EFFECTS OF SURFACE STATES, DEFECTS AND DOPANTS ON THE OPTICAL AND MAGNETIC PROPERTIES OF LOW-DIMENSIONAL MATERIALS

Nanomaterials have attracted the attention of researchers from various fields due to their unique features (that are otherwise absent in the bulk) such as quantum confinement, high surface to volume ratio, ability for surface modification etc. Since the discovery of fullerenes and carbon nanotubes, several synthesis techniques have been developed for nanomaterial growth. However, different control parameters in different synthesis techniques often result in nanostructures with varying defects that may alter their fundamental behavior. Such defects or disorder in the crystal lattice can lead to the disruption of lattice symmetry. The defect-induced symmetry lowering (or breaking) effects play a vital role in the determination of fundamental material characteristics. Thus, it is very important to characterize the defects in order to understand their effects on the nanomaterial properties. This thesis describes the effects of defects in low dimesional systems such as ZnO nanowires, graphene and carbon nanotubes are studied. Firstly, it describes the synthesis and characterization of ZnO nanostructures and discusses the effects of surface states, defects and dopants on their optical and magnetic properties. An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism were lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab-initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations. In the case of grapheme, we have observed new combination modes in the range from 1650 to 2300 cm−1 in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. A peak at 1860 cm−1 (iTALO−) is observed due to a combination of the in-plane transverse acoustic (iTA) and the longitudinal optical (LO) phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent for bulk graphite. The overtone of the out-of-plane transverse optical (oTO) phonon at 1750 cm−1, also called the M band, is suppressed for both SLG and IBLG. In addition, two previously unidentified modes at 2200 and 1880 cm−1 are observed in SLG. The 2220 cm−1 (1880 cm−1) mode is tentatively assigned to the combination mode of in-plane transverse optical (iTO) and TA phonons (oTO+LO phonons) around the K point in the graphene Brillouin zone. Finally, the peak frequency of the 1880 (2220) cm−1 mode is observed to increase (decrease) linearly with increasing graphene layers. Finally, we find that the high curvature in sub-nm SWCNTs leads to (i) an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron-phonon coupling, (ii) an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and (iii) a clear observation of the IR modes

Advanced Mechanical Modeling of Nanomaterials and Nanostructures

This reprint presents a collection of contributions on the application of high-performing computational strategies and enhanced theoretical formulations to solve a wide variety of linear or nonlinear problems in a multiphysical sense, together with different experimental studies

Phononics: Manipulating heat flow with electronic analogs and beyond

The form of energy termed heat that typically derives from lattice vibrations, i.e. the phonons, is usually considered as waste energy and, moreover, deleterious to information processing. However, with this colloquium, we attempt to rebut this common view: By use of tailored models we demonstrate that phonons can be manipulated like electrons and photons can, thus enabling controlled heat transport. Moreover, we explain that phonons can be put to beneficial use to carry and process information. In a first part we present ways to control heat transport and how to process information for physical systems which are driven by a temperature bias. Particularly, we put forward the toolkit of familiar electronic analogs for exercising phononics; i.e. phononic devices which act as thermal diodes, thermal transistors, thermal logic gates and thermal memories, etc.. These concepts are then put to work to transport, control and rectify heat in physical realistic nanosystems by devising practical designs of hybrid nanostructures that permit the operation of functional phononic devices and, as well, report first experimental realizations. Next, we discuss yet richer possibilities to manipulate heat flow by use of time varying thermal bath temperatures or various other external fields. These give rise to a plenty of intriguing phononic nonequilibrium phenomena as for example the directed shuttling of heat, a geometrical phase induced heat pumping, or the phonon Hall effect, that all may find its way into operation with electronic analogs.Comment: 24 pages, 16 figures, modified title and revised, accepted for publication in Rev. Mod. Phy

