236 research outputs found
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Low-dimensional Material: Structure-property Relationship and Applications in Energy and Environmental Engineering
In the past several decades, low-dimensional materials (0D materials, 1D materials and 2D materials) have attracted much interest from both the experimental and theoretical points of view. Because of the quantum confinement effect, low-dimensional materials have exhibited a kaleidoscope of fascinating phenomena and unusual physical and chemical properties, shedding light on many novel applications. Despite the enormous success has been achieved in the research of low-dimensional materials, there are three fundamental challenges of research in low-dimensional materials:
1) Develop new computational tools to accurately describe the properties of low-dimensional materials with low computational cost.
2) Predict and synthesize new low-dimensional materials with novel properties.
3) Reveal new phenomenon induced by the interaction between low-dimensional materials and the surrounding environment.
In this thesis, atomistic modelling tools have been applied to address these challenges. We first developed ReaxFF parameters for phosphorus and hydrogen to give an accurate description of the chemical and mechanical properties of pristine and defected black phosphorene. ReaxFF for P/H is transferable to a wide range of phosphorus and hydrogen containing systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated phosphorene, phosphorus clusters and phosphorus hydride molecules. The potential parameters were obtained by conducting global optimization with respect to a set of reference data generated by extensive ab initio calculations. We extended ReaxFF by adding a 60° correction term which significantly improved the description of phosphorus clusters. Emphasis was placed on the mechanical response of black phosphorene with different types of defects. Compared to the nonreactive SW potential of phosphorene, ReaxFF for P/H systems provides a significant improvement in describing the mechanical properties of the pristine and defected black phosphorene, as well as the thermal stability of phosphorene nanotubes. A counterintuitive phenomenon was observed that single vacancies weaken the black phosphorene more than double vacancies with higher formation energy. Our results also showed that the mechanical response of black phosphorene is more sensitive to defects in the zigzag direction than that in the armchair direction. Since ReaxFF allows straightforward extensions to the heterogeneous systems, such as oxides, nitrides, the proposed ReaxFF parameters for P/H systems also underpinned the reactive force field description of heterogeneous P systems, including P-containing 2D van der Waals heterostructures, oxides, etc.
Based on the evolutionary algorithm driven structural search, we proposed a new stable trisulfur dinitride (S3N2) 2D crystal that is a covalent network composed solely of S-N σ bonds. S3N2 crystal is dynamically, thermally and chemically stable as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations. GW calculations showed that the 2D S3N2 crystal is a wide, direct band-gap (3.92 eV) semiconductor with a small hole effective mass. The anisotropic optical response of 2D S3N2 crystal was revealed by GW-BSE calculations. Our result not only marked the prediction of the first 2D crystal composed of nitrogen and sulfur, but also underpinned potential innovations in 2D electronics, optoelectronics, etc.
Inspired by the discovery of S3N2 2D crystal, we proposed a new 2D crystal, diphosphorus trisulfide (P2S3), based on the extensive evolutionary algorithm driven structural search. The 2D P2S3 crystal was confirmed to be dynamically, thermally and chemically stable by the computed phonon spectrum and ab initio molecular dynamics simulations. This 2D crystalline phase of P2S3 corresponds to the global minimum in the Born-Oppenheimer surface of the phosphorus sulfide monolayers with 2:3 stoichiometry. It is a wide band gap (4.55 eV) semiconductor with P-S σ bonds. The electronic properties of P2S3 structure can be tuned by stacking into multilayer P2S3 structures, forming P2S3 nanoribbons or rolling into P2S3 nanotubes, expanding its potential applications for the emerging field of 2D electronics.
Then we showed that the hydrolysis reaction is strongly affected by relative humidity. The hydrolysis of CO32- with n = 1-8 water molecules was investigated by ab initio method. For n = 1-5 water molecules, all the reactants follow a stepwise pathway to the transition state. For n = 6-8 water molecules, all the reactants undergo a direct proton transfer to the transition state with overall lower activation free energy. The activation free energy of the reaction is dramatically reduced from 10.4 to 2.4 kcal/mol as the number of water molecules increases from 1 to 6. Meanwhile, the degree of the hydrolysis of CO32- is significantly increased compared to the bulk water solution scenario. The incomplete hydration shells facilitate the hydrolysis of CO32- with few water molecules to be not only thermodynamically favorable but also kinetically favorable. We showed that the chemical kinetics is not likely to constrain the speed of CO2 air capture driven by the humidity-swing. Instead, the pore-diffusion of ions is expected to be the time-limiting step in the humidity driven CO2 air capture. The effect of humidity on the speed of CO2 air capture was studied by conducting CO2 absorption experiment using IER with a high ratio of CO32- to H2O molecules. Our result is able to provide valuable insights to designing efficient CO2 air-capture sorbents.
