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UNDERSTANDING AND DESIGNING COMPLEX POTENTIAL ENERGY LANDSCAPES ACROSS THE NANOSCALE
No full text211 pagesIn this thesis, I will discuss four projects I participated during my Ph.D. study, with an emphasis on understanding and designing complex energy landscape between molecules and materials across nanoscale. These research projects are organized into four chapters: Chapter 1: Designer Potential Energy Surfaces via Programmable Magnetic Interactions; Chapter 2: Influence of Pore Size on the van der Waals Interaction in Two-Dimensional Molecules and Materials; Chapter 3: Non-Additivity and Finite-Size Effects in the Polarizabilities and Dispersion Coefficients of the Fullerenes; Chapter 4: Competitive Adsorption as a Route to Area-Selective Deposition. In Chapter 1, we explore how programmable magnetostatic interactions can be used in the rational design of Potential Energy Surfaces (PES) with targeted features. We first explore the PES design space that is accessible with small patterned magnetic arrays via forward and exhaustive enumeration, and characterize the resulting PES by the number, locations, and depths of the PES critical points. This is followed by a detailed investigation into the inverse problem—identification of magnetic patterns that correspond to PES with predefined features—using simulated annealing Monte Carlo (SA-MC) methods. In doing so, we demonstrate a robust theoretical and conceptual paradigm that enables forward and inverse PES engineering with precise control over the critical points and other salient surface features, thereby paving the way towards directed self-assembly using programmable magnetic interactions. As the magnetic interactions are scale-invariant, this approach can essentially scale down to the nanoscale. In Chapter 2, we investigate the influence of void space in porous twodimensional (2D) molecules and materials systems to the van der Waals (vdW) scaling landscape [1]. Analytical and numerical models presented herein demonstrate that the mere presence of a pore leads to markedly different vdW scaling across non-asymptotic distances, with certain relative pore sizes yielding effective power laws ranging from simple monotonic decay to the formation of minima, extended plateaus, and even maxima. These models are in remarkable agreement with first-principles approaches for the 2D building blocks of covalent organic frameworks (COFs), and reveal that COF macrocycle dimers and periodic bilayers exhibit unique vdW scaling behavior that is quite distinct from their non-porous analogs. These findings extend across a range of distances relevant to the nanoscale, and represent a hitherto unexplored avenue towards governing the self-assembly of complex nanostructures from porous 2D molecules and materials. In Chapter 3, we explore the nonadditivity and finite-size effect in a series of popular fullerene molecules [2]. We compute the static isotropic polarizability series (l with l = 1, 2, 3) for the C60–C84 fullerenes using finite-field derivative techniques and density functional theory (DFT), and quantitatively assess the intrinsic non-additivity in these fundamental response properties. By comparing against classical models of the fullerenes as conducting spherical shells (or solid spheres) of uniform electron density, a detailed critical analysis of the derived effective scaling laws (α1~ N^1.2, α2~N^2.0, α3~N^2.7) demonstrates that the electronic structure of finite-sized fullerenes—a unique dichotomy of electron confinement and delocalization effects due to their quasispherical cage-like structures and encapsulated void spaces—simultaneously limits and enhances their quantum mechanical response to electric field perturbations. Corresponding frequency-dependent polarizabilities are obtained byinputting the ` series into the hollow sphere model (within the modified single frequency approximation), and used to compute the molecular dispersion coefficients (Cn with n = 6, 8, 9, 10) need to describe the non-trivial vdW interactions in fullerene-based systems. Using first-order perturbation theory in conjunction with >140,000 DFT calculations, we also computed the non-negligible zero-point vibrational contributions to 1 in C60 and C70, thereby enabling a more accurate and direct comparison between theory and experiment for these quintessential nanostructures. In Chapter 4, we explore the method of competitive adsorption in areaselective deposition (ASD) [3, 4]. ASD has the potential to enable nextgeneration manufacturing and patterning at the 5 nm node and beyond, with direct energy-related applications in solar cells, batteries, fuel cells, supercapacitors, catalysts, and sensors. Well-known for its ability to deposit atomicallythin films with Angstrom scale precision along the growth direction and conformally over complex 3D substrates, ALD has already emerged as a key process in nanomanufacturing. In this regard, the range and scope of ALD-based applications and capabilities can be substantially extended by also controlling the inplane growth timely and significant development that can be realized via area-selective deposition processes that depend on the chemical composition of the underlying surface. In this joint theoretical-experimental work (with the Engstrom Group at Cornell), competitive adsorption strategies will be leveraged to enable AS-ALD by blocking the dissociative chemisorption of the metalcontaining precursor. In this approach, the co-adsorbate must differentiate between two competing surfaces by binding more strongly to one over the other. We computationally identified a series of co-adsorbates that can induce selectivity during chemical vapor deposition (CVD) and ALD process using dispersion inclusive density functional theory, and achieved a deposition of 30 nm of a thin film on the desired growth surface in CVD, and 1:5 nm in ALD. [1] Y. Yang, K. U. Lao, and R. A. DiStasio Jr., Influence of Pore Size on the van der Waals Interaction in Two-DimensionalMolecules and Materials. Phys. Rev. Lett. 122, 026001 (2019). [2] Y. Yang, K. U. Lao, and R. A. DiStasio Jr., Electron Confinement Meet Electron Delocalization: Non-Additivity and Finite-Size Effects in the Polarizabilities and Dispersion Coefficients of the Fullerenes Phys. Chem. Chem. Phys. (2021). 4. T. Suh, Y. [3] Y.Yang, H. W. Sohn, R. A. DiStasio Jr. and J. R. Engstrom, Area-selective atomic layer deposition enabled by competitive adsorption. J. Vac. Sci. A 38, 6 (2020). [4]. T. Suh, Y. Yang, P. Zhao, K. U. Lao, H. Y. Ko, J. Wong, R. A. DiStasio Jr. and J. R. Engstrom, Competitive Adsorption as a Route to Area-Selective Deposition. ACS Appl. Mater. Interfaces 12, 9989 (2020)
