Low-energy Electron Diffraction Pattern Research Articles

Clustering and layering of In adatoms on low and high index silicon surfaces: A comparative study

Using the morphological differences of low and high index surfaces as templates for metal growth, several low dimensional overlayer structures with novel structural and electronic properties can be formed. We present here a first report on submonolayer adsorption and residual thermal desorption studies of In adatoms on reconstructed high index Si (5 5 12)−2 × 1 surface and compare it with the observations on planar Si (111)−7 × 7 surface. The study is done by using in-situ Ultra High Vacuum surface sensitive probes like Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These conventional wide area techniques provide an understanding of atomistic issues involved in the evolution of the interface. We have observed an anomalous growth mode during adsorption at room temperature (RT) above 2ML, which includes adatom layering and clustering on Si (111) surface. This is also manifested during the desorption experiments on both surfaces, and the subtle differences on the two surfaces are discussed. The observation of LEED pattern during the adsorption process shows formation of different superstructural phases on Si (111)−7 × 7 surface. On Si (5 5 12) 2 × 1 surface we observe the sequential 2× (225), 2× (337) and 2× (113) facet formation during adsorption/desorption, which include quasi 1D-nanowire/chain structures. A combination of lattice strain effects, presence of step-edge barrier and quantum size effects are employed to speculate the differences in adsorption and desorption.

Self-limited growth of triangularPtO2 nanoclusters on the Pt(111) surface

The high temperature oxidation of the Pt(111) surface with molecular oxygenhas been studied using scanning tunnelling microscopy and low energy electrondiffraction (LEED). Results indicate a self-limited growth of well-orderedPtO2 nanoclusters which have an O–Pt–O trilayer structure. Each nanocluster has a triangular shapeand nucleates at the Pt(111) surface step edge due to the mobility of Pt atoms. The triangularPtO2 nanoislands on the upper and lower Pt(111) terraces represent two mirror domains withthe mirror plane perpendicular to the substrate and aligned along the direction of the latter. LEED data obtained from the nanostructuredPtO2/Pt(111) surface show acharacteristic (2 × 2) pattern. Different oxidation conditions lead to the formation of chemisorbed oxygen on the Pt(111) surfacealongside PtO2 nanoclusters. Oxygen adsorbs on the surface forming a variety of structures which result in (3 × 3), (5 × 5) and(7 × 7) LEED patterns.

CO adsorption on clean and oxidized Pt 3Ti(111)

CO adsorption on clean and oxidized Pt 3Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt 3Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt 3Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt 3Ti(111) surfaces (< 135 L O 2 exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt 3Ti(111) surfaces (> 220 L O 2 exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.

Structure of chlorine on Ag(111): Evidence of the(3×3)reconstruction

The structure of the chlorine induced reconstruction of Ag(111) has been studied by a combination of low-temperature scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density-functional theory (DFT). We demonstrate that previously observed mysterious LEED pattern arises as a result of diffraction from a system of small $(15--30\text{ }\text{\AA{}})$ triangular antiphase domains with a new $(3\ifmmode\times\else\texttimes\fi{}3)$ superstructure. In our model supported by DFT calculations, within a $(3\ifmmode\times\else\texttimes\fi{}3)$ unit cell the upper silver layer reconstructs forming a couple of three-atom triangles placed in fcc and hcp sites of the substrate. Chlorine atoms occupy fourfold hollow sites between these triangles. The corner holes, which look like depressions in the STM images, are also occupied by chlorine atoms.

