Correct option is D
Nanoscience is the study of the properties of matter that have length scales between 1 and 100 nm. Nanotechnology is the collection of procedures for manipulating matter on this scale in order to build nanosized entities for useful purposes. Some definitions, however, are more restrictive. Thus, a ‘nanomaterial’ is taken to be a solid material that exists over the scale of 1 to 100 nm and exhibits novel properties that are related to its scale.
Semiconducting nanoparticles have been investigated intensively for their optical properties. These particles are often called quantum dots (QD) because quantum effects become important in these three-dimensionally confined particles (dots). Two important effects occur in semiconductors when electrons are confined to tiny regions. First, the HOMO–LUMO energy gap increases from the value observed in bulk crystals. Second, the energy levels of electrons in the LUMOs (and holes—the absence of electrons—in the HOMOs) are quantized, like those of a particle in a box. Both effects play an important role in determining the optical properties of QDs.
Quantum confinement, the trapping of electrons and holes in tiny regions, provides a method of tailoring or engineering the band gap of materials. The crucial feature is that as the critical dimension of a material decreases, the band gap increases. Transitions of electrons between states in the valence band (the so-called HOMO states) and the conduction band (the LUMO states) are called interband transitions, and the minimum energy for these transitions is increased in QDs relative to those in bulk semiconductors.
The optical properties of metallic nanoparticles arise from a complex electrodynamic effect that is strongly influenced by the surrounding dielectric medium. Light impinging on metallic particles causes optical excitations of their electrons. The principal type of optical excitation that occurs is the collective oscillation of electrons in the valence band of the metal. Such coherent oscillations occur at the interface of a metal with a dielectric medium and are called surface plasmons.
Top-down and bottom-up fabrication
There are two basic techniques for the fabrication of nanoscale entities. The first is to take a macroscale (or microscale) object and carve out nanoscale patterns. Methods of this sort are called top-down approaches. In top-down approaches, patterns are first designed on a large scale, and their lateral dimensions are reduced and then used to transfer the nanoscaled features into or on to the bulk material. Physical interactions are used in top-down fabrication approaches, such as photolithography, e-beam lithography, and soft lithography. The most common and well-known approach is photolithography, the technique used to fabricate very large-scale integrated circuits having feature dimensions on the 100 nm scale. The second technique is to build larger objects by controlling the arrangement of their component smaller-scale objects. Methods of this sort are called bottom-up approaches and start with control over the arrangements of atoms and molecules.
