AVS2013 Session SS1-ThA: Organic Layers on Surfaces

We have studied using scanning tunneling microscopy (STM) and density functional theory calculations one of the diarylethene molecules [1] adsorbed on Au(111) and Cu(111). Two isomers of this molecule, open-form and closed-from, are known to be thermally stable, and its isomerization reactions are induced by irradiation of UV and visible light for the cyclization and cycloreversion reaction, respectively, in solution and single crystalline. Based on STM measurements after deposition of either form of the isomer as well as with and without annealing process, we found that the closed-form was more stable than the open-form on both Au(111) and Cu(111), which is opposite to gas phase. We expect that the stability reversal upon adsorption could arise from the partial ionization of the molecule due to the charge transfer between the molecule and the substrate metal, which stabilizes the closed-form isomer more than the open-from. It is also revealed that while small clusters are formed on clean metallic surfaces, a largely extended 2D superstructure can be fabricated when NaCl is co-deposited on Cu(111). Similar to earlier studies of other organic molecules [2,3], x-ray photoelectron spectroscopy data can be reasonably explained by assuming Na ion incorporation into the superstructure and Cl diffusion into the bulk. We propose a superstructure model where a Na ion binds fluorine and sulfur atoms of the adjacent molecules.

[1] M. Irie, S. Kobatake and M. Horichi, Science 291, 1769 (2001).

[2] Ch. Wackerlin et al., Chem. Commun. 47, 9146 (2011).

[3] D. Skomski et al., J. Am. Chem. Soc. 134, 14165 (2012).

As possible active layers in optoelectronic devices, binary blends of organic semiconductors are of high relevance for applications. A detailed understanding of the structure-property relation in these complex systems is crucial for device optimization, but due to the variety of film structures and associated optical properties also a challenge for fundamental research [1,2]. In particular for vacuum deposited thin films, post growth studies are often not sufficient for a detailed characterization due to possible pronounced transient effects observable during film growth [3]. These effects can include film thickness dependent changes in the average molecular tilt angle or intermolecular spacing, which may result in significant variations in the relative intensities or energy position of specific peaks in the absorption spectra [3].

Using organic molecular beam depositon we prepare thin mixed films of the prototypical organic semiconductors perfluoropentacene (PFP, C₂₂F₁₄) and diindenoperylene (DIP, C₃₂H₁₆) and follow the film growth in situ in real-time simultaneously using grazing incidence X-ray diffraction and optical spectroscopy. We discuss the result of the competing effects of strong and favourable donor-acceptor interaction and steric incompatibilities due to the size mismatch of PFP and DIP on the structure formation in mixed films, concentrating on the size of coherently scattering islands, which is related to the structural order within the films. By exploiting the advantages of a simultaneous measurement of structural and optical properties of the samples we are able to relate pronounced changes in the relative intensities of specific peaks in the optical absorption spectra to the observed changes in the structural order. The results of these experiments give insight in the complex mechanisms of structure formation and structure-property relations in mixed systems and point towards the possibility to tune the structural and optical properties for the optimization of opto-electronic devices.

[1] A. Hinderhofer and F. Schreiber, ChemPhysChem, 13, 628 (2012),

[2] A. Aufderheide et al., Phys. Rev. Lett., 109, 156102 (2012),

[3] U. Heinemeyer et al., Phys. Rev. Lett., 104, 257401 (2010),

Organic charge-transfer (CT) complexes are molecular compounds mixing two species with different electron affinities: an electron donor (D) and an electron acceptor (A). Charge transfer processes between D–A complexes and metallic electrodes are at the heart of novel organic optoelectronic devices such as solar cells [1]. In contrast with the existing exhaustive study of the bulk properties of CT solids, very little is known about the thin-film behaviour. The transition from bulk D-A complexes to ultra-thin films of monolayer thickness deposited on metals introduces a new phenomenology related to the organic–inorganic interface [2]. Effects like hybridization, CT with the surface and molecular level alignment become factors that may govern the electronic transport. Hence, the adsorption of an ultra-thin D–A layer on a metal opens a new field of research for the potential application of CT complexes as devices in the nanoscale.

Simultaneous characterization of the interdependent structural and electronic properties is required for a thorough understanding of the D-A complexes under study [3]. Here, by combining both Scanning Tunnelling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS) in situ, we can study the delicate balance that exists between intermolecular and molecule–substrate interactions, as well as the hybridization, and the charge transfer taking place in model donor–acceptor assemblies at metal-organic interfaces.

We will show that the metal surface allows us to expands the variety of such D-A networks, permitting the design of D-A metal-organic frameworks with the formulation of D_mA_n (m,n ≥ 1). By controlling the stoichiometry between tetrathiafulvalene (TTF, electron-donor) and tetracyanoethylene (TCNE, electron-acceptor), we can tune the both the structural and the electronic properties of a donor-acceptor system on Ag(111). We show that this system exhibits various structural phases, depending on the stoichiometry, each leading to different levels of charge transfer. These results demonstrate that atomistic studies on the growth of organic thin films under ultrahigh vacuum (UHV) conditions can lead to the kind of accurate control needed in order to optimize device characteristics.

