Atomic-scale Wires Research Articles

Growth of self-integrated atomic quantum wires and junctions of a Mott semiconductor.

Continued advances in quantum technologies rely on producing nanometer-scale wires. Although several state-of-the-art nanolithographic technologies and bottom-up synthesis processes have been used to engineer these wires, critical challenges remain in growing uniform atomic-scale crystalline wires and constructing their network structures. Here, we discover a simple method to fabricate atomic-scale wires with various arrangements, including stripes, X-junctions, Y-junctions, and nanorings. Single-crystalline atomic-scale wires of a Mott insulator, whose bandgap is comparable to those of wide-gap semiconductors, are spontaneously grown on graphite substrates by pulsed-laser deposition. These wires are one unit cell thick and have an exact width of two and four unit cells (1.4 and 2.8 nm) and lengths up to a few micrometers. We show that the nonequilibrium reaction-diffusion processes may play an essential role in atomic pattern formation. Our findings offer a previously unknown perspective on the nonequilibrium self-organization phenomena on an atomic scale, paving a unique way for the quantum architecture of nano-network.

Phononic heat conductance of gold atomic contacts: Coherent versus incoherent transport

We present here a theoretical method to determine the phononic contribution to the thermal conductance of nanoscale systems in the phase-coherent regime. Our approach makes use of classical molecular dynamics (MD) simulations to calculate the temperature-dependent dynamical matrix, and the phononic heat conductance is subsequently computed within the Landauer-B\"uttiker formalism with the help of nonequilibrium Green's function techniques. Tailored to nanostructures, crucial steps of force constant and heat transport calculations are performed directly in real space. As compared to conventional density functional theory (DFT) approaches, the advantage of our method is two-fold. First, interatomic interactions can be described with the method of choice. Semiempirical potentials may lead to large computational speedups, enabling the study of much larger systems. Second, the method naturally takes into account the temperature dependence of atomic force constants, an aspect that is ignored in typical static DFT-based calculations. We illustrate our method by analyzing the temperature dependence of the phononic thermal conductance of gold (Au) chains with lengths ranging from 1 to 12 atoms. Moreover, in order to evaluate the importance of anharmonic effects in these atomic-scale wires, we compare the phase-coherent approach with nonequilibrium MD (NEMD) simulations. We find that the predictions of the phase-coherent method and the classical NEMD approach largely agree above the Debye temperature for all studied chain lengths, which shows that heat transport is coherent and that our phase-coherent approach is well suited for such nanostructures.

Realization of a Strained Atomic Wire Superlattice.

A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atoms are known to incorporate into the stepped Si(111) surface to form a Au-Si atomic wire array with both one-dimensional (1D) metallic and antiferromagnetic atomic chains. At a reduced density of Au, we find a regular array of Au-Si wires in alternation with pristine Si nanoterraces. Pristine Si nanoterraces impose a strain on the neighboring Au-Si wires, which modifies both the band structure of metallic chains and the magnetic property of spin chains. This is an ultimate 1D version of a strained-layer superlattice of semiconductors, defining a direction toward the fine engineering of self-assembled atomic-scale wires.

Raman spectroscopy as a tool to investigate the structure and electronic properties of carbon-atom wires.

Graphene, nanotubes and other carbon nanostructures have shown potential as candidates for advanced technological applications due to the different coordination of carbon atoms and to the possibility of π-conjugation. In this context, atomic-scale wires comprised of sp-hybridized carbon atoms represent ideal 1D systems to potentially downscale devices to the atomic level. Carbon-atom wires (CAWs) can be arranged in two possible structures: a sequence of double bonds (cumulenes), resulting in a 1D metal, or an alternating sequence of single–triple bonds (polyynes), expected to show semiconducting properties. The electronic and optical properties of CAWs can be finely tuned by controlling the wire length (i.e., the number of carbon atoms) and the type of termination (e.g., atom, molecular group or nanostructure). Although linear, sp-hybridized carbon systems are still considered elusive and unstable materials, a number of nanostructures consisting of sp-carbon wires have been produced and characterized to date. In this short review, we present the main CAW synthesis techniques and stabilization strategies and we discuss the current status of the understanding of their structural, electronic and vibrational properties with particular attention to how these properties are related to one another. We focus on the use of vibrational spectroscopy to provide information on the structural and electronic properties of the system (e.g., determination of wire length). Moreover, by employing Raman spectroscopy and surface enhanced Raman scattering in combination with the support of first principles calculations, we show that a detailed understanding of the charge transfer between CAWs and metal nanoparticles may open the possibility to tune the electronic structure from alternating to equalized bonds.

Quantum thermopower of metallic atomic-size contacts at room temperature.

