Materials consisting of mixtures of strong organic acceptors (A) and donors (D) show a fascinating diversity of physical properties. The first purely organic metals were actually 1:1 mixtures of tetracyanoquinodimethane (TCNQ), a strong organic acceptor, and tetrathiafulvalene (TTF), a strong organic donor. After that, a large number of works have found a plethora of electronic behaviors such as Mott and Peierls insulators, charge- and spin-density wave states and organic superconductors. Whether one particular D:A mixture realizes one or the other of these states depend on their band filling which, in turn, depends on the D:A stoichiometry. Hitherto, however, the stoichiometry of D:A mixtures has been considered to be determined by the chemical nature of the donor and acceptor molecules, so that the only way to modify the D:A ratio in a controlled way would be the chemical functionalization of the molecular backbones. Here we show that the electronic flexibility offered by metal surfaces opens the possibility to choose the stoichiometry of 2D D:A mixtures supported on them from 2:1 to 1:4, as revealed by our STM investigations (see Figure). XPS and NEXAFS measurements together with DFT calculations demonstrate that the possibility of changing the stoichiometry is related to the variable doping of the 2D D:A layer with electrons arising from the substrate. This approach might thus open the possibility to synthesize 2D organic materials with a wide range of tunable properties that can be exploited to fabricate new electronic devices and sensors.

Paper presented at the 31st European Conference on Surface Science (eccos 31), held in Barcelona (Spain) on August 31st - September 4th, 2015.