Tag Archives: quantum dots

In this illustration, the Quantum Dot (QD) spectrometer device is printing QD filters — a key fabrication step. The dots are made by printing droplets. Image: MIT

Spectrometer is small enough to fit in your smartphone

MIT engineers demonstrated a working spectrometer that took a huge leap in scale from a huge, bulky lab gear to a portable piece of equipment that’s small enough to fit in a smartphone. Spectrometer are essential to research nowadays, employed in everything from physics, to biology, to chemistry. To design the spectrometer, the MIT team made use of tiny semiconductor nanoparticles called quantum dots. Having a portable spectrometer could prove to be extremely practical .You can use it to remotely diagnose diseases, detect pollution or food poisoning.

In this illustration, the Quantum Dot (QD) spectrometer device is printing QD filters — a key fabrication step.  The dots are made by printing droplets. Image: MIT

In this illustration, the Quantum Dot (QD) spectrometer device is printing QD filters — a key fabrication step. The dots are made by printing droplets. Image: MIT

The basic function of a spectrometer is to take in light, break it into its spectral components and digitize the signal as a function of wavelength. The information is then read by a computer and shown on a display. Raindrops split a beam of white sunlight into rays of colored light, bending the blueish ones more than the reddish ones to make the well-known arc in the sky. Rain, then, is a brilliant method for separating sunlight. Indeed, the earlier spectrometers consisted of prisms that separate light into its constituent wavelengths, while current models use optical equipment such as diffraction gratings to achieve the same effect. Even so, this kind of equipment is huge. The spectrometer developed at MIT is about the size of a quarter!

The researchers have quantum dots to thank for this achievement. Quantum dots are a type of nanocrystals that absorb light. These are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantum dots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. This is actually a good thing in this case. The quantum dots are made by mixing various  metals such as lead or cadmium with other elements including sulfur, selenium, or arsenic. By controlling the ratio between the materials, you get quantum dots with specific, unique properties.

Nowadays, quantum dots are heavily researched for use in solar panels or for TV displays, since they also fluoresce. While these applications are quite challenging at this stage, quantum dot light absorption is very well studied and as such any spectrometer that uses them can be expected to give out stable results.

The MIT researchers printed hundreds of quantum dots – each absorbing a specific wavelength of light – into a thin film and placed on top of a photodetector such as the charge-coupled devices (CCDs) found in cellphone cameras. An algorithm identifies the percentage of photons absorbed by each dot, then uses this info to compute the intensive and wavelength of the original beam of light. The more quantum dot materials there are, the more wavelengths can be covered and the higher resolution can be obtained. In this case, 200 quantum dots were deployed over a range of 300 nanometers. By adding even more dots, engineers could build a small spectromer that covers the whole range of wavelenghts.

“Using quantum dots for spectrometers is such a straightforward application compared to everything else that we’ve tried to do, and I think that’s very appealing,” says Moungi Bawendi, the Lester Wolfe Professor of Chemistry at MIT and the paper‘s senior author.

Previously, another team from the same MIT unveiled a handheld mass spectrometer. Coupled with this latest news, one might imagine scientists, doctors or hazard control officers using both optical and mass spectrometers in the field quite easily and reliably.

Graphene oxide before (left) and after (right) the new annealing treatment.The graphene sheet is represented by yellow carbon spheres, while the oxygens and hydrogens are represented as red and white spheres. Annealing causes oxygen atoms to form clusters, creating areas of pure graphene (as shown in the right image). This results in increased light absorption, improved conduction of electrons, and efficient light emission. (c) MIT

New graphene treatment may help the wonder material turn mainstream

Graphene, a 2-D array of carbon atoms arranged in a hexagon shape, is one of the most researched material today. We’ve written extensively before about its properties and uses, and indeed the future seems to belong to graphene where it’s sure to dominate the electronics industry. Before this can happen, however, graphene production and manipulation needs to become cheap and efficient. A new method developed at MIT requires much less energy to treat graphene, making it much cheaper, while providing a solid footing for the material to take-off in mainstream industry. Moreover, the resulting graphene sheets have been found to be better than those produced by current, expensive methods.

