Comparison of MNDO to tight-binding,total-energy methods for surface atomic structure determination: Aluminum phosphide (110)

Tight-binding, total-energy (TBTE ) models have been successfully used to calculate the equilibrium surface atomic structures of a variety of materials, but are difficult to apply to substances with complicated interatomic repulsions. In these cases, the modified neglect of diatomic overlap (MNDO ) method, which specifically includes longer-range interactions, may provide an effective alternative. We present new calculations of the surface atomic structure of the AlP (110) surface using both techniques. This surface, whose structure has been determined quantitatively using a low-energy, electron-diffraction intensity analysis, displays a well-known relaxation characteristic of (110) surfaces of zincblende-structure semiconductors. The TBTE model, parameterized to bulk AlP properties, provides a more accurate prediction of the relaxed surface atomic positions, although MNDO , parameterized to small molecules, produces acceptable results. Despite the greater demands placed by MNDO on computer resources, it may prove useful in the study of materials that are difficult to model within a TBTE framework. © 1992 John Wiley & Sons, Inc.     

Structure and oxidation at quasicrystal surfaces

We have investigated the atomic and electronic structure, chemical composition, and oxidation characteristics of the surfaces of icosahedral, Al-rich quasicrystals, using a variety of surface-sensitive techniques (LEED, XPS, STM, AES). We have systematically investigated the way that these traits vary with preparation conditions (e.g. sputtering and then annealing to various temperatures, vs. fracture), with surface symmetry (e.g. 2f vs. 3f vs. 5f surfaces), and with bulk composition (e.g. i-Al–Pd–Mn vs. i-Al–Cu–Fe). We have also compared our results for the quasicrystals with results for crystalline approximants and other related crystalline phases. Our main conclusions are that, under specific conditions of sputter-annealing, the bulk atomic and electronic structures of the clean quasicrystal propagate to the surface. Also, the oxidation chemistry is dominated by that of the primary constituent, aluminum.     

Investigating bonding in small silicon-carbon clusters: exploration of the potential energy surfaces of Si3C4, Si4C3, and Si4C4 using ab initio molecular dynamics

A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si-C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms.     

Electron spectroscopy and adsorption studies: Chemical shifts in X-ray photoelectron and auger spectra for monolayer adsorption

A range of transition metal film substrates (Mo, W, Pt, Ni, Fe, Cu, V, Zn) exhibit no clear chemical shifts in their ESCA or Auger peaks on adsorption of a monolayer equivalent of oxygen or carbon monoxide, though in some cases asymmetric broadening of the peaks may be ascribable to a small chemical shift of a component of the peak. This compares to the large shifts that can occur in these peaks on bulk oxidation to many atomic layers depth. In addition the spectra of the clean substrates show no indication of a difference in the surface layer of atoms. These results are not unexpected, but are in contradiction to some previously reported data. They suggest that attempts to follow chemisorption processes by monitoring chemical shifts of substrate peaks are likely to prove unsuccessful in many cases involving transition metals, particularly for common spectrometer geometries (30° to 60° ejection angle). Changing to grazing exit angle will increase the probability of observing small shifts for the outer substrate atoms by reducing the effective escape depth.     

Sum frequency generation and coherent anti-Stokes Raman spectroscopic studies on plasma-treated plasticized polyvinyl chloride films

