Academic literature on the topic 'Metal Oxide Semiconductor Tin Oxide (SnO2)'

Most research to develop highly transparent and conductive thin films has focused on n-type semiconductors consisting of metal oxides. Historically, transparent conducting oxide (TCO) thin films composed of binary compounds such as SnO2 and In2O3 were developed by means of chemical- and physical-deposition methods. Impurity-doped SnO2 (Sb- or F-doped SnO2, e.g., SnO2:Sb or SnO2: F) and In2O3: Sn (indium tin oxide, ITO) films are in practical use. In addition to binary compounds, ternary compounds such as Cd2SnO4, CdSnO3, and CdIn2O4 were developed prior to 1980, but their TCO films have not yet been used widely.

Tin oxide as a semiconductor metal oxide has revealed great potential in the field of gas sensing due to its porous structure and reduced size. Especially for tin oxide and its composites, inherent properties such as high surface areas and their unique semiconducting properties with tunable band gaps make them compelling for sensing applications. In combination with the general benefits of metal oxide nanomaterials, the incorporation of metal oxides into metal oxide nanoparticles is a new approach that has dramatically improved the sensing performance of these materials due to the synergistic effects. This review aims to comprehend the sensing mechanisms and the synergistic effects of tin oxide and its composites in achieving high selectivity, high sensitivity and rapid response speed which will be addressed with a full summary. The review further vehemently highlights the advances in tin oxide and its composites in the gas sensing field. Further, the structural components, structural features and surface chemistry involved in the gas sensing are also explained. In addition, this review discusses the SnO2 metal oxide and its composites and unravels the complications in achieving high selectivity, high sensitivity and rapid response speed. The review begins with the gas sensing mechanisms, which are followed by the synthesis methods. Further key results and discussions of previous studies on tin metal oxide and its composites are also discussed. Moreover, achievements in recent research on tin oxide and its composites for sensor applications are then comprehensively compiled. Finally, the challenges and scope for future developments are discussed.

In recent years, a transparent conducting oxide (TCO) SnO2 semiconductor have gained considerable attention due to their potential application in gas sensors. More number of studies on TCO oxide have focused on the semiconducting metal oxides in which an intensive argument is that the transparent semiconductors. The SnO2 thin films were deposited at 400 °C and then annealed at 500 °C and 600 °C and its structural, optical and electrical properties were characterized. The doping stoichiometric ratio was maintained as 4% and the resulting solution was sprayed on glass substrate which was kept at nozzle distance of 25 cm and the spray rate was 10 mL/min. The prepared pure SnO2 thin films have been characterized by different methods such as XRD, FESEM, UV-Vis NIR and EDAX analyses. It was found that the nanocrystalline SnO2 grains possesses structural features of the tetragonal rutile structure. Hence the prepared thin films are justified to be nanocrystalline and also the mean crystalline size decreased with respect to annealing temperature.

ABSTRACTWe describe the characteristics of a series of thin film tin oxide films grown by plasma-assisted molecular beam epitaxy on r-plane sapphire substrates over a range of flux and substrate temperature conditions. A mixture of both SnO2 and SnO are detected in several films, with the amount depending on growth conditions, most particularly the substrate temperature. Electrical measurements were not possible on all samples due to roughness related issues with contacting, but at least one film exhibited p-type characteristics depending on measurement conditions, and one sample exhibited significant persistent photoconductivity upon ultraviolet excitation in a metal-semiconductor-metal device structure.

In this paper, a method for determining the doping efficiency of dispersed semiconductor metal oxide materials is proposed proposing to use the dependences of the free charge carrier concentration, normalized to the concentration of the doping impurity (Ne spec.), on the content of this impurity. The possibilities of this method are demonstrated by the example of studying the effect of technological factors on the efficiency of doping of indium oxide with tin and doping of tin oxide with antimony. It is shown that it is impossible to achieve the concentration of free charge carriers in the ITO material, higher than that in ATO materials, due to the lower solubility of tin in the In2O3 lattice, as compared with the solubility of antimony in the SnO2 lattice.

Nanostructured semiconductor metal oxides have played a central role in the gas sensing research field, because of their high sensitivity, selectivity and low response time. Among all the processes, developed for the synthesis of nanostructured metal oxides, gel combustion seems to be the most promising route due to low-cost precursors and simplicity of the process. It combines chemical gelation and combustion, involving the formation of a gel from an acqueous solution and an exothermic redox reaction, yielding to very porous and softly agglomerated nanopowders. In this work, nanostructured tin oxide, SnO2, and titanium oxide, TiO2, have been synthesized through gel combustion. Powders showed nanometric particle size and high specific surface area. The so-obtained TiO2 and SnO2 nanopowders have been used as sensitive element of resistive λ sensor and ethanol sensor respectively, realized depositing films of nanopowders dispersed in water onto alumina substrates provided with Pt contacts and heater. TiO2-based sensors showed at high temperature good response, fast response time, linearity in a wide range of O2 concentration and long-term stability. SnO2-based sensors have shown high sensitivity to low concentrations of ethanol at moderate temperature.

The limited choice of materials for large area electronics limits the expansion of applications. Polycrystalline silicon (poly-Si) and indium gallium zinc oxide (IGZO) lead to thin-film transistors (TFTs) with high field-effect mobilities (>10 cm2/Vs) and high current ON/OFF ratios (IOn/IOff > ~107). But they both require vacuum processing that needs high investments and maintenance costs. Also, IGZO is prone to the scarcity and price of Ga and In. Other oxide semiconductors require the use of at least two cations (commonly chosen among Ga, Sn, Zn, and In) in order to obtain the amorphous phase. To solve these problems, we demonstrated an amorphous oxide material made using one earth-abundant metal: amorphous tin oxide (a-SnOx). Through XPS, AFM, optical analysis, and Hall effect, we determined that a-SnOx is a transparent n-type oxide semiconductor, where the SnO2 phase is predominant over the SnO phase. Used as the active material in TFTs having a bottom-gate, top-contact structure, a high field-effect mobility of ~100 cm2/Vs and an IOn/IOff ratio of ~108 were achieved. The stability under 1 h of negative positive gate bias stress revealed a Vth shift smaller than 1 V.

Tin oxide (SnO2) and titanium dioxide (TiO2) are recognized as attractive energy materials applicable for lead halide perovskite solar cells (PSCs). Sintering is one of the effective strategies for improving the carrier transport of semiconductor nanomaterials. Using the alternative metal-oxide-based ETL, nanoparticles are often used in a way that they are dispersed in a precursor liquid prior to their thin-film deposition. Currently, the creation of PSCs using nanostructured Sn/Ti oxide thin-film ETL is one of the topical issues for the development of high-efficiency PSCs. Here, we demonstrate the preparation of terpineol/PEG-based fluid containing both tin and titanium compounds that can be utilized for the formation of a hybrid Sn/Ti oxide ETL on a conductive substrate (F-doped SnO2 glass substrate: FTO). We also pay attention to the structural analysis of the Sn/Ti metal oxide formation at the nanoscale using a high-resolution transmission electron microscope (HR-TEM). The variation of the nanofluid composition, i.e., the concentration of tin and titanium sources, was examined to obtain a uniform transparent thin film by spin-coating and sintering processes. The maximum power conversion efficiency was obtained for the concentration condition of [SnCl2·2H2O]/[titanium tetraisopropoxide (TTIP)] = 25:75 in the terpineol/PEG-based precursor solution. Our method for preparing the ETL nanomaterials provides useful guidance for the creation of high-performance PSCs using the sintering method.

