Spot Profile Analysis Low Energy Research Articles

Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC.

We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely R0° rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a preoriented template to induce the unconventional orientation. Using spot profile analysis low-energy electron diffraction, angle-resolved photoelectron spectroscopy, and the normal incidence x-ray standing wave technique, we assess the crystalline quality and coverage of the graphene layer. Combined with the presence of a covalently bound graphene layer in the conventional orientation underneath, our surfactant-mediated growth offers an ideal platform to prepare epitaxial twisted bilayer graphene via intercalation.

Surface reconstruction of Pt(001) quantitatively revisited

The complex hexagonal reconstructions of the (001) surfaces of platinum and gold have been under debate for decades. Here, the structural details of the Pt(001) reconstruction have been quantitatively reinvestigated by combining the high resolving power of scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). In addition, LEED simulations based on a Moir\'e approach have been applied. Annealing temperatures around 850 \ifmmode^\circ\else\textdegree\fi{}C yield a superstructure that approaches a commensurable $c(26.6\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}118)$ substrate registry. It evolves from a Moir\'e-like buckling of a compressed hexagonal top layer (hex) where atomic rows of the hex run parallel to atomic rows of the square substrate. Annealing at 920 \ifmmode^\circ\else\textdegree\fi{}C stimulates a continuous rotation of the hex where all angles between \ifmmode\pm\else\textpm\fi{}0.7\ifmmode^\circ\else\textdegree\fi{} are simultaneously realized. At temperatures around 1080 \ifmmode^\circ\else\textdegree\fi{}C, the nonrotated hex coexists with a hex that is rotated by about 0.75\ifmmode^\circ\else\textdegree\fi{}. Annealing at temperatures around 1120 \ifmmode^\circ\else\textdegree\fi{}C yield a locking of the hex in fixed rotation angles of 0.77\ifmmode^\circ\else\textdegree\fi{}, 0.88\ifmmode^\circ\else\textdegree\fi{}, and 0.94\ifmmode^\circ\else\textdegree\fi{}. At temperatures around 1170 \ifmmode^\circ\else\textdegree\fi{}C, the Pt(001)-hex-R 0.94\ifmmode^\circ\else\textdegree\fi{} prevails as the energetically most favored form of the rotated hex.

A combined STM and SPA-LEED study of the “explosive” nucleation and collective diffusion in Pb/Si(111)

A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an “explosive” way, when the Pb deposited amount in the wetting layer is compressed to θc~1.22ML, well above the metallic Pb(111) density. This “explosive” nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was found that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands “explosively”. The SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot.

Desorption of oxygen from alloyed Ag/Pt(111).

We have investigated the interaction of oxygen with the Ag/Pt(111) surface alloy by thermal desorption spectroscopy (TDS). The surface alloy was formed during the deposition of sub-monolayer amounts of silver on Pt(111) at 800 K and subsequent cooling to 300 K. The low-temperature phase of the surface alloy is composed of nanometer-sized silver rich stripes, embedded within platinum-rich domains, which were characterized with spot profile analysis low energy electron diffraction. The TDS measurements show that oxygen adsorption is blocked on Ag sites: the saturation coverage of oxygen decreases with increasing Ag coverage. Also, the activation energy for desorption (Edes) decreases with Ag coverage. The analysis of the desorption spectra from clean Pt(111) shows a linear decay of Edes with oxygen coverage, which indicates repulsive interactions between the adsorbed oxygen atoms. In contrast, adsorption on alloyed Ag/Pt(111) leads to an attractive interaction between adsorbed oxygen atoms.

Pentacene/Cu(110) interface formation monitored byin situoptical spectroscopy

The adsorption of pentacene on the Cu(110) surface has been investigated by reflectance difference spectroscopy (RDS) and spot profile analysis low energy electron diffraction (SPA-LEED). The sensitivity of RDS to electronic transitions involving both surface states and intramolecular transitions allows a close monitoring of the formation of interfacial electronic states and structural phase transitions of the pentacene monolayer on Cu(110).

