In the present work atom probe tomography (APT) was applied to analyze thin films used in semiconductor industry to investigate the capability of atom probe tomography as well as the dopant redistribution in thin silicide layer stacks. Different titanium silicide layer stacks are investigated and titanium diboride precipitates are identified by APT. Arsenic grain boundary segregation is verified by APT in cobalt silicide layer stacks. Furthermore APT measurements are compared to commonly used methods such as TEM and SIMS and found in good agreement. Each method exhibits its own advantages depending on the sample and the question. Atom probe tomography offers some unique features enabling three-dimensional analysis on the nanometer scale as shown on the mentioned thin film layer stacks.

Hydrogen is a carrier energy gas that can be utilized as a clean energy source instead of oil and natural gas. Splitting the water into hydrogen and oxygen is one of the most favorable methods to generate hydrogen. The catalytic properties of molybdenum disulfide (MoS2) could be valuable in this role, particularly due to its unique structure and ability to be chemically modified, enabling its catalytic activity to be further enhanced or made comparable to that of Pt-based materials. In general, these modification strategies may involve either structural engineering of MoS2 or enhancing the kinetics of charge transfer, including by confining to single metal atoms and clusters or integrating with a conductive substrate. We present the results of synergetic integration of MoS2 films with a Si-heterojunction solar cell for generating H2 via the photochemical water splitting approach. The results of the photochemical measurements demonstrated an efficient photocurrent of 36. 3 mA cm-2 at 0 V vs. RHE and an onset potential of 0.56 V vs. RHE. In addition to 25 hours of continuous photon conversion to H2 generation, this study points out that the integration of the Si-HJ with MoS2 is an effective strategy for enhancing the internal conductivity of MoS2 towards efficient and stable hydrogen production. Moreover, we studied the effect of doping an atomic scale of Pt on the catalytic activity of MoS2. The electrochemical results indicated that the optimum single Pt atoms loading amount demonstrated a distinct enhancement in the hydrogen generating, in which the overpotential was minimized to -0.0505 V to reach a current density of 10 mA cm−2 using only 10 ALD cycles of Pt. The Tafel slopes of the ALD Pt/ML-MoS2 electrodes were in the range of 55–120 mV/decade, which indicates a fast improvement in the HER velocity as a result of the increased potential. Stability is another important parameter for evaluating a catalyst. The same (10 ALD cycles) Pt/ML-MoS2 electrode was able to continuously generate hydrogen molecules at for 150 hours. These superior results demonstrate that the low conductivity of semiconductive MoS2 can be enhanced by anchoring the film with Pt SAs and clusters, leading to sufficient charge transport and a decrease in the overpotential.

Interplanetary satellite missions and also satellites orbiting the Earth carry instruments to measure fluxes of neutral atoms, which are plasma key parameters for the investigation of the planets' atmospheres. Neutral atoms entering the mass-spectrometry instrument have to be ionized to be able to guide them using electric and magnetic fields with intent to determine energy and velocity of the atoms and finally their mass. For low energy neutral atoms, the ionization process can be realized by implementing a charge state conversion surface, where-on the atoms are scattered in an angle of grazing incidence and getting ionized.The objective of this work is the characterization and analysis of two diamond-like carbon surface samples by measuring and determining ionization efficiencies and scattering properties and therefore their functionality as charge state conversion surfaces.First, a chemical vapor deposition (CVD) diamond sample is investigated. Furthermore the CVD diamond is compared to another diamond--like carbon surface manufactured by pulsed laser deposition (PLD) technique. All measurements have been accomplished at the ILENA (Imager for Low Energy Neutral Atoms) facility at the Physics Institute, University of Bern in the Department of Space Research and Planetary Sciences.It is discovered that the CVD surface gets electrostatically charged upon scattering of atomic ions. Though this charging effects, a qualitative characterization of the surface can be made. It is shown that the ionization efficiencies of the CVD and the PLD diamond surface are of comparable quality, where on the contrary the scattering properties of the CVD diamond charge state conversion surface are much better. It still has to be investigated in future experiments, if this brilliant scattering properties are due to charging effects or can be assigned to the very smooth surface of the CVD diamond surface. / <p>Validerat; 20120112 (anonymous)</p>

