A paper version of this report is also available following this link PhotonicRoadSME report on quantum dots
Definition of the material category
Quantum dots are a special class of semiconductors materials. Quantum dots, also known as nanocrystals, are a special class of materials known as semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI materials. Semiconductors are a cornerstone of the modern electronics industry and make possible applications such as the Light Emitting Diode and personal computer. Semiconductors derive their great importance from the fact that their electrical conductivity can be greatly altered via an external stimulus (voltage, photon flux, etc), making semiconductors critical parts of many different kinds of electrical circuits and optical applications. Quantum dots are unique class of semiconductor because they are so small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes materials behave differently, giving quantum dots unprecedented tunability and enabling never before seen applications to science and technology.
Quantum dots are particularly significant for optical applications due to their theoretically high quantum yield. In electronic applications they have been proven to operate like a single-electron transistor and show the Coulomb blockade effect. Quantum dots have also been suggested as implementations of qubits for quantum information processing. The ability to tune the size of quantum dots is advantageous for many applications. For instance, larger quantum dots have spectra shifted towards the red compared to smaller dots, and exhibit less pronounced quantum properties. Conversely the smaller particles allow one to take advantage of quantum properties.
Being zero dimensional, quantum dots have a sharper density of states than higher-dimensional structures. As a result, they have superior transport and optical properties, and are being researched for use in diode lasers, amplifiers, and biological sensors. Quantum dots may be excited within the locally enhanced electromagnetic field produced by the gold nanoparticles, which can be observed from the surface Plasmon resonance in the photoluminescence excitation spectrum of (CdSe) ZnS nanocrystals. High-quality quantum dots are well suited for optical encoding and multiplexing applications due to their broad excitation profiles and narrow/symmetric emission spectra. The new generations of quantum dots have far-reaching potential for the study of intracellular processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo observation of cell trafficking, tumor targeting, and diagnostics.
There are several different types of nanomaterials which are described in reports (nanoparticles, quantum dots, and nanotubes) divided by nature, growth methods and industrial application (see table 1) :.
- Polymer nanostructure: 3D Photonic Crystal with designed point defects
- CdTe quantum dots doped with transition metals
- CdTe/ZnTe quantum dots
- GaInAs/GaAs quantum dots
- HgCdTe quantum dots
- InGaAs/GaAs QD gain material
- SiC QDs
A search of the European Patent Database (http://ep.espacenet.com) was conducted. As for papers, there was no distinction made between patents filed in the European Union or elsewhere in the world. The search was performed for patents filed up to 2008 (nowadays). Several search terms were applied for more detailed analysis in the database. All used terms strongly correlated with the quantum dots application areas. The search terms and results are listed in the following table 4. Two different colors were used for indication of areas which have: blue color – above 20 patents to nowadays, yellow color – above 50 patents to nowadays, orange color – above 100 patents to nowadays. It gives possibility to confirm right choice of most intensively developing directions and application areas.
Using the results of the patent search, a few patents were selected to be presented to exemplify possible uses of quantum dots for the different sectors that were investigated.
