InTech-Wide Bandgap Semiconductor One Dimensional Nanostructures for Applications in Nanoelectronics and Nanosensors

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Nanomaterials and Nanotechnology

Wide Bandgap SemiconductorOne-Dimensional Nanostructuresfor Applications in Nanoelectronics

and NanosensorsInvited Review Article

Stephen J. Pearton1,* and Fan Ren2

1 Department of Materials Science and Engineering, University of Florida, Gainesville FL 32611 USA2 Department of Chemical Engineering, University of Florida, Gainesville FL 32611 USA* Corresponding author E-mail: [email protected]

Received 21 November 2012; Accepted 15 January 2013

2013 Pearton and Ren; licensee InTech. This is an open access article distributed under the terms of the CreativeCommons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract Widebandgap semiconductor ZnO, GaN and

InNnanowireshavedisplayedtheabilitytodetectmany

types of gases and biological and chemical species of

interest.Inthisreview,wegivesomerecentexamplesof

using these nanowires for applications in pH sensing,

glucosedetectionandhydrogendetectionatppm levels.

Thewide

bandgap

materials

offer

advantages

in

terms

of

sensingbecauseof their tolerance tohigh temperatures,

environmentalstabilityandthefactthattheyareusually

piezoelectric. They are also readily integrated with

wirelesscommunicationcircuitryfordatatransmission.

KeywordsGaN,ZnO,InN

1.Introduction

The explosion of interest in nanoscience, coupled with

growing

demand

for

reliable,

lowpower

chemical

sensors forawidevarietyof industrialapplications,has

led to a surge in the development of nanostructured

materials for gas detection [15]. Onedimensional (1D)

nanostructures, such as nanowires, nanorods and

nanobelts, are particularly suited for chemical sensing

duetotheirlargesurfacetovolumeratio[611].Interms

ofmaterialselection,widebandgapsemiconductorsare

ideal for gas sensing, having numerous advantageous

properties,including

an

ability

to

operate

at

high

temperatures (or alternatively, a low leakage current at

room temperature), radiation and environmental

stability, and mechanical robustness. The literature of

morethanadecadeofresearchindicatesanimprovement

in consistent growth methods and the potential for the

immediate industrial use of 1D semiconductor

nanostructures. Specific semiconductor materials

including IIInitrides such as GaN [1213] and InN [14

16], metal oxides including ZnO and SnO2 [1718], and

hightemperaturematerialssuchasSiC[19]haveseenthe

greatest interest for chemical gas sensing applications,

chieflyfor

the

detection

of

H2,

O2,

NH3

and

ethanol.

There

isalso interest in applying these tobiomarkerdetection

[2130].

Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional

Nanostructures for Applications in Nanoelectronics and Nanosensors

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There is a strong need formobile, accurate, low power

sensorsthatcanbeusedinapplicationssuchaspersonal

health monitoring, security, perimeter defence, food

spoilage, the monitoring of nuclear materials and gas

leaks. Currently, many of these applications are

monitoredby

lab

based

methods,

such

as

enzyme

linked

immunosorbent assay (ELISA), particlebased flow

cytometric assays or electrochemical measurements.

These methods have impressive sensitivity and

reproducibility. However, many of the applications

mentionedabovewouldbenefit from theability tohave

sensing capabilities inabroader rangeofenvironments.

To move these out of lab environment requires

miniaturization and new approaches to solving power

consumption and handheld capability. In particular, it

wouldbegood tohaveawirelesscapability forsending

sensor data to a remote monitoring site and also to

miniaturizethe

sensors

so

as

to

allow

for

truly

mobile

or

longtermremotemonitoring.Thetechniquesmentioned

above all have significant limitations in terms of using

them outside of controlled lab environments, both in

terms of the size of the components and power

requirements. For longterm monitoring applications,

selfpowering techniques mustbe developed as well as

powercontrolmethodsthatminimizepowerconsumption.

Itisalsodesirabletominimizethesensorweightandsize

requirements and to produce a sensor that is consistent

with other handheld devices. Structures such as

nanowires are attractive candidatesbecause of their ease

and

the

low

cost

of

synthesis,

low

power

consumption

and

compatibilitywithexistingsemiconductorcomponents.

Semiconductorbased sensors can be fabricated using

maturetechniquesfromtheSichipindustryand/ornovel

nanotechnology approaches. Sensors in these harsh

environmentsmust selectivelydetecthydrogen at room

temperature or below while using minimum power.

Several groups have already demonstrated the

application of nitride and oxide semiconductor

nanostructures for exclusive H2 sensing using carbon

nanotubes (CNTs)andZnO.Several issuesmust stillbe

addressed, however, including quantifying sensitivity,

improvingdetection

limits

at

room

temperature,

establishing the reproducibility and stability of the

sensorsandreducingpowerconsumption.Materialssuch

as ZnO show sensitivity to environmental exposure,

particularlyintermsofformingsurfaceconductinglayers

in the presence of oxygen or water vapour exposure.

Thus, the issueofsurfacepassivationandencapsulation

for the sensors is another area thatmustbedeveloped.

Thisisnotexpectedtobealongtermimpediment,since

polymerbased organic electronic and display materials

haveasimilar issuewhichhasbeenmitigated toa large

extentby employing appropriate methods to limit the

exposureof

the

material

to

ambient

conditions.

Therearemanypotentialapplications forwidebandgap

semiconductor nanowire devices because of their

improved carrier confinement over their thin film

counterparts. For GaN nanowires, there are possible

applications in low power and high density fieldeffect

transistors(FETs),

solar

cells,

terahertz

emitters

and

UV

detectors.Thehighsurfacetovolumeratioofnanowires

means that if their surfaces are sensitive to external

stimuliorcanbefunctionalizedtobesensitivetospecific

chemicalsorbiogens,thentheyarelikelytobeattractive

forgasandchemicalsensorarrays.ZnOisapiezoelectric,transparent,widebandgapsemiconductorusedinsurface

acousticwavedevices.Thebandgap canbe increasedby

Mgdoping.ZnOhasbeeneffectivelyusedasagassensor

materialbasedon thenearsurfacemodificationofcharge

distribution with certain surfaceabsorbed species. In

addition, it is attractive forbiosensorsgiven thatZn and

Mgare

essential

elements

for

neurotransmitter

production

andenzymefunctionality.

