Session F20: Focus Session: Mesoscopics - Optics and Plasmonics

Electron Microscopy: an Analytical Tool for Solid State Physicists

For too long the electron microscope has been considered as ``a big magnifying glass.'' Modern electron microscopy however has evolved into an analytical technique, able to provide \textit{quantitative} data on structure, composition, chemical bonding and magnetic properties. Using lens corrected instruments it is now possible to determine atom shifts at interfaces with a precision of a few picometer; chemical diffusion at these interfaces can be imaged down to atomic scale. The chemical nature of the surface atoms can be visualized and even the bonding state of the elements (e.g. Mn$^{2+}$ versus Mn$^{3+}$) can be detected on an atomic scale. Electron microscopy is by principle a projection technique, but the final dream is to obtain atomic info of materials in three dimensions. We will show that this is no longer a dream, but that it is possible using advanced microscopy. We will show evidence of determining the valence change Ce4+ versus Ce$^{3+}$ at the surface of a CeO$^2$ nanocrystal; the atomic shifts at the interface between LaAlO$^3$ and SrTiO$^3$ and the 3D relaxation of a Au nanocrystal.\\[4pt] References:\\[0pt] ``2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy,'' Tan,H., Turner, S., Yucelen, E., Verbeeck, J., Van Tendeloo, G. \textit{Physical Review Letters}, 107, 107602, (2011).\\[0pt] Three-dimensional atomic imaging of crystalline nanoparticles, Van Aert, S., Batenburg, K.J., Rossell, M.D., Erni, R., Van Tendeloo, G. \textit{Nature}, 470, 374 (2011).\\[0pt] ``Advanced Electron Microscopy for Advanced Materials,'' Van Tendeloo, G., Bals, S., Van Aert, S., Verbeeck, J., Van Dyck, D. \textit{Advanced Materials, DOI}: 10.1002/adma.201202107 (2012).\\[0pt] Atomic-scale determination of surface facets in gold nanorods, Goris, B., Bals, S., Van den Broek, W., Carbo-Argibay, E., Gomez-Grana, S., Liz-Marzan, M., Van Tendeloo, G. \textit{Nature Materials},11, 930 ( 2012)\\[0pt] ``Handbook of Nanoscopy'' Eds. G. Van Tendeloo, D. Van Dyck, S.J. Pennycook, Wiley-VCH (2012)   

Spatial mapping of surface plasmons in nanoscale Ag islands on graphite using Scanning Probe Energy Loss Spectroscopy

A scanning STM tip operated at high voltage can be used to obtain localized spectroscopic information about surfaces via energy loss measurements [1]. In this technique, known as Scanning Probe Energy Loss Spectroscopy (SPELS), the STM tip is used as a localized source of field-emitted electrons, which, upon backscattering from a surface, are analyzed by an energy-dispersive detector to obtain localized energy loss spectra. Characteristic surface excitations such as plasmons and excitons (as well as secondary electrons) can be probed with a spatial resolution below 50 nm and an energy resolution approaching 0.3 eV [2]. We report the development of a new generation SPELS instrument utilizing a 400-Channel detector, allowing sufficiently fast sampling of the energy loss spectra to allow us to obtain 2D spatially-resolved maps of energy loss features in a reasonable timeframe. We demonstrate the new instrument by mapping plasmons in (thermally evaporated) Ag nano-islands on the surface of graphite and illustrate the various mechanisms give rise to the contrast obtained in the energy-resolved maps. [1] A. Pulisciano, S.J. Park and R. E. Palmer, Appl. Phys. Lett. 93, 213109 (2008). [2] F. Festy and R. E. Palmer, Appl. Phys. Lett. 85, 5034 (2004).   

Ultrafine and Smooth Full Metal Nanostructures for Plasmonics

Surface plasmon polaritons (SPPs), which are coupled excitations of electrons bound to a metal-dielectric interface, show great potential for application in future nanoscale photonic systems due to the strong field confinement at the nanoscale, intensive local field enhancement, and interplay between strongly localized and propagating SPPs. The fabrication of sufficiently smooth metal surface with nanoscale feature size is crucial for SPPs to have practical applications. A template stripping (ST) method combined with PMMA as a template was successfully developed to create extraordinarily smooth metal nanostructures with a desirable feature size and morphology for plasmonics and metamaterials. The advantages of this method, including the high resolution, precipitous top-to bottom profile with a high aspect ratio, and three-dimensional characteristics, make it very suitable for the fabrication of plasmonic structures. By using this ST method, boxing ring-shaped nanocavities have been fabricated and the confined modes of surface plasmon polaritons in these nanocavities have been investigated and imaged by using cathodoluminescence spectroscopy. The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons, and quality factors can be directly acquired. Numerous applications, such as plasmonic filter, nanolaser, and efficient light-emitting devices, can be expected to arise from these developments.   

