Au Chains Research Articles

Electronic stabilization of the Si(111)5 × 2–Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2–Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2–Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.

Effect of impurities on the mechanical and electronic properties of Au, Ag, and Cu monatomic chain nanowires

In this study, we have investigated the interaction of various different atomic and molecular species (H, C, O, H${}_{2}$, and O${}_{2}$) with the monatomic chains of Au, Ag, and Cu via total-energy calculations using the plane-wave pseudopotential method based on density functional theory. The stability, energetics, mechanical, and electronic properties of the clean and contaminated Au, Ag, and Cu nanowires have been presented. We have observed that the interaction of H, C, or O atoms with the monatomic chains are much stronger than the one of H${}_{2}$ or O${}_{2}$ molecules. The atomic impurities can easily be incorporated into these nanowires; they form stable and strong bonds with these one-dimensional structures when they are inserted in or placed close to the nanowires. Moreover, the metal-atomic impurity bond is much stronger than the metal-metal bond. Upon elongation, the nanowires contaminated with atomic impurities usually break from the remote metal-metal bond. We have observed both metallic and semiconducting contaminated nanowires depending on the type of impurity, whereas all clean monatomic chains of Au, Cu, and Ag exhibit metallic behavior. Our findings indicate that the stability and the electronic properties of these monatomic chains can be tuned by using appropriate molecular or atomic additives.

Conductance of kinked nanowires

The conductance properties of kinked nanowires are studied by first-principles transport calculations within a recently developed complex potential framework. Using prototypical examples of monoatomic Au chains as well as small diameter single-crystalline silicon nanowires we show that transmission strongly depends on the kink geometry and one can tune the conductance properties by the kink angle and other geometrical factors. In the case of a silicon nanowire the presence of a kink drastically reduces the conductance.

Large out-of-plane spin polarization in a spin-splitting one-dimensional metallic surface state on Si(557)-Au

We report unprecedented evidence of a spin split one-dimensional metallic surface state for the system of Si(557)-Au obtained by means of high-resolution spin- and angle-resolved photoelectron spectroscopy combined with first principles calculations. The surface state shows double parabolic energy dispersions along the Au chain structure together with a reversal of the spin polarization with respect to the time-reversal symmetry point as is characteristic from the Rashba effect. Moreover, we have observed a considerably large out-of-plane spin polarization which we attribute to the highly anisotropic wave function of the gold chains.

Surface photovoltage of Ag nanoparticles and Au chains on Si(111)

AbstractWe present surface photovoltage (SPV) measurements of metal nanostructures on Si(111)‐(7 × 7) using scanning tunneling microscopy and spectroscopy (STM/STS). Quantitative results are obtained from spatially resolved I(V) spectra under laser illumination. The SPV data of Ag nanoparticles deposited on Si(111)‐(7 × 7) are compared to results of a Si(111) surface partially covered by the quasi one‐dimensional Si(111)‐(5 × 2)‐Au structure. On the latter we determine the SPV as a function of laser power at different locations. An accurate value for the Fermi‐level pinning position of Si(111)‐(5 × 2)‐Au atomic chains is obtained. Based on these results we establish and discuss a simple model for the band topology. It is confirmed by spatially resolved local work function measurements utilizing the image‐state derived field‐emission resonances.

Structural model of the Au-induced Si(553) surface: Double Au rows

A structural model of Au induced Si(553) surface is proposed. The model is based on the recently revised value of Au coverage of 0.48 monolayers, which suggests the formation of two gold chains on each Si(553) terrace. The resulting structural model, such as the models of other vicinal Si surfaces, features the honey-comb chain, but no buckling of the step edge is observed, unlike in the case of the Si(335)-Au and Si(557)-Au surfaces. The present model is more stable than the models with single Au chain only, and agrees well with existing experimental data. In particular, calculated band structure, featuring two metallic bands coming from the hybridization of gold in both chains with neighboring Si atoms, matches well the photoemission data. Moreover, theoretical scanning tunneling microscopy topographs remain in good agreement with the experiment.

