Yttrium segregation and surface phases of yttria-stabilized zirconia (1 1 1) surface

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.     

Low energy electron diffraction studies of chemisorbed gases on stepped surfaces of platinum

The chemisorption of hydrogen, oxygen, carbon, carbon monoxide and ethylene was studied by low-energy electron diffraction on ordered stepped surfaces of platinum which were cut at angles less than 10° from the (111) face. The chemisorption characteristics of stepped platinum surfaces are markedly different from those of low index platinum surfaces and they are also different from each other. Hydrogen and oxygen which do not chemisorb easily on the (111) and (100) crystal faces chemisorb readily and at relatively low temperatures and pressures on the stepped platinum surfaces used in this study. In contrast to the ordered adsorption of carbon monoxide and ethylene on low index faces, the adsorption was disordered on the stepped surfaces and there is evidence for dissociation of the molecule. Carbon formed several ordered surface structures and caused faceting on the stepped surface, which are not observed on low index platinum surfaces. There appears to be a much stronger interaction of chemisorbed gases with stepped surfaces than with low index planes that must be caused by the differing atomic structures at the steps. Evidence for the differing reactivities of the two stepped surfaces are also discussed.     

Adsorption studies on a stepped Pt(111) surface: O2, CO,C2H4, C2N2

A comparative study of the adsorption of several gases on a Pt(S)-[9(111) × (111)] surface was performed using LEED, Auger spectroscopy, flash desorption mass spectrometry and work function changes as surface sensitive techniques. Adsorption was found to be generally less ordered on the stepped surface than on the corresponding flat surface with the exception of the oxygen, where r well ordered overlayer in registry over many terraces was found. Absolute coverages were determined from flash desorption experiments for O₂, CO and C₂N₂. Similar values were obtained as on flat Pt surfaces. Two different surface species seem to be formed upon adsorption of C₂H₄ depending on the adsorption temperature. Contrary to reports from Pt(111) surfaces conversion between the two surface species is heavily restricted on the stepped surface. Work function changes revealed nonlinear adsorbate effects where the adsorbate is electronegative with respect to the substrate. Various adsorption models are discussed in the light of complementary experimental evidence. The results of this study are compared with data available from flat Pt surfaces and possible influences of steps are discussed. No general trends, however, emerge from this comparison and it seems that eventual influences of steps have to be considered individually for every adsorbate.     

The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.     

Cu台阶面多层弛豫的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法对Cu(311),(511),(331)和(221)四个高指数台阶表面的弛豫结构和弛豫后表面各层的电子特性进行了系统研究.发现四个台阶面的层间弛豫规律依次为-+-…,--+-…,--+-…和---+-…,与其平台-阶梯n(hkl)×(uvw)的表示法2(100)×(111),3(100)×(111),3(111)×(111)和4(111)×(111)中的原子排数n相关,即     

Uncommon dislocation processes at the incipient plasticity of stepped gold surfaces

Gold vicinal surfaces (788), with a high density of steps, along with (111) flat surfaces taken as a reference, have been nanoindented and their resulting penetration curves and related defect structure comparatively analyzed by AFM and atomistic simulations. Stepped surfaces are shown to yield at smaller loads than (111) ones in agreement with calculations of the critical resolved shear stress needed to nucleate a dislocation. In the stepped surfaces, a novel intermediate state is identified in which the penetration curves depart from a Hertzian behavior prior to the appearance of pop-ins. This state is shown to result from heterogeneous nucleation at preexisting surface steps of dislocation loops, most of which retract and vanish when the indenter load is removed.     

High-temperature morphology of stepped gold surfaces

Molecular dynamics simulations with a classical many-body potential are used to study the high-temperature stability of stepped non-melting metal surfaces. We have studied in particular the Au(111) vicinal surfaces in the (M + 1,M− 1,M) family and the Au(100) vicinals in the (M,1,1) family. Some vicinal orientations close to the non-melting Au(111) surface become unstable close to the bulk melting temperature and facet into a mixture of crystalline (111) regions and localized surface-melted regions. On the contrary, we do not find high-temperature faceting for vicinals close to Au(100), also a non-melting surface. These (100) vicinal surfaces gradually disorder with disappearance of individual steps well below the bulk melting temperature. We have also studied the high-temperature stability of ledges formed by pairs of monatomic steps of opposite sign on the Au(111) surface. It is found that these ledges attract each other, so that several of them merge into one larger ledge, whose edge steps then act as a nucleation site for surface melting.     

