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Preparation of Few-Layer Bismuth Selenide by Liquid-Phase-Exfoliation and Its Optical Absorption Properties

Bismuth selenide (Bi2Se3), a new topological insulator, has attracted much attention in recent years owing to its relatively simple band structure and large bulk band gap. Compared to bulk, few-layer Bi2Se3 is recently considered as a highly promising material. Here, we use a liquid-phase exfoliation method to prepare few-layer Bi2Se3 in N-methyl-2-pyrrolidone or chitosan acetic solution. The resulted few-layer Bi2Se3 dispersion demonstrates an interesting absorption in the visible light region, which is different from bulk Bi2Se3 without any absorption in this region. The absorption spectrum of few-layer Bi2Se3 depends on its size and layer number. At the same time, the nonlinear and saturable absorption of few-layer Bi2Se3 thin film in near infrared is also characterized well and further exploited to generate laser pulses by a passive Q-switching technique. Stable Q-switched operation is achieved with a lower pump threshold of 9.3 mW at 974 nm, pulse energy of 39.8 nJ and a wide range of pulse-repetition-rate from 6.2 to 40.1 kHz. Therefore, the few-layer Bi2Se3 may excite a potential applications in laser photonics and optoelectronic devices.

finding, medicine, laser processing, remote sensing and telecommunications 30 . In the field of pulsed lasers, passive Q-switched fiber laser for generating short and large-energy laser pulse is one of most effective ways because of their significant advantages of compactness, simplicity, and flexibility in design 31 . The key element in the passive Q-switched fiber laser is an excellently saturable absorber. Therefore, researchers have never stopped to seek for new saturable absorbers (e.g. semiconductor 32 , carbon nanotubes 33 , graphene 33,34 ). Compared with bulk materials 28,29 , one can expect that the few-layer nanomaterials would possess the more excellent performance of saturable absorption, and could be a potentially saturable absorber. Therefore, we are strongly motivated to develop the pulsed fiber lasers Qswitched with few-layer Bi 2 Se 3 as the saturable absorber.
Here, N-methyl-2-pyrrolidone (NMP), the more promising organic solvent to exfoliate 2D layered materials 24 , is used to exfoliate bulk Bi 2 Se 3 for producing few-layer Bi 2 Se 3 ( Supplementary Fig. S1). Another is chitosan acetic solution (CS-HAc), which possesses the low-toxic, good-biocompatible and environmentally friendly properties 35 . Meanwhile, we also investigated the optical absorption characterization of as-prepared few-layer Bi 2 Se 3 dispersed in solvents in visible light region, and saturable-absorption performance of fewlayer Bi 2 Se 3 thin film in near infrared region. At last, few-layer Bi 2 Se 3 was successfully used as the new fiber-compatibly saturable absorber to attain passive Q-switched fiber laser at 1.53 mm wavelength.

Results
Preparation and exfoliation of bulk Bi 2 Se 3 . Bulk Bi 2 Se 3 was prepared by hydrothermal synthesis and characterized by X-ray diffraction (XRD, Supplementary Fig. S2a). All the labeled peaks can be readily indexed to rhombohedral Bi 2 Se 3 (JCPDS no. 89-2008). The scanning electron microscope (SEM) image in Fig. 1b and transmission electron microscope (TEM) image in Fig. 1c show that the as-synthesized bulk Bi 2 Se 3 exhibits sheet-like structure with a wide size distribution, and is easily to aggregate together.
The thickness of as-synthesized bulk Bi 2 Se 3 is about 40-100 nm determined by atomic force microscopy (AFM) (Supplementary Fig. S2c and d).
