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

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.

Two-Dimensional Metal-Organic Networks as a New Class of Electrocatalysts

Benjamin Wurster

This thesis deals with the electrocatalytic properties of two-dimensional metal organic coordination networks (2D-MOCNs) self-assembled on electrode surfaces and catalytic activity of 2D-MOCNs is demonstrated for the first time. The low-coordinated metal centers within these supramolecular structures resemble the catalytically active sites of metallo-enzymes. The activity of the metal centers is determined by the type of the metal and the coordination environment; both parameters can be carefully adjusted in 2D-MOCNs. In order to study and correlate electrocatalytic properties with the atomic structure of 2D-MOCNs, a scanning tunnelling microscope operating in ultra-high vacuum (UHV) was combined with an electrochemical (EC) setup. Sample preparation and scanning tunneling microscopy (STM) were conducted under UHV conditions. By STM the composition and structure of the networks were controlled and characterized prior to EC experiments. A transfer system between UHV and EC instrumentation was constructed to keep the sample at all times in a clean and controlled environment. It is demonstrated that the mechanism of the oxygen reduction reaction (ORR) can be influenced by the type of the metal center in the network. The networks formed by benzene-tricarboxylic acid (TMA) with either Fe or Mn atoms are structurally identical, but their electrochemical signal differs significantly. Both networks catalyze the reduction of O2 to H2O, but on different pathways. These results emphasize the key role of the unsaturated metal centers in the electrocatalytic reduction. The electrocatalytic response in the ORR can also altered by the distinct coordination environment; this is shown for Fe atoms nitrogen-coordinated by either tetracyanoquinodimethane (TCNQ), bis-pyridyl-bipyrimidine (PBP) or phythalocyanine (Pc). Depending on the specific coordination environment formed by the different ligands the the final product (H2O2 or H2O) and the mechanism of the ORR is changed. In the last part of this thesis the two metal binding sites of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) are used to selectively incorporate different metal centers in a fixed organic environment and create homo- and hetero-bimetallic networks. The coupling of the two metals centers promoted by the organic environment modifies the electrocatalytic response. The correct combination of two metal centers in the right positioning makes an efficient catalyst. In the ORR the peak and onset potential is varied depending on the metal combination. In the case of the oxygen evolution reaction a non-linearly increased catalytic activity by the cooperative bimetallic effect is obtained. The results presented in this thesis demonstrate the high potential of 2D-MOCNs for heterogeneous catalytic chemical conversions. Engineering of the network structure and the distinct coordination environment of the metal centers offers the possibility to tune their catalytic activity. This opens up a new route for the design of a new class of nanocatalyst materials.

EPFL2015

Surface-bound nanostructures

Kyumin Lee

This thesis looks at surface-bound nanowires and nanoparticles, the mechanical and the metrological properties of which are of practical importance in the realization of nanometer-scale electronics. Atomic force microscope (AFM) was the main instrument of research. By bending suspended carbon nanotube structures with the AFM, the Young's modulus of carbon nanotubes has been measured. An efficient new technique that involves an ac-dielectrophoresis preparation of carbon nanotubes on v-groove GaAs substrates and a force-curve measurement of the stiffness has been devised. The Young's modulus of a batch of multiwall carbon nanotubes grown by a single chemical vapor deposition (CVD) process shows a strong diameter dependence, indicating that the small catalyst particles produce crystalline tubes, while the thicker particles produce low-quality tubes with an abundance of structural defects. The experimental result is a strong evidence for the metastable-catalyst growth model of carbon nanotubes in CVD — the growth kinetics of carbon nanotubes is determined by the catalyst's liquid skin, which is more stable for smaller catalysts. As the nanotube study highlighted the importance of the size of catalyst nanoparticles, the topic of accurate nanoparticle sizing by dynamic AFM was then investigated. The measured size of a surface-bound nanoparticle was found to vary with imaging parameters, and a theoretical modeling showed that the non-contact — intermittent-contact mode switching can lead to discrepancies. Experimental results confirmed that the mode switching indeed causes the largest error in size measurements. A discrepancy also exists between the all-non-contact-mode and all-intermittent-contact-mode cases, and this anomaly could be explained by the effects of particle-substrate deformation and capillary forces. Nanoparticles were prepared on surfaces by boiling colloid drops on hot surfaces, a new technique developed for the uniformly dispersed deposition of colloidal nanoparticles and nanowires. Our experiments suggest that the actual deposition occurs through the smooth dewetting of liquid microdrops at elevated temperatures. The method is applicable on a wide range of surfaces and materials. Finally, a general haptic interface for the AFM was realized. The interface can be implemented on different AFM models with little effort, and it supports both the contact- and dynamic-AFM operations. Manipulation of gold nanoparticles has been carried out by raster scanning at different dynamic AFM setpoints, a promising approach for a quantitative study of the nanoparticle-substrate friction.

