Effects of annealing temperature and duration on the morphological and optical evolution of self

18 Pages·2017·9.57 MB·English

RESEARCHARTICLE
Effects of annealing temperature and
duration on the morphological and optical
evolution of self-assembled Pt nanostructures
on c-plane sapphire
MaoSui1,Ming-YuLi1,SundarKunwar1,PuranPandey1,QuanzhenZhang1,
JihoonLee1,2*
a1111111111
1 CollegeofElectronicsandInformation,KwangwoonUniversity,Nowon-guSeoul,SouthKorea,2 Institute
a1111111111 ofNanoscaleScienceandEngineering,UniversityofArkansas,Fayetteville,AR,UnitedStatesofAmerica
a1111111111
a1111111111 *[email protected]
a1111111111
Abstract
Metallicnanostructures(NSs)havebeenwidelyadaptedinvariousapplicationsandtheir
OPENACCESS physical,chemical,opticalandcatalyticpropertiesarestronglydependentontheirsurface
Citation:SuiM,LiM-Y,KunwarS,PandeyP, morphologies.Inthiswork,themorphologicalandopticalevolutionofself-assembledPt
ZhangQ,LeeJ(2017)Effectsofannealing
nanostructuresonc-planesapphire(0001)isdemonstratedbythecontrolofannealingtem-
temperatureanddurationonthemorphological
peratureanddwellingdurationwiththedistinctthicknessofPtfilms.TheformationofPt
andopticalevolutionofself-assembledPt
nanostructuresonc-planesapphire.PLoSONE12 NSsisledbythesurfacediffusion,agglomerationandsurfaceandinterfaceenergyminimi-
(5):e0177048.https://doi.org/10.1371/journal. zationofPtthinfilms,whichreliesonthegrowthparameterssuchassystemtemperature,
pone.0177048
filmthicknessandannealingduration.ThePtlayerof10nmshowstheformationofoverlay-
Editor:YogendraKumarMishra,Instituteof ingNPsbelow650˚CandisolatedPtnanoparticlesabove700˚Cbasedontheenhanced
MaterialsScience,GERMANY
surfacediffusionandVolmer-Webergrowthmodelwhereaslargerwigglynanostructures
Received:February24,2017 areformedwith20nmthickPtlayersbasedonthecoalescencegrowthmodel.Themor-
Accepted:April23,2017 phologiesofPtnanostructuresdemonstrateasharpdistinctiondependingonthegrowth
parametersapplied.Bythecontrolofdwellingduration,thegradualtransitionfromdensePt
Published:May4,2017
nanoparticlestonetworks-likeandlargeclustersisobservedascorrelatedtotheRayleigh
Copyright:©2017Suietal.Thisisanopenaccess
instabilityandOstwaldripening.ThevariousPtNSsshowasignificantdistinctioninthe
articledistributedunderthetermsoftheCreative
CommonsAttributionLicense,whichpermits reflectancespectradependingonthemorphologyevolution:i.e.theenhancementinUV-vis-
unrestricteduse,distribution,andreproductionin ibleandNIRregionsandtherelatedopticalpropertiesarediscussedinconjunctionwiththe
anymedium,providedtheoriginalauthorand
PtNSsmorphologyandthesurfacecoverage.
sourcearecredited.
DataAvailabilityStatement:Allrelevantdataare
withinthepaperanditsSupportingInformation
files.
