Inorganic nanomaterials

CHAPTER 2

Inorganic nanomaterials Francisco Hernandez-Ramirez, Albert Romano-Rodriguez, Joan Daniel Prades MIND—Departament d’Enginyeria Electro`nica i Biome`dica, Universitat de Barcelona, Barcelona, Spain

2.1 Introduction Among the different options used for the realization of inexpensive gas sensors, bare inorganic nanomaterials are considered one of the forerunners thanks to their simplicity and unique properties. In fact, inorganic nanomaterials have turned into perfect test beds of new sensing concepts, paving the way toward advanced operating methods that have been fully widespread in affordable and simple devices afterward [1, 2]. This chapter reviews the use of inorganic nanomaterials as building blocks of gas micro- and nanosensors, covering from the fabrication stage to the operation mode. The discussion of the underlying working principles for these materials is also briefly outlined with a special focus on the applications of new devices setups. The first gas sensors based on inorganic materials have already come a long way with the extensive use in Taguchi sensors of metal oxide semiconductor thin films [3–5]. Moving on from there, nanoscale inorganic materials and specifically one-dimensional (1D) structures, such as nanowires, nanobelts, and nanotubes, have gained tremendous attention within the last two decades due to their potential applications in optoelectronic and electronic devices [1, 2]. Inorganic materials confined in several dimensions at the nanometer scale exhibit properties different to their bulk counterparts [6]. Therefore, the interest of using them for practical applications increases with the deeper understanding of the properties and tailoring of their key parameters. With this in mind, the applicability of 1D inorganic materials for gas and biochemical detection, photonics, and energy harvesting production and storage has been extensively demonstrated and validated in the last few years [1, 7–10]. Here, the use of 1D metal oxide nanomaterials for the realization of inexpensive conductometric gas sensors is specifically presented and described. Despite the fact that other inorganic materials have been used for the same purpose, the formers are the most representative at the time of this writing, and they can be considered a prime example of the state-of-the-art technologies.

Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00002-5

Copyright © 2020 Elsevier Inc. All rights reserved.

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2.2 Operating sensing principles The fundamental principle of the gas-sensing mechanism in inorganic materials relies on changes of the active layer at the surface induced by the analytes, which at the same time result in variations of the measurable physical properties of the working device [11]. In this interaction process, the need for temperature control and the challenges related to achieving it in an efficient manner has usually become paradigmatic [12]. In particular, the operation of conductometric sensors is based on the interaction between adsorbed (physi- or chemisorbed) species and the surface and the effective change of the local charge density in the material, which can be easily monitored by the modulation of the electrical conductivity. Differences in the tendency of physi- and chemisorption processes are dependent on the environmental conditions [11, 13–15], and in general, for an effective and reversible gas response, the activation energy necessary for the complete desorption of analyte species must clearly overcome a specific temperature threshold. This is normally achieved by providing thermal energy at temperatures above 100°C. In the sensing process, ionosorbed oxygen also plays a major role, which interacts with detectable species or competes for the same adsorption sites at the semiconductor surface following complex dynamic mechanisms [16, 17]. Alternatively, the necessary activation energy of these processes can be provided by impinging high-energy photons on the sensing material, which triggers the desorption of gaseous species in a similar way that in the case of the thermal scenario [18]. The working principles of these two operating modes (thermally activated and lightactivated) have been extensively reviewed in the literature despite the fact that, quoting Noboru Yamazoe, the full details of inorganic gas sensors still remain far from having fully understood satisfactorily [13].

