Neal D. Shinn

Metal/metal‐oxide interfaces: A surface science approach to the study of adhesion

Metal‐oxide/metal interfaces play an important role, for example, in the joining of an oxide ceramic to a metal for sealing applications. In order to probe the chemical and physical properties of such an interface, we have performed Auger electron spectroscopic (AES) and temperature programed desorption (TPD) experiments on a model system composed of very thin films of Cr, Fe, Ni, or Cu evaporated onto a very thin thermally grown oxide on a W single crystal. Monolayer films of Fe and Cr were found (by AES) to completely wet the oxide surface upon deposition, and were stable up to temperatures at which the films desorbed (≊1300 K). In contrast, monolayer Ni and Cu films formed three‐dimensional islands exposing the oxidized W surface either upon annealing (Ni) or even upon room‐temperature deposition (Cu). The relative interfacial interaction between the overlayer metal and the oxide, as assessed by TPD, increases in the series Cu

Growth and atomic structure of chromium overlayers on W(110) and W(100)

Abstract The growth, structure and thermal stability of chromium overlayers vapor-deposited onto W(110) and W(100) substrates have been studied using Auger electron spectroscopy, temperature programmed desorption, work function measurements and low energy electron diffraction. Layer-by-layer growth is observed on both substrates in the 120–400 K temperature range, despite a 9% Cr : W bulk lattice mismatch. On W(110), a pseudomorphic (1 × 1) Cr monolayer (ML) forms at 100 K and remains stable up to desorption at 1290 K. A metastable surface structure of (2 × 2) symmetry is observed for ∼2 ML Cr W(110) in the 500 Cr W(100) overlayer exhibits no long range order but remains stable up to 1100 K, while thicker films are thermally unstable above 500 K, forming three-dimensional clusters on top of the pseudomorphic Cr W(100) monolayer.

Oxidation of W(110): valence-band and W(4f) core-level spectroscopy

The initial stages of W(110) oxidation have been characterized by surface-sensitive W(4f) core-level photoemission, with the surface-sensitivity provided by the use of tunable synchrotron radiation for excitation. Specific W(4f) Photoemission features are assigned to W-atoms bonded to oxygen at coverages below and above 1/2 monolayer (ML). The higher coverage oxygen-induced peaks are at a significantly higher binding energy, which is consistent with an increase in the coordination of W to O and, correspondingly, an increase in the extent of charge transfer between W and O. These results are also consistent with previous temperature-programmed desorption (TPD) studies. Notably, the TPD experiments have demonstrated that only atomic oxygen is desorbed for oxygen coverages (θ0) below 1/2 ML, while suboxides of tungsten (WOx, x = 1 – 3) are observed as desorption products for θ01/2 ML. Changes we observe in the valence-band spectra may also be reconciled within this model.

Stimulated desorption from CO chemisorbed on Cr(110)

Abstract Electron stimulated desorption (ESD) experiments using a time-of-flight pulse counting method are reported for molecular CO chemisorbed on the Cr(110) surface at 90 K. Consistent with previous qualitative observations, negligible CO + and O + desorption signals were measured from the α 1 CO overlayer which saturates at 1 4 monolayer. For θ CO > 0.25, a terminally-bonded (α 2 CO) binding mode is populated in addition to the existing ∝ 1 CO binding mode and the ion yield increases sharply. For α 2 CO, both O + and CO + ions are observed; the CO + ions desorb with characteristically lower kinetic energies than O + ions. Near saturation coverages of CO(ads), an observed decrease in the O + yield is attributed to adsorbate-adsorbate interactions which reduce the ion desorption probability, as seen in ESD studies of terminally-bonded CO on other metals. These results are considered in the context of two possible models proposed for the α 1 CO binding state and related ESD observations for CO chemisorbed on Fe(001) and potassium-promoted Ru(001).

Adsorption and thermal stability of Mn on TiO2(110): 2p X-ray absorption spectroscopy and soft X-ray photoemission

Abstract X-ray absorption and photoelectron spectroscopies are used to study the adsorption and reaction of Mn films deposited on TiO 2 (110) at 25°C and after annealing to ∼650°C. Fractional monolayer coverages of Mn at 25°C produce a reactive, disordered interface consisting of reduced Ti cations and oxidized Mn overlayer atoms. Metallic Mn is found only for thicker layers at 25°C. Annealing Mn films to ∼650°C leads to several changes that are largely independent of initial overlayer coverage: Metallic Mn thermally desorbs leaving only Mn 2+ ions; interfacial Ti cations are largely re-oxidized to the Ti 4+ state; and the local order is increased at the interface. The formation of a crystalline ternary surface oxide, MnTiO x , is proposed to account for these chemical and structural changes.

Synchrotron photoemission evidence for ‘‘lying‐down’’ CO on Cr(110)

Synchrotron ultraviolet photoemission spectroscopy (UPS) has been used to identify two sequentially populated molecular CO binding modes on Cr(110) at 90 K. These are distinguished by both intensity and electron‐binding‐energy differences in the CO‐derived valence‐band UPS features. These results support the previously proposed models in which the first binding mode (α1CO) is ‘‘lying down’’ on the surface, with both the carbon and oxygen coordinated to chromium atoms, and the second binding mode (α2CO) is terminally bonded and oriented roughly along the [110] surface normal.