LEED studies of clean high Miller index surfaces of silicon

Clean high Miller index surfaces of silicon were studied by LEED at different temperatures. Three types of surfaces were observed depending on orientation: flat surfaces, stepped ones with the (100) and (111) terraces and with the step heights of one or two interplanar distances, and “hill and valley” or facetted surfaces. All clean surfaces of silicon are reconstructed and contain surface structures with the periodicities different from the periodicities in the bulk. At certain temperatures the reversible order-order and order-disorder transitions take place on clean surfaces of silicon.     

The influence of sulfur adsorption on the step structure of vicinal Mo(100): a LEED and STM study

The effect of sulfur adsorption on the step structure of vicinal Mo(100) surfaces has been studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). STM was used for low densities of steps on a nominally flat (100) surface. LEED was used to study the more highly and regularly stepped (910), (911), and (28,4,1) oriented surfaces. Steps and the sulfur adsorbate were found to have a strong interaction. The presence and orientation of steps on the surface governed the formation of ordered domains of sulfur, and sulfur modified the structure of the steps on the surface. Both techniques show that monatomic steps predominate on the clean surfaces. When sulfur was adsorbed on the surfaces, however, steps coalesced to form groups of steps, double atomic height steps, and multiple height steps, depending on sulfur coverage and initial step density. The results are discussed in the framework of the theory of equilibrium crystal shape and provide information on how the Mo step-step interactions are affected by sulfur adsorption.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

Oxygen dissociation at Pt steps

Using scanning tunneling microscopy, thermal energy atom scattering, and density functional theory we have characterized O (2) dissociation on Pt(111) stepped surfaces at the atomic scale. The most reactive site is at the top of the Pt steps. In both the molecular precursor state (MPS) and the transition state (TS), the O (2) has its axis aligned parallel to the step edge. Controlled step decoration with Ag monatomic chains was used to locally tune the reactivity of Pt step sites. The enhanced reactivity at the Pt step sites is not caused by a decrease of the local dissociation barriers from the MPS but is related to a stabilization of both the MPS and TS.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

Theoretical studies of hydrogen molecule adsorption on flat and stepped platinum surfaces

Theoretical calculations have been applied to the adsorption of hydrogen molecule on flat and stepped platinum surfaces. The method of large unit cell is modified to deal with the stepped surface as well as the flat one. This method is free from the boundary effect which is inevitable in the cluster method. The results calculated for the d-band width and the highest occupied level are in good agreement with the experiments. For the dissociative adsorption, the bottom of the step site is the most favorable, and the result is attributed to the extra orbital interactions at this site.     

Scattering of surface states at step edges in nanostripe arrays

Surface states of noble metal surfaces split into Ag-like and Cu-like subbands in stepped Ag/Cu nanostripe arrays. The latter self-assemble by depositing Ag on vicinal Cu(111). Ag-like states scatter at nude step edges in Ag stripes, leading to umklapp bands, quantum size effects, and peak broadening. By contrast, Ag stripe boundaries become transparent to Cu-like states, which display band dispersion as in flat Cu(111). We find a linear relationship between the quantum size shift and peak broadening that applies in a variety of stepped systems, revealing the complex nature of step barrier potentials.     

