Levitodynamics toward force nano-sensors in vacuum

Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit de CiènciesLevitodynamics addresses the levitation and manipulation of micro- and nanoresonators with the purpose of studying their dynamics. This emerging field has attracted much attention over the last few years owing to unprecedented performances in terms of mechanical quality factors, cooling rates at room temperature, and ultra-high force sensitivities. In this thesis, I establish the use of an optically levitated and electrically driven charged silica nanoparticle as a promising and reliable force sensor in vacuum. The first two experiments discussed in this work seek a deeper knowledge and a higher control of the levitated system. Firstly, I suggest and demonstrate a novel protocol to measure the mass of the particle up to 2% accuracy using its electrically driven motion. This method improves by more than one order of magnitude the state-of-the-art mass measurements in standard optical tweezers schemes. Then, leveraging on these results, a second experiment is performed to address important open issues regarding the morphology of the nanoparticles used, with particular interest in their surface chemistry and in the understanding of mass-losses due to water desorption from the silica spheres. Finally, backed up by extensive theoretical background in nonlinear mechanical oscillators, I investigate the stochastic bistable dynamics of a parametrically driven nanoresonator in the nonlinear regime, discussing the potential of noise-activated stochastic switching and stochastic resonance as unconventional force detection schemes.La levitodinámica estudia la manipulación de micro y nanorresonadores en levitación con el objetivo de controlar su dinámica. Este nuevo campo ha atraído mucha atención en los últimos años gracias a sus prestaciones sin precedentes en términos de factores de calidad mecánica, posibilidad de enfriamiento del centro de masas a temperatura ambiente y altas sensibilidades en la detección de fuerzas. En esta tesis, establezco un sensor de fuerza basado en el uso de una nanopartícula de sílice levitada ópticamente en el vacío que, gracias a su carga, puede ser accionada mediante campos eléctricos. Los dos primeros experimentos discutidos en este trabajo intentan conseguir un conocimiento más profundo y un mayor control del sistema levitado. En primer lugar, se propone y demuestra un nuevo protocolo para la medida de la masa de la partícula en levitación con una precisión del 2% basado en el estudio de la dinámica forzada cuando la partícula es accionada eléctricamente. Este método mejora en más de un orden de magnitud las mediciones de la masa de la partícula en plataformas de pinzas ópticas estándar. Aprovechando este desarrollo, se realiza un segundo experimento para estudiar importantes problemas relacionados con las propiedades físicas y químicas de las nanopartículas utilizadas, con especial interés en su química superficial y en la comprensión de las pérdidas de masa onbservadas debidas a la desorción de agua de las esferas de sílice. Finalmente, gracias a una amplia base teórica en osciladores mecánicos no lineales, investigo la dinámica estocástica biestable de un nanoresonador accionado paramétricamente en el régimen no lineal, discutiendo el potencial de las transiciones estocásticas activadas por ruido externo y la resonancia estocástica como esquemas de detección de fuerza no convencionales.Postprint (published version

Structure and Dynamics of Poly(methyl-methacrylate)/Graphene systems through Atomistic Molecular Dynamics Simulations

The main goal of the present work is to examine the effect of graphene layers on the sructural and dynamical properties of polymer systems. We study hybrid poly(methyl methacrylate) (PMMA)/graphene interfacial systems, through detailed atomistic molecular dynamics (MD) simulations. In order to characterize the interface, various properties related to density, structure and dynamics of polymer chains are calculated, as a function of the distance from the substrate. A series of different hybrid systems, with width ranging between [2.60 – 13.35] nm, are being modeled. In addition, we compare the properties of the macromolecular chains to the properties of the orresponding bulk system at the same temperature. We observe a strong effect of graphene layers on both structure and dynamics of the PMMA chains. Furthermore the PMMA/graphene interface is characterized by different length scales, depending on the actual property we probe: Density of PMMA polymer chains is larger than the bulk value, for polymer chains close to graphene layers up to distances of about [1.0-1.5]nm. Chain conformations are perturbed for distances up to about 2-3 radius of gyration from graphene. Segmental dynamics of PMMA is much slower close to the solid layers up to about [2-3]nm. Finally terminal-chain dynamics is slower, compared to the bulk one, up to distances of about 5-7 radius of gyration

