The main body of this thesis describes how to calculate scanning tunneling microscopy (STM) images from first-principles methods. The theory is based on localized orbital density functional theory (DFT), whose limitations for large-vacuum STM models are resolved by propagating localized-basis wave functions close to the surface into the vacuum region in real space. A finite difference approximation is used to define the vacuum Hamiltonian, from which accurate vacuum wave functions are calculated using equations based on standard single-particle Green’s function techniques, and ultimately used to compute the conductance. By averaging over the lateral reciprocal space, the theory is compared to a series of high-quality experiments in the low- bias limit, concerning copper surfaces with adsorbed carbon monoxide (CO) species and adsorbate atoms, scanned by pure and CO-functionalized copper tips. The theory compares well to the experiments, and allows for further insights into the elastic tunneling regime. A second significant project in this thesis concerns first-principles calculations of a simple chemical reaction of a hydroxyl (oxygen-deuterium) monomer adsorbed on a copper surface. The reaction mechanism is provided by tunneling electrons that, via a finite electron-vibration coupling, trigger the deuterium atom to flip between two nearly identical configurational states along a frustrated rotational motion. The theory suggests that the reaction primarily occurs via nuclear tunneling for the deuterium atom through the estimated reaction barrier, and that over-barrier ladder climbing processes are unlikely.

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Multi-component coatings and high entropy alloys have in recent years attracted great interest for research, since they have shown to exhibit properties greater than the com- ponents of their parts. Today’s climate challenges requires transitioning from fossil fuels to renewable energy sources which demands use of new technology and new innovations. The hydrogen fuel cell is a technology which produces no carbon emissions, and the drive for innovation has led researchers to apply multi-component (high entropy alloys) coatings to invent the next generation hydrogen fuel cells and help the transition to renewable energy sources. This thesis has investigated the process-structure-property relationships of four deposi- tion growth parameters: target current (Itarget), argon pressure (PAr). substrate bias (Vsubstrate) and deposition time (tdeposition) on TiNbZrTa-coatings, grown by magnetron sputtering using an industrial deposition system. The range of the parameters have been: Itarget from 2.5 to 6 A, PAr from 1 to 17 mTorr, Vsubstrate from 30 to 200 V and tdeposition from 3.6 to 12 minutes (depending on Itarget). Coatings have been grown on Si (001) and stainless steel 304 and 316L substrates. The coating microstructure was analyzed by X-ray diffraction and electron microscopy. The results have yielded that all coatings are equimolar and that the coatings exhibit three different morphologies, two different topologies and two different corresponding structures. The different morphologies are wave, coarse columnar and fine columnar morphology. The two topologies are nodular and dune surface topology. The two different structures are a solid solution BCC (110) phase and an amorphous or nanocrystalline phase. The results indicate that parameters affecting the temperature of the substrate (Tsubstrate) is the prime decider for the final morphology of the coatings. High Itarget and Vsubstrate, low PAr and long tdeposition all increases Tsubstrate and results in a coating which exhibits a fine columnar morphology, dune topology and a solid solution BCC phase. These types of coatings have also proven to have improved corrosion resistance compared to the other type of coatings seen in this thesis. The other kind of coating is grown with low Itarget and Vsubstrate, high PAr and short tdeposition, which causes minimal increase of Tsubstrate. These growth parameters result in a coating with coarse columnar morphology, nodular topology and amorphous or nanocrystalline phase, with less corrosion resistance. / FunMat II

