DOE: One small change makes Quantum Dot solar cells more efficient



The quest for more efficient solar cells has led to the search of new materials. For years, scientists have explored using tiny drops of designer materials, called quantum dots.




Now, we know that adding small amounts of manganese decreases the ability of quantum dots to absorb light but increases the current produced by an average of 300%. Under certain conditions, the current produced increased by 700%.

The enhancement is due to the faster rate that the electrons move from the quantum dot to the balance of the solar cell (what the scientists call the electron tunneling rate) in the presence of the manganese atoms at the interface.

Importantly, this observation is confirmed by theory, opening up possibilities for applying this approach to other systems (Applied Physics Letters, “Giant photocurrent enhancement by transition metal doping in quantum dot sensitized solar cells”).

The power conversion efficiency of quantum dot solar cells has reached about 12%. However, the overall efficiency of quantum dot solar cells is relatively low compared to photovoltaic systems in use today that are based on silicon. In addition, quantum dot solar cells are not as efficient as emerging next-generation solar cells.

The results obtained in this work point to a surprisingly straightforward alternative route. Scientists can significantly improve the performance of this family of solar cells by adding small amounts of alternate metals.

In the quest to replace more traditional solar materials, such as silicon, with more efficient and high-performing options, scientists have been studying quantum dot solar cells as an alternative to harvest sunlight for conversion to electricity.

In this solar cell design, quantum dots are used as the material that absorbs sunlight and converts it to electricity. Quantum dots are very small, nanometer-sized, particles, whose solar conversion properties, in this case a characteristic gap in the energy levels of the electrons called the “bandgap,” are tunable by changing the size or chemical composition.

This is in contrast to bulk materials whose bandgap is fixed by the chemical composition or choice of material(s) alone. This size dependence of bandgap makes quantum dots attractive for multi-junction solar cells, whose efficiency is enhanced by using a variety of materials that absorb different parts of the “rainbow” of wavelengths of light found in the solar spectrum.

This research team discovered that adding small amounts of the transition metal manganese (Mn), or “doping,” resulted in a huge enhancement in the efficiency rate of changing light to electricity for lead sulfide (PbS) quantum dot sensitized solar cells.

Relatively small concentrations of Mn (4 atomic percent) cause the current to increase by an average of 300% with a maximum increase of up to 700%.

Moreover, the mechanism by which this occurs cannot be explained by the light absorption alone because both the experimental and theoretical absorption spectra demonstrate a several times decrease in the absorption coefficient on the addition of Mn.

The team proposes that the dramatic increase is due to a mechanism of increased electron tunneling through the atom pairs at the quantum dot interface with the next layer of the solar cell.

The team used ab initio calculations, which is a computational approach that can describe new phenomena without the need to fit or extrapolate experimental data, to confirm this mechanism.

While typical doping approaches focus on improving exciton lifetime and light absorption channels, results obtained in this study provide an alternative route for significant improvement on the efficiency of quantum dot sensitized solar cells.

Source: U.S. Department of Energy, Office of Science

Squashed quantum dots solve a multi-faceted problem – Optoelectronics


Squashed QD cqds.jpg

Oblate-shaped quantum dots

Quantum dots have revolutionized the field of optoelectronics due to their atom-like electronic structure. However, the prospect of colloidal quantum-dot lasers has long been deemed impractical due to the high energies required to induce optical gain. But recent work published in Nature and led by Ted Sargent of the University of Toronto shows that the lasing threshold in cadmium selenide (CdSe)/cadmium sulphide (CdS) core-shell colloidal quantum dots can be lowered by squashing the CdSe core via a clever ligand exchange process.

To instigate optical gain in a semiconductor laser, the difference between the lowest electron level and the highest hole level must be wider than the band gap so that the light emitted when they recombine can stimulate emission in neighbouring nanocrystals. Colloidal quantum dots (CQDs) then, should make ideal candidates for lasing applications, as their atom-like electronic structure means that the electron and hole energy levels are easier to separate.Squashed QD cqds

In practice, however, the energies required to trigger optical gain in CQDs are so high that they can heat up to the point of burning. While electrons tend to occupy one energy state upon excitation, the hole that they leave behind in the valence band can populate one of eight closely spaced states. This degeneracy pushes the hole Fermi level into the band gap and increases the amount of energy required to instigate optical gain.

