NANOSHELLS *
CHAPTER 8 CHAPTER 8
8.3 PROPERTIES OF NANOSHELL PARTICLES
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The metal nanoshells that have plasmon resonance in the IR region are suitable for biological applications, as this electromagnetic range is transparent for biological tissues. Interaction between nanoparticles depends upon the separation between neighboring particles. Thick coating leads to larger separation of the metal particles, whereas thin coating leads to less separation. The dipole to dipole coupling between the particles is responsible for red shifting the plasmon band. If the particles are well separated (thick coating), the dipole to dipole coupling is fully hidden and the plasmon band is located almost at the same position as the individual metal particle. By varying the thickness of the shell by a small value, the color of the core–shell particles can be tuned from one color to another.
At the same time, changes can be monitored spectroscopically by monitoring the SPR bands (Kalele et al., 2006).
8.3.2 LUMINESCENCE PROPERTIES
Semiconductor nanoparticles are fluorescent materials. The coating of silica is applied to them to decrease photo bleaching. The semiconductor nanoparticles coated with another layer of semicon- ductor have proven to be of great importance in enhancing the luminescence of these core–shell assemblies. The choice of shell material is important for localization of the electron hole pair. As in Fig. 8.6, in type I nanostructures such as CdSe/CdS or CdSe/ZnS, the conduction band of the shell material, which is a higher bandgap material, is at higher energy than the core, whereas the valence band of the shell is at lower energy than the core. In these materials, electrons and holes are confined in the core.
In type II nanostructures such as CdSe/ZnTe or CdTe/CdSe, both valence and conduction bands of the core material are at higher or lower energy than in the shell. In this case one carrier is confined in the core and the other in the shell. Type I and type II nanostructures have different properties because of the spatial separation of carriers. The lifetime decay of exciton and quantum yield of core–shell nano- particles is much higher than individual semiconductor nanoparticles. The organic dyes are well-known
FIGURE 8.6
Type i and type ii semiconductor core–shell structures. in type i nanostructures, the conduction band is at a higher energy than the core, while the valence band is at a lower energy. in type ii nanostructures, both valence and conduction bands of the core material are at higher or lower energy than in the shell.
From Kalele, S., Gosavi, S., Urban, J., & Kulkarni, K. (2006). Nanoshell particles: synthesis, properties and applications. Current Science, 15.
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phosphor materials which are similar to semiconductor nanoparticles. They are used as fluorescent bio- logical labels. These dyes are not photostable, thereby quickly bleaching out. Furthermore, some of the dyes cannot be dispersed homogeneously in water. When these dye molecules are entrapped in silica shell, the enhancement of luminescence can be observed. The silica coating on these dye molecules makes them disperse uniformly in water (Kalele et al., 2006).
8.3.3 THERMAL PROPERTIES
The melting point of nanoparticles is lower than that of similar bulk material. This has been attrib- uted to large surface tension in nanoparticles. In order to release this tension, it melts faster than bulk. Encapsulation of silica on these nanoparticles greatly improves the thermal stability of these particles. By changing the thickness of the shell, the variation in melting point can be observed. In some nanoshell assemblies such as metallic shells on dielectric cores, the thermal instabilities can be observed. Complete distortion of the shell can be observed when silica gold nanoshell particles are heated. The melting of nanoshells can be observed at significantly lower temperature. Due to higher surface area of core–shell particles, a higher number of particles is exposed to the surface and is affected by faster melting. When the particles are encapsulated with silica, the enhancement in thermal stability can be observed. It has been proven that 60–70 nm thick coating of silica greatly improves the thermal stability of gold nanoshells. The coating of silica on such shells is a way of preserving the identity of individual core particles because of the high temperature stability of silica (Kalele et al., 2006).
8.3.4 SURFACE CHEMICAL AND CATALYTIC PROPERTIES
The core–shell particles offer high surface area and can be used as efficient catalysts. Titania is a sig- nificant photocatalytic material. It has been established that nanoshells and nanoparticles show differ- ent catalytic behavior from bulk titania. Titania is thermally unstable and loses its surface area readily.
Coating a thin layer of some other stable oxide such as silica on titania can greatly improve its catalytic activity (Kalele et al., 2006).
8.3.5 MAGNETIC PROPERTIES
Stability of magnetic materials is important when studying their magnetic properties. In order to improve the surface characteristics and protect them from reacting with various species to form oxides, they are coated with inert materials. Silica is a good choice because it forms stable dispersions. It is also nonmagnetic and therefore does not interfere with the magnetic properties of the core particles.
Magnetic materials are often susceptible to agglomeration and show anisotropic interactions. Their stable dispersion can be prepared by inducing surface charges on them or adsorbing some organic molecules on their surfaces. Since organic molecules do not form any strong chemical bond such as covalent bond with magnetic particles they can be desorbed. When magnetic particles coated with silica are suspended in the medium, isotropic interactions are observed. Two magnetic materials can be used as core and shell. The magnetic properties can be tailored by varying core to shell dimensions (Kalele et al., 2006).
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