1,721,065 research outputs found
LEAD CHALCOGENIDE QUANTUM DOT ASSEMBLY AND ATTACHMENT ON FLUID INTERFACES
Get PDFThe formation of tiles composed of quantum dots is thought to constitute a new class of self- assembled nanostructured material. Understanding the dynamic physicochemical processes that govern the assembly at a functionalized fluid interface is crucial. We aim to seek design guidance for future advances in improving the assembly result of nanostructured nanocrystals (NCs). What is expected is that a functionalized liquid interface potentially provides more control over the behavior of the nanocrystals (NCs), which increases the complexity of the assembly process. To investigate this self-assembly process, we investigate the insight from the fundamental molecular-level interactions, and we use Molecular Dynamics (MD) simulations to investigate the process. The model system studied here was composed of lead chalcogenide NCs, covered with lead oleate molecules (ligands), and assembled on a monolayer (ML) composed of amphiphile (DPPC) molecules. The impact of ML density and structure of amphiphile molecules are extracted as two main factors of interest.
The simulations aimed to reveal the role of the nature of the monolayer interface on NCs assembly process. Before testing the impact of monolayer parameters on the self-assembly of the NCs, we used density functional theory to confirm that the energy barrier for ligand dissociation from the surface of the NC was high and, as such, are unlikely to detach readily. We studied the degree of NC in-plane orientation and out-of-plane tilt degree during the process as two means of evaluating the NCs’ alignment. We generated simulations focusing on the impact of these two factors on self- assembly that will bring us practical insights into how ML density and the chain length of ML molecules affect the self-assembly performance. We uncover a trend of the analytical attachment among various NCs assembly processes that reflects the contributions that ML density and length of alkyl chain made to the formation of defects in dimer alignments. The simulations and experiments presented in this study provide concrete design rules for the assembly of NCs for further research on superlattice formation
Molecular Modeling of 3D Woven Covalent Organic Frameworks
No full text95 pagesMesostructured porous materials, especially metal-organic framework systems (MOFs), have received widespread attention by researchers over the course of several decades, aimed at exploiting their suitability as sensors, catalysts, adsorption materials, etc. These materials may exhibit high thermal stability for a variety of physical and chemical reactions, benefiting from their unique structural features such as their crystalline order and high internal surface area. Recently, within this class of materials, Covalent Organic Frameworks (COFs) are of particular interest, as they are generally composed of light elements, and offer nanoporous crystalline structure. In this work, we offer an atomic-scale view of the recently synthesized 3D woven COFs, known formally as COF-505-Cu and COF-506-Cu, using molecular simulation techniques, including ab initio Density Functional Theory (DFT) and Molecular Dynamics (MD). First, we explored the reversible demetallation of COF-505-Cu. Serving as the “linkers” of the structure, the covalent bonding between the metal atom and the organic thread is essential to maintain the overall weave of the structure. We proposed two distinct pathways for the demetallation reaction that breaks the covalent bonding, and calculated their respective free energy barriers. We also tested the re-insertion of other, as yet experimentally untested transition metals (Zn and Fe), to the structure, and found that re-formation of the covalent bonding with the metal is strongly affected by the nature of the ionic complex being inserted. Second, we studied the second-generation, non-interpenetrated, COF-506-Cu that was first synthesized in 2018. We adopted the potential function parameters devised for its predecessor (COF-505) to model this material. This force field model was then used to characterize the nature and rate of guest molecule diffusion and uptake in the COF-506-Cu weave. We found that the presence of more guest-accessible space inside the weave greatly increased the diffusion of small molecules, compared to the limited diffusion observed by Acevedo in COF-505. But for other molecules, like the larger THF and methyl orange, movement inside the weave is still constrained through molecular interactions to be tortuous and relatively slow; this was suggested without direct observation in the experimental work. The computationally predicted uptake of THF vapor compared quantitatively well with the experimental data. Those results provided strong validation of the original Acevedo force field model. Looking to the future, the validation of this model could provide a way to predict the efficacy of COF-506, or other variants of this woven COF, ahead of experimental synthesis or applications such as molecule capture (adsorption), separation processes, or catalysis
