Low-energy Electron Diffraction Analysis Research Articles

Degeneracy in Intercalated Pb Phases under Buffer-Layer Graphene on SiC(0001) and Diffuse Moiré Spots in Surface Diffraction.

First-principles density functional theory (DFT) is used to analyze the stability of Pb intercalated phases under buffer layer graphene on SiC(0001) as a function of the supercell size, Pb coverage, and degree of Pb ordering. By comparing the chemical potentials of such two-dimensional Pb structures, we find that there is a family of structurally distinct thermodynamically preferred Pb subsurface configurations with minute stability differences. These differences are comparable to the thermal energies at about 450 °C, where the Pb intercalated phases are grown. High-resolution surface-diffraction experiments using Spot Profile Analysis Low-Energy Electron Diffraction (SPA-LEED) confirm this high degree of degeneracy of the Pb intercalated phases from broad, low-intensity moiré spots observed exclusively from intercalated Pb. The low intensity of the moiré spots implies the coexistence of structurally different subsurface Pb phases.

Growth and atomic structure of twisted bilayer graphene formed on Ni(111) by segregation

Angle-resolved photoelectron spectroscopy and intensity analysis of low-energy electron diffraction were used to explore the growth and atomic structure of twisted bilayer graphene (tBLG) formed on Ni(111) by segregation of dissolved carbon. tBLG was composed of an epitaxial graphene layer in contact with the substrate and an overlaying rotated one with rotation angles of ∼15°. The top rotated graphene layer has been shown to have little effect on the underlying epitaxial graphene/Ni(111) system, indicating that the adjacent graphene layers are effectively decoupled. The process in which the tBLG gradually became dominant on the surface with repeated heating is discussed.

Boron nanostructure formation on Mo(112) surface

Boron-based nanoscale structures are at the vanguard of current research due to multifarious applications in numerous fields. Here we report the formation of ordered boron structure on Mo(112), which was investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While Boron did not form ordered structures at room temperature, it showed a c(2 × 2) structure upon heating at 1500 K. The c(2 × 2) structure shows a regular array of boron clusters on the trench of Mo(112), and STM study does not indicate any alloying. Dynamical LEED analysis and density functional theory reveal that the c(2 × 2) structure is an array of boron clusters, B4. The formation of B4 cluster is also explained by favorable adsorption sites on the trench of Mo(112). The scanning tunneling spectroscopy study shows the metallic nature of this B nanostructure, which will be stimulating for practical applications. This novel network formation by boron clusters indicates strong inter-atomic as well as intra-cluster bonding.

Boron nitride on SiC(0001)

The promising properties of heterostructures of 2D materials call for scalable sample production via epitaxy. The authors focus on surface structures obtained in a surfactant atmosphere in which a boron nitride template layer forms on SiC(0001). Remarkably, they find that this template is a hexagonal B${}_{x}$N${}_{y}$ layer, but not high-quality hBN. Experiments with the normal-incidence x-ray standing wave technique and spot-profile analysis low-energy electron diffraction reveal the $R{0}^{\ensuremath{\circ}}$ orientation of the layer, the presence of an unexpected boron buffer layer, and the interlayer distances with sub-Angstrom precision.

Formation of ordered B structure on W(100)

Adsorption of a metalloid, boron (B) on W(100) was investigated using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) in ultra-high vacuum (UHV) condition. W(100)-c(2 × 2)-B superstructure was formed by annealing at high temperature (1200 K). The best fit structure obtained from LEED analysis and STM observation manifested that B atom positioned at four-fold hollow site with the coverage of 0.5 ML, forming low B concentrated tungsten boride compound, W2B, on W(100) surface. The W atoms beneath B atoms moved downward to form B-W bonds for stabilizing the surface atoms. XPS confirmed B 1s peak shifting towards lower binding energy compared to pure B 1s, which indicated the charge transfer from W to B. No ordered structures other than c(2 × 2) have been observed with the increase of B, which indicates that B atoms diffuse into bulk, not forming ordered B sheets, due to the low diffusion barrier into bulk by annealing.

Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC.

We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely R0° rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a preoriented template to induce the unconventional orientation. Using spot profile analysis low-energy electron diffraction, angle-resolved photoelectron spectroscopy, and the normal incidence x-ray standing wave technique, we assess the crystalline quality and coverage of the graphene layer. Combined with the presence of a covalently bound graphene layer in the conventional orientation underneath, our surfactant-mediated growth offers an ideal platform to prepare epitaxial twisted bilayer graphene via intercalation.

Adsorption of S on Si(111) with M4 × 4 superstructure

The atomic and electronic structures are calculated for the M4 × 4 superstructure of S/Si(111), which was recently identified by Schmidt et al. [Surf. Sci.694 (2020) 121561] by using the spot profile analysis low-energy electron diffraction (SPALEED) and confirmed by scanning tunneling microscopy (STM). The electronic band structures are found to be semiconducting in nature for three different S structures: monomer, Dimer-A and Dimer-B, each of which saturates all the Si dangling bonds. Due to the varying numbers of S atoms per unit cell, we have calculated the lowest energy phase in the accessible S chemical potential range. Dimer-A and Dimer-B are the lower energy phase and their accessible chemical potential value is less than the monomer case, in excellent agreement with the results of Schmidt et al. Dimer-B is slightly more stable geometry than Dimer-A. Application of the nudged elastic band (NEB) method suggests that there is a rotational activation barrier of 40 meV along the Dimer-A–Dimer-B reaction path.

Structural analysis of clean LaB6(100), (111), and (110) surfaces via quantitative low-energy electron diffraction

LaB6 has very important features for electron emission, but its proposed surface structures of the fundamental planes contradict the work function values. In this study, we determined the surface structures of the LaB6 single crystal via quantitative low-energy electron diffraction analysis. On the (100) plane, the surface was terminated by a La atom, which relaxed toward the bulk crystal side. On the (111) and (110) planes, the surfaces were terminated by a La atom and a cluster of B, but the cluster had smaller number of B atoms than the B6 octahedron. Because the B6 octahedron is quite stable, it is likely that the surfaces are terminated by a B6 octahedron. In contrast, our structural analysis revealed that the surface of the (111) plane was terminated by a B3 triangle. The surface of the (110) plane has a c(2 × 2) periodicity, and we obtained three possible models according to our analysis. We cannot tell which of these models exists on the surface, and they can coexist on the surface. These three models have similar structures; however, model 1 has B4 termination, model 2 has B4 and B3 mixed termination, and model 3 has B3 termination. The determined structures for the (111) and (110) planes are very different from previously suggested models. Based on our surface analysis, the work function values of the above surface planes of the LaB6 single crystal can be reasonably explained.

Field-induced resistance peak in a superconducting niobium thin film proximity coupled to a surface reconstructed SrTiO3

Oxygen vacancy is known to play an important role for the physical properties in SrTiO3(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films with different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO2. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.

Adsorbate induced manipulation of 1D atomic wires: Degradation of long-range order in the Si(553)-Au system

Deposition of Au on vicinal Si(553) surfaces results in the self-assembly of one-dimensional (1D) Au atomic wires. Charge transfer from the Au wire to the Si step edge leads to a chain of Si dangling-bond orbitals with a long range ordered threefold periodicity along the steps and finite interchain interaction perpendicular to the steps. Employing spot-profile analysis low-energy electron diffraction (SPA-LEED) we observed a broadening of spot width with time, indicative for the degradation of Si dangling-bond chain and Au wire length. We introduce a new mechanism how adsorbates act as origin for this degradation beyond the intuitive picture of bond saturation. Here, zero-dimensional anti-phase translational domain boundaries are generated which immediately destroy the long-range order along and perpendicular to the steps, respectively. From the temporal evolution of the decreasing coherence length, we conclude that the Au wires are less reactive to adsorption than the Si dangling bond chains.

