Atomic Wires Research Articles

Manipulation of structural, electronic and transport properties of hydrogen-passivated graphene atomic sheet through vacancy defects: first-principles numerical simulations based on density-functional-theory along with tight-binding approximation

Using the first-principles procedure of density-functional-theory within tight-binding approximation and nonequilibrium Green’s function formalism, this paper reports on the impact of vacancy defects on the structural, electronic and transport properties of hydrogen-passivated graphene atomic sheet. After the introduction of vacancy defects in graphene atomic sheet passivated with hydrogen atoms, apart from increase in band gap, a suppression is noted in the intensity of transmission channels and density of states arising from the long array deformations of the graphene sheet and a corresponding shift of the Fermi level. This in turn decreases the conductance of the defected graphene atomic sheet. In case of slow-ion bombardment method, the conductance of the sheet decreases slowly and its value of the order 10−6 S before vanishing the percolation drops to the order 10−10 as the percolation of the sheet is destroyed. But in case of fast bombardment the conductance of the sheet shows a linear drop before vanishing of the percolation of the sheet, and its value of the order 10−6 S before vanishing the percolation drops to the order 10−10 as the percolation of the sheet is destroyed. Furthermore, it is found that the atomic vacancy defects effectively terminate the original smooth sp2-hybrid network of 2D graphene atomic sheet that leads to modify its electronic and transport properties, especially a decrease in its electrical conductance. Interestingly, transmission spectrum of graphene atomic wire with large vacancy defects of 143 attains identical shape to that of a molecular benzene ring.

Plasmon Standing Waves by Oxidation of Si(553)–Au

Self-assembled Au atomic wires on stepped Si surfaces are metallic, as evidenced by a one-dimensionally dispersing plasmonic excitation. Here we investigate the effects of oxidization on metallicity along such Au atomic wires on a regularly stepped Si(553) surface, by employing infrared absorption and high resolution electron energy loss spectroscopies. Our results indicate that only the Si environment undergoes oxidation, which has a remarkably small effect on the plasmon dispersion. Only close to $k_{\parallel}\rightarrow 0$ the plasmon dispersion ends at increasingly higher energies as a function of oxygen exposure, which is attributed to standing wave formation on small sections of Au wires generated by the introduction of O atoms as scattering centers, not to electronic gap opening. This interpretation is in full agreement with the findings by infrared spectroscopy and with low energy electron diffraction.

Fabrication of one-dimensional magic cluster arrays using a vicinal surface as a template

One-dimensional arrays of magic In clusters were grown using the Si(557) surface as a template. We find that magic clusters with an In6Si3 stoichiometry grow on the (111) terraces of the Si(557) surface, which are identical to those formed on the planar Si(111)7 × 7 surface. A preferential formation of the clusters on faulted half unit cells of the 7 × 7 reconstruction enables the growth of two different types of one-dimensional arrays with different cluster-cluster distances. A delayed onset of the cluster growth with an initial formation of atomic wires is found, in contrast to magic In clusters on the planar Si(111) surface.

Structure modulated charge transfer in carbon atomic wires

sp-Hybridized carbon atomic wires are appealing systems with large property tunability. In particular, their electronic properties are intimately related to length, structure, and type of functional end-groups as well as to other effects such as the intermolecular charge transfer with metal nanoparticles. Here, by a combined Raman, Surface Enhanced Raman Scattering (SERS) investigation and first principles calculations of different N,N-dimethylanilino-terminated polyynes, we suggest that, upon charge transfer interaction with silver nanoparticles, the function of sp-carbon atomic wire can change from electron donor to electron acceptor by increasing the wire length. In addition, the insertion into the wire of a strong electrophilic group (1,1,4,4-tetracyanobuta-1,3-diene-2,3-diyl) changes the electron-accepting molecular regions involved in this intermolecular charge transfer. Our results indicate that carbon atomic wires could display a tunable charge transfer between the sp-wire and the metal, and hold promise as active materials in organic optoelectronics and photovoltaics.

