Modelling electronic transport in monocrystalline metal oxide gas sensors: from the surface kinetics to the experimental response

Gas sensing systems and devices based on metal oxides are widely spreading due to their high performance in terms of sensor response and relatively low costs. Despite several experimental studies and molecular simulations are available in the literature, a tool that can quickly predict the macroscopic sensor response, and potentially be used for predictive purposes, is still missing. In this work, we present a modelling approach based on finite-element simulations, using material electrical properties available in the literature. In a first approach, we derive the surface electron trap concentration from fitting the global sensor response. Then, we improve the model by considering the actual time-dependent experimental response. We consider sensors based on single SnO2 nanowires and show how our model predicts with a good agreement the experimental response vs. NO2, as a function of the working temperature and gas concentration, and additionally provides many other physical quantities of interest, such as the conduction band edge bending, the space charge and the width of the depletion layer. We further discuss ideas for improving the model and thus increasing its predictive potential with an engineering perspective

Controlling molecules\u27 momentum with SuMBD: the early stages of Pentacene growth on SiOx/Si

Organic semiconductors are suitable candidates for applications in fields ranging from photonics[1] to devices and sensors realization[2]. Good electrical properties, needed for these applications, can be achieved through molecular assembly in a crystalline structure. On the other hand, the feeble nature of the van der Waals forces between the molecules make this goal not easy to reach in organic thin films and the growth process often gives rise to different polymorphs and molecular orientations that strongly limit the final performances. Such limits need to be overcome by a higher control during the growth process. For this reason, the development of methods that permit to role the molecular assembly through the formation of a crystalline structure is of great interest. Between the vacuum deposition techniques, that up to now give the best results in term of order and purity of the materials, supersonic molecular beam deposition (SuMBD) has shown to improve the control on the growth, giving rise to better morphologies and enhanced electrical properties of the films[3,4]. Using SuMBD we can easily tune the kinetic energy, momentum and internal energy of the impinging molecules[5] influencing the molecules assembly, the island formation and coalescence[6]. The precursor state of the molecule is also important for the activation of new pathways for the adsorption of the molecules on the surface. The possibility to control the different energetic parameters of the impinging molecules and understand how they can influence the molecular assembling is of great importance for the realization of high performances devices. We report a systematic atomic force microscopy study of Pentacene sub-monolayer morphologies on SiOx/Si (60? contact angle), resulting from depositions at room temperature, in different growth conditions. During these early stages of growth, we found that the kinetic energy and the momentum of the impinging molecules play a key role in determining the monolayer morphology. In particular, we investigate the effects induced by changing the parallel and perpendicular momentum of the molecules arriving on the surface. Variations in the energy relaxation mechanisms involving the two component of the momentum activate different adsorption processes leading to modified island fractal dimension, island density and sticking coefficient. The parallel momentum favours a longer mean free path of the molecules adsorbed on the surface giving the possibility for a better assembly and surface energy minimization. Instead, the perpendicular momentum can be fundamental for the activation of deeper adsorption sites on the surface. Our measures show a progressive lowering of the island density when increasing the component of the molecules\u27 momentum parallel to the surface (k//) with the formation of more compact (less fractal) islands, better for the realization of ordered films (Figure1). Large single crystal domains are formed, which is a key aspect for increasing the devices performances

Exploring volatile organic compound emission from thermally modified wood by PTR-ToF-MS

Thermal modification of wood is a well-known industrial process performed to increase the durability and dimensional stability or to change the colour of natural wood. The treatment influences many other properties of wood including the emission of volatile organic compounds (VOCs). VOC release ultimately affects the quality of indoor air and the capability of having low VOC emission is often included as a key parameter for the attribution of quality labels. In the present work, wood from six tree species was subjected to different types of treatment and VOC profiling was carried out on both treated and untreated samples by means of PTR-ToF-MS. Different types of thermal treatment were tested, involving either overpressure or vacuum and the effect of different temperature profiles was evaluated. Hardwood and softwood showed different release profiles under all tested conditions: the headspace of softwood was richer in several VOCs, such as terpenes, phenols and C6-C9 aldehydes and carboxylic acids. Upon thermal treatment, terpene emissions decreased, whereas several other VOCs, such as formic acid, formaldehyde, furfural and acetic acid, were released in higher amounts. With its high sensitivity and throughput, PTR-ToF-MS appears to be a very powerful analytical tool, useful in supporting the selection of wood materials for different end uses

