Real-world graphene devices may have a bumpy ride

In a new article inNature Physics, NIST scientists also say that graphene may be an ideal medium for probing interactions between electric conductors and insulators using ascanning tunneling microscope(STM).

According to NIST Fellow Joseph Stroscio, graphene's ideal properties are only available when it is isolated from the environment.

"To get the most benefit from graphene, we have to understand fully how graphene's properties change when put in real-world conditions, such as part of a device where it is in contact with other kinds of materials,"Stroscio says.

Typical semiconductor chips are a complicated"sandwich"of alternating conducting, semiconducting and insulating layers and structures. To perform their experiment, the NIST group made their own sandwich with a single atomic sheet of graphene and another conductor separated by an insulating layer. When the bottom conductor is charged, it induces an equal and opposite charge in the graphene.

Examined under an STM, which is sensitive to the charged state of the graphene, the highelectron mobilityshould make the graphene look like a featureless plane. But, says NIST researcher Nikolai Zhitenev,"What we found is that variations in the electrical potential of the insulating substrate are interrupting the orbits of the electrons in the graphene, creating wells where the electrons pool and reducing their mobility."

This effect is especially pronounced when the group exposes the substrate-mounted graphene to high magnetic fields. Then theelectrons, already made sluggish by the substrate interactions, lack the energy to scale the mountains of resistance and settle into isolated pockets of"quantum dots,"nanometer-scale regions that confine electrical charges in all directions.

It's not all bad news. Direct access to the graphene with a scanned probe also makes it possible to investigate the physics of other substrate interactions on a nanoscopic scale, something which is less possible in conventional semiconductor devices where the important transport layers are buried below the surface.

"Usually, we cannot study insulators at atomic scale,"says Stroscio."The STM works with a closed loop system that keeps a constant tunneling current by adjusting the tip-sample distance. On an insulator there is no current available, so the system will keep pushing the tip closer to the substrate until it eventually crashes into the surface. Thegraphenelets us get close enough to these substrate materials to study their electrical properties, but not so close that we damage thesubstrateand instrument."

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The peculiar nature of molecular hydrogen vibration under high pressure

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Iitaka, along with colleagues from the Institute ofHigh Performance Computingin Singapore and the University of Saskatchewan in Canada, recently uncovered the physical basis underlying a newly discovered behavior of hydrogen molecules under high pressure.

This behavior was observed in a complex of hydrogen molecules, and hydrogen bound to silicon, which is called silane. Silane’shydrogen atomsare under so-called 'chemical compression' by virtue of their being part of a chemical bond. In 2009, physicists found that the vibrational frequency of hydrogen molecules in silane–hydrogen complexes fell as the applied pressure rose. This anti-correlation was the opposite of previous observations of high-pressure hydrogen.

Iitaka and colleagues modeled the system using molecular dynamics simulations. They first optimized the relative arrangement of hydrogen and silane molecules inside a unit cell, finding that the hydrogen molecules tend to sit at octahedral and tetrahedral sites. They then computed the vibrational frequencies of thehydrogen molecules, and found two groups of vibrational modes, one at high energy and one at low energy.

The frequencies of the lower-energy group decreased monotonically as applied pressure increased. However, the frequencies of the higher-energy group increased with pressure until about 20.1 giga Pascals (GPa), after which they fell. This reproduced the experimentally observed anti-correlation between vibrational frequency and applied pressure, indicating that the simulation was accurate.

The simulations also revealed that this rise and fall in frequencies resulted from interactions between hydrogen and silane molecules. These interactions resulted from the overlap between the filled electron orbitals of one molecule and the empty orbitals of the other molecule. This overlap stabilizes the system, and its strength depends on the distance between the molecules. This distance, in turn, depends on the applied pressure.

The simulation results are another glimpse into the exotic physics that underpins the high-pressure regime, according to Iitaka.“We have shown that there is much more interesting new physics and chemistry to be explored in the world of high pressure.”

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Simple lithium good for many surprises

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At ambient conditions, lithium forms rhombohedral crystals, which at higher temperatures and pressures transform to face-centered cubic and then body-centered cubic crystals– all three among the simplest known crystal structures. But what happens to lithium at very high pressures? In the past few years, intriguing deviations from simple metallic behavior were observed, for example a metal to semiconductor-transition and even superconductivity at 17K.

Nevertheless, the overall picture of the lithium phase diagram remained patchy, motivating a systematic study by researchers from The University of Edinburgh, the ESRF, and the Carnegie Institution of Washington who have mapped the lithium phase diagram at high pressures up to 1.3 Mbar, and over a wide temperature range between 77K and 300K.

