Viera Skákalová

Experimental analysis of charge redistribution due to chemical bonding by high-resolution transmission electron microscopy

The electronic charge density distribution or the electrostatic atomic potential of a solid or molecule contains information not only on the atomic structure, but also on the electronic properties, such as the nature of the chemical bonds or the degree of ionization of atoms. However, the redistribution of charge due to chemical bonding is small compared with the total charge density, and therefore difficult to measure. Here, we demonstrate an experimental analysis of charge redistribution due to chemical bonding by means of high-resolution transmission electron microscopy (HRTEM). We analyse charge transfer on the single-atom level for nitrogen-substitution point defects in graphene, and confirm the ionicity of single-layer hexagonal boron nitride. Our combination of HRTEM experiments and first-principles electronic structure calculations opens a new way to investigate electronic configurations of point defects, other non-periodic arrangements or nanoscale objects that cannot be studied by an electron or X-ray diffraction analysis.

Raman scattering at pure graphene zigzag edges.

Theory has predicted rich and very distinct physics for graphene devices with boundaries that follow either the armchair or the zigzag crystallographic directions. A prerequisite to disclose this physics in experiment is to be able to produce devices with boundaries of pure chirality. Exfoliated flakes frequently exhibit corners with an odd multiple of 30°, which raised expectations that their boundaries follow pure zigzag and armchair directions. The predicted Raman behavior at such crystallographic edges however failed to confirm pure edge chirality. Here, we perform confocal Raman spectroscopy on hexagonal holes obtained after the anisotropic etching of prepatterned pits using carbothermal decomposition of SiO(2). The boundaries of the hexagonal holes are aligned along the zigzag crystallographic direction and leave hardly any signature in the Raman map indicating unprecedented purity of the edge chirality. This work offers the first opportunity to experimentally confirm the validity of the Raman theory for graphene edges.

Electronic conduction in polymers, carbon nanotubes and graphene

In the years since the discovery of organic polymers that exhibited electrical conductivities comparable to some metals, other novel carbon-based conductors have been developed, including carbon nanotubes and graphene (monolayers of carbon atoms). In this critical review, we discuss the common features and the differences in the conduction mechanisms observed in these carbon-based materials, which range from near ballistic and conventional metallic conduction to fluctuation-assisted tunnelling, variable-range hopping and more exotic mechanisms. For each category of material, we discuss the dependence of conduction on the morphology of the sample. The presence of heterogeneous disorder is often particularly important in determining the overall behaviour, and can lead to surprisingly similar conduction behaviour in polymers, carbon nanotube networks and chemically-derived graphene (122 references).

Raman spectroscopy of single-wall carbon nanotubes and graphite irradiated by γ rays

Graphite and single-wall carbon nanotubes irradiated by γ rays of energy of 1.3 MeV were investigated by Raman spectroscopy. Irradiation generates defects in the lattice as confirmed by the increase of the intensity of the defect-induced D line in both materials. On the other hand, the intensity of the radial breathing mode of nanotubes is lowered. The intensity of the G line does not change for graphite but increases for carbon nanotubes. For the latter, this behavior cannot be explained by the defect-mediated double-resonance mechanism. Softening of the q=0 selection rule is suggested as a way to explain the results.

Correlation between resistance fluctuations and temperature dependence of conductivity in graphene

The weak temperature dependence of the resistance R(T) of monolayer graphene1-3 indicates an extraordinarily high intrinsic mobility of the charge carriers. Important complications are the presence of mobile scattering centres that strongly modify charge transport, and the presence of strong mesoscopic conductance fluctuations that, in graphene, persist to relatively high temperatures4,5. In this Letter, we investigate the surprisingly varied changes in resistance that we find in graphene flakes as temperature is lowered below 70 K. We propose that these changes in R(T) arise from the temperature dependence of the scattered electron wave interference that causes the resistance fluctuations. Using the field effect transistor configuration, we verify this explanation in detail from measurements of R(T) by tuning to different gate voltages corresponding to particular features of the resistance fluctuations. We propose simple expressions that model R(T) at both low and high charge carrier densities.

Effect Of Gamma‐Irradiation on Single‐Wall Carbon Nanotube Paper

The mechanical and electrical properties of a bulk material made of single wall carbon nanotubes (SWNT) are, due to weak intermolecular interaction, several orders of magnitude lower than those of the individual molecules themselves. We studied the effect of gamma‐irradiation on SWNT paper in air and under vacuum. For samples irradiated in air, changes in Young modulus and electrical conductivity were observed with maximum value for a dose of 170 kGy. Under vacuum there was only a small effect of irradiation. Raman studies of irradiated samples showed defects formation. Same experiments done with graphite showed similar results. A likely explanation of the results is that cross‐links between nanotubes were induced by irradiation in air.

Dynamic percolation of carbon nanotubes in liquid medium

In this article we have measured the electrical behaviour of carbon nanotubes (CNTs) suspended in electrically insulating liquid. The concentration dependence of conductivity shows a percolation behaviour similar to that observed in electrical composites with an insulating matrix. The value of the critical percolation concentration is strongly determined by the aspect ratio of the fillers forming the network through dynamic percolation. We characterized several single- and multi-wall carbon nanotube materials by the newly proposed method and received a good correlation with the results obtained by methods commonly used for CNT characterization (Raman spectroscopy, transmission electron microscopy and electrical conductivity of free standing papers). As a comparison, fine graphite material has also been evaluated. The electrical properties of the suspensions can be used as a method for CNT characterization. This method can yield important information for CNT producers and for the selection of electrically conducting structures for composites applications.

Shubnikov–de Haas and Aharonov Bohm effects in a graphene nanoring structure

We observed the Shubnikov–de Haas and the Aharonov Bohm oscillations in a graphene nanoring structure of 1 μm in diameter and with a 125 nm channel width. We found a separation of 2ΔVg=17.5 V between electron and hole Landau levels in the plot of longitudinal resistance as a function of gate voltage and magnetic field. This separation can be understood as a result of the transport gap, ΔVt=20 V, in the density of state. The Aharonov Bohm effect was observed in magnetoconductance with poor visibility because of the short phase coherence length of the graphene nanoring structure.

Effects of ion beam heating on Raman spectra of single-walled carbon nanotubes

Free standing films of single-wall carbon nanotubes were irradiated with energetic N+ and C4+ ions. The observed changes in the Raman line shape of the radial breathing mode and the G band of the C4+ irradiated samples were similar to those found for a thermally annealed sample. We ascribe these changes to thermal desorption of volatile dopants from the initially doped nanotubes. A simple geometry of the experiment allows us to estimate the temperature rise by one-dimensional heat conductance equation. The calculation indicates that irradiation-mediated increase in temperature may account for the observed Raman spectra changes.

Conformational transition in polypyrrole at low pressure

Abstract Electrical conductivity and infrared spectra of doped polypyrrole react to variations of pressure below atmospheric pressure, A glass transition, related to the variation of pressure, can be observed in the temperature dependence of the conductivity and infrared spectra. The existence of the glass transition supports the interpretation of the pressure effect as a conformational coil-rod transition.