Evolution of Light-Induced Vapor Generation at a Liquid-ImmersedMetallic NanoparticleZheyu Fang,*,†,! Yu-Rong Zhen,‡ Oara Neumann,†,‡ Albert Polman,§ F. Javier García de Abajo,"

Peter Nordlander,*,†,‡ and Naomi J. Halas*,†,‡

†Department of Electrical and Computer Engineering and ‡Department of Physics and Astronomy and Laboratory forNanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States§Center for Nanophotonics, FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands"IQFR-CSIC, Serrano 119, 28006 Madrid, Spain!School of Physics, State Key Lab for Mesoscopic Physics, Peking University, Beijing 100871, China

*S Supporting Information

ABSTRACT: When an Au nanoparticle in a liquid medium isilluminated with resonant light of su!cient intensity, a nanometerscale envelope of vapor!a “nanobubble”!surrounding theparticle, is formed. This is the nanoscale onset of the well-known process of liquid boiling, occurring at a single nanoparticlenucleation site, resulting from the photothermal response of thenanoparticle. Here we examine bubble formation at an individualmetallic nanoparticle in detail. Incipient nanobubble formation isobserved by monitoring the plasmon resonance shift of anindividual, illuminated Au nanoparticle, when its local environ-ment changes from liquid to vapor. The temperature on thenanoparticle surface is monitored during this process, where adramatic temperature jump is observed as the nanoscale vaporlayer thermally decouples the nanoparticle from the surrounding liquid. By increasing the intensity of the incident light ordecreasing the interparticle separation, we observe the formation of micrometer-sized bubbles resulting from the coalescence ofnanoparticle-“bound” vapor envelopes. These studies provide the "rst direct and quantitative analysis of the evolution of light-induced steam generation by nanoparticles from the nanoscale to the macroscale, a process that is of fundamental interest for agrowing number of applications.KEYWORDS: Plasmonic heating, Au nanoparticle, LSPR, nanobubble, microbubble

Recently it was demonstrated that metallic nanoparticles,suspended in liquids and illuminated by focused sunlight,

can produce steam with remarkably high e!ciency.1 More than80% of the power absorbed by the nanoparticles resulted invaporization, while less than 20% of the absorbed power wentinto heating the liquid.1 This result, occurring at a very low lightintensity threshold (#106 W/m2), has stimulated signi"cantinterest in the fundamental aspects of this process as well as itspossible technological applications.2 To develop and optimizethis e#ect for applications, which range from medical and wastesterilization to water puri"cation and distillation, it is crucial todevelop a fundamental microscopic understanding of thevaporization process. Although no microscopic theory wasproposed to explain the measured e!ciencies, the authorsnoted that the results were consistent with a very simple modelwhere a vapor envelope nucleates around each individualilluminated nanoparticle. The nanoparticle/vapor bubblecomplex expands under further illumination, eventuallyachieving buoyancy, moving toward the liquid/air interfaceand releasing the vapor, after which the nanoparticle reverts

back into the solution and, as long as it is illuminated, repeatsthe process. Clearly an essential step in this process is the initialformation of a vapor layer around the illuminated nanoparticle.Noble metal nanoparticles, when illuminated at a wavelength

corresponding to their plasmon resonance, can serve as highlye!cient localized heat sources at the nanometer-lengthscale.3$18 This is a unique and well-established property thathas enabled numerous applications across a range of "eldsincluding energy,19$21 chemical catalysis,4,8 protein imag-ing,10,12 and biomedicine.14,17 Several studies have alsoinvestigated bubble and vapor formation in ensembles ofmetallic nanoparticles under continuous-wave (CW) andpulsed laser excitation.3$5,13,16,22,23 Despite these studies, theprecise properties of the liquid$vapor phase transition at thenanoparticle surface, such as temperature, internal pressure, andsize of the vapor envelope !the “nanobubble”!formed upon

Received: January 25, 2013Revised: March 11, 2013Published: March 21, 2013

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resonant laser excitation have not been investigated. Quantify-ing these properties at the single nanoparticle level is crucial forunderstanding the vapor generation process.In this Letter, we investigate plasmon-induced vapor

generation from an individual nanoparticle deposited on asubstrate. We combine dark-"eld scattering measurements oflocalized surface plasmon resonance (LSPR) shifts with surface-enhanced Raman scattering (SERS)-based temperature meas-urements on individual, illuminated Au nanoparticles. Thesenano-optic probes were performed during controlled resonantlaser excitation at incident intensities both below and above thethreshold required to induce local vapor formation. Thiscombination of local optical measurements at an individualnanoparticle allows us to determine several key microscopicparameters of this system, such as the nanobubble radiusaround the nanoparticle surface, the internal nanobubblepressure, the nanoparticle surface temperature, and theirdependence on incident light intensity. For dense nanoparticlecoverages and with further increases in incident light intensity,we observe the formation of micrometer sized bubbles due tothe coalescence of nanobubbles centered on adjacent nano-particles. We quantify the energetics of macroscopic bubbleformation based on the incident laser power and the arealdensity of nanoparticle nucleation sites. This series ofexperiments provides a detailed picture of the light-inducedsteam formation process by resonant excitation of metallicnanoparticles which will be useful for the development andoptimization of applications of this unique phenomenon.Gold nanoparticles were chemically synthesized and then

