Abstract
Direct band-gap semiconductors play the central role in optoelectronics. In this regard, monolayer (ML) MX2 (M = Mo, W; X = S, Se) has drawn increasing attention due to its novel optoelectronic properties stemming from the direct band-gap and valley degeneracy. Unfortunately, the more practically usable bulk and multilayer MX2 have indirect-gaps. It is thus highly desired to turn bulk and multilayer MX2 into direct band-gap semiconductors by controlling external parameters. Here, we report angle-resolved photoemission spectroscopy (ARPES) results from Rb dosed MoSe2 that suggest possibility for electric field induced indirect to direct band-gap transition in bulk MoSe2. The Rb concentration dependent data show detailed evolution of the band-gap, approaching a direct band-gap state. As ionized Rb layer on the surface provides a strong electric field perpendicular to the surface within a few surface layers of MoSe2, our data suggest that direct band-gap in MoSe2 can be achieved if a strong electric field is applied, which is a step towards optoelectronic application of bulk materials.
Highlights
Direct band-gap semiconductors play the central role in optoelectronics
As ionized Rb layer on the surface provides a strong electric field perpendicular to the surface within a few surface layers of MoSe2, our data suggest that direct band-gap in MoSe2 can be achieved if a strong electric field is applied, which is a step towards optoelectronic application of bulk materials
Rb dosing on the surface has two main effects on the system: surface electron doping and surface electric field
Summary
Direct band-gap semiconductors play the central role in optoelectronics. In this regard, monolayer (ML) MX2 (M = Mo, W; X = S, Se) has drawn increasing attention due to its novel optoelectronic properties stemming from the direct band-gap and valley degeneracy. We report angle-resolved photoemission spectroscopy (ARPES) results from Rb dosed MoSe2 that suggest possibility for electric field induced indirect to direct band-gap transition in bulk MoSe2. Whether a semiconductor has a direct or indirect band-gap greatly affects its optical properties; excitons in direct band-gap semiconductors, for example, strongly couple to photons. The coupling between the valley degree of freedom and circularly polarized light opened the field of valley Hall effect[5, 6] Such intriguing physics can be realized only in 1 ML 2H-MX2 because multi-layer 2H-MX2 has an indirect band gap and does not strongly couple to light. Our strategy to the issue of inducing a direct gap is to apply a strong electric field perpendicular to the MoSe2 layers by using such alkali metal dosing. Our observation casts a strong possibility for an indirect to direct gap transition under a strong electric field in MoSe2
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