This work explores the physics of micro and nano-scale systems immersed in a fluid. Previous literature has established an understanding of the fluid-solid interaction for systems including cantilevers and doubly clamped beams. Building on these advances, this work extends the theory of doubly clamped beams with an arbitrary amount of tension. Both the driven and stochastic dynamics of a doubly clamped beam are explored. The driven dynamics are investigated for a spatially applied harmonic driving force, and demonstrates quantitative agreement with an experimental beam that is driven electrothermally, in both air and in water. For the stochastic dynamics, the noise spectrum describes the thermal fluctuations at a given frequency. The theoretical model provides an analytical expression for the noise spectrum from an arbitrary number of modes. The noise spectrum of the first eleven modes are computed, and show excellent agreement with the noise spectrum from finite element simulations, which is computed from the deterministic ring down. This agreement is shown across different fluids (air and water), and for multiple measuring points including at the beam midpoint and the quarter point. In addition to exploring the linear dynamics of these systems, the case of large perturbations, resulting in nonlinear dynamics, is explored. This regime is motivated by exploring the theoretical dynamics of a uniformly shrinking doubly clamped beam. The challenges of modeling such a beam using finite element simulations are discussed. As a simpler and more direct alternative to access the nonlinear regime, a virtual beam is defined. The virtual beam controls the nonlinearity of the restoring force by modifying the Young's modulus. This work defines the Young's modulus such that the restoring force is like a Duffing oscillator. Then, the dynamics of this virtual beam are explored in air and water, and it is demonstrated that the Duffing oscillator serves as an appropriate reduced order model for this virtual beam. To understand the stochastic dynamics of the virtual beam, the stochastic Duffing oscillator is solved numerically. The ensemble autocorrelation of the beam dynamics are investigated for nonlinearities varying from linear to strongly nonlinear. The numeric autocorrelation is used to quantify the range of nonlinear strength where a deterministic approach, the ring down, can yield a good approximation. In the strongly nonlinear regime, the stochastic numerical approach is used to determine the autocorrelation. This research was supported by the National Science Foundation, grant number CMMI-2001559, and portions of the computations were conducted using the resources of Virginia Tech's Advanced Research Computing center. / Doctor of Philosophy / This work explores the physics of small systems immersed in a fluid, such as air or water. Previous literature has established an understanding of the force from a fluid acting on solids such as cantilevers and doubly clamped beams. Building on these advances, this work extends theory to doubly clamped beams with any amount of tension. Both the driven and stochastic, or randomly driven, dynamics of a doubly clamped beam are explored. The driven dynamics are developed for a driving force applied over part of the beam, and demonstrates quantitative agreement with an experimental beam, in both air and in water. For the stochastic dynamics, the noise spectrum describes the random thermal fluctuations of the beam at a given frequency. These thermal fluctuations are small, but measureable deviations of the system from equilibrium and are significant for these small scale systems. The noise spectrum can be estimated by computing the statistics from many randomly forced simulations. However, previous literature provides a direct computation of the noise spectrum with a single deterministic ring down. This work provides an analytical expression for the noise spectrum of a doubly clamped beam in tension in fluid for multiple modes. The theoretical noise spectrum shows excellent quantitative agreement with the ring down from finite element simulations. The agreement between theory and simulation is demonstrated in air and water, for a measurement of the noise spectrum at the beam midpoint and at the beam quarter point. In addition to exploring the linear dynamics of these systems, the case of large perturbations, resulting in nonlinear dynamics, is explored. This regime is motivated by exploring the theoretical dynamics of a uniformly shrinking doubly clamped beam. The challenges of modeling such a beam using finite element simulations are discussed. As a simpler and more direct alternative to access the nonlinear regime, a virtual beam is defined. The virtual beam controls the nonlinearity of the restoring force such that the system becomes increasingly stiff as the displacements become larger. This definition results in the restoring force following a Duffing oscillator. Then, the dynamics of this virtual beam are explored in air and water, and it is demonstrated that the Duffing oscillator serves as an appropriate reduced order model for this virtual beam. For varying nonlinear strengths, the stochastic numerical approach is used to quantify the dynamics, and the range of usefulness for the deterministic ring down is investigated. This research was supported by the National Science Foundation, grant number CMMI-2001559, and portions of the computations were conducted using the resources of Virginia Tech's Advanced Research Computing center.