immobilized with an ultrathin layer of poly(4-vinyl pyridine)-(PVP) onto a transparent dielectric substrate at the bottom of amicrochamber (Figure 1a). These nanoparticles were dispersedonto the substrate surface at a very dilute areal coverage,nominally 1 nanoparticle/100 !m2 (Figure 1b shows a SEMimage of a sample of deposited nanoparticles at signi"cantlyhigher areal density, for visualization purposes; more details inthe Methods section). The microchamber was designed to beoptically probed by dark "eld microscopy (for LSPR spectros-copy) during simultaneous illumination by a CW “heating”laser beam. The CW resonant heating laser also served as theRaman pump laser for SERS studies of the heating of thenanoparticle surface. The microchamber could be evacuated forstudies in ambient air or "lled with Deionized (DI) water.Dark-"eld optical microspectroscopy24,25 was used to detect

changes in the individual nanoparticle LSPR scatteringspectrum upon resonant laser illumination (Figure 1c). Themeasured LSPR spectrum in air was found to peak at 563 nm.When the microchamber was "lled with DI water with atemperature of 20 °C, the LSPR was observed to redshift to598 nm. The redshift of 35 nm is in good agreement with theredshift obtained from a Mie calculation for the same particleembedded in air compared with that in water at 20 °C (" =1.778). During laser illumination while the nanoparticle wassurrounded by water, however, the LSPR was observed toblueshift to 581 nm. The observed shift implies a decreasedrefractive index of the medium directly surrounding thenanoparticle, which we interpret as resulting from a watervapor envelope surrounding the nanoparticle.The incident light intensity threshold for the blueshift of the

LSPR is approximately 2.5 ! 1010 W/m2, which is much largerthan the light intensities used in the nanoparticle-enabled solarsteam generation experiment. A likely reason for this di#erenceis that the presence of the substrate forces a nonspherical shape

for the nascent vapor bubble. The increased curvature of thevapor bubble near the nanoparticle/substrate interface shouldresult in substantially increased surface tension, making bubbleformation in this geometry far more di!cult than for anisolated nanoparticle suspended in solution. Other e#ects thatcould be present for nanoparticles suspended in solution, suchas multiple scattering and collective "eld enhancement e#ects,26

may also reduce the energy threshold for an isotropic geometryrelative to a substrate-bound structure.When illuminated by a CW laser, an Au nanoparticle is

heated due to nonradiative plasmon decay. For low incidentlaser power, heat from the nanoparticle is transferred into thesurrounding solution, resulting in a steady-state thermalgradient.6 At higher incident powers (25 mW and above), athin layer of surrounding water vaporizes and forms ananobubble which provides a thermal barrier, reducingsubsequent heat transfer into the surrounding solution. Thesize of the nanobubble is determined from a simple energybalance: the bubble expands until the power dissipated into theliquid becomes equal to the power absorbed by thenanoparticle from the incident laser light. The observed

Figure 1. Light-induced generation of a nanobubble at a singlenanoparticle surface. (a) Schematic showing illumination andplasmonic heating of an individual Au nanoparticle on the transparentglass substrate $oor of our $uidic microchamber. (b) Scanningelectron microscope (SEM) images of a sample with a dense coverageof Au nanoparticles randomly distributed on the glass substrate and atypical nanoparticle (inset). (c) Scattering intensities of a single 100nm diameter nanoparticle in air (black), water (blue), and an envelopeof water vapor produced by laser illumination (red). The blueshiftcorresponds to formation of a "nite vapor envelope of nanometer scalethickness around the nanoparticle. Spectra are shifted vertically forclarity. The red curve corresponds to a vapor envelope thickness of 6.5nm. (d) Dependence of nanobubble-induced LSPR blueshift on Aunanoparticle diameter, with a maximum shift observed for ananoparticle diameter of 100 nm. The error bars represent thenanoparticle-to-nanoparticle variation in the LSPR blueshift observedfor a set of several (<10) nanoparticles. The incident laser power at532 nm was 25 mW focused to a 1 !m diameter spot.

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blueshift of the LSPR is far too large to be consistent withmerely heating the surrounding liquid: for a gold nanoparticleof an equivalent size immersed in 100 °C water ambient, only a3 nm blueshift would be observed (Figure S1, SupportingInformation).The rate of plasmon-induced heat generation in a nano-

particle is proportional to its optical absorption cross sectionmultiplied by the incident optical intensity.3,4 Since theabsorption e!ciency (absorption cross-section divided by thenanoparticle cross-section #RNP

2) varies with nanoparticle size,the dependence of the blueshift on nanoparticle size was alsoinvestigated (Figure 1d). A maximum blueshift of 18 nm wasdetected for Au nanoparticles of 100 nm, decreasing quitestrongly for both smaller and larger nanoparticles. This trendclearly shows an optimal nanoparticle size for steam generationat 532 nm (see Figure S2, Supporting Information). For each

illuminated particle, the light-induced heating power is aproduct of the incident light intensity by the absorption cross-section. For the optimal nanoparticle size of nominally 100 nmdiameter, the laser induced heat generation in each particle is#5 ! 10$4 W. The reason for the reduction in blueshift forlarge nanoparticles is phase retardation with increasing size.The error bars re$ect variations in the resonance blueshiftmeasured for di#erent particles with the same nominal size,which we attribute to the size and shape variation of chemicallysynthesized Au nanoparticles in each size range. This variationsuggests a sensitivity of the steam nucleation threshold todetails of nanoparticle morphology.To directly measure the temperature at the nanoparticle

surface during the bubble formation, we performed SERS onindividual 100 nm diameter Au nanoparticles. The nano-particles were functionalized with a monolayer of para-

