4.1. Surfacemodificationandtreatmentofsubstrate
Interaction of lasers withplamonic NPs allows nanofabrica-tion on various substrates on which the particles are placed or assembled. Ko and co-workers reported the fabrication of nanometer-sizedcratersonapolyimidefilmself-assembledwith AuNPswhenexposedtoa532-nmnanosecond-laserlightthrough an objective lens (3–5ns pulse width, 30–300mJcm−2 power density) [156]. Tsuboi and co-workers observed the nanohole (d<100nm)formationonaAuNP–polymerhybridfilmdeposited onglasssubstratesandirradiatedbyasingle532-nm(∼8ns)laser shot[157].Thesetwostudiesattributedtheoriginofnanoholesto aheat-modefabrication,causedbytheheattransferfromtheNPs.
Theheatmodeofnanofabricationrepresentsthematerial modifi-cationowingtothedamagecausedbymeltingandtheexplosive evaporationofAuatomsandclusters.Thenanoholeswere gen-eratedthroughtherapidoverheatingofstronglyabsorbingNPs
Fig.24.(a)SEMimageofaborosilicateglasssurfaceassembledwith47±8nmAuNPswithasurfacecoverageof30%andexposedtoasingle-shotofa532-nmpulsed-laser light(powerdensity:360mJcm−2pulse−1).TheleftsideoftheimageshowstheglasssurfacefullycoveredwithsmallNPsresultingfromthelasersplittingoftheoriginal particles.Therightsideoftheimagedisplaysabaresurfacefromwhichtheparticleswereremovedafterlaserirradiation.Heremanydarkspotswithaslightlybrightouter ringareformedonthebaresurface.Forcomparison,theinsetdepictstheSEMimageoftheglasssurfacebeforetheirradiation.(b)AFMtopographicalimagesofsample surfacessubjectedtoasingle-shotlaserirradiationatalaserpowerdensityof470mJcm−2.Darkspotsrepresentpittedareascorrespondingtothecraters.Theaveragecrater depthis4–6nm.TheAFMimageswereacquiredforthesurfacesexposedtothelaserirradiationandintheabsenceofPt–Pdcoating.ThefragmentsofAuparticleswere removedbeforethemeasurement.(Reproducedfromref.[116].ReproducedwithpermissionfromtheAmericanChemicalSociety.)
duringa short laser-pulseduration when theinfluence of heat diffusionisminimal.
Hashimoto et al. [116,158] observed the formation of nanocraters on a borosilicate glass surface assembled with 40-nm-diameter Au NPs when irradiated with a single-shot nanosecond-pulsed-laserwithawavelengthof532nm(Fig.24).
Laserpowerdensitiesof twoordersof magnitudesmallerthan thoseofcausingthebreakdownoftheglasswereappliedforthe fabricationofthecraters.
Theaveragediameterofthecraterswas∼20nmwithadepth oflessthan10nmforthesingle-shotexperiment:however,after repeatedirradiations,thesizeofthecratersexpandedtoa100nm scale[158].Thecraterformationwassimultaneouswiththe laser-inducedsplittingofAunanoparticlestogeneratesmallerparticles of15nmdiameter.Thenumber densityofthecratersincreased sharply,dependingonthelaserpowerdensity.Theonsetofthe increase occurred at ∼160–170mJcm−2pulse−1, and reached a valueof twotimes thenumber density oforiginal Auparticles (150±10 particles·m−2)witha weaktendency tolevel off at highpowerdensities (Fig.25a). Theonset of thecrater forma-tioncoincideswiththesplittingofAuNPsbecauseofincreasein thetemperatureabovetheboilingpoint(∼3100K)ofgold result-ingfromtheabsorptionofthelaserenergy.Thus,theexplosive
evaporationofgoldnanoparticlesispostulatedtoplay acrucial roleforthemodificationobservedhere.Thisassumptiongained asupportfromtheestimationoflaserpowerdensitydependent thermoacousticpressuresduetothesuddenevaporationofgold (Fig.25b).
