Redox-Active Ferrocene grafted on H-Terminated Si(111): Electrochemical Characterization of the Charge Transport Mechanism and Dynamics

Abstract

Electroactive self-assembled monolayers (SAMs) bearing a ferrocene (Fc) redox couple were chemically assembled on H-terminated semiconducting degenerate-doped n -type Si(111) substrate. This allows to create a Si(111)|organic-spacer|Fc hybrid interface, where the ferrocene moiety is covalently immobilized on the silicon, via two alkyl molecular spacers of different length. Organic monolayer formation was probed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS) measurements, which were also used to estimate thickness and surface assembled monolayer (SAM) surface coverage. Atomic force microscopy (AFM) measurements allowed to ascertain surface morphology and roughness. The single electron transfer process, between the ferrocene redox probe and the Si electrode surface, was probed by cyclic voltammetry (CV) measurements. CVs recorded at different scan rates, in the 10 to 500 mV s −1 range, allowed to determine peak-to-peak separation, half-wave potential, and charge-transfer rate constant ( K ET ). The experimental findings suggest that the electron transfer is a one electron quasi-reversible process. The present demonstration of surface engineering of functional redox-active organometallic molecule can be efficient in the field of molecular electronics, surface-base redox chemistry, opto-electronic applications.

Spontaneous Grafting of OH-Terminated Molecules on Si−H Surfaces via Si–O–C Covalent Bonding

“…The SAMs on 1 and 2 formed by spontaneous grafting for 12 h at ambient conditions, are characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Figure 3a shows CVs for the formed SAM of 1, at different scan rates, with the characteristic redox peaks of the ferrocene moiety, consistent with voltammetry measurements for monolayers formed on Si-H by other means such as UV light that was reported by Fontanesi et al [36]. The evolution of the peak current versus the scan rates shows a linear relationship, indicating the formation of a surface-bound system.…”

“…While the mechanism of the reaction is not yet well understood, the covalent attachment has been previously demonstrated for thermal grafting of OH-terminated molecules and the spontaneous grafting of graphene oxide films on Si (111) surfaces [39]. In the former, the reaction is thought to be due to a nucleophilic attack of the O lone pair on the Si (111) followed by the loss of H 2 gas to yield Si-O-R monolayers [36]. In the latter, the reaction was explained by a dehydration reaction forming a Si-C=C monolayers [40].…”

“…Khung et al have used thermal and UV-assisted protocols to attach OH-terminated alkynes to Si(111)-H surfaces, showing that thermal energy can catalyze the grafting process through generating reactive silyl radicals that form a Si-O-C linkage via nucleophilic reaction between the OH group of organic molecule and the Si-H surface [35]. Also, Fontanesi et al [36] have used ferrocene methanol to form a redox active monolayer on Si-H surfaces using heat for two hours at 56 • C. Also, in molecular electronics studies, rigid norbornylogous bridge (NB) molecules are widely used. On metals, these NBs form SAMs with restricted molecular rotations [31,33,37], as compared to alkane monolayers, which are the typical molecules used to form monolayers on surfaces.…”

Ferrocene Introduced into 5-Methylresorcinol-Based Organic Aerogels

“…The XPS survey spectrum of Sample 3b is shown in Figure S7 as an example. The main elements detected on the surface of all the studied Fc-aerogel samples were samples or to irradiation by X-ray beam [33]. These results indicate that most of the ferrocene Fe ions in the material were in an oxidation state of +2 (due to the binding energy of 709.7) and the 722.7 eV peak is attributed to the Fe 3+ state of the iron [34].…”

“…The broad structure at a higher binding energy (about 709.7eV) of the main Fe 2p3/2 peak is not a satellite structure but is rather due to Fe 3+ states from oxidation of the surface exposed to air. It could also be related to the presence of ferrocenium, which is typical of these systems, due to manipulation of the samples or to irradiation by X-ray beam [33]. These results indicate that most of the ferrocene Fe ions in the material were in an oxidation state of +2 (due to the binding energy of 709.7) and the 722.7 eV peak is attributed to the Fe 3+ state of the iron [34].…”