Ultrathin Pd Films Research Articles

Hydrogen adsorption and absorption on ultrathin Pd films on Ta(110)

The H 2 adsorption and absorption properties of monolayer and ultrathin Pd films deposited on Ta(110) were studied with AES, LEED, and temperature programmed desorption (TPD). The interaction of H 2 with the fcc (111) Pd monolayer (θ Pd = 1) at 100 K is characterized by an initially high value of the H 2 dissociative sticking coefficient, S ~ 0.6, that decreases rapidly to ~ 0.04 with increasing H 2 uptake. Only a very small amount (< 1% of a monolayer) of hydrogen desorbs from chemisorption sites on the θ Pd = 1 film although there is substantial desorption of hydrogen from bulk absorption sites. We also observed desorption at 150 K from a near-surface hydride or interface state from the fcc (111) Pd monolayer. The absence of any appreciable amount of desorption from a Pd-H chemisorption state on the Pd monolayer is due to destabilization of the Pd-H bond and diffusion of hydrogen into Ta to populate energetically more favorable sites. Increasing the temperature of the Pd monolayer to 500 K caused S to increase to only 0.1 for large H 2 exposures. We attribute this relatively small value of S to the population of hydrogen absorption sites just below the Ta surface which, in effect, create a barrier to further H 2 uptake. LEED observations following H 2 exposures on the θ Pd = 1 film showed that hydrogen, unlike CO, had no effect on the observed LEED pattern, i.e., no large change in the geometric structure of the Pd film occurred. The H 2 chemisorption properties of the pseudomorphic bcc (110) Pd monolayer (θ pd = 0.7–0.8) were almost identical to the fcc (111) Pd monolayer with only a slightly smaller value of S and no desorption peak at ~ 150 K. As the Pd film thickness was increased beyond one monolayer, the H 2 sticking coefficient and desorption from the H-Pd chemisorption state increased. If adsorption experiments were carried out on these thicker Pd films at 100 K, S reached a value of only 0.23 for large H 2 exposures even for films as thick as θ Pd ≈ 50. We propose that slow diffusion into the bulk at this low temperature limits the uptake rate. Increasing the temperature of thick Pd films to 500 K caused a large increase in the H 2 dissociative sticking coefficient to S ≈ 0.4 for θ Pd = 10 and large H 2 exposures.

Hydrogen adsorption and absorption on ultrathin Pd films on Ta(110)

Hydrogen adsorption and absorption on ultrathin Pd films on Ta(110)

Growth and electronic structure of ultrathin epitaxial Pd(111) films on Fe(110) and Co(0001) substrates.

Epitaxial Pd(111) layers on Fe(110) and Co(0001) thick-film substrates atop W(110) are studied using low-energy electron diffraction (LEED), Auger electron spectroscopy, and angle-resolved photoemission with He i, Ne i, and synchrotron-radiation photon sources. Clear LEED patterns obtained for Pd coverages between 0 and 10 atomic layers (AL's) indicate commensurate Pd(111) overlayer structures in both systems. Satellite reflexes for films in the monolayer coverage range indicate long-range coincidences of 10 Pd atoms on 11 substrate atoms along specific directions. Auger peak intensities of overlayer and substrate as a function of film thickness show straight-line segments separated by breaks at the completion of full atomic layers, suggesting layer-by-layer growth up to a Pd coverage of at least 2 AL and no interdiffusion with the substrates. Interface states are identified for monolayer coverages in both systems by observing a saturation of their intensity as a function of film thickness near 1 AL and a vanishing dispersion with the electronic wave-vector component perpendicular to the surface (${\mathit{k}}_{\mathrm{\ensuremath{\perp}}}$). In contrast to bulklike features at higher Pd coverage, the interface states are insensitive to the incident-light polarization (s or p), possibly because of the almost incommensurate structure of the Pd overlayer. The total Pd-induced 4d bandwidth of the monolayer seen in photoemission spectra taken at photon energies near 60--90 eV is approximately 3.5 eV and exhibits noble-metal character, i.e., little intensity near the Fermi level ${\mathit{E}}_{\mathit{F}}$. Monolayer spectral features in both systems show considerable (\ensuremath{\ge}1 eV) dispersion with the electronic-wave-vector component parallel to the surface, ${\mathit{k}}_{\mathrm{\ensuremath{\parallel}}}$, indicating interactions within the overlayer. All Pd-induced features have mainly overlayer character, i.e., all exhibit a Pd-like photon-energy dependence in the region of the Pd 4d Cooper minimum.The transition to a bulklike Pd(111) electronic structure is virtually complete at a Pd coverage of 5 AL, although dispersion with ${\mathit{k}}_{\mathrm{\ensuremath{\perp}}}$ starts at 3 AL and a final-state resonance typical of the bulk is even discernible at 1.5 AL. A ``second-layer'' interface state for Pd coverages between 1.5 and 2.5 AL probably arises through the changed potential caused by the second Pd layer.

