communications Plasmonics
Small Supported Plasmonic Silver Clusters Martin Thämer, Aras Kartouzian, Philipp Heister, Tobias Lünskens, Sabine Gerlach, and Ulrich Heiz* Interest in small metal nanoparticles (NPs) has been rapidly and steadily growing due to their diverse application palette, ranging from fundamental surface physics to ultrafast diagnostics. Among the more contemporary applications of supported noble metal NPs is their use as efficient light harvesting agents and photo-catalysts.[1] Here the strong plasmon resonances, which are characteristic to gold and silver NPs, are of particular importance. The sensitivity of the plasmon peak’s shape and position to the particle's size, shape, and surrounding media makes them tunable over a wide wavelength range and also a very good sensor to probe the aforementioned parameters.[2] In case of silver NPs, the interband transition (transition of 4d electrons to the 5sp band) also has a considerable contribution to the absorption spectrum, which leads to an additional broadband UV absorption.[1b,e] Despite the great interest in the field that has triggered extensive studies on gas phase[3] and matrixembedded small clusters down to the dimers,[4] spectroscopical investigations on supported silver NPs have been limited to particle sizes above 2 nm in diameter.[5] The main reason for this shortcoming is the small surface coverages that are required to avoid agglomeration of NPs in such samples (when grown by vapor deposition), which pushes the conventional surface spectroscopic methods beyond their sensitivity limits. The fact that for any practical applications of metal clusters a kind of support material is required, further demonstrates the importance of experiments on supported metal clusters. In this work, we present the first report on optical properties of supported silver NPs with an average diameter of about 1.5 nm and sub-monolayer surface density, using surface second harmonic generation (s-SHG) spectroscopy. Our observations confirm that plasmon resonance can still be present in such small particles consisting of less than 40 atoms on average. Furthermore, by combining the nonlinear spectroscopic data from s-SHG with the data achieved from highly sensitive surface cavity ringdown (s-CRD) spectroscopy as a linear method, details on the morphology of such small silver NPs could be extracted. Dr. M. Thämer,[+] Dr. A. Kartouzian,[+] P. Heister, T. Lünskens, S. Gerlach, Prof. Dr. U. Heiz Chemistry Department Chair of Physical Chemistry Technical University of Munich Lichtenbergstraße 4, 85748, Garching, Germany E-mail: [email protected] [+] Equally contributing (first authors) DOI: 10.1002/smll.201303158
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Silver NPs in the gas phase possess a comparatively intense and narrow plasmon absorption peak at around 3.5 eV. Previous literature records suggest that supported silver NPs with diameters down to 2 nm still show well resolved plasmonic features that can be well described by the classical Mie theory.[5a,e,6] The collective oscillation of electrons upon interaction with the electromagnetic field has been observed for very small silver cluster cations with as little as 9 atoms in the gas phase.[7] However, the information gained from gas phase studies cannot be simply extended to supported systems due to the interactions between support material and metal particles.[2b,6a,8] Therefore, independent experimental investigations on small supported silver NPs are unavoidable. In transmission mode s-SHG spectroscopy, an intense laser pulse is focused onto the sample and both, the generated second harmonic frequency and the unconverted fundamental beam propagate in the same direction as the incident beam. The frequency doubled s-SHG beam is then isolated prior to detection. Using a tunable laser, the conversion efficiency of the sample (representing the second order susceptibility) as a function of the fundamental frequency can be recorded. The resulting nonlinear spectrum will contain peaks whenever, either the fundamental or the second harmonic frequency matches an optical transition of the sample. This leads to an intrinsic ambiguity in interpretation of the experimentally observed spectral features (see Figure S1). Because of the SHG selection rules this technique is, however, one of the most surface sensitive methods when applied to interfaces of two centro-symmetric or amorphous media, and has been successfully employed to study very low coverages of adsorbates,[9] as well as supported metal NPs.