Oxidation of Pt(1 0 0)-hex-R0.7° by gas-phase oxygen atoms

We utilized temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS), and low energy electron diffraction (LEED) to investigate the oxidation of Pt(1 0 0)-hex-R0.7° at 450 K. Using an oxygen atom beam, we generated atomic oxygen coverages as high as 3.6 ML (monolayers) on Pt(1 0 0) in ultrahigh vacuum (UHV), almost 6 times the maximum coverage obtainable by dissociatively adsorbing O₂. The results show that oxidation occurs through the development of several chemisorbed phases prior to oxide growth above about 1 ML. A weakly bound oxygen state that populates as the coverage increases from approximately 0.50 ML to 1 ML appears to serve as a necessary precursor to Pt oxide growth. We find that increasing the coverage above about 1 ML causes Pt oxide particle growth and significant surface disordering. Decomposition of the Pt oxide particles produces explosive O₂ desorption characterized by a shift of the primary TPD feature to higher temperatures and a dramatic increase in the maximum desorption rate with increasing coverage. Based on thermodynamic considerations, we show that the thermal stability of the surface Pt oxide on Pt single crystal surfaces significantly exceeds that of bulk PtO₂. Furthermore, we attribute the high stability and the acceleratory decomposition rates of the surface oxide to large kinetic barriers that must be overcome during oxide formation and decomposition. Lastly, we present evidence that structurally similar oxides develop on both Pt(1 1 1) and Pt(1 0 0), therefore concluding that the properties of the surface Pt oxide are largely insensitive to the initial structure of the Pt single crystal surface.     

Coadsorption of CO and C6H6 on Co(0 0 0 1)

We have studied the influence of CO on the adsorption of benzene on the Co(0 0 0 1) surface using LEED, XPS, TDS and work function measurements. CO was found to reduce the benzene adsorption, but even at saturation CO exposure no complete blocking was observed. Thermal desorption of the coadsorbed layer featured CO and H₂ peaks indicating partial dehydrogenation of benzene and retaining of the CO bond. Ordered LEED structures were found with all coverages: Pre-adsorption of CO led to patterns already seen for pure carbon monoxide adsorption. Pre-adsorption of benzene showed the known structure of pure benzene also with small CO exposures, but higher CO exposures yielded a mixture of and patterns.     

Anomalous adsorption of Cs on Pt(1 1 1)

The adsorption of Cs on Pt(1 1 1) surfaces and its reactivity toward oxygen and iodine for coverages θ_Cs?0.15 is reported. These surfaces show unusual “anomalous” behavior compared to higher coverage surfaces. Similar behavior of K on Pt(1 1 1) was previously suggested to involve incorporation of K into the Pt lattice. Despite the larger size of Cs, similar behavior is reported here. Anomalous adsorption is found for coverages lower than 0.15 ML, at which point there is a change in the slope of the work function. Thermal Desorption Spectroscopy (TDS) shows a high-temperature Cs peak at 1135 K, which involves desorption of Cs⁺ from the surface.The anomalous Cs surfaces and their coadsorption with oxygen and iodine are characterized by Auger Electron Spectroscopy (AES), TDS and Low Electron Energy Diffraction (LEED). Iodine adsorption to saturation on Pt(1 1 1)(anom)-Cs give rise to a sharp LEED pattern and a distinctive work function increase. Adsorbed iodine interacts strongly with the Cs and weakens the Cs-Pt bond, leading to desorption of Cs_xI_y clusters at 560 K. Anomalous Cs increases the oxygen coverage over the coverage of 0.25 ML found on clean Pt. However, the Cs-Pt bond is not significantly affected by coadsorbed oxygen, and when oxygen is desorbed the anomalous cesium remains on the surface.     

Electronic structure of oxidized SiC(0 0 0 1) studied by inverse photoemission spectroscopy

Starting from the silicon rich (3 × 3) reconstruction of SiC(0 0 0 1) we prepared oxidized surfaces by hydrogen etching as well as by exposure to molecular oxygen. LEED pictures show a (1 × 1)-reconstructed surface with a faint structure being more pronounced for the hydrogen-etched surfaces. Auger spectra reveal a distinct change in the shape of the Si_LVV peak indicating the existence of Si-O bonds on the surface. Inverse photoemission (IPE) is employed to study the electronic structure above the Fermi level of the oxidized samples. On the hydrogen-etched surface difference spectra reveal a surface feature at 1 eV above the Fermi level that presumably originates from an isolated dangling bond on ordered patches of the oxidized surface.     

Formation and hydrogenation of p(2 × 2)-N on Pt(1 1 1)

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N₂ TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ∼ 4 cm⁻¹) peak at 3322 cm⁻¹. The areas of the H₂ TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.     

