Abstract
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 molecularlevel 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 (EA = 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, EB = 0.25−0.40 eV, and pre-exponential factor ( f 0 B ≈ 108−1010 Hz). The biasindependent 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.