This finding may represent a new application of the laser-ablation/fragmentation of nanoparticles to material processing basedonthephotothermalprocess.Veryrecently,Tsuboiand co-workers have demonstrated that this laser-Au NP technique is applicabletotheformationofporesontheshellwallof spheri-calhollowsilicamicroparticlesembeddedwithAuNPs[159].They preparedporesoftunablesizesfrom17to56nmbyvaryingthe sizeoforiginalAuNPsfrom6to39nm.
Inthemeantime,Obaraandco-workersobservedthe forma-tionofnanoholesonthesurfaceofasiliconsubstrateplacedwith 200-nm-diameterAuspheresbyirradiatingasingle femtosecond-laserpulse(800nm)withanintensityof lessthan theablation thresholdofsilicon[160–162].Leidererandcoworkersobserved theablationpatternforregulararraysofgoldtriangleswithaside lengthof450nmandathicknessof25nmwhenthesestructures wereilluminatedwitha150-fslaserpulse(800nm,10mJpulse−1) [163,164]. Holes as small as 5nm were fabricatedat two cor-nersofeachtriangleonasiliconsubstrate.Heltzeletal.observed
0 100 200 300 400 500
fluence / mJ cm
-2pulse
-10 150 300
a
450b
number density / m
-20 100 200 300 400 500
fluence / mJ cm
2pulse
-10 20 40 60
thermoacoustic st
ress / GPa
Fig.25.(a)Numberdensityofcratersasafunctionofthelaserpowerdensityforsingle-shotlaserirradiation.ThenumberdensityoforiginalAuNPswas(150±10)particles
m−2.(b)Thermoacousticpressureestimatedbythevaporpressureofgoldgivenbyp0=
(2m)3/2/(kBT )1/2
03·exp
−L0/kBT
asafunctionofthelaserpowerdensity of45-nm-diameterAuNPsplacedonaglasssubstrate(effectiverefractiveindex:1.12).Intheequation;m,0;L0;andkBrepresenttheatomicmassofgold;latticevibration frequency;sublimationenergy;andBoltzmannconstant,respectively.(Reproducedfromref.[116].ReproducedwithpermissionfromtheAmericanChemicalSociety.)
Fig.26.Nanoscale-ablationsiteanddepthprofile:SEMimagesofthenanorodsbeforeandafterlaserirradiationatalocaleffectivepowerdensityof(a)54mJcm−2,rightat theablationthreshold,and(b)218mJcm−2.Thescalebarscorrespondto75nm,andtheyellowarrowsindicatetheincidentpolarization.(c)Thedepthprofileoftheablation siteshownin(b),alongthelongaxisshownwiththedottedlineasobtainedusingAFM.Notethedifferentscalesforthehorizontalandverticalaxesin(c).(Reproducedfrom ref.[76].ReproducedwithpermissionfromtheOpticalSocietyofAmerica.)
nanoholeswithadiameterslightlysmallerorroughlythesameas thatofthespheresonasiliconsurfacewith250-and40-nmAu nanospheresexposedtoasinglepulseofa532-nmnanosecond laser[165].Theseobservationswereattributedtotheplasmonic enhancementoftheincidentelectricfieldattheboundaryoftheNP andthesubstrate.Thisdeductionwassupportedbytheobservation thatthenanoholeprofilesresembledthelaserintensity distribu-tiononthesubstratesurfaceinthepowerdensityregionlower thantheablationthresholdpowerdensityofthebulksilicon.The assumptionoftheplasmonicenhancementwasalsosupportedby thefactthattheholesproducedbyalinearlypolarizedwaveoflaser radiationexhibitedanelongatedentranceshapeinthedirectionof polarization.Thus,besidestheheatmodeofenhancedmaterial fab-rication,aphoton-modeprocessispostulatedtooccur,inwhichthe electricfieldenhancementduetotheexcitationoftheLSPRband ofNPscancauseamultiphotonabsorption,leadingtotheablation andmodificationofthematerialsurface[160–167].
