Monolayer Of Sn Research Articles

Direct recoil spectroscopy of adsorbed atoms and self-assembled monolayers on Cu(001)

We present results of adsorption experiments of Sn and 1,4-benzenedimethanethiol (BDMT) on Cu(001), which illustrate the capabilities of a new setup specifically designed to perform surface studies by Direct Recoil Spectroscopy (DRS). The system consists of three UHV chambers connected in series with a 1–100 keV ion accelerator. In the main UHV chamber the DRS technique is combined with other more standard techniques such as Auger and Energy Loss Electron Spectroscopy, and Low Energy Electron Diffraction. The capabilities of the instrument are exemplified by two adsorption studies on the (001) face of Cu. First we describe measurements for 0.5 monolayer of Sn adsorbed on Cu that are in agreement with the crystallographic symmetry (32x2) R45° seen by LEED and the appearance of Cu vacancies along the [100] direction. Then we present a study of self-assembling of 1,4-benzenedimethanethiol (BDMT) on Cu(001) from the vapour phase. For this system we show that it is possible to form a standing-up phase at large exposures, of the order of 106 L, and discuss its stability with temperature. We also discuss a S enrichment effect induced during the first adsorption stages of BDMT.

Alloying of Sn in the surface layer of Ag(111)

It is found that 1/3 monolayer (ML) of Sn forms a surface alloy with 2/3 ML of Ag on Ag(111). This highly ordered alloy layer shows a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ structure. By employing experimental and theoretical tools (scanning tunneling microscopy [STM], angle resolved photoelectron spectroscopy, low-energy electron diffraction, and density functional theory), an atomic model has been obtained that reproduces the experimental electronic structure in both real and reciprocal space. Detailed surface band dispersions, constant energy contours, and STM images, obtained experimentally and theoretically, are compared in order to verify the model. Similar, 1-layer-thick alloys on Ag(111) with Pb, Bi, or Sb exhibit measurable spin--orbit interactions. However, no such spin split could be detected in the case of Sn in this study.

Electronic and atomic structures of a3×3surface formed by a binary Sn/Ag overlayer on the Ge(111)c(2×8) surface: ARPES, LEED, and STM studies

The electronic and atomic structures of a well-ordered $3\ifmmode\times\else\texttimes\fi{}3$ periodicity of a binary Sn/Ag overlayer on Ge(111) have been studied. The ordered binary overlayer was formed by depositing 0.75 monolayer of Sn on an Ag/Ge(111) $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surface. Annealing at 330 \ifmmode^\circ\else\textdegree\fi{}C resulted in a low-energy electron diffraction pattern that exhibited sharp spots. A detailed electronic structure investigation was performed by angle-resolved photoelectron spectroscopy. The Sn/Ag/Ge(111) $3\ifmmode\times\else\texttimes\fi{}3$ surface shows a rich band structure. There are seven bands which are positively identified as $3\ifmmode\times\else\texttimes\fi{}3$ surface bands, all within 1.5 eV below the Fermi level (${E}_{F}$). The upper two bands disperse across ${E}_{F}$ exhibiting steep almost linear dispersions down to a minimum energy of \ensuremath{\approx}0.40 eV below ${E}_{F}$ at the $\overline{\ensuremath{\Gamma}}$ point (\ensuremath{\approx}0.30 eV at the $\overline{K}$ point). Constant energy contours have been mapped in the $3\ifmmode\times\else\texttimes\fi{}3$ surface Brillouin zone (SBZ) in order to study an intriguing split observed in the band structure related to the two upper bands. It turned out that the two upper bands are degenerate along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{K}$ and $\overline{M}\ensuremath{-}\overline{K}$ symmetry lines of the $3\ifmmode\times\else\texttimes\fi{}3$ SBZ but separated along $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{M}$. Scanning tunneling microscopy images obtained at \ensuremath{\approx}40 K show essentially a hexagonal structure except for a honeycomb structure in a limited bias range imaging empty states. Core-level spectroscopy shows a narrow Sn 4$d$ spectrum consistent with the high degree of structural order.

Photoemission and LEED study of the Sn/Rh(111) surface—early oxidation steps and thermal stability

We have deposited two monolayers of Sn onto Rh(111) single crystal. After the deposition, no ordered structure was revealed by low energy electron diffraction (LEED). We oxidized the obtained system in a low-pressure oxygen atmosphere at 420 K. The oxidized sample was then gradually heated to study the thermal stability of the oxide layer. We characterized the system by synchrotron radiation stimulated photoelectron spectroscopy and LEED. Valence band and core level photoelectron spectra of rhodium, tin and oxygen were used to study the oxidation of the Sn–Rh(111) surface and its behaviour upon annealing. A low stoichiometric oxide of Sn was created on the surface. The oxidation process did not continue towards creation of SnO2 with higher oxygen dose. The annealing at 970 K caused decomposition of the surface oxide of Sn and creation of an ordered (√3 × √3)R30° Sn–Rh(111) surface alloy.

