Abstract
Alkali-doping is a very efficient way of tuning the electronic properties of active molecular layers in (opto-) electronic devices based on organic semiconductors. In this context, we report on the phase formation and evolution of charge transfer salts formed by 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) in coadsorption with potassium on a Ag(111) surface. Based on an in-situ study using low energy electron microscopy and diffraction we identify the structural properties of four phases with different stoichiometries, and follow their growth and inter-phase transitions. We label these four phases α to δ, with increasing K content, the last two of which (γ and δ-phases) have not been previously reported. During TCNQ deposition on a K-precovered Ag(111) surface we find a superior stability of δ-phase islands compared to the γ-phase; continued TCNQ deposition leads to a direct transition from the δ to the β-phase when the K : TCNQ ratio corresponding to this phase regime is reached, with no intermediate γ-phase formation. When, instead, K is deposited on a surface precovered with large islands of the low density commensurate (LDC) TCNQ phase that are surrounded by a TCNQ 2D-gas, we observe two different scenarios: on the one hand, in the 2D-gas phase regions, very small α-phase islands are formed (close to the resolution limit of the microscope, 10–15 nm), which transform to β-phase islands of similar size with increasing K deposition. On the other hand, the large (micrometer-sized) TCNQ islands transform directly to similarly large single-domain β-phase islands, the formation of the intermediate α-phase being suppressed. This frustration of the LDC-to-α transition can be lifted by performing the experiment at elevated temperature. In this sense, the morphology of the pure TCNQ submonolayer is conserved during phase transitions.
Highlights
The electronic properties of metal–organic interfaces, and the energy level alignment of the metal Fermi level and the molecular orbital states, play a crucial role in determining the characteristics of electronic devices based on organic semiconductors
Structure of the KxTCNQy phases Besides the two coadsorption phases already reported in the literature, which we introduced above and denoted α and β-phase, in the course of this study we have identified two further phases, γ and δ
Beside the incommensurate α-phase (KTCNQ) and the commensurate β-phase (K2TCNQ), which were already reported earlier [10, 11], we found two new phases γ and δ with higher K : TCNQ ratios, and followed the phase transitions occurring upon K or TCNQ deposition
Summary
Anja Haags1,2 , Luke A Rochford, Janina Felter, Phil J Blowey, David A Duncan , D Phil Woodruff and Christian Kumpf
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