We have been studying charge transfer complexes to utilize as conductive wires for nanometer scale molecular devices 1, 2 and also for a new type transistor which is switched by the metal-insulator phase transition induced by an external electric field 3, 4. TTF-TCNQ is one of the typical charge transfer complexes with quasi one-dimensional electric conduction. The most characteristic property of TTF-TCNQ as one-dimensional conductor is the metal-insulator transition at 53K, which is called Peierls transition. Ara-Kato et al., however, reported that an evaporated TTF-TCNQ film exhibits higher transition temperature than that of a single crystal 5. This fact suggests that an evaporated TTF-TCNQ has a structural variation having different electronic structures. Such a variation is undesirable for the application of this material to the nanometer scale devices. Therefore, we have carried out the STM/STS measurements for TTF-TCNQ evaporated films, and investigated the variation of tunneling spectra on different shape grains of evaporated films.
The results are categorized into two phenomena. Just after the film growth, semiconductive spectra were found to be dominant. Thin semiconductive films were presumed to cover the sample surfaces because metallic spectra became dominant after annealing the samples at 60°C in N2 for 1.5 hours. Unique domains showing semiconductive spectra even after the annealing also existed. It was found that the difference of grain shapes were related to the different tunneling spectra. A structural model of the TTF-TCNQ evaporated films will be presented according to the results of the STM/STS and other measurements, such as FT-IR, X-ray diffraction etc.
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