The grain boundary in a computation model at left and a simulated microscope image at center were found to be a near-perfect match for an actual grain boundary, right, depicted in a 2011 Nature paper led by scientists at Cornell. The out-of-place rings highlighted in blue were likely due to distortion caused by irradiation from the microscope's electron beam. Courtesy of the Yakobson Group
Scientists at the Rice University have suggested that sinuous’ grain boundries add strength and predictable semiconducting properties.
Graphene is an atom-thick carbon form. However, it seldom appears as a faultless lattice of six-atom rings that are similar to chicken wires. When chemical vapor deposition is used to grow graphene it is normally made up of individually grown sheets that bloom in the outward direction from hot catalysts until they meet each other. These are like “domains.” At the meeting place, the atoms in regular rows are not essentially aligned, hence they must adjust if a continuous graphene plane is to be formed by them. This adjustment seems like a grain boundary that has irregular five- and seven-atom ring rows that make up for the angular disparity.
Theoretical physicist Boris Yakobson’s lab at Rice University had determined that rings possessing seven carbon atoms could be weak spots that reduced graphene’s strength. Yakobson is Rice’s Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry. However, new studies conducted at Rice demonstrate that sinuous grain boundaries could toughen polycrystalline sheets in certain cases, and this would provide the strength of pure graphene. A band gap, which is a sizable electronic transport gap is also created. Pristine graphene enables ballistic transport of electricity, however, electronics need materials that could initiate and also stop the flow in a controllable manner. This is the property of semiconductors, and researchers have been trying to acquire the ability to control characteristics of semiconductors in 2D materials and graphene.
Yakobson and his team of researchers who conducted the study found that at specific angles, the “sinuous” boundaries that would normally reduce the strength of the sheet actually relieved stress. Zhuhua Zhang, a postdoctoral researcher led this study.
If stress along the boundary were alleviated, the strength of the graphene would be enhanced, but this only applies to sinuous grain boundaries as compared with straight boundaries.
Zhang
The mechanical strength of grain boundaries was calculated by the research team to find out the way each of them influenced the other. Specifically, the boundaries that would possibly bind, and the possible place where they could possibly break under the effect of tensile stress was determined. The interface energy that exists between the sheets could be minimized by the grain boundaries by the formation of dislocations or pairs of rings. Here connected five- and seven-atom units are formed when an atom in a six-member ring moved to its adjacent ring.
Under certain circumstances, the domains’ angles do not control straight boundaries but rather winding boundaries. These sinuous boundaries were simulated by the researchers for measuring the band-gap and tensile strength properties. They found that the small sections were periodic and were applicable to the entire polycrystalline sheet. The periodicity was when the patterns recurred along the boundary’s length.
An astounding discovery was that one of Zhang’s simulations of energetically “preferred” sinuous grain boundaries matched an asymmetric boundary that he had observed in a paper that had been published in the journal Nature in 2011. Imaging with scanning transmission electron microscopy showed that the atomic grain-boundary structure had dislocations that had very similar arrangement. Zhang said that of the hundred rings that were in view, only one pair of rings were not in their designated location, which could possibly be due to a distortion that had occurred due to irradiation from the electron beam emitted by the microscope.
In order to gain benefits from predictions made at the Rice lab, researchers would have to find out the way to grow polycrystalline graphene with accurate misalignment of the components. Yakobson stated that this would be quite difficult.
Graduate students Fangbo Xu, Luqing Wang and Yang Yang are the co-authors of this study.
But this — so far, hypothetically — can be achieved if graphene nucleates at the polycrystalline metal substrate with prescribed grain orientations so that the emergent carbon isles inherit the misalignment of the template underneath.
Yakobson
The U.S. Air Force Office of Scientific Research and the Department of Energy have provided support for this study. The DAVinCI and SUGAR supercomputer clusters that are supported by the National Science Foundation and administered by Rice’s Ken Kennedy Institute for Information Technology were utilized by the researchers.
This study has been published in Advanced Functional Materials.
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