2.3 General overview of gas sensors based on inorganic nanomaterials From a practical point of view, conductometric gas sensors made up with nanomaterials are always thermally activated or light-activated devices. The former ones have been a subject of research for more than two decades [1, 9]: gas sensors made up with either bundles of nanomaterials or individual 1D structures have been extensively studied despite the fact that most of them were not fabricated by means of cost-effective and simple routes. The first and simplest attempts to develop thermally activated gas sensors with 1D nanomaterials were based on a simple idea: bundles of nanowires are bridging the gap between two electrodes, while they were warmed up with the help of an external heater. From a theoretical point of view, the sensing mechanisms of these devices can be described with the same model used for Taguchi sensors, and as a result, the contact areas between nanowires are the main responsible for the sensor response [11]. However, the

Inorganic nanomaterials

random distribution of the nanowires may lead to poor reproducibility of the response, not showing these nanodevices at first glance a clear advantage compared with standard thin-film gas sensors. Besides, the electrical quality of the electrical contacts is not always optimal, which is considered a major drawback to extend the use in industrial applications involving massive deployments [19] (Fig. 2.1).

Fig. 2.1 Schematic diagrams of different types of conductometric gas sensors based on inorganic semiconductors: (A) commercial thin-film metal oxide device formed by a layer of nanoparticles. Here, electrons must go through a network of nanocrystals with different size and shape. From an energy point of view, electrons are to overcome different barriers, which modulate the detection output. (B) Multinanowire sensor. The same conduction model is valid here. (C) Individual-nanowire sensor fabricated with FIB lithography. (Reproduced from F. Hernandez-Ramirez, J.D. Prades, R. Jimenez-Diaz, T. Fischer, A. Romano-Rodriguez, S. Mathur, J.R. Morante, On the role of individual metal oxide nanowires in the scaling down of chemical sensors, Phys. Chem. Chem. Phys. 11(33) (2009) 7105–7110 with permission from the PCCP Owner Societies.)

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To circumvent these first barriers, devices based on individual 1D nanomaterials like nanowires have progressively attracted the major attention of the scientific community. They define a controlled test ground for semiconductors since the geometry and morphology are well-known and in principle controlled. Obviously, the fabrication complexity of one-nanowire devices is significantly higher, and it requires advanced fabrication tools like focused ion beam (FIB) or electron beam lithography (EBL), which exhibits a limited yield and as a result hampers the transfer of these devices to the mass production level [20, 21] (Fig. 2.1). With the help of these devices, it has been possible to establish a direct relationship between the gas response and the nanowire radii as well as modeling and evaluation many other fundamental phenomena occurring during the sensing process (i.e., role of the oxygen diffusion in the response) [17, 22, 23]. Despite the fact that most of these devices were operated with an external heater, their reduced dimensions have allowed the integration in MEMS, reducing the energy consumption in a significant way. Actually, some of these prototypes have successfully merged the advantages of using individual nanowires and industry-ready substrates, but unfortunately, the fabrication process is relatively complex to extend the use on a massive scale [24, 25] (Fig. 2.2). For this reason, self-assembly techniques for the manipulation and positioning of nanowires have been evaluated as a suitable fabrication alternative, showing promising results at the present time [1]. In particular, dielectrophoresis (DEP) or the motion of polarizable particles in a fluid subject to an electric field has been successfully tested with metal oxide nanowires clearly

Fig. 2.2 (A) SEM image of a suspended microhotplate with an integrated heater and an interdigitated electrode. (B) Tin oxide nanowire electrically contacted with FIB. Nanocontacts are made of platinum. (© IOP Publishing. Reproduced with permission from F. Hernandez-Ramirez, J.D. Prades, A. Tarancon, S. Barth, O. Casals, R. Jimenez-Diaz, E. Pellicer, J. Rodriguez, M.A. Juli, A. Romano-Rodriguez, J.R. Morante, S. Mathur, A. Helwig, J. Spannhake, G. Mueller, Portable microsensors based on individual SnO2 nanowires, Nanotechnology 18(49) (2007) 495501. All rights reserved.)

