Extended Space Charge Region and Unoccupied Molecular Band Formation in Epitaxial Tetrafluorotetracyanoquinodimethane Films

Abstract

Generating well-defined molecular structures at inorganic/organic interfaces and within molecular films is fundamental for charge carrier transport and thus the performance of organic molecule-based (opto)­electronic devices. Here we show by means of low-energy electron diffraction that tetrafluorotetracyano­quinodimethane (F4TCNQ) grows in an epitaxial fashion on the Au(111) surface, resulting in a unit cell which consists of one molecule. In this well-ordered crystalline films we found the formation of an extended space charge region and a dispersing unoccupied electronic molecular state using energy- and angle-resolved two-photon photoemission. The latter finding is a clear proof for band formation in the crystalline molecular structure. We suggest that the high electron affinity of F4TCNQ and a bandlike electron transport are responsible for the formation of the space charge region. Using F4TCNQ as a hole injection layer may open the opportunity to manipulate the hole injection barrier in a controlled way via variation of the F4TCNQ layer thickness.

Direct Experimental Evidence for Substrate Adatom Incorporation into a Molecular Overlayer

“…Deposition of F 4 TCNQ onto the Au(111) surface held at room temperature, from an organic molecular beam deposition source, led to the formation of the ordered phase previously identified by STM 1 and LEED. 27 Figure 1 a shows the resulting LEED pattern, obtained using a low incident current multichannel-plate amplified optics (MCP-LEED). Due to the low symmetry of the unit mesh of this phase, the pattern is a sum of the patterns from multiple domains related by the rotational and mirror symmetry elements of the substrate.…”

A Comparative Computational Study of the Adsorption of TCNQ and F4-TCNQ on the Coinage Metal Surfaces

“…The adsorption of TCNQ and F4-TCNQ on different metal surfaces has been the subject of many different experimental and theoretical studies. ,,− From the early results, it was clear that both molecules adsorb flat on most metal surfaces, accepting negative charge from the substrate and causing an increase in the work function of the system. Maybe the only exception is the adsorption of TCNQ on the Au(111) surface.…”

Role of Adatoms for the Adsorption of F4TCNQ on Au(111)

“…To study the role of adatoms, we chose F4TCNQ on Au(111), since multiple independent studies have found that it readily and reproducibly forms well-ordered structures containing adatoms as part of the adlayer. 35 , 44 , 46 , 47 The surface structure consists of only one F4TCNQ molecule and one adatom per unit cell, which reduces the computational effort and facilitates the analysis of this system (compared to larger unit cells). The strong interactions between the adsorbate and substrate lifts the herringbone reconstruction of Au(111), and the F4TCNQ molecule-Au-adatom network is arranged in a Au(111) surface supercell, as shown in Figure 1 .…”