Scientists Dramatically Increase Light from Atomic-Sized Materials

A recent article in the journalScience details how researchers from the Erik Jonsson School of Engineering and Computer Sciencedevised a simple process that dramatically increases light generation from certain atomic-sized materials.

The findings could have a broad impact in the advancement of LED displays, high efficiency solar cells, photo detectors, and nano-electronic circuits and devices.

“The prospect of using atomically thin materials for electronic applications that can detect and control light emission, what is known as optoelectronics, has captured the imagination of scientists and researchers across the world,” said co-author Dr. Robert Wallace, professor of materials science and engineering, and Erik Jonsson Distinguished Chair. 

Alongside a team of international collaborators, the UT Dallas scientists focused their research on a class of materials called transition metal dichalcogenides (TMDs), which can form an atomically thin layer that behaves like a semiconductor switch. For optoelectronic applications, a big hurdle to further development of TMD-based devices has been the low efficiency of their light generation.  

The article in Science shows how the efficiency of light generation in a particular TMD, molybdenum disulfide, can be improved by more than 100 times.

“Typically, this efficiency is captured in a benchmark measurement called ‘quantum yield,’” Wallace said. “Quantum yields are typically less than 1 percent for these atomically thin layered materials, making useful optoelectronic devices impractical. The low yield is thought to originate from imperfections in the TMD layer that results in mediocre electronic quality.”

Despite the typical low yield percentages, the researchers found that a relatively simple wet chemical treatment improves the performance of molybdenum disulfide, resulting in quantum yield of around 95 percent.

Collaborators at the University of California, Berkeley, led by Dr. Ali Javey, originally observed the extremely enhanced quantum yields upon exposure to the chemical treatment, which involves the use of an air-stable, solution-based organic superacid. 

“The improvement observed was intriguing and we were eager to better understand how the treatment improves the efficiency,” Wallace said.

As the chemical treatment is straightforward and easy to adopt, it is expected to significantly impact a wide range of TMD materials and device research and development.

Dr. Kyeongjae “K.J.” Cho, professor of materials science and engineering, said that as computer chips and other components become smaller, that the need for efficient nano-electronic circuits and devices becomes greater.

“In addition to improved optoelectronic devices, such as photodetectors and light-emitting diodes, this discovery will impact performance in other components like transistors,” Cho said.

Other UT Dallas co-authors included graduate students Angelica Azcatl and Dr. Santosh KC, research scientist Dr. Rafik Addou and postdoctoral research associate Dr. Jiyoung Noh. The UT Dallas team was called upon to provide the detailed materials characterization and modeling of the superacid treatment.

“The modeling and characterization study at UT Dallas provides key insights on the underlying mechanisms leading to the improvement in the efficiency,” Cho said.

Work at UT Dallas was supported through the Center for Low Energy Systems Technology, one of six centers supported by the STARnet phase of the Focus Center Research Program — a Semiconductor Research Corporation program sponsored by Microelectronics Advanced Research Corporation and the Defense Advanced Research Projects Agency.

Earlier in this semester, Wallace also was involved in a study that showed TMDs exhibited a quantum mechanical behavior called negative differential resistance, or NDR, at room temperature when grown as a layered structure. NDR is a phenomenon in which electrons, due to their wave nature, tunnel through thin materials with varying resistance.

UT Dallas

A new tool measures the distance between phonon collisions

Tabletop setup provides more nuanced picture of heat production in microelectronics.

Today’s computer chips pack billions of tiny transistors onto a plate of silicon within the width of a fingernail. Each transistor, just tens of nanometers wide, acts as a switch that, in concert with others, carries out a computer’s computations. As dense forests of transistors signal back and forth, they give off heat — which can fry the electronics, if a chip gets too hot.

Manufacturers commonly apply a classical diffusion theory to gauge a transistor’s temperature rise in a computer chip. But now an experiment by MIT engineers suggests that this common theory doesn’t hold up at extremely small length scales. The group’s results indicate that the diffusion theory underestimates the temperature rise of nanoscale heat sources, such as a computer chip’s transistors. Such a miscalculation could affect the reliability and performance of chips and other microelectronic devices.

“We verified that when the heat source is very small, you cannot use the diffusion theory to calculate temperature rise of a device. Temperature rise is higher than diffusion prediction, and in microelectronics, you don’t want that to happen,” says Professor Gang Chen, head of the Department of Mechanical Engineering at MIT. “So this might change the way people think about how to model thermal problems in microelectronics.”

The group, including graduate student Lingping Zeng and Institute Professor Mildred Dresselhaus of MIT, Yongjie Hu of the University of California at Los Angeles, and Austin Minnich of Caltech, has published its results this week in the journal Nature Nanotechnology.

Phonon mean free path distribution

Chen and his colleagues came to their conclusion after devising an experiment to measure heat carriers’ “mean free path” distribution in a material. In semiconductors and dielectrics, heat typically flows in the form of phonons — wavelike particles that carry heat through a material and experience various scatterings during their propagation. A phonon’s mean free path is the distance a phonon can carry heat before colliding with another particle; the longer a phonon’s mean free path, the better it is able to carry, or conduct, heat.

