Structural and chemical analysis of ion beam produced conductive regions on highly resistive organic films

Thin films of both polymeric and nonpolymeric organic solids turn optically dense and electrically conductive on irradiation with high energy ion beams (e.g., 2 MeV Ar⁺). The structural and chemical properties of these films were investigated by ultraviolet (UV) visible, infrared (IR), Raman spectroscopic techniques, electron spin resonance (ESR), electron spectroscopy for chemical analysis (ESCA), and Rutherford backscattering (RBS) measurements. Specifically, in the case of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and nickel phthalocyanine (NiPc), and UV visible, IR, and Raman spectra show the loss of the initial molecular structure at low irradiation doses (10¹³-10¹⁴ cm^{- 2}) followed by the appearance, at high doses, of a spectrum similar to that observed for amorphous carbon. The Raman spectra indicate the absence of any long range graphitic microcrystalline structure and suggest that the films are nearly amorphous at higher doses. The RBS spectra indicate gradual loss of oxygen in PTCDA with increasing irradiation dose. There is negligible oxygen left in the film at high doses and a maximum loss of ∼35% (∼15%) of the carbon atoms in PTCDA (NiPc) is observed. The resistivity of the films decreases with increasing dose, reaching a minimum of ∼5×10^{- 4} Ω cm at a dose of ∼10¹⁷ Ar ⁺/cm². Surprisingly, the resistivity of these films at high doses (∼10¹⁷ Ar⁺/cm²) is considerably lower than that of any amorphous phase of carbon. In the case of NiPc, such a low resistivity is obtained even though 60% of the N and 100% of the Ni originally contained in the films are retained. In situ measurements of the evolution rate of molecular fragments during the bombardment indicate a decrease with dose, suggestive of an irreversible modification of the material with ion bombardment.