In this review, we discuss the progress of nitride and

oxide semiconductor nanostructures for nanoelectronic

devicesandforchemicalandgassensing,specificallyfor

hydrogen.Thefunctionalizingofthesurfacewithoxides,

polymers and nitrides is also useful in enhancing the

detection sensitivity for gases and ionic solutions. The

widebandgaps of these materials make them ideal for

solarblindUVdetection,whichcanbeusedfordetecting

fluorescence from biotoxins. The use of enzymes or

adsorbed

antibody

layers

on

the

semiconductor

surface

leads to thehighlyspecificdetectionofabroadrangeof

antigensofinterestinthemedicalandhomelandsecurity

fields.TheuseofcatalystmetalcoatingsonGaN,InNand

ZnO nanowires hasbeen found to greatly enhance the

detection sensitivity for hydrogen. Pt and Pdcoated

GaN nanowires biased at small voltages show large

changes in currents upon exposure to H2 gas at

concentrationswithin the ppm range. Improvements in

growthtechniquesforInNnanostructureshaveproduced

nanobelts and nanorods capable of hydrogen detection

down to 20 ppm after catalyst coating. Functionalized

ZnO nanorods were also investigated for hydrogen

detection,but

did

not

generate

arelative

response

as

high

asthatforthenitridebasedsensors.

2.GalliumNitride(GaN)nanostructurebasedsensors

GaNsuseasachemical sensor iswelldocumented [26

34].GaNhasahighbreakdownfield,canoperateathigh

temperatures inexcessof400Candhasdecent thermal

conductivity in bulk, lowdefect wafers, making it an

effectivematerial choice forgas sensing.Schottkydiode

devices using thin Pd or Pt contacts have detected

hydrogen at concentrations of hundreds of ppm at

temperaturesas

low

as

100C.

It

is

generally

accepted

that

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H2 isdissociatedwhen adsorbedonPt andPd at roomtemperature.Thereactionisasfollows:

H2(ads)2H++e

Dissociatedhydrogencausesachangeinthechannelandconductancechangeofthenanowirebyalteringthezerobiasdepletiondepth.

While reports on nanostructured GaN gas sensors areincreasing, the difficulty in achieving reproduciblematerial has limited theirnumber in the literature.Thevarious growth techniques used for nanowire synthesisarestillalongwayfromstandardizationand,asaresult,haveproducedawiderangeof1DGaNnanostructuresofvarying crystal quality.The current growthmethods of1D nanostructured GaN include arcdischarge, laser

assistedcatalyticgrowth,thedirectreactionofamixturein a tube furnace and,most recently, chemical vapourdeposition (CVD) from Gacontaining precursors andNH3oncatalystcoatedsubstrates.

The use of CVD for 1D nanostructure preparation isbased on the vapourliquidsolid (VLS) growthmechanism in which solid GaN nanowiresnucleate/precipitate on catalyst nanoparticles of(typically) In,Au,Fe,NiorCoat the substrate surface.Althoughaminimalnanowirediameterisdirectlyrelatedto device sensing capability and the desired optical

characteristics, thediameter is limitedby theminimumsize of catalyticmetalbeading at the surface. PreviousreportshavedemonstratedthegrowthofGaNnanowireswithminimumdiametersof6nmusingmetalcomplexesinstead of elemental metals as surface catalysts fordeposition .GaN nanowire growthby CVD has issueswiththehighmeltingpointofGaprecursors,whichmayencourageGaoxidationwithresidualoxygenintheCVDchamber.AhydrogengascoflowwithNH3preventstheoxidationofGaNnanowiresupondeposition.

For our work, a growth substrate for GaN nanowiredepositionwas preparedby ebeam, evaporating 15 gold onto (100) Siwith 100nm thermally grownoxide.GaNnanowiresweredepositedby thereactionof liquidgallium and NH3 in a 1in. atmospheric quartz tubefurnace.Thesubstratewasheatedto850Candannealedfor15minunderArambientinordertoformdiscreteAunanoparticles on the surface.Growthwasperformed at850Cfor2 5hunderaflowrateof1517sccmNH3and300 sccmH2. The typical length of the produced GaNnanowires was 210 m. The addition of the metalcatalystmakesa largedifference to thesensitivityof thesensors,asshowninFigure1.TheinsetofFigure2(top)shows the morphology of asgrown GaN nanowires

before functionalization. The characterization of thenanowiresshowedahighquality,singlecrystalwurtzite

structure [12,13]. It turns out to be surprisinglystraightforward to achieve good crystal qualitynanowiresusingavarietyofdifferentsynthesismethods.Forsystems suchasZnO, it isalsopossible togrow thenanowires at temperatures around room temperature,

which makes them compatible with a large variety ofsubstrates, including plastic, tape and even paper. Theability to use arbitrary substratesmakes this a versatileapproach.

Figure1.ChangeinresistanceofGaNnanowires,eitherwithorwithoutPd coatings, as a function of gas ambient switched inand out of the test chamber. (Reprintedwithpermission fromRef.110,W.Lim,J.S.Wright,B.P.Gila,J.L.Johnson,A.Ural,T.Anderson,F.RenandS.J.Pearton,Appl.Phys.Lett.93,072110(2008).CopyrightAmericanInstituteofPhysics).

Often, the addition of a functional catalytic layer to thesensing substrate surface is a crucial step in theeffectiveness of wide bandgap sensors. Catalyticfunctional layers work to dissociate molecularcompounds into atomic components which thenbondeasilyontothesensorinterface;forexample,thecatalyticdissociation of H2 into 2Hby Pt metal. Although thismechanism of dissociationby functional layers for H2sensing is wellunderstood, the material choice of thefunctionallayerisstilldisputedforGaN.BothPtandPdhave generally been accepted as choice metals forfunctional coatings on nitridebased sensors; however,

there havebeen few reports comparing the twometalfunctionalities. We would emphasize that the metalcatalyst layer is discontinuous and there is no currentpath from thecontacts to themetal.This factrulesoutachangeintheconductivityofthemetalduetothestorageofhydrogenascontributingtothesensorsignal.

