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Origin of the apparent (2× 1) topography of the Si (100)-c (4× 2) surface observed in low-temperature STM images
Abstract
Low-temperature scanning tunneling microscope (STM) images of the Si(100) surface showing apparent (2×1) atom dimer lines have recently been reported. Using experimental and theoretical approaches, it is demonstrated how those (2×1)-like images result from a c(4×2) surface reconstruction imaged at high bias voltages. In the STM junction, the surface contribution of 3px surface-state electronic resonances relative to the 3pz states is bias voltage dependent. The apparent (2×1) STM images result from an increase in the number of bulk Si electronic channels amplifying Si(100)-c(4×2) surface 3px surface states contribution to the tunneling current with respect to the one of 3pz states.
... Surprisingly, further cooling below ~20 K causes the buckled dimers to appear symmetric again 14,15 . Such symmetric-dimer STM images at low temperature have been explained in terms of various origins such as a dynamical flip-flop motion of buckled dimers 15,16 , local surface charging effects 17 , a possible asymmetric p(2 × 1) reconstruction 18 , an inelastic tunneling mechanism via electron-vibration coupling 19 , and a contribution of bulk states 20,21 . However, the microscopic mechanism underlying the low-temperature symmetric dimer images has remained an open question. ...
... 21 reported that at 7 K the negative bias voltages smaller than − 1.5 V remained a c(4 × 2) reconstruction, but those larger than − 1.5 V produced symmetric dimer images. On the basis of existing low-temperature STM data [15][16][17]21 , the following questions on the appearance of symmetric dimer images can be raised: i.e., Why does the activation barrier (E b ) for the flipping of buckled dimers become much reduced at low temperature? What is the reason why the filled-state and empty-state STM images exhibit symmetric and buckled dimer structures, respectively? ...
... To account for the symmetric dimer images observed from low-temperature STM experiments [15][16][17]21 , we investigate the flip-flop motion of buckled dimers driven by either thermal activation 32 or quantum tunneling. For this, we employ a symmetric double-well potential [see Fig. 1(b)] that describes the potential energy surface of flipping dimers as a function of θ. ...
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.
... Note that the apparition of the c(4 × 2) or p (2 × 2) phase in the STM topography can be slightly shifted near the valence band or near the conduction band as the silicon is p-type or n-type doped, respectively. In fact, a detailed analysis of the local density of states (LDOS) involved in the STM topographies acquired at low biases (i.e., below 2.5 V) reveals that for surface biases outside the range −0.8 to +0.8 V, the acquired STM topographies arise from a mix of the π , σ and π * , σ * states that are located within the first silicon atom layers of the silicon surface [42,43]. While the investigations concerning the Si(1 0 0) surface have fed a long tradition of new impacting results and debates, there is actually no study devoted to a global description of the electronic properties of the Si(1 0 0) surface and in particular one that takes into account the existence of corrugating and anticorrugating states in the Si(1 0 0) silicon surface. ...
... This type of transition can usually occur in confined systems in which the DOS is restricted in 1D or 2D areas such as in graphene sheet [75]. Similar DOS shape has already been related to Van Hove singularities (VHS) [42,76] where critical points allow very specific effects such as resonant electronic transport. The tunnel current jump is observed at the second phase shift energy and is coherently related to a resonant effect as observed in the E 10 energy window. ...
In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the [Formula: see text] direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.
... Therefore, every four dimers on two adjacent rows easily form a stable structure, which is the well-known c(4×2) reconstruction. The c(4×2) reconstruction pattern was found to be dominant (50-60% at room temperature) and stable, consistent with the experiment by Manzano et al. [9]. Therefore, the Si(100)-c(4×2) surface is taken as the most appropriate and applicable research object for DA on the silicon surface. ...
