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1.
To evaluate the interactions between the atoms of Au, Ag and Cu and clean Si(1 1 1) surface, two types of silicon clusters Si4H7 and Si16H20 together with their metal complexes were studied by using hybrid (U)B3LYP density functional theory method. Optimized geometries and energies on different adsorption sites indicate that: (1) the binding energies at different adsorption sites are large (ranging from 1.2 to 2.6 eV depend on the metal atoms and adsorption sites), suggesting a strong interaction between metal atom and silicon surface; (2) the most favorable adsorption site is the on top (T) site. Mulliken population analysis indicated that in the system of on top (T) site, a covalent bond is formed between metal atom and dangling bond of surface Si atom.  相似文献   

2.
Karl Jacobi  Yuemin Wang 《Surface science》2009,603(10-12):1600-1604
The interaction of NO with the O-rich RuO2(1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with 18O. During exposure to NO an O···N–O surface group (NO2-cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO2-cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.  相似文献   

3.
&#x;t pn Pick 《Surface science》2009,603(16):2652-2657
We study the ordered PdSn c(2 × 2), (2 × 1), and PdSn2 (3 × 1) overlayers deposited on Pd(1 1 0) by using first-principles density-functional calculations. It appears that the two PdSn structures are almost degenerate in the energy. Pd–Sn surfaces we consider do not display the marked buckling with Sn atoms displaced towards vacuum that is common for Pt–Sn surfaces. Low-coverage CO chemisorption at these overlayers and on analogous surface structures on Pd3Sn is considered. It is shown that inclusion of an empirical correction to the CO adsorption energy changes the stable adsorption site from the long-bridge to the top one in most cases. The adsorption energy decreases with the number of Sn atoms in the vicinity of the adsorption site, and this property correlates well with the position of the centre of gravity of the local Pd d-electron band, and also with the variation of the local density of d-electron states at the Fermi level. The centre-of-gravity value is used to assess the core-level shifts for Pd atoms in various geometries. Most of the calculated data compare rather well with the recent measurements on Pd–Sn overlayers at Pd(1 1 0) as well as with other data on related bimetallic systems.  相似文献   

4.
The results of a quantitative LEED study of the clean, reconstructed Pt(110)-(1 × 2) surface are described, in which experimental intensity-energy spectra for ten diffracted beams have been compared, by an γ-factor analysis, to intensity spectra calculated using a dynamic theory for various structural models. Despite considerable effort in ensuring the reliability of the experimental and calculated intensities, a satisfactory level of agreement has not been achieved for any of the model structures considered although a “missing-row” model leads to the closest agreement between experimental and calculated intensities.  相似文献   

5.
6.
Surface states are a unique and important class of quantum states that shave an important effect on the electronic properties of Cu(1 1 0) surface. The Cu(1 1 0) surface has been studied using ultraviolet photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), and reflection anisotropy spectroscopy (RAS), and shows a resonance in the RAS spectra at 2.1 eV due to a transition between occupied and unoccupied surface states. The unoccupied surface state involved in the RAS transition at an energy of 1.7 eV at the point of the surface Brillouin zone has been investigated using IPES and the occupied surface state is seen in PES spectra at 0.45 eV below the Fermi level. The energy difference of the surface states, 2.15 eV, is a good match to the transition energy found in the RAS experiments.  相似文献   

7.
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.  相似文献   

8.
Surface segregation of yttria-stabilized zirconia (YSZ) was studied via first-principles computations and thermodynamics. For the cubic YSZ (1 1 1) surface, yttrium can segregate only to a subsurface layer, and these segregation phases are terminated at the surface by defective oxygen layers with honeycomb structure. The segregation is independent of the bulk yttrium concentration at high oxygen partial pressures or low temperatures. At very low oxygen partial pressures and high temperatures there is no surface yttrium segregation and the surface is terminated by O–Zr. Our results provide a reasonable explanation for previous experimental work, and also a framework for extending our understanding of cation segregation in oxide surfaces.  相似文献   

