Nanotechnologists reveal the frictional characteristics of atomically thin sheets

April 01, 2010

PHILADELPHIA -- A team of nanotechnology researchers from the University of Pennsylvania and Columbia University has used friction force microscopy to determine the nanoscale frictional characteristics of four atomically-thin materials, discovering a universal characteristic for these very different materials. Friction across these thin sheets increases as the number of atomic layers decreases, all the way down to one layer of atoms. This friction increase was surprising as there previously was no theory to predict this behavior.

The finding reveals a significant principle for these materials, which are widely used as solid lubricant films in critical engineering applications and are leading contenders for future nanoscale electronics.

Researchers found that friction progressively increased as the number of layers is reduced on all four materials, regardless of how different the materials may behave chemically, electronically or in bulk quantities. These measurements, supported by computer modeling, suggest that the trend arises from the fact that the thinner a material the more flexible it is, just as a single sheet of paper is much easier to bend than a thick piece of cardboard.

Robert Carpick, professor in the Department of Mechanical Engineering and Applied Mechanics at Penn, and James Hone, professor in the Department of Mechanical Engineering at Columbia, led the project collaboratively.

The team tested the nanotribological, or nano-scale frictional properties, of graphene, molybdenum disulfide (MoS2), hexagonal-BN (h-BN) and niobium diselenide (NbSe2) down to single atomic sheets. The team literally shaved off atomic-scale amounts of each material onto a silicon oxide substrate and compared their findings to the bulk counterparts. Each material exhibited the same basic frictional behavior despite having electronic properties that vary from metallic to semiconducting to insulating.

"We call this mechanism, which leads to higher friction on thinner sheets the 'puckering effect,'" Carpick said. "Interatomic forces, like the van der Waals force, cause attraction between the atomic sheet and the nanoscale tip of the atomic force microscope which measures friction at the nanometer scale."

Because the sheet is so thin -- in some samples only an atom thick -- it deflects toward the tip, making a puckered shape and increasing the area of interaction between the tip and the sheet, which increases friction. When the tip starts to slide, the sheet deforms further as the deformed area is partially pulled along with the tip, rippling the front edge of the contact area. Thicker sheets cannot deflect as easily because they are much stiffer, so the increase in friction is less pronounced.

The researchers found that the increase in friction could be prevented if the atomic sheets were strongly bound to the substrate. If the materials were deposited onto the flat, high-energy surface of mica, a naturally occurring mineral, the effect goes away. Friction remains the same regardless of the number of layers because the sheets are strongly stuck down onto the mica, and no puckering can occur.

"Nanotechnology examines how materials behave differently as they shrink to the nanometer scale," Hone said. "On a fundamental level, it is exciting to find yet another property that fundamentally changes as a material gets smaller."

The results may also have practical implications for the design of nanomechanical devices that use graphene, which is one of the strongest materials known. It may also help researchers understand the macroscopic behavior of graphite, MoS2 and BN, which are used as common lubricants to reduce friction and wear in machines and devices.
The study, published in the current edition of the journal Science, was conducted collaboratively by Carpick and Qunyang Li of the Department of Mechanical Engineering in Penn's School of Engineering and Applied Science; Hone, Changgu Lee and William Kalb of the Department of Mechanical Engineering in the Fu Foundation School of Engineering and Applied Science at Columbia; Xin-Zhou Liu of Leiden University in the Netherlands; and Helmuth Berger of Ecole Polytechnique Fédérale de Lausanne in Switzerland.

Research was funded by the National Science Foundation through Penn's Laboratory for Research into the Structure of Matter, Columbia's Nanoscale Science and Engineering Center, the NSF's Directorate for Engineering, the Defense Advanced Research Projects Agency, the Air Force Office of Scientific Research and the New York State Office of Science, Technology and Academic Research.

University of Pennsylvania

Related Mechanical Engineering Articles from Brightsurf:

Best practices for mechanical ventilation in patients with ARDS, COVID-19
A team from pulmonary and critical care medicine at Michigan Medicine outlines 20 evidence-based practices shown to reduce time spent on a ventilator and death in patients with acute respiratory failure and acute respiratory distress -- conditions that have many overlaps with severe COVID-19.

How cells use mechanical tension sensors to interact with their environment
In a painstaking experiment, scientists suspended a single protein filament between two microscopic beads.

Mechanical forces of biofilms could play role in infections
Studying bacterial biofilms, EPFL scientists have discovered that mechanical forces within them are sufficient to deform the soft material they grow on, e.g. biological tissues, suggesting a ''mechanical'' mode of bacterial infection.

How mechanical forces nudge tumors toward malignancy
Researchers studying two forms of skin cancer identified a long-overlooked factor determining why some tumors are more likely to metastasize than others: the physical properties of the tissue in which the cancer originates.

Building mechanical memory boards using origami
Origami can be used to create mechanical, binary switches, and in Applied Physics Letters, researchers report the fabrication of such a paper device, using the Kresling pattern, that can act as a mechanical switch.

Not just light: The sensitivity of photoreceptors to mechanical stimuli is unveiled
''We thought we knew almost everything about photoreceptors, but we have proved that is not the case''.

A mechanical way to stimulate neurons
Magnetic nanodiscs can be activated by an external magnetic field, providing a research tool for studying neural responses.

Cell removal as the result of a mechanical instability
Researchers at Kanazawa University report in the Biophysical Journal that the process of cell removal from an epithelial layer follows from an inherent mechanical instability.

Researchers demonstrate transport of mechanical energy, even through damaged pathways
Researchers from the University of Illinois at Urbana-Champaign's Grainger College of Engineering have experimentally demonstrated a new way to transport energy even through wave-guides that are defective, and even if the disorder is a transient phenomenon in time.

Tissues protect their DNA under mechanical stress
Nuclei and genetic material deform.

Read More: Mechanical Engineering News and Mechanical Engineering Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to