The investigations of in situ evaporated tetra-tert-butyl copper phthalocyanine (TtertBuCuPc) and free-base tetra-tert-butyl phthalocyanine (TtertBuH2Pc) thin films, 3 to 5 monolayer thickness, on Si(111) and SiO2/Si(111) substrates as well as ex situ prepared Langmuir-Blodgett (LB) films of TtertBuCuPc were performed on beamline 4.1 at the SRS storage ring (Daresbury) by means of valence-band X-ray photoelectron spectroscopy (VB XPS) and on beamline SX700 at ISA (Aarhus) by means of near-edge x-ray adsorption fine structure (NEXAFS) spectroscopy. The results obtained were compared with scanning tunnelling microscopy/spectroscopy (STM/STS) studies of TtertBuCuPc thin films on Ag/Si(111) substrate. The angular dependent VB XPS and NEXAFS spectra taken from in situ evaporated thin films of TtertBuCuPc and TtertBuH2Pc and ex situ prepared LB films of TtertBuCuPc have shown that in all cases the phthalocyanine molecules under study form well-ordered films, in which the molecular plane is perpendicular to the substrate surface ("standing" orientation). In the case of a one monolayer film of the TtertBuCuPc prepared on Ag/Si(111)-(√3×√3) substrate, the molecules form a well-ordered dimer-like structure, in which the molecular plane is parallel to the substrate surface ("lying" orientation). The comparison between STS, VB XPS, XAS data and DFT calculations [1] has shown that highest-lying occupied molecular orbital (HOMO) is of b1g symmetry, consists mainly of Cu 3d(x2-y2) electron states and is half occupied. The lowest-lying unoccupied molecular orbital (LUMO) is of eg symmetry and consists of N 2p and C 2p electron states. The HOMO-LUMO band gap was found to be equal to 1.1 eV.
Reference:
(1) M.-S. Liao, S. Scheiner, J. Chem. Phys. 114 (2001) 9780. |