Funding:FinancialsupportfromtheNational Introduction
ResearchFoundationofKorea(no.2011-0030079
Inlastdecade,thePtnanostructures(NSs)havebeenadaptedinwidespreadapplicationssuch
and2016R1A1A1A05005009),andinpartbythe
researchgrantofKwangwoonUniversityin2017is aselectro-catalyticsystems[1–3],hydrogenstorages[4,5],lightemittingdiodes(LEDs)[6],
gratefullyacknowledged. solarcells[7]andphotocatalyticapplications[8,9].Forexample,asacatalystofhighactivity
andselectivity,thePtNSsutilizedintheelectro-chemicalapplicationssuchasfuelcellscan
Competinginterests:Theauthorshavedeclared
thatnocompetinginterestsexist. significantlyimprovethereactionefficiencyfortheoxidationandoxygenreductionbenefitted
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 1/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
bytheenlargedsurfaceareatovolumeratio[1–3].Inaddition,thePtNSscanbeutilizedto
improvethehydrogenstoragecapacityinthecarbon,zeolitesandmetal-organicbasedspill-
overhydrogenstoragesbydissociatinghydrogenmoleculesontoreceptors[4,5].Recently,
duetothelocalizedsurfaceplasmonresonance(LSPR),metallicNSsarereportedinvarious
photochemicalandoptoelectronicapplicationsandtheirsize,densityandconfiguration
dependentlightinteractionscanbeutilizedtotailortheopticalproperties,e.g.spectralabsorp-
tionandscattering[10–12].UnlikethewidelystudiedAgandAuNSswithwell-definedLSPR
absorptionband,thePtNSsexhibitamorebroadenedlightabsorptionintheultraviolent-visi-
bleregions.Profitedbythis,theperformanceofPtNSscanpotentiallyexceedtheAgandAu
NSsinspecificcases.Forinstance,thePtcatalyzedaerobicoxidationofalcoholshasdemon-
stratedaquantumyieldof7.1%undervisiblelightirradiationduetothesignificantlypro-
motedphoto-catalysisactivity,whichisnearlytwicethecaseofAuNSphoto-catalysts[8,9].
Also,anover57%outputpowerincreaseisreportedinanLSPR-enhancedultravioletlight-
emittingdiodewiththeemploymentofPtNSs,sharplyincreasedascomparedwiththeAgNS
caseof20%[6].TheLSPRpropertiescanbeadequatelytunedbythecontrolofNSconfigura-
tion,sizeanddensity,therefore,avitalrequirementisraisedforthecontrolledPtNSfabrica-
tion[13–17].Asaneffectiveapproachofself-assemblythroughphysicalvapordeposition
(PVD),thesolidstatedewetting(SSD)canachieveapreciselymodulatedNSfabrication
onthesubstrateandthushasbeenreportedinthecontrollablefabricationofvariousmetal
NSs,e.g.Au,AgandRh[16,18–20].Meanwhile,sapphirewiththehighthermaldurability,
mechanicalstrengthandchemicalstabilityhaswidelyadaptedinelectronicandoptoelectronic
devices[21–25].ThecontrollablefabricationofvariousPtNSsonsapphirecanprovidethe
essentialreferenceonthepertinentmodulationofsurfacephysical,chemical,opticalandcata-
lyticpropertiesnecessaryforthecorrespondingapplicationsbasedupon.Therefore,inthis
work,westudytheself-assembledPtNSsevolutiononc-planesapphire(0001)basedonthe
controlofannealingtemperatureandduration,inwhichthecoherenteffectofsurfacediffu-
sion,surfaceenergyminimization,RayleighinstabilityandOstwaldripeningleadtothefor-
mationofvariousconfiguration,sizeanddensityofPtNSs.Basedonthemorphologicaland
elementalcharacterizationsbyAFM,SEMandEDS,theevolutionofvariousPtNSsaredem-
onstratedandthegrowthmodelsarediscussedwiththedistinctdepositionamount.Further-
more,theopticalpropertiesofvariousPtNSsareinvestigatedbythereflectancespectraand
theeffectonlatticevibrationisprobedbyRamanspectra.Thecorrespondingvariationof
intensityandpeakpositionsarediscussedalongwiththeNSmorphologyaswellasthesurface
coveragechanges.
