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Liquid exfoliation of two-dimensional PbI2 nanosheets for ultrafast photonics Qun Fan, Jiawei Huang, Ningning Dong, Song Hong, Chao Yan, Yongchao Liu, Jieshan Qiu, Jun Wang, and Zhenyu Sun ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019
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ACS Photonics
Liquid exfoliation of two-dimensional PbI2 nanosheets for ultrafast photonics Qun Fan†,#, Jiawei Huang‡,‖,#, Ningning Dong‡,‖, Song Hong†, Chao Yan§, Yongchao Liu§, Jieshan Qiu†, Jun Wang*,‡,‖ and Zhenyu Sun*,† † State
Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China ‡ Laboratory of Micro-Nano Optoelectronic Materials and Devices, Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, P.R. China ‖ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China § School of Material Science & Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P.R. China # These authors contribute equally to this work. KEYWORDS: PbI2, 2D materials, Liquid exfoliation, Ultrafast photonics
ABSTRACT: Two-dimensional (2D) materials are showing promise in next-generation optoelectronic devices. To this end, largescale production of defect-free 2D nanosheets is vital. Herein, we demonstrate scalable exfoliation and preparation of high-quality 2D PbI2 nanosheets in cyclopentanone. The resulting dispersions can remain good stability over 30 days and can also sustain freezing treatment. The exfoliation kinetics was observed to fit an apparent pseudo-first-order process. Furthermore, the PbI2 nanosheets exhibited excellent saturable absorption properties both at fs pulse and ns pulse excitations in the visible region, thus showing potential application in the field of ultrafast saturable absorbers.
The discovery of graphene (by using a Scotch-tape method in 2004)1 has opened up the door to a variety of other two-dimensional (2D) materials. Over the last decade, research on 2D materials has become increasingly intense owing to their unique structure, superlative physical properties, and various potential applications23 especially in electronics and photics.4 It has been calculated that nearly 2000 compounds characterized with strong in-plane covalent bonding but weak van der Waals force between layers are potentially exfoliable to yield 2D nanosheets.5 Whereas only a small number of such layered structures have been experimentally investigated, such as monoelemental Xene (typically graphene and black phosphorus),6-7 transition metal dichalcogenides (TMDs),8 among others. It is thus highly desirable to further expand the list of prospective 2D crystals to envision their future promising applications. Lead iodine (PbI2) is a well-known transition metal halide (TMH) with a hexagonal I-Pb-I layered structure. It displays high electron (4600 cm2 V−1 s−1) and highly anisotropic hole (3000 cm2 V−1 s−1) mobilities.9-10 Unlike graphene, a zero-overlap semimetal, PbI2 possesses a direct band gap in bulk (2.38 eV) and indirect band gap in monolayer (2.64 eV) due to quantum size effects.11-12 Hence this semiconductor has a great potential as a precursor for the fabrication of lead halide organic-inorganic perovskites13 in solar cells14 and photodetectors15 as well as in optoelectronic devices.16 PbI2 nanosheets were produced by mechanical exfoliation,16 so lution synthesis,17 and conventional physical vapor deposition (PVD).18 However, these aforementioned methods suffer from drawbacks of either low efficiency or extreme reaction conditions, limiting practical applications. It still remains a challenge for mass production of high-quality few-layer PbI2. Direct delamination of
parent layered materials in an appropriate liquid via ultrasonication provides a facile, cost-effective, and potentially scalable route for practical manufacture.19 Nevertheless, liquid-exfoliation of bulk PbI2 has not been explored. This drives efforts to exploit and develop dispersants to attain high-efficiency exfoliation of PbI2 to few layers. Herein, we demonstrate efficient exfoliation and stabilization of atomically thin PbI2 nanosheets in cyclopentanone ((CH2)4CO). Such solvent has not been examined for PbI2 exfoliation. It appears to be more effective than many others studied as shown in Figure S1. The resulting nanosheets remain stably dispersed against aggregation even after freezing treatment. The impacts of initial PbI2 concentration, centrifugation speed, and ultrasonic time were investigated to gain a kinetic understanding of exfoliation. More importantly, we further use our dispersions to explore nonlinear optical properties of PbI2 nanosheets, which show a great potential in ultrafast nonlinear saturable absorbers for Q-switchers, mode-lockers, and so on.
