Probing the interplay between geometric and electronic structure in a two-dimensional K-TCNQ charge transfer network.

P J Blowey,D A Warr,L A Rochford,D P Woodruff,T.-L Lee,D A Duncan,G Costantini

doi:10.1039/c7fd00093f

Abstract

Scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), ultraviolet and soft X-ray photoelectron spectroscopy (UPS and SXPS) have been used to characterise the formation of a coadsorption phase of TCNQ and K on Ag(111), while the normal incident X-ray standing waves (NIXSW) technique has been used to obtain quantitative structural information. STM and LEED show that an ordered incommensurate phase is formed in which the K atoms are surrounded by four TCNQ molecules in a 'windmill' motif, characteristic of other metal/TCNQ phases, in which the nominal TCNQ : K stoichiometry is 1 : 1. UPS and SXPS data indicate the TCNQ is in a negatively-charged state. NIXSW results show that the carbon core of the TCNQ is essentially planar at a height above the Ag(111) surface closely similar to that found without coadsorbed K. In the presence of TCNQ the height of the K ions above the surface is significantly larger than on clean Ag(111), and the ions occupy sites above 'holes' in the TCNQ network. NIXSW data also show that the N atoms in the molecules must occupy sites with at least two different heights above the surface, which can be reconciled by a tilt or twist of the TCNQ molecules, broadly similar to the geometry that occurs in bulk TCNQ/K crystals.

Highlights

The use of thin molecular lms in organic electronic devices (OEDs) has attracted a signi cant amount of interest in recent years, offering a low-cost, transparent, exible and light-weight alternative to typical inorganic semiconductor devices.[1,2] Paper these devices have already seen some commercial success, further optimisation is required to meet the demands of current electronic applications
The molecules assemble into windmill-like structures, consisting of four TCNQ molecules spiralling around a central point
This packing bears a strong resemblance to the bulk crystal structure of K–TCNQ in which layers of TCNQ molecules arrange in similar four-fold coordinated windmill structures in layers on either side of a plane of K+ ions.[26]

Summary

Introduction

The use of thin molecular lms in organic electronic devices (OEDs) has attracted a signi cant amount of interest in recent years, offering a low-cost, transparent, exible and light-weight alternative to typical inorganic semiconductor devices.[1,2] Paper. These devices have already seen some commercial success, further optimisation is required to meet the demands of current electronic applications. One example of this is the two-dimensional (2D) networks that are formed between the strong electron acceptor molecule tetracyanoquinodimethane (TCNQ) and alkali metals.[11,12]

Methods

Results

Conclusion