Physical and Chemical characteristics of nanomaterial(17-18) As compare to conventional bulk particles nanomaterials exhibits some unique physical properties including electrical, catalytic, magnetic, mechanical, thermal, or imaging features that make the nanomaterials a relevant topic in medical, pharmaceutical and different engineering sectors. The nanomaterial possesses some remarkable and specific peculiar properties which may be significantly distinctive from the physical properties of bulk materials.
The specific features of those physical properties are discussed below
Color: There are few examples where the materials show the different color when they are converted to nanoparticles. As per example when the gold materials are converted to nanomaterials they turn into red color. Gold nanoparticles interaction with light is strongly govorened by the particle sizes of the materials. Small particle sizes (~2-150nm) have high surface electron densities which are called as surface plasmons undergo a collective oscillation when they are excited by light at specific wavelengths. This oscillation is dscribed as a surface plasmon resonance (SPR).For small (~30nm) monodisperse gold nanoparticles the surface plasmon resonance phenomona is responsible for an absorption of the blue-green portion of the spectrum (~450 nm) while red light (~700 nm) is reflected, producing a rich red color.(Fig.5)
Fig.5. Suspensions of gold nanoparticles of various sizes.
Melting point: The melting point drastically falls when the particle size of the material approaches to the nanoscale ranges. This phenomenon related to melting point depression is very prominent in nanoscale materials which melt at temperatures hundreds of degrees lower than bulk materials. Melting point depression is most evident in nanowires, nanotubes and nanoparticles, which all melt at lower temperatures than bulk form of the same material. Changes in melting point occur because nanoscale materials have a much larger surface to volume ratio than bulk materials, drastically altering their thermodynamic and thermal properties.
Mechanical strength: All the nanomaterials possess high mechanical strength as compared to their conventional counterparts. The mechanical strength of nanomaterials may be one or two times higher in magnitude than that of single crystals in the bulk form. Defects in the form of atomic vacancies can lower the tensile strength of the materials by up to 85% .Conversion of materials into nanoscale increases crystal perfection or reduction of defects, which would result the enhancement in mechanical strength. Cutting tools which should be harder than the material which is to be cut are made of nano materials, such as tungsten carbide, tatalum carbide, and titanium carbide. These cutting tools are much more erosion-resistant and durable than their conventional large grained bulk materials. In general, hardness of metals (Cu) increases linearly with increase of grain size. However, in case of nanomaterial, the hardness increases linearly with decrease of Particle size (fig 6)
Fig 6. Hardness variation for metal in nanometer and micrometer scale.
Electrical properties of the nanomaterials: This is quite complex phenomenon. Reduction in material's dimensions would have two different contrasting effects on electrical conductivity. By its property nanoparticle product enhance the crystal perfection and as well as it reduce the defects. As a result electron scattering phenomenon due to crystal defects are also reduced and a reduction in resistivity is experienced. However, at room temperature the defect scattering incident contributes a minor influence on the total electrical resistibility of various metals. On the other hand surface scattering phenomenon which is highly increased due to reduction of particle size is one of the prominent reasons for increase of the total resistivity. In addition a reduction in particle size below a critical dimension, i.e. (electron de Broglie wavelength), would result in a modified electronic structure with wide and discrete band gap. The reduction of particle size into this range would result in an increased electrical resistivity.
Optical properties: Optical properties exhibited by nanomaterials are quite different from their bulk counterpart. The reason behind this change in property is mainly due to the effect of the surface plasmon resonance. In addition, the increased energy level spacing is also an important criterion for this changing behavior. Due to increased band gap for semiconductor nanoparticles absorption edge is shifted toward shorter wavelengths. Surface Plasmon resonance effect changes due to change in particle size which in turn changes the color of metallic nanoparticles. The coherent excitation of entire free electrons in the conduction band may produce an in-phase oscillation, called surface plasmon resonance. When the size of a metal nanocrystal is smaller than the wavelength of incident radiation, a surface plasmon resonance is generated. On resonance, light is tightly confined to the surface of the nanostructure, until it gets eventually absorbed inside the metal, or scattered back into photons.
Chemical properties of the materials are also changed when it converts to nano range.Due to increase of exposed surface area of the nanopartclesas compared with conventional bulk objects, reactivity of those particles increase enormosly.
some important features of the nanoparticle chemical properties are given below
1. In case of nanoparticles 50% of all the atoms are surface atoms and as a result electric transport properties of these particles are no longer dependent on solid state bulk phenomenon.Electrical properties are dirctly related to chemical properties.
2. Due to larger proportion of surface atoms, the atoms present in nanomaterials posses a higher energy as compare to atoms present in bulk structure.
3. The interactions between nanoparticles depend on the chemical nature of the surface. Due to large surface area high quantity charge species defects and impurities may be easily attracted to surfaces and interfaces of nano particles and thus chemical nature of the surfaces changes abruptly as compare to their bulk counterpart.
4. Surface properties of the nanoparticles and their interaction can be modified or altered by using molecular monolayer.
18. Vollath D. Nanomaterials: An Introduction to Synthesis,Properties and Application. November/December 2008, Vol. 7, No.6, 865-870.http://omicron.ch.tuiasi.ro/EEMJ.
1.There are some materials like gold shows different type of colour when they are converted to nanomaterials.
2.The melting points of the material are drastically reduced when they are converted into nanoscale.
3.Nanomaterials have higher mechanical strength.
4.Electrical conductivity may reduce or increase when particles are in nanoscale.
5.Optical properties of nanomaterials significantly differ as compare to their conventional counterpart.
6.Chemical properties of nanomaterials are also different from normal size particle.