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The stability of ultrathin metal films on W(110) and W(111)
175
Surface Science 247 (1991) 175-187 North-Holland
The stability of ultrathin metal films on W( 110) and W( 111) * Theodore E. Madey, Ker-Jar Song, Gheng-Zhi Dong Department of Physics and Astronomy nnd Laboratory for Surface Modification, Rutgers University, Piscutaway, NJ 088.50849,
USA
and Richard A. Demmin 1 National Institute of Standark and Technology, Caithersburg, MD 20899, USA Received 5 July 1990; accepted for publication 25 August 1990
Ultrathin films of platinum on tungsten display a rich variety of phenomena related to epitaxial growth. Pt grows on W(110) in a layer-by-layer mode at 300 K, upon annealing multilayers of Pt to T > 800 K, clusters of three-dimensional crystalhtes form on top of a pseudo-morphic Pt monolayer. When oxygen is present on the surface, the Pt monolayer is no longer stable. Annealing coadsorbed Pt and 0 on W(ll0) causes nearly all the Pt to agglomerate into clusters as the oxygen largely replaces the monolayer on the surface. Annealing Pt on W(lll) in the range 800 < T < 1500 K leads to a different behavior: microscopic facets of W having f211) orientation are formed. The faceting appears to be driven by a Pt enhanced anisotropy in the surface free energy. Results based on LEED, Auger, and scanning tunneling microscopy (STM) are presented.
1. Introduction In recent years, there has been widespread interest in the physics and chemistry of ultrat~n metal films on metals. The literature in this field is surveyed in several comprehensive reviews dealing with the kinetics of film growth, the structure, electronic properties and surface chemistry of ultrathin metal films [l-5]. The vast majority of the published papers are concerned with the adsorption of ultrathin metal films on atomically smooth close-packed substrates, e.g., bcc(llO), (100) and fcc(ll1). Relatively few papers deal with metal films on atomically rough substrates [6-81. The purpose of the present work is to determine the structure and reactivity of ultrathin films of Pt and other metals on single crystal * Invited review lecture presented by T.E. Madey.
I Present address: Exxon Research and Engineering Co., Linden, NJ, USA. 0039-6028/91/$03.50
surfaces of tungsten. We focus on a comparison between the atomically smooth W(110) surface and the atomically rough W(lll), and observe striking differences in the thermal stability of the metal-covered surfaces. Tungsten and platinum are chosen for this study because of their different chemisorption and catalytic activities, and their similar nearest-neighbor distances (within 2% in the respective bee and fee bulk forms). Measurements are made using several ultrahigh vacuum surface science methods (including low energy electron diffraction, LEED, Auger electron spectroscopy, AES, ultraviolet photoemission, UPS, and thermal desorption spectroscopy, TDS). In addition, we use scanning tunneling microscopy (STM) to provide a microscopic view of Pt induced surface structures with nanometer level resolutions. Whereas a monolayer of Pt on the close-packed W(110) surface is thermally stable to - 2000 K [9], we have found a surprising result for the
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
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T.E. Madey et at. / The stability of ultrathin metal films on W(I 10) and W(I 1I)
W(111) surface. When W(111) is covered by > 1 X 1015 Pt-atoms/cm and heated in the range 8001600 K, the surface undergoes a massive restructuring to form microscopic facets [8]. At 1200 K, the average facet dimensions are > 300 A, and the dominant facet orientation is W(211). Faceting of W(111) is also induced by a monolayer of adsorbed Au upon heating to - 800 K. The faceting appears to be driven by a Pt or Au enhanced anisotropy in the surface free energy. Faceting of W(111) and other metal surfaces covered with adsorbed gases is well known [lO,ll], but the influence of ultrathin metal films on faceting has not been widely recognized. This paper is organized as follows: The experimental procedures are described in section 2. A summary of the growth and stability of Pt on W(110) are presented in section 3.1, along with a discussion of the surface chemistry, reactivity (to CO and 0,), and electronic properties of Pt/W(llO). In section 3.2, the growth and stability of Pt and Au on W(111) are presented, together with the microscopy of thermally faceted Pt(Au)-covered W(111). The data are discussed in section 4.
2. Experimental procedures Most of the experiments presented here have been performed in an ultrahigh vacuum system with a base pressure of < 2 x lo-” Torr. This system is equipped with a double-pass cylindrical mirror analyzer and glancing-incidence electron gun for AES, four-grid LEED optics and electron gun, and quadrupole mass spectrometer. The samples studied are W single crystals oriented, cut, and polished to expose (110) or (111) faces. A sample is spot-welded to two Ta wires that are, in turn, mounted on MO posts for resistive heating to < 2000 K. Auxiliary heating to higher temperatures is accomplished by electron bombardment using a W filament. The crystal temperature is measured using a W-5%Re/W-26% Re thermocouple spot-welded to the back. Each W crystal is initially cleaned by repeated cycles of heating in oxygen followed by flashing to 2300 K until no contaminants were detected by AES. After the
initial removal of carbon from the bulk of the crystal, subsequent cleanings usually involve only flashing in vacuum to remove CO, 0, or Pt. Occasionally, the sample requires heating in oxygen to remove trace amounts of carbon. Pt (or Au) is deposited from evaporators consisting of resistively heated W wire to which Pt (Au) wire or sheet is wrapped or spot-welded. This W wire is spot-welded to W degassing loops that can be heated independently for outgassing. Pt film thicknesses are based on the deposition time for formation of a Pt monolayer on W(llO), as identified by “breaks” in the AES uptake curves and correlated with thermal desorption data for Pt/W [9]. Au film thicknesses are based on thermal desorption spectroscopy (see section 3.2). The UPS measurements are made in an ultrahigh vacuum chamber using synchrotron radiation at the National Institute of Standards and Technology SURF-II facility. The detector is a double-pass cylindrical mirror analyzer that is masked to detect normal emission (angle of incidence 45 o ). STM measurements of the faceted surface are made using a commercial Digital Instruments Nanoscope II operating in air. A freshly faceted Pt/W(lll) crystal is transferred from the UHV system to the STM, and studies are generally completed within a few hours of breaking vacuum. Good images are obtained over a wide range of tunneling parameters. For most of the STM data shown in this paper, the tunneling current is 0.1 nA and the bias voltage on the Pt/Ir tip is + 150 mV. Control STM experiments on annealed, nonPt-dosed W(111) surfaces show no evidence of facets.
3. Results 3.1. Pt on W(II0) 3.1.1. Growth and stability of films LEED and Auger data for Pt deposited onto W(110) at 300 K indicate that Pt grows in a layer-by-layer fashion, at least for the first few monolayers. A plot of Auger amplitude for Pt as a function of dose time displays distinct breaks in
T. E. Madey et al. / The stability of ultrathin metal films on W(I 10) and W(I 11)
can be due to processes such as evaporation from the surface, agglomeration of the Pt atoms into 3D islands, or interdiffusion at the interface. Thermal desorption data [l] show clearly that there is no Pt desorption below - 1750 K, so Pt evaporation cannot explain the decrease in Pt AES signal upon heating. The bulk phase diagram for Pt/W indicates a low solubility (- 4%) for Pt in W [13], so extensive interface mixing appears unlikely. Also, the stability of a single monolayer seen in fig. 1 (i.e., the relatively small decrease in Pt AES signal for 1.2 ML Pt between 300 and 1800 K) indicates that Pt remains at the surface over a wide temperature range. These results suggest another explanation, i.e., the Pt in excess of 1 ML forms clusters whose size is 2 the electron attenuation length for 64 eV electrons (5-10 A), giving apparent loss of Pt from the surface upon heating the 4 ML. This is also consistent with thermal desorption data, for which a multilayer TDS peak is seen for Pt coverages in excess of 1 ML. Both scanning Auger microscopy (SAM) [14]
slope, indicating completion of monolayers [9]. In contrast, for deposition at 100 K, there are no such breaks in slope. The LEED pattern remains (1 x 1) during Pt deposition at 300 K, consistent with pseudomorphic growth of the Pt layers on W(110); pseudomorphic growth has been reported for Pt on W(100) as well [12]. The thermal stability of the Pt/W system is studied by annealing at different temperatures for 30 s. A plot of the ratio of the Pt (64 eV) and W (169 eV) peaks as a function of annealing temperature, for 1.2 and 4 ML initial coverages of Pt, is shown in fig. 1 [9]. The dashed line is the expected behavior assuming free sublimation of 4 ML bulk Pt, based on vapor pressure data. The significant discrepancy between the dashed line and the experimental curve for 4 ML Pt can be attributed to the fact that Pt overlayers on W(110) are metastable and undergo significant structural changes on annealing at temperatures above 600 K. Above 1500 K the curve flattens, indicating the increased stability of one monolayer. The irreversible rearrangements for Pt coverages greater than 1 ML
1.2
177
ML Pt
/ 500
I
.,.,I,,
1000 1500 Annealing Temperature (K)
Fig. 1. Thermal stability of Pt/W(llO) shown by the variation of the Auger peak-to-peak temperature for 4 and 1.2 ML of Pt on W (the broken line shows the expected behavior pressure
of bulk Pt) [9].
