∼ 3 for the shock. • The galaxy cluster RXCJ 1314.4–2515 is exceptional and unique in hosting two peripheral relic sources, extending on the Mpc scale, and a central small size radio halo. The existence of these sources requires the presence of an ongoing energetic merger. Our combined optical and X–ray investigation suggests that a strong merging process between two or more massive subclumps may be ongoing in this cluster. Thanks to forthcoming optical and X–ray observations, we will reconstruct in detail the merger dynamics and derive its energetics, to be related to the energy necessary for the particle re–acceleration in this cluster. • Finally, RXCJ 2003.5–2323 was found to possess a giant radio halo. This source is among the largest, most powerful and most distant (z=0.317) halos imaged so far. Unlike other radio halos, it shows a very peculiar morphology with bright clumps and filaments of emission, whose origin might be related to the relatively high redshift of the hosting cluster. Although very little optical and X–ray information is available about the cluster dynamical stage, the results of our optical analysis suggest the presence of two massive substructures which may be interacting with the cluster. Forthcoming observations in the optical and X–ray bands will allow us to confirm the expected high merging activity in this cluster. Throughout the present thesis a cosmology with H0 = 70 km s−1 Mpc−1, m=0.3 and =0.7 is assumed.

Planktonic potential CO2 emission calculation: preliminary results by applying an adapted enzymatic methodology to marine ecosystems

[EN]Isocitrate Dehydrogenase (IDH) is a key enzyme in the Krebs cycle, being responsible for the production of one of the three CO2 molecules related to cellular respiration. In order to measure the potential CO2 production linked to the marine planktonic community we have adapted an enzymatic methodology. Preliminary results show that different proportions of autotrophs, heterotrophs and mixotrophs and their metabolic pathways, lead to different relationships between potential CO2 emission and potential O2 consumption during cellular respiration. Although more experiments need to be made, this methodology is leading to a better understanding of cellular respiration in marine samples and their impact on the food chain, vertical Carbon flux and the current sequestering capacity for anthropogenic CO2.

Quantitative Aspekte der Anziehungskraft von Straßenbeleuchtungen auf die Emergenz aus nahegelegenen Gewässern (Ephemeroptera, Plecoptera, Trichoptera, Diptera: Simuliidae, Chironomidae, Empididae) unter Berücksichtigung der spektralen Emission ve

Beim Beobachten von nächtlichen Insektenschwärmen an Straßenbeleuchtungen kommt es zur Frage, inwieweit von einer Bedrohung eines geordneten Naturkreislaufes durch dieses Phänomen gesprochen werden kann. Am Beispiel der bedrohten Artenvielfalt aquatischer Insekten im Bereich anthropogen bedingt verschmutzter Fließgewässer wurde die Problematik umfassend untersucht. Die Freilandtests erstreckten sich von Sommer 1998 bis Sommer 2000. Die vorliegende Arbeit geht mehreren Fragestellungen nach:1. - Wie hoch ist der Anteil aquatischer Insekten, der von dem Ort seiner Emergenz aus von einer naheliegenden Straßenlaterne angezogen wird ?2. - Welcher Lampentyp lockt weniger Insekten an: OSRAM HQL (Weiß-Mischlicht) oder PHILIPPS SON (gelbes Licht) ?3. - Welche Wellenlängenbereiche des Lichts sind beim Anflug an die Lampe von besonderer Präferenz ? zu 1. - Aquatische Insekten wiesen kein einheitliches Verhalten im Anflug an künstliches Licht auf. In den Sommermonaten kam es bei einigen Insektengruppen, wie z.B. den Trichopteren, zu einem massenhaften Anflug an die eine, im Untersuchungsgebiet aufgestellte Straßenlaterne. In dieser Zeit ergaben sich im Mittelwert Fangzahlen in einer Nacht am Licht, die der Emergenz von 25 Metern Bachufer/72 Stunden entsprachen. Bei den Dipteren ergab der mittlere Wert eine Emergenz von knapp 10 Metern Uferlänge/72 Stunden. Aufgrund hoher Fangzahlen konnten folgende Ergebnisse auf Artniveau bei den Chironomiden erzielt werden. Bei den zehn häufigsten Arten aus Emergenz und Lichtfang zeigten sich starke Schwankungen: Der Fang am Licht entsprach im Maximum einer Schlupfrate von 61 Metern Ufer/ 72 Stunden bei einer Art, bei einer anderen Art wurde z.B. ein Lichtfang erzielt, der lediglich die Emergenz von 0,3 Metern/72 Stunden umfaßte.zu 2. - In dem ohne Lichtkonkurrenz stattfindenden Vergleich zwischen PHILIPPS SON 70W und OSRAM HQL 125W ergab sich eine Fangrelation von 1:1,6 (SON:HQL). Bei einem Parallelfang (30 Metern Abstand der Leuchten) mit SON/HQL flogen wesentlich mehr Tiere an die HQL: Hier betrug die Relation 1:3,97 (SON:HQL). Es fand somit bei Lichtkonkurrenz eine Abwanderung der Insekten zur 'attraktiveren' Lampe statt.zu 3. - Zuletzt wurde die Anlockwirkung dreier Farbspektren mit den Intensitätsmaxima einer Wellenlänge von 437nm, 579nm und 599nm getestet. 437nm war die in der HQL gemessene Intensitätsspitze im niedrigwelligen Bereich und nach verbreiteter Auffassung von besonderer Anlockwirkung. Der Wellenlängenbereich um 579nm stellte das Intensitätsmaximum der SON-Lampe dar (gelbes Licht); 599nm war als Alternative für anlockschwache Beleuchtungen ausgewählt worden. Hier ergab sich bei abwechselndem Fang (ohne Lichtkonkurrenz) eine Fangrelation von 1,8 : 3,4 : 1 (437nm : 579nm : 599nm).

