Langmuir 1991, 7, 3019-3026
3019
Faceting of W(11 1) Induced by Ultrathin Pd Films Ker-Jar Song, Cheng-Zhi Dong, and Theodore E. Madey' Department of Physics and Laboratory for Surface Modification, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08855 Received March 4,1991.I n Final Form: April 17,1991 The surface stability of a model bimetallic system, Pd on W(lll), has been studied by using ultrahigh vacuum surface science methods. Fractional monolayer Pd thin films are stable and remain on the surface at all temperatures below -1100 K, at which thermal desorption begins. Above a threshold coverage of about one monolayer, the Pd-covered W(111) surface is structurally unstable; upon annealing above 700 K the W atoms undergo massive rearrangement to expose (211)oriented facets. Multilayer uniform Pd thin films deposited at room temperature are also unstable upon heating to T > 500 K and agglomerate to form three-dimensional Pd clusters on top of the faceted W surface covered by a monolayer of Pd.
I. Introduction The morphology of ultrathin metal films grown on atomically smooth, close-packed metal substrates has long been of interest.' These films have either served as model systems for the study of chemical properties of mixed metal catalysts or have provided specific nanostructures for the investigation of exotic electronic or magnetic properties. A few common thin film growth modes have been Frank-van der Merwe (layer by layer), Volmer-Weber (nucleation of 3d clusters), Stranski-Krastanov (nucleation of 3d clusters after one or a few monolayers), simultaneous multilayers, etc. In most studies of thin films on atomically close-packed metal substrates, the structural rearrangements occur mainly in the overlayer, while the substrate undergoes little or no change. In contrast, recently it has been found3+ in this laboratory that the interaction of Pt and Au ultrathin films with a body centered cubic W(111) surface, which is atomically rough and open, causes the W(111) substrate itself to reconstruct to form microscopic pyramids that expose facets oriented along (2111directions. To investigate the generality of this kind of structural transformation and details of its dependence on the adsorbate coverage, we have extended our study to Pd in the present work. Pd is chosen because of its catalytic importance and because it is likely to induce faceting; its chemical properties and atomic dimensions are close to those of Pt. One big advantage of Pd over Pt is its much lower sublimation energy, which makes calibration of coverage by thermal desorption measurements feasible. We find that ultrathin Pd films can induce faceting of W(111). Above a threshold coverage of about one monolayer, the Pd-covered W( 111) surface is structurally unstable; upon annealing above 700 K the W atoms undergo massive rearrangement to expose (211)oriented facets. We also find evidence of growth of threedimensional Pd crystallites for coverages more than twice the threshold coverage. In the rest of the paper, we first describe our experimental setup, followed by presentation
* Author to whom correspondence should be addressed. (1) Bauer, E. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: New York, 1984; Vol. 3, Part B, p 1. (2) Biberian, J. P.; Somorjai, G. A. J . Vac.Sci. Technol. 1979,16,2073. ( 3 ) Song, K.-J.;Demmin, R. E.; Dong, C.-Z.;Garfunkel, E.; Madey, T. E. Surf. Sci. Lett. 1990, 227, L79. (4) Madey, T. E.; Song, K.-J.; Dong, C.-Z.; Demmin, R. E. Surf. Sci., in press. ( 5 ) Song, K.-J.;Dong, C.; Madey, T. E. The Structure of Surfaces-9; Tong, S. Y., Van Hove, M. A., Xie, X., Takayanagi, K., Eds.; SpringerVerlag: Berlin, 1991.
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and interpretation of thermal desorption, Auger and lowenergy electron diffraction (LEED)results. We then discuss the threshold coverage for facet formation and end with a brief discussion of the energetics driving the faceting phenomena.
11. Experimental Setup Most of the sample preparation and characterization were carried out in a conventional ultrahigh vacuum (UHV)chamber with a base pressure of 2 X 10-loTorr. A Varian four-grid lowenergyelectron diffraction (LEED)system is the major tool used to provide structural information about the surface. The chamber is also equipped with a cylindrical mirror analyzer (CMA) and a grazing incidence electron gun for taking Auger spectra, which provide information about impurities on the sample and the thickness of the film. A quadrupole mass spectrometer is used for both monitoring background residual gas and detecting desorption products in thermal desorption experiments In the latter case, a shutter with an aperture of 0.4 cm diameter is positioned in front of the quadrupole so that only desorption from the central area of the crystal is detected. The sample temperature is measured by a W/5% Re-W/26% Re thermocouple spot-welded to the back of the crystal. Resistive heating of the W(111) sample below 2000 K is controlled by a personal computer which allows the temperature to be programmed as any piece-wise linear function of time. Repetitive oxygen dosing and heating to 2300 K (using an auxiliary electron bombardment filament) is exercised until the Auger peak of carbon no longer appears after annealing for 30 s at 2300 K. The Pd evaporator is composed of parallel but separated sections of 0.5 cm long, 0.5 mm diameter Pd wires spot-welded onto a 0.025 mm thick, 0.5 cm wide Ta foil which is in turn spotwelded to two 0.5 mm diameter Ta wires on each side for resistive heating. The Ta foil was first cleaned by annealing above 1700 K. A constant current source supplying about 8 A is used to obtain a stable deposition rate of about 0.1 monolayer per minute at a distance of about 2.5 cm. Evaporation is by sublimation and the Pd wires never melted during the whole experiment. The rise of total pressure during evaporation is always less than 2 X 10-loTorr for the experiments reported here. After evaporation of Pd onto the sample, no impurities were detected by Auger electron spectroscopy (AES). Upon exposureto the sameresidual gas environment, the growth of the oxygen and carbon Auger peaks is much slower for samples covered with Pd than for bare W samples. This indicates that the sticking probability for residual gases on the W(ll1) surface is reduced by monolayer coverages of Pd.6 A scanning tunneling microscope operated in air has been used to obtain real space images of the surface structure. We choose not to present the images here due to the preliminary nature of these experiments. (6) Presumably, the C and 0 come from residual CO and H20 in the chamber. For CO adsorption on Pd-covered W(110) and 4100) surfaces, see Berlowitz, P. J.; Goodman, D. W. Langmuir 1988,4, 1091.
