Finite element analysis of silicon nanowire array based SAW gas sensor

This work presents the design and finite element analysis of a surface acoustic wave (SAW)-based sensor for the detection of volatile organic compound (VOC) gases. The effect of silicon nanowire array (SiNWA) on a 128º YX-lithium niobate (LiNbO3) substrate for sensing the VOC gases was simulated using COMSOL Multiphysics. The frequency response was investigated in relation to changes in the SiNWA sensitive layer and VOC gas concentration. The resonant frequency of the SAW device was also evaluated, and simulation results were obtained after being exposed to 100ppm concentration of VOC gas. It was determined that the frequency increased, after the sensor was exposed to VOC gases. In general, extending the length of the SiNWA enhances the sensor's sensitivity

Nanostructures and Nanosensors

Cílem této bakalářské práce je představit využití nanotechnologie ve snímacích zařízeních, poskytnout základní informace o nanotechnologii, včetně popisu a klasifikace nanostruktur, a zdůraznit možnost vylepšení výkonu senzoru prostřednictvím nanotechnologie. Tato práce se také zabývá využitím nanodrátu jako součástí senzoru s konfigurací unipolárního tranzistoru. Kvantové tečky a jejich využití v optické detekci jsou také zahrnuty v této práci. Výhody i současná omezení nanosenzorů jsou shrnuty v závěru práce.The aim of this bachelor’s thesis is to introduce the application of nanotechnology in sensing devices, provide fundamental information about nanotechnology including the description and the classification of nanostructures, and emphasise the possibility to enhance sensor operation by means of nanotechnology. The thesis also deals with the application of a nanowire as part of a sensor using the configuration of the field-effect transistor. Quantum dots and their application in optical detection, mainly in the field of nanomedicine, are considered in this thesis as well. The last part is focused on the advantages and current restriction of nanosensors.

Zinc oxide nanostructures with carbon nanotube and gold additives for co gas sensing application

Abstract: Zinc oxide (ZnO) nanostructures were synthesised for gas sensing application. In an attempt to improve the surface area and the electrical conductivity of the ZnO, nanomaterials such as the carbon nanotubes (CNTs) and gold nanoparticles (AuNPs) were used separately to produce CNTs/ZnO and Au/ZnO nanocomposites, respectively. The addition of these nanomaterials onto the ZnO nanostructures significantly improved the gas sensing properties such as the sensitivity and response time. Synthesis of gold nanoparticles was successfully achieved via gold salt (HAuCl4.3H2O) reduction using the Turkevich method. Citrate molecules were used as the stabiliser and to systematically control the sizes of the AuNPs. The sizes of AuNPs were found to increase from 14 nm to 40 nm when the concentration of citrate ions was reduced from 1 mM to 0.3 mM. The size distribution of AuNPs was relatively wider as the particle size increased. The synthesized AuNPs were stable for over a period of 4 weeks. Carbon nanotubes synthesis was achieved using chemical vapour deposition (CVD) method using acetylene gas as the carbon source and ferrocene as the catalyst. An increase in the flowrate of the precursor gas (acetylene) yielded an increase in amorphous carbon, which was attached to the walls of the carbon nanotubes. The optimum flowrate of acetylene was found to be 150 m3/min that yielded CNTs with an average diameter of 95 nm and a relatively narrow size distribution. The hydrothermal chemical precipitation method was used to synthesise ZnO nanostructures. Zinc sulphate (ZnSO4) and sodium hydroxide (NaOH) were used as a metal precursor and reducing agent, respectively. The NaOH concentration of 0.3 M yielded ZnO nanosheets with relatively the highest surface area of 102 m2/g. Gas sensing analysis was conducted using carbon monoxide (CO) gas at 250°C. The sensitivity and response time were calculated to be 9.8% and 114 seconds, respectively, at a CO concentration of 200 ppm. The composites CNTs/ZnO and Au/ZnO were prepared, separately. The average surface area of the Au/ZnO composite was 131 m2/g and that of CNTs/ZnO composite was 153 m2/g. The CNTs/ZnO composite showed an optimum sensitivity of 9.9% and the response time of 49 seconds when exposed to 200 ppm of CO gas at 250°C.M.Tech. (Chemical Engineering

