Open Access
Add to BibSonomyAbstract
Monolayer (ML) transition metal dichalcogenides (TMDCs) are thought to be highly promising materials for the optoelectronic and nanophotonic applications. However, the low absorption cross section and photoluminescence (PL) quantum yield in such atomically thin layers restrict their applications. Considering that the energy transfer in a heterostructure can modulate TMDCs’ optical properties, a type I heterostructure geometry comprising ML TMDCs and lead halide perovskite quantum dots (QDs) has the potential to overcome these shortcomings. In this work, spin-coating the CsPbBr3 QDs on ML WS2 results in ~12.7 times enhancement in the PL intensity of ML WS2 at 295K. This giant enhancement is attributed to the energy transfer process from CsPbBr3 QDs to WS2 with a ~40% energy transfer efficiency and ~2 × 108 s−1 energy transfer rate. Besides, we observed that the internal quantum efficiency of ML WS2 is increased from 6.35% to 29.01%. The result demonstrates the feasibility of using perovskite QDs and ML TMDCs to form a type I heterostructure and improve the performance of the TMDC-based optoelectronic devices.
© 2017 Optical Society of America
1. Introduction
Recently, monolayer (ML) transition metal dichalcogenides (TMDCs), such as WS2, have emerged as highly intriguing materials for the scientific community, due to their extraordinary electronic and optical properties [1–4]. One of the unique properties of TMDCs is their strong light-matter interactions and have optical gaps in the visible and near-infrared spectra range, as they experience a transition from an indirect gap in bulk and few-layer samples to a direct gap in the single-layer regime [5–8].
In spite of their superior optical properties, the practical applications of TMDC-based optoelectronic devices are restricted, as a result of low absorption cross section and photoluminescence (PL) quantum yield in such atomically thin layers [9–11]. With that consideration, various approaches have been developed to modulate the optical properties of ML TMDCs, especially enhancing the PL intensity that is very important for the applications of TMDC-based light-emitting devices. For example, Chorng Haur Sow et al. achieved ~9 times enhancement in the PL intensity of the ML WS2 by laser modification [10]. In addition, low temperature can increase the ML MoS2 internal quantum efficiency from 8.3% (300K) to 45% (83K) [11]. Surface plasmonic effects have also been used to enhance the PL intensity of the ML TMDCs, for example, the presence of the gold nanorods can lead an enhancement factor approximately 6.5-fold for the PL intensity of the ML MoS2 [12]. Substrate engineering also is a feasible method to modify the PL intensity. As the references show [13, 14], substrate SrTiO3 and h-BN could lead to giant PL enhancement in ML TMDCs. Chemical treatment is another way to enhance the PL intensity of ML TMDCs by shielding the defects [15]. However, the effect of laser modification highly depends on the laser power and has the limitation for the maximum laser power. The production cost of the metal nanostructures is high; the cooling process and substrates engineering are complicated for practical applications and fabrication process. And the chemical treatments have the possibility to introduce contaminants into the TMDCs. Therefore, it is worthwhile to develop better methods to improve the PL intensity of the ML TMDCs.
Meanwhile, lead halide perovskite QDs (MPbX3. M = CH3NH3, Cs; X = Cl, Br, I) have been reported as a new family nanomaterials. They profit from size-tunable, broadband absorption, narrow emission spectrum, high-photoluminescence quantum yield and can in some cases accommodates multiexciton generation [16–19]. These excellent optical properties make them a promising alternative for light-harvesting and light-emitting applications.
In this letter, we present an effective method to enhance the PL intensity of ML WS2 by matching band offsets with CsPbBr3 QDs to form a type I heterostructure. In the glovebox, CsPbBr3 QDs solution is spin-coated on the ML WS2 flakes to form CsPbBr3/WS2 heterostructure. 405 nm laser can excite both the ML WS2 and CsPbBr3 and is used to irradiate the materials. Based on the good transport prosperities of the CsPbBr3, such as the long diffusion length and good ambipolar charge transport, the photogenerated electrons and holes in CsPbBr3 QDs are thought to reach the WS2 region through diffusion. Compared with isolated ML WS2, an enhancement about 12.7 times in the PL intensity of WS2 is observed in the CsPbBr3/WS2 heterostructure, suggesting a great portion of the photogenerated carriers (electrons and holes) is transferred from the CsPbBr3 QDs to ML WS2 with a ~40% energy transfer efficiency and ~2 × 108 s−1 energy transfer rate. We also find that the internal quantum efficiency of WS2 is increased from 6.35% to 29.01%.
