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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - NANOFACTORY (Building tomorrowâ€™s nanofactory)

Teaser

Summary

The aim of this project is to translate the concept of production line to the nanoworld to develop what could become tomorrowâ€™s nanofactory. So far, nanostructures are either chemically synthesized or produced using top-down approaches such as nanolithography, but no processes exist to take a few nanostructures and perform the basic operations required to assemble them into a more complex system. This proposal aims at addressing this need by realizing at the nanoscale the different functions that are required for a production line: receiving and moving raw nanomaterial in position, where it can be immobilized and worked on or transformed; combining different elements into more complex systems that support new functionalities. The project uses optical forces generated by plasmonic traps as enabling mechanism to act on raw material and the entire production line will be integrated into microfluidics, which will perform as an advanced conveyor belt. Local electrophoresis and photo-curable polymerization are used to locally modify and assemble raw nanoparticles. In addition to implementing challenging nanotechnologies, such as nanoscale electric contacts and perforated membranes, this project will also explore a fair amount of completely new physics, including the van der Waals interaction â€“ which will be studied numerically and experimentally â€“ the competition between optical and chemical forces or electrostatic attraction, and the detailed determination of the trapping potential produced by plasmonic nanostructures. The foreseen research is very comprehensive, including modelling, nanofabrication and explorations at the nanoscale. The final goal for this project is to demonstrate how additive manufacturing can be implemented at the nanoscale.

Work performed

The work performed during this first period addressed the different facets of the project and the results obtained can be articulated around three main lines of research: 1) Modelling and development of numerical tools; 2) Fabrication of nanostructures and 3) Integration into the experimental platform, as detailed in the following.

1) Modelling and development of numerical tools: here, we have refined the numerical tools able to compute the optical forces produced by complex plasmonic nanostructures. These tools rely on the integral form of Maxwellâ€™s equations â€“ the so-called surface integral equation â€“ and require the computation of several integrals (quadratures) to build the matrices for the system of equations that is finally solved to obtain the electromagnetic field and the resulting forces. In particular, we have developed high order quadratures that use additional discretization points and are more accurate than low order quadratures. This level of accuracy is especially important for the intricate geometries studied in the project, where a nanoscopic object is trapped at extremely short distances from plasmonic nanostructures. Using this approach, we have been able to study in detail the distribution of optical forces at the vicinity of plasmonic nanostructures, identifying the regions where the forces are attractive and those where they are repulsive, thus enabling a diversity of manipulation possibilities. We have also extended the surface integral equation technique to handle dispersive forces like the van der Waals force. This force can become significant between nanoscopic objects when their separation distance is extremely small. In that context, we have made an important discovery: there exist geometries with strong concentrations of van de Waals forces exist. This is similar to the well-known hot spots found in plasmonic systems, which produce strongly localized and amplified optical fields. For van der Waals forces, we have found such forces concentrations between two metal cubes facing each other along an edge.

2) Fabrication of nanostructures. To dispose of plasmonic effects at different wavelengths, it is mandatory to master the fabrication of nanostructures made from different plasmonic metals, including aluminium and silver. Unfortunately, silver is very difficult to fabricate with and silver nanostructures deteriorate extremely rapidly. We have been able to identify the origin of this ageing process: residual water on the surface of the metal. This is a rather surprising result since the general wisdom in the scientific community always considered that oxygen or sulfur are responsible for this process. We have developed a dehydration process that can cure this issue and produces plasmonic nanostructures in silver that are stable over several months in ambient conditions. Finally, we have been able to fabricate samples that combine both very small plasmonic antennas with features in the range of a few tens of nanometre and very large contacts in the range of several microns. This diversity of length scales is challenging for nanofabrication, especially in terms of electron beam lithography, where the exposure dose needs to be adjusted as a function of the structures size and density (proximity effect). It is also challenging to reveal at the end of the process fabricated structures when they combine such large and small parts; to this end, we have introduced a novel approach based on etching of the nanostructures rather than lifting them up. While the latter requires a positive resist, the former uses a negative resist and enables the fabrication of nanostructures with different sizes in a single technological step.

3) Integration into the experimental platform. To be able to conduct the foreseen experiments, different optical instruments have been incorporated into a single platform developed around an inverted microscope. These instruments include a dedicated, tuneable, white light source with

Final results

At this stage, Nanofactory has advanced the state of the art in the following manner. 1) We have developed an extremely accurate modelling framework that can handle optical calculations with unprecedented accuracy, thus enabling computations in very intricate geometries. 2) This general solution of Maxwellâ€™s equations has been extended to compute van der Waals forces in complex metallic systems. We have discovered that there exist in such systems regions of intense van der Waals forces, which represent the equivalent to hot spots in plasmonic systems. 3) We have designed a variety of plasmonic nanostructures that can be used for trapping. Some of these structures exhibit both features in the 10 nm range and in the micron range, which is very challenging to fabricate. 4) We have also been able to stabilise plasmonic nanostructures made from silver, such that this metal can now be used routinely. As opposed to gold â€“ the metal of choice for plasmonics â€“ silver produces stronger field enhancement and can be used in the visible range of the optical spectrum. 5) We are developing an original approach to indirectly measure trapping events based on florescence resonance energy transfer (FRET), which can produce a distinct optical signal when a particle is trapped by a given structure.

Altogether, we have developed different tools that can now be combined to deliver the main objective of the project: building a platform where nanoscopic objects can be assembled together using optical forces.