Quantitative LEED Research Articles

Temperature-dependent segregation on Pt 25Rh 75(111) and (100)

Surface segregation is studied on Pt 25Rh 75(111) and Pt 25Rh 75(100) by LEED intensity analysis and LEIS. Although both equilibrated surfaces are strongly Pt-enriched (up to 80 at.%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The Pt concentration grows continuously on Pt 25Rh 75(111) until 1000°C, whereas it reaches a maximum enrichment around 500°C on Pt 25Rh 75(100) and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the Pt 25Rh 75(100) surface by the interchange of Pt and Rh atoms within the near-surface layers, and on the Pt 25Rh 75(111) surface by a diffusion of Pt atoms from bulk to the near-surface region. The latter leads to an overall Pt enrichment of several layers, and is only observed after annealing at 1100°C. The presence of carbon contamination on the Pt 25Rh 75(100) surface causes a significant reduction of the Pt segregation. There is excellent agreement between the top-layer concentrations derived by LEIS and quantitative LEED.

A quantitative LEED analysis of the oxygen-induced p(3×1) reconstruction of Pt 25Rh 75(100)

Pt 25Rh 75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23 L O 2, 600°C). A previous STM study on O/Pt 50Rh 50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant (“shifted rows”). We present a LEED I( E) analysis of Pt 25Rh 75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I( E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95 Å. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Å, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Å. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt 25Rh 75(100) at low oxygen coverage with STM. We resolve the shifted rows in real-space, and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt 25Rh 75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt 25Rh 75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt 25Rh 75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer.

A structural analysis of the Co(0001) surface and the early stages of the epitaxial growth of Cu on it

The epitaxial growth of copper on Co(0001) was investigated by combined quantitative LEED and STM, including the clean surfaces Cu(111) and Co(0001). The mode of growth is a modified Stranski–Krastanov type, i.e. pseudomorphic and approximately layer-like up to four monolayers of copper; later on, three dimensional islands develop. The stacking of the copper adlayers is exclusively fcc from the very beginning, whereby the ABAB stacking on a given terrace of the substrate is continued as ABABca. Since there are also terraces with an ABA stacking, fcc-twins (e.g. ABAcb) develop in the Cu films. Whilst in the submonolayer coverage regime of copper the cobalt substrate remains hcp stacked, films with more than about two copper layers are found to have induced a stacking fault in the substrate corresponding to a registry shift of the uppermost cobalt layer. As indicated by the analysis of a film slightly above one monolayer coverage, the registry shift seems to be triggered when the terraces of the substrate are fully covered by copper whereby the rearrangement of atoms probably starts at the step edges.

Anti-corrugation and nitrogen c(2 × 2) on Cr(100): STM on atomic scale and quantitative LEED

We present a LEED I- V analysis of c(2 × 2)- N Cr(100) . We found nitrogen residing in fourfold hollow sites and exclude adsorption models in which nitrogne adsorbs on a metal site (on-top, substitutional or second-layer interstitial). We achieved a Pendry R-factor of 0.16 for the best-fit structure. Nitrogen resides at a vertical distance of 0.36 Å above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 Å (7.5% with respect to the bulk value of 1.44 Å). The interlayer spacing between the second and the third layer is contracted to 1.41 Å. The second chromium layer is buckled (0.13 Å). The second-layer chromium atom beneath a nitrogen atom resides deeper in the bulk. The nitrogen bond length to the four first-layer chromium atoms amounts to 2.07 Å, the bond length to the second-layer chromium atom amounts to 1.97 Å. The nitrogen position in c(2 × 2)-N determined by LEED is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 × 1)-Cr(100) are imaged as hillocks and chromium atoms as depressions. This is anti-corrugation of clean Cr(100). Anti-corrugation seems to be related to a surface state of clean Cr(100) and is lifted in p(1 × 1)- N Cr(100) at a (local) nitrogen coverage of 1 monolayer.

Chemical ordering and reconstruction of Pt 25Co 75(100): an LEED/STM study

The surface of a disordered Pt 25Co 75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1–0.4 Å above that of Co, whereas LEED shows that Pt atoms are geometrically ∼0.04 A ̊ higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (〈 C 1〉 = 62.6% Pt, 〈 C 3〉 = 53.5%) and Pt-depleted layers (〈 C 2〉 = 6.9%, 〈 C 4〉 = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L1 2 PtCo 3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.

