Novel electronic structure of inhomogeneous quantum wires on a Si surface

Unveiling the hybridization between the Cr-impurity-mediated flat band and the Rashba-split state of the α-Au/Si(111) surface

Adsorption of foreign atoms onto 2D materials can either lead to ordinary electron doping or the emergence of new electronic effects including topology, superconductivity, and quantum anomalous Hall and Kondo states. We have investigated the effect of Cr doping on the electronic structure of the α-Au/Si(111)- surface and its adsorbate-modified family. It has been found that below a critical coverage of ∼0.05 monolayer, Cr adatoms penetrate beneath the Au and topmost Si layers and induce the occupied resonance flat band in the electronic spectrum as revealed by angle-resolved photoelectron spectroscopy. Further deposition of Cr leads to the growth of the 3D islands spoiling the surface homogeneity. Using density functional theory calculations, we have disclosed the effects of Cr doping on the electronic band structure and revealed the nature of hybridization between the Cr-induced magnetic-split band and the Au-induced Rashba-split surface state. We believe that the synthesized 2D phases and electronic effects produced by magnetic atom doping in the ultimate two-dimensional limit will stimulate further investigations related to the highly correlated phases and will find practical applications in nanoelectronic devices.

Structural and electronic effects of adatoms on metallic atomic chains in Si(111)5 × 2-Au

We investigate the effects of native Si adatoms on structural and electronic properties of the Si(111)5 × 2-Au surface, a representative one-dimensional metal-chain system, by means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. High-resolution STM images of relatively long adatom-free chain segments evidence directly the inherent ×2 reconstruction, which is the essential part of a recently proposed structural model based on a renewed Au coverage of 0.7 monolayer. On the other hand, STM images for chain segments of different lengths reveal that the structural distortion induced by Si adatoms is confined in neighboring unit cells, in good agreement with DFT calculations based on that model. Si adatoms greatly affect the metallic bands of Au chains, one of which becomes fully occupied and represents a tightly confined electronic state to the distortion around Si adatoms, potentially forming short insulating segments within metallic chains. This finding provides an atomic-scale understanding of the observed gradual metal-insulator transition and atomic-scale phase separation induced by Si adatoms.

Electron Quantization in Broken Atomic Wires

We demonstrate using scanning tunneling microscopy and spectroscopy the electron quantization within metallic Au atomic wires self-assembled on a Si(111) surface and segmented by adatom impurities. The local electronic states of wire segments with a length up to 10 nm are investigated as terminated by two neighboring Si adatoms. One-dimensional (1D) quantum well states are well resolved by their spatial distributions and the inverse-length-square dependence in their energies. The quantization also results in the quantum oscillation of the conductance at Fermi level. The present approach provides a new and convenient platform to investigate 1D quantum phenomena with atomic precision.

Metallic Properties of the Si(111) - 5 × 2 - Au Surface from Infrared Plasmon Polaritons and Ab Initio Theory

The metal-atom chains on the Si(111) - 5 × 2 - Au surface represent an exceedingly interesting system for the understanding of one-dimensional electrical interconnects. While other metal-atom chain structures on silicon suffer from metal-to-insulator transitions, Si(111) - 5 × 2 - Au stays metallic at least down to 20 K as we have proven by the anisotropic absorption from localized plasmon polaritons in the infrared. A quantitative analysis of the infrared plasmonic signal done here for the first time yields valuable band structure information in agreement with the theoretically derived data. The experimental and theoretical results are consistently explained in the framework of the atomic geometry, electronic structure, and IR spectra of the recent Kwon-Kang model.

Identification of the au coverage and structure of the Au/Si(111)-(5 × 2) surface

We identify the atomic structure of the Au/Si(111)-(5 × 2) surface by using density functional theory calculations. With seven Au atoms per unit cell, our model forms a bona fide (5 × 2) atomic structure, which is energetically favored over the leading model of Erwin et al. [Phys. Rev. B 80, 155409 (2009)], and well reproduces the Y-shaped and V-shaped (5 × 2) STM images. This surface is metallic with a prominent half filled band of surface states, mostly localized around the Au-chain area. The correct identification of the atomic and band structure of the clean surface further clarifies the adsorption structure of Si adatoms and the physical origin of the intriguing metal-to-insulator transition driven by Si adatoms.

