Low-energy Electron Diffraction Pattern Research Articles

Surface X-ray diffraction study of annealed single-crystal rutile TiO2 (001) surface

A polished rutile TiO2 (001) surface was annealed at 410 °C and studied using low-energy electron diffraction (LEED) and surface X-ray diffraction (SXRD). The LEED pattern revealed a (1 × 1) structure. The crystal truncation rods (CTRs) obtained using SXRD were analyzed to reveal the structure of the annealed TiO2 (001) surface. We found that about 40% of the surface was flat and the remaining 60% consisted of facets. The facets were mainly {011}-oriented. They were three layers deep and contained no terraces. It was reported that TiO2 (001) surface annealed at 410 °C exhibited more moderate photocatalytic activity than the HF-treated surface. Comparing these results, it was concluded that the formation of {011} facet lowered the photocatalytic activity of annealed TiO2 (001) surface than as-received surface.

Oxygen Reduction Reaction Properties of Dry-Process-Synthesized Pt/Graphene/SiC(0001) Model Catalyst Surfaces

Introduction Pt-shell/Pt-M-core (M = Fe, Co, Ni, etc.) nanoparticles have been intensively studied as oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFCs) [1]. However, the M elements easily dissolute into the electrolyte under PEFC’s operating conditions (strong acid and electrochemical potential fluctuations) and, thus, electrochemically-stable core materials are highly desirable to develop future core-shell type ORR catalysts. Because SiC shows high corrosion resistance to acidic solution, SiC is one of the candidates for the core materials. However, because of large lattice mismatch between Pt (0.277nm) and SiC (0.177nm), Pt/SiC interface seems to be unstable. Norimatsu and Kusunoki reported the formation of graphene layers on SiC(0001) substrate surface by high temperature annealing (ca. 1500°C) in vacuum through sublimation of the Si atoms [2]. Because lattice parameter of graphene is 0.246 nm, graphene would work as a buffer layer between Pt-shell and SiC substrate. In this study, we try to fabricate Pt/graphene/SiC(0001) model catalysts in ultra-high vacuum (UHV;~10-8Pa) and investigate the catalyst’s ORR properties. Experimental The Pt/graphene/SiC model catalysts fabrication are conducted in UHV. A nitrogen-doped 4H-SiC (0001) substrate (Nippon Steel and Sumitomo Metal Corporation) was annealed at 1600°C by direct electronic heating in UHV for 1 hour to form graphene layers on the SiC(0001) substrate. Formation of the surface graphene layers was verified with low energy electron diffraction (LEED) and Raman spectroscopy. Then, 1.5nm-thick-Pt was deposited onto the graphene/SiC(0001) substrate by using electron beam evaporation method at a room temperature (RT) (as-prepared catalyst), followed by thermal annealing (annealed catalyst). The UHV-fabricated Pt/graphene/SiC(0001) model catalyst surfaces were observed by scanning tunneling microscopy (STM) in UHV and were transferred to an electrochemical apparatus set in a N2-purged glove box without air exposure. CV and LSV measurements were conducted in N2-purged and O2-saturated 0.1 M HClO4, respectively. The ORR activity was estimated based on j k values at 0.9 V vs. RHE by using Koutecky-Levich equation. Results and Discussion Fig. 1(a) shows Raman spectra of the SiC(0001) substrate before and after the UHV-annealing at 1600°C. Before the UHV-annealing, two bands (1525 and 1695 cm-1) ascribable to the Si-C lattice dominate the spectrum. In contrast, the annealed substrate shows 2D (2720 cm-1) and G (1585 cm-1) bands due to graphene layers, accompanied with remarkable band intensity reductions of the Si-C. The LEED pattern collected after the UHV-annealing (not shown) clearly showed sub-spots caused by graphene layers formation, exhibiting that graphene/SiC(0001) substrate can be generated after the UHV-annealing. Fig. 1(b) shows CVs of the as-prepared (blue) and 400°C-annealed (red) 1.5nm-thick-Pt deposited graphene/SiC(0001) model catalysts. For comparison, CV for the UHV-cleaned Pt(111) (black) is presented in the Fig.1(b). Although both of the Pt/graphene/SiC(0001) model catalysts show adsorption and desorption currents due to hydrogen (0.05 ~ 0.35 V) and oxygen-related-species (0.6 ~ 1.0 V), the CV shapes are clearly different from the clean Pt(111). Especially, double-layer currents for the 400°C-annealed catalyst is much larger than the as-prepared catalyst and clean Pt(111). The results suggest that the graphene/SiC(0001) surface is not completely covered by the deposited Pt for the 400°C-annealing. As for the ORR activity (Fig. 1 (c)), the as-prepared catalyst shows ca. 10 % increase in j k values in comparison to that for the clean Pt(111). In contrast, the j k value for 400°C-model catalyst is 60% lower. The UHV-STM image for the 400°C-annealed catalyst (Fig. 1 (d)) shows that the deposited Pt is aggregated to form hexagonal islands with ca. 20 nm in size. In contrast, the STM image of the as-prepared catalyst shows atomically-rough surface with fine islands (less than 5 nm; Fig. 1(e)). The CVs and STM results suggest that the lower ORR activity for the 400°C-annealed catalyst stems from decrease in electrochemical surface area of the deposited Pt on the graphene/SiC(0001). In contrast, the 10%-activity-increase against clean Pt(111) of the as-prepared catalyst might correlate with surface defects (e.g., (110) steps) that would enhance the ORR activity of Pt(111) epitaxial surface [3]. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan.

