MIT researchers improve quantum-dot performance

QDOTS imagesCAKXSY1K 8New production method could enable everything from more efficient computer displays to enhanced biomedical testing.



Quantum dots — tiny particles that emit light in a dazzling array of glowing colors — have the potential for many applications, but have faced a series of hurdles to improved performance. But an MIT team says that it has succeeded in overcoming all these obstacles at once, while earlier efforts have only been able to tackle them one or a few at a time.
Quantum dots — in this case, a specific type called colloidal quantum dots — are tiny particles of semiconductor material that are so small that their properties differ from those of the bulk material: They are governed in part by the laws of quantum mechanics that describe how atoms and subatomic particles behave. When illuminated with ultraviolet light, the dots fluoresce brightly in a range of colors, determined by the sizes of the particles.

QDot Pix 1


First discovered in the 1980s, these materials have been the focus of intense research because of their potential to provide significant advantages in a wide variety of optical applications, but their actual usage has been limited by several factors. Now, research published this week in the journal Nature Materials by MIT chemistry postdoc Ou Chen, Moungi Bawendi, the Lester Wolfe Professor of Chemistry, and several others raises the prospect that these limiting factors can all be overcome.
The new process developed by the MIT team produces quantum dots with four important qualities: uniform sizes and shapes; bright emissions, producing close to 100 percent emission efficiency; a very narrow peak of emissions, meaning that the colors emitted by the particles can be precisely controlled; and an elimination of a tendency to blink on and off, which limited the usefulness of earlier quantum-dot applications.

Multicolored biological dyes

For example, one potential application of great interest to researchers is as a substitute for conventional fluorescent dyes used in medical tests and research. Quantum dots could have several advantages over dyes — including the ability to label many kinds of cells and tissues in different colors because of their ability to produce such narrow, precise color variations. But the blinking effect has hindered their use: In fast-moving biological processes, you can sometimes lose track of a single molecule when its attached quantum dot blinks off.
Previous attempts to address one quantum-dot problem tended to make others worse, Chen says. For example, in order to suppress the blinking effect, particles were made with thick shells, but this eliminated some of the advantages of their small size.

The small size of these new dots is important for potential biological applications, Bawendi explains. “[Our] dots are roughly the size of a protein molecule,” he says. If you want to tag something in a biological system, he says, the tag has got to be small enough so that it doesn’t overwhelm the sample or interfere significantly with its behavior.

Quantum dots are also seen as potentially useful in creating energy-efficient computer and television screens. While such displays have been produced with existing quantum-dot technology, their performance could be enhanced through the use of dots with precisely controlled colors and higher efficiency.
Combining the advantages
So recent research has focused on “the properties we really need to enhance [dots’] application as light emitters,” Bawendi says — which are the properties that the new results have successfully demonstrated. The new quantum dots, for the first time, he says, “combine all these attributes that people think are important, at the same time.”

The new particles were made with a core of semiconductor material (cadmium selenide) and thin shells of a different semiconductor (cadmium sulfide). They demonstrated very high emission efficiency (97 percent) as well as small, uniform size and narrow emission peaks. Blinking was strongly suppressed, meaning the dots stay “on” 94 percent of the time.

A key factor in getting these particles to achieve all the desired characteristics was growing them in solution slowly, so their properties could be more precisely controlled, Chen explains. “A very important thing is synthesis speed,” he says, “to give enough time to allow every atom to go to the right place.”
The slow growth should make it easy to scale up to large production volumes, he says, because it makes it easier to use large containers without losing control over the ultimate sizes of the particles. Chen expects that the first useful applications of this technology could begin to appear within two years.

Taeghwan Hyeon, director of the Center for Nanoparticle Research at Seoul National University in Korea, who was not involved in this research, says, “It is very impressive, because using a seemingly very simple approach — that is, maintaining a slow growth rate — they were able to precisely control shell thickness, enabling them to synthesize highly uniform and small-sized quantum dots.” This work, he says, solves one of the “key challenges” in this field, and “could find biomedical imaging applications, and can be also used for solid-state lighting and displays.”
In addition to Chen and Bawendi, the team included seven other MIT students and postdocs and two researchers from Massachusetts General Hospital and Harvard Medical School. The work was supported by the National Institutes of Health, the Army Research Office through MIT’s Institute for Soldier Nanotechnologies, and by the National Science Foundation through the Collaborative Research in Chemistry Program.