Polyvinyl chloride (PVC) is a widely used polymer to which various phthalates are extensively applied as plasticizers. PVC materials are often treated with plasma to vary the hydrophobicity or for cleaning purposes, but little is known of the nature of the surface molecular structures after treatment. This research characterizes molecular surface structures of PVC and bis-2-ethylhexyl phthalate (DEHP)-plasticized PVC films in air before annealing, after annealing, and after exposure to air-generated glow discharge plasma using sum frequency generation (SFG) vibrational spectroscopy. In addition, we compare the vibrational molecular signatures on the surfaces of PVC with DEHP (at a variety of percent loadings) to those of the bulk detected using coherent anti-Stokes Raman scattering (CARS). X-ray photoelectron spectroscopy (XPS) and contact angle measurements have been used to analyze PVC surfaces to supplement SFG data. Our results indicate that DEHP was found on the surfaces of PVC films even at low weight percentages (5 wt %) and that DEHP segregates on surfaces after annealing. The treatment of these films with glow discharge plasma resulted in surface-sensitive reactions involving the removal of chlorine atoms, the addition of oxygen atoms, and C-H bond rearrangement. CARS data demonstrate that the bulk of our films remained undisturbed during the plasma treatment. For the first time, we probed the molecular structure of the surface and the bulk of a PVC material using combined SFG and CARS studies on the same sample in exactly the same environment. In addition, the methodology used in this research can be applied to characterize various plasticizers in a wide variety of polymer systems to understand their surface and bulk structures before and after systematic applications of heat, plasma, or other treatments.     

Physical properties of gamma alumina surface hydroxyls revisited through a large scale periodic quantum-chemistry approach

We have studied surface hydroxyls adsorbed onto (001), (011), and (111) gamma alumina surfaces using a quantum-chemistry approach in order to compare with empirical models proposed in the literature. Local electronic structures and geometries in the low OH coverage limit have been evaluated for both ideal and relaxed surfaces with the help of a large scale periodic quantum-chemical code. Hydroxyl groups are adsorbed onto surfaces, and a study of their local electronic structure, vibrational frequencies, charges, and adsorption energies is performed and analyzed as a function of their adsorption site geometry. Our results show that, even on ideal (nonrelaxed) surfaces, OH local environments are more complicated than those stated by empirical models and strongly influence the hydroxyl stretching vibrational mode. Large scale simulation shows that disorder takes place even at 0 K, and the analysis of the vibrational frequencies leads to a revision of Kn?zinger's empirical model. Cationic vacancies in the first surface layers have also been taken into account; they have a significant influence on the surface atomic and electronic structures, modifying the physical properties of adsorbed OH entities. This work emphasizes the necessity to perform an electronic structure calculation to better understand adsorbed OH properties on gamma alumina surfaces and reveals the difficulty to make a one-to-one correspondence between OH stretching frequencies and their other physical properties. Finally, we show that these results agree with some available experimental studies.     

A quartz crystal microbalance study of the adsorption of quaternary ammonium derivates on iron and cementite

The adsorption of alkyl trimethyl ammonium bromide with alkyl groups from C₁₀ to C₁₆ onto solid surfaces (iron and cementite (Fe₃C)) is investigated with and without NaCl. For the first time the adsorption of these surface active compounds onto model carbon steel compounds are measured directly by the use of a quartz crystal microbalance (QCM). The adsorption increases steeply around the critical micelle concentration (cmc) in bulk which was also determined for the homologous series. The adsorption of the higher homologues shows rather rigid surface structures while a detailed interpretation of the isotherms for the lowest homologues is disturbed by high bulk viscosities. Based on the obtained results, and in comparison with similar adsorbents on hydrophilic oxide surfaces, it is concluded that the surfactant is adsorbed patchwise in similar structures as in bulk. Finally, the possibilities for close-packing of the surfactant film are discussed.     

Weak hydrogen bonds as a structural motif for two-dimensional assemblies of oligopyridines on highly oriented pyrolytic graphite: an STM investigation

We present the STM investigation of four different oligopyridines at the liquid/highly oriented pyrolytic graphite interface. The heteroaromatic compounds are constitutional isomers showing the same overall shape regardless of their actual conformation. On the basis of weak intermolecular C-H...N hydrogen-bonding interactions, different nanopatterns are formed following a simple general concept for the two dimensional self-assembly. The molecules arrange either in linear or in cyclic structures. Though the oligopyridines are achiral, the formation of prochiral trimeric superstructures leads to chiral phases due to the immobilization on the surface. Some of the molecules show polymorphic structures depending on the solvent. The large variety of the presented structures formed by self-assembly of the different oligopyridines which retain the same functional heteroaromatic backbone shall open the possibility of exploiting these patterns as templates for the nanostructuring of surfaces with guests such as small molecules or metal ions for intriguing applications in, for example, catalysis.     

