Low-energy Electron Diffraction Pattern Research Articles

Dry cleaning of InSb surfaces by hydrogen molecule exposure in ultrahigh vacuum

Cleaning semiconductor surfaces by atomic hydrogen or hydrogen plasma has gained significant interest because such a dry-cleaning method enables to reduce consumption of chemicals and pure water, and to treat challenging surfaces of three-dimensional semiconductor nanostructures. We have studied effects of mere H2 molecule exposures on (111)B and (110) surfaces of InSb with native oxides in an ultrahigh-vacuum (UHV) chamber. Without any hydrogen cracking, exposure of native-oxide covered InSb(111)B, heated simultaneously at 350 °C, to H2 with a partial pressure of 5∙10−5 mbar decreases amount of surface oxides and carbon, according to x-ray photoelectron spectroscopy, and provides (2×2) low-energy electron diffraction (LEED) pattern. Scanning tunneling microscopy indicates that this InSb(111)B(2×2) surface contains still extra Sb. When the InSb temperature increases to 400 °C during the H2 exposure, LEED changes to (3×3) pattern, which is known to arise from a less Sb-rich surface compared to InSb(111)B(2×2). When InSb(111)B(3×3) is exposed to H2 at the lowered temperature of 300 °C, LEED changes back to (2×2), which is discussed to arise from that InSb(111)B(3×3) contains still oxygen. Experiments for InSb(110) support that the found H2 exposure effects apply to different crystal faces of InSb.

Zinc blende ZnS (001) surface structure investigated by XPS, LEED, and DFT

The formation and stability of the zinc blende ZnS (001) single crystal surface have been examined by x-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED), and density-functional theory (DFT) calculations. LEED patterns obtained from the ZnS (001) surface prepared in ultrahigh vacuum conditions at 1420 K reveal the formation of a (1 × 2) reconstructed structure. Surface chemical characterization performed by XPS indicates that the surface region consists of a Zn-deficient (sulfur-rich) phase. Our DFT theoretical calculations confirm that the ZnS (001) (1x2) surface structure is energetically favorable and consists of a Zn-terminated topmost surface layer stabilized by Zn-vacancies. The calculations also suggest that the remaining Zn-cations on the surface are displaced inward, driven by surface energy minimization related to zinc-blend ZnS (001) polar nature. Notably, the observed surface structural modifications have potential implications for the ZnS physical and chemical properties.

Tin oxide thin films on Ag(111): Thickness and temperature dependent study of surface structure and electronic properties

Growth and structure of tin oxide films on Ag(111) from submonolayer to 10 monolayer equivalent thickness have been studied using low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy techniques for room temperature (RT) and high temperature (HT) (573 K). For RT growth, most of the tin is oxidised to form tin oxide (SnOx) as confirmed by XPS where no metallic component is noticed. However, the grown films are not epitaxial or ordered which can be confirmed by no spots in LEED for higher coverage. Metallic Sn along with two oxidation states of Sn are noticed for HT growth, with the surface mainly composed of SnO2. LEED pattern shows the presence of c(2 × 2) and (3 × 1) along with two different hexagonal multidomain type patterns. The electronic structure for lower coverage resembled the Sn/Ag(111) alloy structures for lower coverage. For higher coverages appearance of three bands below 2 eV binding energy was noticed.

Wafer-scale 30° twisted bilayer graphene epitaxially grown on Cu0.75Ni0.25 (111)

Twisted bilayer graphene (TBG) has been extensively studied because of its novel physical properties and potential application in electronic devices. Here we report the synthesis and characterization of 30° TBG naturally grown on Cu0.75Ni0.25 (111) film and investigate the electronic structure by angle-resolved photoemission spectroscopy. Compared with other substrates, our TBG with a wafer scale is acquired with a shorter growth time. The Fermi velocity and energy gap of Dirac cones of TBG are comparable with those of a monolayer on Cu0.85Ni0.15 (111). The signature of moiré lattices has not been observed in either the low-energy electron diffraction patterns or the Fermi surface map within experimental resolution, possibly due to different Cu and Ni contents in the substrates enhancing the different couplings between the substrate and the first/second layers and hindering the formation of a quasiperiodic structure.