Structural and electronic properties of(32×2)R45°-Sn/Cu(100): Density functional theory and scanning tunneling microscopy

The structural and electronic properties of the $(3\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ phase of 0.5 monolayers of Sn on Cu(100) have been investigated using a combination of scanning tunneling microscopy experiments and total-energy-minimization calculations. Theory and experiment support the missing-row surface-alloy structure as the ground state for this surface [K. Pussi, E. Al Shamaileh, E. McLoughin, A. A. Cafolla, and M. Lindroos, Surf. Sci. 549, 24 (2004)]. The calculated electronic structure reproduces almost perfectly both the experimental scanning tunneling microscopy images and the observed electronic bands. Our results conclusively indicate that the creation of Cu vacancies in the top surface layer produces a significant reduction of the total energy. We also report a metastable $(3\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ phase coexisting with the ground state at room temperature, which we attribute to a two-missing-row surface-alloy structure.

Tailoring the spin density waves in Fe/Cr multilayers by selective inclusion of Sn, V and Mn

The possibility of tailoring the spin density waves in Fe/Cr(001) multilayers through the selective inclusion of Sn, V and Mn monolayers is investigated with the density functional tight-binding linear muffin-tin orbital method in the generalized gradient approximation of the exchange and correlation potential. Despite the non-magnetic character of Sn and V when substituting Cr atoms located at the nodes, the modifications induced on the spin density waves are important due to the strong hybridization. In general we find that V modifies drastically the global features of the spin density waves leading to the onset of a magnetically dead region in the Cr spacer whereas Mn inserted at the nodes rather destroys the density wave working in favor of stabilizing the layered antiferromagnetic structure. The trends obtained are consistent with experimental data when available. Since both the magnetic profile and the position of the nodes at the spacer can be modified, the present results are relevant in the context of the spin-dependent transport through magnetic multilayers in which the magnetoresistance will vary if the scattering regions across the transport direction are modified.

X-ray reflectivity studies of atomic-level surface-segregation in a liquid eutectic alloy of AuSn

X-ray reflectivity studies reveal atomic-level surface-segregation at the free surface of the eutectic ${\mathrm{Au}}_{71}{\mathrm{Sn}}_{29}$ liquid alloy. The surface-segregation extends up to three layers, in which the top layer is almost a pure monolayer of Sn, the second layer is almost a pure monolayer of Au and the third layer appears to be slightly enhanced in Au. Although the surface-segregation concentration profiles can be qualitatively accounted for by the theories of Defay-Prigogine and Strohl-King, they cannot satisfactorily account for the measured surface tension.

Synthesis and characterization of some binary chalcogen compounds by explosive laser irradiation and heat treatment

The thin film preparation of Se, Ge, Sn monolayers and also binary chalcogenide alloys with their various structures and morphologies have been thoroughly studied. A new phase of the composite materials was identified. The short range ordering structure of chalcogens when mixed up with four-fold-coordinated atoms like Ge and Sn produce alloys or compounds with a certain specific characteristic. Laser impulse on these materials caused rapid cooling allowing the materials to form alloys. The optical measurements of the chalcogen films are reported. The transmittance versus wavelength measurement in the UV–VIS–NIR was studied and the optical band gaps of the films were calculated to be 1.48 eV for Ge–Se and 1.65 eV for Sn–Se for the wavelength range 300–1100 nm.

Surface structure and energy bands of 1/3 ML Sn/Ge(111)

The geometrical and electronic structure of 1/3 monolayer of Sn on the Ge(111) surface is investigated within density functional theory. The well known 3 x 3 and √3 x √3 reconstructions are studied in details, performing also band structure calculations and simulating STM images. Our results confirm that the 1U-2D 3 x 3 structure is the more favorable one, not only from an energetic point of view but also from a comparison with STM experiments. On the other hand, the static room temperature electronic band structure of the √3 x √3 hardly compares with the available photoemission data, thus supporting the idea of a dynamical flipping of the Sn ad-atoms.

