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Posts Tagged ‘quantum dots’

Research Alert: January 26, 2015

Monday, January 26th, 2015

Solving an organic semiconductor mystery

Organic semiconductors are prized for light emitting diodes (LEDs), field effect transistors (FETs) and photovoltaic cells. As they can be printed from solution, they provide a highly scalable, cost-effective alternative to silicon-based devices. Uneven performances, however, have been a persistent problem. Scientists have known that the performance issues originate in the domain interfaces within organic semiconductor thin films, but have not known the cause. This mystery now appears to have been solved.

Naomi Ginsberg, a faculty chemist with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory and the University of California (UC) Berkeley, led a team that used a unique form of microscopy to study the domain interfaces within an especially high-performing solution-processed organic semiconductor called TIPS-pentacene. She and her team discovered a cluttered jumble of randomly oriented nanocrystallites that become kinetically trapped in the interfaces during solution casting. Like debris on a highway, these nanocrystallites impede the flow of charge-carriers.

“If the interfaces were neat and clean, they wouldn’t have such a large impact on performance, but the presence of the nanocrystallites reduces charge-carrier mobility,” Ginsberg says. “Our nanocrystallite model for the interface, which is consistent with observations, provides critical information that can be used to correlate solution-processing methods to optimal device performances.”

Ginsberg, who holds appointments with Berkeley Lab’s Physical Biosciences Division and its Materials Sciences Division, as well as UC Berkeley’s departments of chemistry and physics, is the corresponding author of a paper describing this research in Nature Communications. The paper is titled “Exciton dynamics reveals aggregates with intermolecular order at hidden interfaces in solution-cast organic semiconducting films.” Co-authors are Cathy Wong, Benjamin Cotts and Hao Wu.

Ginsberg and her group overcame the challenges by using transient absorption (TA) microscopy, a technique in which femtosecond laser pulses excite transient energy states and detectors measure the changes in the absorption spectra. The Berkeley researchers carried out TA microscopy on an optical microscope they constructed themselves that enabled them to generate focal volumes that are a thousand times smaller than is typical for conventional TA microscopes. They also deployed multiple different light polarizations that allowed them to isolate interface signals not seen in either of the adjacent domains.

“Instrumentation, including very good detectors, the painstaking collection of data to ensure good signal-to-noise ratios, and the way we crafted the experiment and analysis were all critical to our success,” Ginsberg says. “Our spatial resolution and light polarization sensitivity were also essential to be able to unequivocally see a signature of the interface that was not swamped by the bulk, which contributes much more to the raw signal by volume.”

The methology developed by Ginsberg and her team to uncover structural motifs at hidden interfaces in organic semiconductor thin films should add a predictive factor to scalable and affordable solution-processing of these materials. This predictive capability should help minimize discontinuities and maximize charge-carrier mobility. Currently, researchers use what is essentially a trial-and-error approach, in which different solution casting conditions are tested to see how well the resulting devices perform.

“Our methodology provides an important intermediary in the feedback loop of device optimization by characterizing the microscopic details of the films that go into the devices, and by inferring how the solution casting could have created the structures at the interfaces,” Ginsberg says. “As a result, we can suggest how to alter the delicate balance of solution casting parameters to make more functional films.”

Rice-sized laser, powered one electron at a time, bodes well for quantum computing

Princeton University researchers have built a rice grain-sized laser powered by single electrons tunneling through artificial atoms known as quantum dots. The tiny microwave laser, or “maser,” is a demonstration of the fundamental interactions between light and moving electrons.

The researchers built the device — which uses about one-billionth the electric current needed to power a hair dryer — while exploring how to use quantum dots, which are bits of semiconductor material that act like single atoms, as components for quantum computers.

“It is basically as small as you can go with these single-electron devices,” said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.

The device demonstrates a major step forward for efforts to build quantum-computing systems out of semiconductor materials, according to co-author and collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology.

“I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices,” Taylor said.