Structural transition in atomic chains driven by transient doping

A reversible structural transition is observed on Si(553)-Au by scanning tunneling microscopy, triggered by electrons injected from the tip into the surface. The periodicity of atomic chains near the step edges changes from the 1×3 ground state to a 1×2 excited state with increasing tunneling current. The threshold current for this transition is reduced at lower temperatures. In conjunction with first-principles density-functional calculations it is shown that the 1×2 phase is created by temporary doping of the atom chains. Random telegraph fluctuations between two levels of the tunneling current provide direct access to the dynamics of the phase transition, revealing lifetimes in the millisecond range.

Optical fingerprints of Si honeycomb chains and atomic gold wires on the Si(111)-(5×2)-Au surface

The intensively studied Si(111)-(5×2)-Au surface is reexamined using reflectance anisotropy spectroscopy and density functional theory simulations. We identify distinctive spectral features relating directly to local structural motifs such as Si honeycomb chains and atomic gold wires that are commonly found on Au-reconstructed vicinal Si(111) surfaces. Optical signatures of chain dimerization, responsible for the observed (×2) periodicity, are identified. The optical response, together with STM simulations and first-principles total-energy calculations, exclude the new structure proposed very recently based on the reflection high-energy electron diffraction technique analysis of Abukawa and Nishigaya [Phys. Rev. Lett. 110, 036102 (2013)] and provide strong support for the Si honeycomb chain with the triple Au chain model of Erwin et al. [Phys. Rev. B 80, 155409 (2009)]. This is a promising approach for screening possible models of complex anisotropic surface structures.

Confined doping on a metallic atomic chain structure

On Si(111)-(5×2)-Au it is shown that metallic sections of quantum wires between two doping adatoms establish a local electronic structure which is primarily defined by the section length. Such confined doping is a direct consequence of reduced dimensionality and is not observed in higher dimensions. Within a chain segment, the effect of a spatially independent charge-carrier concentration is superimposed by a Coulomb-like interaction due to the positively charged dopants. This offers a natural explanation for the relatively broad photoemission features and the complex appearance in scanning tunneling microscopy and spectroscopy images.

Nanomanipulation and nanofabrication with multi-probe scanning tunneling microscope: from individual atoms to nanowires

The wide variety of nanoscale structures and devices demands novel tools for handling, assembly, and fabrication at nanoscopic positioning precision. The manipulation tools should allow for in situ characterization and testing of fundamental building blocks, such as nanotubes and nanowires, as they are built into functional devices. In this paper, a bottom-up technique for nanomanipulation and nanofabrication is reported by using a 4-probe scanning tunneling microscope (STM) combined with a scanning electron microscope (SEM). The applications of this technique are demonstrated in a variety of nanosystems, from manipulating individual atoms to bending, cutting, breaking carbon nanofibers, and constructing nanodevices for electrical characterizations. The combination of the wide field of view of SEM, the atomic position resolution of STM, and the flexibility of multiple scanning probes is expected to be a valuable tool for rapid prototyping in the nanoscience and nanotechnology.

Correlating electronic transport to atomic structures in self-assembled quantum wires

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.

Recent ARPES experiments on quasi-1D bulk materials and artificial structures

The spectroscopy of quasi-one-dimensional (1D) systems has been a subject of strong interest since the first experimental observations of unusual line shapes in the early 1990s. Angle-resolved photoemission (ARPES) measurements performed with increasing accuracy have greatly broadened our knowledge of the properties of bulk 1D materials and, more recently, of artificial 1D structures. They have yielded a direct view of 1D bands, of open Fermi surfaces, and of characteristic instabilities. They have also provided unique microscopic evidence for the non-conventional, non-Fermi-liquid, behavior predicted by theory, and for strong and singular interactions. Here we briefly review some of the remarkable experimental results obtained in the last decade.

