Evolution of Structural and Optical Properties of ZnO Nanorods Grown on Vacuum Annealed Seed ...

17 Pages·2017·8.2 MB·English

nanomaterials
Article
Evolution of Structural and Optical Properties of ZnO
Nanorods Grown on Vacuum Annealed
Seed Crystallites
WaqarKhan1 ID,FasihullahKhan1,HafizMuhammadSalmanAjmal1,NoorUlHuda1,
JiHyunKim2andSam-DongKim1,*
1 DivisionofElectronicsandElectricalEngineering,DonggukUniversity,Seoul100-715,Korea;
[email protected](W.K.);[email protected](F.K.);[email protected](H.M.S.A.);
[email protected](N.U.H.)
2 SchoolofAdvancedMaterialsScienceandEngineering,SungkyunkwanUniversity,Suwon-Si,
Gyeonggi-Do16419,Korea;[email protected]
* Correspondence:[email protected];Tel.:[phone];Fax:[phone]
Received:26December2017;Accepted:23January2018;Published:26January2018
Abstract: Inthisstudy,theambientconditionfortheas-coatedseedlayer(SL)annealingat350◦Cis
variedfromairornitrogentovacuumtoexaminetheevolutionofstructuralandopticalproperties
ofZnOnanorods(NRs). TheNRcrystalsofhighsurfacedensity(~240rods/µm2)andaspectratio
(~20.3)showgreatlyenhanced(002)degreeoforientationandcrystallinequality,whengrownon
theSLsannealedinvacuum,comparedtothoseannealedinairornitrogenambient. Thisisdueto
thevacuum-annealedSLcrystalsofahighlypreferredorientationtoward(002)andlargegrainsizes.
X-rayphotoelectronspectroscopyalsorevealsthatthehighestO/Znatomicratioof0.89isobtained
inthe caseof vacuum-annealedSL crystals, whichis dueto theeffective desorptionof hydroxyl
groups and other contaminants adsorbed on the surface formed during aqueous solution-based
growthprocess. Nearbandedgeemission(ultravioletrangeof360–400nm)ofthevacuum-annealed
SLsisalsoenhancedby44%and33%ascomparedtothoseannealedinairandnitrogenambient,
respectively,inphotoluminescencewithsignificantsuppressionofvisiblelightemissionassociated
withdeepleveltransition. DuetothisimprovementofSLopticalcrystallinequality,theNRcrystals
grownonthevacuum-annealedSLsproduce~3timeshigherultravioletemissionintensitythanthe
othersamples. Insummary,itisshownthattheZnONRspreferentiallygrowalongthewurtzite
c-axisdirection,therebyproducingthehighcrystallinequalityofnanostructureswhentheygrow
onthevacuum-annealedSLsofhighcrystallinequalitywithminimizedimpuritiesandexcellent
preferred orientation. The ZnO nanostructures of high crystalline quality achieved in this study
canbeutilizedforawiderangeofpotentialdeviceapplicationssuchaslaserdiodes,light-emitting
diodes,piezoelectrictransducersandgenerators,gassensors,andultravioletdetectors.
Keywords: ZnO nanorods; vacuum annealing; photoluminescence; hydrothermal process;
surfacedefects
1. Introduction
Semiconductingmetaloxideshavebeenintensivelydevelopedforoptoelectronicsandsensor
material applications of various transduction platforms in last several decades due their unique
materialpropertiesofopticaltransparencybasedonwideenergybandgapandchemiresistivebehavior
dependingonspecificgasadsorptiononthesurface[1]. Futuremicroelectronicstechnologyutilizing
thesenewfunctionsofmetaloxidewillthereforeleadtoavarietyofchallengingapplicationsand
theirnewconsequentindustrialmarket. Amongmanysemiconductingmetaloxides,ZnOhasbeen
Nanomaterials2018,8,68;doi:10.3390/nano8020068 www.mdpi.com/journal/nanomaterials
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highlightedandextensivelyinvestigatedinrecentyearsforitsdistinctivepropertiesandpotential
applicationsinoptoelectronics,gassensors,andenergyharvestingapplications[1–5]. Ithasadirect
bandgapof~3.37eVatroomtemperature(RT),alargeexcitonbindingenergyof60meV,thermaland
chemicalstability,piezoelectricity,radiationhardness,andtheoptionofwetchemicaletching[4–7].
NumerousmethodshavebeeninvestigatedforthesynthesisofZnOcrystalsthusfar,buttypical
methodsadoptedforthefabricationofZnOnanostructuresincludevaporphaseepitaxy,pulsedlaser
deposition,spraypyrolysis,molecularbeamepitaxy,andchemicalvapordepositiontechniques[3,8–11].
However, most of these process schemes require complicated facilities and high thermal budget
whichultimatelyhinderlow-costandlargescalefabricationonflexiblesubstrates. Amongvarious
growth methods for the ZnO nanorods (NRs), a synthesis technique in aqueous solution such as
hydrothermalmethodisverypromisingbecausetheycanproceedatrelativelylowthermalbudget
allowinglow-costandlarge-scalerolltorollfabrication[4,7,12,13]. ZnOalsoprovideawidevarietyof
nanostructuremorphologiessuchasNRs,nanowires,nano-belts,andnano-flowersformanypractical
applications [8–13]. Fabrication of nano-scale ZnO materials with special morphology and high
crystallinequalityisofgreatinterestformaterialssciencebecauseofitssignificanceinthescientific
researchandpotentialintheminiaturizedtechnologicalapplications.
Drawbackofhydrothermaltechniqueishoweverthepresenceofhugevolumeofdefectsonthe
surfaceofgrownnanostructuresaffectingthestructuralandopticalpropertiesofZnOnanostructures.
