Work Function Change Measurements Research Articles

Adsorption properties of ethene on Pt 3Cu(111)

The adsorption of ethene on the Pt 3Cu(111) surface at 95 K has been studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and work function change measurements. While the TPD, LEED and work function change measurements were presented elsewhere (C. Becker, T. Pelster, M. Tanemura, J. Breitbach, K. Wandelt, submitted to Surf. Sci.), in this communication a full peak assignment of the HREEL spectra is attempted. Two ethene species were found and identified as a weakly bonded π-complex and a stronger bonded di-σ-complex. No ethene conversion/dehydrogenation can be found after heating the precovered surface to temperatures between 220 K and 300 K, surprisingly the di-σ-species cannot be isolated by annealing at subsequently higher temperatures.

Interaction of CO with clean and oxygen covered PdCu(110) single crystal alloy

Compositional changes that may occur upon CO chemisorption on PdCu (110) single crystal alloy have been studied by means of work function change measurements using the Kelvin probe technique. Furthermore, various surfaces of this alloy with different surface compositions were investigated with respect to their reactivity in the CO oxidation applying the same technique. It was found that CO chemisorption, in the case of Cu-rich surfaces, induces the segregation of Pd to the surface. High CO pressures (10-4–10-3 mbar) also cause the surface segregation of still unidentified contamination characterized by an AES signal at 180eV. In the CO oxidation experiments the PdCu alloy showed lower reactivities compared to pure Pd, even though the outermost layer was entirely composed of Pd atoms. The reactivity was found to decrease further with increasing Cu concentrations. The lower reactivity of the PdCu alloy was attributed to the charge transfer from Pd to Cu observed in this system resulting in decreasing the adsorption enthalpy (and probably the sticking coefficient) of CO on “modified” Pd of the alloy surface.

Measurement of work function change with surface segregation of substrate element on a deposited film

Surface composition saturates when metal atoms of substrate diffuse through the deposited film on the substrate and segregate on the surface of the film. The properties of surfaces with segregation are expected to be similar to that with adsorption. Therefore, it seems possible to modify work function by segregation phenomena as well as adsorption. Saturation behavior of surface composition in segregation phenomena is useful for easy fabrication of surfaces with stable work function. Work function change by the surface segregation of Cu on Ti film and of Ti on Cu film has been measured. Work function was decreased approximately 0.3 eV by surface segregation of Cu on Ti film and slightly increased by surface segregation of Ti on Cu. The work function decrease caused by Cu segregation is reproducible when the surface is removed and Cu segregates again by heating. Therefore, the original idea of getting stable work function by using features of segregation phenomenon is verified.

STM measurements on alloy formation during submonolayer growth of Mn on Cu(111)

Stimulated by results from work function change measurements during the epitaxial growth of Mn on Cu(111) at room temperature, this system has been studied using STM under UHV conditions. These measurements show that after reaching a substrate step the Mn atoms are incorporated into the first copper layer. After the initial formation of an alloy brim at the substrate steps this incorporation takes place only into the lower terrace. Further deposition leads to compact alloy islands existing exclusively at substrate steps and with a thickness of at least two layers. These islands exhibit a hexagonal network structure with a periodicity of about 30 Å, while on the atomic scale a (√3×√3)R30° superstructure is found. Annealing such films to 500 K changes the appearance of this network. With increasing temperature the structures become more and more well-ordered and the average size of the unit mesh increases.

Adsorption of carbon dioxide on Cu(110) and on hydrogen and oxygen covered Cu(110) surfaces

The interaction of CO2 with a clean Cu(110) surface and with pre-adsorbed oxygen and hydrogen on this surface has been studied in ultra-high vacuum at temperatures between 20 and 500 K with temperature programmed thermal desorption, low-energy electron diffraction, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy and work function change measurements. CO2 adsorbs only molecularly on the clean and on the hydrogen(1×2) and oxygen(2×1) reconstructed Cu(110) surface, respectively. The initial sticking probability of CO2 is not affected by co-adsorption of oxygen or hydrogen, although the CO2 adsorption is energetically stabilised in this case by 1.3 and 5.4 kJ mol-1, respectively. On clean Cu(110), the isosteric heat of adsorption rises with coverage from ∽13 to 25 kJ mol-1 at saturation. High-resolution electron energy loss spectroscopy suggests that the isolated carbon dioxide molecule is adsorbed in a linear configuration on the clean and on the reconstructed surfaces, while for coverages >0.1 three-dimensional clustering occurs. Our experiments reveal that neither dissociation into oxygen and carbon monoxide nor hydrogenation of carbon dioxide occurs under the experimental conditions.

