In this thesis, we develop a temperature-dependent homogenization technique and implement it into the meshfree particle method for nanoscale continuum simulations. As a hierarchical multiscale method, the nanoscale meshfree particle method is employed to model and simulate nanostructured materials and devices. Recently developed multiscale methods can overcome the limitations of both length and time scales that molecular dynamics has. However, multiscale methods have difficulties in investigating temperature-dependent physical phenomena since most homogenization techniques employed in continuum models have an assumption of zero temperature. A new homogenization technique, the temperature-related Cauchy-Born (TCB) rule, is proposed with the consideration of the free energy instead of the potential energy in this thesis. This technique is verified via stress analyses of several crystalline solids. The studies of material stability demonstrate the significance of temperature effects on nanostructured material stability. Since meshfree particle methods have advantages on simulating the problems involving extremely large deformations and moving boundaries, they become attractive options to be used in the hierarchical multiscale modeling to approximate a large number of atoms. In this thesis, a nanoscale meshfree particle method with the implementation of the developed homogenization technique, i.e. the TCB rule, is proposed. It is shown that numerical simulations in nanotechnology can be beneficial from this technique by saving a great amount of computer time. The nanoscale meshfree particle method is employed to investigate the crack propagation in a nanoplate with the development of cohesive zone model and a thermal-mechanical coupling model. In addition, the nanoscale meshfree particle method is simplified to successfully study mechanisms of nanotube-based memory cells.

For decades, epitaxy is used in nanotechnologies and semiconductor fabrications. So far, it's the only affordable method of high quality crystal growth for many semiconductor materials. Heterostructures developed from these make it possible to solve the considerably more general problem of controlling the fundamental parameters inside the semiconductor crystals and devices. Moreover, as one newly arising study and application branch of epitaxy, selective area growth (SAG) is widely used to fabricate materials of different thicknesses and composition on different regions of a single wafer. All of these new and promising fields have caught the interests and attentions of all the researchers around the world. In this work, we will study the stress and strain analysis of epitaxy in nano-scale materials, in which we seek a methodology to bridge the gap between continuum mechanical models and incorporate surface excess energy effects, which can be obtained by molecular dynamical simulations. We will make a brief description of the elastic behavior of the bulk material, covering the concepts of stress, strain, elastic energy and especially, the elastic constants. After that, we explained in details about the definitions of surface/interface excess energy and their characteristic property tensors. For both elastic constants and surface excess energy, we will use molecular dynamic simulations to calculate them out, which is mainly about curve-fitting the parabola function between the total strain energy density and the strain. After this, we analyzed the stress and strain state in nanoisland during the selective area growth of epitaxy. When the nanoisland is relaxed, the lattice structure becomes equilibrated, which means the total strain energy of system need to be minimized. Compared to other researcher's work, our model is based on continuum mechanics but also adopts the outcome from MD simulations. By combining these microscopic informations and those macroscopic observable properties, such as bulk elastic constants, we can provide a novel way of analyzing the stress and strain profile in epitaxy. The most important idea behind this approach is that, whenever we can obtain the elastic constants and surface property tensors from MD simulations, we can follow the same methodology to analyse the stress and strain in any epitaxy process. This is the power of combining atomistic simulations and continuum method, which can take considerations of both the microscopic and macroscopic factors.

This dissertation is focused on thermal transport at nanometer scale point and line constrictions and in nanowires with sawtooth surface roughness. To better understand thermal transport at a point contact such as that at the tip-sample junction of a scanning probe microscope, a Non Equilibrium Molecular Dynamics (NEMD) method is employed to calculate the temperature distribution and thermal resistance of a nanoscale point constriction formed between two silicon substrates. The simulation reveals surface reconstruction at the two free silicon surfaces and at the constriction. The radius of the heated zone in the cold substrate approaches a limit of about 20 times the average nearest-neighbor distance of boron doping atoms when the constriction radius (a) is reduced below the inter-dopant distance. The phonon mean free path at the constriction is suppressed by diffuse phonon-surface scattering and phonon-impurity scattering. The MD thermal resistance is close to the ballistic resistance when a is larger than 1 nm, suggesting that surface reconstruction does not reduce the phonon transmission coefficient significantly. When a is 0.5 nm and comparable to the dominant phonon wavelength, however, the NEMD result is considerably lower than the calculated ballistic resistance because bulk phonon dispersion and bulk potential are not longer accurate. The MD thermal resistance of the constriction increases slightly with increasing doping concentration due to the increase in the diffusive resistance. The NEMD method is further employed to calculate the temperature distribution and thermal resistance at nanoscale line constrictions formed between two silicone substrates. Similar to the nano point constriction, the thermal resistance at the nano line constriction is dominated by the ballistic resistance for constriction width in the range of 1 nm to 12 nm. An additional question that this dissertation seeks to answer is whether one can engineer the surface roughness on a nanowire to facilitate phonon backscattering so as to reduce the thermal conductivity below the diffuse surface limit. Monte Carlo simulation is used to show that phonon backscattering can occur at sawtooth surfaces of a silicon nanowire, suppressing the thermal conductivity below the diffuse surface limit. Asymmetric sawtooth nanowire surfaces can further cause phonon rectification, making the axial thermal conductance of the nanowire direction dependent. The phonon backscattering and rectification effects can be employed to enhance the thermoelectric figure of merit of nanowires. / text

