Atomic and nanostructures of monolayer c(2 × 2)NiN on Cu(0 0 1)

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.     

Ferromagnetism in Zn1−xCrxTe (x = 0.05, 0.15) films grown on GaAs(1 0 0) substrate

Zn_1−xCr_xTe (x = 0.05, 0.15) films were grown on GaAs(1 0 0) substrate by thermal evaporation method. X-ray diffraction analysis showed the presence of ZnCrTe phase without any secondary phase. The surface was analyzed by high resolution magnetic force microscope and profile measurements showed orientation of magnetic domains in the range of 0.5–2 nm with increase of Cr content. Magnetic moment–magnetic field measurements showed a characteristic hysteresis loop even at room temperature. The Curie temperature was estimated to be greater than 300 K. From the electron spin resonance spectra, the valence state of Cr in ZnTe was found to be +2 with d² electronic configuration. Hall effect study was done at room temperature and the result showed the presence of p-type charge carriers and hole concentration was found to increase from 5.95 × 10¹² to 6.7 × 10¹² m⁻³ when Cr content increases. We deduce the origin of ferromagnetic behavior based on the observed experimental results.     

Structure determination of the Cu(0 0 1)–c(4 × 4)-Sn surface by low-energy electron diffraction

Low-energy electron diffraction (LEED) have been used to determine the Cu(0 0 1)–c(4 × 4)-Sn structure formed at 300 K. It is demonstrated that a structural model suggested by scanning tunneling microscopy observations is correct: The model consists of one substitutional Sn atom and four Sn adatoms in the unit cell. Optimum parameters of the determined c(4 × 4) structure reveal that Sn adatoms laterally are displaced by 0.30 Å away from ideal fourfold-hollow sites along the 〈100〉 directions. It is proposed that such displacements of the Sn adatoms cause the formation of a network of octagonal rings on Cu(0 0 1). The substitutional Sn atom is located at each center of the octagonal rings. The formation conditions of the network are discussed.     

Chemical properties of structurally well-defined PdRu/Ru(0 0 0 1) surface alloys – Interaction with CO

The chemical properties of structurally well-defined PdRu/Ru(0 0 0 1) monolayer surface alloys [H. Hartmann, T. Diemant, A. Bergbeiter, J. Bansmann, H.E. Hoster, R.J. Behm, Surf. Sci. in press, doi:10.1016/j.sucs.2008.10.055.] and a Pd monolayer on Ru(0 0 0 1) were studied by temperature programmed desorption and infrared reflection absorption spectroscopy using CO as probe molecule. IR experiments on the PdRu/Ru(0 0 0 1) surface alloys demonstrate that CO adsorption on Ru sites resembles that on pure Ru(0 0 0 1) (on-top adsorption), while adsorption on the Pd sites occurs on both multifold coordinated and on-top sites, similar to CO on Pd(1 1 1). A significant destabilization of CO adsorption on Pd sites for both, surface alloys and the Pd monolayer film, compared to pure Pd(1 1 1) surfaces is attributed to a combination of geometric strain and vertical electronic ligand effects; an additional variation in the CO adsorption bond strength in the surface alloys is attributed to changes in the neighboring surface atom shell (lateral ligand effects). The chemical modifications introduced by PdRu surface alloy formation are compared with findings for deuterium adsorption on the same surface alloys; effects of the two-dimensional (2D) distribution of surface atoms are illustrated by comparison with CO adsorption on PtRu/Ru(0 0 0 1) surface alloys, where in contrast to the pronounced 2D phase segregation in PdRu/Ru(0 0 0 1) the surface atoms are essentially randomly distributed.     

Selective adsorption of coronene on Si(1 1 1)-(7 × 7)

The adsorption of coronene (C₂₄H₁₂) on the Si(1 1 1)-(7 × 7) surface is studied using scanning tunneling microscopy (STM). Upon room temperature submonolayer deposition, we find that the coronene molecules preferentially adsorb on the unfaulted half of the 7 × 7 unit cell. Molecules adsorbed on different sites can be induced to move to the preferential sites by the action of the tip in repeated image scans. Imaging of the molecules is strongly bias dependent, and also critically depends on the adsorption site. We analyze the results in terms of differential bonding strength for the different adsorption sites and we identify those substrate atoms which participate in the bonding with the molecule.     