Despitedemonstrationsbyseveralgroups,thedirectcauseof plasmon-assistednear-fieldablationisstillnotunderstoodvery well.Ben-Yaker’sgroupconductedfurtherresearchtogainabetter understandingofthephenomena[76].Theyobserved nanoscale-lasermodificationandablationofasilicon(100)surfaceatlocal powerdensitiessignificantlylowerthanthedirect femtosecond-laser-ablationregionofsilicon,i.e.,420mJcm−2.Fig.26showsSEM imagesofablationsitesandadepthprofile,asobservedbyanAFM (atomicforcemicroscopy).
Ben-Yaker’sgroupfoundthatthenanorodablationsiteswere a photo imprint of the nanorod, very similar in size to the nanorod.Theablationsitesdidnotconsistoftwoseparatedcraters, whichisexpectedfromacalculated|E|2enhancementpattern.On thebasis of this observation, theyconcludedthat thePoynting vector enhancement more accurately predicts plasmonic-laser-ablationthanthe|E|2enhancementpattern[76].However,based ontheobservationofadouble-cratershapedhole,Meunier’sgroup stronglyopposedthisview[168].Thisdisputeshouldberesolved byfurtherexperimentalandtheoreticalstudies.
Uenoet al.[28,29,169–171]have demonstrated anew tech-nique of nanogap-assisted surface plasmon nanolithography.
Theyprepared arrays consistingof pairs of closelyspaced gold nanoblocksondielectricsubstratesincludingglass.Theyobserved thetwo-photonabsorption(TPA)-assistedphotopolymerizationof a negative-typephotoresist,SU-8in theintensenear-field sup-posedtoexistinthenanogapbetweentheblocksgeneratedbythe irradiationofahalogenlamp(wavelengthrange:600–1000nm, 0.2Wcm−2), with a CW laser (785nm) or femtosecond laser (800nm)[28].Thephotoresisit,SU-8isknownto photopolymer-izebyexposuretoultravioletlight atwavelengthsshorterthan 360–400nm,andthus,thesimultaneousabsorptionoftwo pho-tonsisprerequisitetoinitiatepolymerization.Fig.27showsthe
SEMimagesofpairsofAunanoblocks,(a)withregionsof polymer-izedSU-8((b)and(c))resultingfromexposuretoafemtosecond Ti:sapphairelaser(120fs,800nm,80MHz).
Whenthelaserbeamwaspolarizedlinearlyalongtheaxisof thepairs(Fig.27b),longitudinalplasmonmodeslocalizedinthe nanogapsinduced significantlocal polymerizationaftera short exposure. Whenthelaser beamwaspolarizedperpendicular to the axis of the pairs, transverse plasmon modes were excited
Fig.27.(a)SEMimageofapairofgoldnanoblocksmeasuring100× 100×40nm3 andseparatedbya5.6-nmwidenanogapbeforeirradiationby anattenuated femtosecond-laserbeam.(b)SEMimageofothernanoblockpairsafter0.01s expo-suretothelaserbeampolarizedlinearlyalongthelongaxisofthepair.(c)SEMimage ofanotherpairafter100sexposuretothelaserbeampolarizedintheperpendicular direction(dande).Theoreticallycalculatednear-fieldpatternsatselectedplanes fortheexcitationconditionsofthesamplesshownin(b)and(c),respectively.In(d), thefieldpatternisshownonthex–yplanebisectingthenanoblocksathalfoftheir height(i.e.,20nmabovethesubstrate),andin(e),thefieldiscalculatedontheplane coincidentwiththelinec–cshownin(d).Thefieldintensityisnormalizedtothat oftheincidentwave,andthereforerepresentstheintensity-enhancementfactor.
(Reproducedfrom[28].ReproducedwithpermissionfromtheAmericanChemical Society.)
and inducedphotopolymerization in thecorresponding regions (Fig.27c).Thecalculatedfield-intensitypatternsshowninFig.27d andecorrespondcloselywiththeexperimental photopolymer-ization pattern. Furthermore, for Aunanoblock structureswith nanogapsnarrowerthan10nm,photopolymerizationrateswere increasedbyordersofmagnitude[169].Thesamegroup[170,171]
alsoused the nanoblock structures for photomasks for contact exposureofapositivephotoresistonglasssubstrates.Asexpected, periodicpatternsofpitswereformedatthenanogapsinthe devel-oped photoresistsurface afterexposureto a femtosecond-laser beam.Itwasshownthattheorderedclustersofnanoparticles sep-aratedbynanogapscanprovideastableandversatileplatformfor furtherdevelopmentofopticalsubwavelengthnanolithography.
