Cadmium sulfide has, like zinc sulfide, two crystal forms. The more stable hexagonal wurtzite structure (found in the mineral Greenockite) and the cubic zinc blende structure (found in the mineral Hawleyite). In both of these forms the cadmium and sulfur atoms are four coordinate. There is also a high pressure form with the NaCl rock salt structure.
Cadmium sulfide is a direct band gap semiconductor (gap 2.42 eV). The proximity of its band gap to visible light wavelengths gives it a coloured appearance.<br /> As well as this obvious property other properties result:
* the conductivity increases when irradiated, (leading to uses as a photoresistor)
* when combined with a p-type semiconductor it forms the core component of a photovoltaic (solar) cell and a CdS/CuS solar cell was one of the first efficient cells to be reported (1954)
* when doped with for example Cu ("activator") and Al ("coactivator") CdS luminesces under electron beam excitation (cathodoluminescence) and is used as phosphor
* both polymorphs are piezoelectric and the hexagonal is also pyroelectric
* electroluminescence
* CdS crystals can act as a gain medium in solid state laser
* In thin-film form, CdS can be combined with other layers for use in certain types of solar cells. CdS was also one of the first semiconductor materials to be used for thin-film transistors (TFTs). However interest in compound semiconductors for TFTs largely waned after the emergence of amorphous silicon technology in the late 1970s.
* Thin films of CdS can be piezoelectric and have been used as transducers which can operate at frequencies in the GHz region.
* Nanoribbons of CdS show a net cooling due annihilation of phonons, during anti-Stokes luminescence at ~510 nm. As a result, a maximum temperature drop of 40 and 15 K has been demonstrated when the nanoribbons are pumped with a 514 or 532 nm laser.