Photoemission Enhancement Induced by Near-Fields via Local

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Photoemission Enhancement Induced by Near-Fields via Local Surface Plasmon Resonance of Silver Nanoparticles on a Hydrogen-Terminated Si(111) Surface Tsuneyuki Nakamura,† Naoyuki Hirata,† Yuji Sekino,† Shuhei Nagaoka,† and Atsushi Nakajima*,†,‡ Department of Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan, and ERATO, Japan Science and Technology Agency (JST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: August 10, 2010

The photoemission enhancement with local surface plasmon resonance (LSPR) was studied by two-photon photoemission (2PPE) spectroscopy for size-selected silver (Ag) and gold (Au) metal nanoparticles (NPs) deposited on a hydrogen-terminated Si(111)-(1 × 1) [H-Si(111)] surface. At 0.0015 monolayer equivalents (MLE) of Ag NPs, the photoemission enhancement was observed at a photon energy of 3.10 eV, which corresponds to the peak energy for the LSPR of isolated Ag NPs. A surface with around 0.005 MLE LSPR of size-selected Ag NPs exhibited three- and four-photon photoemission processes, implying monodispersed Ag NPs on H-Si(111). This enhancement could not be observed for Au NP deposition, even at 1.0 MLE in the photon energy range of 2.90-3.23 eV. Taking into account the polarization and photoemission-angle dependences, the photoemission enhancement could be accounted for by a mechanism involving the nearfields induced by the LSPR of Ag NPs. This mechanism is consistent with an analysis based on the effect of the interparticle distance between Ag NPs on the near-field intensity and polarization. 1. Introduction Nanometer-scale species, such as clusters and nanoparticles (NPs), that show unique electronic, magnetic, and catalytic properties are promising functional candidates for unusual applications of nanomaterials in science and technology. The noble metal NPs, in particular, those of silver (Ag) and gold (Au), have been investigated particularly in recent years because they have a unique optical property called a “local surface plasmon resonance (LSPR)”sa collective oscillation of conduction electrons. When metal NPs are irradiated by an appropriate laser light, electric dipoles are induced by LSPR, which generate enhanced local electric fields generally called “near-fields”. In fact, vibrational spectroscopies based on surface-enhanced Raman scattering (SERS)1-5 and surface-enhanced infrared absorption (SEIRA)6,7 utilize the near-fields induced adjacent to noble metal NPs.4,8-16 As well as the surface-enhanced vibrational spectroscopies, the enhanced photoemission from the noble metal NPs themselves can be observed by electron spectroscopies, such as two-photon photoemission (2PPE) spectroscopy.17-21 However, the role of the near-fields induced by LSPR in promoting enhanced photoemission has not been uncovered because the methodology has not been developed not only for the preparation of well-defined nanoscale species but also for the necessary coverage-controlled deposition. Recently, we have reported Ag-NP-enhanced photoemission studied by the combination of a low-pressure differential mobility analyzer (LP-DMA)22-27 and 2PPE spectroscopy. In this article, we provide a full account of our studies of the photoemission processes of a substrate decorated with metal NPs by 2PPE spectroscopy, in particular, for size-selected Ag * To whom correspondence should be addressed. E-mail: nakajima@ chem.keio.ac.jp. Fax: +81-45-566-1697. † Keio University. ‡ Japan Science and Technology Agency (JST).

