Work function changes in gas sensitive materials: Fundamentals and applications

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Sensors and Actuators B 142 (2009) 470–493

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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ork function changes in gas sensitive materials:undamentals and applications

lexandru Oprea ∗, Nicolae Bârsan, Udo Weimarnstitute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 15, 72076 Tübingen, Germany

r t i c l e i n f o a b s t r a c t

rticle history:vailable online 8 July 2009

eywords:ork function

The paper reviews the main issues concerning the work function changes in gas sensitive materials,giving few representative examples. Starting from an historical perspective, the most relevant field effectgas sensing approaches are presented and discussed in connection with their proven practical or possibleapplications. The specific features of both, conducting and insulating gas sensing materials, are summarilyanalysed and compared with the goal of advancing towards a better understanding of the mechanisms

ffect
ield effectas sensing
responsible for the field e

. Introduction

Nowadays there is an increasing need and, therefore, demand forow power and low cost gas sensors for a wide range of industrialnd domestic applications. The most attractive approaches seem toe the ones based on gas sensing field effect transistors (G-FET) [1].his kind of sensors comprise field effect transistors (FET) as trans-ucers and gas sensing gates able to provide work function changespon ambient atmosphere modifications. Due to their operationrinciple, based on the field/voltage modulation of the channel con-uctance, the FETs can dissipate extremely low power. To, however,aterialise the device potential the designer has to solve a lot of

roblems, starting with the choice of the sensing materials andensor structure and ending up with that of the sensor packaging,unctional regimes and evaluation algorithms.

Many of the concerns accompanying the design are due to theimitations in the sensor materials selection and in their com-atibility with the manufacturing technologies. Here, one has torstly address the conflicting/competing nature of some importanterformance features of the sensing materials. For example highensor sensitivity and selectivity are ensured by strong and spe-ific chemical interaction between the sensing material and thearget analyte while good reversibility and short response/recoveryimes are favoured by week interactions, either physical or chemi-
al. The operation temperature and the total consumed power havehe same opposite character. The power dissipated by the FET itselfctually does not count in the overall energy balance of the sen-or if thermal activated processes, asking for additional heating,
∗ Corresponding author. Tel.: +49 7071 2977633; fax: +49 7071 295960.E-mail address: [email protected] (A. Oprea).URL: http://www.ipc.uni-tuebingen.de/weimar/ (A. Oprea).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.06.043

gas sensing.© 2009 Elsevier B.V. All rights reserved.

are required to produce the modification of the sensing elementoutput parameter—in our case its work function (WF).

Thus, it is a challenge to find/develop sensing materials havinggood sensitive properties at low/room operation temperature, sta-ble, resistant against poisoning and corrosion and, in the same time,compatible with the transducer materials and technologies.

In this context we found it useful to shortly review the pro-gresses made in R&D of materials suited for field effect gas sensing.We will examine especially the WF changes they undergo whenexposed to selected gases and the associated sensing mechanisms.The experimental information used in this analysis has to beacquired with different instruments and setups. Therefore, a relatedsummary on experimental techniques and devices was also pro-vided. In the last part of the review, dedicated to the applications,a restricted set of G-FETs are presented. The selection we made isboth objective and subjective. The main topics are addressed inde-pendent of the personal contributions of the authors but the chosenexamples are mostly selected from the authors’ contributions.

We also considered that it is useful to begin with a historical per-spective on WF measurement, which introduces the ideas, conceptsand methods in use today in the field effect gas sensing.

2. Short historical perspective on work functionmeasurements: From electroscope to field effect transistor

It goes without saying that the field effect gas sensing did notsuddenly emerge and that a long period was necessary to accu-mulate the more or less disparate knowledge finally converging
towards this R&D area; it is, however, amazing to find out the hugeprogress made by the R&D field pioneers. Let us start by recallingthat the WF concept roughly designate the minimal energy neededto bring an electron from an electronic conductor to its surface(more accurate approaches will be given in following and in Section
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Actuators B 142 (2009) 470–493 471

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A. Oprea et al. / Sensors and

) and to focus the beginning of our survey on measurements ded-cated to electrical potential of electronic conductors, either metalsr semiconductors. Depending on the number of the bodies in the

nvestigated system there are two reasonable options to referencehe electrical potential.

For isolated bodies the potential is assumed 0 for positions locatedfar away from them, where a probing charge should experienceno interaction.For two or many bodies only the potential differences between/among them are important. Usually one of the bodies of the sys-tem is taken as reference and the potentials of the other areexpressed in respect to this one. If the system is in equilibrium,having the same value for the chemical potential (CP), the partsare seen as electrically connected and the potentials are expressedas “contact potential differences” (CPDs).

At the beginning of the 19th century, in 1801, Volta [2] reportedo the French Academy his first documented [3] measurementsemonstrating CPDs between dry (non-wet) metals. He used theVolta” condenser, and a leaf electroscope (provided with an addi-ional capacitor) to put in evidence the effects. However, thisriority remained since then less known because of the huge impactf one of his previous discoveries, namely the electrochemical pile.

n the same period, Cavallo [4] (researcher who improved the pithall electroscope), also preformed contact potential measurementsn metals. Repeating 20–30 years later (1820–1829) the experi-ents of Volta, Pfaff [5], confirmed the lack of influence of the

mbient composition on the CPD signals as long as no chemicalnfluence on the sample surface occurred.

Between 1859 and 1861 William Thompson (Lord Kelvin) pro-osed the standard potential zeroing method for the determinationf the contact potential difference [3] of different conducting mate-ials (see Fig. 1). A backing potential from a Danielli cell, tunedith a resistive divider, was used to compensate for the CPD of

he investigated electrode system; the null indicator was a quad-ant electrometer (also Thomson’s invention). Using this apparatus,nown nowadays as Kelvin probe, Thompson reconsidered theeasurement of Volta and Pfaff in connection with many of his

ig. 1. The CPD measurement setup devised by W. Thompson, now known as Kelvinrobe (after [3]).

Fig. 2. The decay of the CPD induced by the oxygen exposure of a Pt plate from aVolta condenser when desorbing in ambient atmosphere. Data were extracted fromthe corresponding table in [3] (after [7], chapter 8). The inset displays the samedependency on logarithmic scale.

ones. In this way he put in evidence, for example, a measurablemetal-metal oxide CPDs. Very important for the future field of gassensing were the studies performed by J. R. Erskine Murray [6] andW. Thompson (in December 1880) on platinum plates exposed togases (hydrogen and oxygen). They revealed visible CPD changessubsequent to the gas exposure events, which tend to recover afterspontaneous desorption of the electrodes in the “normal” atmo-sphere. Fig. 2 reproduces the Thompson’s results reported, in atabular form, on page 119 in Ref. [3].

In spite of the considerable amount of experimental facts gath-ered during years, the cause of the Volta-CPD generation remainedlong time not explained, being treated as an intrinsic property ofthe considered materials. At the beginning of the last century, twosignificant achievements in the physical knowledge provided thegrounds required to understand the origin of the CPD between dif-ferent metals. Here we refer to the Einstein’s explanation of thephotoelectric effect [8], which brought in the foreground the con-cept of WF and to the electronic theory of metals, [9] (see also [10]),which related the WF value in metals to the position of the Fermilevel. The newly gained knowledge finally allowed deducing that,in principle, the CPD is just the difference between the WFs of theinvestigated materials.

To simplify the Kelvin probe operation and reduce the zeroingtime Zisman [11] proposed an equivalent setup based on a vibratingcondenser electrometer operated at a low frequency and havingan earphone as “zero” detector. The sensitivity of the system wasimproved by adding a three stage vacuum tube amplifier.

The evolution of the scientific instrumentation and the appari-tion of the photoelectron related techniques [12], mainly ultravioletphotoemission spectroscopy (UPS) [13] made it possible the directmeasurement of the WF; with the acquired data the rough CPDevaluation of different pairs of electronic conductors reduces to asimple subtraction. The key issue for gas sensing is, in this con-text, the so-called “pressure gap” between the conditions of UPSmeasurements (ultrahigh or at least high vacuum) and the ambi-ent atmospheric conditions encountered in gas sensor operation.Therefore, well performing atmospheric setups were and are stillrequired for CPD estimations on gas sensing materials. The mostused are the KP devices based on Thompson’s and Zisman’s designs,which will be thoroughly addressed in the Section 4.

In contrast to UPS equipment, very expensive, large and compli-cated in exploitation, also simple, small size and low cost devices,
able to detect reduced WF changes came out from the modern sci-entific and technological developments of the last century. Here wemainly refer to the FET. Earliest FET structures have been proposedby Lilienfeld [14], Heil [15] and Shockley [16]. The most employedones, namely the metal oxide semiconductor (MOS) FET [17] (see

472 A. Oprea et al. / Sensors and Actua

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ig. 3. The structure and few functional aspects of the Lundström hydrogen FET.fter the historical paper of Lundström [1].

or example the text book [18]), is also the most interesting for thehemical [19–23] and gas sensing [1,24] because it can easily putn evidence the small electrical voltages appearing between 2 ofts elements (gate and channel, via substrate). These voltages canesult, for example, from the interaction between the gate elec-rode and some orientated dipoles, localised at its interface withhe channel insulating oxide. On this basis, Lundström et al. [1,24]ealised a novel gas sensing device (see Fig. 3), the gas-FET (G-FET),nitially devised for H2 detection (see Section 3.1). Its sensitivityowards hydrogen is provided by a thin Pd gate, permeable to it,specially when heated at elevated temperatures. The H2 moleculesecompose at the hot Pd surface, diffuse in atomic form through theetallic film and finally create a dipolar interfacial layer responsible

or the field effect.Related types of gas sensors have been developed by other

esearchers for different applications. The main technological solu-ions and the associated features will be shortly addressed in thexperimental section together with the materials employed for gasensing through WF changes.

. Using work function for gas sensing: the general frame

The short overview made in the previous section (Section 2) gavehistorical approach on the WF concept and few relevant examplesoncerning the utilisation of WF to detect modifications in the com-osition of the natural or artificial atmosphere. In order to, however,rovide a deeper insight into the field effect gas sensing based onF changes in gas sensitive materials more elaborated approaches

ave to be considered. The present section focuses on some funda-ental features of the WF concept in relation with different classes

f materials that may show WF signals in response to changes in thembient atmosphere. Here, one mainly addresses the establishedodels, supported by comprehensive experimental data.

.1. The work function definition and some principleonsiderations

The WF is the minimal electrical work required to removen electron from an electronic conductor and to bring it, at restzero velocity), just outside of this conductor, explicitly, at a small

acroscopic distance from the surface, but far away enough aticroscopic scale, to avoid any electrostatic interaction [10,25].

his means that the electron should finally reach its first non-ounded state and, therefore, to take the first energy value from itsontinuous spectrum, usually named vacuum level (VL). For non-nteracting bodies VL is usually chosen 0 and it is attained at about0 nm above the surface. When additional electrical fields (weak
nd slowly varying in space) are present the VL state will have aosition dependent energy.
The way from the definition of the WF to the computa-ion/determination of its numerical value it is not always an easyne. Depending on the characteristics of the studied materials, the

tors B 142 (2009) 470–493

composition of the ambient, temperature and the presence of exter-nal fields additional factors have to be taken into account.

