The Pt(110) phase transitions: A study by rutherford backscattering,nuclear microanalysis,LEED and thermal desorption spectroscopy

The adsorbate induced (1×2) (1×1) (2×1)p1g1 phase transitions on Pt(110) have been studied by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED and thermal desorption spectroscopy. RBS data indicate that any displacement of the surface atoms from their expected bulk-like lattice sites in the (1×2) phase is ? 0.002 nm laterally and ? 0.007 nm vertically. This contraint eliminates models for the reconstruction which involve significant lateral displacements (e.g., the paired-atom or hexagonal overlayer models). The RBS data are consistent with both the rumpled model with up/down displacements not exceeding ~0.007 nm and the missing row model with an unrelaxed surface in which the out-of-plane vibrational amplitude is slightly enhanced. A c(8×4) phase, produced by CO (or NO) exposure at T?250 K, has also been characterized by RBS which demonstrated that 0.92×10¹⁵ Pt cm^?2 move on average by ~0.017 nm laterally out-of-registry with the bulk upon formation of this phase. The values of the saturation adsorbate coverages at T?200K were determined by NMA to be 0.92 ± 0.05×10¹⁵, 1.0 ± 0.06×10¹⁵ and 1.07 ± 0.10×10¹⁵ CO molecules, NO molecules and D atoms, respectively, per cm². The value of the saturation coverage by CO (θ = 1.0) supports recent models of the (2×1)p1g1 overlayer. The isosteric heat of adsorption of CO is 160 ± 15 kJ mol^?1 in the range 0.2?θ?0.5.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

The adsorption of CO,O2, and H2 on Pt(100)-(5×20)

The adsorption/desorption characteristics of CO, O₂, and H₂ on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol^?1. These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O₂ fluxes up to coverages of $\text{2}\text{3}$ of a monolayer with a net sticking probability to ssaturation of ? 10^?3. Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O₂, and H₂, the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.     

Oxygen on Ni(110): Surface phases and related absolute coverages

The adsorption of oxygen on Ni(110) was investigated by nuclear reaction analysis (NRA), XPS, Δφ, temperature programmed reaction spectroscopy (TPRS) and LEED. At 423 K, (3 × 1), (2 ×1) and (3 ×1) phases are formed in sequence with increasing O₂ exposure. The coverage in the (2 ×1) phase was determined by NRA, the coverages in the other phases being determined via this calibration by XPS, TPRS and Δφ. Contrary to previous reports, the maximum in the intensity of half-order beams from the (2 × 1) phase is associated with a coverage of (5.6 ± 0.5) × 10¹⁴ O atoms cm⁻² or 0.49 ± 0.05 monolayers, and not 0.25 monolayers. The two (3 ×1) phases are associated with θ = 0.33 ± 0.03 and 0.64 ± 0.06 monolayers respectively. Oxygen adsorbed at 295 K is not at thermodynamic equilibrium. Annealing to T > 400 K causes significant decreases in Δφ and the formation of the (2 ×1) phase for θ > 0.3.     

Stability and surface properties of GeSi alloy films on Si(111) substrate

Several GeSi alloy films with different surface properties were prepared from a 500 Å thick Ge film that had previously been grown on a Si(111)-7×7 substrate by molecular beam epitaxy. The films were prepared by combinations of sputtering, annealing and Ge deposition from an evaporator. The surface properties were studied by Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). A novel LEED system employing position-sensitive detection was used. The Ge film surface gave a superposition of 7×7 and c(2×8) LEED patterns. A 7×7 → 1×1 phase transition was observed at 425±10°C. An irreversible 7×7 → c(2×8) transition was observed when the sample was heated above 500°C. The Ge film melted at 750±30°C and formed a Ge_xSi_1−x (x = 0.85±0.05) alloy whose surface gave a 7×7 LEED pattern. A 7×7 → 1×1 phase transition was observed at 600±0.15°C. Prolonged sputtering and annealing resulted in a Ge_xSi_1−x (x = 0.53±0.05) alloy whose surface gave a 5×5 LEED pattern. An apparent 5×5 → 1×1 phase transition was observed at 870±10°C but at that temperature the film was converted irreversibly to one with a much lower Ge atom fraction (x = 0.025±0.005) whose surface gave a 7×7 LEED pattern. A surface with a 5×5 pattern identical to that for the x = 0.53 alloy was prepared by deposition or Ge on Si. A similar 5×5 surface was prepared by deposition of Ge on a facetted GeSi alloy surface originally showing a superposition of 5×5 and 7×7 patterns. The intensity distributions in all of the 7×7 LEED pattern were found to be similar to those for Si(111)-7×7 at nearly the same electron energies. The characteristics of the 7×7 → 1×1 phase transitions were discussed in direct comparison with those of the Si(111)7×7 → 1×1 and Ge(111)-c(2×8) → 1×1 transitions observed with the same LEED system.     

