Low-energy Electron Diffraction Pattern Research Articles

High-quality PVD graphene growth by fullerene decomposition on Cu foils

We present a new protocol to grow large-area, high-quality single-layer graphene on Cu foils at relatively low temperatures. We use C60 molecules evaporated in ultra high vacuum conditions as carbon source. This clean environment results in a strong reduction of oxygen-containing groups as depicted by X-ray photoelectron spectroscopy (XPS). Unzipping of C60 is thermally promoted by annealing the substrate at 800 °C during evaporation. The graphene layer extends over areas larger than the Cu crystallite size, although it is changing its orientation with respect to the surface in the wrinkles and grain boundaries, producing a modulated ring in the low energy electron diffraction (LEED) pattern. This protocol is a self-limiting process leading exclusively to one single graphene layer. Raman spectroscopy confirms the high quality of the grown graphene. This layer exhibits an unperturbed Dirac-cone with a clear n-doping of 0.77 eV, which is caused by the interaction between graphene and substrate. Density functional theory (DFT) calculations show that this interaction can be induced by a coupling between graphene and substrate at specific points of the structure leading to a local sp3 configuration, which also contribute to the D-band in the Raman spectra.

Preparation of clean surfaces and Se vacancy formation in Bi2Se3 by ion bombardment and annealing

Bismuth Selenide (Bi2Se3) is a topological insulator (TI) with a structure consisting of stacked quintuple layers. Single crystal surfaces are commonly prepared by mechanical cleaving. This work explores the use of low energy Ar+ ion bombardment and annealing (IBA) as an alternative method to produce reproducible and stable Bi2Se3 surfaces under ultra-high vacuum (UHV). It is found that a clean and well-ordered surface can be prepared by a single cycle of 1keV Ar+ ion bombardment and 30min of annealing. Low energy electron diffraction (LEED) and detailed low energy ion scattering (LEIS) measurements show no differences between IBA-prepared surfaces and those prepared by in situ cleaving in UHV. Analysis of the LEED patterns shows that the optimal annealing temperature is ~450°C. Angular LEIS scans reveal the formation of surface Se vacancies when the annealing temperature exceeds 520°C.

Scanning tunneling microscopy study of thin PTCDI films on Ag/Si(111)-√3 × √3.

3,4,9,10-perylene tetracarboxylic diimide molecules were evaporated onto a Ag/Si(111)-√3 × √3 surface and studied by scanning tunneling microscopy/spectroscopy and low energy electron diffraction (LEED). The growth mode was characterized as layer-by-layer growth with a single molecular unit cell in a short range order. The growth of the first two monolayers involves a molecule/substrate superstructure and a molecule/molecule superstructure. At higher coverages, the molecules in each layer were found to align so that unit cells are on top of each other. The experimentally obtained LEED pattern is described as a combination of patterns from the molecular unit cell and the molecule/substrate superstructure. The electronic structure was found to be strongly dependent on the film thickness for the first few layers: Several extra states are found at low coverages compared to higher coverages resulting in a very small pseudo gap of 0.9 eV for the first layer, which widens up to 4.0 eV for thicker films.

Initial stages of Zr–Fe–Si alloy formation on Zr(0001) surface

We studied initial stages of Fe and Si atoms interaction with the Zr(0001) surface. Fe and Si were deposited in-situ on the clean Zr surface up to coverage of one monolayer. Chemical state of Zr as a function of annealing temperature and the surface composition was investigated. Characteristic (1×1) LEED (Low Energy Electron Diffraction) pattern of the clean Zr surface becomes significantly modified when Fe and Si are deposited and annealed at 400°C. The arrangement of new spots of the pattern corresponds to the (2√3×2√3)R30° structure, stable up to annealing at 750°C. Changes in the electronic structure were identified from the images of equi-energy cuts of the k-space covering the entire Brillouin zone. The results imply that hydrogen, typically present in the clean surface, was eliminated either by annealing or upon the deposition of Fe and Si.

