NCAFM2023 Programme Booklet

Wednesday 1100 - 1120

FIRST PRINCIPLES BOND RESOLVED STM OF PTCDA WITH pyFDBM

Emiliano Ventura-Macias 1 , Pablo Pou 1,2 , Ruben Perez 1,2*

1 Dpt. of Condensed Matter Physics Theory, Universidad Autónoma de Madrid, Madrid, Spain 2 Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Madrid, Spain Email: ruben.perez@uam.es

STM measurements with a CO-functionalized tip are a promising technique for intramolecular imaging [1,2,3]. They take advantage of the CO-tip deflection–due to short-range Pauli repulsion–to image bonds inside adsorbed molecules. Furthermore, the use of systems with combined AFM/STM measurements has opened the question of how to model such experiments. Modeling BRSTM introduces two challenges: (1) finding the position of the CO and (2) the contribution of the p x p y orbitals of the CO to the tunneling current. Our previous work showed how we tackled both challenges with the pyFDBM first-principles framework. It uses the Full Density-Based Model (FDBM) to find the CO-tip deflection [4] and the approximation of the signal from the CO p x p y orbitals described in [5]. In this work, we validate our BRSTM simulations with experimental images of PTCDA over Ag(111), and then we investigate a new pinwheel phase of the same system. In the first part, we looked at the importance of including the substrate in AFM and STM simulations. In particular, the Ag substrate modifies the electronic structure of PTCDA significantly, and FDBM can capture these changes. Furthermore, images with separate s and p-wave contributions allowed us to fully identify the origins of the features in the total images. Lastly, state-of-the-art experimental measurements, with precise control of tip-sample distance, corroborated the fitness of our complete framework for these simulations (see Fig.). Then, in the second part, we investigated a pinwheel phase of PTCDA on the same substrate. Four PTCDA molecules arranged around an Ag adatom form this phase, with the oxygen in the same corner of each molecule attached to the adatom. While large-area experimental STM measurements suggested the presence of the adatom in the center of the pinwheel, it was nowhere to be found in up-close BTSTM images. With DFT simulations, we determined that the adatom states are far from the Fermi level and are undetectable to BRSTM with those experimental conditions. Then, BRSTM simulations were able to confirm subtle features that confirmed the presence of the adatom. Furthermore, thanks to the accurate description of CO deflection, we identified a feature that confirms part of the DFT calculated structure. To conclude, with FDBM and the p x p y approximation, we have faithfully reproduced the complex contrast with the tip-sample distance observed in experimental STM with CO-tips.

Fig. Comparison of (top) theoretical and (bottom) experimental BRSTM images of PTCDA over Ag(111). The tip-sample distance increases from right to left

References [1] Kichin, G., et al. (2011). Journal of the American Chemical Society, 133(42), 16847–16851. [2] Hapala, P., et al. (2014). Physical Review B, 90(8), 085421. [3] Song, S., et al. (2021). Surface Review and Letters, 28(8). 2140007. [4] Ellner, M., et al. (2019). ACS Nano, 13(1), 786–795. [5] Gross, L., et al. (2011). Physical Review Letters, 107(8), 086101.

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