The adsorption of silicon tetrachloride (STC, SiCl4) on the silicon surface is a crucial process in polysilicon manufacture. However, the underlying mechanism for the adsorption remains highly uncertain. Here, new dissociative adsorption (DA) reaction pathways involving a flip of a silicon dimer in the first layer and considering physisorption are identified. Different DA patterns, inter-row (IR), inter-dimer (ID), and on-dimer (OD), are confirmed by the density functional theory (DFT) calculations at the PBE-D3(BJ)/TZVP-MOLOPT-GTH level. The stable structures for all minima are searched by global optimization through the artificial bee colony (ABC) algorithm. Findings reveal that the parent molecules dissociate first by breaking one Si-Cl bond, following which the resulting SiCl3 and Cl fragments are attached to adjacent Si-atom sites. Moreover, dimer flipping significantly reduces the energy barrier for chemisorption, mainly due to the change in electronic structure that enhances the interaction of the site with the SiCl3 radical. Physisorption may also be accompanied by dimer flipping to form a stable adsorption structure.
... As a result, the p(2 × 2) and c(4 × 2) structures consisting alternative buckling dimers are seen in the STM images and the intrinsic properties of the clean Si(100) becomes quite complex [32,33]. Upon dosing atomic hydrogen on a clean Si(100) surface at ∼590 K, the π-bond in a bare Si dimer is replaced by two strong Si-H bonds while the dimer structure remains intact, yielding an H:Si(100)-(2 × 1) surface composed of symmetric monohydride dimers (H-Si-Si-H) [29]. ...
A fundamental question for the adsorption of any gas molecule on surfaces is its saturation coverage, whose value can provide a comprehensive examination for the adsorption mechanisms, dynamic and kinetic processes involved in the adsorption processes. This investigation utilizes scanning tunneling microscopy (STM) to visualize the H2O adsorption processes on the Si(100) surface with a sub-monolayers (< 0.05 ML) of chemically-reactive dangling bonds remaining after exposure to 1. a hydrogen atomic beam, 2. H2O, and 3. Cl2gases at room temperature (RT). In all three cases, each of the remaining isolated single dangling bonds (sDB) adsorb and is passivated by either of the two dissociation fragments, the H or OH radical, to form a surface Si-H and Si-OH species. A new adsorption mechanism, termed "dissociative and asynchronous chemisorption", is proposed for the observation presented herein. Upon approaching a sDB site, the H2O molecule breaks apart into two fragments. One is chemisorbed to the sDB. The other attaches to the same or the neighboring passivated dimer to form a transition state of surface diffusion, which then diffuses on the mostly passivated surface and is eventually chemisorbed to another reactive site. In other words, the chemisorption reactions of the two fragments after dissociation occur at different and uncorrelated time and places. This adsorption mechanism suggests that a diffusion transition state can be an adsorption product in the first step of the dissociative adsorption processes.
... 1 In this work, we would like to explore a technique which offers sensing of mechanical vibrations with a sub-nanometer level resolution without the requirement of cryogenics or sophisticated lithographic techniques. Scanning tunneling microscopy (STM) allows real space imaging with an atomic resolution of different surfaces, [9][10][11][12][13] and also allows the study of the density of states of materials. [14][15][16] However, STM can also be used for vibration sensing. ...
Vibration characteristics of a piezo crystal oscillator surface are studied using time series measurements of tunneling current. Using this technique, the fluctuations in the tunneling current between a scanning tunneling microscopy tip and the surface of a piezo crystal oscillator are studied, which reveal sub-nanometer vibrations with a sensitivity of 10⁻²A°Hz . As the excitation frequency applied to the crystal is varied, the vibrations on the oscillator surface exhibit a resonant response. Furthermore, we detected unconventional sub-nanometer perpendicular vibration modes excited on the crystal surface. These vibrations are in a direction transverse to the surface of the crystal oscillator, whose conventional vibration mode is in a horizontal plane parallel to the surface. We also find near resonance higher harmonics of the perpendicular mode. Thus, the piezo crystal oscillator together with the time series tunneling current measurements offer a convenient simultaneous drive and detection system with a wide operating frequency range.
