Photo carrier generation and extraction in photo responsive devices like photodetectors and photovoltaics is generally based on semiconducting properties of the active layer and the electric field generated at the interfaces (p–n junctions, Schottky junctions, etc.). The performance of such devices depends on many parameters like purity of the semiconductors, doping concentrations, interface energy level alignment, and defects. Avoiding such elaborate dependence can be of great interest in futuristic photo responsive devices. Non-centrosymmetric crystals that exhibit photoresponse phenomena based on bulk photovoltaic effect (BPE) can be a viable candidate for developing photo responsive devices with minimum materials and interfaces.11. L. Z. Tan, F. Zheng, S. M. Young, F. Wang, S. Liu, and A. M. Rappe, “ Shift current bulk photovoltaic effect in polar materials–hybrid and oxide perovskites and beyond,” Npj Comput. Mater. 2, 16026 (2016). https://doi.org/10.1038/npjcompumats.2016.26 Unlike our traditional interface photovoltaic effect where the electron–hole pair separation happens at the interface of a p–n junction due to an in-built electric field, in BPE, the separation of charges happens at the bulk due to the non-centrosymmetric nature of the crystal. Thus, in BPE, the whole bulk is an active region, which ensures a much easier way of engineering the fabrication process of photodetectors. Since the BPE does not depend on any in-built electric field as in the p–n junction, the photovoltage can go beyond the bandgap and, thus, could exceed the Shockley–Queisser limit.22. W. Shockley and H. J. Queisser, “ Detailed balance limit of efficiency of p-n junction solar cells,” J. Appl. Phys. 32, 510–519 (1961). https://doi.org/10.1063/1.1736034 The shift-current is one of the dominant contributors of BPE and has been observed in materials like ferroelectrics,3,43. A. Glass, D. V. D. Linde, and T. Negran, “ High-voltage bulk photovoltaic effect and the photorefractive process in linbo3,” in Landmark Papers on Photorefractive Nonlinear Optics ( World Scientific, Singapore, 1995), pp. 371–373.4. K. T. Butler, J. M. Frost, and A. Walsh, “ Ferroelectric materials for solar energy conversion: Photoferroics revisited,” Energy Environ. Sci. 8, 838–848 (2015). https://doi.org/10.1039/C4EE03523B organic crystals,55. T. Ogden and D. Gookin, “ Bulk photovoltaic effect in polyvinylidene fluoride,” Appl. Phys. Lett. 45, 995–997 (1984). https://doi.org/10.1063/1.95444 two-dimensional interfaces,66. M. Nakamura, F. Kagawa, T. Tanigaki, H. Park, T. Matsuda, D. Shindo, Y. Tokura, and M. Kawasaki, “ Spontaneous polarization and bulk photovoltaic effect driven by polar discontinuity in lafeo 3/srtio 3 heterojunctions,” Phys. Rev. Lett. 116, 156801 (2016). https://doi.org/10.1103/PhysRevLett.116.156801 and quantum wells.77. M. Bieler, K. Pierz, U. Siegner, and P. Dawson, “ Shift currents from symmetry reduction and coulomb effects in (110)-orientated gaas/al 0.3 ga 0.7 as quantum wells,” Phys. Rev. B 76, 161304 (2007). https://doi.org/10.1103/PhysRevB.76.161304 It has been found that Weyl semimetals show an anomalous photoresponse due to their topological nature of the band structure. Thus, the current challenge lies in the development of self-powered photodetectors at room temperature based on BPE.Weyl semimetals are topologically non-trivial objects that have a band touching point near the Fermi surface.8,98. S. Jia, S.-Y. Xu, and M. Z. Hasan, “ Weyl semimetals, Fermi arcs and chiral anomalies,” Nat. Mater. 15, 1140–1144 (2016). https://doi.org/10.1038/nmat47879. B. A. Bernevig, “ It's been a Weyl coming,” Nat. Phys. 11, 698–699 (2015). https://doi.org/10.1038/nphys3454 For a material to show Weyl semimetal characteristics, it should be a three-dimensional material and should either break the time-reversal symmetry or inversion symmetry.1010. A. Burkov, “ Topological semimetals,” Nat. Mater. 