STM Data Research Articles

Response to “Stripy Nanoparticles Revisited”

In a paper entitled "Stripy Nanoparticles Revisited", Levy and co-workers contest the interpretation of scanning tunneling microscopy images of monolayer protected gold nanoparticles that our group has presented. We show that the two arguments they use are based on flawed assumptions and contradict each other. We also show new evidence for the existence of stripe-like domains on mixed monolayer-coated gold nanoparticles. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ab initio electronic structure of the Ge(111)-(2 × 1) surface in the presence of surface vacancy. Application to STM data analysis

We present the results of ab initio modeling of Ge(111)-(2 × 1) surface in the presence of atomic vacancy in surface bilayer. We showed that simple crystal structure defect affects surface electronic structure to the extent comparable with the influence of doping atom. We demonstrated the strong difference of surface LDOS structure above surface defects of different kind. We have proved that spatial oscillations of LDOS exist around individual surface vacancy in the same tunneling bias range as in case of donor doping atom on Ge(111)-(2 × 1) surface.

Controlling the width of self-assembled dysprosium silicide nanowires on the Si(001) surface

We present STM data that show that it is possible to use a metal induced 2 × 7 reconstruction of Si(001) to narrow the width distribution of Dy silicide nanowires. This behavior is distinct from the effect of the 7 × 7 reconstruction on the Si(111) surface, where the 7 × 7 serves as a static template and the deposited metal avoids the unit cell boundaries on the substrate. In this case, the 2 × 7 is a dynamic template, and the nanowires nucleate at anti-phase boundaries between 2 × 7 reconstruction domains.

Electronic structure of Ge(111)-(2 × 1) surface in the presence of doping atoms. Ab initio analysis of STM data

We present the result of ab initio modeling of the Ge(111)-(2 × 1) surface electronic structure in the presence of donor doping atom at certain position in the surface bilayer of (2 × 1) reconstruction. We briefly compare these results with the data of experimental low temperature STM investigations. Ab initio calculations demonstrate that doping atom strongly disturbs local electronic structure. The separate state, most probably split off conduction band, appears in the bandgap. Surface LDOS reveals spatial oscillations in vicinity of foreign atom. We also show that the spatial extent of non-negligible inter-atomic interaction between neighboring donor atoms is not less than 70 A.

Adsorption behavior of redox-active suppressor additives: Combined electrochemical and STM studies

The redox chemistry and the related surface phase behavior of Safranine (SAF) and Janus Green B (JGB) have been studied by means of cyclic voltammetry in combination with in situ Scanning Tunneling Microscopy using HOPG (Highly Oriented Pyrolytic Graphite) and single crystalline Cu(1 0 0) as model substrates, both revealing different widths of the accessible potential windows. JGB and SAF serve as prototypical heterocyclic suppressor/leveler additives that are used for the metallization of 3D-TSVs (3D Through Silicon Vias) following a classical “leveling” concept. SAF can be considered as the reductive decomposition product of JGB that is formed at the copper/electrolyte interface upon electroplating. Both additives reveal a pronounced pH-dependent redox-chemistry with redox-transitions lying close to or even beyond the anodic limit of the copper potential window. Affected by these redox-processes are in particular the aromatic cores of those heterocycles that can be (quasi)reversibly reduced by a two electron transfer process within the potential window of copper. Therefore we identify the reduced form of those dyes as the active components for the suppressing/leveling effect in copper plating. STM data clearly shows a dye surface phase behavior that is crucially determined by its potential-dependent redox-chemistry. This will be exemplarily discussed for the SAF dye. On chloride-modified Cu(1 0 0) mono-reduced SAF forms a structurally well-defined monolayer of cationic stacking polymers. However, this coupled anion/cation layer reveals only minor suppressing capabilities with respect to the copper dissolution and deposition processes. Complete reduction of the aromatic heterocycle finally leads to the 3D precipitation of hydrophobic reaction products. 3D clusters of this SAF precipitate are discussed as the active structural motif for the suppressing effect of these dyes.

Surface Vibrational Spectroscopy Study of Benzene and 2,2,2-Trifluoroacetophenone on Pt(111)

High resolution electron energy loss spectroscopy (HREELS) data for C6H6 and C6D6 on Pt(111) were measured as a function of coverage. The spectra show five additional loss peaks with respect to earlier reports that were used in density functional theory (DFT) calculations of the properties of benzene on Pt(111). The spectra display a strong coverage dependence in the CH stretching region. HREELS and STM data for the adsorption of 2,2,2-trifluoroacetophenone (TFAP) on Pt(111) are used in the analysis of the benzene/Pt(111) system. The TFAP spectra display the same coverage dependence as benzene in the CH stretching region. An analysis of STM images of TFAP dimers shows that their azimuthal orientation is not consistent with the occupation of hcp0 sites. The data for benzene adsorption cannot be explained on the basis of existing density functional theory (DFT) calculations. In particular, the spectra are not consistent with either a C2v symmetry bridge adsorption or a combination of adsorption at C2v symmetry and hcp0 sites. The data provide a benchmark for evaluating the ability of emerging computational approaches to predict the chemisorption properties of strongly chemisorbed aromatics. The data for both benzene and TFAP are of relevance to the asymmetric hydrogenation of ketones on chirally modified Pt catalysts.

Evidence of multiple nodeless energy gaps in superconductingBa0.6K0.4Fe2As2single crystals from scanning tunneling spectroscopy

We report on low-temperature scanning tunneling microscopy and spectroscopy (STM) studies of the electronic structure of single-crystalline ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Multiple superconducting gaps are observed in the density of states (DOS) and the sizes of the two dominant gaps ${\ensuremath{\Delta}}_{L}$ and ${\ensuremath{\Delta}}_{S}$ are 7.6 and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra definitely requires the anisotropy. The nodeless gaps with finite anisotropy revealed in our STM data agree well with the expectations of an extended $s$-wave superconductivity.

Performing spin-polarized STM experiments theoretically

By introducing paths on the hypersurface of the band-energy contribution free energy to the magnetic anisotropy energy and of corresponding differences in the zz-like elements of the electric conductivity tensor, viewed as an implicit function of the free energy, experimental spin-polarized scanning tunnel microscope STM data, namely, the recording of a differential current as a function of an externally applied field, can be analyzed and facilitate a direct comparison between theory and experiment. It is shown that along different paths rather different flipping times of the direction of the magnetization can occur. In particular, discussed are in terms of “theoretical experiments” horizontal and vertical movements of the tip, the influence of the magnetic properties of the tip as well as of typical samples. Furthermore, it is claimed that because of different time scales in a presently still fictional time-resolved mode for spin-polarized STM experiments even particular paths on these hypersurfaces could be mapped out.

Entropy Effects in Atom Distribution and Electrochemical Properties of AuxPt1−x/Pt(111) Surface Alloys

We report on the structural and electrochemical properties of Au(x)Pt(1-x) surface alloys prepared by Au vapour deposition onto Pt(111) followed by annealing to 1000 K. Driven by configurational entropy, Pt and Au atoms are distributed homogeneously over the surface. On the nm scale, however, atomically resolved scanning tunnelling microscopy images with chemical contrast reveal the formation of nm-sized Pt-rich and Au-rich aggregates, similar to the behaviour recently reported for Pd(x)Ru(1-x)/Ru(0001) [H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H. E. Hoster, R. J. Behm, Surf. Sci. 2009, 603, 1439]. Based on the STM data, we determine the abundance of specific adsorption sites for different Au contents, and we derive effective pair interaction parameters that allow reproducing the lateral distribution in Monte Carlo simulations. Cyclic voltammograms of the surface alloys have many similarities with Pt(111). H(ad) and OH(ad) related features both decrease with increasing amount of Au. Both seem to adsorb only on Pt sites, but H(ad) requires smaller ensembles of Pt atoms than OH(ad). The onset potential for H(ad)-formation decreases with increasing Au content. This is can be explained by an effect of the Au atoms on the entropy of adsorption.

Plasma nitridation of Ge(100) surface studied by scanning tunneling microscopy

The geometric and electronic structures of the surface species on Ge(100) after plasma nitridation were investigated in this study. An electron cyclotron resonance (ECR) plasma source was used to directly nitride Ge(100), and scanning tunneling microscopy and spectroscopy (STM/STS) were employed to study the structures of the nitrided surface. Nitridation at room temperature generated a large diversity of adsorbate sites on the surface containing N, O, and displaced Ge atoms, differentiated by annealing between 200 °C and 450 °C. Conversely, nitridation at 500 °C produced Ge–N adsorbate sites which formed ordered and disordered structures on the surface free from oxygen. Density functional theory (DFT) simulations were performed focusing on the ordered nitride structure, and the simulated surface structure showed a good correspondence with the STM data. DFT calculations also found an increase of density of states near the Fermi level on the ordered nitride structure, which is consistent with the Fermi level pinning observed in the STS results. The DFT results predict H-passivation can unpin the Fermi level of the nitrided surface by reducing the dangling bonds and the bond strain, but the residual plasma damage and the low nitridation rate in UHV are challenges to obtain complementary experimental results.

