Filled-state Images Research Articles

Scanning Tunneling Microscopy Visualization of Polaron Charge Trapping by Hydroxyls on TiO2(110).

Using scanning tunneling microscopy (STM), we investigate the spatial distribution of the bridging hydroxyl (OHb) bound excess electrons on the rutile TiO2(110) surface and its temperature dependence. By performing simultaneously recorded empty and filled state imaging on single OHbs at different temperatures in STM, we determine that the spatial distribution of the OHb bound excess electrons retains a symmetric four-lobe structure around the OHb at both 78 and 7 K. This indicates that OHbs are much weaker charge traps compared to bridging O vacancies (Ob-vac). In addition, by sequentially removing the capping H of each OHb using voltage pulses, we find that the annihilation of each OHb is accompanied by the disappearance of some lobes in the filled state STM, thus verifying the direct correlation between OHbs and their excess electrons.

Interaction of yttrium atoms with a C60 fullerite film

Using scanning tunneling microscopy (STM) observation, interaction of yttrium (Y) atoms with the C60 fullerite films has been studied. It has been found that adsorbed Y atoms avoid clustering and are incorporated in the C60 film as individual atoms occupying the octahedral voids of the fullerite crystal. As a result of a charge transfer from the Y atom, the three neighboring surface fullerenes look more bright in the filled-state STM images. Yttrium atoms are tightly bound in their sites and neither move laterally nor float to the film surface upon subsequent overgrowth of C60 layers.

Scanning Tunneling Microscope Study on C2H2 Adsorption on a Ge(100) Surface

Adsorption configurations of C2H2 on the Ge(001) surface at room temperature were investigated using scanning tunneling microscope (STM). Two adsorption features, occupying one dimer and two dimers, respectively, were observed as major species. Both of them appear in rounded or elongated bean shapes in filled-state images. In empty-state images, they reveal atomically resolved details: two and four local maxima for the one-dimer and the two-dimer features, respectively. The empty-state double maximum of the one-dimer feature confirms the adsorption of a C2H2 molecule on top of a dimer. Based on the empty-state quadruple maximum of the two-dimer feature, we propose the adsorption of a pair of C2H2 molecules across the ends of two adjacent dimers in the same dimer row (paired end-bridge). This differs from the previous interpretation of the two-dimer feature as a single C2H2 molecule adsorbed in a tetra-σ-bonding configuration. We also observed C2H2 molecules adsorbed in the unpaired end-bridge configuration, but none in tetra-σ-bonding configurations. These observations and interpretations are consistent with the energetics predicted by using theoretical calculations.

Spatially resolved electronic structure of an isovalent nitrogen center in GaAs

Small numbers of nitrogen dopants dramatically modify the electronic properties of GaAs, generating very large shifts in the conduction-band energies with nonlinear concentration dependence, and impurity-associated spatially-localized resonant states within the conduction band. Cross-sectional scanning tunneling microscopy provides the local electronic structure of single nitrogen dopants at the (110) GaAs surface, yielding highly anisotropic spatial shapes when the empty states are imaged. Measurements of the resonant states relative to the GaAs surface states and their spatial extent allow an unambiguous assignment of specific features to nitrogen atoms at different depths below the cleaved (110) surface. Multiband tight binding calculations around the resonance energy of nitrogen in the conduction band match the imaged features. The spatial anisotropy is attributed to the tetrahedral symmetry of the bulk lattice. Additionally, the voltage dependence of the electronic contrast for two features in the filled state imaging suggest these features could be related to a locally modified surface state.

Subsurface Polaron Concentration As a Factor in the Chemistry of Reduced TiO2 (110) Surfaces

Surface reactivity of rutile TiO2 (110) surfaces has long been ascribed to bridging oxygen vacancies (VO), but recently, excess electrons introduced by donor defects are being considered as the main players. However, the spatial distribution of them is not yet clear due to difficulties in interpreting filled state images of scanning tunneling microscopy (STM). In this study, several different images available in the literature are consistently interpreted using density functional theory (DFT). The key factors are polarons in the second layer below Ti5c row (Ti5c-2nd polarons) and a temperature dependence of their concentration. Bright blobs in the experimental images are interpreted as Ti5c-2nd polarons. At 78 K, their concentration reaches 33.3% ML, where 1 ML is defined as the density of (1 × 1) unit cells, regardless of VO coverage. In contrast, at 5 K, it is twice the VO coverage. This discrepancy is understood by the ionization of donor defects other than VO, most probably subsurface Ti interstitials, and subsequent diffusion of polarons to Ti5c-2nd sites at high temperature. This mechanism explains seemingly contradicting reports on oxygen chemisorption on this surface, which suggests that the so-called oxygen-vacancy model needs to be modified at temperature above at least 78 K.

