When using CNTs as electrode material of the Gas ionization sensor, the first mechanism needed to be realized is electron field emission. Electrons would receive enough energy to tunnel through the bended potential barrier caused by the high electric field around sharp tips of CNTs. After their tunneling, electrons might have the impact ionization with neutral molecules and generate lots of charged particles between two electrodes. And electrical gas breakdown would take place when achieving enough amounts of electrons. The total operation principles of gas ionization sensor can be described through the derivation of Townsend’s discharge [32,33] and Paschen’s law [34,35], which illustrate the relationship between Gas ionization sensor breakdown voltages and the gas pressure along with the distance between anode to cathode.
1-3-1 The Mechanism of electron emission
Generally speaking, it’s not straightforward for an electron to escape from the surface of materials. A potential barrier (so-called Fermi level) exists at the surface of materials which prevents the electrons from escape unless certain conditions are satisfied so that electrons upon the metal could emit into the surrounding vacuum or
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gases environment. That is, the binding electron is not able to flee away from the binding energy band of the atom’s surface unless sufficient external energy is provided. The supplied external energy must be higher than the work function of electrode surface material (defined as the work needed to remove an electron from the surface). As electrons receive specific external energy to cross the potential barrier of the work function and reach the vacuum, this phenomenon is called electron escape or electron emission.
In general, the electron emission mechanism can be classified into two types, thermionic emission and field emission. Their operating mechanisms are to use either temperature or electrical field to provide the external energy of electron emission, respectively. These two mechanisms can be described by the band diagram (Fig. 1-7).
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This happens because the thermal energy applied to the carrier overcomes the potential barrier, also known as work function of the metal (Fig. 1-7 (a)). In the beginning, all electrons were bound under the Fermi level (EF) when the temperature is at 0°K. However, part of electrons might acquire kinetic energy from the thermal heating as the temperature increases gradually. When the temperature is high enough, electrons might escape from Fermi level to Vacuum level with high probabilities. This mechanism accomplishes the electron emission without applying any bias voltages at the cost of the thermal energy supplied. In most cases, the thermionic electron is emitted under considerably high temperature, which is based on the different work functions of different materials. The average temperature is about 1500 to 2000 ℃.
On the other hand, the field emission (FE) (also known as Fowler-Nordheim tunneling) is an emission mechanism of electrons induced by external electric fields, which implies that heating was not necessary for the cathode materials. Ab initio,
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most of electrons would remain under the Fermi level at low temperatures. The external electric field would lead to band bending of the Vacuum level, which induces gradual narrowing of the effective potential barrier width (Fig. 1-7 (b)). As soon as it was thin to some extent, the electron tunneling effect occurred and the electron might tunnel to the Vacuum level, namely the well-known Fowler-Nordheim tunneling.
Because this mechanism provides the main escape energy of emitting electrons to the vacuum level by electrical field, it is thus called the field emission.
The throughout thesis will focus on this mechanism of electron emission as our main research issues.
1-3-2 Electron field emission
In quantum mechanical, electron field emission is a tunneling phenomenon of electrons extracted from the conductive solid surface, such as a metal or a
semiconductor, where the surface electrical field is extremely high.
If a sufficient electrical field is applied on the emitter surface, electrons will be emitting through the surface potential barrier into vacuum, even under a very low temperature. On the other hand, thermionic emission is the hot electron emission under high temperature and low electric field. (Fig. 1-8 (a)) demonstrates the band diagram of a metal-vacuum system.
Here W0 is the energy difference between an electron at rest outside the metal and an electron at rest inside, whereas Wf is the energy difference between the Fermi level and the bottom of the conduction band. The work function φ is defined as φ = W0 - Wf. If an external bias is applied, vacuum energy level is reduced and the potential barrier at the surface becomes thinner as shown in (Fig. 1-8 (b)).
Then, an electron having energy “W” has a finite probability of tunneling through
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the surface barrier. Fowler and Nordheim derive the famous F-N equation (eq. 1.2) as follow [36]: the emitting area and β is the local field enhancement factor of the emitting surface, the following equation can be obtained
exp[ ( ) ]
from Eq. (1-6), the slope of a Fowler-Nordheim (F-N) plot is given by
2.97 10 ( )
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The emission area α can be subsequently extracted from a rearrangement of (eq. 1-6)
6.53 10 )
where r is the tip radius of emitter tip, d is the emitter-anode(gate) distance and β` is a geometric correction factor [38].
1-3-3 Operation principles of electron impact ionization
For the application of Gas ionization sensor, an electron would move along the electric field to the anode after it was emitted from the cathode. However, if the distance from anode to cathode is much larger than the average mean free path of an
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electron, the electrical field may induce accelerated electrons to collide with neutral molecules or atoms in its path to anode. Normally, there are three kinds of reactions for an electron to impact on a neutral molecule, which are the dissociation reaction (eq. 1-12), the activation reaction (eq. 1-13) and the ionization reaction (eq. 1-14).
e X 2X e (1-12) e X X e (1-13) e X X＋ 2e (1-14)
The most notable reaction among the above is the ionization reaction (eq. 1-14), which depicts the increment of the number of electrons after the reaction. An ionized electron would be produced whenever an effective collision occurs. Moreover, an ionized positive ion would receive kinetic energy from the acceleration of electric field and collide with the cathode, which would produce more ionized electrons. And the electrical gas breakdown would take place when the amount of ionized electrons was up to a certain quantity.