[10] In this work, s-SHG and s-CRD spectroscopic methods are used to study supported silver NPs with a coverage of approximately 1.5 x 1013 cluster cm-2 (see SI). Jet cooled silver clusters with a distribution ranging from 15 to 65 atoms (Agn, < n > = 37, see Figure S2) corresponding to a mean diameter of d < 1.5 nm[11] were soft-landed on borosilicate (BK7) glass substrates. Figure 1(a) presents the recorded s-SHG spectrum of such samples between 1.4 and 2.8 eV of fundamental photon energy, corresponding to frequency doubled photon energies in the range of 2.8 to 5.6 eV. Also shown in Figure 1(a) is the smoothed version of the spectrum where the contribution of interference effects which appear in these types of measurements (oscillations in the spectrum)[12] are mathematically removed for more clarity. The nonlinear spectrum of silver NPs shows a pronounced resonance peak at 1.84 (3.68) eV of fundamental (second harmonic) photon energy. Intuitively, one would attribute the observed peak to a resonant
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at 3.68 eV, which in turn, strongly suggests its relation to a plasmonic oscillation in the particles. Furthermore, the strong raise in optical loss in the range between 2 to 3 eV that is observed in the linear spectrum (see Figure 1(b)), is not present in the nonlinear spectrum of the clusters shown in Figure 1(a). The s-SHG signal rather decreases monotonously in this spectral region (4 to 6 eV). This hints at the presence of a second SHG inactive absorption band in this range. To further clarify this observation, we conducted photo-stability studies. Samples were irradiated for 20 minutes with laser pulses of similar intensity (∼4mJ/pulse) at two different energies. Once at 1.84 eV, corresponding to the fundamental energy of the peak maximum in s-SHG spectrum, and once at 2.76 eV, which is far from the s-SHG peak but close to the highest measured optical loss by s-CRD spectroscopy (consult Figure 1). At 1.84 eV, the s-SHG signal is continuously monitored during the process. As Figure 2 shows, irradiation with 1.84 eV doesn’t affect the SH intensity generated by the silver NPs. This result shows that the particles are stable during this photon bombardment, which in turn confirms that the linear absorption coefficient at this photon energy must be comparatively low, confirming that the measured nonlinear resonance is indeed located at the frequency doubled energy (3.68 eV). At 2.76 eV, the sample is first irradiated for 20 minutes, and then the s-SHG signal of the sample is recorded again at 1.84 eV to evaluate the impact of the photo-treatment. The result is a dramatic loss of the s-SHG signal intensity of about 70%, indicating that a large fraction of silver clusters have desorbed (Figure 2). The linear absorption coefficient must consequently be large at this fundamental photon energy (2.76 eV), whereas only minor s-SHG activity is observed at the corresponding Figure 1. (a) s-SHG and (b) s-CRD spectra of supported silver NPs with an average diameter of 1.5 nm are presented. In (a), the oscillations on the s-SHG signal are caused by interference effects between the SH contributions of the substrate front surface and the SH contribution of the clusters. The gap around 1.78 eV of fundamental photon energy is due to signal-idler cross-over of the used OPO laser system. The smoothed curve (dotted line) represents the s-SHG signal generated by NPs. A well resolved resonance peak is observed at 1.84 eV. In (b), a monotonous increase in optical loss toward 3.0 eV is observed.
transition at 3.68 eV, having the plasmon resonance energy of small gas phase silver particles at ∼3.5 eV in mind. However, this cannot be unambiguously assigned without complementary information from a linear spectroscopic method.[9a] To this end, s-CRD spectroscopy was conducted on to the same samples to unravel the source of the observed spectral behavior. Figure 1(b) presents the linear absorption spectrum of supported silver clusters in the range from 1.8 to 2.9 eV. The optical loss of silver nano-clusters shows a smooth monotonous raise towards 3.0 eV. The smoothness of this absorption band over more than 1 eV also implies its link to a collective oscillation and not to transitions between discrete energy levels. A brief comparison between the linear and nonlinear spectra in Figure 1 clearly reveals the origin of the observed resonance. At photon energy of 1.84 eV, no spectral features are present in the linear spectrum. Thus, the peak in the nonlinear spectrum can confidently be positioned small 2014, 10, No. 12, 2340–2344
Figure 2. Photo-stability of supported silver NPs at two photon energies are shown. During irradiation at 1.84 eV for 20 minutes, no photodamage is measured. After 20 minutes irradiation at 2.76 eV (shaded area) the intensity of the s-SHG signal measured at 1.84 eV has dropped to 30%.
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M. Thämer et al.