We report conductance and thermopower measurements of metallic atomic-size contacts, namely gold and platinum, using a scanning tunneling microscope (STM) at room temperature. We find that few-atom gold contacts have an average negative thermopower, whereas platinum contacts present a positive thermopower, showing that for both metals, the sign of the thermopower in the nanoscale differs from that of bulk wires. We also find that the magnitude of the thermopower exhibits minima at the maxima of the conductance histogram in the case of gold nanocontacts while for platinum it presents large fluctuations. Tight-binding calculations and Green's function techniques, together with molecular dynamics simulations, show that these observations can be understood in the context of the Landauer-Büttiker picture of coherent transport in atomic-scale wires. In particular, we show that the differences in the thermopower between these two metals are due to the fact that the elastic transport is dominated by the 6s orbitals in the case of gold and by the 5d orbitals in the case of platinum.

Dangling-bond logic gates on a Si(100)-(2 × 1)–H surface

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.

Electronic structure of Ag-induced atomic wires on Si(5 5 7) investigated by STS and angle-resolved photoemission

One-dimensional (1D) superstructures on the Si(5 5 7) surface induced by Ag adsorption have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission. The deposition of ∼0.3 ML of Ag at 450–620 °C yields three different kinds of 1D structures along step edges. These structures form domains of different morphology, whose areal ratio depends on the growth temperature. They commonly share a characteristic atomic-scale wire structure with a ×2 periodicity. These structures are insulating with a band gap of about 0.5 eV as revealed by STS and confirmed consistently by angle-resolved photoemission, in clear contrast to the very recent inverse photoemission result (Phys. Rev. B 77 (2008) 125419).

Nano-architectures by covalent assembly of molecular building blocks

The construction of electronic devices from single molecular building blocks, which possess certain functions such as switching or rectifying and are connected by atomic-scale wires on a supporting surface, is an essential goal of molecular electronics. A key challenge is the controlled assembly of molecules into desired architectures by strong, that is, covalent, intermolecular connections, enabling efficient electron transport between the molecules and providing high stability. However, no molecular networks on surfaces 'locked' by covalent interactions have been reported so far. Here, we show that such covalently bound molecular nanostructures can be formed on a gold surface upon thermal activation of porphyrin building blocks and their subsequent chemical reaction at predefined connection points. We demonstrate that the topology of these nanostructures can be precisely engineered by controlling the chemical structure of the building blocks. Our results represent a versatile route for future bottom-up construction of sophisticated electronic circuits and devices, based on individual functionalized molecules.

Coexistence of Two Different Peierls Distortions within an Atomic Scale Wire: Si(553)-Au

The ground state property of a Au-induced atomic wire array on a stepped Si(553) surface with interesting 1D metallic bands was investigated. Electron diffraction and scanning tunneling microscopy reveal an intriguing coexistence of triple- and double-period lattice distortions at low temperature. Angle-resolved photoemission observes both the nearly 1/3- and 1/2-filled bands to gradually open energy gaps upon cooling. We explain these unusual findings as due to the occurrence of Peierls distortions of triple and double periods on the two different atomic-scale chain elements, respectively, within a single unit wire. The two Peierls distortions are suggested to have different transition temperatures and little lateral correlation between each other.

Surface bismuth removal after Bi nanoline encapsulation in silicon

Recently, several kinds of atomic-scale wires have been reported for nano-scale device and interconnect applications. The Bi nanoline is one of the most promising candidates because of its structural perfection and the capability to be buried in an epitaxial Si layer. A surfactant layer of Bi is necessary for the encapsulation process, and this remains on the sample surface after the burial, but it needs to be removed so that the buried Bi line may be properly characterized by I– V measurements, X-ray diffraction, etc. without perturbations from the surface layer. It is shown by RHEED observation that the thermal desorption is not suitable for this purpose, because the one-dimensional structure of the buried Bi line is disrupted over the same time scale as for removal of the surfactant Bi. Chemical wet etching of the surface layer was found to be a better solution to the removal of the surfactant Bi atoms without destroying the buried Bi lines.

Short range and long range strain fields of Bi nanoline

The practical realisation of nanoscale devices requires the development of practical nanofabrication techniques. The Bi nanolines which self-assemble on Si(001) are promising templates for atomic scale wires, and also have a fascinating subsurface reconstruction. Elastic interactions are often responsible for the limited dimensionality of epitaxial nanoscale structures. The present work examines the elastic strain field around the Bi nanoline in terms of its interaction with other surface features. A short range tensile strain around the Bi nanoline may be identified by the effect on the electronic structure of the neighbouring dimers. A longer range elastic interaction is exhibited in a repulsive interaction between the nanoline and defects and steps. Atomic resolution variable bias scanning tunnelling microscopy (STM) and first principles electronic structure calculations have been used to elucidate these effects, and excellent agreement has been found between experimental observations and theoretical results.

