Formation and thermo-assisted stabilization of luminescent silver clusters in photosensitive glasses

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Formation and thermo-assisted stabilization of

luminescent silver clusters in photosensitive glasses

Kevin Bourhis, Arnaud Royon, Gautier Papon, Matthieu Bellec, Yannick

Petit, Lionel Canioni, Marc Dussauze, Vincent Rodriguez, Laurent Binet,

Daniel Caurant, et al.

To cite this version:

Kevin Bourhis, Arnaud Royon, Gautier Papon, Matthieu Bellec, Yannick Petit, et al.. Formation

and thermo-assisted stabilization of luminescent silver clusters in photosensitive glasses. Materials

Research Bulletin, Elsevier, 2013, 48 (4), pp.1637-1644. �10.1016/j.materresbull.2013.01.003�.

�hal-00809461�

Formation

and

thermo-assisted

stabilization

of

luminescent

silver

clusters

in

photosensitive

glasses

Kevin

Bourhis

a,

*

,

Arnaud

Royon

b

,

Gautier

Papon

b

,

Matthieu

Bellec

b

,

Yannick

Petit

a

,

Lionel

Canioni

b

,

Marc

Dussauze

c

_,

_Vincent

_Rodriguez

c

_,

_Laurent

_Binet

d

_,

_Daniel

_Caurant

d

_,

_Mona

_Treguer

a

_,

Jean-Jacques

Videau

a

,

Thierry

Cardinal

a

a

CNRS,UniversityBordeaux,ICMCB,UPR9048,F-33600Pessac,France b

UniversityBordeaux,LOMA,UMR5798,F-33405Talence,France c

UniversityBordeaux,ISM,UMR5798,F-33405Talence,France d

ENSCP(Chimie-ParisTech),LCMCP,UMRCNRS7574,11ruePierreetMarie,F-75231Pariscedex05,France

1. Introduction

Thephotostructuringofmaterialsconsistsintakingadvantage of their photosensitivity to allow a local modification of the material. The main goal is the elaboration of active photonic devices[1].Short pulsedlasershavebeendemonstratedsincethe late 1990s to be powerful tools allowing surface and bulk modification of glassy material [2]. Choosing the right laser irradiationparametersenablesthree dimensional(3D)processing oftheglassymatrix.Forexample,Glezeretal.usedfemtosecond (fs)laser photo modificationin fusedsilica for 3Ddatastorage application[3].Picosecondandfemtosecondlasersareconsidered tobe‘‘athermal’’sincethepulsewidthismuchshorterthanthe timeofheatdiffusioninglasses,typically1

m

sinfusedsilicafora beamwaistof 2

m

m[4,5]. Howeverwhenprocessing ata high repetitionrate,intheorderofmagnitudeoftheMHz,theabsorbed energycannot bedissipated,resulting in thermalaccumulation pulse after pulse. Eaton and co workers reported that when

operatingwithafslaseratrepetitionratesfrom100kHzto1MHz, thetemperatureelevationwasestimatedtobeequal toseveral thousands ofCelsiusdegrees [6].Heat accumulationby fs laser irradiationatahigh repetition rate(>100kHz)hasbeenrecog nizedasausefuleffectfortheprocessingofglassesinrecentyears

[7 11].Forexample,opticalwaveguideswithsymmetricguiding cross sectionscanbeformedbyisotropicthermal diffusion[7]. Heat accumulationinsideaglassalsoinducestheprecipitationof crystals [10] or the modification of the chemical composition distributionaroundthelaser focalvolume[11,12],which make possibletocontrolthethree dimensionalpropertiesinglasses.

Recently our group showed that irradiating photosensitive glassescontainingsilverionsusinganIRfs laseroperatingata highrepetitionrateresultedintheformationoffluorescentpipes withintheglasswithoutanysignificantvariationoftherefractive index(

D

n<10 4_{)[13].Thesectionofthepipesconsistsinaring}

structure composed of luminescent silver clusters [14]. Its formationwasexplainedbyathree stepmechanism:(i)photo ionization;(ii)thermaldiffusionand(iii)photo dissociation.The proposed model for the fluorescent ring shaped formation stronglysuggestedthatthermaleffectsweremostlyresponsible of the spatial distribution of the silver clusters and of their

ARTICLE INFO Keywords:

A.Opticalmaterials

B.Electronicparamagneticresonance(EPR) B.Luminescence

D.Colorcenters D.Opticalproperties

ABSTRACT

Variousphoto inducedsilverluminescentcentreswereobtainedinphotosensitivezincandphosphate

glassescontainingsilverionsafterexposuretogammaorultravioletnanosecondpulsed laserradiation.

Gamma irradiationoftheglassesresultsmainlyintheformationwithintheglassofelectron trapped

andhole trappedsilvercentresasevidencedbyopticalabsorption,luminescenceandelectronspin

resonancespectroscopies.Forthehighestirradiationdosessilverclustersareobtained.Underultraviolet

nanosecondpulsed laserexposuresimilarspeciesaregeneratedalongthebeampropagationdirection

asprovenbytheanalogousopticalandluminescencesignatures.Inthiscaseforhighirradiationdoses

few silver clusters are created. The evolution of the luminescence spectra with respect to the

temperature and to the durationof the heattreatment after ultraviolet nanosecondpulsed laser

irradiationevidencesthepresenceofpotentialbarriersdeterminingthestabilitylimitsofsomespecies

suchastheAg2+hole trappedcentresortheAgmx+clusterscomposedofsilverionsandsilveratoms.A

heattreatmentofseveralhundredsofdegreesisidentifiedasathekeyparameterfortailoringtheoptical

propertiesandcontrollingtheformationofAgmx+clustersinthephotosensitiveglasses.