σ i ,ext =
ω 1/2 ε −εm ⋅ ε ⋅ V0 c0 m (ε − ε m ) Li + ε m
(1)
where ω is the frequency of excitation light, c0 is the speed of light in vacuum, V0 (1.77 nm3) is the particle volume, εm is the dielectric constant of the surrounding medium (at lowest level of approximation for supported NPs it is given by the weighted mean value between dielectric constants of the support and vacuum,[5c,8b] ε is the complex dielectric function of the particle (here represented by the bulk value after correction for the effect of the [13] and L is the shape paramFigure 3. The overall spectrum consisting of the linear and nonlinear optical response of particle size, i supported silver clusters is presented. Also shown are the simulated extinction spectra as eter for each axes i. For spherical particles predicted by Mietheory. The experimental signals are scaled to match the simulated curves. Lx = Ly = Lz = 1/3. The anisotropic nature The shaded area from 2.25 to 3.0 eV, includes the raising optical loss measured by s-CRD of the surrounding medium is not considspectroscopy. In the corresponding photon energies from 4.5 to above 5.5 eV (also shaded), ered in the above-mentioned model. In a however, no raise in the s-SHG signal is observed. more realistic but still classical approach we include the anisotropy effect by introsecond harmonic energy (5.52 eV) as illustrated in Figure 1. ducing direction dependent dielectric constants for the In conclusion, this absorption is not related to the observed medium (see SI and Figure S3). A deviation of the particle s-SHG active plasmon oscillation. Figure 3 depicts the super- shape from a sphere leads to different shape parameters for position of the linear and nonlinear spectra in correct order different particle axes (see supporting information) and thus to indicate the whole spectral range covered in this study. also to a splitting of the plasmon peak into multiple resoFurther interpretation of the data requires explicit considera- nances at different frequencies. In case of a spheroid there tion of the selection rules in s-SHG spectroscopy as given in would be two plasmon peaks where the resonance frequency the following paragraph. of the oscillation along the short axis is blue shifted and the In s-SHG spectroscopy, only those induced polarization oscillation along the long axes is red shifted with respect oscillations that take place in an asymmetric potential can give to the single spherical plasmon resonance. The experimenrise to second harmonic generation. For supported metal parti- tally observed peak (at 3.68 eV) is blue shifted by 0.37 eV cles, centro-symmetry is only broken along the axis perpendic- with respect to the predicted plasmon resonance energy for ular to the surface. As a result, only oscillations of conduction spherical particles in an isotropic dielectric medium. Since electrons perpendicular to the surface are confined to an asym- this mode is observed in the s-SHG spectrum (i.e., it is SHG metric potential. Therefore, excitations in this direction are active), it must be originating from an oscillation perpenSHG active, whereas charge oscillations parallel to the surface dicular to the surface. Secondly, the blue shift indicates that are SHG inactive and do not appear in the nonlinear spectrum. this is caused by deviation from spherical shape as the aniWe now interpret the experimental data under this light. For sotropy in the surrounding dielectric medium would result a schematic view of the s-SHG active and inactive modes and in a red shift for oscillations perpendicular to the surface the polarization of the probe laser beam consult Figure S4. (see above). We thus conclude that this mode (i.e. the peak Evidently there are two different resonances present observed in s-SHG spectrum) belongs to an oscillation along in the sample, only one of which is detectable by s-SHG the short axis of the oblate NPs, which lie on the surface in a spectroscopy. Based on the s-SHG selection rules that were flattened form. We also conclude that there should be a red introduced earlier, the absence of the resonance band in shifted plasmon resonance corresponding to the oscillation the nonlinear spectrum (that is observed in s-CRD spec- along the long axis of the NPs lying parallel to the surface. trum) suggests that the related electron oscillation occurs According to the aforementioned selection rules, this red parallel to the surface. In addition, the fact that there are shifted resonance is SHG inactive and will not appear in the two different resonances present in the spectra implies s-SHG spectrum. It should, however, be detected in the linear the existence of plasmon splitting effects and thus, devia- spectrum, as the s-CRD method has no preference for any tion of the clusters’ shape from perfect spherical form. Mie of the plasmonic modes. Indeed, the onset of the absorption theory predicts the plasmon frequency of supported spher- band that is observed in the linear spectrum exactly follows ical silver particles with an average size of 1.5 nm to be this description (Figure 3). Due to the high scattering losses located at 3.31 eV based on the following equation for the of BK7 substrates at photon energies above 3 eV, and comabsorption cross section. This equation is generally valid paratively lower mirror qualities in the UV, it is not possible for particles with spheroid shape and assumes an effec- to fully resolve this resonance directly by s-CRD spectrostive homogeneous dielectric constant for the surrounding copy. Using Equation (1), in combination with the knowlmedium:[5c] edge gained from s-SHG spectrum (i.e., the amount of the
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observed blue shift) it is possible to predict the position of the red shifted component. The outcomes of such simulations are graphically included in Figure 3 (see Figure S3 for more details). These simulations are in very good agreement with the experimental data, further supporting the analysis given above. Such particle flattening effects have been also reported earlier for larger supported silver NPs.[5a,c,11] The observed increase in the absorption cross section towards higher energies is due to the broad interband transitions in silver particles. Although these transitions are predicted in simulations for linear absorption cross sections (as is the case here) they do not necessarily contribute to s-SHG spectrum. In summary, the advantages of a linear and a nonlinear surface spectroscopy method are successfully combined to study sub-monolayer coverages of the smallest supported silver NPs to date. Whereas s-SHG spectroscopy compensates for the limited spectral range of s-CRD spectroscopy in the UV, the latter is unavoidable due to the intrinsic ambiguity of s-SHG process. Our results show that well separated borosilicate supported silver NPs with an average diameter of 1.5 nm still can be treated as plasmonic particles. Also plasmon splitting effects are observed for particles with this mean size. Using our purely spectroscopic approach, we found that such small silver particles lie with their short axis perpendicular to the surface. Our analysis based on Mie theory is capable of explaining the experimental findings with compelling agreement, which suggests the appositeness of this classical theory for supported particles of similar size. The refinement of the traditionally applied model by introducing direction dependent dielectric constants and thus reflecting the anisotropic nature of the system (i.e. supported metal particles) leads to a more realistic and reliable analysis. The transition from plasmonic to molecular optical response of supported silver clusters (which is expected to appear for small particle sizes) should therefore be located at still smaller cluster sizes. The high signal to noise ratio in the presented spectra demonstrates the strength of the presented approach. Further studies on size selected silver clusters are on the way.
