Growth and evolution of tetracyanoquinodimethane and potassium coadsorption phases on Ag(111)

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.

The structure of 2D charge transfer salts formed by TCNQ/alkali metal coadsorption on Ag(111)

“…The stoichiometries of these two phases have been identified on the basis of SXP spectra as KTCNQ and K2TCNQ, respectively. A low energy electron microscopy (LEEM) investigation of the development of these two phases also identified two additional, but apparently metastable, phases [17]. STM images from the KTCNQ and K2TCNQ phases are shown in Fig.…”

“…To explore this idea we report here on the full characterisation of the structures formed by coadsorption of TCNQ with Cs and with K on the Ag(111) surface, including quantitative structural data on the adsorption heights using the technique of normal incidence Xray standing waves (NIXSW) [14]. The results of our investigation of TCNQ/K coadsorption phases on this surface, identifying two distinct phases of different TCNQ/K stoichiometry, have already been reported in detail [15,16,17]; these include a particularly complete study of a phase of K2TCNQ stoichiometry, which is commensurate with the substrate and therefore accessible to density functional theory (DFT) calculations. These calculations, aided by the benchmark NIXSW data, gave insight into the nature of the bonding within the overlayer and to the underlying Ag(111) [16].…”

“…Inspection of the associated f values, however, shows that, with the possible exception of the CsTCNQ phase, the N atoms probably occupy sites with more than one height above the surface, which may indicate some twisting of the molecule. However, at least part of the reason for the reduced f values for the CN and N absorbers for K2TCNQ may be due to some co-occupation of KTCNQ domains which does occur during the surface preparation of the higher K coverage phase [25]; these are the atoms with the largest difference in D(111) values for these two phases. The coherent fraction for the Cs atoms in the CsTCNQ phase is also rather low, which also suggests a possible co-occupation of more than one height for this phase.…”

Growth Dynamics and Electron Reflectivity in Ultrathin Films of Chiral Heptahelicene on Metal (100) Surfaces Studied by Spin‐Polarized Low Energy Electron Microscopy

“…[21,22] LEEM has been used to follow the growth of carbon-based and metallic layered materials as well as investigation of electron quantum interference effects that arose between those layers by measuring the energy dependence of electron reflectivity. [23][24][25][26][27][28][29][30][31][32][33] When a spin-polarized electron beam is used for illumination (SPLEEM), the technique becomes sensitive to surface magnetization, thus allowing for complete vector-magnetometric imaging. [34,35] In this contribution we have investigated growth of racemic and enantiopure [7]H on Cu(100) and (12 ML)Ni/Cu(100) substrates, thereby evaluating interface effects such as changes in work function, electron reflectivity, or spin asymmetries.…”

Growth Dynamics and Electron Reflectivity in Ultrathin Films of Chiral Heptahelicene on Metal (100) Surfaces Studied by Spin‐Polarized Low Energy Electron Microscopy

“…[ 21,22 ] LEEM has been used to follow the growth of carbon‐based and metallic layered materials as well as investigation of electron quantum interference effects that arose between those layers by measuring the energy dependence of electron reflectivity. [ 23–33 ] When a spin‐polarized electron beam is used for illumination (SPLEEM), the technique becomes sensitive to surface magnetization, thus allowing for complete vector‐magnetometric imaging. [ 34,35 ]…”