Adsorbed states of iron(II) phthalocyanine on Ag(111) studied by high-resolution electron energy loss spectroscopy Naoka Ohta, a * Ryuichi Arafune, b Noriyuki Tsukahara, a Noriaki Takagi a and Maki Kawai a We investigated the adsorption states of iron(II) phthalocyanine (FePc) molecules on Ag(111) by using high-resolution elec- tron energy loss spectroscopy (HREELS). The HREELS spectra were taken as a function of the coverage of FePc. At 1 monolayer, vibrational modes assigned as A 2u and A 1g representations were observed mainly by the dipole scattering mechanism, which indicates that the first-layer molecules adsorb with the molecular planes parallel to the surface. The vibration associated with the motion of the central Fe 2+ ion perpendicular to the molecular plane shows a considerable shift of the vibration energy. This reflects significant bonding interactions between the FePc molecule and the substrate. In the high coverage regime, mul- tilayers were grown, and the vibration modes attributed to not only A 2u but also E u representation were observed. The FePc molecules in the upper layers tilt the molecular planes to the substrate. The vibrational energy is almost identical to those in the gas phase, indicating weaker interactions between the molecules. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: high-resolution electron energy loss spectroscopy; iron phthalocyanine; Ag(111); thin film; adsorption configuration; vibrational spectra; molecule–substrate interaction Introduction Phthalocyanines (Pc) and metal Pc (MPc) have unique electronic and optical properties, and they are considered as one of the pro- spective materials used for electronic devices, [1] solar cell, [2,3] cat- alyst [4] , etc. The Pc molecule adsorbed on single crystal surface is a model system to understand how the couplings at the mole- cule–substrate interface affect the geometric and electronic structures of the molecule and the performance of the de- vices. [5,6] Thus, the adsorption of Pc and MPc on single crystal sur- face has been studied intensively by various experimental and theoretical methods. [5–7] Surface vibrational spectroscopies are powerful tools to evaluate the adsorption states of molecule at surface and have been applied successfully to the characteriza- tion of bonding at the molecule–substrate interface. In this study, we investigated the adsorbed states of iron(II) Pc (FePc) on Ag(111) by using high-resolution electron energy loss spectroscopy (HREELS). We compare the spectra taken at low coverage with those for high coverages and then discuss the bonding between the first-layer molecule and the substrate together with the intermolecular couplings in the multilayer. Experimental methods The experiments were conducted in an ultra-high vacuum system equipped with an electron spectrometer for HREELS (Omicron, IB-500) and optics for low-energy electron diffraction (LEED). The Ag(111) surface was cleaned by repeated cycles of Ar + ion sputtering and annealing at 720 K. The cleanliness was checked by HREELS. FePc molecules were deposited on the clean Ag(111) surface kept at room temperature by heating the cell of FePc at 590 K. The EELS spectra were measured at room temper- ature with incident electron energy of 4.5 eV in specular and off- specular geometries. The incident angle is set to be 60° with respect to the surface normal. The coverage of FePc on Ag(111) was estimated by Auger electron spectroscopy (AES). AES spectra were measured by using LEED optics. One monolayer (ML) is defined as the surface fully covered with the FePc in the first layer (0.606 molecules/nm 2 ) [8] . The AES intensity of C 1s for the Ag(111) surface saturated with dimethyl disulfide (the density of C atoms is 5.93 atoms/nm 2 ) [9] is used as a reference for determining the coverage of FePc. The estimated error of the coverage is 30%. Results and discussion Figure 1 shows the HREELS spectra of 1 ML and multilayer FePc adsorbed on the Ag(111) surface taken with specular geometry. The spectrum of 1 ML [Fig. 1 (bottom)] shows prominent double peaks around 50 meV (47.5 and 53.5 meV) and 90 meV (89.5 and 94 meV). In addition, small peaks appear at 21, 103, 115, 167, and 189 meV. The gradual increase above ~350 meV comes from * Correspondence to: N. Ohta, Department of Advanced Materials Science, Graduate School of Frontier Science, The University of Tokyo 5-1-5-402 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan. E-mail: 0999273493@mail.ecc.u-tokyo.ac.jp a Department of Advanced Materials Science, Graduate School of Frontier Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan b MANA, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Surf. Interface Anal. 2014, 46, 1253–1256 Copyright © 2014 John Wiley & Sons, Ltd. Special issue article Received: 19 March 2014 Accepted: 26 March 2014 Published online in Wiley Online Library: 15 May 2014 (wileyonlinelibrary.com) DOI 10.1002/sia.5529 1253