Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.

Cerium dioxide nanocubes and truncated octahedra were reduced and oxidized in the scanning transmission electron microscope. The reduction process was stimulated by the electron beam and oxidation was supported by background gases in the microscope environment. High-angle annular dark field imaging is sensitive to local lattice distortions that arise as oxygen vacancies are created and cerium cations reduce enabling high spatial resolution characterization of this process with temporal resolution on the order of seconds. Such measurements enable us to differentiate and infer that the observed behavior between the nanocubes and truncated octahedra may be due to the difference in crystallographic termination of surfaces. In situ measurements taken with different partial pressures of oxygen reveal the cerium oxidation state and the dose rate threshold for the onset of beam reduction are influenced by the environment. Increasing oxygen partial pressure reduces the Ce3+ content and decreases susceptibility to electron beam driven reduction.

Atomic-scale structural dynamics and phase transformation pathways were probed, in situ, during the hydrogen-induced reduction of Fe2O3 nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the α-Fe2O3 → γ-Fe2O3 phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe2O3 and γ-Fe2O3. The progression of the phase transformation into the other half of the bicrystalline Fe2O3 across the bicrystalline boundary led to the formation of a single-crystal phase of γ-Fe2O3 with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe3O4. Further reduction resulted in the coexistence of Fe3O4, FeO, and Fe via the transformation pathway Fe3O4 → FeO → Fe. The series of phase transformations was accompanied by the formation of a Swiss-cheese-like structure, induced by the significant volume shrinkage occurring upon reduction. These results elucidated the atomistic mechanism of the reduction of Fe oxides and demonstrated formation of hybrid structures of Fe oxides via tuning the phase transformation pathway.

is often presented schematically as one or more continuous sheet(s) of planar carbon atoms. In reality the sheets are not planar[1,2] and have contaminants, particularly disordered carbonaceous materials, which are introduced during growth, handling, and transfer to specific substrates. the area of pristine graphene, this contamination must be characterized accurately and reproducibly enough to provide a metric for improvements within a given ‘lot’ of graphene or graphene-based heterostructures on a specific substrate. Since Raman microscopy is relatively insensitive to disordered carbon, and very thin layers might be overlooked by high-resolution transmission electron microscopy (HRTEM), aberration-corrected scanning transmission electron microscopes (ac-STEMs) are the most reliable tools to confirm and quantify the presence of this contamination.

The reduction of metal oxides is of great importance in a large variety of chemical and materials applications ranging from heterogeneous catalysis to electronic device fabrication. However, the fundamental knowledge of the atomic processes underlying the oxide reduction is still very limited. Several reasons contribute to the lack of this fundamental information including the difficulty of measurements of the atomistic processes of the reaction and the longstanding challenge in identifying the reaction mechanisms in heterogeneous systems. By manipulating the surface roughness of a Fe substrate, we recently demonstrated that various morphologies of iron oxide nanostructures, such as nanowires, nanbelts and nanoblades, can be obtained by thermal oxidation of Fe [1, 2]. We previously showed that the reduction of α-Fe2O3 nanowires proceeds via the ordering of oxygen vacancies followed by the transformation pathway of α-Fe2O3  γ-Fe2O3  Fe3O4 [3]. Here we extend the study to the reduction of α-Fe2O3 nanoblades using an environmental transmission electron microscope (ETEM).

Journal Article In situ Atomic-Scale Visualization of CuO Nanowire Growth Get access Wenhui Zhu, Wenhui Zhu Department of Mechanical Engineering & Materials Science and Engineering Program, State University of New York, Binghamton, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Jonathan P Winterstein, Jonathan P Winterstein Center for Nanoscale Science and Technology, National Institute of Standards and Technology,Gaithersburg, MD, USA Search for other works by this author on: Oxford Academic Google Scholar Renu Sharma, Renu Sharma Center for Nanoscale Science and Technology, National Institute of Standards and Technology,Gaithersburg, MD, USA Search for other works by this author on: Oxford Academic Google Scholar Guangwen Zhou Guangwen Zhou Department of Mechanical Engineering & Materials Science and Engineering Program, State University of New York, Binghamton, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 1588–1589, https://doi.org/10.1017/S1431927616008783 Published: 25 July 2016

