Au Chains Research Articles

Gold Nanostructures on TiOx/Mo(112) Thin Films

The structure and composition of Au nanostructures deposited on TiOx/Mo(112) films has been investigated based on first principles DFT calculations. The aim of the calculations is to identify plausible candidates for the structures generated under experimental conditions in the growth of Au/TiOx/Mo(112) films. To this end, both static and molecular dynamics (MD) calculations have been performed. Various Au coverages have been considered, low (below monolayer), medium (monolayer), and high (bilayer) coverages. An ab initio thermodynamic analysis shows that, at the experimental temperature and oxygen partial pressure, the Au monolayer forms a (1 × 1) Au/TiO3/Mo(112) structure. On the basis of the comparison of the computed and experimental valence bands, phonon spectra, and STM images, we propose a tentative assignment to a structure where Au chains are formed at the interface between the Mo support and the TiO3 film. The additional deposition of Au results in the formation of a bilayer (1 × 3) Au/TiO3/Mo(112) structure. Here, we observe the tendency for gold to aggregate and form nanowires with half of the Au atoms in contact with the TiO3/Mo(112) support and the rest forming a second layer. The top and bottom Au layers of the nanowire exhibit a different chemical nature.

Structures and defects of atomic wires on Si(553)-Au: An STM and theoretical study

Room-temperature structure of the Au-induced atomic wire array on the stepped Si(553) surface, which exhibits an exotic double Peierls phase transition at low temperature [J. R. Ahn et al., Phys. Rev. Lett. 95, 196402 (2005)], is investigated by scanning tunneling microscopy (STM) and first-principles calculations. The energetics of various structure models proposed so far are calculated and compared systematically. The most energetically favorable structure is composed of a single Au chain on a terrace and a Si honeycomb chain on a step edge. This model reproduces excellently the high-resolution STM images observed. High-resolution STM images also reveal that there exist several different types of defects on the atomic wires, as categorized by their apparent bias-dependent shapes. The defect with a dominating population is found to be due to water adsorption on the step-edge Si chains. These defects have different effects on the wire lattice at room temperature, that is, only the specific types of defects with minor populations induce a long-range lattice (charge) distortion with a $\ifmmode\times\else\texttimes\fi{}2$ period not on Si step-edge chains, but on Au terrace chains.

Ab initiostudy of toroidal carbon nanotubes with encapsulated atomic metal loops

Toroidal carbon nanotubes can serve as hosts for encapsulated loops of atomic metal wires. Such composite structures have been analyzed using density functional theory for a semiconducting ${\mathrm{C}}_{120}$ torus encapsulating chains of Fe, Au, and Cu atoms. The sheathed metal necklaces form a zigzag structure and drop the highest occupied molecular orbital and/or lowest unoccupied molecular orbital band gap to less than $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The iron composite is ferromagnetic with a magnetic moment essentially the same as that of bcc iron. The azimuthal symmetry of these toroidal composites suggests that they may offer novel electromagnetic properties not associated with straight, metal-encapsulated carbon nanotubes.

Observation of a Mixed-Metal Transition in Heterobimetallic Au/Ag Dicyanide Systems

Crystals of the mixed-metal heterobimetallic Au/Ag dicyanide complex, K[AuxAg1-x(CN)2] (x = 0-->1), were obtained by slow evaporation. The mixed-metal complex K[Au0.44Ag0.56(CN)2] crystallizes in a rhombohedral crystal system, space group R. The crystal structure consists of layers of linear chains of Au(CN)2- and Ag(CN)2- ions and K+ ions that connect the layers through the N atoms. The excitation and emission spectra of single crystals of K[AuxAg1-x(CN)2] were recorded at 4.2-180 K using excitation wavelengths between 230 and 260 nm. Two emission bands due to Ag-Au interactions were observed at 343 and 372 nm. Lifetime measurements indicate the shorter-wavelength emission corresponds to fluorescence and the longer-wavelength band is phosphorescence. These new emission bands are not seen in the pure K[Ag(CN)2] or pure K[Au(CN)2] crystals. Extended Hückel calculations show that the LUMO of the mixed-metal system is bonding while the HOMO is antibonding or very weakly bonding. Moreover, excited-state extended Hückel calculations indicate the formation of exciplexes with shorter metal-metal distances and higher metal-metal overlap populations than the corresponding ground-state oligomers. The luminescence is assigned to a mixed-metal transition from a molecular orbital with Au character to a molecular orbital with Ag character.

