Optical and electrical properties of ion-beam-irradiated films of organic molecular solids and polymers

Abstract

Thin films of several nonpolymeric and polymeric organic compounds become optically dark and electrically conducting on irradiation with energetic particle beams. Using 2‐MeV Ar⁺ ion beam irradiation, we have generated conducting patterns in otherwise high‐resistivity, organic thin films among which are 3, 4, 9, 10‐perylenetetracarboxylic dianhydride, 1, 4, 5, 8‐napthalenetetracarboxylic dianhydride, and Ni phthalocyanine. The room‐temperature resistivity of the films changes by 14 orders of magnitude from its as‐deposited value of ρ>10¹⁰ Ω cm to ρ=5×10⁻⁴ Ω cm at ion doses of 10¹⁷ cm⁻². The temperature (T) dependence of the resistivity follows ρ(T)∝exp(+(T₀/T)^1/2 ) over a wide range of dose and temperature. The characteristic temperature T₀ is found to be a function of dose. The observed behavior of ρ(T) is consistent with charge transport due to hopping between isolated, conducting islands. The dependence on ion energy, ion species, and ion dose rate has been examined. The effectiveness of an ionic species in increasing a film conductivity increases with electronic energy loss of the ion in the film. Compound specific sensitivity towards ion irradiation was demonstrated. Nonlinear effects of ion dose rate were also observed. The final carbon‐rich product has some characteristics similar to those of amorphous carbon; however, the conductivity is higher and the electrical behavior is different.