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Abstract
We fabricated few layer MoSe2 nanosheets via the simple and low-cost ultrasonic-assisted liquid phase exfoliation (UALPE) method. Both the Raman spectrum and the TEM topography confirmed the few-layer structure of the fabricated MoSe2 nanosheets. The modulation depth of the MoSe2 nanosheets was 10.8% and the saturation intensity was 51 kW/cm2. Using the density functional theory (DFT), we investigated the electronics and optical properties, especially the absorption spectrum. As an application, a passively Q-switched laser was realized based on the MoSe2 nanosheets as the saturable absorber. The stable pulse train was produced with an output power of 175 mW. The minimum pulse width we obtained was 217 ns while the pulse energy was 0.36 µJ.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, low dimensional quantum systems are in hot spot for the novel physical phenomena, such as quantum Hall effect [1], quantum anomolous Hall effect [2] and Dirac electrons [3]. The great interest in these nanoscale materials has also been intensified owing to the discovery of the thin layered transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) [4–7] and other 2D nanomaterials [8–11]. Similar with the graphene, 2D TMDs have the hexagonal structure with 2 common structural phases of trigonal prismatic (2H) or octahedral (1 T) coordination of the metal atoms [12]. 2D TMDs can also be easily fabricated with the methods of chemical vapor deposition (CVD) [13–15], pulsed laser deposition (PLD) [16,17], molecular beam epitaxy (MBE) [18] and liquid phase exfoliation (LPE) [19–22]. Among these methods, LPE has attracted much attention due to its simple and low cost features. Ultrasonic assisted LPE (UALPE) could be more attractive since it can easily improve the nano-materials’ physicochemical properties, especially the optically uniform [23–25].
Different with the graphene, TMDs have a relative large bandgap which make TMDs more attractive for the applications as catalysis [26], phototransistors [27] and optoelectronic devices [28]. Among the TMDs family, we put an eye on MoSe2 since the previous studies indicate that selenides take advantages over the sulfides owing to the narrower bandgap, smaller linewidth and tunable excitonic charging effects [14,29–30]. MoSe2 has a bandgap varied from 1.1 eV (bulk material) to ∼ 1.58 eV (monolayer structure) [18]. Up to date, MoSe2 has exhibited excellent chemical, physical and optical properties in the realistic applications [31–33]. Furthermore, MoSe2 shows the nonlinear absorption in the ultrabroad spectrum bands from near-infrared (NIR) to mid-infrared (MIR) [22,34,35]. Some nonlinear optical (NLO) properties have been reported under 1.5 µm (photon energy: ∼ 0.8 eV) laser excitation [21,22]. However, the excitation energy previously reported was much lower than the energy bandgap of few-layer MoSe2 materials. Recently, MoSe2 dispersions are characterized by both open-aperture (OA) and closed-aperture (CA) Z-scan techniques with ultrafast lasers operating at 532–1064 nm, showing excellent NLO properties [33]. However, the investigation on the NLO properties of MoSe2 nanosheets is still in need, because the NLO properties normally vary from the interlayered structures, excitation wavelengths and pulse durations.
In this paper, we fabricated and characterised the few layered MoSe2 nanosheets. The NLO properties were measured under the nanosecond 1 µm laser via the open-aperture Z-scan technique. The modulation depth and saturation intensity of the MoSe2 nanosheets were calculated to be 10.8% and 51 kW/cm2, respectively. In the meantime, we also employed the density functional theory to investigate the electronics and optical properties of the prepared MoSe2 nanosheets. As an application, a passively Q-switch Nd-doped laser at 1 µm with MoSe2 nanosheets as the saturable absorber was demonstrated. The stable pulse train with a pulse duration of 217 ns at a repetition rate of 487 kHz was achieved. Our measurements revealed that the layered MoSe2 nanosheets have excellent NLO properties.
