Enhanced Raman and photoluminescence response in monolayer MoS₂ due to laser healing of defects

Abstract

Bound quasiparticles, negatively charged trions and neutral excitons are associated with the direct optical transitions at the K-points of the Brillouin zone for monolayer MoS 2 . The change in the carrier concentration, surrounding dielectric constant, and defect concentration can modulate the photoluminescence and Raman spectra. Here, we show that exposing the monolayer MoS 2 in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence intensity associated with both trions and excitons, together with ∼3 to 5 times increase of the Raman intensity of first-order and second-order modes. The simultaneous increase of photoluminescence from trions and excitons cannot be understood based only on known scenario of depletion of electron concentration in MoS 2 by adsorption of O 2 and H 2 O molecules. This is explained by laser-induced healing of defect states resulting in reduction of nonradiative Auger processes. This laser healing is corroborated by an observed increase of intensity of both the first-order and second-order longitudinal acoustic Raman modes at the M-point of Brillouin zone by a factor of ∼3 to 5. The A 1g mode hardens by ∼1.4 cm À1, whereas the E 1 2g mode softens by ∼1 cm À1. The second-order longitudinal acoustic Raman mode at the M-point of Brillouin zone at ∼440 cm À1 shows an increase in wavenumber by ∼8 cm À1 with laser exposure. These changes are a combined effect of change in electron concentrations and oxygen-induced lattice displacements.

Photoluminescence Study of B‐Trions in MoS2 Monolayers with High Density of Defects

“…[37] As it was shown in refs. [19,38,39], the reduced electron concentration leads to a blue shift of the A 0 1 peak compared with the peak position in exfoliated high quality monolayer. It was also shown that the nitrogen doping could be an effective way to produce hole doping in MoS 2 monolayers [40] by creating N S acceptor defects.…”

Changes in the Raman spectra of monolayer MoS2 upon thermal annealing

“…Raman spectra were obtained under normal laboratory conditions in backscattering geometry using a Horiba Jobin Yvon LabRAM HR spectrometer equipped with a grating of 1,800 grooves/mm and a cooled charge‐coupled device of 1,024 × 256 pixels. A laser line of 532 nm was focused on the samples with a full width at half maximum (FWHM: the width of the laser light intensity profile between those points at which the intensity is equal to half of the maximum value) of approximately 0.65 μm using a 100× objective lens (numerical aperture = 0.9), and the total laser power on the samples was kept below 300 μW to prevent laser irradiation effect . A Si Raman peak at 520.7 cm −1 was used as an internal reference to calibrate the Raman peaks of MoS 2 .…”

“…A laser line of 532 nm was focused on the samples with a full width at half maximum (FWHM: the width of the laser light intensity profile between those points at which the intensity is equal to half of the maximum value) of approximately 0.65 μm using a 100× objective lens (numerical aperture = 0.9), and the total laser power on the samples was kept below 300 μW to prevent laser irradiation effect. [30] A Si Raman peak at 520.7 cm −1 was used as an internal reference to calibrate the Raman peaks of MoS 2 . Acquisition times varied depending on the samples; otherwise, all other measurement parameters were fixed.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIII

“…Bera et al described enhanced Raman and photoluminescence response in monolayer MoS 2 due to laser healing of defects. They show that exposing monolayer MoS 2 in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence intensity associated with both trions and excitons, together with approximately three to five times increase of the Raman intensity of first‐order and second‐order modes . Bobbitt and Smith used scanning angle Raman spectroscopy to extract interface locations in multilayer polymer waveguide films.…”

“…They show that exposing monolayer MoS 2 in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence intensity associated with both trions and excitons, together with approximately three to five times increase of the Raman intensity of first-order and second-order modes. [83] Bobbitt and Smith used scanning angle Raman spectroscopy to extract interface locations in multilayer polymer waveguide films. The scanning angle Raman method acquires Raman spectra as the incident angle of light upon a prism-coupled thin film is scanned.…”