Electrostatic Interaction and Commensurate Registry at the Heteromolecular F16CuPc–CuPc Interface

Tailoring the properties of molecular thin films and interfaces will have decisive influence on the success of future organic electronic devices. This is equally true for metal–organic and hetero–organic contacts as they occur, for example, in donor–acceptor systems. Here, we report on the structure formation and interaction across such a heteromolecular interface. It is formed by monolayers of F16CuPc and CuPc stacked on a Ag(111) surface. We investigated the lateral and vertical structure using spot-profile analysis low energy electron diffraction and normal incidence X-ray standing waves, and performed pair potential calculations to understand the driving forces for the structure formation. Most surprisingly, for one phase we found a commensurate registry between the two organic layers, usually a sign for a strong (chemisorptive) interaction often involving metallic states of the surface. However, because the organic bilayer is not commensurate with the underlying Ag substrate in our case, the dominating factor must be the intermolecular interaction. Pair potential calculations suggest a site-specific adsorption that leads to a commensurate registry at the heteromolecular interface. The adsorbate system was further characterized by measuring adsorption heights, indicating flat-lying molecules and a CuPc–F16CuPc layer spacing of 3.06 Å.

Epitaxial growth of the topological insulator Bi2Se3 on Si(111): Growth mode, lattice parameter, and strain state

Using spot profile analysis low energy electron diffraction, we studied the growth mode and strain state of ultra-thin epitaxial Bi2Se3(111) films grown by molecular beam epitaxy on Si(111). The first layer grows as complete quintuple layer and covers the Si substrate before the next layer nucleates. Its lateral lattice parameter is increased by 1% compared with the value of a‖ = 4.136 Å for a 6-nm-thick film. With increasing film thickness, a continuous change of the lattice parameter is observed to an asymptotic value, which is explained by a van der Waals-like bonding between the Bi2Se3 film and the Si substrate.

Surface morphology of ultrathin hex-Pr2O3 films on Si(1 1 1)

In this work, the morphology of the surface of hexagonal Pr2O3(0 0 0 1) films grown on Si(1 1 1) is studied by high-resolution low-energy electron diffraction combined with spot profile analysis. For this purpose, praseodymia films prepared by molecular beam epitaxy were capped with protecting amorphous germanium films. After removal of the capping layers due to heating in diluted oxygen atmosphere the surface properties of the oxide film were investigated in situ with Auger electron spectroscopy and spot profile analysis low energy electron diffraction. The removal of the capping layer has no impact on the hexagonal Pr2O3(0 0 0 1) film structure which is shown by x-ray diffraction. Surface sensitive electron diffraction confirms that the surface of the oxide film has hexagonal structure. Diffraction spot profile analysis shows that the film surface has grain structure without any mosaic spread due to the negligible lateral lattice mismatch between hexagonal Pr2O3(0 0 0 1) and Si(1 1 1). In addition, single atomic steps with complete bulk unit cell height are present at the surface. The density of the atomic steps is small pointing again to the high quality of the surface of hexagonal Pr2O3 films compared to cubic Pr2O3 films grown on Si(1 1 1).

Surface structure of Pd(111) with less than half a monolayer of Zn

We have characterized the structural properties of submonolayer amounts of Zn on Pd(111) using scanning tunneling microscopy (STM) and spot-profile analysis low energy electron diffraction (SPA-LEED). Following room temperature deposition of ≈0.06 monolayers (ML) Zn onto Pd(111), we observe the substitution of Zn for Pd in the surface layer. At ≈0.20 ML of deposited Zn, STM reveals a locally ordered phase with a (2/√3 × 2/√3)R30° unit cell located near Zn substitutions; SPA-LEED patterns reveal the same periodicity. We attribute this phase to the metastable bonding of atoms or clusters predominantly in hollow sites surrounding Zn substitutions in the surface layer. At ≈0.4 ML, STM images reveal local (√3 × √3)R30° and (2 × 1) ordering on surfaces annealed to 350 K. At coverages near 0.5 ML, both STM and SPA-LEED show the onset of the formation of the (2 × 1) ordering associated with the Zn : Pd 1 : 1 alloy phase. At all coverages, the surface is dominated by island growth; the islands' size and density is shown to depend critically on annealing at temperatures as low as 350 K. These results provide insight into the structural features of a Zn/Pd(111) coverage regime that has been much debated in recent years.

Lead growth on Si(111) surfaces reconstructed by indium

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis–low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3–In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1–In surface results in metallic Pb(111) crystalline islands after the completion of a double incomplete wetting layer. In addition, the bond arrangement of the adatoms is studied, confirming that In adatoms interact more strongly with the silicon substrate than the Pb ones. This promotes a stronger Pb–Pb interaction and enhances metallization. The onset of the metallization is correlated with the amount of pre-deposited In on the Si(111) surface. The decoupling of the Pb film from the 4 × 1-In interface can also explain the unusual thermal stability of the uniform height islands observed on this interface. The formation of these Pb islands is driven by quantum size effects. Finally, the different results of Pb growth on the two reconstructed surfaces confirm the importance of the interface, and also that the growth morphology, as well as the electronic structure of the Pb film can be tuned with the initial substrate reconstruction.

