A quantum dot (QD), also called a nanocrystal, is a semiconductor nanostructure that can be as small as 2 to 10 nm. The usefulness of quantum dots comes from their peak emission frequency's extreme sensitivity - quantum mechanical in nature - to both the dot's size and composition. QDs have been touted as possible replacements for organic dyes in the imaging of biological systems, due to their excellent fluorescent properties, good chemical stability, broad excitation ranges and high photobleaching thresholds. However, the main drawback of QDs is their toxicity and therefore their application is problematic. If this toxicity problem could be addressed, QDs may one day be safely utilized in many areas. For instance, cadmium telluride (CdTe - which is toxic) QD based nanocomposites can be used as fluorescent probes for biological imaging, they can also be utilized to monitor targeted drug delivery and for controlled modification of structural and functional properties of intracellular components. Scientists in Ireland have been using gelatin during the production of CdTe QDs thereby reducing the toxicity of the particles. Their approach could be useful for the development of other nanoparticle composites with low toxicity as well.
Nanocrystals, also called quantum dots (QD), are artificial nanostructures that can possess many varied properties, depending on their material and shape. For instance, due to their particular electronic properties they can be used as active materials in single-electron transistors. Because certain biological molecules are capable of molecular recognition and self-assembly, nanocrystals could also become an important building block for self-assembled functional nanodevices. The atom-like energy states of QDs furthermore contribute to special optical properties, such as a particle-size dependent wavelength of fluorescence; an effect which is used in fabricating optical probes for biological and medical imaging. So far, the use in bioanalytics and biolabeling has found the widest range of applications for colloidal QDs. Though the first generation of quantum dots already pointed out their potential, it took a lot of effort to improve basic properties, in particular colloidal stability in salt-containing solution. Initially, quantum dots have been used in very artificial environments, and these particles would have simply precipitated in 'real' samples, such as blood. These problems have been solved and QDs are ready for their first real applications.
Semiconductor photonics, electronics and optoelectronics infrastructure is at the core of the information society. As the length scales of electronic devices continue to shrink, the cost of traditional approaches to device fabrication involving lithography is becoming excessive. It is regarded that self-assembled growth methods are a solution to the problem of fabricating smaller devices at a lower cost. Self-assembled quantum dots (QDs) are providing the possibility of new devices for this infrastructure in the short, medium and long term. QDs are ideal for the study of the fundamental properties of nanostructures, which is applicable across the nanotechnology and nanoscience sector. Research in self-assembled semiconductor QDs is therefore characterized by a remarkably well-matched combination of the two main motivations for scientific research, namely academic interest and the potential for industrial applications. As a consequence, there is an intense scientific activity in materials growth, structural characterization, optical and transport spectroscopy, device engineering and computational modeling. The field of self-assembled semiconductor nanostructures started in 1985 in Europe by a French group at the Centre National d'Etudes des Telecommunications - CNET.
Individual quantum dots (QDs) have been widely investigated for the past 15 years, showing their potential applications in quantum computing. However, individual QDs are not enough for practical applications, but preparing and characterizing groups of QDs with controllable crosstalk (quantum dot molecule) is very challenging still. University of Arkansas researchers discovered a simply way to fabricate QD pairs, the most simple QD molecule. This provides a unique opportunity to study carrier interaction among QDs, one step further towards quantum computing.
Carbon nanotubes are attractive materials as the building block of quantum-dot based nanodevices. In particular, single-wall carbon nanotubes (SWCNTs) are interesting because they become metallic and semiconducting, depending on how they are rolled up from the graphene sheet, and they could be applied to various devices such as ultrasmall field-effect transistors, single-electron devices, quantum computing devices, and light-emitting devices. A research group at the Japanese Institute of Physical and Chemical Research (RIKEN) has made extensive experimental efforts to apply SWCNTs to single-electron devices and quantum computing devices (spin qubit) with a single quantum dot as a basic structure.
Researchers in Germany managed to integrate quantum dots (QD) into the walls of nano- and microtubes. This novel structure serves as a quantum light emitter as well as optical waveguide. This represents a major step toward the realization of flexible high quality factor optical resonators based on tubes.