Linear Atomic Chains Research Articles

Plasmon-induced dynamics of H2 splitting on a silver atomic chain

Localized surface plasmon resonances (LSPR) supported in metal nanostructures can be efficiently harnessed to drive photocatalytic reactions, whose atomic scale mechanism remains a challenge. Here, real-time dynamics of H2 photosplitting on a linear silver atomic chain, upon exposure to femtosecond laser pulses, has been investigated using time-dependent density functional theory. The wavelength dependent H2 splitting process is strongly coupled to LSPR excitation in silver chain. We identify that hot electrons produced in the silver chain by plasmon excitation are transferred to the antibonding state of the adsorbed H2 and trigger H2 dissociation, consistent with experimental observations. Increasing illumination intensity and the length of atomic chain promote H2 splitting, thanks to stronger LSPR. Dynamic electronic response can be quantitatively described within the present approach, providing insights towards a complete fundamental understanding on plasmon-induced chemical reactions at the microscopic scale.

Single-photon transport through an atomic chain coupled to a one-dimensional nanophotonic waveguide

We study the dynamics of a single-photon pulse traveling through a linear atomic chain coupled to a one-dimensional (1D) single mode photonic waveguide. We derive a time-dependent dynamical theory for this collective many-body system which allows us to study the real time evolution of the photon transport and the atomic excitations. Our analytical result is consistent with previous numerical calculations when there is only one atom. For an atomic chain, the collective interaction between the atoms mediated by the waveguide mode can significantly change the dynamics of the system. The reflectivity of a photon can be tuned by changing the ratio of coupling strength and the photon linewidth or by changing the number of atoms in the chain. The reflectivity of a single-photon pulse with finite bandwidth can even approach $100%$. The spectrum of the reflected and transmitted photon can also be significantly different from the single-atom case. Many interesting physical phenomena can occur in this system such as the photonic band-gap effects, quantum entanglement generation, Fano-like interference, and superradiant effects. For engineering, this system may serve as a single-photon frequency filter, single-photon modulation, and may find important applications in quantum information.

A Mixed-Valence Tri-Zinc Complex, [LZnZnZnL] (L=Bulky Amide), Bearing a Linear Chain of Two-Coordinate Zinc Atoms.

Reduction of a variety of extremely bulky amido Group 12 metal halide complexes, [LMX(THF)(0,1)] (L=amide; M=Zn, Cd, or Hg; X=halide) with a magnesium(I) dimer gave a homologous series of two-coordinate metal(I) dimers, [L'MML'] (L'=N(Ar(†))(SiMe3), Ar(†)=C6H2{C(H)Ph2}2Pr(i)-2,6,4); and the formally zinc(0) complex, [L*ZnMg((Mes)Nacnac)] (L*=N(Ar*)(SiPr(i)3); Ar*=C6H2{C(H)Ph2}2Me-2,6,4; (Mes)Nacnac=[(MesNCMe)2CH](-), Mes=mesityl), which contains the first unsupported Zn-Mg bond. Two equivalents of [L*ZnMg((Mes)Nacnac)] react with ZnBr2 or ZnBr2(tmeda) to give the mixed valence, two-coordinate, linear tri-zinc complex, [L*Zn(I)Zn(0)Zn(I)L*], and the first zinc(I) halide complex, [L*ZnZnBr(tmeda)], respectively. The analogues [L*ZnMZnL*] (M=Cd or Hg), were also prepared, the Cd species contains the first Zn-Cd bond in a molecular compound. Metal-metal bonding was studied by DFT calculations.

On the interaction of propagating intrinsic localized modes with an isolated impurity in a linear atomic chain

We study the interaction between an intrinsic localized mode and a substitutional defect in a linear chain of atoms interacting with their nearest neighbors through a translational invariant potential function with a harmonic and a quartic anharmonic terms. The classical equations of motion are solved by means of a numerical iterative procedure and moving solutions are sought. The frequencies and atomic displacements from the equilibrium positions are calculated and the interaction between the excitation and the impurity is observed through the changes of the mode profile and the frequency as the breather moves towards and through the impurity.

