Semiconductivity of organic substances. Part 13.—Single crystal and polycrystalline aminoacids

Abstract

A study has been made of semiconduction in valine, glycine, diketopiperazine, aspartic acid, glutamic acid, tryptophan, tyrosine and leucine, using compressed discs, and single crystals for the first three. Guard-ring measurements established a true bulk conduction for glycine. The Δ_ε values were in the range 3.26–4.09 eV, whereas the optical absorption edges were around 5 eV. The log₁₀σ_o values lay between 9 and 17. The results are explained on an electron injection mechanism with temperature variable surface states. “Dry” proteins have markedly lower Δ_ε values, ca. 2.7 eV and σ_o values still explicable by an intrinsic generation of charge carriers. With proteins the discrepancy with the theoretical energy gap of 5 eV for the >CO.H—N< band system remains problematical.