LEED Results Research Articles

A Round Robin Test on the dynamic simulation and the LEED protocol evaluation of a green building

• RRT to evaluate LEED and dynamic simulation results applied by different operators. • Evaluating the influence of different energy simulation software on LEED results. • Assessing if subjectivity of operators affects the final LEED rating score. Green building concept plays a fundamental role in reducing the use of resources and the impacts on human health and environment, during the whole building life cycle. Therefore, a method to measure the building sustainability rate is crucial for comparing various alternatives in terms of use of different materials, energy resources, production processes to reduce energy consumption and environmental impacts. Many protocols have been proposed to perform buildings’ sustainability evaluation; however, different operators applying these certification tools might need to make hypotheses, even different from each other, to complete the whole procedure. Hence, this work aims to evaluate whether and how the hypotheses formulated by each operator can influence the final certification level. To this end, a Round Robin Test among international partners was performed using different versions of LEED sustainability protocol to the same building with the same boundary conditions, comparing and analyzing the results provided by the participants. The final aim was to identify which issues have more influence on the final performance rate, giving to the users a deeper knowledge of the aspects included in these procedures. The results showed the potential of these building environmental certification systems, capable of offering a transversal level of environmental sustainability.

A patterning-free approach for growth of free-standing graphene nanoribbons using step-bunched facets of off-oriented 4H-SiC(0 0 0 1) epilayers

The tunable electronic structure of graphene nanoribbons (GNRs) has attracted much attention due to the great potential in nanoscale electronic applications. Most methods to produce GNRs rely on the lithographic process, which suffers from the process-induced disorder in the graphene and scalability issues. Here, we demonstrate a novel approach to directly grow free-standing GNRs on step-bunched facets of off-oriented 4H-SiC epilayers without any patterning or lithography. First, the 4H-SiC epilayers with well-defined bunched steps were intentionally grown on 4 degree off-axis 4H-SiC substrates by the sublimation epitaxy technique. As a result, periodic step facets in-between SiC terraces were obtained. Then, graphene layers were grown on such step-structured 4H-SiC epilayers by thermal decomposition of SiC. Scanning tunneling microscopy (STM) studies reveal that the inclined step facets are about 13–15 nm high and 30–35 nm wide, which gives an incline angle of 23–25 degrees. LEEM and LEED results showed that the terraces are mainly covered by monolayer graphene and the buffer layer underneath it. STM images and the analysis of their Fourier transform patterns suggest that on the facets, in-between terraces, graphene is strongly buckled and appears to be largely decoupled from the surface.

Interplay between Structural and Electronic Properties in 1,4,5,8-Naphthalenetetracarboxylic Dianhydride Films on Cu(100)

Using various surface science techniques, we have studied the properties of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) thin films on a Cu(100) surface. STM investigations suggest the high mobility of the NTCDA molecules over the surface, leading to the formation of large and well-ordered islands. In line with LEED results, two typical domains with different molecular orientations and brick-wall-like structure are revealed in the STM images. On the other hand, a fingerprint of covalent bonding at the NTCDA/Cu interface is obtained from the core level shifts measured by high-resolution XPS. As a consequence, a charge transfer process develops at the interface and enables a total filling of the π*-like lowest unoccupied orbital, which is the signature of the semiconductor character of the NTCDA layer. The carbon K-edge NEXAFS data confirm the chemisorption nature of the interaction between NTCDA and the Cu(100) surface and indicate an evolution from a flat-lying orientation of NTCDA at 1 ML to a straight-up orientation in a thick layer.

Surface structure of In 2O 3(111) (1 × 1) determined by density functional theory calculations and low energy electron diffraction

The surface structure of In 2O 3(111) has been investigated by dynamical analysis of low energy electron diffraction data, in conjunction with first principles calculations using density functional theory. The experimental data set consisted of eight independent beams whose intensities were measured for incident energies in the range between 25 eV and 250 eV. In fitting the experimental data it was essential to treat the radii of In and O spheres as variable parameters: following this procedure a final Pendry R factor of 0.31 was obtained. The LEED results are compatible with the calculations and both analyses suggest that the surface structure involves only small vertical relaxations in the outermost of the {[ O 2−] 12 24−[ In 3+] 16 48+[ O 2−] 12 24−} quadrupolar units that define the (111) surface. The ab initio slab calculations also confirm that lateral relaxations not considered in fitting the experimental data are of very minor importance.