The atomic-scale reduction mechanism of α-Fe2O3 nanowires by H2 was followed using transmission electron microscopy to reveal the evolution of atomic structures and the associated transformation pathways for different iron oxides. The reduction commences with the generation of oxygen vacancies that order onto every 10th [Formula: see text] plane. This vacancy ordering is followed by an allotropic transformation of α-Fe2O3 → γ-Fe2O3 along with the formation of Fe3O4 nanoparticles on the surface of the γ-Fe2O3 nanowire by a topotactic transformation process, which shows 3D correspondence between the structures of the product and its host. These observations demonstrate that the partial reduction of α-Fe2O3 nanowires results in the formation of a unique hierarchical structure of hybrid oxides consisting of the parent oxide phase, γ-Fe2O3, as the one-dimensional wire and the Fe3O4 in the form of nanoparticles decorated on the parent oxide skeleton. We show that the proposed mechanism is consistent with previously published and our density functional theory results on the thermodynamics of surface termination and oxygen vacancy formation in α-Fe2O3. Compared to previous reports of α-Fe2O3 directly transformed to Fe3O4, our work provides a more in-depth understanding with substeps of reduction, i.e., the whole reduction process follows: α-Fe2O3 → α-Fe2O3 superlattice → γ-Fe2O3 + Fe3O4→ Fe3O4.

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Low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) imaging is presented as a method that is sensitive to the oxidation state of cerium ions in CeO2 nanoparticles. This relationship was validated through electron energy loss spectroscopy (EELS), in situ measurements, as well as multislice image simulations. Static displacements caused by the increased ionic radius of Ce3+ influence the electron channeling process and increase electron scattering to low angles while reducing scatter to high angles. This process manifests itself by reducing the high-angle annular dark field (HAADF) signal intensity while increasing the LAADF signal intensity in close proximity to Ce3+ ions. This technique can supplement STEM-EELS and in so doing, relax the experimental challenges associated with acquiring oxidation state information at high spatial resolutions.

The morphology evolution and associated topotactic relations between MgO nanocubes deposited on electron-transparent Al2O3 substrates were monitored after repeated high-temperature ex situ heat treatments. Owing to the well-defined morphology of MgO smoke cubes and flat basal-plane-oriented substrate, the initial orientation relationship is constrained to be {100}MgO || (0001)sapphire. In this geometry, only one rotational degree of freedom is allowed for MgO particles, and hence, a full set of coincident site lattices are formed, providing the opportunity to examine thermodynamic and kinetic processes and track competing surface and bulk ion-diffusion mechanisms during spinel formation. Crystallographic orientation relationships (ORs) between the sapphire (Al2O3) substrate, the magnesia (MgO) smoke nanoparticles, and the MgAl2O4 spinel reaction products were studied before and after annealing in the temperature range between 1000 and 1100 °C. The ORs adopted between the different pairs of materials were studied using single (0001)-oriented sapphire crystals pre-thinned for transmission electron microscopy (TEM) observations; the spinel/sapphire interface was further investigated on cross-section TEM specimens prepared from bulk samples using the focused ion-beam technique. At temperatures below ~1050 °C, the prevailing OR is \( \left\langle {110} \right\rangle \cdot \left\{ {111} \right\}_{\text{spinel}} //\left\langle {10\bar{1}0} \right\rangle \cdot \left( {0001} \right)_{\text{sapphire}} , \) whereas above that temperature \( \left\langle {110} \right\rangle \cdot \left\{ {100} \right\}_{\text{spinel}} //\left\langle {10\bar{1}0} \right\rangle \cdot \left( {0001} \right)_{\text{sapphire}} \) becomes more common. With the increasing temperature also the morphology of the spinel product is transformed from hexahedral to octahedral. The different ORs and microstructures appear to depend on the reaction temperature and result in different dominating diffusion mechanisms.

Directed self-assembly (DSA) of block copolymers (BCPs) is a rising technique for sub-20 nm patterning. To fully harness DSA capabilities for patterning, a detailed understanding of the three dimensional (3D) structure of BCPs is needed. By combining sequential infiltration synthesis (SIS) and scanning transmission electron microscopy (STEM) tomography, we have characterized the 3D structure of self-assembled and DSA BCPs films with high precision and resolution. SIS is an emerging technique for enhancing pattern transfer in BCPs through the selective growth of inorganic material in polar BCP domains. Here, Al2O3 SIS was used to enhance the imaging contrast and enable tomographic characterization of BCPs with high fidelity. Moreover, by utilizing SIS for both 3D characterization and hard mask fabrication, we were able to characterize the BCP morphology as well as the alumina nanostructures that would be used for pattern transfer.