One-dimensional defect-mediated diffusion of Si adatoms on the Si(111)-(5 x 2)-Au surface

Using scanning tunneling microscopy, we determine that the one-dimensional diffusion of Si adatoms along the Si(111)-(5 x 2)-Au surface reconstruction occurs by a defect-mediated mechanism. Distinctive diffusion statistics, especially correlations between sequential adatom displacements, imply that the displacements are triggered by an interaction with a defect that is localized to the adatom. The defect is intrinsic and thermally activated. The measured diffusion statistics are modeled accurately by a Monte Carlo simulation. The measured adatom diffusion activation barrier is 1.24 +/- 0.08 eV.

The excitation of one-dimensional plasmons in Si and Au-Si complex atom wires

An atom-scale quantum wire array at the Au adsorbed Si(111) surface is studied by electron energy loss spectroscopy. Clear one-dimensional metallicity is verified by the observation of low-energy plasmonic excitation which exhibits a strong anisotropic dispersion. Our theoretical analysis using a quantum-mechanical nonlocal response theory shows that the plasmons are most probably supported in conductive channels made of Si honeycomb wires rather than those made of Au-Si complex wires.

Hopping domain wall induced by paired adatoms on an atomic wire: si(111)-(5 x 2)-Au

We observed an inhomogeneous fluctuation along one-dimensional atomic wires self-assembled on a Si(111) surface using scanning tunneling microscopy. The fluctuation exhibits dynamic behavior at room temperature and is observed only in a specific geometric condition; the spacing between two neighboring adatom defects is discommensurate with the wire lattice. Upon cooling, the dynamic fluctuation freezes to show the existence of an atomic-scale dislocation or domain wall induced by such "unfavorably" paired adatoms. The microscopic characteristics of the dynamic fluctuation are explained in terms of a hopping solitonic domain wall, and a local potential for this motion imposed by the adatoms is quantified.

Zinc porphyrin-driven assembly of gold nanofingers

Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges.

Band-structure engineering of gold atomic wires on silicon by controlled doping

We report on the systematic tuning of the electronic band structure of atomic wires by controlling the density of impurity atoms. The atomic wires are self-assembled on Si(111) by substitutional gold adsorbates and extra silicon atoms are deposited as the impurity dopants. The one-dimensional electronic band of gold atomic wires, measured by angle-resolved photoemission, changes from a fully metallic to semiconducting one with its band gap increasing above 0.3 eV along with an energy shift as a linear function of the Si dopant density. The gap opening mechanism is suggested to be related to the ordering of the impurities.

Roughing titanium quantum wire on patterned monohydride diamond (001) surface

The authors have performed the roughing of titanium (Ti) quantum wires forming on a hydrogen-terminated diamond (001)-2x1 surface patterned with an ordered bare strip array and demonstrated that well-ordered Ti quantum wires are achieved only if the growth conditions (temperature and flux) have optimal values via kinetic Monte Carlo simulations. Considering that a scanning tunneling microscope is capable of selectively desorbing H from diamond (001)-2x1-H surface, they proposed a viable and easy approach to fabricate "ideal quantum wires" on the patterned hydrogen-terminated diamond (001) surface. The physical origin of the Ti quantum wire formation was pursued.

Electronic effects in the length distribution of atom chains

Gold deposited on Si(553) leads to self-assembly of atomic chains, which are broken into finite segments by defects. Scanning tunneling microscopy is used to investigate the distribution of chain lengths and the correlation between defects separating the chains. The length distribution reveals oscillations that indicate changes in the cohesive energy as a function of chain length. We present a possible interpretation in terms of the electronic scattering vectors at the Fermi surface of the surface states. The pairwise correlation function between defects shows long-range correlations that extend beyond nearest-neighbor defects, indicating coupling between chains.

End states in one-dimensional atom chains

End states--the zero-dimensional analogs of the two-dimensional states that occur at a crystal surface--were observed at the ends of one-dimensional atom chains that were self-assembled by depositing gold on the vicinal Si(553) surface. Scanning tunneling spectroscopy measurements of the differential conductance along the chains revealed quantized states in isolated segments with differentiated states forming over end atoms. A comparison to a tight-binding model demonstrated how the formation of electronic end states transforms the density of states and the energy levels within the chains.