**4. Integration of 1D metal oxide nanostructures into chemical sensors**

The chemical sensing capabilities of the MOS have been exploited primarily in electrically-transduced sensors of the resistive and field-effect transistor (FET) types [55]. Chemiresistive sensors or chemiresistors and FET sensors are the most investigated and exploited sensing configurations owing to their simplicity, ease of fabrication and operation, and feasibility of miniaturization. The most widely used architectures of chemiresistors and FET sensors are presented in this section. There follows a discussion on the methods and techniques that allow 1D MOS nanostructures to be integrated as sensitive material into such architectures, paying attention to aspects such as sensor reproducibility and potential for scaling up sensor fabrication.

## **4.1 Sensor architectures**

Chemiresistive sensors are bipolar devices addressed to measure the electrical resistance of a semiconductor (e.g., 1D MOS nanostructure) as the sensing material bridging two electrodes or interdigitated electrodes (IDE) supported by an insulating substrate. Typically, FET sensors consist of a semiconductor (e.g., 1D MOS nanostructure) as the conducting channel connected by the source and drain electrodes. This semiconductor is placed on the top of an insulated gate electrode so that its conductance can be regulated by varying the bias voltage of the gate electrode. This classical architecture for electronic metal-oxide field-effect transistors (MOSFET) is usually similar for FET sensors addressed to gas-phase analytes. In contrast, the architecture of FET sensors for liquid phase analytes differs from the traditional MOSFET, since the gate electrode (or reference electrode) is immersed into the liquid analyte with the conducting channel being sensitive to the ions of the analyte. Therefore, this architecture is known as an ion-sensitive field-effect transistor (ISFET) [37]. **Figure 12** displays a schematic illustration of the two types of sensor architectures (resistive and FET) targeted in this section.

Nowadays, the fabrication of these transducer platforms exploits micro/ nano-fabrication technologies, usually based on silicon as substrate and

*One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

**Figure 12.**

*Schematic illustration of a resistive (a) and FET transducing platform for gas (b) and liquid (c) phase analytes. NW: nanowires, D: drain electrode, S: source electrode, and G: gate electrode.*

#### **Figure 13.**

*(a) Layout of a typical microresistor (lateral view); adapted with permission from [6], Copyright 2018 Authors, licensee MDPI. (b) Optical image of a microhotplate and its indicated components (top view).*

micro-electro-mechanical systems (MEMS) technology. This facilitates their integration as arrays in monolithic microchips as well as the incorporation of microheaters with low thermal losses (the so-called microhotplates) [5, 6, 74]. **Figure 13a**) displays the typical architecture of a microresistor consisting of a microhotplate and a sensing material on its top, and **Figure 13b**) shows an image of a microhotplate. This consists of a thin layer (a few micrometers thick) of a dielectric material, also called a membrane, supported by a silicon substrate at its periphery. The microheater is embedded within the membrane and insulated from the interdigitated electrodes patterned on top of the membrane. The use of silicon and MEMS technologies allows for the incorporation of integrated circuits along with the driving and signal conditioning circuitry or other smart features (e.g., wireless communication) to build electronic noses with potentially low-cost production [152, 153]. However, recently other substrate materials (e.g., polymers) and technologies (e.g., printing) are being explored and optimized to provide also integrated elements driven by the use of optimized active 1D MOS nanostructures that can operate at room temperature or close to it [13, 154].

#### **4.2 Assembly of 1D metal oxide nanostructures on transducer platforms**

To enable practical use of 1D MOS nanostructures, these structures must bring the interdigitated electrodes (chemiresistor) or the source and drain electrodes (FET) into contact. This allows the electrical current to flow and the resistance (or conductance) changes to be monitored. Such connection can be attained either by a single 1D nanostructure or by multiple 1D nanostructures [52, 54, 98]. Due to the requirement of precise alignment between the 1D nanostructure and the patterned electrodes, the fabrication process of the individual 1D nanostructure device is rather complicated, time-consuming and expensive. Therefore, to simplify the fabrication process and electrical signal measurement, the multiple 1D nanostructure devices become the most widely accepted configuration for practical applications.

