Scanning Tunneling Microscopy Results Research Articles

Initial stage of the adsorption of fluorofullerene molecules on Si surface

Spatially resolved images of an individual C60F18 fluorofullerene molecule on Si(100) − 2 × 1 surface have been obtained using scanning tunneling microscopy. Scanning tunneling microscopy results and ab initio calculations show that the fluorofullerene molecules interact with the Si(100) − 2 × 1 surface with F atoms pointing down towards the surface. The adsorption energy of a C60F18 molecule on Si(100) − 2 × 1 surface is ∼12.1 eV, which is much higher than the adsorption energy of the same molecule on Si(111) − 7 × 7 surface (6.65 eV). C60F18 molecules are located in the troughs in-between the dimer rows occupying the four-dimer site on Si(100) − 2 × 1 surface.

Atomic surface structure of Si(1 0 0) substrates prepared in a chemical vapor environment

Subsequent III–V integration by metal-organic vapor phase epitaxy (MOVPE) or chemical vapor deposition (CVD) necessitates elaborate preparation of Si(100) substrates in chemical vapor environments characterized by the presence of hydrogen used as process gas and of various precursor molecules. The atomic structure of Si(100) surfaces prepared in a MOVPE reactor was investigated by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) available through a dedicated, contamination-free sample transfer to ultra high vacuum (UHV). Since the substrate misorientation has a fundamental impact on the atomic surface structure, we selected a representative set consisting of Si(100) with 0.1°, 2° and 6° off-cut in [011] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(100) surfaces indicated two-domain (2×1)/(1×2) reconstructions for lower misorientations implying a predominance of single-layer steps undesirable for subsequent III–V layers. However, double-layer steps developed on 6° misoriented Si(100) substrates, but STM also showed odd-numbered step heights and LEED confirmed the presence of minority surface reconstruction domains. Strongly depending on misorientation, the STM images revealed complex step structures correlated to the relative dimer orientation on the terraces.

Defect Structure of Ultrathin Ceria Films on Pt(111): Atomic Views from Scanning Tunnelling Microscopy

Atomically resolved scanning tunnelling microscopy (STM) images have been obtained on ultrathin films of CeO2(111) supported on Pt(111). The ultrathin films were grown in two ways, by reactive deposition in an oxygen atmosphere and by postoxidation of Ce/Pt surface alloys. STM results are compared with previously reported high-temperature STM and noncontact atomic force microscopy (NC-AFM) images of the native CeO2(111) surface. The similarity between these images is striking and allows a number of defects and adsorbates in our ultrathin film to be assigned. Moreover, the similarity in structure between the native oxide and the ceria ultrathin film indicates that it is an excellent topographic mimic of the native oxide.

Irreversible Change in the NO Adsorption State on Pt(111) under High Pressure Studied by AP-XPS, NEXAFS, and STM

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been applied to the investigation of NO adsorption on Pt(111) under various pressures of NO (up to 1 Torr) at room temperature. Under 10−7 Torr, molecular NO occupies the most stable fcc-hollow sites and partially occupies the energetically unfavorable atop sites. NO reversibly desorbs from the atop sites after evacuation. At NO pressures higher than 10−6 Torr, however, irreversible adsorption of atomic oxygen takes place via NO dissociation, leading to the formation of NO+O domains. This result is consistent with near-edge X-ray absorption fine structure (NEXAFS) and scanning tunneling microscopy (STM) results.

One-dimensional electronic states inGa2Se3on Si(001):As

Gallium selenide Ga2Se3 is a III-VI chalcogenide material whose intrinsic cation vacancy ordering into one dimensional chains has been predicted to result in a one dimensional band at the valence-band maximum VBM. The electronic structure of Ga2Se3 thin films on Si001:As is studied with angle-resolved photoemission spectroscopy. The integrated density of states and the dispersion exhibit good agreement with that predicted by published density-functional theory results. Consistent with a one dimensional state, the electronic states at the VBM show no dispersion perpendicular to the vacancy chains. The Se-Ga bond length from extended x-ray absorption fine structure is 2.34 A. Low-energy electron-diffraction results indicate that the surface is characterized by nanometer-scale 111 facets, consistent with previous scanning tunneling microscopy results.