Structures and electronic transport on silicon surfaces

By utilizing a variety of surface superstructures formed on silicon surfaces and atomic layers grown on them, close correlations between the atomic-scale structures and electrical conduction phenomena at the surfaces have been revealed. State-of-art techniques for analyzing and controlling atomic/electronic structures of surfaces are leading to an understanding of the novel electronic transport properties at surfaces. For example, the electrical conduction through surface-state bands, which are inherent in the surface superstructure, has been confirmed in in-situ measurements. An important phenomenon has also been found, where adatoms donate carriers into the surface-state band, resulting in a remarkable enhancement in electrical conductance. The nucleation of the adatoms diminishes such a doping effect. Furthermore, electrical conduction through atomic layers grown on the surfaces, whose growth structures are sensitive to the substrate surface structures, will be also discussed. In this review, we emphasize that the surface electronic transport properties are closely related to the atomic structures and atomistic dynamics on surfaces. The ultimate two-dimensional electron systems, consisting of the surface-state bands and grown atomic layers, are expected to provide a new stage in surface physics, as well as a precursory stage leading to atomic-scale electronics devices.     

Interplay between subsurface ordering, surface segregation, and adsorption on Pt-Ti(111) near-surface alloys

Using the first-principles cluster expansion (CE) method, we studied the subsurface ordering of Pt/Pt-Ti(111) surface alloys and the effect of this ordering on segregation and adsorption behavior. The clusters included in the CE are optimized by a genetic algorithm to better describe the interactions between Pt and Ti atoms in the subsurface layer. Similar to bulk Pt-Ti alloys, Pt-Ti(111) subsurface alloys show a strong ordering tendency. A series of stable ordered Pt-Ti subsurface structures are identified from the two-dimensional (2D) CE. As an indication of the connection between the 2D and the bulk ordering, the CE predicts a ground-state Pt(8)Ti structure in the (111) subsurface layer, which is the same ordering as the close-packed plane of the bulk Pt(8)Ti compound. We carried out Monte Carlo simulations (MC) using the CE Hamiltonian to study the finite temperature stability of the Pt-Ti subsurface structures. The MC results show that subsurface structures in the Pt-rich range have higher order-disorder transition temperatures than their Ti-rich subsurface counterparts. We calculate the binding energy of different adsorbates (O, S, H, and NO) on Pt-terminated and Ti-segregated surfaces of ordered PtTi and Pt(8)Ti subsurface alloys. The binding of these adsorbates is generally stronger on Ti-segregated surfaces than Pt-terminated surfaces. The adsorption-induced Ti surface segregation is determined by two factors: (i) the unfavorable energy penalty for the Ti atom to segregate to the clean surface and (ii) the favorable energy decrease from stronger adsorbate binding on the Ti-segregated surface. The two factors introduce similar magnitude in energy change for the S and NO adsorption on Ti-segregated surfaces of PtTi subsurface alloys. We predict an adsorption-induced Ti surface segregation that is dependent on the atomic configurations of the Ti-segregated surfaces resulting from the competition of the two factors.     

Direct Formation of C−C Triple‐Bonded Structural Motifs by On‐Surface Dehalogenative Homocouplings of Tribromomethyl‐Substituted Arenes

On‐surface synthesis shows significant potential in constructing novel nanostructures/nanomaterials, which has been intensely studied in recent years. The formation of acetylenic scaffolds provides an important route to the fabrication of emerging carbon nanostructures, including carbyne, graphyne, and graphdiyne, which feature chemically vulnerable sp‐hybridized carbon atoms. Herein, we designed and synthesized a tribromomethyl‐substituted compound. By using a combination of high‐resolution scanning tunneling microscopy, non‐contact atomic force microscopy, and density functional theory calculations, we demonstrated that it is feasible to convert these compounds directly into C?C triple‐bonded structural motifs by on‐surface dehalogenative homocoupling reactions. Concurrently, sp³‐hybridized carbon atoms are converted into sp‐hybridized ones, that is, an alkyl group is transformed into an alkynyl moiety. Moreover, we achieved the formation of dimer structures, one‐dimensional molecular wires, and two‐dimensional molecular networks on Au(111) surfaces, which should inspire further studies towards two‐dimensional graphyne structures.     