Structure determination of the [formula omitted] Bi–Ag(111) surface alloy using LEED I–V and DFT analyses

The deposition of 1/3 of a monolayer of Bi on Ag(111) leads to the formation of BiAg 2 surface alloy with a long range ordered 3 × 3 R 30 ∘ superstructure. A detailed analysis of this structure using LEED I–V measurements together with DFT calculations is presented. We find strong correlation between experimental and calculated LEED I–V data, with the fit between the two data sets having a Pendry's reliability factor of 0.16. The Bi atom is found to replace one top layer Ag atom in each unit cell, forming a substitutional BiAg 2 surface alloy, with the Bi atoms residing approximately 0.6 Å above the Ag atoms due to their size difference. DFT calculations are in good agreement with the LEED results.

Perspectives for surface structure analysis with low energy electron diffraction

The two main methods for surface structure determination, X-ray diffraction and low energy electron diffraction, are briefly compared and two areas are discussed where the application and further development of LEED seems promising. One field is the measurement of thermal vibration with LEED and the second is the analysis of substrate induced distortions in adsorbed organic molecules. As a test case for the analysis of thermal vibrations the results of a temperature dependent LEED I(V) analysis of Cu(1 1 0) is presented showing that LEED is sensitive enough to measure anisotropic vibration amplitudes. As example for organic molecules the results of a LEED I(V) analysis of thiouracil on Ag(1 1 1) are presented and compared to the results of a previous X-ray study. The differences in the structural details can be related to the different diffraction geometries of surface X-ray diffraction and LEED.

Structural and dynamical properties of Ru(0 0 0 1) surface

I performed density functional theory (DFT) calculations combined with MD simulations to study the structural relaxation of Ru(0001) surface. The surface relaxation of the topmost layer is found to be about −4% at absolute zero temperature. Using MD simulations in the temperatures range of 50 K and 900 K, the effect is found to be minor on the surface relaxation as compared to Pd (1 1 1) clean surface. The effect of surface vibration is also investigated using a LEED code and shows no effect of the vibrational level on the IV curves, which rules out any disagreement between proper theory and LEED results of well prepared surfaces.

A first-principles study of K adsorption on Pb(111)

Ab initio total-energy density functional theory calculations with supercell models have been employed to investigate the R30 degrees and (2 x 2) structures of K on the Pb(111) surface. Four "on-surface" sites and a substitutional site were considered. The calculations showed that the substitutional site is more stable than all the on-surface sites, due to its low vacancy formation energy. The calculated R30 degrees geometry agrees well with the LEED results. The density-of-states analysis indicates that the K atom loses part of its loosely bound valence s electron. From the electron density distributions, it was found that the lowering of the work function after the substitutional adsorption can be attributed to the dipole moment, associated with the positively polarized adsorbate atom that is characterized by charge depletion from the K vacuum sides and charge accumulation in the region between K and Pb atoms. Our results indicate that the bonding of K with the Pb(111) surface has a mixed ionic and metallic bond character.

Pseudomorphic growth of Fe monolayers onIr(001)−(1×1): From a fct precursor to a bct film

The epitaxial growth of Fe monolayers on the unreconstructed Ir 001 surface was investigated experimentally by determining film structure, morphology, stress, and magnetism. Our combined quantitative low energy electron diffraction LEED and cantilever stress results indicate pseudomorphic Fe growth with a constant in-plane film strain in an almost perfect layer-by-layer growth mode from 0–10 layers. The combination of the structural analysis by LEED with stress measurements suggests that the first two layers of Fe grow as a face-centered tetragonally fct distorted precursor. Further deposition of Fe leads to the growth of a bodycentered tetragonally distorted phase on top of the fct precursor, which retains its structure. The magnetooptical Kerr effect reveals that the Fe film is ferromagnetic with an in-plane easy magnetization direction along Fe 100 .