To overcome this issue, the researchers took advantage of the fact that CdS imposes a strain on CdSe due to a slight lattice mismatch of 3.9%. By growing an asymmetric CdS shell around a “squashed” oblate CdSe core, they were able to induce a biaxial strain that affected the heavy and light holes of the valence band to different extents, thus lifting the degeneracy.

To produce these asymmetric CQDs the group invented a technique called facet-selective epitaxy, making use of ligands that interact differently with the surfaces of CdSe. One of these ligands, trioctylphosphine sulphide, or TOPS, binds weakly to the (0001) facet of CdSe and not at all to the (0001), while octanethiol interacts similarly with all CdSe surfaces. Therefore, by growing CdS on the (0001) facet with TOPS and then replacing with octanethiol to stimulate epitaxial growth, oblate-shaped CQDs could be made throughout the entire particle ensemble with remarkable uniformity.

The resulting lasers had an unprecedentedly high performance, exhibiting a low lasing threshold of 6.4–8.4 kW cm–2, a seven-fold reduction compared with previous attempts. They also emitted light over a narrow energy range of just 36 meV. Both of these properties can be attributed to the enhanced splitting of the valence band levels that arises due to the oblate CQD shape.

The international team of researchers has certainly proved that continuous-wave CQD lasers are possible, yet there are still some obstacles to be overcome before they are seen on the market. Most importantly, the next step will be exciting the CQDs via electrical rather than optical means, as in standard commercial lasers. Nevertheless, facet-selective epitaxy opens up a whole host of other CQD materials for lasing applications and beyond.

NREL & Colorado School of Mines Researchers Capture Excess Photon Energy to Produce Solar Fuels



Photo shows a lead sulfide quantum dot solar cell. A lead sulfide quantum dot solar cell developed by researchers at NREL. Photo by Dennis Schroeder.




Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a proof-of-principle photoelectrochemical cell capable of capturing excess photon energy normally lost to generating heat.


Using quantum dots (QD) and a process called Multiple Exciton Generation (MEG), the NREL researchers
were able to push the peak external quantum efficiency for hydrogen generation to 114 percent.


The advancement could significantly boost the production of hydrogen from sunlight by using the cell to split water at a higher efficiency and lower cost than current photoelectrochemical approaches.

Details of the research are outlined in the Nature Energy paper Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%, co-authored by Matthew Beard, Yong Yan, Ryan Crisp, Jing Gu, Boris Chernomordik, Gregory Pach, Ashley Marshall, and John Turner.

All are from NREL; Crisp also is affiliated with the Colorado School of Mines, and Pach and Marshall are affiliated with the University of Colorado, Boulder.




Beard and other NREL scientists in 2011 published a paper in Science that showed for the first time how MEG allowed a solar cell to exceed 100 percent quantum efficiency by producing more electrons in the electrical current than the amount of photons entering the solar cell.




“The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current,” Beard said.

“We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds.”

The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption bandedge lost to heat.

The MEG process takes advantages of the additional photon energy to generate more electrons and thus additional chemical or electrical potential, rather than generating heat. QDs, which are spherical semiconductor nanocrystals (2-10 nm in diameter), enhance the MEG process.




In current report, the multiple electrons, or charge carriers, that are generated through the MEG process within the QDs are captured and stored within the chemical bonds of a H2 molecule.

NREL researchers devised a cell based upon a lead sulfide (PbS) QD photoanode. The photoanode involves a layer of PbS quantum dots deposited on top of a titanium dioxide/fluorine-doped tin oxide dielectric stack.

The chemical reaction driven by the extra electrons demonstrated a new direction in exploring high-efficiency approaches for solar fuels.

Funds for the research came from the Department of Energy’s Office of Science.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

MIT: Light-emitting particles (quantum dots) open new window for biological imaging


QD Bio Image V images

‘Quantum dots’ that emit infrared light enable highly detailed images of internal body structures

For certain frequencies of short-wave infrared light, most biological tissues are nearly as transparent as glass. Now, researchers have made tiny particles that can be injected into the body, where they emit those penetrating frequencies. The advance may provide a new way of making detailed images of internal body structures such as fine networks of blood vessels.

The new findings, based on the use of light-emitting particles called quantum dots, is described in a paper in the journal Nature Biomedical Engineering, by MIT research scientist Oliver Bruns, recent graduate Thomas Bischof PhD ’15, professor of chemistry Moungi Bawendi, and 21 others.