Due to the complex nature of phase interfaces, acquiring precise interfacial energies is usually a big challenge for both experimental measurements and computational modelings. In this thesis, we put forward an efficient route for assessing the temperature dependence of the interfacial energy using density functional theory (DFT). For our investigations, we select the Cu-Co binary system as a model with large miscibility gap. Most of the first-principles calculations presented here are carried out using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA), but other alternative DFT methods are also included in the various stages of the project.The first step is to acquire an accurate thermodynamical description of the Cu-Co binary system. We assess the quality of the predicted thermodynamic properties by an effort to reproduce the phase diagram for the entire range of composition using first-principles calculations and alloy theory. The calculations are performed for the random Cu-Co alloys with face-centered cubic (fcc) structure at both ferromagnetic (FM) and paramagnetic (PM) states, depending on the composition. We demonstrate that the equilibrium volumes and magnetic states are crucial for the proper description of the magnetic entropy of the Cu-Co system at elevated temperatures. More specifically, the contribution of magnetic entropy to the free energy in the Cu-rich region obtained for the PM state turns out to be critical. Furthermore, the adopted equilibrium volumes strongly affect the contribution of the vibrational entropy to the free energy. When all effects are properly accounted for, we find that the ab initio phase diagram of the Cu-Co system agrees well with the Thermo-Calc phase diagram and the experimental observations.The Cu-Co system has a large miscibility gap. The interface between the decomposed Cu-rich and Co-rich phases plays critical roles in the precipitation nucleation and growth, therefore having huge effects on the physical and mechanical properties of the alloys. Therefore, adopting the thermodynamical properties of the bulk Cu-Co alloys successfully obtained by our ab initio calculations, we go further and investigate the interfacial properties of the Cu-Co alloys using a coherent interface model. The chemical, magnetic, and strain energy contributions to the formation energy of the interfaces are analyzed separately. We find that the chemical interfacial energies generally decrease with increasing concentrations, namely when the compositions accross the interface become more homogenous. We identify a sizable contribution to the interfacial energies from the magnetic effects. The temperature dependence of the interfacial energy is estimated, to the first-order approximation, through considering how the equilibrium compositions of the two phases vary at different temperatures. Our results show that the temperature dependence of the interfacial energy originates primarily from the temperature-induced increase of the mutual solubility of the alloy constituents and the loss of the magnetic long range order near the Curie temperature. Our ab initio results are compared with the experimental data as well as with those extracted from Thermo-Calc modeling. The present thesis provides an atomic-level description of the bulk and interfacial properties of the Cu-Co binary system using quantum mechanics simulations. This approach is believed to be useful for a complete thermodynamical description of other similar immiscible alloy systems as well from first-principles. / På grund av fasgränssnittens komplexa karaktär är det vanligtvis en stor utmaning att få exakta gränssnittsenergier för både experimentella mätningar och beräkningsmodeller. I denna avhandling presenterar vi en effektiv väg för att bedöma temperaturberoendet för gränssnittsenergin med hjälp av densitetsfunktionell teori (DFT) i ett modellsystem, Cu-Co-legeringar. Våra första principberäkningar är baserade på den exakta muffins-tennorbitalmetoden (EMTO) i kombination med den koherenta potential-approximationen (CPA).Det första steget är att skaffa en noggrann termodynamisk beskrivning för det binära systemet. Vi bedömer kvaliteten på de förutsagda termodynamiska egenskaperna genom ett försök att reproducera fasdiagrammet för hela kompositionen med hjälp av första principberäkningar och legeringsteori. Beräkningarna utförs för de slumpmässiga Cu-Co-legeringarna med ansiktscentrerad kubisk (fcc) struktur vid både ferromagnetiska (FM) och paramagnetiska (PM) tillstånd, beroende på sammansättningen. Vi visar att jämviktsvolymer och magnetiska tillstånd är avgörande för en korrekt beskrivning av den magnetiska entropin i Cu-Co-systemet vid förhöjda temperaturer. Närmare bestämt visar sig den magnetiska entropins bidrag till den fria energin i den Cu-rika regionen som erhålls vid PM-tillståndet vara kritisk. Vidare påverkar de antagna jämviktsvolymerna starkt vibrationsentropins bidrag till den fria energin. När alla effekter är korrekt redovisade kommer vi fram till att ab initio fasdiagrammet för Cu-Co-systemet överensstämmer väl med experimentellt resultat.Cu-Co-systemet har ett stort blandningsgap. Gränssnittet mellan de sönderdelade Cu-rika och Co-rika faserna spelar en avgörande roll för nederbördskärnbildning och tillväxt och har därför enorma effekter på legeringarnas fysiska och mekaniska egenskaper. Här, med de termodynamiska egenskaperna hos bulk-Cu-Co-legeringarna framgångsrikt erhållna med våra ab initio-tillvägagångssätt, går vi vidare och undersöker gränssnittsegenskaperna för Cu-Co-legeringarna med en koherent gränssnittsmodell. De kemiska, magnetiska och stamenergibidragen till gränssnittets bildningsenergi analyseras separat. Vi finner att de kemiska gränssnittsenergierna generellt minskar med ökande koncentrationer, nämligen när kompositionerna över gränssnittet blir mer homogena. Vi identifierar ett betydande bidrag till gränssnittsenergierna från de magnetiska effekterna. Temperaturberoendet för gränssnittsenergin uppskattas, till första ordningens approximation, genom att överväga hur jämviktskompositionerna i de två faserna varierar vid olika temperaturer. Våra resultat visar att temperaturberoendet för gränssnittsenergin främst härrör från den temperaturinducerade ökningen av legeringskomponenternas ömsesidiga löslighet och förlusten av magnetisk långdistansordning nära Curie-temperaturen. Våra ab initio resultat jämförs med experimentella data såväl som med de som extraherats från Thermo-Calc-modellering.Föreliggande avhandling ger en atomnivåbeskrivning av bulk- och gränssnittsegenskaper hos Cu-Co-binära systemet med hjälp av kvantemekaniska simuleringar, vilket antas vara användbart för en fullständig termodynamisk beskrivning av liknande icke-blandbara legeringssystem med exakta initieringsmetoder.

