Review of the gas breakdown physics and nanomaterial-based ionization gas sensors and their applications

Ionization gas sensors are ubiquitous tools that can monitor desired gases or detect abnormalities in real time to protect the environment of living organisms or to maintain clean and/or safe environment in industries. The sensors’ working principle is based on the fingerprinting of the breakdown voltage of one or more target gases using nanostructured materials. Fundamentally, nanomaterial-based ionization-gas sensors operate within a large framework of gas breakdown physics; signifying that an overall understanding of the gas breakdown mechanism is a crucial factor in the technological development of ionization gas sensors. Moreover, many studies have revealed that physical properties of nanomaterials play decisive roles in the gas breakdown physics and the performance of plasma-based gas sensors. Based on this insight, this review provides a comprehensive description of the foundation of both the gas breakdown physics and the nanomaterial-based ionization-gas-sensor technology, as well as introduces research trends on nanomaterial-based ionization gas sensors. The gas breakdown is reviewed, including the classical Townsend discharge theory and modified Paschen curves; and nanomaterial-based-electrodes proposed to improve the performance of ionization gas sensors are introduced. The secondary electron emission at the electrode surface is the key plasma–surface process that affects the performance of ionization gas sensors. Finally, we present our perspectives on possible future directions.

Ionization gas sensors are ubiquitous tools that can monitor desired gases or detect abnormalities in real time to protect the environment of living organisms or to maintain clean and/or safe environment in industries. The sensors' working principle is based on the fingerprinting of the breakdown voltage of one or more target gases using nanostructured materials. Fundamentally, nanomaterial-based ionization-gas sensors operate within a large framework of gas breakdown physics; signifying that an overall understanding of the gas breakdown mechanism is a crucial factor in the technological development of ionization gas sensors. Moreover, many studies have revealed that physical properties of nanomaterials play decisive roles in the gas breakdown physics and the performance of plasma-based gas sensors. Based on this insight, this review provides a comprehensive description of the foundation of both the gas breakdown physics and the nanomaterial-based ionization-gas-sensor technology, as well as introduces research trends on nanomaterial-based ionization gas sensors. The gas breakdown is reviewed, including the classical Townsend discharge theory and modified Paschen curves; and nanomaterial-based-electrodes proposed to improve the performance of ionization gas sensors are introduced. The secondary electron emission at the electrode surface is the key plasma-surface process that affects the performance of ionization gas sensors. Finally, we present our perspectives on possible future directions.
Keywords: gas breakdown, modified Paschen curve, ionization gas sensors, nanomaterial-based gas breakdown characteristics, micro-discharge, plasma, plasma-surface-interactions (Some figures may appear in colour only in the online journal) * Author to whom any correspondence should be addressed.
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Introduction
The current industry is experiencing a sharp increase in the use of hazardous substances, such as toxic, corrosive, dangerously reactive, and flammable gases, which are potentially perilous to the surrounding living things. Gas sensors are used to detect or trace such gases or organic vapors typically in air or low-pressure environments [1][2][3][4][5][6] for a wide range of applications, including medicine [3,7], environmental monitoring [8][9][10], industrial processes [11][12][13], hazardous gas safety [14,15], and aerospace technology [16,17], as shown in table 1. An ideal gas sensor must be able to detect a specific gas in gas mixtures (selectivity) by measuring the electrical signal; gas sensor must respond quickly and sensitively to even small amounts of target gas (i.e. has good response time and sensitivity), return to the initial sensor state after gas detection (reproducibility), and exhibit the same performance consistently, even after multiple operations with high durability (reproducibility and lifetime). Hence a successful gas sensor has to have all these important properties simultaneously: selectivity, response time and sensitivity, reproducibility, and lifetime. Most widely used in the industry commercial gas sensors are based on chemical reactions between gases and detectors. These gas sensors have response times of several seconds and recovery times to the initial state of hundreds of seconds; this time is so long because of slow surface processes involved (e.g. adsorption and dissociation)-these characteristics do not satisfactorily meet the above-mentioned requirements: fast response time and sensitivity, reversibility, and long lifetime [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. On the other hand, in the case of physical gas sensors, such as a quadrupole mass analyzer, it is possible to effectively separate ion species with different mass to charge ratios by ionizing the target gas. However, this conventional physical gas sensor operates at high voltage and has a bulky structure, and therefore cannot be if low power consumption and structural miniaturization is needed.
In this regard, nanomaterial-based ionization gas sensors have attracted attention because of their compact size, low operational power, and fast response and recovery time that can overcome the drawbacks of conventional physical gas sensors [21]. Ionization gas sensors operate based on fingerprinting the gas breakdown voltage of one or more target gases using electrodes composed of nanostructured surface materials [23]. These sensors can operate at low voltages because of the field enhancement at the surface of the sensor electrodes equipped with nanomaterials, which offers great advantages in terms of compactness and power efficiency. Furthermore, they exhibit fast response and recovery time because their operation mechanism is based on gas discharge with time scales determined by ionization and recombination or wall loss of charged particles. Therefore, ionization gas sensors are applicable to space missions, where compactness and low-power operation are essential, and nuclear power plants, where fast detection of toxic gases is crucial for safety management of the facility.
Following the discovery of the field-emission of nanomaterials by de Heer et al [101], designing miniaturized ionization gas sensors using carbon nanotubes [1] represents the most basic electrode configuration for the development of ionization gas sensors. These sensors consist of two parallel electrodes to which a DC voltage is applied, and a nanomaterial is mounted on one side (figure 1). Based on this basic structure, numerous studies are being conducted to improve the performance of these sensors by modifying the nanomaterial deposited on the electrode surface to lower the discharge voltage and increase the current. Eventually, the performance of the any gas sensor is verified by repeated measurements of the Paschen curve showing the relationship between the breakdown voltage as a function of the gap distance and gas pressure.
To improve the above-mentioned sensor performances, complex electrode types have been proposed, differing from previous studies. These studies mark an advancement from conventional parallel plate-type electrodes, and propose a multi-electrode structure [159,160] or electrode structures with high aspect-ratio [161,183], showing improved sensor characteristics. Although gas sensors have seen advancements, the performance evaluation of gas sensors is still limited to observation of changes in the electrical properties of the Paschen curve or voltage-current characteristics, without providing sufficient physical analyses. Advanced gas breakdown physics has thus far demonstrated that numerous effects, e.g. the secondary electron emission, and changes in discharge geometry and power sources, are important to take into account. Therefore, developing a fundamental understanding of gas discharge, which has been overlooked in the development of ionization gas sensors, is an urgent task. A comprehensive review of the fundamentals of gas discharge physics and the performance of ionization gas sensors is beneficial for researchers involved in physics and engineering of gas sensors.
In this review, we describe the state-of-the-art in modern gas breakdown research as well as the nanomaterial-based ionization-gas-sensor technology.
The paper is organized as follows. Section 2 introduces the fundamentals of gas breakdown based on the Townsend theory. Section 3 discusses the modified Paschen curve and the similarity law relevant to the narrow gap and micro discharge in DC and RF gas breakdowns. Section 4 presents the latest research trends in nanomaterial-based ionization gas sensors. Finally, section 5 presents the research challenges and future perspectives involving gas sensors. The breakdown voltage versus H 2 pressure curve in a partially-exhausted tube. The abscissa shows the cube root of gas pressures in a millionth of the atmosphere p · 10 −6 1/3 atm and the ordinate shows the breakdown voltage (V). Reproduced with permission from [22]. [© 2017, Royal Society].

