Topmost Plane Research Articles

Tailoring the germanene-substrate interactions by means of hydrogenation.

Thanks to density functional calculations, the influence of hydrogenation on the interactions between a (2 × 2) germanene layer and a (3 × 3) Al(111) substrate has been investigated. It appears that the Ge-Al inter-atomic distance increases with hydrogen coverage, while the interaction energy and charge transfer between the Ge layer and the Al topmost plane drastically diminish, thus reducing the electrostatic interactions. Moreover, hydrogenation also lowers the electron density at the interface, weakening the chemical interaction between the Ge layer and the Al surface, and opening the door to a possible decoupling of the germanene layer from the Al substrate.

Reconstruction of TiAl Intermetallic Surfaces: A Combined STM and DFT Study

The structure of the γ-TiAl(111) alloy surface was studied with combined experimental and theoretical approaches. The experimental scanning tunneling microscopy (STM) images indicate surface reconstruction with a (2 × 2) superstructure as a result of Al depletion of the alloy surface after preparation. The atomic protrusions form a zigzag pattern oriented along the direction of the shorter surface unit vector of the (2 × 2) supercell, that is, the [101̅] direction. The theoretical investigations performed with molecular dynamics in combination with simulated annealing based on the density functional theory (DFT) show that the surface starts to reconstruct at temperatures higher than 950 K, in agreement with the experimental annealing temperature. Geometric analysis and simulated STM images obtained from periodic DFT calculations show that the atomic protrusions forming the zigzag patterns correspond to Al atoms of the reconstructed topmost plane. The persistence of the reconstructed structure up to the simulated melting temperature of 1800 K shows the high stability of Al vacancies injected in the topmost plane of the alloy.

Atomistic Modeling of Voiding Mechanisms at Oxide/Alloy Interfaces

For the first time, voiding mechanisms resulting from the condensation of atomic vacancies injected at oxide/alloy interfaces by the growth of oxide layers have been studied by means of periodic density functional theory (DFT) calculations. Several interfaces were built by superimposing ultrathin films of alumina on the γ-TiAl(111) surface, and their relative stabilities were compared by calculating the interface energy variation. The formation energy of single Ti or Al vacancies and clustered and dispersed ensembles of 2Ti + 1Al or 2Al + 1Ti vacancies injected into the alloy were calculated. The results show that it is easier to inject the vacancies into the oxide/alloy interface than into the bare alloy surface and into the bulk alloy. The injected vacancies, trapped at the oxide/alloy interface, condense in the topmost plane of the alloy to form 2D clusters. The minimization of the coordination number of the vacancies with metal atoms of the alloy and O atoms of the overlaying oxide favors vacancy condensation and interfacial voiding. The data are relevant for a detailed understanding of the adherence and breakdown of protective oxide films and the lifetime of metallic materials.

Ab initio study of the interaction of chlorides with defect-free hydroxylated NiO surfaces

DFT periodic calculations have been performed to model the adsorption and sub-surface insertion of Cl atoms at increasing coverage on undefective hydroxylated NiO(1 1 1) terraces characteristic of the surface of the passive film on nickel. The results evidence that structural relaxation, splitting the mixed topmost OH/Cl anionic plane, releases the repulsive interactions induced by Cl adsorption, but the effect decreases with increasing Cl coverage with a loss of ∼1.1 eV in stability at surface saturation. Cl sub-surface insertion in the first inner anionic plane also releases the repulsive interactions in the topmost plane but the effect becomes favourable only at high Cl coverage because of inter-layer mixing after surface reconstruction. At low Cl coverage, an unrelaxed diatomic Ni–OH layer weakly bonded to the oxide and likely to dissolve is formed after sub-surface insertion, suggesting a hybrid mechanism of local oxide thinning. Adsorption of Cl by place exchange is energetically favourable but does not seem to promote dissolution. Insertion into the lattice may promote dissolution but it is energetically favourable only at high Cl surface coverage, which is not a likely situation due to Cl repulsive interaction on the surface. For undefective terraces of the passive film, the reported calculations do not confirm the existing hypotheses of adsorption-induced oxide thinning or chloride sub-surface penetration.

Bending Die Design for a Pre-Formed Sheet Metal with a Golden Finger Feature

In this paper, authors proposed an effective quality index of bending operation and a new punch profile design method to prevent defects. The proposed quality index is presented in terms of distance of fracture location with respect to the topmost plane of blank, the maximum von Mises stress, and the maximum shear stress. The Taguchi method with L18 orthogonal array was adopted to evaluate the effects of design parameters and find out the optimum design of punch profile. A new punch feature called “golden finger” was proposed to control the material flow and move the fracture defects out of the trimming line. The results of this study had demonstrated the optimum die design can be achieved with the proposed golden finger feature to obtain a sound product.

Comparison between experimental surface data and bulk structure models for quasicrystalline AlPdMn: Average atomic densities and chemical compositions

We have examined bulk structure models for icosahedral AlPdMn in terms of the densities, compositions, and interplanar spacings for the fivefold planes that might represent physical surface terminations. We focus on four models that contain no partial or mixed occupancies, but some comparison is made to a fifth model containing such sites. Each of the four models contains paired planes (layers) that can be separated into two main families on the basis of three features: the relative densities of the two planes, the gap separating the layer from the nearest atomic plane, and the Pd content in the topmost plane. The experimental data and other arguments lead to the conclusion that the family with no Pd in the top plane is favored. Finally, all models show that correlations should be expected between the heights of steps that delineate terraces and average compositional and/or structural features of the terraces.

Translating the Socialist State: Cultural Exchange, National Identity, and the Socialist World in the Early PRC

Translating the Socialist State:Cultural Exchange, National Identity, and the Socialist World in the Early PRC Nicolai Volland (bio) The founding of the People's Republic of China (PRC) in 1949 marked not only the start of a profound transformation of the Chinese state institutions, the society, and the economy, but also the beginning of a monumental project to redefine the nature of the Chinese nation-state and its position in the world. The establishment of a new government was to give new meaning to the Chinese nation, in its own eyes, and in terms of its interaction with other nations. The politics of "leaning to one side (yi bian dao)," that had been agreed on in 1949,1 meant that the PRC was conceived as a state in the broader framework of the "socialist camp" from its very first hour.2 The integration of the PRC into an emerging socialist world that spanned half of the globe, from Berlin to Pyongyang, from Warsaw to Hanoi, and from Sofia to Novosibirsk—was a momentous event and gave legitimacy to the young regime that the Chinese Communist Party (CCP) was starting to build; it left an imprint on this regime for decades to come. On the topmost plane of politics, the PRC's entry into the socialist camp was translated into a series of bilateral treaties that the Chinese government signed with other socialist nations, and with the Soviet Union in particular. These processes have been relatively well-researched.3 However, the institutionalization of China's cooperation with the socialist nations of Eastern Europe and Asia could be but a first [End Page 51] step in the enormous project of redefining the Chinese nation-state. The next task for the CCP, arguably at least as important as winning diplomatic recognition from its new allies, was to reshape the nation's identity in the minds of its citizens. This new identity had to be both national and international; it was to define the nation-state and at the same time to transcend the national borders. To generate this kind of consciousness among the citizens of the nations in the socialist bloc was key to consolidating the new regimes—a task faced not only by the Chinese government, but by the other socialist nations as well. In this process, cultural factors played a key role, and a fundamental mechanism to create a feeling of cohesiveness and shared goals and values was the promotion of cultural exchanges.4 Soon after the founding of the PRC, the CCP thus began to set up a network of contacts and institutions entrusted with fostering exchanges in the cultural field designed to anchor the PRC firmly in the socialist camp. The government set out to build a cultural diplomacy that would assist and complement the PRC's efforts on the high-level diplomatic fronts and would penetrate deep into the populace to instill identity politics in the people's minds. In contrast to the formation of foreign policy in the early PRC, these efforts in cultural diplomacy have received surprisingly little scholarly attention.5 This article is an effort to explore some of the institutional dimensions and fields of activity of cultural exchange between the PRC and the [End Page 52] Soviet Union and the socialist countries of Eastern Europe, and to relate them to the complex patterns of identity politics in the early Cold War era.6 In the following pages, I will discuss several crucial avenues of cultural exchange that characterized the involvement of the PRC in the cultural diplomacy of the socialist camp. These include mutual visits of orchestras, writers, and drama troupes, the participation of Chinese delegations in international competitions and festivals, and the exchange of students in fields such as arts, drama, and music. Finally, an especially important field was the translation of literature across the socialist camp. In a coordinated effort, representative examples of the national literatures—in particular new works written in the socialist spirit—were translated simultaneously into multiple languages and circulated across the bloc. Readers in Poland thus read the same Russian novels at the same time as their peers in Romania, North Korea, and the PRC, and Chinese...