Lastly, the self-assembly mechanism of one-end-open carbon nanotubes (CNTs) suspended in an aqueous solution was studied by molecular dynamics simulations. It was shown that two one-end-open CNTs with different diameters can coaxially self-assemble into a nanocapsule. The nanocapsules formed were stable in aqueous solution under ambient conditions, and the pressure inside the nanocapsule was much higher than the ambient pressure due to the van der Waals interactions between two parts of the nanocapsule. The effects of the normalized radius difference, normalized inter-tube distance and aspect ratio of the CNT pairs were systematically explored. The electric field response of nanocapsules was studied with ab initio molecular dynamics simulations, which showed that nanocapsules can be opened by applying an external electric field, due to the polarization of carbon atoms. This discovery not only shed light on a simple yet robust nanocapsule self-assembly mechanism, but also underpinned potential innovations in drug delivery, nano-reactors, etc
Quantum Environments: Spin Baths, Oscillator Baths, and applications to Quantum Magnetism
The low-energy physics of systems coupled to their surroundings is understood
by truncating to effective Hamiltonians; these tend to reduce to a few
canonical forms, involving coupling to "baths" of oscillators or spins. The
method for doing this is demonstrated using examples from magnetism,
superconductivity, and measurement theory, as is the way one then solves for
the low-energy dynamics. Finally, detailed application is given to the exciting
recent Quantum relaxation and tunneling work in naomagnets.Comment: Chapter in "Tunneling in Complex Systems" (World Sci., edited T.
Tomsovic); 97 pages. Published in June 199
Modulating electronic structure and reactivity of Cr nitride complexes via xxidation
Metal nitride complexes exhibit interesting structure and bonding properties that are invoked when discussing the reactivity of these systems. The nitride (N3-) moiety can be either nucleophilic or electrophilic based on a variety of factors such as metal identity, oxidation state, and nature of the ancillary ligands. Herein, the electronic tuning of Cr salen nitride complexes is investigated via modulation of phenolate para-R substituents of varying electron donating ability (R = CF3, tBu, NMe2) in order to influence reactivity. Salen ligands can exhibit non-innocent behavior, implying that redox processes can either be metal or ligand-based. This feature allows the ligand to help facilitate difficult substrate transformations uncommon to Earth-abundant first-row metals. Depending on the para-R group, the locus of oxidation in Cr nitride salen complexes (metal vs. ligand) can be influenced. The electronic structure of oxidized compounds is detailed, allowing for rationalization of nitride reactivity based on oxidation locus
Interacting Ions in Biophysics: Real is not Ideal
Ions in water are important in biology, from molecules to organs.
Classically, ions in water are treated as ideal noninteracting particles in a
perfect gas. Excess free energy of ion was zero. Mathematics was not available
to deal consistently with flows, or interactions with ions or boundaries.
Non-classical approaches are needed because ions in biological conditions flow
and interact. The concentration gradient of one ion can drive the flow of
another, even in a bulk solution. A variational multiscale approach is needed
to deal with interactions and flow. The recently developed energetic
variational approach to dissipative systems allows mathematically consistent
treatment of bio-ions Na, K, Ca and Cl as they interact and flow. Interactions
produce large excess free energy that dominate the properties of the high
concentration of ions in and near protein active sites, channels, and nucleic
acids: the number density of ions is often more than 10 M. Ions in such crowded
quarters interact strongly with each other as well as with the surrounding
protein. Non-ideal behavior has classically been ascribed to allosteric
interactions mediated by protein conformation changes. Ion-ion interactions
present in crowded solutions--independent of conformation changes of
proteins--are likely to change interpretations of allosteric phenomena.
Computation of all atoms is a popular alternative to the multiscale approach.
