Single digit parts-per-billion NOx detection using MoS2/hBN transistors

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transistors

Ayaz Ali

^a,b

, Ozhan Koybasi

^c

, Wen Xing

^d

, Daniel N. Wright

^c

, Deepak Varandani

^e

, Takashi Taniguchi

^f

, Kenji Watanabe

^f

, Bodh R. Mehta

^e

, Branson D. Belle

^a,d,∗

aDepartmentofSmartSensorSystems,SINTEFDIGITAL,Oslo,0373,Norway

bDepartmentofElectronicEngineering,UniversityofSindh,Jamshoro,76080,Pakistan

cDepartmentofMicrosystemsandNanotechnology,SINTEFDIGITAL,Oslo,0373,Norway

dDepartmentofSustainableEnergyTechnology,SINTEFINDUSTRY,Oslo,0373,Norway

eDepartmentofPhysics,IndianInstituteofTechnologyDelhi,Delhi-110016,India

fNationalInstituteforMaterialsScience(NIMS),Tsukuba,Ibaraki,305-0044,Japan

a r t i c l e i n f o

Articlehistory:

Received20April2020

Receivedinrevisedform14July2020 Accepted31July2020

Availableonline6August2020

Keywords:

Molybdenumdisulphide(MoS2) Fieldeffecttransistor(FET) Gassensor

NOxdetection Sensitivity

a b s t r a c t

2Dmaterialsofferexcellentpossibilitiesforhighperformancegasdetectionduetotheirhighsurface- to-volumeratio,highsurfaceactivities,tunableelectronicpropertiesanddramaticchangeinresistivity uponmolecularadsorption.Thispaperdemonstratesasimpleﬁeldeffecttransistor(FET)ofmolybdenum disulphide(MoS2)fabricatedonahexagonalboronnitride(hBN)substratethatcandetectNOxdownto concentrationsof6ppbandpossiblyfarbelowatroomtemperature(RT)withasystematicoptimization ofthedevicedesignandfabricationparametersaswellasthedeviceoperatingconditions.Theeffects ofthesubstrate,numberofMoS2layers,channellayoutandbiasingconditionsontheresponseofMoS2

FETstoNOxwereinvestigated,providingdirectionsformaximizingthesensitivity.Thisworkalsosheds lighttheissuesofrecoveryandstabilityandpresentamethodologyforcalibrationofthesensorswhich iscriticalforrepeatableandreliablemeasurements.

1. Introduction

Industrialization and urbanization worldwide has led to increasedlevelsofair-pollutantswhich canseriouslynegatively impacthumanhealthaswellasincreaseglobalenvironmenthaz- ardssuchasozonelayerdeterioration,acidrainandphotochemical smog [1,2]. Theseair-pollutants mainlycontain harmful gasses suchascarbonmonoxide(CO),nitrogenoxides(NOx)orsulphur dioxide(SO₂).IthasbeennotedthatanexposuretoNOxconcen- trationsevenaslowas53parts-per-billion(ppb)canpotentially damagethehumanrespirationsystem[3–6].Existingtechnology todetectthesehazardousgasesisbasedonmetaloxideswhich requireshighoperatingtemperatures(>200^◦C)toactivatethegas sensingprocesses(adsorption/desorption),whichnotonlyrequires fabricationcomplexitybutalsoleadstohighpowerconsumption [7–9].

∗Correspondingauthorat:DepartmentofSmartSensorSystems,SINTEFDIGITAL, Oslo,0373,Norway.

E-mailaddress:Branson.Belle@sintef.no(B.D.Belle).

Conversely,two-dimensional(2D)materialshaveemergedasa verypromisingclassofmaterialswhichhavebeenusedtodevelop ultrahighsensitiveandlow-power-consumptiongassensorsdueto theirunmatchedsurfaceareaperunitvolume,highsurfaceactivi- tiesanduniqueelectronicproperties[10–14].Graphenebasedgas sensorshaveshownremarkableperformancebysensingdownto asinglegasmoleculeatroomtemperature[15].Forhighperfor- mancegassensors,transitionmetaldichalcogenides(TMDs)offer aroutetoobtainsemiconductingpropertieswhichisakeyparam- eter,unlikegraphenewhichhasazerobandgap[16–18].

