The precise determination of the type and concentration of gases is of increasing importance in numerous applications. Despite the diverse operating principles of today´s gas sensors, technological trends can be summarized with the keyword miniaturization, because of the resulting benefits such as integrability and energy efficiency. This work deals with the development and fabrication of novel nanowire based gas sensors, which in comparison to conventional devices have an advantageous combination of high sensitivity and selectivity with low power consumption and small size. On the basis of grown silicon nanowires, sensors based on the functional principle of classical Schottky barrier field effect transistors with abrupt metal-semiconductor contacts are fabricated. The sensing performance of the devices is investigated with respect to the detection of ammonia. Ammonia concentrations down to 170 ppb are measured with a sensor response of more than 160 % and a theoretical limit of detection of 20 ppb is determined. Selectivity investigations show that no cross sensitivity to most common solvents occurring in living spaces exists. Moisture influences on the device are studied and reveal that the sensor responds within seconds, making it potentially suitable as humidity sensor. Moreover, it is shown that a higher relative humidity and higher temperatures decrease the sensor sensitivity. In terms of possible applications, it is a great advantage that the maximum sensitivity is achieved at 25 °C. With respect to sensitivity and selectivity an enhancement is demonstrated compared to most nanosensors known from the literature. Hence, the technology offers the potential to complement conventional measurement systems in future sensor technology especially in portable applications. / Die präzise Bestimmung der Art und Konzentration von Gasen erlangt in zahlreichen Anwendungsgebieten zunehmend an Bedeutung. Trotz der vielfältigen Wirkprinzipien heutiger Gassensoren lassen sich die technologischen Trends mit dem Schlagwort Miniaturisierung zusammenfassen, da sich daraus entscheidende Vorteile wie Integrierbarkeit und Energieeffizienz ergeben. Diese Arbeit beschäftigt sich mit der Entwicklung und Herstellung neuartiger nanodrahtbasierter Gassensoren, welche im Vergleich zu klassischen Sensoren eine vorteilhafte Kombination von hoher Sensitivität und Selektivität bei geringem Stromverbrauch und geringer Größe aufweisen. Auf der Grundlage gewachsener Silizium-Nanodrähte werden Sensoren mit abrupten Metall-Halbleiter-Kontakten hergestellt, welche auf dem Funktionsprinzip klassischer Schottkybarrieren-Feldeffekttransistoren beruhen. Die Eignung der Sensoren wird in Bezug auf die Detektion von Ammoniak untersucht. Dabei kann eine minimale Ammoniakkonzentration von 170 ppb mit einer Signaländerung von mehr als 160 % gemessen werden, wobei die theoretische Nachweisgrenze mit 20 ppb ermittelt wird. Selektivitätsuntersuchungen zeigen, dass keine Querempfindlichkeit gegenüber den am häufigsten in Wohnräumen vorkommenden Lösungsmitteln besteht. Feuchtigkeitseinflüsse auf den Sensor werden untersucht und es wird nachgewiesen, dass der Sensor Ansprechzeiten im Sekundenbereich besitzt, was ihn zu einem potenziell geeigneten Feuchtigkeitssensor macht. Darüber hinaus wird gezeigt, dass eine höhere relative Luftfeuchtigkeit und höhere Umgebungstemperaturen die Sensorsensitivität verringern. In Bezug auf mögliche Einsatzgebiete stellt die maximale Empfindlichkeit bei 25 °C einen großen Vorteil da. Bezogen auf Sensitivität und Selektivität wird somit eine Verbesserung im Vergleich zu den meisten aus der Literatur bekannten Nanosensoren demonstriert. Damit bietet die Technologie das Potential, konventionelle Messsysteme in zukünftiger Sensorik vor allem in portablen Anwendungen zu ergänzen.