YBCO涂层导体用YSZ缓冲层的快速制备研究

基于反应直流溅射法,采用镶嵌有钇粒的金属锆作为靶材,去离子水蒸汽为氧化反应气体,在有Y2O3种子层的双轴织构Ni-5at.%W基带上,系统地研究了温度和卷绕速度对YSZ阻挡层薄膜结构及表面形貌的影响。X射线衍射(XRD)分析表明,生长温度在700℃时制备的薄膜呈现明显的(002)取向;原子力显微镜(AFM)分析显示,该温度下制备的薄膜表面致密、无孔洞、无裂纹。在不同的卷绕速度下,虽然薄膜均为纯c轴取向,但其均方根粗糙度(RMS)和微粒大小均有较大差别。快速制备可达到抑制基片表面氧化、助于薄膜取向改善、提高薄膜制备效率的目的。     

Tailoring confining barriers for surface states by step decoration: CO /vicinal Cu(111)

The influence of CO adsorption on the Shockley type surface state on vicinal Cu(111) surfaces is investigated using angle resolved photoemission. As the steps are decorated with CO the surface state shifts to higher binding energies, which is opposite to the known behavior on flat Cu(111). This is described within a one-dimensional potential model in which clean steps represent repulsive barriers and decorated steps become attractive wells. From the coverage dependence the integrated CO well potential can be quantified. It is U(CO)a = -2.9 eV A on both Cu(332) and Cu(221) surfaces. Density functional calculations reveal that this attractive potential is due to the very local charge transfer from the Cu step atom to the adsorbed molecule.     

Nucleation of Pd dimers at defect sites of the MgO(100) surface

Point defects on oxide surfaces are presumed to be preferential nucleation sites for supported metal clusters. Under typical growth conditions, dimers constitute the first step in island nucleation. First-principles calculations on the formation of Pd dimers on regular and defect sites of the MgO(100) surface show that nucleation occurs with large dimer binding energies at divacancies and charged oxygen vacancies (F+ centers), while it is less favorable on steps and neutral F centers. The extensive database of defect trapping/attachment properties gives a firm basis to rationalize recent atomic-force microscopy findings and provides guidelines valid, in general, for ionic substrates.     

Cation Segregation in an Oxide Ceramic with Low Solubility: Yttrium Doped α-Alumina

The segregation behaviour of a cation (yttrium) with a low solubility in the polycrystalline oxide host (a-Al₂O₃) has been investigated at temperatures between 1450 and 1650°C using analytical scanning transmission electron microscopy. Three distinct segregation regimes were identified. In the first, the yttrium adsorbs to all grain boundaries with a high partitioning coefficient, and this can be modelled using a simple McLean-Langmuir type absorption isotherm. In the second, a noticeable deviation from this isotherm is observed and the grain boundary excess reaches a maximum of 9 Y-cat/nm² and precipitates of a second phase (yttrium aluminate garnet, YAG) start to form. In the third regime, the grain boundary excess of the cation settles down to a value of 6–7 Y-cat/nm² that is in equilibrium with the YAG precipitates. In a material (accidentally) co-doped with Zr, the Zr seems to behave in a similar way to the Y and the Y + Zr grain boundary excess behaves in the same way as the Y grain boundary excess in the pure Y-doped system. In this latter system, Y-stabilised cubic zirconia is precipitated in addition to YAG at higher Y + Zr concentrations.     

Adsorption of tungsten on stepped tungsten surfaces studied by work function measurement

Work function measurements have been performed during the deposition of W on the (110)W plane and several stepped W surfaces with (110) terraces and different terrace width. For each sample the work function decreases with growing coverage. The total work function drop diminishes strongly with decreasing terrace width. The results are interpreted in terms of a reduced nucleation process on stepped surfaces as compared to the flat (110) plane. The step edges act as sinks for the deposited adatonis and cause in their proximity a “dead” zone for nuclei formation. Details of the work function change with coverage are discussed in terms of an edge roughening effect on stepped surfaces.     

Dipole moments associated with edge atoms; A comparative study on stepped Pt,Au and W surfaces

Work function measurements have been performed on stepped Pt and Au surfaces with (111) terraces and on W surfaces with (110) terraces. In each case the work function decreases linearly with increasing step density and depends on the step orientation. The work function changes are attributed to dipole moments associated with the step edges. The dipole moments per unit step length are larger for open edge structures than for densely packed ones. The dipole moments for Pt are about twice as large as for Au and W.