Studies of Dodecanethiol Capped Ag and Au Nanoparticles Using

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Studies of Dodecanethiol Capped Ag and Au Nanoparticles Using Synchrotron Radiation Based Photoelectron Spectroscopy Håkan Rensmo,*,† Karin Westermark,† Donald Fitzmaurice,‡ and Hans Siegbahn† Department of Physics, University of Uppsala, Box 530, S-751 21 Uppsala, Sweden and Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland Received June 20, 2002. In Final Form: September 4, 2002 Differently sized dodecanethiol capped Au and Ag nanoparticles have been investigated using synchrotron radiation based photoelectron spectroscopy (PES). Comparative measurements were also performed on self-assembled monolayers (SAMs) of dodecanethiol at vacuum evaporated Au and Ag surfaces. The series of experiments display important differences in the surface electronic and molecular structure between the different kinds of materials. Specifically, the changes measured in binding energies are discussed with respect to initial and final state effects. Also, a broadening is observed for metal-core levels in the smaller nanoparticles. Thus, surface atoms in a different chemical state than the rest of the surface atoms are detected. In valence-level spectra, the matching between the orbitals centered at the surface-coating molecule and the orbitals centered on the core materials are discussed.

Introduction The realization of methods for the synthesis of nanoscale condensed-phase components (e.g., semiconductor and metal nanoparticles) as well as strategies for assembling these components has enabled the investigation of their use as structural materials, as catalysts, and as critical components in advanced electronic architectures.1-10 Studies of these materials also provide insight into sizedependent scaling laws, the chemistry of size and shape, photophysical processes in confined geometries, and others. Specifically, nanoparticles stabilized by selfassembled monolayers (SAMs) of thiol molecules have attracted much attention because of the simplicity of preparation and their applicability.11-18 Although some properties for these kinds of nanoparticles are linked to * Address correspondence to this author. † University of Uppsala. ‡ University College Dublin. (1) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (2) Alivisatos, A. P. Science 1996, 271, 933. (3) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (4) Herron, N.; Calabrese, J. C.; Farneth, W. E.; Wang, Y. Science 1993, 259, 1426. (5) Schmid, G. Chem. Rev. 1992, 92, 1709. (6) Moritz, T.; Reiss, J.; Diesner, K.; Su, D.; Chemseddine, A. J. Phys. Chem. B 1997, 101, 8052. (7) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371. (8) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (9) Whetten, R. L.; Khoury, J. L.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (10) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (11) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801. (12) Brust, M.; Kiely, C. J. Colloids Surf., A 2002, 202, 175. (13) Korgel, B. A.; Fitzmaurice, D. Phys. Rev. B 1999, 59, 1419114201. (14) Kiely, C.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin D. J. Nature 1998, 396, 444.

their interior composition and symmetry, many functions depend on their surface molecular end electronic structure. Photoelectron spectroscopy (PES) is a route to obtain such information. In this study, synchrotron radiation based PES is used to compare the structures of Ag and Au nanoparticles having different sizes. The measurements are also compared with spectra from SAMs on vacuumevaporated Ag and Au surfaces.19-38 (15) Fink, J.; Kiely, C.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (16) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036-7041. (17) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379-8388. (18) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904-13910. (19) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (20) Yan, C.; Golzhauser, A.; Grunze, M.; Woll, C. Langmuir 1999, 15, 2414. (21) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (22) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (23) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (24) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (25) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (26) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (27) Bourg, M.-C.; Badia, A.; Lennox, R. B. J. Phys. Chem. 2000 104, 6562. (28) Heister, K.; Zharnikov, M., Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (29) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (30) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (31) Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888. (32) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 7, 2408. (33) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333. (34) Yang, Y. W.; Fan, L. J. Langmuir 2002, 18, 1157. (35) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825.

10.1021/la0261040 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/02/2002

Dodecanethiol Capped Ag and Au Nanoparticles

Langmuir, Vol. 18, No. 26, 2002 10373 performed.20,27 In the study below, these samples are referred to as Ag-SAM and Au-SAM. Photoelectron spectra were obtained using synchrotron radiation at beamline I411 of the Swedish National Synchrotron Source MAX-lab.39 The takeoff angle of the photoelectron with respect to the substrate surface was 80° and the angle between the photon polarization and the photoelectron direction was 0°. The quantum chemistry calculations were performed using the program-package HyperChem (ver. 5). The electronic structure was calculated with a single-point calculation at the ab initio 6-31 G* level. To facilitate the comparison, the calculated energy levels were aligned with the lowest binding energy level in the PES experiment.