and Au NPs deposited on a H-Si(111) surface. In addition to the coverage dependence of Ag NPs reported previously, details of the photoemission-angle dependence clearly reveal the intrinsic role of near-fields induced by the LSPR of Ag NPs, via an analysis using the interparticle distance between Ag NPs. 2. Experimental Methods The experimental details of the preparation of size-selected metal NPs and the H-Si(111) substrates and 2PPE measurements have been described previously.25,26,28 Briefly, Ag and Au NPs were produced by vaporizing a rotating Ag or Au disk (diameter ) 50 mm, thickness ) 2 mm, purity ) 99.98%) by the second harmonic of a Nd3+:YAG laser (532 nm, ∼16 mJ/ pulse, 10 ns duration). The NPs produced were then carried into an LP-DMA where NPs of a specific diameter were sorted by controlling the He sheath gas flow and the applied electric field. The size-selected NPs of 4-12 nm in diameter within a typically 10% accuracy were then deposited onto the H-Si(111) substrate through the outlet nozzle of the LP-DMA (inner diameter ) 4.0 mm L). The total amount of the deposited NPs was estimated by monitoring the ion current onto the substrate during the deposition. The size distribution and morphology of the Ag and Au NPs were characterized by a field emission transmission electron microscope (FE-TEM) and a scanning tunnel microscope (STM). The H-Si(111) substrates were prepared in a manner similar to Wade and Chidsey’s method.29 Si(111) wafers were cleaned in 3 parts concentrated sulfuric acid and 1 part 30% by weight aqueous hydrogen peroxide for 40 min at 150 °C. The clean oxidized silicon substrates were immersed in a 5% by weight aqueous hydrofluoric acid for 3 min, then in a 40% by weight aqueous ammonium fluoride solution for 10 min, and finally deoxygenated by purging with argon inserted into the solution for 30 min. The quality of the H-Si(111) substrate (work

10.1021/jp105515t  2010 American Chemical Society Published on Web 09/10/2010

Photoemission Enhancement via LSPR of Ag NPs

Figure 1. TEM images and size distributions of (a) 6.15 and (b) 12.3 nm diameter Ag NPs selected by LP-DMA. Each histogram of the size distributions for over 200 NPs was fitted by a log-normal function.

function ) 4.24 eV)28 was characterized by X-ray photoemission spectroscopy (XPS) and attenuated total reflection infrared spectroscopy (ATR-IR).30 When the Ag (Au)-NP-deposited substrate was transferred from the LP-DMA chamber to the preparation chamber for the 2PPE/UPS chamber, the substrate was stored in a vessel filled with 99.9999% N2 gas. After transportation, the deposited substrate was transferred into the 2PPE/UPS chamber, where the base pressure in the chamber was 40 nm (c < 0.03 MLE) was observed at the same energy with that of H-Si(111), irrespective of the significant photoemission enhancement of 102-103, consistently suggesting that the enhanced photoemission at a low Ag NP coverage of 0-0.05 MLE could originate from the H-Si(111) substrate itself. 3.3.3. Photoelectron Yield against Interparticle Distance. Figure 6a shows the photoelectron yield emitted normal to the substrate (0°) against the averaged interparticle distance for the Ag NPs/H-Si(111) substrate (dNP ) 6.15 ( 1.17 nm). When the Ag NPs are isolated and sufficiently away from one another (the interparticle distance R > 40 nm), the photoelectron yield is enhanced only with p-polarization. For R ) 10-20 nm, however, the photoelectron yield with p-polarization decreases gradually with shortening of the interparticle distance of the Ag NPs. In contrast, as the interparticle distance is decreased below 10 nm, the photoelectron yield increased with both polarizations.

2p2 3

4πε0R'

)

R E0 2πε0R'3

(7)

where R is the polarizability of a metal NP and R′ is the interparticle distance (from center to center). Therefore, the change of the electric dipole moment is expressed as the following equation:

∆p1 ) RE2 )

R2 E0 ) ∆RE0 2πε0R'3

(8)

On the other hand, the intensity of the total electric field is generally expressed as

I ∝ |(p1 + ∆p1) + (p2 + ∆p2)| 2 ≈ 4R2 |E0 | 2 + 8R∆R|E0 | 2 (9) The second term in eq 9 represents the intensity of the nearfield induced from one metal NP. The polarizability, R, is proportional to d3,38 and, therefore, ∆R satisfies the following relation:

∆R ∝

d6 d6 ) R'3 (R + d)3

(10)

