Nanoparticles, nanorods or nanofibres and nanofilms have properties that are normally different from those of the corresponding bulk material.
Semiconductors
A semi conductor has electrical conductivity in between an insulator and a conductor. Electrons have to be shifted into the conduction band from the valence band by supplying appropriate amount of energy called the band gap to be absorbed by the material.
Semiconductor quantum dots
Semiconductor quantum dots (QDs) are nanoparticles or nanorods made of a semiconductor material which has unique properties. But the QDs combine their semiconductor properties with valance and conduction bands in a semiconductor. If the band gap falls in the visible region, then the QD solutions made with particles of different dimensions may show different colors. 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. Because of their unique properties, they can be used in many fields, such as medicine and electronics.
Tailoring QD properties
The control of their dimensions of QDs is obtained using capping agents such as molecules, generally organic ligands, which stop the growth of the nanoparticles, stabilize them and prevent aggregation/agglomeration of the particles themselves.
Further the capping agents can also be used to tune the properties of QDs. By employing a ligand with particular reactivity, QDs can chemically interact with their surroundings. For example, if the capping agent is a long organic chain, the QDs will be hydrophobic, while the use of a polar ligand will make them hydrophilic, thus getting QDs made of the same material but with a different reactivity.
Applications
QDs are currently employed in biology, to detect some particular types of cells. QDs prepared with bioconjugate molecules can be bonded selectively to some cells such as cancer cells or to some harmful bacteria such as E. coli as capping agents. Sample can be treated with QDs and they will show a colored/fluorescent signal when bonded to such molecules or bacteria indicating their presence. This application will help to diagnose a disease, or to establish a possible contamination with dangerous bacterial strains.
QDs are used in fields such as solar cells ito ncrease efficiency of the cell in converting light into energy and also in optoelectronics. CdS or CdTeS QDs can be the used with TiO2 nanowires with a reported efficiency increase of 300% and 350% for CdS and CdTeS respectively. QDs are used for generation of Light Emitting Diodes (LEDs) giving energy-efficiency and also producing brighter colors.
Market
Recent developments in the QD have created a potential market. In the past the commercial use of QDs was limited due to their high cost, but due to the novel discoveries and investments in sectors such as solar energy and optoelectronics the cost of QDs has reduced and the field of application has become wider.