Electron localization in a finite one-dimensional chain

The localization characteristics of the electronic wave functions in a finite one-dimensional chain with the diagonal or the off-diagonal disorder of the potentials have been studied. It has been shown that the eigenfuction at the frontier level is relatively “strong” against the temptation to localize caused by the existence of the random potentials. It has also been pointed out that the spatial behavior of the total density reflects that of the diagonal random potentials, but that under the off-diagonal random potentials the total density is spatially uniform (completely extended).     

Regulating silver nanowire size enables efficient photoelectric conversion

The increasing demand for the state-of-the-art transparent conductive electrodes has received great interest in synthesizing silver nanowires(AgNWs) with a uniform diameter that exhibit excellent conductance, transparency, flexibility and mechanical ductility. Herein, we report the controllable synthesis of ultrathin AgNWs with high aspect ratio via a polyol-assisted process.The diameter of AgNWs can be continuously modulated from 20 to 80 nm by simply adjusting the aging time. The selfassembled films fabricated by using AgNWs with a diameter of 20 nm as building blocks enable an excellent performance and show a sheet resistance of ～30 Ω/sq and an optical transmittance of 94%. In addition, such nanowire-based conductive films can keep the excellent flexibility and resistance after bending 10,000 cycles. Similarly, the intelligent dimming films prepared by this AgNW films also have excellent flexibility and stability.     

Thermodynamic and kinetic size limit of nanowire growth

The universal models of nucleation thermodynamics and growth kinetics were established for nanowire growth upon metal-catalyst-assisted thermal chemical vapor transport on the basis of vapor-liquid-solid (VLS) mechanism. The thermodynamic and kinetic size limit of nanowire growth was deduced from the proposed model. Theoretical predictions are in agreement with experimental data.     

Vibron-polaron in alpha-helices. I. Single-vibron states

The vibron dynamics associated to amide-I vibrations in a three-dimensional alpha-helix is described according to a generalized Davydov model. The helix is modeled by three spines of hydrogen-bonded peptide units linked via covalent bonds. To remove the intramolecular anharmonicity of each amide-I mode and to renormalize the vibron-phonon coupling, two unitary transformations have been applied to reach the dressed anharmonic vibron point of view. It is shown that the vibron dynamics results from the competition between interspine and intraspine vibron hops and that the two kinds of hopping processes do not experience the same dressing mechanism. Therefore, at low temperature (or weak vibron-phonon coupling), the polaron behaves as an undressed vibron delocalized over all the spines whereas at biological temperature (or strong vibron-phonon coupling), the dressing effect strongly reduces the vibrational exchanges between different spines. As a result the polaron propagates along a single spine as in the one-dimensional Davydov model. Although the helix supports both acoustical and optical phonons, this feature originates in the coupling between the vibron and the acoustical phonons only. Finally, the lattice distortion which accompanies the polaron has been determined and it is shown that residues located on the excited spine are subjected to a stronger deformation than the other residues.     

Enhanced critical temperature in a dual-layered molecular superconductor

Single-crystal X-ray diffraction has shown that the high-critical-temperature (T(c)) phase of the filamentary molecular superconductor (BEDT-TTF)(2)Ag(CF(3))(4)(1,1,2-trichloroethane) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] contains layers of BEDT-TTF radical cations with alternating κ- and α'-type packing motifs. This molecule-based superconductor with dual BEDT-TTF packing motifs has a T(c) five times higher than that of its polymorph that contains only κ-type packing.     

Vibron-polaron in alpha-helices. II. Two-vibron bound states

The two-vibron dynamics associated to amide-I vibrations in a three-dimensional (3D) alpha-helix is described according to a generalized Davydov model. The helix is modeled by three spines of hydrogen-bonded peptide units linked via covalent bonds. It is shown that the two-vibron energy spectrum supports both a two-vibron free states continuum and two kinds of bound states, called two-vibron bound states (TVBS)-I and TVBS-II, connected to the trapping of two vibrons onto the same amide-I mode and onto two nearest-neighbor amide-I modes belonging to the same spine, respectively. At low temperature, nonvanishing interspine hopping constants yield a three-dimensional nature of both TVBS-I and TVBS-II which the wave functions extend over the three spines of the helix. At biological temperature, the pairs are confined in a given spine and exhibit the same features as the bound states described within a one-dimensional model. The interplay between the temperature and the 3D nature of the helix is also responsible for the occurrence of a third bound state called TVBS-III which refers to the trapping of two vibrons onto two different spines. The experimental signature of the existence of bound states is discussed through the simulation of their infrared pump-probe spectroscopic response. Finally, the fundamental question of the breather-like behavior of two-vibron bound states is addressed.     

