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Jan 2018

Computing Graphene-Fullerene Junctions in Thermoelectric Devices

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Editorial Feature published in AZONANO on our article “Thermoelectricity in vertical graphene-C60-graphene architectures”- Wu Q., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-10938-2

Recent advances in single-molecule thermoelectricity has isolated and identified different families of high-performance molecules. However, to realize the commercial potential of these molecules and convert them into real-world thin-film energy-harvesting devices, fundamental issues surrounding parallel-aligned junctions within these devices need to be addressed.

A team of Researchers from the UK and Spain have studied a junction composed of two parallel C60 molecules sandwiched between two graphene monolayers, in an attempt to boost the electrical and thermoelectric performance against current single-junction mechanisms.

Molecular devices composed of single or multiple molecules which are bridged by at least two electrodes have gathered a lot of attention from both a theoretical and experimental point of view. Such devices have been found to possess a plethora of properties which facilitate excellent tunability and transport mechanisms, including negative differential resistance (NDR), electrical switching and thermoelectric power generation.

Common thermoelectric materials of the inorganic variety, i.e. Pb, Bi, Co, Sb are toxic and expensive due to finite sources across the globe. So, to circumnavigate the resource shortage, Researchers turned to using single organic molecules, which has worked with great effect so far. But, to prove their commercial value, issues surrounding junctions being placed in parallel need to be resolved.

As a step in the right direction to solving this scientific conundrum, the Researchers believed that a controlled scalability may hold the key. The Researchers have taken to using density functional theory (DFT) calculations to help determine how parallel junctions can be addressed.

The Researchers assembled a four-terminal device at the edges of two graphene sheets which sandwiched two C60 molecules. The terminals were set up as semi-infinite crystalline leads to eliminate edge effects on the graphene sheets. The Researchers used a code called SIESTA to obtain the optimized geometry and density approximation, and used a transport code named GOLLUM to compute the electrical and thermoelectric properties of the devices from the mean-field Hamiltonian and overlap matrices. The Researchers chose a double-z plus polarization (DZP) basis set for their calculations.

The Researchers investigated the properties of the C60 molecules by placing them parallel to each other and sandwiched between the two graphene sheet electrodes. Unlike in classical conductors, the Researchers found that increasing the number of parallel junctions from one to two caused the electrical conductivity to increase by at least a factor of two.

The Researchers also found that the Seebeck coefficient, i.e. the thermoelectric power or thermoelectric sensitivity, is sensitive to the number of molecules sandwiched between the electrodes. In classical conductors, the sensitivity would not change. The Seebeck coefficient sensitivity was also shown to not be proportional to the increase of parallel molecules.

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Sep 2017

Wireless high-speed data and power transfer integrated

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Researchers from North Carolina State University have developed a system that can simultaneously deliver watts of power and transmit data at rates high enough to stream video over the same wireless connection. By integrating power and high-speed data, a true single “wireless” connection can be achieved.

“Recently wireless power as re-emerged as a technology to free us from the power cord,” says David Ricketts, an associate professor of electrical and computer engineering at NC State and senior author of a paper on the work. “One of the most popular applications is in wireless cell phone charging pads. As many know, these unfortunately often require almost physical contact with the pad, limiting the usefulness of a truly ‘wireless’ power source. Recent work by several researchers have extended wireless power to ‘mid-range’ which can supply power at inches to feet of separation. While encouraging, most of the wireless power systems have only focused on the power problem — not the data that needs to accompany any of our smart devices today. Addressing those data needs is what sets our work apart here.”

Wireless power transfer technologies use magnetic fields to transmit power through the air. To minimize the power lost in generating these magnetic fields, you need to use antennas that operate in a narrow bandwidth — particularly if the transmitter and receiver are inches or feet apart from each other.

Because using a narrow bandwidth antenna limits data transfer, devices incorporating wireless power transfer have normally also incorporated separate radios for data transmission. And having separate systems for data and power transmission increases the cost, weight and complexity of the relevant device.

The NC State team realized that while high-efficiency power transfer, especially at longer distances, does require very narrow band antennas, the system bandwidth can actually be much wider.

“People thought that efficient wireless power transfer requires the use of narrow bandwidth transmitters and receivers, and that this therefore limited data transfer,” Ricketts says. “We’ve shown that you can configure a wide-bandwidth system with narrow-bandwidth components, giving you the best of both worlds.”

