Scanning Tunneling Microscopy-Based Inelastic Electron Tunneling Spectroscopy Study of the FePc 2D Lattice on Au(111) at 77 K

We present a study of the localized vibrational property in a two-dimensional (2D) lattice of self-assembled iron(II) phthalocyanine (FePc) on Au(111) by scanning tunneling microscopy (STM)-based inelastic electron tunneling spectroscopy (IETS) at 77 K. The STM images clearly illustrate the adsorption configurations of both the first and second monolayers (MLs). The IETS signal of the FePc at the first ML of FePc lattice is suppressed by the interaction with the Au substrate. The IETS spectra for the molecules at the second ML differ depending on the adsorption configuration; the molecules inside the 2D lattice, which are strongly tilted from the surface normal, show a set of peaks and dips attributed to out-of-plane vibration modes. In contrast, the less tilted molecules at the edge do not show IETS signals. This study demonstrates that the STM-IETS is still sensitive for the local vibrational properties at 77 K.