[1] L. Bartels, Nat. Chem., 2, 87 (2010).

[2] N. Gonzalez-Lakunza, I. Fernández-Torrente, K. J. Franke, N. Lorente, A. Arnau, and J. I. Pascual, Phys. Rev. Lett., 100, 156805 (2008)

[3] D. G. de Oteyza, J. M. Garcıá-Lastra, M. Corso, B. P. Doyle, L. Floreano, A. Morgante, Y. Wakayama, A. Rubio, and J. Enrique Ortega, Adv. Funct. Mater., 19, 3567 (2009)

Molecular materials have a natural tendency toward polymorphism due to the highly anisotropic nature of intermolecular interactions. Because crystal structure dictates key material properties, such as solubility and charge transport, crystallographic phase control is of intense interest to applications ranging from pharmaceuticals to organic electronics. In this talk we describe the kinetic assembly of molecular semiconductors into controlled arrangements. We show how crystalline monolayer films of pentacene (P) and titanyl phthalocyanine (TiOPc) can be produced with varied packings and orientations through controlled deposition. We subject metastable film structures to chemical (fullerene adsorption) tests to create charge-separating interfaces expected in OPV devices. Low-density molecular films rearrange spontaneously under C₆₀ (and C₇₀) deposition, yielding new co-crystalline phases with distinctive electronic features; whereas dense films generally yield abrupt semiconductor interfaces. We further show how highly polarizable fullerenes respond to the electrostatic contours of oriented molecular semiconductor films to form unexpected fullerene arrangements. Structure evolution in functionalized fullerenes further demonstrate how electrostatics impact phase selection. C₆₀- and C₇₀-PCBM molecular ordering is monitored from a glassy monolayer phase, produced by pulsed delivery of solute PCBM from a liquid microaerosol, to neat hcp crystalline arrangements inaccessible through homogeneous PCBM crystallization. We relate kinetic barriers for molecular ordering to solvent expulsion and PCBM reorientation, determining the critical PCBM surface density needed to induce nanocrystallization.

Acknowledgement: Portions of this work have been supported by the National Science Foundation Division of Chemistry-MSN and Division of Materials Research DMR-05-20471.

Organic nanostructures on semiconductor surfaces offer a variety of applications in modern electronic such as light emitting diodes, transistors, solar cells, etc. In most cases well-ordered interfaces are required. Therefore interest in interfaces between organic molecules and inorganic semiconductors surface and the phenomenon of self-organization is still increasing. The growth of thin molecular films with a commensurate structural relationship to the substrate needs substrates that exhibit template structure into which the molecules can be locked. We apply STM, STS, XPS, UPS, and LEED techniques to study adsorption of PTCDI-C8 (N, N'-Dioctyl-3,4,9,10-perylenedicarboximide) on Si(110). The adsorbate of an average thickness from a fraction of monomolecular layer (ML) up to 1.3 ML was vapor deposited on (16x2) reconstructed surface of n-type, Sb-doped substrate. The reconstruction, typical for clean Si(110) surface, shows a stripe-like structure with alternately spaced lower and higher terraces lying along the [-112] and [1-12] directions. The distance between adjacent raised stripes is about 5 nm and the width of the stripes is about 0.2 nm.

Self-assembled monolayers (SAM) became a very powerful tool in the materials sciences, biochemistry, and engineering. In particular SAMs of thiolates are reknown for their ease of formation, their stability and their high order. Nevertheless, not all thiols can form densely packed monolayers and for some cases even the best thiolate monolayers are not well ordered enough.

Based on the idea that a softer anchoring atom would fit better to the soft gold atoms (according to Pearson’s HSAB principle), we prepared a number of selenium-containing precursors and studied their assembly on gold (and in some cases on silver). Data obtained by surface infrared spectroscopy, different photo-electron spectroscopies, and STM hint on the formation of very stable monolayers with improved order.[1-4]

In this talk we will present the preparation methods for the starting materials and the respective monolayers, give details on the surface characterisation, and offer explanations for the superior behaviour of the selenolate SAMs.

References

[1] Szelągowska-Kunstman, K.; Cyganik, P.; Schüpbach, B.; Terfort, A. Phys. Chem. Chem. Phys.12, 4400-4406 (2010)

[2] Weidner, T.; Shaporenko, A.; Müller, J.; Schmid, M.; Cyganik, P.; Terfort, A.; Zharnikov, M. J. Phys. Chem C112, 12495–12506 (2008 )

[3] Cyganik, P.; Szelągowska-Kunstman, K.; Terfort, A.; Zharnikov, M. J. Phys. Chem C 112,15466-15473, (2008 )

[4] Shaporenko, A.; Müller, J.; Weidner, T.; Terfort, A.; Zharnikov, M. J. Am. Chem. Soc.129, 2232-2233 ( 2007 )