OPEN Preparation of Few-Layer Bismuth

SUBJECT AREAS:
Selenide by Liquid-Phase-Exfoliation and
LASERS, LEDS AND LIGHT
SOURCES Its Optical Absorption Properties
FIBRE LASERS
Liping Sun1, Zhiqin Lin1, Jian Peng1, Jian Weng1,3, Yizhong Huang2 & Zhengqian Luo2
SYNTHESIS AND PROCESSING

1
Department of Biomaterials, College of Materials, Xiamen University, Xiamen 361005, China, 2Institute of Optoelectronic
Received Technology, Department of Electronic Engineering, Xiamen University, Xiamen 361005, China, 3ShenZhen Research Institute of
4 November 2013 Xiamen University, Shenzhen 518057, China.
Accepted
8 April 2014 Bismuth selenide (Bi2Se3), a new topological insulator, has attracted much attention in recent years owing to
Published its relatively simple band structure and large bulk band gap. Compared to bulk, few-layer Bi2Se3 is recently
25 April 2014 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
Correspondence and size and layer number. At the same time, the nonlinear and saturable absorption of few-layer Bi2Se3 thin film
requests for materials in near infrared is also characterized well and further exploited to generate laser pulses by a passive
should be addressed to 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,
J.W. (jweng@xmu.
the few-layer Bi2Se3 may excite a potential applications in laser photonics and optoelectronic devices.
edu.cn) or Z.Q.L.
(zqluo@xmu.edu.cn)

T
opological insulators (TIs) as interesting insulators now have become the rising star in physics, chemistry
and materials fields because they are insulating in the bulk phase but possess exotic metal surface state as a
result of the combination of spin-orbit interactions and time-reversal symmetry1–3. In the past few years,
some research groups4–6 achieved great success in the prediction and experimental confirmation of TIs, including
Bi2Se3, Bi2Te3 and Sb2Te3, which have a large band gap and a single Dirac cone. Especially, the remarkable band
gap of Bi2Se3 is approximately up to 0.3 eV (equivalent to 3600 K) that is much larger than the room temperature
energy scale4. It means that Bi2Se3 is able to exhibit topological insulator behavior at room temperature, which is
considered as a promising topological system with a good application prospect7. Recently, most researchers paid
attention to the physical basis8–10, synthesis method11,12 and exploration of the nanostructure13–15 of TIs. However,
it is worth noting that topological properties of Bi2Se3 as three-dimensional (3D) TIs are often covered up by the
bulk state due to high carrier density5,16. Therefore, it is necessary to prepare two-dimensional (2D) Bi2Se3 from its
3D bulk materials in order to acquire the superior performance for some potential applications.
Bi2Se3 possesses stacked layers of laminated structure that are held together by weak van der Waals interac-
tions. Each layer is one quintuple layer (QL) and the five atoms are covalently bonded together along the z axis in
the order of Se-Bi-Se-Bi-Se (Fig. 1a). The thickness of each layer is about 0.96 nm17. It is possible to exfoliate bulk
Bi2Se3 into few-layer nanosheets due to the weak interaction between layers. Up to date, bottom-up synthesis and
top-down exfoliation are two main methods to prepare 2D nanomaterials3. Bottom-up synthesis approach is used
to obtain single-layer or fewer layer 2D nanomaterials by a chemical reaction from the atomic or molecular scale
synthesis18–21. 3D materials held together by weak van der Waals forces can be exfoliated into thin flakes by the
methods of mechanical or chemical exfoliation22–24, which is a top-down process. Therefore, it is possible to obtain
few-layer QLs from bulk Bi2Se3 with ‘‘graphene-inspired’’ exfoliation methods because bulk Bi2Se3 possesses the
graphene-like layered structure. Liquid-phase exfoliation has been used to produce single-layer or few-layer
graphene because it is easier and more convenient than other methods. Furthermore, the as-obtained graphene
could form colloidal dispersions in solvents24,25. Therefore, we attempted to exfoliate bulk Bi2Se3 by liquid-phase
exfoliation method to prepare few-layer Bi2Se3 in solutions.
As a new type of Dirac material, TIs with the unique energy-band structure can induce some fantastically
electronic and optical properties26, opening up many new applications, such as superconductors27 and ultrafast
lasers28,29. Nowadays, these researches are focused on pulsed lasers due to their versatile applications in range