Transistors are the fundamental components of computer processors. The dimensions of transistors used in microprocessors are decreasing every year and the challenge of maintaining this trend now requires nanoscale dimensions. A potential method of achieving nanoscale dimensions is using atomic clusters as building blocks. It is therefore desirable to investigate transistor-like behaviour in cluster devices. Traditionally, transistor devices are made from semiconducting materials. It was therefore proposed that gated behaviour would be observable in devices that are fabricated from germanium clusters. A germanium cluster source was designed and built. Field effects were successfully observed in films of germanium clusters. Immediately after deposition, the gate effect of germanium cluster films was insignificant. As the films slowly oxidized in vacuum, a decrease in the overall carrier concentration was observed which lead to an increase in the gate effect, with a maximum change in resistance observed of 12%. When films of germanium clusters were exposed to air, a resistance decrease was observed, attributed to water vapour adsorbing on the surface. The phenomenon was further investigated and the proposed resistance change mechanism involves water vapour creating surface defects which act as donors and cause the electron concentration in the film to increase. Films of germanium clusters were sensitive to hydrogen concentrations above 1% in air, with up to a factor of 25 decrease in resistance observed at room temperature for 5% hydrogen concentration. Thin films were found to be most sensitive. The higher sensitivity was attributed to the larger surface-to-volume ratio. The proposed mechanism for sensing is that defects are created on the surface of the film, which in turn act as donors which cause the electron concentration in the film to increase. Bismuth is a semimetal and gate effects have previously been observed in bismuth nanowires. Parallel bismuth nanowires of 300nm diameter were successfully deposited at a distance of 200nm apart allowing one of the wires to be used as a gate. The gate effects observed in bismuth cluster structures were weak and inconclusive, with a small gate effect (change in resistance of 0.1%) observed at 11K in some devices.

In this thesis, the author proposed and developed nanostructured materials based Surface Acoustic Wave (SAW) and conductometric transducers for gas sensing applications. The device fabrication, nanostructured materials synthesis and characterization, as well as their gas sensing performance have been undertaken. The investigated structures are based on two structures: lithium niobate (LiNbO3) and lithium tantalate (LiTaO3). These two substrates were chosen for their high electromechanical coupling coefficient. The conductometric structure is based on langasite (LGS) substrate. LGS was selected because it does not exhibit any phase transition up to its melting point (1470°C). Four types of nanostructured materials were investigated as gas sensing layers, they are: polyaniline, polyvinylpyrrolidone (PVP), graphene and antimony oxide (Sb2O3). The developed nanostructured materials based sensors have high surface to volume ratio, resulting in high sensitivity towards di¤erent gas species. Several synthesis methods were conducted to deposit nanostructured materials on the whole area of SAW based and conductometric transducers. Electropolymerization method was used to synthesize and deposit polyaniline nanofibers on 36° YX LiTaO3 and 64° YX LiNbO3 SAW substrates. By varying several parameters during electropolymerization, the effect on gas sensing properties were investigated. The author also extended her research to successfully develop polyaniline/inorganic nanocomposites based SAW structures for room temperature gas sensing applications. Via electrospinning method, PVP fibres and its composites were successfully deposited on 36° YX LiTaO3 SAW transducers. Again in this method, the author varied several parameters of electrospinning such as distance and concentration, and investigated the effect on gas sensing performance. Graphene-like nano-sheets were synthesized on 36° YX LiTaO3 SAW devices. This material was synthesized by spin-coating graphite oxide (GO) on the substrate and then exposin g the GO to hydrazine to reduce it to graphene. X-ray photoelectron spectroscopy (XPS) and Raman characterizations showed that the reduced GO was not an ideal graphene. This information was required to understand the properties of the deposited graphene and link its properties to the gas sensing properties. Thermal evaporation method was used to grow Sb2O3 nanostructures on LGS conductometric transducers. Using this method, different nanoscale structures such as nanorods and lobe-like shapes were found on the gold interdigitated transducers (IDTs) and LGS substrate. The gas sensing performance of the deposited nanostructured Sb2O3 based LGS conductometric sensors was investigated at elevated temperatures. The gas sensing performance of the investigated nanostructured materials/SAW and conductometric structures provide a way for further investigation to future commerciallization of these types of sensors.