This thesis presents the design and theoretical modeling of an aluminum gallium arsenide/gallium arsenide heterostructure from which suspended nanoscale mechanical resonators with embedded two-dimensional electron gas (2DEG) can be made. The mechanical characteristics of the resonator and the piezoelectric actuation scheme are investigated using finite-element modeling. For a 836 nm-long, 250 nm-wide and 164 nm-thick beam with gold electrodes on top, out-of-plane flexural vibrations are verified to be piezoelectrically excited at the beam's fundamental frequency of 925.6 MHz. Fabrication recipes for the making of ohmic contacts to the 2DEG, Hall bars and suspended structures are developed using the designed crystal structure. Electrical properties of the 2DEG are evaluated in both large, unsuspended structures as well as in sub-micron size suspended structures. It is found that the 2DEG has a reasonable electron density of 7.04E11 cm^-2 and electron mobility of 1.72E5 cm^2/(V s). / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2007-09-28 10:13:56.094

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No description available.

We investigate (i) pull-in instabilities in a microelectromechanical (MEM) beam due to the Coulomb force and in MEM membranes due to the Coulomb and the Casimir forces, and (ii) thermomechanical instability in a heat-conducting thermoelastoviscoplastic solid due to thermal softening overcoming hardening caused by strain- and strain-rate effects. Each of these nonlinear multiphysics problems is analyzed by the meshless local Petrov-Galerkin (MLPG) method. The moving least squares (MLS) approximation is used to generate basis functions for the trial solution, and the basis for test functions is taken to be either the weight functions used in the MLS approximation, or the same as for the trial solution. In this case the method becomes Bubnov-Galerkin. Essential (displacement, temperature, electric potential) boundary conditions are enforced by the method of Lagrange multipliers. For the electromechanical problem, the pull-in voltage and the corresponding deflection are extracted by combining the MLPG method with either the displacement iteration pull-in extraction algorithm or the pseudoarclength continuation method. For the thermomechanical problem, the localization of deformation into narrow regions of intense plastic deformation is delineated. For every problem studied, computed results are found to compare well with those obtained either analytically or by the finite element (FE) method. For the same accuracy, the MLPG method generally requires fewer nodes but more CPU time than the FE method; thus additional computational cost is compensated somewhat by the increased efficiency of the MLPG method. / Ph. D.

Un des plus importants profits qu'on peut tirer des M/NEMS est la capacité de les fabriquer en grande masse permettant leur assemblage sous forme de réseau. Toutefois, de nombreux problèmes s'opposent à l'utilisation de ces systèmes tels que la complexité de leur contrôle, la non-uniformité et les couplages entre leurs éléments, les sources de bruits et de non-linéarités, etc.. Il est alors nécessaire de prendre en compte ces différents aspects dès la phase de conception, les corriger ou les exploiter, pour aboutir à des nouvelles architectures qui répondent aux exigences de hautes performances. En se servant d'un large réseau de nano-transducteurs, une contribution au contrôle dynamique robuste d'une micro-surface « intelligente » est développée. La structure continue est ensuite remplacée par un réseau de NEMS dont le modèle est détaillé pour la première fois en tenant compte des dispersions entre les éléments. Des architectures de réseaux couplés sont proposées pour réduire les effets des dispersions, améliorant ainsi la sélectivité des filtres résultants. Basée sur le schéma de transductions distribuées, une nouvelle stratégie d'ajustement du filtre est élaborée par contrôle modal. Ces différents réseaux (couplés ou non) peuvent être utilisés pour des applications capteurs où le système de mesure est modélisé en fonction de la technique utilisée et de la structure adoptée avant d'améliorer les performances par un contrôle approprié. Une nouvelle configuration exploitant les non-linéarités de transduction est proposée pour compenser et mesurer la variation de la fréquence de résonance permettant de réduire la complexité du système global.

