Observation of charge transfer states of F4-TCNQ on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface

We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F₄-TCNQ is 5.24 eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface to the vacuum level is 4.1 eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F₄-TCNQ, characteristic peaks are observed at 1.1 and 2.5 eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F₄-TCNQ. The work function change is increased up to +1.3 eV as a function of F₄-TCNQ coverage. These results support the occurrence of charge transfer into F₄-TCNQ on the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface.     

Ab initio calculations of CO physisorption on ceria(1 1 1)

Applying the method of increments, we have performed MP2 and CCSD(T) calculations for the physisorption of CO on a cerium site on the ceria(1 1 1) surface. Our calculations predict an interaction energy of −0.28 eV. We have compared our calculations to previous CCSD(T) calculations for the physisorption of CO on a cerium site on the ceria(1 1 0) surface and found a difference in the interaction energy that is related to the different structure of the two surfaces. On the ceria(110) surface only 30% of the interaction energy originate from electron correlation effects, but on the ceria(111) surface almost the entire binding energy (80%) is due to electron correlation effects. Analyses of the interaction energy contributions show that most of the electron correlation part originates from the interaction of CO with the O ions in the topmost surface layer.     

Adsorption behaviors of CO on W(1 1 0) and Mo(1 1 0) surfaces in the β-state are still not clear

Adsorption structure of CO on W and Mo at above ～800 K (β-CO) has been extensively studied in the history of surface science. Most of the previous studies concluded that CO is dissociated in the β-CO, and a tilted structure plays a role as a precursor state of the dissociation. We have recently studied valence band spectra of the β-CO on W(1 1 0), oxygen-precovered W(1 1 0) and Mo(1 1 0) using synchrotron radiation. CO-derived states with binding energies close to those of the 4σ-CO can be observed, implying a non-dissociative chemisorption in this high-temperature state. We suggest that still some additional works need to be done in order to understand adsorption structure of β-CO completely.     

Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfaces

We have studied the effect of Zn on hydrogenation of formate to dioxomethylene on the Cu(1 1 1) surface by using a density functional theory–generalized gradient approximation (DFT–GGA)-pseudopotential method. We show that substitutionally adsorbed Zn changes the stability of intermediate states and the activation barrier of the hydrogenation process only slightly. On the other hand, the Zn atom adsorbed on the Cu surface stabilizes all formate, transition state, and dioxomethylene relative to the gas-phase molecules. Our results support a previously proposed reaction scheme that the adsorption state of Zn changes from substitutional to on-surface adsorption during the methanol synthesis.     

Temperature dependent LEED study of Pb{1 1 1}

The thermal behavior of Pb{1 1 1} was studied using low-energy electron diffraction (LEED) in the temperature range 11–323 K. The surface interlayer spacings increase with temperature at about the same rate as the bulk up to 0.5 T_m, and then increase faster. The relaxation of the surface, which is larger than for other fcc {1 1 1} surfaces, is maintained in the temperature range studied. Although Pb has a larger expansion coefficient than other metals, the surface thermal expansion behavior is in line with other surfaces, and is consistent with harmonic interplanar potentials.     

Adsorption of methanol and methoxy on NiAl(1 1 0) and Ni3Al(1 1 1): A DFT study

The adsorption of methanol and methoxy on NiAl(1 1 0) and Ni₃Al(1 1 1) has been investigated using density functional theory (DFT). Optimised adsorption geometries and core level shifts are presented. On both surfaces we find that methanol binds to the Al on-top site via its oxygen atom and with the C–O axis tilted away from the surface normal. Methoxy also shows a preference for Al-dominated sites. On NiAl(1 1 0), we predict that methoxy adsorbs with its oxygen atom in the Al–Al bridge site, while it is determined to be adsorbed with its oxygen atom in a 2Ni + Al hollow site on Ni₃Al(1 1 1), closer to Al than Ni. Surface and adsorbate induced binding energy shifts in the Al 2p states are calculated and found to be in good agreement with experimental high resolution photoelectron spectroscopy results.     

The effect of defects on the hydrogenation in Mg (0 0 0 1) surface

Density-functional theory was presented to investigate the hydrogen dissociation on a pure, Pt-doped, vacancy and oxide Mg(0 0 0 1) surface. Our results show that the energy barriers are 1.05, 0.39, 0.93 and 1.33 eV for H₂ dissociation on the pure, Pt-doped, vacancy and oxide Mg surface, respectively. The calculation results imply that the initial dissociation of H₂ is enhanced significantly for the Pt-doped Mg(0 0 0 1) surface, negligible for the vacancy model and weekend for the oxide model. The density of state results shows that, following the dissociation reaction coordinate, the H–H interactions are weeker for the Pt-doped model while interactions become stronger for the oxide model. It is suggested that the dissociation process is facilitated when Pt atom acts as catalyst and oxide overlayers delay hydrogen adsorption on the Mg layer. The present study will help us understand the defect role being played for the improvement or opposition effect in absorption kinetics of H₂ on the Mg(0 0 0 1) surface.     