Theoptimizationofopticalandstructuralpropertiescanbeachievedbyfullyunderstandingthegrowth
mechanismofnanostructuresandhowthedefectsinthecrystalsbehavethroughthepost-surface
treatment. However,themorphologicalcontrolandcrystalstructureevolutiontoalowdefect-density
nanostructures remain challenging to material scientists. In order to suppress the defects in ZnO
nanostructures, great amount of research effort has been made by using post growth annealings;
for example, Qui et al. post-treated the as-grown ZnO nanowires at 550 ◦C in different ambient
conditionsandreportedtheenhancementofZnOnanocrystalstothehighcrystallinequalitythrough
vacuumannealing[6],howeversufficientunderstandinghowthismethodcontributedtothechange
incrystallinequalitywasnotachieved.
Inthisstudy,weinvestigatehowthepost-annealingambientfortheZnOseedlayer(SL),which
servesasaplatformforthesubsequentnanocrystalnucleation,affectsthecrystallinequalityofZnONR
growth. TodevelophydrothermalmethodtosynthesizehighcrystallinequalityZnOmicrostructures
withasimplepost-annealingprocessatrelativelylowtemperaturewillbehighlydesirabledueto
itseasilycontrollablecondition. Inaddition,thepossibledefectannihilationmechanisminZnONR
crystals during the post-annealing is expected to further investigate. If material properties can be
preciselytunedbydeployingdefectchemistry,wecansatisfythepreciserequirementsforapplications
throughanunderstandingofthedefectformationmechanism. Forthispurpose,weinvestigatedin
thisstudytheroleofannealingambientandtheevolutionofdefectsinas-annealedSLsaswellasZnO
NRcrystalsbyusingvarioussurfacecharacterizationtechniquesincludingX-raydiffraction(XRD),
photoluminescence(PL),X-rayphotoelectronspectroscopy(XPS),fieldemissionscanningelectron
microscopy(FE-SEM),Fouriertransforminfrared(FT-IR)spectroscopy,atomicforcemicroscopy(AFM),
andtransmissionelectronmicroscopy(TEM).
2. ExperimentalProcedure
ZnONRsweregrownonp-Si(100)substrate(2×2cm2,0.01Ω-cmresistivityofborondoping)
usinganaqueoussolutionmethodinthefollowingway. Afterthesurfacecleaningbyacetoneand
isopropylalcoholsequentiallytoremovedustandorganiccontaminants,thesubstrateswererinsedby
de-ionized(DI)wateranddriedwithnitrogen(N )purgefollowedbymoistureremovalonhot-plate
2
at120◦Cfor2min. AqueoussolutionrouteforthegrowthsofZnONRsconsistsofthefollowingtwo
steps. First,acolloidalsol-gelwaspreparedbymixingzincacetatedehydrate[Zn(CH COO) ·2H O]
3 2 2
inanorganicsolventof1-propanoltoforma10mMconcentratesolution. Afterconstantlysonicating
thesolutionfor30min,itwaskeptatroomtemperaturelongerthan5–6hforthesol-gelstabilization.
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tThhei ssoslo-lguetli ostnabfoilrizthateioZnn. OThSiLs sgorlouwtitohnw foars tthhee nZnspOin S-Lco gartoewdtohn wthaes stuhebnst rsaptien-acto3a0t0ed0 ropnm thfeo rsu30bsstraantde
abta 3k0e0d0 artp1m0 0fo◦rC 3f0o sr a1nmd ibna,kaendd atth 1e0c0o °aCti nfogr w1 amsirne, paenadte tdhe1 0cotaimtinegs twoaasc rheipeveaeteadt h1i0c ktinmeesss otof a~c2h0ienvme
aa ftthericaknnneessa loinf g~.2T0h nemSL a-fgtreor wannnseaamlipnlge.s Twheer eSLp-ogsrto-awnnn esaalmedplaets 3w50er◦eC pinosat-aconnnveaelcetdio natf u3r5n0a °cCe wini tah
cthonreveecdtiifofner efunrtnaamceb wienitths tohfrevea cduiuffmere(n5t0 ammTboirern)t,sa otmf voaspcuhuermic (a5i0r ,manTdorNr),2 .atPmoosst-pahnenreica lainirg, sanfodr Nth2e.
Paos-scto-aantendeaSliLnsgsw feorre thcaer arise-cdoaotuetdn SoLtso wnelyret ocarrermiedo voeutt hneoto orgnalyn itco rreesmidouvael sthaen odrguannwica rnetseidduraelasc atinodn
ubnyw-parnotdeudc trsearcetmioani nbiyn-gpirnodthuectcsr yresmtaalsinbiuntga ilns othtoe cinryvsetsatlisg abtuet haolswo ttho eincvryessttaigllaitnee hqouwal itthye ocfrythsetaSllLinies
qinuflaulietyn ceodf btyheth eSLan nise ailinnfgluaemncbeiden tbiyn ctohnen eacntinoenalwinitgh tahmebmieantet riianl cchoanrancetcetriiosnti cswoifthth ethZen OmNatRersiatol
cbheagrarocwternisatfitcesr owfa trhdes .ZnO NRs to be grown afterwards.
