Profile Analysis Low-energy Electron Diffraction Research Articles

Degeneracy in Intercalated Pb Phases under Buffer-Layer Graphene on SiC(0001) and Diffuse Moiré Spots in Surface Diffraction.

First-principles density functional theory (DFT) is used to analyze the stability of Pb intercalated phases under buffer layer graphene on SiC(0001) as a function of the supercell size, Pb coverage, and degree of Pb ordering. By comparing the chemical potentials of such two-dimensional Pb structures, we find that there is a family of structurally distinct thermodynamically preferred Pb subsurface configurations with minute stability differences. These differences are comparable to the thermal energies at about 450 °C, where the Pb intercalated phases are grown. High-resolution surface-diffraction experiments using Spot Profile Analysis Low-Energy Electron Diffraction (SPA-LEED) confirm this high degree of degeneracy of the Pb intercalated phases from broad, low-intensity moiré spots observed exclusively from intercalated Pb. The low intensity of the moiré spots implies the coexistence of structurally different subsurface Pb phases.

Adsorption of S on Si(111) with M4 × 4 superstructure

The atomic and electronic structures are calculated for the M4 × 4 superstructure of S/Si(111), which was recently identified by Schmidt et al. [Surf. Sci.694 (2020) 121561] by using the spot profile analysis low-energy electron diffraction (SPALEED) and confirmed by scanning tunneling microscopy (STM). The electronic band structures are found to be semiconducting in nature for three different S structures: monomer, Dimer-A and Dimer-B, each of which saturates all the Si dangling bonds. Due to the varying numbers of S atoms per unit cell, we have calculated the lowest energy phase in the accessible S chemical potential range. Dimer-A and Dimer-B are the lower energy phase and their accessible chemical potential value is less than the monomer case, in excellent agreement with the results of Schmidt et al. Dimer-B is slightly more stable geometry than Dimer-A. Application of the nudged elastic band (NEB) method suggests that there is a rotational activation barrier of 40 meV along the Dimer-A–Dimer-B reaction path.

Adsorption of sulfur on Si(111)

The adsorption of S on Si(111)- 7 × 7 has been investigated for different preparation schemes and parameters. S was supplied from an electrochemical Ag2S cell. For room temperature adsorption and subsequent annealing, no ordered S induced reconstruction can be observed with spot profile analysis low-energy electron diffraction (SPALEED). S deposition at temperatures above about 400 ∘C, however, leads to a well-ordered reconstruction. Judging from the LEED pattern, the same reconstruction was already observed by Metzner et al. [Surf. Sci. 377–379 (1997) 71–74] who identified it as 4 × 4 reconstruction. The upper temperature limit for the preparation of this superstructure depends on S flux, which is needed to compensate for desorption. Prolonged S exposure leads to surface roughening, as observed with SPALEED and scanning tunneling microscopy (STM), pointing to surface etching by S. From our SPALEED data, we can conclude that the observed reconstruction is not a 4 × 4 reconstruction, but a (2012) superstructure with a rectangular unit cell that exists in three rotational domains, as confirmed by STM. Different structural trial models have been assessed with density functional theory. Among these model structures, a configuration with dimers adsorbed on bridging sites, with a S coverage of 1 monolayer, is most likely, since it is energetically favorable and is in agreement with all experimental results.

Rapid onset of strain relief by massive generation of misfit dislocations in Bi(111)/Si(001) heteroepitaxy

Strain and its relaxation in lattice mismatched heterostructures are crucial for the functionality of modern electronic devices, which are often challenging to determine experimentally. Here, we demonstrate a technique for measuring the strain state during epitaxial growth of Bi(111) films on Si(001) by using the spot profile analysis low-energy electron diffraction. Exploiting two non-equivalent integer-order diffraction spots originated from two Bi sub-lattices, the lattice parameter of the film is determined with high precision, which allows tracing the strain state as the film grows. The sudden and massive generation of misfit dislocations is found at a critical thickness of 4 nm which is explained through the inhomogeneous strain state of films with a thickness below one quarter of the mean distance of the dislocations.

Ga and In adsorption on Si(112): Adsorption sites and superstructure

The adsorption of the two group-III metals Ga and In on Si(112) has strong influence on the morphology of the intrinsically faceted Si(112) surface. Upon Ga or In adsorption, the Si(112) surface is smoothed, and quasi-1D adsorbate structures along the $[1\overline{1}0]$ direction are observed. These structures consist of ($N\ifmmode\times\else\texttimes\fi{}1$) building blocks of different sizes, the periodicity of which can be controlled by surface coverage and deposition temperature, as revealed by spot profile analysis low-energy electron diffraction. From x-ray standing-wave measurements, building blocks consisting of two parallel rows of adsorbate atoms are identified for both Ga/Si(112) and In/Si(112). One adsorption site is identified as a terrace (substitutional) site and the other one as a step-edge (adatom) site. These experimental results are compared to several relaxed model structural configurations obtained from density functional theory calculations. In the case of Ga/Si(112), a previously reported structural model by Snijders et al. [Phys. Rev. B 72, 125343 (2005)], including two Ga vacancies per unit cell is corroborated, while for In/Si(112), existing models by Gai et al. [Phys. Rev. B 61, 9928 (2000)] and by Bentmann et al. [Phys. Rev. B 80, 085311 (2009)] can be ruled out, and a new structural model including only one In vacancy per unit cell in the step-edge site is concluded on, similar to the Al/Si(112) model introduced by Gupta and Batra [Phys. Rev. B 72, 165352 (2005)].