Single crystalline silicene consist of various superstructures using a flexible ultrathin Ag(111) template on Si(111)

Using scanning tunneling microscopy, we studied the formation of silicene on an ultrathin Ag(111) film with a thickness of 6–12 monolayers, which was prepared on a Si(111) substrate. A low-energy electron diffraction pattern with an oval spot indicated that the ultrathin Ag(111) film is more disordered than the single-crystal Ag(111). After Si epitaxy growth, we still measured the classical 4 × 4, √13 × √13, and 2√3 × 2√3 silicene superstructures, which are the same as the silicene superstructure on single-crystal Ag(111). Growing silicene on a single-crystal Ag(111) bulk usually results in the formation of a defect boundary due to the inconsistent orientation of various superstructures. By comparing the angles and boundary conditions between various silicene superstructures on the ultrathin film and single-crystal Ag(111), we discovered that a consistent orientation of various superstructures without obvious boundary defects formed on the ultrathin Ag(111) film. The results indicated single crystalline silicene formation, which was attributed to the domain rotation and lateral shift of the disordered ultrathin Ag(111) film.

Growth and coverage dependent electronic structure of MgO on Ag(001)

Growth and coverage dependent electronic structure of MgO films on Ag(001) have been studied using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopic (ARPES) techniques. The appearance of mosaic structure in the LEED pattern has been observed around 10 ML coverage due to misfit dislocations in the metal-oxide interface. The XPS core level spectra show a substantial amount of binding energy shift relative to their bulk values, indicating the reduction of the on-site Coulomb (U) and change transfer (Δ) energies in oxide layers. This binding energy shift is mainly associated with the image charge screening effect of the highly polarizable metallic substrate. The effect of image charge screening is also reflected in the valence band electronic structure as the bands are shifted to lower binding energies with decreasing film coverage maintaining the bulk MgO band gap till 5 ML. However, for the film coverages below 5 ML, the band gap is reduced substantially in presence of O 2p-Ag 5sp hybridized bands with the finite size effects in proximity to the Ag(001) substrate. Furthermore, our measured valence band electronic structure has been compared with the theoretical calculations and a good agreement has been observed between them for the bulk MgO, however, a more detailed calculation is necessary to explain the monolayer MgO band structure properly.

Surface Structural Analysis of the Layered Perovskite Ca1.9Sr0.1RuO4 by Low Energy Electron Diffraction I-V

Abstract – The atomic structure at surface of the layered perovskite Ca1.9Sr0.1RuO4 has been studied by Low Energy Electron Diffraction (LEED) I-V. The perovskite Ca1.9Sr0.1RuO4 of single crystal was cleaved in ultra high vacuum chamber (the pressure in the chamber was about 1x10-10 Torr). The experiments were conducted at room temperature (T=300 K). The sharp LEED pattern was observed which indicates that the surface of Ca1.9Sr0.1RuO4 is flat and it is a well ordered crystal. LEED I-V data, nine equivalent beams of the layered perovskite Ca1.9Sr0.1RuO4 were recorded at room temperature. LEED I-V calculation was performed to fit experimental data to obtain the surface atomic structure. The LEED I-V analysis reveals that in the surface of the layered perovskite Ca1.9Sr0.1RuO4 the RuO6 octahedra are rotated (in-plane rotation) alternating clockwise and counterclockwise. The in-plane rotation at the surface is 11 degree which is smaller than that in the bulk (13 degree). The Ru – O(1) bond-length at the surface is found to be 1.936 Å which is about the same as in the bulk (1.939 Å). The Ru – O(2) bond length at the surface is 1.863 Å which is much shorter than that in the bulk (2.040 Å). The volume of octahedral Ru-O6 at the surface is reduced by 9% with respect to the bulk. This finding shows that the atomic structure at surface of the layered perovskite Ca1.9Sr0.1RuO4is significantly different than that in the bulk. These lattice distortions strongly influence its electronic properties. Key words: Transition Metal Oxide; Perovskite; Surface Atomic Structure; LEED I-V

