Nanomaterial Synthesis

Molecular precursors can be used for coatings and nanostructures owning different functional properties. In our laboratory, inorganic films are predominantly deposited by chemical vapor deposition (CVD) techniques at reduced pressure. The morphology of the resulting nanoscale coatings is influenced by several parameters, such as precursor feedstock, substrate temperature and the inherent material properties as well as the overall pressure of the system. In addition, solvent-based approaches, such as microwave processing, are used to prepare nanoparticles and nanowires. The structural and chemical analysis of these inorganic structures include XRD, SEM, TEM, EDX, EELS etc..

Nanowire Growth

One-dimensional (1D) nanostructures, including nanowires, nanotubes, nanorods and nanobelts, are of both fundamental and technological interest. They exhibit interesting electronic and optical properties associated with their low dimensionality and a large surface to volume ration associated with their aspect ratio. In addition, these functional 1D nanomaterials represent the critical components in potential nanoscale devices for different applications including energy harvesting and generation, batteries, lighting, sensing, electronics, optoelectronics etc.. Major challenges remain in order to fully exploit the 1D nanostructures: (i) the development of suitable chemical strategies for their rational synthesis as well as (ii) organization and integration of these nanoscale building blocks.

Among the large number of synthetic techniques to prepare single crystalline nanowires, the metal supported growth strategy is very versatile. This synthesis principle requires a metallic growth promoter or nucleation seed to enable 1D-nanocrystal formation. Obvioulsy, the interaction of the metallic particle with the growing material is highly interesting and offers opportunities to enable specific growth scenarios.

Growth studies

We could recently show that defects from a metal can be transferred to a growing nanowire, which is the first example of an information transfer from a metal particle to a nanowire. A solid seed is important for this strategy to be effective. In addition, we could demonstrate that the pre-orientation of a seed material, which is not mixing with the growing nanowire material, can also promote the nanowire growth due to lattice matching of the growth seed and the forming nanowire.

Nanowires of Metastable Materials

Metastable materials, such as Ge/Sn alloys, can also be prepared by our molecule-based approach combined with microwave techniques. Our nanowires are the first examples of this material, which have not been produced via the top-down approach using epitaxially grown layers. Controlling nucleation and metalorganic species are key for a successful and controllable synthesis. This material is extremely interesting due to its direct bandgap, while pure germanium has an indirect bandgap.

Site-selective Growth / Nanowire Sensors

Nanowire networks can be grown on micromembranes for sensor applications. The metal oxide nanowires grow exclusively on the heated area of a micro-machined chip. The same heater is essential for the post-growth sensor operation. A secondary circuit is located on top of the membrane heater and the interdigitates are bridged by the grown nanowires. This configuration allows a direct integration of nanowires in a functional device. The sensors are tested up to 1 month of continuous operation and show long term stability. The gases investigated with this configuration include carbon monoxide and ammonia in different humidity levels.

Thin Film and Nanoparticle growth

Thin films are grown by low pressure chemical vapour deposition (LPCVD) or aerosol-assisted chemical vapour deposition (AACVD) using molecular precursors. A prime example for the impact of the chemical composition of the nanomaterial formed during the thermal decomposition of the thioether functionalised group 13 alkoxides. The thermal decomposition during the usual oxide formation leads to an in situ conversion of the oxide to sulphide species without the formation of sulphates.

Similar studies can be performed in solution phase processing for the preparation of oxide and sulphide nanoparticles.