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Abstract
In the present work, we report the structural properties of the two dimensional (2D) few-layered Ti3C2(OH)2/Ti3C2F2 hybrid MXene synthesized via the HF acid etching method. Various characterizations were exploited to demonstrate the 2D layered structural properties of the hybrid MXene membranes. The density functional theory (DFT) simulation indicated the hybrid MXene possessed the small enough band gap, which could benefit the nonlinear optical applications in the infrared region. By the conventional open-aperture Z-scan technique, typical nonlinear saturable features were measured. Consequently, the hybrid MXene membranes exhibited the excellent saturable absorption properties at 1 and 1.3 µm. As a saturable absorber, passively Q-switched Nd:YVO4 lasers with the prepared hybrid MXene membranes were realized at 1 and 1.3 µm, respectively, producing the stable Q-switching pulse train with a shortest duration of 130 ns.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) materials are widely used due to their unique electronic, mechanical and optical properties in the fields such as catalysis [1,2], energy storage and conversion [3–5], sensors [6–9], photonics and optoelectronic devices [10–14]. Especially, as the saturable absorber, 2D nanomaterials can be applied for the optical pulse generation [15–22]. Up to date, 2D materials are undergoing a fast development. As a new family member of 2D materials, transition metal carbides and carbonitrides MXenes have launched a new wave of research. Typically, MXenes are prepared via the exfoliation of MAX phases in HF acid solution. Here, M denotes the transition metal such as Ti, Sc, Nb elements, A presents the IIIA or IVA element, for instance, Al and X is C or N. The typical formula of MXene is Mn+1XnTx, where T denotes surface termination groups (-O, -OH, -F) [23]. In the past few years, MXenes have been considered as the excellent candidates as the electrode materials for the energy storage devices [24,25].
During the MXenes family, Ti3C2Tx is the most common MXene owing to the low cost, accessibility and wide applications in versatile fields [26–34]. Besides Ti3C2Tx MXene exhibits the excellent electric performance, it also possesses good nonlinear optical properties in the broadband from 1 - 2 µm, demonstrating the potential as the saturable absorber [35]. As the saturable absorber, mode-locking operation at 1066 nm was realized with the Yb-doped fiber laser [35]. With the bulk gain media, passively Q-switched lasers at 1 and 2 µm were reported with the minimum pulse duration of 359 ns [36] and 2.39 µs [37], which was relative broad. While for the mode-locking, a pulse duration of 316 fs was reported from a Yb-doped bulk laser [38]. Typically, the saturable absorber has an important impact on the laser performance. Therefore, improving the 2D materials saturable absorption features is in urgent need for the laser development.
In this paper, we synthesized Ti3C2(OH)2/Ti3C2F2 hybrid MXene as the saturable absorber in a wide spectral band for the short optical pulse generation. The structural characterization was comprehensively explored. Determined by the open aperture Z-scan technology, the Ti3C2(OH)2/Ti3C2F2 hybrid MXene membranes exhibited excellent nonlinear absorption properties with high modulation depth and relative low saturable absorber, which is helpful for the pulse generation as the saturable absorber in the Q-switching and mode-locking regimes. By optimizing the laser crystal and the laser resonator, a minimum pulse duration from the passive Q-switching operation was 130 ns. The maximum pulse energy of 2.45 µJ was obtained in 1.34 µm regime, leading to a peak power of 6.25 W. Our results showed the excellent saturation properties of the hybird MXene in the spectral bands from 1 to 1.34 µm.
2. MXene preparation and characterization
2.1 Preparation of few-layered MXene
Typically, 2D few-layered Ti3C2Tx MXene can be obtained from chemical etching of Ti3AlC2 MAX powder with a purity of 99%. The procedures were similar to the preparation reported in Ref. [35]. The raw Ti3AlC2 MAX powder (∼ 200 meshes) was mixed with the deionized (DI) water to make the 0.05 g/mL solution. Subsequently, the 1 mL DI solution was put into 15 mL 40 wt.% HF for etching Al element. The mixture was stirred at room temperature for 24 hours. Then the suspension was centrifuged at 4500 rpm for 20 minutes to obtain the 2D few-layered Ti3C2Tx MXene. During this step, DI water was added into the deposition to ensure the PH > 6 (∼ 6.5 in our case). As a consequence, the 2D few-layered Ti3C2Tx MXene was prepared. 20 µL Ti3C2Tx MXene dispersion solutions were transferred on the hydrophilic quartz substrate via the spin-coating method. The substrate was rotated at a speed of 1000 rpm in order to disperse the materials uniformly. The thickness of the thin-film was measured as 6.2 µm by a microscopy. Then the Ti3C2Tx hybrid MXene membranes were dried in an oven at 40 °C for 24 hours.