Quasi-one-dimensional metals on semiconductor surfaces with defects

Several examples are known in which massive arrays of metal atomic chains are formed onsemiconductor surfaces that show quasi-one-dimensional metallic electronic structures. Inthis review, Au chains on Si(557) and Si(553) surfaces, and In chains on Si(111) surfaces,are introduced and discussed with regard to the physical properties determined byexperimental data from scanning tunneling microscopy (STM), angle-resolvedphotoemission spectroscopy (ARPES) and electrical conductivity measurements. Theyshow quasi-one-dimensional Fermi surfaces and parabolic band dispersion along the chains.All of them are known from STM and ARPES to exhibit metal–insulator transitionsby cooling and charge-density-wave formation due to Peierls instability of themetallic chains. The electrical conductivity, however, reveals the metal–insulatortransition only on the less-defective surfaces (Si(553)–Au and Si(111)–In), butnot on a more-defective surface (Si(557)–Au). The latter shows an insulatingcharacter over the whole temperature range. Compared with the electronic structure(Fermi surfaces and band dispersions), the transport property is more sensitiveto the defects. With an increase in defect density, the conductivity only alongthe metal atomic chains was significantly reduced, showing that atomic-scalepoint defects decisively interrupt the electrical transport along the atomic chainsand hide the intrinsic property of transport in quasi-one-dimensional systems.

First-principles description of atomic gold chains on Ge(001)

We have performed density-functional theory calculations, including the spin-orbit correction, to investigate atomic gold chains on Ge(001). A set of 26 possible configurations of the Au/Ge(001) system with $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $c(8\ifmmode\times\else\texttimes\fi{}2)$ symmetries is studied. Our data show that the $c(4\ifmmode\times\else\texttimes\fi{}2)$ order results in the lowest energy, which is not in direct agreement with recent experiments. Using total-energy calculations, we are able to explain these differences. We address the electronic band structure and apply the Tersoff-Hamann approach to correlate our data to scanning-tunneling microscopy (STM). We obtain two highly competitive structures of the atomic Au chains for which we report simulated STM images in order to clarify the composition of the experimental Au/Ge(001) surface.

Electronic properties and charge state of gold monomers and chains adsorbed on alumina thin films on NiAl(110)

The adsorption of Au on a thin alumina film on NiAl(110) has been investigated with scanning tunneling microscopy (STM) and spectroscopy as well as density-functional theory (DFT). Gold monomers are identified as the main surface species at low coverage. According to the DFT, the adatoms are negatively charged and exhibit only a single occupied state in the alumina band gap. These results are compatible with the distinct bias-dependent contrast of Au monomers in the STM. With increasing exposure, Au chains of different length develop on the oxide surface. The adchains exhibit a number of maxima in conductance spectroscopy that are assigned with the help of DFT calculations to two sets of quantum well states with $\text{Au}\text{ }6s\text{\ensuremath{-}}{d}_{z2}$ and $\text{Au}\text{ }p$ character. By analyzing the node structure of the highest occupied states, the charge state of the adchains is determined. Chains containing three to seven Au atoms are found to host up to three excess electrons, in accordance with the charge-transfer model developed for Au adsorption on ultrathin oxide films.

Adsorption and activation of O2 at Au chains on MgO/Mo thin films

We have investigated the adsorption of O(2) on Au(n) clusters (n = 1-6) supported by an ultra thin (3ML)MgO(001) film on Mo metal via density functional theory calculations. On thin films, these small clusters have chain like structures and their electronic states resemble 1D quantum well states. The Au(1-3) are charged by one electron whereas the larger Au(4-6) get two electrons from the substrate. This is confirmed both by the symmetries of the HOMO and LUMO states of the clusters and the Bader charge analysis. In contrast to the O(2) adsorption on gas-phase clusters, the adsorption energy of O(2) molecule does not show pronounced oscillations as a function of cluster size. The O(2) is activated (i.e. the O-O bond is elongated and the molecule is charged) in contact with all the cluster sizes. O(2) activation and strong binding is also observed on bare MgO/Mo, whereas the molecule interacts only weakly with bulk MgO. The activation of O(2) is mainly due to charge transferred from the support. The relevance of these results for catalytic activity of small thin-film-supported clusters is discussed.

