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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Structural and chemical bonding changes in nuclear graphite have been investigated during in-situ electron irradiation in a transmission electron microscope (TEM); electron beam irradiation has been employed as a surrogate for neutron irradiation of nuclear grade graphite in nuclear reactors. This paper aims to set out a methodology for analysing the microstructure of electron-irradiated graphite which can then be extended to the analysis of neutron-irradiated graphites. The damage produced by exposure to 200 keV electrons was examined up to a total dose of approximately 0.5 dpa (equivalent to an electron fluence of 5.6x 10 21 electrons cm -2 ). During electron exposure, high resolution TEM images and electron energy loss spectra (EELS) were acquired periodically in order to record changes in structural (dis)order and chemical bonding, by quantitatively analysing the variation in phase contrast images and EEL spectra.

We have measured the binding energy of the image-potential states on Cu(100) and Ag(100) surfaces with two-photon photoemission spectroscopy. We find E& --0.57 (0.18)+0.02 eV for Cu (100) and 0.53 (0.16)+0.02 eV for Ag(100) for the n =1 (n=2) states, respectively. These values are compared with the nearly hydrogenic binding energies of 0.77 -0.83 eV obtained for the (111)surfaces of Cu, Ag, and Ni using the same method. The comparison shows that the binding energy does not depend on the material as long as the surface structure remains constant but changes markedly with the crystal orientation.

what like structure-factor amplitudes from smallmolecule crystals, and estimation of their unknown phases was successfully carried out by statistical direct methods. Reflections to 18 ,~ resolution, which obey rather well the symmetry of space group P321, were merged to produce an asymmetric unit in that space group. I Ghl values for the 34 strongest of these were phased using the small-molecule direct-methods package MITHRIL [Gilmore (1984). J. Appl Cryst. 17,[42][43][44][45][46]. The best phase set was expanded back to the P2~ lattice and negative density was truncated to generate initial phases for all reflections to 18 © 1990 International Union of Crystallography DIRECT PHASE DETERMINATION FOR A MACROMOLECULAR ENVELOPEresolution. Phase refinement by iterative imposition of the local 32 symmetry produced an envelope with convincing features consistent with known properties of the enzyme. The envelope implies that the tryptophanyl-tRNA synthetase dimer is an elongated structure with an axial ratio of about 4: 1, in which the monomers have two distinct domains of unequal size. The smaller of these occurs at the dimer interface, and resembles the nucleotide binding portion of the tyrosyl-tRNA synthetase. It may therefore contain the amino-terminal one hundred or so residues, including all three cysteines, previously suggested to comprise a nucleotide-binding domain in the tryptophanyl enzyme. A purely crystallographic test of the overall features of this envelope was carried out by transporting it to a tetragonal crystal form of the same protein in which the asymmetric unit is a monomer. The small domain fits snugly inside three mercury and one gold heavy-atom binding sites for this crystal form; and symmetry-related molecules provide excellent, but very different, lattice contacts in nearly all directions. IntroductionBacillus stearothermophilus tryptophanyl-tRNA synthetase is a dimer of identical 37 000 dalton subunits (Fersht, Ashford, Bruton, Jakes, Koch & Hartley, 1975). It crystallizes in several different space groups, depending only on the bound ligands (Carter & Carter, 1979;. This crystal polymorphism and complementary fluorescence experiments (Andrews et al., 1985;Merle et al., 1986) suggest that the different crystal forms represent important conformational differences related to catalysis . This very conformational flexibility has been disadvantageous because heavy-atom binding tends to destroy the isomorphism between parent and derivatized crystals, and it has not yet been possible to phase structure factors for any of the crystal forms by isomorphous replacement. Crystal form type II*, grown in the presence of tryptophan and belonging to space group P21, has three enzyme dimers in the asymmetric unit. Evidence for trigonal space group P321 at low resolution suggests that these three dimers are arranged about a local threefold axis nearly parallel to the crystallographic c axis, with molecular dyads along local twofold directions such that the local 32 point group symmetry of the asymmetric uni...

Both photons and electrons may be used to excite surface plasmon polaritons, the collective charge density fluctuations at the surface of metal nanostructures. By virtue of their nanoscopic and dissipative nature, a detailed characterization of surface plasmon (SP) eigenmodes in real space-time ultimately requires joint nanometer spatial and femtosecond temporal resolution.The latter realization has driven significant developments in the past few years, aimed at interrogating both localized and propagating SP modes. In this mini-review, we briefly highlight different techniques employed by our own groups to visualize the enhanced electric fields associated with SPs. Specifically, we discuss recent hyperspectral optical microscopy, tipenhanced Raman nano-spectroscopy, nonlinear photoemission electron microscopy, as well as correlated scanning transmission electron microscopy-electron energy loss spectroscopy measurements targeting prototypical plasmonic nanostructures and constructs. Through selected practical examples from our own laboratories, we examine the information content in multidimensional images recorded by taking advantage of each of the aforementioned techniques. In effect, we illustrate how SPs can be visualized at the ultimate limits of space and time.

The ability to probe the electronic structure of individual nano-objects at high energy resolution using momentum-and space-resolved electron energy loss spectroscopy in the scanning transmission electron microscope is demonstrated through the observation of confinement of the π plasmon in individual single-wall carbon nanotubes. While confinement perpendicular to the tube axis was identified for all investigated tubes, a variable degree of confinement parallel to the tube axis was attributed to the concentration of topological defects. Spatially resolved valence loss spectra allowed for the identification of a loss peak attributed to a chirality-dependent radial interband transition. Furthermore, the importance of a careful consideration of loss peak momentum dispersions for the interpretation of spatially resolved valence loss spectra is discussed.