Fabrication and Characterisation of an Adaptable Plasmonic Nanorod Array for Solar Energy Conversion

The surface plasmonic modes of a side-by-side aligned gold nanorod array supported on a gold substrate has been characterised by electron energy loss spectroscopy (EELS). Plasmonic coupling within the array splits the nanorods' longitudinal mode into a bright mode (symmetrically aligned dipoles) and a dark mode (anti-symmetrically aligned dipoles). We support this observation by means of finite element modelling (FEM)

Contrast reversal in atomic-scale phonon spectroscopic imaging

It was recently shown that phonon spectral mapping of atomic structure is possible in scanning transmission electron microscopy. These results were obtained by deflecting the transmitted beam to detect probe electrons that had been scattered through large momentum transfer into a detector centered at an angle more than twice the radius of the bright-field disk. Atomic-scale spectral maps are still obtained for electrons scattered to angles smaller than the probe convergence angle, albeit at lower contrast and despite the smaller momentum transfers associated with these electrons. It is reported here that apparent reversals of contrast can be observed for these smaller momentum transfers, which is not the case when imaging using probe electrons scattered through larger momentum transfers. This phenomenon can be explained in terms of the channeling of the incident probe. We discuss how such contrast reversals may be avoided and atomic resolution maps reliably obtained

High-resolution monochromated electron energy-loss spectroscopy of organic photovoltaic materials

Advances in electron monochromator technology are providing opportunities for high energy resolution (10 – 200 meV) electron energy-loss spectroscopy (EELS) to be performed in the scanning transmission electron microscope (STEM). The energy-loss near-edge structure in core-loss spectroscopy is often limited by core-hole lifetimes rather than the energy spread of the incident illumination. However, in the valence-loss region, the reduced width of the zero loss peak makes it possible to resolve clearly and unambiguously spectral features at very low energy-losses (<3 eV). In this contribution, high-resolution EELS was used to investigate four materials commonly used in organic photovoltaics (OPVs): poly(3-hexlythiophene) (P3HT), [6,6] phenyl-C61 butyric acid methyl ester (PCBM), copper phthalocyanine (CuPc), and fullerene (C60). Data was collected on two different monochromated instruments – a Nion UltraSTEM 100 MC ‘HERMES’ and a FEI Titan3 60–300 Image-Corrected S/TEM – using energy resolutions (as defined by the zero loss peak full-width at half-maximum) of 35 meV and 175 meV, respectively. The data was acquired to allow deconvolution of plural scattering, and Kramers–Kronig analysis was utilized to extract the complex dielectric functions. The real and imaginary parts of the complex dielectric functions obtained from the two instruments were compared to evaluate if the enhanced resolution in the Nion provides new opto-electronic information for these organic materials. The differences between the spectra are discussed, and the implications for STEM-EELS studies of advanced materials are considered

Bio-inspired fabrication of DNA-inorganic hybrid composites using synthetic DNA

Nucleic acid nanostructures have attracted significant interest as potential therapeutic and diagnostic platforms due to their intrinsic biocompatibility and biodegradability, structural and functional diversity, and compatibility with various chemistries for modification and stabilization. Among the fabrication approaches for such structures, the rolling circle techniques have emerged as particularly promising, producing morphologically round, flower-shaped nucleic acid particles: typically hybrid composites of long nucleic acid strands and inorganic magnesium pyrophosphate (Mg2PPi). These constructs are known to form via anisotropic nucleic acid-driven crystallization in a sequence-independent manner, rendering monodisperse and densely packed RNA or DNA–inorganic composites. However, it still remains to fully explore how flexible polymer-like RNA or DNA strands (acting as biomolecular additives) mediate the crystallization process of Mg2PPi and affect the structure and properties of the product crystals. To address this, we closely examined nanoscale details to mesoscopic features of Mg2PPi/DNA hybrid composites fabricated by two approaches, namely rolling circle amplification (RCA)-based in situ synthesis and long synthetic DNA-mediated crystallization. Similar to the DNA constructs fabricated by RCA, the rapid crystallization of Mg2PPi was retarded on a short-range order when we precipitated the crystals in the presence of presynthesized long DNA, which resulted in effective incorporation of biomolecular additives such as DNA and enzymes. These findings further provide a more feasible way to encapsulate bioactive enzymes within DNA constructs compared to in situ RCA-mediated synthesis, i.e., by not only protecting them from possible denaturation under the reaction conditions but also preventing nonselective association of proteins arising from the RCA reaction mixtures

Spatial distribution of metallic heteroatoms in soot nanostructure mapped by aberration-corrected STEM-EELS

Soot from the thermochemical conversion of solid and liquid fuels can be infused with metallic heteroatoms originating from the fuel – these heteroatoms alter the nanostructure and the reactivity of the soot. Here, we investigate the spatial distribution of metallic heteroatoms in soot generated by biomass gasification, using aberration-corrected Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). The technique allowed for the mapping of heteroatom distribution in soot at the nanoscale, and thereby for the direct correlation of heteroatom concentration with the graphitic nanostructure. Spherical soot particles were coated with a thin layer of silicon, possibly in the form of quartz that may be linked to minor distortions of the nanostructure of the graphitic shell of the particles. Further results on non-spherical soot and inorganic-carbon fused aggregates suggest that the chemistry of formation was affected by the presence of gaseous ash-forming elements, especially calcium, with carbon-oxygen functional groups forming as intermediates in the graphite-inorganic reaction; i.e., prior to the formation of the thermodynamically stable carbonate bonds. The analytical approach demonstrated here can potentially help select fuel additives or aid in the design of fuel blends that minimize the formation of similar, hybrid carbon nanoparticles in combustion or gasification systems

