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Adsorption of molecules on the surface of small metal particles studied by optical spectroscopy
Surface Science 436 (1999) 51–62 www.elsevier.nl/locate/susc
Adsorption of molecules on the surface of small metal particles studied by optical spectroscopy A. Iline, M. Simon, F. Stietz *, F. Tra¨ger Fachbereich Physik, Universita¨t Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel, Germany Received 14 November 1998; accepted for publication 20 April 1999
Abstract The influence of adsorbate layers on surface plasmon excitation of small supported metal particles with mean sizes ranging from R=20 to 40 nm has been investigated. For this purpose, sodium and potassium clusters adsorbed on dielectric substrates served as model systems. Their optical transmission spectra were first recorded under ultra-high vacuum conditions directly after preparing the particles and, subsequently, after dosage of gases such as O , N O, 2 2 CO , H and N . The optical spectra are dominated by two maxima that originate from the excitation of surface 2 2 2 plasmon resonances in the direction of the long and short axis of the oblate particles. Changes are found upon adsorption of as little as 0.04 monolayers on the cluster surface. The results of a series of measurements together with a theoretical treatment using the quasi-static approximation indicate that the variations of the spectra allow one to characterize the strength of the surface chemical bond. In addition, diffusion of the atoms into the bulk of the particles can be followed. Particularly interesting is the observation that the clusters can experience a change in shape if gases such as O or CO react with their surface. © 1999 Elsevier Science B.V. All rights reserved. 2 2 Keywords: Alkali metals; Carbon dioxide; Chemisorption; Clusters; Nitrogen oxides; Oxygen; Visible/ultraviolet absorption spectroscopy
1. Introduction The optical properties of small metal particles have long been the subject of extended experimental and theoretical investigations [1–3]. Often, the optical absorption, scattering and transmission spectra are dominated by pronounced resonant features that are commonly attributed to excitation of plasmon-polaritons, i.e. collective oscillations of the electron gas driven by the electromagnetic field [2–4]. The energetic position, the width and the amplitude of the resonances vary strongly as a function of size and shape of the clusters and * Corresponding author. Fax: +49-561-804-4518. E-mail address: [email protected] (F. Stietz)
thus reflect the influence of these parameters on their optical properties. Not only does this constitute an issue of rather fundamental interest, but it is also attractive for a variety of technical applications. Exploiting the size- and shape-dependent optical properties of such particles or, more generally speaking, of systems with reduced dimensions, opens up the possibility of designing and fabricating novel optical devices, e.g. filters or components for ultrafast switching of light, that utilize their tailor-made absorbance or non-linear susceptibility [1,5,6 ]. Measurements on clusters1 embedded in matri1 The terms ‘cluster’ and ‘small particle’ will be used synonymously in this paper.
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 06 0 7 -X
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ces have shown, that the chemical environment influences the optical properties drastically (see, for example, Refs. [2,3,7–12]). For example, the optical spectra of Ag clusters display an increase in width accompanied by shifts of the plasmon resonance to a lower energy as compared to free clusters in a beam if they are embedded in different matrix materials [2,7–10]. It could be shown that the major part of the peak shifts is brought about by modifications of the Maxwell boundary conditions. However, the observed shifts exceed this classical electrodynamic effect for most investigated materials, and modifications resulting from permanent charge transfer from the metal clusters to the molecules of the surrounding medium must also be taken into account [2,7,13]. The increase in width of the plasmon resonance was interpreted by dynamic charge transfer of conduction electrons from the metal clusters to partially unoccupied levels of the adsorbate and was treated theoretically by Persson et al. [7,13]. Recently, experiments have been started to examine the influence of adsorbed atoms and molecules on the optical spectra of Ag- and Na-clusters [9,10,14]. Also the chemical reactivity of small metal particles has been studied extensively in the past. Most of the work was done for very small metal clusters X with n<50, n denoting the number of n atoms in the particles. The main objective was to investigate the dramatic variations in reactivity as a function of the number of atoms (see, for example, Refs. [15–18]). In addition, supported Pd, Pt, and Cu particles with sizes of about 5 nm have served as model systems for metal catalysts, and chemisorption of CO, O and NO has been exam2 ined [19–24]. It was found that not only the size of the particles but also their shape and morphology determined the chemical reactivity. In this paper, the influence of adsorbate molecules on the optical spectra of supported metal clusters with sizes ranging from 20 to 40 nm is examined systematically. The main motivation of our work is to distinguish between different processes that contribute to adsorbate-induced modifications of the optical properties. On the one hand, a detailed understanding of these effects is essential for correct interpretation of the optical spectra. On the other hand, it opens the door to exploit optical spectroscopy to study chemical
interface reactions on the surface of metal nanoparticles. Certainly, the experiments described in detail below are also of relevance to the design and fabrication of optical components. For example, if the tailor-made absorbance of metal colloids is utilized in optical filters or such particles are embedded in novel integrated systems for ultrafast switching of light, uncontrolled broadening or shifts of the absorption peaks due to chemical reactions with the environment must be excluded. In our experiments Na- and K-particles supported on dielectric substrates served as model systems. They were generated under ultrahigh vacuum conditions by deposition of atoms, subsequent surface diffusion and nucleation, i.e. Volmer–Weber growth. As a second step, the optical transmission spectra have been recorded by using s- or p-polarized light. Subsequently, the clusters were exposed to gases such as O , N O, 2 2 CO , H or N . The dosage being well defined, 2 2 2 different coverages of molecules on the cluster surface could be chosen. After completion of the exposure, the optical transmission spectra were recorded again. The main advantage of the investigations is threefold. First, changes in the optical spectra induced by a single molecular species can be studied. Secondly, the dosage of the gases can be controlled in a sensitive and reproducible manner, making possible a gradual variation of the adsorbate coverage and therefore of the dielectric surrounding of the clusters. In contrast, experiments on clusters embedded in matrices do not offer this possibility. Third, modifications of the axial ratio, i.e. the shape of the oblate clusters, brought about by chemical reactions with adsorbates, can be studied by selective excitation of plasmon polaritons in the direction of the long and short particle axis using s- and p-polarized incident light [25,26 ]. Finally, we mention that optical spectroscopy can easily be applied even under high-pressure conditions to ‘real-world’ catalysts. 2. Experimental 2.1. Set-up and procedure The experimental set-up is illustrated in Fig. 1. It basically consists of an ultra-high vacuum system
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Fig. 1. Schematic representation of the experimental arrangement used for the preparation of supported Na- and K-particles in ultrahigh vacuum as well for measurements of their optical transmission spectra.