Chlorine adsorption on Si(1 1 1) studied by optical methods

The adsorption process of chlorine on Si(1 1 1) has been studied by means of real time surface differential reflectance (SDR) spectroscopy and second harmonic generation (SHG). The structure observed at 3.6 eV in SDR spectra is attributed to transitions including Si–Cl antibonding states. However, the overall feature is due to the removal of the electronic states of the clean surface. Developments of adsorption on Si adatom dangling bonds and breaking of adatom back bonds are obtained from SDR spectra and second harmonic (SH) intensity. They are well fit by the solutions of the rate equations under the assumption of adsorption of atoms without migration, and the initial sticking probability on the dangling bonds and the initial breaking probability of the back bonds are determined. Dependence of the adsorption kinetics on the carrier concentration is briefly reported.     

Formation of a √3 × √3 surface on Si/Ge(1 1 1) studied by STM and LEED

We have performed a detailed study of the formation and the atomic structure of a √3 × √3 surface on Si/Ge(1 1 1) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a √3 × √3 periodicity but unlike the Sn/Ge(1 1 1) and the Sn/Si(1 1 1) surfaces, the Si/Ge(1 1 1) surface is not well ordered. There is no long range order on the surface and the √3 × √3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.     

Hydrogen sorption sites in holmium silicide on silicon(1 1 1)

The hydrogen sorption sites on the surface of holmium silicide grown on Si(1 1 1) have been determined using metastable de-excitation spectroscopy, ultraviolet photoemission spectroscopy and density functional theory calculations. Comparison of calculated and measured surface density of states spectra allow us to locate the position of the second subsurface hydrogen atom in each unit cell to an interstitial site in the layer of rare earth atoms. The hydrogenation energies indicate a reaction pathway that involves concomitant site occupation.     

Sulphur adsorption on Au{1 1 0}: DFT and LEED study

The adsorption of sulphur on clean reconstructed Au{1 1 0}-(1 × 2) surface was studied using density functional theory (DFT) and quantitative low energy electron diffraction (LEED) calculations. The results show that the sulphur atoms form a (4 × 2) ordered structure which preserves the missing row reconstruction of the clean surface. The sulphur atom is found to adsorb on threefold hollow sites, on the {1 1 1} microfacets that border the trenches of the missing rows.     

Optical transitions and carrier dynamics in self-organized InAs quantum dots grown on In0.52Al0.48As/InP(0 0 1)

Optical transitions in self-organized InAs quantum dots (QDs) grown on In_0.52Al_0.48As layer lattice matched to InP(0 0 1) substrate, have been studied by continuous wave (cw) photoluminescence (PL) and time-resolved PL. The dependence of the PL transition on excitation power and photoluminescence excitation measurements clearly shows that the multi-component cw-PL spectrum is related to emission coming from ground and related excited states of QDs with heights varying by monolayer fluctuations. While decay times measured by time-resolved PL are in the nanosecond range for the ground states, shorter decay times related to relaxation of carriers down directly to the ground state are determined for the excited states.     

The electron stimulated chemistry of methyl lactate on Cu(1 1 1)

The electron stimulated chemistry of monolayers of (R)/(S)-methyl lactate ((S)/(R)-MLAc) adsorbed on Cu(1 1 1) has been investigated. Monolayers of MLAc undergo highly efficient electron stimulated processes predominately desorption, but also a significant fraction is converted to an adsorbed alkoxide moiety through the selective cleavage of the O–H bond. The efficiency of the depletion of the adsorbed MLAc state and the absence of significant non-selective fragmentation contrasts with previous studies of the electron beam irradiation of monolayers of oxygen containing organic molecules.     

Metallization of Ge(0 0 1)-p(2 × 1) surface as result of thermal fluctuations

We present a theoretical study of the metallization of Ge(0 0 1)-p(2 × 1) surface which is observed in experimental data. We have considered the connection between thermal fluctuation of this surface structure and its metallic properties. To this end we have performed long-time MD-DFT simulations. The obtained results show that thermal fluctuation of the Ge(0 0 1)-p(2 × 1) structure may cause its metallization which in not necessary connected with a flip-flop motion of dimer atoms. It was shown that the metallization of the Ge(0 0 1)-p(2 × 1) surface takes place when the dimer buckling angle is reduced to around 11°. In the case of our simulations the considered surface system remained in the metallic state for 25% of the simulation time. We have also found that the metallic state of the fluctuating Ge(0 0 1)-p(2 × 1) surface is built up by dangling bonds of the dimer atoms shifted up (D_up) and down (D_down).     

