Saturable and reverse saturable absorption in molybdenum disulfide dispersion and film by defect engineering Download: 772次
1. INTRODUCTION
Unavoidable defects in the growth process of two-dimensional (2D) materials can dramatically affect the performance of the corresponding device. Defect-associated nonradiative recombination will lead to low quantum efficiency () [1], and structure defects will result in lower carrier mobility [2]. Defects can also be a source of merit; for instance, photoluminescence enhancement induced by sulfur (S) vacancy [3] and oxygen bonding at the defect sites [4] in monolayer molybdenum disulfide () and catalytic efficiency in hydrogen evolution reaction improved by S vacancies [5] at edge defects [6]. However, the contribution of these defects to the nonlinear optical (NLO) properties, such as saturable absorption (SA) and reverse saturable absorption (RSA), is not completely clear so far. Recently, different defect types, including effective two-photon absorption (TPA) from zinc (Zn) vacancy [7], tunable NLO absorption derived from the concentration of manganese doping [8], and enhanced modulation depth [9] from localized defect states at grain boundaries, have been introduced to improve the performance of nonlinear photonic devices. Actually, these defects act as different trap centers to capture electrons [10], holes [11], or excitons [12,13], which make it crucial to confirm the type of defect-induced localized states in 2D materials.
is a typical 2D material to design linear optoelectronic devices due to its excellent optical and electronic properties, including high current on/off ratio (), high electron mobility () [14], strong catalytic activity [15], and photoluminescence [16,17]. Importantly, a size-dependent few-layer can be achieved easily using ultrasonic exfoliation [18] combined with the gradient centrifugation method [19]. On the other hand, nonlinear photonic applications including -switching [20], mode-locking [21,22], and an optical limiter [23] have also been designed based on SA and RSA behavior. For example, and graphene were reported with stronger SA [24] than those of and , while BN exhibited a strong RSA effect [25]. SA and RSA can be interpreted as a transmission increasing and decreasing according to pump intensity, respectively. It is determined by a smaller absorption cross section in excited state compared with that in ground state for SA [20,26]. On the other hand, RSA occurs due to larger absorption cross section in excited state compared with that in ground state. Including SA and RSA, some defects can be introduced to improve optical and electronic properties in material. For instance, the defect level of bulk can be tuned from 1.08 to 0.08 eV [27] by S vacancies. The S vacancies can easily transform into an n-type conductor, whereas the Mo vacancies can be used to transform into a p-type conductor [28]. Particularly, these atomic defects are easily formed in small nanosheets compared to those in large nanosheets [29]. Interestingly, the changeover from SA to RSA has been observed in nanosheet dispersions with size using a picosecond laser [30], which is caused by certain edge defect. However, the defect type-size relationship is yet to be identified as well as its related mechanisms in NLO properties. In addition, the reported nonlinear responses are based on liquid environments, and thus it is hard to avoid thermally induced nonlinear scattering [31]. A femtosecond laser source can avoid this condition due to the slow formation of bubbles from the ultrashort pulse laser [32]. It is also easy to solve the problem and design nonlinear optics-based devices by preparing films [33].
In this work, we first present an effective method to introduce S vacancies in by adjusting the nanosheet size. Different nanosheet sizes are obtained through liquid-phase exfoliation by controlling the centrifugation speed, and films with different sizes are fabricated by the vacuum filtration technique. The successful size-separation is identified by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The size-dependent S vacancies are confirmed by Raman spectroscopy and high-resolution X-ray photoelectron spectroscopy (XPS). The influence of S vacancy concentration on NLO properties is studied by a typical Z-scan setup at 800 nm. The changeover from SA to RSA is observed both in films and dispersions. Combined with the S vacancy defect state, which is confirmed by first principle calculations, an energy-level model is carried out based on S vacancy concentration. SA and RSA are strongly related to ground state, excited state, and defect state absorption. This work provides a method to tune nonlinear photonic devices by controlling the defects.
