Nanomaterials are considered superior in many aspects thanks to their large outer surface-to-volume ratio. As we know, more text can be written in a book – a set of sheets – than on a Sumerian clay tablet. Materials consisting of layered structures may yield more surfaces for scientists to engineer. Vanadium pentoxide, which typically has an interlayer spacing of 1 nm, is an example of such a structure and the material is used widely. For example, in rechargeable batteries for trapping and storing mobile lithium ions.

Given the versatile redox-dependent properties of vanadium oxides, the research team of chemists and physicists at National Chung Hsing University (NCHU) in Taichung, Taiwan, inserted free radicals into the interlayer space of the vanadium-oxide nanofibres, each appearing like a 100-sheet book. Using these structures, the group applied nanofabrication techniques to build electronic devices for studying the charge conduction. The radical-intercalation resembles the implant doping process in silicon IC manufacturing and changes the electrical property of the nanofibre to hole conduction due to the radicals' strong electron affinity. Moreover, the scientists discovered that the conduction current changed when the fibre was exposed to different gases, a property that points towards e-nose technology.

Non-vanishing field effect

To examine the behaviour in more detail, the team studied the resistance change when the environment was switched alternately from a specific gas to a vacuum. Intriguingly, the gas molecules can be interlayer-trapped and affect the conduction current by attracting or repelling holes as the gate in a semiconductor transistor.

To the amazement of the scientists, such an effect can be sustained for hours after the surrounding gas has been evacuated. Although molecule gating effects are widely observed in nanomaterials, such a memory property with a long holding time is rare, revealing the superior ability of vanadium oxides for interlayer trapping of molecules.

Future nano-robots?

The group plans to investigate the molecule gating effects from gaseous phase to liquid phase, for trapping different solvable ions. The ion intercalation may induce deformation in the fibre, yielding a potential nano-robot controlled by ion concentration.