Bilayer Islands Research Articles

Magnetization reversal and spin reorientation in Fe/Cu(100) ultrathin films

The magnetization reversal in perpendicular applied magnetic fields in low-temperature grown Fe/Cu(100) ultrathin films was studied in situ by magneto-optical Kerr effect, Kerr-microscopy, and scanning tunneling microscopy. It was found that the onset of long-range ferromagnetic order at \ensuremath{\cong}0.9 monolayers (ML) is related to the coalescence of bilayer islands of Fe. Below 3.8 ML, the magnetization-reversal process takes place in a narrow field range and is characterized by domain-wall motion. While the domain boundaries are rather smooth at thin films, domains get increasingly irregularly shaped above 3 ML, which is assigned to a decrease of the domain-wall energy. At the spin-reorientation transition from out-of-plane to in-plane magnetization at \ensuremath{\cong}3.8 ML, two coexisting metastable spin configurations are found.

Nontraditional models of Ostwald ripening on solid surfaces: from physics to biology

We present Monte Carlo simulations of Ostwald ripening of (i) submonolayer islands on a heterogeneous surface, (ii) lipid bilayer islands, and (iii) 3D supported nm crystallites on a uniform surface. With increasing dimension of the model (from (i) to (iii)), the growth of the average size characterizing phase separation is found to become slower. In particular, the growth law exponent falls from the classical Lifshitz–Slyozov value, x=1/3, obtained for the mono- and bilayer islands, to much lower values, x=0.125±0.015, associated with the nm crystallites. The latter finding is in good agreement with the results of numerous experimental studies of sintering of supported nm crystallites employed in heterogeneous catalysis.

The vibrational spectra of water cluster molecules on Pt(111) surface at 20 K

The adsorption of D 2O on Pt(111) at 20 K has been investigated by infrared reflection absorption spectroscopy (IRAS). Water was found to form as a dimer at low coverages and it grew a tetramer molecule at high coverages. The structures of the cluster water molecules on the surface were derived from the intensities and the frequencies of symmetric and asymmetric ν OD stretching absorptions. The annealing at higher temperatures than 130 K was needed to form an ice bilayer island structure.

Vibrational spectroscopy of water molecules on Pt(111) at submonolayer coverages

The adsorption behavior and vibrational dynamics of H 2O (D 2O) molecules on the Pt(111) surface at surface temperatures, T s, between 20 and 140 K have been investigated using high-resolution helium atom scattering. At T s<40 K, water monomers are adsorbed on the surface, whereas at T s>40 K, the water molecules cluster to form stable dimers or trimers. At higher temperatures of about 100–130 K, water adsorbs in large two-dimensional ice bilayer islands. From time-of-flight measurements the low-frequency external vibrations of the water monomers, dimers and 2D islands were determined, demonstrating substantial differences for these three water-binding configurations.

Relaxation Processes of Isolated Single Silicon Islands on Si(111) Surfaces.

Thermal relaxation of isolated single silicon two dimensional (2D) islands, 2D vacancy islands and pyramid type islands, produced on the Si(111)(7×7) surface by using a tip of a scanning tunneling microscope are observed by temperature variable scanning tunneling microscopy. Thermal decay rates of silicon islands are measured at various substrate temperatures of 650 K-850 K. Effects of the probe tip on the rates are measured and reduced rates are determined without the tip effects. The sizes of islands depend on time with a function form of (t0-t)α. For single bilayer islands, α was 1 without the tip effects and 2/3 with the tip effects. From these results, we conclude that the intrinsic decay process of islands is attachment-detachment limited but is diffusion limited with the tip effect. Activation energies for the intrinsic decay of single 2D islands and 2 D vacancy islands are 1.5 eV and 1.3 eV, respectively. Characteristic (5×5) islands with long life times are found during decay. The (5×5) structure is also observed on pyramid type island surfaces. We discuss the results in terms of two dimensional vapor phase processes.