Figure 2. Temperature at the Au nanoparticle surface during the steam generation process. (a) Power dependence of the Au$S Stokes (black dots)and anti-Stokes (blue squares) mode amplitudes at |390| cm$1 for a pMA-coated single Au NP (100 nm diameter) as a function of incident laserpower. (b) Temperature obtained from the mode amplitudes in a and eq 1, as a function of incident pump laser power. Error bars in panels a and bindicate the uncertainty in the signal due to the low anti-Stokes signal levels in this range of incident laser intensities. (c) Time dependence of Stokes(black dots) and anti-Stokes (blue squares) signals with 25 mW laser excitation. A large intensity jump occurs after 60 s of laser excitation. (d)Temperature jump obtained from the mode amplitudes shown in c and eq 1 occurring after 60 s of 25 mW laser excitation. The 532 nm light wasfocused on a 1 !m diameter spot.

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mercaptoaniline (pMA, 0.3 nm thick), shown previously toyield a robust SERS signal from individual nanoparticles underresonant laser pumping.11 The SERS measurements wereperformed in the water-"lled microchamber.Optical vibrational pumping was observed, showing an

evolution of the Raman response of the system as a functionof incident laser power (Figure 2) (the Stokes and anti-StokesRaman spectra are shown in Figure S3). While several spectralpeaks were observed in the Stokes spectrum, our analysisfocused on the 390 cm$1 mode, which corresponds to the Au$S stretching mode associated directly with the chemical bondbetween the adsorbate molecules and the Au nanoparticlesurface. A quadratic dependence of the anti-Stokes intensity onincident power is observed, as expected in the optical pumpingregime (Figure 2b).27 A temperature was extracted from theratio of the Stokes and anti-Stokes intensities for this vibrationalmode using11,15,18

=+! !

!"##

$%&&

!"#

$%&

II

AE EE E

Ek T

expAS

S

0 ph

0 ph

4ph

B (1)

where IAS and IS are the anti-Stokes and Stokes Ramanintensities for the mode, E0 and Eph are the photon and phononenergies, respectively, and A is a correction factor that accountsfor the ratio of cross sections for Stokes and anti-Stokes Ramanscattering.At low incident laser power (1 mW), the temperature

obtained, assuming A = 1, was 289 K, corresponding withinexperimental error to the ambient temperature of the waterprior to laser illumination (290 K). We therefore assumed thatthe cross sections for Stokes and anti-Stokes scattering aresimilar and set A = 1 for our subsequent analysis, which isreasonable since both Stokes and anti-Stokes cross sections aredescribed by the same matrix element. Stokes and anti-Stokesspectra were obtained from the nanoparticle as incident laserpower was increased, and the integrated areas of the 390 cm$1

mode peaks were converted to e#ective temperatures using eq1 (Figure 2b). The temperature at the nanoparticle surfaceobtained in this manner was observed to increase smoothly andreversibly from 289 to 379 K (±20 K) with increasing laserpower. This reversibility implies that, within this temperaturerange, the temperature of nanoparticle surface responds directlyto the incident power, without any delay e#ects or hysteresisthat would indicate a phase transition such as the formation of avapor envelope around the nanoparticle, or a change innanoparticle morphology or surface chemistry.When the incident laser power was increased to 25 mW,

corresponding to the threshold for nanobubble formationinferred from the LSPR shift, the Raman spectra exhibited largechanges in intensity. Both the Stokes and anti-Stokes intensitiesfor the 390 cm$1 vibrational mode exhibit a dramatic jump after60 s of illumination at this power level (Figure 2c). From theStokes and anti-Stokes data and eq 1, we see that the observedspectral changes correspond to a rapid temperature increasefrom 400 to 465 K (±3 K) (in this regime of higher pumppower levels, the error bars are within the symbols used to plotthe data). This rapid temperature increase is consistent with theformation of a thermally insulating thin vapor layer around thenanoparticle. Once this initial layer is formed, the temperatureof the nanoparticle and its surrounding vapor will increase untila new steady state determined by an energy balance betweenthe incident power and the heat $ow across the vapor/waterinterface of the vapor envelope, is reached. The "nal steady-

state temperature of 465 K, deduced from the Raman spectra,corresponds to the nanoparticle temperature, since the pMA isbound to the Au surface. Because this temperature is measuredindependently from the LSPR experiment and is signi"cantlylower than the temperature onset of 647 K for spinodaldecomposition of water, we do not believe that the measuredLSPR shift is due to a local phase change of the water. TheLSPR probing depth around a nanosphere of 100 nm onlyreaches a few tens of nanometers beyond the nanoparticlesurface. Thus the measured LSPR shift cannot be explained bya large spinodal phase bubble with a weakly reduced refractiveindex but instead requires a substantial reduction of therefractive index in the immediate vicinity of the nanoparticlesurface. Both of these observations support our conclusion thata vapor layer surrounds the nanoparticles.A schematic of the system is shown in Figure 3a (for

simplicity, the substrate is neglected). When the surface

plasmon of the nanoparticle is excited by a laser, a part ofthe incident energy dissipates elastically into scattered photons,while the remainder results in heat. The calculated heat sourcedensity, that is, the generated heating power per unit volumefor a 100 nm diameter Au nanoparticle, immersed in water andilluminated by a linearly polarized plane wave with the incidentE "eld along the y axis, is shown in Figure 3b. The “hottest”regions are at the “north” and “south” poles where the plasmon