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RESULTS AND DISCUSSION
Figure 1. (a) Side view and (b) top view of PbI2 crystal structure. The dark grey and brown atoms represent Pb and I, respectively. (c) Schematic illustration of liquid-exfoliation of 2D PbI2 nanosheets via ultrasonication. Exfoliation and Characterization. As shown in Figure 1a and b, PbI2 has a canonical layered structure with covalently bonded I-PbI repeating units with a separation of 6.98 Å governed by weak van der Waals interactions.20 After being subjected to mild bath ultrasonication followed by centrifugation (CF) (to remove poorly exfoliated aggregates), stable few-layer PbI2 dispersions were readily obtained in the polar aprotic solvent (CH2)4CO. The Tyndall effect of the dispersion indicates its colloidal nature (Figure 1c). Several other solvents such as isopropanol (IPA), propylene carbonate (PC), cyclohexanone, and ethanol were also identified to be effective in exfoliating PbI2 with reasonable concentrations (Figure S1). Thermogravimetric analysis (TGA) (Figure S2) combined with knowledge of the weight of (PbI2 + remaining (CH2)4CO) after evaporation of the solvent for known volumes of dispersions enabled us to determine stock dispersion concentration. A sample of the stock dispersion was serially diluted with (CH2)4CO. A linear trend (R2=0.999) was found for the optical absorbance per unit-cell length at 345 nm (A345/l) against dispersion concentration (CPbI2). The absorption coefficient at λ = 345 nm was derived to be a345 ≈ 19.5 cm-2 mg−1 on the basis of the Lambert−Beer law (A = αCl) (Figure 2a). We used this measured absorption coefficient to estimate the concentrations of all subsequent dispersions. We found that CPbI2 (after centrifugation) increased steadily with the initial concentration of bulk PbI2 (CI), approaching up to 0.11 mg mL−1 at CI = 10.0 mg mL−1 (Figure S3). The yield of PbI2 nanosheets was about 6% at CI 1.0 mg mL−1 after centrifugation but decreased with increasing CI above 1.0 mg mL−1. Similar to exfoliation and dispersion of graphene and black phosphorus, CPbI2 was observed to scale with ω−0.5 (ω is centrifugation rate) (Figures S4 and S5). We further investigated exfoliation kinetics of PbI2 by postulating that 1) starting materials are comprised of a collection of multisheet platelets; 2) each platelet splits into two thinner platelets with equal probability at any of the (n−1) interfaces in an n-sheet platelet; 3) exfoliation is irreversible.21 Exfoliation steps are assumed to take place following a first-order rate law, with pseudofirst-order rate constant k, 𝑑𝐺𝑚 𝑑𝑡
𝑟
= ― (𝑚 ― 1)𝑘𝐺𝑚 +2𝑘∑𝑙 = 𝑚 + 1𝐺𝑙
and r is the maximum sheets amounts. Increases in Gm result from splitting of a flake Gl where l > m. This model is analytically solvable without matrix inversion, permitting solvation of this model with rate constants obtained experimentally.21 Experimental effective optical absorbances of dispersions before CF were plotted versus ultrasonic time (Figure S6). The absorbance growth kinetics seems to behave as we expect for a nominally first order growth process. We further applied a semilogarithmic kinetic model to the data shown in Figure S6 and acquired the result shown in Figure 2b, which discloses first-order time constants when the process governs dynamics over a given time. The long-time kinetics is in accordance with an apparent pseudo-first-order approach with rate constant k = 1.28 × 10-5 s-1 and half-life τ1/2 = 1297 min. The k value is larger than 6.0 × 10-6 s-1 for graphene and 2.0 × 10-6 s-1 for MoS2 exfoliation both in water with a nanolatex stabilizer. If all of the PbI2 in this dispersion were delaminated into few-layer flakes, one would expect to observe an absorbance close to 1.9 per cm, comparing reasonably with the absorption coefficient derived from Figure 2a. The discrepancy may be mainly due to the presence of thicker and readily precipitating PbI2 aggregates in our “before centrifugation” dispersion of Figure S6. When centrifuging to obtain the spectra and absorption data of Figure S7, we settled ∼82% of the PbI2 flakes containing large particulates and possibly a small fraction of few-sheet flakes. Figure 2c displays increases of absorbance versus square root of ultrasonic time (t1/2). Clearly, there are two distinct regions of behaviour: a long time behaviour (t > 9 h) and an original time behaviour. The two regions exhibit slopes of 0.00119 s-1/2 (t > 9 h) and 0.00148 s-1/2 (t < 9 h). These linear regions imply that the kinetic processes are controlled by diffusion and exfoliation is likely rate-limiting. Two similar domains with slopes of 0.0026 and 0.00086 s−1/2 were also found similar to graphene exfoliation in water.21 The ratio of the two distinct slopes 0.00119/0.00086 ≈ 2.21 suggests a ratio of exfoliation diffusion lengths of 2.212 ≈ 4.48 for PbI2 exfoliation in cyclopentanone compared to that for graphene in water. The dispersion stability was evaluated by monitoring concentration versus sedimentation time. The profile as displayed in Figure 2d exhibits an exponential decay and can be well approximated by c(t) = c0 + (cT-c0) e-t/τ, where c0 represents the concentration of the stable phase, cT is the initial concentration of the dispersion, and τ is the sedimentation time constant.19 Fitting the data yielded c0/cT = 0.831 and τ = 92 h, indicating that approximately 83.1% of PbI2 in the dispersion remained stably dispersed without sedimentation over extremely long times. Furthermore, no degradation of the dispersion resulted, even after freezing (-5 °C) for 2 h, which was indicated by the preservation of its absorption spectra after subjected to such treatment (Figure S8).
(1)
where Gm is the number density of m-sheet flakes and presents the thickest flakes in the dispersion. k is the rate constant for splitting
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(a)140
(b)0.0
345 = 19.5 cm-2 mg-1 2
80
(A/l) = (A/l)0 + CPbI2
0
-1.5 -2.0
(c)
0.8
0.
01
48
0.6
Sl
o
=
0
1
e
=
0.4
pe 2
9 11 .0
0.2
Sl op
A345/A345-Max
1.0
0.0 0
20
40
60
t1/2/min1/2
80
=5 .59 *1 0 4
-3.0 0
1000 2000 3000 4000 5000
t/min
0.075
(d) PbI2 Conc./mg mL-1
PbI2 conc./mg mL-1
2
-2.5
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1.2
e
-4
20
op
0
40
-1.0
Sl
*1
60
-0.5
71 7.
In(1-A345/A345-Max)
100
= e1
A345/I/m-1
120 R =0.999
op Sl
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
100
0.060 0.045 0.030 0.015 0.000
C(t) = 0.0448 + 0.0091*exp(-t/92.21) 0
100
200
300
400
500
600
Sedimentation time/h
Figure 2. (a) A345/l versus PbI2 dispersion concentration (tsonic = 6 h; CF: 3000 rpm, 30 min). (b) Semilogarithmic kinetic analysis of ln[1-A345(t)/A345-max]. (c) A345/A345-max versus square root of ultrasonic time. (d) PbI2 dispersion concentration versus sedimentation time (tsonic = 6 h; CF: 3000 rpm, 30 min). The CI in all the cases is 1 mg mL-1.
Exfoliated PbI2 nanosheets were characterized with a comprehensive suite of microscopy and spectroscopy techniques. Typical scanning electron microscopy (SEM) (Figure S9) and transmission electron microscopy (TEM) (Figure 3a) images together with energy dispersive X-ray spectroscopy (EDS) elemental maps and spectrum (Figure 2c-h) show a significant number of thin PbI2 flakes with lateral sizes in the range 100 nm − 1 μm. There are no apparent holes and other faults in the resultant nanosheets by high-resolution TEM (HRTEM) observation of 25 different objects (Figure 3b). HRTEM images and fast Fourier transform (FFT) confirm high crystalline nature of exfoliated PbI2. High-angle annular darkfield (HAADF) scanning TEM (STEM) observations (Figure 3g and h) illustrate that the hexagonal crystal structure of PbI2 was largely preserved after exfoliation. The lattice distance shown in Figure 3g is about 0.22 nm, in good agreement with the theoretical (110) plane spacing of PbI2.16 Atomic force microscopy (AFM) analyses show that most flakes have smooth surfaces (Figures 4 and S9). The lateral dimensions of the flakes agree well with SEM and TEM images. The AFM-measured thickness values of a large number of flakes were in the range of 2 to 3 nm, consistent with flakes 3-4 nanosheets in thickness. Note that apparent AFM heights from liquid-exfoliated samples are usually overestimated because of residual solvent and effects of capillary forces and adhesion as well as elasticity of probe tips or elasticity of the PbI2 material.22 As such, we speculate that a flake with an apparent height of 2 nm is most likely a monolayer or a bilayer. Hence few-layer PbI2 flakes were successfully obtained here.