Relaxation of Nanostructures on the Si(111)(7×7) Surface by High Temperature Scanning Tunneling Microscopy

Single silicon islands have been produced on the Si(111)(7 × 7) surface by a scanning tunneling microscope (STM) tip. Thermal relaxation of the isolated islands is observed by temperature variable scanning tunneling microscopy with strong tip effects. The sizes of islands depend on time t with a functional form of (t0-t)α. It is found that α≃2/3 for single bilayer islands, and α≃1 for three-dimensional ones. During the decomposition of three-dimensional islands, step bunching of over-layers takes place, while the islands have certain facets, like a pyramid just after the creation. At the final stages of the three-dimensional island decompositions, two-dimensional ones with 5 × 5 structure always appear. We have found that characteristic 5 × 5 islands with a long lifetime are formed during relaxation, but the 7 × 7 islands have mostlt a short lifetime. Rotation of small islands is also observed during relaxation. We discuss the results in terms of two-dimensional vapor phase processes.

GROWTH AND ATOMIC STRUCTURE OF ULTRATHIN COBALT LAYERS ON THE Cu(111) FACE CLEAN AND PRECOVERED WITH LEAD

The growth and atomic structure of ultrathin Co layers deposited on the clean Cu(111) face and precovered with about 0.7 ML of Pb were investigated with the use of Auger electron spectroscopy and directional elastic peak electron spectroscopy (DEPES). Auger spectra for the clean Cu (111) face and covered with 1.5 ML of Co indicate the growth of cobalt bilayer islands sunk at one-monolayer depth in the substrate and covered with one monolayer of copper. The ratio of M2,3VV cobalt and copper Auger peak heights increases with cobalt coverage faster for the Pb -precovered Cu(111) face than for the clean surface. DEPES profiles reveal the threefold symmetry of the cobalt layer up to 4.5 ML. For higher coverages, an arrangement of a sixfold symmetry appears, indicating the presence of fcc(111) cobalt domains rotated 60° with respect to the substrate (i.e. growing after a stacking fault abcbac) separated from the abcabc domains by the bcb layer with the hcp structure.

Crystallography and morphology of the early stages of the growth of [formula omitted] by LEED and STM

We report on a quantitative investigation of the structure and morphology of ultrathin films of Co deposited on Cu(111) by an in situ combination of real space (STM) and diffraction (LEED) techniques. The film grows initially as bilayer islands with strict fcc stacking. Part of them are covered by Cu. The lateral distribution of Co- and Cu-terminated domains as well as of uncovered patches is both quantitatively determined by intensity analysis and visualized by STM images. Their simultaneous presence correlates nicely to known magnetic properties of ultrathin films, in particular with the observed magnetic anisotropy.

Growth, morphology and annealing of thin Co layers on [formula omitted

The growth of Co on Au mica has been studied by means of variable temperature scanning tunnelling microscopy and Auger electron spectroscopy. Growth at 300 K (low Co coverage), produces mono- and bilayer islands of about 5 nm in diameter. Rough Co layers are obtained upon further deposition of Co. Direct growth at 430 K leads to much larger and higher Co islands. Their surface density is no longer dictated by the nucleation at the dislocation of the √3 × 22 reconstruction of the Au(111) surface but rather by the growth kinetics of diffusing Co atoms. Annealing at 500 K of films grown at 300 K leads to significant surface smoothing possibly induced by migration of Au on top of the Co film.

Crystallography and morphology of the early stages of the growth of by LEED and STM

We report on a quantitative investigation of the structure and morphology of ultrathin films of Co deposited on Cu(111) by an in situ combination of real space (STM) and diffraction (LEED) techniques. The film grows initially as bilayer islands with strict fcc stacking. part of them are covered by Cu. The lateral distribution of Co- and Cu-terminated domains as well as of uncovered patches is both quantitatively determined by intensity analysis and visualized by STM images. Their simultaneous presence correlates nicely to known magnetic properties of ultrathin films, in particular with the observed magnetic anisotropy.

