AVS2013 Session SS+AS+NS+SP-WeA: Single Molecules at Surfaces: Structure and Dynamics

During the functional lifetime of coated metals, interfacial bonds at polymer/(oxyhydr)oxide/metal joints have to withstand high mechanical forces and corrosive attack. Therefore, it is crucial to control and understand the bonding mechanism originating at polymer/(oxyhydr)oxide/metal interfaces in order to achieve long-term stability. While the type of bonding contributes to the strength of adhesion, the nature of the interface depends on both functionality present in the organic molecules of the coating and the nature of the oxide film e.g. hydration, hydroxyl content, oxide thickness, surface morphology and chemical composition. Those properties might be extremely sensitive to the small changes in the environmental conditions, such as ageing and humidity.

Studying the bonding properties of the polymeric coating to the metal surface is experimentally difficult, because the interface is hard to be reached by analytical techniques due to the relatively high thickness of the polymer coatings. Consequently, the interfacial bonding can be modeled through adsorption of functional groups -representative interfacial adhesive molecules- on differently pretreated surfaces by means of bonding mechanism (FTIR) and affinity (XPS) studies. Furthermore, the buried interface and delamination mechanisms at this location can be studied by Scanning Kelvin Probe (SKP) measurements.

Recent References

In situ study of buried interfacial bonding mechanisms of carboxylic polymers on Zn surfaces, Journal of Physical Chemistry C, Volume: 117, N° in volume: 7, pp: 3374 - 3382, 2013, Taheri p., Flores J., Hannour F., de Wit J., Terryn H., Mol J.

Effects of Zinc Surface Acid-Based Properties on Formation Mechanisms and Interfacial Bonding Properties of Zirconium-Based Conversion Layers, Journal of Physical Chemistry C, Volume: 116, pp: 8426 - 8436, 2012, Taheri p., Lill K., de Wit J., Mol J., Terryn H.

A comparison of the interfacial bonding properties of carboxylic acid functional groups on zinc and iron substrates, Electrochimica Acta, Volume: 56, pp: 1904 - 1911, 2011, Taheri p., Wielant J., Hauffman T., Reyes Flores J., Hannour F., de Wit J., Mol J., Terryn H.

Role of surface oxide properties on the aluminum/epoxy interfacial bonding, Journal of Physical Chemistry C, Volume: 117, N° in volume: 9, pp: 4480 - 4487, 2013, Salgin B., Ozkanat O., Mol J., Terryn H., Rohwerder M.

Bonding mechanisms at buried interfaces between carboxylic polymers and treated zinc surfaces, Journal of Physical Chemistry C, Volume: 117, N° in volume: 6, pp: 2780 - 2792, 2013, Taheri p., Ghaffari M., Flores J., Hannour F., de Wit J., Mol J., Terryn H.

The study of ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS) has been raised to an unprecedented level. By using ex-situ laser focusing and Raman collection optics, optimization of the optical path is achieved without compromising UHV. All sample preparation and tip degassing are performed in-situ, maintaining atomically clean surfaces, greatly enhancing the stability of the tip-sample junction, and ensuring minimal contamination in the field enhancement region beneath the STM tip. At 292K, multiple vibrational modes for copper phthalocyanine (CuPc) adlayers on Ag (111) have been resolved in TER spectra obtained concurrently with molecular resolution UHV Scanning tunneling Microscopy (STM). Then Rhodamine 6G (R6G) molecules were studied as they have larger Raman cross-section. The sample was cooled down to 19 K to decrease the diffusion of R6G on surface at very low coverage. Single R6G molecules and clusters were observed using STM. For the first time, TERS vibrational modes for a few R6G molecules on Ag (111) have been resolved with concurrent molecular resolution STM images at 19 K. In comparison with the TER spectra at 292 K, the sharpening of TERS peaks and the existence of new peaks at 19 K demonstrates the utility of UHV-TERS towards obtaining site-specific chemical information about adsorbed molecules. This study sets the stage for Raman vibrational fingerprinting and correlated sub-molecular resolution topography of site-specific binding of molecules on solid surfaces.

