Simulated Scanning Tunneling Microscopy Research Articles

Theoretical STM images of alkaline-earth metal adsorbed Si(1 1 1)3 × 2 surfaces

We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1–Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg–Si interaction.

Adsorption ofSb4on Ge(001) and Si(001) surfaces: Scanning tunneling microscopy and first-principles calculations

We have performed first-principles total energy calculations and scanning tunneling microscopy experiments to study the energetics and the atomic structures resulting from the adsorption of ${\mathrm{Sb}}_{4}$ molecules on the Ge and $\mathrm{Si}(001)\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}4)$ surfaces. Several metastable arrangements are found: ${\mathrm{Sb}}_{4}$ molecules can adsorb as three-dimensional clusters or as flat tetramers. The latter are more stable and can reside either centered on top of the Ge dimer row or half on-top and half over the trench between dimer rows, and in the trenches between two dimer rows. It is found that it is energetically more favorable for ${\mathrm{Sb}}_{4}$ to split into two ${\mathrm{Sb}}_{2}$ dimers adsorbed parallel and on top of the Ge (Si) dimer rows. Simulated scanning tunneling microscopy images are in excellent agreement with the experimental data. The energetics of the diffusion of the dimers into the first Ge (Si) layer is also discussed.

Cross-sectional imaging of sharp Si interlayers embedded in gallium arsenide

We investigate the electronic properties of the (110) cross-sectional surface of Si-doped GaAs using first-principles techniques. We focus on doping configurations with an equal concentration of Si impurities in cationic and anionic sites, such as occurring in a self-compensating doping regime. In particular we study a bilayer of Si atoms uniformly distributed over two consecutive (001) atomic layers. The simulated cross-sectional scanning tunneling microscopy images show a bright signal at negative bias, which is strongly attenuated when the bias is reversed. This scenario is consistent with experimental results which had been attributed to hitherto unidentified Si complexes.

Study of Interaction between Au and TiO<SUB>2</SUB>(110) at Low Coverage

We have investigated the interaction between Au and a rutile TiO 2 (1 10) surface at low coverage, using density functional theory. We have examined Au adsorption on three types of TiO 2 (110) surface with different stoichiometry and structures; the stoichiometric surface, the surface formed by removing bridging-oxygen (defected surface), and the reconstructed 1 x 2 surface with Ti 2 O 3 rows (added-row surface). For the stoichiometric surface, the most stable site for the Au adsorption is the on-top site above the bridging-oxygen atom. Electrons transfer from the Au adatom to the bridging-oxygen atom after adsorption. For the defected surface, the most stable adsorption site is the bridging-oxygen vacant site. For the added-row surface, the most stable adsorption site is the neighborhood of the Ti 2 O 3 rows. For both the reduced surface, defected and added-row surfaces, electron densities between the Au and the reduced Ti atom increase after adsorption, and it seems that the Au atom covalently interacts with the reduced Ti atoms at the surface. Moreover, we compared the simulated scanning tunneling microscopy (STM) images with the experimental STM images for the added-row surface. The calculated STM images of Au adatom adsorbed near Ti 2 O 3 rows agree with experimental images qualitatively.

First-principles investigation of theCa∕Si(111)2×1surface

Atomic and electronic structure of the $\mathrm{Ca}∕\mathrm{Si}(111)2\ifmmode\times\else\texttimes\fi{}1$ surface has been studied by density functional theory calculations. The Seiwatz chain structure with $\mathrm{Ca}$ adsorbed on the ${T}_{4}$ site, suggested in previous experiments, is found to be the most stable among the structural models considered. This Seiwatz-${T}_{4}$ model produces a semiconducting surface band structure: Two filled and one empty surface-state bands are found in the gap originating from the $\mathrm{Si}$ dangling-bond states on the Seiwatz chain and the $\mathrm{Ca}$ 4$s$ states, respectively, which indicates a complete charge transfer from $\mathrm{Ca}$ to the $\mathrm{Si}$ surface. The calculated band dispersions agree well with photoemission data, but the simulated scanning tunneling microscopy images, empty-state images of bright stripes representing the one-dimensional $\mathrm{Ca}$ chains, and filled-state images of hexagonal lobes representing the $\mathrm{Si}$ dangling bonds, partly differ from experimental images of only bright stripes for both empty and filled states.

