In new quantum-dot LED design, researchers turn troublesome molecules to their advantage

 By nestling quantum dots in an insulating egg-crate structure, researchers at the Harvard School of Engineering and Applied Sciences (SEAS) have demonstrated a robust new architecture for quantum-dot light-emitting devices (QD-LEDs).

Quantum dots are very tiny crystals that glow with bright, rich colors when stimulated by an electric current. QD-LEDs are expected to find applications in television and computer screens, general light sources, and lasers.

Previous work in the field had been complicated by organic molecules called ligands that dangle from the surface of the quantum dots. The ligands play an essential role in quantum dot formation, but they can cause functional problems later on.

Thanks to an inventive change in technique devised by the Harvard team, the once-troublesome ligands can now be used to build a more versatile QD-LED structure. The new single-layer design, described in the journal Advanced Materials, can withstand the use of chemical treatments to optimize the device's performance for diverse applications.

"With quantum dots, the chemical environment that's optimal for growth is usually not the environment that's optimal for function," says co-principal investigator Venkatesh Narayanamurti, Benjamin Peirce Professor of Technology and Public Policy at SEAS.

The quantum dots, each only 6 nanometers in diameter, are grown in a solution that glows strikingly under a black light.

The solution of quantum dots can be deposited onto the surface of the electrodes using a range of techniques, but according to lead author Edward Likovich (A.B. '06, S.M. '08, Ph.D. '11), who conducted the research as a doctoral candidate in applied physics at SEAS, "That's when it gets complicated."

"The core of the dots is a perfect lattice of semiconductor material, but on the exterior it's a lot messier," he says. "The dots are coated with ligands, long organic chains that are necessary for precise synthesis of the dots in solution. But once you deposit the quantum dots onto the electrode surface, these same ligands make many of the typical device processing steps very difficult."

The ligands can interfere with current conduction, and attempts to modify them can cause the quantum dots to fuse together, destroying the properties that make them useful. Organic molecules can also degrade over time when exposed to UV rays.

Researchers would like to be able to use those ligands to produce the quantum dots in solution, while minimizing the negative impact of the ligands on current conduction.

"The QD technologies that have been developed so far are these big, thick, multilayer devices," says co-author Rafael Jaramillo, a Ziff Environmental Fellow at the Harvard University Center for the Environment. Jaramillo works in the lab of Shriram Ramanathan, Associate Professor of Materials Science at SEAS.

"Until now, those multiple layers have been essential for producing enough light, but they don't allow much control over current conduction or flexibility in terms of chemical treatments. A thin, monolayer film of quantum dots is of tremendous interest in this field, because it enables so many new applications."

The new QD-LED resembles a sandwich, with a single active layer of quantum dots nestled in insulation and trapped between two ceramic electrodes. To create light, current must be funneled through the quantum dots, but the dots also have to be kept apart from one another in order to function.

In an early design, the path of least resistance was between the quantum dots, so the electric current bypassed the dots and produced no light.

Abandoning the traditional evaporation technique they had been using to apply insulation to the device, the researchers instead used atomic layer deposition (ALD) -- a technique that involves jets of water. ALD takes advantage of the water-resistant ligands on the quantum dots, so when the aluminum oxide insulation is applied to the surface, it selectively fills the gaps between the dots, producing a flat surface on the top.

The new structure allows more effective control over the flow of electrical current.

"Exploiting these hydrophobic ligands allowed us to insulate the interstices between the quantum dots, essentially creating a structure that acts as an egg crate for quantum dots," says co-author Kasey Russell (A.B. '02, Ph.D. '09), a postdoctoral fellow at SEAS. "The benefit is that we can funnel current directly through the quantum dots despite having only a single layer of them, and because we have that single layer, we can apply new chemical treatments to it, moving forward."

Through Harvard's Office of Technology Development, Likovich and his colleagues have applied for a provisional patent on the device. Beyond the possible applications in computer and TV displays, lights, and lasers, the technology could one day be used in field-effect transistors or solar cells.

The research was supported by the Harvard University Center for the Environment; the Nanoscale Science and Engineering Center at Harvard, which is funded by the National Science Foundation (NSF); and the use of facilities at the Harvard University Center for Nanoscale Systems, a member of the NSF-supported National Nanotechnology Infrastructure Network.

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Journal Reference:

Edward M. Likovich, Rafael Jaramillo, Kasey J. Russell, Shriram Ramanathan, Venkatesh Narayanamurti. High-Current-Density Monolayer CdSe/ZnS Quantum Dot Light-Emitting Devices with Oxide Electrodes. Advanced Materials, 2011; 23 (39): 4521 DOI: 10.1002/adma.201101782

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Nano car has molecular 4-wheel drive: Smallest electric car in the world

 Reduced to the max: the emission-free, noiseless 4-wheel drive car, jointly developed by Empa researchers and their Dutch colleagues, represents lightweight construction at its most extreme. The nano car consists of just a single molecule and travels on four electrically-driven wheels in an almost straight line over a copper surface. The "prototype" can be admired on the cover of the latest edition of Nature.