Probing the Buried Pb/Si(111) Interface with SPA LEED and STM on Si(111)-Pbα√3×√3

High resolution spot profile analysis low energy electron diffraction (SPA-LEED) and variable temperature scanning tunneling microscopy (STM) have been used to observe the growth of Pb on the Pb/Si(111)-α√3×√3 phase, which is driven by quantum size effects (QSE). A change in the rotation of the Pb grown islands with respect to the Si substrate has been observed with increasing coverage θ. At lower coverage, separated two-step islands are grown and are aligned with the [110] axis of the substrate. With increasing coverage above 1.5 ML, of the islands coalesce and form a bilayer, with additional islands grown on top. The preferred Pb island orientation changes to 5.6° with respect to the [110] direction. These changes at the metal/semiconductor buried interface are obtained both with SPA LEED and STM as changes to the period of the Moire pattern. The method of analysis of the corrugation period and rotation angle of the Moire pattern measured with diffraction and STM can be applied to obtain the structure of buried metal/substrate interfaces in other epitaxial systems.

Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni 3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni 3Al and NiAl films, obtained respectively below and above a critical Al initial coverage θ c , differ markedly from those of corresponding bulk alloys. The Ni 3Al ordered films form in a concentration range larger than the stability domain of the L1 2 Ni 3Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases. It also appears that the value of θ c depends on the morphology of the Ni(1 1 1) substrate, increasing from θ c = 4.5 ML for a flat surface to θ c = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.

Structure and stability of cub‐Pr2O3 films on Si(111) under post deposition annealing conditions

AbstractUltra thin heteroepitaxial Pr2O3 films on Si(111) were characterized by X‐ray diffraction and X‐ray reflectometry to determine the film and interface structure. These results exhibit two oxide phases within the praseodymia film after preparation which can be assigned to different oxygen stoichiometries. After post deposition annealing of the films at different temperatures up to 600 °C the surface was studied by spot profile analysis low energy electron diffraction, with regard to the stability of the film surface.The results show that terraces with mono‐atomic step heights and mosaics can be found at the film surfaces after annealing at 300 °C. Higher annealing temperatures do not cause further changes of structure and morphology except of a slight increase of the mosaic spread due to lateral strain effects within the oxide film. These results indicate that the films are stable at temperatures as high as 600 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface phase transition and low-temperature phase of α-Ga(0 1 0) studied by SPA-LEED

The order–disorder phase transition on the α-Ga(0 1 0) ( 2 2 × 2 ) R 45 ° structure was studied by spot-profile analysis low energy electron diffraction (SPA-LEED). A low temperature diffraction pattern reveals a small splitting of the overlayer spots which corresponds to a real-space distance of 81 Å, equivalent to 18 unit cells. The splitting is interpreted as caused by a regular ordering of anti-phase domains of the low-temperature phase. Due to the low symmetry of the surface, the domain boundaries are aligned only in one direction, giving rise to a regular, one-dimensional grid. The temperature dependence of the intensity and width of the reconstruction-induced diffraction spots is also investigated. It suggests that the phase transition takes place at a critical temperature T c = 232 K and that anti-phase boundary proliferation plays a role.

Structural phase transition of ultra thinPrO2 films on Si(111)

Ultra thin heteroepitaxial PrO2 films on Si(111) were annealed under UHV conditions and investigated by x-ray diffraction(XRD), x-ray reflectometry (XRR) and spot profile analysis low energy electrondiffraction (SPALEED) with regard to structural stability and phase transitionsdue to the high oxygen mobility of the oxide. This gives information about themanageability of the material and its application as a model catalyst system in surfacescience. While the samples are stable in UHV at room temperature, annealing at300 °C exhibits a terminatedphase transition from PrO2 and PrO2−Δ to cub-Pr2O3 with an increase in the silicate at the interface and a decrease in the crystalline praseodymialayer mainly due to atomic diffusion of silicon into the oxide film. Strain effects during thephase transition also cause mosaic formation at the surface. Further annealing up to600 °C shows only little change in the film structure. This will finally lead to a model of the filmstructure during the annealing process.