LSPR-assistedlasernanofabricationandnanogap-assisted sur-faceplasmonnanolithographyarepromisingtechniquestobeat thediffraction-limitedresolution(200–300nm).
4.2. Laser-induceddepositionandtreatment
Niidomeandco-workers[137,138,172]reportedthatAuNPs canbedepositedonaglasssurfacebyusingnanosecond-pulsed 532-nmlaserirradiation(83mJcm−2pulse−1)of dodecanethiol-passivated Au NPs (3.2±0.95nm) in cyclohexane. When laser irradiationwascarriedoutusingaphotomask,reddishrectangular patternsbasedonthedepositedAuNPscanbeclearlyobservedby thenakedeye.ThedepositionofAuNPsoccursonlyatthe laser-irradiatedregionofthesubstrate.Theobservationisreminiscent ofthestudyconductedbyItoetal.[118,119]forasingleAuNP(see Section3.2.5).
Adensemonolayerofisolated Ag-and AuNPs40–60nm in averagediameterwerepreparedonmicabyconverting sputter-depositedAg-andAu-islandfilmsbynanosecond-laserirradiation at532nm(∼50mJcm−2)[173].Fig.28shows suchanexample.
Besidestheshapechange,particlegrowthwasobserved.The resul-tantfilmgavestrongLSPRbandswithoutanyseriousbroadening.
Arelativelynarrowrangeoflaserpowerdensitywasemployedfor thepreparation.Thispowerdensityrangecorrespondstothe par-ticletemperaturesbetweenthemeltingandboilingpointsofAu NPs,suggestingthatparticlemeltingtriggersthetransformation observed.
In a higher laser power density regime greater or above 100mJcm−2,acomplicatedmodeoffilmconversionresultingin thestrongdeformation(flattening)oftheLSPRbandbecame dom-inantbeforetheablationmodefinallysetin.The fabricationof sphere-likeAuNPsofsimilarshapesandalignmentsonsapphire, GaN,SiO2,andsiliconsubstratesbyirradiationofafewUVlaser pulsesonaAuthinfilmwasreportedbyothergroups[174–176].
Thenanostructuralchangetoaperiodicallyarrangedline pat-ternwithamodifiedparticlesizeandshapedistributionoccurred whenthinfilmscontainingAuNPswereirradiatedwith femtosec-ondlinearlypolarizedlaserpulses(150fs)[177].Fig.29showsan example.
Alineardependencebetweentheperiodoftheline struc-tures and the laser wavelength (800, 528, 400, and 266nm) wasobserved.Fortheparticlesizeandshapechanges,theatomic diffusionprocess(Ostwaldripening,coalescence/reshaping)was assumed.Photothermalmelting/evaporationcouldbethedriving forceoftheevent.Theperiodicstructureisreminiscentofthe “rip-ple”orLIPSS(laser-induced periodicsurfacestructure)thathas beenformedonnearlyeverykindofsolidmaterial(metals, semi-conductors,glasses,andpolymers)[178–180].Itwasassumedthat theconstructiveinterferencebetweenincidentwavesandsurface wavesgeneratedbythescatteringoftheincidentwavecausedthe observedsurfacestructuralchange[178].Theperiodicallyarranged linepattern couldbeanexample oftheselforganizationofAu NPsresultingfromthelaser-inducedphotothermaleffect.Hereit
ispertinenttonotethat Obaraandco-workers[181]presented computationaland experimental resultsonplasmonic and Mie scatteringcontroloffar-fieldinterferenceforregularripple forma-tiononsemiconductorandmetal.Theyobservedtheinterference ripplepatternonthesiliconsubstrate,whichoriginatesfromthe plasmonicfarfieldbygoldnanospheresirradiatedby femtosecond-laserpulses. Theirresultsuggeststhatinadditiontoplasmonic scattering,theMiescatteringfarfieldalsoplayarolefortheorigin ofsurfacerippleformation.