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Figure 1 | Preparation and exfoliation of as-synthesized bulk Bi2Se3. (a) Schematic of rhombohedral layer structure held together by weak van der Waals
interactions in Bi2Se3. Each QL consists of five covalently bonded atomic sheets along the z axis in the order of Se-Bi-Se-Bi-Se. (b), (c) SEM and
TEM images of as-synthesized bulk Bi2Se3. (d) Photographs of Bi2Se3 dispersed in NMP and CS-HAc before and after sonication.

finding, medicine, laser processing, remote sensing and telecommu- The thickness of as-synthesized bulk Bi2Se3 is about 40–100 nm
nications30. In the field of pulsed lasers, passive Q-switched fiber laser determined by atomic force microscopy (AFM) (Supplementary
for generating short and large-energy laser pulse is one of most Fig. S2c and d).
effective ways because of their significant advantages of compactness, We further exfoliated as-synthesized bulk Bi2Se3 with solution-
simplicity, and flexibility in design31. The key element in the passive phase exfoliation method, which is similar to the exfoliation of
Q-switched fiber laser is an excellently saturable absorber. Therefore, graphite in NMP and CS-HAc35. The as-synthesized Bi2Se3 powders
researchers have never stopped to seek for new saturable absorbers were insoluble in two solvents before sonication (Fig. 1d). After
(e.g. semiconductor32, carbon nanotubes33, graphene33,34). Compared sonication of 30 h, the colors of two solutions were deepened, which
with bulk materials28,29, one can expect that the few-layer nanoma- means that the exfoliated Bi2Se3 had been dispersed in these solvents.
terials would possess the more excellent performance of saturable We also investigated the exfoliation of as-synthesized bulk Bi2Se3 in
absorption, and could be a potentially saturable absorber. Therefore, other solvents (Supplementary Fig. S3). The result shows that NMP
we are strongly motivated to develop the pulsed fiber lasers Q- and CS-HAc are the optimal solvents to exfoliate as-synthesized bulk
switched with few-layer Bi2Se3 as the saturable absorber. Bi2Se3. Therefore, NMP and CS-HAc are selected to investigate the
Here, N-methyl-2-pyrrolidone (NMP), the more promising exfoliation of bulk Bi2Se3. We further investigated the effect of soni-
organic solvent to exfoliate 2D layered materials24, is used to exfoliate cation time on exfoliation of bulk Bi2Se3 (Supplementary Fig. S4).
bulk Bi2Se3 for producing few-layer Bi2Se3 (Supplementary Fig. S1). With increasing ultrasonic time, the color of CS-HAc was deepened,
Another is chitosan acetic solution (CS-HAc), which possesses the but color is already deep dark in NMP at 2 h, which reveals a better
low-toxic, good-biocompatible and environmentally friendly prop- exfoliating effect in NMP. Longer ultrasonic time should produce
erties35. Meanwhile, we also investigated the optical absorption char- higher concentration of few-layer Bi2Se3. However, it needs more
acterization of as-prepared few-layer Bi2Se3 dispersed in solvents in power and time. Therefore, we chose 30 h as the appropriately ultra-
visible light region, and saturable-absorption performance of few- sonic time because Bi2Se3 has already been well dispersed in these
layer Bi2Se3 thin film in near infrared region. At last, few-layer Bi2Se3 two solvents, meeting the requirement of following experiments in
was successfully used as the new fiber-compatibly saturable absorber this study. The exfoliated Bi2Se3 also presented the Tyndall effect of
to attain passive Q-switched fiber laser at 1.53 mm wavelength. the colloidal suspension (Supplementary Fig. S5). The result shows
that the colloidal suspension of exfoliated Bi2Se3 in the two solvents is
Results stable.
Preparation and exfoliation of bulk Bi2Se3. Bulk Bi2Se3 was pre-
pared by hydrothermal synthesis and characterized by X-ray dif- Characterization of few-layer Bi2Se3. The TEM image (Fig. 2a) of
fraction (XRD, Supplementary Fig. S2a). All the labeled peaks can exfoliated Bi2Se3 showed that the as-obtained few-layer Bi2Se3 was
be readily indexed to rhombohedral Bi2Se3 (JCPDS no. 89-2008). extremely thin 2D flake. According to the selected area electron
The scanning electron microscope (SEM) image in Fig. 1b and diffraction (SAED) pattern (Fig. 2b), it could be indexed as a 6-
transmission electron microscope (TEM) image in Fig. 1c show fold symmetry [001] zone axis pattern, which is consistent with the
that the as-synthesized bulk Bi2Se3 exhibits sheet-like structure layered structure along the z axis. Also, it revealed the single-
with a wide size distribution, and is easily to aggregate together. crystalline nature of the thin 2D flake. Furthermore, the distance