Low Loss Plasmonic Oxide Nanocrystals with Controlled Morphology

Localized surface plasmon resonance (LSPR) is a observed in metallic particles and results from the resonant oscillation of free electrons on the particle surface. One can manipulate the resonant frequencies through adjustment of the shape and size of the metal. A series of recent papers report LSPR at NIR and IR frequencies resulting from doped semiconductor nanocrystals. Free carriers in semiconductor particles result from atomic vacancies or through doping with aliovalent cations. While the plasma frequency ($\omega_{p})$ is considered an intrinsic property of metals, through adjustment of dopant concentrations, $\omega_{p}$ can be tuned in plasmonic semiconductors, opening the possibility of producing tunable, low-loss plasmonic nanocrystals to substitute for Au and Ag. We report the size and shape controlled synthesis of plasmonic oxide nanocrystals with highly uniform morphology and shape dependent optical properties. The size, shape, and doping concentration are independently controlled by modifying the synthetic parameters, allowing for precise modulation of optical response. These nanocrystals may be assembled to form superlattices, which function as plasmonic metamaterials, or used as precursors to produce bulk like films with tunable plasma frequencies.   

Aluminum Plasmonic Nanoantennas

We have explored the plasmonic properties of individual Al nanorod antennas fabricated by planar lithography on lightly doped n-type silicon. Energy-resolved cathodoluminescence was used to image the local density of optical states with a spatial resolution of $\sim$ 20 nm and thereby identify the radiative modes of these nanostructures. Al nanoantenna emission exhibited highly tunable plasmonic resonances from the deep UV through the visible region of the spectrum. The dependence of the radiative dipolar and quadrupolar plasmon modes on antenna length and photon energy agreed well with finite difference time domain-based analysis of these nanostructures. The results herald nano-structured aluminum as a practical and highly promising material system for the design and implementation of UV and visible frequency plasmonics, broadening the range of potential applications of plasmonics into areas where complementary metal$-$oxide$-$semiconductor (CMOS) compatibility or low-cost, mass producibility are desired.   

Plasmon coupling between distance-controlled gold nanoparticles

For small enough distances of noble metal nanoparticles in a matrix an additional plasmon-coupled mode is known to appear as a collective excitation between the nanoparticles. We show an approach of combining gold nanoparticles that allows to obtain coupled plasmons that can be dynamically changed, allowing systematic studies of the coupling. Poly-(N-isopropylacrylamide) pNIPAM is a polymer that can be used to produce thermo responsive gels, which have a volume phase transition at around $32^\circ$C. A ligand exchange on Au nanoparticles allows the attachment of the nanoparticles to pNIPAM spheres. The combined hybrid Au-pNIPAM system shows a plasmon-coupled mode above the pNIPAM's phase transition, additionally to the well-known shift and broadening of the fundamental plasmon peak. This plasmon mode can be switched on and off and modified simply by changing the temperature. We present discrete dipole approximation (DDA) calculations that characterize this resonance as a quadrupole Au plasmon mode, which results from close-to-contact-particles within the statistically distributed nanoparticles in the pNIPAM matrix. The presented approach is generalizable and allows to investigate the interaction between different kinds of metal nanostructures.   

Energy concentration of periodic nanoparticle array using Green function formalism

We have studied a periodic array of nanoparticle wires by using the Green function formalism (GFF). When light is incident on the wire, a collective oscillation of the free electrons is excited on the surface of the wires, which is called the coupled surface plasmon. The excitation of coupled surface plasmon can cause an enhancement of the local energy density. By tuning the separation relative to the radius of the wires, an energy concentration can be controlled. When the separation of the wires is small, multipolar effect becomes significant. Dealing with tight-binding model by Park and Stroud (2004) would involve interaction term which appears to be non-existent and the resolution of FDTD is insufficient to resolve the multipole interaction as the multipole field can vary rapidly. We applied GFF to this problem which expresses all interaction in a Greenian within one unit cell. The system was studied under spectral representation and the relation between different resonance modes and the outcoming energy concentration was examined. The energy concentration is largest several hot spots which depend on the incident directions.   

