0.17 Chapter 5_introduction-nanomaterials.  (Page 3/3)

5.2. Classification of Nano-materials.

Nano-materials are classified in exactly the same manner as Bulk Materials. The crystalline materials have some standard Unit Cells (Cubic Cells, FCC, BCC, Hexagonal Unit Cell, tetrahedral structure) which are repeated in all 3-Dimensions.

A Single Crystal of nanometer size less than 100nm is referred to as nano-crystal. It is a single domain crystal with the diameter of the single domain less than 100nm. In Figure 5.7 the nano-crystals are shown.

Figure 5.7. A single-crystal of 100nm size or less is a single domain crystal. It is Quasicrystal if in pyramid form. It is Crystalline if in cubic form. It is super structure if single domain crystals are arranged in a regular periodic form.

5.3. Formation of Stable Nano-structure.

A stable nano-structure is the minimum energy configuration and a minimum energy configuration is obtained in the following manner:

Figure 5.8. One atom surrounded by 6 atoms gives minimum energy configuration hence stable configuration. So a monolayer is stable when we have 7 atoms.

In bulk-materials, minimum energy configuration is spherical. A sphere is a minimum energy configuration from hydro-static equilibrium condition. But this is not true for nano-materials.

In nano materials first stable configuration has 13 atoms as shown in Figure 5.9.

Through inspection it can be shown that stable configuration is achieved by having;

M*(K) atoms = (1/3)(10K ³ +15K ² +11K+3) for K ^th configuration where K = 1,2,3…..

For K= 1, first stable nano-crystal has 13 atoms.

For K= 2, second stable nano-crystal has 55atoms.

For K= 3, third stable nano-crystal has 147 atoms.

For K= 4, fourth stable nano-crystal has 309 atoms.

For K= 5, fifth stable nano-crystal has 561 atoms.

These configurations are shown in Figure 5.10.

Figure 5.9.A 3-D nanocrystal is stable ewhen there are 6 atoms surrounding the core atom and 3-atoms from the top and 3-atoms from the bottom. Meaning by when we have 13 balls then we have first stable 3-D nanocrystal structure though it is not a spherical structure.

Figure 5.10. Stable nano-configuration corresponding to K = 1,2,3,4 and 5 are shown.

In Figure 5.11. it is shown that the surface atoms dominate the crystal structure as we move from Bulk configuration to nano-particle configuration. Because of this dominance by surface atoms in nano-crystal configuration, material properties of nano-particle is completely different from those of the bulk crystal.

Surface to volume ratio:-A 3 nm iron particle has 50% atoms on the surface

-A 10 nm particle 20% on the surface

-A 30 nm particle only 5% on the surface

Figure 5.11. Nano-particle structure for K th Stable, Minimum Energy, Configuration for K = 1, 2, 3, 4, 5 and 7.

Figure 5.12. Calculated Surface Atoms to Bulk Atoms Ratio for Solid Metal Paricles vs the Size of Particle (nm). [Curtsey: Kenneth J.Klabunde, Jane Stark, Olga Koper, et.al. “Nanocrystal as Stoichiometric Regents with Unique Surface Chemistry”, Journal of Physical Chemistry, Vol.100, pp. 12142-12153,(1996)]

As the particle size changes from Bulk – Size to Nano - Size, the overall structure changes and Valence electrons become de-localized. What does this de-localization mean?

We have seen in Band-Theory of Solids that when atoms are far apart the orbital electrons donot interact and the Energy Difference between two consecutive states is at the maximum. But when they are brought close together they start interacting and band gap between consecutive band reduces. What this means that nano-size particle will have a larger band-gap and as nano-size increases to bulk-size, band-gap asymptotically approaches the bulk band-gap. This leads to different physical and chemical properties continuously graded as size increases from nano to bulk size.

The following properties are affected by nano to bulk-size transition:

Optical properties, Bandgap ,Melting point ,Specific heat, Surface reactivity, Magnetic property

and Electrical conduction.