Formation of the BiAg2 surface alloy on lattice-mismatched interfaces

We report on the growth of a monolayer-thick ${\mathrm{BiAg}}_{2}$ surface alloy on thin Ag films grown on Pt(111) and Cu(111). Using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) we show that the surface structure of the $\frac{1}{3}$ ML Bi/$x$-ML Ag/Pt(111) system ($x\phantom{\rule{4pt}{0ex}}\ensuremath{\ge}2$) is strongly affected by the annealing temperature required to form the alloy. As judged from the characteristic ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)$R{30}^{\ensuremath{\circ}}$ LEED pattern, the ${\mathrm{BiAg}}_{2}$ alloy is partially formed at room temperature. A gentle, gradual increase in the annealing temperatures successively results in the formation of a pure ${\mathrm{BiAg}}_{2}$ phase, a combination of that phase with a ($2\ifmmode\times\else\texttimes\fi{}2$) superstructure, and finally the pure ($2\ifmmode\times\else\texttimes\fi{}2$) phase, which persists at higher annealing temperatures. These results complement recent work reporting the ($2\ifmmode\times\else\texttimes\fi{}2$) as a predominant phase, and attributing the absence of ${\mathrm{BiAg}}_{2}$ alloy to the strained Ag/Pt interface. Likewise, we show that the growth of the ${\mathrm{BiAg}}_{2}$ alloy on similarly lattice-mismatched 1 and 2 ML Ag-Cu(111) interfaces also requires a low annealing temperature, whilst higher temperatures result in ${\mathrm{BiAg}}_{2}$ clustering and the formation of a ${\mathrm{BiCu}}_{2}$ alloy. The demonstration that the ${\mathrm{BiAg}}_{2}$ alloy can be formed on thin Ag films on different substrates presenting a strained interface has the prospect of serving as bases for technologically relevant systems, such as Rashba alloys interfaced with magnetic and semiconductor substrates.

Unconventional electronic structure of silicene on Ag/H-Si(111)

We fabricated monolayer silicene on a thin silver (Ag) crystal film grown on hydrogen-terminated Si(111), and studied the crystal and the electronic structure by low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). The fabricated sample exhibits the 3×3 LEED pattern characteristic of silicene and the band dispersions with the periodicity of the silicene Brillouin zone, indicative of successful fabrication of silicene on Ag/H-Si(111). However, we observed no signature for the Dirac-cone state at the K point, while the observed band structure exhibits an unexpected van-Hove singularity with a saddle point at the binding energy of 1eV. Such a singularity has not been reproduced in so-far existing band structure calculations, requesting sophisticated calculations incorporating the complicated strong hybridization between silicene and Ag orbitals.

FromFe3O4/NiO bilayers toNiFe2O4-like thin films through Ni interdiffusion

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni$^{2+}$ ions out of NiO into Fe$_3$O$_4$ ultrathin films resulting in off-stoichiometric nickelferrite-like thin layers. We synthesized epitaxial Fe$_3$O$_4$/NiO bilayers on Nb-doped SrTiO$_3$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400\,$^{\circ}$C, 600\,$^{\circ}$C, and 800\,$^{\circ}$C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogenous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFe$_2$O$_4$. Further evidence comes from low energy electron diffraction patterns indicating a spinel type structure at the surface after annealing. Site and element specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni$^{2+}$ and Fe$^{3+}$ ions and the Fe$^{3+}$ in tetrahedral coordination. We find a quite low coercive field of 0.02\,T, indicating a rather low defect concentration within the thin ferrite films.

Analysis of the Si(111) surface prepared in chemical vapor ambient for subsequent III-V heteroepitaxy

For well-defined heteroepitaxial growth of III-V epilayers on Si(111) substrates the atomic structure of the silicon surface is an essential element. Here, we study the preparation of the Si(111) surface in H2-based chemical vapor ambient as well as its atomic structure after contamination-free transfer to ultrahigh vacuum (UHV). Applying complementary UHV-based techniques, we derive a complete picture of the atomic surface structure and its chemical composition. X-ray photoelectron spectroscopy measurements after high-temperature annealing confirm a Si surface free of any traces of oxygen or other impurities. The annealing in H2 ambient leads to a monohydride surface termination, as verified by Fourier-transform infrared spectroscopy. Scanning tunneling microscopy confirms a well ordered, atomically smooth surface, which is (1×1) reconstructed, in agreement with low energy electron diffraction patterns. Atomic force microscopy reveals a significant influence of homoepitaxy and wet-chemical pretreatment on the surface morphology. Our findings show that wet-chemical pretreatment followed by high-temperature annealing leads to contamination-free, atomically flat Si(111) surfaces, which are ideally suited for subsequent III-V heteroepitaxy.