* Correspondingauthor.Tel.:+33540002543;fax:+33540002761. E-mailaddress:bourhis@icmcb-bordeaux.cnrs.fr(K.Bourhis).

luminescence during the laser irradiation [13]. Bellec et al. evidenced that the luminescence intensity of the structures, suspectedtoberelatedtotheaggregationofthesilverclusters, couldbetailoredonalargeintensityscalebycontrollingthenear IRfs laserparameters[15].Anaccuratecontrolofthelumines cenceintensityatthemesoscopicscalewasachieved.Mastering theaggregationofthesilveratomsandtheiropticalpropertiesin photosensitive materials opens the way to applications for perennialhighdensitydatastorage.Appealingapproacheshave recentlybeendemonstratedinglasseswherethephotolumines cenceoflaser inducedsilverclusters[16]orthewavelengthand polarization dependence of laser elongated silver nanoparticles

[17]wereusedforthedatareadout.Theshapeoftheluminescence spectrawas however dependent on the irradiation conditions, leavingsome uncertaintieson the natureofthe photo induced species. Moreover the influence of the temperature on the formationandonthestabilityofthephoto inducedsilverspecies wasnotdebated.Themajordifficultyistoseparatethethermal effectsfromthephoto ionizationmechanismsrelatedtothelaser energyabsorption.

The optical signatures of the luminescence spectra of such glassesaftergamma,electronandnanosecond laserirradiations attributedtosilverhole andelectron centresandclusters,present verystronganalogieswiththeonesobtainedafterfemtosecond laserirradiation[14,18].Inthis paperanexperimentalstep by step approach is developed. The different phenomena (photo ionizationandthermaleffects)arestudiedseparatelytoallowa progressive comprehension of the mechanisms and to try to identify the different photo produced species. First, gamma irradiationsatthescaleofthebulkglasseswereperformed.The occurring mechanism is mainly theformation of electron and hole centres within the whole bulk material. Second, a UV nanosecond pulsedlaser operatingata low repetitionratewas used for the irradiations. This kind of exposure favours the formationofelectron holepairs,while minimizingthethermal effects. The interaction area is reduced to a cylinder with millimetricsection,relatedtothesize ofthelaser beam,along thethicknessofthevitreoussample.Third,heattreatments(HT) arecarriedoutsubsequentlytotheUVlaserirradiation,inorderto add insights on the thermal effects on the nature and on the concentrationofthephoto inducedspecies.

2. Experimental

2.1. Glasselaborationandpreparation

Glasses withthecomposition 40P2O5 55ZnO 1Ga2O3 4Ag2O

(mol%) were prepared by melt quenching. (NH4)2HPO4, ZnO,

AgNO3 andGa2O3 in powderform wereusedas rawmaterials

andmixedtogetherwiththeappropriateamountina platinum crucible.Aheatingrateof18Cmin 1_{wasconductedupto10008C.}

Themeltwasthenkeptatthislasttemperature(10008C)from24 to48h. Followingthis step,theliquid waspouredinto abrass mouldafterashortincreaseofthetemperatureat11008Cinorder toaccesstheappropriate viscosity.The glasssamples obtained were annealed at 3208C (408C below the glass transition temperature)for3h,cut(0.5 1mm thick)andopticallypolished. The optical properties of the pristine glass were described elsewhere[14].

2.2. Gammairradiation

Gamma irradiations were performed at room temperature usinga137_{Cssourcedeliveringdosesfrom0.18kGyto5.4kGyata}

doserateof0.36kGyh 1_.

2.3. UVnanosecondlaserirradiation

Amode lockedNd:YAGlaser(SureliteContinuumL10)pumped byflashlampswasusedatroomtemperature.Itwasequipped with BBO nonlinear crystals to obtain the 355nm output wavelength.Thepulsedurationwas5 7nsata10Hzrepetition ratefor a 80mJ pulseenergy. Thebeamdiameteron the glass samplewas5mm.Thefluencewas400mJcm 2_{andtheirradiance}

67MWcm 2_.

2.4. Absorptionspectroscopy

Thetransmissionspectrawererecordedatroomtemperature with a Varian Cary 5000 spectrophotometer in double beam configurationbetween200nmand800nmwitha1nmstepanda 2nmspectralbandwidth.Thetransmissionspectrawerecorrected fromtheFresnelreflectionandfromtheglassthickness. 2.5. Luminescencespectroscopy

Themacroscopicluminescence spectra(emissionandexcita tion)onthegammaandUV irradiatedglasseswererecordedat room temperature with a SPEX Fluorolog 2 spectrofluorimeter (Horiba Jobin Yvon).The excitation sourcewasa 450Wxenon lampenablingcontinuousexcitationfrom200nmto800nm.The signal was detected and amplified by a Hamamatsu R298 photomultiplier.

