[Show abstract][Hide abstract] ABSTRACT: We consider theoretically the energy loss of electrons scattered to high angles when assuming that the primary beam can be limited to a single atom. We discuss the possibility of identifying the isotopes of light elements and of extracting information about phonons in this signal. The energy loss is related to the mass of the much heavier nucleus, and is spread out due to atomic vibrations. Importantly, while the width of the broadening is much larger than the energy separation of isotopes, only the shift in the peak positions must be detected if the beam is limited to a single atom. We conclude that the experimental case will be challenging but is not excluded by the physical principles as far as considered here. Moreover, the initial experiments demonstrate that the separation of gold and carbon based on a signal that is related to their mass, rather than their atomic number.
[Show abstract][Hide abstract] ABSTRACT: Beam-induced structural modifications are a major nuisance in the study of materials by high-resolution electron microscopy. Here, we introduce a new approach to circumvent the radiation damage problem by a statistical treatment of large, noisy, low-dose data sets of non-periodic configurations (e.g. defects) in the material. We distribute the dose over a mixture of different defect structures at random positions and with random orientations, and recover representative model images via a maximum likelihood search. We demonstrate reconstructions from simulated images at such low doses that the location of individual entities is not possible. The approach may open a route to study currently inaccessible beam-sensitive configurations.
[Show abstract][Hide abstract] ABSTRACT: In low-dimensional systems, a detailed understanding of plasmons and their dispersion relation is crucial for applying their optical response in the field of plasmonics. Electron energy-loss spectroscopy is a direct probe of these excitations. Here we report on electron energy-loss spectroscopy results on the dispersion of the π plasmons in free-standing graphene monolayers at the momentum range of 0⩽|q|⩽0.5 Å−1 and parallel to the Γ-M direction of the graphene Brillouin zone. In contrast to the parabolic dispersion in graphite and in good agreement with theoretical predictions of a 2D electron gas of Dirac electrons, linear π plasmon dispersion is observed. As with previous EELS results obtained from single-wall carbon nanotubes, this can be explained by local-field effects in the anisotropic 2D system yielding a significant contribution of the low-energy band structure on the high-energy π plasmon response.
[Show abstract][Hide abstract] ABSTRACT: The dependence of high-resolution transmission electron microscopy (HRTEM) image contrast of graphene on the adjustable parameters of an aberration-corrected microscope operated at 80 and 20 kV has been calculated and, for 80 kV, compared with measurements. We used density functional theory to determine the projected atom potential and obtained the image intensity by averaging over the energy distribution of the imaging electrons, as derived from the electron energy loss spectroscopy measurements. Optimum image contrast has been determined as a function of energy spread of the imaging electrons and chromatic aberration coefficient, showing that significant improvement of contrast can be achieved at 80 kV with the help of a monochromator, however at 20 kV only with chromatic aberration correction and bright atom contrast conditions.
[Show abstract][Hide abstract] ABSTRACT: The electron optical performance of a transmission electron microscope (TEM) is characterized for direct spatial imaging and spectroscopy using electrons with energies as low as 20 keV. The highly stable instrument is equipped with an electrostatic monochromator and a C(S)-corrector. At 20 kV it shows high image contrast even for single-layer graphene with a lattice transfer of 213 pm (tilted illumination). For 4 nm thick Si, the 200 reflections (271.5 pm) were directly transferred (axial illumination). We show at 20 kV that radiation-sensitive fullerenes (C(60)) within a carbon nanotube container withstand an about two orders of magnitude higher electron dose than at 80 kV. In spectroscopy mode, the monochromated low-energy electron beam enables the acquisition of EELS spectra up to very high energy losses with exceptionally low background noise. Using Si and Ge, we show that 20 kV TEM allows the determination of dielectric properties and narrow band gaps, which were not accessible by TEM so far. These very first results demonstrate that low kV TEM is an exciting new tool for determination of structural and electronic properties of different types of nano-materials.
[Show abstract][Hide abstract] ABSTRACT: Observations of topological defects associated with Stone-Wales-type
transformations (i.e., bond rotations) in high resolution transmission electron
microscopy (HRTEM) images of carbon nanostructures are at odds with the
equilibrium thermodynamics of these systems. Here, by combining
aberration-corrected HRTEM experiments and atomistic simulations, we show that
such defects can be formed by single electron impacts, and remarkably, at
electron energies below the threshold for atomic displacements. We further
study the mechanisms of irradiation-driven bond rotations, and explain why
electron irradiation at moderate electron energies (\sim100 keV) tends to
amorphize rather than perforate graphene. We also show via simulations that
Stone-Wales defects can appear in curved graphitic structures due to incomplete
recombination of irradiation-induced Frenkel defects, similar to formation of
Wigner-type defects in silicon.
