Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature

Abstract

Thin layers of oligomers with thickness between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti(2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chain. Using an oligo(bisthienylbenzene) based layer, a molecule whose HOMO energy level in a vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e., tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so modest

“…Recently, various groups succeeded in fabricating molecular devices exhibiting rectification ratios (RRs) larger than 10 2 or even 10 5 . 12 – 15 , 23 , 24 , 63 – 68 In view of such achievements, it is hard to discuss “rectification” based on the very modest values RR ≈ 1.5 that characterize CnT junctions. However – and this is the important point we want to make here – these small RR-values are merely a consequence of the fact that the HOMO of CnT does not respond to the entire substrate-tip bias.…”

“…To become part of nanotechnology, molecular electronics should be able to fabricate molecular devices that supersede or at least undertake the basic functions of circuit components employed nowadays in semiconductor-based microelectronics. 1 – 15 Rectification represents such an outstanding function, which has attracted the attention of the community since its inception. 16 In molecular junctions where charge transport is dominated by a single molecular orbital (MO) one may intuitively expect that rectification is directly related to the bias-driven shift of the MO energy 17 – 20 in principle determined by the local electric potential φ , which varies linearly across the junction (“potentiometer rule”) in the absence of screening.…”

Grafting of Diazonium Salts on Surfaces: Application to Biosensors

“…It is also possible to prepare graphene ribbons from diazonium salts. Molecular junctions using aryl oligomers were obtained from diazonium salts [38]. An organic oligomeric aryl layer was grown on a conducting substrate and capped with, for example, a gold layer; this provided a molecular junction between two conducting materials [39].…”

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

“…Many studies has been reported quite recently using diazonium electroreduction to graft conductive oligomers on surfaces [38,[43][44][45][46][47]. Ultrathin layers can be generated since the growth is limited by the conductivity of the organic layer in the potential range required for the electroreduction of the diazonium salt.…”