Work Function Change Measurements Research Articles

I–V LEED study of Neodymium adsorption on W(110) at submonolayer coverage

The adsorption of neodymium (Nd) atoms on W(110) surface was studied by means of the LEED, AES and work function change measurements. It was found out, that Nd atoms form a set of dilute chain structures at coverages below 1 ML similarly to previously investigated rare earth metals (REM) adsorbed on Mo(110). The absence of particular fractional diffraction proved that the atoms in dilute chain structures form a zigzag not only in the case of the Mo(110) substrate but also for adsorption of REM on W(110). As a result of the glide plane symmetry considerations a modified model was proposed with a different configuration of adatoms in the unit cell of the surface forming the zigzag. To support the new hypothesis an I–V LEED analysis was made for the (6 × 2) Nd/W(110) dilute chain system. Several theoretical structures were tested in dynamical scattering simulations, including a centered structure with inequivalent adsorption sites in consecutive chains and a zigzag chain previously discussed in the literature. Ultimately the I–V LEED experiment validated a new modified zigzag chain model.

Thermal desorption and stability of cobalt phthalocyanine on Ag(100)

By performing work function change (ΔWF) measurements, we characterized thermal stability and desorption of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface from sub-monolayer to multilayer coverages. Based on the temperature dependence of the ΔWF we were able to determine the desorption temperature from multilayer. Obtained dependences of ΔWF and a low-energy electron reflectivity (R) for sub- and monolayer reveal that layers with contact with Ag(100) have higher thermal stability and their desorption is accompanied by decomposition of CoPc molecule. Exploring the time evolutions of the ΔWF at various temperatures allowed us to establish effective activation energies and effective frequency prefactors for processes occurring at various temperatures. The effective activation energies remain almost the same, from sub-monolayer to multilayer (2.97 eV – 2.62 eV), whereas the frequency prefactors vary from 1013 s−1 (monolayer) to 1024 s−1 (multilayer). For multilayer only desorption occurs, whereas for layers in contact with reactive Ag(100) surface (monolayer) the decomposition occurs at the same temperature range as desorption. Low-energy electron diffraction was used to describe CoPc molecular arrangements. To the best of our knowledge, we are the first who have observed (5 × 5)R ± 37° structure for CoPc on Ag(100).

Surface alloy formation by adsorption of holmium on Ag/Mo(112) bimetallic surfaces

Work function change measurements, low energy electron diffraction (LEED) and density functional theory (DFT) are used to determine the structures formed on Ag/Mo(112) bimetallic surfaces upon deposition of 0.5 monolayer (ML) of holmium. As the bimetallic surfaces, we have chosen the Mo(112) substrate covered with 1 or 2 ML of Ag. Such surfaces have the same symmetry as the Mo(112) face but different electronic properties. LEED experiment indicates that the c(2 × 2) structure is formed on (1 ML Ag)/Mo(112) bimetallic surface upon deposition of 0.5 ML of Ho. DFT calculations show that a type of Ag–Ho surface alloy is formed, with Ho atoms 0.6 Å below the distorted layer of Ag. This is neither a substitutional nor a subsurface alloy. It is found that the adsorption structure formed on the (2 ML Ag)/Mo(112) bimetallic surface depends on the annealing temperature. After deposition of 0.5 ML of Ho at 300 K, the LEED pattern of p(2 × 2) symmetry is observed. Annealing of the overlayer at 640 K irreversibly changes the p(2 × 2) pattern into a pattern of c(2 × 2) type. The results of DFT computations show that the c(2 × 2) structure of the Ag–Ho surface alloy is energetically most favorable. In this structure, 0.5 ML of Ho is between the two monolayers of Ag, and the symmetry of the topmost layer is changed. The work function change calculated for the c(2 × 2) structure is in a good agreement with the measured value (0.22 eV). The results show that adsorption of Ho on the Ag/Mo(112) bimetallic surfaces is substantially different than on the clean Mo(112).