Fluorescencelifetimesweremeasuredatroomtemperature withtwosetups.Forlifetimeshigherthan10

m

sthemeasure ments were performedwith a SPEXFluorolog 2 spectrofluo rimeter(HoribaJobin Yvon)withdoublemonochromatorsboth inexcitationandemissionintheCzerny Turnerconfiguration. Theexcitationsourcewasapulsedxenonlampemittingpulses of 3

m

sfull widthat half maximum(FWHM). Thesignalwas detectedandamplifiedbyaHamamatsuR298photomultiplier. Thetemporalresolutionwasabout2

m

s.Thelifetimesbetween 50nsand10

m

sweremeasuredwithaNd:YAGlaser(Surelite) emittingpulsesof5 7nsFWHMat355nm.Thediameterofthe beam onthe sample was of 1mm for a deposited energy of 0.4nJ/pulse corresponding to a 320kWcm 2 _{irradiance. The}

emittedfluorescencewascollimatedbylensesandinjectedin anopticalfibre.Atthefibreoutputthelightwasdiffractedbya MS260ispectrometer(Oriel)equippedwithapairofgratingsin theCzerny TurnerconfigurationandanalyzedbyaICCDiStar 720camera(AndorTechnology).Thetemporalresolutionwas about10ns.

2.6. Electronparamagneticresonance(EPR)spectroscopy

TheEPRspectrawererecordedat roomtemperaturewitha Bruker Elexsys E500 spectrometer operating at X band at 9.45GHz and equipped with a SHQ resonator. A 100kHz modulationofthemagneticfieldwasusedforlock indetection, so thatthe EPR signals appear as absorptionderivatives with respecttothemagneticfield.Afewmilligramsofglasspowder werenecessary.Noinfluenceofthetemperature(downto15K) on the shape of the signals after irradiations was observed, indicatingthattheparamagneticcentresobservedarestabilized atroomtemperature.

3. Results

Alltheexperimentswereperformedatroomtemperature.The irradiated glass samples were not studied immediately after irradiationand/orheattreatment,whichmeansthatthetransitory phosphateandsilverspeciescouldnotbeobserved[19,20]. 2

3.1. Gammairradiationinducedcentresandopticalsignatures Theabsorptionspectraoftheglassesaftergammairradiation are presented in Fig. 1a. The photo induced absorption bands appear in the spectral domain between 200nm and 500nm. Several bands are visible around 275nm, 320nm and 370nm. Theirintensityincreaseswiththeirradiationdose.Theradiation inducedspectra(thatisthedifferencebetweenthespectraafter andbefore irradiation) werefitted consideringGaussianenergy contributionsforthephoto inducedspecies,asshowninFig.1bfor the5.4kGy irradiatedglass. Fig.1c shows theevolution of the integratedareasofthethreeGaussianbandsusedforthefit.The assignmentofthosebandsisnotaneasytask.Accordingtothe literature,thebandat320nmwasascribedtotheAg2+_orAg

32+

silver speciesand the bandat around370nmto Ag0 _{atoms in}

various gamma irradiated glass matrices [21 24]. Ershov et al. determinedthetheoreticalabsorptioncharacteristicsofdifferent silverclusters,definedasAgmx+clusters,wheremisthenumberof

ionsandatomsandxtheformalcharge[25].Theycalculatedthat theAg2+specieshaveabsorptionfeaturesataround310 320nm and 360 400nm and that the Ag32+ clusters exhibit intense

absorptionbandsnear260 285nmand540 720nmandalower one at 275 320nm. The calculated position of the absorption bandsofclusterswithalownuclearityareinaccordancewiththe

experimental observations.Asshownin Fig.1c,thearea ofthe band at 275nm shows a nearly linear fast increase with the irradiationdose.Aninflexionoftheslopesforthebandsat320nm and370nm,whentheirradiationdoseincreasesisobserved.Such behaviour maybe related toa slowingdown of the formation reactionsofthedifferentspeciesoraconsumptionofthesespecies. Forexampletherecombinationkineticsoftheelectronswiththe silverionsorthediffusionofAg+_andAg0_{atomstoformtheAg}

2+

aresusceptibletobethekeyfactorshavinganinfluenceonthe resultingabsorptionspectrum.

Tobetterunderstandthespeciesformedafterirradiation,EPR spectroscopywasperformed.TheEPRspectraoftheglassesbefore and after gamma irradiationare represented in Fig. 2a. Before irradiation, no EPR transition is visible, indicating that no paramagnetic centres are observed within the detection limit. Fromthelowest irradiationdoses, severalsignals areobserved. They are similartothose recently measured by Fan etal. in a vitreoussystemwhosecompositionisclosetoours(P2O5Li2O

Al2O3Ag2O) [24]. Several signals corresponding to different

species(electronandholecentres)areobservedonthespectrum oftheglasssubmittedtoa5.4kGyirradiation.Thetransitionsat 3045Gand3675GareassignedtoaAg0_{centre(electroncentre)}

[19,24,27].Thiscentreischaracterizedbyatwo linesignalowing tothehyperfinecouplingwith107_{Ag(I=1/2,naturalabundance}

Fig.1.(a)Absorptionspectraoftheglassesfordifferentirradiationdoses.(b)Fitofthespectrumoftheradiation-inducedspectrumfora5.4kGyirradiationdoseconsidering threeGaussianenergycontributions.(c)EvolutionoftheareasofthethreeGaussianbandsfromFig.1basafunctionoftheirradiationdose.(Forinterpretationofthe referencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.).