Interatomic methods for the dispersion energy derived from the adiabatic connection fluctuation-dissipation theorem
Full text linkpeer reviewedInteratomic pairwise methods are currently among the most popular and accurate ways to include dispersion energy in density functional theory calculations. However, when applied to more than two atoms, these methods are still frequently perceived to be based on ad hoc assumptions, rather than a rigorous derivation from quantum mechanics. Starting from the adiabatic connection fluctuation-dissipation (ACFD) theorem, an exact expression for the electronic exchange-correlation energy, we demonstrate that the pairwise interatomic dispersion energy for an arbitrary collection of isotropic polarizable dipoles emerges from the second-order expansion of the ACFD formula upon invoking the random-phase approximation (RPA) or the full-potential approximation. Moreover, for a system of quantum harmonic oscillators coupled through a dipole-dipole potential, we prove the equivalence between the full interaction energy obtained from the Hamiltonian diagonalization and the ACFD-RPA correlation energy. This property makes the Hamiltonian diagonalization an efficient method for the calculation of the many-body dispersion energy. In addition, we show that the switching function used to damp the dispersion interaction at short distances arises from a short-range screened Coulomb potential, whose role is to account for the spatial spread of the individual atomic dipole moments. By using the ACFD formula, we gain a deeper understanding of the approximations made in the interatomic pairwise approaches, providing a powerful formalism for further development of accurate and efficient methods for the calculation of the dispersion energy. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789814
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Long-range correlation energy calculated from coupled atomic response functions
Full text linkpeer reviewedAn accurate determination of the electron correlation energy is an essential prerequisite for describing the structure, stability, and function in a wide variety of systems. Therefore, the development of efficient approaches for the calculation of the correlation energy (and hence the dispersion energy as well) is essential and such methods can be coupled with many density-functional approximations, local methods for the electron correlation energy, and even interatomic force fields. In this work, we build upon the previously developed many-body dispersion (MBD) framework, which is intimately linked to the random-phase approximation for the correlation energy. We separate the correlation energy into short-range contributions that are modeled by semi-local functionals and long-range contributions that are calculated by mapping the complex all-electron problem onto a set of atomic response functions coupled in the dipole approximation. We propose an effective range-separation of the coupling between the atomic response functions that extends the already broad applicability of the MBD method to non-metallic materials with highly anisotropic responses, such as layered nanostructures. Application to a variety of high-quality benchmark datasets illustrates the accuracy and applicability of the improved MBD approach, which offers the prospect of first-principles modeling of large structurally complex systems with an accurate description of the long-range correlation energy. (C) 2014 AIP Publishing LLC
LARGE-SCALE CONDENSED-PHASE HYBRID DENSITY FUNCTIONAL THEORY BASED AB INITIO MOLECULAR DYNAMICS CALCULATIONS WITH LINEAR SCALING EXACT EXCHANGE: AN INVESTIGATION OF PARAMETER VALUES
No full text34 pagesWithin computational chemistry inherent barriers exist when attempting to model real systems, and the primary barrier for modeling complex systems to a high degree of accuracy is the trade-off between numerical accuracy and computational expense. Large-scale condensed-phase hybrid density functional theory based ab initio molecular dynamics simulations attempt to accurately model real systems, but at high computational cost. Previous work in the DiStasio Group has produced methods to greatly increase the time efficiency of these intensive calculations, successfully reducing a calculation which typically undergoes cubic scaling with system size to a linear scaling regime. The purpose of this study is to investigate how the error of energy calculations changes by varying three parameters within the DiStasio Group’s exact exchange (EXX) program. These variables are c, a scaling factor for the dimensions of an orbital’s force domain, delta (δ), a filter for the amount of overlap required between two orbitals, and epsilon (ϵ), a tolerance factor for how well the bounding box of an orbital covers the orbital’s total electron density. Larger values for c ensure more accurate calculations since all force interactions are included in the energy calculations, but there is likely an optimal value which preserves desired numerical accuracy while reducing the computational expense of the calculation. When the sizes of δ and ϵ are too large the total energy of a system is undercalculated, so smaller values ensure a more complete energy profile. Though due to limitations in the Poisson solver which is implemented in the EXX program, additional issues with smooth calculation convergence exist when the variable δ is set below a certain threshold
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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