IEEE SENSORS JOURNAL, VOL. 8, NO. 2, FEBRUARY 2008 161 A Novel Gas

IEEE SENSORS JOURNAL, VOL. 8, NO. 2, FEBRUARY 2008 161

A Novel Gas Sensor Based on Tunneling-Field-Ionization on Whisker-Covered Gold Nanowires

Ramin Banan Sadeghian, Student Member, IEEE, and Mojtaba Kahrizi, Member, IEEE

Abstract—Typical gas ionization sensors (GISs) work by finger-printing the ionization breakdown voltages of the gases to be iden-tified. In this work, we developed a GIS that operates by field-ion-izing the unknown gas at exceptionally low voltages. The resultantfield-ion current-voltage ( ) characteristic was then usedto identify the gas. Freestanding gold nanowires (AuNW), termi-nated with nanoscale whisker-like features, were employed as field-amplifiers to reduce the field ionization threshold voltages. Syn-thesis of the AuNWs was carried out by the template-assisted tech-nique accompanied by two alterations: 1) polystyrene (PS) micro-spheres were incorporated to reduce the compactness of the pores,thus prevent the nanostructures from collapse, and 2) the templatewas impregnated by HAuCl4 to form gold nanowhiskers duringthe electrochemical nucleation of AuNWs. The sensor was testedin three elemental gases: Ar, N2 and He, in a pressure range of0 01 100 torr. Each gas demonstrated a distinctivecurve, particularly in the field-limited regime. The threshold ion-ization voltages ranged from 1 to 10 V, almost three orders of mag-nitude lower than the voltages used in field-ion-microscopy. Thelow-voltage field ionization was attributed to the field-amplifyingnanoscale whiskers on the AuNW tips, as well as the presence ofresidual amorphous alumina with semiconducting characteristics,due to incomplete removal of the porous anodized alumina (PAA)template.

Index Terms—Field enhancement, field ionization, gas sensor,gold nanowires.

I. INTRODUCTION

GAS SENSOR TECHNOLOGY plays a major role inchemical, medical, and environmental industries, as well

as in protecting the nation against terrorist attacks. Presentportable devices have a limited sensitivity because their elec-trochemical cells cannot detect low gas concentrations. Inaddition, catalyst-based cells suffer from selectivity issues,since different gases when adsorbed may induce similarchanges in the electrical properties. Solid-state resistive ma-terials (e.g., SnO ) are more sensitive but require operation athigh temperatures [1], [2]. Therefore, bringing high sensitivity,selectivity, durability, and low-power operation all together intoa portable instrument is highly desirable.

Manuscript received June 6, 2007; revised September 1, 2007 and October9, 2007; accepted October 12, 2007. This work was supported in part by theNatural Science and Engineering Research Council of Canada and in part bythe Faculty of Engineering and Computer Science, Concordia University. Theassociate editor coordinating the review of this manuscript and approving it forpublication was Prof. Gerald Gerlach.

The authors are with the Department of Electrical and Computer Engineering,Concordia University, Montréal, QC H3G 1M8, Canada (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2007.912788

Of late, miniaturized ionization sensors have been introducedto overcome selectivity issues. These sensors work by finger-printing the ionization breakdown voltage of the unknown gas. Itis known that at constant temperature and pressure every gas dis-plays a unique breakdown electric field [3]–[5]. Since this tech-nique does not involve any chemical reaction, ionization sensorsare not limited by considerations of reversibility and display fastresponse and recovery times. For convenience of discussion,these kinds of sensors will, henceforth, be referred to as “GasBreakdown Ionization Sensors” (GBISs). Miniature GBISs in-corporate an array of high aspect ratio carbon nanotubes (CNT)as field-intensifying elements at one of the electrodes of an ion-ization cell to reduce the breakdown voltages [6]–[12].

Modi et al. [6] and Zhang et al. [7]–[9] utilized multiwalledcarbon nanotubes (MWCNT) at the anode and cathode, respec-tively, and reduced the gaseous breakdown voltages down to arange of 100–450 V. Such voltages are still hazardous to em-ploy. Moreover, CNT-based GBISs operate in corona dischargemode. Very high nonlinear electric fields formed near the tipspromote formation of negative or positive coronas (dependingon the tip polarity) that eventually lead to the electron impactionization breakdown of the gas [3]–[5]. Corona discharges aredifficult to control and they generate excessive heat that may de-stroy sharp and slender CNTs.