2. RESULTS AND DISCUSSION
2.1 A. Methods: Nanosheet Dispersion and Film Preparation
A green ethanol-based solvent exfoliation process was used to prepare few-layer nanosheets [34]. Initially, we chose 40% ethanol mixed with deionized water for 0.9 g in a 300 mL mixed solution. We settled the parameters with 700 W for 3 h using the supersonic machine (Qsonica Q700) to prepare the dispersion. After that, the nanosheet supernatant was acquired with a centrifuge (Centurion Scientific K241) by controlling the rotation speeds with 2000 rpm, 4000 rpm, 6000 rpm, and 8000 rpm for 15 min to remove large nanoparticles. To obtain a larger nanosheet, ultrasonic exfoliation with 70 W for 3 h was used and then let the acquired dispersion stand two weeks with 0 rpm. Subsequently, we can prepare the films with almost the same thickness by controlling filtration volume using the vacuum filtration technique.
2.2 B. Characterization of Dispersions and Films
The nanosheet was successfully exfoliated from optical images of dispersion and presented in Fig.
Fig. 1. (a) Concentrations of dispersion at different rotation speeds with related optical images inserted. (b) Tauc plot of dispersions. (c) Size and height of dispersions at different rotation speeds. (d), (g) AFM image and height profile of at 2000 rpm. (e), (h) AFM image and height profile of at 4000 rpm. (f), (i) AFM image and height profile of at 6000 rpm. The number indicates the different nanosheet.
To further confirm the successful size-separation of , the dispersions were dropped respectively on glass substrate and measured by AFM. Figure
The nanosheet size was also confirmed using TEM as shown in Fig.
Fig. 2. Representative TEM images of dispersions at (a) 0, (b) 2000, (c) 4000, (d) 6000, and (e) 8000 rpm. (f) High-resolution TEM image of nanosheet at 6000 rpm. (g) TEM image of ultrathin layer of defect at 8000 rpm; elemental mapping of (h) Mo (i) S.
XPS was further applied to confirm the S vacancies by analyzing the S:Mo ratio of film in Fig.
Fig. 3. (a) High-resolution XPS spectra of films. (b) XRD patterns of films. (c) Raman spectra of films. (d) Raman shift of and . (e) UV-Vis absorption spectra of films. (f) Tauc plot obtained from UV-Vis absorption spectra of films with an increase in centrifugation speed.
Further XRD experiments are presented in Fig.
2.3 C. Size-Dependent Nonlinear Absorption of Dispersions and Films
In our previous reports, an open aperture Z-scan measurement at the wavelength of 800 nm with a pulse duration of 35 fs and repetition frequency of 1 kHz was equipped. The waist radius was about 30 μm measured by a knife edge method. It was performed to study the NLO property of transition metal dichalcogenides film [34,42]. There is no evident signal from the solution (ethanol) in quartz cuvette at a pulse energy of 0.5 μJ. SA occurs in dispersions at 0 rpm according to the dependence of transmittance peak on pulse energy as presented in Fig.
Fig. 4. Open-aperture Z-scan results of dispersions (a) at 0 rpm with different pulse energies, (b) at 6000 rpm with different pulse energies, and (c) at different rotation speeds. Z-scan results of films (d) at 8000 rpm and different pulse energies, at different rotation speeds (e) with 45 nm thickness and (f) 75 nm thickness.
After fitting the Z-scan curves, the value of all dispersions is roughly as the nanosheets are multilayer with almost the same thickness [30]. This value is on the same scale as the reported value () [45]. Similarly, the of nanosheet dispersion is at the same condition [24]. Meanwhile, is strongly dependent on the nanosheet size, as presented in Fig.
Fig. 5. (a) Calculated results of (red dot) and (blue square), respectively. (b) and FOM from dispersions in red dot and blue square, respectively. (c) Measured (red dot) and (blue square). (d) and FOM from films with different sizes, which are shown using red dot and blue square, respectively.