- WO2008087692 SEMICONDUCTOR MEMORY, SEMICONDUCTOR MEMORY SYSTEM USING THE MEMORY, AND METHOD FOR MANUFACTURING QUANTUM DOT FOR USE IN THE SEMICONDUCTOR MEMORY
- US 2007264778 (A1) Memory device with quantum dot and method for manufacturing the same
- US 2008227230 (A1)Quantum dot vertical cavity surface emitting laser and fabrication method of the same
- US 2008172197 (A1)SINGLE LASER MULTI-COLOR PROJECTION DISPLAY WITH QUANTUM DOT SCREEN
- JP2007318165A QUANTUM DOT LASER
- WO 2008088320 (A1)ZnO nanostructure-based light emitting device
- WO 2008060594 (A2)HIGH LIGHT EXTRACTION EFFICIENCY LIGHT EMITTING DIODE (LED) THROUGH MULTIPLE EXTRACTORS
- WO 2006037226 (A1)Use of quantum dots for biological labels and sensors
Health & Well-Being
- US 2007212746 (A1)QUANTUM DOT NANOPARTICLE-BASED UNIVERSAL NEUROTOXIN BIOSENSOR
- WO 2006083269 (A2)LUMINESCENT NANOSENSORS
Safety & Security
Several barriers related to technological and fabrication aspects have been identified by analysis of the results and expert interviews. Following, a summary of barriers regarding the use of quantum dots for commercial applications is given. The research field of quantum dots is on the way to become mature, there are some few applications or products in on the market, especially quantum dots used as nanophosphors in (bio-) sensors and life science applications (e.g. gene expression analysis, cell analysis, and clinical diagnostics) are commercially available. However, there are other devices and applications that can be employed using quantum dots, especially in the field of ICT. Here, quantum dots based lasers and memories, slow light applications, single photon emitters/detectors (optical buffers), and quantum information processing devices are still in its infancy. The controlled fabrication in regard to size, size distribution, uniformity control, growth control, functionalization and coatings, etc. has still to be improved. Also, for several applications it is necessary to find ways to position single quantum dots in defined areas and spots (e.g. lighting/laser applications), which is highly sophisticated at present.
Trends and future applications
Trends and future applications of quantum dots for the 4 industrial sectors that are to be examined in this report, namely ICT, Environment, Health & Well-being, and Safety & Security, can be identified on the basis of the previous sections.
As mentioned before, the main applications of quantum dots currently are new types of laser system (e.g. VCSELs), waveguide amplifier and system based on laser such as, laser based displays. Papers on the topic are increasingly published (confer section 3), and also, a significant number of patents has been filed in these areas. Laser based on quantum dots, as shown above is end product (Fujitsu) which will use in near future for communication applications. Also should be emphasized that quantum dots laser TV’s as a prototype was shown by Coherent Technology Company in 2007 year, but there is no any market devices at the beginning of 2009 year. Probably in 1 - 2 year quantum dots laser based TV’s will appear on the market for end users. Quantum dots based nanophosphors for display applications will emerge on the market within the next 5 years.
The ICT sector should especially profit from quantum dots laser because these QD laser first of all will use for high speed data transmission. Also waveguide amplifier based on QD will allow increasing distance of telecommunication backbone without repeaters. Further applications for quantum dots in the ICT sector will be quantum information processing and quantum cryptography which are at a very early stage of development and will need more than 10 years to be ready for the market. Also, quantum dot based memories and spintronics for high density data storage applications with fast read/write timesmight be available in 5-10 years. Single photon emitters and detectors based on quantum dots are another application in the ICT sector. Optical gates for optical interconnects based on quantum dots will also emerge on the market in about 5-10 years.
The environmental sector will not benefit as much as the ICT sector. However, some applications will be developed. The improvement of solar cells, for which also a patent has been filed, is a big issue for the environmental sector. However, there is still much development to be performed at this stage to reach the point of application. Furthermore, quantum dots based LEDs (especially ZnO is of interest) will be employed for lighting applications and energy saving. Also, quantum dots can be used as nanophosphors for bio- and chemo-sensors in environmental monitoring.
The health & well-being sector is the one that will largely profit from new types of quantum dots IR laser, because these lasers could be used as surgical [operating] knife, also in ophthalmology. Other lasers for further surgery fields with individually tailored properties will be developed. As mentioned above, a lot of visualization and imaging applications of quantum dots will be realised, such as early diagnosis of tumors and for drug delivery, cell imaging and clinical diagnostics, e.g. quantum dots based IR detectors. Quantum dots with specifically tailored properties will here serve as nanophosphors in sensors/imaging applications, giving fundamental insight in biological processes. This will largely contribute to the improvement of life standard and health care. Also, the development of photodetectors with improved properties will support advancement in patient’s imaging procedures.
There are no strong applications of quantum dots for safety & security sector. However, photodetectors and UV detectors based on quantum dots are developed that might help in surveillance activities. Also, quantum dots might be used as transducers for chemo-sensors in the field of surveillance, monitoring and terrorism defence.