IInn tthhee sseeccoonndd sstteepp,, wwee iimmmmeerrsseedd tthhee ssuubbssttrraatteess ttoo ggrrooww tthhee NNRR ccrryyssttaallss iinn aann eeqquuiimmoollaarr ggrroowwtthh
ssoolluuttiioonn ofo(2f5 m(2M5 )zmincMn)i traztienhce xanhiytrdartaet e(hZenx(aNhOy3d)r2a·6tHe 2O(Z,9n9(%N)Oa3n)d2·6(H252Om,M )9h9e%xa) meatnhdyl en(e2t5e trammMine)
h(HexMamT)e(tChy6Hlen12eNte4t,ra9m9.5in%e) (iHnM25T0) m(Cl6DHI12wNa4,t e9r9.a5s%il)l uins t2r5a0te mdli nDFI iwguarteer1 a.sA iflltuerstcroamtedp lient eFsigtiurrrien 1g. oAfftthere
cgormowptlhetseo sltuitriroinng, tohfe tsheee dg-rgorwowthn ssoulubtsitornat, etshwe esereedp-lgarcoewdnu pssuibdsetrdaotwesn winerae bpelaakceedr fuoprsthideeN dRowgrno wint ha,
baenadktehre fobre tahkee rNwRa gsrsoewaltehd, awnidth thaen baeluakmeirn wumas fsoeiallaendd wpitlhac aend aolnumaihnoutmpl faoteil aatn9d0 p◦lCacfeodr o5n–6 ah h.oFt ipnlaaltley,
atht e90g r°oCw fonrZ 5n–O6 hN. RFisnwalelyre, twhea sghreodwwn iZthnOD INwRast ewrearned wpausrhgeedd wwiitthh DNI2 wgaatse.r and purged with N2 gas.
TToo eexxpplloorree tthhee eevvoollvveedd mmoorrpphhoollooggiieess aanndd ccrryyssttaalllliinnee ssttrruuccttuurreess ooff tthhee aass--ggrroowwnn NNRRss,, wwee
ppeerrffoorrmmeedd FFEE--SSEEMM ((HHiittaacchhii SS--44880000SS,, ooppeerraatteedd aatt 1155 kkVV,, SSuuwwoonn,, KKoorreeaa)) aanndd ccrroossss--sseeccttiioonnaall TTEEMM
((HHiittaacchhii 99550000 aatt 330000 KKVV)) aannaallyyssiiss wwiitthh sseelleecctteedd aarreeaa eelleeccttrroonn ddiiffffrraaccttiioonn ((SSAAEEDD)).. CCrryyssttaalllliinnee qquuaalliittyy
aanndd pprreeffeerrrreedd oorriieennttaattiioonnw weerreei ninvveestsitgigaatetdedb ybyX XRRDD(D (D8A8 dAvdavnacnecsep sepctercotmroemteerteorf Bofr uBkreurkAerX ASXwSi twhiCthu
CKuα 1K.5α4 01.Å54r0a dÅia triaodni,aStieoonu, l,SKeoourel,a )Kfoorreeaa)c hfoSr Leaanchd NSLR asanmd pNleRs psraempparleeds upnredpearrdedif feurnednterc odnidffietiroennst .
cPoLndspiteiocntrso. sPcLo pspye(cMtrFoPsc-o3pDy B(Mio,FPA-s3yDlu Bmio,R Aessyealurcmh ,RSeusewaorcnh,, KSuowreoan),e Kxcoirteead) bexycaite3d2 5b-yn am 32li5n-enmH eliCnde
HlaesCerdw laassearl swoacsa rarliseod coaurrtieadt RoTutt oaet xRaTm tion eexthaemoinpeti ctahlep orpotpiceartli epsroopfethrteiessa mofp tlhees .sCamhepmleisc.a lCbhoenmdiicnagl
baonnddsintogi cahniodm setotricichiaonmaelytrsiics awnearleysdiso nweerbey dXoPnSe ubysi nXgPSP HusIin50g0 P0HVIe r5s0a00P rVoebresa( UPlrvoabce- P(HUIlv,aScu-wPHonI,,
SKuowreoan), sKpoecretrao)m speteecrtrwomithetmer ownoitchh rmoomnaotcohrrAoml Katαor( 1A4l8 6K.6α e(V14)8a6n.6o deeV)( 2a5n.0odWe, (1255.k0V W).,T 1h5e kbVo)n. dTihneg
bcoonnfidginugr actoionnfsigoufrtahteioZnnsO ofS Lthcer yZsntaOls SwLe rceryinsvtaelsst iwgaetreed ibnyvFesTt-iIgRatsepde cbtryo sFcTo-pIRy (sIpFSe6ct6rvo/sscoapnyd H(IFySp6e6rvio/ns
a3n0d00 H, Byrpuekrieorn). 3A00F0M, B(rNu8k-eNr)E. OASF,MB r(uNk8e-rN)EinOnSo, nB-rcuoknetra)c itnm noodne-cwonatsacatl smoocdaerr wieads oaulstot ocaerxraiemdi nouett htoe
esxuarmfacineem thoerp shuorfloacgeie msoofrpthheolSoLgsieasn onfe tahleed SiLns dainffneeraelnetda imn bdiiefnfetrceonnt daimtiobniesn.t conditions.

FFiigguurree 11.. PPrroocceessss ffllooww ooff ZZnnOO nnaannoorrooddss ((NNRRss)) ggrroowwtthh wwiitthh tthhrreeee ddiiffffeerreenntt ppoosstt--aannnneeaalliinngg aammbbiieenntt
ccoonnddiittiioonnss ((vvaaccuuuumm,, aaiirr aanndd NN22)) ffoorr tthhee sseeeedd llaayyeerrss ((SSLLss))..
3. Results and Discussion
3. ResultsandDiscussion
The growth morphology of the ZnO NRs was first examined by FE-SEM, and Figure 2a–c show
ThegrowthmorphologyoftheZnONRswasfirstexaminedbyFE-SEM,andFigure2a–cshow
the top views of the NRs grown on SLs annealed under three different ambient conditions of vacuum,
thetopviewsoftheNRsgrownonSLsannealedunderthreedifferentambientconditionsofvacuum,
aaiirr,, aanndd NN2,, rreessppeeccttiivveellyy.. AAvveerraaggee lleennggtthhss ooff tthhee NNRRss wweerree 11~~11..33 μµmm aass sshhoowwnn iinn tthhee ccrroossss--sseeccttiioonnaall
2
view in Figure 2d, and the NRs grown on the vacuum-annealed SLs exhibited the fastest growth rate.