with a base pressure of 2×10−10 mbar, the sample, a Xe-arc lamp combined with a monochromator for measuring the transmission spectra, a Knudsen cell for evaporating Na- or K-atoms and a gasinlet system. LiF(100) single crystal surfaces were used as substrates for the Na- and K-clusters. Being mounted on a manipulator, the substrate could be cooled to T=80 K and heated to about T=750 K. A thermal atomic beam of Na- or K-atoms with a well-defined flux was generated by the Knudsen cell and directed onto the substrate in order to deposit a predetermined coverage of atoms on the surface held at T=80 K. The flux of the atomic beam was about 1012 atoms per second, and this was measured with a quartz crystal microbalance. The deposited atoms assembled into small particles by surface diffusion and nucleation. The experiments reported here were carried out under conditions where the generated clusters were well separated. For the next step, the optical transmission spectra were recorded. For this purpose, we used a Xe-arc lamp combined with a grating monochromator and a rotatable Glan polarizer. Photon energies ranged from 1.0 to 4.5 eV, and the spectral resolution of the monochromator was normally chosen to be DE=10 meV. The light transmitted through the transparent substrate crystal was collected by a lens and finally registered with a
photodiode. The measured spectra were corrected for the spectral emission profile of the Xe-arc lamp as well as the wavelength-dependent transmission of the optical components and the spectral response of the photodiode. Subsequently, the clusters were exposed to either O , N O, CO , H or N . The gas entered 2 2 2 2 2 the vacuum system through a leak valve, which allowed well controlled and reproducible dosages. These were measured in Langmuir (L), i.e. the product of the chosen pressure rise in 10−6 Torr and the exposure time in seconds. For this purpose, the ionization gauge reading was calibrated by a quadrupole mass spectrometer and corrected for the sensitivity factors of the different gases. The coverage on the cluster surface corresponding to the different dosages can be estimated as follows. The partial pressure of the gases determines how many atoms impinge on the particle surface. For example, a dosage of 1 L of oxygen leads to an impinging rate of 5×1014 atoms/cm2 s. For the sizes of the particles studied here, there are about 3×105 atoms per cluster on the surface. Taking into account the number density of the particles (1010 clusters/cm2, see below ) this gives a number of 3×105 surface atoms/cm2. Therefore, a dosage of 1 L of oxygen corresponds to a coverage of H= 0. 15 ML on the cluster if a sticking coefficient of unity is assumed, and the whole particle surface
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can be covered with adsorbates. After dosage, recording of the optical spectra was repeated. As a final step for each experimental run, the substrate was cleaned by heating to 700 K. 2.2. Sample preparation and characterization As mentioned above, clusters were produced by deposition of thermal atoms followed by surface diffusion and nucleation ( Volmer–Weber growth). The generated particles ‘decorate’ the defects of the substrate surface, as a result of which the number density of the clusters remains constant during growth and is given by the defect density. Its value was determined by taking atomic force microscopy images (AFM ) of different metal clusters and amounts to 1010/cm2 for the used LiF substrate surfaces. As a consequence of the constant density, the average particle size is unequivocally related to the number of deposited atoms that is known from the flux of the atomic beam (see above) and the chosen deposition time. On LiF, for example, the Na coverage of 8.9×1015 atoms/cm2 corresponds to an average cluster radius of 20 nm. Contaminant-free conditions not only follow from the ultrahigh vacuum environment used in our work and the procedure to generate the Na beam but have also been verified independently by laser ablation of the investigated particles. Mass spectrometry of the species released, for example, from Na particles, only gave signals of Na+, Na+ and (with a much 2 lower amplitude) from K+. No species containing oxygen or other contaminants were detectable. It is well known from electron microscopy work (see, for example, Refs. [2,27]) that clusters generated by Volmer–Weber growth have size distributions with a width of about 30–50% of the mean particle size. Furthermore, it is known, for example, from optical spectroscopy [2,25] that clusters in the size range considered here are not spherical but can rather be described as oblate spheroids, with the short axis pointing towards the direction of the surface normal. This is reflected in the optical transmission spectra by two distinct resonances, i.e. surface plasmon-polaritons ( Fig. 2). Their positions depend upon the axial ratio a/b of the particles, which has been determined in the
Fig. 2. Optical extinction of (a) K- and (b) Na-particles (R= 20 nm) as a function of photon energy. The spectra were measured with s- and p-polarized incident light with photon energies ranging from E=1.2 to 5.0 eV.
present work by comparison of the experimental results with theoretical spectra calculated using the T-matrix approach [28]. The influence of the substrate has been taken into account by replacing the dielectric constant of the surrounding by a weighted averaged function e that falls in between m the vacuum value and that of the support material. For details of this procedure, see Ref. [25]. This results in axial ratios of a/b=0.25 and a/b=0.36 for the sodium and potassium particles, respectively. We would also like to mention that the cluster size distribution is reflected in inhomogeneous broadening of the two plasmon resonances. If this effect is taken into account in the theoretical treatment, a width of the cluster size distribution of about 50% is obtained, a value consistent with the above-mentioned results of electron microscopy. In view of the aspherical shape of the particles, the mean sizes quoted in this paper refer to spheres with the same volumes.
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3. Optical spectra of metal clusters without adsorbate coverage Fig. 2 displays transmission spectra measured with s- and p-polarized light for (a) K- and (b) Na-clusters with a mean radius of 20 nm adsorbed on LiF. The light was incident under an angle of 45° with respect to the surface normal. As can be seen from Fig. 2, s-polarized light leads to a single broad maximum, whereas p-polarized light gives rise to two maxima. One of them is located at E =1.53 for K (top), and at E =1.83 eV for Na 1 1 (bottom), the second peak appears at E =2.88 0 and 4.33 eV, respectively. As mentioned above, these resonances are the well-known surface plasmon polaritons, i.e. collective oscillations of the free electron gas driven by the electromagnetic field [2,4,15]. In general, the amplitudes of the two resonances depend on the polarization direction of the incident light. P-polarized light can excite the collective electron oscillations parallel [(1,1)mode] and perpendicular [(1,0)-mode] to the surface, i.e. in the direction of the long and short particle axis, whereas s-polarized light can only stimulate the (1,1)-mode [2,25,26 ]. In addition to the plasmon resonances discussed so far, a small maximum at an energy of 2.21 and 2.80 eV is observed in the spectra of the K- and Na-particles, respectively. T-matrix calculations show that these peaks are brought about by the excitation of octopole resonances [29] in the few very large particles of the distribution. The quadrupole signal is hidden by the relatively broad dipole resonance.
4. Optical spectra of metal clusters covered by adsorbate molecules As an example, Fig. 3 presents spectra of bare Na-particles and particles after different O and 2 CO exposures. The (1,1)-mode shifts from E= 2 1.83 eV for the bare clusters to E=1.74 eV upon an oxygen exposure of 5 L. In contrast, the (1,0)mode shifts to a higher energy in the same exposure range. Furthermore, the width of both resonances increases, and the amplitude shrinks. In contrast to oxygen, adsorption of CO shifts the (1,1)2 mode to a higher energy and the (1,0)-mode to a
Fig. 3. Optical spectra of Na clusters. The upper panel shows experimental spectra of the bare clusters and after O -dosage 2 of 5 L, and the lower panel illustrates the change of the spectrum that occurs after a CO dosage of 10 L. The spectrum measured 2 for 10 L CO is shifted by 3% to lower extinction values in 2 order to illustrate the modification of the peak position of both modes.
lower energy. The width and amplitude remain unchanged. Fig. 4 summarizes the results obtained for Na-particles and displays (a) the peak shift, (b) the width ( FWHM ) and (c) the amplitude of the (1,1)-mode as a function of gas exposure. An oxygen dosage lower than 1.0 L leads to a peak shift to a lower energy by 20 meV and a slight decrease in amplitude. At this stage, the width of the plasmon peak remains unchanged. For exposures ranging from 1.5 to 3 L the energetic shift proceeds further as a function of gas exposure, however, with growing gradient. If the exposure is raised to values above 4 L, the shift ceases. At the same time, the amplitude decreases further, and the width grows drastically in this high-exposure
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Fig. 4. Shift of the center frequency and change of the width and amplitude of the (1,1)-mode of Na clusters as a function of exposure to O , CO , N O, H and N . 2 2 2 2 2
regime. N O induces very similar modifications in 2 the optical spectra. The amplitude of the (1,1)mode decreases, and the width grows for exposures exceeding 2.5 L. In addition, the center frequency is displaced to a lower energy. In sharp contrast to the dosage of O and 2 N O, adsorption of CO gives rise to a shift in the 2 2 (1,1)-mode to higher energies, whereby the width and amplitude of the plasmon peak remain unchanged. Exposure of the particles to N and 2 H does not modify the optical spectra at all. 2 Fig. 5 displays the results obtained for the (1,1)mode of K-clusters. Upon adsorption of oxygen, the following changes are observed. Already after an exposure of 0.25 L, the center frequency is displaced to a lower energy, and the amplitude starts to decline. At this stage, the width remains
Fig. 5. Shift of the center frequency and change of the width and amplitude of the (1,1)-mode of K-clusters as a function exposure to O , CO , H and N . 2 2 2 2
constant. For exposures greater than 1 L, the frequency shift continues as a function of dosage. Simultaneously, the amplitude decreases further, and the width starts to grow. Upon exposure to CO very similar modifications are observed. The 2 peak shifts to a lower energy, the width increases, and the amplitude shrinks. For adsorption of H 2 and N , no changes in the optical spectra have 2 been measured. For the (1,0)-mode, identical modifications of the width and the amplitude, as already discussed for the (1,1)-mode, have been found for sodium and potassium. However, the peak shifts of both modes differ substantially for the reaction of the particles with O and CO . This is illustrated in 2 2 Figs. 6 and 7, which depict the peak shift of the (1,1)- and the (1,0)-mode as a function of O and 2
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Fig. 6. Shift of the center frequency of the (1,1)- and the (1,0)mode of Na-clusters as a function of exposure to (a) O and 2 (b) CO . 2
CO exposure. For sodium ( Fig. 6a) as well as 2 potassium (Fig. 7a) and low oxygen dosages, both modes shift to a lower energy. Above 1 L, the (1,0) mode is, however, displaced to a higher energy, whereas the (1,1) mode shifts further to the red. As a consequence, the energetic separation between both peaks increases. In sharp contrast to oxygen, CO induces a decrease in the energetic 2 separation for sodium clusters already at very low dosages ( Fig. 6b). For potassium, the energetic separation is affected only for exposures to CO 2 of above 10 L, Fig. 7b. We finally mention that we performed experiments similar to those presented above for mean particle sizes ranging from R=20 to 40 nm. No size dependence was found, i.e. clusters with R= 40 nm show very similar changes in amplitude, width and energetic position of the plasmon modes upon, for example, dosage to oxygen, as compared to particles with R=40 nm. Therefore, we have
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Fig. 7. Shift of the center frequency of the (1,1)- and the (1,0)mode of K-clusters as a function of exposure to (a) O and (b) 2 CO . 2
presented exclusively the results obtained for clusters with R=20 nm.
