| 1. |
- Nordstrand, Johan
(author)
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At the Mountains of Modeling : Multiscale Simulations of Desalination by Capacitive Deionization
- 2022
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Doctoral thesis (other academic/artistic)abstract
- More than 2 billion people are living in water-scarce areas. Meanwhile, there are enormous amounts of water in the salty oceans. Capacitive deionization (CDI) rises to the challenge with electrochemical cells for desalinating the water. As the CDI field expands, modeling becomes an increasingly important part of the development landscape. Multiscale modeling could bring innovations from the material scale to pilot plants. The multiscale work in this thesis has been like climbing a mountain. At the start, we investigate the macroscopic device level. One milestone is the electrolytic-capacitor (ELC) model, which can simulate CDI process dynamics. Whereas previous 2D models were unsteady for a single CDIcell, the ELC model could accurately simulate stacks of over 100 cells at a fraction of the time. It also enables simulations of complex upscaled geometries, such as bipolar electrode stacks, ohmic charging, and asymmetric devices. Going up the mountain, the mesoscopic level reveals the local mechanisms behind the macroscopic behavior. One important stepping stone is the dynamic Langmuir (DL) model, which reveals how isotherm-based modeling can crease stable and tractable simulations. Also, developments in isotherm, double-layer, and circuit modeling make it possible to choose what model structures to lean on depending on the conditions. Near the top of the mountain, the microscopic level shows the fundamental atomic mechanisms behind the mesoscopic material properties. These investigations reveal a ladder mechanism of ion transport in crystals of Prussian blue analogs (PBA), meaning the cations climb frames formed by negative groups in the crystal structure.In the end, we plant a flag by combining the developments from the journey into a complete multiscale model. That model demonstrated that we could predict CDI charging trends from the atomic structure of PBA electrodes. Having the full multiscale model also made it possible to backtrack and determine atomic-level mechanisms by comparing the output of different mechanism cases with macroscopic experiment data. The multiscale mountain is massive and has big potential. A dream is that future research will expand these concepts, in CDI and beyond.
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| 2. |
- Ahmadi, Sareh, 1982-
(author)
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Structure-dependent charge transfer at the interafce between organic thin films, and metals and metal oxides
- 2013
-
Doctoral thesis (other academic/artistic)abstract
- The purpose of the research work, presented in this thesis is to offer a detailed atomic level study of interfaces created by adsorption of organic molecules on metals and metal oxides to point out significant impact of substrate, dye structure as well as different mediators on the charge transfer at these interfaces, which is proven to influence the device performance to a great extent.Adsorption of organic photosensitive molecules on metals and metal-oxides is the main focus of this thesis. Phthalocyanines which are organic semiconductors offer a broad range of properties, such as thermal and chemical stability, high charge mobility and strong absorption coefficient in the visible and near-IR regions, which make them very attractive to be applied in various systems and devices. Fuel cells, organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs) and solar cells are examples of phthalocyanine’s applications. The main focus of this work is to characterize the interfaces of Dye Sensitized Solar Cells (DSSCs).DSSC was invented by Michael Grätzel and Brian O’Regan in 1988. At the heart of this cell there is an oxide which is coated by a photosensitive dye. Under illumination, an electron is excited from HOMO to LUMO of the molecule, which can be further transferred to the conduction band of the oxide by a proper energy level alignment. The original state of the dye is regenerated by electron donation via the electrolyte, which usually is an organic solvent containing a redox couple e.g., iodide/triiodide. The iodide is regenerated by reduction of triiodide at the counter electrode. To improve the functionality of the cell, different additives can be added to the electrolyte.To mimic the interfaces of this cell, molecular layers of MPc (M: Fe, Zn, Mg) are adsorbed on both metallic surfaces, Au(111) and Pt(111), and rutile TiO2(110). Layers of iodine were inserted between metallic substrates and dyes to investigate the electronic properties and charge transfer at these multi-interface systems. 4-tert-butyl pyridine is a significant additive to the electrolyte and has proven to enhance the cell’s performance. This molecule was also adsorbed on Pt(111) and TiO2(110). Phthalocyanines were deposited by organic molecular beam deposition and 4TBP was evaporated at room temperature. Surface structures and reconstructions were confirmed by LEED measurements. Surface sensitive synchrotron radiation based spectroscopy methods, XPS and NEXAFS were applied to characterize these surfaces and interfaces. STM images directly give a topographical and electronic map over the surface. All measurements were carried out in UHV condition.When MPc was adsorbed on Au(111) and TiO2(110), charge transfer from molecule to substrate is suggested, while the opposite holds for MPc adsorbed on Pt(111). Moreover, stronger interaction between MPc and Pt(111) and TiO2(110) compared to Au(111) also demonstrates the effect of substrate on the charge transfer at the interface. The stronger interaction observed for these two substrates disturbed the smooth growth of a monolayer; it also resulted in bending of the molecular plane. Interaction of MPc with metallic surfaces was modified by inserting iodine at the interface. Another substrate-related effect was observed when MgPc was adsorbed on TiO2(110); and -cross linked surfaces, where the surface reconstruction directly affect the molecular configuration as well as electronic structure at the interface. Besides, it is shown that the d-orbital filling of the central metal atom in MPc plays an important role for the properties of the molecular layer as well as charge transfer at the interface.Upon adsorption of 4TBP on Pt(111), C-H bond is dissociatively broken and molecules is adsorbed with N atoms down. Modification of surface by iodine, prevent this dissociation. In the low coverage of iodine, there is a competition between 4TBP and iodine to directly bind to Pt(111). Investigation on the adsorption of 4TBP on TiO2(110) illustrated that these molecules in low coverage regime, prefer the oxygen vacancy sites and their adsorption on these sites, results in a downward band bending at the substrate’s surface.
