Electrical contacts to single nanowires: a scalable method allowing multiple devices on a chip. Application to a single nanowire radial p-i-n junction

Semiconductor nanowires are currently at the forefront of research in the areas of nanoelectronics and energy conversion. In all these studies, realising electrical contacts and statistically relevant measurements is a key issue. We propose a method that enables to contact hundreds of nanowires on a single wafer in an extremely fast electron beam lithography session. The method is applied to nanowire-based radial GaAs p-i-n junction. Current-voltage characteristics are shown, along with scanning photocurrent mapping

Nanostructured Microelectrodes for High-Activity Electrochemistry in Biosensing and CO2 Recycling

Electrochemical biosensors are promising candidates for point-of-care (POC) diagnostics, since they require neither costly instrumentation nor expert operators. However, for them to become widely used in a clinical setting, biosensors must be enhanced in sensitivity without increasing costs or compromising the speed of detection. The goal of this thesis is to realize this vision by promoting high-activity redox reactions via rational design of the size, shape and structure of electrodes. Incorporating nanostructures on the surface of microelectrodes greatly increases their surface area and reaction rates, while maintaining efficient diffusion of analytes to electrode surfaces. Here, by coupling optimized electrode morphology with an enzyme-based assay, I report on a new class of ultra-sensitive biosensors. I showcase the applicability of these sensors for POC diagnostics by incorporating them on a platform designed to isolate rare cells in biological samples. The integrated device is the first example of an electrochemical sensing platform capable of detecting cancer cells with clinically-relevant sensitivity and specificity. Decreasing the cost of POC diagnostic devices is vitally important for their adoption in low-resource settings. To that end, I advance a strategy to decrease the costs associated with the fabrication of nanostructured microelectrodes (NMEs). I present an image-reversal soft lithography (IRSL) technique based on an elastomeric stamping protocol, eliminating the need for costly photolithographic fabrication steps. The distinct morphology of the sensors made by IRSL improves access to the surface of the electrodes leading to enhanced performance in the detection of biomolecules. Overall, the strategies presented in this thesis can guide the development of low-cost, sensitive, and rapid diagnostic systems. Outside of biosensing, NMEs have potentially important applications in CO2 recycling. Realizing efficient CO2 conversion to chemical fuels is an important step towards reducing the atmospheric concentration of CO2 and decreasing the carbon footprint of energy. Recently, our group reported highly-efficient electroreduction of CO2 to CO using nano-sharp Au structures. Here, I improve upon the performance of these structures by increasing the density of honed needle-like features on the surface of NMEs and demonstrate a 15-fold improvement in the reaction rates relative to the previously reported best results.Ph.D

In Situ Electrochemical ELISA for Specific Identification of Captured Cancer Cells

Circulating tumor cells (CTCs) are cancer cells disseminated from a tumor into the bloodstream. Their presence in patient blood samples has been associated with metastatic disease. Here, we report a simple system that enables the isolation and detection of these rare cancer cells. By developing a sensitive electrochemical ELISA method integrated within a microfluidic cell capture system, were we able to reliably detect very low levels of cancer cells in whole blood. Our results indicate that the new system provides the clinically relevant specificity and sensitivity needed for a convenient, point-of-need assay for cancer cell counting

High-Density Nanosharp Microstructures Enable Efficient CO₂ Electroreduction

Conversion of CO₂ to CO powered by renewable electricity not only reduces CO₂ pollution but also is a means to store renewable energy via chemical production of fuels from CO. However, the kinetics of this reaction are slow due its large energetic barrier. We have recently reported CO₂ reduction that is considerably enhanced via local electric field concentration at the tips of sharp gold nanostructures. The high local electric field enhances CO₂ concentration at the catalytic active sites, lowering the activation barrier. Here we engineer the nucleation and growth of next-generation Au nanostructures. The electroplating overpotential was manipulated to generate an appreciably increased density of honed nanoneedles. Using this approach, we report the first application of sequential electrodeposition to increase the density of sharp tips in CO₂ electroreduction. Selective regions of the primary nanoneedles are passivated using a thiol SAM (self-assembled monolayer), and then growth is concentrated atop the uncovered high-energy planes, providing new nucleation sites that ultimately lead to an increase in the density of the nanosharp structures. The two-step process leads to a new record in CO₂ to CO reduction, with a geometric current density of 38 mA/cm² at −0.4 V (vs reversible hydrogen electrode), and a 15-fold improvement over the best prior reports of electrochemical surface area (ECSA) normalized current density