Result and Discussion Figure 1. TEM images of two-dimensional hexagonally closepacked monolayers formed by evaporating chloroformic dispersions of size-monodisperse nanoparticles (about 0.5 mg/mL) on a carbon-coated grid. (a) small Au nanoparticle (b) large Au nanoparticles (c) Ag nanoparticles.

Experimental Section Nearly monodisperse chloroform dispersions of dodecanethiol capped Ag nanoparticles were prepared using previously reported methods.12,13 Synthesis of the smaller (2-nm) Au nanoparticles has also been described previously.11 Synthesis of the larger (4.2nm) Au nanoparticles was made by slightly modifying the preparation. In this case, the dodecanethiol was added to a solution of the phase transfer catalyzed stabilized nanoparticles in chloroform14,15 and left to react for 12 h. The nanoparticles were then isolated again by multiple centrifugation and washing (EtOH) cycles and finally redispersed in chloroform. The obtained nanoparticles have earlier been characterized in detail by 1H NMR spectroscopy, elemental analysis, TEM, XRD, and SAXS.13,16-18 The nanoparticles are best described as “soft spheres”, having an effective diameter equal to the diameter of the nanoparticle core and the densely packed dodecanethiol layers. Unlike hard spheressparticles that exhibit a sharp infinitely strong repulsion upon touchingsthe soft spheres employed in the present study exhibit a gradually increasing repulsion because of the forces that act between the stabilizing ligands and an attraction due to the van der Waals (vdW) forces that act between the solid metallic cores.13,17 In this study, 1H NMR (Varian 300 MHz FT-NMR) studies of the dodecanethiol capped Ag and Au nanoparticles confirm the presence of alkane chains at the surface. The resonances of the chemisorbed dodecanethiol molecules can be correlated with the resonances observed for the same compound in solution. The observed broadening is most likely a result of the restricted motion of these chains compared to their solution state. Shown in Figure 1 are representative TEM (JEOL JEL-2000 EX, 80kV) images obtained for the dodecanethiol capped nanoparticles prepared as described above. Two-dimensional closepacked arrays of nanoparticles are observed in which the constituent nanoparticles are separated from their nearest neighbors by the dense alkane monolayer. Sizing these particles, assuming they are spherical, gives average core diameters ((0.5 nm) and average surface-to-surface interparticle spacings. The latter was determined to be about 15 Å for all nanoparticles. Two different particle sizes for both Ag and Au were investigated in detail by PES and are referred to as Au-np-I (4.2 nm in core diameter), Au-np-II (2.0 nm in core diameter), Ag-np-III (7.5 nm in core diameter), and Ag-np-IV (4.5 nm in core diameter). The samples investigated by PES were prepared by solvent evaporation on a carbon-coated conducting substrate. In all cases, nanoparticle multilayers were investigated. In addition, measurements on SAMs of dodecanethiol at flat Ag and Au surfaces (prepared by vacuum evaporation) were (36) Zubra¨gel, Ch.; Schnider, F.; Neumann, M.; Ha¨hner, G.; Wo¨ll, Ch.; Grunze, M. Chem. Phys. Lett. 1994, 219, 127. (37) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, L.; Langmuir 2001, 17, 8. (38) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092.

