Improved recursive Green's function formalism for quasi one-dimensional systems with realistic defects

We derive an improved version of the recursive Green's function formalism (RGF), which is a standard tool in the quantum transport theory. We consider the case of disordered quasi one-dimensional materials where the disorder is applied in form of randomly distributed realistic defects, leading to partly periodic Hamiltonian matrices. The algorithm accelerates the common RGF in the recursive decimation scheme, using the iteration steps of the renormalization decimation algorithm. This leads to a smaller effective system, which is treated using the common forward iteration scheme. The computational complexity scales linearly with the number of defects, instead of linearly with the total system length for the conventional approach. We show that the scaling of the calculation time of the Green's function depends on the defect density of a random test system. Furthermore, we discuss the calculation time and the memory requirement of the whole transport formalism applied to defective carbon nanotubes

The structure and reactivity of heterogenous surfaces and study of the geometry of surface complexes

Issued as Progress report, and Statement of costs report, Project no. G-41-674 (continued by G-41-687 and continues G-41-664

第一原理実空間伝導計算手法の開発とその応用

筑波大学 (University of Tsukuba)201

Propriétés électroniques et thermoélectriques des hétérostructures planaires de graphène et de nitrure de bore

Graphene is a fascinating 2-dimensional material exhibiting outstanding electronic, thermal and mechanical properties. Is this expected to have a huge potential for a wide range of applications, in particular in electronics. However, this material also suffers from a strong drawback for most electronic devices due to the gapless character of its band structure, which makes it difficult to switch off the current. For thermoelectric applications, the high thermal conductance of this material is also a strong limitation. Hence, many challenges have to be taken up to make it useful for actual applications. This thesis work focuses on the theoretical investigation of a new strategy to modulate and control the properties of graphene that consists in assembling in-plane heterostructures of graphene and Boron Nitride (BN). It allows us to tune on a wide range the bandgap, the thermal conductance and the Seebeck coefficient of the resulting hybrid nanomaterial. The work is performed using atomistic simulations based on tight binding (TB), force constant (FC) models for electrons and phonons, respectively, coupled with the Green's function formalism for transport calculation. The results show that thanks to the tunable bandgap, it is possible to design graphene/BN based transistors exhibiting high on/off current ratio in the range 10⁴-10⁵. We also predict the existence hybrid quantum states at the zigzag interface between graphene and BN with appealing electron transport. Finally this work shows that by designing properly a graphene ribbon decorated with BN nanoflakes, the phonon conductance is strongly reduced while the bandgap opening leads to significant enhancement of Seebeck coefficient. It results in a thermoelectric figure of merit ZT larger than one at room temperature.Les excellentes propriétés électroniques, thermiques et mécaniques du graphène confèrent à ce matériau planaire (bi-dimensionnel) un énorme potentiel applicatif, notamment en électronique. Néanmoins, ce matériau présente de sérieux inconvénients qui pourraient limiter son champ d'applications. Par exemple, sa structure de bandes électronique sans bande interdite rend difficile le blocage du courant dans un dispositif. De plus, pour les applications thermoélectriques, sa forte conductance thermique est aussi une forte limitation. Il y a donc beaucoup de défis à relever pour rendre ce matériau vraiment utile pour des applications. Cette thèse porte sur l'étude des propriétés électroniques et thermoélectriques dans les hétérostructures planaires constituées de graphène et de nitrure de bore hexagonal (BN). Différentes configuration de ce nouveau matériau hybride permettent de moduler la bande interdite, la conductance thermique et le coefficient Seebeck. Cette étude a été menée au moyen de calculs atomistiques basés sur les approches des liaisons fortes (TB) et du modèle à constantes de force (FC). Le transport d'électrons et de phonons a été simulé dans le formalisme des fonctions de Green hors équilibre. Les résultats montrent que, grâce à la modulation de la bande interdite, des transistors à base d'hétérostructures de BN et de graphène peuvent présenter un très bon rapport courant passant / bloqué d'environ 10⁴ à 10⁵. En outre, nous montrons l'existence d'états quantiques hybrides à l'interface zigzag entre le graphène et le BN donnant lieu à des propriétés de transport électronique très intéressantes. Enfin, ce travail montre qu'en agençant correctement des nano-flocons de BN sur les côtés d'un nanoruban de graphène, la conductance des phonons peut être fortement réduite alors que l'ouverture de bande interdite conduit à un accroissement important du coefficient Seebeck. Il en résulte qu'un facteur de mérite thermoélectrique ZT plus grand que l'unité peut être réalisé à température ambiante