0 1991 American Chemical Society
Song et al.
3020 Langmuir, Vol. 7, No. 12, 1991
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Temperature (K) Figure 1. Thermal desorption spectra of Pd from a W(111) surface. Heatingrate is 10 K/s for the temperature region shown. Except for the curve with the highest coverage, successive curves have a coverage difference which corresponds to 2 min of Pd deposition. The curve indicated by the arrow corresponds to 10 min of Pd deposition and is at the threshold of building up of the bulklike Pd desorption peak. Note that this coverage threshold also serves as the threshold above which the surfaces will undergo structural transformation upon annealing above 700 K to form new facets. To facilitate the search for ordered structures as a function of coverage, we have deposited “wedge” films of Pd with approximately constant concentration gradient across the sample such that the concentration at one edge is about l / 2 that at the opposite edge. LEED patterns as a function of coverage are easily obtained by moving the sample along the direction of the concentration gradient. The coverage resolution obtained by using this technique is determined by the cross section of the electron beam a t the sample and the concentration gradient. In our case, we estimate the coverage resolution to be l/zoth the coverage of the thickest end. After we obtain a fairly good overall picture using the wedge films, uniform films in each individual coverage region of interest are studied. Since uniform films are less susceptible to diffusion across the surface, determination of coverage tends to be more reliable, especially a t high temperature. The LEED results reported in this paper are based on both the wedge films and the uniform films.
111. Results and Interpretation (A) Thermal Desorption of Pd from W(111). Figure 1 shows a set of thermal desorption spectra for Pd from W(111). For all the experiments shown, heating rates are 20 K/s for temperatures below 1000 K (not shown) and 10K/s above lo00 K. Except for the curve with the highest coverage, successive curves have a coverage difference which corresponds to 2 min of Pd deposition. By integration of the area under the thermal desorption spectroscopy (TDS) curves, it is found that the coverage is linearly dependent on the deposition time. In the submonolayer range, the peak temperature decreases as the coverage increases. This trend is similar to that found for Pd on W(100),7 which was attributed to a repulsive interaction among nearby Pd atoms. For the thicker films, the common leading edges of the multilayer peaks suggest completion of the interface layer(s) and desorption from a Pd bulklike film. Since we do not have independent calibration of the absolute deposition rate, the onset of a bulk-like desorption peak is used as our coverage reference point throughout the rest of this paper. If we define one monolayer as the coverage above which we will see the multilayer peak in the TDS spectra, then 10 min of deposition will result in a film about one monolayer thick. As will be shown below, a 10-min deposition corresponds to the coverage threshold above which faceting occurs upon annealing. (7) Prigge, S.; Roux, H.; Bauer, E. Surf. Sci. 1981, 107, 101.
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l/kT Figure 2. Arrhenius plot of (dO/dt) X Bnfor n = O , I / 3 , and ‘12 for the bulklike desorption of Pd from a Pd film initially about four monolayers thick. 0 is the bulklike Pd coverage which is obtained by integrating the bulklike Pd desorption rate dO/dt (after removal of the desorption rate contributed by the monolayer Pd film). Note that systematicallylarger activation energies are obtained for higher order kinetics. The data are equally well fit by 0- and l/s-order kinetics but less satisfying by assuming kinetics with orders larger than l / 2 .