RecentworksonMoS2basedNOxsensorshavedemonstrated thepotentialofthisclassofmaterialswithexcellentsensitivities rangingfrom100ppmto8ppb[19–31].Intheseworks,elevated temperature,nanoparticles,andredlighthavebeenused,which increasesthepowerconsumptionandcomplexityandaddedsteps indevicefabrication.GiventhattheUnitedStatesEnvironmental Protectionagencyhasstipulated53ppbofNOxasbeingdangerous [6],itisimperativetodeveloplowcosthighlysensitiveNOxsensors.

It isknownthatdevices thatemployhBNas asubstrateare robustandhave increasedperformanceduetothereductionof electron-hole puddlesand scattering sites given theatomically ﬂatsurface[32–34].In thispaper,weemployhighqualityhBN

https://doi.org/10.1016/j.sna.2020.112247

0/).

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Fig.1.Schematicandopticalimage(scalebar10␮m)ofsinglelayerMoS2/hBNtransistor(a,b).MoS2/hBNtransistorcharacteristics(c,d).(c)Transfercharacteristicsatdrain voltage(Vd)of1V.(d)seriesofoutputcharacteristiccurveswithdifferentgatevoltages(Vg=−10Vto60V).

asasubstrateforourmechanicallyexfoliatedMoS₂devices and demonstratesensitivitydowntoatleast6ppbforNOxdetectionat roomtemperaturethroughasystematicoptimizationofthedevice parametersandoperatingconditions.Ourworkprovidessigniﬁ- cantinformationontheimpactofvariousstructuralparameters suchastypeofsubstrate,numberofMoS₂layersandchannelgeom- etryontheresponse.Moreover,wediscussindetail thesensor biasingconditionsandtheiradjustmenttoobtainconsistentand reliablemeasurementresultsaswellasmaximalsensitivity.

2. Experimentalmethods 2.1. Devicefabrication

TheMoS2/hBNVanderWaalsheterostructureswerefabricated bya drytransfertechniqueusingmechanicallyexfoliatedﬂakes [35] (details in supportinginformation Fig. S1). Electron-beam lithography(EBL)wasusedtopatterntheMoS2channelwithsub- sequentshapingviareactiveionetchingusingamixtureofCHF₄ andO₂gases.Finally,electrodeswererealizedbyEBLande-beam evaporationandlift-offofTi/Au(5nm/100nm)metals.

2.2. Ramancharacterization

Raman characterization was carried out using a MonoVista CRS+systemwith532nmlaser.Thelaserpowerwassufﬁciently lowtoavoidsampledamage.Additionally,Ramancharacterization wasperformedonﬂakeofsimilarthicknessnotwithinthechannel ofthedevice.

2.3. Gasmeasurements

Gasmeasurementswerecarriedoutinacustomgaschamberat atmosphericpressure.TheNOxusedwasa50/50mixtureofNO₂ andNO.Pre-dilutedNOx gasbottleswerepurchasedfromLinde GasAS(Norway)withconcentrationof50and5ppmbalancedby argon.For obtainingdesiredgasconcentrationinthis work,the gaseswerefurtherdilutedbymixingwithargonbeforesupplying

tothegaschamber.Thetotalﬂowrateofthegaseswasbetween 40–80ml/min.

