STM imaging and spectroscopy of single copperphthalocyanine molecules

A low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) is used to image individual copperphthalocyanine (CuPc) molecules and to perform local spectroscopic measurements on these molecules. Evaporation of CuPc on liquid-nitrogen-cooled graphite substrates leads to nonclustered individual molecules at random positions. STM images at 35 K show protrusions with a height of about 0.2 nm and a diameter of about 1.0 nm, consistent with CuPc molecules that lie flat on the substrate. Current-voltage curves acquired with the STM tip positioned over CuPc show a pronounced peak in dI/dV vs V at a tip voltage of 0.4 ± 0.1 V. This is attributed to resonant tunneling through a molecular level of CuPc.