David Rossouw

Mapping Bright and Dark Modes in Gold Nanoparticle Chains using Electron Energy Loss Spectroscopy.

We present a scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) investigation of gold nanosphere chains with lengths varying from 1 to 5 particles. We show localized EELS signals from the chains and identify energy-loss peaks arising due to l = 1, 2, 3, 4, and 5 plasmon modes through the use of EELS mapping. We also show the evolution of the energy of these modes as the length of a given chain increases, and we find that a chain containing N particles can accommodate at least N experimentally observable modes, in addition to the transverse mode. As the chain length is increased by the addition of one more gold particle to the chain, the new N + 1 mode becomes the highest energy mode, while the existing modes lower their energy and eventually asymptote as they delocalize along the chain. We also show that modes become increasingly difficult to detect with the EELS technique as l approaches N. The data are compared to numerical simulations.

Resonant optical excitations in complementary plasmonic nanostructures

We compare the plasmonic response of two complementary structures to a scanning electron probe; a silver nanowire and a nanoslot in a silver film of comparable dimensions, desirable for their localized electromagnetic enhancement and enhanced optical transmission respectively. Through electron energy loss spectroscopy, multiple plasmonic resonant harmonics setup in both structures are resolved with inverted phase, in agreement with Babinets principle, and of consequence in the design and fabrication of nanostructures.

Toward 10 meV electron energy-loss spectroscopy resolution for plasmonics.

Energy resolution is one of the most important parameters in electron energy-loss spectroscopy. This is especially true for measurement of surface plasmon resonances, where high-energy resolution is crucial for resolving individual resonance peaks, in particular close to the zero-loss peak. In this work, we improve the energy resolution of electron energy-loss spectra of surface plasmon resonances, acquired with a monochromated beam in a scanning transmission electron microscope, by the use of the Richardson-Lucy deconvolution algorithm. We test the performance of the algorithm in a simulated spectrum and then apply it to experimental energy-loss spectra of a lithographically patterned silver nanorod. By reduction of the point spread function of the spectrum, we are able to identify low-energy surface plasmon peaks in spectra, more localized features, and higher contrast in surface plasmon energy-filtered maps. Thanks to the combination of a monochromated beam and the Richardson-Lucy algorithm, we improve the effective resolution down to 30 meV, and evidence of success up to 10 meV resolution for losses below 1 eV. We also propose, implement, and test two methods to limit the number of iterations in the algorithm. The first method is based on noise measurement and analysis, while in the second we monitor the change of slope in the deconvolved spectrum.

Multicomponent signal unmixing from nanoheterostructures: overcoming the traditional challenges of nanoscale X-ray analysis via machine learning.

The chemical composition of core–shell nanoparticle clusters have been determined through principal component analysis (PCA) and independent component analysis (ICA) of an energy-dispersive X-ray (EDX) spectrum image (SI) acquired in a scanning transmission electron microscope (STEM). The method blindly decomposes the SI into three components, which are found to accurately represent the isolated and unmixed X-ray signals originating from the supporting carbon film, the shell, and the bimetallic core. The composition of the latter is verified by and is in excellent agreement with the separate quantification of bare bimetallic seed nanoparticles.

Tunable Syngas Production from CO2 and H2O in an Aqueous Photoelectrochemical Cell

Syngas, the mixture of CO and H2 , is a key feedstock to produce methanol and liquid fuels in industry, yet limited success has been made to develop clean syngas production using renewable solar energy. We demonstrated that syngas with a benchmark turnover number of 1330 and a desirable CO/H2 ratio of 1:2 could be attained from photoelectrochemical CO2 and H2 O reduction in an aqueous medium by exploiting the synergistic co-catalytic effect between Cu and ZnO. The CO/H2 ratio in the syngas products was tuned in a large range between 2:1 and 1:4 with a total unity Faradaic efficiency. Moreover, a high Faradaic efficiency of 70 % for CO was acheived at underpotential of 180 mV, which is the lowest potential ever reported in an aqueous photoelectrochemical cell. It was found that the combination of Cu and ZnO offered complementary chemical properties that lead to special reaction channels not seen in Cu, or ZnO alone.

The Dark Side of EDX Tomography: Modeling Detector Shadowing to Aid 3D Elemental Signal Analysis.

A simple model is proposed to account for the loss of collected X-ray signal by the shadowing of X-ray detectors in the scanning transmission electron microscope. The model is intended to aid the analysis of three-dimensional elemental data sets acquired using energy-dispersive X-ray tomography methods where shadow-free specimen holders are unsuitable or unavailable. The model also provides a useful measure of the detection system geometry.

