Semiconductor quantum dots (QDs) are nanoparticles or nanorods made of a semiconductor material. Because of their unique properties, they can be used in many fields, such as medicine and electronics. Here, we give a description of how QDs work and of some of their most novel applications.
A material behaves as a semiconductor when its electrical conductivity is in between that of an insulator and that of a conductor.
The mechanism of the conductivity in a semiconductor is shown in the Figure below. Electrons, normally in the valence band, have to be promoted into the conduction band; for this to happen, an appropriate amount of energy has to be absorbed by the material. This value, called the band gap, is different depending on the material.
The peculiarity of QDs is that they combine their semiconductor properties with those of a nanomaterial. A nanomaterial is a material having at least one dimension in the order of nanometers (10-9 m), this usually meaning smaller than about 100 nm. Examples are nanoparticles (particles with a nanoscale diameter), nanorods (rods with all dimensions in the nanoscale) or nanofibres (fibers with a nanoscale diameter), and nanofilms (thin films with a nanoscale thickness). Due to their small dimensions, the properties of nanomaterials are normally different from those of the corresponding bulk material.
In the case of QD nanoparticles, for instance, the value of the band gap can be different depending on the dimensions of the particles. If the band gap falls in the visible region, then the QD solutions made with particles of different dimensions may show different colors.
This can be seen, for instance, in the Figure to the left, where some CdSe QD solutions are shown. Colors go from purple to yellow-brown; this corresponds to increasing nanoparticle diameters.
QDs which do not absorb/emit light in the visible region do not show any color in solution; their energy emissions fall in the ultraviolet or infrared regions of the spectrum.
Tailoring QD properties
When preparing QDs, the control of their dimensions is achieved with the use of capping agents; these are molecules, generally organic ligands, which stop the growth of the nanoparticles, stabilize them and prevent aggregation/agglomeration of the particles themselves.
Further to controlling the dimensions, the capping agents can also be used to tailor the properties of QDs. In fact, by employing a ligand with particular reactivity, QDs can chemically interact with their surroundings. If, for instance, the capping agent is a long organic chain, the QDs will be hydrophobic, while the use of a polar ligand will make them hydrophilic. This process widens remarkably the technical applications of the QDs, as it is possible to obtain QDs made of the same material but with a different reactivity.
Use of QDs in biology
QDs are currently employed in biology, to detect some particular types of cells.
QDs prepared with bioconjugate molecules as capping agents, for instance, can bond selectively to some cells (i.e. cancer cells) or to some dangerous bacteria (i.e. E. coli). When bonded to these molecules or bacteria, QDs will show a colored/fluorescent signal; therefore, if such signal is detected in a sample treated with QDs, this indicates the presence of the cells or of the bacteria. The absence of the signal, on the contrary, will mean that the cells/bacteria are not present in the analyzed sample.
This application of the QDs can help to diagnose a disease, or to establish a possible contamination with dangerous bacterial strains. QDs for this and similar applications are already commercially available.
Novel applications: solar cells and optoelectronics
As reported in Materials Today, more novel applications of the QDs are now being considered in fields such as solar cells and optoelectronics.
The use of QDs within a solar cell matrix can bring a substantial increase in the efficiency of the cell in converting light into energy. An example can be the use of CdS or CdTeS QDs with TiO2 nanowires: Medina-Gonzales et al. recently reported an efficiency increase of 300% and 350% for CdS and CdTeS respectively.
In optoelectronics, QDs are now being considered for a new generation of Light Emitting Diodes (LEDs); these devices, if compared with the standard ones used now, will be more energy-efficient and produce brighter colors. Commercial products are not yet available; many companies, however, are investing in their development. For instance, QD Vision, a company focused on display applications for QDs, and LG Displays, started to work on a joint project to produce novel devices based on QD LEDs.
Considering their potential and the recent developments, the QD market has been increasing rapidly in the last few years.
In the past, for a long period of time, the commercial use of QDs was limited; the greatest obstacle to their wide use was their high cost – between $3000 and $10 000 per gram.
More recent data, however, show how things are changing. The latest data published by the BCC Research Company predict an increase in the global market from $67 million in 2010 to $670 million in 2015. Due to the novel discoveries and investments mentioned above, sectors such as solar energy and optoelectronics are the ones which will have the greatest increase, becoming about 5 and 30 times bigger respectively.
J.A. Smyder, T.D. Krauss: Coming attraction for semiconductor quantum dots. Materials Today, 14(9), 382-387, 2011.
Y. Medina-Gonzalez, W.Z. Xu, B. Chen, N. Farhanghi, P.A. Charpentier: CdS and CdTeS quantum dot decorated TiO2 nanowires. Synthesis and photoefficiency. Nanotechnology, 22, 065603, 2011.
Physics Today. Quantum dots market grows big. Accessed September 2011.
BCC Research. Global Market for quantum dots to grow to $670 million by 2015. Accessed September 2011.