To carry out mechanical work, one usually turns to engines, which transform chemical, thermal or electrical energy into kinetic energy in order to, say, transport goods from A to B. Nature does the same thing; in cells, so-called motor proteins -- such as kinesin and the muscle protein actin -- carry out this task. Usually they glide along other proteins, similar to a train on rails, and in the process "burn" ATP (adenosine triphosphate), the chemical fuel, so to speak, of the living world.

A number of chemists aim to use similar principles and concepts to design molecular transport machines, which could then carry out specific tasks on the nano scale. According to an article in the latest edition of science magazine "Nature," scientists at the University of Groningen and at Empa have successfully taken "a decisive step on the road to artificial nano-scale transport systems." They have synthesised a molecule from four rotating motor units, i.e. wheels, which can travel straight ahead in a controlled manner. "To do this, our car needs neither rails nor petrol; it runs on electricity. It must be the smallest electric car in the world -- and it even comes with 4-wheel drive" comments Empa researcher Karl-Heinz Ernst.

Range per tank of fuel: still room for improvement

The downside: the small car, which measures approximately 4x2 nanometres -- about one billion times smaller than a VW Golf -- needs to be refuelled with electricity after every half revolution of the wheels -- via the tip of a scanning tunnelling microscope (STM). Furthermore, due to their molecular design, the wheels can only turn in one direction. "In other words: there's no reverse gear," says Ernst, who is also a professor at the University of Zurich, laconically.

According to its "construction plan" the drive of the complex organic molecule functions as follows: after sublimating it onto a copper surface and positioning an STM tip over it leaving a reasonable gap, Ernst's colleague, Manfred Parschau, applied a voltage of at least 500 mV. Now electrons should "tunnel" through the molecule, thereby triggering reversible structural changes in each of the four motor units. It begins with a cis-trans isomerisation taking place at a double bond, a kind of rearrangement -- in an extremely unfavourable position in spatial terms, though, in which large side groups fight for space. As a result, the two side groups tilt to get past each other and end up back in their energetically more favourable original position -- the wheel has completed a half turn. If all four wheels turn at the same time, the car should travel forwards. At least, according to theory based on the molecular structure.

To drive or not to drive -- a simple question of orientation

And this is what Ernst and Parschau observed: after ten STM stimulations, the molecule had moved six nanometres forwards -- in a more or less straight line. "The deviations from the predicted trajectory result from the fact that it is not at all a trivial matter to stimulate all four motor units at the same time," explains "test driver" Ernst.

Another experiment showed that the molecule really does behave as predicted. A part of the molecule can rotate freely around the central axis, a C-C single bond -- the chassis of the car, so to speak. It can therefore "land" on the copper surface in two different orientations: in the right one, in which all four wheels turn in the same direction, and in the wrong one, in which the rear axle wheels turn forwards but the front ones turn backwards -- upon excitation the car remains at a standstill. Ernst und Parschau were able to observe this, too, with the STM.

Therefore, the researchers have achieved their first objective, a "proof of concept," i.e. they have been able to demonstrate that individual molecules can absorb external electrical energy and transform it into targeted motion. The next step envisioned by Ernst and his colleagues is to develop molecules that can be driven by light, perhaps in the form of UV lasers.

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The above story is reprinted from materials provided by Empa.

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Journal Reference:

Tibor Kudernac, Nopporn Ruangsupapichat, Manfred Parschau, Beatriz Maciá, Nathalie Katsonis, Syuzanna R. Harutyunyan, Karl-Heinz Ernst, Ben L. Feringa. Electrically driven directional motion of a four-wheeled molecule on a metal surface. Nature, 2011; 479 (7372): 208 DOI: 10.1038/nature10587

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Unique bipolar compounds enhance functionality of organic electronics

 Researchers often work with a narrow range of compounds when making organic electronics, such as solar panels, light emitting diodes and transistors. Professor Tim Bender and Ph.D. Candidate Graham Morse of U of T's Department of Chemical Engineering and Applied Chemistry have uncovered compounds that exhibit unique and novel electrochemical properties.

"Organic solar cell need to absorb light, move electrons and transport holes. Normally you need one compound to do each function. Researchers have found compounds that can do two of the three. Our discovery leads to the potential of achieving all three with a single compound," explains Bender.

During the summer of 2010, Bender gave Morse the very broad task of assessing new compositions of matter. Morse proposed a research hypothesis that led to the discovery of a new class of compounds with phthalimido molecular fragments. Along with fellow U of T collaborators, the pair have shown that their new compounds present the ability to move both holes and electrons in an organic light emitting diode (OLED). Given these compounds absorb sunlight as well, they have the potential to execute all three tasks needed for a functional organic solar cell. Bender and Morse are currently investigating this likelihood.

"Compounds with such electrochemical behaviour are very rare. The knowledge we developed will further an understanding of future compounds and synthesis strategies," says Morse.

An important part of Bender and Morse's work was the use of inexpensive raw materials and scalable synthetic methods so their research could transition smoothly into the next steps for materials development and conceivably a commercial product.