Here, it is noted that there is a difference in the definition between R′ in eq 10 and R. Therefore, the photoelectron yield of 2PPE, Y2PPE, excited by the electric near-field can be obtained as follows from eqs 9 and 10:

Photoemission Enhancement via LSPR of Ag NPs

Y2PPE ∝

{

}

8d9 |E0 | 2 (R + d)3

2

)

64d18 |E0 | 4 (R + d)6

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(11)

In eq 11, the number of the metal NPs excited by the electric near-field is not taken into consideration. Because the number of metal NPs excited is in proportion to the coverage in eq 4, the total photoelectron yield of 2PPE, Y′2PPE, excited by the electric near-field can be obtained as follows:

Y'2PPE ∝ Y2PPE ×

c 1 ≈ 2 (R + d)8

(12)

Exactly speaking, the near-field at a distance r from the particle (along a direction parallel to the incident light polarization) is given by an expansion of all possible multipolar modes; the smaller the distance r (relative to the particle size) where the field needs to be estimated, the higher is the number of terms that need to be included in the near-field expansion.39 Even within the dipole description, furthermore, dipole-dipole interaction between NPs should be additionally taken into account with the reduction in the interparticle distance.40 Indeed, the red shift in the plasmon resonance wavelength maximum caused by the multipolar contribution increases almost exponentially with the reduction in the interparticle distance,41,42 and the broadening of the plasmon resonance absorption takes place with retarded dipole-dipole interaction.39 Because the multipolar interactions between the particles become important at small interparticle distances of R/d < 0.5 (larger than 0.35 MLE),39 the multipolar contribution can be neglected in this work because the coverage studied is less than 0.15 MLE. On the other hand, the dipole-dipole interaction between NPs should be actually taken into account above ∼0.1 MLE.40 Below 0.15 MLE, however, the dipole-dipole interaction contributes little to the electric field of the NP given by eq 7, and moreover, the additional term for the dipole-dipole interaction can be described to be proportional to 1/(R′)3, which is the same interparticle distance dependence with the electric field in eq 7. The total photoelectron yield of 2PPE, Y′2PPE, can then be expressed as eq 12. 3.3.5. Photoelectron Yield Estimation. By using eq 12, the photoelectron yield for s-polarization is fitted as a function of the interparticle distance, as shown as the red curves in Figure 6. The fits are in good agreement even at short interparticle distances (R < 10 nm) for s-polarization, both at emission angles of 0° and 30°. This agreement clearly suggests that the nearfield from one Ag NP excites another Ag NP. This good agreement also suggests that the Ag NPs are monodispersed on the substrate, although possible disagreement could be caused by the interaction of among at least three NPs; one Ag NP could be excited by the near-fields from several Ag NPs. Note that p-polarization contains components both parallel and perpendicular to the substrate because the laser beam is irradiated onto the substrate surface at an incidence angle of 55° in this experiment. Therefore, when the interparticle distance is shorter than about 10 nm, the photoelectron yield is markedly enhanced with both polarizations. As shown in Figure 6, the photoelectron yield for interparticle distances less than 10 nm with p-polarization is roughly fitted by the same curve used for s-polarization. On the other hand, with p-polarization for long interparticle distances (R > 40 nm), there is little interaction between NPs; the photoelectron yield, Y′′2PPE, is proportional to the coverage,

and it can be described from eq 4 by the interparticle distance as follows:

Y′′2PPE ∝

1 (R + d)2

(13)