3. Resultsanddiscussions

AsinglelayerMoS₂/hBNﬁeldeffecttransistor(FET)withachan- nellengthof7␮mandchannelwidthof10␮mwasusedasthe reference device structure for our work(Fig.1(a)). The Raman measurementdataconﬁrmingsingle-layerthicknessoftheMoS2

channelisincludedinsupportinginformationFig.S2.Anoptical imageofthedeviceisshowninFig.1(b).Fig.1(c,d)showsthetrans- ferandoutputcharacteristicsofthefabricatedreferencedevice,

Fig.2.SensorresponseunderthedifferentNOxexposuresatzerogatevoltage (Vg=0V)(a)andNOxgasresponseatfixedtime(35min)fordifferentgasconcen- trations(62,125,250and500ppb)(b).Timedependentresponsechangeunderthe NOxexposureof500ppbandthecorrespondingfittedcurveusingtheexponential decayfunction(c)andextractedadsorptionrateconstant(␶)fromcurvefittingfor differentNOxconcentrations(d).

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whichwasoperatedintwoterminalconﬁguration.AsseeninFig.1, thedeviceshowsn-typebehaviourwheredraincurrentincreases withincreasinggatevoltage.The ﬁeldeffectmobility extracted fromthetransfercharacteristicsis1.16cm²V⁻¹s⁻¹.

Fig.2(a)showsthenormalizedresponsesofthedevicetodiffer- entconcentrationsofNOxrangingfrom5000ppbdownto62ppb atagatevoltageof0V.ThegasresponseisnormalizedasINOx/IAr,

whereI_NOxandI_ArrefertodraincurrentduringtheNO_xexposure andpriortoNOxinAratmosphere,respectively.Uponexposure toNOx,thedraincurrentdecreasesand thresholdvoltage(V_th) shiftstowardpositivegatevoltages,indicating p-typedopingof thechannelwithNOxexposure.Theseattributesconﬁrmthatthe gassensingprocessisbasedonchargetransfermechanism,where NOxcaptureselectronsfromtheMoS2channel.Astheconcentra- tionofNOxincreases,morechargesperunittimearetransferred from MoS₂ to NOx, leading to a steeper decrease in the drain current(Fig.2(a)).Fig.2(b) showsthatourdeviceexhibitsgood response, capable of detectingvery low concentrations of NOx

(62ppb)atroomtemperatureandfollowstheLangmuirisotherm formoleculesadsorbedonthesurface(Fig.2(b)).Itthereforecon- ﬁrms that charge transfer is the sensing mechanism [36]. The resultsalsoshowthattheresponsetimeismuchshorterforhigher NOxconcentration(20sfor5ppm)asshowninFig.2(a)andFig.

S4.Furthermore,theadsorptionrate constant(␶)of oursensor wasextractedwhichrevealstherateoftheNOxadsorptionpro- cessontheMoS2 surface.Fig. 2(c)illustrates thedrain current decreasesassociatedwithNOxadsorption.Thedataisﬁttedwith anexponentialdecayfunctionwhichconﬁrmsthatthereisonly onemechanismassociatedwithNOxadsorption[37].Theadsorp- tionrateconstantasthefunctionofdifferentNOxconcentrations ispresentedinFig.2(d).Itisobservedthat␶decreasesastheNOx

concentrationincreasesandsaturatesforthehigherNOxconcen- trations.

Throughouttherestofthepaper,theeffectsofthedeviceoper- atingconditionsanddevicestructureparametersontheresponse ofthedevicetoNOxweresystematicallyinvestigated.Thisstudy willprovideaninsightintooptimizingtheseparameterstoachieve maximalresponsetoNOxandsensorcalibrationmethodsinorder toobtainconsistentandreliablemeasurementresults.

