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Abstract
In this work, we investigated the nonlinear optical properties of monolayer MoS2 and WS2 modulated by defect engineering via chemical treatment. The results demonstrate that the two-photon luminescence (TPL) and two-photon absorption (TPA) coefficient were remarkably improved after the repair of sulfur vacancies for both monolayer MoS2 and WS2. After the chemical treatment, the nonradiative relaxation path dominant in pristine monolayer MoS2 is significantly alleviated, resulting in enhanced TPL. Our work affords an effective way to tailor the nonlinear absorption, luminescence and relaxation properties of sulfur-based two-dimensional metal dichalcogenides by defect engineering.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, two-dimensional (2D) transition metal dichalcogenides (TMDs) have been extensively studied for their fascinating nonlinear optical (NLO) properties, including ultrafast saturable absorption behavior [1], multiphoton absorption [2] and multiphoton absorption induced luminescence [3]. Among the various NLO properties, TPA and TPL show great potential in applications of optical limiting, optical signal processing, two-photon microscopy, two-photon subband photodetectors, light frequency upconversion, and so on [4–9]. Therefore, it is important to develop TMDs with strong TPA and TPL. Defects such as edges and vacancies have been found to play a key role in determining their NLO properties of 2D TMDs [10]. For example, remarkable resonant NLO susceptibilities have been observed at the boundary and edges of 2D MoS2 [11]. Wang et al. reported the manipulation of a broadband MoS2 saturable absorber by introducing suitable sulfur (S) defects [12]. Very recently, we have shown that the defect of S vacancies will induce saturable absorption (SA) while suppress TPA in few-layered MoS2 [13]. Thus, it is expected to improve the TPA as well as TPL properties of monolayer S-based TMDs by removing or repairing the S vacancies.
Defects in TMDs can be generated naturally or artificially, including vacancy, impurity, adatom, etc [14,15]. According to the previous works, the S vacancies are mainly natural defects in mechanically-exfoliated or chemical vapor deposition (CVD) fabricated TMDs, which essentially limit the performance of TMDs in practical applications [16,17]. A series of methods have been implemented to reduce or mitigate the defects in TMDs to improve its optical properties, such as hydrohalic acid treatment [18], molecular oxygen chemical adsorption [19], substrate selection [20] and encapsulation in hexagonal boron nitride [21]. Among all of these methods, an organic superacid, bis-(trifluoromethane) sulfonimide (TFSI), is reported to be able to effectively repair/passivate the S vacancies in monolayer MoS2, which remarkably improved the room temperature photoluminescence (PL) quantum yield to near 100% [22]. However, few works have been done on how the repairing of S vacancies affects the NLO performances of S-based TMDs, mainly MoS2 and WS2.
In this work, we investigated the effect of this sulfonimide treatment on the NLO properties of monolayer S-based TMDs. The TFSI-treated MoS2 and WS2 show enhanced TPA and TPL performances compared to the untreated counterparts. A remarkable room temperature TPL was observed for the treated monolayer MoS2 and WS2 at a relatively low pump power density of 2.0 × 10−3 GW/cm2 and 1.8 × 10−3 GW/cm2, respectively. In the ultrafast carrier experiments, we found that the carrier relaxation in monolayer MoS2 significantly slows down after sulfonimide treatment, which can be ascribed to the repairing of S vacancies in S-based TMDs.
2. Results and discussion
2.1. Materials characterization
Monolayer MoS2 and WS2 on sapphire substrates were synthesized by CVD method and then transferred with the assistance of poly(methyl methacrylate) (PMMA) to quartz substrates. Figure 1(a) shows the schematic process of this chemical treatment, in which the sample was directly blow dried with nitrogen (N2) without rinsing after taken out from the acid solution as reported previously [22]. No apparent morphological change is observed after TFSI treatment as shown in Figs. 1(b) and 1(c). Both the as-transferred and TFSI-treated MoS2 show a relatively smooth surface with a thickness of 0.86 nm (inset in Figs. 1(b) and 1(c)), confirming that the synthesized MoS2 is monolayer. The slightly larger value than one single layer (~0.67 nm) [23,24] is caused by an air space introduced during the transfer process. The Raman and absorption spectra in Figs. 1(d) and 1(e) reveal that sulfonimide treatment has little effect on the structure and linear optical absorption of monolayer MoS2. The same phenomena were found in the TFSI-treated monolayer WS2. According to the previous reports, the defects in pristine MoS2 and WS2 are mainly S vacancies as illustrated in Fig. 1(f), which has been proved to have significant effect on its NLO performances [9,10]. In order to justify the repairing of S vacancies in TFSI-treated MoS2, X-ray photoelectron spectroscopy (XPS) was carried out as shown in Fig. 1(g). Calculated from the XPS spectra, the S/Mo ratio is higher than that of the untreated one (from 1.812 ± 0.43 to 1.532 ± 0.35), suggesting that the S vacancies in MoS2 are repaired after treatment. Then a small peak at 293 eV was observed, ascribed to -CF3 bonds in TFSI molecule. The results demonstrate that the repairing of S vacancies mainly come from the adsorbed organic acids [32].