Evaluation of the coverage-dependent binding energy of Pd using Bauer’s8 method is hampered by the noise in the TDS data. However, by plotting In ((-dO/dt)/e”)versus l/kTfor various n,gas shown in Figure 2, we find that for the thickest film, the leading edge of the multilayer is best fit by zeroth-order kinetics with an activation energy of 363 f 39 kJ/mol. This can be compared to the heat of sublimation of bulk Pd, which is 376 f 2 kJ/mol.l0 We note that the same data can also be nicely fit by ?/j-order kinetics with a higher activation energy of 378 f 39 kJ/ mol or barely adequately fit by one-half-order kinetics with a still higher activation energy of 386 f 39 kJ/mol. The general trend of a higher activation energy with a higher order kinetics is clearly seen. As will be discussed below in IIIB, multilayer Pd films do form threedimensional Pd clusters on top of the faceted W surface if annealed above 700 K for 3 min. This certainlysuggestdl the possibility of non-zero-order kinetics. For example, if desorption proceeds by evaporation of atoms detached from the three-dimensional cluster, with the detachment from the cluster-substrate boundary as the rate-limiting process, then we expect ‘/s-order kinetics. In many experiments,12the “bulklike” desorption of metal films is analyzed by using zero-orderkinetics. In general, however, for multilayer films that agglomerate, we need to keep in mind the dynamic nature of the morphology during the desorption process and the uncertainty caused by such changes. (B) Auger Spectroscopy: Growth and Thermal Stability of Pd Films. Figure 3a shows the Auger intensities of Pd and W as a function of deposition time. (8) Bauer, E.; Bonczek, E.; Hoppa, H.; Todd, G. Surf. Sci. 1975,53,87. (9) For a recent discussion of this method, see Parker, D. H.; Jones, M. E.; Koel, B. E. Surf. Sci. 1990,233, 6. (10) Honig, R. E.; Kramer, D. A. RCA Reu. 1969, 30, 285. (11) Arthur, J. R.; Cho, A. Y. Surf. Sci. 1973,36,641. These authors
report ‘/2-order kinetics for desorption of low-coverage Au and Cu from graphite; they identify the rate-limiting step in desorption to be detachment of metal atoms from the edges of the two-dimensional metal islands grown on graphite. (12) He, J.-W.; Shea, W.-L.; Jiang, X.; Goodman, D. W. J. Vac. Sci. Technol., A 1990,8,2435. The authors of this paper have measured the activation energies for desorption of “bulklike” Cu, Fe, and Ni films from Re(0001) and Mo(llO! surfaces by using zero-order kinetics. These energies are all lower than the heats of sublimation of pure bulk material. We suggest that this may be partly due to effects of the changing morphology of the film a t the desorption temperature.
Langmuir, Vol. 7, No. 12, 1991 3021
Faceting of W(111)
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are first obtained from Figure 3a and then inverted for finding the effective thickness of a film with a particular Pd/W ratio.
The decrease of the W signal can be fit to an exponentially decaying curve with a decay constant 71 of 14.6 min. Assuming a time dependence proportional to (1.0 exp(-t/.rz)), the increase of the Pd signal can also be fit with a 7 2 of 18.3 min. The longer time constant is attributed to a longer attenuation length for electrons at the Pd Auger energy of 330 eV than that a t the W Auger energy of 169 eV. Although we do not have experimental calibration of the absolute coverage, we note that TDS curves, as shown in Figure 1, indicate that 10 min of evaporation results in a Pd film about one monolayer thick. Assuming this “one monolayer” corresponds to a coverage of one Pd atom per surface unit cell, we estimate13 the attenuation length of the electrons in the Pd to be 1.8 A at 169 eV and 2.3 A at 330 eV. These values are too small compared with the inelastic mean free paths of 5.5 A at 169 eV and 8.3 A a t 330 eV calculated by Tanuma et al.I4 (even if considering the fact that the attenuation length is systematically smaller than the inelastic mean free path by up to 35%le). We will discuss this discrepancy after the discussion of the LEED results and the structure of the film. The exponential behavior of the two curves suggests that the growth of the Pd film is via the “simultaneous multilayer” mode or equivalently, the “stick where they hit” mode. Whatever the name, this mode of growth is an
indication of very limited mobility of the Pd atoms. Careful readers may find the fit to the Pd data not completely satisfying, as there seems to be some small but systematic deviation. To investigate the possibility that the curves may be piecewise linear (layer by layer growth), the absolute values of the derivatives of the Auger intensities are plotted in Figure 3b. Except possibly for the initial 5 min or so, no steplike structure is evident. Even for the initial 5 min, scattering of the data prevents us from asserting whether the film actually forms one complete monolayer first or exhibits simultaneous multilayer growth from the very beginning. We do note that if it has a linear section followed by an exponential, then the break point is somewhere around 5 min, about onehalf of our previous definition of “monolayer” based on TDS. What happens if the films deposited at room temperature are annealed? Figure 4 shows the effective thickn e d 6 of several Pd films as a function of the annealing tempereture. In these experiments, the films were annealed for 3 min at each temperature and cooled down to room temperature for the AES measurement before annealing at the next higher temperature. At or below monolayer coverage, the Pd thin films are stable all the way up to 1000 K as evidenced by the lack of change in the effective thickness. This stability is attributed to a lower surface energy for the Pd-covered W surface than for the bare W surface.’7 Multilayer films, however, are not stable; the effective thickness of Pd decreases monotonically with increasing temperature. There are four possible mechanisms that can cause the apparent disappearance of Pd: (1)desorption of the Pd into vacuum; (2) diffusion of Pd into W to form an alloy; (3) diffusion of Pd along the surface to the side and back of the sample; (4)formatior, of 3d clusters thus reducing the number of Pd atoms within the detection depth of Auger electrons. Since we know from Figure 1that unless heated above 1000 K, Pd will not thermally desorb in any significant amount, thermal desorption plays no role below 1000 K where the reduction of the Pd effective thickness
(13) Since the sample surface is normal to the axis of the cylindrical mirror analyzer, all the Auger electrons collected are assumed to have traversed the P d film a t an angle of 42.3O with respect to the surface normal. The probability of Auger excitation is assumed to be independent of depth. (14) Tanuma, S.;Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11, 571. (15) Jablonski, A. Surf. Sci. 1987, 188, 164.