Redox processes in a tin doped melt with the basic composition 20Na 2O · 80SiO 2 studied by square-wave voltammetry and impedance spectroscopy

Redox processes in a tin doped melt with the basic composition 20Na 2O · 80SiO 2 were studied by square-wave voltammetry and impedance spectroscopy. At temperatures of 1350 and 1450 °C, the recorded square-wave voltammograms exhibited a distinct peak attributed to the reduction of Sn 4+ to Sn 2+. Impedance spectra recorded at the same temperatures could be simulated with a simple equivalent circuit assuming a diffusion controlled electrode reaction. At temperatures T ⩽ 1250 °C, and small step times, the voltammograms exhibited a second peak which was much stronger affected by the frequency (step time) applied than expected from a diffusion controlled electron transfer reaction. In order to fit the impedance spectra at these temperatures, a more complex equivalent circuit was necessary, taking into account adsorption phenomena. In all impedance spectra, the Warburg parameter obtained from simulation, which stands for the diffusion from and to the electrode, exhibited a minimum which was equal to the voltammetric peak at more negative potentials. The voltammetric peak at less negative potentials is caused by the reduction of Sn 4+ to an adsorbed state. Within this reduction step, a monolayer of Sn 2+ species is formed.

Real-space observation of quasicrystalline Sn monolayer formed on the fivefold surface of icosahedralAlCuFequasicrystal

We investigate a thin Sn film grown at elevated temperatures on the fivefold surface of an icosahedral $\mathrm{Al}\mathrm{Cu}\mathrm{Fe}$ quasicrystal by scanning tunneling microscopy (STM). At about one monolayer coverage, the deposited Sn is found to form a smooth film of height consistent with one-half of the lattice constant of the bulk Sn. Analysis based on the Fourier transform and autocorrelation function derived from high-resolution STM images reveals that Sn grows pseudomorphically and hence exhibits a quasicrystalline structure.

Ab initiostudy of quasiperiodic monolayers on a fivefoldi−AlPdMnsurface

We present a structural model for quasiperiodic monolayers formed on the fivefold surface of an icosahedral AlPdMn quasicrystal, based on ab initio density functional calculations. As a starting point we have investigated the relative stability of unsupported triangular, square, and quasiperiodic monolayers. The unsupported quasiperiodic monolayers are shown to be unstable upon relaxation by interatomic forces. This result indicates the important role of the adsorbate/substrate interaction for the stabilization of a quasiperiodic layer. The structural model of a monolayer adsorbed on the fivefold surface has been constructed on the basis of a mapping of the potential-energy landscape of an isolated adatom on the $i\text{\ensuremath{-}}\mathrm{Al}\mathrm{Pd}\mathrm{Mn}$ substrate. The structure of a clean fivefold $i\text{\ensuremath{-}}\mathrm{Al}\mathrm{Pd}\mathrm{Mn}$ surface is well described by a $P1$ tiling, with the vertices of the tiling located in the centers of $B$ (Bergman) clusters and occupied by Pd atoms. The potential-energy mapping emphasizes the important role of the $P1$ skeleton for the stabilization of a quasiperiodic adlayer: adsorption at the vertices of the $P1$ tiling leads to high binding energies of $\ensuremath{\simeq}4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\text{atom}$. The midedge positions of the $P1$ tiling and Mn atoms exposed at the surface are identified as further favorable adsorption sites. Altogether this leads to a structural model of the quasiperiodic adlayer with atoms at the vertices and midedge positions of the $P1$ tiling, centered pentagonal motifs decorating the pentagonal tiles, and one additional atom in the center of the pentagonal star and of the boat tile. For this structure we calculate a surface coverage of $\ensuremath{\simeq}0.09\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{\AA{}}}^{2}$ in perfect agreement with experiment. The diffraction pattern of the adlayer exhibits (pseudo)decagonal symmetry. The structural stability of a $2∕1$ approximant to this model has been tested for Sn, Bi, and Sb monolayers via relaxation by the Hellmann-Feynman forces from ab initio density function theory calculations. The skeleton of adsorbed monolayers based on the $P1$ tiling was found to be stable, although the atomic decoration inside the tiles is partially distorted, albeit without violating the overall symmetry.

Phonon dynamics of theSn∕Ge(111)surface at room temperature and low temperature determined by helium-atom scattering

We report on a high-resolution helium atom scattering study of the surface lattice dynamics, for both the room temperature $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ and the low-temperature $(3\ifmmode\times\else\texttimes\fi{}3)$ structures formed by $1∕3$ of a monolayer of Sn on Ge(111). At room temperature a pronounced lowering of the energy of the transversally polarized phonons along the $\overline{\ensuremath{\Gamma}\mathrm{M}}$ direction is observed for wave vectors in the vicinity of $Q=0.6\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$, the $\overline{\mathrm{K}}$ point of the room temperature $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ structure. The temperature dependence of this mode is analyzed between 140 and 300 K. We conclude that the softening of this mode is related with the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\leftrightarrow}(3\ifmmode\times\else\texttimes\fi{}3)$ phase transition. Phonon distributions curves are determined for both phases and the two high symmetry directions. The experimental data are explained from the backfolding of the slightly modified $\mathrm{Ge}(111)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ Rayleigh wave and from the modes related to the Sn induced superstructures, in good agreement with first-principles calculations.

