Three-Dimensional Crystalline and Homogeneous Metallic Nanostructures Using Directed Assembly of Nanoparticles

Directed assembly of nano building blocks offers a versatile route to the creation of complex nanostructures with unique properties. Bottom-up directed assembly of nanoparticles have been considered as one of the best approaches to fabricate such functional and novel nanostructures. However, there is a dearth of studies on making crystalline, solid, and homogeneous nanostructures. This requires a fundamental understanding of the forces driving the assembly of nanoparticles and precise control of these forces to enable the formation of desired nanostructures. Here, we demonstrate that colloidal nanoparticles can be assembled and simultaneously fused into 3-D solid nanostructures in a single step using externally applied electric field. By understanding the influence of various assembly parameters, we showed the fabrication of 3-D metallic materials with complex geometries such as nanopillars, nanoboxes, and nanorings with feature sizes as small as 25 nm in less than a minute. The fabricated gold nanopillars have a polycrystalline nature, have an electrical resistivity that is lower than or equivalent to electroplated gold, and support strong plasmonic resonances. We also demonstrate that the fabrication process is versatile, as fast as electroplating, and scalable to the millimeter scale. These results indicate that the presented approach will facilitate fabrication of novel 3-D nanomaterials (homogeneous or hybrid) in an aqueous solution at room temperature and pressure, while addressing many of the manufacturing challenges in semiconductor nanoelectronics and nanophotonics.

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

The understanding of material self-assembling and self-organization mechanisms, at mesoscale, is crucial for nanotechnologies development. Such structures, hierarchically organized in superlattices or colloïdal crystals, are observed in inorganic or organo-metallic precipitates. The classical characterization, using electron microscopy and X-ray diffraction, of a synthesized powder is inadequate to describe the hierarchically organized polycrystalline structure of the final particles. Here, we have highlighted the successive steps that lead to the spontaneous formation of a cobalt oxalate dihydrate (COD) colloïdal crystals that precipitate in aqueous media. The characterization of the final particles, made by X-ray and electronic diffraction, has allowed us to determine the controversial crystalline phase of the precipitated COD. The β and γ COD phases has been discriminated by comparing experimental results with simulations of X-ray diffractogram and electron diffraction patterns for these both structures. The precipitated COD corresponds to the γ phase indicating that it comes from solid dehydration of cobalt oxalate tetrahydrate. The final powder characterization, performed by X-ray diffraction (XRD), atomic force microscopy (AFM), low voltage high resolution scanning electron microscopy (LVHRSEM) and transmission electron microscopy (TEM), gives some diverging results about monocrystalline or polycrystalline nature of the precipitates. XRD and AFM analysis show a polycrystalline structure of particles whereas LVHRSEM, TEM and electron diffraction observations indicate a monocrystalline structure. This apparent contradiction has led us to elaborate a new characterization method allowing to follow the growth and the structural evolution of the precipitate as a function of time. The cryo- preparation techniques are well adapted as they allow freezing and direct observation of solid state suspension by electron microscopy. The freeze/drying method has been modified and used for both SEM and TEM, and has allowed us to study the morphological and structural evolution of the COD precipitate from nanoscale to mesoscale. In addition, TEM analysis of samples prepared by classical techniques has been performed to complement cryo-electron microscopy observations. The combination of these two methods show that COD precipitation is a complex multi-step process leading to the formation of a core/shell heterostructure. The anisotropic core is porous and partially crystalline. It is formed by agglomeration of isolated primary particles and agregated ones (15-20 nm sized secondary particles). The crystalline shell corresponds to the final particles faces and is made up by the layer by layer self-assembling and alignment of 5-7 nm sized crystalline primary particles. These nanoparticles are aligned and perfectly ordered in strings of nanograins in the layer structure. The self-assembling occurs in the last step of growth where we have a lower ionic strength and supersaturation inducing slower kinetics of growth and aggregation. Moreover, the crystalline order in the primary and secondary particles increase with the time of reaction consistent with a continuous process of primary particle nucleation and growth. Our results show that self-assembling occurs layer by layer with terraces, kinks and steps that are present on the faces. This reminds the layer by layer crystalline growth models but in this case, the buiding blocks are colloïdal instead of atomic or molecular. Based on these different results, we propose an original model describing the COD precipitate growth. In addition, we have studied the poly(methacrylic acid-sodium salt) effect on the COD precipitation. This additive (PMMA) appeared to be a very good growth inhibitor. The PMMA effect on the final particles morphology is dramatic. Its concentration variation in solution slows down the nanoparticles aggregation rates along a [101] preferential direction. The crystal habit can then be controlled and we have synthesized platelets, cubes and rods with tailored length. The use of cryogenic electron microscopy has been shown to be a powerful tool for the understanding of time dependent aspects of particle growth. The use of such techniques for the study of other systems, where the final precipitate appearance may hide particle substructures, will help to elucidate growth mechanisms and allow finer control of nanostructured materials for many applications. The complex self-assembling and self-organization processes could then be highlighted and studied on a nanometer to micrometer scale.