Strain-induced metal-semiconductor transition observed in atomic carbon chains.

Carbyne, the sp1-hybridized phase of carbon, is still a missing link in the family of carbon allotropes. While the bulk phases of carbyne remain elusive, the elementary constituents, that is, linear chains of carbon atoms, have already been observed using the electron microscope. Isolated atomic chains are highly interesting one-dimensional conductors that have stimulated considerable theoretical work. Experimental information, however, is still very limited. Here we show electrical measurements and first-principles transport calculations on monoatomic carbon chains. When the 1D system is under strain, the chains are semiconducting corresponding to the polyyne structure with alternating bond lengths. Conversely, when the chain is unstrained, the ohmic behaviour of metallic cumulene with uniform bond lengths is observed. This confirms the recent prediction of a metal–insulator transition that is induced by strain. The key role of the contacting leads explains the rectifying behaviour measured in monoatomic carbon chains in a nonsymmetric contact configuration.

Surface effects on the mechanical elongation of AuCu nanowires: De-alloying and the formation of mixed suspended atomic chains

We report here an atomistic study of the mechanical deformation of AuxCu(1−x) atomic-size wires (nanowires (NWs)) by means of high resolution transmission electron microscopy experiments. Molecular dynamics simulations were also carried out in order to obtain deeper insights on the dynamical properties of stretched NWs. The mechanical properties are significantly dependent on the chemical composition that evolves in time at the junction; some structures exhibit a remarkable de-alloying behavior. Also, our results represent the first experimental realization of mixed linear atomic chains (LACs) among transition and noble metals; in particular, surface energies induce chemical gradients on NW surfaces that can be exploited to control the relative LAC compositions (different number of gold and copper atoms). The implications of these results for nanocatalysis and spin transport of one-atom-thick metal wires are addressed.

Origin of the smaller conductances of Rh, Pb, and Co atomic junctions in hydrogen environment

We study theoretically the structural and electronic origins of the smaller conductances (one conductance quantum, G0, and smaller) of Rh, Pb, and Co metal atomic junctions (MAJs) in a hydrogen environment, as were measured in recent experiments. For the Rh MAJs, the 1G0 conductance is attributed to a stable contact bridged by a single hydrogen molecule whose antibonding state provides a single transport channel. For the Pb and Co MAJs the 1G0 conductance is, however, ascribed to a linear atomic chain adsorbing two dissociated H atoms, which largely reduces the density of states at the Fermi energy with respect to the pure ones. On the other hand, the small conductances of 0.3G0 (Rh) and 0.2G0 (Co) are due to H-decorated atomic chains connected to electrodes by a H atom.

NiAl(110) Surface as a Template for Growing Transition Metal Linear Atomic Chains: A DFT Investigation

First-principles calculations based on periodic density functional theory (DFT) have been used to investigate structural, energetic, and electronic properties of different transition metal atoms (Pd, Pt, Cu, Ag, and Au) on the NiAl(110) surface for coverages ranging from 0.25 monolayer up to completing full coverage, with special emphasis on the different possible depositions to form linear atomic chains (LAC). The analysis of the energetic contributions and electronic structure reveals that metal atoms are greatly favored to be aligned along the [001] direction to form LACs. The calculated negative work function changes are interpreted taking into account both the electronegativity and the polarizability of the deposited metal adatoms. This work function change decreases particularly for LACs along the [001] direction and, intriguingly, vanishes for Pt, suggesting an electronic behavior similar to the corresponding free-standing LAC.

An analogy of linear atom chains in seismodynamics of underground pipelines

In the problem of coupled seismic vibrations of an elastic medium and a segmented pipeline equipped with flexible joints, an analogy to a linear chain of lumped masses is discussed. This analogy leads to a simple approach to study the seismodynamics of underground pipelines by reducing the problem for a medium to a one-dimensional one devoted to the shear vibrations of a cylindrical layer. As examples, a number of equations describing the coupled motions of pipelines are proposed with consideration of the viscoelastic properties of the medium or the viscoelastic deformation at the joints.

