The global increase in internet usage requires an upgrade of the existing infrastruc- ture. Lasers are a key proponent to improving existing systems, and engineering better gain materials aids in this effort. (InGa)As is the leading material in this field for 1.55 μm communication wavelengths, but can be improved on by changing the substrate from InP to GaAs. Another improvement would be reducing the losses due to Auger recombination. (InGa)(BiAs) is suggested to alleviate many of these issues, as it can be grown on a GaAs substrate and is capable of decreased Auger recombination. By analyzing prospective alloys (and existing ones) using spatial electron localization, a superior candidate for industrial use can be suggested. The localization captures the disorder introduced by alloying and can be associated with material properties such as the gain characteristics and photoluminescence linewidths. These properties are important factors in determining a successor. The subject of two-dimensional materials is another topic which has shown promise in various applica- tions. Examples include flexible, transparent, and miniaturized electronics. Recent research done by Al Balushi et al. suggests that GaN may be stabilized in a two-dimensional sys- tem. By extending the material modelling approach from the telecommunication application to this system, we were able to show which III-V isoelectronic elements can be substituted into GaN. This two-dimensional system may be the only candidate capable of spanning the visible spectrum. We found Phosphorus to be the strongest candidate for decreasing the band gap. / Thesis / Master of Applied Science (MASc)

Tesis por compendio / [EN] The dimensionality of the system plays a decisive role in the behavior of the electronic dynamics of interacting electrons. In particular, the quasi-2D dimensionality is responsible for the unusual behavior observed in graphene-like materials and layered van-der-Waal systems. Moreover, such effects are also observed for superconducting materials of high critical temperature, even in the normal state, due to their low-dimensionality. The experimental study of graphene triggered a growing attention to respective electronic properties, because the honeycomb lattice defines a band structure with two nodal points in the Brillouin zone which determines a relativistic Dirac-type electronic dynamics. Within a theoretical framework, many properties of single-layer graphene have been studied to allow further characterization of this material. These properties are unconventional due to the unique band structure of graphene, which is described in terms of Dirac fermions, creating links with certain theories of particle physics. In fact, several theoretical groups have employed phenomenological models inspired in quantum cromodynamics (i.e. Nambu-Jona Lassino and Gross-Neveu models) applied to the study of graphene properties. These properties are responsible for the unusual phenomena, such as the fractional Hall effect, which allows the possibility for magnetic catalysis of an excitonic gap, ferromagnetism and superconductivity. The research of high critical temperature superconductors with impurity centers is significant for understanding the underlying physics of such disordered systems. While the cuprate family present insulating properties in the pristine state, the undoped iron pnictides (i.e. LaOFeAs) show a semi-metallic behavior. Inspite these diferences, both compounds are layered structures, where the superconducting state is supported by a quasi-2D square lattice. While for iron pnictides this state is formed by the FeAs layer, the cuprate superconducting state is formed by the CuO layer. The current work focuses on the theoretical study of the structural, electronic and optical properties of graphene-type materials, such as bilayer graphene; and also of s- and d-wave superconductors, more specifically iron pnictides and cuprates, respectively. Furthermore, disordered systems will be focused upon since these (quasi-)2D systems are quite sensitive to disorder. Such properties have major importance for technological device applications, as can be observed in the increasing technological fields of high temperature superconductores and electronic devices. The type of perturbations applied to the systems of interest are chemical impurities and/or external electric bias, and these show variations of the electronic and optical properties when compared to the pristine systems. / [ES] La dimensionalidad de un sistema juega un papel fundamental en la conducta de la dinámica de los electrones que interactúan. En particular, la dimensionalidad cuasi-2D es responsable del comportamiento inusual observado en materiales de tipo grafeno y sistemas laminares basados en enlaces de tipo van der Waals. Además, estos efectos también se observan en materiales superconductores de alta temperatura crítica, incluso en el estado normal, debido a su baja dimensionalidad. El estudio experimental del grafeno provocó una atención creciente a sus propie-dades electrónicas, porque su estructura en forma de panal de abejas da lugar a una estructura de bandas con dos puntos nodales en la zona de Brillouin que determina una dinámica electrónica relativista