2. Fabrication and transfer
A simple and efficient ultrasound-assisted liquid-phase exfoliation (UALPE) method is used to prepare the MoSe2 nanosheets. The bulk MoSe2 powder was firstly mixed in ethanol and sonicated for 20 h at a sonication power of 100 W. After that, these dispersions were centrifuged at 1500 rpm for 20 min to remove the unexfoliated MoSe2 and large-sized powders. Finally we pipetted the top two-thirds of the dispersions into a new container. The abovementioned procedures were repeated several times in order to get more uniform MoSe2 nanosheets.
Indeed, the MoSe2 nanosheets on the substrate would be much more convenient for the realistic applications. To prepare a thin film on the substrate, a drop of the ultrasonized suspension was directly dripped on the surface of the K9 glass plate (2 cm × 2 cm × 0.5 mm). Here K9 glass worked as the substrate. Subsequently, the substrate was rotated at a low speed of 1000 rpm to uniformly disperse the nanosheets. Finally, the substrate was put into an oven in which the temperature was kept at 80 °C for two hours to evaporate the solution. As a consequence, the layered MoSe2 nanosheets were transferred on the K9 glass substrate for the characterizations. The schematic preparation of MoSe2 nanosheets is shown in Fig. 1.
3. Characterization
3.1 Raman spectrum
The Raman spectrum of the MoSe2 nanosheets was measured by a HR-800 Raman spectrometer (LabRam, Horiba Inc.) excited at 488 nm. Raman spectrum at the wavenumber regions of 200-300 cm−1 was exhibited in Fig. 2. The two distinct peaks observed in the Raman spectrum corresponded to the A1g and E2g vibrational modes of the MoSe2 nanosheets, respectively. A1g and E2g peaks located at 240.6 cm−1 and 282.6 cm−1, respectively. While peaks of these modes in the bulk MoSe2 material are 242 cm−1 and 286 cm−1, respectively. Obviously, the peak of A1g mode was red-shifted from bulk to few-layer geometries, consistent with the previous studies [36,37]. The reason for the shift is the Davydov splitting, coming from the multi MoSe2 molecules occurring in one unit cell [37]. What’s more, the vibrational modes A1g and E2g were much broader that those in the bulk MoSe2, also demonstrating the few layered structure of the MoSe2 nanosheets we fabricated. In the meantime, we also noticed that the A1g peak intensity was much stronger than the E2g peaks intensity. As a consequence, we can confirm that the MoSe2 nanosheets transferred on K9 substrate have the few layer structure.
3.2 TEM measurement
The typical TEM images of the MoSe2 nanosheets were shown in Fig. 3, which indicates that the flake size ranges from 0.05 to 1 µm and the MoSe2 nanoflakes are composed of ultrathin nanosheets. The layer number of the nanoflakes was determined from the high-resolution transmission electron microscopy (HRTEM) image of the folded edge and shown in Fig. 3(c). From the HRTEM image, it can be distinctly noticed that the lattice fringe of few-layer MoSe2 was 0.66 nm, identical to the theoretical layer distance [38]. The above results have confirmed that few-layer MoSe2 nanosheets were successfully obtained through UALPE technique.
Fig. 3. TEM images of MoSe2 nanosheets with different resolutions: (a): 200 nm, (b): 50 nm and (c) 2 nm. And (d): the thickness distribution of the nanosheets.
3.3 Absorption properties
Linear absorption. The linear absorption of MoSe2 nanosheets was measured with a PE lambda 750S spectrophotometer (PerkinElmer Inc.). The two absorption peaks located at A (∼690 nm) and B (∼810 nm), corresponding to the excitions from the two spin-orbit split transitions at the K point of the Brillouin zone [34]. The corresponding bandgap was ∼1.51 eV.
