Properties of nanosystems

Lopata, K. ; Neuhauser, D. ; Baer, R. Curve crossing and negative refraction in simulations of near-field coupled metallic nanoparticle arrays. J. Chem. Phys. 2007, 127, 154714–8.Abstract

We extend our previous results [R. Baer et al., J. Chem. Phys. 126, 014705 (2007).] to develop a simple theory of localized surface plasmon-polariton (LSPP) dispersion on regular arrays of metal nanoparticles in the weak-field and weak-damping limits. This theory describes the energy-momentum as well as the polarization-momentum properties of LSPP waves, both of which are crucial to plasmonic device design. We then explicitly compute the dispersion relation for isotropic and anisotropic two-dimensional square lattices, and show curve crossings between all three levels as well as negative refraction where the phase and group velocities (refractive indices), or at least their projection along the main axis, have different signs. The curve crossing implies that scattering between the different polarizations, and therefore different velocities, is easy at the curve crossing momenta, so that a quick change in wave packet direction can be achieved. Time-resolved wave packet dynamics simulations demonstrate negative refraction and the easy scattering over nanometer length scales. This paper also gives some computational schemes for future applications, such as a way to include source terms and how to efficiently treat dissipative effects.

Baer, R. ; Lopata, K. ; Neuhauser, D. Properties of phase-coherent energy shuttling on the nanoscale. J. Chem. Phys. 2007, 126, 014705–13.Abstract

Recently, the possibility of transporting electromagnetic energy as local-plasmon-polariton waves along arrays of silver nanoparticles was demonstrated experimentally [S. A. Maier et al., Nat. Mater. 2, 229 (2003)]. It was shown that dipole coupling facilitates phase-coherent excitation waves, which propagate while competing against decoherence effects occurring within each dot. In this article the authors study the ideal coherent shuttling in such a system, leaving decoherence for future investigation. In the weak field limit, the waves obey a Schrödinger equation, to be solved using either time-dependent wave-packet or energy resolved scattering techniques. The authors study some dynamical characteristics of these waves, emphasizing intuition and insight. Scattering from barriers, longitudinal-transverse coupling and acceleration methods are studied in detail. The authors also discuss briefly two-dimensional arrays and a simple decoherence model

Hod, O. ; Baer, R. ; Rabani, E. Inelastic effects in Aharonov-Bohm molecular interferometers. Phys. Rev. Lett. 2006, 97, 266803.Abstract

Inelastic effects arising from electron-phonon coupling in molecular Aharonov-Bohm (AB) interferometers are studied using the nonequilibrium Green's function method. Results for the magnetoconductance are compared for different values of the electron-phonon coupling strength. At low-bias voltages, the coupling to the phonons does not change the lifetime and leads mainly to scattering phase shifts of the conducting electrons. As a result of these dephasing processes, the magnetoconductance of the molecular AB interferometer becomes more sensitive to the threading magnetic flux as the electron-phonon coupling is increased, opposite to the behavior of an electric gate.

Hod, O. ; Rabani, E. ; Baer, R. Magnetoresistance of nanoscale molecular devices. Acc. Chem. Res. 2006, 39, 109–117.Abstract

Affecting the current through a molecular or nanoscale junction is usually done by a combination of bias and gate voltages. Magnetic fields are less studied because nanodevices can capture only low values of the magnetic flux. We review recent work done with the aim of finding the conditions for magnetic fields to significantly affect the conductance of such junctions. The basic idea is to create narrow tunneling resonances through a molecular ring-like structure that are highly sensitive to the magnetic field. We describe a computational method that allows us to examine atomistic models of such systems and discuss several specific examples of plausible systems, such as the quantum corral, carbon nanotubes, and polycyclic aromatic hydrocarbon molecules. A unique property of the magnetic field, namely, its ability to split degenerate levels on the ring, is shown to allow prototypes of interesting new nanoscale devices, such as the three-terminal parallel logic gate.

Baer, R. ; Neuhauser, D. Theoretical studies of molecular scale near-field electron dynamics. J. Chem. Phys. 2006, 125, 074709–9.Abstract

Near-field scanning microscopy and nonlinear spectroscopy on a molecular scale involve weakly interacting subsystems that dynamically exchange electrons and electromagnetic energy. The theoretical description of such processes requires unified approach to the electron-near-field dynamics. By considering electronic structure and dynamics of two distant clusters or atoms we show that adiabatic local spin-density approximation (ALSDA) fails to describe (even qualitatively) essential details of electron dynamics in weakly interacting systems. A recently developed functional addresses these ailments within a time-dependent setting. With this method we study the spectroscopy of a composite system, namely, two weakly coupled metallic clusters. The near-field (dipole-dipole) coupling and electron transfer display an interesting interplay, producing exponential sensitivity of emission yield to the intercomponent distance.

Baer, R. ; Livshits, E. ; Neuhauser, D. Avoiding self-repulsion in density functional description of biased molecular junctions. Chem. Phys. 2006, 329, 266–275.Abstract

We examine the effects of self-repulsion on the predictions of charge distribution in biased molecular junctions by the local density functional theory methods. This is done using a functional with explicit long-range exchange term effects [R. Baer, D. Neuhauser, Phys. Rev. Lett. 94 (2005) 043002]. We discuss in detail the new density functional, pointing out some of the remaining difficulties in the theory. We find that in weakly coupled junctions (the typical molecular electronics case) local-density functionals fail to describe correctly the charge distribution in the intermediate bias regime. (c) 2006 Elsevier B.V. All rights reserved.