Experimental Section Silver clusters were generated by a pulsed laser vaporization cluster beam source. A quadrupole mass filter (Extrel, 5500 series) was used to narrow the mass distribution of the clusters before soft-landing on the BK7 substrate under ultra-high vacuum (UHV) conditions. The substrate is mounted on a sample holder of home design attached to a rotatable xyz manipulator, which is cooled with liquid nitrogen. Further details of the cluster beam source and UHV chambers are presented in an earlier work.[14] Cluster currents and mass distribution are monitored by a Faraday plate that can be positioned on the axis of the cluster beam. The spectroscopic measurements are performed in situ as described in an earlier publication.[9a,15] Surface coverage was experimentally determined by recording optical loss of the sample while scanning the surface (see Figure S5). Details of data treatment for s-SHG spectroscopy are available elsewhere.[12] For all measurements presented in this work signals are averaged over 200 laser shots.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work has been supported by the BMBF through the project IC4 and the European Research Council (ERC) through an Advanced Research Grant (246645-ASC3).
[1] a) X. Chen, Z. F. Zheng, X. B. Ke, E. Jaatinen, T. F. Xie, D. J. Wang, C. Guo, J. C. Zhao, H. Y. Zhu, Green Chem. 2010, 12(3), 414–419; b) P. V. Kamat, J. Phys. Chem. B 2002, 106(32), 7729–7744; c) S. Sarina, E. R. Waclawik, H. Zhu, Green Chem. 2013, 15(7), 1814–1833; d) P. Christopher, H. L. Xin, S. Linic, Nat. Chem. 2011, 3(6), 467–472; e) W. Hou, W. H. Hung, P. Pavaskar, A. Goeppert, M. Aykol, S. B. Cronin, ACS Catal. 2011, 1(8), 929–936. [2] a) M. A. Cazalilla, J. S. Dolado, A. Rubio, P. M. Echenique, Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61(12), 8033–8042; b) K. L. Kelly, E. Coronado, L. L. Zhao, G. C. Schatz, J. Phys. Chem. B 2003, 107(3), 668–677; c) P. Mulvaney, Langmuir 1996, 12(3), 788–800. [3] a) B. G. Deboer, G. D. Stein, Surf. Sci. 1981, 106(1-3), 84–94; b) K. Laihing, P. Y. Cheng, M. A. Duncan, Z. Phys. D: At., Mol. Clusters 1989, 13(2), 161–169; c) L. D. Socaciu, J. Hagen, U. Heiz, T. M. Bernhardt, T. Leisner, L. Woste, Chem. Phys. Lett. 2001, 340(3–4), 282–288. [4] a) S. Fedrigo, W. Harbich, J. Buttet, Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47(16), 10706–10715; b) W. Harbich, S. Fedrigo, J. Buttet, Chem. Phys. Lett. 1992, 195(5–6), 613–617; c) W. Harbich, S. Fedrigo, J. Buttet, Z. Phys. D: At., Mol. Clusters 1993, 26(1), 138–140; d) F. Conus, J. T. Lau, V. Rodrigues, C. Felix, Rev. Sci. Instrum. 2006, 77(11), 113103–13106; e) I. Rabin, W. Schulze, G. Ertl, C. Felix, C. Sieber, W. Harbich, J. Buttet, Chem.Phys. Lett. 2000, 320(1–2), 59–64. [5] a) A. Hotzel, S. Mathies, D. E. Starr, A. Grujic, M. Wolf, Appl. Phys. B: Lasers Opt. 2011, 103(1), 171–187; b) P. Christopher, S. Linic, ChemCatChem 2010, 2(1), 78–83; c) U. Kreibig, Optical Properties of Metal Clusters, Springer, 1995; d) C. M. Grimaud, L. Šiller, M. Andersson, R. E. Palmer, Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59(15), 9874–9877; e) U. Kreibig, L. Genzel, Surf. Sci. 1985, 156 (Jun), 678–700. [6] a) U. Kreibig, G. Bour, A. Hilger, M. Gartz, Phys. Status Solidi A 1999, 175(1), 351–366; b) G. Mie, Ann. Phys. (Berlin) 1908, 330(3), 377–445. [7] J. Tiggesbaumker, L. Koller, K. H. Meiwesbroer, A. Liebsch, Phys. Rev. A: At., Mol., Opt. Phys. 1993, 48(3), R1749–R1752. [8] a) D. Barreca, A. Gasparotto, C. Maragno, E. Tondello, S. Gialanella, J. Appl. Phys. 2005, 97(5) ; b) A. Hilger, N. Cuppers, M. Tenfelde, U. Kreibig, Eur. Phys. J. D 2000, 10(1), 115–118; c) T. Yamaguchi, S. Yoshida, A. Kinbara, Thin Solid Films 1974, 21(1), 173–187. [9] a) M. Thämer, A. Kartouzian, P. Heister, S. Gerlach, M. Tschurl, U. Boesl, U. Heiz, J. Phys. Chem. C 2012, 116(15), 8642–8648; b) P. Brevet, Surface Second Harmonic Generation, Presses polytechniques et universitaires romandes, 1997; c) K. Kuhnke, M. Epple, K. Kern, Chem. Phys. Lett. 1998, 294(1–3), 241–247; d) Y. R. Shen, Annu. Rev. Phys. Chem. 1989, 40(1), 327–350;
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e) G. J. Simpson, S. G. Westerbuhr, K. L. Rowlen, Anal. Chem. 2000, 72(5), 887–898. [10] a), C. S. Chang, J. T. Lue, Surf. Sci. 1997, 393(1–3), 231–239; b) T. Götz, M. Buck, C. Dressler, F. Eisert, F. Träger, Appl.Phys. A: Mater. Sci. Process. 1995, 60(6), 607–612; c) T. Hayakawa, Y. Usui, S. Bharathi, M. Nogami, Adv. Mater. (Weinheim, Ger.) 2004, 16(16), 1408+; d) R. Srinivasan, Y. Tian, I. I. Suni, Surf. Sci. 2001, 490(3), 308–314. [11] B. Wortmann, K. Mende, S. Duffe, N. Grönhagen, B. von Issendorff, H. Hövel, Phys. Status Solidi B 2010, 247(5), 1116–1121.
