Automated and scalable inline two-stage synthesis process for high quality colloidal quantum dots


By Michael Berger. Copyright © Nanowerk

longpredicte(Nanowerk Spotlight) Colloidal quantum dot (CQDnanocrystals are attractive materials for optoelectronics, sensing devices and  third generation photovoltaics, due to their low cost, tunable bandgap – i.e.  their optical absorption can be controlled by changing the size of the CQD  nanocrystal – and solution processability. This makes them attractive candidate  materials for cheap and scalable roll-to-roll printable device fabrication  technologies.

 

One key impediment that currently prevents CQDs from fulfilling  their tremendous promise is that all reports of high efficiency devices were  from CQDs synthesized using manual batch synthesis methods (in classical  reaction flasks).

 

Researchers have known that chemically producing nanocrystals  of controlled and narrow size-distributions requires stringent control over the  reaction conditions – e.g. temperature and reactant concentration – which is  only practical for small-scale reactions.

 

Such a synthesis is extremely  difficult to scale up, hence very costly to mass produce without severely  compromising quality.   The reason for this is that, just like rain droplets,  nanocrystals form sequentially by ‘nucleation’ and ‘growth’. Both these  phenomena are highly sensitive to temperature and reagent concentration.  Moreover, nucleation and growth must occur at substantially different  temperatures and, in fact, to obtain nanocrystals of uniform sizes, one must be  able to rapidly cool down the reaction from the nucleation temperature to the  growth temperature.

 

Hence, the quality of the product is contingent upon how  well and fast one can homogenize the reactor, both chemically and thermally.   Unfortunately, the only way to scale up batch reactors is by  increasing their volume, whereupon it becomes difficult to homogenize the  reactor and impractical to rapidly cool. The end result is nanocrystals of  low-quality and broad size distributions, which are not useful for fabricating  devices.

Some researchers have sought to circumvent this limitation by  conducting the reactions in narrow fluidic channels (less than a 1 mm in  diameter) while the reactants are continuously pumped through the channels, so  called ‘continuous-flow reactors’.

 

Conceptually, this scheme has several advantages. Narrow-width  channels afford uniform heating and mixing of the reaction, while the reaction  is scalable by simply increasing channel length and pump rate of the reagents.  This sort of scaling does not effect the quality of the product, because the  channel width, and hence the effective reaction volume, remains the same.  Despite these advantages, most attempts to use continuous-flow reactors in the  past have resulted in nanocrystals with a much lower quality than the batch  produced ones.

 

“We have analyzed the nucleation and growth of CQDs in  continuous-flow reactors and realized that, in order to achieve controllable  size and narrow size-distributions, one must employ two temperature stages in  the reactor: one for nucleation, and another for growth,” Osman Bakr, an  assistant professor in the Solar & Photovoltaics Engineering Research Center at King  Abdullah University of Science and Technology (KAUST), tells Nanowerk.

“By  separating these two crucial steps in the formation of the CQDs in time,  temperature, and space, we were able to obtain very high quality nanocrystals,  as good as the best batch synthesis, by a process that is low-cost,  mass-producible, and automated.”

Schematic of a conventional batch synthesis setup and a dual-stage continuous flow reactor setup

 

 

Schematic of (a) a conventional batch synthesis setup and (b) a  dual-stage continuous flow reactor setup with precursor A (Pb-oleate,  octadecene) and precursor B (bis(trimethylsilyl) sulfide in octadecene).  (Reprinted with permission from American Chemical Society)

 

Reporting their findings in ACS Nano (“Automated Synthesis of Photovoltaic-Quality Colloidal Quantum  Dots Using Separate Nucleation and Growth Stages”), Bakr and his team  demonstrated the quality of the CQDs produced by their method by using them to  make CQD-based solar cells that showed very high efficiencies.

 

“In this paper, we developed an automated, scalable, in-line  synthesis methodology of high-quality CQDs based on a flow-reactor with two  temperature-stages of narrow channel coils,” says Professor Ted Sargent from the  University of Toronto who, together with Bakr, led this work. “The flow-reactor  methodology not only enables easy scalability and cheap production, but also  affords rapid screening of parameters, automation, and low reagent consumption  during optimization. 