Epitaxial growth of two-dimensional Pb and Sn films on Al(111)

Single and binary two-dimensional (2D) films of Pb and Sn on Al(111) at room temperature were investigated using low-energy electron diffraction (LEED) and scanning tunneling microscopy. The 2D Pb and Sn films form a (√31×√31)-Pb structure with close-packed six-fold symmetry and a (“2”×√3)-Sn structure with square-like fourfold symmetry, respectively. The 2D bimetallic Pb-Sn films only exhibit (√31×√31)-Pb and/or (“2”×√3)-Sn LEED patterns in any compositional ratio of Pb to Sn. Sub-monolayer Sn deposition on the 2D Pb film reveals (“2”×√3)-Sn LEED pattern, indicating the preferential formation of the 2D Sn film. These results indicate that monolayer Pb and Sn films are immiscible on Al(111).

Growth and characterization of epitaxial Zr(0001) thin films on Al2O3(0001)

The authors report the growth of epitaxial Zr(0 0 0 1) thin films on Al2O3(0 0 0 1) substrates at a temperature of 700 °C via dc magnetron sputtering in an ultrahigh vacuum deposition system equipped with facilities for chemical vapor deposition, low-energy electron diffraction, and Auger electron spectroscopy. Zr layers with a nominal thickness of ∼220 nm are deposited at a rate of ∼0.06 nm/s in 10 mTorr Ar atmosphere. In situ Auger electron spectra of the as-deposited film surface reveal the presence of a Zr peak at 145 eV and Hf peak at 172 eV, the latter due to the presence of Hf impurities in the Zr sputter target. In situ low-energy electron diffraction patterns acquired from the Zr sample show sixfold symmetric spots with an in-plane lattice spacing of 0.31 ± 0.02 nm, characteristic of Zr(0 0 0 1)–(1 × 1) surface. Cross-sectional transmission electron microscopy images reveal columnar growth and the formation of a crystalline, 22 ± 8 nm thick, interfacial layer. Energy dispersive x-ray spectra obtained from this region reveal the presence of both Zr and Al. The authors attribute the formation of this interfacial layer to plasma-induced substrate decomposition during sputtering followed by interdiffusion of Al and Zr at the film–substrate interface. ω-2θ x-ray diffraction data show that the Zr layers are single-phase with hexagonal close-packed structure. Using high-resolution symmetric as well as asymmetric reciprocal space maps, the authors determined that the film is fully relaxed with in-plane and out-of-plane orientation lattice parameters of 0.324 and 0.516 nm, respectively, and identified epitaxial orientation relationships as Zr(0 0 0 4) ‖ Al2O3(0 0 0 12) and Zr(101¯0) ‖ Al2O3(112¯0).

Thermal Stability of N,N′-Bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxdiimide Films on Cu(100)

The thermal stability of N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxdiimide (EP-PTCDI) thin films deposited on Cu(100) has been characterized by using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). For one monolayer absorption, the molecules arrange with a (34−26) symmetry. After annealing the sample at 500 K, XPS measurements show that the molecules dissociate leaving the PTCDI core of the molecule on the surface. According to that, STM measurements show new images of molecules with the size of the PTCDI core, and without the characteristic ethylpropyl end groups. These remaining molecules cover most of the surface with ordered islands, presenting a different molecular arrangement. With the help of the experimental and simulated LEED patterns, the molecular arrangement can be described by a (−563.53) superstructure and its four rotational equivalents. However, complementary measurements of STM images at different tip-to-s...

One-dimensional diffusion of Sr atoms on Sr/Si(111)-3 × 2 reconstruction surface

The electronic and geometric structures of the Sr/Si(111)-3×2 surface were investigated by scanning tunnelling microscopy and scanning tunnelling spectroscopy. The honeycomb-chain (HCC) model may be used to describe the reconstruction structure of the Sr/Si(111)-3×2 surface.Furthermore, one-dimensional (1D) concerted motion of Sr atom chains on the Sr/Si(111)-3×2 surface was observed at room temperature. Three reasons contribute to this 1D self-diffusion: low metal coverage of the Sr/Si(111)-3×2 reconstruction surface, weak interaction between the Sr and Si substrate, and surface vacancies and thermal fluctuation energy at room temperature.From this study, the origin of the long-existing blurred low energy electron diffraction pattern of alkali-earth metal induced-Si(111)3×2 surface was clarified, and the self-diffusion of metal atoms at room temperature also explains the common phase transition phenomenon on these reconstructed surfaces.