As the mean free path can vary from phonon to phonon in a given material — from several nanometers to microns — the material exhibits a mean free path distribution, or range. Chen, the Carl Richard Soderberg Professor in Power Engineering at MIT, reasoned that measuring this distribution would provide a more detailed picture of a material’s heat-carrying capability, enabling researchers to engineer materials, for example, using nanostructures to limit the distance that phonons travel.

The group sought to establish a framework and tool to measure the mean free path distribution in a number of technologically interesting materials. There are two thermal transport regimes: diffusive regime and quasiballistic regime. The former returns the bulk thermal conductivity, which masks the important mean free path distribution. To study phonons’ mean free paths, the researchers realized they would need a small heat source compared with the phonon mean free path to access the quasiballistic regime, as larger heat sources would essentially mask individual phonons’ effects.

Creating nanoscale heat sources was a significant challenge: Lasers can only be focused to a spot the size of the light’s wavelength, about one micron — more than 10 times the length of the mean free path in some phonons. To concentrate the energy of laser light to an even finer area, the team patterned aluminum dots of various sizes, from tens of micrometers down to 30 nanometers, across the surface of silicon, silicon germanium alloy, gallium arsenide, gallium nitride, and sapphire. Each dot absorbs and concentrates a laser’s heat, which then flows through the underlying material as phonons.

In their experiments, Chen and his colleagues used microfabrication to vary the size of the aluminum dots, and measured the decay of a pulsed laser reflected from the material — an indirect measure of the heat propagation in the material. They found that as the size of the heat source becomes smaller, the temperature rise deviates from the diffusion theory.

They interpret that as the metal dots, which are heat sources, become smaller, phonons leaving the dots tend to become “ballistic,” shooting across the underlying material without scattering. In these cases, such phonons do not contribute much to a material’s thermal conductivity. But for much larger heat sources acting on the same material, phonons tend to collide with other phonons and scatter more often. In these cases, the diffusion theory that is currently in use becomes valid.

A detailed transport picture

For each material, the researchers plotted a distribution of mean free paths, reconstructed from the heater-size-dependent thermal conductivity of a material. Overall, they observed the anticipated new picture of heat conduction: While the common, classical diffusion theory is applicable to large heat sources, it fails for small heat sources. By varying the size of heat sources, Chen and his colleagues can map out how far phonons travel between collisions, and how much they contribute to heat conduction.

Zeng says that the group’s experimental setup can be used to better understand, and potentially tune, a material’s thermal conductivity. For example, if an engineer desires a material with certain thermal properties, the mean free path distribution could serve as a blueprint to design specific “scattering centers” within the material — locations that prompt phonon collisions, in turn scattering heat propagation, leading to reduced heat carrying ability. Although such effects are not desirable in keeping a computer chip cool, they are suitable in thermoelectric devices, which convert heat to electricity. For such applications, materials that are electrically conducting but thermally insulating are desired.

“The important thing is, we have a spectroscopy tool to measure the mean free path distribution, and that distribution is important for many technological applications,” Zeng says.

This research was funded in part by in part by MIT’s Solid-State Solar Thermal Energy Conversion Center, which is funded by U.S. Department of Energy.

Source: http://newsoffice.mit.edu/2015/measuring-distance-between-phonon-collisions-0601

Electronics Based on a Two Dimensional Electron Gas

A new material could open the door to a new kind of electronics: researchers at the Vienna University of Technology have created a stable two-dimensional electron gas in strontium titanate.

Usually, microelectronic devices are made of silicon or similar semiconductors. Recently, the electronic properties of metal oxides have become quite interesting. These materials are more complex, yet offer a broader range of possibilities to tune their properties. An important breakthrough has now been achieved at the Vienna University of Technology: a two dimensional electron gas was created in strontium titanate. In a thin layer just below the surface electrons can move freely and occupy different quantum states.

Strontium titanate is not only a potential future alternative to standard semiconductors, it could also exhibit interesting phenomena, such as superconductivity, thermoelectricity or magnetic effects that do not occur in the materials that are used for today’s electronic devices.

The Surface Layer and the Inside

This project closely links theoretical calculations and experiments. Zhiming Wang from Professor Ulrike Diebold’s research team was the leading experimentalist; some of the experimental work was done at the synchrotron BESSY in Berlin. Zhicheng Zhong from Professor Karsten Held’s group studied the material in computer simulations.

Not all of the atoms of strontium titanate are arranged in the same pattern: if the material is cut at a certain angle, the atoms of the surface layer form a structure, which is different from the structure in the bulk of the material. “Inside, every titanium atom has six neighbouring oxygen atoms, whereas the titanium atoms at the surface are only connected to four oxygen atoms each”, says Ulrike Diebold. This is the reason for the remarkable chemical stability of the surface. Normally such materials are damaged if they come into contact with water or oxygen.