Advances in Mechanical Metamaterials for Vibration Isolation: A Review

The adverse effect of mechanical vibration is inevitable and can be observed in machine components either on the long- or short-term of machine life-span based on the severity of oscillation. This in turn motivates researchers to find solutions to the vibration and its harmful influences through developing and creating isolation structures. The isolation is of high importance in reducing and controlling the high-amplitude vibration. Over the years, porous materials have been explored for vibration damping and isolation. Due to the closed feature and the non-uniformity in the structure, the porous materials fail to predict the vibration energy absorption and the associated oscillation behavior, as well as other the mechanical properties. However, the advent of additive manufacturing technology opens more avenues for developing structures with a unique combination of open, uniform, and periodically distributed unit cells. These structures are called metamaterials, which are very useful in the real-life applications since they exhibit good competence for attenuating the oscillation waves and controlling the vibration behavior, along with offering good mechanical properties. This study provides a review of the fundamentals of vibration with an emphasis on the isolation structures, like the porous materials (PM) and mechanical metamaterials, specifically periodic cellular structures (PCS) or lattice cellular structure (LCS). An overview, modeling, mechanical properties, and vibration methods of each material are discussed. In this regard, thorough explanation for damping enhancement using metamaterials is provided. Besides, the paper presents separate sections to shed the light on single and 3D bandgap structures. This study also highlights the advantage of metamaterials over the porous ones, thereby showing the future of using the metamaterials as isolators. In addition, theoretical works and other aspects of metamaterials are illustrated. To this end, remarks are explained and farther studies are proposed for researchers as future investigations in the vibration field to cover the weaknesses and gaps left in the literature

Physics-Based Modeling of Material Behavior and Damage Initiation in Nanoengineered Composites

abstract: Materials with unprecedented properties are necessary to make dramatic changes in current and future aerospace platforms. Hybrid materials and composites are increasingly being used in aircraft and spacecraft frames; however, future platforms will require an optimal design of novel materials that enable operation in a variety of environments and produce known/predicted damage mechanisms. Nanocomposites and nanoengineered composites with CNTs have the potential to make significant improvements in strength, stiffness, fracture toughness, flame retardancy and resistance to corrosion. Therefore, these materials have generated tremendous scientific and technical interest over the past decade and various architectures are being explored for applications to light-weight airframe structures. However, the success of such materials with significantly improved performance metrics requires careful control of the parameters during synthesis and processing. Their implementation is also limited due to the lack of complete understanding of the effects the nanoparticles impart to the bulk properties of composites. It is common for computational methods to be applied to explain phenomena measured or observed experimentally. Frequently, a given phenomenon or material property is only considered to be fully understood when the associated physics has been identified through accompanying calculations or simulations. The computationally and experimentally integrated research presented in this dissertation provides improved understanding of the mechanical behavior and response including damage and failure in CNT nanocomposites, enhancing confidence in their applications. The computations at the atomistic level helps to understand the underlying mechanochemistry and allow a systematic investigation of the complex CNT architectures and the material performance across a wide range of parameters. Simulation of the bond breakage phenomena and development of the interface to continuum scale damage captures the effects of applied loading and damage precursor and provides insight into the safety of nanoengineered composites under service loads. The validated modeling methodology is expected to be a step in the direction of computationally-assisted design and certification of novel materials, thus liberating the pace of their implementation in future applications.Dissertation/ThesisDoctoral Dissertation Aerospace Engineering 201