Theoretically, it is possible to define many model WFs for differ-ent model conductors. For an ideal single crystal, filling the wholespace, no electron extraction from the material is possible, sinceinfinite, but the first free electron state (VL) can be assumed tohave 0 energy and the bound states referred to it. When a finitebody is imaginarily cut from the infinite model body, the previousenergy picture still holds if the lattice and the charge distributionare frozen, that is, not let to relax at the surface. Reducing the ide-ality of the crystal, by letting the boundary lattice and orbitals torelax, shrinkage of the outermost atomic planes will occur togetherwith an outward expansion of the electronic clouds of the outer-most atoms/ions. This process is, in fact, a at-the-“surface”-shift ofthe positive and negative charge gravity centres in respect to eachother which results in a surface dipolar layer. Now, to extract anelectron outside of the conductor will require additional electricalwork, leading to an increase of the WF value. Additional contribu-tions from adsorbed molecules complicate even more the surfaceproperties and the prediction of the material WF for materials sur-rounded by reacting gases.

3.2. The work function of metals and semiconductors

The solids most suited to illustrate the WF concept are the met-als. Due to the fact that in a metal the last occupied level at 0 K isthe Fermi level, which actually represents the chemical potential ofthe electrons at this temperature, the WF should be:

˚ = −EF (1)

if the Fermi energy (EF) is expressed in respect to the vacuum level.At higher temperatures, the chemical potential decreases a lit-

tle (by a factor less than 10−4 until the room temperature, see,for example [10]) but Eq. (1) still holds in the limit of the usualexperimental errors.

If the crystal ends on non-equivalent crystalline planes or itis not uniformly covered with foreign atoms, then, the WF valuewill depend on the macroscopic position because of the differentadditional contributions from the surface. In such cases, potentialgradients outside the metal, determined by electron surface redis-tribution could appear. They will, locally, shift the vacuum level.

The WF concept keeps its meaning in the case of semiconductors,too. There are, however, differences in using it due to some majordifferences between semiconductors and metals, such as:

(a) the empty conduction band (CB) at 0 K;b) the important role played by the energy gap on electrical (and

not only) properties and phenomena;(c) the relatively small concentration of the charge carriers in the

bands at room and even at higher temperatures (where sometypes of gas sensors are usually operated);

d) the possibility of bipolar conduction;(e) the important influence of the surface chemistry on the energy

structure of the semiconductors, at least near the surface.

The features referred above as “a” and “b” lead to a particularphysical picture, i.e., the chemical potential of the electrons has,in most of the cases, values situated in the gap, where no allowedelectronic levels exist. It is, therefore, improper to denote the CP ofthe semiconductors as “Fermi level”. This CP “pseudonym”, mainlyaddressing its 1/2 occupation probability, can still be accepted if cor-
rectly handled. In agreement with it, the symbol EF for the chemicalpotential of electrons in semiconductors will be kept. One has to,however, stress again that the CP occupation number in intrinsicideal semiconductors is, zero, due to zero density of states in thegap

A. Oprea et al. / Sensors and Actuators B 142 (2009) 470–493 473

Fig. 4. The influence of the adsorbate acceptor states or dipoles on the energy structure of a semiconductor having n-type conduction. One observes the evolution of theb sorbind or fref rred ad

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ands in a direction (x), pointing to the bulk of the material, perpendicular to the adipolar layer. The bending of the bands starts at a depth x0 below the semiconduct

ree surface (x = 0). For x > x0 the bands are flat and the value of their edges are refeonor level, ES is the level/band of the surface states, −eVs is the band bending.

The feature “e”, important for gas sensing, will be addressed inections 3.3 and 3.4.

.3. The effect of the surface adsorbate dipoles on the WF oflectronic conductors

The surface dipoles uniformly shift the WF with a constantmount, namely the electrical work wEL,D required to pass an elec-ron throughout the dipolar layer on its way out of the material,owards a free state.

A simple electrostatic picture, which describes the dipolar layers a plan parallel capacitor, allows the straightforward evaluationf the associated voltage drop:

dip = �

C0,dip= �

ε · (1/ıdip)= � · ıdip

ε(2)

here � = �pol is the polarisation superficial charge density, ıpol ishe dipolar charge separation and ε is the electrical permittivity ofhe material (additional considerations are required to evaluate it).he capacity C0,dip is evaluated on area unit.

Using the relation between �pol and the component of the polar-sation perpendicular to the surface (Pn, the dipolar momentum ofhe unit volume):

= �pol = Pn (3)

n conjunction with the dependency of the polarisation on the sur-ace dipole concentration, nS,dip, and normal dipolar momentum,n,ad, of the adsorbate layer:

n = nS,dip · pn,ad

ıdip(4)

ne obtains:

dip = nS,dip · pn,ad

ε(5)

nd a dipolar energy contribution to the WF of:

˚dip = e

ε· nS,dip · pn,ad = wEL,dip (6)

In the case of metals, this is seen directly in the value of the WF:

= ˚0 + wEL,D = ˚0 + e

ε· nS,dip · pn,ad (7)

or the semiconductors, where the electronic affinity, �, is defineds the energy required to bring an electron from the edge of the

g surface (the plane yz in this case). The index “SS” means surface states, while “DL”e surface and attains the maximal slope, indicating the local electrical field, at thes bulk values. The notations are the standard ones: EVAC is the VL, ED is the shallow

conduction band to the vacuum level, the parameters influencedby the dipolar superficial layer are, �:

� = �0 + wEL,D = �0 + e


and, ˚ (see also Fig. 4):

˚ = � + [EC − EF ]b = �0 + [EC − EF ] + e


Here, �0 is the affinity of a clean (adsorbate free) surface and EC,Vare the edges of the conduction/valence band. The b index indicatesthe bulk parameter values (in this case there are the same values atthe surface as well because the presence of the dipolar adsorbatedoes not disturb the flat band situation).

3.4. Surface states and surface chemistry influence on the energystructure of semiconductors

Some semiconductors are very interesting for applications duethe strong dependence of their WF on the chemo-physical state oftheir surface.

Fig. 4 illustrates the evolution of the semiconductor energystructure when localised acceptor states or/and dipoles are presentat the surface. The initial state of the material, i.e., before the gasexposure, is the flat band ones, without any surface states and/ordipoles.

The surface acceptors capture electrons from the conductionband until they will be completely filled or, alternatively, willpin the Fermi level. In any case, the negative charge immobilisedat the surface is exactly balanced by the positive charge of theionised donors located in the region depleted from its free elec-trons (electro-neutrality condition). The depth of the positive spacecharge beneath the semiconductor surface depends on the screen-ing properties of the material, which are gauged by the Debyelength, LD. For a n-type semiconductor, as SnO2, in the Schottkyapproximation [18,26] (fully exhausted) the potential parabolicallydecays from the surface giving rise to an upwards band banding andto a potential energy barrier:

V(x) = V + eNd · [(x − x )2 − x2] (10)
S 2εS0 0
Ebarrier = −eVS (11)

where VS < 0 (the electrons are negatively charged) is the electricalpotential at the surface, Nd is the donor concentration (all donors

4 Actuators B 142 (2009) 470–493

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Fig. 5. The energy structure of an n-type semiconductor near the surface. The sameunidimensional view towards the bulk of the material, as in Fig. 4, has been used.It is important to note the generation of the occupied deep surface levels (acceptorin this example) after a chemical interaction of the gas phase (species X) with the

74 A. Oprea et al. / Sensors and

re supposed ionised), εS is the semiconductor permittivity, Ebarriers the barrier height seen from the semiconductor side and x0 is theepletion layer width:

0 =[− 2εS

eNd· VS

]1/2=

[− 2eVS

kBT

]1/2· LD; LD =

√εkBT

e2ND,A(12)

Because of the static charge separation at the surface, theepleted region behaves as a capacitor with the differential capac-

tance per area unit of [17]:

= εS

x0(13)

The bands bending affects all bounded energy states ending withhe vacuum one due to the fact that a slowly varying macroscopicotential (−e·V(x)) was added to the Hamiltonian of the system.owever, at the equilibrium, CP remains constant throughout theaterial, independent of the distance from the surface. In con-

equence (see Fig. 4) the WF, as the energy difference betweenacuum level at the surface and “Fermi level” value, becomes:

= −EF − e · VS = (EC − EF )b + �0 − e · VS (14)

he index “b” indicates the bulk values. The other symbols arexplained in the caption of Fig. 4.

If the bands bending is very high the Fermi level can cross theid-gap level (intrinsic Fermi level) and produce the inversion [18]

f the surface conduction type, meaning that the concentration ofhe minority charge carriers at the surface will exceed the one ofhe majority charge carriers.

Coming back to the WF value, it is important to recall the con-ribution of surface dipoles – just discussed in the previous section

that has to be added, if the case. Therefore, when both (surfacecceptors and dipolar adsorbate) are present:

= (EC − EF )b + � − e · VS

= (EC − EF )b + �0 − e · VS + e

ε· nS,ad · pn,ad (15)

For gas sensing the interesting surface states are the onesnduced on the sensing material surface by the surrounding atmo-phere through specific reversible chemo-physical effects. There arewo main types of reversible gas surface interactions:

1. Chemisorption, involving net charge transfer and resulting insurface filled states.

. Physisorption, resulting in a dipolar adsorbate layer.

The chemisorption can involve activated mechanisms asking forlevated operating temperatures [27].

Fig. 5 provides a description of the surface states and energycheme for a n-type semiconductor undergoing both chemisorptionnd physisorption processes. The initial material state is no moreupposed to be the flat band one and, therefore, the effect of the gasxposure is seen as changes in the relevant parameters (denoted byhe symbol �).

The most important remarks concerning the surface statesesulting from chemisorption are:

These states appear only if, and as long as, the adsorbate chem-ically bonds on the surface; from this point of view they areof different nature if compared to the intrinsic semiconductorsurface states, whose number is independent of the surface inter-
actions with the ambient atmosphere.These states can capture charge from the semiconductor, e.g., inthe case of ionosorption, interesting for gas sensing, the surfacecharge is built up from free charge carriers originating from thematerial CB or VB by a surface reaction of the adatoms with the
whole solid through its collectivised free electron system (CB). The dipolar layerresults from physisorption (of species Y) and shifts only the semiconductor boundstates. The index “s” indicates the surface parameters.

whole semiconductor body. The strength of the surface reactionand the resulting adsorbate amount are determined by the freecharge availability, the partial pressure of the gaseous oxygen andthe activation energy of the adsorption [28–34]. The key processhere is the physical feedback in the chemical mechanisms pro-vided by the height of the surface barrier. During desorption thecaptured charge carriers are released.

3.5. Electronic conductors in electrical contact

In order to build up a gas sensor, which reads out the WFchanges upon gas exposure with a field effect transducer, a ref-erence potential is required. This reference can be provided byan electronic conductor having its own WF insensitive to theambient atmosphere, if electrically connected to the sensing mate-rial/structure.

When two electronic conductors are in electrical contact, id est,when electrons are allowed to travel between them, the chargeexchange will take place until the CP will reach the same valueoverall in the system. As a consequence, at the interface of the con-ductors localised charges will appear as charge sheets in metals anddepleted regions in semiconductors.

For the gas detection examples discussed in the following para-graphs two cases are important: (1) metal–metal contact and (2)metal–semiconductor contact.

3.5.1. The contact of different metalsThe system composed out of two different metals in electrical

contact reminds the historical system of Volta, (also considered byW. Thompson in his work) (see Fig. 6) As explained in the captionof Fig. 6 and already accounted for in Section 2, the establishmentof the electrical equilibrium results in a CPD that is, in principle, thedifference of the two WFs.

