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Physicists find misaligned carbon sheets yield unparalleled properties

A material composed of two one-atom-thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing — and potentially exploitable — conductive properties.

Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics at The University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with Dr. Fengnian Xia’s group at Yale University in Nature Photonics that describes how the ability of twisted bilayer graphene to conduct electrical current changes in response to mid-infrared light.

From One to Two Layers

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, where each hexagon is formed by six carbon atoms at its vertices. Since graphene’s first isolation in 2004, its unique properties have been intensely studied by scientists for potential use in advanced computers, materials and devices.

If two sheets of graphene are stacked on top of one another, and one layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by a single layer alone or by two aligned layers.

“Graphene has been of interest for about 15 years,” Zhang said. “A single layer is interesting to study, but if we have two layers, their interaction should render much richer and more interesting physics. This is why we want to study bilayer graphene systems.”

A New Field Emerges

When the graphene layers are misaligned, a new periodic design in the mesh emerges, called a moiré pattern. The moiré pattern is also a hexagon, but it can be made up of more than 10,000 carbon atoms.

“The angle at which the two layers of graphene are misaligned — the twist angle — is critically important to the material’s electronic properties,” Wang said. “The smaller the twist angle, the larger the moiré periodicity.”

The unusual effects of specific twist angles on electron behavior were first proposed in a 2011 article by Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this field as a doctoral student in MacDonald’s group.

“At that time, others really paid no attention to the theory, but now it has become arguably the hottest topic in physics,” Zhang said.

In that 2011 research MacDonald and Bistritzer predicted that electrons’ kinetic energy can vanish in a graphene bilayer misaligned by the so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory, finding that offsetting two graphene layers by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current with no resistance and no energy loss.

In a 2019 article in Science Advances, Zhang and Wang, together with Dr. Jeanie Lau’s group at The Ohio State University, showed that when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states, thereby widening the magic angle significantly.

“In our previous work, we saw superconductivity as well as insulation. That’s what’s making the study of twisted bilayer graphene such a hot field — superconductivity. The fact that you can manipulate pure carbon to superconduct is amazing and unprecedented,” Wang said.

New UT Dallas Findings

In his most recent research in Nature Photonics, Zhang and his collaborators at Yale investigated whether and how twisted bilayer graphene interacts with mid-infrared light, which humans can’t see but can detect as heat. “Interactions between light and matter are useful in many devices — for example, converting sunlight into electrical power,” Wang said. “Almost every object emits infrared light, including people, and this light can be detected with devices.”

Zhang is a theoretical physicist, so he and Wang set out to determine how mid-infrared light might affect the conductance of electrons in twisted bilayer graphene. Their work involved calculating the light absorption based on the moiré pattern’s band structure, a concept that determines how electrons move in a material quantum mechanically.

“There are standard ways to calculate the band structure and light absorption in a regular crystal, but this is an artificial crystal, so we had to come up with a new method,” Wang said. Using resources of the Texas Advanced Computing Center, a supercomputer facility on the UT Austin campus, Wang calculated the band structure and showed how the material absorbs light.

The Yale group fabricated devices and ran experiments showing that the mid-infrared photoresponse — the increase in conductance due to the light shining — was unusually strong and largest at the twist angle of 1.8 degrees. The strong photoresponse vanished for a twist angle less than 0.5 degrees.

“Our theoretical results not only matched well with the experimental findings, but also pointed to a mechanism that is fundamentally connected to the period of moiré pattern, which itself is connected to the twist angle between the two graphene layers,” Zhang said.

Next Step

“The twist angle is clearly very important in determining the properties of twisted bilayer graphene,” Zhang added. “The question arises: Can we apply this to tune other two-dimensional materials to get unprecedented features? Also, can we combine the photoresponse and the superconductivity in twisted bilayer graphene? For example, can shining a light induce or somehow modulate superconductivity? That will be very interesting to study.”

“This new breakthrough will potentially enable a new class of infrared detectors based on graphene with high sensitivity,” said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “These new detectors will potentially impact applications such as night vision, which is of critical importance for the U.S. Army.”

In addition to the Yale researchers, other authors included scientists from the National Institute for Materials Science in Japan. The ARO, the National Science Foundation and the Office of Naval Research supported the study.

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Artificial intelligence identifies optimal material formula

Nanostructured layers boast countless potential properties — but how can the most suitable one be identified without any long-term experiments? A team has ventured a shortcut: using a machine learning algorithm, the researchers were able to reliably predict the properties of such a layer.

Porous or dense, columns or fibres

During the manufacture of thin films, numerous control variables determine the condition of the surface and, consequently, its properties. Relevant factors include the composition of the layer as well as process conditions during its formation, such as temperature. All these elements put together result in the creation of either a porous or a dense layer during the coating process, with atoms combining to form columns or fibres. “In order to find the optimal parameters for an application, it used to be necessary to conduct countless experiments under different conditions and with different compositions; this is an incredibly complex process,” explains Professor Alfred Ludwig, Head of the Materials Discovery and Interfaces Team.