Survey Spectra. Figure 2 shows two representative PES survey spectra obtained for Au and Ag nanoparticles measured with a photon energy of 1061 eV. In the figure, the regions of main interest (Ag3d3/2, Au4f7/2, C1s, S2p, and the valence levels) are indicated. The general appearance of these survey spectra is consistent with that expected for the formation of SAMs at Au or Ag surfaces.29 The nanoparticle assemblies studied are prepared from solution by solvent evaporation. The nanoparticles then hexagonally close pack into three-dimensional arrays in which the constituent nanoparticles are separated from their nearest neighbors by the intervening dense alkane monolayers (15 Å). PES is a surface-sensitive technique and the emitted electrons measured in these spectra have an inelastic mean free path of typically 20 Å, that is, of the order of the nanoparticle dimensions. Consequently, electron emission from the top nanoparticle layer largely dominates the PE spectra. This layer contains relatively large volumes from which elastically emitted electrons originate from the intervening capping aliphatic carbon chain only. Therefore, when comparing intensities for the metal relative to carbon (Au3d/C1s or Au4f/C1s), these are expected to be smaller for the nanoparticle assemblies than for the Ag-SAM and Au-SAM and also decrease with decreasing particle size. A comparison between the measured relative amounts of Ag and Au with respect to carbon for 4.5-nm Ag nanoparticles, 7.5-nm Ag nanoparticles, 4.2-nm Au nanoparticles, 2.0-nm Au nanoparticles, and SAMs formed on Ag and Au films are given in Table 1. As seen in the table, the relative intensities follow the expected trends. A quantitative value of the differences in intensities was estimated by calculating the relative surface areas covered by the metal core (see Table 1), that is, the nanoparticles are treated as circles having a diameter of d and are hexagonal close packed with a surface-to-surface interparticle spacing of 15 Å. Such a simplified model qualitatively explains the values reported in Table 1. The survey spectra and the trends in relative intensities are thus compatible with the structures obtained with other techniques as discussed above. A more detailed PES investigation of the nanoparticle surfaces in the selfassembled nanoparticle arrays was then performed. Highresolution spectra of the core levels (C1s, S2p, Ag3d5/2, and Au4f7/2) as well as the valence-band structure were measured and the results are discussed below. C1s. In Figure 3 are shown the C1s spectra of Ag and Au nanoparticles measured with a photon energy of 758 eV. All spectra display a single symmetrical peak having a fwhm of about 0.9 eV. These results are very similar to those obtained for SAMs formed at flat surfaces.28,34,38 (39) Ba¨ssler, M.; Forsell, J. O.; Bjo¨rneholm, O.; Feifel, R.; Jurvansuu, M.; Aksela, S.; Sundin, S.; So¨rensen, S. L.; Nyholm, R.; Ausmees, A.; Svensson, S. J. Electron Spectrosc. Relat. Phenom. 1999, 103, 953.

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Figure 2. Measured survey PE spectra of Ag nanoparticles and Au nanoparticles (hν 1061 eV). Table 1. Change in Relative Intensities of the Metal-Core Level (Au4f and Ag 3d) with Respect to C1sa preparation

experimental intensity relative 2D-SAMs

estimatedb

Ag-SAM Ag-np-III (7.5 nm) Ag-np-IV (4.5 nm) Au-SAM Au-np-I (4.2 nm) Au-np-1I (2.0 nm)

1 0.70 0.34 1 0.42 0.30

1 0.63 0.51 1 0.49 0.30

a The values for the nanoparticles are compared with those obtained for the SAMs on the evaporated metal films. The estimated values are calculated by considering the area covering the metal core for a monolayer of nanoparticles. b See text for details.

For SAMs formed at flat surfaces, trends in the Fermi level referenced C1s binding energy have been observed, such as a continuous shift with respect to the length of the alkanethiol chain35 or the odd-even effect when substituting the terminal methyl group.30,31 In accordance with data previously published for SAMs formed at flat surfaces,28 our Fermi level referenced C1s binding energies for the Ag-SAM (285.20 eV vs EF) and the Au-SAM (284.87 eV vs EF) are clearly different. However, in our vacuum-level referenced C1s data (Table 2), the flatsurface values (Ag-SAM and Au-SAM) are very similar. This indicates the importance of additional factors than those previously discussed28 in deciding observed shifts, for example, work function differences between substrates when comparing different substrate materials. Since our study concerns effects when changing the substrate (by comparing flat surfaces and nanoparticles and by comparing different materials), our analysis of the C1s peak position focuses on the variations of binding energies versus the vacuum level (C1svac), see Table 2. Generally, the measured values are fairly similar within about 0.3 eV. However, some interesting trends are observed when comparing the SAMs formed at the metal

Figure 3. C1s spectra measured for the nanoparticles measured at 748 eV.

films with those of the nanoparticles or when comparing nanoparticles of different core materials. The Ag and Au film both yields values for C1svac of 289.0-289.1 while the values obtained for the nanoparticles are higher and about

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Table 2. Measured C1s Ionization Potentialsa C1s vs vac (eV) Ag-SAM Ag-np-III (7.5 nm) Ag-np-IV (4.5 nm) Au-SAM Au-np-I (4.2 nm) Au-np-1I (2.0 nm)

289.01 289.22 289.17 289.07 289.18 289.26

a Determination of vacuum energies involve a set of measurements, i.e., calibration of the photon energy, using first- and secondorder light, and measurements of the kinetic energies of the C1s electrons using the cutoff edge. The experimental errors in the vacuum energies are therefore somewhat larger than the errors in relative binding energies (estimated to 0.03 eV).

