Enhancement of Inelastic Electron Tunneling Conductance Caused by Electronic Decoupling in Iron Phthalocyanine Bilayer on Ag(111) Naoka Ohta, † Ryuichi Arafune, ‡ Noriyuki Tsukahara, † Maki Kawai, † and Noriaki Takagi* ,† † Department of Advanced Materials Science, Graduate School of Frontier Science, The University of Tokyo, Kashiwa 5-1-5, Chiba, 277-8561 Japan ‡ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Ibaraki, 304-0044 Japan * S Supporting Information ABSTRACT: The effect of electronic decoupling on the inelastic electron tunneling process of iron phthalocyanine (FePc) molecules on Ag(111) was investigated using scanning tunneling microscopy (STM). A drastic difference in the inelastic electron tunneling to individual FePc molecules was found for the first and the second layer molecules grown on Ag(111). The spectrum of the first layer molecule is essentially structureless, whereas the second layer molecules provide giant conductance changes reaching several tens % due to the vibrational excitations. This is the first clear example to demonstrate, by using inelastic tunneling spectroscopy with STM, the enhancement of vibrational inelastic tunneling driven through the electronic decoupling of the molecules from the substrate. 1. INTRODUCTION The vibrational spectrum is utilized for chemical identification as a fingerprint of a molecule so that vibrational spectroscopy is one of the indispensable tools in broad areas of molecular science including biochemistry and physiological chemistry. The advent of scanning tunneling microscopy (STM) enables us to identify chemical species at conducting surfaces on an atomic scale. In particular, the inelastic electron tunneling spectroscopy (IETS) with STM provides vibrational spectra of individual molecules. 1-3 When the sample voltage (V) relative to the STM tip meets the condition of |V| > ℏω/e, where ℏω is the energy of the molecular vibration and e is the elemental charge, not only the elastic tunneling but also the inelastic tunneling contributes to the total tunneling current (I), leading to the conductance change in the dI/dV spectrum at |V| = ℏω/e. The conductance change usually gives rise to a step structure in the dI/dV spectrum. Since the changes associated with the vibrational excitations are usually small (at most a few %), they are often buried in the background noise. As a consequence, it is still demanding to pick up the conduction change experimentally in spite of the current leap in the STM instrumentation. The vibrational excitation in the inelastic tunneling process is understood based on the resonant tunneling mechanism. 4-9 When an electron tunnels from an STM tip to a substrate through a molecule, it is trapped in the molecular state with a certain lifetime. The formation of this transient state leads to a change of internuclear potential which induces the deformation of the molecular structure and then leaving the vibrationally excited state of the molecule in the electronic ground state after the electron escapes into the substrate. Based on the resonant tunneling mechanism, the inelastic excitation process is governed by two factors: One is the lifetime of the transient state and the other is the accessibility to the molecular states from the Fermi level. Since one can elongate the lifetime by decoupling the molecule from the substrate, 10 it should be possible to enhance the conductance change associated with the vibration excitation through tailoring the strength of the coupling at the molecule-substrate interface. Several studies have demonstrated that the progression of vibronic states is observed for individual molecules on metal substrates by electronically decoupling them from the substrate. 11-13 In these studies, the molecules are electronically isolated from the substrate electronic systems by inserting an ultrathin oxide layer 11 and monolayer of organic molecules 12 or by a self-decoupling scheme where a subunit of the molecule itself works as an electronic decoupler. 13 As a result, the lifetime of the temporal anion and/or cation state is sufficiently long that the vibrational ladder of the anion state appears in the density of state spectrum. In contrast, few works have been reported to shed light on the impact of the electronic decoupling on the inelastic excitation of vibration. A molecular bilayer on a metal substrate is a model system to verify the hypothesis about the impact of the electronic decoupling on the IETS process. The first layer works as a buffer layer to isolate the second layer electronically from the substrate electronic system to elongate the lifetime of the electronic state in the second layer molecule. The planar organic molecule such as phthalocyanine (see Figure 1a) and Received: June 26, 2013 Revised: September 24, 2013 Published: October 11, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 21832 dx.doi.org/10.1021/jp406317t | J. Phys. Chem. C 2013, 117, 21832-21837