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[12] A. Kartouzian, P. Heister, M. Thämer, S. Gerlach, U. Heiz, J. Opt. Soc. Am. B 2013, 30(3), 541–548. [13] P. B. Johnson, R. W. Christy, Phys. Rev. B: Condens. Matter Mater. Phys. 1972, 6(12), 4370–4379. [14] A. Kartouzian, M. Thamer, T. Soini, J. Peter, P. Pitschi, S. Gilb, U. Heiz, J. Appl. Phys. 2008, 104(12). [15] A. Kartouzian, M. Thämer, U. Heiz, Phys. Status Solidi B 2010, 247(5), 1147–1151.
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Received: October 2, 2013 Revised: January 23, 2014 Published online: March 10, 2014
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Small Supported Plasmonic Silver Clusters Martin Thämer, Aras Kartouzian, Philipp Heister, Tobias Lünskens, Sabine Gerlach, and Ulrich Heiz* Interest in small metal nanoparticles (NPs) has been rapidly and steadily growing due to their diverse application palette, ranging from fundamental surface physics to ultrafast diagnostics. Among the more contemporary applications of supported noble metal NPs is their use as efficient light harvesting agents and photo-catalysts.[1] Here the strong plasmon resonances, which are characteristic to gold and silver NPs, are of particular importance. The sensitivity of the plasmon peak’s shape and position to the particle's size, shape, and surrounding media makes them tunable over a wide wavelength range and also a very good sensor to probe the aforementioned parameters.[2] In case of silver NPs, the interband transition (transition of 4d electrons to the 5sp band) also has a considerable contribution to the absorption spectrum, which leads to an additional broadband UV absorption.[1b,e] Despite the great interest in the field that has triggered extensive studies on gas phase[3] and matrixembedded small clusters down to the dimers,[4] spectroscopical investigations on supported silver NPs have been limited to particle sizes above 2 nm in diameter.[5] The main reason for this shortcoming is the small surface coverages that are required to avoid agglomeration of NPs in such samples (when grown by vapor deposition), which pushes the conventional surface spectroscopic methods beyond their sensitivity limits. The fact that for any practical applications of metal clusters a kind of support material is required, further demonstrates the importance of experiments on supported metal clusters. In this work, we present the first report on optical properties of supported silver NPs with an average diameter of about 1.5 nm and sub-monolayer surface density, using surface second harmonic generation (s-SHG) spectroscopy. Our observations confirm that plasmon resonance can still be present in such small particles consisting of less than 40 atoms on average. Furthermore, by combining the nonlinear spectroscopic data from s-SHG with the data achieved from highly sensitive surface cavity ringdown (s-CRD) spectroscopy as a linear method, details on the morphology of such small silver NPs could be extracted. Dr. M. Thämer,[+] Dr. A. Kartouzian,[+] P. Heister, T. Lünskens, S. Gerlach, Prof. Dr. U. Heiz Chemistry Department Chair of Physical Chemistry Technical University of Munich Lichtenbergstraße 4, 85748, Garching, Germany E-mail: [email protected] [+] Equally contributing (first authors) DOI: 10.1002/smll.201303158
2340
wileyonlinelibrary.com
Silver NPs in the gas phase possess a comparatively intense and narrow plasmon absorption peak at around 3.5 eV. Previous literature records suggest that supported silver NPs with diameters down to 2 nm still show well resolved plasmonic features that can be well described by the classical Mie theory.[5a,e,6] The collective oscillation of electrons upon interaction with the electromagnetic field has been observed for very small silver cluster cations with as little as 9 atoms in the gas phase.[7] However, the information gained from gas phase studies cannot be simply extended to supported systems due to the interactions between support material and metal particles.