Computationally Probing Atomic Motion Within Crystalline Systems: Defect Migration in Perovskites and Schwoebel Barriers to Admolecule Diffusion on Titanium Dioxide Surfaces
No full textComputational studies of atomic motion in crystalline systems provide critical insights into defect dynamics and surface growth mechanisms, enabling the optimization of materials for energy applications. In metal halide perovskites, the defect tolerance essential to their photovoltaic performance is attributed to self-regulation of free charge carriers via Schottky defect formation. Chapter 2 investigates CsPbBr3, an all-inorganic perovskite, using Nudged Elastic Band Density Functional Theory (NEB-DFT) to quantify migration and activation energies of Br interstitials (Br_i). Calculated migration energies (0.53–0.80 eV) align with the experimentally observed 0.52 eV value. Similarly, titanium dioxide (TiO2) thin films, crucial in photocatalysis and electronics, require precise control of surface morphology influenced by the Ehrlich-Schwoebel barrier. Chapter 3 combines atomic force microscopy (AFM) and ab initio simulations to determine Schwoebel barriers on anatase TiO2, advancing understanding of growth kinetics and enabling tailorable material properties. Together, these computational approaches further link atomic-scale processes with material properties for tunable synthetic control
COVALENT ORGANIC FRAMEWORK AS ARTIFICIAL SOLID ELECTROLYTE INTERFACE FOR LITHIUM METAL ANODE
Get PDFTo combat global climate change and to reduce anthropogenic carbon-emissions, there has been a dire need to transition away from internal combustion engine vehicles. Li-ion batteries, which are used in most electric vehicles, lack the specific energy capacity to deliver desirable driving distances that current internal combustion engine vehicles deliver. Lithium metal battery is a promising next-generation battery with significantly higher theoretical capacity, but it suffers capacity loss and safety issues due to dendrite growth at the lithium metal anode. Covalent Organic Framework (COF) is a porous organic material that has been gaining huge attention in energy storage applications due to its durable structure and versatile structure/function modification strategies.
Here, we demonstrate a simple same-phase interfacial synthesis method to synthesize a free-standing TpPa-SO3H COF membrane at room temperature and pressure. By taking advantage of the anionic sulfonyl functional groups in the pores of the TpPa-SO3H COF membrane, we utilized the TpPa-SO3H membrane as an artificial solid-electrolyte interface as a lithium protective layer to suppress dendrite growth and facilitate uniform lithium deposition. By applying TpPa-SO3H membrane as an aSEI, we demonstrate higher coulombic efficiency and enhanced stability in the Li | Cu half-cell. Furthermore, the simple synthesis method and free-standing nature of the COF membrane synthesized in this work sheds further insight to developing robust yet scalable COF membranes which can be applied to many more applications
A Computational Study of Corrosion in Nickel-Titanium Alloys in Saltwater Conditions
Get PDFCorrosion is a naturally occurring process that can be beneficial when controlled, yet can cause immense damage when allowed to run unchecked. Technologies have been developed to combat unwanted corrosion, with one of the more prominent solutions being thin film coatings of corrosion-resistant materials such as nickel titanium (NiTi). In addition to the corrosion-resistant nature of this alloy, NiTi possesses the unique properties of superelasticity, biocompatibility, and a reversible phase transformation that allows for self-healing. This combination of factors has led to extensive experimental studies on the corrosion of NiTi; however, the cost of materials and analytic equipment can be a sufficient burden to make a computational analysis via density functional theory (DFT) as a viable alternatve. DFT has been used to characterize properties of NiTi in the past, but few DFT studies of corrosion have been performed with this approach.