Effects of ion irradiation damage on the initial interactions of oxygen with polycrystalline gadolinium

The effects of surface and near-surface defects, induced by Ar+ ion irradiation, on the interactions of oxygen with polycrystalline gadolinium surfaces were investigated over the temperature range 140–300K, utilizing Auger electron spectroscopy, direct recoil spectrometry, work function change measurements and density functional theory calculations. This enabled the distinction between the topmost surface and subsurface processes involved in this reaction. It turns out that the formation of bulk gadolinium oxide (at the near-surface region) involves in general three sequential steps: (i) surface random chemisorption, (ii) clustering of the chemisorbed atoms and the formation of a type of “surface oxide”, (iii) dissolution of surface oxygen atoms into the sub-surface and nucleation of the “bulk” oxide. However, the time duration of these steps becomes shorter (i.e. faster kinetics) as the corresponding rate controlling parameters (i.e. surface reactivity and temperature) become more favorable. Consequently, all the above steps are discernable only for the less favorable kinetic parameters (annealed surfaces at low temperatures). For the more active surfaces (sputtered or annealed at higher temperatures) the very initial random surface chemisorption step is practically absent and a two-step process occurs, with an initial stage associated with direct clustering and formation of the “surface oxide”, followed by the subsurface penetration and bulk oxide formation. It was also found that the irradiation induced damage does not affect significantly the topmost surface accumulation of oxygen. However, it accelerates appreciably the penetration of oxygen into the near-surface region and the development of the bulk oxide.

Note: Work function change measurement via improved Anderson method.

We propose the modification to the Anderson method of work function change (Δϕ) measurements. In this technique, the kinetic energy of the probing electrons is already low enough for non-destructive investigation of delicate molecular systems. However, in our implementation, all electrodes including filament of the electron gun are polarized positively. As a consequence, electron bombardment of any elements of experimental system is eliminated. Our modification improves cleanliness of the ultra-high vacuum system. As an illustration of the solution capabilities, we present Δϕ of the Ag(100) surface induced by cobalt phthalocyanine layers.

Interaction of Hydrogen with a Cobalt(0001) Surface

Abstract The interaction of hydrogen and deuterium with the Co(0001) surface has been studied in UHV between 90 and 500 K by means of LEED, temperature-programmed thermal desorption (TPD) and work function change (ΔΦ) measurements. Hydrogen adsorbs spontaneously and dissociatively in two atomic binding states denoted as β 1 and β 2 with a high initial sticking probability. The adsorption energies E(β 1) and E(β 2) are 80 and 100 kJ/mol, respectively, if a first-order desorption kinetics is assumed (a second-order kinetic analysis yields unreasonably low values for both the desorption energies and frequency factors). At sufficiently low temperatures, the adsorbed H atoms form a faint (2 × 2) LEED superstructure, which is best developed after an exposure of ∼20 L. From the temperature dependence of the fractional-order beam intensity at different exposures we determine the critical temperature of the (2 × 2) phase as ∼ 243 ( ± 10) K. Similar to the H-on-Ni(111) system the existence range of the (2 × 2) phase in the temperature–coverage plane is asymmetric; i.e., below the critical coverage Θ crit the respective long-range order has a higher range of stability than above Θ crit. By analogy with H/Ni(111), we assume a similar honeycomb H structure also for the H-on-Co(0001) system and suggest the critical coverage Θ crit to be 0.5, i.e., half a monolayer of H atoms. The H-induced work function change is surprisingly small; it decreases, forms a shallow minimum of −18 meV after ∼20 L exposure around the optimum coverage of the (2 × 2) phase and reaches a saturation value of −10 meV. Our data are discussed and compared with previous work on H/Co(0001) and other close-packed transition metal surfaces, especially with the H-on-Ni(111) system.