Due to the unique combination of electric and optical properties such semiconductor oxides as tin and indium oxides are very perspective multifunctional materials for variety of microelectronic devices production. Experimental studies of these materials allow to define, for example energetic characteristics of the devices created and also to improve existing technologies of films formation, joining of electrocontacts to tin and indium oxides based materials by way of brazing which require additional wetting studies. It should be noted that data on wetting of mentioned oxides by metals are practically absent in literature. Thus a detailed study of the interfacial interaction, adhesion and wetting of ceramic SnO2 and In2O3 materials with some pure metal melts in vacuum was performed by the sessile drop method using foto- and video- fixing including concentration, temporal and temperature dependences of contact angles. It was found that most of the pure metals studied don't wet SnO2 ceramics. However rather intense chemical interaction took place in vacuum at high temperatures in contact of some metals (Sn, Ge, In) with surface of SnO2. It was also shown the effect of the experiment temperature and hold-up time on the values of contact angles. Wetting angles for powdery pressed specimens of In2O3 in the temperature range studied don't change noticeably and vary in a narrow range. For example, for Ga wetting angles vary from about 138 to 128 deg and for Sn  125119 deg, Wetting of SnO2 surface with AgCu melt with different copper content was shown to be insufficient to use it as a brazing alloy, yet this system can be used as a basis for creating a brazing composition. Keywords: indium oxide, tin dioxide, semiconductor, wetting, contact interaction, metal melt.

Halbleiter-Gassensoren auf der Basis von Zinnoxid spielen in der Überwachung, Steuerung und Regelung von Prozessen sowie bei der Kontrolle von Umweltparametern eine wachsende Rolle. Dies liegt daran, dass sie kostengünstig und hoch sensitiv für diverse Gase sind. Der mögliche Anwendungsbereich wird jedoch durch ihre geringe Selektivität eingeschränkt. Daher stellt die Steigerung der Sensitivitäten von SnO2-Sensoren eine bleibende Forderung an die Forschung und Entwicklung dar. In der Literatur werden verschiedene Methoden zur Beeinflussung von Sensitivität und Selektivität beschrieben. Am weitesten verbreitet sind insbesondere die Modifizierung der Betriebsweise und die Veränderung der Zusammensetzung der Zinnoxidschichten. Diese beiden Wege wurden auch in dieser Arbeit beschritten. Zum einen wurde der thermozyklische Betrieb, der gegenüber dem isothermen bereits zu einer Verbesserung der Selektivität geführt hat, systematisch untersucht. Dies erlaubt die Leistungsfähigkeit des thermozyklischen Verfahrens zu optimieren und noch ungeklärte Phänomene des Detektionsprozesses besser zu beschreiben. Zum anderen waren Modifizierungen der Zinnoxidschicht durch Zusatz fester Ionenleiter Gegenstand detaillierter Untersuchungen. Zusätze von Materialien dieses Typs führen ebenso wie Änderungen im Betriebsmodus zu Selektivitätssteigerungen, insbesondere gegenüber Gasen mit bestimmten funktionellen Gruppen, z.B. primären Alkoholen. Folgende Erkenntnisse wurden in dieser Arbeit erlangt: ·        Thermozyklischer Betrieb Der thermozyklische Betrieb führt zu charakteristischen Leitwert-Zeit-Profilen (LZP). Die Form der LZP (Profilstruktur) ist vom Betriebsregime wie der Aufheiz- und Abkühlungsgeschwindigkeit (Temperaturrate), dem Volumenstrom des Messgases, der Schichtdicke der sensitiven Schicht, dem Elektrodenmaterial und vor allem aber von der Art und Konzentration der Gaskomponente sowie dem Feuchtegehalt des Messgases abhängig. Der Temperaturrate kommt hierbei eine besondere Bedeutung zu. Sie beeinflusst die gasspezifischen nicht-stationären Nichtgleichgewichte an der Sensoroberfläche und somit die LZP sowie die Sensitivitäten. Die Sensitivität kann aus den Sensorsignalen (LZP) als Summenparameter für Temperaturzyklen bestimmt werden. Sie lässt sich als mittlere relative Leitwertänderung bei Gasexposition definieren. Ebenso repräsentativ für die gassensitiven Eigenschaften sind Vorfaktor und Exponent (Sensitivitätskoeffizienten A´ und b´) der aus der Konzentrationsabhängigkeit der Leitwertssumme bestimmbaren Potenzfunktion. Da sich jedoch bereits kleinste Mengen an Reaktanten auf das in Luft bestimmte Leitwert-Zeit-Profil signifikant auswirken und daher diese Profile streuen, ist die Darstellung der Parameter der Potenzfunktion und die Bestimmung der Sensitivität als relative Leitwertänderung in Bezug auf eine definierte geringe Gaskonzentration generell zu bevorzugen. Es wurde exemplarisch gezeigt, dass die unter thermozyklischen Bedingungen ermittelten Sensitivitäten gegenüber CO, Propylen sowie Propanol größer sind als die unter isothermen Betriebsbedingungen bestimmten. Der Zusammenhang von Sensitivität und Schichtdicke ist für verschiedene Gase unterschiedlich ausgeprägt. Profilform und Schichtdicken sind nur selten korrelierbar. Abhängigkeiten der spezifischen Größen der LZP-Maxima (Temperatur, Leitwert) von der Schichtdicke wurden nur für Propylen gefunden. Durch die gezielte Variation von Parametern lassen sich indirekt Schlüsse über die Wechselwirkungen und Mechanismen in porösen gassensitiven Schichten ziehen. Dominierende Effekte von Ad- und Desorption bzw. der Reaktion sowie der Diffusion von Gaskomponenten können in Bezug zueinander gesetzt werden. ·        Gassensitive Eigenschaften von SnO2/NASICON-artigen Kompositen Komposite aus SnO2 und NASICON, bei denen das Na+ im NASICON gegen Li+ oder K+ ausgetauscht wurde, haben eine ähnliche Wirkung auf Sensitivität und Selektivität wie die mit Na+. Die Ergebnisse erweitern die in der Literatur beschriebenen Kenntnisse zur Wirkung ionisch leitender Kompositzusätze. Die Einflüsse der Zusätze sind sowohl durch die LZP als auch durch die daraus berechneten Sensitivitäten bzw. Sensitivitätskoeffizienten A´ und b´ darstellbar. Starke Sensitivitätssteigerungen gegenüber primären Alkoholen wurden für steigende Anteile der Alkaliionen in den Kompositen festgestellt. Dagegen ist die Sensitivität gegenüber sekundären Alkoholen bei Kompositschichten im Vergleich zu reinen SnO2-Schichten kaum verändert. Die für 1- und 2-Propanol im thermozyklischen Betrieb gefundenen Sensitivitäten sind in der Tendenz mit denen unter isothermen Betriebsbedingungen erlangten vergleichbar. ·        Einfluss der Elektroden Es wurden Hinweise darauf gefunden, dass bereits in der Literatur diskutierte Einflüsse der Elektroden auf die gassensitiven Eigenschaften im isothermen Betrieb von SnO2-Schichten auch im thermozyklischen Betrieb auftreten. Weiterhin beeinflusst die Art des Elektrodenmaterials, z.B. Gold und Platin sowie das Ausgangsmaterials zur Elektrodenherstellung spezifisch die Sensorsignale im thermozyklischen Betriebsverfahren. Die Einflüsse der Elektroden sind nicht nur gasspezifisch, sondern bewirken auch Unterschiede je nach verwendeter gassensitiver Schicht. Dabei können LZP, abhängig vom Gas, maßgeblich von der Elektrode oder den Schichtzusätzen beeinflusst sein. ·        Diffuse Reflexion Infrarot Fourier-Transformations Spektroskopie (DRIFTS) Isotherme Messungen der Diffusen Reflexion Infrarot Fourier-Transformations Spektroskopie (DRIFTS) in Abhängigkeit der Zusammensetzung der Gasphase und des gassensitiven Schichtmaterials sind geeignet, um adsorbierte Oberflächenspezies zu detektieren und Vorstellungen bezüglich der Oberflächenprozesse zu erlangen. DRIFTS-Untersuchungen in Luft mit verschiedenen Konzentrationen weisen auf eine höhere Reaktivität des 1-Propanol mit adsorbierten HO-Gruppen verglichen mit der des 2-Propanol hin. Die Ergebnisse deuten zudem an, dass sich die Oberflächenprozesse an SnO2/NASICON(x=3)-Kompositen von denen der reinen SnO2-Schicht unterscheiden, auch wenn sich diese jeweils unabhängig von der Art des Alkohols zeigen.