Atomic and electronic structures of the ordered23×23and molten1×1phase on the Si(111):Sn surface

In this work, a selection of unresolved topics regarding the electronic and atomic structures of Si and Ge surfaces, both clean ones and those modified by metal adsorbates, are addressed. The results presented have been obtained using theoretical calculations and experimental techniques such as photoelectron spectroscopy (PES), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Si(001) surfaces with adsorbed alkali metals can function as prototype systems for studying properties of the technologically important family of metal-semiconductor interfaces. In this work, the effect of up to one monolayer (ML) of Li on the Si(001) surface is studied using a combination of experimental and theoretical techniques. Several models for the surface atomic structures have been suggested for 0.5 and 1 ML of Li in the literature. Through the combination of experiment and theory, critical differences in the surface electronic structures between the different atomic models are identified and used to determine the most likely model for a certain Li coverage. In the literature, there are reports of an electronic structure at elevated temperature, that can be probed using angle resolved PES (ARPES), on the clean Ge(001) and Si(001) surfaces. The structure is quite unusual in the sense that it appears at an energy position above the Fermi level. Using results from a combined variable temperature ARPES and LEED study, the origin of this structure is determined. Various explanations for the structure that are available in the literature are discussed. It is found that all but thermal occupation of an ordinarily empty surface state band are inconsistent with our experimental data. In a combined theoretical and experimental study, the surface core-level shifts on clean Si(001) and Ge(001) in the c(4×2) reconstruction are investigated. In the case of the Ge 3d core-level, no previous theoretical results from the c(4×2) reconstruction are available in the literature. The unique calculated Ge 3d surface core-level shifts facilitate the identification of the atomic origins of the components in the PES data. Positive assignments can be made for seven of the eight inequivalent groups of atoms in the four topmost layers in the Ge case. Furthermore, a similar, detailed, assignment of the atomic origins of the shifts on the Si surface is presented that goes beyond previously published results. At a Sn coverage of slightly more than one ML, a 2√3 × 2√3 reconstruction can be obtained on the Si(111) surface. Two aspects of this surface are explored and presented in this work. First, theoretically derived results obtained from an atomic model in the literature are tested against new ARPES and STM data. It is concluded that the model needs to be revised in order to better explain the experimental observations. The second part is focused on the abrupt and reversible transition to a molten 1×1 phase at a temperature of about 463 K. ARPES and STM results obtained slightly below and slightly above the transition temperature reveal that the surface band structure, as well as the atomic structure, changes drastically at the transition. Six surface states are resolved on the surface at low temperature. Above the transition, the photoemission spectra are, on the other hand, dominated by a single strong surface state band. It shows a dispersion similar to that of a calculated surface band associated with the Sn-Si bond on a 1×1 surface with Sn positioned above the top layer Si atoms. There has been extensive studies of the reconstructions on Si surfaces induced by adsorption of the group III metals Al, Ga and In. Recently, this has been expanded to Tl, i.e., the heaviest element in that group. Tl is different from the other elements in group III since it exhibits a peculiar behavior of the 6s2 electrons called the “inert pair effect”. This could lead to a valence state of either 1+ or 3+. In this work, core-level PES is utilized to find that, at coverages up to one ML, Tl exhibits a 1+ valence state on Si(111), in contrast to the 3+ valence state of the other group III metals. Accordingly, the surface band structure of the 1/3 ML √3 x √3 reconstruction is found to be different in the case of Tl, compared to the other group III metals. The observations of a 1+ valence state are consistent with ARPES results from the Si(001):Tl surface at one ML. There, six surface state bands are seen. Through comparisons with a calculated surface band structure, four of those can be identified. The two remaining bands are very similar to those observed on the clean Si(001) surface.

Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and density functional theory calculations

In this work, a selection of unresolved topics regarding the electronic and atomic structures of Si and Ge surfaces, both clean ones and those modified by metal adsorbates, are addressed. The results presented have been obtained using theoretical calculations and experimental techniques such as photoelectron spectroscopy (PES), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Si(001) surfaces with adsorbed alkali metals can function as prototype systems for studying properties of the technologically important family of metal-semiconductor interfaces. In this work, the effect of up to one monolayer (ML) of Li on the Si(001) surface is studied using a combination of experimental and theoretical techniques. Several models for the surface atomic structures have been suggested for 0.5 and 1 ML of Li in the literature. Through the combination of experiment and theory, critical differences in the surface electronic structures between the different atomic models are identified and used to determine the most likely model for a certain Li coverage. In the literature, there are reports of an electronic structure at elevated temperature, that can be probed using angle resolved PES (ARPES), on the clean Ge(001) and Si(001) surfaces. The structure is quite unusual in the sense that it appears at an energy position above the Fermi level. Using results from a combined variable temperature ARPES and LEED study, the origin of this structure is determined. Various explanations for the structure that are available in the literature are discussed. It is found that all but thermal occupation of an ordinarily empty surface state band are inconsistent with our experimental data. In a combined theoretical and experimental study, the surface core-level shifts on clean Si(001) and Ge(001) in the c(4×2) reconstruction are investigated. In the case of the Ge 3d core-level, no previous theoretical results from the c(4×2) reconstruction are available in the literature. The unique calculated Ge 3d surface core-level shifts facilitate the identification of the atomic origins of the components in the PES data. Positive assignments can be made for seven of the eight inequivalent groups of atoms in the four topmost layers in the Ge case. Furthermore, a similar, detailed, assignment of the atomic origins of the shifts on the Si surface is presented that goes beyond previously published results. At a Sn coverage of slightly more than one ML, a 2√3 × 2√3 reconstruction can be obtained on the Si(111) surface. Two aspects of this surface are explored and presented in this work. First, theoretically derived results obtained from an atomic model in the literature are tested against new ARPES and STM data. It is concluded that the model needs to be revised in order to better explain the experimental observations. The second part is focused on the abrupt and reversible transition to a molten 1×1 phase at a temperature of about 463 K. ARPES and STM results obtained slightly below and slightly above the transition temperature reveal that the surface band structure, as well as the atomic structure, changes drastically at the transition. Six surface states are resolved on the surface at low temperature. Above the transition, the photoemission spectra are, on the other hand, dominated by a single strong surface state band. It shows a dispersion similar to that of a calculated surface band associated with the Sn-Si bond on a 1×1 surface with Sn positioned above the top layer Si atoms. There has been extensive studies of the reconstructions on Si surfaces induced by adsorption of the group III metals Al, Ga and In. Recently, this has been expanded to Tl, i.e., the heaviest element in that group. Tl is different from the other elements in group III since it exhibits a peculiar behavior of the 6s2 electrons called the “inert pair effect”. This could lead to a valence state of either 1+ or 3+. In this work, core-level PES is utilized to find that, at coverages up to one ML, Tl exhibits a 1+ valence state on Si(111), in contrast to the 3+ valence state of the other group III metals. Accordingly, the surface band structure of the 1/3 ML √3 x √3 reconstruction is found to be different in the case of Tl, compared to the other group III metals. The observations of a 1+ valence state are consistent with ARPES results from the Si(001):Tl surface at one ML. There, six surface state bands are seen. Through comparisons with a calculated surface band structure, four of those can be identified. The two remaining bands are very similar to those observed on the clean Si(001) surface.