Phosphine adsorption and dissociation on the Si(001) surface: Anab initiosurvey of structures

We report a comprehensive ab initio survey of possible dissociation intermediates of phosphine $({\mathrm{PH}}_{3})$ on the Si(001) surface. We assign three scanning tunneling microscopy (STM) features, commonly observed in room-temperature dosing experiments, to ${\mathrm{PH}}_{2}+\mathrm{H}$, $\mathrm{PH}+2\mathrm{H}$, and $\mathrm{P}+3\mathrm{H}$ species, respectively, on the basis of calculated energetics and STM simulation. These assignments and a time series of STM images which shows these three STM features converting into another, allow us to outline a mechanism for the complete dissociation of phosphine on the Si(001) surface. This mechanism closes an important gap in the understanding of the doping process of semiconductor devices.

Electronic steps and band offset ofSi−Getwo-dimensional superlattices onBi∕Si(111)

The electronic and structural properties of the Bi-covered $\mathrm{Si}\ensuremath{-}\mathrm{Ge}$ two-dimensional superlattices (SLs) have been investigated employing ab initio total energy calculations. The $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ SLs are formed by Ge stripes with a height of one atomic layer, $\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, separated by Si layers on the Si(111) surface covered with a monolayer of Bi trimers. At the equilibrium geometry we find that the Bi trimers on $\mathrm{Ge}∕\mathrm{Si}(111)$ are higher by $0.10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ compared with the Bi trimers on the $\mathrm{Si}∕\mathrm{Si}(111)$ region. In both regions the Bi trimers lie on the $T4$ sites. Our simulated scanning tunneling microscopy images exhibit bias-dependent ``electronic steps,'' which are in accordance with recent experimental findings [Phys. Rev. Lett. 91, 096102 (2003)]. These bias-dependent electronic steps, which can reach up to $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, are due to the different electronic density of states of the Bi trimers adsorbed on the Ge stripes compared with the Bi trimers on the Si stripes. Since the Si and Ge regions of the SL are electronically distinct, we compared the electronic structures of the isolated systems, $\mathrm{Bi}∕\mathrm{Si}∕\mathrm{Si}(111)$ and $\mathrm{Bi}∕\mathrm{Ge}∕\mathrm{Si}(111)$, with the one of the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ SL. We find a valence band offset of $0.21\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ SL, with the highest occupied states lying on the Ge layers. In contrast, for the lowest unoccupied states we find an almost negligible conduction band offset.

Investigation of the geometric and electronic structures of ErSi (2− x) with the density functional theory and comparison with STM images

Study of ErSi (2− x) surfaces system is submitted at one controversy in reason of the STM images bias voltage dependence. In this article, we have investigated by means of calculations issued of the density functional theory (DFT), atomic surface density for 2D and 3D epitaxial erbium silicide on Si(1 1 1). DOS (density of state) in front of the surface, calculated with Wien2k, permits to simulate scanning tunneling microscopy (STM) images in function of the bias voltage and to discriminate the different surface arrangements. The calculations confirm the model of structure accepted and give some new elements in order to close the controversy on the question.

Simple Model of Electronic Density of States of Graphite and Its Application to the Investigation of Superlattices

A model of graphite which is easy to comprehend and simple to implement for the simulation of scanning tunneling microscopy (STM) images is described. This model simulates the atomic density of graphite layers, which in turn correlates with the local density of states. The mechanism and construction of such a model is explained with all the necessary details which have not been explicitly reported before. This model is applied to the investigation of rippling fringes which have been experimentally observed on a superlattice, and it is found that the rippling fringes are not related to the superlattice itself. A superlattice with abnormal topmost layers interaction is simulated, and the result affirms the validity of the moiré rotation pattern assumption. The “odd-even” transition along the atomic rows of a superlattice is simulated, and the simulation result shows that when there is more than one rotated layer at the top, the “odd-even” transition will not be manifest.

A First-Principles Study of (R)- and (S)-PPA Molecules on Cu(111)

The adsorption of (R)- and (S)-2-phenylpropionamide (PPA, C(9)H(11)ON) molecules on a Cu(111) surface has been investigated using the density functional method with supercell models. The adsorption orientations of both (R)- and (S)-PPA molecules on the surface are the same: the phenyl rings are approximately parallel to the Cu(111) surface and positioned in the hollow sites, the amino and methyl groups occupy two-bridge sites, and the carbonyl occupies the top site. After the adsorption, the bond lengths in the two enantiomers are almost unchanged, but the changes for two dihedral angles show differences, especially for (R)-PPA molecule. The first angles between the (N,C9,C7) plane and the (C9,C7,C6) plane are 19.4 and 0.7 degrees for (R)- and (S)-PPA molecules, respectively, and the second angles between the (C8,C7,C6) plane and the (C7,C6,C5) plane are 74.8 and 0.4 degrees for (R)- and (S)-PPA molecules, respectively. The adsorption energies of (R)- and (S)-PPA molecules are calculated to be -34 and -26 kJ mol(-1), respectively. The simulated scanning tunneling microscopy (STM) images of (R)- and (S)-PPA molecules on the Cu(111) surface display different features and are coincident with the experimental ones. The interaction between the adsorption molecule and the metal surface is found to be responsible for the discrimination of (R)- and (S)-PPA molecules on the surface.

