Tuning carrier concentration in a superacid treated MoS2 monolayer

Abstract

The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment on the optical properties of MoS2 monolayers is investigated by means of photoluminescence, reflectance contrast and Raman scattering spectroscopy employed in a broad temperature range. It is shown that when applied multiple times, the treatment results in progressive quenching of the trion emission/absorption and in the redshift of the neutral exciton emission/absorption associated with both the A and B excitonic resonances. Based on this evolution, a trion complex related to the B exciton in monolayer MoS2 is unambiguously identified. A defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to effective passivation of defects on the MoS2 monolayer surface. The passivation reduces the carrier density, which in turn affects the out-of-plane electric field in the sample. The observed tuning of the carrier concentration strongly influences also the Raman scattering in the MoS2 monolayer. An enhancement of Raman scattering at resonant excitation in the vicinity of the A neutral exciton is clearly seen for both the out-of-plane A′1 and in-plane E′ modes. On the contrary, when the excitation is in resonance with a corresponding trion, the Raman scattering features become hardly visible. These results confirm the role of the excitonic charge state plays in the resonance effect of the excitation energy on the Raman scattering in transition metal dichalcogenides.

The optical response of artificially twisted MoS$$_2$$ bilayers

“…Similarly to the aforementioned analysis of the RS, we begin with an examination of the PL spectra of the natural 1 L and 2 L MoS . The 1 L spectrum consists of two narrow emission lines apparent in the vicinity of the optical band gap (so-called A exciton), which can be ascribed to the neutral and charged excitons in accordance with previous reports 44 – 46 . In contrast, the PL spectrum of the 2 L MoS is composed of two distinct emission bands: (1) the transitions in the vicinity of the direct A (X ) and B (X ) excitons formed at the K points of the Brillouin zone (BZ), which are observed correspondingly at about 1.93 eV and 2.07 eV; (2) the significantly much intense transitions, denoted as I and I’, apparent at about 1.5 eV, which are ascribed correspondingly to an indirect recombination process between the and K point in the conduction band (CB) and points in the valence band (VB) of the BZ 32 , 43 , 47 .…”

“…While the energies of direct transitions are hardly affected by the twist angles, the energies of the indirect ones change significantly by about 150 meV. The low-intensity emission bands, denoted with * and apparent between the aforementioned I and X lines, can be described as a remaining part of the defect-related emission, which is reported commonly for 1-L MoS exfoliated on Si/SiO substrates 44 – 46 .…”

The optical response of artificially twisted MoS$_2$ bilayers

“…Similarly to the aforementioned analysis of the RS, we begin with an examination of the PL spectra of the natural 1 L and 2 L MoS 2 . The 1 L spectrum consists of two narrow emission lines apparent in the vicinity of the optical band gap (so-called A exciton), which can be ascribed to the neutral and charged excitons in accordance with previous reports [44][45][46] . In contrast, the PL spectrum of the 2 L MoS 2 is composed of two distinct emission bands: (i) the transitions in the vicinity of the direct A (X A ) and B (X B ) excitons formed at the K ± points of the Brillouin zone (BZ), which are observed correspondingly at about 1.93 eV and 2.07 eV; (ii) the significantly much intense transitions, denoted as I and I', apparent at about 1.5 eV, which are ascribed correspondingly to an indirect recombination process between the Λ and K point in the conduction band (CB) and Γ points in the valence band (VB) of the BZ 32,43,47 .…”

Resonance and antiresonance in Raman scattering in GaSe and InSe crystals

“…the intensity maxima as a function of temperature, has not been reported so far for other layered materials. Particularly, for thin layers of S-TMDs, only a growth or a decrease of the RS signal was observed in a broad temperature range from 5 K to 300 K 22 – 24 . The most interesting result is the strong vanishing of all modes intensities at temperatures above around 280 K under excitations of 1.96 eV and 2.41 eV in GaSe and InSe, respectively.…”

“…This offers supplementary information on the coupling of particular phonons to electronic transitions of a specific symmetry [19][20][21] . The crossover between the non-resonant and resonant conditions can be achieved not only by the variation of the excitation energy but also by the modulation of temperature as it was recently reported [22][23][24] . In such an approach, it is the band structure that changes with temperature allowing for resonance with particular excitation energy.…”