Les progrès technologiques de ces dernières années ont permis une très forte intégration des composants de la microélectronique à l'échelle nanométrique. Face aux limites de la miniaturisation classique, les technologies d'intégration en trois dimensions (3D) ouvrent la voie vers des dispositifs miniaturisés hétérogènes avec de nouvelles générations de puces. En parallèle, de nouveaux concepts tels que les nanofils sans jonction et les nanofils en silicium polycristallins permettent à terme d'imaginer des procédés froids et des dispositifs à faible coût permettant une intégration 3D hyperdense sur un CMOS stabilisé. La fabrication de NEMS à base de nanofils polycristallins pour la détection de masse sur CMOS est donc une nouvelle opportunité « More-Than-Moore ». Les capteurs pourraient être disposés en réseau dense en s'inspirant des architectures mémoires et imageurs. L'adressage individuel de chaque NEMS, la possibilité de les fonctionnaliser à la détection de molécules particulières, et la multiplication des capteurs sur une grande surface (« Very Large Integration » (VLSI)) permettraient la mise en œuvre d'un nouveau genre de capteur multi-physique, compact et ultrasensible. Le but de ces travaux de thèse a donc été la fabrication et l'évaluation des performances de NEMS à base de nanofils en poly-silicium. L'enjeu fut de trouver des procédés avec un budget thermique compatible à une intégration sur back-end. Une étude rigoureuse sur les propriétés physico-chimiques de la couche a été corrélée aux performances électriques, mécaniques, ainsi qu'au rendement des NEMS poly-Silicium, ce qui nous a permis de faire une sélection des meilleurs procédés de fabrication. Les NEMS fabriqués à basse température avec une couche active déposée à température ambiante et recristallisée par laser ont montré des performances, que ce soit au niveau de la transduction (piézorésistivité), ou de la stabilité du résonateur compétitives par rapports aux références monocristallines. / Recently, technological advances lead to a very large scale integration (VLSI) of microelectronics components at the nanoscale. Faced with the traditional miniaturization limits, the three dimensions (3D) integration open the door to heterogeneous miniaturized devices, with new chip generations. At the same time, new concepts such as junctionless nanowires and polycrystalline silicon nanowires allow to imagine low temperature processes and low-cost devices for a 3D integration on a stabilized CMOS. Poly-silicon nanowire based NEMS on CMOS for mass detection is a new "More-Than-Moore" opportunity. The NEMS could be arranged in a dense network like memory and image sensor architectures. The individual addressing of each NEMS, the functionalization for the detection of specific molecules within a large area (VLSI), allow the implementation of a new type of Multi-physics sensors, compact and highly sensitive. The purpose of this thesis has been the manufacturing and the performance evaluation of poly-silicon nanowire based NEMS. The challenge was to find the best processes with a back-end compatible thermal budget. A rigorous study of the layer physicochemical properties has been correlated with the electrical, mechanical performances and the yield of poly-silicon NEMS. This allowed us to make a selection of the best fabrication processes. NEMS manufactured at very low temperature with an active layer deposited at room temperature and recrystallized by a laser annealing exhibited high performances in terms of transduction (piezoresistivity) and frequency stability comparable to monocrystalline references. Polycrystalline silicon.