In this thesis, we investigate the structural and dynamical properties of nano-confined liquids by means of a new AFM-based technique that has the ability to measure normal force, lateral force, and the distance between the AFM tip and the sample simultaneously. Thanks to the mechanical stability of our apparatus, a judicious choice, and a new mechanical drift analysis, we are able to measure the tip-sample distance with sub-angstrom resolution, all the way down to the last liquid layer.

Novel Studies on the Formation and Chemical Reactivity of Compound Clusters and Their Precursors in the Gas and Liquid Phase James A. Bradshaw 139 Pages Directed by Dr. Robert L. Whetten Presented are four separate and unique studies on molecular and nanoscale systems: Atmospheric hydration and aggregation of NaCl clusters, highly water-soluble aurous-thiolate oligomers, water-soluble gold clusters from aurous-thiolate oligomer precursors, and gold iodide clusters. Adsorption of water on cationic and anionic sodium chloride clusters is investigated to elucidate active sites of molecular interaction as well as primary aggregate formation kinetics. Considered an exceptionally abundant atmospheric species, experiments are conducted to further quantify gas phase chemistry and hydration/solvation of alkali halides. Currently the most soluble of all known gold-thiolates, para-mercaptobenzoic acid-based (pMBA) aurous-thiolate oligomers are investigated and physical and chemical properties quantified. Solubility, structural conformation, and poly-dispersity of higher homologs are observed with the goal of further applications in clusters research, medical and biomedical, and industry. Gold thiolate clusters, synthesized using pMBA-based oligomers, are investigated through reductive formation in solution. UV-VIS and UV-VIS-NIR spectroscopy is undertaken to assign structures based on predictions of the HOMO-LUMO gap and other electronic transitions. Gold iodide is investigated in relation to the common thiolate-halide analogy. Synthesis and characterization of a solid precursor as well as anion and cation cluster formation is presented as part of an ongoing collaboration.

Thesis (Ph.D.)--University of Wollongong, 2007. / Typescript. Includes bibliographical references: leaf 206-217.

The objective of this doctoral thesis is to develop, engineer and investigate an amorphous metal tungsten nitride (aWNx) and to study its functionality for applications focused on electromechanical system at the nano-scale. Charge transport based solid state device oriented complementary metal oxide semiconductor (CMOS) electronics have reached a level where they are scaled down to nearly their fundamental limits regarding switching speed, off state power consumption and the on state power consumption due to the fundamental limitation of sub-threshold slope (SS) remains at 60 mV/dec. NEM switch theoretically and practically offers the steepest sub-threshold slope and practically has shown zero static power consumption due to their physical isolation originated from the nature of their mechanical operation. Fundamental challenges remain with NEM switches in context of their performance and reliability: (i) necessity of lower pull-in voltage comparable to CMOS technology; (ii) operation in ambient/air; (iii) increased ON current and decreased ON resistance; (iv) scaling of devices and improved mechanical and electrical contacts; and (v) high endurance. The “perfect” NEM switch should overcome all the above-mentioned challenges. Here, we show such a NEM switch fabricated with aWNx to show (i) sub-0.3-volt operation; (ii) operation in air and vacuum; (iii) ON current as high as 0.5 mA and ON resistance lower than 5 kΩ; (iv) improved mechanical contact; and the most importantly (v) continuous switching of 8 trillion cycles for more than 10 days with the highest switching speed is 30 nanosecond without hysteresis. In addition, tungsten nitride could be the modern life vine by fulfilling the demand of biodegradable material for sustainable life regime. Transient electronics is a form of biodegradable electronics as it is physically disappearing totally or partially after performing the required function. The fabricated aWNx suites this category very well, despite not being a universal bio-element. It has been found that aWNx dissolves in ground water with a rate of ≈ 20-60 nm h-1. This means that a 100 nm thick aWNx disappears in ground water in less than a day and three days are enough to dissolve completely a 300 nm thickness device.