In order to evaluate the performance of the NLO devices, the linear absorption needs to be eliminated. The calculated values are presented in Fig.
By the definition of figure of merit (FOM) for third-order nonlinear absorption, , , and FOM values of nanosheet dispersions are presented in Fig.
2.4 D. Mechanism of Size-Dependent Nonlinear Absorption
For , the appearance of SA can be mainly attributed to band-to-band absorption because the nanosheet bandgap is below the excited photon energy. The excited electrons directly move from the valence band (VB) to the conduction band (CB). Meanwhile, the RSA phenomenon is commonly associated with the aforementioned nonlinear scattering, excited state absorption, and TPA. In our experiments, the nonlinear scattering can be ignored [32] due to the 35-fs laser pulse duration and nanojoule-level pulse energy, which will not induce microbubbles from the thermal effect. The excited state absorption process is associated with the triplet state in organic dyes. However, the triplet state can be neglected because the relaxation time is much longer than the pulse width [34,49]. In addition, the TPA lifetime is very short (in picosecond/femtosecond scale), which is consistent with the ultrashort pulse laser utilized in this work. In essence, TPA mainly occurs when the excited photon energy is lower than the bandgap. In this study, the defect state of S vacancies is introduced for over 2000 rpm. The S vacancies state can act as a stable state and absorb an excited electron. Therefore, the S vacancy state absorption will take part in the optical process, which could also explain the RSA phenomena at high concentration of S vacancies. In other words, the competition between excited and defect state absorption in dispersions and films can be mediated by the concentration of S vacancies.
To confirm the mechanism behind the size-dependent NLO properties of dispersions and films, the band structure and density of states (DOS) of are presented in Fig.
Fig. 6. Band structure and DOS results for monolayer : (a) perfect; (b) with monosulfur vacancy; and (c) with disulfur vacancy.
Fig. 7. (a) Three-energy-level model of few or few-defect . (b) Three-energy-level model combined with defect state of S vacancies in . (c) Three-defect-energy-level diagram at a high concentration of S vacancies in . Fitting results of (d) dispersions at different rotation speeds, films (e) with 45 nm thickness and (f) with 75 nm thickness.
Three-energy-level model (Model 1) in Fig.
Nonlinear propagation equations were defined as a function of pulse energy [22,56] as
Rate equations and nonlinear propagation based on Models 1–3 can be expressed as shown below.
Model 1 is as follows: Model 2 is as follows: Model 3 is as follows:
The dispersion at 0 rpm exhibits a defect-free sample. The films with different sizes and a thickness of 75 nm show SA, which can be described by Model 1. is vital in assessing SA and RSA. RSA occurs if the absorption cross-section of excited state is larger than that of ground state, which results in optical limiting effects [57]. Otherwise, SA occurs [20,26]. Fitting results of dispersions and films, by the Runge–Kutta algorithm [43] based on Model 1, are presented in Figs.
Table 1. Parameters of Dispersions and Films
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3. CONCLUSIONS
In summary, size-dependent S vacancies are successfully introduced in dispersions and films. The size-dependent S vacancies are also confirmed by TEM, Raman spectroscopy, and XPS. The transition from SA to RSA was observed with the increase of S vacancies. Based on the concentration of S vacancies, an energy-level model was developed to understand the dynamic process. The results show that SA and RSA phenomena can be explained by the competition between defect state, excited state, and ground state absorption. To conclude this work, it paves a new way to tune nonlinear photonic devices by defect engineering.
Article Outline
Chunhui Lu, Hongwen Xuan, Yixuan Zhou, Xinlong Xu, Qiyi Zhao, Jintao Bai. Saturable and reverse saturable absorption in molybdenum disulfide dispersion and film by defect engineering[J]. Photonics Research, 2020, 8(9): 09001512.