Silver films grown on a rhenium(0001) surface: a combined TDS, XPS, and Δ Φ study

The energetics and kinetics of Ag thin film growth on Re(0001) were studied by means of temperature-programmed thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and work function change (Δ Φ) measurements. The formation of three individual Ag layers shows up in TDS as three distinct desorption maxima β 1– β 3 appearing between 950 and 1010 K ( β 3), between 900 and 960 K( β 2), and between 870 and 970 K ( β 1). Except in the very low coverage ( Θ) range, in which the desorption is a first-order process, the Ag desorption follows zero-order kinetics. For the first two layers, activation energy of desorption ( E ∗ des ) is strongly Θ dependent: within the first layer, E ∗ des increases almost linearly with Θ from ≈250 kJ mol −1 at Θ=0.05 to about 290 kJ mol −1, reflecting attractive Ag–Ag interactions. From Θ=0.5 to 0.9, E ∗ des rises by only some 10 kJ mol −1. A similar (but much less pronounced) Θ dependence appears for the second monolayer. A detailed shape analysis of the submonolayer TD spectra reveals a phase equilibrium between Ag condensed in islands and individual, mobile Ag atoms (2D gas phase). In XPS, the absence of any energy shift of the Ag and Re core levels underlines the weakness of chemical Ag–Re interactions. Two Ag layers lower the work function of the Re(0001) surface by about 750 meV, with a shallow minimum near the second monolayer. We discuss our data in conjunction with previous STM and LEED results for the same system and compare this system with other Ag-on-metal systems.

Interaction of hydrogen with ultrathin titanium layers on tungsten

Hydrogen adsorption on titanium layers of an average thickness from 0.03 to 50 monolayers deposited on a tungsten substrate was investigated at liquid nitrogen temperature using the field electron emission microscopy techniques. It has been found that the interaction between titanium and hydrogen depends on the thickness and atomic structure of the Ti/W layer. For tightly arranged titanium layers isomorphous with the (011)W face the H–Ti interaction is weak and does not change the morphology and atomic structure of the layer. In the case of titanium layers isomorphous with the (001)W the H–Ti interaction dominates over the Ti–W interaction, beginning from the thinnest titanium layers deposited on the tungsten substrate. This results in a 3-dimensional nucleation of TiH x at the steps of the terraces of the (001)W face. For the titanium layers isomorphous with the (111)W and (112)W faces, the Ti–W interaction is stronger than the H–Ti interaction up to the thickness of 3 to 4 monolayers of titanium. Measurements of the average work function changes as a function of the H 2 exposure indicate that, apart from the hydrogen adsorption state which raises the work function, there exists another one which decreases the work function. The latter state occurs in titanium layers isomorphous with the (111)W face for the coverage in the range from 0.65 to 2.7 titanium monolayers. Measurements of the local work function changes carried out for H 2 adsorption on Ti/(111)W and Ti/(016)W indicate that this adsorption state cannot be recognized as the unstable β + adsorption state observed during the hydrogen adsorption on a surface of bulk titanium [R. Duś, E. Nowicka, Z. Wolfram, Surf. Sci. 269/270 (1992) 545]. The one observed in this experiment is stable and is believed to result from the immersion of the adsorbed hydrogen atoms into the electron gas of the Ti–W interface.

