The quantum future of microscopy: Wave function engineering of electrons, ions, and nuclei

The ability to manipulate particles has always been a fundamental aspect for developing and improving scattering and microscopytechniques used for material investigations. So far, microscopy applications have mostly relied on a classical treatment of the electron-matterinteraction. However, exploiting a particle’s quantum nature can reveal novel information not accessible with conventional schemes. Here,after describing recent methods for coherent wave function engineering, we discuss how quantum manipulation of electrons, He ions, andnuclei can be used to implement low-dose imaging methods, to explore correlated quantum state dynamics in condensed matter, and tomodulate nuclear reactions for energy-related applications and gamma-ray lasers

Finite element modeling and experimental study of brittle fracture in tempered martensitic steels for thermonuclear fusion applications

Pablo Federico Mueller

In this work we have studied brittle fracture in high-chromium reduced activation tempered martensitic steels foreseen as structural materials for thermonuclear fusion reactors. Developing the adequate materials that can withstand the severe irradiation conditions of the burning plasma in a future fusion reactor is one of the major challenges to be solved in order to make profit from the great advantages of thermonuclear fusion as an energy source. High-chromium tempered martensitic steels such as F82H and the most advanced version Eurofer97 are among the main candidate materials for structural applications in future fusion power plants due to low irradiation-induced swelling, good mechanical and thermal properties, and reasonably fast radioactive decay. The most concerning drawback of these kind of steels is irradiation embrittlement, which is manifested by a ductile-to-brittle transition temperature shift to higher temperatures after irradiation whose amplitude depends on the irradiation conditions (temperature, neutron flux, neutron fluence, etc). The aim of this work was to study and model brittle fracture in the ductile-brittle transition region of this kind of steels in the as-received unirradiated conditions. It is necessary to be able to transfer laboratory specimen fracture data to real components and structures in order to assess the performance of these steels in the different operating and transient conditions they could find during the operation life of a fusion reactor. In order to do so, the specimen geometry effects and specimen size effects on measured fracture toughness need to be properly understood, taken into account and predicted with an appropriate model. In particular, specimen size effect on measured toughness is a major concern for the nuclear materials research community owing to the limited irradiation volume in current and planed materials irradiation facilities. The main results of this PhD work are summarized below. The microstructure of Eurofer97 and F82H has been characterized and compared by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy in order to identify microstructural features that could play a role in the measured fracture toughness. Both steels have similar but slightly different chemical composition and final heat-treatments but the prior austenitic grain size measured in F82H is approximately 8 times larger than in Eurofer97. It was shown that the alloying element Tantalum, added to stabilize the austenite grain size, played a different role in both steels. After a careful analysis of the particles present in both steels, it was found that Tantalum in Eurofer97 formed carbides of an average size around 100 nanometers. In contrast in F82H it did not form small carbides but formed big oxide inclusions with a size up to 30 µm. These large particles do not effectively pin the grain boundaries. The different behavior of Tantalum in these steels is believed to be mainly a consequence of the larger content of Oxygen present and the smaller amount of Aluminum in F82H compared to Eurofer97. The Master-Curve ASTM-E1921 standard is a method initially developed to determine the ductile-to-brittle transition reference temperature, T0, in fission reactor ferritic steels from a small number of experiments. In this work the applicability of the Master-Curve method to reduced activation tempered martensitic steels such as Eurofer97 and F82H was studied in detail. Fracture tests with pre-cracked sub-sized compact tension specimens (three different sizes, 0.18T, 0.35T and 0.87T of Eurofer97) were carried out in the temperature range [-196 °C, -40 °C]. The toughness-temperature behavior and scatter were shown to deviate from the ASTM-E1921 standard predictions near the lower shelf. Using the method of maximum likelihood, the athermal component of the Master-Curve was calculated to better fit our fracture toughness data from the lower to the middle transition region. We showed that these Master-Curve adjustments are necessary to make the T0 values obtained near the lower shelf with 0.35T size C(T) specimens consistent with those obtained in the middle transition region with 0.87T C(T) specimens. The ASTM-E1921 specimen size limitations, setting the maximum toughness measurable with a given specimen size, were found to be too lenient for this kind of steels. This problem was especially evident in fracture toughness data of Eurofer97 and F82H obtained in the upper transition temperature range with two different specimen sizes. Thus a more stringent specimen size requirement was proposed to avoid inconsistent transition temperature determinations. A promising local fracture model with the potential of predicting cleavage fracture toughness was studied. Finite element simulations were undertaken for compact specimens, notched specimens and tensile specimens of Eurofer97 steel tested from 20 °C down to -197 °C. Three and two dimensional as well as axisymmetric simulations were run in order to calculate the stress and strain fields at the onset of brittle fracture. For each tested temperature, the calculated load-displacement curves were found to reproduce very well those of the experiments. A local approach fracture criterion was studied. This criterion states that when the maximum principal stress is larger than a critical stress within a critical volume ahead of the crack tip, notch or neck, cleavage is triggered leading to macroscopic fracture of the specimen. It was shown that this model is able to predict the minimum fracture load of the notched specimens with the same values of critical stress and critical volume that were calibrated to predict the lower bound of fracture in compact specimens. This local approach model was also successfully used to predict the strong size effect observed experimentally in pre-cracked compact tension specimens in the upper transition region. The critical fracture stress determined in the standard tensile specimens was found higher than that of the fracture specimens. It was suggested that this difference stems from a significant difference in the stress state between the different specimens (triaxiality). Finally, the comparison between the overall fracture behavior in the transition between F82H and Eurofer97 steels indicated that these two materials are quite similar. A difference in the reference temperature T0 of about 20 °C was found, with a nominal value of -100 °C and -80 °C for F82H and Eurofer97 respectively. However, some excessive scatter in the toughness data was found in F82H with more data points than expected lying below the 1% tolerance bound; this was not observed in Eurofer97. From an engineering point of view, the entire fracture databases of these two steels are encompassed practically by the same lower tolerance bound.