de tipo Dirac. En el plano teórico, muchas propiedades del grafeno de una sola capa se han estudiado para permitir una mayor caracterización de este material. Estas propiedades son poco convencionales debido a la singular estructura de bandas del grafeno, que se describe en términos de fermiones de Dirac, lo que crea vínculos con ciertas teorías de la física de partículas. De hecho, varios grupos teóricos han empleado modelos fenomenológicos inspirados en la cromodinámica cuántica (es decir, los modelos Nambu-Jona Lassino y Gross-Neveu) aplicados al estudio de las propiedades del grafeno. Estas propiedades son responsables de inusuales fenómenos, como el efecto Hall fraccionario, que permite la posibilidad de catálisis magnética de un gap excitónico, ferromagnetismo y superconductividad. La investigación de superconductores de alta temperatura crítica con centros de impurezas es importante para comprender la física subyacente de tales sistemas desordenados. Mientras que la familia de los cupratos presenta propiedades aislantes en estado prístino, los pnictogenuros de hierro sin dopar (es decir, LaOFeAs) muestran un comportamiento semimetálico. A pesar de estas diferencias, ambos compuestos son estructuras en capas, donde el estado superconductor está respaldado por una red cuadrada cuasi-2D. Mientras que para los pnictogenuros de hierro este estado está formado por la capa de FeAs, el estado superconductor de cuprato está formado por la capa de CuO. El presente trabajo se centra en el estudio teórico de las propiedades estructurales, electrónicas y ópticas de los materiales de tipo grafeno, como el grafeno bicapa; y también de superconductores de ondas s y d, más específicamente pnictogenuros y cupratos de hierro, respectivamente. Además, se hace hincapié en sistemas desordenados ya que estos sistemas (cuasi-)2D son bastante sensibles al desorden. Tales propiedades tienen gran importancia para aplicaciones de dispositivos tecnológicos, como se puede observar en la creciente tecnología campos de tensiotrónica y espintrónica. El tipo de perturbaciones aplicadas a los sistemas de interés son las impurezas químicas y campos eléctricos externos. Estas perturbaciones producen variaciones de las propiedades electrónicas y ópticas cuando se comparan con los sistemas prístinos. / [CAT] La dimensionalitat d'un sistema juga un paper fonamental en la conducta de la dinámica dels electrons que interactúen. En particular, la dimensionalitat cuasi-2D és responsable del comportament inusual observat a materials de tipus grafè i sistemes laminars basats en enllaços de tipus van der Waals. A més a més, aquestos efectes també s'observen a materials superconductors d'alta temperatura crítica, inclús al seu estat normal, degut a la seua baixa dimensionalitat. L'estudi experimental del grafè va produir una atenció creixent a les seues propietats electròniques, perque la seua estructura en forma de panal d'abelles dona lloc a una estructura de bandes amb dos punts nodals a la zona de Brillouin que determinen una dinámica electrónica relativista de tipus Dirac. Al planol teòric, moltes propietats del grafè d'una sola capa s'han estudiat per a permetre una major caracterizació d'aquest material. Aquestes propietat són poc convencionals degut a la singular estructura de bandes del grafè, que es descriu mitjançant fermions de Dirac. Aquestos fermions permeten establir víncles amb certes teories de la física de particles. De fet, alguns grups teòrics han empleat models fenomenològics inspirats a la cromodinàmica quàntica (es a dir, els models Nambu-Jona Lassino i Gross-Neveu) aplicats a l'estudi de les propietats del grafè. Aquestes propietats són responsables d'inusuals fenómens, com l'efecte Hall fraccionari, que permet la possibilitat de catálisi magnètica d'un gap excitònic, ferromagnetisme i superconductivitat. La investigació de superconductors d'alta temperatura crítica amb centres d'impureses és important per a comprendre la física subjacent de tals sistemes desordenats. Mentre que la família dels cuprats presenta propietats aïllants en estat pristí, els pnictogenurs de ferro sense dopar (és a dir, LaOFeAs) mostren un comportament semimetálico. Malgrat aquestes diferències, tots dos compostos són estructures en capes, on l'estat superconductor està recolzat per una xarxa quadrada quasi-2D. Mentre que per als pnictogenurs de ferro aquest estat està format per la capa de FeAs, l'estat superconductor dels cuprats està format per la capa de CuO. El present treball es centra en l'estudi teòric de les propietats estructurals, electròniques i òptiques dels materials de tipus grafè, com el grafè bicapa; i també de superconductors d'ones s i d, més específicament pnictogenurs i cuprats de ferro, respectivament. A més a més, es fa emfasi en sistemes desordenats ja que aquestos sistemes (cuasi-)2D són prou sensibles al desordre. Aquestes propietats tenen gran importància per a aplicacions de dispositius tecnològics, com es pot observar a la creixent tecnologia dels camps de la tensiotrònica i l'espintrònica. El tipus de pertorbacions aplicades als sistemes d'interés són les impureses químiques i els camps elèctrics externs. Aquestes pertorbacions produeixen variacions de les propietats electròniques i òptiques quan es comparen amb els sistemes pristins. / César Dos Santos, MJ. (2021). Effects of symmetry breaking in low dimensional materials [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/176058 / Compendio