Formation of Single Cu Atomic Chain in Nitrogen Atmosphere

We study the conductance and geometry of the Cu atomic junction in the presence of N2, through combination of experimental measurement and theoretical calculation. A mechanically controllable break-junction measurement at low temperature reveals N2 molecules stabilize a Cu atomic junction, and reduce its conductance value. The length analysis about the Cu atomic junction indicates that it is elongated with the length of a few atoms, although it is not elongated without molecules. We investigate Cu atomic junction’s geometry by calculating the conductance and total energies of the several models by changing the separation between two Cu electrodes. Through combination of experimental and theoretical study, we show that the Cu linear atomic chain is formed with the support of N2 molecule, and N2 molecule attached on the Cu linear atomic chain. The formation of Cu linear atomic chain is explained that attached N2 molecules reduce the surface energy of the Cu atomic junction or N2 molecule directly supports the Cu–Cu bond.

Acoustical Phonons Transport through a Stepped Quantum Wire

This work presents a theoretical approach for the study of phonon dynamics and scattering properties of an infinite linear atomic chain perturbed by a mono atomic step. The coherent transmittance scattering cross-sections for incident phonons on the atomic waveguide structure are calculated using the Landauer-Buttiker electron scattering description and the matching method formalism with the nearest and next nearest neighbour interactions. Numerical results for different configurations yield an understanding of the chain dynamical properties and the effects on phonon transmittance due to incoming phonons. The reflectance and transmittance coefficients show spectral characteristic features depending on the cut-off frequencies for the propagating phonons. They illustrate the occurrence of Fano resonances in the scattering spectra that result from degeneracy of step localized modes and propagating continuum modes due to the breakdown of the translation symmetry in the propagating direction. Furthermore, the interferences between diffused and reflected waves in the step regions generate Fabry-Perot oscillations whose number is determined by the distance between steps and the number of terraces.

Self-alignment of low-valent octanuclear palladium atoms.

A linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) was used to synthesize linear octapalladium-extended metal atom chains as discrete molecules of [Pd8(μ-dpmppm)4](BF4)4 (1) and [Pd8(μ-dpmppm)4L2](BF4)4 (L=2,6-xylyl isocyanide (XylNC; 2), acetonitrile (3), and N,N-dimethylformamide (dmf; 4)), which are stable in the solution states and show interesting temperature-dependent photochemical properties in the near IR region. Variable temperature NMR studies demonstrated that at higher temperature T≈140 °C the Pd8 chains were dissociated into Pd4 fragments, which were thermodynamically self-aligned to restore the Pd8 chains at lower temperature T<60 °C. The coldspray ionization mass spectra suggested a possibility for further aggregation of the linear tetrapalladium units.

First-principles study of electronic structure and magnetic properties of orthorhombic CrGa2Sb2 and MnGa2Sb2

By the first-principles calculations, we present the results of electronic structure and magnetic properties on bulk CrGa2Sb2 and MnGa2Sb2 in an orthorhombic structure with the linear chains of transition-metal Cr and Mn atoms, using four different exchange correlation potentials: the local density approximation (LDA), the generalized gradient approximation (GGA), GGA + U, and the Tran-Blaha modified Becke-Johnson functional (mBJ). The electronic structure calculations from four exchange correlation potentials show that CrGa2Sb2 is a pseudogap (negative gap) material with very small density of states (DOS) at the Fermi level, while MnGa2Sb2 has notably higher DOS at the Fermi level compared to CrGa2Sb2, exhibiting stronger metallic conductivity, although the mBJ potential obtains lower DOS at the Fermi level than LDA and GGA for both CrGa2Sb2 and MnGa2Sb2. The GGA + U method with a small value (1 eV) of the on-site Coulomb interaction parameter U obtains lower DOS at the Fermi level compared to the large value of U. In agreement with the measurement data, the total energy calculations reveal that both CrGa2Sb2 and MnGa2Sb2 have a stable ferromagnetic ground state with lower energies relative to antiferromagnetic state. Based on the Heisenberg model, the magnetic exchange constants between the nearest-neighbor Cr–Cr and Mn–Mn along transition-metal linear chains are calculated to be 48.6 meV and 27.5 meV for CrGa2Sb2 and MnGa2Sb2, respectively. By the mean-field approximation method, we calculated the Curie temperature of two compounds to be above room-temperature.

