chemical sensing and catalysis by one - dimensional metal - oxide nanostructures

D V A N Andrei Kolmakov and Martin Moskovits Department of Chemistry and Biochemistry, University of California, Santa Barbara, email: akolmakov@chem.ucsb.edu, mmoskovits@ltsc.ucsb.edu K

D V A

N

Andrei Kolmakov and Martin Moskovits

Department of Chemistry and Biochemistry, University of California,

Santa Barbara, email: akolmakov@chem.ucsb.edu, mmoskovits@ltsc.ucsb.edu

Key Words one-dimensional nanostructures, sensors, catalysis

■ Abstract Metal-oxide nanowires can function as sensitive and selective chemical

or biological sensors, which could potentially be massively multiplexed in devices ofsmall size The active nanowire sensor element in such devices can be configuredeither as resistors whose conductance is altered by charge-transfer processes occurring

at that their surfaces or as field-effect transistors whose properties can be controlled

by applying an appropriate potential onto its gate Functionalizing the surface of theseentities offers yet another avenue for expanding their sensing capability In turn, becausecharge exchange between an adsorbate and the nanowire can change the electron density

in the nanowire, modifying the nanowire’s carrier density by external means, such asapplying a potential to the gate, could modify its surface chemical properties andperhaps change the rate and selectivity of catalytic processes occurring at its surface.Although research on the use of metal-oxide nanowires as sensors is still in early stages,several encouraging experiments have been reported that are interesting in their ownright and indicative of a promising future

INTRODUCTION

Chemical and biological sensors have a profound influence in the areas of sonal safety, public security, medical diagnosis, detection of environmental toxins,semiconductor processing, agriculture, and the automotive and aerospace indus-tries (1–4 and references therein) The past few decades has seen the development

per-of a multitude per-of simple, robust, solid-state sensors whose operation is based onthe transduction of the binding of an analyte at the active surface of the sensor to

a measurable signal that most often is a change in the resistance, capacitance, ortemperature of the active element

The evolution of gas sensors closely parallels developments in microelectronics

in that the architecture of sensing elements is influenced by design trends in planarelectronics, and one of the major goals of the field is to design nano-sensors thatcould be easily integrated with modern electronic fabrication technologies For

Figure 1 A cartoon of a nanowire-based electronic nose The nanowire surfaces arefunctionalized with molecule-selective receptors The operation is based on molecularselective bonding, signal transduction, and odor detection through complex patternrecognition.

example, the current goal is to replace the large arrays of macroscopic individualgas sensors used for many years for multicomponent analysis, each having itsassociated electrodes, filters, heating elements, and temperature detection, with

an “electronic nose” embodied in a single device that integrates the sensing andsignal processing functions in one chip (5–8) Multicomponent gas analysis withthese devices is accomplished by pattern recognition analogous to odor identifica-tion by highly evolved organisms (Figure 1) (9–11) By increasing the sensitivity,selectivity, the number of sensing elements, and the power of the pattern recogni-tion algorithms, one can envision a potent device that can detect minute quantities(ultimately one molecule) of an explosive, biohazard, toxin, or an environmentallysensitive substance against a complex and changing background, then signal analert or take “intelligent” action However, this requires an increase in the sensitiv-ity and selectivity of active sensor elements despite the loss of active area and theincreased proximity of neighboring individual sensing elements as the individualcomponents are miniaturized Recent progress in materials science and the manynew sensing paradigms originating out of nanoscience and technology, particu-larly from bottom-up fabrication, makes one optimistic that these goals are withinreach

Metal oxides possess a broad range of electronic, chemical, and physical erties that are often highly sensitive to changes in their chemical environment.Because of these properties metal oxides have been widely studied, and mostcommercial sensors are based on appropriately structured and doped oxides.Nevertheless, much new science awaits discovery, and novel fabrication strate-gies remain to be explored in this class of materials by using strategies based

prop-on nanoscience and technology Traditiprop-onal sensor fabricatiprop-on methods make use

of pristine or doped metal oxides configured as single crystals, thin and thickfilms, ceramics, and powders through a variety of detection and transduction prin-ciples, based on the semiconducting, ionic conducting, photoconducting, piezo-electric, pyroelectric, and luminescence properties of metal oxides (4, 12–14).Chemical and biological sensors having nanostructured metal oxides and espe-cially metal-oxide nanowires benefit from the comprehensive understanding thatexists of the physical and chemical properties of their macroscopic counterparts(15)

