From super-lubricants, to solar cells, to the fledgling technology of valleytronics, there is much to be excited about with the discovery of a unique new two-dimensional semiconductor, rhenium disulfide, by researchers at Berkeley Lab's Molecular Foundry. Rhenium disulfide, unlike molybdenum disulfide and other dichalcogenides, behaves electronically as if it were a 2D monolayer even as a 3D bulk material. This not only opens the door to 2D electronic applications with a 3D material, it also makes it possible to study 2D physics with easy-to-make 3D crystals.

"Rhenium disulfide remains a direct-bandgap semiconductor, its photoluminescence intensity increases while its Raman spectrum remains unchanged, even with the addition of increasing numbers of layers," says Junqiao Wu, a physicist with Berkeley Lab's Materials Sciences Division who led this discovery. "This makes bulk crystals of rhenium disulfide an ideal platform for probing 2D excitonic and lattice physics, circumventing the challenge of preparing large-area, single-crystal monolayers."

Wu, who is also a professor with the University of California-Berkeley's Department of Materials Science and Engineering, headed a large international team of collaborators who used the facilities at the Molecular Foundry, a U.S Department of Energy (DOE) national nanoscience center, to prepare and characterize individual monolayers of rhenium disulfide. Through a variety of spectroscopy techniques, they studied these monolayers both as stacked multilayers and as bulk materials. Their study revealed that the uniqueness of rhenium disulfide stems from a disruption in its crystal lattice symmetry called a Peierls distortion.

"Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces," says Sefaattin Tongay, lead author of a paper describing this research in Nature Communications for which Wu was the corresponding author. The paper was titled "Monolayer behaviour in bulk ReS2 due to electronic and vibrational decoupling." 

"Typically the monolayers in a semiconducting transition metal dichalcogenides, such as molybdenum disulfide, are relatively strongly coupled, but isolated monolayers show large changes in electronic structure and lattice vibration energies," Tongay says. "The result is that in bulk these materials are indirect gap semiconductors and in the monolayer they are direct gap."

What Tongay, Wu and their collaborators found in their characterization studies was that rhenium disulfide contains seven valence electrons as opposed to the six valence electrons of molybdenum disulfide and other transition metal dichalcogenides. This extra valence electron prevents strong interlayer coupling between multiple monolayers of rhenium disulfide.

"The extra electron is eventually shared between two rhenium atoms, which causes the atoms to move closer to one another other, forming quasi-one-dimensional chains within each layer and creating the Peierls distortion in the lattice," Tongay says. "Once the Peierls distortion takes place, interlayer registry is largely lost, resulting in weak interlayer coupling and monolayer behavior in the bulk."

Rhenium disulfide's weak interlayer coupling should make this material highly useful in tribology and other low-friction applications. Since rhenium disulfide also exhibits strong interactions between light and matter that are typical of monolayer semiconductors, and since the bulk rhenium disulfide behaves as if it were a monolayer, the new material should also be valuable for solar cell applications. It might also be a less expensive alternative to diamond for valleytronics.

In valleytronics, the wave quantum number of the electron in a crystalline material is used to encode information. This number is derived from the spin and momentum of an electron moving through a crystal lattice as a wave with energy peaks and valleys. Encoding information when the electrons reside in these minimum energy valleys offers a highly promising potential new route to quantum supercomputing and ultrafast data-processing.

"Rhenium atoms have a relatively large atomic weight, which means electron spin-orbit interactions are significant," Tongay says. "This could make rhenium disulfide an ideal material for valleytronics applications."

The collaboration is now looking at ways to tune the properties of rhenium disulfide in both monolayer and bulk crystals through engineered defects in the lattice and selective doping. They are also looking to alloy rhenium disulfide with other members of the dichalcogenide family. 

Novel approach paves way for new quantum devices

A team of University of Toronto physicists led by Alex Hayat has proposed a novel and efficient way to leverage the strange quantum physics phenomenon known as entanglement. The approach would involve combining light-emitting diodes (LEDs) with a superconductor to generate entangled photons and could open up a rich spectrum of new physics as well as devices for quantum technologies, including quantum computers and quantum communication.