Table 3. Binding Energies for the S2p3/2 Relative to C1S, Ag3d5/2, and Au4f7/2

Ag-SAM Ag-np-III (7.5 nm) Ag-np-IV (4.5 nm) Au-SAM Au-np-I (4.2 nm) Au-np-1I (2.0 nm)

S2p3/2 vs C1s (eV)

S2p vs Ag3d (eV)

-123.27 -123.15 -123.25 -122.93 -122.99 -122.61

-206.30 -206.32 -206.36

S2p vs Au4f (eV)

77.99 78.04 78.20

Table 4. Binding Energies for the Metal-Core Levels Relative to the Fermi Level Ag3d5/2vs EF (eV) Ag-SAM Ag-np-III (7.5 nm) Ag-np-IV (4.5 nm) Au-SAM Au-np-I (4.2 nm) Au-np-1I (2.0 nm)

Figure 4. Ag3d5/2 spectra measured for the Ag nanoparticles and the SAM of dodecanethiol formed at a Ag film (hν ) 510 eV).

Au4f7/2 vs EF (eV)

368.23 368.21 368.21 83.95 83.99 83.92

289.2-289.3 eV. This shift may be a result of differences in surface coverage or surface structure changing the intermolecular interactions and thereby the initial state energy. Such a shift is however probably very small. Another explanation, that we find more likely, is that the shifts originate from a so-called final state effect. When photoionization of the aliphatic carbons in the SAMs occurs, a localized hole in a C1s core level is produced. A corresponding image charge will be formed in the metal substrate that lowers the final state energy of the molecule and consequently the observed C1svac binding energy decreases. Similarities in the values of final state screening of Au and Ag can be expected since they have very similar free-electron densities. However, small particles are known to screen less efficient, which would explain the higher values obtained for the nanoparticles. In particular, this explains that the highest C1svac value is obtained for the smallest nanoparticles (2.0-nm Au nanoparticles). Contributions to the screening may also occur via relaxation of the SAM layer itself. However, for the dense alkane layer coverage studied here, these effects are believed to be similar in all samples. The results emphasize the importance of particle size and monodispersity when interpreting peak positions as well as line profiles for materials of the present kind. The effects of variations in C1svac discussed above should therefore be kept in mind when analyzing small peak shifts using C1s or the Fermi level as a reference signal. The S2p spectra shown below are referenced versus C1s. The spectra for Au4f, Ag3d, and the valence bands are given versus the Fermi level. Ag and Au. For SAMs, the energies of Au4f/Ag3d relative to C1s are often cited; however, the origin of such relative binding energies may be difficult to interpret.

Initial state shifts occur for surface-metal atoms as a consequence of different surface structures or as a consequence of different bonding modes to the sulfur atom. These possibilities have been discussed in the literature.20,26,27 For example, the sulfur atom is expected to withdraw electrons, shifting the peaks with respect to the vacuum energy level toward higher binding energy. On the other hand, the differences in screening when creating a core hole on a Au/Ag atom (see discussion above) also affects the binding energy and shifts the peak position toward lower binding energy for smaller particles. Such final state effects are not often discussed but will also affect peak positions. In this work, we only note that Au4f and Ag3d binding energies versus the Fermi level is almost constant (see Table 4) and instead focus our detailed analysis on the line shapes. These can also be used to highlight the similarities and differences in the electronic structure for different materials, in particular when comparing nanoparticles having different sizes as well as in comparisons with the metallic films. High-resolution PES spectra for Ag3d5/2 and Au4f7/2 are shown in Figure 4 and Figure 5. For both kinds of nanoparticles, the larger particle size (4.2-nm Au and 7.5nm Ag) displays very narrow and symmetric line shapes (fwhm of 0.50 and 0.40 eV, respectively), which are identical to those obtained for the Au and Ag film. Such narrow line width for the Au-SAM and the Ag-SAM implies very similar binding energies for the surface- and bulk-metal atoms, as previously discussed by other workers.31 However, it is here important to realize that already a 4-nm-sized nanoparticle has about a third of its atoms at the surface and that the escape depth (about 15 Å) further accentuate the nanoparticle surface. Therefore, these results strongly indicate that the chemical state for most of the surface-metal atoms are very similar to the state of the bulk atoms both for SAMs formed on metallic films and the larger nanoparticles measured here. In contrast to results for the larger nanoparticles, the smaller nanoparticles display broader peaks. Such effects on the line shape as a result of the particle size have not been discussed previously for these kinds of nanoparticles,

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Figure 5. Au4f7/2 spectra measured for the Au nanoparticles and the SAM of dodecanethiol formed at a Au film. The measurements were performed at different photon energies.