15, 1145–1148 (2016). https://doi.org/10.1038/nmat4788 Non-centrosymmetric Weyl semimetals that break the inversion symmetry have been identified as potential candidates for efficient self-powered photodetectors at room temperature.11,1211. G. B. Osterhoudt, L. K. Diebel, M. J. Gray, X. Yang, J. Stanco, X. Huang, B. Shen, N. Ni, P. J. Moll, Y. Ran et al., “ Colossal mid-infrared bulk photovoltaic effect in a type-i Weyl semimetal,” Nat. Mater. 18, 471–475 (2019). https://doi.org/10.1038/s41563-019-0297-412. J. Ma, Q. Gu, Y. Liu, J. Lai, P. Yu, X. Zhuo, Z. Liu, J.-H. Chen, J. Feng, and D. Sun, “ Nonlinear photoresponse of type-ii Weyl semimetals,” Nat. Mater. 18, 476–481 (2019). https://doi.org/10.1038/s41563-019-0296-5 The diverging and singular nature of the Weyl points in Weyl semimetals results in an anomalous bulk photovoltaic effect at an optical transition in the vicinity of Weyl nodes. The physics of this photodetection property is a matter of debate and, hence, we have explored possibility of employing a type-II Weyl semimetal Td-WTe2 as a self-powered photodetector at room temperature. It is to be noted that the bulk photovoltaic effect was already observed at the edges of Td-WTe2 due to the broken twofold rotational symmetry of the single crystal.1313. Q. Wang, J. Zheng, Y. He, J. Cao, X. Liu, M. Wang, J. Ma, J. Lai, H. Lu, S. Jia et al., “ Robust edge photocurrent response on layered type ii Weyl semimetal WTe2,” Nat. Commun. 10, 5736 (2019). https://doi.org/10.1038/s41467-019-13713-1 Here, we used a methodology of breaking the inversion symmetry of the a–b plane externally by laser illumination within the crystal instead of the edge. Upon laser illumination, Td-WTe2 generates a photothermoelectric (PTE) current due to temperature gradient developed across the crystal. Even though the PTE effect has already been reported in Td-WTe2, they have not been tried to sort out the contributions of BPE and PTE effect individually in the crystal.1414. Q. Wang, C. Yesilyurt, F. Liu, Z. B. Siu, K. Cai, D. Kumar, Z. Liu, M. B. Jalil, and H. Yang, “ Anomalous photothermoelectric transport due to anisotropic energy dispersion in WTe2,” Nano Lett. 19, 2647–2652 (2019). https://doi.org/10.1021/acs.nanolett.9b00513 Since the PTE effect and BPE are equally significant in noncentrosymmetric Weyl semimetals,1111. G. B. Osterhoudt, L. K. Diebel, M. J. Gray, X. Yang, J. Stanco, X. Huang, B. Shen, N. Ni, P. J. Moll, Y. Ran et al., “ Colossal mid-infrared bulk photovoltaic effect in a type-i Weyl semimetal,” Nat. Mater. 18, 471–475 (2019). https://doi.org/10.1038/s41563-019-0297-4 we have performed polarization-photocurrent measurements and observed the contribution of PTE effect and the circular photogalvanic effect separately from the non-linear shift current response in Td-WTe2.Td-WTe2 is a transition metal di-chalcogenide belonging to the Pmn21 space group with a mirror symmetry along the b–c crystal plane and a glide plane symmetry along the a–c crystal plane [as shown in Fig. S1(a) of the supplementary material, Section II].13,1513. Q. Wang, J. Zheng, Y. He, J. Cao, X. Liu, M. Wang, J. Ma, J. Lai, H. Lu, S. Jia et al., “ Robust edge photocurrent response on layered type ii Weyl semimetal WTe2,” Nat. Commun. 10, 5736 (2019). https://doi.org/10.1038/s41467-019-13713-115. C.-L. Lin, R. Arafune, R.-Y. Liu, M. Yoshimura, B. Feng, K. Kawahara, Z. Ni, E. Minamitani, S. Watanabe, Y. Shi et al., “ Visualizing type-ii Weyl points in tungsten ditelluride by quasiparticle interference,” ACS Nano 11, 11459–11465 (2017). https://doi.org/10.1021/acsnano.7b06179 The alignment of the crystal plane and the orientation of the crystal axes were found by using x-ray diffraction analysis (XRD) [Fig. S1(b)] and polarization resolved Raman spectroscopy (Fig. 1), respectively. A significant XRD peak at ∼12.9° [corresponding to (002) of WTe2] confirmed that the crystal is aligned along the a–b plane [Fig. S1(b)]. Angle dependent Raman spectra at ϕ=85° and 185° (angle ϕ is defined as the angle between laser polarization and the edge of the device) in Fig. S2 show significant variation in the intensity of all Raman peaks. Furthermore, the Raman measurement is carried out by varying ϕ values from 0° to 360°, and a polar plot of the