A SILVER NANOWIRE BASED GAS IONIZATION SENSOR EUROCON2009

Nika Azmoodeh, Nicoleta Chivu, Ramin Banan Sadeghian, IEEE and Mojtaba Kahrizi, IEEE Electrical and Computer Engineering, Concordia University, 1455 de Maisonneuve Blvd. West, Montréal, Canada

Abstract: We report fabrication and successful testing of a gas ionization sensor made of silver nanowires (AgNWs) sandwiched between two parallel plates. The device was tested in low pressure argon gas (0.1� P �10-4 torr) where the breakdown voltages (Vb) were recorded against pressure. The new device displayed considerably lower Vb compared to its gold nanowire (AuNW)-based counterparts. The reduction of Vb was justified based on the increase in the secondary electron emission factor on AgNWs due to the lower workfunction of silver. We have also developed a simulation tool to model the device. Our model is based on combined Particle-In-Cell and Monte-Carlo-Collision (PIC-MCC) approaches which provides a platform for further development and optimization of the device. Index Terms: Sensors, nanowires, ionization, Breakdown Voltage I. INTRODUCTION

Gas detectors play a major role in biomedical, chemical and environmental industries. For instance, an electronic nose (E-Nose) contains a number of gas sensing electrochemical cells.

Breakdown ionization sensors, work by fingerprinting the ionization breakdown voltage of the unknown gas (Vb), and therefore, display a good selectivity. In these devices, electron impact ionization mechanism leads to the breakdown of gas [1]. Since Vb is a unique quantity of each gas, by measuring this voltage, the existence and the type of the unknown gases can be found.

Normally, Vb is in the range of several hundred to thousand volts for two parallel electrode plates which makes it difficult and hazardous to employ in many environments[2].

Recently, breakdown ionization sensor has been reported using a film of aligned of CNT arrays [3], and also self standing Au nanowires [1, 4, 5]. In these devices, Vb is reduced considerably compared to that of two flat parallel plates, due to the field

enhancement at the nanowires tips.In this work, we report fabrication of a Gas Ionization Sensor (GIS) using silver nanowires (AgNWs). Breakdown voltages obtained here were considerably less than those measured using gold nanowires (AuNWs) [1, 4].In addition, we have developed a simulation tool to model the device. Our model which is based on combined Particle-In-Cell and Monte-Carlo-Collision (PIC-MCC) approaches provides a platform for further development and optimization of the device. Particle-in-cell (PIC) codes model the plasma using discrete particles, each representing many charged particles. The elastic and inelastic particle collisions are treated using the MCC method. For this we have used an open source simulator XOOPIC which includes Monte Carlo collision (MCC) algorithms for modelling collisions of charged particles with a variety of gases, including such effects as ion/neutral charge exchange, elastic electron scattering, inelastic scattering due to electron impact excitation, and electron impact ionization.

II. THEORY OF OPERATION

Between two parallel electrodes, there are always a few electron ion pairs generated due to cosmic radiation [2]. When an external voltage is applied, the I-V characteristic of the above configuration will demonstrate three distinct discharge regions: The first region is known as the quasi-ohmic region, in which the discharge current I, has a direct dependency on the applied voltage and is defined by the velocity of carriers. The second region is the saturation region, in which all the carriers generated in gap between the parallel plates have arrived at the electrode, therefore I becomes independent of applied voltage. At this stage the saturation current can be given as follow:

dtdneAdI e

sat = , (1)

978-1-4244-3861-7/09/$25.00 ©2009 Crown 1231

where e represents the electron charge, A is the electrode area, d is the gap between the parallel electrodes and dne/dt is the rate of radiation-induced electron production in the gap [2]. As the voltage increases gradually, gas ionization in between two electrodes will occur by electron impact and will give rise to I with an increasing rate till the gas breakdown happens at Vb. Therefore the characteristic between the saturation region and breakdown voltage represent the third pre-breakdown region which refers to Townsend discharge. In this case, the current can be shown as:

)1(1

.−−

= d

dsat

eeII α

α

γ, (2)

where � and γ are Townsend’s primary and secondary ionization coefficients [2, 6]. � shows the number of ionizing collisions an electron made by accelerating for 1 cm in the field direction and γ represents the number of electrons which liberated per incident ion at the cathode. In fact, γ depends on the workfunction of the cathode [6]. Materials with a higher workfunction are expected to demonstrate a lowerγ , since there is more energy needed for the incident ion to liberate the secondary electron.

At the breakdown voltage (Vb), � and γ will be affected in a way that the denominator of (2) becomes zero. Consequently, the current will rise dramatically at Vb and becomes self-sustained. According to the Paschen’s law the breakdown voltage of the gases in a two parallel plates, is dependent on the concentration of the gas N, and the gap distance d [2], and is given by ( )NdfVb = . (3)

In fact the sensitivity of the device can be explained based on its Paschen’s characteristics.

Self-standing nanowires planted on the cathode provide locally enhanced electric field at the sharp tip of the NWs, where the local electric field is given by apploc EE β= , (4)

in which Eapp is the applied electric voltage defined as V/d, and � is the field enhanced factor that is dependent on the device geometry and will increase by increasing the nanowire aspect-ratio and tip sharpness [7]. It is important to note that, we have assumed that the scale of total nanowires length is much smaller than d.

III. EXPERIMENTS

a. Preparation of silver nanowires

In these experiments, the commercial alumina template provided from Whatman Company (ANOPORE®) was used [8]. These templates have two well-defined sides; the bottom side consist of pores with 2r = 20nm diameter and thickness of 2� and the other face consist of pores of 2r = 180nm, pores separation of s = 300 nm, porosity of 109 per cm2 and thickness of 58�m.

To start the electrochemical deposition, one side of the templates must be conductive to provide the required contact of the cathode for electroplating. Therefore, a 100nm layer of gold was sputtered onto the bottom side of the Anopore templates, where this layer does not block the pore completely, but served to convert the bottom surface of the template into the working electrode in a 3-electrode standard electrochemical configuration. In our custom made electrochemical cell, a noble metal (platinum) functions as the auxiliary (counter) electrode, and the saturated Ag/AgCl works as the reference electrode.

The electrodeposition of silver nanowires was carried out galvanostatically with a constant current of 2mA for 100 minutes. The sputter coated side of the template was then attached to silicon wafer initially coated with Ti (10nm) as an adhesion layer, and a second layer of gold (100nm). To obtain free-standing AgNWs, the alumina template was dissolved in 1M NaOH. The morphology of AgNWs was examined by Field Emission Scanning Microscope (FESEM). Fig.1 illustrates a FESEM micrograph of nanowires while the substrate is tilted (45o) to expose the length of the nanowires. The self-standing AgNWs are distributed uniformly with average length of l=10�.

200 nm

Figure 1: FESEM micrograph of the silver nanowire array after dissolution of the alumina template.

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b. Sensor fabrication

As it has been discussed previously, the fundamental architecture of the GIS is made of two parallel plates (electrodes) of metal, in which the gas ionization occur in between of these parallel plates. For this purpose, the self standing AgNWs with its attached substrate was served as cathode and the secondary electrode was made of a double side polished p-type silicon wafer. In order to provide an ohmic junction in the p-type silicon, the wafer was coated with 1�m layer of aluminum on its both sides and then it was annealed in a N2+H2 ambient at 400oC for 40 minutes.

To make the spacing between the two electrodes, a supporting polypropylene ring was attached to the anopore template, while engraving four tiny grooves around the ring to assist the venting flow in between of the parallel plates. Polypropylene is recognized as a material with high electrical strength or high volume resistivity [9].

