Quantum Dots of Many Colors

"Quantum dots" (QDs) of CdSe and similar semiconductors are grown in carefully controlled solution precipitation with controlled sizes in the range L= 4 or 5 nm. It is found that the wavelength (color) of strong fluorescent light emitted by these quantum dots under ultraviolet (uv) light illumination depends sensitively on the size I.

There are enough atoms in this particle to effectively validate the concepts of solid state physics, which include electron bands, forbidden energy band gaps, electron and hole effective masses, and more.

Still, the particle is small enough to be called an "artificial atom", characterized by discrete sharp electron energy states, and discrete sharp absorption and emission wavelengths for photons.

Transmission electron microscope (TEM) images of such nanocrystals, which may contain only 50 000 atoms, reveal perfectly ordered crystals having the bulk

Figure 1.3 Transmission Electron Micrograph (TEM) Image of one 5 nm CdSe quantum dot particle, courtesy Andreas Kornowski, University of Hamburg, Germany crystal structure and nearly the bulk lattice constant. Quantitative analysis of the light emission process in QDs suggests that the bandgap, effective masses of electrons and holes, and other microscopic material properties are very close to their values in large crystals of the same material. The light emission in all cases comes from radiative recombination of an electron and a hole, created initially by the shorter wavelength illumination.

The energy ER released in the recombination is given entirely to a photon (the quantum unit of light), according to the relation ER=hv=hc/X. Here v and X are, respectively, the frequency and wavelength of the emitted light, c is the speed of light 3x 108m/s, and h is Planck's constant h = 6.63x 10~34Js = 4.136x 10"15eVs. The color of the emitted light is controlled by the choice of L, since ER = EG + Ee + Eh, where EG is the semiconductor bandgap, and the electron and hole confinement energies, Ee and Eh, respectively, become larger with decreasing I.

It is an elementary exercise in nanophysics, which will be demonstrated in Chapter 4, to show that these confinement (blue-shift) energies are proportional to 1/L2. Since these terms increase the energy of the emitted photon, they act to shorten the wavelength of the light relative to that emitted by the bulk semiconductor, an effect referred to as the "blue shift" of light from the quantum dot.

coupled to a protein by mercaptoacetic acid. The typical QD core size is 4.2 nm. [8]

Figure 1.4 Schematic of quantum dot with coatings suitable to assure water solubility, for application in biological tissue. This ZnS-capped CdSe quantum dot is covalently

These nanocrystals are used in biological research as markers for particular kinds of cells, as observed under an optical microscope with background ultraviolet light (uv) illumination.

In these applications, the basic semiconductor QD crystal is typically coated with a thin layer to make it impervious to (and soluble in) an aqueous biological environment. A further coating may then be applied which allows the QD to preferentially bond to a specific biological cell or cell component of interest. Such a coated quantum dot is shown in Figure 1.4 [8], These authors say that the quantum dots they use as luminescent labels are 20 times as bright, 100 times as stable against photo-bleaching, and have emission spectra three times sharper than conventional organic dyes such as rhodamine.

The biological researcher may, for example, see the outer cell surface illuminated in green while the surface of the inner cell nucleus may be illuminated in red, all under illumination of the whole field with light of a single shorter wavelength.

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