Temperature-dependent top-layer composition of ultrathin Pd films on Cu(100).

Positron-annihilation-induced Auger electron spectroscopy (PAES), electron-induced Auger electron spectroscopy, and low-energy electron diffraction have been used to study the composition and structure of vapor-deposited Pd films on Cu(100). Our results show that the PAES intensity from Pd saturates while that from Cu attenuates to near zero by one monolayer of Pd deposition at 173 K. This is confirmation of the top-layer selectivity of PAES in metal-on-metal systems. Changes in the PAES intensities provide a direct indication of the intermixing of Cu and Pd in the topmost layer as a Pd film deposited at 173 K is warmed to 303 K, and of the formation of a Cu overlayer upon further warming to 423 K. Lack of agreement between model calculations which assume delocalized positron states and measured PAES intensities at 173 K suggests that positrons are localized at sites containing Pd atoms.

Desorption kinetics of CO from epitaxial Pd films on W(110) in the low coverage limit

The adsorption-desorption kinetics of CO from ultrathin epitaxial Pd films has been studied using modulated molecular beam techniques. We find that the rate constant for desorption, k d, is nearly four orders of magnitude higher for the monolayer Pd film than that for the bilayer film. The rate decreases further by a factor of 7 between 2 and 6 monolayers at which k d attains the value seen on Pd single crystals. We conclude that although a bilayer film is similar to a bulk Pd(111) surface in binding CO, true bulk-like behavior is only reached above six atomic layers of Pd. Temperature programmed desorption at low coverage can be fit using the beam determined kinetic parameters, showing that defects play no major role in the beam experiments. Significant departures in the values of the activation energy for desorption and the preexponential factor for the monolayer film from expected values are attributed to a possible adsorbate induced surface reconstruction.

CO chemisorption properrties of ultrathin Pd films on Ta(110)

We have investigated the CO chemisorption properties of Pd monolayers and ultrathin films of various thicknesses on Ta(110) using TPD, HREELS and LEED. Using Arrhenius plots constructed from CO TPD data, we were able to determine the CO desorption activation energies, E d, on these films. On the pseudomorphic ( θ Pd = 0.65) and the fcc(111) ( θ Pd = 1.0) Pd monolayers, CO desorbs with nearly the same activation energies, E d ∼ 15 kcal/mol, indicating no significant effects of the changes in the Pd monolayer structure on the CO chemisorption properties. These values are in contrast to E d = 35.5 kcal/mol measured on the bulk Pd(111) surface and indicate strong alteration in the chemisorption properties of monolayers of Pd on Ta(110). Our CO TPD results also show that the CO chemisorption properties of the Pd(111) surface are practically recovered at θ Pd = 2.0 if the Pd film is not annealed after deposition at 125 K. For Pd films annealed to 550 K the CO desorption energy increases with the size of the Pd clusters which form on top of the Pd monolayer. HREELS, used to probe the CO adsorption site on these Pd films, shows that CO adsorbs predominantly in atop sites on the pseudomorphic and the fcc(111) Pd monolayers. In addition, a small amount of bridge-bonded CO is detected when the fcc(111) monolayer is exposed to saturation doses of CO. This type of CO corresponds to CO molecules adsorbed on small Pd clusters formed by CO adsorption. This is consistent with our LEED observations which indicate that the fcc(111) structure is reconstructed to the pseudomorphic structure upon exposure to CO.