Annealing Effects after Nitrogen Ion Casting on Monolayer and Multilayer Graphene

The modification of the electronic properties of a coverage-dependent graphene layer by nitrogen ions irradiation was investigated using core-level photoemission spectroscopy (CLPES). Here we describe preparation of monolayer and multilayer epitaxial graphene (EG) functionalized by nitrogen ions irradiation with 100 eV to minimize the damage of graphene layer as we annealed up to 1300 K to track the surface property changes using CLPES. As a result, on the monolayer EG, we found that pyridinic nitrogen mainly existed on the surface. On the multilayer EG, we observed the formation of graphitic nitrogen remaining as a major species confirmed using N 1s core-level spectra. Through a work function change (ΔΦ) measurement, both systems indicated p-type doping properties with 4.71 (monolayer EG) and 4.87 eV (multilayer EG) of work function values after N2 ion irradiation. Interestingly, we observed that monolayer EG maintained its p-type doping character, whereas multilayer EG changed the doping character from p-type to n-type as we annealed both systems up to 1300 K due to the discrepancy of the electron charge transfer. We will systematically demonstrate these variations of electronic properties for both monolayer and multilayer EG.

An Au clusters related spill-over sensitization mechanism in SnO2-based gas sensors identified by operando HERFD-XAS, work function changes, DC resistance and catalytic conversion studies

The role of Au additives in SnO(2)-based thick film gas sensors was investigated by a combination of operando investigation techniques, namely spectroscopic high energy resolved fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and simultaneous DC resistance and work function change measurements. The results have shown that the Au is present in the form of small metallic particles at the surface of the host metal oxide without changing its bulk or surface electronic properties. The sensitization effect of Au can therefore be attributed to the "spill-over effect", meaning that the Au particles enrich the surface of the active metal oxide with oxygen species which consequently react with reducing gases such as CO and H(2). This is in contrast to the effect of Pd and Pt promoters which were found to be distributed at an atomic level on the surface and in the bulk of the supporting sensing material and therefore have a tremendous effect on its bulk and surface electronic properties.

A surface work function measurement technique utilizing constant deflected grazing electron trajectories: Oxygen uptake on Cu(001)

We report on the application of a novel nondestructive in-vacuum technique for relative work function measurements, employing a grazing incidence electron deflection above a sample with a planar surface. Two deflected electron beam detectors are used as a position sensitive detector to control feedback to the sample potential as the sample work function changes. With feedback the sample potential exactly follows the surface sample-size averaged work function variation, so that the deflected beam trajectory remains stable. We also discuss methods to optimize the initial electron trajectories for this method, so as to minimize unwanted effects such as from uncontrolled external magnetic fields. As the electron beam does not impinge on the surface in this new technique electron induced desorption, ionization, dissociation, and/or decomposition is not induced at the interface. Importantly also the technique allows for free access to the surfaces enabling simultaneous deposition/evaporation and/or application of other surface characterization methods. We demonstrate its application in concurrent measurements of helium atom reflectivity and work function changes taking place during molecular oxygen exposure of a Cu(001) surface. A work function measurement sensitivity and stability is demonstrated at ∼10 mV at a sampling rate of 1 Hz and after application of an ∼7 s smoothing routine. In comparison to the helium atom reflectivity measurements, the work function measurements are more sensitive to the initial O uptake, and less so to the final coverage variations and possible surface reordering at higher O coverages.

Interaction of Ethyl Chloride with Amorphous Solid Water Thin Film on Ru(001) and O/Ru(001) Surfaces