Tunable optical excitations in transition-metal doped arrays of noble-metal chains

We apply time-dependent density-functional theory to study the absorption spectrum of arrays of nano-scale pure noble and transition metal (TM) chains. We find that as the number of chains in the noble atom array increases the plasmon peak shifts to higher energies and appears in the visible range for an array of three gold chains, each consisting of more than 10 atoms. We also find collective excitations (plasmons) in arrays of TM chains: a behavior distinct from bulk TM systems. Doping noble metal chains with TM atoms leads to additional plasmon peaks close in energy to the main one for the undoped case. We compare the calculated optical absorption spectrum of the doped chains for several different types of TM atoms at different positions in the chains, and provide rationale for the trends. In the multi-chain case, the response is very sensitive to the position of the doped atoms. We argue that the origin of the additional modes is charge oscillations around the impurity atoms. Finally, we analyze the effect of interaction of excitonic modes created in infinite chains with plasmons in neighboring nanochains, including the possibility of resonance excitations and their trapping by the TM impurity atoms.   

Hollow Shells Of Dipoles: A Group Theoretical Approach

We investigate the plasmonic properties of hollow cylindrical lattices whose constituent elements are modeled as point dipoles. The symmetry of the lattice is described within the framework of group theory, which enables us to obtain the eigenmodes and eigenvalues of the entire polarization field by diagonalizing the dipole interaction part of the Hamiltonian. An incoming plane wave electric field that couples resonantly with the dipole lattice is expanded in terms of cylindrical harmonics, allowing us to precisely determine the contribution of various modes in its response function. The latter can then be obtained analytically for an arbitrary plane wave excitation. This work facilitates the study of cylindrical plasmonic shells with various geometry, as in the case of gold nanoparticles surrounding carbon nanotubes.   

Band edge excitons and trions in CdSe/CdS core/shell nanocrystals

We have developed a theory of positevly and negatively charged excitons (trions) in ``giant'' CdSe/CdS core-shell nanocrystals. The theory describes the energy structure of excitons and trions. We present the results of calculations for the fine structure of the the positively charged trion, the binding energy of the negatively charged trion, and the radiative decay time for excitons and trions. The theoretical results are compared with available experimental data.   

Controlling orientational order of multivalent prisms in superlattice assemblies

Multivalent nanostructures are an increasingly important player in the self-assembly of optically responsive superlattices. Understanding the role nanostructure coordination plays in the ordering of superlattice assemblies is crucial for the plasmonic response of the material. We developed a simple design rule for the assembly of multivalent DNA-Au triangular nanoprisms into 1D ordered superlattices based on both the length of the valent DNA and the size of the prism. Using MD simulations, we describe an order parameter that captures the short-range order of the mesoscale assembly controlled by the design rule. The order parameter shows that even short chains of prisms have a high-degree of orientational order when 1D superlattices are formed. Unlike isotropic polyvalent nanostructures, we find the highly oriented prism superlattices lose orientational order in a multistage fashion through loss of coordination during melting.   

Quantum Size Effects in $\alpha $-Pu (020) Layers

First principles calculations using the projector-augmented wave method and a plane wave basis set as implemented in the Vienna \textit{Ab Initio} Simulation Package (VASP) have been performed for the $\alpha $-Pu (020) layers. Because of severe demands on computational resources, scalar-relativistic computations were performed at the experimental geometry. The surface was assumed to be anti-ferromagnetic (AFM) since previous theoretical studies indicate the ground state of bulk $\alpha $-Pu to be AFM. Up to ten layers have been considered in this study. Work functions and surface energies appear to converge as the number of layers increase. We predict the work function to be around 3.4eV, with the surface energy being approximately 1.6eV. While no experimental results are available for $\alpha $-Pu, experimental results for $\delta $-Pu indicate a work function of approximately 3.2eV and a surface energy of 2.0eV. We will also present results on the magnetic moments and density of states of the layers. Results will be compared with results using the full-potential linearized-augmented-plane-wave method as implemented in the WIEN2k suite of software.