Abstract In this thesis work, crystal and phase structure, chemical composition and gas sensing properties of pulsed laser deposited vanadium oxide thin films were studied. Pulsed laser deposition was used to manufacture vanadium oxide thin films with various crystal structures, film morphologies and phase compositions. Both the well-known vanadium pentoxide V2O5, and a totally new stable phase in a solid-state thin-film form, V7O16, was produced. The existence of these phases was proven by several different characterization methods such as, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The resistive gas sensing measurements of the films with pure V2O5 composition, and mixed phase compositions of V2O5 and V7O16, showed that behaviour of the electrical response to different gases at various measurement temperatures was dependent on the phase composition of the thin films. It was proved that in certain conditions the mixed phase films show p-type semiconducting gas sensing behaviour, instead of the pure n-type behaviour of V2O5. Both types of film compositions were shown to be highly sensitive to ammonia gas, down to 40 ppb-level. The mixed phase composition showed a higher response to ammonia compared to the pure V2O5 phase; however the pure V2O5 showed better long-term stability. Both sensing layer types also showed high selectivity to ammonia in comparison to NO and CO gases. Nanostructured pure V2O5 layers were successfully deposited on commercial microheater platforms and then used as a gas sensor. The V2O5 nanostructures were proven to be very promising candidates as gas sensor material to control the Selective Catalytic Reduction process used in the reduction of NOx gas emissions. The surface valence states of the thin film structures with various phase compositions were studied spectroscopically, and a clear connection between the valence states of the film surfaces and gas sensing properties was found. It was concluded that the pure V2O5 films also had some V4+ ions in the surface, and in the mixed phase thin films, the amount V4+ ions was already quite high, indicating a higher amount of oxygen vacancies in the thin film surface – another proof of the existence of V7O16 phase in the film composition. It is also suggested that the particular quantity of oxygen vacancies is one of the reasons for the high gas-sensing response of the thin films. / Tiivistelmä Tässä työssä tutkittiin pulssilaserkasvatettujen vanadiinioksidiohutkalvojen kide- ja faasirakenteita sekä ominaisuuksia kaasuantureina. Vanadiinioksidiohutkalvoja, jotka omaavat erilaiset kide- ja faasirakenteet, sekä erilaiset morfologiat valmistettiin pulssilaserkasvatuksella. Tunnetun V2O5 -faasin lisäksi myös V7O16 -faasi onnistuttiin valmistamaan ensimmäistä kertaa kiinteän aineen epäorgaanisena faasina ohutkalvorakenteeseen. Näiden erilaisten faasirakenteiden olemassaolo todistettiin käyttämällä useita menetelmiä kuten röntgendiffraktiota, Raman spektroskopiaa ja röntgenfotoelektronispektroskopiaa. Sekä ainoastaan V2O5 -faasia sisältäviä ohutkalvoja, että V2O5 ja V7O16 sekafaasirakenteen omaavia ohutkalvoja tutkittiin kaasuanturina, ja mittaustulokset osoittivat erilaisten kalvojen sähköisten kaasuanturivasteiden ominaisuuksien voimakkaan riippuvuuden kalvojen faasirakenteesta. Havaittiin myös, että sekafaasirakenne omaa tietyissä olosuhteissa p-tyyppisen puolijohteen sähkönjohtavuusmekanismin, toisin kuin puhdas V2O5-rakenne, joka on täysin n-tyyppinen. Molemmat ohutkalvotyypit todennettiin olevan erityisen herkkiä ammoniakki (NH3) kaasulle, jopa 40 miljardisosatasolle. Kalvo, jossa oli sekafaasirakenne, omasi korkeamman sähköisen kaasuvasteen kuin puhtaasta V2O5 faasista koostuva ohutkalvo, joka