Structure Of Overlayers Research Articles

Oxygen driven reconstruction dynamics of Ni(977) measured by time-lapse scanning tunneling microscopy

Time-lapse scanning tunneling microscopy (STM) has been used to observe the oxygen induced reconstruction behavior of Ni(977), a stepped metallic surface. Previous studies using helium atom diffraction resolved the macroscopic kinetics for the reversible step-doubling and -singling of this vicinal surface. Sequential STM imaging recorded at elevated temperature has now elucidated atomic-level mechanistic details for the merging of steps in the presence of small amounts of adsorbed oxygen, less than 2% of a monolayer. Point contact between neighboring steps decorated with chemisorbed oxygen facilitates rapid step coalescence by means of zippering. An optimal oxygen concentration of step edge saturation was found to enable the step merging to proceed most rapidly. Excess oxygen was found to hinder the coalescence of neighboring steps through the possible growth of overlayer structures on the terraces. At sufficiently high temperatures, the surface is driven back to single steps due to oxygen dissolution. The departure of oxygen from the surface through dissolution, as well as the associated presence of oxygen in the selvedge region, may both play a role in destabilizing the double steps. Local step density influences the coalescence behavior by defining the number of available step edge sites. The microscopic details made available by time-resolved STM imaging illuminate some of the mechanistic steps related to the initial stages of metallic oxidation, and the sensitivity of surface morphological transformations to local surface structure and adsorbate coverage.

Adsorption and decomposition of C 60 on Ni(1 1 0) surfaces

In the present paper we study the adsorption of C 60 on Ni(1 1 0) surfaces at RT and 650 K, as well as the desorption/decomposition process. The investigation took place in UHV by means of Auger electron spectroscopy, low energy electron diffraction, thermal desorption spectroscopy and work function (WF) measurements. The observed overlayer structures during C 60 deposition at 650 K are in complete agreement with reported results. The sticking coefficient is nearly the same for both RT and 650 K. Heating of the 1 ML C 60-covered Ni(1 1 0) surface, however, at 750 K, causes a partial fragmentation of C 60 to a 2D graphite-like layer, while the remaining C 60 molecules are rearranged in the (5×3) structure, in contrast to reported results. Further heating to 800 K results in the complete fragmentation of C 60 accompanied by carbon desorption, possibly in molecular C n ( n>10) form. Above 800 K, the C of the graphite-like layer is desorbed in both atomic and molecular form. The WF at saturation coverage was found to be 5.61 eV for both deposition at RT and 650 K. This value is substantially greater than the average value of 5 eV, which has been suggested to hold for C 60 overlayer systems, regardless of the metal substrate.

Growth, structure, and magnetic properties of Fe monolayers on Cu84Al16(100)

We present experimental results on the growth and structure of Fe overlayers deposited on Cu84Al16(100) and discuss the correlation between the structural and magnetic properties of this system. Fe films 1–6 monolayers (ML) thick were grown under molecular beam epitaxy conditions onto the clean substrate at 160 K. Electron diffraction was applied to investigate the structure of the Fe films. The magnetic properties were investigated in situ by surface magneto-optical Kerr effect in the longitudinal geometry. Our results show that the onset of in-plane ferromagnetism around 3.5 ML of Fe coincides with structural changes that suggest a transformation within the Fe films from a fcc-like (100) to a bcc-like (110) structure with increasing Fe thickness, starting from 2.5 ML.

The Adsorption and Reaction of HCl on Pd(111)

HCl adsorbed onto clean Pd(111) produces three distinct HCl desorption states in thermal desorption spectroscopy (TDS), all of which are populated simultaneously, even at exposures as low as 0.33 L. One desorption state emanates from molecularly adsorbed HCl (220 K), while the other two (300 and 470 K) emanate from recombination of H plus Cl atoms. The lowest temperature recombinative peak (β at ∼300 K) belongs to recombination of chlorine adatoms with surface hydrogen. This reaction is competitive with the recombinative desorption of H2. Finally, the highest temperature peak with an onset at 470 K (α) is the recombination of the remaining Cl with hydrogen dissolved in the Pd crystal, which emerges from the bulk onto the surface at approximately 470 K. Some of the chlorine adatoms order on the Pd surface to yield a √3 × √3 R 30° overlayer structure at coverages as low as θ = 0.13 ML. This structure persists through a range of temperatures from 100 to 320 K, when initially adsorbed at 100 K. The order−diso...

