Low-energy Electron Diffraction Research Articles

HBN-Layer-Promoted Heteroepitaxy in Reactively Sputter-Deposited MoSx≈2(0001)/Al2O3(0001) Thin Films: Implications for Nanoelectronics

We show that van der Waals (vdW)-bonded hexagonal boron nitride (hBN) promotes heteroepitaxial growth of semiconducting MoSx(0001) (x = 2.0 ± 0.1) thin films on Al2O3(0001) substrates. hBN layers are grown on Al2O3(0001) via pyrolytic cracking of borazine (∼6 × 104 L) at 1373 K and the MoSx layers are deposited in an ultrahigh-vacuum system via reactive direct-current magnetron sputtering of Mo in an Ar/H2S gas mixture at 1073 K on bare and hBN-covered Al2O3(0001). Using in situ low-energy electron diffraction and Auger electron spectroscopy along with ex situ X-ray diffraction, X-ray photoelectron and Raman spectroscopies, and transmission electron microscopy, we determine the as-deposited MoSx layer composition and crystallinity. We obtain highly 0001-oriented, ∼20-nm-thick, 2H-structured MoSx multilayers with better crystalline quality on hBN/Al2O3(0001) than on Al2O3(0001). We suggest that hBN buffer layer enhances surface diffusion of depositing species, compared to bare Al2O3(0001), leading to an observed improvement in the crystallinity of MoSx layers. We expect that our results are likely to have broad implications in nanoelectronic device fabrication.

Formation of surface states on Pb(111) by Au adsorption

Using low-energy electron diffraction and angle-resolved photoemission spectroscopy, we investigated the lattice and electronic structures of the Pb(111) surface upon the adsorption of Au atoms at the low temperature T = 40 K. Unlike earlier results showing the formation of PbAu-alloy layers at room temperature, we found that Au atoms form a ultra-thin superstructure, Au/Pb(111)-3 × 3, on top of the Pb(111) surface. Moreover, three surface-state bands, S1, S2, and S3, are induced within and immediately adjacent to the Pb bulk projected band gap centered at the surface zone boundary {overline{text{M}}}_{Pb(111)} at the energies of − 0.02, − 1.05, and − 2.56 eV, respectively. First-principles calculation based on Au/Pb(111)-3 × 3 confirms the measured surface-state bands among which the most interesting are the S1 and S3 surface states. They are derived from surface resonances in Pb(111). Moreover, S1, which disperses across Fermi level, exhibits a large anisotropic Rashba splitting with α of 1.0 and 3.54 eVÅ in the two symmetry directions centered at {overline{text{M}}}_{Pb(111)}. The corresponding Rashba splitting of S1 bands in Cu/Pb(111)-3 × 3 and Ag/Pb(111)-3 × 3 was calculated for comparison.

Donor-Acceptor Co-Adsorption Ratio Controls the Structure and Electronic Properties of Two-Dimensional Alkali-Organic Networks on Ag(100).

The results are presented of a detailed combined experimental and theoretical investigation of the influence of coadsorbed electron-donating alkali atoms and the prototypical electron acceptor molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Ag(100) surface. Several coadsorption phases were characterized by scanning tunneling microscopy, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy. Quantitative structural data were obtained using normal-incidence X-ray standing wave (NIXSW) measurements and compared with the results of density functional theory (DFT) calculations using several different methods of dispersion correction. Generally, good agreement between theory and experiment was achieved for the quantitative structures, albeit with the prediction of the alkali atom heights being challenging for some methods. The adsorption structures depend sensitively on the interplay of molecule-metal charge transfer and long-range dispersion forces, which are controlled by the composition ratio between alkali atoms and TCNQ. The large difference in atomic size between K and Cs has negligible effects on stability, whereas increasing the ratio of K/TCNQ from 1:4 to 1:1 leads to a weakening of molecule-metal interaction strength in favor of stronger ionic bonds within the two-dimensional alkali-organic network. A strong dependence of the work function on the alkali donor-TCNQ acceptor coadsorption ratio is predicted.

Early-stage growth of GeTe on Si(111)-Sb

The advent of germanium telluride as a promising ferroelectric Rashba semiconductor for spintronic applications requires the growth of nanometer-thick films of high crystalline quality. In this study, we have elucidated the initial growth stages of GeTe on Si(111)-Sb by scanning tunneling microscopy and low energy electron diffraction. We demonstrate the presence of an initial 0.35-nm-thick GeTe buffer layer followed by the 2D growth of GeTe via Frank-Read sources of atomic steps. As shown by core level spectroscopy, Sb is acting as a surfactant during growth up to a 5-nm-thick film. X-ray diffraction, transmission electron microscopy, and low energy electron microscopy evidence that numerous mirror domains and in-plane misorientations appear early in the growth process and are gradually buried at the film/substrate interface. The use of a miscut Si substrate close to Si(111) allows suppressing these defects from the early beginning of growth.

