Abstract
Alkali metal atoms are frequently used for simple yet efficient n-type doping of organic semiconductors and as an ingredient of the recently discovered polycyclic aromatic hydrocarbon superconductors. However, the incorporation of dopants from the gas phase into molecular crystal structures needs to be controlled and well understood in order to optimize the electronic properties (charge carrier density and mobility) of the target material. Here, we report that potassium intercalation into the pristine 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) monolayer domains on a Ag(111) substrate induces distinct stoichiometry-dependent structural reordering processes, resulting in highly ordered and large KxPTCDA domains. The emerging structures are analyzed by low-temperature scanning tunneling microscopy, scanning tunneling hydrogen microscopy (ST[H]M), and low-energy electron diffraction as a function of the stoichiometry. The analysis of the measurements is corroborated by density functional theory calculations. These turn out to be essential for a correct interpretation of the experimental ST[H]M data. The epitaxy types for all intercalated stages are determined as point-on-line. The K atoms adsorb in the vicinity of the oxygen atoms of the PTCDA molecules, and their positions are determined with sub-Ångström precision. This is a crucial prerequisite for the prospective assessment of the electronic properties of such composite films, as they depend rather sensitively on the mutual alignment between donor atoms and acceptor molecules. Our results demonstrate that only the combination of experimental and theoretical approaches allows for an unambiguous explanation of the pronounced reordering of KxPTCDA/Ag(111) upon changing the K content.
Highlights
T echnologically relevant electric conductivities σ can be achieved through doping, which has become a vital concept for efficient device architectures.[1,2] Alkali metals are frequently employed as electron donors due to their comparatively small ionization energies and relatively straightforward processability.[3−6] Molar doping ratios x on the order of several percent already lead to a substantial increase of σ.4 Higher ratios x have been shown to boost σ by as much as 6 orders of magnitude, as demonstrated for the prototypical dye molecule
The emerging structures are analyzed by low-temperature scanning tunneling microscopy, scanning tunneling hydrogen microscopy (ST[H]M), and low-energy electron diffraction as a function of the stoichiometry
As a starting point for the structural investigations we prepared a pristine perylenetetracarboxylic dianhydride (PTCDA) film on Ag(111), ca. 0.7 MLE thick, which consists of highly ordered, densely packed molecular domains exhibiting the well-known herringbone motif shown in Figure 1a,b.25−28 Part of the silver surface is deliberately left uncovered to allow for possible structural rearrangements without forcing molecules to adopt adsorption sites in the second layer
Summary
T echnologically relevant electric conductivities σ can be achieved through doping, which has become a vital concept for efficient device architectures.[1,2] Alkali metals are frequently employed as electron donors due to their comparatively small ionization energies and relatively straightforward processability.[3−6] Molar doping ratios x (i.e., number of dopants per host molecule) on the order of several percent already lead to a substantial increase of σ.4 Higher ratios x have been shown to boost σ by as much as 6 orders of magnitude, as demonstrated for the prototypical dye molecule. T echnologically relevant electric conductivities σ can be achieved through doping, which has become a vital concept for efficient device architectures.[1,2] Alkali metals are frequently employed as electron donors due to their comparatively small ionization energies and relatively straightforward processability.[3−6] Molar doping ratios x (i.e., number of dopants per host molecule) on the order of several percent already lead to a substantial increase of σ.4. Article van der Waals bound molecular crystals are prone to reordering processes upon intercalation with atoms leading to new crystalline or amorphous phases.[15−21] For doped molecular thin films in the monolayer regime the presence of an interface with the substrate further increases the complexity of the system by providing additional interaction channels and, possibly influencing the physical and electronic structures extensively. Intercalated atoms were inferred only indirectly from the visualization of bonding channels between molecule and dopant.[24]
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