PUB - Publikationen an der Universität Bielefeld

Carbon nanosheets are a new kind of two-dimensional polymeric material that is fabricated by cross-linking aromatic self-assembled monolayers with electrons. Due to their uniform thickness of only about one nanometer, as well as their high chemical, mechanical, and thermal stability, such materials are of high interest for a wide variety of applications. Carbon nanosheets can be released from their original substrate and transferred to arbitrary new substrates. When placed on oxidized silicon wafers, nanosheets can be seen by the naked eye. Folds appear to be darker and blue shifted with respect to the substrate color. This effect was systematically studied for multilayer stacks of nanosheet by means of UV/Vis reflection spectroscopy and simulation. The oxide layer acts as an optical spacer, similar to an interferometer. Thus, the contrast was found to be dependent on the wavelength of light as well as the thickness of the oxide layer. On perforated substrates such as transmission electron microscopy (TEM) grids, large freestanding membranes with the thickness of one monolayer were obtained with an aspect ratio of up to 1:225000. This way, for the first time, the bottom side of the nanosheet became accessible for functionalization, and it was possible to selectively couple fluorescent dyes to the nanosheet; this highlights the versatility of the nanosheets. Freestanding nanosheets have been used as sample supports to image nanoparticles by TEM. Due to the thickness of the nanosheets, the nanoparticles can be observed in intricate detail, with only minimal background from the nanosheet as compared to conventional carbon films. Also, the individual layers of multi-walled carbon nanotubes (MWNT) were resolved. In scanning TEM even single gold atoms were observed. It is possible to deposit more than just nanoparticles onto freestanding nanosheets. The sheets can also be metalized in a variety of different ways either before transfer or afterwards; this was demonstrated by fabricating gold patterns onto freestanding nanosheets. As the nanosheet is stable under an electron beam, patterns can also be written by electron beam induced deposition (EBID). By annealing nanosheets in ultra high vacuum their conductivity can be adjusted flexibly. This is due to a gradual transition to a graphitic phase that reaches a sheet resistivity of only [almost equal to]100 k[Ohm]/sq at [almost equal to]1200 K. The structural changes that are associated with this transition are reflected in the TEM and Raman data. For multilayer stacks it was also possible to observe the transition to graphite by means of UV/Vis spectroscopy. Because of their stability and flexibility, carbon nanosheets will likely find a multitude of applications, including potential use as sensors, filtration membranes, sample supports, and even conductive coatings.