Indirect e-beam evaporation of metal on a cooled substrate that allows making reproducible and gentle electrical contact to molecular films of organic molecules yields strikingly different results with Pd and Au. This is attributed to different growth modes of the metals, which lead to different molecule/metal interactions and to Au penetration in between the molecules. These differences can radically change the effect of the molecules on the resulting junctions.
REFERENCES
The dipole layer effect operates at both forward and reverse bias, as explained in Ref. 3 (cf. Fig. 1 in that reference).
The molecular dipole is taken as positive if the negative pole of the dipole points away from the binding group, viz. away from the surface after binding. As negative dipoles we used , and and as positive dipoles we used and . All values (in parentheses) are in Debye units, for the free molecules.
Complementary experiments showed ICICE-made contacts to have higher effective than geometrical areas, whereas the effective LOFO-made junction areas are up to an order of magnitude smaller. As is clear from Refs. 1 and 3, the larger the effective contact area between deposited metal and molecules, the higher the dipole density due to the interaction of the metal spill-over electron density with the molecules, which is the dipole density that dictates the trend for junctions with LOFO-made contacts and with ICICE-made Pd ones.