Finite-size modelling of electrodes for quantum transport calculations using k-space ab initio techniques

Abstract We present an efficient and easy-to-use numerical treatment to check the necessary size of metallic leads with respect to their quantum-transport properties. The suggested method works entirely in k-space and avoids complicated projections of ab initio band structures onto real-space tight-binding-like Hamiltonians. As an illustrative example, we apply the method to various low-dimensional Au chains and discuss our DFT-based studies with respect to convergence speed and relevance of structural relaxation.

Formation of atomic gold chain on hydrogen terminated Si(001):3×1 surface: A density functional study

Electronic structure calculations are carried out for investigating the formation of a stable atomic gold wire on the hydrogen terminated Si(001):3×1 [H–Si(001):3×1] surface. The H–Si(001):3×1 surface is patterned in various ways by removing desired hydrogen atoms from the surface. Gold atoms are adsorbed on the patterned H–Si(001):3×1 surface at various submonolayer coverage. However, zigzag continuous Au chains are found to be favorable on patterned H–Si(001):3×1 surfaces at a 4/9 monolayer (ML) coverage. The stabilities of continuous atomic gold chains on the patterned H–Si(001):3×1 surfaces are compared. In addition, the Au chains on the patterned H–Si(001):3×1 surfaces are compared with the existing results of the favorable Au chain structures on patterned H–Si(001):1×1 surfaces [Konar et al., Phys. Rev. B 77, 245411 (2008)], and it is found that a continuous Au chain on the H–Si(001):3×1 surface is energetically more favorable. Our calculations also reveal that beyond the 4/9 ML coverage, the additional Au atoms take part in the formation of Au clusters on the surface. However, beyond the 4/9 ML coverage, the gold structures on the surface are found to be less favorable compared to the chain structures at the 4/9 ML coverage. The continuous atomic gold chains supported by a substrate may be used as atomic scale devices, and therefore, experiments may be designed in this direction to produce a continuous atomic Au chain on the patterned H–Si(001):3×1 surface.

The 3[ndσ*(n+1)pσ] Emissions of Linear Silver(I) and Gold(I) Chains with Bridging Phosphine Ligands

The complexes [Au(3)(dcmp)(2)][X](3) {dcmp=bis(dicyclohexylphosphinomethyl)cyclohexylphosphine; X=Cl(-) (1), ClO(4) (-) (2), OTf(-) (3), PF(6) (-) (4), SCN(-)(5)}, [Ag(3)(dcmp)(2)][ClO(4)](3) (6), and [Ag(3)(dcmp)(2)Cl(2)][ClO(4)] (7) were prepared and their structures were determined by X-ray crystallography. Complexes 2-4 display a high-energy emission band with lambda(max) at 442-452 nm, whereas 1 and 5 display a low-energy emission with lambda(max) at 558-634 nm in both solid state and in dichloromethane at 298 K. The former is assigned to the (3)[5dsigma*6psigma] excited state of [Au(3)(dcmp)(2)](3+), whereas the latter is attributed to an exciplex formed between the (3)[5dsigma*6psigma] excited state of [Au(3)(dcmp)(2)](3+) and the counterions. In solid state, complex [Ag(3)(dcmp)(2)][ClO(4)](3) (6) displays an intense emission band at 375 nm with a Stokes shift of approximately 7200 cm(-1) from the (1)[4dsigma*-->5psigma] absorption band at 295 nm. The 375 nm emission band is assigned to the emission directly from the (3)[4dsigma(*)5psigma] excited state of 6. Density functional theory (DFT) calculations revealed that the absorption and emission energies are inversely proportional to the number of metal ions (n) in polynuclear Au(I) and Ag(I) linear chain complexes without close metalanion contacts. The emission energies are extrapolated to be 715 and 446 nm for the infinite linear Au(I) and Ag(I) chains, respectively, at metalmetal distances of about 2.93-3.02 A. A QM/MM calculation on the model [Au(3)(dcmp)(2)Cl(2)](+) system, with Au...Cl contacts of 2.90-3.10 A, gave optimized Au...Au distances of 2.99-3.11 A in its lowest triplet excited state and the emission energies were calculated to be at approximately 600-690 nm, which are assigned to a three-coordinate Au(I) site with its spectroscopic properties affected by Au(I)...Au(I) interactions.

