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Organic/Metal Interfaces

The knowledge of the electronic structure at the interface between organic molecules and a metal surfaces is of fundamental importance for the function of all organic electronics. Especially the progression in miniaturization of structures lead to an increasing influence of the surface properties on the overall characteristics of a device. We investigate very thin layers of organic molecules on top of metal single crystals to examine the electronic structure as well as the temporal dynamics of excitations.
The chosen molecules are mostly well understood in terms of growth properties and physical structure and form commensurate layers by molecular beam epitaxy. However very little is known about the electronic properties. The area of research includes model systems which consists of one molecule type as well as combinations of different organic semiconductors.


Formation of Metal–Organic Interface States

Upon deposition of a monolayer (ML) of 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) on top of Silver (111) (Ag(111)), the partially filled Shockley surface state disappears at its former energetically position and shifts up by 0.62 eV to become an unpopulated interface state (IS).

interfacestate.png

Left: normal emission single-color 2PPE spectra of PTCDA=Ag(111) for different coverage (top) and the clean surface (bottom). The clean surface (black) shows the image potential states, a signal originating from the sp-band and the Shockley-state. Upon adsorption of PTCDA, the Shockley state vanishes and the image potential states are damped. In addition a new state rises at 0.6 eV above EFermi (IS).

Center: Map of the measured dispersion of the interface state (IS) for 1 ML (circles) and 2 ML (diamonds) of PTCDA together with the projected Ag(111) bulk bands (gray shaded area) and the Shockley state of the clean surface (dotted line). The effective mass at the gamma point (meff = 0.46 +- 0.1) is very close to the effective mass of the Shockley state (meff = 0.42 eV) and shows a lifetime of 54 fs at room temperature (right). The short lifetime indicates a high overlap of the IS wave function with the metal, which could be confirmed by DFT calculations.

 

ntcdawavefunction.png

The above figure displays a DFT-calculated wave function of 1,4,5,8-Naphthalenetetracarboxylic dianhydride on Silver (NTCDA/Ag(111)). IS probability density perpendicular to the surface, xy averaged over the unit cell. Inlay: Spatial distribution of the IS probability density at the gamma-point in the vicinity of the molecular plane.

issystematic.png

The formation of interface states was observed for a variety of combinations between organic semiconductors and metal substrates. The energetic position and lifetime show a systematic dependence on the bonding distance.


contact: Andreas Namgalies, Alexander Lerch, Prof. Dr. Ulrich Höfer


Literature

M. Marks, N. L. Zaitsev, B. Schmidt, C. H. Schwalb, A. Schöll, I. A. Nechaev, P. M. Echenique, E. V. Chulkov and U. Höfer
Energy shift and wave function overlap of metal-organic interface states
Phys. Rev. B 84, 081301 (2011). DOI: 10.1103/PhysRevB.84.081301 Abstract Reprint (PDF) (© APS)

C. H. Schwalb, S. Sachs, M. Marks, A. Schöll, F. Reinert, E. Umbach, and U. Höfer
Electron lifetimes in a Shockley-type metal-organic interface state
Phys. Rev. Lett. 101, 146801 (2008). DOI: 10.1103/PhysRevLett.101.146801 Abstract Reprint (PDF) (© APS)

S. Sachs, C. H. Schwalb, M. Marks, A. Schöll, F. Reinert, E. Umbach1 and U. Höfer
Electronic structure at the perylene-tetracarboxylic acid dianhydride/Ag(111) interface studied with two-photon photoelectron spectroscopy
J. Chem. Phys. 131, 144701 (2009). DOI: 10.1063/1.3243851 Abstract Reprint (PDF) (© AIP)

C. H. Schwalb, M. Marks, A. Schöll, F. Reinert, E. Umbach, and U. Höfer
Time-resolved measurements of electron transfer processes at the PTCDA/Ag(111) interface
European Physical Journal B 75, 23 (2010). DOI: 10.1140/epjb/e2010-00106-6 Abstract Reprint (PDF) (© AIP)

M. Marks, A. Schöll, and U. Höfer
Formation of metal-organic interface states studied with 2PPE
J. Electr. Spectrosc.195, 263 (2014). DOI: 10.1016/j.elspec.2014.02.009 Abstract Reprint (PDF) (© Elsevier)

Last modified: 04.04.2016 · armbrusn

 
 
 
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