A team of researchers from Purdue University and Global Foundries semiconductor foundry, led by Associate Professor Zhihong Chen write in Nano Letters regarding the band gap tenability of bilayer MoS₂.

Why bilayer MoS₂

It is well known that bilayer graphene has a tunable band gap, in the range of 0-300meV. However this band gap is a limitation for mid-infrared range applications which require larger gaps. On the contrary, TMDC demonstrate sizeable band gaps with many in the infrared to visible range. The paper, entitled “Electrically tunable bandgaps in bilayer MoS₂” provides the experimental verification of continually tunable bandgap in bilayer MoS₂ by means of a dual-gated transistor geometry.

Transport measurements

Samples were mechanically exfoliated MoS₂ flakes and bilayers were identified with Raman and AFM measurements. A HfO₂ top gated was defined by atomic layer deposition. The high-κ dielectric enables sufficient hole current injection to the MoS₂ layer which shows ambipolar behaviour. The band gap can then be extracted by taking into consideration the applied electric field, the threshold voltages when the Fermi level enters the conduction or valence bands and the applied electrical bias. For monolayer MoS₂, no changes are observed for the band gap, but for bilayer it was found that there is a reduction of 260meV per 1/Vnm displacement field.

Optical measurements

Such tunable bang gaps may also be optically verified from the photoluminescence spectrums. In order to allow optical access to the MoS₂ flake graphene, instead of metal, was used as a top contact. In agreement with the transport measurements, A exciton peak at 1.83eV, changes its possible as the top gate is sweeped. Finally, density functional calculations complete the set by providing excellent agreement between transport, optical measurements and simulations.
Such bandgap tuning of TMDC can be immensely useful in electronic and optoelectronic applications, however, large area, controllable, low-temperature and self-limiting growth of bilayer MoS₂ graphene remains to be addressed before MoS₂ can realise its full potential.