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Figure 2 | Confirmation of few-layer Bi2Se3 exfoliated in NMP. (a) TEM image of few-layer Bi2Se3. (b) SAED pattern of few-layer Bi2Se3. (c) HRTEM
image of few-layer Bi2Se3. (d), (e) AFM image and the corresponding height profile of few-layer Bi2Se3. (f) XRD patterns of few-layer Bi2Se3 and
bulk Bi2Se3.

between the adjacent hexagonal lattice fringes investigated by the the visible light region. However, few-layer Bi2Se3 displayed an
high-resolution TEM (HRTEM) is 0.213 nm for Bi2Se3 (Fig. 2c), absorption band at about 552 nm in CS-HAc and 574 nm in
which is consistent with the lattice space of the (110) plane. The NMP, respectively. The appearance of absorption band after
AFM image (Fig. 2d) also shows that the exfoliated Bi2Se3 is a flake exfoliation is remarkable, which might be due to the exfoliation of
structure and its thickness is about 3–4 nm (Fig. 2e), which nearly bulk Bi2Se3 into nanosheets with a few nanometers thickness. The
equals to 4 layers of Bi2Se317. The XRD pattern (Fig. 2f) of few-layer absorption also increases gradually as the sonication time extended
Bi2Se3 showed a high [006] orientation and some characteristic peaks (Fig. 3b and c), which reveals that more few-layer Bi2Se3 would be
disappeared compared to bulk Bi2Se3, which indicates that bulk obtained with increasing sonication times. The result further
Bi2Se3 had been successfully exfoliated as we expected. At the same suggests that the absorption would be resulted from few-layer Bi2Se3.
time, the bulk Bi2Se3 has successfully been exfoliated to few-layer We further investigate the effect of size and thickness on absorp-
Bi2Se3 in CS-HAc (Supplementary Fig. S6). Besides, Raman tion property. After sonication in NMP, few-layer Bi2Se3 was sepa-
spectrum was also used to further confirm the exfoliation of Bi2Se3 rated in different centrifugal speeds (Fig. 4a). The size distribution
(Supplementary Fig. S7). The A mode of few-layer Bi2Se3 produced a and corresponding height profile of few-layer Bi2Se3 collected at
red shift compared to that of bulk Bi2Se3, which could be attributed to three centrifugation speeds were distinguishing. With centrifugal
the phonon softening36,37. Therefore, we successfully prepared few- speed increasing, the size of few-layer Bi2Se3 was decreased from
layer Bi2Se3 using the solution-phase exfoliation method. 500 to 100 nm and the thickness was also decreased from 10 to
2 nm, and the maximal absorption wavelength was blue-shifted from
Optical absorption characterization of few-layer Bi2Se3. The opti- 613 to 459 nm (Fig. 4b). The similar result is also obtained in CS-
cal absorption properties of few-layer Bi2Se3 were firstly investigated HAc (Supplementary Fig. S8 and S9). The result further suggests that
with ultraviolet-visible (UV-vis) spectra. Interestingly, we found that the broad absorption in the visible light region would be resulted
the dispersion solutions of few-layer Bi2Se3 produced a broad from few-layer Bi2Se3 but not from bulk Bi2Se3.
absorption in the visible light region compared to as-synthesized To further investigate the optical absorption properties of few-
bulk Bi2Se3 (Fig. 3a). The UV-vis spectrum of as-synthesized bulk layer Bi2Se3, we used a spin-coating method to prepare Bi2Se3/
Bi2Se3 showed a nearly straight line without any absorption peak in NMP (few-layer Bi2Se3 exfoliated in NMP) and Bi2Se3/CS-HAc

