LEED Structure Research Articles

A combined LEED and DFT surface structure determination of Cu3Au(001): Evidence of a surface stacking fault

In this work, we have investigated the surface structure of Cu3Au(001) using low energy electron diffraction analysis and ab initio calculations by density functional theory. Our results indicate a different structure from that usually reported in the literature, where we have observed a stacking fault between the first and third layers of the surface. The rippling effect and the interlayer distances belonging to the first six layers of the surface were calculated and compared with other works too. Specifically, a rippling with gold atoms moving towards the vacuum in the first layer of 0.09Å was observed and of 0.02Å in the third layer. A small expansion of the first interlayer distance of 1.3% and a contraction of 0.8% at the second also were observed. Finally, ab initio calculations performed in this work are in good agreement with the experimental results.

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature ( T ⩽ 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2–6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40–300 K).

Reciprocal Space Measurement by Electron Diffractions

New approaches in the measurements and analyses with LEED and RHEED lead us to another view of these techniques as a scan method for a wide-range of crystal reciprocal space. Recent progress of the constant momentum-transfer averaging (CMTA) method for LEED structure analysis and development of Weissenberg RHEED method, in which a principle of a Weissenberg Camera for X-ray crystallography was imported to RHEED measurements, are briefly introduced in this review. Both methods capture a huge number of diffraction patterns, and can survey a crystal reciprocal space in three dimension. It is possible to determine the surface structure directly from a simple analysis based on a Fourier transformation of the obtained reciprocal data.

Zn Adsorption on Pd(111): ZnO and PdZn Alloy Formation

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.

Hydrogen on Cobalt: The Effects of Carbon Monoxide and Sulphur Additives on the D2/Co(0001) System

Hydrogen reaction on catalytic surfaces is an important field of research in fuel cell science. The adsorption of hydrogen (deuterium) on Co(0001) and the influence of carbon monoxide and sulphur on the adsorption were studied by XPS, TDS, WF measurements and LEED. The WF increased due to D2 adsorption, revealing the electronegative character of deuterium. It was found that the deuterium saturation coverage is ~0.17 ML at 320 K and ~0.27 ML at 180 K. The activation energy for desorption was estimated to be 33 kJ/mol. The coadsorption measurements with CO indicated a decrease in the deuterium yield by 50%. A downward shift of 50 K in the deuterium desorption temperature was observed as a consequence of coadsorbed CO, but changes in the CO desorption were minimal. At small CO exposures the (2 × 2) LEED structure of deuterium was seen, while at exposures of above 5 L the (2 √ 3 × 2 √ 3)R30... structure was detected by LEED as with pure CO adsorption. Coadsorption with sulphur led also to a decrease in the D2 yield leading to a complete inhibition of hydrogen adsorption at sulphur saturation. The sulphur (2 × 2) LEED structure did not undergo changes due to deuterium adsorption. As assumed, sulphur worked as a strong poison to the adsorption on Co(0001).

The Growth of Well-ordered Ultra-thin Al2O3 Films on Cu-Al Alloy

The well-ordered ultra-thin Al2O3 film was grown on a Cu-Al alloy for the first time. The growth was achieved by introducing oxygen at high substrate temperature in a ultra-high vacuum. The film showed (7/√3×7/√3)R30o LEED structure. The thermodynamics of the growth mechanism was discussed.

LEED structure analysis of Sb adsorbed Si(0 0 1) surface

Sb adsorbed Si(0 0 1) surfaces have been investigated by LEED and AES. After a few monolayer (ML) deposition at room temperature, the LEED patterns of 1×1, 2×1 and c(4×4) have been observed successively as elevating the annealing temperature. Two structures (1×1 and 2×1) were examined by LEED I– V curve analysis. The genetic algorithm (GA) was operated to search a global optimum structure. For the 1×1 structure, a good R-factor value of 0.22 was obtained for the model in which topmost 1 ML Sb atoms sit on the Si atoms of fourth substrate layer. For the 2×1 structure, two cases of 1 ML and a half ML Sb coverage was examined, and an Sb dimer model with 1 ML coverage gave a better R-factor value.

