<p>A high-pressure CO2 foam was generated with silica nanoparticle dispersion and CO2 for fracturing applications. The effects of different ions and temperature on nanoparticle aggregation were studied. Nanoparticle dispersions were mixed with individual monovalent, divalent ions with varying concentrations, and two synthesized Permian connate water solutions. Samples of nanoparticle dispersions with the presence of NaCl were put into chambers with constant temperature for 14 hours. The peak size of aggregated nanoparticles in each sample was measured. It was found this silica nanoparticle dispersion had a high thermal stability up to 85?. The silica nanoparticle dispersion used in this study maintained a desired stability under an 18% reservoir salinity condition, yet it could be sensitive to high concentrations of Na2SO4 solutions. To investigate foam rheology and stability, high-pressure CO2 foams were generated in a beadpack with different CO2/NP ratios in NaCl solutions. The resulting foam was observed in a sapphire tube. The differential pressure across a capillary tube was recorded to calculate the apparent viscosity of foams. Nanoparticle-stabilized foams could remain stable for days and foam stability decreased with the increasing foam quality. Foam apparent viscosity was found to increase with foam quality and could be 3 times as high as that of the ambient phase. The high stability and fine texture of high-pressure CO2-in-water foams stabilized by silica nanoparticles have broadened the development of foam fracturing, offering a new opportunity for the effective development and stimulation of unconventional reservoirs.

<p> On-chip Interconnection Networks are shifting from multicore to manycore systems and are tending to be heterogeneous with the integrated modules from different vendors of various sizes and shapes. Each module has different properties such as routers, link-width. From a system designer's perspective, making layouts of metal-wired links among interconnection modules for communication will be impractical as it increases the design cost in terms of the communication complexity and power leakage on these links. We can replace all links with wireless or optical links for high-performance, reducing latency. However, it comes with a high-cost. Therefore, we formulate the optimization model to minimize the cost (communication links between subnets) and maximize their data flows in the network-on-chip. </p><p> Since the optimization model using the optimizers such as CPLEX and Gurobi to achieve the best possible solutions, the solution time to a large set of given problems is not acceptable. Hence, we present a mincostflow-based heuristic algorithm (LINCA) that minimizes the quantification of hybrid routers corresponding to the application-specific traffic for manycore systems. LINCA guarantees the performance of hybrid networks on chip. Its results are validated against the manycore system architecture. Our evaluation shows that LINCA can significantly reduce the cost of using hybrid routers (communication links) in the manycore systems. It reduces cost by 84 percent on average across a variety of applications, compared with all of hybrid routers being deployed in the network without using the optimization model. However, we observed that the solution time of LINCA is increased exponentially for large scale networks. We then proposed an efficient predictive framework for optimized reconfiguring on-chip interconnection network. </p><p> The predictive model is built based on the optimization model and learning-based algorithms. As we wish to reduce the communication complexity of the interconnection links in the entire on-chip network, our objective is to minimize those links corresponding to the application-specific traffic demands. Thereby, the overall power dissipation can be mitigated. We believe that our approach will be an essential step when scaling out.</p><p>

<p> At present modulators in communications industry utilize non-linear materials like indium tin oxide (ITO) and DLD-164 as a dielectric, which makes the fabrication process cumbersome and expensive. This thesis discusses the possibility of using only gold and air as conductor and dielectric to characterize a signal modulating device. Both electro-absorption modulation (EAM) and phase change driven modulation is possible with the design. For the change in phase a length of 2.992 &micro;<i>m</i> for the modulating arm of a Mach-Zehnder modulator (MZM) was achieved for operation at 525 <i>nm</i>. High absorptions of electromagnetic (EM) waves was seen at the 480 <i>nm</i> mark allowing a length of just 4.95 &micro;<i>m</i> for EAM. The results suggest that an all plasmonic noble metal modulator utilizing air as a dielectric is possible for operation in the visible 400 <i>nm</i> to 700 <i>nm</i> range. The concept is supported by proof-of-principle based simulations. </p><p> This thesis proposes a novel idea of an all plasmonic modulator driven by changes in free carrier concentration in gold and surface plasmon polariton (SPP) excitations under an applied potential. The prototype model is simulated using a commercial finite difference time domain solver. The simulation enviro<i> nm</i>ent allows Maxwell&rsquo;s equations to be solved in the time domain to investigate light propagation and absorption characteristics under an externally applied electric potential. The free carrier concentration dependent permittivity of gold is exploited to investigate possible applications in nano-photonics and optical communications.</p><p>

