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February 25, 2010

University of Pennsylvania Form Near-Frictionless Diamond Material

This is an SEM image of a silicon microcantilever with an ultrasharp tip of diamond-like carbon with silicon.

Researchers at the University of Pennsylvania, the University of Wisconsin-Madison and IBM Research-Zürich have fabricated an ultra sharp, diamond-like carbon tip possessing such high strength that it is 3,000 times more wear-resistant at the nanoscale than silicon.

The end result is a diamond-like carbon material mass-produced at the nanoscale that doesn’t wear. The new nano-sized tip, researchers say, wears away at the rate of one atom per micrometer of sliding on a substrate of silicon dioxide, much lower than that for a silicon oxide tip which represents the current state-of-the-art. Consisting of carbon, hydrogen, silicon and oxygen molded into the shape of a nano-sized tip and integrated on the end of a silicon microcantilever for use in atomic force microscopy, the material has technological implications for atomic imaging, probe-based data storage and as emerging applications such as nanolithography, nanometrology and nanomanufacturing.

Nature Nanotechnology- Ultralow nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like carbon



Understanding friction and wear at the nanoscale is important for many applications that involve nanoscale components sliding on a surface, such as nanolithography, nanometrology and nanomanufacturing. Defects, cracks and other phenomena that influence material strength and wear at macroscopic scales are less important at the nanoscale, which is why nanowires can, for example, show higher strengths than bulk samples. The contact area between the materials must also be described differently at the nanoscale. Diamond-like carbon is routinely used as a surface coating in applications that require low friction and wear because it is resistant to wear at the macroscale but there has been considerable debate about the wear mechanisms of diamond-like carbon at the nanoscale because it is difficult to fabricate diamond-like carbon structures with nanoscale fidelity. Here, we demonstrate the batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy. This material is known to possess low friction in humid conditions, and we find that, at the nanoscale, it is three orders of magnitude more wear-resistant than silicon under ambient conditions. A wear rate of one atom per micrometre of sliding on SiO2 is demonstrated. We find that the classical wear law of Archard does not hold at the nanoscale; instead, atom-by-atom attrition dominates the wear mechanisms at these length scales. We estimate that the effective energy barrier for the removal of a single atom is ~1 eV, with an effective activation volume of ~1 × 10^−28 m3.

11 pages of supplemental information

The importance of the discovery lies not just in its size and resistance to wear but also in the hard substrate against which it was shown to perform well when in sliding contact: silicon dioxide. Because silicon –- used in almost all integrated circuit devices –- oxidizes in atmosphere forming a thin layer of its oxide, this system is the most relevant for nanolithography, nanometrology and nanomanufacturing applications.

Probe-based technologies are expected to play a dominant role in many such technologies; however, poor wear performance of many materials when slid against silicon oxide, including silicon oxide itself, has severely limited usefulness to the laboratory.

Researchers built the material from the ground up, rather than coating a nanoscale tip with wear-resistant materials. The collaboration used a molding technique to fabricate monolithic tips on standard silicon microcantilevers. A bulk processing technique that has the potential to scale up for commercial manufacturing is available.


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