Development of the optimal estimation method for inversion of stratospheric emission spectra and validation of HNO3 profiles

This doctoral thesis focuses on ground-based measurements of stratospheric nitric acid (HNO3)concentrations obtained by means of the Ground-Based Millimeter-wave Spectrometer (GBMS). Pressure broadened HNO3 emission spectra are analyzed using a new inversion algorithm developed as part of this thesis work and the retrieved vertical profiles are extensively compared to satellite-based data. This comparison effort I carried out has a key role in establishing a long-term (1991-2010), global data record of stratospheric HNO3, with an expected impact on studies concerning ozone decline and recovery. The first part of this work is focused on the development of an ad hoc version of the Optimal Estimation Method (Rodgers, 2000) in order to retrieve HNO3 spectra observed by means of GBMS. I also performed a comparison between HNO3 vertical profiles retrieved with the OEM and those obtained with the old iterative Matrix Inversion method. Results show no significant differences in retrieved profiles and error estimates, with the OEM providing however additional information needed to better characterize the retrievals. A final section of this first part of the work is dedicated to a brief review on the application of the OEM to other trace gases observed by GBMS, namely O3 and N2O. The second part of this study deals with the validation of HNO3 profiles obtained with the new inversion method. The first step has been the validation of GBMS measurements of tropospheric opacity, which is a necessary tool in the calibration of any GBMS spectra. This was achieved by means of comparisons among correlative measurements of water vapor column content (or Precipitable Water Vapor, PWV) since, in the spectral region observed by GBMS, the tropospheric opacity is almost entirely due to water vapor absorption. In particular, I compared GBMS PWV measurements collected during the primary field campaign of the ECOWAR project (Bhawar et al., 2008) with simultaneous PWV observations obtained with Vaisala RS92k radiosondes, a Raman lidar, and an IR Fourier transform spectrometer. I found that GBMS PWV measurements are in good agreement with the other three data sets exhibiting a mean difference between observations of ~9%. After this initial validation, GBMS HNO3 retrievals have been compared to two sets of satellite data produced by the two NASA/JPL Microwave Limb Sounder (MLS) experiments (aboard the Upper Atmosphere Research Satellite (UARS) from 1991 to 1999, and on the Earth Observing System (EOS) Aura mission from 2004 to date). This part of my thesis is inserted in GOZCARDS (Global Ozone Chemistry and Related Trace gas Data Records for the Stratosphere), a multi-year project, aimed at developing a long-term data record of stratospheric constituents relevant to the issues of ozone decline and expected recovery. This data record will be based mainly on satellite-derived measurements but ground-based observations will be pivotal for assessing offsets between satellite data sets. Since the GBMS has been operated for more than 15 years, its nitric acid data record offers a unique opportunity for cross-calibrating HNO3 measurements from the two MLS experiments. I compare GBMS HNO3 measurements obtained from the Italian Alpine station of Testa Grigia (45.9° N, 7.7° E, elev. 3500 m), during the period February 2004 - March 2007, and from Thule Air Base, Greenland (76.5°N 68.8°W), during polar winter 2008/09, and Aura MLS observations. A similar intercomparison is made between UARS MLS HNO3 measurements with those carried out from the GBMS at South Pole, Antarctica (90°S), during the most part of 1993 and 1995. I assess systematic differences between GBMS and both UARS and Aura HNO3 data sets at seven potential temperature levels. Results show that, except for measurements carried out at Thule, ground based and satellite data sets are consistent within the errors, at all potential temperature levels.

