Nanoparticles are defined as small objects that are sized between 1 and 100 nanometers and that behave as a whole unit with respect to their transport and properties. They have size dependent characteristics because the material properties change as their size approaches to the nanoscale, and as percentage of atoms at the material surface becomes significant.
Bulk materials are considered particles larger than one micrometer and they contain insignificant percentage of atoms at the surface in relation to the number of atoms in the bulk of the material, and therefore do not exhibit size-dependent change in their physical properties.
However interesting and unexpected properties of nanoparticles are coming from large surface area of the material, which dominates the contributions made by the small bulk of the material.
Color is an example of a size related property. Nanoparticles often possess unexpected electrical and optical properties as they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep-red to black in solutions. Nanoparticles of yellow gold and grey silicon are red in color. Gold nanoparticles melt at much lower temperatures around 300°C if they are 2.5 nm in size which is very low temperature compared to gold slab that melts at temperature around 1064°C. Size dependent color was utilized by artists of ninth century utilize this feature without knowing for generating glittering effects on the surface of pots or colors in stained glass.
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