Effects of urban air pollution on health and environment have lead researchers to find economic air quality monitoring regulations. Since tin dioxide (SnO2) was demonstrated as a gas sensing device in 1962, tin oxide based thin film sensors have been widely studied due to their high sensitivity and fast response. The main advantages of using tin oxide sensors are their low cost, small size and low power consumption for mobile system applications. But, in order SnO2 based sensors to meet low concentration of gases they should be highly upgraded in sensitivity, selectivity and stability. This study was focused on the capacity of dopants in the SnO2 layer to increase the sensitivity of the sensor in detecting carbon monoxide. 1 wt. % Pd promoted and 0.1 wt. % Na-1 % Pd promoted SnO2 multilayer thin films were produced by sol-gel technique followed by spin coating route on soda-lime glass substrates. The EDX and SEM studies showed the surface composition and the surface structure is homogeneous throughout the films. The film thickness was determined app. 450 nm from the SEM image of the cross-section, after coating 8 layers. The experiments conducted at several temperatures namely 150, 175 and 200oC, in oxygen free and 1% oxygen containing atmospheres showed that the responses at higher temperatures in the presence of oxygen were much sharper with respect to others. Besides, Na promoted test sensors showed larger responses with shorter response time in oxygen free atmospheres at relatively lower temperatures. The results showed that the sensor signal is not directly correlated with the carbon dioxide production in oxygen free atmospheres.

Methods of monodisperse SnO2 crystallites and SnO2-TiO2 composites synthesis were developed. Obtained materials were studied using SEM, TEM, FTIR, XRD methods. Sensors based on these materials were made, and their capability of detecting hydrogen in dry and humid air was investigated. Several different approaches to improvement of sensing devises characteristics were proposed in present study: doping of one metal oxide (SnO2) with another (TiO2) in different ways (mixing or co-precipitating); use of one-dimensional nanowire as a sensing element to detect H2S; application of reactor which converts ammonia to a compound convenient for its detection; analysis of n-dimensional experimental data array with principal components method for acetone selective detection with a single sensor. For every approach, promising results indicating potential for their application are obtained.

Metal Oxide (MO)semiconductors are one of the most widely used materials in commercial gas sensor devices. The basic principle of chemoresistive gas sensor operation stems on the high sensitivity of electrical resistance to ambient gaseous conditions. Depending on whether the oxide is "p type" or "n type", the resistance increases (or decrease), when placed in atmosphere containing reducing (or oxidizing) gases. The study of conductometric metal oxide semiconductor gas sensors has dual importance in view of their technological device applications and understanding fundamental MO-gas interactions. Metal oxides based sensors offer high thermal, mechanical and chemical stability. A large number of MOs show good sensitivities to various gases like CO, NOX, SOX, NH3, alcohols and other Volatile Organic Compounds (VOCs). VOCs are very common hazardous pollutants in the environment. Gas sensors are in great demand for their various applications such as food quality control, fermentation industries, road safety, defence, environmental monitoring and other chemical industries. The aim of the study is to explore the possibility of advancements in semiconducting MO based gas sensor devices through tuning microstructural parameters along with chemical dopants or additives. And further to investigate the underlying mechanism of conductometric MO gas sensors. The novel synthesis method employed is based on the solution combustion method coupled with ultrasonically nebulized spray pyrolysis technique. The well studied SnO2 and relatively unexplored Cr2O3 oxide systems are selected for the study. The non-equilibrium processing conditions result in unique microstructure and defect chemistry. In addition, using this technique MO - Reduced Graphene Oxide (RGO) nanocomposite films has also been fabricated and its application to room temperature gas sensor devices is demonstrated. The thesis comprises of seven chapters. the following section describe the summery of individual chapters. The Chapter 1 describes the introduction and background literature of this technology. A brief review of developments in gas sensor technology so far has been enlisted. This chapter also gives a glimpse of applications of MO semiconductors based sensors. The underlying mechanism involved in the sensing reaction and the primary factors influencing the response of a gas sensor device are enlisted. Further in the later part of the chapter focused the material selection criteria, effect of additives/dopants and future prospects of the technology. The end of this chapter highlights the objective and scope of the work in this dissertation. In the Chapter 2 the the materials selection, characterization techniques and particularly the experimental setups used are elaborated. This includes the deposition method used, which is developed in our group and the the in house built gas sensing system including its working principles and various issues have been addressed. The Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) is a novel deposition method devised, which is a combination of conventional spray pyrolysis and solution combustion technique. Spray pyrolysis is versatile, economic and simple technique, which can be used for large area deposition of porous films. The intention is to exploit the exothermicity of combustion reaction in order to have high crystallinity, smaller crystallite size with high surface area, which are extremely important in gas sensor design and its efficiency. Further the gas sensing system and its operation are discussed in detail including the advantages of vertical sensing chamber geometry, wider analyte concentration range (ppm to percentage) obtained through vapor pressure data and simultaneous multi sensor characterization allowing better comparison. Here in this work, Chromium oxide (Cr2O3) and Tin oxide (SnO2) are selected as gas sensing materials for this work as a p-type and n-type metal oxide semiconductors respectively. Nevertheless Cr2O3 is a less explored gas sensing material as compared to SnO2, which is also being used in many commercially available gas sensor devices. Thus, studying and comparing gas sensing properties of a relatively novel and a well established material would justify the potential of the novel deposition technique developed. In Chapter 3, the effect of exothermic reaction between oxidizer and fuel, on the morphology, surface stoichiometry and observed gas sensing properties of Cr2O3 thin films deposited by UNSPACM, is studied. An elaborative study on the structural, morphological and surface stoichiometry of chromium oxide films is undertaken. Various deposition parameters have been optimized. An extensive and systematic gas sensing study is carried out on Cr2O3 films deposited, to achieve unique microstructure. The crystallinity and microstructure are investigated by varying the deposition conditions. Further, the effect of annealing in oxygen gas atmospheres on the films was also investigated. The gas sensing properties are studied for various VOCs, in temperature range 200 - 375 oC. The possible sensing mechanism and surface chemical processes involved in ethanol sensing, based on empirical results, are discussed. In chapter 4, the effect of 1% Pt doping on gas sensing properties of Cr2O3 thin films prepared by UNSPACM, is investigated. The chemical analysis is done using x-ray photoelectron spectroscopy to find the chemical state of Pt and quantification is done. The gas sensing is done towards gases like NO2, Methane and Ethanol. The enhancement in sensitivity and remarkable reduction in response as well as recovery times have been modeled with kinetic response analysis to study the variation with temperature as well as concentration. Further the analysis of observations and model fittings is discussed. The Chapter 5 deals with the defects induced ferromagnetism and gas sensing studies SnO2 nanoparticles prepared by solution combustion method. The structural, chemical analysis of as-synthesized and annealed SnO2 nanoparticles reveal gradual reduction in defect concentration of as-prepared SnO2. The findings of various characterization techniques along with optical absorption and magnetic studies to investigate the defect structure of the material are presented. As defects play crucial role in gas sensing properties of the metal oxide material, the defect induced room temperature ferromagnetism in undoped SnO2 has been used as a potential tool to probe the evidence of the defects. Finally a correlation is established between observed room temperature ferromagnetism and gas sensing studies and primary role of defects in gas sensing mechanism over microstructure is realized . The Chapter 6 presents the deposition of SnO2 thin films by UNSPACM method on glass substrates for gas sensing application. The readiness of UNSPACM in making sensor materials with unform dopant distribution is demonstrated in order to improve the sensor performance in terms of response and selectivity. The chemical composition, film morphology and gas sensing studies are reported. The SnO2 is doped with Cr and Pt to enhance the sensing properties of the material. The doped Oxide films are found to show enhancement in sensitivity and improve the selectivity of the films towards specific gases like NO2 and CO. Further in Chapter 7 an effort has been made to overcome the problem of high operating temperature of metal oxide gas sensors through use of Reduced Graphene Oxide (RGO) and metal oxide nanocomposite films. Although RGO shows room temperature response towards many toxic and hazardous gases but it exhibits poor sensor signal recovery. This has been successfully solved by making nanohybrids of RGO and SnO2. It not only improves the sensor signal kinetics but it enhances the sensitivity also. Thus this chapter endeavors towards low power consumption gas sensing devices. The key findings and future aspects are summarized in the Chapter 8.