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.

The composition and structure of Cu 3Au(110)-(4 × 1): a low-energy electron diffraction analysis

The structure of the clean Cu 3Au(110)-(4 × 1) surface has been determined using quantitative LEED. It turned out that in the first layer, Cu and Au rows along the [001] direction pair up to build the (4 × 1) periodicity. The second layer contains at least 25%, Au so that the surface region is Au-enriched. The resulting structure can be regarded as a monatomic-thick Au wire embedded in a Cu matrix.

Deep layer oscillatory segregation and relaxation of substitutionally disordered MoxRe1−x(100) surfaces

The surface structure and stoichiometry of the substitutionally disordered alloy MoxRe1−x(100) were determined by quantitative LEED I(E) analyses for bulk concentrations of x=0.75, 0.85 and 0.95. Diffraction intensities were measured and analysed for electron energies up to 600 eV in order to also access values of the physical parameters of deeper layers. An excellent theory-experiment agreement, together with a large data base for each bulk stoichiometry, allow the accurate detection of rather strong oscillatory interlayer relaxations. They extend unusually deep into the bulk, i.e. down to the sixth layer. Additionally, a “sandwich segregation” with Mo termination of the surface and Re enrichment in the second layer is found, which is held responsible for the deeper interlayer relaxations.

Erratum: Surface structure of epitaxial Gd(0001) films on W(110) studied by quantitative LEED analysis

Erratum: Surface structure of epitaxial Gd(0001) films on W(110) studied by quantitative LEED analysis

Photoelectron diffraction determination of the structure of the Cu(100)c - Mn surface phase

The structure of the - Mn surface phase has been determined using scanned-energy mode photoelectron diffraction (PhD) from the Mn core level. The results confirm the earlier finding of a quantitative LEED study, namely that the surface layer comprises an ordered, strongly corrugated two-dimensional alloy in which the Mn atoms occupy substitutional sites. By using near-grazing as well as near-normal emission directions, the PhD data are found to be sensitive to the locations of both the alloy (approximately coplanar) and the underlying substrate Cu atoms relative to the Mn emitter. This can be seen both in direct `projection method' data inversion maps of the emitter environment and in full multiple-scattering simulations. The PhD data indicate a large outward relaxation of the Mn atoms relative to the surrounding alloy-layer Cu atoms , but this is consistent with the previous LEED study within the quoted precisions. There are, however, statistically significant differences between the LEED and PhD conclusions regarding some of the structural parameter values, most notably the layer spacing of the Mn atoms to the underlying pure Cu substrate layer.

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.

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.

The structure of Co films on Cu(111) up to 15 ML

New structural results of the epitaxial growth of Co Cu(111) in the coverage regime 1.5–15 ML are presented as investigated by quantitative LEED and STM. In the low coverage regime there is pure fcc stacking with copper diffused on top of 30% of the domains. The growth is ideally pseudomorphic. At intermediate coverages hcp stacked domains develop on the cost of fcc domains whereby the latter seem to be stabilized by their top copper layer. At high coverages the film is predominantly hcp exhibiting the ideal lattice parameter of Co(0001).

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.

Detecting stacking faults during epitaxial growth by low energy electron diffraction

We study the sensitivity of LEED to stacking faults in the early stages of epitaxial growth. By theoretical simulation of stacking faults in different locations, in particular for Cu/Cu(111), Co/Cu(111), and p(4×4)Pb/Cu/Cu(111), we show that quantitative LEED can detect stacking faults down to three or four layers deep into the substrate. This is also true in the presence of a The modification of the intensity spectra by stacking faults is large enough to be identified by visual comparison with defect-free surfaces. In the cases where different stacking sequences coexist on the surface, a minimum coverage of 30%–50% imperfect stacking sequences is needed in order to be clearly detected in the LEED I- V curves. Therefore, our results provide guidance to conduct experiments where early detection of stacking faults is important.

