Pieter-Jan Demeyer

Wonders of colloidal assembly

In nature the spontaneous formation of ordered structures from molecules, called self-assembly, is a very common process occurring in inorganic matter and living organisms. It is driven by atoms, molecules, particles, granular matter, etc. trying to reach the lowest possible energy state while interacting with each other. Deeper understanding of the subtleties of such interactions will allow mimicking of this kind of behaviour to build custom structures from synthetic molecules. This review attempts to cover the existing techniques for directed self-assembly that are currently used for colloidal crystal growth with a brief explanation of the interactions involved in each technique. It provides examples of the fundamental phenomena occurring in photonic crystals that, in the future, can be exploited in various applications.

A facile way to introduce planar defects into colloidal photonic crystals for pronounced passbands

We demonstrate a facile method for introducing planar defects into colloidal photonic crystals. Firstly, a 2D monolayer of SiO2 microspheres (guest spheres) was fabricated at the air/water interface by compressing the individual microspheres with a surfactant into long-range hexagonal arrays. The floating monolayer, which served as our defect layer, was then transferred onto a pre-deposited colloidal crystal slab consisting of PS@SiO2 microspheres (host spheres). Subsequently, a second colloidal crystal slab of host spheres was deposited on the surface of the defect layer. In comparison to previous methods to introduce planar defects into colloidal photonic crystals, this fabrication results in pronounced passbands in the band gaps of the colloidal photonic crystals. More importantly, the FWHM of the passband in our experiment is just 16 nm, which is narrower than the previously reported results to the best of our knowledge. Furthermore, the defect modes can be engineered by changing the diameter of the guest spheres and/or transforming the host spheres from PS@SiO2 spheres to hollow SiO2 spheres by calcination. The measured defect modes in the spectra match well with the simulated results.

A simple double-bead sandwich assay for protein detection in serum using UV–vis spectroscopy

In this study a double-bead sandwich assay, employing magnetic nanoparticles and gold nanoparticles is proposed. The magnetic nanoparticles allow specific capturing of the analyte in biological samples, while the optical properties of the gold nanoparticles provide the signal transduction. We demonstrated that a major improvement in the assay sensitivity was obtained by selecting an optimal gold nanoparticle size (60 nm). A detection limit of 5-8 ng/mL, a sensitivity of 0.6-0.8 (pg/mL)(-1) and a dynamic range of 3 orders of magnitude were achieved without any further amplification using the detection of prostate specific antigen in serum as a model system. The proposed assay has the ability to be easily implemented within a microfluidic device for point-of-care applications whereby the readout can be executed by a fast and cheap optical measurement.

Sandwich Approach toward Inverse Opals with Linear and Nonlinear Optical Functionalities

Three-dimensionally ordered macroporous materials have unique structural and optical properties, making them useful for numerous applications in catalysis, membrane science, and optics. Accessible and economic fabrication of these materials is essential to fully explore the many possibilities that these materials present. A new templating method to fabricate three-dimensionally ordered macroporous materials without overlayers is presented. The resulting structures are freestanding inverse opals with large-area uniformity. The versatility and power of our fabrication method is demonstrated by synthesizing inverse opals displaying fluorescence, chirality, upconversion, second harmonic generation, and third harmonic generation. This economical and versatile fabrication method will facilitate research on inverse opals in general and on linear and nonlinear optical effects in 3D photonic crystals specifically. The relative ease of synthesis and wide variety of resulting materials will help the characterization and improvement of existing anomalous dispersion effects in these structures, while providing a platform for the discovery and demonstration of novel effects.

Tuning the properties of colloidal magneto-photonic crystals by controlled infiltration with superparamagnetic magnetite nanoparticles