The electrons at/near the Fermi level in the metal with smaller
WF, where they have higher energy, are leaving to go into the metalwith larger WF and lower electron energy at its Fermi level. Thisexchange produces a double layer of localised charge close to thejunction of the metals. Due to very high free electron density itextends over less than one lattice constant on both sides. Therefore,


Fig. 6. The electrical contact of dissimilar metals. The sketch displays the shape of thepotential energy of a single crystal with finite dimensions, relaxed at the surface. Theionic cores are suggested by circles. In the upper panel are shown the independentstats of two dissimilar and non-interacting metals. In the lower panel an electricalconnection (made for example by a wire as W. Thompson did) allow for the electricalequilibrium, bringing the Fermi levels at the same value on behalf of some electrontransfer from Metal 1 to Metal 2 (in the case of W. Thompson Metal 1 was Zn andMetal 2 was Cu, with WF1 of ∼3.7 eV and WF2 of ∼4.7 eV). Between the neighbourfo

ndvaamo

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Fig. 7. The MS structure. The band diagram of the separated metal and semiconduc-tor are given in the upper panel. The lower panel shows the band diagram of the MSsystem. The indices M and S refer to the material type. Dissimilar to Figs. 4 and 5the band profile in the lower panel is no more given along with the axis perpendic-ular to the free material surface, through which the probing electron is imaginary

ree surfaces of the metals, which vacuum level has been shifted by the alignmentf the Fermi levels, drops just the CPD. After [10].

o bands bending will practically occur and the potential has aiscontinuity at the interface that is just the CPD. For 1 V CPDery rough electrostatic estimations lead to a superficial chargemount of about 10 �C/cm2, a charge sheet separation of ∼1 Å anddouble layer capacity of ∼10 �F/cm2. In the calculations genericonovalent metals with a molar mass of 100 g/mol and a density

f 10 g/cm3 have been considered.

.5.2. The metal–semiconductor contactThis metal–semiconductor (MS) structure (Fig. 7) is based on

aterials from different classes and displays new and useful fea-ures. In applications extrinsic semiconductors are used, whoseonductance is due to shallow bulk energy levels obtained by con-rolled impurification. These semiconductors have typically chargearrier concentrations much smaller than the metals with whichhey are in contact. As in the previous case of two different metals,he realisation of the electrical equilibrium in the structure requiresfree charge carrier exchange. This leads to the depletion of a cer-

ain region of the semiconductor from its free electrons or holesetting the ionised dopants without counterbalancing free charge.f one repeats the estimation made for metals but with a free chargeoncentration of 1018 cm−3 (which is frequently the case for SnO2),ne obtains, by using (12), an extent of the space charge in the semi-onductor, x0,S in the range of 1 �m, that is four orders of magnitudearger than in metals.

The MS contact can be hardly influenced from outside, even ifome gaseous species would succeed to diffuse through the MSnterface because the Fermi level is pinned by the metal.

The approach we made, expressing the CPD of MS as the WFs
ifference (known as Schottky–Mott scheme), is not satisfactoryhen interface local states are present. More elaborated treatments
f the MS contact, taking into account the interface particularities,an be found in literature [35,36]. For gas sensing the MS contacts mainly used to provide a voltage reference. The WF readout isypically made with MIS structures.

“extracted” in order to evaluate the WF and its components, but on an axis parallelto this surface, as usual in the solid state standard images. This free surface is addi-tionally supposed to be inert to gases. If any gas adsorption should take place, thiswill happen throughout another free surface of the specimen.

3.5.3. Metal–insulator–semiconductor structures used by thefield effect transducers to detect gas changes in the ambientthrough induced WF change

The CPDs modulation by the gases in simple metal–metal ormetal–semiconductor structures cannot be directly readout withnormal electrical transducers (voltmeters) because of two principlereasons [10,37]:

- the voltmeters measure the differences in the electrochemicalpotential between different points of a system;

- the transducing process needs energy (even though very small)from the measured system.

To illustrate the first assertion, let us suppose a very sensitivemagneto-electric voltmeter having a coil made from a metal M3which connects the free ends of a metal–metal junction consistingof the metals M1 and M2, as shown in Fig. 8.

One would expect that the CPD between M1 and M2 willresult in a small current passing through the coil M3 and pro-ducing the angular deviation of the instrument indicator. This is,actually, not the case; the electrons will shortly flow only acrossthe junctions M1M2, M1M3 and M2M3 in order to equalise thechemical potentials resulting in constant, but not equal, vacuumlevels (electrostatic potentials) for each material, with the inher-ent discontinuities at the interfaces, due to WF differences. Thismeans that no electrical measurement procedure based on cur-rent flow is able to provide the CPD values. The detection of theCPD requires, therefore, electrostatic or non-equilibrium methods.
One can measure the CPD values/shifts by using, for example, theexternal electrical field between the non-free ends of two dissimilarelectronic conductors technologically brought in intimate electricalcontact.

476 A. Oprea et al. / Sensors and Actua

Fig. 8. The impossibility to use a magneto-electric voltmeter for the readout of theCPD. The electrical circuit is suggested in the upper panel. The metals M1, M2 andM3 are shortly indicated by 1, 2 and 3. Metal 3 represents the voltmeter “coil”. ThelN

sMdg

same for the interface with insulator; pn,dip is the dipole momen-

Fuid

ower panel shows position dependency of the vacuum level (electrical potential).o potential drops appear across the voltmeter coil; as well across the other ones.

The metal–insulator–semiconductor (MIS) structure is well
uited to demonstrate the method addressed above. Moreover, theIS structure is, in the same time, the main component of the MIS
iode and MOS-FET, the most utilised transducers for field effectas sensing in industrial applications.

ig. 9. Left panels: The band diagram of a MIS structure in equilibrium, that is, with thepper panel shows the “geometric” configuration, the left middle panel, the band diagra

ndices M and S refer to the metal and respective semiconductor parameters. Right paneipolar layer (DL) at the metal–insulator interface.

tors B 142 (2009) 470–493

In Fig. 9 the band diagram of the non-polarised MIS is sketched.By adding a polarisation voltage in the conducting path it is possibleto modulate the semiconductor depletion level, to bring the semi-conductor in inversion and, even more, to modulate its inversionlevel and, by that, the conductance of the minority free charge sheetformed at the interface with the insulator [18]. The same effect canbe obtained by modifying the metal/semiconductor WFs throughexternal factors such as the interaction of the metal or semicon-ductor surfaces with the gases.

It is important to point out here that the KP works on the basisof the same structure, having as dielectric the ambient atmosphereand, in some cases, a second metal as sensing material, instead ofthe semiconductor. The physical situation in the regions where onehas material discontinuities, i.e., the MIS and MS junctions, is shownin Fig. 9.

The polarisation voltage VP is typically applied by “cutting” themetal M and connecting there a bias source. From the condition thatthe integral of the electrical field along a closed loop throughout thestructure must be zero, one can compute the potential drop over theinsulator layer:

Vi = VSM − VSI − VP + 1ε

· nS,dip · pn,dip (16)

Here VSM denotes the voltage drop corresponding to the semicon-ductor band bending at its interface with the metal and VSI, the

tum perpendicular to the surface. Due to the capacitive distributionof the voltage drop over the semiconductor and insulator at the MISstructure side, the VSI term is negligible in comparison with the VI

term in the case of thick insulators, owing to the big differences

metal and semiconductor electrically connected and without polarisation. The leftm at the MIS side and the left lower panel the band diagram at the MS side. Thels: The same structure in non-equilibrium, due to additional biasing (Vp) and to a


Fi

bs

V

tSSdattrc(g

3

sAssv

ml(

C

wc

sitda

Fig. 11. Different contributions to the gas response of a MOX impedance reflectedby the equivalent circuits fitting the experimental impedance spectra (IS). Changesin the intergranular contact resistance and capacitance, bulk resistance and MOX-electrode contact resistance and capacitance have been identified. Intergranularcontact: the ionosorption of oxygen at the grain surface results in the creation ofpotential barriers and the corresponding depletion layers at the intergranular con-tacts. An intergranular contact can be represented electronically by a resistor Rgb

(due to the high resistive depletion layers) and a capacitor Cgb (due to the sand-wiching of high resistive depletion layers between two high conductive ‘plates’ ofbulk material) in parallel. The electrode contact can also be represented by a (RC)

ig. 10. The processes involved in the CO sensing with SnO2 in dry air: (1) O2

onosorption and (2) CO oxidation (SnO2 film reduction).

etween the insulator width, xi, and the Debye length, LD,S, of theemiconductor.

i = VSM − VP + 1ε

· nS,dip · pn,dip if xi � LD (17)

One frequently encounters this situation in the KP setups, wherehe role of the insulator is played by the instrument air gap (seeection 3.1) and, possibly, by some additional polymer layer (seeection 5.3). For the MIS-FETs, however, the ratio VSI/Vi stronglyepends on the constructive version; it is often close to the unitynd dedicated approaches are required (see for example [18] andhe references therein) to determine the connection between theransducer output parameter (usually its channel conductance,elated to the inversion level of the semiconductor) and the gasontrolled input parameter, usually the WF of the gate electrodethe metal in our generic structure, but a second, semiconductingas sensitive, layer in many gas sensing field effect devices).

.6. The work function role in gas sensing with metal oxides

The metal oxides (MOX) are the best known chemosensitiveemiconductors and, among them, SnO2 is the most utilised one.

cartoon picture of the processes occurring at the SnO2 sensorurface during CO exposure is displayed in Fig. 10. It reproduces thetandard mechanisms widely accepted for this sensing material andalid for the whole MOX class.

The two processes addressed in Fig. 10 build up the simplest, butost acknowledged, reaction scheme [38,39] on the MOX surface

eading to gas sensing (CO in the example) at elevated temperatures200–400 ◦C): They can be described as:

12

Ogas2 + e−

CB + Sk1⇔

k−1

O−ads (18)

Ogas + O−ads

k2→COgas2 + e−

CB + S (19)

here S is an unoccupied adsorption site and k1,2 are the reactiononstants.

Eq. (18) suggests that the atmospheric oxygen thermally dis-
ociates at the semiconductor surface (process 1 in Fig. 10) ands adsorbed as atomic ions by trapping conduction electrons fromhe solid (SnO2). Reducing gases (CO) can undergo a catalytic oxi-ation (towards CO2) as Eq. (19) shows, removing the ad-oxygennd releasing the captured electron back into the CB. Because of
element. The values of the resistor Rc and the capacitor Cc are independent of theambient gas atmosphere. The bulk contribution can be represented by a resistor Rb,whose resistance value is hardly influenced by changes in the ambient atmosphere.After [42].

the O2 adsorption, the crystallites/grains of the sensing film aredepleted, resulting in band banding and barriers at their interfaces(see Fig. 11). Therefore, the MOX resistance increases in compari-son to an initial state supposed to be the one acquired in an inertambient for the same operation temperature. The effect of CO isan opposite one, determining the decrease of the depletion degreeand barrier height. As a consequence, the resistance increase in airis partially canceled, to an extent depending on the analyte con-centration and reaction constants. The first observable effect of an-type MOX exposure to a reducing gas in an oxygen backgroundis, therefore, a resistance decrease. The reaction path in ((18) and(19)) is, however, not necessary the real one, mainly in the pres-ence of the humidity, but it is frequently utilised in quantitativemodelling (see for example [40]). A thorough approach addressingthe physics and chemistry of the sensing mechanisms with MOXscan be found in [41,42].

