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The Good News
2014 November
Pg 3 - The Sunshine Express
First experimental observation of
piezoelectricity in an atomically thin material
- MoS2 - could lead to wearable devices
October 15, 2014: Researchers from Columbia
Engineering and the Georgia Institute of Technol-
ogy report today that they have made the first
experimental observation of piezoelectricity and
the piezotronic effect in an atomically thin mate-
rial, molybdenum disulfide (MoS2), resulting in a
unique electric generator and mechanosensation
devices that are optically transparent, extremely
light, and very bendable and stretchable.
In a paper published online October 15, 2014, in
Nature, research groups from the two institutions
demonstrate the mechanical generation of electric-
ity from the two-dimensional (2D) MoS2 material.
The piezoelectric effect in this material had previ-
ously been predicted theoretically.
Piezoelectricity is a well-known effect in which
stretching or compressing a material causes it to
generate an electrical voltage (or the reverse, in
which an applied voltage causes it to expand or
contract). But for materials of only a few atomic
thicknesses, no experimental observation of piezo-
electricity has been made, until now. The observa-
tion reported today provides a new property for
two-dimensional materials such as molybdenum
disulfide, opening the potential for new types of
mechanically controlled electronic devices.
¡°This material, just a single layer of atoms, could
be made as a wearable device, perhaps integrated
into clothing, to convert energy from your body
movement to electricity and power wearable sen-
sors or medical devices, or perhaps supply enough
energy to charge your cell phone in your pocket,¡±
says James Hone, professor of mechanical engi-
neering at Columbia and co-leader of the research.
¡°Proof of the piezoelectric effect and piezotronic
effect adds new functionalities to these two-
dimensional materials,¡± says Zhong Lin Wang,
Regents¡¯ Professor in Georgia Tech¡¯s School of Ma-
terials Science and Engineering and a co-leader of
the research. ¡°The materials community is excited
about molybdenum disulfide, and demonstrating
the piezoelectric effect in it adds a new facet to
the material.¡±
Hone and his research group demonstrated in
2008 that graphene, a 2D form of carbon, is the
strongest material. He and Lei Wang, a postdoc-
toral fellow in Hone¡¯s group, have been actively
exploring the novel properties of 2D materials like
graphene and MoS2 as they are stretched and
compressed.
Zhong Lin Wang and his research group pioneered
the field of piezoelectric nanogenerators for con-
verting mechanical energy into electricity. He and
postdoctoral fellow Wenzhuo Wu are also develop-
ing piezotronic devices, which use piezoelectric
charges to control the flow of current through the
material just as gate voltages do in conventional
three-terminal transistors.
There are two keys to using molybdenum disul-
fide for generating current: using an odd number
of layers and flexing it in the proper direction.
The material is highly polar, but, Zhong Lin Wang
notes, so an even number of layers cancels out
the piezoelectric effect. The material¡¯s crystalline
structure also is piezoelectric in only certain crys-
talline orientations.
For the Nature study, Hone¡¯s team 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. In
research done at Georgia Tech, Wang¡¯s group
installed measurement electrodes on samples
provided by Hone¡¯s group, then measured cur-
rent flows 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 disap-
peared in samples with an even number of
atomic layers, confirming theoretical predic-
tions published last year. The presence of
piezotronic effect in odd layer MoS2 was also
observed for the first time.
¡°What¡¯s really interesting is we¡¯ve now found
that a material like MoS2, which is not piezo-
electric in bulk form, can become piezoelectric
when it is thinned down to a single atomic
layer,¡± says Lei Wang.
To be piezoelectric, a material must break cen-
tral symmetry. A single atomic layer of MoS2
has such a structure, and should be piezoelec-
tric. However, in bulk MoS2, successive layers
are oriented in opposite directions, and gener-
ate positive and negative voltages that cancel
each other out and give zero net piezoelectric
effect.
¡°This adds another member to the family of
piezoelectric materials for functional devices,¡±
says Wenzhuo Wu.
In fact, MoS2 is just one of a group of 2D semi-
conducting materials known as transition metal
dichalcogenides, all of which are predicted to
have similar piezoelectric properties.
¡°This is the first experimental
work in this area and is an
elegant example of how the
world becomes different when
the size of material shrinks to
the scale of a single atom¡±
- James Hone, professor of mechanical
engineering at Columbia
Piezoelectricity is a well-known effect in which
stretching or compressing a material causes it to
generate an electrical voltage
Researchers Develop World¡¯s
Thinnest Electric Generator
These are part of an even larger family of 2D
materials whose piezoelectric materials remain un-
explored. Importantly, as has been shown by Hone
and his colleagues, 2D materials can be stretched
much farther than conventional materials, particu-
larly traditional ceramic piezoelectrics, which are
quite brittle.
The research could open the door to development
of new applications for the material and its unique
properties.
¡°This is the first experimental work in this area
and is an elegant example of how the world be-
comes different when the size of material shrinks
to the scale of a single atom,¡± Hone adds. ¡°With
what we¡¯re learning, we¡¯re eager to build useful
devices for all kinds of applications.¡±
Ultimately, Zhong Lin Wang notes, the research
could lead to complete atomic-thick nanosystems
that are self-powered by harvesting mechani-
cal 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.
For this study, the research team also worked with
Tony Heinz, David M. Rickey Professor of Opti-
cal Communications at Columbia Engineering and
professor of physics at Columbia¡¯s Graduate School
of Arts and Sciences.
The study was supported by the U.S. Department
of Energy (DOE), Office of Basic Energy Sciences
(BES) (No. DE-FG02-07ER46394) and U.S. Nation-
al Science Foundation (DMR-1122594).
(source: engineering.columbia.edu)