ORNL: Nano-Ribbons may hold Key to ‘on-off’ states for Graphene ~ Applications in Electronics


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“Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.”

 

A new way to grow narrow ribbons of graphene, a lightweight and strong structure of single-atom-thick carbon atoms linked into hexagons, may address a shortcoming that has prevented the material from achieving its full potential in electronic applications.

Graphene nanoribbons, mere billionths of a meter wide, exhibit different electronic properties than two-dimensional sheets of the material.   “Confinement changes graphene’s behavior,” said An-Ping Li, a physicist at the Department of Energy’s Oak Ridge National Laboratory. Graphene in sheets is an excellent electrical conductor, but narrowing graphene can turn the material into a semiconductor if the ribbons are made with a specific edge shape.

Previous efforts to make graphene nanoribbons employed a metal substrate that hindered the ribbons’ useful electronic properties.   Now, scientists at ORNL and North Carolina State University report in the journal Nature Communications that they are the first to grow graphene nanoribbons without a metal substrate. Instead, they injected charge carriers that promote a chemical reaction that converts a polymer precursor into a graphene nanoribbon.

At selected sites, this new technique can create interfaces between materials with different electronic properties. Such interfaces are the basis of semiconductor electronic devices from integrated circuits and transistors to light-emitting diodes and solar cells.   “Graphene is wonderful, but it has limits,” said Li.

“In wide sheets, it doesn’t have an energy gap–an energy range in a solid where no electronic states can exist. That means you cannot turn it on or off.”   When a voltage is applied to a sheet of graphene in a device, electrons flow freely as they do in metals, severely limiting graphene’s application in digital electronics.

GNR 2017 images“When graphene becomes very narrow, it creates an energy gap,” Li said. “The narrower the ribbon is, the wider is the energy gap.”   In very narrow graphene nanoribbons, with a width of a nanometer or even less, how structures terminate at the edge of the ribbon is important too. For example, cutting graphene along the side of a hexagon creates an edge that resembles an armchair; this material can act like a semiconductor. Excising triangles from graphene creates a zigzag edge–and a material with metallic behavior.

To grow graphene nanoribbons with controlled width and edge structure from polymer precursors, previous researchers had used a metal substrate to catalyze a chemical reaction. However, the metal substrate suppresses useful edge states and shrinks the desired band gap.   Li and colleagues set out to get rid of this troublesome metal substrate. At the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, they used the tip of a scanning tunneling microscope to inject either negative charge carriers (electrons) or positive charge carriers (“holes”) to try to trigger the key chemical reaction. They discovered that only holes triggered it. They were subsequently able to make a ribbon that was only seven carbon atoms wide–less than one nanometer wide–with edges in the armchair conformation.

“We figured out the fundamental mechanism, that is, how charge injection can lower the reaction barrier to promote this chemical reaction,” Li said. Moving the tip along the polymer chain, the researchers could select where they triggered this reaction and convert one hexagon of the graphene lattice at a time.   Next, the researchers will make heterojunctions with different precursor molecules and explore functionalities. They are also eager to see how long electrons can travel in these ribbons before scattering, and will compare it with a graphene nanoribbon made another way and known to conduct electrons extremely well.

Using electrons like photons could provide the basis for a new electronic device that could carry current with virtually no resistance, even at room temperature.

“It’s a way to tailor physical properties for energy applications,” Li said. “This is an excellent example of direct writing. You can direct the transformation process at the molecular or atomic level.” Plus, the process could be scaled up and automated.  

Source and top image: Oak Ridge National Laboratory

Rice University: Nanoribbons in solutions mimic nature: Suitable for wide use in Biomimetics


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The tip of an atomic force microscope on a cantilevered arm is used to pull a graphene nanoribbon the same way it would be used to pull apart a protein or a strand of DNA in a Rice University lab. The microscope can be used to measure properties like rigidity in a material as it’s manipulated by the tip. Credit: Kiang Research Group/Rice University

Graphene nanoribbons (GNRs) bend and twist easily in solution, making them adaptable for biological uses like DNA analysis, drug delivery and biomimetic applications, according to scientists at Rice University.

Knowing the details of how GNRs behave in a solution will help make them suitable for wide use in biomimetics, according to Rice physicist Ching-Hwa Kiang, whose lab employed its unique capabilities to probe nanoscale like cells and proteins in wet environments. Biomimetic materials are those that imitate the forms and properties of natural materials.

The research led by recent Rice graduate Sithara Wijeratne, now a postdoctoral researcher at Harvard University, appears in the Nature journal Scientific Reports.

Graphene nanoribbons can be thousands of times longer than they are wide. They can be produced in bulk by chemically “unzipping” carbon nanotubes, a process invented by Rice chemist and co-author James Tour and his lab.