The original aim of the project was not to build a maser, but to explore how to use double quantum dots — which are two quantum dots joined together — as quantum bits, or qubits, the basic units of information in quantum computers.

“The goal was to get the double quantum dots to communicate with each other,” said Yinyu Liu, a physics graduate student in Petta’s lab. The team also included graduate student Jiri Stehlik and associate research scholar Christopher Eichler in Princeton’s Department of Physics, as well as postdoctoral researcher Michael Gullans of the Joint Quantum Institute.

Because quantum dots can communicate through the entanglement of light particles, or photons, the researchers designed dots that emit photons when single electrons leap from a higher energy level to a lower energy level to cross the double dot.

Each double quantum dot can only transfer one electron at a time, Petta explained. “It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person,” he said. “They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned — they are trapped in all three spatial dimensions.”

The researchers fabricated the double quantum dots from extremely thin nanowires (about 50 nanometers, or a billionth of a meter, in diameter) made of a semiconductor material called indium arsenide. They patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, which control the energy levels in the dots.

To construct the maser, they placed the two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero, around minus 459 degrees Fahrenheit. “This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance,” Taylor said.

When the device was switched on, electrons flowed single-file through each double quantum dot, causing them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity to build into a coherent beam of microwave light.

One advantage of the new maser is that the energy levels inside the dots can be fine-tuned to produce light at other frequencies, which cannot be done with other semiconductor lasers in which the frequency is fixed during manufacturing, Petta said. The larger the energy difference between the two levels, the higher the frequency of light emitted.

Claire Gmachl, who was not involved in the research and is Princeton’s Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers, said that because lasers, masers and other forms of coherent light sources are used in communications, sensing, medicine and many other aspects of modern life, the study is an important one.

“In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron,” Gmachl said. “The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources.”

Carbon nanotube finding could lead to flexible electronics with longer battery life

University of Wisconsin-Madison materials engineers have made a significant leap toward creating higher-performance electronics with improved battery life — and the ability to flex and stretch.

Led by materials science Associate Professor Michael Arnold and Professor Padma Gopalan, the team has reported the highest-performing carbon nanotube transistors ever demonstrated. In addition to paving the way for improved consumer electronics, this technology could also have specific uses in industrial and military applications.

In a paper published recently in the journal ACS Nano, Arnold, Gopalan and their students reported transistors with an on-off ratio that’s 1,000 times better and a conductance that’s 100 times better than previous state-of-the-art carbon nanotube transistors.

“Carbon nanotubes are very strong and very flexible, so they could also be used to make flexible displays and electronics that can stretch and bend, allowing you to integrate electronics into new places like clothing,” says Arnold. “The advance enables new types of electronics that aren’t possible with the more brittle materials manufacturers are currently using.”

Carbon nanotubes are single atomic sheets of carbon rolled up into a tube. As some of the best electrical conductors ever discovered, carbon nanotubes have long been recognized as a promising material for next-generation transistors, which are semiconductor devices that can act like an on-off switch for current or amplify current. This forms the foundation of an electronic device.

However, researchers have struggled to isolate purely semiconducting carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and “short” the device. Researchers have also struggled to control the placement and alignment of nanotubes. Until now, these two challenges have limited the development of high-performance carbon nanotube transistors.

Building on more than two decades of carbon nanotube research in the field, the UW-Madison team drew on cutting-edge technologies that use polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.

Previous techniques to align the nanotubes resulted in less-than-desirable packing density, or how close the nanotubes are to one another when they are assembled in a film. However, the UW-Madison researchers pioneered a new technique, called floating evaporative self-assembly, or FESA, which they described earlier in 2014 in the ACS journal Langmuir. In that technique, researchers exploited a self-assembly phenomenon triggered by rapidly evaporating a carbon nanotube solution.