Decomposition of P(CH3)3 on Ru(0001): comparison with PH3 and PCl3

Abstract The decomposition of P(CH3)3 adsorbed on Ru(0001) at 80 K is studied by soft X-ray photoelectron spectroscopy using synchrotron radiation. Using the chemical shifts in the P 2p core levels, we are able to identify various phosphorus-containing surface reaction products and follow their reactions on Ru(0001). It is found that P(CH3)3 undergoes a step-wise demethylation on Ru(0001), P(CH3)3 → P(CH3)2 → P(CH3) → P, which is complete around ∼450 K. These results are compared with the decomposition of isostructural PH3 and PCl3 on Ru(0001). The decomposition of PH3 involves a stable intermediate, labeled as PHx, and follows a reaction of: PH3 → PHx → P, which is complete around ∼190 K. The conversion of chemisorbed phosphorus to ruthenium phosphide is observed and is complete around ∼700 K on Ru(0001). PCl3 also follows a step-wise decomposition reaction, PCl3 → PCl2 → PCl → P, which is complete around ∼300 K. The energetics of the adsorption and the step-wise decomposition reactions of PH3, PCl3 and P(CH3)3 are estimated using the bond order conservation Morse potential (BOCMP) method. The energetics calculated using the BOCMP method agree qualitatively with the experimental data.

Oxygen chemisorption on Cr(110)

Abstract Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O2(ads) vibrational frequency of 1020 cm−1 and a six beam ESDIAD pattern are suggestive of super-oxo O2(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.

Surface chemistry of PH3, PF3 and PCl3 on Ru(0001)

Abstract The adsorption, desorption and decomposition of PH 3 , PF 3 and PCl 3 on Ru(0001) have been studied by soft X-ray photoelectron spectroscopy (SXPS) using synchrotron radiation. Due to large chemical shifts in the P 2p core levels, different phosphorus containing surface species can be identified. We find that PF 3 adsorbs molecularly on Ru(0001) at 80 and 300 K. At 80 K, PH 3 saturates the surface with one layer of atomic hydrogen, elemental phosphorus, subhydride (i.e., PH x (0 x 3 , with a total phosphorus coverage of 0.4 ML. At 300 K, PH 3 decomposes into atomic hydrogen and elemental phosphorus with a phosphorus coverage of 0.8 ML. At 80 K, PCl 3 adsorbs dissociatively into atomic chlorine, elemental phosphorus, PCl and possibly PCl 2 and PCl 3 in the first monolayer. Formation of multilayers of PCl 3 is observed at 80 K. At 300 K, PCl 3 adsorbs dissociatively as atomic chlorine and elemental phosphorus with a saturation phosphorus coverage of 0.1 ML. The variation in total phosphorus uptake at 300 K from PX 3 ( X = H , F and Cl ) adsorption is a result of competition between site blocking by dissociation fragments and displacement reactions. Annealing surfaces with adsorbed phosphorus to 1000 K results in formation of Ru z P ( z = 1 or 2), which is manifested by the chemical shifts in the P2p core level, as well as the P LVV Auger transition. The recombination of adsorbed phosphorus and adsorbed X ( = H , F and Cl ) from decomposition is also observed, but is a minor reaction channel on the surface. Thermochemical data are used to analyze the different stabilities of PX 3 at 300 K, namely, PF 3 adsorbs molecularly and PH 3 and PCl 3 dissociate completely. First, we compare the heat of molecular adsorption and the heat of dissociative adsorption of PX 3 on Ru(0001), using an enthalpy approach, and find results consistent with experimental observations. Second, we compare the total bond energy difference between molecular adsorption and complete dissociation of PX 3 on Ru(0001). In particular, we apply Shustorovichs bond-order conservation-Morse potential (BOC-MP) method to estimate the heat of adsorption for PH 3 and PCl 3 and the bond energies of the relaxed P-X bonds of the adsorbed PX 3 on the surface. The bond strength difference among the relaxed P-X bonds (i.e., the relaxed P-F bond ( 475 kJ mol ) is much stronger than either the relaxed P-H bond ( 287 kJ mol ) or the relaxed P-Cl bond ( 288 kJ mol )) suggests that PF 3 is more stable than PH 3 and PCl 3 on Ru(0001) at 300 K. These values are used to evaluate the total bond energy differences between molecular adsorption and complete dissociation for each of the PX 3 , and the results agree with the experimental trends.

Core-level photoemission and alkali ion scattering structure analysis of Ni adsorption and surface alloying on W(001)

Abstract The growth and structural evolution of Ni overlayers deposited on W(001) are studied by low energy alkali ion scattering and W 4 f 7 2 core-level photoemission. The composition and thermal stability of the interfacial layers are found to vary with nickel coverage. For sub-monolayer Ni coverages deposited near 300 K, annealing to 1000 K leads to a random alloy in the surface layer with W atoms protruding above the Ni atoms by approximately 0.022 nm. At Ni coverages near one monolayer, the surface layer is a nearly pure, pseudomorphic nickel overlayer 0.125 nm above the underlying W substrate. A second pseudomorphic nickel layer can be grown at 300 K but annealing results in intermixing at the buried interface. At 2 monolayers (ML) coverage, the uppermost two atomic layers are mostly nickel. Fitting of the W 4 f 7 2 photoemission spectra requires three components: an invariant bulk component, a surface component at lower binding energy, and a third component at intermediate binding energy. The surface component intensity decreases linearly to zero as the nickel coverage approaches 1 ML, indicating that this component originates from coordinatively unsaturated W atoms in the alloyed top layer. The intermediate component, present at all coverages, is assigned to coordinatively saturated atoms in the layer below the alloyed top layer for coverages between 0–1 ML, and below the Ni layer at 1 ML. Variation in the binding energy of the intermediate component is observed during growth of the second pseudomorphic Ni overlayer. The correlation of structural and electronic information shows that coordination and interfacial strain dominate charge transfer in determining core-level shifts.