Metal Oxide (MO)semiconductors are one of the most widely used materials in commercial gas sensor devices. The basic principle of chemoresistive gas sensor operation stems on the high sensitivity of electrical resistance to ambient gaseous conditions. Depending on whether the oxide is "p type" or "n type", the resistance increases (or decrease), when placed in atmosphere containing reducing (or oxidizing) gases. The study of conductometric metal oxide semiconductor gas sensors has dual importance in view of their technological device applications and understanding fundamental MO-gas interactions. Metal oxides based sensors offer high thermal, mechanical and chemical stability. A large number of MOs show good sensitivities to various gases like CO, NOX, SOX, NH3, alcohols and other Volatile Organic Compounds (VOCs). VOCs are very common hazardous pollutants in the environment. Gas sensors are in great demand for their various applications such as food quality control, fermentation industries, road safety, defence, environmental monitoring and other chemical industries. The aim of the study is to explore the possibility of advancements in semiconducting MO based gas sensor devices through tuning microstructural parameters along with chemical dopants or additives. And further to investigate the underlying mechanism of conductometric MO gas sensors. The novel synthesis method employed is based on the solution combustion method coupled with ultrasonically nebulized spray pyrolysis technique. The well studied SnO2 and relatively unexplored Cr2O3 oxide systems are selected for the study. The non-equilibrium processing conditions result in unique microstructure and defect chemistry. In addition, using this technique MO - Reduced Graphene Oxide (RGO) nanocomposite films has also been fabricated and its application to room temperature gas sensor devices is demonstrated. The thesis comprises of seven chapters. the following section describe the summery of individual chapters. The Chapter 1 describes the introduction and background literature of this technology. A brief review of developments in gas sensor technology so far has been enlisted. This chapter also gives a glimpse of applications of MO semiconductors based sensors. The underlying mechanism involved in the sensing reaction and the primary factors influencing the response of a gas sensor device are enlisted. Further in the later part of the chapter focused the material selection criteria, effect of additives/dopants and future prospects of the technology. The end of this chapter highlights the objective and scope of the work in this dissertation. In the Chapter 2 the the materials selection, characterization techniques and particularly the experimental setups used are elaborated. This includes the deposition method used, which is developed in our group and the the in house built gas sensing system including its working principles and various issues have been addressed. The Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) is a novel deposition method devised, which is a combination of conventional spray pyrolysis and solution combustion technique. Spray pyrolysis is versatile, economic and simple technique, which can be used for large area deposition of porous films. The intention is to exploit the exothermicity of combustion reaction in order to have high crystallinity, smaller crystallite size with high surface area, which are extremely important in gas sensor design and its efficiency. Further the gas sensing system and its operation are discussed in detail including the advantages of vertical sensing chamber geometry, wider analyte concentration range (ppm to percentage) obtained through vapor pressure data and simultaneous multi sensor characterization allowing better comparison. Here in this work, Chromium oxide (Cr2O3) and Tin oxide (SnO2) are selected as gas sensing materials for this work as a p-type and n-type metal oxide semiconductors respectively. Nevertheless Cr2O3 is a less explored gas sensing material as compared to SnO2, which is also being used in many commercially available gas sensor devices. Thus, studying and comparing gas sensing properties of a relatively novel and a well established material would justify the potential of the novel deposition technique developed. In Chapter 3, the effect of exothermic reaction between oxidizer and fuel, on the morphology, surface stoichiometry and observed gas sensing properties of Cr2O3 thin films deposited by UNSPACM, is studied. An elaborative study on the structural, morphological and surface stoichiometry of chromium oxide films is undertaken. Various deposition parameters have been optimized. An extensive and systematic gas sensing study is carried out on Cr2O3 films deposited, to achieve unique microstructure. The crystallinity and microstructure are investigated by varying the deposition conditions. Further, the effect of annealing in oxygen gas atmospheres on the films was also investigated. The gas sensing properties are studied for various VOCs, in temperature range 200 - 375 oC. The possible sensing mechanism and surface chemical processes involved in ethanol sensing, based on empirical results, are discussed. In chapter 4, the effect of 1% Pt doping on gas sensing properties of Cr2O3 thin films prepared by UNSPACM, is investigated. The chemical analysis is done using x-ray photoelectron spectroscopy to find the chemical state of Pt and quantification is done. The gas sensing is done towards gases like NO2, Methane and Ethanol. The enhancement in sensitivity and remarkable reduction in response as well as recovery times have been modeled with kinetic response analysis to study the variation with temperature as well as concentration. Further the analysis of observations and model fittings is discussed. The Chapter 5 deals with the defects induced ferromagnetism and gas sensing studies SnO2 nanoparticles prepared by solution combustion method. The structural, chemical analysis of as-synthesized and annealed SnO2 nanoparticles reveal gradual reduction in defect concentration of as-prepared SnO2. The findings of various characterization techniques along with optical absorption and magnetic studies to investigate the defect structure of the material are presented. As defects play crucial role in gas sensing properties of the metal oxide material, the defect induced room temperature ferromagnetism in undoped SnO2 has been used as a potential tool to probe the evidence of the defects. Finally a correlation is established between observed room temperature ferromagnetism and gas sensing studies and primary role of defects in gas sensing mechanism over microstructure is realized . The Chapter 6 presents the deposition of SnO2 thin films by UNSPACM method on glass substrates for gas sensing application. The readiness of UNSPACM in making sensor materials with unform dopant distribution is demonstrated in order to improve the sensor performance in terms of response and selectivity. The chemical composition, film morphology and gas sensing studies are reported. The SnO2 is doped with Cr and Pt to enhance the sensing properties of the material. The doped Oxide films are found to show enhancement in sensitivity and improve the selectivity of the films towards specific gases like NO2 and CO. Further in Chapter 7 an effort has been made to overcome the problem of high operating temperature of metal oxide gas sensors through use of Reduced Graphene Oxide (RGO) and metal oxide nanocomposite films. Although RGO shows room temperature response towards many toxic and hazardous gases but it exhibits poor sensor signal recovery. This has been successfully solved by making nanohybrids of RGO and SnO2. It not only improves the sensor signal kinetics but it enhances the sensitivity also. Thus this chapter endeavors towards low power consumption gas sensing devices. The key findings and future aspects are summarized in the Chapter 8.