Inorganic nanomaterials

leading to a higher yield in the production of nanosensors. DEP has been applied to manipulate different types of 1D nanomaterials, such as SnO2 nanobelts and nanowires; GaN, ZnO, InP, and Ga2O3 nanowires; and carbon nanotubes, enabling the fabrication of sensing and electronic devices like current rectifiers; field-effect transistors; and photonic, thermal, and gas sensors [26–33]. It is noteworthy to remark that DEP is compatible with standard CMOS manufacturing process and wafer-scale assembly, validating that this technique is suitable for the mass production of devices [1]. On the other hand, light-activated gas sensors have also attracted attention since their study has made a nice contribution to further enhance our knowledge of the fundamental properties of the semiconductors’ surfaces [18, 34]. Besides, they can extend the use of gas sensors in applications with explosive and inflammable atmospheres. Unfortunately, their configuration is far from being simple despite some promising attempts to design affordable systems, and for this reason, the use remains mainly restrained for academic and research purposes: standard inorganic nanomaterials like metal oxide semiconductors are wide-bandgap materials (i.e., SnO2, Eg > 3.9 eV), which makes necessary UV light to perform sensing experiments at room temperature. This is a major drawback due to the energy consumption constraints, the need for coupling a light source to the sensor, and very specific control requirements during the operation.

2.4 Toward cost-effective gas sensors based on inorganic materials The massive use of gas sensors based on inorganic nanomaterials will become a tangible reality only when simple, reliable, and easy-to-operate devices are finally developed. On this basis, two different strategies are usually followed to reduce the final cost and the complexity of the new devices: (i) the adoption of automated fabrication routes and (ii) the simplification of the operation methods. The first ones focus on innovative strategies specifically designed to automate the growth and assembling of the nanomaterials in the new devices, skipping the use of complex fabrication techniques that may be affected by a limited yield. The goal is, therefore, moving toward an industrial production level keeping the cost low during the fabrication stage. In detail, in situ controlled growth of nanomaterials on substrates and electrodes [35–37] and self-assembly techniques, such as DEP, have shown promising results in the last years, as commented before. On the other hand, the smart use of newly discovered phenomena at the nanoscale can be employed to work with sensors relying on nanomaterials in a simplified way that results in lowering the operational costs during the device life span. In this sense, selfheating effect has become paradigmatic after a decade of active research in the field [38]. This section details the two previously mentioned strategies followed to obtain costeffective sensors, paying special attention to self-heated miniaturized devices since they have proven to be competitive enough in terms of gas detection performance. It goes

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without saying that in the real world, both cost-cutting strategies should be successfully combined to attain mature industry-ready technologies.

2.4.1 Automated fabrication routes The possibility to obtain cost-effective gas sensors always begins with an adequate fabrication route. In this sense, any methodology with options for becoming an industrialization solution needs at least to be automated, give reproducible results, and provide a high production yield. In this sense, the major challenge associated with the fabrication of devices based on nanomaterials rests on how to position them in the proper locations. To accomplish this, two different strategies are usually followed: (i) in situ controlled growth at some specific parts of the device and (ii) the use of postgrowth alignment techniques, such as DEP. As far as the first one is concerned, the literature is full of examples involving the application of templates and etching or epitaxial metal-promoted techniques [1], among other methodologies specifically designed to attain the nanomaterial production at well-defined positions of the devices. This has allowed some interesting results such as the systematic Ti-catalyzed growth of Si nanowires to form bridges across several micron-wide trenches [39]. In the pursuit of developing inexpensive gas sensors that integrate semiconductor nanomaterials, the site-selective direct growth of inorganic nanowires on top of CMOS-compatible MEMS [i.e., suspended microheaters with integrated additional interdigitated electrodes (IDE) on the top] has however been shown as one of the most promising approaches to attain this ambitious goal. Going a step further of the works in which the indiscriminate growth of semiconductor elements over the whole substrates was reported [40], the localized production of nanomaterials on the top of MEMS in a similar way as in the thin-film case [41] has been successfully reported in the literature with quite promising results for silicon nanowires [36], germanium nanowires [35], carbon nanotubes [42], tin oxide nanotubes [43], and cupric oxide nanowires [37], as an example. The main advantage of the here-presented technique relies on achieving localized growth on top of the membranes (heated area) by using the power dissipation of the heater only to that end. Actually, it is the effect of the heater that directly activates the semiconductor growth mechanism, typically based on CVD or thermal oxidation mechanisms [35, 37], removing the need for huge reactors with high-energy consumption requirements [35]. This involves a dramatic reduction of the energy demands during the synthesis process and by extension leads to a drop of the total production costs. It is noteworthy that the sensing performance of gas sensors obtained with the in situ growth controlled strategy has shown promising outputs, such as CO monitoring for concentrations as low as 1 ppm [37], even though they are not optimized yet (Fig. 2.3).

