News & Videos: Manufacturing complex 3D metallic structures at nanoscale made possible - Aalto University Finland

Manufacturing complex 3D metallic structures at nanoscale made possible

18.10.2012

The fabrication of many objects, machines, and devices around us rely on the controlled deformation of metals by industrial processes such as bending, shearing, and stamping. Is this technology transferrable to nanoscale? Can we build similarly complex devices and machines with very small dimensions?

Scientists from Aalto University in Finland and the University of Washington in the US have just demonstrated this to be possible. By combining ion processing and nanolithography they have managed to create complex three-dimensional structures at nanoscale.

The discovery follows from a quest for understanding the irregular folding of metallic thin films after being processed by reactive ion etching.

– We were puzzled by the strong-width-dependent curvatures in the metallic strips. Usually initially-strained bilayer metals do not curl up this way, explains Khattiya Chalapat from Aalto University.

The puzzle began to unravel when Chalapat noticed, together with Dr. Hua Jiang, that the Ti peak was absent from the EDX spectra of folded Ti/Al bilayers.

Further experiments at the O.V. Lounasmaa Laboratory confirmed that the strips bend upward with strong width-dependent curvatures if the bottom layer of the strips is made more reactive to ions than the top surface.

In nature, similar geometrical effects take place in self-organization directly observable to the human eye. When dandelion flowers bloom, one may try cutting the flower stem into small strips; put them in water, and the strips will fold with observable width-dependent curvatures due to differences in the water absorption between the inside and outside parts of the stem.

– Our idea was to find a way to adapt these natural processes to nanofabrication. This led us to an incidental finding that a focused ion beam can locally induce bending with nanoscale resolution.

The technology has various applications in the fabrication of nanoscale devices. The structures are surprisingly resilient:­ the team found them to be quite sturdy and robust under a variety of adverse conditions, such as electrostatic discharge and heating.

– Because the structures are so small, the coupling and the magnitude of typical nanoscale forces acting on them would be commensurately small, reminds Docent Sorin Paraoanu, the leader of the Kvantti research group, Aalto University.

– As for applications, we have demonstrated so far that these structures can capture and retain particles with dimensions of the order of a micrometer. However, we believe that we are just scratching the tip of the iceberg: a comprehensive theory of ion-assisted self-assembly processes is yet to be reached, notes Paraoanu.

The research has been recently published in the Early View edition of Advanced Materials.

Khattiya Chalapat and Sorin Paraoanu would like to give credit to the Aalto University research facilities for microfabrication and imaging at Micronova Centre for Micro and Nanotechnology and the Nanomicroscopy Center in Finland.

The article online (onlinelibrary.wiley.com)

Kvantti research group (ltk.tkk.fi)

Further information:
Khattiya Chalapat, Ph.D. student
O. V. Lounasmaa Laboratory, Aalto University
khattiya.chalapat@aalto.fi

Sorin Paraoanu, Docent, Group Leader
O. V. Lounasmaa Laboratory, Aalto University
sorin.paraoanu@aalto.fi

News: Manufacturing complex 3D metallic structures at nanoscale made possible - Aalto University

Tribology_At the Nanoscale, Graphite Can Turn Friction Upside Down_fromNIST

If you ease up on a pencil, does it slide more easily? Sure. But maybe not if the tip is sharpened down to nanoscale dimensions. A team of researchers at the National Institute of Standards and Technology (NIST) has discovered that if graphite (the material in pencil "lead") is sticky enough, as measured by a nanoscale probe, it actually becomes harder to slide a tip across the material's surface as you decrease pressure—the exact opposite of our everyday experience.

Technically, this leads to an effectively "negative coefficient of friction," something that has not been previously seen, according to team leader Rachel Cannara. Graphite, Cannara explains, is one of a special class of solids called "lamellar" materials, which are formed from stacks of two-dimensional sheets of atoms. The sheets are graphene, a single-atom-thick plane of carbon atoms that are arranged in a hexagonal pattern. Graphene has a number of exotic electrical and material properties that make it attractive for micro- and nanoelectromechanical systems with applications ranging from gas sensors and accelerometers to resonators and optical switches.

Zhao Deng, a University of Maryland postdoctoral researcher at NIST's Center for Nanoscale Science and Technology, noted some odd data while experimenting on graphite with an atomic force microscope (AFM). Deng was measuring the friction forces on the nanoscale tip of an AFM tracking across the graphite as he modified the "stickiness" of the surface by allowing tiny amounts of oxygen to adsorb to the topmost graphene layer.

Deng found that when the adhesive force between the graphene and the stylus became greater than the graphene layer's attraction to the graphite below, reducing the pressure on the stylus made it harder to drag the tip across the surface—a negative differential friction.

Backed by theoretical simulations performed by collaborators from NIST and Tsinghua University in Beijing, Cannara's team found that, after the AFM tip has been pressed into the graphite surface, if the attractive force is high enough, the tip can pull a small localized region of the surface layer of graphene away from the bulk material, like raising a nanoscale bubble from the surface. Pushing that deformation around takes more work than sliding over a flat surface. Therefore, whenever the researchers pressed the AFM tip against the sticky graphite surface and then tried to pull the two apart, they measured an increase in friction force with a sensitivity in the tens of piconewtons.

"Once we have a complete model describing how these graphene sheets deform under repeated loading and sliding at the nanoscale—which we're working on now—friction force microscopy may be the most direct way to measure the energy that binds these layered materials together. And, since it's nondestructive, the measurement can be performed on working devices," Cannara says. Understanding how the sheets interact with each other and with other parts of a device would help quantify the energy required to produce individual sheets from bulk material, assess device operation, and assist in formulating new structures based on layered materials, she says.

* Z. Deng, A. Smolyanitsky, Q. Li, X.-Q. Feng and R. J. Cannara. Adhesion-dependent negative friction coefficient on chemically modified graphite at the nanoscale. Nature Materials. Published online: 14 October 2012 | doi:10.1038/nmat3452.

Slip Sliding Our Way: At the Nanoscale, Graphite Can Turn Friction Upside Down