5. Discussion We first note that measurement of the optical spectra obviously provides a sensitive technique to detect adsorbates on the surface of small particles. As can be seen from Figs. 4–7, even dosages of as little as 0.25 L can induce measurable shifts in the peak position and noticeably attenuate the amplitude of the plasmon polariton. Since 1 L corresponds to H=0.15 ML (see Section 2.1), optical spectroscopy provides a sensitivity of H<0.04 ML. In order to suggest an interpretation of the changes of the energetic position, the amplitude and the width of the plasmon resonances upon adsorption of molecules, the different processes that may in
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principle contribute to the modifications will be summarized briefly below. 5.1. Energetic position of the plasmon resonance In general, red and blue shifts of the plasmon modes can occur if adsorbate shells are formed. To distinguish between the different processes contributing to these shifts, calculations have been performed using the quasi-static approximation. Thus, features in the optical spectra that are brought about by excitation of multipoles and by retardation are neglected. Therefore, the theoretical results have not been used here to fit the spectra quantitatively, but are only exploited to qualitatively investigate trends of the shifts of the plasmon polaritons. We would like to mention in addition, that calculations of the optical spectra based on a model by Yamaguchi et al. [30] have shown that electrodynamic coupling among the clusters is negligible for the experimental conditions considered here. Fig. 8a–c illustrate schematically how the
Fig. 8. Schematic representation of supported oblate spheroidal Na-clusters ( left-hand side) together with their calculated absorption cross-sections for surface plasmon excitation (righthand side). The Na clusters are embedded in a medium with dielectric constant e and have a Na O shell with, from top to m 2 bottom, increasing thickness.
formation of an overlayer on the particle surface has been treated theoretically. In the calculations, a mean diameter of 30 nm and a height of 7.2 nm are assumed. This corresponds to the axial ratio of a/b=0.24. The shell material is taken into account by a dielectric constant e . The whole shell particle consisting of the metallic core and the overlayer is embedded in the medium e (e =1.37), the value of which accounts for the m m asymmetric surrounding consisting of the vacuum and the LiF(100) substrate (see above). Moreover, we have assumed that the adsorbate shell is allowed to cover the whole particle surface rather than to leave the particle substrate interface unaffected because of the following three reasons. First, the exact size of the contact area of the particle and the substrate surface is not known. Second, it cannot be ruled out that the particle/substrate interface is affected by adsorption of atoms brought about, for example, by diffusion into this interface region. Third, calculations assuming that one-third of the cluster surface remains uncovered upon shell formation showed identical trends as compared to those presented here. Only the amount of the peak shifts depends on the fractions of the covered and uncovered surface areas. The theoretical results can be summarized as follows. Red shifts of the (1,1) and the (1,0) mode are observed if the dielectric constant of the adsorbate layer is higher than that of the surrounding. Additional shifts to lower energy can occur if charge transfer from the metal cluster to the adsorbate shell takes place [10,13]. Blue shifts for both peaks occur, however, if the dielectric constant of the adsorbed shell is lower than that of the surrounding medium and/or charge transfer from the adsorbate to the metal cluster occurs. In addition to these shifts, which are identical for both modes, changes in their energetic separation take place if the shape of the particles varies upon adsorption of molecules. In principle, two possibilities exist to modify the shape of the clusters due to adsorption. First, the molecular overlayer may change the surface tension [19,31,32] and thus give rise to a variation of the axial ratio. It can increase or decrease, depending on the sign of the change of surface tension. Second, the adsorbate can react in such a way with the metal clusters that the atoms not only adsorb on the
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surface but also diffuse into the interior of the particles and react there. As a result, a shell forms consisting of the compound material. Fig. 8a–c show a Na cluster with an adsorbate shell, the thickness of which increases from 1 to 3 nm. Since the reaction of oxygen with the Na and K clusters will be discussed in more detail below, the dielectric function of Na O was chosen for the adsorbate 2 shell. The optical constants were taken from Ref. [33]. As a result of diffusion and compound formation, not only the size of the metallic core but also its axial ratio decreases. Fig. 8d–f show the corresponding absorption cross-sections. A considerable red shift of the (1,1) mode and a blue shift of the (1,0) mode are observed if the thickness of the layer is increased. This greater energetic separation between the modes results mainly from the decreasing axial ratio brought about by the increasing thickness of the Na O overlayer. 2 Similar calculations have also been performed for different clusters sizes (20 nm
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if the cluster Fermi level intersects partially unoccupied adatom or admolecule levels [2,7,13]. The electrons tunneling to the adsorbate and, after a certain residence time, returning to the cluster are scattered out of phase of the collective excitation. As a result, the width of the plasmon resonance increases, an effect known as ‘chemical interface damping’ [2,7,13]. In the discussion of the experiments presented in the next section, it has to be taken into consideration that this effect is most pronounced for small metal clusters, i.e. particles with diameters below 10 nm [2]. Since larger Na and K clusters have been examined here, this effect seems to be of minor importance for the present investigations. Although the experimentally observed changes integrate over the size and shape distributions, the spectra recorded for different experimental conditions together with the theoretical results described above allow us to suggest the following interpretation of the variations of the amplitude, position and width of the surface plasmon resonance upon adsorption of molecules. 5.4. Sodium From measurements using electron energy loss spectroscopy ( EELS), X-ray photoelectron spectroscopy ( XPS) and low-energy electron diffraction (LEED) [25–27], O is known to react 2 very effectively with sodium surfaces. Oxygen is chemisorbed dissociatively on Na(110)-surfaces and thin Na-films with considerable charge transfer to the adsorbed oxygen atom [34–36 ]. Therefore, it is not surprising that already, oxygen coverages lower than 0.1 monolayer induce pronounced changes in the optical spectra of the Na clusters. Following an oxygen exposure of 0.25 L, the amplitude of the plasmon peak starts to decrease. This indicates that the number of conduction electrons in the Na cluster is reduced because of a charge transfer to the adsorbed oxygen molecules. Most likely, Na O is formed by chemisorption of oxygen 2 on the cluster surface [35,37]. Furthermore, already in the lowest exposure regime, a peak shift to a lower energy is observed. As mentioned above, this can be caused by (1) the modified chemical environment, (2) changes in the shape of the Na
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clusters and (3) charge transfer to the oxygen atoms. Since the energetic separation between the (1,1)- and (1,0)-mode remains constant below 1 L (Fig. 6a), changes in the shape of the particles do not seem to play a role in this exposure range. Moreover, a comparison between calculated spectra of the bare Na clusters (not shown here) and the clusters with a Na O-shell of 1 nm thick2 ness ( Fig. 8d ) indicates that the modified chemical environment is also of minor importance for the peak shift. In fact, the calculations rather give a plasmon peak shift to a slightly higher energy. This results from the difference between the dielectric function of the Na O-shell and the surrounding 2 medium e without adsorbate. This displacement m is in contrast to the experimentally observed red shift, and we therefore interpret the measured peak shift by charge transfer from the sodium clusters to the adsorbed oxygen atoms. This overcompensates the blue shift brought about by the modified environment. Exposures between 1.5 and 4 L give rise to an even larger displacement of the (1,1) mode as a function of oxygen exposure. In addition to charge transfer, a second effect obviously contributes to the measured red shift. In this exposure regime, the (1,0)-mode, however, starts to shift to a higher energy, i.e. the energetic separation of both peaks increases. Our calculations of the peak position as a function of increasing Na O shell thickness 2 reproduce a growing energetic separation between the plasmon modes ( Fig. 8). Therefore, we interpret the additional red shift of the (1,1)-mode accomplished by the blue shift of the (1,0) mode as a decrease in the axial ratio of the (metallic) Na cluster brought about by the increasing Na O 2 overlayer thickness. For exposures above 4 L, the amplitudes of the plasmon resonances drop off, and the widths increase drastically. This can be understood as follows. As already mentioned above, the adsorbed oxygen does not simply form an overlayer on the metal particles. Rather, the oxygen penetrates into the clusters, thus gradually transforming the metal into Na O. This interpretation is corroborated by 2 two arguments. First, the disappearance of the Na plasma resonance goes hand in hand with the growth of a new distinct resonance, which is