Materialsandmethods
PreparationofsubstrateandPtnanostructurefabrication
ThePtNSswerefabricatedonc-planesapphire(0001)waferswithathicknessof430μmand
off-axis±0.1(iNexusInc.,SouthKorea).Initially,thewafersweredicedinto6×6mm2squares
usingamachinesaw.Inordertopreparethesubstrateforthedeposition,eachsamplewas
degassedat650˚Cfor900sunderavacuumbelow1×10−4Torr.Duringthedegassing,an
Inconelblankwasmountedonthebacksideofsampletoestablishastablethermalconduction
fromahalogenlamp.Afterthedegassing,thesapphiresurfaceshowedasmoothmorphology
withtheheightdistributionof±0.5nmasshowninS1Fig.Inaddition,theRamanspectraof
baresapphireareshowninS1Fig.Rightafterthedegassing,thePtlayersweredepositedina
sputterchamberwithagrowthrateof0.05nm/sandionizationcurrentof3mAunder1×10−1
Torratambienttemperature.ThetemperatureeffectonthePtNSfabricationwasinvestigated
withthreedistinctdepositionthickness,i.e.3,10and20nm,whichwereseparatelydeposited
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 2/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
bythecontrolofsputteringdurations.AfterthePtfilmdeposition,thesamplesshoweduniform
surfacemorphologiesandthesurfaceroughnesswasslightlyincreasedalongwiththePtthick-
nessasshowninS2Fig.Subsequently,thesamplesweretransferredtoapulsedlaserdeposition
(PLD)chamberforannealingatvarioustemperaturebetween500and950˚Cwithaconstant
rampingrateof4˚C/sunder1×10−4Torr.Afterreachingthetargettemperature,adwelling
durationof450swasequallyappliedtoeachfabricationtoguaranteeasufficientsurfacediffu-
sion.Theeffectofannealingdurationwasinvestigatedbetween0and3600sat800˚Cwiththe
Ptdepositionthicknessof15and20nm.Meanwhile,therampingratewasacceleratedto10˚C/
sinordertominimizetheripeningeffectduringthetemperature-riseprocess[26,27].After
eachgrowth,thesubstratetemperaturewasimmediatelyquencheddowntotheambientto
eliminatetheundesiredripening.
CharacterizationofPtnanostructures
AfterthefabricationofPtNSs,morphological,elementalandopticalcharacterizationswere
carriedout.Anatomicforcemicroscope(AFM)(ParkSystems,SouthKorea)non-contact
modewasutilizedtoobtainthethreedimensionalsurfacetopographyandascanningelectron
microscope(SEM)(CoXEM,SouthKorea)wasemployedforthelargerscale2Dimages.The
AFMtips(NSC16/AIBS,μmasch,USA)utilizedwere17–21μminlengthwithacurvature
radiuslessthan10nm.Thespringconstantandresonantfrequencyofcantileverswere~40
N/mand~300kHzrespectively.TheoriginaldatafromAFMwasprocessedbytheXEIsoft-
ware(ParkSystems)toobtainthetop-andside-views,cross-sectionalline-profiles,RMSsur-
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
faceroughness(R )andsurfacearearatio(SAR).TheR andSARaregivenby 1 n y2and
q q n i¼1 i
ðAGA(cid:0)GASÞ(cid:2)100½%(cid:138)where,theyiisheightateachpixel,ASissurfacearea(2D)andAGisgeomet-
ricarea(3D).Theelementalanalysiswasperformedbyanenergy-dispersiveX-rayspectro-
scope(EDS)system(ThermoFisherNoranSystem7,USA).ThereflectanceandRaman
spectrawereacquiredbytheUNIRAMIIsystem(UniNanoTechCo.Ltd.,SouthKorea).The
lightsourceforreflectancemeasurementwasthecombinationofdeuterium(250(cid:20)λ(cid:20)450
nm)andhalogen(450(cid:20)λ(cid:20)1100nm)lampsandtheRamansignalwasexcitedbya532nm
laseratambienttemperature.
Resultsanddiscussion
Fig1showsthemorphologicalevolutionofself-assembledPtnanostructures(NSs)onc-plane
sapphirebythevariationofannealingtemperature(AT)between550and950˚CwiththePt
thicknessof10nm.Thedetailedmorphologicalandopticalcharacterizationsarepresentedin
Figs2–4andthecorrespondingAFMside-viewsof3×3μm2areprovidedinS3Fig.Gener-
ally,throughthegraduallyenhancedsurfacediffusionbythethermalenergy,theformationof
PtnanoclustersfirstappearsontopofthePtlayersandthentheisolatedPtnanoparticles
(NPs)aregraduallyformedfromthecontinuousPtlayeralongwiththeincreasedAT.De-
pendingontheAT,thePtadatomsurfacediffusioncanmainlyoccurthroughtwotypical
paths:(i)onthePtsurfaceatlowerATand(ii)onthesapphiresurfaceathigherAT.Thesur-
facediffusionofadatomscanbedescribedbytherelation[23]:
(cid:18) (cid:19)
E
D ¼D exp (cid:0) ; ð1Þ
S 0 kT
wheretheD isthepre-exponentialfactor,kisBoltzmannconstant,Tisthesystemtempera-
0
tureandEistheactivationenergyofPtatoms[28].FromtheEq(1),itcanbeinfferedthatthe
D isdirecltydependentontheT.Thus,forasinglePtadatom,ithasahighpossibilitytobe
S
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 3/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
Fig1.Annealingtemperatureeffectontheevolutionofself-assembledPtnanoparticles(NPs)on
sapphire(0001).Eachsamplewasdepositedwith10nmthickPtlayerandannealedbetween550and950˚C
fortheidenticaldurationof450s.(a)—(i)Atomicforcemicroscope(AFM)top-viewsof3×3μm2.