Formation and STM tip-induced reduction of ultra thin SnO film on Au(1 1 1)

This Letter presents a study on the oxidation of electrochemically deposited Sn monolayer on Au(1 1 1) surface and STM tip-induced reduction of as-prepared ultra thin SnO film. A threshold bias of 0.6 V (tip negative) at a low tunneling current of ∼50 pA is required to image the as-formed SnO thin film by STM, typical of a semiconductor characteristic. Increasing the tunneling current to ∼2 nA leads to the reduction of the SnO back to Sn. Based on the energy level calculation for the SnO, a mechanism involving direct electron tunneling is proposed to account for the tip-induced reduction.

Transition from alloying layer growth to island growth of Sn on a Ga-covered Si [formula omitted] surface

Atomic depth distribution and growth modes of Sn on an Si(1 1 1)-√3×√3-Ga(1 ML) surface at room temperature were studied after each monolayer deposition of Sn by using reflection high-energy electron diffraction (RHEED) and characteristic X-ray spectroscopy measurements as functions of the glancing angle θ g of the incident electron beam. Intermixing between Sn and Ga occurred after deposition of one monolayer (ML) of Sn, resulting in the formation of a flat film of Ga–Sn alloy. Above a critical Sn coverage of about 1 ML, the growth mode changed significantly. That is, the alloying between Sn and Ga was suppressed, and additional Sn grew into islands on the Sn–Ga alloy.

Origin of induced Sn magnetic moments in thin Fe/Cr/Sn/Cr multilayers

The origin of magnetic moment induced on one monolayer of Sn embedded within thin layers of Cr in Fe/Cr/Sn/Cr and Cr/Sn multilayers is investigated by first-principle spin-polarized electronic structure calculation. The structures consist of 1.15 nm (8 monolayers) and 1.3 nm (9 monolayers) of Fe, and 0.86 nm (6 monolayers) to 2.4 nm (17 monolayers) of Cr. The position of the Sn monolayer in Cr is either symmetric or asymmetric with respect to the Fe layers. It is found that in all these multilayers the Sn atoms get a small magnetic moment which is predominantly $5d$ in character. This is induced by the exchange interaction with the $3d\ensuremath{-}\mathrm{Cr}$ orbitals at the Cr/Sn interfaces. We find that the presence of Fe layers reduces the magnetic moment of Cr atom at the Cr/Sn interface, and therefore the induced magnetic moment on Sn atom decreased from $0.067{\ensuremath{\mu}}_{B}/\mathrm{atom}$ in Cr/Sn structure to $0.044{\ensuremath{\mu}}_{B}/\mathrm{atom}$ in Fe/Cr/Sn/Cr structure. These predictions are in strong correlation with M\ossbauer observations at the ${}^{119}\mathrm{Sn}$ site: the measured hyperfine field which in turn is affected by magnetic moment of surrounding Cr atoms, decreases from 13 T for Cr/Sn to 2 T for Fe/Cr/Sn/Cr multilayers. It is also found that the magnetic moment of a Cr atom depends strongly on its distance from the Fe layer, and therefore the induced magnetic moment of Sn is also influenced by the thickness of Cr multilayers. Our results show that the magnetic moments on Cr sites decrease smoothly away from the Fe layer and then increase abruptly at the Cr/Sn interface.

Clustering and Anisotropy in Monolayer Formation under Potential Control: Sn on Au(111)

We present an in situ scanning tunneling microscopy study on Sn underpotential deposition (UPD) on reconstructed and unreconstructed Au(111) electrodes. Unusual behaviors in the formation of Sn (sub)monolayer at high underpotential shift are observed on both surfaces. On the unreconstructed Au(111) surface, Sn forms size-confined two-dimensional clusters of 1−2 nm. Decreasing potential leads to immersion of the Sn clusters into the substrate surface lattice forming surface alloying. While on reconstructed Au(111) surface, Sn prefers to nucleate in the face-centered cubic regions followed by anisotropic growth along the [112̄] direction of the reconstruction rows. The nuclei expand toward the hexagonal close packed regions to build up deposit domains which develop until a final gap of 3−6 nm is reached between two adjacent domains. Decreasing potential leads to a structural transformation to cordlike structures aligning all in one direction by rotation of 18−21° with respect to the [112̄] direction, involving surface alloying. The results are in contrast to the previously observed UPD adlayer structure and dynamics. The nanoscale properties of the Sn deposit and the influence from the UPD alloying are discussed as the two possible causes for the unusual Sn UPD phenomena.