2. Results and discussion
The ML WS2 were synthesized on clean sapphire (Al2O3) substrates via low-pressure chemical vapor deposition (LPCVD) reaction of WO3 and sulfur, according to the reference [20]. The scanning electron microscopy (SEM) image of the ML WS2 is shown in lighter color in Fig. 1(a). Raman spectra and atomic force microscopy (AFM) measurement were performed to characterized the thickness of the WS2. In Fig. 1(b), the Raman frequency difference of (in-plane vibration) and (out-of-plane vibration) is 61.39 cm−1, suggesting the monolayer nature of the synthesized WS2 flake [20, 21]. In the Fig. 1(c), AFM shows the subnanometer thickness, confirming the monolayer thickness. The PL spectra of ML WS2 for different 405 nm laser power is shown in Fig. 1(d), with the peak wavelength at ~616 nm. Later transient absorption (TA) measurement shows that its internal quantum efficiency is just 6.35% under 400 nm pulse laser irradiation.
Fig. 1 Characterization of ML WS2 and CsPbBr3. (a) SEM image of the ML WS2 in lighter color. (b) Typical Raman spectra of the ML WS2 under 457 nm laser. (c) AFM of the ML WS2 showing a height less than 1 nm. (d) PL spectra of ML WS2 flake and CsPbBr3 QDs (0.1 mg/ml) on sapphire plates for different incident power of 405 nm laser. (e) Statistical distribution of the length along the CsPbBr3 long edges. The average length of the long edges is about 11.07 nm. The inset picture is TEM image of the CsPbBr3 QDs. (f) Schematic of theoretically predicted band alignments of CsPbBr3/WS2 heterostructure and possible charge separation, transfer and recombination pathways. The gray dashed line represents the mid-gap states. The straight upward arrows (purple) represent the excitation by 405 nm laser. Those straight downward arrows (red) represent the radiative recombination. All the measurements were carried out at 295K under ambient atmospheric conditions.
CsPbBr3 QDs (in solvent of n-hexane) are synthesized as previously reported [19]. The lengths of the long edges of the CsPbBr3 QDs are summarized in Fig. 1(e), showing an average size of 11.07 nm. The inset picture is the TEM image of the CsPbBr3 QDs. PL spectra excited by 405 nm laser is centered at 511 nm (2.43 eV), with a full width at half-maximum (FWHM) of ~18.7nm (89 meV), as shown in Fig. 1(d).
To fabricate the CsPbBr3/WS2 type I heterostructure, 125 µL/cm2 of CsPbBr3 QDs solution (0.5 mg/ml in n-hexane, 600 rpm) was spin-coated on the ML WS2 flake. Figure 1(f) illustrates the band alignment of ML WS2 and CsPbBr3 QDs. As previously reported [17, 18, 22, 23], ML WS2 and CsPbBr3 QDs have bandgaps of 1.98 eV and 2.50 eV, respectively. And ML WS2’s conduction band minimum (CBM) is 490 meV lower than that of CsPbBr3 QDs. ML WS2’s valence band maximum (VBM) is 30 meV higher than that of CsPbBr3 QDs. Consequently, the heterostructure forms a type I heterostructure (neglecting the hybridization of electronic states in the WS2 layer and CsPbBr3 QDs) [24]. Under 405 nm laser excitation, electron-hole pairs are created in CsPbBr3 QDs and WS2. Carriers in the narrow-gap material, namely ML WS2, are prohibited from interlayer transfer due to their lower energies. Thus, electrons (holes) will transfer from CsPbBr3 QDs’ CBM (VBM) to WS2’s CBM (VBM) and mid-gap states quickly, which may enhance the PL intensity of WS2 enormously, and decrease the PL intensity of CsPbBr3 QDs.