Water Adsorption on TiO2

Scanning Tunneling Microscopy (STM) studies and Density Functional Theory (DFT) investigations of the interaction of water with the rutile TiO2 (110) surface are summarized. From high-resolution STM the following reactions have been revealed: water adsorption and diffusion in the Ti troughs, water dissociation in bridging oxygen vacancies, assembly of adsorbed water monomers into rapidly diffusing water dimers, and formation of water dimers by reduction of oxygen molecules. The STM results are rationalized based on DFT calculations, revealing the bonding geometries and reaction pathways of the water species on the TiO2 (110) surface.

Nano-faceting of the Ru surface

Nano-faceting of Ru (112¯0) induced by NO2 is studied by scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The surface becomes fully faceted and covered with oxygen upon annealing the sample at T ⩾ 650 K in NO2 (10−8 Torr) and the faceted surface is stable in ultra high vacuum (UHV) for sample temperatures <850 K. The kinematical LEED simulations and STM results show that the faceted surface consists of uniform long ridges running along [0 0 0 1] direction with four facets (101¯1),(011¯1),(101¯1¯), and (011¯1¯) on the ridges.

Hard repulsive barrier in hot adatom motion during dissociative adsorption of oxygen on Ag(100)

Random pairing simulation and low temperature scanning tunneling microscopy (STM) are used to investigate the detailed O(2) dissociative adsorption processes at 200 K for various coverages. The distribution of oxygen adatoms shows a strong repulsion between the adsorbates with a radius of approximately 0.8 nm. The comparison between STM results and simulation reveals two prominent pairing distances of 2 and 4 nm and their branching ratio is about 2:1. These findings shed new light on the origin of the large intrapair distances found and on the process behind the empirical "eight-site rule."

Bidentate chemisorption of acetic acid on aSi(001)−(2×1)surface: Experimental and theoretical investigations

Adsorption of acetic acid on the $\text{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface is studied by x-ray photoelectron spectroscopy (XPS), scanning-tunneling microscopy (STM), and ab initio calculations. We find that $\text{C}\text{ }1s$ line of the acetic acid on the surface is composed of mainly methyl and adsorbed carboxyl groups. The filled-state STM images indicate that protrusions of the adsorbate are located on center of single Si dimer. The on-top configuration with a vertical $\text{Si-O-C}({\text{CH}}_{3})\text{-O-Si}$ linkage to the Si dimer is supported by these XPS and STM results. The ab initio calculations show a plausible reaction pathway from the initial adsorption structure with O-H cleavage to the on-top configuration.

STM Investigation of Temperature-Dependent Two-Dimensional Supramolecular Architectures of C60 and Amino-tetraphenylporphyrin on Ag(110)

Multicomponent supramolecular self-assemblies of exceptional long-range order and low defectivity are obtained if C(60) and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on Ag(110) by sequential evaporation in the submonolayer range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. A (+/-2 -3, 6 +/- 3) array consisting of supramolecular stripes of a 1:1 C(60)/TPP-NH2 2D adduct develops at 410 K (the low temperature, LT, phase). If the LT phase is annealed at 470 K, then a 3:1 fullerene/TPP-NH2 (+/-3 -5, 5 +/- 5) nanoporous array (the HT phase) forms, with each pore containing a single porphyrin molecule. Phase separation occurs by annealing the HT phase at 520 K. Structural models are proposed and discussed on the basis of the experimental scanning tunneling microscopy results.