We further exfoliated as-synthesized bulk Bi 2 Se 3 with solutionphase exfoliation method, which is similar to the exfoliation of graphite in NMP and CS-HAc 35 . The as-synthesized Bi 2 Se 3 powders were insoluble in two solvents before sonication (Fig. 1d). After sonication of 30 h, the colors of two solutions were deepened, which means that the exfoliated Bi 2 Se 3 had been dispersed in these solvents. We also investigated the exfoliation of as-synthesized bulk Bi 2 Se 3 in other solvents ( Supplementary Fig. S3). The result shows that NMP and CS-HAc are the optimal solvents to exfoliate as-synthesized bulk Bi 2 Se 3 . Therefore, NMP and CS-HAc are selected to investigate the exfoliation of bulk Bi 2 Se 3 . We further investigated the effect of sonication time on exfoliation of bulk Bi 2 Se 3 ( Supplementary Fig. S4). With increasing ultrasonic time, the color of CS-HAc was deepened, but color is already deep dark in NMP at 2 h, which reveals a better exfoliating effect in NMP. Longer ultrasonic time should produce higher concentration of few-layer Bi 2 Se 3 . However, it needs more power and time. Therefore, we chose 30 h as the appropriately ultrasonic time because Bi 2 Se 3 has already been well dispersed in these two solvents, meeting the requirement of following experiments in this study. The exfoliated Bi 2 Se 3 also presented the Tyndall effect of the colloidal suspension ( Supplementary Fig. S5). The result shows that the colloidal suspension of exfoliated Bi 2 Se 3 in the two solvents is stable.
Characterization of few-layer Bi 2 Se 3 . The TEM image (Fig. 2a) of exfoliated Bi 2 Se 3 showed that the as-obtained few-layer Bi 2 Se 3 was extremely thin 2D flake. According to the selected area electron diffraction (SAED) pattern (Fig. 2b), it could be indexed as a 6fold symmetry [001] zone axis pattern, which is consistent with the layered structure along the z axis. Also, it revealed the singlecrystalline nature of the thin 2D flake. Furthermore, the distance  between the adjacent hexagonal lattice fringes investigated by the high-resolution TEM (HRTEM) is 0.213 nm for Bi 2 Se 3 (Fig. 2c), which is consistent with the lattice space of the (110) plane. The AFM image (Fig. 2d) also shows that the exfoliated Bi 2 Se 3 is a flake structure and its thickness is about 3-4 nm (Fig. 2e), which nearly equals to 4 layers of Bi 2 Se 3 17 . The XRD pattern (Fig. 2f) of few-layer Bi 2 Se 3 showed a high [006] orientation and some characteristic peaks disappeared compared to bulk Bi 2 Se 3 , which indicates that bulk Bi 2 Se 3 had been successfully exfoliated as we expected. At the same time, the bulk Bi 2 Se 3 has successfully been exfoliated to few-layer Bi 2 Se 3 in CS-HAc ( Supplementary Fig. S6). Besides, Raman spectrum was also used to further confirm the exfoliation of Bi 2 Se 3 ( Supplementary Fig. S7). The A mode of few-layer Bi 2 Se 3 produced a red shift compared to that of bulk Bi 2 Se 3 , which could be attributed to the phonon softening 36,37 . Therefore, we successfully prepared fewlayer Bi 2 Se 3 using the solution-phase exfoliation method.
Optical absorption characterization of few-layer Bi 2 Se 3 . The optical absorption properties of few-layer Bi 2 Se 3 were firstly investigated with ultraviolet-visible (UV-vis) spectra. Interestingly, we found that the dispersion solutions of few-layer Bi 2 Se 3 produced a broad absorption in the visible light region compared to as-synthesized bulk Bi 2 Se 3 (Fig. 3a). The UV-vis spectrum of as-synthesized bulk Bi 2 Se 3 showed a nearly straight line without any absorption peak in the visible light region. However, few-layer Bi 2 Se 3 displayed an absorption band at about 552 nm in CS-HAc and 574 nm in NMP, respectively. The appearance of absorption band after exfoliation is remarkable, which might be due to the exfoliation of bulk Bi 2 Se 3 into nanosheets with a few nanometers thickness. The absorption also increases gradually as the sonication time extended (Fig. 3b and c), which reveals that more few-layer Bi 2 Se 3 would be obtained with increasing sonication times. The result further suggests that the absorption would be resulted from few-layer Bi 2 Se 3 .
We further investigate the effect of size and thickness on absorption property. After sonication in NMP, few-layer Bi 2 Se 3 was separated in different centrifugal speeds (Fig. 4a). The size distribution and corresponding height profile of few-layer Bi 2 Se 3 collected at three centrifugation speeds were distinguishing. With centrifugal speed increasing, the size of few-layer Bi 2 Se 3 was decreased from 500 to 100 nm and the thickness was also decreased from 10 to 2 nm, and the maximal absorption wavelength was blue-shifted from 613 to 459 nm (Fig. 4b). The similar result is also obtained in CS-HAc (Supplementary Fig. S8 and S9). The result further suggests that the broad absorption in the visible light region would be resulted from few-layer Bi 2 Se 3 but not from bulk Bi 2 Se 3 .