Im ersten Teil der vorliegenden Arbeit werden die physikalischen Eigenschaften, insbesondere die elektrische Leitfähigkeit, von Zink-Zinn-Oxid Dünnschichten sowie darauf basierenden Schottky-Dioden in Abhängigkeit von der Kationenkomposition bestimmt. Zur Herstellung dieser Dünnschichten wurde ein Verfahren genutzt, welches die Herstellung von kontinuierlichen Kompositiongradienten im Rahmen eines gepulsten Laserabscheidungsprozesses bei Raumtemperatur ermöglicht. Erster Schwerpunkt der Diskussion ist die Abhängigkeit elektrischer Eigenschaften der Dünnschichten sowie die Diodeneigenschaften vom Kationenverhältnis. Des Weiteren wird die Langzeitstabilität der Schottky-Dioden und der Einfluss der Sauerstoffzufuhr während der Kontaktherstellung auf die Eigenschaften der Schottky-Dioden herausgestellt. DieErgebnissetiefenaufgelösterRöntgenphotoelektronenspektroskopiewerden diskutiert und ein Mechanismus, welcher zu einer Verbesserung der Schottky-Dioden über die Zeit führt, wird vorgestellt. Die Erkenntnisse über die optimale Kationenkomposition und den Einfluss des Sauerstoffs auf die Eigenschaften von Schottky-Dioden wurden genutzt, um Metall-Halbleiter-Feldeffekttransistoren herzustellen, welche im zweiten Teil der vorliegenden Arbeit beschrieben werden. In einem ersten Schritt wurden hierfür die Abscheidebedingungen in der Sputterkammer optimiert und eine neue Abscheiderezeptur für die Herstellung von Feldeffekttransistoren eingeführt. Auch hier finden alle Abscheidungen bei Raumtemperatur statt. Die Abscheidung mittels Sputtern wurde gewählt, da diese Abscheidemethode größere industrielle Relevanz als die gepulste Laserabscheidung hat. Metall-Halbleiter-Feldeffekttransistoren mit zwei verschiedenen Gate-Typen werden vorgestellt und jeweils der Einfluss der Kanalschichtdicke auf die Transistoreigenschaften untersucht. Der Einfluss des durch die Herstellung erzeugten Sauerstoffreservoirs in dem Schottky-Gate Kontakt auf die Eigenschaften der Feldeffekttransistoren wird ebenso gezeigt wie der Einfluss eines thermischen Ausheizprozesses auf die Schaltgeschwindigkeit der Feldeffekttransistoren. Außerdem werden einfache Inverter, welche auf zwei gleichartigen Feldeffekttransistoren basieren, vorgestellt. Ebenfalls werden SchottkyDioden Feldeffekttransistoren Logik basierte Inverter vorgestellt und charakterisiert. AbschließendwerdenRingoszillatoren,aufgebautausmehrereninReihegeschaltetenSchottkyDiodenFeldeffekttransistorenLogikbasiertenInverternvorgestellt. DerEinflussderKanalschichtdicke und der Gate-Geometrie auf die Oszillationsfrequenz wird diskutiert.:Contents 1 Introduction 2 Theoretical Descriptions 2.1 The Amorphous Semiconductor Zinc Tin Oxide 2.2 Schottky Barrier Diodes 2.3 Field-Effect Transistors 2.4 Inverter 2.5 Inverter Chain and Ring Oscillator 3 Methods 3.1 Growth and Structuring Techniques 3.1.1 Pulsed Laser Deposition 3.1.2 Sputtering Deposition 3.1.3 Photolithography 3.2 Characterization Techniques 3.2.1 Hall Effect Measurements 3.2.2 XRD and XRR Measurements 3.2.3 Static and Dynamic Current-Voltage Measurements 3.2.4 Further Characterization Techniques 4 Physical Properties of Amorphous Zinc Tin Oxide 4.1 Characterization of Pulsed Laser Deposited Zinc Tin Oxide Thin Films Having a Continuous Composition Spread 4.2 Properties of Schottky Barrier Diodes in Dependence on the Cation Composition 4.3 Long Term Stability of Schottky Barrier Diodes 4.4 ImportantRoleofOxygenfortheFormationofHighlyRectifyingContacts 4.5 Processes Governing the Long Term Stability 5 Demonstration and Characterization of Zinc Tin Oxide Based Devices 5.1 Implementation of a New Sputtering Recipe 5.1.1 CharacterizationandElectricalOptimizationoftheZincTinOxide Thin Films .1.2 Optimization of the Gate Contact 5.2 Devices with PtOx/Pt Gate Contact 5.2.1 Variation of the Channel Thickness 5.2.2 Influence of the Oxygen Reservoir on the Performance and Long Term Stability of Devices 5.2.3 Tuning of the Electron Mobility 5.2.4 Frequency Dependent Switching of Transistors 5.3 Devices with i-ZTO/PtOx/Pt Gate Contact 5.3.1 Transistors with Varying Channel Thickness 5.3.2 Simple Inverter 5.3.3 SDFL Inverter 5.3.4 Inverter Chain 5.3.5 Ring Oscillators 5.4 Comparison to Literature 6 Summary and Outlook Abbreviations List of Symbols Bibliography List of Own and Contributed Articles Appendix
In the first part of the present work the physical properties, especially the electrical properties, of zinc tin oxide thin films as well as Schottky diodes based thereon are determined as a function of the cation composition. For film growth, a room temperature pulsed laser deposition process was used, which allows the realization of a continuous composition gradient within one sample. First focus of the discussion is the dependence of electrical properties of thin films as well as diode properties on the cation ratio. Furthermore, the long-term stability of the Schottky diodes and the influence of the oxygen supply during contact fabrication on the properties of the Schottky diodes are highlighted. The results of depth-resolved Xray photoelectron spectroscopy measurements are discussed and a mechanism leading to an improvement of the Schottky diodes over time is elucidated. The findings on the optimal cation composition and the influence of oxygen on the properties of Schottky diodes were used to produce metal-semiconductor field-effect transistors, which are described in the second part of this thesis. In a first step, the deposition conditions in the sputter chamber were optimized and a new deposition recipe for the fabrication of field effect transistors was developed. Here, too, all depositions take place at room temperature. Sputter deposition was chosen because this deposition method has greater industrial relevance than pulsed laser deposition. Metal-semiconductor field-effect-transistors with two different gate types are presented and the influence of the channel layer thickness on the transistor properties is investigated. The influence of the oxygen reservoir in the Schottky gate contact on the properties of the field-effect-transistors is shown as well as the influence of a thermal annealing process on the switching speed of the field-effect-transistors. In addition, simple inverters based on two identical field-effect-transistors are demonstrated. Also Schottky diode field-effect-transistor logic based inverters are presented and characterized. Finally, ring oscillators consisting of several series-connected Schottky diode field-effecttransistor logic based inverters are presented. The influence of channel layer thickness and gate geometry on the oscillation frequency is discussed.:Contents 1 Introduction 2 Theoretical Descriptions 2.1 The Amorphous Semiconductor Zinc Tin Oxide 2.2 Schottky Barrier Diodes 2.3 Field-Effect Transistors 2.4 Inverter 2.5 Inverter Chain and Ring Oscillator 3 Methods 3.1 Growth and Structuring Techniques 3.1.1 Pulsed Laser Deposition 3.1.2 Sputtering Deposition 3.1.3 Photolithography 3.2 Characterization Techniques 3.2.1 Hall Effect Measurements 3.2.2 XRD and XRR Measurements 3.2.3 Static and Dynamic Current-Voltage Measurements 3.2.4 Further Characterization Techniques 4 Physical Properties of Amorphous Zinc Tin Oxide 4.1 Characterization of Pulsed Laser Deposited Zinc Tin Oxide Thin Films Having a Continuous Composition Spread 4.2 Properties of Schottky Barrier Diodes in Dependence on the Cation Composition 4.3 Long Term Stability of Schottky Barrier Diodes 4.4 ImportantRoleofOxygenfortheFormationofHighlyRectifyingContacts 4.5 Processes Governing the Long Term Stability 5 Demonstration and Characterization of Zinc Tin Oxide Based Devices 5.1 Implementation of a New Sputtering Recipe 5.1.1 CharacterizationandElectricalOptimizationoftheZincTinOxide Thin Films .1.2 Optimization of the Gate Contact 5.2 Devices with PtOx/Pt Gate Contact 5.2.1 Variation of the Channel Thickness 5.2.2 Influence of the Oxygen Reservoir on the Performance and Long Term Stability of Devices 5.2.3 Tuning of the Electron Mobility 5.2.4 Frequency Dependent Switching of Transistors 5.3 Devices with i-ZTO/PtOx/Pt Gate Contact 5.3.1 Transistors with Varying Channel Thickness 5.3.2 Simple Inverter 5.3.3 SDFL Inverter 5.3.4 Inverter Chain 5.3.5 Ring Oscillators 5.4 Comparison to Literature 6 Summary and Outlook Abbreviations List of Symbols Bibliography List of Own and Contributed Articles Appendix