Structure of Relaxed Monolayers of Arachidic Acid Studied by X-ray Reflectometry and Atomic Force Microscopy

Following the constant pressure relaxation on different subphases and the deposition on solid substrates, the structure and morphology of arachidic acid monolayers were studied by X-ray reflectometry and atomic force microscopy. The growth of three-dimensional (3D) phase nuclei was observed from the first minutes of the relaxation process in a monolayer relaxed on a subphase of low pH ≤ 3 with or without metal ions. With an increase in the relaxation time, the granule-like nuclei were transformed into platelike multilayer islands. The structure of 3D islands was commensurate with the B and C forms of bulk arachidic acid, with the B phase being prevalent after a long period of relaxation. During the storage of the samples in air, the granule-like islands were unstable and, after 2 months, transformed into the more stable platelike islands. The relaxation process was much slower in the case of binding the monolayer to cadmium or lead cations in a subphase of pH 5.3. After about 1 h of relaxation, only low folds arrayed with a hexagonal symmetry were observed on the monolayer of cadmium arachidate. On monolayers of lead arachidate, 1-2 bilayer islands emerged with hexagonal boundaries after a relaxation of some 3 h.

On reverse 2D nucleation—growth—collision in the electroreduction of incomplete HgS bilayers at the mercury/electrolyte interface

The dissolution of incomplete HgS bilayers has been investigated by means of current-time ( i-t) transients using the potential step technique. The transients can be theoretically well described by a model which involves a three-stage process. The first stage corresponds to an electrodesorption of those regions of the first monolayer which are not covered by the islands of the second layer. Here, the HgS monolayer consists of a condensed and an expanded phase and only the expanded phase can be electrodesorbed. The dissolution of the condensed phase clusters at their edges is much faster than the electrodesorption of the expanded phase. The electrodesorption of the first monolayer proceeds up to the time when all monolayer clusters are dissolved. Then as the second stage of the overall process, the dissolution of the remaining bilayer islands starts according to a reverse two-dimensional instantaneous nucleation—growth—collision, i.e. a dissolution of the bilayer islands at their edges. The third stage of the HgS bilayer dissolution begins when all bilayer islands are dissolved and can be theoretically well described with a first-order electrodesorption process of the admolecules of the first monolayer.

Initial growth mode of Cr on Cu(100) and Ag(100): a comparison

The initial growth mode of Cr on Cu(100) and Ag(100) surfaces is studied by photoemission methods. In particular photoemission is used as a titration method allowing to determine the clean surface area left after Cr deposit. For Cu(100) it is found that at room temperature, or above, body centered cubic (bcc) Cr grows initially in the form of bilayer or multilayer islands with (110) orientation and with 90% substrate coverage by ∼ 4 monolayers. This contrasts with the growth mode observed on Ag(100) where, at moderate substrate temperatures (430–450 K), only monolayer bcc Cr islands with (100) orientation are formed initially. This behaviour is confirmed by Cr 2p core level photoemission measurements which indicate that in the monolayer range Cr is already in bulk-like environment on Cu(100) in contrast to Ag(100) where a flat monolayer with strongly enhanced magnetism is obtained.

Double period RHEED oscillations during MBE growth of GaAs and AlAs on the GaAs(110) surface

Molecular beam epitaxy (MBE) of GaAs and AlAs on the GaAs(110) surface has been studied in situ using reflection high-energy electron diffraction (RHEED). Additionally ex situ atomic force microscopy (AFM) has been employed to investigate the surface topography after the growth. During the growth complex RHEED oscillations with a single and a double period can be measured, depending on the diffraction feature that is monitored. The AFM images of the surface reveal mono- and bilayer high steps and islands, reflecting a layer-by-layer growth with the participation of both at the same time. A simple model that leads to a qualitative understanding of the experimental observation is presented.