AFM was employed to characterize the surface morphology of the CsPbBr3/WS2 heterostructure. According to the color bar in Fig. 2(a), the maximum fluctuation is about 12 nm, similar to the average size of the CsPbBr3 QDs. Raman spectra of CsPbBr3/WS2 heterostructure and ML WS2 are shown in Fig. 2(b), no obvious difference is observed. PL spectra of isolated WS2 and CsPbBr3/WS2 heterostructure for different 405 nm incident power are shown in Fig. 2(c). The isolated WS2 shows PL peaks at 2.01 eV, corresponding to its A-exciton resonance. As expected, PL signal of ML WS2 is enhanced nearly 12.7 times in the CsPbBr3/WS2 heterostructure. Besides, an obvious red-shift of the WS2 PL emission was observed. The red-shift could be attributed to the existence of mid-gap states in the bandgap of WS2, which needs further experiments and analysis to confirm whether it is caused by trapping states, trion states or other mechanisms. Compared to the isolated CsPbBr3 QDs in Fig. 1(d), it is obvious that the PL intensity of CsPbBr3 QDs in heterostructure is much weaker, even if its concentration (0.5 mg/ml) is higher than 0.1 mg/ml in Fig. 1(f). The decrease of CsPbBr3 QDs PL and the enhancement of WS2 PL suggest an efficient energy transfer process exists in the heterostructure [25], as the Fig. 1(f) shows.
Fig. 2 Characterization of the isolated ML WS2 and ML WS2 covered by CsPbBr3 QDs with a concentration of 0.5 mg/ml. (a) AFM image of the CsPbBr3/WS2 heterostructure. (b) Raman spectra of isolated ML WS2 and CsPbBr3/WS2 heterostructure under 457 nm CW laser. The laser can excite both the WS2 and CsPbBr3. (c) Typical PL spectra of an isolated ML WS2 and CsPbBr3/WS2 heterostructure under different incident powers of 405 nm laser. The magenta line represents the isolated WS2 PL signal after multiplying 5. All the measurements were carried out at 295K under ambient atmospheric conditions.
To obtain more detailed information about the energy transfer process, we performed time correlated single photon counting (TCSPC) measurements. The PL decay curves of isolated CsPbBr3 QDs and CsPbBr3 QDs in heterostructure for different excitation fluence are shown in Fig. 3. The PL intensity of the isolated CsPbBr3 QDs decays with a biexponential function. The lifetime of the exciton PL of the isolated CsPbBr3 QDs is 3.5 ns on average, shorter than ~4 ns as we previously reported [16, 26], which may be caused by the trapping states of defects induced during the spin-coating process. The PL lifetime of CsPbBr3 QDs decreases obviously when it is spin-coated on the surface of ML WS2, indicating decrease of the CsPbBr3 QDs PL by energy transfer to WS2, as previously reported [27]. The energy transfer efficiency can be given by Eq. (1). in which is the native 1/e lifetime of the donor QDs on sapphire (in the absence of WS2) and the 1/e lifetime of the QDs on top of the WS2 acceptor. We found that all of the energy transfer efficiencies under these four different incident fluence are close to 40%. The energy transfer rates are given by Eq. (2), and they are all close to 2 × 108 s−1.
Fig. 3 PL decay curves of isolated CsPbBr3 QDs and CsPbBr3 QDs in heterostructure under 405 nm pulse laser for different excitation fluence in air. The gray lines are the IRF of the system with a resolution (FWHM of IRF) about 300 ps.
Previous optical spectroscopy study [28] has shown that the PL lifetime in ML WS2 samples at room temperature is about 800 ps, which nearly has the same order with the FWHM of the instrument response function (IRF) in our TCSPC. Due to this limit, the PL decay curve of WS2 cannot be measured accurately by TCSPC. To further confirm there is an energy transfer process from QDs to WS2, femtosecond broadband TA spectroscopy is used to directly probe the ultrafast dynamics of isolated WS2 and WS2 in heterostructure. A 400 nm femtosecond pulse laser (≈100 fs, 1 kHZ) with a fluence of 160µJ/cm2 excites the isolated WS2, CsPbBr3 QDs and CsPbBr3/WS2 heterostructure, respectively, and the photoinduced changes in the absorption spectrum (∆A) is probed by a laser-generated supercontinuum light after controlled time delays. Typical results are shown in Fig. 4(a)-4(b).
Fig. 4 Transient absorption (TA) spectra of WS2, CsPbBr3 QDs and CsPbBr3/WS2 heterostructure upon excitation at 3.1 eV with a fluence of 160µJ cm−2 at 295 K. (a) and (b) 2D plots of TA from WS2 and CsPbBr3/WS2 heterostructure, respectively. (c) TA spectra for isolated WS2, CsPbBr3 QDs and CsPbBr3/WS2 heterostructure at 700 fs pump-probe delay. (d) Evolution of TA signals at the WS2 A-exciton resonance in isolated WS2 and CsPbBr3/WS2 heterostructure.