The team’s most recent advance also brings the field closer to realizing carbon nanotube transistors as a feasible replacement for silicon transistors in computer chips and in high-frequency communication devices, which are rapidly approaching their physical scaling and performance limits.

“This is not an incremental improvement in performance,” Arnold says. “With these results, we’ve really made a leap in carbon nanotube transistors. Our carbon nanotube transistors are an order of magnitude better in conductance than the best thin film transistor technologies currently being used commercially while still switching on and off like a transistor is supposed to function.”

The researchers have patented their technology through the Wisconsin Alumni Research Foundation and have begun working with companies to accelerate the technology transfer to industry.

Research Alert: March 4, 2014

Tuesday, March 4th, 2014

SRC and MIT extend high-resolution lithography

MIT researchers sponsored by Semiconductor Research Corporation have introduced new directed self-assembly (DSA) techniques that promise to help semiconductor manufacturers develop more advanced and less expensive components.

The MIT study demonstrates that complex patterns of lines, bends and junctions with feature sizes below 20nm can be made by block copolymer self-assembly guided by a greatly simplified template. This study explained how to design the template to achieve a desired pattern. Electron-beam lithography was used to produce the template serially, while the block copolymer filled in the rest of the pattern in a parallel process. This hybrid process can be five or more times faster than writing the entire pattern by electron beam lithography.

sembly to produce dense, high resolution patterns was proposed and demonstrated several years ago, but there was no systematic way to design templates to achieve a complex block copolymer pattern. The MIT study developed a simple way to design a template to achieve a specific block copolymer pattern over a large area. Although the work used electron-beam lithography to define the template, other methods such as photolithography with trimming could be used to produce the templates.

Block copolymer lithography is already on the semiconductor industry roadmap as directed self-assembly, but the process is still in its infancy. Although DSA patterning has been demonstrated on 300 millimeter wafers, these early trials used templates fabricated by photolithography with limited resolution and limited control of the feature geometry. The MIT process offers a path to far more complicated geometries using relatively simple templates. Next steps involve the research being shared with semiconductor companies for further studies.

How 19th century physics could change the future of nanotechnology

Researchers at the University of Cincinnati have found that their unique method of light-matter interaction analysis appears to be a good way of helping make better semiconductor nanowires.

“Semiconductor nanowires are one of the hottest topics in the nanoscience research field in the recent decade,” says Yuda Wang, a UC doctoral student. “Due to the unique geometry compared to conventional bulk semiconductors, nanowires have already shown many advantageous properties, particularly in novel applications in such fields as nanoelectronics, nanophotonics, nanobiochemistry and nanoenergy.”

Wang will present the team’s research “Transient Rayleigh Scattering Spectroscopy Measurement of Carrier Dynamics in Zincblende and Wurtzite Indium Phosphide Nanowires” at the American Physical Society (APS) meeting to be held March 3-7 in Denver. Nearly 10,000 professionals, scholars and students will attend the APS meeting to discuss new research from industry, universities and laboratories from around the world.

Key to this research is UC’s new method of Rayleigh scattering, a phenomenon first described in 1871 and the scientific explanation for why the sky is blue in the daytime and turns red at sunset. The researchers’ Rayleigh scattering technique probes the band structures and electron-hole dynamics inside a single indium phosphide nanowire, allowing them to observe the response with a time resolution in the femtosecond range – or one quadrillionth of a second.

JILA physicists discover “quantum droplet” in semiconductor

JILA physicists used an ultrafast laser and help from German theorists to discover a new semiconductor quasiparticle—a handful of smaller particles that briefly condense into a liquid-like droplet.

Quasiparticles are composites of smaller particles that can be created inside solid materials and act together in a predictable way. A simple example is the exciton, a pairing, due to electrostatic forces, of an electron and a so-called “hole,” a place in the material’s energy structure where an electron could be, but isn’t.