Figure 3. Nanobubble theoretical modeling. (a) Schematic ofnanobubble formation around a nanoparticle under 532 nm resonantlaser illumination. The nanobubble is a dielectric spacer layer of outerradius RB between the nanoparticle surface (with radius RNP) and thesurrounding water medium. (b) Simulated heat-source density for anilluminated 100 nm diameter Au nanoparticle immersed in water.Light is incident along the z axis and linearly polarized along y. (c)Near-"eld intensity enhancement for a 100 nm diameter Aunanoparticle in water, relative to incident "eld intensity, under thesame incidence and polarization conditions as in b. (d) Mie calculationof the scattering cross sections for a 100 nm diameter Au NP in air(black), surrounded by a steam bubble with outer radium RB = 60 nm(red), and in water (blue). The simulation for the steam layer is ingood agreement with the experimental observations in Figure 1c.

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induced local currents are maximal. This is quite distinct fromthe regions of the nanoparticle surface with the largestplasmon-induced "eld enhancements, which occur on thesides of the sphere where the plasmon induced charge density ismaximal (Figure 3c). Although the heat source density is notspherically uniform, the thermalization and heat di#usion in thegold particle are extremely fast relative to the heat di#usionthrough the steam and water, and thus we expect the metalnanoparticle to be e#ectively at a uniform temperature.Since the observed LSPR blueshift of the nanoparticle is

smaller than that for an in"nite embedding medium of watervapor, we can use this information to estimate the bubble radiusRB. Within an electrostatic dipole model,28,29 the redshift of thedipolar surface plasmon resonance is proportional to therelative permittivity change:

!!

""

" "" "

"" = "

" # !!

B

w

B(eff)

w

B(eff)

air

w air (2)

where !$B is the experimental redshift of the plasmonresonance for a nanoparticle in a "nite nanobubble, !$w isthe redshift of the plasmon resonance in water relative to itsvalue for steam, !"B(eff) is the permittivity increase for a "nitenanobubble relative to steam, and !"w is the permittivityincrease for water relative to steam. The superscript “e#”denotes the e#ective permittivity change experienced by thenanosphere. Assuming the geometry depicted in Figure 3a, thevalues of " = 1.0 for steam (veri"ed below), and " = 1.77 forwater at room temperature, we performed Mie calculations30,31

to obtain theoretical scattering spectra for di#erent values of thebubble radius. From comparison of the measured value of !$B/!$w = 0.51 (Figure 1c) to the calculated spectra, we deducethat the thickness of the vapor shell around a single Aunanoparticle is nominally 6.5 nm (RB = 56.5 nm) (seeSupporting Information, Figure S4). The LSPR measurementprovides strong and consistent evidence that a nanosize bubbleis generated. The spectral shift, calculated from Mie theory,agrees with the experimental scattering spectra using theconcentric steam-water shell model where RB = 56.5 nm(Figure 3d).The conventional model for plasmon induced bubble

formation has recently been challenged.1 In the conventionalmodel, the temperature increase at the particle surface iscalculated as !T = %I/(4#kRNP), where % is the particleabsorption cross section, I is the incident intensity, and k is thethermal conductivity of embedding material.26 By consideringthe presence of the glass substrate and using the thermalconductivity (#1.6 W m$1 K$1) of glass around 500 K, thismodel leads to an estimated particle temperature of around 774K. If the thermal conductivity of water is used, an even highertemperature results, in direct con$ict with the observed 465 Kfrom our Raman experiment. We notice that the calculation oftemperature in this model is based on energy dissipation by atotal conductive heat transfer process. However, it is well-known that other heat transfer processes such as convectioncan also increase the e#ective thermal conductivity. Thereforewe speculate that the discrepancy between our measuredtemperature and the result from the conventional model is dueto the neglect of $uid convection. The conventional modelassumes a stationary $uid and only conductive heat transfer.Once the bubble is formed, there is an alternative approach

for determination of the temperature by using the equation ofstate for saturated steam: Clausius$Clapeyron relation (CCR).

The CCR approach does not depend on the balance betweenthe heating power and the dissipated power and thus isindependent of the complexity of combinations of various heattransfer channels (through water, glass, convection, etc.).Instead, the CCR approach depends on only one unknownparameter: the bubble size which has been directly determinedfrom our LSPR experiments. With the nanobubble radiusdetermined, the temperature and pressure of the con"nedsteam within the nanobubble can be estimated as follows. Weassume that the system reaches steady state after bubbleformation, as suggested in Figure 2c and d. At equilibrium, theLaplace pressure due to the surface tension at the water-steaminterface is balanced by the vapor pressure of the steam, whichis a function of temperature given by the CCR. Hence, we haveC exp(h/kBTs) = p0 + 2&(Ts)/RB, where C is a constant, h is theenthalpy of water evaporation, Ts is the temperature ofsaturated steam inside the bubble, p0 is the atmosphericpressure (1 atm), and & is the surface tension coe!cient (seeSupporting Information, Text S1). The calculated steamtemperature Ts within the nanobubble is 464 K, with acorresponding internal pressure of 1.49 MPa (14.7 atm). Thislarge pressure is essentially a consequence of the large surfacetension for a bubble of nanoscale dimensions. At this pressureand temperature, the dielectric permittivity of steam is close to1.0, consistent with the assumption made in the Mie calculationof steam layer thickness.The temperature of the nanoparticle can also be calculated