(16) The effective thickness of any film is defined to be the thickness of that uniform film which gives rise to the same Auger intensity ratio (Pd/W) as the film being considered. The relation between the P d / W ratio and the thickness is obtained by using data as shown in Figure 3a. (17)Mezey, L. Z.; Giber, J. J p n . J . Appl. Ph>s. 1982, 21, 1569. According to this calculation, surface enorgies of Pd and W are 2.04and 3.47 J/m2. Assuming the Pd covered W to have a surface energy similar to that of Pd, it will be much lower than that of hare W.
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Deposition Time (min) Figure 3. (a) Auger intensity of Pd and W as a function of deposition time with the W(111) substrate kept at room temperature during successive deposition of Pd. Solid lines are fits with exponential time dependence. (b)Derivatives of Pd and W Auger signal with respect to deposition time. Except for the initial 5 min, there is no apparent steplike structure. See text for explanation.
3022 Langmuir, Vol. 7, No. 12, 1991
occurs. If it is caused by diffusion of the Pd into the W bulk, we would expect the tails of the TDS curvesto extend to higher temperatures as coverages increase.l8 Instead, TDS curves as shown in Figure 1show a build up of multilayer peaks, which is most easily accounted for if Pd simply stayed on top of the surface. The convergence of all the high temperature tails to the same curve also supports the idea that large concentrations of Pd atoms do not diffuse into the W bulk. This is compatible with the bulk phase diagramlg of the Pd-W system which indicates no stable compound and a very small solubility of Pd in W (12% in the temperature range we studied). The third possibility of massive diffusion along the surface is related to the large surface concentration gradient at the edge of the crystal. While submonolayer Pd films are relatively immobile, the monolayer edge of a multilayer Pd film strip has been observedz0to move 0.5 mm in 3 min at 1070 K on a W(110) surface. Although we do not know what happens to multilayer Pd on W(lll),zl clearly it is not justified to eliminate the third possibility. One important but not so obvious point, however, is the conservation of mass. The thickest film shown in Figure 4 is about four monolayers. If diffusion to the back of the crystal is the only thing that happens, then at equilibrium, we would expect a two monolayer thick uniform film. The fact that the effective thickness of Pd decrease well below two monolayers indicates that another mechanism has to be invoked. That the Pd atoms stay on top of the surface, but with an effective thickness decreasing toward that of a monolayer film upon annealing even at temperatures as low as 500 K, leads us to conclude that Pd must be agglomerating to form 3d clusters. Diffusion along the surface to the edge and/or the back of the crystal seems likely to occur, but we do not know to what extent. (C)Low-Energy Electron Diffraction (LEED) Results. For Pd/W(111), we have identified three different structures in three different coverage regimes below four monolayers. For submonolayer films deposited at room temperature, LEED patterns show essentially the W(111) 1X 1pattern, although increase in the background intensity is evident. After annealing above 800 K, some faint spots surrounding the 1X 1spots can be recognized, indicating some kind of superstructure. A typical example can be found in Figure 5a. The surface, however, remains flat, as indicated by the way the LEED beams move as the incident electron energy is varied (cf. discussion in following paragraphs). Very dramatic changes occur for films thicker than a threshold coverage of about one monolayer. A t room temperature, LEED patterns from these films are similar to those of submonolayer films. Once annealed above 700 K, however, the original W(111) 1 X 1 spots totally disappear and new patterns form. LEED patterns of W(111) surfaces that have Pd coverages equivalent to 9, 10, and 11min of deposition and annealed at 1000 K for 3 min are shown in parts a, b, and c of Figure 5. The correspondingly labeled thermal desorption spectra of Pd from these three surfaces are shown in Figure 5e. It is clear by comparison of parts a, b, and c of Figure 5 that parts a and 5c show totally different structures, while part b shows a transition stage composed of mixtures of both (more part c than a). It is also clear by examination of (18) Campbell, C.; Goodman, D. W. J. Phys. Chem. 1988, 92,2569. (19) Shank, F.A.Constitutionof Binary Alloys, Second Supplement; McGraw-Hill: New York, 1969. (20) Wagner,H.SurfaceMobilitiesonSolidMaterials;VuThienBinh, Ed.; Plenum Press: New York, 1983. Note that the boundary does not move at a constant speed but proceeds instead with the square root of time. (21) More precisely, on a W surface composed of facets of (211) orientations. As will be shown later, W(111)surface with multilayer Pd coverage become faceted after annealing above 700 K.
