Browsing by Author "Marco Buongiorno Nardelli, Committee Member"

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Atomic Structure Optical Properties and Electron Transport in Self-assembled Monolayers on Surfaces
(2007-07-18) Wang, Shuchun; Marco Buongiorno Nardelli, Committee Member; Wenchang Lu, Committee Member; Jerry Bernholc, Committee Chair; Chris Roland, Committee Member; Zhilin Li, Committee Member
Adsorbate-induced modification of semiconductor or metal surfaces creates a nano-scale quantum structure which offers a rich vein of exotic physical phenomena for investigation. Human desire to harness these properties for technological or scientific purposes has led to extensive experimental and theoretical investigations. This dissertation focuses on the ab initio simulations of atomic, electronic, optical, and transport properties of nano-scale systems. The calculated results for indium nanowires on the Si(111) surface identify their atomic structure and reveal a phase transition at low temperature. Transport simulations on the self-assembled monolayer of ferrocenyl-alkanethiolate on Au(111) surface show negative difference resistance, which is in very good agreement with experimental observations. This opens a new opportunity for applications in nanoscale molecular devices.
Effect of Confinement on Chemical Reactions
(2008-09-17) Santiso, Erik Emilio; Keith E. Gubbins, Committee Chair; Marco Buongiorno Nardelli, Committee Member; Carol K. Hall, Committee Member; H. Henry Lamb, Committee Member
Electronic Structure Calculations of Bi2Te3/Sb2Te3 Superlattices for Thermoelectric Applications
(2009-11-13) Krishnan, Mandayam Gomatam; Wengchang Lu, Committee Member; Marco Buongiorno Nardelli, Committee Member; Mark Johnson, Committee Member; Michael Paesler, Committee Member
The electronic structure of Bi2Te3/Sb2Te3/BiSbTe3 in quintuple layers is calculated at the atomic level for applications in thermo electric applications of micro refrigeration and power generation. The trigonal lattice structure is used for these combinations of materials in bulk and the hexagonal lattice approximation is used for quintuple layers of these materials (Te1- Bi-Te2-Bi-Te1) for super-lattice structures. Electronic Structure Calculations on various combinations of alternating quintuple layers (1:1, 1:2, 2:1, 1:3, 3:1, 1:4, 2:3, 3:2) are made using Local Density Approximations (LDA) for band gaps and charge distributions and the results compare well with other published methods such as Linearized Augmented Plane Waves (LAPW) used as a reference. The results for (2:1, 1:3, 3:1, 1:4, 2:3, 3:2) super lattices are new for this work without any references.
First Principles Studies of Interface Dielectric Properties of Polymer/metal-oxide Nanocomposites}
(2009-07-29) Yu, Liping; Wenchang Lu, Committee Member; Zhilin Li, Committee Member; Marco Buongiorno Nardelli, Committee Member; Jerry Bernholc, Committee Chair
This thesis is devoted to studying interface dielectric properties of polymer nanocomposites from first principles. We aim to understand at atomic scale the role of interface effects and the dielectric finite size effects of nanoparticles in determining the effective dielectric properties of polymer nanocomposites. To study surface effects from first principles, we first investigate the two common methods, namely dipole correction and Coulomb cutoff, used to eliminate the artificial effects introduced by using the supercell approximation. We implement Coulomb cutoff technique in a plane-wave-based density functional theory code and compare it with dipole correction for the same system under the same conditions. By comparison, both methods are shown to be equivalent and able to remove the artificial effects of periodic images very accurately. We also find that a combination of these two methods offers an easy way to distinguish the localized bound states of interest from highly delocalized unoccupied states while using a relatively small supercell, and to ascertain the convergence of the results with respect to supercell size. To understand the dielectric properties at the atomic scale, we develop a new nanoscale averaging model to connect the macroscopic quantities to the corresponding microscopic ones. This model allows us to compute the spatially resolved local dielectric permittivity, including the critically important ionic contributions, for interfaces and other complex structures. In this model, a simple way of evaluating real-space decay length of the nonlocal dielectric functions is also proposed. By using the dipole correction and our averaging model in supercells, we calculate the optical and static local dielectric permittivity profiles for polymer (polypropylene) / metal-oxide (PbTiO$_3$ and alumina) nanocomposites. Our {em ab-initio} results show that metal-oxide/polymer interface effects are very localized and are mostly confined to the metal-oxide surface side, and that nanoscale metal-oxide slabs can on average retain the macroscopic value of bulk dielectric permittivity. These findings suggest that classical mixing laws associated with macroscopic composites can be applied to model the overall dielectric constant of a real polymer/metal-oxide nanocomposite system.
