Highly Photoluminescent Monolayer MoS₂ and WS₂ Achieved via Superacid Assisted Vacancy Reparation and Doping Strategy

Abstract

Chemical treatment of transition-metal dichalcogenides (TMDs) by organic superacid bis(trifluoromethane) sulfonamide TFSI, has been proved as an effective route to reduce the density of sulfur vacancies and improve photoluminescence quantum yield (PL QY) of monolayer TMDs (1L-TMDs). However, such kind of functional organic superacid is only limited to the TFSI so far, and the underlying mechanism of PL enhancement remains elusive. Here, a novel organic superacid trifluoromethanesulfonic (TFMS), which can significantly increase the PL QYs of 1L-MoS 2 and 1L-WS 2 to 14.1% and 56.7% respectively, the relatively higher PL QY values for chemical vapor deposition grown 1L-TMDs is explored. Through in-depth study and analysis with scanning transmission electron microscopy and density functional theory calculation, PL enhancement mechanism is attributed to a synergistic effect of TFMS-induced p-type doping and sulfur vacancy reparation. Especially, the respective function of [CF 3 SO 3 ] − and H + of TFMS played in superacid-induced PL enhancement is clearly identified. The results not only enrich the functional superacid family that can remarkably improve the luminescence performance of TMDs, but also clearly reveal the PL enhancement mechanism of superacid-treated TMDs.

Defect-suppressed submillimeter-scale WS2 single crystals with high photoluminescence quantum yields by alternate-growth-etching CVD

“…The N e , which is estimated by spectral weight of X A − ( I A − / I total ), 28,31 was calculated to further discuss the elimination of vacancies by AGE-CVD. The equation can be written as: 24 where I A − and I A 0 are the PL integrated intensities of X A − and X A 0 , respectively. The total emission intensity is defined as I total = I A − + I A 0 .…”

“…20–22 However, uncontrollable vacancies inevitably formed during the thermodynamic non-equilibrium CVD processes. 15,23,24 Vacancies in CVD-grown WS 2 will introduce localized donor states inside the band gap, which largely lower its carrier mobility and photoluminescence (PL) emission. 25,26 Although these vacancies can be repaired by some strategies, such as introducing iron oxide during CVD or post-processing with organic superacids after CVD growth, 27–30 these treatments are usually time-consuming, complicated and may introduce uncontrollable chemical contaminants into TMDs.…”

Superacid Treatment on Transition Metal Dichalcogenides

“…Unfortunately, the PL QY of TMDC monolayers was quite low, countering the expected high performance of the materials. After the first demonstration of TFSI treatment on MoS 2 , the TFSI treatments were also applied to other TMDC monolayers (figure 3) [21,53,[58][59][60][61][62][63][64]. The PL QYs of WS 2 monolayers increased to near unity after the treatment, showing similar results as MoS 2 monolayers.…”

“…Currently, widespread research has focused on TFSI treatment, but the surface chemistry of the mechanism requires more in-depth research. One probable scenario would be defect passivation of sulfur vacancies by sulfur adatoms [60,63,95] reported by analysis via transmission electron microscope [60,95]; conversely, PL behavior indicates the generation of defective sites even after the TFSI treatment process, which is identified by the bound-exciton-related signal at low temperatures [23]. Or TFSI treatment procedure may be concluded as a simple doping effect between the TFSI molecules and TMDCs.…”

Lattice Vacancy Induced Energy Renormalization of Photonic Quasiparticles in Two-Dimensional Semiconductors

“…An intuitive notion, based on the quasiparticle recombination physics, is that the PL enhancement relies on the suppression of nonradiative Auger processes through vacancy repair or passivation, which is supported by the microscopic evidence from lattice images . The lattice vacancies are thus believed to be negative in this scenario. ,,,− In contrast, another opinion deduces that the lattice defects are critical, rooted in the population modulation of photonic quasiparticles due to the charge transfer from physically or chemically molecular adsorption. , Recently, a third view concluded that the role of lattice vacancies is negligible even in the presence of high densities of them. , To date, no works have clarified the realistic effect of lattice vacancies on the energy renormalization of bound photonic quasiparticles or unraveled their imitate correlation.…”