3.1. Thresholdshift,recoveryandgatevoltagedependenceofthe response

Asmentionedabove,NOxexposureleadstoasigniﬁcantshiftof thedevice’sthresholdvoltagetowardpositivegatevoltages.Vari- oustechniquescanbeemployedtorecoverMoS₂gassensorsafter NOx exposure.Onemethodisbyapplyinga negativegatevolt- agepulsetorefreshthesensor[38].Theothermethodsthatcan beusedtorecoverthesensorafterNO_xexposurearetoannealat elevatedtemperatures(100–200^◦C)[39–41]orlightillumination, wherephotogeneratedholes reactwithadsorbedgasmolecules

andgiverisetodesorption[42].Therecoverytimeofthesensor stronglydependsontheintensityofthelight.Sensorrecoveryat roomtemperaturebyilluminatingwithwhitelightfromalight emittingdiode(LED)wasdemonstratedwhichshiftsthethreshold backtooriginalstate(Fig.S3).Thisroomtemperaturesensorrecov- erysuggeststhatphysisorptionofNOxmoleculesonmechanically exfoliatedMoS2isthemoredominant.Inaddition,thecyclictestof sensorresponseandrecoveryshowsgoodrepeatabilityasshown insupportinginformationFig.S4.Inadditiontogasexposure,envi- ronmental effects suchashumidity/watermolecules couldalso shiftthesensor’sthresholdvoltage[43,44].Apropercalibration of thesensorafter each measurementis therefore essentialfor repeatabilityandreliabilityofthemeasurementresults.Thefol- lowingexperimentwasconductedtostudytheconsequencesof uncalibratedoperation.Weﬁrstmeasuredthegatecharacteristics andthenresponseofthesensortoaNOxconcentrationof62ppbat agatevoltageof-10V.Afterthismeasurement,werecoveredthe sensorusingwhiteLEDwhichshiftedthethresholdvoltageback totheoriginalstate.Next,thesensorwaskeptinambientairfora fewdaysandthenthegatecharacteristicsofthesensorwasmea- suredagain.Thiswasimmediatelyfollowedbythemeasurementof thesensorresponsetoaNOxconcentrationof125ppbatthesame gatevoltageof-10V.Ascanbeseenfromtheresultspresented inFig.3,thedeviceshowslowerresponsetoaconcentrationof 125ppbascomparedtoaconcentrationof62ppb.Lowerresponse tohigherconcentrationisattributedtothethresholdshiftdueto environmentaleffectsbeforethemeasurementataconcentration of125ppb.Thisisevidentfromthetransfercharacteristicsmea- suredrightbeforeeachgasexposure(Fig.3(b)).Thetransfercurve hadshiftedby−25Vbeforeexposureto125ppbofNOx.Thisshows thatbyoperatingthesensoratthesamegatevoltageof-10Vdur- ingthemeasurementoftheresponseto125ppbofNOx,thedevice isnolongerinthesameconditionthatitwaswhenmeasuringthe responseto62ppb.Torestorethesameoperatingconditionsas theﬁrstmeasurementwiththegateat-10V,thedevicemustbe operatedwiththegateat−35Vduringthesecondmeasurement.

To study the effect of gate voltage on the sensor response andexploretheoptimumoperatingconditions,wemeasuredthe response ofthereference devicetoNOx atthree differentgate voltages.AsindicatedinFig.4,eachofthechosengatevoltagescor- respondstoadifferentregionofthetransfercharacteristicscurve (i.e.,regionsofdifferenttransconductances):i)subthresholdregion (V1),ii)transitionfromsubthresholdtolinearorquadraticregion (V₂),andiii)linearregion(V₃).Duetothethresholdshift,theval- uesofV₁,V₂,andV₃ arecalibratedbeforeeachmeasurementto ensurethatthedeviceisoperatedatthesamecondition,i.e.,the samedopinglevel.Fig.5showsthegatevoltagedependenceof theresponsetodifferentconcentrationsofNOx.Theresultsshow thatthemaximalsensitivityisachievedwhenthesensorisoper- atedinthesubthresholdregion(V₁).Thisisbecauseatsubthreshold regime,thedeviceisdepletedofchargecarriersandhencethedrain

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Fig.4. Transfercharacteristiccurveinlogarithmicscaleshowingdifferentregions (i)subthreshold(V1),ii)subthresholdtolinear(V2),andiii)linear(V3)).