II-VI Semiconductor Nanowire Array Sensors Based on Piezotronic, Piezo-Phototronic and Piezo-Photo-Magnetotronic Effects

With the rapid progress of nanotechnologies, there are two developing trends for the next generation of sensors: miniaturization and multi-functionality. Device miniaturization requires less power consumption, or even self-powered system. Multi-functional devices are usually based on multi-property coupling effects. Piezoelectric semiconductors have been considered to be potential candidates for self-powered/multi-functional devices due to their piezotronic coupling effect. In this dissertation, ZnO and CdSe nanowire arrays have been synthesized as the piezoelectric semiconductor materials to develop the following self-powered/multi-functional sensors: (1) self-powered gas sensors of ZnO/SnO2, ZnO/In2O3, ZnO/WO3 and CdSe nanowire arrays have been assembled. All these gas sensors are capable of detecting oxidizing gas and reducing gas without any external power supply owing to piezotronic effect which can convert mechanical energies to electrical energy to power the sensors; (2) a self-powered ZnO/ZnSe core/shell nanowire array photodetector has been fabricated. This photodetector is able to detect the entire range of the visible spectrum as well as UV light because of its type II heterostructure. The absolute sensitivity and the percentage change in responsivity of the photodetector were significantly enhanced resulting from the piezo-phototronic effect. The photodetector also exhibited self-powered photodetection behavior; (3) three dimensional nanowire arrays, such as ZnO and ZnO/Co3O4, have been synthesized to investigate piezo-magnetotronic and piezo-photo-magnetotronic effects. Under magnetic field, the magnetic-induced current of ZnO nanowire array decreased as magnetic field increased, and the current difference was magnified by one order of magnitude caused by piezo-magnetotronic effect through applying a stress. In contrast, under UV light illumination, the current response increased with an increment of magnetic field. The current difference was enhanced by at least two orders of magnitude attributed to piezo-photo-magnetotronic effect. Furthermore, ZnO/Co3O4 core/shell structure was employed to further improve the magnetic-induced current difference. This phenomenon projects a potential for multi-functional piezo-magnetotronic and piezo-photo-magnetotronic device development

Breath analysis of Volatile Organic Compounds for early-stage disease detection using nanomaterials