Near-infrared imaging for research on biological tissues, with wavelengths between 700 and 900 nanometers (billionths of a meter), is widely used, but wavelengths of around 1,000 to 2,000 nanometers have the potential to provide even better results, because body tissues are more transparent to that light. “We knew that this imaging mode would be better” than existing methods, Bruns explains, “but we were lacking high-quality emitters” — that is, light-emitting materials that could produce these precise wavelengths.

QD bio Image II imagesLight-emitting particles have been a specialty of Bawendi, the Lester Wolf Professor of Chemistry, whose lab has over the years developed new ways of making quantum dots. These nanocrystals, made of semiconductor materials, emit light whose frequency can be precisely tuned by controlling the exact size and composition of the particles.

The key was to develop versions of these quantum dots whose emissions matched the desired short-wave infrared frequencies and were bright enough to then be easily detected through the surrounding skin and muscle tissues. The team succeeded in making particles that are “orders of magnitude better than previous materials, and that allow unprecedented detail in biological imaging,” Bruns says. The synthesis of these new particles was initially described in a paper by graduate student Daniel Franke and others from the Bawendi group in Nature Communications last year.

The quantum dots the team produced are so bright that their emissions can be captured with very short exposure times, he says. This makes it possible to produce not just single images but video that captures details of motion, such as the flow of blood, making it possible to distinguish between veins and arteries.

QD Bio Image IV GAAlso Read About

Graphene Quantum Dots Expand Role In Cancer Treatment And Bio-Imaging

 

 

The new light-emitting particles are also the first that are bright enough to allow imaging of internal organs in mice that are awake and moving, as opposed to previous methods that required them to be anesthetized, Bruns says. Initial applications would be for preclinical research in animals, as the compounds contain some materials that are unlikely to be approved for use in humans. The researchers are also working on developing versions that would be safer for humans.QD Bio Image III 4260773298_1497232bef

 

The method also relies on the use of a newly developed camera that is highly sensitive to this particular range of short-wave infrared light. The camera is a commercially developed product, Bruns says, but his team was the first customer for the camera’s specialized detector, made of indium-gallium-arsenide. Though this camera was developed for research purposes, these frequencies of infrared light are also used as a way of seeing through fog or smoke.

Not only can the new method determine the direction of blood flow, Bruns says, it is detailed enough to track individual blood cells within that flow. “We can track the flow in each and every capillary, at super high speed,” he says. “We can get a quantitative measure of flow, and we can do such flow measurements at very high resolution, over large areas.”

Such imaging could potentially be used, for example, to study how the blood flow pattern in a tumor changes as the tumor develops, which might lead to new ways of monitoring disease progression or responsiveness to a drug treatment. “This could give a good indication of how treatments are working that was not possible before,” he says.

###

The team included members from MIT’s departments of Chemistry, Chemical Engineering, Biological Engineering, and Mechanical Engineering, as well as from Harvard Medical School, the Harvard T.H. Chan School of Public Health, Raytheon Vision Systems, and University Medical Center in Hamburg, Germany. The work was supported by the National Institutes of Health, the National Cancer Institute, the National Foundation for Cancer Research, the Warshaw Institute for Pancreatic Cancer Research, the Massachusetts General Hospital Executive Committee on Research, the Army Research Office through the Institute for Soldier Nanotechnologies at MIT, the U.S. Department of Defense, and the National Science Foundation.

Additional background

ARCHIVE: A new contrast agent for MRI http://news.mit.edu/2017/iron-oxide-nanoparticles-contrast-agent-mri-0214

ARCHIVE: A new eye on the middle ear http://news.mit.edu/2016/shortwave-infrared-instrument-ear-infection-0822

ARCHIVE: Chemists design a quantum-dot spectrometer http://news.mit.edu/2015/quantum-dot-spectrometer-smartphone-0701

ARCHIVE: Running the color gamut http://news.mit.edu/2014/startup-quantum-dot-tv-displays-1119

ARCHIVE: Fine-tuning emissions from quantum dots http://news.mit.edu/2013/fine-tuning-emissions-from-quantum-dots-0602

IBM Scientists: Quantum transport goes ballistic ~ ‘Q & A’ with NIST


 