This review is limited primarily to semiconducting devices with dimensional nanostructures such as nanowires and nanobelts Likewise, we restrictourselves to two related device configurations: conductometric elements and field-effect transistors A few issues relating to real-world sensors and sensor arrays arealso discussed

quasi-one-Numerous quasi-one-dimensional oxide nanostructures with useful properties,compositions, and morphologies have recently been fabricated using so-calledbottom-up synthetic routes Some of these structures could not have been createdeasily or economically using top-down technologies A few classes of these newnanostructures with potential as sensing devices are summarized schematically inFigure 2 These achievements in oxide one-dimensional nanostructure synthesisand characterization were recently reviewed by Xia et al (16) and others elsewhere(17–19) Much work has also been published on the use of carbon nanotubes,individually or as arrays, as sensors (20–25) Although we do not refer to thiswork (which has also been thoroughly reviewed) (26–30), the great progressmade to date in understanding the electronic properties of carbon nanotubes, their

Figure 2 A schematic summary of the kinds of quasi-one-dimensional

metal-oxide nanostructures already reported (see reviews 16, 17) (A) nanowires and nanorods; (B) core-shell structures with metallic inner core, semiconductor, or metal-oxide; (C) nanotubules/nanopipes and hollow nanorods; (D) heterostructures; (E) nanobelts/nanoribbons; (F) nanotapes, G-dendrites, H-hierarchical nanostructures; (I) nanosphere assembly; (J) nanosprings.

reactivity toward gases, photochemical properties, junction effects, and mance when configured as transistors certainly informs the discussion of all quasi-one-dimensional systems We therefore acknowledge the great debt we owe to thatliterature in establishing and clarifying many of the key questions pertaining toquasi-one-dimensional nanostructures.

perfor-The properties of bulk semiconducting oxides have been extensively studiedand documented Not so those of quasi-one-dimensional oxide nanostructures (i.e.,systems with diameters below∼100 nm), which are expected to possess novelcharacteristics for the following reasons:

(a) A large surface-to-volume ratio means that a significant fraction of the

atoms (or molecules) in such systems are surface atoms that can participate

in surface reactions

(b) The Debye length λD(a measure of the field penetration into the bulk)for most semiconducting oxide nanowires is comparable to their radiusover a wide temperature and doping range, which causes their electronicproperties to be strongly influenced by processes at their surface As a result,one can envision situations in which a nanowire’s conductivity could varyfrom a fully nonconductive state to a highly conductive state entirely on thebasis of the chemistry transpiring at its surface This could result in bettersensitivity and selectivity For example, sensitivities up to 105-fold greaterthan those of comparable solid film devices have already been reportedfor sensors on the basis of individual In2O3nanowires (31) The signal-to-noise ratio obtained indicates that∼103molecules can be reliably detected

on a 3-µm-long device By shortening the conductive channel length to

∼30 nm, the adsorption of as few as 10 molecules could, in principle, bedetected

(c) The average time it takes photo-excited carriers to diffuse from the interior

of an oxide nanowire to its surface (∼10−12–10−10s) is greatly reducedwith respect to electron- to-hole recombination times (∼10−9–10−8s) Thisimplies that surface photoinduced redox reactions (Figure 3) with quan-tum yields close to unity are routinely possible on nanowires (assumingreactants reach the surfaces rapidly enough and interfacial charge transferrates are not limiting) The rapid diffusion rate of electrons and holes to thesurface of a nanostructure provides another opportunity as well The recov-ery and response times of conductometric sensors are determined by theadsorption-desorption kinetics that depends on the operation temperature.The increased electron and hole diffusion rate to the surface of the nanode-vice allows the analyte to be rapidly photo-desorbed from the surface (∼afew seconds) even at room at temperature