First principles total energy calculations of the adsorption of germane and digermane on Si(0 0 1)-c(2 × 4)

First principles total energy studies are performed to investigate the energetics, and the atomic structure of the adsorption of germane (GeH₄), and digermane (Ge₂H₆) on the Si(0 0 1)-c(2 × 4) surface. It has been observed experimentally that adsorption of Ge₂H₆ is a dissociative process, which first yields GeH₃ and then GeH₂ fragments as products. We first study the adsorption of GeH₂ considering two different models; the intra-row and the on-dimer geometries. Our results show that the on-dimer site is more stable than the intra-row geometry by 0.44 eV. This is not a surprise since in the absence of H atoms, adsorption in the on-dimer site leaves no dangling bonds. In contrast, when the GeH₂ fragment is considered together with two H atoms, the intra-row geometry is favored energetically as compared with the on-dimer site, in good agreement with experiment. Similar results have been previously obtained for the adsorption of SiH₂ on Si(0 0 1). Digermane adsorption is explored according to two different geometries. In the first one, we have considered the adsorption as two GeH₃ fragments, while in the second, we have considered the adsorption as two GeH₂ fragments plus 2 H fragments. In good agreement with experiments, it is found that the latter geometry is energetically more favorable.     

Reversible and irreversible reactions of three oxygen precursors on InAs(0 0 1)-(4 × 2)/c(8 × 2)

The substrate reactions of three common oxygen sources for gate oxide deposition on the group III rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface are compared: water, hydrogen peroxide (HOOH), and isopropyl alcohol (IPA). Scanning tunneling microscopy reveals that surface atom displacement occurs in all cases, but via different mechanisms for each oxygen precursor. The reactions are examined as a function of post-deposition annealing temperature. Water reaction shows displacement of surface As atoms, but it does not fully oxidize the As; the reaction is reversed by high temperature (450 °C) annealing. Exposure to IPA and subsequent low-temperature annealing (100 °C) show the preferential reaction on the row features of InAs(0 0 1)-(4 × 2)/c(8 × 2), but higher temperature anneals result in permanent surface atom displacement/etching. Etching of the substrate is observed with HOOH exposure for all annealing temperatures. While nearly all oxidation reactions on group IV semiconductors are irreversible, the group III rich surface of InAs(0 0 1) shows that oxidation displacement reactions can be reversible at low temperature, thereby providing a mechanism of self-healing during oxidation reactions.     

Topmost-surface-sensitive Si-2p photoelectron spectra of clean Si(1 0 0)-2 × 1 measured with photoelectron Auger coincidence spectroscopy

Topmost-surface-sensitive Si-2p photoelectron spectra of a clean Si(1 0 0)-2 × 1 surface have been measured using Si-2p photoelectron Si-L₂₃VV Auger coincidence spectroscopy (Si-2p–Si-L₂₃VV PEACS). The escape depth of the PEACS electrons is estimated to be ～1.2 Å. The results support the assignments of the Si up-atoms, the Si down-atoms, the Si 2nd-layer, and the Si bulk proposed in previous researches. The Si-2p component with a binding energy of ?0.23 eV relative to the bulk Si-2p_3/2 peak, is shown to originate mainly from the topmost surface. Site selectivity of PEACS is indicated to be achieved to some degree by carefully selecting the kinetic energy of the Auger electrons. Since PEACS can be applied to any surface, the present study opens a new approach to identify PES components.     

H adsorption at Ag/Si interfaces in epitaxially grown Ag(1 1 1) films on Si(1 1 1)7 × 7 substrates

The interaction of atomic H with Ag(1 1 1)/Si(1 1 1)7 × 7 surfaces was studied by thermal desorption (TD) spectroscopy and scanning tunneling microscopy (STM) at room temperature. TD spectroscopy revealed an intense peak from mono H–Si bonds, even though the Si surface was covered by the Ag atoms. This peak was not observed from Ag-coated SiO₂/Si substrates. STM observation showed no clear change of the Ag surface morphology resulting from H exposure. All these results indicate that the atomic H adsorbs at neither the Ag surfaces nor Ag bulk sites, but at the Ag/Si interface by diffusing through the Ag film.     