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Figure 3 | UV-vis absorption spectra of few-layer Bi2Se3. (a) UV-vis absorption spectra of as-synthesized bulk and few-layer Bi2Se3 suspension.
(b) UV-vis absorption spectra of few-layer Bi2Se3 suspension prepared with different sonication times in NMP. (c) UV-vis absorption spectra of few-layer
Bi2Se3 suspension prepared with differrent ultrasonic times in CS-HAc. The upper part of resulting suspension of each sample after sonication was
collected, and then centrifuged for 30 min at 1000 rpm to receive the supernatant as the measurement solution.

(few-layer Bi2Se3 exfoliated in CS-HAc) thin films on quartz plate, Generation of Q-switched laser pulses using the saturable absorp-
respectively. As shown in Fig. 5a, we measured the linear absorption tion of few-layer Bi2Se3. As well as known, the optically saturable
spectra of the two films by a spectrophotometer scanning from 300 to absorption can be used to efficiently generate the laser pulses by the
2000 nm. One can clearly see that both of the films have the relatively passive Q-switching or mode-locking techniques42,43. The lower Isa of
flat transmission curves in the UV-to-near infrared (NIR) region, e.g. few-layer Bi2Se3 may be very helpful for developing the low-
the transmittance of Bi2Se3/NMP varies only from 0.67 to 0.84 in the threshold Q-switched/mode-locked lasers. In this section, to testify
broad wavelength range of 350 , 2000 nm. It indicates that the few- the performance of few-layer Bi2Se3, we will exploit the saturable
layer Bi2Se3 would be a promising broadband optical material. In absorption of few-layer Bi2Se3 to passive Q-switch erbium-doped
order to compare the nonlinear absorption of our few-layer Bi2Se3 fiber laser (EDFL) for generating laser pulses. Supplementary Fig.
with that of bulk Bi2Se3 (.50 layers) previously reported38,39, we also S10 shows the experimental setup of Q-switched EDFL using few-
used the same Z-scan technique to measure the nonlinear transmis- layer Bi2Se3 as a saturable absorber. In order to clearly evaluate the
sion responses of the two few-layer Bi2Se3 films. When the two significance of few-layer Bi2Se3 to Q-switching operation, we pur-
samples were strongly excited by a femtosecond Ti: sapphire laser posely performed the following control experiments. At first, when
with the highest optical intensity of 2.6 GW/cm2 (Fig. 5b and c), the as-synthesized bulk Bi2Se3 was deliberately inserted into the laser
open-aperture Z-scan transmission curves of Bi2Se3/NMP and cavity, we found that the Q-switching operation at 1530.2 nm was
Bi2Se3/CS-HAc were obtained, respectively. One can obviously see extremely unstable with a large pulse-intensity and repetition-rate
that the two samples possess the saturable absorption, i.e. the optical fluctuation (see the Supplementary Fig. S11 for more details).
transmittance is different under differently optical intensity. The Moreover, the Q-switching operation has a high pump threshold
modulation depths (dT) are 3.8% for Bi2Se3/NMP and 3.7% for of 22.1 mW, a broad pulse duration of 22.8 ms and a small
Bi2Se3/CS-HAc, respectively, which is comparable to that of gra- operating range of pump power (22.1 , 67.5 mW). In contrast, a
phene40,41. Furthermore, by carefully fitting the curves in Fig. 5b very stable Q-switching operation was produced when the few-layer
and 5c, the produced saturable intensities (Isa) are 53 MW/cm2 for Bi2Se3 was placed in the laser cavity to replace as-synthesized bulk
Bi2Se3/NMP and 41 MW/cm2 for Bi2Se3/CS-HAc, respectively. It is Bi2Se3 as followed.
very interesting that the Isa values are much less than that of bulk As increasing the pump power, we found that the laser with few-
Bi2Se3 reported previously38,39, mainly benefiting from the few-layer layer Bi2Se3 as saturable absorber reached its threshold at the pump
structure of exfoliated Bi2Se3. In the field of passive Q-switched or power of 9.3 mW only, and the stable Q-switching operation
mode-locked lasers, the lower Isa of saturable absorber, the easier the occurred simultaneously. The pump threshold for Q-switching is
start of Q-switching/mode-locking operation is, implying that few- much lower than that of as-synthesized bulk Bi2Se3 (22.1 mW),
layer Bi2Se3 might be very helpful for developing the low-threshold and other saturable absorber-based pulsed EDFLs reported prev-
Q-switched/mode-locked lasers. iously29,38,39, mainly benefiting from the lower Isa of few-layer