OurGaNnanowireswerefunctionalizedforH2detectionwith thin Pt or Pd films (~100 thick) depositedbysputtering.Deposited functional layerswerechecked fordiscontinuityat thesurfacebySEM.ThemorphologyoffunctionalizedGaN nanowires is shown in the inset ofFigure2.Al/Pt/AuOhmicelectrodescontactingbothendsof multiple nanowires were deposited by sputteringusing a shadow mask. Au wires werebonded to the



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contact pad for currentvoltage (IV) measurements

performedat25150CinH2atvaryingconcentration(10

3000ppm)inN2ambience.Notethatnounderlyingthin

film of GaN was observed for the conditions used to

growthetestednanowires.

Figure2.Relative responsesofmetalcoatedGaNnanowires tovaryinghydrogenconcentrationsatroomtemperature.Top)Pt

coating.Bottom)Pdcoating.InsetsshowGaNnanowiresbefore

(asgrown;top)andafterPtdeposition(bottom).

Figure3.RelativeresponsesofPtcoatedInNnanobeltstovaryinghydrogenconcentrationsatroomtemperature.

2.1HydrogenDetectionUsingGalliumNitrideNanostructuresFigure 3 shows the timedependant relative response

(R/R)of themetalcoatedGaNnanowiresensorsas the

gas ambient is switched from air to varying hydrogenconcentrations inN2 ambience and thenback to air.As

shown from the difference in Figure 3 (top) and 3

(bottom), PdcoatedGaN nanowires produced a higher

relative response over the same nanowires as when

functionalized with Pt. Comparatively, Pdcoated GaN

nanowiresshowedrelativeresponsesof~7.4%at200ppm

up to ~9.1% at 1500 ppm.The relative responses ofPt

coatednanowireswere~1.65%at200ppmup to~1.85%

at 2000 ppm. Uncoated nanowires had little or no

effective response to changes inmeasurement ambient,

i.e.,theyshowednosignificantdetectionofhydrogen.

It is clear that both Pt and Pd are effective in their

catalytic dissociation of molecular H2 into atomic

hydrogen. Although it takes >5 min for hydrogen to

saturate relative to the responseofPtcoatednanowires,

the initial resistance change is noticed in less than 2

seconds ofH2 exposure forbothmetalcoatednanowire

sensors. This suggests that the diffusion of hydrogen

throughthemetalcoatingisnotthelimitingfactorinthe

timeresponseof thesensors,butrather that it is limited

by the mass transport of gas into the enclosure. This

factorwasconfirmedbyalteringtheH2introductionrate

into the test chamber. While the resistance change

depended on hydrogen gas concentration, variations to

change upon hydrogen exposure were small at H2

concentrations above 800 ppm. The rate of resistance

change decreases as the nanowire surface becomes

saturated with hydrogen in all cases. In contrast to

previous functionalized GaN nanowire sensors, these

devicesexhibitnearperfectreversibility,asshownbythe

nearcompleterecoveryoftheoriginalcurrentresponsein

the air upon the removal of hydrogen. The repeatable

detectiveabilityof these sensorsoverpreviousoneuse

only functionalized devices is promising for longterm

applications.

AlthoughthedetectionmechanismforhydrogenonGaN

nanowires is still not fully understood, previous

suggestions for sensing include the desorption of

adsorbedhydrogen at the surface andgrainboundaries

(in polycrystalline material), an exchange of charges

betweenabsorbedgasspeciesandtheinterfaceleadingto

a change in thedepletiondepth, or else changes to the

conduction at the surface by gas adsorption and

desorption.HydrogengassensingonfunctionalizedGaN

nanosurfaceshaspreviouslybeen explainedas follows.

As the sensor surface is exposed to hydrogen gas,

hydrogenmolecules are catalytically dissociated on thefunctionalmetal surface into atomic hydrogen . These

0 200 400 600 800 1000 1200

0

1

2

3

4

|dR|/R(%)

Time (sec)

N2

20 ppm H2

50 ppm H2

100 ppm H2200 ppm H2

300 ppm H2



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hydrogenatomsareabsorbedontothemetalsurfaceand

subsequentlydiffuse through thenanoparticulatemetal

coating until they are adsorbed at the metal/GaN

interface. Hydrogen atoms reaching the GaN surface

formadipolelayerattheinterface,causingameasurable

change in the electrical fieldacross thedepletion region

directly below the GaN surface. This change in the

depletionlayerleadstoamodulationofthebarrierheight

and adifference in themeasured forward and reverse

biascurrents.

Theactivationenergycalculated forbothGaNnanowire

sensors was higher than expected for a typical H

diffusionprocess inPt,PdorGaN.Assuch, thisenergy

was attributed to the chemiadsorption of H to the

metal/semiconductor interface. The calculated values

were~7.3kcalmol1forPtcoatedGaNand~2.2kcalmol1

for Pdcoated GaN. Both Pt and Pdcoated GaNnanowires showed no change in resistance upon

exposuretoothermeasurementambiences,includingO2,

CO2,NH3andC2H6.

Figure4.RelativeresponsesofPdcoatedInNnanobeltstovaryinghydrogenconcentrationsat130C.

3.IndiumNitrideNanostructurebasedSensors

InNhasreceived significantattentionrecentlydue to its

narrow direct bandgap (0.70.8 eV) and superiorelectronic transportcharacteristics. This,combinedwith

its easily integrated structure with other nitrides,

includingGaN andAlN,makes it apromisingmaterial

forhighefficiencyIRemitters,detectorsandsolarcells,as

well as for use in high frequency electronic devices.

Thoughtheresearchisincreasing,reportsonthegrowth,

propertiesandapplicationsofInNnanostructuresremain

intheirinfancy.

ThegrowthofInNnanostructuresbyconventionalmetal

organic chemical vapour deposition (MOCVD) hasmet

withdifficultybecauseofthelowthermaldecomposition

temperature of InN (


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TheactivationenergyforHdetectiononbothPt andPdcoatedInNnanobeltswascalculatedfromtheslopeoftheArrhenius plot of natural log (dR/dt) versus 1/T. Pdcoatednanobeltswerefoundtohaveanactivationenergyof0.097eV,whereasPtcoatednanobeltshadanenergy

of0.12eV.Thesevaluesarelowbutstilllargerthanthoseexpected for a typicalhydrogen diffusion in InN,Pt orPd.Thus,andaswiththeresultsfoundforfunctionalizedGaNnanowirehydrogen sensors, thedominant sensingmechanism is likely tobe thechemisorptionofH to themetalnitrideinterface.

