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Langmuir 1998, 14, 7324-7326
Surface Defects and Homogeneous Distribution of Silver Particles on HOPG A. Stabel,†,‡ K. Eichhorst-Gerner,§ J. P. Rabe,§ and A. R. Gonza´lez-Elipe*,† Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla) and Department Q. Inorga´ nica. Avda, Americo Vespucio s/n, 41092 Sevilla, Spain, and Institut fu¨ r Physik, Physik von Makromoleku¨ len (Humboldt-Universita¨ t zu Berlin), Invalidenstrasse 110, D-10115 Berlin, Germany Received April 15, 1998. In Final Form: September 25, 1998
Introduction Owing to their chemically inert character, graphite and, in particular, highly oriented pyrolitic graphite (HOPG) have been widely used as a support to study the deposition of metals.1 Several techniques have been used for the characterization of the particle size and shape, including TEM and SFM.1-3 Using the latter techniques, several authors had shown that evaporated metals, such as Ag and Au, tend to form small particles when evaporated on HOPG.3,4 A general behavior of silver and other metals on this substrate is that the particles concentrate at the step edges existing at the surface of the graphite. At these locations, the particles grow in size with the amount of evaporated metal and with the temperature of the graphite substrate during evaporation. However, because of such agglomeration processes, on cleaved HOPG surfaces it is not a straightforward issue to get a homogeneous distribution of particles. It has been proposed to use thermal oxidation to generate surface defects that may act as nucleation centers to obtain a homogeneous distribution of particles on HOPG.5 Also, sputtering followed by thermal oxidation has been used to generate surface defects that serve as nucleation centers to fix the particles.6 Here we propose to use Ar plasma and ulterior thermal oxidation to introduce a controlled number of surface defects on HOPG. In previous papers,7,8 it has been shown that by this treatment it is possible to control the concentration of such defects. Typically, they have a thickness of one atomic layer and a diameter between 10 and 20 nm. The results of the present paper show that such a modified HOPG is a good substrate for the formation of a homogeneous distribution of silver particles deposited by evaporation. * Corresponding author. E-mail:
[email protected]. Telephone: 34-954489528. Fax: 34-954460665. † CSIC-Universidad de Sevilla. ‡ Present address: VAW Aluminium AG, Georg-von-BoeselagerStrasse 25, 53117 Bonn, Germany. § Humboldt-Universita ¨ t zu Berlin. (1) Ohto, M.; Yamaguchi, S.; Tanaka, K. Jpn. J. Appl. Phys. 1995, 34, L694. (2) Zoval, J. V.; Stiger, R. M.; Biernacki, P. R.; Penner, R. M. J. Phys. Chem. 1996, 100, 837. (3) Ahn, Y.-O.; Seidl, M. J. Appl. Phys. 1995, 77, 5558. (4) Francis, G. M.; Goldby, I. M.; Kuipers, L.; Issendorff, B.; Palmer, R. E. J. Chem. Soc., Dalton Trans. 1996, 665. (5) Chang, H.; Bard, A., J. Am. Chem. Soc. 1991, 113, 5588 (6) Ho¨vel, H.; Becker, Th.; Bettac, A.; Reihl, B.; Tschudy, M.; Williams, E. J. J. Appl. Phys. 1997, 81, 154. (7) Tracz, A.; Kalachev, A. A.; Wegner, G.; Rabe, J. P. Langmuir 1995, 11, 2840. (8) Zhong, X. Q.; Luniss, D.; Elings, V. Surf. Sci. 1993, 290, 688.
