Beth S. Guiton

Correlated optical measurements and plasmon mapping of silver nanorods.

Plasmonics is a rapidly growing field, yet imaging of the plasmonic modes in complex nanoscale architectures is extremely challenging. Here we obtain spatial maps of the localized surface plasmon modes of high-aspect-ratio silver nanorods using electron energy loss spectroscopy (EELS) and correlate to optical data and classical electrodynamics calculations from the exact same particles. EELS mapping is thus demonstrated to be an invaluable technique for elucidating complex and overlapping plasmon modes.

Single-Molecule Surface-Enhanced Raman Scattering: Can STEM/EELS Image Electromagnetic Hot Spots?

Since the observation of single-molecule surface-enhanced Raman scattering (SMSERS) in 1997, questions regarding the nature of the electromagnetic hot spots responsible for such observations still persist. For the first time, we employ electron-energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to obtain maps of the localized surface plasmon modes of SMSERS-active nanostructures, which are resolved in both space and energy. Single-molecule character is confirmed by the bianalyte approach using two isotopologues of Rhodamine 6G. Surprisingly, the STEM/EELS plasmon maps do not show any direct signature of an electromagnetic hot spot in the gaps between the nanoparticles. The origins of this observation are explored using a fully three-dimensional electrodynamics simulation of both the electron-energy-loss probability and the near-electric field enhancements. The calculations suggest that electron beam excitation of the hot spot is possible, but only when the electron beam is located outside of the junction region.

Spontaneous Compositional Nanopatterning in Li-Containing Perovskite Oxides

A structure designed to show functionality on the nanometer length scale ideally would spontaneously form periodic nanometer-scale patterns comprising regions with contrasting properties. Here we report the synthesis of three new oxides that spontaneously form a variety of nanostructures with a periodic arrangement of phases with compositional and functional contrast. This is achieved through the partial substitution of Ti by Al, Cr, and Mn in periodically phase-separated (Nd(2/3-x)Li(3x))TiO(3) nanochessboard structures. The generality of this spontaneous compositional nanopatterning is promising for an array of exotic bulk and nanostructural properties.

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.

Direct observation of Li diffusion in Li-doped ZnO nanowires

The direct observation of Li diffusion in Li-doped zinc oxide nanowires (NWs) was realized by using in situ heating in the scanning transmission electron microscope (STEM). A continuous increase of low atomic mass regions within a single NW was observed between 200 °C and 600 °C when heated in vacuum, which was explained by the conversion of interstitial to substitutional Li in the ZnO NW host lattice. A kick-out mechanism is introduced to explain the migration and conversion of the interstitial Li (Lii) to Zn-site substitutional Li (LiZn), and this mechanism is verified with low-temperature (11 K) photoluminescence measurements on as-grown and annealed Li-doped zinc oxide NWs, as well as the observation of an increase of NW surface roughing with applied bias.

Reply to 'Nanoscale phase separation in perovskites revisited'

in particular energy-filtered TEM can be affected by diffraction effects as well. This can complicate data interpretation, see for example ref. 7. To minimize the strain contribution, we carried out STEM imaging in thin sample areas. The dark-field STEM images did not show any pattern, indicating that strain effects can be suppressed. On the other hand, the bright-field STEM signal highlighted a faint rectangular line pattern, which represents the true domain structure. Using powder synchrotron X-ray diffraction (ID31 beamline, European Synchrotron Radiation Facility) and neutron diffraction (D2B diffractometer, Institut LaueLangevin) in combination with the TEM data, we performed a complete quantitative structure solution of the (3+2)-dimensional incommensurately modulated Nd0.617Li0.15TiO3 structure. Rietveld refinement revealed that the primary modulation is related to rectangular domains with a quasiperiodic two-dimensional perturbation of the tilting pattern of the TiO6 octahedra, including an in-phase tilting component around the c axis6. This extra component has not been considered in any of the previous models3,8,9. The refinement undoubtedly demonstrated the absence of both the compositional modulation and the claimed phase separation. The simulated ED pattern in Fig. 1b demonstrates that the refined structure is in agreement with the experiment. We conclude that the previously proposed compositionally driven phase-separation model3 of (Nd2/3–xLi3x)TiO3 is not supported by the experimental data. The domain structure of (Nd2/3–xLi3x)TiO3 is not due to a compositional variation but merely due to displacive modulations with a primary contribution of the frustrated TiO6 octahedra tilting. ❐