Water Adsorption on a Co(0001) Surface

We have investigated the adsorption of water on a Co(0001) surface by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 130 K, the interaction between adsorbed water (H2O(a)) and Co(0001) is quite weak, and water adsorption forms an intact, nonwetting, and three-dimensional layer on Co(0001). The monolayer and multilayer of H2O(a) could be distinguished by XPS, and they desorb molecularly upon heating but only giving a single water desorption peak. An ordered p(2 × 2) LEED pattern was observed at low exposures of water. At room temperature, water adsorbs dissociatively on Co(0001), forming chemisorbed atomic oxygen (O(a)) and atomic hydrogen (H(a)). Upon heating, H(a) recombines into H2 desorbing from the surface, whereas O(a) remains on the surface. The interaction of water with Co(0001) is greatly influenced by the presence and nature of the oxygen species on Co(0001). Water adsorption on Co(0001) precovered with 0.45 ML O(a) at 130 K forms a mixture layer of OH(a) and H(a) via the reaction of 2H2O(a) + O(a) → 3OH(a) + H(a). However, on oxide-like Co(0001) surface, the water decomposition is completely passivated even at room temperature. These results broaden our fundamental understanding of water interaction with metal surfaces and provide insights into the water-involved catalytic reactions catalyzed by cobalt.

Oxidation of ultra-thin zinc films on Rh(100) surface

The oxidation of submonolayer zinc films on Rh(100) surface by O 2 gas has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). With a zinc coverage of 0.8 ML, an atomically flat ultra-thin zinc oxide film formed at an oxygen partial pressure of 2 × 10 − 8 mbar and a temperature of 150 °C. The zinc oxide film showed a c(16 × 2) LEED pattern. The high resolution STM image of the zinc oxide film showed single dotted spots and double dotted spots arranged linearly and periodically along the [01¯1] direction. We propose an atomic arrangement model of the film accounting for the LEED pattern, the STM image, and the atomic arrangement of the bulk ZnO(0001) surface.

Electronic structure of the [formula omitted] surface: Angle-resolved photoemission study

The electronic structure of Ni 2 P ( 10 1 ̄ 0 ) has been investigated by photoemission spectroscopy (PES) utilizing synchrotron radiation. The surface prepared by Ar + ion sputtering (3 kV, 15 min) and annealing (400 °C) gave a c(4 ×2) low-energy electron diffraction pattern. In the PES spectra measured for this surface, a Ni 3d–P 3p hybrid band (main band) and a satellite were observed at 0–4 eV and at 8 eV, respectively. Resonant PES study showed that the satellite is associated with the photoemission process leading to a two-hole bound state. The angle-resolved PES measurements showed that there were at least two surface states; one was observed at about 0.4 eV around the Γ ¯ point and the other was observed at about 0.6 eV along the boundary of the surface Brillouin zone along the [0001] direction.

STM studies on the reconstruction of the Ni 2P (101̅0) surface

The surface structure of Ni 2P (101̅0), a model for highly active hydrodesulfurization catalysts, was studied using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) in order to understand the reconstruction of the surface layers. Annealing at 573 K revealed a (1 × 1) LEED pattern which changed to a c(2 × 4) arrangement by further heating to 723 K. Atomic scale STM images were obtained for both the (1 × 1) and c(2 × 4) structures. Bright spots observed in the STM images were interpreted to be due to surface phosphorus atoms and this was supported by a density functional theory (DFT) simulation. Several possible models for the c(2 × 4) reconstructed structures were proposed including a P-dimer defect model, a missing-row model and a missing-row + added-row model. The last model gave the best explanation for the c(2 × 4)structure. The mechanism for the c(2 × 4) reconstruction on the Ni 2P (101̅0) surface is discussed.

Structure of the (010) surface of the orthorhombic complex metallic alloyT-Al3(Mn,Pd)

The atomic and electronic structures of the (010) surface of the $\mathrm{T}{\text{-Al}}_{3}$(Mn,Pd) complex metallic alloy is investigated by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), x-ray photoelectron diffraction (XPD), and ab initio calculations. While structural imperfections are observed at the surface and out of the various possible terminations, the puckered P2 layer is identified as the only surface termination, thus pointing out the existence of a well-defined minimum in the surface energy landscape. The measured step heights correspond to distances between identical planes along the [010] direction in the bulk model, i.e., $b/2$. A bias dependency of the STM topography is found. The XPD and LEED patterns confirm the pseudotenfold symmetry of the sample. XPS and UPS show a more metallic signature of the $\mathrm{T}$ phase compared to Al-based quasicrystalline phases.