<p dir="ltr">Porous silicon (po-Si) is a unique form of silicon (Si) that features tunable nanopores distributed throughout its bulk structure. While crystalline Si (c-Si) already boasts technological advantages, po-Si offers an additional key aspect with its large surface area relative to its small volume, making it highly conducive to surface chemistry. In this research, our focus centers on the design of a synaptic device based on po-Si, exploring its potential for neuromorphic hardware applications.</p><p><br></p><p dir="ltr">To begin, we delve into the analysis of several electrical properties of po-Si using density functional theory (ab initio/first principles) calculations. Notably, we discover the presence of intra-pore dangling states within the bandgap region of po-Si. Although po-Si is known for its higher bandgap compared to c-Si, resulting in low carrier density and increased resistance, the existence of these dangling states significantly impacts its electronic transport.</p><p><br></p><p dir="ltr">Additionally, we investigate the electric-field driven modulation of dangling bonds through controlled intra-pore Si-H bond dissociation. This modulation enables precise control over the density of dangling states, facilitating the tunability of po-Si conductance. Theoretically evaluating the current-voltage characteristics of our proposed po-Si based synaptic devices, we determine the potential range of obtainable conductivity.</p><p><br></p><p dir="ltr">Finally, we evaluate the performance by integrating porous silicon nanoelectronics devices into neural networks. These devices exhibit superior synaptic plasticity, faster response times, and reduced power consumption compared to other synapses. The research indicates that poroussilicon devices are highly effective in neuromorphic systems, paving the way for more efficient and scalable neural networks. These advancements have significant practical and cost-effective implications for a wide range of applications, including pattern recognition, machine learning, and artificial intelligence.</p><p><br></p><p dir="ltr">Overall, our analyses reveal that the integration of po-Si based synaptic devices into the neural fabric offers a path towards achieving significantly denser and more energy-efficient neuromorphic hardware. With its tunable properties, large surface area, and potential for controlled conductance, po-Si emerges as a promising candidate for the development of advanced silicon based nano-electronic devices tailored for neuromorphic computing. As we delve deeper into the potentials of po-Si, the era of cognitive computing, inspired by the elegance of bio-mimetic neural networks, edges closer to becoming a reality.<br><br></p>

From computation to sensing, two-dimensional materials are revolutionizing the field of nanoscale electronics and devices. They enable the engineering of membranes, circuits and coatings with tailored electronic properties at ultimate, atomic thinness. Yet, the manufacturing processes to obtain these materials are not sufficiently advanced to meet industrial demands. The next step for them to push into the consumer market is the successful, large-scale integration with existing silicon technology. For many two-dimensional materials, this proves challenging due to high synthesis temperatures or low mechanical stability in transfer processes. Not so for two-dimensional noble-metal chalcogenides: PtSe2 is an exemplary candidate because it can be grown at temperatures below 500 ℃, rendering it suitable for facile integration at the back end of the line. Additionally, it features very high stability with respect to moisture, irradiation, and mechanical strain, high carrier mobilities, and electronic properties that can be fine-tuned with the number of layers. These properties collectively make it a very promising material for free-standing nanoelectromechanical sensors, such as piezoresistive pressure sensors and motion detectors for the Internet of Things. Unfortunately, one cannot have their cake and eat it too: The broadly tunable properties of PtSe2 lead to challenges in fabricating devices with reproducible performance. This issue can be overcome with sufficient understanding and control of the nanostructure of PtSe2 thin films. The aim of this thesis is to study these nanostructures in depth, employing state-of-the-art density-functional theory and a machine learning approach to get closer to modelling PtSe2 under realistic conditions. Firstly, the family of noble-metal dichalcogenides is introduced and discussed. Secondly, the role of stacking disorder in PtSe2 is taken into account and its impact on electromechanical properties is analyzed. Lastly, a machine learning approach is employed to study edges and surfaces of nanoplatelets of PtSe2, which are the building blocks of polycrystalline thin films. Through these studies, crucial parameters have been identified that need to be controlled during the manufacturing process of PtSe2, and the groundwork to built up large-scale models has been laid out.

