Adsorption and electronic properties of PTCDA molecules on Si(111)-(7 X 7): Scanning tunneling microscopy and first-principles calculations

Scanning tunneling microscopy (STM) experiments and densityfunctional theory (DFT) calculations are combined to unravel the complex shifts and splittings of molecular orbitals (MOs) for the prototype system of a single pi conjugated molecule bonded to a semiconductor surface. Intramolecular resolution in STM images of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on Si(111) (7 X 7) cannot be understood as resulting from a simple rigid shift of the MOs of the free molecule. DFT calculations and simulations of STM images with realistic tips show large splittings of the original MOs that contribute in a complex way to the tunnel current and are understood under symmetry and chargetransfer arguments. The system is characterized by a strong, partially ionic covalent bonding involving the carboxyl groups of the PTCDA and the Si dangling bonds.