Acoustic Vibrations in Bimetallic Au@Pd Core ... - ACS Publications

Letter pubs.acs.org/JPCL

Acoustic Vibrations in Bimetallic Au@Pd Core−Shell Nanorods M. Fernanda Cardinal,†,‡ Denis Mongin,§ Aurélien Crut,*,§ Paolo Maioli,§ Benito Rodríguez-González,† Jorge Pérez-Juste,† Luis M. Liz-Marzán,† Natalia Del Fatti,§ and Fabrice Vallée§ †

Departamento de Quimica Fisica, Universidade de Vigo, 36310 Vigo, Spain International Iberian Nanotechnology Laboratory, Braga, 4710229, Portugal § FemtoNanoOptics Group, Université Lyon 1, CNRS, LASIM, 43 Bd du 11 Novembre, 69622 Villeurbanne, France ‡

S Supporting Information *

ABSTRACT: The acoustic vibrations of gold nanorods coated with palladium were investigated as a function of Pd amount using ultrafast pump−probe spectroscopy. Both the extensional and breathing vibrational modes of the nanorods were coherently excited and detected. This permits precise determination of their periods, which were found to decrease and increase with Pd deposition, for the extensional and vibrational modes, respectively. These opposite behaviors reflect changes of the nanoparticle size and mechanical properties, in agreement with numerical simulations. Comparison of experimental and computed periods yields information on the amount of deposited Pd, providing a novel tool to characterize bicomponent nano-objects for small fractions of one of the components (Pd/Au atomic fraction down to 5%).

SECTION: Nanoparticles and Nanostructures

T

bimetallic spherical objects whose time domain response is strongly dominated by their fundamental breathing mode,5,6,10,20,21 two modes, the extensional and breathing modes, are coherently excited and detected, as in pure nanorods.2,22 As they correspond to different mechanical movements (mostly along the rod long and short axes, respectively), their periods are differently affected by Pd deposition on the gold nanorods, that is, by the concomitant changes of nanorod size and mechanical properties. Comparison with numerical simulations permits us to analyze the contributions of these effects and to obtain information on the amount of deposited palladium and on its location, even for a very small amount of Pd (a few % in volume). Pd-coated Au nanorods (Au@Pd-NRs) were prepared from the same initial colloidal solution of gold nanorods (Au-NRs) with an average aspect ratio η of 3.5 (sample a, Table 1). These Au-NRs were synthesized through the seeded growth method23 by reduction of HAuCl4 with ascorbic acid (AA) in the presence of hexadecyltrimethylammonium bromide (CTAB) and AgNO3 (additional information about the chemicals used in the synthesis can be found in the Supporting Information). The as-prepared Au-NR solution was washed and concentrated by centrifugation to a final 2.2 mM gold concentration, estimated from the absorbance at 400 nm. Subsequently, the Au-NRs were coated with palladium in an aqueous solution containing CTAB, Na2PdCl4, and AA.24 Five different growth

he enhanced role of interfaces and the concomitant increased impact of material interfacing are key effects in the physical and chemical properties of nano-objects. As their optical response,1 the characteristics of their low-frequency acoustic vibrations do not only depend on their morphology2−5 but also on their interface conditions (e.g., matrix, surfacedeposited material, or bound molecules).6−10 This acoustic signature has been used in the past to obtain information about the coupling of nano-objects made of a single material with their surrounding environment6,10−13 or to design mechanical nanosensors that detect the frequency shift of a nano-object vibration mode due to mass deposition at its surface.14,15 This nanobalance concept can also be used to analyze the nano-objects themselves when they are formed by two or more materials but has only been scarcely exploited in this context.9,16,17 In nanoparticles made from materials of different nature (e.g., metal and dielectric), vibration of one component dominates the observed vibrational response. Its analysis as a function of the particle composition yields information on the mechanical contact of the material components within the particles, as recently shown in core−shell nanospheres.9,18,19 In contrast, in nanoparticles formed by materials of similar nature, such as bimetallic ones, the acoustic response of the full object is detected16,17 and is thus expected to contain information on the characteristics of the composite nanoparticles, that is, their size, composition, and material distribution. Using time-resolved pump−probe spectroscopy, we have investigated the acoustic modes of bimetallic core−shell nanorods comprised of gold nanorod cores surrounded by different amounts of palladium (Au@Pd nanorods). Conversely to © 2012 American Chemical Society

