51
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Tizei LHG, Mkhitaryan V, Lourenço-Martins H, Scarabelli L, Watanabe K, Taniguchi T, Tencé M, Blazit JD, Li X, Gloter A, Zobelli A, Schmidt FP, Liz-Marzán LM, García de Abajo FJ, Stéphan O, Kociak M. Tailored Nanoscale Plasmon-Enhanced Vibrational Electron Spectroscopy. NANO LETTERS 2020; 20:2973-2979. [PMID: 31967839 PMCID: PMC7227010 DOI: 10.1021/acs.nanolett.9b04659] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/12/2019] [Revised: 01/08/2020] [Indexed: 05/24/2023]
Abstract
Atomic vibrations and phonons are an excellent source of information on nanomaterials that we can access through a variety of methods including Raman scattering, infrared spectroscopy, and electron energy-loss spectroscopy (EELS). In the presence of a plasmon local field, vibrations are strongly modified and, in particular, their dipolar strengths are highly enhanced, thus rendering Raman scattering and infrared spectroscopy extremely sensitive techniques. Here, we experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. We finely tune the energy of surface plasmons in metallic nanowires in the vicinity of hexagonal boron nitride, making it possible to monitor and disentangle both strong phonon-plasmon coupling and plasmon-driven phonon enhancement at the nanometer scale. Because of the near-field character of the electron beam-phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.
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Affiliation(s)
- Luiz H. G. Tizei
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Vahagn Mkhitaryan
- The
Barcelona Institute of Science and Technology, ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels (Barcelona), Spain
| | - Hugo Lourenço-Martins
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Leonardo Scarabelli
- CIC
biomaGUNE and Ciber-BBN, Paseo de Miramón 182, 20014 Donostia-San Sebastián, Spain
- Department
of Chemistry and Biochemistry, University
of California, Los Angeles, Los
Angeles, California 90095, United States
| | - Kenji Watanabe
- National
Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
| | - Takashi Taniguchi
- National
Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
| | - Marcel Tencé
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Jean-Denis Blazit
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Xiaoyan Li
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Alexandre Gloter
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Alberto Zobelli
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | | | - Luis M. Liz-Marzán
- CIC
biomaGUNE and Ciber-BBN, Paseo de Miramón 182, 20014 Donostia-San Sebastián, Spain
- Ikerbasque,
Basque Foundation for Science, 48013 Bilbao, Spain
| | - F. Javier García de Abajo
- The
Barcelona Institute of Science and Technology, ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels (Barcelona), Spain
- ICREA-Institució
Catalana de Recerca i Estudis Avanats, Passeig Lluís Companys 23, 08010 Barcelona, Spain
| | - Odile Stéphan
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
| | - Mathieu Kociak
- Laboratoire
de Physique des Solides, Université
Paris-Saclay, CNRS, 91405, Orsay, France
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52
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Wallum A, Nguyen HA, Gruebele M. Excited-State Imaging of Single Particles on the Subnanometer Scale. Annu Rev Phys Chem 2020; 71:415-433. [DOI: 10.1146/annurev-physchem-071119-040108] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials.
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Affiliation(s)
- Alison Wallum
- Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
| | - Huy A. Nguyen
- Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
| | - Martin Gruebele
- Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
- Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
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53
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Hage FS, Radtke G, Kepaptsoglou DM, Lazzeri M, Ramasse QM. Single-atom vibrational spectroscopy in the scanning transmission electron microscope. Science 2020; 367:1124-1127. [PMID: 32139541 DOI: 10.1126/science.aba1136] [Citation(s) in RCA: 74] [Impact Index Per Article: 18.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2019] [Accepted: 02/03/2020] [Indexed: 11/02/2022]
Abstract
Single-atom impurities and other atomic-scale defects can notably alter the local vibrational responses of solids and, ultimately, their macroscopic properties. Using high-resolution electron energy-loss spectroscopy in the electron microscope, we show that a single substitutional silicon impurity in graphene induces a characteristic, localized modification of the vibrational response. Extensive ab initio calculations reveal that the measured spectroscopic signature arises from defect-induced pseudo-localized phonon modes-that is, resonant states resulting from the hybridization of the defect modes and the bulk continuum-with energies that can be directly matched to the experiments. This finding realizes the promise of vibrational spectroscopy in the electron microscope with single-atom sensitivity and has broad implications across the fields of physics, chemistry, and materials science.