Breakdown of the bi-dimensional symmetry in bct Fe layers by epitaxy on Co([formula omitted]) surface

The structure of thin Fe overlayers on the Co(112̄0) surface has been investigated by low energy electron diffraction (LEED) and by primary-beam diffraction modulated electron emission (PDMEE), as a function of the Fe coverage (6–50 ML) and of the substrate temperature (between room temperature and 300°C). The structure of the layers was the tetragonally distorted bct modification of the equilibrium bcc phase, with the (001)Fe||(112̄0)Co and [11̄0]Fe||[11̄00]Co relationships with the Co substrate. The Fe(001)–Co(112̄0) system exhibits a very large misfit asymmetry in the interface plane. As a consequence, a breakdown of the long-range order occurs in the Fe film along the [11̄00] substrate direction.

Structure and Thermal Stability of the Sulfide Overlayers on CH3CSNH2-Passivated GaAs(100) Surfaces

Structure and Thermal Stability of the Sulfide Overlayers on CH3CSNH2-Passivated GaAs(100) Surfaces

An approach to the atomic structure of bulk metallic overlayers using tip-induced layer-by-layer dissolution

Well-grown 3-D copper clusters were obtained on a Pt(111) surface at intermediate overpotentials from a sulfuric acid solution. The hexagonal and triangular clusters grew layer-by-layer and were aligned to the orientation of the substrate, indicating epitaxial growth without rotation. By using the tip as an anodic electrode, the atomic structure of each layer of the cluster was monitored locally during dissolution. The results show that in the copper clusters the lattice constant relaxes directly to the bulk lattice constant in the second layer (at least after deposition of the third layer). Additional discussion on the dissolution kinetics is also included. The same method has been used to elucidate the structure of the Cu overlayer on Pt(100).

Electronic structure effects from hydrogen bonding in the liquid phase and in chemisorption: an integrated theory and experimental effort.

A closely integrated theoretical and experimental effort to understand chemical bonding using X-ray spectroscopic probes is presented. Theoretical techniques to simulate XAS (X-ray absorption spectroscopy), XES (X-ray emission spectroscopy), RIXS (resonant inelastic X-ray scattering) and XPS (X-ray photoelectron spectroscopy) spectra have been developed and implemented within a density functional theory (DFT) framework. In combination with new experimental techniques, such as high-resolution XAS on liquid water under ambient conditions and XES on complicated surface adsorbates, new insight into e.g. hydrogen-bonded systems is obtained. For the (3x2) overlayer structure of glycine/Cu(110), earlier work has been extended to include adsorbate-adsorbate interactions. Structures are optimized for large cluster models and for periodic boundary conditions. It is found that specific features in the spectra arise from hydrogen-bonding interactions, which thus have important effects at the molecular-orbital level. XAS on liquid water shows a pronounced pre-edge feature with significant intensity, while the spectrum of ice shows only little intensity in this region. Theoretical spectrum calculations, based on instantaneous structures obtained from molecular-dynamics (MD) simulations, show that the pre-edge feature in the liquid is caused by water molecules with unsaturated hydrogen bonding. Some aspects of the theoretical simulations will be briefly discussed.

Nickel ferrocyanide modified electrodes as active cation-exchange matrices: real time XRD evaluation of overlayer structure and electrochemical behavior

The dynamic variations in the structure of nickel ferrocyanide modified electrodes was determined using in situ diffractoelectrochemical techniques. X-ray powder diffraction peaks from the nickel ferrocyanide modified electrode were recorded during electrochemical oxidation/reduction of the surface overlayer and reversible changes in the unit cell lattice parameter observed. The electrochemical changes that occur during oxidation/reduction in various alkali cation electrolytes are correlated with changes in the lattice parameter. It is postulated that micro-domains form when two cations having markedly different radii such as cesium and potassium are intercalated into the nickel ferrocyanide structure so that distinct regions of the nickel ferrocyanide layer containing cesium exist separated from regions with the lighter alkali cations.