Halbleiter-Gassensoren auf der Basis von Zinnoxid spielen in der Überwachung, Steuerung und Regelung von Prozessen sowie bei der Kontrolle von Umweltparametern eine wachsende Rolle. Dies liegt daran, dass sie kostengünstig und hoch sensitiv für diverse Gase sind. Der mögliche Anwendungsbereich wird jedoch durch ihre geringe Selektivität eingeschränkt. Daher stellt die Steigerung der Sensitivitäten von SnO2-Sensoren eine bleibende Forderung an die Forschung und Entwicklung dar. In der Literatur werden verschiedene Methoden zur Beeinflussung von Sensitivität und Selektivität beschrieben. Am weitesten verbreitet sind insbesondere die Modifizierung der Betriebsweise und die Veränderung der Zusammensetzung der Zinnoxidschichten. Diese beiden Wege wurden auch in dieser Arbeit beschritten. Zum einen wurde der thermozyklische Betrieb, der gegenüber dem isothermen bereits zu einer Verbesserung der Selektivität geführt hat, systematisch untersucht. Dies erlaubt die Leistungsfähigkeit des thermozyklischen Verfahrens zu optimieren und noch ungeklärte Phänomene des Detektionsprozesses besser zu beschreiben. Zum anderen waren Modifizierungen der Zinnoxidschicht durch Zusatz fester Ionenleiter Gegenstand detaillierter Untersuchungen. Zusätze von Materialien dieses Typs führen ebenso wie Änderungen im Betriebsmodus zu Selektivitätssteigerungen, insbesondere gegenüber Gasen mit bestimmten funktionellen Gruppen, z.B. primären Alkoholen. Folgende Erkenntnisse wurden in dieser Arbeit erlangt: ·        Thermozyklischer Betrieb Der thermozyklische Betrieb führt zu charakteristischen Leitwert-Zeit-Profilen (LZP). Die Form der LZP (Profilstruktur) ist vom Betriebsregime wie der Aufheiz- und Abkühlungsgeschwindigkeit (Temperaturrate), dem Volumenstrom des Messgases, der Schichtdicke der sensitiven Schicht, dem Elektrodenmaterial und vor allem aber von der Art und Konzentration der Gaskomponente sowie dem Feuchtegehalt des Messgases abhängig. Der Temperaturrate kommt hierbei eine besondere Bedeutung zu. Sie beeinflusst die gasspezifischen nicht-stationären Nichtgleichgewichte an der Sensoroberfläche und somit die LZP sowie die Sensitivitäten. Die Sensitivität kann aus den Sensorsignalen (LZP) als Summenparameter für Temperaturzyklen bestimmt werden. Sie lässt sich als mittlere relative Leitwertänderung bei Gasexposition definieren. Ebenso repräsentativ für die gassensitiven Eigenschaften sind Vorfaktor und Exponent (Sensitivitätskoeffizienten A´ und b´) der aus der Konzentrationsabhängigkeit der Leitwertssumme bestimmbaren Potenzfunktion. Da sich jedoch bereits kleinste Mengen an Reaktanten auf das in Luft bestimmte Leitwert-Zeit-Profil signifikant auswirken und daher diese Profile streuen, ist die Darstellung der Parameter der Potenzfunktion und die Bestimmung der Sensitivität als relative Leitwertänderung in Bezug auf eine definierte geringe Gaskonzentration generell zu bevorzugen. Es wurde exemplarisch gezeigt, dass die unter thermozyklischen Bedingungen ermittelten Sensitivitäten gegenüber CO, Propylen sowie Propanol größer sind als die unter isothermen Betriebsbedingungen bestimmten. Der Zusammenhang von Sensitivität und Schichtdicke ist für verschiedene Gase unterschiedlich ausgeprägt. Profilform und Schichtdicken sind nur selten korrelierbar. Abhängigkeiten der spezifischen Größen der LZP-Maxima (Temperatur, Leitwert) von der Schichtdicke wurden nur für Propylen gefunden. Durch die gezielte Variation von Parametern lassen sich indirekt Schlüsse über die Wechselwirkungen und Mechanismen in porösen gassensitiven Schichten ziehen. Dominierende Effekte von Ad- und Desorption bzw. der Reaktion sowie der Diffusion von Gaskomponenten können in Bezug zueinander gesetzt werden. ·        Gassensitive Eigenschaften von SnO2/NASICON-artigen Kompositen Komposite aus SnO2 und NASICON, bei denen das Na+ im NASICON gegen Li+ oder K+ ausgetauscht wurde, haben eine ähnliche Wirkung auf Sensitivität und Selektivität wie die mit Na+. Die Ergebnisse erweitern die in der Literatur beschriebenen Kenntnisse zur Wirkung ionisch leitender Kompositzusätze. Die Einflüsse der Zusätze sind sowohl durch die LZP als auch durch die daraus berechneten Sensitivitäten bzw. Sensitivitätskoeffizienten A´ und b´ darstellbar. Starke Sensitivitätssteigerungen gegenüber primären Alkoholen wurden für steigende Anteile der Alkaliionen in den Kompositen festgestellt. Dagegen ist die Sensitivität gegenüber sekundären Alkoholen bei Kompositschichten im Vergleich zu reinen SnO2-Schichten kaum verändert. Die für 1- und 2-Propanol im thermozyklischen Betrieb gefundenen Sensitivitäten sind in der Tendenz mit denen unter isothermen Betriebsbedingungen erlangten vergleichbar. ·        Einfluss der Elektroden Es wurden Hinweise darauf gefunden, dass bereits in der Literatur diskutierte Einflüsse der Elektroden auf die gassensitiven Eigenschaften im isothermen Betrieb von SnO2-Schichten auch im thermozyklischen Betrieb auftreten. Weiterhin beeinflusst die Art des Elektrodenmaterials, z.B. Gold und Platin sowie das Ausgangsmaterials zur Elektrodenherstellung spezifisch die Sensorsignale im thermozyklischen Betriebsverfahren. Die Einflüsse der Elektroden sind nicht nur gasspezifisch, sondern bewirken auch Unterschiede je nach verwendeter gassensitiver Schicht. Dabei können LZP, abhängig vom Gas, maßgeblich von der Elektrode oder den Schichtzusätzen beeinflusst sein. ·        Diffuse Reflexion Infrarot Fourier-Transformations Spektroskopie (DRIFTS) Isotherme Messungen der Diffusen Reflexion Infrarot Fourier-Transformations Spektroskopie (DRIFTS) in Abhängigkeit der Zusammensetzung der Gasphase und des gassensitiven Schichtmaterials sind geeignet, um adsorbierte Oberflächenspezies zu detektieren und Vorstellungen bezüglich der Oberflächenprozesse zu erlangen. DRIFTS-Untersuchungen in Luft mit verschiedenen Konzentrationen weisen auf eine höhere Reaktivität des 1-Propanol mit adsorbierten HO-Gruppen verglichen mit der des 2-Propanol hin. Die Ergebnisse deuten zudem an, dass sich die Oberflächenprozesse an SnO2/NASICON(x=3)-Kompositen von denen der reinen SnO2-Schicht unterscheiden, auch wenn sich diese jeweils unabhängig von der Art des Alkohols zeigen.