Received: January 26, 2012 Accepted: February 14, 2012 Published: February 14, 2012 613

dx.doi.org/10.1021/jz3000992 | J. Phys. Chem. Lett. 2012, 3, 613−619

The Journal of Physical Chemistry Letters

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Table 1. Pd2+/Au0 Ratios Used in the Synthesis Solution of the Different Au@Pd Samples and TEM Statistics (average and standard deviation) for Nanorod Length (L), Width (W) and Aspect Ratio (η = L/W)a samples

[Pd+2]/[Au0]

Au-NRs a Au@Pd-NRs b c d e f

0 0.11 0.22 0.44 0.89 1.78

length L (nm) 33.7 32.6 33.4 34.9 33.7 34.2

± ± ± ± ± ±

2.4 2.8 2.8 2.9 3.2 3.6

width W (nm) 9.7 9.5 9.8 10.6 11.1 13.4

± ± ± ± ± ±

aspect ratio η = L/W

0.7 0.9 0.8 0.9 1.1 1.9

3.5 3.5 3.4 3.3 3.0 2.5

± ± ± ± ± ±

0.3 0.4 0.4 0.4 0.4 0.4

estimated VPd/VAu 0 0.05 0.11 0.21 0.43 0.86

± ± ± ± ±

0.04 0.07 0.15 0.30 0.60

estimated Pd thickness e (nm) 0 0.11 0.23 0.44 0.85 1.55

± ± ± ± ±

0.08 0.15 0.29 0.53 0.92

a

The volume fraction of the palladium shell estimated from TEM characterization of sample f (see main text) and its corresponding thickness e under the assumption of uniform Pd coverage (Figure 4c) are also given.

solutions were prepared with [Pd2+]/[Au0] molar ratios of 0.11, 0.22, 0.44, 0.89, and 1.78, referred to as samples b−f in Table 1. To each growth solution containing 0.09 mM Au0 in 15 mM CTAB, a certain amount of Na2PdCl4 was added (according to the [Pd2+]/[Au0] molar ratio) together with AA, satisfying the [AA]/[Pd2+] = 25 molar ratio. After 30 min of reaction at 27 °C, the samples were centrifuged and redispersed in water twice before further characterization. Transmission electron microscopy (TEM) images (Figure 1b) show that core−shell

difficult to exploit for samples b−e because of their small Pd fractions and the uncertainties affecting determination of average sizes (assuming uniform coating, the Pd thickness layer is estimated to be between 0.1 and 1.5 nm for the investigated Au@Pd-NRs, Table 1). Therefore, we first compared TEM size statistics of samples a (bare Au-NRs) and f (Au@Pd-NRs with thicker Pd shell) to estimate the Pd volume deposited in the latter case, assuming cylindrical rod sections. We subsequently estimated the deposited Pd volume for samples b−e, assuming that it was proportional to the concentration of Pd in the solution used for the growth reaction (Table 1). To confirm metal segregation within the nanoparticles as well as their estimated atomic percentages of Pd and Au, elemental mapping was performed using scanning TEM and X-ray energy dispersive spectroscopy (STEM-XEDS). Images obtained in STEM mode using a high angle annular dark field (HAADF) detector show distinct areas with different mass−thickness contrast. They indicate that Pd indeed grows around the Au-NR cores, rather than forming an alloy (Figure S1 of the Supporting Information for samples d and f). This is in agreement with STEM-XEDS elemental maps, also showing Au−Pd segregation, with Pd on the Au-NR surfaces (Figure S1, Supporting Information). The average Pd/Au atomic fractions estimated using XEDS analysis of several single particles were (0.34 ± 0.12) for sample d and (0.91 ± 0.41) for sample f. These values are consistent with those obtained from the volumes deduced with the TEM-based method (Table 1), which correspond to Pd/Au atomic fractions of (0.24 ± 0.10) and (0.99 ± 0.42) for samples d and f, respectively. Further characterization was carried out with high-resolution TEM (HRTEM) to analyze the crystalline structure of the nanoparticles, which can impact their vibrational response.3,22 Given the similarity between the face-centered cubic (fcc) lattice parameters of Au and Pd (4.08 and 3.89 Å, respectively),25 palladium is expected to grow epitaxially on the gold surface, as previously reported.24 This was confirmed in our HRTEM studies, which show that most of the Au@Pd-NRs are indeed single crystals. This is illustrated in Figure S2 of the Supporting Information for particles prepared with [Pd2+]/[Au0] = 1.78 (sample f) through the corresponding fast Fourier transform (FFT) showing a spot pattern characteristic of a fcc single crystal oriented on the [100] zone axis. The presence of areas with clear contrast difference in the HRTEM images (Figure S2, Supporting Information) also confirms that Pd grows on top of the Au-NR core. The optical extinction spectra of the different samples a−f are shown in Figure 1a. The longitudinal localized surface plasmon resonance (LSPR) was measured at about 750 nm for bare Au-NRs, as expected from their mean aspect ratio.26 Pd deposition was found to shift and strongly broaden the LSPR

