Nanogap structures for molecular electronics and biosensing

Molecular transport characterization is an active part of the research field in nanotechnology. In this interesting branch the self-assembly approach is highly exploited; it consists in spontaneous formation of highly ordered monolayers on various substrate surfaces. Self-assembled monolayers (SAMs) have found their applications in various areas, such as nanoelectronics, surface engineering, biosensing, etc. An important area in biosensing is the electrochemical detection, that enables sensing of dierent biomarkers with an important role, for many dierent applications in biomedical diagnostics or in monitoring of biological systems. Various test structures have been developed in order to carry out characterizations of self-assembled molecules, and numerous reports have been published in the past several years on the transport characteristics. This thesis' purpose is the single protein biomolecular sensing, that could become the starting point for monitoring drugs, developing clean energy systems, realizing bio-opto-electronic transistors... The possibility to cover so many fields is related to the kind of proteins, molecules, bioelements that will be inserted inside sensors. Biomolecular sensing has to be thought in order to reach a result with the better compromise between instrumentation versatility and measurements precision. The main underlying idea is to use single molecules as active elements in nano-devices. As a consequence, the proper realization of a molecule-electrode contact is a crucial issue. What is needed by author is something versatile, precize, cheap, at single molecule level and able to record measurements in few time in order to do statistical characterizations. The final goal of this work is a platform system adapt for both industry and research field. Electrical nanogap devices are the main character of this work. They have proven good performances as element for detecting small quantities of biomolecules, allowing direct transduction of biomolecular signals into useful electrical ones such as resistance/impedance, capacitance/dielectric, or field effect. Nanogaps are now one of the most busy area of research in the nanotechnology world. Moreover, these structures do not require feedback to maintain the mutual arrangement (comparing with conducting tip AFM) and are less stochastic with respect to electrochemical cells. Several techniques can be applied to nanogap fabrication: mechanically broken or positioned junctions, nano-scale lithography by Synchrotron radiation sources, electrochemical deposition and etching, and electromigration. None of these techniques is presently able to give precise control as to thefinal gap size. In this thesis the electromigration approach has been choosen, because of several useful characteristics. It is cost eective, because of the relatively low complexity of the required equipment. It can be embedded into a lab-on-chip system, thus exploiting the possibility to tailor the gap formation process by means of a digital loop control system. To this end, it just requires a conventional microchip fabrication process. It allows the parallelization with a smart packaging through which it is possible to produce more probes at the same time and perform many measurements in contemporary. The employment of nanogaps, as an instrumentation for the molecular charac- terization, has also some issues that have to be considered in order to obtain useful measurements. To characterize molecules the leakedge must be not higher than some pA to avoid the noise overcome the signal. Nanogap platform is perfect for molecular electronics. The experiments have been developed in dry way, as a consequence the solutions were evaporated before the measurement starting. This brought several problems cause biochemical analysis requires liquid solution in order to avoid an untimely death of the bio-elements tha has to be characterized. Considering a future developement, an improvement is necessary in terms of a system able to work with salty solutions without damaging the microchip's probes. Therefore it is a necessary a set-up allowing the anchorage of a microfluidic part. At the same time it is necessary to keep in mind that the presence of a new system has to not overcome the molecule signal, maintaining the leakedge under some tens of pA.