Vacuum deposited thin layers of phthalocyanine and its derivatives carry a lot of interest due to their application in different opto-electronic devices. The applications based on these molecules require a clear understanding of their electronic structure. In this paper we show the experimental and theoretical understanding of the tunneling spectra of nickel-phthalocyanine (NiPc). An ultra-thin layer (monolayer) of NiPc is prepared using Organic Molecular Beam Epitaxy (OMBE) on HOPG under UHV conditions at room temperature. HOPG was selected to minimize the interaction of the molecules with the substrate, which is helpful to understand the electronic structure of single molecules.
Typical current-voltage characteristics and the normalized differential characteristics were obtained for NiPc adsorbed on HOPG. I-V shows a region with no current around 2 eV between the onsets. This is typically expected for the semiconductor-type molecules and arises due to the HOMO-LUMO gap. Normalized differential conductivity shows distinct peaks corresponding to the molecular resonances. Separate Gaussian fit of spectrum show clearly three peaks. The peaks are assigned to HOMO-1, HOMO and LUMO. The origin of these peaks was understood in terms of the single molecule electronic structure calculated using Gaussian 03 with B3LYP methods. Different basis sets were used to optimize the structure and the DOS of single molecules are compared with the experiments.
The tunneling spectra were recorded at different current set-points and reproduced the spectrum again. The HOMO and HOMO-1 becomes well resolved in to separate peaks however the HOMO shifts towards the Fermi energy with increasing set points. The position of the LUMO is pinned and therefore the HOMO-LUMO gap decreases with increasing set-current. The scenario is discussed by a local charging of molecular orbitals due to the tunnel current originating from the tip. A similar HOMO-LUMO gap shrinking with LUMO shift was observed for Naphthalocyanine [1]. The difference in the electronic nature of these molecules and the interface between molecule and substrate were found to be the reason for tip-induced either HOMO or LUMO shift.
[1] T. G. Gopakumar, F. Müller, M. Hietschold, J. Phys. Chem. B. 2006, 110(12), 6060. |