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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 lasers44, the sideband structure appeared in the
optical spectrum was due to the multimode oscillation and the cavity
perturbations of Q-switching44. As shown in Fig. 6d, we also mea-
sured 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 Bi2Se3 (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 loss45.

Figure 4 | Few-layer Bi2Se3 in NMP collected in different centrifugal Discussion

speeds. (a) TEM and AFM images, and the corresponding height profiles of In this work, we attempted to exfoliate as-synthesized bulk Bi2Se3 for
few-layer Bi2Se3 in NMP collected in different centrifugal speeds. Firstly, preparing few-layer Bi2Se3 by liquid-phase exfoliation method, and
the stock solution was centrifuged at 2000 rpm for 30 min, and the the result shows that it is viable. In the process of preparation, ten
precipitate was collected as sample one (top). Then, the remaining solvents were used to exfoliate Bi2Se3 with same ultrasonic time and
supernatant was centrifuged at 8000 rpm for 20 min, and the precipitate concentration in order to find the optimal solvents to exfoliate Bi2Se3.
was collected as sample two (middle). At last, the supernatant collected in With the aid of ultrasound wave, few-layer Bi2Se3 has successfully
second step was further centrifuged at 13000 rpm for 12 min, and the been prepared in NMP and CS-HAc. The exfoliation of as-synthe-
precipitate was collected as sample three (bottom). (b) UV-vis absorption sized bulk Bi2Se3 is attributed to the energy provided by the ultra-
spectra of few-layer Bi2Se3 in NMP. The absorption band was blue-shifted sound wave which overcomes the van der Waals force between Bi2Se3
with decreasing thickness and size of few-layer Bi2Se3. QLs. With the increasing of ultrasonic time, higher concentration of
few-layer Bi2Se3 was produced. However, the increasing amount of
Bi2Se3. Fig. 6 summarizes the output characteristics of the Q- few-layer Bi2Se3 is not obvious after 30 h. Considering the efficiency
switched pulses. Fig. 6a and 6b give the typical oscilloscope trace of of preparation, 30 h is selected as the appropriate ultrasonic time to
the Q-switched pulse trains and the single pulse envelope at the prepare few-layer Bi2Se3. In NMP solvent, many materials held by
pump power of 87.2 mW, respectively. The Q-switching pulse van der Waals forces could be exfoliated to produce 2D nanosheets
output with the repetition rate of 23.8 kHz was stable, and no sig- due to its appropriate surface tension46. Bi2Se3 has a similar structure
nificant pulse jitter was observed on the oscilloscope. The measured held together via van der Waals forces between QLs, so it is possible

Figure 5 | Optical absorption of few-layer Bi2Se3. (a) The linear absorption of few-layer Bi2Se3. (b) The nonlinear optical absorption (i.e. saturable
absorption) of Bi2Se3/NMP. (c) The nonlinear optical absorption (i.e. saturable absorption) of Bi2Se3/CS-HAc.