Fig. 1 (a) Schematic of the TFSI treatment procedure. AFM topography of (b) the as-transferred and (c) TFSI-treated monolayer MoS2 on quartz substrate. Insets show the height profiles. (d) Absorption, (e) Raman spectra of the as-transferred and TFSI-treated monolayer MoS2. (f) Schematic structures of the monolayer MoS2 with S vacancies. (g) XPS spectra of the C 1s and Mo 3d core levels of monolayer MoS2 before and after the treatment.
Previous work demonstrates that the PL properties of TMDs are strongly related with defects in TMDs [25–28], which can induce the nonradiative recombination, causing a low quantum yield [29]. Here, steady-state PL spectra is measured to investigate whether the TFSI treatment can repair the defects in TMDs. Figure 2(a) shows the PL spectra of monolayer MoS2 before and after TFSI treatment at a low excitation power density of 5.4 W·cm−2. Obviously, the PL intensity is significantly enhanced after sulfonimide treatment, which is about 50 times higher than that of the as-transferred counterpart as shown in the inset of Fig. 2(a). The remarkably enhanced PL demonstrates that the TFSI treatment has successfully repaired the defects in MoS2.
Fig. 2 (a, b) PL spectra for monolayer MoS2 and WS2 before and after the TFSI treatment. Inset in (a) shows the normalized spectra. Inset in (b) shows the stability of PL intensity and peak shift with time. (c,d) TPL spectra for TFSI-treated monolayer MoS2 and WS2 on quartz substrate pumped by 1030-nm fs laser pulses, respectively. Inset shows a quadratic power dependence of the TPL emission.
The presence of defects can form defect energy levels within the bandgap making most photoexcited carriers recombine nonradiatively and decrease the PL quantum yield of pristine monolayer MoS2 and WS2. Once the defect is repaired or removed, the nonradiative recombination will be suppressed, leading to the PL enhancement.
A noticeable blue shift for the peak energy is observed in our experiment. According to the previous report, the PL spectra of MoS2 can be divided into trion (X−; 1.82 eV) and exciton (X; 1.88 eV) peaks (Fig. 3), using Gaussian band shapes. A negative trion is composed of two electrons and a hole formed by a neutral exciton (a photogenerated electron-hole pair) binding with an electron. In pristine monolayer MoS2, the trion peak is dominant due to a high electron concentration caused by the rich electron-donor defects, which are mainly S vacancies [30]. As shown in Fig. 3, the trion peak is suppressed greatly with the exciton peak increased after treatment, suggesting the defects in MoS2 are repaired. The repairing of S vacancies will cause a decrease of the electron concentration, leading to that the neutral excitons recombine rather than form negative trions. The blue shift was also observed in a poly(4-styrenesulfonate)-treated system [31]. We measured the PL intensity of the TFSI-treated MoS2 over different times and found that the PL intensity decreased gradually and almost went back to the level of the untreated one after one week. When treated by the TFSI for another time, the PL can be enhanced again. This shows that the PL modulation is reversible. Similar PL enhancement is observed in monolayer WS2 [Fig. 2(b)], which is consistent with the previous reports [32]. Differently, the PL enhancement is much lower than that in monolayer MoS2, which is mainly due to the low defect density in monolayer WS2. Our results show that TFSI treatment can also lead to a blue shift in WS2. Similar to the PL intensity, the PL blue shift in WS2 and MoS2 will eventually disappear or slightly attenuate with the fading of treatment effect over a certain time, probably due to the subsequent desorption of the acid molecules [31].
2.2. Two-photon absorption and two-photon excitation photoluminescence
Based on the above results, the defect of S vacancies in monolayer MoS2 and WS2 have been repaired. Thus, the TPA and TPL properties are expected to be enhanced at the same time.
