Charge Transfer Processes in Atom-Surface Collisions

Theoretical methods for describing charge transfer processes in atom-surface collisions will be reviewed. Special emphasis will be on the resonant tunneling mechanism, which normally is expected to be the dominant decay mechanism of excited states near metal surfaces. Recent theoretical calculations have shown that the lifetimes for excited atomic states near metal surfaces can be much longer than what previously has been believed. This finding has important consequences for the interpretation and modeling of charge transfer processes in atom-surface scattering events. In particular, it means that excitations in a desorbing species formed at the time of impact or near the surface may survive the passage through the surface region. In the region close to the surface, it is important to describe the hybridization between the atomic and the surface levels as well as effects of impurities on the local electronic structure. It is shown that such effects can be particularly strong when alkali atoms are coadsorbed on the surface. At finite alkali coverage, the energies of atomic levels will appear corrugated along the surface. It is shown that this effect can drastically influence the probability for a charge exchange process in an atom-surface scattering event. A dynamical theory for describing ion/atom-surface charge exchange processes that takes into account the low tunneling rates as well as non-image-like level shirts and lateral corrugations of the surface potential at small atom-surface separations is presented. The results are applied to recent experimental sputtering, desorption and ion-surface neutralization data. Good agreement between experiments and theoretical predictions is found indicating that the theoretical model is accurate

Plasmonic heating in Au nanowires at low Temperatures: The role of thermal boundary resistance

Inelastic electron tunneling and surface-enhanced optical spectroscopies at the molecular scale require cryogenic local temperatures even under illumination - conditions that are challenging to achieve with plasmonically resonant metallic nanostructures. We report a detailed study of the laser heating of plasmonically active nanowires at substrate temperatures from 5 to 60 K. The increase of the local temperature of the nanowire is quantified by a bolometric approach and could be as large as 100 K for a substrate temperature of 5 K and typical values of laser intensity. We also demonstrate that a $\sim 3\times$ reduction of the local temperature increase is possible by switching to a sapphire or quartz substrate. Finite element modeling of the heat dissipation reveals that the local temperature increase of the nanowire at temperatures below $\sim$50 K is determined largely by the thermal boundary resistance of the metal-substrate interface. The model reproduces the striking experimental trend that in this regime the temperature of the nanowire varies nonlinearly with the incident optical power. The thermal boundary resistance is demonstrated to be a major constraint on reaching low temperatures necessary to perform simultaneous inelastic electron tunneling and surface enhanced Raman spectroscopies.Comment: 20 pages, 5 figures + 17 pages supporting materia

Bridging quantum and classical plasmonics with a quantum-corrected model

Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems

Work Function Dependence of Charge Transfer in Desorption and Sputtering of Atoms from Surfaces

Using a recently developed many-electron theory, we investigate the work function dependence of charge transfer during desorption and sputtering of atoms from metal surfaces. We investigate the effects of substrate bandwidth, atomic velocity and level degeneracy on the charge transfer. We show that many-electron interactions introduce relatively small but measurable effects on the work function dependence of the charge transfer. We find that these effects can be stronger for negative ion states than for positive ion states. The reason is that for negative ions, a strongly correlated Kondo state may be formed near the surface

Transient currents and universal timescales for a fully time-dependent quantum dot in the Kondo regime

Using the time-dependent non-crossing approximation, we calculate the transient response of the current through a quantum dot subject to a finite bias when the dot level is moved suddenly into a regime where the Kondo effect is present. After an initial small but rapid response, the time-dependent conductance is a universal function of the temperature, bias, and inverse time, all expressed in units of the Kondo temperature. Two timescales emerge: the first is the time to reach a quasi-metastable point where the Kondo resonance is formed as a broad structure of half-width of the order of the bias; the second is the longer time required for the narrower split peak structure to emerge from the previous structure and to become fully formed. The first time can be measured by the gross rise time of the conductance, which does not substantially change later while the split peaks are forming. The second time characterizes the decay rate of the small split Kondo peak (SKP) oscillations in the conductance, which may provide a method of experimental access to it. This latter timescale is accessible via linear response from the steady stateand appears to be related to the scale identified in that manner [A. Rosch, J. Kroha, and P. Wolfle, Phys. Rev. Lett. 87, 156802 (2001)].Comment: Revtex with 15 eps figures. Compiles to 11 page

Nanorice Particles: Hybrid Plasmonic Nanostructures

A new hybrid nanoparticle, i.e., a nanorice particle, which combines the intense local fields of nanorods with the highly tunable plasmon resonances of nanoshells, is described herein. This geometry possesses far greater structural tunability than previous nanoparticle geometries, along with much larger local field enhancements and far greater sensitivity as a surface plasmon resonance (SPR) nanosensor than presently known dielectric-conductive material nanostructures. In an embodiment, a nanoparticle comprises a prolate spheroid-shaped core having a first aspect ratio. The nanoparticle also comprises at least one conductive shell surrounding said prolate spheroid-shaped core. The nanoparticle has a surface plasmon resonance sensitivity of at least 600 nm RIU(sup.-1). Methods of making the disclosed nanorice particles are also described herein

Multipolar plasmon resonances in individual Ag nanorice

6 páginas, 4 figuras.-- El pdf del artículo es la versión pre-print.We study the optical excitation of high-order surface plasmon resonance modes in individual Ag nanorice particles using dark-field scattering spectroscopy. We analyze the results by model calculations using the boundary element method. Symmetry breaking caused by oblique illumination makes the even order resonance modes observable in the optical spectrum. All the resonance peaks are found to redshift with increasing length of the particle.This work is supported by NSFC Grant Nos. 10625418 and 10874233, and MOST Grant Nos. 2006DFB02020, 2007CB936800, and 2009CB930700, “Bairen Project” of CAS. P.N. acknowledges financial support from The Robert A Welch Foundation, grant C-1222. J.A. and A.R. acknowledge financial support from Etortek project inanoGUNE from the Basque Government and project FIS2007-66711-C01-01 from the Spanish Ministry of Science and Innovation.Peer reviewe