Nanostructures and phase diagrams of ABC star triblock copolymers in pore geometries

The nanostructures and phase diagrams of ABC star triblock copolymers in pore geometries are investigated using the real-space self-consistent field theory in two-dimensional space. Two types of pores with neutral surfaces, namely, pores with small and large diameters, are considered. A rich variety of nanostructures are exhibited by the ABC star triblock copolymers in these two types of pores, which differ from those observed in bulk and in other confinements. These structures include perpendicular undulating lamellae, concentric core-shell cylinders, polygonal tiling with cylindrical arrangements, and other complex structures. Triangular phase diagrams for the ABC star triblock copolymers are constructed. The small pores clearly affect the corner and central space of the phase diagrams by distorting the bulk structures into concentric arrangements. Meanwhile, the large pores induce the transformation of bulk structures into concentric structures in most of the phase space, but slightly affect the structures at the center of the phase diagrams. Furthermore, the order-order and order-disorder phase transitions, as well as the stable and metastable phases, in the triangular phase diagrams are examined by analyzing their free energies. These observations on the ABC star triblock copolymers in the pore geometries provide a deeper insight into the behavior of macromolecules in a confined system.     

Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals

Electron-hole excitation and relaxation in the bulk, at interfaces, and surfaces of solid state materials play a key role in a variety of physical and chemical phenomena that are important for surface photochemistry, particle-surface interactions, and device physics. Information on charge carrier relaxation in metals can be obtained through analysis of linewidths measured by photoemission and related techniques, which give an estimate of the upper limit for electron and hole relaxation; however, many factors can contribute to spectral broadening, thus it is difficult to extract specific information on electronic relaxation processes. With femtosecond lasers it is possible to probe directly in a time-resolved fashion the charge carrier dynamics in metals by a variety of linear and nonlinear optical techniques. Femtosecond time-resolved two-photon photoemission has attracted particularly strong interest because it incorporates many of the surface analytical capabilities of photoemission and inverse photoemission — the traditional probes for surface and bulk band structures of solid state materials — with time-resolution that is approaching the fundamental response of electrons to optical excitation. Advances in the direct measurements of electron-hole excitation, charge carrier relaxation, and dynamics of intrinsic and adsorbate induced surface states are reviewed. With femtosecond lasers it also is possible to probe a variety of coherent phenomena, and even to control the charge carrier dynamics in metals through the optical phase of the excitation light. Pioneering experiments in this new field also are discussed.     

The Structure of Sub-nm Platinum Clusters at Elevated Temperatures

Little is known about metallic clusters consisting merely of a dozen of atoms or even less, despite of their importance in catalysis and crystal nucleation. Scanning transmission electron microscopy (STEM) provides direct atomic structure information but has inherently suffered from limited time resolution. We employ fast dynamic STEM combined with a spatio-temporal image denoising algorithm to explore the structure and stability of Pt clusters on carbon, which represents a highly relevant catalysis system. At room temperature, dynamic amorphous 2D structures are found, while above ≈300 °C, the clusters transform into a crystalline state. Our experimental and theoretical data reveal an unexpected strong trend of the crystalline clusters to exhibit the face-centered cubic, bulk structure of Pt with cuboidal geometries being most prominent.     

