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

Claudio Fontanesi,Luca Pasquali,Prakash Chandra Mondal,Walter Giurlani,Davide Vanossi,Maria Cannio,Massimo Innocenti,Monica Montecchi,Enrico Da Como

doi:10.1038/s41598-019-45448-w

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 (KET). 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.

Highlights

Over the past decades, surface-confined nanometric electroactive molecular assemblies have been the subject of key research, which play a major role in understanding redox processes[1,2,3,4,5,6,7]
We obtained an effective thickness, d = 32 Å and 16 Å for the Si−Me−Fc and Si−UA−Fc surfaces, respectively. This result may suggest, by comparison with optimized structures and relevant geometrical parameters obtained by DFT results, that the coverage with Si−Me−Fc is about 2 to 3 monolayers, while the coverage corresponds to a single monolayer in the case of Si−UA−Fc
From the experimental point of view, our electrochemical experimental results are in line with the evidence reported by Simonet[52], Zannoni et al.[41,49] and Calborean[21], when dealing with similar hybrid interfaces

Summary

Introduction

Surface-confined nanometric electroactive molecular assemblies have been the subject of key research, which play a major role in understanding redox processes[1,2,3,4,5,6,7]. Among the redox-active moieties, ferrocene-based derivatives form a class of attractive organic compounds, which can be considered as model systems This is due to ferrocene structural stability, aromaticity, ease of modification, associated to a reversible single electron transfer process (Fc+/Fc), low oxidation/reduction potential, and easy to prepare self-assembled monolayers (SAMs)[20,21]. In view of the above fascinating behaviour, ferrocene based SAMs could be exploited as memory elements, where the ferrocene redox centre is used as the charge storage unit and considering the molecule in the neutral or oxidized form as the two states of a bit[22]. Particular attention is devoted to draw a parallelism between the striking differences observed for the ET dynamics, probed by CV measurements, in the case of ferrocene covalently immobilized, or in bulk solution, in the case of SAMs of comparable thickness

Methods

Results

Conclusion