Empty-state Scanning Tunneling Microscopy Images Research Articles

Imaging empty states on the Ge(100) surface at 12 K

Our understanding of bias-dependent scanning-tunneling-microscopy (STM) images is complicated not only by the multiplicity of the surface electronic structure, but also the manifold tunneling effects in probing semiconductor surfaces having directional dangling- and covalent-bond orbitals. Here we present a refined interpretation of empty-state STM images from the model semiconductor surface, Ge(100), on the basis of measurements at low temperature (12 K) combined with density-functional-theory calculations. In the lower-bias regime ($\ensuremath{\le}1.6\phantom{\rule{0.16em}{0ex}}\mathrm{V}$), the electron tunneling is found to occur predominantly in antibonding dangling-bond or/and dimer-bond states (${\ensuremath{\pi}}_{1}^{*},{\ensuremath{\pi}}_{2}^{*}$, and ${\ensuremath{\sigma}}^{*}$) of Ge(100) at the surface-parallel wave vector ${k}_{\ensuremath{\parallel}}=0$, leading to the tunneling current maxima located directly on the dimer rows. At higher biases (e.g., at 2 V), the current maxima are shifted to the position in the troughs between the dimer rows, because the tunneling occurs efficiently in the ${\ensuremath{\pi}}_{2}^{*}$ states at ${k}_{\ensuremath{\parallel}}\ensuremath{\ne}0$ associated with the dimer-up atoms of two adjacent dimer rows, i.e., because of increased sideways tunneling. Thus, the empty-state STM images of Ge(100), albeit strongly bias-dependent, reflect the dimer arrangement rather than the backbonds and surface resonances at all experimental conditions used. The results are also discussed in comparison with the counterpart system of Si(100).

Electronic and atomic structures of the Ag induced [formula omitted] superstructure on Ge(111)

The Ag/Ge(111) 3×3 surface together with Ag/Si(111) 3×3 constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional 3×3 superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced 3×3 structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the M¯-point of the 3×3 surface Brillouin zone. ii) The Ag/Ge(111) 3×3 surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (LT) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) 3×3 for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈100K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.

Scanning tunneling microscopic and spectroscopic studies on a crystalline silica monolayer epitaxially formed on hexagonal SiC(0001¯) surfaces

An epitaxial silicon-oxide monolayer of chemical composition of Si2O3 (the Si2O3 layer) formed on hexagonal SiC(0001¯) surfaces has been observed by scanning tunneling microscopy (STM). Filled- and empty-state STM images with atomic resolution support the previously reported model. Typical structural defects in the Si2O3 layer are found to be missing SiOn (n = 1, 2, 3) molecules. The band gap of the Si2O3 layer obtained by point tunneling spectroscopy is 5.5±0.5 eV, exhibiting considerable narrowing from that of bulk SiO2, 8.9 eV. It is proposed that the Si2O3 layer is suitable as a relevant interface material for formation of SiC-based metal-oxide-semiconductor devices.

Emergence of state at Fermi level due to the formation of In-Sn heterodimers on Si(100)-2×1

Structure and electronic properties of one-dimensional bimetallic In-Sn chains formed by codeposition on a Si(100)-2$\ifmmode\times\else\texttimes\fi{}$1 surface are studied experimentally by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy and theoretically using density-functional theory. The codeposition of In with a small amount of Sn allows separation of various In-Sn structures and their identification in empty-state STM images. A 16$\phantom{\rule{0.28em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.28em}{0ex}}$2 supercell is employed to model an indium atomic chain in which one or two Sn atoms are embedded. This atomic model is used to identify unambiguously various In-Sn structures observed experimentally. At low Sn:In ratio the codeposition results in strongly preferential formation of isolated heterogeneous In-Sn dimers. The In-Sn dimer induces tilting of the neighboring homogeneous In-In dimer accompanied with a charge transfer. Consequently a localized state at Fermi level appears. These results contribute to a discussion on possible transport of electric charge along one-dimensional atomic chains of metals.

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.

Structural and electronic properties of the Co-induced Si(111) [formula omitted] surface reconstruction

Scanning Tunneling Microscopy (STM) observations of the Si(111) 13×13−R13.9°-Co surface reconstruction show that two types of clusters coexist on the surface appearing as dark and bright on the images. The atomic structure of the dark ones has been recently proposed by L. Chaput et al. (L. Chaput, F. Dulot, M. C. Hanf, P. Wetzel, Surf. Sci. 604 (2010) 513) [34]. We found that each dark cluster consists of three Co atoms located on substitutional sites of the Si(111) surface with an overlying triangle of six Si adatoms. Here, we present the determination of the bright clusters atomic structure. Combining high-resolution filled and empty-state STM images and DFT simulations, we show that bright clusters consist of additional Si adatoms located on top of the six Si atoms terminating the dark clusters. These Si adatoms are in number of one, two or three. Energy stability of these three configurations and description of the electronic states are discussed in detail.

Electronic effects of single H atoms on Ge(001) revisited

The adsorption of isolated H atoms on the Ge(001) surface is studied using density functional theory (DFT) and scanning tunneling microscopy (STM). Two stable adsorption positions that are found in DFT correspond to H atom attachment to an up-or down-buckled Ge dimer atom, respectively. Surprisingly, in the case where H bonds to the down-buckled Ge atom, we find that there is a redistribution of a unit of charge which leaves the net charge of the doubly occupied dangling bond of the unreacted Ge atom intact. This configuration is found to be the more stable of the two structures. Comparison to filled- and empty-state STM images confirms that this lowest energy structure is observed at room temperature. These results represent a fundamentally different picture of the physics and chemistry of H adsorption to Ge(001) compared with previous work.

Adsorption State of 4,4′′-Diamino-p-terphenyl through an Amino Group Bound to Si(111)-7 × 7 Surface Examined by X-ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy

The adsorption of 4,4′′-diamino-p-terphenyl (DAT) on Si(111)-7 × 7 at room temperature (RT) is examined using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). At low coverages of DAT, the XPS peak from N(1s) in two amino groups of a DAT is decomposed into two peaks with almost the same intensity, corresponding to the binding state of a N atom in Si-NH-terphenyl and to that in NH2-terphenyl, respectively. This indicates that the DAT is chemically bonded to the Si surface even at RT through one amino group at one end of the DAT, and the other amino group at the other end is intact. In empty-state STM images, round protrusions of ∼0.3 nm high are found over center Si adatom sites, possibly attributed to the binding site of DAT. The positions of some protrusions in the empty-state STM images slightly shift in the filled-state images. To compare the empty and the filled-state STM images, we propose a model for the DAT aslant bound to the Si surface through an amino group at the one end of DAT with a preferential orientation within the half cell of 7 × 7.

Interaction of acetone with the Si(0 0 1) surface

The adsorption of acetone [(CH 3) 2CO] on the Si(0 0 1) surface has been investigated using density functional theory (DFT) and scanning tunneling microscopy (STM). Three distinct features have been observed in the experimental STM images – one single-dimer and two two-dimer wide features. We have considered 32 different possible adsorbate configurations corresponding to the [2 + 2] cycloaddition, dative bonded, α-Hydrogen cleavage, C H bond cleavage, methyl cleavage, cross-dimer bridging, O-dimer insertion, OH cleavage, alcohol-like, end-bridging and dimer-bridging structures. The optimised geometry, binding energy, and simulated filled- and empty-state STM images for each of these possible configurations are presented. These results have facilitated the identification of the three experimentally-observed acetone adsorbate structures.

First-principles investigation of an epitaxial silicon oxynitride layer on a6H‐SiC(0001)surface

In a recent publication, Shirasawa et al. [Phys. Rev. Lett. 98, 136105 (2007)] have experimentally shown that incorporation of nitrogen at the interface of a silicate adlayer on $6H\text{\penalty1000-\hskip0pt}\mathrm{Si}\mathrm{C}(0001)$ leads to the formation of a well ordered, highly stable epitaxial silicon oxynitride (SiON) layer without dangling bond states. We investigate the structural and electronic properties of this intriguing system by employing density functional theory with self-interaction-corrected pseudopotentials. Our results corroborate the structural model inferred from low-energy electron diffraction. In addition, our calculated filled- and empty-state scanning tunneling microscopy images are in excellent agreement with the experimental data, clearly revealing that O and Si surface states of the silicate double layer on top of the system, respectively, give rise to the observed images. The calculated surface band structure exhibits a surface band gap of $9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in very good agreement with the scanning tunneling spectroscopy data. The physical origin of this amazingly large gap is clarified.

