The (2 × 2) reconstructions on the SrTiO3 (001) surface: A combined scanning tunneling microscopy and density functional theory study

Scanning tunneling microscopy study showed that the (2 × 2) reconstruction on the (001) surface of SrTiO₃ should have a surface structure with a 4-fold symmetry. The previously proposed solution for the (2 × 2) reconstruction with the p2gm symmetry only has a 2-fold symmetry. In this study density functional theory study was carried out to propose a possible surface structure with the p4mm surface symmetry which matches the scanning tunneling microscopy images and suggests that two different (2 × 2) surface structures exist. The formation of the (2 × 2) reconstruction with the p4mm symmetry may be due to the kinetics as it has slightly higher surface energy than the one with the p2gm symmetry.     

Hydrogen chemisorption on Si(111)√ 3×√ 3R30∘-B passivated surface studied by thermal desorption and scanning tunneling microscopy

Atomic H chemisorption on the Si(111)√ 3×√ 3R30^°-B surface has been studied by thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The B-modified Si surface is known to be inert towards adsorbates, since the surface dangling bonds of Si adatoms are passivated by B atoms sitting in sub-surface sites. However, it was found that even on a perfectly passivated surface, H is adsorbed on the surface by destroying the original √ 3 × √ 3 structure. STM observations revealed that H exposure led to the creation of defects at surface sites, and H was subsequently adsorbed as Si-monohydride at these sites. H exposure also caused cluster island formation at the top surface. The islands are composed of hydrogenated amorphous Si atoms or B-hydrogen complexes.     

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.     

3C-SiC(001)-3 × 2 reconstructed surface analyzed by high-resolution medium energy ion scattering

The atomic structure of the 3C-SiC(001)-3 × 2 reconstructed surface was analyzed precisely by high-resolution medium energy ion scattering (MEIS). The present MEIS analysis unambiguously shows that the (3 × 2) surface consists of Si adatoms (1/3 ML, 1 ML = 1.05 × 10¹⁵ atoms/cm²) on top and underlying Si adlayer (2/3 ML) on the bulk truncated Si plane. As the result, the most probable structure is focused on the Two Adlayer Asymmetric Dimer Model predicted by ab initio calculations and the modified versions with alternating long and short dimers in the 2nd adlayer proposed by photoelectron diffraction (PED) and by grazing incidence X-ray diffraction (GIXRD) analyses. Observed MEIS spectra are well reproduced by the structure relatively close to that determined by PED rather than GIXRD. Interestingly, the first principle calculations using VASP (Vienna ab initio simulation package) prefer symmetric dimers in the second Si adlayer and non-relaxed interplanar distance between the top Si and 2nd C plane of the bulk-truncated surface, which are, however, unable to reproduce the observed MEIS spectra. The distorted 2nd adlayer (asymmetric dimers) may correlate with the compressed interplanar distance between the underlying Si and C planes.     

Scanning tunneling microscopy imaging of NiO(100)(1 × 1) islands embedded in Ag(100)

The imaging of NiO(100)(1 × 1) islands embedded in Ag(100) by scanning tunneling microscopy is addressed. As a function of tunneling conditions and tip termination it is possible to resolve the NiO–vacuum interface, the second oxide layer as well as the NiO-substrate interface with atomic contrast. We find that for sub-monolayer coverages of NiO the oxide islands consist of an essentially defect-free surface layer at the vacuum interface with a number of NiO second layer patches incorporated into the Ag substrate underneath. The oxide layer is surrounded by a rim of a NiO bilayer of monoatomic width. A reduction of the density of states between a NiO monolayer and local NiO bilayer stackings is suggested to be responsible for the observed appearance of mosaic patches at the island surface.     

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.     

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.     

Vacancy creation on the Si(111)−7 × 7 surface due to sulfur desorption studied by scanning tunneling microscopy

Ultra high vacuum scanning tunneling microscopy (STM) has been used to study the adsorption and subsequent thermal desorption of a sulfur overlayer deposited in situ, at room temperature, on the Si(111)-7 × 7 surface. Little ordering is visible in the STM images of this overlayer 1–2 monolayers thick, with the underlying silicon surface retaining the (7 × 7) reconstruction with weakened low energy electron diffraction (LEED) spot intensity. STM images of this surface following a thermal anneal at 375°C revealed the presence of a number of monolayer deep “holes” or voids in the (7 × 7) surface. The appearance of these voids is consistent with a coalescence of vacancy defects induced by the sulfur desorption process as also observed for oxygen induced etching of the silicon surface. In addition, we have observed small regions of the (√3 × √3)R30° and c(4 × 2) reconstructions within areas exhibiting a high degree of surface disorder, with the metastable (5 × 5) reconstruction also being found. Both reconstructions and disorder are attributed to vacancy diffusion to step edges causing a redistribution of surface silicon atoms.     

