Semiconductor nanocrystal

Lead(II) sulfide

Lead(II) sulfide (also spelled sulphide, see Sulfur#Spelling) is a chemical compound , most often purified from the mineral galena. PbS, PbSe and PbTe lead salts are semiconductors of the IV-VI family. Lead(II) sulfide is toxic due to the lead content, see lead poisoning.

Applications

Lead sulfide(II) and several other lead compounds are used as detection element material in various infra-red sensors. Of these, PbS (lead sulfide) is one of the oldest and most commonly used. It is used for eg. infrared detectors.

When used for infrared, these detection element are classified as photon detectors: unlike thermal detectors (the other major type), IR detectors respond directly to the incident radiation. Thermal detectors respond only to the material's temperature change caused by the energy from the photons. Because of this difference, radiation can then be measured in two ways, either by detecting the tiny photocurrent or by measuring the change in the materials electrical resistance. Measuring the resistance change is the more commonly used method.

At room temperature, PbS elements are sensitive to radiation at wavelengths between approximately 1 and 2.5 μm. This range corresponds to the shorter wavelengths in the IR band, and is called near-IR, a term which refers to the fact that it is close to the wavelengths of visible light. Only very hot objects emit radiation in these wavelengths.

Cooling the PbS elements, for example using pressurised or liquified gas or a Peltier element system shifts its sensitivity range to between approximately 2 and 4 μm. Objects which emit radiation in these wavelengths still have to be quite hot; several hundred degrees Celsius; but not as hot as those which are detectable by uncooled sensors.

Other compounds used for this purpose include indium antimonide (InSb) and HgCdTe, which have somewhat better properties for detecting the longer IR wavelengths.

The high dielectric constant of PbS leads to relatively slow detectors (compared to silicon, germanium, InSb or HgCdTe).

PbS can be formed as nanocrystals.

Astronomy

Elevations above 2.6 km (1.63 mi) on the planet Venus are coated with a shiny substance. Though the composition of this coat is not entirely certain, one theory is that Venus "snows" crystallized lead sulfide much as Earth snows frozen water. If this is the case, it would be the first time the substance was identified on a foreign planet. Other less likely candidates for Venus' "snow" are bismuth sulfide and tellurium.

See also

References

  • Lead Sulphide - An Intrinsic Semiconductor, E H Putley and J B Arthur, Proc. Phys. Soc. B, 64 pp. 616–618 (1951)
  • Absorption Spectra of Lead Sulphide at Different Temperatures, W. Paui and R. V. Jones, Proc. Phys. Soc. B, 6 (3) pp. 194–200 (1953).
  • Electronic and Vibrational Properties of Lead Sulphide Nanocrystals, Janet Lynn Machol, Ph.D. Thesis, Cornell Univ. (1993).
  • Nonlinear optical properties of lead sulfide nanocrystals in polymeric coatings, S W Lu et al, Nanotechnology 13 669-673 (2002)

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