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Research Alert: February 17, 2015

A new spin on spintronics

A team of researchers from the University of Michigan and Western Michigan University is exploring new materials that could yield higher computational speeds and lower power consumption, even in harsh environments.

Most modern electronic circuitry relies on controlling electronic charge within a circuit, but this control can easily be disrupted in the presence of radiation, interrupting information processing. Electronics that use spin-based logic, or spintronics, may offer an alternative that is robust even in radiation-filled environments.

Making a radiation-resistant spintronic device requires a material relevant for spintronic applications that can maintain its spin-dependence after it has been irradiated. In a paper published in the journal Applied Physics Letters, from AIP Publishing, the Michigan research team presents their results using bulk Si-doped n-GaAs exposed to proton radiation.

How Does Spintronics Work?

Modern electronic devices use charges to transmit and store information, primarily based upon how many electrons are in one place or another. When a lot of them are at a given terminal, you can call that ‘on.’ If you have very few of them at the same terminal, you can call that ‘off,’ just like a light switch. This allows for binary logic depending on whether the terminal is ‘on’ or ‘off.’ Spintronics, at its simplest, uses the ‘on/off’ idea, but instead of counting the electrons, their spin is measured.

“You can think of the spin of an electron as a tiny bar magnet with an arrow painted on it. If the arrow points up, we call that ‘spin-up.’ If it points down, we call that ‘spin-down.’ By using light, electric, or magnetic fields, we can manipulate, and measure, the spin direction,” said researcher Brennan Pursley, who is the first author of the new study.

While spintronics holds promise for faster and more efficient computation, researchers also want to know whether it would be useful in harsh environments. Currently, radioactivity is a major problem for electronic circuitry because it can scramble information and in the long term degrade electronic properties. For the short term effects, spintronics should be superior: radioactivity can change the quantity of charge in a circuit, but should not affect spin-polarized carriers.

Studying spintronic materials required that the research team combine two well established fields: the study of spin dynamics and the study of radiation damage. Both tool sets are quite robust and have been around for decades but combining the two required sifting through the wealth of radiation damage research. “That was the most difficult aspect,” explains Pursley. “It was an entirely new field for us with a variety of established techniques and terminology to learn. The key was to tackle it like any new project: ask a lot of questions, find a few good books or papers, and follow the citations.”

Technically, what the Michigan team did was to measure the spin properties of n-GaAs as a function of radiation fluence using time-resolved Kerr rotation and photoluminescence spectroscopy. Results show that the spin lifetime and g-factor of bulk n-GaAs is largely unaffected by proton irradiation making it a candidate for further study for radiation-resistant spintronic devices. The team plans to study other spintronic materials and prototype devices after irradiation since the hybrid field of irradiated spintronics is wide open with plenty of questions to tackle.

Long term, knowledge of radiation effects on spintronic devices will aid in their engineering. A practical implementation would be processing on a communications satellite where without the protection of Earth’s atmosphere, electronics can be damaged by harsh solar radiation. The theoretically achievable computation speeds and low power consumption could be combined with compact designs and relatively light shielding. This could make communications systems faster, longer-lived and cheaper to implement.

Novel solid-state nanomaterial platform enables terahertz photonics

Compact, sensitive and fast nanodetectors are considered to be somewhat of a “Holy Grail” sought by many researchers around the world. And now a team of scientists in Italy and France has been inspired by nanomaterials and has created a novel solid-state technology platform that opens the door to the use of terahertz (THz) photonics in a wide range of applications.

During the past decade, materials research has played an essential role in filling the THz gap, beginning with the development of THz quantum cascade lasers, which rely heavily on semiconductor heterostructured artificial nanomaterials. The development of THz spectroscopy, nanospectroscopy and THz imaging expanded the range of powerful tools for the characterization of a broad range of materials — including one-dimensional or two-dimensional semiconductors, biomolecules and graphene.

The missing piece? A complementary detection technology capable of fulfilling THz application-oriented needs in fields such as biomedical diagnostics, security, cultural heritage, quality and process controls, and high data-rate wireless communications that require ad hoc integrated generation and detection systems.

