Vibrational and structural study of PTCDA, P2O, and P4O on Ag(111) and Ag(110)
In this thesis, the vibrational and structural properties of three conjugated organic molecules (COMs) - Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), 6,13-pentacenequinone (P2O), and 5,7,12,14-pentacenetetrone (P4O) - grown on Ag(111) and Ag(110) are presented. PTCDA has been extensivel...
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| Format: | Doctoral Thesis |
| Language: | English |
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Philipps-Universität Marburg
2025
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| Online Access: | PDF Full Text |
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| Summary: | In this thesis, the vibrational and structural properties of three conjugated organic molecules (COMs) - Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), 6,13-pentacenequinone (P2O), and 5,7,12,14-pentacenetetrone (P4O) - grown on Ag(111) and Ag(110) are presented.
PTCDA has been extensively studied on various substrates, including Ag(111) and Ag(110). Focus has been on the structure models of the Herringbone (HB) monolayer (possible on both substrates) and the Brick Wall (BW) monolayer (possible on Ag(110)). We find that the layer quality and conversion rates reported in previous studies can be significantly improved. This thesis presents a method for preparing a 100\% pure HB phase using high-resolution FT-IRAS (Fourier Transform - Infrared Absorption Spectroscopy) and SPA-LEED (Spot Profile Analysis - Low Energy Electron Diffraction). It is shown that the presence of the bilayer does not affect the stability of the monolayer, and important conclusions on the commensurability of the HB monolayer are compared with existing literature using simulations.
P2O and P4O, oxidized modifications of pentacene, show significant improvements due to ketone groups. They lower the HOMO-LUMO gap, improve photostability by altering the optical absorption and decreasing photo-induced degradation, increase chemical stability by making the molecule less reactive to oxidative and other chemical attacks, and can potentially improve electron transport properties, making them more suitable for various organic electronic applications. Layer growth characteristics are shown using SPA-LEED and FT-IRAS, highlighting lateral interactions in P2O and island growth in P4O. High-resolution IRAS determines the coverage needed for lateral interaction. Lateral ordering in both species is driven by orientation-dependent interactions, including steric hindrance and hydrogen bonding. P2O and P4O show characteristic LEED reflexes indicating long-range ordering, preserved up to bilayer coverages. In heterolayers, ordered contact layer reflexes are maintained. P2O's ketone groups at the 6 and 13 positions, and P4O's at the 5, 7, 12, and 14 positions, lead to different types of lateral interactions, with P4O showing attractive island growth and P2O displaying repulsive interactions between rows.
While the self-assembly behavior, electronic properties, and adsorption heights are addressed in literature on Ag(111), this thesis gives specific focus on molecule-molecule and molecule-substrate interactions at sub-monolayer coverages, monolayer growth phases, and interfacial charge transfer dynamics. Furthermore, this thesis advances the understanding of molecular behavior of P2O and P4O on an anisotropic substrate Ag(110), which is underexplored in literature. By leveraging high-resolution techniques like SPA-LEED and FT-IRAS, we are able to note a shift to a commensurate growth pattern for P2O, which is in contrast to its behavior on Ag(111), as well as directional molecular alignment and anisotropy-driven structural arrangements for both P2O and P4O.
Interfacial dynamical charge transfer (IDCT) is the rapid movement of charge carriers between a planar COM and a metal substrate, crucial for organic electronic devices like OFETs, OPVs, and OLEDs. PTCDA's strong electron-accepting properties arise from its extended $\pi$-conjugated system and anhydride groups. P2O and P4O also act as electron acceptors due to their oxygen-based functional groups. IR spectra reveal a dramatic reduction in IDCT for PTCDA on Ag(110) compared to Ag(111), attributed to stronger molecule-metal interactions and an energy shift in the LUMO (Lowest Unoccupied Molecular Orbital), reducing its DOS (density of states) at the Fermi level. This enhanced coupling eliminates the asymmetric nature of the vibrational line shapes typical of weakly interacting systems. Prominent IDCT is observed for P4O vs. P2O on Ag(111) due to a higher DOS of the P4O-LUMO at $\epsilon_{F}$. Molecule-metal interaction is stronger on Ag(110) for both molecules. PTCDA on Ag(110) shows a difference in the intensities of the IDCT-induced vibrational modes, with HB phase showing a stronger effect, indicating a further downward shift of LUMO w.r.t. the $\epsilon_{F}$ for the BW phase.
These findings provide valuable insights into the influence of substrate symmetry and molecular functionalization on IDCT, offering pathways to optimize organic electronic device interfaces. The comprehensive analysis of vibrational properties, molecular packing, and charge transfer dynamics on Ag(111) and Ag(110) advances our understanding of COM behavior, paving the way for improved material design and application. |
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| DOI: | 10.17192/z2025.0110 |