Electronic, optical, mechanical and thermoelectric properties of graphene

Graphene, a two-dimensional allotrope of graphite with sp2 bonded carbon atoms, is arranged in honeycomb structure. Its quasi one-dimensional form is graphene nanoribbon (GNR). Graphene related materials have been found to display excellent electronic, chemical, mechanical properties along with uniquely high thermal conductivity, electrical conductivity and high optical transparency. With excellent electrical characteristics such as high carrier transport properties, quantum Hall effect at room temperature and unusual magnetic properties, graphene has applications in optoelectronic devices. Electronically, graphene is a zero bandgap semiconductor making it essential to tailor its structure for obtaining specific band structure. Narrow GNRs are known to open up bandgap and found to exhibit variations for different chiralities i.e., armchair and zigzag. Doping graphene, with p- or n- type elements, is shown to exhibit bandgap in contrast to pristine graphene. In this study, optical properties including dielectric functions, absorption coefficient, transmittance, and reflectance, as a function of wavelength and incident energy, are studied. Refractive index and extinction coefficient of pristine graphene are presented. A key optical property in the infrared region, emissivity, is studied as a function of wavelength for various multilayered configurations having graphene as one of the constituent layers. Application of such a structure is in the fabrication of a Hot Electron Bolometer (a sensor that operates on the basis of temperature-dependent electrical resistance). Graphene is found to have very high elastic modulus and intrinsic strength. Nanoindentation of graphene sheet is simulated to study the force versus displacement curves. Effects of variation of diameter of indenter, speed of indentation and number of layers of graphene on the mechanical properties are presented. Shrinking size of electronic devices has led to an acute need for thermal management. This prompted the study of thermoelectric (TE) effects in graphene based systems. TE devices are finding applications in power generation and solid state refrigeration. This study involves analyzing the electronic, thermal and electrical transport properties of these systems. Electronic thermal conductivity, of graphene based systems (κe), is found to be negligible as compared to its phonon-induced lattice thermal conduction (κp). Variations in κp of graphene and GN Rs are evaluated as a function of their width and length of their edges, chiralities, temperature, and number of layers. The interdependence of transport parameters, i.e., electrical conductivity (σ), thermoelectric power (TEP) or Seebeck coefficient (S), and κ of graphene are discussed. The thermoelectric performance of these materials is determined mainly by a parameter called Figure-of-Merit. Effective methods to optimize the value of Figure-of-Merit are explored. Reducing the thermal conductivity and increasing the power factor of these systems are found to improve the Figure-of-Merit significantly. This involves correlation of structure and transport properties. Effects of doping on σ, κ and Hall coefficient are discussed

Mechanical properties of one-dimensional nanostructures, experimental measurement and numerical simulation

One-Dimensional (1D) nanostructures are generally defined as having at least one dimension between 1 and 100 nm. Investigations of their mechanical properties are important from both fundamental study and application point of view. Different methods such as in-situ tensile test and Atomic Force Microscopy (AFM) bending test have been used to explore the mechanical properties of 1D nanostructures. However, searching for reliable measurement of 1D nanostructures is still under way. In this dissertation, two methods, Atomic Force Acoustic Microscopy (AFAM)-based method and nanoindentation, were explored to realize reliable study of mechanical properties of two kinds of energy conversion-related nanomaterials: single crystalline rutile TiO 2 nanoribbons and alkaline earth metal hexaboride MB6 (M=Ca, Sr, Ba) 1D nanostructures. The work principle of AFAM-based method is: while an AFM cantilever is in contact with a tested nanostructure, its contact resonance frequencies are different from its free resonance frequencies. The cantilever resonant frequency shift is correlated to the Young's modulus of the tested nanostructure based on Hertz contact mechanics. The measured modulus of BaB6 nanostructures was 129 GPa, which is much lower than the value determined using the nanoindentation method. Due to the small load (120 nN) applied on the nanostructure during the experiment, the AFAM-based method may actually measure the mechanical property of the outside oxidation layers of BaB6 nanostructures. Nanoindentation is capable of giving insights to both Young's modulus and hardness of bulk elastic-plastic materials. The assumptions behind this method are that the material being tested is a homogeneous half-space. Cares must be taken to extract properties of tested materials when those assumptions are broken down. Nanoindentation on a 1D nanostructure is one of such cases that those assumptions are invalid. However, this invalidity was not realized in most published work on nanoindentation of 1D nanostructures, resulting in unreliable data on mechanical properties of 1D nanostructures. In this work, factors which could affect measured nanostructure-on-substrate system modulus such as the selection of a substrate to support the nanostructure, the cross section of a nanostructure, the width-to-thickness ratio (or diameter) of a nanostructure, and the nanostructure-substrate contact mechanism were first subjected to a systematic experimental investigation. A Finite Element Modeling (FEM)-based data inverse analysis process was then proposed to extract the intrinsic modulus of nanostructures from measured system modulus. This data inverse process solved the intrinsic modulus of nanostructures by equalizing the simulated nanostructure-on-substrate modulus with the experimentally measured system modulus. In finite element simulation, another important aspect: the experimental indenter area function in addition to aforementioned other factors was carefully considered. Based on systematic experimental and numerical investigations, the Young's modulus of rutile TiO2 nanoribbons, CaB6 nanostructures, SrB6 nanostructures and BaB6 nanostructures was determined to be 360, 175-365, 300-425 and 270-475 GPa, respectively. These numbers are the first reported mechanical properties for these nanomaterials. Besides the finite element simulation, an "analytical" solution to obtain a nanostructure-on-substrate system modulus is also presented. Compared to the finite element simulation, the solution could significantly reduce processing time for the data inverse method. It is applicable to a nanostructure with a width to thickness ratio larger than 4. This part of dissertation work clearly demonstrates that both experimental and numerical investigations are needed for studying of mechanical properties of 1D nanostructures by nanoindentation

