Properties and Applications
Nanocrystals in which so-called quantum effects occur due to their extremely small diameter (in the range of a few nanometers) are called quantum dots. These do not consist of a uniform material, but describe an entire class of materials. Quantum effects cause extremely interesting optical, magnetic and electronic properties in nanocrystals. For example, they can shine (fluoresce) with the aid of light, supply electricity very efficiently or serve as a super-small memory or processor elements in IT.
Schemtic overview of the set-up of Epitactic Quantum Dots © C. Steinbach
With a size of about 1-100 nm, quantum dots consist mostly of semiconductor materials. They are made of either one or different materials, which follow a construction principle of core and shell. Often different materials are used for the core and shell, whereby several coating layers are also possible. Both the electronic and optical properties of the quantum dots can be precisely adjusted with these so-called core-shell structures, which make them very interesting for a number of applications.
For free metallic quantum dots there is a theoretical risk that they can self-ignite because of their large surface area. However, as they are usually only processed embedded in liquids or plastics and used in very small quantities, spontaneous combustion is very unlikely.
There are three main types of quantum dots:
- III-V-semiconductors: made of elements of main group III of the periodic table of the elements (boron, aluminium, gallium, indium) and main group V (nitrogen, phosphorus, arsenic, antimony, bismuth)
- II-VI- semiconductors: made of elements of transition metal group II (zinc, cadmium) and main group VI (oxygen, sulphur, selenium, tellurium)
- Silicon (Si), the standard material of the semiconductor and chip industry
The best-known representative of III-V semiconductors is gallium arsenide (GaAs). In the field of optical data processing, it serves primarily as a light source and is also used as an amplification medium in lasers. However, gallium arsenide appears to be restricted to special applications and does not compete with silicon in the semiconductor industry.
The most prominent representatives of the II-VI semiconductor quantum dots are cadmium selenide (CdSe) and cadmium telluride (CdTe). Zinc oxide (ZnO), which is already widely used in the form of micro and nanoparticles, is also increasingly being used as a material in quantum dots. Thanks to their outstanding fluorescence properties, II-VI semiconductor materials are used in electronics, photonics, photovoltaics and biomedicine. Cadmium selenide-based quantum dots are preferably found in lighting applications and displays based on quantum dot LEDs.
Schematic structure of a free core-shell quantum dot © C. Steinbach
In thin-film solar cells, the use of cadmium telluride-based quantum dots is currently being tested. They promise a significant increase in efficiency. However, since these quantum dot materials contain toxic cadmium, further research is being conducted into alternatives. II-VI quantum dots are used as biomarkers for the detection of biomolecules in medical samples.
Silicon quantum dots are currently not as advanced as III-V and II-VI semiconductors. However, they promise great potential for integration into current silicon electronics, e.g. as a component of optical chips, processors, optical sensors or in photovoltaics for achieving major advances in efficiency. Due to the current high price, such silicon materials are mainly used in the space industry.
Quantum dots are still a major topic for research. They are currently only used sporadically in consumer-oriented products. Many concepts and effects need to be examined in more detail.
QD ©Leo / Fotolia.com
Quantum dots were first discovered in the 1980s. Today's production methods, including chemical processes in solutions, photolithography or molecular beam epitaxy, vary depending on the starting material.
The starting material for silicon quantum dots is silicon dioxide. Further silicon ions are introduced into a corresponding matrix of silicon dioxide and then heated at high temperatures for a longer period of time until the desired nanocrystals form.
In electron beam lithography, quantum dots are "written" onto an appropriate substrate via an electron beam and then exposed by a suitable etching process. In a simplified description of the complex process a special lacquer is applied to a surface, which contains the components for the generation of the desired quantum dots. The spot-like electron beam converts the components into quantum dots at the very small points, where it hits the lacquer surface. The excess paint residues are then removed. Disadvantages of this method are poor reproducibility and high effort.
Molecular beam epitaxy is used to produce quantum dots from III-V semiconductors such as gallium arsenide. Here, the two metals gallium and arsenic are vaporized simultaneously and then shot at a surface. Alternatively, they can also be produced from organometallic compounds by so-called gas phase deposition. In this procedure the quantum dots are created directly on the substrate required for the respective application.
Finally, colloidal quantum dots can also be produced from III-V semiconductors and II-VI semiconductors using chemical processes in solutions, usually for use in biological media. The quantum dots developed in the mid-1990s, for example, consisted of a core of cadmium selenide with a shell of zinc sulfide. First, cadmium selenide nanocrystals are precipitated from a cadmium salt solution with selenide anions. Using the same principle, a zinc sulfide coating is then grown on these nanocrystals.