Welcome to Class Number Eight, Session Number One. This session will focus on the production and the classification of metal oxide nanostructures that would be a futility for sensing applications. One of the most outstanding properties of metal oxide is their stability at elevated temperatures of hundreds of centigrees. These features make them excellent candidates for sensing species that exist in environments with elevated temperatures, as well as for sensing species that include oxygen in combustion control, especially in automobile engines. These sensors actually operate at high temperatures where exchange of oxygen between the lattice and the surrounding gas atmosphere modified the ball conductivity. Given the fact that these sensors rely on surface processes, their sensitivity increases dramatically when the particle size becomes comparable to the depletion length. Therefore, nanoscale effects are extremely important for these kind of sensors. The nanoscale effect can be considered since a wide range of nanostructures can be produced. Semiconductor gas sensors have been investigated extensively for the purpose of practical applications, such as gas leak detectors, and environmental monitoring, part of which we will discuss in the upcoming slides. [SOUND] Three types of metal oxide sensors can be distinguished according to their morphology. The first type is mezoporous, thick film sensors which are typically produced by screen printing. This type can be referred to as the first generation of sensors, which is also known as the tagochi type of sensors which comprise thick film sensors. This gas sensor usually utilizes a porous center block body consisting of polycristalline particles of semiconducting of side. Examples of block or film type devices and a polycristalline particle involved in these devices are illustrated in this slide. The second generation of metal oxide gas sensors is the thin film gas sensors. These sensors are usually fabricated by chemical vapor deposition or physical vapor deposition. As we will describe later on. Many mechanisms are, are involved in the sensing process, and not all of them are completely understood yet in these thin film gas sensors. Nevertheless, in the presented example on the screen, thin films of titanium dioxide, which have a thickness of around 100 to 200 nanometers are deposited at room temperature by RF reacting sputtering from a pure titanium source in an environment of argon and oxygen. The suggested sensing mechanism, in this case, it divided to two main stages. During the first stage, can use option of oxygen on the surface of Titanium Dioxide surface, is followed by kinetics. Where the change in surface absorbent concentration with time follows algorithm reaction. Following surface absorption, the second stage involves diffusion of the Oxygen into the reduced nonstoichiometric titanium dioxide film, giving rise to an oxidized layer growth with relatively low electrical conductivity. Commercial sensors are commonly based on thin layers of metal oxide semi conductors. Although this design has become a success story from a commercial point of view, it presents two major drawbacks. The first drawback is related to the high power consumption. Something that hinder actually the use of the metal oxide films in portable and autonomous systems. And the second drawback is the poor stability of the metal oxide films that is derived from multiple causes, such as grand boundary contribution among the nano particles inside the sens singular itself. The third generation of metal oxide gas sensors are based on nano wire or nanofibers. Metal oxide nanowires have unique properties, such as well-defined, geometry, high surface to volume ratio, and good crystallinity. The underlying sensing mechanism for sensors that are based on metal oxide nanowire arise from the chemo absorption or charge transfer interactions at the surface. That is exposed to the gas phase. Nevertheless, there are some differences in the transduction mechanism, as pictured in these cartoons. And, as will be explained in further details in the upcoming sessions. One of the main advantages of the use of individual nanowires as building blocks for sensors is there to provide a deeper comprehension of the fundamental absorption mechanisms of the gas molecules onto the metal oxides. [SOUND] There are several techniques for the fabrication of metal oxide nanostructure. One common method is the so called sol-gel synthesis. The sol-gel process is a versatile, is a versatile solution based process for making advanced materials. Including ceramics and organic, and organic hybrids. In general, the Sol-Gel process involves the transition of a solution system from a liquid, sol, mostly colloidal, into the solid gel phase. In this context and before we proceed further, it's important to clarify a few definitions that will be quite relevant to the discussed topic. First of all, sol stands for stable suspension of colloidal solid particles, or polymers in a liquid. Second, gel stands for porous, three-dimensional continuous solid network surrounding a continuous liquid phase, and third colloidal gels stands for agglomeration of dense colloidal particles, and fourth polymeric gels stand agglomeration of polymeric particles made from sub colloidal units, and fifth agglomeration. It stands for covalent bonds. Hydrogen bonds, polymeric chain, eh, eh, eh, polymeric chain interactions and more and more. Several studies have shown that thin films with a variety of properties can be deposited on a substrate by spin coating or deep coating. When the salt is cast into a mold, a wet gel is formed. With further drying and heat, the gel is converted into dense material. If the liquid is a wet gel, and is extracted in super critical conditions, a highly porous, and extremely low density material, called air gel is obtained. Without going into the exact chemistry details of this whole gel process. It can be normally evidenced that the starting material used in the preparation of the sol are usually inorganic metal salts. Then, the precursor is subjected to a series of hydro lasers and polymerization reactions to form the colloidal suspension or the so-called sol. A wide variety of nanostructures can be produced using sol-gel synthesis. A few examples are presented on the screen. In the upper left images, you can see zinc-oxide nano sheets grown at the air liquid interface to form a large zinc-oxide film by zinc ion supply from the aqua solution. The zinc oxide film has sufficiently high strength to freely stand alone and to show high z-axis orientation. The film can be pasted into a desired substrate, such as polymer film or a glass substrate. In the upper right images you can see zinc oxide particles having a hexagonal cylinder shape, long ellipse shape, or hexagonal symmetry radial whiskers that are prepared in aqueous solution. The morphology is controlled, in this case, by changing the supersaturation degree. In the bottom right images, you can see the [INAUDIBLE] titanium dioxide particles with high surface area, that are prepared at 50 centigrade. A lot of the new approach for the production of