located at 3.8 eV and characteristic of particles consisting of the formed Na O [14]. Secondly, the 2 dielectric constant of Na O being known, theoreti2 cal spectra of Na O clusters could be computed 2 and, in fact, also show a peak at 3.8 eV [14]. In summary, the following model for the reaction of oxygen with Na clusters is proposed. In the low-exposure regime, up to 1–2 L, chemisorption occurs, whereby the oxidation reaction is limited to the very surface region of the cluster. As a consequence, a slight decrease in amplitude and a small shift in the energetic positions of the plasmon resonances at constant width are observed. In the regime, between 2 and 4 L, oxygen starts to diffuse into the cluster. The thickness of the Na O shell gradually increases. Trans2 formation of the Na cluster into Na O begins. In 2 the high-exposure regime, this reaction continues, and all of the sodium is gradually oxidized. As a result, the plasmon resonance of metallic Na disappears. At the same time, a resonance characteristic of Na O emerges. 2 N O induces similar changes in the optical 2 spectra, the main difference being that the observed modifications occur at higher gas exposures as compared to oxygen. Molecular beam experiments have shown that N O dissociates into nitrogen and 2 oxygen if the molecules adsorb on Na clusters [33], a reaction also observed, for example, on single crystal surfaces of transition metals. We interpret the modifications of the optical spectra studied here also by dissociation of N O into N 2 2 and O, the oxygen atoms being chemisorbed on the Na cluster surface. As a consequence, the spectra exhibit similar modifications, as already discussed for oxygen dosage. The higher exposures needed to induce comparable changes result from the smaller dissociation cross-section of N O as 2 compared to O [33]. However, the nitrogen mole2 cules split off from the N O most likely do not 2 remain on the cluster surface. This is confirmed by experiments in which the Na clusters were exposed to N and in which no changes of the 2 optical spectra were found ( Fig. 4). A very interesting further observation is the decrease in energetic separation of the two resonance modes upon exposure of the Na particles to CO ( Figs. 3 and 6b). This clearly indicates a 2
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change in the axial ratio of the Na clusters, which tend to become more similar to spheres. Since the amplitude and width of the plasmon resonances remain unchanged, noticeable charge transfer and formation of a compound layer do not seem to occur. Therefore, the observed shape changes are most likely brought about by a modification of surface tension of the metal particles due to adsorption of CO -molecules. Similar changes in 2 the equilibrium shape have been found theoretically for metal particles of a different size and have been observed experimentally for reaction of CO 2 with different metal particles either during or after the growth process of the clusters [19,38,39]. 5.5. Potassium In analogy to sodium, exposure of K-clusters to oxygen molecules influences the amplitude, the width and the peak position of both plasmon modes for coverages far below a single monolayer (Figs. 5 and 7). As for sodium surfaces (Section 5.1), oxygen is known to chemisorb dissociatively on thin K films accompanied by a considerable charge transfer from the potassium to the adsorbed oxygen atoms [36 ]. Therefore, it is again not surprising that, already, small amounts of oxygen exposure induce changes in the optical spectra of the K-clusters, Fig. 5. Upon oxygen exposure of 0.25 L, the center frequencies of the (1,1)- and the (1,0)-modes are displaced to a lower energy, and the amplitudes start to decrease. Similar to sodium, this can be interpreted by chemisorption of oxygen atoms on the particle surface most likely forming K O. The energetic 2 separation of both plasmon resonances remains constant in this range, indicating that no shape changes take place. For exposures ranging from 1 to 8 L, the amplitudes of the plasmon modes decrease further. In addition, the energetic separation of the (1,1)- and the (1,0)-mode increases gradually, i.e. the (1,1) mode shifts to a lower energy, and the (1,0) mode shifts to a higher energy ( Fig. 7a). Both observations can be interpreted by increasing the thickness of the K O shell. As already discussed for sodium, 2 formation of a compound layer of growing thickness is accompanied by a decreasing axial ratio
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and shrinking size of the K core. This results not only in a lower plasmon resonance amplitude, because of less absorbing and scattering metallic material, but also in an increasing energetic difference between both plasmon modes as a function of oxygen exposure. For exposures greater than 10 L, the resonances vanish. Again, this can be interpreted by transformation of the particles into K O. 2 CO induces very similar modifications of the 2 center frequency of the (1,1) mode as well as their amplitude and width. Fairly similar to the dissociative chemisorption of N O on Na-particles, we 2 interpret the observed changes also by dissociation of CO on the K-surface into carbon and oxygen, 2 the latter being chemisorbed. As a consequence, the measured optical spectra exhibit similar modifications, as discussed for oxygen. However, the dosages necessary for comparable changes in the spectra are larger as compared to oxygen, reflecting the smaller probability of dissociative chemisorption. Therefore, only very small changes in the energetic separation and the width of the plasmon modes can be observed ( Figs. 5 and 7). Obviously, the reaction of CO on K-clusters is 2 very different compared to Na-particles. This again demonstrates the sensitivity of variations of the optical spectra towards different reaction mechanisms and will have to be investigated in more detail in the future. Finally, the measurements show that exposure of the clusters to N and H does not influence 2 2 the optical spectra at all. At present, we do not know, however, whether these molecules are not adsorbed on the particle surface or whether the induced modifications are below the detection limit.
6. Conclusions In conclusion, the results presented here show that optical spectroscopy can be effectively used for the detection and characterization of chemical interface reactions on supported metal clusters. Upon exposure to molecules, the optical spectra change in a variety of ways, depending on the nature of the adsorption reaction. The measure-
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ments not only provide submonolayer sensitivity but also allow one to distinguish between weak interactions, e.g. CO with sodium, and strong 2 reactions, e.g. O with Na and K. Even diffusion 2 of atoms into the interior of the clusters and subsequent transformation of the clusters into Na O and K O can be detected. In addition, shape 2 2 changes brought about either by the formation of a compound layer with increasing thickness or by changes in surface tension have been observed. In future experiments, it would be very useful to extend the experiments to smaller clusters, where strong size-dependent reaction rates are expected upon adsorption of various molecules, and to investigate changes in the optical spectra as a function of cluster size.
Acknowledgements Financial support of the Deutsche Forschungsgemeinschaft (DFG) through the Graduiertenkolleg ‘Materialien und Komponenten der Mikrosystemtechnik’ and the Fond der Chemischen Industrie is gratefully acknowledged.
References [1] P. Jena, S.N. Khanna, B.K. Rao (Eds.), Science and Technology of Atomically Engineered Materials, World Scientific, Singapore, 1995. [2] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer Series in Material Science Vol. 25 Springer, Berlin, 1994. [3] H. Haberland ( Ed.), Clusters of Atoms and Molecules II, Springer Series in Chemical Physics 56, Springer, Berlin, 1994. [4] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley, New York, 1993. [5] R.F. Haglund Jr., in: R.E. Hummel, P. Wißligmuth ( Eds.), Optics of Small Particles, Interfaces and Surfaces, CRC Press, Boca Raton, FL, 1996. [6 ] R.F. Haglund Jr., D.H. Osborne, R.H. Magruder, C.W. White, R.A. Zuhr, D.E. Hole, P.D. Townsend, F. Gonella, P. Mazzoldi, in: P. Jena, S.N. Khanna, B.K. Rao ( Eds.), Science and Technology of Atomically Engineered Materials, World Scientific, Singapore, 1995. [7] H. Ho¨vel, S. Fritz, A. Hilger, U. Kreibig, M. Vollmer, Phys. Rev. B 48 (1993) 18178. [8] K.-P. Charle, W. Schulze, B. Winter, Z. Phys. D 12 (1989) 471.