(d) Semiconducting oxide nanowires are usually stoichiometrically better

de-fined and have a greater level of crystallinity than the multigranular oxidescurrently used in sensors, potentially reducing the instability associatedwith percolation or hopping conduction

Figure 3 A summary of a few of the electronic, cal, and optical processes occurring on metal oxides thatcan benefit from reduction in size to the nanometer range.

chemi-(e) Nanowires are easily configurable as field-effect transistors (FETs) and

potentially integratable with conventional devices and device fabricationtechniques Configured as a three-terminal FET, the position of the Fermilevel within the bandgap of the nanowire could be varied and thus used toalter and control surface processes electronically

(f) Finally, as the diameter of the nanowire is reduced, or as its materials

prop-erties are modulated either along its radial or axial direction, one can expect

to see the onset of progressively more significant quantum effects (32)

Surface Reactions on One-Dimensional Oxides,

Gas Sensing, and Catalysis

The exploration of the metal-oxide nanostructures as a platform for chemical ing is a recent event Yang and coworkers fabricated and tested the performance

sens-of individual SnO2single-crystal nanoribbons configured as four-probe tometric chemical sensors both with and without concurrent UV irradiation (33).Photoinduced desorption of the analyte can lead to rapid detection and reversibleoperation of a sensor even at room temperature A detection limit∼3 ppm and re-sponse/recovery times of the order of seconds were achieved for NO2 Comparingthe performance of the ohmic nanoribbon sensors with those that showed rectifi-cation led the authors to conclude that the nanoribbons themselves dominate thephoto-chemical response and not the phenomena occurring at the Schottky barriers

conduc-Figure 4 Top: TEM, HRTEM, SEM images of an individual SnO2 nanoribbon: (A) low magnification, (B) atomically resolved, and (C) deposited on previously prepared

Au electrodes Bottom: The conductance response of the nanoribbon to NO2pulses inair with simultaneous 365 nm irradiation (after Law et al 33)

A wide array of potentially useful one-dimensional metal-oxide nanostructures,including nanobelts, were synthesized and characterized in Wang’s group (19, 34)and in other laboratories (see 16, 17 and references therein) Comini et al (35)configured groups of the SnO2nanobelts between platinum interdigitated elec-trodes and assessed their behavior at 300–400◦C under a constant flux of syntheticair The nanobelt sensors showed excellent sensitivity toward CO, ethanol, and

NO2 NO2could be detected down to a few parts per billion

Individual SnO2and ZnO2single-crystalline nanobelts (30–300 nm width and10–30 nm thickness) (34) were configured as FETs and studied by Arnold et al

(36) The electrical properties of these individual nanobelts in vacuum, in air,and under oxygen, as a function of thermal treatment, suggested that the oxygenadsorption and desoption dynamics depends sensitively on the concentration ofsurface oxygen vacancies, which, in turn, affect the electron density in the nanobelt.CdO nanowires, nanobelts, and nanowhiskers are prospective active elementsfor LEDs and lasers from nanostructures The Zhou group (37) showed that

in vacuum, as-prepared CdO nanowires have a carrier concentration of

∼1.3 × 1020cm−3arising from oxygen vacancies and interstitial Cd dependent conductance measurements indicate an activated process with Ea∼ 13.3meV at high temperature, switching to tunneling conductance below 30 K Theconductance of single nanowires exposed to 200 ppm of NO2(an oxidizing gas)

Temperature-at room temperTemperature-ature dropped by∼30%

Kolmakov et al used nanoporous alumina as a template for synthesizing arrays

of parallel Sn nanowires, which were converted to polycrystalline SnO2nanowires

of controlled composition and size (38) Conductance measurements on these dividual nanowires were carried out in inert, oxidizing, and reducing environments

in-in the temperature range∼25–300◦C (39) At high temperatures and under an inert

or reducing ambient, the nanowires behaved as highly doped semiconductors orquasi-metals with high conductances that depended weakly on temperature Whenexposed to oxygen, the nanowires were transformed to weakly doped semiconduc-tors with a high conductance activation energy The switching between the highand low conductance states of the nanowires was fully reversible at all tempera-tures Configured as a CO sensor, a detection limit of ∼a few 100 ppm for CO

in dry air and at 300◦C was measured with these SnO2 nanowires, with sensorresponse times of∼30 s

