Optical spectroscopy and scanning tunneling microscopy of thin films of the metal-tetracyanoquinodimethane derivatives

Semiconducting thin films of charge-transfer complexes which exhibit a novel field-induced phase change have been examined by optical spectroscopy and scanning tunneling microscopy (STM). We have demonstrated a field-induced, charge-transfer reaction driven by the electric field at the microscope tip when the field generated by the microscope exceeds the switching threshold of the organic charge-transfer complex: CuTCNQCl2 (TCNQ = tetracyanoquinodimethane). The threshold of phase transition induced by the scanning tunneling microscope for CuTCNQCl2 is 7.5 ×105 V/cm, whereas for the parent compound CuTCNQ it is 1.0 × 105 V/cm. These threshold values are proportional to the strength of the acceptor, i.e., the first half-wave reduction potential E1 of the acceptor molecule. We used an acetonitrile-vapor-assisted film preparation method to deposit weak donor-acceptor complexes (such as 2,5-dialkyl- and 2,5-dialkoxy-substituted TCNQ derivatives). These complexes do not react with copper metal by the conventional post-heating method. These deposited films have been analyzed by UV-Vis and FT-IR spectroscopy and the surfaces have been imaged using STM. This STM-induced solid-state chemical reaction is being explored for possible development as a high-density molecular-based information storage media.