Entanglement occurs when particles become correlated in pairs to predictably interact with each other regardless of how far apart they are. Measure the properties of one member of the entangled pair and you instantly know the properties of the other. It is one of the most perplexing aspects of quantum mechanics, leading Einstein to call it "spooky action at a distance."

"A usual light source such as an LED emits photons randomly without any correlations," explains Hayat, who is also a Global Scholar at the Canadian Institute for Advanced Research. "We've proved that generating entanglement between photons emitted from an LED can be achieved by adding another peculiar physical effect of superconductivity - a resistance-free electrical current in certain materials at low temperatures."

This effect occurs when electrons are entangled in Cooper pairs – a phenomenon in which when one electron spins one way, the other will spin in the opposite direction. When a layer of such superconducting material is placed in close contact with a semiconductor LED structure, Cooper pairs are injected in to the LED, so that pairs of entangled electrons create entangled pairs of photons. The effect, however, turns out to work only in LEDs which use nanometre-thick active regions – quantum wells.

"Typically quantum properties show up on very small scales – an electron or an atom. Superconductivity allows quantum effects to show up on large scales – an electrical component or a whole circuit. This quantum behaviour can significantly enhance light emission in general, and entangled photon emission in particular," Hayat said.

Computer security systems may one day get a boost from quantum physics, as a result of recent research from the National Institute of Standards and Technology (NIST). Computer scientist Yi-Kai Liu has devised away to make a security device that has proved notoriously difficult to build—a "one-shot" memory unit, whose contents can be read only a single time.

The research, which Liu is presenting at this week's Innovations in Theoretical Computer Science conference, shows in theory how the laws of quantum physics could allow for the construction of such memory devices. One-shot memories would have a wide range of possible applications such as protecting the transfer of large sums of money electronically. A one-shot memory might contain two authorization codes: one that credits the recipient's bank account and one that credits the sender's bank account, in case the transfer is canceled. Crucially, the memory could only be read once, so only one of the codes can be retrieved, and hence, only one of the two actions can be performed—not both.

"When an adversary has physical control of a device—such as a stolen cell phone—software defenses alone aren't enough; we need to use tamper-resistant hardware to provide security," Liu says. "Moreover, to protect critical systems, we don't want to rely too much on complex defenses that might still get hacked. It's better if we can rely on fundamental laws of nature, which are unassailable."

Unfortunately, there is no fundamental solution to the problem of building tamper-resistant chips, at least not using classical physics alone. So scientists have tried involving quantum mechanics as well, because information that is encoded into a quantum system behaves differently from a classical system.

Liu is exploring one approach, which stores data using quantum bits, or "qubits," which use quantum properties such as magnetic spin to represent digital information. Using a technique called "conjugate coding, "two secret messages—such as separate authorization codes—can be encoded into the same string of qubits, so that a user can retrieve either one of the two messages. But as the qubits can only be read once, the user cannot retrieve both.

The risk in this approach stems from a more subtle quantum phenomenon: "entanglement," where two particles can affect each other even when separated by great distances. If an adversary is able to use entanglement, he can retrieve both messages at once, breaking the security of the scheme.

However, Liu has observed that in certain kinds of physical systems, it is very difficult to create and use entanglement, and shows in his paper that this obstacle turns out to be an advantage: Liu presents a mathematical proof that if an adversary is unable to use entanglement in his attack, that adversary will never be able to retrieve both messages from the qubits. Hence, if the right physical systems are used, the conjugate coding method is secure after all.

"It's fascinating how entanglement—and the lack thereof—is the key to making this work," Liu says. "From a practical point of view, these quantum devices would be more expensive to fabricate, but they would provide a higher level of security. Right now, this is still basic research. But there's been a lot of progress in this area, so I'm optimistic that this will lead to useful technologies in the real world."

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