The above observations can be largely accounted for in terms of mechanismsdeveloped over many years to explain the function of polycrystalline metal-oxidegas sensors (40–43) This mechanism is outlined below, using SnO2 nanowires

in the presence of oxygen (an electron acceptor) and CO (an electron donor) as amodel system for oxide semiconductor systems more generally Specific departuresfrom this general picture are pointed out for individual cases and for other surfaceadsorbate molecules when necessary

The surface of stoichiometric tin oxide (a large bandgap semiconductor) is atively inert Even moderate annealing in vacuum, or under an inert or reducingatmosphere, causes some of the surface oxygen atoms to desorb, leaving behindoxygen vacancy sites (Figure 5) Likewise, exposure to UV results in oxygen pho-todesorption (or of other surface species) even at low temperatures Essentially, allexperiments carried out to date on metal-oxide nanowires (or other nanostructures)indicate that the role of oxygen vacancies dominates their electronic propertiesalong much the same lines as they do in bulk systems Each vacancy results in theformation of a filled (donor) intragap state lying just below the conduction band

rel-edge (Figure 5c) The energy interval between these states (or at least some) and

the conduction band is small enough that a large fraction of the electrons in thedonor states is ionized even at low temperatures, thus converting the material into

Figure 5 (a) Stoichiometric SnO2(110) surface, (b) partially reduced SnO2withmissing bridging oxygens Molecular oxygen binds to the vacancy sites as an electronacceptor CO molecules react with preadsorbed oxygens Electron are released back

to the nanowire [a,b after Kohl (14) with modifications] (c) Oxygen vacancies make

SnO2into an n-type semiconductor (d ) When the Debye length is comparable to the

radius of the nanowire, adsorption of electron acceptors shifts the position of the Fermilevel away from the conduction band

an n-type semiconductor At a given temperature the conductance of the nanowire,

G = πR2eµn/L, is determined by the equilibrium conditions determining the

rel-ative concentrations of (singly or doubly) ionized surface vacancy states, whichdetermine the electron concentration in the bulk of the material (Surface defectscan also migrate into the interior resulting in bulk defects that are clearly much lessresponsive to surface processes, and their low diffusion constant implies that theyare normally not important participants in the material’s sensing action, which re-quires a response time faster than the inverse diffusion rate However, bulk defects

do contribute to a sensor’s long-term stability.)

The conductance of SnO2changes rapidly with gas adsorption as a result of a(usually) multistep process wherein the first is the adsorption of a molecule (for ex-ample, with O2, might dissociate into two surface oxygen ions after chemisorption)with a consequent molecule-to-SnO2charge transfer (or vice versa) With oxygen

as the adsorbate, the afore-mentioned surface vacancies are partially repopulated,

which results in ionized (ionosorbed) surface oxygen of the general form O βS −α.The resulting (equilibrium) surface oxygen coverage,θ, depends on the oxygen

partial pressure and the system temperature through the temperature-dependent

adsorption/desorption rate constants, k , on the concentration of itinerant

electrons, n, and the concentration of unoccupied chemisorption (vacancy) sites, N s.

(whereα, β = {1,2} accounts for the charge and molecular or atomic nature of the

chemisorbed oxygen (44)) In forming ionosorbed oxygen, electrons become calized on the adsorbate, creating a∼30–100-nm-thick, electron-deficient surfacelayer corresponding approximately to the Debye length for SnO2(in the tempera-ture range 300–500 K), which results in band bending in the surface region of bulksamples For 10–100 nm diameter nanowires, the charge-depletion layer encom-passes the entire nanowire resulting in a so-called flat-band conditions whereinthe relative position of the Fermi level shifts away from the conduction band edge

lo-not only at the surface but throughout the nanowire (Figure 5d ) Ultimately, a new

kinetic equilibrium among the free electrons and the neutral and ionized vacancies

is re-established Under these nearly flat-band conditions at moderate tures and for electron momenta directed radially, electrons can reach the surface

tempera-of the nanowire with essentially no interference from the low electrostatic barrier