... The development of passivation methods with hydrogen of the silicon surface has often been motivated by fundamental studies of the hydrogenated surface properties. Already, the bare Si(100) surface, although deeply investigated since the past decades, can always surprise us when new electronic or surface structure properties can still be discovered [47,48]. Indeed, at the nanoscale, the study of the Si(100):H surface properties and its use has regularly influenced the passivation methods with hydrogenation. ...
In this chapter, we give a short review about the methods that are commonly used to passivate the Si(100) surface with hydrogen atoms. The wet technique is discussed in terms of surface pollution and surface roughness. A basic recipe is given. A second part is devoted to the methods commonly used with vacuum techniques. A discussion is done on the hydrogenation parameters to improve the surface quality at the atomic scale, in particular the one that concerns the formation of various phases such as the 3×1 and dihydride. A third method is also detailed and permits the surface hydrogenation at the atomic scale allowing to design some patches on the Si(100) surface of a few nanometer.
... The symmetric appearance of the Y molecule positioned over a DB dimer seems to arise from either (1) oscillations of the DB dimer caused by tunneling electrons or (2) the filtering of the DB states by the bulk states of the Ge substrate, which provide an averaged symmetric contribution, similar to the recently reported symmetric appearance of buckled silicon dimers on a Si(001)-c(4Â2) surface, where the buckling of the dimer is not apparent from the image when imaging is performed at a higher voltage. 28 To reproduce these effects in the calculated image, the images of the Y molecule on a DB dimer with two buckling configurations are calculated, and the average of the two images is obtained. This results in a symmetric image (Figure 5b) that agrees reasonably well with the experimental image. ...
Fabrication of single-molecule logic devices requires controlled manipulation of molecular states with atomic-scale precision. Tuning molecule-substrate coupling is achieved here by the reversible attachment of a prototypical planar conjugated organic molecule to dangling bonds on the surface of a hydrogenated semiconductor. We show that the ground electronic state resonance of a Y-shaped polyaromatic molecule physisorbed on a defect-free area of a fully hydrogenated surface cannot be observed by scanning tunneling microscopy (STM) measurements because it is decoupled from the Ge bulk states by the hydrogen-passivated surface. The state can only be accessed by STM if the molecule is contacted with the substrate by a dangling bond dimer. The reversibility of the attachment processes will be advantageous in the construction of surface atomic-scale circuits composed of single molecule devices interconnected by the surface dangling bond wires.
... 15 However, this process implies evacuating the hydrogen from the STM chamber prior to applying the pulse. 22 To further highlight that the observed HD's features are local hydrogenated silicon dimers was modeled, although the c(4 × 2) phase is known to be the ground-state structure of the bare silicon surface [33][34][35] [the calculation of the c(4 × 2) phase is detailed in the Supplemental Material]. 22 As observed in Fig. 4, the local density of states (LDOS) shows a depression at the hydrogenated site for the occupied states, which clearly reproduces the observed HD [Figs. ...
The control of the dissociative adsorption of individual hydrogen molecules is performed on the silicon surface at the atomic scale. It is achieved using the tip of a low-temperature (9 K) scanning tunneling microscope (STM) exposed to 10−6 torr of H2 and by probing the bare Si(100)-2 × 1 surface at a positive bias. This effect is very localized and is induced by the tunnel electrons. The statistical study of this process reveals an activation energy threshold matching the creation of H2− at the surface of the STM tip. Our results are supported by ab inito density functional calculations of a hydrogenated silicon dimer.