Comparison of sulfur interaction with hydrogen on Pt(1 1 1), Ni(1 1 1) and Pt3Ni(1 1 1) surfaces: The effect of intermetallic bonding

Adsorption strengths of hydrogen and sulfur both individually and together as co-adsorbates were investigated on Pt(1 1 1), Ni(1 1 1) and Pt₃Ni(1 1 1) surfaces using density functional theory in order to determine the effect of metal alloying on sulfur tolerance. The adsorption strengths of H and S singly follow the same trend: Ni(1 1 1) > Pt(1 1 1) > Pt₃Ni(1 1 1), which correlates well with the respective d-band center positions of each surface. We find that the main effect of alloying is to distort both the sub-layer structure and the Pt overlayer resulting in a lowered d-band. For all three surfaces, the d-band shifts downward non-linearly as a function of S coverage. Nearly identical decreases in d-band position were calculated for each surface, leading to an expectation that subsequent adsorption of H would scale with surface type similarly to single species adsorption. In contradiction to this expectation, there was no clearly discernable difference between the energies of coadsorbed H on Pt(1 1 1) and Ni(1 1 1) and only a slightly lowered energy on Pt₃Ni(1 1 1). This provides evidence that coadsorbed species in close proximity interact directly through itinerant mobile electrons and through electrostatic repulsion rather than solely through the electronic structure of the surface. The combination of the lowered d-band position (arising from distorted geometry) and direct co-adsorbate interactions on Pt₃Ni(1 1 1) leads to a lower energy barrier for H₂S formation on the surface compared to pure Pt(1 1 1). Thus, alloying Pt with Ni both decreases the likelihood of S adsorption and favors S removal through H₂S formation.     

XPS and SRUPS study of oxygen adsorption on Cd0.9Zn0.1Te (1 1 1)A surface

Synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS) and X-ray photoelectron spectroscopy (XPS) have been applied to investigate oxygen adsorption on a cadmium zinc telluride (CZT) (1 1 1)A surface. The surface chemical composition and the surface oxidation process were monitored by recording the Te 3d, O 1s, Zn 2p, Cd 4d core level peaks, and the Cd MNN Auger peak. The CZT (1 1 1)A surface was effectively oxidized by dosing oxygen directly. The typical surface state of the clean CZT (1 1 1)A surface was identified. After oxygen exposure, this surface state disappeared and a signal due to the formation of O–CZT appeared. In addition, the work function of CZT decreased with the increasing oxygen exposure.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Laser-induced photo-desorption in the Na/Cu(1 1 1) system

Desorption in the Na/Cu(1 1 1) system induced by an electronic excitation is studied using a quantal approach. The system is excited by a laser pulse in the fs range to the Na¹ state corresponding to the transient capture of an electron by the alkali adsorbate. The present quantal approach describes on an equal footing the laser-induced vibrational excitation of the adsorbate in the adsorption well and the photo-desorption process. It confirms earlier results using a semi-classical input. It also allows a discussion of the photo-desorption probability with the photon energy: the maximum of the desorption probability per absorbed photon occurs off-resonance in the high-energy wing of the electronic transition. This feature is related to the dynamics of the laser-induced process.     

Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

Atomic and electronic properties of furan on the Si(0 0 1)-(2 × 2) surface

The atomic and electronic properties of the adsorption of furan (C₄H₄O) molecule on the Si(1 0 0)-(2 × 2) surface have been studied using ab initio calculations based on pseudopotential and density functional theory. We have considered two possible chemisorption mechanisms: (i) [4 + 2] and (ii) [2 + 2] cycloaddition reactions. We have found that the [4 + 2] interaction mechanism was energetically more favorable than the [2 + 2] mechanism, by about 0.2 eV/molecule. The average angle between the CC double bond and Si(1 0 0) surface normal was found to be 22°, which is somewhat smaller than the experimental value of 28°, but somewhat bigger than other theoretical value of 19°. The electronic band structure, chemical bonds, and theoretical scanning tunneling microscopy images have also been calculated. We have determined a total of six surface states (one unoccupied and five occupied) in the fundamental band gap. Our results are seen to be in good agreement with the recent near edge X-ray absorption fine structure and high resolution photoemission spectroscopy data.