Instability monitoring and fermi level pinning in phosphate buffer saline/self assembled monolayer/Si electrode system

This paper presents a study of the various methods for monitoring the long-term instabilities, i.e. flat band voltage shifts or capacitance variations, in electrolyte/self-assembled-monolayer/Si capacitor. Such instabilities affect the use of such devices as biosensors. The flat-band voltage shift is typically observed when the capacitor is dipped in biological solutions such as Phosphate Buffer Saline (PBS) solution or other common buffers, even before adding the biomolecules target. Capacitance drifts are observed over a time scale of a few hours at room temperature. It was found that the instability figures were depended on the measurement method. A comparative study was conducted between common flat band voltage and capacitance monitoring methods. The flat band voltage was extracted by the Mott-Schottky plot or extracted from the measured Capacitance vs. Voltage (CV) curve using the calculated flat band capacitance. The capacitance at fixed bias was extracted using electrochemical impedance spectroscopy (EIS) by fitting the measurement to a model assuming a capacitor in series to resistor. Impedance spectroscopy under very low current was found to yield the most stable results, while the other methods showed a significant drift. It was found that the flat band voltage instabilities were lower for a higher concentration solution. Moreover, the flat band values became almost constant at about ∼0.5 V (with an error of about 20-30 mV) after 60 minutes of measurement. Those results indicate on the generation of electrically active fast state at the interface causing Fermi level pinning; this effect occurs during the capacitance voltage scan. Note that, this is the first case that Fermi level pinning behavior is observed in the context of biosensors, while the device is dipped in a biological solution. The Fermi pinning phenomenon of semiconductor in electrolyte has been observed also in the past in the context of semiconductor/redox electrolyte interfaces. In summary, it is suggested that it is preferable to measure capacitance at fixed bias, and that the most stable operating point is at, or near, the point of zero current.