Growth and characterization of ultrathin epitaxial MnO film on Ag(001)

We present here a comprehensive growth procedure to obtain a well-ordered MnO(001) ultrathin film on Ag(001) substrate. Depending upon the oxygen partial pressure during the growth, different phases of manganese oxide have been detected by Low Energy Electron Diffraction (LEED) and X-ray Photoelectron Spectroscopic (XPS) studies. A modified growth scheme has been adopted to get well-ordered and stoichiometric MnO(001) ultrathin film. The detailed growth mechanism of epitaxial MnO film on Ag(001) has been studied step by step, using LEED and XPS techniques. Observation of sharp (1×1) LEED pattern with a low inelastic background, corresponds to a long-range atomic order with low defect densities indicating the high structural quality of the film. The Mn 2p and Mn 3s core-level spectra confirm the oxidation state as well as the stoichiometry of the grown MnO films. Apart from the growth optimization, the evolution of strain relaxation of the MnO(001) film with film thickness has been explored.

Low temperature removal of surface oxides and hydrocarbons from Ge(100) using atomic hydrogen

Germanium is a group IV semiconductor with many current and potential applications in the modern semiconductor industry. Key to expanding the use of Ge is a reliable method for the removal of surface contamination, including oxides which are naturally formed during the exposure of Ge thin films to atmospheric conditions. A process for achieving this task at lower temperatures would be highly advantageous, where the underlying device architecture will not diffuse through the Ge film while also avoiding electronic damage induced by ion irradiation. Atomic hydrogen cleaning (AHC) offers a low-temperature, damage-free alternative to the common ion bombardment and annealing (IBA) technique which is widely employed. In this work, we demonstrate with X-ray photoelectron spectroscopy (XPS) that the AHC method is effective in removing surface oxides and hydrocarbons, yielding an almost completely clean surface when the AHC is conducted at a temperature of 250°C. We compare the post-AHC cleanliness and (2×1) low energy electron diffraction (LEED) pattern to that obtained via IBA, where the sample is annealed at 600°C. We also demonstrate that the combination of a sample temperature of 250°C and atomic H dosing is required to clean the surface. Lower temperatures prove less effective in removal of the oxide layer and hydrocarbons, whilst annealing in ultra-high vacuum conditions only removes weakly bound hydrocarbons. Finally, we examine the subsequent H-termination of an IBA-cleaned sample using XPS, LEED and ultraviolet photoelectron spectroscopy (UPS) in order to examine changes in the work function of Ge(100) upon hydrogenation.

Preparation and structure of ultra-thin GaN (0001) layers on In0.11Ga0.89N-single quantum wells

Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1–10nm thickness, covering a 3nm thick In0.11Ga0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760°C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification.

The Surface Structure of Cu2O(100)

Despite the industrial importance of copper oxides, the nature of the (100) surface of Cu2O has remained poorly understood. The surface has previously been subject to several theoretical and experimental studies, but has until now not been investigated by atomically resolved microscopy or high-resolution photoelectron spectroscopy. Here we determine the atomic structure and electronic properties of Cu2O(100) by a combination of multiple experimental techniques and simulations within the framework of density functional theory (DFT). Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) characterized the three ordered surface structures found. From DFT calculations, the structures are found to be energetically ordered as (3,0;1,1), c(2 × 2), and (1 × 1) under ultrahigh vacuum conditions. Increased oxygen pressures induce the formation of an oxygen terminated (1 × 1) surface structure. The most common termination of Cu2O(100) has previously been described by a (3√2 × √2)R45° unit cell exhibiting a LEED pattern with several missing spots. Through atomically resolved STM, we show that this structure instead is described by the matrix (3,0;1,1). Both simulated STM images and calculated photoemission core level shifts compare favorably with the experimental results.