2000 ratio (Pt(64 eV)/W(169 for a 4 ML Pt-coverage,
eV)) with annealing based on the vapor
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and scanning tunneling microscopy (STM) [15] have been applied to this study and provide direct evidence for the formation of Pt clusters on W( 110) after annealing. In these experiments, W(110) surfaces were dosed under UHV conditions with - 10 ML Pt at 300 K, and then heated to 1600 K for 1 min. The samples were removed from the vacuum system and transported to the SAM and STM systems. Both microscopic investigations provide0 direct evidence that clusters of Pt form (d - 100 A), and that steps of atomic dimensions on the W(110) substrate are decorated preferentially. We have observed many images where there are large, planar regions (the stable Pt monolayer on W(110)) and other regions where clusters predominate frequently along surface steps. LEED studies of the annealed surface demonstrate that the Pt clusters are actually 3D crystallites having (111) surfaces in epitaxial registry with W(110). For Pt deposited at 300 K and annealed to 1700 K, the clusters assume the NishiyamaWasserman orientation [l]. For Pt deposited onto a heated surface (1100 K) before annealing to 1700 K, the clusters have the Kurdjumov-Sachs orientation [ 11. In summary, annealing several monolayers of Pt on W(110) causes all but the first layer to agglomerate into 3D clusters on the surface of the tungsten. The first layer remains dispersed in a single atomic layer with the crystal structure of the underlying tungsten and requires a higher temperature for evaporation than does the excess platinum. 3.1.2. Surface chemistry and reactivity The effects of the interaction between platinum and tungsten is manifested in the surface chemistry of the platinum-covered surface in the presence of reactive gases such as carbon monoxide and oxygen [16,17]. Temperature-programmed desorption experiments show that molecular CO is more weakly bound to a monolayer Pt film deposited at 90 K than to either bulk Pt or the W substrate, similar to conclusions drawn from experiments on other metal thin films [18-201. Carbon monoxide is also weakly adsorbed on films annealed to 1500 K, even for initial Pt coverages much greater than one monolayer. This is ad-
ditional evidence for substantial thermally induced structural changes in multilayer films that result in a W surface that is covered by a monolayer Pt film with unique CO chemisorption properties. Platinum films of at least one monolayer also prevent the dissociative adsorption of CO normally occurring on the W(110) surface. For submonolayer films annealed to 1500 K, the total amount of dissociative adsorption of CO decreases linearly with increasing Pt coverage, reaching zero at one monolayer of Pt. This implies that the inhibition of CO dissociation by Pt is a localized, site-blocking process. LEED and AES have also been used to characterize Pt overlayers coadsorbed with oxygen on W(110) [17]. The pseudomorphic monolayer of Pt that is stable to high temperatures on clean W(110) is no longer stable in the presence of oxygen. Irrespective of which is dosed first, Pt coadsorbed with oxygen appears to agglomerate into 3D clusters at temperatures above - 500-1000 K. The evidence for this is shown in fig. 2, in which the AES signals for coadsorbed Pt and oxygen are monitored as a function of temperature for 30 s annealings at successively higher temperatures. The open symbols represent the annealing experiment performed on a surface predosed with 1.4 ML Pt, annealed at 1600 K to form a single Pt layer (with a small fraction of the surface covered by Pt crystallites) and then dosed with 100 L 0, at 300 K prior to further annealing. The closed symbols represent measurements performed on a surface first dosed with 0, and annealed to form an ordered LEED structure. The dashed line in fig. 2a indicates the level of 1 ML where the Pt/W Auger peak height ratio remains in the absence of oxygen until desorption at > 1800 K. Adsorbed oxygen has a profound effect on the stability of the Pt monolayer: the Pt Auger peak height drops dramatically below the level for 1 ML as the temperature is raised from 500 to 1500 K. It is clear that Pt is not present as a monolayer film on the W, in contrast to the case without oxygen. Above 1500 K, the Pt AES peak height rises rapidly back to the level for 1 ML Pt before dropping again due to desorption of the Pt. The changes in the oxygen Auger peak height during this procedure are recorded in fig. 2b, in
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500
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I
1500 1000 Annealing Temperature (K)
.
I.,
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I
”
2000 ”
I
,
E 40 .o, I” % 30a” 2 a5 20(5)
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Annealing Temperature
(K)
Fig. 2. Auger electron spectroscopy (AES) signals of Pt and oxygen as a function of annealing temperature for coadsorbed Pt + O/W(llO): (a) Pt/W AES peak height ratio versus temperature, and (b) oxygen AES peak height versus temperature. Coverages indicates Pt dosed first, Pt/O/W indicates oxygen dosed first, O/W refers to oxygen adsorption alone are all - 1 ML; O/Pt/W 1171.
which the oxygen level is observed to remain relatively constant until - 1500 K, where it begins a rapid decline and becomes undetectable above 1800 K. Because the oxygen remains on the surface up to this high temperature, it can be concluded that it is bound to the W substrate, and not Pt, after annealing. If the oxygen were bound to the Pt clusters, the expected desorption temperature would be - 800 K [21]. The rapid decline in the Pt peak height with
annealing is due to the oxygen displacing it on the W surface. However, instead of desorbing, as is usually the case when one adsorbate is displaced by another, the strong interatomic attraction of the Pt atoms causes them to aggregate into clusters of Pt metal that remain on the surface. Oxygen begins to desorb from the surface at 1500 K and the Pt clusters spread out once again to form a single layer on the W(110) surface, the normal configuration of Pt/W(llO) in the absence of
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Adsorbed
Oxygen
Pt Monolayer
J
1
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of ultrathin
1500 K
Oxygen
and Pt
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Pt Cluster
metal
films
on W(I IO) and W(1 I I)
emission intensity 1-2 eV below the Fermi level is greatly reduced from clean W(110) or bulk Pt, suggesting an electronic structure that is noblemetal-like. These results are similar to UPS results for Pt and Pd films on Ta and Nb substrates 119,241. The noble-metal-like electronic structure of the Pt monolayer seems to correlate qualitatively with the weakened CO binding energy observed for this surface. 3.2. Pt on W(IlI)
1
1800 K Pt Monolayer
/ J
///////////////
7’
W(110) Substrate
&z
Fig. 3. Schematic diagram of morphology of O/Pt/W annealing sequence of fig. 2 [17].
during
oxygen. A schematic depiction of the morphological changes during annealing is shown in fig. 3. Similar results have been observed for other conditions of 0 and Pt coadsorption, although there are slight variations in the temperatures and extent of changes [17]. Oxygen-induced clustering of Pd on W(100) has been reported by Gomer et al.
1221. The mechanism for oxygen-induced clustering can be described as follows. A monolayer of oxygen lowers the surface energy of W(llO), and the surface energy of Pt is greater than that of O/W(llO). The total energy of the Pt/O/W(llO) system is lowered when clusters of Pt coexist with large regions of O/W(llO). The preferential desorption of oxygen from Pt/O/W(llO) at 17001800 K is explained by a thermodynamic argument (171. 3.1.3. Electronic properties of Pt / W(ilO) Synchrotron photoemission studies for 0 to 6 monolayer thick films of Pt on W(110) have been performed using 50 eV photons [23]. A single monolayer of Pt yields a valence band spectrum that is unlike those of subsequent layers, i.e., the
3.2. I. Growth und stability of films: formation of facets The behavior of Pt on W(ll1) is very different from that on W(110): The Pt-covered W(lll) surface is unstable upon heating, and reconstructs to form facets. Evidence for facet formation is based on a combination of LEED and STM studies. When Pt is deposited onto W(111) at 300 K, no distinct new features are seen in LEED; the background increases and the substrate spots become less sharp. Measurements of the Pt AES signal as a function of Pt dose indicate that Pt film growth is not layer-by-layer. The AES uptake curve is smooth, with no signs of breaks in slope, indicating that adsorption occurs in a random fashion without the completion of well-defined monolayers. The LEED pattern for Pt/W(lll) remains (1 x 1) until the surface is heated to T > 800 K. The LEED patterns observed after annealing above 800 K are a function of both Pt coverage and annealing conditions (time, temperature). A typical sequence of LEED patterns (E = 31 eV) as a function of Pt coverage at constant temperature is shown in fig. 4. In each case, the Pt is dosed at 300 K onto clean W(111); the sample is then annealed at 1200 K for 3 min before cooling to 300 K to observe the pattern. With increasing Pt coverage, the following sequence is observed: (1 X 1) - fig. 4a; (3 X 3) - fig. 4b; (1 X 1) - not shown; facets + (1 X 1) - fig. 4~; completely faceted - fig. 4d. The basis for identifying the faceted surface is as follows: The (1 x 1) and (3 x 3) patterns are due to long-range order on a planar Pt/W(lll) surface: as the LEED beam energy increases, the diffrac-
T.E. Madey et al. / The stability of ultrathin rneta~~~~
on W/110) and Flit)
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Fig. 4. Sequence of LEED patterns for different coverages of Pt/W(lll) after annealing to 1200 K. For patterns (a) to (d), the electron energy E = 31 eV, the W(111) specular (0,O) beam is at - 9 o’clock, and Pt coverages range from 0.50, to > 0,. For pattern (e) the W(111) specular (0, 0) beam is in the center, 0 > iI,, and the electron energy is varied from 80 to 190 eV in a time exposure demonstrating the motion of beams due to faceting. The white arrow indicates a specular beam from a W(211) facet. See text for details.