To further investigate the optical absorption properties of fewlayer Bi 2 Se 3 , we used a spin-coating method to prepare Bi 2 Se 3 / NMP (few-layer Bi 2 Se 3 exfoliated in NMP) and Bi 2 Se 3 /CS-HAc (few-layer Bi 2 Se 3 exfoliated in CS-HAc) thin films on quartz plate, respectively. As shown in Fig. 5a, we measured the linear absorption spectra of the two films by a spectrophotometer scanning from 300 to 2000 nm. One can clearly see that both of the films have the relatively flat transmission curves in the UV-to-near infrared (NIR) region, e.g. the transmittance of Bi 2 Se 3 /NMP varies only from 0.67 to 0.84 in the broad wavelength range of 350 , 2000 nm. It indicates that the fewlayer Bi 2 Se 3 would be a promising broadband optical material. In order to compare the nonlinear absorption of our few-layer Bi 2 Se 3 with that of bulk Bi 2 Se 3 (.50 layers) previously reported 38,39 , we also used the same Z-scan technique to measure the nonlinear transmission responses of the two few-layer Bi 2 Se 3 films. When the two samples were strongly excited by a femtosecond Ti: sapphire laser with the highest optical intensity of 2.6 GW/cm 2 ( Fig. 5b and c), the open-aperture Z-scan transmission curves of Bi 2 Se 3 /NMP and Bi 2 Se 3 /CS-HAc were obtained, respectively. One can obviously see that the two samples possess the saturable absorption, i.e. the optical transmittance is different under differently optical intensity. The modulation depths (dT) are 3.8% for Bi 2 Se 3 /NMP and 3.7% for Bi 2 Se 3 /CS-HAc, respectively, which is comparable to that of graphene 40,41 . Furthermore, by carefully fitting the curves in Fig. 5b and 5c, the produced saturable intensities (I sa ) are 53 MW/cm 2 for Bi 2 Se 3 /NMP and 41 MW/cm 2 for Bi 2 Se 3 /CS-HAc, respectively. It is very interesting that the I sa values are much less than that of bulk Bi 2 Se 3 reported previously 38,39 , mainly benefiting from the few-layer structure of exfoliated Bi 2 Se 3 . In the field of passive Q-switched or mode-locked lasers, the lower I sa of saturable absorber, the easier the start of Q-switching/mode-locking operation is, implying that fewlayer Bi 2 Se 3 might be very helpful for developing the low-threshold Q-switched/mode-locked lasers.
Generation of Q-switched laser pulses using the saturable absorption of few-layer Bi 2 Se 3 . As well as known, the optically saturable absorption can be used to efficiently generate the laser pulses by the passive Q-switching or mode-locking techniques 42,43 . The lower I sa of few-layer Bi 2 Se 3 may be very helpful for developing the lowthreshold Q-switched/mode-locked lasers. In this section, to testify the performance of few-layer Bi 2 Se 3 , we will exploit the saturable absorption of few-layer Bi 2 Se 3 to passive Q-switch erbium-doped fiber laser (EDFL) for generating laser pulses. Supplementary Fig.  S10 shows the experimental setup of Q-switched EDFL using fewlayer Bi 2 Se 3 as a saturable absorber. In order to clearly evaluate the significance of few-layer Bi 2 Se 3 to Q-switching operation, we purposely performed the following control experiments. At first, when as-synthesized bulk Bi 2 Se 3 was deliberately inserted into the laser cavity, we found that the Q-switching operation at 1530.2 nm was extremely unstable with a large pulse-intensity and repetition-rate fluctuation (see the Supplementary Fig. S11 for more details). Moreover, the Q-switching operation has a high pump threshold of 22.1 mW, a broad pulse duration of 22.8 ms and a small operating range of pump power (22.1 , 67.5 mW). In contrast, a very stable Q-switching operation was produced when the few-layer Bi 2 Se 3 was placed in the laser cavity to replace as-synthesized bulk Bi 2 Se 3 as followed.
