Graphene balloons to identify noble gases

Graphene balloons to identify noble gases
Credit: TU Delft/Makars Šiškins

New research by scientists from Delft University of Technology and the University of Duisburg-Essen uses the motion of atomically thin graphene to identify noble gasses. These gasses are chemically passive and do not react with other materials, which makes it challenging to detect them. The findings are reported in the journal Nature Communications.

Graphene is an ultimately thin material consisting of only one layer of carbon atoms. Its atomic thickness makes it a perfect filter material for gasses and liquids: graphene by itself it is not permeable, but small perforations make it very permeable. Moreover, the material is among the strongest known and withstands high stresses. Together, these two traits provide the perfect basis for new types of gas sensors.

Nano balloons

The scientists use microscopic balloons made of bilayer graphene (with a thickness of 0.7 nm), with very small nanopore perforations with diameters down to 25 nm, to detect gasses. They use a laser to heat the gas inside the balloon and make it expand. The pressurized gas then escapes through the perforation. “Picture a balloon that deflates when you let the air run out,” says TU Delft researcher Irek Rosłoń, “We measure the time it takes the balloon to deflate. At such a small scale, this happens very quickly—within around 1/100.000th of a second—and interestingly, the length of time depends strongly on the type of gas and the size of the pores. For example helium, a light gas with high molecular velocity, escapes five times faster than krypton, a heavy and slowly moving gas.” The method allows to distinguish gasses based on their mass and molecular velocity, which normally requires big mass spectrometers.

Gas pumping

The graphene balloons are continuously driven by an optothermal force at high-frequencies of 100 kHz, causing gas to be pumped in and out through the nano-pores very rapidly. The permeation of the gas can be studied by looking at the mechanical motion of the graphene. At low pumping frequencies, the gas has plenty of time to escape and does not affect the motion of the graphene significantly. However, the membrane experiences a large amount of drag at increased pumping frequencies, in particular when the period of pumping corresponds to the typical time it takes for the gas to leave the balloon. “By measuring at various frequencies, we can find that peak in the drag. The frequency at which a peak is observed corresponds to the permeation speed of the gas.”

The researchers extended this idea to study gas flow through nano-channels. Connecting the balloon to a long channel makes it much harder for the gas to escape. The increase in the deflation time gives experimental insight into the gas flow mechanics within the nano-channels. Altogether, this work shows how the extraordinary properties of graphene can be used to study gas dynamics at the nanoscale, as well as to engineer new types of sensors and devices. In the future, this can enable small, low-cost and versatile sensor devices to

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Reliable quality-control of graphene and other 2-D materials is routinely possible, researchers say

Reliable quality-control of graphene and other 2D materials is routinely possible
New experiments confirm that the Bell-Shaped-Component (BSC) is a reliable diagnostic of the quality of graphene growth. Credit: U.S. Department of Energy, Ames Laboratory

Graphene and other single-atom-thick substances are a category of wonder materials, with researchers the world over investigating their electronic properties for potential applications in technologies as diverse as solar cells, novel semiconductors, sensors, and energy storage.

The greatest challenge for the design of these single-layer or 2-D materials into all their myriad potential uses is the need for an atom-by-atom perfection and uniformity that can be difficult and painstaking to achieve at such small scales, and difficult to assess as well.

“We are trying to be more clever than nature in assembling these materials,” said Michael C. Tringides, a senior scientist at the U.S. Department of Energy’s Ames Laboratory and professor of physics at Iowa State University, who investigates the unique properties of 2-D materials and metals grown on graphene, graphite, and other carbon coated surfaces. “And to do so, we’re forcing atoms to assemble in ways they normally would not. One of the major challenges of the field is to reliably produce high quality graphene and other materials like it.”

Tringides and other scientists at Ames Laboratory have discovered and confirmed a method which could serve as an easy but reliable way to test the quality of graphene and other 2-D materials. It takes advantage of the very broad background in surface electron diffraction, named the Bell-Shaped-Component (BSC) which strongly correlates to uniformly patterned, or “perfect” graphene.

Understanding the correlation has implications for reliable quality control of 2-D materials in a manufacturing environment.