Organometallic reactions are a fundamental class of chemical transformations. The mechanisms of organometallic reactions are routinely modeled by calculating intermediates and transition-state structures on a potential energy surface with density functional theory (DFT). The translation of these calculated structures to a reaction mechanism is typically done under the umbrella of statistical transition state theory. This dissertation reports the use of DFT calculations and quasiclassical direct dynamics trajectories to explore the possibility of nonstatistical dynamic effects in organometallic reactions. Chapter 1 provides a brief review of potential energy surfaces, transition state theory, dynamics trajectories, and a review of previous dynamics studies of organometallic reactions. Chapter 2 reports dynamics trajectories of an organometallic β–hydride transfer reaction with Rh, Ir, and Co metal centers. This chapter was previously published as Dalton Trans. 2020, 49, 7747-7757. Chapters 3 reports the potential energy surface and structures for benzene reductive elimination for dimethyl silyl-bridged W and Mo metallocene complexes. Chapter 4 reports gas-phase and explicit solvent dynamics trajectories for this benzene reductive elimination reaction.

Transition-metal based molecules and point defects in wide-bandgap semiconductors have been of particular interest lately due to their potential quantum information science applications. To accurately calculate the electronic properties of these systems from first principles, it is important to appropriately account for electronic correlation effects. Density functional theory (DFT) has been one of the most popular methods to perform calculations on correlated systems, due to the combination of numerical efficiency and precision in many applications. However, traditional DFT methods fail to accurately calculate the important electronic and magnetic properties, such as bandgaps in semiconductors or magnetic ordering in defects, to name a few. This dissertation focuses on two areas in which traditional DFT methods are likely to produce inaccurate predictions. The first area is connected to an error that is intrinsic to most DFT formulations due to the approximate nature of the exchange-correlation functional, known as the self-interaction error (SIE). It is known to cause the underestimation of bandgaps in solids, underestimated reaction barriers in molecules, and incorrect dissociation curves. The second area is the case where the ground and/or excited states are described by multiconfigurational wavefunctions rather than a single Slater determinant. Chapter 1 of the thesis provides a brief overview of various electronic-structure methods, as well as the Fermi-Löwdin Orbital Self-Interaction Correction method (FLOSIC) which is used to remedy the SIE in DFT. Chapter 2 reports on the application of the FLOSIC-DFT on a Cu-based molecule, and its effects on the predicted electronic properties. Chapter 3 describes the application of the FLOSIC-DFT to the computation of the hyperfine coupling terms, which are crucial for the realization of spin qubits and for interpreting electron paramagnetic resonance experiments. Chapter 4 turns to the application of a multiconfigurational method to describe the electronic properties of a silicon vacancy in silicon carbide, a potential point-defect qubit. / Doctor of Philosophy / Computational electronic structure methods, which attempt to predict optical, electronic, and other properties of molecules and materials just by solving the Schrödinger equation for the wavefunction of the electrons in them, have been instrumental in many areas of research, including the design of semiconductors, drug discovery, improved solar panel design, and discovering systems that can work as qubits for quantum-computing purposes, to name a few. One of the most successful sets of these methods, known as Density Functional Theory (DFT) methods, makes solving for electronic wavefunctions (and from them, other materials properties) computationally efficient, while maintaining the accuracy of such predictions by accounting for the complex quantum-mechanical interactions between the particles. For this, DFT was the subject of the 1998 Nobel Prize in chemistry. However, there