We have investigated thickness dependence of electronic states of single metal-free Phthalocyanine (H₂Pc) on NaCl insulating films on Au(111), using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Our aim is to understand how the thickness of NaCl films affect the molecular properties using the well-known and widely studied molecule.[1] NaCl films are known to weaken the molecular-metal interaction,[2] but we found the electronic properties of the H₂Pc adsorbed on 2 ML and 3 ML-thick NaCl are indeed different. Isolated H₂Pc molecules are adsorbed on both 2 ML and 3 ML thick NaCl islands, which are grown from Au(111) step edges. Appearances of H₂Pc are similar for both islands, four-lobe or eight-lobe depending on bias voltages. STS spectra acquired at the centers of molecules on both islands show one peak in occupied and one in unoccupied states. By visualizing spatial distribution of density of states at the peak bias, we found that these two peaks corresponded to the highest occupied molecular orbital (HOMO), and degenerated lowest unoccupied molecular orbitals (LUMO and LUMO+1). A difference between 2ML and 3 ML is the peak position of degenerated LUMO and LUMO+1 states and thus HOMO-LUMO gap. Our observation implies that the effect of NaCl is not just to decouple metal electronic states, but it indeed affects the electronic states of adsorbed molecules, which might arise from the metal-insulator interaction such as interfacial dipole.

[1] T. Komeda, H. Isshiki, and J. Liu, Sci. Tech. Adv. Mater.11, 054602 (2010).

[2] J. Repp, G. Meyer, S. Stojković, A. Gourdon, and C. Joachim, Phy. Rev. Lett.94 (2005).

The stability of chemical bonding between molecule and substrate is one of the key factor considering diverse applications of Self-Assembled Monolayers (SAMs). Addressing this issue experimentally or theoretically is difficult considering complex interplay of the molecule-substrate and molecule-molecule interactions in SAMs. So far most of experiments have been performed for thiol based SAMs on Au(111) substrate and far less is known for equally important Ag(111) substrate.

In this presentation we report exchange and desorption experiments probing the effect of S to Se substitution on stability of molecule-substrate interface in SAMs on Ag(111) substrate by using two homologue series of molecules in the form BPnS/Ag(111) (CH₃-(C₆H₄)₂-(CH₂)_n-S-Ag, n = 2-6) and BPnSe/Ag(111) (CH₃-(C₆H₄)₂-(CH₂)_n-Se-Ag, n =2-6).¹ Such experiments were possible since both analogs form similar structures on Ag(111) substrate² and thus observed differences in stability between these two types of SAMs could be related to the molecule-substrate interface stability. Quantitative control of the exchange process was obtained using IRRAS (Infrared Reflection Absorption Spectroscopy). Additionally, ion-induced desorption experiments by means of SIMS (Secondary Ion Mass Spectrometry) were performed. Obtained results will be discussed together with analogical experiments^3-5 performed previously for Au(111) substrate enabling comparison of molecule substrate-interface stability for thiol and selenol based SAMs on Ag(111) and Au(111) substrates.

References

(1) J. Ossowski, P. J. Rysz, A. Terfort and P. Cyganik in preparation.

(2) T. Weidner, A. Schaporenko, J. Muller, M. Schmid, P. Cyganik, et al. J. Phys. Chem. C 2008, 112, 12495.

(3) K. Szelagowska-Kunstman, P. Cyganik, et al. Phys. Chem. Chem. Phys.2010, 12, 4400.

(4) F. Vervaecke, S. Wyczawska, P. Cyganik, et al. ChemPhysChem(Communication)2011, 12, 140. (5) S. Wyczawska, P. Cyganik, A. Terfort, P. Lievens, ChemPhysChem(Communication)2011, 12, 2554.

Tryptanthrins represent a class of compounds of interest for their anti-parasitic properties. Particularly, they have shown in-vitro efficacy versus organisms that cause malaria, leishmania, trypanosomiasis, tuberculosis, and fungal infections. However, little is known of their mode(s) of action at the molecular level. To investigate their geometric and electronic behavior, STM has been used to observe molecular monolayers of these compounds. Sub-molecular resolution has allowed the direct observation of individual lobes of the HOMO and LUMO states. The parent compound forms rows that alternate in contrast (light/dark), which is attributed to adsorption-induced stereoisomerization, with each row comprised of one enantiomer. Also excellent lobe-to-lobe resolution is observed for the LUMO of the parent compound as well as the HOMO of 8-fluorotryptanthrin (which similarly displays adsorption-induced stereoisomerization). Additionally, the authors have undertaken measurements of the tunneling barriers of these molecules, with a quantitative structure-activity relationship (QSAR) correlation as the goal. Sub-molecular quantitative measurements of the tunneling barriers have been accomplished for the parent compound, and show an interesting correlation to its known chemistry. Also, preliminary correlations of biological efficacy and molecular barrier heights have been observed over several analogs. Generally speaking, increased anti-parasitic efficacy corresponds to lower measured barrier heights, which is consistent with a possible tunneling mechanism.