Electronic functionalisation of graphene via external doping and dosing
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There exist many reports on functionalisation of graphene on a non-spatially resolved scale; this report concentrates on reviewing atomic-scale interactions of functionalising agents, i.e. on the electronic behaviour of single atoms, which are introduced as adatoms or lattice site impurities for the purpose of external doping and dosing to achieve bandgap engineering and electrical contacting of graphene; it also reviews the associated defects. Emphasis is put on visualisation of such interactions by advanced imaging in conjunction with localised spectroscopy techniques. Whereas the existing literature describing the development of such techniques in the application to graphene warrants a review in its own right, here the authors focus on observations, with modelling support, of the interaction phenomena themselves and not on the evaluation of measurements by such techniques. Atomic resolution transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS) in imaging and scanning mode, as well as scanning tunnelling microscopy (STM) are the most frequently applied techniques in aid of revealing topography and defect assisted interactions of graphene with foreign atoms and molecules. Electron-probe based investigations additionally lead to electron beam assisted interactions of foreign species with graphene. The graphene metal interaction observed in a transmission electron microscope is a prevalent example of how reactions occurring between metals and graphene can be emphasised and thereby assessed: metal-mediated etching of graphene has proven to be a common phenomenon after metal dosing, e.g. to fabricate electrical contacts. The review reports furthermore on investigations revealing atomic position, bonding and dynamics of non-metal p- and n-dopants as well as on revealing the functionalisation of graphene via molecular self-assembly, intercalation and nano-sculpting. Literature till the end of 2013/begin of 2014 is reviewed.