Gaseous breakdown requires a self-sustaining electron multi-plication in the gap that is initiated from the cathode. The break-down condition in uniform fields is explained by Townsend’scriterion

(1)

where is the gap spacing, and and are Townsend’s pri-mary and secondary ionization coefficients, respectively [3].is defined as the number of ionizing collision an electron makesby traveling the unit distance towards the anode and is given incm . represents the number of electrons generated in

the gap by secondary factors such as ion bombardment at thecathode and photoelectron emission, per primary electron [3],[4]. Both and depend on the ratio of electric field to gasconcentration , a quantity known as the reduced field.In (1), is the factor representing the exponential growth ofelectrons throughout the gap. In nonuniform fields, the product

is replaced by [4].Alternatively, the field-enhancement effect of sharp tips can

be employed to field-ionize gas molecules. Instead of the break-down voltage, it is possible to use the tunneling field-ionizationcharacteristic as a signature to fingerprint the unknown gas typeand measure its concentration. Sensors that utilize the field-ioncurrent are, henceforth, referred to as “Gas Field Ionization Sen-sors” (GFISs). Note that the field strengths required for field ion-

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162 IEEE SENSORS JOURNAL, VOL. 8, NO. 2, FEBRUARY 2008

ization of typical gases are much higher than their breakdownfields. However, contrary to impact ionization, a high electricfield is required “locally” at the tip, where tunneling ioniza-tion takes place. In other words, in order that field ionizationoccur before breakdown, electron multiplication must beinsufficient to satisfy (1). The above conditions impose geomet-rical constraints on the device. Particularly, the field ionizationsites at the anode (i.e., high aspect ratio protrusions) must havesharper tips. In addition, itself must be as small as possible toreduce the amount of electron multiplication, thus increase thebreakdown voltage according to the Paschen’s law [4].

Madou and Morrison [13], for the first time, used an arrayof micrometer-sized tips to field-ionize formic acid (HCOOH)in atmospheric air. Spindt [14] applied microfabrication tech-nology to fabricate a multipoint field-ionization source. Inhis microvolcano array structure, gas was introduced to theionization region through submicron orifices of microvolcanos.The microvolcano field-ionizer was tested in 0.2 torr of tolueneC H , where the threshold of detectable field-ionization

was 45 V. Ghodsian et al. [15] developed a low-cost siliconmicromachining process and fabricated a single-tip GFIS. Theysuccessfully employed the self-aligned technique to efficientlyminimize the tip-to-counter electrode distance, hence increasethe local field. Field ionization experiments were carried outin vaporized acetic acid CH COOH , where their sensorsdemonstrated a minimum detectable concentration of 14 ppm( 0.01 torr) under an applied voltage of 5 V.

The GISs mentioned above, were all used to field-ionize or-ganic molecules. It is imperative to note that the occurrence offield ionization, and the amplitude of the resultant current, bothdepend rigorously on the ionization potential of the gas particle.As a result, field ionization of organic molecules with ioniza-tion energies of 9–12 eV, entail lower fields than elemental gasmolecules with ionization energies above 12 eV. Moreover, itis desirable to have a larger number of field ionizing tips at theanode to increase the produced current. However, the density ofsuch tips fabricated using standard silicon micromachining pro-cesses is limited due to lithography constraints. Therefore, in-troducing a new structure that provides a higher density of tips,with reduced cost and simpler fabrication processes, is the goalof this research. Such a device must also be capable of ionizingnonorganic species.

Freestanding nanowire arrays are promising candidates asfield ionization sources, due to their high aspect ratio and den-sity. Previously, we reported on the fabrication of a GBIS usingfreestanding nanowires [16]. In this work, we developed andsuccessfully tested a GFIS using gold nanowires (AuNWs) withsharp features at their tips.

The template-assisted electrochemical method has been acommon approach to grow high aspect ratio nanostructures[17]. This technique is mainly known for its ease and low cost.However, it suffers from the fact that embedded nanowiresarrays with high aspect ratio, normally bundle or collapseafter the template has been removed. This happens because ofthe surface tension force exerted on the nanowires during theevaporation of the solvent. We developed a method to preventour AuNWs from collapsing. Vertically aligned AuNWs weregrown in porous anodized alumina (PAA) templates, and thenincorporated at the anode of a miniature cylindrical cell with

Fig. 1. Field ionization of an atom under a positive applied field at the vicinityof (a) metal and (b) semiconductor. The semiconductor work function at thesurface increases by the amount of band bending � . Inclusion of the surfacestates allows tunneling to occur into them.

a flat cathode. Field ionization tests were performed usinga number of test gases. Anomalous low ionization thresholdvoltages were recorded.