Study of Si(0 0 1)4 × 2-Ga structure by scanning tunneling microscopy and ab initio calculation

We have studied Si(0 0 1)–Ga surface structures formed at Ga coverages of slightly above 0.50 monolayer (ML) at 250 °C by scanning tunneling microscopy (STM). 4 × 2-, 5 × 2-, and 6 × 2-Ga structures were observed in a local area on the surface. The 4 × 2-Ga structure consists of three protrusions, as observed in filled- and empty-state STM images. The characters of these structures are clearly different from those of other Si(0 0 1)–Ga structures. We also performed an ab initio calculation of the energetics for several possible models for the 4 × 2-Ga structure, and clarified that the three-orthogonal-Ga-dimer model is the most stable. Also, the results of comparing the simulated STM images and observation images at various bias voltages indicate that this structural model is the most favorable.

A scanning tunneling microscopy study of PH 3 adsorption on Si(1 1 1)-7 × 7 surfaces, P-segregation and thermal desorption

PH 3 adsorption on Si(1 1 1)-7 × 7 was studied after various exposures between 0.3 and 60 L at room temperature by means of scanning tunneling microscopy (STM). PH 3-, PH 2-, H-reacted, and unreacted adatoms can be identified by analyzing empty-state STM images at different sample biases. PH x -reacted rest-atoms can be observed in empty-state STM images if neighboring adatoms are hydrogen terminated. Most of the PH 3 adsorbs dissociatively on the surface, generating H- and PH 2-adsorbed rest-atom and adatom sites. Dangling-bonds at rest-atom sites are more reactive than adatom sites and the faulted half of the 7 × 7 unit cell is more reactive than the unfaulted half. Center adatoms are overwhelmingly preferred over corner adatoms for PH 2 adsorption. The saturation P coverage is ∼0.18 ML. Annealing of PH 3-reacted 7 × 7 surfaces at 900 K generates disordered, partially P-covered surfaces, but dosing PH 3 at 900 K forms P/Si(1 1 1)- 6 3 surfaces. Si deposition at 510 K leaves disordered clusters on the surface, which cannot be reordered by annealing up to 800 K. However, annealing above 900 K recreates P/Si(1 1 1)- 6 3 surfaces. Surface morphologies formed by sequential rapid thermal annealing are also presented.

Addimer chain structures: Metastable precursors to island formation on Ge–Si(0 0 1)-(2 × n) alloyed surface

We have identified addimer chain structures as metastable precursors to compact epitaxial islands on the (2 × n) reconstructed SiGe wetting layer, using polarity-switching scanning tunneling microscopy (STM). These chain structures are comprised of 2–12 addimers residing in the troughs of neighboring substrate dimer rows. The chain structures extend along equivalent 〈1 3 0〉 directions across the substrate dimer rows in a zigzag fashion, giving rise to kinked and straight segments. We measure a kink-to-straight ratio of nearly 2:1. This ratio corresponds to a free energy difference of 17 ± 4 meV, favoring the formation of kinked segments. The chain structures convert to compact epitaxial islands at elevated temperatures (⩾90 °C). This conversion suggests that the chain structures are a precursor for compact island formation on the SiGe wetting layer. We digitally process filled- and empty-state STM images to distinguish chain structures from compact islands. By monitoring the populations of both species over time, the chain-to-island conversion rates are measured at substrate temperatures ranging from 90 to 150 °C. The activation energy for the conversion process is measured to be 0.7 ± 0.2 eV with a corresponding pre-exponential factor of 5 × 10 4±2 s −1.

Scanning tunneling microscopy study of the Eu-inducedGe(111)−(3×2)∕(3×4)reconstruction

Eu-induced $(3\ifmmode\times\else\texttimes\fi{}2)$ reconstruction of the Ge(111) surface has been investigated by scanning tunneling microscopy (STM). The empty-state STM images show the chainlike atomic structure that is similar to those of the metal-induced $\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surfaces with an adsorbate coverage of $1∕6$ monolayer (ML). The filled-state STM images combined with the empty-state images at the low bias voltage reveal that the Ge arrangement of $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ can be well interpreted in terms of the honeycomb chain-channel (HCC) model with the characteristic $\mathrm{Ge}\mathrm{Ge}$ double bond and slightly modified Ge honeycomb chains which are similar to those of the $1∕6$-ML HCC structure of $\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$. In addition, the $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surface is found to have a local $\ifmmode\times\else\texttimes\fi{}4$ periodicity along Eu chains, which can be explained, based on the analysis of STM line profiles, with two nonequivalent adsorption sites occupied by the Eu atoms in the empty channels of the HCC structure. The structural modifications of the Ge honeycomb chains as well as the origin of the $\ifmmode\times\else\texttimes\fi{}2$ and $\ifmmode\times\else\texttimes\fi{}4$ chains of Eu atoms in the HCC structure on the $\mathrm{Eu}∕\mathrm{Ge}(111)$ surface are discussed.

