NANO MATERIALS

 

Here is the intro and some properties of nano materials..

INTRODUCTION

The word “nano” derives from the latin word “nanus” and Greek word “𝜈 ́𝛼νς,” Both indicating a person of very low height, i.e. a dwarf.  Nanomaterials are conventionally defined as materials having at least a dimension between 1 and 100 nm.

Common types of nanomaterials include nanofilaments, nanotubes, nanowires, nanocables, nanothin film, dendrimers, quantum dots, composite materials, and other materials besides nanopowder. Because of the unique nanoscale (1–100 nm) size, nanomaterials are different from microscopic atoms molecules and macro-objects in terms of their physical, chemical, electrical, and magnetic properties.

On the basis of reduction in size of materials in different dimensions, nanomaterials are classified into three groups.

Surface effects

Nanomaterials possess a large fraction of surface atom per unit volume. This dramatic increase in the ratio of the surface atom to interior atom in nanomaterials is responsible for great change in physical and other properties of the materials. When the size of the object is reduced to a nanometric range the proportion of surface atom increased leading to substantially more reactive surface sites. Thus nanomaterials possess high surface area over volume ratio, which leads them to interact with the environment more effectively compared to bulk materials. The surface atoms have less coordination that lead them to lesser stability and highly reactive than the interior atoms. A further consequence of this lower stability of atoms or molecules at the surface is the lower melting point of surface layers. 

Properties of nanomaterials:

High surface area and quantum effects at the nanoscale are responsible for different properties of nanomaterials compared to bulk materials. The quantum confinement effects describe electrons in terms of energy levels, valence bands, conduction bands, and electron energy band gaps. The confinement means restricting the motion of randomly moving electrons to specific energy levels (discreteness). When the particle size is reduced to nanoscale, the band gap is increased as compared with the bulk materials, and it leads to wider in the separation between the energy levels. Hence, properties such as melting point, fluorescence, electrical conductivity, magnetic permeability, and chemical reactivity change as a function of the size of the particle.

 Electrical properties :

The electrical conductivity of nanomaterials is generally lower than the bulk materials due to the increase of the band gap energy with a decrease in particle size of the nanomaterials. In bulk metals, the valence and conduction bands overlap, while in metal nanoparticles there is a gap between these bands. The gap observed in metal nanoparticles can be similar in size to that seen in semiconductors (< 2 eV) or even insulators (> 2 eV). This results in a metal becoming a semiconductor.  

 Optical properties: 

The optical properties such as reflection, transmission, absorption, and light emission of

the nanomaterials are completely dependent on their electronic structure.  At nanoscale level, nanoparticles are so small that electrons in them are not as much as free to move as in case of bulk material. Due to this quantum confinement of electrons, nanoparticles react differently with light compared to the bulk material.  Optical properties such as emission and adsorption occur when electron transition occurs between the highest occupied molecular orbital, which is a valence band, and the lowest unoccupied molecular orbital, which is essentially the conduction band. This optical bandgap increases with the decrease in particle size. Tuning the nanosize of a semiconductor means tuning of bandgap leading to light of certain wavelength being emitted. Thus the same material emits different colours depending on its size. 

Example: 

It is well known that gold has the characteristic yellow, brilliant color, but when gold is produced in the nanosized dimension range, its color invariably changes to red. In other words, gold shows

a color depending on the size of the gold particles. This phenomenon has an impressive example in the so-called Lycurgus cup now preserved at the British Museum in London. 

This cup was probably created in Rome during the fourth century and shows a different color changing the light from opaque green to bright red. This is the most famous example of the so-called dichroic glass. Actually, it presents some tiny amount of nanosized gold and silver that gives these unusual optical properties. This happens because at the nanoscale range, the electron cloud located on the surface of a gold nanoparticle has the possibility to resonate with different wavelengths of light depending on their frequency. 

Magnetic properties:

Compared to bulk materials nanomaterials show a variety of unusual magnetic behavior due to the surface or interface effects. The magnetic properties of a magnet are described by its magnetisation curve (called hysteresis curve which is the variation of intensity of magnetisation with applied magnetic field). These curves show the property like whether a magnet can be used as permanent magnet or a electromagnet etc…. The size of the material can change the property of a magnet by changing the magnetisation curves. Thus nanostructuring of bulk magnetic materials leads to changes in the curves which can produce soft or hard magnets with improved properties. Thus the nanomaterials may become superparamagnetic, even though their corresponding bulk materials are not magnetic. For example, Fe3O4 nanoparticles showed superparamagnetic-like behavior, even though bulk iron oxide (Fe3O4) is ferromagnetic. Exceptional surface energy and the flipped orientation of the spin electrons of the Fe3O4 nanoparticles are responsible for this phenomenon.  Superparamagnetic nanoparticles are not magnetic when located in a zero magnetic field, but they quickly become magnetized after an external magnetic field is applied. When they are below the superparamagnetic diameter, the nanoparticles can revert quickly to a nonmagnetized state after an external magnet is removed

 Mechanical properties 

 Mechanical properties of nanomaterials may reach the theoretical strength, which are one or two orders of magnitude higher than that of single crystals in the bulk form. The enhancement in mechanical strength is simply due to the reduced probability of defects. Carbon nanotubes are 100 times stronger than steel but six times lighter.