Nanoscale materials often have stochastic material properties due to random distribution of material defects and lack of sufficient number of defects to ensure a consistent average. Current methods to measure the mechanical properties employ microelectromechanical systems based tensile loading platforms. The nanoscale specimens are typically mounted manually onto the loading platforms with external integration techniques so the boundary conditions have random variations, complicating the experimental measurement of the stochasticity in the natural state of the material properties. In this Ph. D. thesis, we show methods for batch-compatible (i.e. monolithic) integration of nanoscale specimens cofabricated with the tensile loading platforms. The specimens are gold nanowires of 40 nm thickness, 350 nm to 410 nm width (depending on the specimen), and 7-micrometer length. The uniaxial micro tensile loading platforms are interdigitated electrode electrostatic actuators. The experiments are performed in a scanning electron microscope and digital image correlation is employed to measure displacements to determine nominal stress and nominal strain. The ultimate tensile strength of the nanocrystalline gold nanowires approach 1 GPa, consistent with the smaller-is-stronger paradigm. The batch-compatible integration method is designed to microfabricate uniaxial micro tensile testing platforms that are suitable for transmission electron microscope experiments. This batch-compatible integration method is designed also to create nominally identical nanoscale specimens and boundary conditions for a broad range of nanoscale materials provided the nanoscale materials of interest are compatible with the etchants used in the microfabrication processes. Furthermore, in addition to the batch-compatible integration method, a generalized external integration method that can be applied to free-standing thin-films is developed. Using this method, mechanical behavior of single crystal gold metal, and single crystal gold-silver alloy nanoscale specimens are extracted. For the extraction of the mechanical properties, similar procedures followed for batch-compatible integrated nanoscale specimens are followed. For single crystal gold nanoscale specimen, a Young's modulus of 33.42 GPa, and ultimate tensile strength of 0.48 GPa is obtained. For single crystal gold-silver alloy nanoscale specimen, a Young's modulus of 64.47 GPa, and ultimate tensile strength of 0.67 GPa is obtained.

Made of only one sheet of carbon atoms, graphene is the thinnest yet strongest material ever exist. Since its discovery in 2004, graphene has attracted tremendous research effort worldwide. Guaranteed by the superior electrical and excellent mechanical properties, graphene is the ideal building block for NanoElectroMechanical Systems (NEMS). In the first parts of the thesis, I will discuss the fabrications and measurements of typical graphene NEMS resonators, including doubly clamped and fully clamped graphene mechanical resonators. I have developed a electrical readout technique by using graphene as frequency mixer, demonstrated resonant frequencies in range from 30 to 200 MHz. Furthermore, I developed the advanced fabrications to achieve local gate structure, which led to the real-time resonant frequency detection under resonant channel transistor (RCT) scheme. Such real-time detection improve the measurement speed by 2 orders of magnitude compared to frequency mixing technique, and is critical for practical applications. Finally, I employed active balanced bridge technique in order to reduce overall electrical parasitics, and demonstrated pure capacitive transduction of graphene NEMS resonators. Characterizations of graphene NEMS resonators properties are followed, including resonant frequency and quality factor ($Q$) tuning with tension, mass and temperatures. A simple continuum mechanics model was constructed to understand the frequency tuning behavior, and it agrees with experimental data extremely well. In the following parts of the thesis, I will discuss the behavior of graphene mechanical resonators in applied magnetic field, {i.e.} in Quantum Hall (QH) regime. The couplings between mechanical motion and electronic band structure turned out to be a direct probe for thermodynamic quantities, {i.e.}, chemical potential and compressibility. For a clean graphene resonators, with quality factors of $1 \times 10^4 $, it underwent resonant frequency oscillations as applied magnetic field increases. The chemical potential of graphene shifts smoothly within each LL, causing the resonant frequency to change in an explicit pattern. Between LLs, the finite compressibility caused the resonant frequency changing dramatically. The overall oscillations of resonant frequency with the applied magnetic field could be fitted with only disorder potential as free parameter. Compared with conventional electronic transport technique, such mechanical measurements proven to be a more direct and powerful tool, which we used o study the properties of graphene's ground states in broken symmetry states. In the last part this thesis, I will present the study of graphene NEMS oscillators with positive feedback loop. The demonstrated oscillators are self-sustained (without external radio frequency, RF, stimulus), and the oscillation frequencies can be controlled by tension{i.e.}, (applied gate voltage). I also carefully studied the influence of feedback gain and phase, as well as linewidth compression as function of temperature.