https://doi.org/10.1371/journal.pone.0177048.g001
activatedanddiffuseonthesurfaceatincreasedATandtheamountofactivatedPtatomscan
besignificantlyincreasedathigherATfromamacropespective.AsdescribedinFig2(B),with
anincreasedthermalenergy,thetopmostadatomsonthePtlayercanbefirstactivatedand
diffuseonthedevelopingoverlayingPtlayer.Subsequently,thecollisionsamongthediffusing
atomscantakeplaceandresultinthenucleationoftinynanoclustersatincreasedAT.Asindi-
catedinFig2(C),asthediffusionlengthl canbeenhancedatanincreasedtemperature[29,
D
30],thediffusingPtadatomscanhavehighpossibilitytotravelafurtherdistanceandbe
adsorbedbythenanoclustersdrivenbythesurfaceenergyminimizationmechanism[31,32].
Finally,atasufficienthighAT,thePtadatomdiffusionandnucleationwoulddirectlyoccur
onthesapphiresubstrateasseeninFig2(D),resultingintheformationofPtislands(NPs)
accordingtotheVolmer–Webergrowthmodel[33,34].Withthestrongerbindingenergy
betweenPtadatoms(γ )ascomparedtothatbetweenPtandsubstrateatoms(γ),namelyγ
Pt I Pt
>γ,thePtadatomstendtobindwitheachotherandgrowin3-dimensionalislandsand
I
therefore,thecontinousPtlayercangradaullydevelopintotheindividualPtNSsalongwith
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 4/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
Fig2.PtNPsnucleationandgrowthatlowerannealingtemperaturerangebetween550and750˚C
withthe10nminitialfilmthicknessand450sannealingduration.(a)–(d)Schematicsofdiffusion
processdependingontemperature.(e)–(g)AFMside-viewsof1×1μm2andtop-viewsof200×200nm2in
(e-1)–(g-1).(e-2)–(g-2)Cross-sectionalline-profilesacquiredfromthelocationsindicatedbythebluelinesin
(e-1)–(g-1).
https://doi.org/10.1371/journal.pone.0177048.g002
theincreasedAT.AsshownbytheAFMside-andtop-viewsinFig2(E)and2(E-1),theforma-
tionoftinynanoclustersadoptingirregularconfigurationwasobservedwithhighdensitydue
tothelimitedsurfacediffusionandtheheightoftypicalnanoclusterswas~2nm.Then,when
theATwasincreasedto650˚C,thedimensionofNSswasobviouslyincreasedwiththe
enhancedsurfacediffusion,i.e.NSsheightreachedover15nm.Thenat750˚C,thePtlayer
wassignificantlydewettedbytheincreasedthermalenergy,resultingintheformationofiso-
latedNSsonsapphiresurfaceasseeninFig2(G).AsevidencedbythelineprofilesinFig2(F-
2)and2(G-2),thePtNSsformedat750˚Cshowedapreferentiallateralgrowthwiththeslight
decreaseinheight.Theconfigurationalvariationcanbecausedbythesurfaceenergydiffer-
encebetweenPtandsapphire.AsthePtNSsformeddirectlyonthesapphiresubstrate,the
dewettingangleθcanbecorrespondinglyvariedaccordingtotheYoung’sequationanddeter-
minedbythevariedinterfacialenergies[35].Between750and850˚C,thenanoclusterswere
graduallydevelopedintomoreregularshapethroughtheseparationinducedbytheRayleigh
instability[36,37].Atthesametime,withtheenhancedsurfacediffusion,thesurfacemor-
phologyevolvedintermsofheightanddimensionbetween850and950˚Candshowedmore
preferentialverticalgrowth.Asexhibitedbythediameter(measuredalongshorterdirections)
distributionhistograminFig3(D),themedianofdiameterwasfirstincreasedfrom850to
900˚C,whichcanbeattributedtothefurthershapedevelopment,andthenanoparticlecounts
oflargediameterswereclearlyincreasedat900˚C,i.e.above125nm.However,at950˚Cslight
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 5/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
Fig3.EvolutionofPtNPsonc-planesapphirebetween850and950˚Cwiththe10nmthickness.