Single-photon modulation by the collective emission of an atomic chain

We study the collective spontaneous emission of a linear atomic chain excited by a single photon. The interaction between the atoms and the common vacuum field can significantly change the eigenenergy and the spontaneous emission rate of the system. Due to the dipole-dipole interactions, the system prepared in a single-photon timed Dicke state is the superposition of superradiant and subradiant eigenstates that can have a nonexponential decay dynamics. We can tune the frequency and linewidth of the superradiant and subradiant emission from a timed Dicke state by changing the direction of the atomic dipole moment or the atomic separation. In addition, the emission direction of the superradiant and subradiant photons also depends on the polarization of the atoms.

Species fractionation in atomic chains from mechanically stretched alloys

Bettini et al (2006 Nat. Nanotechnol. 1 182–5) reported the first experimental realization of linear atomic chains (LACs) composed of different atoms (Au and Ag). The different contents of Au and Ag were observed in the chains from what was found in the bulk alloys, which raises the question of what the wire composition is, if it is in equilibrium with a bulk alloy. In this work we address the thermodynamic driving force for species fractionation in LACs under tension, and we present the density-functional theory results for Ag–Au chain alloys. A pronounced stabilization of the wires with an alternating Ag–Au sequence is observed, which could be behind the experimentally observed Au enrichment in LACs from alloys with high Ag content.

Dynamic HAADF-STEM observation of a single-atom chain as the transient state of gold ultrathin nanowire breakdown.

Ultrathin chemically grown gold nanowires undergo irremediable structural modification under external stimuli. Thanks to dynamic high-angle annular dark-field imaging, electron-beam-induced damage was followed, revealing the formation of linear chains of gold atoms as well as reactive clusters on the side, opening fascinating prospects for applications in both catalysis and electronic transport.

Structural and electronic properties of endohedral doped SWCNTs: A DFT study

The structural and electronic properties of semiconductors (Si and Ge) and metal (Au and Tl) atoms doped armchair (n, n) and zigzag (n, 0); n=4–6, single wall carbon nanotubes (SWCNTs) have been studied using an ab-initio method. We have considered a linear chain of dopant atoms inside CNTs of different diameters but of same length. We have studied variation of B.E./atom, ionization potential, electron affinity and HOMO–LUMO gap of doped armchair and zigzag CNTs with diameter and dopant type. For armchair undoped CNTs, the B.E./atom increases with the increase in diameter of the tubes. For Si, Ge and Tl doped CNTs, B.E./atom is maximum for (6, 6) CNT whereas for Au doped CNTs, it is maximum for (5, 5) CNTs. For pure CNTs, IP decreases slightly with increasing diameter whereas EA increases with diameter. The study of HOMO–LUMO gap shows that on doping metallic character of the armchair CNTs increases whereas for zigzag CNTs semiconducting character increases. In case of zigzag tubes only Si doped (5, 0), (6, 0) and Ge doped (6, 0) CNTs are stable. The IP and EA for doped zigzag CNTs remain almost independent of tube diameter and dopant type whereas for doped armchair CNTs, maximum IP and EA are observed for (5, 5) tube for all dopants.