Fig.2.(a)EPRspectraoftheglassesaftergammairradiation(Inset:magnificationofthespectraaround3425G).IntensityoftheRPEsignalsat(b)3250Gand(c)3675Gasa functionoftheirradiationdose.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

51.8%)and109_{Agnuclei(I=1/2,naturalabundance48.2%).Foreach}

hyperfineline, thesplitting due to thetwo isotopes of theAg nucleusisnotresolved.Thisspectrumissimilarinshapetotheone observedby Yokotaand Imawagaafter stabilizationof theAg0

centreatroomtemperature[19].Sincetheunpairedelectronis mainlyinans typeorbital(5s1_{configurationforAg}0_{),theg factor}

andthehyperfineinteractionarebothdominantlyisotropic.The valuesdeterminedbysimulationofthespectrumobtainedafter irradiation at 5.4kGy are: giso=1.965 for the g factor and

Aiso=550G for the average hyperfine coupling for 107Ag and 109_{Ag.The transitions at 2840G and 3250G correspond to the}

parallelcomponentgjj2.36and theperpendicularcomponent

g?2.06of anAg2+ion (hole centre) in anaxial symmetryas

alreadyreportedbyseveralauthors[19,24,27].Itisinterestingto notethat the phosphorous oxygen hole centre (POHC), whose formationisreportedinliteratureafterX orgamma irradiationof phosphateglasses[20,27 29],wasnotdetectedinourirradiated samples.ThiscentreisassociatedtoanintenseEPRsignalwithag factorcloseto2.008andAiso=4G[20].Theabsenceofthiscentre

at room temperature in our irradiated glass samples can be explainedbythefactthat,accordingtotheliterature,thePOHC centreisstableatroomtemperatureafterX orgamma irradiation ofphosphateglasses(i)thatdonotcontainAg2O[20,26]and(ii)

that containAg2O and that are moderatelydepolymerised (i.e.

phosphateglasseswithO/P3)[26].Inourcase,thephosphate network is too depolymerised (O/P=3.275>3) to enable the formationof stable POHCand only the morestable Ag2+_hole

centresaredetectedatroomtemperature.Itisalsointerestingto pointoutthat,becauseoftheoccurrenceofAg2Oinourglasses,

phosphateelectroncentresarenotdetectedafterirradiation[18]

andonlythemostefficientAg0electron centresaredetected. To evaluatethechangein theamounts oftheAg0 _{and Ag}2+

centreswiththeirradiationdose,theintensitiesoftheperpendic ularcomponentat3250GoftheAg2+_{centreandtheintensityof}

thehigh field hyperfine line at 3675G of theAg0 _{centre were}

measured.The intensitiesof theselines wereapproximated by Eq.(1)[30].

I/Ymaxð

D

HppÞ2 (1)

whereYmaxisthepeak to peakamplitudeofthelineand

D

Hppthe

peak to peaklinewidth.

Theintensityofthesignalsat3250Gand3675Garereportedin

Fig.2bandcasafunctionoftheirradiationdose.Theintensityof the3250Gline,attributedtotheAg2+_{hole centres,increaseswith}

theirradiationdose.Itisdifficultinaccordancewiththeerrorbars toconcludebetweenalinearoranonlinearevolution.Regarding theevolutionoftheintensityofthelineat3675G,associatedto Ag0_{electron centres,thesituationisclearer.Theintensitystrongly}

increasesupto1.08kGyandhasalowerslopefrom1.08kGyto 5.4kGy.ThegrowthoftheEPRsignalintensityat3675Gversus theirradiationdosesiscomparabletotheoneobservedforthe 370nmopticalabsorptionbandareas(Fig.1c),which wasalso attributedtoAg0_{atoms.Theslopebreakobservedinbothcasesfor}

thesameirradiationdose(1.08kGy)mayindicatetheconsump tionofAg0_{atomsandtheirclusteringatthehighestirradiation}

doses.TheintensityofthetransitionassociatedtotheAg2+_centre

at3250Gincreasesbyafactor4aftera5.4kGyirradiationdose compared to a 0.18kGy irradiation dose. This is the same behaviouras fortheincrease of theopticalabsorptionband at 370nmand320nm.Forhigherdoses,additionalEPRsignalsare observedinthe3300 3600Gmagneticfieldrange,asshownby theinsetinFig.2a.ThesesignalsaregenerallyassignedtoAgmx+

(x<m) clusterswitha lownuclearity [19,27,31].Theirabsence fromtheEPRspectraoftheglassesafterirradiationat0.18kGyand 0.36kGyseemstoindicatethat,fortheseirradiationdoses,the amount of Ag0 _{locally generated is not sufficient to allow a}

significantformationofAgmx+clusters.

The normalized excitation (

l

em=700nm) and emission

(

l

exc=325nm)spectraoftheirradiatedglassesarerepresented

in Fig.3a. The emission maximum of theirradiated glasses is locatedat

l

=630nmandisaccompaniedbyabandnear500nm. Theintensityofbothbandsincreaseswiththeirradiationdose.The intensities of the different contributions to the emission is dependent on the excitation wavelength. Indeed for a 405nm excitation wavelength, the emission band at around 500nm becomespredominant(notshownhere).Theexcitationspectrafor the 0.18kGyand 5.4kGydoses are represented in Fig. 3b for

l

em=700nm.Theselectionofthe700nmemissionwavelength

allowstominimizethepristineglasscontributionwhichhasan emission spectrum ranging from 350nm to 600nm for an excitationwavelengthataround260nm[13,14].Severalexcita tionbandsarevisible between250nmand500nm.Anintense broadbandis observednear 320nmfor the0.18kGy irradiated glass.Itisaccompaniedbyshouldersat275nmand370nm.The excitationspectracanbedecomposedbythethreeGaussianbands alreadyidentifiedintheabsorptionspectraat370nm,320nmand 275nmrespectively.Theintensityofthebandsincreaseswiththe irradiationdose.Thefitoftheluminescencedecaycurvesforan excitation at325nm andan emission at700nm(Fig.3c) bya