Les systèmes intelligents sont le résultat combiné de différentes avancées en microélectronique et en particulier de l’augmentation des puissances de calcul, la diminution des consommations d’énergie, l'ajout de nouvelles fonctionnalités et de moyens de communication et en particulier à son intégration et application dans notre vie quotidienne. L'évolution du domaine des systèmes intelligents est prometteuse, et les attentes sont élevées dans de nombreux domaines : pour la surveillance dans l'industrie, les transports, les infrastructures et l'environnement, ainsi que dans le logement, l'électronique grand public et les services de soins de santé, mais aussi dans les applications pour la défense et l’aérospatial. Aujourd’hui, l'intégration de plus en plus de fonctions dans les systèmes intelligents les conduisent vers un problème énergétique où l'autonomie devient le principal problème. Par conséquent, il existe un besoin croissant en capteurs autonomes et sources d'alimentation. Le développement de dispositifs de récupération d’énergie et de capteurs autoalimentés est une façon de répondre à ce problème énergétique. Parmi les technologies étudiées, la piézoélectricité a l'avantage d'être compatible avec l'industrie des MEMS. De plus elle génère des tensions élevées et elle possède un fort couplage direct entre les physiques mécaniques et électriques. Parmi les matériaux piézoélectriques, les nanofils (NFs) semi-conducteurs piézoélectriques pourraient être une option prometteuse car ils présentent des propriétés piézoélectriques plus importantes et une plus grande gamme de flexion.Parmi les différents NFs piézoélectriques, les NFs de ZnO et de GaN sont les plus étudiés. A l'échelle nanométrique leurs propriétés piézoélectriques sont plus que doublées. Ils ont l'avantage d'être compatible avec l’industrie microélectronique et raisonnablement synthétisable par des approches top-down et bottom-up. En particulier, nous avons étudié la croissance par voie chimique de NFs de ZnO. Pour les utiliser correctement, nous avons étudié le comportement des NFs de ZnO. Nous avons effectué une étude analytique et des simulations par éléments finis (FEM) d'un NF de ZnO en flexion. Ces études décrivent la distribution du potentiel piézoélectrique en fonction de la force et permettent d’établir les règles d'échelle et de dimensionnement. Ensuite, nous avons développé la caractérisation mécanique par AFM du module de Young de NFs de ZnO et de GaN, puis nous avons effectué des caractérisations piézoélectriques par AFM de ces NFs pour vérifier leur comportement sous des contraintes mécaniques de type flexion. Une fois leur comportement physique compris, nous discutons des limites de notre modèle de NFs piézoélectriques en flexion et nous développons un modèle plus réaliste et plus proche des configurations expérimentales. En utilisant ce nouveau modèle, nous avons évalué le potentiel des NFs de ZnO pour les capteurs de force et de déplacement en mesurant le potentiel généré sous une contrainte, puis, sur la base d’expériences, nous avons évalué l'utilisation de NFs de GaN pour les capteurs de force en mesurant le courant au travers des NFs contraints. De même, nous avons évalué le potentiel de ces NFs pour les applications de récupération d'énergie liées aux capteurs autonomes. Pour bien comprendre la problématique, nous avons étudié l’état de l’art des nano générateurs (NG) et leurs architectures potentielles. Nous analysons leurs avantages et inconvénients, afin de définir une structure de NG de référence. Après une brève étude analytique de cette structure pour comprendre son fonctionnement et les défis, nous avons effectué plusieurs simulations FEM pour définir des voies d'optimisation pour les NG utilisé en mode de compression ou de flexion. Enfin la fabrication de prototypes et leurs caractérisations préliminaires sont présentées. / Smart systems are the combined result of different advances in microelectronics leading to an increase in computing power, lower energy consumption, the addition of new features, means of communication and especially its integration and application into our daily lives. The evolution of the field of smart systems is promising, and the expectations are high in many fields: Industry, transport, infrastructure and environment monitoring as well as housing, consumer electronics, health care services but also defense and space applications. Nowadays, the integration of more and more functions in smart systems is leading to a looming energy issue where the autonomy of such smart systems is beginning to be the main issue. Therefore there is a growing need for autonomous sensors and power sources. Developing energy harvesters and self-powered sensors is one way to address this energy issue. Among the technologies studied, piezoelectricity has the advantage to be compatible with the MEMS industry, it generates high voltages and it has a high direct coupling between the mechanic and electric physics. Among the piezoelectric materials, semiconductor piezoelectric nanowires (NWs) could be a promising option as they exhibit improved piezoelectric properties and higher maximum flexion.Among the different piezoelectric NWs, ZnO and GaN NWs are the most studied, their piezoelectric properties are more than doubled at the nanoscale. They have the advantage of being IC compatible and reasonably synthesizable by top-down and bottom-up approaches. Especially we studied the hydrothermal growth of ZnO NWs. In order to use them we studied the behavior of ZnO NWs. We performed analytical study and FEM simulations of a ZnO NW under bending. This study explains the piezoelectric potential distribution as a function of the force and is used to extract the scaling rules. We have also developed mechanical AFM characterization of the young modulus of ZnO and GaN NWs. Following we perform piezoelectric AFM characterization of these NWs, verifying the behavior under bending stresses. Once physics understood, we discuss limitation of our piezoelectric NWs models and a more realistic model is developed, closer to the experimental configurations. Using this model we evaluated the use of ZnO NW for force and displacement sensors by measuring the potential generated, and from experiments, the use of GaN NW for force sensor by measuring the current through the NW. But energy harvesting is also necessary to address the energy issue and we deeper investigate this solution. To fully understand the problematic we study the state of the art of nanogenerator (NG) and their potential architectures. We analyze their advantages and disadvantages in order to define a reference NG structure. After analytical study of this structure giving the basis for a deeper understanding of its operation and challenges, FEM simulations are used to define optimization routes for a NG working in compression or in bending. The fabrication of prototypes and theirs preliminary characterization is finally presented.