Summary of European and national R&D funded projects
In this section, a list of some representative European and National initiatives (Germany, France, Spain, Finland, Austria, Switzerland, Poland, UK) in the field of quantum dots during the last years is presented. A project search was conducted using following keywords:
Following databases have been searched:
- DFG Germany: http://gepris.dfg.de/gepris/
- BMBF Germany: http://foerderportal.bund.de/foekat/jsp/SucheAction.do?actionMode=searchmask
- Madri+D database Spain: http://www.madrimasd.org/Investigadores/buscador-proyectos-investigacion/default.asp
- FWF Austria: http://www.fwf.ac.at/de/projects/projekt_datenbank.asp
- ARAMIS Switzerland: http://www.aramis.admin.ch/Default.aspx
- Ministry of Science and Education Poland (2007 - ): http://nauka-polska.pl/dhtml/raportyWyszukiwanie/wyszukiwaniePraceBadawcze.fs?lang=pl
- Ministry of Science and Education Poland (2006 - 2007): http://www.nauka.opi.org.pl/granty/zaawansowane.htm
- Pôle Optique & Photonic France: http://www.popsud.org/
- Ministère de l'Enseignement supérieur et de la Recherche France: http://www2.enseignementsup-recherche.gouv.fr/appel/index.htm
- FinNano 2005-2010 Finland: http://akseli.tekes.fi/opencms/opencms/OhjelmaPortaali/ohjelmat/NANO/en/etusivu.html
- Engineering and Physical Sciences Research Council EPSRC database UK: http://gow.epsrc.ac.uk/ListProgrammes.aspx
- Cordis Project database for FP7: http://cordis.europa.eu/fp7/projects_en.html
- Cordis Project database for FP6: http://cordis.europa.eu/fp6/projects.htm
A total of 95 national and international R&D funded projects have been identified, examined and summarized. The table shown below gives an idea of the domain of applications of the projects mentioned (indicated in grey color) as well as the origin of funding.
- PARSEM "Interfacial Phenomena at Atomic Resolution and Multiscale Properties of Novel III-V Semiconductors (PARSEM)"
- "Quantum Dots incorporated into Nanowires"
- ACDIQDOTS "Asymmetric Cell Division Imaging with Quantum Dots"
- NANOQUANTA"Nanoscale Quantum Simulations for Nanostructures and Advanced Materials"
- QPHOTON "High-Q semiconductor Nanostructures for single Photon emission, detection and manipulation"
- "Chaotic nanometric devices: bifurcations, scars and quantization ()"
- NANO - DFT "Electronic structure of nanosystems: Density Functional Theory approach"
- "Applications for semiconductor nanoparticles: from biomedicine to optics"
- POISE "Physics of intersubband semiconductor emitters"
- CAMEL "Control of quantum dynamics of atoms, molecules and ensembles by light"
- SELF ASSEMBLY "Structure and electronic properties of low-dimensional systems and molecular assemblies"
- QOQIP "Quantum Optics for Quantum Information Processing"
- FLUOROMAG "Multi-parameter sensing for high sensitivity diagnostics using fluorescent and magnetic nanoparticles"
- SPINCQED "Circuit QED with electron spins in a semiconductor quantum dots"
- MULTIQUANTUM "Combining multi-photon microscopy and quantum dot technology to study molecular dynamics on single cells in vivo"
- DOTSWITCH "Quantum Dots for All-Optical Switching in Optical Data Communication Networks"
- EMALI "Engineering, manipulation and characterization of Quantum States of matter and light"
- QDOTS "Novel Quantum Dot Imaging technologies for the study of morphogenesis and other biological processes"
- NANOELECTROPHOTONICS "Nanoelectronics and Nanophotonics: Cooperation and Accentuation of Quantum Functionality and Lasing"
- "Synthesis of magnetic quantum dots"
- "Spontaneous Emission of Semiconductor Quantum Dots Controlled in Optical Microcavities"
- STRONG CORR "Strong electronic correlations in low dimensional systems"
- DOMINO "Antimonide Quantum Dots for Mid-Infrared Nano-Photonic Devices"
- NANO UB-SOURCES "Ultra-broad bandwidth light sources based on nano-structuring devices"
- ZODIAC "Zero Order Dimension based Industrial components Applied to telecommunications"