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Time (x1000 s) Fig. 2.3 (A) Schematic drawing of a microhotplate with an integrated buried heater and an external circuit containing interdigitated electrodes (IDE) used to the in situ growth of semiconductor nanowires. (B) SEM images of (left) suspended microhotplates used to grow nanowires. (Right) The micromembrane shows nanowires grown in the device (brighter area in the center). (C) Response of the device shown in (B) to pulses of different concentrations of CO in synthetic air at 260°C. The heating power was 48 mW, the same value required to heat micromembrane to 700°C under vacuum. (Reproduced from B. Sven, J.-D. Roman, S. Jordi, D.P. Joan, G. Isabel, S. Joaquin, C. Carles, R.-R. Albert, Localized growth and in situ integration of nanowires for device applications, Chem. Commun. 48 (2012) 4734–4736 with permission from The Royal Society of Chemistry.)

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This suggests that there is still room for improvement in terms of the final devices’ functionalities. Nevertheless and in spite of the intrinsic advantages of the in situ growth approach, the technology has still a limited range and impact beyond the academic ecosystem, and it has been basically restrained to study the fundamentals of gas sensors based on very specific materials. Regarding the postgrowth alignment and position of nanomaterials, there are several methods that have been described so far to successfully control the fabrication of nanodevices. Apart from some exotic approaches, such as the “blown-bubble film technique” [44], which involves expanding a bubble from a homogeneous suspension of stable epoxy solutions containing surface-modified nanomaterials, resulting in their alignment for the later transfer to both rigid and flexible substrates, the most mature and commonly used technique is DEP, as stated in the previous section. DEP has been successfully used to assemble nanowires from a solution without the need for surface modification. Actually, the electrokinetic motion of dielectrically polarized materials in nonuniform electric fields can be used for the self-alignment of nonsymmetric nanostructures and in particular nanowires and nanotubes. In this respect, an external field is induced between metal microelectrodes by applying an AC voltage, resulting in a well-defined space-charge region. This nonmechanical manipulation technique allows the assembly of different quantities of 1D structures, depending on the concentration of the nanostructures in the solvent, the magnitude and frequency of the applied field, and the gap distance between the electrodes. The alignment process can be optimized for single-nanostructure bridging devices leading to reliable singlenanostructure diodes [45], multiwire FETs [46], and gas sensors [31]. Appropriate bus bar spacing and very high frequencies allow almost exclusive alignment of single nanostructures between electrodes [47]. On the other hand, medium frequencies attract the most wires; however, they partially adhere to the buses or the aligned wires are accompanied by additional wires. This high dependency of the DEP result on the experimental conditions is usually studied with the help of theoretical tools such as finite element simulation that facilitate to understand the role of each key parameter and improve the geometry design of the sensor substrate (i.e., microelectrodes) to minimize undesired effects like the attraction of unaligned elements. Nevertheless and after all, DEP is a technique marked by the need for trial-and-error procedures: the optimization of this nanofabrication strategy involves controlling many different parameters with a direct impact on the position and alignment yields, such as (i) density of nanomaterials in the solution, (ii) electrical properties of the materials, (iii) viscosity of the solvent, (iv) properties of the applied electric field (i.e., amplitude and frequency), (v) flow of the solution, and (vi) geometry of the electrodes. It goes without saying that reaching the full control of all of them is not straightforward taking into account that there are cross dependencies with some of them. For this reason, DEP