are several areas where DFT is typically not successful in producing accurate predictions. One of the areas is connected to the approximated term in all DFT methods that often erroneously accounts for electrons interacting on themselves (an effect known as the self-interaction error). The other area is related to systems that must be described with superposition of several electronic configurations. Chapter 1 of the thesis provides a brief overview of various electronic-structure methods, as well as the Fermi-Löwdin Orbital Self-Interaction Correction method (FLOSIC) which suggests a modification to the standard DFT methods that aims to remove the self-interaction error. Chapter 2 reports on the application of the FLOSIC-DFT on a Cu-based molecule, and its effects on the predicted electronic properties. Chapter 3 describes the application of the FLOSIC-DFT to describe the interaction between the magnetic moments of electrons and nuclei, known as the hyperfine coupling, which is crucial for the realization of spin qubits and for interpreting some experimental results. Chapter 4 turns to the application of a multiconfigurational method to describe the electronic properties of a silicon vacancy in silicon carbide, a potential point-defect qubit.

The following dissertation is divided into two parts. Part I deals with the modeling of helium trapping at oxide-iron interfaces in nanostructured ferritic alloys (NFAs) using density functional theory (DFT). The modelling that has been performed serves to increase the knowledge and understanding of the theory underlying the prevention of helium embrittlement in materials. Although the focus is for nuclear reactor materials, the theory can be applied to any material that may be in an environment where helium embrittlement is of concern. In addition to an improved theoretical understanding of helium embrittlement, the following DFT models will provide valuable thermodynamic and kinetic information. This information can be utilized in the development of large-scale models (such as kinetic Monte Carlo simulations) of the microstructural evolution of reactor components. Accurate modelling is an essential tool for the development of new reactor materials, as experiments for components can span decades for the lifetime of the reactor. Part II of this dissertation deals with the development, and use of, kinetic Monte Carlo (KMC) simulations for improved efficiency in investigating catalytic chemical reactions on surfaces. An essential technique for the predictive development and discovery of catalysts relies on modelling of large-scale chemical reactions. This requires multi-scale modelling where a common sequence of techniques would require parameterization obtained from DFT, simulation of the chemical reactions for millions of conditions using KMC (requiring millions of separate simulations), and finally simulation of the large scale reactor environment using computational fluid dynamics. The tools that have been developed will aid in the predictive discovery, development and modelling of catalysts through the use of KMC simulations. The algorithms that have been developed are versatile and thus, they can be applied to nearly any KMC simulation that would seek to overcome similar challenges as those posed by investigating catalysis (such as the need for millions of simulations, long simulation time and large discrepancies in transition probabilities). / Ph. D.

No / 4-Chloro-2(H)-pyran-2-one undergoes thermal Diels−Alder cycloaddition with electron-deficient dienophiles to afford, without any significant selectivity, 6-endo- and 5-endo-substituted bicyclic lactone cycloadducts. In contrast to 3- and 5-bromo-2(H)-pyran-2-one, 4-chloro-2(H)-pyran-2-one does not undergo thermal cycloadditions with electron-rich dienophiles. The regio- and stereochemical preferences of the cycloadditions of 4-chloro-2(H)-pyran-2-one and other related 2(H)-pyran-2-ones are investigated computationally. Calculations were carried out on the transition states leading to the four possible regio- and stereoisomeric cycloadducts using density functional theory (B3LYP/6-31G*). These studies allow prediction of the regio- and stereoselectivity in these reactions which are in line with experimental observations.