Phonon Spectroscopy at Atomic Resolution

Advances in source monochromation in transmission electron microscopy have opened up new possibilities for investigations of condensed matter using the phonon-loss sector of the energy-loss spectrum. Here, we explore the spatial variations of the spectrum as an atomic-sized probe is scanned across a thin flake of hexagonal boron nitride. We demonstrate that phonon spectral mapping of atomic structure is possible. These results are consistent with a model for the quantum excitation of phonons and confirm that Z-contrast imaging is based on inelastic scattering associated with phonon excitation

Elemental distribution within the long-period stacking ordered structure in a Mg-Gd-Zn-Mn alloy

High angle annular dark field scanning transmission electron microscope imaging and electron energy loss spectroscopy was used to elucidate the elemental distribution (Gd, Zn, Mn) within the long-period stacking ordered (LPSO) structure in a Mg-15Gd-0.8Zn-0.8Mn (wt%) alloy. While Gd and Zn enrichment was observed within the LPSO structure, no significant enrichment in Mn was observed. After averaging over a large region, a very weak Mn signal was resolved but no significant variations in Mn signal were observed over this region, suggesting that Mn is indeed present. These results provide useful information to support the future development of high performance Mg alloys

Solute clustering and precipitation in an Al–Cu–Mg–Ag–Si model alloy

Solute clustering and precipitation in an Al–Cu–Mg–Ag–Si model alloy has been investigated by atom probe tomography (APT) as well as high-angle annular dark-field (HAADF) imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). Nine types of solute clusters (Cu, Ag, Mg–Cu, Mg–Ag, Mg–Cu–Si, Mg–Ag–Si, Mg–Ag–Cu, Cu–Ag–Si and MgAgCuSi) were observed by APT in both the as-quenched alloy and after ageing the alloy at 180 °C for 1 h. Three types of precipitates (Ω (AlCuMgAg), θ (Al2Cu) and Mg2Si) were observed by APT and HAADF-STEM after further ageing at 180 °C for 24 h and 100 h. We propose that MgAgCu and MgAgCuSi clusters are likely to be responsible for the formation of the Ω (AlCuMgAg) phase. Furthermore, we also suggest that the θ (Al2Cu) phase forms from Cu clusters and the Mg2Si phase forms from the decomposition of MgAgSi and MgAgCuSi clusters by losing Ag to Ω phase growth. Many early binary clusters (Mg–Cu, Mg–Ag) do not seem to undergo a significant further growth during ageing; these are more likely to be transformed into complex ternary and quaternary clusters and be subsequently consumed during the growth of large clusters/precipitates. Furthermore, it is proposed that the plate-like Ω (AlCuMgAg) precipitates evolve continuously from the MgAgCu and MgAgCuSi clusters, rather than via heterogeneous nucleation on their precursors (i.e. MgAgCu and MgAgCuSi clusters). More interestingly, even after ageing at 180 °C for 100 h, the Ω (AlCuMgAg) precipitates remain coherent with the α-Al matrix, indicating that these precipitates have a high thermal stability. This can mainly be attributed to the presence of a single Mg–Ag-rich monolayer observed at the interface between the Ω precipitate and the α-Al matrix, significantly improving the coarsening resistance of the Ω (AlCuMgAg) precipitates. Our results thus reveal links between a variety of solute clusters and the different types of precipitates in the Al–Cu–Mg–Ag–Si model alloy. Such information can in the future be used to control the precipitation by tailoring solute clustering

Theory of momentum-resolved phonon spectroscopy in the electron microscope

We provide a theoretical framework for the prediction and interpretation of momentum-dependent phonon spectra due to coherent inelastic scattering of electrons. We complete the approach with first-principles lattice dynamics using periodic density functional theory and compare to recent electron energy-loss measurements on cubic and hexagonal boron nitride performed within a scanning transmission electron microscope. The combination of theory and experiment provides the ability to interpret momentum-dependent phonon spectra obtained at nanometer spatial resolution in the electron microscope

Position and momentum mapping of vibrations in graphene nanostructures in the electron microscope

Propagating atomic vibrational waves, phonons, rule important thermal, mechanical, optoelectronic and transport characteristics of materials. Thus the knowledge of phonon dispersion, namely the dependence of vibrational energy on momentum is a key ingredient to understand and optimize the material's behavior. However, despite its scientific importance in the last decade, the phonon dispersion of a freestanding monolayer of two dimensional (2D) materials such as graphene and its local variations has still remained elusive because of experimental limitations of vibrational spectroscopy. Even though electron energy loss spectroscopy (EELS) in transmission has recently been shown to probe the local vibrational charge responses, these studies are yet limited to polar materials like boron nitride or oxides, in which huge signals induced by strong dipole moments are present. On the other hand, measurements on graphene performed by inelastic x-ray (neutron) scattering spectroscopy or EELS in reflection do not have any spatial resolution and require large microcrystals. Here we provide a new pathway to determine the phonon dispersions down to the scale of an individual freestanding graphene monolayer by mapping the distinct vibration modes for a large momentum transfer. The measured scattering intensities are accurately reproduced and interpreted with density functional perturbation theory (DFPT). Additionally, a nanometre-scale mapping of selected momentum (q) resolved vibration modes using graphene nanoribbon structures has enabled us to spatially disentangle bulk, edge and surface vibrations