"Our initial goal was high yield and the selective production of helical SiC/SiO2 nanowires," researcher Haifeng Zhang told nanotechweb.org. "When we modified the experimental set-up and changed the conditions, we found large quantities of helical SiO2 nanosprings on the silicon substrate."
To make the springs, Zhang and colleagues used a dual flow-tube furnace. They placed a silicon wafer substrate in the inner tube, and iron and Si/SiO2 powders inside the outer flow tube. Synthesis of the springs took about two hours at 1160° C in the presence of methane. The researchers do not know what part the methane played in the growth process, but its presence was essential for the nanosprings to form.
In this way, the researchers made amorphous helical SiO2 nanosprings up to 8 µm long with diameters of 80-140 nm. The nanosprings, which looked like coiled telephone cords, were joined at either end to straight amorphous SiO2 nanowires. The ribbon making up the nanosprings was rectangular, rather than cylindrical, in cross-section.
The team tested the spring-like qualities of the helical wires by focusing the electron beam of a transmission electron microscope (TEM) onto various points along their length. This caused local heating and expansion of the spring, with corresponding contraction of the spring away from the heating point. Zhang and colleagues also applied pressure to the nanosprings using the tip of an atomic force microscope (AFM), an experience that bent and slightly stretched the nanosprings.
"The in situ TEM heating experiment and the AFM manipulation suggest that the helical nanosprings possess the same characteristics as micro/macro springs, such as the storage of mechanical energy and flexibility," said Zhang. "This is an important step for the use of nanosprings in nanodevices."
Zhang and colleagues are now looking to further characterize the nanosprings' physical properties, build useful nanospring-containing devices such as nanoswitches, nanosensors, nano-sized magnetic field producers and detectors, and improve control of the growth process.
The researchers reported their work in Nano Letters.