The adsorption of ethyl chloride (EC) on clean and oxygen covered Ru(001) surfaces and its interaction with coadsorbed amorphous solid water (ASW) is reported based on temperature-programmed desorption (TPD) and work function change (DeltaPhi) measurements. Adsorption of EC is characterized by a decrease of 2.1 eV in work function at monolayer coverage. Flipped adsorption geometry on average (ethyl facing the surface) is found in the second layer with a 0.3 eV increase in the work function. On ruthenium substrates the orientation of EC molecules could be controlled chlorine down or up by varying the oxygen coverage, as indicated by work function change measurements. DeltaP-TPD from clean Ru(001) surface reveals a complete dissociation of the EC molecules on the clean Ru(001) surface at coverages up to 0.1 +/- 0.05 ML. At higher coverage it leads to two distinct TPD peaks: at T =175 and 120-130 K, for the monolayer and multilayer coverage, respectively. Compression of preadsorbed 0.3 ML EC molecules into small islands on the surface occurs when ASW layers at coverage in the range 0.5-6 bilayers (BL) are adsorbed on top of the EC covered surface, associated with a shift in peak desorption from 190 K down to 125 K. A Caging process of EC molecules within the layers of ASW takes place by adsorbing more than 6 BL until a fully caged system has developed above 20 ASW BL. A reversed TPD peak shift of the trapped EC molecules occurs from its compressed phase at 125 K up to 165 K, the onset for ASW transition to crystalline ice and desorption. The detachment process leads to partial flipping over the geometry of the EC at the second layer, as determined by DeltaPhi measurement. The distance of the lifted and caged EC molecules from the Ru surface has been determined to reside 1.0 +/- 0.2 nm above the solid surface, based on the thermal dissociation and hydrogen uptake measurements of sandwiched EC molecules between a variable thickness ASW film and a fixed 10 BL layer, on top of clean Ru(001). We conclude that the caged EC molecules are trapped in a micelle-like geometry with the ethyl groups pointing inward. This conclusion is based on photodecomposition studies of the caged EC molecules at 193 nm, as a function of EC initial coverage.

Coadsorbate effects on adsorbate vibrational properties

A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability α v of an adsorbate mode and of the dielectric screening ϵ due to this extra species is presented. Four (ternary) coadsorption systems consisting of the Ru(0 0 0 1)-(2 × 2)-(X + CO + O) layer with additional coadsorbates X = H, NO, CO, or O (all of them occupying the remaining empty fcc site) have been studied with FT-IRAS, TDS, LEED and work function change measurements. On-top CO is thereby used as a probe molecule to monitor coadsorbate effects on the dielectric properties of the layer. The vibrational polarizability α v associated with the internal C–O stretch mode ( ν C–O) of on-top CO is lowered by all coadsorbates. The dielectric screening ϵ within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ϵ is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of α v and dielectric screening ϵ, as well as the involved line shifts of ν C–O and ν Ru–CO can be understood in terms of the standard Blyholder backbonding model, i.e. CO 5 σ charge donation to the metal combined with a backdonation to electronic states with 2π∗ character.

Deformations of charge-density wave crystals under electric field

Abstract We report the effects of electric field induced deformations of quasi one-dimensional conductors with charge-density wave (CDW). The most pronounced sort of deformation is torsional strain (TS). The TS is found to comprise two contributions. The features of the 1st—the larger one—are threshold hysteretic dependence on electric field and high relaxation time τ : For o-TaS 3 τ ∼10 −2 s at T =80 K and falls as exp(900 K/ T ) with increasing T . The 2nd contribution is linear in electric field and does not drop with frequency increase. The amplitude of this contribution falls abruptly with T approaching the Peierls transition temperature T P from below. Similar features of TS are demonstrated for other CDW compounds: (TaSe 4 ) 2 I, K 0.3 MoO 3 and NbS 3 type II, for which T P ≈360 K. We attribute the 1st and the 2nd contributions to large (hysteretic) and small (near-equilibrium) CDW deformations, respectively, likely—shear at the surface. The TS is observed also above T P : For TaS 3 and (TaSe 4 ) 2 I typical torsional amplitude is 10 −1 °/V in the resonance regimes, corresponding to the piezomodulus ∼10 −9 m/V. A separate study of TS was performed at room temperature with AFM technique. Apart from this (‘intrinsic’) effect, we observe electrostatic contribution to the TS. In contrast to the intrinsic response, the electrostatic one is proportional to the potential either over the sample, or over an additional electrode (‘gate’) placed nearby, but not to the difference of potentials between the sample ends. It is typically 2 orders of magnitude less. The intrinsic TS reveals a new electromechanical effect at room temperature, presumably associated with the excitations of the pinned mode of the CDW fluctuations. Its observation opens prospects for application of quasi one-dimensional conductors as micro- and nano-actuators. Basing on the electrostatic-induced TS we propose prototypes of electrometers and a technique for measurements of work function changes. Using a cryogenic transmission electronic microscope we observe also flexural deformation of TaS 3 in the double-clamped mode, visible both in resonant and static modes.