碩士
國立海洋大學
電機工程學系
88
Abstract In this thesis, the low cost IR furnace chemical vapor deposition system is used to obtain the ZnSe epilayer. The analysis of the ZnSe epilayers quality including the crystallographic properties was demonstrated by XRD, the analysis of the surface morphologies and the thickness was observed by SEM, the composition depth profiles was measured by SIMS and the discussion of optical characteristics was determined by PL measurement. Planar metal-semiconductor-metal (MSM) photodetector have been noted recently, however, the photocurrent and photoresponsivity of such photodetector are limited due to the blocking of incident light by conventional Schottky metals. To improve the photoresponsivity of ZnSe photodetector, the tin-doped indium oxide (ITO) transparent electrode is used as electrode in this paper. In this experiment, the ITO film was deposited at room temperature by RF magnetron sputtering on the top of ZnSe as an electrode. After deposited, the ITO film is annealed in an oxygen ambience at 400oC. After annealing, the transmittance of ITO film from 82.5﹪raise to 90.1﹪and the electrical resistivity of ITO film form 4.5E-1 low to 2.8E-3. C-V measure is used to calculate the carrier’s concentration of ZnSe epilayer. The carrier concentration of the as grown ZnSe epilayer is 2.91×1013cm-3. The barrier height of the Au/ZnSe/PSL Schottky barrier can also be estimated is 1.4eV and the barrier height of the ITO/ZnSe/PSL Schottky barrier is 1.35eV. The Au ZnSe MSMPD and ITO ZnSe MSMPD show a very high performance of spectral response on short-wavelength region. The photo–responsivity of the Au ZnSe MSMPD is 4.61A/W and the photo–responsivity of the ITO ZnSe MSMPD is 6.94A/W at 470nm light wavelength with fix incident light 0.367mW. The impluse response of the MSMPD is 23ns for a bias voltage of 10V and we calculate a bandwidth of about 23.5 MHz. In this thesis, we studied and fabricated the metal-semiconductor– metal photodetector devices based on ZnSe material using low cost IR furnace CVD system. To improve the performance of photodetector, we employ indium tin oxide as transparent electrodes with high Schottky barrier contacts with ZnSe. It will be helpful to obtain cheaper short wavelength blue light photodetectors and high responsivity blue light photodetectors. In recent year, a great deal of efforts has been devoted to the development of OEIC technique. The wide bandgap ZnSe epilayer was grown on Si substrate by using low cost CVD system will show a great potential for the applications of the short wavelength OEIC.

