Scanning tunneling microscopy of TTF-TCNQ single crystal and thin film

We have studied single crystals and thin films deposited on alkali halides of one-dimensional organic conductor, tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), by scanning tunneling microscope (STM). The molecular structures of both samples imaged by STM were similar and the I-V (tunneling current—bias voltage) curves measured were both metallic at room temperature. However, when thin films were cooled from room temperature, some parts of films showed to be insulating at ≈200K which is much higher than the critical temperature of the metal-insulator phase transition which was observed for the bulk single crystal (53K). The results may suggest that the charge density wave occurs at higher temperature in thin films as in the case of (BEDT-TTF)2I3 films.