With this wider bandwidth, the NC State team then envisioned the wireless power transfer link as a communication link, adapting data-rate enhancement techniques, such as channel equalization, to further improve data rate and data signal quality.

The researchers tested their system with and without data transfer. They found that when transferring almost 3 watts of power – more than enough to power your tablet during video playback — the system was only 2.3 percent less efficient when also transmitting 3.39 megabytes of data per second. At 2 watts of power, the difference in efficiency was only 1.3 percent. The tests were conducted with the transmitter and receiver 16 centimeters, or 6.3 inches, apart, demonstrating the ability of their system to operate in longer-distance wireless power links.

“Our system is comparable in power transfer efficiency to similar wireless power transfer devices, and shows that you can design a wireless power link system that retains almost all of its efficiency while streaming a movie on Netflix,” Ricketts says.

Source: North Carolina State University, 18 September 2017.

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Sep 2015

CINN researchers develope a new microscopy technique for improving the performace of magnetic storage devices

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The technology of magnetic storage of digital data has progressed enormously in the last two decades thanks to the scientific advances in nanomagnetism. At present, magnetic bits have dimensions of few tens of nm and magnetic devices are based in stacks of ultrathin (few nm) films of different magnetic characteristics. The fundamental understanding of the magnetic interactions between the layers, the practical realization and the detailed characterization of the magnetic properties are necessary steps to achieve magnetic devices with unprecedented dimensions and performances. Magnetic domain imaging is a major field of activity to visualize magnetic structures from few tens of nm down to atomic scale, not only at the material surface but also at buried layers.

A team of researchers from Universidad de Oviedo and Centro de Investigación en Nanomateriales y Nanotecnología (CSIC-Universidad de Oviedo) in collaboration with scientists from Universidad de Oporto (Portugal), ALBA Synchrotron (Barcelona) and Centro Nacional de Microelectrónica (Barcelona) has succeeded in analyzing in detail the magnetic characteristics of domains at a nanometric scale of both simple and buried layers of NdCo alloys with perpendicular anisotropy. The experimental method used is Magnetic Transmission X-ray microscopy with lateral resolution of 40 nm. Depending on the specific X-ray energies used it is possible to identify the magnetic response of each chemical element and obtain the image of buried layers with different chemical composition from the surface.

The studied NdCo layers present characteristic magnetic stripe domain patterns in which the magnetization points alternatively up and down, giving rise to an image of dark/bright bands that can be seen in the figure (panel a). In this experiment, the films were mounted on a goniometer which allowed to vary the angle of incidence of the photon beam relative to the surface of the films. Thus, contrast changes as a function of the incident X-ray direction could be measured and analyzed in a quantitative manner. By acquiring series of images at different angles at exactly the same location on the films it was possible to determine the angle of the magnetization with the surface both in Ndco single layers and in NdCo/Permalloy bilayers.

Magnetic singularities were observed, localized at some of the dislocation cores within the magnetic stripe pattern (see Green ellipse in panel a). They have been identified as meron-like topological defects (i.e. ½ skyrmions) with a magnetic structure as sketched in panel b. In these regions, the magnetization performs a single turn helix with a well defined chirality which is the smallest possible reversed domain in this system. Micromagnetic calculations confirmed the above findings which provide an additional insight of the magnetization reversal in magnetic heterostructures.

Graphical abstract-Nature Comm Sept2015

Panel a: Magnetic transmission x-ray microscopy image of stripe domains in a 55 nm thick NdCo film buried under a 40 nm thick permaloy overlayer, obtained at ALBA synchrotron. Green ellipse contains a dislocation with a meron-like magnetic structure localized at its core. Panel b: Sketch of the micromagnetic structure of a meron-like topological defect (1/2 skyrmion).

Article and authors
“Nanoscale Imaging of Buried Topological Defects with Quantitative X-Ray Magnetic Microscopy” por C.Blanco-Roldán, C. Quirós, A. Sorrentino, A. Hierro-Rodríguez, L. M. Álvarez-Prado, R. Valcárcel, M. Duch, N. Torras, J. Esteve, J. I. Martín, M. Vélez, J. M. Alameda, E. Pereiro, and S. Ferrer. Nature Communications, September 4th, 2015.

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