### *4.2.1 Coating methods*

The assembly of multiple 1D nanostructures for chemical sensors usually involves the use of a two-step process [52], following any of the routes sketched in **Figure 14a**). In the first route (top-electrode architecture), either 1D nanostructures are synthesized directly on a blank substrate or pre-synthetized 1D nanostructures are transferred on the substrate. Then, the electrodes are deposited by sputtering on the substrate with the 1D nanostructures on their top with the help of a mask. Conversely, in the second route (bottom-electrode architecture), the electrodes are deposited firstly on the blank substrate and the 1D nanostructures are either synthesized or transferred on the substrate with the patterned electrodes on its top. Technologically, bottom-electrode architectures are preferred for chemical sensors, as they may facilitate the direct integration of 1D nanostructures. This type of architecture also prevents the introduction of contaminants into the sensitive materials as the processing steps for the definition of the electrode are performed before the integration of the sensitive material.

The synthesis of 1D MOS nanostructures directly onto the sensor substrate [155], either a blank substrate or a substrate with patterned electrodes, is the preferred choice, since it reduces the fabrication time and costs of the sensors. Also, it reduces the incorporation of contaminants by avoiding the use of transfer media (often liquids) that tend to degrade the surface properties of the sensitive structures. On the other hand, the direct integration of 1D MOS nanostructures demands substrate materials such as silicon (Si), alumina (Al2O3), quartz, or hightemperature-resistant polymers (e.g., polyimide PI) [156] capable to withstand the thermal steps required for the synthesis of the MOS nanostructures and the

#### **Figure 14.**

*(a) Schematic diagram of the architectures (top- or bottom-electrodes) used in multiple 1D nanostructures based chemical sensors: (1) blank substrate, (2) nanowires transferred onto the blank substrate, (3) electrodes patterned onto the substrate with nanowires, (4) electrodes patterned onto the blank substrate, (5) nanowires transferred onto the substrate with patterned electrodes; adapted with permission from [52], Copyright 2021 Authors, licensee MDPI. (b) Schematic illustration of wet-coating methods for the transfer of nanomaterials from a liquid dispersion to a substrate; adapted from [158].*

#### *One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

high operating temperatures of the MOS chemical sensors (>200 °C). However, since the most advanced functional nanomaterials for chemical sensing based on MOS suggest that the next generation of MOS chemical sensors could work at room temperature, the need for heating elements could be omitted in the future, opening the possibility to use more abundant substrate materials that are cheaper and easier to process compared to those above. In this respect, steps are already being taken towards the synthesis of 1D MOS nanostructures on tiny substrates of flexible stretchable soft polymers (e.g., polyethylene terephthalate PET, polytetrafluoroethylene PTFE, polyaniline PANI, polyimide PI), textiles, or paper [157], which are of major interest for use in wearable sensor devices for emerging applications (e.g., healthcare). To achieve this, the processing temperatures are required to be below the glass transition temperature of polymers (<400 °C PI) or the thermal degradation temperature of the substrates (<100 °C textiles, paper). This limits the application of some synthetic procedures as those based on high-temperature chemical vapor deposition or electrospinning, as they demand temperatures of at least 400°C for either precursor decomposition or sintering steps. Then, the transfer of the pre-synthesized 1D nanostructures on flexible substrates seems to be the only feasible alternative to date.

The transfer of the pre-formed 1D MOS nanostructures to the sensor substrate may be a complex, time-consuming and expensive approach. In transfer methods, firstly, the 1D nanostructures need to be detached from the substrate on which they were synthesized and subsequently be dispersed in a liquid, usually a volatile organic solvent. There is a diversity of wet-coating techniques to transfer nanomaterials in suspension in a liquid to a substrate. The most common ones are illustrated in **Figure 14b**) [158]. Basically, the choices are either immersing the sensor substrate in the dispersion (dip-coating) or dosing the dispersion in form of droplets (drop coating, spin coating, spray coating) that settle on the substrate. The liquid evaporates from the in-flight droplets and/or the substrate, which can be heated at a constant temperature during or at the end of the transfer process.

The two approaches described previously result in multiple 1D MOS nanostructures forming a mat- or web-like deposit; this is a stacked network of 1D MOS nanostructures lying randomly in all directions of space. The deposits often display a bi-modal pore size distribution with relatively large pores (sub-micron to a few microns) side by side with nanosized pores (e.g., electrospun nanofibers), facilitating effective gas transport into the sensing layer. In addition, there are many cross-points between the 1D nanostructures in the network, which may favor the current percolation and the whole conductivity of the film. Nevertheless, several weaknesses have been identified in these mat-type sensitive layers that bring into question their usefulness for chemical sensors.