IBM QT download

IBM scientists have shot an electron through an III-V semiconductor nanowire integrated on silicon for the first time (Nano Letters, “Ballistic One-Dimensional InAs Nanowire Cross-Junction Interconnects”). This achievement will form the basis for sophisticated quantum wire devices for future integrated circuits used in advanced powerful computational systems. (A Q&A with NIST)

NIST: The title of your paper is Ballistic one-dimensional InAs nanowire cross-junction interconnects. When I read “ballistic” rather large missiles come to mind, but here you are doing this at the nanoscale. Can you talk about the challenges this presents?
Johannes Gooth (JG): Yes, this is very similar, but of course at a much different scale. Electrons are fired from one contact electrode and fly through the nanowire without being scattered until they hit the opposed electrode. The nanowire acts as a perfect guide for electrons, such that the full quantum information of this electron (energy, momentum, spin) can be transferred without losses. 

Quantum Trnp goes Ballistic id46328_1IBM scientist Johannes Gooth is focused on nanoscale electronics and quantum physics.

 

We can now do this in cross-junctions, which allows us to build up electron pipe networks, where quantum information can perfectly be transmitted. The challenge is to fabricate a geometrically very well defined material with no scatterers inside on the nano scale. The template-assisted selective epitaxy or TASE process, which was developed here at the IBM Zurich Lab by my colleagues, makes this possible for the first time.
NIST: How does this research compare to other activities underway elsewhere?
JG: Most importantly, compared to optical and superconducting quantum applications the technique is scalable and compatible with standard electronics and CMOS processes.
NIST: What role do you see for quantum transport as we look to build a universal quantum computer?
JG: I see quantum transport as an essential piece. If you want to exercise the full power of quantum information technology, you need to connect everything ballistic: a quantum system that is fully ballistically (quantum) connected has an exponentially larger computational state space compared to classically connected systems.
Also, as stated above, the electronics are scalable. Moreover, combining our nanowire structures with superconductors allows for topological protected quantum computing, which enables fault tolerant computation. These are major advantages compared to other techniques.IBM QT II 51XVJRfhbNL._SX348_BO1,204,203,200_
NIST: How easily can this be manufactured using existing processes and what’s the next step?
JG: This is a major advantage of our technique because our devices are fully integrated into existing CMOS processes and technology.
NIST: What’s next for your research?
JG: The next steps will be the functionalization of the crosses, by means of attaching electronic quantum computational parts. We will start to build superconducting/nanowire hybrid devices for Majorana braiding, and attach quantum dots.
Source: NIST

 

Quantum Dots ~ “On the Move” ~ Illuminating Applications for photovoltaic cells, computers and drug delivery


The quantum dots used by the researchers are particles of semi-conducting material just a few nanometres wide, and are the subject of great interest because of their potential for use in photovoltaic cells or computers.

“The great thing about these particles is that they absorb light and emit it in a different colour,” explains research leader Lukas Kapitein. “We use that characteristic to follow their movements through the cell with a microscope.”

But to do so, the quantum dots had to be inserted into the cell. Most current techniques result in dots that are inside microscopic vesicles surrounded by a membrane, but this prevents them from moving freely.

However, the researchers succeeded directly delivering the particles into cultured cells by applying a strong electromagnetic field that created transient openings in the cell membrane.

In their article (“Probing cytoskeletal modulation of passive and active intracellular dynamics using nanobody-functionalized quantum dots”), they describe how this electroporation process allowed them to insert the quantum dots inside the cell.

The various transport processes that can be Sstudied using quantum dots

The various transport processes that can be Sstudied using quantum dots. Cyan: rapid diffusion. Red: slow diffusion in an actin network. Green: active transport by motor proteins. (Image: Anna Vinokurova)

Extremely bright

Once inserted, the quantum dots begin to move under the influence of diffusion. Kapitein: “Since Einstein, we have known that the movement of visible particles can provide information about the characteristics of the solution in which they move.

“Previous research has shown that particles move fairly slowly inside the cell, which indicates that the cytoplasm is a viscous fluid. But because our particles are extremely bright, we could film them at high speed, and we observed that many particles also make much faster movements that had been invisible until now.

“We recorded the movements at 400 frames per minute, more than 10 times faster than normal video. At that measurement speed, we observed that some quantum dots do in fact move very slowly, but others can be very fast.”

Kapitein is especially interested in the spatial distribution between the slow and fast quantum dots: at the edges of the cell, the fluid seems to be very viscous, but deeper in the cell he observed much faster particles.