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Affiliation(s)
- F S Hage
- SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, UK
| | - G Radtke
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, 75005 Paris, France.
| | - D M Kepaptsoglou
- SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, UK.,York Nanocentre and Department of Physics, University of York, Heslington, York YO10 5DD, UK
| | - M Lazzeri
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, 75005 Paris, France
| | - Q M Ramasse
- SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, UK. .,School of Chemical and Process Engineering and School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
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54
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Hong J, Senga R, Pichler T, Suenaga K. Probing Exciton Dispersions of Freestanding Monolayer WSe_{2} by Momentum-Resolved Electron Energy-Loss Spectroscopy. PHYSICAL REVIEW LETTERS 2020; 124:087401. [PMID: 32167311 DOI: 10.1103/physrevlett.124.087401] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/16/2019] [Accepted: 01/27/2020] [Indexed: 06/10/2023]
Abstract
Excitons, as bound electron-hole paired quasiparticle, play an essential role in the energy transport in the optical-electric properties of semiconductors. Their momentum-energy dispersion relation is a fundamental physical property of great significance to understand exciton dynamics. However, this dispersion is seldom explored especially in two-dimensional transition metal dichalcogenides with rich valleytronic properties. In this work, momentum resolved electron energy-loss spectroscopy was used to measure the dispersions of excitons in freestanding monolayer WSe_{2}. Besides the parabolically dispersed valley excitons, a subgap dispersive exciton was observed at nonzero momenta for the first time, which can be introduced by the prolific Se vacancies. Our work provides a paradigm to directly probe exciton dispersions in 2D semiconductors and could be generalized to many low-dimensional systems.
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Affiliation(s)
- Jinhua Hong
- Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
| | - Ryosuke Senga
- Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
| | - Thomas Pichler
- Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria
| | - Kazu Suenaga
- Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
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55
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Collins SM, Kepaptsoglou DM, Hou J, Ashling CW, Radtke G, Bennett TD, Midgley PA, Ramasse QM. Functional Group Mapping by Electron Beam Vibrational Spectroscopy from Nanoscale Volumes. NANO LETTERS 2020; 20:1272-1279. [PMID: 31944111 DOI: 10.1021/acs.nanolett.9b04732] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Vibrational spectroscopies directly record details of bonding in materials, but spatially resolved methods have been limited to surface techniques for mapping functional groups at the nanoscale. Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope presents a route to functional group analysis from nanoscale volumes using transmitted subnanometer electron probes. Here, we now use vibrational EELS to map distinct carboxylate and imidazolate linkers in a metal-organic framework (MOF) crystal-glass composite material. Domains <100 nm in size are observed using vibrational EELS, with recorded spatial resolution <15 nm at interfaces in the composite. This nanoscale functional group mapping is confirmed by correlated EELS at core ionization edges as well as X-ray energy dispersive spectroscopy for elemental mapping of the metal centers of the two constituent MOFs. These results present a complete nanoscale analysis of the building blocks of the MOF composite and establish spatially resolved functional group analysis using electron beam spectroscopy for crystalline and amorphous organic and metal-organic solids.
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Affiliation(s)
- Sean M Collins
- Department of Materials Science and Metallurgy , University of Cambridge , 27 Charles Babbage Road , Cambridge CB3 0FS , United Kingdom
| | - Demie M Kepaptsoglou
- SuperSTEM Laboratory , SciTech Daresbury Campus , Daresbury WA4 4AD , United Kingdom
- Department of Physics , University of York , Heslington, York YO10 5DD , United Kingdom
| | - Jingwei Hou
- Department of Materials Science and Metallurgy , University of Cambridge , 27 Charles Babbage Road , Cambridge CB3 0FS , United Kingdom
| | - Christopher W Ashling
- Department of Materials Science and Metallurgy , University of Cambridge , 27 Charles Babbage Road , Cambridge CB3 0FS , United Kingdom
| | - Guillaume Radtke
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC , 75005 Paris , France
| | - Thomas D Bennett
- Department of Materials Science and Metallurgy , University of Cambridge , 27 Charles Babbage Road , Cambridge CB3 0FS , United Kingdom
| | - Paul A Midgley
- Department of Materials Science and Metallurgy , University of Cambridge , 27 Charles Babbage Road , Cambridge CB3 0FS , United Kingdom
| | - Quentin M Ramasse
- SuperSTEM Laboratory , SciTech Daresbury Campus , Daresbury WA4 4AD , United Kingdom
- School of Chemical and Process Engineering and School of Physics , University of Leeds , Leeds LS2 9JT , United Kingdom
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56
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Zeiger PM, Rusz J. Efficient and Versatile Model for Vibrational STEM-EELS. PHYSICAL REVIEW LETTERS 2020; 124:025501. [PMID: 32004041 DOI: 10.1103/physrevlett.124.025501] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/11/2019] [Revised: 11/18/2019] [Indexed: 06/10/2023]
Abstract
We introduce a novel method for the simulation of the impact scattering in vibrational scanning transmission electron microscopy electron energy loss spectroscopy simulations. The phonon-loss process is modeled by a combination of molecular dynamics and elastic multislice calculations within a modified frozen phonon approximation. The key idea is thereby to use a so-called δ thermostat in the classical molecular dynamics simulation to generate frequency dependent configurations of the vibrating specimen's atomic structure. The method includes correlated motion of atoms and provides vibrational spectrum images at a cost comparable to standard frozen phonon calculations. We demonstrate good agreement of our method with simulations and experiments for a 15 nm flake of hexagonal boron nitride.