Figure 4(a)-4(b) present two two-dimensional (2D) plots of TA spectra of isolated WS2 and CsPbBr3/WS2 heterostructure, where the horizontal axis, vertical axis and color scale represent the pump-probe time delay, probe wavelength, and magnitude of changes in transmission spectrum (∆T/T) which is proportional to the change in absorption coefficient, respectively. The Fig. 4(a)-4(b) show prominent resonant features in TA spectra centered on 616 and 614 nm (gray horizontal dashed lines), respectively. The detailed TA spectra at 700 fs time delay is shown in Fig. 4(c). Both TA spectra of isolated WS2 and CsPbBr3/WS2 heterostructure are dominated by a low energy feature around 2.01 eV (616 nm), and a higher energy feature around at 2.39 eV (518 nm), which correspond to the A and B exciton of WS2, respectively [29]. The A and B peaks are well-studied excitonic states derived from the transition at the K point from the split valence band to the conduction band,due to spin-orbit coupling [30]. However, we did not observe the ∆A feature of CsPbBr3 QDs in the spectra of CsPbBr3/WS2 heterostructure, which needs further studies to explain.
The TA kinetics at the WS2 A-exciton resonance in Fig. 4(d) are fitted to a third-order multiexponential decay. The results are listed in Table 1. The time constants in Table 1 represent the recovery of the global ground state after the excitation. According to previous reports [31], only the final component t3 is related to radiative recombination, which has been reported to be from hundreds of picoseconds to nanoseconds at room temperature. Similar to the definition in reference [11], the ratio of radiative recombination here could also be defined as internal quantum efficiency.
Table 1. Fitting results of the TA kinetics resonance in isolated WS2 and CsPbBr3/WS2 at WS2 A-exciton.
For isolated WS2, observation of the t3 reveals that it accounts for less than 10% of the signal intensity, which matches well with the low-quantum yield of PL often observed from ML TMDCs [11,32,33]. However, the radiative recombination time constant t3 in CsPbBr3/WS2 heterostructure accounts for 29.01%. Compared to isolated WS2, it is increased by a factor about 4.6, which is attributed to the energy transfer from CsPbBr3 QDs to WS2.
In addition, in Fig. 4(c)-4(d), we can find that in the first few tens of picoseconds, the signal from CsPbBr3/WS2 heterostructure (black solid sphere) is larger than that from the isolated WS2 (red solid sphere) under the same condition. This is another evidence of energy transfer, since without such a process, the electrons in QDs would not contribute to the signal (according to the previous report, the electrons are more efficient in inducing a differential transmission signal than the holes in the same layer) [34].
3. Conclusion
In conclusion, we have shown that the PL intensity of WS2 is enhanced by a factor of 12.7 after spin-coated CsPbBr3 QDs (0.5 mg/ml) under 405 nm laser irradiation. This obvious enhancement of the ML WS2 PL intensity is attributed to the energy transfer mechanism from CsPbBr3 QDs to WS2. Besides, using TCSPC, we find the energy transfer efficiency is about 40%, and the energy transfer rate is about 2 × 108 s−1. With the TA measurement, we find that under such an experimental condition, the internal quantum efficiency of WS2 is increased from 6.35% to 29.01%, namely, the probability of the radiative recombination is increased. Our study suggests a simple but feasible method to improve the performance of the TMDC-based optoelectronic and photonics devices by fabricating type I heterostructure with lead halide perovskite QDs. In future work, a great effort is needed to improve the energy transfer efficiency and rate from lead halide perovskite QDs to ML TMDCs in the type I heterostructure, which may be useful to obtain stronger enhancement.
Funding
Scientific Researches Foundation of National University of Defense Technology (No. zk16-03-59).