EPFL2008

Stimuli-responsive nanostructured surfaces

Ana Maria Popa

The coupling of stimuli-responsive macromolecules to nanostructured surfaces opens the perspective for the fabrication and integration of "smart" micro- and nano-electro-mechanical systems (MEMS&NEMS) in which the smallest motile unit is the polymeric chain. The goal of this work was to develop thermoresponsive, durable nanoporous surfaces as a first step toward the fabrication of functional nanocontainers and nanovalves. The proposed strategy involves in a first step the design and implementation of a clean-room compatible, reproducible and up-scalable method for defining organized nanoporous arrays in MEMS compatible surfaces, such as silicon, silicon oxide and silicon nitride. The main objective of the second step was the covalent immobilization of responsive macromolecules on the obtained structures, as to provide the later with a "smart" behavior. The nanostructuring step was realized by block-copolymer assisted lithography. This newly developed technique enables the definition of nanoscale features on semiconductor substrates over large surface areas and in a cost effective manner. A novel method for patterning thin metal films by a lift-off approach, using spin coated block-copolymer micelles monolayers as a template, was investigated. The obtained nanoporous mask was used as resist for the structuring of hard substrates in plasma-assisted processes and structures with aspect ratios as high as 1:10 were be obtained. By using different block-copolymer micelles and spin-coating conditions, the proposed approach enables the variation of the nanopores' diameter from 40 to 80 nm and a surface coverage between 11.7% and 25.5 % in an independent manner. This technique was successfully integrated in a standard microfabrication process for the batch production of thin nanoporous silicon nitride membranes. The 100 nm thick membranes span over thousands of square micrometers and present porosities higher than 109 pores/cm2. They are thus competitive in terms of porosity with the commercially available polymeric membranes. In addition they are two orders of magnitude thinner than polymeric membranes, this making them extremely interesting for applications where permeation rate or response time is important. The rapid flow of water soluble small molecular species through the nanopores was demonstrated in a qualitative manner in this work. Another series of investigations targeted the functionalization of the available nanoporous surfaces with responsive molecules. Poly(N-isopropyl acrylamide) (PNIPAAM) was chosen as a model system. Its coupling to silicon surfaces by a "grafting-to" approach from melt, using an epoxy-functionalized silane as anchoring layer was studied. The analysis of the yielded grafting density showed results superior to those obtained by classic grafting-to approaches. The relation between the polymer's weight and its responsiveness was investigated in liquid media by atomic force microscopy and by dynamic force spectroscopy. Polymers with more than 500 monomers in the back bone were shown to exhibit enhanced responsiveness. Additionally, two bioengineered polymers from the class of elastin-like polymers (ELPs) were investigated as possible candidates for the fabrication of stimuli responsive nanovalves. The molecules were synthesized in collaboration with the Bioforge group (Universidad de Valladolid, Spain) and details of the bioproduction process are presented. A method for the covalent photoattachment of macromolecules using a benzophenone-functionalized silane, was tested in this work. First the capacity of the benzophenone silane to graft synthetic thermo-responsive polymers such as PNIPAAM was demonstrated. The temperature-induced morphology change of the photografted PNIPAAM was not affected by this anchoring strategy and was monitored by tapping AFM in liquid media. The investigations were then extended to the photo-immobilization of ELPs on nanoporous silicon surfaces and the XPS measurements revealed the success of this step. However no conformation modification of the proteins could be evidenced by tapping-mode AFM in liquid media under different ionic strengths.

EPFL2009