Cadmium Selenide

When atoms are arranged in nanometer-size clusters of diameters from less than 100 nanometers to as large as 700 nanometers, interesting optical properties can be demonstrated. For example, nano phase versions of pure cadmium selenide can be made almost any color in the spectrum simply by changing its cluster size. Indeed, some types of lipstick are made in many different colors even though the predominant light-scattering molecule is the same in all color versions. What is the physics here?

Answer

The wavelength of visible light is comparable to nanophase cluster sizes. For example, greenish light has a wavelength of about 580 nanometers, five to ten times the nanophase cluster sizes. Clusters behaving as particles ranging from about 1 nanometer to 50 nanometers in diameter are too small to have any significant scattering of visible light, so these materials are effectively transparent. Clusters of sizes comparable to particular wavelength ranges of visible light are subject to quantum confinement restrictions.

Quantum mechanics predicts the correct behavior at the small cluster sizes. The smaller the nanophase cluster size becomes, the greater are the energy spacings for the electron states. Which colors of light are absorbed and emitted are determined by these energy spacings. If the energy spacings are too great, the incoming light will not be absorbed, and light of that wavelength and longer will not be scattered. For example, a typical semiconductor is cadmium selenide. When the size of the cluster is 1.5 nanometers, the cadmium selenide appears yellow, but when the size is 4 nanometers, it will appear red. And larger clusters appear black. Therefore, the observed color of the clusters in the nanophase depends on their actual sizes.