- N2T2 DEVICES "Novel Nano-Template Technology And Its Applications To The Fabrication Of Novel Photonic Devices"
- DOMINO "Antimonide Quantum Dots for Mid-Infrared Nano-Photonic Devices"
- NANO UB-SOURCES "Ultrabroad bandwidth light sources based on nano-structuring devices"
- CUSMEQ "Coherent ultra-fast spectroscopy and manipulation of excitonic Q-bits"
- HERODOT "Heterogeneous quantum rod and quantum dot nanomaterials, towards a novel generation of photonic devices"
- GOSPEL "Governing the speed of light"
- FWMIMAGING "Study of coherent non-linear optical response of nanoparticles and application to multiphoton imaging in cell biology"
- NANOSMARTS "Smart nondimensional biosensors for detection of tumor cells and cytotoxic amyloids intermediates"
- FAST-DOT "Compact ultrafast laser sources based on novel quantum dot structures"
- IBPOWER "Intermediate band materials and solar cells for photovoltaics with high efficiency and reduced cost"
- DELIGHT "Development of low-cost technologies for the fabrication of high-performance telecommunication lasers"
- DOTSENSE "Group III-nitride quantum dots as optical transducers for chemical sensors"
- "Optical investigations of semiconductors quantum dots with modulation and high lateral resolution technique"
- "GaAs laser with quantum dots active layer"
- "Correlation between photons emitted from single quantum dot CdTe/ZnTe"
- "MBE growth and optical spectroscopy of semi magnetic semiconductor QD"
- "Dephasing of optical excitations and coupling of Quantum Dots"
- "Modelling of dynamics of Quantum-Dot Lasers and Amplifiers"
- "Ultrafast nano-optics with semiconductor quantum dots"
- "Pure-optical signal processing with quantum dot based semiconductor amplifiers"
- "Plasmonic coupling of individual quantum dots"
- "ERA NanoSci - Optical interface for gate-controlled quantum dots"
- "Nanostructuring of homo- and heteroepitaxial light emitters for enhanced quantum efficiency using photonic crystal components"
- MISTRAL "Monolithically integrated silicon based photonic devices for telecommunication applications"
- MONALISA "Epitaxy of monolithically integrated III-V materials on silicon as lightemitter"
- nanoQUIT "Self-organised MBE-growth of InAs-Quantum Dots on structured substrates"
- AlGaInP "Quantum Dots on GaP-substrate for efficient luminescence in the green spectral range"
- INNOTRANS "Nanostructured comb laser and nano-photonic chip for optical communications"
- QPENS "GaAs and GaN based quantum for semiconductor emitter and optimized verification electronics for system-investigations for quantum cryptography transmission protocols"
- "Semiconductor quantum dots doped glass fibers as nonlinear optical switches"
- "Entangles photons from quantum dot components"
- "Quantum optics in artificial molecules"
- "Controlled positioning of self-organised SiGe islands"
- "Electroluminescent cooling for photonic heat transfer applications"
- "Advanced III-V semiconductor materials for high-efficient solar cells"
- DAUNTLESS "Development of vanguard semiconductor sources for single and entangled photon emission"
- "Nanophotonics - Extension"
- LIGHTCAVITI "Localization of Light in Optical Nanocavities"
- NEONATE "New Compound Semiconductor Materials for Optoelectronic Devices"
- TERAPEPO "THz sources"
- ONLIGAN2 "Optical non-linearities of GaN"
- "Optimisation of fluorescence emission of sole CdSe nanocrystals by controlling their structure and vicinity"
- "Studies of optolectronic and electrochemical properties of novel TiO2 based photosensitive sol-gels for realization of photovoltaic cells of the 3rd generation and for realization of photo-batteries"
- "Quantum dots for photovoltaic energy production"
- "Long-wavelength high-performance quantum dot lasers and amplifiers"
- "Quantum dot light emitters grown on patterned substrates"
- "Active Photonic Crystals Incorporating Site-Controlled Quantum Dots"
- "Microcavity quantum dot light emitters at 1,3 um wavelength"
- "GaAs-based long wavelength quantum wire and quantum dot lasers"