[Show abstract][Hide abstract] ABSTRACT: While crystalline two-dimensional materials have become an experimental reality during the past few years, an amorphous 2D material has not been reported before. Here, using electron irradiation we create an sp2-hybridized one-atom-thick flat carbon membrane with a random arrangement of polygons, including four-membered carbon rings. We show how the transformation occurs step by step by nucleation and growth of low-energy multivacancy structures constructed of rotated hexagons and other polygons. Our observations, along with first-principles calculations, provide new insights to the bonding behavior of carbon and dynamics of defects in graphene. The created domains possess a band gap, which may open new possibilities for engineering graphene-based electronic devices.
[Show abstract][Hide abstract] ABSTRACT: We have performed a near-edge X-ray absorption fine-structure (NEXAFS) and a transmission electron microscopy (TEM) investigation of freely suspended graphene oxide (GO) sheets. We utilized a photoemission electron microscope to identify GO membranes and to acquire C K and O K absorption spectra. The overall line shape of C K-edge spectra demonstrates that the honeycomb carbon network of graphene is the scaffold of GO. However, the intensity ratio of π∗ and σ∗ resonances, and a broad feature at about 20eV from the edge, indicate the presence of new carbon bonds. The O K-edge spectra show that oxidized regions are made of carbonyl, epoxide, and hydroxyl groups attached to the plane of graphene, while carboxyl groups might also be present at the edges. Further, our study indicates the presence of ordered arrangements of oxygen atoms in GO sheets. Our investigation provides a new and efficient route to study the electronic structure of suspended membranes.
[Show abstract][Hide abstract] ABSTRACT: Specially designed transmission electron microscopy (TEM) sample carriers have been developed to enable atomically resolved studies of the heat-induced evolution of adsorbates on graphene and their influence on electrical conductivity. Here, we present a strategy for graphene-based carrier realization, evaluating its design with respect to fabrication effort and applications potential. We demonstrate that electrical current can lead to very high temperatures in suspended graphene membranes, and we determine that current-induced cleaning of graphene results from Joule heating.
Journal of Physics D Applied Physics 01/2011; 44(5):055502. DOI:10.1088/0022-3727/44/5/055502 · 2.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: From combined Raman spectroscopy and electron diffraction studies on several freestanding single-walled carbon nanotubes (SWNTs), we define Raman criteria which correlate the main features of the Raman spectrum (radial breathing mode and G modes) and the optical transition energies with the structure of the SWNT under investigation. On this basis, we discuss the possibilities to determine the (n,m) indices of an individual SWNT from a single wavelength Raman experiment. We show the efficiency of this approach in assigning the (n,m) structure of different individual nanotubes including all types of achiral SWNTs. Finally, the limits and the accuracy of the method are discussed.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t We report the polarization-dependent C K photoabsorption spectra of single-and few-layer graphene (FLG) samples produced by micromechanical cleavage of highly ordered pyrolytic graphite (HOPG) on a SiO 2 substrate. We show that the unoccupied r density of states of graphene and FLG strongly reflects the one measured on bulk HOPG, demonstrating the two-dimensional character of r states as well as the very-weak interlayer couplings between planes. Moreover, our spectra taken with different polarization allow us to show the predicted hybrid nature of the interlayer state.
[Show abstract][Hide abstract] ABSTRACT: We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single-layer graphene we observe a splitting of the pi resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.
[Show abstract][Hide abstract] ABSTRACT: We describe the synthesis of very thin sheets (between a few and ten atomic layers) of hexagonal boron nitride (h-BN), prepared either on a SiO2 substrate or freely suspended. Optical microscopy, atomic force microscopy, and transmission electron microscopy have been used to characterize the morphology of the samples and to distinguish between regions of different thicknesses. Comparison is made to previous studies on single- and few-layer graphene. This synthesis opens the door to experimentally accessing the two-dimensional phase of boron nitride.
[Show abstract][Hide abstract] ABSTRACT: Graphene is a single atomic layer of graphite that has only recently become experimentally accessible in an isolated form
. We prepare graphene into freestanding membranes [2,3], i.e., a crystalline foil with a thickness of only one carbon atom.
These membranes are highly promising for TEM studies, since (a) this membrane itself is of tremendous scientific interest,
(b) adsorbates can be studied against a highly transparent and crystalline background, and (c) the precisely known structure
is an ideal tuning and calibration tool for electron microscopy developments. Graphene membranes are very stable at low acceleration
voltages, we and present results obtained on a conventional TEM (Jeol 2010 at 100kV) as well as initial tests on the new aberration-corrected
TEAM 0.5 microscope operated at its lower limit of 80kV. The TEAM 0.5 microscope achieves sub-Angstrom resolution even at
80kV, thus resolving every single carbon atom in the graphene lattice.