[2b,6a,8] Therefore, independent experimental investigations on small supported silver NPs are unavoidable. In transmission mode s-SHG spectroscopy, an intense laser pulse is focused onto the sample and both, the generated second harmonic frequency and the unconverted fundamental beam propagate in the same direction as the incident beam. The frequency doubled s-SHG beam is then isolated prior to detection. Using a tunable laser, the conversion efficiency of the sample (representing the second order susceptibility) as a function of the fundamental frequency can be recorded. The resulting nonlinear spectrum will contain peaks whenever, either the fundamental or the second harmonic frequency matches an optical transition of the sample. This leads to an intrinsic ambiguity in interpretation of the experimentally observed spectral features (see Figure S1). Because of the SHG selection rules this technique is, however, one of the most surface sensitive methods when applied to interfaces of two centro-symmetric or amorphous media, and has been successfully employed to study very low coverages of adsorbates,[9] as well as supported metal NPs.[10] In this work, s-SHG and s-CRD spectroscopic methods are used to study supported silver NPs with a coverage of approximately 1.5 x 1013 cluster cm-2 (see SI). Jet cooled silver clusters with a distribution ranging from 15 to 65 atoms (Agn, < n > = 37, see Figure S2) corresponding to a mean diameter of d < 1.5 nm[11] were soft-landed on borosilicate (BK7) glass substrates. Figure 1(a) presents the recorded s-SHG spectrum of such samples between 1.4 and 2.8 eV of fundamental photon energy, corresponding to frequency doubled photon energies in the range of 2.8 to 5.6 eV. Also shown in Figure 1(a) is the smoothed version of the spectrum where the contribution of interference effects which appear in these types of measurements (oscillations in the spectrum)[12] are mathematically removed for more clarity. The nonlinear spectrum of silver NPs shows a pronounced resonance peak at 1.84 (3.68) eV of fundamental (second harmonic) photon energy. Intuitively, one would attribute the observed peak to a resonant
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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at 3.68 eV, which in turn, strongly suggests its relation to a plasmonic oscillation in the particles. Furthermore, the strong raise in optical loss in the range between 2 to 3 eV that is observed in the linear spectrum (see Figure 1(b)), is not present in the nonlinear spectrum of the clusters shown in Figure 1(a). The s-SHG signal rather decreases monotonously in this spectral region (4 to 6 eV). This hints at the presence of a second SHG inactive absorption band in this range. To further clarify this observation, we conducted photo-stability studies. Samples were irradiated for 20 minutes with laser pulses of similar intensity (∼4mJ/pulse) at two different energies. Once at 1.84 eV, corresponding to the fundamental energy of the peak maximum in s-SHG spectrum, and once at 2.76 eV, which is far from the s-SHG peak but close to the highest measured optical loss by s-CRD spectroscopy (consult Figure 1). At 1.84 eV, the s-SHG signal is continuously monitored during the process. As Figure 2 shows, irradiation with 1.84 eV doesn’t affect the SH intensity generated by the silver NPs. This result shows that the particles are stable during this photon bombardment, which in turn confirms that the linear absorption coefficient at this photon energy must be comparatively low, confirming that the measured nonlinear resonance is indeed located at the frequency doubled energy (3.68 eV). At 2.76 eV, the sample is first irradiated for 20 minutes, and then the s-SHG signal of the sample is recorded again at 1.84 eV to evaluate the impact of the photo-treatment. The result is a dramatic loss of the s-SHG signal intensity of about 70%, indicating that a large fraction of silver clusters have desorbed (Figure 2). The linear absorption coefficient must consequently be large at this fundamental photon energy (2.76 eV), whereas only minor s-SHG activity is observed at the corresponding Figure 1. (a) s-SHG and (b) s-CRD spectra of supported silver NPs with an average diameter of 1.5 nm are presented. In (a), the oscillations on the s-SHG signal are caused by interference effects between the SH contributions of the substrate front surface and the SH contribution of the clusters. The gap around 1.78 eV of fundamental photon energy is due to signal-idler cross-over of the used OPO laser system. The smoothed curve (dotted line) represents the s-SHG signal generated by NPs. A well resolved resonance peak is observed at 1.84 eV. In (b), a monotonous increase in optical loss toward 3.0 eV is observed.