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of ultrathin metalfilms on W(I 10) and W(I I I)
tion beams move inward, converging on the specular (0, 0) beam for planar W(lll). In contrast, a characteristic of LEED from a faceted surface is the fact that diffraction beams appear to converge on the specular (0, 0) beams for the facets, in directions away from the macroscopic surface normal, as the electron kinetic energy increases. The electron energy is varied from 80 to 190 eV in the time exposure of fig. 4e, corresponding to LEED for a fully faceted surface. Most of the streaks due to the motion of LEED beams do not converge on the center, but move in various directions away from the center. The beams converge on (211) poles; one of the three symmetrically disposed (211) poles is indicated by the arrow. This beam is identified as the specular reflection from a (211)
facet based on a Laue diffraction pattern (to identify the azimuth) and by measurement of the polar angle 28, where 8 2: 18.5 k lo. The (211) surface is inclined at 19“ from the (111). There are a number of conclusions that can be drawn from the sequence in fig. 4 and similar data. First, there is a critical coverage 13,necessary to form the completely faceted surface. For coverages much less than 0,, the Pt atoms may remain in W lattice sites and only a (1 X 1) pattern is observed following annealing at all temperatures. - 0.5 e,, the (3 x 3) ordered For Pt coverages structure is observed upon annealing. As the Pt coverage increases further, the (3 x 3) disappears and a (1 x 1) reappears. For higher coverages but still slightly below SC, a mixture of facets and
annealed for 3 minutes at each temperature
\
Annealing
Temperature(K)
Fig. 5. Thermal stability of Pt/W(lll) showing the Auger peak height ratio (Pt(64 eV)/W(169 eV)) as a function of annealing temperature for different initial coverages of Pt/W(lll). 6’, is the coverage required for complete faceting upon annealing to Ts 1100 K.
T E. Madey et al. / The stability of ultrathin metal filmr on W(I 10) and W(I 11)
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planar (1 x 1) is seen (800 < T < 1200 K), while for coverages 2 0,, the surface becomes completely faceted (800 < T c 1200 K). Even the completely faceted surface undergoes a phase separation into faceted and planar regions above 1200 K. The LEED spots from the faceted surfaces are sharp and well-defined. This indicates that typical facet dimensions are 2 the LEED transfer width, which is - 100 A [25]. Based on a calibration procedure described in [8], we estimate the critical coverage 8, to be - 1 x lOI5 Pt atoms/cm2. This can be compared with the coverage of the topmost atom layer on W(lll), 5.7 X lOI4 atoms/cm’, and the surface atom coverage on W(211) is 8.1 x 1014 atoms/cm’. For the completely faceted surface, the surface area increases by 6% over the area of planar W(111). An important issue in this study concerns the thermal stability of the Pt film as the facets form. Plots of the Pt/W Auger intensity ratio as a function of annealing temperature are given in fig. 5, and provide insights. For initial Pt coverages > 6, at 300 K, the Pt/W Auger intensity ratio decreases slightly upon heating to - 1600 K; rapid desorption occurs as the temperature approaches 2000 K. The near-constancy of the Auger ratio in the range 300-1600 K suggests that the majority of Pt remains at the surface in this range. There may be some 2D alloy formation, but the majority of Pt does not prefer subsurface sites. For initial Pt coverages > f3, at 300 K, the Pt/W Auger intensity ratio drops with increasing temperature, reaching a limiting value at 1400-1700 K. This behavior is very similar to that shown in fig. 1 for Pt/W(llO), where the Pt in excess of 1 ML is known to form 3D clusters. On W(lll) a similar process may occur, but further investigation is required.
Fig. 6. Series of scanning tunneling micrographs (top-view) for Pt/W(lll), after annealing to different temperatures. For all micrographs, the field of view is 350 nm X 350 nm; the vertical gray-scales are different. Annealing temperatures: top: 880 K; middle: 1200 K; bottom: 1400 K.
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3.2.2. Microscopy of facets for Pt/ W(1 II) Direct evidence for the formation of facets on annealed, Pt-dosed W(111) is provided using the scanning tunneling microscope (STM), as demonstrated in the series of STM micrographs in fig. 6. For each experiment, the clean W(lll) surface was dosed at 300 K with a Pt coverage of 1.5 to 2 times f?,, and annealed under ultrahigh vacuum conditions to the indicated temperature. After removal from the vacuum system, it was transported to the STM for characterization in air. Three images are shown in figs. 6a to 6c, corresponding to Pt/W(lll) surfaces annealed to 880, 1200 and 1400 K, respectively. The images are all top-views showing the triangular shapes of the pyramidal facets; the gray (vertical) scale is related to height. All lateral scales are the same, but vertical scales are different. All scans of different regions of the surfaces show similar results. However, the backside of the W(lll) crystal that is not Pt-dosed remains planar, and facets do not develop. Fig. 6 shows clearly that size and density of pyramidal facets depend very much on annealing temperature. The linear dimensions of the facets increase with annealing temperature, from - 10 nm at 880 K to - 100 nm at 1400 K, and correspondingly, the density of pyramids decreases with at temperature, from - 4 x 10” pyramids/cm2 at 1400 K. 880 K to - 6 X lo9 pyramids/cm’ Several of the faceted surfaces have been examined also with a high resolution scanning electron microscopy (SEM) having a field emission source (beam diameter approximately a few nanometers). The large pyramidal facets formed after the 1400 K anneal are resolved, but there is not sufficient contrast to allow the smaller facets (1200 K) to be resolved. 3.2.3. Gold on W(Il1) Metal-induced faceting of W(lll) is not limited to Pt; it occurs also for another metal, Au, which has a much lower melting temperature and surface energy than Pt. Au does not grow in a layer-by-layer fashion on W(lll); rather, Au forms a stable monolayer upon which Au clusters form at higher coverages, even at 300 K (Stranski-Krastanov growth). Upon heating to > 800 K, for Au coverages greater than
3
600I
d
.i
400-
i
x
Temperature Fig.
(K)