Fig.3.(a)Normalizedemission(lexc=325nm)andexcitation(lem=700nm)spectraforirradiationdosesof0.18kGy(squares)and5.4kGy(circles).(b)Excitationspectrum fora0.18kGyirradiationdose(lem=700nm)fittedbythreeGaussianbands(sameparametersasforthefitinFig.1b)and(c)Luminescencedecay(lexc=325nm,

lem=700nm)fittedbyabi-exponentiallaw.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

bi exponential law gives lifetimes equals to

t

1=(82)

m

s and

t

2=(30080)

m

s.The

t

1lifetimeisobservedinallthefitteddecays

for excitation wavelengths varying from 325nm to 405nm and emissionwavelengthincludedbetween400nmand700nm.The

t

2

lifetime,muchlonger,ismostprobablyassociatedtoAg+Ag+pairs alreadypresentinlowquantityinthepristineglass[33].Indeeda luminescencelifetimeof125

m

swasalreadymeasuredinunirradi atedglasseswithsimilarcompositionbyBelharouaketal.[32].Itwas attributed tosilver dimers (Ag+_Ag+_{) aftercomparison withsilver}

polyphosphatecrystalsinwhichtheX raydiffractiondataindicated somepairingofsilverionsinneighbouringcells[33].Fortheseglasses a broad emission band peaking near 550nm for an excitation wavelength at around 325nm was observed. It is reasonable to assumethatthisbandcontributestotheoneobservedinourcase near500nm.Howevertheincreaseofitsintensityandthemeasured lifetimecannotbe explainedbytheuniquepresenceofthesilver dimercentresreportedbyBelkarouaketal.Regardingtheshorter

t

1

lifetime,luminescence lifetimesranging from1

m

s to 30

m

s were previously reported in silver activated radiophotoluminescent glasses after gamma exposure [34]. Such lifetimes are hardly compatiblewiththeemissionofsilverclusterswhichareconsidered topresentshorterlifetimesontheorderofafewnanosecondsorless

[15,35 38]. Therefore, we propose that the

t

1 lifetime of a few

microsecondsisattributedtotheemissionoftheAg2+_species.Such

assignmentforthe630nmemissionbandwasalreadyproposedby Hsuetal.[39].

Inconclusionforthegammairradiationstudy,twomainphoto inducedcentreswereidentified(Ag0andAg2+)andthepresenceof Agmx+species(x<m)inlowquantitywasstronglysuggested.The

luminescencemeasurementsconcludedonthepresenceofseveral emittingcentres.Theemissionbandnear630nmisassociatedto theexcitationbandnear325nmandthelifetimeof8

m

sismost probably related to a d d transition of the Ag2+ _{ion. One can}

supposethattheexcitationbandnear325nmisnotuniqueand additional bands may be possible near 275nm or 370nm, correspondingtothe3d orbitalsplitting.Forthehighirradiation doses additional absorption bands are observed near 280nm, 320nm and 350nm (Fig. 1a) which may be related to the formationofAgmx+clusters.

3.2. IdentificationofthespeciesinducedbyUVnanosecondlaser irradiation

The absorption spectra of the glasses before and after irradiationby10 180,000pulses areshownin Fig.4a.After 10 pulses,asmallvariationoftheabsorptioncoefficientisobservedin the270 550nm spectral range.Twoshoulders arevisible near

325nmand380nm.Theirintensitiesstronglyincreasewhenthe number ofpulsesreaches severalthousands.Below 300nmthe spectrumofthe10 pulsesirradiatedglassislessintensethanthat ofthepristineglassone.This‘‘negative’’differencemaysignifya localconsumptionofthelocalAg+ions(absorbingbelow300nm). Whenincreasingthenumberofpulses,thedifferencebetweenthe spectraoftheirradiatedglassesandthepristineglassdecreases untilitbecomes‘‘positive’’.Thisobservationmeansthatabsorbing species are generated, which might be related to theoptically absorbing silver clusters (near 290nm), also detected by EPR spectroscopy after gamma irradiation (Figs. 1a and 2a). EPR investigation on theUV laser irradiated glass sampleswas not successfulduetothesmallirradiatedvolumelimitingtheamount ofUV producedspecies.Indeedinthecaseofgammairradiation thewholeglassisexposedtothebeamwhicheasesthedetection ofphoto producedspecies.ForUV laserirradiations,theexposed zone is reducedtoa cylindrical area alongthe5 mm diameter beampath.Thephoto producedclustersarebelievedtobemuch lessnumeroussincetheabsorptioncoefficientafter180,000pulses (Fig. 4a) is still 10 times lower than the one of the 5.4kGy irradiationdose(Fig.1a),forwhichtheEPRspectroscopyallows thedetectionofmagneticsignaturesattributedtosilverclusters (Fig.2a)TheemissionspectraoftheUVlaser irradiatedglassesare represented in Fig.4b fora 325nmexcitation wavelength.The spectra are dominated by an intense band centred at about 630nm,attributedtoAg2+_{speciesbyHsuandco workers[39].For}

