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Atomic force microscopy images of as-deposited (left) and laser-annealed (right) reduced graphene oxide (rGO) thin films. The entire “pulsed laser annealing” process is done at room temperature and atmospheric pressure, using high-power laser pulses to convert p-type rGO material into n-type and completed in about one fifth of a microsecond. (credit: Anagh Bhaumik and Jagdish Narayan/Journal of Applied Physics)
Researchers at North Carolina State University (NC State) have developed a layered material that can be used to develop transistors based on graphene — a long-sought goal in the electronics industry.
Graphene has attractive properties, such as extremely high conductivity, meaning it conducts the flow of electrical current really well (compared to copper, for example), but it’s not a semiconductor, so it can’t work in a transistor (aside from providing great connections). A form of graphene called “graphene oxide” is a semiconductor, but it does not conduct well.
However, a form of graphene oxide called “reduced graphene oxide” (rGO) does conduct well*. Despite that, rGO still can’t function in a transistor. That’s because the design of a transistor is based on creating a junction between two materials: one that is positively charged (p-type) and one that is negatively charged (n-type), and native rGO is only a p-type.
The NC State researchers’ solution was to use high-powered laser pulses to disrupt chemical groups on an rGO thin film. This disruption moved electrons from one group to another, effectively converting p-type rGO to n-type rGO. They then used the two forms of rGO as two layers (a layer of n-type rGO on the surface and a layer of p-type rGO underneath) — creating a layered thin-film material that could be used to develop rGO-based transistors for use in future semiconductor chips.
The researchers were also able to integrate the rGO-based transistors onto sapphire and silicon wafers across the entire wafer.
The paper was published in the Journal of Applied Physics. The work was done with support from the National Science Foundation.
* Reduction is a chemical reaction that involves the gaining of electrons.
Abstract of Conversion of p to n-type reduced graphene oxide by laser annealing at room temperature and pressurePhysical properties of reduced graphene oxide (rGO) are strongly dependent on the ratio of sp2 to sp3hybridized carbon atoms and the presence of different functional groups in its structural framework. This research for the very first time illustrates successful wafer scale integration of graphene-related materials by a pulsed laser deposition technique, and controlled conversion of p to n-type 2D rGO by pulsed laser annealing using a nanosecond ArF excimer laser. Reduced graphene oxide is grown onto c-sapphire by employing pulsed laser deposition in a laser MBE chamber and is intrinsically p-type in nature. Subsequent laser annealing converts p into n-type rGO. The XRD, SEM, and Raman spectroscopy indicate the presence of large-area rGO onto c-sapphire having Raman-active vibrational modes: D, G, and 2D. High-resolution SEM and AFM reveal the morphology due to interfacial instability and formation of n-type rGO. Temperature-dependent resistance data of rGO thin films follow the Efros-Shklovskii variable-range-hopping model in the low-temperature region and Arrhenius conduction in the high-temperature regime. The photoluminescence spectra also reveal less intense and broader blue fluorescence spectra, indicating the presence of miniature sized sp2 domains in the vicinity of p* electronic states, which favor the VRH transport phenomena. The XPS results reveal a reduction of the rGO network after laser annealing with the C/O ratio measuring as high as 23% after laser-assisted reduction. The p to n-type conversion is due to the reduction of the rGO framework which also decreases the ratio of the intensity of the D peak to that of the G peak as it is evident from the Raman spectra. This wafer scale integration of rGO with c-sapphire and p to n-type conversion employing a laser annealing technique at room temperature and pressure will be useful for large-area electronic devices and will open a new frontier for further extensive research in graphene-based functionalized 2D materials.
While all three of these options bring attractive properties to the table—most importantly, a very high theoretical capacity—those properties are lost in the real world. Silicon electrodes crack and break after just a short number of charge/discharge cycles. Meanwhile, the use of graphene on electrodes is limited because graphene’s attractive surface area is only possible in single stand-alone sheets, which don’t provide enough volumetric capacitance. Layer the graphene sheets on top of each other to gain that volumetric capacity, and you begin to lose that attractive surface area.
“Silicon combined with graphene is better than a bulk silicon electrode,” explained Gurpreet Singh, an associate professor at KSU and one of the researchers, in an e-mail interview with IEEE Spectrum. “However, nano-silicon/graphene electrodes fail to satisfy key requirements for any practical applications.” Among other things, they have poor volumetric capacity, high cost, and low cycling efficiency—too much lithium is lost irreversibly with each charge-discharge cycle. What’s more, their mechanical and chemical instability that can lead to rapid capacity decay.
To overcome this, the KSU researchers turned to the high temperature glass ceramic, silicon oxycarbide. In research described in the journal Nature Communications, the KSU team created a self-standing anode material consisting of silicon oxycarbide glass particles embedded into a chemically modified graphene oxide matrix.
A heated silicon resin decomposes so that “the constituent silicon, carbon, and oxygen atoms are arranged in a random 3-D structure, and any excess carbon precipitates out into string-like or cellular regions. Such an open 3-D structure renders large sites for reversible lithium storage and smooth channels for solvated lithium-ion transportation from the electrolyte.”
This stands in stark contrast to crystalline silicon, which undergoes an alloying reaction with lithium that results in enormous volume changes and also an irreversible reaction with the electrolyte that leads to chemical instability and fading capacity as the charge-discharge cycles add up.
The KSU researchers claim that the electrode has a capacity of approximately 600 miliampere-hour per gram or 400 miliampere-hour per cubic centimeter of the electrode after 1020 cycles. The researchers expect that the power density (the maximum amount of power that can be supplied per unit mass) will be more than three times that of today’s Li-ion batteries.
