Tin dioxide based semiconducting gas sensors

Práce se zabývá polovodičovými senzory plynů, jejichž aktivní vrstva je tvořena oxidem ciničitým (SnO2). V první části jsou rozděleny senzory plynů dle principu jejich funkčnosti. Dále je popsán princip fungování polovodičových SnO2 senzorů a možné modifikace této aktivní vrstvy. Praktická část komplexně shrnuje senzor plynů od jeho návrhu až po samotnou výrobu, testování a charakterizaci. Při konstrukci senzoru bylo využito několika mikroelektronických technologií, jako je například tenkovrstvá, tlustovrstvá, LTCC, spray-coating nebo wire-bonding technologie. V samotném závěru jsou shrnuty vlastnosti vyrobených senzorů a rozdíly ve funkčnosti modifikovaného a nemodifikovaného senzoru.This project is aimed at semiconductive gas sensors based on tin dioxide. In the first part, gas sensors are divided depending on their principal of functionality. Next part is about functionality of tin dioxide gas sensors and the possibilities of active layer modification. Experimental describes gas sensor from its drawing until construction, testing and characterization. Several microelectronic technologies such as thin-film, thick-film, LTCC, spray-coating or wire-bonding were used for constructing the sensor. In the last part properties of gas sensors and differences in functionality between modified and unmodified gas sensor are summarized.

Infinite selectivity of wet SiO2 etching in respect to Al

We propose and demonstrate an unconventional method suitable for releasing microelectromechanical systems devices containing an Al layer by wet etching using SiO2 as a sacrificial layer. We used 48% HF solution in combination with 20% oleum to keep the HF solution water-free and thus to prevent attack of the Al layer, achieving an outstanding etch rate of thermally grown SiO2 of 1 µm·min1. We also verified that this etching solution only minimally affected the Al layer, as the chip immersion for 9 min increased the Al layer sheet resistance by only 7.6%. The proposed etching method was performed in an ordinary fume hood in a polytetrafluorethylene beaker at elevated temperature of 70 °C using water bath on a hotplate. It allowed removal of the SiO2 sacrificial layer in the presence of Al without the necessity of handling highly toxic HF gas

Downsizing the Channel Length of Vertical Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as While the majority of OECTs use simple planar geometry, there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF6. Both variants show different promising features, leading the way in terms of transconductance (gm), with the measured peak gm up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 m. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of gm; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 104. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired

Direct measurement of oxygen reduction reactions at neurostimulation electrodes

Objective. Electric stimulation delivered by implantable electrodes is a key component of neural engineering. While factors affecting long-term stability, safety, and biocompatibility are a topic of continuous investigation, a widely-accepted principle is that charge injection should be reversible, with no net electrochemical products forming. We want to evaluate oxygen reduction reactions (ORR) occurring at different electrode materials when using established materials and stimulation protocols. Approach. As stimulation electrodes, we have tested platinum, gold, tungsten, nichrome, iridium oxide, titanium, titanium nitride, and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). We use cyclic voltammetry and voltage-step amperometry in oxygenated versus inert conditions to establish at which potentials ORR occurs, and the magnitudes of diffusion-limited ORR currents. We also benchmark the areal capacitance of each electrode material. We use amperometric probes (Clark-type electrodes) to quantify the O-2 and H2O2 concentrations in the vicinity of the electrode surface. O-2 and H2O2 concentrations are measured while applying DC current, or various biphasic charge-balanced pulses of amplitude in the range 10-30 mu C cm(-2)/phase. To corroborate experimental measurements, we employ finite element modelling to recreate 3D gradients of O-2 and H2O2. Main results. All electrode materials support ORR and can create hypoxic conditions near the electrode surface. We find that electrode materials differ significantly in their onset potentials for ORR, and in the extent to which they produce H2O2 as a by-product. A key result is that typical charge-balanced biphasic pulse protocols do lead to irreversible ORR. Some electrodes induce severely hypoxic conditions, others additionally produce an accumulation of hydrogen peroxide into the mM range. Significance. Our findings highlight faradaic ORR as a critical consideration for neural interface devices and show that the established biphasic/charge-balanced approach does not prevent irreversible changes in O-2 concentrations. Hypoxia and H2O2 can result in different (electro)physiological consequences

CMOS compatible piezoelectric resonator with FET structure for graphene monolayer properties modulation

Práce je zaměřena na výzkum nové struktury umožňující charakterizaci fyzikálních vlastností grafenu při přesně řízených podmínkách. Návrh spojuje MEMS piezoelektrický rezonátor spolu s Hall Bar/FET strukturou. Tento přístup umožňuje měnit vlastnosti grafenu odděleně nebo společně dvěma metodami. Mechanický způsob je založen na relativní deformaci způsobené rezonátorem, na kterém je umístěna grafenová monovrstva. Navrhovaná struktura umožňuje měřit vlastnosti grafenu vyvolané pouze změnou mechanického pnutí a frekvencí nucených kmitů bez vlivu vnějšího elektrického pole. Druhý přístup přidává možnost ovládat fyzikální vlastnosti grafenu pomocí elektrického pole FET struktury. Tato technika využívá grafenovou monovrstvu jako laditelný sensor pro molekulární detekci. Měření koncentrace v jednotkách ppb není konstrukčně ničím limitováno. Realizované frekvenčně laditelné piezoelektrické MEMS rezonátory s monovrstvou grafenu budou využitelné v mnoha oblastech pro detekci na molekulové úrovni. Výsledné struktury budou vyrobeny v souladu s požadavky na bio- a CMOS kompatibilitu.This work proposes a new structure allowing characterization of graphene monolayer properties under precisely specified conditions. It combines MEMS piezoelectric resonator with Hall Bar/FET structure. This approach allows changing graphene properties separately or together via two methods. The mechanical way is based on induced strain from the resonator which is graphene monolayer situated on. It brings the opportunity to measure graphene properties induced by the changes of mechanical strain and frequency of forced vibrations without the influence from external electric field. The second way uses FET structure to influence graphene monolayer using an electric field from bottom gate. There is no limit to measure concentration in units of ppb in terms of structure design. This approach of fabrication CMOS-compatible and biocompatible tunable frequency-modulated piezoelectric MEMS resonators with graphene monolayer can be very useful in many fields for molecule level detection.