currentchangesmoreabruptlyuponexposuretoagivenconcentra- tionofNOxmolecules.However,whenthesensorisexposedtothe highconcentrationsofNOx(5000ppb),theeffectofthegatevolt- agebecomesnegligible.Ourexperimentalresultsareinagreement withthesimulationworkpresentedbyRaoetal.[17]

3.2. Effectofthesubstrate

Tostudytheeffectofthesubstrateontheperformanceofthe gassensor,asinglelayerMoS2transistoronbareSiO2(MoS2/SiO2) wasfabricatedandcomparedwiththereferencedevicewhichis onhBN(MoS₂/hBN).Fig.S5inthesupportinginformationshows theopticalimageofthesinglelayerMoS2transistorfabricatedon SiO2 substrate.Beforeconductingsensingcomparison, thetran- sistorperformancecomparisonofthesedeviceswascarriedout.

Foreffectivecomparison,bothdeviceswerechosenwiththesame dimensions(channellengthandwidth).Thetransfercharacteristic curvesofbothdevicesarepresentedinFig.6.Theﬁeldeffectmobil-

Fig.6.TransfercharacteristiccurveofsinglelayerMoS2/SiO2(blackcurve)andsin- glelayerMoS2/hBNtransistor(redcurve),respectively.(Forinterpretationofthe referencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversion ofthisarticle).

ityofthedeviceswasextractedas1.16cm²V⁻¹s⁻¹forMoS₂/hBN and0.25cm²V⁻¹s^-1forMoS2/SiO2transistor,respectively.Inaddi- tion, thecurrent on/off ratioof 10⁴ and 10³ was observed for MoS₂/hBNandMoS₂/SiO₂device,respectively.Thisperformance differencecanbeattributedtofewerinterfacetrapsbetweenMoS2

andhBNcomparedtoMoS₂andSiO₂.Inaddition,hBNisanatom- icallyﬂatsubstratewithfewerscatteringsites.

For the comparison of NOx gas sensing, both transistors (MoS2/hBNand MoS2/SiO2) wereoperated in the subthreshold region(Fig.4),andtheirresponsesweremeasured.Fig.7shows thatuponexposuretolowerconcentrationsofNOx(25ppb),the MoS2/hBN deviceexhibits higher response as compared tothe MoS₂/SiO₂device.However,theeffectofsubstratebecomesnegli-

Fig.5. GatevoltagedependentresponseofthesinglelayerMoS2/hBNgassensor.Thesensorwasoperatedatthreedifferentgatevoltages(V1,V2andV3)andtheresponse tovariousNOxexposures(62,125,250,500,1000and5000ppb)wasmeasured.

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Fig.8.ThresholdvoltageshiftupondifferentgasexposuresforMoS2/SiO2device (a)andforMoS2/hBNdevice(b),respectively.

Fig.9.EffectivemobilitychangeasafunctionofNOxconcentrations(25,62,125, 500and1000ppb)forMoS2/SiO2device(blackcurve)andMoS2/hBNdevice(red curve).(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereader isreferredtothewebversionofthisarticle).

giblewhenthedeviceisexposedtothehigherNO_xconcentrations (500ppb).

To understand the mechanism behind the substrate effect, transfercharacteristiccurveswererecordedaftereachgasmea- surementandthecorrespondingthresholdvoltageshiftandchange inthemobilityfortheMoS₂/SiO₂deviceandMoS₂/hBNdevicewere extractedandshowninFigs.8&9.Arelativelylargethreshold voltageshiftisobservedfortheMoS₂/hBNdeviceatlowNOxcon- centrations(25ppb)whileforhigherNOx concentration,almost same threshold voltage shift is observed for both devices. The thresholdvoltageshiftreﬂectstheamountofchargetransferupon gasexposure;hence,itsuggeststhattheMoS₂/hBNdeviceismore sensitivetoultra-lowconcentrationsofNOx(25ppb).Furthermore, achangeinmobilityisalsoobservedforbothdevicesunderNO_x exposure(Fig.9).InthecaseofMoS₂/hBN,asharpdecreaseinthe mobilityisobservedforlowconcentrationgasexposures.Thiscan beexplainedbytheincreaseinscatteringsitesonadsorptionof NOxmoleculesduetodevicebeingmorehomogeneousonaﬂatter substrate.However,incaseoftheMoS2/SiO2 device,thechange

