crystalnano5

1DirectlyProbingLightAbsorptionEnhancementofSingleHierarchicalStructureswithEngineeredSurfaceRoughnessJingweiWang1,RunShi1,WeijunWang1,NianduoCai1,PengchengChen1,DejunKong1,AbbasAmini2&ChunCheng1Hierarchicalnanostructuresareidealarchitecturestoharvestsolarenergy.
Theunderstandingoflightabsorptioninsinglehierarchicalstructuresisemergentlyimportantandgreatlyhelpfulinenhancingmultiscaleopticalphenomenaandlightmanagement.
However,duetothegeometricalcomplexityofhierarchicalarchitectures,theoreticalandexperimentalstudiesoflightabsorptionhavefacedsignificantchallenges.
Here,wedirectlyquantifylightabsorptioninsinglehierarchicalstructuresforthefirsttimebyutilizingVO2-basednearfieldpowermeter.
Itisfoundthatlighttrappingissignificantlyenhancedinroughmicrowireswhentheroughnessamplitudeiscomparabletotheincidentlightwavelength.
TheroughnessenhancedlightabsorptionisverifiedasageneralphenomenononbothVO2andSihierarchicalstructures.
Therefore,ourworknotonlyprovidesasimpleandquantitativemethodofmeasuringlightabsorptionuponsinglegeometricallycomplexstructuresinmicro/nanoscale,butalsocontributesageneralruletorationallydesignofhierarchicalstructuresforenhancedperformanceinphotoelectricandphotochemicalapplications.
Conversionofsunlighttoelectricalandchemicalenergyisapromisingandprovenstrategyforlarge-scalepro-ductionofenergyfromarenewablesource.
Toeffectivelyharnesssolarenergy,aphotovoltaiccellorphoto-electrochemicalelectrodemustabsorbmostofthesolarspectrumandcollectthephoton-generatedcarrierswithminimallossestorecombination.
Forplanardevices,thistaskcanbedifficultbecausetherequiredthick-nessofmaterialforadequateabsorptionoflightisoftengreaterthanthedistanceoverwhichphoton-generatedchargescanbeefficientlycollected.
Semiconductormicro/nanostructuresoffernewapproachestomeettheserequirementsoflightabsorptionandchargecollection1–3.
Specifically,hierarchicalarchitecturesthatarecon-structedbymultiscalemicro/nanostructureshaveattractedintensiveattentionssincetheyareidealcandidatesforhigh-performancesolarenergyharvestingdevices4–8.
Thesestructurespossessspecificconfigurationsofmicros-calebackbonesandone-dimensional(1D)nanostructurearrayssurroundedthebackbones.
Thedirectpathwayalong1Dcrystallinenanostructuresdiminishesthepossibilityofchargerecombinationandoffersarelativelylargeaspectratioforrapidelectron-holeseparationandchargetransport,aswellaselectrochemicalreactions9–11.
Mostimportantly,high-densitytreelikebranchednanowirearraysofhierarchicalstructuresprovidelongopticalpathsforefficientlighttrappingandthuspromotetheutilizationoflightsignificantly12–16.
Plentyofeffortshavebeendevotedtothesynthesisandapplicationsofhierarchicalstructures,whilehowtheirstructuralparameters,suchasthesizeofmultiscalenanostructures,affectlightabsorptionisyettobesystematicallystudied.
Thus,quantificationoflightabsorptionuponsinglehierarchicalstructureshastobesettledurgently.
Lightabsorptionandpropagationatthesub-micrometerscalearethekeypointsformanycriticaltechnolo-giesandapplications.
Lightabsorptionof1Dnanostructurearraysisawellunderstoodphenomenonexperimen-tally,analytically(Maxwellequation),andnumerically(finiteelementmethod).
However,thelightabsorptionofsinglesub-micronsolidsisstillhardtobequantitativelydeterminedbyexperiments16–18.
Thisislargelyduetodifficultiesinaccurateanddirectcharacterizationofenergyflowandtemperaturedistributionatthisscale.
1DepartmentofMaterialsScienceandEngineering,SouthernUniversityofScienceandTechnology,Shenzhen,518055,China.
2CenterforInfrastructureEngineering,WesternSydneyUniversity,Kingswood,NewSouthWales,2751,Australia.
