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

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• 作者：Christian Godet ; Alain-Bruno Fadjie-Djomkam ; Soraya Ababou-Girard ; Sylvain Tricot ; Pascal Turban ; Yan Li ; Sidharam P. Pujari ; Luc Scheres ; Han Zuilhof ; Bruno Fabre
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：April 3, 2014
• 年：2014
• 卷：118
• 期：13
• 页码：6773-6787
• 全文大小：718K
• 年卷期：v.118,no.13(April 3, 2014)
• ISSN：1932-7455

文摘

The relaxation dynamics of surface-bound n-alkyl chains was studied by broadband admittance spectroscopy (10 mHz鈥?0 MHz) measured at low temperature (130鈥?00 K) in the reverse bias regime of rectifying Hg//organic monolayer (OML)鈥?i>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 鈮?10²鈥?0⁵ Hz) with very small activation energy (E_A = 20鈥?0 meV) and pre-exponential factor (f ⁰_A 鈮?10³鈥?0⁶ Hz). With increasing T, peak B also appears, with higher values of activation energy, E_B = 0.25鈥?.40 eV, and pre-exponential factor (f ⁰_B 鈮?10⁸鈥?0¹⁰ Hz). The bias-independent relaxation mechanism A, with very low activation energy typical of dipole鈥揹ipole 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.