With exceptional carrier mobilities, mechanical strength, and optical transparency, graphene is a leading material for next-generation electronic devices. However, for most applications, graphene will need to be integrated with other materials, which motivates efforts to understand and tune its surface chemistry. In recent work, published in the scientific journal Small, scientists from the University of Groningen in the Netherlands studied surface chemistry and self assembly of organic molecular wires on Graphenea's CVD graphene.
Since all atoms in graphene are surface atoms, a natural approach to tune graphene’s electronic properties is to use surface interactions. In particular, the modification of graphene via functionalization with organic molecules holds promise for tuning the electronic properties of graphene, controlling interfaces with other materials, and tailoring surface chemical reactivity.
Recently, a number of studies on self-assembly of organic molecules on graphene have been reported, albeit with few reports combining both the surface organization and its effect on the electrical performance of graphene devices. The scarcity of combined studies is likely to be due to the specific limitations presented by each source of graphene used. For example, mechanically exfoliated flakes of graphene on an insulating substrate, such as the commonly used silicon dioxide, allow fabrication of electronic devices, but are very challenging to approach with the tip of a scanning tunneling microscope (STM) due to their small dimensions. On the other hand, large-area graphene grown epitaxially from silicon carbide or grown by chemical vapor deposition (CVD) on a metal is suitable for studying self-assembly, but is not readily used in field-effect transistors due to the lack of a back gate electrode in the substrate.
In contrast, graphene grown by CVD and subsequently transferred to silicon/silicon dioxide wafers combines the accessibility of large-area graphene with the utility of a back gate present in the substrate. Furthermore, waferscale CVD graphene transferred to silicon/silicon dioxide has become widely available. In the work detailed in the Small publication, the team led by Prof. Ben L. Feringa used our commercially available CVD graphene on silicon/silicon dioxide. High quality CVD graphene on this substrate is available from Graphenea on wafer sizes ranging from 10mm x 10mm to a full 4'' diameter wafer, or on any other custom-sized wafer or substrate material.
Figure: Molecular wires grown on a graphene layer, imaged by scanning tunneling microscopy (from Small, a Wiley publication)
The research team discovered several surprising features of molecular wire self-assembly on graphene. Notably, it is found that the wires grow in patches oriented at 60 degrees to each other (see figure, part b), indicating the the atomic structure of graphene (a honeycomb lattice with 60 degree symmetry) plays a role in the self-assembly process, reflecting on the macroscopic topology of the wires. A second surprising finding is that the performance (doping level and mobility) of graphene transistors becomes better when they are covered by the layer of organic molecular wires. The performance increase is attributed mostly to cleaning of the graphene by the solvents used in the chemistry process.
In conclusion, this top-ranking publication confirms the importance of surface properties of graphene, studying the important interaction of the graphene surface with organic molecules. Importantly, the results show that CVD graphene is compatible with graphene transistor technology, touting a quality high enough to investigate surface chemistry effects. Combined with the ability to grow wafer-scale layers, CVD graphene is so far the most serious contender for fast graphene electronics and precise sensors.