Molecular switches in which a molecular property such as the conformation is changed in response to an external stimulus (e.g. light or current) are promising candidates for both memory and logic applications. Our research aims at the understanding of fundamental differences in the isomerization behaviour of free molecules and those adsorbed at surfaces (e.g. Ag(111)) which has been observed for some azobenzene-derived molecular switches.

Condensation of molecular hydrogen in the junction of a low temperature STM strongly affects the tunnelling and leads to the appearance of a new type of geometric image contrast. In the framework of this project we study STM transport through condensed gas media from a fundamental point of view. At the same time we utilise the newly discovered effects in the development of a new STM imaging technique - the scanning tunnelling hydrogen microscopy (STHM).

New J. Phys. 10, 053012 (2008)

Fundamental understanding of single molecule transport experiments requires close interaction with theory. In this project we aim for the highest possible degree of experimental control over STM-based single molecules transport junctions; this allows us to provide direct input for ab-initio transport calculations. One of the project goals is exploring the limits of the single-particle picture in realistic molecular transport junctions.

Nanotechnology 19, 065401 (2008)

The controlled fabrication of self-organized nanostructures with dimensions in the single digit nanometer range is becoming possible. However, the measurement of charge transport through such nanostructures is still a challenge. The main problem is to provide contacts to the nanostructures. Our approach is to provide such a contact by a multi-tip STM in order to enable charge transport and scanning potentiometry measurements at self-assembled (surface-) nanostructures.

Rev. Sci. Instr. 77, 093701 (2006)

The functionality of organic field effect transistors resides in their interfaces, both organic metal (charge carrier injection) and organic insulator (charge transport). Highly ordered molecular adsorbate layers provide an excellent tool to study the relevant physical properties of these interfaces. Within the DFG priority program 1121 "Organic field effect transistors: structure and dynamics" (2001-2007) we have investigated fundamental interface properties of organic field effect transistors. The results are summarized and reviewed in phys. stat. sol. (a) 205, 511 (2008) (Feature Article and Review).

Self-assembled growth of semiconductor nano-structures is a bottom-up approach to build nano-structures with the help of nature. Recently, nano-wires have attracted a lot of interest because they are required to interconnect functional units in nano- and molecular electronics. We found a way of controlled fabrication of Si and Ge nanowires with a width down to 3 nm and a thickness of only one atomic layer (0.3 nm).

Phys. Rev. Lett. 98, 166104 (2007)

This project aims at a wide-ranging characterization of nanometer-sized functional inorganic fullerene- and metal-oxo-based cluster molecules and their aggregates. The targeted molecules are in the focus of interest as molecular building blocks for nanoelectronics and spintronics due to their unique structural, supramolecular, redox, and magnetic properties. Moreover, these features can be synthetically tuned to a large extent.

Appl. Phys. A 87 (3), 475 (2007)

Nanoparticles and quantum dots are of great interest due to their optical, electronic and structural properties which are dominated by quantization effects. For ultra-small particles (< 5 nm) a precise structural characterization is very difficult. We utilize x-ray diffraction at synchrotron sources combined with a novel data analysis method for powder-diffraction data to investigate size, shape, structure (including impurities and stacking faults) stress, relaxation effects, etc. Also ensemble properties like size distributions can be analysed.

Appl. Phys. A 85 (4), 337 (2006)

For small quantum dots grown on surfaces (just like for free-standing nanoparticles) a determination of structural properties is difficult. We utilize different diffraction and imaging techniques (XRD, AFM, LEEM-PEEM) in order to investigate size and shape of the quantum dots, but also composition and intermixing of different materials. In future, special emphasis will be placed on in-situ observation of growth and formation of the quantum dots.

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 π-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)

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, article in press.

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 α-polymorphic chains but also its complex formation with coadsorbed water characterized by a proton exchange.

Appl. Phys. A 87, 435 (2007)

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)

Functional π-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.

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)

In-situ methods which allow observing the growth of molecular islands and films on surfaces appear to be extremely useful. This shall be enabled within this project by installing an instrument for Low Energy Electron Microscopy (LEEM) and Photo Emission Electron Microscopy (PEEM). The aberration corrected machine will not only allow microscopy at a resolution of a few nanometers, but also high-resolution electron spectroscopy with a variety of different UV-, optical and x-ray sources.

Local studies of subtle many-body effects occurring during electron transport processes demand instruments reliably operating in sub-Kelvin temperature range. In the course of this project a new mK-STM fulfilling the temperature stability conditions is developed.

The construction of an UHV chamber including lab facilities for microscopic, spectroscopic and diffraction analysis is in progress. An ESCA setup based on the Scienta R4000 analyser and equipped with monochromated X-ray, ultraviolet and laser excitation sources will be combined with LT-STM, MCP-LEED, and an HREELS analyzer. Low temperature sample transfer will be enabled.