| 1. |
- Helmer, Pernilla, 1992-
(författare)
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A Computational Venture into the Realm of Laminated Borides and their 2D Derivatives
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Daily life in modern society is highly dependent on many different materials and techniques for manipulating them, and the technological forefront is constantly pushed further by new discoveries. Hence, materials science is a very important field of research. The field of 2D materials is a rather young subfield within materials science, sprung from the realisation of the first 2D material graphene. 2D materials have, due to their 2D morphology, a very high surface-to-weight ratio, which makes them clearly attractive for applications where the material surface is an important characteristic, such as for energy storage and catalysis.The family of 2D materials called MXenes contrast to other 2D materials through the methods used to synthesise them. Traditionally, 2D materials are mechanically exfoliated from a 3D bulk structure in which the 2D sheets are only kept together by weak van der Waals forces, while MXenes are instead chemically exfoliated by selectively etching the A element from a member of the MAX phase family. A MAX phase is a hexagonal nanolaminated crystal structure on the formula Mn+1AXn, with n = 1 – 4, where the M indicates one or several transition metals, A stands for an "A element", commonly a metalloid, and X stands for C or N. After etching away the A element from the MAX phase the Mn+1Xn-layers are left, making up the MXene. MXenes thus show an unusual structural and chemical diversity, and the composition spectra is even further expanded by atoms and small molecules, called surface terminations, attaching to the MXene surface upon etching. These terminations in turn also influence the properties of the MXene. Hence, the MXene family shows great potential for property tailoring towards many different applications.Besides MAX phases there are many other nanolaminated materials which can not be mechanically exfoliated like graphene, and the natural question arises: can other nanolaminated materials be etched into completely new 2D materials? This thesis is concerned with the so called MAB phases – a family of laminated materials similar to MAX phases, but with B instead of C or N – and their 2D derivatives from a computational perspective. More specifically, paper I concerns the quaternary out-of-plane-ordered MAB (o-MAB) phase Ti4MoSiB2 – which has been etched into a 2D titanium oxide – and its related ternary counterparts Mo5SiB2 and Ti5SiB2. In paper II the properties and possible termination configurations of a 2D MXene-analogue named boridene is studied.Both projects concern novel materials that have recently been experimentally realised, and the main aim of the first principles calculations presented here has been to complement and explain the experimental results. In paper I bonding characteristics of Ti4MoSiB2, Mo5SiB2 and Ti5SiB2 are studied, with the goal of better understanding why the two former are experimentally realisable while the latter has never been reported. In Ti4MoSiB2 Ti and Mo populate two symmetrically inequivalent lattice sites, and the bond between these two sites was found to display a large peak of bonding states just below the Fermi level. This peak is fully populated in Ti4MoSiB2 and Mo5SiB2, but only partially populated in Ti5SiB2, which was identified to be the key difference causing Ti5SiB2 to be unstable.Paper II instead focuses on the 2D material boridene, derived from a 3D MAB phase with in-plane ordering (i-MAB). The i-MAB phase is similar in structure to i-MAX phases, and the boridene show similar structure and properties as the corresponding i-MXene etched from i-MAX, including a high activity for the hydrogen evolution reaction (HER). The boridene surface was experimentally found to be terminated by O, F and OH species, and the first principles investigations were aimed at screening the possible termination compositions using dynamical stability analysis, and how the electronic properties of boridene are influenced by the terminations. It was found that the terminations are critical to the dynamical stability of boridene, while the specific composition is less important. For termination with only a single species, the material was predicted to be a small bandgap semiconductor with varying bandgap for different species, while for termination with mixed species, the material was found to be metallic.Hence, this thesis has slightly expanded the theoretical knowledge of MAB phases and their first 2D derivative, boridene, by detailed first principles characterisation. Hopefully, these studies can contribute in further development of the considered and related materials, and bring meaningful insight into the behaviour and properties of MAB phases and their 2D derivatives.