Studies on the electrical transport properties of carbon nanotube composites

This work presents a probabilistic approach to model the electrical transport properties of carbon nanotube composite materials. A pseudo-random generation method is presented with the ability to generate 3-D samples with a variety of different configurations. Periodic boundary conditions are employed in the directions perpendicular to transport to minimize edge effects. Simulations produce values for drift velocity, carrier mobility, and conductivity in samples that account for geometrical features resembling those found in the lab. All results show an excellent agreement to the well-known power law characteristic of percolation processes, which is used to compare across simulations. The effect of sample morphology, like nanotube waviness and aspect ratio, and agglomeration on charge transport within CNT composites is evaluated within this model. This study determines the optimum simulation box-sizes that lead to minimize size-effects without rendering the simulation unaffordable. In addition, physical parameters within the model are characterized, involving various density functional theory calculations within Atomistix Toolkit. Finite element calculations have been performed to solve Maxwell\u27s Equations for static fields in the COMSOL Multiphysics software package in order to better understand the behavior of the electric field within the composite material to further improve the model within this work. The types of composites studied within this work are often studied for use in electromagnetic shielding, electrostatic reduction, or even monitoring structural changes due to compression, stretching, or damage through their effect on the conductivity. However, experimental works have shown that based on various processing techniques the electrical properties of specific composites can vary widely. Therefore, the goal of this work has been to form a model with the ability to accurately predict the conductive properties as a function physical characteristics of the composite material in order to aid in the design of these composites

INVESTIGATION OF ADSORPTION, REACTION AND CONFINEMENT OF MOLECULES IN SINGLE WALL CARBON NANOTUBES

Adsorption of simple molecules (CF4, Xe, CO2, NO and H2O) inside single wall carbon nanotubes has been investigated by means of infrared spectroscopy. It was demonstrated that confinement has a profound effect of the IR spectra of the internally adsorbed species. The spectral changes relate to the enhanced binding of the adsorbates to the nanotube interior walls and to the spatial limitations that prohibit formation of bulk-like structures.It was found that CF4 exhibits a 15 cm-1 redshift in its í3 symmetric stretching mode when adsorbed on the exterior surface of closed SWNTs. Adsorption on the nanotube is accompanied by adsorption in the interior in the case of opened SWNTs and the í3 mode is redshifted 35 cm-1. In addition it was shown that confined CF4 does not exhibit LO-TO splitting observed in the bulk phase. Physisorption of NO inside of carbon nanotubes results in cis-(NO)2 dimer formation for almost all adsorbed NO, indicating that confinement shifts the equilibrium according to Le Chatelier's Principle. In all cases Xe was used as a displacing agent to verify the internal adsorption. It was shown that Xe preferentially adsorbs inside nanotube displacing high coverage CF4 molecules. The externally bound adsorbates do not form a full monolayer and therefore Xe adsorbs non-competitively on empty external sites. Confinement of H2O in the nanotube interior leads to appearance of a sharp mode at 3507 cm-1 that is indicative of a weaker hydrogen bond relative to hydrogen bonding in bulk ice. Molecular simulations show that the confined water forms stacked ring structures with bulk-like intra-ring and weaker inter-ring hydrogen bonds. The spectroscopy studies of adsorption in nanotubes were accompanied by nitrogen volumetric adsorption studies of bulk nanotubes. It was demonstrated that n-nonane can be utilized as a nanotube interior blocking agent. The oxidation of SWNTs by ozone, followed by heating to remove oxidized carbon atoms as carbon oxides occurs preferentially on the outer surface of bulk samples of nanotubes. The high surface reactivity of O3 at the outer surface of a bulk nanotube sample causes this effect.It was found that CF4 exhibits a 15 cm-1 redshift in its í3 symmetric stretching modewhen adsorbed on the exterior surface of closed SWNTs. Adsorption on the nanotube isaccompanied by adsorption in the interior in the case of opened SWNTs and the í3 mode isredshifted 35 cm-1. In addition it was shown that confined CF4 does not exhibit LO-TO splittingobserved in the bulk phase.Physisorption of NO inside of carbon nanotubes results in cis-(NO)2 dimer formation foralmost all adsorbed NO, indicating that confinement shifts the equilibrium according to LeChatelier's Principle.In all cases Xe was used as a displacing agent to verify the internal adsorption. It wasshown that Xe preferentially adsorbs inside nanotube displacing high coverage CF4 molecules.The externally bound adsorbates do not form a full monolayer and therefore Xe adsorbs noncompetitivelyon empty external sites