The sample resistance is not the only parameter that is affectedby the gas exposure. Fig. 11 also indicates the intergrain barriercapacitance and semiconductor WF as possible candidates for theoutput parameter role. The conductometric setup is the most sen-sitive when the current is passed across the intergrin barriersof the sample. Usually, the capacitances are measured with acbridges/capacimeters or impedance spectrometers [43]. The exper-imental data are fitted with equivalent electrical circuits, expressingthe sample properties and the processes experienced by it.

A careful analysis of the model sensing layer depicted in Fig. 11
[42] reveals, however, that the contributions to the dc conduc-tance and equivalent ac resistances and capacitances also reflectthe components of the material WF. So, the conductance Ggb, ofthe back to back intergranular barriers (gb) depends on the con-centration of the charge carriers able to overcome these energy

4 Actuators B 142 (2009) 470–493

bIcplt

G

tti

C

d

p

gcern

samtndt

3m

ltetiotfrOdt

drct(ou


arriers, Ebarrier = −e·VS, seen from the inner side of the grains.n non-degenerated semiconductors it is proportional to the bulkoncentration, nb, through the Boltzmann activation factor. The pro-ortionality constant is determined by the conduction mechanism

imiting the electrical transport in this region (diffusion or barrierhermal escalation) [41]:

gb ∝ nb exp[−|eVS |

kBT

]; Rgb = 1

Ggb∝ 1

nbexp

[ |eVS |kBT

](20)

The capacitive response comes from the space charge capaci-ance, which is proportional to the reciprocal of the square root ofhe potential barrier height [18] (Schottky approximation). For thentergrain barriers one has (see Eq. (13)):

gb = εS

x0∝

√εS

|eVS | (21)

Similar contributions bring the barriers between the semicon-ucting layer and the contacts, GC(RC) and CC:

GC ∝ nb exp[−�˚C

kBT

]; RC = 1

GC∝ 1

nbexp

[�˚C

kBT

];

CC ∝√

ε

�˚C(22)

They are not sensitive to the gases because of the Fermi levelinning by the metal (see Section 3.4.2).

Also, one has a conductance contribution from the bulk of therains, which is proportional to the bulk concentration of the freeharge carriers, Gb ∝ nb [10]. Gb is almost insensitive to the ambi-nt especially when the grains are sufficiently large (the depletedegion is much smaller then the grain size, �, and its variation doesot effectively modify the grain core diameter).

Summarising, one has to note that all gas interaction with theensing layers leading to electrical response (and not only), areccompanied by the modification of the WF. However, not all WFodifications are “visible” for all types of transducers. A change of

he WF determined by a modification of the electron affinity doesot by itself influences the resistance or capacitance of the layer. Toetect such effects, only field effect devices are suited, because ofheir sensitivity to the whole WF changes.

.7. Gas sensing with organic semiconductors through WFodification

The use of organic semiconductors (OS) for gas sensing has fol-owed the same directions, bringing into play the same types ofransducers. The sensing mechanisms are, however, quite differ-nt. Owing to their large molecules with different side chains orermination radicals the OS interaction with the surrounding gasess more complex. Notable limitations are encountered in the casef thermally activated sensing, where neither the physical proper-ies of the materials nor the suitable surface reactions are met. Thisact either reduces the chances to have reversible chemisorption oresults in long response and recovery times. In compensation, theSs allow new technological solutions evolving towards full organicevices integrated on plastic foils. Very enticing as transducers arehe organic FETs (O-FET).

The semiconducting properties of the organic materials areue to the delocalisation of their � molecular orbitals, normallyesulting in p-type conduction. The transport of the free charge
arriers is hindered by the reduced electric charge mobility (10−8
o 10−3 cm2/Vs) resulting in space charge limited currents (SCLCs)see, for phthalocyanines, [44,45]). Therefore, the production of fullrganic sensing FET is still challenging and the standard way tose OS for field effect gas sensing is to further deposit them as gas

Fig. 12. CuPc interaction with NO2. (a) Free CuPc and (b) CuPc in interaction withNO2.The LDA modelling indicates gas molecule bonding at the middle of the CuPcmolecule with a significant distortion of the molecular plane. From [55,56].

sensing gates in hybrid FET structures [46–48], based on inorganic(Si) semiconductors. Very popular for such applications are thephthalocyanines (Pc) and the porphyrines [49–52]. Besides them,also organic conductors have been utilised in hybrid FET configu-ration as sensing materials [53].

The WF changes of the OS are induced by gas adsorption throughmechanisms that are often matter of scientific debate; differentinvestigation approaches indicate different interaction sites andpaths. The CuPc infrared spectroscopy data [54] suggests the �rings as bonding places for NO2 while UPS spectra and the relatedmodelling point to the bonding of the nitrogen dioxide to the cen-tral metallic atom of the Pc [55–58], followed by a distortion ofthe molecule (see Fig. 12). We think that the photoelectron spec-troscopy information is more suited to explain the effects observedin field effect gas sensing, despite the large pressure gap (see alsothe considerations in Sections 2 and 4.1).

3.8. Gas sensing with non-conducting organic materials in WFsensitive structures

There is no reason to get WF responses from insulators them-selves. Lacking the electronic conduction they cannot establish anelectrical equilibrium with the electronic conductors but, surpris-ingly, significant voltage signals in KP setups have been reported[56,59,60]. The analysis of the acquired data revealed that in thiscase responsible for the WF gas responses are, in fact, the metallicelectrodes [61,62]. Initially thought to only “contact” the organicfilms to the measuring instrument, the conducting substrates areproved to have a quite different role in the behaviour of the inves-tigated samples, that is, to actually provide the WF shifts detectedin the experiments.

Because of the operation in the presence of humidity, the ques-tion of a whole/partial electrochemical origin of the response givenby the wet specimens needs also to be taken into account. This
concern was particularly legitimate in the case of the organic com-pounds with some ionic character, e.g., the polyacrylic acid (PAA)extensively studied for field effect gas sensing [56,61–66]. Com-bined electrical, electrochemical, gravimetric and spectroscopicmethods, to be addressed in more detail in Section 5.2, allowed


Fig. 13. The sketch of the zeroing CPD—measurement. In the upper panel is depicted the energy band scheme in 3 different circumstances while the lower panel illustratesthe associated “equivalent circuits”. The switch position indicates the connected/disconnected state of the electrode system. (a) The electrodes are not connected and donot interact (are separated and at large distances from each other); the electrostatic potential/vacuum level (Evac) of both materials is the same (0) and the Fermi levels laya als) arv differa its inp lue of

tca“

3

gctrbFritot

4i

tpgeitrgwtss

4

tma

KP operation and KP measurements [37,69–75]. A recent overviewon KP techniques and their application to chemoresistive gas sen-sors is given in chapter 8 of [7].

The instrument is, in fact, a vibrating capacitor electrometerreading the potential drop U0 across the air gap between its

t depths given by the values of the corresponding WFs. (b) the electrodes (materiacuum level is no more the same; the difference of the vacuum levels expresses thepplying an appropriate backing potential (VB) the vacuum level can be brought tootential indicator (the quadrant electrometer in the W. Thompson’s setup). The va

o exclude the electrochemical contributions to the observed WFhanges in the case of PAA based samples. Other dielectric materi-ls producing KP signals, where no electrochemical effects can besuspected”, will also be shortly mentioned there.

.9. Gas sensing with ionic conductors and field effect transducers

This kind of materials does not exactly fit the topic of WF basedas sensors. The gas responses they provide are due, in fact, to ioniconduction and electrochemical effects [67], usually described byhe Nernst equation [68]. Independent of their origins, these gasesponses, alike to the CPDs generated in electronic conductors, cane also detected with the electrometric transducers such as KP orET. Care is, therefore, required when interpreting the informationelated to materials that could be electrochemical active under thenfluence of the ambient atmosphere in order to discern betweenhe contributions having electronic origin and those having an ionicne. In this respect, the examples and discussions in Section 5.2 andhe references indicated there are very useful.

. Getting the experimental input: experimental tools andnvestigated samples

In the following, the investigation tools and the investigationarget being under consideration for WF based gas sensing will beresented. The investigation tools can be divided into two mainroups: devices and instruments on the one hand and complexxperimental setups on the other one. The investigation targets,d est the materials and the samples, will be tackled accordingo the material classes they belong to. When needed, the mate-ial preparation, deposition technologies, layer morphology, sampleeometry and sample structure will be addressed. The order inhich the topics are handled, beginning with the experimental

ools and techniques, favours the understanding of the sampletructure and preparation conditions requested by the WF mea-urements.

.1. Investigation tools

From the expertise gained during many decades, see Section 2,wo investigation tools seem extremely suited for WF measure-

ents in operation or operation-like ambients: the Kelvin probend the Gas-FET. The results obtained with UPS have reduced rel-

e in electrical contact; both materials have the same Fermi level and therefore theence in the electrostatic potential and is the CPD of the materials. (c) By additionallyitial value (0 overall); the correct tuning, that is VB = −CPD, is realised with a zeroCPD is thus the opposite of the balanced backing potential.

evance in gas sensing because of the pressure gap (see Section 2);moreover, UPS usually provides lower resolution than the normalpressure instruments.

Fig. 13 presents the operating steps of an atmospheric zeroingCPD measurement method based on different electronic conduc-tors (metal and semiconductor), such as the generic MIS structureanalysed in Section 3.5.

Thick insulators/air gaps are supposed and, therefore, the bandsbending in the semiconductor are no more visible at the figure scale(according to Section 3.5). The functionality of the setup remainsthe same when the semiconductor is replaced by a different metalin respect to the initial one. The main idea of the setup is to bringthe vacuum level of he semiconductor, modified with the CPD valueafter the electrical contact of the electrodes, back to its initial valueby using an additional backing potential VB (see the caption ofFig. 13).

4.1.1. The atmospheric Kelvin probe: principle construction,operation and types

The modern atmospheric KPs are based on Thomson [3] andZisman [11] designs (see Fig. 14) and follow the operating principlesketched above. Many papers and review papers are addressing the

Fig. 14. The schematics of the standard KP. The reference electrode (KP grid or tip) iselectromechanically actuated following a harmonic (sinus type) time dependency.

480 A. Oprea et al. / Sensors and Actuators B 142 (2009) 470–493

F rationc in thed

eo3

U

H

qpf

i

waif

tiT

i

wd

nr(

4

tta

I

aaB

tv

The operation principle is illustrated in Fig. 16. In fact, thenumeric procedure has to be repeated for each measured CPD valueby an algorithm performing on a computing system (microcon-troller or personal computer).

ig. 15. The schematics and the picture of the lock-in Besocke [70] KP 07 in opeontinuously measured, through VB, by using the condition of zero total voltage dropetector and the backing potential generator.

lectrodes. Therefore, U0 represents in fact the voltage drop Vi,KPn the insulation layer of the MIS structure discussed in Section.5. From Eq. (17), U0 results to be:

0 = Vi,KP = VSM + 1ε

· nS,dip · pn,dip − VP = CPD − VB (23)

ere CPD has a larger meaning, including also dipolar effects.If the mechanical oscillation is harmonic and small, which is fre-

uently the case, through the wiring of the KP an alternating currentroportional to the constant voltage U0 is flowing (see Appendix 1

or details and exact calculations):

(t) ∼= a2

s20

· ε · ω · U0 sin(

ωt + �

2

)= −I0 sin

(ωt + �

2

)(24)

here s0, a, ε, ω, t are, the static (rest) spacing of the electrodes, themplitude and angular frequency of the oscillation, the permittiv-ty of the atmosphere surrounding the KP electrodes, the angularrequency and, respectively, the time.