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Figure 6 | Performance of few-layer Bi2Se3 to passively Q-switch erbium-doped fiber laser. (a) The typical oscilloscope trace of Q-switched pulses at the
pump power of 87.2 mW. (b) The single pulse envelope. (c) The typical optical spectrum of Q-switching operation. (d) The RF output spectrum.
(e) The pulse repetition rate and the pulse energy vs the pump power. (f) The pulse duration as a function of the pump power.

to obtain few-layer Bi2Se3 after sonication in NMP. Really, the is necessary to synthesize nanosheets that are monodispersed in both
expected results have been obtained as we suppose so. Another aque- size and thickness, but it remains a challenge by liquid-phase-exfo-
ous surfactant solution, CS-HAc, was also used to prepare few-layer liation method at this stage.
Bi2Se3 through hydrophobic interaction of main chains of chitosan The blue-shift of UV-vis absorption of few-layer Bi2Se3 with its
and the surface of Bi2Se3. Few-layer Bi2Se3 can stably be size decreasing is similar to the results for many semiconductor
dispersed in chitosan dispersion, which is due to the electrostatic nanoparticles47, which the small dimensions result in differently
repulsion35 between NH41 in the side chains of chitosan adsorped physical properties compared with their corresponding bulk materi-
on the surface of Bi2Se3. Therefore, liquid-phase-exfoliation of als. Therefore, we also studied the optical band gap (Eg) of few-layer
as-synthesized bulk Bi2Se3 allows production of few-layer Bi2Se3 Bi2Se3 according to their optical absorption spectra in solution. The
suspensions in NMP or CS-HAc, which might be a simple and con- optical absorption properties of few-layer Bi2Se3 with different size
venient method to prepare few-layer Bi2Se3 for further investigating and thickness in solutions were further investigated in UV–Vis–NIR
its properties and exploring the promising applications. spectral region (Supplementary Fig. S12 and 13). There is not obvi-
The optical absorption spectrum of few-layer Bi2Se3 in solution ous absorption peak in NIR region. The recorded absorption spectra
exhibits a strong absorption band in the visible light region, which is were mathematically processed to acquire the values of Eg48. The
different from as-synthesized bulk Bi2Se3 without any absorption optical absorption is calculated using the following equation:
peak in this region. The few-layer Bi2Se3 with a small size can be

ðahvÞn ~B hv{Eg ð1Þ
considered as a ‘‘quantum dot’’ that would result in quantum con-
finement, leading to resonance that can be tuned with size. Therefore, where a is the absorption coefficient, h is Planck constant, n is the
the optical absorption of few-layer Bi2Se3 is size-dependent (Fig. 4). frequency of photon, Eg is the band gap and B is a constant. For the
The products in this study consist of nanosheets with different sizes direct band gap semiconductor Bi2Se3, n is 2. The (ahn)2 vs. hn curves
and thicknesses, and it is difficult to prepare the sample of fixed size for all samples were shown in Supplementary Fig. S12 and 13. The Eg
with different thickness or fixed thickness with different sizes. of few-layer Bi2Se3 with different sizes and thicknesses obtained by
Therefore, for systematic and in-depth investigating the effect of different centrifugation speeds in NMP were determined by extra-
thickness and size on the optical absorption of few-layer Bi2Se3, it polating the straight portion of the plot to the energy axis. The Eg