The TPL was investigated with the excitation of 1030nm fs laser pulse with the repetition of 500 kHz at room temperature. Figure 2(d) shows the TPL intensity as a function of the excitation power. The spectral shape and the peak position (~1.9 eV) match quite well with those of single photon luminescence at 532 nm excitation [Fig. 1(a)]. The PL intensity exhibits a quadratic dependence on the excitation intensity [inset in Fig. 2(c)], confirming the TPL nature. Similarly, we observed room-temperature TPL in the TFSI-treated monolayer WS2 [Fig. 2(d)]. The better TPL signal-to-noise ratio of WS2 is due to its lower defect density and higher quantum yield than MoS2 [33]. As far as we know, only low-temperature TPL was observed for monolayer WS2 at a temperature of 10 K by Ye et al. [34]. The realization of room-temperature TPL is of great significance for pushing its practical applications. The remarkable TPL observed in TFSI-treated monolayer MoS2 and WS2 at room temperature at a low excitation power density of 2.0 × 10−3 GW/cm2 and 1.8 × 10−3 GW/cm2, respectively, can be attributed to the suppressed nonradiative recombination due to the repairing of the S vacancies.
A micro I-scan system [35] was used to investigate the nonlinear absorption properties of the monolayer MoS2 on quartz substrate before and after treatment, using 340 fs pulses from a mode-locked fiber laser operating at 1030 nm with a repetition rate of 1 kHz. As shown in Figs. 4(a) and 4(b), the monolayer MoS2 and WS2 exhibit typical TPA behaviors by absorbing two exciting photons with energy of 1.2 eV. The TPA saturation was also observed due to the saturation of photon-excited carriers and depletion of the electron population in the ground state [36]. In this process, the TPA coefficient decreases with increasing pulse intensity as shown in Fig. 4(c).
Fig. 4 Nonlinear transmittance versus incident pulse peak irradiance for (a) monolayer MoS2 and (b) monolayer WS2 before and after TFSI treatment. (c) TPA coefficient versus incident pulse intensity for monolayer MoS2 before and after TFSI treatment. (d) The schematic representation of TPA influenced by defect state in monolayer MoS2. (e) Pump–probe results of as-transferred and TFSI-treated MoS2 monolayer. (f) A single plot of the data shows three different temporal regions in monolayer MoS2.
As shown in Fig. 4(d), the TPA and SA coexist in S-based TMDs under our experimental conditions. Our previous study on few-layer MoS2 samples [13] revealed that the influence of SA on the I-scan curves is negligible. The I-scan results were fitted using a model including nonsaturable linear absorption:
Where , , and z are the one photon absorption coefficient, the TPA coefficient and propagation distance in the sample, respectively.Generally, the TPA coefficient β0 of nonsaturable TPA is constant. For the TPA saturation process in our experiment [37], the TPA coefficient is related to the incident light intensity. According to the previous reports, monolayer MoS2 should be a homogeneously broadened system in the process of TPA saturation. The model is as follows:
Where is the TPA saturation intensity. In our experiments, no obvious nonlinear response was observed from the quartz substrate.As shown in Figs. 4(a) and 4(b), the I-scan data are well fitted with Eqs. (1) and (2), with the TPA coefficients and saturation intensity given in Table 1. The TPA coefficient is ~1.42 × 104 cm/GW for the as-transferred monolayer MoS2, similar to the previous report [35]. After the TFSI treatment, the TPA coefficient of monolayer MoS2 was significantly increased to ~8.50 × 104 cm/GW, nearly six times higher than that of the untreated counterpart with the TPA saturation intensity decreasing to 12.1 GW/cm2 from 35.5 GW/cm2 (Table 1). When left in the air for 120 h, the TPA coefficient decreased to half of that of the freshly treated MoS2, which is consistent with the PL results. According to the XPS results, the acid molecules are physically adsorbed on the surface of MoS2, and then desorbed over certain time [38], resulting in the TPA coefficient decreasing. Besides, defect repairing is also expected to tailor the other nonlinear parameters of TMDCs, such as the nonlinear refraction, which will be studied in our next work.
Table 1. Parameters Obtained from the Fitting of I-scan and Pump−Probe Results.