As a result, the electrons become distributed homogeneously throughout the entirevolume of the nanowire Accordingly, the charge conservation condition simplifiesto

Ns · θ = R

2 · (n − n m), where n m is the density of itinerant electrons remaining in the nanowire afterexposure to the adsorbate The accompanying electron depletion n = 2Nsθ/R

results in a significant drop in conductance:

in-Upon adsorbing a reducing gas such as CO, the following surface reaction takesplace with the ionoadsorbed oxygen

β · CO gas + O −α

βS → β · CO gas

2 + α · e−which results in the reformation of the adsorption (defect) sites and the redonation

of electrons to the SnO2(Figure 5b) It can be shown that under flat-band

condi-tions the increase in electron concentration, nC O ∼ p α+1 β , and therefore in the

conductivity of the nanowire, GCO ∼ e · n CO (T ) · µ(T ), increases

mono-tonically with CO partial pressure (44) This was confirmed experimentally on

nanowires assuming O−(α, β = {1}) to be the dominant reactive surface species

(39) The foregoing simple mechanism is able to account for the operation oftin-oxide nanowire sensors under ideal ambients consisting of dry oxygen and acombustible gas such as CO In a real-world environment, a large array of othermolecules (chief among them, water) complicates the picture Surface hydrox-yls and hydrocarbons can temporarily or permanently react with adsorption sitesmodifying or adding to the possible reaction pathways

A consequence of being able to shift the position of the Fermi level of the oxidenanowire by applying an external field or by doping the nanowire is the possibility

of controlling molecular adsorption onto its surface (resulting in the oscillation ofthe adsorbate between an electron donor and acceptor) An interesting instance ofthis was reported recently (46) with In2O3nanowires exposed to NH3 For nano-wires with a low density of oxygen vacancies (corresponding to a Fermi levellower in energy within the bandgap), the adsobate behaved as an electron donorcausing the resistivity of the nanowire to increase upon exposure to ammonia.With a higher oxygen vacancy density (the Fermi level nearer to the lower edge ofthe conduction band) the NH3behaved as an acceptor, quenching the nanowire’sconductance (Figure 6)

Single Nanowire FETs

The architecture of a typical nanowire-based FET is shown in Figure 7 Thenanowire acts as a conducting channel that joins a source and drain electrode Theentire assembly rests on a thin oxide film, which, itself, lies on top of a conducting

(in this case p-type Si) gate electrode (This is a so-called back gate configuration.

A top gate can also be deposited on the nanowire as an alternative.) Tuning ananowire’s properties by configuring it as the conductive channel of a FET was

Figure 6 Alternating donor (right plot) versus acceptor (left plot) behavior of

NH3adsorbate as a function of the doping level of an In2O3nanowire (taken fromZhang et al 46)

Figure 7 A schematic traces the response of the nanowire’s conductance

to the charge state of the adsorbed molecules [adapted with permission from

Nano Lett 2002 Copyright Am Chem Soc (49)].

recently reported with single metal-oxide nanowires and nanobelts (36, 47) Thepioneering effort in this regard is from Lieber, who has also reported the opera-tion of nanowire sensors in an aqueous medium (48) This FET device consists

of an individual Si nanowire acting as the conductive channel whose thin, nativeoxide skin, used as the gate oxide, is functionalized with target-specific receptors.These receptors change their charge state when bonded to their target species Thelayer of molecular or ionic receptors essentially acts as a polarized gate electrodemodifying the carrier density inside the Si nanowire and, therefore, its conduc-tance By terminating the surface with 3-aminopropyltriethoxysilane, calmodulin,

or biotin receptors, the nanowire was used, respectively, as a pH monitor, a Ca2+ion concentration detector, and as a sensor for a variety of biomolecules pH sens-ing resulted from the change in the charge state of the amine as it gained or lostprotons in response to the pH of the surrounding medium A similar approach was

used by this group to create bistable switches (49) By covering a semiconductingnanowire with an oxide layer of variable thickness (Figure 7, top panel) and thenbonding cobalt phthalocyanine, which is capable of existing in two or more redoxstates to the oxide, the layer of molecules acted as a virtual gate electrode alteringthe carrier concentration in the nanowire by changing the redox state of the cobalt

phthalocyanine (Figure 7a,b) These molecules bonded to the oxide could also be

made to accept or donate electrons either by varying the potential applied to aglobal back gate or by pulsing the bias voltage of the nanowire