[9] U. Kreibig, M. Gartz, A. Hilger, H. Ho¨vel, in: P. Jena, S.N. Khanna, B.K. Rao ( Eds.), Science and Technology of Atomically Engineered Materials, World Scientific, Singapore, 1995. [10] U. Kreibig, M. Gartz, A. Hilger, Ber. Bunsenges. Phys. Chem. 101 (1997) 1593. [11] M.A. Smithard, Solid State Commun. 14 (1974) 407. [12] J. Hecht, J. Appl. Phys. 50 (1979) 7186. [13] B.N.J. Persson, Surf. Sci. 281 (1993) 153. [14] T. Brandt, W. Hoheisel, A. Iline, F. Stietz, F. Tra¨ger, Appl. Phys. B 65 (1997) 793. [15] A. Kaldor, D.M. Cox, M.R. Zakin, Adv. Chem. Phys. 70 (1986) 211. [16 ] S.J. Riley, Clusters of Atoms and Molecules II, H. Haberland ( Ed.), Springer Series in Chemical Physics 56, Springer, Berlin, 1994. [17] L. Holmgren, M. Andersson, A. Rosen, J. Chem. Phys. 109 (1998) 3232. [18] S.L. Andersson, Clusters of Atoms and Molecules II, H. Haberland ( Ed.), Springer Series in Chemical Physics 56, Springer, Berlin, 1994. [19] C.R. Henry, Surf. Sci. Rep. 31 (1998) 231. [20] M. Thomas, J.T. Dickinson, H. Poppa, G.M. Pound, J. Vac. Sci. Technol. 15 (1978) 568. [21] E. Gillet, S. Channakhone, V. Matolin, M. Gillet, Surf. Sci. 152/153 (1985) 603. [22] E.J. Altmann, R.J. Gorte, Surf. Sci. 172 (1986) 71. [23] H.J. Freund, B. Billmann, D. Ehrlich, M. Hassel, R.M. Jaeger, K. Kuhlenbeck, C.A. Ventrice Jr., F. Winkelmann, S. Wohlrab, C. Xu, Th. Bertrams, H. Brodde, H. Neddermeyer, J. Mol. Catal. 82 (1993) 143. [24] D.W. Goodman, Chem. Rev. 95 (1995) 523. [25] T. Go¨tz, W. Hoheisel, M. Vollmer, F. Tra¨ger, Z. Phys. D 33 (1995) 133. [26 ] H. Houmlvel, A. Hilger, L. Nusch, U. Kreibig, Z. Phys. D 42 (1997) 203. [27] J.-A. Venables, Surf. Sci. 299/300 (1994) 798. [28] P.W. Barber, S.C. Hill, Light Scattering by Small Particles: Computational Methods, Advanced Series in Applied Physics 2, World Scientific, Singapore, 1990. [29] A. Iline, M. Simon, F. Stietz, F. Tra¨ger, to be published. [30] T. Yamaguchi, S. Yoshida, A. Kinbara, Thin Solid Films 21 (1973) 173. [31] H. Ibach, Surf. Sci. Rep. 29 (1997) 193. [32] R. Abermann, Thin Solid Films 186 (1990) 233. [33] J. Hecht, W.P. Brest, H.P. Broida, J. Chem. Phys. 71 (1979) 3132. [34] L. Surnev, G. Rangelov, E. Bertel, F.P. Netzer, Surf. Sci. 184 (1987) 10. [35] S. Andersson, J.B. Pendry, P.M. Echenique, Surf. Sci. 65 (1977) 539. [36 ] W. Maus-Friedrich, S. Dieckhoff, M. Wehrhahn, S. Pu¨lm, V. Kempter, Surf. Sci. 271 (1992) 113. [37] O. Hampe, G.M. Koretsky, M. Gegenheimer, T. Bergen, M.M. Kappes, Z. Phys. D 40 (1997) 62. [38] A. Shi, Phys. Rev. B 36 (1987) 9068. [39] M. Ba¨umer, M. Frank, J. Libuda, S. Stempel, H.-J. Freund, Surf. Sci. 391 (1997) 204.
Adsorption of molecules on the surface of small metal particles studied by optical spectroscopy A. Iline, M. Simon, F. Stietz *, F. Tra¨ger Fachbereich Physik, Universita¨t Kassel, Heinrich-Plett-Str. 40, D-34132 Kassel, Germany Received 14 November 1998; accepted for publication 20 April 1999
Abstract The influence of adsorbate layers on surface plasmon excitation of small supported metal particles with mean sizes ranging from R=20 to 40 nm has been investigated. For this purpose, sodium and potassium clusters adsorbed on dielectric substrates served as model systems. Their optical transmission spectra were first recorded under ultra-high vacuum conditions directly after preparing the particles and, subsequently, after dosage of gases such as O , N O, 2 2 CO , H and N . The optical spectra are dominated by two maxima that originate from the excitation of surface 2 2 2 plasmon resonances in the direction of the long and short axis of the oblate particles. Changes are found upon adsorption of as little as 0.04 monolayers on the cluster surface. The results of a series of measurements together with a theoretical treatment using the quasi-static approximation indicate that the variations of the spectra allow one to characterize the strength of the surface chemical bond. In addition, diffusion of the atoms into the bulk of the particles can be followed. Particularly interesting is the observation that the clusters can experience a change in shape if gases such as O or CO react with their surface. © 1999 Elsevier Science B.V. All rights reserved. 2 2 Keywords: Alkali metals; Carbon dioxide; Chemisorption; Clusters; Nitrogen oxides; Oxygen; Visible/ultraviolet absorption spectroscopy
1. Introduction The optical properties of small metal particles have long been the subject of extended experimental and theoretical investigations [1–3]. Often, the optical absorption, scattering and transmission spectra are dominated by pronounced resonant features that are commonly attributed to excitation of plasmon-polaritons, i.e. collective oscillations of the electron gas driven by the electromagnetic field [2–4]. The energetic position, the width and the amplitude of the resonances vary strongly as a function of size and shape of the clusters and * Corresponding author. Fax: +49-561-804-4518. E-mail address: [email protected] (F. Stietz)
thus reflect the influence of these parameters on their optical properties. Not only does this constitute an issue of rather fundamental interest, but it is also attractive for a variety of technical applications. Exploiting the size- and shape-dependent optical properties of such particles or, more generally speaking, of systems with reduced dimensions, opens up the possibility of designing and fabricating novel optical devices, e.g. filters or components for ultrafast switching of light, that utilize their tailor-made absorbance or non-linear susceptibility [1,5,6 ]. Measurements on clusters1 embedded in matri1 The terms ‘cluster’ and ‘small particle’ will be used synonymously in this paper.
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 06 0 7 -X
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ces have shown, that the chemical environment influences the optical properties drastically (see, for example, Refs. [2,3,7–12]). For example, the optical spectra of Ag clusters display an increase in width accompanied by shifts of the plasmon resonance to a lower energy as compared to free clusters in a beam if they are embedded in different matrix materials [2,7–10]. It could be shown that the major part of the peak shifts is brought about by modifications of the Maxwell boundary conditions. However, the observed shifts exceed this classical electrodynamic effect for most investigated materials, and modifications resulting from permanent charge transfer from the metal clusters to the molecules of the surrounding medium must also be taken into account [2,7,13]. The increase in width of the plasmon resonance was interpreted by dynamic charge transfer of conduction electrons from the metal clusters to partially unoccupied levels of the adsorbate and was treated theoretically by Persson et al. [7,13]. Recently, experiments have been started to examine the influence of adsorbed atoms and molecules on the optical spectra of Ag- and Na-clusters [9,10,14]. Also the chemical reactivity of small metal particles has been studied extensively in the past. Most of the work was done for very small metal clusters X with n<50, n denoting the number of n atoms in the particles. The main objective was to investigate the dramatic variations in reactivity as a function of the number of atoms (see, for example, Refs. [15–18]). In addition, supported Pd, Pt, and Cu particles with sizes of about 5 nm have served as model systems for metal catalysts, and chemisorption of CO, O and NO has been exam2 ined [19–24]. It was found that not only the size of the particles but also their shape and morphology determined the chemical reactivity. In this paper, the influence of adsorbate molecules on the optical spectra of supported metal clusters with sizes ranging from 20 to 40 nm is examined systematically. The main motivation of our work is to distinguish between different processes that contribute to adsorbate-induced modifications of the optical properties. On the one hand, a detailed understanding of these effects is essential for correct interpretation of the optical spectra. On the other hand, it opens the door to exploit optical spectroscopy to study chemical
interface reactions on the surface of metal nanoparticles. Certainly, the experiments described in detail below are also of relevance to the design and fabrication of optical components. For example, if the tailor-made absorbance of metal colloids is utilized in optical filters or such particles are embedded in novel integrated systems for ultrafast switching of light, uncontrolled broadening or shifts of the absorption peaks due to chemical reactions with the environment must be excluded. In our experiments Na- and K-particles supported on dielectric substrates served as model systems. They were generated under ultrahigh vacuum conditions by deposition of atoms, subsequent surface diffusion and nucleation, i.e. Volmer–Weber growth. As a second step, the optical transmission spectra have been recorded by using s- or p-polarized light. Subsequently, the clusters were exposed to gases such as O , N O, 2 2 CO , H or N . The dosage being well defined, 2 2 2 different coverages of molecules on the cluster surface could be chosen. After completion of the exposure, the optical transmission spectra were recorded again. The main advantage of the investigations is threefold. First, changes in the optical spectra induced by a single molecular species can be studied. Secondly, the dosage of the gases can be controlled in a sensitive and reproducible manner, making possible a gradual variation of the adsorbate coverage and therefore of the dielectric surrounding of the clusters. In contrast, experiments on clusters embedded in matrices do not offer this possibility. Third, modifications of the axial ratio, i.e. the shape of the oblate clusters, brought about by chemical reactions with adsorbates, can be studied by selective excitation of plasmon polaritons in the direction of the long and short particle axis using s- and p-polarized incident light [25,26 ]. Finally, we mention that optical spectroscopy can easily be applied even under high-pressure conditions to ‘real-world’ catalysts. 2. Experimental 2.1. Set-up and procedure The experimental set-up is illustrated in Fig. 1. It basically consists of an ultra-high vacuum system
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Fig. 1. Schematic representation of the experimental arrangement used for the preparation of supported Na- and K-particles in ultrahigh vacuum as well for measurements of their optical transmission spectra.