Findings yielded by such experiments are so-called structure zone diagrams, from which the surface of a certain composition resulting from certain process parameters can be read. “Experienced researchers can subsequently use such a diagram to identify the most suitable location for an application and derive the parameters necessary for producing the suitable layer,” points out Ludwig. “The entire process requires an enormous effort and is highly time consuming.”

Algorithm predicts surface

Striving to find a shortcut towards the optimal material, the team took advantage of artificial intelligence, more precisely machine learning. To this end, PhD researcher Lars Banko, together with colleagues from the Interdisciplinary Centre for Advanced Materials Simulation at RUB, Icams for short, modified a so-called generative model. He then trained this algorithm to generate images of the surface of a thoroughly researched model layer of aluminium, chromium and nitrogen using specific process parameters, in order to predict what the layer would look like under the respective conditions.

“We fed the algorithm with a sufficient amount of experimental data in order to train it, but not with all known data,” stresses Lars Banko. Thus, the researchers were able to compare the results of the calculations with those of the experiments and analyse how reliable its prediction was. The results were conclusive: “We combined five parameters and were able to look in five directions simultaneously using the algorithm — without having to conduct any experiments at all,” outlines Alfred Ludwig. “We have thus shown that machine learning methods can be transferred to materials research and can help to develop new materials for specific purposes.”

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A tech jewel: Converting graphene into diamond film

Can two layers of the “king of the wonder materials,” i.e. graphene, be linked and converted to the thinnest diamond-like material, the “king of the crystals”? Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) have reported in Nature Nanotechnology the first experimental observation of a chemically induced conversion of large-area bilayer graphene to the thinnest possible diamond-like material, under moderate pressure and temperature conditions. This flexible, strong material is a wide-band gap semiconductor, and thus has potential for industrial applications in nano-optics, nanoelectronics, and can serve as a promising platform for micro- and nano-electromechanical systems.

Diamond, pencil lead, and graphene are made by the same building blocks: carbon atoms (C). Yet, it is the bonds’ configuration between these atoms that makes all the difference. In a diamond, the carbon atoms are strongly bonded in all directions and create an extremely hard material with extraordinary electrical, thermal, optical and chemical properties. In pencil lead, carbon atoms are arranged as a pile of sheets and each sheet is graphene. Strong carbon-carbon (C-C) bonds make up graphene, but weak bonds between the sheets are easily broken and in part explain why the pencil lead is soft. Creating interlayer bonding between graphene layers forms a 2D material, similar to thin diamond films, known as diamane, with many superior characteristics.

Previous attempts to transform bilayer or multilayer graphene into diamane relied on the addition of hydrogen atoms, or high pressure. In the former, the chemical structure and bonds’ configuration are difficult to control and characterize. In the latter, the release of the pressure makes the sample revert back to graphene. Natural diamonds are also forged at high temperature and pressure, deep inside the Earth. However, IBS-CMCM scientists tried a different winning approach.

The team devised a new strategy to promote the formation of diamane, by exposing bilayer graphene to fluorine (F), instead of hydrogen. They used vapors of xenon difluoride (XeF2) as the source of F, and no high pressure was needed. The result is an ultra-thin diamond-like material, namely fluorinated diamond monolayer: F-diamane, with interlayer bonds and F outside.

For a more detailed description; the F-diamane synthesis was achieved by fluorinating large area bilayer graphene on single crystal metal (CuNi(111) alloy) foil, on which the needed type of bilayer graphene was grown via chemical vapor deposition (CVD).

Conveniently, C-F bonds can be easily characterized and distinguished from C-C bonds. The team analyzed the sample after 12, 6, and 2-3 hours of fluorination. Based on the extensive spectroscopic studies and also transmission electron microscopy, the researchers were able to unequivocally show that the addition of fluorine on bilayer graphene under certain well-defined and reproducible conditions results in the formation of F-diamane. For example, the interlayer space between two graphene sheets is 3.34 angstroms, but is reduced to 1.93-2.18 angstroms when the interlayer bonds are formed, as also predicted by the theoretical studies.

“This simple fluorination method works at near-room temperature and under low pressure without the use of plasma or any gas activation mechanisms, hence reduces the possibility of creating defects,” points out Pavel V. Bakharev, the first author and co-corresponding author.

Moreover, the F-diamane film could be freely suspended. “We found that we could obtain a free-standing monolayer diamond by transferring F-diamane from the CuNi(111) substrate to a transmission electron microscope grid, followed by another round of mild fluorination,” says Ming Huang, one of the first authors.

Rodney S. Ruoff, CMCM director and professor at the Ulsan National Institute of Science and Technology (UNIST) notes that this work might spawn worldwide interest in diamanes, the thinnest diamond-like films, whose electronic and mechanical properties can be tuned by altering the surface termination using nanopatterning and/or substitution reaction techniques. He further notes that such diamane films might also eventually provide a route to very large area single crystal diamond films.

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Ultra-thin optical elements directly measure polarization

For the first time, researchers have used ultra-thin layers of 2D structures known as metasurfaces to create holograms that can measure the polarization of light. The new metasurface holograms could be used to create very fast and compact devices for polarization measurements, which are used in spectroscopy, sensing and communications applications.