Although the primary goal of this particular study was to create a nanoscalelogic device, the results demonstrate the subtle interplay between the chemistry

at the surface of the oxide and the nanowire conductance Sweeping the voltagefrom negative to positive values then back again often shows significant hysteresisand other memory effects These effects normally arise from trapping of charges

at various interface (and other) sites, frequently residing there for a long time, or(in the absence of heating or irradiation with light) indefinitely This also leads to

a better appreciation of the variety of effects that a nanowire sensor must contendwith when operating in a real-world environment

Arnold et al (36) succeeded in creating working FETs out of pristine, individualSnO2 and ZnO2 single-crystalline nanobelts and investigated their operation inair, in vacuum, and after admitting low concentrations of oxygen and nitrogen

in vacuum chamber The performance of the devices as a function of thermalpretreatment in air and in vacuum was also reported Nanobelts with conductingchannels as short as 100 nm and as long as 6µm were used These devices exhibited

excellent switching ratios (the resistance ratios between the ON and OFF states)

up to 106and electron mobilities as high as 125 cm2V−1s−1at room temperature

in air The channel conductance and the threshold voltage (defined as the value ofthe gate potential required to turn the device on) of the devices were found to besensitive to the gas environment and thermal pretreatment

For example, nanobelts annealed in vacuum were found to have such highelectron densities that they could not be gated at reasonable values of the gatevoltage However, exposure to even low concentrations of oxygen dramaticallydepleted the electron density in the nanobelt and shifted its threshold potentialtoward more positive Vgvalues, as previously indicated should be the case for an

n-type semiconductor The switching ratio of the nanobelt FET was also found to

be strongly dependent on the channel length For channel lengths in the range of

100 to 500 nm, no significant modulation of the conductance with gate voltagewas observed, implying some, as yet, poorly understood size dependence on theperformance of nanowire-based FET devices

Zhou and coworkers (31) characterized and explored the room-temperatureelectronic properties of individual In2O3nanowires (∼10 nm diameter) configured

as FET sensors IDS(VDS) measured in atmospheres consisting of trace amounts

of NO2or NH3mixed in Ar showed significant conductance decreases when thetarget gas was introduced (Figure 8) Both trace gases behaved as oxidizers Apartfrom the dramatic increase of the resistance, exposure to oxidizing gases induced

Figure 8 Top: IDS(VDS) measured in pure Ar before and after 1% NH3is admitted

at room temperature Note different left and right scales Bottom: the corresponding

I(VG) (after Li et al 31)

positive shifts in the threshold voltage to values exceeding 15 V, consistent with anappreciable reduction in the carrier density inside the nanowire The dynamic range

of the resistivity change, Rgas IN/Rinert, due to 100 ppm of NO2, was measured to be

∼106, an impressive increase Concentrations as low as 500 ppb and response times

of the order of a few seconds (with 100 ppm of NO2) were reported Recovery ofthe nanowire’s original conductance following the normally irreversible adsorption

of the target molecule was accomplished using UV irradiation At high values

of VDS, IDSis often observed to saturate, occasionally with portions of the I(V)curve showing negative differential resistance (Figure 8 top panel) The observedcurrent saturation at high-bias voltage (the so-called pinch-off effect) is typicalfor FETs It results from the electron depletion near the drain electrode when it

is at a high potential The negative differential resistance, on the other hand, is anintriguing observation The authors tentatively ascribed this effect to bias-inducedredistribution of electron density in the conduction channel closer to the nanowiresurface, where increased scattering probability degrades the electron mobility

One can see from above discussion that setting the gate potential at an priate value increases the sensitivity S = Ranalyte IN/Rambientof a nanowire sensorconfigured as a FET As an example, for the In2O3 nanowire sensor shown inFigure 8, setting Vgto−10 V results in a value of S ∼ 2 for 1% of NH3, whereasunder the same conditions the sensitivity of the device increases 105-fold at Vg =

appro-−30 V This electronic control over sensitivity is one of the major promisingcharacteristics of nanowire-based FET sensors, especially when they are incor-porated into arrays in their eventual real-world applications The question arises:Can one control the selectivity of a FET sensor using the same general approach?