Surface Characterization of NbO Islands Formed on Nb(100) by X-Ray Photoelectron Diffraction

An ordered thin film of niobium oxide (NbO) was formed on the Nb(100) surface by heating the niobium single crystal at 1023 K for 2 min under an oxygen atmosphere. The surface composition was analyzed by X-ray photoelectron spectroscopy (XPS), and the surface structure was characterized by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) generated by synchrotron radiation. Annealing at 1223 K in ultra-high vacuum yielded a reconstructed (3 × 1) LEED pattern. XPS measurements indicated that the surface comprised of metallic and oxidized niobium species. The comparison between experimental and theoretical XPD results indicated that 60% of the Nb(100) surface was covered by two-layered nanoislands of NbO, and 40% had bare patches.

Growth, structural evolution and electronic properties of ultrathin films of Sn on W(110)

Growth and structure of Sn on W(110) from submonolayer to thick film (up to 8 monolayers) coverages at room temperature (RT) and as-well-as for higher temperatures (HT) (up to 673 K) have been studied using low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) techniques. Surface structural evolution was studied where (3 × 1), (1 × 3) and (1 × 4) LEED patterns were observed for RT whereas (3 × 1) and (1 × 4) LEED patterns were observed for growth at 673 K due to the energetically available adsorption sites for Sn atoms on W(110) surface. The absence of alloy formation between Sn and W was confirmed by the XPS data where surface core level shift of Sn 3d was observed whereas no shift was observed for W 4f levels. Detailed electronic structures were studied along Γ¯ - H¯ and Γ¯ - N¯ directions for Sn deposited on W(110) with the help of ARPES. ARPES data for RT and HT case is mostly similar. A pair of splitted bands was observed due to the deposition of Sn near the surface projected band gap.

Studies of early stages of Mn/GaN(0001) interface formation using surface-sensitive techniques

Non-doped GaN(0001) crystals are used as substrates in this study, on which Mn films are vapour deposited in situ under ultrahigh vacuum (UHV). The early stages of the Mn/GaN(0001) interface formation at room temperature are determined by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). The electron affinity for the already cleaned GaN(0001)-(1 × 1) surface, achieved by thermal cleaning, is 3.5 eV. The binding energy (BE) of the Ga-3d core level line shifts from 20.3 eV to 19.8 eV and the Mn-3p state from 47.6 eV to 47.2 eV upon stepwise Mn deposition due to charge transfer, e.g. Schottky barrier (SB) formation. The splitting of the Mn-2p doublet amounts to 11.2 eV and the Mn-2p3/2 peak has a BE of 638.7 eV, these values corresponding to metallic manganese. The work function of the Mn films with the thickness of 1 nm is 3.4 eV and slightly decreases to 3.2 eV with further overlayer thickness. The SB height of the interface is determined to be 1.2 eV. The Mn films show no LEED patterns. The XPS results, generally, show the absence of any interfacial compound formation.

Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/Al2O3(0001) thin films.