7. Temperature-programmed desorption spectra Au/W(lll) for a wide range of initial coverages.
for
- 1 ML, facets having (211) orientation are observed using LEED. The facets develop over a rather narrow temperature range, 800 to 950 K. Above 950 K, the surface reverts to planar W(lll)-(3 x 3), but the facets reappear (reversibly) when the sample is again annealed in the range 800 to 950 K. A series of Au thermal desorption spectra for Au/W(lll) is shown in fig. 7. Thermal desorption data have been reported also for Au/W(llO) [26] and Au/Ru(OOl) [27]. The peak at - 1420 K is due to the first Au monolayer, and the peak at - 1200 K is due to multiple layers of Au. The critical coverage of Au needed to induce faceting of W(lll) corresponds to saturation of the first monolayer. 4. Discussion 4.1. Pt on W(i10)
and W(1II)
The most striking aspect of this work is the contrast between the thermal stability of Pt on
T.E. Madey et al. / The ~tab;l;fy of ufrraihin me~ai~i~
W(110) and W(lll). The close-packed W(110) surface is a relatively rigid template in its interactions with Pt. The W(110) substrate is thermodynamically stable and remains essentially planar at all coverages of Pt, up to the desorption temperature of Pt. A single Pt monolayer forms a continuous, pseudomo~hic film on W(ll0) up to high temperatures. Pt coverages in excess of one monolayer are sufficiently mobile above 700-800 K that Pt clusters form. The clusters grow as microcrystallites having Pt(ll1) planes in epitaxy with the W(110) substrate. In contrast, the planar W(ll1) surface is not thermodynamically stable when covered with Pt > 1 x lOI and annealed. For Pt coverages atoms/cm2 and after annealing in the range 8001600 K, the W(lll) surface undergoes a massive reconstruction to form arrays of three-sided pyramidal facets having sides of W(211) planes. The linear dimensions of the bases of the faceted pyramids increase with temperature in the range 880-1400 K, from 2 10 nm at 800 K to - 100 nm at 1400 K. The facets disappear at higher temperatures, and are completely absent when the Pt desorbs from the surface. In view of the macroscopic roughness of the surfaces evident in the STM micrographs (fig. 6), it is surprising to note how little W must be “moved” in order to transform from a planar W(lll) surface to the faceted surface. Diffusion of as few as (2-3) x 1015 Watoms/cm2 can produce the pyramidal facets. Note that the pyramids are relatively shallow: the angle between the W(lll) and W(211) planes is 19”. In these studies, the growth of pyramidal Ptcovered facets on the W(111) substrate is not an equilibrium process. (On the other hand, the Au/W system may be in equilibrium during facet growth; see below.) We have not observed a reversibility of morphology for Pt/W, that is, we do not see the same size of facets appear when we approach a given temperature from both above and below in the range 800-14~ K: even for B > S,, after heating to > 1400 K, the completely faceted surface does not reappear when heating at T < 1200 K. This absence of equilibrium suggests that we are observing the growth form of the microcrystals rather than the equilibrium form. During the growth of the surface microcrystals
on W~Ii~) and W(I I l)
185
the morphology is dictated by kinetic processes (primarily diffusion of the present case). The shape during growth is related to the polar plot of the growth rate at low supersaturation [28,29]; the equilibrium crystal shape (ECS) is determined by the polar plot of the surface free energy (the y-plot) [ll]. The growth shape will generally have sharper edges and fewer exposed facets than the equilibrium crystal shape. Because W(211) facets appear in the growth form for Pt/W, we can be sure that (211) is a stable facet in the ECS. However, we cannot be certain whether or not other facets, not appearing in the growth form, may be present in the ECS. As an example, the stable ciose-packed Pt-covered W(110) surface is almost certainly present in the ECS. Perhaps at equilibrium, this surface also appears on the facets grown on W(lll). Even though true equilibrium is not present for Pt/W(lll), we can gain insights into the faceting process by considering equilibrium concepts, i.e., the surface energies of Pt and Pt/W, and the polar plot of surface energy (the y-plot). For clean W, the ECS contains small facets of (llO), (100) and (112) planes. The anisotropy in surface energy (y - 3500 erg/cm* for W) is small, with a maximum value of - 3% derived from studies of the equilibrium shape of a field ion emitter [30]. The (111) region is disordered in thermally annealed field ion images, but LEED studies confirm that macroscopic W(lll) is a planar surface. The faceting of Pt-covered W( 111) appears to be driven by several energetic considerations, viz., the average surface energy of the Pt-covered W surface is lower than for clean W, and the anisotropy in surface energy for different crystallographic directions is greater for Pt/W than for clean W. The fact that a Pt monolayer “wets” the W substrate is consistent with the lower surface energy. The increase in anisotropy of surface energy insures that a nearby densely-packed low-energy surface (W(211) at 19” or W(110) at 35” with respect to W(lll)) will appear in the ECS in the 11111 direction. Although calculations of the influence of Pt on the surface energy of W are not available, Weinert et al. [31] have performed total energy calculations of the energetics of Pd film growth on Nb(100)
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and Nb(ll0). Their results demonstrate that: (a) the surface energy y of Pd is lower than that of Nb; (b) the anisotropy in the surface energy of clean Pd and Nb is relatively small, < 10% (comparing (100) and (110) planes); and (c) the surface energy for a monolayer of Pd/Nb(llO) is about 30% less than the surface energy for Pd/Nb(lOO), that is, there is an increase in anisotropy of surface energy for monolayer Pd films on Nb. Based on the above, we suggest that for an ultrathin film of Pt (8 >, 1 x lOI atoms/cm’) on W, the surface energy y of Pt/W(211) is sufficiently less than y for Pt/W(lll) that the surface energy /y d A is minimized by the growth of (211) facets at the expense of the planar (111) surface. The surface energy of Pt/W(llO) is sufficiently low that faceting does not occur on this surface.
4.2. Au on W(III)
Our initial studies of faceting induced by Au on W(111) suggest a behavior somewhat different from the Pt/W(lll) system. The growth and disappearance of (211) oriented facets is reversible with temperature in the range 800-950 K, for > 1 ML of Au. Up to now, our STM studies of Au/W(lll) confirm the occurrence of microfacets, and further studies are underway.
5. Conclusion
The observation of Pt- and Au-induced faceting of W(111) may have far-reaching implications for surface structural effects in many bimetallic systems, particularly for atomically rough surfaces with (relatively) high surface energy. Until the present, most studies of metals-on-metals have been for atomically close-packed substrates (bcc(llO), fcc(lll), fcc(100)) that do not appear to form facets. However, atomically rough surfaces, or small particles (e.g., catalysts) with high defect concentration may well undergo structural rearrangements to new morphologies that are different from the uncoated substrates.
Acknowledgements The authors acknowledge valuable discussions with Eric Garfunkel. One of the authors (TEM) acknowledges valuable discussions with D. Nenow. This work was supported in part by the US Department of Energy, Division of Basic Energy Sciences.
References 111 E. Bauer, Appl. Surf. Sci. 11/12 (1982) 479. Physics of Solid Surfaces and 121 E. Bauer, The Chemical Heterogeneous Catalysis, Vol. 3B, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1984) p. 1. 131 C. Argile and G.E. Rhead, Surf. Sci. Rep. 10 (1989) 277. and J.E. Houston, Science 236 (1987) [41 D.W. Goodman 404. J. 151 J.-W. He, W.-L. Shea, X. Jiang and D.W. Goodman, Vat. Sci. Technol. A 8 (1990) 2435 (61 A. Cetronio and J.P. Jones, Surf. Sci. 40 (1973) 227. [71 S. Mrbz and E. Bauer, Surf. Sci. 169 (1986) 394. and 181 K.-J. Song, R.A. Demmin, C.-Z. Dong, E. Garfunkel T.E. Madey, Surf. Sci. Lett. 227 (1990) L79. R.A. Demmin and T.E. Madey, Thin [91 S.M. Shivaprasad, Solid Films 163 (1988) 393. WI J.C. Tracy and J.M. Blakely, Surf. Sci. 13 (1968) 313; H. Niehus, Surf. Sci. 87 (1979) 561. and L.D. Schmidt, Prog. Surf. 1111 M. Flytzani-Stephanopoulos Sci. 9 (1979) 83; J.M. Blakely and M. Eizenberg, The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 1, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1981) p. 1. WI R.W. Judd, M.A. Reichelt, E.G. Scott and R.M. Lambert, Surf. Sci. 185 (1987) 529. of Binary Phase Diagrams I131 W.G. Moffatt, The Handbook (Genium, Schenectady, NY, 1986). 1141 N.H. Turner and R.A. Demmin, unpublished. and T.E. Madey, I151 K.-J. Song, R.A. Demmin, E. Garfunkel unpublished. and T.E. Madey, Lang1161 R.A. Demmin, S.M. Shivaprasad muir 4 (1988) 1104. u71 R.A. Demmin and T.E. Madey, J. Vat. Sci. Technol. A 7 (1989) 1954. WI D. Prigge, W. Schlenk, E. Bauer, Surf. Sci. 123 (1982) L698. P91 X. Pan, M.W. Ruckman and M. Strongin, Phys. Rev. B 35 (1987) 3734. Langmuir 4 (1988) 1201 P.J. Berlowitz and D.W. Goodman, 1091. WI J.L. Gland, B.A. Sexton and G.B. Fisher, Surf. Sci. 95 (1980) 587. P21 N. Shamir and R. Gomer, Surf. Sci. 246 (1989) 49.
T. E. Madey et al. / The stability of ultrathin metal films on W(I IO) and W(I I I) [23] R.A. Denunin, R.L. Kurtz, R.L. Stockbauer, T.E. Madey, D.R. Mueller and A. Shih, in press. [24] M.W. Ruckman and M. Strongin, Phys. Rev. B 29 (1984) 7105. (251 D.P. Woodruff and T.A. Delchar, Modem Techniques of Surface Science (Cambridge University Press, Cambridge, 1986). (261 J. Kolaczkiewicz and E. Bauer, Surf. Sci. 175 (1986) 508.
187
[27] C. Harendt, K. Christmann and W. Hirschwald, Surf. Sci. 165 (1986) 413. [28] D. Nenow and A. Trayanov, Surf. Sci. 213 (1989) 488. [29] D. Nenow, Prog. Cryst. Growth Charact. 9 (1984) 185. [30] M. Drechsler and A. Mtiller, J. Cryst. Growth 3/4 (1968) 518. [31] M. Weinert, R.E. Watson, J.W. Davenport and G.W. Fernando, Phys. Rev. B 39 (1989) 12585.