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| 2. |
- Chen, Xin, 1992-
(författare)
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Theoretical Investigations of Two-Dimensional Materials : Studies on Electronic, Magnetic, Mechanical, and Thermal Properties
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two-dimensional (2D) materials have been paid enormous attention since the first realization of graphene in 2004, in connection to high-speed flexible electronics, 2D magnetism, optoelectronics, and so on. Apart from graphene, many new 2D materials with special properties have been predicted and synthesized. For the understanding of several interesting phenomena and prediction of new 2D materials, materials-specific density functional theory (DFT) plays a very important role.In this thesis, based on first-principles calculations, structural, magnetic, electronic, mechanical, and thermal transport properties of two kinds of 2D systems are investigated.The first kind of 2D materials is based on the synthesized material or the predicted structure with ultralow energy. These materials were functionalized by adsorbing transition metal atoms or oxygen atoms, which makes a significant difference in the properties. A part of the thesis covers the study of the self-assembly process of 3d transition metal hexamers on graphene with different defects. Interestingly, it is found that the easy axis of magnetization can be tuned between in-plane and out-of-plane directions in the presence of an external electric field. The second subsection is the oxygen functionalized form of 2D honeycomb and zigzag dumbbell silicene. Interestingly, both the structures are Dirac semimetal.The other kind of 2D materials discussed in this thesis are new materials which were never reported before. Starting from a global structure search, we predicted several structures with ultrahigh stability and novel properties. One work is about a new allotrope of graphene, namely PAI-graphene. It is a new structural motif, which is energetically very close to graphene with interesting properties. PAI-graphene is a semimetal with distorted Dirac cones. By applying tensile strain, three different topological phases can be achieved. The second subsection is the work about new 2D structural forms of A2B (A=Cu, Ag, Au, and B=S, Se). Our obtained square-A2B (s-A2B) structures are energetically more favored than all the reported 2D structures for A2B. s-A2B structures are direct bandgap semiconductors with high carrier mobilities. All the s-A2B structures have unusually low lattice thermal conductivities. Moreover, s-A2B monolayers have ultra-low Young’s moduli and in-plane negative Poisson’s ratios. The third work is about the phase transition in s-A2B monolayers. We proposed two new s-A2B structure, s(I)- and s(II)-Au2Te. S(I)-Au2Te is an auxetic direct-gap semiconductor, while s(II)-Au2Te is a topological insulator. By applying strain or using thermal means, we can achieve a structural phase transition between the two phases.
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| 3. |
- Ektarawong, Annop
(författare)
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First-principles study of configurational disorder in icosahedral boron-rich solids
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is a theoretical study of configurationally disordered icosahedral boronrich solids, in particular boron carbides, using density functional theory and alloy theory. The goal is to resolve discrepancies, regarding the properties of boron carbides, between experiments and previous theoretical calculations which have been a controversial issue in the field of icosahedral boron-rich solids. For instance, B13C2 is observed experimentally to be a semiconductor, meanwhile electronic band structure calculations reveal a metallic character of B13C2 due to its electron deficiency. In B4C, on the other hand, the experimentally observed band gap is unexpectedly smaller, not the usual larger, than that of standard DFT calculations. Another example is given by the existence of a small structural distortion in B4C, as predicted in theoretical calculations, which reduces the crystal symmetry from the experimentally observed rhombohedral (R3m) to the based-centered monoclinic (Cm). Since boron carbide is stable as a single-phase over a broad composition range (~8-20 at.