Adsorption of S on Ir(1 0 0) surface from first-principles calculations

The adsorption of adatom S on the unreconstructed Ir(1 0 0) surface is studied using projector-augmented-wave method based on density-functional theory. It is confirmed that S energetically favors the fourfold hollow site for both the p(2 × 2) and c(2 × 2) phases. The optimized geometries are in reasonable agreement with the LEED results. Also, vibrational frequencies, work-function change and density of states are presented for adatom S in the hollow site.

STM and LEED studies of atomically ordered terraced Si(557) surfaces

We report results of LEED, STM, and photoemission studies of the atomic and electronic structure of atomically ordered terraced Si(hhm) surfaces. LEED and STM data demonstrate the possibility of fabricating atomically accurate terraced structures based on Si(557) with different periodicities depending on the thermal treatment procedure. Atomically resolved STM images reveal (7 × 7) terrace ordering and triple step structure. Comparative photoemission studies of the valence band and Si 2p core level electronic structure have been done on clean stepped surfaces with different terrace widths (namely, Si(557), Si(556)) and flat Si(111)-(7 × 7).

First-principles investigation of the role of registry relaxations on stepped Cu(100) surfaces

The multilayer relaxations of the 311 , 511 , 711 , and 911 Cu surfaces are investigated using the all-electron full-potential linearized augmented plane-wave FLAPW method. We found relaxation sequences like −+− , −−+− , −−−+− , and −−−−+− for 311 , 511 , 711 , and 911 , respectively, where the − and + signs indicate contraction and expansion, respectively, of the interlayer spacing. Furthermore, we found that the first-neighbor distances between the Cu atoms in the step edges do not depend on the surface termination, i.e., dSC−CC is the same for all studied surfaces. Our FLAPW relaxation sequences are in full agreement with quantitative low-energy electron diffraction LEED results, as well as with the multilayer relaxationcoordination trend proposed recently. However, large discrepancies are found for the magnitude of the interlayer relaxations, particularly for those involving atoms at the step edges. From our calculations, we suggest that these discrepancies are due to the fact that the atomic displacements parallel to the surface were not take into account in the quantitative analysis of the LEED intensities, which we found to play an important role for a quantitative description of the stepped Cu 2n−1, 11 surfaces.

Multilayer relaxations of (3 1 1), (3 3 1) and (2 1 0) fcc transition metal surfaces studied by pseudopotential DFT calculations

The multilayer relaxations of (3 1 1), (3 3 1) and (2 1 0) surfaces of Ni, Cu, Rh, Pd, Ag, Ir and Pt have been investigated by pseudopotential DFT calculations. Our calculations show a relaxation sequence of − +⋯ for the interlayer spacings of all (3 1 1) surfaces and − − + ⋯ for all (3 3 1) and (2 1 0) surfaces. We also found that the surfaces of Ir and Pt had more significant relaxations than other surfaces. Comparisons with LEED results, where possible, are also made in this paper.

The adsorption of xenon on Ru(1 0 [formula omitted] 0): an angle-resolved UV photoemission study

Recently, we examined the interaction of xenon with a ruthenium(1 0 1 ̄ 0) surface between 29 and 300 K and determined adsorbed phases, binding states as well as adsorption/desorption kinetics and work function changes. Here we present our UV photoemission results, which support and complement the previous results and shed light on the electronic structure of the adsorbed Xe phases. Up to the completion of the first monolayer we observe the typical 4d and 5p emissions of a single Xe species: The Xe 5p 1/2 and 4d 3/2 signals appear at a binding energy of 7.33 and 63.5 eV below E F, respectively. Other emissions due to adsorption at step sites or to two-dimensional phase transitions (solid–gas) were not observed, indicating a good surface homogeneity in agreement with our previous findings. In the entire Xe coverage range studied, we note an m j splitting of the Xe 5p 3/2 level, which is particularly pronounced around the first monolayer coverage. In this saturated monolayer phase, we find a mutual Xe distance of 4.2 (±0.1) Å, in analogy to our previous LEED results. A detailed comparison between the saturated (compressed) monolayer ( Θ=1.0) and the relaxed monolayer ( Θ=0.95) is performed, and our data are discussed in view of the properties of one- and two-dimensionally ordered systems known from the literature.