Sub-Nanometer Channels Embedded in Two-Dimensional Materials

Two-dimensional (2D) materials are among the most promising candidates for next-generation electronics due to their atomic thinness, allowing for flexible transparent electronics and ultimate length scaling. Thus far, atomically-thin p-n junctions, metal-semiconductor contacts, and metal-insulator barriers have been demonstrated. While 2D materials achieve the thinnest possible devices, precise nanoscale control over the lateral dimensions is also necessary. Here, we report the direct synthesis of sub-nanometer-wide 1D MoS2 channels embedded within WSe2 monolayers, using a dislocation-catalyzed approach. The 1D channels have edges free of misfit dislocations and dangling bonds, forming a coherent interface with the embedding 2D matrix. Periodic dislocation arrays produce 2D superlattices of coherent MoS2 1D channels in WSe2. Using molecular dynamics simulations, we have identified other combinations of 2D materials where 1D channels can also be formed. The electronic band structure of these 1D channels offer the promise of carrier confinement in a direct-gap material and charge separation needed to access the ultimate length scales necessary for future electronic applications.Comment: 22 pages main manuscript and methods, 4 main figures, 30 pages supplementary materials, 16 extended figure

Surface softening in metal-ceramic sliding contacts: An experimental and numerical investigation

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation

Avoidance and Coalescence of Delamination Patterns

Delamination of coatings and thin films from substrates generates a fascinating variety of patterns, from circular blisters to wrinkles and labyrinth domains, in a way that is not completely understood. We report on large-scale numerical simulations of the universal problem of avoidance and coalescence of delamination wrinkles, focusing on a case study of graphene sheets on patterned substrates. By nucleating and growing wrinkles in a controlled way, we are able to characterize how their interactions, mediated by long-range stress fields, determine their formation and morphology. We also study how the interplay between geometry and stresses drives a universal transition from conformation to delamination when sheets are deposited on particle-decorated substrates. Our results are directly applicable to strain engineering of graphene and also uncover universal phenomena observed at all scales, as for example in geomembrane deposition

Predicting the Mechanical Properties of Nanocomposites Reinforced with 1-D, 2-D and 3-D Nanomaterials

Materials with features at the nanoscale can provide unique mechanical properties and increased functionality when included as part of a nanocomposite. This dissertation utilizes computational methods at multiple scales, including molecular dynamics (MD) and density functional theory (DFT), and the coupled atomistic and discrete dislocation multiscale method (CADD), to predict the mechanical properties of nanocomposites possessing nanomaterials that are either 1-D (carbyne chains), 2-D (graphene sheets), or 3-D (Al/amorphous-Si core-shell nanorod). The MD method is used to model Ni-graphene nanocomposites. The strength of a Ni-graphene nanocomposite is found to improve by increasing the gap between the graphene sheet and a crack embedded in the Ni matrix. Ni-graphene nanocomposites also show substantially greater strength than pure Ni, depending on the loading direction and crack orientation relative to the graphene sheet. Moreover, polycrystalline graphene may serve as a better reinforce in Ni-graphene nanocomposites due to its improved interfacial shear stress with the Ni matrix compared to pristine graphene. This work develops a patchwork quilt method for generating polycrystalline graphene sheets for use in MD models. Carbyne-based nanocomposites are modeled from first principles using DFT. This research finds that carbyne can only serve as an effective reinforcement in Ni-based nanocomposites when it is dielectrically screened from the Ni matrix, otherwise the carbyne structure is lost. When graphene is used as a dielectric screen, the local stiffness of the nanocomposite improves with the number of carbyne chains present. Specific stiffness is introduced as an alternative to elastic stiffness for characterizing low-dimensional materials because it is not dependent on volume when derived using an energy vs. strain relation. A two-material formulation of CADD is developed to model Al/a-Si core-shell nanorods under indentation/retraction. The structural deformation behavior is found to be dependent on the geometry of both core and shell. When present, the a-Si shell protects the Al core by delocalizing forces produced by the indenter. It is also found that substrate deformation becomes important for core-shell structures with sufficiently small cores. This work can help guide experimental and computational work related to the discussed 1-D, 2-D and 3-D nanomaterials and aid in future nanocomposite design

Thin Film Mechanics

This doctoral thesis details the methods of determining mechanical properties of two classes of novel thin films suspended two-dimensional crystals and electron beam irradiated microfilms of polydimethylsiloxane (PDMS). Thin films are used in a variety of surface coatings to alter the opto-electronic properties or increase the wear or corrosion resistance and are ideal for micro- and nanoelectromechanical system fabrication. One of the challenges in fabricating thin films is the introduction of strains which can arise due to application techniques, geometrical conformation, or other spurious conditions. Currently, inadequate models exist to model strain within thin films, making it difficult to produce structurally robust thin films and to prevent premature failure of a coating ordevice. It is thus imperative to understand and quantify thin film behavior under strain, both to aid in the development of new materials and processing techniques, as well as to enable the implementation of thin films into new designs. Chapters 2-4 focus on two dimensional materials. This is the intrinsic limit of thin films-being constrained to one atomic or molecular unit of thickness. These materials have mechanical, electrical, and optical properties ideal for micro- and nanoelectromechanical systems with truly novel device functionality. As such, the breadth of applications that can benefit from a treatise on two dimensional film mechanics is reason enough for exploration. This study explores the anomylously high strength of two dimensional materials. Furthermore, this work also aims to bridge four main gaps in the understanding of material science: bridging the gap between ab initio calculations and finite element analysis, bridging the gap between ab initio calculations and experimental results, nano scale to microscale, and microscale to mesoscale. A nonlinear elasticity model is used to determine the necessary elastic constants to define the strain-energy density function for finite strain. Then, ab initio calculations-density functional theory-is used to calculate the nonlinear elastic response. Chapter 2 focuses on validating this methodology with atomic force microscope nanoindentation on molybdenum disulfide. Chapter 3 explores the convergence criteria of three density functional theory solvers to further verify the numerical calculations. Chapter 4 then uses this model to investigate the role of grain boundaries on the strength of chemical vapor deposited graphene. The results from these studies suggest that two dimensional films have remarkably high strength-reaching the intrinsic limit of molecular bonds. Chapter 5 explores the viscoelastic properties of heterogeneous polydimethylsiloxane (PDMS) microfilms through dynamic nanoindentation. PDMS microfilms are irradiated with an electron beam creating a 3 m-thick film with an increased cross-link density. The change in mechanical properties of PDMS due to thermal history and accelerator have been explored by a variety of tests, but the effect of electron beam irradiation is still unknown. The resulting structure is a stiff microfilm embedded in a soft rubber with some transformational strain induced by the cross-linking volume changes. Chapter 5 employs a combination of dynamic nanoindentation and finite element analysis to determine the change in stiffness as a function of electron beam irradiation. The experimental results are compared to the literature. The results of these experimental and numerical techniques provide exciting opportunities in future research. Two dimensional materials and flexible thin films are exciting materials for novel applications with new form factors, such as flexible electronics and microfluidic devices. The results herein indicate that you can accurately model the strength of two dimsensional materials and that these materials are robust against nano-scale defects. The results also reveal local variation of mechanical properties in PDMS microfilms. This allows one to design substrates that flex with varying amounts of strain on the surface. Combining the mechanics of two dimensional materials with that of a locally irradiated PDMS film could achieve a new class of flexible microelectromechanical systems. Large-scale growth of two dimensional materials will be structurally robust-even in the presence of nanostructural defects-and PDMS microfilms can be irradiated to vary strain of the electromechanical systems. These systems could be designed to investigate electromechanical coupling in two dimensional films or for a substitute to traditional silicon microdevices