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were 1.22 eV for 2000 rpm, 1.39 eV for 8000 rpm and 1.50 eV for fluctuation. Further exploiting the few-layer Bi2Se3 with the saturable
13000 rpm (more details in supplementary Table S1 and S2), absorption, we have successfully obtained the few-layer Bi2Se3-based
respectively, which is higher than that of as-synthesized bulk passive Q-switched EDFL. Compared with as-synthesized bulk
Bi2Se3 (1.08 eV). Meanwhile, the Eg of few-layer Bi2Se3 increases Bi2Se3, we have revealed that few-layer Bi2Se3 is more favorable for
with the centrifugation speed increasing (size and thickness of few- stable Q-switching. The reason why few-layer Bi2Se3 for Q-switched
layer Bi2Se3 decreasing), which indicates a blue-shift phenomenon. pulsed laser is superior to bulk Bi2Se3 could be explained as follows. It
The reason for this larger Eg of few-layer Bi2Se3 might be due to the is well known that most of unique characteristics of topological insu-
well-known quantum confinement effect by shifting the conduction lator (including optical and electrical ones) originate from the metal-
and valence band edges in opposite directions49–51. It is worth noting lic states on the surfaces or edges. As illustrated in Supplement Fig.
that the Eg of as-synthesized bulk Bi2Se3 is larger than the theoretical S15, because bulk Bi2Se3 can be exfoliated to many few-layer Bi2Se3
value (0.3 eV) calculated by first-principle electronic structure. The sheets, in this process the surfaces/edges can be sharply increased.
reason is that the as-synthesized ‘‘bulk’’ Bi2Se3 with a thickness of 40– Therefore, one can think that the metallic states of few-layer Bi2Se3
100 nm and a size of 100–500 nm (Supplementary Figure S2) is should be stronger than that of bulk Bi2Se3. As is well known, the
smaller than those of the generally bulk Bi2Se3 (thickness and electrons at the surface, such as metals, are very active with very low
size $ 10 mm). That is to say, the as-synthesized ‘‘bulk’’ Bi2Se3 is surface energy, and they are readily excited by externally electromag-
nanosheet and not the real bulk Bi2Se3. The higher Eg of as-synthe- netic (e.g. lightwave) or thermal fields. According to this way, one can
sized ‘‘bulk’’ Bi2Se3 (1.08 eV) is attributed to the quantum size effect easily understand that under light excitation, the surface electrons of
as also considered by Gorer and Hodes52. Therefore, it is reasonable few-layer Bi2Se3 can be transited more readily, because few-layer
that the theoretical Eg of Bi2Se3 is smaller than the experimental Eg Bi2Se3 possesses more metallic surfaces/edges in comparison with
because the Eg growing is nearly inversely proportional to the lateral the bulk one. Thus, the optically saturable absorption of few-layer
size53. Bi2Se3 is more excellent than that of bulk Bi2Se3. Also, the few-layer
For a 2D crystallite, the band gap shift, DEg, is described by the Bi2Se3 can significantly enlarge the surface-to-volume ratio, and can
equation54,55 be considered as a ‘‘quantum dot’’ that would result in quantum
confinement. This could lead to the easier occurrence of the saturable
h2 h2 absorption which has been partially verified by the lower saturable
DEg ~ z ð2Þ
4mxy L2xy 8mz L2z intensity (53 and 41 MW/cm2) in Fig. 5. Therefore, few-layer Bi2Se3
can generate the stable Q-switching operation compared to the
where mxy and mz are the reduced effective masses of electron-hole unstable operation with bulk Bi2Se3. The Q-switched laser based
pairs in parallel (xy) and perpendicular (z) directions, respectively, on few-layer Bi2Se3 has the low pump threshold of 9.3 mW, the pulse
and Lxy and Lz are the corresponding dimensions of the crystallite. energy of 39.8 nJ, the pulse duration of 4.9 ms and the wide range of
For the ideally thin nanosheets, Lxy (0.1–1 mm) is much larger than pulse-repetition-rate from 6.2 to 40.1 kHz, comparable to those
Lz (0.96 nm for Bi2Se3 QL), so the first term in eq. 2 can be neglected. reported fiber lasers Q-switched by other saturable absorbers (e.g.
Consequently, the band gap shift depends only on Lz. As shown in graphene60, carbon nanotubes44, and semiconductor38,39). The prom-
Supplementary Fig. S12, we can get an approximate DEg 5 Eg ising results might have been due to the unique energy-band struc-