(a)–(c)AFMside-viewsof1×1μm2.(a-1)–(c-1)Correspondingline-profiles.(d)Diameterand(e)height
distributionhistograms.Summaryplotsof(f)diameterandheight,(g)rootmeansquaredroughness(Rq)
and(h)surfacearearatio(SAR)alongwiththeannealingtemperature.Errorrange:±5%in(f).(i)Energy
dispersivex-rayspectroscope(EDS)elementalcharacterizationof550and950˚Csamplesinthe10nmset,
showingwellmatchedspectra.(Inset)Enlargedrangebetween1.4and2.6keVdepictsPtMα1peak.
https://doi.org/10.1371/journal.pone.0177048.g003
decreaseonthenanoparticlediameterwasobservedastheNSsadoptedmorepackedconfigu-
rationasseeninFig3(C),preferentialverticalgrowth.Meanwhile,intermsoftheheight,a
continuouslyincreasingtrendwasobservedasdisplayedinFig3(E)thatdepictstheheight
evolutionofNPs.Ontheotherhand,thenumberofshortNPsweresignificantlydecayed
alongwiththeincreasedtemperaturewhichcanbelikelyduetotheabsorptionbythelarge
NPsinordertolowerthesurfacefreeenergy.AsshownbythesummaryplotsinFig3(F),the
diameterwas99.9nmat850˚C,thenincreasedby1.16timesto116.3nmat900˚Candfinally
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 6/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
Fig4.ReflectanceandRamanspectraofthePtNPsonsapphire(0001)fabricatedbetween500and
950˚Cwiththe10nmfilmthickness.(a)–(b)UV-VIS-NIRreflectancespectraofcorrespondingsamples.(c)
Reflectancespectrumofbaresapphire.(d)Averagereflectanceplottedwithrespecttotheannealing
temperature.(e)CorrespondingRamanspectraoftheA peaks.Summaryplotsof(f)peakintensity,(g)
1g
peakpositionand(h)FWHM.
https://doi.org/10.1371/journal.pone.0177048.g004
decreasedby1.07timesto105nmat950˚C.Meanwhile,theheightwaskeptincreasingfrom
7.4to10.2nmby1.37timesbetween850and950˚C.Inaddition,theR andSARaresumma-
q
rizedinFig3(G)and3(H)andspecificvaluesareprovidedinS2Table.TheR andSARare
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q
P
givenby n1 ni¼1yi2andðAGA(cid:0)GASÞ(cid:2)100½%(cid:138)where,theyiisheightateachpixel,ASissurface
area(2D)andA isgeometricarea(3D).Between500and650˚C,theR andSARweregradu-
G q
allyincreasedduetothenucleationandgrowthofPtnanoclusters.Between650and750˚C,
boththeR andSARgraduallydecreasedduetothepreferentiallateralgrowthasmentioned
q
above.Finally,alongwiththeseparationandgrowthofisolatedNSs,theR andSARwere
q
graduallyincreasedagainbetween750and950˚C.Theenergy-dispersiveX-rayspectroscopy
(EDS)spectraof950and550˚CsamplesareshowninFig3(I)andthespectraofothersamples
areprovidedinS4Fig.Asseeninthespectra,KαpeaksforC,OandAlwereobservedat0.28,
0.53and1.49keVrespectively.Meanwhile,thePtMα1peakwasobservedat2.05keV.Asevi-
dencedbytheinset,similarcountswereobservedforallsamplesbetween950and550˚C,indi-
catingtheequalPtcontentswithoutsublimationatrelativelyhightemperature.