The co-induced 1 × 2 ↔ 1 × 1 phase transition of Pt(110) studied by leed and work function measurements

The adsorption of CO on the reconstructed 1 × 2-Pt(110) surface causes a lifting of the reconstruction which has been studied by LEED and work function measurements. The work function initially decreases until at γ = 0.2 the lifting of the reconstruction begins. A comparison with Pt(100) and Pt(111) shows that the similar behaviour of CO-induced work function changes on all three low-index planes of Pt appears to be related with a similar binding geometry of CO adsorbed on top of a (quasi-)hexagonal configuration of Pt atoms. No hysteresis is observed in the adsorption/desorption equilibrium of CO on Pt(110). Although the energetics of the CO-induced phase transition on Pt(110) appear to be analogous to those of the phase transition 1 × 1 ⇹ hex of Pt(100), a number of differences exist between the two surfaces which can be explained by the different structural properties of the various surface phases involved.     

The surface reconstructions of the (100) crystal faces of iridium,platinum and gold: I. Experimental observations and possible structural models

The structures of the reconstructed Ir(100), Pt(100) and Au(100) surfaces have been investigated. Low energy electron diffraction (LEED) patterns are analyzed and LEED intensity versus energy data are measured. A variety of structures is observed by LEED: Ir(100) exhibits a relatively simple (1 × 5) pattern; Pt(100) shows a series of closely related patterns, a typical representative of which has a $\text{(}\text{14}\text{1}\text{?1}\text{5}\text{)}$ structure; Au(100) usually exhibits a c(26 × 68) pattern, often inaccurately described in the literature as a (20 × 5) pattern. The reconstruction of Au(111) is also considered for comparison. Various plausible structural models are discussed, while laser simulation is used to lessen the number of these models. The analysis is completed in a companion paper where LEED intensity calculations are reported to determine the atomic locations.     

Absolute coverage measurements of deuterium on Ni (110)

The absolute coverage of deuterium adsorbed on Ni(110) at temperatures below 170 K to the formation of a (1 × 2) LEED pattern has been determined by nuclear microanalysis (NMA). The result, θ_D = 0.96 ± 0.08, is consistent with a saturation coverage of one full monolayer. Heating the crystal above ～ 190 K is shown to result in a gradual loss of deuterium from the system, accompanied by streaking of the LEED pattern, with complete desorption above ～ 340 K. The low-temperature (2 × 1)-D phase was found to correspond to θ_D = 0.64 ± 0.05 monolayers. The results are expected to be valid also for the equivalent phases obtained by hydrogen adsorption.     

The adsorption of Xe and CO on Au(100)

The adsorption of Xe and CO on Au(100) has been studied by LEED, Auger electron spectroscopy, electron energy loss spectroscopy (EELS) and surface potential measurements. The physical adsorption of xenon showed successive stages preceding the completion of a monolayer. The heat of adsorption was 22 (±2) kJ mol^?1 and the maximum surface potential was 0.45 V. Carbon monoxide gave a surface potential of 0.85 V at the highest coverage reached. The heat of adsorption showed a continuous fall from an initial value of 58 (±3) kJ mol^?1 as the coverage increased. Ordered adsorption structures were not observed in LEED for either Xe or CO. The EEL spectrum of clean Au(100) agreed well with spectra of polycrystalline gold. New loss features observed with adsorbed Xe and CO are discussed.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

Dynamical behaviour of PtRh(100) alloy surface during dissociative adsorption of NO and reaction of NO with H2

When a clean Pt-Rh(100) alloy surface was exposed to NO at T > 440 K, the LEED pattern changed sequentially as p(1 × 1) → c(2 × 2) → c(2 × 2) + p(3 × 1) → p(3 × 1), where the c(2 × 2) pattern appeared immediately after the exposure to NO. In contrast to this, the appearance time for the p(3 × 1) depends strongly on the initial Rh concentration on the surface adjusted by annealing. When the p(3 × 1) surface was exposed to H₂ by mixing H₂ into NO gas, the AES intensity of O(a) decreased and of N(a) increased markedly and the LEED pattern changed from p(3 × 1) to c(2 × 2). These results suggest that N(a) has equal affinity to Pt and Rh atoms so that the N(a) does not distinguish the Pt and Rh sites on the alloy surface. On the other hand, O(a) makes a stronger bond with Rh atoms so that Rh atom segregation onto the surface is induced. By reacting randomly distributed Rh atoms on the Pt-Rh(100) surface with oxygen, a surface compound in a p(3 × 1) arrangement is built on the surface.     

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the $\text{(}\text{14}\text{1}\text{1}\text{5}\text{)}$ reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).     

