Most of the research in the field of 2D materials has largely focused on graphene until now, but the fact that this material lacks a direct electronic bandgap means that scientists are now actively looking at other 2D layered materials too. MoS2, for its part, is a semiconducting crystal with a sizeable electronic bandgap and many other attractive physical properties that include the fact that it is highly elastic and strong (like graphene).

The Case researchers, led by Philip Feng, have now made the first MoS2 vibrating nanodevices using an exfoliation procedure that allows them to make very small drumhead diaphragms of the material suspended on micron-sized cavities predefined on a thin silicon dioxide substrate layer. The suspended diaphragms have diameters in the 1–5 micron range and can be made just one monolayer thick. This monolayer consists of a hexagonal plane of Mo atoms covalently bonded and sandwiched between two planes of S atoms in a trigonal prismatic structure.

Sheets vibrate thanks to Brownian motion

At room temperature, and without any external driving force, thermodynamic fluctuations cause these ultrathin sheets to vibrate thanks to Brownian motion, explained Feng. "We have now shown that we can make high-frequency resonant-mode nanoelectromechanical systems (NEMS) based on these suspended MoS2 nanostructures, something that has not been done before because of difficulties in nanofabrication and the much greater challenge of detecting and reading out the motion of the ultrasmall MoS2 structures."

The team succeeded in monitoring the motion of the MoS2 sheets thanks to a highly sensitive technique that allowed them to read out the miniscule signals induced by the nanostructures. "We were able to detect the device’s movements by carefully engineering a custom-built laser interferometry system that boasts a very small spot size and ultralow-noise signal transduction," Feng told "We found that we could measure displacements with a resolution of just 30 fm/Hz1/2 (1 fm=10-15m) at room temperature".

"By carefully tuning this very 'quiet' measurement, we could precisely monitor the thermally agitated Brownian motion of the MoS2 devices," he added. "These vibrating MoS2 nanodevices pave the way towards novel NEMS based on atomically thin 2D semiconducting crystals that could be ideal for use in innovative new sensors, transducers and nanomachines capable of detecting and converting Brownian-type motions at very small scales into electronic and optical signals," he said. "This work shows that graphene is no longer the only 2D crystal that we can investigate for making such nanodevices."

The team published its results in ACS Nano DOI: 10.1021/nn4018872.

Further reading

Molybdenite transistor is a first (Feb 2011)
MoS2 transistors get bendy (May 2013)
Nanoresonators feel noise (Apr 2011)
Drastically different 1D and 2D nanomaterials integrated in test device (Aug 2012)
Nanocomposites enable ultrathin metallic cantilevers (Jun 2006)