Metasurfaces are optical elements with nanoscale features and an overall thickness that is less than 1/50th that of a human hair. They can be made with standard microelectronics fabrication techniques, enabling mass production, and can be easily integrated into wafer-scale optical systems. Despite these promising features, they are not yet used in many practical applications.

In Optica, The Optical Society’s journal for high impact research, a multi-institutional group of researchers report using metasurface holograms to effectively and quickly determine polarization at near-infrared to visible wavelengths. The new work represents a step toward functional metasurface-based devices to support a range of applications from telecommunications to chemical analysis.

“Holograms made from metasurfaces are an efficient and effective way to generate high-quality images with subwavelength resolution,” said research team leader Xueqian Zhang from Tianjin University, China. “Our work uniquely applies metasurface holograms to polarization measurements, which could enable camera-size devices that measure polarization in one step without moving parts.”

Measuring polarization directly

Although sunlight and most household light sources emit unpolarized light that oscillates in all directions, optical components such as filters can be used to produce polarized light that propagates in just a single plane — typically vertical or horizontal. Analytical instruments such as spectrometers can measure how light polarization changes after interacting with a material to determine its physical properties. Different light polarizations can also be used to send multiple signals through optical fibers for telecommunications applications.

Conventional methods for determining polarization often require multiple measurements, bulky optical setups or precise adjustment of high-quality optical components to indirectly determine the polarization state. In the new work, the researchers instead used a metasurface to determine polarization directly by comparing the amplitude and phase of light waves that are polarized at right angles to themselves.

The metasurface generates two overlapping holographic images, one that is left-handed circularly polarized (LCP) and another that is right-handed circularly polarized (RCP). Circularly polarized light features an electric field oscillation plane that rotates to the left or right in a plane perpendicular to the direction of the wave.

“The overlapping images can be simply and quickly captured using a CCD camera,” said Zhang.

“By analyzing the interference of the two holographic images, we can obtain the amplitude contrast and phase difference between the LCP and RCP components of the incident beam, thus identifying the polarization state.”

Key to the new technique was an algorithm called Gerchberg-Saxton, which is widely used in holographic research. The researchers figured out how to modify this algorithm so that it could be used to identify the phase difference between the LCP and RCP components of the incident light in the overlapping holographic images.

Effective polarization measurements

The researchers demonstrated their new metasurface holographic approach by using it to measure the polarization states of illuminating light beams with known polarizations. The measured polarization states matched well with the known ones, confirming the effectiveness of the approach. In the future, the metasurface could be incorporated into a camera’s photosensitive area to make a compact device for measuring polarization.

The metasurface the researchers used is based on the Pancharatnam-Berry phase (also known as geometric phase) method, which features relative phase responses that do not exhibit any dispersion. This allows the metasurface holograms to work over a broad range of wavelengths.

“Our method can be extended to many potential applications requiring polarization measurement, such as polarization spectroscopy, sensing and communications,” said Zhang. “Polarization-encoded holography could also be used for security information transmission because only a receiver who knows the desired polarization states could decode information from the final holographic images.”

Now that they have proved the concept, the researchers plan to improve the efficiency of the method and will compare its performance with conventional commercial instruments used to measure polarization.

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Conductivity at the edges of graphene bilayers

The conductivity of dual layers of graphene greatly depends on the states of carbon atoms at their edges; a property which could have important implications for information transmissions on quantum scales.

Made from 2D sheets of carbon atoms arranged in honeycomb lattices, graphene displays a wide array of properties regarding the conduction of heat and electricity.

When two layers of graphene are stacked on top of each other to form a ‘bilayer’, these properties can become even more interesting. At the edges of these bilayers, for example, atoms can sometimes exist in an exotic state of matter referred to as the ‘quantum spin Hall’ (QSH) state, depending on the nature of the interaction between their spins and their motions, referred to as their ‘spin-orbit coupling’ (SOC). While the QSH state is allowed for ‘intrinsic’ SOC, it is destroyed by ‘Rashba’ SOC. In an article recently published in EPJ B, Priyanka Sinha and Saurabh Basu from the Indian Institute of Technology Guwahati showed that these two types of SOC are responsible for variations in the ways in which graphene bilayers conduct electricity.

For nanoribbons of bilayer graphene, whose edge atoms are arranged in zigzag patterns, the authors showed that the bands of electron energies which are allowed and forbidden are significantly different to those found in monolayer graphene. For intrinsic SOC, the QSH state even caused atoms in the zigzag to have a gap between these bands, which disappeared in odd atoms. However, this asymmetry disappeared for Rashba SOC, which changed the relationship between the energy required to add an electron to the bilayer, and its conductivity.

This conduction sensitivity to the states of edge atoms shows that graphene bilayers could be particularly useful for spintronics applications. This field studies how quantum spins can be used to efficiently transmit information, which is of particular interest to researchers in fields like quantum computing. Sinha and Basu also found that the characteristic SOC behaviours they uncovered persisted with or without voltage across the bilayers, which dispelled theories that this aspect could prevent the QSH state from forming. Their work furthers our knowledge of graphene bilayers, potentially opening up new areas of research into their intriguing properties.

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