[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: Transmission electron microscopy has witnessed rampant development and
surging point resolution over the past few years. The improved imaging
performance of modern electron microscopes shifts the bottleneck for image
contrast and resolution to sample preparation. Hence, it is increasingly being
realized that the full potential of electron microscopy will only be realized
with the optimization of current sample preparation techniques. Perhaps the
most recognized issues are background signal and noise contributed by sample
supports, sample charging and instability. Graphene provides supports of single
atom thickness, extreme physical stability, periodic structure, and ballistic
electrical conductivity. As an increasing number of applications adapting
graphene to their benefit emerge, we discuss the unique capabilities afforded
by the use of graphene as a sample support for electron microscopy.
Solid State Communications 04/2012; 152(15). · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an accurate measurement and a quantitative analysis of
electron-beam induced displacements of carbon atoms in single-layer graphene.
We directly measure the atomic displacement ("knock-on") cross section by
counting the lost atoms as a function of the electron beam energy and applied
dose. Further, we separate knock-on damage (originating from the collision of
the beam electrons with the nucleus of the target atom) from other radiation
damage mechanisms (e.g. ionization damage or chemical etching) by the
comparison of ordinary (12C) and heavy (13C) graphene. Our analysis shows that
a static lattice approximation is not sufficient to describe knock-on damage in
this material, while a very good agreement between calculated and experimental
cross sections is obtained if lattice vibrations are taken into account.
[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: 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. · 2.53 Impact Factor
[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: A micro-Raman investigation was carried out on several flakes of monolayer (1 L) graphene obtained by the micro-mechanical exfoliation technique and, then, put on a c-Si wafer coated by a SiO(2) thin film. Some of the 1 L zones show a remarkable dispersion of the 2D-overtone wavenumber as a function of the position within the graphene sheet, and, in some case, a significant broadening of the E(2g) phonon (G band) is associated to this wavenumber shift of 2D-band. Such effects were studied, in particular, for a 1 L zone characterized by a rather strong lattice disorder, as revealed by the strong D/G band intensity ratio, and for other zones quite ordered, showing a vanishing intensity of the D band. Moreover, by moving along different directions within 1 L graphene sheets, different trends for 20 wavenumber and E(2g) phonon bandwidth vs. position were observed. All these reported behaviours are explained in terms of different distributions of intrinsic uniaxial strain occurring within the 1 L graphene sheets. (C) 2010 Elsevier B.V. All rights reserved.
Diamond and Related Materials 01/2010; 19(5-6):608-613. · 1.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an atomic-resolution observation and analysis of graphene constrictions and ribbons with sub-nanometer width. Graphene membranes are studied by imaging side spherical aberration-corrected transmission electron microscopy at 80 kV. Holes are formed in the honeycomb-like structure due to radiation damage. As the holes grow and two holes approach each other, the hexagonal structure that lies between them narrows down. Transitions and deviations from the hexagonal structure in this graphene ribbon occur as its width shrinks below one nanometer. Some reconstructions, involving more pentagons and heptagons than hexagons, turn out to be surprisingly stable. Finally, single carbon atom chain bridges between graphene contacts are observed. The dynamics are observed in real time at atomic resolution with enough sensitivity to detect every carbon atom that remains stable for a sufficient amount of time. The carbon chains appear reproducibly and in various configurations from graphene bridges, between adsorbates, or at open edges and seem to represent one of the most stable configurations that a few-atomic carbon system accomodates in the presence of continuous energy input from the electron beam. Comment: 12 pages, 4 figures
New Journal of Physics 05/2009; · 4.06 Impact Factor
[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.
Chemical Physics Letters 04/2009; 475(s 4–6). · 2.15 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: 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 present a detailed transmission electron microscopy and electron diffraction study of the thinnest possible membrane, a single layer of carbon atoms suspended in vacuum and attached only at its edges. Membranes consisting of two graphene layers are also reported. We find that the membranes exhibit random microscopic curvature that is strongest in single-layer membranes. A direct visualization of the roughness is presented for two-layer membranes where we used the variation of diffracted intensities with the local orientation of the membrane.
Solid State Communications 04/2007; · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By combining, on the same freestanding single-walled carbon nanotubes,
electron diffraction and Raman experiments, we were able to obtain the
resonance energy of unambiguously (n,m) -identified single-walled
carbon nanotubes. We focus on the analysis of the first optical
transition of metallic tubes (E11M) and the
third and fourth transitions of semiconducting tubes (
E33S and E44S ,
respectively) in comparison with calculated values using a nonorthogonal
tight-binding approach. For semiconducting tubes, we find that the
calculated energies E33S and
E44S have to be corrected by
non-diameter-dependent (rigid) shifts of about 0.43eV and 0.44eV ,
respectively, for tubes in the 1.4 2.4-nm -diameter range. For metallic
tubes in the 1.2 1.7-nm -diameter range, we show that a rigid shift
(0.32eV) of the calculated transition energy also leads to a good
estimation of E11M . The rather large and
non-diameter-dependent shifts for the third and fourth transitions in
semiconducting tubes question a recent theoretical study, which relates
the shifts to electron-electron correlation and exciton binding energy
and suggest that the exciton binding is very small or missing for the
higher transitions E33S and
E44S , contrary to the lower transitions
E11S and E22S .
Physical Review B 03/2007; 75(15):155432. · 3.66 Impact Factor