Donor-Acceptor Type Reduced Graphene-Oxide and a Tin-Selenide Nanohybrid With Broad and Ultrafast Optical Limiting Properties

The non-linear absorption properties of reduced graphene-oxide (RGO) have been studied extensively but the optical limiting (OL) performance of RGO was always confined to visible light. In this study, by anchoring SnSe nanosheets onto the surface of RGO, the reduced graphene-oxide and a tin-selenide (SnSe/RGO) nanohybrid shows a broader reverse saturable absorption (RSA), ranging from 400 to 800 nm, and an enhanced non-linear optical (NLO) response. The improvement of the NLO absorption response is attributed to a multiphoton-absorption process and electron-transfer effect in this artificially constructed donor-acceptor system. Pump-probe experiments suggest a response time of ~1.7–8 ps for the SnSe/RGO nanohybrid.

Once the input-beam radiation exceeds a threshold, the optical limiting (OL) material strongly attenuates the intensity of the output beam. This ensures no damage is done to human eyes and avoids potential damage to optical sensors that are exposed to high energy lasers. The range of conventional OL materials is diverse, ranging from porphyrins, phthalocyanines [23], and mixed metal-complexes [24] to carbon-based nanomaterials such as fullerenes and carbon nanotubes [25]. Recently, the OL properties of emerging 2D materials were studied in detail for MoS 2 quantum dots [26], black phosphorus [27], and TiO 2 /RGO [28]. Carbon-based materials are a typically used representative 2D-system for optical limiters. Due to its unusual 2D Dirac-like band-structure, graphene exhibits zero bandgap properties. In contrast, RGO is a semiconductor with a tunable bandgap (0.5-6 eV) [29]. The non-linear absorption coefficient of RGO was determined as 2.67 cm/GW at a wavelength of 532 nm [30]. This suggests a considerable RSA response in the visible region. To endow RGO-based OL materials with enhanced OL properties, an RGO-related nanohybrid was built using several methods. These methods include: PEG-OPE-RGO hybrids [PEG: poly(ethylene glycol), OPE: oligo (phenylene ethynylene)] [31], and SWNT/RGO hybrids (SWNTs: single-walled carbon nanotubes) [32]. Apart from the expected high OL performance, broadband OL materials are needed for many OL applications. It would be highly beneficial if such materials could be obtained by constructing an RGO-based hybrid.
2D SnSe nanosheets are an ideal candidate for the construction of RGO-based hybrids that can extend their OL properties into the near infrared region. The SnSe exhibit a narrow indirect bandgap of 0.9 eV, which results in a broad absorption-spectrum (covering the near-infrared region). Moreover, it is stable under ambient conditions with low toxicity [33]. These properties make 2D SnSe interesting for use in many optoelectronic devices, such as memory switching [34], light-emitting devices [35], and solar cells [36]. In a previous report, the RSA properties in the near infrared band were confirmed and studied [37].
In this paper, we constructed a SnSe/RGO nanohybrid using a hydrothermal method, and we confirmed a broadband OL property of an as-prepared nanohybrid, ranging from 400 to 800 nm. The dominating OL mechanism of the as-prepared hybrid was found to be a multi-photon absorption process. Moreover, using pump-probe measurements, we confirmed the optical bleaching phenomena in the as-prepared nanohybrid and revealed a response-time on the scale of picoseconds. Further, the donor-acceptor model was proposed to get insight into the underlying mechanisms of the enhanced OL performance of the nanohybrid.

