Hydromel
Structure de mise en forme 2 colonnes

Work performed on the assembly of nanowires

Nanotechnology is a driver for device innovation and one of the most important key technologies for markets in the 21st century.  Nanophotonic and Nanoelectronic technologies contribute significantly to vital societal objectives, such as achieving information society, improving health care and prevention, saving energy, providing safety and security for the citizens and, last but not least, creating employment and growth.  The innovative and competitive capability of many important European industries, such as ICT, lighting, health care and life-sciences, space and defence as well as the transport and automotive sector largely rely on progress and development in nanotechnologies. 

Nanowires based on semiconductor and metallic materials are key components for future nanophotonic and sensor technologies.  Conjugated polymer materials, e.g., poly(9,9-dioctylfluorene), (PFO) which exhibit many unique properties, such as solution processability and emission colour tunability, are only beginning to be extensively explored in the context of these nanowire technologies.

 

Metallic nanowires, as active elements in electrochemical based sensors for example, offer a number of enhancements compared to macroelectrodes due to their many advantageous properties: low background charging, high current density due to enhanced mass transport.  However, widespread take-up of nanowires as active device elements has been limited by the lack of practical and effective methodologies for assembly and/or fabrication of robust nanowire arrays.

In Hydromel, researchers in the Nanotechnology Group at the Tyndall National Institute (Cork, Ireland) are addressing this fabrication challenge by developing low cost self-assembly technologies for the assembly of nanowires at technologically relevant substrates, i.e., silicon chips.  Two different fabrication methodologies being explored: In the first method, long range forces such as, e.g., electric force fields are used to drive the on-chip self-assembly of freestanding nanowires to form ordered nanowire arrays.  In this approach, an AC voltage bias is applied to microelectrodes patterned on a silicon chip substrate and followed by dispensing a suspension droplet of polymer or metal nanowires on top of the chip.  The applied voltage establishes an electric field between the electrodes. The field is non-uniform due to the 3-D nature of the electrodes and field is strongest at the electrode edges and decreases to a minimum at the central point between the electrodes.  The nanowires in solution become polarised (charge separated) in the presence of the electric field.  By tailoring the frequency of the applied electric field and by careful selection of the suspension medium, nanowires ae induced to migrate to the areas of highest field strength thorough a process known as dielectrophoresis.  In this manner the nanowires spontaneously self-assemble at the electrode edges and orient in the direction of the electric field to form well ordered nanowire arrays. 

 

 

 

 

In the second method, thin metal films ~ 100 nm of, e.g., gold, are evaporated as a single layer onto an epoxy substrate and the substrates further encapsulated with epoxy to form an epoxy block.  The epoxy block is then thinly sliced, as little as 50 nm thick, perpendicular to the evaporation direction using an ultramicrotome to yield nanomembranes each containing a nanowire.  Placement methods such as self-assembly approaches are employed to assemble the nanomembranes at specific locations at pre-fabricated silicon chips and the epoxy is then chemically removed to release the nanowire.  Following release, metal contacts are made to the nanowires, using standard microelectronic fabrication techniques, to allow electrical characterisation of the nanowire and enable them to be used as electrochemical sensors.

For more information on the above approaches please contact Dr. Alan O’Riordan alan.oriordan@tyndall.ie at the Tyndall National Institute.

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