10 pulses, the630nm bandis unique. The signalobserved for wavelengthsbelow500nmarisesfrombackgroundnoisedueto the very low intensity of the emission signal combined with emissionofthepristineglass[32].Forseveralthousandsofpulses, the630nmbandintensitystronglyincreasesandanotherweak bandisobservednear450nm,similarlytothoseobservedinthe caseofgammairradiation(Fig.3a).Thecorrespondingexcitation spectrumfora700nmemissionwavelengthisshowninFig.4b. For10pulsestwobandsnear275nmand320nmareobserved withashoulderoflowintensityaround370nm.Whenthenumber of pulses increases, the relative intensity of the 320nm and 370nm strongly rises whereas thecontribution below 275nm banddecreases.Thislastbandwasattributedtothefree Ag+_ion

exhibitinganexcitationbandmaximumataround265nmandan emissionbandmaximumataround380nminthepristineglass

[14]. Its contribution is significant for low numbers of pulses becausethetailoftheemissionofthefree Ag+_{ionextendsupto}

750nm.Itsrelativeintensitydecreaseswiththenumberofpulses sincetheintensitiesoftheemissionbandsofthephoto produced speciesnear320nmand370nmbecomedominant.Thefitofthe excitation spectra withthe same Gaussian energy distribution

Fig.4.(a)AbsorptionspectraoftheUVlaser-irradiatedglassesfordifferentnumberofpulses.(b)Normalizedemission(lexc=325nm)andexcitation(lem=700nm)spectra fordifferentnumberofpulses.(c)Excitationspectrum(lem=700nm)oftheglassexposedto180,000pulsesfittedbythreeGaussianbands(sameparametersasforthefitin

Fig.1b).(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

parametersasinthecaseofthegammairradiationwassuccessful, asdemonstratedinFig.4cfora UV laserirradiationof180,000 pulses. The bands at 320nm and 370nm seem to be linked togetherandanexcitationatthesewavelengthsleadstoaunique emission band at 630nm, most probably related to the Ag2+

species.Thefitoftheluminescencedecays(notshownhere)for threeexcitationwavelengths(325nm,355nmand405nm)and the600nmemissionwavelengthindicatedauniquelifetimeequal to (93)

m

s. To detect shorter lifetimes, measurements were performedwiththeUVnanosecondlaserasexcitingpulsedsource. Thetime resolvedemissionspectraofthe180,000pulses irradiated glass is shown in Fig. 5a for different delays after the 355nm excitationpulse.Aftera100nsdelay,thespectrumcollectedshowsa broadbandcentrednear630nm.Forfurtherdelays,theintensityof the630nm band decreasesprogressively whilekeepingthesame spectralshape. After15

m

s,thefluorescencesignal merges tothe background. The luminescence decay lifetimes were deduced by plottingtheintensityofthebandat630nmversusthedelayafterthe pulse. A typical luminescence decay for

l

exc=355nm and

l

em=630nmisrepresentedinFig. 5b.Twolifetimesweredeter

minedafterfittingofthedecaysbyabi exponentiallaw.Thefirstone isabout(0.70.3)

m

sandthesecond(52)

m

s.Thelongerlifetime is compatible with the ones measured previously on gamma irradiatedsamples andis thereforeassignedto Ag2+_{centres. The}

shorter lifetime might be related to luminescence extinction or energytransferphenomena.

InconclusionfortheUVnanosecondlaserirradiationstudy,the excitationbandsat325nmand380nmarelinkedtotheemission bandat630nmwithalifetimeontheorderof5

m

sattributedto theAg2+_{species.Theweakemissionbandnear500nmisrelated}

mainlytoanexcitationbandnear325nmandmayalsoarisefrom excitationsatlowerwavelengths(275nm).Itismostprobably relatedtoAgmx+silverclusterswithalownuclearityasinthecase

ofthegammairradiationstudy.

3.3. StabilityandformationoftheUV laserphoto inducedcentres afterheattreatment

The evolution of the photo induced species versus HT was investigatedusing theUV laserirradiated samplesafter18,000 pulses. Several temperatures from 2008C up to 4008C were appliedtotheUV laserirradiatedglassesfordifferentdurations from2minto10min.Thistemperaturerangewaschosengiven thenumericalsimulationsperformedbyBellecetal.[13].Fs laser

irradiation at a 10MHz repetition rate of a glass of the same composition as the one studied in this paper provokes a temperature increase which reaches 4008C after 104 _pulses,

supposedtobethedrivingforceleadingtotheformationofstable silverclusters[15].Thistemperatureisreachedin1msgiventhe highrepetitionrateofthelaserandismostprobablyhigherwhen the number of pulses increases to 105 ₁₀7_{. However it is not}

possibletoproceedtolongheattreatmentatsuchatemperature, whichis208ChigherthantheTgoftheglass,becausetheglassis

softenedandstartstoundergophysicaldeformationbeyondtheTg.

Theopticalabsorptionspectraoftheglassesas irradiatedand afterHTarepresentedinFig.6a.After2minofHTat4008C,the absorptioncoefficientisdividedbytwointhe280 500spectral range.Fora10minHT,thespectrumexhibitsalmostnodeviation ascomparedtotheonecollectedforthepristineglass,exceptfora shoulderdistinguishableataround320nm.