The fit using eq 13 shown as a blue curve in Figure 6a is also in good agreement with the photoelectron yield at low coverages with p-polarization. The discrepancy from the fit for interparticle distances shorter than 40 nm is probably caused by the decrement of the near-field component on the substrate, as mentioned later. 3.4. Enhanced Photoemission by Near-Fields. 3.4.1. Relation between Near-Fields and Interparticle Distance of Ag NPs. The near-field intensity is particularly enhanced under the Ag NPs (between the Ag NPs and H-Si(111) substrate), and thus, the magnitude and spatial distribution of the near-field effect on the substrate are strongly dependent on the interparticle distance.43,44 It has been reported that the near-field intensity increases for the component perpendicular to the substrate surface, until Ag NP separation becomes close to the incident wavelength, but the perpendicular component decreases as Ag NPs approach each other more closely and finally becomes a minimum at zero interparticle distance. This is the reason why the photoelectron yield decreases as the Ag NPs come closer together (10 nm < R < 40 nm) in Figure 6a. On the other hand, when Ag NPs closely approach each other (R < 10 nm), a Ag NP can be excited by the near-field emitted from another Ag NP. Here, only a near-field having a component parallel to the substrate surface can excite the electrons of the Ag NP because Ag NPs align in the lateral direction (parallel to the substrate surface). In particular, the near-field intensity of components in the direction parallel to the substrate surface is most enhanced at zero interparticle distance (touching each other) by LSPR coupling.43,44 3.4.2. Spatial Distribution of Near-Fields. The excitation by the near-field of LSPR discussed above is schematically summarized in Figure 7, in which the monodispersed Ag NPs interact with an external light field on a H-Si(111) substrate. The direction of the induced near-field depends strongly on the polarization of the incident light. When LSPR occurs in metal NPs excited by the incident light, the induced near-field exhibits an inhomogeneous intensity distribution: incident light with p-polarization can effectively induce a near-field in the direction perpendicular to the substrate surface, whereas that with s-polarization can do so in the parallel direction. It should be noted that only a near-field having a component perpendicular to the substrate surface could excite the electrons of the H-Si(111) substrate. As shown in Figure 4b, the photoelectron yield with ppolarized light becomes maximum not at 0° but at 5-10°. At a low coverage of 0.03 MLE, photoemission could be derived from H-Si(111). When photoelectrons from H-Si(111) are generated by the near-field of Ag NPs, the photoelectrons emitted normal to the substrate would be considerably disturbed because Ag NPs are located right above Si and H atoms in the photoemission direction. However, the photoelectrons from the H-Si(111) substrate could be emitted without a disturbing effect of the Ag NPs, when the photoemission angle is slightly tilted at 5-10°. Thus, the photoelectron yield does not become maximum at 0°, which confirms that the enhanced photoemission is attributed to the supporting H-Si(111) substrate. Finally, the properties of the near-field emitted from Ag NPs are discussed. As shown in the dependences of Ag NP coverage

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Figure 7. Schematic picture of the LSPR of uniform-sized spherical Ag NPs on the H-Si(111) substrate. (a) Ag NPs are isolated; therefore, from the spatial distribution of the induced near-fields, the enhanced photoemission from H-Si(111) occurs only with the electric field component perpendicular to the substrate. (b) Ag NPs are close to each other; the photoelectrons from Ag NPs could be emitted by the near-field induced with the electric field parallel to the substrate. (c) Ag NPs contact with each other, which induces the strong enhancement of the photoemission from Ag NPs.

and photoemission angle, the near-field is well polarized along the excitation polarization, whereas the near-field is nonuniform spatially. Because the electron excitations in Ag NPs could be induced without any strict propensity rules, such as the symmetry of the wave function for electronic states, coherent multiphoton excitation could occur rather easily.45 In our study, therefore, four-photon photoemission could occur with help from the LSPR of Ag NPs. 4. Conclusions We have investigated 2PPE spectra of Ag and Au NPs on H-Si(111) and found that the photoemission could be enhanced not only from Ag NPs but also from the supporting H-Si(111) substrate, together with the finding of multiphoton photoemission involving four photons. The photoemission enhancement of 0.0015 MLE Ag NPs was observed at a photon energy of 3.10 eV, which corresponds to the peak energy for the LSPR of isolated Ag NPs, whereas no enhancement could be observed