- "Development of novel optical near-field probes utilizing semiconductor quantum dots as nanoscopic light ermitters/detectors"
- PHOTODOT "Photonic light emitting devices based on quantum dot semiconductors"
- "Actively manipulating electronic excitations in nanocrystals"
- "Fabrication of first 337 nm laser diodes for biological applications"
- "Growth and Electronic Properties of InN and N-rich Alloys"
- "High-efficiency Hybrid Solar Cells for Micro-generation"
- "Investigation of Hybrid III-V/II-VI structures grown by Liquid Phase Epitaxy for Mid-infrared Optoelectronic Devices"
- "Magnetic field- and pressure- optical effects in CuInSe2, CuGaSe2 and CuInS2"
- "Materials Challenges in GaN-based Light Emitting Structures"+
- NEDQIT "NanoEngineered Diamond for Quantum Information Technology"
- "Photoreactivity of nanostructured semiconducting oxides"
- "Semiconductor-based hybrid structures for ultraviolet micro-devices"
- "Soliton Formation through Self-Induced Transparency in Semiconductor Microcavities"
- "Spectroscopy and Applications of Nitride Quantum Dots"
- "Support for the EPSRC National Centre for III-V Technologies at Sheffield"
- "Coupling of single quantum dots to two-dimensional systems"
- "Spin Ping Pong - Towards a Quantum Dot spin QuBit"
- "Nanostructures for magneto-electronics devices: synthesis and properties MBE"
- "Semiconductor lasers self-assembled quantum rings for applications in telecommunications"
This part of the review is mainly based on two novel handbooks: “Handbook of Nanotechnology”, 2nd edition, Bhushan editor, Springer, 2007 ; “Nanoparticle Technology Handbook”, edited by Masuo Hosokawa, Kiyoshi Nogi, Makio Naito, Toyokazu Yokoyama, Elsevier, 2007 , as mentioned before. Also some more useful handbook wrote in last 6 years took into account, such as: “Handbook of nanoscience engineering and technology”, edited by William A. Goddard III, Donald W. Brenner, Sergey E. Lyshevski, Gerald J. Iafrate, 2002, CRC Press, 2002 ; “Introduction to Nanotechnology” Charles P., Jr. Poole, Frank J. Owens, Wiley, 2003 .
Based on these materials and different review papers published in last 3 - 5 years should be emphasize following directions in which quantum dots are intensively studying, using and have strong applications perspectives, such as:
- Solar cells;
Most of review articles which were used for this review focused on properties of quantum dots which can be used for different application areas which will describe below. The following table 3 (Publications overview) gives an overview of the number of papers in the ScienceDirect database (http://www.sciencedirect.com/) concerning certain topics. Three different colors were used for indication of areas which have: blue color – above 20 publications since 2002 year to 2008 year, yellow color – above 50 publications since 2002 year to 2008 year, orange color – above 100 publications since 2002 year to 2008 year. It gives possibility to determine most perspectives directions and application areas.
Light emitting devices (Quantum dot laser)
There are several inquiries into using quantum dots as light-emitting diodes to make displays and other light sources: "QD-LED" displays, and "QD-WLED" (White LED). In June, 2006, QD Vision announced technical success in making a proof-of-concept quantum dot display. Quantum dots are valued for displays, because they emit light in very specific gaussian distributions. This can result in a display that can more accurately render the colors that the human eye can perceive. Quantum dots also require very little power since they are not color filtered. Additionally, since the discovery of "white-light emitting" QD, general solid-state lighting applications are closer than ever. A liquid crystal display (LCD), for example, is powered by a single fluorescent lamp that is color filtered to produce red, green, and blue pixels. Displays that intrinsically produce monochromatic light can be more efficient, since more of the light produced reaches the eye.