II. THEORY OF OPERATION

A. Field Ionization (FI) Formalism on Metals andSemiconductors

Field ionization (FI) occurs when a valance electron of a gasatom or molecule (hereafter referred to as gas particle) havingan ionization potential of , tunnels through a potential barrierinto a vacant energy state of the conduction band of a metal witha work function of [Fig. 1(a)] [18]–[23]. As is specific toany gas particle, the resulted tunneling current can be used tofingerprint the unknown gas type. FI action requires very highthreshold fields in the order of V cm ,achievable only at the vicinity of very sharp tips [18], [19]. FIcannot occur when the gas particle is at a distance from the sur-face less than the critical distance given by

(2)

because the electron energy level in the particle would lie belowthe metallic Fermi level, where there are no available states fortunneling to take place.

In the case of semiconductors, the basic metal-tip FI theoryrequires some modification. Field penetration into the semi-conductor, may cause significant upward band bending andmake the near surface region to become -type degenerate[19], [24]–[27]. As shown in Fig. 1(b), the electron may thentunnel into the empty states of the valence band above . Theeffective work function at the surface increases by the amountof band bending . Hence, the critical distance is given by

(3)

BANAN SADEGHIAN AND KAHRIZI: A NOVEL GAS SENSOR BASED ON TUNNELING-FIELD-IONIZATION ON WHISKER-COVERED GOLD NANOWIRES 163

where is proportional to the penetrated field according to

(4)

where is the relative permittivity of the semiconductor andis the penetration depth. Inclusion of surface states associ-

ated with impurities and dangling bonds at the semiconductorsurface affects the band bending potential even in a field-freecase [19], [24]–[27]. Under an applied field, charging of thesestates yields some space charge and reduces the band bending.In this case, tunneling may occur to these surface states abovethe Fermi level, even if there is no inversion. Hence, the criticaldistance is controlled by the altered Fermi level [25]. Variousauthors have measured values considerably lower than theequivalent values for metallic counterparts [24]–[27]. At a con-stant applied field, the resulting is usually smaller comparedwith the case of metal anodes. This would imply a higher tun-neling probability and therefore a higher FI current.

Depending on the field strength, the field-ion charac-teristic consists of three distinctive regimes: In the field-limitedregime where the field is relatively low, the total rate of ioniza-tion is small compared with the rate of arrival. The current isproportional to the equilibrium number of particles near the tip,which exceeds the zero-field value due to the polarization effectin the high-field region. The ion current in this case is approxi-mately given by

(5)

where is the metal tip curvature, is the electron charge, isdefined previously, is the orbital frequency of the tunnelingelectron, is the tunneling probability given by

(6)

and is the equilibrium gas concentration near the tip given by

(7)

where is the concentration far from the ionization zone,and are the gas and tip temperatures, respectively, is theBoltzmann constant, and is the polarization energy of parti-cles defined as

(8)

where is the atomic (molecular) polarizability [18]. The ioncurrent in the field-limited regime rises steeply with the electricfield [22].

At sufficiently high fields, nearly all particles approachingthe tip become ionized before reaching it. The current is, there-fore, limited by the gas supply into the ionization zone and risesmildly with the field. The supply-limited ionization current isgiven by [22]

(9)

where is the supply function, or the number of particles im-pinging on a unit surface per unit time. It also exceeds the gas

kinetic value due to the attraction of particles by polarizationforces and is given by

(10)

where is the gas pressure, and is the atomic (molecular)weight of the gas particle.

At intermediate fields between these extremes, the situationbecomes more complicated and the ion current must be cal-culated from detailed kinetic considerations. A simplified casewould be when the kinetic energy of gas particles rebounding tothe tip , exceeds . The particles will then escapethe ionization region and hoping trajectories from the AuNWshank can be neglected [18], [22]. In this case, particles ap-proach to the ionization region only from the gas phase, wherethe arrival rate is determined by the supply function of (10).