Mechanical and Transport Properties of Carbon Nanotube Systems
(2004-02-05) Zhao, Qingzhong; Jerry Bernholc, Committee Chair; Frank Mueller, Committee Member; Christopher Roland, Committee Member; Marco Buongiorno Nardelli, Committee Member
The mechanical and transport properties of carbon nanotube systems are studied by large-scale ab initio, tight-binding and classical molecular dynamics simulations. The ultimate strength of carbon nanotubes is investigated theoretically. While the formation energy of strain-induced topological defects determines the thermodynamic limits of the elastic response and of mechanical resistance to applied tension, it is found that the activation barriers for the formation of such defects are much larger than estimated previously. The theoretical results indicate a substantially greater resilience and strength, and show that the ultimate strength limit of carbon nanotubes has yet to be reached experimentally. Carbon nanotubes are indeed the strongest material known. The electronic transport in a new type of carbon nanotube material: carbon nanotube-metal cluster assembly is investigated for gas absorption. For an Al cluster attached to a metallic nanotube, we have observed that its electrical response dramatically changes upon NH3 adsorption onto the metal cluster. For a semiconducting nanotube-Al cluster assembly, the same gas adsorption enhances the system's conductivity. The results of our ab initio simulations explain the observed behavior in terms of interactions between the molecular species and the nanotube-cluster system, where successive charge transfers between the components tailor the electronic and transport properties. Carbon nanotubemetal cluster assemblies could be a new type of nanotube-based chemical/biological sensors.
Organizational and Interfacial Examination of Adenine Films on Ag(111): Self-Assembly and Shockley State Modification
(2009-10-14) Andrews, Katie Marie; Daniel Dougherty, Committee Member; Marco Buongiorno Nardelli, Committee Member; Thomas P. Pearl, Committee Chair
The scope of this research has been two-fold: to quantify the strength of intermolecular hydrogen bonds that dictate self-assembly of single nucleobases on a metallic platform and to identify the resultant interfacial electronic influence of an organized organic film on a surface with a Shockley-type surface state. A model system of weakly bound, physisorbed adenine molecules on the (111) face of crystalline silver has been chosen, where the dosing parameters of the molecules from a solid source are limited to the low, sub-monolayer regime when exposed to a room temperature substrate. STM topographic imaging at 300, 83, and 15 K reveal highly organized domains of two orientations composed of dimerized adenine molecules aligning with particular 2-dimensional substrate lattice vectors. In addition, this system has shown high molecular mobility and STM tip induced dissociation when the sample is either imaged at room temperature, or annealed at 340 K and subsequently imaged at 83 K. By systematically varying the bias voltage (and therefore the tunneling gap) and rescanning the same area, the small regions of alternate domains have been shown to dissociate at larger tunneling gaps than molecules in the primary domain orientation of an island. Using this qualitative study of hydrogen bond strength along with our high-resolution images, a proposed molecular structure is given with adenine dimer pair models and energy characteristics taken from the current literature. In order to probe the interfacial structure of this system and the interaction of an adsorbed nucleobase film on a surface with a known Shockley-type surface state near the Fermi level, differential conductance maps and point spectroscopy curves were taken at low temperature. The point spectra reveal an upward energetic shift of +152 meV to +82.5 Ã‚Â± 2 meV of the Shockley state when taken over adenine islands. A series of differential conductance maps taken over a range of energies shows free-electron like scattering in the film. A parabolic dispersion curve was obtained for the film regions and when compared to the experimentally measured dispersion curve over bare silver, gives an effective mass ratio between the two of m*A/Ag/m*Ag = 1.1 Ã‚Â± 0.05. Considering the characteristics of the dispersion curve and the signature of the adenine films in the point spectra, the observed interface state is believed to be a film induced modification of the native Shockley state from bare Ag(111). The chemical structure of adenine includes Ã â‚¬-orbitals protruding from the molecular plane as well as a large gap (~4 eV for the free molecule) between the HOMO and LUMO. It is therefore hypothesized that the mechanism behind the upward energetic shift of the surface state is a modification of the silver Shockley state, however, further investigation, both experimentally and theoretically, must be undertaken before conclusive remarks about the mechanism behind this phenomenon can be made.