Song et al. Figure 5e that the transition is completed in a very narrow coverage region no more than a tenth of a monolayer. Also, as indicated by the labeled curve in Figure 1 (which corresponds to a film obtained by 10 min of deposition), the transition occurs at a coverage of about one monolayer according to our definition. What is the structure of the new phase after the transition? The most distinctive characteristic of the new structure is the way the LEED spots move when the incident energy is varied. Simple kinematics considerations indicate that when the incident energy is increased, all the LEED spots will move and converge toward the fixed spot that corresponds to the specularly reflected beam. If the surface is flat, as is the case before the structural transition, all spots converge toward the center as a result of normal incidence. However, after the structural transition, the spots no longer converge toward the center where the original specular spot was. Instead, they move sideways, as can be seen from Figure 5d which shows the time exposure of the LEED pattern while the incident electron energy is increased from 8 to 120 eV. This indicates formation of new specular directions, i.e. formation of large enough patches of surfaces oriented differently from the original surface. Since these newly formed LEED patterns are almost identical with those found in the case of Pt/W(lll)314 under similar situations, they are easily identified23 to be due to diffraction from (2111oriented facets with 1X 1unit cells. From the size of the sharpest diffraction spots, we estimate the aver e linear dimension of the facets to be larger than loo?. The profiles of the LEED spots are elliptical, with the long axis pointing along the trough direction,24 probably indicating some disorder along the trough direction. We note also that although the sharpness of the LEED spots depends strongly on the highest temperature to which the surface has been annealed, no discernible dependence on the coverage of Pd has been observed. Thus, the facet size distribution does not seem to depend on the Pd coverage so long as the coverage is above the threshold coverage. What is the origin of these facets? There are at least two possibilities: (1)a special Stranski-Krastanov growth mode (formation of 3d bcc Pd6925clusters after completion cf the first monolayer) with all the 3d bcc Pd clusters exposing (211)surfaces; (2) a massive restructuring of the W(111) surface to expose (211)facets due to the presence of a monolayer of Pd on the surface. In the following paragraphs, we discuss arguments that are against the first possibility and in favor of the second. One observation against the hypothesis of Pd pyramid formation is the small amount of Pd needed to induce the formation of facets. From simple geometrical considerations, the amount of Pdneeded to keep most of the surface covered26by Pd pyramids is a linear function of the average size cif the pyramids. For a surface covered with Pd (22) Probably not very well-known, this simple but very effective technique has been used by Henrich, V. E. Surf. Sci. 1976,57,385;and Niehus, H.Surf. Sci. 1979, 87, 561. (23) By comparison with the LEED patterns of the faceted surface generated by a kinematic calculation. (24) The bcc (211) sueace consista of parallel close-packed rows of atoms runningalongthe (111)direction with neighboringrows separated by 1.633d, where d is the nearest neighbor distance. (25) Skrivsr, H. L. Phys. Reo. B: Condens. Matter 1985, 31, 1909. Calculations in this paper have shown that for Pd, the total energy difference between the face-centeredcubic (fcc)and body-centeredcubic (bcc) structures are very small, on the order of a few kJ/mol. Experimentally, the authorsin ref 6 reported formation of thin Pd(100) bcc film grown pseudomorphicallyon W(100). Thus there is no a priori reason why 3d bcc Pd clusters cannot grow on a W(111) surface precoveredwith a monolayer of Pd. (26) Since Figure 4a indicates no sign of diffraction spots from (111) oriented patches, most of the surfacehas to either be coveredby pyramids of Pd or become faceted.
Faceting of W(l11)
pyramids with average lateral size larger than 100 A, the average thickness of the Pd film is more than 5 A, or at least 5.5 Pd atoms per 1 X 1surface unit cell. The TDS curve (c) in Figure 5e of Pd from a surface which has an average patch size larger than 100 A (the surface with the LEED pattern in Figure 5c), however, indicates that the Pd coverage is only slightly more than one monolayer. Although we have not measured the absolute coverage of this TDS monolayer, a plausible model (as discussed in section IV) suggests it to be three Pd atoms per unit cell, which is much less than the 5.5 Pd atoms per unit cell needed for pyramid formation. Also, for Pd pyramid formation, we would expect a significant decrease (increase) of the Pd (W) Auger signal as the initially uniform Pd film agglomerates to form 3d clusters. However, this is not what we have observed. As shown by the curve labeled with empty squares in Figure 4, which corresponds to a sample with Pd coverage slightly above the threshold coverage, the Pd effective thickness does not show significant change as the annealing temperature is raised from room temperature (when the film is flat) to loo0 K (after which the surface is well faceted). Finally, for pyramid formation, it is difficult to reconcile the linear dependence of the amount of Pd required on the average size of the Pd pyramids with either the independence of the patch size distribution on the Pd coverage or the sharp structural transition as a function of coverage. In the case of restructuring of the tungsten surface to expose (211)oriented facets, the rationale is that the (211) faceted form of the surface becomes more stable than the planar form once a threshold amount of Pd is on the surface. We note that so long as the surface is totally faceted, i.e. consists of only (211) oriented patches, the total surface area remains constant irrespective of the average facet size. Thus, the same threshold amount of Pd can stabilize the faceted surface irrespective of the facet size distribution. The TDS results indicate this threshold coverage to be about one monolayer. Since the threshold amount of Pd needed to cover the faceted surface is independent of the size of the facets, it is not surprising that the transition occurs in a very narrow coverage range. The invariance of the Auger signal as the surface changes from a planar to a faceted form is again consistent with faceting since the Pd remains in the form of a thin film covering the faceted W. Finally, the independence of the facet size on Pd coverage is also understandable as the size of the facets depends only on how much tungsten has moved. Thus, we conclude that the formation of (211) oriented facets is due to a Pd-induced restructuring of the tungsten substrate, as in the case of Pt/W(111).3t4 In the case of P t / W ( l l l ) , it has been found also that the facets do not distribute themselves in an ordered way and the facet sizes are not u n i f ~ r m . ~We ~ ~expect ? ~ ~ the same to happen in the case of Pd/W(111). A schematic of the faceted surface containing pyramidal shaped extrusions and pits is shown in Figure 6. We note that the angle between (211)and (111)oriented surfaces is sin-' (1/3) = 19.5'. For surfaces covered with more than two monolayers of Pd and annealed at loo0 K, LEED spots additional to those coming from the (211)facets appear. These spots can be seen in Figure 7a. As the energy of the incident electrons is increased, these spots do not converge toward either the specular direction of the (111)surface or the specular directions of the (211) surfaces. Instead, they converge toward points near the specular direction of the (110) surfaces, as shown in Figure 7b. The position of these extra spots, however, does not match the LEED (27) Dong, C. Z.; Song, K.-J.; Madey, T. E. Unpublished result.
