Molecular nanoplasmonics : origin, simulation and its application

Abstract

Different methods from classical to quantum mechanical region are developed to simulate the plasmonic phenomena of noble metal in time domain with further possible interaction with organic molecule. The Raman spectrum is modeled as an important application of localized surface plasmon enhanced field and various mechanisms of enhancement are researched in detail related to unique features of Raman signal while hot electron induced reaction is another possible application field.

First, electromagnetic method following Maxwell's equations is developed in the formalism of FVM to simulate the plasmonic metal in time domain. Light source composed by sheets of current density is added to generate planewave-like fields with mixed boundary to support the propagation. Far-field properties concerning the spectrum measurement in experiment are briefly introduced. To demonstrate its applicability, this method is employed to simulate the Au/Ag nanoparticle with different radii. The feature of plasmonic peak and its broadening and shift with the size are discussed. The electric field distribution is also shown to indicate the local enhancement around metal surface. Possible correction in dielectric function is added to account for the mean free path of electrons in small dimension.

Second, quantum mechanical method is employed to simulate the plasmonic metal and demonstrate its origin and evolution in time domain. Various sizes of Au cluster are modeled by TDDFTB to show the evolution of plasmon peak from discretized states to collective oscillation states. With quantum dynamical simulation of local field, the plasmon peak can be used in comparison with EM results to indicate the differences and features. In addition to the Au cluster, its combined system with pyridine is employed for study of plasmon and charge transfer states.

Next, calculation of Raman spectrum is established from Placzek's polarizability theory and extended to resonance case under short time approximation. The differential cross section of Raman spectrum is derived under the KHD framework and reduced to classical form by perturbation theory and appropriate approximations. Additionly, SERS is demonstrated with full quantum treatment of metallic cluster and organic molecule. Possible enhancement mechanisms are discussed with focus on the charge transfer state and its impact on the overall changing of Raman spectrum. Numerical experiments on Au cluster and pyridine are carried out and their repsonse under different excitation energy is examined. Then, multiscale method involving EM and QM simulation is developed with information exhange through interace and boundary. Electromagnetic equation in potential formalism is revisited with coupling to quantum regime. A dissipative quantum transport theory is built on NEGF formalism with AWBL approximation of lead self-energy. A set of equation of motion is given for time domain propagation. Its coupling with EM simulation is built though effective charge and solving poisson equation with potential provided by macroscopic modelling. Numerical examples regarding Raman spectrum in open system are discussed.

Last, temporal evolution of hot carrier in noble metal nanoparticle is illustrated. Concurrently, through the real-time dynamics of the AuNP-\ce{H2} system, a negative ion trasient state and subsequent disocciation are confirmed.