Kirstein, Stefan, PD Dr.
Rabe, Jürgen P., Prof. Dr.
Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin
The main objective of this project is to investigate quasi one-dimensional hybrid inorganic/organic systems (HIOS) consisting of silver - and eventually also semiconductor - nanowires coated by a shell of conjugated organic molecules. The synthesis is carried out in aqueous solutions by means of self-assembled nanotubular J-aggregates of cyanine dyes as photochemically active templates (see Figure 1), and AgNO3 salt that is photocatalytically reduced to elementary silver. This synthesis allows filling of the inner part of the nanotubular J-aggregates with crystalline silver nanowires with a diameter of less than 10 nm and lengths of more than 1 µm (see Figure 2). The colloidal structures are processable from solution, providing high volume concentrations of metal/organic molecule interfaces and allowing deposition of individual HIOS onto solid substrates. The approach of using colloidal quasi one-dimensional HIOS structures complements the projects within the CRC dealing with planar structures for future device applications.
The first objective is to achieve control over the growth of metallic and possibly semiconducting nanowires within photocatalytically active nanotubular dye aggregates that act as organic templates. Recently, we applied tubular J aggregates as templates to grow extremely thin and smooth silver nanowires from salt solutions, exhibiting unprecedented diameters of less than 7 nm and lengths exceeding micrometers. The nanotubular aggregates consist of amphiphiphilic dyes in aqueous solution which allow the reduction of silver cations by photoexcitation and charge transfer. Questions we want to address in this project concern the nucleation process, the subsequent growth and the resulting crystallinity of the nanowires. Furthermore, this process shall be applied to explore the possible synthesis of semiconducting materials such as, e.g., ZnO. Finally, we want to coat the nanowires with layers of other conjugated molecules to obtain cylindrical objects consisting of a metallic or semiconducting core coated with a shell of conjugated organic material.
The second objective is to correlate the structure of the cylindrical hybrid inorganic/organic structures with their optical properties. For metallic wires these are determined by plasmonic excitations of the metal and their interactions with the dielectric coating. The nanowires grown here provide an unparalleled opportunity to study plasmons of single and isolated thin metal wires with very high aspect ratios embedded into a conjugated organic material. Due to the small diameter of the nanowire (less than 10 nm) the plasmonic field is expected to extend largely into the surrounding material. This effect shall be used to optically excite the surrounding shell of the wires that consists of conjugated organic material. Alternatively, the wires shall be placed on films of organic molecules or semiconducting material to create enhanced and local optical excitations within the film via the plasmon resonances of the wires. The coupling between plasmon modes of neighboring nanowires shall be investigated with respect to tailoring of antennae for increased field amplification. It is also expected that there is resonant coupling of the plasmons in the infrared region with molecular vibrations in the shell, which may be useful for ultrasensitive infrared detection. In the case of successful synthesis of semiconducting wires (ZnO) the excitonic coupling between the inorganic wires and a surrounding conjugated organic material shall be studied.
 D.M. Eisele, J. Knoester, S. Kirstein, J.P. Rabe, D.A. Vanden Bout, „Individual nanotubular J-aggregates with highly uniform exciton fluorescence on solid substrates”, Nature Nanotechnology, 4 (2009), 658-663
 D.M. Eisele, H. von Berlepsch, C. Böttcher, K.J. Stevenson, D.A. Vanden Bout, S. Kirstein, J.P. Rabe, „Photoinitiated growth of sub-7 nm silver nanowires within a chemically active organic nanotubular template”, J. Am. Chem. Soc. 132, 2104 (2010).
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Figure 1: (a) Chemical structure of amphiphilic cyanine dye C8S3 with hydrophilic and hydrophobic groups enhanced by colors as used in the sketch of the aggregate structure in (b). The tubes exhibit a bilayer structure with the hydrophilic groups pointing towards the inner and outer region of the tube. (c) The UV/vis absorption spectra of the tubular aggregates show red-shifted exciton bands (J-aggregation) (taken from ).
Figure 2: (a) Cryo-TEM image of a silver nanowire with a width of (6.4+0.5) nm partially filling a supramolecular dye nanotube, and a silver nanoparticle on the nanotube’s outside, 15 min after adding AgNO3 to the solution and exposure to white light for 20 s. (b) line scans across the template filled with silver (black), and unfilled template (red). (c) TEM image of silver nanowires immobilized on a solid substrate 72 h after adding AgNO3 and exposure of the solution to white light for 90 min (taken from ).