Obviously, the field-limited current depends strongly onthrough both the tunneling probability and the critical dis-tance , and also on . This dependence can be employed tofingerprint different gas species, by measuring their field ioniza-tion current. However, to obtain measurable FI currents in lowapplied voltages, an array of sharp tips is required at the anode.Freestanding AuNWs with nanoscale sharp whiskers are pro-posed in this work as field ionizers.

B. Geometrical Field Enhancement

As mentioned earlier, the electric field strengths required toproduce FI of common gases are achievable at the apex of sharpprotrusions. In geometries resembling a capacitor like parallel-plate structure, the applied field is defined as

(11)

where is the applied voltage between the electrodes and isthe interelectrode separation. The local field which de-termines the tunneling barrier is the field close to the apex of aprotrusion placed on the anode. is often related to bya field enhancement factor, defined as

(12)

has been the subject of analytical, computational, and exper-imental investigations and several formulas have been obtainedto predict a value for it based on the geometrical model for themicroprotrusion or nanoprotrusion [28]–[30]. These geomet-rical models include “hemisphere on a plane,” “hemisphere ona post,” and “hemi-ellipsoid on plane.” For instance, the “hemi-sphere on a plane” model predicts that for ultrahigh aspect ratiostructures such as carbon nanotubes, is almost proportional tothe ratio of the protrusion length to the base radius or tipcurvature

(13)

The nominal range of values predicted for high aspect ratiostructures such as carbon nanotubes lie within[28], [29]. However, it is well established that if the protrusion


Fig. 2. SEM micrographs of the PAA pores prior to electrochemical replication. (a) The top face of the PAA after covering with PS microspheres. Some of themicrospheres have penetrated into the pores. (b) Cross-section image of the pore walls showing the blocking spheres. (c) Au nanoparticle aggregates on the porewalls after the impregnation step.

is of atomic or subatomic scale, the models mentioned above arenot accurate, and the distribution of electron charge needs to becalculated using quantum mechanics, and the field distributiondeduced from the atomic-level charge distribution [19], [20]. Inthese cases, the up-limit of the field amplitude at the apex of ahemisphere can be used as an approximate for , which isgiven by

(14)

where is the applied potential, is the apex radius, and (thefield factor) is a constant which depends on the taper angle of theprotrusion, with an approximate value ranging from 3 to 8 (often

is used) [18], [19]. For instance, to produce ionization atan applied voltage of 1 kV, it is necessary to make the metallicspecimens with end radii of 5–10 nm.

Presence of whisker-like protrusions on blunt-tip shanks im-proves the overall field enhancement effect. These structures areoften referred to as “super tips” [31]. The resulting is roughlyestimated by the product of the field enhancement factors of theshank and the whiskers individually. However, such estimationdoes not always agree with experimental data [32].

III. EXPERIMENTAL

A. Synthesis of Vertically Aligned AuNW Arrays

Porous Anodic Alumina (PAA) membranes have been widelyused in fabricating metallic and metal-oxide semiconductornanostructures by electrochemical replication of their nanoscalepores [17], [33]–[35]. Commercially available PAA membranes(Anodisc 25) [36], were employed here as templates. Thesemembranes are ca. 60 m thick that contain 58 m long uncon-nected pores with an average diameter of 180 nm. The pores arewider at the top side of the membrane and have slender wallsbetween them, while they become slightly narrower at the otherside with thicker walls. The remaining 2 m of the bottomconsists of branched and interconnected pores corresponding tothe sizes claimed by the company (20 nm) [35]. The PAAs areperipherally bonded to an annular polypropylene ring for easeof handling. Because the pores are perpendicular to the PAAsurface, the embedded nanostructures will be highly orderedand vertically aligned.

Electrochemical replication of the pores was performed basedon earlier works [33]–[35] with a few modifications to producearrays of freestanding AuNWs. Usually, after the PAA has beenchemically removed, these high aspect ratio structures collapse

due to the surface tension force exerted on them during the evap-oration of the solvent. To prevent the wires from collapsing, thePAA was first immersed into a 0.05% (w/v) water-based sus-pension of polystyrene (PS) microspheres (100 nm in diameter)and then dried in air. As a result, about 25% of the pores wereblocked. This method ensures that the resulting array is sparseenough so that the wires would not stick together after removalof the embedding template.

