# Your search: "author:Strubbe, David A"

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## Scholarly Works (33 results)

Low-dimensional materials have attracted significant attention in the field of materials science and technology. These materials, characterized by their limited dimensions in one, two or three directions, exhibit exceptional properties different from those of their bulk counterparts. This deviation results from the interaction of quantum effects, surface interactions, and unique electronic structures, creating a wide range of innovative applications in various fields such as electronics, photonics, energy storage and more. This study not only reveals the scientific complexities of their behavior but also paves the way for disruptive innovations that could transform industries and transform our interactions with the world around us. In this work, I have studied different classes of low-dimensional materials such as bulk and monolayer transition-metal dichalcogenides, one-dimensional organic-metal halide hybrids, organic metal halide nanoribbons, Bi$_2$Te$_3$ and HfTe$_5$.

In the opening chapter, I lay the groundwork by discussing fundamental theories crucial for our subsequent calculations. These theories encompass density functional theory, density functional perturbation theory, GW and Bethe-Salpeter equations and other fundamental concepts on Raman, and calculation methods for elastic tensors. Chapter 2 delves into a comprehensive analysis of the impact of introducing Ni into the bulk phases of MoS$_2$, with a particular focus on the mechanism supporting low wear for lubrication applications. This chapter also presents systematic doping studies conducted through first-principles calculations. Chapter 3 narrows the focus to examine the consequences of Ni doping on the monolayer phases of MoS$_2$, revealing the induction of various known and novel distorted phases. These induced phases bring forth unique properties, including those of ferroelectric materials, multiferroic semimetals, ferromagnetic polar metals, and the presence of multiple gaps in conduction bands. These findings have promising implications for applications in spintronics, intermediate band solar cells, and non-linear optics. In Chapter 4, the attention shifts to a low-energy nine-layer phase within transition-metal dichalcogenides, where we explore methods for characterizing this phase through Raman spectroscopy and powder diffraction. Additionally, we investigate its potential applications in piezoelectricity and valleytronics. Next Chapter 5 delves into understanding the anisotropic properties of one-dimensional organic-metal halide hybrids and the mechanism of broadband emission. This chapter introduces an innovative approach for determining exciton-phonon coupling and the self-trapped exciton structure using excited state forces within a GW/Bethe-Salpeter framework. Chapter 6 offers a concise overview of my contributions to other collaborative works on low-dimensional materials, encompassing organic-metal halide hybrids, organic metal halide nanoribbons, Bi$_2$Te$_3$ and HfTe$_5$. Finally, I conclude with reflections on the potential of low-dimensional materials, their current progress, and future prospects.

Hybrid organic metal-halide perovskites are promising materials for next generation solar cell application. Methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$), sometimes called MAPI, is one of the favorable perovskites for making solar cells. Most of the research in the last decade about this material is aimed towards improving its photoconversion efficiency (PCE) and stability. In this work, I have done a detailed study on the three phases (orthorhombic, tetragonal, and cubic) of MAPI to understand how these different phases behave under stress. The total work is divided into 5 chapters. In chapter 1, I give an overview on perovskites for solar cell applications and discuss briefly about the theories that are involved in my calculations. In chapter 2, I investigate the effect of uniaxial strain on the pseudo-cubic structure and identify the most favorable vibrational modes to measure local strain using IR and Raman spectroscopy. In chapter 3, I investigate the same for low-temperature orthorhombic and room-temperature tetragonal phases. In addition to this, I explained about an improvement I made to the Quantum ESPRESSO code to enable these calculations. In chapter 4, I discussed how an analytical method we developed can help to understand hidden symmetries in the tetragonal perovskite and can be useful for any approximately symmetric structure to use symmetry for spectroscopic studies. In chapter 5, the last chapter, I studied the elastic properties of all three phases in detail and tried to determine the root cause behind the discrepancies in earlier published results. We also provide accurate reference values and an appropriate general methodology for elastic properties of metal halide perovskites. This work opens a way for a standard non-destructive bench-top characterization method to be usable for analyzing the critical role of local strain in hybrid perovskite photovoltaics. It provides an analytical method to calculate irreducible representations of vibrational modes for any approximately symmetric crystal structure which can be helpful for spectroscopic studies. Calculated detailed elastic properties of metal-halide perovskite will be useful for future reference for commercial application of perovskites for solar cell and flexible electronics.