The relation (24) also holds if the voltage between the KP elec-rodes (U(t) this time) is no more constant but only slowly changingn time (see Appendix 1 for details and additional explanations).herefore, Eq. (24) can be used for the determination of the CPD:

(t) = −a2

s20

· ε · ω · U(t) sin(

ωt + �

2

)= −gKP · U(t) sin

(ωt + �

2

)

= −I(t) sin(

ωt + �

2

)(25)

here gKP is a device specific parameter with dimension of a con-uctance, in the following named KP conductance.

In order to easily obtain the value of the CPD and, simulta-eously, ensure high immunity against noise there are two mainegions in which the KP can be operated: (a) zero KP current andb) high KP current.

.1.2. Zeroing/locking Kelvin probesAs the name indicates, the zeroing KPs adjust the backing poten-

ial (VB(t)) to exactly compensate the CPD between the sample andhe reference electrode when, according to Eq. (25), the currentmplitude should be zero:

(t) ≡ 0 ⇔ U(t) = CPD(t) − VB(t) ≡ 0 ⇔ CPD(t) = VB(t) (26)

Usually the detection of the true balance is made with a lock-inmplifier and therefore the KPs employing this working regime are
lso known as locking KPs. The schematics of such an instrument,esocke KP from Besocke GmbH [70], is presented in Fig. 15:
This KP type is very simple and stable in operation. The modifica-ion of the static spacing in time does not directly influence the CPDalue, but only the accuracy and corresponding noise. Indeed, the

at the premises of IPC Tuebingen. VB denotes the backing potential. The CPD isKP circuit. The balance is realised through automatic feedback between the lock-in

modification of the KP capacitance will not modify the balance con-dition. Smaller KP capacitance, however, means smaller currentsin the unbalanced situations with effects on the zeroing precision.The main drawback is the possible systematic error when operatingwith non-harmonic KP currents, arising either from non-harmonicactuation or from reduced static spacing (assumption a s0 in Eq.(24) no more valid). In both cases, the lock-in detector will pro-vide the correct zero condition only for the selected fundamentalfrequency, “loosing” the input from harmonics and giving impre-cise/wrong CPDs.

4.1.3. Non-locking Kelvin probesThe non-locking KPs are designed to overcome the above men-

tioned disadvantages. The instrument is operated with increasedbaking potentials, many times larger then the measured CPDs,resulting in relatively high KP currents. On the basis of the acquireddata, the current amplitude dependency on the backing potentialis derived. According to Eq. (25), it is a straight line:

I(t) = gKP u(t) = gKP[CPD(t) − VB(t)] (27)

whose intercept with the abscissa correspond to the zeroing con-dition of the locking method:

I(t) ≡ 0 ⇒ CPD(t) = VB(t) (28)

Fig. 16. The dependency of the KP current on the applied backing voltage for ahypothetical pair of electrodes having 1 V CPD. The insets suggest the instant KPcurrent as resulting from Eq. (25). It is important to remark the proportionality ofthe KP current amplitude with the applied voltage (CPD–VB) and the 180◦ change ofthe phase when going from positive to negative values of the applied voltage.


of th

cfndtmucmM

4

sgpawtmma

aWt(tcirs

t

host a dipolar adsorbat layer modifying the value of its own electron

Fit

Fig. 17. The functional schematic of a non-locking KP and the picture

An example of non-locking KP sketch, roughly describing theommercial apparatus KPSP from KP Technology Ltd. and KP6500rom McAllister Technical Services, is shown in Fig. 17. Theon-locking KP has also a disadvantage. The KP conductance isepending on the electrode static spacing and the permittivity ofhe surrounding atmosphere (see Eq. (25)). Accidental/unintended

odifications of these parameters induce errors in the CPD val-es. Therefore, such instruments need an electronic feedback toompensate for the KP conductance variation during the measure-ents. Practical solutions are described by KP Technology Ltd., andcAllister Technical Services in their manuals.

.1.4. The “gas” field effect transistorThe G-FET realised by Lundström et al. [1,24,76–78] has been

hortly presented in Section 2 together with the associated hydro-en sensing mechanism, also proposed by Lundström. Due torinciple and practical interests for G-FETs, the device function-lity and the sensitive parameters deserve more insight. Here, weill mainly focus on the constructive types and transducing func-

ions of the G-FETs. The gas sensing mechanisms generating theeasured CPDs have been shortly discussed in Section 3; they areainly depending on the sensing material/sample structure. Some

pplication examples will be given in Sections 5 and 6.In the standard Lundström G-FET configuration, see Fig. 18, the

nalyte must diffuse through the gate in order to influence the metalF, seen from the MI interface, in agreement with the general func-

ional mechanism of field effect gas detection with MIS structuresSection 3.5, Fig. 9). This requirement restricts the applicability ofhe original G-FET to permeable gates and diffusive analytes. Theompetition between different gases, present in the ambient, and
nevitable gas phase or surface reactions can strongly modify theesponse of this type of devices (see for example the role of atmo-pheric oxygen in hydrogen sensing [24,78,79]).
In principle the G-FET output parameter is the conductance ofhe semiconductor inversion layer (transistor channel) in the linear

ig. 18. The schematic cross section of the G-FET with the polarisation and measurementt in the optimal operating point. VDD ensures the external energy necessary to supply the she semiconductor band banding in inversion regime.

e McAllister KP 6500, in operation at the premises of IPC Tuebingen.

operation region [18]:

gch = · CI · w

L· [VGS − Vth] (29)

where is the mobility of the minority carriers in the semicon-ductor, CI is the insulator layer capacity, w and L are the channelwidth and length, VGS is the gate source voltage and finally, Vth isthe threshold voltage.

The threshold voltage depends on the metal–semiconductorCPD and reflects the changes in the metal WF induces by the targetgases [18]:

Vth = ˚M − ˚S + 2e

|EF,S − Ei,S | +√

4εSND · |EF,S − Ei,S |CI

(30)

˚M/˚S are the metal/semiconductor WFs, EF,S is the semiconductorFermi level, Ei,S is the semiconductor mid-gap, εS is the semicon-ductor permittivity and ND is the semiconductor doping level.

For gases with large molecules suitable gates are, usually, notavailable because of the lack of electronic conductors having therequired gas permeability. Therefore, the suspended gate (SG) FETstructure developed by Janata for ion sensitive FETs [22,23] hasbeen adapted for gases in hybrid processing version [80,81]. Theschematic of a SG-FET with polarisation is shown in Fig. 20.

The device functionality remains, roughly, the same. A few dif-ferences have to be noticed: the gate can be either a metal or asemiconductor; the metallic gates usually ask for an additionalsensing film, able to provide a dipolar layer under the analyte influ-ence (Fig. 19, left); the semiconductor gates can either providethemselves a band bending in response to the target gases or to

affinity (Fig. 19, right).Independent of the gate material being chosen, the sensitivity of

the SG-FET is decreased by the decreasing of the effective insulatorcapacitance, which now includes the contribution from the channelpassivation and device air gap, connected in series.

circuits. VGG is providing the polarisation voltage of MIS structure, in order to bringensor output signal, namely, the source drain current controlled by the gas through


Fig. 19. Enhancement SG-FET for gas sensing: schematic representation of the functional elements: S-source, D-drain, SG-suspended gate, SL-sensing layer, and Ch-channel.T s beinc ence od el prel

sge

eteim(tmsttai

4

tbhattai

ptscbrvtt

Fcr

he device polarisation is standard, the drain (VDD) and gate (VGG) supplying voltagehannel conductance is produced by the modification of the gate WF under the influipolar layer location in the case of an insulating sensing layer, while the right pan

ayer band bending contributions are also possible (see Section 3.5).

The SG-FET is versatile and allows for different constructive ver-ions; among them, the flip-chip hybrid structure – with the sensingate substrate used as device support [82] – has found the largestxtent in applications.

The SG-FET gas sensors are susceptible to damages due to thexposure of the transducer part, the FET chip, to the target gases inhe same time with the sensing part, the gate structure. In order toliminate this drawback, the SG-FET was further improved. Follow-ng the design employed for the buried gate FET, common in static

emory circuits, a new FET transducer, the capacitively controlledCC) FET [83] was devised. It contains two sections: a capaci-ive divider including the gas sensing gate structure, externally

ounted in suspended configuration, and a buried-in MOS-FETharing its buried gate with the capacitive divider (see Fig. 42 in Sec-ion 6.3). Only the SG and the divider side of the chip are “seeing”he ambient. The transducing FET, providing the WF informationcquired by the sensing gate, remains protected from the negativenfluences of the surrounding atmosphere.

.2. Investigation setups

The tools presented above are often not able to deliver, alone,he expected data concerning the WF of the studied materialsecause of the complexity the interaction with the target gasesas. Moreover, in many cases, WF is not enough, by itself, to givethorough functional description of the gas sensing properties of

he considered systems and additional means are required to fulfilhe evaluation tasks. From these needs was born the approach welmost always use in our researches: complex/combined operandonvestigations.

By complex measurements we understand two or more com-lementary investigation techniques simultaneously performed onhe same sample. Fig. 20 gives an example for a planar MOX gasensor: the KP provides the WF shifts of the sensing film, as CPDhanges, while the film resistance provides insight on the bands
ending. Due to the dc polarisation of the semiconductor layer,equired by the resistivity measurement, the accuracy of the CPDalues obtained in these conditions could be questioned. However,he theoretical analysis and the experiments indicated and, respec-ively, proved that no systematic errors larger than the random
ig. 20. Combined KP and resistance measurements. The dc polarisation of the semi-onductor film required by the resistance evaluation is “seen” by the KP and, possibly,eflected by the CPD values (see the text).

g positive in respect with the source (for a n-type channel). The modulation of thef the dipolar layer created through the gas adsorption. The left panel illustrates the

sents a SG-FET containing a semiconducting sensing layer. For the semiconducting

ones affect the parameter values obtained in simultaneous KP andresistivity tests. Pioneering results have been provided by this tech-nique about three decades ago [84]. Interesting newer results andthe importance of simultaneous measurements are presented anddiscussed in Section 5.1.

Now one has to shortly address the “operando” investigations[85]. The term “operando” was introduced and is used in relationwith the catalysis experiments. There, and in the field of gas sensingas well, the surface phenomena and the temperature are determin-ing for the properties and behaviour of the analysed systems. Inorder to gain useful information, the test samples have to be inves-tigated in conditions similar to those in which the real sensorswill work. Thus, the “operando” concept synthesises this vision,equally referring to the ambient conditions and sample operatingregimes encountered in the applications. The combined/complexoperando investigations actually merge the two partial approachesjust addressed before. Fig. 21 provides the principle sketch of anoperando setup for gas sensing.

One of the most important parts of the system is the com-puter controlled gas mixing station supplying the measuring unitwith the artificial atmosphere needed during the evaluation of thesamples. Equally important are also the afferent gas analytics, theacquisition algorithms and the acquisition rates of the examinedfeatures.

4.3. The targets of the investigations: materials and samples

Three main classes of electronic conductors are known toundergo work function changes under gas exposure: the inor-ganic semiconductors, the organic semiconductors and the metals.The effects on the materials from the last category, excepting thenoble metals, are weak and not stable in chemically active envi-ronments [86–88]; however, polymer-metal bimorph structuresshowing effects comparable with those from the first two classeshave been reported [56,59,60]. The causes of these effects and thesensing mechanisms will be discussed later in Section 5.2.