The newly developed quantum dot laser realized high-speed operation of 10Gbps at wavelengths of 1.3 micrometers which are used for optical transmission systems, for a temperature range from 20°C to 70°C without drive current adjustments. The achieved 10Gpbs high-speed operation is the world's fastest for a quantum dot laser for use in optical telecommunication systems.
Optical output characteristics of the new quantum dot laser were measured and recorded in increments of 10°C. The optical output characteristics were nearly stable independent of temperature, for a 20°C to 50°C range. Even for temperatures exceeding 50°C, efficiency (slope of the characteristics curve) was constant, with minimal variances up to 70°C. In comparison to the performance of conventional strained quantum-well lasers of the past, the new quantum dot laser achieves significantly higher stability of temperature.
10Gbps modulation waveforms of the new quantum dot laser at 20°C and 70°C without constant drive current were verified. Although the current used for the laser was the same at both temperatures, a distinct light output diagram is achieved with the extinction ratio (*4) of 7 decibels (7dB). Waveforms for a strained quantum-well laser under the same conditions showed output degradation and pattern disfigurement, with indistinct waveforms.
Average optical output variances measured for 10Gbps modulation operation across various temperatures depicted that for strained quantum-well lasers, the average optical output dropped significantly at higher temperatures, while the average light output variance for the new quantum dot laser was less than 5% and minimal.
The results achieved with the newly developed quantum dot laser will enable major simplification of circuits to drive lasers, thereby paving the way for optical transmitters that are compact, low-cost, and low power-consuming, for future optical metro-access systems and high-speed optical LANs.
Quantum dots may have the potential to increase the efficiency and reduce the cost of today's typical silicon photovoltaic cells. According to experimental proof from 2006, quantum dots of lead selenide can produce as many as seven excitons from one high energy photon of sunlight (7.8 times the band gap energy). This compares favourably to today's photovoltaic cells which can only manage one exciton per high-energy photon, with high kinetic energy carriers losing their energy as heat. This would not result in a 7-fold increase in final output however, but could improve the maximum theoretical efficiency from 25% to about 70%. A common scheme to improving the work done per photon in solar cells is to stack junctions of increasing band gap on top of each other. Practically only two or three junctions may be stacked. The introduction of nanostructures potentially allows for a greater number of energy gaps to be introduced through quantum confinement. With an infinite number of band-gaps available the maximum theoretical efficiency of photovoltaics is 69 % .
The earliest work on multiple band gap photovoltaics involving nanostructures was regarding the use of quantum wells . Aroutiounian et al. have suggested that quantum dots could be placed in a thick intrinsic region of a n-i-p hetrojunction . The authors propose using a GaAs/InAs structure since it can generate a variety band-gaps well matched to the solar spectrum. Aroutiounian suggest epitaxy as a growth mechanism using the phenomenon of self-assembly through lattice mismatch strain. Furthermore, if the quantum dots are stacked closely enough, the strain fields of the lower dots will extend through the bulk substrate which encourages quantum dots to vertically align themselves into columns. In essence, the quantum dots are encouraged to form into weak quantum wires; this of critical importance due to the need to extract charge carriers in order to generate photocurrent. Coupling between the quantum dots allows the generated electrons and holes to be injected from the high absorption quantum dots into the n and p regions. The theoretical model of this system demonstrates a 5.5 % improvement in efficiency over the same system without the inclusion of quantum dots.
Impact ionization is essentially the inverse of the Auger recombination process. In impact ionization a photon with energy greater than twice the band-gap creates an electron-hole pair with considerable excess energy. Normally the electron would then thermalize through phonon emission back to the bottom of the conduction band. However, it is possible also for the electron to relax by generating another electron-hole pair instead. In this manner is it possible to generate more than one free carrier per photon, resulting in greater photocurrent. The resulting product of two electron-hole pairs is sometimes known as a biexciton.