Nanosecond-andfemtosecond-laser-induceddetachmentwas observedfor Aunanostructuresprepared ona silicon orquartz substrate by nanosphere lithography [182–184]. Laser-induced melting was considered to be responsible for the nanosecond excitation because of the contraction of the liquid toward a sphere [182]. The detachment was observed for femtosecond-laser excitationin both air andliquid. The ejectionmechanism in air was consideredto involve theablation of surface atoms from a gold particle, which generated intense pressure at the particle–substrate interface[183]. In contrast, in a liquid envi-ronment, a mechanism was postulated that involves energy transfer from a photoexcited nanoprism to the solvent within cavities anddefectsat theparticle–substrateinterface[184].In this instance, the hot-solvent molecules result in an intense pressureattheparticle–substrateinterface,resultingin particle ejection.
4.3. Miscellaneousapplications
Theoptical-limiting effectthat strongly decreasesthe trans-missionoflight at higherlaser intensitieswasobserved forAg nanoclusters and AuNPs dispersed in solution [185–190]. The optical-limiting behavior is applicable to devices that protect humaneyesandsolid-statesensorsfromintenseopticalbeams.
Fig.30shows anexampleofa dendrimernanocompositeofAg clusters,{Ag(0)}E.
Thenonlineartransmissionwasobservedat532nm,withinthe LSPRresonancespectral range.Thethresholdpowerdensityfor opticallimitingwas2.0Jcm−2.Themechanismofopticallimiting wasascribedtotheformationofnanobubbles.Theoptical-limiting performanceof{Ag(0)}Eiscomparedwellwiththeresultsobtained withcarbonnanotubes.Morerecentnanosecondstudiesexpressed rathercontroversialopinions[188–190].WhilePolavarapu[188]
concluded that nonlinear scattering plays an important role in theoptical-limitingbehaviorofoleylamine-cappedAuNPs,Sun’s group [189,190] postulated the dominant contributionof free-carrierabsorptionforAuNPaggregates.
Becauseofchemicalandthermalstabilitieswithless cytotoxi-city,plasmonicNPshavebeenpopularinbiomedicalapplications.
In particular, the sharp molecular-scale confinement of high-temperature regions is very beneficial as a nanoheater. For example, Takedaet al. [191,192]demonstrated that the degra-dation of proteins and nucleic acids can result from 532-nm nanosecond-laserexcitation(10ns,94mJcm−2pulse−1)ofAuNPs anaqueoussolution.Forinstance,theymixedAuNPs(diameter:
∼20nm)withtwokindsofproteinmolecules:bovineserum albu-min(BSA)andlysozyme.ProteinswereselectivelyadsorbedonAu NPsbyadjustingthepHofthesolution.Theyobservedselective degradationbecausetheAuNPcreatesahigh-temperatureregion initsclosevicinitybythelaserirradiation.Atthelaserpower den-sitytheyemployed,thefragmentation/sizereductionofNPsisalso likely.Ontheotherhand,theyassumedthattheformationofthe laser-induced bubbleis unlikely. Beforethesestudies, asimilar studyhadbeenconductedbyHuttmannetal.[193].Under irradia-tionwithnano-andpicosecond-laserpulses,theenzymesalkaline phosphataseand chymotrypsinboundtothesurfaceof 15-nm-diameterAuNPswereinactivated.Inthiscase,clearevidenceof
Fig.28. (a)AFMimage(dimensions:1000× 1000nm2)ofanas-depositedAu-islandfilmconsistingofflat-shapedparticlesassimulatedbelowtheimage.(b)1000×1000nm2 AFMimagetakenafterlaserirradiation(single-shotat∼50mJcm−2),clearlyprovidingevidenceforconversiontogoodsphericalnanoparticles,withatypicalheightprofile andsizedistributionshownbelowtheimageandontheright,respectively.(Reproducedfrom[173].ReproducedwithpermissionfromtheAmericanChemicalSociety.)
