Geometric and electronic structure of positively and negatively poled LiNbO3 (0 0 0 1) surfaces

The effect of ferroelectric poling direction on the structure and electronic properties of the LiNbO₃ (0 0 0 1) surface was characterized. Low energy and reflection high energy electron diffraction indicated that both the positively and negatively poled surfaces were (1 × 1) with no evidence of longer range periodic reconstructions. Low energy ion scattering spectra from both surfaces were dominated by scattering from oxygen atoms. X-ray and ultraviolet photoelectron spectra also showed little difference between the positively and negatively poled surfaces, with the exception of a high binding energy shoulder on the O 1s core level of the negative surface. Exposure of the surfaces to atomic hydrogen caused reduction of the surface Nb rather than an increase in intensity on the high binding energy side of the O 1s peak, indicating that the shoulder on the O 1s peak on the negative surface was not due to surface hydroxyl groups. Temperature programmed desorption measurements indicated that the nearly stoichiometric LiNbO₃ samples were susceptible to loss of Li₂O starting at temperatures as low as 500 K, independent of the poling direction. An adatom/vacancy model is proposed in which oxygen ad-anions accumulate on one side of the crystal while oxygen anion vacancies are created on the opposite surface. This model can explain the apparent oxygen termination of both surfaces and the observed (1 × 1) periodicity of the surfaces, and also effectively screens the thickness dependent electric field associated with the polar orientation of the crystal.     

CO adsorption on the reduced RuO2(1 1 0) surface

The O-bridge atoms on a stoichiometric RuO₂(1 1 0) surface were removed by reaction with CO. The resulting reduced surface was then further exposed to CO. By means of thermal desorption spectroscopy and high-resolution electron energy-loss spectroscopy three adsorbed CO states were identified on bridge sites and assigned to double-bonded, single-bonded, and single-bonded species in the vicinity of O-bridge residues, respectively.     

Interaction of H2O with the RuO2(1 1 0) surface studied by HREELS and TDS

The adsorption of water on a RuO₂(1 1 0) surface was studied by using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). The first thermal desorption peak observed between 350 and 425 K is attributed to molecular water adsorbed on fivefold coordinated Ru_cus sites. Higher coverages of water give rise to TDS peaks between 190 and 160 K, which we attribute to water in the second layer bound to bridge oxygen, and multilayers, respectively. HREELS shows that H₂O chemisorbs on Ru_cus sites through oxygen inducing a slight red shift of the vibrational frequency of O_bridge atoms. Molecular adsorption is also confirmed by the presence of both the scissor and the libration modes showing the expected isotopic shift for D₂O. The water adsorbed on the Ru_cus sites also forms hydrogen bonds with the bridge oxygen indicated by the broad intensity at the lower frequency side of the O-H stretch mode. HREELS and TDS results suggest that on the perfect RuO₂(1 1 0) surface water dissociation is almost negligible.     

Comparison of the interaction of Pd with positively and negatively poled LiNbO3(0 0 0 1)

To investigate the possibility of manipulating the surface chemical properties of finely dispersed metal films through ferroelectric polarization, the interaction of palladium with oppositely poled LiNbO₃(0 0 0 1) substrates was characterized. Low energy ion scattering indicated that the Pd tended to form three-dimensional clusters on both positively and negatively poled substrates even at the lowest coverages. X-ray photoelectron spectroscopy (XPS) showed an upward shift in the binding energy of the Pd 3d core levels of 0.9 eV at the lowest Pd coverages, which slowly decayed toward the bulk value with increasing Pd coverage. These shifts were independent of the poling direction of the substrate and similar to those attributed to cluster size effects on inert supports. Thus, the spectroscopic data suggested that Pd does not interact strongly with LiNbO₃ surfaces. The surface chemical properties of the Pd clusters were investigated using CO temperature programmed desorption. On both positively and negatively poled substrates, CO desorption from freshly deposited Pd showed a splitting of the broad 460 K desorption peak characteristic of bulk Pd into distinct peaks at 270 and 490 K as the Pd coverage was decreased below 1.0 ML; behavior that also resembles that seen on inert supports. It was found that a small fraction of the adsorbed CO may dissociate (<2%) for Pd on both positively and negatively poled substrates. The thermal response of the smaller Pd clusters on the LiNbO₃ surfaces, however, was different from that of inert substrates. In a manner similar to Nb₂O₅, when CO desorption experiments were carried out a second time, the adsorption capacity decreased and the higher temperature desorption peak shifted from 490 K to below 450 K. This behavior was independent of the substrate poling direction. Thus, while there was evidence that LiNbO₃ does not behave as a completely inert support, no significant differences between positively and negatively poled surfaces were observed. This lack of sensitivity of the surface properties of the Pd to the poling direction of the substrate is attributed to the three-dimensional Pd clusters being too thick for their surfaces to be influenced by the polarization of the underlying substrate.     