Etching of semiconductors by halogens is of vital importance in device manufacture. A greater understanding of the relevant processes at the atomistic level can help determine optimal conditions for etching to be carried out. Supersaturation etching is a seemingly counter-intuitive process where the coverage of the etchant molecules on the surface to be etched is > 1. Here we use density functional theory computations of reaction pathways and barriers to suggest that supersaturation etching of Si(001) by Br2 should be more effective than conventional etching by Br2, as well as both conventional and supersaturation etching by Cl2. Analysis of our results shows that this is due in part to the larger size of bromine atoms, and partly due to Br-Si bonds being weaker than Cl-Si bonds. We also show that for both conventional and supersaturation etching, the barrier for the rate limiting step of desorption of SiX2 units is lower when the halogen X is Br rather than Cl. This contributes to the overall reaction barrier for supersaturation etching being lower for Br2 than Cl2.
The objective of this thesis is to explore the control of electronically induced processes in various functionalized molecules adsorbed on the surface of silicon (100). In the context of molecular nanoscience, this work has been carried out using a scanning tunneling microscope operating at low temperature (9K). We used an approach combining statistical study and theoretical modelling in order to explore the physics of the various observed processes. This thesis begins with the study of the Hexaphenylbenzene (HPB) molecule for which the lateral phenyl rings enable the molecule-silicon surface electronic decoupling. Thanks to this effect, we could achieve a directive and reversible diffusion control of physisorbed HPB molecules along the SA silicon step edge through a process combining the joint actions of tunnel electrons and the local STM tip induced electrostatic field. These first results allowed considering the study of a couple of metaltetraphenyl porphyrin molecules adsorbed on the Si(100)-2x1 surface. Similarly to the HPB molecules, the two chosen metalloporphyrins: NiTPP and CuTPP, have lateral phenyl rings. Several adsorption conformations for these molecules were characterized and their response to electronic excitation has been studied. In the case of NiTPP, this led to the control of the reversible activation of an intra-molecular bistable despite the partial chemisorption of the molecule on the silicon surface. As for CuTPP molecule, our study revealed hysteresis behavior on the I(V) conduction curves associated with reversible conformation changes which represents the realization of a memory function. Following the study of each molecule apart, we performed the co-adsorption of the two molecules on the Si(100) surface to study molecular pairs. Various pairs of molecules have been studied. On one of them, we were able to activate an inter-molecular excitation transfer process by locally exciting one molecule and observing a conformation change of the second molecule of the pair. This result thus shows the electronic control of a bi-molecular device getting rid of substrate mediated electronic process. Finally, as a perspective, this thesis presents a novel technique allowing the controlled local hydrogenation of the Si(100) surface. This is achieved thanks to the passivation of the STM tip by molecular hydrogen at 9K. The tunnel electrons are then used to induce the intra-dimer dissociative adsorption of H2 molecules on the Si(100) surface. This technique could be considered for the passivation of Si(100) or to locally modify molecular circuits.
Dissociative adsorption of H2O, NH3, CH3OH and CH3NH2 polar molecules on the Si(100) surface results in a 1:1 mixture of two adsorbates (H and multi-atomic fragment A = OH, NH2, CH3O, CH3NH, respectively) on the surface. By using density functional theory (DFT) calculations, the adsorption geometry, the total energies and the charge densities for various possible ordered structures of the mixed adsorbate layer have been found. Analyzing the systematic trends in the total energies unveils concurrently the nearest-neighbor interactions ENN and the next nearest-neighbor interactions ENNN between two polar adsorbates A. In going from small to large polar adsorbates, ENN's exhibit an attractive-to-repulsive crossover behavior, indicating that they include competing attractive and repulsive contributions. Exploration of the charge density distributions allows the estimation of the degree of charge overlapping between immediately neighboring A's, the resulting contribution of the steric repulsions, and that of the attractive interactions to the corresponding ENN's. The attractive contributions to nearest neighboring adsorbate–adsorbate interactions between the polar adsorbates under study are shown to result from hydrogen bonds or dipole–dipole interactions.