Treating graphene

 

Graphene oxide before (left) and after (right) the new annealing treatment.The graphene sheet is represented by yellow carbon spheres, while the oxygens and hydrogens are represented as red and white spheres. Annealing causes oxygen atoms to form clusters, creating areas of pure graphene (as shown in the right image). This results in increased light absorption, improved conduction of electrons, and efficient light emission.  (c) MIT

Graphene oxide before (left) and after (right) the new annealing treatment.The graphene sheet is represented by yellow carbon spheres, while the oxygens and hydrogens are represented as red and white spheres. Annealing causes oxygen atoms to form clusters, creating areas of pure graphene (as shown in the right image). This results in increased light absorption, improved conduction of electrons, and efficient light emission. (c) MIT

Graphene by itself lacks some key electrical properties to reach its full potential. Typically, scientists dope graphene sheets with oxygen atoms – this is done by treating the graphene with harsh chemicals at high temperatures (700 to 900 degrees Celsius), which gobbles up a lot of energy. Furthermore, the treatment leaves oxygen atoms unevenly distributed across the graphene sheets causing performance issues.

MIT professors Jeffrey Grossman and Angela Belcher, along their doctoral students, devised a new treating method for graphene that involves much lower temperatures – just 50 to 80 degrees Celsius. This low temperature approach arranges the oxygen atoms in clusters, leaving areas of pure graphene between them. This helps create a more orderly structure, leading to better electrical properties.

“We’ve been very interested in graphene, graphene oxide, and other two-dimensional materials for possible use in solar cells, thermoelectric devices, and water filtration, among a number of other applications,” says Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering.

A better way to make graphene

The orderly structure thus creates oxygen clusters that insulate current, while the graphene pure areas in between freely conduct. In fact, the researchers claim the electrical resistance dropped four to five orders of magnitude, all through a method that is more environmentally friendly since it uses much less energy and disposes of harsh chemicals extensively used by other methods.

Another plus is that the pure graphene regions essentially work as quantum dots which are great light absorbents.

“It produces a 38 percent improvement in the collection of photons,” Grossman says, compared to untreated graphene oxide, “which is a significant improvement that could be important for its use in a number of applications, such as solar cells.”

Graphene solar cells are being researched thoroughly, and the MIT method may provide a means to further increase efficiency.  Besides solar cells, the scientists envision the method becoming very useful for producing graphene destined for thermoelectric devices, solar thermal fuels, and desalination filters.

“It is surprising that new processing regimes are still being identified,” says Mark Hersam, director of Northwestern University’s Materials Research Center, who was not involved in this work. This research “shows that there is still much work to be done on fully understanding and exploiting graphene,” he says.

The method was described in a paper published in the journal Nature Chemistry

The quantum dot device structure shown with a transmission electron microscopy (TEM) image of a cross-section of a real device.

Forget incandescent light bulbs, make way for quantum dot LEDs

The quantum dot device structure shown with a transmission electron microscopy (TEM) image of a cross-section of a real device.

The quantum dot device structure shown with a transmission electron microscopy (TEM) image of a cross-section of a real device.

Capable of illuminating in a wide array of pure colors and operating at high efficiency, quantum dot LEDs are set to become the future’s foremost illuminating medium. However, at this time, these fantastic quantum dot light emitting diodes are limited by a physical effect which triggers after a certain photon barrier is crossed, becoming highly inefficient thereafter. This has made them commercially nonviable, however recent work by the Nanotechnology and Advanced Spectroscopy team at Los Alamos National Laboratory could provide a working solution that might usher in a new age of lighting.

Quantum dots are nano-sized semiconductor particles whose emission color can be tuned by simply changing their dimensions. They feature near-unity emission quantum yields and narrow emission bands, which result in excellent color purity.