Growth of MnxAu1−x Films on Cu(001) and Ag(001) Single‐Crystal Substrates

The growth, morphology, and structure of films on Cu(001) and Ag(001) are studied by means of low‐energy electron diffraction (LEED), medium‐energy electron diffraction, Auger electron spectroscopy, and scanning tunnelling microscopy. Different concentrations x from about 0.5 to 1 and thicknesses from 0.2 to 12.9 ML of are examined. For several values of x, exhibits a c(2 2) superstructure pattern on Cu(001) when the total thickness is around or above 0.5 ML. Above 1 ML, LEED patterns of can be only observed on Ag(001), but not on Cu(001). LEED‐I(V) is employed to deduce the vertical interlayer distance for as‐grown and post‐annealed films on Ag(001). Above 500 K, Ag from the substrate segregates into the films.

Selective Fabrication of Bismuthene and α-Bi on Hydrogen-Terminated SiC(0001).

Nanomaterials based on monoatomic bismuth (Bi) are attracting particular attention because they are candidates of two-dimensional (2D) topological insulators and Rashba metals useful for spintronic applications. We report convenient selective fabrication of two different types of ultrathin Bi films, bismuthene and α-Bi on hydrogen-terminated SiC(0001), by combining the molecular-beam-epitaxy (MBE) method and the low-temperature and low-pressure hydrogen chemical etching of SiC. We have succeeded in selectively fabricating these two different Bi phases by simply tuning the substrate temperature during the MBE process. We observed that while bismuthene and α-Bi showed a similar low-energy electron diffraction pattern of the (√3 × √3)R30° periodicity, angle-resolved photoemission spectroscopy revealed a sizable difference in the band structure; bismuthene shows a massive Dirac cone, a signature of 2D topological insulators, whereas α-Bi exhibits an insulating behavior with a large band gap of more than 1.8 eV. We discuss the underlying mechanism of selective fabrication in terms of hydrogen desorption from the hydrogen-terminated SiC substrate.

Band energy diagrams of n-GaInP/n-AlInP(100) surfaces and heterointerfaces studied by X-ray photoelectron spectroscopy

Lattice matched n-type AlInP(100) charge selective contacts are commonly grown on n-p GaInP(100) top absorbers in high-efficiency III–V multijunction solar or photoelectrochemical cells. The cell performance can be greatly limited by the electron selectivity and valance band offset at this heterointerface. Understanding of the atomic and electronic properties of the GaInP/AlInP heterointerface is crucial for the reduction of photocurrent losses in III–V multijunction devices. In our paper, we investigated chemical composition and electronic properties of n-GaInP/n-AlInP heterostructures by X-ray photoelectron spectroscopy (XPS). To mimic an in-situ interface experiment with in-situ stepwise deposition of the contact material, 1 nm –50 nm thick n-AlInP(100) epitaxial layers were grown on n-GaInP(100) buffer layer on n-GaAs(100) substrates by metal organic vapor phase epitaxy. We observed (2 × 2)/c(4 × 2) low-energy electron diffraction patterns with characteristic diffuse streaks along the [011¯] direction due to PP dimers on both AlInP(100) and GaInP(100) as-prepared surfaces. Atomic composition analysis confirmed P-rich termination on both surfaces. Angle-resolved XPS measurements revealed a surface core level shift of 0.9 eV in P 2p peaks and the absence of interface core level shifts. We assigned the surface chemical shift in the P 2p spectrum to PP bonds on a surface. We found an upward surface band bending on the (2 × 2)/c(4 × 2) surfaces most probably caused by localized mid-gap electronic states. Pinning of the Fermi level by localized electronic states remained in n-GaInP/n-AlInP heterostructures. A valence band offset of 0.2 eV was derived by XPS and band alignment diagram models for the n-n junctions were suggested.

