Quantum transport modeling of atomic nanostructures on silicon

Surface effects can adversely influence the performance of a nanoelectronic device,but may also lead to new functionality. The focus of this thesis is to theoreticallystudy the role of surfaces in nanoelectronics. Our theoretical analysis is from atomicfirst principles achieved by combining density functional theory with the Keldyshnonequilibrium Green's function approach. This technique allows for all atoms in asystem to be treated on an equal footing without any phenomenological parameters.The first part of the thesis considers conduction through a molecule with no substrateto illustrate the sort of system typically modeled in transport calculations. Two Auelectrodes are bridged by a substituted benzenediamine molecule (R = CH3, NH2,OH) where an H atom is removed to form a radical that may behave as a spin filter,depending on the R group. Next is considered a π–stacked line of ethylbenzenemolecules on the Si(100) surface, where the Si atoms are explicitly included in thecalculation. Although the molecules conduct electrons at certain energies, a channeloccurs through the substrate, which can dominate the conductance. The use ofsubstituent groups to modulate the electron transport properties of such wires is alsoinvestigated, showing that the conductance of the molecular wire could be tuned todominate over the substrate. Finally, the conductance of the Si(111)–7 × 7 metallicsurface is studied. Inspired by experiments suggesting that atomic steps reduce thesurface conductance, the atomic structure and transport properties of such steps areexamined, revealing that dimer atom buckling along the step edges is the primaryculprit since it leads to an opening of a local band gap at the step.Les effets de surface peuvent affecter la performance d'un dispositif nanoélectronique, mais peuvent aussi conduire à de nouvelles fonctionnalités. L'objectif de cette thèse est d'effectuer une étude théorique sur le rôle des surfaces en nanoélectronique. Notre analyse, de type premiers principes atomiques, est effectuée en combinant la théorie de la fonctionnelle de la densité avec les fonctions de Green hors-équilibre. Cette technique permet de traiter tous les atomes de manière égale sans utiliser de paramètres phénoménologiques. La première partie de cette thèse considère la conduction à travers une molécule sans substrat, afin d'illustrer le genre de systèmes typiquement modélisés dans les calculs de transport. Deux électrodes en Au sont mises en contactavec une molécule benzènediamine substituée (R = CH3, NH2, OH), où un atome H est retiré pour former un radical qui peut se comporter comme un filtre de spin, dépendant du groupe R. Ensuite, nous nous concentrons sur une ligne formée d'éthylbenzènes empilées–π sur la surface de Si(100), où les atomes de silicium sont explicitement inclus dans le calcul. Quoique les molécules permettent le transport d'électronsà certaines énergies, un canal se forme à travers le substrat qui peut dominer la conductance. Nous étudions aussi comment certains substituants peuvent moduler les propriétés de transport électronique de ces fils moléculaires. Nous trouvons que la conductance du fil moléculaire peut être modifiée pour dominer l'effet du substrat.Enfin, la conductance de la surface métallique Si(111)–7 × 7 est analysée. Dans lebut d'expliquer théoriquement les expériences suggérant que les marches atomiques réduisent la conductance de la surface, la structure atomique et les propriétés de transport de ces marches ont été examinées. Les résultats révèlent que c'est la déformation atomique des dimères le long des marches qui cause ce phénomène, en raison de la formation d'une bande interdite localisée proche de la marche

Ethylene Carbonate-Based Electrolyte Decomposition and Solid–Electrolyte Interphase Formation on Ca Metal Anodes

The formation of a solid–electrolyte interphase (SEI) in multivalent ion batteries, resulting from the decomposition of organic solvents at the anode interface, is a major bottleneck to their development as it prevents ionic diffusion and reversible stripping and plating. To gain insight into SEI formation in these systems, we investigate the decomposition of pure ethylene carbonate (EC) and an EC/Ca­(ClO₄)₂ electrolyte on a Ca metal surface using density functional theory and <i>ab initio</i> molecular dynamics calculations. We first find that CaCO₃, CaO, and Ca­(OH)₂ are all primary inorganic SEI components. We then investigate the reaction mechanisms of this decomposition, finding that although a fast two-electron reduction producing CO₃^2– and C₂H₄ is thermodynamically and kinetically favorable, a reaction producing C₂H₄O₂^2– and CO dominates when multiple EC molecules are considered. Finally, we find similar results upon the inclusion of Ca­(ClO₄)₂ salt

Conduction modulation of \u3c0-stacked ethylbenzene wires on Si(100) with substituent groups

For the realization of molecular electronics, one essential goal is the ability to systematically fabricate molecular functional components in a well-controlled manner. Experimental techniques have been developed such that \u3c0-stacked ethylbenzene molecules can now be routinely induced to self-assemble on an H-terminated Si(100) surface at precise locations and along precise directions. Electron transport calculations predict that such molecular wires could indeed carry an electrical current, but the Si substrate may play a considerable role as a competing pathway for conducting electrons. In this work, we investigate the effect of placing substituent groups of varying electron donating or withdrawing strengths on the ethylbenzene molecules to determine how they would affect the transport properties of such molecular wires. The systems consist of a line of \u3c0-stacked ethylbenzene molecules covalently bonded to a Si substrate. The ethylbenzene line is bridging two Al electrodes to model current through the molecular stack. For our transport calculations, we employ a first-principles technique where density functional theory (DFT) is used within the non-equilibrium Green\u2019s function formalism (NEGF). The calculated density of states suggest that substituent groups are an effective way to shift molecular states relative to the electronic states associated with the Si substrate. The electron transmission spectra obtained from the NEGF\u2013DFT calculations reveal that the transport properties could also be extensively modulated by changing substituent groups. For certain molecules, it is possible to have a transmission peak at the Fermi level of the electrodes, corresponding to high conduction through the molecular wire with essentially no leakage into the Si substrate.Peer reviewed: YesNRC publication: Ye