2.2 Characterization
During the etching process, Al element from the bulk MAX phase was replaced by at least three different surface termination groups (OH, O, and F), which can be utilized to change the electronic band structure and also, enrich variety of the MXene family. In fact, the MXene membranes with different terminations (OH-, F-) have the similar structure. Here, we selected the Ti3C2(OH)2 MXene as the structural model, which is shown in Fig. 1(a). The top view indicates that MXene belongs to the hexagonal system, which can be further described by the P63/mmc space group. From the side view, it is easily to see that MXene features an ABC stacking, where carbon atoms occupy the octahedral sites between two titanium atom layers. In fact, since the sample was fabricated in the solution circumstance, it is hard for us to purify the MXene. For surveying the surface morphology of the as-prepared few-layer Ti3C2Tx MXene, scanning electron microscope (SEM) measurements were operated, shown in Fig. 1(b) and Fig. 1(c). With different spatial resolution in both cases, the page-like layered structure can be clearly seen, indicating good etching effect. Furthermore, the atomic components and corresponding ratio of the as-prepared few-layer MXene were measured by the energy dispersive spectrometer (EDS), shown in Fig. 1(d). The percentage of Ti, C, O, F, and Al was 45.8%, 26.28%, 14.71%, 11.09% and 2.12% in our MXene sample, respectively. As a consequence, we can conclude that the as-prepared Ti3C2(OH)2/Ti3C2F2 hybrid MXene consists of roughly 56 at.% Ti3C2(OH)2 MXene and 44 at.% Ti3C2F2 MXene. However, we noticed the Ti and C ratio is not the stoichiometric (3:2). The possible reasons may be the followings: (1) The EDS cannot detect the H element, which may change the element ratio we obtained; (2) There are some impurities in our sample, such as Ti3Al, Ti2AlC and others [39], which also result in the slight ratio difference. The side-view high-resolution transmission electron microscopy (HRTEM) image (Fig. 1(e)) shows the delaminated MXene, in which the inter-layer distance is 0.98 nm. Then, the top-view HRTEM with a higher resolution was further carried out to present a hexagonal arrangement of the few-layered MXene, which is can be seen in Fig. 1(f). In addition, the corresponding selected area electron diffraction (SAED) image displayed a 6-fold symmetry feature also demonstrating the high crystal quality of the as-prepared few-layered MXene. Figure 2 shows the detailed element distribution.
Fig. 1. (a) Top and side views of the structural model of Ti3C2(OH)2 MXene. (b), (c) SEM images of MXene with different solution. (d) Element distribution of MXene. (e) Side-view HRTEM image. (f) Top view HRTEM image of MXene and SAED partten (inset).
The linear optical absorption properties of few-layer Ti3C2(OH)2/Ti3C2F2 hybrid MXene were measured by UV-VIS-IR absorption spectrometer. It is worth noting that the hybrid MXene showed the wide spectral absorption from visible to MIR regimes, making it possible to modulate the laser in terms of Q-switching and mode-locking. Meantime, we also measured the photoluminescence (PL) spectrum of the as-prepared hybrid MXene film. Under the excitation wavelength of 400 nm, as shown in Fig. 3(a), the PL spectrum is peaked at 485 nm with a FWHM of > 100 nm, demonstrating the nanostructures of the hybrid Ti3C2(OH)2/Ti3C2F2 MXene membranes. In addition, to ensure the layered structure, we utilized the Raman microscopy to investigate the structure and vibrational modes of the few-layer hybrid Ti3C2(OH)2/Ti3C2F2 MXene at room temperature with the excitation wavelength of 532 nm. The measurement result is shown in Fig. 3(b). Four active resonant Raman modes of A1g(2) (202 cm−1), Eg(5) (302 cm−1), Eg(7) (374 cm−1) and Eg(4) (617 cm−1) can be observed from 100 to 800 cm−1, indicating the layered nanostructure of the as-prepared Ti3C2(OH)2/Ti3C2F2 hybrid MXene.
Fig. 3. (a): Linear absorption spectrum (black) and Photoluminescence spectrum (blue). (b): Raman spectrum of few-layered Ti3C2Tx hybrid MXene.
2.3 First-principle simulation of band structures
The theoretical simulation was based on the density functional theory (DFT) with the projector-augmented wave (PAW) method as implemented in the Vienna ab initio simulation package (VASP). The exchange-correlation function was treated within the generalized gradient approximation (GGA) and parameterized using the Perdew-Burke-Ernzerhof (PBE) formula. A plane wave basis was set with a cutoff energy of 500 eV and a 5×5×1 Gamma-centered k-points grid. Exfoliation was modeled by first removing Al atoms from the Ti3AlC2 lattice. The outer Ti of the remaining Ti3C2 layers were linked with OH or F terminal groups, followed by full geometry optimization until all components of the residual forces became less than 0.01 eV / Å and the total energy convergence criterion was set as 10−6 eV. In addition, the minimum vacuum region between the slabs was set to 20 Å in the z-direction to avoid super-slab interactions and the periodic influence [18,40,41].