Whereas the melting point of a material usually rises under pressure, and even the lightest gaseous elements, hydrogen and helium, melt at 1000K and 0.5 Mbar, lithium remains liquid at this pressure down to temperatures as low as 190K. This is by far the lowest melting temperature observed for any material at this pressure.

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Phase diagram of lithium (red) and sodium (blue). The lithium phase diagram indicates the various solid states and also the liquid state

There are more surprises: Above 0.6 Mbar, lithium adopts three novel, complex crystal structures not previously observed in any element with 40, 88, and 24 atoms per unit cell. The most complex ones, with 40 and 88 atoms, were even never predicted theoretically. The scientists suggest that the overall appearance of thelithiumphase diagram and particularly the anomalously low melting temperatures are due to quantum effects starting to play the dominant role at high compressions. They also speculate that a ground metallic liquid state, which has been predicted but never observed for hydrogen, and which should exhibit highly unusual properties, might be constructed on the basis of the lithium-rich compounds.

The experiments were carried out using powder diffraction and single-crystal diffraction at the ESRFHigh-PressureBeamline ID09A, and at the High Pressure Collaborative Access Team (HP-CAT) 16-ID-B beamline at the APS, where, in addition, critical cryogenic techniques were refined.

The research team had to address several experimental challenges: handling a diamond anvil cell in a cryostat; ensuring a wide opening angle to allow for single-crystal diffraction; and last but not least, coping with the high reactivity of lithiumm in particular when in the liquid phase.

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Supercomputer unravels structures in DVD materials

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Information is stored in a DVD in the form of microscopic bits (each less than 100 nanometres in size) in a thin layer of a polycrystalline alloy containing several elements. The bits can have a disordered, amorphous or an ordered, crystalline structure. The transition between the two phases lasts only a few nanoseconds and can be triggered by a laser pulse. Common alloys for storage materials such as DVD-RAMs or Blu-ray Discs contain germanium (Ge), antimony (Sb) und tellurium (Te) and are known as GST after the initials of the elements. The most popular alloys for DVD-RW are AIST alloys, which contain small amounts of silver (Ag) and indium (In) as well as antimony (Sb) and tellurium (Te).

"Both alloy families contain antimony and tellurium and appear to have much in common, but the phase change mechanisms are quite different", explains Dr. Robert Jones of Forschungszentrum Julich, who has collaborated with an international team on the problem. In addition to experimental data and x-ray spectra from the Japanese synchrotron SPring-8, the world's most powerful x-ray source, the team used extensive simulations on the Jülich supercomputer JUGENE. The combination of experiment and simulations has enabled the structures of both phases to be determined for the first time and allowed the development of a model to explain the rapid phase change.

The phase change in AIST alloys proceeds from the outside of the bit, where it adjoins the crystalline surroundings, towards its interior. InNature Materials, the team explains this using a"bond exchange model", where the local environment in the amorphous bit is changed by small movements of an antimony atom (see figure). A sequence of many such steps results in reorientation (crystallization), without requiring empty regions or large motions. The antimony atoms, stimulated by the laser pulse, have simply exchanged the strengths of the bonds to two neighbours, hence the name"bond exchange"model.

The team had clarified the phase transition in GST materials in earlier work (DOI: 10.1103/PhysRevB.80.020201). Here the amorphous bit crystallizes via nucleation, i.e. small crystallites formed in the interior grow rapidly until they covered the whole bit. The speed of the transition can be explained by observing that amorphous and crystalline phases contain the same structural units,"ABAB"rings. These four-membered rings contain two germanium or antimony atoms (A) and two tellurium atoms (B) and can rearrange in the available empty space without breaking many atomic bonds.

The calculation of the structure of amorphous AIST is the largest yet performed in this area of research, with simulations of 640 atoms over the comparatively long time of several hundred picoseconds. Some 4000 processors of the Julich supercomputer JUGENE were used for over four months in order to obtain the necessary precision. In addition to sheer computing power, however, experience in scientific computing and the simulation of condensed matter is essential. Jones notes:"Forschungszentrum Jülich is one of the few places where all these aspects come together."

The deeper theoretical understanding of the processes involved in writing and erasing a DVD should aid the development ofphase changestorage media with longer life, larger capacity, or shorter access times.

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How do you make lithium melt in the cold?

Lithium is the first metal in theperiodic tableand is the least dense solid element at room temperature. It is most commonly known for its use in batteries for consumer electronics, such as cell phones and laptop computers. And, with only threeelectronsper atom, lithium should behave like a model, simple metal.