In experiments the reconstructed Si(100) surface shows silicon dimers pointing along the [011] direction. However, the origin of the dimer formation is still unclear. Our theoretical studies on dynamics and energetics show that the reconstruction process depends crucially on the initial local surface morphology: starting from different local tilting scenarios various reconstruction domains can appear as a result of thermal excitation. Molecular dynamics simulations show that c(4 x 2) and asymmetric p(2 x 1) reconstructions can appear quite easily while p(2 x 2) domains are less likely to be found even though they are energetically favorable. The latter is consistent with experimental findings of p(2 x 2) domains being observed quite rarely. The simulations show further that spontaneous dimer-flipping in asymmetric p(2 x 1) domains is possible at about 100 K and driven by the stress within the aligned atoms below the tilted dimers. This can result in a transition to c(4 x 2) reconstruction which is consistent with this reconstruction dominating in experiments above 80 K.
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.
Isolated, paired, and clustered dangling bonds are prepared on a Si(100) surface as well-defined chemically reactive sites for chemisorption of iodine diatomic molecules. The surrounding dangling bonds around a designed reactant configuration are passivated by hydrogen- and iodine-termination. Following exposure to I2 at room temperature, the adsorbate configurations on these reactive sites have been examined using scanning tunneling microscopy (STM). On clean Si(100), three types (type-I, type-II, and type-M) of adsorption pathways have been identified, consistent with previous findings. The results from H- and I-masked Si(100) show that at least two dangling bonds in the same row and in close proximity (<4 Å) are needed to trigger chemisorption and that dissociative adsorption is the dominant mechanism. Contrary to its major role on clean Si(100), type-II adsorption is not observed when the two needed dangling bonds are surrounded by H-adatoms. These findings indicate that a seemingly simple chemisorption reaction on reactive sites involves not only the sites themselves but also the relevant surrounding bonds and adatoms.
The mechanical switching of a single pentacene molecule chemisorbed in a planar configuration along a dimer row of the Si(100)-(2 × 1) surface was performed experimentally using the tip apex of a scanning tunneling microscope. The mechanical switching reaction path was identified theoretically on the ground state potential energy surface of the pentacene/Si(100)-(2 × 1) system. A low-temperature scanning tunneling microscope as well as semiempirical ASED+ molecular mechanical and elastic scattering quantum chemistry (ESQC) calculations were employed to perform the studies. Pushing with the STM tip apex and at zero bias voltage exactly at the center of the chemisorbed pentacene molecule induces a mechanical conformation change of the pentacene from its metastable to its surface stable conformation on the Si(100)-(2 × 1) surface.
Using a low temperature, ultrahigh vacuum scanning tunneling microscope (STM), dI/dV differential conductance maps were recorded at the tunneling resonance energies for a single Cu phthalocyanine molecule adsorbed on an Au(111) surface. We demonstrated that, contrary to the common assumption, such maps are not representative of the molecular orbital spatial expansion, but rather result from their complex superposition captured by the STM tip apex with a superposition weight which generally does not correspond to the native weight used in the standard Slater determinant basis set. Changes in the molecule conformation on the Au(111) surface further obscure the identification between dI/dV conductance maps and the native molecular orbital electronic probability distribution in space.
Atomic-scale Boolean logic gates (LGs) with two inputs and one output (i.e. OR, NOR, AND, NAND) were designed on a Si(100)-(2 × 1)-H surface and connected to the macroscopic scale by metallic nano-pads physisorbed on the Si(100)-(2 × 1)-H surface. The logic inputs are provided by saturating and unsaturating two surface Si dangling bonds, which can, for example, be achieved by adding and extracting two hydrogen atoms per input. Quantum circuit design rules together with semi-empirical elastic-scattering quantum chemistry transport calculations were used to determine the output current intensity of the proposed switches and LGs when they are interconnected to the metallic nano-pads by surface atomic-scale wires. Our calculations demonstrate that the proposed devices can reach ON/OFF ratios of up to 2000 for a running current in the 10 µA range.