Phonon Transport across Dissimilar Material Interfaces and in Nanostructured Materials

Nano-scale thermal conduction has been an interesting research topic over the past two decades due to the intriguing electron and phonon physics in the low-dimensional materials, the ever-increasing challenges in the thermal management, and the potentials in using nanostructures for enhanced energy conversion, storage and thermal management. Atomistic simulation methods, such as molecular dynamics (MD) and atomistic Green\u27s function (AGF), have been powerful theoretical tools for understanding and predicting the phonon transport and thermal conduction in nanostructured materials and across dissimilar interfaces. The research conducted in this thesis aims at further developing these atomistic simulation methods and broadening the understanding of phonon transport in nanostructured materials and across the interfaces of dissimilar materials. There is strong artificial simulation-domain-size effect on the prediction of thermal conductivity using equilibrium MD (EMD) for high thermal conductivity materials. In this thesis, spectral method is further developed to avoid the simulation domain size effect and obtain the intrinsic phonon thermal conductivity of materials from EMD simulations. This method is then applied for the study of strain effects on thermal conductivity of low-dimensional nanostructures. We show that thermal conductivity of nanostructures can be greatly tuned by applied mechanical strains. The underlying mechanism on strain/stress tuning of the one-dimensional and two-dimensional nanostructures of carbon and silicon are explained. Phonon transport across dissimilar material interfaces plays a critical role in determining the effective thermal properties of nanostructures. Although AGF has been widely used for the prediction of frequency-dependent phonon transmission across material interfaces, most of the past works have been limited to lattice-matched interfacial structures. In this thesis, an integrated MD and recursive AGF approach is developed to study the frequency-dependent phonon transmission across lattice-mismatched dissimilar interfaces. Due to the induced atomic disorder, lattice mismatch greatly affects the phonon transmission across dissimilar material interface. The reduction of thermal conductance is correlated with adhesion energy at the interface. Further studies are performed for phonon transport across interfaces with defects and specie diffusion. The numerical tools developed in this thesis can be useful for the understandings of phonon transport in nanostructured materials