Fig.29.(a)TEMimages(upper:cross-sectionalview,lower:overheadview)ofapolymerfilmcontaininggoldnanoparticlesafterlaserirradiation(=400nm)and(b)TEM imagesofanonirradiatedpolymerfilmcontaininggoldnanoparticles.(Reprintedwithpermissionfromref.[177].Copyright2005,AmericanInstituteofPhysics.)
Fig.30.(a)Theoptical-limitingbehaviorofthe{Ag(0)}Enanocompositeat532nmfora10-Hzpulse-repetitionrateobservedbyirradiatingnanosecond-laserpulses.The solidlineillustratesthecaseofaconstanttransmissionof∼60%(thetransmissionvalueatlowpowerdensity).Theinsetshowsthecorrespondingtransmissionvariation withtheinputpowerdensity.(b)AschematicrepresentationofdendrimernanocompositeofAgclusters,{Ag(0)}Earchitecture.(Modifiedfromref.[185].Reproducedwith permissionfromtheAmericanChemicalSociety.)
thefragmentationwasobtainedbyTEMmeasurement.However, theyreservedthejudgmentwithregardtobubbleformationasa damagemechanism.Theinvolvementofbubblegenerationwith proteindeactivationisstillnotclear.
Another example of a biomedical approach is the applica-tionofnanobubblestotheranostics(diagnosticsandtherapeutics) [194,195].Adistinct goaloftheranostics istoselectively target specific(diseased)tissuesorcellstoincreasediagnosticand thera-peuticaccuracy.InanexampleshowninFig.31[194],asingle-shot laser-pulse excitation of transient and localized vapor bubbles
(definedasPNB[45])causeddamagetolivingcells(lungcarcinoma cellsA549)usingAuNPs.
For each cell, brightfield and side scatteringoptical images wereregisteredbeforeandafterthepumppulse.InFig.31, pic-ture(a)shows thebright-field imagebeforeirradiationand (c) shows it after theirradiation. Also, (b) is the scatteringimage of PNBs generated inside the cell. The detection of cell dam-age was based on the observation of blebbing bodies in the bright-field image of the cell after the pulse (c). The study revealed conditions necessary for the precise excitation and
Fig.31.Invivogenerationanddetectionofplasmonicnanobubble(PNB)inAuNP-treatedA549cells:(a)bright-fieldmicroscopyimageoftwocellspriortotheirexposure toasinglepump-laserpulse(532nm,0.5ns,200mJcm−2);(b)time-resolvedscatteringimageoftheintracellularPNBsobtainedwiththe10-nsdelayed(relativetothe pump-laserpulse)pulsed-probelaser(690nm,0.5ns),wherethelinesshowthecontoursofthecellsasin(a);(c)bright-fieldmicroscopyimageoftwocellsobtained 120saftertheirexposuretoasinglepump-laserpulseandthegenerationofPNBs;(d)timeresponseobtainedduringtheexposuretoasinglepump-lasershowingbubble expansionandcollapse.(Reproducedfromref.[194].ReproducedwithpermissionfromIOPPublishing.)
Fig.32.Upperpanel:dark-fieldlight-scatteringimagesofAuNP-dopedzeoliteLcrystals.Lowerpanel:plotsofthenumberofAuNPsperzeolitecrystal(occupancy)versus abundancefordifferentquantitiesofAuNPsinzeolitecrystals.Barchartsshowexperimentalresultsforapproximately200zeolitecrystals;lineplotsshowcalculatedcurves basedonaPoissondistributionforvariousexperimentalaverageoccupancies:(a)1.8;(b)4.2.(ReproducedfromRef.[197].ReproducedwithpermissionfromtheAmerican ChemicalSociety.)
detectionofplasmoniceffectsinvivoaroundAuNPswithnanoscale sensitivity.
Finally,weshowanexampleofcomposite-materialfabrication basedonlaser-ablationgenerationofAuNPsinliquids[101–105].
Finally,weshowanexampleofcomposite-materialfabrication basedonlaser-ablationgenerationofAuNPsinliquids[101–105].