Mn clusters on Si(1 1 1) surface: STM investigation

Using scanning tunneling microscopy (STM), the early stages of the Mn/Si(1 1 1) interface formation upon annealing of 0.05–0.30 ML of Mn at 320–550 °C have been studied. It has been found that depending on the growth conditions several kinds of clusters develop at the surface, including “large” clusters 50–115 Å in size and “small” clusters, which occupy single 7 × 7 unit cell halves. Two kinds of the “small” clusters have been distinguished. These are pyramidal clusters built of ∼10 Mn atoms and Mn-ring-clusters, which are seen in the empty-state STM images as a group of six protrusions forming a ring with a diameter of ∼7 Å and a center at the on-top site. The structural properties of the “small” clusters are discussed in some detail.

Long-Range Surface Reconstruction:Si(110)−(16×2)

A variety of reconstruction models is studied for the Si(110)-(16 x 2) surface using first-principles calculations. Assuming appropriate rebonding of edge atoms and surface chains buckled in antiphase, we show that steps along the [112] direction yielding a trench indeed lower the surface energy. We explain the long-range surface reconstruction and develop a geometry model based on steps, adatoms, tetramers, and interstitials. The model is able to explain the stripes of paired pentagons seen obviously in empty-state scanning tunneling microscopy images.

Empty-state scanning tunneling microscopy image of C2H2 on Si(0 0 1)––new evidence for paired end-bridge di-σ configuration

New evidence of the paired end-bridge configuration in the room-temperature adsorption geometries of C2H2 molecules on Si(001) is presented by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. The distinct four-leaved feature occupying two adjacent Si dimer sites in the experimental empty-state STM images are well reproduced by simulations of the paired end-bridge adsorption configuration. Calculated energetics suggests that the Si(001) surface is covered by paired end-bridge structures at the saturation coverage of 1 ML, in agreement with the existing experiments.

3×2reconstruction of the Sm/Si(111) interface

The initial formation of the samarium/silicon interface is studied by combining scanning tunneling microscopy (STM), low-energy electron diffraction, and ab initio pseudopotential calculations. A $\mathrm{Si}(111)3\ifmmode\times\else\texttimes\fi{}2$-Sm reconstruction is formed at a Sm coverage of 1/6 monolayer. High-resolution STM images reveal a strong bias-voltage dependence for the $3\ifmmode\times\else\texttimes\fi{}2$ reconstruction. In the empty-state STM images, the rows with a $\ifmmode\times\else\texttimes\fi{}2$ periodicity are shown at high bias voltages, and attributed to Sm atoms. At low bias voltage, an additional double row feature with a $\ifmmode\times\else\texttimes\fi{}1$ periodicity appears and dominates the empty-state image as the bias voltage decreases. The paired protrusions with the $\ifmmode\times\else\texttimes\fi{}1$ periodicity in the double rows in the empty-state image are attributed to Si atoms. In the filled-state STM images, double rows of protrusions forming zigzag chains with a $3\ifmmode\times\else\texttimes\fi{}1$ structure are observed and assigned to Si atoms. We propose a honeycomb chain-channel model for the $3\ifmmode\times\else\texttimes\fi{}2$ phase, common to alkali metals and alkaline-earth metals on Si(111). Simulated STM images based on this model are in excellent agreement with experiment.

Dopant-pair structures segregated on a hydrogen-terminated Si(100) surface.

Novel atomic structures on a H-terminated Si(100)-(2x1)-H surface were found using scanning tunneling microscopy (STM). The structures are distinguishable only from Si dimers in empty-state STM images. They were observed on arsenic- and phosphorus-doped substrates, but not on boron-doped substrates. Surface density of these structures was found to be proportional to the dopant density in the substrate. First-principles calculations clarify that they are consisting of dopant pairs that are segregated from the bulk material. Hydrogen atoms attached to the dopant pair are found to flip between two positions on the surface due to a quantum effect.