Orbital magnetism in finite size systems

The orbital magnetism in atoms is described in terms of Larmor diamagnetic and van Vleck paramagnetic contributions. The orbital magnetism in metals is described by Landau diamagnetism. Here, a discussion of the intermediate, mesoscopic regime is presented using a simple free-particle-in-a-box approximation. It is argued that, in general, one cannot separate Larmor and van Vleck contributions, and that the total susceptibility is expected to be small. The conclusions are illustrated on some experimental results.     

Quantum decoherence in finite size exciton-phonon systems

Based on the operatorial formulation of the perturbation theory, the properties of a confined exciton coupled with phonons in thermal equilibrium is revisited. Within this method, the dynamics is governed by an effective Hamiltonian which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. Special attention is thus paid for describing the time evolution of the excitonic coherences at finite temperature. As in an infinite lattice, temperature-enhanced quantum decoherence takes place. However, it is shown that the confinement softens the decoherence. The coherences are very sensitive to the excitonic states so that the closer to the band center the state is located, the slower the coherence decays. In particular, for odd lattice sizes, the coherence between the vacuum state and the one-exciton state exactly located at the band center survives over an extremely long time scale. A superimposition involving the vacuum and this specific one-exciton state behaves as an ideal qubit insensitive to its environment.     

Electronic perturbation in a molecular nanowire of [IrCl5(NO)]- units

The nitrosyl in [IrCl5(NO)]- is probably the most electrophilic known to date. This fact is reflected by its extremely high IR frequency in the solid state, electrochemical behavior, and remarkable reactivity in solution. PPh4[IrCl5(NO)] forms a crystal in which the [IrCl5(NO)]- anions are in a curious wire-like linear arrangement, in which the distance between the N--O moiety of one anion and the trans chloride of the upper one nearby is only 2.8 A. For the same complex [IrCl5(NO)]- but with a different counterion, Na[IrCl5(NO)], the anions are stacked one over the other in a side-by-side arrangement. In this case the electronic distribution can be depicted as the closed-shell electronic structure Ir III-NO+, as expected for any d(6) third-row transition metal complex. However, in PPh4[IrCl5(NO)] an unprecedented electronic perturbation takes place, probably due to NO*-Cl- acceptor-donor interactions among a large number of [IrCl5(NO)]- units, favoring a different electronic distribution, namely the open-shell electronic structure Ir IV-NO*. This conclusion is based on XANES experimental evidence, which demonstrates that the formal oxidation state for iridium in PPh4[IrCl5(NO)] is +4, as compared with +3 in K[IrCl5(NO)]. In agreement, solid-state DFT calculations show that the ground state for [IrCl5(NO)]- in the PPh4+ salt comprises an open-shell singlet with an electronic structure which encompasses half of the spin density mainly localized on a metal-centered orbital, and the other half on an NO-based orbital. The electronic perturbation could be seen as an electron promotion from a metal-chloride to a metal-NO orbital, due to the small HOMO-LUMO gap in PPh4[IrCl5(NO)]. This is probably induced by electrostatic interactions acting as a result of the closeness and wire-like spatial arrangement of the Ir metal centers, imposed by lattice forces due to pi-pi stacking interactions among the phenyl rings in PPh4+. Experimental and theoretical data indicate that in PPh4[IrCl5(NO)] the Ir-N-O moiety is partially bent and tilted.     