Organic heterostructure approach for multifunctional light-emitting field-effect transistors

The possibility of combining different functionalities in a single device is of great relevance for further development of organic electronics in integrated components and circuitry. Organic light-emitting transistors (OLETs) have been demonstrated to be able to combine in a single device the electrical switching functionality of a field-effect transistor and the capability of light generation. A novel strategy in OLET realization is the tri-layer vertical hetero-junction. This configuration is similar to the bi-layer except for the presence of a new middle layer between the two transport layers. This “recombination” layer presents high emission quantum efficiency and OLED-like (Organic Light-Emitting Diode) vertical bulk mobility value. The key idea of the vertical tri-layer hetero-junction approach in realizing OLETs is that each layer has to be optimized according to its specific function (charge transport, energy transfer, radiative exciton recombination). Clearly, matching the overall device characteristics with the functional properties of the single materials composing the active region of the OFET, is a great challenge that requires a deep investigation of the morphological, optical and electrical features of the system. As in the case of the bi-layer based OLETs, it is clear that the interfaces between the dielectric and the bottom transport layer and between the recombination and the top transport layer are crucial for guaranteeing good ambipolar field-effect electrical characteristics. Moreover interfaces between the bottom transport and the recombination layer and between the recombination and the top transport layer should provide the favourable conditions for the charge percolation to happen in the recombination layer and form excitons. Organic light emitting transistor based on the tri-layer approach with external quantum efficiency out-performing the OLED state of the art has been recently demonstrated [Capelli et al., Nat. Mater. 9 (2010) 496-503] widening the scientific and technological interest in this field of research.

Surface-functionalized latex particles as additives in the mineralization of zinc oxide

Polystyrene latex particles modified at the surface with different hydrophilic functional groups were prepared by miniemulsion polymerization and applied to control the crystallization of zinc oxide in aqueous medium. The effects of both latex structure and concentration on the crystal growth, morphology, crystalline structure, and properties of the resulting zinc oxide were analyzed. Depending on the latex additive used, micro- and submicrosized crystals with a broad variety of morphologies were obtained. Among the studied latexes, the carboxyl-derived particles were shown to be a convenient system for further quantitative investigations. In this case, as the additive concentration increases, the aspect ratio of the crystals decreases systematically. Latex particles are assumed to adsorb preferentially onto the fast growing {001} faces of ZnO, interacting with the growth centers and reducing the growth rate in [001]. When zinc oxide is precipitated in the presence of latex, the polymer particles become incorporated into the growing crystals and polymer–inorganic hybrid materials are obtained. These materials are composed of an inorganic and largely undisturbed crystalline matrix in which organic latex particles are embedded. Increasing amounts of latex become incorporated into the growing crystals at increasing overall concentration in the crystallizing system. Photoluminescence (PL) spectra were measured to obtain information on defect centers. Emission spectra of all samples showed a narrow UV peak and a broad band in the green-yellow spectral region. The former emission is attributed to exciton recombination, whereas the latter seems to be related with deep-level donors. Latex appears to be a quencher of the visible emission of zinc oxide. Thus, compared to pure zincite, ZnO–latex hybrid materials show a significantly lower PL intensity in the visible range of the spectrum. Under continuous photoexcitation, a noticeable dynamic behavior of the PL is observed, which can be related to a photodesorption of adsorbed oxygen. These surface-adsorbed oxygen species seem to play a crucial role for the optical properties of the materials and may mediate the tunneling of electrons from the conduction band to preexisting deep-level traps, probably related to intrinsic defects (oxygen vacancies or interstitial zinc). The polymer particles can block the sites where oxygen adsorbs, and the disappearance of the “electron-shuttle” species leads to the observed quenching of the visible emission. Electron paramagnetic resonance (EPR) provided additional information about crystal defects with unpaired electrons. Spectra of all samples exhibit a single signal at g ≈ 1.96, typical for shallow donors. Contrary to the results of other authors, no correlation was possible between the EPR signal and the visible range of PL spectra, which suggests that centers responsible for the visible emission and the EPR signal are different.