Epitaxially grown quantum dots (QDs) make up a significant portion of nanoscale semiconductor research, yet precise solutions for their eigenstates in complex geometries are often unknown. Eigenstates are extremely relevant as they impact the emission wavelength, performance, and stability of many optoelectronic devices. In this thesis, atomic force microscopy, transmission electron microscopy, and atom probe tomography (APT) are used to assess and compare QD size and core concentration. APT by means of isosurface reconstruction provides the most accurate ensemble averaged quantum dot size and core concentration. High-angle annular dark-field imaging quantifies core concentration very well, but fails in comparison to precisely quantify QD size. Ensemble averaging is discarded in favour of using the raw APT data to devise a model that can solve the Schrödinger equation in 3-dimensional space and can be expanded upon to include non-trivial quantum dot geometries of any kind. The electron and hole eigenstates for an entire quantum dot ensemble are solved using this model. Hybridized eigenstates between neighbouring quantum dots are realized and found to experience both bonding and anti-bonding of the charge carriers. The existence of a degenerate state is also discovered. The simulated eigenenergies are compared to the photoluminescence emission spectrum and found to accurately represent the exciton recombination energy. This makes it possible to obtain very realistic 3-D eigenstate representations for a variety of complex structures. The modelling technique outlined in this thesis is not constrained to just QDs, but can also be applied to an array of many other nanoscale structures. / Thesis / Master of Applied Science (MASc)

Indiana University-Purdue University Indianapolis (IUPUI) / Alkyl radicals are powerful intermediates for the generation of carbon-carbon bonds, which play an indispensable role in the synthesis of natural products, pharmaceuticals, and pesticides. Traditionally, there are two main methods for the generation of alkyl radicals. The first is C-H bond functionalization via hydrogen-atom-transfer (HAT). HAT processes have been used as an effective approach for selectively activating C-H bonds via radical pathways. The other strategy to explore the generation of alkyl radicals is C-X bond functionalization via halogen-atom-transfer (XAT). Alkyl halides are one of the largest classes of building blocks in synthesis and they can be obtained from the corresponding alcohols. The most straightforward and effective way to form such alkyl radicals is the direct homolytic cleavage of C-X bonds. In past decades, photoredox catalysis has emerged as a powerful and greener tool for the synthesis of radicals under mild reaction conditions, which has brought tremendous attention. Although remarkable success has been made in this field, some methods still require costly transition metal catalysts or toxic reagents. Herein, we display a series of visible light-induced approaches under transition-metal free conditions or using earth-abundant metals. These novel photo-induced transformations and corresponding mechanistic work will be discussed in the following order: We will first present our work on metal-free visible-light-promoted C(sp3)-H functionalization of aliphatic cyclic ethers using trace O2. This reaction uses a trace amount of aerobic oxygen as the sole green oxidant under blue light at room temperature to achieve the synthesis of sulfone and phosphate derivatives in good to excellent yields using cyclic ethers and vinyl sulfones. Then, we report on a photo-induced C(sp3)-H chalcogenation of amide derivatives and ethers via a ligand-to-metal charge-transfer. This reaction converts secondary and tertiary amides, sulfonamides, and carbamates into the corresponding amido-N,S-acetal derivatives in good yields, using an earth abundant metal catalyst under mild conditions. Finally, we present a photoredox polyfluoroarylation of alkyl halides via halogen atom transfer. This method converts primary, secondary, and tertiary unactivated abundant alkyl halides into the corresponding polyfluoroaryl compounds in good yields and has good functional group compatibility.