“QD-LEDs can potentially provide many advantages over standard lighting technologies, such as incandescent bulbs, especially in the areas of efficiency, operating lifetime and the color quality of the emitted light,” said Victor Klimov of Los Alamos.

Chances having that you’re using at least one incandescent light bulb in your home or office right now. The truth is these should be called incandescent radiators, since these only generate 10% light from provided power, while the rest of 90% is lost as heat! Using LEDs, however, electricity is converted directly into light resulting in a much more efficient process.

QD-LEDs are particularly attractive due to their spectrally narrow, tunable emission, and ease of processing. This is why scientists in solar cell research are so seduced by them. In the past few years massive leaps forward in the QD-LED research have been made, however one major obstacle hindered their market introduction. At high current QD-LEDs perform poorly, a problem known as ‘droop’. This means you can only use them at low currents or low brightness, obviously not very attractive for most people who need vivid light.

The Los Alamos scientists found that the ‘droop’ is caused by an effect called Auger recombination. In this process, instead of being emitted as a photon, the energy from recombination of an excited electron and hole is transferred to the excess charge and subsequently dissipated as heat. In their paper, published in the journal  Nature Communications, the researchers describe the mechanics efficiency losses in QD-LEDs and propose two possible solutions for circumventing the problem.

The first approach is to reduce the efficiency of Auger recombination itself, which can be done by incorporating a thin layer of cadmium selenide sulfide alloy at the core/shell interface of each quantum dot. A second strategy tackles charge imbalance by better controlling the flow of extra electrons into the dots themselves. This can be accomplished by coating each dot in a thin layer of zinc cadmium sulfide, which selectively impedes electron injection.

“This fine tuning of electron and hole injection currents helps maintain the dots in a charge-neutral state and thus prevents activation of Auger recombination,”  Jeffrey Pietryga, a chemist in the nanotech team.

quantum-dots-cell

Quantum dots with nanowires yield greater solar cell efficiency

For some years now, scientists have been exploring the use of quantum dots as the basis for a novel type of solar cell. The advantages over conventional solar photovoltaic cells are numerous, minus one aspect: efficiency, which is actually the most important one. A new technique developed at MIT labs that uses quantum dots in conjunction with nanowires has lead to a significant increase in efficiency, lending hope that sometime they might become viable enough to enter mass production and become a sound alternative to current solar energy capturing technology.

A quantum dot is a particle of matter so small that the addition or removal of an electron changes its properties in some useful way. All atom are, of course, quantum dots, but multi-molecular combinations can have this characteristic as well. For some time, quantum dots have been the subject of research in the field of solar energy as scientists have been interested in using them as the  absorbing photovoltaic material, instead of bulk materials like the most commonly used one, silicon.

What makes quantum dots extremely appealing is their wide range energy bandgaps which allows them to  absorb light over a much wider range of wavelengths than conventional devices. Moreover, if proven efficient quantum dots-based solar cells would be at a significant logistical and economic advantage over conventional solutions since they can be manufactured at room temperature, saving a lot of energy (translating in less money spent and less harm done to the environment) that would normally be traded off for the high temperature processing of silicon and other PV materials. Also, quantum dots can be made out of just about any kind of material that do not require extensive purification, as silicon does. Quantum dot PV cells can also be applied on virtually any kind of surface, be them flexible like clothing or rigid like roof tops.  A while ago we mentioned a nifty quantum dot paint that could be used to turn any kind of surface into a solar cell that works.

A new generation of solar cells?

quantum-dots-cell

Vertical arrays of ZnO nanowires can decouple light absorption from carrier collection in PbS quantum dot solar cells and increase power conversion efficiencies by 35%. The resulting ordered bulk heterojunction devices achieve short-circuit current densities in excess of 20 mA cm−2 and efficiencies of up to 4.9%. (c) MIT

If quantum dot PV cells are so awesome, why aren’t we hearing more about them? Why aren’t they everywhere? The short answer is because they’re really, really poor at transferring energy. Efficiency range somewhere in the range of 2-3%, at best.