Substantially low desorption barriers in recombinative desorption of deuterium from a Si(1 0 0) surface

We have studied desorption kinetics of deuterium molecules from a Si(1 0 0) surface by means of temperature-programmed desorption (TPD) spectra and isothermal desorptions.Three desorption components, denoted as β_1,A,β_1,B, and C, can be distinguished in semi-logarithmic plots of the TPD spectra.Their peak positions and intensities are strongly affected by the surface preparation methods employed, either with or without annealing to control the initial D coverage .Peak C appears at the leading edge of the TPD peak.It accounts for only about 5% of the TPD peak at and it diminishes rapidly with decreasing , vanishing at .In contrast, together the β_1,A and β_1,B peaks account for the whole TPD peak at any less than 1.0 ML. The maximum of the β_1,A peak is nearly constant at around the maximum temperature of the TPD peak.On the other hand, the β_1,B peak appears on the high-temperature side of the TPD peak and it systematically shifts to higher temperatures with decreasing .These results imply that first- and second-order kinetics are operating for the β_1,A and β_1,B desorptions, respectively.Isothermal desorption experiments confirm the above predicted kinetics for a limited region, namely .From the results for the rate curve analysis, the desorption barriers are evaluated to be 1.6 ± 0.1 eV and 1.8 ± 0.1 eV for the β_1,A and β_1,B desorptions, respectively.These values are substantially lower than the widely accepted value of ∼2.5 eV. To reproduce the measured TPD spectra we take the Arrhenius-type rate equation containing the first- and second-order rate terms for the β_1,A and β_1,B desorptions.The TPD spectra measured for can be reasonably fit with the proposed rate equation when the values given above for E_d,A and E_d,B are used. For , however, the TPD curves are not fit with the same values; rather, the best-fit curves require values for E_d,A and E_d,B larger than those given above. Combining the present kinetics results with those obtained by STM along with the studies, the β_1,A and β_1,B peaks may be attributed to desorption along the 2H path, while peak C may be attributed to desorption along the 4H path. The atomistic desorption mechanism as well as the energy relationship between the desorption barrier and isosteric heat of adsorption are discussed.     

In situ XPS study of Pd(1 1 1) oxidation at elevated pressure, Part 2: Palladium oxidation in the 10 mbar range

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 0.4 mbar O₂. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430 K, the Pd(1 1 1) surface was covered by a 2D oxide and by a supersaturated O_ads layer. The supersaturated O_ads layer transforms into the Pd₅O₄ phase upon heating and disappears completely at approximately 470 K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655 K, the bulk PdO phase appears and this phase decomposes completely at 815 K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4 mbar O₂, but the Pd₅O₄ phase appears below 745 K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.     

Oxygen induced surface structure of Mo(1 1 0) studied by scanning tunneling microscopy

The oxygen induced surface structures formed on Mo(1 1 0) by oxygen exposure at 1300 K in UHV has been studied by scanning tunneling microscopy (STM). Two kinds of oxygen-adsorbed surface structures are observed. One consists of one-dimensional rows running along or directions at substrate molybdenum lattices, and another shows more complex structure including discrete arrangement of large protrusions and zig-zag alignments of small protrusions. This complex structure is probably a further oxygen-adsorbed structure than the well-known p(2 × 2) structure of 0.3 ML coverage. On the basis of STM image, an atomic model is proposed, where adsorbed oxygen atoms occupy both long-bridge and the quasi-threefold sites of molybdenum lattice (0.4 ML coverage). This structure is presumed to be a transient state during site-conversion with increase of oxygen exposure.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

The transition from surface to bulk oxide growth on Pt(1 0 0): Precursor-mediated kinetics

We investigated the kinetics governing the transition from surface (2D) to bulk (3D) oxide growth on Pt(1 0 0) in ultrahigh vacuum as a function of the surface temperature and the incident flux of an oxygen atom beam. For the incident fluxes examined, the bulk oxide formation rate increases linearly with incident flux (Φ_O) as the oxygen coverage increases to about 1.7 ML (monolayer) and depends only weakly on the surface temperature in the limit of low surface temperature (T_S < 475 K). In contrast, in the high temperature limit (T_S > 525 K), the bulk oxide formation rate increases with for oxygen coverages as high as 1.6 ML, and decreases with increasing surface temperature. We show that the measured kinetics is quantitatively reproduced by a model which assumes that O atoms adsorb on top of the 2D oxide, and that this species acts as a precursor that can either associatively desorb or react with the 2D oxide to form a 3D oxide particle. According to the model, the observed change in the flux and surface temperature dependence of the oxidation rate is due to a change in the rate-controlling steps for bulk oxide formation from reaction at low temperature to precursor desorption at high temperature. From analysis of flux-dependent uptake data, we estimate that the formation rate of a bulk oxide nucleus has a fourth-order dependence on the precursor coverage, which implies a critical configuration for oxide nucleus formation requiring four precursor O atoms. Considering the similarities in the development of surface oxides on various transition metals, the precursor-mediated transition to bulk oxide growth reported here may be a general feature in the oxidation of late transition metal surfaces.     