Single nanowire gas sensor able to distinguish fish and meat and evaluate their degree of freshness

A non-invasive, small, and fast device is needed for food freshness monitoring, as current techniques do not meet these criteria. In this study, a resistive sensor composed of a single semiconductor nanowire was used at different temperatures, combining the responses and processing them with multivariate statistical analysis techniques. The sensor, very sensitive to ammonia and total volatile basic nitrogen, proved to be able to distinguish samples of fish (marble trout, Salmo trutta marmoratus) and meat (pork, Sus scrofa domesticus), both stored at room temperature and 4 °C in the refrigerator. Once separated, the fish and meat samples were classified by the degree of freshness/degradation with two different classifiers. The sensor classified the samples (trout and pork) correctly in 95.2% of cases. The degree of freshness was correctly assessed in 90.5% of cases. Considering only the errors with repercussions (when a fresh sample was evaluated as degraded, or a degraded sample was evaluated as edible) the accuracy increased to 95.2%. Considering the size (less than a square millimeter) and the speed (less than a minute), this type of sensor could be used to monitor food production and distribution chain

Detection of mackerel fish spoilage with a gas sensor based on one single SnO2 nanowire

A chemosensor consisting of one single tin oxide nanowire is used to determine the freshness status of mackerel fish (Scomber scombrus) in a quick and non-invasive way. The tiny chemoresistive sensor is first tested with pure ammonia and then used to measure the total volatile basic nitrogen from different samples of fish at different degrees of freshness. The sensor has proved capable of determining the freshness of a sample in few seconds compared to traditional methods such as microbial count and chromatography, which take hours. The sensor response is well correlated with the total viable count (TVC), proving that the total volatile basic nitrogen is a good way to quickly test the bacterial population in the sample. After calibrating the sensor (following the degradation of the fish during almost two days), it has been tested with random double blind samples, proving that it can well discriminate the degree of freshness of the fish preserved at different temperature

Quantitative assesment of trout fish spoilage with a single nanowire gas sensor in a thermal gradient

The response of a single tin oxide nanowire was collected at different temperatures to create a virtual array of sensors working as a nano-electronic nose. The single nanowire, acting as a chemiresistor, was first tested with pure ammonia and then used to determine the freshness status of trout fish (Oncorhynchus mykiss) in a rapid and non-invasive way. The gas sensor reacts to total volatile basic nitrogen, detecting the freshness status of the fish samples in less than 30 s. The sensor response at different temperatures correlates well with the total viable count (TVC), demonstrating that it is a good (albeit indirect) way of measuring the bacterial population in the sample. The nano-electronic nose is not only able to classify the samples according to their degree of freshness but also to quantitatively estimate the concentration of microorganisms present. The system was tested with samples stored at different temperatures and classified them perfectly (100%), estimating their log(TVC) with an error lower than 5%

Physical Vapour Deposition Techniques for Producing Advanced Organic Chemical Sensors

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The UA9 experimental layout

The UA9 experimental equipment was installed in the CERN-SPS in March '09 with the aim of investigating crystal assisted collimation in coasting mode. Its basic layout comprises silicon bent crystals acting as primary collimators mounted inside two vacuum vessels. A movable 60 cm long block of tungsten located downstream at about 90 degrees phase advance intercepts the deflected beam. Scintillators, Gas Electron Multiplier chambers and other beam loss monitors measure nuclear loss rates induced by the interaction of the beam halo in the crystal. Roman pots are installed in the path of the deflected particles and are equipped with a Medipix detector to reconstruct the transverse distribution of the impinging beam. Finally UA9 takes advantage of an LHC-collimator prototype installed close to the Roman pot to help in setting the beam conditions and to analyze the efficiency to deflect the beam. This paper describes in details the hardware installed to study the crystal collimation during 2010.Comment: 15pages, 11 figure, submitted to JINS

