Interband Transitions in Metals

Observed values of (a) N and (b) K for vapor deposited Cu (solid lines) compared with theoretical free electron values (dashed lines). (N and K values taken from Lynch, D.W. and Hunter, W.R. in Palik, E.D., Ed., Handbook of Optical Constants of Solids, Vols. 1-3, Academic Press, 1997.) 

Gold s yellow color is due to a similar mechanism. Silver has a band structure similar to Cu except the d-bands are 4 eV below the Fermi level hence the loss of reflectance occurs in the UV region. 

Thin transparent conductive films such as indium tin oxide are crucial to the fabrication of electroluminescent panels and to liquid crystal displays. 

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The simple free electron model (the Drude model) developed in Section 4.4 for metals successfully explains some general properties, such as the filter action for UV radiation and their high reflectivity in the visible. However, in spite of the fact that metals are generally good mirrors, we perceive visually that gold has a yellowish color and copper has a reddish aspect, while silver does not present any particular color that is it has a similarly high reflectivity across the whole visible spectrum. In order to account for some of these spectral differences, we have to discuss the nature of interband transitions in metals. 

The absorption spectrum of continuous thin film Ag is compared to that of Ag nanocrystals of much the same sizes as thickness of the film [17]. At high energy of radiation both spectra are practically identical, because absorption of such radiation results from interband transitions in metal [17]. 

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