9.
G. Cilpa  M. Guitou  G. Chambaud   《Surface science》2008,602(17):2894-2900
A model system consisting of a cluster of 13 Ag atoms and n (n = 1, 2, 3) H2 molecules has been used to study, by ab initio methods, the structural and energetic characteristics of the chemi- and physisorption processes of H2 on a (1 0 0) surface of silver. The dissociative chemisorption of a first H2 molecule is analyzed in terms of hydrides formation and it is shown that several electronic states are interacting in the vicinity of the activation barrier leading to complex electronic processes. The energy of the physisorption interaction of the first H2 molecule for different orientations and that of further H2 molecules coming directly on top of the first chemisorbed one are determined with highly correlated wavefunctions. As for the (H2)nCu13 system, already studied with similar approaches, it is found for the (H2)nAg13 system that the physisorption energy of the second layer is enhanced by a factor close to two compared to that of the first layer due to dipolar interactions with the polarized surface. The physisorption energy of the third and further layers tends to the van der Waals H2/H2 interaction energy.  相似文献   

10.
The interaction of xenon atoms with the TiO2(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO2(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO2(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance.  相似文献   

11.
《Solid State Ionics》2006,177(1-2):105-112
Five compositions of Li[Co1 −2x(Li1 / 3Mn2 / 3)x(Ni1 / 2Mn1 / 2)x]O2 solid solutions ( x = 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized using a sol–gel method with three end members of LiCoO2, Li2MnO3(Li[Li1 / 3Mn2 / 3]O2), and Li[Ni0.5Mn0.5]O2. The compositions of metals in transition metal sites were changed to see the effect of them on electrochemical behavior of the solid solutions. All the samples were nano-sized semi-spherical shaped particles with a layered structure. The reduction of cobalt content (the increase of other metals) in the sites increases the lattice parameters, a and c, resulting in the shift of Raman and XRD peak positions. The discharge capacity fading turned serious at higher Co contents, but it was significantly diminished with the decrease of Co content. At lower Co contents, the capacity increased with cycle numbers. The most stable voltage profile was obtained from the composition of Li[Li1 / 15Co3 / 5Ni1 / 10Mn7 / 30]O2 (x = 0.2).  相似文献   

12.
《Surface science》2006,600(8):1654-1658
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 (Dup) and down (Ddown).  相似文献   

13.
The chemisorption of isocyanate (NCO) species on Pd(1 0 0) was studied within the density functional formalism (DFT) using a periodic slab model. The NCO was adsorbed on top, bridge and hollow sites of the metal surface at different coverages. At low coverages, the adsorption energies are in the range of ?2.5/?3.0 eV, indicating an important adsorbate–substrate interaction for the three sites studied. The lateral repulsive interaction between neighboring NCO species is almost negligible or weak at lower and intermediate coverages, and very strong at complete monolayer. At low coverages, both bridge and hollow sites are energetically preferred; yet the bridge site becomes the only favoured site at intermediate and complete coverages. Work function and dipole moment calculations can be interpreted by an important charge transfer from the metal surface to NCO. Interestingly, while on hollow site the charge taken by NCO is essentially the same over all the range of coverage, an increasing depolarization is observed on bridge and top sites as the coverage increases. Symmetric and asymmetric NCO stretching modes were also calculated and compared with recent infrared spectroscopy measurements.  相似文献   

14.
We study the dynamics of HD and H2 molecules interacting with Pd(1 1 1) and Cu(1 1 0) using the classical trajectory method based on potential energy surfaces obtained from Density Functional Theory calculations. Our results predict a negligible isotopic effect on the dissociative adsorption probability on Pd(1 1 1) whereas on Cu(1 1 0), the adsorption probability for HD(νi=0) is slightly lower than for H2(νi=0), mainly due to its lower initial vibrational zero point energy. The final rotational energy distribution of scattered HD and H2 molecules are very similar. This shows that the asymmetric mass distribution of HD, barely affects the fraction of initial translational energy transferred to rotation during the scattering process. Our calculations point to the larger number of open rotational excitation channels for HD, as the main cause of rotational excitation probabilities larger than for H2. The theoretical apparent rotational temperature, Trot, of HD molecules scattered from Pd(1 1 1) at impact energy , is in good agreement with the experimental value. In contrast, for Cu(1 1 0) the theoretical Trot is much lower than the value measured for Cu(1 0 0). Possible reasons for such a discrepancy between theory and experiments are discussed.  相似文献   