Fig. 2 depicts a schematic view of GIS made of a plate of nanowires film and an Al coated silicon wafer as the counter electrode.

Figure 2: Schematic illustration of GIS[1] .

IV. RESULTS AND DISSCUTIONS

a. Pre-breakdown current

To measure the pre-breakdown discharge current (I) of the GIS at a very low pressure (P = 10-6 Torr), a sweep voltage up to ±200V (�t = 1s, �V = 1V) was applied. Fig. 3 illustrates the I–V curve of the anode and cathode measured at P = 10-6, while the AgNWs were configured as the cathode. It is clear that I(Anode) is remarkably higher than I(Cathode).

-1E-10

0

1E-10

2E-10

3E-10

4E-10

5E-10

0 50 100 150 200

Discharge current (A)

Applied voltage (V)

I(Anode)I(Cathode)

Figure 3: Room temperature pre-breakdown discharge currents of both of the electrodes in low pressure air (P = 10-

6 torr), for AgNWs film as the cathode.

As shown in Fig. 3 the anodic current increases with the field in the quasi-ohmic region until it saturates. The saturation region is similar to that of uniform field conditions in parallel plate configurations where there is no impact ionization. At this stage the current remains voltage-independent, as indicated in (1), up to the point where the Townsend discharge starts at higher voltages.

Since the pressure is kept constant during the sweep, the slight increase in Isat can be solely attributed to abnormal electron repulsion from the AgNWs tips [4]. These electrons can trigger secondary ionizations in the gap at very low gas pressures, therefore, increases dn�dt (see (1)).

b. Breakdown Voltage in the GIS The breakdown voltage of ultra-pure argon was measured at the pressures in the range of 10-4

< P <10-1 torr, at room temperature. The I–V curves of ultra-pure argon taken at different

gas pressures are shown in Fig. 4. It is clear that Vb decreases by increasing P, which indicates that the tested gas pressures were less than the Paschen minimum. Above 0.1 torr, Vb is expected to remain unchanged [4].

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Comparing the value of Vb in these devices with two parallel-plate structures, the Vb is significantly reduced, considering the same spacing and gas pressure. As it was explained in the theoretical section, the sharp tip of the nanowires enhances the local electric field (Eloc) at the tips by a factor of �, and will provoke ionization of the gas molecules at a lower Eapp. Table.1 represents a comparison between Ar gas breakdown voltage between GIS fabricated with AuNWs and AgNWs, with the same sensor structure and gas pressure. As it can be seen, Vb in AgNW-GIS is considerably decreased, especially in the left side of the Paschen’s curve.

Table 1: Breakdown voltages of Ar

Ar gas pressure

(torr) 10-4 10-3 10-2 10-1

Vb in AgNWs as cathode (V)

128 118 76 68

Vb in AuNWs as cathode (V)

>400 >400 187 163

c. Device Simulation

A Particle-in-Cell–Monte-Carlo-Collision (PIC-MCC) was used to model our device. An open source simulator, XOOPIC is designed based on PIC-MCC to simulate plasmas in a well-defined geometry using a variety of boundary conditions. We have incorporated the circuit elements such as the series resistance, R, dc voltages and the contribution of field-emission from AgNWs tips to the discharge process.

The nanowire arrays were modelled by parallel rectangular conductive sheets along the 3rd imaginary

dimension (z) with unity length. Simulations were carried out at P = 0.01, 0.05, 0.1, 0.5 and 1 torr of argon. At each pressure, it was assumed that a constant fraction of gas particles in the medium were singly ionized due to cosmic radiation. In addition to primary electrons, the contribution of secondary and field-emitted electrons to the discharge was considered. The secondary electrons were set to have the same collision model as the primary electrons, so the secondary ionization coefficient, γ, was generated self-consistently.

At each pressure, the minimum voltage that led to formation of an avalanche from the nanowire tips was recorded as the corresponding Vb. Fig. 5 shows the simulated Vb–P curve obtained in the pressure range of 0.01 � P � 1 torr of Ar.