The new quasiparticle, described in the Feb. 27, 2014, issue of Nature and featured on the journal’s cover, is a microscopic complex of electrons and holes in a new, unpaired arrangement. The researchers call this a “quantum droplet” because it has quantum characteristics such as well-ordered energy levels, but also has some of the characteristics of a liquid. It can have ripples, for example. It differs from a familiar liquid like water because the quantum droplet has a finite size, beyond which the association between electrons and holes disappears.

Although its lifetime is only a fleeting 25 picoseconds (trillionths of a second), the quantum droplet is stable enough for research on how light interacts with specialized forms of matter.

The JILA team created the new quasiparticle by exciting a gallium-arsenide semiconductor with an ultrafast red laser emitting about 100 million pulses per second. The pulses initially form excitons, which are known to travel around in semiconductors. As laser pulse intensity increases, more electron-hole pairs are created, with quantum droplets developing when the exciton density reaches a certain level. At that point, the pairing disappears and a few electrons take up positions relative to a given hole. The negatively charged electrons and positively charged holes create a neutral droplet. The droplets are like bubbles held together briefly by pressure from the surrounding plasma.

The Week In Review: February 28, 2014

Friday, February 28th, 2014

MIT researchers sponsored by Semiconductor Research Corporation have introduced new directed self-assembly (DSA) techniques that promise to help semiconductor manufacturers develop more advanced and less expensive components.

Renesas Electronics Corporation unveiled the RX64M Group of microcontrollers (MCUs), its first product in the flagship RX Family of 32-bit MCUs to be fabricated in a 40nm process.

EV Group and Brisbane Materials Technology introduced a new anti-reflective (AR) coating solution based on BMT’s innovative XeroCoat materials. The jointly developed manufacturing solution enables lumen output increases of up to eight percent.  The AR coating manufacturing solution can be seamlessly integrated with established production schemes, allowing the coating of LED components at room temperature and atmospheric pressure.

International Rectifier, IR, announced that the company has commenced initial production at its new ultra-thin wafer processing facility in Singapore (IRSG). Wafer thinning, metallization, testing and additional proprietary wafer level processing are undertaken at the new 60,000 square foot manufacturing site which receives processed wafers from IR’s internal fabs and foundry partners. The facility, which will employ approximately 135 people in the initial phase, will process a variety of products, including the company’s latest generation power MOSFETs and IGBTs.

PLACYD, an EU funded consortium of industrial and academic collaborators and led by Arkema will establish a dedicated material manufacturing facility that allows the production of block copolymers meeting the rigorous standards required for their use in industry as nanolithographic templates.  PLACYD brings together researchers and industries to allow for the first time the integration of synthesis through to wafer scale production and system/device characterization. Partners include: CEA-Leti, STMicroelectronics, Intel IPLS, Mentor Graphics, ASML and other leading EU companies and research organizations.

JILA physicists used an ultrafast laser and help from German theorists to discover a new semiconductor quasiparticle—a handful of smaller particles that briefly condense into a liquid-like droplet. Quasiparticles are composites of smaller particles that can be created inside solid materials and act together in a predictable way. A simple example is the exciton, a pairing, due to electrostatic forces, of an electron and a so-called “hole,” a place in the material’s energy structure where an electron could be, but isn’t.

Research Alert: Feb. 18, 2014

Tuesday, February 18th, 2014

Breakthrough development of flexible 1D-1R memory cell array

With the introduction of curved smartphones, flexible electronic goods are gradually moving to the center stages of various markets. Flexible display technology is the culmination of the latest, cutting-edge electric cell device technology. Developing such products, however, requires not only a curved display, but also operational precision of other parts, including the memory, in a flexible state.

Dr. Tae-Wook Kim at KIST announced their successful development of a 64-bit memory array using flexible and twistable carbon nano material and organo-polymer compound, which can accurately store and delete data.

