Effect of Adsorption of Gases on the Semiconductive Properties of All-trans β-Carotene

Abstract

The presence of various adsrobed gases increases the semiconduction currents in crystalline powder of β-carotene by several orders of magnitude and decreases the activation energy for semiconduction. The current increase depends on the amount and nature of the adsorbed gas. The process is reversible simply by desorbing the gases. The kinetics of the conductivity rise follow the Roginsky-Zeldovich equation for the adsorption-time correlation. It has been noted that the adsorbed gases have no effect on the σ0 term, i.e., the mobilities of the charge carriers and the number of conducting states do not change. A value of σ0 ≈ 1.0 × 10−3(Ω·cm)−1 has been measured. Assuming a value for the radius of polarization field, R = 5.3 Å and using the measured values of the dielectric constant, values for the activation energies of β-carotene with adsorbed O2 and methanol, and also the increment in current expected, have been predicted. The measured values are in good agreement with these. We conclude that the changes in the effective dielectric constant (possibly due to formation of donor-acceptor complexes) and the consequent changes in the activation energy can satisfactorily account for the electrical behavior of all-trans β-carotene crystals when various gases are adsorbed on it.