EPFL2009

Realization and study of InAs/GaAs quantum dot superluminescent diodes emitting at 1.3µm

Marco Rossetti

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons and valence band holes in all three spatial directions, thus creating fully discrete energy levels. The confinement in the InAs/GaAs material system is generated by the bandgap difference for the two materials (∼ 0.4 eV for InAs, and ∼ 1.4 eV for GaAs) which provides a minimum of potential energy for both electrons and holes inside the InAs nanoparticle. The appearance of new nanoscale effects modifies the electro-optical properties of such a system which makes possible the improvement of existing device performance or even fascinating novel device applications. For the creation of coherent structures with very high purity and high density, as it is required for application in light emitting devices, self-assembling is a very important pathway. The quantum dot creation mostly relies on the relaxation of the strain accumulated during the growth of lattice mismatched materials, which happens in a non-equlibrium condition. This results in a statistical distribution of all the parameters characterizing the InAs nanocrystals as size, composition and strain, whose direct manifestation is the inhomogeneously broadened light emission deriving from quantum dot ensembles. At the dawning of the quantum dot development the broadening was considered as a limiting factor for application in semiconductor lasers. Today, it is considered favorably for all the applications where a broad gain spectrum is required as optical amplifiers, monolithic and external-cavity tunable lasers, and superluminescent diodes. The objective of this thesis is the application of InAs/GaAs self-assembled quantum dots to the active region of high power and broad-band superluminescent diodes emitting in the 1.3 μm wavelength region. Superluminescent diodes are edge-emitting semiconductor light sources based on the amplification of spontaneous emission along a waveguide where optical gain is achieved through current injection. They combine the high power and brightness of laser diodes with the low coherence of LEDs. The latter is a fundamental feature for the application in optical coherence tomography medical imaging, where the image resolution is related to the spectral bandwidth of the light source used. The achievement of bandwidths larger than 100 nm are demonstrated in this work through the optimization of molecular beam epitaxy growth conditions. This may be done optimizing the statistical size-dispersion of the dot ensemble, or through the use of different dot layers emitting at slightly different wavelengths. The large bandwidth emission is obtained also due to the peculiar energy structure of this material, for which a multi-state emission appears under particular injection conditions. As a term of comparison, best commercial single-chip superluminescent diodes for the imaging application at 1.3 μm show bandwidths around 60 nm. Moreover, the discrete nature of the energy levels in InAs quantum dots together with the implementation of a GaAs-based technology can be beneficial for the device temperature stability (the competing technology, based on InP, suffers from strong thermal degradation). The analysis and understanding of the device temperature characteristics will be detailed in the last chapter of the manuscript. Even though standard devices show an important temperature dependence, a better stability may be achieved through the use of p-doped active regions. In the last few years quantum dot devices have shown outstanding properties if compared to the competing quantum well technology. Low threshold currents, high temperature stability and low chirp in lasers, large bandwidths and low gain saturation in amplifiers have been demonstrated. However, in spite of the large interest among the scientific community, many of the microscopic processes at the origin of the electro-optical characteristics of this material are not completely understood yet. The physics of the quantum dot active material, will be modeled in this work through the use of systems of rate equations with different complexity depending on the system they are applied to. We will provide a mean-field rate equation model allowing to get insights about carrier dynamics in QD lasers. Also, we will develop two different traveling-wave rate equation models, one for the modeling of the L-I characteristics and the other for the modeling of the spectral characteristics of quantum dot superluminescent diodes. Considering the carrier and photon density distributions across the device cavity is essential in superluminescent diodes, where the high single pass gain results in large non-homogeneities of photon and carrier distributions. The work is motivated by an industrial cooperation with EXALOS AG, a leading company in the development, manufacturing, and sales of broadband superluminescent diodes.