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| 3. |
- Berntsen, Magnus H.
(author)
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Consequences of a non-trivial band-structure topology in solids : Investigations of topological surface and interface states
- 2013
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Doctoral thesis (other academic/artistic)abstract
- The development and characterization of experimental setups for angle-resolved photoelectron spectroscopy (ARPES) and spin- and angle-resolved photoelectron spectroscopy (SARPES) is described. Subsequently, the two techniques are applied to studies of the electronic band structure in topologically non-trivial materials.The laser-based ARPES setup works at a photon energy of 10.5 eV and a typical repetition rate in the range 200 kHz to 800 kHz. By using a time-of-flight electron energy analyzer electrons emitted from the sample within a solid angle of up to ±15 degrees can be collected and analyzed simultaneously. The SARPES setup is equipped with a traditional hemispherical electron energy analyzer in combination with a mini-Mott electron polarimeter. The system enables software-controlled switching between angle-resolved spin-integrated and spin-resolved measurements, thus providing the possibility to orient the sample by mapping out the electronic band structure using ARPES before performing spin-resolved measurements at selected points in the Brillouin zone.Thin films of the topological insulators (TIs) Bi2Se3, Bi2Te3 and Sb2Te3 are grown using e-beam evaporation and their surface states are observed by means of ARPES. By using a combination of low photon energies and cryogenic sample temperatures the topological states originating from both the vacuum interface (surface) and the substrate interface are observed in Bi2Se3 films and Bi2Se3/Bi2Te3 heterostructures, with total thicknesses in the ultra-thin limit (six to eight quintuple layers), grown on Bi-terminated Si(111) substrates. Band alignment between Si and Bi2Se3 at the interface creates a band bending through the films. The band bending is found to be independent of the Fermi level (EF) position in the bulk of the substrate, suggesting that the surface pinning of EF in the Si(111) substrate remains unaltered after deposition of the TI films. Therefore, the type and level of doping of the substrate does not show any large influence on the size of the band bending.Further, we provide experimental evidence for the realization of a topological crystalline insulator (TCI) phase in the narrow-band semiconductor Pb1−xSnxSe. The TCI phase exists for temperatures below the transition temperature Tc and is characterized by an inverted bulk band gap accompanied by the existence of non-gapped surface states crossing the band gap. Above Tc the material is in a topologically trivial phase where the surface states are gapped. Thus, when lowering the sample temperature across Tc a topological phase transition from a trivial insulator to a TCI is observed. SARPES studies indicate a helical spin structure of the surface states both in the topologically trivial and the TCI phase.
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| 4. |
- Skoglund Lindberg, Peter Anders, 1980-
(author)
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Electron-Impact Liquid-Jet Water-Window X-ray Sources
- 2010
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Licentiate thesis (other academic/artistic)abstract
- This Thesis describes the development and characterization of a soft x-ray liquidjet-anode electron-impact source. With a water-jet target the primary emission is the O Kα line at 525 eV. This is close to the lower edge of the water-window, a spectral region lacking simple laboratory sources. In the hard x-ray regime electronimpact microfocus sources have matured and are simple, stable, reliable, and inexpensive. It would be beneficial if this source concept could be used also for soft x-ray generation. Spectral measurements of a 120 W, 30 keV electron beam focused on a 20 μm water jet show an x-ray intensity of up to 3.2 × 1012 ph/(s×sr×line). Combined with source size measurements up to 50 W a maximum brightness of 3.5 × 109 ph/(s×μm2×sr×line) is reported. This makes the brightness comparable to the compact discharge-plasma sources presently used for soft x-ray microscopy. The source appears to be scalable another order of magnitude which would make the brightness equal to that of the laser-plasma sources.