Pd(111) thin films, ∼245 nm thick, are deposited on Al2O3(0001) substrates at ≈0.5Tm, where Tm is the Pd melting point, by ultrahigh vacuum dc magnetron sputtering of Pd target in pure Ar discharges. Auger electron spectra and low-energy electron diffraction patterns acquired in situ from the as-deposited samples reveal that the surfaces are compositionally pure 111-oriented Pd. Double-axis x-ray diffraction (XRD) ω-2θ scans show only the set of Pd 111 peaks from the film. In triple-axis high-resolution XRD, the full width at half maximum intensity Γω of the Pd 111 ω-rocking curve is 630 arc sec. XRD 111 pole figure obtained from the sample revealed six peaks 60°-apart at a tilt angles corresponding to Pd 111 reflections. XRD ϕ scans show six 60°-rotated 111 peaks of Pd at the same ϕ angles for 11[Formula: see text]3 of Al2O3 based on which the epitaxial crystallographic relationships between the film and the substrate are determined as [Formula: see text]ǁ[Formula: see text] with two in-plane orientations of [Formula: see text]ǁ[Formula: see text] and [Formula: see text]ǁ[Formula: see text]. Using triple axis symmetric and asymmetric reciprocal space maps, interplanar spacings of out-of-plane (111) and in-plane (11[Formula: see text]) are found to be 0.2242 ± 0.0003 and 0.1591 ± 0.0003 nm, respectively. These values are 0.18% lower than 0.2246 nm for (111) and the same, within the measurement uncertainties, as 0.1588 nm for (11[Formula: see text]) calculated from the bulk Pd lattice parameter, suggesting a small out-of-plane compressive strain and an in-plane tensile strain related to the thermal strain upon cooling the sample from the deposition temperature to room temperature. High-resolution cross-sectional transmission electron microscopy coupled with energy dispersive x-ray spectra obtained from the Pd(111)/Al2O3(0001) samples indicate that the Pd-Al2O3 interfaces are essentially atomically abrupt and dislocation-free. These results demonstrate the growth of epitaxial Pd thin films with (111) out-of-plane orientation with low mosaicity on Al2O3(0001).

Investigation of the atomic and electronic structures of highly ordered two-dimensional germanium on Au(111)

Low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and photoelectron spectroscopy have been used to study an ordered structure formed by Ge atoms deposited onto the Au(111) surface. Based on a careful analysis of STM images and LEED patterns, we propose a new unit cell for the atomic structure of the Ge layer. Core level data indicate that some Ge atoms diffuse into the Au(111) crystal during annealing after deposition at room temperature. This is further corroborated by angle resolved photoelectron spectroscopy measured for different amounts of Ge remaining after sputtering and annealing. The results of the ARPES study clearly exclude the interpretation, in the literature, of a parabolic band as part of a Dirac cone of germanene.

Properties of Pt on Cu(111) revealed by AES, LEED, and DEPES

The investigations of Pt on Cu(111) were carried out to reveal the growth mode and crystalline structure of the adsorbate with the use of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and directional elastic peak electron spectroscopy (DEPES). Auger signal recorded during the continuous Pt adsorption on Cu(111) at 330 K confirms the layer by layer growth mode. LEED patterns observed at 1ML of Pt show the formation of the compressed adlayer. At higher coverages such as 3ML and 6ML the increase of the lattice constant of the topmost Pt layer, which approaches the bulk value of Pt(111), is noted. DEPES investigations performed for 1ML of platinum indicate the nucleation of adsorbate domains characterized by A/CBA and B/CBA stacking sequences at the Pt/Cu interface, which at higher coverages lead to the nucleation of mutually rotated by 180° Pt islands. Experimental DEPES data were compared to theoretical results obtained with the use of the multiple scattering (MS) calculations for different adsorption geometries including the Pt and Cu intermixing at the interface. The quantitative analysis of the data made by R-factor calculations enabled the determination of Pt domain populations already at 1 ML and 6 ML.

Growth of h-BN on copper (110) in a LEEM

Hexagonal boron nitride (h-BN) was grown by borazine vapour deposition on single crystalline Cu (110) substrates at 740 °C. The growth was investigated in situ using a Low-Energy Electron Microscope (LEEM). Substrates were prepared ex situ by mechanical and electrochemical methods and once in the LEEM system, by annealing in a H2 atmosphere resulting in a reconstructed surface. Exposure to borazine vapour resulted in the nucleation of well-aligned trigonal h-BN islands, which merged to ribbons along surface steps, and into larger, more irregularly shaped features. A coverage of up to 60% was achieved with an exposure of 3900 L. A diffraction ring in the low energy electron diffraction pattern was observed with a preferential alignment along the Cu 〈 111 〉 directions of the underlying substrate. Low-energy electron reflectivity scans, as well as x-ray photoelectron and Raman spectroscopies, confirmed the presence of a partial monolayer of h-BN on the surface.

Adsorption of silicon atoms on the surface of the Au/W(110)

The possibility of formation of an ordered silicene-like structure on Au/W(110) surface has been considered using angle-resolved photoelectron spectroscopy and X-ray photoelectron spectroscopy. It is shown that the addition of silicon atoms results in a considerable distortion of the electron structure of the initial substrate, and the resulting electron structure cannot be attributed to silicene. The configuration of reflections in the low energy electron diffraction pattern indicates the formation of two-dimensional ordered silicon structures with a large number of multidirectional domains.