Configurations of discharge sources
The first experiment on gas breakdown was reportedly conducted approximately 140 years ago-at that time, the gas breakdown was observed between two parallel metal plates by applying DC voltage. The arguably first documented graph correlating the discharge voltage and operating pressure was published in 1880 by Rue and Muller [22]. In the experiment, the authors used partially-exhausted tubes, and a certain minimum breakdown voltage was recorded while reducing the background gas pressure. The breakdown voltage as a function of pressure at a constant interelectrode gap or as a function of interelectrode gap width at a constant pressure was measured (figure 2). In 1889, Paschen used spherical electrodes with variable spacing and measured the breakdown voltage at different gas pressures and interelectrode spacings [23]. An empirical relation for the breakdown voltage, suggested by Paschen, states that the electrical breakdown voltage is a function of the product of the background gas pressure, p, and the inter-electrode distance, d.
Unlike earlier studies of the gas breakdown conducted with simple configurations of discharge sources, recent studies have been conducted using more complicated geometries of electrode shape, complex electrode material, and driving voltage, as shown in figure 3.
Gas breakdown in low-pressure RF discharge is an important research topic and is widely used in semiconductor and display processes [24,25]. Specifically, an RF discharge source with a distance of less than several centimeters between planar-round electrodes (called capacitively coupled plasma in industry, see figure 3(b)), is favored for use in surface treatment and etching of semiconductor materials. The RF discharges have different breakdown curves depending on the discharge geometry, and materials of the chamber body and electrodes [26]. These differences are caused by changes in electron heating, plasma generation, and wall losses of charged particles affected by the RF electric field. In particular, in the left-branch of the Paschen curve (the breakdown curve in the low-pressure range shown in figure 9), the loss of electrons to electrodes plays a dominant role due to the large amplitude of electron oscillations in the applied RF electric fields; and the Paschen curve can be multi-valued [27].
The strong confinement of charged particles by applied magnetic field aids in producing a high-density plasma, and affects low-pressure gas breakdown, (see schematic in figure 3(c)). The applications that utilize strong magnetic field are the nuclear fusion [28], electric propulsion [29], ion implantation [30], and deposition of thin films [31]. Effects of magnetic field plays a crucial role of the particle transport and gas ionization in the electrical breakdown phenomenon in magnetized discharges. Many studies have investigated the gas breakdown in the presence of magnetic field [32,33,[45][46][47][48][49]. In the configurations using crossed electric and magnetic field, it was found that the magnetic field affects more the left-branch of the Paschen curve [33].
Gas breakdown in the configuration with an electrode different from that of a normal parallel plate as shown in figure 3(d) exhibits a change in the Paschen curve. The presence of a gas distributor plate/showerhead located in the gas inlet system and gas cooling system of the electrostatic chuck in the plasma process becomes an undesired source of gas breakdown, because of a structure with a high aspect-ratio (figure 3(d)) exposed to high voltage. Eventually, this leads to reduction of the process efficiency and can generate impurities due to sputtering of the materials. To understand and prevent such unwanted breakdown phenomena, the discharge occurring inside a hole with a high aspect ratio has been studied [34].
Gas discharge in a long tube structure is mainly affected by the radial diffusion. Consequently, the discharge voltage curve shows an increase in breakdown voltage and the Paschen minimum shifts to the right with the aspect ratio increase. In the case of hollow cathode discharge, an electron oscillates back and forth reflecting from the cathode sheath, this enhanced radial confinement of fast electrons results in a lower breakdown voltage.
The interest in micro discharges, which have an interelectrode gap of less than a few millimeters, has increased significantly in recent decades. Often micro discharges that are used in plasma processes develop a microstructure with a sharp surface morphology; this leads to the enhancement of the local electric field [35], causing unwanted gas breakdown due to field emission in unwanted locations. In contrast to micro discharges, the field emission process plays a key positive role in enabling the ionization gas sensor to be used at low power and scaled down to a compact size device [1]. Research on the development of electrode structures and the morphology of nanomaterials beneficial for ionization gas sensors has been actively conducted recently.

Townsend discharge theory
Townsend defined the steps for initiating discharge and introduced the breakdown theory (Townsend discharge theory) [36][37][38]. He identified two processes necessary to maintain a discharge: volumetric ionization and the secondary electron emission, mostly induced by accelerated ions bombarding the cathode . The process is described by the Townsend's first ionization coefficient, α, related to the ionization rate per unit length produced by the electron avalanche in an electric field [38].
The ionization coefficient, α,is conventionally described by an empirical formula [38]: where A and B are experimentally determined constants for different gases (figure 4 and table 2). Townsend's second ionization co-efficient, γ, accounts for the secondary electron emission by all particles incident on the cathode, including electrons, ions, metastable atoms and molecules, photons, and even fast neutrals.  The breakdown condition is given by [50,51] Finally, the Townsend breakdown condition is obtained by combining equation (2) with equation (1), This is known as Paschen's law. The minimum breakdown voltage V B and the corresponding value of pd can be obtained readily from equation (3): This expression indicates that the value of (pd) min is the function of the electrode material and background gas, as demonstrated by the graph of breakdown voltage for various gases shown in figure 5(a). More detailed experimental data on the gas breakdown voltage up to 160 kV for the left and 12 kV and for the right branch of the Paschen curve are shown in figures 5(b) and (c) [52]. Different dashed and solid lines for CO 2 and air in figure 5(b) show previously published data (see references there in). The discrepancy is probably due to the faults in the design and experimental procedure [52], demonstrating that the gas breakdown voltage can be greatly influenced by the experimental procedure, such as the surface condition of electrode, temperature of electrodes and chamber wall, and rise time of applied voltage as well as the discharge configuration.

Limitations of the Townsend theory
Although the Townsend discharge theory describe the Paschen curve correctly for simple two-plate electrode configuration at medium range of pd parameters, the gas breakdown phenomena outside of this range of pd and more complex discharge geometry cannot be completely described by this theory, as explained in the following examples. Townsend, McCallum, and Miller measured neon's DC breakdown curve at different gas pressures [53][54][55], and discovered that the breakdown voltage at a large gap is considerably higher than that at a small gap on a scaled pd plot. These results imply that the Townsend theory is valid only for a limited range of interelectrode distances between two parallel electrodes. Furthermore, a deviation from the Townsend theory was reported in the breakdown of mercury for the left branch [56] and in krypton and xenon breakdown with γ being dependent on the electrode distance [57]. Lisovskiy studied low-pressure gas discharges in cylindrical tubes with various aspect ratios [58][59][60][61][62][63], and showed that the gas breakdown in a DC electric field can be explained by modified relations, V B = f (pL, L/R), where L and R are the length and the radius of the electrode, respectively. A similar study of narrow gap discharges (range of μm) suggested that adequate theory must account for the effects of field emission by an intense electric field [64]. Furthermore, secondary electron emission can be caused not only by ion bombardment but also by fast neutrals and energetic electrons on the anode in DC breakdown or the electrode in RF breakdown 184].