transition at 3.68 eV, having the plasmon resonance energy of small gas phase silver particles at ∼3.5 eV in mind. However, this cannot be unambiguously assigned without complementary information from a linear spectroscopic method.[9a] To this end, s-CRD spectroscopy was conducted on to the same samples to unravel the source of the observed spectral behavior. Figure 1(b) presents the linear absorption spectrum of supported silver clusters in the range from 1.8 to 2.9 eV. The optical loss of silver nano-clusters shows a smooth monotonous raise towards 3.0 eV. The smoothness of this absorption band over more than 1 eV also implies its link to a collective oscillation and not to transitions between discrete energy levels. A brief comparison between the linear and nonlinear spectra in Figure 1 clearly reveals the origin of the observed resonance. At photon energy of 1.84 eV, no spectral features are present in the linear spectrum. Thus, the peak in the nonlinear spectrum can confidently be positioned small 2014, 10, No. 12, 2340–2344
Figure 2. Photo-stability of supported silver NPs at two photon energies are shown. During irradiation at 1.84 eV for 20 minutes, no photodamage is measured. After 20 minutes irradiation at 2.76 eV (shaded area) the intensity of the s-SHG signal measured at 1.84 eV has dropped to 30%.
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M. Thämer et al.
σ i ,ext =
ω 1/2 ε −εm ⋅ ε ⋅ V0 c0 m (ε − ε m ) Li + ε m
(1)
where ω is the frequency of excitation light, c0 is the speed of light in vacuum, V0 (1.77 nm3) is the particle volume, εm is the dielectric constant of the surrounding medium (at lowest level of approximation for supported NPs it is given by the weighted mean value between dielectric constants of the support and vacuum,[5c,8b] ε is the complex dielectric function of the particle (here represented by the bulk value after correction for the effect of the [13] and L is the shape paramFigure 3. The overall spectrum consisting of the linear and nonlinear optical response of particle size, i supported silver clusters is presented. Also shown are the simulated extinction spectra as eter for each axes i. For spherical particles predicted by Mietheory. The experimental signals are scaled to match the simulated curves. Lx = Ly = Lz = 1/3. The anisotropic nature The shaded area from 2.25 to 3.0 eV, includes the raising optical loss measured by s-CRD of the surrounding medium is not considspectroscopy. In the corresponding photon energies from 4.5 to above 5.5 eV (also shaded), ered in the above-mentioned model. In a however, no raise in the s-SHG signal is observed. more realistic but still classical approach we include the anisotropy effect by introsecond harmonic energy (5.52 eV) as illustrated in Figure 1. ducing direction dependent dielectric constants for the In conclusion, this absorption is not related to the observed medium (see SI and Figure S3). A deviation of the particle s-SHG active plasmon oscillation. Figure 3 depicts the super- shape from a sphere leads to different shape parameters for position of the linear and nonlinear spectra in correct order different particle axes (see supporting information) and thus to indicate the whole spectral range covered in this study. also to a splitting of the plasmon peak into multiple resoFurther interpretation of the data requires explicit considera- nances at different frequencies. In case of a spheroid there tion of the selection rules in s-SHG spectroscopy as given in would be two plasmon peaks where the resonance frequency the following paragraph. of the oscillation along the short axis is blue shifted and the In s-SHG spectroscopy, only those induced polarization oscillation along the long axes is red shifted with respect oscillations that take place in an asymmetric potential can give to the single spherical plasmon resonance. The experimenrise to second harmonic generation. For supported metal parti- tally observed peak (at 3.68 eV) is blue shifted by 0.37 eV cles, centro-symmetry is only broken along the axis perpendic- with respect to the predicted plasmon resonance energy for ular to the surface. As a result, only oscillations of conduction spherical particles in an isotropic dielectric medium. Since electrons perpendicular to the surface are confined to an asym- this mode is observed in the s-SHG spectrum (i.e., it is SHG metric potential. Therefore, excitations in this direction are active), it must be originating from an oscillation perpenSHG active, whereas charge oscillations parallel to the surface dicular to the surface. Secondly, the blue shift indicates that are SHG inactive and do not appear in the nonlinear spectrum. this is caused by deviation from spherical shape as the aniWe now interpret the experimental data under this light. For sotropy in the surrounding dielectric medium would result a schematic view of the s-SHG active and inactive modes and in a red shift for oscillations perpendicular to the surface the polarization of the probe laser beam consult Figure S4. (see above). We thus conclude that this mode (i.e. the peak Evidently there are two different resonances present observed in s-SHG spectrum) belongs to an oscillation along in the sample, only one of which is detectable by s-SHG the short axis of the oblate NPs, which lie on the surface in a spectroscopy. Based on the s-SHG selection rules that were flattened form. We also conclude that there