Effects of heat annealing on the film characteristics and gas sensing properties of substituted and un-substituted copper phthalocyanine films

Copper phthalocyanine and tetra-tert-butyl copper phthalocyanine were vacuum deposited onto gold electrode. The effects of substrate materials (glass and gold) and post-deposition annealing on the film morphology and crystalline structure were studied. The film characteristics were related to their sensing propertied to NO₂. The results show that the growth behaviors of CuPc on glass and gold surfaces were different in the early growth stage, but their surface morphologies were similar at the later stage. The CuPc films have structures of amorphous and β form crystallite, respectively, on gold and glass surfaces. After heat annealing, the grains grow larger and a β form structure were obtained on both surfaces. The films without heat treatment show a short response time and higher sensitivity to NO₂ due to the higher surface area of their loosely packed fine-grain structure. But this characteristic also causes in-diffusion of NO₂ deeply into the bulk crystal and thus, the recovery process is slow and not completely reversible. Comparing with CuPc films, ttb-CuPc films have smoother morphology both on glass and gold and thus, the gas sensitivity is much smaller due to their smaller exposing area for the adsorption of NO₂. The ttb-CuPc films have a diffraction peak at D = 16.04 angstroms after heat annealing. The substitution of tetra-tert-butyl group into the phthalocyanine ring leads to a larger lattice spacing of crystal, which may be the reason for ttb-CuPc films to have faster recovery process. Because no apparent variation of the crystal morphology was found on the heat-treated ttb-CuPc films, the gas sensing properties vary little due to the heat treatment.