Journal Article Analysis of TEM tomography artifacts with experiments on model specimens Get access Jonathan Winterstein, Jonathan Winterstein NIST Center for Nanoscale Science and Technology, Gaithersburg, MD 20899 Search for other works by this author on: Oxford Academic Google Scholar Joshua Schumacher, Joshua Schumacher NIST Center for Nanoscale Science and Technology, Gaithersburg, MD 20899 Search for other works by this author on: Oxford Academic Google Scholar J Alexander Liddle J Alexander Liddle NIST Center for Nanoscale Science and Technology, Gaithersburg, MD 20899 Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 1603–1604, https://doi.org/10.1017/S143192761500879X Published: 23 September 2015

We demonstrate quantitative core-loss electron energy-loss spectroscopy of iron oxide nanoparticles and imaging resolution of Ag nanoparticles in liquid down to 0.24 nm, in both transmission and scanning transmission modes, in a novel, monolithic liquid cell developed for the transmission electron microscope (TEM). At typical SiN membrane thicknesses of 50 nm the liquid-layer thickness has a maximum change of only 30 nm for the entire TEM viewing area of 200×200 µ m.

Journal Article Measuring Gas Adsorption on Individual Facets of a Nanoparticle by a Surface Plasmon Nanoprobe Get access Pin Ann Lin, Pin Ann Lin Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USAMaryland Nanocenter, University of Maryland, College Park, MD 20742, USA Search for other works by this author on: Oxford Academic Google Scholar John M Kohoutek, John M Kohoutek Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USAMaryland Nanocenter, University of Maryland, College Park, MD 20742, USA Search for other works by this author on: Oxford Academic Google Scholar Jonathan Winterstein, Jonathan Winterstein Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA Search for other works by this author on: Oxford Academic Google Scholar Henri Lezec, Henri Lezec Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA Search for other works by this author on: Oxford Academic Google Scholar Renu Sharma Renu Sharma Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 2053–2054, https://doi.org/10.1017/S1431927615011046 Published: 23 September 2015

Reduction of metal oxides is a reaction of removing lattice oxygen and plays an role in producing active materials for a variety of applications ranging from catalysis to electronic devices [1]. The reduction of iron oxides has been investigated intensively because of its vital role in heterogeneous catalysis. The variable valence states of Fe cause a rather complicated Fe-O phase diagram, leading to the possibility of multiple phases being formed during redox reactions. It has, therefore, been a longstanding challenge in understanding the reduction mechanism of Fe 2 O 3 . Here, we present observations of an iron oxide superlattice structure resulting from oxygen vacancy ordering and stacking fault formation as well as the phase transformation pathway during the reduction of α-Fe 2 O 3 nanowires The α-Fe 2 O 3 nanowires were prepared by the thermal oxidation of pure iron The reduction was conducted in a vacuum chamber at T = 500 ℃ with the pressure of H 2 gas (99.999% purity) at 270 Pa. The morphologies of the nanowires both before and after the H 2 reduction were examined using a scanning electron microscope (SEM). The crystal structure and valence state of the nanowires were characterized by TEM imaging, diffraction, and electron energy-loss spectroscopy using an aberration-corrected TEM operated at 300 kV.

Journal Article Staining Block Copolymers using Sequential Infiltration Synthesis for High Contrast Imaging and STEM tomography Get access T Segal-Peretz, T Segal-Peretz Institute for Molecular Engineering, University of Chicago, Chicago, USAMaterials Science Division, Argonne National Laboratory, Argonne, USA Search for other works by this author on: Oxford Academic Google Scholar J Winterstein, J Winterstein Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, USA Search for other works by this author on: Oxford Academic Google Scholar M Biswas, M Biswas Energy Systems Division, Argonne National Laboratory, Argonne, USA Search for other works by this author on: Oxford Academic Google Scholar JA Liddle, JA Liddle Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, USA Search for other works by this author on: Oxford Academic Google Scholar Jeffrey W Elam, Jeffrey W Elam Energy Systems Division, Argonne National Laboratory, Argonne, USA Search for other works by this author on: Oxford Academic Google Scholar N J Zaluzec, N J Zaluzec Electron Microscopy Center, NST Division, Argonne National Laboratory, Argonne, USA Search for other works by this author on: Oxford Academic Google Scholar PF Nealey PF Nealey Institute for Molecular Engineering, University of Chicago, Chicago, USAMaterials Science Division, Argonne National Laboratory, Argonne, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 611–612, https://doi.org/10.1017/S1431927615003852 Published: 23 September 2015

In situ environmental transmission electron microscopy (ETEM) experiments require specimen heating holders to study material behavior in gaseous environments at elevated temperatures. In order to extract meaningful kinetic parameters, such as activation energies, it is essential to have a direct and accurate measurement of local sample temperature. This is particularly important if the sample temperature might fluctuate, for example when room temperature gases are introduced to the sample area. Using selected-area diffraction (SAD) in an ETEM, the lattice parameter of Ag nanoparticles was measured as a function of the temperature and pressure of hydrogen gas to provide a calibration of the local sample temperature. SAD permits measurement of temperature to an accuracy of ±30°C using Ag lattice expansion. Gas introduction can cause sample cooling of several hundred degrees celsius for gas pressures achievable in the ETEM.