The most common memory cell today uses a silicone (Si)-based hard, inorganic matter, however, in order to be flexible, the materials were fabricated with a carbon-based organic compound. The recently developed memory cell uses a technology, which arranges such organic material in a single configuration at room temperature and places the material on a desired spot on the substrate. This is the core technology in enlarging the storage capacity of memory, an unprecedented discovery until now. In particular, demonstrating this on a bendable substrate required highly sophisticated technology, which signified difficulties in fabrication. The research team developed a technology with the above characteristics to make the electric current flow in one direction so that the data could be rewritable even in a curved state.

Leeds researchers build world’s most powerful terahertz laser chip

University of Leeds researchers have taken the lead in the race to build the world’s most powerful terahertz laser chip.

A paper in the Institution of Engineering and Technology’s (IET) journal Electronics Letters reports that the Leeds team has exceeded a 1 Watt output power from a quantum cascade terahertz laser.

The new record more than doubles landmarks set by the Massachusetts Institute of Technology (MIT) and subsequently by a team from Vienna last year.

Terahertz waves, which lie in the part of the electromagnetic spectrum between infrared and microwaves, can penetrate materials that block visible light and have a wide range of possible uses including chemical analysis, security scanning, medical imaging, and telecommunications.

Widely publicised potential applications include monitoring pharmaceutical products, the remote sensing of chemical signatures of explosives in unopened envelopes, and the non-invasive detection of cancers in the human body.

However, one of the main challenges for scientists and engineers is making the lasers powerful and compact enough to be useful.

Professor Edmund Linfield, Professor of Terahertz Electronics in the University’s School of Electronic and Electrical Engineering, said: “Although it is possible to build large instruments that generate powerful beams of terahertz radiation, these instruments are only useful for a limited set of applications. We need terahertz lasers that not only offer high power but are also portable and low cost.”

The quantum cascade terahertz lasers being developed by Leeds are only a few square millimetres in size.

In October 2013, Vienna University of Technology announced that its researchers had smashed the world record output power for quantum cascade terahertz lasers previously held by Massachusetts Institute of Technology (MIT). The Austrian team reported an output of 0.47 Watt from a single laser facet, nearly double the output power reported by the MIT team. The Leeds group has now achieved an output of more than 1 Watt from a single laser facet.

Professor Linfield said: “The process of making these lasers is extraordinarily delicate. Layers of different semiconductors such as gallium arsenide are built up one atomic monolayer at a time. We control the thickness and composition of each individual layer very accurately and build up a semiconductor material of between typically 1,000 and 2,000 layers. The record power of our new laser is due to the expertise that we have developed at Leeds in fabricating these layered semiconductors, together with our ability to engineer these materials subsequently into suitable and powerful laser devices.”

Professor Giles Davies, Professor of Electronic and Photonic Engineering in the School of Electronic and Electrical Engineering, said: “The University of Leeds has been an international leader in terahertz engineering for many years. This work is a key step toward increasing the power of these lasers while keeping them compact and affordable enough to deliver the range of applications promised by terahertz technology.”

Quantum dots provide complete control of photons

A semiconductive materials research group led by Professor Per Olof Holtz is now presenting an alternative method where asymmetrical quantum dots of a nitride material with indium is formed at the top of microscopic six-sided pyramids. With these, they have succeeded in creating light with a high degree of linear polarization, on average 84%. The results are being published in the Nature periodical Light: Science & Applications.

“We’re demonstrating a new way to generate polarized light directly, with a predetermined polarization vector and with a degree of polarization substantially higher than with the methods previously launched,” Professor Holtz says.

In experiments, quantum dots were used that emit violet light with a wavelength of 415 nm, but the photons can in principle take on any colour at all within the visible spectrum through varying the amount of the metal indium.

“Our theoretical calculations point to the fact that an increased amount of indium in the quantum dots further improves the degree of polarization,” says reader Fredrik Karlsson, one of the authors of the article.

The micropyramid is constructed through crystalline growth, atom layer by atom layer, of the semiconductive material gallium nitride. A couple of nanothin layers where the metal indium is also included are laid on top of this. From the asymmetrical quantum dot thus formed at the top, light particles are emitted with a well-defined wavelength.