In an effort to further understand electronic and thermoelectric phenomenon at the nanometer scale, we have studied the transport properties of single molecule junctions. To carry out these transport measurements, we use the scanning tunneling microscope-break junction (STM-BJ) technique, which involves the repeated formation and breakage of a metal point contact in an environment of the target molecule. Using this technique, we are able to create gaps that can trap the molecules, allowing us to sequentially and reproducibly create a large number of junctions. By applying a small bias across the junction, we can measure its conductance and learn about the transport mechanisms at the nanoscale. The experimental work presented here directly probes the transmission properties of single molecules through the systematic measurement of junction conductance (at low and high bias) and thermopower. We present measurements on a variety of molecular families and study how conductance depends on the character of the linkage (metal-molecule bond) and the nature of the molecular backbone. We start by describing a novel way to construct single molecule junctions by covalently connecting the molecular backbone to the electrodes. This eliminates the use of linking substituents, and as a result, the junction conductance increases substantially. Then, we compare transport across silicon chains (silanes) and saturated carbon chains (alkanes) while keeping the linkers the same and find a stark difference in their electronic transport properties. We extend our studies of molecular junctions by looking at two additional aspects of quantum transport - molecular thermopower and molecular current-voltage characteristics. Each of these additional parameters gives us further insight into transport properties at the nanoscale. Evaluating the junction thermopower allows us to determine the nature of charge carriers in the system and we demonstrate this by contrasting the measurement of amine-terminated and pyridine-terminated molecules (which exhibit hole transport and electron transport, respectively). We also report the thermopower of the highly conducting, covalently bound molecular junctions that we have recently been able to form, and learn that, because of their unique transport properties, the junction power factors, GS2, are extremely high. Finally, we discuss the measurement of molecular current-voltage curves and consider the electronic and physical effects of applying a large bias to the system. We conclude with a summary of the work discussed and an outlook on related scientific studies.

The use of nanoparticles has grown tremendously in the past 25 years in virtually all ndustries from alternative energy formulations to drug delivery development and from semiconductor fabrication to cosmetic research. The main goal of this thesis is to shed light on the surface behavior of these universally used components. The thesis explores controlling surface reactivity of nanoparticles to great detail and concludes with a proven method to organize the nanoparticles using self-assembly. The consistent and reproducible organization of the nanoparticles has tremendous value in all industries using nanoparticles in lowering production and processing costs and time.The surface reactivity of the nanoparticles is found to be well-controlled. In Chapter 3, we show a method to control alkyne reactivity on nanoparticles using a mixture of organosilane monolayers. These surfaces have the unique ability to react with azide-terminated materials through the well-known copper catalyzed azide alkyne cycloaddition `click' reaction. We also put to use a new method to ensure that the mixed surface formed is reactively homogeneous; this novel technique will sure help research endeavors as this has not been demonstrated on surfaces of nanoparticles before. We extend our surface reactivity studies with the azide-functional surface in Chapter 4. Since we were unable to create a homogeneous surface using the methods described in Chapter 3, we looked to creating a mixed monolayer by kinetic control. This involved fabrication of a bromine-terminated surface and subsequent incomplete substitution of the bromide to azide. This method of creating mixed monolayers was shown to be universally applicable to surfaces of different chemical nature and different surface geometries with the same reaction kinetics. We also contend that this method of creating mixed monolayers is novel and it provides for an additional in the control of other surface reactivity groups. Chapter 5 provides the grand finale of the thesis with the intended use of the mixed monolayers surfaces to organize nanoparticles. We show that by carefully controlling the amount of polymer chemisorbed onto a surface, the self-assembly behavior of the particles is changed. In such a method we constructed a phase diagram showing how polymeric coverage controls selfassembly. We also ensured that the phases formed were indeed equilibrated structures by studying the formation of the phases under different preparation conditions. We encountered sheets, rods, and agglomerates and were able to consistently create these structures as well as study them using image analysis.