Investigation of Effects of Diameter, Doping and Vacancy Defects on the Band Structure and Transport Properties of Silicon Nanowires for Potential Applications in Field-Effect Transistors

Silicon nanowires (SiNWs) with unique band structure and transport properties are considered potential candidates for future nanoelectronics devices such as field-effect transistors (FETs). We present a model of a SiNW-FET comprising $$\langle100\rangle$$ silicon atomic wires with a cylindrical-shaped metallic gate wrapped around the wires. For this purpose, we report on the energy band structure and density of states of SiNWs of diameters 5.93 A, 9.71 A and 13.55 A with $$\langle100\rangle$$ cleavage orientation by employing generalized gradient approximation and meta-generalized gradient approximation as well as the semi-empirical extended-Huckel model. Moreover, the transmission and transport properties of doped and undoped SiNWs of diameter 5.93 A with and without vacancy defects are explored using a non-equilibrium green function approach with self-consistent calculations. The corresponding I–V characteristics of the proposed cylindrical-shaped metallic-gate SiNW-FET under a specific gate voltage are presented. Our results show that the undoped SiNWs with vacancy defects on the surface are more suitable candidates for nanoelectronic device applications such as FETs in contrast to their counterparts with vacancies at the center.

Multiple Electronic Components and Lifshitz Transitions by Oxygen Wires Formation in Layered Cuprates and Nickelates

There is growing compelling experimental evidence that a quantum complex matter scenario made of multiple electronic components and competing quantum phases is needed to grab the key physics of high critical temperature (Tc) superconductivity in layered cuprates. While it is known that defect self-organization controls Tc, the mechanism remains an open issue. Here we focus on the theoretical prediction of the multiband electronic structure and the formation of broken Fermi surfaces generated by the self-organization of oxygen interstitials Oi atomic wires in the spacer layers in HgBa2CuO4+δ, La2CuO4+δ and La2NiO4+δ, by means of self-consistent Linear Muffin-Tin Orbital (LMTO) calculations. The electronic structure of a first phase of ordered Oi atomic wires and of a second glassy phase made of disordered Oi impurities have been studied through supercell calculations. We show the common features of the influence of Oi wires in the electronic structure in three types of materials. The ordering of Oi into wires leads to a separation of the electronic states between the Oi ensemble and the rest of the bulk. The wire formation first produces quantum confined localized states near the wire, which coexist with, Second, delocalized states in the Fermi surface (FS) of doped cuprates. A new scenario emerges for high Tc superconductivity, where Kitaev wires with Majorana bound states are proximity-coupled to a 2D d-wave superconductor.

On topology-tuned thermoelectric properties of noble metal atomic wires

We have carried out a systematic first principles study of thermal and thermoelectric transport properties of different noble metal (viz. Ag, Cu, Au, and Pt) atomic wires modelled in different topologies by using the Landauer-Büttiker approach in the linear response regime. Both electronic and phononic contributions are considered, with the respective transmission functions determined using the non-equilibrium Green's function approach, based upon the density functional theory and the general utility lattice program, respectively. To explore the role of topology, we have considered the linear, ladder, zigzag (ZZ), and double zigzag (DZZ) topologies for each of the four wires. Numerical results are presented for the electronic κel and phononic κph thermal conductance, thermopower S, and total figure of merit ZT. In addition, the validity of the Wiedemann-Franz law in these atomic wires has been studied. Quite generally, we find a significant dependence of these properties on the wire topology. Under ambient conditions, the valence electrons are found to be the dominant carriers of heat energy in all the wires, except for the Ag, Cu, and Au wires in the DZZ topology due to their semi-metallic nature. In relative terms, the phononic contribution is minimum (∼ 4%) for the Pt wire in linear topology, while maximum (∼ 15%) for the Cu wire in ZZ topology. However, the phonon contribution increases steadily with decreasing temperature to become as large as about 30% at T = 100 K (for Cu wire in ZZ topology). In overall, the Pt wire is found to have the maximum total thermal conductance in all topologies. S and ZT, apart from depending on topology and type of metal wire, exhibit an oscillatory behaviour as a function of chemical potential μ of electrodes. S has been found to change even its sign at characteristic μ, with a maximum value of ∼±1.5 mV/K for the Au wire in DZZ topology at μ = ∓0.15 eV. Also, ZT is found to be maximum (∼ 3) for this wire, with the presence of two pronounced peaks at μ ∼±0.6 eV, apparently located at somewhat higher μ as compared to S. An analysis of our results reveals that the change in thermoelectric properties with topology stems primarily from a change in electronic band structure and phonon spectra.