Growth mechanism and structure of ultrathin Pd films vapor-deposited on Ta(110)

The structure and growth of Pd films formed by vapor-deposition on Ta(110) have been studied using AES and LEED over a wide range of substrate temperatures. The thermal stability of monolayer, bilayer and trilayer Pd was examined also. At 300 K, the growth of Pd on Ta(110) is best described by a Frank-van der Merwe (layer-by-layer) growth mode. Initially, the first Pd layer grows in 2D pseudomorphic islands up to a Pd coverage of θ Pd = 0.65 (defined relative to the atomic density of the (111) surface of bulk Pd, θ Pd = 1). Subsequent deposition of Pd atoms induces a phase structural phase transition; for 0.65 < θ Pd< 1.0 the monolayer growth proceeds through formation of 2D islands of Pd fcc (111) structure. At 125 K, the growth mode of Pd/Ta(110) is similar, but seems to deviate slightly from the layer-by-layer mode. For a substrate temperature of 500 K the growth mechanism changes. The first layer is still pseudomorphic initially, but the phase transition to the Pd fcc(111) structure occurs at θ Pd = 0.82. After completion of the Pd monolayer, additional Pd deposition results in the formation of 3D crystallites 2–3 layers thick on top the first Pd layer, i.e., a Stranski-Krastanov growth mode occurs at 500 K. Annealing studies show that the pseudomorphic Pd monolayer is stable to very high temperatures (up to 1350 K), but the growth of Pd crystallites occurs upon heating thicker Pd layers above 370 K.

Ｐｄ／Ｍ（Ｍ：Ｗ，Ｍｏ，Ｔａ，Ｎｂ）原子界面とＰｄ成長層のＦＩＭ観察

The growth processes and the atomic structure of Pd ultra-thin films vapor deposited on the substrate metals (W, Mo, Ta, Nb), and a hydrogen interaction with the Pd monolayer film were investigated by a field ion microscope (FIM) with atomically high resolution. The Pd monolayer on the substrate metals exhibits a pseudomorphic structure which has an atomic arrangement identical to the substrate and is strongly bound with the substrate metal atoms. The desorption field strengths of the pseudomorphic Pd monolayer on W (011), Mo (011), Ta (011) and Nb (011) planes were measured. The fields depend strongly on the evaporation fields of the substrate metals and are higher than the evaporation field of Pd. In the growth of the Pd films, the two different Pd structures are observed. The observed structures are the Pd overlayer grown with the Pd inherent atomic structure and the stable pseudomorphic Pd underneath. Hydrogen interactions with the pseudomorphic Pd monolayer grown on W (011) and Mo (011) planes were also studied.

Chemisorption of hydrogen on ultrathin Pd films on Mo (100)

The H 2 adsorption properties of ultrathin Pd films deposited on Mo(100) have been studied with Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD). The pseudomorphic monolayer of Pd on Mo(100) has a greatly reduced H 2 uptake capacity at 150 K compared to bulk Pd. Desorption from the Pd monolayer occurs from a chemisorption state with a desorption temperature of about 290 K. Thicker Pd films also have a reduced H 2 uptake, until θ Pd≥5.5. H 2 desorption from the low-temperature hydride state that is characteristic of bulk Pd only occurs for Pd films with θ Pd≥5.5.

Ultrathin films of Pd on Au(111): Evidence for surface alloy formation.

Pd ultrathin films on Au(111) have been prepared by vapor deposition and studied over the substrate temperature range 150--650 K using Auger electron spectroscopy (AES), low-energy ion-scattering spectroscopy (LEISS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). With regard to assessing the growth mode of Pd adlayers on Au(111), AES uptake plots (AES signal intensity versus deposition time) are of limited utility. LEISS data are critical for establishing the surface composition of the deposited film and the existence of alloying. The growth mode of vapor-deposited Pd films on Au(111) at 150 K is described well by an epitaxial layer-by-layer mechanism for the first few layers. However, for Pd deposition at substrate temperatures of 300 K and above, LEISS data taken using 1 keV Ne{sup +} ions indicate surface alloy formation, with the extent of Au interdiffusion increasing with temperature. Other experiments involving annealing monolayer Pd films show interdiffusion (alloying) at temperatures as low as 240 K. XPS data indicate very little ({lt}0.2 eV) or no shifts in the Pd(3{ital d}), Au(4{ital f}), or Pd x-ray-excited Auger ({ital M}{sub 5}{ital VV}) regions as a function of Pd coverage or substrate temperature. We discuss the surface stoichiometry in relation to priormore » published work on this system, the relevance of this system for studies of Pd monolayer and thin film properties, and our results pertaining to the surface chemistry of small molecule adsorbates on Pd films deposited on Au(111).« less

Chemical and electronic properties of ultrathin metal films: The Pd/Re(0001) and Pd/Ru(0001) systems.

