Pre-exponential factor in semiconducting vitamin A (alcohol and acetate).

概要

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The semiconductive properties of vitamin A (alcohol and acetate) on adsorption of various vapors have been studied. The adsorbed vapors increase the semiconduction currents by several orders of magnitude and decrease the semiconduction activation energies. Such change depends on the chemical nature and also on the amount of vapor adsorbed. Semiconducting vitamin A follows the three-constant equation σ(T)=σ0′exp(E⁄2kT0)exp(−E⁄2kT) where the conventional pre-exponential factor σ0 has been replaced by σ0′exp(E⁄2kTo) (the so called compensation effect). Here T0 and σo′ are constants for the substance and T0 is called the characteristic temperature. Various methods used for evaluating these constants have yielded consistent results with T0≈402 K and σ0′≈2.8×10−9 Ω−1 cm−1 for vitamin A alcohol and T0≈335 K and σ0′1.5×10−10 Ω−1 cm−1 for vitamin A acetate. Excellent correlation obtained between the relevant parameters in semiconducting vitamin A indicates that σ0 and E are physically related. Various models for conduction mechanism leading to compensation effect have been discussed. The measured activation energies on adsorption of same amount of various vapors show a linear relationship with the ionization potential of the adsorbed molecules suggesting that charge-transfer interaction is responsible for the semiconductivity enhancement.
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