High-temperature phonons in h-BN: momentum-resolved vibrational spectroscopy and theory

Vibrations in materials and nanostructures at sufficiently high temperatures result in anharmonic atomic displacements, which leads to new phenomena such as thermal expansion and multiphonon scattering processes, with a profound impact on temperature-dependent material properties including thermal conductivity, phonon lifetimes, nonradiative electronic transitions, and phase transitions. Nanoscale momentum-resolved vibrational spectroscopy, which has recently become possible on monochromated scanning-transmission-electron microscopes, is a unique method to probe the underpinnings of these phenomena. Here we report momentum-resolved vibrational spectroscopy in hexagonal boron nitride at temperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) that reveals temperature-dependent phonon energy shifts and demonstrates the presence of strong Umklapp processes. Density-functional-theory calculations of temperature-dependent phonon self-energies reproduce the observed energy shifts and identify the contributing mechanisms.Comment: 21 pages, 4 figures, 2 tables, 3 supplemental figures, 3 supplemental table

Vibrational spectroscopy of water with high spatial resolution

The ability to examine the vibrational spectra of liquids with nanometer spatial resolution will greatly expand the potential to study liquids and liquid interfaces. In fact, the fundamental properties of water, including complexities in its phase diagram, electrochemistry, and bonding due to nanoscale confinement are current research topics. For any liquid, direct investigation of ordered liquid structures, interfacial double layers, and adsorbed species at liquid–solid interfaces are of interest. Here, a novel way of characterizing the vibrational properties of liquid water with high spatial resolution using transmission electron microscopy is reported. By encapsulating water between two sheets of boron nitride, the ability to capture vibrational spectra to quantify the structure of the liquid, its interaction with the liquid‐cell surfaces, and the ability to identify isotopes including H2O and D2O using electron energy‐loss spectroscopy is demonstrated. The electron microscope used here, equipped with a high‐energy‐resolution monochromator, is able to record vibrational spectra of liquids and molecules and is sensitive to surface and bulk morphological properties both at the nano‐ and micrometer scales. These results represent an important milestone for liquid and isotope‐labeled materials characterization with high spatial resolution, combining nanoscale imaging with vibrational spectroscopy.Microscopy research performed as part of a user proposal at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility (J.A.H, J.C.I.). This research was conducted, in part, using instrumentation within ORNL's Materials Characterization Core provided by UT‐Battelle, LLC under Contract No. DE‐AC05‐00OR22725 with the U.S. Department of Energy. This manuscript was authorized by UT‐Battelle, LLC under Contract No. DE‐AC05‐00OR22725 with the U.S. Department of Energy. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid‐up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe‐public‐access‐plan). ​Acquisition of UIC JEOL ARM200CF was supported by an MRI‐R2 grant from the National Science Foundation (DMR‐0959470). The Gatan Quantum GIF acquisition at UIC was supported by an MRI grant from the National Science Foundation (DMR‐1626065). Theoretical simulations were supported by Spanish Ministry project FIS2016‐80174‐P.Peer reviewe

Vibrational spectroscopy in the electron microscope

Vibrational spectroscopies using infrared radiation In the past two decades, the performance of electron microscopes has been greatly improved by the introduction of multipole-based aberration correction technology Because of recent progress, we are now able to answer the question in the positive. The progress has taken place on three principal fronts: (1) the energy resolution of EELS carried out in the electron microscope has been improved to around 10 meV; (2) the EELS-STEM instrument has been optimized so that the electron probe incident on the sample contains a current sufficient to perform EELS experiments even when the energy width of the probe is ,10 meV and its size ,1 nm; and (3) the tail of the intense zero loss peak (ZLP) in the EELS spectrum has been reduced so that it does not obscure the vibrational features of interest. The innovations responsible for the progress are (1) a monochromator of a new design 17 , which is able to reach an energy resolution comparable to the highest resolution attained previously The observation of vibrational peaks due to hydrogen in TiH 2 and in the epoxy resin is especially interesting. In TiH 2 , hydrogen is mobile and bound only weakly, which results in the relatively low (for hydrogen) vibrational energy of 147 meV. In epoxy resin, hydrogen is mostly bound to carbon, and 360 meV (2,900 cm 21 ) is a typical C-H stretch vibrational energy 1-3 . Up to now, hydrogen has been essentially invisible in electron microscopes, its presence typically inferred from the modified electron distribution due to the electron it contributes to the sample's electron density distribution. Its unambiguous detection by vibrational spectroscopy promises to provide a general technique for hydrogen Macmillan Publishers Limited. All rights reserved ©2014 detection in the many hydrogen-containing materials studied by electron microscopy. Another attractive prospect involves analysing the types of covalent hydrogen bonding present in microscopic amounts of matter, with H-C, H-N, H-O and other types of hydrogen bonds giving distinct vibrational frequencies 1-3 . The width of the vibrational peaks shown in The vibrational signal was obtained by subtracting the background under the peak at 138 meV in all the spectra, which were similar to the spectrum shown in Aloof beams losing energy to delocalized electronic excitations (such as surface plasmons) have been studied extensively in low-loss EELS The spatial resolution obtainable with the aloof signal is comparable to that of tip-enhanced optical spectroscopy 7 , without needing to have a sharp tip in the vicinity of the examined structure. Because the interaction distance for the signal can be much larger than the diameter o