% C), substitution of boron and carbon atoms for one another is conceivable. For this reason, the discrepancies have been speculated in the literature, without a proof, to originate from configurational disorder induced by substitutional defects. However, owing to its complex atomic structure, represented by 12-atom icosahedra and 3-atom intericosahedral chains, a practical alloy theory method for direct calculations of the properties of the relevant configurations of disordered boron carbides, as well as for a thermodynamic assessment of their stability has been missing.In this thesis, a new approach, the superatom-special quasirandom structure (SA-SQS), has been developed. The approach allows one to model configurational disorder in boron carbide, induced by high concentrations of low-energy B/C substitutional defects. B13C2 and B4C are the two stoichiometries, mainly considered in this study, as they are of particular importance and have been in focus in the literature. The results demonstrate that, from thermodynamic considerations, both B13C2 and B4C configurationally disorder at high temperature. In the case of B13C2, the configurational disorder splits off some valence states into the band gap that in turn compensates the electron deficiency in ordered B13C2, thus resulting in a semiconducting character. As for B4C, the configurational disorder eliminates the monoclinic distortion, thus resulting in the restoration of the higher rhombohedral symmetry. Configurational disorder can also account for an excel lent agreement on elastic moduli of boron carbide between theory and experiment. Thus, several of the previous discrepancies between theory and experiments are resolved.Inspired by attempts to enhance the mechanical properties of boron suboxide by fabricating boron suboxide-boron carbide composites, as recently suggested in the literature, the SA-SQS approach is used for modeling mixtures of boron suboxide (B6O) and boron carbide (B13C2), denoted by pseudo-binary (B6O)1–x(B13C2)x alloys. The knowledge of configurational disorder, gained from the previous studies of boron carbide, is applied to model the mixing alloys. By investigating the thermodynamics of mixing between B6O and B13C2, the phase diagram of the (B6O)1–x(B13C2)x alloys is outlined and it reveals the existence of a miscibility gap at all temperatures up to the melting point, indicating the coexistence of B6O-rich and either ordered or disordered B13C2-rich domains in (B6O)1–x(B13C2)x alloys under equilibrium condition. However, a limited intermixing of B6O and B13C2 to form solid solutions at high temperature is predicted, e.g. a solid solution of ~5% B13C2 in B6O and ~20% B6O in B13C2 at 2000 K.
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| 4. |
- Wang, Duo
(författare)
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Ab initio studies of advanced functional materials with complex magnetism
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For centuries, magnetism of materials has been an inevitable part of human civilization. Only in the last century, the mysteries of magnetism started to unfold thanks to the development of quantum theory of solids. Nevertheless, even today, new exotic phenomena related to magnetism keep on surprising us and provide an enormous playground for theoreticians and experimentalists to unravel the complexities. In this thesis, the magnetic properties of materials are studied from different aspects by using first-principle density functional theory. Specifically, we investigated the substituted quadruple perovskite compounds ACu2Fe2Re2O12 (A=Ca, Sr, Ba, Pb, Sc, Y, La). Seven different A-site doped structures are studied, including divalent and trivalent charge substitutions. We found that all these compounds are half-metallic ferrimagnets with large magnetization and high transition temperatures (above 405K). Interestingly, the trivalent atom doping at the A-site can significantly increase the transition temperature. The exchange mechanism is explained by the super-exchange in the Re-Cu and Re-Fe pairs. Moreover, we investigated three different two-dimensional magnets, CrI3, FeS2, and CrO. For the first project, we studied stacking dependent magnetic properties of CrI3. It was found that the magnetic ground state can be tuned by the stacking sequences. In the second project, we studied the monolayer FeS2. The results show that the structures with FM and AFM configuration are close in energy. By performing further spin-spiral calculations, we found that the ground state magnetic configurations are different with different crystal structures. This structure dependent magnetic property indicates the existence of spin-lattice coupling in this material. In the third project, we predicted a monolayer CrO, which is a Weyl semimetal with antiferromagnetism up to room temperature. Finally, a heterostructure structure with G-type SrMnO3 supported on SrTiO3 substrate is investigated. We found that with a 2.9% tensile strain introduced by the substrate, the SrMnO3 keeps as G-type AFM. Moreover, oxygen vacancy intends to stay at the surface. Interestingly, this vacancy induces the AFM-FM transition on the specific layer due to the double exchange mechanism.
