Dynamics of Substituted Alkyl Monolayers Covalently Bonded to Silicon: A Broadband Admittance Spectroscopy Study

The relaxation dynamics of surface-bound n-alkyl chains was studied by broadband admittance spectroscopy (10 mHz–10 MHz) measured at low temperature (130–300 K) in the reverse bias regime of rectifying Hg//organic monolayer (OML)–n-doped Si tunnel junctions. To obtain molecular-level information on the structure and dynamics of grafted monolayers, carboxyl or amide dipolar moieties were located either at the top free surface with variable acid concentration (0%, 5%, and 100%) or at the inner position in the alkyl backbone (100% amide units). Two classes of dipolar relaxation mechanisms are found with different thermally activated behavior. At low T, only peak A is observed (f ≈ 102–105 Hz) with very small activation energy (E A = 20–40 meV) and pre-exponential factor (f 0 A ≈ 103–106 Hz). With increasing T, peak B also appears, with higher values of activation energy, E B = 0.25–0.40 eV, and pre-exponential factor (f 0 B ≈ 108–1010 Hz). The bias-independent relaxation mechanism A, with very low activation energy typical of dipole–dipole interaction, is attributed to extrinsic relaxation of adventitious H2O molecules in hydrogen-bond clusters. Mechanism B is attributed to intrinsic relaxation of the alkyl chain assembly. In the acid series, the relative intensity of peak B is consistent with the acid group coverage given by XPS, in contrast with peak A, and its activation energy reveals increased motional constraints in the acid-substituted OML. The shape of dipolar relaxation peaks, discussed in the framework of Dissado-Hill/Jonscher theories for many-body interactions, is useful to discriminate near-substrate and molecular tail relaxations through order/disorder effects.