Theutilityof InNnanorods forhydrogendetectionwasalsoinvestigated.InNnanorodsweregrownonpolishedcAl2O3substratesbyHMOVPE.Withthismethod,TMInwas reactedwithHCl to form chlorinated indium.Thiswas then combined with NH3 to form InN on the

substratedownstream.Thesourcezonetemperaturewaskeptbelow 300C to prevent TMIn dissociationbeforereactionwithHCl.Growthwascompletedatatmosphericpressure inN2ambient ina temperature rangebetween600650C at aN/Inmole ratio of 250 and aHCl/TMIninletmole ratio of 4 to 5. Finally, InN nanorodswerefunctionalizedwithathin,discontinuousPtfilm(~100)bysputtering.

GlancingIncidenceXRayDiffraction(GIXD)scansoftheInN nanostructures are shown in Figure 5. Since theorientation of each nanostructure was largely random,

multiple

wurtzite

InN

peaks

were

observed

for

both

InN

nanobeltsandInNnanorods.Ofnoteherearethelarger,sharperpeaks for thenanorods as against those for thenanobelts. This difference in the XRD profile maybeattributed to a thicker, denser nanostructured surface,andissupportedbythevariationinsurfacemorphologyas shown in the SEM inset photos. The nanorods aretypically larger, longer and denser over the substratesurface.

Thehigherqualityof these InNnanorods (as comparedwiththepreviouslydiscussedInNnanobelts)resultsinalarger relative response upon exposure to hydrogen.Figure 6 shows the relative resistance changeof thePtcoatedmultiple InNnanorodsuponhydrogenexposure.100ppmH2inN2ambienceproducedarelativeresponseof ~10%,while 250 ppmH2 gave a response of ~12%.Unlike the immediate response of the InN nanobelts tohydrogen gas, however, the initial change in theresistanceofInNnanorodsuponhydrogenexposurewasquite slow, taking up to fiveminutes exposurebeforeexhibitingacurrentresponse.Thisslowerresponsecouldbe due to the Pt functionalization layerbeing coveredwith native oxide, which is removed by exposure tohydrogen.Thehighersurfacedensityofthenanorodsas

comparedwith thepreviousnanobeltsmay also inhibitan immediate reaction to hydrogen exposure by the

reductionofthenanostructuresurfacearea.Itiscertainlytrue that producing higher quality nanostructuresmitigates the effects of defects in thebulk or on thesurface to thesensing response. Itappears thatmanyofthe initial results in the literature concerning the gas

sensing properties of nitride and oxide nanorods havebeen dominated by surface trapping/detrappingphenomena rather thanby the true intrinsic responseofthematerialitself.

Figure5.GIXD2 scanofInNnanomaterials.A)InNnanobeltsonSiNXcoatedSi.B)InNnanorodsoncAl2O3.InsetsshowSEMmicrographsoftherespectivedepositedInNnanomaterials.

4.ZincOxide(ZnO)nanostructurebasedsensors

There is a great deal of literature dedicated to thesynthesis and characterization of various types ofnanostructuredZnOmorphologies.Areviewofthemanyforms ofnanosizedZnO, includingnanobelts, cages, combsand wiresisprovidedbyWang[11].Thiseaseofnanofabrication, combinedwith the high compatibilitywith Sibased microelectronics afforded tosemiconductingmetaloxides,makesZnOaparticularlyinterestingcandidateforsolidstatechemicalgassensing.

ZnO is a chemically and thermally stable, inherentlyntypesemiconductorwitha largeexcitonbindingenergy



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andwidebandgap.ZnObasedgassensorshavealreadybeen developed from thin films, nanoparticles andnanowires. Additionally, ZnO easily forms aheterostructuresystemwithMgOandCdOoveralimitedmiscibility range that is promising for blue/UV

optoelectronics,transparentelectronicsandsensors.

Figure 6. Relative responses of Ptcoated InN nanorods tovarying hydrogen concentrations at room temperature.(Reprintedwithpermission fromRef.111,W.Lim,J.S.Wright,B.P.Gila,S.J.Pearton,F.Ren,W.Lai,L.C.Chen,M.HuandK.H.Chen,Appl.Phys.Lett. 93,202109 (2008).CopyrightAmericanInstituteofPhysics).

4.1HydrogendetectionusingZincOxidenanostructuresForourwork, thesiteselectivegrowthofZnOnanorodswas achieved by Molecular Beam Epitaxy (MBE) onAl2O3. The growth timewas ~2 h at 600C, producingsinglecrystalnanorodswitha typical lengthof210 mand having a diameter between 30150 nm. Al/Ti/Auelectrodesweredepositedvia ebeamwith a separationdistanceof~3 m.Finally,Auwireswerebondedtothecontact pads for IV measurements performed attemperatures between 25150C in 10% H2 in N2ambience. No current was measured through thediscontinuousAu islands andno thin film ofZnOwasobservedwiththegrownconditionsofnanorods.Insomecases,nanorodswerefunctionalizedusingPd,Pt,Au,Ni,

Ag or Ti discontinuous thin films (~100 thick)depositedbysputtering.

AlthoughtherewasnodetectablechangeincurrentuponH2 exposure at room temperature, ZnO nanorods didreflect changes in current,beginning at ~112C.Whilechange incurrentonlyapproaches16nAat200C, thesechangesare readilydetectedby conventional ammeters;however, an onchip heater wouldbe needed for thepracticalapplicationofZnOnanorodsforthedetectionofhydrogen.Nonetheless, these results show the possiblecapability of ZnO nanorods for use in hydrogen gas

sensorswithoutthedepositionofanadditionalfunctionallayer.Inthiscase,thegassensingmechanismisbelieved

toderivefromthedesorptionofabsorbedsurfaceoxygenandgrainboundaries inpolyZnOorelse fromchangesin surface or grain boundary conduction by gasadsorption/desorption.Sincethenanowiresaretypicallyasinglecrystal,thelattereffectisnotlikelytocontribute.

Whendetectinghydrogenwithsimilarthinfilmrectifiers,the observed changes in current were consistent withchanges in the near surface doping of the ZnO,suggesting the introductionofashallowdonor stateviahydrogenadsorption.Thenanorodsalsoshowedastrongphotoresponse abovebandgap UV light (366 nm). Thequick photoresponse indicated that the changes inconductivity due to the injection of carriers wasbulkrelatedandnotduetosurfaceeffects.