In future research, the KSU team aims to produce electrode materials with larger dimensions. As a benchmark, the researchers are looking at today’s pencil cell battery that uses a graphite-coated copper foil electrode, which is more than 30 cm in length.
Singh added: “We are also looking at batteries as structural materials, such as load bearing batteries that can be charged and discharged while under dynamic loads.”
(From left) T. Phanindra Sai, Amogh Kinikar, Arindam Ghosh resorted to mechanical exfoliation to make graphene conduct current along the edge.
The new way of making graphene with a perfect edge structure was the key to success Researchers from the Indian Institute of Science (IISc), Bengaluru have been able to experimentally produce a new type of electrical conductor that was theoretically predicted nearly 20 years ago.
A team led by Arindam Ghosh from the Department of Physics, IISc successful produced graphene that is single- or a few-layers thick to conduct current along one particular edge — the zigzag edge. The zigzag edge of graphene layer has a unique property: It allows flow of charge without any resistance at room temperature and above.
“This is the first we found the perfect edge structure in graphene and demonstrated electrical conductance along the edge,” says Prof. Ghosh. The results of the study were published in the journal Nature Nanotechnology.
A few-layers-thick graphene that conducts current along one edge does not experience any resistance and so can lead to realising power-efficient electronics and quantum information transfer, even at room temperature.
Getting an edge
Many groups over the world have been trying to access these edges since the emergence of graphene in 2004, but have been largely unsuccessful because when current flows through graphene, it flows through both the edge as well as the bulk. “We succeeded in this endeavour by creating the bulk part of graphene extremely narrow (less than 10 nanometre thick), and hence highly resistive, thus forcing the current to flow through the edge alone,” he says.
“While the bulk is totally insulating, the edge alone has the ability to conduct because of the unique quantum mechanics of the edge. Because of the zigzag orientation of carbon atoms [resulting from the hexagonal lattice], the electron wave on each carbon atom overlaps and forms a continuous train of wave along the edge. This makes the edge conducting,” explains Prof. Ghosh. The edge will remain conductive even if it is very long but has to be chemically and structurally pristine.
In the past, others researchers had tried making narrow graphene through chemical methods. But the use of chemicals destroys the edges. So the IISc team resorted to mechanical exfoliation to make graphene that are single- and few-layers thick. They used a small metal robot to peel the graphene from pyrolytic graphite. “If you take a metal tip and crash it on graphite and take it back, a part of the graphite will stick to the tip. The peeling was done slowly and gradually (in steps of 0.1 Å),” says Amogh Kinikar from the Department of Physics at IISc and the first author of the paper.
Effect of chemicals
The exfoliation was carried out at room temperature but under vacuum and the electrical conductance was measured at the time of exfoliation before the pristine nature of the edge was affected. The unsatisfied bonds of the carbon atoms make them highly reactive and they tend to react with hydrogen present in the air. “The edges conduct without any resistance as long as the edges don’t come in contact with any chemicals,” says Prof. Ghosh. “It is very easy to passivate [make the surface unreactive by coating the surface with a thin inert layer] the edges to prevent contamination [when narrow graphene is used for commercial purposes].”
As the carbon atoms have a hexagonal structure, exfoliation is by default at 30 degree angle and one of the edges has a zigzag property. “The steplike changes observed for small values of conductance when other variables were changed were surprising. Through theoretical work we were able to link this to edge modes in graphene,” says Prof. H.R.Krishnamurthy from the Department of Physics, IISc and one of the authors of the paper.
There are currently several chemical methods to produce very narrow graphene nanoribbons. But these chemicals tend to destroy the edges. “So the challenge is to produce graphene nanoribbons using chemicals that do not destroy the edges,” Prof. Ghosh says. “We believe that this successful demonstration of the dissipation-less edge conduction will act as great incentive to develop new chemical methods to make high-quality graphene nano-ribbons or nano-strips with clean edges.”
I was looking for any concrete laced with graphene news, and I found this story. I'm posting it because it has a graphite company, Zenyata (ZEN on the Israeli exchange) that is developing graphene for concrete, and is investable if you have access to the Israeli market (I don't).
I am not sure how these graphite to graphene plays will work out against the other mass production forms that don't use graphite and have showed promise recently, but there are a bunch of them out there.
Zenyatta Ventures Graphene – Wonder material makes concrete stronger
By now most investors have heard of the new wonder material called graphene. This single carbon atom super material can be derived from natural graphite. Graphene can truly be called the material of the future as it is 200 times stronger than steel but very flexible, and it conducts electricity better than any other material. Graphene is so thin that it may be thought of as two-dimensional and can be mixed with all manner of materials to increase strength. Due to these and other properties, it has the potential to literally revolutionize several industries. Thousands of researchers all over the world are working to develop many disruptive and game changing graphene applications. One such revolutionary application is the addition of graphene to cement creating a strong and environmentally “green” concrete.
Zenyatta Ventures Ltd. (TSXV-ZEN) partnership with Ben-Gurion University (‘BGU’) and Larisplast Ltd. is putting the junior mining company on the leading edge of a graphene concrete application because of its purity and crystallinity derived from a rare igneous (or volcanic) style of graphite. The deposit, called Albany, is especially well suited to form graphene from graphite as noted by scientists from BGU and Lakehead University in Thunder Bay.
Zenyatta is traded on the US OTC under ticker ZENYF. Company is actually Canadian-based (their Albany graphite deposit is located in Northern Ontario). Its main listing is on the Toronto Stock Exchange. The NR you made reference to is regarding a partnership with an Israeli University named Ben-Gurion. Here's the 5-year chart on ZENYF (been quite a ride!)...there's a number of active SI Zenyatta boards that you can also easily find if interested: