Quantum dots, also called semiconductor nanocrystals, are tiny particles of semiconductor material that have very special properties due to their small size. They represent a distinct class of materials that is intermediate between molecular-sized inorganic complexes or clusters and the bulk semiconductors we find in computer chips. A transmission electron micrograph of cadmium selenide particles prepared in our group is shown in Figure 1.

Figure 1. Transmission electron micrograph of cadmium selenide quantum dots prepared in our laboratory. The average particle diameter is approximately 5 nm.

The most notable characteristics of quantum dots are their size dependent optical spectra. When electrons are confined to very small spaces (like the interior of a semiconductor quantum dot), their allowed energy levels spread out in energy as shown in Figure 2.

Figure 2. Nano-sized semiconductor particles have larger energy spacings between allowed levels than the bulk (macroscopic) material.

The result is that the amount of energy required to promote such an electron to the next higher level increases. This means that a shorter wavelength of light (blue rather than red, for example) will be needed to excite that electron. Thus, smaller dots have a tendency to absorb and emit shorter wavelength light than large dots, leading to distinct changes in the observed optical spectra (see Figure 3). These spectral shifts lead to striking color changes in the quntum dots as we change their size (see Figure 4). This shifting of energy levels with size is well understood on the basis of elementary quantum mechanics and the classic conceptual model called the "particle-in-a-box." The squeezing of these electrons that results in the energy shifts is commonly called "quantum confinement."

	Figure 3. Optical spectra (absorption, solid lines and photoluminescence, dashed lines) of cadmium selenide quantum dots of different sizes.
	Figure 4. Cadmium selenide quantum dots suspended in solvents. The top photo shows their appearance in ordinary room light, and the bottom photo shows fluorescence from these dots under black light illumination. The materials in the vials have the same chemical composition. The only difference is the sizes of the particles.

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