This article is a thorough tutorial on the traditional approach to Bardeen's tunnelling
theory in the context of scanning tunnelling microscopy. We follow Duke's interpretation of
Bardeen's theory as the tunnelling analogue of Oppenheimer's perturbation theory for field
ionization of atomic hydrogen. We explain Oppenheimer's ideas, Fermi's golden rule, and
Bardeen's formulae; and we derive the celebrated Tersoff–Hamann formula using an
argument due to Chen. Finally, we discuss the application of Bardeen's theory
to scanning tunnelling microscopy beyond the Tersoff–Hamann approximation.
The scientific and technical challenges involved in building the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) are discussed. The practical, laboratory scale approach explored today to assemble a multi-access atomic scale precision interconnection machine is presented. Depending on the surface electronic properties of the targeted substrates, two types of machines are considered: on moderate surface band gap materials, scanning tunneling microscopy can be combined with scanning electron microscopy to provide an efficient navigation system, while on wide surface band gap materials, atomic force microscopy can be used in conjunction with optical microscopy. The size of the planar part of the circuit should be minimized on moderate band gap surfaces to avoid current leakage, while this requirement does not apply to wide band gap surfaces. These constraints impose different methods of connection, which are thoroughly discussed, in particular regarding the recent progress in single atom and molecule manipulations on a surface.
Differential conductance (dI/dV) images taken with a low-temperature scanning tunneling microscope enabled the first observation of the electron probability distribution of the molecular orbitals of a pentacene molecule directly adsorbed on a metal surface. The three highest occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) and the lowest unoccupied molecular orbital are imaged. Thus dI/dV imaging without any intervening insulating layer permits the visualization of a large variety of molecular orbitals in the electronic cloud of a wide-gap molecule physisorbed on a metal surface.
It is demonstrated that the silicon atom dangling bond (DB) state serves as a quantum dot. Coulomb repulsion causes DBs separated by less, similar2 nm to exhibit reduced localized charge, which enables electron tunnel coupling of DBs. Scanning tunneling microscopy measurements and theoretical modeling reveal that fabrication geometry of multi-DB assemblies determines net occupation and tunnel coupling strength among dots. Electron occupation of DB assemblies can be controlled at room temperature. Electrostatic control over charge distribution within assemblies is demonstrated.
Using scanning tunneling microscopy (STM), we studied the dimer structure of the Si(001)2 × 1 surface at low temperature (< 10 K). Asymmetric (buckled) dimer structure, locally forming c(4 × 2) or p(2 × 2) periodicity, was observed with positive sample bias voltages, while most of the dimers appear symmetric with negative bias voltages. Our observation indicates that actual dimer structure is asymmetric and that the apparent symmetric dimer observation is due to an artifact induced by STM imaging. Since a transition temperature between the buckled- and symmetric-dimer imaging, which is found to be ∼40 K, corresponds to the temperature where the carrier density changes dramatically from intrinsic to saturation range, the apparent symmetric-dimer imaging should be related with the reduced carrier density and ensuing charging effect.
The Hu¨ckel theory, with an extended basis set consisting of 2s and 2p carbon and 1s hydrogen orbitals, with inclusion of overlap and all interactions, yields a good qualitative solution of most hydrocarbon conformational problems. Calculations have been performed within the same parametrization for nearly all simple saturated and unsaturated compounds, testing a variety of geometries for each. Barriers to internal rotation, ring conformations, and geometrical isomerism are among the topics treated. Consistent σ and π charge distributions and overlap populations are obtained for aromatics and their relative roles discussed. For alkanes and alkenes charge distributions are also presented. Failures include overemphasis on steric factors, which leads to some incorrect isomerization energies; also the failure to predict strain energies. It is stressed that the geometry of a molecule appears to be its most predictable quality.
We report on dynamical flipping of buckled dimers on the Si(001) surface at 5 K. A c(4×2) ordering of the buckled dimers has been observed to fluctuate at 5 K by using low-temperature scanning tunneling microscopy (STM). The flip-flop motion of the buckled dimers on the Si(001) surface exhibits symmetrical appearance in the STM images, independent of sample bias voltages and tunneling current.