Nonlinear absorption. We note that the photon energy of 1064 nm was low than that of 1.51 eV (∼810 nm). The UV-VIS-IR linear absorption has proven that our MoSe2 nanosheets possessed the absorption at 1064 nm. The reason was the edge-effect of the nanoflakes. The previous studies demonstrated that owing the edge effect, the nonlinear absorption properties could be also expected [34]. In our case, the nonlinear transmittance of the MoSe2 nanosheets was determined by the open aperture Z-scan method. The pump source was a nanosecond laser emitting the central wavelength at 1064 nm with pulse duration of 4 ns, and repetition rate of 10 Hz. The relative transmission versus the relative position can be seen in Fig. 5. The transmission peaked at the symmetric center (Z = 0) which is a typical phenomenon in the OA Z-scan. In consideration of the beam size, we could estimate the incident intensity at the corresponding position. As shown in Fig. 5(b), the nonlinear transmission of MoSe2 film on K9 substrate increases with the incident optical intensity. The experimental data can be fitted by the following equation:
Fig. 5. Relative transmission versus the relative position. Insert: Nonlinear transmission versus the incident intensity.
4. Simulation
4.1 Calculation details
First principles plane-wave calculations based on the density functional theory (DFT) were performed using the Vienna ab initio simulation package (VASP) code [39]. The exchange-correlation potential was approximated by the generalized gradient approximation (GGA), in our case, we considered the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functionals [40]. Before the electronic and the optical properties calculation, all structure was geometrically relaxed until the unbalanced force components converge below 0.01 eV/Å. The energy cutoff was 500 eV and the electronic ground state convergence condition was 10−6 eV for all systems. The Brillouin zone was sampled by 15×15×1 Gamma-Centered grid point. In the present calculations, the slab-supercell approach is adopted, and a large vacuum slab of more than 15 Å that separates the neighboring slabs is added in the direction perpendicular to the atomic planes. The periodic boundary condition was employed along the MoSe2 layer. Moreover, in order to simplify the simulation, we only considered the monolayer and bilayer MoSe2 nanosheets which can represent the direct and in-direct bandgaps.
4.2 Electronic properties
In fact, the electronic properties of the monolayer MoSe2 have been investigated via the first principles in the previous works [41,42]. The lattice constant a of MoSe2 was 3.319 Å. Herein, in order to study the electronic properties of monolayer and bilayer of MoSe2, we investigated the density of states (DOS) and the band structures. The partial density of states (PDOS) showed that the Mo 4d and Se 4p orbitals played a decisive role in the properties of MoSe2. As shown in Fig. 6, the bandgap of the monolayer MoSe2 is direct with the gap energy of 1.393 eV (KV-KC). However, the bandgap of bilayer MoSe2 became indirect with a gap energy of 0.865 eV (ΓV-KC).
4.3 Optical properties
Having the real and imaginary parts of the dielectric function $\epsilon (\omega )= {\epsilon _1}(\omega )+ \textrm{i}{\epsilon _2}(\omega )$, based on the Kramers-Kronig relations, one can obtain the absorption coefficient and the loss spectrum. The real part ${\epsilon _1}(\omega )$ represents the response of the materials to the incident photon, while the imaginary part ${\epsilon _2}(\omega )$ corresponds to the energy absorbed by the materials. In our case, only the direct bandgap was taken into account for the sake of simplicity, ruling out the influence of the phonon. Due to the geometric symmetry in the two directions of x and y (along the MoSe2 plane), we studied the optical properties along the x-direction (along the plane) and the z-direction (perpendicular the plane). Figure 7(a) shows the imaginary part of the dielectric function in x- and z-directions for the monolayer MoSe2 and Fig. 7(b) shows the real part, respectively.
The absorption coefficient of the monolayer MoSe2 is depicted in Fig. 8. The optical absorption spectrum was directly connected to the imaginary part of the dielectric function. The first critical point of absorption was near 900 nm, which was close to the calculated band gap 1.393 eV (890 nm). This confirms the direct optical transitions between the KV and KC. Please note in the simulation curve in Fig. 8, there are two peaks at 654 nm and 750 nm, presenting the similar absorption properties when compared to the linear optical absorption measurement shown in Fig. 4.