However, this research has shown that under pressure ranging between about 395,000 atmospheres (40 GPa) and about 592,000 atmospheres (60 GPa), lithium behaves in a manner that's anything but simple. Not only does it become a liquid atroom temperature, but it then refuses to freeze until the temperature reaches a chilly -115o F. At pressures above about 592,000 atmospheres (60 GPa), when lithium does eventually solidify, it is into a range of highly complex, crystalline states. The highest pressure reached in the study was about 1.3 million atmospheres (130 GPa).

The research team, including Malcolm Guthrie, Stanislav Sinogeikin and Ho-kwang (Dave) Mao, of Carnegie's Geophysical Laboratory, believe that this exotic behavior is directly due to the exceptionally low mass of the lithium atom. An elementary result ofquantum physicsis that atoms continue to move, even when cooled to the lowest possible temperature. As the mass of an atom decreases, the importance of this residual, so called 'zero-point,' energy increases. The researchers speculate that, in the case oflithium, the zero-point energy increases with pressure to the point that melting occurs. This work raises the possibility of uncovering a material which never freezes. The prospect of a metallic liquid at even the lowest temperatures raises the intriguing possibility of an entirely novel material, a superconducting liquid, as proposed previously by theorists for hydrogen at very high pressure.

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New glass tops steel in strength and toughness

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"These results mark the first use of a new strategy formetallic glassfabrication and we believe we can use it to makeglassthat will be even stronger and more tough,"says Robert Ritchie, a materials scientist who led the Berkeley contribution to the research.

The new metallic glass is a microalloy featuring palladium, a metal with a high"bulk-to-shear"stiffness ratio that counteracts the intrinsic brittleness of glassy materials.

"Because of the high bulk-to-shear modulus ratio of palladium-containing material, the energy needed to form shear bands is much lower than the energy required to turn these shear bands into cracks,"Ritchie says."The result is that glass undergoes extensive plasticity in response to stress, allowing it to bend rather than crack."

Ritchie, who holds joint appointments with Berkeley Lab's Materials Sciences Division and the University of California (UC) Berkeley's Materials Science and Engineering Department, is one of the co-authors of a paper describing this research published in the journalNature Materialsunder the title"A Damage-Tolerant Glass."

Co-authoring theNature Materialspaper were Marios Demetriou (who actually made the new glass), Maximilien Launey, Glenn Garrett, Joseph Schramm, Douglas Hofmann and William Johnson of Cal Tech, one of the pioneers in the field of metallic glass fabrication.

Glassy materials have a non-crystalline, amorphous structure that make them inherently strong but invariably brittle. Whereas the crystalline structure of metals can provide microstructural obstacles (inclusions, grain boundaries, etc.,) that inhibit cracks from propagating, there's nothing in the amorphous structure of a glass to stop crack propagation. The problem is especially acute in metallic glasses, where single shear bands can form and extend throughout the material leading to catastrophic failures at vanishingly small strains.

In earlier work, the Berkeley-Cal Tech collaboration fabricated a metallic glass, dubbed"DH3,"in which the propagation of cracks was blocked by the introduction of a second, crystalline phase of the metal. This crystalline phase, which took the form of dendritic patterns permeating the amorphous structure of the glass, erected microstructural barriers to prevent an opened crack from spreading. In this new work, the collaboration has produced a pure glass material whose unique chemical composition acts to promote extensive plasticity through the formation of multiple shear bands before the bands turn into cracks.

"Our game now is to try and extend this approach of inducing extensive plasticity prior to fracture to other metallic glasses through changes in composition,"Ritchie says."The addition of the palladium provides our amorphous material with an unusual capacity for extensive plastic shielding ahead of an opening crack. This promotes a fracture toughness comparable to those of the toughest materials known. The rare combination of toughness and strength, or damage tolerance, extends beyond the benchmark ranges established by the toughest and strongest materials known."

The initial samples of the new metallic glass were microalloys of palladium with phosphorous, silicon and germanium that yielded glass rods approximately one millimeter in diameter. Adding silver to the mix enabled the Cal Tech researchers to expand the thickness of the glass rods to six millimeters. The size of the metallic glass is limited by the need to rapidly cool or"quench"the liquid metals for the final amorphous structure.

"The rule of thumb is that to make a metallic glass we need to have at least five elements so that when we quench the material, it doesn't know what crystal structure to form and defaults to amorphous,"Ritchie says.

The new metallic glass was fabricated by co-author Demetriou at Cal Tech in the laboratory of co-author Johnson. Characterization and testing was done at Berkeley Lab by Ritchie's group.

"Traditionally strength and toughness have been mutually exclusive properties in materials, which makes these new metallic glasses so intellectually exciting,"Ritchie says."We're bucking the trend here and pushing the envelope of the damage tolerance that's accessible to a structural metal."

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