Intertwined Solitons and Impurities in a Quasi-One-Dimensional Charge-Density-Wave System: In/Si(111).

Recent studies on a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) system, In atomic wires on Si(111), have brought intriguing issues on topological solitons in quasi-1D systems: the existence of few-atom-sized solitons, chirality of solitons, and the realization of logic exploiting the chirality switching. Using scanning tunneling microscopy and first-principles calculations, we show that the previously reported "short" phase-flip defects are In adatoms and thus have nonsolitonic nature, resolving the controversy over the existence of highly localized solitons. The observed "long" phase-flip and phase-slip defects are genuine solitons with and without chirality, respectively. While achiral solitons (phase-slip defects) can exist on the pristine CDW (8×2) surface, chiral solitons (phase-flip defects) cannot due to their breakage of 8×2 ordering. The chiral solitons can exist only when they are trapped by In adatoms and constitute a part of a closed-loop domain wall. The intertwinement of chiral solitons and In adatoms implies the limitations of the previously proposed logic utilizing soliton chirality, but it provides an opportunity to realize this by controlling the In-adatom defect.

Scanning tunneling microscopy and Raman spectroscopy of polymeric sp-sp2 carbon atomic wires synthesized on the Au(111) surface.

Long linear carbon nanostructures based on sp-hybridization can be synthesized by exploiting on-surface synthesis of halogenated precursors evaporated on Au(111), thus opening a way to investigations by surface-science techniques. By means of an experimental approach combining scanning tunneling microscopy and spectroscopy (STM and STS) with ex situ Raman spectroscopy we investigate the structural, electronic and vibrational properties of polymeric sp-sp2 carbon atomic wires composed by sp-carbon chains connected through phenyl groups. Density-functional-theory (DFT) calculations of the structure and the electronic density of states allow us to simulate STM images and to compute Raman spectra. The comparison of experimental data with DFT simulations unveil the properties and the formation stages as a function of the annealing temperature. Atomic-scale structural information from STM complement the Raman sensitivity to the single molecular bond to open the way to detailed understanding of these novel carbon nanostructures.

Alloying-induced spin Seebeck effect and spin figure of merit in Pt-based bimetallic atomic wires of noble metals.

We have investigated using first principles the occurrence and tunability of the spin Seebeck effect in Pt-based bimetallic wires of noble metals (viz. XPt, X = Cu, Ag, and Au) modelled in linear, ladder, and double zigzag (DZZ) topologies. The spin figure of merit ZsT and its charge counterpart ZcT are calculated by considering both electronic and phononic contributions to the thermal conductance. For this endeavour, we have employed the Landauer-Büttiker approach, with the requisite electron τel(E) and phonon τph(E) transmission functions obtained using the non-equilibrium Green's function approach, based on density functional theory and the general utility lattice program, respectively. We find that alloying and/or topological tailoring bring in quantitative as well as qualitative changes in the transport properties. Unlike the pristine wires, τel(E) now depends (except for the CuPt wire in the ladder topology) markedly on spin, thus resulting in an unequal current in the two spin channels and hence a non-zero spin Seebeck coefficient Ss. Remarkably, the AgPt wire in the linear topology and all wires in the DZZ1 and DZZ3 configurations of the DZZ topology exhibit half-metallic conduction, with a sizable gap in the density of ↑-spin states at the Fermi level. Alloying also introduces energy gap(s) in the phonon density of states. Consequently, we find a significant reduction in the electronic and phononic thermal conductance. Interestingly, Ss is of the same order as its charge counterpart Sc, and both can be tuned via the topology and chemical potential μ of the electrodes. This together with the reduced thermal conductance results in a significantly high room-temperature ZcT (∼33) and ZsT (∼25) at μ ∼ 0.24 eV in the AgPt wire in the DZZ3 topology, with about a ten-fold increase in ZcT as compared to the pristine wires. Furthermore, there exists a characteristic μ in some bimetallic wires for which Sc approaches zero, but Ss remains quite appreciable. This result arises due to the emergence of bipolar thermal conduction, which under criticality makes S↑ + S↓ = 0. Importantly, such a situation may be exploited to generate a pure thermal spin voltage. Our results can be explicated on the basis of changes in the electronic band structure and phononic spectra due to alloying and topological effects.