Adsorption of small molecules on a (2 × 1) PdZn surface alloy on Pd(1 1 1)

The adsorption of methanol, formaldehyde, methoxy, carbon monoxide and water on a (2 × 1) PdZn surface alloy on Pd(1 1 1) has been studied using DFT calculations. The most stable adsorption structures of all species have been investigated with respect to the structure and the electronic properties. It was found that methanol is only weakly bound to the surface. The adsorption energy only increases with higher methanol coverage, where chain structures with hydrogen bonds between the methanol molecules are formed. The highest adsorption energy was found for the formate species followed by the methoxy species. The formaldehyde species shows quite some electronic interaction with the surface, however the stable η₂ formaldehyde has only an adsorption energy of about 0.49 eV. The calculated IR spectra of the different species fit quite well to the experimental values available in the literature.     

Interaction of Ag with the Si(1 1 1)1 × 1–H surface

Synchrotron radiation based photoemission spectroscopy (SRPES) and low energy electron diffraction (LEED) are used to study the interaction between Ag atoms and the Si(1 1 1)1 × 1–H surface. At an Ag coverage of 0.063 monolayers (ML) on the Si(1 1 1)1 × 1–H surface, the Si 2p component corresponding to Si–H bonds decreases, and an additional Si 2p component appears which shifts to a lower binding energy by 109 meV with respect to the Si bulk peak. The new Si 2p component is also observed for 0.25 ML Ag on the Si(1 1 1)7 × 7 surface. These findings suggest that Ag atoms replace the H atoms of the Si(1 1 1)1 × 1–H surface and form direct Ag–Si bonds. Contrary to the widely accepted view that there is no chemical interaction between Ag particles and the H-passivated Si surface, these results are in good agreement with recent first-principles calculations.     

Structural,electronic and energetic properties of water adsorbed on β-Si3N4 (0 0 0 1) surface: First-principles calculations

Structural, energetic and electronic properties of water molecules adsorbed on β-Si₃N₄ (0 0 0 1) surface, at various coverages, are investigated using density functional theory. At low coverages (θ ? 0.5), it is found that all H₂O molecules undergo spontaneous dissociation forming hydroxyl (OH) and imino (NH) groups where the reactive sites are identified, a result shown for the first time using ab initio theory. For higher coverages (θ > 0.5), only partial dissociation takes place where some of the molecules stay intact being bound via H-bond in good agreement with experimental findings. The driving force for the water dissociation has been identified to be dangling bonds on lower coordinated N and Si surface atoms showing that not all surface atoms are reactive corroborating with previous experimental findings.     

Raman scattering study of IrxRu1−xO2 mixed metal oxide nanowires (0 ≤ x ≤ 1)

Recent interests in mixed metal oxide nanostructured materials especially Ir_xRu_1−xO₂ compounds have been mainly driven by the technological application as electrocatalyst and electrode materials. We present room temperature Raman scattering results of single crystalline Ir_xRu_1−xO₂ (0 ≤ x ≤ 1) nanowires grown by atmospheric pressure chemical vapor deposition. We observed that the E_g, the A_1g, and the B_2g phonon modes of a single Ir_xRu_1−xO₂ nanowire are blue-shifted linearly with respect to the Ir contents from which we could get stoichiometry information. We also observed that the asymmetric lineshape and the broadening of the full width at half maximum of the E_g mode that involves the out-of-plane oxygen vibration. The unusual asymmetric broadening of the E_g phonon can be explained by the activation of the non-zone-center phonons due to substitutional disorder present in the system. We also found that there is a mixed mode of the A_1g and the B_2g phonons due to the substitutional disorder, in the range of 630–750 cm⁻¹.     