Electronic structure of one-dimensional indium chains on Si(111)

The driving force for the phase transition of quasi-one-dimensional (1D) indium chains on the $\mathrm{Si}(111)\text{\ensuremath{-}}4\ifmmode\times\else\texttimes\fi{}1$ surface has been controversial. Using first-principles density-functional calculations we investigate the surface band structure of the low-temperature phase including a periodic lattice distortion. We find that the surface states ${m}_{2}$ and ${m}_{3}$ hybridize to yield a band-gap opening, while the surface state ${m}_{1}$ crosses the Fermi level. The simulated scanning tunneling microscopy (STM) images reproduce the features observed in the STM measurements, such as a double periodicity and an out-of-phase distribution in the filled- and empty-state images. The present results agree in many respects with recent photoelectron spectroscopy and STM experiments but do not support a 1D charge-density-wave mechanism accompanying a metal-insulator transition.

Two-Dimensional Carbon Incorporation into Si(001): C Amount and Structure ofSi(001)−c(4×4)

The C amount and the structure of the Si(001)-c(4 x 4) surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The c(4 x 4) phase is found to contain 1/8 monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The c(4 x 4) phase constitutes the subsurface Si(0.875)C(0.125) delta layer with two-dimensionally ordered C atoms.

Tip and Surface Determination from Experiments and Simulations of Scanning Tunneling Microscopy and Spectroscopy

We present a very efficient and accurate method to simulate scanning tunneling microscopy images and spectra from first-principles density functional calculations. The wave functions of the tip and sample are calculated separately on the same footing and propagated far from the surface using the vacuum Green function. This allows us to express the Bardeen matrix elements in terms of convolutions and to obtain the tunneling current at all tip positions and bias voltages in a single calculation. The efficiency of the method opens the door to real time determination of both tip and surface composition and structure, by comparing experiments to simulated images for a variety of precomputed tips. Comparison with the experimental topography and spectra of the Si111-(7 x 7) surface shows a much better agreement with Si than with W tips, implying that the metallic tip is terminated by silicon.

Coadsorption phases ofCOand oxygen on Pd(111) studied by scanning tunneling microscopy

The adsorption of $\mathrm{C}\mathrm{O}$ on an oxygen precovered Pd(111) surface was investigated between 60 and 300 K. Applied methods were variable temperature scanning tunneling microscopy (STM) and video STM to analyze the coadsorption structures. The STM data are compared with simulated STM images for the various surface phases in order to identify the appropiate structural model for each case. Low-energy electron diffraction and reaction isotherms by means of mass spectrometry were used to correlate the phases with the reaction yielding $\mathrm{C}{\mathrm{O}}_{2}$. The video-STM data recorded during $\mathrm{C}\mathrm{O}$ adsorption at 300 K on the ${(2\ifmmode\times\else\texttimes\fi{}2)}_{\mathrm{O}}$ phase show a fast phase transition into the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}_{\mathrm{O}}^{\mathrm{\ensuremath{\circ}}}$ structure, followed by reaction to $\mathrm{C}{\mathrm{O}}_{2}$. The reaction only starts after completion of the phase transition, indicating that the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}_{\mathrm{O}}^{\ensuremath{\circ}}$ structure plays a crucial role for the reaction. At temperatures between 170 and 190 K the phase transition is slow enough to be monitored with STM. The experimental images of both the ${(2\ifmmode\times\else\texttimes\fi{}2)}_{\mathrm{O}}$ and the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}_{\mathrm{O}}^{\ensuremath{\circ}}$ structures are well reproduced by the simulations. Further $\mathrm{C}\mathrm{O}$ adsorption caused a second phase transition into a $p{(2\ifmmode\times\else\texttimes\fi{}1)}_{\mathrm{O}}$ structure. The STM simulations strongly support a pure oxygen $p(2\ifmmode\times\else\texttimes\fi{}1)$ structure, rather than a mixed $\mathrm{O}+\mathrm{C}\mathrm{O}$ structure, in contrast to previous experimental work. The $\mathrm{C}\mathrm{O}$ molecules form the same structures between the $\mathrm{O}$ islands that are known from the pure Pd(111)/$\mathrm{C}\mathrm{O}$ system. At lower temperatures, between 110 and 60 K, a so far unknown $(2\ifmmode\times\else\texttimes\fi{}2)$ phase was observed. The formation of this structure, and its imaging by the STM, show that it constitutes a mixed $p{(2\ifmmode\times\else\texttimes\fi{}2)}_{\mathrm{O}+\mathrm{C}\mathrm{O}}$ structure, where the oxygen atoms remain unchanged, and the $\mathrm{C}\mathrm{O}$ molecules occupy hcp sites between the $\mathrm{O}$ atoms.