Fig.10.EffectoftheMoS2layerthicknessonsensorresponse.Gassensingresponse ofmonolayer,bilayerandfour-layerMoS2devicesundervariousNOxexposures(62, 125,250,500,1000and5000ppb).Thesensorswereoperatedatsaturatedregime (Vg=30V)inordertokeepthedopinglevelsame.

in mobilityin thepresenceof NOx ismuch lower forlow con- centrationswhichshowsthateffectofsurfaceroughnessismore dominant.Thiscanbeseenfromthepristinedevicecharacteris- tics(Fig.6),wheretheMoS₂/hBNdeviceexhibitshighmobilityand currenton/offratioascomparetoMoS2/SiO2.

3.3. Effectofnumberoflayers

TheeffectofMoS2layerthicknessontheresponseofthesensor toNOxwasalsostudied.Inadditiontothereferencedevice,wealso fabricatedFETswithbilayerandfour-layerMoS₂onhBN.Thetrans- fercharacteristicscurvesofthedeviceswithdifferentnumberof MoS₂layersareshowninﬁg.S6.Thethresholdvoltageisdifferent forFETswithdifferentnumberofMoS₂layersasthecarrierconcen- trationvarieswithMoS2thickness.Hence,operatingthedevices atsamegatevoltagewouldnotprovideacorrectcomparisonof theirresponsetoNOx.Instead,thedeviceswereoperatedatdif- ferentgatevoltagesthatleadtothesamedopinglevelorthesame effectivepointofoperationonthetransfercharacteristicscurveas discussedinSection3.1.weoperatedallthreesensorsatthelinear regimeofthetransfercharacteristicscurve(Vg=30V)andmea- suredtheresponseofthedevicesundervariousNOxgasexposures asshowninFig.10.Ourresultsshowthattheresponsedecreases withincreasingnumberofMoS₂layers.Thiscanbeattributedtothe screeningeffect[45].InmonolayerMoS2FET,NOxinteractswith thesurfacechargecarriersasthechannelismonolayer.Inthecase ofmulti-layerMoS₂,ontheotherhand,NOxinteractslesswithcar- riersfurtherawayfromthetoplayer.Furthermore,ascanbeseen inFig.10,theeffectofnumberofMoS₂layersbecomesmoresig- niﬁcantathighergasconcentrationswheretheresponsediffersby ordersofmagnitude.

3.4. Effectofchanneldimensions

To demonstrate the effect of MoS2 channel dimensions on response,wefabricatedhallbarshapedchannel(Fig.S7).Forcom- parison,weusedtwodifferentchannellengths(CHL1=12␮mand CHL2=24␮m)withsamechannelwidth(4␮m)andmeasuredthe

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layerMoS2/hBNdevicebeforeandafterofanultra-lowexposureofNOx(6ppb)(a).

TimeresponseofsinglelayerMoS2/hBNdeviceunderanultra-lowexposureofNOx

(6ppb)(b).Thesensorwasoperatedatsub-thresholdgatevoltageof−5V.

electricalcharacteristicsforeachchannellengthinordertochoose thesameoperatingconditionsforthedevices.Thetransfercurvesof bothchannellengthsshowthesamethresholdvoltage(Fig.S8),and thereforeoperatingthesensorsatsamegatevoltagegivesacorrect comparison.Fig.S9showstheresponseofthedeviceswithCHL1 andCHL2underﬁxedgatebiasof-10VtovariousNOxgasconcen- trations(62,125,250,500,1000and5000ppb).Thedevicewith shorterchannellength(CHL1)showsarelativelyhigherresponse ascomparedtolongchannel(CHL2).Thisisexpectedasthecharges undergolessscatteringduringtransportthroughashorterchannel.