Formation and structural characterization of two-dimensional wetting water layer on graphite (0001).

Understanding the structure and wettability of monolayer water is essential for revealing the mechanisms of nucleation, growth, and chemical reactivity at interfaces. We have investigated the wetting layer formation of water (ice) on the graphite (0001) surface using a combination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). At around monolayer coverages, the LEED pattern showed a (2 × 2) periodicity and STM revealed a hydrogen-bonded hexagonal network. The lattice constant was about 9% larger than that for ice Ih/Ic crystals, and the packing density was 0.096 Å-2. These results indicate that an extended ice network is formed on graphite, different from that on metal surfaces. Graphite is hydrophobic under ambient conditions due to the airborne contaminant but is considered inherently hydrophilic for a clean surface. In this study, the hydrophilic nature of the clean surface has been investigated from a molecular viewpoint. The formation of a well-ordered commensurate monolayer supports that the interaction of water with graphite is not negligible so that a commensurate wetting layer is formed at the weak substrate-molecule interaction limit.

From high temperature phase formation to transition metal substitution in the Fe/Al9Co2(001) system

We report the formation of several complex intermetallics as surface alloys upon the adsorption of Fe on Al9Co2(001) for different dosing conditions. Up to 4 monolayers equivalent (MLE) of Fe deposited on the substrate held at 593 K, the low energy electron diffraction pattern consists of a (2×2)R45° phase with two additional domain types rotated by ±8∘ from it. The scanning tunneling microscopy (STM) measurements show that these three types of domains have the same crystallographic structure. The lattice parameters and structural motifs point towards the formation of the high-temperature Al8Fe5 phase (γ-brass of Zn8Cu5-type structure). For Fe deposition between 593 K and 873 K, we have identified the formation of two phases, tentatively assigned to a ternary disordered Al9(Co,Fe)2 overlayer and to the monoclinic Al13Fe4(100). The stoichiometric evolution of the grown structures have been characterized by angle-resolved x-ray photoelectron spectroscopy measurements. Density functional theory based calculations have been performed to model the Al8Fe5(100)/Al9Co2(001) interface, its structural stability and to simulate the corresponding STM images.

Evidence of sp2-like Hybridization of Silicon Valence Orbitals in Thin and Thick Si Grown on α-Phase Si(111)√3 × √3R30°-Bi.

One-monolayer (ML) (thin) and 5-ML (thick) Si films were grown on the α-phase Si(111)√3 × √3R30°-Bi at a low substrate temperature of 200 °C. Si films have been studied in situ by reflection electron energy loss spectroscopy (REELS) and Auger electron spectroscopy, as a function of the electron beam incidence angle α and low-energy electron diffraction (LEED), as well as ex situ by grazing incidence X-ray diffraction (GIXRD). Scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) were also reported. The REELS spectra, taken at the Si K absorption edge (~1.840 KeV), reveal the presence of two distinct loss structures attributed to transitions 1s→π* and 1s→σ* according to their intensity dependence on α, attesting to the sp2-like hybridization of the silicon valence orbitals in both thin and thick Si films. The synthesis of a silicon allotrope on the α-phase of Si(111)√3 × √3R30°-Bi substrate was demonstrated by LEED patterns and GIXRD that discloses the presence of a Si stack of 3.099 (3) Å and a √3 × √3 unit cell of 6.474 Å, typically seen for multilayer silicene. STM and STS measurements corroborated the findings. These measurements provided a platform for the new √3 × √3R30° Si allotrope on a Si(111)√3 × √3 R30°-Bi template, paving the way for realizing topological insulator heterostructures from different two-dimensional materials, Bi and Si.

