Metal instability achieves energy-efficient nanotechnology

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“Nano-electromechanical resonators are used in all sorts of modern 
technology. You may not see them, but they can be found in robotics, medi-
cal tools and environmental sensors,” says Osaka University Professor 
Hidezaku Tanaka, who is developing new nanotechnologies. 
Earlier this year, Tanaka and his research team reported a freestanding 
nanowire that could reduce the power demands of nano-resonators by a fac-
tor of one hundred. 
“Transition metals undergo an insulator to metal transition. We made 
freestanding nanowires made of vanadium dioxide (VO2) that had high per-
formance at low power.” 
The phase transition can occur by injecting electrical power into 
VO2 crystals. Because the mechanical response to the power is non-linear, 
Tanaka showed that unprecedented low levels of power could be used to 
generate a disproportionally strong mechanical response. Tanaka found that 
the freestanding character of the wire is key, as otherwise the non-linearity 
and thus the energy efficiency was much less. 
“Building the freestanding nanowire was not easy. Metal oxides are 
very stiff and brittle. We could fabricate the nanowires by growing them on 
magnesium oxide (MgO) and then etching the MgO layer away.” 
In his latest publication, the collaboration team by Tanaka group, Pro-
fessor Daniele Marré group in Italy and Dr. Nicola Manca in the Nether-
lands group to determine just how simple the construction of nano-
resonators using his VO2 freestanding nanowires could be. Because of the 
electro-mechanical properties of VO2 crystals and his freestanding design, 
the nanowires could generate mechanical oscillations at MHz frequencies 
using nothing more than a simple DC power source. This efficient conver-
sion of electrical energy to mechanical work reduces the need for dedicated 
electronic devices, thus enabling the creation of even smaller nano-
electromechanical systems (NEMS) than currently used. 
The nanowires depend on the spontaneous oscillations in the electrical 
signal caused by phase transitions in the VO2. These electrical oscillations 
cause the VO2 nanowires to also oscillate, but the non-linear electro-
mechanical coupling means that that power on the nano-scale can generate 
VO2 oscillations at MHz frequencies. The team showed the additional ener-
gy for the crystal oscillations comes in the form of heat caused by the elec-
trical power. 

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“We set up our design so that a Joule effect was localized to a gap of 
exposed VO2. We found the source of energy for the mechanical response 
is dominated by thermal dissipations and not electrical power,” said 
Tanaka. 
Designing an NEMS that efficiently exploits heat generated by the 
phase transitions gives a new paradigm for energy-efficient technologies. 
“Our system is simple and scalable. It opens the possibility of realizing 
NEMS that have fast switching and powered by a DC power source.” 
Materials provided by Osaka University: 
https://www.sciencedaily.com/releases/2017/07/170731090945.htm 
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