by applying Fourier’s law at the gold$vapor interface: Pabs =Gg,sSNP(TNP $ Ts), where Pabs is the power absorbed by the AuNP, Gg,s is the e#ective interfacial thermal conductance, SNP isthe surface area of the nanoparticle, and TNP is the NP surfacetemperature. The temperature di#erence (TNP $ Ts) isestimated to be approximately 72 K from a gold-steaminterfacial thermal conductance of 275 MW/(m2 K) (seeSupporting Information, Text S1, for a detailed calculation),resulting in an estimated nanoparticle surface temperature ofTNP = 536 K. The estimated average temperature of themolecular layer at the gold$vapor interface then was found tobe 500 K, which is a bit higher but close to the 465 K obtainedfrom our SERS measurements. This di#erence is most likelydue to our neglect of the dielectric substrate, which serves as aheat sink as compared with water. It is worth noting that theanalysis of the interfacial thermal conductance (SupportingInformation) is an essential step for "nal determination of thesurface temperature of the gold nanosphere from the steamtemperature calculated in our approach. In fact, there are twocompeting cooling rates: one controlled by the e#usivity of the$uid and one determined by the interface e#ect as discussed byGe et al.32 Such a competition is very important for transienttemporal evolution of temperature, i.e., in a pulsed-laserstudy.32 However, it is not relevant here because we use CWlaser illumination, and the system is in steady state after thebubble is formed. In steady state, the heating power is balancedby the power dissipated through $uid e#usivity and heattransfer across the interfaces. There is no need to consider thecompetition between these two mechanisms since in a steadystate we are not interested in the cooling rates but thetemperature di#erence across the interface.To study the transition from discrete nanobubbles localized

on their individual, parent nanoparticles to optically observable,micrometer sized bubbles, the incident light intensity needs tobe increased above the 25 mW threshold and/or thenanoparticle density on the substrate needs to be increased.

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A time series of images under these new conditions, resulting inthe formation of a microbubble, is shown in Figure 4a. After

illumination (t = 10.6 s), a 1 !m bright spot (bubble) appearsin the image, and after t = 13.0 s it has expanded to amicrobubble of 10 !m diameter, with a volume encompassingapproximately 5000 nanoparticles. We believe that the origin ofthe observed microbubbles is the coalescence of individualnanobubbles centered on adjacent nanoparticles, as illustratedin Figure 4b. With increasing light intensity, the radii of thenanobubbles increase. If the interparticle spacing is su!cientlysmall, nanobubbles centered on adjacent nanoparticles willtouch and coalesce, resulting in the formation of a largermicrobubble encompassing several nanoparticles. Similarattraction-and-coalescence has also been observed for micro-bubbles.The "nal size of the microbubble is a function of both

incident power and nanoparticle separation. This is shown inthe contour plot in Figure 4c, where the color map represents"nal bubble size as a function of these two parameters (theincident laser spot size is 5 !m). This contour plot is a result ofan extended series of experimental measurements for multipleinterparticle distances and input laser powers (SupportingInformation, Table 1). Generally speaking, higher incidentpowers (%400 mW) and shorter interparticle separations (&0.4!m) result in a larger bubble size. We observe a strongsensitivity to interparticle distance: for large interparticleseparations, only small microbubbles can be observed, even atthe highest laser powers. This re$ects the fact that the totalenergy absorbed within the laser spot of 5 !m is due to the total

number of nanoparticles within the beam spot, a number whichgrows as the inverse square of the interparticle separation. In anisotropic environment we would anticipate that, for a speci"cbubble radius (single-color contours of Figure 4c), the inputpower would scale with the inverse square of the interparticledistance. The data measured here deviates from that simplescaling, most likely due to the additional loss introduced by thenanoparticle substrate.In conclusion, we have examined the phenomenon of light-

induced vapor generation at a plasmonic nanoparticle surface,from the initial formation of a nanoscale vapor envelope aroundan individual nanoparticle to the formation of micrometer-scalebubbles due to coalescence of the expanded vapor envelopes.Our single-particle studies of vapor generation enable a detailedcharacterization of the size, pressure, and temperature of thenascent nanobubble formed at a nanoparticle surface. Thecombination of quantitative measurements at the singlenanoparticle level and rigorous analysis provides us with the"rst detailed picture of the vapor generation process at its initialstages. Our approaches for characterizing vapor formationshould prove to be useful in the study of other local energeticprocesses, such as catalysis and local chemical or material-growth reactions.

Methods. Sample Preparation. Au nanoparticles withdi#ering diameters ranging from 30 to 200 nm (NanoXact,Ted Pella, Inc.) were randomly dispersed and immobilized onthe microchamber bottom surface, which was functionalizedwith poly(4-vinyl pyridine)(PVP). For single nanoparticledetection, a transmission electron microscope (TEM) samplegrid was used to fabricate "duciary marks by e-beamevaporation. To prevent the in$uence from other Aunanoparticles, the selected nanoparticle was the only onefound within the 10 !m ! 10 !m sampling area. Themicrochamber was fabricated using 3 M tape with a diameter of1 cm and thickness of 80 !m. The chamber was sealed by a thinglass cover slide. The temperature of the microchamber and ofthe injected water was kept constant at 285 K. A thermographwas used to monitor any temperature change during theexperimental measurements.