for Au NP deposition, even at 1.0 MLE, in the photon energy range of 2.90-3.23 eV. The dependences on photoemission angle and interparticle distance suggest that the photoemission enhancement could be originated by the near-fields of Ag NPs, which is well polarized with the incident light. Depending on the interparticle distance, the spatial distribution of the nearfields strongly affects the photoemission mechanism. A nearfield can enhance the photoemission from a H-Si(111) surface at a low coverage of Ag NPs, while it can do so from Ag NPs at higher coverages, >0.12 MLE. We have experimentally demonstrated that the near-field from the LSPR of metal NPs plays an intrinsic role in enhancing the electronic excitation probability for the supporting surface. Acknowledgment. This work is partly supported by a grantin-aid for scientific research (A) (No. 19205004) from the Ministry of Education, Culture, Sports, Science and Technology

Photoemission Enhancement via LSPR of Ag NPs (MEXT). T.N. expresses his gratitude for the research fellowship from Japan Society for the Promotion of Science for Young Scientist. References and Notes (1) Kerker, M.; Wang, D. S.; Chew, H. Appl. Opt. 1980, 19, 4159. (2) Inoue, M.; Ohtaka, K. J. Phys. Soc. Jpn. 1983, 52, 3853. (3) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (4) Lal, S.; Grady, S.; Goodrich, N. K.; Halas, G. P. Nano Lett. 2006, 6, 2338. (5) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569. (6) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (7) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (8) Bayer, D.; Wiemann, C.; Gaier, O.; Bauer, M.; Aeschlimann, M. J. Nanomater. 2008, 249514. (9) Krenn, J. R.; Weeber, J. C.; Dereux, A.; Bourillot, E.; Goudonnet, J. P.; Schider, B.; Leitner, A.; Aussenegg, F. R.; Girard, C. Phys. ReV. B 1991, 60, 5029. (10) Busolt, U.; Cottancin, E.; Ro¨hr, H.; Socaciu, L.; Leisner, T.; Wo¨ste, L. Appl. Phys. B: Lasers Opt. 1999, 68, 453. (11) Wiemann, C.; Bayer, D.; Rohmer, M.; Aeschlimann, M.; Bauer, M. Surf. Sci. 2007, 601, 4714. (12) Gunnarsson, L.; Rindzevicius, T.; Prikulis, J.; Kasemo, B.; Kall, M.; Zou, S. L.; Schatz, G. C. J. Phys. Chem. B 2005, 109, 1079. (13) Shimada, T.; Imura, K.; Hossain, M. K.; Okamoto, H.; Kitajima, M. J. Phys. Chem. C 2008, 112, 4033. (14) Okamoto, H.; Imura, K. Jpn. J. Appl. Phys. 2008, 47, 6055. (15) Zhang, P.; Smith, S.; Rumbles, G.; Himmel, M. E. Langmuir 2005, 21, 520. (16) Takimoto, B.; Nabika, H.; Murakoshi, K. J. Phys. Chem. C 2009, 113, 11751. (17) Merschdorf, M.; Pfeiffer, W.; Thon, A.; Voll, S.; Gerber, G. Appl. Phys. A: Mater. Sci. Process. 2000, 71, 547. (18) Evers, F.; Rakete, C.; Watanabe, K.; Menzel, D.; Freund, H.-J. Surf. Sci. 2005, 43, 593. (19) Pfeiffer, W.; Kennerknecht, C.; Merschdorf, M. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 1011. (20) Watanabe, K.; Menzel, D.; Nilius, N.; Freund, H.-J. Chem. ReV. 2006, 106, 4301. (21) Nilius, N.; Ernst, N.; Freund, H.-J. Phys. ReV. Lett. 2000, 84, 3994. (22) Knutson, E. O.; Whitby, K. T. J. Aerosol Sci. 1975, 6, 443. (23) Seto, T.; Kawakami, Y.; Suzuki, N.; Hirasawa, M.; Aya, N. Nano Lett. 2001, 1, 315.

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