First-principles study of the atomic structures,electronic properties,and surface stability of BaTiO3 (001) and (011) surfaces

The atomic structures, electronic properties, and surface stability of (001) and (011) surfaces of BaTiO₃ are studied by first-principles calculations. Four differently terminated BaTiO₃ surfaces are considered in this study, including (001)-BaO, (001)-TiO₂, (011)-BaTiO, and (011)-O₂ terminations. The relaxations and rumplings are calculated and discussed, finding that the first layer relaxes inwards, while the second layer relaxes outwards for (001) and (110) surfaces. The data obtained for electronic properties show that O_2p states in (001)-BaO/(001)-TiO₂ termination shift to the lower/higher energy region, leading to a wide/narrow band gap. And the new produced surface states are observed in (011) surface terminations, which is mainly attributed to the supplied electrons from outermost surface atoms, even O atoms are oxidized. Furthermore, the (001) surface of BaTiO₃ is found to be more stable than the (011) surface according to the predicted surface energy which is 0.86 and 2.92 J/m² for (001) and (011) surfaces, respectively. Of which, BaO termination is predicted to be more likely to cleavage from the (001) direction than the TiO₂ termination is.     

Structure and stability of the (001) alpha-quartz surface

The structure and surface energies of the cleaved, reconstructed, and fully hydroxylated (001) alpha-quartz surface of various thicknesses are investigated with periodic density functional theory (DFT). The properties of the cleaved and hydroxylated surface are reproduced with a slab thickness of 18 atomic layers, while a thicker 27-layer slab is necessary for the reconstructed surface. The performance of the hybrid DFT functional B3LYP, using an atomic basis set, is compared with the generalised gradient approximation, PBE, employing plane waves. Both methodologies give similar structures and surface energies for the cleaved and reconstructed surfaces, which validates studying these surfaces with hybrid DFT. However, there is a slight difference between the PBE and B3LYP approach for the geometry of the hydrogen bonded network on the hydroxylated surface. The PBE adsorption energy of CO on a surface silanol site is in good agreement with experimental values, suggesting that this method is more accurate for hydrogen bonded structures than B3LYP. New hybrid functionals, however, yield improved weak interactions. Since these functionals also give superior activation energies, we recommend applying the new functionals to contemporary issues involving the silica surface and adsorbates on this surface.     

Evidence of surface reconstructions and incorporation of oxygen into the oxide framework on the hydroxylated La2O3(001) surface

By performing first-principles Molecular Dynamics simulations at 300 K, we show that water dissociates on the A-La2O3(001) surface giving rise to one exclusive type of hydroxyl-group, which is associated with a surface reconstruction, incorporating an additional oxygen ion into the oxide subsurface, yielding a surface structure that is oxygen rich.     

Al (110) surface oxide thermal stability in ultrahigh vacuum

This research characterizes the stability of the Al₂O₃ surface oxide on Al (110) as a function of temperature and within an ultrahigh vacuum environment (p < 5 × 10^?12 Torr). Auger electron spectroscopy and temperature desorption spectroscopy were used to correlate the change in oxygen and carbon surface concentration. The surface oxide was observed to remain stable up to 350–400 °C. Above this temperature, the oxide began to dissociate resulting in a CO desorption peak at 425 °C followed by extensive dissolution of the C and O into the Al bulk. A second and much smaller CO desorption peak was observed at 590 °C in concert with complete oxide breakdown and the virtual disappearance of surface carbon and oxygen. Extrapolation of the Auger electron spectral ratios of C_KLL and O_KLL peaks to the sum of the Al⁰_LVV and Al³⁺_LVV peak suggests that the surface concentration of each approaches zero at ~640 °C. The predominant mechanism for reduction of the surface oxide occurs by dissolution into the bulk instead of desorption. Copyright © 2013 John Wiley & Sons, Ltd.     

On the stability of the W(001) surface

Recently, the present authors have suggested that some of the basic features of the unstable W(001)-(1×1) surface can be explained by the competition between the direct attractive and indirect repulsive interactions present at the surface. To understand this mechanism in more detail, number of surface structures (steps in the (0,1) and (1,1) direction, vacancy, adatoms etc.) have been investigated within a simple LCAO recursion scheme. The direct interactions (two-body term) and the indirect ones (three-body terms) are approximately additive for surface atoms and adatoms having four nearest neighbours. The unreconstructed (1×1) surface is under compressive (repulsive) stress and we suggest that, for example, steps should expand near their edges.     