polarization angle and the ratio of the intensities of the peaks at 164 and 212 cm−1 of the Raman spectrum is shown in Fig. 1(a). The intensity ratio variation shows the anisotropic behavior of the Td-WTe2 single crystal in the a–b plane. The I164/I212 intensity ratio in Raman spectra is calculated and found to be maximum when the laser polarization is along the long edge of the device, which reveals that the a-axis of the crystal is aligned in the long edge of the device, as illustrated in Fig. 1(b).1616. Q. Song, X. Pan, H. Wang, K. Zhang, Q. Tan, P. Li, Y. Wan, Y. Wang, X. Xu, M. Lin et al., “ The in-plane anisotropy of wte 2 investigated by angle-dependent and polarized Raman spectroscopy,” Sci. Rep. 6, 29254 (2016). https://doi.org/10.1038/srep29254A two-terminal photodetector device with a planar structure is fabricated as mentioned in the supplementary material, section (I). As shown in the schematic [Fig. S3(a)], an optoelectronic measurement setup using a linearly polarized laser of wavelength (λ) = 640 nm as the light source is used. The current–voltage (I–V) characteristics are measured using a computer controlled source measure unit (SMU). The measurements are done by aligning the crystallographic a-axis of the device along the polarization direction of the incident laser. As shown in Fig. S3(b), photoresponse in the range of ∼0.3 nA at an unbiased condition is obtained. Since there is no in-built electric field to separate the photo-generated carriers, the observed enhancement in the current under illumination can be due to the BPE, in particular, the shift current mechanism. The shift current mechanism relies on the bulk inversion symmetry of the material, and the driving force for the carrier separation in this mechanism is the coherent evolution of electron and hole wave functions. Figure 2 shows the I–V characteristics of the device in the presence of light (λ= 640 nm) and in the dark conditions. The linear and symmetric nature of the I–V characteristic with respect to the origin confirms that the electrical contacts are Ohmic in nature. This rules out the contribution of photocurrent originating from the Schottky junction.The origin of such a photocurrent is studied in detail by changing the polarization property of the illumination laser source using a quarter wave plate (QWP) placed in between the linearly polarized laser source and the photodetector device. The angle between the fast axis of the QWP and the initial laser polarization is set as θ. When the laser's polarization direction is parallel or perpendicular to the fast axis of QWP, the polarization does not change. At an angle, θ=π/4 the linearly polarized light becomes circularly polarized light due to a phase difference of ±π/2 between the parallel and perpendicular components. At all other intermediate angles, the linearly polarized light becomes elliptically polarized. The measurements are done using a 640 nm laser source with minimum incident light power (20 μW) to reduce the local heating. Figure 3(a) displays the polar plot of photoresponse under unbiased conditions and is highly anisotropic in nature. Figure 3(b) shows the plot of photocurrent vs θ. The curves are fitted with a function, A+B sin θ+CT cos 4θ+CS sin 4θ+D cos 2θ. Under the localized laser illumination, the temperature gradient can be generated on the device which further creates a potential via Seebeck effect. The shift current parallel to the developed electric field (EDC) due to Seebeck effects is referred to as longitudinal shift current, and that perpendicular to EDC as transverse shift current. Here, the coefficient of cos 4θ represents the contribution of the longitudinal shift current and the photothermoelectric current (PTE) due to Seebeck effects along the a-axis, whereas the coefficient of sin 4θ represents the contribution of the transverse shift current with periodicity π/2. The coefficient of cos 4θ and sin 4θ is obtained as represented by the red and green bar, respectively, as an inset in Fig. 3(b). As expected, since the photocurrent measurement was along the crystallographic a-axis, the PTE current and the longitudinal shift current obtained were four times larger than the transverse shift current. The observed photoresponse depends on the different generation mechanisms corresponding to the various processes that contribute to the photocurrent. Thus, the second- and third-order photoresponses are possible in this material. The contribution of the second order shift current is ruled out as the Td-WTe2 crystal is aligned in the a–b plane. This can be inferred from the fact that Td-WTe2 is a Pmn21 space group with an a–b plane that is centrosymmetric and the other two crystal planes with a broken inversion symmetry.1313. Q. Wang, J. Zheng, Y. He, J. Cao, X. Liu, M. Wang, J. Ma, J. Lai, H. Lu, S. Jia et al., “ Robust edge photocurrent response on layered type ii Weyl semimetal WTe2,” Nat. Commun. 10, 5736 (2019). https://doi.org/10.1038/s41467-019-13713-1 The second-order shift current response is given by1111. G. B. Osterhoudt, L. K. Diebel, M. J. Gray, X. Yang, J. Stanco, X. Huang, B. Shen, N. Ni, P. J. Moll, Y. Ran et al., “ Colossal mid-infrared bulk photovoltaic effect in a type-i Weyl semimetal,” Nat. Mater. 18, 471–475 (2019). https://doi.org/10.1038/s41563-019-0297-4 Jα=σ(2)αβγEβE*γ+c.c.(1)Here, σ(2)αβγ is the second-order responsivity tensor of rank three, and Eβ is the component of a linearly polarized electromagnetic field along the direction of the crystallographic axis. However, the inversion symmetry of the a–b plane can be broken using an in-built electric field developed from the thermal gradient due to Seebeck effects under localized illumination [photothermoelectric (PTE) effect] or due to the difference in the work function of the metal-semimetal contact.17,1817. N. M. Gabor, J. C. Song, Q. Ma, N. L. Nair, T. Taychatanapat, K. Watanabe, T. Taniguchi, L. S. Levitov, and P. Jarillo-Herrero, “ Hot carrier–assisted intrinsic photoresponse in graphene,” Science 334, 648–652 (2011). https://doi.org/10.1126/science.121138418. D. Sun, G. Aivazian, A. M. Jones, J. S. Ross, W. Yao, D. Cobden, and X. Xu, “ Ultrafast hot-carrier-dominated photocurrent in graphene,” Nat. Nanotechnol. 7, 114–118 (2012). https://doi.org/10.1038/nnano.2011.243 Since a gold (Au) contact was used on both sides, the latter can be neglected. The PTE effect originates from the difference in temperature between the decoupled charge carriers and the WTe2 lattice. In layered materials, this effect is common because of the inefficient interaction of phonons with the carriers. When an excited electron–hole pair undergoes non-radiative recombination, the amount of energy approximately equal to the band separation is transferred to the crystal lattice, which, in turn, increases the local temperature of the lattice. Under localized illumination, a non-uniform temperature distribution is established, and a voltage (EDC) is generated which is proportional to the temperature difference. In our case, it is rather difficult to quantify EDC as both photothermal current and shift current exist. Therefore, under our experimental configurations, the presence of shift current indicates that the inversion symmetry is broken by the electric field generated due to PTE effect. In effect the broken inversion symmetry leads to a non-zero second-order DC optical response along the a–b plane defined by a rank four tensor σαaβγ. In the rank four tensor, the second index represents the direction of the field EDC. Since the whole shift current phenomena are based on the developed EDC and a rank four tensor, the whole process becomes a third-order nonlinear shift current.1212. J. Ma, Q. Gu, Y. Liu, J. Lai, P. Yu, X. Zhuo, Z. Liu, J.-H. Chen, J. Feng, and D. Sun, “ Nonlinear photoresponse of type-ii Weyl semimetals,” Nat. Mater. 18, 476–481 (2019). https://doi.org/10.1038/s41563-019-0296-5 This accelerating DC field places the system in a non-equilibrium state, which acts as a source for accelerating the Bloch electrons by displacing the Fermi distribution. Thus, the photocurrent observed can be attributed to the third order shift current and PTE current generated in Td-WTe2. The third-order nonlinear shift current based on the semi-quantitative analysis is represented as1212. J. Ma, Q. Gu, Y. Liu, J. Lai, P. Yu, X. Zhuo, Z. Liu, J.