Dirk Obergfell

Observations of Chemical Reactions at the Atomic Scale: Dynamics of Metal-Mediated Fullerene Coalescence and Nanotube Rupture**

the chemicalreactivity of their interior has been considered to be very low.TEM is the only method that allows direct visualizationandstudyofthemoleculesinsidenanotubes.Wearecurrentlyexploiting the capabilities of an aberration-corrected TEM,which can record atomic resolution images in a time muchshorter than needed for the electron beam to promotestructural transformations inside SWNTs. Under our imagingconditions,thesetransformationsusuallyoccuronatimescaleof seconds, which enables us to capture atomic images of theintermediates. These images can then be combined into amovie to follow the chemical transformations as they happen.Both walls and interiors of SWNTs can be seen on TEMmicrographs, such that the positions and orientations of theencapsulated molecules can be readily determined from theimages (Figure 1c). Accelerated electrons interact with thespecimen and transfer their energy and momentum to theatoms of the specimen, causing knock-on damage, ionization,and/or heat-induced damage, the extent of which depends onthe nature of the material. With this in mind, we carried outour TEM studies with an accelerating voltage of only 80 kV,which is well below the threshold for knock-on damage incarbon nanotubes (86 kV).

Atomically resolved mechanical response of individual metallofullerene molecules confined inside carbon nanotubes

The hollow core inside a carbon nanotube can be used to confine single molecules and it is now possible to image the movement of such molecules inside nanotubes. To date, however, it has not been possible to control this motion, nor to detect the forces moving the molecules, despite experimental and theoretical evidence suggesting that almost friction-free motion might be possible inside the nanotubes. Here, we report on precise measurements of the mechanical responses of individual metallofullerene molecules (Dy@C82) confined inside single-walled carbon nanotubes to the atom at the tip of an atomic force microscope operated in dynamic mode. Using three-dimensional force mapping with atomic resolution, we addressed the molecules from the exterior of the nanotube and measured their elastic and inelastic behaviour by simultaneously detecting the attractive forces and energy losses with three-dimensional, atomic-scale resolution.

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.

Transmission electron microscopy and transistor characteristics of the same carbon nanotube

A technique is presented which allows one to combine TEM investigations with transport measurements and potentially a wide range of other investigations on the same nanoobject. Using this technique, we have obtained high-resolution transmission electron microscopy images and transport investigations including transfer characteristics on the same single-walled carbon nanotube. The transfer characteristics show ambipolar transport. This observation is discussed taking into account TEM information on tube diameter, number of tubes in the bundle, and possible tube filling with fullerenes (peapods).

Graphene−Metal Interface: Two-Terminal Resistance of Low-Mobility Graphene in High Magnetic Fields

The two-terminal magnetotransport of a single graphene layer was investigated up to a field of 55 T. The dependence of the electron transmission probability at the organo-metallic interface between the graphene and the metal electrodes was studied as a function of filling factor and electron density. A resistance-plateau spanning several tens of tesla width was observed. We argue that this plateau originates from an augmented sublattice spin-splitting due to the high surface-impurity concentration of the graphene layer. At electron densities close to the Dirac point, fingerprints of a thermally activated energy gap were observed.

Effects of Charge Impurities and Laser Energy on Raman Spectra of Graphene

The position and width of the Raman G-line was analyzed for unintentionally doped single-layered graphene samples. Results indicate a significant heating of the monolayer by the laser beam. Moreover, a weak additional component was resolved in the G-band. The position of the line is independent of the level of doping of the sample. We conclude that this new component is due to the phonons coupled to the intraband electronic transitions.

Atomic-resolution three-dimensional force and damping maps of carbon nanotube peapods

Atomic force microscopy (AFM) has become a versatile and powerful method for imaging both insulating and conducting objects down to the atomic scale. By extending the high spatial resolution and sensitivity of AFM to the force spectroscopy dimension, oscillations of individual molecules can be studied with atomic resolution. Using three-dimensional mapping of the force and damping fields we address individual Dy@C(82) metallofullerene molecules confined inside single-walled carbon nanotubes (so-called metallofullerene peapods) and reveal their oscillatory behaviour via attractive interactions with the AFM probe tip. The damping energy DeltaE signals, generated in very close proximity of the tip and nanotube peapod, show a close relationship with hysteresis in the short-range forces, thereby indicating that a soft vibrational (phonon) mode is site-specifically (i.e., atom-by-atom) induced by the AFM tip.

Application of 80kV Cs-corrected TEM for nanocarbon materials

Being one of the main methods for studying the atomic structure of carbon materials, high resolution TEM suffers from a serious flaw as the energy of electrons, necessary to obtain atomic resolution (200–300 keV), is far above the kick-off damage threshold for graphitic carbon (about 60 keV) [1], which means that carbon nanostructures are always observed with a structure more or less distorted by electron beam. Practical implementation of Cs correctors in the last decade [2] opened the possibility to decrease acceleration voltage of the microscopes significantly, yet preserving the resolution at reasonable level. Decreasing the energy of the electron beam, besides decreasing radiation damage, has the advantage of increasing the phase contrast [3]. Considering the possibility to visualize one single carbon atom, the increase of the contrast reduces quadratically the electron dose necessary for this.

Novel freestanding nanotube devices for combining TEM and electron diffraction with Raman and Transport

A versatile procedure for combining high‐resolution transmission electron microscopy (TEM) and electron diffraction with Raman spectroscopy and transport measurements on the very same nanotube is presented. For this we prepare free‐standing structures on the corner of a substrate by electron beam lithography and an etching process. Further, this procedure makes possible a TEM quality control of nanotubes grown directly on the substrate.

Freestanding nanostructures for TEM‐combined investigations of nanotubes

We present a method which allows to design almost arbitrary freestanding nanostructures by lithography in such a way that TEM investigations are possible in combination with various other measurements on the same carbon nanotube.