As increasing the pump power, we found that the laser with fewlayer Bi 2 Se 3 as saturable absorber reached its threshold at the pump power of 9.3 mW only, and the stable Q-switching operation occurred simultaneously. The pump threshold for Q-switching is much lower than that of as-synthesized bulk Bi 2 Se 3 (22.1 mW), and other saturable absorber-based pulsed EDFLs reported previously 29,38,39 , mainly benefiting from the lower I sa of few-layer   Fig. 6 summarizes the output characteristics of the Qswitched pulses. Fig. 6a and 6b give the typical oscilloscope trace of the Q-switched pulse trains and the single pulse envelope at the pump power of 87.2 mW, respectively. The Q-switching pulse output with the repetition rate of 23.8 kHz was stable, and no significant pulse jitter was observed on the oscilloscope. The measured single-pulse envelope shows the good symmetry and has the pulse duration of 5.4 ms. The typical laser spectrum of the Q-switching operation depicted in Fig. 6c has the central wavelength of 1530.3 nm with the 10-dB bandwidth of 2.2 nm. As usually observed in Q-switched fiber lasers 44 , the sideband structure appeared in the optical spectrum was due to the multimode oscillation and the cavity perturbations of Q-switching 44 . As shown in Fig. 6d, we also measured the RF output spectrum of Q-switching pulses at the same pump power of 87.2 mW. The pulse repetition rate is 23.8 kHz.
The RF signal-to-noise ratio is more than 50 dB, and the 20-dB RF linewidth is less than 10 Hz (limited by the RF resolution bandwidth of 10 Hz), further indicating the good stability of the Q-switching operation. Moreover, the stability of the Q-switching is excellent in our testing period of 4 h, and the stable Q-switching is available in the large range of pump power (9.3 , 150.1 mW), which is superior to that of bulk Bi 2 Se 3 (22.1 , 67.5 mW). Fig. 6e plots the pulse repetition rate and the pulse energy as a function of the pump power. As increasing the pump power from 9.3 to 150.1 mW, one can see that: 1) the pulse repetition rate linearly increases from 6.2 to 40.1 kHz; and 2) the pulse energy monotonically increases in the lower pump power, but slightly saturates after exceeding the pump power of 100 mW. The maximum pulse energy obtained in our experiment is 39.8 nJ, corresponding to the average output power of 1.6 mW at the pump power of 150.1 mW. In addition, we also recorded the evolution of pulse duration in different pump powers. As shown in Fig. 6f, the pulse duration can be significantly narrowed from 24.0 to 4.9 ms with the increase of pump power. The pulse duration might be further reduced by shortening the cavity length and optimizing the cavity loss 45 .

Discussion
In this work, we attempted to exfoliate as-synthesized bulk Bi 2 Se 3 for preparing few-layer Bi 2 Se 3 by liquid-phase exfoliation method, and the result shows that it is viable. In the process of preparation, ten solvents were used to exfoliate Bi 2 Se 3 with same ultrasonic time and concentration in order to find the optimal solvents to exfoliate Bi 2 Se 3 . With the aid of ultrasound wave, few-layer Bi 2 Se 3 has successfully been prepared in NMP and CS-HAc. The exfoliation of as-synthesized bulk Bi 2 Se 3 is attributed to the energy provided by the ultrasound wave which overcomes the van der Waals force between Bi 2 Se 3 QLs. With the increasing of ultrasonic time, higher concentration of few-layer Bi 2 Se 3 was produced. However, the increasing amount of few-layer Bi 2 Se 3 is not obvious after 30 h. Considering the efficiency of preparation, 30 h is selected as the appropriate ultrasonic time to prepare few-layer Bi 2 Se 3 . In NMP solvent, many materials held by van der Waals forces could be exfoliated to produce 2D nanosheets due to its appropriate surface tension 46 . Bi 2 Se 3 has a similar structure held together via van der Waals forces between QLs, so it is possible  to obtain few-layer Bi 2 Se 3 after sonication in NMP. Really, the expected results have been obtained as we suppose so. Another aqueous surfactant solution, CS-HAc, was also used to prepare few-layer Bi 2 Se 3 through hydrophobic interaction of main chains of chitosan and the surface of Bi 2 Se 3 . Few-layer Bi 2 Se 3 can stably be dispersed in chitosan dispersion, which is due to the electrostatic repulsion 35 between NH 4 1 in the side chains of chitosan adsorped on the surface of Bi 2 Se 3 . Therefore, liquid-phase-exfoliation of as-synthesized bulk Bi 2 Se 3 allows production of few-layer Bi 2 Se 3 suspensions in NMP or CS-HAc, which might be a simple and convenient method to prepare few-layer Bi 2 Se 3 for further investigating its properties and exploring the promising applications.