Large-Scale Synthesis and Comprehensive Structure Study of δ-MnO2

Layered δ-MnO2 (birnessites) are ubiquitous in nature and have also been reported to work as promising water oxidation catalysts or rechargeable alkali-ion battery cathodes when fabricated under appropriate conditions. Although tremendous effort has been spent on resolving the structure of natural/synthetic layered δ-MnO2 in the last few decades, no conclusive result has been reached. In this Article, we report an environmentally friendly route to synthesizing homogeneous Cu-rich layered δ-MnO2 nanoflowers in large scale. The local and average structure of synthetic Cu-rich layered δ-MnO2 has been successfully resolved from combined Mn/Cu K-edge extended X-ray fine structure spectroscopy and X-ray and neutron total scattering analysis. It is found that appreciable amounts (∼8%) of Mn vacancies are present in the MnO2 layer and Cu2+ occupies the interlayer sites above/below the vacant Mn sites. Effective hydrogen bonding among the interlayer water molecules and adjacent layer O ions has also been observed for the first time. These hydrogen bonds are found to play the key role in maintaining the intermediate and long-range stacking coherence of MnO2 layers. Quantitative analysis of the turbostratic stacking disorder in this compound was achieved using a supercell approach coupled with anisotropic particle-size-effect modeling. The present method is expected to be generally applicable to the structural study of other technologically important nanomaterials.

Shell-Induced Ostwald Ripening: Simultaneous Structure, Composition, and Morphology Transformations during the Creation of Hollow Iron Oxide Nanocapsules

The creation of nanomaterials requires simultaneous control of not only crystalline structure and composition but also crystal shape and size, or morphology, which can pose a significant synthetic challenge. Approaches to address this challenge include creating nanocrystals whose morphologies echo their underlying crystal structures, such as the growth of platelets of two-dimensional layered crystal structures, or conversely attempting to decouple the morphology from structure by converting a structure or composition after first creating crystals with a desired morphology. A particularly elegant example of this latter approach involves the topotactic conversion of a nanoparticle from one structure and composition to another, since the orientation relationship between the initial and final product allows the crystallinity and orientation to be maintained throughout the process. Here we report a mechanism for creating hollow nanostructures, illustrated via the decomposition of β-FeOOH nanorods to nanocapsules of α-Fe2O3, γ-Fe2O3, Fe3O4, and FeO, depending on the reaction conditions, while retaining single-crystallinity and the outer nanorod morphology. Using in situ TEM, we demonstrate that the nanostructured morphology of the starting material allows kinetic trapping of metastable phases with a topotactic relationship to the final thermodynamically stable phase.

Understanding Hollow Metal Oxide Nanomaterial Formation with in situ Transmission Electron Microscopy

Nanomaterials have been studied intensely for several decades, in large part for potential applications such as catalysis, energy storage and sensors,[1–3] which rely on their high surface area. Hollow nanostructures with their especially high surface areas, low densities and large capacities are particularly promising, and the properties of hollow nanomaterials are often found to be favorable as compared to their solid counterparts.[4] Hollow iron oxides are of particular interest, having already been shown to perform well as photocatalysts, super capacitors, and anodes for lithium ion batteryies.[5–7] In order to fully exploit the potential of these hollow iron oxide nanomaterials, however, it is critical to understand and control the crystal structure (for the many different iron oxide phases) and morphology, and to understand the resulting structure-property relationships. Much work has been done to understand mechanisms leading to hollow nanostructures, such as the Kirkendall effect, and Oswald ripening, but these mechanisms alone cannot account for every scenario. Here we discuss the in situ characterization of series of phase transformations as solid FeOOH nanorods evolves to produce hollow nanostructures of α-Fe2O3, γ-Fe2O3, Fe3O4 and FeO. In situ observation of a single individual FeOOH nanorod in the transmission electron microscopy (TEM), shows a new mechanism for the formation of a hollow capsule

Direct Observation of Hafnia Structural Phase Transformations

1. Department of Chemistry, University of Kentucky, Lexington, Kentucky, USA 2. Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY, USA 3. Department of Chemistry, Texas A&M University, College Station, TX, USA 4. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA 5. Materials Science and Technology Division, Oak Ridge National Lab, Oak Ridge, TN, USA