The results of the research are opening up possibilities, for example for more energy-effective polarized light-emitting diodes in the light source for LCD screens. As the quantum dots can also emit one photon at a time, this is very promising technology for quantum encryption, a growing technology for wiretap-proof communications.

Research Alert: Feb. 4, 2014

Tuesday, February 4th, 2014

Diamond defects boost quantum technology

New research shows that a remarkable defect in synthetic diamond produced by chemical vapor deposition allows researchers to measure, witness, and potentially manipulate electrons in a manner that could lead to new “quantum technology” for information processing.

An “NV-” center can be created within a diamond’s scaffold-like structure by replacing a missing carbon atom with a nitrogen atom (N)that has trapped an electron making the center negatively charged. Scientists can monitor the center’s behavior and thereby provide a window for understanding how electrons respond to different conditions. The center has the potential to serve as a qubit in future quantum computers.

For the first time, Struzhkin and his team, led by Marcus Doherty of the Australian National University, observed what happens to electrons in these NV- centers under high-pressure and normal temperatures. Coauthor of the study, Viktor Struzhkin at the Carnegie Institution for Science, explained: “Our technique offers a powerful new tool for analyzing and manipulating electrons to advance our understanding of high-pressure superconductivity, as well as magnetic and electrical properties.”

Struzhkin and team subjected single-crystal diamonds to pressures up to 600,000 times atmospheric pressure at sea level (60 gigapascals, GPa) in a diamond anvil cell and observed how electron spin and motion were affected. They optically excited the NV- centers with light and scanned microwave frequencies in a process called optically detected magnetic resonance to determine any changes. The NV- center is very sensitive to magnetic fields, electrical fields, and stress.

Until now, researchers thought that the orbits of the electrons that contribute to the defect’s electronic structure and spin dynamics were localized to the area immediately surrounding the vacancy. Doherty explained: “Our team found instead that the electrons also orbit more distant atoms and that the span of their orbits contract with increasing pressure.”

Quantum dots provide complete control of photons

A semiconductive materials research group led by Professor Per Olof Holtz is now presenting an alternative method where asymmetrical quantum dots of a nitride material with indium is formed at the top of microscopic six-sided pyramids. With these, they have succeeded in creating light with a high degree of linear polarization, on average 84%. The results are being published in the Nature periodical Light: Science & Applications.

“We’re demonstrating a new way to generate polarized light directly, with a predetermined polarization vector and with a degree of polarization substantially higher than with the methods previously launched,” Professor Holtz says.

In experiments, quantum dots were used that emit violet light with a wavelength of 415 nm, but the photons can in principle take on any color at all within the visible spectrum through varying the amount of the metal indium.

“Our theoretical calculations point to the fact that an increased amount of indium in the quantum dots further improves the degree of polarization,” says reader Fredrik Karlsson, one of the authors of the article.

The micropyramid is constructed through crystalline growth, atom layer by atom layer, of the semiconductive material gallium nitride. A couple of nanothin layers where the metal indium is also included are laid on top of this. From the asymmetrical quantum dot thus formed at the top, light particles are emitted with a well-defined wavelength.

The results of the research are opening up possibilities, for example for more energy-effective polarized light-emitting diodes in the light source for LCD screens. As the quantum dots can also emit one photon at a time, this is very promising technology for quantum encryption, a growing technology for wiretap-proof communications.

New theory may lead to more efficient solar cells

A new theoretical model developed by professors at the University of Houston (UH) and Université de Montréal may hold the key to methods for developing better materials for solar cells.

Eric Bittner, a John and Rebecca Moores Professor of Chemistry and Physics in UH’s College of Natural Sciences and Mathematics, and Carlos Silva, an associate professor at the Université de Montréal and Canada Research Chair in Organic Semiconductor Materials, say the model could lead to new solar cell materials made from improved blends of semiconducting polymers and fullerenes.