Negative Differential Resistance of Au-MgB2-Au Nanoscale Junctions

The electron transport of linear atomic chain of MgB2 sandwiched between Au(100) electrodes was investigated by using the density functional theory with the non-equilibrium Green’s function method. We have calculated the corresponding cohesion energy and conductance of junctions in different distance. It is found that, at the equilibrium position, the Au−B bond-length is 1.90 A, the B−Mg bond-length is 2.22 A, and the equilibrium conductance is 0.51G0 (G0=2e2 /h). The transport channel is almost formed by the π antibonding orbitals, which was made up of the px and py orbital electrons of B and Mg atoms. In the voltage range of −1.5 to 1.5 V, the junctions show the metallic behaviors. When the voltage is larger than 1.5 V, the current decreases gradually and then negative differential resistance appears almost symmetrically on both positive and negative bias.

Plasmonlike resonances in atomic chains: A time-dependent density-functional theory study

We studied plasmonlike resonances in one-dimensional (1D) atomic chain systems by using time-dependent density-functional theory (TDDFT) and local density functional theory. Recent TDDFT studies have shown the coexistence of longitudinal and transverses collective plasmonlike resonances in the atomic chains of simple and noble metals. Such atomic chains contain only a few atoms. The induced polarization occurs along the atomic chain in longitudinal mode and perpendicular to the atomic chain in transverse mode. To understand the emergence of plasmonlike resonance in 1D atomic chains better, we studied carbon chains in which plasmonic resonances are not expected to occur. We used TDDFT to study the emergence of collective resonances in various forms of carbon chains, cumulenes ${\mathrm{C}}_{n}{\mathrm{H}}_{4}$, polyynes ${\mathrm{C}}_{n}{\mathrm{H}}_{2}$, and alkenes ${\mathrm{C}}_{n}{\mathrm{H}}_{n+2}$. The excitation energy and dipole oscillation strengths of these systems were determined through TDDFT by using the turbomole package. We determined how collective plasmonlike resonances arise from single-electron excitations when the number of electrons increases as the carbon chain lengthens. The collective excitation behavior is then compared with that of metallic atomic chains. Our TDDFT results showed longitudinal collective modes for cumulenes and polyynes, as well as for finite-length chains. These collective excitations exhibit the same behavior as that of longitudinal ``plasmon'' previously identified in sodium and silver chains, although polyynes are gapped in the long chain limit. Such longitudinal excitations are absent in alkenes. However, unlike metal atomic chains, carbon chains exhibited no transverse collective mode. The band structure of periodic atomic chains was calculated by using the standard local density functional method. These structures were used to interpret the results and to relate the single-electron excitation to the collective plasmonlike response. Within the one-particle quantum-well picture, the longitudinal mode in the linear atomic chain arises from intraband transition with $\ensuremath{\Delta}q=1$, where $q$ is the quantum number of quantum wells. $\ensuremath{\Delta}q=1$ intraband transitions can be found in metallic (e.g., Na) chains and in carbon chains (cumulenes and polyynes), such longitudinal collective mode is rather ``generic''. Meanwhile, the transverse modes of the sodium chains are attributed to interband transitions with an even $\ensuremath{\Delta}q$ (dominated by $\ensuremath{\Delta}q=0$), and such transverse collective excitations only form if the allowed $\ensuremath{\Delta}q=0$ transitions occur between bands that are parallel to each other. Such bands can be found in simple metals, but not in carbon chains.

Quadrupole plasmon excitations in finite-size atomic chain systems

The existence and the nature of a new mode of electronic collective excitations (quadrupole plasmons) in confined one-dimensional electronic systems, used to mimic finite-size linear metal atomic chains, have been predicted by an eigen-equation method. The eigen-equation based on the time-dependent density-functional theory is presented for calculating the collective excitations in confined systems. With this method, all modes of collective excitations in the 1D systems can be found out. These modes include dipole plasmons and quadrupole plasmons. The dipole plasmon mode corresponds to the antisymmetric oscillation of induced charge, and can be shown as a resonance of the dipole response. In the quadrupole plasmon modes, the induced charge distribution is symmetric, and the dipole response vanishes. The motion of the electrons in the quadrupole modes is similar to the vibration of atoms in the breathing mode of phonons. This type of plasmon can be shown as a resonance of the quadrupole response, and has to be excited by a non-uniform field.