Cette thèse s'inscrit dans la thématique de la nano-optomécanique et de l'emploi de nanorésonateurs mécaniques comme sonde de force ultrasensible pour étudier leur interaction avec la lumière. Pour cela, un nanofil de carbure de silicium est positionné dans un faisceau laser fortement focalisé. Cela permet, en mesurant les fluctuations de l'intensité transmise, d'observer avec grande dynamique et une sensibilité proche de la limite quantique standard le mouvement Brownien du nanorésonateur. La grande sensibilité en force des nanofils, inhérente à leur très faible masse, permet d'étudier l'action en retour de la mesure, c'est-à-dire la force exercée par le laser focalisé sur le nanofil. L'exploitation de la légère levée de dégénérescence observée entre les deux polarisations mécaniques transverses permet de réaliser une cartographie vectorielle bidimensionnelle du champ de force optique, avec une sonde de diamètre sub-longueur d'onde. Cette mesure permet également de mettre en évidence le caractère non-conservatif de l'interaction lumière-matière, dont la signature emblématique est l'existence de vorticité dans le champ de force mesuré. Ce dernier présente de très fortes variations spatiales, qui modifient profondément la dynamique du nanofil. Cette action en retour de la mesure est responsable d'un fort couplage entre les deux polarisations mécaniques du mode fondamental du nanofil. Le caractère bidimensionnel du couplage ainsi que la topologie non-conservative du champ de force conduisent à une bifurcation et à une instabilité dynamique du nanofil. Cette nouvelle instabilité optomécanique est observée avec des forces optiques instantanées, qui suivent instantanément les variations d'intensités vues par le nanofil. En présence d'absorption, le cas plus général d'un champ de force partiellement retardé par les constantes de temps thermiques est également étudié, conduisant à un refroidissement, spécifique et accordable en position, des deux polarisations mécaniques. Enfin l'interaction lumière-matière entre le laser et le nanofil et la grande variété des propriétés optomécaniques accessibles à cette approche sont développés. Ces développements démontrent la possibilité d'observer et de contrôler optiquement des nanorésonateurs mécaniques de très grande sensibilité, proche de l'attonewton, pour des mesures vectorielles ultrasensibles de champ de force. / This thesis is related to the field of nano-optomechanics and the use of nanomechanical resonators as ultrasensitive force sensor to study their interaction with light. A silicon carbide nanowire is positioned in a tightly focused laser beam. This enables, by measuring the transmitted intensity fluctuations, to observe with great dynamics and with a sensitivity close to the standard quantum limit the Brownian motion of the nanoresonator. The huge force sensitivity of the nanowires, due to their ultra low mass, permits to study the measurement backaction, which is induced by the force exerted on the nanowire by the focused laser beam. The exploitation of the slight degeneracy observed between the two transverse mechanical polarizations enables to realize a vectorial bidimensional cartography of the optical force field, with a probe of sub-wavelength diameter. This measurement highlights the non-conservative feature of the light-matter interaction, a symbolic signature being the existence of vorticity in the measured force field. The latter shows strong spatial variations, which modify deeply the nanowire dynamics. This measurement backaction is responsible of a strong coupling between the two mechanical polarizations of the nanowire fundamental mode. The bidimensional feature of the coupling and the force field non-conservative topology lead to a bifurcation and to a dynamical instability of the nanowire. This new optomechanical instability is observed with instantaneous optical forces, which follows instantaneously the intensity variations seen by the nanowire. In presence of absorption, a more general case of a force field partially delayed by the thermal time constants is studied, leading to a cooling, specific and tunable with the position, of the two mechanical polarizations. Then, the light-matter interaction between a laser and the nanowire and the great variety of optomechanical properties accessible with this approach are developed. These developments demonstrate the ability to observe and control optically nanomechanical resonators with a huge sensitivity, close to the attonewton, for ultrasensitive measurements of vectorial force fields.