Surface roughness and LEED crystallography: Analysis of flat and vicinal W(110)

Low-energy electron diffraction intensity vs voltage (LEED I-V) measurements and analysis are used to determine the multilayer surface relaxation of W(110). Measurements and analysis are presented for both flat and vicinal surfaces, demonstrating that surface roughness leads to only small errors in LEED structure determinations. Flat, clean W(110) exhibits first-(d{sub 12}) and second-(d{sub 23}) layer relaxations of {minus}3.0{plus_minus}1.3{percent} and +0.2{plus_minus}1.3{percent}, respectively, relative to the bulk lattice spacing d{sub 0}=2.237h{Angstrom}. This experimentally determined surface relaxation of W(110) is compatible with a recent combined density-functional theory calculation and LEED study [M. Arnold, G. Hupfauer, P. Bayer, L. Hammer, K. Heinz, B. Kohler, and M. Scheffler, Surf. Sci. {bold 382}, 288 (1997)]. Surface roughness (in the present case, uniform atomic height steps) is found to produce a small apparent increase in the measured value of d{sub 12} when determined using standard (flat surface) LEED I-V methodology. However, for low step densities ({lt}20 atoms/step) the apparent change in d{sub 12} is small compared to other sources of error, so it is unlikely that surface roughness is a significant source of error in LEED structure determinations. {copyright} {ital 1999} {ital The American Physical Society}

The structure of the underpotential deposition of copper on to Pt(111) in the presence of chloride anions: a LEED structure analysis

The underpotential deposition (UPD) of copper on to Pt(111) in the presence of chloride anions was studied by full-dynamical LEED-intensity analyses. A deformed (4×4) LEED pattern was observed at an emersion potential of 0.36 V vs. Ag/AgCl, right in between the two characteristic and well-separated peaks of the cyclic voltammogram (CV). Corresponding LEED intensity measurements and simulations of the integral-order beams indicate that the UPD of Cu forms a single pseudomorphic Cu layer on Pt(111).

The STRUCTURE OF THE c(2×2) Mn/Ni(001) SURFACE ALLOY BY PHOTOELECTRON DIFFRACTION

Surface alloys are two-dimensional metallic systems that can have structures that are unique to the surface, and have no counterpart in the bulk binary phase diagram. A very unusual structure was reported for the Mn–Ni system, based on a quantitative LEED structure determination, which showed that the Mn atoms were displaced out of the surface by a substantial amount. This displacement was attributed to a large magnetic moment on the Mn atoms. The structure of the Mn–Ni surface alloy was proposed to be based on a bulk termination model. Magnetic measurements on Mn–Ni, however, showed conclusively that the magnetic structure of these surface alloys is distinct from the bulk alloy analogs. For example, bulk MnNi is an antiferromagnet, whereas the surface alloy is ferromagnetic. This suggests that the proposed structure, based on bulk termination, is not complete. Photoelectron diffraction techniques were used to investigate this structure, using both a comparison to multiple scattering calculations and photoelectron holography.

High-resolution core-level photoemission of the CO-induced Pd(110) surface reconstruction

The complex behaviour of the CO/Pd(110) adsorption system has been followed with high-resolution XPS of both the C 1s and the surface core-level shift (SCLS) of the Pd 3d features, from initial exposures to a monolayer coverage. For low exposures (<0.45 ML) the SCLS shifts from −0.5 to +0.43 eV. The C 1s emission first appears as a low-energy band at around 284.8 eV characteristic of CO in high coordination sites, indicating that reconstruction begins with initial exposure. On increasing exposure up to saturation at room temperature ( ϴ CO=0.75 ML) with its (4×2) CO LEED structure, a second higher binding-energy band assignable to two-fold bridged molecules appears and becomes dominant; concomitant with this is a further change in the SCLS to +0.63 eV. The features observed for ϴ CO=0.75 ML coverage are fully consistent with the model of CO adsorption on a (1×2) missing-row reconstructed surface where the C 1s peaks at 286.3 and 286.1 eV binding energy are assigned to two-fold bridge sites on the ridges and in the grooves, respectively. In the process of reconstruction, the latter appear before the former. At low temperatures the (2×1)p2mg “herring-bone” CO overlayer on the unreconstructed (110) surface is characterised by a large SCLS of +0.94 eV and a single C 1s feature at 286.3 eV, proving that this structure involves CO adsorbed in two-fold bridge sites.