Experimental Setup and Measurements. The sample wasplaced on a piezoelectric positioning stage (Nanonics Co.) heldby a xyz coarse adjustment stage (Newport Co.). A longworking distance (12 mm) objective lens (Mitutoyo NIR100X) with a numerical aperture 0.5 was used to focus a 532CW laser beam onto the Au NP at normal incidence. The focusdiameter was measured by moving a razor edge with thepiezoelectric positioning stage, assuming a Gaussian beampro"le. A white light source (MI-150, Edmond Optics) wasused to illuminate the sample at oblique incidence. The dark-"eld scattering signals passed through the same objective lensand were detected by a monochromator (Acton SP2150, PI)with a CCD camera (PIXIS400, PI). For the Ramanmeasurements, a monolayer of p-mercaptoaniline (pMA)(#0.3 nm) was bound to the Au nanoparticle surface, andSERS measurements were performed with a microchamber"lled with pure water. The signal was recorded by a Ramanspectrometer (Acton SpectraPro2300i, PI) along the sameexperimental optical path. Microbubble generation was imagedusing a CCD camera.

Figure 4. Coalescence of nanobubbles into micrometer-sized bubbles.(a) Time series of microbubble generation. The nanoparticle arealdensity is 3 ! 108 cm$2 (average NP$NP separation #0.6 !m). (b)Schematics depicting microbubble formation due to the coalescence ofneighboring nanobubbles produced on individual Au nanoparticles.(c) Experimental diagram of the dependence of "nal bubble size andformation time on incident laser power and average nanoparticleseparation, where the color indicates "nal bubble diameter (scale baron right). The 532 nm light was focused on a 5 !m diameter spot.

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! ASSOCIATED CONTENT*S Supporting InformationFigure S1: The surface plasmon resonance spectrum of a 100nm nanoparticle in liquid water at 20 and 100 °C; Figure S2:Calculated Mie absorption e!ciency for di#erent size Aunanoparticles; Figure S3: Raman spectra for Figure 2a and c inthe main text; Figure S4: Calculation of nanobubble thickness;Text S1: Calculation of temperature inside a nanobubble; TableS1: Experimental data for Figure 4c in main text; Movie S1:Microbubble coalescence. This material is available free ofcharge via the Internet at http://pubs.acs.org.

! AUTHOR INFORMATIONCorresponding Author*E-mail: zhyfang@rice.edu; nordland@rice.edu; halas@rice.edu.Author ContributionsZ.F. and Y.R.-Z. contributed equally.NotesThe authors declare no competing "nancial interest.

! ACKNOWLEDGMENTSWe thank the following agencies for their support of thisresearch: The Robert A. Welch Foundation (C-1220, NH) and(C-1222, PN), The Air Force O!ce of Scienti"c Research(FA9550-10-1-0469, NH), the DoD NSSEFF (N00244-09-1-0067, NH and PN), the NIH NCI (U01 CA151886-01, NHand PN), and the Bill and Melinda Gates Foundation(OPP1044571, NH). Work at AMOLF is part of the researchprogram of FOM and is also supported by the EuropeanResearch Council.

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Nano Letters Letter

dx.doi.org/10.1021/nl4003238 | Nano Lett. 2013, 13, 1736$17421742

1

Supplementary Information

Evolution of light-­induced vapor generation at a

liquid-­immersed metallic nanoparticle

Zheyu Fang1,5 *, Yu-­Rong Zhen2 , Oara Neumann 1,2, Albert Polman3, F. Javier García de Abajo 4,

Peter Nordlander1,2, and Naomi J. Halas1,2*

Department of Electrical and Computer Engineering, 2Department of Physics and Astronomy, and

Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, USA

Center for Nanophotonics, FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The

Netherlands

IQFR -­ CSIC, Serrano 119, 28006 Madrid, Spain

School of Physics, State Key Lab for Mesoscopic Physics, Peking University, Beijing 100871, China

Contents

F igure S1. The Surface plasmon resonance spectrum of a 100 nm nanoparticle in

liquid water at 20 and 100 degrees Celsius

F igure S2. Calculated Mie absorption efficiency for different size Au nanoparticles

F igure S3. Raman spectra for Figure 2a and 2c in the main text.

F igure S4. Calculation of nanobubble thickness

T ext S1. Calculation of temperature inside a nanobubble

Table S1. Experimental data for Figure 4c in main text

Movie S1. Microbubble coalescence

2

F igure S1. The Surface plasmon resonance spectrum of a 100 nm nanoparticle in

liquid water at 20 and 100 degrees Celsius

F I G UR E S1. Simulated Mie scattering cross-section for an Au NP (100 nm diameter) immersed in

20 and 100 ºC water. The resonance peak of the NP in 100 ºC water has a ~3 nm blue-shift

compared with the one in 20 ºC water.

The permittivity of water at 20 ºC and 100 ºC is 1.7778 and 1.7387 S1, respectively, thus

11

)293(

)373(

)293(

)373(

Kwater

Kwater

K

K

can be calculated as 0.95, which means there is only a 5% difference (2.5 nm)

between the scattering redshift with respect to air ambient (peak at 530 nm) and peak wavelengths in

water for 20 and 100 ºC.