Adsorbate-substrate bonding and the growth of naphthalene thin films on Ag(1 1 1)

The adsorption kinetics, energetics and growth of naphthalene thin films, from submonolayer to about 10 layers, on a Ag(1 1 1) surface at low temperature in a ultrahigh vacuum chamber are examined by using temperature programmed desorption spectroscopy. The first layer adsorption occurs with a desorption energy of 85 ± 5 kJ/mole and results in an interface dipole of 5 ± 1 D, from charge transfer of approximately 0.2 e from naphthalene to Ag. The surface dipole induced inter-adsorbate repulsion causes the lowering of the adsorption energy within the first layer near the saturation coverage so that the second layer deposition begins before the completion of the first layer. The second layer is a metastable phase with desorption energy, 74 ± 3 kJ/mole, smaller than the multilayer desorption energy of 79 ± 5 kJ/mole. Fractional order desorption kinetics were found for both the metastable and the multilayer phases, suggesting desorption from 2-D islanding and 3-D islanding, respectively.     

Autocatalytic partial reduction of FeO(1 1 1) and Fe3O4(1 1 1) films by atomic hydrogen

The interaction of atomic hydrogen with thin epitaxial FeO(1 1 1) and Fe₃O₄(1 1 1) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of 1/4 monolayer (ML) of oxygen with a 2 × 2 pattern appearing in LEED. This FeO_0.75 film is passive against further reduction. The first reduction step saturates after removal of ∼0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2 × 2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe₃O₄ film, irregular desorption bursts of H₂O and H₂ were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts.     

The chemistry of C2 perfluoroalkyl iodide on the Cu(1 1 1) single crystal surface

The thermal chemistry of perfluoroethyl iodide (C₂F₅I) adsorbed on Cu(1 1 1) has been investigated by temperature-programmed reaction/desorption (TPR/D), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). I 4d and F 1s XPS spectra show that dissociative adsorption of C₂F₅I to form the surface-bound perfluroethyl (Cu-C₂F₅) moieties occurs at very low temperature (T < 90 K), while the C-F bond cleavage in adsorbed perfluroethyl (Cu-C₂F₅) begins at ca. 300 K. XPS and TPR/D studies further reveal that the reactions of ^βCF₃^αCF_2(ad) on Cu(1 1 1) are strongly dependent on the surface coverage. At high coverages (?0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) evolves, via α-F elimination, into the surface-bound tetrafluoroethylidene moieties (CuCF-CF₃) followed by a dimerization step to form octafluoro-2-butene (CF₃CFCFCF₃) at 315 K as gas product. The surface-bound (Cu-C₂F₅) decomposes preferentially, at low coverages (?0.04 L), via consecutive α-F abstraction to afford intermediate, trifluoroethylidyne (CuCCF₃), resulting in the final coupling reaction to yield hexafluoro-2-butyne (CF₃CCCF₃) at 425 K. However, at middle coverages (ca. 0.08-0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) first experiences an α-F elimination and then prefers to loss the second F from β position to yield the intermediate of Cu-CF₂-CFCu (μ-η,η-perfluorovinyl), which may further evolve into hexafluorocyclobutene (CF₂CFCFCF₂) at 350 K through cyclodimerization reaction. Our results have also shown that the surface reactions to yield the products, CF₃CFCFCF₃ and CF₃CCCF₃, obey first-order kinetics, whereas the formation of CF₂CFCFCF₂ follows second-order kinetics.     