5. Application for Q-switching
As an application, in this section we demonstrated a passively Q-switched laser based on the abovementioned MoSe2 nanosheets. The laser resonator was a simple linear cavity, shown in Fig. 9. The pump source was a fiber-coupled diode laser (FAP system, Coherent inc., USA) emitting the wavelength at 808 nm. The diamter of the coupled fiber was 400 µm and the numerical aperture (NA) was 0.22. Through an optical refocus module, the incident pump light was focused into the laser crystal with a beam radius of 200 µm. The resonator was 50 mm long. M1 was a plane mirror with anti-reflection coating at 808 nm and high-reflection coating at 1064 nm. The output mirror M2 was also a flat mirror and with a transmittance of 6.8% at 1 µm. The 10 mm long c-cut 0.1 at.% Nd:GdVO4 crystal was used as the gain medium. To reduce the thermal load, the laser crystal was held by a copper block and kept at a temperature of 10 °C. A laser power meter (MAX 500 AD, Coherent, USA) was used to record the output power of the laser.
A digital oscilloscope (1 GHz bandwidth, Tektronix DPO 7104C, USA) was used to record the laser pulses. As shown in Fig. 10, the average output power augments almost linearly with the increase of input power. The maximum output power was 175 mW at the incident pump power of 4.02 W. In our case, the slope efficiency was about 9.3%. The reason may come from the followings: (1). The gain crystal was only polished, which would lead to some extra losses, resulting in the low output power; (2). The output coupler had a large transmission of 6.8%, which also led to the low net gain in the laser resonator; (3). The large nonsaturation losses of 13% and the low pump level would also make the output power very low. We did not further increase the incident pump power to prevent the possible damage and the instability of the MoSe2 nanosheets. At the maximum pump level, the operation wavelength was 1063.2 nm with a FWHM of 0.5 nm. The dependence of pulse duration and repetition rate on the input power were presented in Fig. 10. With the enhancement of input power from 2.26 W to 4.02 W, the pulse duration decreased from 505 ns to 217 ns, while the repetition rate increased from 105 kHz to 487 kHz. When the incident pump power was 4.02 W, the maximum peak power and pulse energy were 1.65 W and 0.36 µJ, respectively. The typical and stable Q-switching operation with MoSe2 nanosheets as the saturable absorber was illustrated in Fig. 11. According to Fig. 11, the timing jitter RMS we estimated was 0.1 ns while the amplitude RMS was 4.6%.
Fig. 10. (a) Output power, (b) the operation wavelength and (c) the pulse repetition rate and pulse width versus incident pump power.
6. Conclusions
In conclusion, we fabricated and characterized home-made few layer MoSe2 nanosheets. The thickness of the prepared MoSe2 nanosheets was estimated as ∼ 4 nm, corresponding to 5-6 layers. The optical and electronic properties of MoSe2 were performed by a commercial VASP code based on the density function theory. The modulation depth and the saturation intensity of the MoSe2 nanosheets were determined by the open-aperture Z-scan technique as 10.8% and 51 kW/cm2, respectively. The nonsaturable loss was as high as 13%. These parameters would lead to a short pulse duration even at a low pump level in passive Q-switching. In order to confirm the nonlinear optical properties, we demonstrated a passively Q-switched laser based on the home-made MoSe2 nanosheets as the saturable absorber. The shortest pulse duration was 217.5 ns and the highest pulse energy was 0.36 µJ.
Funding
National Natural Science Foundation of China (NSFC) (21872084, 61575109); Shandong University (2018TB044).