should be a red introduced earlier, the absence of the resonance band in shifted plasmon resonance corresponding to the oscillation the nonlinear spectrum (that is observed in s-CRD spec- along the long axis of the NPs lying parallel to the surface. trum) suggests that the related electron oscillation occurs According to the aforementioned selection rules, this red parallel to the surface. In addition, the fact that there are shifted resonance is SHG inactive and will not appear in the two different resonances present in the spectra implies s-SHG spectrum. It should, however, be detected in the linear the existence of plasmon splitting effects and thus, devia- spectrum, as the s-CRD method has no preference for any tion of the clusters’ shape from perfect spherical form. Mie of the plasmonic modes. Indeed, the onset of the absorption theory predicts the plasmon frequency of supported spher- band that is observed in the linear spectrum exactly follows ical silver particles with an average size of 1.5 nm to be this description (Figure 3). Due to the high scattering losses located at 3.31 eV based on the following equation for the of BK7 substrates at photon energies above 3 eV, and comabsorption cross section. This equation is generally valid paratively lower mirror qualities in the UV, it is not possible for particles with spheroid shape and assumes an effec- to fully resolve this resonance directly by s-CRD spectrostive homogeneous dielectric constant for the surrounding copy. Using Equation (1), in combination with the knowlmedium:[5c] edge gained from s-SHG spectrum (i.e., the amount of the
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Small Supported Plasmonic Silver Clusters
observed blue shift) it is possible to predict the position of the red shifted component. The outcomes of such simulations are graphically included in Figure 3 (see Figure S3 for more details). These simulations are in very good agreement with the experimental data, further supporting the analysis given above. Such particle flattening effects have been also reported earlier for larger supported silver NPs.[5a,c,11] The observed increase in the absorption cross section towards higher energies is due to the broad interband transitions in silver particles. Although these transitions are predicted in simulations for linear absorption cross sections (as is the case here) they do not necessarily contribute to s-SHG spectrum. In summary, the advantages of a linear and a nonlinear surface spectroscopy method are successfully combined to study sub-monolayer coverages of the smallest supported silver NPs to date. Whereas s-SHG spectroscopy compensates for the limited spectral range of s-CRD spectroscopy in the UV, the latter is unavoidable due to the intrinsic ambiguity of s-SHG process. Our results show that well separated borosilicate supported silver NPs with an average diameter of 1.5 nm still can be treated as plasmonic particles. Also plasmon splitting effects are observed for particles with this mean size. Using our purely spectroscopic approach, we found that such small silver particles lie with their short axis perpendicular to the surface. Our analysis based on Mie theory is capable of explaining the experimental findings with compelling agreement, which suggests the appositeness of this classical theory for supported particles of similar size. The refinement of the traditionally applied model by introducing direction dependent dielectric constants and thus reflecting the anisotropic nature of the system (i.e. supported metal particles) leads to a more realistic and reliable analysis. The transition from plasmonic to molecular optical response of supported silver clusters (which is expected to appear for small particle sizes) should therefore be located at still smaller cluster sizes. The high signal to noise ratio in the presented spectra demonstrates the strength of the presented approach. Further studies on size selected silver clusters are on the way.
Experimental Section Silver clusters were generated by a pulsed laser vaporization cluster beam source. A quadrupole mass filter (Extrel, 5500 series) was used to narrow the mass distribution of the clusters before soft-landing on the BK7 substrate under ultra-high vacuum (UHV) conditions. The substrate is mounted on a sample holder of home design attached to a rotatable xyz manipulator, which is cooled with liquid nitrogen. Further details of the cluster beam source and UHV chambers are presented in an earlier work.[14] Cluster currents and mass distribution are monitored by a Faraday plate that can be positioned on the axis of the cluster beam. The spectroscopic measurements are performed in situ as described in an earlier publication.[9a,15] Surface coverage was experimentally determined by recording optical loss of the sample while scanning the surface (see Figure S5). Details of data treatment for s-SHG spectroscopy are available elsewhere.[12] For all measurements presented in this work signals are averaged over 200 laser shots.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work has been supported by the BMBF through the project IC4 and the European Research Council (ERC) through an Advanced Research Grant (246645-ASC3).
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Received: October 2, 2013 Revised: January 23, 2014 Published online: March 10, 2014
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