The (2×2) reconstructions on the surface of cobalt silicides: Atomic configuration at the annealed Co/Si(111) interface

We have used scanning tunneling microscopy (STM) and ab initio total-energy calculations to characterize surface and interfacial structure of Co-Si(111) system. It has been found experimentally that two different types of the (2×2) surface structures occur. The coexistence of two phases is demonstrated by the example of STM image of the surface formed at the early stages of cobalt silicide formation under moderate annealing temperatures (500°C). The measured height difference between the adjacent (2×2) reconstructed patches equal to about 1.0Å (as determined from the filled-state STM images). In addition, the shift of the atomic rows by half of the row spacing is observed. Two adatom models of the (2×2) surface structures are developed. According to our data, these structures are assigned to CaF2-type CoSi2 and CsCl-type CoSi with a (2×2) array of Si adatoms on their surfaces. If the latter is the case, it has а coherent double interface CoSi/CoSi2/Si(111) with a two-layer CoSi2. Both of these interfaces are characterized by the eightfold cobalt coordination and incorporate a grown-in stacking fault.

Origin of Symmetric Dimer Images of Si(001) Observed by Low-Temperature Scanning Tunneling Microscopy.

It has been a long-standing puzzle why buckled dimers of the Si(001) surface appeared symmetric below ~20 K in scanning tunneling microscopy (STM) experiments. Although such symmetric dimer images were concluded to be due to an artifact induced by STM measurements, its underlying mechanism is still veiled. Here, we demonstrate, based on a first-principles density-functional theory calculation, that the symmetric dimer images are originated from the flip-flop motion of buckled dimers, driven by quantum tunneling (QT). It is revealed that at low temperature the tunneling-induced surface charging with holes reduces the energy barrier for the flipping of buckled dimers, thereby giving rise to a sizable QT-driven frequency of the flip-flop motion. However, such a QT phenomenon becomes marginal in the tunneling-induced surface charging with electrons. Our findings provide an explanation for low-temperature STM data that exhibits apparent symmetric (buckled) dimer structure in the filled-state (empty-state) images.

Adsorption and self-assembled structures of sexithiophene on the Si(111)-√3×√3-Ag surface.

The adsorption and self-assembled structures of α-sexithiophene (α-6T) have been investigated on a Si(111)-Ag surface using scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations. The adsorbed α-6T molecules are arranged into unidirectional molecular rows with a side-by-side orientation. The molecular rows reveal three kinds of appearances in the filled-state STM images, which reflect the distinct adsorption sites. From tunneling spectroscopy, we find that the filled-state STM images of α-6T should be influenced by the surface states of Si(111)-Ag. At one monolayer coverage, sequentially ordering of the triple molecular rows results in the close-packed arrangement of the α-6T overlayer.

Si(100)-2 × 1-H dimer rows contrast inversion in low-temperature scanning tunneling microscope images

Detailed low temperature scanning tunneling microscope images of the Si(100)-2×1-H surface show a remarkable contrast inversion between filled- and empty-state images where the hydrogen dimer rows appear bright for filled-state images and dark for empty-state images. This contrast inversion originates from the change in the dominant surface states and their coupling to the tip apex and the bulk silicon channels as a function of the bias voltage: dimer SiSi bonding states dominate the filled-state images and valley states associated with SiSi anti-bonding states dominate the empty-state images. Care is required when constructing and interpreting the atomic structure of dangling-bond structures on the Si(100)-2×1-H surface.

