Tuning the Electronic Communication between Redox Centers Bound to Insulating Surfaces

Ferrocene-terminated monolayers covalently bound to hydrogen-terminated silicon surfaces. Toward the development of charge storage and communication devices

The combination of monocrystalline silicon's well-defined structure and the ability to prepare hydrogen-terminated surfaces (Si-H) easily and reproducibly has made this material a very attractive substrate for immobilizing functional molecules. The functionalization of Si-H using the covalent attachment of organic monolayers has received intense attention due to the numerous potential applications of controlled and robust organic/Si interfaces. Researchers have investigated these materials in diverse fields such as molecular electronics, chemistry, and bioanalytical chemistry. Applications include the preparation of surface insulators, the incorporation of chemical or biochemical functionality at interfaces for use in photovoltaic conversion, and the development of new chemical and biological sensing devices. Unlike those of gold, silicon's electronic properties are tunable, and researchers can directly integrate silicon-based devices within electronic circuitry. Moreover, the technological processes used for the micro- and nanopatterning of silicon are numerous and mature enough for producing highly miniaturized functional electronic components. In this Account, we describe a powerful approach that integrates redox-active molecules, such as ferrocene, onto silicon toward electrically addressable systems devoted to information storage or transfer. Ferrocene exhibits attractive electrochemical characteristics: fast electron-transfer rate, low oxidation potential, and two stable redox states (neutral ferrocene and oxidized ferrocenium). Accordingly, ferrocene-modified silicon surfaces could be used as charge storage components with the bound ferrocene center as the memory element. Upon application of a positive potential to silicon, ferrocene is oxidized to its corresponding ferrocenium form. This redox change is equivalent to the change of a bit of information from the "0" to "1" state. To erase the stored charge and return the device to its initial state, a low potential must be applied to reduce the whole generated ferrocenium. In this type of application, the electron is transferred from the ferrocene headgroups to the underlying conducting silicon surface by a tunneling process across the monolayer. To produce a stable and reproducible electrical response, this process must be efficient, fast, and reversible. The stability, charge density, and capacitance performances of high-quality ferrocene-terminated monolayers could compete with those of the existing semiconductor-based memory devices, such as dynamic random access memories, DRAMs. Moreover, we provide experimental evidence that a series of immobilized ferrocene centers can efficiently communicate via a lateral electron hopping process. Using these modified interfaces, we demonstrate that the thin redox-active monolayer can behave as a purely conducting material, highlighting an unprecedented very fast electron communication between immobilized redox groups. Perhaps more importantly, the surface coverage of ferrocene allows us to precisely control the rate of this process. Such characteristics are relevant not only for electrocatalytic reactions but also for widening the potential applications of these assemblies to novel molecular electronic devices (e.g. chemiresistors, chemically sensitive field-effect transistors (CHEMFETs)) and redox chemistry on insulating surfaces.