H 2O adsorption on alkali (Li, Na and K) precovered Ni(775)

The coadsorption of H 2O and alkalis (Li, Na and K) on a stepped Ni(s)(111) surface with nominal (775) orientation was studied by UPS (UV-light induced photoelectron spectroscopy), LEED, TDS (thermal desorption spectroscopy) and work function change measurements (Δ Φ). On clean Ni(775) five H 2O desorption states denoted as A ( T m=160 K), B (175 K), C (225 K), D (260 K) and E (340 K) were detected. The A and B states are also observed at similar desorption temperatures on flat Ni(111) and are therefore attributed to H 2O adsorption on the (111) terraces of Ni(775) ( B state) or to the desorption of ice multilayers ( A state). Consequently, the C, D and E states are ascribed to the adsorption and a partial dissociation at step sites. The C state is associated with a “five peak” UPS spectrum interpreted as due to the overlap of the 1b 1, 3a 1 and 1b 2 orbitals of molecular water and 2 σ and 1 π emission from OH groups. A preferred adsorption of the alkalis at step sites for low coverages is deduced from the Δ Φ changes. For Li adsorption the saturation of the step sites is accompanied by a significant change in the slope of the Δ Φ versus coverage curve. The increase of the effective dipole moment (less negative for low coverages, adsorption at step sites) is caused by screening of the positively charged alkalis in a step-down adsorption geometry. For the coadsorption of H 2O and alkalis (at low alkali precoverages) only H 2O desorption states C, D and E from step sites are influenced. The C state shifts to lower peak temperature, whereas the D and E states (recombination from dissociation products) decrease in intensity with increasing alkali coverage. Alkali coadsorption on Ni(775) is accompanied by the dissociation of water, depending on the nature and the precoverage of the alkali. The tendency to induce dissociation is highest for Li and decreases with increasing ionic radius from Na to K.

Adsorption of CO on the ordered Ni 3Al(111) surface

The adsorption of CO on the ordered Ni 3Al(111) surface between 100 and 375 K has been studied using HREELS, TDS, LEED, and work function change measurements (Δ φ). With LEED no additional CO structure could be observed, only a variation of the substrate spot intensities. Three distinct CO desorption states at 440, 370, and 290 K are found, two of which can be directly correlated with adsorption on three-fold ( T des=440 K, ν C–O=226 meV) and on-top sites ( T des=370 K, ν C–O=254 meV), respectively. Below 190 K additional loss peaks appear and are attributed to defect/step-like sites. Both the desorption temperatures and the loss energies are in close agreement with those for CO on Ni(111) and Ni(100). The aluminium atoms in the surface merely act as inert spacers.

Comparison of the initial oxidation of polycrystalline indium and ultrathin deposits of indium on the Au (111) surface

Previous studies have shown that alloying occurs when indium deposits equivalent to a few monolayers are evaporated onto Au (111). This article compares the initial oxidation of the ultrathin alloy to that of polycrystalline bulk indium. Auger electron spectroscopy, electron energy loss spectroscopy, low energy electron diffraction, and work-function change measurements were conducted under ultrahigh vacuum conditions at room temperature. Oxidation of the polycrystalline sample follows first-order Langmuir kinetics, and saturates by 1×103 L (1 L=10−6 Torr s), to form a 4.5 Å thick oxide layer. The stoichiometry of the compound is In2O3. The oxygen uptake of the alloy saturates with less than two layers of In–Au alloy present, and an oxide layer of different stoichiometry is generated, 4.2 Å thick. It is suggested that gold may be participating in the alloy stoichiometry. The results are compared to another study of oxidation of bulk indium.

Interaction of Hydrogen with Vanadium Layers Preadsorbed on Tungsten Field Emitter Tip

Interaction of hydrogen with vanadium layers preadsorbed on a thermally cleaned tungsten field emitter was studied at room temperature and 78 K through measurements of the total work function changes. An increase in the work function followed by its slight decrease at higher exposure can be understood taking into account the possibility of negatively (β ) and positively (β+) polarized adspecies formation on thin vanadium layer. This process leads to vanadium hydride formation. The work function results suggest that hydrogen diffusion into the vanadium layer is meaningful at room temperature. Thermal desorption of hydrogen adsorbate carried out within the temperature range 409-461 K from thin va^iadium layer (θv = 40) provided a value of 127 ± 6 kJ/mol for the activation energy for desorption.

Reaction of nitrogen with fcc- and bcc-iron films on copper(100)

Up to ∼10 ML epitaxial iron films grow on Cu(100) with fcc and beyond ∼10 ML with a bcc structure. As revealed by UPS and work function change (Δ φ) measurements, even at T ad=75 K, nitrogen adsorption leads to spontaneous and complete N 2 dissociation on all of these iron films, followed by the formation of a merely chemisorbed nitrogen layer on iron films of <1.5 ML, but of both chemisorbed and incorporated nitrogen on iron films of >1.5 ML. This phase change is associated with the availability of octahedrally iron-coordinated interstitial sites. The critical exposure for nitrogen incorporation (1) increases with increasing iron-film thickness and (2) is no longer reached at T ad≥150 K due to a decreased N 2 sticking coefficient. Bcc films show a reduced tendency for nitrogen incorporation compared to fcc films.

