Patchy Surfaces

Tailoring solid surfaces attracts a number of technological and scientific interest with numerous applications. One method to modify a solid surface is via the formation of a molecular film. The adsorbed molecules assemble spontaneously (self assemble) forming a monolayer that is called self-assembled monolayer (SAM). When dissimilar molecules are employed to form a SAM on a surface, separation into domains takes place, but the domain shapes that have been reported vary from irregular patches with highly non-uniform size distributions to disordered stripes or worm-like domains. It was postulated that the various morphologies observed to date were due to kinetic trapping. The scope of this thesis was to investigate the thermodynamic equilibrium for such monolayers. High quality one- and two-component thiol SAMs on flat Au(111) was produced and studied. The thiols that were used were Octanethiol (OT), 4-Cyano-1-butanethiol (CN4T), para-nitrothiophenol (NB4M) and 3-mercaptopropionic acid (MPA). The SAMs were produced via solution immersion. XPS and contact angle measurements verified the SAM formation. Cyclic voltammetry has been employed for the reductive desorption of the bounded thiols. The reductive potentials of the thiols have been measured at -0.97 V for OT, -0.79 V for CN4T, -0.75 V for the NB4M and -0.78 V for the MPA. The difference in the peak potentials facilitates our studying of binary SAMs by this method. Binary SAMs of OT:CN4T, OT:NB4M and OT:MPA at different feed ratios was produced via solution immersion. The surface composition of the OT:CN4T and OT:NB4M binary SAMs was determined by XPS. The CV analysis of the binary SAMs showed two distinct separated peaks at the reductive potentials of the two constituting thiols, which indicates the existence of phase separated macro domains onto the Au(111) surface. A large series of one- and two-component SAMs were thermally treated inside neat solvent at 60°C (annealing) for various amounts of time. XPS and contact angle measurements verified that the annealing process did not cause any alterations on the composition or the quality of the SAMs. Upon annealing, the evolution of the binary SAM surfaces was found to lead to new nanoscale thermodynamic phases as indicated by the voltammograms. The two-peak profiles that were seen at the non-annealed binary SAMs changed to three-peak profiles, with the appearance of a new peak at an intermediate potential. As the annealing time increased a single peak profile was received. The one-peak CV profiles remained unaltered (one peak at an intermediate potential) upon further annealing. The new phase is therefore the thermodynamically equilibrium phase. The new equilibrium phase was identified by STM. The new phases were found to depend on the choice of ligand and the composition. STM imaging elucidated further that the new phase can morphologically be stripes with an average width of ~3 nm for annealed SAMs of OT:CN4T and OT:NB4M, prestripes for OT:NB4M binary SAMs with higher OT concentration, while OT:CN4T at 20 days of annealing and OT:MPA 10:90 led to micellar domains in the size range from 4 to 8 nm as the thermodynamic phase. The work presented in this thesis has shown how the process of annealing leads to new thermodynamically equilibrium phases in binary SAMs. Due to this process, nanostructured surfaces with a variety of domains can be achieved in binary SAMs of thiols on flat Au(111) surfaces

Probing the interaction of nanoparticles with small molecules in real time via quartz crystal microbalance monitoring

Y. Y. acknowledges University College London (UCL) for the Overseas Research Scholarship and the Graduate Research Scholarship. The project received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement no. 633635 (DIACHEMO) and the EPSRC (grant number EP/J500549/1).Despite extensive advances in the field of molecular recognition, the real-time monitoring of small molecule binding to nanoparticles (NP) remains a challenge. To this end, we report on a versatile approach, based on quartz crystal microbalance with dissipation monitoring, for the stepwise in situ quantification of gold nanoparticle (AuNPs) immobilisation and subsequent uptake and release of binding partners. AuNPs stabilised by thiol-bound ligand shells of prescribed chemical composition were densely immobilised onto gold surfaces via dithiol linkers. The boronate ester formation between salicylic acid derivatives in solution and boronic acids in the AuNP ligand shell was then studied in real time, revealing a drastic effect of both ligand architecture and Lewis base concentration on the interaction strength. The binding kinetics were analysed with frequency response modelling for a thorough comparison of binding parameters including relaxation time as well as association rate constant. The results directly mirror those from previously reported in-depth studies using nuclear magnetic resonance spectroscopy. By achieving quantitative characterisation of selective binding of analytes with molecular weight below 300 Da, this new method enables rapid, low cost, rational screening of AuNP candidates for molecular recognition.Publisher PDFPeer reviewe

Electrochemical Behavior of Carbon Electrodes for In Situ Redox Studies in a Transmission Electron Microscope

