Use of Diode Analogy in Explaining the Voltammetric Characteristics of Immobilized Ferrocenyl Moieties on a Silicon Surface

Abstract

The creation of a model that explains the dependency of the voltammetric characteristics of ferrocene-terminated Si (Si-Fc) samples on the type of substrate (n-or p-type) would be helpful in understanding the electronic characteristics of these materials. To explain the dependency, Si-Fc samples are treated like diodes. As diodes, the samples may allow charge flow in a certain direction while inhibiting the opposite flow. The treatment of a sample as a diode is done to facilitate analysis of charge flow within the sample, thus enabling easy prediction of its electrochemical characteristics. Likewise, the trend of the anodic peak potential versus light intensity plot (of the samples with n-type substrate) is also associated with the sample's diode characteristics. Our proposed model opens many scientific possibilities, especially in relating the voltammetric characteristics of electroactive molecules on a Si surface with the properties of a diode (e.g., open-circuit voltage).Ferrocene-containing moieties on semiconductor or semiconductor oxide surfaces have been the subject of many reports and have potentials as memory devices. [6,12,13,17] The Fe atom in the ferrocenyl (Fc) moiety has two stable and reversible oxidation states. Its oxidation state can increase from + 2 to + 3 upon the introduction of potential and return back to + 2 when it is removed. These two reversible oxidation states could correspond to the bits of memory devices. However, the electrochemical characteristics of Fc-terminated Si (Si-Fc) depend on whether the silicon substrate is n-type or p-type. Cyclic voltammetry (CV) measurements of Si-Fc samples with an n-type substrate (n Si-Fc) show the presence of cathodic and anodic peaks only when the measurements are done under illumination. [4,6,8,18] This is in contrast with those with a p-type substrate (p Si-Fc), which show the presence of cathodic and anodic peaks with or without illumination. We believe that the creation of a model that can describe these phenomena would lead to a better understanding of the electronic properties of these materials and this could be very helpful in the construction of molecular memory devices.For n Si-Fc samples in the dark, the depletion condition [6,8,18] existing within the sample has been associated to the absence of anodic and cathodic peaks during CV measurements. Meanwhile, the occurrence of CV peaks on p Si-Fc samples with or without illumination has been related to accumulation condition. Both the depletion and accumulation conditions are consequences of the bending of energy bands due to an applied bias and are related to the existence of a potential barrier at the Si substrate-molecular layer interface (in the equilibrium condition). To be able to construct a more holistic model, we generalized that the cause of the above phenomenon is the existence of a potential barrier at the Si-molecular layer interface, which makes the sample behave like a diode in the presence of an external bias (e.g. CV measurements). Because diodes have already been well ...

Functionalization of Oxide-Free Silicon Surfaces with Redox-Active Assemblies

“…Upon illumination with a sufficient light intensity (higher than ca. 30 mW cm –2 ), the full electrochemical reversibility of the bound Fc + /Fc couple could be reached owing to the fast oxidation of Fc by the photogenerated electron–hole pairs. As a result of the photogenerated electrons-induced activation of the redox process, the redox potential of Fc bound to n -type silicon under illumination is between 200 and 400 mV lower than that observed for Fc bound to p -type silicon in the dark or under illumination (Figure ).…”

A Gauss’s law analysis of redox active adsorbates on semiconductor electrodes: The charging and faradaic currents are not independent

“…Second, the presented approach does not require finite-difference approximations of differential equations, nor does it rely on forcing the data to conform to the diode equations ( 68 ) commonly used in the solid-state device literature. There are two advantages to these features.…”