Kapitein: “We have shown that the slow movement occurs because the particles are caught in a dynamic network of protein tubules called actin filaments, which are more common near the cell membrane. So the particles have to move through the holes in that network.”

Motor proteins

In addition to studying this passive transport process, the researchers have developed a technique for actively moving the quantum dots by binding them to a variety of specific motor proteins. 
These motor proteins move along microtubuli, the other filaments in the cytoskeleton, and are responsible for transport within the cell.

This allowed them to study how this transport is influenced by the dense layout of the actin network near the cell membrane. They observed that this differs for different types of motor protein, because they move along different types of microtubuli.

Kapitein: “Active and passive transport are both very important for the functioning of the cell, so several different physics models have been proposed for transport within the cell. Our results show that such physical models must take the spatial variations in the cellular composition into consideration as well.”

Source: Utrecht University

Simultaneous detection of multiple spin states in a single quantum dot – The Promise of Quantum Computing


Quantum dots are very small particles that exhibit luminescence and electronic properties different from those of their bulk materials. As a result, they are attractive for use in solar cells, optoelectronics, and quantum computing. Quantum computing involves applying a small voltage to quantum dots to regulate their electron spin state, thus encoding information.

While traditional computing is based on a binary information system, electron spin states in quantum dots can display further degrees of freedom because of the possibility of superposition of both states at the same time. This feature could increase the density of encoded information.

A scanning electron microscope image of a quantum dot experimental setup


A scanning electron microscope image of the quantum dot used in this research. We formed the quantum dot by applying voltage to surface gate electrodes. Electron spin states can be read out by measuring the electric current flowing nearby the dot (white arrow). (Image: Osaka University)

Readout of the electron spin of quantum dots is necessary to realize quantum computing. Single-shot spin readout has been used to detect spin-dependent single-electron tunneling events in real time. The performance of quantum computing could be improved considerably by single-shot readout of multiple spin states.

A Japanese research collaboration based at Osaka University has now achieved the first successful detection of multiple spin states through single-shot readout of three two-electron spin states of a single quantum dot. They reported their findings in Physical Review Letters (“Single-Shot Ternary Readout of Two-Electron Spin States in a Quantum Dot Using Spin Filtering by Quantum Hall Edge States”).

Comparison Between Binary Spin Readout and Ternary Spin Readout

This is a comparison between binary spin readout and ternary spin readout. (Image: Osaka University)

To read out multiple spin states simultaneously, the researchers used a quantum point contact charge sensor positioned near a gallium arsenide quantum dot. The change in current of the charge sensor depended on the spin state of the quantum dot and was used to distinguish between singlet and two types of triplet spin states.

“We obtained single-shot ternary readout of two-electron spin states using edge-state spin filtering and the orbital effect,” study first author Haruki Kiyama says.

That is, the rate of tunneling between the quantum dot and electron reservoir depended on both the spin state of the electrons and the interaction between electron spin and the orbitals of the quantum dot. The team identified one ground state and two excited states in the quantum dot using their setup.

The researchers then used their ternary readout setup to investigate the spin relaxation behavior of the three detected spin states.

“To confirm the validity of our readout system, we measured the spin relaxation of two of the states,” Kiyama explains. 
“Measurement of the dynamics between the spin states in a quantum dot is an important application of the ternary spin readout setup.”

Binary Spin Readout


This is the results of binary spin readout using previous and new schemes, and that of ternary spin readout by combining these two binary-readout schemes. (Image: Osaka University)

The spin relaxation times for the quantum dot measured using the ternary readout system agreed with those reported, providing evidence that the system yielded reliable measurements. This ternary readout system can be extended to quantum dots composed of other materials, revealing a new approach to examine the spin dynamics of quantum dots and representing an advance in quantum information processing.

Source: Osaka University

Also Read About: Graphene Doubles Up Quantum Dots’ Promise for Quantum Computing

NASA & MIT Collaborate to develop space-based quantum-dot spectrometer ~ Quantum Dots in Space “simplifying instrument integration.”


A NASA technologist has teamed with the inventor of a new nanotechnology that could transform the way space scientists build spectrometers, the all-important device used by virtually all scientific disciplines to measure the properties of light emanating from astronomical objects, including Earth itself.