Carbon Oxidation at the Atomic Level: A Computational Study on Oxidative Graphene Etching and Pitting of Graphitic Carbon Surfaces

In order to understand the oxidation of solid carbon materials by oxygen-containing gases, carbon oxidation has to be studied on the atomic level where the surface reactions occur. Graphene and graphite are etched by oxygen to form characteristic pits that are scattered across the material surface, and pitting in turn leads to microstructural changes that determine the macroscopic oxidation behavior. While this is a well-documented phenomenon, it is heretofore poorly understood due to the notorious difficulty of experiments and a lack of comprehensive computational studies. The main objective of the present work is the development of a computational framework from first principles to study carbon oxidation at the atomic level. First, the large body of literature on carbon oxidation is examined with regards to experimental observations of the pitting phenomenon as well as relevant theoretical studies on different aspects of the mechanistic details of carbon oxidation. Next, a comprehensive, atomic-scale kinetic mechanism for carbon oxidation is developed, which comprises only elementary surface reactions with reaction rates derived from first principles. The mechanism is then implemented using the Kinetic Monte Carlo (KMC) method. This framework for the first time allows the simulation of oxidative graphene etching at the atomic scale to relevant time- and lengthscales (up to seconds and hundreds of nanometers), and in a wide range of conditions (temperatures up to 2000 Kelvin, pressures ranging from vacuum to atmospheric pressure). The numerical results reveal information about the pitting process in heretofore unattained detail: Pit growth rates (and therefore intrinsic oxidation rates) are calculated and validated against a set of different experimental data at a wide range of conditions. Such information is crucial for modelling of material behavior on meso- and macroscales. The dependence of the pit geometry (hexagonal vs. circular) on temperature and gas pressure is assessed. This is important for utilizing oxidative etching as a manufacturing technique for graphene-based nanodevices. More subtle phenomena like pit inhibition at low pressures and temperatures are also discussed. Moreover, all these findings are examined with respect to the underlying reaction mechanism. This unveils the fundamental reasons for the observed reaction behavior, in particular different activation energies and reaction orders at low and high temperatures, as well as the transition of the pit geometry. The present work is a first step in an ongoing effort to develop predictive models for carbon oxidation in Thermal Protection Systems (TPS), with the ultimate goal of improved safety for hypersonic flight vehicles