Inorganic nanomaterials

usually requires a systematic calibration work to optimize the process prior to the sensor production stage, which can be easily nullified with slight changes of any of the experimental parameters. From a practical point of view, this is a major drawback hampering the application of this technique beyond lab-class devices. Besides, the electrical quality of the contacts between aligned nanomaterials and prepatterned microelectrodes on the substrates is not always optimal after the entire process, and it usually requires the postprocessing of the device, involving an additional optical lithography step in some cases. Despite these shortcomings, DEP has allowed the fabrication of gas sensors based on nanowires with very good performance characteristics, being some of them integrated into functional CMOS operating circuitry [28].

2.4.2 Simplified operation methods: Self-heated nanosensors The final cost of new technologies is linked not only to the fabrication process but also to the operating conditions. Gas sensors require controlling the latter accurately to thus obtain repeatable and long-term stable readouts. In the case of systems based on nanomaterials, at first glance, this means complex working methodologies and the use of increasingly miniaturized heaters. However, the coupling of inorganic semiconductors and MEMS microhotplates for the realization of affordable gas sensors had successfully started a long time ago with metal oxide thin films [48]. Actually, microhotplates can follow standard IC fabrication procedures, allowing series production of portable and reliable devices at low cost. Moreover, due to their low thermal mass, fast dynamics and low power consumption are simultaneously achieved in an easy way [49]. Capitalizing these opportunities, intensive research efforts have been devoted to reach microhotplates fully compatible with SOI CMOS technologies during more than two decades [50], since the combination of metal oxide sensing layers operated between 300°C and 500°C with standard silicon processes could be defined at the very least as troubling. From this point, the race to integrate nanomaterials and microhotplates became accelerated leading to operational hydrogen [51], carbon monoxide [24], and other pollutant sensors [52]. Quoting Meier et al. [43], these devices have resulted on paper in excellent building blocks of “electronic noses” with potentially different sensors integrated into a single microchip, all of them morphologically different and with complementary sensing characteristics. As a result of this activity, the interest to commercially exploit the new generation of microhotplates also grew with the emergence of new companies like AMS Sensors [53]. All in all, this optimization at the expense of putting together nanomaterials and MEMS has allowed reducing the power consumption up to tens of milliwatts in advanced

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devices [24]. Unfortunately, despite being a remarkable jump ahead, these values are still excessive for power-limited systems, if continued operation is required. Therefore, there is not a clear advantage in moving from standard inorganic thin films to nanomaterials, especially considering the problems related to the fabrication steps when we talk about nanodevices. Alternatively, nanotechnology enables the use alternative methods for conductometric-like sensing operation that feature zero power consumption [54–56]. These systems are mostly based on integrating a source of energy collection/harvesting together with the sensing element in a way that no further energy is needed to activate the gas-sensor interactions, providing even an energy surplus that can be used to read the sensor signal. However, their architectures are far from being simple, making the broad adoption difficult. In this context, research with inorganic nanomaterials has revealed an additional and unforeseen advantage when electrically driven. They can reach relatively high temperatures going through electrical tests (i.e., electrical resistance measurements), even with the small amounts of electrical power dissipated during the electrical probing [20, 21]. This so-called self-heating effect makes it possible to reduce the power consumption of nanoscale devices down to the microwatt regime. In the case of conductometric gas sensors, this is a factor 1000 lower than state-of-the-art microsensors mentioned earlier. From a fundamental point of view, the self-heating effect is just the consequence of the Joule dissipation of power at a very small scale. Since 1847, it has been well-known that the electrical power dissipated in a resistive component leads to a temperature increase of the same [57]. The self-heating effect is nothing else than this effect, brought to such small scale that the temperature increase is remarkable, even when only small amounts of electrical power are applied. Simple figures about the power dissipated per unit volume (i.e., the power density) can help to realize the dramatic differences when the device scale is reduced. As a matter of fact, one nanowire in self-heating operation can easily reach power densities 10,000 times larger than a conventional domestic heater (assuming 1 μW in a (100 nm)2
10 μm nanowire and 1 kW in a (1 mm)2
1-m heater filament). The reason why self-heating is so efficient relays on the large power densities directly dissipated in the material that needs to be heated (i.e., a gas-sensitive nanowire). This is a huge difference with conventional device architectures based on an independent heating element that warms up the sensor material. Also, the heater needs to be electrically insulated from the sensor material, which adds even more material to heat. Therefore, the advantage of the self-heating approach is double: (i) removing the need for an external heating element and (ii) reduction of the power needs by eliminating the energy consumption associated with the external heater. A decade of research on self-heated devices has shown that this is a quite general phenomenon in nanowire-like structures with huge potential for efficient heating in miniaturized devices, which paves the way for many different applications in several fields of sensing and actuation.