A scanning tunneling microscopy investigation of the 1 × 2 ⇌ 1 × 1 structural transformation of the pt(110) surface

Lifting of the reconstruction of the clean Pt(110) surface under the influence of adsorbed CO proceeds at 300 K through homogeneous nucleation of small holes (with about 10 × 10 Ų size). At 350 K more correlated displacements of [110] strings take place, a characteristic shared with the reverse process, namely the restoration of the 1× 2 phase after desorption.     

Metallization of the β-SiC(100) 3 × 2 surface: A DFT investigation

Using density functional theory (DFT) we report results for the electronic structure and vibrational dynamics of hydrogenated silicon carbide (001) (3 × 2) surfaces with various levels of hydrogenation. These results were obtained using density functional theory with a generalized gradient exchange correlation function. The calculations reveal that metallization can be achieved via hydrogen atoms occupying the second silicon layer. Further increase of hydrogen occupation on the second silicon layer sites results in a loss of this metallization. For the former scenario, where metallization occurs, we found a new vibrational mode at 1870 cm^? 1, which is distinct from the mode associated with hydrogen atoms on the first layer. Furthermore, we found the diffusion barrier for a hydrogen atom to move from the second to the third silicon layer to be 258 meV.     

Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1 − x) K0.5Na0.5NbO3 − xLiNbO3 ceramics

(1 ? x) K_0.5Na_0.5NbO₃ ? xLiNbO₃ (where x = 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) (KNLN) perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that single phase was formed for pure KNN while a small amount of second phase (K₆Li₄Nb₁₀O₃₀, ～3%) was present in LN doped KNN ceramics. Phase analysis indicated the change in the crystal structure from orthorhombic to tetragonal with increase in LN content. The electrical behavior of the ceramics was studied by impedance spectroscopy technique in the high temperature range. Impedance analysis was performed using an equivalent circuit model. The impedance response in pure KNN and KNLN ceramics could be deconvoluted into two contributions, associated with the bulk (grains) and the grain boundaries. Activation energies for conductivity were found to be strongly frequency dependent. The activation energy obtained from dielectric relaxation data was attributed to oxygen vacancies. From PFM we found that the composition with 6.5 wt.% LN displays stronger piezocontrast as compared to pure KNN implying an evidence of a pronounced piezoelectric coefficient.     

Microstructural analysis of La2/3 − xLi3xTiO3 single crystals and quenched samples observed by high resolution transmission electron microscopy

The La ordering and domain structures of La_2/3 ? xLi_3xTiO₃ single crystals and quenched samples have been observed by high resolution electron microscopy. In single crystals, the grown single crystals are not real single crystals with mono-domain, but crystals with 90° oriented domains with 20–100 nm sizes. A remarkable difference of the domain size between polycrystals and single crystals has not been observed at the same composition. In quenched samples, the La disordering has been observed. Furthermore, domain structures of quenched samples are different from those of furnace-cooled ones. Domain structures change with the La disordering. The understanding of microstructures is necessary to clarify the Li-ion conduction mechanism.     

Reaction of tert-butyl isocyanate and tert-butyl isothiocyanate at the Ge(100) − 2 × 1 Surface

The adsorption of tert-butyl isothiocyanate and tert-butyl isocyanate at the Ge(100) ? 2 × 1 surface was probed using multiple internal reflection Fourier transform infrared (FTIR) spectroscopy X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling. Results indicate that there are multiple surface products for each molecule. FTIR studies of tert-butyl isothiocyanate reveal adsorption through an S-dative bonded state, while XPS studies further suggest a reactive desorption product which leaves excess sulfur atoms at the surface. Studies of tert-butyl isocyanate indicate that the molecule dissociatively adsorbs at the surface, resulting in tert-butyl and germyl isocyanate groups, as the major pathway, in addition to forming several minor products, including a [2 + 2] cycloaddition product across the C=N bond. DFT was used to simulate vibrational spectra and map the reaction pathways, and confirms that the assigned products are energetically favorable.     