Even if the above techniques allow for a high degree of control of the parameters relevant to the synthesis or transfer of the 1D nanostructures, properties such as the size, thickness, porosity, or nano/microstructure vary greatly from one deposit to another, which lead to differences in the chemical sensing performance between sensors. To overcome this problem, wire structures connected in parallel are the ideal architecture to achieve a well-defined conduction channel that is easy to modulate by the interactions of the analyte and surface. In such a structure the grain boundaries or nanowire-nanowire interfaces are removed and thus the sensitivity of the system depends only on the nanowire surface due to an efficient transfer of charge with a lower probability of recombination. Usual approaches to achieve such integration of 1D nanostructures involve alignment methods based on the Langmuir-Blodgett technique or electric and magnetic field-assisted orientation techniques [159] such as dielectrophoresis [160]. For instance, a recent report based on the dielectrophoresis method proposed the use of a nanoelectrode array

**Figure 15.**

*(a) View of a nanoelectrode array containing several single-nanowires connected in parallel. (b), (c) Detailed views of a pair of nanoelectrodes with a nanowire interconnected across them; reprinted from [161].*

system for the integration of various single-nanowires connected in parallel [161]. This system allowed for the selective integration of single-nanowires connected across various faced electrodes using the dielectrophoresis method, as can be observed in **Figure 15**.

Several methods have also been developed to orient and align electrospun fibers, either mechanically (e.g., rotating drum, rotating sharp disk) [84, 162–164] or by the action of electric fields [165–167]. In the latter, gaps of an insulating material (e.g., air, quartz, polystyrene) are introduced on the surface of a conductive substrate, and patterned electrodes (e.g., interdigitated electrodes) can also be used to obtain oriented nanofibers. When an insulating gap is introduced into the collector, it changes the structure of the external electric field. As a result, the directions of the electrostatic forces acting on a fiber that is sitting across the gap will be altered. In addition, once the charged fiber has moved into the vicinity of the electrodes, charges on the fiber will induce opposite charges on the surface of the electrodes. These opposite charges will further attract the fiber to the electrodes. These two types of electrostatic forces simultaneously pull the fiber towards the edge of the two electrodes and acting on different portions of a fiber will eventually lead to its uniaxial alignment through the gap, as can be seen in **Figure 16** [167].

It has been observed that the electrospun MOS nanofibers have a weak interfiber interaction and poor adherence to the electrodes and the substrate, resulting in high contact resistance and inferior mechanical properties. Thus, various treatments have been applied to the as-spun polymer/inorganic precursor fibers before sintering. These are traditional treatments such as irradiation with UV-light [168] or hot-pressing [169, 170], and a novel nanoscale welding technique [171]. The latter is simple and easy to apply, does not require specific equipment, preserves the interconnected structure of nanofibers, and is also the most efficient in terms of enhancement of the interfiber connections and interfacial adhesion. All this makes it the optimal treatment for application in chemical sensors.

Usually, conventional wet-coating techniques do not allow for highly-localized highly-dispersed coating of very small areas. Ideally, the sensing material should cover only the zone spanned by the electrodes, this is the active surface of the sensor whose area in the case of the microsensors is typically less than 0.2 mm2 . In practice, however, it is necessary to use masks to keep the region around the electrodes clean, particularly the bond pads of the electrodes and microheater. In this regard, the electrohydrodynamic jet printing technique [172–174], also called e-jet printing, is a precise way to transfer nanomaterials in a liquid on a substrate with greater resolution and repeatability than standard wet-coating and printing techniques (e.g., screen printing, aerosol-jet printing, ink-jet printing). In e-jet printing,

*One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

**Figure 16.**

*(a) Layout of the setup to electrospin fibers as uniaxially aligned arrays, where the collector consists of two conductive strips separated by a void gap. SEM micrographs of uniaxially aligned electrospun nanofibers of (b) TiO2 and (c) Sb-doped SnO2; the insets show enlarged SEM micrographs of the nanofibers. Adapted with permission from [167], Copyright 2003 American Chemical Society.*

droplets of a liquid suspension, also called an ink, are pulled out from a nozzle by the action of an external electric field. Charges accumulate at the liquid surface and the coulombic repulsion causes the liquid meniscus at the nozzle tip to deform into a conical shape (Taylor cone). When the electric field exceeds a critical value, the stress from the surface charge repulsion at the cone apex exceeds the surface tension and a small droplet is emitted towards the substrate. The key to high-resolution droplet printing is to use electric potentials below those required for droplet atomization (electrospray) and small nozzles (<100 μm). Then, the deposited droplets can be as small as hundreds of nanometers.