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| 5. |
- Claesson, Thomas, 1976-
(author)
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Investigations of Transition Metal Systems : Angle-Resolved Photoelectron Spectroscopy at Low and High Energy
- 2007
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Doctoral thesis (other academic/artistic)abstract
- The work presented in this thesis aims at achieving an increased understanding of the electronic structure of materials whose properties are to a large extent governed by transition metals. All studies are based on data from angle-resolved photoelectron spectroscopy (ARPES) employed on the valence band of the studied systems. In some cases spectroscopic data have also been combined with numerical electronic structure studies performed using a density functional theory computer code. Five of the studies make extensive use of the benefits provided by performing angle-resolved photoelectron spectroscopy at high photon energies. When probed with soft x-ray photons, the near Fermi edge distribution of spectral weight in Nd2-xCexCuO4 and La2-x-yNdySrxCuO4 display significant differences as compared to similar studies performed in the 20-100 eV range of photon energies. This effect can be attributed to the approximate two-fold increase in probing depth as compared to the low photon energy case. However, high photon energies not only give increased probing depth, it also enables a straightforward interpretation of ARPES data from fully three-dimensional compounds in terms of valence band energy dispersion. This principle has been used to study the dispersions in the transition metal oxides CoO and Cu2O as well as in a proof-of-principle study, where copper was used as a test case. CoO exhibits a magnetic phase transition at the Néel temperature, from a paramagnetic state into an antiferromagnetically ordered state. The presented data show a significant shift of spectral weight between different band structure regions as the sample temperature is elevated across the Néel temperature. The Cu2O ARPES data have confirmed the existence of a hybridized Cu 3d - Cu 4s state. Through comparision with theoretically calculated results, a refined value of the Hubbard potential U has been determined. An extensive ARPES dataset from copper, covering the entire Brillouin zone, has in a proof-of-principle study been used to derive the binding-energy resolved valence band charge density. Two ARPES studies of the La2-x-yNdySrxCuO4 system have been performed in the cnventional low photon energy range. The high energy resolution has enabled a detailed study of dispersive features in the near Fermil level region of the valence band.
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| 6. |
- Ji, Shaozheng
(author)
-
Photo-induced Structural Dynamics in Transition Metal Dichalcogenides
- 2020
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Doctoral thesis (other academic/artistic)abstract
- Ultrafast electron microscope (UEM), a combination of transmission electron microscopy and laser-based pump-probe techniques, facilitates ultrafast imaging, diffraction, and electron-spectroscopy with high spatial resolution. The unique advantages of UEM enable local ultrafast dynamic studies in materials, nano-system, and biology. The performance of UEM, such as its temporal and energy resolutions and coherence, is largely determined by the quality of electron beam. In this thesis, the beam dynamics in our UEM with a thermionic gun was studied. The influence of cathode geometry and Wehnelt bias voltage on the electron pulse dynamics is determined through experiments and finite element simulations. A guard ring cathode can effectively address the problem of shank-emitted electrons in traditional truncated tip geometries, allowing UEM operation at minimum Wehnelt bias and improving the temporal resolution under realistic conditions. A sub-ps temporal resolution can be reached with few electrons in one pulse. Compared to the 300 fs laser pulse width, the temporal duration of the electron pulse is nevertheless elongated during the propagation in the UEM column. The simulations show that the initial energy spread and the angular distribution from the photoemission process are the dominant factors in this temporal dispersion.Utilizing our UEM, the structural dynamics including photo-induced phase transitions and coherent phonon excitation were studied in two transition metal dichalcogenides (TMDs), 1T-TaSe2 and Td-WTe2. 1T-TaSe2 is a room temperature commensurate charge density wave (C-CDW) material. The C-CDW phase undergoes a phase transition to an incommensurate charge density wave (IC-CDW) at 473 K featured by a rotation of the superstructure. Under photoexcitation, the C-CDW in 1T-TaSe2 can be suppressed within sub-ps time scale. A recovery time-constant of ~0.7 ps is observed for the commensurate periodic lattice distortion (PLD) at a pump fluence insufficient to drive a phase transition into the IC-CDW phase. At higher pump fluence, sufficient to drive nucleation of the IC-CDW phase, there is a ~1 ps delay between the extinction of the C-CDW phase and the onset for formation of the IC-CDW phase. Within the ~1 ps, a transient unreconstructed state may exist. The ~1 ps delay time for the nucleation of the IC-CDW phase implies that a phononic thermalization is involved in the decay of this highly perturbed photoinduced transient state. During the nucleation of the IC-CDW phase, a