Evidence of substrate metallization by Li adsorption on the Si(001) surface

We report an evidence of substrate metallization induced by Li adsorption on the Si(001) surface, based on the combined results of electron energy-loss (EEL) and angle-resolved photoemission (ARP) measurements. The metallic surface at a low dose of Li manifests itself as a loss peak due to an intraband surface plasmon in EEL spectra and a metallic peak in ARP spectra. These peaks are coherently understood in terms of substrate metallization, where electrons from Li adatoms partially occupy the empty substrate surface bands. Furthermore, the unique negative dispersion of the plasmon reveals that local field effects may cause such an anomalous dispersion.     

Confining barriers for surface state electrons tailored by monatomic Fe rows on vicinal Au111 surfaces

We fabricated monatomic Fe wires on vicinal Au(111) surfaces and found that decoration of step edges with Fe adatoms has a significant influence on the behavior of surface state electrons confined between regularly arranged steps. On a surface with Fe monatomic rows, angle-resolved photoemission spectra measured in the direction perpendicular to the steps shows parabolic dispersion, in contrast to one-dimensional quantum-well levels observed on a clean surface. Simple analysis using a one-dimensional Kronig-Penney model reveals potential barrier reduction from 20 to 4.6 eV A, suggesting an attractive nature of the Fe adatoms as scatterers.     

Negative‐Imaging of Citrate Layers on Gold Nanoparticles by Ligand‐Templated Metal Deposition: Revealing Surface Heterogeneity

Surface heterogeneity of a metal nanoparticle is typically regarded as boundary defects and various crystalline facets. While organic capping ligands of a single type are assumed to be homogeneously distributed on the nanoparticle surface, heterogeneous surface coverage of citrate molecules on individual facets of gold nanoparticles (AuNPs) is revealed. Pt metallic clusters with 2 nm in diameter, epitaxially grown on the surface of AuNPs by chemical reaction and imaged by high‐resolution transmission electron microscopy, are utilized as negative‐imaging probes for densely packed adlayers where the underneath gold surface may not be accessible for Pt deposition. At pH > 5.0, citrate anions form only a loosely packed layer. At pH 4.5, citrates and citric acids form both loosely packed and densely packed layers that appear phase separated, and the densely packed domain as small as 5 nm × 5 nm is likely composed of fully protonated citric acids. IR spectra indicate that citric acid binds to a surface Au adatom through the oxygen atom of the central hydroxyl group, and similarly, citrate anions bind to Au adatoms through the carboxylate oxygen atom. This study also reveals the role of Au adatom in the adsorption of citrate species on the metallic surface of AuNPs.     

Gold-adatom-mediated bonding in self-assembled short-chain alkanethiolate species on the Au(111) surface

Microscopic evidence for Au-adatom-induced self-assembly of alkanethiolate species on the Au(111) surface is presented. Based on STM measurements and density-functional theory calculations, a new model for the low-coverage self-assembled monolayer of alkanethiolate on the Au(111) surface is developed, which involves the adsorbate complexes incorporating Au adatoms. It is also concluded that the Au(111) herringbone reconstruction is lifted by the alkanethiolate self-assembly because the reconstructed surface layer provides reactive Au adatoms that drive self-assembly.     

Array of double Au-Ag chains on the Si(5 5 7) surface

Using scanning tunneling microscopy we study the topographic properties of Ag structure on the Au induced, highly ordered Si(5 5 7) surface. Topography measurements show that a small amount of Ag (0.25 ML) deposited on that surface leads to considerable modifications of the one-dimensional structure induced by Au atoms. In particular, we observe two different chains on each terrace, which are identified as Si adatoms and Ag chain structures. The STM topography of those chains strongly depends on the bias voltage, indicating an important role of electronic effects in this system.     

Molecular dynamics simulation of nanoscale surface diffusion of heterogeneous adatoms clusters

Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters(hexamer and above), the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster,vacancy formation, filling, and shifting can be observed from the results.     

On the charge transfer in the “single-sheet graphene-intercalated metal layer-SiC substrate” system

The proposed scheme for the consideration of charge transfer in the three-layer Gr/Me/SiC system (where Gr is a single-sheet graphene, Me is an intercalated metal layer, and SiC is a substrate) contains three stages. At the first stage, a metal monolayer adsorbed on silicon carbide is considered and the charge of adatoms in this monolayer is calculated. At the second stage, the shift of the Dirac point of free-standing single-layer graphene in an electrostatic field induced by charged adatoms of the monolayer is estimated. At the third stage, a weak interaction between Me/SiC and free-standing graphene is included, which allows electrons to tunnel but does not significantly distort the density of states of free-standing graphene. Estimations are performed for n- and p-type 6H-SiC(0001) substrates and Cu, Ag, and Au layers. The charge state of the graphene sheet and the shift of the Dirac point with respect to the Fermi level of the system are calculated. A comparison with the available experimental and theoretical results shows that the proposed scheme works quite satisfactorily.     

