This image illustrates how surface area increases as the size is reduced. Image Credit: NASA Ames Center for Nanotechnology
Properties of materials at the nanoscale are different in many cases from the properties of materials observed in other 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 4nm 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 conductance band. This property distinguishes various semiconductors such as Silicon, Germanium and Gallium Arsenide on their electron transport characteristics and field of applicability. 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 very small nanowires or nanoparticles. (A nanowire is a wire-like structure with diameter of the order of a nanometer). For example, a silicon nanowire with diameter of 1.3 nm exhibits 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 by electrons in that material; the unabsorbed part of the light remains visible.
On most smooth metal surfaces, light is entirely reflected by the very high density of electrons; this is why the surfaces of slabs of metal have mirror-like appearance. In contrast, small particles absorb some of the light, leading to the appearance of color. This property depends on size.
Chinese Ruby color pottery using gold particles.
For example, Gold exhibits a different color depending on its particle size. Extremely tiny particles of Gold have been used to color glasses since the very early days of glass making. Ruby vases (with color that is pink to blood-red) were made using finely dispersed Gold particles for centuries.
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 are more important.