Theemissionspectrafora325nmwavelengthexcitationare depictedinFig.6b.AssoonastheHTisperformed,adecreaseofthe intensityoftheglobalfluorescenceisobserved,accompanyingthe diminishingoftheopticalabsorptionbandsasdescribedabove.A strongincreaseofthebandnear500nmisnoticed.Ashoulderof weak intensityis visible in thetailofthe bandbelow400nm. WhenthedurationoftheHTrises,severalphenomenaoccur.The intensityof the630nmband undergoesa strong decreaseand becomesquasi negligibleafter10minofHT.Abroadbandcentred at500nmdominatesthewholespectrum.Theshoulderobserved below400nmisstronglyamplifiedandexhibitsamaximumnear 380nm.Itsintensitybecomesequaltothatofthe500nmband. The excitation spectra for a 700nm emission wavelength for samplesheat treatedat4008CareshowninFig.6b.Asannounced previously,thephoto inducedabsorptionandemissionintensities decrease after heat treating the samples. For comparison, the spectra were normalized in Fig. 6b. The fit of the different excitation spectra was conducted using Gaussian energy con tributionspeakingrespectively at370nm,320nmand 275nm, likeforgammaandUV laserirradiations,asshowninFig.6c.Fora 2min HT, the intensity of the bands at 320nm and 370nm decreases while a band at around 275nm grows in relative intensity.For a 10minHT, this band dominatesthe spectrum whilethecontribution at370nm hascompletelydisappeared. The band at 320nm is still present. The luminescence decay measurements performed on the microsecond setup (Spex Fluorolog 2 Jobin Yvon) for several excitation and emission wavelengths indicate the presence of a lifetime of about

Fig.5.(a)Time-resolvedemissionspectra(lexc=355nm)oftheUV-laserirradiatedglassafter180,000pulsesforseveralpulsedelays.(b)Fitoftheluminescencedecayfor

lexc=355nmandlem=630nmbyabi-exponentiallaw.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthis article.)

(83)

m

s.Noshorterlifetimescouldbedetectedwithanysetup giventhelowdensityofphoto inducedcentresandthelowintensity oftheassociatedluminescence.

TheobservationsrelatedtotheHTsat4008Carerepresentative of the tendencies observed for all the other temperatures and duration.Thestrongintensitydecreaseintheabsorptionspectra between280nmand500nmindicatesthedisappearanceofmost ofthecolourcentres.Thebandat380nmattributedtoAg0_species

is no longer visible. The luminescence spectra show that the emissionbandat630nmprogressivelydecreasesunderheating. ThesedataareconsistentwiththedisappearanceoftheAg0_and

Ag2+ photo produced centres. The new luminescent bands at 380nmand500nmformainexcitationat275nmand320nmare mostlikelyresultingintherelaxationoftheelectronsandholes trappedontheAg0_andAg2+_{centres.Theevolutionofthoselast}

bandsversusthetemperatureisdifferentandsuggeststhatthey arise from different emitting centres. The centre emitting at around380nmwasobservedonlyinthecaseofthe4008CHTfor durationsontheorderof10min.

AtemperatureclosetotheTgresultsinincreasingthemobility

ofthespeciesincludingelectronsandholesbutalsoions.TheHTis likelytoovercomethepotentialbarrierrequiredfortheformation ofAgmx+silverclusterscomposedofbothAg+andAg0species.The

emission band near 500nm in silver containing glasses was attributedbyBelharouakandco authors[32]tomolecularcentres whoserelatedshortlifetimeontheorderofthenanosecondwould belinkedto5s$5pallowedtransitions[40].Thecalculationsof theopticaltransitions energiesforsomechargedAgmx+clusters

(with m14 atoms) in aqueous solution were performed by Ershovetal.[25].Someofthemexhibitseveralintenseabsorption bandsbetween250nmand400nm,especiallyclusterswithalow nuclearity.ForexamplefortheAg2+ion,thecalculationshowstwo

transitions near 310 320nm (s plevels) and 360 400nm (s s levels),fortheAg32+twotransitionsoccurnear260 285nm(s p*

levels) and 275 320nm (s s levels) and for the Ag42+ two

transitions near270 293nm(s p*levels) and 320 390nm (s s levels).Itisthereforestronglybelievedthatsuchsilverclusters maybegeneratedduringtheHTprocess.

4. Discussion

GammaandUVnanosecondlaserirradiationsmainlyresultin theformationofcolourcentresliketheAg2+_{hole centresandthe}

Ag0electroncentre.Theabsorptionbandat380nmwasassigned toAg0_{atoms.Theemissionat630nmforthemainexcitationband}

around320nmwasattributedtoAg2+_{ions,withanassociated}

lifetimerangingfrom5to8

m

s.Itwasshowninthecaseofthe

gammairradiationsthatforthehigherdosesthedensityofphoto produced centres allows theformation of weakly concentrated Agmx+silverspecies.Severalconvergingproofstendtoconfirmthis

trendinthecaseofUV laserirradiations,thoughthisirradiation mode producesalsomainlycolourcentres.ForUV nanosecond laserirradiationsfollowedbyHTs,asignificantproductionofsilver clustersis observed.Such clustersareformedfor temperatures rangingfrom2008Ctotemperaturesbeyondtheglasstransition temperature(Tg=3808C).Itis probablethatbeyond theTgthe

growthoftheclustersisacceleratedsincethecrystallizationratein glasses generallyreaches a maximum for temperaturesslightly superiortotheTg.Thepresenceofthe380nmemissiononlyfor

HTs at 4008C means that the associated species have a high activationbarrierwhichpreventsitsformationatlowertempera tures. Syutkin et al. assumed that the cluster formation was governedbythediffusionofAg+_{ions[31],butrequiresthetransfer}

ofanelectroncomingfromaphosphategrouptowardsanAg+_ion

toinitiatetheclusteringreactions[41].

