Single PTCDA molecules on planar and stepped KCl and NaCl(100) surfaces
Introduction
A great interest has been recently devoted to the adsorption of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules (C24H8O6) on different metallic and isolated surfaces [1], [2], [3], [4]. This molecule shows a remarkable ability of forming well-defined long-range ordered arrangements as well as good epitaxial growth properties on various surfaces [5]. These properties can be ascribed to the non-uniform internal charge distribution within the molecular structure [6], shown schematically in Fig. 1.
On ionic surfaces, like KCl and NaCl, the adsorption of PTCDA molecules is mainly dominated by van-der-Waals and electrostatic forces [7], [8], which arise between the partially charged atomic sites of the molecules and the ions of the surface [9], [10], [11]. Recent studies have shown that PTCDA molecules form commensurable long-range ordered monolayers on these surfaces [12], [13].
Here, first principles calculations on the adsorption of single PTCDA molecules on both KCl(100) and NaCl(100) surfaces are presented. The paper is organized as follows: after introducing the methodology, we compare the adsorption of single PTCDA molecules on planar KCl(100) and NaCl(100) surfaces. Then, the translational as well as the rotational diffusion mechanisms of the molecule are studied. Finally, we discuss the adsorption of the molecules at step edges, which are energetically favorable.
Total-energy density functional calculations have been performed using the Vienna Ab Initio Simulation Package (VASP) [14]. Within the generalized gradient approximation (GGA), the formulation of the PW91 [15] has been used to model the electron exchange and correlation interactions. The projector-augmented wave (PAW) [16] method has been used to described the electron–ion interaction. The electronic wave functions are expanded into plane waves up to a kinetic energy of 400 eV. A semi-empirical scheme based on the London dispersion formula was used to approximately account for the influence of dispersion interactions [17]. Total-energy DFT calculations with this correction (DFT-D) yielded reliable results for various molecular adsorption systems (cf. Refs. [18], [19], [20], [21], [22]). Using the DFT-D approximation, our calculations predict the bulk lattice constants of KCl and NaCl within an error bar less than 1%. The integration of the Brillouin zone was done using a Γ-centered 2 × 2 × 1 k-point mesh with a convergence criterion of 10− 5 eV for the total energy. Six layer slabs were used to model the substrate in the supercell model. During structure optimization, the lowest two atomic layers were fixed to bulk positions, whereas, if not stated otherwise, the adsorbate and the four uppermost surface layers were allowed to relax freely until the forces were lower than 0.03 eV/Å. A vacuum layer of 40 Å separate the material slabs. Supercells of dimensions (4 × 4) surface unit cells have been used to model the single molecule adsorption in order to suppress unwanted interactions between periodic images of the molecule. The same setup was used to model the step edges in a saw tooth-like model system, described in [21]. Adsorption energies (Eads) are calculated as:where Esys is the energy of the adsystem, Esur and Emol are the energies of the clean surface and the energy of the molecule in gas phase, respectively.
Section snippets
PTCDA on planar surfaces
For single PTCDA molecules on planar KCl and NaCl surfaces, various adsorption geometries have been tested and named as SQ1, SQ2, R1 and R2 (Fig. 2). In the SQ1 and SQ2 (R1 and R2) structures, the molecular long-axis is parallel to the [110] ([100]) direction of the surface. The center of the molecule is on top of an anion site in SQ1 and R1, while it is above a cation site in SQ2 and R2. The carboxyl oxygen atoms of the molecule are above two cation atoms of the surface in SQ1 and R1, while
Adsorption at step-edges
The adsorption of PTCDA molecules at NaCl and KCl step edges has been studied experimentally by Karacuban et al. [6] and Guo et al. [22]. Defective step edges oriented along <100> have been proven to be preferred adsorption sites for single PTCDA molecules on both KCl and NaCl surfaces. This result agrees with fluorescence spectroscopy experiments [23] and DFT calculations (cf. Ref. h-aldahhak-2013 and guo2014). At both KCl and NaCl steps, the so-called vacancy site has been identified to be
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