Nanotechnology Now - Press Release: Tin selenide nanosheets enables to develop wearable tracking devices

If you have ever used a microphone, told time on a quartz watch, or listened to a speaker, you have been witness to the piezoelectric effect, or converting mechanical stress into electricity (or vice versa).

Electric charge can accumulate in the crystal lattices of certain solid materials simply by applying a mechanical force to them, such as squeezing or pushing. This remarkable property does not affect all solids, but only occurs when a solid�s crystal structure is such that when mechanical stress is applied, some atoms get pushed together or further apart, destabilizing the balance between positive and negative electric charge. This deformation makes the crystal capable of conducting an electric current.

This piezoelectric effect plays a key role in functional sensors, wearable electronics and microelectromechanical systems. But the materials used for these �piezotronic devices� tend to be large, bulk volumes of crystal structure, limiting their flexibility.

Recently, however, so-called 2D materials�those that are just a handful of atoms thick�have emerged as a promising candidate for flexible piezotronics. The flexibility comes from the atomic-scale thinness of these piezoelectric nanogenerators, or �PENGs�, and theoretical analysis suggests that 2D materials exhibit an even stronger piezoelectric effect than commonly used bulk piezoelectric materials such as quartz.

�But what is predicted by theory in a computer runs into trouble once such materials are used in the real world,� said Prof. Jianhua Hao, a material scientist at Hong Kong Polytechnic University and corresponding author of the paper.

A considerable amount of effort has gone into exploring the piezoelectric properties of many materials, including 2D semiconductors such as hexagonal boron nitride and transition-metal dichalcogenides. But it turns out that the piezoelectricity only occurs when these materials exist in single layers (or �monolayers�). The piezoelectric effect disappears as one layer�s electronic polarization gets cancelled out by its adjacent layers� polarization in the opposite direction. This severely limits the applicability of 2D piezotronics due to the poor mechanical durability of monolayers.

Some researchers have tried to get around this problem by various strategies, such as using novel stacking configurations for the layers.

�But the piezoelectric performance is extremely weak,� added Prof. Hao. �And perhaps even more importantly, the engineering process is just very complicated and not easily controllable, making them not particularly economically competitive with existing piezotronic materials for most practical applications.�

But one group of 2D materials, monochalcogenides from groups IV-VI of the periodic table (binary compounds that pair tin or germanium with selenium or sulfur), have intrinsically high piezoelectric capability. This impressive performance comes from a buckled or �puckered� structure of the crystal. Tin selenide in particular combines good piezoelectric capability with robust mechanical durability.

So many researchers have put their eyes on developing SnSe films using mechanical exfoliation and solution mediated growth technique. These were then used for PENGs in self-powered pH sensors and finger-force monitors.

�Unfortunately, these synthesis methods ran into some troubles with sample quality too,� continued Prof. Hao. �The technique limits the lateral size, or area, of PENG that can be produced, so it�s not very scalable. And it�s not very controllable either.�

So the researchers wanted to see if chemical vapor deposition (CVD) might work. This technique, commonly used in the conventional semiconductor industry, and enjoys strong scalability, controllability and thus capacity for mass production, involves exposure of the given substance (in this case the tin selenide compound) to a vapor that reacts with the substance and subsequently produces a thin film deposit.

The researchers deployed this technique to produce multi-layer SnSe nanosheets of varying thicknesses and then tested their performance via a piezoresponse force microscope. The best performing one achieved energy conversion efficiency far higher than PENGS made of all previously used materials. This was then developed into a PENG and used in a self-powered sensor to monitor human motion�a device attached to the finger and wrist of a human tester, thus testing its flexibility.

The device not only continued its excellent piezoelectric output, but demonstrated the requisite flexibility and durability for a wearable device.

The research team now hope to take their proof of concept and develop it into commercially viable self-powered and wearable health-tracking devices.

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