Adsorption of Sm and Gd on the Mo(1 1 1) face

Adsorption and thermal stability of Sm and Gd thin layers on Mo(1 1 1) surface was investigated using Auger electron spectroscopy (AES), low electron energy diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoemission spectroscopy and work function change measurements (Δ ϕ). At 300 K the first three layers of Sm grows in the monolayer-by-monolayer mode. Sm forms initially a pseudomorphic layer of trivalent atoms, which is probably completed to a densely packed layer in part by divalent Sm atoms. In contrast, the second Sm monolayer is entirely composed of divalent atoms. Gd also grows in the monolayer-by-monolayer mode in the investigated coverage range. At higher temperatures (800 K for Gd and 530 K for Sm) only the first layer is thermally stable. Both adsorbates did not cause surface reconstruction or faceting

Hydrogen adsorption on rhodium: Hydride formed on the surface of thin rhodium films

H 2 interaction with thin Rh films deposited on Pyrex glass under UHV conditions has been studied by simultaneous measurement of work function changes Δ Φ and hydrogen pressure P, at selected constant temperatures: 78 and 298 K. Prior to the adsorption experiments the thin film topography was illustrated using the AFM and STM methods. The influence of hydrogen adsorption on the resistance of thin Rh film was examined in the course of an independent experiment. The number of sites accessible for adsorption on the thin Rh film surface was found determining population of oxygen adatoms within the monolayer at 78 K, when incorporation of these adspecies below the surface is negligible. It was established that at all examined temperatures hydrogen adsorption led to coverage Θ approaching 1 under equilibrium pressure below 10 −3 Pa, increasing the work function. Under higher H 2 pressure an additional uptake of hydrogen leading to Θ ∼ 1.68 at 298 K, and to Θ ∼ 2 at 78 K is reached. On this surface at low temperatures there exist weakly bound, reversibly adsorbed, positively charged adspecies characteristic for hydrogen adsorption on transition metal hydrides. The change of thin Rh film resistance caused by hydrogen adsorption was not measurable.

The Chemistry of Trimethylamine on Ru(001) and O/Ru(001)

The interaction and reactivity of trimethylamine (TMA) has been studied over clean and oxygen-covered Ru(001) under UHV conditions, as a model for the chemistry of high-density hydrocarbons on a catalytic surface. The molecule adsorbs intact at surface temperature below 100 K with the nitrogen end directed toward the surface, as indicated from work function change measurements. At coverage less than 0.05 ML (relative to the Ru substrate atoms), TMA fully dissociates upon surface heating, with hydrogen as the only evolving molecule following temperature-programmed reaction/desorption (TPR/TPD). At higher coverage, the parent molecule desorbs, and its desorption peak shifts down from 270 K to 115 K upon completion of the first monolayer, indicating a strong repulsion among neighbor molecules. The dipole moment of an adsorbed TMA molecule has been estimated from work function study to be 1.4 D. Oxygen precoverage on the ruthenium surface has shown efficient reactivity with TMA. It shifts the surface chemistry toward the production of various oxygen-containing stable molecules such as H2CO, CO2, and CO that desorb between 200 and 600 K, respectively. TMA at a coverage of 0.5 ML practically cleans off the surface from its oxygen atoms as a result of TPR up to 1650 K, in contrast to CO oxidation on the O/Ru(001) surface. The overall reactivity of TMA on the oxidized ruthenium surface has been described as a multistep reaction mechanism.