Decagonal Sn clathrate on d -Al-Ni-Co

Decagonal quasiperiodic ordering of Sn thin films on $d$-Al-Ni-Co is shown based on scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory (DFT). Interestingly, the decagonal structural correlations are partially retained even up to a large film thickness of 10 nm grown at $165\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The nucleation centers called ``Sn white flowers'' identified by STM at submonolayer thickness are recognized as valid patches of the decagonal clathrate structure with low adsorption energies of these motifs. Due to the excellent lattice matching (to within 1%) between columns of Sn dodecahedra in the clathrate structure and pentagonal motifs at the $d$-Al-Ni-Co surface, the interfacial energy favors clathrate over the competing Sn crystalline forms. DFT study of the Sn/Al-Ni-Co composite model shows good mechanical stability, as shown by the work of separation of Sn from Al-Ni-Co slab that is comparable to the clathrate self-separation energy. The relaxed surface terminations of the ${\mathrm{R}}_{2}{\mathrm{T}}_{4}$ clathrate approximant are in self-similarity correspondence with the motifs observed in the STM images from monolayer to the thickest Sn film.

Dirac cones in graphene grown on a half-filled 4d-band transition metal

New opportunities for structural and electronic properties engineering of graphene can be achieved by tuning the interfacial interaction, which is ruled by the interplay between d-band filling and geometry of the support. Here, is demonstrated the growth of graphene, featuring Dirac cones around the Fermi level, on the rectangular (110) surfaces of Rh, a half-filled 4d-band transition metal element. The analysis of the structural properties by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) shows that domains with a continuum of possible graphene-substrate orientations with angular scatter of around 10° coexist in graphene/Rh(110) surfaces. Within each domain, surface structure is characterized by a distinct stripe-like moiré pattern. The interfacial chemistry analysis, by microprobeX-ray photoelectron spectroscopy (μ-XPS), of all the rotational domains studied, demonstrates the existence of two main levels of interfacial interaction strength, similar to previously reported graphene-metal systems characterized by the absence of Dirac cones around the Fermi level. However, the band structures of these domains probed by micro angle resolved photoelectron spectroscopy (μ-ARPES) present Dirac cones, with Fermi velocities comparable with those previously reported on weakly coupled graphene layers. Both the unique properties of graphene/Rh(110) surfaces and the prospect to obtain novel graphene-metal interfaces through the interplay between d-band filling and geometry, are expected to open new opportunities to study phenomena up to now masked behind the interaction with the substrate.

C60 Adsorbed on Ni(111) and Co(0001) Surfaces

Carbon-60 molecules were deposited on the fcc Ni(111) surface and, for the first time, the surface of bulk hcp Co(0001) and measured using low-energy electron diffraction and scanning tunneling microscopy. An adlayer with predominantly (4 × 4) domains is formed in each case. Other domains exemplify chiral epitaxial degeneracy. Annealing produces films with bright and dim molecules, with differing details per substrate. For C60 adsorption atop Ni(111), annealing results not only in vacancy formation beneath dim molecules but also in adatom nucleation below bright molecules.

Bonding, retention and thermal stability of shallow nitrogen in diamond (100) by low-energy nitrogen implantation

We investigate the sub-surface bonding, retention, and thermal stability of nitrogen in diamond (100) implanted with 200 and 800 eV N2+ at room temperature (RT) and 600 °C at an ion dose of 3.4×1014 ions/cm2. Upon 200 eV N2+ implantation at RT and 600 °C, nitrogen is incorporated in the diamond lattice mainly in a C-N/C=N bond bonding configuration alongside a small CN(nitrile-like) component. 800 eV N2+ implantation at both RT and 600 °C, the implanted nitrogen can occupy multiple bonding configurations, including C-N/C=N and CN(nitrile-like) bonds. Implantation at 600 °C resulted in higher thermal stability of the implanted nitrogen compared to RT implantation due to dynamic annealing assisted defect curing and desorption of weekly bonded species (possibly nitrogen bonded to defects) during the high temperature implantation. The long-range order of the N2+ implanted diamond surfaces was also investigated using low-energy electron diffraction measurements.