15.
Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the atomic and electronic structure of the group-IV adsorbates (C, Si, Ge, Sn, and Pb) on the GaAs(0 0 1)-(1 × 2) surface considered in two different models: (i) non-segregated Ga-IV-capped structure and (ii) segregated structure in which the group-IV atoms occupying the second layer while the As atom floats to the surface. The non-segregated structure is energetically more favorable than the segregated structure for Sn and Pb, whereas it is the other way around for C, Si, and Ge.  相似文献   

16.
《Solid State Ionics》2006,177(11-12):1033-1040
LiMn2−xTixO4 compounds with 0  x  1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3¯m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d0 Ti4+ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO4 were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process.  相似文献   

17.
We present theoretical and experimental study of the structural and electronic properties of the Ti/W(1 1 1) adsorption system. Atomic arrangements of the considered surfaces and their electronic structures have been obtained from first-principles pseudopotential calculations based on the density functional theory in a plane-wave-basis implementation. The corresponding experimental data have been provided by photofield emission spectroscopy. Investigations of the clean and Ti-covered W(1 1 1) surfaces indicate substantial structural relaxations deep into the substrate, and a noticeable modification of the surface electronic properties of the system induced already by a thin film of titanium. Configuration with adatoms positioned in substrate-lattice-continuation (i.e., deep-hollow) sites is found to be energetically most favorable. A good agreement between the measured photofield emission spectra and the computed local-density-of-states distributions confirms our theoretical predictions for a clean W(1 1 1) substrate as well as Ti coverages of 0.25 and 1 ML.  相似文献   

18.
The formation of melamine–PTCDI bimolecular networks deposited on Ag(1 1 1) is studied by means of first principle calculations. Emphasis is placed on the interplay of the inter-molecular hydrogen bonds and the molecule–substrate contacts. Our simulations show rather strong distortions of the adsorbed molecules near the contact points due to the influence the hydrogen bonds. Despite this, the charge transfer from the substrate to a PTCDI molecule remains almost the same (0.9 e) as obtained for an isolated PTCDI molecule. A detailed analysis of the topological features of the electronic density reveals that the charge transfer modifies the two types of hydrogen bonds in opposite ways, weakening the central bond and strengthening the two lateral ones, while roughly keeping a constant binding energy. Altogether, the influence of the substrate on the molecular network is proved to be weak.  相似文献   

19.
V.M. Bermudez 《Surface science》2010,604(7-8):706-712
The adsorption of dimethyl methylphosphonate (DMMP) on the (0 1 0) surface of anatase TiO2, which is isostructural with the (1 0 0), has been studied using density functional theory and two-dimensionally-periodic slab models. The experimentally-observed faceting of this surface has, for the first time, been included in the modeling. The relaxations of bare surfaces both with and without faceting are similar, leading to an atomic-scale roughening due to inward (outward) displacement of fivefold-coordinated Ti5c (sixfold-coordinated T6c) atoms together with outward displacement of threefold-coordinated O3c atoms. Molecular adsorption occurs by formation of a Ti5c?OP dative bond with one or more CH?O2c bonds between CH3 groups and unsaturated, twofold-coordinated (O2c) sites. The energies for molecular adsorption, obtained using the B3LYP functional, are virtually identical (about ?21.0 kcal/mol) for the two surfaces and are also close to those found elsewhere for the rutile (1 1 0) and anatase (1 0 1) surfaces. A possible first step in the dissociative adsorption of DMMP has also been modeled and is found to be thermodynamically favored over molecular adsorption to a degree which depends on faceting.  相似文献   

20.
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.  相似文献   

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