and as expected the effect is especially clear for the smallest nanoparticles studied (i.e., the 2.0-nm Au nanoparticles) displaying an asymmetry toward higher binding energies (fwhm of about 1.0 eV). For these 2.0-nm large Au nanoparticles, about two-thirds of the atoms are expected to be surface atoms. Together with the spectra measured on S2p (see below), these results indicate that for the small (2 nm) Au nanoparticles a fraction of the surface atoms are in a different chemical state than the rest of the surface Au atoms. The Au4f peak was investigated in more detail by measuring it at lower and higher photon energies. As seen from Figure 5, the asymmetric peak broadens even more when using 150 eV photon energy. This trend of the peak width further supports this conclusion. S2p. The chemical state of the sulfur chemisorbed at Ag and Au surfaces as well as on nanoparticles has been discussed in the literature.26,40-42 The state depends on preparation procedure (i.e., time for the 2D self-assembled layer to form) and the structure is sensitive to temperature and X-ray radiation damage.26,37,43 For the end-station used here, the spot of the synchrotron radiation is very small allowing for very large intensity. Therefore, after prolonged measurements, X-ray damage was observed on the S2p peak and care was taken to perform the measurements at fresh spots. (40) Bensebaa, B.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf. Sci 1998, 405, L472. (41) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (42) Walczak, M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396. (43) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods B 1997, 131, 245, 103.

Figure 6. S2p spectra measured for the Ag nanoparticles and the SAM of dodecanethiol formed at Ag film. Ag-np-III (510 eV), Ag-np-IV (251 eV), Ag-SAM (251 eV).

A summary of the peak positions for the different materials is given in Table 3. Peak positions are given relative to Ag3d5/2 and Au4f7/2 as well as with respect to C1s. For the Ag materials, a detailed analysis of peak shapes is shown in Figure 6. While the Ag film is expected to contain relatively large flat regions having a close packed self-assembled monolayer, the contrary is expected for the nanoparticles. However, neither from peak positions nor from the line shapes could any difference in chemical state be observed. The comparison therefore indicates that the chemical states of thiols at nanoparticle surfaces (larger than 4 nm in diameter) are very similar to that of Ag-SAM. Also for the larger Au nanoparticles, a single chemical state of surface adsorbed sulfur atoms is observed, see Figure 7. However, for the smaller nanoparticles, the peaks display a clear asymmetry and can be deconvoluted using two spin-orbit split pairs. Since such an asymmetry was also observed for the Au4f peak, this indicates that they originate from the same Au-S adsorption site. Finally, comparing sulfur adsorption for the two materials by calibrating S2p versus C1s, a lower binding energy is measured for the silver surface. Such a calibration is expected to partly compensate for final state shift variations because of differences in particle size.

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Figure 7. S2p spectra measured for the Au nanoparticles and the SAM of dodecanethiol formed at Au film. Au-np-I (251 eV), Au-np-II (251 eV), Au-SAM (251 eV).

This result therefore suggests that the sulfur adsorption onto Ag results in a larger charge transfer, an effect that is independent of the structure of the metal material (i.e., it applies both to nanoparticles and films). Valence Levels. The valence bands contain overlapping molecular orbital bands built up by metal d or s energy levels (Au5d/Ag4d and Au6s/Ag5s) as well as C2p, C2s, and S3p energy levels.36,44-47 The measured structures are generally complicated, depending on factors such as photon energy (variation in cross section), as well as angular symmetries (X-ray polarization vs takeoff angle) and such effects were clearly observed. The effects measured for the valence level electron structures are discussed by comparing a set of measured valence bands with that of Ag nanoparticles measured at a photon energy of 150 eV. For measurements performed at 150 eV, the Ag4d levels (clearly observed using a photon energy of 510 eV) are suppressed and the photoemission from the alkane chains dominate, see Figure 8. A calculation of the dodecanethiol electronic structure is also given in Figure 8. To highlight the S3p contributions in the frontier orbital structure, the calculated atomic orbital structures for the four highest occupied molecular orbitals are also shown. At higher (44) Duwez, A.-S.; Di Paolo, S.; Ghijsen, J.; Riga, J. J. Phys. Chem. B 1997, 101, 884. (45) Leavitt, A. J.; Beebe, T. P. Surf. Sci. 1994, 314, 23. (46) Bru¨ckner, M.; Heinz, B.; Morgner, H. Surf. Sci. 1994, 319, 370. (47) Riely, H.; Price, N. J.; White, R. G.; Blyth, R. I. R.; Robindon, A. W. Surf. Sci. 1995, 331-333, 189.

