"The well-established nature of the substrates and the processes to create surface acoustic waves makes the novel technique facile and ready to be applied."Ludwig Bartels

An international team from University of California Riverside and the University of Augsburg in Germany has taken a new approach to unraveling the properties of novel two-dimensional semiconductors, materials with unique properties that could offer improved integration of optical communication with standard silicon-based devices. As part of the groundswell of research into new materials for electronic and optoelectronic applications, the work helps improve our knowledge of monolayer films.

The study, which was reported in Nature Communications [Preciado et al. Nat. Commun. (2015) DOI: 10.1038/ncomms9593], involved the development of a single-atomic-layer-thin film of molybdenum disulfide on a substrate of lithium niobate, which is employed in a range of electronic devices that use high-frequency signals, such as cell phones and radar installations. Lithium niobate is the archetypical ferroelectric material and the key substrate for many applications, including surface acoustic wave radio frequencies devices and integrated optics. Although it offers a unique combination of properties, its lack of optical activity and semiconducting transport have up to now hampered its application in optoelectronics.

On applying electrical pulses to the lithium niobate, the team produced very high-frequency sound waves – "surface acoustic waves" – that run along the surface of lithium niobate like tremors. These surface waves allowed them to listen to how the illumination of lithium niobate by laser light changes the electric properties of molybdenum disulfide.

Cell phones use resonances of these surface waves to filter electric signals in the same way that a glass can resonate when tapped at exactly the right pitch. As a glass fills up with liquid, the tone at which it resonates alters, and this tone can help identify how full the glass is. Similarly, the team could “hear” the lithium niobate sound waves and were able to infer how much current the laser light was allowing to flow in the molybdenum disulfide. In addition, they fabricated transistor structures onto the molybdenum disulfide films that proved the accuracy of the analysis.

Their prototypical device presents electrical characteristics that are competitive with molybdenum disulfide devices on silicon, and the surface acoustic waves allowed them to realize a sound-driven battery and an acoustic photodetector, which could lead to new ways to non-invasively investigate the electrical properties of monolayer films. As UC Riverside team leader Ludwig Bartels points out, “The well-established nature of the substrates and the processes to create surface acoustic waves makes the novel technique facile and ready to be applied. In particular, even remote, wireless sensing applications appear to be within reach.”