Quantum dots are tiny nanocrystals that glow when stimulated by an external source such as ultraviolet (UV) light. How many atoms are included in the quantum dot determines their size and the size of the quantum dot determines the colour of light emitted.
Quantum Dot Materials
Quantum dots can be made from a range of materials, currently the most commonly used materials include zinc sulphide, lead sulphide, cadmium selenide and indium phosphide. Many of the promising applications for quantum dots will see them used within the human body. In order to avoid toxic materials leaching from the quantum dots, they are also coating in a protective polymer.
How Quantum Dots Work
When energy is applied to an atom, electrons are energised and move to a higher level. When the electron returns to it’s lower and stable state, this additional energy is emitted as light corresponding to a particular frequency. Quantum dots work in much the same way but a quantum dot crystal acts as one very large atom. The energy source used to stimulate a quantum dot is commonly ultraviolet light. The frequency or colour of light given off is not related to the material used in the quantum dot, but by the size of the quantum dot.
Quantum Dot Size and Colour Relationship
Large quantum dots produce light with a long wavelength and small quantum dots produce light with small wavelengths. In terms of colour in the visible spectrum, this means large quantum dots produce red light and small quantum dots produce blue light – sizes in between account for all the other colours in the spectrum. By combining a range of sizes of quantum dots in the same sample, the entire light spectrum can be produced simultaneously and appears as white light.
Quantum dots can be manufactured by a number of processes from colloidal synthesis to chemical vapour deposition (CVD). The cheapest and simplest method is benchtop colloidal synthesis. Electrochemical techniques and CVD can be used to create ordered arrays of quantum dots on a substrate material.
Quantum dots show promise for use in a wide range of applications from the quantum computers of the future, to medical applications, high resolution television screens and household lighting.
Medical Applications and Cancer Treatments
Quantum dots can be encased within a shell tuned to mimic organic receptors within the body. These receptors can correspond to particular diseases, viruses or other items. The quantum dots will then seek out and attach to the disease en masse. Due to the fluorescent nature of quantum dots the site of the problem is then made easily visible. The number of receptors required on the surface of the dot is small compared to the surface area of the dot itself. This leaves a large amount of room to place other things on the dot. This can include various drugs for treating a disease the quantum dot has been tuned to find.
In this manner, quantum dots can be tuned to seek out cancer cells and deliver chemotherapy drugs directly to the cancer cells. This avoids poisoning healthy cells and therefore the awful side effects associated with cancer treatments.
The energy emitted from quantum dots as light, is close to 100% of the energy put into the system. This exceptionally high efficiency make quantum dots appealing for use in lights and as individual colour pixels in vibrant colour flat panel displays. For use in lighting, a layer of quantum dots can be sandwiched in between two electrically conductive layers. A current applied directly to the quantum dots between these layers will cause them fluoresce and will be an extremely high efficiency light source.