-H. Chen, J. Feng, and D. Sun, “ Nonlinear photoresponse of type-ii Weyl semimetals,” Nat. Mater. 18, 476–481 (2019). https://doi.org/10.1038/s41563-019-0296-5 Jα=σ(3)αaβγ(0,ω,−ω;EDCa)Eβ(ω)Eγ(−ω)+σ(3)αaβγ(0,−ω,ω;EDCa)Eβ(−ω)Eγ(ω),(2)where σ(3)αaβγ(0,ω,−ω;EDCa) is the rank four responsivity tensor and the second term is the complex conjugate of the first term. The photocurrent measurements are done along the a-axis as mentioned previously with EDC aligned along the crystallographic a-axis.Furthermore, power dependent photocurrent measurements are done to understand the higher order contribution to the photocurrent. A quadratic photocurrent-power curve for both λ= 640 and 520 nm illumination is obtained as depicted in Fig. 4. The curve is fitted with a function f(P)=AP2+BP+C, where A, B, and C are some constants and P is the power of the laser source used. The quadratic nature of the power-photocurrent curve confirms that the origin of photocurrent can be a combined effect of second-order shift current, third-order shift current, and the PTE effect. The quadratic term in the function corresponds to the nonlinear third order bulk photovoltaic phenomena. As given in Eq. (2), the third order photocurrent depends on the square of the incident excitation light field and an additional electric field term EDC. Since the field EDC originates due to the Seebeck effect, it varies linearly as we increase the power of the incident light. In effect, the third order BPE effect has a quadratic dependence of power, which corresponds to the term AP2 in f(P). The linear term in the function f(P) arises due to the second order phenomena and the PTE effect, as it depends on the square of the electric field polarization of the laser.1212. J. Ma, Q. Gu, Y. Liu, J. Lai, P. Yu, X. Zhuo, Z. Liu, J.-H. Chen, J. Feng, and D. Sun, “ Nonlinear photoresponse of type-ii Weyl semimetals,” Nat. Mater. 18, 476–481 (2019). https://doi.org/10.1038/s41563-019-0296-5 Since the symmetry does not permit the second-order shift current to get effectively collected at the electrodes, the observed photocurrent is a contribution from the PTE effect. At the end, the quadratic power photocurrent curve confirms the presence of response due to PTE effect and third order nonlinear BPE. The quadratic power dependence of photocurrent is verified in a long range of laser power using a source of wavelength 640 nm [see the supplementary material, section (V), Figs. S4 and S5]. The device shows better photoresponsivity under an optical illumination of λ= 640 nm compared to 520 nm. The dominant nature of the photoresponsivity under the illumination of λ= 640 nm is due to the occurrence of the carrier excitation mainly in the vicinity of Weyl nodes compared to that of the optical transition under λ = 520 nm illumination. Figure S6 depicts the band structure of Td-WTe2 computed using quantum expresso by turning on the spin–orbit coupling.19,2019. P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. D. Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari, and R. M. Wentzcovitch, “ Quantum espresso: A modular and open-source software project for quantum simulations of materials,” J. Phys.: Condens. Matter 21, 395502 (2009). https://doi.org/10.1088/0953-8984/21/39/39550220. P. Giannozzi, O. Andreussi, T. Brumme, O. Bunau, M. B. Nardelli, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, M. Cococcioni et al., “ Advanced capabilities for materials modelling with quantum espresso,” J. Phys.: Condens. Matter 29, 465901 (2017). https://doi.org/10.1088/1361-648X/aa8f79 The possible optical transitions from the valence band at 520 nm is more toward the vicinity of the Weyl nodes on comparing the possible transitions at 640 nm. The diverging Berry curvature near the Weyl nodes enhances the separation of the electron–hole pair.The injection current or the circular photogalvanic effect (CPGE) is another contributor of