Stacked nanoparticle layers shine new light on optical thin films

Posted: November 5, 2012

Stacked nanoparticle layers shine new light on optical thin films(Nanowerk Spotlight) The refractive index is the property of a material that changes the speed of light and describes how light propagates through the material. The refractive index is an important property of solar cells – the higher it is, the more incident light gets reflected and is not converted to a photocurrent.

Air for instance, has a low refractive index very close to 1.0; but silicon, still the most common material used in today’s commercial solar cells, has a high refractive index which causes more than 30% of incident light to be reflected back from the surface of the silicon crystals.Solar cell manufacturers have therefore developed various kinds of anti reflection coatings to reduce the unwanted reflective losses (read more in our Nanowerk Spotlight“Moth eyes inspire self-cleaning antireflection nanotechnology coatings”). The purpose of these optical thin-films is to minimize the differences in the refractive indices between the ambient medium and the solar cells (or other opto-electronic devices).”For both solar cells and LEDs, coating with nano-particles can enhance the performance without harming the electrical properties of the devices, as can occur with etching or lithographic processing,” Hsuen-Li Chen, a professor in the Department of Materials Science and Engineering at National Taiwan University, tells Nanowerk.

nanoparticle multilayer stacksSchematic representation of: a) graded-refractive-index nanoparticle multilayer stacks, and b) scattering particles on graded-refractive-index nanoparticle stacks. (Reprinted with permission from Wiley-VCH Verlag)

In new work, reported in the October 16, 2012 online edition of Advanced Functional Materials (“Nanoparticle Stacks with Graded Refractive Indices Enhance the Omnidirectional Light Harvesting of Solar Cells and the Light Extraction of Light-Emitting Diodes”), Chen and his team have not only demonstrated this advantageous feature but also provided a strategy for optimizing the types and sizes of nanoparticles for use in both solar cells and LED’s. “Previous research did not mainly focus on the refractive indices of nanoparticles” says Chen. “Therefore, we wanted to know how these nanoparticles behave if they were spin-coated onto substrates. We assumed that nanoparticle stacks can be seen as optical thin films with refractive indices because of their little roughness and we successfully used both simulation and experimental measurement to prove our hypothesis.”The team’s main motivation has been to develop an easy and inexpensive method to construct optical thin films.

Traditionally, multi-layer optical thin-films with graded refractive indices were fabricated by PVD (physical vapor deposition) or CVD (chemical vapor deposition). However, using vacuum systems is both time consuming and expensive. In order to save money and processing time, Chen’s team therefore decided to spin-coat dielectric nanoparticle stacks with suitable refractive index to fabricate graded refractive indices multi-layers.”Our assumption was that, if the sizes of the nanoparticles are far less than the wavelength,they can be treated as optical thin films with effective refractive indices after they have been spin-coated onto the substrate,” Chen explains.

“Our rapid, low-cost, solution-based method allows the construction of graded-refractive-index nanoparticle stacks that function as broadband, omnidirectional antireflection coatings. This technique can minimize the reflectance of the silicon-air interface and increase the efficiency of silicon solar cells.””Moreover” he says, “if the sizes of the particles were to be close to the wavelength of incident light, these particles will behave as scattering centers, changing the direction of incident light and roughening the surface.

Different optoelectronic devices require different surface morphologies; some need a flat surface to avoid scattering and retain their electrical properties, while others need a moderately rough surface to enhance the light extraction or increase the optical path through a scattering effect.”Therefore, in their experiments, the researchers prepared both types of system readily through the selection of the types and size of the nanoparticles and their subsequent spincoating onto the substrates.Chen points out that the thickness of each spin-coated layer is critical and can be controlled by the rotational speed. “However, it took time to optimize them because we constructed multi-layers. The interference between individual layers as well as the solvent we chose would influence the thickness and roughness significantly. Fortunately, we were able to overcome these problems.”Because spin-coating is a rapid and cheap process, the main issue in this work was to choose nanoparticles with suitable refractive indices.