Cooperative phenomena in self-assembled nucleation of 3 × 4-In/Si(100) surface magic clusters

Using statistical analysis of the scanning tunneling microscopy images of the 3 × 4-In surface magic cluster (SMC) arrays on Si(100)1 × 2 substrate at low In coverages, the main regularities of the space distribution of the nucleated SMCs have been established. It has been found that a nucleated SMC perturbs the surface potential relief within a limited zone around itself in a way that some of the sites in the zone demonstrate a great preference for nucleating a new cluster, while in the other sites the nucleation is greatly suppressed. Outside the zone, the nucleation probability is close to that of the random nucleation simulated using Monte-Carlo technique. Energetic background of the observed phenomena has been proved using density-functional theory calculations, which clearly demonstrate that the lower the formation energy the higher the occurrence probability of a given cluster-pair configuration.     

Bonding geometry of Mn-wires on the Si(100)(2 × 1) surface

The bonding geometry of monoatomic Mn-wires, which form on the reconstructed Si(100)(2 × 1) surface at room temperature, was investigated with scanning tunneling microscopy (STM). The Mn-wire structures are always perpendicular to the Si-dimer rows and the images exhibit a strong modulation of their apparent height as a function of bias voltage. The Mn-wire structures appear as depressions in the empty state images for bias voltages around 0.7 V, and as protrusions for all other bias voltages. It is suggested that the wire-images are defined by mixed Mn-Si states, either through a hybridization between the Mn d-states and the Si-p states, or backbonding from Mn-d electrons into the broken Si-dimer bond. The dominant bonding geometry shows that the Mn-wire maxima are positioned in between the Si-dimer rows, and a small percentage of about 20% is in registry with the Si-dimer rows, and might be described as defective wires. The experimental STM images cannot currently be described in a satisfactory manner with theoretical bonding models from the literature.     

Passivation effect of allylamine molecule on the electronic structure of a Si(001) − (2 × 1) surface

The chemisorption of the allylamine molecule, which contains two functional groups (ethenyl and hydroxyl), on a Si(001) ? (2 × 1) surface was studied using density functional theory (ab-initio DFT) based on the pseudopotential approach. In particular, we focused on the determination of the most stable position of the CC double bond in the ethenyl group and observation of the passivation effect of allylamine on the electronic structure of the clean Si(001) ? (2 × 1) phase. For this purpose, all of the possible interaction mechanisms occurring at the interface were considered: (i) dissociative bonding where the CC bond is parallel to the silicon surface, (ii) dissociative bonding where the CC bond is perpendicular to the silicon surface, and (iii) the [2 + 2] CC cycloaddition reaction. From our total energy calculations, it was found that the bifunctional allylamine molecule attached to the Si(001) ? (2 × 1) surface through the amino functional group, by breaking the N–H bond and forming a Si–H bond and Si–NHCH2CHCH2 surface fragments. During this process, the ethenyl functional group remains intact, and so can be potentially used as an extra reactive site for additional chemical interactions. In addition to these findings, the nudged elastic band method (NEB) calculations related with the reaction paths showed that the parallel position of the CC bond with respect to the surface of the substrate is more favorable. In order to see the influence of the chemisorbed allylamine molecule on the surface states of the clean Si(001) – (2 × 1), we also plotted the density of states (DOS), in which it is seen that the clean Si(001) – (2 × 1) surface was passivated by the adsorption of allylamine.     

Monitoring surface reactions with scanning tunneling microscopy: CO oxidation on p(2 × 1)-O pre-covered Cu(110) at 400 K

Scanning tunneling microscopy (STM) is used to study the oxidation of CO on O pre-covered Cu(110) in the steady state at 400 K at the atomic level. There is a strong preference for CO to react with oxygen along the p(2 × 1) oxygen rows. The reaction appears to occur initially at the outer edge of an oxygen island, creating defects in the overlayer structure. Once created, oxygen overlayer defects are more reactive than non-defect sites and play a dominant role in sustaining the reaction. Copper atoms that make-up the p(2 × 1) oxygen overlayer structure add to terrace edges after oxygen supply is exhausted and do not form small Cu islands that could eventually lead to surface roughening.