Figure 1. (a) Extinction spectra of the initial Au-NR solution (sample a) and of the Au@Pd ones synthesized with increasing [Pd2+]/[Au0] molar ratios (samples b−f; see Table 1). (b) Representative TEM images corresponding to samples a (left), c (middle), and e (right) (scale bars: 50 nm).

Au@Pd-NRs with different Pd amounts were obtained. For small Pd fractions, the particles keep the regular rod shape of bare Au-NRs (samples b−d, Table 1). A rod-like morphology is still obtained for larger amounts of Pd (samples e and f) but with more irregular shapes. The mean deposited Pd volumes in each sample can a priori be estimated from the nanoparticle size statistics (Table 1) based on TEM images (Figure 1b). However, this approach is 614



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band (samples b−f). These effects are associated with modification of both the shape and the dielectric characteristics of the particle upon Pd deposition and are consistent with previous investigations in Au@Pt nanorods.27 In particular, LSPR broadening is due to optical absorption in the Pd layer, that is, to the large imaginary part of the Pd dielectric function in this spectral range as compared to that of gold.28 Consequently, the LSPR peak is almost washed out for the sample with the highest Pd concentration (sample f), which presents a featureless optical response similar to that of palladium nanoparticles.29 The acoustic vibrations of Au@Pd-NRs were investigated using time-resolved pump−probe spectroscopy. In this technique, a first (pump) pulse excites the nanoparticles, while a second time-delayed (probe) pulse follows the relative change of the sample optical transmission ΔT/T.30 The femtosecond pump and probe pulses were generated using an amplified Ti:Sapphire laser delivering 150 fs pulses at 800 nm with a repetition rate of 250 kHz. Part of the pulse train is frequencydoubled in a 200 μm thick beta barium borate (BBO) crystal to create the pump beam at 400 nm. The other part is used to generate the wavelength-tunable (in the 480−700 nm range) probe pulse using optical parametric amplification (OPA) of part of a white light supercontinuum created in a sapphire plate. To test the possible impact of the probe wavelength, λpr, on the experimental data, additional measurements were performed using a homemade Ti:Sapphire oscillator delivering ∼20 fs pulses at 860 nm with a repetition rate of 76 MHz. Nearinfrared probe pulses at the fundamental wavelength were then used, selecting part of the output pulse train, the other part being frequency-doubled to generate 430 nm pump pulses. Both femtosecond sources were associated with the same pump− probe setup, where the pump−probe time delay is controlled by a translation stage on the probe beam optical path. High sensitivity ΔT/T measurements were achieved using mechanical chopping of the pump beam at 40 kHz and synchronous differential detection of the probe transmission. Time-dependent ΔT/T signals measured in bare (sample a) and Pd-coated (sample e) Au-NR solutions are shown in Figure 2. For pump pulses at 400 nm, electronic interband absorption takes place in both the Au and Pd parts of the nanoparticles, driving the electrons out of equilibrium in both components. Electron−electron scattering and electron diffusion redistribute the energy, leading to a thermalized hot electron distribution within a few hundred femtoseconds,30 a duration much shorter than the time scale investigated here. This electron heating is reflected in a change of the sample optical transmission, which is monitored by the probe pulse at λpr = 700 nm, close to the LSPR of bare Au-NRs (Figure 1). This is reflected in both mono- and bimetallic systems by a fast transient in the ΔT/T signal, which decays as electrons cool down by energy transfer to the particle ionic lattice. In bare Au-NRs, this fast (few picoseconds) response is consistent with that previously reported for these systems.31 It is mostly due to the LSPR red shift induced by electron heating, which dominates the few-picosecond nonlinear response around the LSPR (λLSPR ≈ 750 nm for bare gold nanorods, Figure 1).31 Although detailed modeling has not been performed for Au@ Pd-NRs, the observed transient peak was expected to have a similar electronic origin, that is, to reflect nonequilibrium electron kinetics. In both systems, the ΔT/T signal subsequently reaches a plateau, after about 50 ps, of different sign in bare and Pd-coated Au-NRs. The rise or decay time to this plateau reflects external cooling of the nanoparticles by energy transfer