Surface alloy formation by adsorption of holmium on Ag/Mo(112) bimetallic surfaces

Work function change measurements, low energy electron diffraction (LEED) and density functional theory (DFT) are used to determine the structures formed on Ag/Mo(112) bimetallic surfaces upon deposition of 0.5 monolayer (ML) of holmium. As the bimetallic surfaces, we have chosen the Mo(112) substrate covered with 1 or 2 ML of Ag. Such surfaces have the same symmetry as the Mo(112) face but different electronic properties. LEED experiment indicates that the c(2 × 2) structure is formed on (1 ML Ag)/Mo(112) bimetallic surface upon deposition of 0.5 ML of Ho. DFT calculations show that a type of Ag–Ho surface alloy is formed, with Ho atoms 0.6 Å below the distorted layer of Ag. This is neither a substitutional nor a subsurface alloy. It is found that the adsorption structure formed on the (2 ML Ag)/Mo(112) bimetallic surface depends on the annealing temperature. After deposition of 0.5 ML of Ho at 300 K, the LEED pattern of p(2 × 2) symmetry is observed. Annealing of the overlayer at 640 K irreversibly changes the p(2 × 2) pattern into a pattern of c(2 × 2) type. The results of DFT computations show that the c(2 × 2) structure of the Ag–Ho surface alloy is energetically most favorable. In this structure, 0.5 ML of Ho is between the two monolayers of Ag, and the symmetry of the topmost layer is changed. The work function change calculated for the c(2 × 2) structure is in a good agreement with the measured value (0.22 eV). The results show that adsorption of Ho on the Ag/Mo(112) bimetallic surfaces is substantially different than on the clean Mo(112).

Direct observation for atomically flat and ordered vertical {111} side-surfaces on three-dimensionally figured Si(110) substrate using scanning tunneling microscopy

A three-dimensional Si{111} vertical side-surface structure on a Si(110) wafer was fabricated by reactive ion etching (RIE) followed by wet-etching and flash-annealing treatments. The side-surface was studied with scanning tunneling microscopy (STM) in atomic scale for the first time, in addition to atomic force microscopy (AFM), scanning electron microscopy (SEM), and low-energy electron diffraction (LEED). AFM and SEM showed flat and smooth vertical side-surfaces without scallops, and STM proved the realization of an atomically-flat 7 × 7-reconstructed structure, under optimized RIE and wet-etching conditions. STM also showed that a step-bunching occurred on the produced {111} side-surface corresponding to a reversely taped side-surface with a tilt angle of a few degrees, but did not show disordered structures. Characteristic LEED patterns from both side- and top-reconstructed surfaces were also demonstrated.

Nanoscale characterisation of TiO2(110) annealed in air

Abstract Rutile titanium dioxide (TiO2) (110) surfaces annealed in air were examined by frequency modulation atomic force microscopy (FM-AFM), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) techniques. The terraces separated by monatomic steps showed an atomically irregular appearance in the obtained FM-AFM images, while the surface produced a (1 × 1) LEED pattern that indicated atomic ordering of the top few surface layers. Subsequent annealing in atmospheric-pressure O2 produced several ten-nanometre-sized pits with irregular shapes on the terraces. Molecule-sized spots were recognised on the subsequently O2-annealed surface. Half-order spots along the 〈001〉 directions emerged in the LEED pattern. We propose the rutile substrate covered with hydroxylated amorphous TiO2 of monatomic thickness as the structure of the air-annealed surface. The amorphous TiO2 was dehydroxylated by subsequent O2 annealing and was transformed into the thermodynamically stable rutile structure. The molecule-sized spots were attributed to hydroxyl (OH) and hydroperoxyl (OOH) groups that were produced on the area with the (1 × 1) structure. A part of the OH and OOH groups were arranged in a (2 × 1) structure and produced the half-order spots in the LEED pattern.