Orbital-Driven Rashba Effect in a Binary Honeycomb Monolayer AgTe.

The Rashba effect is fundamental to the physics of two-dimensional electron systems and underlies a variety of spintronic phenomena. It has been proposed that the formation of Rashba-type spin splittings originates microscopically from the existence of orbital angular momentum (OAM) in the Bloch wave functions. Here, we present detailed experimental evidence for this OAM-based origin of the Rashba effect by angle-resolved photoemission (ARPES) and two-photon photoemission experiments for a monolayer AgTe on Ag(111). Using quantitative low-energy electron diffraction analysis, we determine the structural parameters and the stacking of the honeycomb overlayer with picometer precision. Based on an orbital-symmetry analysis in ARPES and supported by first-principles calculations, we unequivocally relate the presence and absence of Rashba-type spin splittings in different bands of AgTe to the existence of OAM.

Adsorption of sulfur on Si(111)

The adsorption of S on Si(111)- 7 × 7 has been investigated for different preparation schemes and parameters. S was supplied from an electrochemical Ag2S cell. For room temperature adsorption and subsequent annealing, no ordered S induced reconstruction can be observed with spot profile analysis low-energy electron diffraction (SPALEED). S deposition at temperatures above about 400 ∘C, however, leads to a well-ordered reconstruction. Judging from the LEED pattern, the same reconstruction was already observed by Metzner et al. [Surf. Sci. 377–379 (1997) 71–74] who identified it as 4 × 4 reconstruction. The upper temperature limit for the preparation of this superstructure depends on S flux, which is needed to compensate for desorption. Prolonged S exposure leads to surface roughening, as observed with SPALEED and scanning tunneling microscopy (STM), pointing to surface etching by S. From our SPALEED data, we can conclude that the observed reconstruction is not a 4 × 4 reconstruction, but a (2012) superstructure with a rectangular unit cell that exists in three rotational domains, as confirmed by STM. Different structural trial models have been assessed with density functional theory. Among these model structures, a configuration with dimers adsorbed on bridging sites, with a S coverage of 1 monolayer, is most likely, since it is energetically favorable and is in agreement with all experimental results.

Structural investigation and magnetic properties of oxygen adsorption on ultrathin Fe(110) film

We have investigated oxygen induced surface structures on Fe(110) thin film and their magnetic properties using low energy electron diffraction (LEED) and surface magneto-optic Kerr effect (SMOKE). We have determined two chemisorbed structures, p(2 × 2) at 0.25 monolayer (ML) and p(3 × 2) at 0.33 ML using a dynamical LEED analysis. We found that oxygen atoms adsorb in long bridge sites on p(2 × 2) and move to three fold hollow sites on p(3 × 2) due to a steric repulsion. During chemisorption, proceeding the formation of FeO, the topmost surface of Fe film is ferromagnetic and with further oxygen adsorption the surface loses ferromagnetism in accordance with the oxide formation.

Growth-mode investigation of epitaxial EuS on InAs(100)

A persistent challenge in the field of spintronics is the search for suitable materials that enable the circumvention of the impedance mismatch preventing efficient spin-injection from metallic ferromagnetic conductors into semiconductors. One promising material is europium sulfide (EuS), a ferromagnetic semiconductor below the Curie temperature of 16.5 K. Investigation and optimization of the conditions required for high-quality growth of epitaxial EuS films on suitable substrates are thus of particular interest for the creation of efficient devices. We present the results of a growth-mode study employing atomic force microscopy and spot-profile analysis low-energy electron diffraction (SPA-LEED) of epitaxial EuS thin films deposited by electron-beam evaporation on InAs(100) substrates with varying combinations of, respectively, growth and annealing temperatures, Tg and Ta, from room temperature to 400 °C. We observed Stranski-Krastanov-like growth featuring low-roughness surfaces with root mean square values between 0.4 – 0.9 nm for all temperature combinations. An increased tendency for nucleation into grains and islands was observed for higher Ta from 300 – 400 °C. The corresponding nucleation mode, defined by varying degrees of 2D and 3D nucleation, was dependent on Tg. A 2D island growth mode was observed for Tg = 150 °C and Ta = 400 °C featuring a sharp and bright SPA-LEED pattern. This suggests the formation of a highly ordered, smooth surface for these growth conditions thereby providing a good starting point for optimization attempts for potential future devices.

Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride on Bi-Terminated Silicon

The epitaxial quality of thin films crucially depends on their interaction with the substrate. Up to now, Ag-terminated Si(111) has been employed as the model substrate for the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on semiconductors. In this study, we will show that Bi termination results in PTCDA films of superior epitaxial quality. We have studied the growth of PTCDA on bismuth-passivated Si(111) in detail by means of spot profile analysis of low-energy electron diffraction (SPA-LEED), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The XPS results reveal the presence of intact PTCDA molecules on the surface upon adsorption. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. STM shows a very smooth growth front of the PTCDA film, preserving the step structure of the substrate. High-resolution SPA-LEED data demonstrate the presence of a multidomain surface with a rich variety of PTCDA surface structures, which were identified to be most prominently herring-bone polytypes. However, in the monolayer range, quadratic brick-wall structures and a nearly square-like structure as well as a perylene-like structure have also been found. Despite the simultaneous presence of multiple domains, the individual domains show excellent lateral ordering, with larger domain sizes as compared to the case of Ag-terminated Si(111).

Rapid onset of strain relief by massive generation of misfit dislocations in Bi(111)/Si(001) heteroepitaxy

Strain and its relaxation in lattice mismatched heterostructures are crucial for the functionality of modern electronic devices, which are often challenging to determine experimentally. Here, we demonstrate a technique for measuring the strain state during epitaxial growth of Bi(111) films on Si(001) by using the spot profile analysis low-energy electron diffraction. Exploiting two non-equivalent integer-order diffraction spots originated from two Bi sub-lattices, the lattice parameter of the film is determined with high precision, which allows tracing the strain state as the film grows. The sudden and massive generation of misfit dislocations is found at a critical thickness of 4 nm which is explained through the inhomogeneous strain state of films with a thickness below one quarter of the mean distance of the dislocations.

Thermal stability and interlayer exchange processes in heterolayers of TiOPc and PTCDA on Ag(1 1 1)

Organic–organic interfaces, in particular donor–acceptor type heterointerfaces, represent central elements of organic electronic devices. Understanding and controlling their physical properties is key to improve and modify their performance. For this purpose we have investigated the structural properties and thermal evolution of molecular heterointerface model systems. Specifically, various stacked titanyl-phthalocyanine (TiOPc)–3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) heterolayers grown on Ag(1 1 1) have been examined using Fourier transform infrared absorption spectroscopy and spot-profile analysis low-energy electron diffraction. An accurate description of the thermal evolution and prevalence of the various phases of TiOPc and PTCDA is derived from the spectral signatures of the two molecular species explored in previous studies. Exceptionally high thermal stability is found for stacked heterolayers comprising TiOPc double layers with characteristic up–down arrangement of its axial Ti–O unit and single layers of PTCDA. Thereby, interlayer exchange is negligible in a wide temperature range ( K). This is ascribed to attractive intermolecular interaction (dipole–dipole interaction, hydrogen bonding) among the TiOPc molecules oriented face-to-face within the TiOPc bilayer sheet. Stacked layers of comprising single layers of TiOPc and of PTCDA turn out to be thermally much less stable and more heterogeneous. In the course of their thermal evolution, a number of processes such as interlayer exchange, TiOPc double-layer formation and dewetting are encountered.