Size-selective investigations of spectral and emission properties of small particles and nanotubes

In dieser Arbeit wurde die Elektronenemission von Nanopartikeln auf Oberflächen mittels spektroskopischen Photoelektronenmikroskopie untersucht. Speziell wurden metallische Nanocluster untersucht, als selbstorganisierte Ensembles auf Silizium oder Glassubstraten, sowie ferner ein Metall-Chalcogenid (MoS2) Nanoröhren-Prototyp auf Silizium. Der Hauptteil der Untersuchungen war auf die Wechselwirkung von fs-Laserstrahlung mit den Nanopartikeln konzentriert. Die Energie der Lichtquanten war kleiner als die Austrittsarbeit der untersuchten Proben, so dass Ein-Photonen-Photoemission ausgeschlossen werden konnte. Unsere Untersuchungen zeigten, dass ausgehend von einem kontinuierlichen Metallfilm bis hin zu Clusterfilmen ein anderer Emissionsmechanismus konkurrierend zur Multiphotonen-Photoemission auftritt und für kleine Cluster zu dominieren beginnt. Die Natur dieses neuen Mechanismus` wurde durch verschiedenartige Experimente untersucht. Der Übergang von einem kontinuierlichen zu einem Nanopartikelfilm ist begleitet von einer Zunahme des Emissionsstroms von mehr als eine Größenordnung. Die Photoemissions-Intensität wächst mit abnehmender zeitlicher Breite des Laserpulses, aber diese Abhängigkeit wird weniger steil mit sinkender Partikelgröße. Die experimentellen Resultate wurden durch verschiedene Elektronenemissions-Mechanismen erklärt, z.B. Multiphotonen-Photoemission (nPPE), thermionische Emission und thermisch unterstützte nPPE sowie optische Feldemission. Der erste Mechanismus überwiegt für kontinuierliche Filme und Partikel mit Größen oberhalb von mehreren zehn Nanometern, der zweite und dritte für Filme von Nanopartikeln von einer Größe von wenigen Nanometern. Die mikrospektroskopischen Messungen bestätigten den 2PPE-Emissionsmechanismus von dünnen Silberfilmen bei „blauer“ Laseranregung (hν=375-425nm). Das Einsetzen des Ferminiveaus ist relativ scharf und verschiebt sich um 2hν, wenn die Quantenenergie erhöht wird, wogegen es bei „roter“ Laseranregung (hν=750-850nm) deutlich verbreitert ist. Es zeigte sich, dass mit zunehmender Laserleistung die Ausbeute von niederenergetischen Elektronen schwächer zunimmt als die Ausbeute von höherenergetischen Elektronen nahe der Fermikante in einem Spektrum. Das ist ein klarer Hinweis auf eine Koexistenz verschiedener Emissionsmechanismen in einem Spektrum. Um die Größenabhängigkeit des Emissionsverhaltens theoretisch zu verstehen, wurde ein statistischer Zugang zur Lichtabsorption kleiner Metallpartikel abgeleitet und diskutiert. Die Elektronenemissionseigenschaften bei Laseranregung wurden in zusätzlichen Untersuchungen mit einer anderen Anregungsart verglichen, der Passage eines Tunnelstroms durch einen Metall-Clusterfilm nahe der Perkolationsschwelle. Die elektrischen und Emissionseigenschaften von stromtragenden Silberclusterfilmen, welche in einer schmalen Lücke (5-25 µm Breite) zwischen Silberkontakten auf einem Isolator hergestellt wurden, wurden zum ersten Mal mit einem Emissions-Elektronenmikroskop (EEM) untersucht. Die Elektronenemission beginnt im nicht-Ohmschen Bereich der Leitungsstrom-Spannungskurve des Clusterfilms. Wir untersuchten das Verhalten eines einzigen Emissionszentrums im EEM. Es zeigte sich, dass die Emissionszentren in einem stromleitenden Silberclusterfilm Punktquellen für Elektronen sind, welche hohe Emissions-Stromdichten (mehr als 100 A/cm2) tragen können. Die Breite der Energieverteilung der Elektronen von einem einzelnen Emissionszentrum wurde auf etwa 0.5-0.6 eV abgeschätzt. Als Emissionsmechanismus wird die thermionische Emission von dem „steady-state“ heißen Elektronengas in stromdurchflossenen metallischen Partikeln vorgeschlagen. Größenselektierte, einzelne auf Si-Substraten deponierte MoS2-Nanoröhren wurden mit einer Flugzeit-basierten Zweiphotonen-Photoemissions-Spektromikroskopie untersucht. Die Nanoröhren-Spektren wiesen bei fs-Laser Anregung eine erstaunlich hohe Emissionsintensität auf, deutlich höher als die SiOx Substratoberfläche. Dagegen waren die Röhren unsichtbar bei VUV-Anregung bei hν=21.2 eV. Eine ab-initio-Rechnung für einen MoS2-Slab erklärt die hohe Intensität durch eine hohe Dichte freier intermediärer Zustände beim Zweiphotonen-Übergang bei hν=3.1 eV.