Superfluidity and localization in Bosonic glasses

Bosonic excitations within long-range ordered, but strongly inhomogeneous phases have been studied in quite some detail. My thesis focuses instead on understanding the insulating, localized phase of disordered bosonic systems. In particular I study localization properties of strongly interacting bosons and spin systems in a disorder potential at zero temperature. I focus on simple, prototypical spin models (Ising model and XY model) in random fields on a Cayley tree with large connectivity. Regarding the nature of the quantum phase transition in strong disorder I find the following results: i) With a uniformly distributed disorder non-extensive excitations in the disordered phase are all localized. ii)Moreover, I find that the order arises due to a collective condensation, which is qualitatively distinct from a Bose Einstein condensation of single particle excitations into a delocalized state. In particular, in non-frustrated Bose glasses, I do not find evidence for a boson mobility edge in the Bose glass. These results are qualitatively different from claims in the recent literatures . Considering that (many body) localization of bosons is a kind of quantum glass transition, it is an interesting question to ask what phenomena occur, if the ingredients for more conventional (classical) glassy physics are added to a disordered bosons system, namely: random, frustrated interactions between the bosons. One can still think about such a system as bosons in a disordered potential, where the disordered potential is, at least partly, self-generated by random frustrated interactions between the bosons. This question takes us to the study of another type of disordered systems: glassy systems. Those are typically characterized by low temperature phases with an inhomogeneous density or magnetization pattern, which is extremely long-lived due to the occurrence of non-trivial ergodicity breaking. I study a solvable model of hard core bosons (pseudospins) subject to disorder and frustrating interactions. This solvable model provides insight into the possibility of coexistence of super uidity and glassy density order, as well as into the nature of the coexistence phase (the superglass). In particular, for the considered mean field model I prove the existence of a superglass phase. This complements the numerical evidence for such phases provided by quantum Monte Carlo investigations in finite dimensions and on random graphs. Those were, however, limited to finite temperature, and could thus not fully elucidate the structure of the phases at T = 0. In contrast, my analytical approach allows one to understand the quantum phase transition between glassy superfluid and insulator, and the non-trivial role played by glassy correlations. When the frustrated interactions are strong enough, the superfluid order may be destroyed. As I will show in a mean field model, this happens within the glass phase of the system, where a disorder induced superfluid-insulator phase transition takes place to give way to a frustrated Bose glass. The glassy background on top of which this happens leads to many interesting phenomena which seem not to have been noticed before. To understand the nature of the glassy superfluid-insulator quantum phase transition at zero temperature and the transport properties on the insulating, Bose glass side of the transition is the goal of the third part of my thesis. To address the above questions, I studied an exactly solvable model of a glassy superfluid-insulator quantum phase transition on a Bethe lattice geometry with high connectivity. My main results can be summarized as follows: i) I found that the superfluid-insulator transition is shifted to stronger hopping. This is a result of the pseudo gap in the density of states of the glass state, which tends to strongly disfavor the onset of superfluidity. ii) In the glassy insulator, the discrete local energy levels become broadened due to the quantum fluctuations.The level-broadening process appears as a phase transition which has strong similarities with an Anderson localization transition, and has implications on many body localization. By using the locator expansion for bosons I found that, the glassy insulator has a finite mobility edge for the bosonic excitations, which, however, does not close upon approaching the SI quantum phase transition point. This finding helps to understand the nature of the superfluid-to-frustrated Bose glass transition: the superfluid emerges as a collective phase ordering phenomenon at zero temperature, and not as a condensation in to a single particle delocalized state, in contrast to opposite predictions in the recent literatures. The existence of a mobility edge in the insulator suggests the possibility of phononless, activated transport in the bosonic insulator, which might be a candidate explanation for the experimentally seen activated transport, which has remained a mystery for a long time

Electrons, excitons et polarons dans les systèmes organiques : approches ab initio à N-corps de type GW et Bethe-Salpeter pour le photovoltaïque organique