This is due to a sort of paradoxical structure solar cells need to have in order to be efficient, namely: the absorbing layer needs to be thin enough to allow charged particles absorbed from solar energy to pass directly through wires that carry the current away, but it needs to be thick enough to absorb light efficiently. So you get better in one place, only to find you’ve worsen the other. It’s because of this discrepancy that researchers usually turn to tradeoffs in designing their devices.

Joel Jean, a doctoral student in MIT’s Department of Electrical Engineering and Computer Science (EECS), is the lead author of recently published paper in the journal Advanced Materials where a novel technique is used to boast quantum dots PV cells efficiency by employing nanowires – extremely thin wires with a fantastic length to width ratio which conduct electricity through wires that are 2000 times thinner than a strand of hair.

These nanowires are conductive enough to extract charges easily, but long enough to provide the depth needed for light absorption, Jean says. Using a bottom-up growth process to grow these nanowires and infiltrating them with lead-sulfide quantum dots produces a 50 percent boost in the current generated by the solar cell, and a 35 percent increase in overall efficiency, Jean says.

“If you shine light along the length of the nanowires, you get the advantage of depth,” he says. But also, “you decouple light absorption and charge carrier extraction, since the electrons can hop sideways onto a nearby nanowire and be collected.”

Tests of the device have rendered efficiency in the order of 5 percent, among the greatest ever recorded for quantum dots PVs. The researchers hope that with a little more effort and tuning they could increase this to 10 percent, which is currently the minimal efficiency threshold with which commercial solar cells are produced today. Further research will, among other things, explore using longer nanowires to make thicker films, and also work on better controlling the spacing of the nanowires to improve the infiltration of quantum dots between them.

Expect to hear more and more from quantum dots PV cells, though. Much more…

As cells squeeze through a narrow channel, tiny holes open in their membranes, allowing large molecules such as RNA to pass through. Image: Armon Sharei and Emily Jackson

New method allows large molecules to get squeezed through cell membranes

A group of researchers at MIT have devised a new method for infiltrating cells with large molecules such as nanoparticles or proteins that is a lot more non-intrusive and doesn’t damage the cell. Imaging target cells or growing more stable stem cells might thus be possible with this method.

As cells squeeze through a narrow channel, tiny holes open in their membranes, allowing large molecules such as RNA to pass through. Image: Armon Sharei and Emily Jackson

As cells squeeze through a narrow channel, tiny holes open in their membranes, allowing large molecules such as RNA to pass through.
Image: Armon Sharei and Emily Jackson

Every cell has a membrane, which is put to great use as it protects the cell’s inner environment by regulating what gets in and what gets out. Typically, you don’t want foreign molecules entering your cells, but sometimes you do. Various methods have been employed to breach cell membranes and introduce other bodies, however these tend to be intrusive and sometimes can lead to the damaging and even destruction of the cell.

The MIT method of introducing large molecules in cell is a lot safer and efficient and implies squeezing the cell through a narrow construction just enough for tiny, yet temporary, gaps to surface. Prior to squeezing the cell, large molecules – be it RNA, proteins or nanoparticles – are tasked to float outside cell, such that when the holes pop these slide through the membrane instantly.

Through this technique the MIT researchers were able to deliver reprogramming proteins which turned the target cells into pluripotent stem cells – notoriously difficult to generate efficiently – with a success rate 10 to 100 times better than any other existing method. A simply massive advancement. Also, they’ve also tested the method with other large molecules like special nanoparticles, like carbon nanotubes or quantum dots, to image cells and thus monitor their activity.

“It’s very useful to be able to get large molecules into cells. We thought it might be interesting if you could have a relatively simple system that could deliver many different compounds,” says Klavs Jensen, the Warren K. Lewis Professor of Chemical Engineering, professor of materials science and engineering, and a senior author of a paper describing the new device in this week’s issue of the Proceedings of the National Academy of Sciences.

The team’s fantastic research builds upon previous work, when Jensen and Robert Langer, the David H. Koch Institute Professor at MIT and also a study lead author, forced molecules into cells as they flowed through a microfluidic device. The process was slow and not very effective, but it was during this time that the researchers learned that if you squeeze a cell just right now, tiny holes will appear – pure windows of opportunity.

Capitalizing on this, the scientists then proceeded to adjust their set-up and devised some rectangular microfluidic chips, no larger than a quarter, fitted with 40 to 70 parallel channels.  Cells are suspended in a solution with the material to be delivered and flowed through the channel at high speed — about one meter per second. Halfway through the channel, the cells pass through a constriction about 30 to 80 percent smaller than the cells’ diameter. The cells don’t suffer any irreparable damage, and they maintain their normal functions after the treatment.

“This appears to be a very broadly applicable approach for loading a diversity of different compounds into a diversity of different cells,” says Mark Prausnitz, a professor of chemical and biomolecular engineering at Georgia Tech, who was not part of the research team. “It’s a really nice example of taking devices from the world of engineering and microelectronics and using them in quite different ways to solve problems in medicine that could have really great impact.”

 

source: MIT

No, this is not a DJ spinning your favorite groove, although there's a laser show. The photo depicts Brown University physicist Cuong Dang directing a green laser onto a film of colloidal quantum dots, which reemit RGB light. All using a single laser.

First single RGB laser devised using quantum dots

No, this is not a DJ spinning your favorite groove, although there's a laser show. The photo depicts Brown University physicist Cuong Dang directing a green laser onto a film of colloidal quantum dots, which reemit RGB light. All using a single laser.

No, this is not a DJ spinning your favorite groove, although there's a laser show. The photo depicts Brown University physicist Cuong Dang directing a green laser onto a film of colloidal quantum dots, which reemit RGB light. All using a single laser.

Most digital devices today, like displays or blue-ray disks, use lasers which emit the colors red, green and blue, which when combined can render any color in the visible spectrum of light. However, current technology requires a separate laser for each color, since they produce monochromatic light. A team of researchers at Brown University has successfully managed to produce a RGB laser which emits visible light of varied colors using a single device, by employing nanoscale single crystals called colloidal  quantum dots.

“Today in order to create a laser display with arbitrary colors, from white to shades of pink or teal, you’d need these three separate material systems to come together in the form of three distinct lasers that in no way shape or form would have anything in common,” said Arto Nurmikko, professor of engineering at Brown University and senior author of a paper describing the innovation in the journal Nature Nanotechnology. “Now enter a class of materials called semiconductor quantum dots.”

Colloidal quantum dots (CQDs) produce light by quantum excitation, which allow for precise control and whose size determine the emitted color. By overlaying many thin quantum dots films of variable size, the researchers researchers observed broad-spectrum emission.

The idea of leveraging the properties of the thin film isn’t new, but past attempts to use CQDs in semiconductor lasers have failed because the necessary energy tends to wind up as heat instead of light. The Brown University researchers chose to use a different semi-conducting alloy, made of of zinc, cadmium, sulfur and proprietary organic molecular glue. The latter element is highly important because it reduces an excited electronic state requirement for lasing and protects the nanocrystals from a kind of crosstalk that makes it hard to produce laser light.

This alloy coating allowed the researchers to build a device which directed the excitations within the material to make light rather than heat the primary output. Thus, the scientists concluded the coated pyramids require 10 times less pulsed energy or 1,000 times less power to produce laser light than previous attempts at the technology.

The team of researchers demonstrated their setup in an experiment which used a monochromatic laser directed onto the thin coated layers. The short laser pulses stimulated the three different CQDs to re-emit light of red, green, and blue wavelengths, after applied filtering.