Biogenic emission of halocarbons

Within this doctoral thesis, biogenic emissions of several globally relevant halocarbons (methyl chloride, methyl bromide, methyl iodide, dibromomethane, chloroform and bromoform) have been investigated in different environments. An airborne study was focused on the tropical rainforest ecosystem, while shipborne measurements investigated naturally occurring oceanic plankton blooms. Laboratory experiments using dried plant material were made to elucidate abiotic production mechanisms occurring in organic matter. Airborne measurements over the tropical rainforest of Suriname and French Guyana (3 - 6 °N, 51 - 59 °W) revealed net fluxes of 9.5 (± 3.8 2σ) µg m-2 h-1 methyl chloride and 0.35 (± 0.15 2σ) µg m-2 h-1 chloroform emitted in the long dry season (October) 2005. An extrapolation of these numbers to all tropical forests helped to narrow down the range of the recently discovered and poorly quantified methyl chloride source from tropical ecosystems. The value for methyl chloride obtained (1.5 (± 0.6 2σ) Tg yr-1) affirms that the contribution of the tropical forest ecosystem is the major source in the global budget of methyl chloride. The extrapolated global net chloroform flux from tropical forests (56 (± 23 2σ) Gg yr-1) is of minor importance (5 - 10 %) compared to the global sources. A source of methyl bromide from this region could not be verified. The abiotic formation of methyl chloride and methyl bromide from dead plant material was tested in a laboratory study. The release from plant tissue representative of grassland, deciduous forest, agricultural areas and coastal salt marshes (hay, ash, tomato and saltwort) has been monitored. Incubations at different temperatures (25 - 50 °C) revealed significant emissions even at ambient temperature, and that the emissions increased exponentially with temperature. The strength of the emission was found to be additionally dependent on the availability of halide and the methoxyl group within the plant tissue. However, high water content in the plant material was found to inhibit methyl halide emissions. The abiotic nature of the reaction yielding methyl halides was confirmed by its high activation energy calculated via Arrhenius plots. Shipborne measurements of the atmospheric mixing ratios of methyl chloride, methyl bromide, methyl iodide, dibromomethane and bromoform have been conducted along a South Atlantic transect from the 27.01. - 05.02.2007 to characterize halocarbon emissions from a large-scale natural algae bloom encountered off the coast of Argentina. Mixing ratios of methyl chloride and methyl bromide were not significantly affected by the occurrence of the phytoplankton bloom. Emissions of methyl iodide, dibromomethane and bromoform showed pronounced mixing ratio variations, triggered by phytoplankton abundance. Methyl iodide was strongly correlated with dimethyl sulfide throughout the sampled region. A new technique combining satellite derived biomass marker (chlorophyll a) data with back trajectory analysis was successfully used to attribute variations in mixing ratios to air masses, which recently passed over areas of enhanced biological production.

Main characteristics of the emission from elliptical galaxies

Elliptical galaxies are one of the most characteristic objects we can find in the sky. In order to unveil their properties, such as their structure or chemical composition, one must study their spectral emission. In fact they seem to behave rather differently when observed with different eyes. This is because their light is mainly brought by two different components: optical radiation arises from its stars, while the X emission is primarly due to a halo of extremely hot gas in which ellipticals seem to be embedded. After a brief classification, the two main processes linked to these phenomena will be described, together with the informations we can collect thanks to them. Eventually, we will take a quick look at the other regions of the electromagnetic spectrum.