On a Si(001)-(2×1)-H substrate, electrons tunneling through hydrogen atomic junctions fabricated between two surface dangling-bond (DB) wires are theoretically investigated using the elastic-scattering quantum-chemistry method. The surface states introduced in the Si band gap by removing H atoms from a Si(001)-(2×1)-H surface were calculated and also analyzed using a simple tight-binding model. The two-channel surface conductance of a DB wire results from a combination of through-space and through-lattice electronic couplings between DB states. The conductance of the DB wire-H-junction-DB wire structure decreases exponentially with the length of H junction with an inverse decay rate ranging from 0.20 to 0.23 Å−1, depending on the energy. When the DB wire-H-junction-DB wire structure is contacted by Au nanoelectrodes, the transmission resonances corresponding to the DB wire states split, demonstrating a coupling of the DB wires through short surface hydrogen atomic junctions. This splitting decreases with the length of H junction between the DB wires with an inverse decay length ranging from 0.22 to 0.44 Å−1, indicating that such an atomic scale surface tunneling junction is not a very good insulator.
We revisit and refine the interpretation of the scanning tunneling microscopy (STM) images of the Si(100) dimers, based on results from an extensive set of STM observations carried out at low temperature (80 K) and total-energy calculations of Si(100) surfaces. The interpretation addresses some unresolved questions and brings much experimental and theoretical research into unanimous agreement. We show that tunneling from surface resonances and bulk states seriously contributes to the STM images within usual tunneling conditions. In the empty state, tunneling from these states overwhelms tunneling from the localized π* surface state, which STM is generally believed to observe.
Using scanning tunneling microscopy (STM), we studied the dimer structure of the Si(001)2×1 surface at low temperature (<10 K). Asymmetric (buckled) dimer structure, locally forming c(4×2) or p(2×2) periodicity, was observed with positive sample bias voltages, while most of the dimers appear symmetric with negative bias voltages. Our observation indicates that actual dimer structure is asymmetric and that the apparent symmetric dimer observation is due to an artifact induced by STM imaging. Since a transition temperature between the buckled- and symmetric-dimer imaging, which is found to be ∼40K, corresponds to the temperature where the carrier density changes dramatically from intrinsic to saturation range, the apparent symmetric-dimer imaging should be related with the reduced carrier density and ensuing charging effect.
Scanning tunneling microscopy (STM) and spectroscopy were used to study the structural change between c(4×2) and p(2×2) on highly doped n-type Si(100) surfaces at 6 K. Sample voltage control during STM imaging allowed us to manipulate the surface structure between c(4×2) and p(2×2). We found that the sample voltage for producing p(2×2) [c(4×2)] depends upon the tip-sample distance and dopant concentration. Coinciding with that, energy shifts of the π* (π) state in tunneling spectra were observed. These results suggest that the structural change caused through STM was due to electron (hole) injection into the π* (π) state. Also, the difference in how the c(4×2) and p(2×2) domains emerge, when electrons or holes are injected into the surface, can be understood by considering the electronic features of the π and π* states.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
The resistance of a conductor measured in a four-probe setup is invariant if the exchange of the voltage and current sources is accompanied by a magnetic field reversal. We present a derivation of this theorem. The reciprocity of the resistances is linked directly to the microscopic reciprocity of the S-matrix, which describes reflection at the sample and transmission through the sample. We demonstrate that this symmetry holds for a conductor with an arbitrary number of leads. Since leads act like inelastic scatterers, consideration of a many-probe conductor also implies that the reciprocity of resistances is valid in the presence of inelastic scattering. Various conductance formulae are discussed in the light of the reciprocity theorem. Finally, we discuss some implications of our results for the nature of a voltage measurement and point to the difference between chemical potentials and the local electric potential.