Surface Science 247 (1991) 175-187 North-Holland
The stability of ultrathin metal films on W( 110) and W( 111) * Theodore E. Madey, Ker-Jar Song, Gheng-Zhi Dong Department of Physics and Astronomy nnd Laboratory for Surface Modification, Rutgers University, Piscutaway, NJ 088.50849,
USA
and Richard A. Demmin 1 National Institute of Standark and Technology, Caithersburg, MD 20899, USA Received 5 July 1990; accepted for publication 25 August 1990
Ultrathin films of platinum on tungsten display a rich variety of phenomena related to epitaxial growth. Pt grows on W(110) in a layer-by-layer mode at 300 K, upon annealing multilayers of Pt to T > 800 K, clusters of three-dimensional crystalhtes form on top of a pseudo-morphic Pt monolayer. When oxygen is present on the surface, the Pt monolayer is no longer stable. Annealing coadsorbed Pt and 0 on W(ll0) causes nearly all the Pt to agglomerate into clusters as the oxygen largely replaces the monolayer on the surface. Annealing Pt on W(lll) in the range 800 < T < 1500 K leads to a different behavior: microscopic facets of W having f211) orientation are formed. The faceting appears to be driven by a Pt enhanced anisotropy in the surface free energy. Results based on LEED, Auger, and scanning tunneling microscopy (STM) are presented.
1. Introduction In recent years, there has been widespread interest in the physics and chemistry of ultrat~n metal films on metals. The literature in this field is surveyed in several comprehensive reviews dealing with the kinetics of film growth, the structure, electronic properties and surface chemistry of ultrathin metal films [l-5]. The vast majority of the published papers are concerned with the adsorption of ultrathin metal films on atomically smooth close-packed substrates, e.g., bcc(llO), (100) and fcc(ll1). Relatively few papers deal with metal films on atomically rough substrates [6-81. The purpose of the present work is to determine the structure and reactivity of ultrathin films of Pt and other metals on single crystal * Invited review lecture presented by T.E. Madey.
I Present address: Exxon Research and Engineering Co., Linden, NJ, USA. 0039-6028/91/$03.50
surfaces of tungsten. We focus on a comparison between the atomically smooth W(110) surface and the atomically rough W(lll), and observe striking differences in the thermal stability of the metal-covered surfaces. Tungsten and platinum are chosen for this study because of their different chemisorption and catalytic activities, and their similar nearest-neighbor distances (within 2% in the respective bee and fee bulk forms). Measurements are made using several ultrahigh vacuum surface science methods (including low energy electron diffraction, LEED, Auger electron spectroscopy, AES, ultraviolet photoemission, UPS, and thermal desorption spectroscopy, TDS). In addition, we use scanning tunneling microscopy (STM) to provide a microscopic view of Pt induced surface structures with nanometer level resolutions. Whereas a monolayer of Pt on the close-packed W(110) surface is thermally stable to - 2000 K [9], we have found a surprising result for the
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
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W(111) surface. When W(111) is covered by > 1 X 1015 Pt-atoms/cm and heated in the range 8001600 K, the surface undergoes a massive restructuring to form microscopic facets [8]. At 1200 K, the average facet dimensions are > 300 A, and the dominant facet orientation is W(211). Faceting of W(111) is also induced by a monolayer of adsorbed Au upon heating to - 800 K. The faceting appears to be driven by a Pt or Au enhanced anisotropy in the surface free energy. Faceting of W(111) and other metal surfaces covered with adsorbed gases is well known [lO,ll], but the influence of ultrathin metal films on faceting has not been widely recognized. This paper is organized as follows: The experimental procedures are described in section 2. A summary of the growth and stability of Pt on W(110) are presented in section 3.1, along with a discussion of the surface chemistry, reactivity (to CO and 0,), and electronic properties of Pt/W(llO). In section 3.2, the growth and stability of Pt and Au on W(111) are presented, together with the microscopy of thermally faceted Pt(Au)-covered W(111). The data are discussed in section 4.
2. Experimental procedures Most of the experiments presented here have been performed in an ultrahigh vacuum system with a base pressure of < 2 x lo-” Torr. This system is equipped with a double-pass cylindrical mirror analyzer and glancing-incidence electron gun for AES, four-grid LEED optics and electron gun, and quadrupole mass spectrometer. The samples studied are W single crystals oriented, cut, and polished to expose (110) or (111) faces. A sample is spot-welded to two Ta wires that are, in turn, mounted on MO posts for resistive heating to < 2000 K. Auxiliary heating to higher temperatures is accomplished by electron bombardment using a W filament. The crystal temperature is measured using a W-5%Re/W-26% Re thermocouple spot-welded to the back. Each W crystal is initially cleaned by repeated cycles of heating in oxygen followed by flashing to 2300 K until no contaminants were detected by AES. After the
initial removal of carbon from the bulk of the crystal, subsequent cleanings usually involve only flashing in vacuum to remove CO, 0, or Pt. Occasionally, the sample requires heating in oxygen to remove trace amounts of carbon. Pt (or Au) is deposited from evaporators consisting of resistively heated W wire to which Pt (Au) wire or sheet is wrapped or spot-welded. This W wire is spot-welded to W degassing loops that can be heated independently for outgassing. Pt film thicknesses are based on the deposition time for formation of a Pt monolayer on W(llO), as identified by “breaks” in the AES uptake curves and correlated with thermal desorption data for Pt/W [9]. Au film thicknesses are based on thermal desorption spectroscopy (see section 3.2). The UPS measurements are made in an ultrahigh vacuum chamber using synchrotron radiation at the National Institute of Standards and Technology SURF-II facility. The detector is a double-pass cylindrical mirror analyzer that is masked to detect normal emission (angle of incidence 45 o ). STM measurements of the faceted surface are made using a commercial Digital Instruments Nanoscope II operating in air. A freshly faceted Pt/W(lll) crystal is transferred from the UHV system to the STM, and studies are generally completed within a few hours of breaking vacuum. Good images are obtained over a wide range of tunneling parameters. For most of the STM data shown in this paper, the tunneling current is 0.1 nA and the bias voltage on the Pt/Ir tip is + 150 mV. Control STM experiments on annealed, nonPt-dosed W(111) surfaces show no evidence of facets.
3. Results 3.1. Pt on W(II0) 3.1.1. Growth and stability of films LEED and Auger data for Pt deposited onto W(110) at 300 K indicate that Pt grows in a layer-by-layer fashion, at least for the first few monolayers. A plot of Auger amplitude for Pt as a function of dose time displays distinct breaks in
T. E. Madey et al. / The stability of ultrathin metal films on W(I 10) and W(I 11)
can be due to processes such as evaporation from the surface, agglomeration of the Pt atoms into 3D islands, or interdiffusion at the interface. Thermal desorption data [l] show clearly that there is no Pt desorption below - 1750 K, so Pt evaporation cannot explain the decrease in Pt AES signal upon heating. The bulk phase diagram for Pt/W indicates a low solubility (- 4%) for Pt in W [13], so extensive interface mixing appears unlikely. Also, the stability of a single monolayer seen in fig. 1 (i.e., the relatively small decrease in Pt AES signal for 1.2 ML Pt between 300 and 1800 K) indicates that Pt remains at the surface over a wide temperature range. These results suggest another explanation, i.e., the Pt in excess of 1 ML forms clusters whose size is 2 the electron attenuation length for 64 eV electrons (5-10 A), giving apparent loss of Pt from the surface upon heating the 4 ML. This is also consistent with thermal desorption data, for which a multilayer TDS peak is seen for Pt coverages in excess of 1 ML. Both scanning Auger microscopy (SAM) [14]
slope, indicating completion of monolayers [9]. In contrast, for deposition at 100 K, there are no such breaks in slope. The LEED pattern remains (1 x 1) during Pt deposition at 300 K, consistent with pseudomorphic growth of the Pt layers on W(110); pseudomorphic growth has been reported for Pt on W(100) as well [12]. The thermal stability of the Pt/W system is studied by annealing at different temperatures for 30 s. A plot of the ratio of the Pt (64 eV) and W (169 eV) peaks as a function of annealing temperature, for 1.2 and 4 ML initial coverages of Pt, is shown in fig. 1 [9]. The dashed line is the expected behavior assuming free sublimation of 4 ML bulk Pt, based on vapor pressure data. The significant discrepancy between the dashed line and the experimental curve for 4 ML Pt can be attributed to the fact that Pt overlayers on W(110) are metastable and undergo significant structural changes on annealing at temperatures above 600 K. Above 1500 K the curve flattens, indicating the increased stability of one monolayer. The irreversible rearrangements for Pt coverages greater than 1 ML
1.2
177
ML Pt
/ 500
I
.,.,I,,
1000 1500 Annealing Temperature (K)
Fig. 1. Thermal stability of Pt/W(llO) shown by the variation of the Auger peak-to-peak temperature for 4 and 1.2 ML of Pt on W (the broken line shows the expected behavior pressure
of bulk Pt) [9].