Figure 3. (a) Low-magnification HAADF STEM image of PbI2 nanosheets. (b) HRTEM image of several stacked flakes. The inset shows corresponding FFT of the image. (c) HAADF STEM image of an individual PbI2 flake. EDS elemental maps of (d) Pb, (e) I, and (f) EDS spectrum taken from the region shown in (c). (g) Highresolution HAADF STEM image of PbI2 nanosheets with a lattice spacing of approximately 0.22 nm. The brown circle illustrates the iodine atom, while the silvery center circle is Pb. (h) Quasi-color image of g to guide eye. Inset shows a sketch of the PbI2 structure with darker atoms as iodine atoms and light shaded atoms as Pb atoms.
Figure 4. Tapping-mode AFM images of exfoliated PbI2 nanosheets deposited on a SiO2/Si wafer.
The X-ray diffraction (XRD) patterns (Figure 5a) of bulk PbI2 and exfoliated nanosheets both show typical diffraction
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ACS Photonics peaks of 12.67, 25.91, 34.27, and 39.52°, which can be ascribed to (001), (101), (102), and (110), respectively, of hexagonal PbI2 with a 2H type crystal structure (JCPDS file no.07-0235). Compare with bulk PbI2, PbI2 nanosheets exhibit a dramatically weaker (001) diffraction peak, suggesting loss of orientation during exfoliation. 2H–PbI2 with the space group D33d has three atoms and one layer with in a primitive unit cell (cell constants are a = 4.559 Å and c = 6.987 Å). As expected, two Raman active modes A1g and Eg, and two infrared active modes A2u and Eu were observed before and after exfoliation (Figure 5b).23 Specifically, Raman bands at frequencies of 73 and 95 cm-1 can be assigned to the Eg vibration mode due to shear type deformation of PbI2 layers and A1g vibrational mode arising from breathing deformation, respectively. It was found that after exfoliation, the intensity of all Raman modes became weaker and the peaks’ full widths at half-maximum (FWHMs) were broadened. Such occurrence is indicative of reduction of thickness of PbI2 as a result of exfoliation.24-25 XPS has been carried out to probe the chemical status of elements of the PbI2 nanosheets. The peaks with binding energies (BEs) at 138.2 and 143.0 eV (Figure 5c) can be assigned to Pb2+ 4f7/2 an Pb2+ 4f5/2, respectively, while the peaks with BEs at 619.2 and 630.7 eV (Figure 5d) correspond to I3d5/2 and I- 3d3/2, consistent with previously reported values for PbI2.26 This suggests high quality of the exfoliated PbI2 nanosheets. (a)
PbI2 nanosheets
(b)
A1g
30
Intensity/a. u.
(110)
(101)
(102)
(001)
(c)
20
40
PbI2 (#07-0235) 50
60
2Theta/°
70
Eg Pb I
PbI2 nanosheets
90
100
200
300
Raman shift/cm Pb 4f
Pb 4f7/2 138.2 eV
80
A1g
Bulk PbI2
Eg
Pb 4f5/2 143.0 eV
134 136 138 140 142 144 146 148
Binding energy/eV
(d)
-1
400
I 3d
I 3d5/2 619.2 eV
Intensity/a.u.
Intensity/a.u.
Bulk PbI2
615
I 3d3/2 630.7 eV
620
625
630
Binding energy/eV
criterion that L < z0, where L is sample thickness and z0 (𝑧0 = π 𝜔02/𝜆 , ω0 is beam waist radius and λ is wavelength of incident light) is beam diffraction length.27 To evaluate the NLO responses, the dispersions were adjusted to have a comparable linear transmittance of 43% at 515 nm and 51% at 532 nm, corresponding to the absorption coefficient α0 of 8.5 cm-1 and 6.6 cm-1, respectively.