Scanning tunneling microscopy at theNbSe3surface: Evidence for interaction betweenq1andq2charge density waves in the pinned regime

We have investigated both ${\mathbf{q}}_{\mathbf{1}}$ and ${\mathbf{q}}_{\mathbf{2}}$ charge density wave (CDW) states taking place in ${\text{NbSe}}_{3}$ by means of low-temperature scanning tunneling microscopy (STM) under ultrahigh vacuum on the in situ cleaved $(\mathbit{b},\mathbit{c})$ surface. High-resolution topographical images with atomic lattice resolution were obtained in the temperature range between 5 and 140 K. The careful and thorough analysis of the dependence of the STM images on bias polarity, energy, and temperature allowed us to identify unambiguously the three different types of chains composing the ${\text{NbSe}}_{3}$ unit cell at all temperatures, resolving contradictions from previous STM results. From two-dimensional Fourier transform of the STM images, we show that at the surface plane both CDW's wave vectors are in very good agreement with bulk reported values projected on the $(\mathbit{b},\mathbit{c})$ plane. The ${\mathbf{q}}_{\mathbf{1}}$ CDW has, for wave vector, ${\mathbf{q}}_{\mathbf{1}}=0.24{\mathbit{b}}^{\ensuremath{\ast}}$. Spatially, the ${\mathbf{q}}_{\mathbf{1}}$ modulation is essentially developed on type III chains with a weak contribution on type II neighboring chains. The ${\mathbf{q}}_{\mathbf{2}}$ CDW has, for wave vector, the projected value ${\mathbf{q}}_{\mathbf{2}\mathbit{p}}=0.26{\mathbit{b}}^{\ensuremath{\ast}}+0.5{\mathbit{c}}^{\ensuremath{\ast}}$. This modulation is mainly developed on type I chains but surprisingly has an important contribution on type III chains with an amplitude similar to the ${\mathbf{q}}_{\mathbf{1}}$ contribution on these chains. This simultaneous double modulation on chain III leads to a beating phenomenon between the ${\mathbf{q}}_{\mathbf{1}}$ and ${\mathbf{q}}_{\mathbf{2}\mathbit{p}}$ periodicities and gives rise to a new domain superstructure developed along the chain axis which is characterized by the vector $\mathbit{u}=2\ifmmode\times\else\texttimes\fi{}({\mathbf{q}}_{\mathbf{2}\mathbit{p}}\ensuremath{-}{\mathbf{q}}_{\mathbf{1}})\ensuremath{-}{\mathbit{c}}^{\ensuremath{\ast}}=2\ifmmode\times\else\texttimes\fi{}(0.26\ensuremath{-}0.24){\mathbit{b}}^{\ensuremath{\ast}}$. We believe that these new features give a clue of the coupling between the ${\mathbf{q}}_{\mathbf{1}}$ and ${\mathbf{q}}_{\mathbf{2}}$ CDWs in the pinned regime. Whereas most studies investigated the various characteristics of both CDWs by probing the Nb atoms properties, our results are consistent with the interpretation according to which the electronic local density of states probed by STM is mostly that of the surface Se atoms.

Intrarow Adsorption Structure of Glycine on Ge(100)

The adsorption structure of glycine on Ge(100) was investigated using scanning tunneling microscopy (STM), density functional theory (DFT) calculations, and high-resolution core-level photoemission spectroscopy (HRCLPES). We found a major adsorption feature of glycine on Ge(100) in the STM images. This feature appeared as a bright protrusion between two dimer rows with a dark adjacent dimer. The position of the bright protrusion located in the middle of the two dimer rows indicates a multibonding adsorption structure. The results of the theoretical calculations confirm that the adsorption structure of glycine on Ge(100) (between two possible multibonding adsorption structures) is an "intrarow O-H dissociated and N dative bonded structure". In the HRCLPES experiments, we found an N 1s peak (at 399.5 eV) and two O 1s peaks (at 531.1 and 532.0 eV), which represent strong evidence that the adsorption configuration of glycine on Ge(100) is composed of both O-H dissociation and N dative bonding. All our STM, DFT, and HRCLPES results suggest that the adsorption structure of glycine molecules on Ge(100) is an "intrarow O-H dissociated and N dative bonded structure".