Mass sensing with graphene and carbon nanotube mechanical resonators

In recent years, carbon nanotube and graphene mechanical resonators have attracted considerable attention because of their unique properties. Their high resonance frequencies, high quality factors and their ultra-low mass turn them into exceptional sensors of minuscule external forces and masses. Their sensing capabilities hold promise for scanning probe microscopy, magnetic resonance imaging and mass spectrometry. Moreover, they are excellent probes for studying mechanical motion in the quantum regime, investigating nonlinear dynamics and carrying out surface science experiments on crystalline low-dimensional systems. A goal for fully exploiting the potential of mechanical resonators remains: Reaching the fundamental limit of the resolution of mass sensing imposed by the thermomechanical noise of the resonator. Currently, limitations are typically due to noise in the motion transduction. Nanotube and graphene resonators are particularly sensitive to noise in the detection since their intrinsically small dimensions result in minuscule transduced electrical or optical signals. This thesis researches ways for improving the mass resolution of the intrinsically smallest mechanical resonator systems, which are based on suspended graphene and carbon nanotubes. For this, we follow two complementary pathways. We first see how far we can go in terms of mass resolution with graphene resonators by reducing their size. We fabricate double clamped graphene resonators with submicron lengths and measure their mechanical properties at 4.2 K. The frequency stability of the resonators allows us to evaluate their mass resolution. We show that the frequency stability of graphene resonators is limited by the imprecision of the detection of the mechanical motion. We then develop a new electrical downmixing scheme to read-out the mechanical motion with a lower noise compared to previous techniques. It utilizes a RLC resonator together with an amplifier based on a high electron mobility transistor operated at 4.2 K. The signal to noise ratio is improved thanks to signal read-out at higher frequency (1.6 MHz compared to 1-10 kHz) and low temperature amplification. We observe an improved frequency stability measuring a carbon nanotube mechanical resonator with this read-out. The stability is no longer limited by the measurement instrumentation noise but by the device itself. Observing the intrinsic fluctuations of the resonator allows in future experiments to study surface science phenomena. We present some preliminary results that hint to the observation of the diffusion of xenon atoms on the surface of the resonator and to the adsorption of single fullerene molecules.En los últimos años, los resonadores mecánicos de nanotubos de carbono y grafeno han atraído una atención considerable debido a sus propiedades únicas. Sus altas frecuencias de resonancia, sus factores de calidad altos y su masa extremadamente baja los convierten en sensores excepcionales de fuerzas externas y masas minúsculas. Sus capacidades de detección son prometedoras para la microscopía con sonda de barrido, la tomografía por resonancia magnética y la espectrometría de masas. Además, son sondas excelentes para estudiar el movimiento mecánico en el régimen cuántico, investigar la dinámica no lineal y llevar a cabo experimentos de ciencia de superficie en sistemas cristalinos de baja dimensión. La explotación de todo el potencial de los resonadores mecánicos sigue siendo un objetivo: alcanzar el límite fundamental de la resolución de la detección de masas impuesta por el ruido termomecánico del resonador. Actualmente, las limitaciones se deben normalmente al ruido en la transducción de movimiento. Los resonadores de nanotubos y grafeno son particularmente sensibles al ruido en la detección, ya que sus dimensiones intrínsecamente pequeñas producen señales eléctricas u ópticas transducidas minúsculas. Esta tesis investiga formas de mejorar la resolución de masa de los sistemas de resonadores mecánicos intrínsecamente más pequeños, que se basan en grafeno suspendido y nanotubos de carbono. Para esto, seguimos dos caminos complementarios. Primero vemos hasta dónde podemos llegar en términos de resolución de masa con resonadores de grafeno al reducir su tamaño. Fabricamos resonadores de grafeno de doble sujeción con longitudes submicrométricas y medimos sus propiedades mecánicas a 4,2 K. La estabilidad de la frecuencia de los resonadores nos permite evaluar su resolución de masa. Mostramos que la estabilidad de la frecuencia de los resonadores de grafeno está limitada por la imprecisión de la detección del movimiento mecánico. Luego desarrollamos un nuevo esquema de downmixing eléctrico para leer el movimiento mecánico con un ruido más bajo en comparación con las técnicas anteriores. Utiliza un resonador RLC junto con un amplificador basado en un transistor de alta movilidad de electrones operado a 4,2 K. La relación señal / ruido se mejora gracias a la lectura de la señal a mayor frecuencia (1,6 MHz en comparación con 1-10 kHz) y a la amplificación a temperatura baja. Observamos una mejor estabilidad de la frecuencia midiendo un resonador mecánico de nanotubos de carbono con esta lectura. La estabilidad ya no está limitada por el ruido de la instrumentación de medición, sino por el propio dispositivo. Observar las fluctuaciones intrínsecas del resonador permite en futuros experimentos estudiar fenómenos de ciencia de superficie. Presentamos algunos resultados preliminares que apuntan a la observación de difusión de átomos de xenón en la superficie del resonador y a la adsorción de moléculas individuales de fulereno