The non-additive kinetic potential functional is a key element in density-dependent embedding methods. The correspondence between the ground-state density and the total effective Kohn-Sham potential provides the basis for various methods to construct the non-additive kinetic potential for any pair of electron densities. Several research groups used numerical or analytical inversion procedures to explore this strategy which overcomes the failures of known explicit density functional approximations. The numerical inversions, however, apply additional approximations/simplifications. The relations known for the exact quantities cannot be assumed to hold for quantitiesobtained in numerical inversions. The exact relations are discussed with special emphasis on such issues as: the admissibility of the densities for which the potential is constructed, the choice of densities to be used as independent variables, self-consistency between the potentials and observables calculated using the embedded wavefunction, and so forth. The thesis focuses on how these issues are treated in practice. The inverted potentials are calculated for weakly overlapping pairs of electron densities – the case not studied previously.

The behaviour of Vnad at the vicinity of the nuclei has been questioned since the beginning. Available computational tools and methods in the past led to a cusp at nuclei in Vnad calculations. I analysed existence and non-existence properties of the cusp in Vnad analytically, and compare against nuclear cusps condition for the ground-state density and resulting cusp in the Kohn-Sham potential. I showed the agreement of numerical calculations with this fact for various diatomic model systems of two and four electrons. The results are compared to the von Weizsäcker functional (exact for one orbital) and other kinetic energy functionals.

In addition, I found that the well-known step structure of Vxc associated with molecular dissociation also appears in Vnad, even in local and semi-local functionals in the region where the two subsystems' densities weakly overlap.

- 1 supplemental PDF

Microstructure defects are the source of many interesting physical phenomena in materials. The influence of microscopic imperfections are visible among a background of a crystalline material and can noticeably change macroscopic materialproperties. Ab initio computations are a wide range of computational methods used to study materials at such microscopic levels. Classical and quantum-level computations are often viewed as competing methods, but we instead take a complementary approach. We combine strengths of both methods to study two classes of materials—silicon and the two-dimensional (2D) transition metal dichalcogenide (TMD) molybdenum disulfide (MoS2). In MoS2, we interrogate the effects of the presence of dopants. Conversely in Si, we take amorphization as a disruption to the base crystalline structure. In both, we are interested in how these changes to microstructure affect energetic efficiencies. In a-Si, the photovoltaic efficiency is hampered by the presence of dangling bonds, while in MoS2, mechanical efficiencies of lubrication are enhanced when doped.

Both systems pose their unique challenges and deviations from standard workflows. We will review the overarching strategies used to study them in Ch. 2. Throughout our studies, we have developed several techniques to overcome specific challenges which are detailed in their respective sections. With a-Si, a classical potential Monte Carlo code is used to generate realistic, non-biased, fully amorphous coordinates faster than what could be achieved by currently available quantum-mechanical methods. Then, density functional theory is used to relax the structures. With MoS2, higher-accuracy DFT energy computations are used to parametrize a classical force-field for the computation of larger systems which would otherwise be difficult with DFT alone.

We cover a wide range of analyses between both materials. We have developed methodologies for analyzing the amorphous and 2D material systems using classical methods and density functional theory. We have computed a range of material properties for MoS2 and amorphous Si and hydrogenated a-Si:H. With Si, we modified the Wooten-Winer-Weaire algorithm to produce amorphous networks with included voids by application of initial strain. The size of the voids is somewhat controlled by the strain value. These voids emerge naturally as a part of the amorphization process.

We find that the amorphous networks generated by simple Keating springs when applied in the WWW method is retained when relaxed by DFT. Structure-scale approximations of the Keating potential yields the result that ∆θ explains a large portion of the structure’s energy, and this holds even in density functional theory.

With 2D materials, we outline a specific multi-step method to quantify the sliding of defected materials in Ch. 6. Successive increases in the degrees of freedom while sliding allow us to access different components of sliding—namely the potential barrier differences, low-energy sliding pathways, and slip planes. The sliding potential in MoS2, even while intercalated, is composed of pairwise interactions of the MoS2 interfaces. This means that computing arbitrarily sized systems can be theoretically computed by only considering interactions of their interfaces.

We find tetrahedrally intercalated Ni-doped MoS2 to be stable and thus more important than is considered in the literature. When intercalated, we find Ni can bind layers together, explaining the material’s increased resistance to wear, or material loss during sliding.

Re-doped MoS2 has shown an increase in friction with an increasing layer count as measured with atomic force microscopy. This is counter to typical 2D materials. We find that intercalated Re can explain this relationship as an alteration to outof-plane stiffness. For this material, in computing the vibrational spectroscopy we overcame a difficulty in computing its Raman spectra. It has a metallic character, thereby limiting our Raman intensity computation due to an infinite dielectric constant. We developed a method to circumvent this computation and approximate the spectra by substitution of the atomic Raman tensor.