Particle and Phase Thicknesses from XPS Analysis of Supported Bimetallic Catalysts: Calcined Co-Rh/Nb 2O 5

The surface structure and elemental composition of a series of calcined Co-Rh/Nb 2O 5 bimetallic catalysts have been investigated using X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). New formulae for the quantitative analysis of XPS intensities for supported bimetallic catalysts involving up to three separate, layered phases on spherical support particles are used for the first time. These apply an average takeoff angle for photoelectrons from spherical particles whose radii are large compared to the electron attenuation length. Calcined monometallic Co/Nb 2O 5 and Rh/Nb 2O 5, and four calcined bimetallic Co-Rh/Nb 2O 5 catalysts with similar Co loadings (≍ 1.9 wt%) and variable Rh loadings (0.3 to 2.3 wt%) were examined. Reference spectra for pure CoNb 2O 6 (columbite) are also presented here for the first time. The catalysts were prepared by incipient wetness impregnation and calcined at 673 K to generate the oxide precursors. The XPS lineshapes and the Co (2p) spinorbit splitting indicated the presence of two Co species, Co 3O 4 and Co +2, on all calcined Co-containing catalysts. The measured XPS Co/Nb and Rh/Nb atomic ratios for the catalysts were factors of 1.5-2 and 2-4.5, respectively, greater than the bulk atomic ratios, showing that both Co and Rh oxides were surface-enriched. The measured XPS peak intensities were compared to the values predicted from several different structural models of the oxide particles. These models assumed large spherical Nb 2O 5 particles (diameter ≍ 60 nm to match the BET area) with the Co and Rh oxides covering fractions of this support′s surface in several particle arrangements. The best agreement, according to the minimum least squares criteria, was found for a model in which the Co +2 phase is adsorbed on the Nb 2O 5 support surface, whereas the Co 3O 4 forms thick (>2.5 nm) islands covered by Rh2O3 on approximate to 3.6% of the support surface. The thickness of the Rh 2O 3 overlayer increased to 2.6 nm as the Rh/Co bulk atomic ratio increased to 0.72. The Co +2 phase was present at submonolayer concentrations. Between 78 and 90 mole% of the total Co was present as Co 3O 4 in this model. The reduction temperature of the Co 3O 4 strongly decreased as the Rh/Co bulk atomic ratio increased, while the reduction temperature of the Rh 2O 3 was not strongly influenced by the presence of Co 3O 4. Thus, the TPR results are consistent with this bilayer island model, with Rh 2O 3 on top of Co 3O 4, The Co +2 species was not obvious in TPR due to its low concentration.

Growth kinetics of CaF2/Si(111) heteroepitaxy: An x-ray photoelectron diffraction study.

Kinetic variations of the initial stages of CaF[sub 2] growth on Si(111) by molecular-beam epitaxy are studied with the [ital in] [ital situ] combination of x-ray photoelectron spectroscopy and diffraction. After the formation of a chemically reacted interface layer, the morphology of the subsequent bulk layers is found to be dependent on the substrate temperature and incident flux rate, as well as the initial interface structure. For substrate temperatures above [similar to]600 [degree]C, subsequent layers do not easily wet the interface layer, and a transition is observed from a three-dimensional island formation at low flux to a laminar growth following the coalescence of bilayer islands at higher flux. At lower substrate temperatures ([similar to]450 [degree]C), a different stoichiometry and structure of the interface layer leads to laminar growth at all fluxes, but with a different bulk nucleation behavior. Crystalline heteroepitaxy is not observed when growth initiates at room temperature, but homoepitaxy does proceed at room temperature if the first few layers are deposited at a high temperature. The different growth regimes are discussed in terms of a kinetic model separating step and terrace nucleation where, contrary to homoepitaxy, step nucleation leads to islanded growth.

Scanning-tunneling-microscopy study of the growth of cobalt on Cu(111).

Cobalt grows initially on Cu(111) in the form of bilayer islands with the shape of triangles of alternate orientation. This is probably related to the initial stacking sequence of the Co layers. The islands do not coalesce upon further deposition and the resulting film, although epitaxial and crystalline to the extent tested by diffraction techniques, is, actually, granular. Thus, microscopic evidence is provided for the existence of an intrinsic defect, namely, twinned fcc crystallites and their associated grain boundaries, which may be the much sought cause of discrepancy on antiferromagnetic coupling data in Co/Cu(111) superlattices grown in ultrahigh vacuum.