If so, a more subtle, and potentially chemically more interesting effect might beobservable Because many target gases are detected through a catalytic reaction(for example, CO detection is carried out using the catalytic oxidation of CO to

CO2 on the surface of SnO2), gate control of sensor selectivity implies control

of surface reactivity, including catalysis, by changing the potential applied to thegate Recent experiments on the oxidation of CO on SnO2nanowires suggest thatthis is indeed possible (50, 51) This effect is not unique with nanowires Indeed,such concepts are integral aspects of the electronic theory of catalysis and gassensing by semiconductors (52) The effect of an electrostatic field on the sur-face chemistry has also been reported for thin film semiconductors and FET-basedchemical sensors fabricated by traditional technologies (53–57) The differencebetween what one expects with bulk systems and with nanowires, however, isone of extent The combination of factors such as the comparability of the Debyelength and nanowire radius, the small number of carriers present in the nanowire(typically∼105electrons), the high surface-to-bulk ratio, and its small capacitancesuggests that a relatively small number of adsorbed molecules can alter the car-rier concentration significantly Reciprocally, the removal or addition of a smallnumber of electrons can alter its surface chemistry measurably For example, be-cause chemisorbed oxygen intermediates are required in the catalytic oxidation of

CO, and a small number of electrons is required to create these adsorbed oxygenspecies, the catalytic oxidation of CO should then be significantly controllableusing relatively small values of the gate voltage (Figure 9)

The source-drain current, ISD, of a SnO2nanowire configured as a back-gatedFET was measured at constant VSD as a function of the gate-to-source voltageunder flowing gas with various partial pressures of nitrogen, oxygen and CO.(Figure 9, top panel) Baseline values of IDSwere established through prolongedexposure of the system to dry N2 while maintaining the system at the selected

gate potential and temperature At time t 1, 10 sccm of oxygen gas were mixed into

the 100 sccm nitrogen flow This was followed at time t 2by the addition of CO(5 sccm) into the gas flowing into the cell The steady-state value of the source-drain conductance in the dry nitrogen atmosphere, GN2, decreases monotonicallyand significantly as the gate potential becomes more negative and increases alittle and then saturates at positive values of the gate potential (Figure 9, bot-tom panel) This behavior is expected SnO2is an n-type semiconductor hence

a negative gate voltage will cause the electron density in the nanowire to crease Also plotted in Figure 9 is the conductance decrease, Gox y, following

de-Figure 9 (a) Response of a SnO2nanowire’s current to the sudden addition of oxygen

to the flowing nitrogen gas at time t1followed by the addition of CO at time t2atvarious values of the gate potentials at 553 K GN2is the steady-state current in a drynitrogen environment; GO2(CO), the values of the conductance decrease (increase)when O2(CO) gas is sequentially admitted into the gas cell (b) The reactivity of

oxygen, G oxy, and CO, G CO, as a function of gate voltage as measured by the total change in conductance determined from the response curves shown in (a) The

nanowire conductance GN2under dry nitrogen is included for comparison Also shown

is the extent of reaction of CO through a putative second-reaction channel that doesnot involve ionosorbed oxygens as a reagent (51)

recently reported with single metal- oxide nanowires and nanobelts (36, 47) Thepioneering effort in this regard is from Lieber, who has also reported the opera-tion of nanowire sensors in an... class="page_container" data-page="12">

used by this group to create bistable switches (49) By covering a semiconductingnanowire with an oxide layer of variable thickness (Figure 7, top panel) and thenbonding