Langmuir, Vol. 7,No.12,1991 3023 pattern of (110)facets grown on W(111). In fact, they do not match any of the LEED patterns of a whole setz8of surfaces with other facet orientations. Furthermore, the intensity of these spots increases very slowly as the Pd coverage is increased from two to four monolayers and is always weak compared with the intensity of the coexisting spots which originated from the (211)facets. Considering the narrow coverage range over which the transition from a planar surface to a (211) faceted form occurs, we believe the slow buildup of small intensity to be evidence against the hypothesis of formation of differently oriented tungsten facets which are more stable at higher coverages. Instead, as Auger results indicate agglomeration of Pd into 3d clusters in this coverage range, we suggest that these spots come from the exposed surfaces of the 3d Pd clusters. The slow buildup (as a function of coverage) of small intensities of the new spots coexisting with bright spots of (211) facets is then rationalized as a result of the slow buildup of the 3d Pd covering a small fraction of the (211) faceted surface. As shown in Figure 7a, the new spots always come in pairs. We believe this to be a result of two different but equally possible epitaxial relations between the Pd clusters and the faceted W substrate. Details of the epitaxial relations and the structure of the Pd clusters are currently under investigation.
IV. Discussion
(A)Critical Coverage for Faceting: Definition of a Monolayer. If we construct hard sphere models to reproduce the W bcc or the Pd fcc lattice, the diameter of the W spheres is 2.74 A while that for Pd spheres is 2.75 A. The almost identical hard sphere diameters suggest that deposited Pd atoms may simply occupy positions so as to continue the W lattice. What is the consequence of such a model? Figure 8a shows the top view of a hard sphere model of the W(111) surface. This surface is very loosely packed. Figure 8b shows the side view of a Pdcovered W(111) substrate seen from the [liO] direction. The unlabeled spheres represent the W atoms while the labeled spheres represented the Pd atoms. Three geometrical layers (each denoted by a, b, and c) of Pd are shown. It is apparent from Figure 8 that only for Pd spheres above the third geometrical layer will Pd not be in direct contact with any of the substrate W spheres. The numbers of W spheres as nearest neighbors and next nearest neighbors for Pd spheres are shown in Table I as a function of position of the Pd spheres. As mentioned before, the reduction of the binding energy of Pd on W(100) as Pd coverage increases has been attributed7to repulsion among Pd atoms. In the present case, if we assume that the more W neighbors a Pd atom has the more strongly it is bound, then the binding energy of the Pd atom will decrease as Pd coverage increases beyond the first geometrical layer (i.e. the layer labeled a). Thus, due to the very open nature of the W(111) surface, the decreasing binding energy of Pd may simply reflect occupation of sites with reducing number of W neighbors. Note also that only those Pd atoms sitting on top of three layersm of Pd, such as the one labeled d in Figure 8b, will have a binding energy similar to that of Pd on a (28) More specifically,all surfaces in the ( i l l ) , (loo),and (il0) zones. (29) We do not have any direct evidence against the possibility of formation of Pd/W alloy pyramids. However, TDS does not wem to indicate formation of alloy. (30) Incontrast,for'Pd"spheresstackedonboth W(110)andW(loO), Pd spheres in the second layer cannot touch W spheres at all. Thus, in those c a w , the physical monolayer is the same as the geometric monolayer.
3024 Langmuir, Vol. 7, No. 12, 1991
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temperature Figure 5. (a, b, and c) Pictures of LEED patterns at an incident energy of 101 eV for W(111) surfaces with Pd coverages of (a) 0.9, (b! 1.0, (c) 1.1monolayers that have been annealed a t 1000 K for 3 min. (d) Time exposure of LEED patterns while the incident energy is increased from 8 to 120 eV. Same surface as in part c. The sidewaysmotion indicates formation of facets oriented differently from the average surface. One of the spots due to specular reflection from 1211) oriented facets is indicated by the arrow. (e) Thermal desorption spectra of Pd from the surfaces whose LEED patterns are shown in parts a, b, and c (correspondingly labeled). The structural transition occurs in a very narrow coverage range of about 0.1 monolayer.