face-centered cubic (FCC) like stacking order is observed already at ~4 ps after photoexcitation. Such rapid stacking order formation indicates that the nucleation of the IC-CDW phase in the adjacent layers is not independent but coupled together. We can infer that the nucleation of the IC-CDW is inherently 3-dimensional (3D). The highly 3D feature of CDW in 1T-TaSe2 indicates a strong interlayer interaction that establish long range out-of-plane stacking order.Both in 1T-TaSe2 and Td-WTe2, a coherent shear phonon is observed by photoexcitation. In 1T-TaSe2, the coherent shear mode is along the stacking direction of the C-CDW phase. We analyze the launching mechanism in terms of hot/cold spots on the Se-sublattice that result from the rapid melting of the PLD. During the melting, a difference in Se-phonon amplitudes results in shear forces between the layers. For a perfect trigonal stacking, the force will be compensated. However, there always remain uncompensated restoring forces in stacking-order direction because of the domain structure in out-of-plane direction. The excitation of a coherent shear phonon is even stronger in Td-WTe2. The shear direction is along the b axis where there is a stacking displacement for the adjacent layers.In Td-WTe2, a photo-induced phase transition from orthorhombic Td to orthorhombic T* phase is observed which involves a stacking order change in the out-of-plane direction by a layer shear displacement along the b axis direction. Upon photoexcitation with pump fluence higher than a critical value, the change in interlayer potential results in the formation of a new metastable phase with a ~4 ps time constant. The shear displacement of the adjacent layers increases linearly with the increase of pump fluence and stabilize at ~ 8 pm when the pump fluence is higher than ~2 mJ/cm2. The photo-induced phase transition in Td-WTe2 can be influenced by local defect structures. In a ripple defect rich sample, a new phase transition from orthorhombic T* to monoclinic T’ phase will occur following the Td to T* phase transition. It can be inferred that strain fields in the sample can modulate the photo-induced phase stability. This effect has potential application in strain engineering of 2 dimensional TMDs.The observed photo-induced phase transition and coherent shear phonon in 1T-TaSe2 and Td-WTe2, demonstrate the importance of inter-layer interaction in TMDs.
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| 7. |
- Månsson, Martin, 1976-
(author)
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Adatoms, Quasiparticles & Photons : The Multifaceted World of Photoelectron Spectroscopy
- 2007
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Doctoral thesis (other academic/artistic)abstract
- The experimental work presented in this thesis is based on a wide assortment of very advanced and highly sophisticated photoelectron spectroscopy (PES) techniques. The objective of the present study has been to reveal and understand the electronic structure and electron dynamics in a broad spectrum of materials, ranging from wide band gap oxides, via semiconductors along with metals, and finally high-temperature superconductors. The first part of the thesis concerns laser-based pump-and-probe PES. This unique experimental technique has permitted a study of the excited electronic structure and the electron dynamics of several semiconductor surfaces. An insight into details of the adatom to restatom charge-transfer of the Ge(111)c(2x8) surface is presented, as well as an estimate for the timescale in which the dynamic adatoms of the Ge(111):Sn(sqrt3xsqrt3)R30deg surface operate. Further results comprise a novel unoccupied surface state at the GaSb(001) surface as well as a time-resolved study of the charge accumulation layer at the InAs(111)A/B surfaces. In the second part, high-resolution synchrotron based angle-resolved PES (ARPES) data from the cuprate high-temperature superconductor La(2-x)Sr(x)CuO(4) (LSCO) is presented. This extensive study, reveals detailed information about how the Fermi surface and electronic excitations evolve with doping in the superconducting state. The results comprise support for a connection between high- and low-energy electronic responses, the characteristics of the superconducting gap, and indication of a quantum phase transition between two different superconducting phases. In the third group of experiments we move away from the two-dimensional systems and concentrate on fully three-dimensional compounds. By the use of soft x-ray ARPES it is possible to extract the three-dimensional electronic structure in a straightforward manner with increased k(perpendicular)-resolution. As a result the first high-quality ARPES data from Cu2O is presented, as well as a novel method for extracting the (real space) electron density by ARPES. These experiments clearly display the advantages of using soft x-ray ARPES. If the material and type of experiment is chosen wisely, the benefit of the increased k||-window and the free electron final state, surpass the drawbacks of decreased count-rate and inferior energy resolution. Finally we return to the high-temperature superconductors (NCCO & Nd-LSCO) and make use of the increased bulk-sensitivity. From an evident change in the shape of the Fermi surface when moving from low to high photon energies, the durface to bulk difference in electronic structure is highlighted.
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