Highly integrated arrays of MEMS gas sensors have been prepared by e-jet printing [175, 176], as depicted in **Figure 17**. Firstly, long electrospun MOS nanofibers were fragmented into smaller pieces by ultrasonication and dispersed in a liquid. The electrodes were then coated with the fragments of the MOS nanofibers by using an e-jet printing system including a pulse-modulated voltage supply and a moving stage to control the droplet size and droplet position, respectively. The nozzle used was less than 100 μm in inner diameter and the dot pattern size of the MOS nanofibers was in the order of 40–60 μm. It is worth mentioning that e-jet printing was accomplished without nozzle clogging and that all the microsensors survived the process, in contrast to the standard wet-coating techniques, where the failure of the microsensor is frequently observed owing to the impact of big droplets, typically of tens of micrometers, causing the fragile dielectric membrane to crack. Furthermore, using a smaller nozzle, adjusting the applied voltage and pulse width, and tightly controlling the X-Y moving platform a continuous jetting regime was

#### **Figure 17.**

*(a) Layout of a typical setup for e-jet printing mounted on an X-Y positioning stage, (b) Optical microscope micrograph of the microhotplate (MHP), (c) SEM micrograph of the MHP coated with fragments of Pd-loaded SnO2 nanofibers (inset) by using the e-jet printing setup in (a). Adapted with permission from [175], Copyright 2018 Authors.*

#### **Figure 18.**

*(a) Sketch of a typical electrohydrodynamic direct-writing (EDW) setup; reprinted with permission from [186], Copyright 2006 American Chemical Society. SEM images of (b) PVP/Zn fiber obtained by EDW over a silicon substrate and (c) ZnO nanofiber obtained by sintering of the PVP/Zn fiber in (b); reprinted with permission from [189], Copyright 2013 Elsevier.*

achieved, enabling to print line patterns less than 40μm in width with a maintained geometry and homogeneous distribution of the fragments of MOS nanofibers [176].

#### *4.2.2 Patterning methods*

Conventional methods based on micro- and nano-fabrication top-down processes that include steps of deposition, lithography, and etching may be used to pattern 1D MOS nanostructures and assist the growth of 1D MOS nanostructures in selected areas of a device. These processes are usually employed to define the area, density, and size of 1D nanostructures by patterning catalyst seeds that assist the growth of 1D nanostructures via VLS growth mechanisms. Other approaches also include forming templates to assist the directional growth of 1D nanostructures with subsequent template etching to free the 1D nanostructures. Hence, these processes generally lead to vertical aligned 1D nanostructures that are bridged with top and bottom contact as resistors or as FETs, for instance, by adding a vertical surrounded-gate [177, 178]. These approaches are not discussed in this chapter and more details can be found in the following literature [55, 158, 179, 180].

The electrospinning technique is more suited for the formation of nanofiber mats on areas that largely exceed the active area of the chemical sensors. Generally, the size of the area covered with the electrospun fibers decreases with the decreasing nozzle-to-substrate distance, which is accompanied by a reduction of the applied potential to preserve the electric field and prevent the onset of electrical discharges. The near-field electrospinning (NFES) method, also known as the electrohydrodynamic direct-writing (EDW) method, exploits this behavior [181–186]. The working principle of EDW is similar to that of the traditional electrospinning but with some distinctive features, namely the small nozzle diameter (<50 μm), the low applied voltage (<1 kV), the short distance between the nozzle and the substrate (0.5–5 mm), and the use of a moving nozzle and/or substrate, which facilitates position-tunable alignment and on-demand patterning of fibers on the substrate. EDW uses the stability region of the liquid jet and supplies discrete droplets of the polymer solution, in the same way as a dip pen does. This technique has been proven for direct-writing individual MOS nanofibers on silicon substrate for use in both chemiresistive [187, 188] and FET [189] sensor devices. As an example, **Figure 18** shows images of a ZnO nanofiber obtained by sintering a PVP/ Zn nanofiber written by EDW on a substrate.