Shown in Figure 2e–g are the histograms of NR diameter distributions, and it was shown that the
Nanomaterials2018,8,68 4of17
view in Figure 2d, and the NRs grown on the vacuum-annealed SLs exhibited the fastest growth
Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 17
rate. ShowninFigure2e–garethehistogramsofNRdiameterdistributions,anditwasshownthat
tshtaetissttaitciss toicfs NofRN dRiadmiaemtere twerewree raelsaol ssoigsnigifnicifiancatlnyt layffaefcfetecdte dbyb yththe eSSLL aannnneeaalilningg ccoonnddiittiioonn. .TTaabbllee 11
ssuummmmaarriizzeedd tthhee mmoorrpphhoollooggyy ssttaattiissttiiccss ooff tthhee NNRRss ggrroowwnn uunnddeerr tthhrreeee ddiiffffeerreenntt ccoonnddiittiioonnss.. TThhee NNRRss
ggrroowwnn oonn SSLLss aannnneeaalleedd iinn vvaaccuuuumm sshhoowweedd tthhee ssmmaalllleesstt mmeeaann ddiiaammeetteerr ooff 6655 nnmm;; oonn tthhee ootthheerr hhaanndd,,
tthhee llaarrggeerr ddiiaammeetteerrss ooff 8800 aanndd 111155 nnmm wweerree mmeeaassuurreedd iinn tthhee ccaasseess ooff aannnneeaalliinnggss iinn aaiirr aanndd NN22 aammbbiieenntt
ccoonnddiittiioonn,, rreessppeeccttiivveellyy.. AAssppeecctt rraattiioo ooff tthhee NNRRss wwaass ddeeccrreeaasseedd ffrroomm 2200..33 ((vvaaccuuuumm)) ttoo 1133..77 ((aaiirr)) aanndd
88..88 ((NN22)) ddeeppeennddiinngg oonn tthhee aannnneeaalliinngg aammbbiieenntt.. TThhee NNRRss aallssoo eexxhhiibbiitteedd tthhee mmaaxxiimmuumm ssuurrffaaccee ddeennssiittyy
((~~224400 rrooddss//µμmm22)) iinn ththeec acsaesoe fovfa cvuaucmuu-amn-naenanlienagli,nbgu,t bthuet dtehnes idtyenwsaitsys iwgnaisfi csaignntliyficreadnutlcye drebdyu3c3e–d6 2b%y
i3n3–th6e2%ca isne sthoef cdaifsfeesr eonf tdaimffebrieenntt acmonbdieitniot ncos.nditions.
(a) (b)
(c) (d)

(e) (f)
Figure2.Cont.
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(g)
(g)
Figure 2. Field emission scanning electron microscopy (FE-SEM) (top view) of the ZnO NRs grown
FFiigguurree 22.. FFiieelldd eemmiissssiioonn ssccaannnniinngg eelleeccttrroonnm micicroroscsocoppyy( F(EFE-S-ESEMM)()t o(tpopv ivewiew)o)f otfh ethZen ZOnON RNsRgsr ogwronwonn
on SLs annealed in (a) vacuum; (b) air; and (c) nitrogen. Cross-sectional views of the ZnO NRs are
oSnL sSaLnsn aenanleedaliend( ain) v(aac) uvuamcu;u(bm);a (ibr;) aanidr; (acn)dn i(tcro) gneitnr.oCgerons. sC-sreocstsio-sneacltivoinewals voifewthse oZfn tOheN ZRnsOa rNesRhso awren
shown in (d). Histogram for the diameter distribution of NRs grown on SLs annealed in (e) vacuum;
sinho(wd)n. Hini s(tdo)g. rHaimstofogrratmhe fdoira tmhee tdeiradmisettreirb duitsiotrnibouftNioRn sogf rNoRwsn gornowSLns oann SnLesa laendniena(leed) vinac (ueu) vma;c(ufu)amir;,
(f) air, and (g) nitrogen.
(afn) dai(rg, )anndit r(ogg) enni.trogen.
Table 1. Morphologies of the ZnO NRs (measured by FE-SEM) prepared under three different SL
TTaabbllee 11.. MMoorrpphhoollooggiieess ooff tthhee ZZnnOO NNRRss ((mmeeaassuurreedd bbyy FFEE--SSEEMM)) pprreeppaarreedd uunnddeerr tthhrreeee ddiiffffeerreenntt SSLL
annealing ambient conditions.
annealingambientconditions.
annealing ambient conditions.
Annealing Diameter Mean Standard Deviation of NR Density Aspect
AnAnenanleianlgin g DiaDmiaemteert er MeManea n StanSdtaanrdda DrdeDvieavtiiaotnio nof NNRRD Deennsistyity AspeActsRpaeticot
Ambient for SL Range (nm) Diameter (nm) Diameter (nm) (rods/μm2) Ratio of NR
AmAbmiebnite nfotrf oSrLS L RanRgaen g(enm(nm) ) DiaDmiaemteert e(rn(mnm) ) DoifaDmiaemteert (enrm(n)m ) ((rroodds/sµ/μmm2)2) RoaftNioR of NR
Vacuum 50–110 65 11.8 ~240 20.3
VacVuaucumu m 50–5101–011 0 65 65 111.18. 8 ~~224400 20.230.3
Air 60–130 80 12.0 ~160 13.7
AirA ir 60–6103–013 0 80 80 121.20. 0 ~~116600 13.173.7
Nitrogen 70–190 115 20.4 ~90 8.8
NitNroitgreonge n 70–7109–019 0 1151 15 202.04. 4 ~~9900 8.88.8
We obtained well aligned NRs normal to the substrates with pure wurtzite hexagonal faces as
WWee oobbttaaiinneedd wweellll aalliiggnneedd NNRRss nnoorrmmaall ttoo tthhee ssuubbssttrraatteess wwiitthh ppuurree wwuurrttzziittee hheexxaaggoonnaall ffaacceess aass
shown in each micrograph of Figure 2a–c. The degree of NR c-axis alignment was more prominent
sshhoowwnn iinn eeaacchh mmiiccrrooggrraapphh ooff FFiigguurree2 2aa––cc..T Thheed deeggrereeeo offN NRRc -ca-xaixsisa laiglingmnmenetnwt wasams moroerper pomroimneinnetnint
in the case of vacuum-annealing than the other cases. Figure 3a,b respectively represent the θ-2θ XRD
itnh ethcea csaesoef ovf avcaucuumum-a-nannenaelainligngth tahnanth theeo oththeerrc caasseess..F Fiigguurree 33aa,,bb rreessppeeccttiivveellyy rreepprreesseenntt tthhee θθ--22θθ XXRRDD
spectra obtained from the SLs annealed under three different ambient conditions and the NRs grown
ssppeeccttrraa oobbttaaiinneedd ffrroomm tthhee SSLLss aannnneeaalleedd uunnddeerr tthhrreeee ddiiffffeerreenntt aammbbiieenntt ccoonnddiittiioonnss aanndd tthhee NNRRss ggrroowwnn
atop each SL.