The band structures of the monolayer Ti3C2, Ti3C2F2 and Ti3C2(OH)2 are shown in Fig. 4. As displayed in Fig. 4, the monolayer Ti3C2 exhibits the semi-metallic properties, while the monolayer Ti3C2F2 and Ti3C2(OH)2 possess the little band gap. For the single-layered Ti3C2F2 MXene, the band gap was ∼ 20 meV, and with respect to the single layer Ti3C2(OH)2 MXene, the gap energy was approximately 0.1 eV, which coincided with the previous reports [42,43]. In fact, when the layers increased, the theoretical simulation showed that both few-layered Ti3C2F2 and Ti3C2(OH)2 MXene are semi-metallic, no matter which the terminal group is. The theoretical results implied that the Ti3C2F2 and Ti3C2(OH)2 hybrid MXene could be applied in the infrared region.
Fig. 4. Band structures of monolayer (a) Ti3C2, (b) Ti3C2F2 and (c) Ti3C2(OH)2. Fermi level was set to 0
2.4 NLO features of hybrid MXene
A conventional open-aperture Z-scan technology with the nanosecond excitation laser was employed to investigate the nonlinear absorption properties of the prepared few-layered 2D Ti3C2Tx hybrid MXene membranes. In the experiment, the balanced twin power measurement system was implemented to rule out the influence of the power fluctuations or the environmental perturbations, such as temperature, humidity or vibrations. The typical nonlinear transmission curves versus the position Z were shown in Fig. 5, which were fitted by the equation [19]:
Fig. 5. (a) Typical nonlinear transmission of the hybrid MXene versus position at 1 µm, (b) The nonlinear absorption properties at 1 µm, (c) Typical nonlinear transmission versus position at 1.34 µm, (d) The nonlinear absorption properties at 1.34 µm.
The modulation depth, the nonlinear saturation loss, the saturation intensity and the nonlinear absorption coefficient were obtained by fitting experimental data with the following formula [19]:
With the nanosecond pulse excitation at 1 µm, the modulation depth was 29.8% with the saturation intensity of 1.3 MW/cm2 and the nonsaturable loss of 12.5%, while the nonlinear absorption coefficient was −1.39 cm/MW. When excited with the nanosecond 1.34 µm pulses, the modulation depth was 26.2%, and the saturable intensity was 0.82 MW/cm2 with a large nonsaturable loss of 21%. The nonlinear absorption coefficient was −1.21 cm/MW at 1.34 µm band. Table 1 summarizes the previous investigations on the nonlinear absorption features of the typical 2D nanomaterials. As shown in Table 1, when compared with the recent developed 2D saturable absorbers (SAs), the modulation depth of the hybrid MXene is relative large, while the saturation intensity is quite low in the spectral band from 1 to 1.34 µm. Therefore, with Ti3C2Tx hybrid MXene membranes as the SA, the shorter pulse duration could be expected, even at the lower pump level.
Table 1. Parameters comparison between few-layered Ti3C2Tx hybrid MXene and other typical 2D materials.
3. Saturable absorption properties
In order to confirm the wideband saturable absorption properties of the as-prepared hybrid MXene sample, the passively Q-switched lasers were demonstrated with the few-layered MXene membranes as the saturable absorber at 1 and 1.34 µm. The schematic configuration of the diode-pumped passively Q-switched laser with MXene-SA is shown in Fig. 6. The pump source was a commercial fiber-coupled diode laser with a maximum output power of 40 W and the output wavelength of 808 nm. A simple plano-plano cavity was employed with the physical length of 20 mm to realize the passive Q-switching operation at 1.06 and 1.34 μm with the hybrid MXene. The laing crystal was a 0.3 at.% a-cut Nd:YVO4 with a dimension of 3×3×10 mm3. The laser crystal was wrapped with indium foil and mounted on water-cooled copper heat-sink maintained at 17 °C to efficiently remove the thermal load during the laser operation. In the experiment, the output pulse characteristics were detected by a fast InGaAs photodetector (EOT, ET-5000, USA) with a response time of 35 ps and monitored by a digital oscilloscope (1 GHz bandwidth, DPO 7104, Tektronix Inc., USA). The average output power was measured by a laser power, MAX 500AD (Coherent., USA).