Raman spectroscopy of surface phonons on Sb-terminated Si(001)

We study the surface structure of Sb-terminated singular (on-axis) and vicinal (4° offcut in [110]) Si(001) by Raman spectroscopy (RS). In the Raman spectra three surface phonons with A 1-symmetry are identified at 80 cm −1, 130.5 cm −1 and 162 cm −1. The polarisation dependent Raman scattering intensities for the singular and vicinal samples are found to be correlated with the different surface reconstructions. After deposition of Sb at 260°C and further annealing to 400°C and 600°C, a nearly balanced ratio of (2×1):(1×2) domains for the singular sample and a predominantly (2×1) reconstructed surface for the vicinal sample is derived from RS, in agreement with LEED results. In contrast, after Sb deposition at 600°C, a balanced ratio of (2×1) and (1×2) reconstructed domains is found for vicinal as well as for singular Si(001).

Core-level photoemission study of the BiGaAs(111)A interface

Core-level photoemission spectra of the As 3d, Ga 3d and Bi 5d core levels were recorded for the BiGaAs (111)A-(2×2) surface. From the early stages of Bi growth, there are two distinct chemical environments for Bi. Surface Bi is bonded to both As and Ga atoms. The deposition of approximately 1 ML of Bi removes the dangling bonds from the surface As and Ga atoms, which are associated with the (2×2) vacancy-buckling structure. Annealing to 350°C reverses this process and surface dangling bonds for Ga reappear. When the Bi had fully desorbed at 425°C, photoemission and LEED results showed that the surface recovers the (2×2) vacancy structure but with a larger degree of surface disorder than that found before the deposition of Bi.

Structure determination of Ag(111) by low-energy electron diffraction

A quantitative structure determination of the Ag(111) clean surface has been performed using low-energy electron diffraction. Optimisation of the outermost layer spacings and vibrational amplitudes was achieved using two different dynamical scattering computational methods, one using the LEEDFIT code and the second exploiting a Simulated Annealing algorithm implemented on the conventional Van Hove/Tong code. The results obtained show that a bulk-terminated structural model describes the data best (Δ d 12=0.00±0.02 Å and Δ d 23=0.00±0.03 Å), and in particular, the analysis excludes a 2.5% contraction of the outermost layer spacing, which was recently reported on the basis of ion-scattering measurements. The LEED result of no contraction is consistent with prior results for the (111) face of several other fcc metals.

LEED, AES, and STM study of an ideal Si(111)–(1×1):H surface prepared by in situ atomic hydrogen exposure of the Pb induced (√3×√3)R30°-β phase: first experimental evidence of a non reversible transformation

STM monitored topographical observations of the growth mode of Pb on a Si(111) surface passivated in situ by atomic hydrogen exposure using for the first time a group IV metal as intermediate adsorbate, are presented. A tri-dimensional growth mode of Volmer–Weber type is confirmed for the initial condensation phase of Pb atoms on the clean (1×1):H unreconstructed surface. A non reversible transformation (√3×√3→1×1→7×7) is clearly evidenced on the basis of LEED and Auger results.

Imaging the polar [formula omitted] and non-polar [formula omitted] surfaces of ZnO with STM

Scanning tunneling microscopy has been used to image the non-polar (10 1 ̄ 0) and polar (000 1 ̄ ) -O surfaces of zinc oxide. Atomic resolution of one sublattice is achieved in the case of ZnO(10 1 ̄ 0) . Images of the (000 1 ̄ ) surface evidence a high density of (10 1 ̄ 0) step edges that locally have the same structure. They occur in four forms, two being as triangular pits and protrusions on flat terraces. They also appear as regular step edges, which alternate with sawtooth step edges. Together these form a double-step, which provides an explanation for earlier LEED results.

The role of surface relaxations in determining the STM images of sulfur adatoms and clusters on Re(0001): theory versus experiment

We apply our recent Green's function based STM formalism to study the adsorption of S monomers and clusters on Re(0001). Starting from previous STM experimental data and LEED results, we consider surface relaxations common to the different S structures observed on this system. The role of such relaxations on the final shape of the theoretical STM images is then examined, and we find that the sensitivity of the images to these structural parameters can be considerable in certain cases. The consistency of proposed relaxed models is also confirmed after a theory–experiment comparison of the STM images.