Electrochemical liquid cell transmission electron microscopy (TEM) is a unique technique for probing nanocatalyst behavior during operation for a range of different electrocatalytic processes, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), or electrochemical CO2 reduction (eCO(2)R). A major challenge to the technique's applicability to these systems has to do with the choice of substrate, which requires a wide inert potential range for quantitative electrochemistry, and is also responsible for minimizing background gas generation in the confined microscale environment. Here, we report on the feasibility of electrochemical experiments using the standard redox couple Fe(CN)(6)(3-/4)(-) and microchips featuring carbon-coated electrodes. We electrochemically assess the in situ performance with respect to flow rate, liquid volume, and scan rate. Equally important with the choice of working substrate is the choice of the reference electrode. We demonstrate that the use of a modified electrode setup allows for potential measurements relatable to bulk studies. Furthermore, we use this setup to demonstrate the inert potential range for carbon-coated electrodes in aqueous electrolytes for HER, OER, ORR, and eCO(2)R. This work provides a basis for understanding electrochemical measurements in similar microscale systems and for studying gas-generating reactions with liquid electrochemical TEM

A review of molecular phase separation in binary self-assembled monolayers of thiols on gold surfaces

Binary self-assembled monolayers (SAMs) on gold surfaces have been known to undergo molecular phase separation to various degrees and have been subject to both experimental and theoretical studies. On gold nanoparticles in particular, binary SAMs ligand shells display intriguing morphologies. Consequently, unexpected behaviors of the nanoparticles with respect to their biological, chemical, and interfacial properties have been observed. It is critical that the phase separation of binary SAMs be understood at both molecular and macroscopic level to create, and then manipulate, the useful properties of the functionalized surfaces. We look into the current understanding of molecular phase separation of binary SAMs on gold surfaces, represented by Au(111) flat surfaces and Au nanoparticles, from both theoretical and experimental aspects. We point out shortcomings and describe several research strategies that will address them in the future

Bimodal atomic force microscopy for the characterization of thiolated self-assembled monolayers

Surface coatings are becoming an integral part of materials. In recent years, molecular coatings have found larger acceptance and uses. Among them, self-assembled monolayers (SAMs) are attractive due to their inherent versatility, manufacturability, and scale up ease. Understanding their structure-properties relationships in realistic conditions remains a major challenge. Here we present a methodology based on simultaneous topographical and nanomechanical characterization of SAMs using a commercially available setup for bimodal atomic force microscopy (AFM). It allows for accurate and quantitative measurement of surface elasticity, which is correlated to molecular ordering through topographical imaging. Our results indicate that effective surface elasticity (E*) scales with monolayer formation-time and ligand-length, parameters known to affect ligand ordering. The method developed, is extended to provide localization of the chemical species present in thiolated binary SAMs. Within the systems tested phase separation down to approximate to 10 nm domains could be observed both in the topography and in the elasticity channel

Combined and Distinct Contributions of Different Carbon Nano-Forms in Polypropylene

In the present work a thorough investigation of how small amounts of multiwall carbon nanotubes (CNTs), nanodiamonds (NDs), and combinations between them affect the properties and performance of polypropylene (PP) is presented. An improvement on the mechanical properties of the nanocomposites as a consequence of the incorporation of CNTs and NDs is detected which is also related to their crystalline characteristics as well as melting and crystallization kinetics. In specific ratios, property enhancements caused by the combined incorporation of the nanofillers exceed the corresponding reinforcements induced by each type of filler separately and are probably linked with their dispersion state. The incorporation of CNTs and NDs does also increase effectively the thermal conductivity of PP, while the two fillers affect this property distinctly. The detected cumulative trends of the separate effects of the fillers confirm their distinct contributions on heat capacity and thermal diffusivity, marking a notable difference in the reinforcement mechanism of the mechanical versus thermal properties

Additive micro-manufacturing of crack-free PDCs by two-photon polymerization of a single, low-shrinkage preceramic resin

Additive manufacturing (AM) methods are being integrated in ceramics fabrication either as the main manufacturing tool or for auxiliary purposes. By using polymers, powders and preceramic formulated materials, AM techniques are pushing towards higher resolution, lower shrinkage and shorter building time. Herein, we present the fabrication of ceramic microstructures (<200x200x200 μm3) with sub-micrometer resolution based on two-photon polymerization (TPP). 3D structuring of a preceramic resin by photopolymerization produces a so-called green body. The final ceramic part is obtained after pyrolysis of the green body. The high-resolution 3D shaped structures that we demonstrated could be employed as tools or components for microdevices. We report a lower linear shrinkage of 30% of TPP green bodies from a polysiloxane precursor with low porosity, no cracks and no significant shape distortion after pyrolysis, which implies the potential for highly controllable manufacturing of micro-ceramic parts based on commercially available chemical compounds. The protocol for preparing, fabricating and developing the resin is detailed