Figure 2. Time-dependent transmission ΔT/T changes (black lines) measured with pump and probe wavelengths of 400 and 700 nm, respectively in (a) the bare Au-NR solution (sample a) and (b) a Au@ Pd-NR one (sample e). The red lines are fits taking into account electron−phonon thermalization in the nanorods, their cooling, and their extensional and breathing vibrations (see main text). The insets are zooms highlighting the small period oscillations due to the nanorod breathing mode (green box in the main plots).

to the surrounding medium.32,33 The residual long delay is thus ascribed to both residual heating of the particle and local heating of the surrounding liquid, which reduces its refractive index.33,34 The slightly negative background ΔT/T signal measured in bare AuNPs suggests a dominant contribution of the latter effect that leads to a blue shift of the LSPR.35 The different long delay responses observed in Au@Pd-NRs indicate a different impact of heating the particles and their surroundings on the optical properties of the system. Their quantitative interpretation would require full modeling of the optical extinction spectrum of the colloidal solutions (Figure 1) and of the temperature dependence of the dielectric functions of the different involved materials (as reported for monometallic particles),30,31,33 which is out of the scope of this Letter. We focus here on the pronounced oscillations overlapping the background signal and its rise in both bare and Pd-coated gold nanorods (Figure 2). In both cases, short- and long-period components were observed, clearly detectable on a short (less than 20 ps) and long time scale, respectively. In bare Au-NRs, following previous experiments,2,22,36,37 the observed oscillations are ascribed to the rod fundamental extensional and breathing acoustic modes. These two modes involve mechanical displacements mostly along and perpendicular to the rod axis, respectively, so that their periods, Text and Tbr, are mostly determined by the rod length and width, respectively2 (see below). As extensively discussed for different monometallic nano-objects,3,10,30 the excitation of these modes in time-resolved experiments is a consequence of fast heating of the particle lattice due to hot electron− lattice energy transfer. This heating imposes particle dilation, which impulsively launches acoustic modes whose displacements are associated with changes of the particle volume. This 615



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mechanical movement modulates the dielectric function of the nanoparticles (i.e., their optical properties), which translates into a modulation of the sample transmission, monitored by the probe pulse. Similarly, the observed time domain oscillations in Au@PdNRs are ascribed to their fundamental breathing and extensional modes. In such bimetallic nanoparticles, the pump energy absorbed by the electrons is expected to be quickly redistributed by the electrons in the whole particle volume. Though different electron−lattice energy-transfer times in the Au and Pd parts might lead to inhomogeneous lattice heating, both components are expected to be sufficiently heated, and/or lattice heat redistribution over the small (nanometric) size of each particle is expected to be sufficiently fast to trigger full particle dilation. This launches fundamental vibration modes of the whole core− shell nano-objects, as for homogeneous heating in monometallic particles.10,16−18,30 Note that this is in contrast with the case of nanoparticles formed by materials of different nature, such as metallic and dielectric, where selective heating of the metallic part may lead to the preferential activation of higher-order core−shell modes.9,18,38 To extract the vibration mode periods, we fitted the timedependent ΔT/T signals by the sum of two damped sinusoidal functions and two decaying exponentials (the latter describing the above-discussed electron−phonon thermalization and global cooling of the nanoparticles). As in most measurements on ensembles of nanoparticles in solution, oscillation damping is dominated by dephasing of the vibrational contributions of the different nanoparticles due to their size and shape dispersions10,20 and will not be discussed here. Extensional and breathing mode periods of Text = (40.5 ± 0.8) ps and Tbr = (3.9 ± 0.1) ps were thus extracted for the bare Au-NRs, and values of Text = (36.6 ± 1.2) ps and Tbr = (4.8 ± 0.2) ps were extracted for the Pd-coated ones (sample e, Figure 2). Such impact of the presence of Pd on the nanoparticle vibrations has been observed in all of the investigated samples. Whereas Tbr shows the expected size dependence (increase with the bimetallic rod width, as for monometallic rods), Text decreases with Pd addition (Figure 3). This is in stark contrast with the dependence of Text on the length of monometallic rods, which would predict an opposite behavior, that is, a linear increase of Text with rod length (see Figure 3 and the discussion below). Because the optical spectra are also modified by the presence of Pd (Figure 1), the observed period changes might also be influenced by the different way the samples are optically probed. Such influence is known to take place when probe wavelengths close to the LSPR of the nanoparticles are used,30,31 leading to size, shape, or composition selections in the investigated particle population.36 To check for the possible influence of this effect, similar measurements were performed with the femtosecond oscillator system to probe at 860 nm. With this wavelength, probing is performed on the red side of the average LSPR wavelength (Figure 1), instead of its blue side as in the previous configuration. Though this should lead to different particle population selection, the deduced periods are only slightly modified, yielding Text = (42.2 ± 1.2) ps and Tbr = (3.6 ± 0.2) ps for bare Au-NRs (as compared to (40.5 ± 0.8) and (3.9 ± 0.1) ps for λpr = 700 nm). Similar systematic slight differences were also observed for Au@Pd samples, demonstrating a moderate effect of the choice of probe wavelength. The measured period shifts with increasing the Pd content of the Au@Pd-NRs can thus be ascribed to modification of their average vibrational response.