The optical absorption spectrum of few-layer Bi 2 Se 3 in solution exhibits a strong absorption band in the visible light region, which is different from as-synthesized bulk Bi 2 Se 3 without any absorption peak in this region. The few-layer Bi 2 Se 3 with a small size can be considered as a ''quantum dot'' that would result in quantum confinement, leading to resonance that can be tuned with size. Therefore, the optical absorption of few-layer Bi 2 Se 3 is size-dependent (Fig. 4). The products in this study consist of nanosheets with different sizes and thicknesses, and it is difficult to prepare the sample of fixed size with different thickness or fixed thickness with different sizes. Therefore, for systematic and in-depth investigating the effect of thickness and size on the optical absorption of few-layer Bi 2 Se 3 , it is necessary to synthesize nanosheets that are monodispersed in both size and thickness, but it remains a challenge by liquid-phase-exfoliation method at this stage.
The blue-shift of UV-vis absorption of few-layer Bi 2 Se 3 with its size decreasing is similar to the results for many semiconductor nanoparticles 47 , which the small dimensions result in differently physical properties compared with their corresponding bulk materials. Therefore, we also studied the optical band gap (E g ) of few-layer Bi 2 Se 3 according to their optical absorption spectra in solution. The optical absorption properties of few-layer Bi 2 Se 3 with different size and thickness in solutions were further investigated in UV-Vis-NIR spectral region (Supplementary Fig. S12 and 13). There is not obvious absorption peak in NIR region. The recorded absorption spectra were mathematically processed to acquire the values of E g 48 . The optical absorption is calculated using the following equation: where a is the absorption coefficient, h is Planck constant, n is the frequency of photon, E g is the band gap and B is a constant. For the direct band gap semiconductor Bi 2 Se 3 , n is 2. The (ahn) 2 vs. hn curves for all samples were shown in Supplementary Fig. S12 and 13. The E g of few-layer Bi 2 Se 3 with different sizes and thicknesses obtained by different centrifugation speeds in NMP were determined by extrapolating the straight portion of the plot to the energy axis. The E g  supplementary Table S1 and S2), respectively, which is higher than that of as-synthesized bulk Bi 2 Se 3 (1.08 eV). Meanwhile, the E g of few-layer Bi 2 Se 3 increases with the centrifugation speed increasing (size and thickness of fewlayer Bi 2 Se 3 decreasing), which indicates a blue-shift phenomenon. The reason for this larger E g of few-layer Bi 2 Se 3 might be due to the well-known quantum confinement effect by shifting the conduction and valence band edges in opposite directions [49][50][51] . It is worth noting that the E g of as-synthesized bulk Bi 2 Se 3 is larger than the theoretical value (0.3 eV) calculated by first-principle electronic structure. The reason is that the as-synthesized ''bulk'' Bi 2 Se 3 with a thickness of 40-100 nm and a size of 100-500 nm (Supplementary Figure S2) is smaller than those of the generally bulk Bi 2 Se 3 (thickness and size $ 10 mm). That is to say, the as-synthesized ''bulk'' Bi 2 Se 3 is nanosheet and not the real bulk Bi 2 Se 3 . The higher E g of as-synthesized ''bulk'' Bi 2 Se 3 (1.08 eV) is attributed to the quantum size effect as also considered by Gorer and Hodes 52 . Therefore, it is reasonable that the theoretical E g of Bi 2 Se 3 is smaller than the experimental E g because the E g growing is nearly inversely proportional to the lateral size 53 . For a 2D crystallite, the band gap shift, DE g , is described by the equation 54 where m xy and m z are the reduced effective masses of electron-hole pairs in parallel (xy) and perpendicular (z) directions, respectively, and L xy and L z are the corresponding dimensions of the crystallite. For the ideally thin nanosheets, L xy (0.1-1 mm) is much larger than L z (0.96 nm for Bi 2 Se 3 QL), so the first term in eq. 2 can be neglected. Consequently, the band gap shift depends only on L z . As shown in Supplementary Fig. S12, we can get an approximate DE g 5 E g (13000 rpm) 2 E g (bulk) 5 0.42 eV. Therefore, the calculated m z is 0.24 m e (m e : electron mass). The Bohr radius R of exciton can be calculated by the following equation 