Two-dimensional interaction of spin chains in the Si(553)-Au nanowire system

Adsorption of Au on Si(553) results in the self-assembly of highly ordered step arrays of one-dimensional (1D) Au atomic wires along the step direction. Charge transfer from the terrace to the step edge causes every third Si atom at the step edge to exhibit a partially filled dangling bond hosting a single fully spin-polarized electron which forms in an ordered 1D spin chain along the step. The interstep correlation of this threefold periodicity in neighboring Si step edges and the geometry of the unit cell has been determined by means of high-resolution spot profile analysis low-energy electron diffraction, scanning tunneling microscopy, and density functional theory. While the twofold periodicity of the Au wires exhibits a weak interwire interaction, leading to streaks in the diffraction pattern, the correlation of the Si step edge atoms is by far a stronger interaction, resulting in clear spots. The corresponding unit cell spanned by threefold ordered step edge atoms can be described as a centered structure which is magnetically frustrated and may stabilize a (two-dimensional) quantum spin liquid.

Isotropic thin PTCDA films on GaN(0 0 0 1)

The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

Structural and Vibrational Properties of CuPc/Ag(111) Ultrathin Films

The initial stages of copper(II)–phthalocyanine (CuPc) thin film growth on Ag(111) have been investigated using Fourier transform infrared absorption spectroscopy (FT-IRAS), spot profile analysis low-energy electron diffraction (SPA-LEED), and thermal desorption spectroscopy (TDS). Starting at (sub)monolayer coverages up to 5 monolayers (ML), a number of ordered overlayers are found. Vibrational spectroscopy shows characteristic spectroscopic signatures for the individual submonolayer phases as well as for the bilayer, trilayer, and multilayers. Highly asymmetric line shapes of the in-plane vibrational modes of submonolayer CuPc provide unequivocal evidence for interfacial dynamical charge transfer between the metal electronic states and CuPc molecular orbitals, indicative for a partially filled LUMO at the Fermi energy as well as the prevalence of severe nonadiabaticity in the electron–vibron coupling. Growth of the second and third CuPc layers proceeds in a layer-by-layer fashion (Frank van der Merwe gr...

The quantum nature of C-H···metal interaction: vibrational spectra and kinetic and geometric isotope effects of adsorbed cyclohexane.

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (∼20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C6 D12 ) is lower than that of C6 H12 (inverse kinetic isotope effect). In addition, the work function change by adsorbed C6 D12 is smaller than that by adsorbed C6 H12 . These results indicate that C6 D12 molecules are slightly more distant from the surface than C6 H12 molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.

Surface reconstruction of Au(001): High-resolution real-space and reciprocal-space inspection

The hexagonal reconstruction of Au(001) has been a model system for the understanding of complex metal surface reconstructions, and it has been intensively studied over almost five decades. Nevertheless, details of the reconstruction regarding the top layer to substrate matching and alignment are not unambiguously identified due to restricted resolution of previously available analytical tools as well as nonuniform sample quality. We therefore quantitatively reinvestigated the system by applying high-resolution real-space and reciprocal-space inspection using scanning tunneling microscopy and spot profile analysis low-energy electron diffraction in analyzing a high-quality Au(001) sample. We show that the Au(001) reconstruction consists of two rotational domains of a commensurate $c(28\ifmmode\times\else\texttimes\fi{}48)$ superstructure. It results from a Moir\'e-like buckling of a quasihexagonal top layer which is characterized by lattice vectors having a length of $0.9655a$ and $0.9581a$, respectively, where $a$ is the interatomic distance of Au(001). The former vector runs exactly along [110] or [\ensuremath{-}110], whereas the latter one deviates by 59.75\ifmmode^\circ\else\textdegree\fi{} from those directions. ${\mathrm{Ar}}^{+}$ ion bombardment at elevated temperatures induces a rotation of the top layers up to angles of \ifmmode\pm\else\textpm\fi{}0.83\ifmmode^\circ\else\textdegree\fi{}. Sample annealing yields a turning back into the initial top layer alignment. Rotation and reorientation proceeds continuously, i.e. all angles between 0 and \ifmmode\pm\else\textpm\fi{}0.83\ifmmode^\circ\else\textdegree\fi{} are observed. Using simple hard sphere models, the main characteristics of the reconstruction are explained. This includes even structural details of the reconstruction at step edges for nonrotated and rotated quasihexagonal top layers.