taas toisaalta omasi paremman stabiiliuden pidemmällä aikavälillä. Molemmat kaasuanturimateriaalit havaittiin selektiiviseksi NH3 -kaasulle verrattuna NO- ja CO-kaasuihin. Puhdas V2O5 nanorakenne onnistuttiin myös kasvattamaan kaupalliselle anturialustalle, ja käyttämään menestyksekkäästi herkkänä NH3- kaasuanturina. Lisäksi puhtaan V2O5 nanorakenteen todennettiin olevan erittäin lupaava kaasuanturimateriaali hyödynnettäväksi NOx-kaasupäästöjen vähentämiseen käytettävän SCR-katalyysiprosessin (Selective Catalytic Reduction) ohjauksessa. Ohutkalvotyyppien pinnan sähköistä rakennetta tutkittiin röntgenspektroskopiamenetelmillä, ja selvä yhteys materiaalien pintojen valenssitilojen ja kaasuanturiominaisuuksien välillä havaittiin. Huomattiin, että myös puhdas V2O5 ohutkalvo omaa pinnallaan pienen määrän V4+ -ioneja, ja että ohutkalvossa, jossa on sekafaasirakenne, V4+ -ionien määrä on suuri, ollen yksi todiste lisää V7O16 faasin olemassaoloon kalvon rakenteessa. Tästä johtuva happivakanssien olemassaolo on yksi syy näiden ohutkalvojen korkeaan kaasuherkkyyteen.

Two-Dimensional (2D) materials often exhibit distinguished properties as compared to their 3D counterparts and offer great potential to advance technology. However, even graphene, the first synthesized 2D material, still faces several challenges, despite its high mobility and high thermal conductivity. Similarly, germanene and silicene face challenges due to readily available semiconducting properties to be used in electronics, photonics or photocatalysis applications. Here, we propose two approaches to tune the band gap: One is by forming nanoribbon and edge functionalization and another by doping using inorganic nanoparticle’s interaction with 2D nanomaterials. Edge functionalization of armchair germanene nanoribbons (AGeNRs) has the potential to achieve a range of band gaps. The edge atoms of AGeNRs are passivated with hydrogen (-H and -2H) or halogen (-F, -Cl,-OH, -2F,-2Cl) atoms. Using density functional theory calculations, we found that edge-functionalized AGeNRs had band gaps as small as 0.012 eV when functionalized by -2H and as high as 0.84 eV with -2F. Doping can change the semiconducting behavior of AGeNRs to metal due to the half-filled band making it useful for negative differential resistance (NDR) devices. In the case of zigzag germanene nanoribbons (ZGeNRs), single N or B doping transformed them from anti-ferromagnetic (AFM) semiconducting to ferromagnetic (FM) semiconductor or half-metal. Lastly, formation and edge free energy studies revealed the feasibility of chemical synthetization of edge-functionalized and doped germanene. Electronic, optical and transport properties of the graphene/ZnO heterostructure have been explored using first-principles density functional theory. The results show that Zn12O12 can open a band gap of 14.5 meV in graphene, increase its optical absorption by 1.67 times, covering the visible spectrum and extended to the infra-red (IR) range, and create slight nonlinear I-V characteristics depending on the applied bias. This agrees well with collaborative experimental measurement of a similar system. In conclusion, we have successfully studied the potential use of edge functionalization, band gap periodicity in nanoribbon width, and doping in germanene nanoribbons. Structural stability was also studied to investigate the feasibility for experimental synthesization. Inorganic nanoparticle’s interaction with graphene envisages the possibility of fabricating photo-electronic device covering visible spectrum and beyond. Finally, graphene complexes were merged with naturally available direct band gap of monolayer MoS2 to build efficient energy harvesting and photo detecting devices.