An example of chemistry-morphology interaction: making up for the geometric and energetic heterogeneities of the (1 0 0) surface of single crystalline silicon by high-temperature treatments in H2

A lot of work has been carried out to prepare chemically homogeneous (1 0 0) silicon surfaces. The hydrogen-terminated (1 0 0) silicon surfaces are the most promising ones, especially in view of their remarkable environmental stability. The simplest way to produce hydrogen-terminated surfaces (attack in water solution of HF of a sacrificial, thermally grown, oxide) results in strongly heterogeneous rough surfaces (although with prevailing dihydride terminations). These surfaces can, however, be flattened and homogenized by treating them in H₂ at high temperature (>850 C). The morphological and chemical changes undergone by the surface during the treatment are studied X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunnelling microscopy, infrared absorption spectroscopy in the attenuated total reflection mode, reflection high energy electron diffraction and thermal programmed desorption, and the mechanisms responsible for them are discussed.     

Adsorption and reaction of bicyclic hydrocarbons at Pt(1 1 1) and Sn/Pt(1 1 1) surface alloys: trans-decahydronaphthalene (C10H18) and bicyclohexane (C12H22)

Adsorption and desorption of trans-decahydronaphthalene (C₁₀H₁₈) and bicyclohexane (C₁₂H₂₂) can be used to probe important aspects of non-specific dehydrogenation leading to surface carbon accumulation and establish better estimates of activation energies for C-H bond cleavage at Pt-Sn alloys. This chemistry was studied on Pt(1 1 1) and the (2 × 2)-Sn/Pt(1 1 1) and (√3 × √3)R30°-Sn/Pt(1 1 1) surface alloys by using temperature programmed desorption (TPD) mass spectroscopy and Auger electron spectroscopy (AES). These hydrocarbons are reactive on Pt(1 1 1) surfaces and fully dehydrogenate at low coverages to produce H₂ and surface carbon during TPD. At monolayer coverage, 87% of adsorbed C₁₀H₁₈ and 75% C₁₂H₂₂ on Pt(1 1 1) desorb with activation energies of 70 and 75 kJ/mol, respectively. Decomposition of C₁₀H₁₈ is totally inhibited during TPD on these Sn/Pt(1 1 1) alloys and decomposition of C₁₂H₂₂ is reduced to 10% of the monolayer coverage on the (2 × 2)-Sn/Pt(1 1 1) alloy and totally inhibited on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy. C₁₀H₁₈ and C₁₂H₂₂ are more weakly chemsorbed on these two alloys compared to Pt(1 1 1) and these molecules desorb in narrow peaks characteristic of each surface with activation energies of 65 and 73 kJ/mol on the (2 × 2) alloy and 60 and 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy, respectively. Alloyed Sn has little influence on the monolayer saturation coverage of these two molecules, and this is decreased only slightly on these two Sn/Pt(1 1 1) alloys. The use of these two probe molecules enables an improved estimate of the activation energy barriers E* to break aliphatic C-H bonds in alkanes on Sn/Pt alloys; E* = 65-73 kJ/mol on the (2 × 2)-Sn/Pt(1 1 1) alloy and E* ? 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy.     

Adsorption of ethene on Pt(1 1 1) and ordered PtxSn/Pt(1 1 1) surface alloys: A comparative HREELS and DFT investigation

The adsorption of ethene (C₂H₄) on Pt(1 1 1) and the Pt₃Sn/Pt(1 1 1) and Pt₂Sn/Pt(1 1 1) surface alloys has been investigated experimentally by high-resolution electron energy loss spectroscopy and temperature programmed desorption. The experimental results have been compared with density functional theory (DFT) calculations allowing us to perform a complete assignment of all vibration modes and loss features to the species present on the surfaces. On Pt(1 1 1) as well as on the Pt-Sn surface alloys an η² di-σ-bonded conformation of ethene has been found to be the most stable adsorbed form. In addition to this majority species a minor amount of π-bonded ethene has been identified, which is more abundant on the Pt₂Sn surface alloy than on the other surfaces. Additionally the HREELS spectra of ethene on Pt(1 1 1) and the Pt-Sn surface alloys differ only slightly in terms of the energetic positions of the loss peaks.     