Figure 3. Experimental periods of the extensional (a) and breathing (b) modes (black squares) and computed ones assuming palladium deposited either along the side of gold nanorods (blue triangles, Figure 4a), at their tips (green circles, Figure 4b), or uniformly (red squares, Figure 4c). The vertical error bars were evaluated from different fits to different experimental signals, and the horizontal ones reflect estimated uncertainties on mean Pd volumes.

To analyze this effect, we have modeled the vibration modes of a bimetallic Au@Pd-NR as a function of Pd volume and spatial localization on the gold rod surface. Modeling was carried out by extending the models developed for free (the impact of a liquid environment on mode periods being small)6 monometallic rods. As a first approximation, the periods Text and Tbr of a pure nanorod can be estimated by describing it as a cylinder of very large aspect ratio η. For such a cylinder, Text and Tbr can then be analytically computed and are directly proportional to the rod length L and width W, respectively2,39

Text = 2L Tbr =

πW τ

ρ Y

(1)

ρ(1 + ν)(1 − 2ν) Y (1 − ν)

(2)

where Y is the Young’s modulus of the material, ν its Poisson’s ratio, ρ its density, and τ the smallest root of τJ0(τ) = (1 − 2ν)/ (1 − ν)J1(τ), J0 and J1 being Bessel functions. Although these expressions were established assuming a mechanically isotropic material, they can be extended to an anisotropic one, replacing Y by its value along the cylinder axis. For a cubic crystal, the elastic properties are described by three independent elastic constants C11, C12, and C44 (186, 157, and 42 GPa for gold, respectively), and Y along the unitary vector direction (nx,ny,nz) is given by22,40 ⎡ C11 + C12 Y[nxnynz] = ⎢ ⎢⎣ (C11 + 2C12)(C11 − C12) ⎤−1 ⎛ 1 ⎞ 2 ⎟(nx2ny2 + ny2nz2 + nx2nz2)⎥ +⎜ − ⎥⎦ (C11 − C12) ⎠ ⎝ C44 (3)

For the [100] growth direction of our Au-NRs (Figure S1, Supporting Information), one obtains Y[100] = 42 GPa41 (significantly smaller than the 79 GPa direction-averaged value for a polycrystal). Text = 45.7 ps and Tbr = 5.7 ps are thus estimated for the bare Au-NRs (using their mean dimensions, 616