Adsorption of gold on hydrogen terminated Si(0 0 1): Formation of chain structure

Possible formation of stable Au atomic wire on the hydrogen terminated Si(0 0 1): 3 × 1 surface is investigated under the density functional formalism. The hydrogen terminated Si(0 0 1): 3 × 1 surface is patterned in two different ways by removing selective hydrogen atoms from the surface. The adsorption of Au on such surfaces is studied at different sub-monolayer coverages. At 4/9 monolayer (ML) coverage, zigzag continuous Au chains are found to be stable on the patterned hydrogen terminated Si(0 0 1): 3 × 1 surface. The reason for the stability of the wire structures at 4/9 ML coverage is explained. It is to be noted that beyond 4/9 ML coverage, the additional Au atoms may introduce clusters on the surface. The continuous atomic gold chains on the substrate may be useful for the fabrication of atomic scale devices.

Coverage-dependent faceting of Au chains on Si(557)

The structural and electronic phase diagrams of Au on Si(557) are established using scanning tunneling microscopy (STM) and angle-resolved photoemission. Five phases consisting of altogether seven facets are observed in the submonolayer regime. With increasing Au coverage one observes $\text{Si}(111)7\ifmmode\times\else\texttimes\fi{}7+\text{Si}(112)$, Si(557)-Au, $\text{Si}(111)5\ifmmode\times\else\texttimes\fi{}2\text{-Au}+\text{Si}(335)\text{-Au}$, $\text{Si}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-Au}+\text{Si}(335)\text{-Au}$, and $\text{Si}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{-Au}+\text{Si}(5 5 11)\text{-Au}$. The relative surface areas of the five phases and seven facets are determined accurately by depositing a Au wedge ranging from 0 to 0.7 monolayer and performing automatic pattern recognition on large-scale STM images. Angle-resolved photoemission spectra are decomposed into contributions from the five phases. The Fermi wave vectors and the band filling of various facets are identified. Using Si(557)-Au as reference we find a coverage of three Au chains per unit cell for the frequently studied $\text{Si}(111)5\ifmmode\times\else\texttimes\fi{}2\text{-Au}$ surface (instead of the widely used value of two Au chains). Likewise a coverage of two Au chains per unit cell is found for Si(553)-Au (instead of one Au chain), in agreement with x-ray diffraction. A structural model with three Au rows per unit cell is developed for $\text{Si}(111)5\ifmmode\times\else\texttimes\fi{}2\text{-Au}$ using density-functional-theory calculations.

Growth and structure of gold films on a Re() surface

We have deposited Au films in ultra-high vacuum onto a rhenium () surface in submonolayer and multilayer concentrations and studied them by means of low- andmedium-energy electron diffraction in the temperature range between 300 and 800 K. In thesubmonolayer range, Au forms several low-energy electron diffraction (LEED) phases, namely, a(1 × 3), a(1 × 4),a (1 × 5)and a (1 × 6) phase, consisting of one-dimensionally ordered Au chains in the [] direction, until the formation of a complete pseudomorphic monolayer is indicated by a(1 × 1) LEED phase. In themultilayer regime, a (1 × 8) LEED phase appears over a surprisingly wide coverage range from about two to atleast eight monolayers, which we interpret as a hexagonal uniaxially compressedreconstructed Au overlayer on pseudomorphically grown hexagonal close-packed goldlayers. In order to get access to absolute Au coverages, we have performed LEED(I,V) measurements and carried out a LEED structure determination for the(1 × 1) phase. We propose the formation of a full Au monolayer in which both the Re trough andtop-row sites are being covered by Au atoms. The data are discussed and compared withthose from previous studies on related systems.

