Cup-shaped (above) and cone-shaped (below) nanoparticles. Nanoparticles have many applications in chemistry, biosensors, solar cells or photochemical devices. Depending on their size, shape and position, different results can be obtained. Nevertheless, it is not possible to control all these parameters with the current production methods.
Nowadays, the chemical synthesis of nanoparticles is well consolidated. It enables controlling the nanoparticles composition -which is interesting for chemical and cosmetics products- but not their shape and position on a surface. The latter can be done with physical methods, but the result is not the best and these methods don’t allow either a low cost mass production. Therefore, these methods are restricted to scientific research.
Plasmonics phenomena
Tunning the size and position of nanoparticles is especially interesting in plasmonics. Plasmons happen when a light beam reaches the interface of a dielectric (material with low conductivity, like the air) and a metallic material, which triggers phenomena like electric field concentration or light absorption enhancement. These are especially relevant for the development of sensors and photovoltaic panels.
Plasmonic resonance depends on the material of the nanoparticle, its shape, the surrounding medium, its orientation and the possible interaction between nanostructures. But to carry plasmonic devices out of the laboratory requires a mass production method which enables the production of arranged nanoparticles modelling their geometry. Now this is possible with a method developed by scientists at the CSIC’s Instituto de Microelectrónica de Barcelona (IMB-CNM). The technique enables producing nanoparticles with an accurate control of their shape and orientation, and at low cost.
Flexible and low cost
The method is based on the combination of nanoimprint lithography and several techniques of metal deposition. Firstly, a silicon oxide mask is created on a substrate by nanoimprint lithography, which allows for nanometric resolution. Secondly, the created patterns are transferred to the polymer layer which has been previously deposited below the mask. Through metal deposition the metallic nanoparticles are formed and take the shape of the cavities between the polymer and the mask. Controlling the thickness of the polymer is sufficient to control the height of the structures obtained. Also, depending on the technique used for the nanoparticles metallization, and tuning the directionality of the deposition, different shapes using the same mask are obtained: for example the empty cup-shaped or, on the contrary, the solid cone-shaped nanoparticles.
This method can be adapted to obtain virtually any shape and combination of materials, which enables the adaptation of the design to enhance the plasmonic resonance of nanoparticles. Besides, it can be applied to many other fields in nanotechnology, where one the main issues is the mass production at low cost.
This research has been developed by group of Nanofabrication of the IMB-CNM, led by the CSIC researcher Francesc Perez Murano, and the group of magnetic nanomaterials of the University of Barcelona. The results have been published in the Nanotechnology journal, where the scientists showed some of the cup-shaped nanoparticles obtained in gold, and videos that demonstrate their plasmonic resonance. This research has been part of the doctoral thesis of PhD students Nerea Alayo and Ana Conde-Rubio, the latter one is currently continuing the work as part of her PhD.
Reference article:
Nanoparticles with tunable shape and composition fabricated by nanoimprint lithography. Nerea Alayo, Ana Conde-Rubio, Joan Bausells, Xavier Borrisé, Amilcar Labarta, Xavier Batlle and Francesc Pérez-Murano. Nanotechnology, Volume 26, Number 44 doi: 10.1088/0957-4484/26/44/445302
Overview at the IOP website: http://iopscience.iop.org/0957-4484/labtalk-article/63030
Videos:
https://www.youtube.com/watch?v=-DkLHZfV0XU&feature=player_embedded
https://www.youtube.com/watch?v=zXpTVOFDPII&feature=player_embedded