Figure 8. (a) Valence-level spectra of Ag-np-III measured at two different photon energies. Also shown are the energies calculated by quantum chemistry calculations. (b) Calculated orbital structure for dodecanethiol. To highlight the main S3p contribution, the orbital structure for the four highest molecular orbitals are shown.

binding energies, the calculated orbitals were largely dominated by the C2p orbitals. Supported by these calculations and other studies,44,45 the large main peak at 6 eV and shoulder at about 8 eV are assigned mainly to different combinations of the atomic C2p orbitals. The orbitals responsible for the bonding to the metal surface (π* backbone having large S3p contributions) are observed at lower energies (2 eV), while the region around the Fermi edge is dominated by Ag4s. At higher binding energy (13 eV), a relatively welldefined band is observed. In earlier theoretical work44 comparing different molecular structures of aliphatic chains, such a pronounced feature is consistent with a zigzag alkane structure. This feature was observed for all measurements performed at low-photon energies (highsurface sensitivity, e.g., hν)150 eV) and thus indicates a close-packed SAM structure for all preparations (see Figures 8-9).

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Figure 9. Valence-level spectra of Ag-np-III, Ag-np-IV, AgSAM, and an unmodified evaporated Ag film measured with a photon energy of 150 eV.

A detailed comparison between the differently sized nanoparticles and the Ag-film is displayed in Figure 9. The spectra are normalized versus Ag5s as shown from the inset. We first note the differences in relative intensities in the C2p binding energy region (5-10 eV). The relatively higher intensity observed for the smaller nanoparticles is a consequence of their size, that is, for the smaller nanoparticles there are relatively more carbons in the top layer. The intensity in the region around 2 eV is very similar in all measurements (see the inset in Figure 9). This region is expected to have a large S3p contribution and the similarity in intensity is therefore merely a confirmation that all samples have a very similar ratio between surface silver atoms and sulfur atoms. The small variations observed could result from differences in average adsorption configurations which cannot however be established from the present set of data. The fact that the structure at 2 eV has a very small contribution of Ag5s is seen in Figure 8. Here, we measured the same nanoparticles (Ag-np-III nanoparticles) with two different photon energies thus changing the relative cross section between the Ag 5s and the binding orbitals (largely S3p). The spectra therefore show an energy difference of about 2 eV between the uppermost metal-centered electrons and those binding the sulfur linker atom. Finally, Figure 10 shows the valence band of Agnanoparticles compared with those obtained for the Aunanoparticle. At 150 eV (very surface sensitive mode), the obtained structures are very similar to those obtained for Ag. This is expected since both spectra are dominated by the dodecanethiol layer. The comparison using two photon energies, corresponding to Figure 8 for Ag- nanoparticles, is more difficult to make because of the smaller energy distance between the Au6s and Au5d levels. An estimate of the position of the linker orbitals could therefore not be made in this case.

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Figure 10. Valence-level spectra of Au-np-I, Au-np-II, and Ag-np-IV measured with a photon energy of 150 eV. A valencelevel spectrum of Au-np-II measured with a photon energy of 510 eV is also included.

Conclusion The PES spectra for the dodecanethiol capped nanoparticles show strong similarities to the SAMs of dodecanethiol at flat surfaces; however, some important differences were observed. Measurements of the C1s vacuum energy level indicated differences related to the nanoscale of the material. Thus, the smaller nanoparticles show a varying screening of the photoinduced core hole, shifting the C1s energy levels. That the sulfur bind to the metal surface is established independent of material and particle size. However, the charge transfer due to the bonding is different when comparing the Au materials with the Ag materials. In a comparison with SAMs at flat surfaces, no difference in line shape for the metal core energy levels was observed for the larger nanoparticles. However, for smaller nanoparticles, the line shape of the metal core energy levels was slightly broader. In line with the S2p spectra, the extra broadening in Au4f was interpreted to arise from a fraction of the surface atoms. The valence structure is in accordance with what is expected for SAMs at the surface. Specifically, a feature at low binding energy (about 2 eV) versus the Fermi level was interpreted as the metal-sulfur bond. Acknowledgment. This work was supported by the Swedish Research Council (VR), the Foundation for Strategic Research, Sweden (SSF), STINT (The Swedish Foundation for International Cooperation in Research and Higher Education), and the Go¨ran Gustafsson Foundation. Dr. Declan Ryan is gratefully acknowledged for valuable discussions. LA0261040