In an effort to further understand electronic and thermoelectric phenomenon at the nanometer scale, we have studied the transport properties of single molecule junctions. To carry out these transport measurements, we use the scanning tunneling microscope-break junction (STM-BJ) technique, which involves the repeated formation and breakage of a metal point contact in an environment of the target molecule. Using this technique, we are able to create gaps that can trap the molecules, allowing us to sequentially and reproducibly create a large number of junctions. By applying a small bias across the junction, we can measure its conductance and learn about the transport mechanisms at the nanoscale. The experimental work presented here directly probes the transmission properties of single molecules through the systematic measurement of junction conductance (at low and high bias) and thermopower. We present measurements on a variety of molecular families and study how conductance depends on the character of the linkage (metal-molecule bond) and the nature of the molecular backbone. We start by describing a novel way to construct single molecule junctions by covalently connecting the molecular backbone to the electrodes. This eliminates the use of linking substituents, and as a result, the junction conductance increases substantially. Then, we compare transport across silicon chains (silanes) and saturated carbon chains (alkanes) while keeping the linkers the same and find a stark difference in their electronic transport properties. We extend our studies of molecular junctions by looking at two additional aspects of quantum transport - molecular thermopower and molecular current-voltage characteristics. Each of these additional parameters gives us further insight into transport properties at the nanoscale. Evaluating the junction thermopower allows us to determine the nature of charge carriers in the system and we demonstrate this by contrasting the measurement of amine-terminated and pyridine-terminated molecules (which exhibit hole transport and electron transport, respectively). We also report the thermopower of the highly conducting, covalently bound molecular junctions that we have recently been able to form, and learn that, because of their unique transport properties, the junction power factors, GS², are extremely high. Finally, we discuss the measurement of molecular current-voltage curves and consider the electronic and physical effects of applying a large bias to the system. We conclude with a summary of the work discussed and an outlook on related scientific studies.

The physical properties of materials at the nanoscale are controlled to a large extent by their interfaces. While much knowledge has been acquired about the properties of material in the bulk, there are many new and interesting phenomena at the interfaces that remain to be better understood. This is especially true at the scale of their constituent building blocks - atoms and molecules. Studying materials at this intricate level is a necessity at this point in time because electronic devices are rapidly approaching the limits of what was once thought possible, both in terms of their miniaturization as well as our ability to design their behavior. In this thesis I present our explorations of the interplay between mechanical properties, electronic transport and binding energetics of single atomic contacts and single-molecule junctions. Experimentally, we use a customized conducting atomic force microscope (AFM) that simultaneously measures the current and force across atomic-scale junctions. We use this instrument to study single atomic contacts of gold and silver and single-molecule junctions formed in the gap between two gold metallic point contacts, with molecules with a variety of backbones and chemical linker groups. Combined with density functional theory based simulations and analytical modeling, these experiments provide insight into the correlations between mechanics and electronic structure at the atomic level. In carrying out these experimental studies, we repeatedly form and pull apart nanoscale junctions between a metallized AFM cantilever tip and a metal-coated substrate. The force and conductance of the contact are simultaneously measured as each junction evolves through a series of atomic-scale rearrangements and bond rupture events, frequently resulting in single atomic contacts before rupturing completely. The AFM is particularly optimized to achieve high force resolution with stiff probes that are necessary to create and measure forces across atomic-size junctions that are otherwise difficult to fabricate using conventional lithographic techniques. In addition to the instrumentation, we have developed new algorithmic routines to perform statistical analyses of force data, with varying degrees of reliance on the conductance signatures. The key results presented in this thesis include our measurements with gold metallic contacts, through which we are able to rigorously characterize the stiffness and maximum forces sustained by gold single atomic contacts and many different gold-molecule-gold single-molecule junctions. In our experiments with silver metallic contacts we use statistical correlations in conductance to distinguish between pristine and oxygen-contaminated silver single atomic contacts. This allows us to separately obtain mechanical information for each of these structural motifs. The independently measured force data also provides new insights about atomic-scale junctions that are not possible to obtain through conductance measurements alone. Using a systematically designed set of molecules, we are able to demonstrate that quantum interference is not quenched in single-molecule junctions even at room temperature and ambient conditions. We have also been successful in conducting one of the first quantitative measurements of van der Waals forces at the metal-molecule interface at the single-molecule level. Finally, towards the end of this thesis, we present a general analytical framework to quantitatively reconstruct the binding energy curves of atomic-scale junctions directly from experiments, thereby unifying all of our mechanical measurements. I conclude with a summary of the work presented in this thesis, and an outlook for potential future studies that could be guided by this work.