Inorganic nanomaterials

It is evident that the self-heating effect in nanomaterial-based devices can damage or destroy them, similarly to well-known equivalent effects in microelectronic components (Fig. 2.4). That is the reason why the effect was first described and regarded as a threat or as an experimental challenge [20, 21]. In fact, suppliers of electronic instruments for research offer the know-how and the developed specific tools to avoid the problem (i.e., pulsed-operation modules) [23, 58–60]. In 2007, it was demonstrated for the first time that the self-heating effect could be controlled, not only with costly lab-class equipment but also with inexpensive commercial electronic components [24], providing an opportunity to use nanowire devices in consumer electronic products. In that work, self-heating was controlled and minimized in constant current operation. Today, we also know that a similar level of control can be achieved in constant voltage operation. In any case, it was shown that special care must always be

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Fig. 2.4 SEM image of a tin oxide nanowire after a few hours under working conditions. The contact area (inset) is destroyed due to self-heating. Melting temperature of tin oxide is close to T ¼ 1100°C. (Reprinted with permission from F. Hernandez-Ramirez, A. Tarancon, O. Casals, E. Pellicer, J. Rodriguez, A. Romano-Rodriguez, J.R. Morante, S. Barth, S. Mathur, Electrical properties of individual tin oxide nanowires contacted to platinum electrodes, Phys. Rev. B 76 (2007) 085429. Copyright (2007) by the American Physical Society.)

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taken during device manipulation, connection, and start-up stages to avoid static discharge effects and power peaks, which may cause irreparable damage to the nanomaterials. One year later, works of Strelcov et al. [61] and Prades et al. [62] reported the first chemical gas sensors based on self-heating effect, both working with individual SnO2 nanowires. The former demonstrated that the bias conditions influenced the response to gases and suggested that it was related to the self-heating mechanism. The latter proved that different electrical biases lead to gas responses fully equivalent to those obtained at different temperatures controlled with an external heater, showing that it was a temperature-related effect practicable to be controlled. This second work also demonstrated that the electrical power required to reach thermal conditions suitable for sensing was remarkably small: already in the range of tens of microwatts to temperature increases of hundreds of Kelvin. The result was a significant step forward to validate the advantage of working with nanomaterials instead of traditional thin-film layers for gas-sensing purposes. From the onset of these pioneering works, major strides have been made: the first reports on self-heating in nanomaterials, basically nanowires, lead to the misconception that the benefits of this operating method (i.e., low power consumption, fast thermal response time, and no need of an additional heater [38]) could only be achieved in devices based on a single nanowire. Consequently, self-heating in nanowires became closely associated to nanofabrication and to the challenges of gaining electrical access to individual nanostructures. Later, Chinh et al. thoroughly investigated the self-heating effect in multiple wire systems [63]. Interestingly, they observed that self-heating also appeared in devices based on a few nanowires but being slightly less efficient. This kind of system is relatively simple to fabricate as it is based on the deposition of random wires followed by a metallization step with conventional lithography methods. Due to the random nature of the few nanowires involved, the dispersion in the electrical properties and in the self-heating behavior was very large, and to palliate this aspect, Guilera et al. reported the use of the alreadyexplained DEP methodology to orienting the wires between a pair of electrodes. In principle, this measure should contribute to increase the degree of order and reduce the dispersion between samples while keeping efficient the self-heating properties [64]. Recent works have finally shown that the self-heating effect can be also extended to devices based on large random networks of 1D nanostructures exhibiting relatively good efficiencies [65, 66], considering the simplicity of both the fabrication and operating methodologies (Fig. 2.5). Actually, these systems are extremely easy to produce (i.e., by drop casting of nanostructures dispersed in a solvent [65] or just by the direct growth of the 1D nanomaterials on the top of microelectrodes [66]) and exhibit good reproducibility. As a matter of fact, in contrast with small arrangements of a few nanowires, these networks contain thousands of wires, and they are large enough to offer a macroscopic average of the distribution and properties of the individual elements [67].