Electrode system of thick-film electrochemical sensors optimization

Tato práce se zabývá optimalizací tříelektrodového elektrochemického senzoru pro detekci látek v roztocích vyrobeného technologií tlustých vrstev. Teoretická část práce popisuje technologii tlustých vrstev, úvod do elektrochemie a elektroanalytické metody pro detekci látek v roztocích. V praktické části je řešena optimalizace velikosti povrchu referenční a pomocné elektrody tříelektrodového systému. Závěr práce obsahuje zhodnocení zjištěných výsledků a popis ideálního tříelektrodového systému.This thesis is about optimization of three-electrode electrochemical sensor for detection of substances in solutions fabricated using standard thick film technology. The theoretical part of this thesis describes the thick film technology, introduction to electrochemistry and electro analytical methods for the detection of substances in solutions. The surface optimization of reference and auxiliary electrode of a three-electrode system is solved in the practical part. In conclusions results of the evaluation and description of the ideal three-electrode system are discussed.

Modifications of Parylene by Microstructures and Selenium Nanoparticles: Evaluation of Bacterial and Mesenchymal Stem Cell Viability

Parylene-based implants or coatings introduce surfaces suffering from bacteria colonization. Here, we synthesized polyvinylpyrrolidone-stabilized selenium nanoparticles (SeNPs) as the antibacterial agent, and various approaches are studied for their reproducible adsorption, and thus the modification of parylene-C-coated glass substrate. The nanoparticle deposition process is optimized in the nanoparticle concentration to obtain evenly distributed NPs on the flat parylene-C surface. Moreover, the array of parylene-C micropillars is fabricated by the plasma etching of parylene-C on a silicon wafer, and the surface is modified with SeNPs. All designed surfaces are tested against two bacterial pathogens, Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). The results show no antibacterial effect toward S. aureus, while some bacteriostatic effect is observed for E. coli on the flat and microstructured parylene. However, SeNPs did not enhance the antibacterial effect against both bacteria. Additionally, all designed surfaces show cytotoxic effects toward mesenchymal stem cells at high SeNP deposition. These results provide valuable information about the potential antibacterial treatment of widely used parylene-C in biomedicine

A New Method for 2D Materials Properties Modulation by Controlled Induced Mechanical Strain

This paper proposes a new method for characterization of 2D materials under the precisely specified conditions. It is achieved by integration of a 2D material as a field effect transistors structures with a piezoelectric resonator. Properties of the 2D material can be mechanically adjusted by the resonator. It results in the independent and precise control of an amplitude of induced mechanical strain, its modulating frequency, which all influence the 2D material properties. The electrical field required to measure 2D material field effect transistors will not be affected by the vibrations, thus giving us a chance to perform the precise measurement of the electrical properties of the 2D material. This approach has a great potential for measuring and monitoring cells, enzymes, nucleic acids, deoxyribonucleic acid and ribonucleic acid. It can be also used for measurement of toxic, combustive or waste gases

Mechanical strain and electric-field modulation of graphene transistors integrated on MEMS cantilevers

This work proposes a structure which allows characterization of graphene monolayers under combined electric field and mechanical strain modulation. Our approach is based on a cantilever integrated into a two-dimensional graphene-based Field effect transistor (FET). This allows us to change graphene properties either separately or together via two methods. The first way involves electric field induced by the gate. The second is induction of mechanical strain caused by external force pushing the cantilever up or down. We fabricated devices using silicon-on-insulator wafer with practically zero value of residual stress and a high-quality dielectric layer which allowed us to precisely characterize structures using both mentioned stimuli. We used the electric field/strain interplay to control resistivity and position of the charge neutrality point often described as the Dirac point of graphene. Furthermore, values of mechanical stress can be obtained during the preparation of thin films, which enables the cantilever to bend after the structure is released. Our device demonstrates a novel method of tuning the physical properties of graphene in silicon and/or complementary metal-oxide-semiconductor technology and is thus promising for tunable physical or chemical sensors. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Grantová Agentura České Republiky, GA ČR: GJ18-06498Y; Vysoké Učení Technické v Brně, BUT: FEKT-S-20-6206; Central European Institute of Technology, CEITEC: LM2015041Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [GJ18-06498Y]; CEITEC Nano Research Infrastructure (MEYS CR, 2016 2019) CEITEC Brno University of Technology [LM2015041]; Brno University of Technology [FEKT-S-20-6206