Mahmooda Sultana, a research engineer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, now is collaborating with Moungi Bawendi, a chemistry professor at the Cambridge-based Massachusetts Institute of Technology, or MIT, to develop a prototype imaging spectrometer based on the emerging quantum-dot technology that Bawendi’s group pioneered. NASA’s Center Innovation Fund, which supports potentially trailblazing, high-risk technologies, is funding the effort.

Introducing Quantum Dots

Quantum dots are a type of semiconductor nanocrystal discovered in the early 1980s. Invisible to the naked eye, the dots have proven in testing to absorb different wavelengths of light depending on their size, shape, and chemical composition. The technology is promising to applications that rely on the analysis of light, including smartphone cameras, medical devices, and environmental-testing equipment.

“This is as novel as it gets,” Sultana said, referring to the technology that she believes could miniaturize and potentially revolutionize space-based spectrometers, particularly those used on uninhabited aerial vehicles and small satellites. “It really could simplify instrument integration.”

Absorption spectrometers, as their name implies, measure the absorption of light as a function of frequency or wavelength due to its interaction with a sample, such as atmospheric gases.

After passing through or interacting with the sample, the light reaches the spectrometer. Traditional spectrometers use gratings, prisms, or interference filters to split the light into its component wavelengths, which their detector pixels then detect to produce spectra. The more intense the absorption in the spectra, the greater the presence of a specific chemical.

While space-based spectrometers are getting smaller due to miniaturization, they still are relatively large, Sultana said. “Higher-spectral resolution requires long optical paths for instruments that use gratings and prisms. This often results in large instruments. Whereas here, with quantum dots that act like filters that absorb different wavelengths depending on their size and shape, we can make an ultra-compact instrument. In other words, you could eliminate optical parts, like gratings, prisms, and interference filters.”

Just as important, the technology allows the instrument developer to generate nearly an unlimited number of different dots. As their size decreases, the wavelength of the light that the quantum dots will absorb decreases. “This makes it possible to produce a continuously tunable, yet distinct, set of absorptive filters where each pixel is made of a quantum dot of a specific size, shape, or composition. We would have precise control over what each dot absorbs. We could literally customize the instrument to observe many different bands with high-spectral resolution.”

Prototype Instrument Under Development

nasa-symbolWith her NASA technology-development support, Sultana is working to develop, qualify through thermal vacuum and vibration tests, and demonstrate a 20-by-20 quantum-dot array sensitive to visible wavelengths needed to image the sun and the aurora. However, the technology easily can be expanded to cover a broader range of wavelengths, from ultraviolet to mid-infrared, which may find many potential space applications in Earth science, heliophysics, and planetary science, she said.

Under the collaboration, Sultana is developing an instrument concept particularly for a CubeSat application and MIT doctoral student Jason Yoo is investigating techniques for synthesizing different precursor chemicals to create the dots and then printing them onto a suitable substrate. “Ultimately, we would want to print the dots directly onto the detector pixels,” she said.

“This is a very innovative technology,” Sultana added, conceding that it is very early in its development. “But we’re trying to raise its technology-readiness level very quickly. Several space-science opportunities that could benefit are in the pipeline.”

NASA

Energy-collecting Windows Dream One-Step Closer


window-solar-id45905

Silicon-based luminescent solar concentrator. While most of the light concentrated to the edge of the silicon-based luminescent solar concentrator is actually invisible, we can better see the concentration effect by the naked eye when the slab is illuminated by a “black light” which is composed of mostly ultraviolet wavelengths. (image: Uwe Kortshagen, University of Minnesota)

February 20, 2017

Researchers at the University of Minnesota and University of Milano-Bicocca are bringing the dream of windows that can efficiently collect solar energy one step closer to reality thanks to high tech silicon nanoparticles.

The researchers developed technology to embed the silicon nanoparticles into what they call efficient luminescent solar concentrators (LSCs). These LSCs are the key element of windows that can efficiently collect solar energy. When light shines through the surface, the useful frequencies of light are trapped inside and concentrated to the edges where small solar cells can be put in place to capture the energy.

The research is published today in Nature Photonics (“Highly efficient luminescent solar concentrators based on Earth-abundant indirect-bandgap silicon quantum dots”).