with a base pressure of 2×10−10 mbar, the sample, a Xe-arc lamp combined with a monochromator for measuring the transmission spectra, a Knudsen cell for evaporating Na- or K-atoms and a gasinlet system. LiF(100) single crystal surfaces were used as substrates for the Na- and K-clusters. Being mounted on a manipulator, the substrate could be cooled to T=80 K and heated to about T=750 K. A thermal atomic beam of Na- or K-atoms with a well-defined flux was generated by the Knudsen cell and directed onto the substrate in order to deposit a predetermined coverage of atoms on the surface held at T=80 K. The flux of the atomic beam was about 1012 atoms per second, and this was measured with a quartz crystal microbalance. The deposited atoms assembled into small particles by surface diffusion and nucleation. The experiments reported here were carried out under conditions where the generated clusters were well separated. For the next step, the optical transmission spectra were recorded. For this purpose, we used a Xe-arc lamp combined with a grating monochromator and a rotatable Glan polarizer. Photon energies ranged from 1.0 to 4.5 eV, and the spectral resolution of the monochromator was normally chosen to be DE=10 meV. The light transmitted through the transparent substrate crystal was collected by a lens and finally registered with a
photodiode. The measured spectra were corrected for the spectral emission profile of the Xe-arc lamp as well as the wavelength-dependent transmission of the optical components and the spectral response of the photodiode. Subsequently, the clusters were exposed to either O , N O, CO , H or N . The gas entered 2 2 2 2 2 the vacuum system through a leak valve, which allowed well controlled and reproducible dosages. These were measured in Langmuir (L), i.e. the product of the chosen pressure rise in 10−6 Torr and the exposure time in seconds. For this purpose, the ionization gauge reading was calibrated by a quadrupole mass spectrometer and corrected for the sensitivity factors of the different gases. The coverage on the cluster surface corresponding to the different dosages can be estimated as follows. The partial pressure of the gases determines how many atoms impinge on the particle surface. For example, a dosage of 1 L of oxygen leads to an impinging rate of 5×1014 atoms/cm2 s. For the sizes of the particles studied here, there are about 3×105 atoms per cluster on the surface. Taking into account the number density of the particles (1010 clusters/cm2, see below ) this gives a number of 3×105 surface atoms/cm2. Therefore, a dosage of 1 L of oxygen corresponds to a coverage of H= 0. 15 ML on the cluster if a sticking coefficient of unity is assumed, and the whole particle surface
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can be covered with adsorbates. After dosage, recording of the optical spectra was repeated. As a final step for each experimental run, the substrate was cleaned by heating to 700 K. 2.2. Sample preparation and characterization As mentioned above, clusters were produced by deposition of thermal atoms followed by surface diffusion and nucleation ( Volmer–Weber growth). The generated particles ‘decorate’ the defects of the substrate surface, as a result of which the number density of the clusters remains constant during growth and is given by the defect density. Its value was determined by taking atomic force microscopy images (AFM ) of different metal clusters and amounts to 1010/cm2 for the used LiF substrate surfaces. As a consequence of the constant density, the average particle size is unequivocally related to the number of deposited atoms that is known from the flux of the atomic beam (see above) and the chosen deposition time. On LiF, for example, the Na coverage of 8.9×1015 atoms/cm2 corresponds to an average cluster radius of 20 nm. Contaminant-free conditions not only follow from the ultrahigh vacuum environment used in our work and the procedure to generate the Na beam but have also been verified independently by laser ablation of the investigated particles. Mass spectrometry of the species released, for example, from Na particles, only gave signals of Na+, Na+ and (with a much 2 lower amplitude) from K+. No species containing oxygen or other contaminants were detectable. It is well known from electron microscopy work (see, for example, Refs. [2,27]) that clusters generated by Volmer–Weber growth have size distributions with a width of about 30–50% of the mean particle size. Furthermore, it is known, for example, from optical spectroscopy [2,25] that clusters in the size range considered here are not spherical but can rather be described as oblate spheroids, with the short axis pointing towards the direction of the surface normal. This is reflected in the optical transmission spectra by two distinct resonances, i.e. surface plasmon-polaritons ( Fig. 2). Their positions depend upon the axial ratio a/b of the particles, which has been determined in the
Fig. 2. Optical extinction of (a) K- and (b) Na-particles (R= 20 nm) as a function of photon energy. The spectra were measured with s- and p-polarized incident light with photon energies ranging from E=1.2 to 5.0 eV.
present work by comparison of the experimental results with theoretical spectra calculated using the T-matrix approach [28]. The influence of the substrate has been taken into account by replacing the dielectric constant of the surrounding by a weighted averaged function e that falls in between m the vacuum value and that of the support material. For details of this procedure, see Ref. [25]. This results in axial ratios of a/b=0.25 and a/b=0.36 for the sodium and potassium particles, respectively. We would also like to mention that the cluster size distribution is reflected in inhomogeneous broadening of the two plasmon resonances. If this effect is taken into account in the theoretical treatment, a width of the cluster size distribution of about 50% is obtained, a value consistent with the above-mentioned results of electron microscopy. In view of the aspherical shape of the particles, the mean sizes quoted in this paper refer to spheres with the same volumes.
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3. Optical spectra of metal clusters without adsorbate coverage Fig. 2 displays transmission spectra measured with s- and p-polarized light for (a) K- and (b) Na-clusters with a mean radius of 20 nm adsorbed on LiF. The light was incident under an angle of 45° with respect to the surface normal. As can be seen from Fig. 2, s-polarized light leads to a single broad maximum, whereas p-polarized light gives rise to two maxima. One of them is located at E =1.53 for K (top), and at E =1.83 eV for Na 1 1 (bottom), the second peak appears at E =2.88 0 and 4.33 eV, respectively. As mentioned above, these resonances are the well-known surface plasmon polaritons, i.e. collective oscillations of the free electron gas driven by the electromagnetic field [2,4,15]. In general, the amplitudes of the two resonances depend on the polarization direction of the incident light. P-polarized light can excite the collective electron oscillations parallel [(1,1)mode] and perpendicular [(1,0)-mode] to the surface, i.e. in the direction of the long and short particle axis, whereas s-polarized light can only stimulate the (1,1)-mode [2,25,26 ]. In addition to the plasmon resonances discussed so far, a small maximum at an energy of 2.21 and 2.80 eV is observed in the spectra of the K- and Na-particles, respectively. T-matrix calculations show that these peaks are brought about by the excitation of octopole resonances [29] in the few very large particles of the distribution. The quadrupole signal is hidden by the relatively broad dipole resonance.
4. Optical spectra of metal clusters covered by adsorbate molecules As an example, Fig. 3 presents spectra of bare Na-particles and particles after different O and 2 CO exposures. The (1,1)-mode shifts from E= 2 1.83 eV for the bare clusters to E=1.74 eV upon an oxygen exposure of 5 L. In contrast, the (1,0)mode shifts to a higher energy in the same exposure range. Furthermore, the width of both resonances increases, and the amplitude shrinks. In contrast to oxygen, adsorption of CO shifts the (1,1)2 mode to a higher energy and the (1,0)-mode to a
Fig. 3. Optical spectra of Na clusters. The upper panel shows experimental spectra of the bare clusters and after O -dosage 2 of 5 L, and the lower panel illustrates the change of the spectrum that occurs after a CO dosage of 10 L. The spectrum measured 2 for 10 L CO is shifted by 3% to lower extinction values in 2 order to illustrate the modification of the peak position of both modes.