Their size means they can operate on the scale of biological components like proteins and DNA, Kiang said. “We study the mechanical properties of all different kinds of materials, from proteins to cells, but a little different from the way other people do,” she said. “We like to see how materials behave in solution, because that’s where biological things are.” Kiang is a pioneer in developing methods to probe the energy states of proteins as they fold and unfold.

She said Tour suggested her lab have a look at the of GNRs. “It’s a little extra work to study these things in solution rather than dry, but that’s our specialty,” she said.

Nanoribbons are known for adding strength but not weight to solid-state composites, like bicycle frames and tennis rackets, and forming an electrically active matrix. A recent Rice project infused them into an efficient de-icer coating for aircraft.

But in a squishier environment, their ability to conform to surfaces, carry current and strengthen composites could also be valuable.

“It turns out that graphene behaves reasonably well, somewhat similar to other biological materials. But the interesting part is that it behaves differently in a solution than it does in air,” she said. The researchers found that like DNA and proteins, nanoribbons in solution naturally form folds and loops, but can also form helicoids, wrinkles and spirals.

Kiang, Wijeratne and Jingqiang Li, a co-author and student in the Kiang lab, used to test their properties. Atomic force microscopy can not only gather high-resolution images but also take sensitive force measurements of nanomaterials by pulling on them. The researchers probed GNRs and their precursors, graphene oxide nanoribbons.

The researchers discovered that all nanoribbons become rigid under stress, but their rigidity increases as oxide molecules are removed to turn graphene oxide nanoribbons into GNRs. They suggested this ability to tune their rigidity should help with the design and fabrication of GNR-biomimetic interfaces.

“Graphene and graphene oxide materials can be functionalized (or modified) to integrate with various biological systems, such as DNA, protein and even cells,” Kiang said. “These have been realized in biological devices, biomolecule detection and molecular medicine. The sensitivity of graphene bio-devices can be improved by using narrow graphene materials like nanoribbons.”

Wijeratne noted are already being tested for use in DNA sequencing, in which strands of DNA are pulled through a nanopore in an electrified material. The base components of DNA affect the electric field, which can be read to identify the bases.

The researchers saw nanoribbons’ biocompatibility as potentially useful for sensors that could travel through the body and report on what they find, not unlike the Tour lab’s nanoreporters that retrieve information from oil wells.

Further studies will focus on the effect of the nanoribbons’ width, which range from 10 to 100 nanometers, on their properties.

Explore further: Graphene nanoribbons get metallic

More information: Sithara S. Wijeratne et al, Detecting the Biopolymer Behavior of Graphene Nanoribbons in Aqueous Solution, Scientific Reports (2016). DOI: 10.1038/srep31174

 

 

Rice De-icer Graphene Nano Ribbons gains anti-icing properties


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Rice University scientists have modified their graphene-based de-icer to resist the formation of ice well below the freezing point and added superhydrophobic capabilities. The robust film is intended for use in extreme environments as well …more

Rice University scientists have modified their graphene-based de-icer to resist the formation of ice well below the freezing point and added superhydrophobic capabilities. The robust film is intended for use in extreme environments as well …more
Rice University scientists have advanced their graphene-based de-icer to serve a dual purpose. The new material still melts ice from wings and wires when conditions get too cold. But if the air is above 7 degrees Fahrenheit, ice won’t form at all.

 

 

The Rice lab of chemist James Tour gave its de-icer superhydrophobic (water-repelling) capabilities that passively prevent water from freezing above 7 degrees. The tough film that forms when the de-icer is sprayed on a surface is made of atom-thin graphene nanoribbons that are conductive, so the material can also be heated with electricity to melt ice and snow in colder conditions.
The material can be spray-coated, making it suitable for large applications like aircraft, power lines, radar domes and ships, according to the researchers. The study was published this month in the American Chemical Society journal ACS Applied Materials and Interfaces.

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“We’ve learned to make an ice-resistant material for milder conditions in which heating isn’t even necessary, but having the option is useful,” Tour said. “What we now have is a very thin, robust coating that can keep large areas free of ice and snow in a wide range of conditions.”
Tour, lead authors Tuo Wang, a Rice graduate student, and Yonghao Zheng, a Rice postdoctoral researcher, and their colleagues tested the film on glass and plastic.
Rice de-icer gains anti-icing properties.

 
Test samples show a graphene nanoribbon-based film developed at Rice University with both passive and active de-icing properties. When the film at left is heated without an application of lubricants, ice melts but water droplets remain on …more
Materials are superhydrophobic if they have a water-contact angle larger than 150 degrees.

The term refers to the angle at which the surface of the water meets the surface of the material. The greater the beading, the higher the angle. An angle of 0 degrees is basically a puddle, while a maximum angle of 180 degrees defines a sphere just touching the surface.
The Rice films use graphene nanoribbons modified with a fluorine compound to enhance their hydrophobicity. They found that nanoribbons modified with longer perfluorinated chains resulted in films with a higher contact angle, suggesting that the films are tunable for particular conditions, Tour said. Warming test surfaces to room temperature and cooling again had no effect on the film’s properties, he said.