Moreover, the CQDs are as good in quality and device  performance as the best CQDs that are produced in the traditional batch  methodology.”   The team also developed a general theory for how one can use the  flow-reactors to finely tune the quality and size distribution of the CQDs, and  explained why previous attempts of using flow-reactors based on a  single-temperature-stage, as opposed to a dual-temperature-stage, necessarily  produce CQDs of low-quality and broad size distribution.

 

This work paves the way towards the large-scale and affordable  synthesis of high-quality CQD nanocrystals in tunable sizes, enabling  photovoltaics, light-emitting diodes, photodetectors, and biological tagging  technologies that take advantage of the nanoscale properties of those promising  materials.

 

“Over the last ten years we have seen tremendous advancements in  software and computer integration, in items that we use in our everyday lives,”  says Bakr. “Flow-reactors as a platform are ideally placed to take advantage of  this trend. Software that automates the routines of flow-reactors already  exists. In the near future, researchers will be able to run and monitor hundreds  of experiments to produce CQDs from home using a mobile app.

 

Moreover, because  flow-reactors contain very few moving parts, essentially just programmable  pumps, I expect that it will become an automated research platform that most  labs studying nanocrystals can afford.”   “Our work has shown that flow-reactors can produce nanocrystals  that are as good as the best batch produced reactions, with exquisite control  over reaction conditions,” he adds. “We believe that this will encourage the  nanomaterials community to take advantage of the enormous productivity gains in  R&D afforded by flow-reactors, which other chemical industries, such as  pharmaceuticals, are currently utilizing earnestly.”

Read more: http://www.nanowerk.com/spotlight/spotid=32945.php#ixzz2j2YbZvu8

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Solar paint paves the way for low-cost photovoltaics


072613solar(Nanowerk Spotlight) Using quantum dots as the basis  for solar cells is not a new idea, but attempts to make such devices have not  yet achieved sufficiently high efficiency in converting sunlight to power. The  latest advances in  quantum dots photovoltaics have recently resulted in solar  cell power conversion efficiencies exceeding 7% (see for instance: “Graded Doping for Enhanced Colloidal Quantum Dot  Photovoltaics”).

 

Although these performance levels are promising, all  high-performing device results to date have relied on a multiple-layer-by-layer  strategy for film fabrication rather than employing a single-layer deposition  process.    The attractiveness of using quantum dots for making solar cells  lies in several advantages over other approaches: They can be manufactured in an  energy-saving room-temperature process; they can be made from abundant,  inexpensive materials that do not require extensive purification, as silicon  does; and they can be applied to a variety of inexpensive and even flexible  substrate materials, such as lightweight plastics.

 

In new work, reported in the August 12, 2013 online edition of  Advanced Materials (“Directly Deposited Quantum Dot Solids Using a  Colloidally Stable Nanoparticle Ink”), a research team from the University  of Toronto and King Abdullah University of Science and Technology (KAUST)  developed a semiconductor ink with the goal of enabling the coating of large  areas of solar cell substrates in a single deposition step and thereby  eliminating tens of deposition steps necessary with the previous layer-by-layer  method.

 

“We sought an approach that would achieve highly efficient  utilization of CQD materials,” says Professor Ted Sargent from the  University of Toronto, who, together with Osman Bakr, an  assistant professor in the Solar & Photovoltaics Engineering Research Center at KAUST,  led the work. “To achieve this, we made a solar cell ink that can be deposited  in a single step which makes it an excellent material for high-throughput  commercial fabrication.”

 

The team’s ‘solar paint’ is composed of semiconductor  nanoparticles synthesized in solution – so-called colloidal quantum dots (CQDs).  They can be used to harvest electricity from the entire solar spectrum because  their energy levels can be tuned by simply changing the size of the particle.    Previously, films made from these nanoparticles were built up in  a layer-by-layer fashion where each of the thin CQD film deposition steps is  followed by curing and washing steps to densify the film and form the final  semiconducting material.