References
1. Y. Zhang, Y. Tan, H. Stormer, and P. Kim, “Experimental observation of the quantum Hall effect and Berry's phase in graphene,” Nature 438(7065), 201–204 (2005). [CrossRef]
2. C. Chang, J. Zhang, X. Feng, J. Shen, Z. Zhang, M. Guo, K. Li, Y. Ou, P. Wei, L. Wang, Z. Ji, Y. Feng, S. Ji, X. Chen, J. Jia, X. Dai, Z. Fang, S. Zhang, K. He, Y. Wang, L. Lu, X. Ma, and Q. Xue, “Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator,” Science 340(6129), 167–170 (2013). [CrossRef]
3. A. Geim, “Graphene: Status and Prospects,” Science 324(5934), 1530–1534 (2009). [CrossRef]
4. P. Joensen, F. Frindt, and S. Morrison, “Single-layer MoS2,” Mater. Res. Bull. 21(4), 457–461 (1986). [CrossRef]
5. Y. Peng, Z. Meng, Z. Chang, J. Lu, W. Yu, Y. Jia, and Y. Qian, “Hydrothermal Synthesis and Characterization of Single-Molecular-Layer MoS2 and MoSe2,” Chem. Lett. 30(8), 772–773 (2001). [CrossRef]
6. T. Lei, W. Chen, J. Huang, C. Yan, H. Sun, C. Wang, W. Zhang, Y. Li, and J. Xiong, “Multi-Functional Layered WS2 Nanosheets for Enhancing the Performance of Lithium–Sulfur Batteries,” Adv. Energy Mater. 7(4), 1601843 (2017). [CrossRef]
7. A. Allain and A. Kis, “Electron and Hole Mobilities in Single-Layer WSe2,” ACS Nano 8(7), 7180–7185 (2014). [CrossRef]
8. Y. Chen, C. Zhao, S. Chen, J. Du, P. Tang, G. Jiang, H. Zhan, S. Wen, and D. Tang, “Large Energy, Wavelength Widely Tunable, Topological Insulator Q-Switched Erbium-Doped Fiber Laser,” IEEE J. Sel. Top. Quantum Electron. 20(5), 315–322 (2014). [CrossRef]
9. Y. Wang, W. Huang, C. Wang, J. Guo, F. Zhang, Y. Song, Y. Ge, L. We, J. Liu, J. Li, and H. Zhang, “An All-Optical, Actively Q-Switched Fiber Laser by an Antimonene-Based Optical Modulator,” Laser Photonics Rev. 13(4), 1800313 (2019). [CrossRef]
10. J. Li, H. Luo, B. Zhai, R. Lu, Z. Guo, H. Zhang, and Y. Liu, “Black phosphorus: a two-dimension saturable absorption material for mid-infrared Q-switched and mode-locked fiber lasers,” Sci. Rep. 6(1), 30361 (2016). [CrossRef]
11. Y. Wang, F. Zhang, X. Tang, X. Chen, Y. Chen, W. Huang, Z. Liang, L. We, Y. Ge, Y. Song, J. Liu, D. Zhang, J. Li, and H. Zhang, “All-Optical Phosphorene Phase Modulator with Enhanced Stability Under Ambient Conditions,” Laser Photonics Rev. 12(6), 1800016 (2018). [CrossRef]
12. S. Manzeli, D. Ovchinnikov, D. Pasquier, O. Yazyer, and A. Kis, “2D transition metal dichalcogenides,” Nat. Rev. Mater. 2(8), 17033 (2017). [CrossRef]
13. Y. Xie, Z. Wang, Y. Zhan, P. Zhang, R. Wu, T. Jiang, S. Wu, H. Wang, Y. Zhao, T. Nan, and X. Ma, “Controllable growth of monolayer MoS2 by chemical vapor deposition via close MoO2 precursor for electrical and optical applications,” Nanotechnology 28(8), 084001 (2017). [CrossRef]
14. X. Wang, Y. Gong, G. Shi, W. Chow, K. Keyshar, G. Ye, R. Vajtai, J. Lou, Z. Liu, E. Ringe, and B. Tay, “Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe2,” ACS Nano 8(5), 5125–5131 (2014). [CrossRef]