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Agradecemos a compreensão. on 2016-11-10T12:39:58Z (GMT) / Submitted by CAMILA PAIXÃO SANTOS (camila.paixao@yahoo.com.br) on 2016-11-11T18:37:20Z No. of bitstreams: 1 DEFESA_FINAL.pdf: 2077990 bytes, checksum: 7c050271b8596995c2ed6deeef0aca4e (MD5) / Approved for entry into archive by LUIZA DE MENEZES ROMANETTO null (luizaromanetto@hotmail.com) on 2016-11-16T19:53:18Z (GMT) No. of bitstreams: 1 santos_cp_me_guara.pdf: 2077990 bytes, checksum: 7c050271b8596995c2ed6deeef0aca4e (MD5) / Made available in DSpace on 2016-11-16T19:53:18Z (GMT). No. of bitstreams: 1 santos_cp_me_guara.pdf: 2077990 bytes, checksum: 7c050271b8596995c2ed6deeef0aca4e (MD5) Previous issue date: 2016-09-03 / Não recebi financiamento / Este trabalho apresenta e discute o uso do óxido de cério na fabricação de filmes espessos por “screen printing” para aplicações em sensores de gás. Nesse estudo o CeO2 puro foi obtido pelo método dos precursores poliméricos utilizando como resina precursora o citrato de céria. O “puff” – espuma resultante da primeira fase do tratamento térmico da resina- foi calcinado a 550, 600, 700 e 750°C. O pó foi caracterizado por termogravimetria (TG) e as propriedades estruturais, morfológicas foram avaliadas por difratometria de raios X (DRX), espectroscopia Raman, área de superfície por isotermas Brunauer, Emmett e Taller (BET) e microscopia eletrônica de varredura (MEV). A resposta sensora foi estudada em uma câmara de teste construída no Laboratório de Catálise e Superfícies do Instituto de Ciência de Tecnologia de Materiais (INTEMA) da Universidade de Mar del Plata. A TG mostrou a formação de óxido de cério a 550°C, temperatura relativamente baixa quando comparada com outros métodos. Mediante DRX todas as amostras mostraram picos correspondentes à fase pura de CeO2 o qual cristaliza em uma estrutura cúbica do tipo fluorita, entretanto, maiores temperaturas de calcinação mostraram aumento da cristalinidade e tamanho do cristalito. No espectro Raman, um forte pico em torno do 461 cm-1 foi detectado, atribuído às vibrações simétricas do Ce-O. A área de superfície BET dos pós foi de 301, 77 m2/g o que evidencia a formação de partículas muito pequenas e altamente reativas. As micrografias obtidas por MEV mostram a presença de diferentes tamanhos na forma de aglomerados. A caracterização da resposta sensora mostrou que o sensor fabricado a partir de pós de CeO2 puro apresenta um bom tempo de resposta, alcançando a melhor performance com temperatura de trabalho de 400 °C, tanto em atmosferas redutoras e oxidantes. A característica principal observada foi que os resultados são dependentes dos ciclos anteriores, a reprodutibilidade do sistema é garantida quando se apaga a “memória” do sistema, expondo-o ao vácuo. / This paper presents and discusses the use of cerium oxide in the production of thick films for "screen printing" for applications in gas sensors. In this study the pure CeO2 was obtained by the polymeric precursor method using as a precursor resin citrate ceria. The "puff" - resulting foam from the first stage of thermal treatment of the resin-calcined at 550, 600, 700 and 750 °C. The powder was characterized by thermogravimetry (TG) and structural, morphological were evaluated by X-ray diffraction (XRD), Raman spectroscopy, isothermal Brunauer surface area, Emmett and Taller (BET) and scanning electron microscopy (SEM) . The sensor response was studied in a test chamber built in the Laboratory of Catalysis and Surface Materials Technology Institute of Science (INTEMA), University of Mar del Plata. The thermogravimetry showed the formation of cerium oxide at 550° C, relatively low temperature compared with other methods. Upon XRD all samples showed peaks corresponding to pure CeO2 phase which crystallizes in a cubic fluorite type structure, however, higher calcination temperatures showed increased crystallinity and crystallite size. In the Raman spectrum, a strong peak around 461 cm-1 was detected, assigned to symmetric vibrations of the Ce-O. The BET surface area of the powders was 301, 77 m2 /g which shows the formation of very small and highly reactive particles. The SEM micrographs show the presence of different sizes in the form of agglomerates. The characterization of the sensor response showed that the sensors manufactured from pure CeO2 powder has a good response time, achieving better performance at 400 °C working temperature in both reducing and oxidizing atmospheres. The main feature observed was that the results are dependent on previous cycles, the system reproducibility is guaranteed when it deletes the "memory" of the system, exposing it to vacuum.