Aswith toGaNand InN,previous reportshave shownthat theadditionofmetallicnanoparticleson thesurface

increases the detection sensitivity for H2. Accordingly,andaswiththeGaNnanowiresandInNnanobelts,metalcatalyst coatings were deposited to ZnO nanorods toincreasethedetectionefficiencyforhydrogengas.Figure9 shows the time dependence of the relative resistancechange of metalcoated multiple ZnO nanorods withvariousmetal functionalization layersupon exposure to500 ppmH2. Themeasuredbias voltagewas 0.5V. Ptcoatednanorodsexhibitedarelativeresponseofupto8%atroomtemperatureuponexposureto500ppmhydrogenconcentration inN2 ambience.Unlike the results for thenitridebased nanostructures, this was a factor of two

largerthanobtainedwithPdcoatings.ThePtcoatedZnOnanorodseasilydetectedhydrogendownto100ppm(theexperimental limit),with relative responses of 4% at thisconcentration after a 10min exposure. All of the othermetals functional layers showed very little relativeresponsestohydrogenexposure(Figure7).

Differencesintherelativeresponsetohydrogenexposurebetween Pt and Pdcoated sampleswere suggested ascomingfromthecatalyticpropertiesofthemetals.Pdhasahigherpermeability thanPt,but thesolubilityofH2 islargerinPd.Additionally,bondingstudiesofHtoNi,Ptand Pd surfaces have shown that adsorption energy islowest on Pt. As shown, however, the calculatedactivationenergy forGaNnanowiresand InNnanobeltswas lower forPdcoated samples rather thanPt.This ismostlikelyduetotheadsorptionofhydrogenatanoxideinterfacebetween thenitride surface and themetal andnotfromtheadsorptionofhydrogenatthemetalcoating.The existenceand importanceof thisoxide interface forhydrogen sensing on nitridebased semiconductors iswelldocumented. Because the hydrogen adsorption ontheoxidesideofaZnObasedsensorwouldcontributetothe creation of a shallow donor state, the sensingmechanismforhydrogendetectiononmetalcoatedZnO

nanostructuresisprobablydifferentfromthatofGaNorInN.



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Figure 7.Relative responses of ZnO nanorods to 500 ppmH2using various metal functionalizations. (Reprinted with

permission from Ref.89, Y.W.Heo,D.P.Norton, L.C. Tien, Y.

Kwon,B.S.

Kang,

F.

Ren,

S.J.

Pearton

and

J.R.

LaRoche,

Mat.

Sci.

Eng.R47,1(2004).CopyrightElsevier).

Similar to nitridebased nanostructured sensors is the

rapid, initial responseofZnOmultiplenanorod sensors

to hydrogen exposure. Effective nanorod resistance

continues to change for >15 minute of exposure. This

suggests that the chemisorption of hydrogen to the

metal/ZnO interface is the ratelimiting step in

conductancechanges to theZnO.Therecoveryof initial

resistance upon the removal of hydrogen from the

ambience was rapid (290nmare

called

solar

blind

and

are

useful

in

applications that need to detect UV photons in the

presence of sunlight, such as flame sensors, missile

detectors and aircraft detection. Si detectors have very

poor solarblind performance while wide bandgap

systemsofferimprovedspeedandlowerdarkcurrents.

Currently,the

most

commonly

used

wide

bandgap

semiconductor system forUVdetection isGaN/AlGaN.

There is also interest in developing ZnO/ZnMgO

nanowireUV detectors as a complementary technology

forUVdetection,with the followingadvantagesrelative

tothenitrides.

1.TheZnObasedmaterials offer similarbandgaps to

the nitrides, but can be grown at much lower

temperaturesonawider rangeofsubstrates, including

large area Si or cheap transparent materials, such as

glass.Thenanowirescanbetransferredtoanysubstrate

forintegration

with

other

sensors

and

are

compatible

withlowtemperaturematerialssuchaspolymers.

2.ThenanowireUVdetectorsoperateatverylowpower

levelscomparedwithexistingnitrideUVdetectors.

3. The fabrication approach developed previously for

ZnOnanowiregassensorsallowsforasimple,lowcost,

singlestepapproachtorealizingrobustUVdetectors.

4. ZnO nanowire UV detectors can be readily

integrated with onchip wireless circuits to provide

data transmission to a central monitoring location.

Thus, it is possible to have either single detectors or

arrays of detectors that operate at very lowpower

levels

and

do

not

need

constant

monitoring

by

humans.

UV detectors have applications in space exploration in

providing imaging and spectroscopic data of nearby

galaxies.Becausetheuniverseisexpanding,UVradiation

emittedbydistantgalaxiesisredshiftedandreachesour

galaxyasvisibleor infraredradiation.Galaxiescloser to

theMilkyWay canbe analysedwithUV radiation and

comparisons can be made with visible and infrared

imagestoascertainhowtheuniverseformedandchanges

withtime.

a)UVPhotoresponseofSingleZnONanowiresZnO

nanowires

grown

by

site

selective

MBE

were

single

crystals and typically conductingwith a carrier density

within the 10171018 cm3 range.These nanowires canbe

removedby sonication from theiroriginal substrateand

thentransferredtoarbitrarysubstrates,wheretheycanbe

contacted atboth endsbyAl//Pt/AuOhmic electrodes.

The currentvoltage and photoresponse characteristics

wereobtainedboth inthedarkandwithUV(254or366

nm)illumination.Thecurrentvoltage(IV)characteristics

are Ohmic under all conditions, with nanowire

conductivity under UV exposure of 0.2 Ohm.cm. The

photoresponse showed only a minor component, with

longdecay

times

(tens

of

seconds)

thought

to

originate

from surface states. Recent reports have shown the

0 5 10 15 20 25 30

0

2

4

6

8500ppm H

2Air

Time(min)

|deltaR|/R(%

)

Pt

Pd

Au

Ag

Ti

Ni



9/15

sensitivityofZnOnanowirestothepresenceofoxygenin

the measurement ambient and to UV illumination. The

slowphotoresponseof thenanowireswas suggested as

originating in the presence of surface states, which

trappedelectronswithreleasetimeconstantsfromm.sec

tohours.

In

sharp

contrast

to

these

results,

we

have

demonstrated that the photoresponse characteristics of

singleZnOnanowiresgrownbysiteselectiveMolecular

BeamEpitaxy(MBE)havearelativelyfastphotoresponse

and display electrical transport dominated by bulk

conduction.