Fig4showsthereflectanceandRamanspectraofthePtNSsonsapphire(0001)fabricated
bythevariationofATwiththe10nmPtthickness.Thereflectancespectrawereanalyzedover
UV,visibleandNIRregionsandgenerally,thefabricationofPtNSsshowedanenhancement
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 7/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
onthereflectanceintensityascomparedtothebaresapphireasshowninFig4(A),4(B)and4
(C).Inthemeantime,thespectralshapeevolutionbytheformationofshouldersanddips
wereobservedalongwiththevariationofsurfacemorphology.Theshouldersanddipscanbe
relatedtothelocalizedsurfaceplasmonresonance(LSPR)inducedbythePtNSs.Thereso-
nancecanbegeneratedbyvariousmodes,e.g.dipoleandquandrupole[38,39].AsseeninFig
4(A)and4(B),asmallshoulderwasobservedintheUVregionwithacenterpositionat~380
nm,whichcanberelatedtothequandrupolarresonance.Meanwhile,thedipolarresonance
canbealsogenerated,however,asthelightabsorptionofPtisweakerandbroadascompared
withAgandAu[18,19,40],nocleardipolarpeaksweredirectlyobservedinthereflectance
evolution.AsseeninFig4(A)and4(B),thereflectancewasgraduallyincreasedbetween500
and700˚Cwhereasitwasgraduallyattenuatedabove750˚C.Also,cleardistinctionswere
observedbetweenUV-VISregionandnearIRregions.Inspecific,theintensityincrease
between350and800wasgraduallyenhanced,however,intheNIRregion,thereflectancewas
graduallydecreasedabove750˚C.Toobservethespectralevolutionclearly,theaveragereflec-
tancetrendinUV-VISandNIRregionswereseparatelyinvestigatedasshowninFig4(D).As
evidencedbyFig4(D),thereflectanceinUV-VISandNIRregionwasconsistentlyincreased
upto750˚CwiththeformationofoverlayingNPsanddenseisolatedNPs.Between750and
850˚C,thereflectanceintensityintheUV-VISregionwerealmostsimilar(~19%),whichcan
beinducedbytheenhancementfromPtnanostructureLSPRintheVISregionandat900˚C,
thereflectancebetween400and600nmwasfurtherenhanced,showingapeakthatmatches
thePtdipolarpositionrecordedbetween500and550nm.Lastly,thedipolarpeakwasstill
observedat950˚CasseeninFig4(B).Meanwhile,duetotheslightlyreducedaveragediameter
aswellasthenanostructurecoverage,theoverallreflectanceintensitywasdecreased.Fig4(E)
displaystheRamanspectralevolutionofA vibrationalmodepeaksofthesamples[41]and
1g
thecorrespondingintensity,peakpositionandfullwidthathalfmaximum(FWHM)aresum-
marizedinFig4(F),4(G)and4(H)accordingly.ThespecificvaluesofRamanpeakintensity,
positionandFWHMaresummarizedinS4Table.TheRamanspectralvariationwasalso
observedwiththemorphologicalevolutionofPtNSs.AsseeninFig4(F),thedecreaseofpeak
intensitywaswitnessedbetween600and750˚CwiththeformationofisolatedPtNSs.Asdis-
cussedabove,owingtothesurfaceplasmoneffect,thelightabsorptioncanbeenhancedwhich
canresultintheattenuationofpeakintensity.Consequently,theintensitywasweakened
between750and900˚C,whichmatcheswiththeUV-visreflectanceshowninFig4(D).At
950˚C,asthereflectancewasdecreased,theRamanpeakintensitywasrecoveredasseeninFig
4(E)and4(F),whichcanbealsoinducedbytheslightlydecreasednanostructurecoverage.
Withrespecttothepeakposition,theA peaksappearedat~417.6cm-1withtheminorshift
1g
within0.1cm-1,suggestingthesapphirelatticestraininducedbythePtNSsfabricationstayed
inalowlevel[42,43].Inaddition,theFWHMsweredistributedstablybetween8and8.5cm-1.
Fig5showstheevolutionofself-assembledPtNSswiththeincreasedPtthicknessof20nm
bytheATvariationbetween500and950˚C.ThecorrespondingAFMtop-viewsof3×3μm2
andSEMimagesareprovidedinS5andS6FigsandthedetailedanalyseswiththeAFMtop-,
side-viewsandcross-sectionallineprofilesarepresentedinS7Fig.Ascomparedtotheprevi-
ousset,asimilarevolutiontrendwasobservedatarelativelylowertemperaturerangebetween
550and800˚C,however,atincreasedtemperaturebetween850and950˚CtheNSsshoweda
distinctevolutionwiththelateralgrowthandcoalescence.Forexample,thenucleationoftiny
voidsandgrowthofoverlayingPtnanoclusterswereobservedbetween550and650˚Cas
clearlyshowninFig5(A),5(B)and5(C)andtheheightofatypicalnanoclusterwas~7nm.