Bismuth-stabilizedc(2×6)reconstruction on a InSb(100) substrate: Violation of the electron counting model

By means of scanning tunneling microscopy/spectroscopy (STM/STS), photoelectron spectroscopy, and first-principles calculations, we have studied the bismuth (Bi) adsorbate-stabilized InSb(100) substrate surface which shows a $c(2\ifmmode\times\else\texttimes\fi{}6)$ low-energy electron diffraction pattern [thus labeled $\text{Bi}/\text{InSb}(100)c(2\ifmmode\times\else\texttimes\fi{}6)$ surface] and which includes areas with metallic STS curves as well as areas with semiconducting STS curves. The first-principles phase diagram of the Bi/InSb(100) surface demonstrates the presence of the Bi-stabilized metallic $c(2\ifmmode\times\else\texttimes\fi{}6)$ reconstruction and semiconducting $(4\ifmmode\times\else\texttimes\fi{}3)$ reconstruction depending on the chemical potentials, in good agreement with STS results. The existence of the metallic $c(2\ifmmode\times\else\texttimes\fi{}6)$ phase, which does not obey the electron counting model, is attributed to the partial prohibition of the relaxation in the direction perpendicular to dimer rows in the competing reconstructions and the peculiar stability of the Bi-stabilized dimer rows. Based on (i) first-principles phase diagram, (ii) STS results, and (iii) comparison of the measured and calculated STM and photoemission data, we show that the measured $\text{Bi}/\text{InSb}(100)c(2\ifmmode\times\else\texttimes\fi{}6)$ surface includes metallic areas with the stable $c(2\ifmmode\times\else\texttimes\fi{}6)$ atomic structure and semiconducting areas with the stable $(4\ifmmode\times\else\texttimes\fi{}3)$ atomic structure.

Self-assembled rows of Ni porphyrin dimers on the Ag(111) surface

The growth and ordering of 5-(10,15,20-triphenylporphyrinatonickel(ii))dimer (NiTPP-dimer) molecules on the Ag(111) surface have been investigated using scanning tunnelling microscopy/spectroscopy (STM/STS) and low-energy electron diffraction (LEED). At one monolayer (ML) coverage the NiTPP-dimer forms a well-ordered close-packed molecular layer in which the porphyrin molecules have a flat orientation with the molecular plane lying parallel to the substrate. STM and LEED data obtained from one monolayer of the NiTPP-dimer on the Ag(111) surface show the formation of three domains which grow along the main crystallographic directions of the substrate. Scanning tunnelling spectroscopy data obtained from the NiTPP-dimer on the Ag(111) surface show good agreement with optical band gap measurements and density functional theory calculations.

The origin of inter-dimer-row correlated adsorption for NH 3 on Si(0 0 1)

We discuss the interaction between adsorbing ammonia molecules and pre-adsorbed ammonia fragments on the Si(0 0 1) surface, searching for experimental evidence of a H-bonded precursor state predicted by modelling. While correlations along dimer rows have already been identified, these mix substrate-mediated effects due to dimer buckling with ammonia–adsorbate effects. Correlations between fragments on neighbouring dimer rows are not affected by substrate effects (in this system), allowing an analysis of direct ammonia–adsorbate effects. We present an analysis of cross-row correlations in existing high-coverage STM data which shows significant correlations between NH 2 groups on neighbouring dimer rows over a significant range, providing evidence for the H-bonded precursor state with a range of around 10 Å. We discuss implications for the interpretation of STM images of ammonia on Si(0 0 1).

Magnetic shape anisotropy calculations of Fe nanostructures at the Si/Fe(1 1 1) interface

The influence of Si capping layers on the magnetic properties of thin Fe films grown on Si(1 1 1) has been studied by means of shape anisotropy calculations. Fe surface morphology simulations are realized using experimental STM data. Surface modifications induced by the interaction between the Si overlayer and the Fe surface are performed in agreement with the model proposed in a previous work by Stephan et al. [J. Magn. Mater. 293 (2005) 746]. Calculations of the uniaxial anisotropy energy constant K u are then performed on the modified Fe surface morphology for different Si deposition geometries as proposed in the model. The relevant data deduced by this method such as anisotropy constants and their related easy axis direction, are directly compared to the experimental ones obtained by ex situ magneto-optical Kerr effect (MOKE) measurements at room temperature using the transverse bias initial inverse susceptibility and torque (TBIIST) method. We show that a very good agreement between those results leads to a confirmation of the proposed model.