Experimental Section Silver has been deposited on HOPG by resistive evaporation from a metal source consisting of a silver filament wrapped around a tungsten wire. The source was carefully tested for linearity as a function of the evaporation time. A warming up period prior to deposition was necessary to achieve these steady-state conditions. Evaporation was carried out in the pretreatment chamber of a photoelectron spectrometer under a residual pressure of 10-8 Torr. Two types of graphite surfaces have been used: surfaces obtained by cleaving HOPG with scotch tape and defective HOPG surfaces prepared by exposure of the previous graphite surfaces to an Ar plasma and subsequent thermal oxidation. This method has been proven to be very efficient for the production of HOPG surfaces with a controlled density of homogeneous defects. They consist of monolayer thick holes produced by the removal of the carbon atoms in a circular surface region of a diameter of approximately 10-20 nm. The surface concentration of defects in these samples ranged between 700 and 1700 µm-2. More details about the production and morphology of these defective HOPG surfaces can be found in a previous publication.7 Examination by SFM of the silver particles has been carried out on four samples where silver was evaporated for the same period of time (i.e. 2.5 min) at 298 or 463 K, as the temperature of the substrate. Accordingly, the samples will be called HOPG-298, -463 and p-HOPG-298, -463 for the cleaved and Ar plasma-modified substrates, respectively. The XPS spectra have been recorded with an ESCALAB 210 spectrometer working in the pass energy constant mode at a value of 50 eV. The Mg KR radiation has been used as excitation source. Sensitivity factors supplied with the spectrometer have been used for the determination of Ag/C ratios from the intensity of the Ag 3d and C 1s photoelectron peaks. SFM images have been taken with a Nanoscope III multinode (Digital Instruments) in the tapping mode in air. The cantilever had resonance frequencies in the range 200-300 kHz. Recent results in the literature have shown that, under similar conditions, silver clusters are not knocked out from their nucleation centers by the SFM tip.9 This point was systematically checked in our samples by repeat imaging of the selected zones of observation.
Results and Discussion The deposition of silver on HOPG was calibrated by XPS. Typically, photoelectron spectra were characterized by the C 1s and C Auger peaks of the substrate and the Ag AES and Ag photoelectron peaks. In some cases, contamination of the surface by oxygen was also detected by the appearance of a small O 1s peak in the spectra. The maximum intensity of this peak corresponded to less than 0.1 monolayers. The deposition of silver on the HOPG was followed by measuring the intensity of the Ag 3d peak as a function of the evaporation time on a freshly cleaved sample. Figure 1 shows the evolution of the intensity of this peak expressed as the measured Ag/C ratio (i.e. ratio between the intensities of the Ag 3d and C 1s peaks corrected by the respective sensitivity factors) as a function of the evaporation time. The shape of the curve defined by the points in Figure 1 is not linear, indicating that silver does not extend as a monolayer on the surface of HOPG but agglomerates in the form of particles.10 It is worth mentioning that during the deposition experiment the position of the Ag 3d5/2 peak shifted slightly from 368.6 to 368.4 eV, for the lowest and highest depositions, respectively. Simultaneously, the width of the peak decreased slightly. This behavior is typical for some (9) Mahoney, W.; Schaefer, D. M.; Patil, A.; Andres, R. P.; Reifenberger, R. Surf. Sci. 1994, 316, 383. (10) Gollion, Ph.; Grenet G. Surf. Interface Anal. 1995, 23, 404.
10.1021/la9804334 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/13/1998
Notes
Langmuir, Vol. 14, No. 25, 1998 7325
Figure 1. Plot of the Ag/C ratio determined by XPS for a series of successive evaporations of Ag on HOPG (b). Included in the figure are similar ratios for the four samples analyzed by SFM (O).
metals deposited on HOPG and other supports and has been widely discussed in the literature for small metal particles deposited on various substrates.11 The four samples examined by SFM were also analyzed by XPS, and their Ag/C ratios are also included in Figure 1. From these four ratios only that corresponding to the HOPG-298 sample fits within the line in Figure 1 for the calibration experiment. On the contrary, the ratio of sample HOPG-463 is smaller and those of samples p-HOPG-298 and p-HOPG-463 are higher than that expected from the calibration experiment. These values suggest that while in sample HOPG-298 the dispersion of silver is equivalent to that obtained during the calibration experiment, the metal is more agglomerated in sample HOPG-463, while it is more dispersed in the two p-HOPG samples. Also, the agglomeration degree of silver is higher in sample p-HOPG-463 than in sample p-HOPG-298 and similarly higher in HOPG-463 than in HOPG-298. These results show that the temperature contributes to the agglomeration of silver to bigger particles. A similar behavior has been found by others before.4 This assessment of the dispersion degree of silver based on XPS measurements was confirmed by direct inspection of the particle size and shape by SFM. Figure 2 presents the SFM images of samples HOPG-298 and HOPG-463. These images correspond to graphite substrates which have been cleaved by scotch tape prior to evaporation and where some step edges are the typical surface defects observed by SFM. For convenience, the images in Figure 2 correspond to selected zones with a relatively high concentration of such defects. They show that silver particles form exclusively at the edge defects of the substrate, the flat surface plane remaining free from silver particles. These two images prove that silver particles tend to form at defective zones of the surface, in this case characterized by step edges. A similar result has been reported previously by several authors.3,4,12 The particle size in sample HOPG-298 was rather heterogeneous with a distribution profile between 10 and 30 nm. On the contrary, big clusters of typically about 30 nm dominate sample HOPG-463. This increase in the average particle size with temperature confirms the XPS result above in the sense that an increase of the temperature of the substrate produces an increase of the agglomeration degree of silver particles deposited by evaporation on (11) Wertheim, G. K. Z. Phys. D 1989, 12, 319. (12) Metois, J. J.; Heyrand, J. C.; Kern, R. Surf. Sci. 1978, 78, 191.
