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The electronic band structure of atomically thin semiconductors can be tuned by applying a perpendicular electric field. The principle was demonstrated experimentally shortly after discovering graphene by opening a finite band gap (~100 meV) in graphene bilayers [1-3], which naturally are zero-gap semiconductors. So far, however, the same principle could not be employed to control a broader class of materials because the required electric fields are beyond reach in current devices.
The limitation, however, has been recently overcome in double ionic gated transistors that enable the application of huge electric fields [4]. This talk will acquaint the audience with the basics of the ionic gating technique [5] and how it is implemented in the new type of transistors [4] that enable continuous bandgap tuning of few-layer semiconducting transition metal dichalcogenides (from bilayer to heptalayer WSe2).

[1] Oostinga, J. B., Heersche, H. B., Liu, X., Morpurgo, A. F. & Vandersypen, L. M. K. Gate-induced insulating state in bilayer graphene devices. Nature Materials 7, 151–157 (2008)
[2] Zhang, Y. et al. Direct observation of a widely tunable bandgap in bilayer graphene. Nature 459, 820–823 (2009)
[3] Mak, K. F., Lui, C. H., Shan, J. & Heinz, T. F. Observation of an Electric-Field-Induced Band Gap in Bilayer Graphene by Infrared Spectroscopy. Phys. Rev. Lett. 102, 256405 (2009)
[4] D. Domaretskiy, M. Philippi, M. Gibertini, N. Ubrig, I. Gutiérrez-Lezama, A.F. Morpurgo. Quenching the band gap of 2D semiconductors with a perpendicular electric field. arXiv:2108.06117
[5] Gutiérrez-Lezama, I., Ubrig, N., Ponomarev, E. & Morpurgo, A. F. Ionic gate spectroscopy of 2D semiconductors. Nature Reviews Physics 3, 1–12 (2021)

Location: Stuckelberg, Ecole de Physique
Time: Friday 8 April 2022, 12:00 for pizza, 12:30 start discussion

As usual, we will provide pizza to share for lunch. Please register on this doodle before Thursday 7 April, 19:00, so we can order enough pizza!