Magnetism of rough overlayers – an augmented space recursive study

The use of the augmented space recursion (ASR) coupled with tight-binding linearized muffin tin orbital method as an ideal methodology for the study of electronic and magnetic structures of rough surfaces, interfaces and overlayers is proposed here. The present method deals satisfactorily with roughness, short-ranged clustering effects and interdiffusion. We present here results on magnetism of Fe overlayer on Cu, Ag and Au substrates.

Epitaxy and Molecular Organization on Solid Substrates

The recent emergence of molecular films as candidates for functional electronic materials has prompted numerous investigations of the underlying mechanisms responsible for their structure and formation. This review describes the role of epitaxy in molecular organization on crystalline substrates. A much-needed grammar of epitaxy is presented that classifies the various modes of epitaxy according to transformation matrices that relate the overlayer lattice to the substrate lattice. The different modes of epitaxy can be organized hierarchically to reflect the balance of overlayer–substrate and molecule–molecule energies. In the case of molecular overlayers, the mismatch of overlayer and substrate symmetries commonly leads to coincident epitaxy in which some of the overlayer lattice points do not reside on substrate lattice points. Analyses of numerous reported epitaxial molecular films reveal that coincidence is quite common even though, based on overlayer–substrate interface energies alone, not as energetically favorable as commensurism. The prevalence of coincidence can be attributed to overlayer elastic constants, associated with molecule–molecule interactions within the overlayer, that are larger than the elastic constants of the overlayer–substrate interface. This condition facilitates prediction of the epitaxial configuration and overlayer structure through simple and comparatively efficient geometric modeling that does not require the input of potential energies, while revealing the role of phase coherence between the overlayer and substrate lattices.

Magnetic structure of manganese overlayers on semi-infinite chromium substrate

We report a study of the magnetic behavior of ultrathin Mn films deposited on a Cr substrate using a real space tight binding approach. The magnetism is described in the Hartree–Fock approximation of the Hubbard Hamiltonian. The density of states is calculated within the recursion technique. The magnetic moments of Mn–Cr system up to three Mn overlayers for different crystallographic orientations {(001), (011), (111)} are calculated. The onset of magnetism depends on the faces considered. A sizable induced moment appears in the Cr interface layer. The calculations show ferromagnetic sheets of both metals for the planes {(001) and (111)} and magnetic c(2×2) configuration for the (011) plane. This behavior is consistent with the antiferromagnetic ground states of the constituents.

Surface morphology changes during Pb deposition on Cu(1 0 0): evidence for surface alloyed Cu(1 0 0)–c(2×2) Pb

Using low energy electron microscopy (LEEM), we have followed Cu(1 0 0) surface morphology changes during Pb deposition at different temperatures. Surface steps advance and two-dimensional (2-D) islands nucleate and grow as deposited Pb first alloys, and then dealloys, on a 125°C Cu(1 0 0) surface. From LEEM images, we determine how much Cu is being displaced at each stage and find that the amount of material added to the top layer for a complete Pb/Cu(1 0 0) c(4×4) reconstruction (a surface alloy) is consistent with the expected c(4×4) Cu content of 0.5 monolayer. However, as the surface changes to the Pb/Cu(1 0 0) c(2×2) overlayer, we find that the displaced material from surface dealloying, 0.22 ML, is more than a factor of two lower than expected based on a pure Pb c(2×2) overlayer. Further, we find that in the 70–130°C range the amount of Cu remaining in c(2×2) increases with increasing substrate temperature during the deposition, showing that surface Cu is alloyed with Pb in the c(2×2) structure at these temperatures. When holding the sample at 125°C, the transformation from the c(2×2) structure to the higher coverage c(5√2×√2) R45° overlayer structure displaces more Cu, confirming the c(2×2) surface alloy model. We also find the c(2×2) surface has characteristically square 2-D islands with step edges parallel to the (1 0 0) type crystallographic directions, whereas the c(5√2×√2) R45° structure has line-like features which run parallel to the dislocation double rows of this surface’s atomic structure and which expand into 2-D islands upon coarsening.

