Jannik C. Meyer

Raman Spectrum of Graphene and Graphene Layers

Graphene is the two-dimensional (2d) building block for carbon allotropes of every other dimensionality. It can be stacked into 3d graphite, rolled into 1d nanotubes, or wrapped into 0d fullerenes. Its recent discovery in free state has finally provided the possibility to study experimentally its electronic and phonon properties. Here we show that graphenes electronic structure is uniquely captured in its Raman spectrum that clearly evolves with increasing number of layers. Raman fingerprints for single-, bi- and few-layer graphene reflect changes in the electronic structure and electron-phonon interactions and allow unambiguous, high-throughput, non-destructive identification of graphene layers, which is critically lacking in this emerging research area.

The structure of suspended graphene sheets

The recent discovery of graphene has sparked much interest, thus far focused on the peculiar electronic structure of this material, in which charge carriers mimic massless relativistic particles. However, the physical structure of graphene—a single layer of carbon atoms densely packed in a honeycomb crystal lattice—is also puzzling. On the one hand, graphene appears to be a strictly two-dimensional material, exhibiting such a high crystal quality that electrons can travel submicrometre distances without scattering. On the other hand, perfect two-dimensional crystals cannot exist in the free state, according to both theory and experiment. This incompatibility can be avoided by arguing that all the graphene structures studied so far were an integral part of larger three-dimensional structures, either supported by a bulk substrate or embedded in a three-dimensional matrix. Here we report on individual graphene sheets freely suspended on a microfabricated scaffold in vacuum or air. These membranes are only one atom thick, yet they still display long-range crystalline order. However, our studies by transmission electron microscopy also reveal that these suspended graphene sheets are not perfectly flat: they exhibit intrinsic microscopic roughening such that the surface normal varies by several degrees and out-of-plane deformations reach 1 nm. The atomically thin single-crystal membranes offer ample scope for fundamental research and new technologies, whereas the observed corrugations in the third dimension may provide subtle reasons for the stability of two-dimensional crystals.

Raman Modes of Index-Identified Freestanding Single-Walled Carbon Nanotubes

Using electron diffraction on freestanding single-walled carbon nanotubes, we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3 nm. On the same freestanding tubes, we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7 nm), these measurements confirm previously established radial breathing mode frequency versus diameter relations and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.

Synthesis of individual single-walled carbon nanotube bridges controlled by support micromachining

Single-walled carbon nanotubes (SWNTs) were directly grown onto poly-crystalline silicon grids by catalytic thermal chemical vapour deposition. We demonstrate that simple micromachining of the catalyst-covered support can influence the number, location and alignment of suspended SWNTs. Sharp apexes formed by over-etching circular microstructures enable the scalable, cost-efficient formation of mostly individual, straight SWNT bridges, as verified by Raman scattering and electron diffraction.

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).

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.

Transport and TEM on the same individual carbon nanotubes and peapods

For the first time, full transistor characteristics — both output and transfer characteristics — in field‐effect transistor configuration have been recorded and TEM investigations have been performed on the same individual nanotubes and metallofullerene peapods. Usual approaches for combining transport and TEM on the same nanotube only allow for measuring the current voltage characteristics, but no gate dependence can be acquired. Applying our new method of underetching a Si/SiO2 substrate from the edge of a chip after the transport measurements, we can additionally get the transfer characteristics Isd(Vg), i.e. the gate response of the current, which provides crucial information about the electronic properties of the system investigated. After the transport measurements and the etching process the samples can be viewed in the TEM, which enables us to check, whether a contacted nanotube is really a single tube or a thin bundle and whether a tube is filled with fullerenes indeed. Gaining this additional st...

Electrical Transport in Dy Metallofullerene Peapods

If endohedral metallofullerenes are inserted into a single‐walled carbon nanotube (SWNT), a linear chain of metallofullerenes will form in the interior of the tube, the resulting structure is named metallofullerene peapod. C82 molecules each containing a single Dysprosium atom have been filled into single‐walled carbon nanotubes. The success of filling was verified by transmission electron microscopy (TEM). Transport measurements in field‐effect transistor configuration were performed on (Dy@C82)n@SWNT metallofullerene peapods. Several (Dy@C82)n@SWNT structures exhibit so‐called ambipolar Isd(Vg) characteristics at 4.2 K.