Molecular adsorption and growth of naphthalene films on Ag(1 0 0)

The adsorption of naphthalene, vacuum deposited on a Ag(1 0 0) surface, was comprehensively investigated by means of low-energy electron diffraction (LEED), temperature-programmed thermal desorption (TPD) spectroscopy, X-ray photoelectron spectroscopy (XPS), and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the mono- and multilayer regime. A growth of long-range ordered monolayer at 140 K is observed with LEED. The polarization-dependent C 1s NEXAFS shows that the naphthalene molecules in the monolayer lie almost parallel to the Ag(1 0 0) surface. With increasing film thickness, the molecular orientation turns to upright position. Furthermore, NEXAFS measurements show that in the multilayer regime the molecular orientation depends on the substrate temperature during deposition.     

Photoelectron spectroscopy study of Pb/Ag(1 1 1) in the submonolayer range

The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Growth and structure of vanadium oxide on anatase (1 0 1) terraces

The interaction of vanadium oxide with epitaxial anatase films exposing (1 0 1) terraces was characterized. The TiO₂ films were grown on vicinal LaAlO₃ (1 1 0) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE); reflection high energy and low energy electron diffraction (RHEED and LEED) indicated that the films exposed (1 0 1) terraces of the anatase TiO₂ polymorph. When a vanadium oxide monolayer was deposited onto the anatase surface by OPA-MBE at 725 K, only (1 × 1) RHEED and LEED patterns were observed. The V X-ray photoelectron spectroscopy (XPS) peak intensities indicated that the monolayer wetted the anatase surface and so the diffraction patterns were attributed to an epitaxial vanadia layer. Analysis of the vanadium oxide monolayer by X-ray and ultraviolet photoelectron spectroscopies revealed that the V was predominantly 5+. When the vanadia coverage was increased at 725 K, Auger electron spectra showed only very slow attenuation of the anatase Ti peaks while spots began to develop in RHEED patterns recorded along the LaAlO₃ direction; both indicative of 3-D cluster formation. In the orthogonal direction, the RHEED patterns showed unusual diagonal streaks. Meanwhile, the (1 × 1) LEED pattern persisted even after 30 nm of vanadia was deposited. This was attributed to gaps between the 3-D clusters exposing the epitaxial monolayer. Core level XPS spectra of the 3-D clusters revealed a broad V 2p_3/2 peak that was centered at the position expected for V⁴⁺ but could be deconvoluted into three peaks corresponding to V³⁺, V⁴⁺, and V⁵⁺. It is shown that crystallographic shear that accommodates such variations in the oxygen content of V oxides can lead to the diagonal streaks in RHEED patterns recorded along the LaAlO₃ [0 0 1] direction even as the pattern in the orthogonal direction shows sharp transmission spots. The results show that vanadia growth on anatase (1 0 1) proceeds through the Stranski-Krastanov mode with a strong vanadia-titania interaction stabilizing a dispersed vanadia monolayer. The results are compared with previous data for vanadia growth on anatase (0 0 1) where smooth, epitaxial VO₂ films grow ad infinitum.     

Ultra-thin films and surface alloying of Pd on Cu(1 1 1) investigated by medium energy ion scattering

The structure of Pd films on Cu(1 1 1) and the alloying between the films and the substrate have been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ ions. Data are presented for the and alignments (nominal one- and three-layer alignments, respectively). It is found that beyond 1 ML the Pd grows in a twinned fcc structure, the incommensurate nature of which increases the visibility of the Cu(1 1 1) substrate to MEIS. Deposition of 0.2 ML of Pd produces a structure in which Pd mostly occupies the top two layers which have interlayer distances d₁₂ = 208 ± 4 pm and d₂₃ = 211 ± 4 pm. Some twinning is also present in this structure. Upon annealing 1.6 ML of Pd to 600 K for 1 min, the copper and palladium interdiffuse leaving around 0.4 ML of visible palladium. Energy plots show that there are several layers with an altered structure present over at least part of the surface. This may be due to large scale interdiffusion or alloy island formation. Incremental annealing to successively higher temperatures shows that the structural transformation begins around 500 K.     

Dynamics of NF3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF₃ on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF₃ adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF₃ is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F⁺ central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF₃ (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F⁺, N⁺, NF⁺, and ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the and ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF₃ molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF₃ crystal. In this process, some NF₃ molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.     

Growth and electronic structure of Mn on Al(1 1 1)

Growth and electronic structure of a 3d transition metal Mn on a free-electron-like metal Al(1 1 1) have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). A LEED is observed at 6 ML Mn coverage, that is related to the α phase of bulk Mn. From the intensity of the adlayer and substrate core-level peaks and angle dependent studies, evidence of primarily layer by layer growth is observed. The Mn 2p core-level for 0.1 ML Mn coverage appears at 0.3 eV lower binding energy with respect to bulk Mn. With increasing Mn coverage, the Mn 2p binding energy increases. An extra component is observed at the lower binding energy side of the Al 2p spectra, that is related to interface alloying. The asymmetric line-shape of bulk-like Mn becomes symmetric for lower Mn coverages.