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.     

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.     

Strained c(4 × 2) CoO(1 0 0)-like monolayer on Pd(1 0 0): Experiment and theory

We report on an interface-stabilized strained c(4 × 2) phase formed by cobalt oxide on Pd(1 0 0). The structural details and electronic properties of this oxide monolayer are elucidated by combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4 × 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(1 0 0) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (～9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(1 0 0) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 × 2) structures have been previously found for nickel and manganese oxides on the same substrate.     

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.     

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.     

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.     

Real time dynamics of Si magic clusters mediating phase transformation: Si(111)-(1 × 1) to (7 × 7) reconstruction revisited

Using Scanning Tunneling Microscope (STM), we show that the surface undergoes phase transformation from disordered “1 × 1” to (7 × 7) reconstruction which is mediated by the formation of Si magic clusters. Mono-disperse Si magic clusters of size ~ 13.5 ± 0.5 Å can be formed by heating the Si(111) surface to 1200 °C and quenching it to room temperature at cooling rates of at least 100 °C/min. The structure consists of 3 tetra-clusters of size ~ 4.5 ? similar to the Si magic clusters that were formed from Si adatoms deposited by Si solid source on Si(111)-(7 × 7) [1]. Using real time STM scanning to probe the surface at ~ 400 °C, we show that Si magic clusters pop up from the (1 × 1) surface and form spontaneously during the phase transformation. This is attributed to the difference in atomic density between “disordered 1 × 1” and (7 × 7) surface structures which lead to the release of excess Si atoms onto the surface as magic clusters.     

TiO2(110)-(1 × 1) supported Cu particles: An FT-RAIRS investigation

The morphology of TiO₂(110)-(1 × 1) supported Cu particles has been investigated by Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS), employing adsorbed CO as a probe molecule sensitive to local surface structure. For Cu coverage (deposited at 300 K) less than 2.85 MLE nucleated Cu particles in the range 2 nm–4 nm are formed, as indicated by a final state shift in the core level Cu(2p_3/2) binding energy and by the existence of only transmission bands in the FT-RAIRS spectra for adsorbed CO. ν_S(CO) indicates that these small particles expose sites similar to those of the stepped Cu surfaces Cu(211), Cu(311), and Cu(755). At Cu coverages in the range of 6 MLE and above, corresponding to particle sizes above 4.6 nm, ν_S(CO) indicates the predominance of (110), (100) and (111) adsorption sites. Annealing the Cu layers to 650 K results in the slight growth of the particle sizes, and transformation of the CO adsorption sites corresponding to the close packed facets. The transformation of the local dielectric from that of titania to that dominated by the Cu particle is shown to take place between 3.7 and 4.2 nm, and this change is also to a smaller extent sensitive to the dispersion of the particles.     

Coverage dependence of glycine adsorption on the Ge(100) − 2 × 1 surface

A combination of infrared spectroscopy, X-ray photoelectron spectroscopy and density functional theory has been used to investigate the adsorption behavior of glycine at the Ge(100) ? 2 × 1 surface under ultrahigh vacuum conditions. Comparison of experimental and simulated IR spectra indicates that at 310 K, glycine adsorbs on Ge(100) ? 2 × 1 via O–H dissociation, with some fraction of the products also forming an N dative bond to a neighboring germanium atom. O–Ge dative bonding is not observed. As coverage increases, the surface concentration of the monodentate O–H dissociated adduct increases, while that of the N dative-bonded species appears constant. XPS data support and clarify the IR findings and reveal new insights, including the presence at higher coverage of a minor product that has undergone dual O–H and N–H dissociation. These findings are supported by the calculated energy diagrams, which indicate that the reaction of a glycine molecule on the Ge(100) ? 2 × 1 surface via O–H dissociation and interdimer N dative bonding is both kinetically and thermodynamically favorable and that N–H dissociation of this adduct is feasible at room temperature given incomplete thermal accommodation along the reaction pathway.     

Pressure and compositional dependence of electric conductivity in the (Mg1 − xFex)1 − δO (x = 0.01–0.40) solid-solution

(Mg_1 ? xFe_x)_1 ? δO (x = 0.01–0.43) single crystals (~ 8 mm in diameter) were made by a melt-growth method. Electrical conductivity measurements were carried out as functions of temperature and frequency by a complex impedance method under pressure (~ 43 GPa and ~ 673 K and at 0.1 MPa and ~ 1400 K). Our experimental results show a change in charge transport mechanism in the (Mg_1 ? xFe_x)_1 ? δO solid solution at high temperature. The temperature of inflection point of the slope in Arrhenius plots depend greatly on both composition and extrinsic factors of crystals. The low-temperature conduction mechanism in (Mg_1 ? xFe_x)_1 ? δO solid solution is small polaron. Pressure effect of the electric conductivity was observed and the conductivity increased to 0.5 at log scale of S/m with increasing pressure up to 43.4 GPa. The activation energy was decreased linearly with increasing pressure. Chemical composition and homogeneity of specimen rather than pressure greatly influence the electric conductivity. The activation energy of 2.37(4) eV for the (Mg_0.99Fe_0.01)_1 ? δO solid solution might correspond to a migration enthalpy of O ions through thermally formed defects. It is proposed that a possible dominant electrical conduction mechanism in ferropericlase under the lower mantle conditions, at least in the higher temperature region, is super ionic conduction.     

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.     

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.     

Tin-stabilized (1 × 2) and (1 × 4) reconstructions on GaAs(100) and InAs(100) studied by scanning tunneling microscopy,photoelectron spectroscopy,and ab initio calculations

Tin (Sn) induced (1 × 2) reconstructions on GaAs(100) and InAs(100) substrates have been studied by low energy electron diffraction (LEED), photoelectron spectroscopy, scanning tunneling microscopy/spectroscopy (STM/STS) and ab initio calculations. The comparison of measured and calculated STM images and surface core-level shifts shows that these surfaces can be well described with the energetically stable building blocks that consist of Sn–III dimers. Furthermore, a new Sn-induced (1 × 4) reconstruction was found. In this reconstruction the occupied dangling bonds are closer to each other than in the more symmetric (1 × 2) reconstruction, and it is shown that the (1 × 4) reconstruction is stabilized as the adatom size increases.     

Ga-induced restructuring of Si(5 5 12) − 2 × 1 reconstructed surface at room temperature

Motivated by the need to form 1D-nanostructured dopants on silicon surfaces, we have attempted to grow Ga on the high index Si(5 5 12) surface which has a highly trenched (1D) morphology. The evolution of the interface with Ga adsorption in the monolayer regime has been probed by in situ AES, LEED and EELS. Controlling the kinetics by changing the Ga flux rates shows an interesting difference in the 1.0 to 1.5 ML region. The low flux rate (0.03 ML/minute) results in a Frank van der Merwe (layer by layer) growth mode up to 2 ML, while the higher flux rate (0.1 ML/minute) shows a transient island formation after the completion of 1 ML. The low rate shows the formation of 2 × (3 3 7) and (2 2 5) superstructures, while only the 2 × (3 3 7) is observed in a wide coverage range for the higher rate. The results demonstrate the ability to kinetically control the surface phases with different electronic properties of this technologically important interface.     

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.