Figure 2. SFM images of samples HOPG-298 (top) and HOPG463 (bottom).
HOPG. A similar effect of temperature has been observed by STM/SFM for different metals deposited on HOPG.4 Figure 3 shows pictures similar to those in Figure 2, but corresponding to samples p-HOPG-298, -463. In contrast to the images in Figure 2, those in Figure 3 show a homogeneous distribution of particles of a similar size deposited on the HOPG surface treated with a plasma. Only in the case of sample p-HOPG-463 can a slight enrichment in the concentration of particles be observed at step edges. Also, it is observed in this case that the size of the particles at the step edge is relatively larger than that on the graphite planes. In sample p-HOPG-298, the silver particles have a very narrow size distribution around 10-15 nm and the particle density is ∼1350 µm-2, which is within a factor of 2 equal to the pit density. Thus, it seems that most of the pits generated on the surface of HOPG are acting as nucleation centers. This suggests that formation of silver particles proceeds by agglomeration of the evaporated atoms that, once deposited on the graphite surface, migrate to the pit centers, where they agglomerate into the observed particles. The rather sharp distribution of particle size found in this case also supports this view. A similar mechanism has been proposed recently for the formation of silver particles formed selectively on the pits produced by Ne+ bombardment of HOPG and where such a behavior has been attributed to the high mobility of the incoming atoms that would condense exclusively on the pit centers.6 In sample p-HOPG-463, the average particle size has a value of about 30 nm, while the island density is of the order of 180 µm-2. So, it seems clear that heating the substrate at 463 K
7326 Langmuir, Vol. 14, No. 25, 1998
Notes
It must be noted that, since in all cases the size of the particles is much higher than the mean free path of the electrons in silver (≈1.5 nm as determined according to Seah and Dench13), large changes in particle size such as those found in our case (e.g. from 10 to 30 nm) are not expected to produce large changes in the intensity ratios measured by XPS. In this sense the values of 1.1 and 1.2 for the Ag/C ratio determined for samples p-HOPG-298 and -463 are in good agreement with any assessment based on the dependence of XPS intensities on particle size and shape. However, this is not the case when comparing samples HOPG-298 and -463, where the XPS intensity ratios are 0.9 and 0.5, respectively. In this case the agglomeration of the particles in a restricted space around the step edge regions of the surface allows the particles to shadow each other when observed by XPS (i.e., a particle very close to others can prevent the ejected photoelectrons from reaching the detector), thus leading to a drastic attenuation of the spectrum. As a summary, from the previous results it can be concluded that a common effect in the four samples observed by SFM is that the silver particles form in all cases at surface defects that act as nucleation centers for the growing metal particles. In samples where only edge defects exist (e.g. HOPG-298, -463), the particles concentrate around these edges. However, in those samples where the defects, in the form of pit holes, are homogeneously distributed on the substrate plane, silver particles form and are distributed all over the whole graphite surface. In this respect the increase in particle size observed at high temperature is an additional effect that does not modify substantially the previous conclusion. Figure 3. SFM images of samples p-HOPG-298 (top) and p-HOPG-463 (bottom).
leads to a large agglomeration of the silver particles, whose size increases considerably. The fact that the concentration of particles is much smaller at 463 K than at 298 K suggests that in this case not only diffusion of evaporated atoms is contributing to the formation of the silver particles. In addition, small clusters or particles initially formed at pit sites might migrate at this high temperature, contributing to the formation of bigger particles associated with some of the pits only and, eventually, the step edges.
Acknowledgment. It is a pleasure to acknowledge the technical support of E. Poblenz (HU Berlin). The project has been supported by the Deutsche Forschungsgemeinshaft of Germany (RA 482/3-1) and the DGICYT of Spain (PB96-0863-C02-02). One of us (A.S.) thank the Alexander von Humboldt foundation for a postdoctoral grant at the ICMSE. LA9804334 (13) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.
