Effects of the Nanometre Length Scale
- When the size of a material is reduced to the nanometre scale (1–100 nm), its physical and electronic properties change significantly. These changes mainly occur due to quantum confinement effects, which influence the energy band structure and atomic arrangement of the material.
- At very small dimensions, electrons are confined within a limited space. This confinement alters the density of electronic states and changes the behaviour of electrons compared with those in bulk materials.
- According to the free electron model, the energy of electronic states is inversely proportional to the square of the system dimension (1/L²), where L represents the size of the system. As the system size decreases, the spacing between energy levels increases.
- In bulk materials, atoms form closely spaced molecular orbitals and energy bands, but when the number of atoms decreases in nanoscale systems, these energy levels become widely separated.
- As the size of the material decreases, the energy bands become narrower, and the electronic behaviour begins to resemble the particle-in-a-box model rather than the behaviour of electrons in a continuous solid.
- This phenomenon is known as quantum confinement, where electrons behave more like those in localized molecular bonds rather than in delocalized metallic states.
- The change in electronic energy levels modifies the total energy of the system, which affects the thermodynamic stability of nanoscale materials compared with bulk crystals.
- Due to this change in total energy, nanomaterials may adopt different crystal structures from their bulk counterparts. For example, some metals that normally form hexagonal close-packed (HCP) structures may adopt face-centered cubic (FCC) structures in nanoscale systems.
- Reduction in size can also influence chemical reactivity, because the arrangement and occupation of outer electronic energy levels change.
- Many physical properties such as electrical conductivity, thermal conductivity, optical properties, and magnetic behaviour also change due to modifications in the electronic structure.
- In some cases, nanoscale metallic systems may undergo a metal–insulator transition, where a band gap forms and the material loses its metallic conductivity.
- Transport properties may also become quantized rather than continuous, because electron energy levels become discrete at very small dimensions.
- Mechanical properties such as strength and elasticity may change because electronic structure affects the interaction between atoms and the interatomic spacing.
- Another important effect of nanoscale size reduction is the increase in the surface area to volume ratio (S/V).
- As particle size decreases, the specific surface area increases dramatically, especially for particles smaller than 100 nm.
- For example, spherical nanoparticles with diameters of about 2 nm can have a specific surface area close to 500 m²/g, which is extremely large compared with bulk materials.
- This large surface area increases the surface energy of the system, which strongly affects the stability and structure of nanomaterials.
- As a result, nanomaterials may stabilize metastable structures or show lattice relaxation, such as expansion or contraction of the crystal lattice.
- At the surface of a nanoparticle, atoms have fewer neighbouring atoms, which leads to differences in bonding and electronic structure. This effect is responsible for surface tension and surface energy.
- In very small nanoparticles, a large fraction of atoms exist on or near the surface. For example, in a 5 nm particle about 30–50% of atoms are located at the surface, whereas in a 100 nm particle only a few percent of atoms are affected by surface effects.
- Similar effects occur in nanocrystalline materials, where many atoms are located at grain boundaries instead of within the crystal interior.
- Because of these structural differences, nanomaterials often exhibit properties very different from those of bulk materials, including changes in chemical reactivity, mechanical strength, electrical conductivity, and optical behaviour.
