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.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)