Physicochemical properties of the Sb/p-SiC interface

A comprehensive study of the chemical composition, structural and electronic properties of antimony adsorption layers on the p–type SiC(0001) surfaces are presented in this report. All processes and measurements were performed in situ under ultrahigh vacuum conditions. Electron affinity of the cleaned substrate, determined by ultraviolet photoelectron spectroscopy (UPS), amounted to 2.8 eV. The (1 × 1) structure of p-SiC was revealed by low-energy electron diffraction (LEED). After adsorption of antimony, intermixing of Sb with p-SiC substrate was excluded by X-ray photoelectron spectroscopy (XPS). The work function of the Sb film showing no LEED pattern was 4.55 eV. The Schottky barrier height of the Sb/SiC interface was calculated to be 1.2 eV. After Sb desorption at the temperature above 300 °C a small amount of Sb in the form of SbOx compound remained on the surface causing a reduction in the vacuum level by 0.4 eV and increasing the electron affinity up to 3.1 eV.

Effects of the deposition rate on growth modes of Ag islands on the hydrogen-terminated Si(111)-(1 × 1) surface: The role of surface energy and quantum size effect

We have investigated the early stage of Ag island growth at 2 monolayer (ML) coverage on the hydrogen-terminated Si(111)–(1 × 1) surface using low-energy electron-diffraction (LEED) and scanning tunneling microscopy (STM) at room temperature. First, it is found that the Ag(10) LEED pattern varies from arc-like spots to three spots by changing the Ag deposition rate from 1.0×10−1 (a fast deposition rate) to 1.1×10−4 (a slow deposition rate) ML/s. Second, STM observation reveals that adsorbed Ag atoms grow into dome-like three dimensional (3D) clusters at the fast deposition rate and flat-top two dimensional (2D) islands at the slow deposition rate. Third, most abundant 2D islands show the 8 atomic layer height, which coincides with that obtained from the quantum size effect. The side structures of 2D islands agree well with those calculated from Wulff theory. We will discuss the exact nature of 3D clusters and 2D islands of Ag grown on the hydrogen-terminated Si(111)–(1 × 1) surface and these results indicate the possibility of using kinetic controlled growth to investigate the physics of crystal growth.

Growth of ultrathin titanium oxide films on Ag(110)

Titanium oxide films grown on Ag(110) have been investigated by low-energy electron diffraction (LEED), photoelectron spectroscopy (PES), and X-ray absorption spectroscopy (XAS). As Ti atoms were deposited on Ag(110) in O2 at 5.0 × 10−6 Torr and the surface was subsequently annealed at 500 °C for 30 min, a film with a (1×1) periodicity with respect to Ag(110) was formed. The core-level PES study showed that the film was composed of TiO2 in which 1/6 of Ti atoms are reduced to Ti3+. The film gave a (1×1) LEED pattern when the thickness was less than 1 nm. The O K-edge and Ti L-edge XAS measurements showed that each Ti atom in the film was coordinated by 6 O atoms forming a distorted octahedron, as in the case of rutile and anatase TiO2. Analyses of XAS results suggested that the long-range order of the film was similar to that of anatase TiO2.

Stabilization of polar Mn3O4(001) film on Ag(001): Interplay between kinetic and structural stability

Stabilization processes of polar surfaces are often very complex and interesting. Understanding of these processes is crucial as it ultimately determines the properties of the film. Here, by the combined study of Low Energy Electron Diffraction (LEED), X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoemission Spectroscopy (UPS) techniques we show that, although there can be many processes involved in the stabilization of the polar surfaces, in case of Mn3O4(001)/Ag(001), it goes through different reconstructions of the Mn2O4 terminated surface which is in good agreements with the theoretical predictions. The complex surface phase diagram has been probed by LEED as a function of film thickness, oxygen partial pressure and substrate temperature during growth, while their chemical compositions have been probed by XPS. Below a critical film thickness of ∼ 1 unit cell height (8 sublayers or 3 ML) of Mn3O4 and oxygen partial pressure range of 2 × 10−8 mbar < P(O2) ≤ 5 × 10−7 mbar, different surface structures are detected and beyond this thickness a constant evolution of apparent p(2 × 2) structure have been observed due to the coexistence of p(2 × 1) and c(2 × 2) structures. Similar apparent p(2 × 2) structure has also observed by the oxidation of Ag(001)-supported MnO(001) surface. Our study also shows that the substrate temperature during growth plays a crucial role in determining the final structure of the polar Mn3O4 film and as a consequence of that a strong interplay between structural and kinetic stability in the Mn3O4 film has been observed. Further, stripe-like LEED pattern has been observed from the Mn3O4(001) surface, for the film grown at higher oxygen partial pressure (> 5 × 10−7 mbar) and higher temperature UHV annealing. The origin of these stripes has been explained with the help of UPS results.