Thionicotinamide SAM on Gold: Adsorption Studies and Electroactivity

AbstractSTM and impedance results of the self‐assembled monolayer (SAM) formed with thionicotinamide (TNA) on gold indicate the presence of defects that increase with the immersion time of the electrode in the TNA solution affecting the SAM electroactivity toward the electron transfer reaction of the cytochrome c metalloprotein and [Fe(CN)6]4− and [Ru(NH3)6]3+ complexes. It was observed that this electroactivity was also affected by the pH of the electrolyte solution. SERS and STM data indicate sulfur coordination to the surface with contribution of the NH2 group. From the dependence of the TNA surface coverage on the temperature and concentration in solution, thermodynamic parameters of adsorption were determined.

Pb chains on reconstructed Si(335) surface

The structural and electronic properties of Si(335)-Au surface decorated with Pb atoms are studied by means of density-functional theory. The resulting structural model features Pb atoms bonded to neighboring Si and Au surface atoms, forming monoatomic chain located 0.2 nm above the surface. The presence of Pb chain leads to a strong rebonding of Si atoms at the step edge. The fact that Pb atoms occupy positions in the middle of terrace is consistent with STM data, and also confirmed by simulated STM images. The calculated band structure clearly shows one-dimensional metallic character. The calculated electronic bands remain in very good agreement with photoemission data.

A surface X-ray diffraction study of TiO 2(1 1 0)(3 × 1)–S

Surface X-ray diffraction has been used to investigate the structure of TiO 2(1 1 0)(3 × 1)–S. In concert with existing STM and photoemission data it is shown that on formation of a (3 × 1)–S overlayer, sulphur adsorbs in a position bridging 6-fold titanium atoms, and all bridging oxygens are lost. Sulphur adsorption gives rise to significant restructuring of the substrate, detected as deep as the fourth layer of the selvedge. The replacement of a bridging oxygen atom with sulphur gives rise to a significant motion of 6-fold co-ordinated titanium atoms away from the adsorbate, along with a concomitant rumpling of the second substrate layer.

Instanton Sector of Correlated Electron Systems as the Origin of Populated Pseudo-gap and Flat “Band” Behavior: Analytic Solution

Finite temperature instantons between meta-stable vacua of correlated electronic system are solved analytically for quasi one-dimensional Hubbard model. The instantons produce dynamic symmetry breaking and connect metallic state with the dual vacua: superconducting (SC) and spin-density wave (SDW) states. The instantons spread along the Matsubara’s imaginary time and possess the structure similar to the coordinate-space solitonic lattices previously discovered in quasi one-dimensional Peierls model. On the microscopic level the inter-vacua excursion is described by mutual transformations between the “resonating quartets” of the couples of electron–hole and Cooper pairs. Spectral properties of the electrons in the “instantonic crystal” reveal pseudo-gap (PG) behavior, with finite fermionic density of states in the center of the PG and “flat-band” outside of it. Analytically derived inverse temperature scaling of the pseudo-gap and the densities of the SC and SDW condensates is discussed in the context of ARPES and STM data in high-Tc cuprates.

An atomistic view of structural and electronic properties of rare earth ensembles on Si(0 0 1) substrates

Synergistic experimental and theoretical studies of low coverage adsorption geometries of rare earth adatoms on Si(0 0 1) were performed. Density functional calculations showed charge transfer from adatoms to Si(0 0 1) and explained observed bias dependence in scanning tunneling microscopy images. Comparison of STM simulations with empty states STM data revealed a direct correlation between coverage and surface reconstructions; the (4 × 8) reconstruction is a low coverage precursor to (2 × 4) reconstruction. Charge transfer from adatom to substrate was also confirmed by Kelvin probe force microscopy; contact potential difference measurements of Dy/Si(0 0 1) reveal a 0.28 eV higher surface potential than that of (2 × 1) reconstructed Si.

Electronic states of Fe nanostructures on Pt(997) surface

A variety of one dimensional atomic chains can be synthesized on stepped surfaces, which provide opportunities for systematically tailoring the surface properties. The electronic structure of the high vicinal surface plays an essential role in determining the physical properties of the atomic chains. The authors have used surface analysis techniques to study the surface of Pt(997) single crystal. Angle resolved ultraviolet photoemission spectroscopy was performed to characterize the electronic states of Pt(997) surface by using a synchrotron radiation light source. Different from the expected free electron model on the flat Pt(111) surface, electron confinement is observed with a wave vector perpendicular to the step direction. Fe was carefully deposited onto Pt(997) substrate at room temperature. The STM data show that Fe atomic chains were formed along the step edges due to the step decoration growth. The exchange splitting of Fe 3d bands is estimated according to the photoemission spectroscopy data of Fe/Pt.