Recent Advances in OLED Optical Design

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials.

Hydroxypropyl-cellulose (HPC), a derivative of naturally abundant cellulose, can self-assemble into helical nanostructures that lead to striking colouration from Bragg reflections. The helical periodicity is very sensitive to pressure, rendering HPC a responsive photonic material. Recent advances in elucidating these HPC mechano-chromic properties have so-far delivered few real-world applications, which require both up-scaling fabrication and digital translation of their colour changes. Here we present roll-to-roll manufactured metre-scale HPC laminates using continuous coating and encapsulation. We quantify the pressure response of the encapsulated HPC using optical analyses of the pressure-induced hue change as perceived by the human eye and digital imaging. Finally, we show the ability to capture real-time pressure distributions and temporal evolution of a human foot-print on our HPC laminates. This is the first demonstration of a large area and cost-effective method for fabricating HPC stimuli-responsive photonic films, which can generate pressure maps that can be read out with standard cameras.

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Roll-to-roll fabrication of touch-responsive cellulose photonic laminates.

Photonic crystal based biosensors: Emerging inverse opals for biomarker detection

We present our results on the fabrication of large area colloidal photonic crystals on flexible poly(ethylene terephthalate) (PET) film using a roll-to-roll Langmuir–Blodgett technique. Two-dimensional (2D) and three-dimensional (3D) colloidal photonic crystals from silica nanospheres (250 and 550 nm diameter) with a total area of up to 340 cm2 have been fabricated in a continuous manner compatible with high volume manufacturing. In addition, the antireflective properties and structural integrity of the films have been enhanced via the use of a second roll-to-roll process, employing a slot-die coating of an optical adhesive over the photonic crystal films. Scanning electron microscopy images, atomic force microscopy images, and UV–vis optical transmission and reflection spectra of the fabricated photonic crystals are analyzed. This analysis confirms the high quality of the 2D and 3D photonic crystals fabricated by the roll-to-roll LB technique. Potential device applications of the large area 2D and 3D col...

Large Area 2D and 3D Colloidal Photonic Crystals Fabricated by a Roll-to-Roll Langmuir-Blodgett Method.

We report here (i) a method for the synthesis of highly monodisperse silica@gold@silica core–shell–shell (CSS) particles (<6% deviation in size) with coherent outer silica shells of directly tunable thickness and porosity, (ii) the fabrication of three unit cell thick (12 layers) metallodielectric photonic crystals (MDPCs) from these particles and (iii) their angle-resolved optical reflectance properties demonstrating the influence of dielectric contrast modification and metal plasmonic modes on photonic band gap properties. The synthetic route presented describes the use of two alkylamines, the first, 1,3-diaminopropane (DAP), as a surface manipulating agent, and the second, N,N-dimethyldodecylamine (DMDDA), as a catalyst for the outer silica shell formation, for the reproducible synthesis of monodisperse CSS particles avoiding the formation of the unwanted side-products such as core-free silica particles and aggregated particles with multiple cores. A rational chemical reaction mechanism describing the pathways involved in the formation of coherent silica shells, demonstrating the roles of both the amines, is proposed. The creation of partial wetting/de-wetting sites on gold nanoparticle surfaces by the adsorbed DAP molecules, the surface silanol modification by DAP and the slow hydrolysis–condensation reactions catalyzed by DMDDA are found to be responsible for the selective deposition of silica (heterogeneous nucleation) only onto the existing silica@gold core–shell particles leading to the formation of monodisperse CSS particles. The optical reflectance of the MDPC formed from these CSS particles shows an angle-dependent blue-shift that perfectly obeys the Bragg–Snell law predictions for photonic crystals. The dielectric contrast modification and the possible metal plasmonic mode-coupling in the MDPC also result in a high gap-to-midgap ratio (23%) as compared to the PC made of neat silica particles of comparable size (9%).

A facile method for the synthesis of highly monodisperse silica@gold@silica core–shell–shell particles and their use in the fabrication of three-dimensional metallodielectric photonic crystals

Electrochemical properties of opal-V6O13 composites

The bottom-up colloidal synthesis of photonic band gap (PBG) materials or photonic crystals (PC) has attracted considerable interest as compared to so-called top-down lithographic approaches due to the simple processing steps involved and the prospect of the economically viable production of complex 3-dimensional optical materials from simple colloidal particles. To date self-assembly techniques constitute the most popular approach to fabricate 3D photonic crystals from colloidal particle suspensions. Based on the natural tendency of monodisperse colloidal particles to organise into ordered arrays, this method represent the best option due to the ease of fabrication, ability to produce larger area samples and cost. Here we report on the fabrication of long range three-dimensional (3D) ordered poly (methyl methacrylate) (PMMA)-silica PC structures and the subsequent fabrication of robust silica inverse opals using self-assembly methods. The optical properties of these materials are described and discussed in terms of potential applications of these materials.

Preparation and Properties of Silica Inverse Opal via Self-Assembly

A polymer waveguide is fabricated on top of an inverted opal photonic crystal structure to demonstrate an air clad functioning waveguide for the first time to the best of our knowledge. An optically thick layer with a refractive index (n = 1.01) close to air was realized on a silicon wafer by first co-forming a self-assembled 3-D photonic crystal structure with PMMA spheres and a silica backbone. Following the fabrication of polyimide waveguides on this surface by micro-molding, the PMMA spheres were removed to leave behind an inverted opal structure underneath the waveguides with refractive index close to air. Broadband, polarization independent fundamental mode optical waveguiding from 1300 nm to 1600 nm wavelengths was obtained. This original approach overcomes some of the drawbacks associated with conventional polymer waveguides and can be the basis for a range of optical interconnection and sensing applications.

Joseph McGrath

Papers

Large Area 2D and 3D Colloidal Photonic Crystals Fabricated by a Roll-to-Roll Langmuir-Blodgett Method.

A facile method for the synthesis of highly monodisperse silica@gold@silica core–shell–shell particles and their use in the fabrication of three-dimensional metallodielectric photonic crystals

Electrochemical properties of opal-V6O13 composites

Preparation and Properties of Silica Inverse Opal via Self-Assembly

Air-clad broadband waveguide using micro-molded polyimide combined with a robust, silica-based inverted opal substrate