EPFL2007

Improving Sensitivity in Solid State NMR

Sensitivity in Nuclear Magnetic Resonance (NMR), especially in solid-state NMR, has always been a challenging and important issue and thus a motivation for new developments. The magnetic field B0, the gyromagnetic ratios of the observed nuclei, as well as the preparation of the samples are important for optimizing the sensitivity. Often only a small amount of sample is available and the nuclei of interest have low gyromagnetic ratios, which makes solid-state NMR extremely time consuming and, in some cases, practically impossible. In addition, strong interactions can cause extensive line-broadening, thus making it difficult to excite or detect high resolution spectra. In this thesis two main aspects concerning sensitivity in solid-state NMR are addressed. The first part reveals the difficulties of the excitation and detection of quadrupolar nuclei, in particular of the highly abundant nucleus nitrogen-14, and presents an indirect detection method combined with a more efficient excitation and reconversion scheme using Delays Alternating with Nutations for Tailored Excitation (DANTE). The second part presents a new experimental set-up for Dynamic Nuclear Polarization (DNP), which has evolved into a very successful method at the Ecole Polytechnique Fédérale de Lausanne. It is shown in this work that the DNP method combined with solid-state NMR allows one to enhance NMR signal intensities by one or two orders of magnitude, thus opening the doors towards a great variety of new exciting applications. In solids spinning at the magic angle, the indirect detection of single-quantum (SQ) and double-quantum (DQ) 14N spectra (I = 1) via spy nuclei S = 1/2 such as protons can be achieved in the manner of heteronuclear single- or multiple-quantum correlation (HSQC or HMQC) spectroscopy. The coherence transfer is achieved by recoupling the heteronuclear dipolar interactions, i.e., with symmetry-based sequences. The line broadening caused by the homogeneous decay of the transverse terms of the spy nuclei S can be reduced by implementing suitable dipolar decoupling schemes during the evolution period. The lack of efficiency of the excitation and reconversion can be overcome by replacing the conventional rectangular pulses applied to the nitrogen-14 nuclei by a train of short rotor-synchronized pulses in the manner of DANTE. These pulse trains excite a large number of crystallites in the sample uniformly and allow one to efficiently excite magnetization over large bandwidths, limited only by the quality factor of the probe. When combining N interleaved DANTE schemes, obtained by applying N pulses per rotor period, leading to so-called "DANTE-N" sequences, one obtains methods that greatly improve the uniformity of the excitation and lead to higher signal intensity in less rotor periods than basic DANTE. Applications to nitrogen-14 with fast magic angle spinning (typically νrot ≥ 60 kHz), using either direct or indirect detection, are backed up by simulations that provide insight into the properties of basic and interleaved DANTE sequences. DNP exploits the polarization transfer, driven by microwave irradiation, from unpaired electrons, usually carried by polarization agents such as TEMPO or TOTAPOL, to the target nuclei. This transfer results in enhancements of the signal-to-noise ratios and thus opens many new possibilities for challenging samples. In this thesis the experimental requirements are demonstrated and important parameters like the temperature dependence are investigated. Various sample preparation methods are proposed, e.g., by attaching the radical directly to the molecules under investigation, instead of preparing a solution that should forma glass upon freezing. The combination of indirect detection of 14N with DNP improves the signal-to-noise ratio dramatically and shortens the recovery delay between subsequent experiments. Besides samples of biological interest, it was also shown that the NMR signals of molecules on surfaces of various materials can be greatly enhanced by DNP. A characterization of the distribution of surface binding modes and interactions in a series of functionalized materials could be demonstrated. The bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions could be identified and compared. Furthermore, the remarkable gain in time afforded by DNP allows the facile acquisition of two-dimensional correlation spectra. The surfaces were characterized by transferring the polarization from protons to other nuclei such as 13C, 29Si and 27Al by means of cross-polarization.

EPFL2012