In this communication, we present a theoretical analysis of the position of molecular levels at reactive metal/organic interfaces. We have considered several hydrocarbon oligomers and studied the energy level alignment at interfaces of these organic materials and gold electrodes. The molecules are chemically bound to the metal substrate through thiol end groups, resulting in strongly-interacting, chemisorptive interfaces.
The relative position of the molecular levels with respect to the metal Fermi level has important consequences for interface energetics and transport properties through such molecular wires. The elastic and inelastic conductance of these molecules has already been investigated using state-of-the-art Density-Functional Theory — Non-Equilibrium Green’s Function (DFT-NEGF) methods [1]. However, since DFT methods do not yield accurate values for molecular resonances (in particular, the HOMO-LUMO gap is greatly underestimated), we discuss the impact on molecular transport of molecular level positions at the interface.
We present a method, following the ideas developed in Ref. [2], to calculate molecular level positions beyond the results of DFT. Our approach makes many-body corrections to the molecular energy when an electron or hole is added to the system; the energy corrections associated with these charge excitations result in shifts of the molecular levels and increase the molecular gap with respect to DFT. The effect of these corrections has been analyzed in the context of weakly-interacting metal/organic junctions [3]; here we study strongly-coupled interfaces.
Our results aim in the direction of a systematic improvement of one of the inherent deficiencies of DFT such as the position of molecular levels, which could have important consequences for molecular transport calculations.
References:
[1] M. Paulsson, T. Frederiksen and M. Brandbyge, Nano Lett. 6, 258 (2006).
[2] R. Oszwaldowski, H. Vázquez, P. Pou, J. Ortega, R. Pérez and F. Flores, J. Phys.: Condens. Matter 15 S2665 (2003).
[3] H. Vázquez, R. Oszwaldowski, P. Pou, J. Ortega, R. Pérez, F. Flores and A. Kahn, Europhys. Lett. 65 802 (2004), H. Vázquez, F. Flores, R. Oszwaldowski, J. Ortega, R. Pérez and A. Kahn Appl. Surf. Sci. 234 107 (2004). |