ANALYSIS OF THERMAL VIBRATIONS AND INCOMMENSURATE LAYERS BY LOW ENERGY ELECTRON DIFFRACTION

The multiple scattering theory of LEED is briefly reviewed, and recent developments concerning the analysis of thermal vibrations with LEED and the analysis of lattice modulations in incommensurate layers are discussed. Usually only isotropic thermal vibrations have been considered in LEED structure analyses. This restriction can be overcome by an extension of the theory to anisotropic and anharmonic vibrations, allowing not only a higher precision in the determination of structure parameters but also the study of dynamical processes with LEED. In the case of incommensurate layers the satellite reflections arise from multiple diffraction as well as from modulations in the adsorbate or substrate lattice. It is shown that an approximation can be introduced in the multiple scattering formalism to calculate the satellite intensities. The method can be applied to incommensurate layers as well as to higher order commensurate layers.

AES and LEED investigation of Al segregation and oxidation of the (100) face of Fe 85 Al 15 single crystals

The surface segregation and oxidation behavior of Fe85Al15(100) were investigated by means of AES and LEED. Sputter cleaning of the surface causes preferential Al removal and leads to an Al depleted surface layer. The segregation of Al to the Fe85Al15(100) surface was studied in the temperature range from 300 to 800°C. At 375 to 400°C a weak c(2 × 2) LEED pattern is found. At temperatures in excess of 600°C thermodynamic equilibrium is approached very rapidly. At such temperatures Al segregation leads to a well-ordered (1 × 1) LEED structure with bright and sharp spots at a low background intensity. Oxidation at room temperature leads to disordered oxygen adsorption, whereas at 700°C a (6 × 6) superstructure is observed in addition to the matrix spots. This superstructure is attributed to the formation of a thin Al2O3 overlayer on the Fe85Al15(100) surface.

Pseudomorphic growth of Ni films on Cu(001): a quantitative LEED analysis

We present LEED structure determinations of ultra-thin epitaxial Ni films on Cu(001) for coverages of 3, 5 and 11 ML. From full dynamical intensity analyses, a tetragonal distortion of all films can be deduced, in good accordance with FMR results. The structural parameters of the 5 and 11 ML films are practically identical. So, obviously there are no structural changes at about 7 ML, where the orientation of the magnetization is reported to switch from in-plane to out-of-plane. The film growth is pseudomorphic with an in-plane lattice parameter a p = 2.53 A ̊ at coverages of 3 and 5 ML. This value, which is slightly reduced with respect to the copper bulk, was recently also found to produce the best fit for a similarly prepared clean Cu(100) surface. With further increasing coverage, a p tends to approach the value of the nickel bulk (2.49 Å) but does not fully reach it even at 11 ML ( a p = 2.51 A ̊ ). The films at 5 and 11 ML (and probably also in the regime between) show considerable tetragonal distortions. The top Ni layer shows an outward relaxation of 8% relative to the distances between deeper layers, which, due to the tetragonal distortion, are reduced compared to the ideal value of the nickel bulk.

Recent advances in LEED surface crystallography

After the arrival of fast, reliable and computer controlled measurement techniques the main obstacle in LEED structure determination was the vast computational work necessary to calculate dynamical intensities and to retrieve the correct structure. This has been largely overcome by structural search procedures and in particular by the introduction of the perturbation method tensor LEED. The power of this development is reviewed with emphasis on recent extensions to describe the easy chemical substitution of atoms and their anharmonic and anisotropic vibrations. Additional to surfaces of considerable geometrical complexity ( geometrical tensor LEED), also fast access to substitutionally disordered compounds with the layer dependent determination of their surface stoichiometry ( chemical tensor LEED) is allowed. Also, there is access to surface dynamics ( thermal tensor LEED). Additionally, tensor LEED can be inverted to establish a direct method for the direct determination of the surface stoichiometry and the geometrical refinement of structures without application of trial-and-error procedures.

