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7/28/2019 InTech-Wide Bandgap Semiconductor One Dimensional Nanostructures for Applications in Nanoelectronics and Nan
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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
1www.intechopen.com
ARTICLE
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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
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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)
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Nanostructures for Applications in Nanoelectronics and Nanosensors
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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)
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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
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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)
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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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