In order to obtain correct WF results during sample specificinterrogation (KP or G-FET in operando setups) a good coating(without holes or significant pinholes) of the sample substrates(base electrode) is needed. For the KP measurements, the thick-ness and the roughness of the investigated layers are not critical;they extend from 50 nm to 100 �m or even more, depending onthe material type and properties. Thicker films could, however,present longer response/recovery times if diffusion processes arepresent. The sensitive gates of G-FET have to meet the specificdevice requirements; for certain types of G-FETs (SG-FETs) the
roughness is limited to a maximum of a few �m.
4.3.1. The inorganic semiconductorsThe inorganic semiconductors for gas sensors are prepared

through several methods: sol–gel, hydrothermal, spray pyrolysis,


Fig. 21. The sketch of an “operando” measuring system. One has to distinguish the measuring unit under artificial atmosphere, the reference gas analytics, the computercontrolled gas mixing station (consisting of gas sources, pipelines, mass flow controllers (MFCs), electro-valves, and driving computer) and the data acquisition compartment(computer and IEEE 488 connected instruments).

F es not

rapeo

Fb

ig. 22. SnO2 sample for KP and resistance measurements. The structure sketch do

heotaxial growth and thermal oxidation (RGTO), etc. [89,90]. Theyre deposited as thin/thick polycrystalline or granular (sinteredowders) films on substrates provided with at least one metalliclectrode, usually Pt or Au. For heating, the substrates have theirwn heating element, typically, a Pt meander.

ig. 23. PAA layer morphology and the outline of the PAA based sample for KP and SG-FEut the KP setup requires an additional holder, shown in the right panel.

respect the real proportion which are, however, visible on the TEM micrograph.

The sensing layer morphology is diverse and depends on thematerial nature and its preparation route. For standard KP measure-ments a surface roughness below 10 �m is more than enough. Scan-ning KP microscopy (not addressed by this paper) requires a higherspatial resolution and therefore a better surface quality. Fig. 22

T evaluations. The bimorph structures are identical for both interrogation methods,

4 Actuators B 142 (2009) 470–493

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4

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tafip

4

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5r

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Fig. 24. Change of the relative conductance of the �-Fe2O3 based sensors (�-Fe O –In O (Fe:In = 9:1) and �-Fe O ) as a function of ethanol and propanal


hows a planar sample suited for combined resistance and WF mea-urements by using the setup in Fig. 20. Here, one has to additionallyemark that it is strongly indicated to preserve the sensor geometrynd structure along with all measurements of a complex investiga-ion when consistent information on certain sensing material/layerre required. Changing the geometry and nature of the electrodesnd of the substrate can result in wrong/irrelevant inputs due toifferences in the target gas adsorption and conversion, the tem-erature and electrical field distribution, and the ratios betweenhe morphological and geometrical parameters of the layer.

.3.2. The organic semiconductorsThe organic semiconductors have been not addressed together

ith the inorganic ones, because of some major differences in thehysical and chemical properties, deposition methods and allowedperation conditions in sensor structures.

The sensing layers of the experimental samples, for both KP andG-FETs, are realised by either vacuum or low pressure physicalechniques (thermal evaporation and plasma coating) or, when pos-ible, through liquid phase procedures: spray coating, spin coating,et printing. They cover the contact layers (Pt; Au) present on theest substrates, usually made from alumina or silicon. The sampleayout is similar to the one presented in Fig. 23 for non-conductingolymers, the only difference being the nature of the organic coat-

ng.Due to their molecular architecture allowing for different cen-

ral metal atoms and side chains, the phthalocyanines have beennd still are the most investigated organic semiconductor class foreld effect gas sensing [45]. Nearby, other compounds such as theorphyrines have also been considered [75].

.3.3. The bimorph polymer–electronic conductor structuresThis kind of field effect sensing structures are based on noble

r corrosion stable electrodes (Pt; Au; stainless steel, conductingdoped) Si) covered with suitable polymers such as: polyacryliccid (PAA), poly(4-vinylphenol), poly(acrylonitrile-co-butadiene),olystyrene, etc. Here we include both the organic materials with-ut any electrical conduction and those with some residual ioniconduction, but without electrochemical activity. The metals areither deposited by adequate methods (thermal vacuum deposi-ion, screen printing, thermal treatment, etc.) or self-supportedamples on metallic substrates are utilised. The polymer technol-gy is much simpler, often spray, drop or spin coating proceduresre used. Semiautomatic or automatic deposition systems improvehe uniformity and reproducibility of the layers [64].

. Selected experimental results: work function signals inesponse to ambient composition in illustrative examples

The, more or less general, perspective on field effect gas sens-ng provided by the previous sections will be now completed witheveral interesting examples concerning unusual material featuresnder gas exposure or even unexpected gas responses.

.1. Metal oxides

.1.1. Surface conduction type change upon gas exposureThe conduction type, n or p, is an alternative way to refer to the

olarity of the majority carriers in a semiconductor. For the MOXmployed in the gas sensing the classification of conduction types based on the standard interaction scheme (see Section 3.6), of
he MOX with the oxygen, and the reducing and oxidising gases,n connection with the direction of the resistance change upon theas exposure. For example, a semiconductor showing a resistanceecrease when reacting with a reducing gas (as CO) will be classifieds n because, according to the reaction standard scheme, this is the
2 3 2 3 2 3

concentrations in dry air: G0, conductance in the dry synthetic air; G, conductancein the presence of the test gas. Measurements were performed on two identicalsensors, differentiated with full and open symbols on the graphs. After [92].

response to be expected from an n-type semiconductor. For a p-typeone would expect the opposite behaviour and so on (for all casessee [39]).

The conduction type established in this way is not always thematerial characteristic one. There are materials for which the con-duction type is changing with temperature, depending on the depthand state density of the local levels or on the mobility values (seethe references in [91,92]). The granular and porous materials usedfor gas sensing have another very strong reason to undergo suchconduction switches. Due to their huge free surface they easilyreact with the ambient, often through mechanisms involving thefree charge carrier from the bands, as discussed in Section 3.4. As aconsequence, depleted regions or even inversion layers can appearbecause of the adsorption processes. In line with the classificationcriteria of the conduction, an inversion will result in a change of thesurface conduction type. Here, the scientific issue is to distinguishbetween the possible chemical and physical origins of the observedeffects and to find out the mechanisms responsible for them.

We will give an example for the case of �-Fe2O3 based sen-sor materials [91,92]. An overview of the results of dc-conductancemeasurements for screen printed thick films is given in Fig. 24.

One observes that for low target gas concentrations all G/G0 val-ues of the relative conductance (in respect with the initial value)are lower than 1 indicating a p-type conduction. Under certainconditions, visible in the figure, the conduction type changes to n.To explain the causes simultaneously WF and resistance measure-ments have been performed. Fig. 25 shows the results.

A SnO2:Pd thick film sensor, presenting n-type conduction underall measurement conditions employed in the studies [42], wasadded as witness. For separated CO and humidity exposures thebehaviour of �-Fe2O3–In2O3 is that of a p-type semiconductor. Theethanol containing ambient can already determine the switch to n-type conduction if the ethanol concentration exceeds 25 ppm in drysynthetic air. Indeed, at 25 ppm ethanol the variation of the sampleresistance changes the direction. In a background of 50% relativehumidity the p–n switch happens already from the first test step of13 ppm ethanol. No resistance increase step is present anymore.

For all cases, the WF and the deduced band banding (−e·VS),monotonically decreases with the gas exposure “strength”, startingfrom the dry synthetic air condition, which was taken as refer-ence (in Fig. 27 the CPD values are depicted, which correspondingly
increase). Due to the continuous bands bending evolution underexposure, one can conclude that no changes in the nature of sur-face reactions are taking place. Hence, the chemistry of the surfaceis involved in the conduction type and conductance values only


F ) senst

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FsotuA

ig. 25. Changes of the resistance (1) and CPD (2) of the �-Fe2O3–In2O3 (Fe:In = 9:1he reference Pd/SnO2 sensor. After [92].

hrough the amount of charge being exchanged between the surfacedsorbate states and the semiconductor conduction and valenceands. Indeed, the slight p character in the synthetic dry air back-round is due to the weak inversion of the MOX film resulted fromhe oxygen adsorption (Section 3.4, Eq. (18)) as Fig. 26 demonstratesor the case of the precipitated �-Fe2O3 [91] samples.

In dry nitrogen and at very low partial pressures of dry oxy-en in dry nitrogen the material has n-type conduction. This facts easily observable because of the same direction of the conduc-ance change upon O2 concentration with that of the SnO2 witness
ample. Starting with ∼1000 Pa O2 in N2 the oxygen surface stateoncentration, having an acceptor character (see Section 3.4), isigh enough to determine a surface inversion layer visible in Fig. 26s conductance type inversion; the �-Fe2O3 and, respectively, SnO2
ig. 26. Changes of the CPD (1) and conductance G (2) of the precipitated �-Fe2O3

amples [91] under pure dry oxygen in dry nitrogen exposure. The conductance (3)f the witness Pd/SnO2 sample which is exposed to the same atmosphere and knowno show only n-type behaviour under the conditions of the measurements. In thepper part of the panel is displayed the profile of the oxygen partial pressure pO2 .fter [91].

or under exposure of water, CO and ethanol in dry and humid air; (3) resistance of

specimens are evolving in opposite directions when the oxygen par-tial pressure further increases. All this time the WF (see Fig. 26)continuously rises with the oxygen concentration indicating theincrease of the surface negative charge, surface barrier and deple-tion degree, ending up with the surface inversion. In syntheticair (∼20 kPa O2 partial pressure) the �-Fe2O3 and �-Fe2O3–In2O3should be well in inversion because of the clear p-character theyshow. Therefore, the initial state of the �-Fe2O3 based samples,when heated in air, is the inversion.

This initial state is marked with A in Fig. 27 and corresponds toband diagram (a) in Fig. 28. The weak interaction with the reduc-ing gases (CO) partially consumes the adsorbed oxygen releasing
some captured electrons in the CB. Consequently the inversion isreplaced by strong depletion only and the semiconductor reachesa compensate state (point B in Fig. 27; EF equalises the mid-gap inFig. 28b). Increasing the strength of the reaction with the reducing
Fig. 27. The conductance of �-Fe2O3–In2O3 (Fe:In = 9:1) sensor as a function of theband bending change (here �[−eVS] = (−eVS) − (−eVS,0) = e[VS,0 − VS] < 0), deter-mined by simultaneous work function and conductivity measurements at ∼320 ◦C.After [92].


Fig. 28. Schematic energy-level diagram of an n-type semiconductor. One starts with pure dry nitrogen atmosphere, where one can consider a flat band situation (case Oon the sketch). Oxygen adsorption (a) leads to the surface states which are occupied by the electrons, resulting or in a surface inversion layer (the Fermi level EF lies belowt educic matert

gmntbpb

5

rbFte

st

-

-

Fs

he intrinsic level Ei) and p-type surface conductivity. Weak interactions with the rharacter are observable. The strong interactions with reducing gases (c) bring thehe CB (0.078 eV), was shifted for visibility reasons.

ases (curves B and C in Fig. 27), the surface reduction becomesore pronounced, resulting in a material with low depletion and

-type surface conduction (as its bulk). This state corresponds tohe panel (c) in Fig. 28. On the basis of the conduction model foripolar conduction in MOX proposed by Bârsan [91], reference [93]resents an analysis of the conductance dependency on the bandsending giving more general view on the features analysed above.

.2. Organic materials

Few years ago significant gas responses from insulating mate-ials PAA, polystyrene, (poly(4-vinylphenol), poly(acrylonitrile-co-utadiene), interrogated with KP setups or SG-FET transducers (seeig. 29) have been reported. At the beginning the effect was ascribedo a standard WF chemo-physical modulation under the target gasxposure [56,59,60].