Mechanical properties of water desalination and wastewater treatment membranes

Applications of membrane technology in water desalination and wastewater treatment have increased significantly in the past few decades due to its many advantages over other water treatment technologies. Water treatment membranes provide high flux and contaminant rejection ability and require good mechanical strength and durability. Thus, assessing the mechanical properties of water treatment membranes is critical not only to their design, but also for studying their failure mechanisms, including the surface damage, mechanical and chemical ageing, delamination and loss of dimensional stability of the membranes. The various experimental techniques to assess the mechanical properties of wastewater treatment and desalination membranes are reviewed. Uniaxial tensile test, bending test, dynamic mechanical analysis, nanoindentation and bursting tests are the most widely used mechanical characterization methods for water treatment membranes. Mechanical degradations induced by fouling, chemical cleaning as well as membrane delamination are then discussed. Moreover, in order to study the membranes mechanical responses under similar loading conditions, the stress-state of the membranes are analyzed and advanced mechanical testing approaches are proposed. Some perspectives are highlighted to study the structure-properties relationship for wastewater treatment and water desalination membranes

Doctor of Philosophy

dissertationUltra-thin multilayer diamond-like carbon (DLC) coatings are used in precision engineering applications (including hard disk drives (HDDs)); their high hardness, chemical stability, and low friction coefficient in a range of environments allow protecting delicate substrate materials from damage, wear, and corrosion. A critical challenge when designing ultra-thin DLC coatings is understanding how they deform and delaminate from the substrate as a function of operating and coating design parameters including coating layer thickness and composition. We use molecular dynamics simulations of the ultra-thin multilayer DLC coatings used in HDD recording heads, which consist of stacked layers of DLC and amorphous silicon (a-Si) on a Ni substrate, to quantify the effect of coating design parameters on the mechanical properties of the coating, plastic deformation of the substrate, and adhesion of the coating to the substrate. Based on the physical understanding gained from the simulations we derive design guidelines for ultra-thin multilayer DLC coatings. We find that the hardness and Young's modulus of the coating increase with increasing DLC layer thickness and decreasing a-Si layer thickness because DLC and a-Si are the hardest and softest materials in the coating, respectively. We observe that plastic deformation of the Ni substrate for a constant mechanical load increases with increasing coating hardness because plastic deformation is increasingly preferential to the substrate with increasing coating hardness, causing the DLC coating to bend like a plate into the plastically deformed substrate. We show that the presence of an intermediate a-Si layer is critical for improving adhesion of the DLC coating to the Ni substrate because bonding between Ni and DLC distorts the Ni lattice more than bonding between a-Si and Ni. Similarly, we observe that that an intermediate layer comprised of low sp3-fraction DLC improves adhesion of high sp3-fraction DLC to Si but not Ni substrates compared to coatings without an intermediate layer. For coatings with an intermediate a-Si layer, adhesion improves with decreasing a-Si layer thickness because less a-Si is present to plastically deform during loading, which displaces the coating failure region from the a-Si layer into the Ni substrate

Enhancing the Stretchability of Two-Dimensional Materials through Kirigami: A Molecular Dynamics Study on Tungsten Disulfide

In recent years, the 'kirigami' technique has gained significant attention for creating meta-structures and meta-materials with exceptional characteristics, such as unprecedented stretchability. These properties, not typically inherent in the original materials or structures, present new opportunities for applications in stretchable electronics and photovoltaics. However, despite its scientific and practical significance, the application of kirigami patterning on a monolayer of tungsten disulfide (WS2), a van der Waals material with exceptional mechanical, electronic, and optical properties, has remained unexplored. This study utilizes molecular dynamics (MD) simulations to investigate the mechanical properties of monolayer WS2 with rectangular kirigami cuts. We find that, under tensile loading, the WS2 based kirigami structure exhibits a notable increase in tensile strain and a decrease in strength, thus demonstrating the effectiveness of the kirigami cutting technique in enhancing the stretchability of monolayer WS2. Additionally, increasing the overlap ratio enhances the stretchability of the structure, allowing for tailored high strength or high strain requirements. Furthermore, our observations reveal that increasing the density of cuts and reducing the length-to-width ratio of the kirigami nanosheet further improve the fracture strain, thereby enhancing the overall stretchability of the proposed kirigami patterned structure of WS2.Comment: 19 pages, 5 figure