Windows that can collect solar energy, called photovoltaic windows, are the next frontier in renewable energy technologies, as they have the potential to largely increase the surface of buildings suitable for energy generation without impacting their aesthetics—a crucial aspect, especially in metropolitan areas. LSC-based photovoltaic windows do not require any bulky structure to be applied onto their surface and since the photovoltaic cells are hidden in the window frame, they blend invisibly into the built environment.

The idea of solar concentrators and solar cells integrated into building design has been around for decades, but this study included one key difference—silicon nanoparticles. Until recently, the best results had been achieved using relatively complex nanostructures based either on potentially toxic elements, such as cadmium or lead, or on rare substances like indium, which is already massively utilized for other technologies. Silicon is abundant in the environment and non-toxic. It also works more efficiently by absorbing light at different wavelengths than it emits. However, silicon in its conventional bulk form, does not emit light or luminesce.

“In our lab, we ‘trick’ nature by shrinking the dimension of silicon crystals to a few nanometers, that is about one ten-thousandths of the diameter of human hair,” said University of Minnesota mechanical engineering professor Uwe Kortshagen, inventor of the process for creating silicon nanoparticles and one of the senior authors of the study. “At this size, silicon’s properties change and it becomes an efficient light emitter, with the important property not to re-absorb its own luminescence. This is the key feature that makes silicon nanoparticles ideally suited for LSC applications.”

Using the silicon nanoparticles opened up many new possibilities for the research team.

“Over the last few years, the LSC technology has experienced rapid acceleration, thanks also to pioneering studies conducted in Italy, but finding suitable materials for harvesting and concentrating solar light was still an open challenge,” said Sergio Brovelli, physics professor at the University of Milano-Bicocca, co-author of the study, and co-founder of the spin-off company Glass to Power that is industrializing LSCs for photovoltaic windows “Now, it is possible to replace these elements with silicon nanoparticles.”

Researchers say the optical features of silicon nanoparticles and their nearly perfect compatibility with the industrial process for producing the polymer LSCs create a clear path to creating efficient photovoltaic windows that can capture more than 5 percent of the sun’s energy at unprecedented low costs.

“This will make LSC-based photovoltaic windows a real technology for the building-integrated photovoltaic market without the potential limitations of other classes of nanoparticles based on relatively rare materials,” said Francesco Meinardi, physics professor at the University of Milano-Bicocca and one of the first authors of the paper.

The silicon nanoparticles are produced in a high-tech process using a plasma reactor and formed into a powder.

“Each particle is made up of less than two thousand silicon atoms,” said Samantha Ehrenberg, a University of Minnesota mechanical Ph.D. student and another first author of the study. “The powder is turned into an ink-like solution and then embedded into a polymer, either forming a sheet of flexible plastic material or coating a surface with a thin film.”

The University of Minnesota invented the process for creating silicon nanoparticles about a dozen years ago and holds a number of patents on this technology. In 2015, Kortshagen met Brovelli, who is an expert in LSC fabrication and had already demonstrated various successful approaches to efficient LSCs based on other nanoparticle systems. The potential of silicon nanoparticles for this technology was immediately clear and the partnership was born. The University of Minnesota produced the particles and researchers in Italy fabricated the LSCs by embedding them in polymers through an industrial based method, and it worked.

“This was truly a partnership where we gathered the best researchers in their fields to make an old idea truly successful,” Kortshagen said. “We had the expertise in making the silicon nanoparticles and our partners in Milano had expertise in fabricating the luminescent concentrators. When it all came together, we knew we had something special.”

Source: University of Minnesota

 

Graphene Quantum Dots: Introduction and Market News


 

quantum-dots-fluorescent-image-img_assist-400x198

What are quantum dots?

Quantum dots, or QDs, are semiconductor nanoparticles or nanocrystals, usually in the range of 2-10 nanometers (10-50 atoms) in size. Their small size and high surface-to-volume ratio affects their optical and electronic properties and makes them different from larger particles made of the same materials. Quantum dots confine the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. Quantum dots are also sometimes referred to as ‘artificial atoms’, a term that emphasizes that they are a single object with bound, discrete electronic states, similarly to naturally occurring atoms or molecules.

gqds-iv-images
Image: Grapehene Quantum Dots 

Many types of quantum dot are fluorescent – they emit light of specific frequencies if electricity or light is applied to them. These frequencies can be tuned by changing the dots’ size, shape and material, opening the door to diverse applications. Generally speaking, smaller dots appear blue while larger ones tend to be more red. Specific colors also vary depending on the exact composition of the QD.
Applications

Thanks to their highly tunable properties, QDs are attracting interest from various application developers and researchers. Among these potential applications are displays, transistors, solar cells, diode lasers, quantum computing, and medical imaging. Additionally, their small size enables QDs to be suspended in solution, which leads to possible uses in inkjet printing and spin-coating. These processing techniques may result in less-expensive and less time consuming methods of semiconductor fabrication. Quantum dots are considered especially suitable for optical applications, thanks to their ability to emit diverse colors, coupled with their high efficiencies, longer lifetimes and high extinction coefficient.