Insects are not mere reflex machines. Instead, they adapt their behaviour flexibly to changing environmental contingencies. Among the insects, honeybees (Apis mellifera) possess an impressive repertoire of cognitive abilities, despite their limited number of neurons. Thanks to the standardization of behavioral, neurobiological, neuroimaging, and genetic methods, bees became a widely used invertebrate model in research. Importantly, the study of their capacities allows us to integrate evidence from an invertebrate species into broader scientific frameworks - often based on vertebrate studies - supporting a deeper understanding of the evolution of certain cognitive mechanisms and their universality. Honeybees can process different information from their environment, such as the numerousness of an array or the relationships – both perceptual and abstract – between objects. Once identified, such relationships allow bees to form distinct categories to which they will refer to implement adaptive choices. An ongoing debate focused on whether numerical abilities in bees are supported by a unified neural mechanism – as for vertebrates - or if multiple segregated mechanisms are involved. Additionally, there is interest in further expanding our knowledge about the extent of bees’ categorization capacities in different contexts. This thesis aims to address these questions, providing evidence that can shed light on the neural organization and limits of honeybees’ cognitive abilities, as well as on potential similarities or differences with other species. In the first two studies, the existence of a general mechanism for the estimation of quantity in honeybees was investigated. Specifically, I addressed the issue of whether bees’ numerical abilities are supported by a general magnitude mechanism that estimates continuous (e.g., space, time, size) and discrete (i.e., number) quantities. In the first study, we investigated the bees' ability to transfer learning from numerical to size dimension. Using appetitive-aversive conditioning, independent groups of free-flying foragers were trained to discriminate between larger and smaller visual numerousness (i.e., 2 vs. 4, 2 vs. 3, 4 vs. 8, 4 vs. 6; 0.5 or 0.67 ratio difference). We then tested the bee's generalization ability with a comparison between stimuli with different sizes and identical numerosity (e.g., 4 larger elements vs. 4 smaller elements). Honeybees spontaneously chose the congruent size with respect to their training. No effect of numerical contrast and ratio difference experienced was found as bees previously reinforced toward the larger numerosity, chose the relatively larger size, and vice versa. These results demonstrated the ability of this insect species to make a transfer from the numerical to the size dimension. Given the possibility of asymmetric relationships between magnitudes, we sought to explore whether honeybees possess the capacity to make the reverse transfer as well, from a continuous (size) to a discrete (number) dimension. Similar to the previous study, free-flying foragers were trained to discriminate between relatively larger vs. smaller squares or diamonds. Their generalization ability over novel shapes (i.e., circles) and novel dimensions (i.e., number) was subsequently tested. Our results confirmed the ability of bees to transfer size discrimination to novel shapes. Moreover, when presented with a 4 vs. 8 elements comparison, bees spontaneously selected the congruent numerosity with respect to their training (i.e., bees trained to select the smaller/larger size, selected the smaller/larger numerosity, respectively). To check for any perceptual cue involvement in bees’ decision-making, different continuous variables covarying with numerosity were controlled for (i.e., total area, contour length, stimulus size, convex hull). Subsequent analyses also revealed no role of spatial frequency in the bees’ behavior. The results revealed a bee’s capacity to transfer between numerical and size dimensions, suggesting the universality of the magnitudes coding mechanism and highlighting the presence of a unified circuit supporting discrete and continuous quantity processing. The second aim of this thesis was to enlarge our knowledge of the ability of bees to spontaneously encode regularities from the physical world. To this purpose, I tested bees' ability to extrapolate the structure of temporally defined odor sequences. In a series of six experiments, the spontaneous and trained ability of bee foragers to learn, memorize, and generalize an odor sequence composed of three distinct odors was tested. A proboscis extension response (PER) conditioning paradigm was employed (i.e., absolute, differential, and generalization). The first two experiments investigated honeybees’ ability to learn an arbitrary odor sequence. Bees were trained to respond to a specific sequence of three odors and then tested for their spontaneous ability to generalize their response to novel sequences with a similar structure but composed of novel odors and to reject novel configurations although composed of familiar odors. The role of a particular odor position in the sequence, the odor-reward temporal closeness, and their possible effects on memory were also investigated in the third experiment. The fourth and fifth experiments aimed to understand the effect of differential conditioning on bees’ learning ability. Lastly, we determined whether a conditioning procedure favouring a generalization strategy could lead to the spontaneous encoding of the internal sequence structure. In general, the results highlighted an early tendency of bees to encode the single odor properties, instead of learning the entire sequence structure, together with a significantly increased response towards the novel odor configurations composed of familiar odors. No effect of the odor’s position or temporal closeness with the reward was apparent. During absolute and differential conditioning, bees likely employed two strategies to memorize the dyad of the first and second elements of the sequence, together with a more general response to novelty. However, the use of a transfer paradigm potentially revealed a weak spontaneous generalization over similar structures one hour after the training, irrespective of the single-element properties. Overall, these results shed light on the strategies employed by bees to solve an odor abstraction task, highlighting the crucial role of the type of conditioning to let them emerge. Altogether, the thesis provides new evidence on honeybees’ cognition. The findings have implications not only for the study of bees’ behavior but also for broader investigations into the universal development of basic cognitive mechanisms and the convergent evolution of similar abilities in small and large brains.

