Nanoscale Investigations of Thermal and Momentum Transport in Graphene – Water Systems

Demand for miniaturized electronic devices has given rise to new challenges in thermal management. Integration with graphene, a two-dimensional (2D) material with excellent thermal properties, allows for further reduced sizes and combats thermal management issues within novel devices. Moreover, due to its wide availability and adequate thermal properties, liquid water is commonly used within traditional thermal systems to enhance cooling performance; as such, water is expected to yield similar performance in smaller-scale applications. However, at reduced sizes descending to the nanoscale realm, system behaviors deviate from traditional macroscale-based theory as interfacial effects become amplified. Employing insight provided by molecular dynamics simulations, this thesis investigates momentum and thermal transport characteristics, stemming from interfacial interactions, of graphene/water systems to unravel their nanoscale contributions on system-wide thermal performance. The convective heat transfer process for a laminar flow of liquid water in graphene nanochannels is emphasized as a joint assessment of momentum and thermal transport, with understandings obtained from initial investigations. In preliminary momentum transport analysis, wettability assessments identified graphene/water system behavior as highly dependent on interfacial surface interactions. Extension to flow simulations further revealed that surface interactions significantly impact momentum transport of flowing water behavior and slip development; attributing to the anatomically smooth nature of 2D graphene, slip flow is observed even in cases of extreme hydrophilicity. In thermal transport assessments, increasing surface interactions are shown to enhance heat transfer due to decreased interfacial thermal resistance. In convection heat transfer analysis, momentum and thermal transport are found to be strongly correlated; however, thermal transport was determined to be more influential on resultant system characteristics than momentum transport. Additionally, system size dependence on momentum and thermal transport is observed, with convective performance suggested as the ratio of thermal slip length to system size. Findings presented in this thesis are expected to enhance knowledge of the physics behind solid/liquid interfacial phenomena and establish more accurate descriptions of nanoscale momentum and thermal transport. Although constrained by limited dimensional/time scales, this work is anticipated to aid in laying the ground work for understanding nanoscale thermal characteristics, with aim at developing novel thermal systems

Nanoscale Investigation of Metamorphic Processes

This doctoral thesis presents micro to nanoscale investigations of metamorphic processes taking place within Earth’s lower crust. Atom probe tomography, together with a wide range of cutting-edge techniques, has been applied to well-preserved rocks that have been affected by interactions with fluids in different geological contexts and under different extent of deformation. The integrated outcome of this research provides unprecedented insights into the complex interplay between rock, fluids, and deformation

Support-based transfer and contacting of individual nanomaterials for in-situ nanoscale investigations

Although in-situ transmission electron microscopy (TEM) of nanomaterials has been gaining importance in recent years, difficulties in sample preparation have limited the number of studies on electrical properties. Here, a support-based preparation method of individual 1D and 2D materials is presented, which yields a reproducible sample transfer for electrical investigation by in-situ TEM. Using a mechanically rigid support grid allows the reproducible transfer and contacting to in-situ chips by focused ion beam with minimum damage and contamination. The transfer quality is assessed by exemplary studies of different nanomaterials, including a monolayer of WS2. Preliminary results from in-situ test experiments give an overview of possible studies, which concern the interplay between structural properties and electrical characteristics on the individual nanomaterial level as well as failure analysis under electrical current or studies of electromigration, Joule heating and related effects. The TEM measurements can be enriched by additional correlative microscopy techniques, which allow the study with a spatial resolution in the range of a few microns. Although developed for in-situ TEM, the present transfer method is also applicable to transferring nanomaterials to similar chips for performing further studies or even for using them in potential electrical/optoelectronic/sensing devices.Comment: 23 pages, 15 figure

Magnetic circular dichroism in EELS: Towards 10 nm resolution

We describe a new experimental setup for the detection of magnetic circular dichroism with fast electrons (EMCD). As compared to earlier findings the signal is an order of magnitude higher, while the probed area could be significantly reduced, allowing a spatial resolution of the order of 30 nm. A simplified analysis of the experimental results is based on the decomposition of the Mixed Dynamic Form Factor S(q,q',E) into a real part related to the scalar product and an imaginary part related to the vector product of the scattering vectors q and q'. Following the recent detection of chiral electronic transitions in the electron microscope the present experiment is a crucial demonstration of the potential of EMCD for nanoscale investigations.Comment: 12 pages, 6 figures, submitted to Ultramicroscop

Nanoscale investigations of surface phenomena in the water teatment industry using the atomic force microscope