aattoopp eeaacchh SSLL..

(a) (b)
(a) (b)
Figure 3. θ-2θ X-ray diffraction (XRD) patterns of (a) the SLs annealed in three different ambients;
Figure 3. θ-2θ X-ray diffraction (XRD) patterns of (a) the SLs annealed in three different ambients;
aFnigdu (rbe) 3th.eθ -Z2nθOX N-raRy gdroifwfrnac otino nea(cXhR dDif)fepraetntte rSnLs. of(a)theSLsannealedinthreedifferentambients;
and (b) the ZnO NR grown on each different SL.
and(b)theZnONRgrownoneachdifferentSL.
From the XRD patterns, it was shown that all reflections were in perfect agreement with the
From the XRD patterns, it was shown that all reflections were in perfect agreement with the
reporFterodm intdheexeXsR oDf JpCaPttDerSn fsi,leits w(caasrds hnouwmnbethr a3t6a-1ll45re1fl) efoctri othnes hweexraegionnaple prfheacstea gZrneOem. Ienn etawchit hcatshee,
reported indexes of JCPDS files (card number 36-1451) for the hexagonal phase ZnO. In each case,
trheep omrtoesdt iinntdeenxsees poefaJkCs PaDloSngfi l(e0s02(c)a orrdiennutamtiboenr w36e-r1e4 o5b1s)efrovretdh efrohmex athgeo nSaLlsp, ahnadse thZins Ore.pInreeseanchts ctahsaet,
the most intense peaks along (002) orientation were observed from the SLs, and this represents that
othuer mZonsOt isneteends cerpyesataklsliatelos nhga(v0e0 2a) porrieefnetraretido nowrieenretaotbiosner valeodnfgr ocm-axthise. SELssp,eacniadlltyh,i sthreep dreesgernetes othf a(t0o0u2)r
our ZnO seed crystallites have a preferred orientation along c-axis. Especially, the degree of (002)
preferred orientation was the most strong from the SLs annealed in vacuum (at 2θ = 34.44°) with no
preferred orientation was the most strong from the SLs annealed in vacuum (at 2θ = 34.44°) with no
visible reflection from other planes, while fairly significant (100) reflections were also observed from
visible reflection from other planes, while fairly significant (100) reflections were also observed from
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ZnOseedcrystalliteshaveapreferredorientationalongc-axis. Especially,thedegreeof(002)preferred
orientation was the most strong from the SLs annealed in vacuum (at 2θ = 34.44◦) with no visible
reflectionfromotherplanes,whilefairlysignificant(100)reflectionswerealsoobservedfromtheSLs
whenannealedinotherambientconditions[14]. Thedegreeoforientation,F(hkl),canbegivenbythe
followingequation:[15]
(P(hkl)− P (hkl))
F(hkl) = 0 (1)
(1− P (hkl))
0
whereP(hkl)=I(hkl)/ΣI(hkl),P (hkl)=I (hkl)/Σ(I (hkl),I(hkl)isthemeasuredpeakintensityfrom
0 0 0
(hkl)plane,andI (hkl)isthereferencepeakintensityof(hkl)planegivenbyJCPDScardNo. 36-1451.
0
AssummarizedinTable2,thehighestvalueofF(002)wasobtainedfromtheSLsannealedinvacuum,
whereasmuchlowerdegreeoforientationsweremeasuredfromtheSLcrystalsannealedinairandN
2
ambients. TheexcellentF(002)valueinthecaseofvacuumannealingsuggeststhatthegasmolecules
and/orcontaminantspresentintheannealingambientcanplayaveryimportantroleintheSLcrystal
growthandgraincoalescenceprocess[14–16].
Theaveragegrainsizedwascalculatedfromfullwidthathalfmaximum(FWHM)of(002)peak
byusingDebye-Schererformula:[15,16]
d = (0.94λ)/(βCosθ) (2)
whereλistheX-raywavelength(0.154nm),βisFWHMinradians,andθistheBragg’sdiffraction
angle. ThegrainsizesofSLcrystalscalculatedbythismethodwere55.6nm(vacuum),30.4nm(air),
and28.3nm(N )ineachdifferentannealingambientconditionasshowninTable2. Annealingin
2
vacuumcanpromotethegraingrowthandintergranularcoalescenceoftheSLcrystalsthroughthe
grainboundarydiffusion,therebyproducinglargegrainsizesduringthepost-annealing. However,
gasmoleculesandcontaminantsinthegrainboundariesintroducedduringtheannealinginairorN
2
ambientscansuppresstheintergranulardiffusionfortheevolutionarySLcrystalgrowth[12,15,16].
Table2.ParametersoftheZnOSLcrystalsextractedbyXRDanalysis.