Electronic stabilization of the Si(111)5 × 2-Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2-Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2-Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.     

Diffusion of single adatoms of platinum,iridium and gold on platinum surfaces

Diffusion of single adatoms of platinum iridium and gold on platinum surfaces has been studied experimentally using field ion microscopy and theoretically using Morse potentials to describe the adatom bonding. The order of increasing activation energies for the surfaces studied was (111), (113), (011), (133), (001) and calculated activation energies agreed satisfactorily with the experimental values. On (113) and (011) platinum the activation energies for the adsorbates increased in the order Au, Pt, Ir. The calculations show that this variation arises largely from differences in the adsorbate binding energy, with the differences in adatom size having little effect. Diffusion of Au on (011) platinum occurred only along the direction of the surface channels, as is to be expected from the surface structure. In contrast, Pt and Ir adatoms were found to diffuse in two dimensions on (011). Since direct inter-channel jumps by adatoms should require large activation energies, it is proposed that the observed inter-channel adatom transfers proceed by a surface vacancy mechanism.     

Medium energy ion scattering investigation of methylthiolate-induced modification of the Au(111) surface

100 keV H⁺ scattering has been used to investigate the structure of the methylthiolate/Au(111) interface in the Au(111)(√3 × √3)R30° phase. Adsorption of the thiolate onto the clean Au(111) surface leads to a large drop in the scattered ion yield due to the lifting of the clean surface ‘herring-bone’ reconstruction, but the thiolate-covered surface shows an ion yield higher than that of an unreconstructed Au(111) surface, providing direct evidence of a significant number of Au atoms that are displaced from their bulk-terminated positions at the buried interface. Simulations for two different Au adatoms models at the interface, namely, the Au-adatom-monothiolate (AAM) and Au-adatom-dithiolate (AAD) models, show significant sensitivity to the exact values of interlayer spacings and atomic vibrational amplitudes, but the comparison with experimental results appears to favour the AAD model with 0.17 ML Au adatoms in bridging sites at the interface.     

Point defects along metallic atomic wires on vicinal Si surfaces: Si(5 5 7)–Au and Si(5 5 3)–Au

Point defects on the metallic atomic wires induced by Au adsorbates on vicinal Si surfaces were investigated using scanning tunneling microscopy and spectroscopy (STM and STS). High-resolution STM images revealed that there exist several different types of defects on the Si(5 5 7)–Au surface, which are categorized by their apparent bias-dependent images and compared to the previous report on Si(5 5 3)–Au [Phys. Rev. B (2007) 205325]. The chemical characteristics of these defects were investigated by monitoring them upon the variation of the Au coverage and the adsorption of water molecules. The chemical origins and the tentative atomic structures of the defects are suggested as Si adatoms (and dimers) in different registries, the Au deficiency on terraces, and water molecules adsorbed dissociatively on step edges, respectively. STS measurements disclosed the electronic property of the majority kinds of defects on both Si(5 5 7)–Au and Si(5 5 3)–Au surfaces. In particular, the dominating water-induced defects on both surfaces induce a substantial band gap of about 0.5 eV in clear contrast to Si adatom-type defects. The conduction channels along the metallic step-edge chains thus must be very susceptible to the contamination through the electronic termination by the water adsorption.     

Scattering of Cu(1 0 0) image state electrons from single Cu adatoms and vacancies: A comparative study

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of defects (Cu adatoms and Cu vacancies) is presented for low defect density at the surface. A wave packet propagation approach is employed for the electron scattering calculations, where the defect induced potentials are obtained from an ab initio density functional study. Scattering of the image state electron by a defect induces inter-band and intra-band transitions leading, respectively, to the population decay and to the dephasing of the image states. Comparison of the respective effects of adatoms and vacancies shows that Cu adatoms are much more efficient in inducing population decay and dephasing of the image potential states. Present results for the case of Cu adatoms are compared with available time-resolved two-photon photoemission data.     

Structure of quantum wires in Au/Si(557)

The structure of the Au/Si(557) surface is determined from three-dimensional x-ray diffraction measurements, which directly mandate a single Au atom per unit cell. We use a "heavy atom" method in which the Au atom images the rest of the structure. Au is found to substitute for a row of first-layer Si atoms in the middle of the terrace, which then reconstructs by step rebonding and adatoms. The structure is consistent with the 1D metallic behavior seen by photoemission.     

