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Trapping and controlling light at the interface of atomically thin nanomaterials

Posted By Graphene Council, Friday, August 21, 2020
Light can partake in peculiar phenomena at the nanoscale. Exploring these phenomena can unlock sophisticated applications and provide useful insights into the interactions between light waves and other materials.

In a recent study, scientists at Cornell University propose a novel method by which nanoscale light can be manipulated and transported. These special modes of light transport are known to arise at finely tuned interfaces between slightly different nanomaterials. Minwoo Jung, lead researcher on this study, illustrates this concept through a simple analogy: "A floating tube has a hole in the middle, but a normal balloon doesn't. No matter how you squeeze the round balloon, it cannot be reshaped like a donut-at least not without popping the balloon, re-knitting the rubber, and re-injecting the air. Thus, a tube and a balloon are distinct in their topology because they are not connected through a smooth deformation."

Jung further explains that physicists have been interested in gluing two topologically distinct materials side by side so that one of them acts like a balloon and the other like a tube. This means that, at their interface, a process that connects these two materials must occur, much like the poking/popping/re-knitting/re-injecting from a balloon to a tube. Under the right conditions, this process can give rise to a strong channel for transmitting energy or information along the interface. Because this process can be applied to light (which acts as a carrier of energy or information), this branch of physics is called topological photonics.

Jung and his team combined the fascinating concept of topological photonics with an innovative technique that traps light in an atomically thin material. This method brought together two rapidly emerging fields in applied and fundamental physics: graphene nanolight and topological photonics. Jung says, "Graphene is a promising platform for storing and controlling nanoscale light and could be key in the development of on-chip and ultracompact nanophotonic devices, such as waveguides and cavities."

The research team ran simulations involving a graphene sheet layered on a nanopatterned material that functions as a metagate. This honeycomb-like metagate consists of a solid layer of material with holes of different sizes, centered at the vertices of the hexagons. The varying radii of these holes affect the way in which the photons pass through the material. The scientists found that strategically "gluing" together two different metagates creates a topological effect that confines photons at their interface in a predictable, controllable manner.

Different choices of metagate designs demonstrate the dimensional hierarchy of the device's topology. Specifically, depending on the metagate geometry, nanolight can be made to flow along one-dimensional edges of the topological interface or can be topologically stored at zero-dimensional (point-like) vertices. Moreover, the metagate allows for on-and-off electric switching of these waveguides or cavities. Such battery-operated topological effects can benefit the technological adoption of topological photonics in practical devices.

Jung's team is optimistic that the synergistic combination of graphene nanolight and topological photonics will spur advances in relevant research areas, like optics, material sciences, and solid-state physics. Their graphene-based material system is simple, efficient, and suitable for nanophotonic applications: a step forward in harnessing the full potential of light.

Tags:  Cornell University  Graphene  Minwoo Jung  photonics 

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Graphene sensors find subtleties in magnetic fields

Posted By Graphene Council, Friday, August 21, 2020
As with actors and opera singers, when measuring magnetic fields it helps to have range. Cornell researchers used an ultrathin graphene “sandwich” to create a tiny magnetic field sensor that can operate over a greater temperature range than previous sensors, while also detecting miniscule changes in magnetic fields that might otherwise get lost within a larger magnetic background.

The group’s paper, “Magnetic Field Detection Limits for Ultraclean Graphene Hall Sensors,” published Aug. 20 in Nature Communications.

The team was led by Katja Nowack, assistant professor of physics in the College of Arts and Sciences and the paper’s senior author.

Nowack’s lab specializes in using scanning probes to conduct magnetic imaging. One of their go-to probes is the superconducting quantum interference device, or SQUID, which works well at low temperatures and in small magnetic fields.

“We wanted to expand the range of parameters that we can explore by using this other type of sensor, which is the Hall-effect sensor,” said doctoral student Brian Schaefer, the paper’s lead author. “It can work at any temperature, and we’ve shown it can work up to high magnetic fields as well. Hall sensors have been used at high magnetic fields before, but they’re usually not able to detect small magnetic field changes on top of that magnetic field.”

The Hall effect is a well-known phenomenon in condensed matter physics. When a current flows through a sample, it is bent by a magnetic field, creating a voltage across both sides of the sample that is proportional to the magnetic field.

Hall-effect sensors are used in a variety of technologies, from cellphones to robotics to anti-lock brakes. The devices are generally built out of conventional semiconductors like silicon and gallium arsenide.

Nowack’s group decided to try a more novel approach. The last decade has seen a boom in uses of graphene sheets – single layers of carbon atoms, arranged in a honeycomb lattice. But graphene devices often fall short of those made from other semiconductors when the graphene sheet is placed directly on a silicon substrate; the graphene sheet “crumples” on the nanoscale, inhibiting its electrical properties.

Nowack’s group adopted a recently developed technique to unlock graphene’s full potential – sandwiching it between sheets of hexagonal boron nitride. Hexagonal boron nitride has the same crystal structure as graphene but is an electrical insulator, which allows the graphene sheet to lie flat. Graphite layers in the sandwich structure act as electrostatic gates to tune the number of electrons that can conduct electricity in the graphene.