Carbon monoxide adsorption on Co deposited Pt(1 0 0)-hex: IRRAS and LEED investigations

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Pt(1 0 0) surfaces deposited with Co layers with different thicknesses. Pt(1 0 0) surfaces cleaned in ultrahigh vacuum showed surface reconstruction, i.e., Pt(1 0 0)-hex: two absorption bands ascribable to adsorbed CO on the 1 × 1 surface and hex domains emerge at 2086 and 2074 cm −1, respectively, after 1.0 L CO exposure. Deposition of a 0.3-nm-thick-Co layer on Pt(1 0 0)-hex at 333 K changes the low-energy electron diffraction (LEED) pattern from hex to p(1 × 1), indicating that the deposited Co lifts the reconstruction. The IRRAS spectrum for 1.0-L-CO-exposed Co 0.3 nm /Pt(1 0 0)-hex fabricated at 333 K yields a single absorption band at 2059 cm −1. For Co 0.3 nm /Pt(1 0 0)-hex fabricated at 693 K, the LEED pattern shows a less-contrasted hex and the pattern remains nearly unchanged even after CO exposure of 11 L, although only 1.0 L CO exposure to Pt(1 0 0)-hex lifts the surface reconstruction. A Co 0.3 nm /Pt(1 0 0)-hex surface fabricated at 753 K exhibits an absorption band at 2077 cm −1, which is considered to originate from CO adsorbed on the Pt-enriched surface alloy. Co 0.3 nm /Pt(1 0 0)-hex surfaces fabricated above 773 K show a clear hex-reconstructed LEED pattern, and the frequencies of the adsorbed CO bands are comparable to those of Pt(1 0 0)-hex, indicating that the deposited Co atoms are diffused near the surface region. The outermost surface of the 3.0-nm-thick-Co-deposited Pt(1 0 0)-hex is composed of Pt–Co alloy domains even at a deposition temperature of 873 K. Based on the LEED and IRRAS results, the outermost surface structures of Co x /Pt(1 0 0)-hex are discussed.

Atomic Roughness of an Intrinsically Chiral Surface Orientation of an fcc Metal: Cu{531}

Intrinsically chiral metal surfaces would potentially offer a route toward enantioselective heterogeneous catalysis. A bulk-terminated {531} surface of an fcc metal has an intrinsically chiral unit mesh: it thus appears to be an attractive candidate for fundamental studies of the interaction of such an intrinsically chiral metal surface with chiral adsorbates. Using atomically resolved scanning tunnelling microscopy at 77 K, we show that the real Cu{531} surface departs strongly from ideal bulk termination, exhibiting a high degree of atomic-scale roughness. We examine the impact of the roughness on the low-energy electron diffraction pattern, discuss the origins and nature of the roughness in the context of thermal roughening, and comment upon the implications of the roughness for adsorption of chiral molecules.

Preparation and photoemission investigation of bulklikeα-Mnfilms on W(110)

We report the successful stabilization of a thick bulk-like, distorted $\alpha$-Mn film with (110) orientation on a W(110) substrate. The observed $(3\times3)$ overstructure for the Mn film with respect to the original W(110) low-energy electron diffraction pattern is consistent with the presented structure model. The possibility to stabilize such a pseudomorphic Mn film is supported by density functional total energy calculations. Angle-resolved photoemission spectra of the stabilized $\alpha$-Mn(110) film show weak dispersions of the valence band electronic states in accordance with the large unit cell.

In situ observation of change in surface atomic arrangement of Sc–O∕W(100) system during phase transition at high temperature

In situ observation of low-energy electron diffraction (LEED) patterns during the phase transition of the Sc–O∕W(100) surface at high temperature was performed to investigate the correlation between the surface atomic arrangement, coverage of surface atoms, and work function during the phase transition. For this purpose, a LEED system enabling LEED observation at high temperatures of 1500–1900K was developed. The temperature dependence of the intensity of diffraction spots was measured, and the Debye temperatures were estimated for relevant surfaces. The changes in the intensity of diffraction spots during the phase transition were found to correlate with the changes in the coverage of surface atoms and work function. The present results revealed that surface phenomena such as surface segregation, oxidation, oxygen desorption, and diffusion of ScO complexes predominate the surface properties of the Sc–O∕W(100) system.