EPFL2004

Fabrication of Functional Silicon Surfaces using Metallic Nanostructures Produced by Block Copolymer Self-Assembly

Li Wang

Self-assembly is the autonomous organization of components into patterns or structures without human intervention. At the molecular level, self-assembly usually involves non-covalent interactions of various natures, such as van der Waals, electrostatic and hydrophobic interactions, hydrogen and coordination bonds. Thus, this process provides one solution to the fabrication of ordered aggregates from components with typical sizes in the submicron scale, which is a key to applications in nanotechnology. The work presented in this thesis aims at producing hybrid and functional nanostructured silicon surfaces using self-assembled metallic nanostructures. As self-assembling tool, we use polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymers, which are amphiphilic molecules able to form well-defined nanostructures and to guide the positioning of metallic components into periodic arrangements. Various approaches were explored to reach this goal. First, self-assembled monolayers of PS-b-P2VP copolymer micelles were used as nanoreactors for the chemical synthesis of metal nanostructures of tunable dimensions and morphologies. This was achieved via a post-loading method, where the micelle cores are impregnated with metal salts after their deposition on silicon surfaces. Gold, silver and iron oxide nanoparticle (NP) arrays of controlled sizes and periodicities were produced. We also exploited the responsive properties of PS-b-P2VP micelles to create ordered arrays of gold nanorings having feature sizes below 100 nm. Surface plasmon resonance properties were studied and showed a significant change of optical properties between the gold particle and nanoring morphologies. Then, we focused our efforts on the controlled positioning of preformed gold NPs using PS-b-P2VP micelles as guiding templates. Two methods were developed. In the first, hierarchical positioning of NPs was achieved in one step during the spontaneous micelle phase inversion into nanoring structures. In the second, we used block copolymer-assisted lithography for producing periodic amino-silane nanopatterns, which then acted as electrostatic traps for the selective immobilization of gold NPs in aqueous solution. For both approaches, we showed the formation of ordered arrangements of individual gold NPs or clusters on silicon surfaces, whose morphologies, densities and periodicities could be tuned by simply varying the initial block copolymer molecular weights or deposition conditions. In a last part, we demonstrated the fabrication of functional silicon surfaces with high aspect ratio and controlled morphologies using self-assembled metallic patterns. Using a simple and efficient process called metal-assisted chemical etching (MAC-Etch), both highly porous silicon surfaces and ultra long silicon nanowires were produced. In particular, we reported for the first time the possible fabrication of free-standing nanoporous silicon membranes by this method and demonstrated the effective transport of molecules across the pores. Finally, the catalytic properties of self-assembled gold NPs were evaluated to grow vertically aligned silicon nanowires in a Low Pressure Chemical Vapor Deposition (LPCVD) reactor via a Vapor-Liquid-Solid (VLS) growth mechanism.

EPFL2011

Cutting Force Modelling for Drilling of Fiber-Reinforced Composites

Mihai-Bogdan Lazar

Although used in a very large variety of applications, drilling is one of the most complex and least understood manufacturing processes. Most of the research on drilling was done in the field of metal cutting for mechanical parts since, in this case, high precision and quality are needed. The use of composite materials in engineering applications has increased in recent years, and in many of these applications drilling is one of the most critical stages in the manufacturing process. This is because it is among the last operations in the manufacturing plan of composite parts. Delamination and extensive tool wear are among the problems which drilling of composite materials are currently facing. A major difference between metallic and composite plates is their structure: isotropic for metals and anisotropic for composite materials; meaning that while for metallic materials all the structure will respond in a similar manner under the machining loads, the composite structure will have localized responses from the same loads, leading to defects in the internal structure of the remaining work-piece material (i.e. delamination). Delamination can lead to failure in use and parts with such defects are usually discarded. Delamination is not usually visually detectable and special testing is necessary, affecting the costs of the final parts. Delamination during drilling was found to occur at tool entry (peel-up) or tool exit (push-out) and depends on the loads at inter-laminar level. The work presented in the current thesis focuses in providing reliable information about the thrust and torque distribution along the drill radius (and work-piece thickness) during drilling for varying cutting parameters, drill geometry and work-piece material. Such data should assist in the development of delamination models capable of capturing the influence of the drill geometry and cutting parameters on delamination onset and propagation during both exit and entry of the drill in the work-piece. A cutting force model is proposed to obtain the elementary cutting force distribution along the drill radius which is able to account for changes in axial feed rate and drill geometry. Based on oblique cutting, forces are considered on both rake and relief faces. A generic relationship in the form of a transformation matrix is developed to relate oblique cutting to drilling, valid for any drill geometry. The mathematical description of the drill geometry in the scope of cutting force modeling has been revised. The kinematics of the drilling process is now taken into account for (i) all geometrical parameters of the drill and for (ii) the elementary cutting forces decomposition. Additionally, a new drill type and its geometric features have been described mathematically and the definition of the geometrical parameters has been generalized so that other drills types or variations could be easily implemented into the model. It proved therefore possible to adopt simpler expressions for the empirical force coefficients of the cutting force model. Up to four empirical coefficients are used, which are calculated from experiments for each work-piece material and drill type. Most experimental investigations on drilling fiber reinforced composites analyze only the total thrust and torque generated during drilling or separately the forces caused by the chisel edge and cutting lips by drilling with or without a pilot hole. The later type of analysis suggested that is possible to obtain more detailed information about the distribution of the loads in drilling from the analysis of the forces variation during tool entry into the work-piece. Pursuing this direction, an experimental analysis method is proposed to obtain the axial and tangential elementary cutting force distribution along the tool radius or work-piece thickness. The cutting force distribution obtained experimentally was used to calibrate the cutting force model, rather than the total thrust and torque. The experimentally obtained cutting force distribution can also be used alone for analyzing the drilling process (i.e. the loads distribution among the plies of the composite laminate and how this load is influenced by changes in the drill geometry and the cutting conditions).

EPFL2012