Typically, the fabrication of graphene devices first involves locating individual flakes pre-deposited on a substrate. Because graphene is a single atomic layer thick, this warrants the use of an atomic force microscope (AFM). In addition, a special substrate with markers is necessary so that operators can refer to the same location later in the process to contact the flakes with metal electrodes. The fabrication of markers and electrodes is carried out by electron beam lithography (EBL), which requires the use of a second instrument. Furthermore, a third instrument (such as a Raman spectrometer) is needed in most cases to determine if the graphene flake is just one layer thick.
All-in-one solution
The new method invented by the Max Planck scientists is based on a phenomenon called fluorescence quenching. Here, the fluorescence emitted from dyes is found to be suppressed by the underlying graphene flake.
The procedure starts with the coating of substrate (containing pre-deposited graphene sheets) with a photoresist, which is actually a fluorescent dye. Next, the sample is loaded into a confocal microscope, which uses a laser to image the substrate surface. At positions where the graphene sheets are present, the fluorescence intensity is lower and the flake is immediately located. In a subsequent step, a second laser (different wavelength) in the same instrument is used to perform lithography.
Multiple benefits
As is apparent, the approach is based on a single instrument and avoids the use of markers on the substrate. What's more, the magnitude of the fluorescence intensity can be used to identify the number of layers in the graphene flake.
A full description of the work can be found in the journal Nanotechnology.