Paper SS+AS+NS+SP-WeA4
Electronic Structure of H₂Pc on Ultrathin NaCl Films with Various Thickness

Wednesday, October 30, 2013, 3:00 pm, Room 202 A

We have investigated thickness dependence of electronic states of single metal-free Phthalocyanine (H₂Pc) on NaCl insulating films on Au(111), using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Our aim is to understand how the thickness of NaCl films affect the molecular properties using the well-known and widely studied molecule.[1] NaCl films are known to weaken the molecular-metal interaction,[2] but we found the electronic properties of the H₂Pc adsorbed on 2 ML and 3 ML-thick NaCl are indeed different. Isolated H₂Pc molecules are adsorbed on both 2 ML and 3 ML thick NaCl islands, which are grown from Au(111) step edges. Appearances of H₂Pc are similar for both islands, four-lobe or eight-lobe depending on bias voltages. STS spectra acquired at the centers of molecules on both islands show one peak in occupied and one in unoccupied states. By visualizing spatial distribution of density of states at the peak bias, we found that these two peaks corresponded to the highest occupied molecular orbital (HOMO), and degenerated lowest unoccupied molecular orbitals (LUMO and LUMO+1). A difference between 2ML and 3 ML is the peak position of degenerated LUMO and LUMO+1 states and thus HOMO-LUMO gap. Our observation implies that the effect of NaCl is not just to decouple metal electronic states, but it indeed affects the electronic states of adsorbed molecules, which might arise from the metal-insulator interaction such as interfacial dipole.

[1] T. Komeda, H. Isshiki, and J. Liu, Sci. Tech. Adv. Mater.11, 054602 (2010).

[2] J. Repp, G. Meyer, S. Stojković, A. Gourdon, and C. Joachim, Phy. Rev. Lett.94 (2005).