The present thesis aims at exploring the properties and merits of the ab initio Green's function many-body perturbation theory (MBPT) GW and Bethe-Salpeter formalisms, in order to provide a well-grounded and accurate description of the electronic and optical properties of condensed matter systems. While these approaches have been developed for extended inorganic semiconductors and extensively tested on this class of systems since the 60 s, the present work wants to assess their quality for gas phase organic molecules, where systematic studies still remain scarce. By means of small isolated study case molecules, we want to progress in the development of a theoretical framework, allowing an accurate description of complex organic systems of interest for organic photovoltaic devices. This represents the main motivation of this scientific project and we profit here from the wealth of experimental or high-level quantum chemistry reference data, which is available for these small, but paradigmatic study cases.This doctoral thesis came along with the development of a specific tool, the FIESTA package, which is a Gaussian basis implementation of the GW and Bethe-Salpeter formalisms applying resolution of the identity techniques with auxiliary bases and a contour deformation approach to dynamical correlations. Initially conceived as a serial GW code, with limited basis sets and functionalities, the code is now massively parallel and includes the Bethe-Salpeter formalism. The capacity to perform calculations on several hundreds of atoms to moderate costs clearly paves the way to enlarge our studies from simple model molecules to more realistic organic systems. An ongoing project related to the development of discrete polarizable models accounting for the molecular environment allowed me further to become more familiar with the actual implementation and code structure.The manuscript at hand is organized as follows. In an introductory chapter, we briefly present the basic mechanisms characterizing organic solar cells, accentuating the properties which seek for an accurate theoretical description in order to provide some insight into the factors determining solar cell efficiencies. The first chapter of the main part is methodological, including a discussion of the principle features and approximations behind standard mean-field techniques (Hartree, Hartree-Fock, density functional theory). Starting from a description of photoemission experiments, the MBPT and quasiparticle ideas are introduced, leading to the so-called Hedin's equations, the GW method and the COHSEX approach. In order to properly describe optical experiments, electron-hole interactions are included on top of the description of inter-electronic correlations. In this context, the Bethe-Salpeter formalism is introduced, along with an excursus on time-dependent density functional theory. Chapter 2 briefly presents the technical specifications of the GW and Bethe-Salpeter implementation in the FIESTA package. The properties of Gaussian basis sets, the ideas behind the resolution of the identity techniques and finally the contour deformation approach to dynamical correlations are discussed. The third chapter deals with the results obtained during this doctoral thesis. On the electronic structure level, a recent study on a paradigmatic dipeptide molecule will be presented. Further, also its optical properties will be explored, together with an in-depth discussion of charge-transfer excitations in a family of coumarin molecules. Finally, by means of the Buckminster fullerene C60 and the two-dimensional semi-metal graphene, we will analyze the reliability of two many-body formalisms, the so-called static COHSEX and constant-screening approximation, for an efficient calculation of electron-phonon interactions in organic systems at the MBPT level. After a short conclusion, the Appendix containing details and derivations of the formalisms presented before closes this work.Cette thèse se propose d'explorer les mérites d'une famille d'approches de simulation quantique ab initio, les théories de perturbation à N-corps, pour l'exploration des propriétés électroniques et optiques de systèmes organiques. Nous avons étudié en particulier l'approximation dite de GW et l'équation de Bethe-Salpeter, très largement utilisées dès les années soixante pour les semiconducteurs de volume, mais dont l'utilisation pour les systèmes organiques moléculaires est très limitée. L'étude de quelques cas d'intérêt pour le photovoltaïque organique, et en particulier de petites molécules pour lesquelles sont disponibles des données expérimentales ou des résultats issus d'approches de chimie quantique, nous ont permis de valider ces approches issues de la physique du solide.Ce doctorat s'inscrit dans le cadre du développement d'un outil de simulation quantique spécifique (le projet FIESTA) dont l'objectif est de combiner les formalismes GW et Bethe-Salpeter avec les techniques de la chimie quantique, c'est-à-dire en particulier l'utilisation de bases localisées analytiques (bases gaussiennes) et des approches de type «résolution de l'identité» pour le traitement des intégrales Coulombiennes. Ce code est aujourd'hui massivement parallélisé, permettant, au delà des études de validation présentées dans ce travail de thèse, l'étude de systèmes complexes comprenant plusieurs centaines d'atomes. En cours de développement, l'incorporation d'approches hybrides combinant mécanique quantique et écrantage à longue portée par des approches modèles de milieu polarisable m'a permis d'une part de me familiariser avec le code et le développement méthodologique, et permet d'autre part d'envisager l'étude de systèmes réalistes en couplage avec leur environnement.Le manuscrit s‘ouvre sur une introduction au photovoltaïque organique afin de mettre en lumière les questionnements spécifiques qui requièrent le développement de nouveaux outils théoriques à la fois fiables en terme de précision et suffisamment efficaces pour traiter des systèmes de grande taille. Le premier chapitre est d'ordre méthodologique et rappelle les fondements des techniques ab initio de type champ-moyen (Hartree, Hartree-Fock et théorie de la fonctionnelle de la densité). En partant des principes de la photoémission, les théories de perturbation à N-corps et la notion de quasi-particule sont ensuite introduites, conduisant aux équations de Hedin et aux approximations GW et COHSEX. De même, à partir de la compréhension d'une expérience d'optique, le traitement des interactions électron-trou est présenté, menant à l'équation de Bethe-Salpeter. Le chapitre 2 introduit brièvement les spécificités techniques liées à l'implémentation des formalismes GW et Bethe-Salpeter. Les propriétés analytiques des bases gaussiennes et les principes mathématiques derrière les techniques de type «résolution de l'identité» et «déformation de contour», sont brièvement décrites. Le troisième chapitre présente les résultats scientifiques obtenus durant cette thèse. Le cas paradigmatique d'un polypeptide model nous permettra de discuter des spécificités de l'approche GW appliquée à des systèmes moléculaires afin d'obtenir des énergies de quasiparticule de bonne qualité. De même, l'utilisation de l'équation de Bethe-Salpeter pour l'obtention du spectre optique de ce système sera présentée, ainsi que le cas d'une famille de colorants d'importance pour les cellules de Graetzel (les coumarines). Finalement, nous explorons dans le cas du fullerène C60 et du graphène le calcul des termes de couplage électron-phonon dans le cadre de l'approche GW, c'est-à-dire au delà des approches standards de type théorie de la fonctionnelle de la densité. Notre étude vise à vérifier si une approximation statique et à écrantage constant au premier ordre permet de garder la qualité des résultats GW pour un coût numérique réduit. Après la conclusion, les appendices donnent le détail de certaines dérivations