Vibrational properties of the compressed and the relaxed 1,4,5,8-naphthalene-tetracarboxylic dianhydride monolayer on Ag(111)

Layers of 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) grown on Ag(111) at about 80 K have been investigated using Fourier transform infrared spectroscopy, spot profile analysis low-energy electron diffraction, and temperature programmed desorption. Specifically, the compressed and the relaxed monolayer, as well as the transformation between the two ordered phases have been analyzed in detail. It is found that the two monolayer phases display distinctly different vibrational spectra and can thus be discriminated with high accuracy and sensitivity. For the NTCDA/Ag(111) monolayer strong in-plane vibrational modes point at a marked dynamic charge transfer between molecule and metal substrate and provide clear evidence for an efficient electronic coupling to the Ag(111) surface in conjunction with a partially filled electronic level at the Fermi energy. The bilayer, on the other hand, is largely electronically decoupled from the substrate and, according to the vanishing infrared-active in-plane vibrational modes, is oriented parallel to the surface. On the basis of spectroscopic data the metastable nature of the bilayer phase is identified as such, leading to an improved understanding of processes encountered in the course of layer preparation and resolving inconsistencies reported in the literature.

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.

Lost in reciprocal space? Determination of the scattering condition in spot profile analysis low-energy electron diffraction

The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions.

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.

Effects of laser irradiation on the morphology of Cu(110)

The effects of pulsed laser irradiation on the morphology of the Cu(110) surface were investigated by means of reflectance difference spectroscopy (RDS) and spot profile analysis low-energy electron diffraction (SPA-LEED). The laser light induces surface defects (adatoms and islands) as well as subsurface dislocation lines. The high surface mobility leads to efficient annealing of the surface defects even at room temperature, whereas the subsurface dislocation lines persist up to temperatures $T>800\text{ }\text{K}$. SPA-LEED profiles of the (00) diffraction spot from the laser irradiated surface suggest an anisotropic distribution of the subsurface line defects related to the geometry of the fcc easy glide system, which is corroborated by STM measurements. Comparative experiments using conventional Ar ion bombardment point out the distinctiveness of the morphological changes induced by laser irradiation.

Crystallographic phase transition and island height selection in In/Si(111) growth

In/Si(111) has been studied with spot profile analysis low-energy electron diffraction, scanning tunneling microscope, and first-principles total energy calculations to identify its growth morphology at low temperatures. Of the different substrate interfaces used, only In growth on Si(111)-$\text{Pb-}\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3$ has resulted in uniform height fcc (111) four-layer islands. A transition to the bulk bct (101) oriented islands is favored at higher temperatures $T>250\text{ }\text{K}$ and/or larger coverages $\ensuremath{\theta}>5\text{ }\text{ML}$. These results suggest two stabilizing effects for the preferred morphologies, i.e., quantum size effects and orientation dependent surface and interface energies. These stabilizing effects are suppported from first-principles calculations.

Epitaxial Bi(111) films on Si(001): Strain state, surface morphology, and defect structure

Smooth and epitaxial thin bismuth (Bi) films with low defect density were grown on Si(001) by molecular beam epitaxy. The film quality is characterized by in situ spot profile analysis low-energy electron diffraction and scanning tunneling microscopy, and ex situ atomic force microscopy and X-ray diffraction. The complete process is accomplished in three steps. Firstly, a template of a strained 6 nm Bi(111) film is grown at 150 K. Secondly, during annealing to 450 K the strain is relieved by the formation of an ordered array of misfit dislocations at the interface. Finally, additional Bi is deposited at 450 K up to the desired thicknesses of the Bi film. The film consists of 90° rotated and twinned μm size crystallites with a terrace size larger than 100 nm and an overall roughness of only 0.6 nm. A 25 nm thick Bi film is relaxed to Bi bulk lattice constant which is confirmed by X-ray diffraction.

Oxygen chemisorption on Cu(19 19 1) studied by spot profile analysis low-energy electron diffraction

Cu(110) and the vicinal Cu(19 19 1) surfaces were characterized by recording maps of the reciprocal space by means of spot profile analysis low-energy electron diffraction (SPA-LEED). For both surfaces, kinematic simulations were performed to get insight into the main features of the experimental data. Furthermore, it is shown that chemisorption of oxygen and subsequent annealing lead to the formation of a Cu-CuO stripe phase and induce faceting of the Cu(19 19 1) surface. The evolution from the clean Cu(19 19 1) surface to the coexistence of the (110) and (111) facets with increasing oxygen exposure was characterized by SPA-LEED.

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.

Influence of the deposition angle on the magnetic anisotropy in thin Co films on Cu(001)

Off-normal molecular beam epitaxy of Co on Cu(001) has been found to result in a uniaxial magnetic anisotropy. The easy magnetization axis is perpendicular to the plane of incidence for deposition angles between $10\ifmmode^\circ\else\textdegree\fi{}$ and $80\ifmmode^\circ\else\textdegree\fi{}.$ The uniaxial magnetic anisotropy is related to the formation of a uniaxial surface morphology during off-normal growth. Spot profile analysis low-energy electron diffraction measurements reveal the presence of elongated adatom structures in contrast to square ones growing at normal incidence. The long sides of the adatom structures are oriented perpendicular to the plane of incidence, i.e., parallel to the easy magnetization axis. The formation of elongated adatom structures is due to steering. Steering originates from long-range attractive forces between incident atoms and substrate atoms and leads to preferential arrival of atoms on top of adatom structures.