A more careful analysis [62] of the investigated specimenshowed the impossibility to really get WF outputs from PAA forwo reasons:

the WF cannot be defined for such a material, which is missingthe electronic conductivity;

−14 −12
the residual ionic [62,94] conductance, of 10 to 10 inthe absence of the humidity or at reduced humidity, is notlarge enough to allow KP responses at the operating frequency(∼200 Hz). Indeed, under the given conditions, the electric relax-ation time of the PAA layer is � = RC ≥ 1012 10−12 F = 1 s, while the
ig. 29. The calibration curve and the signals (inset) delivered by a 0.2 �m PAA KPample in 200 sccm synthetic air at 50% relative humidity and 30 ◦C. From [62].

ng gases (b) partially recover the band banding; for our samples both slight p or nial to pronounced n character. The real position of the donor level, much closer to

KP period only 10 ms, so that the specimen could not follow theKP tip even if, to some extent, conducting (see also the discussionat the beginning of Section 3.1.1).

For the origin of the KP signal few other possibilities remained:

- The polymer undergoes a standard sorption process [95] and themodification of the permittivity is detected by the KP setup as adielectric response.

- Due to the target gas effect a dipolar layer appears at the polymersurface.

- The structure polymer–electrode experiences electrochemicaleffects; this was suggested, in part, by the logarithmic depen-dency of the KP voltage on the concentration, which reminds theNernst equation (see Fig. 29).

- Due to the ambient composition a dipolar layer appears at thepolymer interface with the electrode.

The first two hypotheses have been excluded because:

- The dielectric contribution to the KP signals estimated for thegiven PAA layer characteristics is too small [62].

- The KP output saturates at a thickness of the PAA sample [62] ofabout 1 �m, which would be not the case if volume phenomenawere involved.

- The temperature dependency of the sensitivity [64] is weak andnear linear, not exponential, as would require a volume sorptioncontrolled by the partition coefficient.

- The magnitude [62], the time constants [61] and their ratios underdifferent exposure conditions for the gravimetric responses arecompletely different from the KP ones.

According to the considerations provided in Sections 3.8 and3.9, the electrochemical activity of the PAA KP samples was takeninto consideration, in a first step only from the principle viewpoint[62]. This analysis did not find strong reason for an electrochemicalorigin of the KP signals but could not exclude it. Therefore additionalexperimental evidence was sought. The cyclic voltammograms ofPAA specimens under different exposure conditions (see Fig. 30)displayed no electrode charge transfer reactions, so, also the third
potential reason for the KP response has been ruled out [61].
One has to prove now that the remaining possibility is the realone and, more importantly, to reveal the mechanisms behind it.Obviously, the material producing the KP signals, in the frame of thislast hypothesis, is the electrode of the sample; it can, in principle,


Fig. 30. Upper panel: The cyclic voltammogram of a PAA layer shows no dependenceon the ammonia concentration with only dry air in the background. Lower panel: Thecthr

r3

aaer

alfih

FaKlsh

Fig. 32. In humid air the gold surface is covered with adsorbed water molecules (A).

yclic voltammogram dependency on the humidity and ammonia concentration forhe same sample. In the presence of ammonia the diagram is similar to the one inumid air at higher levels of humidity. The absence of the electrode charge transfereaction is proved by the lack of the voltammogram peaks. From [61].

espond with WF changes to the ambient composition (see Section.3).

The scarce experimental data from literature address only neg-tive WF responses of Au to ammonia [86] or humidity [87,88]nd only in separated exposures. Combined exposure of the barelectrode (Au usually) to ammonia and humidity, gave interestingesults (see also [61]), shown in Fig. 31.

This behaviour is due to the competitive adsorption [61] of the
mmonia and water dipoles at the metallic surface. At low humidityevels there are still enough free adsorption sites on the gold sur-ace to accept more NH3, resulting in an additional WF decrease,.e. the downwards peaks of ammonia at 0% relative humidity. Atigh humidity levels (90% r.h. for example) the Au surface is pretty
ig. 31. The response of an uncoated gold substrate to ammonia pulses in dry air, 50%nd 90% relative humidity. The measurements were performed with the McAllisterP6500. It is important to remark the switch of the KP signal sign when going from

ow humidity (dry air) to high humidity. At 50% relative humidity the signal is stilllightly downwards but it will is switch the orientation to become upwards for higherumidity. From [61].

Ammonia interacts with these water molecules forming ammonia water complexesvia hydrogen bonds (B) which may desorb from the gold surface (C). Afterwards, thefree sorption sites can be occupied by either ammonia or water molecules (D). From[61].

hydrated and the ammonia rather interacts with H2O moleculesthan with the metal. Some water dipoles are removed and, par-tially, replaced by NH3 resulting in a net increase of the WF inrespect with the initial state with only 90% r.h., but without ammo-nia, owing to the dipolar momentum difference (1.85 D for waterand only 1.42 D for ammonia). Because in the KP measurement thesignal is currently referred to the initial state of the system at thegiven background, the observed NH3 pulses appear as positive athigh humidity. The case of 50% r.h. represents, for the given sample,the border between the two functional regions, additive and com-petitive, with a still slight unbalance in the favour of the additiveones, as the negative direction of the signal pulses indicates. Thewhole mechanism is sketched in Fig. 32.

At this stage it is important to address the role played by thepolymer in the sensing process [96]. Already in [62] PAA was seenas a concentrator of the gaseous ammonia from the ambient at itsinterface with the metal and, in the same time, as protection layer,able preserve the intrinsic sensitivity towards NH3 of the metallicelectrode. This mechanism should be valid in, both, dry or humidatmosphere.

Due to the interaction between ammonia molecules fixed at thegold surface and the carboxyl or carboxylate groups of the polymera second sensing mechanism is also possible [61]; it will be, now,discussed for the case of dry atmosphere. In the presence of the goldsubstrate, because of the dispersion effects, the free electron pairof ammonia undergoes mirror charge interactions with the metal-lic surface, resulting in a small internal charge transfer and, moreimportantly, in some localisation and bonding of this electron pair.Owing to the formed ammonia/gold complex, the strong Lewis basecharacter of the NH3 molecule diminishes, making it able to pro-vide hydrogen bonds to an oxygen atom in the polymer, in spiteof the fact that it usually does not act as a hydrogen bond donor[97]. As Fig. 33 shows, such processes increase the charge densityat the nitrogen atom directed towards the gold surface. This causesan enhancement of the local dipole moment at the metal surfaceand, consequently, the negative Kelvin probe signal increases inmodulus.

The effect of the ambient humidity has to be considered in theframe provided by some recent papers that thoroughly analyse thegaseous ammonia and humidity interaction with the PAA and itssalts [65,66,94]. They document the high ability of PAA to retainlarge amounts of water through inner hydrogen bonds avoiding thewetting of the electrode surface. Under these conditions the signalsof KP samples covered with PAA films display always negative WFresponses to ammonia, even at high ambient humidity. However, aslight decrease of the KP signal with the increase of the humidityalways occurs [64].

The mechanisms and the trends addressed above are more gen-eral than the system for which they have been analysed. Otherpolymers (poly(4-vinylphenol), poly(acrylonitrile-co-butadiene),polystyrene) and other electrodes (Ag, Fe, Si) are also providing KPsignals with significant amplitudes. Fig. 34 shows the behaviour of


Fig. 33. Interaction of ammonia with an uncoated gold surface (a) and the inter-action change if the ammonia additionally forms hydrogen bonds to a polymer asPAA covering the gold surface (b). Delta symbolises the dipolar charge of NH3; thedoubling of the signs (“+” and “−”) indicates the onset of the ammonia interactionwith the mirror charge in the metal accompanied by a small charge transfer insideof the NH3 molecule and the possible contributions from the neighbourhood. From[61].

Fig. 34. The response of an uncoated and PAA coated Ag substrate to ammonia pulsesin dry air and 50% relative humidity. The measurements were performed with theBesocke 07 KP. It is important to remark the “positive” contribution of the NH3 inhumid ambient.

Fig. 35. Schematic view of Lundstrom-FET/SGFET d

Fig. 36. Hydrogen sensitivity of Lundstrom-FET and SG-FET with Pt gate at low H2

concentrations. From [98].

the bare and PAA coated Ag, where the same general trends areobserved despite of the slow baseline drifts in the left panel.

6. Applications: using WF responses towardscustomised/dedicated gas sensors

6.1. Lundström field effect transistor (FET)

The original G-FET, devised by Lundström, with different designparticularities [98] using new materials [99] was improved dur-ing the years towards higher sensing parameters, reduced workingtemperature and other target gases. As an example we present acombined Lundström and SG FET sensor structure (see Fig. 35). TheLundström sensor is more sensitive and responds better at low H2concentrations, while the SG-FET has a near linear calibration curveup to the few percent range. Fig. 36 shows the combined sensorsignals.

Another development direction of the Lundström-FET, widelyemployed in gas sensing is the MIS diode; both I–V and C–V opera-tion modes are used [100–103].

6.2. Suspended gate FETs (SG-SET)

As underlined in Section 4.1.2 suspended gate FETs raised in rel-atively short time a lot of interest due to their high versatility. Goodresults for the detection of gaseous ammonia and nitrogen diox-ide have been obtained [50,63], by using p-channel FETs mountedwith polymer bumps through the flip-chip technology [82] on gas

ouble sensor in CMOS technology. From [98].


Fig. 37. Schematics of a Flip-chip SG-FET for gas sensing. The air gap is established bythe height of a SiO2 profile at 1–5 �m. A passivation layer of silicon nitride depositedover the channel oxide reduces the parasitic influence of the ambient humidity fluc-tuations. Even so differential operation is required to obtain pertinent results. Asrti

s1piGaipntptpwo

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films coated on separated Si wafers (∼1 mm × 2 mm). They are usedas SG in the hybrid transistor structure. In normal operating con-ditions, that is, at room temperature, the platforms dissipate less

econd channel without sensing layer on the suspended gate is used to provide theeference signal. The transistor well is a buried in n-type Si in which the FET struc-ure is realised. The whole chip substrate is made of p-type silicon. By underlinedtems are indicated the sensor elements shown also on the panel. From [50].

ensitive gates. At the isothermal operation point (drain current.2 mA) the FETs have a gate transconductance of 15 �A/V. A sim-lified sketch of a sensor from the third generation is presented

n Fig. 37. The sensor platforms have been produced by MicronasmbH in C-MOS technology and include the read-out electronicss well. In order to avoid the influences from the ambient humid-ty fluctuations the channel oxide was covered with a silicon nitrideassivation layer. Moreover, the sensor operates fully differentially;ear each sensing FET, having the SG coated with a sensing film,here is a reference FET, only used to compensate for the externalarasitic effects. The isothermal operating point is kept constanthrough a feedback circuitry controlling the transistor well (TW)otential [50,63] in respect with the ground (see also Fig. 37). In thisay the output parameter of the sensor is the amplified difference

f the TW voltages from the sensing FET and reference FET.By using PAA sensing films, such as those described in Section

.2 and the first generation of transducers (without an integratedmplification stage), gaseous ammonia sensing SG-FETs workingt room temperature were obtained. They have logarithmic gateensitivity of 25–30 mV/decade of concentration and about 3 ppmH3 detection limit (see Fig. 38), slightly worse than the valuesbtained with similar layers in KP setups.