Sensing performance of thermal electronic noses: a comparison between ZnO and SnO2 nanowires

In recent times, an increasing number of applications in different fields need gas sensors that are miniaturized but also capable of distinguishing different gases and volatiles. Thermal electronic noses are new devices that meet this need, but their performance is still under study. In this work, we compare the performance of two thermal electronic noses based on SnO2 and ZnO nanowires. Using five different target gases (acetone, ammonia, ethanol, hydrogen and nitrogen dioxide), we investigated the ability of the systems to distinguish individual gases and estimate their concentration. SnO2 nanowires proved to be more suitable for this purpose with a detection limit of 32 parts per billion, an always correct classification (100%) and a mean absolute error of 7 parts per millio

Optimization of nanostructured materials towards gas sensing

As its title announces, the general aim of this doctoral thesis is to investigate the growth and use of nanostructured materials in order to make them suitable for sensoristics. Sensors applications have become very important in the last years because of a new sensibility towards pollution of the urban world and its effects on human health. Only very recently people and countries discovered the importance of environment preservation and monitoring. After a period of fast and uncontrolled industrial progress, we are now aware of this danger. Thus we need to monitor the environment and the changes which are happening directly or indirectly because of human presence. During the last decades, solid-state gas sensors have played an important role in environmental monitoring and chemical process control. The strong investigation which followed, made clear that the field of science and sensor technology cannot search for new sensor materials which are ideal, because different applications (e.g. different transformations of energy and different goals for sensors) require different materials. However materials are important drivers in sensor technology. The combination of the right materials (new or existing) to the right application can result in smarter, cheaper, or more reliable sensors. In order to give a contribution to this important evolving situation, during these three years the PhD candidate investigated two of the most important areas related to nanostructured materials used in sensing applications. On one side, the recent interesting field of metal oxide nanowires has been studied, both in terms of fundamentals (growth mechanism and structural properties) and sensor properties towards different gases. On the other side, the less exploited (in terms of sensor devices) field of organic thin films has been investigated, in terms of growth and fundamental properties (charge carriers mobility) which are required to use them as sensors. While nanostructured metal oxides are already in use in commercial sensors (usually in the form of porous thick or thin films), organic materials are still in a prototypal phase, and need further investigation in order to be effectively used. This different evolution step is reflected also in the present thesis: in which zinc and tin oxide nanowires are characterized as gas sensing devices, while molecular materials are only optimized towards a better order and a higher carrier mobility, which is one of the bottlenecks towards a higher response. For this reason, the chapters concerning metal oxide nanowires will give a wide picture, from their growth mechanism to their structure until their use (in different architectures) in sensing applications. Oxide nanowires have been used as passive (resistive) sensors (they have been used also as active sensors, but such data are still under analysis) both in order to develop new real sensors, and to better understand the sensing mechanism behind the high response of such nanostructured materials. Their nanoscale dimensions, comparable to the depletion layer, makes them almost ideal intrinsic on-off devices, and this can be exploited to fabricate a new generation of sensors characterized by a huge response. The problems of metal oxide sensors are however their poor selectivity and high working temperature. In this direction goes the investigation of the molecular materials. Concerning the organic complement in this thesis, the aim of the experimental work was the optimization of the overall field effect mobility of carriers (holes) along the whole device, which means several microns (tens of microns, due to the impossibility to use standard lithography techniques on organic delicate materials). This meant the minimization of grain boundaries, that are one of the steps hindering the charge carrier mobility, and even the recently found domain boundaries. Exploiting the high kinetic energy achievable by SuMBD, we found that it is partially transformed in surface mobility, increasing the order of the fundamental building blocks inside each monolayer, and decreasing the grain and domain boundary density (because of wider and less fractal grains). At the end of the thesis we will show a first combination of the two families of materials, just as a sample of what the exploitation of the best features of each family (high response for metal oxides and good selectivity for organic materials) can provide