Ag segregation and epitaxial Cr growth on Ag(100)

The crystallographic and electronic structure of Cr films deposited on an Ag (100) crystal is studied by angle resolved photoemission. Both valence band and photoelectron forward scattering data show that body-centered-cubic Cr grows epitaxially on Ag (100) held at room temperature during deposition. Yet the Cr film is not uniform and exhibits a poor long range order. As soon as bilayer islands are formed and up to coverages as high as ∼60 layer equivalent (LE), the growth of Cr films is invariably accompanied by surface segregation of Ag. Apparently monolayer islands formed initially are free of segregated Ag. This is confirmed by a study of the flat well-ordered Cr monolayer formed when one LE is deposited on the substrate heated at ∼440 K [C. Krembel, M. C. Hanf, J. C. Peruchetti, D. Bolmont, and G. Gewinner, J. Magn. Magn. Mater. 93, 529 (1991); C. Krembel, M. C. Hanf, J. C. Peruchetti, D. Bolmont, and G. Gewinner, Phys. Rev. B 44, 8407, 11472 (1991)]. In spite of the higher substrate temperature during deposition we could not find any evidence for the presence of segregated Ag on top of the ordered Cr monolayer. Possible reasons for this peculiar behavior, namely that Ag segregation takes place on bi- or multilayers of Cr but not on the monolayer, are discussed.

Nucleation and growth of ultrathin Fe and Au films on Cu(100) studied by scanning tunneling microscopy

Cu(100) surfaces modified by depositing Au and Fe in the monolayer (ML) thickness regime are studied with the scanning tunneling microscope (STM). STM results confirm that Au deposition of 0.5 ML at 300 K creates a CuAu c(2×2) alloy monolayer, with nearly regular disruptions of structure that relieve misfit strain. The alloy forms by Au replacement of Cu atoms in the surface. Submonolayer Fe deposition at 300 K yields monolayer islands and additional features interpreted as Fe patches in the surface Cu layer. This suggests that Fe atoms replace Cu atoms in the surface. Deposition of ∼4 ML Fe yields a more homogeneous surface with height variations that suggest structural disorder in the Fe layer. Fe deposition at ∼110 K and warming to 300 K yields many monolayer and bilayer islands, with no evidence of Fe incorporation into the surface. Unusual tip behavior is also discussed which appears to yield STM sensitivity to the chemical difference between Fe and Cu areas.

Adsorption of H 2O on Ru(001) : I. Bilayer and clusters

The adsorption of H 2O on Ru(001) was investigated by photoemission spectroscopy (UPS and XPS), LEED and workfunction measurements in the temperature range 120–300 K. The H 2O coverage was calibrated via O1s core level intensity by comparison with a p(2 × 2)-O layer corresponding to a coverage of 0.5. The O1s spectra of adsorbed H 2O layers always showed a contribution near 531.3 eV binding energy although the main intensity was located at 532.7–533.9 eV depending on coverage. Annealing of adsorbed layers to 173–193 K caused an O1s double peak to appear with binding energies at 531.3 and 532.7 eV. These peaks are assigned to islands of a H 2O bilayer whereby the binding energies 531.3 and 532.7 eV belong to the first and second H 2O layer, respectively. Higher order H 2O layers have an O1s binding energy of at least 533.2 eV. The layer-dependent binding energy of H 2O is suggested to be due to a distance-dependent screening effect of the O1s core hole, aside from the first layer H 2O molecules which are chemically bonded to Ru surface atoms via the oxygen lone pair orbital. The formation of perfect bilayer islands is thermally activated and does not occur during low temperature adsorption at 120 K. Up to 15% of the first bilayer of H 2O molecules are found to dissociate after a slow anneal to 210 K whereas dissociation is negligible for a fast flash.