Atomic and electronic structures of extended defects, disorder, and reactions in solids

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Xie, Weiyu
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Electronic thesis
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As the physicist Sir Charles Frank said, `Crystals are like people: it is the defects in them that make them interesting.' Although tiny in size, the defects in the microscopic scale would determine the macroscale functionality of materials. For the purpose to design, functionalize and engineer materials, the fundamental understanding and tailoring defects and other disorder features are paramount. This thesis focuses on the extended defects studies in solids, through first-principles calculations and other theoretical methods, concerning their atomic and electronic related properties. The thesis is organized in three parts: Part I: Extended defects in inorganic materials; PART II: Disorders in organic conducting polymers/molecule crystals; PART III: Nanosize effect on the defect-mediated reactant diffusion for catalysis reaction. In Part I, we are mainly focusing on the extended defects in inorganic materials, with their forms as dislocations, grain boundaries, and etc. This part starts with the general introduction to extended defects in Chapter 1, followed by three different case studies from Chapter 2 to 4. Chapter 2 presents the multi-scale simulation of small-angle grain boundaries in Si. As electrically-benign, low-cost, near-single crystalline semiconductors is crucial for large-scale applications, understanding the electrical behavior of small-angle grain-boundaries (GBs), consisting of array of dislocations, becomes important. However, a quantitative microscopic theory is still prohibitively difficult. Here, by developing a multiscale approach combining Monte-Carlo simulation, elasticity theory, first-principles calculation, and grand-canonical statistical modeling, we quantitatively explain the recent experimental observation showing the disappearance of hole transport barrier in poly-Si when the GB angle is below a critical value of a few degrees. It reveals the microscopic origin for the observation as a transition from electrically harmful dislocations to electrically benign dislocations. Chapter 3 presents our developed modular approach for calculations of an epitaxial interface formed between materials with a large misfit (f ≥ 10%). Two different types of epitaxial interfaces, metal-semiconductor with covalent bonds at interfaces and semiconductor-semiconductor with Coulombic interactions at interfaces, are studied through this method. Both of them demonstrate good agreements with experimental results. Via the electron localization function (ELF), we observe and quantify the directional bonding density ρb at the interface. For the metal-semiconductor interfaces (Al/Si, Cu/Si), the ρb is found responsible for the experimentally observed epitaxial relationships. For the semiconductor-semiconductor interface formed between CdTe film and As-passivated Ge with no dangling bonds on the Ge substrate surface, a weaker interface binding with ρb = 0 is obtained. Chapter 4 goes beyond the extended defects in three-dimensional (3D) materials, focusing on the extended defects in 2D heterostructures of graphene and hexagonal boron nitride (h-BN) monolayers. Guided by the Clar's sextet rule, the stable heterostructures at different misorientaion angles are modeled and calculated. Based on these structures, the Schottky barrier height (SBH) for 2D graphene/h-BN lateral heterostructures is computed using first-principles methods. The calculated SBH is remarkably insensitive to the misorientation angle between graphene and h-BN and close to the ideal SBH that follows the Schottky-Mott limit. This unexpected result can be quantitatively reproduced by an infinitely long dipole line model developed for 2D systems. The quickly decaying of interface dipole induced potential difference is responsible for the universal SBH. The present results raise the possibility of fabricating high-quality transport devices using current graphene/h-BN heterostructures and establish the principle that any 2D heterostructure follows the Schottky-Mott limit when the device size is large than several nanometers. In Part II, we discuss disorders in organic materials, specifically, the conducting polymers/molecule crystals. Compared with inorganic crystals, organic polymers or molecules crystals have higher degree of freedom and thus more room for disorders. Furthermore, because of the technical difficulty of simulating the weak van der Waals (vdW) interaction in these systems, this disorder problem was hard to be accurately described before. In this part, we will introduce our developed DFT+LAP method that is capable to describe vdW interactions accurately. Two prototype conducting inorganic materials P3HT and DNTT(-C10) are studied. In Chapter 5, the crystal structure of poly(3-hexylthiophene) (P3HT) has been studied by first-principles calculations based on density functional theory. The generalized gradient approximation is employed and van der Waals interactions are treated accurately by the recently proposed local atomic potential (LAP) approach. A variety of different models were tested, and the model having the lowest energy is a non-interdigitated structure having an orthorhombic cell with a = 17.2 Å, b = 7.7 Å, and c = 7.8 Å, where a, b, and c are the lengths of the lattice vectors perpendicular to the lamallae, in the π-π stacking direction, and along the thiophene backbone, respectively. These values are in reasonably good agreement with experiment. The P3HT polymer is not invariant under inversion and therefore exhibits directionality. Our calculations suggest that a likely structural defect occurring in P3HT is one in which one of the polymer backbones within a lamella runs in the direction opposite to the majority. Such defects may form in the process of self-assembly of the non-interdigitated lamellae and may be an important source of π-π stacking disorder. A possible explanation for a recently observed structural phase transition in polythiophene is proposed. In Chapter 6, we present a comparison based on first-principles calculations of the electronic structure of non-alkylated and alkylated dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene: DNTT and DNTT-C10. The calculations show that the addition of alkyl chains decreases intermolecular distances, in agreement with experiments. Calculations indicate that effective masses are reduced by the addition of alkyl chains, and within a simple deformation potential model, this translates into higher mobility for DNTT-C10. The shorter intermolecular distances found in DNTT-C10 are attributed to van der Waals interactions between alkyl chains. In the final part of this thesis (Part III), we study the interesting "side-effect" of disorders. For example, how would the nano-size effect in reactant affect the catalytic reaction that is caused by catalysts? It is found that chemical reactions involve competing pathways. Here, two pathways for H desorption from MgH2 are identified by first-principles calculations: one involves H diffusion in bulk Mg, while the other involves H vacancy diffusion at the MgH2 surface. The latter is sensitive to the size of the reactant MgH2 and self-terminates in bulk material as dehydrogenation eventually eliminates exposed MgH2. However, this surface vacancy pathway can maximize the catalyst effect of Pd by decoupling the kinetics of H desorption from that of H diffusion. When the surface-to-bulk ratio is large as in the case of MgH2 nanostructures, H desorption will take place primarily via the low-barrier surface vacancy pathway. Our picture attributes the experimentally observed size effect of MgH2 on H desorption, beyond the quantum size regime, to a synergy between the nanosize of the reactant and the catalyst.
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Rensselaer Polytechnic Institute, Troy, NY
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