 

These additional steps are required to exchange the  long ligands that keep the CQDs stable in solution for short ligands that allow  efficient charge transport. However, this means that many steps are required to  build a thick enough film to absorb enough sunlight.   “We simplified this process by engineering the CQD surfaces with  short organic molecules in the solution phase to enable a stable colloidal  solution and reduce the film formation to a single step,” Bakr explains to  Nanowerk. “At the same time, the post processing steps are reduced  significantly, since the semiconducting material is formed in solution.  This  means that CQD films can be deposited quickly and at low cost, similar to a  paint or ink.”

 

       colloidal quantum dot solar cell fabrication methods

 

a)  Schematic of the standard layer-by-layer spin-coating process with active  materials usage yield and required total material indicated. b) Schematic of the  single-step film process with active materials usage yield and required total  material indicated. (Reprinted with permission from Wiley-VCH Verlag)  

 

 

Besides the reduction in processing steps, the new process is  also much more efficient in terms of materials usage. While the layer-by-layer,  solid-state treatment approach provides less than 0.1% yield in its application  of CQD materials from their solution phase onto the substrate, the new approach  achieves almost 100% use of available CQDs.

 

“This means that for the same amount of CQD material, we could  make a thousand-fold larger area of solar cells compared with conventional  methods,” Bakr points out.  “Our technology paves the way for low-cost  photovoltaics that can be fabricated on flexible substrates using roll-to-roll  manufacturing, similar to a printing press,” adds Lisa Rollny, a PhD candidate  in Sarget’s group and a co-author of the paper. “Our ink is also useful in  biological applications, e.g. in biosensors and tracing agents with an infrared  response.”  

 

“In previous work, we found new routes of passivating the CQD  surface using a combination of organic and inorganic compounds in a solid state  approach with large improvements in efficiency,” says Rollny. “We intend to  integrate this knowledge with our solar CQD ink to further improve the  performance of this material, especially in terms of how much solar energy is  converted into usable electrical energy.”  

 

Although the team have developed an effective method for  producing a CQD film in a single step, the electronic properties of the  resulting films are not optimized yet. This is due to the very small  imperfections on the CQD surface that reduce the usable electricity output of a  solar cell. Through careful engineering of CQD surfaces in solution, the  researchers  plan to eliminate these unwanted surface sites in order to make  higher quality, higher efficiency CQD solar cells using their single step  process.

 

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=31922.php#ixzz2dkjBHzZG

Global Quantum Dots (QD) Market worth of $7480.25 Million by 2022


Montreal, Canada | Posted on May 21st, 2012

QDOTS imagesCAKXSY1K 8ELECTRONICS.CA PUBLICATIONS, the electronics industry market research and knowledge network, announces the release of a comprehensive global report on Quantum Dots market.

 

 

According to the report titled, “Quantum Dots (QD) Market – Global Forecast & Analysis 2012 – 2022”  the total market for Quantum dots is expected to reach $7480.25 million ($7.48 Billion) by 2022, at a CAGR of 55.2% from 2012 to 2022.
Quantum Dots (QD) is the most advanced area of “semiconductor nanoparticles”, which is undergoing massive research. QDs are semiconductor nanoparticles, and, as the name suggests, have size from 2 nm to 10 nm. Due to their miniature property; they are highly versatile and flexible. The uniqueness of QD material lays in the fact that its power intensity depends on the input source and size of QD.

