Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms

Abstract

Chemical treatment using bis(trifluoromethane) sulfonimide (TFSI) was shown to be particularly effective for increasing the photoluminescence (PL) of monolayer (1L) MoS, suggesting a convenient method for overcoming the intrinsically low quantum yield of this material. However, the underlying atomic mechanism of the PL enhancement has remained elusive. Here, we report the microscopic origin of the defect healing observed in TFSI-treated 1L-MoS through a correlative combination of optical characterization and atomic-scale scanning transmission electron microscopy, which showed that most of the sulfur vacancies were directly repaired by the extrinsic sulfur atoms produced from the dissociation of TFSI, concurrently resulting in a significant PL enhancement. Density functional theory calculations confirmed that the reactive sulfur dioxide molecules that dissociated from TFSI can be reduced to sulfur and oxygen gas at the vacancy site to form strongly bound S-Mo. Our results reveal how defect-mediated nonradiative recombination can be effectively eliminated by a simple chemical treatment method, thereby advancing the practical applications of monolayer semiconductors.

Measurement of Quantum Yields of Monolayer TMDs Using Dye-Dispersed PMMA Thin Films

“…All 1L-TMDs were exfoliated on quartz substrates using bulk single crystals (2D Semiconductors Inc. Scottsdale, AZ, USA). 1L-MoS 2 samples were grown using the chemical vapor deposition (CVD) method on Si/SiO 2 substrates and transferred onto quartz substrates using the wet transfer method, wherein hydrofluoric acid (HF) was used as the SiO 2 etchant, according to previously reported methods [13,15,34,35]. The CVD-grown 1L-MoS 2 samples were chemically treated using TFSI (Sigma-Aldrich, St. Louis, MO, USA) molecules after being transferred onto the quartz substrates and further optical characterizations were conducted [13,14].…”

“…In these figures, it is shown that contribution of the trion recombination is ~25% for the overall emissions at all power ranges measured in this study. The PL spectra of the pristine, CVD-grown 1L-MoS2 are depicted in Figure S4 For 1L-MoS 2 , the effect of chemical treatment using TFSI to increase the QY was investigated because TFSI is known to enhance the QY of 1L-TMDs via the repair of the sulfur vacancies [13,14]. The laser-intensity-dependent PL spectra of the TFSI-treated CVD-grown 1L-MoS 2 on a quartz substrate are depicted in Figure 4a (for the Raman spectra, see the Supplementary Materials, Figure S2c).…”

“…However, the photoluminescence (PL) quantum yields (QYs) of pristine 1L-TMDs are typically less than 1%, significantly limiting their application in optoelectronic devices [8]. A high density of structural defects in these systems has been proven to be the main cause of the low QYs of 1L-TMDs [9][10][11][12][13]. Methods to improve the QYs of 1L-TMDs through the treatment of these defects using bis(trifluoromethane) sulfonimide (TFSI) [13][14][15][16], hydrohalic acids (HBr) [17], poly(4-styrenesulfonate) [18], and light illumination [19][20][21] have been reported.…”

“…The PL intensity and absorption data of the reference sample are then measured using microscopes for comparison with data obtained from the 1L-TMD samples using the same experimental conditions [27][28][29][30]. In this method, the reference samples are usually thin films made of fluorescent organic dyes, such as 3-borylbithiophene derivatives [27][28][29][30] or rhodamine 6G (R6G) [1,13,14]. The thicknesses and emission profiles of the reference films must be spatially uniform at micro-scale dimensions to permit comparison with the local PL of micro-sized 1L-TMDs.…”

Defect Engineering of Two‐Dimensional Molybdenum Disulfide

“…Among them, solution‐based exfoliation techniques such as LPE, CE and ECE hold great promise in terms of low‐cost, scalability and convenience for multicomponent hybridization. However, the exfoliation procedures would inevitably introduce additional defects to the MoS 2 lattice . For example, sulfur vacancies (SVs) have been widely observed in exfoliated MoS 2 nanosheets using high resolution microscopy .…”