Iron multilayers on Cu(100) - a case of complex reconstruction investigated by quantitative LEED

Epitaxial growth of iron on Cu(100) leads to the formation of fcc iron whereby as function of film thickness a variety of different structures is observed. As detected by full dynamical LEED structure analyses using tensor LEED these structures are rather complex. In the coverage regime below 5 monolayers all iron layers are reconstructed both vertically and in-plane. Above 5 monolayers only the top layer is reconstructed until a martensitic transformation at about 10 monolayers leads to the formation of a bcc(110) surface. The structures found correlate nicely with known magnetic properties. Surprisingly, the in-plane lattice parameter of the fcc iron layers was found to be 2.52 Å, i.e. smaller than the value of 2.55 Å known for bulk copper layers. However, a re-investigation of clean Cu(100) also yields a 1% in-plane contraction of copper surface layers compared to bulk layers, i.e. the clean copper surface - at least after conventional preparation procedures - relaxes under its surface stress. In conclusion, the growth of Fe on Cu(100) has to be regarded as pseudomorphic.

Segregation-induced surface structures of carbon-doped Mo 0.75Re 0.25(100)

The carbon segregation at the Mo 0.75Re 0.25(100) surface was investigated by means of AES and quantitative LEED structure determination. The equilibrium surface carbon concentration is found to be a function of the bulk concentration and the annealing temperature. With increasing surface carbon concentration four different phases have been observed: c(2 × 2), (3 × 3)diag, p(1 × 1), (1 × 1) s. The two (1 × 1) phases differ substantially both by Auger amplitudes and LEED spectra which makes evident that they indeed correspond to different structural phases. Quantitative LEED structure determination of the c(2 × 2) and (1 × 1) phases reveals the occupation of top layer hollow sites by carbon. In the case of the (1 × 1) s phase carbon atoms reside additionally in subsurface octahedral interstitial sites. There is good theory-experiment agreement as mirrored by a best fit Pendry R-factor of ≤ 0.23 for the three structures.

Interaction of CO with the ordered Ni3Al(111) surface

The interaction of CO with the structurally and chemically ordered (2 × 2) Ni3Al(111) surface was studied with TDS, UPS and LEED. At low coverages, a CO-induced (2 × 2)-LEED structure and the same activation energy of CO desorption as well as the same 5σ/1π)−(4σ) splitting of chemisorbed CO as found on Ni(111) suggest (a) an initial CO adsorption on the Ni-trimer sites in the alloy surface and (b) no significant electronic influence from the adjacent Al atoms. With increasing coverage, the (2 × 2) CO-pattern fades away and two more chemisorbed CO states are detected suggesting a less-ordered CO layer and the population of inequivalent (probably “mixed”) adsorption sites.

Structure and stability of fcc Fe films on Cu(100) for coverages below five monolayers

The strong correlation between magnetism, growth and structure of ultrathin fcc Fe films on Cu(100) has caused much interest in these films recently. However, whilst the film structure has been extensively studied in the coverage regime 5 <Θ < 10 ml, the structural properties for lower coverages are only poorly known. We present LEED structure determinations of the 4 × 1 and 5 × 1 superstructures observed at coverages 2 and 4 ml, respectively. The results show that different from current assumptions the entire Fe films are heavily reconstructed. Atomic rows in the [110] direction buckle and shift in all iron layers leading to a complex film structure with atomic layers everything else but flat fcc (100) layers. So, in the coverage regime Θ < 5 ml flat fcc (100) layers seem to be unstable and exhibit a highly distorted fcc structure. In addition, we find that all Fe interlayer distances are expanded which is nicely related to the ferromagnetic state of the films.

Cs and CO coadsorbed on Ru(0001): low-energy electron diffraction analysis

The adsorption of CO on a precovered Cs(2 × 2)-Ru(0001) structure (θ(Cs) = 0.25) leads to the formation of two distinct (2 × 2)-Cs-CO phases with one and two CO molecules in the unit cell, respectively. The LEED structure analyses showed that Cs remains in the on-top position while CO resides in the hcp position for CO coverage below 0.25 and subsequently fills up the fcc sites. Compared to the clean CO-(√3 × √3)R30° surface, the CO molecule switches its adsite from being on-top to threefold hollow under the influence of coadsorbed Cs. The CO axis is oriented normal to the surface; no tilting has been found. In addition to the short-range interaction, the electrostatic (long-range) attraction between Cs and CO is manifested by the island growth observed for Cs coverages below 0.25 down to 0.08.