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The resonance frequency of a silicon nitride (SiN) nano-electromechanical systems (MEMS/NEMS) can be measured precisely due to their large quality factor that is associated to low thermomechanical fluctuations. While these properties enable the fabrication of high performance sensors, their use will eventually raise questions regarding their long-term stability, notably for calibration purposes. The long-term frequency stability and aging of SiN are less studied than the short-term fluctuations such as thermomechanical noise. Long-term aging studies exist for quartz clocks as well as MEMS silicon clocks and accelerometers, but not for SiN resonators with high quality factors. Thus, in this work we conduct the aging study of SiN membranes fabricated by our lab, by constantly tracking changes of the resonance frequency of the device over a long period. The evolution of the frequency drift is tracked, by optical interrogation, continuously for 135 days with a digital phase locked loop (PLL). Our device is placed in a cell under high vacuum to suppress air damping on our resonating membrane. Furthermore, due to its high sensitivity to temperature changes, our silicon nitride resonator and vacuum chamber are placed in an air bath providing a stable temperature (within 0.5 K over 135 days in the present case). To compensate further the frequency drifts induced by temperature changes, a multimeter measures the resistance of a calibrated thermistor placed inside the vacuum environment. The measured frequency drift for the aging periods of 135 days was of 300 parts per million (ppm) and was consistent with previously reported double logarithmic models for quartz oscillators. The initial stage of negative frequency drift, in our aging data, is consistent with the behaviour expected from the desorption of water due to the transition from ambient air environment to high vacuum. We review models explaining how water adsorption/desorption impacts our membrane's frequency by (1) inducing chemical reaction stresses (most important effect), (2) through the contribution of the water surface tension stress (non-negligible effect), and (3) through mass loading from water molecules (weakest effect). After this initial negative trend, the membrane frequency drift inverts and increases almost linearly, in a fashion consistent with loss of mass from desorption of other chemical species. To identify these chemical species, X-ray photoelectron spectroscopy measurements were conducted on a reference membrane stored in an ambient setting and on our membrane placed under vacuum during our aging studies. The aged membrane, compared to its reference counterpart, contained substantially less alkaline ion contaminants (i.e., sodium, calcium and potassium), most likely due to desorption of these species during the aging measurement, and to the increase in adsorption occurring on the reference membrane concurrently. We therefore hypothesize that trapped negative charges, which is a typical phenomenon within dielectric materials such as SiN, might progressively attract positive ion contaminants over time when the device is exposed to ambient air.

This dissertation develops uncertainty quantification methodologies for modeling, response analysis and optimization of diverse dynamical systems. Two distinct application platforms are considered pertaining to engineering dynamics and precision medicine. First, the recently developed Wiener path integral (WPI) technique for determining, accurately and in a computationally efficient manner, the stochastic response of diverse dynamical systems is employed for solving a high-dimensional, nonlinear system of stochastic differential equations governing the dynamics of a representative model of electrostatically coupled micromechanical oscillators. Compared to alternative modeling and solution treatments in the literature, the current development exhibits the following novelties: a) typically adopted linear, or higher-order polynomial, approximations of the nonlinear electrostatic forces are circumvented; and b) stochastic modeling is employed, for the first time, by considering a random excitation component representing the effect of diverse noise sources on the system dynamics. Further, the WPI technique is enhanced and extended based on a Bayesian compressive sampling (CS) treatment. Specifically, sparse expansions for the system response joint PDF are utilized. Next, exploiting the localization capabilities of the WPI technique for direct evaluation of specific PDF points leads to an underdetermined linear system of equations for the expansion coefficients. Furthermore, relying on a Bayesian CS solution formulation yields a posterior distribution for the expansion coefficient vector. In this regard, a significant advantage of the herein-developed methodology relates to the fact that the uncertainty of the response PDF estimates obtained by the WPI technique is quantified. Also, an adaptive scheme is proposed based on the quantified uncertainty of the estimates for the optimal selection of PDF sample points. This yields considerably fewer boundary value problems to be solved as part of the WPI technique, and thus, the associated computational cost is significantly reduced. Second, modeling and analysis of the physiological mechanism of dynamic cerebral autoregulation (DCA) is pursued based on the concept of diffusion maps. Specifically, a state-space description of DCA dynamics is considered based on arterial blood pressure (ABP), cerebral blood flow velocity (CBFV), and their time derivatives. Next, an eigenvalue analysis of the Markov matrix of a random walk on a graph over the dataset domain yields a low-dimensional representation of the intrinsic dynamics. Further dimension reduction is made possible by accounting only for the two most significant eigenvalues. The value of their ratio indicates whether the underlying system is governed by active or hypoactive dynamics, indicating healthy or impaired DCA function, respectively. The reliability of the technique is assessed by considering healthy individuals and patients with unilateral carotid artery stenosis or occlusion. It is shown that the proposed ratio of eigenvalues can be used as a reliable and robust biomarker for assessing how active the intrinsic dynamics of the autoregulation is and for indicating healthy versus impaired DCA function. Further, an alternative joint time-frequency analysis methodology based on generalized harmonic wavelets is utilized for assessing DCA performance in patients with preeclampsia within one week postpartum, which is associated with an increased risk for postpartum maternal cerebrovascular complications. The results are compared with normotensive postpartum individuals and healthy non-pregnant female volunteers and suggest a faster, but less effective response of the cerebral autoregulatory mechanism in the first week postpartum, regardless of preeclampsia diagnosis.