The sandwich technique was pioneered by co-author Lei Wang, a former postdoctoral researcher with the Kavli Institute at Cornell for Nanoscale Science. Wang also worked in the lab of co-senior author Paul McEuen, the John A. Newman Professor of Physical Science and co-chair of the Nanoscale Science and Microsystems Engineering (NEXT Nano) Task Force, part of the provost’s Radical Collaboration initiative.

“The encapsulation with hexagonal boron nitride and graphite makes the electronic system ultraclean,” Nowack said. “That allows us to work at even lower electron densities than we could before, and that’s favorable for boosting the Hall-effect signal we are interested in.”

The researchers were able to create a micron-scale Hall sensor that functions as well as the best Hall sensors reported at room temperature while outperforming any other Hall sensor at temperatures as low as 4.2 kelvins (or minus 452.11 degrees Fahrenheit).

The graphene sensors are so precise they can pick out tiny fluctuations in a magnetic field against a background field that is larger by six orders of magnitude (or a million times its size). Detecting such nuances is a challenge for even high-quality sensors because in a high magnetic field, the voltage response becomes nonlinear and therefore more difficult to parse.

Nowack plans to incorporate the graphene Hall sensor into a scanning probe microscope for imaging quantum materials and exploring physical phenomena, such as how magnetic fields destroy unconventional superconductivity and the ways that current flows in special classes of materials, such as topological metals.

“Magnetic field sensors and Hall sensors are important parts of many real-world applications,” Nowack said. “This work puts ultraclean graphene really on the map for being a superior material to build Hall probes out of. It wouldn’t be really practical for some applications because it’s hard to make these devices. But there are different pathways for materials growth and automated assembly of the sandwich that people are exploring. Once you have the graphene sandwich, you can put it anywhere and integrate it with existing technology.”

Tags:  Cornell University  Graphene  Katja Nowack  Semiconductors  Sensors 

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Origami-inspired robots that could fit in a cell

Posted By Graphene Council, The Graphene Council, Thursday, February 6, 2020

Imagine robots that can move, sense and respond to stimuli, but that are smaller than a hair’s width. This is the project that Cornell professor and biophysicist Itai Cohen, who gave a talk on Wednesday, January 29 as a part of Duke’s Physics Colloquium, has been working on with and his team. His project is inspired by the microscopic robots in Paul McEuen’s book Spiral. Building robots at such a small scale involves a lot more innovation than simply shrinking all of the parts of a normal robot. At low Reynolds number, fluids are viscous instead of inertial, Van der Waals forces come into play, as well as other factors that affect how the robot can move and function. 

To resolve this issue, Cohen and his team decided to build and pattern their micro robots in 2D. Then, inspired by origami, a computer would print the 2D pattern of a robot that can fold itself into a 3D structure. Because paper origami is scale invariant, mechanisms built at one scale will work at another, so the idea is to build robot patterns than can be printed and then walk off of the page or out of a petri dish.

However, as Cohen said in his talk last Wednesday, “an origami artist is only as good as their origami paper.” And to build robots at a microscopic scale, one would need some pretty thin paper. Cohen’s team uses graphene, a single sheet of which is only one atom thick. Atomic layer deposition films also behave very similarly to paper, and can be cut up, stretch locally and adopt a 3D shape. Some key steps to making sure the robot self-folds include making elements that bend, and putting additional stiff pads that localize bends in the pattern of the robot. This is what allows them to produce what they call “graphene bimorphs.”

Cohen and his team are looking to use microscopic robots in making artificial cilia, which are small leg-like protrusions in cells. Cilia can be sensory or used for locomotion. In the brain, there are cavities where neurotransmitters are redirected based on cilial beatings, so if one can control the individual beating of cilia, they can control where neurotransmitters are directed. This could potentially have biomedical implications for detecting and resolving neurological disorders. 

Right now, Cohen and his lab have microscopic robots made of graphene, which have photovoltaics attached to their legs. When a light shines on the photovoltaic receptor, it activates the robot’s arm movement, and it can wave hello. The advantage of using photovoltaics is that to control the robot, scientists can shine light instead of supplying voltage through a probe—the robot doesn’t need any tethers. During his presentation, Cohen showed the audience a video of his “Brobot,” a robot that flexes its arms when a light shines on it. His team has also successfully made microscopic robots with front and back legs that can walk off a petri dish. Their dimensions are 70 microns long, 40 microns wide and two microns thick. 

Cohen wants to think critically about what problems are important to use technology to solve; he wants make projects that can predict the behavior of people in crowds, predict the direction people will go in response to political issues, and help resolve water crises. Cohen’s research has the potential to find solutions for a wide variety of current issues. Using science fiction and origami as the inspiration for his projects reminds us that the ideas we dream of can become tangible realities.

Tags:  2D materials  Cornell University  Graphene  graphene bimorphs  Itai Cohen 

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