Luttinger liquid and charge density wave phases in a spinless fermion wire on a semiconducting substrate

An interacting spinless fermion wire coupled to a three-dimensional (3D) semiconducting substrate is approximated by a narrow ladder model (NLM) with varying number of legs. We compute density distributions, gaps, charge-density-wave (CDW) order parameters, correlation functions, and the central charge using the density-matrix renormalization group method. Three ground-state phases are observed: a one-component Luttinger liquid, a quasi-one-dimensional (1D) CDW insulator, and a band insulator. We investigated the convergence of the NLM properties with increasing number of legs systematically and confirm that the NLM is a good approximation for the quasi-1D phases (Luttinger liquid and CDW) of the 3D wire-substrate model. The quantum phase transitions between these phases are investigated as function of the coupling between wire and substrate. The critical nearest-neighbor interaction increases with increasing coupling between wire and substrate and thus the substrate stabilizes the Luttinger liquid in the wire. Our study confirms that a Luttinger liquid or CDW insulator phase could occur in the low-energy properties of atomic wires deposited on semiconducting substrates.

Au-induced atomic wires on stepped Ge(hhk) surfaces

Au adsorption on high-index Si surfaces is known to form quasi-one-dimensional surface reconstructions with interesting physical properties such as Rashba-split bands and ordered arrays of local magnetic moments. Here we report on a novel family of Au-induced chain systems, hosted on Ge(hhk) substrates. Our study includes Ge(553), Ge(557), and Ge(335) for both bare and Au-adsorbed surfaces, respectively. We employ scanning tunneling microscopy and low-energy electron diffraction to characterize the topography and the occurrence of superstructures. Stable bare surfaces with regularly distributed steps are found for the (553) and (335) orientations. For nominal Ge(557) substrates a refaceting to Ge(223) is observed. Addition of Au tends to promote a change in the surface index which creates Ge(221)-Au, Ge(557)-Au, and Ge(335)-Au surfaces. Their STM images show elements that are reminiscent of the well-understood Si(hhk)-Au surfaces but also reveal important differences.

Charge Mobility and Recombination Mechanisms in Tellurium van der Waals Solid.

Trigonal tellurium is a small band gap elemental semiconductor consisting of van der Waals bound one-dimensional helical chains of tellurium atoms. We study the temperature dependence of the charge carrier mobility and recombination pathways in bulk tellurium. Electrons and holes are generated by irradiation of the sample with 3 MeV electrons and detected by time-resolved microwave conductivity measurements. A theoretical model is used to explain the experimental observations for different charge densities and temperatures. Our analysis reveals a high room temperature mobility of 190 ± 20 cm2 V–1 s–1. The mobility is thermally deactivated, suggesting a band-like transport mechanism. According to our analysis, the charges predominantly recombine via radiative recombination with a radiative yield close to 98%, even at room temperature. The remaining charges recombine by either trap-assisted (Shockley–Read–Hall) recombination or undergo trapping to deep traps. The high mobility, near-unity radiative yield, and possibility of large-scale production of atomic wires by liquid exfoliation make Te of high potential for next-generation nanoelectronic and optoelectronic applications, including far-infrared detectors and lasers.

How To Probe the Limits of the Wiedemann-Franz Law at Nanoscale.