Ballistic conductance calculation of atomic-scale nanowires of Au and Co

Ballistic conductance calculations on infinite one-atom zigzag chains of Co and Au, andmulti-atom Au nanowires, were computed using the Landauer–Büttiker formalismwithin the local-density approximation (LDA). In the cobalt case, the majorityspin channel exhibits quantized conductance, but because of the close spacingof the d levels, the minority channel does not. The calculated conductance issomewhat larger than what is observed. We suggest that the discrepancy is anindication that the Landauer–Büttiker formalism breaks down in this narrow d bandmaterial. In contrast, Au (which was determined not to be spin polarized) does showwell-behaved quantization in the case of the infinite zigzag chain. By increasing thedimension of the cross section of the chain, we find that quantization is lost once thelateral dimensions of the wire exceed two or so atoms. Finally, we show that thezigzag chain of Au can have a Peierls instability, making the chain insulating.

Ligand‐Unsupported Au(I) Chains with Short Au(I)×××Au(I) Contacts.

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Pulling gold nanowires with a hydrogen clamp: Strong interactions of hydrogen molecules with gold nanojunctions

In this paper an experimental study of the interaction of hydrogen molecules with gold nanowires is presented. Our results show that chains of Au atoms can also be pulled in hydrogen environment; however, in this case the conductance of the chain is strongly reduced compared to the perfect transmission of pure Au chains. The comparison of our experiments with recent theoretical prediction for the hydrogen welding of Au nanowires implies that a hydrogen molecule can even be incorporated in the gold nanocontact, and this hydrogen clamp is strong enough to pull a chain of gold atoms.

Influence of the Metal Substrate on the Adsorption Properties of Thin Oxide Layers: Au Atoms on a Thin Alumina Film on NiAl(110)

Thin oxide films grown on metal substrates are widely used in surface science to model bulk oxides, assuming their chemical and electronic properties to be similar. In some cases, however, this might not be justified as the present scanning tunneling microscopy studies demonstrate for Au atoms on a thin alumina film on NiAl(110). Au atoms were evaporated onto the oxide film at a sample temperature of approximately 10 K. At low coverage, this leads to the formation of one-dimensional clusters with unusually large Au-Au distances of 5.6-6.0 A. A direct interaction between the Au atoms can be excluded, and a substrate-mediated mechanism is supposed instead. This assumption is strengthened by the finding that the Au chains exhibit a preferential orientation: They are almost aligned with the [001] direction of the NiAl(110) substrate, clearly indicating that the metal substrate participates in the binding of the Au atoms.

Ligand-unsupported Au(i) chains with short Au(i)⋯Au(i) contacts

A new series of solids with ligand-unsupported Au(I) chains with short Au...Au contacts were synthesized; as Ag compounds with the same structure are known, the new phases now allow unbiased comparison of Ag...Ag and Au...Au metallophilic bonds not supported by bridging ligands, which shows the latter to be consistently shorter by 0.03-0.04 A.

Computer simulations in the study of gold nanowires: the effect of impurities

Suspended gold nanowires have recently been made in an ultra-high vacuum ambient and were imaged by electron microscopy. Two puzzles were presented: one atom thick wires are produced and some of the atomic distances between these atoms before their breaking were too large. Simulations using realistic molecular dynamics method were able to unveil some processes to explain the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. The calculations showed how defects induce the formation of constrictions that eventually will form the one-atom chains. Atomically thin chains, five atoms long were obtained, before breaking. The results were in excellent agreement with experimental results except for the large Au-Au distances. In fact no theoretical calculation of pure gold nanowires have been able to produce such large distances. Light impurities that cannot be imaged in these experiments may be responsible for these large Au-Au distances. Using ab initio total energy calculations based on the density functional theory, we have studied the effect of H, C, O, N, B, S, CH, CH2, and H2 impurities on the nanowire’s electronic and structural properties, in particular how they affect the rupture of the nanowire. We find that the impurities tend to locally increase the nanowire’s strength, in such a way that its rupture always occurs at an Au-Au bond and never at an Au-X bond (X being an impurity). In particular, oxygen seems to form very stable bonds that may be used to pull longer Au chains. Regarding the observed large Au-Au bond lengths, it was found, based on quasi-static calculations, that the best candidate to explain the large distances is H. However, some particular experimental conditions may lead to different results.