α-Sexithiophene on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O: An STM and NEXAFS study

The adsorption of α-sexithiophene (6T) on Cu(1 1 0), Cu(1 1 0)-(2 × 1)O and the mesoscopically patterned Cu-O striped surface have been studied by STM (scanning tunnelling microscopy), XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure). The molecular resolution of the STM allowed to determine the orientation and local order of the molecules in the submonolayer and monolayer regime. It is shown that the 6T molecules align with their long molecular axis along the densely packed copper rows on Cu(1 1 0) and along the Cu-O rows on the Cu(1 1 0)-(2 × 1)O surface. On the striped phase with alternating copper and Cu-O regions the molecules adsorb first on the Cu regions and after complete filling of these regions, on the Cu-O. The orientation is the same on both areas as on the respective pristine surfaces with the only exception that the molecules reorient by 90° if the width of the copper regions is smaller than the molecular length. The NEXAFS measurements allowed for a determination of the adsorption geometry of the molecules: while 6T lies flat on the surface on clean copper, the molecular planes are inclined with an angle as high as 39° with respect to the substrate on (2 × 1)O. For the latter, this inclination angle is 4° higher than in the bulk crystal structure of 6T observed for thicker films to release stress and allow commensurability with the substrate lattice, while for the former it is a result of the aromatic system bonding to the Cu(1 1 0) surface, as confirmed by XPS.     

Growth of 〈1 0 0〉 directed ADP crystal with slotted ampoule

Good quality ammonium dihydrogen phosphate single crystals have been grown by: (i) Sankaranarayanan–Ramasamy (SR) method and (ii) SR method with slotted ampoule. The grown crystals were subjected to UV–Vis spectroscopy, high-resolution X-ray diffractometer, dielectric, piezoelectric and laser damage threshold studies. Compared to the (1 0 0) plane of the conventional method grown ADP crystal and 〈1 0 0〉 directed SR method grown ADP crystal, the crystal grown by SR method with slotted ampoule has higher growth rate, higher optical transparency, high crystalline perfection, low dielectric loss, high piezoelectric charge coefficient and high laser damage threshold due to diffusion of segregated impurities away from the growing crystal in the slotted ampoule growth.     

Atomic structure of the (3 × 2) Si–GaAs (0 0 1) reconstructed surface: A clue to δ doping mechanism derived from in situ grazing incidence X-ray diffraction data

In view of understanding silicon incorporation in the δ doping process of GaAs (0 0 1), Si atoms have been deposited, under UHV, on a α(2 × 4) arsenic terminated substrate. In the low coverage regime, a transition to a less As rich (3 × 2) reconstructed Si–GaAs (0 0 1) surface was observed whose atomic structure has been investigated by grazing incidence X-ray diffraction performed in situ. Silicon is found to occupy not only a Ga substitutional site, precursor of a donor dopant but also to form nuclei for neutral clusters, on a template made by the (3 × 2) GaAs (0 0 1) reconstructed surface observed by Martrou et al. [Phys. Rev. B 72 (2005) 241307®]. The maximum surface concentration of donor-like silicon is estimated at 1.04 × 10¹⁴ cm^?2 (1/6th monolayer).     

Preparation,surface state and band structure studies of SrTi(1 − x)Fe(x)O(3 − δ) (x = 0–1) perovskite-type nano structure by X-ray and ultraviolet photoelectron spectroscopy

In this report, SrTi_(1 ? x)Fe_(x)O_(3 ? δ) photocatalyst powder was synthesized by a high temperature solid state reaction method. The morphology, crystalline structures of obtained samples, was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM), respectively. The electronic properties and local structure of the perovskite STF_x (0 ≤ x ≤ 1) systems have been probed by extended X-ray absorption fine structure (EXAFS) spectroscopy. The effects of iron doping level x (x = 0–1) on the crystal structure and chemical state of the STF_x have been investigated by X-ray photoelectron spectroscopy and the valence band edges for electronic band gaps were obtained for STF_x by ultraviolet photoelectron spectroscopy (UPS). A single cubic perovskite phase of STF_x oxide was successfully obtained at 1200 °C for 24 h by the solid state reaction method. The XPS results showed that the iron present in the STF_x perovskite structure is composed of a mixture of Fe³⁺ and Fe⁴⁺ (SrTi_(1 ? x)[Fe³⁺, Fe⁴⁺]_(x)O_(3 ? δ)). When the content x of iron doping was increased, the amount of Fe³⁺ and Fe⁴⁺ increased significantly and the oxygen lattice decreased on the surface of STF_x oxide. The UPS data has confirmed that with more substitution of iron, the position of the valence band decreased.