Behaviors of surfactant atoms on Si(001) surface

The behaviors of surfactant atoms for thin film growth on the Si(001) surface formed by molecular beam epitaxy were observed. At first, heterointerfaces on Si/Ge and Ge/Si without surfactant effect were investigated and are discussed on the basis of thermodynamics of surfaces and surface diffusion. The mechanism of island formation as a later stage of Stranski-Krastanov growth mode is also discussed. Surfactant effects of Bi atoms on the heteroepitaxial structures were investigated experimentally. Behavior of Sb as a surfactant was also observed and compared with the results of Bi. Mechanisms for improvements in the abruptness at heterointerfaces of Si/Ge/Si as well as suppression of island formation are discussed. The formation of atomic-scale wires of Bi was observed. The structure of Bi wires at high temperatures is described.

Coherent electron-phonon coupling and polaronlike transport in molecular wires

We present a technique to calculate the transport properties through one-dimensional models of molecular wires. The calculations include inelastic electron scattering due to electron-lattice interaction. The coupling between the electron and the lattice is crucial to determine the transport properties in one-dimensional systems subject to Peierls transition since it drives the transition itself. The electron-phonon coupling is treated as a quantum coherent process, in the sense that no random dephasing due to electron-phonon interactions is introduced in the scattering wave functions. We show that charge carrier injection, even in the tunneling regime, induces lattice distortions localized around the tunneling electron. The transport in the molecular wire is due to polaron-like propagation. We show typical examples of the lattice distortions induced by charge injection into the wire. In the tunneling regime, the electron transmission is strongly enhanced in comparison with the case of elastic scattering through the undistorted molecular wire. We also show that although lattice fluctuations modify the electron transmission through the wire, the modifications are qualitatively different from those obtained by the quantum electron-phonon inelastic scattering technique. Our results should hold in principle for other one-dimensional atomic-scale wires subject to Peierls transitions.

Transmission through Peierls distorted one-dimensional atomic wires: quantum coherent electron–phonon coupling

One-dimensional atomic-scale wires are subject to intense research both experimentally and theoretically. Such highly confined electronic systems exhibit strong electron–phonon coupling which would be expected to have an important effect on electron transport in the coherent regime. In this paper, we present an approach to calculate the inelastic effects on the electronic transport through heterojunctions of the type metal/molecular wire/metal. The electron–phonon coupling inside the molecular wire is explicitly considered on the quantum level. We show that the propagation of a tunneling electron inside the wire induces lattice distortions representative of the formation of a polaron. Such a polaron can also interact with a pre-existing soliton in the wire. The coherent tunneling electron–lattice deformations have non-negligible effects on the transmission properties through the molecular wire.

Scanning tunneling microscopy/spectroscopy of dangling-bond wires fabricated on the Si(100)-2×1-H surface

We present a scanning tunneling microscopy/spectroscopy (STM/STS) study of atomic-scale dangling-bond (DB) wires on a hydrogen-terminated Si(100)-2×1-H surface. In the case of DB wires made of paired DBs, the STS shows semiconductive electronic states with a band gap of approximately 0.5 eV. The DB wires made of both single and paired DBs show a finite density of states at Fermi energy and do not show semiconductive band gaps. We have succeeded in making an atomic-scale wire that has a finite density of states at Fermi energy on a semiconductive surface. The results are in good agreement with a recent first-principles theoretical calculations.

Conductance through a single atom

In this paper we present an analysis of conduction through a single atom between two metal electrodes. Based on ab initio total-energy and electronic-structure calculations, and molecular-dynamics simulations using the embedded-atom model, we show that the conductance through an atom depends on the electronic structure of both the single atom and the metal electrodes, as well as the binding structure between the single atom and the surfaces of the metal electrodes. Our results enable us to interpret experimental results obtained by using a mechanical break junction on atomic-scale wires.

Fabrication of atomic wires based on self-organization

One-dimensional chain-like structures consisting of rows of single chromium adatoms have been obtained by annealing a submonolayer chromium film deposited under ultra-high vacuum (UHV) conditions onto a Cr(100) single crystal substrate. The self-organization of the Cr atoms under the influence of the substrate surface potential leads to two kinds of atomic rows orientated in orthogonal directions, thereby reflecting the symmetry of the Cr(100) surface potential. The formation of these atomic-scale wires has been monitored in-situ by means of a UHV scanning tunneling microscope (STM).

Towards the Synthesis of Atomic Scale Wires

ABSTRACTRecent work1 has highlighted the possibility that through the introduction of metals into the one-dimensional channels of zeolite L, it may be feasible to engineer charge transport along the channels to produce a unique compound comprising a precise, assembled array of ultrafine, atomic-scale conducting wires embedded within the aluminosilicate framework. Using electron spin resonance (ESR), and microwave cavity perturbation measurements, we examine the properties of these remarkable materials as a function of composition as they approach the insulator to metal transition.