Silicon-based nanostructures are essential building blocks for nanoelectronic devices and nano-electromechanical systems (NEMS). As the silicon device size continues to scale down, the surface to volume ratio becomes larger, rendering the properties of surfaces and interfaces more important for improving the properties of the nano-devices and systems. One of those properties is the friction, which is important in controlling the functionality and reliability of the nano-device and systems. The goal of this dissertation is to investigate the deformation and friction behaviors of single crystalline silicon nanolines (SiNLs) using nanoindentation techniques. Following an introduction and a summary of the theoretical background of contact friction in Chapters 1 and 2, the results of this thesis are presented in three chapters. In Chapter 3, the fabrication of the silicon nanolines is described. The fabrication method yielded high-quality single-crystals with line width ranging from 30nm to 90nm and height to width aspect ratio ranging from 10 to 25. These SiNL structures have properties and dimensions well suited for the study of the mechanical and friction behaviors at the nanoscale. In Chapter 4, we describe the study of the mechanical properties of SiNLs using the nanoindentation method. The loading-displacement curves show that the critical load to induce the buckling of the SiNLs can be correlated to the contact friction and geometry of SiNLs. A map was built as a guideline to describe the selection of buckling modes. The map was divided into three regions where different regions correlate to different buckling modes including Mode I, Mode II and slidingbending of SiNLs. In Chapter 5, we describe the study of the contact friction of the SiNL structures. The friction coefficient at the contact was extracted from the loaddisplacement curves. Subsequently, the frictional shear stress was evaluated. In addition, the effect of the interface between the indenter and SiNLs was investigated using SiNLs with surfaces coated by a thin silicon dioxide or chromium film. The material of the interface was found to influence significantly the contact friction and its behavior. Cyclic loading-unloading experiments showed the friction coefficient dramatically changed after only a few loading cycles, indicating the contact history is important in controlling the friction behaviors of SiNLs at nanoscales. This thesis is concluded with a summary of the results and proposed future studies. / text

L'étude de la dynamique des parois de domaines dans les nano-systèmes ferromagnétiques est cruciale pour le développement des dispositifs de stockage de l'information basés sur le déplacement et le contrôle des parois. Ces dispositifs ont plusieurs avantages : non-volatilité, rapidité d'exécution, haute densité de stockage, et faible consommation de l'énergie. En utilisant des méthodes micro-magnétiques et analytiques, nous avons constaté que l'interaction entre deux parois affectait les processus de dépiégeage sous champ magnétique, dans des nanofils en nickel à géométrie cylindrique et planaire. Nous avons mis en évidence des comportements non linéaires de la dynamique d'une paroi piégée, qui varient selon le matériau et le type de piège utilisé. Les diagrammes de phases représentant l'exposant de Lyapunov ont permis la distinction entre des zones chaotiques et périodiques, en fonction de la fréquence et de l'amplitude d'une excitation harmonique. Nous avons présenté des résultats sur la manipulation précise d'une paroi transverse sous impulsions de courant dans un nanofil planaire en nickel, structuré par une multitude de défauts artificiels. Nous avons montré que le positionnement exact de la paroi à température ambiante est possible uniquement pour des impulsions symétriques de très courte durée. Des effets inertiels pouvant s'opposer au couple de transfert de spin, ou au contraire l'amplifier ont été observés. Ces derniers résultats ouvrent une route vers le déplacement des parois dans les deux directions par des impulsions unipolaires de courant. / The study of the domain wall dynamics in ferromagnetic nano-systems is crucial for the developement of data-storage devices based on control and displacement of the domain walls. These devices have several advantages : non-volatility, fast execution time, high density, and low power consumption. Using micromagnetics and analytical methods, we have shown that the interaction between two domain walls influences the depinning process under magnetic field, in cylindrical and planar shaped nickel nanowires. We highlighted the nonlinear behaviour of the dynamics of a pinned domain wall, which varies with the material properties and the type of the pinning sites. The Lyapunov phase diagrams display chaotic and periodic regions function of the amplitude and frequency of a harmonic excitation. We have also presented results about the precise manipulation of transverse domain walls by current pulses in a nickel planar nanowire with artificial defects. We have shown that exact positioning of the domain walls at room temperature is possible only for very short symmetric current pulses. We observed inertial effects which can oppose or amplify the spin transfert torque effect. These results open a route to domain wall displacement in both directions with unipolar current pulses.

Many material properties depend on specific details of microstructure and both optimal material performance and material reliability often correlate directly to microstructure. In nano- and micro-systems, the material's microstructure has a characteristic length scale that approaches that of the device in which it is used. Fundamental understanding and prediction of material behavior in nano- and micro-systems depend critically on methods for computing the effect of microstructure. Methods for including the physics and spatial attributes of microstructures are presented for a number of materials applications in devices. The research in our group includes applications of computation of macroscopic response of material microstructures, the development of methods for calculating microstructural evolution, and the morphological stability of structures. In this review, research highlights are presented for particular methods for computing the response in: 1) rechargeable lithium ion battery microstructures, 2) photonic composites with anisotropic particulate morphologies, 3) crack deflection in partially devitrified metallic glasses. / Singapore-MIT Alliance (SMA)