Bismuth-stabilizedc(2×6)reconstruction on a InSb(100) substrate: Violation of the electron counting model

By means of scanning tunneling microscopy/spectroscopy (STM/STS), photoelectron spectroscopy, and first-principles calculations, we have studied the bismuth (Bi) adsorbate-stabilized InSb(100) substrate surface which shows a $c(2\ifmmode\times\else\texttimes\fi{}6)$ low-energy electron diffraction pattern [thus labeled $\text{Bi}/\text{InSb}(100)c(2\ifmmode\times\else\texttimes\fi{}6)$ surface] and which includes areas with metallic STS curves as well as areas with semiconducting STS curves. The first-principles phase diagram of the Bi/InSb(100) surface demonstrates the presence of the Bi-stabilized metallic $c(2\ifmmode\times\else\texttimes\fi{}6)$ reconstruction and semiconducting $(4\ifmmode\times\else\texttimes\fi{}3)$ reconstruction depending on the chemical potentials, in good agreement with STS results. The existence of the metallic $c(2\ifmmode\times\else\texttimes\fi{}6)$ phase, which does not obey the electron counting model, is attributed to the partial prohibition of the relaxation in the direction perpendicular to dimer rows in the competing reconstructions and the peculiar stability of the Bi-stabilized dimer rows. Based on (i) first-principles phase diagram, (ii) STS results, and (iii) comparison of the measured and calculated STM and photoemission data, we show that the measured $\text{Bi}/\text{InSb}(100)c(2\ifmmode\times\else\texttimes\fi{}6)$ surface includes metallic areas with the stable $c(2\ifmmode\times\else\texttimes\fi{}6)$ atomic structure and semiconducting areas with the stable $(4\ifmmode\times\else\texttimes\fi{}3)$ atomic structure.

Carbon Monoxide Adsorption on Cobalt-Deposited Platinum Single Crystal Surfaces Investigated by IR Reflection-Absorption and Low-Energy Electron Diffraction

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Co deposited Pt(111) (Co/Pt(111)) and Pt(100) (Co/Pt(100)) (Co thickness = 0.15, 0.3 and 0.6 nm) bimetallic surfaces. The 1.0-L-CO exposure to a clean Pt(111) at room temperature yielded linear-bonded and bridge-bonded CO-Pt bands at 2092 and 1850 cm-1. The Co depositions onto the Pt(111) at 333 K brought about the small hexagons of satellites on the low-energy electron diffraction (LEED) patterns, even for the Co thickness of 0.15 nm. The CO-Pt IRRAS band intensities for the 333-K-fabricated Co/Pt(111) surfaces decreased with increasing Co thickness. The CO-Pt bands almost disappeared for the 6nm-thick-Co/Pt(111) surface and the band at 2000 cm-1 that ascribable to CO-Co bonds dominated the spectra. The results indicated that the deposited Co grows epitaxially on the 333-K-Pt(111) substrate. In contrast, the 693-K-fabricated 0.3nm-thick-Co/Pt(111) which showed a clear p(1×1) LEED pattern yielded a single absorption band at 2067 cm-1 originated from adsorbed CO on the Pt site influenced by neighbored Co atoms. As for the cleaned Pt(100) substrate surface, the LEED pattern showed a hex-reconstructed pattern and the 1.0-L-CO exposed surface arose two absorption bands at 2086 and 2076 cm-1, which can be ascribed respectively to linearly bonded CO on Pt in the 1×1 and hex domains. The Co depositions onto the 333-K-Pt(100)-hex substrate surface lift the hex reconstruction. The adsorbed CO on the 333-K-fabricated 0.3nm-thick-Co/Pt(100)-hex revealed a single absorption band at 2059 cm-1. In contrast to the 333-K-fabricated surface, the 693-K-fabricated 0.3nm-thick-Co/Pt(100)-hex showed the hex-like reconstruction: the 1.0-L-CO exposure to the 693-K-fabricated surface resulted in an adsorbed CO band at 2073 cm-1 accompanied with a shoulder at 2059 cm-1. The results obtained suggest that the deposited Co atoms are easy to incorporated into the Pt(100)-hex substrate surface in comparison to the Pt(111). [DOI: 10.1380/ejssnt.2010.161]