Migrating Oxygen Atoms

Something remarkable happens when the material is irradiated with high-energy electromagnetic waves: “The radiation can remove oxygen atoms from the surface”, Ulrike Diebold explains. Then other oxygen atoms from within the bulk of the material move up to the surface. Inside the material, an oxygen deficiency builds up, as well a surplus of electrons.

“These electrons, located in a two dimensional layer very close to the surface, can move freely. We call this an electron gas”, says Karsten Held. There has already been some evidence of two dimensional electron gases in similar materials, but until now the creation of a stable, durable electron gas at a surface has been impossible. The properties of the electrons in the gas can be finely tuned. Depending on the intensity of the radiation, the number of electrons varies. By adding different atoms, the electronic properties can also be changed.

“In solid state physics, the so-called band structure of a material is very important. It describes the relationship between the energy and the momentum of the electrons. The remarkable thing about our surface is that it shows completely different kinds of band structures, depending on the quantum state of the electron”, says Karsten Held.

The electron gas in the new material exhibits a multitude of different electronic structures. Some of them could very well be suitable for producing interesting magnetic effects or superconductivity. The promising properties of strontium titanate will now be further investigated. The researchers hope that, by applying external electric fields or by placing additional metal atoms on the surface, the new material could reveal a few more of its secrets.

Source: http://www.tuwien.ac.at/en/news/news_detail/article/8663/

Self-aligning DNA wires for application in nanoelectronics

Since continuous miniaturization in microelectronics is already starting to reach the physical limits, researchers are seeking new methods for device fabrication. One promising candidate is the DNA origami technique in which individual strands of the biomolecule self-assemble into arbitrarily shaped nanostructures. 

The formation of entire circuits, however, requires the controlled positioning of these DNA structures on a surface – something which previously has only been possible using very elaborate techniques. 

Now, researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have come up with a simpler strategy which combines DNA origami with self-organized pattern formation. The researchers' method is featured in the scientific journal Nanoscale’s current issue (DOI: 10.1039/C3NR04627C)

Dr. Adrian Keller of the HZDR Institute of Ion Beam Physics and Materials Research describes the new method: “Its beauty lies with the fact that we're allowing nature to simply run its course as soon as we've created the necessary framework.” In the DNA origami technique, the DNA structures self-assemble as long strands of the biomolecule fold into complex, predefined nanoscale shapes by pairing with multiple smaller DNA strands. The physicists used the technique to produce small tubes with lengths of 412 nanometers and diameters of six nanometers. These structures can be used as scaffolds for manufacturing nanoelectronic components like nanowires.

In order to align these nanotubes on the surface, the researchers drew on a principle of self-organization that is actually quite common in nature. Wind may for instance form ordered patterns on a sandy beach. "Similar processes are at work here," explains Keller. "We irradiate the surface onto which we want to place the nanostructures - in our case, the silicon wafers - with ions. This results in the spontaneous appearance of ordered nanopatterns resembling miniature sand dunes. At that point, our job is pretty much done as natural processes are taking over and doing all the work."

Through electrostatic interactions between the charged DNA nanostructures and the charged surface, the nanotubes align themselves in the valleys of the dunes. Says Keller: "This technique works so well that not only do the small tubes follow the wavy patterns, they even replicate occasional pattern defects. Meaning this technique should also allow for production of curved nanocomponents." The maximum degree of alignment the Dresden researchers were able to obtain was at a pattern wavelength of 30 nanometers. "True, we're only looking at a total yield of 70 percent of nanotubes that perfectly follow the pattern," concedes Keller. "But it's still impressive considering the natural process we used."

Because unlike previous approaches, according to Keller, the new technique is quick, cheap, and simple. "Until now, we had to draw on lithographic techniques plus treat the surface with chemicals in order to align the DNA nanostructures. Although this does produce the desired outcome, it nonetheless complicates the processes. Our new technique offers a much simpler alternative." Since aligning the small tubes is based exclusively on electrostatic interaction with the prestructured surface, using this particular method the nanotubes could also be arranged into more complex arrays such as electronic circuits. Keller is convinced that they can be attached to individual transistors, for instance, and connect them electrically: "This way, DNA based nanocomponents could be integrated into technological devices and contribute to further miniaturization."

Developing electronic circuits based on such self-organization principles is the subject of research at the HZDR-coordinated International Helmholtz Research School NanoNet (www.ihrs-nanonet.de). The international Ph.D. program trains junior scientists in molecular electronics as part of DRESDEN-concept – an alliance between the HZDR, the TU Dresden, and several partners from science. The focus of the program is on techniques which functionalize atoms, molecules, and artificial nanostructures to enable information exchange among them and eventually build electronic building blocks like a transistor. The long term vision of this scientific approach is the development of components that spontaneously assemble into electronic circuits.

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Publication:

Teshome, B., Facsko, S. & Keller, A. (2014). Topography-controlled alignment of DNA origami nanotubes on nanopatterned surfaces. Nanoscale, 6,1790, DOI: 10.1039/C3NR04627C

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Source: http://www.hzdr.de/db/Cms?pNid=99&pOid=40853