AVS 59th Annual International Symposium and Exhibition
    Applied Surface Science Friday Sessions
       Session AS+TF+VT-FrM

Paper AS+TF+VT-FrM2
Differences in the Electronic Structure Highly-Oriented Films of H2-, Fe-, Co-, and Cu-Phthalocyanines Revealed by NEXAFS Spectroscopy

Friday, November 2, 2012, 8:40 am, Room 20

Session: Surface Analysis using Synchrotron Techniques
Presenter: M. Bagge-Hansen, Lawrence Livermore National Laboratory
Authors: T.M. Willey, Lawrence Livermore National Laboratory
M. Bagge-Hansen, Lawrence Livermore National Laboratory
J.R.I. Lee, Lawrence Livermore National Laboratory
R. Call, Lawrence Livermore National Laboratory
L. Landt, Lawrence Livermore National Laboratory
T. van Buuren, Lawrence Livermore National Laboratory
C. Colesniuc, University of California, San Diego
C.M. Monton, University of California, San Diego
I. Schuller, University of California, San Diego
Correspondent: Click to Email
Phthalocyanines are extensively studied as molecular semiconductor materials for chemical sensors, dye-sensitized solar cells, and other applications. Pthalocyanines offer high tunability through the choice of metal center atom; nearly all transition metals and many other heaver elements can reside at the relatively stable square planar center of the phthalocyanines. H2-, Fe-, Co-, and Cu-phthalocyanine molecules in films deposited on gold substrates show prostrate orientation, as opposed sapphire substrates, where phthalocyanines stand in a more upright conformation under deposition conditions used. Angular dependence in NEXAFS, commonly attributable to π* and σ* resonances, in both carbon and nitrogen K-edges, quantify the orientational order. H2-phthalocyanine shows the cleanest angular dependence, with nearly no intensity in the π* regime with normal beam incidence. Metal L-edges in prostrate films, on the other hand, have dramatic variation in angular dependence of resonances into empty states. Fe- and Co- resemble the K-edges; StoBe DFT shows that the lowest-energy allowed resonances are indeed molecular π* states, with a high degree of mixing with the dxz and dyz orbitals of the metals. In contrast, the intense, in-plane resonance of the Cu-PC L-edge LUMO resembles a molecular σ* state. Confirmed by StoBe, the dx2-y2 character at the Cu center is responsible for this intense in-plane resonance. NEXAFS thus directly probes the electronic structure, illuminating the uniqueness of Cu- compared to H2, Fe-, and Co- phthalocyanines.