“This discovery challenges conventional wisdom, but the correlation between this strange phenomenon and high quality graphene is unmistakable. In practical application, we see it extending to other high-interest 2-D materials that are similar to graphene in having similar uniformity of a single layer,” said Tringides.

Last year, Ames Laboratory researchers discovered through low energy electron diffraction— a technique commonly used in physics to study the crystal structure of the surfaces of solid materials— that broad diffraction patterns are an indicator that reliably demonstrates a 2-D material’s high quality. It was a feature of high quality graphene that essentially lurked in the background, and had been overlooked in published research because it was the exact opposite of what is generally accepted from diffraction studies—that only sharp, bright diffraction spots should be present. Because that finding was counterintuitive, further investigation was required under different experimental conditions and to understand the origin of the BSC, said Tringides.

First, the scientists grew graphene by annealing, or heating it, through a range of high temperatures, and comparing the growth of the BSC diffraction along with the growth of the other, generally accepted indicator of sharp diffraction spots. The evolution of the broad diffraction background closely mirrored that of the sharper spot, which proved that they are correlated. Secondly, the group then experimented with depositing metal atoms (in this case dysprosium) on the surface and underneath the graphene.

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On-surface synthesis of graphene nanoribbons could advance quantum devices

On-surface synthesis of graphene nanoribbons could advance quantum devices
Scientists synthesized graphene nanoribbons, shown in yellow, on a titanium dioxide substrate, in blue. The lighter ends of the ribbon show magnetic states. The inset drawing shows how the ends have up and down spin, suitable for creating qubits. Credit: ORNL, U.S. Dept. of Energy

An international multi-institution team of scientists has synthesized graphene nanoribbons—ultrathin strips of carbon atoms—on a titanium dioxide surface using an atomically precise method that removes a barrier for custom-designed carbon nanostructures required for quantum information sciences.

Graphene is composed of single-atom-thick layers of carbon taking on ultralight, conductive and extremely strong mechanical characteristics. The popularly studied material holds promise to transform electronics and information science because of its highly tunable electronic, optical and transport properties.

When fashioned into nanoribbons, graphene could be applied in nanoscale devices; however, the lack of atomic-scale precision in using current state-of-the-art “top-down” synthetic methods—cutting a graphene sheet into atom-narrow strips—stymie graphene’s practical use.

Researchers developed a “bottom-up” approach—building the graphene nanoribbon directly at the atomic level in a way that it can be used in specific applications, which was conceived and realized at the Center for Nanophase Materials Sciences, or CNMS, located at the Department of Energy’s Oak Ridge National Laboratory.

This absolute precision method helped to retain the prized properties of graphene monolayers as the segments of graphene get smaller and smaller. Just one or two atoms difference in width can change the properties of the system dramatically, turning a semiconducting ribbon into a metallic ribbon. The team’s results were described in Science.

ORNL’s Marek Kolmer, An-Ping Li and Wonhee Ko of the CNMS’ Scanning Tunneling Microscopy group collaborated on the project with researchers from Espeem, a private research company, and several European institutions: Friedrich Alexander University Erlangen-Nuremberg, Jagiellonian University and Martin Luther University Halle-Wittenberg.

ORNL’s one-of-a-kind expertise in scanning tunneling microscopy was critical to the team’s success, both in manipulating the precursor material and verifying the results.

“These microscopes allow you to directly image and manipulate matter at the atomic scale,” Kolmer, a postdoctoral fellow and the lead author of the paper, said. “The tip of the needle is so fine that it is essentially the size of a single atom. The microscope is moving line by line and constantly measuring the interaction between the needle and the surface and rendering an atomically precise map of surface structure.”

In past graphene nanoribbon experiments, the material was synthesized on a metallic substrate, which unavoidably suppresses the electronic properties of the nanoribbons.

“Having the electronic properties of these ribbons work as designed is the whole story. From an application point of view, using a metal substrate is not useful because it screens the properties,” Kolmer said. “It’s a big challenge in this field—how do we effectively decouple the network of molecules to transfer to a transistor?”

The current decoupling approach involves removing the system from the ultra-high vacuum conditions and putting it through a multistep wet chemistry process, which requires etching the metal substrate away. This process contradicts the

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