Angle-Resolved and Resonant Photoelectron Spectroscopy Study of Ni2P (10-10) Single-Crystal Surface

The electronic structure of Ni2P (10−10) has been investigated by photoemission spectroscopy (PES) utilizing synchrotron radiation. The surface which was cleaned by Ar+ ion sputtering (3 kV, 15 min) and annealing (400oC) gave a c (2×4) low-energy electron diffraction pattern. In the PES spectra measured for this surface, a Ni 3d-P 3p hybrid band (main band) and a satellite were observed at 0—4 eV and at 8 eV, respectively. Resonant PES (RPES) study showed that the satellite is associated with the photoemission process leading to a two-hole bound final state. The angle-resolved PES measurements showed that there were at least two surface states ; one was observed at about 0.4 eV around the Γ− point and the other was observed at about 0.6 eV along the boundary of the surface Brillouin zone along the [0001] direction. The results of RPES measurements suggested that the latter state should have little contribution of the Ni 3d component.

Atomic and nanostructures of monolayer c(2×2)NiN on Cu(001)

Abstract Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.

Coverage dependence and surface atomic structure ofMn/Si(111)−3×3studied by scanning tunneling microscopy and spectroscopy

Thin manganese silicide films of different thicknesses on Si(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and scanning tunneling spectroscopy (STM/STS). Up to a Mn coverage of 3--4 monolayers (ML), island formation is favored. For higher Mn coverages up to 12 ML uniform film growth is found. The silicide film morphology at low coverages supports a layered Mn-Si film structure. The silicide surfaces displayed a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ LEED pattern. STM images recorded from the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ surfaces mostly show a hexagonal pattern but a honeycomb pattern has also been observed. A surface atomic structure based on chained Mn triangles is proposed. Our STM results are in good agreement with a recent theoretical model. The high-quality STS spectra recorded from the different surfaces show a clear metallic character at 1.5 ML and higher coverages. The filled-state features in the STS spectra at surfaces with 3--4 ML Mn coverages are similar to earlier published angle-resolved photoelectron spectroscopy data.

Low energy photoelectron diffraction analysis at high angular resolution of Cu and Mn/Cu surfaces

X-ray photoelectron diffraction simulations using a real-space approach are shown to accurately produce the extraordinarily detailed photoelectron diffraction pattern from Cu{111} at an electron kinetic energy of 523.5 eV. These same simulations show that most sensitivity is obtained when using low energy electrons at high angular resolution. Structural differences are observed to be greatest around a kinetic energy of ∼100 eV and many of the features observed in the photoelectron diffraction patterns may be directly related to phenomena observed in low energy electron diffraction patterns from the same surface. For Cu{100}, simulations of buckled surfaces with a Mn overlayer predict that low energy photoelectron diffraction can easily discriminate chemical and structural differences. Even the effects of the relaxed surface of Cu{100} is indeed observable along azimuthal scans around a kinetic energy of 100 eV. Our results show that low energy photoelectron diffraction is extremely sensitive to changes in surface structure if high resolution patterns are acquired.

Experimental study of the interfacial cobalt oxide inCo3O4/α−Al2O3(0001)epitaxial films

A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular beam epitaxy on a-Al2O3(0001) single crystals is reported. The experimental results show that the cobalt oxide films become progressively more disordered with increasing thickness, starting from the early stages of deposition. Low energy electron diffraction patterns suggest that the unit cell remains similar to that of a-Al2O3(0001) up to a thickness of 17 A, while at larger thicknesses a pattern identified with that of Co3O4(111) becomes visible. X-ray photoelectron spectroscopy reveals sudden changes in the shape of the Co 2p lines from 3.4 to 17 A cobalt oxide thickness, indicating the transition from an interfacial cobalt oxide layer towards [111]-oriented Co3O4. In particular, the absence of characteristic satellite peaks in the Co 2p lines indicates the formation of a trivalent, octahedrally coordinated, interfacial cobalt oxide layer during the early stages of growth, identified as the Co2O3 corundum phase.