3

F igure S2. Calculated Mie absorption efficiency for different size Au

nanoparticles.

F I G UR E S2. Calculated Mie absorption efficiency (absorption cross-section divided by the

cross-section area 2NPR ) for Au nanoparticles in water with diameters varying from 30 nm to 200

nm, at the laser excitation wavelength of 532 nm. The squares indicate calculated absorption

efficiencies for the nanoparticle diameters at which the measurements were performed (see Fig. 1d).

The main reason for decreasing absorption efficiency for Au NP with diameter larger than ~70 nm is

that the resonance peak of the NP redshifts with increasing radius, such that the excitation

wavelength of 532 nm is no longer on resonance. In addition, with larger size, the scattering

becomes more pronounced relative to absorption.

4

F igure S3. Raman spectra for F igure 2a and 2c in the main text.

F I G UR E S3. Top: Raman response of a pMA-­coated single Au NP (in CCD counts) as a function

of Raman shift (x-­axis) under different incident laser powers. Bottom: Raman spectra at 25 mW

incident power for different excitation times. Each characteristic Stokes spectrum exhibits strong

optical pumping of the Au-­S vibrational mode at 390 cm-­1.

5

F igure S4. Determination of nanobubble thickness.

F I G UR E S4. Ratio of wavelength redshift for a gold nanosphere with a diameter of 100 nm

embedded in a vapour bubble to that in water, B/ w, calculated using Mie scattering theory. The

thickness of the nanobubble layer surrounding the Au nanoparticle responsible for the observed

blueshift is deduced to be 6.5 nm (RB=56.5 nm, as indicated by the arrow) by direct comparison of

the calculated B/ w with the experimental value of 0.51 (indicated by the dashed line). We use )373( K

water =1.74S1 in the calculation for the Au nanosphere in a vapour bubble to account for the

surrounding hot (373K) water when a steam bubble is present;; )293( Kwater =1.78S1 was used for the Au

nanosphere embedded in cold water;; a i r=1.00 was used for the Au nanosphere embedded in air.

The empirical (Johnson and Christy) dielectric functionS2 for Au was used in all Mie calculations.

T E X T S1. Calculation of temperature inside a nanobubble

a) Steam temperature To determine the temperature inside the nanobubble, we assume that the steam generated inside the

bubble is saturated. The pressure of the saturated steam is determined by the vapor pressure of water

6

as a function of temperature. We assume that, once formed, the nanobubble is in steady-­state and the

steam pressure is balanced by the Laplace pressure due to surface tension at the water-­steam

interface. The Clausius-­Clapeyron Relation (CCR) is used to represent the relation between the

vapor pressure of water and the temperature. We therefore have

BssB

RTpTk

hC /)(2)exp( 0 , (S8)

where the left side is the CCR and the right side is the Laplace pressure, Ts is the absolute

temperature (in Kelvin) of saturated steam inside the bubble, 0p is the atmosphere pressure (set to 1

atm), Bk is Boltzmann constant, h is the enthalpy of water evaporation per water molecule, and

is the surface tension coefficient, which is a function of temperature ( 2.1422222.0 TT

mN/m S3). The parameters in the CCR for water are C=95GPa and h=7082×10-­20J. With RB=56.5

nm, Ts is solved from Eq. (S8) to be 464K with a corresponding steam pressure of 1.49MPa. The

refractive index of steam at the state of a temperature of 464K and a pressure of 1.49MPa is

calculated to be n=1.002 for the wavelength of laser excitation 532 nm, using Eq. A1 in Ref. S1 as

well as the reference therein. This result is consistent with our previous assumption that the refractive

index of steam is set to 1, which was used in our determination of the vapour shell thickness.

b) NP surface temperature When the system is in steady state, the law of energy conservation must be observed, which means

the absorbed power must be equal to the dissipated power for the heat

transfer at the Au-­steam interface, we have

sNPNPsgabs TTSGP , , (S9)

where a bsP is the optical input power absorbed by the Au NP, sgG , is the effective thermal

boundary conductance at the Au-­steam interface, 24 NPNP RS is the surface area of the

nanoparticle, and N PT is the surface temperature of the nanoparticle. Similarly, at the steam-­water

interface, we have

wsBwsabs TTSGP , , (S10)

where wsG , is the effective thermal boundary conductance at the steam-­water interface, 24 BB RS

is the outer surface area of the nanobubble, and wT is the temperature of the surrounding hot water,

assumed to be 373K.

Noting that APP i nca bs , where is the effective absorption cross-­section of the Au

nanoparticle after nanobubble formation, i n cP is the incident power of the laser, and 4/2DA is

7

the laser spot area with a diameter D = 1 µm, we obtain a bsP = 0.62 mW for a single Au NP with 100

nm diameter, given that i n cP' = 24 mW (a 25mW input power multiplied by 96% transmission of the

glass cover slide), and = 0.0202 µm2 as calculated using Mie scattering theory.

Measuring the thermal boundary conductance/resistance is a topic of fundamental importanceS4.

By inserting a bsP = 0.62 mW, RB = 56.5 nm, Ts = 464 K, and Tw = 373 K into Eq. (S10), we

obtain wsG , , the effective thermal boundary conductance, to be 170 MW/(m2K). Interestingly, this

conductance is found to be quite comparable to the reported nanoscale thermal conductance of the

Au-­water interfaceS5 ( wgG , =105 MW/(m2K)) and the platinum-­water interfaceS6 (130 MW/(m2K)).

