Nanomaterials
Generally nanomaterials are considered as those with one of their structural features of size less than 100nm. nanomaterials have a wide variety of potential applications in biomedical, optical, and electronic fields. Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. Many of these nanomaterials are made directly as dry powders, but they will rapidly aggregate through a solid bridging mechanism in as little as a few seconds. To keep the nanoparticles as such they are prepared and stored in a liquid medium in order to have sufficient interparticle repulsive forces and stay without aggregation. The composition of the nanoparticle is briefly explained below.
Composition
Nanoparticle consists of two main parts, a core and an outer organic stabilizing layer.
Inner core
The core can be made out of a variety of materials and it is responsible for the optical and electrical properties as it confines electrons to the physical dimensions of the particle. This leads to well known quantum “confinement” effects dictated by quantum mechanics. The core is nearly identical to the crystal structure of the parent (bulk) material and may be thought of as a much smaller fragment of the bulk lattice. NP may be spherical, cubes, rods and other non-spherical shapes.
Outer organic shell
The second major aspect of the NP is its outer organic shell. In the case of the solution-phase synthesis of colloidal nanomaterials, structure is stabilized in order to prevent aggregation of the particles. For some of the NPs the stabilizing layer will consist of simple organic surfactants which provide steric stabilization of the particles. Nanoparticles located in close proximity undergo a repulsive potential arising from surface bound organic molecules. Alternatively, they can be stabilized by electrostatic means, based on the Coulomb repulsion of like charges to prevent particle agglomeration.
General structure
The general structure of the organic passivating ligand consists of a head group that “sticks” to the NP surface via dative bonds, actual covalent bonds or electrostatic attraction. The surfactant molecule also possesses a “tail” which points away from the nanoparticle surface and extends into the surrounding liquid medium. The polar/nonpolar nature dictates the NP solubility within surrounding organic or aqueous media. The organic shell plays a relevant role even in the quantum efficiency of the nanoparticles, their stability in different media and prevents high electrical conduction.
Stabilization
For many chemically synthesized NPs, their primary solubility will be within organic solvents. The surfactant molecules also provide electronic stabilization of the NP by coordinating to dangling bonds on the surface. These dangling bonds stem from the abrupt termination of the NP core. If not taken into account, they may lead to defect related contributions to the NP optical and electrical properties. As a consequence, for this and other abovementioned reasons, the synthesis and development of colloidal NPs is as much about the growth of the core as it is about the choice of organic surfactants passivating their surfaces.