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Table 1, and bulk gold Poisson’s ratio ν = 0.42). They differ from the experimental periods by ∼10 and ∼30%, respectively, a discrepancy mostly due to the approximation of very large aspect ratio objects that was used to establish eqs 1 and 2. More precise calculations have thus been performed using a numerical method based on finite element modeling (FEM).3 The nanorod shape was described as a cylinder capped by two hemispheres, resembling the typical geometry observed in TEM (Figure 1b). For the investigated aspect ratio η of our nanorods, Text and Tbr are found to be mostly set by L and W, respectively, with a smaller dependency on the other dimension. Taking into account crystalline anisotropy, the computed periods Text = 41.9 ps and Tbr = 4.1 ps are in excellent agreement with the experimental ones, (40.5 ± 0.8) and (3.9 ± 0.1) ps for λpr = 700 nm and (42.2 ± 1.2) and (3.6 ± 0.2) ps for λpr = 860 nm (Figure 2). This experimental variation is consistent with the probe wavelength selection discussed above, using a larger λpr increasing the relative contribution of high aspect ratio rods, which, for a given volume, exhibit larger Text and smaller Tbr (eqs 1 and 2). Agreement between the numerically computed and measured periods, as also obtained in single nanorod experiments,37 validates the modeling approach and model nanorod geometry. The mode periods Tbr and Text of Au@Pd-NRs were computed using similar analytical or numerical methods. The former generalizes the large aspect ratio cylinder model2,39 to the core−shell geometry,18 yielding a qualitative interpretation of the experimental results. We will focus here on the numerical FEM simulations that permit quantitative comparisons and tests of different Pd localizations, that is, full nanorod dimensions. Following the bare Au-NR modeling, the Au core of the Au@Pd particle was modeled as a cylinder capped by hemispheres. The Pd layer was also assumed to exhibit a cylindrical geometry, but different localizations were used to analyze its impact on the measured mode periods. Either a constant length or width of the Au@Pd-NRs (i.e., Pd deposited along the Au nanorod sides, Figure 4a, or at its ends, Figure 4b) or a uniform Au@Pd rod size increase (i.e., deposition of a uniform thickness Pd layer, Figure 4c) was considered. Following the structure deduced by HRTEM (Figure S2, Supporting Information), the Au and Pd parts were assumed to be monocrystalline, with a [100] direction along the rod axis. Tabulated density and elastic constants of Pd were used (ρ = 12020 kg/m3, C11 = 227 GPa, C12 = 176 GPa, and C44 = 72 GPa).42,43 The Text and Tbr periods computed for each Au@Pd rod geometry are shown in Figure 4a−c as a function of the Pd fraction parametrized by the maximum Pd thickness e. To discriminate shape and composition effects, calculations were also carried out for pure Au or Pd rods of the same full size (Figure 4). In the latter case, the computed periods were larger for gold than those for palladium due to the smaller density and larger elastic constants of the latter (see eqs 1 and 2 for long cylinders). Their dependencies on the parameter e reflect the expected results for a monometallic rod due to the concomitant changes of its length and width. More precisely, matter addition on the nanorod side (i.e., increase only of W) increases Tbr while weakly affecting Text (Figure 4a). Conversely, addition on the tips (i.e., increase only of L) leads to a Text increase with almost no Tbr change (Figure 4b). Uniform coating results in concomitant increase of Text and Tbr (Figure 4c). The periods computed for the Au@Pd-NRs evolve from those of a pure Au-NR (for e = 0) to those of a pure Pd object of the same geometry with increasing Pd fraction, except for the

Figure 4. (Top) Au@Pd-NR cylindrical shape models used in the simulations corresponding to palladium deposition (a) on the sides of a gold nanorod, (b) at its tips, and (c) uniformly. The initial Au-NR shape is modeled as a cylinder capped by hemispheres (yellow area, of 33.7 nm length and 9.7 nm width). The deposited Pd is shown by the blue area, and its amount is quantified by its maximum thickness e. (Bottom) Numerically computed periods of the extensional, Text, and breathing, Tbr, modes of Au@Pd-NRs for the three model shapes (squares) and for pure Au and Pd rods of the same shape and size (up and down triangles, respectively).