This thesis attempts to fill this gap in knowledge by simulating the adsorption of two ionic species (Cl and OH) found in salt water onto the low-index surfaces of NiTi and calculating how the adsorption of these species affect the energy landscape of the surface. We begin with pristine surface terminations on the (110), (100), and (111) planes of NiTi and demonstrate that our methodology can replicate existing literature values of the electron density of states and surface energies on this alloy. Cl and OH adsorbates are then introduced into the system and allowed to relax onto the NiTi surface. We show that OH will tend to bind to all surfaces with greater strength than Cl and also show that both adsorbates will prefer to bind at sites that maximize bonding to Ti atoms over Ni atoms in the surface. The addition of either adsorbate reduces the surface energy of NiTi, which is supported by Gibbs adsorption theorem. Overall, we demonstrate a methodology to study corrosion that can be adapted to study other surfaces of NiTi, other materials, or other adsorbates to recreate different environments and solutions
Uncovering the Fundamental Mechanisms leading to Nucleation and Growth in the Solution Processing of Hybrid Organic Inorganic Perovskites
Get PDFPower conversion efficiencies of hybrid organic-inorganic perovskite (HOIP) solar cells now rival those of traditional silicon-based solar cells. Unlike silicon, HOIPs can be processed directly from solution, leading to low-cost and energy-efficient fabrication. While many studies have shown that the composition of these solutions ultimately affects the cell’s efficiency, the underlying physics governing the solution processing of the final crystalline product is very poorly understood and under-investigated due to the overwhelming complexity of the system. Despite the importance of understanding this correlation between processing and performance, the many choices of species and the processing “recipe” cannot be fully explored by either an experimental or a computational trial-and-error approach. We need to understand the key rules that underlie the complexation and nucleation processes to help guide this exploration. To start this process, we have performed accurate ab initio Density Functional Theory (DFT) calculations of important moieties in solution and, critically, identified the Mayer Bond Order (MBO) as a metric of complexation effectiveness in solutions containing the building blocks of lead halide salts. We provide clear evidence, through a proof of concept involving the additive THTO, that the high bonding power and Lewis basicity, measured by the MBO, provides a computationally efficient and accurate way to determine solvent performance and screen currently unused experimental solvents for their effectiveness, hence precluding the need for experimental trial-and-error. For the second step in the process, determining the mechanism of complexation that leads to perovskite nucleation, we have studied the effect of the bath solvents and anti-solvents on the complexation of lead salts to the chaperone cation (methylammonium, formamidinium or cesium) in solution. Using quantum mechanically modeled systems, we have found that, for each choice of lead-halide (iodine, bromine, chlorine), paired with one of these three cations, the permittivity of each solvent or anti-solvent has a direct effect on the binding energy between the PbX3 and B-site cation motifs. This work shows that the solvating strength of the bath solvents hinders the formation of the smallest perovskite building blocks (PbX3M), while the use of the anti-solvents does not. Specifically, our ab initio calculations showed that the binding energy between the two major building blocks that go to make up the final perovskite crystal structure can be increased using a relatively low dielectric solvent. The goal of solution processing is, first, to dissolve the lead salts, and, second, to allow nucleation to occur in solution. Our work suggests that these two concurrent events can be controlled to maximize the objectives of each event over the course of the solution processing cycle to achieve a high nucleation density and large-grain crystal growth
A Computational Study of Corrosion in Nickel-Titanium Alloys in Saltwater Conditions
No full textCorrosion is a naturally occurring process that can be beneficial when controlled, yet can cause immense damage when allowed to run unchecked. Technologies have been developed to combat unwanted corrosion, with one of the more prominent solutions being thin film coatings of corrosion-resistant materials such as nickel titanium (NiTi). In addition to the corrosion-resistant nature of this alloy, NiTi possesses the unique properties of superelasticity, biocompatibility, and a reversible phase transformation that allows for self-healing. This combination of factors has led to extensive experimental studies on the corrosion of NiTi; however, the cost of materials and analytic equipment can be a sufficient burden to make a computational analysis via density functional theory (DFT) as a viable alternatve. DFT has been used to characterize properties of NiTi in the past, but few DFT studies of corrosion have been performed with this approach.