“We have managed to show that it’s possible to create not only light, but laser light,” Nurmikko said. “In principle, we now have some benefits: using the same chemistry for all colors, producing lasers in a very inexpensive way, relatively speaking, and the ability to apply them to all kinds of surfaces regardless of shape. That makes possible all kinds of device configurations for the future.”

The team’s prototypes are the first lasers of their kind, however the researchers warrant that the solution is far from practical for use in commercial products. The findings, described in a paper published in the journal Nature Nanotechnology, represent a milestone in the march towards a single-material multi-wavelength laser, which might provide important technological advances. Full color holograms? Well just have to wait and see…

source: Ars Technica 

Solar paint applied on a transparent surface. ©2011 American Chemical Society

Solar paint promises to turn any surface into a solar cell

Researchers have successfully managed to create a “solar paint” made out of quantum dots, which exhibits similar properties to multifilm solar cell architectures. The later are sophisticated, expensive and require a lot of time to deploy; the paint can be easily applied to basically any surface, like a house’s roof, and prepare it to easily generate photocurrent.

 

Solar paint applied on a transparent surface. ©2011 American Chemical Society

Solar paint applied on a transparent surface. ©2011 American Chemical Society

Quantum dots, simply put, are crystal semi-conductors that display unique optical and electrical properties that are different in character to those of the corresponding bulk material – its discrete properties translates into quantum behavior. These tiny crystals, which range from 2 to 10 nanometers in diameter, were added to dye mixtures. The researchers used three separate quantum dots dyes, based on CdS, CdSe, and TiO2, respectively. The later is used to make most of today’s commercial dyes, however instead of painting a fence in a certain color, the quantum dots based paint will allow it to exhibit optical and electrical properties.

“Quantum dots are semiconductor nanocrystals which exhibit size-dependent optical and electronic properties,” said Prashant V. Kamat of the Radiation Laboratory and Department of Chemistry and Biochemistry at the University of Notre Dame in Indiana, one of the lead authors of the study. “In a quantum dot sensitized solar cell, the excitation of semiconductor quantum dot or semiconductor nanocrystal is followed by electron injection into TiO2 nanoparticles. These electrons are then transferred to the collecting electrode surface to generate photocurrent. The holes that remain in the semiconductor quantum dot are removed by a hole conductor or redox couple and are transported to a counter electrode.”

Quantum dots have been used in transistors, lasers, LEDs or solar cells. Solar cells made with quantum dots require a number of time intensive steps for layering the tiny crystals, and is typically considered an expensive technology. In contrast, the paint, researchers say, can be applied and dried like any other conventional paint. Of course, its efficiency is quite low – only 1%, compared to at least 5% in the case of multi-layered quantum dot solar cells.

On a related note, researchers from the US Department of Energy’s National Renewable Energy Laboratory (NREL) have for the first time built a solar cell, based on quantum dots, with an external quantum efficiency of over 100 percent.

“If we can optimize the paint preparation, it should be possible for anyone to open a bottle (or a can in the long run) and apply it to a conducting surface,” he said. “This will decrease the variability between lab to lab or person to person as one encounters in a multi-step process. Having fewer fabrication steps and ambient preparative conditions should provide an economically viable transformative technology.”

Scientists are now working on the next critical step, improving efficiency (they hope to do this by using other semiconductor materials for higher absorption range) and the other two components of a solar cell array; a hole conducting layer and a counter electrode network.

I think this kind of technology is amazing, although its performances aren’t competitive enough yet, however the researchers hoping to make it commercially available in the near future. Perfect for DIY projects or large-scale, cheap solar energy programs; whole neighborhoods filled with solar paint.

“The goal is to prepare a solar paint that has long shelf life,” Kamat said. “In our laboratories we have tested the performance for a few days to a week, and we find it stable as long as it is stored in the dark. Additional tests are underway to investigate long-term stability of paints with different compositions.”

physorg

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