Growth and morphology of thin epitaxial Pd films on Au(001) film grown on Fe-buffered MgO(001) substrate

We have investigated the growth of thin Pd films on a Au(001) surface, which was a Au(001) film grown on a Fe-buffered MgO(001) substrate, at 290 K up to 4 monolayer (ML) thickness and the effect of post-annealing (PA) at 470 K. The surface morphology and structure are determined using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The LEED patterns of the as-grown and PA films indicate that the Pd(001) films grow epitaxially on the Au(001) surface with noticeable in-pain lattice expansion of 4.5% relative to the bulk Pd, indicating a tensely strained tetragonal deformation. The Pd film does not grow in the layer-by-layer growth mode at 290 K. Instead, numerous Pd islands grow anisotropically along the 〈110〉 direction. The surface morphology of the films improves significantly after the PA at 470 K due to the coalescence of the elongated Pd islands. Consequently, the room temperature growth of the Pd layers followed by PA at 470 K enables us to fabricate the high-quality epitaxial Pd(001) film, which has large and atomically flat terraces without Au segregation. Additional Pd growth up to 8 ML thickness onto such 4 ML PA films kept at 470 K is also studied by STM. This specific three-step method facilitates the quasi-layer-by-layer growth and opens a chance for practical use in the future.

An efficient route to prepare suspended monolayer for feasible optical and electronic characterizations of two‐dimensional materials

AbstractTwo‐dimensional (2D) materials are highly sensitive to substrates, interfaces, and the surrounding environments. Suspended 2D materials are free from substrate‐induced effects, thus an ideal approach to study their intrinsic properties. However, it is very challenging to prepare large‐area suspended 2D materials with high efficiency. Here we report a universal method, based on pretreatments of densely patterned hole array substrates with either oxygen‐plasma or gold film deposition, to prepare large‐area suspended mono‐ and few‐layer 2D materials. Multiple structural, optical, and electrical characterization tools were used to fully evaluate the improved performance of various suspended 2D layers. Some of these observations reported in this study are: (1) Observation of a new Raman low frequency mode for the suspended MoS2; (2) Significantly stronger photoluminescence (PL) and second harmonic generation (SHG) signals of suspended WSe2, which enables the study of new optical transition processes; (3) The low energy electron diffraction pattern on suspended MoS2 also exhibits much sharper spots than that on the supported area; and (4) The mobility of suspended graphene device approaches 300 000 cm2 V−1 s−1, which is desirable to explore the intrinsic properties of graphene. This work provides an innovative and efficient route for fabricating suspended 2D materials, and we expect that it can be broadly used for studying intrinsic properties of 2D materials and in applications of hybrid active nanophotonic and electronic devices.image

Quantum size effects in Ag thin films grown on the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal: Influence of the growth temperature

We have studied the growth and electronic structure of Ag thin films on the fivefold surface of the icosahedral (i)-Al-Cu-Fe quasicrystal using scanning tunneling microscopy, low energy electron diffraction (LEED), ultraviolet photoemission spectroscopy, and density functional theory. Upon deposition at 400 K, Ag islands grow to form crystallites with a preferred thickness for a given coverage. LEED patterns reveal five rotational domains of Ag crystallites with (111) orientation for coverages larger than approximately seven monolayers. Quantum well states are observed in the photoemission spectra of Ag/i-Al-Cu-Fe ranging from 5 to 35 monolayers, indicating electron confinement within the film thickness and, thus, confirming electronic growth of Ag thin films on quasicrystalline surfaces. Electronic structure calculations have been performed to discuss the possible origins of the confinement at the film-substrate interface.

Molecular beam epitaxy growth of monolayer hexagonal MnTe2 on Si(111) substrate* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11604366, 11634007, 21872099, and 22072102) and the National Natural Science Foundation of Jiangsu Province, China (Grant No. BK 20160397). F. S. L. acknowledges support from the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No.