Two PAA templates were treated as described above (here-after referred to as PAA #1 and PAA #2). In order to form goldnucleation sites, the PAA pore walls were impregnated with di-luted hydrogen tetrachloroaurate HAuCl . This technique isan advantage for producing a number of whisker-like nanopar-ticles on the AuNW tip that can boost the field enhancementeffect. A 1% (w/w) HAuCl -acetone solution was prepared asdescribed in [37] and dropped on PAA #1. It is well-known thatHAuCl is converted to gold nanoparticles in contact with a sur-face having hydroxyl groups like amorphous anodic alumina[38]. PAA #2 was not impregnated, for comparison reasons.

Fig. 2(a) shows an SEM image of the PAA pores after treat-ment with PS microspheres. Blocking of pores by the spheres isdiscernable in Fig. 2(b). The SEM image of the pore walls afterthe impregnation step [Fig. 2(c)] clearly shows the Au nanopar-ticle aggregates are randomly dispersed inside the channels andanchored to the pore walls.

The PAA templates were installed in a custom-made fixtureto secure and cover the holding ring and the bottom side of themembranes, while exposing a circular area (1.75 cm in diam-eter) at the top side. A thin Au layer was then sputtered on theexposed area of the PAAs. The sputtered Au film is normallyaccumulated at the pore edges without plugging them. It servesas the working electrode of a standard three-electrode electro-chemical configuration.

Electroplating was performed galvanostatically in acustom-made electrochemical cell containing Orotemp 24 Tgold solution (Technic Inc.) at room temperature without agi-tation ( , ). Aplatinum gauze was used using as the auxiliary electrode andthe redox potential was V versus a saturatedAg/AgCl reference electrode. Since the sputtered gold cannotseal the pores completely, the electrodeposition takes placebilaterally, that is both through the pores and the exterior of thePAA [39]. Inside the pores, the reduction takes place on thewalls first and eventually the pores are completely sealed.

The PAAs were rinsed in distilled water and attached to anSi substrate coated with Ti (5 nm) and Ag (100 nm), using di-luted conductive paste. The structures were immersed into a


Fig. 3. (a) SEM image of the array of AuNWs grown in PAA #1. AuNW tips extruded from the embedding channels can be clearly seen. (b) A close-up view ofthe AuNW tips. The arrows show nanoscale whisker-like features.

Fig. 4. (a) SEM image of the array of AuNWs grown in PAA #2. (b) A close-up view of a regular AuNW tip.

2 M NaOH solution to dissolve the template. PAA #1 was etchedfor 30 min, and PAA #2 for 1 h. The substrates were then rinsedin distilled water, leaving vertically aligned AuNWs.

Figs. 3 and 4 show the SEM images of the AuNW arraysgrown in PAA #1 and PAA #2, respectively. It is apparent fromFig. 3(a) that PAA #1 has not been completely removed andonly the tips of AuNWs have extruded from the embedding porechannels. Fig. 3(b) shows a close-up view of the correspondingtips on which whisker-like nanostructures are formed due tothe local field enhancement at Au nanoparticle sites. The sur-rounding alumina membrane is evident in this figure as well.The AuNWs grown in PAA #2, on the other hand, are alu-mina free and are not terminated with whiskers (Fig. 4). Afterthe etching treatment, we performed Micro-Raman analysis onthe AuNW film of PAA#1 to examine the composition of theAuNWs. Fig. 5 shows the surface Raman spectrum. The spec-trum exhibits peaks at 987, 381, and 280 cm (apart from thecarbon peaks at 1351 and 1540 cm ). These Raman peaks havebeen assigned to phosphate and acid phosphate ions, contam-inants from the PAA membrane, and confirm the presence ofresidual alumina [40], [41].

B. Device Fabrication

The AuNW films of PAA #1 and PAA #2 were incorporated atthe anode of two ionization cells, hereafter referred to as GFIS#1 and GFIS #2, respectively. Gas vents were created by en-graving the polypropylene ring to facilitate flow of gas into the

Fig. 5. The Raman spectrum recorded from the AuNW film grown in PAA #1.

ionization region between the electrodes. A 1-inch -type Siwafer as the counter electrode was mounted on the ring. Thewafer was initially processed in HF solution to remove the na-tive oxide, coated with Al using PVD technique at both sides,and annealed at 450 C for 30 min in an N H ambient toform an ohmic contact. Using optical profilometry, the resultinggap spacing between the nanowire tips and the counter electrodewas estimated to be m. A schematic illustration of theGFIS cell and its companion measurement setup are presentedin Fig. 6.


