Cadmium selenide

Cadmium selenide (CdSe) is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium (II) selenide, cadmium selenide, and cadmoselite.

Cadmium selenide is a semiconducting material, but has yet to find many applications in manufacturing. This material is transparent to infra-red (IR) light, and has seen limited use in windows for instruments utilizing IR light.

Much current research on cadmium selenide has focused on nanoparticles. Researchers are concentrating on developing controlled syntheses of CdSe nanoparticles. In addition to synthesis, scientists are working to understand the properties of cadmium selenide, as well as apply these materials in useful ways.

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Production

The production of cadmium selenide has been carried out in two different ways. The preparation of bulk crystalline CdSe is done by the High-Pressure Vertical Bridgman method or High-Pressure Vertical Zone Melting.^[1]

However, cadmium selenide in the bulk form is not very interesting. The most interesting form of cadmium selenide is known as nanoparticles. (see applications for explanation) Several methods for the production of CdSe nanoparticles have been developed: arrested precipitation in solution, synthesis in structured media, high temperature pyrolysis, sonochemical, and radiolytic methods are just a few.^[2]

Production of cadmium selenide by arrested precipitation in solution is performed by introducing alkylcadmium and trioctylphosphine selenide (TOPSe) precursors into a heated solvent under controlled conditions.^[3]

Me₂Cd + TOPSe → CdSe + (byproducts).

Synthesis in structured environments refers to the production of cadmium selenide in liquid crystal or surfactant solutions. The addition to surfactants to solutions often results in a phase change in the solution leading to a liquid crystallinity. A liquid crystal is similar to a solid crystal in that the solution has long range translational order. Examples of this ordering are layered alternating sheets of solution and surfactant, micelles, or even a hexagonal arrangement of rods.

High temperature pyrolysis synthesis is usually carried out using an aerosol containing a mixture of volatile cadmium and selenium precursors. The precursor aerosol is then carried through a furnace with an inert gas, such as hydrogen, nitrogen, or argon. In the furnace the precursors react to form CdSe as well as several by-products.^[2]

Applications

Cadmium selenide in its wurtzite crystal structure is an important II-VI semiconductor. As a semiconductor CdSe has a band gap of 1.74 eV at 300 K. It is an n-type semiconductor, which is difficult to dope p-type, however p-type doping has been achieved using nitrogen.^[4] CdSe is also being developed for use in opto-electronic devices, laser diodes, nanosensing, and biomedical imaging.^[5] They are also used being tested for use in high-efficiency solar cells^[6][7][8]

Most of the usefulness of CdSe stems from nanoparticles. Nanoparticles are just what the name implies, particles of CdSe that are 1–100 nm (1 nm = 10⁻⁹ m) in size. CdSe particles of this size exhibit a property known as quantum confinement. Quantum confinement results when the electrons in a material are confined to a very small volume. Quantum confinement is size dependent, meaning the properties of CdSe nanoparticles are tunable based on their size.^[9]

Since CdSe nanoparticles have a size dependent fluorescence spectrum, they are finding applications in optical devices such as laser diodes. Using these particles, engineers are able to manufacture laser diodes that cover a large part of the electromagnetic spectrum.^[10]

Along similar lines, doctors are developing these materials for use in biomedical imaging applications. Human tissue is permeable to far infra-red light. By injecting appropriately prepared CdSe nanoparticles into injured tissue, it may be possible to image the tissue in those injured areas.^[11]

Safety information

Cadmium is a toxic heavy metal and appropriate precautions should be taken when handling it and its compounds. Selenides are toxic in large amounts. See MSDS^[12].

References

1. ^ http://www.sttic.com.ru/lpcbc/Basics.html
2. ^ ^{a b} Didenko, Y. Y.; Suslick K. S. Chemical Aerosol Flow Synthesis of Semiconductor Nanoparticles. J. Am. Chem. Soc.; (Communication); 2005; 127(35); 12196-12197
3. ^ Bawendi et al. Synthesis of CdE Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706-8715
4. ^ T Ohtsuka, J Kawamata, Z Zhu, T Yao, Applied Physics Letters, 65, 466-468, (1994)
5. ^ Christopher Ma et al. Single-Crystal CdSe Nanosaws. J. AM. CHEM. SOC. 2004, 126, 708-709
6. ^ Direct carrier multiplication due to inverse Auger scattering in CdSe quantum dots. Appl. Phys. Lett., Vol. 84, No. 13, 29 March 2004.
7. ^ Effect of electronic structure on carrier multiplication efficiency: Comparative study of PbSe and CdSe nanocrystals. Appl. Phys. Lett. 87, 253102 (2005).
8. ^ Direct Observation of Electron-to-Hole Energy Transfer in CdSe Quantum Dots. PRL 96, 057408 (2006).
9. ^ http://www.ringsurf.com/info/Technology_/Nanotechnology/Structures/
10. ^ Colvin, V. L.; Schlamp, M. C.; Alivisato, A. P. Nature 1994, 370, 354.
11. ^ (a) Chan, W. C.; Nie, S. M. Science 1998, 281, 2016. (b) Bruchez, M.;Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013.
12. ^ Additional safety information available at www.msdsonline.com, search 'cadmium selenide.'

Related materials

• Cadmium sulfide
• Cadmium telluride
• Zinc selenide
• Mercury selenide