Abstract
The few layer transition metal dichalcogenides are two dimensional materials that have an intrinsic gap of the order of ≈2 eV. The reduced screening in two dimensions implies a rich excitonic physics and, as a consequence, many potential applications in the field of opto-electronics. Here we report that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size. We show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. This constancy of binding energy is both a striking exception to the universal reduction in binding energy with gap size that all materials are believed to follow, as well as evidence of a degree of control over inter-layer excitons not found in their well studied intra-layer counterparts.
Highlights
As in a previous work[21], we find that excitonic binding energies are very sensitive to the number of k-points
The electric field is included as a scalar potential in the Kohn-Sham Hamiltonian. This potential has a saw-tooth shape, a fact that follows from the imposition of periodic boundary conditions, and gradient of this scalar potential, which is constant across the unit-cell, is the electric field
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Summary
Dilna Azhikodan[1], Tashi Nautiyal[1], Sam Shallcross2 & Sangeeta Sharma[3] received: 02 August 2016 accepted: 24 October 2016 Published: 14 November 2016. We report that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size We show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. A recent experimental investigation reports an intriguing excitonic physics of MoS2 in the presence on a layer perpendicular electric field, with intra- and inter-layer excitons appearing to behave quite differently[5] It is the purpose of the present paper to apply the full “tool kit” of many-body ab-initio science (GW in conjunction with the BSE) to this system, and to fully reveal the remarkable nature of this excitonic physics of MoS2 in an electric field
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