Modified Paschen curve
In certain ranges of pressure and electrode gap, numerous experimental and theoretical studies have demonstrated that the gas breakdown voltage cannot be solely explained by the Paschen curve based on the classical Townsend theory. Gas breakdown in devices with various electrode shapes, gap distances, and driving frequencies proceed with a more complicated profile of the electric field that modifies the mechanism of heating and loss of charged particles, resulting in a strong modification of the Paschen curve. Table 3 shows some representative experimental and theoretical studies of the modified Paschen curve. In particular, the radial diffusion and the oscillations in the RF electric field become the main factors in the modified Paschen curve of DC discharge (long tube) and RF discharge, respectively. In the case of micro-to nanoscale discharge breakdown, the electric-field-induced electron emission plays a significant role in the breakdown process.
Specifically, the electron field emission occurs due to the strong electric field in the micro-to nano-scale gap discharges, and the electron field emission can be further enhanced by positive ions approaching the cathode (i.e. ion-enhanced field emission) [35], resulting in strong modification of the left branch of the Paschen curve (the breakdown curve in the low-pressure range shown in figure 13).
The similarity laws of scaling are useful in that they can allow for estimation of the physical parameters of interests in a scaled system [91,119,129]. The similarity laws of scaling can be applied to understand the modified Paschen curves in DC and RF discharges. In the case of DC gas breakdown, the effect of the non-uniform electric field for a given discharge configuration can be analyzed as the similarity law in which the aspect ratio is taken into account, whereas in the RF gas breakdown, the geometrical factor and driving frequency are contributing quantities in a scaling law for the RF electric field.

Similarity law in DC gas breakdown
In this chapter, we discuss the DC breakdown similarity law that deviates from the classical Townsend breakdown criterion. In addition to the classical Paschen curve, which states that the breakdown voltage depends on the gas pressure and gap length, recent studies on narrow gap discharge emphasize that the radial diffusion of plasma and non-uniform electric field must also be considered [58,[92][93][94]. Discharge in a long tube whose length exceeds its diameter is one of the conventional forms of gas discharges. Lisovskiy et al conducted experiments and theoretical studies by changing L, R, and the cathode material keeping the electric field uniform, and showed that the discharge initiation voltage is a function of pL and L/R [58], suggesting a formula of the modified Paschen curve. When L/R converges to zero, the curve follows the conventional Paschen curve (figure 6), indicating that the Paschen curve can be used for the basis of the similarity law.
Gas discharge in a long tube structure may be affected by the charged particles' diffusion escape to the tube wall (see figure 7) and by the non-uniformly of the electric field profile. Most studies have focused on the diffusion loss of charged particles towards the tube wall [34,[58][59][60][61][62], and the effects of non-uniform electric fields have not been sufficiently investigated. Recently, Lisovskiy [93] demonstrated that the gas breakdown criterion is a function of pL, L/R el , and L/R tube , Figure 7. Paschen curves for varying N 2 pressure and a fixed inter-electrode distance L of 72 mm and radius R el of 6 mm and two values of inner radii of the discharge tube R tube (6.5 mm and 28 mm). The breakdown voltage for large R tube is remarkably lower than that for the narrow tube, indicating that radial diffusion to near the tube wall increases the breakdown voltage. Reprinted from [93], Copyright (2017), with permission from Elsevier.
where the non-uniformity of the electric field between the electrodes is described by L/R el (R el and R tube are the electrode radius and the tube radius, respectively), and the diffusion to the tube walls is described by R tube . However, conducting research by independently controlling the non-uniformity of the electric field and the diffusion of charged particles is challenging. Fu pointed out that the dimension of the electrode was changed in Lisovskiy's experiment and succeeded in performing an experiment that altered only the radius of the dielectric tube; the breakdown voltage reached a lower value for a highly non-uniform electric field with fixed pd [94]. Additional theoretical and experimental supports are needed to investigate the effects of non-uniform electric fields.
In the hollow cathode geometry shown in figure 8(a), the reduced wall losses of high-energy electrons and the efficient collection of ions in the hollow cathode cause a decrease in the breakdown voltage at a critical gas pressure [95][96][97][98][99][100]185]. Figure 8(b) shows a typical dependency of discharge voltage in helium hollow cathode discharge at different values of discharge current. The current-voltage characteristics of each current has a minimum at low pressures around 25 Pa, indicating efficient glow discharge via the hollow cathode effect [97]. The variations in the discharge voltage and current in the hollow cathode discharge was described using the similarity law by considering the width or diameter of the cathode [100].