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| 5. |
- Elgammal, Karim, 1986-
(författare)
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Density Functional Theory Calculations of Graphene based Humidity and Carbon Dioxide Sensors
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene has many interesting physical properties which makes it useful for plenty of applications. In this work we investigate the possibility of using graphene as a carbon dioxide and humidity sensor. Carbon dioxide and water adsorbates are modeled on top of the surface of a graphene sheet, which themselves lie on one of two types of silica substrates or sapphire substrate. We evaluate the changes in the electronic and structural properties of the graphene sheet in the presence of the described adsorbates as well as the accompanying substrate. We perform the study using ab-initio calculations based on density functional theory (DFT), that allows fast, accurate and efficient investigations. In particular, we focus our attention on investigating the effects of defects in the substrate and how it influences the properties of the graphene sheet. The defects of the substrate contribute with impurity bands leading to doping effects on the graphene sheet, which in turn together with the presence of the adsorbates result in changes of the electronic charge distribution in the system. We provide charge density difference plots to visualize these changes and also determine the relaxed minimum distances of the adsorbates from the graphene sheet together with the respective minimum energy configurations. We also include the density of states, Löwdin charges and work functions for further investigations.
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| 6. |
- Mahani, Mohammad Reza
(författare)
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Magnetic solotronics near the surface of a semiconductor and a topological insulator
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Technology where a solitary dopant acts as the active component of an opto-electronic device is an emerging field known as solotronics, and bears the promise to revolutionize the way in which information is stored, processed and transmitted. Magnetic doped semiconductors and in particular (Ga, Mn)As, the archetype of dilute magnetic semiconductors, and topological insulators (TIs), a new phase of quantum matter with unconventional characteristics, are two classes of quantum materials that have the potential to advance spin-electronics technology. The quest to understand and control, at the atomic level, how a few magnetic atoms precisely positioned in a complex environment respond to external stimuli, is the red thread that connects these two quantum materials in the research presented here.The goal of the thesis is in part to elucidate the properties of transition metal (TM) impurities near the surface of GaAs semiconductors with focus on their response to local magnetic and electric fields, as well as to investigate the real-time dynamics of their localized spins. Our theoretical analysis, based on density functional theory (DFT) and using tight-binding (TB) models, addresses the mid-gap electronic structure, the local density of states (LDOS) and the magnetic anisotropy energy of individual Mn and Fe impurities near the (110) surface of GaAs. We investigate the effect of a magnetic field on the Mn acceptor LDOS measured in cross-sectional scanning tunneling microscopy, and provide an explanation of why the experimental LDOS images depend weakly on the field direction despite the strongly anisotropic nature of the Mn acceptor wavefunction. We also investigate the effects of a local electrostatic field generated by nearby charged As vacancies, on individual and pairs of ferromagnetically coupled magnetic dopants near the surface of GaAs, providing a means to control electrically the exchange interaction of Mn pairs. Finally, using the mixed quantum-classical scheme for spin dynamics, we calculate explicitly the time evolution of the Mn spin and its bound acceptor, and analyze the dynamic interaction between pairs of ferromagnetically coupled magnetic impurities in a nanoscaled semiconductor.The second part of the thesis deals with the theoretical investigation of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi2Se3 TI, using an approach that combines DFT and TB calculations. Our analysis clarifies the crucial role played by the spatial overlap and the quasi-resonant coupling between the Mn-acceptor and the topological surface states inside the Bi2Se3 band gap, in the opening of a gap at the Dirac point. Strong electronic correlations are also found to contribute significantly to the mechanism leading to the gap, since they control the hybridization between the p orbitals of nearest-neighbor Se atoms and the acceptor spin-polarization. Our results explain the effects of inversion-symmetry and time-reversal symmetry breaking on the electronic states in the vicinity of the Dirac point, and contribute to clarifying the origin of surface-ferromagnetism in TIs. The promising potential of magnetic-doped TIs accentuates the importance of our contribution to the understanding of the interplay between magnetic order and topological protected surface states.
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