metal oxide nano structures, rely on electro spinning. Electrospinning has been recognized as an efficient and highly versatile method, which can be further developed for mass production of uniform, ultra thin, and continuous fibers, with nanometer to micrometer-size diameter. The principle of this method is as follows. Solution is ejected from metallic needle. When a sufficiently high voltage is applied to a liquid droplet, the body of the liquid becomes charged and electrostatic repulsion counteracts the surface tension and the droplet is stretched. At a critical point, a steam and a stream of liquid erupts from the surface. This point of eruption is known as the Taylor cone. On increasing the applied voltage further, a charged liquid jet is ejected from the Taylor Cone, and attracted to the Earth collector which is positioned at a fixed distance from the needle. During this process, the solvent evaporates from the solution, leaving dry and very thin fibers with micron to sub-micron diameters on the collector. The viscosity of the solution is controlled by polymer concentration and the effect of the behavior on the jet is well-controlled by tailoring the rest of the device para, parameters. The nanofibers generated by the electrospinning have diameters ranging from tens of nanometers to several micrometers. The power, which expressed amongst the rest by the voltage, and the current injection rate, muzzle capacity, collector design, and other environmental factors such as temperature and the humidity are possible variables that must be controlled during the synthesis process. Above all, the type of solution, and the fluid properties, such as viscosity surface tension and therefore pressure should be carefully adjusted to form a continuous and homogenous-sized fibers. In the previous slides, we have described the main weight, or chemical methods for the production of metal oxide nanostructures. Now we will move to the dry or physical techniques that are available for the production of the discussed nanostructures. CVD, or chemical vapor deposition, and MO CVD, which is metal or metallurgic chemical vapor deposition are methods to produce thin layers or defined films of metals or metal oxides on a substrate, namely the target. These techniques are often employed to design materials for microelectronics, such as high K dielectrics for most devices. Basically, acidity apparatus consists of several basic components. The first are sources and feed lines for gases. It includes also mass and flow controllers for metering the gases into the system, it includes also a reaction chamber, or reactor. And also a system for heating up the wafer on which the film is to be deposited and temperature sensors. The fundamental principle of the CVD process is that the chemical reactants, called precursors, are in the gas or vapor state when they arrive at the base material or substrate. A chemical reaction which is usually activated by heat occurs on the substrate surface then. And the substrate temperature, in this case, is quite critical, and can influence what reactions, and where reactions will take place. In this slide, crystal quality of zinc oxide nanocubes are presented as represented in the examples for metal oxide nano structures. Formed by CVD technique. In the presented case, zinc oxide nanocubes growth was performed in a furnace, consisting of quartz crystal, vacuum chamber, and a mixture of zinc oxide graphite, was used as the precursor materials The temperature of the furnace was raised to 1,100 Centigrades. As seen in the presented images some of these nanotubes have single tubular channel as shown in the upper images. But others of these nanotubes have multiple channels. The differences between these channels is tailor-made by the parameters which are operated during the CVD process. Many parameters can indeed control during the CVD process. For example, the longer the time of the CVD process, the longer the nanowires that will form in the presented example on the screen. Another example is the change of the growth modifier, and the concentration during the process of the growth. The seed layer strongly influenced zinc oxide volume fraction and alignment of the nanotubes. For example, an oriented zinc oxide seed layer [INAUDIBLE] highly aligned zinc oxide nanotubes [INAUDIBLE] as could be presented on the screen of the current slide. Similar metal oxide nanostructures to those presented in the previous slides have been achieved by physical depo, deposition, which is appropriated by PVD. This technique differs from CVD in that the precursors are solid, with the material to be deposited being vaporized from a solid target and deposited onto the sub strait. Variance of PVD system include, cathodic arc deposition in which a high power electric arc discharged at the target or the source material blast away some into highly ionized vapor to be deposited into the work piece. The other type is called electrical beam physical vapor deposition. In which the material to be deposited is heated to a high vapor pressure by electron. In high vacuum, and is transported by diffusion to be deposited by condensation into cooler piece. Another form of this technique is the operative position in which the material to be deposited is heated to a high level of pressure. By electrically resistive heating in low vacuum other promising technique of PVD called Pulsed Laser Deposition, in which high power lazer ablaze material from the target into the vapor and from there to the substrate. And, of course, there is the so called desputtered position, in which a glow plasma discharge bombards the material and sputtering some away as vapor from substrate to in the position. The advantages of the PVD process are, PVD coating are sometimes harder and more corrosion resistant than coatings applied by the electroplating process. Most coatings have high temperature and good impact strength. It's not oppression, resistance, and are also durable that protective top coats are almost never necessary. Other advantage is the ability to utilize virtually any type of inorganic, and some organic coating materials or an equally diverse group of substrates and surfaces using wide variety of finishes. Further advantage include that this PVD approach is more environmentally friendly than the traditional coating processes such as electroplating and painting. And also more than one technique can be used to deposit the given film in the case of the PVD approach. A separate technique, besides the advantages, there are also disadvantages. And, this is advantages of the PVD approach are that the specific technologies can impose constraints, for example, line of site transfer is typical for most PVD coding techniques. However, there are methods that allow full coverage of complex geometries. Another disadvantage is that some PVD technologies typically operate at very high temperatures and vacuums, requiring sufficient attention by the operating personnel. And finally, the other advantage is that it requires a cooling water system that this big large heat loves. We have now reached the end of class number eight, session number one. Thank you.