Είναι ευρέως αποδεκτό πως η διάγνωση μιας ασθένειας σε πρώιμο στάδιο είναι εξαιρετικής σημασίας, καθώς αυξάνει την πιθανότητα επιτυχούς θεραπείας, σε αντίθεση με την διάγνωση σε προχωρημένο στάδιο. Η μειωμένη συμμόρφωση των ασθενών προς τις υπάρχουσες διαγνωστικές μεθόδους, ωστόσο, παρεμποδίζει την έγκαιρη διάγνωση, καθιστώντας την ανάπτυξη μη-επεμβατικών διαγνωστικών μέσων επιτακτική ανάγκη. Η ανάλυση του εκπνεόμενου αέρα αποτελεί μια από τις πιο υποσχόμενες μη-επεμβατικές διαγνωστικές μεθόδους, προσελκύοντας το ερευνητικό ενδιαφέρον τα τελευταία χρόνια. Οι Πτητικές Οργανικές Ενώσεις που περιέχονται στον εκπνεόμενο αέρα θεωρούνται πιθανοί βιοδείκτες, εξαιρετικής σημασίας, για ένα πλήθος διαφορετικών ασθενειών. Η διαγνωστική ικανότητα διαφορετικών συνόλων πτητικών οργανικών ενώσεων έχει αναδειχθεί, τόσο με τη χρήση αναλυτικών τεχνικών όσο και – κυριότερα – με τη χρήση αισθητήρων αερίων. Η όλο και εντονότερη ανάπτυξη καινοτόμων νανοϋλικών, κατάλληλων για την δημιουργία αισθητήριων μέσων, επιτρέπει την ανάπτυξη αποτελεσματικότερων διαγνωστικών αισθητήρων, αποτελώντας, συνεπώς, ένα σημαντικό σύγχρονο ερευνητικό πεδίο. Η συγκεκριμένη εργασία στοχεύει, αρχικά, στην παρουσίαση των διαφόρων τύπων νανοϋλικών και αισθητήρων που μελετώνται για διαγνωστικές εφαρμογές. Εν συνεχεία – λαμβάνοντας υπόψιν την σημασία της συνετής και οργανωμένης επιλογής των αισθητήριων υλικών ενός αισθητήρα – παρουσιάζεται μια επισκόπηση των παραγόντων που επηρεάζουν την αλληλεπίδραση των πτητικών οργανικών ενώσεων με πολυμερικά υμένια – ένα κοινό συστατικό των αισθητήριων μέσων. Τέλος, προτείνεται μια σειρά πολυμερών, τα οποία θα μπορούσαν να χρησιμοποιηθούν για την δημιουργία χημικών αισθητήρων αντίστασης, βασισμένων σε μεταλλικά σωματίδια επικαλυμμένα με πολυμερικά υμένια, με στόχο την ανίχνευση εκπνεόμενων πτητικών οργανικών ενώσεων που έχουν χαρακτηριστεί επαναλαμβανόμενα ως πιθανοί βιοδείκτες για το Άσθμα, την Χρόνια Αποφρακτική Πνευμονοπάθεια, τον Καρκίνο του Πνεύμονα και τον Καρκίνο του Μαστού.Undoubtedly, early-stage disease diagnosis is of particular importance, increasing the chances for effective treatment, in comparison to advanced-disease stages. Lack of patient compliance for the existing diagnostic methods, however, limits prompt diagnosis, rendering the development of non-invasive diagnostic tools mandatory. One of the most promising non-invasive diagnostic methods that has attracted the research interest during the last years is breath analysis. Volatile Organic Compounds (VOCs) contained in the exhaled breath are considered as important potential biomarkers of various types of diseases. The diagnostic ability of VOC-patterns detection using analytical techniques and, especially, sensors, has been demonstrated. The progressive development of novel nanomaterials, suitable for sensing element creation, enhances the development of effective diagnostic sensors, comprising a major topic of current research. The current thesis aims, firstly, to present an overview of the various types of nanomaterials and sensors investigated for diagnostic sensors development. Further on, taking into consideration the importance of sensible sensing-material selection, the parameters affecting the interactions of VOCs with polymers – a common component of sensing elements – are summarized. Last but not least, a series of polymers that could potentially detect repeatedly identified VOCs as potential biomarkers of asthma, COPD, lung and breast cancer, using a polymer coated-MNSs based chemiresistor are proposed

Gas Sensors Based on Semiconducting Nanowire Field-Effect Transistors

One-dimensional semiconductor nanostructures are unique sensing materials for the fabrication of gas sensors. In this article, gas sensors based on semiconducting nanowire field-effect transistors (FETs) are comprehensively reviewed. Individual nanowires or nanowire network films are usually used as the active detecting channels. In these sensors, a third electrode, which serves as the gate, is used to tune the carrier concentration of the nanowires to realize better sensing performance, including sensitivity, selectivity and response time, etc. The FET parameters can be modulated by the presence of the target gases and their change relate closely to the type and concentration of the gas molecules. In addition, extra controls such as metal decoration, local heating and light irradiation can be combined with the gate electrode to tune the nanowire channel and realize more effective gas sensing. With the help of micro-fabrication techniques, these sensors can be integrated into smart systems. Finally, some challenges for the future investigation and application of nanowire field-effect gas sensors are discussed