Fig. 6. Schematic illustration of the GFIS cell and the measurement setup. Thecounter electrode is mounted on the peripheral supporting ring. I denotes thefield-ion current.

C. Measurement Setup and Methodology

Ohmic connections were made to both electrodes andthe device was placed in a vacuum chamber with electricalfeedthroughs. The chamber was pumped down to a base pres-sure of torr prior to each test. As shown in Fig. 6,characterization was conducted in differential mode using twosource measure units (SMUs) of a HP4155 semiconductor pa-rameter analyzer connected in series. Guarded triax cables wereused to minimize the capacitive leakage current. In addition, aparasitic charging current may be added to the field-ion currentsduring the staircase voltage sweep due to the finite capacitanceof the GIS cells. The maximum capacitive current is given by

(15)

where is the cell capacitance, and and are the voltagestep and the pulse rise time, respectively. To ensure that diesoff prior to sampling at the end of each sweep cycle (becomesnegligible compared with the real FI currents), a sweep delayof ms was chosen in the differential mode

V . The and pressure data were recorded simultane-ously by a LabView code into excel sheets.

IV. RESULTS AND DISCUSSIONS

Different characteristics were observed for each of thedevices comprising of different AuNW films. However, onlyGFIS #1 displayed measurable FI action. The low-voltage FIof the tested gases were initially attributed to the abundance ofnanoscale whiskers on the AuNW tips of GFIS #1. Neverthe-less, computations showed that the factor of the AuNW tipswas not exclusively sufficient to produce field strengths closeto the expected values for metallic specimens (for instance,see [23, Table 1]) within the studied voltage range. It was, hence,suggested that FI takes place by tunneling into the surface statesof the residual amorphous alumina on the AuNWs at a reduced

. Yet, the FI threshold voltage was reduced due to the pres-ence of the field amplifying features.

It is well established that thin layers of alumina exhibit semi-conductive properties with band gaps ranging from less than1 to 10 eV [42]–[47]. These values are strongly diminishedwith respect to the bulk values due to the defect induced states

Fig. 7. Room temperature FI I � V curves of helium in different pressures(P = 0:01, 0.1, 1, 10, and 100 torr) obtained by GFIS #1. The curves areplotted in log–log coordinates to distinguish the four different regimes, namely:1) ohmic (pre-ionization); 2) field-limited; 3) intermediate; and 4) supply-lim-ited. The curves are redrawn in linear coordinates at the inset to show their vari-ation with pressure in the field-limited zone.

encroached in the band gap. For instance, diffusion of Al inthe oxide during the anodization process gives rise to impuritylevels close to the conduction band [43], [44]. Au precipitates,on the other hand, form states close to the valence band [44].

FI tests were performed using both of the devices withthe AuNWs at the anode. Fig. 7 shows the room temperaturefield-ion characteristics of the GFIS #1 measured inhelium within a pressure range of torr,corresponding to a concentration range of

mol liter .The curves exhibit four distinct regimes as indicated by the

block arrows. At zone 1, where the field strength is less than theionization threshold, current flows by the movement of existingradiation-generated electron-ion pairs. The current density is,therefore, determined by the speed at which carriers travel andcorresponds to the ohmic region of discharge explained by

(16)

where is the gas conductivity, is the average effec-tive electric field between the electrodes, and are the elec-tron and ion concentrations, and and are the mobilitiesassuming to be constant at low fields [4]. In the ohmic region,

is almost independent of the gas type and constant at sim-ilar ambient conditions (i.e., temperature, humidity, and amountof radiation).

As the voltage increases towards zone 2, the ionization isinitiated in the field-limited regime and the current increasessharply according to (5) [22]. As shown in the inset of Fig. 7,field-limited currents increase with gas pressure (except at

torr, where generation of corona at V led to sev-eral avalanches). The increase in current was not however, pro-portional to (or ) as predicted by (5), mainly because gasparticle accommodation on the tip does not follow the concen-tration far from it, and/or the effective increases upon creationof a multilayer of ad-particles [18], [48]. Note that (5) is validonly when the current determined from it is much less than that


Fig. 8. Room temperature FI I � V curves of Ar, N , and He, measured byGFIS #1 at P = 0:01 torr. The arrows indicate the approximate ionizationthreshold points. The table in the inset shows the field-limited ionization cur-rents of the tested gases at V = 10 V.

given by (9). At intermediate fields (zone 3), as described ear-lier, the exact ionization mechanism was difficult to quantify.