Figure8showsthechangeincurrentatafixedbiasofthe

nanowiresinthedarkandunderilluminationby366nm

light.Theconductivity isgreatly increasedasaresultof

the illumination,asevidencedby thehighercurrent.No

effectwasobserved for illumination forbelowbandgap

light.Transport

measurements

show

that

the

ideality

factorofPtSchottkydiodes formedon thosenanowires

exhibiting an ideality factor of 1.1,which suggests that

thereislittlerecombinationoccurringinthenanowire.It

alsoexhibitstheexcellentOhmicityofthecontactstothe

nanowire, even at lowbias.Onblanket films of ntype

ZnOwithcarrierconcentrationwithinthe1016cm3range,

weobtainedacontactresistanceof35x105Ohm.cm2for

these contacts. In the case of ZnO nanowires madeby

thermal evaporation, the IV characteristics were

rectifying in the dark and onlybecame Ohmic during

abovebandgap illumination. The conductivity of the

nanowire

during

illumination

with

366

nm

light

was

0.2

Ohm.cm.

Thephotoresponseof thesingleZnOnanowireatabias

of 0.25 V under pulsed illumination by a 366 nm

wavelength Hg lamp in Figure 8 shows that the

photoresponseismuchfasterthanthatreportedforZnO

nanowires grown by thermal evaporation from ball

milled ZnO powders and is likely due to the reduced

influenceof the surface states seen in thatmaterial.The

generallyquotedmechanism forphotoconduction is the

creation of holes by illumination that discharge the

negatively charged oxygen ions onto the nanowire

surfacewith

the

de

trapping

of

electrons

and

transit

to

the electrodes. The recombination times in high quality

ZnOmeasuredby timeresolvedphotoluminescence are

short oftheorderoftensofps whilethephotoresponse

measurestheelectrontrappingtime.Thereisalsoadirect

correlation reported between the photoluminescence

lifetimeandthedefectdensityinbothbulkandepitaxial

ZnO.Inournanowires,theelectrontrappingtimesareof

theorderoftensofseconds,andthesetrappingeffectsare

onlyasmallfractionofthe totalphotoresponserecovery

characteristic.Onceagain,weseeanabsenceof thevery

long time constants for recovery seen in nanowires

preparedby

thermal

evaporation.

Figure 8. SEM micrograph of ZnO nanowire and timedependence of photocurrent as the 366nm light source is

modulated.

4.2.2pHmeasurementThere are a number of applicationswhere an ability to

determine the pH of a solution is desirable in order to

understand the level of acidity or alkalinity. These

includedeterminingsalinitylevelsinbodiesofwater,the

industrialmonitoringofsludgesorrunoffand,ofcourse,

bloodinhumans.Manyoxidesaresensitivetochangesin

the pH of solutions touching their surfaces through a

changeinsurfaceconductivity.Thismakesmaterialssuch

asZnO, SnO2 and related compounds attractive for pH

sensingapplications.

Of particular interest indetermining the ability ofZnO

nanorods for acting as pH sensors is the need for a

method of introducing the test solution in a controlled

andreproduciblefashiontothenanowiresurface.Thisis

usually accomplished by the use of a microchannel

fabricated in a polymer. Small volumes of the solution

canbe introducedwith a syringe autopipette.A typical

geometry for the nanowire sensor and integrated

microchannel and the conductance of the nanowire is

shown in Figure 9. The measurementswere performed

both in the dark or under UV illumination (365 nm

wavelength) at room temperature with the nanorod

biasedat

afixed

voltage

of

0.5

V

0 100 200 300 4000

200

400

600

800

Current(nA)

Time (sec)



9www.intechopen.com


10/15

Figure 9.The SEM of an integratedmicrochannel across aZnOnanorod contacted at both ends by Ohmic contacts. The

conductanceofthenanorodasafunctionofthepHofthesolution

whichflowedacrossitisshownatthebottomofthefigure.

Thenanorodsshowedaverystrongphotoresponse.The

conductivity is greatly increased as a result of the

illumination, as evidenced by the higher current. No

effectwasobservedforilluminationwithbelowbandgap

light.Theadsorptionofpolarmoleculesonthesurfaceof

ZnO affects the surface potential and device

characteristics.

The data in Figure 9 shows that the integratedmicrochannel/nanorod can indeed provide a sensitive

measurementofpHoverarangeofat least2 to12.The

sensitivitywas independentofwhether theexperiments

werecarriedout in thedarkorunder illumination.The

slope of the conductance/pHplotwas 8.5nS/pH in the

darkand20nS/pHunder illumination.The resolutionof

themeasurementswastypically~0.1pH,whichcompares

wellwithothermethodsforpHdetermination.

4.2.3BiomedicalApplications

AlGaN/GaNHEMTs canbe used formeasurements ofpHinEBCandglucose,throughtheintegrationofthepH

and glucose sensor onto a single chip and with the

additional integration of the sensors into a portable,

wireless package for remote monitoring applications.Figure 10 shows an optical microscopy image of an

integrated pH and glucose sensor chip and cross

sectional schematics of the completed pH and glucose

device.Thegatedimensionof thepHsensordeviceand

glucosesensorswas2050 m2.

For theglucosedetection,ahighlydensearrayof2030

nmdiameterand2 mtallZnOnanorodswasgrownon

the2050 m2gatearea.ThelowerrightinsetinFigure

10 showscloserviewof theZnOnanorodarraysgrown

onthegatearea.ThetotalareaoftheZnOwasincreased

significantlywith theZnOnanorods.TheZnOnanorod

matrix provides a microenvironment for immobilizing

negatively charged GOx while retaining its bioactivity,

andpasseschargesproducedduringtheGOxandglucose

interaction to theAlGaN/GaNHEMT.TheGOxsolutionwas preparedwith a concentration of 10mg/mL in 10

mM phosphate buffer saline (pH value 7.4, Sigma

Aldrich).After fabricating the device, a 5 lGOx (~100

U/mg,SigmaAldrich)solutionwasprecisely introduced

tothesurfaceoftheHEMTusingapicolitreplotter.The

sensorchipwaskeptat4oC in the solution for48hours

for GOx immobilization on the ZnO nanorod arrays,

followed by extensive washing to remove the un

immobilizedGOx.