Structure of the liquid-vapor interface of a dilute ternary alloy: Pb and Sn in Ga

We report the results of grazing incidence x-ray diffraction and x-ray reflectivity studies of the liquid-vapor interfaces of three dilute ternary alloys, composed of a small constant amount of Pb (0.072 at. %) and a small variable amount of Sn (0.12, 3.4, and 7.7 at. %) dissolved in Ga. The solutes Pb and Sn compete to segregate in the liquid-vapor interfaces of these alloys; in each case, over the temperature range studied (30--120 \ifmmode^\circ\else\textdegree\fi{}C), the composition of the outermost layer of the interface is dominated by excess Pb. In the case of the alloy with Sn concentration of 0.12 at. %, the excess Pb in the liquid-vapor interface forms a crystalline monolayer that melts at 67.8 \ifmmode^\circ\else\textdegree\fi{}C. In the case of the alloy with Sn concentration of 3.4 at. %, the excess Pb in the liquid-vapor interface again forms a crystalline monolayer, but that crystalline monolayer melts at 40.5 \ifmmode^\circ\else\textdegree\fi{}C. In the case of the alloy with Sn concentration of 7.7 at. %, the excess Pb in the liquid-vapor interface forms a two-dimensional liquid over the entire accessible temperature range. When the excess Pb in the liquid-vapor interface forms a two-dimensional crystal, it fully covers the surface of the liquid; at higher temperatures, when the Pb monolayer is liquid, it only partially covers the liquid alloy surface, with the remainder covered by a partial monolayer of liquid Sn. Two-dimensional liquid Pb and two-dimensional liquid Sn appear to be immiscible under the conditions of our experiments. In the case of the sample with the smallest concentration of Sn, the correlation length obtained from the width of the diffraction peak drops almost discontinuously at 67.8 \ifmmode^\circ\else\textdegree\fi{}C to a smaller value. It then further decreases continuously until about 90 \ifmmode^\circ\else\textdegree\fi{}C, thereafter becoming constant. These data suggest the existence of an intermediate disordered phase stable between 67.8 and 90 \ifmmode^\circ\else\textdegree\fi{}C. Our experiments fall short of providing unambiguous evidence for the existence and character of this disordered phase. We speculate that between 67.8 and about 90 \ifmmode^\circ\else\textdegree\fi{}C the Pb monolayer has a hexatic structure and that above 90 \ifmmode^\circ\else\textdegree\fi{}C it is liquid, as is implied by the results of recent simulations of the temperature dependence of the structure of a quasi-two-dimensional layer of Pb.

STM observation of Si(111)-α−3×3-Sn at low temperature

We carefully investigated the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase on Si(111) surface covered by 1/3 monolayer of Sn using scanning tunneling microscopy (STM) at room temperature (RT), 70 K, and 6 K, with reflection high-energy electron diffraction (RHEED) at RT and 120 K. While weak streaks of the $3\ifmmode\times\else\texttimes\fi{}3$ periodicity was observed in RHEED at 120 K, STM showed a long-range ordered $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase only, without $3\ifmmode\times\else\texttimes\fi{}3$ domains, over the whole temperature range investigated. The $3\ifmmode\times\else\texttimes\fi{}3$ streaks were found to originate from local $3\ifmmode\times\else\texttimes\fi{}3$ modulations around defects. The present result indicates that there is no phase transition from the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ to $3\ifmmode\times\else\texttimes\fi{}3$ on cooling down to 6 K, which contradicts to a prediction from core-level photoemission study and challenges the thermal fluctuation picture for the RT phase.

Electronic structure and magnetic properties of Cr/Sn multilayers

Electronic structure and magnetic properties of Cr/Sn metallic multilayers have been investigated using spin-polarized first principles tight-binding linear muffin-tin orbital (TB-LMTO) method within atomic sphere approximation (ASA). Calculations for an Fe/Cr multilayer have also been done for comparison. The experimentally observed reduced magnetic moment of thin Cr layer sandwiched between Sn or Fe layers has been explained from our band structure calculations. For Cr/Sn multilayer, self-consistent calculations have been done to determine the lattice relaxation for growth of multiple layers of Cr over one monolayer of Sn.