Characterization of SnSe/RGO Nanohybrid
The predecessors used the liquid-phase exfoliation method to produce 2D ultrathin structural materials, and proved that the crystal characteristics did not change during the stripping process [38,39]. In this article, we added a hydrothermal intercalation process to prepare SnSe nanosheets based on the liquidphase exfoliation method. Highly pure SnSe powder (99.5%) was purchased from Alfa Co. Inc. The 2D SnSe nanosheets were prepared via facile hydrothermal intercalation and liquid exfoliation [40,41]. Particularly, the 0.5 g of SnSe was put into 60 ml of ethylene glycol (EG) solution containing 1 g of LiOH and kept in 50-ml of Teflon-lined autoclave at 220 • C for 24 h. Then, the powder was collected by centrifugation and, following the sonication process, a few layers of SnSe nanosheets were obtained. After collecting by centrifugation and washing with water and ethanol, the desired few layers of SnSe nanosheets were obtained and dispersed in EG solution. 2D RGO was purchased from Aladdin Co. Ltd. SnSe/RGO nanocomposite was prepared using the hydrothermal method. The details can be found in our previous works [42].  It can be seen that some SnSe nanosheets (red arrow) are anchored on the surface of the RGO nanosheets. Figure 1B shows the energy dispersive x-ray (EDX) spectroscopy result for the as-prepared SnSe/RGO nanohybrid. The EDX of the as-prepared SnSe/RGO nanohybrid displays an Se peak at 1.39 keV and Sn peaks at 3.0 keV, which are characteristic EDX peaks for Se and Sn. The phase purity and crystal structure of the as-prepared SnSe/RGO nanohybrid were identified using X-ray diffraction (Bruker D8 Advance) (see Figure 1C). The peak at 2θ = 23.4 • is consistent with a lattice plane (002) of RGO [43]. The other diffraction peaks match the standard orthorhombic phase of SnSe well (JCPDS card no. 32-1382) [44]. Figure 1D shows the UV-vis absorption spectra (Cary60, Agilent) of the RGO nanosheets, the SnSe/RGO nanohybrid, and the SnSe nanosheets. Interestingly, after anchoring the SnSe nanosheets onto RGO, the SnSe/RGO hybrid shows broadband absorption covering both the visible and near infrared range. The enhancement of linear absorption increases the concentration of the photo-excited carrier, which may benefit charge transfer at the interface of the nanohybrid and induce the enhanced third-order non-linear optical response of the 2D nanohybrid.