JingweiWangandRunShicontributedequallytothiswork.
CorrespondenceandrequestsformaterialsshouldbeaddressedtoC.
C.
(email:chengc@sustc.
edu.
cn)Received:29March2018Accepted:12July2018Published:xxxxxxxxOPEN2Incaseofgeometricallycomplicatednanostructures,i.
e.
,hierarchicalstructures,simulationalorexperimentalevaluationsoflightabsorptionbecomeevenmoredifficult.
Recently,wehavedevelopedopticallyreadablenear-fieldpowermeters(NFP)basedonthemetal–insulatortransition(MIT)insinglecrystalvanadiumdioxide(VO2)micro/nanobeamswhichenablesdirectquantificationoflightabsorptionofanysinglemicro/nanowires19,20.
Here,wepresentthequantitativestudyoflightabsorptionofsinglehierarchicalstructuresbyapplyingthisuniqueNFPtechnique.
ThesurfaceofVO2andSimicrowireswereengineeredbyatop-downapproachwithfocusionbeam(FIB)techniquetoformhierarchicalstructureswithcontrollabletrenchsizes.
LongVO2wiresweresuspendedfromasubstrateasNFPs.
ThehierarchicalwiresbondedtotheNFPswerelocallyheatedusingafocusedlaserwhichstimulatesthephasetransition.
Theresultantdomainstructurescanbeopticallyimaged.
Byusingtheheattransporttheory,wedeterminedthelightabsorp-tionasafunctionoftrenchdepthandspacing.
Itwasfoundthatlighttrappingwassignificantlyenhancedforhierarchicalstructures(roughmicro/nanowires)whentheamplitudeofsurfaceroughnesswascomparabletothelightwavelength.
Fromtheresults,anenhancedlightabsorptionwasobtainedfromthesinglehierarchicalstruc-turespreparedbysurfaceroughnessengineering.
ItisfoundthattheenhancedlightabsorptioncouldbeobtainedbysurfaceroughnessengineeringonbothVO2andSimaterialsystemswhichthusprovidesauniversalstrategyformodernhierarchicalstructuredesign.
MethodsFortheopticalabsorbancemeasurementsofsingleVO2andSimicrowireswithpatternedtrenches,experimentswerecarriedoutinachamberwithaquartzwindow(SeetheSupplementaryFilesforthedetailsofsampleanddevicepreparation)19–21.
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EffectofsurfaceroughnessontheopticalabsorptionofasingleSimicrowire.
(a)AsurfaceroughnessmodifiedSimicrowirebondedtoaVO2NFP.
TheleftpanelshowstheopticalimagesoftheSi-VO2systemwithlaseroffandon.
AM/IdomainwallistriggeredwhenthelaserisonandheatunidirectionallytransfersfromSimicrowiretotheVO2NFP.
(b)TheenlargedSEMimageandopticalimageofthepatternedsectionsoftheSimicrowire.
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AcknowledgementsThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(GrantNo.
51776094),theGuangdong-HongKongjointinnovationproject(GrantNo.
2016A050503012),theGuangdongNaturalScienceFundsforDistinguishedYoungScholars(GrantNo.
2015A030306044),andtheTrainingProgramforOutstandingYoungTeachersatHigherEducationInstitutionsofGuangdongProvince(GrantYQ2015151),Climbingproject(pdjh2017c0030),StudentInnovationTrainingProgram(2017X40).
ThestartinggrantsfromSouthernUniversityofScienceandTechnologyareacknowledged.
TheauthorsalsoappreciatethevaluablesuggestionsfromProf.
JunqiaoWufromUniversityofCalifornia,Berkeley.
AuthorContributionsJ.
W.
andR.
S.
carriedouttheexperimentanddesignedthestudy.
C.
C.
draftedthearticle.
W.
W.
,N.
C.
,P.
C.
,D.
K.
,A.
A.
performedcriticalrevision.
Allauthorsreadandapprovedthefnalmanuscript.
AdditionalInformationSupplementaryinformationaccompaniesthispaperathttps://doi.
org/10.
1038/s41598-018-29652-8.
CompetingInterests:Theauthorsdeclarenocompetinginterests.
Publisher'snote:SpringerNatureremainsneutralwithregardtojurisdictionalclaimsinpublishedmapsandinstitutionalaffiliations.
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