The detailed findings of their study were recently published in Applied Materials and Interfaces, an interdisciplinary journal designed to report on the function and development of new cutting-edge materials and their applications. The journal falls under the American Chemical Society -- a top tier publisher in the field of chemistry and its application.

Journal Reference:

Graham E. Morse, Jeffery S. Castrucci, Michael G. Helander, Zheng-Hong Lu, Timothy P. Bender. Phthalimido-boronsubphthalocyanines: New Derivatives of Boronsubphthalocyanine with Bipolar Electrochemistry and Functionality in OLEDs.. ACS Applied Materials & Interfaces, 2011; 3 (9): 3538 DOI: 10.1021/am200758w

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Chemistry: New insight into 100-year-old Haber-Bbosch process of converting nitrogen to ammonia

For the past 100 years, the Haber-Bosch process has been used to convert atmospheric nitrogen into ammonia, which is essential in the manufacture of fertilizer. Despite the longstanding reliability of the process, scientists have had little understanding of how it actually works. But now a team of chemists, led by Patrick Holland of the University of Rochester, has new insight into how the ammonia is formed. Their findings are published in the latest issue of Science.

Holland calls nitrogen molecules "challenging." While they're abundant in the air around us, which makes them desirable for research and manufacturing, their strong triple bonds are difficult to break, making them highly unreactive. For the last century, the Haber-Bosch process has made use of an iron catalyst at extremely high pressures and high temperatures to break those bonds and produce ammonia, one drop at a time. The question of how this works, though, has not been answered to this day.

"The Haber-Bosch process is efficient, but it is hard to understand because the reaction occurs only on a solid catalyst, which is difficult to study directly," said Holland. "That's why we attempted to break the nitrogen using soluble forms of iron."

Holland and his team, which included Meghan Rodriguez and William Brennessel at the University of Rochester and Eckhard Bill of the Max Planck Institute for Bioinorganic Chemistry in Germany, succeeded in mimicking the process in solution. They discovered that an iron complex combined with potassium was capable of breaking the strong bonds between the nitrogen (N) atoms and forming a complex with an Fe3N2 core, which indicates that three iron (Fe) atoms work together in order to break the N-N bonds. The new complex then reacts with hydrogen (H2) and acid to form ammonia (NH3) -- something that had never been done by iron in solution before.

Despite the breakthrough, the Haber-Bosch process is not likely to be replaced anytime soon. While there are risks in producing ammonia at extremely high temperatures and pressures, Holland points out that the catalyst used in Haber-Bosch is considerably less expensive than what was used by his team. But Holland says it is possible that his team's research could eventually help in coming up with a better catalyst for the Haber-Bosch process -- one that would allow ammonia to be produced at lower temperatures and pressures.

At the same time, the findings could have a benefit far removed from the world of ammonia and fertilizer. When the iron-potassium complex breaks apart the nitrogen molecules, negatively charged nitrogen ions -- called nitrides -- are formed. Holland says the nitrides formed in solution could be useful in making pharmaceuticals and other products.

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The above story is reprinted from materials provided by University of Rochester.

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Journal Reference:

M. M. Rodriguez, E. Bill, W. W. Brennessel, P. L. Holland. N2 Reduction and Hydrogenation to Ammonia by a Molecular Iron-Potassium Complex. Science, 2011; 334 (6057): 780 DOI: 10.1126/science.1211906

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Making chemicals from biogas instead of burning it

 Combustible gases generated by organic matter in landfill sites or from biomass are commonly burned to generate electricity. However, a Finnish team, writing in a forthcoming issue of the International Journal of Sustainable Economy, suggests that such biogas might be more usefully used as an alternative feedstock for the chemical industry. They explain that using biogas in this way would reduce our dependency on oil and gas-derived products and is commercially and technically viable.

Jouko Arvola of the University of Oulu and colleagues there and at Oulu University of Applied Sciences point out that environmental pressure has turned our focus to reducing carbon emissions by the employment of renewable energy sources instead of fossil fuels. Biomass can be readily converted to usable energy mostly in the form of methane through anaerobic fermentation, they point out. Rather than simply burning this biogas, the team suggests that at the local level it would be beneficial in terms of resources and pollution to utilise this valuable carbon source as an industrial feedstock. They have now examined the viability of such an approach to industrial sites in Finland and demonstrated, in theory at least, that this is a serious alternative to natural gas or oil-derived resources.

To initiate such a switch to biogas from landfill and other sources, there may have to be subsidies akin to those implemented in food production. However, as the price of raw fossil materials -- oil and gas -- continues to rise, biogas will become a more competitive alternative feedstock and government support could gradually be reduced.

"The use of biogas can be promoted by identifying existing industrial sites currently using fossil-based gas as raw material and by analysing whether they can utilise biogas," the team says. "By constructing biogas producing unit at industrial sites potentially enables development of other biogas applications. Building pipelines to other biogas users, or vehicle uses, are potential options," they add.

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