Neste trabalho, foram desenvolvidos filmes finos de composição SrTi1-xFexO3 (0,00≤ x ≤0,150) nanoestruturados visando sua aplicação como sensor de gás ozônio. Os filmes finos foram depositados através da técnica de deposição por feixe de elétrons (EBD) cujo alvo utilizado foram pastilhas obtidas a partir do pó cristalino SrTi1-xFexO3 (0,00≤ x ≤0,150) sintetizadas através do método dos precursores poliméricos. Foi observado a partir das analises termogravimétrica e térmica diferencial que a incorporação de ferro no sistema diminui a temperatura de queima do pó precursor, sugerindo que o ferro atua como catalisador na cadeia polimérica. Os filmes depositados por DFE apresentam-se no estado amorfo sendo necessário um tratamento térmico ex-situ para que a fase cristalina desejada seja obtida. Todos os filmes apresentaram uma boa aderência aos diferentes tipos de substratos utilizados. Após o processo de cristalização, os filmes depositados sobre diferentes substratos foram caracterizados através das técnicas de difração de raios-X, espectroscopia UV-Vis e microscopia de força atômica (MFA). Foi observado que um aumento na temperatura de tratamento térmico dos filmes leva a um aumento no grau de cristalização e a uma diminuição no valor da energia de gap, calculada a partir dos espectros UV-VIS. A analise por MFA mostrou que a mudança do tipo de substrato utilizado não influencia as propriedades estruturais e microestruturais dos filmes. Através da realização de medidas de resistência elétrica, observou-se que os filmes cristalinos submetidos a um tratamento térmico ex-situ a 500oC por 4 horas apresentaram uma boa sensibilidade ao gás ozônio sendo possível detectar a presença de até 75 ppb de ozônio. / In this study, nanostructured thin films of SrTi1-xFexO3 (0.00 ≤ x ≤ 0.150) compositions were prepared looking their application as ozone gas sensor. The thin films were deposited using the technique of electron beam deposition (EBD) whose targets were obtained from polycrystalline SrTi1-xFexO3 (0.00 ≤ x ≤ 0.150) powders synthesized by the polymeric precursor method. It was observed from the thermogravimetric and differential thermal analysis that the incorporation of iron in the system decreases the calcination temperature of the precursor powders, suggesting that the iron acts as a catalyst in the polymer chain. The as obtained films deposited by EBD present an amorphous state being necessary a ex-situ heat treatment to obtain the desired crystalline phase. All films showed good adhesion to different substrates. After the crystallization process, the films deposited on different substrates were characterized through X-ray diffraction, UV-Vis spectroscopy and atomic force microscopy (AFM) trechniques. It was observed that an increase in the annealing temperature of the film results in an increase in the degree of crystallization and a decrease in the value of band gap energy, which was calculated from the UV-VIS spectra. The AFM analysis showed that changing the type of substrate does not influence the structural properties and microstructure of the films. By carrying out measurements of electrical resistance, it was observed that the crystalline films subjected to a ex-situ heat treatment at 500oC for 4 hours showed a good sensitivity to the ozone gas being possible to detect the presence of up to 75 ppb ozone.