The nature of the electronic and chemical properties of ultrathin Pd films on Re(0001) and Ru(0001) has been studied using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and CO chemisorption. The results indicate that the Pd(3${\mathit{d}}_{5/2}$) binding energy for a monolayer (ML) of Pd on Re(0001) and Ru(0001) is perturbed by +0.60 and +0.30 eV, respectively, from that of the surface atoms of Pd(100). These electronic perturbations induce large changes in the chemical properties of the Pd films. TPD results indicate that the desorption temperature of CO from 1 ML of Pd on Re(0001) and Ru(0001) is \ensuremath{\sim}120 K lower than the corresponding desorption temperature from Pd(100). The XPS and CO-TPD data indicate that Pd transfers charge to the Re and Ru substrates, becoming electron deficient and less efficient at \ensuremath{\pi} backdonation toward CO. By comparison of these results with those reported previously for Pd, Ni, and Cu adlayers, a correlation is observed among the electronic perturbations of the adlayers, the cohesive metal-substrate bond strength, the ability of the film to chemisorb CO, and the CO-induced shift in the metal core-level binding energy. In general, the results indicate that the formation of a metal-metal bond at a surface leads to a gain of electron density by the element initially having the greater fraction of empty states in its valence band. This behavior is completely contrary to that seen in bulk alloys, likely a consequence of the anisotropic character of a surface that changes the relative electronegativities of the metal atoms. On the basis of these results, a qualitative scale of surface electronegativities is developed, showing trends that are very different from those found in three-dimensional bulk alloys.

Chemisorption of CO on ultrathin films of Pd on Mo(100)

The altered bonding that exists at bimetallic interfaces can affect the properties of metal monolayers and thin films. We have probed the chemisorptive properties of ultrathin films of Pd on Mo(100) by studies of CO adsorption on these surfaces. Our investigations were carried out using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). The heat of adsorption of CO is reduced from 36.5 kcal/mol on Pd(100) single crystal surfaces to 20 kcal/mol on the pseudomorphic Pd monolayer. In addition, a 34% reduction in the initial sticking probability of CO occurs on the Pd monolayer at 150 K relative to thick (20 layers) Pd films. Both the heat of adsorption and the sticking probability of CO increase with Pd film thickness indicating that the chemistry of these surfaces can be “tuned”. However, the origin of this tuning is not clear. On the Pd monolayer, HREELS shows that hollow and bridging sites are populated first, followed by atop sites at higher CO coverages. This is consistent with the site preference on bulk Pd(100) surfaces. The bonding of CO at bridging sites is clearly destabilized on the Pd monolayer and to a lesser extent on thicker films. The vibrational spectra of chemisorbed CO are shown to be a sensitive probe of the structure of Pd films of this type. The alterations in CO adsorption on the Pd monolayer and thin films on Mo(100) compared with that on bulk Pd surfaces are consistent with those observed for ultrathin Pd films on other early transition metal substrates such as Nb, Ta, and W. CO bonding on the pseudomorphic monolayer of Pd on Mo(100), which has a highly strained PdPd lattice, is not weakened as much as on the relatively less strained Pd monolayers on Nb(110) and Ta(110), indicating that lattice strain is not sufficient to account for the origin of the weakened CO interactions.

Ultrathin Pd films on Cu(100): An investigation using the Finnis-Sinclair potentials of Rafii-Tabar and Sutton.

The energy per atom of various surface configurations, when a half monolayer of Pd is deposited on Cu(100), is calculated by using the Finnis-Sinclair potentials of Rafii-Tabar and Sutton. Long-range order in the alloyed Cu/Pd surface is also calculated as a function of temperature. The results of the energy calculations are in agreement with existing experimental observations, while the results for the long-range-order calculations suggest that future experimental data may exhibit two transition temperatures.

