Interface Properties of Graphene Paves Way for New Applications


201306047919620Researchers from North Carolina State University and the University of Texas have revealed more about graphene’s mechanical properties and demonstrated a technique to improve the stretchability of graphene – developments that should help engineers and designers come up with new technologies that make use of the material.

Graphene is a promising material that is used in technologies such as transparent, flexible electrodes and nanocomposites. And while engineers think graphene holds promise for additional applications, they must first have a better understanding of its mechanical properties, including how it works with other materials.

“This research tells us how strong the interface is between graphene and a stretchable substrate,” says Dr. Yong Zhu, an associate professor of mechanical and aerospace engineering at NC State and co-author of a paper on the work. “Industry can use that to design new flexible or stretchable electronics and nanocomposites. For example, it tells us how much we can deform the material before the interface between graphene and other materials fails. Our research has also demonstrated a useful approach for making graphene-based, stretchable devices by ‘buckling’ the graphene.”

The researchers looked at how a graphene monolayer – a layer of graphene only one atom thick – interfaces with an elastic substrate. Specifically, they wanted to know how strong the bond is between the two materials because that tells engineers how much strain can be transferred from the substrate to the graphene, which determines how far the graphene can be stretched.

The researchers applied a monolayer of graphene to a polymer substrate, and then stretched the substrate. They used a spectroscopy technique to monitor the strain at various points in the graphene. Strain is a measure of how far a material has stretched.

Initially, the graphene stretched with substrate. However, while the substrate continued to stretch, the graphene eventually began to stretch more slowly and slide on the surface instead. Typically, the edges of the monolayer began to slide first, with the center of the monolayer stretching further than the edges.

“This tells us a lot about the interface properties of the graphene and substrate,” Zhu says. “For the substrate used in this study, polyethylene terephthalate, the edges of the graphene monolayer began sliding after being stretched 0.3 percent of its initial length. But the center continued stretching until the monolayer had been stretched by 1.2 to 1.6 percent.”

The researchers also found that the graphene monolayer buckled when the elastic substrate was returned to its original length. This created ridges in the graphene that made it more stretchable because the material could stretch out and back, like the bellows of an accordion. The technique for creating the buckled material is similar to one developed by Zhu’s lab for creating elastic conductors out of carbon nanotubes.

The paper, “Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate,” was published online Aug. 1 in Advanced Functional Materials. Lead author of the paper is Dr. Tao Jiang, a postdoctoral researcher at NC State. The paper was co-authored by Dr. Rui Huang of the University of Texas. The research was funded by the National Science Foundation (NSF) and the NSF’s ASSIST Engineering Research Center at NC State.

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Note to Editors: The study abstract follows.

“Interfacial Sliding and Buckling of Monolayer Graphene on a Stretchable Substrate”

Authors: Tao Jiang and Yong Zhu, North Carolina State University; Rui Huang, University of Texas at Austin

Published: Aug. 1 2013, Advanced Functional Materials

Researchers Create ‘Nanoflowers’ for Energy Storage, Solar Cells


Release Date: 10.11.2012

Researchers from North Carolina State University have created flower-like structures out of germanium sulfide (GeS) – a semiconductor material – that have extremely thin petals with an enormous surface area. The GeS flower holds promise for next-generation energy storage devices and solar cells.

 

 

 

 

 

Matt Shipman |

Dr. Linyou Cao |

The GeS “nanoflowers” have petals only 20-30 nanometers thick, and provide a large surface area in a small amount of space. (Click to enlarge image.)

“Creating these GeS nanoflowers is exciting because it gives us a huge surface area in a small amount of space,” says Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State and co-author of a paper on the research. “This could significantly increase the capacity of lithium-ion batteries, for instance, since the thinner structure with larger surface area can hold more lithium ions. By the same token, this GeS flower structure could lead to increased capacity for supercapacitors, which are also used for energy storage.”

To create the flower structures, researchers first heat GeS powder in a furnace until it begins to vaporize. The vapor is then blown into a cooler region of the furnace, where the GeS settles out of the air into a layered sheet that is only 20 to 30 nanometers thick, and up to 100 micrometers long. As additional layers are added, the sheets branch out from one another, creating a floral pattern similar to a marigold or carnation.

“To get this structure, it is very important to control the flow of the GeS vapor,” Cao says, “so that it has time to spread out in layers, rather than aggregating into clumps.”

GeS is similar to materials such as graphite, which settle into neat layers or sheets. However, GeS is very different from graphite in that its atomic structure makes it very good at absorbing solar energy and converting it into useable power. This makes it attractive for use in solar cells, particularly since GeS is relatively inexpensive and non-toxic. Many of the materials currently used in solar cells are both expensive and extremely toxic.

The paper, “Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS,” is published online in the journal ACS Nano. The paper was co-authored by Cao; Dr. Chun Li, a former postdoctoral researcher at NC State, now a professor at the University of Electronic Science and Technology of China; Liang Huang, a former visiting Ph.D. student at NC State; Gayatri Pongur Snigdha, a former undergraduate student at NC State; and Yifei Yu, a Ph.D. student at NC State. The work was supported by the U.S. Army Research Office.

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Note to Editors: The study abstract follows.

“Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS”

Authors: Chun Li, Liang Huang, Gayatri Pongur Snigdha and Linyou Cao, North Carolina State University

Published: Online, ACS Nano

Abstract: We report a synthesis of single crystalline two-dimensional (2D) GeS nanosheets using vapor deposition processes, and show that the growth behavior of the nanosheet is substantially different from those of other nanomaterials and thin films grown by vapor depositions. The nanosheet growth is subject to strong influences of the diffusion of source materials through the boundary layer of gas flows. This boundary layer diffusion is found to be the rate-determining step of the growth under typical experimental conditions, evidenced by a substantial dependence of the nanosheet’s size on diffusion fluxes. We also find that high quality GeS nanosheets can only grow in the diffusion-limited regime, as the crystalline quality substantially deteriorates when the rate-determining step is changed away from the boundary layer diffusion. We establish a simple model to analyze the diffusion dynamics in experiments. Our analysis uncovers an intuitive correlation of diffusion flux with the partial pressure of source materials, the flow rate of carrier gas, and the total pressure in synthetic setup. The observed significant role of boundary layer diffusions in the growth is unique for nanosheets. It may be correlated to the high growth rate of GeS nanosheets, ~3-5 [micrometer]/min, which is one order of magnitude higher than other nanomaterials (such as nanowires) and thin films. This fundamental understanding on the effect of boundary layer diffusions may generally apply to other chalcogenide nanosheets that can grow rapidly. It can provide useful guidance for the development of general paradigms to control the synthesis of nanosheets.