Stereochemical Effects on the Organizational and Electronic Structure of the Tartaric Acid/Ag(111) System
(2009-08-07) Santagata, Nancy Marie; Marco Buongiorno Nardelli, Committee Member; Lin He, Committee Member; Christopher Gorman, Committee Co-Chair; Thomas P. Pearl, Committee Co-Chair
This dissertation aims to develop an understanding of the forces that drive the organization of organic molecules at metallic surfaces, namely intermolecular structure, in the limit of weak molecule-surface interactions. A model system, composed of a chiral molecule, tartaric acid (C4H6O6), and a metallic surface with unique electronic properties, Ag(111), is employed. The interfacial organizational and electronic structures of the tartaric acid/Ag(111) system has been studied in detail with low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (STM) and spectroscopy (STS), differential conductance (dI/dV) mapping, and density functional theory (DFT). Molecularly resolved STM images of both (R,R)- and (S,S)-tartaric acid on Ag(111) in the submonolayer coverage regime reveal the role of intermolecular hydrogen bonding in stereospecific domain formation. Global chirality is expressed upon the deposition of enantiopure tartaric acid; therefore, directional and anisotropic lateral interactions involving molecular chiral centers dictate domain organization despite a weak interaction with the underlying Ag(111) lattice. Further, these enantiopure films are characterized by adsorbates whose molecular axis lies parallel to the plane of the surface. In contrast, films deposited from a racemic mixture do not separate laterally into homochiral domains with global chirality. Molecularly resolved STM images, in combination with DFT simulations, confirm that these racemic films are composed of a unique paired (S,S)-/(R,R)-tartaric acid basis whose combined molecular axis is oriented perpendicular to the plane of the surface. The differences in adsorption geometry for enantiopure versus racemic tartaric acid films is therefore controlled by intermolecular interactions involving chiral centers, indicating that chirality can be utilized in the directed two-dimensional assembly of molecular components. This dissertation also describes the modification of the Ag(111) Shockley-type surface state as a signature of tartaric acid adsorbate structure. Shockley-type surface states exist on several metal surfaces and are characterized by electron confinement by the vacuum barrier on one side and a band gap in the bulk on the other. The proximity of the surface state to the Fermi level (-67 meV for this particular surface) amplifies its role in surface chemistry, bonding, and organization. The adsorption of both enantiopure and racemic tartaric acid in the submonolayer regime induces a positive shift of the Ag(111) surface state energy after the adsorption of both forms of tartaric acid. The magnitude of the shift differs, however, for films composed of either enantiopure (E(R,R) TA=813.9 Ã‚Â± 2.9 meV) or racemic (ÃŽâ€ EDL TA=54.5 Ã‚Â± 3.5 meV) domains. These film-dependent modifications of the Shockley-type surface state are attributed to unique the adsorbate geometries, which are controlled by intermolecular interactions involving chiral centers. In sum, the combined experimental and theoretical results presented herein indicate that internal molecular structure (chirality) can be exploited for the design of rational nanostructures that possess tailored structural and electronic properties.