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Thesis advisor: Peter X. Zhang / Thesis advisor: James P. Morken / Radical reactions have attracted continuous research interest in recent year considering their diverse reactivities. Hydrogen-atom abstraction (HAA), as one type of the most well-explored radical reactions, has been identified as one of powerful tools for C–H functionalization. Reactions involving 1,4-HAA, which is typically a challenging process both entropically and enthalpically, are rather scarce, while 1,5-HAA have been well demonstrated for variety of synthetic applications. Guided by the concept of metalloradical catalysis (MRC), 1,4-HAA was for the first time utilized as the key step to achieve asymmetric construction of chiral ring structures: cyclobutanones, azetidines and tetrahydropyridines. The design of different D2-symmetric chiral amidoporphyrin as the supporting ligand is the key to all these transformations. The reactions can be conducted under mild conditions, affording corresponding ring structure in good yields with excellent selectivity. Furthermore, The combined computational and experimental studies have shed light on the mechanistic details of these new asymmetric radical intramolecular C–H alkylation processes, which are fundamentally different from existing catalytic systems involving metallocarbenes for concerted C–H insertion. We envision that these asymmetric radical processes via Co(II)-based MRC could become an alternative method for important chiral ring structures synthesis and potentially provide new opportunities for complex molecule construction. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The flavin-containing monoamine oxidases (MAO) A and B catalyze the oxidative deamination of primary and secondary amines. The overall process involves a two electron oxidation of the amine to the iminium with concomitantreduction of the flavin. Based on extensive studies with a variety of chemical probes, Silverman and colleagues have proposed a catalytic pathway for the processing of amine substrates and inactivators by MAO-B that is initiated by a single electron transfer (SET) step from the nitrogen lone pair to the oxidized flavin followed by α-proton loss from the resulting amine radical cation that leads to a carbon radical. Subsequent transfer of the second electron leads to the reduced flavin and the iminium product. In the case of N-cyclopropylamines, the initially formed amine radical cation is proposed to undergo rapid ring opening to form a highly reactive primary carbon centered radical that is thought to be responsible for inactivation of the enzyme. In this thesis we have exploited the unique substrate and inactivator properties of 1,4-disubstituted tetrahydropyridine derivatives to probe the mechanism of MAO-B catalysis. Reports of the parkinsonian inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) as a structurally unique substrate of MAO-B initiated these studies. Consistent with the SET pathway, the N-cyclopropyl analog of MPTP proved to be an efficient time and concentration dependent inactivator but not a substrate of MAO-B. On the other hand, the 4-benzyl-1-cyclopropyl analog is both a substrate and inactivator of MAO-B. These properties may not be consistent with the obligatory formation of a cyclopropylaminyl radicalcation intermediate. In an attempt to gain further insight into the mechanism associated with the MAO catalyzed oxidation of 1,4-disubstituted tetrahydropyridines, deuterium isotope effects studies on both the substrate and inactivation properties of the 4-benzyl-1- cyclopropyl derivative were undertaken. A series of 1-methyl- and 1-cyclopropyltetrahydropyridine derivatives bearing various heteroaro-matic groups at C-4 also have been examined. The MAO-B substrate properties, inactivator properties and partition ratios for these compounds together with preliminary results from chemical model studies are discussed in terms of the MAO-B catalytic pathway. / Ph. D.