(13000 rpm) 2 Eg(bulk) 5 0.42 eV. Therefore, the calculated mz is ture of few-layer Bi2Se3. This performance of the Q-switched laser
0.24 me (me: electron mass). The Bohr radius R of exciton can be shows good prospects of few-layer Bi2Se3 as an excellently saturable
calculated by the following equation56 absorber in the future.
h2
R~ ð3Þ Methods
mz pe2 Synthesis of bulk Bi2Se3. Polyvinyl pyrrolidone (0.9 g) was dissolved in ethylene
glycol (EG, 36 mL). Then bismuth oxide powder (Bi2O3, 1 mmol), selenium powder
where e is the dielectric constant at optical frequencies. The dielectric (Se, 3 mmol) and ethylenediamine tetraacetic acid powder (4 mmol) were added into
constant for Bi2Se3 can be typically set to be 100e057,58, where e0 is the above-mentioned EG solution. The resulting suspension was stirred vigorously and
vacuum permittivity. e is electron charge, 1.062 3 10219 C. The 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
calculated R from eq. 3 is about 21.79 nm. Therefore, the calculated high-speed centrifugation, washed several times with deionized water and absolute
R is much larger than the thickness of few-layer Bi2Se3 at 13000 rpm ethanol, and finally dried at 60uC for 96 h in an oven.
(0.96 3 2 5 1.92 nm), suggesting that electron-hole pairs would be
physically confined in few-layer Bi2Se3. The calculated R values for Preparation of few2layer of Bi2Se3. The as-synthesized bulk Bi2Se3 was dispersed in
NMP or stock solution of chitosan (0.2 mg?mL21) that was prepared in 0.5% acetic
other few-layer Bi2Se3 were listed in Supplement Table S1 and S2. It is acid aqueous solution at a concentration of 1 mg?mL21 by sonication in a sonic bath
well-known that semiconductors perform dramatic quantization for 30 h (KQ2250 DB). The upper part of the resulting suspension after leaving to
effect when charge carriers (electrons and holes) are confined by stand for 24 h was collected and centrifuged for 30 min at 1000 rpm. Subsequently,
potential barriers to small regions of space59. Or equivalently, the 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
thickness of few-layer Bi2Se3 is less than twice the Bohr radius of phials and dispersed in the solvent used above for further characterization.
excitons in the bulk material. In a word, the blue-shift phenomenon
implies that the Eg would increase with decreasing thickness, espe- Characterization. Powder X-ray diffraction system (Rigaku Ultima IV XRD)
cially for the molecularly thin nanosheets by quantum size effect. 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 Bi2Se3. The
Under strong light excitation (Supplement Fig. S14), the electrons sample was prepared by dropping the dispersive solutions on the surface of glass slid
in the valence band become depleted while the finial state in the which had been etched a groove, then drying with an infrared lamp. Again and again
conduction band is partially occupied, and further excitation from to depositing a film on the fluted glass was named a continuous drop-dry process.
the valence band is blocked and no further absorption is induced, 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
leading to a saturable absorption effect. The saturable intensity of then dried at room temperature. Energy dispersive X-ray spectrum pattern was
few-layer Bi2Se3 thin film is much less than that of bulk Bi2Se3 (.50 acquired through spreading as-synthesized Bi2Se3 powders on sample stage directly.
layers). Therefore, the saturable absorption of few-layer Bi2Se3 is The micrographs of samples were taken using a transmission electron microscope
further exploited to Q-switch fiber laser, experimentally confirming (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
the advantage of few-layer Bi2Se3 as a broadband saturable absorber lacey carbon film and then dried under room temperature at atmospheric pressure.
because the Q-switching operation of as-synthesized bulk Bi2Se3 was AFM images were obtained in the tapping mode in air using an Agilent 5500 atomic
extremely unstable with the large pulse-intensity and repetition-rate force microscope. The samples were prepared by dropping their dispersions on mica

SCIENTIFIC REPORTS | 4 : 4794 | DOI: 10.1038/srep04794 7

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