Growth mechanism and structure of ultrathin palladium films formed by deposition on Mo(100)

The growth mechanism, structure and thermal stability of monolayer and ultrathin Pd films formed by vapor deposition on Mo(100) were studied using AES, LEED, and TPD. Pd film growth at 150 K is described well by a Frank-van der Merwe (FM) or layer-by-layer growth mechanism with a small amount of layer disorder and/or non-ideal layering. The Pd monolayer is pseudomorphic with the Mo(100) substrate lattice as shown by LEED. Pd films deposited on Mo(100) at 450 and 600 K grow by forming three-dimensional (3D) islands on top of an initially formed Pd monolayer, i.e., a Stranski-Krastanov growth mode. Alloying could also explain the AES curves at these temperatures. Thermal desorption of Pd from multilayer films begins at 1250 K with an activation energy of 100 kcal mol . This is 7 kcal mol higher than the bulk sublimation energy of palladium due to interaction with the molybdenum substrate and was observed for films as thick as 20 layers. Pd desorption is kinetically limited by decomposition of a Pd-Mo alloy and/or diffusion of Pd from the subsurface layers of Mo to the surface. Annealing studies show that the Pd monolayer is stable to 1200 K, but that agglomeration of Pd into 3D islands and possibly alloy formation occurs upon heating thicker films above 400 K.

Coverage and structure of ultrathin Pd films on Cu(100).

Coverage and structure of ultrathin Pd films on Cu(100).

The interaction of ultrathin films of Ni and Pd with W(110): an XPS study

The interaction of ultrathin films of Ni and Pd with W(110) has been examined using X-ray photoelectron spectroscopy (XPS) and the effects of annealing temperature and adsorbate coverage (film thickness) are investigated. The XPS data show that the atoms in a monolayer of Pd or Ni supported on W(110) are electronically perturbed with respect to the surface atoms of Pd(100) and Ni(100). The magnitude of the electronic perturbations is larger for Pd than for Ni adatoms. Our results indicate that the difference in Pd(3 d 5 2 ) XPS binding energies between a pseudomorphic monolayer of Pd on W(110) and the surface atoms of Pd(100) correlates with the variations observed for the desorption temperature of CO (i.e., the strength of the PdCO bond) on these surfaces. A similar correlation is seen for the Ni(2 p 3 2 ) XPS binding energies of Ni/W(110) and Ni(100) and the CO desorption temperatures from the surfaces. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of: (1) variations that occur in the NiNi and PdPd interactions when Ni and Pd adopt the lattice parameters of W(110) in a pseudomorphic adlayer; and (2) transfer of electron density from the metal overlayer to the W(110) substrate upon adsorption. Measurements of the Pd(3 d 5 2 ) XP binding energy of Pd/W(110) as a function of film thickness indicate that the PdW interaction affects the electronic properties of several layers of Pd atoms.

Experimental probe for thin-film magnetism in p(1 x 1) Pd and V on Ag(100).

We report high-sensitivity experiments utilizing the magneto-optic Kerr effect which probe for ferromagnetism in ultrathin epitaxial films of Pd and V on Ag(100) substrates. Our experiments fail to detect unambiguous manifestations of ferromagnetism in these two thin-film systems.

Rutherford backscattering experiments on percolating Pd films

Rutherford backscattering experiments are performed on ultrathin percolating Pd films during irradiation at 300 K with Xe ions. The spectra are sensitive to the topology of the films, allowing us to follow the appearance of a percolating structure.

Growth and electronic structure of ultra-thin Pd and Ag films on Al(111)

Pd and Ag films vapor-deposited onto Al(111) were studied by AES, LEED and angle-resolved UPS. Both metals grow in the Stranski-Krastanov mode. With the substrate at 520 K Ag grows epitaxially. For Pd no ordered structures were found in the monolayer region. For Ag mainly two bands were found for the 4d states showing a maximum dispersion of 0.4 eV and a splitting into three bands in the outer parts of the two-dimensional Brillouin zone. In the monolayer the Pd4d emission lies 4 eV below the Fermi edge, which means that it maintains its atomic 4d 105s 0 configuration.

Growth and electronic structure of ultra-thin Pd and Ag films on Al(111)

Abstract Pd and Ag films vapor-deposited onto Al(111) were studied by AES, LEED and angle-resolved UPS. Both metals grow in the Stranski-Krastanov mode. With the substrate at 520 K Ag grows epitaxially. For Pd no ordered structures were found in the monolayer region. For Ag mainly two bands were found for the 4d states showing a maximum dispersion of 0.4 eV and a splitting into three bands in the outer parts of the two-dimensional Brillouin zone. In the monolayer the Pd4d emission lies 4 eV below the Fermi edge, which means that it maintains its atomic 4d 10 5s 0 configuration.