Effects of heat annealing on the ®lm characteristics and gas sensing properties of substituted and un-substituted copper phthalocyanine ®lms

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 ®lm morphology and crystalline structure were studied. The ®lm 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 ®lms have structures of amorphous and b form crystallite, respectively, on gold and glass surfaces. After heat annealing, the grains grow larger and a b form structure were obtained on both surfaces. The ®lms without heat treatment show a short response time and higher sensitivity to NO2 due to the higher surface area of their loosely packed ®ne-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 ®lms, ttb±CuPc ®lms 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 ®lms have a diffraction peak at D ˆ 16:04 AÊ 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 ®lms to have faster recovery process. Because no apparent variation of the crystal morphology was found on the heat-treated ttb±CuPc ®lms, the gas sensing properties vary little due to the heat treatment. # 2001 Elsevier Science B.V. All rights reserved.