Atomic processes during Cl supersaturation etching ofSi(100)−(2×1)

Supersaturation etching starts with Cl insertion into Si-Si bonds of Si(100) and leads to the desorption of ${\text{SiCl}}_{2}$ pairs. During etching, insertion occurs through a ${\text{Cl}}_{2}$ dissociative chemisorption process mediated by single dangling bond sites created by phonon-activated electron-stimulated desorption of atomic Cl. Based on scanning tunneling microscopy results, we identify a surface species, describe its involvement in supersaturation etching, and explore the energetics that control this process. In doing so, we show that insertion occurs at room temperature and that paired dangling bonds of bare dimers also mediate this process.

Observation of All the Intermediate Steps of a Chemical Reaction on an Oxide Surface by Scanning Tunneling Microscopy

By means of high-resolution scanning tunneling microscopy (STM), we have revealed unprecedented details about the intermediate steps for a surface-catalyzed reaction. Specifically, we studied the oxidation of H adatoms by O(2) molecules on the rutile TiO(2)(110) surface. O(2) adsorbs and successively reacts with the H adatoms, resulting in the formation of water species. Using time-lapsed STM imaging, we have unraveled the individual reaction intermediates of HO(2), H(2)O(2), and H(3)O(2) stoichiometry and the final reaction product-pairs of water molecules, [H(2)O](2). Because of their different appearance and mobility, these four species are discernible in the time-lapsed STM images. The interpretation of the STM results is corroborated by density functional theory calculations. The presented experimental and theoretical results are discussed with respect to previous reports where other reaction mechanisms have been put forward.

Characterization of size-quantized PbTe thin films synthesized by an electrochemical co-deposition method

Size-quantized thin films of PbTe were electrodeposited on Au (1 1 1) substrates using a practical electrochemical method, based on the simultaneous underpotential deposition of Pb and Te from the same solution containing ethylenediamine tetraacetic acid, Pb 2+, and TeO 3 2− at a constant potential. These thin films were characterized by X-ray diffraction (XRD), scanning tunneling microscopy (STM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), and reflection absorption-FTIR (RA-FTIR). AFM, STM, and XRD results indicate that the growth of PbTe thin films follows the nucleation and two dimensional growth mechanism, resulting in high crystalline films of PbTe (2 0 0) in cubic structure, which was grown at a kinetically preferred orientation on Au (1 1 1). The EDS analyses of the films reveal that Pb and Te are present in an atomic ratio of approximately 1:1. The quantum-confined effect of the PbTe thin films are confirmed by the RA-FTIR measurements.

Surface study of the (100) and (010) faces of the quasicrystalapproximant Al4(Cr, Fe)

Abstract Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) results are used to study the pseudo-6-fold nature of the (100) surface of the orthorhombic quasicrystal approximant Al4(Cr, Fe). LEED patterns are also presented from the pseudo-10-fold (010) surface of this material. In each case the results are compared with the known bulk structure of this complex metallic alloy.

Core level photoemission and STM characterization of Ta/Si(1 1 1)-7 × 7 interfaces

We present the results of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) of the Ta/Si(1 1 1)-7 × 7 system after deposition of Ta at substrate temperatures from 300 to 1250 K. The coverage of Ta varied from 0.05 up to 2.5 of a monolayer (ML). STM shows that at 300 K and coverage less than 1 ML, a disordered chemisorbed phase is formed. Deposition on a hot surface (above 500 K) produces round 3D clusters randomly distributed on the surface. Cluster height and their diameter are found to change drastically with annealing temperature and the Ta coverage. Analysis of photoemission data of the Si 2p core levels shows that at room temperature and at coverage ⩽1 ML core level binding energy shifts and intensity variations of Si surface related components are observed, which clearly indicate that the reaction starts already at 300 K. Shifts in the binding energy, changes of the peak shapes and intensity of the Ta 4f doublet at higher temperatures can be explained by the formation of stable silicide on the surface.

Surface structure of Sn-doped In2O3 (111) thin films by STM

High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ∼1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 × 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.