Inorganic nanomaterials

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Fig. 2.5 Resistance record of a carbon nanofiber (CNF) film operated with self-heating and with an external heater. The variations in temperature due to the resistance variations caused by the gas pulses were estimated to be in the order of 1°C. Therefore, the coupling between sensing and heating effects in self-heating operation was nearly negligible in this kind of material. (Reprinted from C. Fàbrega, O. Casals, F. Hernández-Ramírez, J.D. Prades, A review on efficient self-heating in nanowire sensors: prospects for very-low power devices, Sens. Actuators B: Chem. 256 (2018) 797–811. Copyright (2018), with permission from Elsevier.)

According to further investigations, the reason why the temperature of large networks of nanowires can be elevated with just a few milliwatts [66, 68] is that heat dissipation concentrates in certain regions of the network: the so-called hot spots. In fact, the most effective regions for this effect seem to be the most resistive segments of the entire structure. In other words, the use of 1D nanostructures leads to mesh-like structures that define current paths in which efficient heating is locally possible (Fig. 2.6). Therefore, despite the macroscopic dimensions of the system, efficiency values fully comparable with those obtained in difficult-to-fabricate one-nanowire systems are possible and, above all, easy to obtain and operate. From the point of view of the sensing signal, these works have also demonstrated that the hot spots are the ones that generate most of the sensor output, leading to a consistent control of the sensor temperature, precisely in the points relevant for sensing.

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Fig. 2.6 (A) Experimental observations (thermal micrographs) of hot spots in random networks of nanofibers (CNF) operated (left) in self-heating mode and (right) with an external heater. Clearly, the temperature pattern in self-heating mode concentrates in some central regions of the nanowire network. This is also seen as disperse high-temperature spikes in the blue histogram of the micrographs earlier, corresponding to the self-heating operation. (B) 3D model of the simulated temperature profile in a random network of nanowires (d ¼ 60 nm, σ ¼ 100 S m1, and κ ¼ 1 W m1 K1) operated at 6.8 μW. The nanowire portions in direct contact with the substrate remain cooler than those located further away from the substrate. (Reprinted from C. Fàbrega, O. Casals, F. Hernández-Ramírez, J.D. Prades, A review on efficient self-heating in nanowire sensors: prospects for very-low power devices, Sens. Actuators B: Chem. 256 (2018) 797–811. Copyright (2018), with permission from Elsevier.)