Author index volumes 111–120

Adsorption of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment μ = (0.18 ± 0.02) × 10^?30C m and polarizability α = (6.3 ± 1.3) × 10^?40J V^?2m². Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2 × 2), p(2 × 2) and (1 × 1) structures. Bismuth is thought to behave in a similar way, but, unlike lead, forms a second monolayer which replicates the first. Electron spectroscopy shows that sub-monolayer bismuth removes the surface state (Swanson) peak and at monolayer coverage a new peak emerges and shifts with increasing coverage. Using Gadzuk's theory, this peak is tentatively attributed to the ²P level in bismuth adatoms which form a p(2 × 2) structure in the first and second monolayers. Its shift with coverage is ascribed to an increase in the local surface field. There remains the difficulty of reconciling the proposed occupation of the ²P level with the observed small positive charge on the bismuth adatom.     

Iodine adsorption on a platinum stepped surface: Pt(s)[6(111) × (111)]

I₂ adsorption on Pt(s)[6(111) × (111)] surfaces under vacuum and atmospheric pressure conditions was studied by LEED, AES and thermal desorption. In contrast to smooth Pt(111), the surface structures were composed of multiple phase domains having (3 × 3) or ($\text{3}\text{×}\text{3}$)R30° local geometry and structural coincidence of the adjacent terraces. No special stability or instability of iodine adsorption at steps was observed.     

The adsorption of Xe and CO on a Cu (311) single crystal surface

LEED, electron energy loss spectroscopy and surface potential measurements have been used to study the adsorption of Xe and CO on Cu (311). Xe is adsorbed with a heat of 19 ± 2 kJ mol^/t-1. The complete monolayer has a surface potential of 0.58 V and a hexagonal close-packed structure with an interatomic distance of 4.45 ± 0.05 Å. CO gives a positive surface potential increasing with coverage to a maximum of 0.34 V and then falling to 0.22 V at saturation. The heat of adsorption is initially 61 ± 2 kJ mol^?1, falling as the surface potential maximum is approached to about 45 kJ mol^?1. At this coverage streaks appear in the LEED pattern corresponding to an overlayer which is one-dimensionally ordered in the [011̄] direction. Additional CO adsorption causes the heat of adsorption to decrease further and the overlayer structure to be compressed in the [011̄] direction. At saturation the LEED pattern shows extra spots which are tentatively attributed to domains of a new overlayer structure coexisting with the first. Electron energy loss spectra (EELS) of adsorbed CO show two characteristic peaks at 4.5 and 13.5 eV probably arising from transitions between the electronic levels of chemisorbed CO.     

Chemisorption of CO on the unreconstructed Pt(100) surface

The chemisorption of CO on the clean, unreconstructed Pt(100)-1 × 1 surface was investigated by LEED and XPS. Three LEED patterns, c(2 × 2), (√2 × 3√2) R45° and c(4 × 2), were observed with increasing CO exposure and structure models corresponding to these LEED patterns were proposed. The absolute coverage of CO was determined by combining the O(1s) XPS data with coverage information derived from LEED. The maximum CO coverage thus obtained was θ = 0.75 and the initial sticking coefficient was determined to be s₀ = 0.6. This coverage calibration can also be utilized for other oxygen containing molecules by comparing the corresponding O(1s)it peak intensities.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Cosegregation of tungsten and nitrogen on Fe-9%W-N(100)

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) have been used to study the cosegregation of tungsten and nitrogen on ferritic Fe-9%W-N(100) single crystals with nitrogen contents ranging from c_N = 11 to 51 wt-ppm. Cosegregation occurs at temperatures T≲600°C depending on the nitrogen content. The thickness of the cosegregated surface layer is estimated by means of Ar⁺ ion depth profiling as being less than two atomic layers. The LEED pattern of the tungsten and nitrogen covered Fe-9%W-51ppmN(100) substrate shows a sharp (1 × 1) structure at a low background intensity indicating epitaxial stabilization of the cosegregated tungsten nitride layer on the bcc (100) surface. The tungsten and nitrogen covered Fe-9%W-11ppmN(100) substrate exhibits a c(2 × 2) structure. On Fe-9%W-51ppmN(100) a temperature-driven phase transition between the cosegregated (1 × 1) and c(2 × 2) surface phases is observed.     

Evidence of epitaxial growth of bcc Co on Cr(100)

We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 × 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.     

The coadsorption of I and Cl on Pt(111)

Using LEED and AES, the coadsorption of iodine and chlorine on Pt(111) was studied. Five surface structures were observed: (1 × 1), (√3 × √3)R30°, (√7 × √7)R±19.1°, (3 × 3) and c(2 × 4). Fractional monolayer coverages for both Cl and I are assigned to these structures. The importance of the adatom-adatom steric interactions is discussed.