COMPUTATIONAL DESIGN OF ALLOY MATERIALS FOR USE AS TRANSPARENT CONDUCTORS AND PHOTOCATALYSTS

Ph.DDOCTOR OF PHILOSOPH

The Role of Electrostatic Interaction between Free Charge Carriers and Counterions in Thermoelectric Power Factor of Conducting Polymers: From Crystalline to Polycrystalline Domains

One of the most burning problems in organic thermoelectrics is the lack of deep understanding on the key limiting factors of thermoelectric efficiency at different length scales for conducting polymers. Here, by examining a prototypical pi-conjugated polymer, poly(3-hexylthiophene) from molecular level to crystalline and polycrystalline domains, and on the basis of first-principles calculations, new insights are presented into the thermoelectric transport in conducting polymers, and new material design guidelines are provided. It is proved that in the crystalline domains of conducting polymers, due to the strong electrostatic interactions between free charge carriers and counterions, the power factor within a wide range of doping level is governed by the counterion-induced electronic scattering. It is corroborated that in the polycrystalline domains, although the short mean free path prevents the holes undergoing grain-boundary scatterings, the crystallite orientations relative to the conduction path and the grain sizes strongly affect the power factor, leading to the modulation of the power factor by at least two orders of magnitude

Computational Design of Perovskite Ba_<i>x</i>Sr_1–<i>x</i>SnO₃ Alloys as Transparent Conductors and Photocatalysts

Using a first-principles-based multiscale computational approach involving density functional theory and the cluster expansion method, we produced the structural evolution for the perovskite Ba_<i>x</i>Sr_1–<i>x</i>SnO₃ system in relation to its Ba:Sr composition from the formation energies of different alloy configurations and demonstrated their use as tunable alloy transparent conductors and photocatalysts via structural, electronic, and optical studies. The predicted phase diagram revealed the transformation of the structure of Ba_<i>x</i>Sr_1–<i>x</i>SnO₃ from orthorhombic to tetragonal and finally to cubic with increasing <i>x</i>, forming disordered solid solutions for 0 < <i>x</i> < 1 that is entropically stabilized against phase segregation. This trend is similarly observed in the published experiments. A special quasirandom structure approach is used to model the disordered solid solutions of the Ba_<i>x</i>Sr_1–<i>x</i>SnO₃ alloys. Structural analyses have indicated that the decrease in Ba:Sr ratio is associated with the decrease in unit cell volume, and also the increased distortion of the (Ba,Sr)­O₁₂ cuboctahedra, while the SnO₆ octahedra remained relatively undistorted and underwent tilting to accommodate the smaller Sr atoms. Electronic and optical studies have shown the Ba_<i>x</i>Sr_1–<i>x</i>SnO₃ alloys to possess transparent conducting, photocatalytic water splitting and CO₂-reduction capabilities, which can be tailored via compositional engineering. The results should serve as a guide for the investigations of structure–property relationships of perovskite-based alloys

A Dual-Surface Mechanism of Oxidant-Free Pyrrole Polymerization in the Two-Dimensional Titanium Carbide (MXene) Interlayer Nanospace

The influence of confinement in acidic nanotemplates on the polymerization reaction of pyrrole was previously reported. Similarly, an in situ oxidant-free polymerization of pyrrole on and in between the Ti3C2 MXene layers has been demonstrated experimentally. The newly formed PPy/MXene (PPy/polypyrrole) interface showed high electrical conductivity and supercapacitor features with excellent cycling stability. However, the polymerization mechanism remains unclear. In this study, the pyrrole polymerization mechanisms on Ti3C2 MXene surfaces and in between MXene interlayers with different terminations were investigated based on first-principles calculations. Key factors for such oxidant-free polymerization were identified, including the configuration of surface hydrogen-bonded pyrrole, surface acidity, confinement effect, and charge transfer between MXene surfaces and pyrrole monomers. By controlling these factors, one can design covalently bonded conducting organic molecules/polymers on or in between MXene interlayers for electronic applications. These findings not only uncover the mechanism for a proton-assisted pyrrole polymerization on and in between MXene interlayers but also provide a general mechanistic understanding and guideline for other possible in situ polymerizations on and in between two-dimensional MXene interlayers

Effect of substituents in sulfoxides on the enhancement of thermoelectric properties of PEDOT:PSS: experimental and modelling evidence

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), being the most popular conductive polymer, has been doped with various additives with the aim of improving its thermoelectric performance. Among all additives, dimethyl sulfoxide (DMSO) has been widely used for various treatments. In this work, we designed and synthesized a series of aliphatic- and aromatic-substituted sulfoxides as dopants to improve the thermoelectric properties of PEDOT:PSS. It was found that the substituents in the sulfoxides played a vital role in controlling the thermoelectric properties. Sulfoxides with relatively longer alkyl chains and large phenyl groups increased the electrical conductivity of PEDOT:PSS to more than 200 S cm(-1)compared to the pristine PEDOT:PSS film. The sulfoxide with 4-nitrophenyl substituents, however, led to negligible changes in electrical conductivity but increased the Seebeck coefficient from 22 to 56 mu V K-1. In contrast, the sulfoxide with 4-hydroxyphenyl substituents remarkably improved both the electrical conductivity and Seebeck coefficient, leading to a power factor of up to 69 mu W m(-1)K(-2), much higher than that of the PEDOT:PSS film which was obtained by simply mixing with DMSO. Several simulation methods were used to evaluate various interactions between sulfoxides, PEDOT, and PSS to elucidate the mechanisms, revealing that the sulfoxide with 4-hydroxyphenyl groups exhibited additional interaction with the PSS phase, while the sulfoxide with 4-nitrophenyl groups showed strong interaction with the PEDOT phase instead and hence disrupted electrical conductivity. Our findings would uncover the mechanism of electrical conductivity enhancement, providing a general strategy for designing promising alternative additives for PEDOT:PSS treatment and eventually achieving better thermoelectric properties

Reconfiguring crystal and electronic structures of MoS2 by substitutional doping.

Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors

Cu(I)/Cu(II) Creutz-Taube Mixed-Valence 2D Coordination Polymers

Graphene-like two-dimensional (2D) coordination polymers (GCPs) have been of central research interest in recent decades with significant impact in many fields. According to classical coordination chemistry, Cu(II) can adopt the dsp2 hybridization to form square planar coordination geometry, but not Cu(I); this is why so far, there has been no 2D layered structures synthesized from Cu(I) precursors. Herein we report a pair of isostructural GCPs synthesized by the coordination of benzenehexathiol (BHT) ligands with Cu(I) and Cu(II) ions, respectively. Various spectroscopic characterizations indicate that Cu(I) and Cu(II) coexist with a near 1:1 ratio in both GCPs but remain indistinguishable with a fractional oxidation state of +1.5 on average, making these two GCPs a unique pair of Creutz-Taube mixed-valence 2D structures. Based on DFT calculations, we further uncovered an intramolecular pseudo-redox mechanism whereby the radicals on BHT ligands can oxidize Cu(I) or reduce Cu(II) ions upon coordination, thus producing isostructures yet with distinct electron configurations. For the first time, we demonstrate that using Cu(I) or Cu(II), one can achieve atomically isostructural 2D structures, indicating that a neutral periodic structure can host a different number of total electrons as ground states, which may open a new chapter for 2D materials