Zone 4 corresponds to the supply-limited ion-ization regime. According to (10), at a constant pressure, thesupply function is proportional to via . Therefore, thesupply-limited characteristic given by (9) shows an ohmic be-havior. In this regime, as shown in Fig. 7, the versus

curves converged to the unity slope line, confirming theohmic behavior similar to zone 1.

Fig. 8 shows the field-ion curves of ultra pure Ar, He,and , all obtained at using the GFIS #1. It isnoticeable that these gases displayed quite distinctive currents,particularly in the field-limited ionization regime. For instance,note the wide difference among the field-limited current valuesof these gases at , as represented at the table in inset ofFig. 8. According to (5), and consistent with its higher ionizationpotential and low polarizability, helium displayed the highestthreshold voltage. The FI current of helium was also smaller,even though its pre-ionization current was noticeably higher be-cause of its higher mobility compared with and Ar. At el-evated voltages, all of the three curves approached each otherin the supply-limited regime, because in accordance to (9), thecurrent no longer depends on the ionization energy of thegases.

It was impossible to accurately measure the FI currents atlower pressures though, because the operation of ion-pump atpressures below 0.01 torr generated a substantial amount ofelectron-ion pairs in the chamber that superimpose the field-ioncurrent. Otherwise, it is expected that the device is capable ofmeasuring concentrations down to or less,similar to typical gas concentrations employed atom probe fieldion microscopy [19]–[21].

Literature reports a threshold field of Vcm for FI of helium using a tungsten specimen [23]. In orderto estimate the factor of the AuNWs in the GFIS #1 and ex-plain the enhanced ionization mechanism, we used the threshold

Fig. 9. Discharge I � V curves of argon, obtained by GFIS #2. No FI actionwas observed. The characteristics obey that of regular gas discharges, wherethere are sufficient electron-ion pairs in the gap. At P � 0:01 torr there was noionization in the chamber, or in the gap.

ionization voltage of helium taken from the first knee of the cor-responding curve in Fig. 8 V . At such a lowthreshold voltage, even if we assume that the maximum possiblelocal field is obtained at the ionization sites (i.e., whiskers at theAuNW tips), that is the up limit value given by (14) as

(17)

an average value of is obtained for the radii ofwhiskers which is unrealistic because it is even smaller than theatomic radius of gold (1.79 ) [49]. The conventional field en-hancement formalisms that incorporate the applied field in par-allel-plate structures such as the one given by (13), would pre-dict astonishing geometries as well. In the cylindrical configu-ration of the GFIS #1, for instance, the applied field betweenthe electrodes is . Thisfield must be amplified times togenerate field ionization of helium. According to (13), a protru-sion height-to-base radius ratio as high as isrequired. This value appears to be far higher than the featuresobserved in Fig. 3. Therefore, the actual fields at the ionizationsites must be lower than expected. It is concluded that GFIS #1field-ionizes gas particles at field strengths considerably lowerthan the reported values on metal tips. As discussed earlier, wesuggest that the ionization is governed by tunneling into the sur-face states of semiconductive anodic alumina scales that remainon the AuNW tips due to incomplete dissolution of PAA #1.Presence of surface states and band bending due to field pen-etration, both may increase the effective work function at thesurface of the semiconductor with respect to the bulkvalue. As a result, field ionization in this case entails lower fieldsthan the values reported for field ionization on metal specimens,although the penetrated field is reduced by a factor of . Theexact values of , , , and therefore , are difficult to quan-tify because there is no knowledge on the impurity concentra-tion of the amorphous alumina after dissolution in NaOH, noron the nature and quantity of the surface states.

Fig. 9 shows the curves obtained by GFIS #2 in argon.Note that no field ionization was observed using this device.


At low pressures, where the operation of the ion-pump reg-ularly generates excessive electron-ion pairs, the current flowobeyed general discharge rules with distinct ohmic and satu-ration regimes [4], [16]. An average factor for the blunt-tipAuNWs of this device can be obtained by comparing the dis-charge current in the ohmic zone, to that of a similar parallel-plate ionization cell [16]. The stochastic avalanches observedat V were due to creation of positive coronas thatsurround the AuNW tips. Coronas may hasten the breakdownprocess by formation of electron avalanches or plasma streamersthat bridge the gap between the electrodes. As mentioned ear-lier, the likelihood of breakdown can be reduced by reducingthe interelectrode spacing , because according to Townsend’stheory, the electron-impact multiplication factor depends expo-nentially on [3]–[5]. At higher pressures torr, whenion pump was off there was no current flow, except the noisycapacitive leakage current.

V. CONCLUSION

We demonstrated the use of low-cost template-assistednanowire growth method to implement a gas ionization sensor.The proposed device operates based on fingerprinting thetunneling field-ion current of the unknown gas at relatively lowvoltages. Field-amplifying erectile AuNWs were synthesizedby replicating the nanoscale pores of commercially availablePAA templates, and then removing the temple selectively. Thetemplates were modified to produce freestanding structures withnanoscale features at their tips that boost the field enhancementeffect. It was shown that low-voltage field ionization occurs onthe AuNWs terminated with whisker-like nanoparticles, if theembedding template was treated in the etchant for 30 mins. Nomeasurable ionization was observed on blunt-tip AuNWs, orwhen the template underwent longer etching periods or a moreconcentrated NaOH solution was used, so that the PAA wasdissolved completely.

Characterization of the sensor was performed at room tem-perature in Ar, He, and , resulting distinctive curvesparticularly in the field-limited regime. The detectable range ofconcentration demon-strated by our device was about two orders of magnitude smallerthan the detection range of the CNT-based GBIS [6], suggestingan improved sensitivity.

The FI action occurred at surprisingly low voltages (e.g.,V for helium), that are three orders of magnitude smaller than

typical voltages in field-ion microscopes and one order of mag-nitude less than the breakdown voltages in CNT-based GBISs.The enhanced FI behavior could be explained by the combi-nation of two effects: a) the geometrical field enhancement ofnanoscale whisker-like features at the AuNW tips and b) re-duced ionization field due to the presence of amorphous aluminaresidues that are semiconductive in character. Field penetrationand band bending at the surface of this layer and the existenceof surface states due to Al and Au contaminants, would facili-tate tunneling even if there is no inversion [19]. Some evidenceof the presence of the amorphous alumina was found throughMicro-Raman spectroscopy.

ACKNOWLEDGMENT

The authors would like to thank Dr. H. Vali from McGillUniversity for providing SEM facilities for this project, andDr. Y. Djaoued and Dr. S. Balaji from the Université de Monctonfor performing the MicroRaman measurements.

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Ramin Banan Sadeghian (S’06) was born inTehran, Iran, in 1975. He received the B.Sc. degreein electronic engineering from the Sharif Universityof Technology, Tehran, in 1998, the M.S. degree inelectronic engineering from the K. N. Toosi Univer-sity of Technology, Tehran, in 2002, and the Ph.D.degree in electrical engineering from ConcordiaUniversity, Montreal, QC, Canada, in 2007.

During 1997–2000, he was with the Faragir-Fan-Rayaneh, Inc., as an Electronic Engineer, spe-cializing in Industrial automation and biomedical

instrumentation. In 2000, he joined the R&D Center of the Iran KhodroIndustrial Group as an Electronic Engineer, where he worked on automotiveelectronics and vehicle security systems and contributed to the design ofSamand (codename X7). He is presently a part-time Faculty Member and Re-search Assistant with the Department of Electrical and Computing Engineering,Concordia University. He has published 15 articles in the area of nanosensorsin refereed journals and conference proceedings. His research interests includefabrication and characterization of nanoscale electron devices, gas sensors, andSOI technology.

Dr. Banan Sadeghian is a member of the Canadian Center for Biorecognitionand Biosensors (CBB).

Mojtaba Kahrizi (M’02) received the Ph.D. degree in applied solid-statephysics from Concordia University, Montréal, QC, Canada.

After spending five years at St. Francis Xavier University, Nova Scotia,Canada, as a Research Associate and Assistant Professor, in 1991, he returned tothe Department of Electrical and Computer Engineering, Concordia University.He established the Micro Devices and Microfabrication Laboratories in thatdepartment. In June 2001, he joined the department as an Associate Professor,where he is involved with teaching and research focused on fundamental issuesrelated to micro and nanofabrications, in particular, with devices related tooptical communications and health related issues.

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IEEE SENSORS JOURNAL, VOL. 8, NO. 2, FEBRUARY 2008 161 A Novel Gas