Formation of surface adspecies at furan and tetrahydrofuran adsorption on the Pt surface studied by work function changes and Auger electron spectroscopy

The adsorption of tetrahydrofuran (THF) and furan (F) on polycrystalline Pt-foil has been studied by work function change (Δ φ) measurements and Auger electron spectroscopy (AES). The results were compared with those obtained earlier for the adsorption of cyclic hydrocarbons. The general character of the adsorption process (i.e. the dependence of ∣Δ φ∣ and C/Pt ratio on temperature and exposure, and the ∣Δ φ∣ vs. C/Pt correlation) is similar to the one observed during adsorption of cyclic hydrocarbons. THF produces σ-bonded adspecies in a similar manner to cyclopentane. F forms first a π-bonded surface complex, as do benzene and toluene. The π-adsorbed species are then transformed into σ-bonded ones under exposure to excess F. The numeric values characterizing the adsorption of the two heterocycles (mainly the C/Pt values) differ substantially from those measured for the adsorption of cyclic hydrocarbons.

Measurement and modeling of work function changes during low energy cesium sputtering

In this paper, a H-terminated silicon wafer was bombarded by low energy cesium ions during ToF-SIMS analysis and work function variations of the target were measured for different analysis conditions. This measurement was performed by measuring the shift of the secondary ions energy distributions with a reflectron type analyzer. At first, the silicon’s work function change was found to be −2.3 eV during 500 eV Cs + bombardment at 45°. This effect is due to the creation of a dipolar layer at the surface of the silicon by the implanted cesium. Then the work function variation was measured at 300 eV for varying cesium surface concentrations. The work function was found to decrease monotonously with the increasing cesium surface concentration, as during cesium adsorption experiments. The results were modeled following three different approaches and the value of the effective polarizability α of cesium was found to be equal to 1.9 × 10 −39 C m 2/V. Finally, the effect of the bombardment energy on the work function variation was studied for beams with energies ranging from 250 to 2000 eV. The effective polarizability of cesium was found to increase with increasing Cs beam energy.

Basics of oxygen and SnO 2 interaction; work function change and conductivity measurements

The interaction of oxygen and nanocrystalline SnO 2 has been investigated by means of simultaneous work function change and resistance measurements. Various oxygen concentrations between 50 ppm and 6000 ppm have been dosed at two different temperatures. At 400 °C, only ionic species have been identified. However, at 200 °C dipolar species play the major role reflected by a massive decrease in electron affinity.

Chemical and structural components of work function changes in the process of palladium hydride formation within thin Pd film

Surface phenomena which occur in the process of palladium hydride PdH x formation during H 2 interaction “in situ” with thin palladium films were studied by means of two methods: (i) measurements of work function changes Δ Φ , and (ii) determination of topographical images using atomic force microscopy (AFM). A very strong decrease of the work function up to ∼ 2 V was registered in the course of the reaction at 78 K when thin Pd films deposited on Pyrex glass were applied. However a small Δ Φ was noticed when using Pd films deposited on glass precovered with a thick platinum film. AFM topography exhibited a net of distinct protrusions in the first case, but the topography was almost unchanged in the second. We suggest that the above unusually high decrease of the work function in the process of PdH x formation within a Pd/glass system is caused by the change of thin Pd film topography, and not by very strong polarization of hydrogen adspecies on PdH x surface.

Fundamental studies on SnO 2 by means of simultaneous work function change and conduction measurements

Simultaneous work function change and conduction measurements have been performed on undoped SnO 2 thick film sensors in order to investigate the surface reactions of CO under dry and humid conditions. On their basis, changes in the electron affinity have been observed for the reaction with water and the reaction with CO in the background of water. Additionally, by comparing the results for the changes in band bending, one can conclude that both water and CO compete for the same reaction sites, i.e. preadsorbed oxygen species. The kinetics of work function and band bending changes show that in the case of humid air more than one dipole species is involved in the reaction with CO.