The Influence of High Doping on Electronic and Optical Properties in Tungsten Oxide

Perovskites are a well-known class of materials with rich physics and a wide variety of applications. One such perovskite is tungsten oxide, which is a well-known chromogenic material used in smart windows and other display technologies. Due to its open crystal structure, it is possible to incorporate high concentrations of dopants. In this dissertation, we seek to understand the influence of dopants on atomic and electronic structure, as well as transport and optical properties using density functional theory.First, we examine alkali doping and incorporating the oxygen vacancy in cubic and monoclinic tungsten oxide. We investigate the relative stabilities of different charge states and its implications on electrical properties, such as conductivity and electrochromism. Our results suggest that both alkali dopants and oxygen vacancies are shallow donors, and we discuss its implications for device development. Next, we study the effect of charge doping. Tungsten trioxide has been experimentally shown to transform from the monoclinic symmetry to cubic symmetry with increasing monovalent doping. Our calculations show that electron doping primarily drives the phase transformation. We characterize the phase transformation from low to high symmetry, quantify the energetics of this transformation, and elucidate on the mechanism of these structural changes. Building on our insights on the electronic behavior of dopants and defects, we study the influence of doping concentration on transport. Understanding the transport properties of these carriers is critical in many of the device applications for which tungsten oxide is used. We investigate the role of electron-phonon scattering in electron transport, and discuss the effects of spin-orbit coupling. Finally, we examine the influence of doping concentration and structural distortions on optical absorption. We explore crystalline and disordered structures to demonstrate why these structural changes can enhance absorption at a microscopic level. Carrier-induced direct absorption is shown to wholly explain the drop in transmittance and coloration in electrochromism. Our findings shed light on how electronic features can be optimized for improved display and energy technologies

Towards structural health monitoring in carbon nanotube reinforced composites

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 51-56).An experimental investigation was conducted to understand the non-destructive evaluation (NDE) capabilities of carbon nanotubes (CNTs) of several network architectures towards structural health monitoring (SHM). As heterogeneous composite structures become increasingly common in industry, detecting mechanical damage and damage accumulation becomes increasingly difficult as many modes of failure occur below the external surface. Traditional SHM techniques may be time consuming and costly; however, CNTs are a unique material that shows promise as a strain or damage sensor. Three different laminate samples types with various CNT network architectures were tested in open-hole tension. Samples tested were quasiisotropic carbon fiber, carbon fiber prepreg with unidirectional knocked-down CNT surface patch, and fuzzy fiber reinforced plastic (FFRP) samples, which consist of radially grown CNTs on a woven ceramic fiber substrate. Mechanical load and electrical resistance were simulataneously measured using three different probes configurations with respect to the tensile direction that measured either surface or through thickness resistance changes. Measurements were taken near and away from the stress concentration. Results indicated that different CNT network architectures influenced the consistency and efficacy of indicating damage acculumation. Changes in electrical resistance correlated strongly with sample mechanical damage accumulation for unidirectional knocked-down CNTs, but had more consistent values and readings for the FFRP samples, indicating that CNT network architecture beyond the inherent piezoresistivity of the CNT heavily influences the NDE capabilities of using CNTs as strain or damage sensors. Results also suggest that CNT network architecture must be further optimized to achieve reliable NDE and SHM, and may depend on the desired application.by Wennie Wang.S.B

PyCDFT: A Python package for constrained density functional theory

We present PyCDFT, a Python package to compute diabatic states using constrained density functional theory (CDFT). PyCDFT provides an object-oriented, customizable implementation of CDFT, and allows for both single-point self-consistent-field calculations and geometry optimizations. PyCDFT is designed to interface with existing density functional theory (DFT) codes to perform CDFT calculations where constraint potentials are added to the Kohn-Sham Hamiltonian. Here we demonstrate the use of PyCDFT by performing calculations with a massively parallel first-principles molecular dynamics code, Qbox, and we benchmark its accuracy by computing the electronic coupling between diabatic states for a set of organic molecules. We show that PyCDFT yields results in agreement with existing implementations and is a robust and flexible package for performing CDFT calculations. The program is available at https://github.com/hema-ted/pycdft/.Comment: main text: 27 pages, 6 figures supplementary: 7 pages, 2 figure