 
The researchers discovered that below 7 degrees, water would condense within the structure’s pores, causing the surface to lose both its superhydrophobic and ice-phobic properties. At that point, applying at least 12 volts of electricity warmed them enough to retain its repellant properties.

 
Applying 40 volts to the film brought it to room temperature, even if the ambient temperature was 25 degrees below zero. Ice allowed to form at that temperature melted after 90 seconds of resistive heating.

 
The researchers found that while effective, the de-icing mode did not remove water completely, as some remained trapped in the pores between linked nanoribbon bundles. Adding a lubricant with a low melting point (minus 61 degrees F) to the film made the surface slippery, sped de-icing and saved energy.

 

 

 
Explore further: Graphene nanoribbons an ice-melting coat for radar
More information: Tuo Wang et al, Passive Anti-icing and Active Deicing Films, ACS Applied Materials & Interfaces (2016). DOI: 10.1021/acsami.6b03060
Journal reference: ACS Applied Materials and Interfaces
Provided by: Rice University

 

Graphene Nanoribbons for high-speed electronics


High Speed Elec Graph Nono Ribs id42275An international research team at Tohoku University’s Advanced Institute of Materials Research (AIMR) succeeded in chemically interconnecting chiral-edge graphene nanoribbons (GNRs) with zigzag-edge features by molecular assembly, and demonstrated electronic connection between GNRs (ACS Nano, “Self-Assembly Strategy for Fabricating Connected Graphene Nanoribbons”). The GNRs were interconnected exclusively end to end, forming elbow structures, identified as interconnection points (Fig. 1a).
This configuration enabled researchers to demonstrate that the electronic architecture at the interconnection points between two GNRs (Fig. 1b) is the same as that along single GNRs; evidence that GNR electronic properties, such as electron and thermal conductivities, are directly extended through the elbow structures upon chemical GNR interconnection.
Electronically Connected Graphene Nanoribbons Foresee High-Speed Electronics
Figure 1 shows interconnected graphene nanoribbons (GNRs). The interconnection points are observed as elbow structures. The inset of (a) shows the chemical structure of an elbow interconnection point of two chiral-edge GNRs. The top panel of (b) shows the scanning tunneling microscopy topograph, highlighting a single GNR and a pair of connected GNRs (elbow). The bottom panel of (b) shows the local density of states (LDOS) of these two structures share the same electronic architecture, including the elbow interconnection point. This indicates that electronic properties, such as electron and thermal conductivities, should be comparable between termini 1-2 and termini 3-4. (Image: Patrick Han)
This work shows that future development of high-performance, low-power-consumption electronics based on GNRs is possible.
Graphene has long been expected to revolutionize electronics, provided that it can be cut into atomically precise shapes that are connected to desired electrodes. However, while current bottom-up fabrication methods can control graphene’s electronic properties, such as high electron mobility, tailored band gaps and s pin-aligned zigzag edges, the connection aspect of graphene structures has never been directly explored. For example, whether electrons traveling across the interconnection points of two GNRs would encounter higher electric resistance remains an open question. As the answers to this type of questions are crucial towards the realization of future high-speed, low-power-consumption electronics, we use molecular assembly to address this issue here.
“Current molecular assemblies either produce straight GNRs (i.e., without identifiable interconnection points), or randomly interconnected GNRs,” says Dr. Patrick Han, the project leader. “These growth modes have too many intrinsic unknowns for determining whether electrons travel across graphene interconnection points smoothly. The key is to design a molecular assembly that produces GNRs that are systematically interconnected with clearly distinguishable interconnection points.”
To reach this goal, the AIMR team used a Cu substrate, whose reactivity confines the GNR growth to six directions, and used scanning tunneling microscopy (STM) to visualize the GNR electronic structures. By controlling the precursor molecular coverage, this molecular assembly connects GNRs from different growth directions systematically end to end, producing elbow structures–identified as interconnection points (Fig. 1a). Using STM, the AIMR team revealed that the delocalization of the interconnected GNR *-states extends the same way both across a single straight GNR, and across the interconnection point of two GNRs (periodic features in Fig. 1b, bottom panel). This result indicates that GNR electronic properties, such as electron and thermal conductivities, should be the same at the termini of single GNRs and that of two connected GNRs.
“The major finding of this work is that interconnected GNRs do not show electronic disruption (e.g., electron localization that increases resistance at the interconnection points),” says Han. “The electronically smooth interconnection demonstrates that GNR properties (including tailored band gaps, or even spin-aligned zigzag edges) can be connected to other graphene structures. These results show that finding a way to connect defect-free GNRs to desired electrodes may be the key strategy toward achieving high-performance, low-power-consumption electronics.”
Source: Tohoku University

 

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