“Scientists don’t fully understand what is going on inside the materials that make up solar cells. We were trying to get at the fundamental photochemistry or photophysics that describes how these cells work,” Bittner said.

“There is a theoretical limit for the efficiency of the ideal solar cell – the Shockley-Queisser limit. The theory we published describes how we might be able to get above this theoretical limit by taking advantage of quantum mechanical effects,” Bittner said. “By understanding these effects and making use of them in the design of a solar cell, we believe you can improve efficiency.”

Silva added, “In polymeric semiconductors, where plastics form the active layer of solar cells, the electronic structure of the material is intimately correlated with the vibrational motion within the polymer chain. Quantum-mechanical effects due to such vibrational-electron coupling give rise to a plethora of interesting physical processes that can be controlled to optimize solar cell efficiencies by designing materials that best exploit them.”

Research Alert: Jan. 28, 2014

Tuesday, January 28th, 2014

New quantum dots herald a new era of electronics operating on a single-atom level

Physicists from the Institute of Experimental Physics at the Faculty of Physics at the University of Warsaw (FUW) have successfully created and studied two completely new types of the structures. The materials and elements used in the process make it wholly likely that solotronic devices may come into widespread use in the future.

The results, the Warsaw physicists have just published in Nature Communications, pave the way for developing the field of solotronics.

The researchers have presented two new systems with single magnetic ions: CdTe quantum dots with a cobalt atom, and cadmium selenide (CdSe) dots with a manganese atom.

The discovery made by physicists at the University of Warsaw demonstrates that other magnetic elements – such as chromium, iron and nickel – can be used in place of manganese. These elements do not have nuclear spin, which should make quantum dots that contain them easier to manipulate.

In quantum dots where tellurium is replaced by the lighter selenium, researchers observed that the duration for which information was remembered increased by an order of magnitude. This finding suggests that using lighter elements should prolong the time quantum dots containing single magnetic ions store information, perhaps even by several orders of magnitude.

“We have demonstrated that two quantum systems that were believed not to be viable in fact worked very effectively. This opens up a broad field in our search for other, previously rejected combinations of materials for quantum dots and magnetic ions,” concludes Dr. Wojciech Pacuski (FUW).

Cooling microprocessors with carbon nanotubes

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a “process friendly” technique that would enable the cooling of microprocessor chips through carbon nanotubes.

Frank Ogletree, a physicist with Berkeley Lab’s Materials Sciences Division, led a study in which organic molecules were used to form strong covalent bonds between carbon nanotubes and metal surfaces. This improved by six-fold the flow of heat from the metal to the carbon nanotubes, paving the way for faster, more efficient cooling of computer chips. The technique is done through gas vapor or liquid chemistry at low temperatures, making it suitable for the manufacturing of computer chips.

“We’ve developed covalent bond pathways that work for oxide-forming metals, such as aluminum and silicon, and for more noble metals, such as gold and copper,” says Ogletree, who serves as a staff engineer for the Imaging Facility at the Molecular Foundry, a DOE nanoscience center hosted by Berkeley Lab. “In both cases the mechanical adhesion improved so that surface bonds were strong enough to pull a carbon nanotube array off of its growth substrate and significantly improve the transport of heat across the interface.”

Although the approach used by Ogletree, Kaur and their colleagues substantially strengthened the contact between a metal and individual carbon nanotubes within an array, a majority of the nanotubes within the array may still fail to connect with the metal. The Berkeley team is now developing a way to improve the density of carbon nanotube/metal contacts. Their technique should also be applicable to single and multi-layer graphene devices, which face the same cooling issues.

“Part of our mission at the Molecular Foundry is to help develop solutions for technology problems posed to us by industrial users that also raise fundamental science questions,” Ogletree says. “In developing this technique to address a real-world technology problem, we also created tools that yield new information on fundamental chemistry.”

This work was supported by the DOE Office of Science and the Intel Corporation.


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