ENGRI111_HW3 - Yuri Andrade Sylvester [email protected]

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Yuri Andrade Sylvester [email protected] September 26, 2004 ENGRI 111 Homework #3 Due Wednesday, September 29, 2004 1. The band gap of several materials is given below. What is the color of each material (explain your reasoning briefly ) ? Material Eg/eV Color C(diamond) 5.4 Colorless (transparent) ZnS 3.6 Colorless (transparent - whitish) ZnO 3.2 Colorless (transparent) CdS 2.4 Yellow / orange HgS 2.1 Red GaAs 1.4 Dark gray In order to understand this question we need to remember that the band gap energies for the visible spectrum run from 1.77eV and up to 3.2eV and their colors are red and blue, respectively. Carbon in the diamond lattice configuration has a transparent color because all light energies (of the visible spectrum) can be absorbed due to its large band gap. In this case, light from all frequencies is reflected or transmitted. This is the typical behavior of a good insulator (diamond being a good example). Pure zinc sulfide is transparent (whitish) because its band gap energy is 3.2eV, which is not enough to absorb and reemit frequencies of the visible spectrum, but it is close enough to the threshold, that is why if impurities are present one can notice a with hue being transmitted. Zinc oxide is colorless (transparent) because all of the energies of the visible light spectrum are transmitted. It is important to notice that these substances are transparent only in their pure form
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