In this project, I investigate the photoconductivenature of photochromic rare-earth metal oxy hydrides (REMHO). Such materials have received increasingscientific attention since they show a color-neutralphotochromic effect that can be applied, e.g., in smartwindows or chromogenic devices. Photochromicmaterials reversibly turn opaque from transparentunder illumination with light of optical wavelength. Inrecent studies it was found that these materials alsoshow an instant decrease in resistivity whenilluminated which can be used in optical sensors. Tounderstand the nature of this photoconductive effect,I grew yttrium oxy hydride thin films by reactivemagnetron sputtering. I measured the resistivity forillumination from front and substrate side, opticaltransmission and compositions of the samples andrelated the results to photoconductivity. I show thatphotoconductivity is a bulk effect and not directlyrelated to photochromism. Samples that almost lostphotochromism due to aging, still show strongphotoconductivity. Moreover, it was observed that theresistance increased faster during bleaching for frontillumination than for back illumination.

The magnetocaloric effect (MCE) is the temperature change in a magnetic material due to a change in an applied magnetic field. How the MCE behaves in different magnetic materials and at different phase transitions is fundamental to understand. The driver of the MCE is a change in entropy which has multiple contributions: magnetic, lattice, and electron. In this thesis the MCE is studied in a simple antiferromagnetic (AFM) model andin a realistic noncollinear spin glass Neodymium model using Monte Carlo and Atomistic Spin Dynamics simulations. For the simple AFM system, no clear results were achieved, indicating that MCE in AFM materials is not due to a change solely in the magnetic entropy. For the complex magnetic material Nd, a more clear result is seen, indicating that frustration in the system might be important to the MCE in noncollinear materials. Nd results also signify more phase transitions than previously reported.

Detta projekts mål har varit att undersöka tre olika 2D-materials triboelektriska egenskaper och därmed placera dem i en triboelektrisk serie. Detta utfördes genom att använda en triboelektrisk nanogenerator (TENG) och mäta den resulterande spänningen. Tio stycken motmaterial applicerades mot varje 2D-material på nanogeneratorn. Utifrån resultatet var det möjligt uppmärka typiska vågformer för en TENG, alltså kunde resultatet från mätningen antas vara från den triboelektriska effekten. 2D-materialen placerades tillsammans med dess motmaterial i en triboelektrisk serie och sorterades sedan för att bestämma dess elektronaffinitet. För de tre 2D-materialen hade de gemensamt att ETFE och FEP tillhör den positiva sidan av den triboelektriska serien relativt de 2D-materialen. Resten, alltså: cellofan, kapton, LDPE, nylon, PEEK, PEI, polypropylene och PTFE, placerades negativt i deras respektives 2D-materials serie. Dock blev resultatet ej som förväntat, då ordningen på motmaterialen i serien kunde antas vara samma för alla 2D-material, men detta var inte vad som hittades. Anledningen till detta kan möjligtvis vara ytladdningar som kan ha överförts till materialen medans de hanterades, eller på grund av ytstrukturen av 2D-materialen. Därför föreslås att detta arbete kan förbättras genom mer varsam hantering och spridning av materialen över dess plattform.

Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the easurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical diversity and ambient temperature low cost manufacturing. In this thesis we investigate the switching and conducting properties of photochromic dithienylethene derivatives. Such molecules change their conformation in solution when acted upon by light. Photochromic molecules are attractive candidates for use in molecular electronics because of the switching between different states with different conducting properties. The possibility of optically controlling the conductance of the molecule attached to leads may lead to new device implementations. The switching reaction is investigated with potential energy calculations for different values of the reaction coordinate between the closed and the open isomer. The electronic and atomic structure calculations are performed with density functional theory (DFT). It is concluded that there is a large potential energy barrier separating the open and closed isomer and that switching between open and closed forms must involve excited states. The conducting properties of the molecule inserted between gold leads is calculated within the Non Equilibrium Green Function theory. The transmission function is calculated for the two isomers with different basis sizes for the gold contacts, as well as the electrostatic potential, for finite applied bias voltages. We conclude that a Au 6s basis give qualitatively the same result as a Au spd basis close to the Fermi level. The transmission coefficient at the Fermi energy is around 10 times larger in the closed molecule compared to the open. This will result in a large difference in conductivity. It is also found that the large difference in conductivity will remain for small applied bias voltages. The results are consistent with earlier work. / QC 20101119

In the present work, growth and structural characterization of CrB2/TiB2 superlattices on (0001) Al2O3 substrate is studied. The superlattices are grown using a direct current magnetron sputtering (DCMS) system with a base pressure of <9E-7 Torr. For structural characterization X-ray diffraction (XRD), X-ray reflectivity (XRR), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and scanning transmission electron microscopy (STEM) are used. Hardness is measured using nanoindentation technique. For growth of CrB2/TiB2 superlattices it is shown that the sputtering gas pressure of PAr= 4 mTorr, and substrate temperature of T= 600 °C are optimized parameters for growing well-structured superlattices with good interface quality. Superlattices with a layer thickness ratio of Γ= 0.43 (Γ= DTiB2/DCrB2+DTiB2) and a total thickness of 1 μm are deposited with different modulation periods Λ=1, 2, 6, 8, and 10 nm to see the layer-thickness affect on the quality of the structures. XRD and TEM results show that by increasing the modulation period, the quality of superlattices with smooth interfaces increases. The superlattices with modulation period Λ=8 nm is shown to be the best structure having coherent lattice and smooth interfaces up to ~20 periods. The STEM analysis shows that after about 20 periods, grains started to grow at slightly different orientations. A superlattice of TiB2/CrB2(having TiB2 as the first deposited layer) with modulation period Λ=8 nm shows an epitaxial growth of TiB2 on Al2O3 (0001) substrate, however, no big difference between the structure of TiB2/CrB2 and CrB2/TiB2 superlattices is seen. EDX maps and line profiles show that there is a diffusion of CrB2 into TiB2 layers which is a serious problem for obtaining sharp interfaces. STEM also shows that for a small modulation period of Λ=1 nm, there is a faint layered structure, whereas EDX, SAED and XRD indicates a homogenous textures Ti-Cr-B film in this sample. Finally, the hardness measurement shows a hardness value of 29-34 GPa for different modulation periods. The lowest hardness value is related to the sample with modulation period of Λ=1 nm with about 29 GPa, and the highest hardness is related to the sample with Λ=8 nm ith around 34 GPa.

This thesis investigates how the low-energy magnetic configuration of a mesoscopic chain can be tuned by geometrical modifications. The magnetic arrays made by single-domain stadium shaped elements positioned side-by-side were fabricated by patterning into a sputtered ferromagnetic thin film. The thickness of the thin film was determined by X-ray reflectivity measurements while Scanning Electron Microscopy and Atomic Force Microscopy were used to characterize the surface morphology of the nanostructures. Magnetic Force Microscopy was used to image the magnetic configuration of mesoscopic chains after applying a thermal annealing protocol and a field demagnetization protocol. By gradually modifying the geometrical arrangement of the half of mesospins, the magnetic chain is found to exhibit a transition from antiferromagnetic to dimer antiferromagnetic configuration after the thermal annealing treatment. After the field demagnetization protocol, both antiferromagnetic and dimer antiferromagnetic domains are formed. Micromagnetic simulations were performed to investigate how the interaction between the mesospins is affected by the geometrical modifications and a qualitative method was invented to examine the theoretical low-energy state of the magnetic chains. It is found that the low-energy magnetic configuration of the mesoscopic arrays is formed after the competition and collaboration of different interactions and is the one observed after the thermal annealing treatment.