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Affiliation(s)
- Paul M Zeiger
- Department of Physics and Astronomy, Uppsala University, P.O. Box 516, Uppsala 75120, Sweden
| | - Ján Rusz
- Department of Physics and Astronomy, Uppsala University, P.O. Box 516, Uppsala 75120, Sweden
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57
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Radtke G, Taverna D, Menguy N, Pandolfi S, Courac A, Le Godec Y, Krivanek OL, Lovejoy TC. Polarization Selectivity in Vibrational Electron-Energy-Loss Spectroscopy. PHYSICAL REVIEW LETTERS 2019; 123:256001. [PMID: 31922788 DOI: 10.1103/physrevlett.123.256001] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/19/2019] [Indexed: 06/10/2023]
Abstract
Orientation-dependent aloof-beam vibrational electron-energy-loss spectroscopy is carried out on uniaxial icosahedral B_{12}P_{2} submicron crystals. We demonstrate that the high sensitivity of the signal to the crystal orientation allows for an unambiguous determination of the symmetry of normal modes occurring at the Brillouin zone center of this anisotropic compound. The experimental results are assessed using first-principles quantum mechanical calculations (density functional theory) of the dielectric response of the specimen. The high spatial resolution inherent to this technique when implemented in the transmission electron microscope thus opens the door to nanoscale orientation-dependent vibrational spectroscopy.
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Affiliation(s)
- G Radtke
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
| | - D Taverna
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
| | - N Menguy
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
| | - S Pandolfi
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
| | - A Courac
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
| | - Y Le Godec
- Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 75005 Paris, France
| | - O L Krivanek
- Nion Co., 11511 NE 118th Street, Kirkland, Washington 98034, USA
| | - T C Lovejoy
- Nion Co., 11511 NE 118th Street, Kirkland, Washington 98034, USA
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58
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Polman A, Kociak M, García de Abajo FJ. Electron-beam spectroscopy for nanophotonics. NATURE MATERIALS 2019; 18:1158-1171. [PMID: 31308514 DOI: 10.1038/s41563-019-0409-1] [Citation(s) in RCA: 98] [Impact Index Per Article: 19.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/28/2018] [Revised: 05/04/2019] [Accepted: 05/14/2019] [Indexed: 05/22/2023]
Abstract
Progress in electron-beam spectroscopies has recently enabled the study of optical excitations with combined space, energy and time resolution in the nanometre, millielectronvolt and femtosecond domain, thus providing unique access into nanophotonic structures and their detailed optical responses. These techniques rely on ~1-300 keV electron beams focused at the sample down to sub-nanometre spots, temporally compressed in wavepackets a few femtoseconds long, and in some cases controlled by ultrafast light pulses. The electrons undergo energy losses and gains (also giving rise to cathodoluminescence light emission), which are recorded to reveal the optical landscape along the beam path. This Review portraits these advances, with a focus on coherent excitations, emphasizing the increasing level of control over the electron wavefunctions and ensuing applications in the study and technological use of optically resonant modes and polaritons in nanoparticles, 2D materials and engineered nanostructures.