3.5. Maximalsensitivity

The results obtained from the above studies allowed us to choosethebest parameters for maximum sensitivity. We have demonstratedthathighersensitivitiestoNOxcanbeachievedwith monolayerMoS2onhBNsubstratewithshortchannellengthand byoperatingatthesubthresholdregime.Byimplementingallopti- mizationsbasedonstudiesreportedintheprevioussections,single digitppb sensitivitytoNOx wasobtained. Fig.11(a)showsthe transfercharacteristicsbeforeandaftertheexposureoftheopti- mizedsensorto6ppbofNOx.Asigniﬁcantshiftinthetransfercurve wasobservedevenaftersuchlowexposures.Moreover,oneorder ofmagnitudechangeintheresistanceofMoS2wasobservedwithin twohoursofultra-lowNOxexposure(6ppb)asshownFig.11(b).

Thesignaltonoiseratioisonetheimportantparametersforthe high-performancegassensor,especiallyforlowconcentrationgas sensors.Oursensorshowsanorderofmagnitudechangeinresis- tance atultra-low exposure(6ppb). Our experimentalsetup is currentlynotcapableofprovidinggasconcentrationsbelow6ppb, andhencewewerenotabletoexploretheultimateNO_x detec- tionlimitofMoS₂transistors.However,theremarkableresponse to6ppbofNOxindicatesthatthedetectionlimitcanbewell-below 6ppb.

4. Conclusionandoutlook

WeperformedacomprehensivestudyontheresponseofMoS₂ transistorstoNOxexposure.Inthisstudy,wedemonstratedhow thedevicestructureparametersandoperatingconditionsimpact theresponse. Ourfindings showthattheresponseof theMoS₂ transistorstoNOxcanbesignificantlyenhancedbyusinghBNsub- strateinsteadofSiO₂,usingmonolayerMoS₂channel,andreducing thechannellength.Thebiasvoltageusedforthesensoroperation alsosignificantlyaffectstheresponseandourresultsshowthatthe maximalresponseisobtainedwhenthedeviceisoperatedinthe subthresholdregimeofthetransfercharacteristicscurve.Aknown characteristicofMoS2transistorsasgassensoristheirpoorself- recoveryatroomtemperature,which wasalsoobservedinour work.Wehaveshownthatitisextremelyimportanttocalibrate thedevicebyadjustingthebiasvoltagebeforeeachoperationin

gasspeciesistodecoratetheMoS2transistorswithnanoparticles ofspeciﬁcworkfunctionswhichiscurrentlyunderinvestigation byourgroup.

CRediTauthorshipcontributionstatement

AyazAli:Conceptualization,Methodology,Investigation,Writ- ing-originaldraft,Writing-review&editing,Visualization.Ozhan Koybasi: Conceptualization,Methodology, Validation, Writing - review&editing,Supervision. WenXing:Methodology,Investi- gation,Resources.DanielN.Wright:Methodology,Investigation, Resources. Deepak Varandani: Project administration, Funding acquisition.TakashiTaniguchi:Methodology,Resources,Writing -originaldraft.KenjiWatanabe:Methodology,Resources,Writing -originaldraft.BodhR.Mehta:Conceptualization,Methodology, Writing-review&editing,Supervision,Fundingacquisition.Bran- sonD.Belle:Conceptualization,Methodology,Validation,Writing -review&editing,Supervision,Fundingacquisition.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingﬁnan- cialinterestsorpersonalrelationshipsthatcouldhaveappearedto inﬂuencetheworkreportedinthispaper.