Inorganic nanomaterials

Be that as it may, these latter multinanowire devices are an important first step in pursuit of cost-effective gas sensors based on inorganic nanomaterials, which combines a simple fabrication step and cost-effective operation leading to ultralow power needs. Despite how to optimize the performance of these sensors is still under research and needs to be systematized [38], self-heated bundle nanowire sensors are among the most promising alternatives to attain at the mid-term industrial gas detectors based on nanomaterials only. Anyhow, to build up a real sensor system, other components in demand of power must be considered. Typically, at least processing and communication units are always needed [69]. Even in the most austere configurations (e.g., with aggressive duty cycling), these components demand at least a few tens of microwatt [70]. Therefore, from a fullsystem perspective, the advantages of using nanomaterials instead of thin films for gassensing purposes should always be analyzed from the proper perspective, and self-heating is just one optimization route of the new generation of gas sensors relying on inorganic nanomaterials.

2.5 Conclusions Inorganic nanomaterials are among the forerunners for the development of new devices and in particular gas nanosensors. After more than two decades of active research in this field, the acquired knowledge about the fundamentals of the sensing mechanisms in 1D nanomaterials has exponentially grown, showing that these devices have clear advantages compared with their microcounterparts. The fabrication processes and running conditions of most of them are unfortunately complex, demanding in some cases highly skilled operators and controlled environments, which may incur elevated costs of the final systems. This undoubtedly results in a major drawback that hampers the transition of the new technologies from the lab to the real world. When it comes down to it, the gas sensor industry is a mature sector quite reluctant to accept any innovative concept that involves the loss of reproducibility and the rising of costs. For this reason, the search of new strategies to increase the production yield and simplify the operating conditions has become a high priority since the advent of the first gas nanosensors based on semiconductor nanomaterials a few years ago. In this context, two parallel strategies have steadily evolved with the aim to improve the throughput of the devices, making them affordable and ultimately simpler. The first one focuses on the fabrication stage of the devices and seeks to obtain them in an automated way to keep the production costs low. To that end, two different approaches have shown promising results up to now: (i) the in situ growth of 1D nanomaterials in welldefined locations of the sensor substrate, or alternatively (ii) the postgrowth positioning in well-defined geometry through self-assembly techniques. Regardless of the fact that both once have allowed obtaining quite promising results and the almost-automated fabrication of nanosensors has become a living reality, these new technologies still remain in their infancy, and a huge optimization effort is pending

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before the industry adoption. Actually, once the conceptual stage of these new technologies has been successfully completed with quite interesting proof-of-concept devices, it is time to carry out more systematic studies to attain transparent methodologies independent of individual cases that could lead to real cost reductions in the coming years. On the other hand, the attractiveness of gas sensors relying on semiconductor nanomaterials is significantly constrained by the demanding requirements of the running conditions. The first devices needed well-controlled experimental parameters difficult to transfer to real applications, but surprisingly, the use of the well-known Joule effect has opened an extremely interesting path toward the reduction of energy needs and the configuration specifications of these devices. In fact, self-heating in nanoscale materials becomes extremely important at low-bias conditions with dramatic influence on their final electrical behavior. It has been demonstrated that gas response fully equivalent responses to those monitored with the help of an external heater are monitored with devices excited only through this phenomenon, while the last results have successfully validated that even very easy-to-fabricate device geometries containing messy nanomaterials are suitable to operate in such conditions. This is a major step forward that allows combining nanodevices with consumer-class electronics keeping the power consumption extremely low without risk of damaging the sensing system. All in all, the self-heating effect has revealed itself as a direct route toward the cost drop of sensor operation. As discussed in this chapter, the different techniques presented herein are the first tangible results toward the systemization of gas sensors based on nanomaterials. From a practical point of view, they should not be examined individually. Once they are mature enough, it is conceivable that they will be combined to attain the expected cost reduction and the simplification level.

Acknowledgments This work has been partially supported by the Spanish Ministerio de Economı´a y Competitividad, through project TEC2013-48147-C6 (AEI/FEDER, European Union), and by the European Research Council, under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 336917. J.D. Prades acknowledges the support of the Serra Hu´nter Program.

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