Colloidal Photonic Crystals:Fabrication Via Magnetically Induced Self-assembly And Applications

Magnetically induced self-assembly of superparamagnetic colloid nanoparticles into periodic structure is a fast preparation technique of photonic crystals, which is developed in recent years. Compared with conventional colloidal assembly methods, magnetically induced self-assembly has several advantages. First, conventional self-assembly approaches that involve gravitational force, centrifugal force, ejectrophoretic deposition, electrostatic interaction and capillary force need a long time to fabricate photonic structure. They may take hours to days or even months to produce colloidal crystals of centimetre scale and the low production efficiency of such self-assembly processes can’t satisfy the requirements of practical application. However, with the magnetically induced self-assembly technique, superparamagnetic colloid nanoparticles can self-assemble into photonic structures in one second and intensely diffract the visible light. Second, through the modulation of the magnetic field strength and direction, it is convenient to control the formed one-dimensional photonic structures and further realize the change of diffraction color. Finally, unlike the photonic structures fabricated with conventional self-assembly technique, the formation of photonic structures based on the magnetically induced self-assembly technique is reversible. Through the application and withdrawal of the magnetic fields, the photonic structures can be formed and decomposed. The recent developments in the techniques have enabled convenient modulation of photonic properties and easy fabrication of sophisticated photonic structures.Based on the huge advantages of the magnetically induced self-assembly technique, the main objective of this dissertation is to explore the mechanism of magnetically induced self-assembly of magnetic nanoparticles and to fabricate functional photonic crystals based on magnetically induced self-assembly technique, such as photonic wordpad、photonic humidity sensor and photonic anti-counterfeiting labels. Furthermore, a novel invisible photonic printing is studied in detail. The main parts of the as-obtained results are summarized below:1A novel photonic wordpad was fabricated through the fast magnetically induced self-assembly of carbon-encapsulated superparamagnetic colloidals, followed by an instant radical polymerization to fix the photonic crystal structures inside a polyacrylamide glycol gel matrix. Distilled water is used as ink to write letters or Chinese characters with a different color on the wordpad because it can swell the polymer matrix and then increase the interparticle spacing within the chains, and as a result a red-shift in optical diffraction occurs. The photonic wordpad not only has an excellent reusability because the ink mark can be written or erased by introducing or removing the water in the surface layer of the photonic wordpad, but also is environmentally friendly and low-carbon owing to the reusability of the wordpad and non-toxicity of the ink. In addition, due to the use of the recent development of magnetically tunable assembly techniques, the processing time of this photonic wordpad is greatly reduced compared to using conventional assembly methods, which has an important significance in the large-scale industrial production in the future.2A visually readable and highly stable self-display photonic humidity sensor has been fabricated through the fast magnetically induced self-assembly of carbon-encapsulated superparamagnetic colloidals, followed by an instant radical polymerization. The photonic humidity sensor can self-display brilliant colors from navy blue to light red as the relative humidity changes from11%to97%. The whole shift of the reflection wavelength is nearly160nm, which almost covers the whole visual region and its intensity is readable by the naked eye. In addition, cycle tests demonstrate that the photonic humidity sensor has a good stability and reproducibility in its diffraction signal. Therefore, coupled with low cost and no power consumption, the photonic humidity sensor system can realize colorimetric detection of humidity, similarly to pH indicator paper.3Based on the recently developed magnetic-induced self-assembly techniques, we designed a novel, simple and low-cost method to fabricate a special class of photonic crystals with double photonic band-gap hetero-structures, and eventually achieved the purpose of modulating the optical diffraction color of the structural colors. The method greatly simplifies the fabrication of photonic crystals with multiple photonic band-gap hetero-structures and extends the modulation means of the optical diffraction color of structural colors. Furthermore, it is worth noting that due to the resulting structural colors that are derived from the double photonic band-gap hetero-structures consisting of double diffraction peaks and presenting a magnetic switching effect through the application and withdrawal of the magnetic fields (0.05T), which is more difficult to be imitated by those of chemical dyes and pigments, a kind of novel photonic anti-counterfeiting label has been prepared with these structural colors. Due to the widespread counterfeiting of various commercial objects and the urgent requirements of forgery protection, the photonic anti-counterfeiting labels will undoubtedly find applications in anti-counterfeiting fields.4Based on the magnetically induced self-assembly technique and our previous work, we have developed a novel photonic printing technique that enables fast and convenient preparation of invisible photonic prints with good tenability and reproducibility. The showing and hiding of the latent photonic prints is instantaneous, and the only required instrument is a magnet, which is easy to obtain and harmless to human body. This invisible photonic printing allows end-user control of the structural design utilizing simple software on a PC and is also applicable to a handwritten-pattern printing method. In addition, the invisible photonic prints were found to be stable for at least three months at room temperature and can be used after prolonged storage. These advantages reveal the excellent flexibility and practicality of this novel invisible printing technique.