Metal oxide nanostructures for sensor applications

Electrorheological fluids have been paying a lot of attention due to their potential use in active control of various devices in mechanics, biomedicine or robotics. An electrorheological fluid consisting of polarizable particles dispersed in a non-conducting liquid is considered to be one of the most interesting and important smart fluids. This work presents the effect of the dopant, camphorsulphonic acid or citric acid, on the electrorheological behaviour of suspensions of doped polyaniline nanostructures dispersed in silicone oil, revealing its key role. The influence of carbon nanoparticle concentration has also been studied for these dispersions. All the samples showed an electrorheological effect, which increased with electric field and nanostructure concentration and decreased with silicone oil viscosity. However, the magnitude of this effect was strongly influenced not only by carbon nanoparticle concentration but also by the dopant material. The electrorheological effect was much lower with a higher carbon nanoparticle concentration and doped with citric acid. The latter is probably due to the different acidities of the dopants that lead to a different conductivity of polyaniline nanostructures. Furthermore, the effect of the carbon nanoparticles could be related to its charge trapping mechanism, while the charge transfer through the polymeric backbone occurs by hopping. Polyaniline/camphorsulphonic acid composite nanostructures dispersed in silicone oil exhibited the highest electrorheological activity, higher than three decades increase in apparent viscosity for low shear rates and high electric fields, showing their potential application as electrorheological smart materials.authorsversionpublishe

Gas Microsensors Based on Self-Organized 3D Metal-Oxide Nanofilms

Tato disertační práce se zabývá vývojem a výrobou mikrosenzorů plynů s využitím nové trojrozměrné (3D) nanostrukturované polovodivé vrstvy z oxidů kovů, která efektivně využívá výhod anorganických materiálů připravených snadno dostupnými elektrochemickými výrobními technikami bez použití litografie. Citlivá vrstva je vytvořena anodizací kovové dvojvrstvy Al/Nb naprášené na SiO2/Si substrátu skrze masku z porézní anodizované aluminy. Běžně se skládá z 200 nm tlusté vrstvy NbO2 propojující pole k ní kolmo orientovaných a prostorově oddělených Nb2O5 nanosloupků o šířce 50 nm, délce až 900 nm a celkovým počtem přibližně 8109 sloupkůcm-2. Nanosloupky fungují jako polovodivé nanokanálky jejichž rezistivita je vysoce citlivá na reakce probíhající na povrchu a rozhraní vrstvy. V rámci nového uspořádání senzoru byly na povrchu pole nanosloupků připraveny Pt nebo Au strukturované elektrody realizované pomocí běžných mikrotechnologických nebo elektrochemických depozičních technik, následované selektivním odleptáním vrstvy aluminy. Pro charakterizaci detekčních schopností byla struktura senzoru upevněna do standardního pouzdra TO-8 a elektricky propojena s využitím techniky drátového kontaktování. Elektrické charakterizace 3D nanovrstvy z oxidů niobu odhalily asymetrickou závislost vodivosti způsobenou Schottkyho bariérou vzniklou na rozhraní Pt/Nb2O5 nebo Au/Nb2O5. Při detekci vodíku a amoniaku byl zkoumán vliv struktury a složení citlivé vrstvy na elektrické a detekční schopnosti se zaměřením na citlivost, selektivitu, detekční limity a rychlost odezvy/zotavení. Rychlá a intenzivní odezva na H2 potvrdila potenciál využití 3D nanovrstvy z oxidů niobu jako citlivé vrstvy pro senzorické aplikace. Na počítači provedené mikrofluidní simulace difuze plynů do 3D nanovrstvy předpovídají možnost značného zlepšení detekčních schopností použitím perforované horní elektrody, a to optimalizací morfologie vrstvy, úpravou krystalické struktury a vhodnou úpravou návrhu elektrody. Předběžné experimenty prokázaly, že i další 3D nanovrstvy připravené z kovových dvojvrstev jako Al/W mají vysokou perspektivu při využití v pokročilých senzorických aplikacích. Techniky a materiály využité v této práci jsou vhodné pro vývoj technologicky jednoduchého, ekonomického a pro životní prostředí ohleduplného řešení mikro- a nanozařízení, kde použití definovaných nanokanálků pro nosiče náboje a povrchové reakce může přinést značné výhody.This dissertation concerns the development, fabrication and integration in a gas sensing microdevice of a novel 3-dimensional (3D) nanostructured metal-oxide semiconducting film that effectively merges the benefits of inorganic nanomaterials with the simplicity offered by non-lithographic electrochemistry-based preparation techniques. The film is synthesized via the porous-anodic-alumina-assisted anodizing of an Al/Nb metal bilayer sputter-deposited on a SiO2/Si substrate and is basically composed of a 200 nm thick NbO2 layer holding an array of upright-standing spatially separated Nb2O5 nanocolumns, being 50 nm wide, up to 900 nm long and of 8109 cm2 population density. The nanocolumns work as semiconducting nano-channels, whose resistivity is greatly impacted by the surface and interface reactions. Either Pt or Au patterned electrodes are prepared on the top of the nanocolumn array using an innovative sensor design realized by means of microfabrication technology or via a direct original point electrodeposition technique, followed by selective dissolution of the alumina overlayer. For gas-sensing tests the film is mounted on a standard TO-8 package using the wire-bonding technique. Electrical characterization of the 3D niobium-oxide nanofilm reveals asymmetric electron transport properties due to a Schottky barrier that forms at the Au/Nb2O5 or Pt/Nb2O5 interface. Effects of the active film morphology, structure and composition on the electrical and gas-sensing performance focusing on sensitivity, selectivity, detection limits and response/recovery rates are explored in experimental detection of hydrogen gas and ammonia. The fast and intensive response to H2 confirms the potential of the 3D niobium-oxide nanofilm as highly appropriate active layer for sensing application. A computer-aided microfluidics simulation of gas diffusion in the 3D nanofilm predicts a possibility to substantially improve the gas-sensing performance through the formation of a perforated top electrode, optimizing the film morphology, altering the crystal structure and by introducing certain innovations in the electrode design. Preliminary experiments show that a 3D nanofilm synthesized from an alternative Al/W metal bilayer is another promising candidate for advanced sensor applications. The techniques and materials employed in this work are advantageous for developing technically simple, cost-effective and environmentally friendly solutions for practical micro- and nanodevices, where the well-defined nano-channels for charge carriers and surface reactions may bring unprecedented benefits.