Dynamical motions of lipids and a finite size effect in simulations of bilayers

Molecular dynamics (MD) simulations of dipalmitoylphosphatidylcholine bilayers composed of 72 and 288 lipids are used to examine system size dependence on dynamical properties associated with the particle mesh Ewald (PME) treatment of electrostatic interactions. The lateral diffusion constant Dl is 2.92 x 10(-7) and 0.95 x 10(-7) cm2/s for 72 and 288 lipids, respectively. This dramatic finite size effect originates from the correlation length of lipid diffusion, which extends to next-nearest neighbors in the 288 lipid system. Consequently, diffusional events in smaller systems can propagate across the boundaries of the periodic box. The internal dynamics of lipids calculated from the PME simulations are independent of the system size. Specifically, reorientational correlation functions for the slowly relaxing phosphorus-glycerol hydrogen, phosphorus-nitrogen vectors, and more rapidly relaxing CH vectors in the aliphatic chains are equivalent for the 72 and 288 lipid simulations. A third MD simulation of a bilayer with 72 lipids using spherical force-shift electrostatic cutoffs resulted in interdigitated chains, thereby rendering this cutoff method inappropriate.     

Bottom-up assembly of high density molecular nanowire cross junctions at a solid/liquid interface

A monoalkylated tetrathiafulvalene derivative forms multilayer structures at the solid-liquid interface, with high density of cross junctions, which are interesting for molecular electronic circuit self-assembly.     

The critical molecular weight for resisting slow crack growth in a polyethylene

An ethylene-hexene copolymer was fractionated into five fractions and the density of short-chain branches was measured for each fraction. The slow crack growth behavior was measured on each fraction by sandwiching the small amount of fractionated resin of about 0.2 g between polyethylene grips. The resistance to slow crack growth was negligible for the three fractions whose M_w was less than 1.5 × 10⁵. For the fourth fraction with M_w greater than 1.5 × 10⁵, the resistance to slow crack growth was very high, being greater than that for the whole resin even though its density of short-chain branches was less than that of the whole resin. It is concluded that a molecular weight greater than 1.5 × 10⁵ is required to create the number of tie molecules that is necessary to produce a high resistance to slow crack growth in this particular copolymer. © 1996 John Wiley & Sons, Inc.     

Temperature dependence of ice critical nucleus size

We present a molecular dynamics study of ice growth from supercooled water. By performing a series of simulations with different initial conditions, we have quantitative established the relationship existing between the critical nucleus size and the temperature. The results show that ice embryos containing hundreds or thousands of molecules are needed for the system to crystallize macroscopically, even at high degrees of supercooling. Our findings explain the difficulty in observing spontaneous ice nucleation in atomistic simulations and the relative ease with which water droplets can be supercooled under controlled experiments.     

Direct measurement of critical nucleus size in confined volumes

In crystallization, the critical nucleus size is of pivotal importance. Above this size, it is favorable for the new crystalline phase to form; below this size, the clusters will tend to dissolve rather than grow. To date, there has been no direct method for measuring the critical nucleus size. Instead, the size is typically calculated from the variation of crystallization rates with temperature. This involves using bulk values of the interfacial tension and enthalpy of fusion, which are inappropriate for small critical nucleus sizes. Here, we present a direct method for measuring the size of the critical nucleus, based on observing crystallization temperatures of materials within microemulsions. Using this approach, the number of molecules in the critical nucleus can be found simply by measuring the droplet size. Data on the freezing of water in water-in-oil microemulsions with and without the nucleating agent, heptacosanol, are presented to support our hypothesis. The results show that the critical nucleus contains 90-350 ice molecules for water pool radii of approximately 1.2-1.8 nm for the heptacosanol-doped microemulsions in which heterogeneous nucleation is initiated at the droplet interface. For the microemulsions without heptacosanol, the critical nucleus contains 70-210 ice molecules for water pool radii of approximately 1.2-1.8 nm. The smaller values arise because homogeneous nucleation occurs and therefore the crystallization temperatures are lower. We can also determine how bulk properties are perturbed at the nanoscale, and we find that the ratio of the ice-water interfacial tension to the enthalpy of fusion decreases significantly for water pool radii that are <2 nm.     