2000 ratio (Pt(64 eV)/W(169 for a 4 ML Pt-coverage,
eV)) with annealing based on the vapor
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and scanning tunneling microscopy (STM) [15] have been applied to this study and provide direct evidence for the formation of Pt clusters on W( 110) after annealing. In these experiments, W(110) surfaces were dosed under UHV conditions with - 10 ML Pt at 300 K, and then heated to 1600 K for 1 min. The samples were removed from the vacuum system and transported to the SAM and STM systems. Both microscopic investigations provide0 direct evidence that clusters of Pt form (d - 100 A), and that steps of atomic dimensions on the W(110) substrate are decorated preferentially. We have observed many images where there are large, planar regions (the stable Pt monolayer on W(110)) and other regions where clusters predominate frequently along surface steps. LEED studies of the annealed surface demonstrate that the Pt clusters are actually 3D crystallites having (111) surfaces in epitaxial registry with W(110). For Pt deposited at 300 K and annealed to 1700 K, the clusters assume the NishiyamaWasserman orientation [l]. For Pt deposited onto a heated surface (1100 K) before annealing to 1700 K, the clusters have the Kurdjumov-Sachs orientation [ 11. In summary, annealing several monolayers of Pt on W(110) causes all but the first layer to agglomerate into 3D clusters on the surface of the tungsten. The first layer remains dispersed in a single atomic layer with the crystal structure of the underlying tungsten and requires a higher temperature for evaporation than does the excess platinum. 3.1.2. Surface chemistry and reactivity The effects of the interaction between platinum and tungsten is manifested in the surface chemistry of the platinum-covered surface in the presence of reactive gases such as carbon monoxide and oxygen [16,17]. Temperature-programmed desorption experiments show that molecular CO is more weakly bound to a monolayer Pt film deposited at 90 K than to either bulk Pt or the W substrate, similar to conclusions drawn from experiments on other metal thin films [18-201. Carbon monoxide is also weakly adsorbed on films annealed to 1500 K, even for initial Pt coverages much greater than one monolayer. This is ad-
ditional evidence for substantial thermally induced structural changes in multilayer films that result in a W surface that is covered by a monolayer Pt film with unique CO chemisorption properties. Platinum films of at least one monolayer also prevent the dissociative adsorption of CO normally occurring on the W(110) surface. For submonolayer films annealed to 1500 K, the total amount of dissociative adsorption of CO decreases linearly with increasing Pt coverage, reaching zero at one monolayer of Pt. This implies that the inhibition of CO dissociation by Pt is a localized, site-blocking process. LEED and AES have also been used to characterize Pt overlayers coadsorbed with oxygen on W(110) [17]. The pseudomorphic monolayer of Pt that is stable to high temperatures on clean W(110) is no longer stable in the presence of oxygen. Irrespective of which is dosed first, Pt coadsorbed with oxygen appears to agglomerate into 3D clusters at temperatures above - 500-1000 K. The evidence for this is shown in fig. 2, in which the AES signals for coadsorbed Pt and oxygen are monitored as a function of temperature for 30 s annealings at successively higher temperatures. The open symbols represent the annealing experiment performed on a surface predosed with 1.4 ML Pt, annealed at 1600 K to form a single Pt layer (with a small fraction of the surface covered by Pt crystallites) and then dosed with 100 L 0, at 300 K prior to further annealing. The closed symbols represent measurements performed on a surface first dosed with 0, and annealed to form an ordered LEED structure. The dashed line in fig. 2a indicates the level of 1 ML where the Pt/W Auger peak height ratio remains in the absence of oxygen until desorption at > 1800 K. Adsorbed oxygen has a profound effect on the stability of the Pt monolayer: the Pt Auger peak height drops dramatically below the level for 1 ML as the temperature is raised from 500 to 1500 K. It is clear that Pt is not present as a monolayer film on the W, in contrast to the case without oxygen. Above 1500 K, the Pt AES peak height rises rapidly back to the level for 1 ML Pt before dropping again due to desorption of the Pt. The changes in the oxygen Auger peak height during this procedure are recorded in fig. 2b, in
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500
r..
I
1500 1000 Annealing Temperature (K)
.
I.,
”
I
”
2000 ”
I
,
E 40 .o, I” % 30a” 2 a5 20(5)
PVOnV
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r
Annealing Temperature
(K)
Fig. 2. Auger electron spectroscopy (AES) signals of Pt and oxygen as a function of annealing temperature for coadsorbed Pt + O/W(llO): (a) Pt/W AES peak height ratio versus temperature, and (b) oxygen AES peak height versus temperature. Coverages indicates Pt dosed first, Pt/O/W indicates oxygen dosed first, O/W refers to oxygen adsorption alone are all - 1 ML; O/Pt/W 1171.
which the oxygen level is observed to remain relatively constant until - 1500 K, where it begins a rapid decline and becomes undetectable above 1800 K. Because the oxygen remains on the surface up to this high temperature, it can be concluded that it is bound to the W substrate, and not Pt, after annealing. If the oxygen were bound to the Pt clusters, the expected desorption temperature would be - 800 K [21]. The rapid decline in the Pt peak height with
annealing is due to the oxygen displacing it on the W surface. However, instead of desorbing, as is usually the case when one adsorbate is displaced by another, the strong interatomic attraction of the Pt atoms causes them to aggregate into clusters of Pt metal that remain on the surface. Oxygen begins to desorb from the surface at 1500 K and the Pt clusters spread out once again to form a single layer on the W(110) surface, the normal configuration of Pt/W(llO) in the absence of
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Adsorbed
Oxygen
Pt Monolayer
J
1
1
of ultrathin
1500 K
Oxygen
and Pt
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emission intensity 1-2 eV below the Fermi level is greatly reduced from clean W(110) or bulk Pt, suggesting an electronic structure that is noblemetal-like. These results are similar to UPS results for Pt and Pd films on Ta and Nb substrates 119,241. The noble-metal-like electronic structure of the Pt monolayer seems to correlate qualitatively with the weakened CO binding energy observed for this surface. 3.2. Pt on W(IlI)
1
1800 K Pt Monolayer
/ J
///////////////
7’
W(110) Substrate
&z
Fig. 3. Schematic diagram of morphology of O/Pt/W annealing sequence of fig. 2 [17].
during
oxygen. A schematic depiction of the morphological changes during annealing is shown in fig. 3. Similar results have been observed for other conditions of 0 and Pt coadsorption, although there are slight variations in the temperatures and extent of changes [17]. Oxygen-induced clustering of Pd on W(100) has been reported by Gomer et al.
1221. The mechanism for oxygen-induced clustering can be described as follows. A monolayer of oxygen lowers the surface energy of W(llO), and the surface energy of Pt is greater than that of O/W(llO). The total energy of the Pt/O/W(llO) system is lowered when clusters of Pt coexist with large regions of O/W(llO). The preferential desorption of oxygen from Pt/O/W(llO) at 17001800 K is explained by a thermodynamic argument (171. 3.1.3. Electronic properties of Pt / W(ilO) Synchrotron photoemission studies for 0 to 6 monolayer thick films of Pt on W(110) have been performed using 50 eV photons [23]. A single monolayer of Pt yields a valence band spectrum that is unlike those of subsequent layers, i.e., the
3.2. I. Growth und stability of films: formation of facets The behavior of Pt on W(ll1) is very different from that on W(110): The Pt-covered W(lll) surface is unstable upon heating, and reconstructs to form facets. Evidence for facet formation is based on a combination of LEED and STM studies. When Pt is deposited onto W(111) at 300 K, no distinct new features are seen in LEED; the background increases and the substrate spots become less sharp. Measurements of the Pt AES signal as a function of Pt dose indicate that Pt film growth is not layer-by-layer. The AES uptake curve is smooth, with no signs of breaks in slope, indicating that adsorption occurs in a random fashion without the completion of well-defined monolayers. The LEED pattern for Pt/W(lll) remains (1 x 1) until the surface is heated to T > 800 K. The LEED patterns observed after annealing above 800 K are a function of both Pt coverage and annealing conditions (time, temperature). A typical sequence of LEED patterns (E = 31 eV) as a function of Pt coverage at constant temperature is shown in fig. 4. In each case, the Pt is dosed at 300 K onto clean W(111); the sample is then annealed at 1200 K for 3 min before cooling to 300 K to observe the pattern. With increasing Pt coverage, the following sequence is observed: (1 X 1) - fig. 4a; (3 X 3) - fig. 4b; (1 X 1) - not shown; facets + (1 X 1) - fig. 4~; completely faceted - fig. 4d. The basis for identifying the faceted surface is as follows: The (1 x 1) and (3 x 3) patterns are due to long-range order on a planar Pt/W(lll) surface: as the LEED beam energy increases, the diffrac-
T.E. Madey et al. / The stability of ultrathin rneta~~~~
on W/110) and Flit)
181
Fig. 4. Sequence of LEED patterns for different coverages of Pt/W(lll) after annealing to 1200 K. For patterns (a) to (d), the electron energy E = 31 eV, the W(111) specular (0,O) beam is at - 9 o’clock, and Pt coverages range from 0.50, to > 0,. For pattern (e) the W(111) specular (0, 0) beam is in the center, 0 > iI,, and the electron energy is varied from 80 to 190 eV in a time exposure demonstrating the motion of beams due to faceting. The white arrow indicates a specular beam from a W(211) facet. See text for details.