Structural characterisation of Fe layers on Co(112̄0)

The structure of thin Fe overlayers on the Co(112̄0) surface has been investigated by low energy electron diffraction (LEED) and by primary-beam diffraction modulated electron emission (PDMEE). Films were prepared by slow Fe deposition at room temperature, up to a nominal coverage of 14 ML. The structure of the layers was the tetragonally distorted bct modification of the equilibrium bcc phase, with the (001)Fe||(112̄0)Co and [11̄0]Fe||[11̄00]Co relationships with the Co substrate. In the low coverage range (0–2 ML) iron forms 3 ML thick islands, that by further Fe deposition increase in thickness and finally coalesce at a coverage as large as 5 ML. The occurrence of a highly distorted bct phase is detected at a nominal coverage of about 2 ML. The final, 14 ML thick film exhibits a 3% contraction along the growth axis with respect to the equilibrium phase. The Fe(001)–Co(112̄0) system is a rarely encountered example in which the substrate and overlayer unit meshes exhibit a very large misfit asymmetry in the interface plane. As a consequence, a breakdown of the long-range order occurs in the Fe film along the [11̄00] substrate direction.

Ordered alloying of Pd with the Mo(100) Surface

The interaction of Pd with the Mo(100) surface has been studied with low-energy electron microscopy and diffraction, and first-principles total-energy calculations. Deposition of Pd at elevated temperature leads initially to the development of an intense $c(2\ifmmode\times\else\texttimes\fi{}2)$ diffraction pattern. Theoretical calculations demonstrate that a $c(2\ifmmode\times\else\texttimes\fi{}2)$ substitutional alloy is energetically favored compared to an overlayer structure at half monolayer coverage. The creation of a large number of islands upon Pd deposition is consistent with the formation of a substitutional alloy. Accommodation of Pd in excess of the ideal $c(2\ifmmode\times\else\texttimes\fi{}2)$ coverage leads to the formation of a $c(2\ifmmode\times\else\texttimes\fi{}n)$ structure $(n=8,10),$ although with little apparent further change of surface morphology. Proliferation of antiphase domain walls in the $c(2\ifmmode\times\else\texttimes\fi{}2)$ alloy is proposed to explain the $c(2\ifmmode\times\else\texttimes\fi{}n)$ periodic structure. Theoretical calculations indicate that a pseudomorphic Pd overlayer is the most stable configuration at 1 monolayer coverage, and that the Pd-covered Mo(100) surface may become unstable with respect to faceting to the {112} orientations if pseudomorphic growth can be realized up to 2 monolayer coverage. However, the $c(2\ifmmode\times\else\texttimes\fi{}n)$ structure is found experimentally to be stable at the interface between Mo and thicker Pd overlayers. These results suggest that there is a kinetic limitation to the formation of the pseudomorphic structure and the faceting instability is preempted by the formation of more complex surface alloy structures.

Adsorbate-induced reconstruction and overlayer structures of sulfur on Ir(110): An STM study

The adsorption of sulfur on the Ir(110) surface was investigated by scanning tunnel microscopy, low energy electron diffraction (LEED), and Auger electron spectroscopy. Upon annealing to 700--1000 K, sulfur induces a missing-row reconstruction of the substrate at minimum coverages of 0.15\ifmmode\pm\else\textpm\fi{}0.05 ML (Monolayer). At 0.5 ML sulfur coverage a $p(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}2S$ structure with $p2\mathrm{mg}$ symmetry is observed, which is compatible with an earlier LEED analysis of that structure. At higher coverages up to saturation, a $c(2\ifmmode\times\else\texttimes\fi{}4)\ensuremath{-}6\mathrm{S}$ structure develops for which a structural model is proposed. At saturation coverage, the LEED pattern exhibits additional streaks at $\frac{1}{4}$ positions in the $[1\ifmmode\bar\else\textasciimacron\fi{}10]$ azimuth. The streaks are due to sulfur dimers in a second adsorbate layer, which can be desorbed by annealing at 1100 K. Surprisingly, the atoms in the dimers seem to be located in on-top positions over sulfur atoms of the first layer, leading to long-range order in the $[1\ifmmode\bar\else\textasciimacron\fi{}10]$ direction only.