Pulse width
λ (nm)
T0
α0 (cm-1)
αNL (cm/GW)
Imχ(3) (esu)
340 fs
515
0.43
8.5 0
-0.104 ±0.02
-(3.20 ±0.82)×10-14
6 ns
532
0.51
6.6 4
-21.5 ±7.7
-(6.86 ±2.48)×10-12
FOM (esu cm) (3.76 ±0.96) ×10-15 (1.03 ±0.37) ×10-12
Isat (GW/cm2) 28.6 0.20 ±0.02
Figure 6. Open-aperture Z-scan results of the PbI2 dispersions in IPA for (a) femtosecond pulses excitation at 515 nm and (b) nanosecond pulses excitation at 532 nm. The solid lines represent corresponding theoretical results. The gray dots represent the results of pure IPA solvent with an input pulse energy of (a) 120 nJ and (b) 100 μJ.
Eu+A2u
10
Intensity/a.u.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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635
Figure 5. (a) The XRD patterns and (b) Raman spectra of bulk PbI2 and PbI2 nanosheets. (c) Pb 4f and (d) I 3d XPS spectra of PbI2 nanosheets.
Application in Ultrafast Photonics. Nonlinear optical absorption (NLO) properties of PbI2 nanosheets were investigated through an open-aperture Z-scan method.27 In our work, mode-locked fiber laser and Q-switched Nd: YAG laser, whose pulse widths are about 340 fs and 6 ns, were operated at their frequency-doubled modes at 515 nm and 532 nm, respectively. The pulse repetition rate of the nanosecond pulsed laser was adjusted to 1 Hz, which is low enough to neglect the acoustic shock waves and cumulatively thermal effects.27-30 While the femtosecond pulsed laser was illuminated with a frequency of 100 Hz taking into account that the thermal effect from femtosecond pulse is weak.27 The laser beams were focused by a 15 cm focus lens, and beam waist radii were about 15 μm at 515 nm and 35 μm at 532 nm. All PbI2 dispersions were measured in 1 mm quartz cells, which strictly satisfied the
The open-aperture Z-scan data of the PbI2 dispersions both at fs and ns pulses excitation are shown in Figure 6. Saturable absorption responses which mainly originate from Pauli blocking or phase space filling were observed.31 As we can see from Figure 6a, the transmission change was about 15% at an excitation pulse energy of 60 nJ, corresponding to a peak irradiance of 24.3 GW /cm2, while it enlarged to 40% at 100 nJ with a peak irradiance of 40.4 GW /cm2. For the ns pulse excitation (Figure 6b), the transmission variation changed from 4% at 10 μJ (43.4 MW/cm2) to 23% at 20 μJ (86.8 MW/cm2). As a control, higher excitation energies were employed on pure IPA solvent (gray dots) and no obvious NLO behaviour was found. Therefore, we could conclude that the SA responses arose from PbI2 nanosheets in our experiments. According to the NLO theory, the Z-scan results can be solved through the propagation equation,27, 32-33 𝑑𝐼(𝑧)
= ―𝛼(𝐼)𝐼 (2) where I is the incident intensity, z is the propagation deep in the sample. In order to obtain the saturation intensity (Isat), the absorption coefficient can be written as31, 34 𝑑𝑧
𝛼0
(3) It is worth noting that Eq. (3) is built on the steady-state condition, i.e., the excited pulse width 𝜏p should be much longer than the intraband relaxation time to ensure that the excited electrons in the conduction band have sufficient time to achieve band-filling within the pulse duration. A few to tens of picoseconds’ decay time in PbI2 dispersion has been reported by Zhang et al.,35 which is much shorter than our ns pulse (6 ns) and longer than the fs pulse (340 fs). Therefore, we could safely 𝛼(𝐼) = 1 + 𝐼/𝐼𝑠𝑎𝑡
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ACS Photonics employ Eq. (3) to estimate the Isat for ns excitation. For fs pulse excitation,36 we could use the equation 𝐼𝑠𝑎𝑡 = ℏω/στp,37-38 where σ is the absorption cross-section and ℏω is the incident photon energy. The obtained saturation intensities were 28.6 and 0.2 GW/cm2 for the fs and ns pulses, respectively. To estimate the third-order nonlinear optical susceptibility, the total absorption can be expressed in the form of 𝛼(𝐼) = 𝛼0 + 𝛼𝑁𝐿𝐼, where 𝛼𝑁𝐿 is the nonlinear optical absorption coefficient. The imaginary part of the third order nonlinear susceptibility (Imχ(3)) which is related to αNL, could be obtained ―7 2 using the relation that Imχ(3) = [10 𝑐𝜆𝑛 96𝜋2]𝛼𝑁𝐿, where c is the velocity of light and n is the refractive index.27 For comparison, we have calculated the figure of merit (FOM), defined as |Imχ(3)/𝛼0|, to remove the discrepancy caused by the linear absorption α0. The fitting results were summarized in Table 1. It can be clearly seen that the αNL, Imχ(3), and FOM values at 532 nm ns pulses are about 2 orders larger than those at 515 nm fs pulses, suggesting that PbI2 nanosheets exhibit superior SA performances under ns pulses excitation. Table 1. Linear and nonlinear optical parameters of PbI2 dispersions in IPA.