Metallic and Semiconducting Single-Walled Carbon Nanotubes: Differentiating Individual SWCNTs by Their Carbon 1s Spectra

The C 1s inner shell excitation spectra of individual metallic and semiconducting single-walled carbon nanotubes (SWCNTs) were measured using high-resolution electron energy loss spectroscopy in an aberration-corrected transmission electron microscope (TEM-EELS). On the basis of its diameter, the metallic SWCNT is most likely a (10,10) sample, whereas (11,12) and a number of other chiral vectors are consistent with the diameter of the semiconducting SWCNTs. The C 1s X-ray absorption spectra of the same electronically pure SWCNT materials were measured as individual bundles or agglomerations of bundles by scanning transmission X-ray microscopy. Spectral differences in the C 1s → π* transitions of metallic and semiconducting species, related to differences in the van Hove singularities in their unoccupied states, are observed by both methods. The fine structure of the C 1s → π* transitions is similar to that recently reported from nonspatially resolved X-ray absorption spectroscopy of ensemble samples of high-purity metallic and semiconducting SWCNTs. The quality of the TEM-EELS spectra of individual SWCNTs is such that the line shape can be used to identify if they are metallic or semiconducting, thereby opening up the possibility to interrogate the electronic state of single-SWCNT devices. A strong X-ray linear dichroism in the C 1s → π* band of both types of SWCNTs was observed.

Mixed-quantum-dot solar cells

Colloidal quantum dots are emerging solution-processed materials for large-scale and low-cost photovoltaics. The recent advent of quantum dot inks has overcome the prior need for solid-state exchanges that previously added cost, complexity, and morphological disruption to the quantum dot solid. Unfortunately, these inks remain limited by the photocarrier diffusion length. Here we devise a strategy based on n- and p-type ligands that judiciously shifts the quantum dot band alignment. It leads to ink-based materials that retain the independent surface functionalization of quantum dots, and it creates distinguishable donor and acceptor domains for bulk heterojunctions. Interdot carrier transfer and exciton dissociation studies confirm efficient charge separation at the nanoscale interfaces between the two classes of quantum dots. We fabricate the first mixed-quantum-dot solar cells and achieve a power conversion of 10.4%, which surpasses the performance of previously reported bulk heterojunction quantum dot devices fully two-fold, indicating the potential of the mixed-quantum-dot approach.Solution processed colloidal quantum dots are emerging photovoltaic materials with tuneable infrared bandgaps. Here, Yang et al. create a class of quantum dot bulk heterojunction solar cell via ligand design, enabling longer photocarrier diffusion lengths for greater photocurrent and performance.

Electron Energy Loss Spectroscopy Investigation into Symmetry in Gold Trimer and Tetramer Plasmonic Nanoparticle Structures.

We present a combined scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) investigation into the mode symmetries of plasmonic nanoparticle trimer and tetramer structures. We obtain nanometer-resolved energy loss spectra for both trimer and tetramer structures and compare these to boundary element method simulations. We show that EELS, in conjunction with eigenmode simulations, offers a complete characterization of the individual superstructures, and we trace the evolution of both optically dark and bright modes and identify multipolar mode contributions. We then apply this technique to tetramer structures that exhibit an expanded range of mode symmetries for two-dimensional and three-dimensional self-assembled geometries. These findings provide a comprehensive experimental account of the available photonic states in self-assembled nanoparticle clusters.

Self-Similarity of Plasmon Edge Modes on Koch Fractal Antennas

We investigate the plasmonic behavior of Koch snowflake fractal geometries and their possible application as broadband optical antennas. Lithographically defined planar silver Koch fractal antennas were fabricated and characterized with high spatial and spectral resolution using electron energy loss spectroscopy. The experimental data are supported by numerical calculations carried out with a surface integral equation method. Multiple surface plasmon edge modes supported by the fractal structures have been imaged and analyzed. Furthermore, by isolating and reproducing self-similar features in long silver strip antennas, the edge modes present in the Koch snowflake fractals are identified. We demonstrate that the fractal response can be obtained by the sum of basic self-similar segments called characteristic edge units. Interestingly, the plasmon edge modes follow a fractal-scaling rule that depends on these self-similar segments formed in the structure after a fractal iteration. As the size of a fractal structure is reduced, coupling of the modes in the characteristic edge units becomes relevant, and the symmetry of the fractal affects the formation of hybrid modes. This analysis can be utilized not only to understand the edge modes in other planar structures but also in the design and fabrication of fractal structures for nanophotonic applications.