Computational studies of electronic and thermal properties of low dimensional materials

The control of low dimensional materials holds potential for revolutionizing the electronic, thermal, and thermoelectric materials engineering. Through strategic manipulation and optimization of these materials, unique properties can be uncover which enable more efficient and effective materials development. Towards the determination of nanoscale strategies to improve the electronic and phononic devices, computational simulations of modified low dimensional materials have been carried in this research. First, the electronic properties of chemically func tionalized phosphorene monolayers are evaluated with spin-polarized Density Functional Theory, as a potential method to tune their electronic properties. The functionalization not only leads to formation of additional states within the semiconducting gap, but also to the emergence of local magnetism. The magnetic ground state and electronic structure are investigated in dependence of molecular coverage, lattice direction of the molecular adsorption and molecule type functionalization. Furthermore, the physical and transport properties of phosphorene grain boundaries under uniaxial strain are evaluated by the use of Density Functional based Tight Binding method in combination with Landauer theory. In both grain boundary types, the electronic bandgap decreases under strain, however, the respective thermal conductance is only weakly affected, despite rather strong changes in the frequency-resolved phonon transmission. The combination of both effects results in an enhancement in the thermoelectric figure of merit in the phosphorene grain boundary systems. Finally, the thermoelectric properties of carbon nanotubes peapod heterostructures are studied and compared to pristine nanotubes using also the Density Functional based Tight Binding method and Landauer theory. It is found that the fullerene encapsulation modifies the electron and phonon transport properties, causing the formation of electronic channels and the suppression of vibrational modes that lead to an improvement of the thermoelectric figure of merit. The results of this thesis highlight the potential of strategic manipulation and optimization of low dimensional materials in improving their unique electronic and thermal properties, revealing promising avenues for improving electronic and phononic devices.:ABSTRACT i ZUSAMMENFASSUNG ii ACKNOWLEDGEMENT iv LIST OF FIGURES ix LIST OF TERMS AND ABBREVIATIONS xviii 1 Introduction 1 1.1 Motivation 1 1.2 Objectives and outline 6 2 Computational Methods 8 2.1 Density Functional Theory 8 2.1.1 The Many-Body System Hamiltonian and the Born-Oppenheimer approximation 9 2.1.2 Thomas-Fermi-Dirac approximation model 10 2.1.3 The Hohenberg-Kohn theorems 12 2.1.4 The Kohn-Sham orbitals equations 13 2.1.5 Exchange-correlation functionals 15 2.2 Density Functional Based Tight Binding method 16 2.2.1 Tight-binding formalism 17 2.2.2 From DFT to DFTB 20 2.2.3 Parametrization 22 2.3 Atomistic Green’s functions 23 2.3.1 Non-Equilibrium Green’s functions for modeling electronic transmission 23 2.3.2 Non-equilibrium Green’s function for modeling thermal transmission 27 3 Tuning the electronic and magnetic properties through chemical functionalization 3.1 Introduction 33 3.1.1 Black phosphorus as a 2D material 33 3.1.2 Chemical Functionalization of low dimensional systems 35 3.1.3 Bipolar Magnetic Semiconductors 36 3.2 Computational approach 38 3.3 Interface effects in phosphorene by OH functionalization 39 3.3.1 Single molecule functionalization 39 3.3.2 Lattice selection 43 3.3.3 Coverage 45 3.4 Chiral functionalization effect in phosphorene 48 3.5 Functionalizing