Non-linear Optical Response
A femtosecond-pulse laser was used with a pulse width of 35 fs and pulse repetition rate of 2 kHz. Z-scan technology has higher sensitivity and stronger functionality [45]. We used the OA Z-scan technology to investigate the non-linear characteristics of materials. In the OA Z-scan measurement system, the incident femtosecond laser is focused by a lens, with a focal length of 150 mm. A cuvette, containing as-prepared samples, moves near the focus in the direction of the laser-light propagation (z-axis). Then, the transmitted intensity through the sample was recorded using a power meter.
Figures 2A-C shows the OA Z-scan results of SnSe, RGO, and SnSe/RGO at multiple wavelengths. The pristine SnSe shows the typical saturable absorption (SA) at 400 and 600 nm. When the excitation wavelength changes to near infrared (800 nm), the SnSe nanosheets exhibit a significant RSA response, which is similar to previous reports by Ye et al. [37]. Interestingly, for RGO, there is an RSA response with the excitation wavelength at 400 and 600 nm, which is consistent with previous studies [30]. The NLO absorption response exhibits an SA response, when the excitation wavelength is 800 nm. Such Pauli-blocking-induced optical bleaching saturable absorption has been reported previously [46]. As expected, as-prepared SnSe/RGO exhibits an RSA response that ranges from visible to the near infrared. A reasonable physical mechanism of broadened and enhanced RSA will be discussed, based on non-linear absorption parameters and the photoexcited carrier dynamic lifetime of material, in the following section.
To quantitatively analyze the OA Z-scan results, the NLO parameters of the mentioned materials were extracted by fitting the OA Z-scan curve. Based on the spatial transient Gaussian pulse model, the transmitted light intensity in the OA Z-scan experiment is [47].
where, z0 is the diffraction length of the beam, q 0 = α NL I 0 (t)L eff /(1 + z 2 /z 2 0 ), I 0 (t) is the intensity of the light at focus, α NL is the third order non-linear absorption coefficient, L eff = [1 − exp(−α 0 l)]/α 0 is known as the effective length of the sample, which is defined in terms of the linear absorption coefficient, α 0 , and the effective optical path length through the sample, l. For the OA Z-scan results in Figures 2A-C, we found that the non-linear absorption coefficient of SnSe/RGO is higher than in pristine SnSe for visible light. The α NL of SnSe/RGO were one order of magnitude larger than that of pristine RGO. For instance, there is an increase from 0.24 ± 0.01 cm/GW (RGO) to 2.71 ± 0.05 cm/GW (SnSe/RGO) at 400 nm. In addition, when the excitation wavelength changes to near infrared light (800 nm), the non-linear absorption coefficients were −5.30 ± 0.17 cm/GW, 11.61 ± 0.28 cm/GW, and 125.09 ± 2.73 cm/GW for pristine RGO, pristine SnSe, and SnSe/RGO, respectively. To determine the intrinsic non-linear absorption response, we extracted the imaginary part of the third-order non-linear optical susceptibility, Imχ (3) , and the figure of merit (FOM) [48]. All extracted NLO parameters are summarized in Table 1.
For a better quantitative comparison, the OL parameters of this series of materials were extracted, including the OL threshold and the onset fluence. The OL threshold is defined  [53], Au nanoparticle ∼7.5 J/cm 2 [54], etc. The lower limiting threshold indicates that nanomaterials show better optical activity and a more effective OL performance [55]. In terms of onset fluence, lower values were obtained than for pristine RGO or SnSe. Accordingly, SnSe/RGO is a potential OL candidate that may be used to shield sensor devices from high-intensity light.
To discover the dominate mechanism of such a broadened RSA performance, we performed further analyzation on the OA Z-scan results. In a conventional semiconductor system, the multi-photon absorption process usually dominates such a reverse saturable absorption. As shown in Figure 3D, the plot of Ln(1-T Norm ) vs. Ln(I) for different wavelengths confirmed multi-photon absorption induced RSA in the present system [47,56]. The slope values for SnSe/RGO are 1.42, 1.16, and 1.18, respectively, which suggests two photon and three photon absorption takes place during the laser excitation. Additionally, the pump-probe measurements were exploited to find out the timescale of the OL response and the mechanism of significantly enhanced OL for SnSe/RGO. To exclude the effect from the solvent bubble non-linear optical scattering effect [22], all the experiments were conducted under threshold value that the solvent bubble is generated.
As shown in Figures 4A-C, the pump-probe results, at multiple wavelengths, for RGO, SnSe, and SnSe/RGO are shown. Using SnSe/RGO as an example, we can observe a typical negative signal near time zero in the photo-excited carrier dynamic relaxation. The significant decrease in transmittance indicates a multi-photon-absorption-induced RSA response. Moreover, we extracted the lifetimes of the photo-excited carriers of RGO, SnSe, and SnSe/RGO at different wavelengths. For pristine RGO, the photo-excited lifetime ranges from 0.3 to 0.65 ps for different wavelengths. For pristine SnSe, on the other hand, the lifetime changed from several picoseconds (2.51 ps at 800 nm) to several tens of picoseconds (13.47 ps at 600 nm; 34.01 ps at 400 nm). As shown in Figure 4D, when we summarized the photoexcited carrier lifetimes, a desired median lifetime of SnSe/RGO was observed at three measurement wavelengths. This is strong evidence for electrontransfer in a donor-acceptor system. We believe that favorable energy-level alignment facilitates the above-mentioned electron transfer effect in the inset of Figure 4D [57,58]. In a conventional organic polymer molecule system, the charge transfer induced enhancement third-order non-linearity of the composite was observed [59]. Moreover, these electron transfers  [57] and RGO [58].
increase the non-linear absorption process, which has also been clearly observed in other similar donor-acceptor 2D material systems [27].

CONCLUSION
In conclusion, the non-linear absorption and carrier dynamics of SnSe/RGO were determined using an OA Z-scan and pumpprobe experiments at different wavelengths. The RSA response was obtained for the whole wavelength region, ranging from visible to near infrared. For pristine RGO this was done only for the visible range (400-600 nm). The broad RSA response of the RGO/SnSe nanohybrid can be attributed to a multi-photon absorption mechanism. The limit thresholds for the RGO/SnSe nanohybrid were 0.25 mJ/cm 2 , 0.6 mJ/cm 2 , and 0.055mJ/cm 2 at 400, 600, 800 nm, respectively. These are one order of magnitude below pristine RGO or SnSe, which suggests that SnSe/RGO has a stronger OL performance than pristine SnSe and RGO. The pump-probe measurements confirmed multi-photon absorption induced optical bleaching in the SnSe/RGO hybrid. The photoexcited carrier lifetime of the SnSe/RGO hybrid occurs on a picosecond timescale. We also compared the carrier lifetimes of the different samples, and the median lifetime of SnSe/RGO provides reasonable evidence for the electron transfer effect in the hybrid system, which contributes significantly to the improved OL performance of SnSe/RGO. Our results provide new opportunities to construct novel OL material systems.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.