There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. In general, quantum wires, wells, and dots are grown by advanced epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.
In the present scenario of QD technology market, Healthcare is the only industry, which has gained significant market share. Healthcare needs high precision in tissue labeling, cancer therapy, tumor detection, etc. and QD-based devices work for the same.
Lighting industry is huge; and after the introduction of efficient lighting like LED, this industry has taken a huge leap. LED lighting and fixtures market is growing by leaps and bounds since the last few years and expected to expand further. Now companies are looking for the alternate technology for LED lighting. QD lighting will fulfill the need; it is highly efficient and cost-effective. QD Vision has collaborated with Nexxus Lighting to launch its first QD LED light, and soon it will capture the market. Likewise, the company is also working on QD display.
QD technology will play a crucial role in solar energy-oriented industry as well. Researchers have developed QD-based solar cell, which is 50% as efficient as conventional solar cell. University of Toronto has achieved an efficiency of 4.2% conversion with solar cell based on colloidal QDs (CQD). Researchers are also working on QD-based paint that can be applied to panels or walls to capture solar energy.
Global Quantum Dots Market for technology-products and applications is expected to reach $7480.25 million by 2022, at an estimated CAGR of 55.2% from 2012 to 2022. Americas are holding a leadership position in QD technology market on the whole; followed by Europe and APAC. In the market of ROW, Middle East and Africa are the largest contributors.
Details of the new report, table of contents and ordering information can be found on Electronics.ca Publications’ web site.  View the report:

http://www.electroiq.com/articles/sst/2012/05/quantum-dots-see-55-cagr-on-led-display-other-applications.html

 

How far can charge carriers travel in CQD films?


QDOTS imagesCAKXSY1K 8Researchers at the University of Toronto in Canada have succeeded in directly measuring the diffusion length of charge carriers in colloidal quantum dot films for the first time. The work will help in improving these technologically important materials and allow us to better understand charge transport in quantum dot solids in general.

Colloidal quantum dots (CQDs) could be ideal as the light-absorbing component in inexpensive highly efficient solar cells. Photons having energies equal to or greater than the band gap of the photovoltaic material can produce excited charge carriers (electrons and holes) that can then be harvested to generate power. CQDs can absorb light over a wide spectrum of wavelengths thanks to the fact that the bandgaps in quantum dots can be tuned over a large energy range simply by changing the size of the dots during wet chemical synthesis – something that is not possible for bulk semiconductors. This is useful for building multiple junction solar cells to maximize power conversion efficiency.

The problem is, however, that only a limited number of CQD materials have been studied in any great detail for use in solar cells – with two common examples being lead or cadmium-based crystals. Charge carriers in these compounds have modest diffusion lengths (that is, the distance that carriers can travel before recombining). A long diffusion length is important for solar cells because it allows photo-generated electrons and holes to be collected by the device and produce useful current before they recombine.

Illuminating a CQD film

To better understand charge transport in CQDs, researchers have been busy trying to actually measure the diffusion length in these materials, but most techniques to do so have only been indirect until now. For example, diffusion length can be estimated by combining mobility measurements (in field-effect transistor test structures) with separately measured charge carrier lifetimes. Unfortunately, such techniques invariably produce values that are too high.

Now, a team led by Edward Sargent has put forward not one, but two new methods to directly measure the diffusion length in CQDs. “We employ what we refer to as the ‘donor-acceptor’ scheme where charges preferentially flow from a large band-gap CQD layer (the donor) into an adjacent smaller band-gap layer (the acceptor), where they ultimately recombine and produce bright photoluminescence (PL) that we can measure,” explains Sargent.

pic2

1D and 3D methods

In their first, “1D” method, the Toronto researchers use a donor layer whose thickness can be varied, capped with a thin acceptor layer. By measuring the PL signal from the acceptor as a function of donor thickness, they are able to directly extract the diffusion length. In the second, “3D” method, they use a mix of donor and acceptors, where acceptor CQDs are uniformly mixed into a donor matrix. “With this technique, we can deconstruct diffusion length into two components – charge mobility and lifetime,” Sargent told nanotechweb.org.

The researchers have succeeded in measuring a diffusion length of as high as 80 nm for lead sulphide CQDs using both methods.

The new techniques, which are detailed in the journal ACS Nano, are an easy way to measure the diffusion length in CQDs without having to make full photovoltaic devices first, says Sargent. And, they will allow researchers to make such measurements quickly and simply in a host of solid CQD materials.