15. N. Perea-Lopez, Z. Lin, N. Pradhan, A. Iniguez-Rabago, A. Elias, A. McCreary, J. Lou, P. Ajayan, H. Terrones, L. Balicas, and M. Terrones, “CVD-grown monolayered MoS2 as an effective photosensor operating at low-voltage,” 2D Mater. 1(1), 011004 (2014). [CrossRef]
16. M. Serna, S. Yoo, S. Moreno, Y. Xi, J. Oviedo, H. Choi, H. Alshareef, M. Kim, M. Minary-Jolandan, and M. Quevedo-Lopez, “Large-Area Deposition of MoS2 by Pulsed Laser Deposition with In Situ Thickness Control,” ACS Nano 10(6), 6054–6061 (2016). [CrossRef]
17. F. Ullah, T. Nguyen, C. Le, and Y. Kim, “Pulsed laser deposition assisted grown continuous monolayer MoSe2,” CrystEngComm 18(37), 6992–6996 (2016). [CrossRef]
18. Y. Zhang, T. Chang, B. Zhou, Y. Cui, H. Yan, Z. Liu, F. Schmitt, J. Lee, R. Moore, Y. Chen, and H. Lin, “Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2,” Nat. Nanotechnol. 9(2), 111–115 (2014). [CrossRef]
19. M. Zhang, R. Howe, R. Woodward, E. Kelleher, F. Torrisi, G. Hu, S. Popov, J. Taylor, and T. Hasan, “Solution processed MoS2-PVA composite for sub-bandgap mode-locking of a wideband tunable ultrafast Er:fiber laser,” Nano Res. 8(5), 1522–1534 (2015). [CrossRef]
20. R. Woodward, R. Howe, G. Hu, F. Torrisi, M. Zhang, T. Hasan, and E. Kelleher, “Few-layer MoS2 saturable absorbers for short-pulse laser technology: current status and future perspectives,” Photonics Res. 3(2), A30–A42 (2015). [CrossRef]
21. Z. Luo, Y. Li, M. Zhong, Y. Huang, X. Wan, J. Peng, and J. Weng, “Nonlinear optical absorption of few-layer molybdenum diselenide (MoSe2) for passively mode-locked soliton fiber laser,” Photonics Res. 3(3), A79–A86 (2015). [CrossRef]
22. W. Liu, M. Liu, H. Han, S. Fang, H. Teng, M. Lei, and Z. Wei, “Nonlinear optical properties of WSe2 and MoSe2 films and their applications in passively Q-switched erbium doped fiber lasers,” Photonics Res. 6(10), C15–21 (2018). [CrossRef]
23. T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P. Tan, A. Rozhin, and A. Ferrari, “Nanotube-polymer composites for ultrafast photonics,” Adv. Mater. 21(38-39), 3874–3899 (2009). [CrossRef]
24. M. Zhang, G. Hu, G. Hu, R. Howe, L. Chen, Z. Zheng, and T. Hasan, “Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber,” Sci. Rep. 5(1), 17482 (2015). [CrossRef]
25. M. Zhang, Q. Wu, F. Zhang, L. Chen, X. Jin, Y. Hu, Z. Zheng, and H. Zhang, “2D Black Phosphorus Saturable Absorbers for Ultrafast Photonics,” Adv. Opt. Mater. 7(1), 1800224 (2019). [CrossRef]
26. A. Laursen, S. Kegnaes, S. Dahl, and I. Chorkendorff, “Molybdenum sulfides—efficient and viable materials for electro-and photoelectrocatalytic hydrogen evolution,” Energy Environ. Sci. 5(2), 5577–5591 (2012). [CrossRef]
27. Y. Yoon, K. Ganapathi, and S. Salahuddin, “How Good Can Monolayer MoS2 Transistors Be?” Nano Lett. 11(9), 3768–3773 (2011). [CrossRef]
28. H. Zeng, J. Dai, W. Yao, D. Xiao, and X. Cui, “Valley polarization in MoS2 monolayers by optical pumping,” Nat. Nanotechnol. 7(8), 490–493 (2012). [CrossRef]