References and links
1. X. Xu, W. Yao, D. Xiao, and T. F. Heinz, “Spin and pseudospins in layered transition metal dichalcogenides,” Nat. Phys. 10(5), 343–350 (2014). [CrossRef]
2. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides,” Nat. Nanotechnol. 7(11), 699–712 (2012). [CrossRef] [PubMed]
3. C. Ruppert, A. Chernikov, H. M. Hill, A. F. Rigosi, and T. F. Heinz, “The role of electronic and phononic excitation in the optical response of monolayer WS2 after ultrafast excitation,” Nano Lett. 17(2), 644–651 (2017). [CrossRef] [PubMed]
4. V. Vega-Mayoral, D. Vella, T. Borzda, M. Prijatelj, I. Tempra, E. A. A. Pogna, S. Dal Conte, P. Topolovsek, N. Vujicic, G. Cerullo, D. Mihailovic, and C. Gadermaier, “Exciton and charge carrier dynamics in few-layer WS2.,” Nanoscale 8(10), 5428–5434 (2016). [CrossRef] [PubMed]
5. W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P.-H. Tan, and G. Eda, “Evolution of electronic structure in atomically thin sheets of WS2 and WSe2.,” ACS Nano 7(1), 791–797 (2013). [CrossRef] [PubMed]
6. X. Fu, J. Qian, X. Qiao, P. Tan, and Z. Peng, “Nonlinear saturable absorption of vertically stood WS2 nanoplates,” Opt. Lett. 39(22), 6450–6453 (2014). [CrossRef] [PubMed]
7. T. Cao, G. Wang, W. Han, H. Ye, C. Zhu, J. Shi, Q. Niu, P. Tan, E. Wang, B. Liu, and J. Feng, “Valley-selective circular dichroism of monolayer molybdenum disulphide,” Nat. Commun. 3, 887 (2012). [CrossRef] [PubMed]
8. O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, “Ultrasensitive photodetectors based on monolayer MoS2.,” Nat. Nanotechnol. 8(7), 497–501 (2013). [CrossRef] [PubMed]
9. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, and M. Chhowalla, “Photoluminescence from Chemically Exfoliated MoS2.,” Nano Lett. 11(12), 5111–5116 (2011). [CrossRef] [PubMed]
10. A. Venkatakrishnan, H. Chua, P. Tan, Z. Hu, H. Liu, Y. Liu, A. Carvalho, J. Lu, and C. H. Sow, “Microsteganography on WS2 monolayers tailored by direct laser painting,” ACS Nano 11(1), 713–720 (2017). [CrossRef] [PubMed]
11. O. Salehzadeh, N. H. Tran, X. Liu, I. Shih, and Z. Mi, “Exciton kinetics, quantum efficiency, and efficiency droop of monolayer MoS2 light-emitting devices,” Nano Lett. 14(7), 4125–4130 (2014). [CrossRef] [PubMed]
12. K. C. J. Lee, Y.-H. Chen, H.-Y. Lin, C.-C. Cheng, P.-Y. Chen, T.-Y. Wu, M.-H. Shih, K.-H. Wei, L.-J. Li, and C.-W. Chang, “Plasmonic gold nanorods coverage influence on enhancement of the photoluminescence of two-dimensional MoS2 monolayer,” Sci. Rep. 5, 16374 (2015). [CrossRef] [PubMed]
13. Y. Li, Z. Qi, M. Liu, Y. Wang, X. Cheng, G. Zhang, and L. Sheng, “Photoluminescence of monolayer MoS2 on LaAlO3 and SrTiO3 substrates,” Nanoscale 6(24), 15248–15254 (2014). [CrossRef] [PubMed]
14. M. K. L. Man, S. Deckoff-Jones, A. Winchester, G. Shi, G. Gupta, A. D. Mohite, S. Kar, E. Kioupakis, S. Talapatra, and K. M. Dani, “Protecting the properties of monolayer MoS2 on silicon based substrates with an atomically thin buffer,” Sci. Rep. 6(1), 20890 (2016). [CrossRef] [PubMed]
15. M. Amani, P. Taheri, R. Addou, G. H. Ahn, D. Kiriya, D.-H. Lien, J. W. Ager 3rd, R. M. Wallace, and A. Javey, “Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides,” Nano Lett. 16(4), 2786–2791 (2016). [CrossRef] [PubMed]
16. K. Wei, Z. Xu, R. Chen, X. Zheng, X. Cheng, and T. Jiang, “Temperature-dependent excitonic photoluminescence excited by two-photon absorption in perovskite CsPbBr3 quantum dots,” Opt. Lett. 41(16), 3821–3824 (2016). [CrossRef] [PubMed]
17. K. Wu, G. Liang, Q. Shang, Y. Ren, D. Kong, and T. Lian, “Ultrafast interfacial electron and hole transfer from CsPbBr3 perovskite quantum dots,” J. Am. Chem. Soc. 137(40), 12792–12795 (2015). [CrossRef] [PubMed]