Spurred on by its results, the team says that it will now be looking at improving the critical parameters (such as charge carrier lifetimes) that the diffusion length hinges on. “This will ultimately lead to improvements in CQD photovoltaic device efficiency,” adds Sargent.

About the author

Belle Dumé is contributing editor at nanotechweb.org.

Quantum dots for superior solar cells


QDOTS imagesCAKXSY1K 8

Researchers at the University of Toronto in Canada and KAUST in Saudi Arabia have made a solar cell out of colloidal quantum dot (CQD) films that has a record-breaking efficiency of 7%. This is almost 40% more efficient than the best previous devices based on CQDs.

CQDs are semiconductor particles only a few nanometres in size. They can be synthesized in solution, which means that films of the particles can be deposited quickly and without fuss on a wide range of flexible or rigid substrates – just like paint or ink can.

QD Solar Chip

CQD photovoltaics

CQDs could be used as the light-absorbing component in cheap, highly efficient inorganic solar cells. In a solar cell, high-energy photons hitting the photovoltaic material can produce excited electrons and holes (charge carriers) that have energies at least equal to or greater than the band gaps of the material. Another advantage of using CQDs as the photovoltaic material is that they absorb light over a wide spectrum of wavelengths thanks to the fact that the band gap can be tuned over a large energy range by simply changing the size of the nanoparticles.

There is a snag, however; the high surface area to volume ratio of nanoparticles results in bare surfaces that can became “traps” in which electrons invariably get stuck. This means that electrons and holes have time to recombine instead of producing useful current, something that inevitably reduces the efficiency of devices made from CQD films.

Surface passivation

A team led by Edward Sargent at Toronto may now have come up with a solution to this annoying problem. The researchers have succeeded in passivating the surface of CQD films by completely covering all exposed surfaces using a chlorine solution that they added to the quantum-dot solution immediately after it was synthesized. “We employed chlorine atoms because they are small enough to penetrate all of the nooks and crannies previously responsible for the poor surface quality of the CQD films,” explained Sargent.

QD Solar Chip 2

In the lab

The team then spin cast the CQD solution onto a glass substrate that was covered with a transparent conductor. Next, an organic linker was used to bind the quantum dots together. This final step in the process results in a very dense film of nanoparticles that absorbs a much larger amount of sunlight.

Reducing traps

“Our hybrid passivation scheme employs chlorine atoms to reduce the number of traps for electrons associated with poor CQD film surface quality while simultaneously ensuring that the films are dense and highly absorbing thanks to the organic linkers,” Sargent told nanotechweb.org.

Electronic spectroscopy measurements confirmed that the films hardly contained any electron traps at all, he adds. Synchrotron X-ray scattering measurements at sub-nanometre resolution performed by the scientists at KAUST corroborated the fact that the films were highly dense and contained very closely packed nanoparticles.

Low-cost photovoltaics on the horizon

“Most solar cells on the market today are made out of heavy crystalline materials,” explained Sargent, “but our work shows that light and versatile materials like CQDs could potentially become cost-competitive with these traditional technologies. Our results also pave the way for low-cost photovoltaics that could be fabricated on flexible substrates, for example, using roll-to-roll manufacturing (in the same way that newspapers are printed in mass quantities).”

The team is now looking at further reducing electron traps in CQD films for even higher efficiency. “It turns out that there are many organic and inorganic materials out there that might well be used in such hybrid passivation schemes,” added Sargent, “ so finding out how to reduce electron traps to a minimum would be good.”

The researchers say that they are also interested in using layers of different-sized quantum dots to make a multi-junction solar cell that could absorb over an even broader range of light wavelengths.

The current work is detailed in Nature Nanotechnology.

 

Major Breakthroughs in Solar Technology for 2013


QDOTS imagesCAKXSY1K 8Despite a tough market leading to widespread cost reductions and negative returns for many operators in the photovoltaic sector in 2012, solar technology nonetheless took major strides and achieved a number of landmark breakthroughs in key research areas.

 

In materials research, the North Carolina State University (NCSU) in Raleigh, North Carolina used cutting-edge nanotechnology to develop slimmer and more affordable solar cells.