Atomic structure of the surface alloy formed by a room-temperature-deposited Mn monolayer on Ag(001)

The determination of the atomic structure of a Mn monolayer deposited at room temperature on Ag(001) was achieved by using surface extended x-ray absorption fine structure. Keeping in mind the previous results of x-ray photoelectron diffraction, several structural models involving the two topmost layers of the Ag substrate were tested in ab initio polarization-dependent XAFS calculations. Among these models, the most consistent one with the experimental data is made of a mixture of two different local environments that correspond to two different structures: first an inverted layer of Mn atoms substituting Ag ones underneath the topmost plane in fcc Ag; second a ${\mathrm{Ag}}_{0.5}{\mathrm{Mn}}_{0.5}$ surface alloy confined to the two topmost layers of Ag substrate. This latter arrangement corresponds to the observed $c(2\ifmmode\times\else\texttimes\fi{}2)$ low-energy electron diffraction long-range order. Accurate values for the Mn-Ag and Mn-Mn first interatomic distances were derived from standard analysis and found to be $2.86\ifmmode\pm\else\textpm\fi{}0.02$ and $2.88\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ respectively. This definitely establishes that, in this surface alloy, Mn adopts an unusually large atomic volume, essentially the same as Ag. This finding is consistent with a high spin state of the Mn similar to the one in dilute Ag-based alloys or in the atomic ${}^{6}{S}_{5/2}$ ground state.

Surface hydroxylation and local structure of NiO thin films formed on Ni(111)

X-ray photon spectroscopy (XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) have been combined to study the surface hydroxylation and the structure of NiO thin films formed on Ni(111) at 300 and 500 K. After oxidation at 300 K, the thin film consists of NiO(111) grains (3 ± 1 nm large) in parallel epitaxy with the substrate and homogeneously distributed on the substrate. The average thickness is three to four NiO(111) layers with local variations measured on different grains [ca 1 NiO(111) plane] and at boundaries between grains (up to 0.6 nm). The film is terminated by 0.85 ± 0.1 monolayer (ML) of hydroxyl groups (OH − according to XPS) stabilizing the unreconstructed (1 × 1) oxide surface structure. Exposing this film to 150 L (1 Langmuir = 10 −6 Torr s −1) of water leads to the formation of a second layer of hydroxyl groups with a total coverage of 1.5–2 ML with no significant variation of the structure of the film. The XPS data are well fitted with a terminal sequence (OHNiOH) equivalent to one layer of β-Ni(OH) 2 with partial filling of the topmost OH plane. After oxidation at 500 K, the surface is covered at 93 ± 3% by extended and atomically flat NiO(100) areas. Triangular NiO(111) grains (8 ± 2 nm large) in parallel epitaxy are present within the NiO(100) layer and cover 7 ± 3% of the surface. They are covered by a full monolayer of OH − groups also stabilizing the (1 × 1) structure. The NiO(100) layer is characterized by (2 × 2), (2 × 3), (3 × 2) and (3 × 3) local superstructures assigned to local deviations from the average NiO stoichiometry in the topmost plane. These areas do not adsorb hydroxyl groups on regular sites but possibly on defect sites (steps). Exposing this film to 150 L of water leads to the lateral extension of the triangular NiO(111) grains. A preferential reorientation of hydroxyl-free NiO(100) to OH −-covered NiO(111) upon water exposure is observed. This reorientation affects the NiO(100) matrix only at the perimeter of the NiO(111) grains, showing that the hydroxyl groups do not induce nucleation of NiO(111) in the NiO(100) layer but cause the growth of the existing NiO crystallites.