56 where e is the dielectric constant at optical frequencies. The dielectric constant for Bi 2 Se 3 can be typically set to be 100e 0 57,58 , where e 0 is the vacuum permittivity. e is electron charge, 1.062 3 10 219 C. The calculated R from eq. 3 is about 21.79 nm. Therefore, the calculated R is much larger than the thickness of few-layer Bi 2 Se 3 at 13000 rpm (0.96 3 2 5 1.92 nm), suggesting that electron-hole pairs would be physically confined in few-layer Bi 2 Se 3 . The calculated R values for other few-layer Bi 2 Se 3 were listed in Supplement Table S1 and S2. It is well-known that semiconductors perform dramatic quantization effect when charge carriers (electrons and holes) are confined by potential barriers to small regions of space 59 . Or equivalently, the thickness of few-layer Bi 2 Se 3 is less than twice the Bohr radius of excitons in the bulk material. In a word, the blue-shift phenomenon implies that the E g would increase with decreasing thickness, especially for the molecularly thin nanosheets by quantum size effect.
Under strong light excitation (Supplement Fig. S14), the electrons in the valence band become depleted while the finial state in the conduction band is partially occupied, and further excitation from the valence band is blocked and no further absorption is induced, leading to a saturable absorption effect. The saturable intensity of few-layer Bi 2 Se 3 thin film is much less than that of bulk Bi 2 Se 3 (.50 layers). Therefore, the saturable absorption of few-layer Bi 2 Se 3 is further exploited to Q-switch fiber laser, experimentally confirming the advantage of few-layer Bi 2 Se 3 as a broadband saturable absorber because the Q-switching operation of as-synthesized bulk Bi 2 Se 3 was extremely unstable with the large pulse-intensity and repetition-rate fluctuation. Further exploiting the few-layer Bi 2 Se 3 with the saturable absorption, we have successfully obtained the few-layer Bi 2 Se 3 -based passive Q-switched EDFL. Compared with as-synthesized bulk Bi 2 Se 3 , we have revealed that few-layer Bi 2 Se 3 is more favorable for stable Q-switching. The reason why few-layer Bi 2 Se 3 for Q-switched pulsed laser is superior to bulk Bi 2 Se 3 could be explained as follows. It is well known that most of unique characteristics of topological insulator (including optical and electrical ones) originate from the metallic states on the surfaces or edges. As illustrated in Supplement Fig.  S15, because bulk Bi 2 Se 3 can be exfoliated to many few-layer Bi 2 Se 3 sheets, in this process the surfaces/edges can be sharply increased. Therefore, one can think that the metallic states of few-layer Bi 2 Se 3 should be stronger than that of bulk Bi 2 Se 3 . As is well known, the electrons at the surface, such as metals, are very active with very low surface energy, and they are readily excited by externally electromagnetic (e.g. lightwave) or thermal fields. According to this way, one can easily understand that under light excitation, the surface electrons of few-layer Bi 2 Se 3 can be transited more readily, because few-layer Bi 2 Se 3 possesses more metallic surfaces/edges in comparison with the bulk one. Thus, the optically saturable absorption of few-layer Bi 2 Se 3 is more excellent than that of bulk Bi 2 Se 3 . Also, the few-layer Bi 2 Se 3 can significantly enlarge the surface-to-volume ratio, and can be considered as a ''quantum dot'' that would result in quantum confinement. This could lead to the easier occurrence of the saturable absorption which has been partially verified by the lower saturable intensity (53 and 41 MW/cm 2 ) in Fig. 5. Therefore, few-layer Bi 2 Se 3 can generate the stable Q-switching operation compared to the unstable operation with bulk Bi 2 Se 3 . The Q-switched laser based on few-layer Bi 2 Se 3 has the low pump threshold of 9.3 mW, the pulse energy of 39.8 nJ, the pulse duration of 4.9 ms and the wide range of pulse-repetition-rate from 6.2 to 40.1 kHz, comparable to those reported fiber lasers Q-switched by other saturable absorbers (e.g. graphene 60 , carbon nanotubes 44 , and semiconductor 38,39 ). The promising results might have been due to the unique energy-band structure of few-layer Bi 2 Se 3 . This performance of the Q-switched laser shows good prospects of few-layer Bi 2 Se 3 as an excellently saturable absorber in the future.

