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How Can Quantum Dots Be Used?

Photonics Spectra
Jul 2005
Quantum dots are so versatile that most of their applications have not yet been realized. A few in development are:

Solar cells. Solar cells made from silicon can be stable and efficient, but they are expensive to make. Solar cells that employ organic semiconductors are less expensive but suffer from poor efficiency. Quantum dots can be used to make solar cells by combining colloidal dots with organic semiconductors.

The dots improve the efficiency of these devices and enable the manufacturer to control their absorptive properties. The quantum dots can be formed into an ordered 3-D array with interdot spacing that is small enough for strong electronic coupling, and minibands form that allow long-range electron transport. This increases the likelihood that each photon impinging on the cell will cause electricity to flow, increasing the voltage. Quantum dots also can produce impact ionization, where one energetic carrier can split into two, increasing the current.

Anticounterfeiting. Two aspects of quantum dots give them the ability to act as an anticounterfeiting measure: narrow and specific emission peaks, and an emission intensity that is dependent on the excitation wavelength. With these traits, several sizes of dots can be combined with several wavelengths of excitation light to create an almost infinite variety of emission spectra. By changing the number of dots, their individual concentrations, their emission peaks or their excitation wavelength, a vast variety of spectral codes can be inserted into many different materials.

Counterfeiting the precise signatures would require the industrial capabilities of a semiconductor laboratory.

Paints and powders. Relative to organic dyes, quantum dots have a very long photostability and emit for long periods of time without decaying. When mixed into paint, infrared-emitting quantum dots could be applied to military uniforms to identify friendly forces using night-vision equipment. Because of the specificity of quantum dot emissions, enemy combatants would stand little chance of detecting the signature.

In powdered form, quantum dots mimic naturally occurring dust and can adhere unnoticed to passersby or surfaces for days, enabling the material to act as an IR tag for tracking intruders or hostile personnel. This could make quantum dots useful in security or espionage applications.

Telecommunications products. Quantum dots can overcome some limitations of conventional semiconductor materials in the telecom market. For example, absorption saturation is a nonlinear effect that will occur when a sharp change in transmittivity follows an increase in the intensity of light incident on a material. Quantum dot structures have too few electrons to absorb energy quickly, so they are highly transmissive at high intensities of light.

This effect is specific to quantum dot materials because traditional materials have far more electrons in their energy bands. This strong nonlinear effect means that quantum dots could be used to make extremely fast optical devices that switch between high and low transmittivities, resulting in devices capable of more efficient information transfer.

Electroluminescent devices. Quantum dots can improve the efficiency of organic LEDs. For example, researchers at MIT in Cambridge, Mass., demonstrated an organic LED that contains one layer of quantum dots between two organic thin films. This resulted in a twenty-fivefold improvement in the luminescent power efficiency over previous quantum dot LEDs. Today’s organic materials produce relatively broad emission peaks, yielding less than optimal color saturation.

Quantum dots, on the other hand, offer much sharper peaks, extremely high quantum efficiencies and the ability to tune the color emission by changing the size of the dots. Besides being used for thin and bright flat panel displays, such organic LEDs may find use in scientific wavelength calibration and robotic vision.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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