Low-coverage sulfur induced reconstruction of Ni(111)

STM, LEED and AES data are reported for S segregated on Ni(111) between 675 and 1175 K. Two chemisorbed phases corresponding to different coverages (√39 for Θ S = 0.22 ML and (5√3 × 2)rect for Θ S = 0.40 ML) have been observed which are identical to phases produced by adsorption and post-annealing of H 2S. STM data obtained for the (5√3 × 2)rect-S phase confirm the missing row reconstruction of the first Ni plane in this phase. For the √39-S phase, STM and LEED data give evidence of reconstruction of the topmost Ni plane to generate 4-fold coordinated S adsorption sites. Two models are proposed, one with a (100) arrangement of the first Ni plane to give an overstructure similar to that of the Ni(100)-(2 × 2)-S phase, the other with a clockwise-anticlockwise reconstruction of the (100) arrangement of the first Ni plane similar to that reported for C and N adsorption on Ni(100). STM data provide direct evidence of a preferential formation of the √39-S phase at substrate monoatomic steps, on the upper terraces. Nucleation takes place preferentially at step edges where the lower coordination of Ni atoms allows an easier reconstruction of the upper terrace to produce the topmost plane of the √39 phase. The growth can proceed then on both the upper and the lower terraces.

On the superconducting gap structure of high-temperature superconductors by STM/STS

The tunneling spectra observed by STM on the HTSC's have been converging in terms of the superconducting gap energy values: 2 Δ/ k B T c=6 to 9. There are, however, still disagreements between the details of various results. Both d- and s-wave-like features have been claimed for the superconducting gap structure. Furthermore, both flat and V-shaped structures have been observed in the background conductance curve outside the superconducting gap. The major differences among various reports seem to be attributable to the difference in the top layer and in the carrier density of the sample. It is noted that those who observe on the BiO topmost plane in a Bi2212 crystal or on the CuO-chain plane of YBCO seem to be claiming the d-wave, and those who observe on different top planes the s-wave symmetry.

Probing bimetallic surfaces with photoelectron diffraction: Au/Cu(001) and Fe/Cu(001)

Photoelectron diffraction has been used to investigate Au/Cu(001) and Fe/Cu(001). In the case of Au/Cu(001), a structural determination of the 1/2 ML, c(2×2) surface has been performed, indicating that it is a surface alloy with Au and Cu atoms together in the topmost plane and pure Cu(001) underneath. Moreover, the Au/Cu(001) system has been studied as a function of temperature and concentration, allowing a detailed picture of surface alloying and dealloying.

LEED I-V curve analysis for the structure of iron films on Si(111) surfaces

Iron films deposited on Si(111)7×7 surfaces grow layer-by-layer and the structure of iron films of more than half a monolayer (12 ML) average thickness changes from a 1×1 to a 2×2 superstructure after annealing above 400°C. LEED I-V curves of several beams have been measured, and for a film of 1 ML average thickness, experimental curves are compared with curves obtained by dynamical calculations. Consequently, it is most probable for the 1×1 structure that iron atoms sit in 3-fold hollow sites on a Si substrate which is missing the topmost plane.