Plasmon-Induced Energy Transfer and Photoluminescence

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Plasmon induced energy transfer and photoluminescence manipulation in MoS2 with different number of layers Jiahao Yan, Churong Ma, Pu Liu, and Guowei Yang ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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Plasmon induced energy transfer and photoluminescence manipulation in MoS2 with different number of layers

Jiahao Yan, Churong Ma, Pu Liu & Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China. *Corresponding author: [email protected]

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ABSTRACT Molybdenum disulfide (MoS2) monolayer or few-layer with intriguing physical properties enables a wide range of applications such as photocatalysis and photodetection. The controllable light-matter interaction in MoS2 nanoflakes with different number of layers is critical for developing new optoelectronic functionalities. Recently, plasmonic nanostructures are used to obtain strong near-field enhancement for the effective photoluminescence (PL) manipulation of MoS2. However, it is still unclear whether the PL manipulation is dominated by electron injection processes or the strong field induced Purcell effect, so far. Here, we investigate the PL manipulation of MoS2 nanoflakes with different number of layers using Au nanoparticles with different aggregate states. Combining the measured PL and scattering spectra, the Kelvin probe force microscopy (KPFM) images at single-particle level and the numerical simulations, we figure out how the electron injection and strong-field enhancement contribute to the PL manipulation and why PL quenching occurs in few-layer flakes but PL enhancement in thicker flakes. These findings would give us more profound understanding on the interaction between two-dimensional (2D) materials and plasmonic nanostructures. KEYWORDS: molybdenum disulfide, photoluminescence manipulation, gold nanoparticles, plasmon

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MoS2 as a member of 2D transition metal dichalcogenides shows intriguing physical properties and promising applications in optoelectronic.1 And their optical properties such as PL emission can be greatly modified through generating strong field enhancement. Specifically, plasmonic nanostructures which can achieve light harvesting and localized field enhancement are very suitable to realize effective PL manipulation of MoS2.2 Different kinds of plasmonic nanoantenna arrays like nanodiscs and nanorods have been used to enhance the PL emission from MoS2 monolayer.3-5 Besides, Fano resonances transition

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and hot electrons induced structural phase

have been observed in MoS2-plasmon hybrid systems. To design a

sub-micro-scale optical platform, some efforts were made to realize the effective PL manipulation based on a single plasmonic nanoantenna.10-11 And the characterization on single-particle level gives us an opportunity to explore different coupling situations between plasmonic nanostructures and MoS2.12 Although plasmonic nanostructures with strong localized electric field are expected to enhance the PL emission from MoS2 nanoflakes, PL quenching arising from the plasmonic effect has also been reported recently. Both the far-field directional emission effect13 and the inverse electron injection mechanism were proposed to explain PL quenching of MoS2.14 However, it remains unclear which effect is dominant and why both PL enhancement and quenching can be observed in the plasmon-MoS2 hybrid system. To reveal the law of energy transfer or electron transfer between nanoantennas and MoS2 thoroughly, we fabricated MoS2 nanoflakes with different thickness by mechanical exfoliation15 to build the plasmon-MoS2 3


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hybrid structures.16,17 Here, through investigating the interaction between MoS2 nanoflakes with different thickness and Au aggregates with different assembling structures, we revealed the rule of PL manipulation in plasmon-MoS2 hybrid structures. For monolayer or few-layer MoS2 (d15 nm), the PL intensities were significantly enhanced because of the strong near-field enhancement and the hot electron injection from Au aggregates to MoS2. And for MoS2 nanoflakes with thickness between 5 nm and 15 nm, the inverse electron injection effect (from MoS2 to Au) which contributes to the PL quenching and the near-field enhancement which leads to the PL enhancement would reach equilibrium, so the PL intensities almost remain unchanged. The simulated field distributions at Au-MoS2 interfaces confirm the variation trend of field enhancement versus the number of layers, and the KPFM images obtained under illumination or dark state reveal the relative variation of Fermi levels, which indicates the injection direction of electrons 18,19 in the Au-MoS2 hybrid structures. Studying the interaction between plasmonic nanostructures and MoS2 nanoflakes with different thickness has important significance, because it directs us to design effective hybrid functional devices in photodetection20-23 and photocatalysis24,25 where not only monolayer but also multilayer MoS2 are widely used.