Realization of a Particle-in-a-Box: Electron in an Atomic Pd Chain

Well-defined Pd chains were assembled from single atoms on a NiAl(110) surface with the tip of a scanning tunneling microscope. The electronic properties of the chains were determined by spatially resolved conductance measurements, revealing a series of quantum well states with parabolic dispersion. The particle-in-a-box states in Pd chains show higher onset energy and larger effective mass than those in Au chains investigated before, reflecting the influence of elemental composition on one-dimensional electronic systems. The intrinsic widths and spectral intensities of Pd induced states provide information on lifetime and spatial localization of states in the atomic chain.

Adsorption-induced constraint on delocalization of electron states in an Au chain on NiAl(110)

We have carried out a density functional study of the localized constraint on the delocalized, unoccupied resonance states in a mono atomic Au chain on a NiAl(110) surface by adsorption of a CO molecule on one of the adatoms in the chain. This constraint was observed recently by scanning tunnelling microscopy and spectroscopy. The repulsive interaction of the occupied 5${\sigma}$ molecular state with the unoccupied, resonance state in a Au adatom in the chain is found to break the degeneracy of the Au adatom-induced resonance states and to disrupt their delocalization in the chain.

Batch Preparation of Linear Au and Ag Nanoparticle Chains via Wet Chemistry

We present a simple, wet-chemical method for fabricating linear chains of Au and Ag nanoparticles by selectively etching alternating segments from striped metal nanowires. Nanowires composed of sacrificial Ni segments as well as Au and/or Ag segments were prepared by templated electrodeposition in the pores of alumina membranes. After removal from the membrane, wires were coated with SiO2, and selective etching revealed the nanoparticle chains. Extinction spectra are presented as a function of interparticle spacing.

First principles theory of artificial metal chains on NiAl(110) surface

Using first-principle calculations we have studied the stable phases of metal-atom chains on the (110) surface of NiAl. Our investigation is mainly focused on the technologically relevant case of Au chains, but the results will be analyzed in the broader framework of a family of metallic systems. We demonstrate that the nature of the adatom (Au, Mn, Ni, Cu, Al, H, C, Na, K, and Ca) is responsible for different levels of interaction with the substrate and gives rise to a variety of electronic behaviors. With some transition metals (such as Au, Mn, Ni, and Cu) the NiAl surface acts as a simple structural template for the formation of the artificial one-dimensional system and does not affect the electronic properties of the chains. With other atomic species (H, C, Na, K, and Ca) we observe substantially different couplings and stronger interactions. We demonstrate that the different electronic properties of the various adatoms are responsible for different couplings with the substrate and compare our findings with the existing experimental results. Finally, in the case of Au chains we have investigated the role of adatom-adatom interactions in the formation of such one-dimensional structures.

Quantum Transport Effects in Copper(II) Phthalocyanine Sandwiched between Gold Nanoelectrodes

The electrical transmission of copper(II) phthalocyanine (CuPc) sandwiched between gold nanoelectrodes is studied on the basis of the Green function formalism coupled with the Gaussian-broadening technique. In the Au-CuPc-Au junction, broadened density of states (DOS) of the Au chains is defined as continuous DOS of electrodes to calculate the Green function of the electrodes. Two peaks of the transmission function found in the vicinity of the Fermi level are analyzed in terms of molecular orbitals (MOs). A convenient procedure to analyze MO contribution to a transmission peak is proposed. It is found that (I) symmetry-matched interactions between CuPc and the gold nanoelectrodes are important to the enhancement of the transmission function and (II) the nanoelectrodes have almost no effect on the electronic states of CuPc.

Scanning tunneling microscopy study of self-organized Au atomic chain growth on Ge(001)

Gold deposition onto Ge(001) at $675\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ led to self-organized atomic chains in $(4\ifmmode\times\else\texttimes\fi{}2)$ and $\mathrm{c}(8\ifmmode\times\else\texttimes\fi{}2)$ surface reconstructions. The chains were separated by $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and ran up to several hundred nanometers long. The Au-induced domains showed alternating white and gray chains in STM images that could be explained by $\mathrm{Au}\text{\ensuremath{-}}\mathrm{Au}$ and mixed $\mathrm{Au}\text{\ensuremath{-}}\mathrm{Ge}$ dimer rows, respectively. The chains showed a zigzag pattern attributed to dimer buckling. Bias-dependent STM imaging suggested that the Au chains were metallic nanowires. The chains, however, were not defect-free and contained missing dimer vacancies. The Ge terraces adjacent to the Au nanowire domains contained a high density of $(1+2+1)$ dimer vacancy defects that tended to run along [100] and [310] directions. The above results show strong similarities with those obtained for Pt on Ge(001) but are very different from those for Ag, which only weakly interacted with Ge(001), and thus support models suggesting stronger bonding of low-coordinated atoms of the $5d$ metals compared to the corresponding $4d$ metals.

Electronic properties of artificial Au chains with individual Pd impurities

Artificial Au atomic chains with individual Pd impurities were assembled from single metal atoms with a scanning tunneling microscope on a NiAl(110) surface. Scanning tunneling spectroscopy (STS) revealed an electronic resonance 2.15 eV above the Fermi energy localized within 4 A of single Pd atom impurities and two electronic resonances 2.25 eV and 2.95 eV above the Fermi energy localized within 8 A of Pd dimer impurities. The emergence of these localized resonances was studied by STS at each stage of the atom-by-atom assembly. Additionally, conductance images of the chains revealed delocalized electronic density oscillations in the pure Au segments of the chains.

Computational study of electron states in Au chains on NiAl(110)

We have carried out a density functional study of unoccupied, resonance states in a single Au atom, dimers, a trimer, and infinite Au chains on the NiAl(110) surface. Two inequivalent orientations of the ad-chains with substantially different interatomic distances were considered. From the study of the evolution of the electron states in an Au chain from being isolated to adsorbed, we find that the resonance states derive from the $6s$ states of the Au atoms, which hybridize strongly with the substrate states and develop a $p$-like polarization. The calculated resonance states and the local density of states (LDOS) images were analyzed in a simple tight-binding, resonance model. This model clarifies (1) the physics of direct and substrate-mediated adatom-adatom interactions and (2) the physics behind the enhancements of the LDOS at the ends of the adatom chains, and (3) the physical meaning of the ``particle-in-box'' model used in the analysis of observed resonance states. The calculated effective mass and band bottom energy are in good agreement with experimental data obtained from scanning tunnelling spectroscopy.

Nonmetallic transport of a quasi-one-dimensional metallicSi(557)−Ausurface

The sheet conductivity of a $\mathrm{Au}$-covered $\mathrm{Si}(557)$ facet surface was measured by microscopic four-point probe methods using an independently driven four-tip scanning tunneling microscope and temperature-variable monolithic probes. This surface is composed of a periodic array of $\mathrm{Au}$ chains and known to have a quasi-one-dimensional metallic band structure. Its surface conductivities parallel to the $\mathrm{Au}$ chains $({\ensuremath{\sigma}}_{\ensuremath{\Vert}})$ and perpendicular to them $({\ensuremath{\sigma}}_{\ensuremath{\perp}})$, were obtained separately at room temperature (RT), and the anisotropy ${\ensuremath{\sigma}}_{\ensuremath{\Vert}}∕{\ensuremath{\sigma}}_{\ensuremath{\perp}}$ was $\ensuremath{\sim}3$. The temperature dependence of the surface conductivity showed a semiconductive character below RT with an activation energy of $\ensuremath{\sim}55\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Then it can be concluded that the transport along the Au chains is not metallic band conduction.

Electronic and transport properties of artificial gold chains.

We study the electronic and transport properties of artificial Au atomic chains on a NiAl(110) surface template using state-of-the-art first principles calculations. Au chains display remarkable one-dimensional electronic properties that can be tuned by the selective adsorption of small molecules: a single CO group is shown to modulate the electronic wave functions, acting as a "chemical scissor" along the chain, to strongly modify the coherent transport properties of the system, and to help design one-dimensional nanodevices through artificial profiling of energy barriers.