lower energy. The width and amplitude remain unchanged. Fig. 4 summarizes the results obtained for Na-particles and displays (a) the peak shift, (b) the width ( FWHM ) and (c) the amplitude of the (1,1)-mode as a function of gas exposure. An oxygen dosage lower than 1.0 L leads to a peak shift to a lower energy by 20 meV and a slight decrease in amplitude. At this stage, the width of the plasmon peak remains unchanged. For exposures ranging from 1.5 to 3 L the energetic shift proceeds further as a function of gas exposure, however, with growing gradient. If the exposure is raised to values above 4 L, the shift ceases. At the same time, the amplitude decreases further, and the width grows drastically in this high-exposure
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Fig. 4. Shift of the center frequency and change of the width and amplitude of the (1,1)-mode of Na clusters as a function of exposure to O , CO , N O, H and N . 2 2 2 2 2
regime. N O induces very similar modifications in 2 the optical spectra. The amplitude of the (1,1)mode decreases, and the width grows for exposures exceeding 2.5 L. In addition, the center frequency is displaced to a lower energy. In sharp contrast to the dosage of O and 2 N O, adsorption of CO gives rise to a shift in the 2 2 (1,1)-mode to higher energies, whereby the width and amplitude of the plasmon peak remain unchanged. Exposure of the particles to N and 2 H does not modify the optical spectra at all. 2 Fig. 5 displays the results obtained for the (1,1)mode of K-clusters. Upon adsorption of oxygen, the following changes are observed. Already after an exposure of 0.25 L, the center frequency is displaced to a lower energy, and the amplitude starts to decline. At this stage, the width remains
Fig. 5. Shift of the center frequency and change of the width and amplitude of the (1,1)-mode of K-clusters as a function exposure to O , CO , H and N . 2 2 2 2
constant. For exposures greater than 1 L, the frequency shift continues as a function of dosage. Simultaneously, the amplitude decreases further, and the width starts to grow. Upon exposure to CO very similar modifications are observed. The 2 peak shifts to a lower energy, the width increases, and the amplitude shrinks. For adsorption of H 2 and N , no changes in the optical spectra have 2 been measured. For the (1,0)-mode, identical modifications of the width and the amplitude, as already discussed for the (1,1)-mode, have been found for sodium and potassium. However, the peak shifts of both modes differ substantially for the reaction of the particles with O and CO . This is illustrated in 2 2 Figs. 6 and 7, which depict the peak shift of the (1,1)- and the (1,0)-mode as a function of O and 2
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Fig. 6. Shift of the center frequency of the (1,1)- and the (1,0)mode of Na-clusters as a function of exposure to (a) O and 2 (b) CO . 2
CO exposure. For sodium ( Fig. 6a) as well as 2 potassium (Fig. 7a) and low oxygen dosages, both modes shift to a lower energy. Above 1 L, the (1,0) mode is, however, displaced to a higher energy, whereas the (1,1) mode shifts further to the red. As a consequence, the energetic separation between both peaks increases. In sharp contrast to oxygen, CO induces a decrease in the energetic 2 separation for sodium clusters already at very low dosages ( Fig. 6b). For potassium, the energetic separation is affected only for exposures to CO 2 of above 10 L, Fig. 7b. We finally mention that we performed experiments similar to those presented above for mean particle sizes ranging from R=20 to 40 nm. No size dependence was found, i.e. clusters with R= 40 nm show very similar changes in amplitude, width and energetic position of the plasmon modes upon, for example, dosage to oxygen, as compared to particles with R=40 nm. Therefore, we have
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Fig. 7. Shift of the center frequency of the (1,1)- and the (1,0)mode of K-clusters as a function of exposure to (a) O and (b) 2 CO . 2
presented exclusively the results obtained for clusters with R=20 nm.
5. Discussion We first note that measurement of the optical spectra obviously provides a sensitive technique to detect adsorbates on the surface of small particles. As can be seen from Figs. 4–7, even dosages of as little as 0.25 L can induce measurable shifts in the peak position and noticeably attenuate the amplitude of the plasmon polariton. Since 1 L corresponds to H=0.15 ML (see Section 2.1), optical spectroscopy provides a sensitivity of H<0.04 ML. In order to suggest an interpretation of the changes of the energetic position, the amplitude and the width of the plasmon resonances upon adsorption of molecules, the different processes that may in
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principle contribute to the modifications will be summarized briefly below. 5.1. Energetic position of the plasmon resonance In general, red and blue shifts of the plasmon modes can occur if adsorbate shells are formed. To distinguish between the different processes contributing to these shifts, calculations have been performed using the quasi-static approximation. Thus, features in the optical spectra that are brought about by excitation of multipoles and by retardation are neglected. Therefore, the theoretical results have not been used here to fit the spectra quantitatively, but are only exploited to qualitatively investigate trends of the shifts of the plasmon polaritons. We would like to mention in addition, that calculations of the optical spectra based on a model by Yamaguchi et al. [30] have shown that electrodynamic coupling among the clusters is negligible for the experimental conditions considered here. Fig. 8a–c illustrate schematically how the
Fig. 8. Schematic representation of supported oblate spheroidal Na-clusters ( left-hand side) together with their calculated absorption cross-sections for surface plasmon excitation (righthand side). The Na clusters are embedded in a medium with dielectric constant e and have a Na O shell with, from top to m 2 bottom, increasing thickness.
formation of an overlayer on the particle surface has been treated theoretically. In the calculations, a mean diameter of 30 nm and a height of 7.2 nm are assumed. This corresponds to the axial ratio of a/b=0.24. The shell material is taken into account by a dielectric constant e . The whole shell particle consisting of the metallic core and the overlayer is embedded in the medium e (e =1.37), the value of which accounts for the m m asymmetric surrounding consisting of the vacuum and the LiF(100) substrate (see above). Moreover, we have assumed that the adsorbate shell is allowed to cover the whole particle surface rather than to leave the particle substrate interface unaffected because of the following three reasons. First, the exact size of the contact area of the particle and the substrate surface is not known. Second, it cannot be ruled out that the particle/substrate interface is affected by adsorption of atoms brought about, for example, by diffusion into this interface region. Third, calculations assuming that one-third of the cluster surface remains uncovered upon shell formation showed identical trends as compared to those presented here. Only the amount of the peak shifts depends on the fractions of the covered and uncovered surface areas. The theoretical results can be summarized as follows. Red shifts of the (1,1) and the (1,0) mode are observed if the dielectric constant of the adsorbate layer is higher than that of the surrounding. Additional shifts to lower energy can occur if charge transfer from the metal cluster to the adsorbate shell takes place [10,13]. Blue shifts for both peaks occur, however, if the dielectric constant of the adsorbed shell is lower than that of the surrounding medium and/or charge transfer from the adsorbate to the metal cluster occurs. In addition to these shifts, which are identical for both modes, changes in their energetic separation take place if the shape of the particles varies upon adsorption of molecules. In principle, two possibilities exist to modify the shape of the clusters due to adsorption. First, the molecular overlayer may change the surface tension [19,31,32] and thus give rise to a variation of the axial ratio. It can increase or decrease, depending on the sign of the change of surface tension. Second, the adsorbate can react in such a way with the metal clusters that the atoms not only adsorb on the
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surface but also diffuse into the interior of the particles and react there. As a result, a shell forms consisting of the compound material. Fig. 8a–c show a Na cluster with an adsorbate shell, the thickness of which increases from 1 to 3 nm. Since the reaction of oxygen with the Na and K clusters will be discussed in more detail below, the dielectric function of Na O was chosen for the adsorbate 2 shell. The optical constants were taken from Ref. [33]. As a result of diffusion and compound formation, not only the size of the metallic core but also its axial ratio decreases. Fig. 8d–f show the corresponding absorption cross-sections. A considerable red shift of the (1,1) mode and a blue shift of the (1,0) mode are observed if the thickness of the layer is increased. This greater energetic separation between the modes results mainly from the decreasing axial ratio brought about by the increasing thickness of the Na O overlayer. 2 Similar calculations have also been performed for different clusters sizes (20 nm
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if the cluster Fermi level intersects partially unoccupied adatom or admolecule levels [2,7,13]. The electrons tunneling to the adsorbate and, after a certain residence time, returning to the cluster are scattered out of phase of the collective excitation. As a result, the width of the plasmon resonance increases, an effect known as ‘chemical interface damping’ [2,7,13]. In the discussion of the experiments presented in the next section, it has to be taken into consideration that this effect is most pronounced for small metal clusters, i.e. particles with diameters below 10 nm [2]. Since larger Na and K clusters have been examined here, this effect seems to be of minor importance for the present investigations. Although the experimentally observed changes integrate over the size and shape distributions, the spectra recorded for different experimental conditions together with the theoretical results described above allow us to suggest the following interpretation of the variations of the amplitude, position and width of the surface plasmon resonance upon adsorption of molecules. 5.4. Sodium From measurements using electron energy loss spectroscopy ( EELS), X-ray photoelectron spectroscopy ( XPS) and low-energy electron diffraction (LEED) [25–27], O is known to react 2 very effectively with sodium surfaces. Oxygen is chemisorbed dissociatively on Na(110)-surfaces and thin Na-films with considerable charge transfer to the adsorbed oxygen atom [34–36 ]. Therefore, it is not surprising that already, oxygen coverages lower than 0.1 monolayer induce pronounced changes in the optical spectra of the Na clusters. Following an oxygen exposure of 0.25 L, the amplitude of the plasmon peak starts to decrease. This indicates that the number of conduction electrons in the Na cluster is reduced because of a charge transfer to the adsorbed oxygen molecules. Most likely, Na O is formed by chemisorption of oxygen 2 on the cluster surface [35,37]. Furthermore, already in the lowest exposure regime, a peak shift to a lower energy is observed. As mentioned above, this can be caused by (1) the modified chemical environment, (2) changes in the shape of the Na