Transition metal carbides (TMCs) are of increasing interest for catalytic processes. Their performance and stability under common oxidative conditions in catalytic reactions are crucial for several applications, including catalysis and electrochemical reactions. In this work, we report a detailed XPS study of the interactions of stoichiometric NbC and TaC surfaces with common oxidizing agents like O2 and H2O, which are important media in many chemical processes. Experimental results showed that NbC reacts with O2 to produce Nb sub-oxrides, while TaC is inert to O2 exposure. TaC surfaces are more sensitive to H2O vapor, with a greater surface oxidation and hydroxylation. Atmospheric oxidation of NbC and TaC was also studied, and results showed that both films oxidized yielding to the formation of Nb2O5 and Ta2O5, hydroxylated/ oxide carbon species, and some adventurous carbon build-up. TMCs are catalytically active in many reactions, especially those involving electrochemical nitrogen reduction reactions (NRR) to ammonia. Experimental and DFT calculations were used to provide insight on how carbide surface structures change electrochemically and how that evolution relates to NRR activity. Results showed that NbC has NRR activity at pH 3.2 after immersion in 0.3 M NaOH, leaving niobium suboxides. However, photoemission data showed that the Nb2O5 overlayer is restored after polarization to -1.3 V vs. Ag/AgCl, inhibiting NRR activity. TaC, on the other hand, is inactive for NRR at potentials more positive than -1.0 V, as NaOH treatment fails to remove the Ta2O5 surface layer induced by ambient exposure. The study also found that the formation and stabilization of intermediate oxidation states on the surface of transition metal ions are crucial for N≡N bond activation and NRR activity.

In dieser Arbeit beschreiben wir wie nicht-störungstheoretischer Wärmetransport im Rahmen von ab initio-Simulationen und linearer Antworttheorie formuliert werden kann. Die daraus resultierende ab initio-Green-Kubo-Methode ermöglicht die Simulation von Wärmetransport in Festkörpern beliebiger Anharmonizität und ist besonders geeignet um "stark anharmonische" Systeme zu beschreiben in denen störungstheoretische Ansätze unzuverlässig werden. Um die systematische Unterscheidung von harmonischen und anharmonischen Materialien zu ermöglichen führen wir ein "Anharmonizitätsmaß" ein, welches die anharmonischen Beiträge zu den interatomaren Kräften unter thermodynamischen Bedingungen quantifiziert. Mit diesem Anharmonizitätsmaß untersuchen wir typische dynamische Effekte die in stark anharmonischen Materialien auftreten, sowie die Grenzen störungstheoretischer Methoden zur Berechnung von Wärmetransporteigenschaften. Wir zeigen, dass eine negative Korrelation des Anharmonizitätsmaßes mit der Wärmeleitfähigkeit einfacher Kristalle besteht, was die intuitive Auffassung bestärkt, wonach harmonische Materialien bessere Wärmeleiter sind und umgekehrt. Auf diesen Erkenntnissen aufbauend identifizieren wird anharmonische Materialien als Kandidaten für Wärmetransport-Simulationen auf der Suche nach neuen thermischen Isolatoren. Auf diesem Wege identifizieren wir mehrere neue thermische Isolatoren welche potentielle technologische Relevanz als thermische Barrieren oder Thermoelektrika aufweisen könnten, und schlagen diese zur experimentellen Untersuchung vor. / In this work, we describe how a non-perturbative heat transport formalism for solids emerges in the framework of ab initio simulations coupled with linear response theory. The resulting ab initio Green Kubo method allows for studying heat transport in solids of arbitrary anharmonic strength, and is particularly suited to describe “strongly anharmonic” systems where per- turbative approaches become unreliable. In order to discern harmonic from anharmonic materials in a systematic way, we introduce an “anharmonicity measure” which quantifies the anharmonic contribution to the interatomic forces under thermodynamic conditions. Using this anharmonicity measure, we investigate typical dynamical effects occurring in strongly anharmonic compounds and investigate the limits of perturbative approaches for the study of thermal transport. We show that this measure negatively correlates with bulk thermal conductivities in simple solids, supporting the intuitive notion that more harmonic materials are better heat conductors and vice versa. Based on these findings, we identify anharmonic compounds as candidates for thermal transport simulations in the search for novel thermal insulators. In this way, we identify several new thermal insulators of potential technological relevance as thermal barriers or thermoelectric materials which we suggest for experimental study.