Everything, everywhere, all at once - Micro- and Nano-fabrication for sensitive, homogeneous and spatio-temporally-resolved raman and infrared spectroscopy sensors
Ketki Srivastava is a PhD student in the Department Biomedical and Environmental Sensorsystems. (Co)Promotors are prof.dr.ir. M. Odijk and prof.dr.ir. A. van den Berg from the Faculty of Electrical Engineering, Mathematics and Computer Science and dr. W. van der Stam from Utrecht University.
Catalysis is a critical process in the chemical industry with applications spanning in fields such as petroleum refining, environmental protection, and pharmaceuticals. Catalysts particles are used to accelerate chemical reactions and understanding their structure-function dynamics is vital to improving their performance for better efficiency and throughput. Multiscale studies to obtain kinetic and real-time information such as reaction rates and intermediates of catalytic reactions is therefore, crucial for researchers in the catalysis field as this data access not only aids in designing better catalysts, but also enhances the efficiency of many industrially used catalytic reactions. To achieve this, advancements in miniaturized devices capable of single catalyst particle screening are necessary. Furthermore, improved signal-to-noise ratios and enhanced resolution are important for the vibrational spectroscopy techniques widely employed in studying catalyst particles to attain the desired characteristics. To address these needs, enhancement methods are utilized, often involving patterned nanostructures that provide high electric field intensities for probing the desired analyte or molecule.
The primary aim of this thesis is to explore various micro- and nano-fabrication tools to fabricate Raman and infrared active substrates that can be used for probing catalysts under their native reaction conditions (i.e., to monitor chemical reactions in situ). These substrates should be capable of offering high enhancement factors with low variance for sensitive and homogeneous detection. Additionally, the sensors should also be able to obtain kinetic information in microreactors for spatio-temporal characterization.
This thesis is broadly divided into two sections: a) Surface-enhanced Raman spectroscopy (SERS) devices focusing on lithographic tools for homogeneous sensing and b) Infrared spectroscopy (IR) for obtaining chemical information in microreactors. A short description of the chapters included in this thesis is given below:
Chapter 1 introduces the research topic and provides motivation for this thesis. The title of this thesis and main goals of the thesis are explained in this chapter.
Chapter 2 provides a theoretical background on the concepts discussed in this thesis. Topics such as surface-enhanced Raman spectroscopy, infrared spectroscopy, microfabrication and microfluidics are covered in details.
Chapter 3 discusses the state of the art of various lithographic tools to fabricate lithographic SERS substrates. The text delves deeply into established lithographic methods, including electron beam lithography and displacement Talbot lithography, as well as emerging techniques like ink-jet printing and dip-pen lithography. It offers a comparative overview and discusses the current state of the field in relation to commercially available substrates.
Chapter 4 reports a combination of displacement Talbot lithography and glancing angle deposition to create uniform SERS sensing substrates composed of gold nanoparticles on a silicon nanocone array. Various parameters for the glancing angle deposition are optimized, resulting in an enhancement factor in the order of 108, as determined using benzenethiol as a probe molecule. The uniformity of the sensing capabilities is demonstrated by measuring a 100 µm² area, with a recorded variance of only ~4%.
Chapter 5 investigates lithographic techniques for fabricating SHINER substrates, which provide a more uniform and reproducible sensing platform compared to randomly deposited metal nanoparticles. In this work, we studied three case to demonstrate alternative methods with which SHINER substrates can be fabricated. For lithographic fabrication, we used a combination of electron beam lithography and atomic layer deposition to form uniform, pin-hole free shell-isolated nanostructures.
In Chapter 6, the limitations of SHINER substrates used in Chapter 5 are improved on. One of the drawbacks leading to a lower enhancement of SHINER substrates in Chapter 5 was the far-field coupling of the nanostructures. In Chapter 6, improvements to the electron beam lithography process are made to achieve nanostructure spacings of ~ 20 nm. The method is used in combination with nanoimprint lithography to reveal low-cost and reproducible nanostructures.
Chapter 7 discusses the various process steps that were employed in the optimization of a negative electron beam resist – AR n7520.18. Dose test consisting of nanopillar arrays with varying pitch, lattice organization and diameters were conducted and the optimal conditions were determined to fabricate nanopillars that can be further processed to fabricate gold hollow nanopillars.
Chapter 8 reports the fabrication of an ATR integrated microreactor to monitor chemical reactions. In this work, the use a single-bounce ATR accessory allows one to spatially and temporally resolve a chemical reaction. As a proof of concept, we monitored an SN2 chemical reaction to determine the diffusion coefficients of the reactants and the rate constant of the chemical reaction. Here, we used a combination of synchrotron infrared measurements and numerical simulations to determine these values.
Chapter 9 improves on the functionalities of the devices fabricated in Chapter 8. In this work, the ATR-microreactor is integrated with a 6-unit rotational gradient micromixer to achieve ~ 100% mixing efficiency at flowrates as high at 30 µL/min. The mixing was confirmed using experimental results in addition to numerical simulations. The chapter focuses on the fabrication of the upgraded devices exploring various fabrication methods such as Bosch-based etching processes, maskless-laser writing and thermal oxidation.
Chapter 10 is compilation of work dedicated to the improvements in the fabrication of nanostructures for surface-enhanced infrared absorption spectroscopy (SEIRAS). Various process flows are optimized in this work to achieve SEIRAS nanostructures on silicon substrates and silicon-rich nitride membranes.
Finally, Chapter 11 provides a conclusion to the work done in this theses with a short summary of the individual thesis chapters. This is followed by the funding and contributions and the scientific output of this thesis.