RESULTS AND DISCUSSION 4


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MoS2 nanoflakes with different number of layers were obtained by mechanical exfoliation and then transferred on a Si/SiO2 substrate. Au nanoparticle colloid fabricated by laser ablation in liquid (LAL) was deposited on the substrate, and Au aggregates with different sizes were formed on MoS2 nanoflakes during the evaporation process. The PL spectra of different Au aggregates were measured under the 514 nm laser excitation as shown in Fig. 1a (see Methods). Optical microscopy and atomic force microscopy (AFM) were used to confirm the layer number of MoS2 nanoflakes. And scanning electron microscope (SEM) images, AFM images and optical microscopy images are matched together to clarify different Au aggregates. In Fig. 1b, the SEM image of a typical few-layer MoS2 is presented. And the large magnification SEM images show a single Au nanoparticle and an Au aggregate. It should be noted that the nanoparticles fabricated by LAL have a broad size distribution26-28 which is beneficial for supporting broadband multimodal plasmon resonances. The AFM image (Fig. 1c) and the height profiles (Fig. 1d) taken along the red and blue lines in Fig. 1c indicate the thickness of this MoS2 flake is 4.8 nm (6 layers). Through the bright field optical microscopy image (Fig. 1e) and increasing the brightness, we can observe the “black dots” which represent the Au aggregates. Switching the laser beam on the “black dots”, the PL spectra at Point A, B and C (see Fig. 1b) can be measured precisely as shown in Fig. 1f. The PL intensities with a single Au nanoparticle and without Au are almost the same. Then we increased the area of Au nanoparticles to enhance the PL signals arising from the interaction between Au and MoS2. Just as the PL intensity at Point A with the Au aggregates, the 5


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PL intensity is slightly weakened. This phenomenon is a little counterintuitive since stronger field enhancement was expected to generate in Au aggregates rather than in a single Au nanoparticle, which would be beneficial for larger PL enhancement. The dark field microscopy image and the dark field scattering spectra at Point A and C were also obtained as shown in Fig. 1e and 1g. Au aggregates exhibit broader scattering peak and can generate much stronger scattering at λ = 675nm , but the narrow exciton peak from MoS2 layer cannot be observed at both Point A and C which means the PL emission from MoS2 layer is still weak. Using the same method, we also observed the PL quenching caused by Au aggregates on MoS2 nanoflakes with two layers and four layers (see details in Supporting Information Fig. S1 and S2). All experimental results indicate that pure MoS2 nanoflakes without Au exhibit the strongest PL emission while the PL intensities are weakened when increasing the size of Au aggregates. The bandgap of MoS2 will change and the direct excitonic transition will weaken when increasing the layer number.29 Since Zhen Li et al30 have reported the indirect band PL enhancement in metal/thick MoS2 heterojunctions, we only focus on how plasmon effect influences the direct excitonic transition of multilayer MoS2. To investigate the interaction between Au aggregates and thicker MoS2 nanoflake, two typical MoS2 flakes with thickness of 6.4 nm and 8 nm were chosen as shown in Fig. 2a and b. The specific morphologies of Au aggregates are shown in Fig. S3 and 4. Au aggregates with different sizes from dimer, trimer to large oligomers were selected to measure their PL spectra. To find the MoS2 flakes with desired thickness preliminarily, 6


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we examined the color of different flakes as shown in Fig. S3 and 4. The thickness of these two flakes was confirmed by AFM images and the height profiles in Fig. 2d and d. Following the track marked in Fig. 2a and b, we measured the PL spectra of pure MoS2, small aggregates and large aggregates as shown in Fig. 2e and f. Unexpectedly, the PL intensity from MoS2 almost remained unchanged no matter how many Au nanoparticles place on it. In Fig. S5 (Supporting Information), we studied the interaction between Au aggregates and the MoS2 nanoflake with the thickness of 16.8 nm. From the measured PL spectra at different points, we can see large Au aggregates (Point D and E in Fig. S5d) can enhance the PL emission from the MoS2 flake. And when shrinking the aggregates (Point F), the PL intensity decreases gradually. Finally, for the single Au nanoparticles (Point A and B), the PL intensity is nearly equal to that of pure MoS2 flake without Au. This PL manipulation situation is totally different from that of few-layer MoS2 as shown in Fig. 1. The PL enhancement caused by Au aggregates becomes more obvious in the Au-MoS2 hybrid system when further increasing the thickness of MoS2. As shown in Fig. 3a, a large and thick MoS2 flake was chosen. From the AFM image and the height profile in Fig. 3b, we can confirm that the thickness is about 24 nm. The large magnification SEM images in Fig. 3c reveal different Au aggregates like Au dimer (Point B and F), small size oligomers (Point G) and large aggregates (Point A and D) on this MoS2 flake. The bright field and dark field optical microscopy images are presented in Fig. S6. After matching different “black dots” in bright field optical 7