We investigate, with scanning tunneling microscopy, the Si(001) surface at low temperatures. This surface consists of asymmetric (buckled) dimers at 110 K as has been reported previously. At 20 K, however, symmetric dimers dominate the surface regardless of the carrier type or the dopant concentration. Asymmetric dimers are observed only near defects at 20 K. Our results indicate that the Si(001) surface is not c(4×2)-symmetric, but rather p(2×1)-symmetric at the zero temperature limit, and suggest a reconsideration of its ground state. It is unusual that the higher symmetric phase is more stable than the less symmetric one.
Avoiding the Bardeen—Tersoff—Hamann approximation we present calculations of the scanning tunneling microscope (STM) image of benzene or Rh(111). The tunneling current between the tip and the substrate through the benzene is calculated from the generalized Landauer formula. The electronic structure of the complete system made of the tip, of the substrate and of the benzene adsorbate is obtained with its molecular orbitals from an extended Hückel molecular orbital calculation. Experimental and calculated images are in good agreement. This leads to a detailed analysis of the image using the molecular orbital approach in through-space and through-bond tunneling processes.
The atomic structure of the Si(001) surface has been examined with use of scanning tunneling microscopy (STM). The STM images reveal a dimer-type reconstruction and are inconsistent with chain and vacancy models. Both buckled and nonbuckled dimers are observed, giving rise to regions of (2×1), c(4×2), and p(2×2) symmetry. The surface has a high density of vacancy-type defects, which appear to induce or stabilize buckling of the dimers at room temperature. The STM images also reveal the atomic structure at steps and defects. At high annealing temperature the step density increases dramatically, eventually leading to faceting.
The ability of a rupture of translational invariance in a one-dimensional chain to control the through chain electron propagation is studied on the model system ...AAABAAA... . A discrete formulation of the one-dimensional scattering theory is considered with the help of the transfer-matrix technique. A tight-binding Hamiltonian, with a single orbital per center, is used to represent the chemical nature of the perturbated chain. The electronic transmission coefficient and the change in the chain electronic density of states induced by the defect B are analytically calculated from the scattering matrix S. The complete dependence on the electronic transmission of the energetic position of the impurity orbital and of its couplings with the chain is analyzed in detail. Possible applications of the model to the design of molecular switches are described by means of the effective Azbel's transmission coefficient.
We present ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local-density approximation at each molecular-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using subspace alignment. This approach avoids the instabilities inherent in quantum-mechanical molecular-dynamics calculations for metals based on the use of a fictitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows us to perform simulations over several picoseconds.
The first low-temperature scanning-tunneling-microscope (STM) images of Si(001) are presented. It is observed that on cooling to 120 K the number of buckled dimers increases, confirming that dimers have an asymmetric character. Buckled-dimer domains of c(4×2) order are bounded by p(2×2) regions. Defects pin nearby dimers into a buckled configuration and act to smear out the transition to order. At room temperature dimers rapidly switch orientation leading to an averaged symmetric appearance in STM images.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
We have investigated the basic surface reconstruction of Si(100) on well defined surfaces fabricated on various substrates at low temperatures (-80 K) by scanning tunneling microscopy. Below 40 K, the single p(2x2) phase, a phase never observed before, was observed exclusively on n-type substrates doped in the range of 0.002 to 0.017 Omega cm. We also exclude the possibility of the (2x1) symmetric dimer commonly observed at low temperature (-10 K) being the basic surface reconstruction by showing that a buckled dimer can be flip-flopped by the tunneling tip.
We report scanning tunneling microscopy (STM) studies of the technologically important Si(100) surface that reveal at 5 K the coexistence of stable surface domains consisting of the p(2 x 1) reconstruction along with the c(4 x 2) and p(2 x 2) reconstructions. Using highly resolved tunneling spectroscopic measurements and tight binding calculations, we prove that the p(2 x 1) reconstruction is asymmetric and determine the mechanism that enables the contrast variation observed in the formation of the bias-dependent STM images for this reconstruction.