Fast orbital localization scheme in molecular fragments resolution

The method for localizing orbitals on a set of predefined molecular fragments is introduced. Regional localized molecular orbitals (RLMO) are obtained through block diagonalization of the one-electron density matrix and further refinement of the resulting eigenvectors. The algorithm is fast and reliable, as is illustrated by a few examples. Potential applications range from conceptual insight into a chemical bonding to reduced scaling computational techniques. RLMOs are particularly well suited for fragmentation computational methods and for exploiting the locality of electronic correlation in post-HF methods.     

An atomistic model and key parameters for devising single molecular nanowire sensors

Based on one impurity model Hamiltonian describing a nanowire upon adsorption of a molecule, we obtain an analytical formula of the conductance which is governed clearly by modulating key parameters. The formula shows that the conductance change in nanowire upon adsorption of a molecule is mainly controlled by three factors, electron hopping between adsorbed molecule and nanowire, chemical potential, and the change of atomic configurations of the nanowires near the adsorption site. Conductance is very sensitive to the choice of these key parameters; therefore, a proper nanowire system that renders matched chemical potential as well as hopping strength between the nanowire and the adsorbed molecule should be devised for the sensor applications. Our model calculations give similar conductance features to the conductance obtained by the first principle calculations for a singe-molecule-adsorbed molecular wire. It is worthy of note that the system can be in antiresonance, which is characterized by a quick drop in conductance when a molecule is adsorbed on the nanowires.     

The critical size of clusters of water molecules on a carbon surface

The experimental adsorption isotherms of water and nitrogen vapors on graphitized carbonaceous adsorbents with large pore size prepared from ultradispersive technical carbon black have been compared with those on the surface of non-porous graphitized carbon black. The saturation value of water vapor adsorption has been shown to be proportional to the concentration of primary adsorption centers. At low concentrations of these centers the saturation value corresponds to the formation of fractions of a dense monolayer on the surface. The maximum size of clusters of water molecules on a carbonaceous adsorbent surface has been estimated.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 54–56, January, 1993.     

Hydrodynamic fractionation of finite size gold nanoparticle clusters

We demonstrate a high-resolution in situ experimental method for performing simultaneous size classification and characterization of functional gold nanoparticle clusters (GNCs) based on asymmetric-flow field flow fractionation (AFFF). Field emission scanning electron microscopy, atomic force microscopy, multi-angle light scattering (MALS), and in situ ultraviolet-visible optical spectroscopy provide complementary data and imagery confirming the cluster state (e.g., dimer, trimer, tetramer), packing structure, and purity of fractionated populations. An orthogonal analysis of GNC size distributions is obtained using electrospray-differential mobility analysis (ES-DMA). We find a linear correlation between the normalized MALS intensity (measured during AFFF elution) and the corresponding number concentration (measured by ES-DMA), establishing the capacity for AFFF to quantify the absolute number concentration of GNCs. The results and corresponding methodology summarized here provide the proof of concept for general applications involving the formation, isolation, and in situ analysis of both functional and adventitious nanoparticle clusters of finite size.     

Exciton-plasmon interaction in a composite metal-insulator-semiconductor nanowire system

We report about the synthesis and optical properties of a composite metal-insulator-semiconductor nanowire system which consists of a wet-chemically grown silver wire core surrounded by a SiO2 shell of controlled thickness, followed by an outer shell of highly luminescent CdSe nanocrystals. With microphotoluminescence (micro-PL) experiments, we studied the exciton-plasmon interaction in individual nanowires and analyzed the spatially resolved nanocrystal emission for different nanowire length, SiO2-shell thickness, nanocrystal shape, pump power, and emission polarization. For an SiO2 spacer thickness of approximately 15 nm, we observed an efficient excitation of surface plasmons by excitonic emission of CdSe nanocrystals. For nanowire lengths up to approximately 10 microm, the composite metal-insulator-semiconductor nanowires ((Ag)SiO2)CdSe act as a waveguide for 1D-surface plasmons at optical frequencies with efficient photon outcoupling at the nanowire tips, which is promising for efficient exciton-plasmon-photon conversion and surface plasmon guiding on a submicron scale in the visible spectral range.