phosphorene towards BMS 51 3.6 Summary 53 4 Tuning transport properties through strain and grain bound-aries 4.1 Introduction 54 4.1.1 Strain in low dimensional materials 54 4.1.2 Grain boundaries 56 4.2 Computational approach 58 4.2.1 Molecular systems 58 4.2.2 Electron and phonon transport and thermoelectric figure of merit 58 4.3 Structural modification by strain in GB systems 60 4.4 Electronic structure modification by strain in GB systems 63 4.5 Thermal transport modification by strain in GB systems 65 4.6 Thermoelectric figure of merit of strained GB systems 68 4.7 Summary 71 5 Tuning transport properties through hybrid nanomaterials: CNT peapods 73 5.1 Introduction 73 5.1.1 Carbon-based nanostructures 73 5.1.2 CNT peapods as hybrid nanomaterials 76 5.2. Computational details 77 5.2.1 CNT peapod model 77 5.2.2 Quantum transport methodology 78 5.3 Structural properties of CNT peapods 79 5.4 Electronic properties of CNT peapods 80 5.5 Thermal properties of CNT peapods 83 5.6 Thermoelectronic properties of CNT peapods 85 5.7 Summary 88 6 Conclusions and outlook 91 Appendices Appendix A Supplementary information to phosphorene functionalization A.1 Spin resolved density of states of 1-OH system 96 A.2 Spin valve model 97 Appendix B Supplementary information to phosphorene grain boundaries 98 B.1 Projected Phonon Density of States in GB1 98 B.2 Thermoelectric transport properties of GB2 99 Appendix C Supplementary information to CNT peapods 101 C.1 Geometry optimization of CNT peapods with larger CNT diameter 101 C.2 Additional analysis of electron transport properties 102 C.3 Phonon band structure of different CNT structures 104 C.4 Additional analysis of thermoelectric performance 105 REFERENCES 105 LIST OF PUBLICATIONS 131 PRESENTATIONS 132Die Kontrolle niedrigdimensionaler Materialien birgt das Potenzial für eine Revolutionierung der elektronischen, thermischen und thermoelektrischen Technologien. Durch strategische Manipulation und Optimierung dieser Materialien können einzigartige Eigenschaften aufgedeckt werden, die eine effizientere und effektivere Materialentwicklung ermöglichen. Um Strategien im Nanobereich zur Verbesserung elektronischer und phononischer Bauelemente zu ermitteln, wurden in dieser Forschungsarbeit rechnerische Simulationen modifizierter niedrigdimensionaler Materialien durchgeführt. Zunächst werden die elektronischen Eigenschaften von chemisch funktionalisierten Phosphoren-Monoschichten mit Hilfe der spinpolarisierten Dichtefunktionaltheorie als potenzielle Methode zur Abstimmung ihrer elektronischen Eigenschaften bewertet. Die Funktionalisierung führt nicht nur zur Bildung zusätzlicher Zustände innerhalb der halbleitenden Lücke, sondern auch zum Auftreten von lokalem Magnetismus. Der magnetische Grundzustand und die elektronische Struktur werden in Abhängigkeit von der molekularen Bedeckung, der Gitterrichtung der molekularen Adsorption und der Funktionalisierung des Moleküls untersucht. Darüber hinaus werden die Transporteigenschaften von Phosphoren-Korngrenzen unter uniaxialer Belastung mit Hilfe der auf Dichtefunktionen basierenden Tight-Binding-Methode in Kombination mit der Landauer-Theorie untersucht. In beiden Korngrenzentypen nimmt die elektronische Bandlücke unter Dehnung ab, die jeweilige Wärmeleitfähigkeit wird jedoch nur schwach beeinflusst, trotz ziemlich starker Änderungen in der frequenzaufgelösten Phononentransmission. Die Kombination bei der Effekte führt zu einer Erhöhung der thermoelektrischen Leistungszahl in den Phosphorkorngrenzensystemen. Schließlich werden die thermoelektrischen Eigenschaften von Kohlenstoffnanoröhren-Peapod-Heterostrukturen untersucht und mit denen von reinen Nanoröhren verglichen, wobei auch die auf Dichtefunktionen