What's more, the excited atom of (eq. 1-13) would emit photons and then return to ground state which could be expressed as (eq. 1-15),
X X hν (1-15) where h is the Plank Constant and ν is the frequency of radiation light. That’s the reason why spark and glare can be seen when the electric gas breakdown occurs on the microstructure of electrodes with high voltages applied (Fig. 1-8)[39].
1-3-4 The fundamental mechanism of gas ionization breakdown
The magnitude of breakdown voltage of gas ionization sensors is based on Paschen’s law, which points out that the breakdown characteristics of a gap between two electrodes are a function (generally not linear) of the product of the gas pressure and the gap length, which is usually denoted as Vbr= f( pd ). Nevertheless, before this
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law was deduced, the mechanism of applying high voltage among anode and cathode has to be described below.
From (Fig. 1-10)[40], it shows that the processes of gas ionization via positive nanotip could be separated into three parts, which are molecule ionization via nanotip, impact ionization effect, and electron recombination along with γ-process[41]. As one can see in (Fig. 1-10), in process 1, when CNTs were used as positive electrode, the high voltage among anode and cathode would generate a strong local electric field around nanotips, where neutral molecule was ionized and an electron was released simultaneously. The ion would move to the negative electrode under the electrostatic force and the electron would be absorbed concurrently on the positive nanotip. Here suppose that the ionization rate induced by the ion’s moving to the negative electrode is small so that it could be ignored. On the surface of the negative electrode, the ion would recombine with an electron and revert to molecule. However, the release energy of recombination and the kinetic energy of the ion would promote the electron emission from the negative electrode in the process III. This process is the so-called γ-process. The electron released from the negative electrode would get enough energy to make other gas molecule ionized during its moving to the positive electrode. This is the electron impact ionization effect (process II). Thus, the pre-breakdown current was generated mainly by the molecule ionization via positive nanotips when utilizing CNTs as the positive electrode. The current could be continued and magnified in processes I and II to result in high current to cause electric breakdown. Therefore, the application of CNTs as positive electrode could enhance the gas ionization process, lower the working voltage, and improve the sensitivity of the sensor.
When CNTs were used as negative electrode, as shown in (Fig. 1-11), in process I, electrons would emit via negative nanotips. Then they move to the positive
electrode while obtaining kinetic energy from electrical field. As soon as they receive
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enough energy to make a molecule ionized, the impact ionization occurs and produces more electrons and ions as desired. This is the process which we mentioned above (process II). Similarly, the ions also induce recombination of electron and γ-process (process III). These three processes would duplicate unceasingly and the breakdown would occur when the number of generated electrons was sufficient. Therefore, the application of CNTs as negative electrode would enhance the emission current, lower the working voltage, and improve the sensitivity of the sensor.
1-3-5 Townsend discharge and Paschen’s law
As mentioned above, an electron moves beyond the average mean free path along the electrical field might acquire sufficient kinetic energy to ionize a neutral molecule into a positive ion and a free electron. This means that the number of free electrons get doubled if we take the original colliding electron into account. As this mechanism proceeds again, these two electrons might collide with two neutral molecules and become two positive ions and four electrons in totality. Therefore, the number of charged particles increases exponentially like a snow avalanche as it repeats continually, which is the well-known Avalanche breakdown effect.
As depicted in (Fig. 1-12), assume the number of electrons after impact ionization is Ne, then
Ne 2 、2、2、2、2、2、2、2… (1-16) Note that the number of collisions is related to average mean free path λ. Suppose that the distance of electron to the anode is X, then Ne can be described as
Nex 2
λe
λe
α(1-17), where α is the Townsend’s first ionization coefficient, which tells the average number of ionizing collisions are made by an electron as it travels 1cm in the direction of the
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electric field.
And
α
λAp exp
!" (1-18).Here A and B are the constants which depend on gaseous species. E is electrical field strength and p is gaseous pressure. From (eq. 1-17), one can easily realize the
exponential relationship between Ne and α. That is, Ne will grow exponentially as the value of α increase. This is the well-known Townsend discharge which illustrates that initially a very small amount of free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by the avalanche multiplication.
In order to describe the mechanism of gas breakdown, we have to assume that γeα#$ 1 electrons will be generated when eα#$ 1 positive ions impact on the cathode. They are referred to the Secondary electrons.
Now, if the electrical field flux is Γ&, the unit volume density is n& and ν& is the electron velocity, then
Γ& n&ν& (1-19), The unit of Γ& is [m· s], n& is [m], ν& is [m · s], and
ν& µ&· E (1-20), where µ& is the electron mobility, E is the magnitude of electrical field between two electrodes.
