Sensors Based on Chemicurrents
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Selective and nonselective solid-state chemical sensors have been developed for a variety of applications in process control, environmental monitoring, and hazardous substance detection. Most rely on an indirect detection mechanism whereby electronic or electro-optical properties of the device are altered by the substance of interest allowing for a measurable response. Sensors based on analyte–sensor interactions, which modify capacitance, conductivity, and refractive index, operate in this manner.
Recently, a means of directly monitoring charged carriers produced from gas–surface interactions has been described whereby “hot” electrons/holes (e–h) from gas–surface reactions are themselves detected using ultrathin film metal-semiconductor (MS) Schottky diodes. When an adsorbate binds to the surface with a relatively large adsorption energy, the energy may appear as an energetic e–h pair generated at the metal surface; the excited electron may travel ballistically through the thin metal film and transverse the Schottky barrier (SB) if the film is thin compared with the ballistic mean free path and if the kinetic energy of the electron is larger than the barrier height. Once injected into the conduction band of the semiconductor (inverse electron emission, IEE), the electron is detected as a “chemicurrent” analogous to the photocurrent in a photodiode.
The energetic charge carriers have a ballistic mean free path of tens of nanometers and thus for small-dimension metal structures, the “hot” charge carriers persist at energies well above the Fermi level. Hot electrons have interesting and unusual reactivity and transport properties which may be taken advantage of in Schottky-barrier metal-semiconductor (MS) and metal-oxide-semiconductor (MOS) diode sensors. Using ultrathin metal films on MS and MOS device structures, we have investigated the “chemielectronic” phenomena associated with a variety of molecular and atomic interactions with transition metal surfaces (Ag, Au, Pt, Pd) and found distinct differences in the mechanism of signal production between the highly energetic atomic and molecular species and more weakly interacting species such as xenon and hydrocarbons. In this review, the basis for the chemical sensitivity of these diode structures will be addressed, emphasizing the importance of oxide interfacial states in the detection of electronic signals from weakly interacting gases. Experimental evidence linking the surface chemistry to the electronic processes giving rise to signals in MS and MOS diode sensors will be presented.