Targeted Dy intercalation under graphene/SiC for tuning its electronic band structure

Metal intercalation of graphene is a promising method to tune its electronic band structure and generate novel electronic and topological phases. The tuning depends critically on the ability to bond the intercalated atoms at predesigned, subsurface interlayer locations because the emerging band structure depends on metal location. We have studied Dy intercalation under single-layer graphene (SLG) on SiC using spot profile analysis--low-energy electron diffraction and scanning tunneling microscopy (STM). The experimental work is complemented with density-functional theory (DFT) analysis. Because different diffraction spots originate from different subsurface interlayer regions, it is possible to identify changes in the intercalation location by monitoring the spot intensity as a function of growth conditions. DFT calculations of the chemical potential as a function of intercalated Dy coverage support the variation of the stability of the intercalated phase at different intercalated locations. The preferred location is confirmed from STM studies showing the removal of the $6\ifmmode\times\else\texttimes\fi{}6$ moir\'e corrugation at the preferred location, observed at higher Dy coverage.

First steps of silicene growth on an insulating thin-film: effect of the substrate temperature

Silicene is a two-dimensional (2D) material with very promising electronic properties for applications in silicon modern technology. However, the first experimental synthesis of silicene on metallic surfaces shows strong interactions between the silicene and its substrate, which can alter its electronic properties. Here, we report on the first steps of silicene growth on an insulating surface (NaCl) using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angle-resolved photoemission spectroscopy (ARPES). We demonstrate the importance of temperature annealing in the growth of silicene on NaCl. Indeed, after deposition of silicon on the NaCl/Ag(110) surface, we observe the following stages: (i) at room temperature, the silicon atoms accumulate on top of the NaCl layer without any given order. (ii) At 60 °C, silicon dimers start to grow on the NaCl. (iii) At 140 °C, these dimers form a 2D silicon chains on the surface. (iv) After a post-annealing at 200 °C, evident 2D silicon nanoribbons with a honeycomb-like structure were observed. Our results of the first silicene growth stages on an insulating surface are a necessary step for exploring its growth mechanism further.

Ag and Pb diffusion on Ge(111)-3x2sqrt(3)sqrtSn surface

Ag and Pb diffusion on Ge(111)-3x2sqrt(3)sqrtSn surface has been studied by Auger electron spectroscopy and low energy electron diffraction. The mechanism of diffusion of atoms of these elements along the Ge(111)-3x2sqrt(3)sqrtSn surface is determined, and the temperature dependences of the diffusion coefficients are measured. The diffusion parameters of Ag along the Ge(111)-3x2sqrt(3)sqrtSn and Si(111)-2sqrt(3)sqrtSn surfaces are compared. Keywords: surface, surface diffusion, Ge(111), Ag, Pb, Sn.

Chiral self-organized single 2D-layers of tetramers from a functional donor-acceptor molecule by the surface template effect.

The ability to control the structural properties of molecular layers is a key for the design and preparation of organic electronic devices. While microscopic growth studies of planar, rigid and symmetric π-conjugated molecules have been performed to a larger extent, this is less the case for elongated donor-acceptor molecules with flexible functional groups, which are particularly interesting due to their high dipole moments. Prototypical molecules of this type are merocyanines (MCs), which have been widely studied for the use as efficient absorbers in organic photodetectors. For maximized light absorption and optimized electronic properties the molecular arrangement which is affected by the initial assembly of the films at the supporting substrate interface is decisive. The situation deserves special attention, when the surface nucleation leads to so far not known and bulk-unlike aggregates. Here, we report on the growth of a typical MC (HB238) on the Ag(100) surface, serving as the substrate. In the energetically preferred phase, the molecules adsorb in a face-on geometry and organize in tetramers with a circular dipole arrangement. The tetramers further self-order in large, enantiopure domains with a periodicity that is commensurate to the Ag(100) surface, likely due to a specific bonding of the thiophene and thiazol rings to the Ag surface. Using scanning tunneling microscopy (STM) in combination with low energy electron diffraction we derive the detailed structure of the tetramers. The center of the tetramer, which is most prominent in STM images, consists of four upward pointing tert-butyl groups from four molecules. It is encircled by a ring of four hydrogen bonds between terminal CN-groups and thiophene rings on neighboring molecules. In parallel, the surface interaction modifies the intramolecular dipole, which is revealed from photoemission spectroscopy. Hence, this example shows how the surface template effect leads to an unforeseen molecular organization which is considerably more complex compared to that in the bulk phases of HB238, which feature paired dipoles.

Growth of a quasicrystal-related structure and superstructure for ultrathin Ce-Ti-O films on Pt(111).