Ontheotherhand,thecolourcentres(Ag0_andAg2+_)exhibitalow

thermalstabilitysincetheybegintodisappearprogressivelyfrom 2008C.Thereforeweconcludethatthetrappingoftheelectronsand holes by these colour centres is of low energy and that their relaxation may occur as soonas their vibrational levels, whose energyisclosetothatoftheconduction/valenceband,arethermally filled.However,thesilvercolourcentresarestillmorestablethan thePOHCs.EvenifsomePOHCspre existatroomtemperatureinthe pristine phosphateglasses[20]oraregeneratedbytheUV laser irradiations,theyrelaxduetoelectrontransferfromtheAg+ion

[42,43].Inourcasethemeasurementsatroomtemperaturedidnot allowthecharacterizationofsuchtransientspecies,thoughthey maybeobservedatlowtemperature[19,20].

The evolution of the different silver species after laser irradiation and HT canbe described bya two step mechanism (Fig.7).Thefirststepconsistsintheformationofcolourcentres (Ag0 _{and Ag}2+_{)consecutively tothe formationof electron hole}

pairsduringthelaserirradiation.Whentheirradiationdose(i.e. thenumberofpulses)ishighenough,theconcentrationofcolour centresissufficienttoallowtheirclusteringinalowquantityof Agmx+ species. The second step probably combines two effects

relatedtotherelaxationofthecolourcentrestoAg+_ionsanda

diffusion process of the electrons and ions inside the medium leadingtotheformationoftheclusters.Itseemsalsoobviousthat fortemperaturesofsomehundredsofCelsiusdegrees,mostofthe ‘‘free’’electronandholesrecombinate.Itisstronglyprobablethat thenuclearityoftheAgmx+clustersistemperature dependent,as

shownbytheapparitionofthebandnear380nmintheemission spectrum. More investigations are necessary to identify the

Fig.6.(a)Absorption,(b)Emission(lexc=325nm)andexcitation(lem=700nm)spectraoftheUV-laserirradiatedglassesafter18,000pulsesbeforeandafterHTat4008C. (c)Fitoftheexcitationspectrum(lem=700nm)fora10minHTat4008C.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheweb versionofthisarticle.)

relationbetweenthecolourcentredensity,whichislinkedtoboth thequantityofelectron holepairsgeneratedduringtheirradia tionandthetemperatureincrease,andtheamountofstabilized Agmx+silverclusters.

5. Conclusions

Thegoalofthisworkwastostudyseparatelytheeffectsofthe photo absorptionand ofthetemperatureon theformation and consumptionof silver species in photosensitive zinc and phos phate glasses containing silver ions. This study allowed the progressivecomprehensionoftheinvolvedmechanismsandthe identificationofthedifferentphoto producedspecies.Theglasses were exposed either to gamma or to ultraviolet nanosecond pulsed laserradiationsfollowedbyaheattreatmentinthecaseof thelaser irradiation.

Thegamma irradiationstudyhaspointedoutthatAg0_andAg2+

species are formed from the very low irradiation doses. High irradiationdosesarerequiredtoobservetheapparitionofAgmx+

clusters,mostprobablylinkedtoaclusteringofthephoto induced specieswhentheirconcentration overtakesacertainconcentra tion.Theopticalandmagneticsignaturesoftheproducedspecies havebeenidentified.

The effect of a UV nanosecond laser irradiation of the photosensitiveglasseshasbeeninvestigated.Basedontheresults obtained after the gamma irradiations, it is concluded that it resultedmainlyintheformationoftheAg0_andAg2+_centresandof

asmallamountofAgmx+clusters.

Consecutiveheat treatmentsrangingfrom2008Cto4008Cfor durationsupto10minleadtotheconclusionthattheAg0_andAg2+

photo inducedcentres have a lowthermal stability.Raising the temperatureresultedinadramaticdiminutionoftheabsorptionand luminescencesignalsrelatedtothesespecies,leadingalsoprobably totherecombinationofmostelectronsandholes.Itisbelievedthat theincreaseofthetemperatureisresponsibleforahighermobility oftheAg0andAg2+species,resultingintheirclustering.

On the basis of the results presented herein, appealing perspectivesforthedevelopmentofthisworkconcernon going studieson the time resolvedluminescence spectroscopy of the silver species photo induced after fs laser irradiation at the micrometricscale. Ina futurework,themain resultspresented abovewillbecomparedtotheopticalsignaturesoffemtosecond laserirradiationsinglasses,confirmingthesimilarnatureofthe photo inducedspecies.Animproveddescriptionofthemechanism at the mesoscopic scale may also be achieved by near field scanning optical microscopy, which allows a sub wavelength resolution.Inthiscase,therepartitionofthedifferentsilverspecies withinthefocalvolumecouldbeobtained.

Acknowledgements

ThisworkhasbeensupportedbytheGISAdvancedMaterialsin Aquitaine’’,theRegionAquitaineandtheANR(grantsBLAN94603 andBLAN94604).

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Fig.7.Schematicrepresentationoftheevolutionofthedifferentsilverspeciesbeforeirradiation(left),afterhighdoseirradiation(middle)andafterheattreatment(right).