The dioxides of titanium (TiO2), manganese (MnO2), and zirconium (ZrO2) are important materials because of their technological uses. TiO2 is used mainly as white pigment. Because of its semiconducting properties, TiO2, in its nanomaterial form, is also used as an active component of photocells and photocatalysis for self-cleaning glasses and cements . MnO2 is used primarily in electrode materials. ZrO2 is used in refractory ceramics, abrasive materials, and stabilized zirconia as ionic conductive materials stable at high temperature. Many of these properties are, of course, dependent on particle size and shape (§ Chap. 1). Dioxides of other tetravalent elements with interesting properties have been studied elsewhere in this book, especially VO2, which exhibits a metal–isolator transition at 68°C, used, for instance, in optoelectronics (§ 4.1.5), and silica, SiO2 (§ 4.1.4), which is likely the most ubiquitous solid for many applications and uses. Aqueous chemistry is of major interest in synthesizing these oxides in the form of nanoparticles from inorganic salts and under simple, cheap, and envi­ronmental friendly conditions. However, as the tetravalent elements have re­stricted solubility in water (§ 2.2), metal–organic compounds such as titanium and zirconium alkoxides are frequently used in alcoholic solution as precursors for the synthesis of TiO2 and ZrO2 nanoparticles. An overview of the conversion of alkoxides into oxides is indicated about silica formation (§ 4.1.4), and since well-documented works have already been published, these compounds are not considered here. The crystal structures of most MO2 dioxides are of TiO2 rutile type for hexacoordinated cations (e.g., Ti, V, Cr, Mn, Mo, W, Sn, Pb) and CaF2 fluorite type for octacoordinated, larger cations (e.g., Zr, Ce), but polymorphism is common. Some dioxides of elements such as chromium and tin form only one crystal­line phase. So, hydrolysis of SnCl4 or acidification of stannate [Sn(OH)6]2− leads both to the same rutile-type phase, cassiterite, SnO2. Many other dioxides are polymorphic, especially TiO2, which exists in three main crystal phases: anatase, brookite, and rutile; and MnO2, which gives rise to a largely diversified crystal chemistry.

Supercapacitors evolved as a breakthrough to the existing shortages in energy resources because of its enhanced capacitive performance, long-term stability, and high power density. Transition metal oxides (TMOs), a redox active material in energy storage applications, showing high specific capacitance (100–2000 F/g) than the electrical double-layer capacitor (EDLC) material has been reviewed a lot. Among various TMOs, nickel oxide (NiO), tin oxide (SnO2), manganese dioxide (MnO2), tungsten oxide (WO3), vanadium pentoxide (V2O5) are widely used by researchers due to their high theoretical capacitance, low cost, and long cycle life. The limitations of TMO-based electrode material includes low electrical conductivity, ion mobility, and low energy density. It is thus important to develop proper combination of TMO with other transition metals, TMOs, transition metal dichalcogenides (TMDs), conducting polymers (CPs) and carbon-based materials (graphene oxide (GO), activated carbon (AC) and reduced GO (rGO)). This chapter focuses on ongoing development in six TMO-based electrode material (NiO, ZnO, MnO2, SnO2, WO3, V2O5) fabrication for the enhancement of electrochemical performance, their synthesis method and then review about the recent progress in studying the supercapacitor performance of the material. The limitations of each TMOs listed separately, providing new insights for future energy storage applications.

We investigated the effect of electrode design parameters on the performance of hydrogen microsensors. The sensors with varying electrode parameters were fabricated integrating micromachined interdigitated electrodes with indium oxide (In2O3) doped polycrystalline tin dioxide (SnO2) nanoparticles and tested in a controlled gas environment. It was observed that the sensitivity was closely related to the gap between, and a ratio of the gap to the width in interdigitated electrodes (IDE).

Chemical Spray Pyrolysis (CSP) of ZnO and SnO2 is of interest for gas sensor applications. The structural properties of the deposited film can be strongly influenced by deposition conditions. In this work, two solutions consisting of Tin Chloride and Zinc Chloride was sprayed on a heated substrate, where temperature was varied from 400° C to 450° C for ZnO, and from 350° C to 500° C for SnO2. X-ray diffraction and scanning electron microscopy, indicating a non-homogenous-structured film formed at low temperature for both oxides. At 450° C, a porous structure is observed for SnO2. This structure becomes homogenous at higher temperature. It was also found that at temperatures lower than 450° C, substrate temperature has significant impact on the composition of the synthesized films.

Tin oxide was the first transparent conductor to have achieved significant commercialization. SnO2 is an n-type semiconductor with an optical band gap of about 3.6 eV in poly crystalline form. One of the main reasons for the wide use is its rather desirable characteristic of having both, high optical transmittance and high electrical conductivity. Under optimum deposition conditions, tin oxide crystallizes in the tetragonal (rutile) structure. In this study, nano crystalline thin oxide conductive thin films has been manufactured by thermal evaporation techniques onto steel substrates using metallic tin targets and oxidation kinetics have been studied after D.C. plasma oxidation by using XRD (X-Ray Diffraction). The activation energy of SnO and SnO2 from Sn phase transformations has also been studied.

Single-crystalline tin dioxide (SnO2) nanobelts have been assembled with microfabricated suspended heaters as low-power, sensitive gas sensors. With less than 4 mW power consumption of the micro-heater, the nanobelt can be heated up to 500°C. The electrical conductance of the heated nanobelt was found to be highly stable and sensitive to toxic and inflammable gas species including dimethyl methyl phosphonate (DMMP), nitrogen dioxide (NO2), and ethanol. The experiment is a step towards the large scale integration of nanomaterials with microsystems, and such integration via a directed assembly approach can potentially enable the fabrication of low-power, sensitive, and selective integrated nanosensor systems.

Abstract In laser-based manufacturing, processing setup customization is one of the popular approaches used to enhance diversity in material processing using a single laser. In this study, we propose setup design modification of an ultrafast laser system to demonstrate both Tin Oxide (SnO2) nanoparticle synthesis from bulk metal, and post printing of said nanoparticles using Laser Induced Forward Transfer (LIFT) method. Using the Pulse Laser Ablation in Liquid (PLA-L) method, nanoparticles were synthesized from a bulk tin metal cube submerged in distilled water. Such nanoparticles dispersed in water can form colloidal ink that can be used for different printed electronics applications. Pulse energy was varied to investigate the influence on morphological properties of the nanoparticles. It was observed that a decrease in average particle size, and an increase in the number of particles synthesized occurred as the pulse energy was increased. In our study, we adapted the same laser system to enable LIFT operation for printing of the SnO2 nanoparticles. The colloidal ink prepared was then used in LIFT method to study feasibility of printing the synthesized nanoparticles. By varying not only the laser parameters but process parameters such as coating thickness and drying time, printed results can be improved. Experimental results show great potential for both synthesizing and printing of the nanoparticles using a single laser system. This study serves as a proof of concept that a single laser system can turn bulk metal into nanoparticles-based applications without the need for extra processing from other machines/systems, opening the door to highly customizable prints with reduced lead times.

Rutile tin oxide (SnO2) is a wide band gap (3.6 eV at 300K [1]) n-type semiconductor material. It is widely used as sensing elements in gas sensors [2]. The sensing mechanism is generally attributed to the significant change in the electrical resistance of the material associated with the adsorption/desorption of oxygen on the semiconductor surface [3]. The formation of oxygen adsorbates (O2− or O−) results in an electron-depletion surface layer due to the electron transfer from the oxide surface to oxygen [4]. Recent studies [5, 6] have shown that use of tin oxide nanocrystals significantly improves the dynamic response and the sensitivity of sensors since the electron depletion may occur in the whole crystallite. Here we report on the fabrication and characterization of a miniaturized gas sensor based on tin oxide nanocrystals. A simple, convenient and low-cost mini-arc plasma source is used to synthesize high-quality tin oxide nanoparticles in aerosol phase at atmospheric pressure. The nanoparticle sensor is then fabricated by electrostatic assembly of product tin oxide nanoparticles onto e-beam lithographically patterned interdigitated electrodes. The microfabricated nanoparticle sensor exhibits good sensitivity and dynamic response to low-concentration ethanol vapor and hydrogen gas diluted in air.