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of ultrathin metalfilms on W(I 10) and W(I I I)
tion beams move inward, converging on the specular (0, 0) beam for planar W(lll). In contrast, a characteristic of LEED from a faceted surface is the fact that diffraction beams appear to converge on the specular (0, 0) beams for the facets, in directions away from the macroscopic surface normal, as the electron kinetic energy increases. The electron energy is varied from 80 to 190 eV in the time exposure of fig. 4e, corresponding to LEED for a fully faceted surface. Most of the streaks due to the motion of LEED beams do not converge on the center, but move in various directions away from the center. The beams converge on (211) poles; one of the three symmetrically disposed (211) poles is indicated by the arrow. This beam is identified as the specular reflection from a (211)
facet based on a Laue diffraction pattern (to identify the azimuth) and by measurement of the polar angle 28, where 8 2: 18.5 k lo. The (211) surface is inclined at 19“ from the (111). There are a number of conclusions that can be drawn from the sequence in fig. 4 and similar data. First, there is a critical coverage 13,necessary to form the completely faceted surface. For coverages much less than 0,, the Pt atoms may remain in W lattice sites and only a (1 X 1) pattern is observed following annealing at all temperatures. - 0.5 e,, the (3 x 3) ordered For Pt coverages structure is observed upon annealing. As the Pt coverage increases further, the (3 x 3) disappears and a (1 x 1) reappears. For higher coverages but still slightly below SC, a mixture of facets and
annealed for 3 minutes at each temperature
\
Annealing
Temperature(K)
Fig. 5. Thermal stability of Pt/W(lll) showing the Auger peak height ratio (Pt(64 eV)/W(169 eV)) as a function of annealing temperature for different initial coverages of Pt/W(lll). 6’, is the coverage required for complete faceting upon annealing to Ts 1100 K.
T E. Madey et al. / The stability of ultrathin metal filmr on W(I 10) and W(I 11)
183
planar (1 x 1) is seen (800 < T < 1200 K), while for coverages 2 0,, the surface becomes completely faceted (800 < T c 1200 K). Even the completely faceted surface undergoes a phase separation into faceted and planar regions above 1200 K. The LEED spots from the faceted surfaces are sharp and well-defined. This indicates that typical facet dimensions are 2 the LEED transfer width, which is - 100 A [25]. Based on a calibration procedure described in [8], we estimate the critical coverage 8, to be - 1 x lOI5 Pt atoms/cm2. This can be compared with the coverage of the topmost atom layer on W(lll), 5.7 X lOI4 atoms/cm’, and the surface atom coverage on W(211) is 8.1 x 1014 atoms/cm’. For the completely faceted surface, the surface area increases by 6% over the area of planar W(111). An important issue in this study concerns the thermal stability of the Pt film as the facets form. Plots of the Pt/W Auger intensity ratio as a function of annealing temperature are given in fig. 5, and provide insights. For initial Pt coverages > 6, at 300 K, the Pt/W Auger intensity ratio decreases slightly upon heating to - 1600 K; rapid desorption occurs as the temperature approaches 2000 K. The near-constancy of the Auger ratio in the range 300-1600 K suggests that the majority of Pt remains at the surface in this range. There may be some 2D alloy formation, but the majority of Pt does not prefer subsurface sites. For initial Pt coverages > f3, at 300 K, the Pt/W Auger intensity ratio drops with increasing temperature, reaching a limiting value at 1400-1700 K. This behavior is very similar to that shown in fig. 1 for Pt/W(llO), where the Pt in excess of 1 ML is known to form 3D clusters. On W(lll) a similar process may occur, but further investigation is required.
Fig. 6. Series of scanning tunneling micrographs (top-view) for Pt/W(lll), after annealing to different temperatures. For all micrographs, the field of view is 350 nm X 350 nm; the vertical gray-scales are different. Annealing temperatures: top: 880 K; middle: 1200 K; bottom: 1400 K.
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3.2.2. Microscopy of facets for Pt/ W(1 II) Direct evidence for the formation of facets on annealed, Pt-dosed W(111) is provided using the scanning tunneling microscope (STM), as demonstrated in the series of STM micrographs in fig. 6. For each experiment, the clean W(lll) surface was dosed at 300 K with a Pt coverage of 1.5 to 2 times f?,, and annealed under ultrahigh vacuum conditions to the indicated temperature. After removal from the vacuum system, it was transported to the STM for characterization in air. Three images are shown in figs. 6a to 6c, corresponding to Pt/W(lll) surfaces annealed to 880, 1200 and 1400 K, respectively. The images are all top-views showing the triangular shapes of the pyramidal facets; the gray (vertical) scale is related to height. All lateral scales are the same, but vertical scales are different. All scans of different regions of the surfaces show similar results. However, the backside of the W(lll) crystal that is not Pt-dosed remains planar, and facets do not develop. Fig. 6 shows clearly that size and density of pyramidal facets depend very much on annealing temperature. The linear dimensions of the facets increase with annealing temperature, from - 10 nm at 880 K to - 100 nm at 1400 K, and correspondingly, the density of pyramids decreases with at temperature, from - 4 x 10” pyramids/cm2 at 1400 K. 880 K to - 6 X lo9 pyramids/cm’ Several of the faceted surfaces have been examined also with a high resolution scanning electron microscopy (SEM) having a field emission source (beam diameter approximately a few nanometers). The large pyramidal facets formed after the 1400 K anneal are resolved, but there is not sufficient contrast to allow the smaller facets (1200 K) to be resolved. 3.2.3. Gold on W(Il1) Metal-induced faceting of W(lll) is not limited to Pt; it occurs also for another metal, Au, which has a much lower melting temperature and surface energy than Pt. Au does not grow in a layer-by-layer fashion on W(lll); rather, Au forms a stable monolayer upon which Au clusters form at higher coverages, even at 300 K (Stranski-Krastanov growth). Upon heating to > 800 K, for Au coverages greater than
3
600I
d
.i
400-
i
x
Temperature Fig.
(K)