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image with different points in Fig. 3a, we could measure the PL spectra of different points as shown in Fig. 3d. Pure MoS2 flake shows the lowest PL intensity, and PL intensities are slightly increased after placing Au nanoparticle dimers on it. For the Au oligomers containing more nanoparticles (Point G), the PL intensities can be increased by 45% compared with that of pure MoS2. And for large Au aggregates (Point A and D), the PL intensities nearly triple compared with that of pure MoS2. The dark field scattering spectra of different aggregates were also measured as shown in Fig. 3e. Considering the multimodal plasmon resonant modes generated in Au nanoparticles with different sizes 31 and the plasmon hybridization induced mode shift and broadening,32 larger Au aggregates show much broader and stronger plasmon resonant peaks, which are favorable to the PL enhancement. The scattering spectra of Point A, D and G show obvious MoS2 excitonic peak at λ = 681nm , and this excitonic peak cannot be observed in the scattering from Au aggregates on few-layer MoS2 (see Fig. 1g). It thus indicates that the coupling and enhancement effect is much more obvious in the Au-thicker (>30 layers) MoS2 heterostructures than in the Au-thinner ( EF − MoS 2 − EF − Au . However, the hot electrons transferred from Au aggregates to MoS2 are much less than the electrons injected from MoS2 to Au, because efficient generation of hot electrons appears only in small nanoparticles with diameters below 20 nm,39 and such small contact area prevents effective hot electron injection.40 To confirm our theoretical prediction above, the KPFM was used to analyze the changes of surface potential at nanometer scale. A typical thicker MoS2 flake on the indium tin oxide (ITO) substrate is shown in Fig. 5a. From the inset height profile, we 12


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can see the thickness is 18.4 nm. The KPFM surface potential images of this Au-MoS2 heterostructure in the dark and under white light illumination are shown in Fig. 6b. To analysis the potential differences between Au aggregates and MoS2 quantificationally, we plot the surface potential profiles (see Fig. 6c) along the orange and blue lines marked in Fig. 6b. The potential differences between Au and MoS2 along blue and orange lines are 68 mV and 56 mV respectively in the dark. Under illumination, the potential differences are 69 mV and 58 mV respectively which nearly remain unchanged. The electron injection from Au aggregates to MoS2 layer would lift the Fermi level of MoS2 and then narrow the potential difference, while the electron injection from MoS2 to Au would lift the Fermi level of Au and then widen the potential difference.18 So the unchanged potential differences mean that the charge transport between Au and this MoS2 flake is unobvious or counterbalanced. In theory, the indirect (1.3-1.4eV) PL emission is dominated in this 23-layer MoS2 flakes (see Fig. 6e), so the conduction band is lower than that of few-layer MoS2 and the electrons on direct band are much less than that of few-layer MoS2. Therefore, the electron injection process from thicker MoS2 to Au is greatly weakened compared with that of few-layer MoS2. And this electron injection from MoS2 and the hot electrons from Au reach a balanced state. So we can conclude that it is the field enhancement mechanism that dominates the PL enhancement of thicker (>20 layers) MoS2. A typical few-layer MoS2 was investigated as shown in Fig. 6d. Since MoS2 nanoflakes were transferred on ITO substrates, surface is a bit rough. In our case, we 13


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thus measured the height (4.8 nm) through the lowest points in height profile. The schematic diagram in Fig. 6e illustrates that the electrons mainly stimulate to the direct conduction band under excitation for few-layer MoS2 nanoflakes. So the electron injection from MoS2 to Au could be much more prominent than that of the Au-thicker MoS2 heterostructures. This electron injection effect can be demonstrated by KPFM surface potential images in the dark and under illumination as shown in Fig. 6f. Two Au nanoparticles were investigated, and the potential line profiles were shown in Fig. 6g. Compared with 23-layer MoS2, the potential difference between Au and few-layer MoS2 is reduced, because the Fermi level lifts in 6-layer MoS2. The potential differences between Au and MoS2 along blue and orange lines are 22 mV and 21 mV respectively in the dark. Under illumination, the potential differences increase to 30 mV and 29 mV respectively which represent effective electron injection from MoS2 to Au aggregates lifts Au aggregates’ Fermi level. As for the value of surface potential difference in our case, it’s a normal value and very common in other published works,18,41 although this kind of Au-MoS2 hybrid structure is not an efficient carrier injection system like P-N junction or other semiconductor heterostructures. What’s more, the cantilever is above the top surface of nanoparticles whose surface potential changes much less than the interface between nanoparticles and MoS2, because the electron injection is not significant at top surface. If we put large size Au aggregates on monolayer or few-layer MoS2, much larger contact area can be generated and much more electrons can be generated under strong near field. So large amount of electrons injecting from the direct conduction band to Au 14


