Resumen de Inkjet-printed devices for chemical and biosensing applications
Inkjet printing is a nascent technology that is evolving from text and graphic printing into a major topic of scientific research and R&D, where it can be used as a highly reproducible non-contact patterning technique to deposit liquid functional materials at high speed on either small or large areas of flexible and non-flexile substrates. It is a low cost technique because minimizes the waste products and reduces the process steps. Moreover, the development of organic materials such as polymers with electrical features has boosted the fabrication of new organic printed electrical devices.
In this framework, this thesis is devoted to a development of novel organic devices for chemical and biosensing applications for a low-cost cost devices using inkjet printing technology. A Metal-Insulator-Semiconductor (MIS) diode was developed as a NO2 gas sensor and a Biological Field-Effect Transistor (BioFET) was fabricated and optimized to operate correctly under aqueous media and to detect biomolecules.
This thesis presents the fabrication and characterization of a novel inkjet-printed MIS diode on flexible plastic substrates. The structure consists of a polymeric insulator/semiconductor interface sandwiched between two silver electrodes. It is proposed that the rectification properties are due to a voltage controlled leakage current across the insulator/semiconductor interface. The current across the insulator is caused by the formation of a semiconductor brush-like morphology into the underneath porous insulator layer. The carrier injection into the insulator follows a thermionic emission model. Temperature dependent measurements reveal an interfacial barrier height between 0.97 eV and 0.36 eV depending on the morphology and type of insulator layer used. MIS based diodes show rectification ratios up to 150 at |10V| and a current density up to approximately 1 μAcm−2. The simple fabrication process of the diodes also makes it advantageous for scaling up to roll-to-roll production. Moreover, it has been shown that the MIS diode shows a good selectivity to detect NO2.
Moreover, in this thesis biosensors based on Organic Field-Effect Transistors (OFETs) have developed due to the possibility of rapid, label-free, and inexpensive detection. However, top-gate BioFETs using a polymer as insulator and an amorphous polymer as organic semiconductor have been seldom reported. Therefore in this thesis, a systematic investigation in terms of topographical and electrical characterization has been carried out in order to find the optimal fabrication process for obtaining a reliable polymer insulator. Moreover, a simple immobilization protocol was used to ensure the proper attachment of a model biomolecule onto the Cytop's hydrophobic surface whilst keeping its remarkable insulating properties with gate current in the range of dozens of pico Ampers. The top-gate BioFETs used in this study operated at voltages in the range of few volts and showed durability even when they were exposed to oxygen plasma, wet amine functionalization treatment, and aqueous media. As the final stage, the BioFET was fabricated using inkjet printing technology. Drain/Source electrodes and the organic semiconductor were inkjet-printed achieving very thin films which are primordial for the correct electrical performance. Furthermore, the electrical characteristics were studied showing a good performance. Finally, as a preliminary result, the both top-gate BioFETs developed in the first and second demonstrated the capability of detecting the presence of biomolecules through changes in the drain current, which opens the way for further use in the immunosensing field.