 

Their small size also means that electrons do not have to travel as far as with larger particles, thus electronic devices can operate faster. Examples of applications that take advantage of these electronic properties include transistors, solar cells, quantum computing, and more. QDs can greatly improve LED screens, offering them higher peak brightness, better colour accuracy, higher color saturation and more. QDs are also very interesting for use in biomedical applications, since their small size allows them to travel in the body, thus making them suitable for applications like medical imaging, biosensors, etc.
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What is graphene?

 

Graphene is a material made of carbon atoms that are bonded together in a repeating pattern of hexagons. Graphene is so thin that it is considered two dimensional. Graphene’s flat honeycomb pattern gives it many extraordinary characteristics, such as being the strongest material in the world, as well as one of the lightest, most conductive and transparent. Graphene has endless potential applications, in almost every industry (like electronics, medicine, aviation and much more).

 

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Graphene structure photo
The single layers of carbon atoms provide the basis for many other materials. Graphite, like the substance found in pencil lead, is formed by stacked graphene. Carbon nanotubes are made of rolled graphene and are used in many emerging applications from sports gear to biomedicine.

 

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Graphene quantum dots

The term graphene quantum dots (GQDs) is usually used to describe miniscule fragments, limited in size, or domains, of single-layer to tens of layers of graphene. GQDs often possess properties like low toxicity, stable photoluminescence, chemical stability and pronounced quantum confinement effect, which make them attractive for biological, opto-electronics, energy and environmental applications.

 

tour-gqds-iii-1206_coal-5-webPhoto: Dr. James M. Tour: Rice University: 

The synthesis of graphene quantum structures, such as graphene quantum dots, has become a popular topic in recent years. While graphene usually does not have a bandgap – which is a problem for many applications – graphene quantum dots do contain a bandgap due to quantum confinement and edge effects, and that bandgap modifies graphene’s carrier behaviors and can lead to versatile applications in optoelectronics. GQDs were also found to have four quantum states at a given energy level, unlike semiconductor quantum dots, which have only two. These additional quantum states, according to researchers, could make GQDs beneficial for quantum computing.
Additional properties of GQDs such as high transparency and high surface area have been proposed for energy and display applications. Because of the large surface area, electrodes using GQDs are applied for capacitors and batteries. Various techniques have been developed to produce GQDs. Top-down methods include solution chemical, microwave, and ultrasonic methods. Bottom-up methods include hydrothermal and electrochemical methods.

 

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Graphene Quantum Dots in the News

Dotz Nano secures first order of graphene quantum dots:

In January 2017, Dotz Nano, a nanotechnology company focused on the development, manufacture and commercialization of graphene quantum dots (GQDs), signed a marketing agreement with Strem Chemicals, a manufacturer and distributor of specialty chemicals headquartered in the U.S.

Strem Chemicals will aim to facilitate sales of Dotz’s GQDs to academic, industrial and government research and development laboratories, as well as commercial businesses using GQDs for research purposes.

 
Fuji Pigment announces graphene and carbon QD manufacturing process:

In April 2016, Fuji Pigment announced the development of a large-scale manufacturing process for carbon and graphene quantum dots (QDs). Fuji Pigment stated that its toxic-metal-free QDs exhibit a high light-emitting quantum efficiency and stability comparable to the toxic metal-based quantum dots.

 
Samsung developed graphene quantum dots based flash memory devices:

In June 2014, researchers from Samsung Electronics (and Korea’s Kyung Hee University) developed flash devices based on graphene quantum dots (GQDs). The performance of such a device is promising, with an electron density that is comparable to semiconductor and metal nanocrystal based memories. Those flash memory can also be made flexible and transparent.

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Read More: Graphene ‘artificial atom’ opens door to quantum computing

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