Pd surface. Thus, it is plausible that the “monolayer” previously defined based on TDS results really corresponds to the completion of the first “physical monolayer” 31 but not the “geometric monolayer”. By the first “physical
monolayer”, we mean all the Pd atoms that are in direct contact with the W substrate, Le. all three geometric layers labeled a, b, and c in Figure 8b. In contrast, a geometric layer, such as that labeled a (or b or c), consists of atoms
Faceting of W(ll1)
Langmuir, VoZ. 7,No.12, 1991 3025
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Figure 6. Schematic of a partially faceted (111) surface. Three pyramids and two pits are shown. Note the relative orientation of the pyramids and the pits. For clarity, these pyramids and pits are shown separated. For the case of Pt on W(111),%our STM studies show that they usually come side by side.
at the same depth with respect to the surface and is characterized by one atom per surface unit cell. For Pd/ W(lll),onephysicalmonolayercontains1.7 X 1015atoms/ cm2; one geometrical monolayer contains 5.7 x 1014atoms/ cm2. As mentioned previously, by assuming the TDS “monolayer”to correspond to a coverageof one geometrical layer (i.e. one Pd atom per surface unit cell), the estimated attenuation length of the electrons in the Pd is much shorter than expected. If we reassign the TDS monolayer as a physical monolayer and redo the calculation, we get 5.4 at 169 eV and 6.8 A at 330 eV,12in better agreement with the calculation by Tanuma et al.13 which predicted inelastic mean free path values of 5.5 and 8.3 A at 169 and 330 eV, respectively. Thus, assuming the accuracy of the calculation by Tanuma et al., the measured attenuation length of the Auger electrons supports the hypothesis that the TDS monolayer is the physical monolayer instead of the geometric layer.
It is interesting to note that the atom density corresponding to one physical monolayer of Pd on W(111) will provide exactly one physical monolayer of Pd for the faceted (211) surfaces (the effect of increased area is included). For the (211)surfaces, one physical monolayer consists of two geometric layers. Thus, one geometriclayer of Pd on W (111)can only provide two-thirds of a geometric layer of Pd for the faceted (211)surface. However, since the LEED pattern of the faceted surface clearly exhibits a 1 x 1 structure, the threshold coverage has to provide no less than one geometric monolayer of Pd covering the (211) faceted surface. This fact argues against the hypothesis that the TDS monolayer corresponds to one geometric layer of the (111)surface. Is the TDS monolayer equal to one or two geometric layers on the (211) faceted surface? Again, due to the open structure of the (211)surface, we tend to favor the hypothesis that only for coverages above two geometric layers will the Pd desorb in a bulklike fashion, and the TDS monolayer should be one physical monolayer. Another intriguing property of this system is the fact that one physical monolayer of Pd on W( 111) will provide (31) By “physical”,the emphasis is on the direct interaction between neighboring layers instead of the geometric position with respect to the surface. In any real experiment, it is always the physical monolayer that counta. For a close-packed surface, the geometric monolayer is identical with the physical monolayer. However, for looselypacked surfaces (most notably, surfaces vicinal to a close packed surface), we believe it is important to emphasize the concept of a physical monolayer.
Figure 7. (a) Picture of LEED pattern at 21 eV of a W(111) surface with four monolayers of Pd and annealed at 1000 K for 3 min. Two of the spots originating neither from the (111) nor the (211)surfaces are indicated by the arrow. Other corresponding spots are located at positions symmetric to the major symmetry axis. These “extra” spots come in pairs. Their movements as the incident energy changes are shown in part b, which is a time exposure of the LEED pattern while the incident energy varies from 13 to 27 eV.
exactly32 one physical monolayer of Pd for all faceted surfaces within a continuous range33of facet orientations about (111). Thus, so long as the average surface remains ( l l l ) , no matter how the surface morphology changes, the same amount of Pd (one physical monolayer) will be just enough to cover the whole surface34so long as the exposed (32) Adopting our definitionof the “physicalmonolayer”,such equality is a result of the surface being normal to a close-packed atomic row. The general proof is lengthy and will be published elsewhere, but the reader can easily convince himself by checking some special cases such as faceting of bcc (111) surface into bcc (110)or (loo]. The general proof applies to any crystal structure with one atom per unit cell. (33) This range depends on the lattice structure. For each crystal structure, there are several close-packed directions. The whole 4r of solid angle is separated into equal shares of ranges for each of these closepacked directions. (34) Assuming the Pd atoms to occupy positions so as to continue the W lattice.
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3026 Langmuir, Vol. 7, No. 12,1991
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Figure 8. A hard sphere model of the W(111) surface. (a) Top view of a bare W(111) surface; the first three layers of W atoms are clearly exposed. (b) Side view of the W(111) surface covered with three layers of P d atoms. The surface is cut open along the [112] direction, as indicated by the line in part a, and viewed from the [110] direction. The P d atoms are assumed to occupy positions so as to continue the W lattice. The first three layers of Pd, labeled a, b, and c, are all in contact with the W substrate. See text for the implications of this model. Table I. Number of Neighboring Tungsten Atoms for Pd Atoms in a Hard Sphere Model (See Text for Detai1)a Pd Pd Pd Pd atoms in atoms in atoms in atoms in 1st layer 2nd layer 3rd layer 4th layer no. of W atoms as 4 1 1 0 nearest neighbor no. of W atoms as 2nd 3 3 0 0 nearest neighbor a Pd atoms in the third layer are still in touch with substrate W atoms.