The surface structure of Fe3O4(1 1 1) films as studied by CO adsorption

We have studied adsorption of CO on Fe₃O₄(1 1 1) films grown on a Pt(1 1 1) substrate by temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (HREELS). Three adsorption states are observed, from which CO desorbs at ∼110, 180, and 230 K. CO adsorbed in these states exhibits stretching frequencies at ∼2115-2140, 2080 and 2207 cm⁻¹, respectively. The adsorption results are discussed in terms of different structural models previously reported. We suggest that the Fe₃O₄(1 1 1) surface is terminated by 1/2 ML of iron, with an outermost 1/4 ML consisting of octahedral Fe²⁺ cations situated above an 1/4 ML of tetrahedral Fe³⁺ ions, in agreement with previous theoretical calculations. The most strongly bound CO is assigned to adsorption to Fe³⁺ cations present on the step edges.     

Steric effect of methyl, methoxy, and ethyl substituents on the excimer formation of naphthalene on Al2O3(0 0 0 1)

Structural relaxation of molecules on surfaces can be monitored by observing the spectral evolution as increasing amount of thermal energy is made available. Naphthalene forms excimers in the excited state when amorphously prepared, but relaxes to a more ordered state when heated. Substituent groups on the naphthalene can substantially alter the available pathways during the thermally induced structural relaxation, and is reflected in the formation of excimer or trap fluorescence. A general correlation was determined for the naphthalene substitution with methyl, methoxy, and ethyl groups on the 1- and 2-positions with the pathway taken by the molecular adlayer.     

Comparison of the adsorption of N2 on Ru(1 0 9) and Ru(0 0 1) - A detailed look at the role of atomic step and terrace sites

We present a direct side-by-side comparison of the adsorption and desorption of nitrogen on the atomically-stepped Ru(1 0 9) surface and the atomically-flat Ru(0 0 1) surface. Both infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) are employed in this study, along with density functional theory (DFT). We find that the chemisorptive terminal binding of N₂ is stronger on the atomic step sites than on the terrace sites of Ru(1 0 9) as indicated by TPD and by a reduction of the singleton vibrational frequency, ν(N₂), by ∼9 cm⁻¹, comparing steps to terraces. In addition, we find that metal-metal compression effects on the terrace sites of Ru(1 0 9) cause stronger binding of N₂ than found on the Ru(0 0 1) surface, as indicated by a reduction of the terrace-N₂ singleton vibrational frequency by ∼11 cm⁻¹ when compared to the singleton N₂ mode on Ru(0 0 1). These spectroscopic results, comparing compressed terrace sites to Ru(0 0 1) sites and confirmed by TPD and DFT, indicate that N₂ bonds primarily as a σ-donor to Ru. Using equimolar ¹⁵N₂ and ¹⁴N₂, it is found that dynamic dipole coupling effects present at higher N₂ coverages may be partially eliminated by isotopically detuning neighbor oscillators. These experiments, considered together, indicate that the order of the bonding strength for terminal-N₂ on Ru is: atomic steps > atomic terraces > Ru(0 0 1). DFT calculations also show that 4-fold coordinated N₂ may be stabilized in several structures on the double-atom wide steps of Ru(1 0 9) and that this form of bonding produces substantial decreases in the N₂ vibrational frequency and increases in the binding energy, compared to terminally-bound N₂. These highly coordinated N₂ species are not observed by IRAS.     

Reactivity of monolayer V2O5 films on TiO2(1 1 0) produced via the oxidation of vapor-deposited vanadium

The growth, and reactivity of monolayer V₂O₅ films supported on TiO₂(1 1 0) produced via the oxidation of vapor-deposited vanadium were studied using X-ray photoelectron spectroscopy and temperature programmed desorption (TPD). Oxidation of vapor-deposited vanadium in 10⁻⁷ Torr of O₂ at 600 K produced vanadia films that contained primarily V³⁺, while oxidation in 10⁻³ Torr at 400 K produced films that contained primarily V⁵⁺. The reactivity of the supported vanadia layers for the oxidation of methanol to formaldehyde was studied using TPD. The activity for this reaction was found to be a function of the oxidation state of the vanadium cations in the film.     