Monolayer MnTe2 stabilized as 1T structure has been theoretically predicted to be a two-dimensional (2D) ferromagnetic metal and can be tuned via strain engineering. There is no naturally van der Waals (vdW) layered MnTe2 bulk, leaving mechanical exfoliation impossible to prepare monolayer MnTe2. Herein, by means of molecular beam epitaxy (MBE), we successfully prepared monolayer hexagonal MnTe2 on Si(111) under Te rich condition. Sharp reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) patterns suggest the monolayer is atomically flat without surface reconstruction. The valence state of Mn4+ and the atom ratio of ([Te]:[Mn]) further confirm the MnTe2 compound. Scanning tunneling spectroscopy (STS) shows the hexagonal MnTe2 monolayer is a semiconductor with a large bandgap of ∼ 2.78 eV. The valence-band maximum (VBM) locates at the Γ point, as illustrated by angle-resolved photoemission spectroscopy (ARPES), below which three hole-type bands with parabolic dispersion can be identified. The successful synthesis of monolayer MnTe2 film provides a new platform to investigate the 2D magnetism.

Graphene-substrate decoupling by S segregation. A LEEM/LEED study

The growth of graphene on Ni(110) is studied with low energy electron microscopy, low energy electron diffraction (LEED) and associated methods at several sulfur coverages obtained by surface segregation and compared with growth on the clean surface. On the clean surface graphene grows nearly epitaxial with small azimuthal alignment fluctuations. The segregated S reduces the interaction between graphene and Ni, which results in the formation of several azimuthal orientations. The reduced film-substrate interaction is evident in the details of the LEED pattern, in the work function change and the energy dependence of the specular reflected intensity. This study clarifies the differences between earlier studies of graphene growth on Ni(110) and suggests using impurity surface segregation as means for decoupling of graphene from the substrate.

Making ferromagnetic metal MnSi ultrathin films semiconductor

Atomically flat MnSi films were fabricated on Si(111)-7 × 7 reconstructed surface by molecular beam epitaxy(MBE). Both scanning tunneling microscopy (STM) images and low energy electron diffraction (LEED) patterns demonstrate a well-defined (3×3)R30o structure reconstruction. A thickness-driven metal–semiconductor transition in MnSi ultrathin films was observed with decreasing the thickness down to 6 ML (monolayers). The temperature dependence of the resistance and the negative magnetoconductivity suggest the MnSi ultrathin films with thickness lower than 6ML exhibit weak anti-localization (WAL) of two-dimensional (2D) electron systems. This finding that not only advances our understanding of the mechanism of thickness-driven metal–semiconductor transition, but also provides a new strategy to use ferromagnetic semiconductor as spin injector in spintronic devices.

A New Method for Evaluating Li-Ion Battery Anode Materials Based on Surface Compositional and Structural Characterization

There is a global effort to develop safer lithium-ion batteries (LIB) with a high energy density and long lifetime. Material properties, their mutual interactions, and active interfaces are key factors in battery performance. Popular methods involve the mixing of different ingredients and development of a suitable fabrication process. Significant effort has been spent to understand the reaction mechanism of battery materials, but this is very challenging due to high system complexity. Different techniques have been developed to follow these reactions, and most significantly, the reactions between Li and electrode materials. Our team has developed a new method to characterize Li thermal diffusion and structural interaction on the surfaces of potential anode and cathode materials. This method is based on surface crystallographic and compositional characterization of deposited ultra-thin film of Li on potential single crystal or polycrystalline anode material. Surface crystallography characterization is done with w Low Energy Electron Diffraction (LEED) and surface composition with Auger Electron Spectroscopy (AES). This characterization monitors Li thermal diffusion into the electrode surface at room temperature and changes in surface crystalline structure and composition. The data provides a strong link between Lithium thermal diffusion and lithiation processes in LIB. The dynamic interaction of Li is monitored at the nanoscale level and gives the direct response how Li atoms interact with the substrate. The diffusion of evaporated Li on single crystals of HOPG, Si(111)-(211)-(100), SiC-6H, CVD-Diamond and LiNbO3 have been performed. The results shows that HOPG is the excellent reference substrate for this method as Li diffuses at room temperature and surface crystallography of HOPG remains unchanged. The structural reaction of Li with Si is very strong causing LEED pattern to disappear and there is no Li diffusion at room temperature as indicated from AES data. The data for SiC-6H and LiNbO3 shows better Li diffusional behavior than on Si but not as good as on HOPG. This method of using a single crystal substrate and basic surface characterization techniques is providing simple nanoscale access into the chemical reactions between Li and the substrate material that will result in better understanding of Lithium diffusion in the fabricated battery electrodes. The initial material classification for “native” Lithium thermal diffusion properties is proposed.