Totakeadvantageofthequickresponse(lessthan1sec)

of the HEMT sensor, a realtime exhaled breathcondensate (EBC)collectorisneeded.Theamountof the

EBC required to cover theHEMT sensing area is very

small. Eachtidalbreathcontainsaround3loftheEBC.

ThecontactangleofEBConSc2O3hasbeenmeasuredas

being less than 45o, and it is reasonable to assume a

perfecthalfsphereofanEBCdropletformedtocoverthe

sensingareaofthe450 m2gatearea.Thevolumeofa

halfspherewithadiameterof50 m isaround31011

litres.Therefore,100,00050 mdiameterdropletsofEBC

canbeformedfromeachtidalbreath.

To condense 3lofwatervapour,only ~ 7Jof energy

need to be removed for each tidal breath, which can

easily be achieved with a thermal electric module a

Peltierdevice.The figure also shows aphotograph and

schematic of the system for collecting the EBC. TheAlGaN/GaNHEMTsensorisdirectlymountedonthetop

of the Peltier unit (TB80.451.3 HT 232, Kryotherm),

whichcanbecooledtoprecisetemperaturesbyapplying

known voltages and currents to the unit. During our

measurements, the hotter plate of the Peltier unitwas

kept at 21oC and the colder platewas kept at 7oCby

applying abias of 0.7V at 0.2A.The sensor takes less

than 2 sec to reach thermal equilibriumwith thePeltier

unit. This allows the exhaled breath to immediately

condenseonthegateregionoftheHEMTsensor.

2 3 4 5 6 7 8 9 10 11 12

0

50

100

150

200

250

300

Conductance(nS)

pH

non UV

UV(365nm)



11/15

Figure10.SEM imageofan integratedpHandglucose sensor.The insetsshowaschematiccrosssectionof thepHsensorand

an SEMof theZnOnanorodsgrown in the gate regionof the

glucosesensor.

Theuse ofPeltier cooling allows for a compact and low

power approach tobuilding the sensor unit.Theseunits

are inexpensive and very reliable, and can be used to

controltheamountofvapourcondensationontothesensor

gatearea.Thismightalsobeused in future toselectively

control thecondensationofparticulargases inamixture,

based on their different vapour pressures. An example

wouldbemakingsurethattheexhaledbreathcondensate

isnotsimplyairfromthebackofthenasalcavitiesor the

mouth as opposed to thenecessary air fromdeep in thelungs.ThesimultaneousdeterminationofthepHcanhelp

inensuringthetestvapourisindeedfromthelungs.

TheHEMT sensorsonly exhibiteda response tohuman

breathandnottocalibrationinjectionsofN2gas.TheN2

did not cause any change of drain current while the

increaseofexhaledbreath flow ratedecreased thedrain

current.For every tidalbreath, thebeginningportionof

theexhalationisfromthephysiologicdeadspaceandthe

gases in this space do not participate in CO2 and O2

exchangeinthelungs.Therefore,thecontentsinthetidal

breatharedilutedbythegasesfromthisdeadspace.For

higher flow rate exhalation, this dilution effect is less

effective. Once the exhaled breath flow rate is above

1L/min, the sensor current change reaches a limit.As a

result, the test subjectexperienceshyperventilationand

thedilutionbecomesinsignificant.Thesensorisoperated

at 50 Hz and a 10% duty cycle, which produces heat

duringoperation.ItonlytakesafewsecondsfortheEBC

tovaporize from the sensingareaand causes the spike

like response. The principal component of the EBC is

watervapour,whichrepresentsnearlyallof thevolume

(>99%) of the fluid collected in theEBC.Themeasured

current changeof the exhaledbreath condensate shows

that thepHvaluesarewithin the rangeofpH78.This

rangeisthetypicalpHrangeofhumanblood.

Theglucosewas sensedbyZnOnanorod functionalized

HEMTswithaglucoseoxidaseenzyme localizedon the

nanorods,as shown inFigures 1113.This catalyzes the

reactionofglucoseandoxygentoformgluconicacidand

hydrogenperoxide.Figure12showstherealtimeglucose

detectioninaPBSbuffersolutionusingthedraincurrent

change in theHEMT sensorwith a constantbiasof250

mV.Nocurrentchangecanbeseenwiththeadditionofa

buffersolutionataround200sec,showingthespecificity

andstabilityofthedevice.Bysharpcontrast,thecurrent

change showed a rapid responseof less than 5 seconds

whenthetargetglucosewasaddedtothesurface.Sofar,

the glucose detection using an Au nanoparticle, ZnO

nanorodandnanocomborcarbonnanotubematerialwith

GOx immobilization is based on electrochemical

measurement . Since there is a reference electrode

requiredinthesolution,thevolumeofthesamplecannot

be easily minimized. The current density is measuredwhen a fixed potential is applied between the nano

materialsandthereferenceelectrode.Thisisafirstorder

detectionandtherangeofdetectionlimitofthesesensors

is0.570 M.EventhoughtheAlGaN/GaNHEMTbased

sensor used the same GOx immobilization, the ZnO

nanorods were used as the gate of the HEMT. The

glucosesensingwasmeasuredthroughthedraincurrent

ofHEMT,withachangeofthechargesontheZnOnano

rods,andthedetectionsignalwasamplifiedthroughthe

HEMT.AlthoughtheresponseoftheHEMTbasedsensor

is similar to that of an electrochemicalbased sensor, a

much lowerdetection limit of 0.5nMwas achieved fortheHEMTbased sensordue to thisamplification effect.

Since there is no reference electrode required for the

HEMTbased sensor, the amount of the sample only

dependsupontheareaofthegatedimensionandcanbe

minimized.Thesensorsdonotrespondtoglucoseunless

theenzymeispresent,asshowninFigures12and13.

Althoughmeasuring the glucose in the EBC is a non

invasive and convenient method for the diabetic

application,theactivityoftheimmobilizedGOxishighly

dependent on the pH value of the solution. The GOx

activitycanbereducedto80%forpH=5to6.

Bywayofcontrast,whentheglucosesensorwasusedin

a pHcontrolled environment, the drain current stayed

fairlyconstant. In thisexperiment,50lofPBS solution

was introduced on the glucose sensor to establish the

baseline of the sensor, as in the previous experiment.

