Nanotechnology Lect.4

Optical properties
Some nanomaterials display very different optical properties, such as colour and transparency, compared to bulk materials. In this section, the reason for this behaviour is discussed and some examples provided. Before going into detail, some fundamentals are reviewed.
? Interaction of light with matter
The ?colour‘ of a material is a function of the interaction between the light and the object. If a material absorbs light of certain wavelengths, an observer will not see these colours in the reflected light. Only reflected wavelengths reach our eyes and this makes an object appear a certain colour. For example, leaves appear green because chlorophyll, which is a pigment, absorbs the blue and red colours of the spectrum and reflects the green.
In general light (I) incident on a material can be transmitted (T), absorbed (A) or reflected (R):
I = T+A+R
As the size of the materials is reduced, scattering (S) of light can also contribute to its colour (or transparency). A short summary of each process follows.
? Reflection (R) occurs when lightstrikes a smooth surface and the incident wave is directed back into the original medium. The reflected wave has same geometrical structure as the incident wave.
Babylon University/College of Engineering /Chemical Engineering Department/Nanotechnology
Page 26 of 130
Lecturer: Hassanin Mehsin Ali Fourth Stage 2017/2018
? Absorption (A) is a process that involves energy transformation. The energy levels of a substance determine the wavelengths of light that can be absorbed. It is a molecular phenomenon, dependent on the chemical identity and structure of the substance (not on the size of the molecules or clusters), and involves electronic transitions, vibrations and rotations.
? Transmission (T) is the ability of light to pass through a material: it is complementary to absorption. Transmission of light is what is left after reflection, scattering and absorption have occurred