Similarity law in RF gas breakdown
Unlike DC discharge, which maintenance requires the production of secondary electrons on the surface by the incident ions, this is not necessary for a RF discharge. In an RF discharge, the oscillatory motion of electrons depends on the driving frequency, which leads to a specific shape of the RF breakdown curve. When the displacement of the electrons caused by the RF electric field is greater than the gap at a low pressure, the electrons incident on the electrode becomes important, initiating breakdown due to secondary electron emission. The  (1), 50 (2), 100 (3), 150 (4), and 200 mA (5). Reproduced from [97]. © IOP Publishing Ltd. All rights reserved.  [27,77], respectively. The data of multi-valued region are obtained by fixing a certain RF voltage at a sufficiently low pressure, and then the pressure in the chamber is increased slowly until breakdown occurs. Reproduced from [27]. © IOP Publishing Ltd. All rights reserved. Ar RF discharges. The product of RF driving frequency and inter-electrode length is identical for two curves in the plot. Reproduced from [80]. © IOP Publishing Ltd. All rights reserved.  Paschen curve contains a multi-valued region which is a distinctive feature of the RF breakdown. The RF gas breakdown curves and detailed experimental techniques to measure them were studied by Gutton and Kirchner [69,70]. The dependence of the driving frequency and discharge initiation voltage on gap widths and gas pressures were analyzed in references [71][72][73], and the two-minima in the breakdown curve in RF discharge studies were discovered [74,75]. The theoretical research commenced with the Hale's modeling research [76] and Kihara's analytical treatment [77], which highlighted the importance of balance between the ionization rate and the electron diffusion or drift loss to the walls. Recently, because of the development of better measuring devices, the precision of RF breakdown measurements has improved, and the similarity law in RF breakdown has been established based on a large amount of data.
The RF breakdown curve is explained by dividing the discharge into several regimes based on L and electron displacement amplitude A = eE RF /mν en ω RF in the RF electric field, where E RF denotes the amplitude of the RF field, e the electron charge, m the electron mass, ν en the electron-neutral frequency, and ω RF the RF frequency. The first regime is typical right branch of a Paschen curve with A < L, where the effect of the secondary electron emission by electrons is negligible ( figure 9(a)). When A ∼ L/2, a significant number of electrons is lost to electrodes and the breakdown curve shows a pressure of turning point p t . In the case of A > L the pressure ranging between p min (pressure of a minimum point) and p inf (pressure of an inflection point), the surface processes play significant roles in determining the shape of breakdown curves. In this case, electron-electrode collisions produce greater effect in electron multiplication than the volumetric ionization (electron-neutral gas collisions), resulting in multi-valued regions.
For narrower gaps with A L (the Paschen curves of (4) and (5) in figure 9(b)), the RF gas breakdown evolves similarly to the one in the DC field at low pressure. Thus, the RF breakdown curve contains a second minimum corresponding to the DC Paschen curve located at pressures lower than p inf (approximately 1 and 2 Torr of curves (4) and (5), respectively, in figure 9(b)).
Jones proposed the similarity law in an RF breakdown [78], assuming that the discharge initiation voltage depends on the product of pressure and gap width pL and the ratio of RF frequency to pressure f /p determine the discharge initiation voltage. Moon et al identified the role of the driving frequency on the gas breakdown voltage and discharge mode transition (i.e. α to γ mode transition) in capacitively coupled plasma in helium atmospheric-pressure [79]. Lisovskiy described changes in discharge characteristics using the product of RF frequency and gap length fL in the modified Paschen curve depending on the aspect ratio (L/R) [80]. The similarity law for the RF breakdown voltage, which is a function of pL, L/R, and fL, considers the electron displacement amplitude and electron drift velocity in RF electric fields, various collision processes, and the radial and axial diffusion of electrons [80]; the validity of similarity law for the RF breakdown was verified for H 2 and Ar discharges, as shown in figure 10.
Furthermore, a theoretical study of the modified Paschen curve in the presence of RF field should be studied at the kinetic level [81,82]. Based on the consideration of the electron oscillation amplitude, the particle-in-cell Monte Carlo collisions (PIC-MCC) simulation aided in developing a modified breakdown condition for the breakdown voltage as a function of the driving frequency and pressure in the modified Paschen curve (figure 11). It is crucial that the electron oscillation amplitude is considered in the simulation of RF breakdown. When the electron oscillation amplitude is smaller than the electrode gap ( figure 12(a)), the charge reproduction rate in gas phase should be considered while the surface processes by electrons begin to play a role at low pressures ( figure 12(b)). Further, the theoretical study of the modified Paschen curve in a RF electric field has been extended to the microwave (mw) frequency range [83,84,184], and a universal breakdown theory encompassing the RF and mw frequency ranges has been proposed [85]. Note that these complex processes in the gas breakdown in AC electric fields can be analyzed making use of the nonlocal electron kinetics approach [25,[86][87][88], because the ionization process is a global (nonlocal) in an entire discharge volume in the nonlocal kinetic discharge regime [86][87][88][89][90].

Electric-field-induced secondary electron emission
In gas discharges at micro-to nano-scale, the electron tunneling effect at the cathode plays a significant role in a series of processes called field-emission effect [101]. Field emission is induced by a high electric field at the sharp tip of the microstructures/protrusions and can serves as a major additional current source in the breakdown. A local electric field can be created at the end of the nanostructure because of the uneven distribution of charged carriers. When a strong electric field is applied to a solid surface, the potential energy barrier at the surface is modified, and the electrons at the Fermi level can break through the energy barrier by tunneling and get released into the vacuum [102]. As the strength of the electric field increases, the thickness of the potential energy barrier reduces, resulting in an increase in the field Smooth transition in the current-voltage is observable only in the narrow gap experiment. The authors suggested that the field emission stabilizes the transition from pre-breakdown to self-sustained discharge. Reprinted from [118], with the permission of AIP Publishing.  [64], and is expressed as follows: where A and B are the empirical constants, and v and t are functions describing the image-charge effects. The current emission density equation implies that the current can be increased in several ways to maximize the field emission effect, i.e. by decreasing φ w or increasing E l . The local electric field is primarily a function of the aspect ratio of protrusions at the surface, i.e. nanowires, if nanowires are attached to the surface to increase field-emission effect. Unlike pure vacuum field emission, gas discharge creates a significant amount of charged particles that changes the existing electric field. This effect is particularly crucial when the positive ions are generated in the discharge at the cathode ( figure 13(a)). The ions not only bombard the cathode surface and induce the secondary electron emission from the surface of cathode, but also approaching ions greatly change the electric field near the cathode, allowing the electrons to easily pass through the potential barrier [109]. This is termed as the ion-enhanced field emission, which further accelerates the field emission. This mechanism adds the Fowler-Nordheim field emission equation to the Townsend theory, eventually leading to the expansion of the modified Paschen curve for the micro discharges. Kisliuk and Boyle conducted a theoretical study of field emissions and provided the concept of ion-enhanced field emission with an effective secondary emission coefficient in relation to the breakdown phenomenon [110][111][112].
The concept of modified Paschen curve originated from Germer's works on arcs with a small electrode gap [103][104][105][106]. Germer identified a modified arc discharge in a sub-micro gap (∼ 0.1-0.5 μm), where the field emission was dominant. The study of the Paschen curve in the micro gap was carried out by Torres and Dhariwal [107,108]. They presented a deviation of the Paschen curve when the gap dimensions reduce to less than ∼10 μm at a left branch of the voltage-gap curve ( figure 13(b)). Go et al established a mathematical model of the modified Paschen curve for microscale discharge, and posited that the mode change dominated by an ion-enhanced field emission breakdown occurs at a gap distance of less than 15 μm discharge for atmospheric air [35]. The mathematical description of the micro discharge explains the transition that occurs between the Townsend and field emission effects, contributing to the development of various theoretical formulas that provide solutions for the breakdown voltage [113,114].
Venkattraman et al completed a model considering the inherent relation between the ion-enhanced field emission and the ion-induced secondary electron emission using a simplified form of the Fowler-Nordheim equation [115,186]. In the models, the field-enhancement factor in the Fowler-Nordheim equation is shown to be the most dominant parameter with its increase leading to a significant drop in the breakdown voltage. In addition, the Fowler-Nordheim equation was used to describe an electron current source in PIC-MCC studies, and it was revealed that self-sustained discharges with significant ion-enhanced field emission acts as a major charge generation mechanism in small gaps [116,117].
Bilici et al [118] reported a reversible smooth transition from the field emission to a self-sustained plasma for a discharge with an electrode with a microstructure and observed hysteresis in the current-voltage characteristics for a large electrode gap where the field emission was negligible ( figure 14). The research includes analytical theory for gas breakdown in micro gap discharges [119], gas breakdown in discharge driven by pulsed voltages [120,121], sub-atmospheric pressure discharges [122], effect of electrode-surface condition on microscale breakdown [123],  confirms that the recorded current is dominated by the field emission. From [101]. Reprinted with permission from AAAS. and discharges with asymmetric electrodes [124]. Agreements between theoretical predictions and experimental data therefore explaining the phenomena observed in different experimental studies have been achieved, and the combination of the field emission and Townsend theory has been developed into a universal theory that describes well various discharge breakdown mechanisms including the range of quantum spacecharge-limited emission to classical gas breakdown [119,125]. Advanced studies describing changes in the secondary electron emission and field enhancement factors caused by the material properties of electrodes, such as morphology and roughness, are expected to provide a generalized universal theory. Although the field emission effect is a dominant factor to initiate the breakdown for small gaps, the effect of microparticles (or even viruses and bacteria) suspended in the discharge area should not be overlooked because they enhance the electric field between the microparticles and cathode in mm-scale N 2 gap discharge [126]. Recent studies have reviewed the detailed theoretical and experimental works focusing on the description of field-emission-driven microscale gas breakdowns [127,128] and contributions of thermionic and space-chargelimited emissions [129].