Herein, oxide quasicrystal-related (OQC-R) structure and Ce-Ti-O-(3 × 3) superstructure ultrathin films were prepared on Pt(111) and characterized using scanning tunneling microscopy (STM) and low-energy electron diffraction. The OQC-R structure with dodecagonal clusters consisting of triangles, squares, and rhombuses was observed in STM images. The first discovery of the OQC-R structure with a magnetic rare earth metal expands the possibility of discovering new oxide quasicrystals with novel magnetism or superconductivity. By depositing Ti on an OQC-R ultrathin film and post-annealing, a honeycomb lattice of the Ce-Ti-O-(3 × 3) superstructure was prepared. From X-ray photoelectron spectroscopy (XPS) and resonant-photoelectron spectroscopy, the chemical states of the Ce and Ti atoms in the OQC-R structure corresponded to the Ce3+ and Ti2+ states, while those for the Ce-Ti-O-(3 × 3) superstructure corresponded to the Ce3+, Ti3+, and Ti2+ states. The phase transformation from the OQC-R structure to the Ce-Ti-O-(3 × 3) honeycomb superstructure likely occurred when the amount of Ti increased and was more oxidized. The elemental atomic density was also calibrated using XPS and Rutherford backscattering spectroscopy. These results propose tentative structural models of the OQC-R structure as Ce18Ti14O41 and the Ce-Ti-O-(3 × 3) superstructure as CeTi6O9.

Unravelling the surface structure of β-Ga2O3 (100).

The present work is on a comprehensive surface atomic structure investigation of β-Ga2O3 (100). The β-Ga2O3 single crystal was studied by a structural model system in the simulations and in situ characterization via X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) allowed for probing the outermost layers' properties. In situ XPD characterization allows for the collection of valuable element-specific short-range information from the β-Ga2O3 surface, and the results were compared to a systematic and precise multiple scattering simulation approach. The experiments, characterizations, and simulations indicated strong evidence of considerable structural variations in the interatomic layer's distances. Such atomic displacement could clarify the electronic phenomena observed in theoretical studies. The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the β-Ga2O3 surface has two possible terminations. The best fits to the photoelectron diffraction curves are used to calculate the interplanar relaxation in the first five atomic layers. The results show good agreement with previous density functional theory calculations, establishing XPD as a useful tool for probing the atomic structure of oxide surfaces.

Growth and electronic properties of Co-doped Mn3O4 thin films: a combined experimental and theoretical investigation.

In this study, we have prepared and investigated the electronic properties of a new and promising cobalt doped Mn3O4 oxide surface by site-selective and element-sensitive X-ray-absorption (XAS) and photoemission spectroscopy (XPS and resonant PES) combined with density functional theory (DFT) calculations. The crystallinity of both pristine and Co doped thin films was ensured by low energy electron diffraction measurements, in which similar diffraction patterns were obtained for both films suggesting the inclusion of the dopant species within the crystalline Mn3O4 thin film. According to our combined experimental data and theoretical calculation results, XAS measurements and replace energy calculations could identify Co impurities adopting preferentially a 2+ oxidation state substituting a Mn2+ cation on tetrahedral sites (80%) as well as the Mn3+ on octahedral sites (20%). Direct evidence of these findings could be found by comparing the pristine Mn3O4 electron absorption and photoemission spectral features with those of the doped ones. For instance, the formation of oxygen vacancies related to the formation of Co2+ in an octahedral site could be directly observed. Remarkably, the valence band spectrum of Co-Mn3O4 thin films presents additional spectral features close to the Fermi edge that can be directly attributed to Co states when compared to the PDOS obtained by the DFT calculations. It is noteworthy that the formation and stabilization of these Co dopant species in the host Mn3O4 surface could potentially affect and obviously modulate its capability for adsorption of molecular species and transfer of electrons, which makes the cobalt doped Mn3O4 surface potentially promising for catalytic applications.

Obtaining Niobium Nitride on n-GaN by Surface Mediated Nitridation Technique

In this work the n-GaN(1000) surface is used as a source of nitrogen atoms in order to obtain niobium nitride film by a surface-mediated nitridation technique. To this end, the physical vapor deposition of the niobium film on GaN is followed by sample annealing at 1123 K. A thermally induced decomposition of GaN and interfacial mixing phenomena lead to the formation of a niobium nitride compound, which contains Nb from thin film and N atoms from the substrate. The processes allowed the obtaining of ordered NbNx films on GaN. Structural and chemical properties of both the GaN substrate and NbNx films were studied in-situ by surface-sensitive techniques, i.e., X-ray and UV photoelectron spectroscopies (XPS/UPS) and a low-energy electron diffraction (LEED). Then, the NbNx/GaN surface morphology was investigated ex-situ by scanning tunneling microscopy (STM).