RF MEMS/NEMS RESONATORS FOR WIRELESS COMMUNICATION SYSTEMS AND ADSORPTION-DESORPTION PHASE NOISE

During the past two decades a considerable effort has been made to develop radio-frequency (RF) resonators which are fabricated using the micro/nanoelectro-mechanical systems (MEMS/NEMS) technologies, in order to replace conventional large off-chip components in wireless transceivers and other high-speed electronic systems.The first part of the paper presents an overview of RF MEMS and NEMS resonators, including those based on two-dimensional crystals (e.g. graphene). The frequency tuning in MEMS/NEMS resonators is then analyzed. Improvements that would be necessary in order for MEMS/NEMS resonators to meet the requirements of wireless systems are also discussed.The analysis of noise of RF MEMS/NEMS resonators and oscillators is especially important in modern wireless communication systems due to increasingly stringent requirements regarding the acceptable noise level in every next generation. The second part of the paper presents the analysis of adsorption-desorption (AD) noise in RF MEMS/NEMS resonators, which becomes pronounced with the decrease of components' dimensions, and is not sufficiently elaborated in the existing literature about such components. Finally, a theoretical model of phase noise in RF MEMS/NEMS oscillators will be presented, with a special emphasize on the influence of the resonator AD noise on the oscillator phase noise

Chemical Self-Assembly Strategies Toward the Design of Molecular Electronic Circuits

The field of molecular electronics is generally divided into one of two major categories, the first focusing on the unique functionalization of single molecules to produce electronic behavior, the other utilizing large assemblies of molecules to produce electronic behavior. The former approach is largely attributed to the seminal paper by Aviram and Ratner in which they proposed a molecular donor-bridge-acceptor (D-B-A) type architecture could lead to single molecule rectification producing electronic effects similar to conventional semiconductor based diodes. Extensive research has been carried out in both fields as it is foreseen that new approaches to electronics miniaturization will be necessary in the near future. In the following research, the focus turns to a seemingly overlooked area of molecular electronics, this being the necessity for designed interconnects of nanoscale electrodes. The approach to problem utilized the well studied oligomerization properties of 1,4-phenylene diisocyanide (PDI), which upon exposure to gold incorporates gold adatoms to form conductive one-dimensional oligomers of the form -(Au-PDI)n- Monte Carlo simulations along with conductivity studies of nanoparticle arrays both suggest the oligomerization is inherently self-limiting, providing a potential avenue toward controlled interconnection of nanoelectrodes and design of molecular electronic circuits

Focused electron- and ion-beam induced processes:in situ monitoring, analysis and modeling

Focused electron- and ion-beam induced processing are well established techniques for local deposition and etching that rely on decomposition of precursor molecules by irradiation. These high-resolution nanostructuring techniques have various applications in nanoscience including attach-and-release procedures in nanomanipulation and fabrication of sensors (magnetic, optical and thermal) for scanning probe microscopy. However, a complete physical and chemical understanding of the process is hampered by the lack of suitable means to monitor and to access the numerous interrelated and time-varying process parameters (deposition and etch rate, yield, molecule flux and adsorption/desorption). This thesis is a first attempt to fill this gap. It is based on experimental and simulative approaches for the determination of process conditions and mechanical properties of deposited materials: Mass and force sensors: The use of tools merging micromechanical cantilever sensors and scanning electron microscopy was demonstrated for in situ monitoring and analysis. A cantilever-based resonant mass sensing setup was developed and used for real-time mass measurements. A noise level at the femtogram-scale was achieved by tracking the resonance frequency of a temperature stabilized piezoresistive cantilever using phase-locking. With this technique the surface coverage and residence time of (CH3)3PtCpCH3 molecules, the mass deposition rate, the yield, and the material density of corresponding deposits were measured. In situ cantilever-based static force sensing and mechanical modal vibration analysis were employed to investigate the Young's modulus and density of individual high aspect ratio deposits from the precursor Cu(hfac)2. Precursor supply simulations and experiments: A prerequisite to understand and quantify irradiative precursor chemistry is the knowledge of the local flux of molecules impinging on the substrate. Therefore, Monte Carlo simulations of flux distributions were developed and gas flows injected into a vacuum chamber were analyzed experimentally for the precursors Co2(CO)8, (hfac)CuVTMS, and [(PF3)2RhCl]2. The process parameters extracted from the mentioned approaches are valuable input for numerical focused electron- and ion-beam induced process models (Monte Carlo, continuum). We evaluated the precursor surface diffusion coefficient and the electron impact dissociation cross-section by relating deposit shapes to a continuum model