Nanoparticles possess a variety of shapes and their names are characterized by their different shapes. For example, there are nanospheres that are spherical, nanoreefs, nanoboxes, nanoclusters, nanotubes etc. These shapes or morphologies sometimes arise spontaneously as an effect of a templating or directing agent during synthesis for example during miscellar emulsions or anodized alumina pores, or from the innate crystallographic growth patterns of the materials themselves.
Types of nanoparticle
The morphologies of nanoparticles help serve their various purposes such as long carbon nanotubes being used to bridge an electrical junction. Amorphous particles usually adopt a spherical shape or nanospheres and anisotropic microcrystlaline whiskers correspond to their particular crystal shape. Small nanoparticles usually form clusters. These may be of various shapes like rods, fibers, and cups etc. The study of fine particles is called micromeritics.
Why is morphology important?
Controlling the morphology of nanoparticles is of key importance for exploiting their properties for their use in several emerging technologies. Optical ﬁlters and bio-sensors are among the many applications that use optical properties of gold nanoparticles and it requires anisotropy of the particle shape as larger shapes produce greater plasmon losses.
Despite the great importance of the morphology of nanoparticles, it is generally not well characterized and practically never controlled. However, this is of prime importance. For example, this is important in magnetic devices where well-defined magnetization axes and switching fields are required to store or to process information.
The morphological characteristics to be taken into account are:
- Aspect ratio
Nanoparticle aspect ratios
Nanoparticles with high- and low-aspect ratio are classified separately. High aspect ratio nanoparticles include nanotubes and nanowires, with various shapes, such as helices, zigzags, belts, or perhaps nanowires with diameter that varies with length.
Small-aspect ratio morphologies include spherical, oval, cubic, prism, helical, or pillar. Collections of many particles exist as powders, suspension, or colloids.
Dimensionality and classification of nanoparticles
Nanoparticles are generally classified based on their dimensionality, morphology, composition, uniformity, and agglomeration. Classification based on dimensionality is a generalization of the concept of aspect ratio.
These are one dimensional in the nanometer scale are typically thin films or surface coatings, and include the circuitry of computer chips and the antireflection and hard coatings on eyeglasses. These have been used in electronics, chemistry, and engineering.
Two-dimensional nanomaterials have two dimensions in the nanometer scale. These include 2D nanostructured films, with nanostructures firmly attached to a substrate, or nanopore filters used for small particle separation and filtration. Asbestos fibers are an example of 2D nanoparticles.
Materials that are nanoscaled in all three dimensions are considered 3D nanomaterials. These include thin films deposited under conditions that generate atomic-scale porosity, colloids, and free nanoparticles with various morphologies.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)