Within the third generation of FETs, nitrogen dioxide sensors,ased on copper phthalocyanine (CuPc), have been developed.s shown below (Fig. 39), they are very sensitive (20 mV/decade

ogarithmic slope). The response time constant of 1 min and the

ig. 38. The responses of a PAA flip-chip SG-FET under ammonia exposure at roomemperature. First generation of FET platform, using external electronics was utilised.rom [63].

Fig. 39. The calibration curve of aCuPc flip-chip SG-FET exposed to NO2 at roomtemperature. The sensors are based on the third generation of FET platform, havingdifferential operation and integrated amplifying stage. After [50].

recovery time constant of only about 10 min are making themappropriate for alarm purposes or measurements in ambientswhere the expected NO2 concentration variation is slow. Con-centrations as low as 100 ppb have been measured in repetitiveexposures and a theoretical detection limit in ∼20 ppb range hasbeen deduced from the calibration curve and the noise level of theFET.

6.3. Capacitively controlled FETs (CC-FET)

The sensor based on CC-FET platforms are arrays containing 3pairs of p-channel MOS-FETs. One FET in each pair is employed formeasuring, while the other one acts as reference, like in the case ofthe CuPc-FETs described above. Each pair is connected to a differ-ential output stage resulting in a 3 signals “measuring path”. Thetransducer structure is given in Fig. 40. With 2.4 �m channel length,and 8 �m channel width it needs only ∼10 �A to reach the isother-mal operation point. Optimal air gaps of 1–5 �m were realised bypatterning some appropriate spacing layers. The chips contain addi-tional circuit elements such as: active heaters; gap thermometer,differential readout stages that make it very flexible during thetests.

The sensing elements of the sensors are thin organic or inorganic

than 1 mW.

Fig. 40. SG-CC-FET structure. On the left side is located the capacitive voltage dividerwhile in the right side the p-channel MOSFET. The buried gate connects electricallythe two parts. The gas sensitive film is deposited on the top electrode (SG) andseparated by the air gap from the passivation layer covering the buried gate. Byunderlined items are indicated the sensor elements shown also on the panel. From[49].


FtpF

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ig. 41. The responses of the SG-CC-FETs to gate potential changes (baseline arbi-rary set). The records are coming from many platforms and different measuringath. The gates were already covered with the sensing material, TiN in this case.rom [49].

A rough picture of the platform performance, here mainly refer-ing the gate sensitivity and its reproducibility among the differentas path and platforms is given by Fig. 41.

Two types of sensing materials have been employed: TiN formmonia and, respectively, CuPc for NO2 detection. The sensoresponses reproduce again the behaviour obtained with similar lay-rs in KP measurements [49]. Figs. 42 and 43 display calibrationurves of the sensors.

An interesting developing direction of SG-CC-FETs makes thebject of [104], which proposes and demonstrates a new principleo measure the hydrogen concentration in air. Instead of a sim-
le Pt catalytic gate, a bimorph structure poly-methylmetacrylate–latinum (PMMA/Pt) is employed as gate in a SG-CC-FET structure.he polymer controls the diffusion of the ambient gases, mainlyydrogen and oxygen, towards the buried Pt electrode and, by that,he chemo-physical state of its surface. If the permeability of PMMA
ig. 42. CuPc–CC-FETs responses to NO2 at 25 ◦C at 40% relative humidity (up) andheir calibration curves (down). Two different path of a platform are displayed. Theimilarity of the signals is remarkable.

Fig. 43. TiN FETs responses to NH3 at 25 ◦C at 0% and 40% relative humidity (up) andtheir calibration curves (down). The ambient humidity reduces the device sensitivity.From [49].

increases only to oxygen but not to hydrogen when the temperatureincreases, which is very plausible due to the high intrinsic diffu-sivity of the hydrogen, a discontinuity in the WF evolution uponhydrogen concentration will occur (see Fig. 44). The process is seen

Fig. 44. Upper panel: Work function change of the PMMA/Pt bimorph if exposedto different hydrogen concentrations at 25 and 75 ◦C. Lower panel: Resulting sensortemperature, if operated in the temperature controlled mode. From [104].

Actua

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y the authors as a “phase transition” dependent on temperaturend corresponds to a sudden enrichment of the Pt surface withydrogen. It is fully reversible, as Fig. 44 shows. Through a pro-rammed temperature variation one can bring the device in theF “jump” condition, easily detectable by suited electronics. The

alibration curve of the sensor operated in this regime is quite lin-ar over the broad concentration range between 0.1–4% H2, as theignals in Fig. 44 indicate. Due to its functional particularities, theuthors named this type of G-FET temperature controlled phaseransition FET, that is, TPT-FET.

. Summary

The literature overview and the few new results presentedere show, once more, the potential of the work function relatedxploratory techniques for gas sensing purposes. This fact is inde-endent of the nature of the investigated materials, the type ofhe used transducers or the sensing principles. It was interestingo observe the long and successful way laying behind the actualtage of the field effect gas sensing and the still open prospects. Thensight given by the review highlights the central role played by the

ork function modifications under gas exposure in understand-ng the fundamentals of some of the most important gas sensing

echanisms, indicating the best suited approaches for practicalpplication. It was shown that the work function response orig-nates from the surface chemo-physical processes, which involvehe whole electronic system of the electronic conductors acting asensing elements, either through charge transfer processes withhe bands or by shifting their energy spectrum. Depending on thearticular type of the surface interaction with the ambient atmo-phere, more or less complex contributions to the work functionre available for the sensing process and, therefore, only somelasses of transducers are appropriate. Among them, the field effectransducers (Kelvin probe, mainly for laboratory investigations, andeld effect transistors, as sensor transducers first of all) are theost versatile, providing signals for any work function modifica-

ion, independent of its origin. The completed overview also revealsome of the functional limitation indicating, however, the solutionsvailable to overcome them by using new types of devices or newperating regimes for the established ones. Moreover, it confirmshe increasing interest for the field effect gas sensing with the mainocus on the room temperature sensing mechanisms, low poweronsumption, integrated solution, autonomous operation, and fulllastic devices.

ppendix A. Kelvin probe operation

The Kelvin probe (see Fig. 14 in the main text) consists of aeference electrode vibrating due to some electromechanical actu-tion over a conducting sample. Both form a plan parallel capacitorVolta condenser) with the capacitance C depending on the elec-rode common area A, the spacing between the electrodes, s, and theermittivity ε of the medium in which the KP is immersed (usuallyir):

= ε · A

s(A1)

For a harmonic oscillation of the reference electrode in respectith its rest position one gets for the dynamic spacing s(t):

(t) = s0 + a sin(ωt) (A2)

here s0, a, ω, t are, in order, the static (rest) spacing of the elec-rodes, the amplitude and angular frequency of the oscillation and,espectively, the time.

In the following, the harmonically time depending parametersill be symbolised by small characters while the non-harmonically

tors B 142 (2009) 470–493 491

and slowly varying parameters will be addressed through capi-tals.

The corresponding time dependent KP capacitance will be:

c(t) = ε · A

s0 + a sin(ωt)= ε · A

s0· 1

1 + (a/s0)sin(ωt)

= C0 · 11 + (a/s0)sin(ωt)

(A3)

For small vibration amplitudes a in respect with s0 the capacitancebehaves almost harmonically:

c(t) = C0 · 11 + (a/s0)sin(ωt)

∼= C0 ·[

1 − a

s0sin(ωt)

](A4)

Let us suppose that between the electrodes of the KP condenserthere is a constant voltage difference U0; it can be, for example,only the CPD existing between the electrodes, due to their differ-ent nature, or the algebraic sum of CPD and a constant backingpotential, VB.

Under these conditions each electrode owns a charge q(t) of:

q(t) = c(t) · U0 = C0 · U0 · 11 + (a/s0)sin(ωt)

= Q0 · 11 + (a/s0)sin(ωt)

(A5)

Because of the variation undergone by the capacitance, thecharge q(t) has to periodically modify its value so that to keep con-stant the voltage U0, as it was assumed. This is only possible if someelectrons leave the negative electrode when c(t) is decreasing (theseparation s(t) of the electrodes is increasing) and pass through theexisting circuit to the other electrode, where neutralise a part ofthe positive charge existing there. When, in turn, s(t) decreases, thecapacitance has to increase together with the electrical field E(t)between the electrodes:

E(t) = U0

s(t)(A6)

In order to satisfy the requirements of the Gauss low (firstMaxwell equation) in this new electrical situation, the electronswhich previously left must come back to their initial electrode. Aslong as the electrodes are moving in respect to each other a chargeexchange between them will continuously occur. The key of thisprocess is the constancy of the voltage U0. If no dissipative elementsobstruct the way of the moving electrons (the involved conductorsare ideal) no electrical energy will be lost. Because this actuallynever happens some mechanical energy of the vibration will bespent to compensate the losses. It is also important to point outhere that the electrostatic picture of the KP operation being used iscorrect due to the relatively large value of the KP vibration period(more than 1 ms) when compared with the charge relaxation timeinside the electrodes (typically in ps range for metals and ns rangefor semiconductors).

Coming back to the KP functionality one has to observe that theperiodical charge exchange between the KP electrodes through theconnecting wires and other circuit elements (zero detector, backingpotential source) is actually nothing else than an electrical alternat-ing current:

i(t) = d q(t)dt

= Q0 · ddt

[1

1 + (a/s0)sin(ωt)

]

= −a

s20

· ε · ω · U0 · cos(ωt)

[1 + (a/s0)sin(ωt)]2

= −I0 · sin(ωt + (�/2))

[1 + (a/s0)sin(ωt)]2(A7)

4 Actua

Tttvgioct

i

wcm

R


he relation (A7) also holds if the voltage u(t) between the KP elec-rodes is no more constant but only slowly changing in time, that is,he characteristic modification times of U(t) are mach larger the KPibration period. This is frequently the case for the CPDs related toas sensing, where the gas sensing KP sample follows the changesn the composition of the ambient atmosphere not too fast, becausef limited response time (usually more than 1 s). Therefore Eq. (A7)an be utilised for the determination of the CPD, if explicitly rewrit-en to describe the setup in Fig. 14:

(t) = −a2

s20

· ε · ω · U(t) · sin(ωt + (�/2))

[1 + (a/s0)sin(ωt)]2

= −gKP · U(t) · sin(ωt + (�/2))

[1 + (a/s0)sin(ωt)]2(A8)

here gKP is a device specific parameter with dimension of aonductance, let us name it KP conductance (KPC). Additional infor-ation and references are provided by [7].

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Biographies

Alexandru Oprea received the diploma in physics from the University of Bucharestin 1976 and the PhD in solid state physics from the Central Institute of Physics,Bucharest, Romania in 1996. He is senior scientist in the Gas Sensor Group of theUniversity of Tübingen, Germany. The research fields: thin films solar cells, highfield electroluminescent devices, polymer and metal oxide gas sensors.

Nicolae Bârsan received in 1982 his diploma in physics from the Faculty of Physics ofthe Bucharest University and in 1993 his PhD in solid state physics from the Instituteof Atomic Physics, Bucharest, Romania. Since 1995 he is a researcher at the Instituteof Physical Chemistry of the University of Tübingen and actually is in charge with

Udo Weimar received his diploma in physics 1989, his PhD in chemistry 1993 andhis habilitation 2002 from the University of Tübingen. He is currently the head ofGas Sensors Group at the University of Tübingen. His research interest focuses onchemical sensors as well as on multicomponent analysis and pattern recognition.

Work function changes in gas sensitive materials: Fundamentals and applications

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