As the scientists report in the journal APL Materials, from AIP publishing, by using an approach that exploits the excitation of plasma waves in the channel of field-effect transistors (FET), they were able to create the first FET detectors based on semiconductor nanowires, designed in a plethora of architectures — including tapers, heterostructures and metamaterial-antenna coupled. While they were at it, they also developed the first THz detectors made of mono- or bi-layer graphene.

“Our work shows that nanowire FET technology is versatile enough to enable ‘design’ via lithography of the detector’s parameters and its main functionalities,” explained Miriam Serena Vitiello, lead author of the paper as well as research scientist and group leader of Terahertz Photonics Group in the Nanoscience Institute at CNR and Scuola Normale Superiore in Pisa, Italy.

What’s the nanowire detector capable of? It offers “a concrete perspective of application-oriented use, since it operates at room temperature — reaching detection frequencies greater than 3 THz, with maximum modulation speed in the MHz range, and noise equivalent powers that are already competitive with the best commercially available technologies,” Vitiello said.

In terms of applications, because the nanodetectors can be tapped for large-area fast imaging across both the THz and the sub-terahertz spectral ranges, don’t be surprised to see them commercialized in the near future for a variety of spectroscopic and real-time imaging applications — possibly even in the form of fast multi-pixel THz cameras.

Next, the scientists’ goals are to “push the device’s performance in the ultrafast detection realm, explore the feasibility of single photon detection by using novel architectures and material choices, develop compact focal plane arrays, and to integrate on-chip the nanowire detectors with THz quantum cascade microlasers,” noted Vitiello. “This will allow us to take THz photonics to a whole new level of ‘compactness’ and versatility, where it can finally begin to address many killer applications.”

Novel crumpling method takes flat graphene from 2-D to 3-D

Researchers at the University of Illinois at Urbana-Champaign have developed a unique single-step process to achieve three-dimensional (3D) texturing of graphene and graphite. Using a commercially available thermally activated shape-memory polymer substrate, this 3D texturing, or “crumpling,” allows for increased surface area and opens the doors to expanded capabilities for electronics and biomaterials.

“Fundamentally, intrinsic strains on crumpled graphene could allow modulation of electrical and optical properties of graphene,” explained SungWoo Nam, an assistant professor of mechanical science and engineering at Illinois. “We believe that the crumpled graphene surfaces can be used as higher surface area electrodes for battery and supercapacitor applications. As a coating layer, 3D textured/crumpled nano-topographies could allow omniphobic/anti-bacterial surfaces for advanced coating applications.”

Graphene–a single atomic layer of sp2-bonded carbon atoms–has been a material of intensive research and interest over recent years. A combination of exceptional mechanical properties, high carrier mobility, thermal conductivity, and chemical inertness, make graphene a prime candidate material for next generation optoelectronic, electromechanical, and biomedical applications.

“In this study, we developed a novel method for controlled crumpling of graphene and graphite via heat-induced contractile deformation of the underlying substrate,” explained Michael Cai Wang, a graduate student and first author of the paper, “Heterogeneous, Three-Dimensional Texturing of Graphene,” which appeared in the journal Nano Letters. “While graphene intrinsically exhibits tiny ripples in ambient conditions, we created large and tunable crumpled textures in a tailored and scalable fashion.”

“As a simpler, more scalable, and spatially selective method, this texturing of graphene and graphite exploits the thermally induced transformation of shape-memory thermoplastics, which has been previously applied to microfluidic device fabrication, metallic film patterning, nanowire assembly, and robotic self-assembly applications,” added Nam, whose group has filed a patent for their novel strategy. “The thermoplastic nature of the polymeric substrate also allows for the crumpled graphene morphology to be arbitrarily re-flattened at the same elevated temperature for the crumpling process.”

“Due to the extremely low cost and ease of processing of our approach, we believe that this will be a new way to manufacture nanoscale topographies for graphene and many other 2D and thin-film materials.”

The researchers are also investigating the textured graphene surfaces for 3D sensor applications.

“Enhanced surface area will allow even more sensitive and intimate interactions with biological systems, leading to high sensitivity devices,” Nam said.

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