Understanding the interaction between surfaces at the intermolecular level in ambient conditions is not only a fundamental science, but is of increasing value to water treatment systems. Here the uses of the atomic force microscopy (AFM) modified with particles of interest are assessed, and compared to bench-scale experimental techniques. In the first part of this study, the results from force measurements performed with calcite-modified probes in synthetic hard water (SHW) on selected substrates showed there was no correlation with macroscale scaling rate experiments. However, unmodified tips showed some correlation with non-metal substrates, where carbon coatings (Dymon-iC and Graphit-iC) were least adhesive. Although unmodified tips were unlikely to represent one of the surfaces of interest in water treatment systems, the findings suggest they can be used to screen materials with Ra < 50 nm. Contact angle measurements complemented force data, indicating the origin of repulsive forces on carbon coatings was due to hydrophilic repulsion because carbon and calcite were highly basic. Enhanced adhesion was caused by hydrophobic attraction and the presence of acidic surface groups. In the 2nd part of this study, force measurements were performed on natural organic matter (NOM) polyanions such as humic acid fraction (HAF), fulvic acid fraction (FAF) and hydrophilic acid (HPIA) using modified and unmodified tips. The results showed in symmetric NOM-NOM interactions with modified tips, HPIA-HPIA dominated both adhesion and detachment lengths, while FAF-FAF and HAF-HAF gave similar adhesion profiles. It is thought these intermolecular interactions can be transferred to floc size data, where HPIA flocs were bigger than FAF flocs. In non-symmetric systems adhesion between FAF-NOM was indiscriminate, compared to HAF and HPIA polyanions, indicating FAF polyanions were most likely to control coagulation performance during NOM removal

NanoSQUID magnetometry of individual cobalt nanoparticles grown by focused electron beam induced deposition

We demonstrate the operation of low-noise nano superconducting quantum interference devices (SQUIDs) based on the high critical field and high critical temperature superconductor YBa$_2$Cu$_3$O$_7$ (YBCO) as ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have been patterned by focused ion beam milling. This allows to precisely define the lateral dimensions of the SQUIDs so as to achieve large magnetic coupling between the nanoloop and individual MNPs. By means of focused electron beam induced deposition, cobalt MNPs with typical size of several tens of nm have been grown directly on the surface of the sensors with nanometric spatial resolution. Remarkably, the nanoSQUIDs are operative over extremely broad ranges of applied magnetic field (-1 T $< \mu_0 H <$ 1 T) and temperature (0.3 K $< T<$ 80 K). All these features together have allowed us to perform magnetization measurements under different ambient conditions and to detect the magnetization reversal of individual Co MNPs with magnetic moments (1 - 30) $\times 10^6\,\mu_{\rm B}$. Depending on the dimensions and shape of the particles we have distinguished between two different magnetic states yielding different reversal mechanisms. The magnetization reversal is thermally activated over an energy barrier, which has been quantified for the (quasi) single-domain particles. Our measurements serve to show not only the high sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these sensors are exceptional magnetometers for the investigation of the properties of individual nanomagnets

Chemically Patterned Surfaces as Test Platforms to Study Magnetic and Solvent-Responsive Properties at the Nanoscale: Investigations Using Scanning Probe Microscopy

Chemically patterned surfaces were fabricated using a combination of molecular self-assembly and particle lithography to generate billions of nanostructures of organosilane self-assembled monolayers (SAMs). Monodisperse mesospheres were used as surface masks to prepare nanostructures on flat surfaces using the simple benchtop chemistry steps of mixing, centrifuging, evaporation, and drying. Periodic arrays of well-defined organosilane nanostructures serve as discrete surface sites for the selective deposition of polymers and magnetic nanoparticles. In this dissertation, particle lithography approaches for surface patterning provide new directions for studying surface chemistry at the molecular-level using high resolution investigations with scanning probe microscopy (SPM). Atomic force microscopy (AFM) can be used to analyze samples in ambient and liquid environments. The solvent responsive nature of OTS nanostructures were investigated using in-situ liquid imaging with AFM. AFM provides unique capabilities for molecular visualization and ultrasensitive measurements of changes in heights, widths and surface coverage of the swollen OTS nanostructures with nanoscale resolution. Ring nanostructures of OTS presented a 3D interface for studying the interaction of solvents at the molecular level. The vibrational response of patterned magnetic Fe3O4 nanoparticles in response to an applied external magnetic field was detected using magnetic sample modulation AFM (MSM-AFM). The vibration of Fe3O4 nanoparticles can be detected with a nonmagnetic AFM tip operated in continuous contact mode. In MSM-AFM, an AC current applied to the wire coil solenoid within the special sample plate drives the actuation of magnetic nanomaterials that are attached to surfaces. The magnetic Fe3O4 nanoparticles were induced to vibrate in the presence of externally applied electromagnetic field. Parameters such as frequency and magnetic field strength can be tuned in-situ to study dynamic changes in the vibrational response of samples. The AFM tip serves as a force and motion sensor for mapping the vibrational response of magnetic nanomaterials. The information acquired from MSM images includes the distribution of individual magnetic domains as well as spectra of the characteristic resonance frequencies of the vibrating magnetic nanomaterials