BPE in Weyl semimetals.21,2221. J. Rioux, G. Burkard, and J. E. Sipe, “ Current injection by coherent one-and two-photon excitation in graphene and its bilayer,” Phys. Rev. B 83, 195406 (2011). https://doi.org/10.1103/PhysRevB.83.19540622. M. Bieler, K. Pierz, and U. Siegner, “ Simultaneous generation of shift and injection currents in (110)-grown ga as/al ga as quantum wells,” J. Appl. Phys. 100, 083710 (2006). https://doi.org/10.1063/1.2360380 The Fourier transform of the polarization-dependent photocurrent measurement was analyzed to get the CPGE response. In Fig. 5(a), the photocurrent with frequency 1/π denotes the injection current, which depends on the circular polarization. The photocurrent with frequency 2/π represents the in-plane anisotropic response, and the photocurrent with zero frequency shows the component of photocurrent independent of the polarization. The CPGE response from the Weyl cone is due to the chirality selection rule as illustrated in Fig. 5(b), explained based on the conservation of angular momentum.23,2423. Q. Ma, S.-Y. Xu, C.-K. Chan, C.-L. Zhang, G. Chang, Y. Lin, W. Xie, T. Palacios, H. Lin, S. Jia et al., “ Direct optical detection of Weyl fermion chirality in a topological semimetal,” Nat. Phys. 13, 842–847 (2017). https://doi.org/10.1038/nphys414624. R. Yu, H. Weng, Z. Fang, H. Ding, and X. Dai, “ Determining the chirality of Weyl fermions from circular dichroism spectra in time-dependent angle-resolved photoemission,” Phys. Rev. B 93, 205133 (2016). https://doi.org/10.1103/PhysRevB.93.205133 As a consequence of the chiral selection rule and the tilt of the Fermi level due to the in-built field EDC, the carriers are injected asymmetrically into the conduction band, giving rise to an imbalance in the carriers that move to the right and left. Since the angular interval between two consecutive right circular polarizations of a QWP is π, the CPGE response is periodic with an angular frequency 1/π.

In summary, self-powered bulk photovoltaic effects in non-centrosymmetric type-II Weyl semimetal Td-WTe2 are observed using a laser of excitation λ= 640 nm at room temperature. The photocurrent observed is in a range of ∼0.3 nA. The component of transverse shift current of periodicity π/2 was separated from the longitudinal shift current and the PTE. It is impossible to separate the PTE current from the longitudinal shift current due to the symmetric properties of the a–b plane in WTe2. Also, the contribution of circular photogalvanic effect was sorted out from the linear photogalvanic effect by Fourier analysis of the polarization-dependent photocurrent.

See the supplementary material for the experimental methods, x-ray diffraction, Raman spectroscopy, time-resolved photocurrent, and atomic force microscopic data. It also includes the experimental methods, band structure computation of WTe2 using quantum expresso, and an explanation for different possible photoresponses.

The authors acknowledge Dr. Madhu Thalakulam, Mr. S. Hari Krishnan, and Mr. A. Muhammed Raees for lithographic patterning. V.K.P. acknowledges the Council of Scientific and Industrial Research (CSIR), India for financial support. The authors acknowledge financial support from the Scheme for Transformational and Advanced Research in Sciences (STARS) (Grant No. STARS/APR2019/PS/308/FS) funded by the Ministry of Education (Government of India) and the Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM), Kerala, India.

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Albert Mathew: Conceptualization (lead); Data curation (equal); Formal analysis (lead); Investigation (lead); Methodology (equal); Software (lead); Visualization (equal); Writing – original draft (lead). Vijith K. Pulikodan: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (lead); Writing – review & editing (equal). Manoj A. G. Namboothiry: Conceptualization (equal); Funding acquisition (lead); Project administration (lead); Resources (lead); Supervision (lead); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.

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