18. V. K. Ravi, G. B. Markad, and A. Nag, “Band edge energies and excitonic transition probabilities of Colloidal CsPbX3(X = Cl, Br, I) perovskite nanocrystals,” ACS Energy Lett. 1(4), 665–671 (2016). [CrossRef]
19. L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh, and M. V. Kovalenko, “Nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut,” Nano Lett. 15(6), 3692–3696 (2015). [CrossRef] [PubMed]
20. Y. Zhang, Y. Zhang, Q. Ji, J. Ju, H. Yuan, J. Shi, T. Gao, D. Ma, M. Liu, Y. Chen, X. Song, H. Y. Hwang, Y. Cui, and Z. Liu, “Controlled growth of high-quality monolayer WS2 layers on sapphire and imaging its grain boundary,” ACS Nano 7(10), 8963–8971 (2013). [CrossRef] [PubMed]
21. H. Zeng, G.-B. Liu, J. Dai, Y. Yan, B. Zhu, R. He, L. Xie, S. Xu, X. Chen, W. Yao, and X. Cui, “Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides,” Sci. Rep. 3, 1608 (2013). [CrossRef] [PubMed]
22. J. Kang, S. Tongay, J. Zhou, J. Li, and J. Wu, “Band offsets and heterostructures of two-dimensional semiconductors,” Appl. Phys. Lett. 102(1), 012111 (2013). [CrossRef]
23. C. Gong, H. Zhang, W. Wang, L. Colombo, R. M. Wallace, and K. Cho, “Band alignment of two-dimensional transition metal dichalcogenides: application in tunnel field effect transistors,” Appl. Phys. Lett. 103(5), 053513 (2013). [CrossRef]
24. M. Z. Bellus, M. Li, S. D. Lane, F. Ceballos, Q. Cui, X. C. Zeng, and H. Zhao, “Type-I van der Waals heterostructure formed by MoS2 and ReS2 monolayers,” Nanoscale Horiz. 2(1), 31–36 (2017). [CrossRef]
25. X. Hong, J. Kim, S.-F. Shi, Y. Zhang, C. Jin, Y. Sun, S. Tongay, J. Wu, Y. Zhang, and F. Wang, “Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures,” Nat. Nanotechnol. 9(9), 682–686 (2014). [CrossRef] [PubMed]
26. K. Wei, X. Zheng, X. Cheng, C. Shen, and T. Jiang, “Observation of ultrafast exciton–exciton annihilatioN in CsPbBr3 quantum dots,” Adv. Opt. Mater. 4(12), 1993–1997 (2016). [CrossRef]
27. F. Prins, A. J. Goodman, and W. A. Tisdale, “Reduced dielectric screening and enhanced energy transfer in single- and few-layer MoS2.,” Nano Lett. 14(11), 6087–6091 (2014). [CrossRef] [PubMed]
28. L. Yuan and L. Huang, “Exciton dynamics and annihilation in WS2 2D semiconductors,” Nanoscale 7(16), 7402–7408 (2015). [CrossRef] [PubMed]
29. C. Cong, J. Shang, X. Wu, B. Cao, N. Peimyoo, C. Qiu, L. Sun, and T. Yu, “Synthesis and optical properties of large-area single-crystalline 2D semiconductor WS2 monolayer from chemical vapor deposition,” Adv. Opt. Mater. 2(2), 131–136 (2014). [CrossRef]
30. G. L. Frey, R. Tenne, M. J. Matthews, M. S. Dresselhaus, and G. Dresselhaus, “Optical properties of MS2 (M = Mo, W) inorganic fullerenelike and nanotube material optical absorption and resonance Raman Measurements,” J. Mater. Res. 13(9), 2412–2417 (1998). [CrossRef]
31. S. H. Aleithan, M. Y. Livshits, S. Khadka, J. J. Rack, M. E. Kordesch, and E. Stinaff, “Broadband femtosecond transient absorption spectroscopy for a CVD MoS2 monolayer,” Phys. Rev. B 94(3), 035445 (2016). [CrossRef]
32. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS2: a new direct-gap semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010). [CrossRef] [PubMed]
33. M. Amani, D.-H. Lien, D. Kiriya, J. Xiao, A. Azcatl, J. Noh, S. R. Madhvapathy, R. Addou, S. Kc, M. Dubey, K. Cho, R. M. Wallace, S.-C. Lee, J.-H. He, J. W. Ager 3rd, X. Zhang, E. Yablonovitch, and A. Javey, “Near-unity photoluminescence quantum yield in MoS2,” Science 350(6264), 1065–1068 (2015). [CrossRef] [PubMed]
34. F. Ceballos, M. Z. Bellus, H.-Y. Chiu, and H. Zhao, “Ultrafast charge separation and indirect exciton formation in a MoS2-MoSe2 van der Waals heterostructure,” ACS Nano 8(12), 12717–12724 (2014). [CrossRef] [PubMed]