Evolution of surface antiferromagnetic Néel temperature with film coverage in ultrathin MnO films on Ag(001)

The surface antiferromagnetic ordering of MnO(001) ultrathin films has been probed using Low-energy Electron Diffraction (LEED) and Angle-Resolved Photoemission spectroscopy (ARPES) techniques. We observe coherent exchange scattered half-order spot in LEED which is only visible at low electron beam energy (<40 eV) and arises below a certain temperature with a periodicity of the magnetic unit cell of MnO(001) surface, confirms its magnetic origin. A thickness dependent evolution of surface antiferromagnetic (AFM) Néel temperature has been observed for the MnO films. Interestingly, we found the Néel temperature of the MnO films exceed its bulk Néel temperature even in the ultrathin limit (∼2 ML). Detailed LEED analysis reveals that the increase of the Néel temperature is due to the presence of strain and the image charge screening effect in the film while, the competition between the finite size effects, surface strain, and substrate polarizability ultimately determine the observed Néel temperature. The surface magnetic critical exponent (β1) varies from 0.16 ± .03 to 0.81 ± .03, upon changing film thickness from 3 to 10 ML, indicates the transition from nearly 2D Ising system to 3D Ising/Heisenberg system. Furthermore, upon paramagnetic (PM)-AFM transition, ARPES energy distribution curves (EDCs) show, opening an energy gap of 150 meV with an increased occupation in the eg levels is good agreement with the theoretical prediction of the presence of AFM-II type ordering in the film.

Atomic and electronic structure of doped Si(111)(23×23)R30∘ -Sn interfaces

The hole doped Si(111)(2root3x2root3)R30(degrees)-Sn interface exhibits a symmetry-breaking insulator-insulator transition below 100 K that appears to be triggered by electron tunneling into the empty surface-state bands. No such transition is seen in electron-doped systems. To elucidate the nature and driving force of this phenomenon, the structure of the interface must be resolved. Here we report on an extensive experimental and theoretical study, including scanning tunneling microscopy and spectroscopy (STM/STS), dynamical low-energy electron diffraction (LEED) analysis, and density functional theory (DFT) calculations, to elucidate the structure of this interface. We consider six different structure models, three of which have been proposed before, and conclude that only two of them can account for the majority of experimental data. One of them is the model according to Tornevik et al. [C. Tornevik et al., Phys. Rev. B 44, 13144 (1991)] with a total Sn coverage of 14/12 monolayers (ML). The other is the 'revised trimer model' with a total Sn coverage of 13/12 ML, introduced in this work. These two models are very difficult to discriminate on the basis of DFT or LEED alone, but STS data clearly point toward the T\"ornevik model as the most viable candidate among the models considered here. The STS data also provide additional insights regarding the electron-injection-driven phase transformation as well as the critical role of valence band holes in this process. Similar processes may occur at other metal/semiconductor interfaces, provided they are non-metallic and can be doped. This could open up a new pathway toward the creation of novel surface phases with potentially very interesting and desirable electronic properties.

Epitaxial growth of Al9Ir2 intermetallic compound on Al(100): Mechanism and interface structure

The adsorption of Ir adatoms on Al(100) has been investigated under various exposures and temperature conditions. The experimental and theoretical results reveal a diffusion of Ir adatoms within the Al(100) surface selvedge already at 300 K. Above 593 K, two domains of a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.{6}^{\ensuremath{\circ}}$ phase are identified by low energy electron diffraction (LEED) and scanning tunneling microscopy measurements. This phase corresponds to the initial growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound at the Al(100) surface. The ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic domains are terminated by bulklike pure Al layers. The structural stability of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) grown on Al(100) has been analyzed by density functional theory based calculations. Dynamical LEED analysis is consistent with an Ir adsorption leading to the growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic compound. We propose that the epitaxial relationship ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}(001)\ensuremath{\parallel}\mathrm{Al}$(100) and ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}[100]\ensuremath{\parallel}\mathrm{Al}[031]/[013]$ originates from a matching of Al atomic arrangements present both on Al(100) and on pure Al(001) layers present in the ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound. Finally, the interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ precipitates and the Al matrix has been characterized by transmission electron microscopy measurements. The cross-sectional observations are consistent with the formation of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) compounds. These measurements indicate an important Ir diffusion within Al(100) near the surface region. The coherent interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ and the Al matrix is sharp.