In a breakthrough invention,
scientists have successfully executed an experiment of piezoelectricity and the
piezotronic effect in an atomically thin material, molybdenum disulfide (MoS2),
resulting in a unique electric generator and mechanosensation devices; although
the piezoelectric effect in this material had already been predicted theoretically.
Piezoelectricity is defined as
a phenomenon in which pressure generates an electrical voltage in a material or
vice-versa. But no experimental observation of piezoelectricity has been made yet
for few atom thickness material. The observation of molybdenum disulfide
material has unfolded the potential for new types of mechanically controlled
electronic devices.
In an interesting application of this phenomenon,
this material could be made as a wearable device, integrated into clothing, to
convert energy from your body movement to electricity and power wearable
sensors or medical devices, or perhaps supply enough energy to charge cell
phone.
There are two keys to using molybdenum disulfide
for generating current: using an odd number of layers and flexing it in the
proper direction. The material is highly polar, but an even number of layers
cancels out the piezoelectric effect. The material's crystalline structure also
is piezoelectric in only certain crystalline orientations.
Group of researchers placed thin flakes of MoS2
on flexible plastic substrates and determined how their crystal lattices were
oriented using optical techniques. They then patterned metal electrodes onto the
flakes and measure current flow as the samples were mechanically deformed. They
monitored the conversion of mechanical to electrical energy, and observed
voltage and current outputs.
The researchers also noted that the output
voltage reversed sign when they changed the direction of applied strain, and
that it disappeared in samples with an even number of atomic layers, confirming
theoretical predictions published last year. The presence of piezotronic effect
in odd layered MoS2 was also observed for the first time.
To be piezoelectric, a material must break
central symmetry. A single atomic layer of MoS2 has such a
structure, and should be piezoelectric. However, in bulk MoS2,
successive layers are oriented in opposite directions, and generate positive
and negative voltages that cancel each other out and give zero net
piezoelectric effect.
In fact, MoS2 is just one of a group
of 2D semiconducting materials known as transition metal dichalcogenides, all
of which are predicted to have similar piezoelectric properties. These are part
of an even larger family of 2D materials whose piezoelectric materials remain
unexplored. The research could lead to complete atomic-thick nanosystems that
are self-powered by harvesting mechanical energy from the environment. This
study also reveals the piezotronic effect in two-dimensional materials for the
first time, which greatly expands the application of layered materials for
human-machine interfacing, robotics, MEMS, and active flexible electronics.