breathing mode of end-coated rods (Figure 4). Actually, for all geometries, the Tbr changes mostly reflect an increase of the rod width W. For geometry a or c, W increases with the Pd layer thickness e, leading to an almost linear evolution of Tbr with e. The composition change only translates in different slopes as compared to the single-metal case (Figure 4a,c). For the constant width geometry b, Tbr is almost constant and does not reach the Pd breathing mode period for the investigated e values as a consequence of almost full mechanical decoupling of the Au and Pd parts for movements perpendicular to the rod axis. In contrast, evolution of Text exhibits a more complex behavior with Pd addition. For a constant rod length (geometry a), Text decreases with e, mostly reflecting a change of the mean elastic properties of the bimetallic rod (i.e., its stiffening as compared to that of pure gold). In contrast, it increases with e for the constant width case (geometry b), mostly reflecting an increase of L, with only a correction due to stiffening. For a uniform Pd layer (geometry c), both effects take place. Stiffening first dominates for a weak Pd fraction, leading to a decrease of Text with e, although the rod length actually increases. For a large Pd fraction, the vibrational response is essentially determined by the Pd part, and Text becomes almost identical to the pure Pd case rising with L or e (Figure 4c). For large Pd content, transition from cylindrical to square cross section rods has been experimentally observed and ascribed to a dependence of Pd growth efficiency on the different crystalline facets of the Au-NRs.24 Similar FEM simulations were thus performed assuming a square section for the Pd shell (keeping the gold nanorod shape unchanged, Figure S3 in the Supporting Information). The same trends as those for cylindrical shapes were obtained (Figure S3, Supporting Information), confirming the impact of both Pd content and localization on the acoustic response of the bimetallic rod. 617



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The computed evolution of the mode periods for uniform Pd deposition (Figure 4c) is in good agreement with the experimental results (Figure 3; the values of Pd thickness e indicated in Table 1 were used in the simulations). In particular, the counterintuitive reduction of Text with Pd addition (due to rod stiffening) is very well reproduced, together with the increase of Tbr (with increasing W). The Pd amounts used here are insufficient to reach the e values where Text increases again (Figure 4c). A good reproduction of the experimental data was also obtained for Pd deposition on Au-NRs sides (Figure 4a), a geometry that leads to similar values for Text and Tbr. The similarity between the predictions of these two models is a consequence of the fact that the Pd mass is predominantly located on Au-NRs sides in both cases (Figure 4c). In contrast, the computed periods for the end-coating geometry (Figure 4b) are incompatible with the experimental results. Validation of these results by TEM analysis is difficult to perform here because of the small Pd fractions used in our study (average Pd thickness between 0.1 to 1.5 nm), leading to large particle to particle variations of Pd amount and location as well as to difficulties in their quantification (Table 1). However, the reproduction of the ensemble-averaged vibrational response of the Au@Pd-NRs using a uniform Pd layer thickness is consistent with the identical average increases of L and W, which were measured using electron microscopy in a previous study,24 involving larger Pd amounts (Pd/Au atomic fraction up to 2.8, compared to less than 1 in our case). In conclusion, the fundamental extensional and breathing acoustic modes of Au@Pd-NRs were investigated as a function of the amount of Pd using time-resolved spectroscopy. Starting from bare nanorods, the periods were found to decrease and increase, respectively, with deposition of small amounts of Pd (Pd/Au atomic fraction down to 5%). Comparing the experimental data to the results of numerical simulations, these opposite behaviors were ascribed to the different sensitivity of the vibrational modes toward the full dimensions of the bimetallic rods and to the change of their elastic properties with their composition. This double dependence is similar to that of the optical LSPR of bimetallic nanoparticles, affected by both the full particle geometry and the dielectric responses of its components,27 but can be more precisely tested in the vibrational domain as two different modes are observed. Excellent agreement with the measured data was obtained by assuming that the deposited Pd forms a uniform shell, in agreement with a previous structural study of Au@Pd-NRs for larger Pd/Au fractions.24 Conversely, our experimental observations are incompatible with a preferential growth of the Pd shell at Au-NR ends. These results show that the acoustic response of bimetallic nano-objects can yield information on their composition and mean spatial repartition of their constituting materials, a generally challenging task in the presence of a small amount of one of the components. As ensemble studies were performed, only average information over a large number of nanoparticles was obtained. Extension to individual bimetallic nanoparticles should bring new insights on the acoustic properties of these systems and provide information on the dispersion of their composition and material distribution. Furthermore, the double-mode investigations performed here circumvent a common problem of nanobalances, namely, the dependence of frequency shift signals on both the deposited mass and its position,14,15 and could therefore be exploited in other contexts for nanoweighting applications.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information about Au@Pd-NR synthesis, characterization, and modeling. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

N.D.F. thanks the Institut Universitaire de France (IUF). M.F.C. acknowledges a Ph.D. student scholarship from INL, Braga. The authors acknowledge support from Acciones Integradas MICINN, FR2009-0034XX projects, and Ministère des Affaires étrangères et européennes, Projet PICASSO 22930NE.

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