According to the above discussion, we could perform the slow SA model for 340 fs pulse and the fast SA model for 6 ns pulse excitation. The fitting results are shown in Figure 7 and the fitting parameters are summarized in Table 2. To evaluate the ability of PbI2 as a saturable absorber, we consider the ratio of σes and σgs instead of their absolute values as they can vary from the fabrication procedure rather than the nature of material.41 The σes/σgs ratio of PbI2 dispersions are 0.012 for 515 nm fs pulse and 0.015 for 532 nm ns pulse illuminating. σes is the source of loss due to residual absorption in high intensity in laser cavity, which is fairly smaller than the σgs for saturable absorbers. As a result, the ratio of σes/σgs < 1 is reasonable. A smaller σes/σgs ratio indicates less excited-state loss for saturable absorber. This is crucial for enhanced passive SA performances.39 Table 2. The parameters used for fitting based on the fast and slow saturable absorber models. SA Pulse Wavelength N Tmax σgs σes σes/σgs width (cm-3) (cm2) (cm2) 515 nm 2.21 0.990 3.85 4.57 0.012 340 ×10+17 ×10×10Slow fs 17 19 532 nm Fast
6 ns
In addition to saturation intensity, the excited-state absorption (ESA) cross section σes and ground-state absorption (GSA) cross section σgs are fundamental features to evaluate the saturability of a material, which are also essential for the application of passive laser Q-switching. Fast and slow SA models have been employed to estimate the σes and σgs of PbI2 dispersions at 515 nm fs and 523 nm ns excitation, respectively.39 In the case that the incident pulse width is shorter than the excited-state decay time, the slow SA model analyzed by Frantz-Nodvik equation can be employed39-40 𝑇 = 𝑇0 +
𝑇𝐹𝑁 ― 𝑇0
(𝑇
𝑚𝑎𝑥 1 ― 𝑇0 ―𝑁𝜎𝑔𝑠𝐿 𝑇0 = 𝑒 ,
(4)
― 𝑇0) ―𝑁𝜎𝑒𝑠𝐿
𝑇𝑚𝑎𝑥 = 𝑒 where , 𝑇0 is the linear transmission, L is the sample thickness, N is the absorber density, and Tmax is the maximum transmission . TFN is the transmission of an “ideal” saturable absorber (σes ≡ 0) which 𝜎 𝐸(0) ― 1]}/𝜎𝑔𝑠𝐸(0), can be described as 𝑇FN = 𝑙𝑛{1 + 𝑇0[𝑒 𝑔𝑠 where 𝐸(0) is the total pulse energy (photons per unit area). In the case of the pulse duration longer than the excited-state decay time, fast SA model can be used and the expression is named as Hercher’s equation 𝑇
[
𝑆 + 𝑇 ∙ 𝐼(0) 𝐷 𝑆 + 𝐼(0)
]
= 𝑇0
(5)
where 𝐷 = (𝜎𝑔𝑠 ― 𝜎𝑒𝑠)/𝜎𝑒𝑠, S = 1/𝜏𝜎𝑔𝑠, and τ denotes the interband relaxation time. 𝐼(0) is the input photons per unit time per unit area.