Therefore, the current between two electrodes (I) is
I $|e|n&µ& $|e|Γ& (1-21), One can easily understand the physical meaning through (Fig. 1-13),
The total electron flux on positive electrode Γ&d is the combination of Γ&eα# and Γ&eα#, that is
18 it implies that i# will approach infinity and the electrical breakdown occurs when 91 $ γ0eα#$ 11: 0.
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This is the Paschen’s law which was first stated in 1889, named after Friedrich Paschen. It depicts the breakdown characteristics of a gap are a function (generally not linear) of the product of the gas pressure and the gap length, usually written as V=
f( pd ). As we plot (eq. 1-30) with x axis being the breakdown voltages versus the products of pressure and distance as y axis, one can obtain graphs like (Fig. 1-14), which is the so-called Paschen’s curve.
1-4 Motivation
1-4-1 Stability issue
Gas sensors operate on a variety of different fundamental mechanisms [42], and they play an important part in monitoring the environmental changes, controlling chemical processes, preventing from terrorism, and in the application of medical and agricultural behaviors. Gas sensors can be classified into two types, a chemical type operated by gas absorption and a physical type operated by ionization [43-44,27].
Since the electrical conductance of CNTs is highly sensitive to certain gas molecules, they have been used to fabricate the chemical sensors with a fast response
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time than conventional materials like metal-oxide, polymer, porous silicon, etc.
[45-48]. The sensing mechanism involves detecting conductance change of CNTs induced by charge transfer from gas molecules adsorbed on their surfaces. However, all gas-adsorptive types of sensors have several limitations. For instance, they are unable to detect gases with low adsorption energy, and also challenging to use electrical conductance measurement to distinguish between gases in a mixture, i.e., gases with different concentrations can produce the same output signal as that for a single pure gas. Also, gas sensors of chemical type are sensitive to environmental conditions like moisture, temperature, and gas flow velocity. Besides, chemisorptions can cause irreversible changes in nanotube conductivity [26]. Thus, CNT-based gas ionization sensors are expected to overcome these disadvantages.
Gas ionization sensors are physical mechanisms that work by fingerprinting the ionization characteristics of distinct gases. However, conventional ionization sensors are limited by the huge and bulky architecture, risky high-voltage operation and high power consumption. Many investigations have studied on the improvement of these issues. Carbon nanotubes with relatively low work function, very sharp nanotips, and structural and chemical stability under high electrical field, were known to be the best field emitters over many conventional field emitting metals like Mo and W. The usage of CNTs for the improvement of the characteristic of gas ionization sensors has been addressed in recent years [49-52]. Modi et al. [4] proposed the fabrication and successful testing of ionization micro-sensors (Fig. 1-15) featuring the electrical breakdown of a range of gases and gas mixtures at carbon nanotube tips. The sharp tips of nanotubes generate very high electric fields at relatively low voltages, lowering breakdown voltages several-fold in comparison with traditional electrodes. Moreover, S J Kim et al. [50] fabricated a physical type gas sensor (Fig. 1-16) based on an electrical discharge theory known as Paschen’s law. The gas sensor works by figuring
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changes of the discharge current depending on the concentration of gases which can be ionized through collisions with electrons emitted from CNT emitters.
Therefore, the field emission electrons play an important role in the operational mechanism of gas ionization sensors. As shown in (Fig. 1-11)[40], when CNTs were used as the negative electrode, the mechanism can be classified into three processes, electron emission via nanotips, impact ionization, and recombination of electron along with γ process. Process II and Process III were associated with the characteristics of target gas molecules such as the diameter of molecules and the ionization energy of gases. However, few studies emphasized on the mechanism of Process I which indicates that the material, surface morphology and crystallinity of the electrode are the most critical factors in the breakdown characteristics of gas ionization sensors.
In the beginning of this thesis, the effect of different surface morphology of CNTs film on the gas breakdown characteristics is presented. In first, a random oriented and a uniform CNTs film are fabricated individually by sputtering catalyst on the substrate and thermal chemical vapor deposition (TCVD) method to grow the CNTs. The effect of protrusions on the breakdown characteristics is investigated.
Then, the synthesis of CNT-based film with co-deposition catalyst is obtained to improve the stability issues.
1-4-2 The reduction of breakdown voltage
Moreover, several researchers [53-55] have suggested that the field emission properties of CNT films have a strong dependence on the density and morphology of the CNT deposit. Electrostatic screening effect provoked by the proximity of neighboring tubes is a crucial factor that might reduce the field enhancement and thus the number of emitted electrons. Therefore, another aim of this thesis is to find out an optimal morphology of pillar array of aligned CNT bundles to the application of gas
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ionization sensors. To explore the effect of the pillar arrangement on its breakdown characteristic of gas ionization sensors, different spacer height ratio (R/H) as a function of H is investigated by changing H while maintaining R. An optimal R/H ratio that has a lowest breakdown voltage would lessen the high operating-voltage and high power consumption issues of the ionization sensors.
Finally, the pillar array of aligned CNT bundles with optimal R/H ratio is utilized to explore its gas ionization characteristics under different gas environment. Paschen’s curve for distinct gases was obtained by experiments to approach a wide and appropriate breakdown voltage window that provide enough space to distinguish different gases.