Electronic effects of isolated halogen atoms on the Ge(001) surface

The asymmetric slab approach and density functional theory (DFT) are used to discuss the interaction of isolated halogen atoms (F, Cl, Br and I) with the Ge(001)c(4×2) surface. For each adsorbate on thick slabs we find two stable adatom configurations separated by ~0.1eV. In both configurations, the unsaturated dangling bonds (DB) of the chemisorbed dimers are fully charged due to the interaction of the DB-derived surface states with the bulk states. This behavior is found to be insensitive to whether the calculations are performed using standard ground-state DFT, or DFT supplemented by MBJLDA and spin-orbit corrections to produce an accurate surface electronic structure and energy gap. On chemisorbed ultrathin Ge slabs, however, these unsaturated DB's are empty for the lowest energy configurations. This results in different geometries and energetics for the chemisorbed dimers. A detailed analysis of the predicted scanning tunneling microscopy images of halogen-chemisorbed surfaces is also presented. It is predicted that the halogen atoms may image darkly, yielding filled-state images similar to that of the clean surface, while the empty-state images should be noticeably different.

Oxygen vacancies versus fluorine at CeO2(111): a case of mistaken identity?

We propose a resolution to the puzzle presented by the surface defects observed with STM at the (111) surface facet of CeO 2 single crystals. In the seminal paper of Esch et al. [Science 309, 752 (2005)] they were identified with oxygen vacancies, but the observed behavior of these defects is inconsistent with the results of density functional theory (DFT) studies of oxygen vacancies in the literature. We resolve these inconsistencies via DFT calculations of the properties of both oxygen vacancies and fluorine impurities at CeO2(111), the latter having recently been shown to exist in high concentrations in single crystals from a widely used commercial source of such samples. We find that the simulated filled-state STM images of surface-layer oxygen vacancies and fluorine impurities are essentially identical, which would render problematic their experimental distinction by such images alone. However, we find that our theoretical results for the most stable location, mobility, and tendency to cluster, of fluorine impurities are consistent with experimental observations, in contrast to those for oxygen vacancies. Based on these results, we propose that the surface defects observed in STM experiments on CeO2 single crystals reported heretofore were not oxygen vacancies, but fluorine impurities. Since the similarity of the simulated STM images of the two defects is due primarily to the relative energies of the 2p states of oxygen and fluorine ions, this confusion might also occur for other oxides which have been either doped or contaminated with fluorine.

Two-Dimensional Self-Assembled Gold Silicide Honeycomb Nanonetwork on Si(111)7×7

The growth evolution of Au on Si(111)7×7 at room temperature under ultrahigh vacuum conditions is studied by using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Both STM filled-state and empty-state imaging show that on the 7×7 surface two distinct Au layers form one after another, each by connecting patches of adjoining Au clusters, before individual islands start to grow on top. XPS measurements of the same coverages reveal that Au exists as gold silicide in the two interfacial distinct layers and as metallic Au in the islands. The critical thickness of the gold silicide interface region is found to be two monolayers, which marks the transition from layer-by-layer to island growth. These results provide direct observation and chemical-state characterization of Au growth on Si(111)7×7 in the Stranski–Krastanov mode. Of special interest is the formation of the gold silicide honeycomb nanonetwork at 0.76 monolayer coverage, which is made up of six triangular Au clusters (around each corner hole) interconnected to one another at the dimer rows of the Si(111)7×7 substrate. With the corner holes of the 7×7 surface exposed, this new gold silicide nanonetwork, in effect, forms a two-dimensional template of nanopores (∼1 nm in pore size) for molecular trapping application. The gold silicide honeycomb nanonetwork also offers a new conducting phase of fundamental interest to semiconductor device fabrication and to potential applications in biofunctionalization.

Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy

We study subsurface arsenic dopants in a hydrogen-terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant-related features that fall into two classes, which we call As1 and As2. When imaged in occupied states, the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionization when imaged under empty-state conditions, similar to previous observations of acceptors in GaAs. Density functional theory calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction-band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled-state images and an isotropic depression in empty-state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

Surface [4 + 2] Cycloaddition Reaction of Thymine on Si(111)7×7 Observed by Scanning Tunneling Microscopy

The early stage of thymine (Thy) adsorption on Si(111)7×7 surface is studied with scanning tunneling microscopy (STM) and density functional theory (DFT)-based computational method. Bright protrusions corresponding to the adsorbed thymine molecules are observed in both empty-state and filled-state STM images. These bright protrusions in the empty-state images exhibit three different degrees [lower (L), medium (M), and higher (H)] of intensities. The L and M protrusions are found on the adatom–restatom pairs in the 7 × 7 unit cell, indicating bidentate configurations for Thy adsorption. A free Thy molecule has been found to interact with the Si surface by a two-step process: Thy first undergoes keto–enol tautomerization to form the more stable dienol tautomer, which then binds to the Si adatom–restatom pair via the [4 + 2] cycloaddition reaction, leading to two different adproducts. Our DFT/B3LYP/6-31++G(d,p) calculations show three plausible cycloaddition products, with the 1,4-cyclohexadiene adproduct being more stable than 3,6- and 2,5-cyclohexadienes. Our calculations also suggest the viability of formation of a Thy molecule hydrogen-bonded with the bidentate Thy adproducts already on the surface. Statistical analysis for three different exposures of Thy on the 7 × 7 surface reveals that the L protrusion has the highest relative surface concentration (80%), with the M (16%) and H features (4%) being significantly less popular. These results lead us to attribute the L and M protrusions to the 1,4- and the 3,6-cycloaddition products, respectively, with the least popular H protrusion assigned to a Thy molecule hydrogen-bonded to the bidentate Thy, all attached to an adatom–restatom pair. The observation of [4 + 2] cycloaddition products on the Si surface confirms the formation of precursor dienol Thy tautomer. This surface-mediated two-step reaction pathway for Thy is unique for surfaces, in contrast to keto–enol tautomerization that is mainly observed in the solution phase either by acid- or base-catalyzed pathways. Our STM study coupled with our separate XPS work have demonstrated that this type of tautomerization can also be observed on Si surfaces, and this can initiate the subsequent cycloaddition reactions of Thy molecule with the surface.

Self-Directed Growth of Aligned Adenine Molecular Chains on Si(111)7×7: Direct Imaging of Hydrogen-Bond Mediated Dimers and Clusters at Room Temperature by Scanning Tunneling Microscopy

The early stage of adsorption of adenine on Si(111)7×7 is studied by scanning tunneling microscopy (STM). Bright protrusions are observed in both empty-state and filled-state STM images, indicative of molecular adsorption of adenine through dative bonding. The majority of these bright protrusions appear as dimer pairs formed by hydrogen bonding at the initial adsorption stage. The formation of dative bonds between the substrate and adenine and the feasibility of the H-bond mediated dimers are supported by ab initio DFT/B3LYP/6-31G++(d,p) calculations, and are in excellent accord with our recent X-ray photoemission data. Remarkably, these dimers are found to undergo self-organization into aligned superstructures, evidently with common link arrangements, including straight, offset, and zigzag chains, square quartets, double quartets, and other multiple dimer structures. As the exposure of adenine increases, the populations of dimers as well as the self-organized double dimer and other higher-order structures also increase. The end-to-end dimer links are found to be most prominent in the growth of adenine molecular chains, most notably aligned along the Si dimer-wall or [-1 1 0] direction of the 7×7 unit cell. The self-aligned adenine dimer molecular chains offer a natural template for catch-and-release biosensing, lithography, and molecular electronic applications.

Electronic structures of a Zn vacancy on the ZnO(10 $\bar 1$ 0) surface: Density functional theory calculations

We perform spin-polarized density functional theory calculations for a Zn vacancy on the ZnO(10 $\bar 1$ 0) surface. Two stable configurations of the surface Zn vacancy are found, and the activation energy barrier is estimated to be ∼0.01 eV. The lower energy configuration has a newly formed surface Zn-O bond to restore the bulk-like structure on the surface. Due to the newly formed bond, the vacancy state in the band gap is characterized by a complicated hybridization of neighboring surface and subsurface atoms and by a more extended electron density. Despite such a hybridization, the surface Zn vacancy is found to have a robust magnetic moment of 1 μ B , implying that surface Zn vacancies may be responsible for the ferromagnetism observed in ZnO thin films and nanoparticles. Simulated scanning tunneling microscope images show that the two structures of the surface Zn vacancy can be distinguished in the filled-state images.