This thesis attempts to fill this gap in knowledge by simulating the adsorption of two ionic species (Cl and OH) found in salt water onto the low-index surfaces of NiTi and calculating how the adsorption of these species affect the energy landscape of the surface. We begin with pristine surface terminations on the (110), (100), and (111) planes of NiTi and demonstrate that our methodology can replicate existing literature values of the electron density of states and surface energies on this alloy. Cl and OH adsorbates are then introduced into the system and allowed to relax onto the NiTi surface. We show that OH will tend to bind to all surfaces with greater strength than Cl and also show that both adsorbates will prefer to bind at sites that maximize bonding to Ti atoms over Ni atoms in the surface. The addition of either adsorbate reduces the surface energy of NiTi, which is supported by Gibbs adsorption theorem. Overall, we demonstrate a methodology to study corrosion that can be adapted to study other surfaces of NiTi, other materials, or other adsorbates to recreate different environments and solutions
COVALENT ORGANIC FRAMEWORK AS ARTIFICIAL SOLID ELECTROLYTE INTERFACE FOR LITHIUM METAL ANODE
No full textTo combat global climate change and to reduce anthropogenic carbon-emissions, there has been a dire need to transition away from internal combustion engine vehicles. Li-ion batteries, which are used in most electric vehicles, lack the specific energy capacity to deliver desirable driving distances that current internal combustion engine vehicles deliver. Lithium metal battery is a promising next-generation battery with significantly higher theoretical capacity, but it suffers capacity loss and safety issues due to dendrite growth at the lithium metal anode. Covalent Organic Framework (COF) is a porous organic material that has been gaining huge attention in energy storage applications due to its durable structure and versatile structure/function modification strategies.
Here, we demonstrate a simple same-phase interfacial synthesis method to synthesize a free-standing TpPa-SO3H COF membrane at room temperature and pressure. By taking advantage of the anionic sulfonyl functional groups in the pores of the TpPa-SO3H COF membrane, we utilized the TpPa-SO3H membrane as an artificial solid-electrolyte interface as a lithium protective layer to suppress dendrite growth and facilitate uniform lithium deposition. By applying TpPa-SO3H membrane as an aSEI, we demonstrate higher coulombic efficiency and enhanced stability in the Li | Cu half-cell. Furthermore, the simple synthesis method and free-standing nature of the COF membrane synthesized in this work sheds further insight to developing robust yet scalable COF membranes which can be applied to many more applications
Determining The Energy Profile And Polymorphs Of 6,13-Tips Pentacene In Vacuo And In Solvent
No full textOrganic semiconducting materials are gaining popularity as solar cell materials, in large part because of their ability to be processed onto flexible substrates and to be manufactured more cheaply. However, organic materials are not nearly as efficient as silicon in regards to charge transport. To circumvent this, a process called solution shearing on 6,13-bis(triisopropylsilylethynyl) pentacene (6,13-TIPS pentacene) significantly reduces the pi-orbital overlap, thereby enhancing the charge transport by an 80% increase. To fully appreciate what is happening at an atomic level and how shearing the unit cell to conform to a configuration that is both energetically favorable and conducive to charge transport, an atomically explicit model of molecules of 6,13-TIPS pentacene were analyzed via ab initio and Molecular Dynamics techniques. In vacuo, the parameters of the unit cell [a,b,[gamma]] were varied and subjected to minimization calculations, which yielded an energy profile. From this profile, we were able to identify the five lowest energy configurations which were well aligned with the five polymorphs determined experimentally. Closer examination of the molecular configurations suggested that the fluctuations in the energy profile were due to the bending and twisting of the acene backbone, the wagging of the silylethynyl groups, and the rotating of the methyl groups. Compared to an equilibrium structure, there is a 4.5° deviation in the acene backbone and a nearly 10° difference in the silylethynyl angle across this energy landscape. However, when comparing the changes in these different angles against the energy profile, the minima for both are closely aligned. The crystal was also studied in ten different solvents represented by a Lennard-Jones atom, which varied only in size. The new structure was submitted to energy minimization calculations. From this data, it was determined that as the solvent size increases, the stability of the unit cell decreases. Based on the data gathered, the energy profile of 6,13-TIPS pentacene is significantly affected by the rotating, wagging, and bending of the various side groups and the acene backbone. Whether these deviations occur in sync or are a result of a "domino effect" is not yet clear. The size of the solvent also impacts the energy profile; as the solvent radius increases, the molecules in the unit cell can be seen to almost push at the solvent in an effort to expel it from the unit cell
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