Growth modes of organic semiconductor thin films using organic molecular beam deposition: epitaxy,van der Waals epitaxy,and quasi-epirtaxy

We compare the ultrahigh vacuum growth of several different archetype organic molecular compounds on a variety of substrates, and find that the resulting thin films can form epitaxial, van der Waals epitaxial (vdWE) or quasi-epitaxial (QE) structures. Epitaxial and vdWE films form lattices commensurate with that of the substrate, where epitaxy refers to chemisorbed films while vdWE refers to purely vdW-bonded, physisorbed films. In contrast, QE films are incommensurate with the substrate lattice. In spite of this incommensurability, QE films can exhibit long range order, and have a unique orientational relationship with the substrate lattice. Quasi-epitaxial growth can result in strained structures, where the strain energy is relieved by variations in the internal lattice degrees of freedom. It has been found that QE growth can thus result in stable, quasi-equilibrium film structures which are different than the bulk structure of that same material. Organic thin films such as the phthalocyanines, 3,4,9,10-perylenetetracar☐ylic dianhydride, and the fullerenes grown on graphite, alkali halide crystals, Au, semiconductors and other substrates are all discussed.     

ZrC(111)弛豫表面电子结构及其吸附氢的理论研究

采用第一性原理的方法对ZrC（111）清洁和氢吸附表面的电子结构进行了研究 。构型优化结果表明，从表面深入到体相一定深度，层间距存在交替“收缩”和“ 扩张”的现象，其紧邻E_F以下的DOS峰对应于活性表面态，主要成分为表面Zr原子 4d_(xz)/d_(yz)轨道；与理想表面相比，表面弛豫对该表面态影响不大。对于氢吸 附表面，计算结果表明氢原子倾向于吸附在表面孔洞上方位置（正对着第三层Zr原 子），此时H的1s态从体相的电子态中分离出来。此外，本文中H/ZrC（111）和 H/NbC（111）体系的H 1s诱导态变化进行了解释，并对清洁的吸附表面的芯能级位 移以及功函的变化进行了探讨。     

Effect of Two‐Dimensional Crystal Orbitals on Fermi Surfaces and Electron Transport in Three‐Dimensional Perovskite Oxides

Perovskite oxides are candidate materials in catalysis, fuel cells, thermoelectrics, and electronics, where electronic transport is vital to their use. While the fundamental transport properties of these materials have been heavily studied, there are still key features that are not well understood, including the temperature‐squared behavior of their resistivities. Standard transport models fail to account for this atypical property because Fermi surfaces of many perovskite oxides are low‐dimensional and distinct from traditional semiconductors. In this work, the low‐dimensional Fermi surfaces of perovskite oxides are chemically interpreted in terms of two‐dimensional crystal orbitals that form the conduction bands. Using SrTiO₃ as a case study, the d/p‐hybridization that creates these low‐dimensional electronic structures is reviewed and connected to its fundamentally different electronic properties. A low‐dimensional band model explains several experimental transport properties, including the temperature and carrier‐density dependence of the effective mass, the carrier‐density dependence of scattering, and the temperature dependence of resistivity. This work highlights how chemical bonding influences semiconductor transport.     

Surface X-ray scattering of stepped surfaces of platinum in an electrochemical environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO(4) at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)-(100), are reconstructed to a (1 × 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 × 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.     

Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix.     

The Structure of Sub‐nm Platinum Clusters at Elevated Temperatures

Little is known about metallic clusters consisting merely of a dozen of atoms or even less, despite of their importance in catalysis and crystal nucleation. Scanning transmission electron microscopy (STEM) provides direct atomic structure information but has inherently suffered from limited time resolution. We employ fast dynamic STEM combined with a spatio‐temporal image denoising algorithm to explore the structure and stability of Pt clusters on carbon, which represents a highly relevant catalysis system. At room temperature, dynamic amorphous 2D structures are found, while above ≈300 °C, the clusters transform into a crystalline state. Our experimental and theoretical data reveal an unexpected strong trend of the crystalline clusters to exhibit the face‐centered cubic, bulk structure of Pt with cuboidal geometries being most prominent.