Figure 5: Simulated Vb–P curve in Ar.

Fig. 6 shows the evolution of total number of carriers with time for P = 1 torr. During the first few picoseconds, the existing radiation-generated electrons are depleted. When the Argon ions gain sufficient energy, secondary electrons are liberated and an avalanche is initiated at the space close to the tip. The higher total number of electrons compared to ions, shows that field emitted electrons contribute to the formation of an avalanche.

0E+0

5E-9

1E-8

2E-8

2E-8

3E-8

3E-8

4E-8

0 25 50 75 100 125 150

Current (A)

Applied voltage(V)

10 -1

10 -2

10 -3

10 -4

Figure 4: The I–V curves obtained in argon(10-4 � P � 0.1 torr).

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Figure 6: The evolution of the total number of carriers in argon (P=1 torr).

Fig. 7 shows the cathode current evolution of the device for Ar gas at P = 1 Torr.

Figure 7: The cathode current at Vb (P=1 torr).

Once the total number of carriers built up at the top

of nanowires tip with respect to time an avalanche of electrons was happened. As a result, the constant currents suddenly fall to zero, indicating the breakdown of the gas. This breakdown was happened at Vb = 70V at the cathode in time t = 2.4×10-9 seconds.

V. CONCLUSIONS

A gas ionization sensor was fabricated using AgNWs at the cathode, and tested at low pressures (P � 0.1 torr). The new device showed improvement compared to its previous AuNWs counterpart, as the breakdown voltages were further reduced. The reduction of Vb was attributed to the lower workfunction of silver compared to that of gold. A MCC-PIC code was developed to simulate the breakdown mechanism in the sensor. The simulated results were in agreement with the breakdown voltage measurements.

ACKNOWLEDGMENTS

This work was partially supported by funding from the Natural Science and Engineering Research Council of Canada (NSERC) and by the faculty of engineering and computer Science of Concordia university.

REFERENCES [1] R. Banan-Sadeghian and M. Kahrizi, "A

novel miniature gas ionization sensor based on freestanding gold nanowires," Sensors

and Actuators A: Physical, vol. 137, pp. 248-255, 2007.

[2] A. M. Howatson, An introduction to gas discharges, 2nd ed. Oxford: Pergamon Press, 1976.

[3] A. Modi, N. Koratkar, E. Lass, B. Wei, and P. M. Ajayan, "Miniaturized gas ionization sensors using carbon nanotubes," Nature, vol. 424, pp. 171-174, 2003.

[4] R. Banan-Sadeghian and M. Kahrizi, "A Low Pressure Gas Ionization Sensor Using Freestanding Gold Nanowires," in IEEE ISIE, Vigo, Spain, 2007, pp. 1387-1390.

[5] R. Banan-Sadeghian and M. Kahrizi, "A Low Voltage Gas Ionization Sensor based on Sparse Gold Nanorods," in IEEE Sensors Conf., Atlanta, GA, 2007, pp. 648-651.

[6] J. M. Meek and J. D. Craggs, Electrical Breakdown of Gases. New York: John Wiley & Sons, 1978.

[7] R. G. Forbes, C. J. Edgcombe, and U. Valdre, "Some comments on models for field enhancement," Ultramicroscopy, vol. 95, pp. 57-65, 2003.

[8] "Anopore® inorganic membranes, product datasheet," Whatman Inc., Florham Park, NJ, Available: http://www.whatman.com/products/?pageID=7.57.293 as of 8/29/2007.

[9] C. Rodehed, A. Gustafsson, and U. W. Gedde, "Electrical strength and chemical surface analysis of polypropylene after exposure to external partial discharges," Electrical Insulation, IEEE Transactions on [see also Dielectrics and Electrical Insulation, IEEE Transactions on], vol. 25, pp. 1066-1076, 1990.

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