Adsorption of iso-butane on ZnO(0 0 0 1)-Zn

Presented are thermal desorption spectroscopy (TDS) and adsorption probability measurements of iso-butane on the Zn-terminated surface of ZnO. The initial adsorption probability, S₀, decreases linearly from 0.57 to 0.22 (±0.02) with impact energy, E_i = 0.74-1.92 eV, and is independent of adsorption temperature, T_s = 91-114 K (±5 K), indicating non-activated molecular adsorption. The coverage, Θ, dependent adsorption probabilities, S(Θ), show a cross-over from adsorbate-assisted adsorption (S increases with Θ) to Kisliuk-like dynamics at about the desorption temperature of iso-butane bi-layers (∼110 K). Thus, the adsorption dynamics are precursor-mediated. The enhanced (gas-surface) mass-match, caused by forming a second layer of the alkane, leads to adsorbate-assisted adsorption. A direct fitting procedure of the TDS data yields a pre-exponential factor of 2.5 × 10¹³/s and a coverage dependent heat of adsorption of E_d(Θ) = 39 − 6 ∗ Θ + 2.5 ∗ exp(−Θ/0.07) kJ/mol.     

Adsorption and thermal decomposition of diethylaluminum hydride on Si(1 0 0)-2 × 1

Chemistry of organoaluminum compounds on silicon surfaces forms a foundation of chemical vapor deposition (CVD) for the formation of metal-semiconductor interconnects. We have applied multiple internal reflection Fourier-transform infrared spectroscopy and thermal desorption mass spectrometry to analyze the chemistry of one of the promising Al-CVD precursors, diethylaluminum hydride, on a Si(1 0 0)-2 × 1 surface. Diethylaluminum hydride adsorbs molecularly on this surface both at room temperature and at 100 K. Thermally induced surface reaction consumes the monolayer of adsorbed organoaluminum molecule. The only hydrocarbon product is ethylene desorbing from the silicon surface around 600 K. Despite a clean reaction that removes carbon from the surface, aluminum deposition is not significant because of the formation of alane products.     

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.     

Effects of potassium on the reaction pathway of C3H7 species over Mo2C/Mo (1 0 0)

The adsorption and surface reactions of propyl iodide on clean and potassium-modified Mo₂C/Mo(1 0 0) surfaces have been investigated by thermal desorption spectroscopy (TPD), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) in the 100-1200 K temperature range. This work is strongly related to the better understanding of the catalytic effect of Mo₂C in the conversion of hydrocarbons. Potassium was found to be an effective promoter: it induced the rupture of C-I bond in the adsorbed C₃H₇I even at 100 K. The extent of C-I bond scission varied approximately linearly with the concentration of K coverage at the adsorption temperature of 100 K. As revealed by HREELS and TPD measurements the primary products of the dissociation are C₃H₇ and I. The former one was stabilized by potassium and underwent dehydrogenation and hydrogenation to give propene and propane. The desorption of both compounds is reaction-limited process. A fraction of propyl groups was converted into di-σ-bonded propene, which was stable up to ∼380 K. The coupling reaction of propyl species was also facilitated by potassium and resulted in the formation of hexane and hexene with T_p ∼ 230-250 K. Hydrogen was released with T_p = 390 K, indicative of a desorption limited process. The effect of potassium was explained by the extended electron donation to adsorbed propyl iodide in one hand, and by the direct interaction between potassium and I on the other hand. This was reflected by the shift of the desorption of potassium from the coadsorbed layer at and above 1.0 ML to higher temperature, and by the coincidal T_p values (∼700 K) of potassium and iodine. The formation of KI was also supported by the appearance of a loss feature at 650 cm⁻¹ in the HREEL spectra attributed to a phonon mode of KI.     

Ethylene hydrogenation on Ni(1 0 0) surface

The hydrogenation of ethylene on Ni(1 0 0) surface has been studied by TDS. The decrease in the bonding energy with increasing coverage is revealed for both of adsorbed hydrogen and ethylene by the shift of desorption to lower temperatures. Ethane formation is only observed on the preadsorbed hydrogen coverage exceeding 0.5 monolayer (ML), coupled with the growth of H₂ shoulder peak at lower temperatures. Further increase of H coverage to saturation reduces the bonding energy of subsequently adsorbed ethylene by 15 kJ/mol and decreases the saturation coverage of ethylene to about one-third on the clean surface. This leads to the shift of ethane desorption from 250 to 220 K and an appearance of additional ethane peak at 180 K. The latter ethane formation coincides with the hydrogenation of surface ethyl species derived from ethyl iodide as a precursor. It indicates that the rate of ethyl formation on the surface would be comparable to that of subsequent hydrogen addition to the surface ethyl species in the hydrogenation of ethylene when the preadsorbed hydrogen coverage approaches 1.0 ML.