Such computations involve formidable challenges. Biological systems exist on
very different scales from atomic motion. Biological systems exist in ionic
mixtures (extracellular/intracellular solutions), and usually involve flow and
trace concentrations of messenger ions (e.g., 10-7 M Ca2+). Energetic
variational methods can deal with these characteristic properties of biological
systems while we await the maturation and calibration of all atom simulations
of ionic mixtures and divalents
Investigations of Metal/Organic Interfaces and Metalation Reactions of Organic Semiconductors
Modern electronic devices are increasingly based on organic semiconductors. The performance of such devices crucially depends on the properties of the interface between the organic semiconductors and the metal contacts. Understanding the influence of the topology of the organic semiconductor’s conjugated pi-electron system on the interface interaction could greatly improve the device’s performance. Furthermore, the knowledge about reactions of heteroatomic organic semiconductors with metal atoms during electrode fabrication may lead to enhanced lifetimes of such devices. This cumulative dissertation comprises several publications and a number of so far unpublished results, addressing metal/organic interface interactions and metalation reactions of heteroatomic organic semiconductors. The properties of the interfaces are tailored by investigating the alternant aromatic molecules naphthalene and pyrene as well as the nonalternant aromatic molecules azulene and azupyrene on different metallic singlecrystal surfaces. Investigations by means of temperature-programmed desorption reveal stronger desorption energies for the non-alternant molecules on both Ag(111) and Cu(111). The biggest difference is observed on Cu(111), on which azulene and azupyrene are chemisorbed, whereas naphthalene and pyrene are physisorbed. The enhanced interface interaction of the non-alternant molecules is associated with the formation of surface dipoles that lead to stronger intermolecular repulsion between the adsorbed molecules. These results are supported by additional surface science methods, such as X-ray photoelectron spectroscopy or near-edge X-ray absorption fine structure spectroscopy, as well as density functional theory calculations conducted by group members and external collaboration partners. Detailed quantitative analysis of temperature-programmed desorption data of benzene on Cu(111) and Ag(111) yields experimental desorption energies that can be used as a benchmark for theoretical adsorption energies derived by density functional theory calculations. The interactions of metal/organic interfaces are compared with organic/inorganic interfaces in the case of pentacene and its fluorinated derivative perfluoropentacene on Au(111) as well as on bulk and two-dimensional MoS2 in a collaboration project. Organic semiconductors often interact weakly with inorganic surfaces, e.g., the thermal desorption of the first molecular layer is indistinguishable from multilayer desorption. No monolayer desorption peaks are observed as is mostly the case on metal surfaces. However, monolayer desorption of pentacene and perfluoropentacene on MoS2 occurs at significantly higher temperatures than the multilayer desorption. Detailed analysis reveals that the monolayers of both molecules are entropically stabilized. Codeposition of both molecules results in strong attractive intermolecular interactions on MoS2, while these interactions are weaker on Au(111). Metalation reactions of organic semiconductors with metal atoms, e.g., Co on tetraphenylporphyrin and Ca on alpha-sexithiophene, during interface preparation were investigated by means of hard X-ray photoelectron spectroscopy and temperature-programmed desorption mass spectrometry. The thickness of the reaction zone is changed by variation of experimental properties during interface formation. It is found that only the sample temperature during metal atom deposition and the metal atom flux in the case of Ca have an impact on the reaction depth, which is usually limited to few nanometers. In contrast to Co and Ca, Li atoms readily diffuse into the organic bulk and react
with tetraphenylporphyrin over several tens of nanometers, forming dilithium tetraphenylporphyrin or monolithium monohydrogen tetraphenylporphyrin depending on the deposited Li amount. Furthermore, the transmetalation reaction of lead(II) tetraphenylporphyrin with Cu atoms on the Cu(111) surface was proven by temperature-programmed desorption. In addition, the Ullmann coupling reaction of bromo- and iodobenzene on Cu(111) was examined. While bromobenzene molecules desorb intact from the Cu(111) surface, iodobenzene molecules dissociate into iodine atoms and phenyl radicals. The latter form biphenyl that desorbs in three distinct desorption peaks at different temperatures. In a collaborative project, the oxidation state and electronic structure of Pb atoms in the newly synthesized Pb3F8 were studied by hard X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy giving evidence for the presence of Pb(II) and Pb(IV) species. The experimental results are complemented by constructional work to improve the temperatureprogrammed desorption setup. Moreover, two Igor Pro 8 scripts were written to quickly import data from different experimental setups and speed up the data treatment
A Century
This book was published on the occasion of celebration of 100 years of IACS in 1976. This is an important and enriched document to know the formative period and history of IACS. The coverage of this book had been classified into four distinct periods: 1. The Early Perid 2. The Period of Prof. C V Raman 3. The Period of Prof. K S Krishnan 4. Post Independence PeriodFinancial assistance was given from Govt. of India & Govt. of W.B. to the Steering Committee for the Centenary Celebration of IACS under Prof. D. Basu, Directo
The 1991 research and technology report, Goddard Space Flight Center
The 1991 Research and Technology Report for Goddard Space Flight Center is presented. Research covered areas such as (1) earth sciences including upper atmosphere, lower atmosphere, oceans, hydrology, and global studies; (2) space sciences including solar studies, planetary studies, Astro-1, gamma ray investigations, and astrophysics; (3) flight projects; (4) engineering including robotics, mechanical engineering, electronics, imaging and optics, thermal and cryogenic studies, and balloons; and (5) ground systems, networks, and communications including data and networks, TDRSS, mission planning and scheduling, and software development and test
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Laboratory directed research and development annual report. Fiscal year 1994
The Department of Energy Order DOE 5000.4A establishes DOE`s policy and guidelines regarding Laboratory Directed Research and Development (LDRD) at its multiprogram laboratories. This report represents Pacific Northwest Laboratory`s (PNL`s) LDRD report for FY 1994. During FY 1994, 161 LDRD projects were selected for support through PNL`s LDRD project selection process. Total funding allocated to these projects was 35K or less. The projects described in this report represent PNL`s investment in its future and are vital to maintaining the ability to develop creative solutions for the scientific and technical challenges faced by DOE and the nation. The report provides an overview of PNL`s LDRD program, the management process used for the program, and project summaries for each LDRD project
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