Size does matter for nanoscale friction

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2010

New research reveals that atomic-scale interactions play a major role in determining how friction behaves.

KEY CONCEPTS
• Frictional effects among atomically thin sheets of materials with different structures and electronic properties were studied.
• An atomic force microscope determined that friction decreases as the thickness of atomic layers increases from one to around four and then levels off.
• A single sheet of atoms produces more friction because it more readily bends to conform to an AFM tip, leading to more contact area and higher friction.

Researchers continue working to gain a better understanding of friction at the nanoscale because it has an impact on how materials are prepared for such applications as nanoscale data storage systems, nanocomposites and nanoelectromechanical devices. One thing that has been learned is that atomic-scale interactions are very important in determining how friction behaves at the nanoscale.

In a previous TLT article, work was discussed that correlates friction at the nanoscale with friction at the macroscale by taking into account that surfaces at the nanoscale are not smooth but, in fact, are rough with topography similar to a mountain range (1). Atoms and molecules are present at the tops of peaks and can rub against each other in a fashion similar to frictional behavior at the macroscale.

In looking at materials on the nanoscale, two-dimensional interactions become very important. Graphene is one such material that is being closely examined because it consists of individual atomic layers of carbon and is a precursor for graphite. A previous TLT article highlighted work done to show that graphene is the strongest material ever examined (2).

One objective in working with materials at the atomic level is to determine the frictional effects seen with small numbers of atomic layers. Such work has not been carried out until now.

ATOMICALLY THIN SHEETS
STLE-member Robert Carpick, professor in the department of mechanical engineering and applied mechanics at the University of Pennsylvania in Philadelphia, and James Hone, professor in the department of mechanical engineering at Columbia University in New York, headed up a group of researchers that evaluated how friction occurs among atomically thin materials. The materials evaluated are hexagonal boron nitride, graphene, molybdenum disulfide and niobium diselenide.

Carpick says, “Our objective was to evaluate a range of layered materials that have different lattice constants, elastic constants and electronic properties in order to see if we could find some commonality for frictional behavior that is independent of their different structures and electronic properties.” 

Graphene and molybdenum disulfide are certainly familiar materials widely used as solid lubricants. Hexagonal boron nitride is also utilized as a solid lubricant in applications such as a coating for cutting tools. The final material, niobium diselenide, is not widely known. Carpick says, “Niobium diselenide is an exotic material of interest for its low temperature, superconductivity. As far as we know, there are no known tribological applications.”

The researchers used an atomic force microscope (AFM) in a technique known as friction force microscopy to measure the frictional forces encountered with atomically thin sheets of these materials. The sheets of these four materials were prepared by exfoliating them from a bulk source onto a silicon oxide substrate. The AFM also was used in a topographic mode to determine the thickness of the atomic sheets.

The researchers found that the friction generated for all four materials increases with decreasing atomic thickness. This result may not seem intuitive, but Carpick provides an explanation.

“When a material gets thin, it also becomes very flexible,” he says. “A macroscale example is the fact that bending back a single sheet of paper is far easier than bending a piece of wood. As the AFM tip approaches a single sheet of atoms, an attractive Van der Waals’ force occurs between the tip and the single sheet, causing the sheet to bend as it conforms to the tip. This means more contact area and, thus, higher friction.” 

The researchers also observed that the friction force exhibited a stick-slip motion, slipping once for every unit cell of the lattice for graphene and molybdenum disulfide. The force required to slip was seen to build up for the first 1-2 nanometers of sliding, consistent with the idea of deforming the puckered sheet as sliding begins.”

Carpick continues, “In contrast, thicker sheets of atoms do not conform as readily because they are stiffer, leading to a lower level of friction. The bending effect of the single sheet around the tip is also known as a puckering.” 

Figure 1 shows the puckering effect as a single atomic layer of a graphene sheet adheres to an AFM tip. Graphene atoms moving out of the plane to conform to the tip are shown in blue and red.


Figure 1. Adhesion of a sliding AFM tip on a single layer of graphene atoms leads to higher friction than on thicker layers due to a puckering effect. Atoms depicted in blue and red adhere to the tip and move out of the plane. (Courtesy of the University of Pennsylvania)

The researchers estimate that a monolayer sheet causes 20% greater friction than bilayers of atoms and 2-3 times higher friction than bulk sheets of atoms. This phenomenon was seen regardless of changes in scanning speeds, applied forces, AFM tips made from different materials and humidity changes. In the latter case, a reduction in humidity from 30% to 5% did lead to overall lower friction, but the variation of friction as a function of thickness remained constant.

Carpick indicates that friction does not continue to decrease as the thickness of atomic layers increases. He says, “Friction decreases as the number of atomic sheets increases to around four but then levels off. The friction seen for four atomic layers is comparable to friction seen for 50 layers or when the material is present in a bulk state.” 

The researchers then placed the materials on a surface that they strongly adhere to in order to assess the frictional effect. The surface used was muscovite mica, which forces the materials to be atomically flat when facing the tip of the AFM.

The result is that no increase in friction is seen with a decrease in atomic thickness for any of the materials. Carpick says, “When the AFM tip encounters the atomic layers of the materials, they do not come off the substrate and adhere in the same fashion as when they are on the silicon oxide surface.”

A second follow-up experiment determined how the materials perform if suspended over 300-nanometer holes placed in the silicon oxide substrate. Friction did increase as the thickness declined in a similar fashion to when the materials were placed on the silicon oxide substrate.

These results clearly suggest that the thickness dependence of friction at the nanoscale applies to all thin materials that are either attached loosely to a substrate or suspended over a substrate. Carpick is hoping that this research will aid the design and performance of nanoscale mechanical devices.

Further information can be found in a recent article (3) and by contacting Carpick at carpick@seas.upenn.edu. 

REFERENCES
1. Canter, N. (2009), “Understanding Friction Laws at the Nanoscale and their Relation to the Macroscale,” TLT, 65 (7), pp. 10–11.
2. Canter, N. (2009), “Graphene: The Strongest Material ever Examined,” TLT, 65 (2), pp. 28–29.
3. Lee, C., Qunyang, L., Kalb, W., Liu, X., Berger, H., Carpick, R. and Hone, J., “Frictional Characteristics of Atomically Thin Sheets,” Science, 328 (5974), pp. 76–80.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.