Other effects on secondary electron emission effects
Under the conditions of high voltage and low-pressure breakdown, additional elementary processes associated with fast ions and fast neutrals produced in charge transfers collisions are necessary to take into account to the experimental data. In addition to the electron impact ionization and secondary emission of electrons by ions, different elementary processes can affect breakdown at low pressures. These processes create completely different turning points on the left-hand branch of the Paschen curve, and are mainly observed in helium or mercury vapor discharges [130][131][132]. Hartmann et al investigated the breakdown in low-pressure helium gas considering the ion impact ionization at high electric fields and secondary electron emissions from the cathode by fast neutral atoms [132]. The results showed that both the fast atoms initiate processes, and He + impact ionization has a significant effect on the breakdown curve ( figure 15).
In recent years, the left-hand branch of the Paschen curve for helium has been studied. Experimental studies and simulations [133] of helium breakdown in the range of 100-1000 kV at pd < 1 Torr cm ( figure 16) and a quasi-analytical model for voltages ranging between 10 kV and 1000 kV [134] have been performed. Both studies revealed that the anisotropic scattering of ions and fast atoms plays a major role in the dynamics of breakdown and the formation of Paschen curve.
In summary, sections 2 and 3 discussed the status of theoretical and experimental research of the gas breakdown phenomena. We introduced the Townsend theory, which is the foundation of gas breakdown physics, and presented the relevant discharge characteristics observed in DC, RF, and micro discharges. Gas breakdown essentially requires the following processes: volumetric ionization and secondary electron emission. The mechanism of particle generation and loss can be modified by the discharge configuration, structure and morphology of electrodes, and driving frequency of the applied voltage form. Analyzing the discharge characteristics using the similarity laws or scaling laws allows for better understanding the modified Paschen curve. These principles are immensely beneficial to understanding of the operations of ionization gas sensors and improving of their performance, because these sensors are essentially scaled down to micro/nanometer scales. In addition, knowledge of the characteristics of nanomaterials and the change in discharge characteristics caused by their use in electrodes are considered as essential elements required for developing ionization gas sensors.

Gas sensors (ionization gas sensors)
Ionization gas sensors that operate based on fingerprinting the breakdown voltage of a target gas using nanoscale materials are being rapidly developed for academic and industrial applications. To ensure the safety of industrial processes and preserve daily life, the most important performance expected of gas sensors is the fast and accurate selective detection of corrosive, flammable, explosive, spontaneously combustible, and toxic gases. The technology behind ionization gas sensors utilizes many of the discharge effects that were introduced previously. Therefore, to detect various types of gases, the ionization gas sensors must be designed considering the Townsend discharge theory and the modified Paschen curve, which were reviewed in previous chapters.
Among the operating characteristics of nanomaterial-based ionization gas sensors, ion-induced field emission is one of the most dominant factors affecting performance of the ionization gas sensors. The advantages of minimizing the required driving power by using the field emission and unique breakdown voltage characteristics of each gas can be combined, enabling  1 μm). The results indicate that the gap-sensitivity of thresholds can be an additional factor to design ionization gas sensors with multiple arrays. Reprinted from [152], with the permission of AIP Publishing. the ionization gas sensors to selectively and efficiently detect various types of gases. Therefore, increasing the field emission is a key performance objective. Accordingly, inventing new nanomaterials and surface structures has been the core of research and development (table 4).
Numerous studies have shown that the physicochemical properties of materials play a significant role in improving the performance of ionization gas sensors. Identifying controllable materials and optimizing the effect of field emission to determine appropriate discharge characteristics are crucial steps in achieving optimal performances. Following the initial discovery of field-emission-based ionization gas sensors using carbon nanotubes, several studies have been conducted to enhance the performance of gas sensors (e.g. sensitivity, selectivity, etc) by integrating nanowires or nanorods made from various materials such as metals, metal oxides, and silicon (table 4).
In the following chapters, we discuss the development trends of nanomaterial-based ionization gas sensors. These sections are organized according to nanomaterial type. In each section we discuss the sensor operation characteristics related to the gas breakdown theory.

Carbon nanotube-based ionization gas sensors
Field-emission electron effect using carbon nanotubes was initially proposed by de Heer et al (figure 17) [101]; since then, numerous studies have reported the yield of nanotubes, their aspect ratios and wall thicknesses [135][136][137][138], growth mechanisms [139,140], and field emission properties [141][142][143][144][145][146]. The ability to emit cold electrons at a low voltage, as well as good mechanical stability, have contributed to research development in the application fields of field-emission displays, vacuum microelectronic devices, and x-ray sources [147][148][149].  By adopting the field emission effect, Modi et al developed a miniaturized ionization gas sensor using carbon nanotubes [1], which overcame the limitations of low power efficiency and bulky structure of conventional ionization gas sensors. The anode and cathode of an ionization gas sensor consist of vertically aligned multiwalled nanotubes and Al plates separated by a glass insulator at the edges. Application of multiwalled nanotubes to an electrode lowers the breakdown voltage of He, Ar, CO 2 , N 2 , O 2 , and NH 3 gases to several hundred volts ( figure 18). Since then, follow-up studies focusing on the breakdown voltage have been widely conducted. Kim proposed a fabrication technology for carbonnanotube-based ionization gas sensors and succeeded in developing a sensor with negligible current change for 24 h at a constant vacuum pressure and static voltage (1 kV), ensuring sensor compactness and reliability [150]. Carbon nanotubes using micro-electromechanical systems (MEMS) have contributed to achieving low voltage operations through sensor miniaturization. Hou et al [151,152] introduced MEMS in the form of hollow slot electrodes with carbon nanotube sidewalls ( figure 19). The discharge initiation voltage of mixed gases with He, CO 2 , and air at a gap interval of 6 μm was less than 20 V (figure 20), thereby exhibiting an enhanced integration compatibility accompanied by high reproducibility [152].
Without the assistance of MEMS, gap distance was controlled by polyimide films (spacers) [153], which resulted in the discharge of NH 3 with a breakdown voltage of only 18.8 V and at a gap spacing of 7 μm ( figure 21).
Tunneling field-ionization characteristics can be used as indicators for fingerprinting various gases based on the Fowler-Nordheim emission theory. The performance indicators of ionization gas sensor are evaluated in the corona discharge of the dark discharge regime, which is a non-self-sustaining discharge regime. Gas sensing through Fowler-Nordheim plots has high power efficiency and sensitivity because the process does not require an applied voltage till the attainment of breakdown voltage [154,155]. In particular, pre-breakdown current measurements were used to detect gases that require high ionization energy [154]. By using the Fowler-Nordhiem plots, the performances of carbon nanotube arrays grown on porous silicon were analyzed [156,157]. Li et al tried to enhance the uniformity of a carbon nanotube array on a substrate using a glass substrate with phosphor coated as an anode [156]. The emission current achieved was 1 mA cm −2 at a field of 9.5 V μm −1 . The fluctuation of the emission current density reduced to less than 5%, emphasizing the possible development of carbon nanotubes into stable semiconductor devices.
The field emission properties of multiwalled nanotubes can be used to effectively detect humidity by observing the prebreakdown current variation at relative humidity levels. Hui et al utilized a data analysis system using the cubic spline interpolation algorithm to evaluate the performance of the ionization gas sensor [158]. The study evaluated inter-electrode distances, environmental factors, and gas mixture detections.
The new analysis method suggested that the sensitivity of a sensor corresponding to environmental factors (temperature and relative humidity) can be accurately measured. The proposed data analysis method effectively observed the elevation of breakdown voltage caused by a large number of negative ions generated by water vapors.
Recently, ionization gas sensors of various structures have been proposed. The gas sensors are composed of a multiwalled carbon nanotube cathode, an extracting electrode, and a collecting electrode [159,160]. Each electrode had opposite polarity based on the extracting electrode, and the ion currents accelerated by the collecting electrode were measured ( figure 22). As NO concentration is increased, metastable states of N 2 were consumed by quenching, and this leads to the decrease of positive ion density [160]. In addition, the electron emission was reduced by NO gas adsorption on multiwalled nanotubes, that strongly affected the field emission. As a result, the ionization rate is reduced, and a slow response of gas sensor is exhibited. Eventually, the extracted ion current collected by the collecting electrode decreased with the increase in NO concentration, and it was considered as an indicator of the sensor.
micrographs of multiwalled carbon nanotubes with 20 nm diameter and 5 μm length. (c) A simplified circuit diagram showing the collection of positive ions and minimization of ion bombardment on the carbon nanotube. (d) Gas sensing properties in the N 2 -NO mixture at atmospheric pressure with varying extraction voltage range of 80 V to 150 V and with fixed colling voltage of 10 V. Collecting current versus NO concentration graph shows intrinsic sensitivity to NO. Reproduced with permission from reference [160]. Copyright 2015 AIP Publishing.
Further, the sensor with tripolar-electrode structure distinguishes gas species through a non-self-sustaining discharge current with desirable stability, exhibiting excellent performance in terms of lifetime, and stability without damaging (c) The emission current density versus electric field property was investigated several times; the current density was stabilized because of the field annealing effect. The Fowler-Nordheim plot (inset) exhibited a linear behavior. Reprinted from [170], with the permission of AIP Publishing. effect of breakdown. Contrary to the conventional gas sensors such as chemiresistors or field effect transistors with response and recovery time of hundreds of seconds, the gas sensors of tripolar-electrode type show a stabilization time of several seconds at room temperature [162]. After all, the gas discharge time is fast, but it needs a relatively long time for stabilization due to adsorption compared to other gas detection. Accordingly, for faster time characteristics, it is expected that the effect of NO gas adsorption on multiwalled nanotubes should be minimized.
A room-temperature deposition method is developed to obtain uniform carbon nanotube networks. Vacuum filtration, which is a room-temperature deposition method [163], enables the creation of uniform carbon nanotube networks. In this method, a carbon nanotube film is deposited on a nitrocellulose membrane via vacuum filtration, and then O 2 plasma etching and wet etching procedures are performed. Overcoming the disadvantage (i.e. hysteresis characteristics in the detection of NO 2 ) of existing sensors with a non-suspended architecture, humidity sensors with suspended nanotube beams exhibit excellent response and recovery times that are nearly three times lower than that of the former without any chemical modification to the nanotubes [163] ( figure 23). In addition, the horizontally aligned billions of carbon nanotube beams are suspended over a metal electrode at a height of 3.6 μm, and the sensor exhibits the ability to detect He, N 2 , Ar, and air mixture gases [164].