The commensurate-to-incommensurate phase transition of an organic monolayer: A high resolution LEED analysis of the superstructures of NTCDA on Ag(1 1 1)

The structural order of 1,4,9,10-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) monolayers on Ag(1 1 1) has been investigated by spot profile analysis low energy electron diffraction (SPA-LEED). For increasing coverage, we find a sequence of three highly ordered structures: a commensurate structure (α), a uniaxially incommensurate structure (α 2), and an incommensurate structure (β) with coverages of 0.9 ML, 0.95 ML, and 1 (saturated) monolayer (ML), respectively. In the high coverage regime, the structures coexist and a coverage increase causes a change of their relative fractions. The α and β structures were known before [U. Stahl, D. Gador, A. Soukopp, R. Fink, E. Umbach, Surf. Sci. 414 (1998) 423], but the β structure was proposed as commensurate, since its very small misfit with respect to a commensurate structure could not be resolved. This misfit leads to a periodic modulation, causing additional Moiré satellites in the diffraction pattern. This finding demonstrates the importance of high resolution methods for the geometry determination of large organic adsorbates.

Highly ordered thin films of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on Ag(1 0 0)

We report experiments performed on vacuum deposited perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) films on Ag(1 0 0) by spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). Up to a coverage of 1 ML, PTCDA forms a commensurate c(8 × 8) structure with two molecules per primitive unit cell, which are perpendicularly oriented to each other (four molecules per c(8 × 8) unit cell). The PTCDA islands exhibit an equilibrium shape that corresponds to the symmetry of the Ag(1 0 0) surface. The structural order is stable up to a temperature of ∼670 K, at which a phase transition to a disordered phase occurs. Upon cooling back to room temperature, the order is partly re-obtained, although there is evidence that molecules have partly desorbed and dissociated at the high temperature. At coverages above 1 ML, a second layer orders in a herringbone structure on top of the c(8 × 8)-ordered first layer. The corresponding unit cell is very similar to that of the (1 0 2) plane of PTCDA bulk crystals.

Formation ofMn3O4(001)on MnO(001): Surface and interface structural stability

X-ray absorption and photoemission spectroscopies, high-resolution electron energy loss spectroscopy, spot profile analysis low energy electron diffraction, and density functional theory calculations are employed to study the growth of (001) oriented ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ surfaces on a Pd(100)-supported MnO(001) substrate, with the Hausmannite planar lattice constants aligned along the [110] direction of the underlying MnO(001) support. We show that despite the rather large lattice mismatch, abrupt interfaces may exist between rocksalt MnO and Hausmannite. We argue that this process is facilitated by the relatively low computed strain energy and we propose realistic models for the interface. An atop site registry between the Mn(O) atoms of the oxygen rich ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ termination and the MnO(001) O(Mn) atoms underneath is found to be the energetically most favorable configuration. The significant planar expansion is accompanied by a large compression of the ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ vertical lattice constant, yielding structural distortion of the O-Mn-O octahedral axis. Spot profile analysis low energy electron diffraction experiments show that the conversion reaction proceeds easily in both directions, thus indicating the reversible redox character of the transition.

Solid phase epitaxy of Ge films on CaF 2/Si(1 1 1)

At room temperature deposited Ge films (thickness < 3 nm) homogeneously wet CaF 2/Si(1 1 1). The films are crystalline but exhibit granular structure. The grain size decreases with increasing film thickness. The quality of the homogeneous films is improved by annealing up to 200 °C. Ge films break up into islands if higher annealing temperatures are used as demonstrated combining spot profile analysis low energy electron diffraction (SPA-LEED) with auger electron spectroscopy (AES). Annealing up to 600 °C reduces the lateral size of the Ge islands while the surface fraction covered by Ge islands is constant. The CaF 2 film is decomposed if higher annealing temperatures are used. This effect is probably due to the formation of GeF x complexes which desorb at these temperatures.

Unusual thermal stability of quantum size effect Pb islands grown on Si(111)-In(4×1)

Pb islands grown on the anisotropic Si(111)-In(4×1) were studied using scanning tunneling microscopy and spot profile analysis–low energy electron diffraction. Anisotropic wire shapes with a uniform island height of four layers due to quantum size effects are observed as well as a preferred width of 6.6nm. Unlike islands grown on clean and Pb covered Si(111), Pb islands grown on the Si(111)-In(4×1) reconstructed surface maintain a height of four layers to room temperature. The increased temperature stability enhances the potential use of the grown structures for technological applications.