PhD Thesis Colloquium

Date: 30 July 2015

Plasmon-Phonon Coupled Dynamics of Nanocrystalline Structures

Brahmanandam Javvaji

Supervisors: D. Roy Mahapatra(AE) and S. Raha (SERC)

Abstract

Excitation of solids with electromagnetic radiation induces many-body interactions of quasiparticles such as plasmons, phonons etc. Plasmons and phonons are discrete energy packets of collective oscillations of free electron gas and lattice mechanical vibrations. A coupled resonance of plasmons and phonons occur when the energies of these quasiparticles become comparable. A detailed understanding of the coupling phenomena is important in nanoscale structures since the coupling is substantially stronger and is different than that in bulk. This type of coupling is significant in explaining physical behaviour like light scattering, structural transitions and transport of energy. Nanocrystalline structures like nanoscale thin films, nanowires, nanotubes, quantum dots etc. are important elements in micro and nano electronic devices. Inelastic energy loss spectroscopy and Raman spectroscopy are experimental tools to estimate the plasmon-phonon coupling in nanocrystalline structures. Theoretical understanding of the coupling is possible using both analytical and first principle based methods of calculation. Existing analytical method allows calculation of plasmon and phonon frequencies separately and uses a dielectric function to obtain plamson-phonon coupling. Application of analytical methods to nanocrystalline structures is quite easy provided the required parameters are possible to estimate accurately. Numerical methods use wave function based energy transitions to estimate the coupling, whereas it is limited by size of the nanocrystal and boundary conditions. Interestingly, several techniques have been developed to incorporate the electronic information into empirical inter-atomic potentials defined over the given atomic arrangements. With this approach, nanostructures up to few hundreds of nanometers can be modelled. However, energy exchange between the atomic field variables and electromagnetic field variables are difficult to establish with this approach. In the present thesis, a semi-classical atomic-continuum approach is developed by identifying several energy contributions from both atomistic and continuum representation of the problem defined above. A Lagrangian framework involving these energies are established with the help of variational calculus, which results in a set of governing differential equations for the atomic-continuum field variables. The governing equations of the atomic model are solved by creating a lattice representation of the nanocrystal. The equations of the continuum model are solved at nodes in the finite element representation of the nanocrystal. At each point, there exists an energy exchange via field variables of atoms and the finite element nodes, which is determined by an interpolation function and by considering the principle laws of conservation. The developed simulation framework is employed to investigate the phonon and plasmon characteristics for nanocrystalline copper thin films and carbon nanotubes. Several fundamental properties are estimated which are found in agreement with experimental and first principle calculations. The simulation framework is further applied to analyse the coupling behaviour in a photonic device model and a waveguide model involving various different nanostructures and different material types. The materials and device models concentrated here include nanocrystalline copper, free standing structurally engineered graphene, pure carbon nanotubes, macromolecular carbon nanotube composite structure, engineered graphene on a silicon substrate and zinc oxide on a zinc substrate. Intense coupled modes are identified using these device models to demonstrate the usefulness of the developed framework and simulation results. Several interesting excitation modes are found to be participating in the coupling process, which enhances the coupling characteristics compared to its constituting structural parts. Examples are shown wherein the coupled mode excitation is possible to control by engineering a graphene nanostructure with various structural defects, and scattering of various coupled modes leading to local heating nanostructure.