The cells are comprised of sandwiched nanostructures which not only cut down on material usage and expenditures but also improve solar absorption and raise conversion efficiency.

As an added bonus, the manufacturing processes for the new technology are compatible with techniques currently employed throughout the industry for the production of thin-film solar cells.

In terms of government-funded initiatives, the National Renewable Energy Laboratory (NREL), a research arm of the US Department of Energy, teamed up with Natcore Technology to create the most absorbent solar cell ever devised, capable of capturing some 99.7 per cent of available sunlight.

 The new technology resulting from this collaborative effort between the government and private sectors could reduce the cost of solar cells by around two to three per cent while lifting energy output by up to 10 per cent. The black silicon used for the cells is also far cheaper than standard anti-reflection technologies.
nanotechnology-solar-cells-1A key area of research for 2012 was improved storage techniques for renewable energies, with scientists from Houston’s Rice University in Texas developing a remarkable “paintable” battery which can be applied to any tractable surface. The rechargeable battery opens a new vista of possibilities for the convenient storage of solar energy.

In the field of flexible thin-film cells, a joint undertaking between scientists from Canada and Saudi Arabia smashed the world record for solar efficiency, surpassing the ousted place holder by a staggering 37 per cent. The colloidal quantum dot (CQD) thin-film solar cell, developed by scientists from Canada’s University of Toronto and the King Abdullah University of Science & Technology in Saudi Arabia, achieved a world-record efficiency level of seven per cent via the application of a “hybrid passivation scheme.”

The new technology could potentially be applied to the cheap, mass manufacture of thin-film solar cells by using flexible substrates to “print” the devices in a process akin to that traditionally employed for the production of newspapers.paintable-battery-rice-university

 

 

‘Quantum dot’ solar cells offer bright future with reliable, low cost energy


London, July 30 (ANI): Researchers have made a breakthrough in the development of colloidal quantum dot (CQD) films, leading to the most efficient CQD solar cell ever.

Researchers from the University of Toronto (U of T) and King Abdullah University of Science and Technology (KAUST) created a solar cell out of inexpensive materials that was certified at a world-record 7.0 percent efficiency.

“Previously, quantum dot solar cells have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult,” said Dr. Susanna Thon, a lead co-author of the paper.

“Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces,” Dr. Thon stated.

Quantum dots are semiconductors only a few nanometres in size and can be used to harvest electricity from the entire solarspectrum – including both visible and invisible wavelengths. Unlike current slow and expensive semiconductor growth techniques, CQD films can be created quickly and at low cost, similar to paint or ink.

The researchers, led by U of T Engineering Professor Ted Sargent, paves the way for solar cells that can be fabricated on flexible substrates in the same way newspapers are rapidly printed in mass quantities.

The U of T cell represents a 37 percent increase in efficiency over the previous certified record. In order to improve efficiency, the researchers needed a way to both reduce the number of “traps” for electrons associated with poor surface quality while simultaneously ensuring their films were very dense to absorb as much light as possible. The solution was a so-called “hybrid passivation” scheme.

“By introducing small chlorine atoms immediately after synthesizing the dots, we’re able to patch the previously unreachable nooks and crannies that lead to electron traps. We follow that by using short organic linkers to bind quantum dots in the film closer together,” explained doctoral student and lead co-author Alex Ip.

Work led by Professor Aram Amassian of KAUST showed that the organic ligand exchange was necessary to achieve the densest film.

“The KAUST group used state-of-the-art synchrotron methods with sub-nanometer resolution to discern the structure of the films and prove that the hybrid passivation method led to the densest films with the closest-packed nanoparticles,” stated Professor Amassian.

The advance opens up many avenues for further research and improvement of device efficiencies, which could contribute to a bright future with reliable, low cost solar energy.