Oxygen Adsorption on the Fe(110) Surface: The Old System – New Structures

Adsorption of oxygen on the (110) surface of epitaxial iron films on tungsten (110) was studied using low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), and Auger electron spectroscopy within an exposure range of 0–300 Langmuir (L). Selected oxygen adsorption structures on Fe(110) reported in the literature were critically compared and revised in reference to the present study. The initial adsorption of 1/4 oxygen monolayer resulting in the commonly observed (2 × 2) structure was followed by a structure that was frequently termed as (3 × 1). Its complex LEED pattern was ultimately resolved and interpreted as originating from two structural domains of a large oblique unit cell (eight times larger than the substrate unit cell) and 3/8 oxygen coverage. A new (3 × 2) structure was identified at a coverage of 2/3. The domain interpretation of last two structures was verified by LEEM and confirmed by density functional theory calculations. The onset of oxygen–iron bonding formation was recognized by the change in the symmetry of the LEED pattern and the shape of the iron AES signal. Finally, the formation of an iron oxide FeO(111) monolayer was evidenced at the oxygen exposure of ∼300 L.

Growth and termination of a rutile IrO2(100) layer on Ir(111)

We investigated the oxidation of Ir(111) by gas-phase oxygen atoms at temperatures between 500 and 625K using temperature programmed desorption (TPD), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEISS) and density functional theory (DFT) calculations. We find that a well-ordered surface oxide with (√3×√3)R30° periodicity relative to Ir(111) develops prior to the formation of a rutile IrO2(100) layer. The IrO2(100) layer reaches a saturation thickness of about four oxide layers under the oxidation conditions employed, and decomposes during TPD to produce a single, sharp O2 desorption peak at ~770K. Favorable lattice matching at the oxide-metal interface is likely responsible for the preferential growth of the IrO2(100) facet during the initial oxidation of Ir(111), with the resulting coincidence lattice generating a clear (6×1) moiré pattern in LEED. Temperature programmed reaction spectroscopy (TPRS) experiments reveal that CO and H2O molecules bind only weakly on the IrO2(100) surface and LEISS measurements show that the oxide surface is highly enriched in O-atoms. These characteristics provide strong evidence that the rutile IrO2(100) layer is oxygen-terminated, and thus lacks reactive Ir atoms that can strongly bind molecular adsorbates. Oxygen binding energies predicted by DFT suggest that on-top O-atoms will remain adsorbed on IrO2(100) at temperatures up to ~625K, thus supporting the conclusion that the rutile IrO2 layer grown in our experiments is oxygen-terminated. As such, the appearance of only a single O2 TPD peak indicates that the singly coordinate, on-top O-atoms remain stable on the IrO2(100) surface up to temperatures at which the oxide layer begins to thermally decompose.

電界放出電子線を用いた低速電子回折装置の開発

Low-energy electron diffraction (LEED) apparatuses with field-emission (FE) tips were developed. The FE tips were fabricated by field-assisted gas etching to obtain an atomically sharp tip apex. These tips emit single-spot electron beams. Using the FE beams in conjunction with a lensless system, we observed backscattered electrons at the surface of few-layer graphene grown on SiC(0001). The obtained hexagonal patterns at sample biases of 93 V, 54 V, and 28 V were interpreted as diffraction spots derived from the graphene, the SiC(0001) substrate, and a buffer layer, respectively. On the basis of the opening angle of FE, the irradiated areas were estimated to be 350 nmφ when the sample bias was 100 V. Since the FE beams were easily focused by a magnetic lens, a LEED apparatus with the focused FE beams was also examined. This approach might be useful for improving the sharpness of LEED patterns.

Growth of cerium tungstate epitaxial layers: influence of temperature

Thin films of cerium tungstate prepared in situ by cerium deposition in oxygen atmosphere onto the W(100) single‐crystal were investigated by means of photoelectron spectroscopy and low‐energy electron diffraction (LEED). The studied temperature range was 173–1073 K. It was found that the temperature necessary for the oriented growth of Ce6WO12(100) was 673 K, and at higher temperatures, the LEED pattern improved. Photoemission data revealed the partial formation of CeO2 on the surface at preparation temperatures below 473 K due to limited diffusion of tungsten atoms from the substrate. Copyright © 2015 John Wiley & Sons, Ltd.