Nanostructural and electrical properties of functionally terminated self-assembled monolayers on silicon surfaces
Self-assembled monolayers (SAMs) having alkyl chains with typically 18 CH2 units and with different functional end groups (methyl, thiol, thiophene, phenoxy, and biphenyl) have been attached to hydroxylated (100) silicon surfaces. Their layer structure has been studied using grazing incidence x-ray reflectometry. An excellent data analysis is possible on the basis of a two layer model. One layer with constant thickness (about 10.8 A˚) for all the SAMs investigated is associated with the alkyl chain/silicon interface, whereas the second layer is associated with the functional end group. Its dimension changes with the size and nature of the end group. The layer dimension increases from about 22 A˚ for the smallest end group (methyl) to about 32 A˚ for the largest one (biphenyl). The experimental layer thickness values are in good agreement with those expected from molecular modeling. The electrical properties of the SAM layers have been studied using Au/Al contacts deposited on the functional end groups. Of particular interest are the insulating properties of the alkyl chain and the breakdown voltages which exhibit very high values of typically 16 MV/cm. A lateral in-plane conductance along the end groups has been measured in the case of an I2-doped biphenyl end group. Iodine doping can increase the conductivity by a factor of 12-14. This suggests the possibility of a nanomolecular transistor with the functional end group as an active layer without any additional deposition of an organic conducting layer on the SAM dielectric layer.