7.78 ×10+18
0.990
8.53 ×10-
1.29 ×10-
19
20
0.015
Figure 8. The (a) FOM, (b) saturation intensity and (c) σgs/σes ratio comparisons between PbI2 dispersion and other two-dimensional semiconductor materials in the visible region.9, 1443
Albeit with lower SA properties as compared with those obtained for organic-inorganic lead based 2D perovskites,42-43 our PbI2 dispersions showed comparable FOM values to emerging 2D graphene,44 BP,33 and MoS244 under 515 nm 340 fs excitation (Figure 8). The σes/σgs ratios of PbI2 dispersions are remarkably smaller than those of BP both at fast and slow pulse excitations. As for the Isat, the value of PbI2 is about two times of BP, whereas PbI2 is much more stable than BP in the air. The most common application of SA feature is in laser technology for high-speed optical modulators of optical losses placed in laser cavities.45 Materials with low Isat, small σes/σgs ratio (low nonsaturable loss), and large FOM (strong NLO response) are highly desirable as an excellent saturable absorber. The high stability along with low nonsaturable loss renders PbI2 useful as a suitable optical material candidate in passive Q-switch, modelocking, and other ultrafast optoelectronic devices in the visible regime.
CONCLUSION Figure 7. Normalized transmissions of PbI2 dispersions in IPA depending on the incident light fluence and intensity fitted by using (a) a slow SA model and (b) a fast SA model. The insets are enlargements of their counterparts.
In summary, high-quality 2D PbI2 nanosheets were obtained by simply employing an ultrasonication-assisted liquid-phase exfoliation process. An unexplored solvent cyclopentanone was discovered, which afforded more efficient exfoliation and dispersion of few-layer PbI2 than many others. We found that PbI2 exfoliation kinetics fitted an apparent pseudo-first-order
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process. We further investigated the SA properties of PbI2 nanosheets both at fs pulse of 515 nm and ns pulse of 532 nm. PbI2 nanosheets were found to exhibit stronger SA response at 6 ns pulse excitation than that at 340 fs pulse excitation. ESA and GSA cross sections were estimated, which suggested that PbI2 could serve as ultrafast nonlinear saturable absorbers for Q-switchers, mode-lockers, among others.
(5) Mounet, N.; Gibertini, M.; Schwaller, P.; Campi, D.; Merkys, A.; Marrazzo, A.; Sohier, T.; Castelli, I. E.; Cepellotti, A.; Pizzi, G., Two-Dimensional Materials from High-Throughput Computational Exfoliation of Experimentally Known Compounds. Nat. Nanotechnol. 2016, 13, 246–252. (6) Geim, A. K., Graphene: Status and Prospects. Science 2009, 324, 1530-1534. (7) Zhang, Y. Q.; Dong, N. N.; Tao, H. C.; Yan, C.; Huang, ASSOCIATED CONTENT J. W.; Liu, T. F.; Robertson, A. W.; Texter, J.; Wang, J.; Sun, Supporting Information. “This material is available free of charge Z. Y., Exfoliation of Stable 2D Black Phosphorus for Device via the Internet at http://pubs.acs.org.” Fabrication. Chem. Mater. 2017, 29, 6445−6456. (8) Wang, Q. H.; Kalantarzadeh, K.; Kis, A.; Coleman, J. N.; AUTHOR INFORMATION Strano, M. S., Electronics and Optoelectronics of TwoCorresponding Author Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. * E-mail:
[email protected]. (9) Wu, Y. Z.; Islam, A.; Yang, X. D.; Qin, C. J.; Liu, J.; * E-mail:
[email protected]. Zhang, K.; Peng, W. Q.; Han, L. Y., Retarding the ORCID Crystallization of PbI2 for Highly Reproducible PlanarQun Fan: 0000-0002-5487-7321 Structured Perovskite Solar Cells via Sequential Deposition. Jiawei Huang: 0000-0001-5108-9904 Energy Environ. Sci. 2014, 7, 2934-2938. Ningning Dong: 0000-0002-8722-1599 (10) Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Yang, W. S.; Seo, Jun Wang: 0000-0003-0064-3551 J.; Yun, J. S.; Ho-Baillie, A.; Huang, S.; Green, M. A.; Zhenyu Sun: 0000-0001-5788-9339 Seidel, J., Beneficial Effects of PbI2 Incorporated in OrganoAuthor Contributions Lead Halide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1502104. ‡Q.F. and ‡J.H. contributed equally to this study and share first authorship. (11) Matuchova, M.; Zdansky, K.; Zavadil, J., Lead Iodide Crystals Prepared Under Stoichiometric and Notes Nonstoichiometric Conditions. Mater. Sci. Eng., B 2009, 165, 60-63. The authors declare no competing financial interest. (12) Zhong, M.; Zhang, S.; Huang, L.; You, J.; Wei, Z.; Liu, ACKNOWLEDGMENT X.; Li, J., Large-Scale 2D PbI2 Monolayers: Experimental Realization and Their Indirect Band-Gap Related Properties. This work was supported by the State Key Laboratory of OrganicNanoscale 2017, 9, 3736–3741. Inorganic Composites (No. oic-201503005); Beijing Natural (13) Yaffe, O.; Chernikov, A.; Norman, Z. M.; Zhong, Y.; Science Foundation (No. 2192039); Beijing National Laboratory for Molecular Sciences (BNLMS20160133); the State Key Velauthapillai, A.; van der Zande, A.; Owen, J. S.; Heinz, T. Laboratory of Separation Membranes and Membrane Processes F., Excitons in Ultrathin Organic-Inorganic Perovskite (Tianjin Polytechnic University, No. M2-201704). J.W. thanks the Crystals. Phys. Rev. B 2015, 92, 045414-045420. support from NSFC (61675217, 61875213 and 61522510), the (14) Manser, J. S.; Christians, J. A.; Kamat, P. V., Intriguing Strategic Priority Research Program of CAS (XDB16030700), the Optoelectronic Properties of Metal Halide Perovskites. Key Research Program of Frontier Science of CAS (QYZDBChem. Rev. 2016, 116, 12956-13008. SSW-JSC041), the Program of Shanghai Academic Research (15) Zhong, M. Z.; Huang, L.; Deng, H. X.; Wang, X. T.; Li, Leader (no. 17XD1403900), and the Youth Innovation Promotion B.; Wei, Z. M.; Li, J. B., Flexible Photodetectors Based on Association, CAS. Phase Dependent PbI2 Single Crystals. J. Mater. Chem. C REFERENCES 2016, 4, 6492-6499. (16) Wang, Y.; Pei, H.; Sun, H.; Zhu, X. H.; Yang, D. Y.; (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, Gao, X. Y., Mechanical Exfoliation and Raman Spectra of D. A.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, Ultrathin PbI2 Single Crystal. Mater. Lett. 2016, 168, 68-71. A. A., Electric Field Effect in Atomically Thin Carbon (17) Wang, G.; Li, D.; Cheng, H. C.; Li, Y.; Chen, C. Y.; Films. Science 2004, 306, 666-669. Yin, A.; Zhao, Z.; Lin, Z.; Wu, H.; He, Q., Wafer-Scale (2) Sun, Z. Y.; Ma, T.; Tao, H. C.; Fan, Q.; Han, B. X., Growth of Large Arrays of Perovskite Microplate Crystals Fundamentals and Challenges of Electrochemical CO2 for Functional Electronics and Optoelectronics. Sci. Adv. Reduction Using Two-Dimensional Materials. Chem 2017, 2015, 1, e1500613. 3, 560-587. (18) Zhang, J. Y.; Song, T.; Zhang, Z. J.; Ding, K.; Huang, (3) Sun, Z. Y.; Talreja, N.; C., Tao, H. C.; Texter, J.; Muhler, F.; Sun, B. Q., Layered Ultrathin PbI2 Single Crystal for M.; Strunk, J.; Chen, J. F., Catalysis of Carbon Dioxide High Sensitivity Flexible Photodetector. J. Mater. Chem. C Photoreduction on Nanosheets: Fundamentals and 2015, 3, 4402-4406. Challenges. Angew. Chem. Int. Ed. 2017, 57, 7610-7627. (19) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; (4) Tao, H. C.; Zhang, Y. Q.; Gao, Y. N.; Sun, Z. Y.; Yan, Sun, Z. Y.; De, S.; Mcgovern, I. T.; Holland, B.; Byrne, M.; C.; Texter, J., Scalable Exfoliation and Dispersion of TwoGun'Ko, Y. K., High-Yield Production of Graphene by Dimensional Materials - An Update. Phys. Chem. Chem. Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. Phys. 2016, 19, 921-960. 2008, 3, 563-568. ACS Paragon Plus Environment
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