29. J. Ross, S. Wu, H. Yu, N. Ghimire, A. Jones, G. Aivazian, J. Yan, D. Mandrus, D. Xiao, and W. Yao, “Electrical control of neutral and charged excitons in a monolayer semiconductor,” Nat. Commun. 4(1), 1474 (2013). [CrossRef]
30. S. Tongay, J. Zhou, C. Ataca, K. Lo, T. Matthews, J. Li, J. Grossman, and J. Wu, “Thermally Driven Crossover from Indirect toward Direct Bandgap in 2D Semiconductors: MoSe2 versus MoS2,” Nano Lett. 12(11), 5576–5580 (2012). [CrossRef]
31. C. Tsai, K. Chan, F. Abild-Pedersen, and J. Norskov, “Active edge sites in MoSe2 and WSe2 catalysts for the hydrogen evolution reaction: a density functional study,” Phys. Chem. Chem. Phys. 16(26), 13156–13164 (2014). [CrossRef]
32. S. Larentis, B. Fallahazad, and E. Tutuc, “Field-effect transistors and intrinsic mobility in ultra-thin MoSe2 layers,” Appl. Phys. Lett. 101(22), 223104 (2012). [CrossRef]
33. K. Wang, Y. Feng, C. Chang, J. Zhan, C. Wang, Q. Zhao, J. Coleman, L. Zhang, W. Blau, and J. Wang, “Broadband ultrafast nonlinear absorption and nonlinear refraction of layered molybdenum dichalcogenide semiconductors,” Nanoscale 6(18), 10530–10535 (2014). [CrossRef]
34. R. Woodward, R. Howe, T. Runcorn, G. Hu, F. Torrisi, E. Kelleher, and T. Hasan, “Wideband saturable absorption in few-layer molybdenum diselenide (MoSe2) for Q-switching Yb-, Er- and Tm-doped fiber lasers,” Opt. Express 23(15), 20051–20061 (2015). [CrossRef]
35. Z. Yan, G. Li, T. Li, S. Zhao, K. Yang, S. Zhang, M. Fan, L. Guo, and B. Zhang, “Passively Q-switched Ho, Pr: LiLuF4 laser at 2.95 µm using MoSe2,” IEEE Photonics J. 9(5), 1–7 (2017). [CrossRef]
36. P. Tonndorf, R. Schmidt, P. Boettger, X. Zhang, J. Boerner, A. Liegig, M. Albrecht, C. Kloc, O. Gordan, D. Zahn, S. de Vasconcellos, and R. Bratschitsch, “Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2,” Opt. Express 21(4), 4908–4916 (2013). [CrossRef]
37. T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura, and E. Matsuura, “Raman scattering and infrared reflectance in 2H-MoSe2,” J. Phys. Soc. Jpn. 49(3), 1069–1077 (1980). [CrossRef]
38. J. Yang, J. Zhu, J. Xu, C. Zhang, and T. Liu, “MoSe2 Nanosheet Array with Layered MoS2 Heterostructures for Superior Hydrogen Evolution and Lithium Storage Performance,” ACS Appl. Mater. Interfaces 9(51), 44550–44559 (2017). [CrossRef]
39. G. Kresse and J. Furthmueller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set,” Phys. Rev. B 54(16), 11169–11186 (1996). [CrossRef]
40. J. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett. 77(18), 3865–3868 (1996). [CrossRef]
41. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M = Mo, Nb, W, Ta; X = S, Se, Te) monolayers,” Phys. B 406(11), 2254–2260 (2011). [CrossRef]
42. Y. Ma, Y. Dai, M. Guo, C. Niu, J. Lu, and B. Huang, “Electronic and magnetic properties of perfect, vacancy-doped, and nonmetal adsorbed MoSe2, MoTe2 and WS2 monolayers,” Phys. Chem. Chem. Phys. 13(34), 15546–15553 (2011). [CrossRef]
