Nanoscale Properties

Properties of materials at the nanoscale are different in many cases from the properties of materials observed on other length scales.

Consider, for example, the melting point of metals. Nanoparticles often exhibit a lower melting point than the corresponding metals in bulk, and these melting points depend on size. For example, bulk gold melts at 1064 degrees Celsius, but a 4 nm gold particle melts at roughly 850 degrees Celsius.

In semiconductors such as Silicon, the band gap changes with the size. The band gap is the energy needed to move an electron from the valence band to the conduction band. This characteristic affects the electrical and optical properties of various semiconductors (such as Silicon, Germanium and Gallium Arsenide) and impacts where these materials are used. The band gaps of these three materials are 1.12, 0.67 and 1.42 electron volts (eV) respectively in bulk form. Studies show that the band gap increases when these materials are made in the form of nanowires or nanoparticles. (A nanowire is a wire-like structure with diameter of the order nanometers). For example, a silicon nanowire with diameter of 1.3 nm has a very wide band gap of 3.5 eV.

The color of a material can also be size dependent. The appearance of color is caused by the partial absorption of light primarily by electrons in that material; light that isn’t absorbed remains visible.

On many smooth metal surfaces, light is reflected by the very high density of conduction electrons; this is why the surfaces of slabs of metal can have a mirror-like appearance. In contrast, small particles absorb some of the light, leading to the appearance of color. This property depends on the size of nanoparticles.

Nanosystems are not large enough for many classical laws of physics to apply. For example, Ohm’s law, which describes the relation between current and voltage in a conductor, does not describe current conduction through a tiny nanowire. Here other effects, known as quantum mechanical effects become important.