Corner hole adatom stacking fault structure of Bi on Au(1 1 1)

The surface atomic structure of Bi on Au(1 1 1) is studied with scanning tunneling microscopy. At about 0.5 monolayer of Bi, a well-ordered 6 × 6 atomic structure is observed. The structure has three notable features: corner holes, Bi adatoms, and stacking faults, very similar to a semiconductor surface of Si(1 1 1)-7 × 7. Out of 18 Bi surface atoms in a unit cell, six atoms are at hollow sites and are adatoms, and another six atoms are near-bridge sites. The last six atoms surround corner holes and are lower than other surface atoms by about 0.2 Å. A possible atomic model is proposed based on our observation.     

Scattering by Cu adatoms between image-potential bands on Cu(001)

With the increasing resolution and sensitivity of photoelectron spectroscopy, the influence of defects is becoming more and more obvious. Scattering processes induced by adsorbate atoms can be studied by time- and angle-resolved two-photon photoemission. We have examined the dynamics of electrons in image-potential states on the Cu(001) surface for statistically distributed Cu adatoms and have identified different scattering mechanisms. Scattering of electrons from the second (n=2) to the bottom of the first (n=1) image-potential band is observed, which we attribute to inelastic interband scattering with electrons in the bulk. At energies above the bottom of the n=2 band, resonant interband scattering from the n=2 to the n=1 image-potential band is found. The rate for these processes can be determined by modeling the time-resolved measurements via optical Bloch equations of a four-level system. Comparison of the transition and decay rates reveals that the decay rate of the n=2 electrons is almost exclusively changed by additional resonant interband-scattering processes upon adsorption. PACS 73.20.At; 79.60.Ht; 68.49.Jk     

Theoretical calculations of dissociative adsorption of CH4 on an Ir(111) surface

The activation energy for chemisorption of CH(4) at an Ir(111) surface is determined using density functional theory combined with an estimate of the long range dispersion interaction. The results are found to be in good agreement with published results of bulb and beam experiments analyzed with a precursor model. A surprisingly large surface relaxation is found where an Ir surface atom is displaced outwards by as much as 0.6 A. A strongly bound molecular state at kinks and adatoms involving eta(2)-H,H bonding was also found.     

Effect of atomic impurities on the helical surface states of the topological insulator Bi2Te3

The effect of atomic impurities including N, O, Na, Ti and Co on the surface states of the topological insulator (TI) Bi(2)Te(3) is studied using pseudopotential first principles methods. The robustness of the TI surface states is particularly investigated against magnetic and non-magnetic atomic adsorption by calculating the electronic band structure, charge transfer, and magnetic moments. Interestingly, it is found that a non-magnetic nitrogen atom has produced a residual magnetic moment and opens a gap in the surface states whereas Na and O atoms preserve the Dirac-like dispersion. The charge transfer from the adatoms produces an electric dipole field that causes Rashba splitting in the surface bands. For atomic impurities with 3d orbitals (Ti and Co), the TI surface states are destroyed and two spin-resolved resonance peaks are developed near the Fermi level in the DOS.     

Formation of dispersive hybrid bands at an organic-metal interface

An electronic band with quasi-one-dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.     

Dislocation injection, reconstruction, and atomic transport on {001} Au terraces

High-resolution electron microscopy investigations of Au films show that adatoms on (100) surfaces insert into the underlying terrace to form surface dislocations. This injection readily occurs when the number of adatoms on a terrace is approximately 20 atoms or less. The surface dislocation glides along the terrace, but is repelled from the edges. The dislocation escapes by squeezing out in the dislocation line direction (not gliding out the terrace edge). Atomistic simulations confirm the dislocation stability, easy glide along the terrace and trapping at the terrace edge. These results have profound implications for film growth.     

A surface core level shift study of the nitrogen-induced reconstruction on W{100}

Evidence for the occurrence of a nitrogen-induced reconstruction of W{100} is presented from a surface core level shift (SCLS) study of the W 4f $\text{7}\text{2}$ level using a new method of analysing core shift data, which relates the centre of gravity of the spectrum to the coverage.We infer that at low coverages N adatoms cause a local displacement of surface W atoms, forming small contracted islands. At θ ⩾ 0.3 monolayers all surface W atoms are found to be bonded to at least one N adatom, and the appearance of large contracted domains deduced from LEED patterns in the range 0.3⩽θ ⩽0.4 is consistent with the present data. Removal of surface reconstruction is observed between 0.4 and 0.5 monolayers. The SCLS induced in surface W atoms by bonding to one, two and three N adatoms are identified, and are consistent with charge transfer to the N.