The performance of magnetic-field sensors and optical isolators is largely determined by the efficiency of the active materials. This efficiency could be dramatically increased by integrating Faraday materials in photonic crystals. For this purpose, monodisperse nanospheres were self-assembled into a colloidal photonic crystal and magnetic functionality was introduced by dipping the photonic crystal in a suspension containing superparamagnetic nanoparticles. Reflection and absorbance measurements of these magneto-photonic crystals revealed clear relationships between the time spent in suspension and the position and strength of the photonic band gap. When additional magnetic material was introduced, the band gap was red shifted and the strength of the band gap was decreased. Using Braggs law and the Maxwell-Garnet approximation for effective media, the filling fraction of the magneto-photonic crystals was calculated from the observed red shift. While superparamagnetic nanoparticles did confer magneto-optical properties to the photonic crystal, they also increased the absorption, which can be detrimental as the Faraday effect is measured in transmission. Therefore a trade-off exists in the optical regime between the amount of Faraday rotation and the absorption. By carefully controlling the filling fraction, this trade-off was investigated and optimized for photonic crystals with different band gaps. Both polystyrene and silica photonic crystals were filled with superparamagnetic nanoparticles. In case of the polystyrene photonic crystals, it was found that the maximum achievable filling fraction was influenced by the size of the polystyrene nanospheres. Smaller polystyrene nanospheres gave rise to smaller pore diameters and a faster onset of pore blocking when filled with superparamagnetic nanoparticles. As a result, the maximum achievable filling fraction was also lower. Pore blocking was found to be negligible in silica photonic crystals. Together with a higher mechanical strength, this makes silica photonic crystals more suited for the fabrication of colloidal magneto-photonic crystals. In this paper, a nanoscale engineering approach is described to carefully control the filling fraction of magneto-photonic crystals. This allows fine-tuning the absorption and the position and strength of the photonic band gap. By tailoring the properties of magneto-photonic crystals, the means for application-specific designs and a better description of Faraday effects in 3D magneto-photonic crystals are provided.

Introducing high-quality planar defects into colloidal crystals via self-assembly at the air/water interface

We demonstrate a facile method for fabrication of colloidal crystals containing a planar defect by using PS@SiO2 core-shell spheres as building blocks. A monolayer of solid spheres was embedded in core-shell colloidal crystals serving as the defect layer, which formed by means of self-assembly at the air/water interface. Compared with previous methods, this fabrication method results in pronounced passbands in the band gaps of the colloidal photonic crystal. The FWHM of the obtained passband is only ~16nm, which is narrower than the previously reported results. The influence of the defect layer thickness on the optical properties of these sandwiched structures was also investigated. No high-cost processes or specific equipment is needed in our approach. Inverse opals with planar defects can be obtained via calcination of the PS cores, without the need of infiltration. The experimental results are in good agreement with simulations performed using the FDTD method.

Control of Photon Emission by Photonic Bandgap Engineering in Colloidal Crystals

Photonics is generally regarded as the most promising industry to continue the technological evolution when electronic devices reach their fundamental limits on operating speeds and bandwidth. Nonetheless, more efficient manipulation of the optical processes is required to harness its full potential. Many optical devices, e.g. lasers, (organic) light emitting diodes, and (organic) photovoltaics, are based on excited state processes. To optimize the efficiency of these devices, the flow of energy should be controlled both spatially and energetically. Photonic crystals are seen as a promising class of materials that are able to fulfill both roles. In telecommunication, photonic crystal fibers have already proven their merits in providing spatial control on the optical signals. Moreover, photonic crystals have the potential to guide light on a microscopic scale and in three dimensions, which will be important in future optical devices.

Fabrication of polymer inverse opals with linear and nonlinear optical functionalities using a sandwiching approach

Three-dimensionally (3D) ordered macroporous materials combine interesting structural and optical properties. Accessible and economic fabrication is essential to fully explore the unique possibilities these materials present. A common method to fabricate 3D ordered macroporous materials is by self-assembling colloids, resulting in so-called opals. A templating strategy is then often used to introduce additional functionality inside the porous structure, giving rise to inverse opals. In this work, we developed an easy and versatile method to fabricate highly uniform polymer inverse opals without overlayers. Briefly, our approach consists of sandwiching a resin melt between two opal templates, forcing all material inside or between the macroporous structures. The opal voids are fully filled and the superfluous melt material is extruded before curing the resin. Finally, the opal templates are removed by chemical etching. The resulting structures are freestanding 3D macroporous films with large-area uniformity, displaying strong photonic properties due to their structural order. Additionally, many applications require specific optical functionalities. The versatility of our templating method is uniquely suited for this purpose as it allows doping of the melt before infiltration. Therefore, we can incorporate a large variety of optical functions in the inverse opals using a single approach We believe this method will help the systematic investigation and improvement of existing effects in these structures, while providing a platform for the discovery and demonstration of novel effects. As this method combines 3D ordered macroporous materials with linear and nonlinear optical materials, it is even possible to tune optical interactions, which could be technologically relevant for OLEDs, solar cells, lasers, electro-optical modulators and optical switches.