The adsorption of Al on Pd(001) and of O and CO on an interfacial Al-alloyed Pd(001) surface

The adsorption of ultrathin Al films on Pd(001) and adsorption of oxygen and carbon monoxide on an interfacial Al-alloyed Pd(001) surface have been examined via low energy He + ion scattering, low energy electron diffraction and work function change (ΔΦ) measurements. For low Al coverages at 325 K, interaction between Al and the Pd substrate is observed, resulting in the loss of near surface order by an Al coverage of 0.5 monolayer (ML). The adsorption of Al results in a work function decrease. Annealing Al films with initial coverages from 0.5 ML to larger than 1 ML at 750–950 K leads to interfacial alloying and a stable (2 × 2)p4g LEED pattern dominates the surface structure. This structure has been determined to be a clock rotated (001) Pd top layer above a mixed c(2 × 2) AlPd underlayer. Adsorption of oxygen on the (2 × 2)p4g surface at 325 K induces Al segregation and lifts the reconstruction. In contrast, the interaction of CO with the (2 × 2)p4g surface dose not lift the reconstruction. The implications of these results for relative stability of AlO bond strength are discussed.

Growth and structure of lead overlayers on nickel(100) thermal stability of the adlayer

The adsorption of Pb on Ni(100) has been characterized by Auger electron spectroscopy, low energy electron diffraction, measurements of work function changes (Δφ) and crystal current measurements (ic). The lead layer grows by the Stranski-Krastanov mode. In the submonolayer range four well-organized two-dimensional phases have been found (C(2 × 2), ▪, ▪ and ▪. Isothermal desorption experiments show two distinct zero-order kinetics: the first one is connected to the desorption of the ▪ and ▪ phases, the second one to the desorption of the ▪ phase. In the range of temperature investigated (591–613 K) no desorption of the C(2 × 2) phase is found.

Growth and thermal stability of Pd and Pt adsorption layers on the Ta (211) face

The initial growth of Pd and Pt on Ta(211) was studied at several temperatures by low-energy electron diffraction, Auger electron spectroscopy and work function change measurements. At room temperature Frank-van der Merwe-type growth was observed for Pd. It has been found that at temperature T > 800 K and coverage θ > 2 ML the agglomeration of adsorbate and the formation of large crystallites occurs. In both cases, of Pd and Pt adatoms, the formation of an alloy cannot be excluded. Formation of chains at low coverages has not been observed for the investigated metals.

Adsorption and Reactivity of CO2 on Potassium‐Covered Re(001)

AbstractThe interaction of CO2 with potassium‐covered Re(001) has been investigated. This system has been studied by means of work function (Δϕ), optical second harmonic generation (SHG), and temperature‐programmed desorption (TPD) measurements. Strong electronic interaction between carbon dioxide and potassium is observed upon adsorption at 90 K. This is indicated by a rapid quenching of the SHG signal of K following postadsorption of CO2, with a quenching cross section of 70 Å2. Work function change measurements are consistent with such interaction, evidenced by an undepolarization effect, namely, further decrease of the work function upon CO2 adsorption, below the minimum obtained by pure potassium. In the presence of potassium, the dissociation probability of 0.5 ML adsorbed carbon dioxide increases from 0.5 on the clean metal surface to 0.85 on 1 ML potassium‐covered Re(001), information obtained from TPD measurements following heating to 1250 K. It is concluded that a K–CO2 surface compound is formed upon adsorption at 95 K on the potassium‐covered surface.