Molecular Self-Assembly on Surfaces of Organic Conducting Salts: Epitaxial Monolayer Films of Naphthalene on the (001) Surface of (TMTSF)2PF6

Pristine and naphthalene-adsorbed (001) surfaces of the organic conducting salt (TMTSF)(2)PF6 were examined by scanning tunneling microscopy (STM) to determine the structure of epitaxial monolayer films of naphthalene on the (001) surface. The observed STM images were interpreted by calculating partial electron density plots of the surface, and for naphthalene-adsorbed surfaces, by carrying out Monte Carlo simulations for the adsorption of a naphthalene molecule on the surface. The STM images recorded for the pristine (001) surface correspond to the cation layer of (TMTSF)2PF6, and each circular bright spot of the molecular-resolution STM images represents the three hydrogen atoms of the most protruding methyl group of a TMTSF molecule on the (001) surface. Naphthalene molecules adsorbed on this surface form a pseudomorphic (1 x 1) overlayer structure with respect to the underlying substrate. The naphthalene overlayer shows mechanical stability against etching by the scanning tip. An identical overlayer structure of naphthalene was obtained from several different preparation methods. On the cation-layer (001) surface naphthalene is adsorbed on each four-methyl-step'' defined by four methyl groups of two adjacent TMTSF molecules within each TMTSF stack.

Angle-resolved ESCA methods: Distribution and molecular conformation of perfluoropolyether lubricant

Simple but useful methods for interpreting angle-resolved ESCA data from real-world samples are presented. First is a two-tiered overlayer model allowing thinly covered and thickly covered substrate. It can be productively applied, even in a manual fashion, with common office spreadsheets. Next is a triangle-wave roughness model that can reasonably approximate substrate polishing texture. Finally, a two-slab overlayer model is presented. With the help of canned minimization routines, it can bring out conformation subtleties in overlayer structures and even isolate a deeply buried monatomic layer. The following are presented as examples. Perfluoropolyether (Zdol) lubricant on hard disk carbon overcoat shows coexisting monolayer and multilayer regions consistent with structures identified by surface energy and ellipsometric surface diffusion measurements. Polishing texture of disks is detected but leads to little change in the two-tiered model results. Layering within the lubricant shows significant perfluoropolyether backbone adjacent to the overcoat surface accompanied by a discernible excess of ether oxygen. The latter indicates that the inverted-U conformation sometimes sketched for bonded lubricants is an exaggeration, certainly for the system studied.

The structure of copper phthalocyanine and perylene-3,4,9,10-tetracarboxylic dianhydride overlayers on the (111)A surfaces of InAs and InSb

Low energy electron diffraction has been used to investigate the ordered structures formed when copper phthalocyanine (CuPc) and perylene 3,4,9,10-tetracarboxylic dianhydride (PTCDA) are vapour deposited onto the unpassivated, In-terminated InAs(111)A-(2×2) and InSb(111)A-(2×2) reconstructed surfaces. A ( 12 × 12 ) R30° structure is observed on both surfaces after 1–2 monolayers CuPc deposition and a commensurate, diamond-shaped lattice is formed with intermolecular spacing of 1.48 and 1.59 nm on InAs and InSb, respectively. The similarity between the structures, and the unusually large intermolecular spacing, is thought to be related to the presence of exposed second layer group V atoms in the (2×2) reconstruction of the substrate with which the CuPc molecules interact and by which they are pinned. Deposition of PTCDA also leads to the formation of ordered overlayer structures on both surfaces. The diffraction patterns support the presence of six equivalent domains of a rectangular PTCDA lattice that has two molecules per unit cell, the molecular arrangement based on a herringbone structure. The results are consistent with growth occurring with the bulk (102) plane parallel to the substrate surface.

Metal/semiconductor incommensurate structure with a rare domain configuration exhibitingp31msymmetry

An incommensurate overlayer structure on a Ga/Si(111) surface, which can be identified with the $6.3\sqrt{3}\ifmmode\times\else\texttimes\fi{}6.3\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ superstructure observed by the electron diffraction method, has been directly observed using scanning tunneling microscopy. The domain-wall pattern of this structure consists mainly of an array of isolated triangles, in contrast to the meshlike network usually observed on an incommensurate overlayer. A particular feature of this structure is that it assumes a rare domain configuration with $p31m$ symmetry, rather than the $p3m1$ symmetry of the underlying Si(111) double layer, to relax the strain induced by its relatively high Ga coverage.