Abstract
The current study generates profound atomistic insights into doping-induced changes of the optical and electronic properties of the prototypical PTCDA/Ag(111) interface. For doping K atoms are used, as KxPTCDA/Ag(111) has the distinct advantage of forming well-defined stoichiometric phases. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combine state-of-the-art density-functional theory calculations with optical differential reflectance data, photoelectron spectra, and X-ray standing wave measurements. In combination with the full structural characterization of the KxPTCDA/Ag(111) interface by low-energy electron diffraction and scanning tunneling microscopy experiments (ACS Nano2016, 10, 2365–2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal–organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications. The combination of the employed complementary techniques allows us to understand the peculiarities of the optical spectra of K2PTCDA/Ag(111) and their counterintuitive similarity to those of neutral PTCDA layers. They also clearly describe the transition from a metallic character of the (pristine) adsorbed PTCDA layer on Ag(111) to a semiconducting state upon doping, which is the opposite of the effect (degenerate) doping usually has on semiconducting materials. All experimental and theoretical efforts also unanimously reveal a reduced electronic coupling between the adsorbate and the substrate, which goes hand in hand with an increasing adsorption distance of the PTCDA molecules caused by a bending of their carboxylic oxygens away from the substrate and toward the potassium atoms.
Highlights
Alkali-metal-doped organic semiconductor films have demonstrated a wide variety of interesting properties and have been used in numerous applications ranging from superconductivity[1−5] to hydrogen storage[6,7] and batteries.[8]
The dopinginduced charge transfer significantly influences the optical properties of organic molecules,[16] where the nature of the formed intragap states depends on the amount of induced charges rather than the type of the dopant used.[17]
During the growth of PTCDA and the subsequent deposition of K we performed differential reflectance spectroscopy (DRS) experiments in real time to determine the optical properties of the samples.[28]
Summary
Alkali-metal-doped organic semiconductor films have demonstrated a wide variety of interesting properties and have been used in numerous applications ranging from superconductivity[1−5] to hydrogen storage[6,7] and batteries.[8]. K-doped perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on Ag(111) is a well-suited system for studying the impact of doping on a molecular monolayer in the presence of interfacial charge transfer with a metal substrate The reasons for this are (i) that PTCDA on Ag(111) is undisputedly the best investigated interface between a noble metal surface and an organic semiconductor molecule[22−26] and (ii) that only specific phases of KxPTCDA with well-defined stoichiometries exist on Ag(111), as known from a previous study:[19] At comparably low K-doping levels (in the following referred to as K2PTCDA/Ag(111)), the well-known herringbone structure of PTCDA on Ag(111)[22] (see Figure 1a) spontaneously converts into another highly ordered phase with two K atoms and one PTCDA molecule per adsorbate unit cell. At even higher potassium content has another well-ordered phase been detected, which is, unstable and degrades within an hour at room temperature by segregation of K, thereby forming the K4PTCDA/Ag(111) phase with a relatively high defect concentration.[19]
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