While the Wiedemann-Franz law is known to be robust for many bulk materials, possible violations have been actively discussed for certain classes of bulk materials such as heavy Fermion materials. At nanoscale on the other hand the limits of the Wiedemann-Franz law and how to probe and control them remains an open question. Therefore, we propose here a systematic way to elucidate the limits of the Wiedemann-Franz law at nanoscale. Using first-principles calculations, we examine the Wiedemann-Franz law in nanoscale conductors, namely in gold and platinum-based atomic wires. We explain the recently observed experimental evidence of the Wiedemann-Franz law in atomic-point contacts, but conversely we show that in regimes not discussed in these experiments notable violations of the Wiedemann-Franz law emerge. Depending on the temperature and gate potential as well as chemical properties and conformation, the violations reach up to 30% for gold and for platinum they can even exceed 350%.

Electronic and Magnetic Properties of Fe Atomic Chain and Fe Atomic Plane: Anab initioStudy

The electronic and magnetic properties of Fe atomic wire and atomic plane have been theoretically investigated from full potential linearized augmented plane wave (FPLAPW) method within a frame work of density functional theory (DFT). This work is based on the comparative study of number of Fe nanochains with infinite length and infinitely spread Fe nanosheet. A most commonly adopted GGA approximation is used for electron exchange correlation. In our calculation, the property of Fe-chain is predicted to be magnetic metal with the presence of deep valley (in Spin-up DOS) and a peak (in Spin-down DOS) at Fermi level ([Formula: see text]) shows the antisymmetric DOS. The presence of antisymmetric DOS is a signature of exchange splitting between the degenerated d-states. The splitting between t[Formula: see text] states is very prominent in Fe-chain which enhances the magnetic moment. The magnetic moment decreases with the increase in number of Fe-chains.

Thermoelectric voltage switching in gold atomic wire junctions

We explore the thermoelectric properties of gold atomic chains bridging gold electrodes by means of \textit{ab initio} and semi-empirical calculations, and heuristic reasoning. We predict the thermoelectric voltage induced by a temperature difference across such junctions to oscillate, repeatedly changing sign, as a function of the number of atoms $N$ making up the atomic chain. We also predict the amplitude of the oscillations to be proportional to $N$ for long atomic chains. Further we predict the thermoelectric voltage to change sign, in some cases, if the junction is stretched without changing $N$. Our predictions apply regardless of whether regular or irregular electrode grain boundaries are present and whether the electrodes are symmetric or asymmetric. Our findings may pave the way to the realization of new mechanically controllable voltage switches enabling direct conversion of heat into electricity in energy harvesting applications.

Ultrafast switching in an atomic wire system at surfaces

Abstract

Distributions of atomic wire lengths

Controlled growth of metal monatomic wires would enable on-demand tuning of electronic and magnetic properties in this new class of materials. Prior to this work, it was believed that the binding energy can be determined from the distribution of the lengths of the wires. Another misconception was that the antiripening mechanism or quantum effects are responsible for the growth of wires during postdeposition annealing. Combining kinetic Monte Carlo and first-principles density-functional theory calculations, we study the growth of one-dimensional atomic wires on the steps of vicinal surfaces. We show that for a large value of the bond energy, the antiripening mechanism and quantum effects do not affect the length of metal monatomic wires. The conditions under which wires with magic length appear are determined. The observed mechanism of wire growth will be useful both for explaining the experimental data and for creating atomic wires with a given length.

Physics insight and first-principles calculation of atomic conductance: implications for the future interconnects

There are still many unknowns and open questions concerning the properties of atomic wires, which can be very different from the bulk case. A physics-based compact model framework for analysis of atomic-scale conductors in the presence of point defects is described, and the results of the compact model are compared with First-principle DFT calculations. The calculations indicate that the typically more resistive bulk metals can emerge as lower-resistance options over Al and Cu at atomic dimensions. In the proposed model, the electrons are considered to be localized at their host atoms. The discussion topics include low-temperature resistance of atomic wires from different materials, and the influence of elementary point defects, such as atomic voids on electron transport. It is argued that the concept of resistivity should be used with care when applied to low-dimensional metals, for example the effective resistivity of small conductors is increased even in the absence of surface and grain-boundary scattering. The related concept of electron ‘mean free path’ needs also to be revisited. These questions could have significant implications in terms of the selection of most appropriate material systems for tera-scale metallization schemes.

Carbyne: from the elusive allotrope to stable carbon atom wires

Abstract