Review articleNanoparticle processing for optical applications – A review
Introduction
Interest in nanoparticles, which are defined as particles with diameters ranging from a few to 100 nm, is rapidly increasing because nanoparticles have unprecedented chemical and physical properties that differ markedly from those of the bulk material [1], [2]. Nanoparticles have great potential for use in various industries. For example, nanoparticles have been used in such varied applications as electronics, catalysts, drug carriers, sensors, pigments, magnetic and optical materials, etc. In particular, the unique optical properties associated with nanoparticles and their composite materials include a high- or low-refractive index, high transparency, novel photoluminescence properties, photonic crystal, and plasmon resonance.
It is important to develop methods for nanoparticle synthesis that allow for control of nanoparticle characteristics, including size distribution, morphology, crystallinity, purity, and composition. Several methods for the synthesis of nanoparticles and their composite materials have been reported previously, including liquid-phase-, solid-phase- and gas-phase-processes [2]. However, to be feasible for use in industry, a process needs to be simple, inexpensive, and able to operate continuously with a high production rate. Thus, this review focuses on aerosol spray methods, such as flame-assisted methods, spray pyrolysis, and spray drying, because these methods are simple, and inexpensive, and operate continuously to produce high-purity nanoparticle materials at a rapid rate [1].
On the other hand, production of a nanocomposite material with novel optical properties, such as a high refractive index for a transparent material, often requires incorporation of nanoparticles with different optical properties into a matrix material to create either a polymer solution (melt) or an elastomer. For instance, nanoparticle-polymer composites can be prepared either by synthesis of nanoparticles within a polymer matrix (an in situ process) or by dispersion of nanoparticles in a monomer and subsequent polymerization of the monomer. However, typically, in situ processes have high production costs and produce materials with a high impurity content. To overcome these problems, dispersion of nanoparticles in a monomer and subsequent polymerization of the monomer is better suited to use in industry, especially because many nanoparticles are commercially available. However, use of commercially available nanoparticles also has limitations. The most common problem is nanoparticle agglomeration in most solvents due to the strong forces of attraction among nanoparticles in a liquid suspension. Therefore, development of techniques, such as bead milling, for dispersion of nanoparticles in either a solvent or monomer is important.
Other optical properties associated with nanoparticles, such as photonic crystal, plasmon resonance, and transparent conductive material, are also of interest. To obtain these optical properties, manipulation of nanoparticles into one- (1D), two- (2D) or three-dimensional (3D) nanostructured materials must be possible. 1D nanostructures, (wires, fibers, and rods), 2D nanostructures (monolayer particle film), and 3D nanostructures (ordered porous films) have become the focus of intensive research.
This article provides an overview of current research on nanoparticle processing for optical applications. The main text of this review is organized into four sections. Section 2 discusses the use of aerosol spray methods for the production of nanoparticles and nanostructured particles for optical applications, such as photoluminescent and transparent conductive materials. The preparation of particles with ordered porous morphology and their optical properties are also reviewed in this section. Section 3 discusses techniques for dispersion of agglomerated nanoparticles into a solvent or a monomer. Sections 4 Coating process for making nanoparticle films, 5 Electrospinning method for nanoparticles derived fibers describe coating methods (i.e., spin- and dip-coating) and electrospinning methods for the preparation of nanostructured films and fibers from a nanoparticle dispersion precursor.
Section snippets
Aerosol spray methods for the synthesis of nanoparticles and nanostructured particles
Aerosol spray methods, such as spray pyrolysis, spray drying, and the flame spray method, have the advantage of being continuous, low-cost, and single-step methods for the preparation of nanoparticles and nanoparticle composites that are spherical, chemically homogeneous, and comprised of multiple materials. A representative aerosol spray system is shown schematically in Fig. 1 [1]. The primary components of the system are as follows: (i) an atomizer or nebulizer that converts the solution into
Nanoparticle dispersing method
As mentioned in the introduction, production of a composite material with high transparency and either new or high quality optical properties often requires incorporation of nanoparticles with different optical properties into a matrix. Therefore, the development of techniques for dispersion of nanoparticles into a solvent or monomer is important.
Many mechanical processing methods have been developed for dispersion of agglomerated particles in liquids, including agitator disks, colloid mills,
Coating process for making nanoparticle films
Materials with 1D, 2D, or 3D structures have received considerable attention from both theorists and experimentalists during the past decade. These materials hold promise for use in optical applications. Of note, nanoparticles can be used to obtain novel or higher performance optical materials. Various methods of nanoparticle processing for the production of photonic crystals and enhancement of LED films are described below.
Electrospinning method for nanoparticles derived fibers
As mentioned earlier, indium tin oxide (ITO) is the most frequently used transparent conducting oxide (TCO), and has many applications, which include flat panel displays, solar cells, light-emitting diodes, and sensors [17], [18], [19], [20]. In recent years, the preparation of 1D nanostructures, such as nanofibers, nanowires, nanorods, nanobelts, and nanotubes, from various materials have been the subject of intensive research due to their unique properties and intriguing application in many
Conclusion
When nanoparticle becomes a great topic in enhancing the optical properties, its preparation, dispersion and manipulation processing become very important. This review presented an overview of current research on the processing of nanoparticles for optical applications, including an aerosol spray method for nanoparticle processing, a nanoparticle dispersion method, a coating method for making films, and an electrospinning method for making fibers from nanoparticle dispersion. Even the presented
Acknowledgements
The author acknowledges the support of the Ministry of Education, Culture, Sports, Science and the Technology (MEXT) of Japan. The author thanks Prof. Kikuo Okuyama at Hiroshima University for the advice and productive discussions. The author also thanks to Mr. M. Miftahul Munir at Hiroshima University for the assistantship.
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