basierende Tight-Binding-Methode und die Landauer-Theorie verwendet werden. Es wird festgestellt, dass die Fullereneinkapselung die Elektronen- und Phononentransporteigenschaften modifiziert und die Bildung von elektronischen Kanälen und die Unterdrückung von Schwingungsmoden bewirkt, was zu einer Verbesserung der thermoelektrischen Leistungszahl führt. Die Ergebnisse dieser Arbeit verdeutlichen das Potenzial der strategischen Manipulation und Optimierung niedrigdimensionaler Materialien zur Verbesserung ihrer einzigartigen elektronischen und thermischen Eigenschaften und zeigen vielversprechende Wege zur Verbesserung elektronischer und phononischer Bauteile auf.:ABSTRACT i ZUSAMMENFASSUNG ii ACKNOWLEDGEMENT iv LIST OF FIGURES ix LIST OF TERMS AND ABBREVIATIONS xviii 1 Introduction 1 1.1 Motivation 1 1.2 Objectives and outline 6 2 Computational Methods 8 2.1 Density Functional Theory 8 2.1.1 The Many-Body System Hamiltonian and the Born-Oppenheimer approximation 9 2.1.2 Thomas-Fermi-Dirac approximation model 10 2.1.3 The Hohenberg-Kohn theorems 12 2.1.4 The Kohn-Sham orbitals equations 13 2.1.5 Exchange-correlation functionals 15 2.2 Density Functional Based Tight Binding method 16 2.2.1 Tight-binding formalism 17 2.2.2 From DFT to DFTB 20 2.2.3 Parametrization 22 2.3 Atomistic Green’s functions 23 2.3.1 Non-Equilibrium Green’s functions for modeling electronic transmission 23 2.3.2 Non-equilibrium Green’s function for modeling thermal transmission 27 3 Tuning the electronic and magnetic properties through chemical functionalization 3.1 Introduction 33 3.1.1 Black phosphorus as a 2D material 33 3.1.2 Chemical Functionalization of low dimensional systems 35 3.1.3 Bipolar Magnetic Semiconductors 36 3.2 Computational approach 38 3.3 Interface effects in phosphorene by OH functionalization 39 3.3.1 Single molecule functionalization 39 3.3.2 Lattice selection 43 3.3.3 Coverage 45 3.4 Chiral functionalization effect in phosphorene 48 3.5 Functionalizing phosphorene towards BMS 51 3.6 Summary 53 4 Tuning transport properties through strain and grain bound-aries 4.1 Introduction 54 4.1.1 Strain in low dimensional materials 54 4.1.2 Grain boundaries 56 4.2 Computational approach 58 4.2.1 Molecular systems 58 4.2.2 Electron and phonon transport and thermoelectric figure of merit 58 4.3 Structural modification by strain in GB systems 60 4.4 Electronic structure modification by strain in GB systems 63 4.5 Thermal transport modification by strain in GB systems 65 4.6 Thermoelectric figure of merit of strained GB systems 68 4.7 Summary 71 5 Tuning transport properties through hybrid nanomaterials: CNT peapods 73 5.1 Introduction 73 5.1.1 Carbon-based nanostructures 73 5.1.2 CNT peapods as hybrid nanomaterials 76 5.2. Computational details 77 5.2.1 CNT peapod model 77 5.2.2 Quantum transport methodology 78 5.3 Structural properties of CNT peapods 79 5.4 Electronic properties of CNT peapods 80 5.5 Thermal properties of CNT peapods 83 5.6 Thermoelectronic properties of CNT peapods 85 5.7 Summary 88 6 Conclusions and outlook 91 Appendices Appendix A Supplementary information to phosphorene functionalization A.1 Spin resolved density of states of 1-OH system 96 A.2 Spin valve model 97 Appendix B Supplementary information to phosphorene grain boundaries 98 B.1 Projected Phonon Density of States in GB1 98 B.2 Thermoelectric transport properties of GB2 99 Appendix C Supplementary information to CNT peapods 101 C.1 Geometry optimization of CNT peapods with larger CNT diameter 101 C.2 Additional analysis of electron transport properties 102 C.3 Phonon band structure of different CNT structures 104 C.4 Additional analysis of thermoelectric performance 105 REFERENCES 105 LIST OF PUBLICATIONS 131 PRESENTATIONS 13