The geometry of Bi nanolines on Si(0 0 1)

A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab initio theoretical technique and scanning tunnelling microscopy (STM). Our calculations demonstrate decisively that the recently proposed Haiku geometry is a lower energy configuration than any of the previously proposed line geometries. Furthermore, we have made comparisons between STM constant-current topographs of the lines and Tersoff–Haman STM simulations. Although the Haiku and the Miki geometries both reproduce the main features of the constant-current topographs, the simulated STM images of the Miki geometry have a dark stripe between the dimer rows that does not correspond well with experiment.

Localization of the Cu(111) Surface State by Single Cu Adatoms

The Cu adatom-induced localization of the two-dimensional Shockley surface state at the Cu(111) surface was identified from experimental and simulated scanning tunneling microscopy spectra. The localization gives rise to a resonance located just below the surface state band edge. The adatom-induced surface state localization is discussed in terms of the existence theorem for bound states in any attractive two-dimensional potential. We also identify adatom-induced resonance states deriving from atomic orbitals in both experimental and simulated spectra.

Novel water overlayer growth on Pd(111) characterized with scanning tunneling microscopy and density functional theory.

Scanning tunneling microscopy (STM) images of water submonolayers on Pd(111) reveal quasiperiodic and isolated adclusters with internal structure that would ordinarily be ascribed to icelike puckered hexagonal units. However, density functional theory and STM simulations contradict this conventional picture, showing instead that the water adlayers are composed mainly of flat-lying molecules arranged in planar water hexagons. A new rule for two dimensional (2D) water growth is offered that generates the structures observed experimentally from planar hexamer units.

Simulated scanning tunneling microscopy images of three-dimensional clusters:H8Si8O12onSi(100)−2×1

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Structural elements on reconstructed Si andGe(110)surfaces

We present ab initio studies of possible reconstruction elements on Si and $\text{Ge}(110)$ surfaces. Using $2\ifmmode\times\else\texttimes\fi{}2$, $3\ifmmode\times\else\texttimes\fi{}2$, and $6\ifmmode\times\else\texttimes\fi{}2$ unit cells we optimize models with buckled atomic chains, dimers, different adatom distributions, and interstitial atoms which may exist on the larger $\text{Si}(110)16\ifmmode\times\else\texttimes\fi{}2$ or $\text{Ge}(110)16\ifmmode\times\else\texttimes\fi{}2∕c(8\ifmmode\times\else\texttimes\fi{}10)$ surface reconstructions. We show that adatom reconstructions gain energy. Only the adatom model which seemingly leaves no dangling bonds cannot occur on Si and $\text{Ge}(110)$ surfaces. An adatom-rest atom electron transfer mechanism is more favorable. An adatom-tetramer-interstitial $3\ifmmode\times\else\texttimes\fi{}2$ model also stabilizes the Si and Ge surfaces and leads to a semiconducting behavior (at least for Si). Simulated scanning tunneling microscopy (STM) images of empty states of this reconstruction look like the pentagon structures observed on $\text{Si}(110)16\ifmmode\times\else\texttimes\fi{}2$. A $6\ifmmode\times\else\texttimes\fi{}2$ reconstruction with five-membered adclusters is energetically completely unfavorable, though it also reproduces the empty-state pentagonlike STM images.

Bi-covered InAs(110) surfaces: An ab initio study

Abstract In this work we have performed an ab initio investigation of the Bi-covered InAs(1 1 0) surface. We have considered two different structural models viz. ECLS and (1 × 2) missing row model. Our total energy results indicate that the (1 × 2) model is energetically more favourable than the ECLS model, thus, supporting the recent experimental results by Betti et al. [Phys. Rev. B 59 (1999) 15760]. However, the calculated equilibrium atomic geometry indicates some disagreement with respect to the experimental measurements. The “surface electronic topology” of the Bi/InAs(1 1 0) surface was investigated through simulations of scanning tunneling microscopy (STM) images, where we inferred a semimetallic character along the Bi-chains of the (1 × 2) model.