An Analysis of the Si(111)-5*2 (s)-Au Surface Structure -The Ordered Structure of the Intermediate State of One and Two Dimensional Structures-

In order to exactly analyze a crystal structure it is necessary to consider from two viewpoints of long range order (LRO) and short range order (SRO). The information on the reciprocal lattice which is obtained by electron diffraction or X-ray diffraction concerns mainly on the long range order. On the other hand, the images observed by STM (tunnel scanning microscopy) concern mainly with the short range order. For the analysis of the STM images, we must construct the structure which does not contradict to the diffraction patterns. We observe sometimes the streak reflections in LEED (low energy electron diffraction) and RHEED (reflection high energy electron diffraction) patterns taken from the surface structures. When such streak patterns appear, we are apt to deal with it as the case that the crystallinity is not good. There are actually such cases, but some cases suggest a deep meaning on the structure. In this paper, from the standpoint of diffraction theory, we consider the streak reflection patterns which were observed often in 5×2 (s)-Au surface structure formed on the Si(111) surface when Au atoms are deposited on the Si(111) surface. Since by coexisting with the streak reflections and the sharp diffraction points of 5 times structure in the pattern of this surface structure, it is the case of that we cannot simply neglect it by saying that the crystallinity is not good. When this diffraction pattern is examined in detail, an important hint of the structure is obtained which shows an intermediate state of one dimensional and two dimensional lattices. A suggested structure is that for one dimensional lattice of Au atoms (Au chain), the position of the neighboring chain is decided with a connection probability α. The calculation method of diffracted intensity of the streaks in the 5×2 (s)-Au structure is a typical example of diffraction theory and worthwhile to consider in details and instructive also to understand many other diffraction phenomena.

Atomic gold chain on hydrogen terminatedSi(001):1×1surface: A density functional theory study

The possibility for the formation of stable Au atomic wire structures on the patterned hydrogen terminated $\text{Si}(001):1\ifmmode\times\else\texttimes\fi{}1$ surface is investigated using density functional theory based total energy calculations. The hydrogen terminated $\text{Si}(001):1\ifmmode\times\else\texttimes\fi{}1$ surface is patterned in two different ways (pattern 1 and pattern 2) by removing desired hydrogen atoms from the surface. The adsorption of Au on such surfaces is studied at different submonolayer coverages. Eventually, it is found that at 2/3 monolayer (ML) coverage, stable Au chain structure is obtained on both the patterned hydrogen terminated $\text{Si}(001):1\ifmmode\times\else\texttimes\fi{}1$ surfaces. The reason for the stability of the wire structures at 2/3 ML coverage is explained. Most importantly, the Au chain on the patterned hydrogen terminated $\text{Si}(001):1\ifmmode\times\else\texttimes\fi{}1$ surface having pattern 1 may be metallic in nature, and therefore, experiments may be designed in this direction to produce metallic wires supported by semiconducting substrates. However, beyond 2/3 ML coverage, the additional Au atoms may form clusters due to the lack of dangling bonds on the surface. Hence, Au coverage should be controlled at 2/3 ML for the formation of Au chain on the patterned hydrogen terminated $\text{Si}(001):1\ifmmode\times\else\texttimes\fi{}1$ surface.

Counting Electrons Transferred through a Thin Alumina Film into Au Chains

Low-temperature STM measurements combined with density functional theory calculations are employed to study the adsorption of gold on alumina/NiAl(110). The binding of Au monomers involves breaking of an oxide Al-O bond below the adatom and stabilizing the hence undercoordinated O ion by forming a new bond to an Al atom in the NiAl. The adsorption implies negative charging of the adatom. The linear arrangement of favorable binding sites induces the self-organization of Au atoms into chains. For every ad-chain, the number of transfer electrons from the support is determined by analyzing the node structure of the corresponding highest occupied molecular orbital.

半導体結晶表面上の擬１次元金属

Several examples are known in which massive arrays of metal atomic chains are formed on semiconductor surfaces that show quasi-one-dimensional electronic structures. Here, Au chains on Si(557) and Si(553), and In chains on Si(111) surfaces are introduced and discussed with experimental data of scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and electrical conductance measurements. They show quasi-one-dimensional Fermi surfaces and parabolic band dispersion along the wires. All of them exhibit metal-insulator transitions and charge-density wave formation due to Peierls instability of the metallic chains. The electrical conductivity, however, reveals the metal-insulator transition only on the less-defective surfaces, while a defective surface does not. The importance of atomic-scale defects on quasi-one-dimensional systems is demonstrated by showing the dramatic decrease of conductivity along the metallic wires with increase of the defect density.