Buckled layer structure for atomic adsorption on W{100}: the (√2 × √2)R45° nitrogen structure from ATLEED

Adsorption of 0.5 monolayer of N adatoms on W{100} results in a sharp (√2 × √2)R45° LEED pattern. The only previous quantitative LEED study of this system gave a simple overlayer model with a Pendry R-factor of 0.55. An exhaustive search has been made of possible structures, including a novel vacancy reconstruction, displacive reconstructions and underlayer adsorption. From this work a new overlayer structure is derived with an R p value of 0.22, displaying a considerable buckling of 0.27 ± 0.05 A ̊ within the second W layer and consequently involving large changes in the interlayer spacings of the surface. The N adatom is pseudo-five-fold coordinated to the W surface, bonding to a second-layer W atom with a nearest-neighbour bond length of 2.13 Å and with the four next-nearest-neighbour W atoms in the surface plane at 2.27 Å. The structure does not resolve the work function anomaly observed on this surface.

LEED study of Pt 25Co 75(111)

A quantitative LEED study of the Pt 25Co 75(111) surface (in the substitutionally disordered state) shows weak relaxations of interlayer spacings ( Δd 12/ d bulk , Δd 23/ d bulk , Δd 34/ d bulk ) = (000-1.9, −1.4, 3.6)%, no st but a significant oscillatory composition profile ( C 1, C 2, C 3) = (46, 3, 37) at% Pt. This behaviour is very close to the ge observed for PtNi(111) surfaces as a function of the bulk concentration C b and in good agreement with the predictions of the tight binding Ising model of Treglia, Legrand and Ducastelle and of the embedded atom method results of Lundberg.

The (110) surface of the disordered Pt 25Co 75 alloy: a sandwich of almost pure monoatomic layers

The (110) surface structure of the Pt 25Co 75 alloy in the substitutionally disordered state was studied by quantitative LEED. In a way similar to Pt x Ni 1− x (110) surfaces, on the Ni rich side, the composition profile versus depth is strongly oscillating with a pure cobalt top layer; however, the oscillation is stronger and goes deeper for Pt 25Co 75(110) than for the previously studied surfaces of Pt 10Ni 90(110) and Pt 50Ni 50(110). Actually the selvedge, which can be easily produced by simple standard argon cleaning and annealing UHV techniques, is a sequence of three almost pure layers: CoPtCo. This provides an interesting system to be studied: the monolayer limit of PtCo sandwiches.

Determination of the surface relaxation of Sc(0001) by video LEED analysis

The authors describe a low-cost system of video data acquisition as an extension to qualitative LEED studies that allows quantitative LEED intensity-voltage (I-V) analysis. The type of equipment used and the procedures developed to align the samples, acquire the data, extract the LEED spot intensities and determine the surface layer spacing from a simple kinematic analysis are outlined. As a test case, the authors determine the surface layer spacing d12 of the rare earth metal Sc(0001) using kinematic analysis of I-V spectra of the (00) LEED spot. Values derived from spectra taken on different days are reproducible to within 0.5% (about 1 pm), and the mean value of d12 is found to be 0.2633+or-0.0005 nm. This compares to the spacing for bulk Sc of 0.2634 nm, indicating that there is no surface relaxation.

Angle-resolved photoemission and LEED from rare-earth metals

Angle-resolved photoemission and LEED studies of single-crystal surfaces of the rare-earth metals are still relatively few in number. The reasons for this are outlined and comparisons are made between the procedures used to obtain clean and well-ordered surfaces suitable for study. Valence-band photoemission spectra are compared with surface electronic structure and photocurrent calculations in order to identify surface and bulk features. Core-level spectroscopy indicates that the 5p levels of the lanthanides show intensity variations with emission angle that are not explicable in terms of a simple core-level shift. Although the 4p levels of Y have some band character, detailed comparison of the 4p photoemission lineshape with those produced by photocurrent calculations enables the surface core-level shift to be determined. Quantitative LEED studies of close-packed (0001) surfaces show essentially a bulk-terminated structure for Sc, a structural reconstruction of the divalent surface of Sm and a magnetic reconstruction of ferromagnetic Gd. The more open (11 2 0) and (10 1 0) surfaces of all the rare-earth metals studied undergo extensive reconstructions into a close-packed structure almost identical to that of a (0001) surface. Angle-resolved photoemission and LEED have yet to distinguish between these structures, indicating a reconstruction depth of at least five atomic layers.