Since our effective thermal conductance at the steam-­water interface is on the same order as that at

the metal-­water interface, we speculate that such a high thermal conductance may be induced by the

condensation-­evaporation balance at the interface between the saturated steam and the surrounding

water. While an unsaturated steam layer is a thermal insulator, this saturated steam layer may be

perceived as a good thermal conductor. Although acoustic mismatch and phonon scattering are often

used to account for interfacial thermal behavior, a microscopic nanoscale heat transfer theory

involving interfacial phase-­transition dynamics would be required to explain such a high value

of wsG , , comparable to its metal-­water counterpart.

The effective interfacial thermal conductance at the Au-­steam interface can be estimated by the

following two methods. First, since solids usually have well-­defined densities of states of phonons

relative to liquids, we can suspect that phonon coupling at the Au-­steam interface is better than at the

steam-­water interface;; therefore a moderate enhancement wssg GG ,, 2 = 340 MW/(m2K) can be

expected. On the other hand, based on the hypothesis that the interfacial heat transfer in this system

is a series of processes analogous to serial resistances in an electrical circuit, we can write

yxyx GGG /1/1/1 , , where xG and yG can be any combination of sG , wG , gG , accounting

for the bulk (nanoscale) contributions to the interfacial conductance from steam, water, and Au. If

we consider that saturated steam possesses ideal thermal conducting and interfacial coupling

properties, sG can be assumed to be much larger than the other two conductances. Therefore

wsw GG , = 170 MW/(m2K) results, leading to sgg GG , = 275 MW/(m

2K). Both conductances

estimated by the above two methods are similar, however, we choose the lesser value sgG , = 275

MW/(m2K). As a result, for NPR = 50 nm, a bsP = 0.62 mW, and Ts = 464 K, and we obtain TNP = 536

K from Eq. (S9).

Nevertheless, because the temperature obtained in the SERS experiment is for an introduced

pMA layer coated on the NP surface, even with Ts and TNP determined, it is still challenging to

8

estimate the temperature theoretically, due to a lack of detailed knowledge of the thermal coupling

properties of the Au/water-­molecule and water-­molecule/steam interfaces. We assume that the

perturbation of the molecular layer to the entire temperature field is negligible, hence an average

temperature for the molecular layer is calculated to be (TNP+ Ts)/2=500 K, not far from the

experimental temperature (465 K) determined from our SERS measurements.

Table S1. Experimental data for F igure 4c in the main text.

NP distance ( m)

Power (mW)

0.05 0.1 0.2 0.4 0.8 1.6

1000 (96,0) (80,0) (55,0) (35,0.1) (8,32) (0, ∞)

900 (92,0) (74,0) (52,0) (33,0.1) (5,45) (0, ∞)

800 (87,0) (69,0) (48,0) (31,0.5) (3,60) (0, ∞)

700 (82,0) (62,0) (45,0.1) (28.5,1.2) (1, ∞) (0, ∞)

600 (75,0) (57,0) (42,0.5) (26,2.5) (1, ∞) (0, ∞)

500 (70,0.1) (55,0.3) (38,1) (22.5,4.1) (0, ∞) (0, ∞)

400 (64,0.5) (50,0.5) (34,2) (21,6.5) (0, ∞) (0, ∞)

300 (53,1) (42,1) (28,3.5) (18.5,8.2) (0, ∞) (0, ∞)

200 (42,1.3) (35,2.5) (25,6.8) (14.7,11.9) (0, ∞) (0, ∞)

100 (30,3.2) (23,6.4) (16,9) (10.2,15.3) (0, ∞) (0, ∞)

Table 1. Experimental data of final bubble size and bubble start-­up time depending on laser power and average NP surface-­to-­surface separation. The data format is (x, y), such that the generated bubble is of diameter x m, with start-­up time y sec.

By interpolating the experimental data in Table 1, we present in Fig. 4c a color map of the final bubble diameter, as well as contour lines of the start-­up time. Three regions can be clearly identified within the color map, as delimited by the assisting contour lines.

9

M O V I E S1. Microbubble coalescence

This movie shows the coalescence of microbubbles resulting in a larger microbubble. The laser has

an incident power of 150 mW.

References

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S2 Johnson, P. B. & Christy, R. W. Optical-­Constants of Noble-­Metals. Phys Rev B 6, 4370-­4379 (1972).

S3 The surface tension of the liquid water-­water vapor interface at saturation pressure (source: NIST) http://chemweb.calpoly.edu/jhagen/446%20handout%204%20water%20surface%20tension.pdf

S4 Swartz, E. T. & Pohl, R. O. Thermal-­Boundary Resistance. Rev Mod Phys 61, 605-­668 (1989).

S5 Plech, A., Kotaidis, V., Gresillon, S., Dahmen, C. & von Plessen, G. Laser-­induced heating and melting of gold nanoparticles studied by time-­resolved x-­ray scattering. Phys Rev B 70, 195423 (2004).

S6 Wilson, O. M., Hu, X. Y., Cahill, D. G. & Braun, P. V. Colloidal metal particles as probes of nanoscale thermal transport in fluids. Phys Rev B 66, 224301 (2002).