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Affiliation(s)
- Albert Polman
- Center for Nanophotonics, AMOLF, Amsterdam, the Netherlands.
| | - Mathieu Kociak
- Laboratoire de Physique des Solides, Université de Paris-Sud, Orsay, France
| | - F Javier García de Abajo
- ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels (Barcelona), Spain
- ICREA-Institució Catalana de Reserca I Estudis Avançats, Barcelona, Spain
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59
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Zachman MJ, Hachtel JA, Idrobo JC, Chi M. Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials. Angew Chem Int Ed Engl 2019. [DOI: 10.1002/ange.201902993] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Affiliation(s)
- Michael J. Zachman
- Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge TN 37831 USA
| | - Jordan A. Hachtel
- Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge TN 37831 USA
| | - Juan Carlos Idrobo
- Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge TN 37831 USA
| | - Miaofang Chi
- Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge TN 37831 USA
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60
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Zachman MJ, Hachtel JA, Idrobo JC, Chi M. Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials. Angew Chem Int Ed Engl 2019; 59:1384-1396. [PMID: 31081976 DOI: 10.1002/anie.201902993] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2019] [Revised: 05/01/2019] [Indexed: 11/10/2022]
Abstract
Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.
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Affiliation(s)
- Michael J Zachman
- Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
| | - Jordan A Hachtel
- Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
| | - Juan Carlos Idrobo
- Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
| | - Miaofang Chi
- Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
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61
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Yan X, Liu C, Gadre CA, Dai S, Gu L, Yu K, Aoki T, Wu R, Pan X. Unexpected Strong Thermally Induced Phonon Energy Shift for Mapping Local Temperature. NANO LETTERS 2019; 19:7494-7502. [PMID: 31517496 DOI: 10.1021/acs.nanolett.9b03307] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Measuring temperature in nanoscale is crucial for the research and development of microelectronic devices. Plasmon resonance has been utilized to map local temperature gradient in metallic materials (Al) due to their large coefficients of thermal expansion. However, most semiconductors (including Si and SiC) possess much smaller coefficients of thermal expansion due to their strong covalent bonding in crystal structure, for which the plasmon-based temperature measurement becomes unreliable. Here, we report an unexpected strong, thermally induced phonon energy shift in SiC by spatially resolved vibrational spectroscopy in transmission electron microscopy with in situ heating, demonstrating that this shift can be applied as a useful tool for measuring nanoscale temperature. When a bulk phonon spectrum is used, the spatial resolution of vibrational spectroscopy can be as high as one nanometer. Molecular dynamics simulations reveal that lattice expansion only contributes a small fraction of phonon energy shift and that vibrant motions away from the bonds are predominate factors. This study gains deeper insight into the understanding of dynamic behaviors of the phonon and provides a new avenue to measure local temperature in nanodevices.
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62
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Position and momentum mapping of vibrations in graphene nanostructures. Nature 2019; 573:247-250. [PMID: 31406319 DOI: 10.1038/s41586-019-1477-8] [Citation(s) in RCA: 66] [Impact Index Per Article: 13.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2018] [Accepted: 06/03/2019] [Indexed: 11/08/2022]
Abstract
Propagating atomic vibrational waves-phonons-determine important thermal, mechanical, optoelectronic and transport characteristics of materials. Thus a knowledge of phonon dispersion (that is, the dependence of vibrational energy on momentum) is a key part of our understanding and optimization of a material's behaviour. However, the phonon dispersion of a free-standing monolayer of a two-dimensional material such as graphene, and its local variations, have remained elusive for the past decade because of the experimental limitations of vibrational spectroscopy. Even though electron energy loss spectroscopy (EELS) in transmission has recently been shown to probe local vibrational charge responses1-4, such studies are still limited by momentum space integration due to the focused beam geometry; they are also restricted to polar materials such as boron nitride or oxides1-4, 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 spectroscopy5-7 or EELS in reflection8,9 do not have any spatial resolution and require large microcrystals. Here we provide a new pathway to determine phonon dispersions down to the scale of an individual free-standing graphene monolayer by mapping the distinct vibrational modes for a large momentum transfer. The measured scattering intensities are accurately reproduced and interpreted with density functional perturbation theory10. Additionally, a nanometre-scale mapping of selected momentum-resolved vibrational modes using graphene nanoribbon structures has enabled us to spatially disentangle bulk, edge and surface vibrations. Our results are a proof-of-principle demonstration of the feasibility of studying local vibrational modes in two-dimensional monolayer materials at the nanometre scale.
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