Adsorption and Oxidation of CO on Co3O4/Ir(100) Thin Films

The combination of noble metals and reducible metal oxides often displays superior catalytic properties in low-temperature CO oxidation reactions. In the present study, we investigated the adsorption and activation of CO on submonolayer and monolayer CoOx films deposited on Ir(100) at low temperature under ultra-high vacuum (UHV) conditions. Using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), we found that the CoOx films have a (3 × 1) structure and consist of a Co–O bilayer and an O–Co–O trilayer associated with Co2+ and Co3+ states, respectively. A combination of XPS, temperature-programmed desorption (TPD), and infrared reflection absorption spectrum (IRAS) reveals that the presence of the interface of the submonolayer CoOx film and Ir significantly enhances the catalytic activity of cobalt oxide for CO oxidation. In contrast, a monolayer film of CoOx without exposing CoOx–Ir interface sites is hardly active for CO oxidation. CO adsorbs on the bridge sites of Ir atoms on the submonolayer CoOx/Ir(100) surface at 100 K. Upon heating, CO partially reacts with oxygen on the surface to produce CO2, which desorbs at 215 K, while unreacted CO molecules adsorbed on Ir bridge sites migrate to top sites. Increasing the temperature leads to CO2 desorption at 295 and 325 K, arising from the reaction of CO with chemisorbed oxygen on the Ir surface and oxygen of cobalt oxide, respectively. These results reveal the importance of the interface between Ir and cobalt oxide in the oxidation of CO.

2D honeycomb transformation into dodecagonal quasicrystals driven by electrostatic forces

Dodecagonal oxide quasicrystals are well established as examples of long-range aperiodic order in two dimensions. However, despite investigations by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), photoemission spectroscopy as well as density functional theory (DFT), their structure is still controversial. Furthermore, the principles that guide the formation of quasicrystals (QCs) in oxides are elusive since the principles that are known to drive metallic QCs are expected to fail for oxides. Here we demonstrate the solution of the oxide QC structure by synchrotron-radiation based surface x-ray diffraction (SXRD) refinement of its largest-known approximant. The oxide QC formation is forced by large alkaline earth metal atoms and the reduction of their mutual electrostatic repulsion. It drives the n = 6 structure of the 2D Ti2O3 honeycomb arrangement via Stone–Wales transformations into an ordered structure with empty n = 4, singly occupied n = 7 and doubly occupied n = 10 rings, as supported by DFT.

Structure and electronic properties of stable facets in the 2D material hexagonal boron nitride (hBN) on curved platinum

<h2>Abstract</h2> A hexagonal boron nitride (hBN) monolayer was grown on a curved crystal c-Pt(331) substrate including all vicinal surfaces between Pt(111) and Pt(110) faces. The surface structure has been studied by a combination of low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). We observed that the hBN monolayer covers the entire curved crystal, but induces a faceting subsequent to the growth. We encountered (111), (221), (441), (991), and (110) as stable facets. We assign this faceting to two factors i) a better lattice coincidence and ii) the existence of local covalent bonds and non-local Van der Waals interactions. The electronic structure was characterized by near-edge x-ray absorption fine structure (NEXAFS), X-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy (ARPES). The hBN/Pt(111) has the weakest interacting overlayer system, while the hBN on vicinal surfaces is more strongly bound to the Pt substrate. Additionally, we determined using angle-resolved photoemission measurements (ARPES) that the π band shifts downward when going away from the hBN/Pt(111) surface towards hBN/Pt(110). In the latter case, the π band shift is the largest.

Electronically ordered ultrathin Cr2O3 on Pt(1 1 1) in presence of a multidomain graphene intralayer

In the last decade, reducing the dimensionality of materials to few atomic layers thickness has allowed exploring new physical properties and functionalities otherwise absent out of the two dimensional limit. In this regime, interfaces and interlayers play a crucial role. Here, we investigate their influence on the electronic properties and structural quality of ultrathin Cr2O3 on Pt(111), in presence of a multidomain graphene intralayer. Specifically, by combining Low-Energy Electron Diffraction, X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy, we confirm the growth of high-quality ultrathin Cr2O3 on bare Pt, with sharp surface reconstructions, proper stoichiometry and good electronic quality. Once a multidomain graphene intralayer is included at the metal/oxide interface, the Cr2O3 maintained its correct stoichiometry and a comparable electronic quality, even at the very first monolayers, despite the partially lost of the morphological long-range order. These results show how ultrathin Cr2O3 films are slightly affected by the interfacial epitaxial quality from the electronic point of view, making them potential candidates for graphene-integrated heterostructures.