3.1 Q-switching at 1 µm
In this case, the input mirror M1 was HT coated at 808 nm and HR coated at 1064 nm, while the output coupler M2 had a transmission of 4% at 1064 nm. The continuous wave (CW) Nd:YVO4 laser was investigated for comparison with the Q-switched operation performance in Fig. 7(a). With the increase of the pump power, the CW output power increased linearly with slope efficiencies of 32.4%. A maximum CW output power of 0.62 W was obtained at the pump power of 2.45 W. With the MXene inserted into the resonator, the passive Q-switching operation was achieved by aligning the cavity mirrors carefully. A maximum average output power of 0.3 W was obtained at the pump power of 2.45 W, corresponding to the slope efficiency of 15.9%. Figure 7(b) shows the dependence of pulse repetition rate and pulse duration versus incident pump power. The pulse duration decreased monotonically from 280 to 130 ns while the pulse repetition rate increased from 79 to 508 kHz. The augments of pulse energy and pulse peak power versus incident pump power are depicted in Fig. 7(c). Under the maximum pump power of 2.45W, pulses energy achieves 0.6 μJ, with the peak power achieving 4.35 W. The temporal pulse profile and the corresponding pulse train under the incident pump power of 2.45 W are shown in Fig. 7(d), demonstrating excellent amplitude stability.
Fig. 7. (a) Average CW output power (black dots) and Q-switching output power at 1 µm (red dots) versus the incident pump power; (b) Pulse repetition rate and Pulse width at 1064 nm versus the incident power; (c) Single pulse energy and Peak power at 1.06 µm versus the incident pump power; (d) Typical pulse profile with a duration of 130 ns and the corresponding pulse train (inset).
3.2 Q-switching at 1.34 µm
The input mirror M1 was HT coated at 808 nm and HR coated at 1340 nm, while the output coupler M2 had a transmission of 3.8% at 1340 nm. Similar, the CW operation at 1.34 µm was investigated first, and the CW power was depicted in Fig. 8(a). With the increase of the pump power, CW output power increased linearly with slope efficiencies of 33.8%. A maximum CW output power of 0.7 W was obtained at the pump power of 2.65 W. Subsequently, MXene saturable absorber was inserted into the resonator, and the passive Q-switching operation was achieved by aligning the cavity mirrors carefully. A maximum average output power of 0.48 W was obtained at the pump power of 2.65 W, corresponding to the slope efficiency of 26.1%. From Fig. 8(b), the pulse duration decreases monotonically from 476 to 390 ns while the pulse repetition rate augments from 71 to 195 kHz. Under the maximum pump power of 2.65W, pulses energy was achieved as 2.45 μJ with the peak power was 6.25 W, showing in Fig. 8(c). Figure 8(d) shows the typical temporal pulse profile and the pulse train.
Fig. 8. (a) Average CW output power (black dots) and Q-switching output power at 1.34 µm (red dots) versus the incident pump power; (b) Pulse repetition rate and Pulse width at 1340 nm versus the incident power; (c) Single pulse energy and Peak power at 1.34 µm versus the incident pump power; (d) Typical pulse profile with a duration of 390 ns and the corresponding pulse train (inset).
Table 2 summarized the laser performance of hybrid MXene and other 2D materials. It can be seen that the Q-switching laser with MXene as saturable absorber generated pulses with short pulse width. In addition, during the experiments, the material possessed the good characteristics. After being placed for many days, it can still work normally. In the laser experiment, after a long time of high power excitation, it can still maintain good modulation characteristics. The sample possesses good chemical and thermal stability. In summary, the as-prepared Ti3C2Tx hybrid MXene sample is a good saturable absorber for NIR region.
Table 2. Q-switching performance with few-layered Ti3C2Tx hybrid MXene and other typical 2D materials.
4. Conclusion
In conclusion, we synthesized the 2D few-layered Ti3C2Tx hybrid MXene by HF acid chemical etching and successfully transferred Ti3C2Tx hybrid MXene on the quartz substrate as the saturable absorber. The comprehensive investigation on the structural properties was exploited in this work. By using the Z-scan technology, excellent nonlinear absorption properties were proved in Ti3C2Tx hybrid MXene membranes, which could be applied in Q-switching and the mode-locking. In the actual use field, passively Q-switched lasers were realized by taking Ti3C2Tx hybrid MXene membranes as the saturable absorber at 1 and 1.34 µm, which shows the potential application in the broad spectral band. The shortest pulse duration we could obtain was 130 ns, while the largest output power was 300 mW at 1.06 µm. It can convince us that the well-designed MXene saturable absorber could be excellent saturable absorber for the 100-nanosecond pulses generation in the near-infrared band.
Funding
National Natural Science Foundation of China (12004213, 21872084, 61575109); Fundamental Research Fund of Shandong University (2018TB044).
Acknowledgments
H. C. would like to thank the financial support from the Young Scholar Program of Shandong University.
Disclosures
The authors declare no conflicts of interest.
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