Electronic and geometric structure of heterorganic interfaces
Organic hererojunctions, i.e., the interfaces between different organic materials, are very important for the field of organic electronics since interface properties are dominant for the functionality of the entire electronic device. We have recently achieved evidence for a partly chemisorptive bonding between single monolayers of different organic molecules, i.e., across an organic heterojunction. Copper-II phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules, which are stacked on a Ag(111) surface, show such an unexpectedly strong interaction. The results are published in a recent research article in Physical Review Letters.
Phys. Rev. Lett. 108, 106103 (2012)
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Adsorption behavior of metal-phthalocyanines on metal surfaces
Metal-phtalocyanine molecules represent an exceptional model system for studying all variations of molecule-molecule and molecule-substrate interactions. We are investigating these molecules in contact with noble metal surfaces. One recent result concerns the interaction strength of CuPc with Au, Ag and Cu(111), which is directly correlated with the different structure formation on these surfaces. By selecting the substrate properly, the interaction can be arbitrarily tuned from weak physisorption to strong chemisorption as well as from attractive to repulsive intermolecular interaction.
J. Chem. Phys. 135, 234703 (2011).
Phys. Rev. B 83, 195414 (2011).
Phys. Rev. B 83, 085416 (2011).
Nature Physics 5, 153 (2009).
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Interaction of ?-conjugated organic molecules with surfaces
For improving electronic devices based on organic materials, well-ordered defect-free films with "perfect" interfaces are needed. The growth of such films is dominantly influenced by the formation of the first molecular layer on the substrate which is investigated in this project. In particular, we study molecule-substrate interaction of large ?-conjugated molecules on metal surfaces and the consequences for the local adsorption geometry (chemisorption or physisorption, molecular orientation, bending, etc.) and the formation of the films.
Nature 444, 350 (2006);
New J. Phys. 9, 50 (2007);
Prog. Surf. Sci. 82, 479 (2007) (Review)
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Order/Disorder phenomena of sub-monolayer organic films
Ordering phenomena in organic films are a result of the fine balance of molecule-substrate and molecule-molecule interaction. The latter usually is attractive due to van-der-Waals forces. However, metal-phthalocyanines show repulsive interaction upon adsorption on a Ag(111) surface. The origin is a donation/backdonation effect of electronic charge which leads to a substrate-mediated Pauli-repulsion. The effect can be utilized to grow extremely large domains and crystalline grains which might become very useful for organic electronics.
Nature Physics 5, 153 (2009)
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Complex molecular architectures at surfaces
In this project we study the self-organisation and formation of structurally complex phases of organic materials which may offer new functionalities. Several exemplary systems are in focus of research: interface-stabilized organic layers (e.g. the tetracene ?-phase on Ag(111)), multinary organic phases (e.g. tetracene and DH4T on Ag(111)), or ordered arrays of chiral and pro-chiral molecules (e.g. amino acids).
Langmuir 22, 9572 (2006),
J. Phys Chem. B 110, 23756 (2006)
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Surface chemistry of organic adsorbates
When adsorbed, molecules with functional groups can interact chemically with surfaces and with other molecules. In this project aspects of this interaction are investigated. For example, surface spectroscopy indicates that H bonding does not only control the adsorption of formic acid leading to molecular flat lying ?-polymorphic chains but also its complex formation with coadsorbed water characterized by a proton exchange.
Appl. Phys. A 87, 435 (2007)
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Functional ?-conjugated molecules such as biphenyls form periodic well ordered structures on single crystal surfaces. A precise determination of the unit cell can be gained from LEED patterns obtained using a microchannelplate MCP LEED system operated in the nondestructive nA current regime. Together with complementary molecularly resolved STM images this leads to reliable structure models.
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Precision measurements of adsorption height and molecular conformation of complex organic adsorbates
The prime source of information on complex molecular adsorbates is the STM. However, the STM cannot provide structural parameters such as adsorption height and molecular conformation. Since the latter are important for the analysis of the molecule-substrate bonding, we apply the normal incidence X-ray standing wave technique NIXSW to gain access to vertical structural parameters of complex organic-inorganic interfaces.
Phys. Rev. Lett. 100, 136103 (2008);
Phys. Rev. Lett. 94, 036106 (2005)
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Charge Transport through Surfaces
The increasing importance of surface conductance compared to conductance through the (semiconductor) bulk in modern nanoelectronic devices calls for a reliable determination of the surface conductivity We use distance dependent four-probe measurements in order to disentangle surface from bulk conductivity. Alternatively tunneling potentiometry (STP) allows to map the potential landscape while a current flows through the surface / nanostructure under study. Potentiometry maps give insight into fundamental transport properties, such as the influence of defects on the local electric transport.
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