7. Temperature-programmed desorption spectra Au/W(lll) for a wide range of initial coverages.
for
- 1 ML, facets having (211) orientation are observed using LEED. The facets develop over a rather narrow temperature range, 800 to 950 K. Above 950 K, the surface reverts to planar W(lll)-(3 x 3), but the facets reappear (reversibly) when the sample is again annealed in the range 800 to 950 K. A series of Au thermal desorption spectra for Au/W(lll) is shown in fig. 7. Thermal desorption data have been reported also for Au/W(llO) [26] and Au/Ru(OOl) [27]. The peak at - 1420 K is due to the first Au monolayer, and the peak at - 1200 K is due to multiple layers of Au. The critical coverage of Au needed to induce faceting of W(lll) corresponds to saturation of the first monolayer. 4. Discussion 4.1. Pt on W(i10)
and W(1II)
The most striking aspect of this work is the contrast between the thermal stability of Pt on
T.E. Madey et al. / The ~tab;l;fy of ufrraihin me~ai~i~
W(110) and W(lll). The close-packed W(110) surface is a relatively rigid template in its interactions with Pt. The W(110) substrate is thermodynamically stable and remains essentially planar at all coverages of Pt, up to the desorption temperature of Pt. A single Pt monolayer forms a continuous, pseudomo~hic film on W(ll0) up to high temperatures. Pt coverages in excess of one monolayer are sufficiently mobile above 700-800 K that Pt clusters form. The clusters grow as microcrystallites having Pt(ll1) planes in epitaxy with the W(110) substrate. In contrast, the planar W(ll1) surface is not thermodynamically stable when covered with Pt > 1 x lOI and annealed. For Pt coverages atoms/cm2 and after annealing in the range 8001600 K, the W(lll) surface undergoes a massive reconstruction to form arrays of three-sided pyramidal facets having sides of W(211) planes. The linear dimensions of the bases of the faceted pyramids increase with temperature in the range 880-1400 K, from 2 10 nm at 800 K to - 100 nm at 1400 K. The facets disappear at higher temperatures, and are completely absent when the Pt desorbs from the surface. In view of the macroscopic roughness of the surfaces evident in the STM micrographs (fig. 6), it is surprising to note how little W must be “moved” in order to transform from a planar W(lll) surface to the faceted surface. Diffusion of as few as (2-3) x 1015 Watoms/cm2 can produce the pyramidal facets. Note that the pyramids are relatively shallow: the angle between the W(lll) and W(211) planes is 19”. In these studies, the growth of pyramidal Ptcovered facets on the W(111) substrate is not an equilibrium process. (On the other hand, the Au/W system may be in equilibrium during facet growth; see below.) We have not observed a reversibility of morphology for Pt/W, that is, we do not see the same size of facets appear when we approach a given temperature from both above and below in the range 800-14~ K: even for B > S,, after heating to > 1400 K, the completely faceted surface does not reappear when heating at T < 1200 K. This absence of equilibrium suggests that we are observing the growth form of the microcrystals rather than the equilibrium form. During the growth of the surface microcrystals
on W~Ii~) and W(I I l)
185
the morphology is dictated by kinetic processes (primarily diffusion of the present case). The shape during growth is related to the polar plot of the growth rate at low supersaturation [28,29]; the equilibrium crystal shape (ECS) is determined by the polar plot of the surface free energy (the y-plot) [ll]. The growth shape will generally have sharper edges and fewer exposed facets than the equilibrium crystal shape. Because W(211) facets appear in the growth form for Pt/W, we can be sure that (211) is a stable facet in the ECS. However, we cannot be certain whether or not other facets, not appearing in the growth form, may be present in the ECS. As an example, the stable ciose-packed Pt-covered W(110) surface is almost certainly present in the ECS. Perhaps at equilibrium, this surface also appears on the facets grown on W(lll). Even though true equilibrium is not present for Pt/W(lll), we can gain insights into the faceting process by considering equilibrium concepts, i.e., the surface energies of Pt and Pt/W, and the polar plot of surface energy (the y-plot). For clean W, the ECS contains small facets of (llO), (100) and (112) planes. The anisotropy in surface energy (y - 3500 erg/cm* for W) is small, with a maximum value of - 3% derived from studies of the equilibrium shape of a field ion emitter [30]. The (111) region is disordered in thermally annealed field ion images, but LEED studies confirm that macroscopic W(lll) is a planar surface. The faceting of Pt-covered W( 111) appears to be driven by several energetic considerations, viz., the average surface energy of the Pt-covered W surface is lower than for clean W, and the anisotropy in surface energy for different crystallographic directions is greater for Pt/W than for clean W. The fact that a Pt monolayer “wets” the W substrate is consistent with the lower surface energy. The increase in anisotropy of surface energy insures that a nearby densely-packed low-energy surface (W(211) at 19” or W(110) at 35” with respect to W(lll)) will appear in the ECS in the 11111 direction. Although calculations of the influence of Pt on the surface energy of W are not available, Weinert et al. [31] have performed total energy calculations of the energetics of Pd film growth on Nb(100)
186
T. E. Madey et al. / The stability of ultrathin metal films on W(I IO) and W(1 I I)
and Nb(ll0). Their results demonstrate that: (a) the surface energy y of Pd is lower than that of Nb; (b) the anisotropy in the surface energy of clean Pd and Nb is relatively small, < 10% (comparing (100) and (110) planes); and (c) the surface energy for a monolayer of Pd/Nb(llO) is about 30% less than the surface energy for Pd/Nb(lOO), that is, there is an increase in anisotropy of surface energy for monolayer Pd films on Nb. Based on the above, we suggest that for an ultrathin film of Pt (8 >, 1 x lOI atoms/cm’) on W, the surface energy y of Pt/W(211) is sufficiently less than y for Pt/W(lll) that the surface energy /y d A is minimized by the growth of (211) facets at the expense of the planar (111) surface. The surface energy of Pt/W(llO) is sufficiently low that faceting does not occur on this surface.
4.2. Au on W(III)
Our initial studies of faceting induced by Au on W(111) suggest a behavior somewhat different from the Pt/W(lll) system. The growth and disappearance of (211) oriented facets is reversible with temperature in the range 800-950 K, for > 1 ML of Au. Up to now, our STM studies of Au/W(lll) confirm the occurrence of microfacets, and further studies are underway.
5. Conclusion
The observation of Pt- and Au-induced faceting of W(111) may have far-reaching implications for surface structural effects in many bimetallic systems, particularly for atomically rough surfaces with (relatively) high surface energy. Until the present, most studies of metals-on-metals have been for atomically close-packed substrates (bcc(llO), fcc(lll), fcc(100)) that do not appear to form facets. However, atomically rough surfaces, or small particles (e.g., catalysts) with high defect concentration may well undergo structural rearrangements to new morphologies that are different from the uncoated substrates.
Acknowledgements The authors acknowledge valuable discussions with Eric Garfunkel. One of the authors (TEM) acknowledges valuable discussions with D. Nenow. This work was supported in part by the US Department of Energy, Division of Basic Energy Sciences.
References 111 E. Bauer, Appl. Surf. Sci. 11/12 (1982) 479. Physics of Solid Surfaces and 121 E. Bauer, The Chemical Heterogeneous Catalysis, Vol. 3B, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1984) p. 1. 131 C. Argile and G.E. Rhead, Surf. Sci. Rep. 10 (1989) 277. and J.E. Houston, Science 236 (1987) [41 D.W. Goodman 404. J. 151 J.-W. He, W.-L. Shea, X. Jiang and D.W. Goodman, Vat. Sci. Technol. A 8 (1990) 2435 (61 A. Cetronio and J.P. Jones, Surf. Sci. 40 (1973) 227. [71 S. Mrbz and E. Bauer, Surf. Sci. 169 (1986) 394. and 181 K.-J. Song, R.A. Demmin, C.-Z. Dong, E. Garfunkel T.E. Madey, Surf. Sci. Lett. 227 (1990) L79. R.A. Demmin and T.E. Madey, Thin [91 S.M. Shivaprasad, Solid Films 163 (1988) 393. WI J.C. Tracy and J.M. Blakely, Surf. Sci. 13 (1968) 313; H. Niehus, Surf. Sci. 87 (1979) 561. and L.D. Schmidt, Prog. Surf. 1111 M. Flytzani-Stephanopoulos Sci. 9 (1979) 83; J.M. Blakely and M. Eizenberg, The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 1, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1981) p. 1. WI R.W. Judd, M.A. Reichelt, E.G. Scott and R.M. Lambert, Surf. Sci. 185 (1987) 529. of Binary Phase Diagrams I131 W.G. Moffatt, The Handbook (Genium, Schenectady, NY, 1986). 1141 N.H. Turner and R.A. Demmin, unpublished. and T.E. Madey, I151 K.-J. Song, R.A. Demmin, E. Garfunkel unpublished. and T.E. Madey, Lang1161 R.A. Demmin, S.M. Shivaprasad muir 4 (1988) 1104. u71 R.A. Demmin and T.E. Madey, J. Vat. Sci. Technol. A 7 (1989) 1954. WI D. Prigge, W. Schlenk, E. Bauer, Surf. Sci. 123 (1982) L698. P91 X. Pan, M.W. Ruckman and M. Strongin, Phys. Rev. B 35 (1987) 3734. Langmuir 4 (1988) 1201 P.J. Berlowitz and D.W. Goodman, 1091. WI J.L. Gland, B.A. Sexton and G.B. Fisher, Surf. Sci. 95 (1980) 587. P21 N. Shamir and R. Gomer, Surf. Sci. 246 (1989) 49.
T. E. Madey et al. / The stability of ultrathin metal films on W(I IO) and W(I I I) [23] R.A. Denunin, R.L. Kurtz, R.L. Stockbauer, T.E. Madey, D.R. Mueller and A. Shih, in press. [24] M.W. Ruckman and M. Strongin, Phys. Rev. B 29 (1984) 7105. (251 D.P. Woodruff and T.A. Delchar, Modem Techniques of Surface Science (Cambridge University Press, Cambridge, 1986). (261 J. Kolaczkiewicz and E. Bauer, Surf. Sci. 175 (1986) 508.
187
[27] C. Harendt, K. Christmann and W. Hirschwald, Surf. Sci. 165 (1986) 413. [28] D. Nenow and A. Trayanov, Surf. Sci. 213 (1989) 488. [29] D. Nenow, Prog. Cryst. Growth Charact. 9 (1984) 185. [30] M. Drechsler and A. Mtiller, J. Cryst. Growth 3/4 (1968) 518. [31] M. Weinert, R.E. Watson, J.W. Davenport and G.W. Fernando, Phys. Rev. B 39 (1989) 12585.