Metal and metal oxide-based ionization gas sensors
Sadeghian and Kahrizi developed the nanowire or nanorodbased ionization gas sensor, which complements the shortcomings of the oxidation process and durability of carbon nanotube-based sensors [165]. The electrochemically grown gold nanowire-based ionization gas sensor consists of two parallel plates containing an electrode equipped with a freestanding array of metal nanowires ( figure 24). Similar to carbon nanotubes, gold nanowires with high aspect ratios generate strong non-linear electric fields. The change in dark current observed through the current growth graph indicated that the field emission dominates a non-self-sustaining Townsend discharge [166]. The gold nanowire detected low gas concentrations (pressures) with a small breakdown voltage, indicating higher sensitivity than carbon nanotubes.
To lower the field ionization threshold voltages, freestanding gold nanowires or rods terminated with nanoscale whiskerlike features were developed [166,167]. Experiments with helium gas confirmed the field-limited ionization current at approximately 10 V, which was less than the breakdown voltages in carbon nanotube-based sensors. The enhanced field emission was interpreted as the result of the combination of whisker-like features and the presence of amorphous alumina residues. In addition, sensors based on the low work function of Au exhibit exceptional field emission properties [168]. Gold nanowires possess an advantage of being resistant to degradation by oxidation. However, they are cost-intensive and require complex synthesis processes in terms of commercialization of the technology. The monolayers of diamond can dramatically enhance field emission properties to metal surfaces through the stable radial cation of diamondoid [169]. The diamondoid-modified Au surfaces overcoming the limitations of the poor conductivity of the diamond film showed an exceptionally low work function of approximately 1.6 eV with excellent moisture-and air-stability and high thermal stability (figure 25).
Zinc oxide (ZnO) has desirable qualities in terms of oxidation, high electric fields, and cost-effectiveness, lack of which are the disadvantages of previous materials. Zinc oxide has a wide band gap of 3.37 eV at room temperature with an exceptional chemical stability; thus, various studies have investigated gas sensors using ZnO nanowires. Lee et al performed detailed studies on the field emission properties of ZnO The stability test of a ZnO nanowire and carbon nanotube. The voltage fluctuation of ZnO nanowire was less than 5% while that of the carbon nanotube was more than 200%. The SEM image of ZnO nano array (d) did not exhibit significant changes whereas that of carbon nanotube array (e) collapsed. Reproduced from [180]. © IOP Publishing Ltd. All rights reserved. nanowires grown on a silicon substrate [170]. The nanowires showed an intense emission current density of 1 mA cm −2 at 11 V μm −1 . The emission current-voltage characteristics analyzed using the Fowler-Nordheim equation showed a turn-on voltage of 6.0 V μm −1 at a density of 0.1 μA cm −2 ( figure 26). This study focused on the brightness of ZnO field emitters for an efficient display usage (glass-sealed field emission display).
Liao et al conducted a detailed study on ZnO properties and considered ZnO as the primary element for ionization gas sensors [180]. Experimental studies were performed on the gas species, gap length, He concentration, and stability. The results were compared with those of carbon-nanotube-based sensors. The results of studies on He, NO 2 , H 2 , CO, air, and O 2 showed that the overall breakdown voltage was appreciably higher than that of carbon-nanotube-based sensors; however, the voltage decreased as the inter-electrode spacing decreased from 25 μm to 15 μm (figure 28). The stability and anti-oxidation factor of the ZnO sensor were superior to those of carbonnanotube-based sensors, even though the breakdown voltage was high. The results indicated ZnO nanowires as a preferrable candidate for ionization gas sensors.
Wang et al indicated that the breakdown voltage was higher than that of the carbon nanotube because of the weak fieldemission effect caused by the large radii of ZnO [181]; they proposed an ionization gas sensor that used Pd nanoparticlecapped ZnO nanorods as an anode to reduce the relatively high breakdown voltage of the ZnO nanowire-based ionization gas sensor. The aspect ratio of the nanorod was 50, similar to that of the ZnO wire in the previous study. The newly proposed sensors achieved a reduction in breakdown voltage of approximately 10% and exhibited a stable performance even in mixture gases ( figure 29).
An ionization sensor using a conductive yttrium-doped ZnO nanorod array was developed [182]. The aspect ratio of the nanorod was changed through the Y/Zn molar ratio, and a nanorod with a higher aspect ratio was generated through the increase in dopant concentration. The discharge start voltage was measured in air, O 2 , N 2 , CO 2 , CH 4 , and Ar. A maximum voltage reduction of approximately 20% was achieved. Sensitivity and stability were also improved, which was explained by an increase in the conductivity of the Y-doped ZnO and the inertness of metal oxide materials.
ZnO nanowires grown on microelectrodes were effective in reducing the breakdown voltage and maximizing the field emission effect [161]. To maximize the field emission effect, tungsten was coated on the nanowire, which reduced the corona inception voltage and increased the ion concentration of the sensor by 10 17 m −3 (figures 30 and 31).  MEMS-based gas sensors with ZnO nanowires achieved a low operating voltage with fast response and recovery times (figure 32). The fabrication of the planar ionization sensor was based on the selective and seedless growth on Au electrodes. The semi-linear response of the sensor with the addition of NH 3 to pure N 2 from 0 ppm to 1000 ppm has performed excellently as an ammonia gas sensor [183].
Based on the observation that the ZnO wire-based sensor had a relatively high discharge initiation voltage, a study on field emission properties and their application to ionization gas sensors based on the high aspect ratio of CuO and Cu 2 O nanowires was conducted [187][188][189]. These exhibited short response and recovery times, which were attributed to the rapid chemical process of gases on the wire. Specifically, the current-voltage curve in the mixed gas possessed two transition regions (figure 33). Molybdenum oxide wire was used as a field emission source. The measured turn-on field was approximately 3.5 MV m −1 , which was higher than that of carbon nanotube or SiC nanowires [190]. However, the wire was favorably competitive with high durability and conductivity. Tin dioxide (SnO 2 ) can also be produced as a wide band gap semiconductor (3.6 eV at room temperature), and its field emission properties resemble metal wires in various morphologies [191].
Very recently, the discharge characteristics of ionization gas sensors using manganese (Mn) sculptured thin films were investigated [192][193][194]. Mn helical nano-sculptured thin films with nano-flower are fabricated on top of helical nanosculptured stem and pillars [192]. Then, changes in the breakdown voltage are investigated with various gap distances in various gases (air, O 2 , N 2 , CO 2 , and Ar), as well as a wide range of gas pressures (0.75-750 Torr). Ionization sensors using Mn nano-flower sculptured thin film as a cathode and a stainless-steel ball as an anode have lower breakdown voltage and better selectivity than existing devices using carbon nanotube, Cu, Au, Ag, ZnO as electrode material at medium ranges of gas pressure (0.015-0.15 Torr) [194].

Silicon-based ionization gas sensors
Generally, research on the field emission and ionization gas sensors focuses on metal oxides with high durability and high field ionization current. Nevertheless, the fabrication of nano-scale ionization gas sensors is an essential basis for silicon substrates. Recently, field emission properties have been investigated by etching silicon substrates. Therefore, it is essential to consider the field emission phenomenon in the silicon nano structures of various configurations. The field emission characteristics based on the Fowler-Nordheim equation of silicon nanowire were investigated by Au et al [195]. As the diameter of Si nanowires was reduced from 30 nm to 10 nm, the turn-on field decreased to 13 V μm −1 and was comparable to the value reported for carbon nanotubes. Zeng et al demonstrated the increase in electrical contact and mechanical bonding between the Si nanowires and substrate through annealing at high temperatures. The nanowires and substrate were improved, and the turn-on voltage was reduced to 3.4 V μm −1 [196]. The field emission property for oriented SiC nanowire was 10 mA cm −2 at low applied fields of 2.5-3.5 V μm −1 [197]. Recently, the dense array nano fabrication of high-aspect-ratio silicon nanowires has achieved a high current density (100 A cm −2 ) with long operating lifetime (>100 h) [198], and advanced fabrication and driving methods have been applied to various applications. The high performance of silicon nanowire is recognized in [199][200][201][202][203]. Wang mentioned that the recent gas sensor studies were limited to low-pressure operations; a gas sensor utilizing a silicon microneedle array for applications under practical conditions was proposed. The gas sensor showed fast recovery rates targeting high concentration volatile organic compounds [204].
Karaagac and Islam reported that the field ionization was enhanced in gold-coated ultra-sharp silicon nanowires produced through an Ag-assisted electroless etching technique [205]. Sadeghian and Islam [2] observed an anomalous enhancement of field-ion current in gold-catalyzed whiskered silicon nanowires ( figure 34). Compared to other metallic tips, the required electric field and measured current were weak in this case (maximum of 0.018 × 10 8 V cm −1 ). These current and electric field characteristics were described using geometrical field enhancements and surface-state models based on the field amplification effect of suspended gold nanoparticles present on the whiskered tips.