Nozik originally proposed that impact ionization might be enhanced in semiconductor nanocrystals . This was later shown experimentally by Schaller and Klimov . While impact ionization requires photons with a minimum energy of at least twice the band-gap, the process occurs most frequently at photons with even higher energy. For the PbSe nanocrystals studied by Schaller and Klimov the onset was not seen until E ~ 3 Eg. The peak internal efficiency found was 118 % for photons with energy of 3.8 Eg. (External quantum efficiency is the measured energy efficiency from the current-voltage produced outside the device; internal (quantum) efficiency is the probability of a photon creating an electron-hole pair. For impact ionization internal quantum efficiency can be greater than 100 %.) Construction of a working photovoltaic prototype with improved performance through impact ionization or some other down conversion method has not yet been achieved. The proof of principle will require demonstrating that the electron-hole pairs generated by the nanocrystals can be extracted to provide electrical current. This may be difficult because the relaxation time of electron-hole pairs generated through this method is very short. While regular electron-hole pairs in the PbSe nanocrystals were shown to have a relaxation time on the order of a microsecond, the so called biexciton created through impact ionization tends to decay via Auger recombination on the picosecond scale. If this occurs then the system returns to its original state with one high-energy electron hole pair. Quantum dot photovoltaics would theoretically be cheaper to manufacture, as they can be made "using simple chemical reactions."
Biology (luminescent and magneto-luminescent markers)
In modern biological analysis, various kinds of organic dyes are used. However, with each passing year, more flexibility is being required of these dyes, and the traditional dyes are often unable to meet the expectations. To this end, quantum dots have quickly filled in the role, being found to be superior to traditional organic dyes on several counts, one of the most immediately obvious being brightness (owing to the high quantum yield) as well as their stability (much less photodestruction). For single-particle tracking, the irregular blinking of quantum dots is a minor drawback [71, 127, 128].
The use of quantum dots for highly sensitive cellular imaging has seen major advances over the past decade. The improved photostability of quantum dots for example, allows the acquisition of many consecutive focal-plane images that can be reconstructed into a high-resolution three-dimensional image. Another application that takes advantage of the extraordinary photostability of quantum dot probes is the real-time tracking of molecules and cells over extended periods of time. Researchers were able to observe quantum dots in lymph nodes of mice for more than 4 months.
Semiconductor quantum dots have also been employed for in vitro imaging of pre-labeled cells [127, 128]. The ability to image single-cell migration in real time is expected to be important to several research areas such as embryogenesis, cancer metastasis, stem-cell therapeutics, and lymphocyte immunology.
Scientists have proven that quantum dots are dramatically better than existing methods for delivering a gene-silencing tool, known as siRNA, into cells.
First attempts have been made in using quantum dots for tumor targeting under in vivo conditions. There exist two basic targeting schemes: active targeting and passive targeting. In the case of active targeting, quantum dots are functionalized with tumor specific binding sites to specifically bind to tumor cells. Passive targeting utilizes enhanced permeation and retention of tumor cells for the delivery of quantum dot probes. Fast growing tumor cells typically have more permeable membranes than healthy cells, allowing the leakage of small nanoparticles into the cell body. Moreover, tumor cells lack an effective lymphatic drainage system, which leads to subsequent nanoparticles accumulation.
One of the remaining issues with quantum dot probes is their in vivo toxicity . CdSe nanocrystals for example are highly toxic to cultured cells under UV illumination. The energy of UV irradiation is close to the covalent chemical bond energy of CdSe nanocrystals. As a result, semiconductor particles can be dissolved, in a process known as photolysis, to release toxic cadmium ions into the culture medium. In the absence of UV irradiation, however, quantum dots with a stable polymer coating have been found to be essentially nontoxic. Then again, only little is known about the excretion process of polymer-protected quantum dots from living organisms. These and other questions must be carefully examined before quantum dot applications in tumor or vascular imaging can be approved for human clinical use.
Another cutting edge application of quantum dots is also being researched as potential artificial fluorophore for intra-operative detection of tumors using fluorescence spectroscopy.
Quantum dot technology is one of the most promising candidates for use in solid-state quantum computation. By applying small voltages to the leads, one can control the flow of electrons through the quantum dot and thereby make precise measurements of the spin and other properties therein. With several entangled quantum dots, or qubits, plus a way of performing operations, quantum calculations might be possible. Also, quantum dots based memories will help to provide much more data storage capacities for archiving of large data amounts for the future.
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