Hod, O. ; Rabani, E. ; Baer, R. Magnetoresistance Devices Based on Single Walled Carbon Nanotubes. J. Chem. Phys. 2005, 123, 051103.Abstract

We demonstrate the physical principles for the construction of a nanometer-sized magnetoresistance device based on the Aharonov-Bohm effect [Phys. Rev. 115, 485 (1959)]. The proposed device is made of a short single-walled carbon nanotube (SWCNT) placed on a substrate and coupled to a tip/contacts. We consider conductance due to the motion of electrons along the circumference of the tube (as opposed to the motion parallel to its axis). We find that the circumference conductance is sensitive to magnetic fields threading the SWCNT due to the Aharonov-Bohm effect, and show that by retracting the tip/contacts, so that the coupling to the SWCNT is reduced, very high sensitivity to the threading magnetic field develops. This is due to the formation of a narrow resonance through which the tunneling current flows. Using a bias potential the resonance can be shifted to low magnetic fields, allowing the control of conductance with magnetic fields of the order of 1 T.

Hod, O. ; Rabani, E. ; Baer, R. A Parallel Electromagnetic Molecular Logic Gate. J. Am. Chem. Soc. 2005, 127, 1648. hod2005a.pdf
Hawthorne, M. F. ; Zink, J. I. ; Skelton, J. M. ; Bayer, M. J. ; Liu, C. ; Livshits, E. ; Baer, R. ; Neuhauser, D. Electrical or Photocontrol of the Rotary Motion of a Metallacarborane. Science 2004, 303, 1849–1851.Abstract

Rotary motion around a molecular axis has been controlled by simple electron transfer processes and by photoexcitation. The basis of the motion is intramolecular rotation of a carborane cage ligand (7,8-dicarbollide) around a nickel axle. The Ni(III) metallacarborane structure is a transoid sandwich with two pairs of carbon vertices reflected through a center of symmetry, but that of the Ni(IV) species is cisoid. The interconversion of the two provides the basis for controlled, rotational, oscillatory motion. The energies of the Ni(III) and Ni(IV) species are calculated as a function of the rotation angle.

Hod, O. ; Baer, R. ; Rabani, E. Feasible nanometric magnetoresistance devices. The Journal of Physical Chemistry B 2004, 108, 14807–14810. hod2004.pdf
Collepardo-Guevara, R. ; Walter, D. ; Neuhauser, D. ; Baer, R. A Hückel study of the effect of a molecular resonance cavity on the quantum conductance of an alkene wire. Chem. Phys. Lett. 2004, 393, 367–371.Abstract

{We use Huckel theory to examine interference effects on conductance of a wire when a ‘lollypop’ side-chain is bonded to it, acting as a resonance cavity. A clear signature of interference is found at these ballistic conducting systems, stronger in small systems. Gating effects are enhanced by the presence of the loop, where the electronic wavefunctions can experience large changes in phase. Using an ‘interference index’

Baer, R. ; Neuhauser, D. ; Weiss, S. Enhanced absorption induced by a metallic nanoshell. Nano Lett. 2004, 4 85–88.Abstract

Nanoshells have been previously shown to have tunable absorption frequencies that are dependent on the ratio of their inner and outer radii. Inspired by this, we ask: can a nanoshell increase the absorption of a small core system embedded within it? A theoretical model is constructed to answer this question. A core, composed of a “jellium” ball of the density of gold is embedded within a jellium nanoshell of nanometric diameter. The shell plasmon frequency is tuned to the core absorption line. A calculation based the time-dependent density functional theory was performed showing a 10 fold increase in core excitation yield.

Hod, O. ; Rabani, E. ; Baer, R. Carbon nanotube closed-ring structures. Phys. Rev. B 2003, 67, 195408.Abstract

We study the structure and stability of closed-ring carbon nanotubes using a theoretical model based on the Brenner-Tersoff potential. Many metastable structures can be produced. We focus on two methods of generating such structures. In the first, a ring is formed by geometric folding and is then relaxed into minimum energy using a minimizing algorithm. Short tubes do not stay closed. Yet tubes longer than 18 nm are kinetically stable. The other method starts from a straight carbon nanotube and folds it adiabatically into a closed-ring structure. The two methods give strikingly different structures. The structures of the second method are more stable and exhibit two buckles, independent of the nanotube length. This result is in strict contradiction to an elastic shell model. We analyze the results for the failure of the elastic model.

Baer, R. ; Neuhauser, D. Ab initio electrical conductance of a molecular wire. Int. J. Quantum Chem. 2003, 91, 524–532. baer2003c.pdf
Baer, R. ; Neuhauser, D. ; Zdanska, P. ; Moiseyev, N. Ionization and high-harmonic generation in aligned benzene by intense circularly polarized light. Phys. Rev. A 2003, 68, 043406. baer2003e.pdf