Future challenges and perspectives
Ionization gas sensors developed using various materials and structures of nanomaterials have been introduced to satisfy the requirements of low-voltage operation with a high signal-to-noise ratio and to improve the long-term stability, repeatability, sensitivity, and selectivity. Recent advancements in fabrication technologies have facilitated the creation of complicated surface structures. Theoretical and experimental studies on discharge characteristics, including all electron emission properties of nano-and micro-sized electrodes with complex shapes, are being conducted. Although numerous theoretical and experimental studies are being conducted to develop micro discharge and ionization gas sensors, several challenges and perspectives remain that must be addressed to realize effective and practical applications.
The operating mechanism of ionization gas sensors that require a constant power supply acts as a potential challenge for applications in long-term missions or in the field of compact wearable devices. The technology for energy harvesting systems can be applied to the ionization gas sensor operating at low power. The nanogenerators (for example, piezoelectricity [206,207], pyroelectricity [208], and triboelectricity [209,210]) can be utilized as electricity sources for the miniaturization of ionization gas sensors. For example, a triboelectric generator, which converts the mechanical energy    into electrical energy, can be a representative example that unfolds the possibility of being used as a self-powered sensor (figures 35 and 36). In the generation and operation environments of high breakdown voltages and low field emission currents, there is a need to deepen the research and consider power generation efficiency, time stability, and durability of nanogenerators [211][212][213][214][215]. Eventually, an in-depth analysis of microscale discharge can contribute to the development of gas sensors and high-performance self-powered sensors.
In manufacturing nanomaterials, understanding plasma physics is essential for improving the performance of the ionization gas sensors and simplifying the manufacturing process. Plasma-enhanced chemical vapor deposition (PECVD) or plasma etching has the advantage of being able to fabricate nanomaterials at a relatively low synthesis temperature [140,[216][217][218]. In particular, the PECVD process that utilizes catalyst particles helps in the uniform growth of nanomaterials. Knowledge of the sheath formation at the boundary between the plasma and material acts as an essential factor in predicting the nanomaterial growth and analyzing the process results.
For instance, in the fabrication of vertically aligned carbon nanocone arrays, the PECVD process in a nickel-catalyzed silicon substrate requires adequate knowledge of the catalyst particles and the sheath electric field formation in the nanocone (figure 37).
Classical molecular dynamics simulations provide specific plasma effects and allow for prediction of the growth process in PECVD of single-walled carbon nanotube on Ni catalyst particles [219,220]. For example, Neyts et al demonstrated that the applied electric field has a strong influence on the catalyst-reactant interaction (figure 38). They showed that the applied electric field (hundreds of kV cm −1 ) results in alignment of the growing of carbon nanotube, and transport of positively charged carbon ions along vertical nanowalls becomes enhanced by local electric field in the sheath, which also promotes the vertical growth of carbon nanotubes.
Typically, a novel process for nanoparticle creation is required in advance to grow vertical nanomaterials. Understanding the discharge kinetics of plasma enables the (a1)-(c1) Display images of different plasma environments, such as discharge types (inductively coupled plasma (ICP) or capacitively coupled plasma (CCP)) and different gases (Ar or O 2 ). (a2)-(c2) The ion energy distribution function for each plasma environment. The ICP presents a low ion energy bombardment on the amorphous carbon layer wafer, while the ion energy distributions have a bimodal shape with a high energy peak in case of the CCPs. (a3)-(c3) Cross-sectional view of FE-SEM images of amorphous carbon layers after plasma etching for 7 min in each plasma environment. (a4)-(c4) Top tilted view of FE-SEM images in each plasma environment. The quantum-dots (nano-dots) were obtained with the O 2 ICP, while the vertical nanotips were generated with the Ar CCP with a high ion energy distribution. Interestingly, nanowhiskers were produced in the O 2 CCP. These results indicate the electric field of the sheath is a crucial factor to the fomation of the vertical nanotips, which are essential to the electrode structure of the ionization gas sensors. Reprinted from [221], Copyright (2020), with permission from Elsevier.
controlling of various structures (e.g. quantum dot, nanowhisker, and sharp-tip) of nanomaterials without the aid of any catalyst through self-structuration during etching of an amorphous carbon layer (figure 39) [221]. Controlling the plasma environment results in independent or combined effects of ion energy and radicals on the formation of nanostructure [222] and in modulation of the surface morphology, chemical components, and overall structure of nanomaterials [223,224].
Therefore, it is believed that the interdisciplinary studies of the fundamental physics and chemistry of plasma sources and advanced material engineering are needed for future progress in this area.
The performance of an ionization sensor is related to the field emission and Townsend avalanche, where the secondary electron emission coefficient of the nanomaterial plays a significant role. The interpretation of the experimental data and a theoretical approach for nanomaterial-based breakdown inevitably requires an emission coefficient suitable for each plasma environment. However, most studies consider only the secondary electron emission coefficients for high ions or neutrals with high energy. The electron induced secondary electron emission (EISEE) due to the energy of tens of eV has an emission factor of more than 1 [225][226][227]. Thus, database of the electron energy dependent EISEE is strongly required. Moreover, ion induced and/or electron induced secondary emission coefficients vary depending on the surface material characteristics (roughness, surface shape, and oxidation degree), and a database obtained through quantitative measurements is highly required.
Gas sensors can maintain safe environmental conditions for workers by establishing a hazardous gas monitoring system in various industries such as mining, oil and gas, food manufacturing, and pharmaceuticals. Because of the expansion of application fields, gas sensors must ensure the performance stability in extreme conditions such as temperature, pressure, humidity, and radiation. As the use of hazardous substances and toxic gases continues to increase with the development of high-tech industries of semiconductors and nuclear power plants, the social awareness regarding safety and environmental protection is being increasingly emphasized, and accordingly, the regulations and safety standards are becoming increasingly stringent. Hence, the gas detection system of the future must quickly and accurately measure low concentrations of the target gas and consistently provide reliable operation performance for a long time using an efficient power supply source. In the case of conducting polymer gas sensors, stability has been proven through space station experiments over several years, while studies on the stability of ionization gas sensors are limited to several hundred hours of tests. Thus, long-term evaluation of ionization gas sensors is necessary for the practical usage at a commercial level.
Differences in performance indicators in each group and the absence of a consistent evaluation standard can act as institutional weaknesses in the development of ionization gas sensors. The foremost priority must be the standardization of the evaluation criteria such as low detection limit, reproducibility, precision, and response time via field emission properties. Therefore, an urgent need exists for a standardized measurement technology that can quantitatively evaluate the quality of ionization sensors.

Conclusions and outlook
The gas breakdown physics and the associated features of nanomaterial-based ionization gas sensor technology were reviewed. Based on the similarity law of scaling, theoretical and experimental studies on the discharging mechanism were systematically introduced from the Paschen curve of a macroscopic scale of DC discharge to the modified Paschen curve of microscopic scale of RF discharge. Among the various characteristics of gas breakdown physics, local field enhancement in nano-and micro-morphologies has greatly contributed to reducing the breakdown voltage, overcoming the limitation of conventional ionization gas sensors. By discussing ionization gas sensors developed using materials ranging from carbon nanotubes to silicon microneedles, the basic research trends on field emission properties of each material and the latest research trends on applications of ionization gas sensors were reviewed. In response to the demand for developing ionization gas sensors that highlight both the engineering advantages and economical aspects, such as cost-effectiveness and durability, we introduced metal-oxide-based ionization gas sensors with their various nanostructures. Finally, we presented the future research challenges, such as requirements for efficient power systems, investigation of advanced nanomaterials based on the fundamental understanding of the plasma physics and source science, and standardization of evaluation criteria. The development of an optimal nano-micro ionization gas sensor, with the aim of diversifying detection gases and operating environments, will improve the versatility of ionization gas sensors and aid in improving the quality and safety standards of life.