Cadmium selenide is an inorganic compound with the formula CdSe. It is a
black to red-black solid that is classified as a II-VI semiconductor of
the n-type. Much of the current research on cadmium selenide is focused on its
nanoparticles. Structure
Three crystalline forms of CdSe are known: wurtzite, sphalerite and
rock-salt. The sphalerite CdSe structure is unstable and converts to the wurtzite
form upon moderate heating. The transition starts at about 130 °C, and
at 700 °C it completes within a day. The rock-salt structure is only observed
under high pressure. 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.
Cadmium selenide may also be produced in the form of nanoparticles. 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.
Production of cadmium selenide by arrested precipitation in solution is
performed by introducing alkylcadmium and trioctylphosphine selenide
precursors into a heated solvent under controlled conditions.
Me2Cd + TOPSe → CdSe + CdSe nanoparticles can be modified by
production of two phase materials with ZnS coatings. The surfaces can be
further modified, e.g. with mercaptoacetic acid, to confer
solubility. Synthesis in structured environments
refers to the production of cadmium selenide in liquid crystal or surfactant
solutions. The addition of 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. CdSe nanoparticles
CdSe-derived nanoparticles with sizes below 10 nm 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. One type of CdSe nanoparticle is a CdSe quantum dot. This discretization of
energy states results in electronic transitions that vary by quantum dot
size. Larger quantum dots have closer electronic states than smaller quantum
dots which means that the energy required to excite an electron from HOMO
to the LUMO is lower than the same electronic transition in a smaller
quantum dot. This quantum confinement effect can be observed as a red shift in
absorbance spectra for nanocrystals with larger diameters.
CdSe quantum dots have been implemented in a wide range of applications
including solar cells, light emitting diodes, and biofluorescent tagging.
CdSe-based materials also have potential uses in biomedical imaging. 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. Surface Chemistry of CdSe Quantum Dots
CdSe quantum dots are semiconductor nanocrystals composed of a CdSe core and
a ligand shell. Ligands play important roles in the stability and solubility of
the nanoparticles. During synthesis, ligands stabilize growth to prevent
aggregation and precipitation of the nanocrystals. These capping ligands also
affect the quantum dot’s electronic and optical properties by passivating
surface electronic states. An application that depends on the nature
of the surface ligands is the synthesis of CdSe thin films. The density of the
ligands on the surface and the length of the ligand chain affect the separation
between nanocrystal cores which in turn influence stacking and conductivity.
Understanding the surface structure of CdSe quantum dots in order to
investigate the structure’s unique properties and for further
functionalization for greater synthetic variety requires a rigorous description
of the ligand exchange chemistry on the quantum dot surface.
A prevailing belief is that trioctylphosphine oxide or
trioctylphosphine, a neutral ligand derived from a common precursor used in
the synthesis of CdSe dots, caps the surface of CdSe quantum dots. However,
results from recent studies challenge this model. Using NMR, quantum dots have
been shown to be nonstoichiometric meaning that the cadmium to selenide
ratio is not one to one. CdSe dots have excess cadmium cations on the surface
that can form bonds with anionic species such as carboxylate chains. The CdSe
quantum dot would be charge unbalanced if TOPO or TOP were indeed the only type
of ligand bound to the dot. The CdSe ligand shell may contain both X
type ligands which form covalent bonds with the metal and L type ligands that
form dative bonds. It has been shown that these ligands can undergo exchange
with other ligands. Examples of X type ligands that have been studied in the
context of CdSe nanocrystal surface chemistry are sulfides and thiocyanates.
Examples of L type ligands that have been studied are amines and phosphines.
A ligand exchange reaction in which tributylphosphine ligands were displaced
by primary alkylamine ligands on chloride terminated CdSe dots has been
reported. Stoichiometry changes were monitored using proton and phosphorus
NMR. Photoluminescence properties were also observed to change with ligand
moiety. The amine bound dots had significantly higher photoluminescent
quantum yields than the phosphine bound dots.
Applications CdSe material is transparent to
infra-red light and has seen limited use in photoresistors and in windows for
instruments utilizing IR light. The material is also highly luminescent.
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. Cadmium selenide is a known carcinogen
to humans and medical attention should be sought if swallowed or if contact
with skin or eyes occurs. References
= Related materials = Cadmium sulfide
Cadmium telluride Zinc selenide
Mercury selenide Additional links
National Pollutant Inventory - Cadmium and compounds
Nanotechnology Structures - Quantum Confinement
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