Structural and electronic properties of the Pb/Ge(111)-β(3×3)R30∘surface studied by photoelectron spectroscopy and first-principles calculations

We have studied structural and electronic properties of a Ge(111) surface covered with a monatomic Pb layer [Pb/Ge(111)-$\ensuremath{\beta}$] by means of core-level photoelectron spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), and a first-principles band structure calculation. There has been a controversy about the surface structure of Pb/Ge(111)-$\ensuremath{\beta}$ between a close-packed model with a coverage of 4/3 monolayers and a trimer model with a coverage of 1 monolayer. This problem has been examined by analyzing the line shape of a Pb 5$d$ core-level spectrum and comparing the experimental band structure with those calculated for two models. The line shape of the core-level spectrum agrees with a close-packed model. The valence band structure observed by ARPES has been well reproduced by the calculation employing the close-packed model. The close-packed model therefore describes correctly the surface structure of Pb/Ge(111)-$\ensuremath{\beta}$. The scanning-tunneling microscopy (STM) image simulated for the close-packed model is in good agreement with the experimental filled-state STM image, in which three protrusions per unit cell were observed.

Bidentate Surface Structures of Glycylglycine on Si(111)7×7 by High-Resolution Scanning Tunneling Microscopy: Site-Specific Adsorption via N–H and O–H or Double N–H Dissociation

The early adsorption stage of glycylglycine on Si(111)7×7 surface has been studied by scanning tunneling microscopy (STM). Filled-state imaging shows that glycylglycine adsorbs dissociatively in a bidentate fashion on two adjacent Si adatoms across a dimer wall or an adatom-restatom pair, with the dissociated H atoms on neighboring restatoms. The present STM result validates our hypothesis that both bidentate configurations involving N-H and O-H dissociation and double N-H dissociation are equally probable. Our STM results further show that the relative surface concentrations of the five bidentate configurations follow a specific ordering. This suggests that N-H dissociation at a center adatom site would likely be followed by N-H dissociation at an adjacent restatom, while N-H dissociation at a corner adatom site would be succeeded by O-H dissociation at an adatom across the dimer wall. Evidently, the strong bidentate interactions also inhibit surface diffusion of the adsorbed glycylglycine fragment, and the adsorption apparently follows random sequential adsorption statistics. The random nature of adsorption is also supported by the similar relative occupancies of the center adatom and corner adatom sites, indicating that the relative reactivities of these adatom sites do not play a significant role. Our DFT computational study shows that all three bidentate (Si-)NHCH(2)CONHCH(2)COO(-Si) adatom-adatom configurations (center-center, corner-corner, center-corner) have similar adsorption energies for a double adatom-adatom pair across the dimer wall, while the (Si-)NHCH(2)CON(-Si)CH(2)COOH bidentate adatom-restatom configuration is energetically favorable. The free -CONH- and -COOH groups remaining on the respective bidentate adstructures could facilitate adsorption of the second adlayer through the formation of hydrogen bonding.

Initial oxidation structure of chlorinated Si(001)

The Cl-saturated Si(001) surface is investigated by employing density functional theory calculations. From the electronic density of states, the low- and the high-bias filled-state images and the empty-state images are found to originate from Cl(3p) states directed in-plane, in parallel with the Si dimer row, and perpendicular to the surface (z), respectively. The bright triplet, which is a characteristic scanning tunneling microscopy (STM) feature of an unchlorinated bare Si dimer, is attributed to an occupied dangling bond state of the bare Si dimer that is hybridized with the neighboring Cl(3pz) orbital. The previously observed split-dimer feature is confirmed to be the dimer-bond oxidized structure. Finally, we suggest that the initial back-bond oxidation does not occur on the chlorinated Si(001) surface because it is energetically not favored compared with dimer-bond oxidation and because its STM simulation does not match experimental images.

Subsurface Incorporation of Co Atoms into Si(100)

The initial adsorption structures of Co on Si(100) were investigated using scanning tunneling microscopy (STM). Filled-state STM images showed bright protrusions at Si dimers. Density functional th...