Summary of Two-dimensional piezoelectric material forms basis of world’s thinnest electric generator
Researchers observed piezoelectricity in atomically thin molybdenum disulfide (MoS2), demonstrating the world's thinnest electric generator. By placing single- and few-layer MoS2 flakes on flexible substrates, determining crystal orientation, patterning metal electrodes, and measuring current under mechanical strain, they confirmed that odd-layered MoS2 exhibits piezoelectric voltage that reverses with strain direction while even-layered samples show no effect. The result validates prior theoretical predictions and shows bulk-nonpiezoelectric MoS2 becomes piezoelectric when reduced to a single atomic layer due to broken inversion symmetry.
Parts used in the World's Thinnest Electric Generator:
- Molybdenum disulfide (MoS2) thin flakes (single and few layers)
- Flexible plastic substrates
- Optical equipment for crystal orientation determination
- Metal electrodes (patterned onto MoS2 flakes)
- Mechanical deformation apparatus for applying strain
- Electrical measurement equipment to record current and voltage
Researchers from Columbia University and the Georgia Institute of Technology are laying claim to having observed piezoelectricity in an atomically thin material for the first time. The effect was demonstrated in the world’s thinnest electric generator made from a two-dimensional molybdenum disulfide (MoS2) material, which had previously been predicted to exhibit such properties.
Co-leader of the research, Zhong Lin Wang, has previously been involved in developing the world’s first practical piezoelectric nanogenerator, followed by the first self-powered nanodevice with wireless data transmission, and a low-cost, large-scale nanogenerator. Now, with colleagues including fellow co-leader James Hone, Wang has been involved in developing the world’s thinnest electric generator.
The device was created by placing thin layers of MoS2, a material made up of a single layer of atoms, on flexible plastic substrates and using optical techniques to determine how the material’s crystal lattices were oriented. This process is required because MoS2‘s crystalline structure makes the material piezoelectric only in certain orientations. It is also highly polar, meaning that an odd number of atomic layers are needed to ensure the piezoelectric effect isn’t canceled out.
Metal electrodes were then patterned onto the flakes and the current flows as the samples were mechanically deformed were measured. Confirming theoretical predictions published last year, the team found that the output voltage reversed sign when the direction of applied strain was changed, with the voltage disappearing entirely in samples with an even number of layers.
“What’s really interesting is we’ve now found that a material like MoS2, which is not piezoelectric in bulk form, can become piezoelectric when it is thinned down to a single atomic layer,” says Lei Wang, a postdoctoral fellow in Hone’s group. This is because bulk MoS2 is made up of successive layers that are oriented in opposite directions, thereby generating positive and negative voltages that cancel each other out.
For more detail: Two-dimensional piezoelectric material forms basis of world’s thinnest electric generator
- What material was used to make the world's thinnest electric generator?
Two-dimensional molybdenum disulfide (MoS2) thin flakes were used. - How was the piezoelectric effect detected in the device?
By patterning metal electrodes on MoS2 flakes and measuring current and voltage as samples were mechanically deformed. - Does MoS2 show piezoelectricity in bulk form?
No, bulk MoS2 is not piezoelectric because successive layers are oriented in opposite directions and cancel out the effect. - Can a single atomic layer of MoS2 be piezoelectric?
Yes, a single atomic layer of MoS2 becomes piezoelectric due to broken inversion symmetry. - Why do even-numbered MoS2 layers not show piezoelectric voltage?
Even-numbered layers have opposing layer orientations that produce voltages which cancel each other, so no net piezoelectric voltage appears. - How does the output voltage behave when the direction of applied strain is changed?
The output voltage reverses sign when the direction of applied strain is changed. - What role did optical techniques play in the experiment?
Optical techniques were used to determine the crystal lattice orientations of MoS2 flakes, which is required because piezoelectricity depends on orientation. - What substrates supported the MoS2 flakes in the device?
Flexible plastic substrates were used to support the MoS2 flakes.
