We report the synthesis and characterization of molecular rectifying diodes on silicon using sequential grafting of self-assembled monolayers of alkyl chains bearing a pi group at their outer end (Si/sigma-pi/metal junctions). We investigate the structure-performance relationships of these molecular devices, and we examine the extent to which the nature of the pi end group (change in the energy position of their molecular orbitals) drives the properties of these molecular diodes. Self-assembled monolayers of alkyl chains (different chain lengths from 6 to 15 methylene groups) functionalized by phenyl, anthracene, pyrene, ethylene dioxythiophene, ethylene dioxyphenyl, thiophene, terthiophene, and quaterthiophene were synthesized and characterized by contact angle measurements, ellipsometry, Fourier transform infrared spectroscopy, and atomic force microscopy. We demonstrate that reasonably well-packed monolayers are obtained in all cases. Their electrical properties were assessed by dc current-voltage characteristics and high-frequency (1-MHz) capacitance measurements. For all of the pi groups investigated here, we observed rectification behavior. These results extend our preliminary work using phenyl and thiophene groups (Lenfant et al., Nano Lett. 2003, 3, 741). The experimental current-voltage curves were analyzed with a simple analytical model, from which we extracted the energy position of the molecular orbital of the pi group in resonance with the Fermi energy of the electrodes. We report experimental studies of the band lineup in these silicon/alkyl pi-conjugated molecule/metal junctions. We conclude that Fermi-level pinning at the pi group/metal interface is mainly responsible for the observed absence of a dependence of the rectification effect on the nature of the pi groups, even though the groups examined were selected to have significant variations in their electronic molecular orbitals.

The electrodeposition mechanisms of poly(3,4-ethylenedithiathiophene) (PEDTT), which is the sulfur analogue of the well-known poly(3,4-ethylenedioxythiophene) (PEDOT), is investigated in acetonitrile solution by means of potentiostatic methods. By analyzing the current transients within electrocrystallization theory, we observe that the electrodeposition process is a combination of two mechanisms: progressive nucleation, followed by a diffusion-controlled three-dimensional growth (PN3DD); and an instantaneous nucleation, followed by a three-dimensional growth mechanism with charge transfer as the rate-limiting factor (IN3DC). This trend is contrary to PEDOT electrodeposition mechanisms. Cyclic voltametric measurements show important differences between PEDOT and PEDTT. The most unexpected result is that, although 3,4-ethylenedithiathiophene (EDTT) has a lower oxidation potential than 3,4-ethylenedioxythiophene (EDOT), the polymer PEDTT presents a higher oxidation potential and larger band gap than PEDOT. Density functional theory (DFT) calculations reveal important structural and electronic differences between some oligomers of EDTT and EDOT. We analyze these results in terms of the electron-donating effect of the S atom, the difference in the reactivity of the radical-cations of the monomers, and the difference in the geometries of the oligomers.