An interdisciplinary team of engineers from the University of Pennsylvania has made a discovery regarding the surface properties of graphene, the Nobel-prize winning material that consists of an atomically thin sheet of carbon atoms.
On the macroscale, adding fluorine atoms to carbon-based materials makes for water-repellant, non-stick surfaces, such as Teflon. However, on the nanoscale, adding fluorine to graphene had been reported to vastly increase the friction experienced when sliding against the material.
Through a combination of physical experiments and atomistic simulations, the Penn team has discovered the mechanism behind this surprising finding, which could help researchers better design and control the surface properties of new materials.
The research was led by postdoctoral researcher Qunyang Li, graduate student Xin-Zhou Liu and Robert Carpick, professor and chair of the Department of Mechanical Engineering and Applied Mechanics in Penn's School of Engineering and Applied Science. They collaborated with Vivek Shenoy, a professor in the Department of Materials Science and Engineering. The Penn contingent also worked with researchers from the Naval Research Laboratory and Brown University.
The work was published in Nano Letters.
Besides its applications in circuitry and sensors, graphene is of interest as a super-strong coating. As components of mechanical and electrical systems get smaller, they are increasingly susceptible to wear and tear. Made up of fewer atoms than their macroscale counterparts, each atom is that much more important to the component's overall structure and function.
"One of the major failure mechanisms for these small-scale devices is friction and adhesion," Liu said. "Because graphene is so strong, thin and smooth, one of its potential applications is to reduce friction and increase the lifespan of these devices. We wanted to better understand the fundamental mechanisms of how the addition of other atoms influences the friction of graphene."
"Generally speaking," Carpick said, "fluorine makes surfaces more water-repellent and non-stick. Gore-Tex and Teflon, for example, get their properties from fluorine. Teflon is a fluorinated carbon polymer, so we thought fluorinated graphene might be like two-dimensional Teflon."
To test the friction properties of this material, the Penn researchers collaborated with Paul Sheehan and Jeremy Robinson of the Naval Research Laboratory. Sheehan and Robinson were the first to discover fluorinated graphene and are experts in producing samples of the material to specification.
"This meant we were able to systematically vary the degree of fluorination in our graphene samples and quantify it precisely," Liu said. "That let us make accurate comparisons when we tested the friction of these different samples with an atomic force microscope, an ultra-sensitive instrument that can measure nanonewton forces."
The researchers were surprised to find that adding fluorine to graphene increased the material's friction but could not immediately explain the mechanism responsible. Another group of researchers had simultaneously made the same observation; they also showed that the addition of fluorine increased the stiffness of the graphene samples and hypothesized this was responsible for the increased friction.
The Penn researchers, however, thought another mechanism must be at work. They turned to Shenoy, whose expertise is in developing atomic scale simulations of mechanical action, to help explain what the addition of the fluorine was doing to the graphene's surface.
"We don't have a microscope that can visualize what's happening at this small scale," Shenoy said, "but there are few enough atoms that we can model how they behave with a high degree of accuracy."
"It turns out that by adding fluorine," Liu said, "we're changing the energy corrugation landscape of the graphene. We're essentially introducing electronic roughness, which at the nanoscale, can act like physical roughness in increasing friction."
In fluorinated graphene, the fluorine atoms do stick up out of the plane of carbon atoms, but the physical changes in height paled in comparison to the changes of local energy each fluorine atom produced.
"At the nanoscale," Carpick said, "friction isn't just determined by the placement of atoms but also how much energy is in their bonds. Each fluorine atom has so much electronic charge that you get tall peaks and deep valleys in between them, compared to the smooth plane of regular graphene. You could say it's like trying slide over a smooth road versus a bumpy road."
"Every material interacts with the world through its surface," Carpick said, "so understanding and manipulating surface properties -- friction, adhesion, interactions with water, catalysis -- are major, ongoing areas of scientific research. Seeing that fluorine increases friction in graphene isn't necessarily a bad thing, since it may give us a way to tailor that property to a given application. It also will help us understand how the addition of other elements, like hydrogen or oxygen, might influence those properties."
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