Reactivity and thermal stability of oxidized copper clusters on the tantalum(V) oxide surface

Oxidized copper clusters 2–5 nm in size have been obtained by RF discharge sputtering of a copper wire in an oxygen atmosphere. Isolated CuO clusters or, at long deposition times, their agglomerates form on the support. The thermal stability of the oxidized nanoparticles in a vacuum and their reactivity toward CO in relation to the deposition time have been investigated by X-ray photoelectron spectroscopy. The asprepared clusters show low reactivity (10^?7?10^?9), but their activation by reduction and subsequent reoxidation in an oxygen medium raises their reactivity to ～10^?5. This is due to the appearance of weakly charged oxygen species on the surface. The reactivity of the CuO clusters has been compared to the reactivity of earlier studied nanosized copper oxide model objects.     

Electronic effects induced by single hydrogen atoms on the Ge(001) surface

The properties of an isolated dangling bond formed by the chemisorption of a single hydrogen atom on a dimer of the Ge(001) surface are investigated by first-principles density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements. Two stable atomic configurations of the Ge-Ge-H hemihydride with respect to the neighboring bare Ge-Ge dimers are predicted by DFT. For both configurations, the unpaired electron of the HGe(001) system is found to be delocalized over the surface, rendering the isolated dangling bond of the hemihydride unoccupied. However, local surface charge accumulation, such as may occur during STM imaging, leads to the localization of two electrons onto the hemihydride dangling bond. The calculated surface densities of states for one of the charged Ge-Ge-H hemihydride configurations are found to be in good agreement with atomic-resolution STM measurements on n-type Ge(001). Comparison with a Si-Si-H hemihydride of the Si(001) surface shows similarities in structural properties, but substantial differences in electronic properties.     

Interaction of triatomic germanium with lithium atoms: electronic structure and stability of Ge3Lin clusters

Quantum chemical calculations were applied to investigate the electronic structure of mono-, di-, and tri- lithiated triatomic germanium (Ge3Lin) and their cations (n = 0-3). Computations using a multiconfigurational quasi-degenerate perturbation approach (MCQDPT2) based on complete active space CASSCF wavefunctions, MRMP2 and density functional theory reveal that Ge3Li has a 2A' ground state with a doublet-quartet gap of 24 kcal/mol. Ge3Li2 has a singlet ground state with a singlet-triplet (3A' '-1A1) gap of 30 kcal/mol, and Ge3Li3 a doublet ground state with a doublet-quartet (4A' '-2A') separation of 16 kcal/mol. The cation Ge3Li+ has a 1A' ground state, being 18 kcal/mol below the 3A' state. The computed electron affinities for triatomic germanium are EA(1) = 2.2 eV (experimental value is 2.23 eV), EA(2) = -2.5 eV, and EA(3) = -5.9 eV, for Ge3-, Ge32-, and Ge33-, respectively, indicating that only the monoanion is stable with respect to electron detachment, in such a way that Ge3Li is composed of Ge3-Li+ ions. An atoms in molecules (AIM) analysis shows the absence of a Ge-Ge-Li ring critical point in Ge3Li. An electron localization function (ELF) map of Ge3Li supports the view that the Ge-Li bond is predominantly ionic; however, a small covalent character could be anticipated from the Laplacian at the Ge-Li bond critical point. The ionic picture of the Ge-Li bond is further supported by the natural bond orbital (NBO) results. The calculated Li affinity value for Ge3 is 2.17 eV, and the Li+ cation affinity value for Ge3- amounts to 5.43 eV. The larger Li+ cation affinity of Ge3- favors an electron transfer, resulting in a Ge3-Li+ interaction.     

Effects of adsorbed bismuth on Si(001) surface electronic states

It is shown that depositing Bi on an Si(001) surface fills the free broken-bond surface states, whose concentration decreases linearly as the bismuth coating 8 increases up to _st, = 0.6 monolayer. The bismuth desorption activation energy is constant < _st, (E_d = 2.77 ± 0.1 eV) and decreases for > _st.Taras Shevachenko Kiev National University, ul. Vladimirskaya 64, 252601 Kiev-17, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 168–171, May–June, 1996. Original article submitted September 19, 1995.     

Textural and adsorption properties of silica modified with germanium(IV) oxide

Nitrogen low-temperature adsorption-desorption shows that the incorporation of germanium(IV) oxide into a silicate matrix under the conditions of the sol-gel transition gives rise to the formation of silica with a uniform mesoporous texture that has a predominating pore diameter of about 4 nm.     

Different adsorption structures of pyridine on Si(001) and Ge(001) surfaces

The adsorption and reaction of pyridine on the Si(001) and Ge(001) surfaces are investigated by first-principles density-functional calculations within the generalized gradient approximation. On both surfaces the N atom of pyridine initially reacts with the down atom of the dimer, forming a single bond between the N atom and the down atom. On Ge(001) such an adsorption configuration is most favorable, but on Si(001) a further reaction with a neighboring dimer occurs, resulting in formation of a bridge-type configuration. Especially we find that on Ge(001) the bridge-type configuration is less stable than the gas phase. Our results provide an explanation for a subtle difference in the adsorption structures of pyridine on Si(001) and Ge(001), which was observed from recent scanning tunneling microscopy experiments.     

Tuning the thermal stability of copper(II) hexacyanoferrate(II) nanoparticles

Journal of Thermal Analysis and Calorimetry - It is well known that the physical properties of nanoparticles can be tuned by controlling synthetic factors such as pH, temperature, reactant ratio or...     

Structure and binding energies of unsaturated hydrocarbons on Si(001) and Ge(001)

The adsorption of acetylene, ethylene, and benzene on the Si(001) and Ge(001) surfaces is investigated by first-principles density-functional calculations within the generalized-gradient approximation. We find that the adsorption energies of the three hydrocarbons containing a triple bond, a double bond, and a pi-conjugated aromatic ring decrease as the sequence of C2H2>C2H4>C6H6. We also find that the bondings of acetylene, ethylene, and benzene to Ge(001) are much weaker than those to Si(001). As a result, benzene is weakly bound to Ge(001) while it is chemisorbed on Si(001), consistent with temperature-programmed desorption data.     

High purity chemical etching and thermal passivation process for Ge(001) as nanostructure template

An advanced two-step cleaning process of the Ge(001) surface for nanoscience requirements is presented. First, wet-chemical etching with a variant of the Piranha solution (H(2)SO(4), H(2)O(2), H(2)O) is used to remove contaminants as well as the native oxide layer. Second, passivation of the surface is achieved by a rapid thermal oxidation step, leading to a homogeneous protective oxide layer. The thickness of the oxide layer is tuned to be thick enough to protect the surface, yet thin enough to be completely removed by thermal treatment in ultra-high vacuum. The application of this recipe results in an outstandingly clean and atomically flat surface, with carbon contamination at the detection limit of x-ray photoelectron spectroscopy. Scanning tunneling microscopy and electron diffraction reveal a long range ordered surface with typical terrace diameters of ~100 nm, suitable for the growth of atomic-scale nanostructures.     

乙炔在Ge(001)表面吸附的反应路径

采用第一性原理方法研究了乙炔分子在Ge(001)表面的吸附反应.通过系统考察0.5和1.0ML覆盖度时形成di-σ和end-bridge构型的反应路径,研究在表面形成di-σ和paired-end-bridge构型的反应几率.除了表面反应以外,本文还涉及了亚表层Ge原子参与的吸附反应,乙炔在亚表层原子上吸附形成的亚稳态结构sub-di-σ,是形成end-bridge结构的第二条途径,此反应机理对于表面吸附结构的形成起重要的作用.与乙炔分子不同的是,表面以下原子参与时乙烯分子的吸附反应为吸热反应.综合热力学和动力学的分析表明,paired-end-bridge构型是乙炔分子吸附的主要构型,此结论解释了乙炔分子在Ge(001)表面吸附构型的实验结果.对于乙烯和乙炔两分子在Ge(001)表面吸附的分析比较揭示了导致两者之间差异的原因.     

Simultaneous Improvements of Thermal Stability and Mechanical Properties of Poly（propylene carbonate） via Incorporation of Environmental-friendly Polydopamine

Inspired by the photoprotection, radical scavenging of melanin together with versatile adhesive ability of mussel proteins, polydopamine（PDA） nanoparticles were successfully prepared and incorporated into environmentally friendly polymer, poly（propylene carbonate）（PPC） via solvent blending. The prepared composites exhibited excellent thermal stability in air and nitrogen atmosphere and extraordinary mechanical properties. The composites displayed eminent increase of temperature at 5% weight loss（T5%） by 30-100 K with 0.3 wt%-2.0 wt% loadings, meanwhile, the tensile strength and Young＇s modulus were significantly improved from 11.5 MPa and 553.7 MPa to 40.5 MPa and 2411.2 MPa, respectively. The kinetic calculation indicated that improvement of T5% is presumably derived from suppressing chain-end unzipping. The glass transition temperature（Tg） of the PPC/PDA composites increased by 8-10 K. This is probably due to hydrogen bonding interaction since the abundant proton donors along PDA chains would interact with proton acceptors like C = O and C―O―C in PPC which would cause restriction of segmental motion of PPC chains.     

Tuning the electronic properties of N-heterocyclic carbenes

The electron-donating properties of N-heterocyclic carbenes ([N,N'-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4'-R-substituted aryl rings (4,4'-R=NEt2, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.     

Dynamic surface rearrangement and thermal stability of nitrogen functional groups on carbon nanotubes

Dynamic surface rearrangement and thermal stability of N-functional groups on carbon nanotubes (CNTs), obtained by functionalization of pristine CNTs with NH(3), were studied by temperature-programmed XPS and MS: a link between the stability of the functional group and decomposition temperature have been established and a conversion into graphitic nitrogen was observed.