facets are within a certain orientational range. This peculiar property may have certain physical implications. For example, we know that W has a lower surface energy if covered by Pd. But will the total energy along the transition path be so low as to make such a transition path more favorable and the W surface always remain covered by Pd while the morphology is changing? Is the threshold coverage only needed for tipping the thermodynamic equilibrium or is it needed mainly for overcoming the kinetic barrier? More experiments are being planned to investigate these questions and to calibrate absolutely the threshold Pd coverage for faceting of W(111). (B) Surface Energy Anisotropy. As discussed prev i o ~ s l yfaceting ,~ of surfaces is driven by the anisotropy in surface energy y as a function of crystallographic orientation. In general, since the anisotropy in y is small for clean metals, clean surfaces of pure metals are thermally stable35and remain planar upon annealing, irrespective of their crystallographic orientations (e.g., for clean W, the maximum anisotropy, Ay/y, is ~ 3 % ~ ~ ) . The presence of adsorbed layers on metals can lower the surface energy y and can cause an increase in the anisotropy of y. When facets are formed, the total energy Sy dA (where A is the surface area) is lowered by the formation of low-energy facets even though the total surface area i n ~ r e a s e s . ~ ~ , ~ 8 (35)To our knowledge, Au may be an exception, see Darby, T. P.; Guan, D. Y.; Balluffi, R. W. Surf. Sci. 1978,72,357. (36)Drechsler, M.; Mueller, A. J. Cryst. Growth 1968,3/4,518. (37)Williams, E.D.; Bartelt, N. C. Ultramicroscopy 1989,31,36.
Whereas there have been many studies of faceting induced by adsorbed gases,35,39*40 facetingof metal surfaces induced by ultrathin metal films has been seen in only a few c a ~ e s . ~ -Recently, ~$~l Weinert et al.42performed total energy calculations for films of Pd and Ag on Nb( 100) and Nb(ll0) that provide insights into the faceting process. They find that a single monolayer of Pd or Ag lowers the surface energy y of both surfaces and also increases the 30%. We suggested5 that anisotropy of y to Ay/y similar processes occur for Pt and Au on W(111), which induce the formation of stable (211) surfaces. This suggestion is supported by a recent calculation performed by Chex1,4~who studied the total energy for films of Pd, Pt, and Au on W(lOO), -(llO), -(111)and -(112) surfaces by using an embedded atom method. Chen finds a lowering of the surface energy for all surfaces and a large increaseof the anisotropy when the W surfacesare covered by one monolayer of Pd, Pt, or Au films. Just any metal cannot similarly enhance the surface energy anisotropy of W, though. For example, our preliminary study of Ni on W(111) indicates no faceting of W.44 We note that Pt, Pd, and Au all have hard sphere diameters that closelymatch45 that of W, whereas Ni is much smaller. The W(111) unit cell has a linear dimension about 60% larger than those of the fcc (111)unit cells of Pt, Pd, or Au. The W(211) surface, however, consists of parallel close packed rows separated by 1.63d, where d is the hard sphere diameter. This is rather similar to the fcc (110) surface with parallel close-packed rows separated by 1.414d. Thus it may seem that for Pt, Pd, and Au, faceting to 12111 surfaces can improve the “epitaxy”, but for Ni, switching to the (211) surface does not help. The verificationof such speculations is certainly a worthwhile goal for further investigations using both experimental and theoretical approaches.
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V. Summary
For Pd adsorbed on W(111), there is a threshold coverage, above which the surface will form (211) facets upon annealing above 700 K. The bulklike thermal desorption of Pd also appears above this threshold coverage, which we have tentatively identifiedas one “physicalmonolayer’’. For even thicker Pd films, three-dimensional Pd clusters form on the faceted surface upon annealing. LEED pattern from well-defined crystalline surfaces of these Pd clusters can be seen if the Pd coverage is larger than two physical monolayers. Acknowledgment. The authors acknowledgevaluable discussions with Dr. S. P. Chen of Los Alamos National Laboratory and the technical assistance of Professor Eric Garfunkel. This work has been supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences. Registry No. Pd, 7440-05-3; W, 7440-33-7. (38)Flytzani-Stephanopoulos,M.; Schmidt,L. D.Prog. Surf. Sci. 1979, 9,83. (39)Tracy, J. C.; Blakely, J. M. Surf. Sci. 1968,13,313. (40)Taylor, N.J. Surf. Sci. 1964,2,544. (41)Cetronio, A.; Jones, J. P. Surf. Sci. 1973,40,227.Mroz, S.;Bauer, E.Surf. Sci. 1986,169,394. (42)Weinert, M.; Watson, R. E.; Davenport, J. W.; Fernando, G. W. Phys. Rev. B: Condens. Mutter 1989,B39,12585. (43)Chen, S. P. To be submitted for publication. (44)Dong, C.-Z.; Song, K.-J.; Madey, T. E. To be submitted for
publication. (45)Hard sphere diameters for W, Pt, Pd, Au, and Ni are 2.74,2.77, 2.75,2.88,and 2.49 A, respectively.