Then,between700and800˚C,alongwiththeenhancedsurfacediffusionandvoidgrowth,the
PtlayersgraduallyevolvedintowigglyNSsat800˚CasseeninFig5(F).Finally,between850
and950˚C,thewigglyNSswereevolvedduetothesignificantvoidcoalescencealongwiththe
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 8/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
Fig5.AnnealingtemperatureeffectonthesurfacemorphologyevolutionofPtnanostructureswith
20nm-thickPtdepositionannealedforconstant450satdifferenttemperatureaslabelled.(a)–(i)AFM
side-viewsof3×3μm2.(j)–(l)EnlargedAFMtop-viewsalongwiththecorrespondingline-profiles.
https://doi.org/10.1371/journal.pone.0177048.g005
agglomerationofPtatomsinordertoreducethesurfaceandinterfaceenergy[44].Asevi-
dencedbytheline-profilesshowninFig5(J)and5(L),between750and950˚C,thelateral
dimensionofNSswasclearlyexpandedwhiletheNSheightwasslightlyincreasedfrom~15to
22nm.Noticeably,aspresentedinFig5(L),theNSswereeventuallyformedwithaflattop,
whichmightbe(111)planeduetothecrystalstructureofPtandsapphire.Aswellknown,the
single-crystalsapphirespossesstherhombohedralstructureandonthec-plane,eachAl(O)
atomcanbesurroundedbysixO(Al)atoms,formingahexagonalarrangement[15].Mean-
while,asthecrystalstructureofPtisfacecenteredcube(fcc),thegrowthofPtNSscanfollow
[111]direction,alongwhichthearrangementofPtatomscanmatchthehexagonalarrange-
mentofsapphiresubstrate.Inaddition,asthe(111)planecontainsthelowestsurfaceenergy
amongvariouscrystalorientations[45]andthustheflattopcouldbepreferentiallygrown
(111)plane.Inaddition,theR andSARarepresentedinFig6(F)and6(G)andthecorre-
q
spondingvaluesaresummarizedinS2Table.TheR andSARweregraduallyincreasedalong
q
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 9/18
Morphologicalandopticalevolutionofself-assembledPtnanostructures
Fig6. (a)ReflectancespectraofthePtnanostructuresonsapphirefabricatedwiththevariationofannealing
temperatureswiththe20nmPtdeposition.(b)Reflectancespectrumofbaresapphire.(c)Averagereflectance
plottedasafunctionofannealingtemperature.(d)CorrespondingRamanspectraoftheA peak.(e)Summary
1g
plotofA peakintensities.Summaryplotsof(f)Rqand(g)SAR.(h)EDSelementalcharacterizationofthe
1g
samplesat950and500˚C.(Inset)Theenlargedenergyrangebetween1.4and2.6keVshowingPtpeaks.
https://doi.org/10.1371/journal.pone.0177048.g006
withtheATfrom500to950˚CsuggestingtheevolutionofenlargedPtNSs.Fig6(H)displays
theEDSspectraof950and500˚CsamplesandenlargedPtMα1peaksarepresentedinthe
inset.TheintensityofPtMα1peakswassimilarbetween500to950˚C,whichconfirmsthe
equalamountofPtineachsampledespiteoftheindifferentmorphologywithoutanysublima-
tionlossathightemperature.Furthermore,theNSsfabricatedwith3nmPtdepositionare
presentedinS9–S13Figs,likewise,representedwiththeAFMtop-,side-views,Rq,SARand
EDSspectra.Theevolutionof3nmsetwasdistinctfrom10and20nmsetsinthattheforma-
tionofround-domeshapedPtNPswereobservedthroughoutthewholeATrangebetween
500and950˚C.Duetosufficientdewettingoflowthicknessfilm,themetalNSstendtoreduce
PLOSONE|https://doi.org/10.1371/journal.pone.0177048 May4,2017 10/18