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aggregates leads to PL quenching. While for single Au nanoparticles, the weaker field enhancement and smaller contact area make the quenching effect become unobvious. One difference between PL measurement and KPFM measurement is the substrates we used. To make substrate conductive, we used ITO glass as a substrate in KPFM measurement. In Fig. S12, we also measured the PL spectra of Au-MoS2 hybrid structures on the ITO substrate to make sure the PL manipulation phenomena is independent of substrates. From the discussion above, it can be easily understood why PL intensities nearly remain unchanged in Fig. 2. For MoS2 flakes with the thickness from 6.4 nm to 10 nm, the electron injection effect and the field enhancement effect reach a balanced state. That is, the plasmon induced field enhancement increases the PL intensities while the electron injection from MoS2 to Au quenches the PL intensities, so the PL intensities of MoS2 remain unchanged no matter which kinds of Au aggregates on it.

CONCLUSIONS In summary, the PL enhancement or quenching can be achieved in the Au-MoS2 hybrid structrure through tuning the thickness of MoS2 or the size of Au aggregate. The competition between electrons transfer and plasmon effect lead to PL quenching in monolayer and few-layer (less than 6 layers) MoS2, while it contributes to the PL enhancement in thicker (more than 20 layers) MoS2 flakes. This work thus demonstrated two important mechanisms to manipulate the PL intensities of MoS2 nanoflakes. Through analyzing the field enhancements at the interfaces between Au aggregates and MoS2 with different thickness and the surface potential differences 15


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obtained from KPFM, we found out that the near field enhancement dominates the PL enhancement process and the electron injection from MoS2 to Au dominates the PL quenching process. Therefore, our findings open up a strategy for realizing the desired PL manipulation in the plasmon-2D materials hybrid system with different number of layers and different aggregation states, which can find more applications in novel optoelectronic devices and photoelectrochemical devices.

METHODS Fabrication of MoS2 flakes and Au aggregates. MoS2 nanoflakes were mechanically exfoliated from their bulk crystals (SPI Supplies) on Si/SiO2 substrates with a 200 nm thick oxide layer. Before exfoliation, the substrates were cleaned by acetone, ethanol and deionized water for 30 min, followed by oxygen plasmon treatment for 60s. Au nanoparticles with wide size distribution were fabricated by femtosecond laser ablation in deionized water on an Au target using a Legend Elite Series ultrafast laser (Coherent Inc.). After 15 min of focused laser ablation process, we obtained a gold colloid suspension. Dilute this colloid suspension to some degree, and then transfer one drop of the solution onto MoS2 flakes loaded on Si/SiO2 substrate. During the evaporation process, different kinds of Au aggregates can be formed. Finally, the Au-MoS2 hybrid structures were annealed in an argon environment at atmospheric pressure at 250oC for 2h to improve the contact quality between Au and MoS2. Optical characterization and simulation. Samples were excited by a Renishaw 16


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inVia confocal Raman microscope using a 514 nm laser and a 50x objective lens (NA=0.75) at room temperature. The spot size of focused laser beam is about 2μm (Fig. S13) which can cover most Au aggregates. The signals of PL were collected using the same objective lens and then displayed using a monochromator with a grating of 1,800 lines/mm. The spectra with a spectral resolution of ~1 cm–1 were finally obtained and recorded by a CCD detector. The typical integration time and excitation power were maintained on the order of 10 s and 1 mW, respectively, without any observable heating effect. And the scattering spectra of different Au aggregates were collected using a dark-field optical microscope (Olympus BX51) integrated with a monochromator (Princeton Instruments, ISOPlane 160) and a charge-coupled device (CCD) camera (Princeton Instruments, Pixis 400B_eXcelon). The oblique incident white light was illuminated on the aggregates, and the scattered light was collected by the same dark-field objective on top (NA=0.80). The simulated near field distributions were calculated by using the finite-difference time-domain method (FDTD Solutions 8.6.0, Lumerical Solutions, Inc.). Au nanoparticles were illuminated with a linear polarized normal incident TFSF (total-field scattered-field) source (300-90 nm). SEM, AFM and KPFM Characterization. SEM images were obtained by Auriga-4525 (Carl Zeiss). AFM and KPFM measurements were conducted using a Bruker Dimension Icon. PtIr-coated silicon cantilevers were used as a conductive tip. Hence, the surface potential (SP) values of Au-MoS2 samples measured in dark and under illumination by KPFM are calculated as SP=SPAu-MoS2-SPPtIr. A 100W xenon 17


