Photographs (top left) of colored light reflected from 5 x 5 mm MGNT arrays at different tilt angles and scanning electron micrographs of 500 nm diameter MGNT array at various magnifications.Researchers have fabricated metallic glass nanotubes (MGNTs) in regular patterns on the surface of silicon substrates for the first time [Chen et al., Materials Today (2017), doi: 10.1016/j.mattod.2017.10.007]. Like biological nanostructured surfaces, MGNTs show some surprising water repelling and attracting properties.
Metallic glasses (MGs) possess remarkable mechanical and thermophysical properties, as well as high strength and biocompatibility. Their amorphous structure and lack of grain boundaries account for their unusual characteristics, but also mean that in the bulk they show little or no plastic deformation. This shortcoming makes bulk MGs (BMGs) brittle and impossible to work at room temperature.
Thin film MGs (TFMGs), however, fabricated using the sputter deposition technique whereby material is ejected from a target onto a substrate, are ductile and retain the attractive mechanical attributes of BMGs. The team from National Taiwan University of Science and Technology has used this approach produce individual MGNTs on Si substrates. A coating of Zr55Cu30Al10Ni5 is sputter-deposited over photoresist templates using radio frequency magnetron sputtering. The photoresist templates are then removed by ultrasonic vibration of the substrate in a solvent.
“We successfully fabricated the first-ever metallic glass nanotubes on a Si substrate by a simple lithography and sputter deposition process for very large-scale integration,” explains Jinn P. Chu.
The nanotubes are 500-750 nm tall and 500-750 nm in diameter, with wall thicknesses of 44-103 nm depending on the deposition time. The researchers found that as the nanotube walls become thicker, the MGNT-coated surface becomes more hydrophobic, repelling water.
“The hydrophobicity is due to air trapped within the tubes, which prevents the intrusion of water into the nanostructures,” explains Chu. “We also observed that surface cooling produces negative pressure within the nanochambers, creating a sucking force against the water droplets. Conversely, surface heating produces positive pressure within the nanochambers, which pushes off the droplets.”
By heating and cooling the MGNT array, water can be repelled and attached to the surface in turn. The researchers demonstrate that this thermally response wetting/dewetting behavior is reversible over at least five cycles between 25°C and 55°C.
“The MGNT array represents a biomimetic analog with a switchable contact interface, the behavior of which can be controlled simply by altering the surface temperature,” points out Chu.
The combination of properties offered by MGNT arrays could be useful in solar cells, optical sensors, and biosensors, suggest the researchers.
“The invention of cost-effective MGNTs will bring nanotechnology into a new era,” says Chu.