AnnealingAmbient (002)2θ (002) GrainSize (002)Spacing Degreeof(002)
forSL (◦) FWHM(◦) (nm) (Å) Orientation
Vacuum 34.44 0.156 55.6 2.600 0.98
Air 34.53 0.303 30.4 2.594 0.66
Nitrogen 34.54 0.304 28.3 2.593 0.61
TheXRDpatternsfortheNRsgrownontheSLsannealedunderdifferentenvironmentwerealso
showninFigure3b. Singlestrongpeak(2θ=34.47◦)along(002)orientationwasobservedfromthe
NRsgrownontheSLsannealedinvacuum, andnootherpeaklinkedtoanydifferentorientation
wasfound. ThisrevealsthattheZnONRcrystals,inthiscase,growdominantlyalongc-axisinthe
verticaldirectiontothesubstrate. Ontheotherhand,weobtainedweakbutvisibleadditionalpeaks
from(100)and(101)planesat31.7◦ and36.4◦,respectively,withadominantpeakfrom(002)fromthe
NRsgrownontheSLsannealedinairandN . Theintensityof(002)peakmeasuredfromthevacuum
2
annealingwassignificantlyreducedby2.5and7.6timesinthecasesofairandN ambientannealing,
2
respectively. Thisdepictsthatthec-axisalignmentoftheZnONRgrowthwasconsiderablyeffected
bytheannealingambientconditionfortheSLs. Aswasdiscussedearlier,theSLcrystalsannealedin
vacuumstatehaveahighlypreferredorientationtoward(002)andlargegrainsizes. Assuggestedby
manyformerresearchers,eachcrystallinesurfaceoftheSLgrainactsasanucleiforthegrowthofNRs,
andtheZnONRstendtodominantlygrowalongthe[001]directionbecauseofitslowersurfacefree
energy(1.6J/m2)thanthoseof(100)(3.4J/m2)and(101)(2.0J/m2)planes[17,18]. Thiscanexplain
whywehaveastrongc-axisalignmentalongverticaldirectiontothesubstratewhentheZnONRs
grownonthevacuum-annealedSLs.
Nanomaterials2018,8,68 7of17
TEMdark-field(DF)analyseswerecarriedoutfortheNRstructuresgrownunderthreedifferent
SLannealingconditions. Figure4a–cshowstheDFimagesoftheNRcrystalsinthebottomrows,and
cleardifferencesincontrastwerefoundfromeachNRcrystalcorrespondingtotheirplaneindexes
diffracted on the SAED patterns. As shown in Figure 4a, most of the NR crystals grown on the
Nanomaterials 2018, 8, x FOR PEER REVIEW 7 of 17
vacuum-annealedSLswerebrightasobservedunder(002)diffractioncondition,whereasonlyfew
NNRRss sshhoowweedd tthhee bbrriigghhtt ddiiffffrraaccttiioonn ccoonnttrraasstt uunnddeerr ((110000)) oorr ((110022)) ddiiffffrraaccttiioonn ccoonnddiittiioonn.. TThhee ssaammpplleess
pprreeppaarreedd uunnddeerr ddiiffffeerreenntt aannnneeaalliinngg aammbbiieennttss ((ssuucchh aass aaiirr oorr NN22)) sshhoowweedd qquuiittee ddiiffffeerreenntt ccoonnttrraasstt
ddiissttrriibbuuttiioonn iinn NNRRss ddeeppeennddiinngg uuppoonn tthhee SSLL aannnneeaalliinngg ccoonnddiittiioonn aass sshhoowwnn iinn FFiigguurree 44bb,,cc.. DDeessppiittee tthhee
lliimmiittaattiioonn ooff TTEEMM aannaallyyssiiss iinn vveerryy llooccaall aarreeaass,, tthhiiss oobbsseerrvvaattiioonn wwaass iinn ggoooodd aaggrreeeemmeenntt wwiitthh tthhee rreessuullttss
ooff XXRRDD aannaallyyssiiss ooff hhiigghheerr ssttaattiissttiiccaall rreelliiaabbiilliittyy..

(a)

(b)
Figure4.Cont.
Nanomaterials2018,8,68 8of17
Nanomaterials 2018, 8, x FOR PEER REVIEW 8 of 17
Nanomaterials 2018, 8, x FOR PEER REVIEW 8 of 17

(c)
(c)
Figure 4. Cross-sectional TEM micrographs of the ZnO NRs grown on SLs annealed in (a) vacuum,
Figure4. Cross-sectionalTEMmicrographsoftheZnONRsgrownonSLsannealedin(a)vacuum,
(bFi)g auirre, a4n. dC r(ocs) sn-sietrcotigoenna la TmEbMie nmtsic. rBorgirgahpt-hfsie oldf tihme aZgneOs aNnRd st hgreoirw sne loenct eSdL sa arenan eealelecdtr oinn (da)i fvfraaccutiuomn ,
(b)air,and(c)nitrogenambients.Bright-fieldimagesandtheirselectedareaelectrondiffraction(SAED)
(S(bA)E aDir), paantdte r(cn)s nairter orgeesnp eacmtivbeielyn tssh. oBwring hint- f(iteoldp -liemfta)g aens da n(tdo pth-reigirh st)e.l eDcaterdk faireelda eimleactgreosn a dreif fsrhaocwtionn
patternsarerespectivelyshownin(top-left)and(top-right).Darkfieldimagesareshownaccordingto
a(cScAorEdDin) gp atott eerancsh adrief freersepnet cdtiivfferlayc tsihoonw cnon idni t(itoonp -olef fpt)l aanned in(tdoepx-r iingh (tb)o. tDtoamrk rfoiewlds) iwmiatghe (st oapre-c sehnotewr)n
eachdifferentdiffractionconditionofplaneindexin(bottomrows)with(top-center)theircomposite
thaceciro rcdoimngp otsoi teea vcihe wdisf fienr tehnrte de idffirfafecrteionnt ccoolnodrsit. ion of plane index in (bottom rows) with (top-center)
viewsinthreedifferentcolors.
their composite views in three different colors.
As shown in Figure 5, we performed AFM for each scanning area (20 × 20 μm2) of the SLs
As shown in Figure 5, we performed AFM for each scanning area (20 × 20 µm2) of the SLs
anneaAlesd sihno wdinff eirne nFti gaumreb i5e,n tws.e Tpheer fAorFmMe dc hAarFaMct efroirz aetaiocnhs srceavnenailnegd asrigean i(f2ic0a n×t l2y0 rμedmu2c) eodf stuhref aScLes
annealedindifferentambients. TheAFMcharacterizationsrevealedsignificantlyreducedsurface
raonungehanleedss ifnr odmif ftehree nStL afmilmbise nantsn. eTahleed A inF Mva ccuhuarma cctoermizpaatrioends t ore tvheoaslee da nsnigenaliefidc ainnt alyir roerd Nuc2e. dT hseu rrfoaocte
roughnessfromtheSLfilmsannealedinvacuumcomparedtothoseannealedinairorN . Theroot
mroeuangh snqeusasr fer oromu gthhen eSsLs foilfm SLs aannnneeaalleedd iinn vvaaccuuuumm cwoamsp faoruendd t oto t hboe s1e. 3a4n nnmea,l ewdh iinle a tihr oosre N o22f. STLh efi lrmoost
meansquareroughnessofSLannealedinvacuumwasfoundtobe1.34nm,whilethoseofSLfilms
amnneeaanl esdq uianr ea irro aungdh nNe2s sa mofb SiLen atn wneearele d1. 8in4 vaancdu u2.m05 wnams ,f oreusnpde cttoiv beel y1.. 3T4h nism e,n whahnilcee tmhoenset ooff SsLu rffialmces
annealedinairandN ambientwere1.84and2.05nm, respectively. Thisenhancementofsurface
samnonoetahlende sisn fariorm a ntdh eN 22v aacmubuimen-ta nwneerael e1d.8 4S La ncdr y2s.0ta5l sn mca, nr ebspe ecdtuivee ltyo. Tthheis heinghhaenr ce(0m0e2n) td oefg srueerf aocfe
smoothnessfromthevacuum-annealedSLcrystalscanbeduetothehigher(002)degreeoforientation
osrmieonotathtinoens as s forobmse rvtheed bvya couuurm X-RaDnn aenaalelyds iSs L[1 9c]r.y stals can be due to the higher (002) degree of
asobservedbyourXRDanalysis[19].
orientation as observed by our XRD analysis [19].
(a)
(a)
Figure5.Cont.
Nanomaterials2018,8,68 9of17
Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 17
(b)
(c)
Figure 5. Atomic force microscopy (AFM) images of ZnO SLs annealed in (a) vacuum; (b) air, and (c)
Figure 5. Atomic force microscopy (AFM) images of ZnO SLs annealed in (a) vacuum; (b) air,
Nitrogen ambient.
and(c)Nitrogenambient.
Wide scan XPS spectra for the SLs annealed in different ambient conditions are shown in Figure
6aW inid ae rsacnagneX oPfS 0s–p1e1c5t0r aeVfo.r MthaeinS Lcsonasntniteuaelnedts ionf dZifnf earnendt Oam exbhieinbittc voanrdioituios npshaorteoeshleocwtronni npFeaigkus roef6 a
inadrifafnergeenot fco0r–e1-1le5v0eelsV .aMnda sinpicno-norsbtiittuael nsptsliottfinZgns aans dwOelle axsh Aibuitgvera rpieoaukssp, ahnodto seolmecetr tornacpese aokf scaorfbdoinff aerree nt
corael-sloe vdeeltsecatnedd isnp tihne- oSrLb cirtaylstsaplsl.i tFtiignugrse a6sb wsheolwl ass twAou cgleearrp ceoarke-sl,evaenld Zsno pmeaektsr oafc e2sp1o/2f (1c0a4r4b.o4n eVa)r eanadls o
det2epc3t/e2 d(1i0n21th.3e eSVL) scerpyastraaltse.dF biyg usprein6-borsbhitoawl ssptlwittoincgl efoarr ecaocrhe -SleLv cerlyZstnalp aenankesaolefd2 ipn1 /v2ac(u10u4m4,. 4aierV, a)nadn d
2p3N/22. (T1h02e 1p.3eaekV l)ocsaetpioanras,t esdymbymseptriinc -sohrabpitea olfs pthleit tpienagkfso, ranedac shpiSnL ocrrbyitsatla slpalnitntienagl evdaliunev (2a3c.u1u emV), aoifr t,haen d
N .ZTnh 2epp deoaukblolecta ctoionnfisr,msys mthmate tZrnic issh parpeeseonftt hase Zpnea2+k csh,eamndicaspl isntaoter binit aZlnsOpl istttoinicghvioamlueetr(y2 3in.1 aelVl )caosfetsh e
2
Zn[21p6]d. Houobwleetvceor,n 2fipr1m/2 spethaka toZf nthies SpLr easnennetaalsedZ inn2 +vacchueummic walasst ashteifitnedZ bnyO 0s.1to7i ecVhi otom heitgrhyeirn eanlelrcgays,e asn[d1 6].
Hotwhiesv sehri,f2t pis attpriebaukteodf ttoh ae hSLighanern oexaildedizaintiovna csutautem owf Zans sehleimfteedntb iyn 0th.1e7 ZenVOt coohreig mheartreixn.e rgy,andthis
1/2
shiftisaTtotr eibxuamteidneto thae hcihgahnegreo oxfi cdhiezmatiicoanl ssttaattee ionf SZLn ZenlOem creynsttainls,t hwee ZobnsOercvoerde mmoarter icxlo.sely O1s peaks
deTcoonevxoalmutiende itnhteoc thharneeg eseopfacrhateem siactaelllsittea tpeeianksS LofZ OnaO, Ocbr,y asntadl sO,cw aes sohboswernv eind Fmigourerec 6loc–seel. yOOa, 1wshpiechak s
is the lowest energy peak at ~530.3 eV, corresponds to the O2− ion in the wurtzite structure of
deconvolutedintothreeseparatesatellitepeaksofO ,O ,andO asshowninFigure6c–e.O ,whichis
a b c a
thehleoxwaegsotneanl eZrgnyOp. eBaekcaauts~e 5O30a .i3s eaV ,gcooordre mspeoansudrset ooft hsetoOic2h−ioimonetirnict hoexywguenrt zpirteessetnrucec tiunr ethoef wheuxratzgioten al
ZnOstr.uBcetucareu, sewOe evisalauagtoedo dthme esatsouicrheioomf settoryic hoifo meaecthr icZnoOxy gSeLn cpryrestsaelns cebyi nutshinegw ∫uOrat/∫zZitne isntr uecatcuhr e,
a
corresponding annealing condition [16,20–22], where ∫Oa and ∫Zn(cid:82) respe(cid:82)ctively represent the peak
weevaluatedthestoichiometryofeachZnOSLcrystalsbyusing O / Znineachcorresponding
a
curve integration of Oa and Zn. It is o(cid:82)bserved i(cid:82)n Table 3 that the percentage contribution of Oa in Ot
annealingcondition[16,20–22],where O and Znrespectivelyrepresentthepeakcurveintegration
(Ot = Oa + Ob + Oc) is the highest in the caase of vacuum-annealing. Each percentage value of Oa, Ob,
ofO andZn. ItisobservedinTable3thatthepercentagecontributionofO inO (O =O +O +O )
anad Oc in Table 3 was obtained by ∫Ok/∫Ot, where ∫Ok is the curve integration aof inditvidutal Oa1s satebllite c
isthehighestinthecaseofvacuum-annealing. EachpercentagevalueofO ,O ,andO inTable3was
peak (k = (cid:82)a, b, o(cid:82)r c), and ∫Ot(cid:82) is the curve integration of total O1s peak. aAlthbough ZncO is oxygen-
obtainedby O / O,where O isthecurveintegrationofindividualO1ssatellitepeak(k=a,b,
deficient matekrial int nature, butk the highest O/Zn atomic ratio of 0.89 was found in the case of
(cid:82)
orcv)a,caunudm-aOntniesatlhede cSuLr vcreyisnttaelsg,r aasti oshnoowfnto itna lTOa1bslep 3e,a wk.hAiclhth ios uegvhenZ nhOighisero xthyagne nth-doesefi coife notthmera taesr-ial
innaantnuerael,edb uZtntOhe nhaingohceryststOal/s Zgnroawtonm biyc hraytdirooothfe0r.m89alw maesthfooudns d[2i3n].t hOeb c(~a5s3e1o.2f evVac) uisu mtho-aungnhet atloe dbeS L
crystals,asshowninTable3,whichisevenhigherthanthoseofotheras-annealedZnOnanocrystals
Nanomaterials2018,8,68 10of17
grownbyhydrothermalmethods[23]. O (~531.2eV)isthoughttobeoriginatedfromO2– ionsin
b
theoxyNgaenonmadteerifialcs i2e01n8t, 8r, ex gFOioRn PEwERit RhEiVnIEtWh e ZnOmatrix[20,21]. ThehigherbindingenergyO1c0 ocfo 1m7 ponent
(~532.1eV)isassociatedwiththepresenceoflooselyboundoxygensuchas–CO ,–OHspeciesonthe
3
originated from O2– ions in the oxygen deficient region within the ZnO matrix [20,21]. The higher
surfaceofZnOcrystals. OneinterestingobservationisthattheSLcrystalsannealedinvacuumshow
binding energy Oc component (~532.1 eV) is associated with the presence of loosely bound oxygen
significantreductioninO contributioncomparedtothoseannealedinotherambientsasshownin
such as –CO3, –OH speccies on the surface of ZnO crystals. One interesting observation is that the SL
Figurec6rcy–setalasn adnnTeaableled 3i.n Tvhaicsuhumig hshsouwp psrigensisfiicoannto freOduccptieornc einn tOagc ecoonftrtihbeutvioanc ucuommp-aarnend etaol etdhossaem plesis
mostlikanenlyeadleude itno otthheere fafmecbtiievnetsd aess oshrpowtion nino fFOigu2,reC 6Oc–3e, oarnOd HTagblreo u3.p Tshaids shoigrbh esdupopnretshseiosnu orff aOcec formed
duringpoeurcrenwtaegteo orfg tahne ivcascuoulumt-iaonnn-ebaalesde dsapmrpolcese siss m[2o3s]t .liAkeslys duume mto athriez eefdfecitnivTea dbelseor3p,ttiohne oOf Oc2p, eCrOce3, ntageof
or OH groups adsorbed on the surface formed during our wet organic solution-based process [23].
thevacuum-annealedsampleshavealmosthalvedinvaluecomparedtothoseofthesamplesannealed
indiffeAresn stuammmbaireiznetds .in Table 3, the Oc percentage of the vacuum-annealed samples have almost halved
in value compared to those of the samples annealed in different ambients.
(a) (b)
(c) (d)
(e)
Figure 6. (a) Wide scan X-ray photoelectron spectroscopy (XPS) spectra of ZnO SLs annealed in three
Figure6.(a)WidescanX-rayphotoelectronspectroscopy(XPS)spectraofZnOSLsannealedinthree
different ambients; (b) High resolution spectra of Zn-2p of the SLs; O1s core level spectra obtained
differentambients;(b)HighresolutionspectraofZn-2poftheSLs;O1scorelevelspectraobtained
fro mtheZnOSLsannealedin(c)vacuum;(d)air,and(e)nitrogenambientarede-convolutedinto
threedistantsatellitepeaks.