Methods
Synthesis of bulk Bi 2 Se 3 . Polyvinyl pyrrolidone (0.9 g) was dissolved in ethylene glycol (EG, 36 mL). Then bismuth oxide powder (Bi 2 O 3 , 1 mmol), selenium powder (Se, 3 mmol) and ethylenediamine tetraacetic acid powder (4 mmol) were added into above-mentioned EG solution. The resulting suspension was stirred vigorously and subsequently sealed in a steel autoclave. The autoclave was heated to 200uC in 30 min and maintained this temperature for 20 h. The as-obtained product was collected by high-speed centrifugation, washed several times with deionized water and absolute ethanol, and finally dried at 60uC for 96 h in an oven.
Preparation of few2layer of Bi 2 Se 3 . The as-synthesized bulk Bi 2 Se 3 was dispersed in NMP or stock solution of chitosan (0.2 mg?mL 21 ) that was prepared in 0.5% acetic acid aqueous solution at a concentration of 1 mg?mL 21 by sonication in a sonic bath for 30 h (KQ2250 DB). The upper part of the resulting suspension after leaving to stand for 24 h was collected and centrifuged for 30 min at 1000 rpm. Subsequently, the supernatant was decanted to another centrifuge tube. After centrifuging the supernatant at 10000 rpm for 10 min, the as-obtained product was collected into phials and dispersed in the solvent used above for further characterization.
Characterization. Powder X-ray diffraction system (Rigaku Ultima IV XRD) equipped with Cu Ka radiation (l 5 1.542 Å ) over the 2h range of 10-80u was used to characterize the crystal structure of as-synthesized bulk and few-layer Bi 2 Se 3 . The sample was prepared by dropping the dispersive solutions on the surface of glass slid which had been etched a groove, then drying with an infrared lamp. Again and again to depositing a film on the fluted glass was named a continuous drop-dry process. SEM images were obtained on LEO-1530 operated at 20 kV. SEM samples were prepared by depositing a small drop of solution on small pieces of silicon wafer and then dried at room temperature. Energy dispersive X-ray spectrum pattern was acquired through spreading as-synthesized Bi 2 Se 3 powders on sample stage directly. The micrographs of samples were taken using a transmission electron microscope (JEOL JEM-1400, JEM-2100) at an accelerating voltage of 200 kV. To prepare the TEM samples, a small drop of sample was deposited onto copper grids coating with lacey carbon film and then dried under room temperature at atmospheric pressure. AFM images were obtained in the tapping mode in air using an Agilent 5500 atomic force microscope. The samples were prepared by dropping their dispersions on mica www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 4794 | DOI: 10.1038/srep04794 substrates. Raman spectra (XploRA,Jobin-Yvon) were recorded with a diode laser at the excitation wavelength of 532 nm. The UV-vis absorption spectrum was measured on UV-vis spectrometer (UV-2550, Shimadzu). The UV-vis-NIR absorption spectrum was recorded on a Varian Cary 5000. The linear absorption spectra were measured by a spectrophotometer (Perkinelmer Lambda 7500) scanning from 300 to 2000 nm. The output laser spectrum was monitored by an optically spectral analyzer (Advantest Q8384) with the spectral resolution of 0.01 nm. The pulsed characteristics of this laser were detected by a 10 GHz photodetector (Nortel PP-10G) together with a digital storage oscilloscope (DSO, Agilent MSO7104A) and a radio-frequency (RF) spectrum analyzer (Gwinstek GSP-930).