The interaction of carbon monoxide with the pure and potassium-promoted Cu(332) surface

The adsorption behaviour of the KCO coadsorbate system has been studied at an adsorption temperature of 95 K on the vicinal Cu(332) surface. Thermal desorption spectroscopy experiments show that the K adatoms disturb the electrostatic potential induced by the steps of the vicinal surface, and therefore the CO molecules adsorbed on step sites are the first to be influenced by the coadsorbed alkali. With K present on the surface in a metal-like form, a strong KCO interaction is evidenced by a stabilization of the adlayer up to temperatures >500 K. Work-function change measurements reveal a change in the bonding of CO towards the precovered surface: the results cannot be interpreted using a simple Blyholder model, but imply a direct interaction of potassium with the 1π orbital of CO. The existence of this direct interaction gives rise to an abolition of the (1π + 5σ) degeneration in the CO-induced states in the UV-photoelectron spectra.

Methyl Formate on Ag(111). 2. Electron-Induced Surface Reactions

Methyl formate, HCOOCH3, molecularly adsorbed at 120 K on Ag(111), readily undergoes electron-induced chemistry. Working mainly with monolayer coverages and 50 eV electrons, we have characterized the products ejected during electron irradiation and desorbed thermally after irradiation. Controlled electron irradiation at 120 K dissociates HCOOCH3, but more than 95% of the C and O is retained, even when the original monolayer is completely dissociated. H2, CH4, CH2O, CO2, and HOCH2CHO (glycolaldehyde) desorb as the temperature is raised. Heating to 450 K leaves a clean Ag(111) surface as measured by X-ray photoelectron spectroscopy and work function change measurements. Primary products are themselves altered by electron irradiation; no new products appear in desorption, but the distribution changes. The data support a two-channel electron dissociation model, one forming methyl (H3C(a)) and formates (HCOO(a) and HCO(a)O(a)), the second forming methoxy (CH3O(a)) and formyl (HC(a)O).

The interaction of methyl chloride with a silicon(111) surface

AbstractIn the technical Müller‐Rochow synthesis, dimethyl dichlorsilane (DMD) is fabricated from methyl chloride which interacts with a ‘contact mass’ consisting of elemental Si and Cu (Cu being a catalyst). In order to better understand some of the elementary steps of this reaction, we have followed the adsorption of methyl chloride on a (7×7) reconstructed Si(111) surface in the temperature range between 100 and 1000 K by means of LEED, Auger electron spectroscopy (AES), temperature‐programmed thermal desorption (TPD), vibrational loss spectroscopy (HREELS), and work function change (Δϕ) measurements. CH3C1 adsorbs effectively in a single weakly chemisorbed state. The adsorption energy is strongly coverage‐dependent and decreases from initially ˜ 40 kJ/mol to ˜ 30 kJ/mol near the monolayer. During adsorption, the work function of the Si(111) surface first increases sharply by 0.36 eV and saturates, after an intermediate maximum of ˜ 40 eV near the monolayer, at ˜ 0.61 eV. CH3C1‐derived vibrational losses appear at 88, 176, 340, and 370 meV and can be associated with known gas phase vibrations of CH3C1. An additional band at 260 meV is tentatively ascribed to the Si‐H stretching vibration, pointing to competing dissociation of adsorbed CH3C1 into hydrogen and methylene chloride CH2C1. Otherwise, CH3C1 does not react with Si(111) under our UHV conditions (CH3C1 partial pressures ≤10−5 mbar, 100 ≤T≤300 K), as the absence of any C1‐ and/or Si‐containing mass fragments in the TPD spectra proves. We deduce from our investigation that the combined chlorination and methylation of Si in the technical Müller‐Rochow process is dictated by processes which run only at much larger (atmospheric) pressures and/or elevated temperatures.

GROWTH INVESTIGATIONS OF ULTRATHIN Ag FILMS ON Pt(111) USING STM AND DYNAMICAL WORK FUNCTION MEASUREMENTS

The growth of Ag on Pt(111) has been studied with in situ work function change measurements and STM at deposition temperatures of 350 K and 520 K. The strain of the compressively stressed Ag monolayer is relieved in the submonolayer regime by the development of two different reconstructions. One of these reconstructions occurs only at higher coverages (Θ&lt;0.85 ML), partially displacing the other reconstruction which is found on all submonolayer Ag films. For coverages beyond one monolayer the first monolayer Ag becomes pseudomorphic with the Pt(111) substrate. Based on a different work function behavior for a deposition at 350 K and 520 K, respectively, we suggest different mechanisms for the disappearance of these reconstructions during the completion of the first Ag layer and the initial growth of the second layer.