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lamp was chosen as the light source.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org Detailed characterization on few-layer MoS2 flakes (Figure S1,S2), microscope and SEM images of typical MoS2 flakes (Figure S3,S4), detailed characterization on thick MoS2 flakes (Figure S5-S7), comparison of PL intensities among different Au-MoS2 samples (Figure S8), simulated field distribution at MoS2-Au interfaces (Figure S9, S10), simulated radiation patterns (Figure S11), detailed characterization of the MoS2 sample used in KPFM measurement (Figure S12) and microscope image of a typical MoS2 flake under laser beam (Figure S13).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The National Basic Research Program of China (2014CB931700), the National Natural Science Foundation of China (91233203) and State Key Laboratory of Optoelectronic Materials and Technologies supported this work. 18


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Figure Captions

Figure 1. PL quenching in a six-layer MoS2. (a) Schematic representation of the Au-MoS2 heterostructure on the Si/SiO2 substrate under laser excitation. (b) SEM images of the investigated MoS2 flake. High magnification SEM images correlate to the points marked in the lower magnification image. The scale bars in high magnification SEM images are 100 nm. (c) AFM measurement on this MoS2 flake. (d) The corresponding height profiles along lines marked in (c) showing the thickness is 4.8 nm. (e) Bright field and dark field optical images. (f) PL intensity comparison of different points marked in (b) excited at 514 nm. (g) Dark field scattering spectra at Point A and C. The red bar shows the location of exciton peak of MoS2.

Figure 2. PL manipulation of MoS2 nanoflakes ( 6.4 nm < d < 10 nm ) by Au aggregates. (a) and (b) SEM images of two typical MoS2 flakes. (c) and (d) AFM images and the inserted height profiles showing the thickness of MoS2 flakes is 6.4 nm and 8 nm respectively. (e) and (f) PL intensity comparison of different points marked in (a) and (b) excited at 514 nm.

Figure 3. PL enhancement of a 30-layer MoS2 flake. (a) The SEM image of a typical thick MoS2 flake. (b) The AFM image and the inserted height profile showing the thickness of MoS2 flake is 24 nm. (c) High magnification SEM images of different Au aggregates on MoS2. The scale bars are all set to 200 nm. (d) The PL spectra of 25


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different points marked in (a) excited at 514 nm. (e) Dark field scattering spectra of different Au aggregates. The red bar shows the location of exciton peak of MoS2.

Figure 4. PL intensity versus the area of Au aggregates and the thickness of MoS2. (a) The change of PL intensity versus area of Au aggregates on MoS2 layers with different thickness (4.8 nm, 6.4 nm, 8 nm and 24 nm). (b) PL spectra of MoS2 with different number of layers without Au aggregates. Arrows indicate the variation trend of A and B peaks.

Figure 5. Calculated electric field distributions at the Au-MoS2 interface. (a) (b) Electric field enhancement distributions at the interface between Au aggregates and MoS2 with thickness of 3 nm at λ = 514nm and λ = 682nm (exciton peak). (c) (d) The field enhancement distributions at the interface between Au aggregates and MoS2 with thickness of 25 nm at λ = 514nm

and λ = 682nm . (e) (f) The field

enhancement distributions of single Au nanoparticles. (g) The field enhancement distribution of an Au dimer. (h) The field enhancement distribution of an Au tetramer.

Figure 6. KPFM measurement of the Au-MoS2 heterostructures. (a) The AFM image of a thick MoS2 flake with the thickness of 18.4 nm. (b) The KPFM surface potential images under white light illumination and in the dark. (c) Line scanned potential profiles along the blue and orange lines marked in (b). (d) The AFM image of a thin MoS2 flake with the thickness of 4.8 nm. (e) Schematic showing the electron 26


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transports in the Au-thick MoS2 heterostructures and Au-thin MoS2 heterostructures. (f) The KPFM surface potential images of thin MoS2 under white light illumination and in the dark. (g) Line scanned potential profiles along the blue and orange lines marked in (f).

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Figure 2

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Figure 4

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Title: Plasmon induced energy transfer and photoluminescence manipulation in MoS2 with different number of layers Authors: Jiahao Yan, Churong Ma, Pu Liu & Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China. *Corresponding author: [email protected]

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