Toxic Gas Sensing on Nanoporous Carbons

Activated carbons, either synthetic, developed in our laboratory, or commercial, were prepared or further modified, in order to introduce specific heteroatoms such as oxygen, nitrogen and sulfur to their matrices. Chips coated with thin layers of the carbon samples were used for the sensing of gaseous ammonia. They were exposed to continuous cycles of various ammonia concentrations (10-500 ppm), and changes in normalized resistance were analyzed. In all cases linear responses were recorded and the chips reached sensitivities as high as 31%, which are comparable to those of modified graphene-based sensors. The applied specific surface chemical modifications were an effective means to control the type of the charge carriers (electrons or holes), and thus the electronic and transport properties. The mechanism of the reversible sensing was governed by several processes including specific interactions between the surface functional groups and the molecules of the target gas, pore-filling with ammonia (especially of pores smaller than 0.7 nm), electron–hole conductivity, and charge transport through ionic conductivity. Strongly acidic carboxylic and sulfonic groups played an important role in ammonia sensing by promoting charge transport via ionic conductivity, due to the formation of NH4+. Among all N-containing groups, nitrogen located in six-membered rings (pyridines and quaternary nitrogen), rather than nitrogen on the periphery (amines, amides) played the most important role in sensing. An important aspect was the conversion of the conduction type from predominantly p- to predominantly n- upon oxidation of the carbon surface due to introduction of electron withdrawing nitro groups to the matrix. Owing to the high porosity of the oxidized carbon and the polarity of the formed -NO2 groups present in the pore system, opposite signal changes compared to the initial counterpart were recorded (decrease instead of increase in the normalized resistance). Interesting changes in the electrical response were noticed for S- and N-dual-doped carbons. Their ability to activate oxygen and generate superoxide ions resulted in oxidation of ammonia to nitrogen dioxide. NO2 adsorbed in the pore system caused an increase in the population of holes (h+) as charge carriers in the matrix, which led to a conductivity increase upon ammonia exposure. The surface chemical and structural features of the carbons acted either synergistically or competitively. When the chips were exposed to H2S, they showed a very low sensitivity to this gas. A high surface acidity of the carbons enhanced their affinity towards NH3 adsorption, contributing to a selective ammonia detection. The role of the specific chemical arrangement of the heteroatoms on ammonia sensing is extensively examined and analyzed