Ultra-Thin and Highly Insulating Aromatic Monolayers by N-Heterocyclic Carbenes

The efficiency of organic electronic devices relies on application of organic gate dielectric materials. Such organic films should exhibit high chemical/thermal stability, aromatic functionality compatible with organic semiconductors, and low gate leakage currents in combination with low thickness to reduce the operating voltage. An interesting class of materials for such applications are self-assembled monolayers (SAMs) among which the N-heterocyclic carbenes (NHC) are known for their high chemical/thermal stability. The conductivity of NHC SAMs, however, has been sparsely explored and their electrical properties remain controversial. Here we report conductivity analysis for a well-defined series of aromatic NHC SAMs. Our data show that all analyzed monolayers are highly insulating and in particular the shortest possible NHC of just ∼3.3 Å is by 5 orders of magnitude more insulating than standard insulators based on alkanethiolate SAM of the same length. Our calculations indicate the absence of destructive quantum interference (DQI) effect which has been considered responsible for suppression of conductivity in aromatic molecules. The suppression of SAMs conductivity just via selection of the imidazolium-based bonding group is conceptually simpler opening possibility of using NHC SAMs as an ultra-thin, and exceptionally insulating, aromatic monolayers for functionalization of the gate electrodes.