Designable Semiconductors

A team of researchers stacked two single-atom-thick layers of molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) — common semiconductor materials — on top of each other to better understand how interactions between different layers affect semiconducting behaviour, adding to the pool of basic knowledge about how semiconductors work¹.

Semiconductors are essential components of all modern electronic circuits.

The ability to tailor them layer-by-layer to achieve desired properties promises a wide-range of applications in the future, including higher-temperature superconductors and ultrathin electronics. 

However, the available repertoire of stable two-dimensional lattices at ambient conditions, as well as knowledge about how their arrangement affects their overall properties, remain sparse for practical purposes.

Within individual layers of such artificial layered semiconductors, atoms are held together by strong covalent bonds. Between layers, they are held by weak but cumulative Van der Waals (VdW) interactions.

A team of researchers from the USA, Taiwan, China and Saudi Arabia, set out to investigate how the strength of such VdW interactions between layers affects semi-conducting behaviour, in order to introduce an additional, adjustable parameter for rational design.

In MoS2 layers, the atomic distance is different from that in WSe2 layers.  Even when the two layers are rotationally aligned, the individual atoms between the layers are not. Rather, a higher or lower number of atoms (corresponding to higher or lower VdW interactions) are encountered periodically if one follows a straight line laterally along the lattice. 

The researchers used this property of periodically varying VdW strength to look at how VdW strength affects the distribution of electrons in the semiconductor with powerful microscopes, which in turn shows what kind of semi-conductor it is.  

They found that it behaves as what is called a direct band-gap semiconductor — where electrons can be excited and released directly upon interaction with light. Their findings can later help as an additional design concept for new layered semiconductors.

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