In general terms, this thesis is about tuning coupled electronic and nuclear (or mechanical) dynamics in the nanoscale. With “tuning” we mean changing parameters to achieve a specific phenomenon or functionality. This is not a trivial task in this context, because the dynamics of the systems we consider depend nontrivially on the parameters. To be more concrete, we consider two systems which are “complimentary” in many aspects. We start by studying nonradiative decay of an electronic excitation in a minimal example from supramolecular chemistry: a molecular dimer. Each monomer in our model has two electronic states and the respective potential energy surfaces (PESs) are harmonic. Electronic de-excitation occurs in the monomeric level through well-localized regions in the nuclear space which we call ``NRD channels\'\'. The monomers interact via transition dipole-dipole interaction. The decay dynamics of the monomer are trivial due to its harmonic PESs and simple NRD channel. However, the dimer shows distorted and nontrivially coupled PESs conferring rather complex decay dynamics on it. Depending on the position of the NRD channel, we find that the NRD lifetime can exhibit a completely different dependence on the intermolecular-interaction strength. The extension to larger aggregates and the implications to the quantum yield of molecular systems will be discussed. Our findings suggest design principles for molecular systems where a specific fluorescence quantum yield is desired. The most part of this thesis is about a nanoscale rotor driven by charge tunneling. The rotor consists of electronic islands linked to a bearing via insulating arms. The islands can exchange electrons via tunneling with flanking electronic leads. An uniform electrostatic field brings about the coupling between electronic and mechanical degrees of freedom. Moreover, coupling to an environment lead to dissipation in the mechanical dynamics. In the literature one can identify two generic models of this type of rotor [1-3], which we refer to as “mean-field” and “stochastic” models in this thesis. In the mean-field model the system is described by a set of deterministic differential equations involving the average charge on the electronic islands, and therefore charge fluctuations are not taken into account. In the stochastic model the rotor is described by Fokker–Planck equations which fully take into account the charge fluctuations. We start by showing and comparing the dynamics of these models. The models show interesting phenomenology and predict useful functionality to the rotor. However, it is often unclear which assumptions are made upon the system when using these models. To clarify this matter we derived the models using the “orthodox” theory of single electron tunneling [4]. Next, we go on and propose experimental devices which can be described by these models. The parameter ranges accessible using these devices are estimated. Turning our attention back to functionality, we show how to introduce a preferred direction of rotation, which is useful in the context of motors. In the outlook we also show how to recast the system as a current rectifier. [1] A. Y. Smirnov, S. Savel’ev, L. G. Mourokh and F. Nori; Phys. Rev. E 78 031921 (2008). [2] A. Croy and A. Eisfeld; EPL (Europhysics Lett. 98 68004 (2012). [3] A. Smirnov, L. Murokh, S. Savel’ev and F. Nori; Bio-mimicking rotary nanomotors; volume 7364 (2009); doi:10.1117/12.821567; URL http://dx.doi.org/10.1117/12. 821567. [4] B. L. Altshuler, P. A. Lee and W. R. Webb; Mesoscopic phenomena in solids; volume 30; Elsevier (2012).