Metal-molecule-metal junctions have become a widely used test-bed for the study of nanoscale electronic phenomena. Single-molecule junctions in particular have provided a deeper understanding of charge transport across interfaces, and single-molecule electronic components have been proposed as a successor for silicon technology. This thesis presents an experimental approach for controlling the electronic properties of single-molecule junctions by manipulating the environment about the junction. With this tunable functionality, we are able to demonstrate single-molecule variants of transistors and diodes. We begin our work by probing charge transport through single-oligomers of commonly used molecules in organic electronic devices. We focus on these systems due to their narrow band gaps, giving them the potential for exhibiting high molecular conductances. Single-molecule junctions are formed using the Scanning Tunneling Microscope-based break junction (STM-BJ) technique. We first consider a family of oligothiophenes, ranging in length from 1 to 6 units. We find that this family of molecules exhibits an anomalous conductance decay with molecular length; this is mainly due to conformational effects. These conformational effects also result in very broad conductance distributions, further preventing oligothiophenes from being useful in molecular electronic devices. However, we find that thiophene dioxides are particularly well-suited for single-molecule devices, primarily due to exceptionally narrow band gaps. Oligothiophene dioxides also constitute a unique system where the dominant conductance orbital changes with molecular length. Specifically, we find that the shorter oligomers have transport dominated by the highest occupied molecular orbital (hole-type transport), but longer oligomers have transport dominated by the lowest unoccupied molecular orbital (electron-type transport). We next demonstrate a method for gating single-molecule junctions. In order to over- come the difficulty of lithographically defining a gate electrode in close enough proximity to the molecular junction so that the gate voltage impacts the electrostatics of the junction, we turn to measurements in electrolytic solutions. Ions in these solutions form compact layers of charge at metal surfaces, and these electric double layers can be controlled by the gate electrode; such electrolytic gating results in high gating efficiencies. Using this technique, we show that we are able to continuously modulate the conductance of non-redox active molecular junctions. Using ionic environments, we next develop a new technique for creating a single-molecule diode. Performing break junction measurements in electrolytic solutions without the presence of a gate electrode, we show that we still have control of the junction’s electrostatic environment. In particular, if the source and drain electrodes are of considerably different areas, we find that we asymmetrically control this environment. Using this technique, we demonstrate single-molecule diodes created from otherwise symmetric molecular junctions. Combining this with measurements on thiophene dioxide oligomers, we show single-molecule diodes with the highest reported rectification ratios to date. This technique has the potential for application in nano-scale systems beyond single-molecule junctions. These results constitute another step toward the development of single-molecule devices with commercial applications. Finally, the methods presented in this thesis offer further insights into the electronic structure of molecular junctions. We show that we can assess energy-level alignment at metal molecule interfaces– this alignment is a crucial parameter controlling the proper- ties of the interface. We also demonstrate that we can probe large regions ( 2eV) of the transmission function which governs charge transport through the junction. By being able to control level alignment, we are also able to offer preliminary studies on single-molecule junctions in the resonant transport regime. Combined, the results presented in this thesis grant new insights into electron transport at the nanoscale and provide new routes for the development of functional single-molecule devices.