Density Functional Theory Modeling of MnO₂ Polymorphs as Cathodes for Multivalent Ion Batteries

Multivalent ion batteries (MVIBs) provide an inexpensive and energy-dense alternative to Li-ion batteries when portability of the battery is not of primary concern. However, it is difficult to find cathode materials that provide optimal battery characteristics such as energy density, adequate charge/discharge rates, and cyclability when paired with a multivalent ion. To address this, we investigate six MnO₂ polymorphs as cathodes for MVIBs using density functional theory calculations. We find voltages as high as 3.7, 2.4, 2.7, 1.8, and 1.0 V for Li, Mg, Ca, Al, and Zn, respectively, and calculate the volume change due to intercalation. We then focus specifically on Ca and compute the energy barriers which are associated with the diffusion of the ion throughout the materials. Our findings show that the α-phase displays the most rapid diffusion kinetics for a Ca ion, with a diffusion barrier as low as 190 meV. We then investigate the potential for the five polymorphs exhibiting the highest voltage to intercalate additional atoms and demonstrate that it is energetically favorable for each to accept at least one additional Ca ion; furthermore, two of the phases can accept more than two Ca ions. However, in each case, there is also a corresponding drop in the voltage as further atoms are intercalated. We also utilize a crystal-chemistry approach to detail the structural evolution of each phase by computing the bond valence sum and effective coordination of the Mn⁴⁺ ions upon intercalation of increasing numbers of Ca ions. Finally, by computing the electronic density of states, Bader charges, and real space charge density, we describe how the additional electrons from the Ca ions are distributed throughout the unit cell. These insights provide guidance in selecting a MnO₂ polymorph with the traits necessary for the realization of MVIBs

Ba₄B₈TeO₁₉: A UV Nonlinear Optical Material

We report a new noncentrosymmetric barium tellurium borate, Ba₄B₈TeO₁₉ that has potential ultraviolet (UV) nonlinear optical (NLO) applications. Ba₄B₈TeO₁₉ was synthesized by a flux method and crystallizes in the noncentrosymmetric space group <i>Cc.</i> The material exhibits a framework structure of [B₈O₁₇]_∞ double layers connected to distorted TeO₆ octahedra. Second harmonic generation (SHG) measurements at 1064 and 532 nm on polycrystalline Ba₄B₈TeO₁₉ indicate that the title compound is phase-matchable (type I) with a moderate SHG response (1 × KH₂PO₄ at 1064 nm and 0.2 × β-BaB₂O₄ at 532 nm). In addition, a short absorption edge (210 nm) was measured. Using density functional theory calculations, we show that the SHG response originates from contributions from O 2p and Te 5s states at the valence and conduction band edges. Finally, by computing the linear optical properties, we find that this compound displays a moderate birefringence of 0.055 at 1064 nm and 0.059 at 532 nm, necessary conditions for phase-matching in UV NLO materials

Towards graphyne molecular electronics

Photograph of competitors in the 1997 National Cutting Horse Association Summer Cutting Spectacular held at Will Rogers Coliseum in Fort Worth, Texas

Effect of Anchoring Groups on Single Molecule Charge Transport through Porphyrins

Controlling charge transport through individual molecules and further understanding the effect of anchoring groups on charge transport are central themes in molecule-based devices. However, in most anchoring effect studies, only two, or at most three nonthiol anchoring groups were studied and compared for a specific system, i.e., using the same core structure. The scarcity of direct comparison data makes it difficult to draw unambiguous conclusions on the anchoring group effect. In this contribution, we focus on the single molecule conductance of porphyrins terminated with a range of anchoring groups: sulfonate (−SO₃^–), hydroxyl (−OH), nitrile (−CN), amine (−NH₂), carboxylic acid (−COOH), benzyl (−C₆H₆), and pyridyl (−C₆H₅N). The present study represents a first attempt to investigate a broad series of anchoring groups in one specific molecule for a direct comparison. It also is the first attempt, to our knowledge, to explore single molecule conductivity with two novel anchoring groups sulfonate (−SO₃^–) and hydroxyl (−OH). Our experimental results reveal that the single molecule conductance values of the porphyrins follow the sequence of pyridyl > amine > sulfonate > nitrile > carboxylic acid. Electron transport calculations are in agreement that the pyridyl groups result in higher conductance values than the other groups, which is due to a stronger binding interaction of this group to the Au electrodes. The finding of a general trend in the effect of anchoring groups and the exploration of new anchoring groups reported in this paper may provide useful information for molecule-based devices, functional porphyrin design, and electron transfer/transport studies