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clusters and (3) charge transfer to the oxygen atoms. Since the energetic separation between the (1,1)- and (1,0)-mode remains constant below 1 L (Fig. 6a), changes in the shape of the particles do not seem to play a role in this exposure range. Moreover, a comparison between calculated spectra of the bare Na clusters (not shown here) and the clusters with a Na O-shell of 1 nm thick2 ness ( Fig. 8d ) indicates that the modified chemical environment is also of minor importance for the peak shift. In fact, the calculations rather give a plasmon peak shift to a slightly higher energy. This results from the difference between the dielectric function of the Na O-shell and the surrounding 2 medium e without adsorbate. This displacement m is in contrast to the experimentally observed red shift, and we therefore interpret the measured peak shift by charge transfer from the sodium clusters to the adsorbed oxygen atoms. This overcompensates the blue shift brought about by the modified environment. Exposures between 1.5 and 4 L give rise to an even larger displacement of the (1,1) mode as a function of oxygen exposure. In addition to charge transfer, a second effect obviously contributes to the measured red shift. In this exposure regime, the (1,0)-mode, however, starts to shift to a higher energy, i.e. the energetic separation of both peaks increases. Our calculations of the peak position as a function of increasing Na O shell thickness 2 reproduce a growing energetic separation between the plasmon modes ( Fig. 8). Therefore, we interpret the additional red shift of the (1,1)-mode accomplished by the blue shift of the (1,0) mode as a decrease in the axial ratio of the (metallic) Na cluster brought about by the increasing Na O 2 overlayer thickness. For exposures above 4 L, the amplitudes of the plasmon resonances drop off, and the widths increase drastically. This can be understood as follows. As already mentioned above, the adsorbed oxygen does not simply form an overlayer on the metal particles. Rather, the oxygen penetrates into the clusters, thus gradually transforming the metal into Na O. This interpretation is corroborated by 2 two arguments. First, the disappearance of the Na plasma resonance goes hand in hand with the growth of a new distinct resonance, which is
located at 3.8 eV and characteristic of particles consisting of the formed Na O [14]. Secondly, the 2 dielectric constant of Na O being known, theoreti2 cal spectra of Na O clusters could be computed 2 and, in fact, also show a peak at 3.8 eV [14]. In summary, the following model for the reaction of oxygen with Na clusters is proposed. In the low-exposure regime, up to 1–2 L, chemisorption occurs, whereby the oxidation reaction is limited to the very surface region of the cluster. As a consequence, a slight decrease in amplitude and a small shift in the energetic positions of the plasmon resonances at constant width are observed. In the regime, between 2 and 4 L, oxygen starts to diffuse into the cluster. The thickness of the Na O shell gradually increases. Trans2 formation of the Na cluster into Na O begins. In 2 the high-exposure regime, this reaction continues, and all of the sodium is gradually oxidized. As a result, the plasmon resonance of metallic Na disappears. At the same time, a resonance characteristic of Na O emerges. 2 N O induces similar changes in the optical 2 spectra, the main difference being that the observed modifications occur at higher gas exposures as compared to oxygen. Molecular beam experiments have shown that N O dissociates into nitrogen and 2 oxygen if the molecules adsorb on Na clusters [33], a reaction also observed, for example, on single crystal surfaces of transition metals. We interpret the modifications of the optical spectra studied here also by dissociation of N O into N 2 2 and O, the oxygen atoms being chemisorbed on the Na cluster surface. As a consequence, the spectra exhibit similar modifications, as already discussed for oxygen dosage. The higher exposures needed to induce comparable changes result from the smaller dissociation cross-section of N O as 2 compared to O [33]. However, the nitrogen mole2 cules split off from the N O most likely do not 2 remain on the cluster surface. This is confirmed by experiments in which the Na clusters were exposed to N and in which no changes of the 2 optical spectra were found ( Fig. 4). A very interesting further observation is the decrease in energetic separation of the two resonance modes upon exposure of the Na particles to CO ( Figs. 3 and 6b). This clearly indicates a 2
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change in the axial ratio of the Na clusters, which tend to become more similar to spheres. Since the amplitude and width of the plasmon resonances remain unchanged, noticeable charge transfer and formation of a compound layer do not seem to occur. Therefore, the observed shape changes are most likely brought about by a modification of surface tension of the metal particles due to adsorption of CO -molecules. Similar changes in 2 the equilibrium shape have been found theoretically for metal particles of a different size and have been observed experimentally for reaction of CO 2 with different metal particles either during or after the growth process of the clusters [19,38,39]. 5.5. Potassium In analogy to sodium, exposure of K-clusters to oxygen molecules influences the amplitude, the width and the peak position of both plasmon modes for coverages far below a single monolayer (Figs. 5 and 7). As for sodium surfaces (Section 5.1), oxygen is known to chemisorb dissociatively on thin K films accompanied by a considerable charge transfer from the potassium to the adsorbed oxygen atoms [36 ]. Therefore, it is again not surprising that, already, small amounts of oxygen exposure induce changes in the optical spectra of the K-clusters, Fig. 5. Upon oxygen exposure of 0.25 L, the center frequencies of the (1,1)- and the (1,0)-modes are displaced to a lower energy, and the amplitudes start to decrease. Similar to sodium, this can be interpreted by chemisorption of oxygen atoms on the particle surface most likely forming K O. The energetic 2 separation of both plasmon resonances remains constant in this range, indicating that no shape changes take place. For exposures ranging from 1 to 8 L, the amplitudes of the plasmon modes decrease further. In addition, the energetic separation of the (1,1)- and the (1,0)-mode increases gradually, i.e. the (1,1) mode shifts to a lower energy, and the (1,0) mode shifts to a higher energy ( Fig. 7a). Both observations can be interpreted by increasing the thickness of the K O shell. As already discussed for sodium, 2 formation of a compound layer of growing thickness is accompanied by a decreasing axial ratio
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and shrinking size of the K core. This results not only in a lower plasmon resonance amplitude, because of less absorbing and scattering metallic material, but also in an increasing energetic difference between both plasmon modes as a function of oxygen exposure. For exposures greater than 10 L, the resonances vanish. Again, this can be interpreted by transformation of the particles into K O. 2 CO induces very similar modifications of the 2 center frequency of the (1,1) mode as well as their amplitude and width. Fairly similar to the dissociative chemisorption of N O on Na-particles, we 2 interpret the observed changes also by dissociation of CO on the K-surface into carbon and oxygen, 2 the latter being chemisorbed. As a consequence, the measured optical spectra exhibit similar modifications, as discussed for oxygen. However, the dosages necessary for comparable changes in the spectra are larger as compared to oxygen, reflecting the smaller probability of dissociative chemisorption. Therefore, only very small changes in the energetic separation and the width of the plasmon modes can be observed ( Figs. 5 and 7). Obviously, the reaction of CO on K-clusters is 2 very different compared to Na-particles. This again demonstrates the sensitivity of variations of the optical spectra towards different reaction mechanisms and will have to be investigated in more detail in the future. Finally, the measurements show that exposure of the clusters to N and H does not influence 2 2 the optical spectra at all. At present, we do not know, however, whether these molecules are not adsorbed on the particle surface or whether the induced modifications are below the detection limit.
6. Conclusions In conclusion, the results presented here show that optical spectroscopy can be effectively used for the detection and characterization of chemical interface reactions on supported metal clusters. Upon exposure to molecules, the optical spectra change in a variety of ways, depending on the nature of the adsorption reaction. The measure-
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ments not only provide submonolayer sensitivity but also allow one to distinguish between weak interactions, e.g. CO with sodium, and strong 2 reactions, e.g. O with Na and K. Even diffusion 2 of atoms into the interior of the clusters and subsequent transformation of the clusters into Na O and K O can be detected. In addition, shape 2 2 changes brought about either by the formation of a compound layer with increasing thickness or by changes in surface tension have been observed. In future experiments, it would be very useful to extend the experiments to smaller clusters, where strong size-dependent reaction rates are expected upon adsorption of various molecules, and to investigate changes in the optical spectra as a function of cluster size.
Acknowledgements Financial support of the Deutsche Forschungsgemeinschaft (DFG) through the Graduiertenkolleg ‘Materialien und Komponenten der Mikrosystemtechnik’ and the Fond der Chemischen Industrie is gratefully acknowledged.
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