2.3. Physical mechanism for the enhanced TPA and TPL performances
The influence of TFSI treatment on the dynamics of carrier recombination was studied on the example of MoS2 by a nondegenerate ultrafast optical pump–probe technology. In order to make the probe transmission relevant with the intraband absorption of generated electrons and holes, we choose 520 nm as the pump and 1040 nm as the probe wavelengths with the probe wavelength much lower than the exciton line. Figure 4(e) shows the pump-probe result of the monolayer MoS2 before and after the treatment in which the probe pulse intensity is ~2.62 GW/cm−2. Obviously, the ΔT/T is negative for monolayer MoS2 before and after treatment. The maximum |ΔT/T| become larger for the treated monolayer MoS2 compared with that of the untreated counterpart, indicating that the TPA is enhanced after treatment. The single photon SA introduced by the S vacancy states will be weakened significantly with the removing/repairing of the S vacancies resulting in enhanced TPA [12,17]. The pump-probe data were fitted using a three-exponential model with the autocorrelation of the pump and probe pulse taken into account,
where g(t) is the pump–probe signal, D1, D2 and D3 are the relative amplitudes, erfc is the integral error function, σ is the laser pulse duration (350 fs), τ1, τ2 and τ3 are the lifetimes of the sample. The fastest component (τ1) was found to be less than 1 ps for the MoS2 samples, which is attributed to the quenching of the MoS2 exciton by carrier trapping.The second component τ2 is considered to be associated with the exciton-phonon scattering process, which will be affected greatly by the defects. The third component τ3 is related with the radiative recombination of the exciton [39].
The corresponding fitting parameters are summarized in Table 1. The experimental decay curves in Fig. 4(e) can be fitted well with the three processes. After the TFSI treatment, the carrier relaxation (τ2) for monolayer MoS2 becomes slower, which means that the defects are repaired and fewer channels for defect-assisted recombination are available. This conclusion is confirmed by our transient absorption spectra in Figs. 5(a) and 5(b). The spectra of MoS2 before and after TFSI treated are dominated by the bleaching of B-exciton at 606 nm and A-exciton at 655 nm. In the transient absorption map, the negative signal in the blue region represents the reduced absorption (photobleaching) at the stimulus resonance and the positive signal in the corresponding red region is the red-shifted photoinduced absorption [40]. It can be observed that the relaxation lifetime of TFSI-treated samples is extended compared to that of the as-transferred counterpart in Figs. 5(c) and 5(d), which can be corroborated by Amani’s luminescence lifetime measurement [22]. The variance in the kinetic data could be considered as an extent to which defects play a role in the time dynamics of monolayer MoS2.
Fig. 5 Transient absorption spectra for as-transferred (a) and TFSI-treated (b) MoS2 monolayer upon excitation at 365 nm pump pulse (~8 nJ/pulse). (c, d) Decay curves for the two bleaching pits of A and B for the monolayer MoS2 before and after the TFSI treatment, respectively.
A typical band structure of monolayer MoS2 with S vacancies is afforded as shown in the inset in Fig. 4(d). It is well known that the exciton ground-state energy level (E1s) of monolayer MoS2 is located at ∼1.8 eV, which is independent of the layer number. Theoretical and experimental work has demonstrated rich excitonic dark states below the edge of the conduction band in monolayer MoS2. Both TPA and SA participate in the NLO performances of monolayer MoS2 under our experimental conditions, competing with each other. Li et al. have theoretically and experimentally demonstrated that new bands appear within the bandgap near the Fermi level in MoS2 when S-vacancies are introduced [41], which locates lower than one photon energy (1.2 eV) resulting in inevitable SA. After the TFSI treatment, the S vacancies in S-based TMDs were effectively repaired with the defect energy level removed. Therefore, the SA can be effectively suppressed, resulting in enhanced TPA.
3. Conclusions
In summary, we investigated the influence of defect repairing by the TFSI treatment on the TPA, TPL and relaxation properties of monolayer MoS2 and WS2. The TFSI treated MoS2 shows the significant TPA performance with the TPA coefficient nearly six times compared to the untreated counterpart. A remarkable TPL was observed at room temperature in both TFSI-treated monolayer MoS2 and WS2. The enhanced TPA and TPL performance can be ascribed to the reduction of the defect density caused by defect repairing, which plays an important role in tailoring the band structure of S-based TMDs. Our work shed light on how the defects tailor the nonlinear absorption and luminescence properties of S-based TMDs, which is of great importance for their NLO applications.
Funding
National Natural Science Foundation of China (NSFC) (61675217, 61875213, 11874370); Strategic Priority Research Program of Chinese Academy of Sciences (CAS) (XDB16030700); Key Research Program of Frontier Science of CAS (QYZDB-SSW-JSC041); Program of Shanghai Academic Research Leader (17XD1403900); Natural Science Foundation of Shanghai (18ZR1444700); Youth Innovation Promotion Association, CAS; President’s International Fellowship Initiative of CAS (2017VTB0006, 2018VTB0007).
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