Electron donor–acceptor interactions and surface semiconductivity in molecular crystals as a function of ambient gas

Abstract

Quantitative studies of the magnitude, rate and reversibility of changes in surface semiconductivity of single crystals of phthalocyanines, perylene, tetracyanoquinodimethane (TCNQ) and molecular complexes as a function of ambient gas are reported. NO₂+ N₂O₄ increases the surface conductivity of phthalocyanines, by factors of up to 10⁸. At low pressures, the magnitude of the increase follows the Freundlich adsorption isotherm, while the rate obeys the Elovich equation. The saturation conductivity at high pressures is similar for all the phthalocyanines and corresponds to complete surface coverage, each adsorbed molecule producing one ionised state. Reversibility on heating in vacuo depends on the metal : (metal free, Ni, Cu, Zn) > (Co, Mn) > Pb. In all cases, treatment with low pressures of NH₃ gives rapid reversal of the effects. BF₃ gives smaller, irreversible enhancements. The surface conductivity of perylene is enhanced by a factor up to 10⁸ in BF₃ and the effect is reversed on treatment with NH₃. NO₂+ N₂O₄ produces smaller effects (10⁴), easily reversible. Perylene—TCNQ shows small conductivity increases (10²) with both NO₂+ N₂O₄ and NH₃, while TCNQ shows similar effects only with NH₃. Semiconduction activation energies in the presence of gases enhancing conductivity are reduced to values comparable with those obtained from temperature dependence of photoconduction (0.1–0.2 eV). Conductivity changes are interpreted in terms of production of ionised states following weak chemisorption involving donor–acceptor interactions. The magnitude and reversibility of the changes depends on the nature of the orbitals involved in these interactions, and provides scope for selective gas detection.