In order to give you an idea of how the bulk materials physical properties change as you
go down the periodic column 16, I have presented some data from wikipedia.com and
webelements.com. Cadmium semi-conductor bulk properties:
Bulk Properties:
Density 8.150 g/cm3
Melting point 1427°C
Boiling point 1559°C
Lattice constant 4.6958Å
Crystal structure Cubic
Band gap 2.5 eV at 300 K
Bulk Properties:
Density 4.82 g/cm3
Melting point 1750°C at 100 bar
Boiling point 980°C
Crystal structure Hexagonal
Band gap of 2.42 eV at 300 K
Bulk Properties:
Bulk Properties:
Density 5.816 g/cm3
Melting point 1268 °C (1541 K)
Crystal structure Hexagonal (wurtzite)
Band gap of 1.74 eV at 300 K
Density: 5.850 g/cm3.
Melting Point: 1314 K
Lattice constant: 0.648 nm at 300K
Crystal structure Cubic
The direct band gap is 1.56 eV at 300 K.
The fluorescent nature, as well as other size-tunable properties, of CdE (cadmium E=
oxide (O), sulfide(s), selenide (Se), and telluride (Te)) nanoparticles has been
significantly researched for over 15 years. The intense research of nucleation,
morphology, and composition has given rise to several robust and simplistic synthesis
routes to these various materials. The nucleation time given for a reaction will clearly
control the size of the nanoparticles, and also the morphology of the particles. Typically,
the morphology of the CdE, or chalcogens (O→Te), form quantum dots, rods, or wires;
however, CdSe and CdTe can take a tetrapod morphology as well, as seen in Figure 1 and
2.
Figure 1. Growth time influences morphology of CdSe, TEM images left to right with
increasing reaction time, 15s, 660s, 7200s. J. Mater. Chem., 13, 1705 (2003)
Figure 2. Growth time influences morphology of CdSe, TEM images left to right with
increasing reaction time, 15s, 1660s, 3600s. J. Mater. Chem., 13, 1705 (2003)
As said before, the interest in CdE nanoparticles is in the ease of tuning the quantum
confinement effects. Quantum confinement occurs when one or more of the dimensions
of a nanocrystal are made very small such that it approaches the size of an exciton in the
bulk crystal, called the Bohr exciton radius. (You may remember the simplistic equation
for finding the bohr radius (ao) from the mid-term test: ao = 4пεoћ/(me2), ao~5nm ). CdE
has of course been experimented in every configuration of quantum confinement effect:
2-D confinement is only restricted in one dimension, and the result is a quantum well (or
plane). These are what most lasers are currently built from. 1-D confinement occurs in
nanowires. 0-D confinement is found only in the quantum dot.
The tunable size of these quantum dots has been significantly researched and allows for
the direct control over the nanoparticles luminescent emission wavelength (absorbance),
see Figure 3. Studies as early as 1993, by Murray and Bawendi, showed clear
experimental evidence of the relation between size and their corresponding fluorescent
emission, see Figure 4. Since then, numerous articles on the theory and physical
relationships of the fluorescent emissions have been published. Below the figures is
several very important papers that reflect the several uses of advanced quantum physics
theory to model CdE properties.
Figure 3. Fluorescence induced by exposure to ultraviolet light in vials containing CdSe
quantum dots that are increasing in size (left to right). http://en.wikipedia.org/wiki/Fluorescence
Figure 4. Optical absorption spectra of CdSe nanocrystals dispersed in hexane and
corresponding TEM avg. size distribution (Å). J. Am. Chem. Soc., 115, 8706 (1993)
A classic paper that corresponds to our basic discussions in class and uses extensive
equations starting with the iterative Hartree scheme . This paper accounts for many of the
advanced topics in quantizing CdE interactions of electron-hole pairing, bonding orbitals,
and crystal system.
J. Chem. Phys., vol. 66, no. 2, 946 (1992)
A paper reporting on luminescence in CdSe that is probed via several studies involving
time-, wavelength-, temperature-, polarization-resolved luminescence. Overall a good
paper outlining experiments and the resulting data.
Another paper heavy in theoretical calculations of CdSe QD and a comparison to
obtained experimental results. A good reading into the lowest excitation states of QDs
and possible reasons as to the shifting of these energy states.
In order to give you an idea of how the bulk materials physical properties change as you
go down the periodic column 16, I have presented some data from wikipedia.com and
webelements.com. Cadmium semi-conductor bulk properties:
Bulk Properties:
Density 8.150 g/cm3
Melting point 1427°C
Boiling point 1559°C
Lattice constant 4.6958Å
Crystal structure Cubic
Band gap 2.5 eV at 300 K
Bulk Properties:
Density 4.82 g/cm3
Melting point 1750°C at 100 bar
Boiling point 980°C
Crystal structure Hexagonal
Band gap of 2.42 eV at 300 K
Bulk Properties:
Bulk Properties:
Density 5.816 g/cm3
Melting point 1268 °C (1541 K)
Crystal structure Hexagonal (wurtzite)
Band gap of 1.74 eV at 300 K
Density: 5.850 g/cm3.
Melting Point: 1314 K
Lattice constant: 0.648 nm at 300K
Crystal structure Cubic
The direct band gap is 1.56 eV at 300 K.