As long the refractive indices of the materials of optoelectronic devices are known, it is not difficult to coat nanoparticle stacks onto them to reduce the interfacial reflectance.The result of the team’s study is a novel strategy for arranging dielectric nanoparticles of various types and sizes to enhance both the omnidirectional light harvesting of solar cells and the light extraction of LEDs; and because the fabrication approach involves rapid and simple spin-coating and does not require any etching, the electrical properties of the devices will remain unimpaired.

“The underlying strategy of our process is to match the optical constants while considering the effects of scattering under different wavelengths,” Chen sums up the study. “We are convinced that our nanoparticle-based technique has great potential for application to other optoelectronic devices, including thin-film solar cells, organic solar cells, transparent conductors, and OLEDs.

By Michael Berger. Copyright © Nanowerk

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New Power Generation Technique with a Hybrid Nanomaterial

Physics team demonstrates new power generation technique with a hybrid nanomaterial
(Nanowerk News) A University of Texas at Arlington physics professor has helped create a hybrid nanomaterial that can be used to convert light and thermal energy into electrical current, surpassing earlier methods that used either light or thermal energy, but not both.
Working with Louisiana Tech University assistant professor Long Que, UT Arlington associate physics professor Wei Chen and graduate students Santana Bala Lakshmanan and Chang Yang synthesized a combination of copper sulfide nanoparticles and single-walled carbon nanotubes.
The team used the nanomaterial to build a prototype thermoelectric generator that they hope can eventually produce milliwatts of power. Paired with microchips, the technology could be used in devices such as self-powering sensors, low-power electronic devices and implantable biomedical micro-devices, Chen said.
“If we can convert both light and heat to electricity, the potential is huge for energy production,” Chen said. “By increasing the number of the micro-devices on a chip, this technology might offer a new and efficient platform to complement or even replace current solar cell technology.”
In lab tests, the new thin-film structure showed increases by as much at 80 percent in light absorption when compared to single-walled nanotube thin-film devices alone, making it a more efficient generator.
Copper sulfide is also less expensive and more environment-friendly than the noble metals used in similar hybrids.
In October, the journal Nanotechnology published a paper on the work called “Optical and thermal response of single-walled carbon nanotube–copper sulfide nanoparticle hybrid nanomaterials “. In it, researchers also say also found that they could enhance the thermal and optical switching effects of the hybrid nanomaterial as much as ten times by using asymmetric illumination, rather than symmetric illumination.
Coauthors on the Nanotechnology paper from Louisiana Tech include Yi-Hsuan Tseng, Yuan He and Que, all of the school’s Institute for Micromanufacturing.
“Dr. Chen’s research with nanomaterials is an important advancement with the potential for far-reaching applications,” said Pamela Jansma, dean of the UT Arlington College of Science. “This is the kind of work that demonstrates the value of a research university in North Texas and beyond.”
Chen is currently receiving funding from the U.S. Department of Defense to develop nanoparticle self-lighting photodynamic therapy for use against breast and prostate cancers. In 2010, he was the first to publish results in the journal Nanomedicine demonstrating that near infrared light could be used to heat copper sulfide nanoparticles for photothermal therapy in cancer treatment, which destroys cancer cells with heat between 41 and 45 degrees Celsius.
Next month, the Journal of Biomedical Nanotechnology will publish Chen’s work successfully coupling gold nanoparticles with the copper sulfide nanoparticles for the photothermal therapy. Such a material would be less costly and potentially more effective than using gold particles alone, Chen said. The new paper is called “Local field enhanced Au/CuS nanocomposites as efficient photothermal transducer agents for cancer treatment.”
Chen is also leading a UT Arlington team exploring ways to develop various nanoparticles for radiation detection. That work is funded by a $1.3 million grant from the National Science Foundation and the U.S. Department of Homeland Security.
Source: University of Texas at Arlington