Acknowledgements

ThisworkwassupportedbytheResearchCouncilofNorway (Project no. 280788). The Research Council of Norway is also acknowledgedforthesupporttotheNorwegianMicro-andNano- FabricationFacility,NorFab,projectnumber245963/F50.Growth ofhexagonalboronnitridecrystalswassupportedbytheMEXT ElementStrategyInitiativetoFormCoreResearchCenter,Grant NumberJPMXP0112101001andtheCREST(JPMJCR15F3),JST.

AppendixA. Supplementarydata

Supplementarymaterial relatedto this articlecanbe found, in the online version, at doi:https://doi.org/10.1016/j.sna.2020.

112247.

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Biographies

Dr.AyazAlireceivedhisBSdegreeinElectronicsfromUniversityofSindh,Jamshoro, in2009,MSandPhDdegreeinMicroelectronicsfromZhejiangUniversity,China in2014and2018respectively.Currently,heisapostdocfellowatSINTEF,Oslo.

HisresearchinterestfocusesonCMOSintegrationofgraphenelike2Dmaterials (BN,BPandTMDs)forhigh-performanceheterostructuredevicessuchasbroadband photodetectorsandemergingsmartsensorsfortheinternet-ofthingsandﬂexible electronics.

Dr.OzhanKoybasiisaSeniorScientistatSINTEFinthedepartmentofMicrosystems andNanotechnology.HeobtainedhisMSdegreein2010andPhDdegreein2011 fromPurdueUniversityinelectricalengineeringandphysics,respectively.Hejoined SINTEFin2013,andsincethen,hehasbeenleadingR&Dandproductionprojects onsemiconductor-basedradiationdetectorsandphotodetectors.Hisotherresearch interestsincludeapplicationof2Dmaterialsandrelatedmaterialsindifferenttypes ofsensorssuchasgassensorsandphotodetectors.

Dr.WenXingcurrentlyworksasresearchscientistattheDepartmentofThin FilmandMembraneTechnology,SINTEF.HeobtainedhisPhDin2013atUni- versityofOslo,Norway.Hiscurrentresearchinterestsareelectrochemistryin hightemperatureenergyconvertingmaterials,gasseparationmembranesand2D materials.

Dr.DanielNilsenWrightreceivedhisMCheminchemicalphysicsfromLiverpool Universityin2001andhisPhDinphysicsfromtheUniversityofOsloin2008.He iscurrentlyaresearchscientistatSINTEFworkingasaprojectleaderforprojects involvingsilicondevicefabricationandadvancedelectronicpackaging.

Dr.DeepakVarandaniisaSeniorScientistinThinFilmLaboratory,Department ofPhysics,IndianInstituteofTechnologyDelhi.Hiscurrentresearchinterestsare synthesisandcharacterizationofnanomaterials.

Dr.TakashiTaniguchiisResearcherintheNationalInstituteforMaterialsScience, Japan.Hiscurrentresearchinterestsaresynthesis,opticalandelectricalcharacter- izationofnovel2Dmaterials.

Dr.KenjiWatanabeisResearcherintheNationalInstituteforMaterialsScience, Japan.Hiscurrentresearchinterestsaresynthesis,opticalandelectricalcharacter- izationofnovel2Dmaterials.

Prof.BodhR.MehtaiscurrentlyDean(R&D),SchlumbergerChairProfessorand memberoftechnicalcommitteeonsensorbasedambientairmonitoringsystems (CentralPollutionControlBoard,India).Heleadsagroupwhichhasextensiveexpe- rienceintheﬁeldofthinﬁlmandnanomaterialsforgassensing,SolarCell,Resistive Memory,Thermoelectricandphotoelectrochemicalapplications.

Dr.BransonBelleisaSeniorScientistatSINTEFinthedepartmentofRenewable EnergyTechnology.HeobtainedhisPhDin2007fromtheUniversityofManchester intheﬁeldofMagnetism.Hiscurrentresearchinterestsarethedevelopmentof sensorsanddevicesbasedon2Dmaterialheterostructuresaswellasatomicforce microscopy.