Probing the geometrical and electronic structure of two-dimensional charge transfer networks on metal surfaces

Authors: Philip Blowey (University of Warwick)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: June 2018
Diamond Proposal Number(s): 17261 , 14884 , 15899 , 15122 , 12975 , 11310 , 9459 , 8940

Abstract: Due to its ability to form conductive organic salts, the prototypical electron acceptor molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ) has attracted considerable interest in the field of organic electronics. This has motivated numerous surface science studies of TCNQ and related molecules, with an aim to understand the molecule-substrate interface and, in particular, the nature of any charge transfer. Although charge transfer is strongly dependent on subtle aspects of the molecular adsorption geometry, there is a dearth of detailed structural investigations for these systems. In this thesis, a variety of surface science techniques were used to characterise model systems of TCNQ adsorbed on coinage metal substrates with the aim to identify key relationships between the adsorption structure and the electronic properties of the surface. Particular focus was given to studying two-dimensional charge-transfer networks formed by TCNQ and alkali metals on the surface of Ag. Scanning tunnelling microscopy and low energy electron diffraction were used to characterise the packing and ordering of molecules and to ascertain whether the adsorbed layer is commensurate with respect to the underlying substrate. X-ray and ultraviolet photoelectron spectroscopy were used to provide complementary information on the chemical composition and electronic properties of the surface. Most significantly, the normal incident X-ray standing wave (NIXSW) technique was used to obtain precise quantitative structural measurements of the surface. On the surfaces of coinage metals, TCNQ is generally believed to adsorb in a significantly bent conformation, with all four cyano groups pointing down towards the substrate. The NIXSW measurements in this thesis show that the conformation adopted by TCNQ on Ag(100) is consistent with this, but on Ag(111), TCNQ adopts a considerably different conformation that was found, through comparison with density functional theory calculations, to result from the participation of Ag adatoms within the surface structure. These results also highlight the need for using both experimental and theoretical quantitative structural methods to obtain a reliable understanding of metal-organic interfaces and that some previously studied systems may need to be re-investigated. On both the (111) and (100) surfaces of Ag, a wide variety of TCNQ/alkali metal network structures were formed with Cs, K and Na. NIXSW measurements obtained from a subset of these structures show that the alkali metals adsorb at elevated heights above the TCNQ molecules. In comparable structures, K adsorbs closer to the surface than Cs and causes a smaller shift to the surface work function. The alkali metal adsorption height was also found to decrease as its coverage relative to TCNQ increased.

Subject Areas: Physics

Instruments: I09-Surface and Interface Structural Analysis

Added On: 27/09/2018 11:45

Discipline Tags:

Technical Tags: