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Highly Ordered Light-manipulators by Self-assembly

Periodic Reporting for period 1 - HOLES (Highly Ordered Light-manipulators by Self-assembly)

Reporting period: 2015-03-01 to 2017-02-28

Photonic technologies rely on the controlled propagation of ‘photons’, i.e. the quantum of light. These have various advantages compared to devices using electronic charge carriers (electronic devices), the most important are speed and the fact that photons can intersect without interacting. As a consequence, data transmitted by light can travel longer distances, faster, and most often with considerably lower losses and interferences. Applications of photonics would, thus, be ubiquitous, including areas from everyday life to the most advanced science uses, i.e. from telecommunications, information processing, sensing, medicine, and the like. Unfortunately, the development of photonic technologies has hitherto been slow. One of the main reasons resides in the lack of industrially applicable methods to process materials of the required optical characteristics.

So far, inorganic systems are selected for the fabrication of photonic crystals. Whilst they can exhibit high refractive indices - a number that describes how light propagates through that medium, it cannot be varied in a controlled way and is fixed for a given material. Therefore, in most cases many processing steps need to be changed when already one of the constituent of a multicomponent systems is varied, which is not an ideal situation from a technological point of view. Similar problems arise from more standard lithographic methods, which have traditionally been adopted for photonic structures manufacturing. They are difficult to be scaled to industrial level due to their high-cost and small patternable area. Therefore, a “photonic revolution” can be foreesen, if the development of easy-to-process materials and cost-effective, high-throughput processing methods for production of photonic elements is realised. Thereby, this is currently one of the hottest scientific topics from both scientific and social perspectives, as contributing to such development of photonics will generate knowledge, growth, services and more and better jobs that will deliver economic and social welfare.

My project was designed to produce such a step change through the development of versatile, rapid, low cost, processing routes that allow efficient and controlled deposition of optical materials into photonic structures. I proposed to produce a step change in the photonics field through the employment of the Dynamic Templating Process, a solution-processing method, which exploits the self-assembly of water microdroplets to create microscale honeycombs in a polymer-based material of suitable optical properties. Due to these properties and the honeycomb-like morphology induced, these materials would present photonic properties, so that they would allow manipulating the flow of light. Hence, this project was designed to generate new knowledge as well as a new disruptive technology to lay the foundations for a next generation low-cost photonic devices that will contribute to strengthen Europe´s long-standing position in manufacturing.
This research project explored a novel route for the fabrication of two-dimensional photonic crystals. It aimed to evaluate the potential of the breath figure method for this purpose. Specifically, the aim was to patten hexagonal arrays of holes (honeycombs) in a high refractive index polymer based material. Due to the symmetry and the high refractive index contrast of the material system, an optical band gap would be openned, thus allowing the manipulation of the electromagnetic radiation in the material.

My new approach has been to employ nanocomposites made of solution processable commodity – low-cost - polymers (soluble in organics like polystyrene, PS) and ultrafine inorganic compounds having high refractive indexes, such as MoS, which, in addition can be exfoliated from bulk MoS2 and, thus, can be obtained in large quantities,
In order to achieve high-refractive index composited from these materials a novel procedure was developed that allows MoS2 dispersions in organic solvents, such as chloroform.

Employing the MoS2 dispersion developed and starting from the breath figure deposition conditions used for neat PS, periodic micro-honeycombs were produced in the high-refractive index nanocomposite (see attached Figure). Their periodicity was assessed by the Voronoi diagram method. The systematic study of the processing variables allowed to tune the characteristic dimensions of the honeycomb. Moreover, we found that under certain humidity and temperature conditions, regions with square pore arrangements were formed, a result that has not hitherto been reported in the literature. Areas up to 100 cm2 were patterned using this method.

I aimed to to produce complex optical devices by coupling the photonic structures with a light emitting microcavity. As a light emmiting material, we selected MEH–PPV, which has demonstrated striking optical properties when the molecule adopts a planar conformation, i.e. in the so called “red phase”.We processed red-phase MEH-PPV in UHMWPS pseudogels and demonstrated that the red-phase was mainteined. The first demonstration of this approach was developed in our lab by employing MEH-PPV/ UHMWPS gels and PBMA honeycombs. Preliminary results are encouraging, the work is still on going but will lead to impactful results in very near future.

The main findings of HOLES and other parallel side projects carried out through collaborations established thanks to this fellowship have been disseminated in 7 publications in high-impact journals, such as Materials Horizons, Chem. Mater., J. Mater. Chem A, etc. Additional 5 papers have been already submitted for publication and we anticipate that an additional 3 papers will result from this body of work. 2 review papers have been written (one of them already published in Progress in Polymer Science, the polymers-related journal having the highest impact factor, IF=27) as well as a book chapter. I have also communicated our findings in 2 oral presentations at the 2015 MRS Fall Meeting & Exhibit (Boston, USA) and 2016 Energy Materials Nanotechnology conference in Duvrovnik (Croatia, INVITED TALK). A poster was presented at the conference on Synchrotron Radiation in Polymer Science in Madrid. 3 international seminars have been delivered in KAUST (Saudi Arabia), Intitute of Polymer Science and Technology (Madrid, Spain), and POLYMAT foundation (San Sebastian, Spain).
The photonic estructures developed in HOLES are novel, and will lead to exciting exploration into the development of new solution processed optical structures. In addition, the processing protocols established in this work are entirely adaptable to a high-throughput manufacturing process. As such, the work is industrially relevant and competes well with existing technology, and I expect the photonic structures and processing methods developed in this work to be easily incorporated into different technologies and products.
Micro-honeycomb made of the high-refractive index composite