Then,glucoseat a10nM concentrationprepared in the

PBS solution was introduced to the gate area of the

glucose sensor through amicroinjector. Therewas no

glucose in the 50 l PBS solution and the PBS solution

wasaddedat20and30min.Ittooktimefortheglucose

solutiontodiffusetothegateareaofthesensorthrough

theblankPBS,andthedraincurrentgraduallyincreased

correspondingtotheglucosediffusionprocess.Sincethe



11www.intechopen.com


12/15

fresh glucosewas continuously provided to the sensor

surfaceand thepHvalueof theglucosewascontrolled,

oncetheconcentrationoftheglucosereachedequilibrium

atthegateoftheglucosesensor,thedraincurrentofthe

glucoseremainedconstant,except in thepresenceof the

glucosesolution,whichwastakenoutfrom timetotime

using a micropipette. Small oscillations of the drain

current were observed, which could be eliminated by

usingamicrofluidicdeviceforthisexperiment.

Figure 11. Schematic of a ZnO nanorod functionalizedHEMT(top)andaSEMofnanorodsonthegatearea(bottom).

Figure 12. Plot of drain current versus time with successiveexposureofglucosefrom500pMto125Min10mMphosphate

buffersalinewithapHvalueof7.4,bothwithandwithout theenzymelocatedonthenanorods.

Figure 13. Change in drainsource current in HEMT glucosesensors,bothwithandwithoutthelocalizedenzyme.

ThehumanpHvalue canvary significantly,depending

onhealth.SincewecouldnotcontrolthepHvalueofthe

EBCsamples,weneededtomeasurethepHvaluewhile

determining theglucose concentration in theEBC.With

the fast response time and low volume of the EBC

required forHEMTbased sensor, a handheld and real

timeglucosesensingtechnologycouldberealized.

4.2.4LacticAcidAnother application for functionalized AlGaN/GaN

sensors is thedetection and quantificationof lactic acidcontent inbreath orblood. This is important in sports

training, where the level of exertion and recovery of

swimmers and sprinters is necessary to optimize their

training practices. It is also important in situations like

monitoring of the effectiveness of anaesthetics during

medical procedures or for individuals with conditions

suchasdiabetesandchronicrenalfailure.

A ZnO nanorod array,whichwas used to immobilize

lactateoxidase (LOx),wasselectivelygrownon thegate

areausinglowtemperaturehydrothermaldecomposition

(Figure 14, top). The array of onedimensional ZnO

nanorodsprovided a large effective surface areawith a

high surfacetovolume ratio and a favourable

environment for the immobilization of LOx. The

AlGaN/GaNHEMTdrainsourcecurrentshowedarapid

response when various concentrations of lactate acid

solutionswere introduced to thegateareaof theHEMT

sensor. The HEMT could detect lactate acid

concentrations from 167 nM to 139 M. Figure 14

(bottom) shows a realtime detection of lactate acidby

measuring theHEMTdrain currentata constantdrain

sourcebiasvoltageof500mVduringtheexposureofthe

HEMT sensor to solutionswith different concentrations

oflactateacid.Thesensorwasfirstexposedto20lof10

mMPBS andno current change couldbedetectedwith

100

101

102

103

104

105

0

5

10

15

20

no enzyme

with enzyme

Change

ofId

s(A)

Concentration (nM)



13/15

theadditionof10lofPBSatapproximately40seconds,

showing the specificity and stability of the device. By

contrast, a rapid increase in the drain current was

observedwhen the target lactateacidwas introduced to

thedevicesurface.

Ascomparedwiththeamperometricmeasurementbased

lactateacidsensors,ourHEMT sensorsdonot requirea

fixed reference electrode in the solution tomeasure the

potential applied between the nanomaterials and the

reference electrode. Although the time response of the

HEMTsensorsissimilartothatofelectrochemicalbased

sensors, a significant change of drain current was

observed in exposing theHEMT to the lactateacidat a

low concentration of 167 nM due to this amplification

effect. In addition, the amount of the sample,which is

dependentupon the areaof thegatedimension, canbe

minimizedfor

the

HEMT

sensor

due

to

the

fact

that

no

reference electrode is required. Thus,measuring lactate

acid in the exhaled breath condensate (EBC) can be

achievedasanoninvasivemethod.

Figure 14. Schematic crosssectional view of theZnO nanorodgated HEMT for lactic acid detection (top) and plot of drain

currentversustimewithsuccessiveexposurestolactateacidfrom

167 nM to 139 M (bottom). (Reprinted with permission from

Ref.31B.H.Chu,B.S.Kang,F.Ren,C.Y.Chang,Y.L.Wang, S.J.

Pearton,A.V.

Glushakov,

D.M.

Dennis,

J.W.

Johnson,

P.

Rajagopal,

J.C.Roberts,E.L.Piner,andK.J.Linthicum,Appl.Phys.Lett.93,

042114(2008).CopyrightAmericanInstituteofPhysics).

5.SummaryandOutlook

Nitrideandoxidesemiconductornanostructuresareideal

materials for chemical sensing. Material properties,

including high chemical and thermal stability coupled

withahigh

surface

to

volume

ratio,

give

these

nanostructured sensors high sensitivity with detection

capabilitiesdowntothelowppmlevelsforgasessuchas

hydrogen. While hydrogen sensing devices have been

demonstrated on single ZnO nanowires, there are

advantagesinthedevelopmentofwidebandgapsensors

involvingcontactingmultiplenanowiresheets.Although

the power requirement for usingmultiple nanowires is

higher than that for single nanowire devices, the

simplicity of fabrication, including minimal processing

stepsformultiplenanowiresensors,ishighlyfavourable

forthefuturemassproductionofthesesensors.

Theselectivesensingofhydrogeninnitrogenambientshas

beenshownusingnanostructuredZnO,GaNandInNdown

to50ppmatroomtemperature.Sensorswerefunctionalized

using Pt or Pd in order to facilitate the dissociation of

molecularhydrogenand improvesensingefficiency.Allof

thesensorsdisplayedexcellentresponsecharacteristicsand

decentrecoveryuponexposuretoairorpureoxygen.

There is also great promise for using nanowires in

conjunctionwithHEMTsensorstoenhancethedetection

sensitivity for glucose and lactic acid. Once again, the

high

surface

area

of

nanowires

provides

an

ideal

approach for enzymatic detection of biochemically

importantsubstances[1530].

6.Acknowledgments

The work at UF was partially supported by NSF

(J.M.Zavada).7.References

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