In-plane strain-free stanene on a Pd2Sn(111) surface alloy

Here we report the growth of laterally strain-free stanene, that is a two-dimensional (2D) honeycomb structure of tin atoms, on a Pd(111) crystal terminated by a ${\mathrm{Pd}}_{2}\mathrm{Sn}$ surface alloy. The atomic geometry and the electronic structure have been thoroughly investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. The STM images clearly reveal the epitaxial growth of honeycomb stanene on the preformed ${\mathrm{Pd}}_{2}\mathrm{Sn}$ surface alloy. LEED patterns clearly show commensurate (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} spots, corresponding to a lattice constant of 0.47 nm, in perfect accord with the cell size of free-standing stanene. The measured very low buckling, within 20 pm, is derived from section profiles of atomic-scale STM images. This contrasts the significantly larger buckling of free-standing stanene, possibly due to a STM tip effect as well as a non-negligible interaction with the underlying surface alloy. The electronic structure exhibits a characteristic 2D band with parabolic dispersion, which is in good accordance with electronic structure calculations in density functional theory.

Highly Biaxially Strained Silicene on Au(111).

Many of graphene’s remarkable properties arise from its linear dispersion of the electronic states, forming a Dirac cone at the K points of the Brillouin zone. Silicene, the 2D allotrope of silicon, is also predicted to show a similar electronic band structure, with the addition of a tunable bandgap, induced by spin–orbit coupling. Because of these outstanding electronic properties, silicene is considered as a promising building block for next-generation electronic devices. Recently, it has been shown that silicene grown on Au(111) still possesses a Dirac cone, despite the interaction with the substrate. Here, to fully characterize the structure of this 2D material, we investigate the vibrational spectrum of a monolayer silicene grown on Au(111) by polarized Raman spectroscopy. To enable a detailed ex situ investigation, we passivated the silicene on Au(111) by encapsulating it under few layers hBN or graphene flakes. The observed spectrum is characterized by vibrational modes that are strongly red-shifted with respect to the ones expected for freestanding silicene. By comparing low-energy electron diffraction (LEED) patterns and Raman results with first-principles calculations, we show that the vibrational modes indicate a highly (>7%) biaxially strained silicene phase.

New atomic-scale insights into the I/Ni(100) system: phase transitions and growth of an atomically thin NiI2 film.

We use a traditional surface science approach to create and study an atomically thin NiI2 film (a promising two-dimensional ferromagnetic material) formed on nickel substrate as a result of molecular iodine adsorption. The I/Ni(100) system was examined with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory calculations. We found out that the iodine adsorption on Ni(100) at 300 K leads to the formation of non-equilibrium phases, whereas the adsorption at elevated temperature (≥390 K) gives rise to the thermodynamically stable phases. In both cases, a simple p(2 × 2) structure is formed at 0.25 ML. As more iodine is adsorbed at 300 K, the p(2 × 2) phase is replaced by the small coexisting domains of c(3 × 2) and c(6 × 2) phases both corresponding to the coverage of 0.33 ML, while adsorption at elevated temperature results in the formation of only one c(3 × 2) phase. At further iodine adsorption the c(3 × 2) phase transforms into the c(5 × 2) one, while the c(6 × 2) phase - into the one both corresponding to the coverage of 0.40 ML. In addition to simple chemisorbed phases, a new shifted-row reconstruction of Ni(100) induced by iodine adsorption was discovered. At coverages exceeding 0.40 ML, we observed complex LEED patterns and superstructures in STM and assigned them to specific surface reconstructions. We also found that prolonged iodine dosing leads to the nucleation of nickel iodide islands and the growth of a 2D atomically thin iodide film partially exfoliated from the substrate.