Their work featured in a letter published in Nature Nanotechnology. (ANI)

 

Why quantum dots can join every aspect of everyday life


 

Sheet of semiconductor crystals

Tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable properties that scientists think they will soon be used in everything from light bulbs to the design of ultra-efficient solar cells. Photograph: Science Photo Library

The properties of a material were once thought to be defined only by its chemical composition. But size matters too, especially for semiconductors. Make crystals of silicon small enough – less than 10 nanometres – and their tiny dimensions can start to dictate how the atoms behave and react in the presence of other things.

These tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable, novel properties that scientists think they will soon be used in everything from light bulbs to imaging of cancer cells or in the design of ultra-efficient solar cells.

Semiconductors such as silicon or indium arsenide are chosen to build electronic circuits because of the discrete energy levels at which they can give off electrons or photons. This makes them useful in building switches, transistors and other devices. It was once thought these energy levels – known as band gaps – were fixed. But shrinking the physical size of the semiconductor material to quantum-dot level seems able to change the band gaps, altering the wavelengths of light the material can emit or changing the energy it takes to change a material from an insulator to a conductor.

Instead of looking for brand new materials to build different devices, then, quantum dots make it possible to use a single type of semiconductor to produce a range of different characteristics. Researchers could tune dots made from silicon to emit a range of different colours in different situations, for example, instead of having to use a range of materials with different chemical
compositions.

“The main application for quantum dots at the moment is biological tagging of cells,” says Paul O’Brien, a professor of inorganic materials at the University of Manchester and co-founder of Nanoco Technologies a quantum dot manufacturer also based in Manchester. They are used in the same way as fluorescent dyes, to label agents, he says, but with the advantage that a single laser source can be used to illuminate many different tags each with a specific wavelength.

By attaching different types of quantum dots to proteins that target and attach to specific cell types in the body, these bits of semiconductor can be used by doctors to monitor different kinds of cells. When a laser is then directed on to tagged cells, doctors can see what colour they glow.

The ability to shine also makes quantum dots well suited to produce white light. Existing white bulbs based on low energy light emitting diode (LED) technology tend to produce a garish and bluish form of light that notoriously feels cold, says O’Brien. This is because these LEDs use a phosphor that produces an artificial white light that contains less red wavelengths than natural white light. By embedding quantum dots into a film that is placed over a bulb containing blue LEDs, it is possible to get a much warmer colour of white light. The blue light from the LED stimulates the quantum dots which, in turn, emit light in a range of colours. Provided you have chosen your dots carefully, these will combine to form white light.

The first of these quantum dot lights hit the market in 2010, a partnership between QD Vision, an MIT spinout in Lexington, Massachusetts, and Nexxus Lighting of Charlotte, North Carolina.

Backlights for laptops, tablets and mobile devices are next in line, and they should appear in products before the end of 2012 says VJ Sahi, head of corporate development at materials design company Nanosys of Palo Alto, California. Besides the colour advantages, quantum-dot-based backlights can be three times more efficient than traditional backlights.

Eventually, says Sahi, quantum dots will do more than just light up displays. The long-term aim is use them to create each red, green and blue sub-pixel that makes up a coloured display. This should produce much brighter colours and consume less power than LCD or even the latest state-of-the-art organic LED (OLED) displays. They should also have no problems with viewing angles, he adds.

The interesting properties of quantum dots come from the fact that they behave like tuning forks for photons, a result of a phenomenon called confinement. At less than 10 nanometres in size – about 50 atoms – they fall within the dimensions of a critical quantum characteristic of the material known as the exciton Bohr radius. The energy levels of electrons within the material’s atoms are constrained and, when a photon or electron hits an atom and excites it, the atom re-emits the energy as a photon of a very specific energy level.

Quantum dots also have another trick up their sleeve. Besides converting photons of one energy into photons of another, they can also be used to release electrons and create electrical currents: in other words they can be used to make solar cells. Arthur Nozik at the National Renewable Energy Laboratory in Boulder, Colorado, says that quantum-dot solar cells would be much more efficient at converting the energy from photons and therefore boost the amount of power they can produce.

Such applications are many years from becoming commercial reality. But they serve to demonstrate that no material technology stands still; sometimes all you have to do is cut it down to size.